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Keywords: RTP, end-to-end, network, audio, video, RTCP
Network Working Group Audio-Video Transport Working Group
Request for Comments: 1889 H. Schulzrinne
Category: Standards Track GMD Fokus
S. Casner
Precept Software, Inc.
R. Frederick
Xerox Palo Alto Research Center
V. Jacobson
Lawrence Berkeley National Laboratory
January 1996
RTP: A Transport Protocol for Real-Time Applications
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This memorandum describes RTP, the real-time transport protocol. RTP
provides end-to-end network transport functions suitable for
applications transmitting real-time data, such as audio, video or
simulation data, over multicast or unicast network services. RTP does
not address resource reservation and does not guarantee quality-of-
service for real-time services. The data transport is augmented by a
control protocol (RTCP) to allow monitoring of the data delivery in a
manner scalable to large multicast networks, and to provide minimal
control and identification functionality. RTP and RTCP are designed
to be independent of the underlying transport and network layers. The
protocol supports the use of RTP-level translators and mixers.
Table of Contents
1. Introduction ........................................ 3
2. RTP Use Scenarios ................................... 5
2.1 Simple Multicast Audio Conference ................... 5
2.2 Audio and Video Conference .......................... 6
2.3 Mixers and Translators .............................. 6
3. Definitions ......................................... 7
4. Byte Order, Alignment, and Time Format .............. 9
5. RTP Data Transfer Protocol .......................... 10
5.1 RTP Fixed Header Fields ............................. 10
5.2 Multiplexing RTP Sessions ........................... 13
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RFC 1889 RTP January 1996
5.3 Profile-Specific Modifications to the RTP Header..... 14
5.3.1 RTP Header Extension ................................ 14
6. RTP Control Protocol -- RTCP ........................ 15
6.1 RTCP Packet Format .................................. 17
6.2 RTCP Transmission Interval .......................... 19
6.2.1 Maintaining the number of session members ........... 21
6.2.2 Allocation of source description bandwidth .......... 21
6.3 Sender and Receiver Reports ......................... 22
6.3.1 SR: Sender report RTCP packet ....................... 23
6.3.2 RR: Receiver report RTCP packet ..................... 28
6.3.3 Extending the sender and receiver reports ........... 29
6.3.4 Analyzing sender and receiver reports ............... 29
6.4 SDES: Source description RTCP packet ................ 31
6.4.1 CNAME: Canonical end-point identifier SDES item ..... 32
6.4.2 NAME: User name SDES item ........................... 34
6.4.3 EMAIL: Electronic mail address SDES item ............ 34
6.4.4 PHONE: Phone number SDES item ....................... 34
6.4.5 LOC: Geographic user location SDES item ............. 35
6.4.6 TOOL: Application or tool name SDES item ............ 35
6.4.7 NOTE: Notice/status SDES item ....................... 35
6.4.8 PRIV: Private extensions SDES item .................. 36
6.5 BYE: Goodbye RTCP packet ............................ 37
6.6 APP: Application-defined RTCP packet ................ 38
7. RTP Translators and Mixers .......................... 39
7.1 General Description ................................. 39
7.2 RTCP Processing in Translators ...................... 41
7.3 RTCP Processing in Mixers ........................... 43
7.4 Cascaded Mixers ..................................... 44
8. SSRC Identifier Allocation and Use .................. 44
8.1 Probability of Collision ............................ 44
8.2 Collision Resolution and Loop Detection ............. 45
9. Security ............................................ 49
9.1 Confidentiality ..................................... 49
9.2 Authentication and Message Integrity ................ 50
10. RTP over Network and Transport Protocols ............ 51
11. Summary of Protocol Constants ....................... 51
11.1 RTCP packet types ................................... 52
11.2 SDES types .......................................... 52
12. RTP Profiles and Payload Format Specifications ...... 53
A. Algorithms .......................................... 56
A.1 RTP Data Header Validity Checks ..................... 59
A.2 RTCP Header Validity Checks ......................... 63
A.3 Determining the Number of RTP Packets Expected and
Lost ................................................ 63
A.4 Generating SDES RTCP Packets ........................ 64
A.5 Parsing RTCP SDES Packets ........................... 65
A.6 Generating a Random 32-bit Identifier ............... 66
A.7 Computing the RTCP Transmission Interval ............ 68
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A.8 Estimating the Interarrival Jitter .................. 71
B. Security Considerations ............................. 72
C. Addresses of Authors ................................ 72
D. Bibliography ........................................ 73
1. Introduction
This memorandum specifies the real-time transport protocol (RTP),
which provides end-to-end delivery services for data with real-time
characteristics, such as interactive audio and video. Those services
include payload type identification, sequence numbering, timestamping
and delivery monitoring. Applications typically run RTP on top of UDP
to make use of its multiplexing and checksum services; both protocols
contribute parts of the transport protocol functionality. However,
RTP may be used with other suitable underlying network or transport
protocols (see Section 10). RTP supports data transfer to multiple
destinations using multicast distribution if provided by the
underlying network.
Note that RTP itself does not provide any mechanism to ensure timely
delivery or provide other quality-of-service guarantees, but relies
on lower-layer services to do so. It does not guarantee delivery or
prevent out-of-order delivery, nor does it assume that the underlying
network is reliable and delivers packets in sequence. The sequence
numbers included in RTP allow the receiver to reconstruct the
sender's packet sequence, but sequence numbers might also be used to
determine the proper location of a packet, for example in video
decoding, without necessarily decoding packets in sequence.
While RTP is primarily designed to satisfy the needs of multi-
participant multimedia conferences, it is not limited to that
particular application. Storage of continuous data, interactive
distributed simulation, active badge, and control and measurement
applications may also find RTP applicable.
This document defines RTP, consisting of two closely-linked parts:
o the real-time transport protocol (RTP), to carry data that has
real-time properties.
o the RTP control protocol (RTCP), to monitor the quality of
service and to convey information about the participants in an
on-going session. The latter aspect of RTCP may be sufficient
for "loosely controlled" sessions, i.e., where there is no
explicit membership control and set-up, but it is not
necessarily intended to support all of an application's control
communication requirements. This functionality may be fully or
partially subsumed by a separate session control protocol,
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which is beyond the scope of this document.
RTP represents a new style of protocol following the principles of
application level framing and integrated layer processing proposed by
Clark and Tennenhouse [1]. That is, RTP is intended to be malleable
to provide the information required by a particular application and
will often be integrated into the application processing rather than
being implemented as a separate layer. RTP is a protocol framework
that is deliberately not complete. This document specifies those
functions expected to be common across all the applications for which
RTP would be appropriate. Unlike conventional protocols in which
additional functions might be accommodated by making the protocol
more general or by adding an option mechanism that would require
parsing, RTP is intended to be tailored through modifications and/or
additions to the headers as needed. Examples are given in Sections
5.3 and 6.3.3.
Therefore, in addition to this document, a complete specification of
RTP for a particular application will require one or more companion
documents (see Section 12):
o a profile specification document, which defines a set of
payload type codes and their mapping to payload formats (e.g.,
media encodings). A profile may also define extensions or
modifications to RTP that are specific to a particular class of
applications. Typically an application will operate under only
one profile. A profile for audio and video data may be found in
the companion RFC TBD.
o payload format specification documents, which define how a
particular payload, such as an audio or video encoding, is to
be carried in RTP.
A discussion of real-time services and algorithms for their
implementation as well as background discussion on some of the RTP
design decisions can be found in [2].
Several RTP applications, both experimental and commercial, have
already been implemented from draft specifications. These
applications include audio and video tools along with diagnostic
tools such as traffic monitors. Users of these tools number in the
thousands. However, the current Internet cannot yet support the full
potential demand for real-time services. High-bandwidth services
using RTP, such as video, can potentially seriously degrade the
quality of service of other network services. Thus, implementors
should take appropriate precautions to limit accidental bandwidth
usage. Application documentation should clearly outline the
limitations and possible operational impact of high-bandwidth real-
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time services on the Internet and other network services.
2. RTP Use Scenarios
The following sections describe some aspects of the use of RTP. The
examples were chosen to illustrate the basic operation of
applications using RTP, not to limit what RTP may be used for. In
these examples, RTP is carried on top of IP and UDP, and follows the
conventions established by the profile for audio and video specified
in the companion Internet-Draft draft-ietf-avt-profile
2.1 Simple Multicast Audio Conference
A working group of the IETF meets to discuss the latest protocol
draft, using the IP multicast services of the Internet for voice
communications. Through some allocation mechanism the working group
chair obtains a multicast group address and pair of ports. One port
is used for audio data, and the other is used for control (RTCP)
packets. This address and port information is distributed to the
intended participants. If privacy is desired, the data and control
packets may be encrypted as specified in Section 9.1, in which case
an encryption key must also be generated and distributed. The exact
details of these allocation and distribution mechanisms are beyond
the scope of RTP.
The audio conferencing application used by each conference
participant sends audio data in small chunks of, say, 20 ms duration.
Each chunk of audio data is preceded by an RTP header; RTP header and
data are in turn contained in a UDP packet. The RTP header indicates
what type of audio encoding (such as PCM, ADPCM or LPC) is contained
in each packet so that senders can change the encoding during a
conference, for example, to accommodate a new participant that is
connected through a low-bandwidth link or react to indications of
network congestion.
The Internet, like other packet networks, occasionally loses and
reorders packets and delays them by variable amounts of time. To cope
with these impairments, the RTP header contains timing information
and a sequence number that allow the receivers to reconstruct the
timing produced by the source, so that in this example, chunks of
audio are contiguously played out the speaker every 20 ms. This
timing reconstruction is performed separately for each source of RTP
packets in the conference. The sequence number can also be used by
the receiver to estimate how many packets are being lost.
Since members of the working group join and leave during the
conference, it is useful to know who is participating at any moment
and how well they are receiving the audio data. For that purpose,
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each instance of the audio application in the conference periodically
multicasts a reception report plus the name of its user on the RTCP
(control) port. The reception report indicates how well the current
speaker is being received and may be used to control adaptive
encodings. In addition to the user name, other identifying
information may also be included subject to control bandwidth limits.
A site sends the RTCP BYE packet (Section 6.5) when it leaves the
conference.
2.2 Audio and Video Conference
If both audio and video media are used in a conference, they are
transmitted as separate RTP sessions RTCP packets are transmitted for
each medium using two different UDP port pairs and/or multicast
addresses. There is no direct coupling at the RTP level between the
audio and video sessions, except that a user participating in both
sessions should use the same distinguished (canonical) name in the
RTCP packets for both so that the sessions can be associated.
One motivation for this separation is to allow some participants in
the conference to receive only one medium if they choose. Further
explanation is given in Section 5.2. Despite the separation,
synchronized playback of a source's audio and video can be achieved
using timing information carried in the RTCP packets for both
sessions.
2.3 Mixers and Translators
So far, we have assumed that all sites want to receive media data in
the same format. However, this may not always be appropriate.
Consider the case where participants in one area are connected
through a low-speed link to the majority of the conference
participants who enjoy high-speed network access. Instead of forcing
everyone to use a lower-bandwidth, reduced-quality audio encoding, an
RTP-level relay called a mixer may be placed near the low-bandwidth
area. This mixer resynchronizes incoming audio packets to reconstruct
the constant 20 ms spacing generated by the sender, mixes these
reconstructed audio streams into a single stream, translates the
audio encoding to a lower-bandwidth one and forwards the lower-
bandwidth packet stream across the low-speed link. These packets
might be unicast to a single recipient or multicast on a different
address to multiple recipients. The RTP header includes a means for
mixers to identify the sources that contributed to a mixed packet so
that correct talker indication can be provided at the receivers.
Some of the intended participants in the audio conference may be
connected with high bandwidth links but might not be directly
reachable via IP multicast. For example, they might be behind an
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application-level firewall that will not let any IP packets pass. For
these sites, mixing may not be necessary, in which case another type
of RTP-level relay called a translator may be used. Two translators
are installed, one on either side of the firewall, with the outside
one funneling all multicast packets received through a secure
connection to the translator inside the firewall. The translator
inside the firewall sends them again as multicast packets to a
multicast group restricted to the site's internal network.
Mixers and translators may be designed for a variety of purposes. An
example is a video mixer that scales the images of individual people
in separate video streams and composites them into one video stream
to simulate a group scene. Other examples of translation include the
connection of a group of hosts speaking only IP/UDP to a group of
hosts that understand only ST-II, or the packet-by-packet encoding
translation of video streams from individual sources without
resynchronization or mixing. Details of the operation of mixers and
translators are given in Section 7.
3. Definitions
RTP payload: The data transported by RTP in a packet, for example
audio samples or compressed video data. The payload format and
interpretation are beyond the scope of this document.
RTP packet: A data packet consisting of the fixed RTP header, a
possibly empty list of contributing sources (see below), and the
payload data. Some underlying protocols may require an
encapsulation of the RTP packet to be defined. Typically one
packet of the underlying protocol contains a single RTP packet,
but several RTP packets may be contained if permitted by the
encapsulation method (see Section 10).
RTCP packet: A control packet consisting of a fixed header part
similar to that of RTP data packets, followed by structured
elements that vary depending upon the RTCP packet type. The
formats are defined in Section 6. Typically, multiple RTCP
packets are sent together as a compound RTCP packet in a single
packet of the underlying protocol; this is enabled by the length
field in the fixed header of each RTCP packet.
Port: The "abstraction that transport protocols use to distinguish
among multiple destinations within a given host computer. TCP/IP
protocols identify ports using small positive integers." [3] The
transport selectors (TSEL) used by the OSI transport layer are
equivalent to ports. RTP depends upon the lower-layer protocol
to provide some mechanism such as ports to multiplex the RTP and
RTCP packets of a session.
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Transport address: The combination of a network address and port that
identifies a transport-level endpoint, for example an IP address
and a UDP port. Packets are transmitted from a source transport
address to a destination transport address.
RTP session: The association among a set of participants
communicating with RTP. For each participant, the session is
defined by a particular pair of destination transport addresses
(one network address plus a port pair for RTP and RTCP). The
destination transport address pair may be common for all
participants, as in the case of IP multicast, or may be
different for each, as in the case of individual unicast network
addresses plus a common port pair. In a multimedia session,
each medium is carried in a separate RTP session with its own
RTCP packets. The multiple RTP sessions are distinguished by
different port number pairs and/or different multicast
addresses.
Synchronization source (SSRC): The source of a stream of RTP packets,
identified by a 32-bit numeric SSRC identifier carried in the
RTP header so as not to be dependent upon the network address.
All packets from a synchronization source form part of the same
timing and sequence number space, so a receiver groups packets
by synchronization source for playback. Examples of
synchronization sources include the sender of a stream of
packets derived from a signal source such as a microphone or a
camera, or an RTP mixer (see below). A synchronization source
may change its data format, e.g., audio encoding, over time. The
SSRC identifier is a randomly chosen value meant to be globally
unique within a particular RTP session (see Section 8). A
participant need not use the same SSRC identifier for all the
RTP sessions in a multimedia session; the binding of the SSRC
identifiers is provided through RTCP (see Section 6.4.1). If a
participant generates multiple streams in one RTP session, for
example from separate video cameras, each must be identified as
a different SSRC.
Contributing source (CSRC): A source of a stream of RTP packets that
has contributed to the combined stream produced by an RTP mixer
(see below). The mixer inserts a list of the SSRC identifiers of
the sources that contributed to the generation of a particular
packet into the RTP header of that packet. This list is called
the CSRC list. An example application is audio conferencing
where a mixer indicates all the talkers whose speech was
combined to produce the outgoing packet, allowing the receiver
to indicate the current talker, even though all the audio
packets contain the same SSRC identifier (that of the mixer).
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End system: An application that generates the content to be sent in
RTP packets and/or consumes the content of received RTP packets.
An end system can act as one or more synchronization sources in
a particular RTP session, but typically only one.
Mixer: An intermediate system that receives RTP packets from one or
more sources, possibly changes the data format, combines the
packets in some manner and then forwards a new RTP packet. Since
the timing among multiple input sources will not generally be
synchronized, the mixer will make timing adjustments among the
streams and generate its own timing for the combined stream.
Thus, all data packets originating from a mixer will be
identified as having the mixer as their synchronization source.
Translator: An intermediate system that forwards RTP packets with
their synchronization source identifier intact. Examples of
translators include devices that convert encodings without
mixing, replicators from multicast to unicast, and application-
level filters in firewalls.
Monitor: An application that receives RTCP packets sent by
participants in an RTP session, in particular the reception
reports, and estimates the current quality of service for
distribution monitoring, fault diagnosis and long-term
statistics. The monitor function is likely to be built into the
application(s) participating in the session, but may also be a
separate application that does not otherwise participate and
does not send or receive the RTP data packets. These are called
third party monitors.
Non-RTP means: Protocols and mechanisms that may be needed in
addition to RTP to provide a usable service. In particular, for
multimedia conferences, a conference control application may
distribute multicast addresses and keys for encryption,
negotiate the encryption algorithm to be used, and define
dynamic mappings between RTP payload type values and the payload
formats they represent for formats that do not have a predefined
payload type value. For simple applications, electronic mail or
a conference database may also be used. The specification of
such protocols and mechanisms is outside the scope of this
document.
4. Byte Order, Alignment, and Time Format
All integer fields are carried in network byte order, that is, most
significant byte (octet) first. This byte order is commonly known as
big-endian. The transmission order is described in detail in [4].
Unless otherwise noted, numeric constants are in decimal (base 10).
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RFC 1889 RTP January 1996
All header data is aligned to its natural length, i.e., 16-bit fields
are aligned on even offsets, 32-bit fields are aligned at offsets
divisible by four, etc. Octets designated as padding have the value
zero.
Wallclock time (absolute time) is represented using the timestamp
format of the Network Time Protocol (NTP), which is in seconds
relative to 0h UTC on 1 January 1900 [5]. The full resolution NTP
timestamp is a 64-bit unsigned fixed-point number with the integer
part in the first 32 bits and the fractional part in the last 32
bits. In some fields where a more compact representation is
appropriate, only the middle 32 bits are used; that is, the low 16
bits of the integer part and the high 16 bits of the fractional part.
The high 16 bits of the integer part must be determined
independently.
5. RTP Data Transfer Protocol
5.1 RTP Fixed Header Fields
The RTP header has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first twelve octets are present in every RTP packet, while the
list of CSRC identifiers is present only when inserted by a mixer.
The fields have the following meaning:
version (V): 2 bits
This field identifies the version of RTP. The version defined by
this specification is two (2). (The value 1 is used by the first
draft version of RTP and the value 0 is used by the protocol
initially implemented in the "vat" audio tool.)
padding (P): 1 bit
If the padding bit is set, the packet contains one or more
additional padding octets at the end which are not part of the
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payload. The last octet of the padding contains a count of how
many padding octets should be ignored. Padding may be needed by
some encryption algorithms with fixed block sizes or for
carrying several RTP packets in a lower-layer protocol data
unit.
extension (X): 1 bit
If the extension bit is set, the fixed header is followed by
exactly one header extension, with a format defined in Section
5.3.1.
CSRC count (CC): 4 bits
The CSRC count contains the number of CSRC identifiers that
follow the fixed header.
marker (M): 1 bit
The interpretation of the marker is defined by a profile. It is
intended to allow significant events such as frame boundaries to
be marked in the packet stream. A profile may define additional
marker bits or specify that there is no marker bit by changing
the number of bits in the payload type field (see Section 5.3).
payload type (PT): 7 bits
This field identifies the format of the RTP payload and
determines its interpretation by the application. A profile
specifies a default static mapping of payload type codes to
payload formats. Additional payload type codes may be defined
dynamically through non-RTP means (see Section 3). An initial
set of default mappings for audio and video is specified in the
companion profile Internet-Draft draft-ietf-avt-profile, and
may be extended in future editions of the Assigned Numbers RFC
[6]. An RTP sender emits a single RTP payload type at any given
time; this field is not intended for multiplexing separate media
streams (see Section 5.2).
sequence number: 16 bits
The sequence number increments by one for each RTP data packet
sent, and may be used by the receiver to detect packet loss and
to restore packet sequence. The initial value of the sequence
number is random (unpredictable) to make known-plaintext attacks
on encryption more difficult, even if the source itself does not
encrypt, because the packets may flow through a translator that
does. Techniques for choosing unpredictable numbers are
discussed in [7].
timestamp: 32 bits
The timestamp reflects the sampling instant of the first octet
in the RTP data packet. The sampling instant must be derived
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from a clock that increments monotonically and linearly in time
to allow synchronization and jitter calculations (see Section
6.3.1). The resolution of the clock must be sufficient for the
desired synchronization accuracy and for measuring packet
arrival jitter (one tick per video frame is typically not
sufficient). The clock frequency is dependent on the format of
data carried as payload and is specified statically in the
profile or payload format specification that defines the format,
or may be specified dynamically for payload formats defined
through non-RTP means. If RTP packets are generated
periodically, the nominal sampling instant as determined from
the sampling clock is to be used, not a reading of the system
clock. As an example, for fixed-rate audio the timestamp clock
would likely increment by one for each sampling period. If an
audio application reads blocks covering 160 sampling periods
from the input device, the timestamp would be increased by 160
for each such block, regardless of whether the block is
transmitted in a packet or dropped as silent.
The initial value of the timestamp is random, as for the sequence
number. Several consecutive RTP packets may have equal timestamps if
they are (logically) generated at once, e.g., belong to the same
video frame. Consecutive RTP packets may contain timestamps that are
not monotonic if the data is not transmitted in the order it was
sampled, as in the case of MPEG interpolated video frames. (The
sequence numbers of the packets as transmitted will still be
monotonic.)
SSRC: 32 bits
The SSRC field identifies the synchronization source. This
identifier is chosen randomly, with the intent that no two
synchronization sources within the same RTP session will have
the same SSRC identifier. An example algorithm for generating a
random identifier is presented in Appendix A.6. Although the
probability of multiple sources choosing the same identifier is
low, all RTP implementations must be prepared to detect and
resolve collisions. Section 8 describes the probability of
collision along with a mechanism for resolving collisions and
detecting RTP-level forwarding loops based on the uniqueness of
the SSRC identifier. If a source changes its source transport
address, it must also choose a new SSRC identifier to avoid
being interpreted as a looped source.
CSRC list: 0 to 15 items, 32 bits each
The CSRC list identifies the contributing sources for the
payload contained in this packet. The number of identifiers is
given by the CC field. If there are more than 15 contributing
sources, only 15 may be identified. CSRC identifiers are
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inserted by mixers, using the SSRC identifiers of contributing
sources. For example, for audio packets the SSRC identifiers of
all sources that were mixed together to create a packet are
listed, allowing correct talker indication at the receiver.
5.2 Multiplexing RTP Sessions
For efficient protocol processing, the number of multiplexing points
should be minimized, as described in the integrated layer processing
design principle [1]. In RTP, multiplexing is provided by the
destination transport address (network address and port number) which
define an RTP session. For example, in a teleconference composed of
audio and video media encoded separately, each medium should be
carried in a separate RTP session with its own destination transport
address. It is not intended that the audio and video be carried in a
single RTP session and demultiplexed based on the payload type or
SSRC fields. Interleaving packets with different payload types but
using the same SSRC would introduce several problems:
1. If one payload type were switched during a session, there
would be no general means to identify which of the old
values the new one replaced.
2. An SSRC is defined to identify a single timing and sequence
number space. Interleaving multiple payload types would
require different timing spaces if the media clock rates
differ and would require different sequence number spaces
to tell which payload type suffered packet loss.
3. The RTCP sender and receiver reports (see Section 6.3) can
only describe one timing and sequence number space per SSRC
and do not carry a payload type field.
4. An RTP mixer would not be able to combine interleaved
streams of incompatible media into one stream.
5. Carrying multiple media in one RTP session precludes: the
use of different network paths or network resource
allocations if appropriate; reception of a subset of the
media if desired, for example just audio if video would
exceed the available bandwidth; and receiver
implementations that use separate processes for the
different media, whereas using separate RTP sessions
permits either single- or multiple-process implementations.
Using a different SSRC for each medium but sending them in the same
RTP session would avoid the first three problems but not the last
two.
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5.3 Profile-Specific Modifications to the RTP Header
The existing RTP data packet header is believed to be complete for
the set of functions required in common across all the application
classes that RTP might support. However, in keeping with the ALF
design principle, the header may be tailored through modifications or
additions defined in a profile specification while still allowing
profile-independent monitoring and recording tools to function.
o The marker bit and payload type field carry profile-specific
information, but they are allocated in the fixed header since
many applications are expected to need them and might otherwise
have to add another 32-bit word just to hold them. The octet
containing these fields may be redefined by a profile to suit
different requirements, for example with a more or fewer marker
bits. If there are any marker bits, one should be located in
the most significant bit of the octet since profile-independent
monitors may be able to observe a correlation between packet
loss patterns and the marker bit.
o Additional information that is required for a particular
payload format, such as a video encoding, should be carried in
the payload section of the packet. This might be in a header
that is always present at the start of the payload section, or
might be indicated by a reserved value in the data pattern.
o If a particular class of applications needs additional
functionality independent of payload format, the profile under
which those applications operate should define additional fixed
fields to follow immediately after the SSRC field of the
existing fixed header. Those applications will be able to
quickly and directly access the additional fields while
profile-independent monitors or recorders can still process the
RTP packets by interpreting only the first twelve octets.
If it turns out that additional functionality is needed in common
across all profiles, then a new version of RTP should be defined to
make a permanent change to the fixed header.
5.3.1 RTP Header Extension
An extension mechanism is provided to allow individual
implementations to experiment with new payload-format-independent
functions that require additional information to be carried in the
RTP data packet header. This mechanism is designed so that the header
extension may be ignored by other interoperating implementations that
have not been extended.
Schulzrinne, et al Standards Track [Page 14]
RFC 1889 RTP January 1996
Note that this header extension is intended only for limited use.
Most potential uses of this mechanism would be better done another
way, using the methods described in the previous section. For
example, a profile-specific extension to the fixed header is less
expensive to process because it is not conditional nor in a variable
location. Additional information required for a particular payload
format should not use this header extension, but should be carried in
the payload section of the packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| defined by profile | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extension |
| .... |
If the X bit in the RTP header is one, a variable-length header
extension is appended to the RTP header, following the CSRC list if
present. The header extension contains a 16-bit length field that
counts the number of 32-bit words in the extension, excluding the
four-octet extension header (therefore zero is a valid length). Only
a single extension may be appended to the RTP data header. To allow
multiple interoperating implementations to each experiment
independently with different header extensions, or to allow a
particular implementation to experiment with more than one type of
header extension, the first 16 bits of the header extension are left
open for distinguishing identifiers or parameters. The format of
these 16 bits is to be defined by the profile specification under
which the implementations are operating. This RTP specification does
not define any header extensions itself.
6. RTP Control Protocol -- RTCP
The RTP control protocol (RTCP) is based on the periodic transmission
of control packets to all participants in the session, using the same
distribution mechanism as the data packets. The underlying protocol
must provide multiplexing of the data and control packets, for
example using separate port numbers with UDP. RTCP performs four
functions:
1. The primary function is to provide feedback on the quality
of the data distribution. This is an integral part of the
RTP's role as a transport protocol and is related to the
flow and congestion control functions of other transport
protocols. The feedback may be directly useful for control
of adaptive encodings [8,9], but experiments with IP
Schulzrinne, et al Standards Track [Page 15]
RFC 1889 RTP January 1996
multicasting have shown that it is also critical to get
feedback from the receivers to diagnose faults in the
distribution. Sending reception feedback reports to all
participants allows one who is observing problems to
evaluate whether those problems are local or global. With a
distribution mechanism like IP multicast, it is also
possible for an entity such as a network service provider
who is not otherwise involved in the session to receive the
feedback information and act as a third-party monitor to
diagnose network problems. This feedback function is
performed by the RTCP sender and receiver reports,
described below in Section 6.3.
2. RTCP carries a persistent transport-level identifier for an
RTP source called the canonical name or CNAME, Section
6.4.1. Since the SSRC identifier may change if a conflict
is discovered or a program is restarted, receivers require
the CNAME to keep track of each participant. Receivers also
require the CNAME to associate multiple data streams from a
given participant in a set of related RTP sessions, for
example to synchronize audio and video.
3. The first two functions require that all participants send
RTCP packets, therefore the rate must be controlled in
order for RTP to scale up to a large number of
participants. By having each participant send its control
packets to all the others, each can independently observe
the number of participants. This number is used to
calculate the rate at which the packets are sent, as
explained in Section 6.2.
4. A fourth, optional function is to convey minimal session
control information, for example participant identification
to be displayed in the user interface. This is most likely
to be useful in "loosely controlled" sessions where
participants enter and leave without membership control or
parameter negotiation. RTCP serves as a convenient channel
to reach all the participants, but it is not necessarily
expected to support all the control communication
requirements of an application. A higher-level session
control protocol, which is beyond the scope of this
document, may be needed.
Functions 1-3 are mandatory when RTP is used in the IP multicast
environment, and are recommended for all environments. RTP
application designers are advised to avoid mechanisms that can only
work in unicast mode and will not scale to larger numbers.
Schulzrinne, et al Standards Track [Page 16]
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6.1 RTCP Packet Format
This specification defines several RTCP packet types to carry a
variety of control information:
SR: Sender report, for transmission and reception statistics from
participants that are active senders
RR: Receiver report, for reception statistics from participants that
are not active senders
SDES: Source description items, including CNAME
BYE: Indicates end of participation
APP: Application specific functions
Each RTCP packet begins with a fixed part similar to that of RTP data
packets, followed by structured elements that may be of variable
length according to the packet type but always end on a 32-bit
boundary. The alignment requirement and a length field in the fixed
part are included to make RTCP packets "stackable". Multiple RTCP
packets may be concatenated without any intervening separators to
form a compound RTCP packet that is sent in a single packet of the
lower layer protocol, for example UDP. There is no explicit count of
individual RTCP packets in the compound packet since the lower layer
protocols are expected to provide an overall length to determine the
end of the compound packet.
Each individual RTCP packet in the compound packet may be processed
independently with no requirements upon the order or combination of
packets. However, in order to perform the functions of the protocol,
the following constraints are imposed:
o Reception statistics (in SR or RR) should be sent as often as
bandwidth constraints will allow to maximize the resolution of
the statistics, therefore each periodically transmitted
compound RTCP packet should include a report packet.
o New receivers need to receive the CNAME for a source as soon
as possible to identify the source and to begin associating
media for purposes such as lip-sync, so each compound RTCP
packet should also include the SDES CNAME.
o The number of packet types that may appear first in the
compound packet should be limited to increase the number of
constant bits in the first word and the probability of
successfully validating RTCP packets against misaddressed RTP
Schulzrinne, et al Standards Track [Page 17]
RFC 1889 RTP January 1996
data packets or other unrelated packets.
Thus, all RTCP packets must be sent in a compound packet of at least
two individual packets, with the following format recommended:
Encryption prefix: If and only if the compound packet is to be
encrypted, it is prefixed by a random 32-bit quantity redrawn
for every compound packet transmitted.
SR or RR: The first RTCP packet in the compound packet must always
be a report packet to facilitate header validation as described
in Appendix A.2. This is true even if no data has been sent nor
received, in which case an empty RR is sent, and even if the
only other RTCP packet in the compound packet is a BYE.
Additional RRs: If the number of sources for which reception
statistics are being reported exceeds 31, the number that will
fit into one SR or RR packet, then additional RR packets should
follow the initial report packet.
SDES: An SDES packet containing a CNAME item must be included in
each compound RTCP packet. Other source description items may
optionally be included if required by a particular application,
subject to bandwidth constraints (see Section 6.2.2).
BYE or APP: Other RTCP packet types, including those yet to be
defined, may follow in any order, except that BYE should be the
last packet sent with a given SSRC/CSRC. Packet types may appear
more than once.
It is advisable for translators and mixers to combine individual RTCP
packets from the multiple sources they are forwarding into one
compound packet whenever feasible in order to amortize the packet
overhead (see Section 7). An example RTCP compound packet as might be
produced by a mixer is shown in Fig. 1. If the overall length of a
compound packet would exceed the maximum transmission unit (MTU) of
the network path, it may be segmented into multiple shorter compound
packets to be transmitted in separate packets of the underlying
protocol. Note that each of the compound packets must begin with an
SR or RR packet.
An implementation may ignore incoming RTCP packets with types unknown
to it. Additional RTCP packet types may be registered with the
Internet Assigned Numbers Authority (IANA).
Schulzrinne, et al Standards Track [Page 18]
RFC 1889 RTP January 1996
6.2 RTCP Transmission Interval
if encrypted: random 32-bit integer
|
|[------- packet -------][----------- packet -----------][-packet-]
|
| receiver reports chunk chunk
V item item item item
--------------------------------------------------------------------
|R[SR|# sender #site#site][SDES|# CNAME PHONE |#CNAME LOC][BYE##why]
|R[ |# report # 1 # 2 ][ |# |# ][ ## ]
|R[ |# # # ][ |# |# ][ ## ]
|R[ |# # # ][ |# |# ][ ## ]
--------------------------------------------------------------------
|<------------------ UDP packet (compound packet) --------------->|
#: SSRC/CSRC
Figure 1: Example of an RTCP compound packet
RTP is designed to allow an application to scale automatically over
session sizes ranging from a few participants to thousands. For
example, in an audio conference the data traffic is inherently self-
limiting because only one or two people will speak at a time, so with
multicast distribution the data rate on any given link remains
relatively constant independent of the number of participants.
However, the control traffic is not self-limiting. If the reception
reports from each participant were sent at a constant rate, the
control traffic would grow linearly with the number of participants.
Therefore, the rate must be scaled down.
For each session, it is assumed that the data traffic is subject to
an aggregate limit called the "session bandwidth" to be divided among
the participants. This bandwidth might be reserved and the limit
enforced by the network, or it might just be a reasonable share. The
session bandwidth may be chosen based or some cost or a priori
knowledge of the available network bandwidth for the session. It is
somewhat independent of the media encoding, but the encoding choice
may be limited by the session bandwidth. The session bandwidth
parameter is expected to be supplied by a session management
application when it invokes a media application, but media
applications may also set a default based on the single-sender data
bandwidth for the encoding selected for the session. The application
may also enforce bandwidth limits based on multicast scope rules or
other criteria.
Schulzrinne, et al Standards Track [Page 19]
RFC 1889 RTP January 1996
Bandwidth calculations for control and data traffic include lower-
layer transport and network protocols (e.g., UDP and IP) since that
is what the resource reservation system would need to know. The
application can also be expected to know which of these protocols are
in use. Link level headers are not included in the calculation since
the packet will be encapsulated with different link level headers as
it travels.
The control traffic should be limited to a small and known fraction
of the session bandwidth: small so that the primary function of the
transport protocol to carry data is not impaired; known so that the
control traffic can be included in the bandwidth specification given
to a resource reservation protocol, and so that each participant can
independently calculate its share. It is suggested that the fraction
of the session bandwidth allocated to RTCP be fixed at 5%. While the
value of this and other constants in the interval calculation is not
critical, all participants in the session must use the same values so
the same interval will be calculated. Therefore, these constants
should be fixed for a particular profile.
The algorithm described in Appendix A.7 was designed to meet the
goals outlined above. It calculates the interval between sending
compound RTCP packets to divide the allowed control traffic bandwidth
among the participants. This allows an application to provide fast
response for small sessions where, for example, identification of all
participants is important, yet automatically adapt to large sessions.
The algorithm incorporates the following characteristics:
o Senders are collectively allocated at least 1/4 of the control
traffic bandwidth so that in sessions with a large number of
receivers but a small number of senders, newly joining
participants will more quickly receive the CNAME for the
sending sites.
o The calculated interval between RTCP packets is required to be
greater than a minimum of 5 seconds to avoid having bursts of
RTCP packets exceed the allowed bandwidth when the number of
participants is small and the traffic isn't smoothed according
to the law of large numbers.
o The interval between RTCP packets is varied randomly over the
range [0.5,1.5] times the calculated interval to avoid
unintended synchronization of all participants [10]. The first
RTCP packet sent after joining a session is also delayed by a
random variation of half the minimum RTCP interval in case the
application is started at multiple sites simultaneously, for
example as initiated by a session announcement.
Schulzrinne, et al Standards Track [Page 20]
RFC 1889 RTP January 1996
o A dynamic estimate of the average compound RTCP packet size is
calculated, including all those received and sent, to
automatically adapt to changes in the amount of control
information carried.
This algorithm may be used for sessions in which all participants are
allowed to send. In that case, the session bandwidth parameter is the
product of the individual sender's bandwidth times the number of
participants, and the RTCP bandwidth is 5% of that.
6.2.1 Maintaining the number of session members
Calculation of the RTCP packet interval depends upon an estimate of
the number of sites participating in the session. New sites are added
to the count when they are heard, and an entry for each is created in
a table indexed by the SSRC or CSRC identifier (see Section 8.2) to
keep track of them. New entries may not be considered valid until
multiple packets carrying the new SSRC have been received (see
Appendix A.1). Entries may be deleted from the table when an RTCP BYE
packet with the corresponding SSRC identifier is received.
A participant may mark another site inactive, or delete it if not yet
valid, if no RTP or RTCP packet has been received for a small number
of RTCP report intervals (5 is suggested). This provides some
robustness against packet loss. All sites must calculate roughly the
same value for the RTCP report interval in order for this timeout to
work properly.
Once a site has been validated, then if it is later marked inactive
the state for that site should still be retained and the site should
continue to be counted in the total number of sites sharing RTCP
bandwidth for a period long enough to span typical network
partitions. This is to avoid excessive traffic, when the partition
heals, due to an RTCP report interval that is too small. A timeout of
30 minutes is suggested. Note that this is still larger than 5 times
the largest value to which the RTCP report interval is expected to
usefully scale, about 2 to 5 minutes.
6.2.2 Allocation of source description bandwidth
This specification defines several source description (SDES) items in
addition to the mandatory CNAME item, such as NAME (personal name)
and EMAIL (email address). It also provides a means to define new
application-specific RTCP packet types. Applications should exercise
caution in allocating control bandwidth to this additional
information because it will slow down the rate at which reception
reports and CNAME are sent, thus impairing the performance of the
protocol. It is recommended that no more than 20% of the RTCP
Schulzrinne, et al Standards Track [Page 21]
RFC 1889 RTP January 1996
bandwidth allocated to a single participant be used to carry the
additional information. Furthermore, it is not intended that all
SDES items should be included in every application. Those that are
included should be assigned a fraction of the bandwidth according to
their utility. Rather than estimate these fractions dynamically, it
is recommended that the percentages be translated statically into
report interval counts based on the typical length of an item.
For example, an application may be designed to send only CNAME, NAME
and EMAIL and not any others. NAME might be given much higher
priority than EMAIL because the NAME would be displayed continuously
in the application's user interface, whereas EMAIL would be displayed
only when requested. At every RTCP interval, an RR packet and an SDES
packet with the CNAME item would be sent. For a small session
operating at the minimum interval, that would be every 5 seconds on
the average. Every third interval (15 seconds), one extra item would
be included in the SDES packet. Seven out of eight times this would
be the NAME item, and every eighth time (2 minutes) it would be the
EMAIL item.
When multiple applications operate in concert using cross-application
binding through a common CNAME for each participant, for example in a
multimedia conference composed of an RTP session for each medium, the
additional SDES information might be sent in only one RTP session.
The other sessions would carry only the CNAME item.
6.3 Sender and Receiver Reports
RTP receivers provide reception quality feedback using RTCP report
packets which may take one of two forms depending upon whether or not
the receiver is also a sender. The only difference between the sender
report (SR) and receiver report (RR) forms, besides the packet type
code, is that the sender report includes a 20-byte sender information
section for use by active senders. The SR is issued if a site has
sent any data packets during the interval since issuing the last
report or the previous one, otherwise the RR is issued.
Both the SR and RR forms include zero or more reception report
blocks, one for each of the synchronization sources from which this
receiver has received RTP data packets since the last report. Reports
are not issued for contributing sources listed in the CSRC list. Each
reception report block provides statistics about the data received
from the particular source indicated in that block. Since a maximum
of 31 reception report blocks will fit in an SR or RR packet,
additional RR packets may be stacked after the initial SR or RR
packet as needed to contain the reception reports for all sources
heard during the interval since the last report.
Schulzrinne, et al Standards Track [Page 22]
RFC 1889 RTP January 1996
The next sections define the formats of the two reports, how they may
be extended in a profile-specific manner if an application requires
additional feedback information, and how the reports may be used.
Details of reception reporting by translators and mixers is given in
Section 7.
6.3.1 SR: Sender report RTCP packet
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| RC | PT=SR=200 | length | header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| NTP timestamp, most significant word | sender
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ info
| NTP timestamp, least significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sender's packet count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sender's octet count |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_1 (SSRC of first source) | report
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
| fraction lost | cumulative number of packets lost | 1
-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extended highest sequence number received |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| interarrival jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| last SR (LSR) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| delay since last SR (DLSR) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_2 (SSRC of second source) | report
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
: ... : 2
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| profile-specific extensions |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The sender report packet consists of three sections, possibly
followed by a fourth profile-specific extension section if defined.
The first section, the header, is 8 octets long. The fields have the
following meaning:
Schulzrinne, et al Standards Track [Page 23]
RFC 1889 RTP January 1996
version (V): 2 bits
Identifies the version of RTP, which is the same in RTCP packets
as in RTP data packets. The version defined by this
specification is two (2).
padding (P): 1 bit
If the padding bit is set, this RTCP packet contains some
additional padding octets at the end which are not part of the
control information. The last octet of the padding is a count of
how many padding octets should be ignored. Padding may be needed
by some encryption algorithms with fixed block sizes. In a
compound RTCP packet, padding should only be required on the
last individual packet because the compound packet is encrypted
as a whole.
reception report count (RC): 5 bits
The number of reception report blocks contained in this packet.
A value of zero is valid.
packet type (PT): 8 bits
Contains the constant 200 to identify this as an RTCP SR packet.
length: 16 bits
The length of this RTCP packet in 32-bit words minus one,
including the header and any padding. (The offset of one makes
zero a valid length and avoids a possible infinite loop in
scanning a compound RTCP packet, while counting 32-bit words
avoids a validity check for a multiple of 4.)
SSRC: 32 bits
The synchronization source identifier for the originator of this
SR packet.
The second section, the sender information, is 20 octets long and is
present in every sender report packet. It summarizes the data
transmissions from this sender. The fields have the following
meaning:
NTP timestamp: 64 bits
Indicates the wallclock time when this report was sent so that
it may be used in combination with timestamps returned in
reception reports from other receivers to measure round-trip
propagation to those receivers. Receivers should expect that the
measurement accuracy of the timestamp may be limited to far less
than the resolution of the NTP timestamp. The measurement
uncertainty of the timestamp is not indicated as it may not be
known. A sender that can keep track of elapsed time but has no
notion of wallclock time may use the elapsed time since joining
Schulzrinne, et al Standards Track [Page 24]
RFC 1889 RTP January 1996
the session instead. This is assumed to be less than 68 years,
so the high bit will be zero. It is permissible to use the
sampling clock to estimate elapsed wallclock time. A sender that
has no notion of wallclock or elapsed time may set the NTP
timestamp to zero.
RTP timestamp: 32 bits
Corresponds to the same time as the NTP timestamp (above), but
in the same units and with the same random offset as the RTP
timestamps in data packets. This correspondence may be used for
intra- and inter-media synchronization for sources whose NTP
timestamps are synchronized, and may be used by media-
independent receivers to estimate the nominal RTP clock
frequency. Note that in most cases this timestamp will not be
equal to the RTP timestamp in any adjacent data packet. Rather,
it is calculated from the corresponding NTP timestamp using the
relationship between the RTP timestamp counter and real time as
maintained by periodically checking the wallclock time at a
sampling instant.
sender's packet count: 32 bits
The total number of RTP data packets transmitted by the sender
since starting transmission up until the time this SR packet was
generated. The count is reset if the sender changes its SSRC
identifier.
sender's octet count: 32 bits
The total number of payload octets (i.e., not including header
or padding) transmitted in RTP data packets by the sender since
starting transmission up until the time this SR packet was
generated. The count is reset if the sender changes its SSRC
identifier. This field can be used to estimate the average
payload data rate.
The third section contains zero or more reception report blocks
depending on the number of other sources heard by this sender since
the last report. Each reception report block conveys statistics on
the reception of RTP packets from a single synchronization source.
Receivers do not carry over statistics when a source changes its SSRC
identifier due to a collision. These statistics are:
SSRC_n (source identifier): 32 bits
The SSRC identifier of the source to which the information in
this reception report block pertains.
fraction lost: 8 bits
The fraction of RTP data packets from source SSRC_n lost since
the previous SR or RR packet was sent, expressed as a fixed
Schulzrinne, et al Standards Track [Page 25]
RFC 1889 RTP January 1996
point number with the binary point at the left edge of the
field. (That is equivalent to taking the integer part after
multiplying the loss fraction by 256.) This fraction is defined
to be the number of packets lost divided by the number of
packets expected, as defined in the next paragraph. An
implementation is shown in Appendix A.3. If the loss is negative
due to duplicates, the fraction lost is set to zero. Note that a
receiver cannot tell whether any packets were lost after the
last one received, and that there will be no reception report
block issued for a source if all packets from that source sent
during the last reporting interval have been lost.
cumulative number of packets lost: 24 bits
The total number of RTP data packets from source SSRC_n that
have been lost since the beginning of reception. This number is
defined to be the number of packets expected less the number of
packets actually received, where the number of packets received
includes any which are late or duplicates. Thus packets that
arrive late are not counted as lost, and the loss may be
negative if there are duplicates. The number of packets
expected is defined to be the extended last sequence number
received, as defined next, less the initial sequence number
received. This may be calculated as shown in Appendix A.3.
extended highest sequence number received: 32 bits
The low 16 bits contain the highest sequence number received in
an RTP data packet from source SSRC_n, and the most significant
16 bits extend that sequence number with the corresponding count
of sequence number cycles, which may be maintained according to
the algorithm in Appendix A.1. Note that different receivers
within the same session will generate different extensions to
the sequence number if their start times differ significantly.
interarrival jitter: 32 bits
An estimate of the statistical variance of the RTP data packet
interarrival time, measured in timestamp units and expressed as
an unsigned integer. The interarrival jitter J is defined to be
the mean deviation (smoothed absolute value) of the difference D
in packet spacing at the receiver compared to the sender for a
pair of packets. As shown in the equation below, this is
equivalent to the difference in the "relative transit time" for
the two packets; the relative transit time is the difference
between a packet's RTP timestamp and the receiver's clock at the
time of arrival, measured in the same units.
Schulzrinne, et al Standards Track [Page 26]
RFC 1889 RTP January 1996
If Si is the RTP timestamp from packet i, and Ri is the time of
arrival in RTP timestamp units for packet i, then for two packets i
and j, D may be expressed as
D(i,j)=(Rj-Ri)-(Sj-Si)=(Rj-Sj)-(Ri-Si)
The interarrival jitter is calculated continuously as each data
packet i is received from source SSRC_n, using this difference D for
that packet and the previous packet i-1 in order of arrival (not
necessarily in sequence), according to the formula
J=J+(|D(i-1,i)|-J)/16
Whenever a reception report is issued, the current value of J is
sampled.
The jitter calculation is prescribed here to allow profile-
independent monitors to make valid interpretations of reports coming
from different implementations. This algorithm is the optimal first-
order estimator and the gain parameter 1/16 gives a good noise
reduction ratio while maintaining a reasonable rate of convergence
[11]. A sample implementation is shown in Appendix A.8.
last SR timestamp (LSR): 32 bits
The middle 32 bits out of 64 in the NTP timestamp (as explained
in Section 4) received as part of the most recent RTCP sender
report (SR) packet from source SSRC_n. If no SR has been
received yet, the field is set to zero.
delay since last SR (DLSR): 32 bits
The delay, expressed in units of 1/65536 seconds, between
receiving the last SR packet from source SSRC_n and sending this
reception report block. If no SR packet has been received yet
from SSRC_n, the DLSR field is set to zero.
Let SSRC_r denote the receiver issuing this receiver report. Source
SSRC_n can compute the round propagation delay to SSRC_r by recording
the time A when this reception report block is received. It
calculates the total round-trip time A-LSR using the last SR
timestamp (LSR) field, and then subtracting this field to leave the
round-trip propagation delay as (A- LSR - DLSR). This is illustrated
in Fig. 2.
This may be used as an approximate measure of distance to cluster
receivers, although some links have very asymmetric delays.
Schulzrinne, et al Standards Track [Page 27]
RFC 1889 RTP January 1996
6.3.2 RR: Receiver report RTCP packet
[10 Nov 1995 11:33:25.125] [10 Nov 1995 11:33:36.5]
n SR(n) A=b710:8000 (46864.500 s)
---------------------------------------------------------------->
v ^
ntp_sec =0xb44db705 v ^ dlsr=0x0005.4000 ( 5.250s)
ntp_frac=0x20000000 v ^ lsr =0xb705:2000 (46853.125s)
(3024992016.125 s) v ^
r v ^ RR(n)
---------------------------------------------------------------->
|<-DLSR->|
(5.250 s)
A 0xb710:8000 (46864.500 s)
DLSR -0x0005:4000 ( 5.250 s)
LSR -0xb705:2000 (46853.125 s)
-------------------------------
delay 0x 6:2000 ( 6.125 s)
Figure 2: Example for round-trip time computation
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| RC | PT=RR=201 | length | header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_1 (SSRC of first source) | report
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
| fraction lost | cumulative number of packets lost | 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extended highest sequence number received |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| interarrival jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| last SR (LSR) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| delay since last SR (DLSR) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_2 (SSRC of second source) | report
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
: ... : 2
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| profile-specific extensions |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Schulzrinne, et al Standards Track [Page 28]
RFC 1889 RTP January 1996
The format of the receiver report (RR) packet is the same as that of
the SR packet except that the packet type field contains the constant
201 and the five words of sender information are omitted (these are
the NTP and RTP timestamps and sender's packet and octet counts). The
remaining fields have the same meaning as for the SR packet.
An empty RR packet (RC = 0) is put at the head of a compound RTCP
packet when there is no data transmission or reception to report.
6.3.3 Extending the sender and receiver reports
A profile should define profile- or application-specific extensions
to the sender report and receiver if there is additional information
that should be reported regularly about the sender or receivers. This
method should be used in preference to defining another RTCP packet
type because it requires less overhead:
o fewer octets in the packet (no RTCP header or SSRC field);
o simpler and faster parsing because applications running under
that profile would be programmed to always expect the extension
fields in the directly accessible location after the reception
reports.
If additional sender information is required, it should be included
first in the extension for sender reports, but would not be present
in receiver reports. If information about receivers is to be
included, that data may be structured as an array of blocks parallel
to the existing array of reception report blocks; that is, the number
of blocks would be indicated by the RC field.
6.3.4 Analyzing sender and receiver reports
It is expected that reception quality feedback will be useful not
only for the sender but also for other receivers and third-party
monitors. The sender may modify its transmissions based on the
feedback; receivers can determine whether problems are local,
regional or global; network managers may use profile-independent
monitors that receive only the RTCP packets and not the corresponding
RTP data packets to evaluate the performance of their networks for
multicast distribution.
Cumulative counts are used in both the sender information and
receiver report blocks so that differences may be calculated between
any two reports to make measurements over both short and long time
periods, and to provide resilience against the loss of a report. The
difference between the last two reports received can be used to
estimate the recent quality of the distribution. The NTP timestamp is
Schulzrinne, et al Standards Track [Page 29]
RFC 1889 RTP January 1996
included so that rates may be calculated from these differences over
the interval between two reports. Since that timestamp is independent
of the clock rate for the data encoding, it is possible to implement
encoding- and profile-independent quality monitors.
An example calculation is the packet loss rate over the interval
between two reception reports. The difference in the cumulative
number of packets lost gives the number lost during that interval.
The difference in the extended last sequence numbers received gives
the number of packets expected during the interval. The ratio of
these two is the packet loss fraction over the interval. This ratio
should equal the fraction lost field if the two reports are
consecutive, but otherwise not. The loss rate per second can be
obtained by dividing the loss fraction by the difference in NTP
timestamps, expressed in seconds. The number of packets received is
the number of packets expected minus the number lost. The number of
packets expected may also be used to judge the statistical validity
of any loss estimates. For example, 1 out of 5 packets lost has a
lower significance than 200 out of 1000.
From the sender information, a third-party monitor can calculate the
average payload data rate and the average packet rate over an
interval without receiving the data. Taking the ratio of the two
gives the average payload size. If it can be assumed that packet loss
is independent of packet size, then the number of packets received by
a particular receiver times the average payload size (or the
corresponding packet size) gives the apparent throughput available to
that receiver.
In addition to the cumulative counts which allow long-term packet
loss measurements using differences between reports, the fraction
lost field provides a short-term measurement from a single report.
This becomes more important as the size of a session scales up enough
that reception state information might not be kept for all receivers
or the interval between reports becomes long enough that only one
report might have been received from a particular receiver.
The interarrival jitter field provides a second short-term measure of
network congestion. Packet loss tracks persistent congestion while
the jitter measure tracks transient congestion. The jitter measure
may indicate congestion before it leads to packet loss. Since the
interarrival jitter field is only a snapshot of the jitter at the
time of a report, it may be necessary to analyze a number of reports
from one receiver over time or from multiple receivers, e.g., within
a single network.
Schulzrinne, et al Standards Track [Page 30]
RFC 1889 RTP January 1996
6.4 SDES: Source description RTCP packet
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| SC | PT=SDES=202 | length | header
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC/CSRC_1 | chunk
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1
| SDES items |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC/CSRC_2 | chunk
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2
| SDES items |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
The SDES packet is a three-level structure composed of a header and
zero or more chunks, each of of which is composed of items describing
the source identified in that chunk. The items are described
individually in subsequent sections.
version (V), padding (P), length:
As described for the SR packet (see Section 6.3.1).
packet type (PT): 8 bits
Contains the constant 202 to identify this as an RTCP SDES
packet.
source count (SC): 5 bits
The number of SSRC/CSRC chunks contained in this SDES packet. A
value of zero is valid but useless.
Each chunk consists of an SSRC/CSRC identifier followed by a list of
zero or more items, which carry information about the SSRC/CSRC. Each
chunk starts on a 32-bit boundary. Each item consists of an 8-bit
type field, an 8-bit octet count describing the length of the text
(thus, not including this two-octet header), and the text itself.
Note that the text can be no longer than 255 octets, but this is
consistent with the need to limit RTCP bandwidth consumption.
The text is encoded according to the UTF-2 encoding specified in
Annex F of ISO standard 10646 [12,13]. This encoding is also known as
UTF-8 or UTF-FSS. It is described in "File System Safe UCS
Transformation Format (FSS_UTF)", X/Open Preliminary Specification,
Document Number P316 and Unicode Technical Report #4. US-ASCII is a
subset of this encoding and requires no additional encoding. The
Schulzrinne, et al Standards Track [Page 31]
RFC 1889 RTP January 1996
presence of multi-octet encodings is indicated by setting the most
significant bit of a character to a value of one.
Items are contiguous, i.e., items are not individually padded to a
32-bit boundary. Text is not null terminated because some multi-octet
encodings include null octets. The list of items in each chunk is
terminated by one or more null octets, the first of which is
interpreted as an item type of zero to denote the end of the list,
and the remainder as needed to pad until the next 32-bit boundary. A
chunk with zero items (four null octets) is valid but useless.
End systems send one SDES packet containing their own source
identifier (the same as the SSRC in the fixed RTP header). A mixer
sends one SDES packet containing a chunk for each contributing source
from which it is receiving SDES information, or multiple complete
SDES packets in the format above if there are more than 31 such
sources (see Section 7).
The SDES items currently defined are described in the next sections.
Only the CNAME item is mandatory. Some items shown here may be useful
only for particular profiles, but the item types are all assigned
from one common space to promote shared use and to simplify profile-
independent applications. Additional items may be defined in a
profile by registering the type numbers with IANA.
6.4.1 CNAME: Canonical end-point identifier SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CNAME=1 | length | user and domain name ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The CNAME identifier has the following properties:
o Because the randomly allocated SSRC identifier may change if a
conflict is discovered or if a program is restarted, the CNAME
item is required to provide the binding from the SSRC
identifier to an identifier for the source that remains
constant.
o Like the SSRC identifier, the CNAME identifier should also be
unique among all participants within one RTP session.
o To provide a binding across multiple media tools used by one
participant in a set of related RTP sessions, the CNAME should
be fixed for that participant.
Schulzrinne, et al Standards Track [Page 32]
RFC 1889 RTP January 1996
o To facilitate third-party monitoring, the CNAME should be
suitable for either a program or a person to locate the source.
Therefore, the CNAME should be derived algorithmically and not
entered manually, when possible. To meet these requirements, the
following format should be used unless a profile specifies an
alternate syntax or semantics. The CNAME item should have the format
"user@host", or "host" if a user name is not available as on single-
user systems. For both formats, "host" is either the fully qualified
domain name of the host from which the real-time data originates,
formatted according to the rules specified in RFC 1034 [14], RFC 1035
[15] and Section 2.1 of RFC 1123 [16]; or the standard ASCII
representation of the host's numeric address on the interface used
for the RTP communication. For example, the standard ASCII
representation of an IP Version 4 address is "dotted decimal", also
known as dotted quad. Other address types are expected to have ASCII
representations that are mutually unique. The fully qualified domain
name is more convenient for a human observer and may avoid the need
to send a NAME item in addition, but it may be difficult or
impossible to obtain reliably in some operating environments.
Applications that may be run in such environments should use the
ASCII representation of the address instead.
Examples are "doe@sleepy.megacorp.com" or "doe@192.0.2.89" for a
multi-user system. On a system with no user name, examples would be
"sleepy.megacorp.com" or "192.0.2.89".
The user name should be in a form that a program such as "finger" or
"talk" could use, i.e., it typically is the login name rather than
the personal name. The host name is not necessarily identical to the
one in the participant's electronic mail address.
This syntax will not provide unique identifiers for each source if an
application permits a user to generate multiple sources from one
host. Such an application would have to rely on the SSRC to further
identify the source, or the profile for that application would have
to specify additional syntax for the CNAME identifier.
If each application creates its CNAME independently, the resulting
CNAMEs may not be identical as would be required to provide a binding
across multiple media tools belonging to one participant in a set of
related RTP sessions. If cross-media binding is required, it may be
necessary for the CNAME of each tool to be externally configured with
the same value by a coordination tool.
Application writers should be aware that private network address
assignments such as the Net-10 assignment proposed in RFC 1597 [17]
may create network addresses that are not globally unique. This would
Schulzrinne, et al Standards Track [Page 33]
RFC 1889 RTP January 1996
lead to non-unique CNAMEs if hosts with private addresses and no
direct IP connectivity to the public Internet have their RTP packets
forwarded to the public Internet through an RTP-level translator.
(See also RFC 1627 [18].) To handle this case, applications may
provide a means to configure a unique CNAME, but the burden is on the
translator to translate CNAMEs from private addresses to public
addresses if necessary to keep private addresses from being exposed.
6.4.2 NAME: User name SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NAME=2 | length | common name of source ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This is the real name used to describe the source, e.g., "John Doe,
Bit Recycler, Megacorp". It may be in any form desired by the user.
For applications such as conferencing, this form of name may be the
most desirable for display in participant lists, and therefore might
be sent most frequently of those items other than CNAME. Profiles may
establish such priorities. The NAME value is expected to remain
constant at least for the duration of a session. It should not be
relied upon to be unique among all participants in the session.
6.4.3 EMAIL: Electronic mail address SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EMAIL=3 | length | email address of source ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The email address is formatted according to RFC 822 [19], for
example, "John.Doe@megacorp.com". The EMAIL value is expected to
remain constant for the duration of a session.
6.4.4 PHONE: Phone number SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PHONE=4 | length | phone number of source ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The phone number should be formatted with the plus sign replacing the
international access code. For example, "+1 908 555 1212" for a
number in the United States.
Schulzrinne, et al Standards Track [Page 34]
RFC 1889 RTP January 1996
6.4.5 LOC: Geographic user location SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LOC=5 | length | geographic location of site ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Depending on the application, different degrees of detail are
appropriate for this item. For conference applications, a string like
"Murray Hill, New Jersey" may be sufficient, while, for an active
badge system, strings like "Room 2A244, AT&T BL MH" might be
appropriate. The degree of detail is left to the implementation
and/or user, but format and content may be prescribed by a profile.
The LOC value is expected to remain constant for the duration of a
session, except for mobile hosts.
6.4.6 TOOL: Application or tool name SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOOL=6 | length | name/version of source appl. ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A string giving the name and possibly version of the application
generating the stream, e.g., "videotool 1.2". This information may be
useful for debugging purposes and is similar to the Mailer or Mail-
System-Version SMTP headers. The TOOL value is expected to remain
constant for the duration of the session.
6.4.7 NOTE: Notice/status SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NOTE=7 | length | note about the source ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following semantics are suggested for this item, but these or
other semantics may be explicitly defined by a profile. The NOTE item
is intended for transient messages describing the current state of
the source, e.g., "on the phone, can't talk". Or, during a seminar,
this item might be used to convey the title of the talk. It should be
used only to carry exceptional information and should not be included
routinely by all participants because this would slow down the rate
at which reception reports and CNAME are sent, thus impairing the
performance of the protocol. In particular, it should not be included
Schulzrinne, et al Standards Track [Page 35]
RFC 1889 RTP January 1996
as an item in a user's configuration file nor automatically generated
as in a quote-of-the-day.
Since the NOTE item may be important to display while it is active,
the rate at which other non-CNAME items such as NAME are transmitted
might be reduced so that the NOTE item can take that part of the RTCP
bandwidth. When the transient message becomes inactive, the NOTE item
should continue to be transmitted a few times at the same repetition
rate but with a string of length zero to signal the receivers.
However, receivers should also consider the NOTE item inactive if it
is not received for a small multiple of the repetition rate, or
perhaps 20-30 RTCP intervals.
6.4.8 PRIV: Private extensions SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PRIV=8 | length | prefix length | prefix string...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... | value string ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This item is used to define experimental or application-specific SDES
extensions. The item contains a prefix consisting of a length-string
pair, followed by the value string filling the remainder of the item
and carrying the desired information. The prefix length field is 8
bits long. The prefix string is a name chosen by the person defining
the PRIV item to be unique with respect to other PRIV items this
application might receive. The application creator might choose to
use the application name plus an additional subtype identification if
needed. Alternatively, it is recommended that others choose a name
based on the entity they represent, then coordinate the use of the
name within that entity.
Note that the prefix consumes some space within the item's total
length of 255 octets, so the prefix should be kept as short as
possible. This facility and the constrained RTCP bandwidth should not
be overloaded; it is not intended to satisfy all the control
communication requirements of all applications.
SDES PRIV prefixes will not be registered by IANA. If some form of
the PRIV item proves to be of general utility, it should instead be
assigned a regular SDES item type registered with IANA so that no
prefix is required. This simplifies use and increases transmission
efficiency.
Schulzrinne, et al Standards Track [Page 36]
RFC 1889 RTP January 1996
6.5 BYE: Goodbye RTCP packet
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| SC | PT=BYE=203 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC/CSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ... :
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| length | reason for leaving ... (opt)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The BYE packet indicates that one or more sources are no longer
active.
version (V), padding (P), length:
As described for the SR packet (see Section 6.3.1).
packet type (PT): 8 bits
Contains the constant 203 to identify this as an RTCP BYE
packet.
source count (SC): 5 bits
The number of SSRC/CSRC identifiers included in this BYE packet.
A count value of zero is valid, but useless.
If a BYE packet is received by a mixer, the mixer forwards the BYE
packet with the SSRC/CSRC identifier(s) unchanged. If a mixer shuts
down, it should send a BYE packet listing all contributing sources it
handles, as well as its own SSRC identifier. Optionally, the BYE
packet may include an 8-bit octet count followed by that many octets
of text indicating the reason for leaving, e.g., "camera malfunction"
or "RTP loop detected". The string has the same encoding as that
described for SDES. If the string fills the packet to the next 32-bit
boundary, the string is not null terminated. If not, the BYE packet
is padded with null octets.
Schulzrinne, et al Standards Track [Page 37]
RFC 1889 RTP January 1996
6.6 APP: Application-defined RTCP packet
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| subtype | PT=APP=204 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC/CSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| name (ASCII) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| application-dependent data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The APP packet is intended for experimental use as new applications
and new features are developed, without requiring packet type value
registration. APP packets with unrecognized names should be ignored.
After testing and if wider use is justified, it is recommended that
each APP packet be redefined without the subtype and name fields and
registered with the Internet Assigned Numbers Authority using an RTCP
packet type.
version (V), padding (P), length:
As described for the SR packet (see Section 6.3.1).
subtype: 5 bits
May be used as a subtype to allow a set of APP packets to be
defined under one unique name, or for any application-dependent
data.
packet type (PT): 8 bits
Contains the constant 204 to identify this as an RTCP APP
packet.
name: 4 octets
A name chosen by the person defining the set of APP packets to
be unique with respect to other APP packets this application
might receive. The application creator might choose to use the
application name, and then coordinate the allocation of subtype
values to others who want to define new packet types for the
application. Alternatively, it is recommended that others
choose a name based on the entity they represent, then
coordinate the use of the name within that entity. The name is
interpreted as a sequence of four ASCII characters, with
uppercase and lowercase characters treated as distinct.
Schulzrinne, et al Standards Track [Page 38]
RFC 1889 RTP January 1996
application-dependent data: variable length
Application-dependent data may or may not appear in an APP
packet. It is interpreted by the application and not RTP itself.
It must be a multiple of 32 bits long.
7. RTP Translators and Mixers
In addition to end systems, RTP supports the notion of "translators"
and "mixers", which could be considered as "intermediate systems" at
the RTP level. Although this support adds some complexity to the
protocol, the need for these functions has been clearly established
by experiments with multicast audio and video applications in the
Internet. Example uses of translators and mixers given in Section 2.3
stem from the presence of firewalls and low bandwidth connections,
both of which are likely to remain.
7.1 General Description
An RTP translator/mixer connects two or more transport-level
"clouds". Typically, each cloud is defined by a common network and
transport protocol (e.g., IP/UDP), multicast address or pair of
unicast addresses, and transport level destination port. (Network-
level protocol translators, such as IP version 4 to IP version 6, may
be present within a cloud invisibly to RTP.) One system may serve as
a translator or mixer for a number of RTP sessions, but each is
considered a logically separate entity.
In order to avoid creating a loop when a translator or mixer is
installed, the following rules must be observed:
o Each of the clouds connected by translators and mixers
participating in one RTP session either must be distinct from
all the others in at least one of these parameters (protocol,
address, port), or must be isolated at the network level from
the others.
o A derivative of the first rule is that there must not be
multiple translators or mixers connected in parallel unless by
some arrangement they partition the set of sources to be
forwarded.
Similarly, all RTP end systems that can communicate through one or
more RTP translators or mixers share the same SSRC space, that is,
the SSRC identifiers must be unique among all these end systems.
Section 8.2 describes the collision resolution algorithm by which
SSRC identifiers are kept unique and loops are detected.
Schulzrinne, et al Standards Track [Page 39]
RFC 1889 RTP January 1996
There may be many varieties of translators and mixers designed for
different purposes and applications. Some examples are to add or
remove encryption, change the encoding of the data or the underlying
protocols, or replicate between a multicast address and one or more
unicast addresses. The distinction between translators and mixers is
that a translator passes through the data streams from different
sources separately, whereas a mixer combines them to form one new
stream:
Translator: Forwards RTP packets with their SSRC identifier intact;
this makes it possible for receivers to identify individual
sources even though packets from all the sources pass through
the same translator and carry the translator's network source
address. Some kinds of translators will pass through the data
untouched, but others may change the encoding of the data and
thus the RTP data payload type and timestamp. If multiple data
packets are re-encoded into one, or vice versa, a translator
must assign new sequence numbers to the outgoing packets. Losses
in the incoming packet stream may induce corresponding gaps in
the outgoing sequence numbers. Receivers cannot detect the
presence of a translator unless they know by some other means
what payload type or transport address was used by the original
source.
Mixer: Receives streams of RTP data packets from one or more sources,
possibly changes the data format, combines the streams in some
manner and then forwards the combined stream. Since the timing
among multiple input sources will not generally be synchronized,
the mixer will make timing adjustments among the streams and
generate its own timing for the combined stream, so it is the
synchronization source. Thus, all data packets forwarded by a
mixer will be marked with the mixer's own SSRC identifier. In
order to preserve the identity of the original sources
contributing to the mixed packet, the mixer should insert their
SSRC identifiers into the CSRC identifier list following the
fixed RTP header of the packet. A mixer that is also itself a
contributing source for some packet should explicitly include
its own SSRC identifier in the CSRC list for that packet.
For some applications, it may be acceptable for a mixer not to
identify sources in the CSRC list. However, this introduces the
danger that loops involving those sources could not be detected.
The advantage of a mixer over a translator for applications like
audio is that the output bandwidth is limited to that of one source
even when multiple sources are active on the input side. This may be
important for low-bandwidth links. The disadvantage is that receivers
on the output side don't have any control over which sources are
Schulzrinne, et al Standards Track [Page 40]
RFC 1889 RTP January 1996
passed through or muted, unless some mechanism is implemented for
remote control of the mixer. The regeneration of synchronization
information by mixers also means that receivers can't do inter-media
synchronization of the original streams. A multi-media mixer could do
it.
[E1] [E6]
| |
E1:17 | E6:15 |
| | E6:15
V M1:48 (1,17) M1:48 (1,17) V M1:48 (1,17)
(M1)-------------><T1>-----------------><T2>-------------->[E7]
^ ^ E4:47 ^ E4:47
E2:1 | E4:47 | | M3:89 (64,45)
| | |
[E2] [E4] M3:89 (64,45) |
| legend:
[E3] --------->(M2)----------->(M3)------------| [End system]
E3:64 M2:12 (64) ^ (Mixer)
| E5:45 <Translator>
|
[E5] source: SSRC (CSRCs)
------------------->
Figure 3: Sample RTP network with end systems, mixers and translators
A collection of mixers and translators is shown in Figure 3 to
illustrate their effect on SSRC and CSRC identifiers. In the figure,
end systems are shown as rectangles (named E), translators as
triangles (named T) and mixers as ovals (named M). The notation "M1:
48(1,17)" designates a packet originating a mixer M1, identified with
M1's (random) SSRC value of 48 and two CSRC identifiers, 1 and 17,
copied from the SSRC identifiers of packets from E1 and E2.
7.2 RTCP Processing in Translators
In addition to forwarding data packets, perhaps modified, translators
and mixers must also process RTCP packets. In many cases, they will
take apart the compound RTCP packets received from end systems to
aggregate SDES information and to modify the SR or RR packets.
Retransmission of this information may be triggered by the packet
arrival or by the RTCP interval timer of the translator or mixer
itself.
A translator that does not modify the data packets, for example one
that just replicates between a multicast address and a unicast
address, may simply forward RTCP packets unmodified as well. A
Schulzrinne, et al Standards Track [Page 41]
RFC 1889 RTP January 1996
translator that transforms the payload in some way must make
corresponding transformations in the SR and RR information so that it
still reflects the characteristics of the data and the reception
quality. These translators must not simply forward RTCP packets. In
general, a translator should not aggregate SR and RR packets from
different sources into one packet since that would reduce the
accuracy of the propagation delay measurements based on the LSR and
DLSR fields.
SR sender information: A translator does not generate its own sender
information, but forwards the SR packets received from one cloud
to the others. The SSRC is left intact but the sender
information must be modified if required by the translation. If
a translator changes the data encoding, it must change the
"sender's byte count" field. If it also combines several data
packets into one output packet, it must change the "sender's
packet count" field. If it changes the timestamp frequency, it
must change the "RTP timestamp" field in the SR packet.
SR/RR reception report blocks: A translator forwards reception
reports received from one cloud to the others. Note that these
flow in the direction opposite to the data. The SSRC is left
intact. If a translator combines several data packets into one
output packet, and therefore changes the sequence numbers, it
must make the inverse manipulation for the packet loss fields
and the "extended last sequence number" field. This may be
complex. In the extreme case, there may be no meaningful way to
translate the reception reports, so the translator may pass on
no reception report at all or a synthetic report based on its
own reception. The general rule is to do what makes sense for a
particular translation.
A translator does not require an SSRC identifier of its own, but may
choose to allocate one for the purpose of sending reports about what
it has received. These would be sent to all the connected clouds,
each corresponding to the translation of the data stream as sent to
that cloud, since reception reports are normally multicast to all
participants.
SDES: Translators typically forward without change the SDES
information they receive from one cloud to the others, but may,
for example, decide to filter non-CNAME SDES information if
bandwidth is limited. The CNAMEs must be forwarded to allow SSRC
identifier collision detection to work. A translator that
generates its own RR packets must send SDES CNAME information
about itself to the same clouds that it sends those RR packets.
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BYE: Translators forward BYE packets unchanged. Translators with
their own SSRC should generate BYE packets with that SSRC
identifier if they are about to cease forwarding packets.
APP: Translators forward APP packets unchanged.
7.3 RTCP Processing in Mixers
Since a mixer generates a new data stream of its own, it does not
pass through SR or RR packets at all and instead generates new
information for both sides.
SR sender information: A mixer does not pass through sender
information from the sources it mixes because the
characteristics of the source streams are lost in the mix. As a
synchronization source, the mixer generates its own SR packets
with sender information about the mixed data stream and sends
them in the same direction as the mixed stream.
SR/RR reception report blocks: A mixer generates its own reception
reports for sources in each cloud and sends them out only to the
same cloud. It does not send these reception reports to the
other clouds and does not forward reception reports from one
cloud to the others because the sources would not be SSRCs there
(only CSRCs).
SDES: Mixers typically forward without change the SDES information
they receive from one cloud to the others, but may, for example,
decide to filter non-CNAME SDES information if bandwidth is
limited. The CNAMEs must be forwarded to allow SSRC identifier
collision detection to work. (An identifier in a CSRC list
generated by a mixer might collide with an SSRC identifier
generated by an end system.) A mixer must send SDES CNAME
information about itself to the same clouds that it sends SR or
RR packets.
Since mixers do not forward SR or RR packets, they will typically be
extracting SDES packets from a compound RTCP packet. To minimize
overhead, chunks from the SDES packets may be aggregated into a
single SDES packet which is then stacked on an SR or RR packet
originating from the mixer. The RTCP packet rate may be different on
each side of the mixer.
A mixer that does not insert CSRC identifiers may also refrain from
forwarding SDES CNAMEs. In this case, the SSRC identifier spaces in
the two clouds are independent. As mentioned earlier, this mode of
operation creates a danger that loops can't be detected.
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BYE: Mixers need to forward BYE packets. They should generate BYE
packets with their own SSRC identifiers if they are about to
cease forwarding packets.
APP: The treatment of APP packets by mixers is application-specific.
7.4 Cascaded Mixers
An RTP session may involve a collection of mixers and translators as
shown in Figure 3. If two mixers are cascaded, such as M2 and M3 in
the figure, packets received by a mixer may already have been mixed
and may include a CSRC list with multiple identifiers. The second
mixer should build the CSRC list for the outgoing packet using the
CSRC identifiers from already-mixed input packets and the SSRC
identifiers from unmixed input packets. This is shown in the output
arc from mixer M3 labeled M3:89(64,45) in the figure. As in the case
of mixers that are not cascaded, if the resulting CSRC list has more
than 15 identifiers, the remainder cannot be included.
8. SSRC Identifier Allocation and Use
The SSRC identifier carried in the RTP header and in various fields
of RTCP packets is a random 32-bit number that is required to be
globally unique within an RTP session. It is crucial that the number
be chosen with care in order that participants on the same network or
starting at the same time are not likely to choose the same number.
It is not sufficient to use the local network address (such as an
IPv4 address) for the identifier because the address may not be
unique. Since RTP translators and mixers enable interoperation among
multiple networks with different address spaces, the allocation
patterns for addresses within two spaces might result in a much
higher rate of collision than would occur with random allocation.
Multiple sources running on one host would also conflict.
It is also not sufficient to obtain an SSRC identifier simply by
calling random() without carefully initializing the state. An example
of how to generate a random identifier is presented in Appendix A.6.
8.1 Probability of Collision
Since the identifiers are chosen randomly, it is possible that two or
more sources will choose the same number. Collision occurs with the
highest probability when all sources are started simultaneously, for
example when triggered automatically by some session management
event. If N is the number of sources and L the length of the
identifier (here, 32 bits), the probability that two sources
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independently pick the same value can be approximated for large N
[20] as 1 - exp(-N**2 / 2**(L+1)). For N=1000, the probability is
roughly 10**-4.
The typical collision probability is much lower than the worst-case
above. When one new source joins an RTP session in which all the
other sources already have unique identifiers, the probability of
collision is just the fraction of numbers used out of the space.
Again, if N is the number of sources and L the length of the
identifier, the probability of collision is N / 2**L. For N=1000, the
probability is roughly 2*10**-7.
The probability of collision is further reduced by the opportunity
for a new source to receive packets from other participants before
sending its first packet (either data or control). If the new source
keeps track of the other participants (by SSRC identifier), then
before transmitting its first packet the new source can verify that
its identifier does not conflict with any that have been received, or
else choose again.
8.2 Collision Resolution and Loop Detection
Although the probability of SSRC identifier collision is low, all RTP
implementations must be prepared to detect collisions and take the
appropriate actions to resolve them. If a source discovers at any
time that another source is using the same SSRC identifier as its
own, it must send an RTCP BYE packet for the old identifier and
choose another random one. If a receiver discovers that two other
sources are colliding, it may keep the packets from one and discard
the packets from the other when this can be detected by different
source transport addresses or CNAMEs. The two sources are expected to
resolve the collision so that the situation doesn't last.
Because the random identifiers are kept globally unique for each RTP
session, they can also be used to detect loops that may be introduced
by mixers or translators. A loop causes duplication of data and
control information, either unmodified or possibly mixed, as in the
following examples:
o A translator may incorrectly forward a packet to the same
multicast group from which it has received the packet, either
directly or through a chain of translators. In that case, the
same packet appears several times, originating from different
network sources.
o Two translators incorrectly set up in parallel, i.e., with the
same multicast groups on both sides, would both forward packets
from one multicast group to the other. Unidirectional
Schulzrinne, et al Standards Track [Page 45]
RFC 1889 RTP January 1996
translators would produce two copies; bidirectional translators
would form a loop.
o A mixer can close a loop by sending to the same transport
destination upon which it receives packets, either directly or
through another mixer or translator. In this case a source
might show up both as an SSRC on a data packet and a CSRC in a
mixed data packet.
A source may discover that its own packets are being looped, or that
packets from another source are being looped (a third-party loop).
Both loops and collisions in the random selection of a source
identifier result in packets arriving with the same SSRC identifier
but a different source transport address, which may be that of the
end system originating the packet or an intermediate system.
Consequently, if a source changes its source transport address, it
must also choose a new SSRC identifier to avoid being interpreted as
a looped source. Loops or collisions occurring on the far side of a
translator or mixer cannot be detected using the source transport
address if all copies of the packets go through the translator or
mixer, however collisions may still be detected when chunks from two
RTCP SDES packets contain the same SSRC identifier but different
CNAMEs.
To detect and resolve these conflicts, an RTP implementation must
include an algorithm similar to the one described below. It ignores
packets from a new source or loop that collide with an established
source. It resolves collisions with the participant's own SSRC
identifier by sending an RTCP BYE for the old identifier and choosing
a new one. However, when the collision was induced by a loop of the
participant's own packets, the algorithm will choose a new identifier
only once and thereafter ignore packets from the looping source
transport address. This is required to avoid a flood of BYE packets.
This algorithm depends upon the source transport address being the
same for both RTP and RTCP packets from a source. The algorithm would
require modifications to support applications that don't meet this
constraint.
This algorithm requires keeping a table indexed by source identifiers
and containing the source transport address from which the identifier
was (first) received, along with other state for that source. Each
SSRC or CSRC identifier received in a data or control packet is
looked up in this table in order to process that data or control
information. For control packets, each element with its own SSRC,
for example an SDES chunk, requires a separate lookup. (The SSRC in a
reception report block is an exception.) If the SSRC or CSRC is not
Schulzrinne, et al Standards Track [Page 46]
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found, a new entry is created. These table entries are removed when
an RTCP BYE packet is received with the corresponding SSRC, or after
no packets have arrived for a relatively long time (see Section
6.2.1).
In order to track loops of the participant's own data packets, it is
also necessary to keep a separate list of source transport addresses
(not identifiers) that have been found to be conflicting. Note that
this should be a short list, usually empty. Each element in this list
stores the source address plus the time when the most recent
conflicting packet was received. An element may be removed from the
list when no conflicting packet has arrived from that source for a
time on the order of 10 RTCP report intervals (see Section 6.2).
For the algorithm as shown, it is assumed that the participant's own
source identifier and state are included in the source identifier
table. The algorithm could be restructured to first make a separate
comparison against the participant's own source identifier.
IF the SSRC or CSRC identifier is not found in the source
identifier table:
THEN create a new entry storing the source transport address
and the SSRC or CSRC along with other state.
CONTINUE with normal processing.
(identifier is found in the table)
IF the source transport address from the packet matches
the one saved in the table entry for this identifier:
THEN CONTINUE with normal processing.
(an identifier collision or a loop is indicated)
IF the source identifier is not the participant's own:
THEN IF the source identifier is from an RTCP SDES chunk
containing a CNAME item that differs from the CNAME
in the table entry:
THEN (optionally) count a third-party collision.
ELSE (optionally) count a third-party loop.
ABORT processing of data packet or control element.
(a collision or loop of the participant's own data)
IF the source transport address is found in the list of
conflicting addresses:
THEN IF the source identifier is not from an RTCP SDES chunk
containing a CNAME item OR if that CNAME is the
participant's own:
Schulzrinne, et al Standards Track [Page 47]
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THEN (optionally) count occurrence of own traffic looped.
mark current time in conflicting address list entry.
ABORT processing of data packet or control element.
log occurrence of a collision.
create a new entry in the conflicting address list and
mark current time.
send an RTCP BYE packet with the old SSRC identifier.
choose a new identifier.
create a new entry in the source identifier table with the
old SSRC plus the source transport address from the packet
being processed.
CONTINUE with normal processing.
In this algorithm, packets from a newly conflicting source address
will be ignored and packets from the original source will be kept.
(If the original source was through a mixer and later the same source
is received directly, the receiver may be well advised to switch
unless other sources in the mix would be lost.) If no packets arrive
from the original source for an extended period, the table entry will
be timed out and the new source will be able to take over. This might
occur if the original source detects the collision and moves to a new
source identifier, but in the usual case an RTCP BYE packet will be
received from the original source to delete the state without having
to wait for a timeout.
When a new SSRC identifier is chosen due to a collision, the
candidate identifier should first be looked up in the source
identifier table to see if it was already in use by some other
source. If so, another candidate should be generated and the process
repeated.
A loop of data packets to a multicast destination can cause severe
network flooding. All mixers and translators are required to
implement a loop detection algorithm like the one here so that they
can break loops. This should limit the excess traffic to no more than
one duplicate copy of the original traffic, which may allow the
session to continue so that the cause of the loop can be found and
fixed. However, in extreme cases where a mixer or translator does not
properly break the loop and high traffic levels result, it may be
necessary for end systems to cease transmitting data or control
packets entirely. This decision may depend upon the application. An
error condition should be indicated as appropriate. Transmission
might be attempted again periodically after a long, random time (on
the order of minutes).
Schulzrinne, et al Standards Track [Page 48]
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9. Security
Lower layer protocols may eventually provide all the security
services that may be desired for applications of RTP, including
authentication, integrity, and confidentiality. These services have
recently been specified for IP. Since the need for a confidentiality
service is well established in the initial audio and video
applications that are expected to use RTP, a confidentiality service
is defined in the next section for use with RTP and RTCP until lower
layer services are available. The overhead on the protocol for this
service is low, so the penalty will be minimal if this service is
obsoleted by lower layer services in the future.
Alternatively, other services, other implementations of services and
other algorithms may be defined for RTP in the future if warranted.
The selection presented here is meant to simplify implementation of
interoperable, secure applications and provide guidance to
implementors. No claim is made that the methods presented here are
appropriate for a particular security need. A profile may specify
which services and algorithms should be offered by applications, and
may provide guidance as to their appropriate use.
Key distribution and certificates are outside the scope of this
document.
9.1 Confidentiality
Confidentiality means that only the intended receiver(s) can decode
the received packets; for others, the packet contains no useful
information. Confidentiality of the content is achieved by
encryption.
When encryption of RTP or RTCP is desired, all the octets that will
be encapsulated for transmission in a single lower-layer packet are
encrypted as a unit. For RTCP, a 32-bit random number is prepended to
the unit before encryption to deter known plaintext attacks. For RTP,
no prefix is required because the sequence number and timestamp
fields are initialized with random offsets.
For RTCP, it is allowed to split a compound RTCP packet into two
lower-layer packets, one to be encrypted and one to be sent in the
clear. For example, SDES information might be encrypted while
reception reports were sent in the clear to accommodate third-party
monitors that are not privy to the encryption key. In this example,
depicted in Fig. 4, the SDES information must be appended to an RR
packet with no reports (and the encrypted) to satisfy the requirement
that all compound RTCP packets begin with an SR or RR packet.
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UDP packet UDP packet
------------------------------------- -------------------------
[32-bit ][ ][ # ] [ # sender # receiver]
[random ][ RR ][SDES # CNAME, ...] [ SR # report # report ]
[integer][(empty)][ # ] [ # # ]
------------------------------------- -------------------------
encrypted not encrypted
#: SSRC
Figure 4: Encrypted and non-encrypted RTCP packets
The presence of encryption and the use of the correct key are
confirmed by the receiver through header or payload validity checks.
Examples of such validity checks for RTP and RTCP headers are given
in Appendices A.1 and A.2.
The default encryption algorithm is the Data Encryption Standard
(DES) algorithm in cipher block chaining (CBC) mode, as described in
Section 1.1 of RFC 1423 [21], except that padding to a multiple of 8
octets is indicated as described for the P bit in Section 5.1. The
initialization vector is zero because random values are supplied in
the RTP header or by the random prefix for compound RTCP packets. For
details on the use of CBC initialization vectors, see [22].
Implementations that support encryption should always support the DES
algorithm in CBC mode as the default to maximize interoperability.
This method is chosen because it has been demonstrated to be easy and
practical to use in experimental audio and video tools in operation
on the Internet. Other encryption algorithms may be specified
dynamically for a session by non-RTP means.
As an alternative to encryption at the RTP level as described above,
profiles may define additional payload types for encrypted encodings.
Those encodings must specify how padding and other aspects of the
encryption should be handled. This method allows encrypting only the
data while leaving the headers in the clear for applications where
that is desired. It may be particularly useful for hardware devices
that will handle both decryption and decoding.
9.2 Authentication and Message Integrity
Authentication and message integrity are not defined in the current
specification of RTP since these services would not be directly
feasible without a key management infrastructure. It is expected that
authentication and integrity services will be provided by lower layer
protocols in the future.
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10. RTP over Network and Transport Protocols
This section describes issues specific to carrying RTP packets within
particular network and transport protocols. The following rules apply
unless superseded by protocol-specific definitions outside this
specification.
RTP relies on the underlying protocol(s) to provide demultiplexing of
RTP data and RTCP control streams. For UDP and similar protocols, RTP
uses an even port number and the corresponding RTCP stream uses the
next higher (odd) port number. If an application is supplied with an
odd number for use as the RTP port, it should replace this number
with the next lower (even) number.
RTP data packets contain no length field or other delineation,
therefore RTP relies on the underlying protocol(s) to provide a
length indication. The maximum length of RTP packets is limited only
by the underlying protocols.
If RTP packets are to be carried in an underlying protocol that
provides the abstraction of a continuous octet stream rather than
messages (packets), an encapsulation of the RTP packets must be
defined to provide a framing mechanism. Framing is also needed if the
underlying protocol may contain padding so that the extent of the RTP
payload cannot be determined. The framing mechanism is not defined
here.
A profile may specify a framing method to be used even when RTP is
carried in protocols that do provide framing in order to allow
carrying several RTP packets in one lower-layer protocol data unit,
such as a UDP packet. Carrying several RTP packets in one network or
transport packet reduces header overhead and may simplify
synchronization between different streams.
11. Summary of Protocol Constants
This section contains a summary listing of the constants defined in
this specification.
The RTP payload type (PT) constants are defined in profiles rather
than this document. However, the octet of the RTP header which
contains the marker bit(s) and payload type must avoid the reserved
values 200 and 201 (decimal) to distinguish RTP packets from the RTCP
SR and RR packet types for the header validation procedure described
in Appendix A.1. For the standard definition of one marker bit and a
7-bit payload type field as shown in this specification, this
restriction means that payload types 72 and 73 are reserved.
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11.1 RTCP packet types
abbrev. name value
SR sender report 200
RR receiver report 201
SDES source description 202
BYE goodbye 203
APP application-defined 204
These type values were chosen in the range 200-204 for improved
header validity checking of RTCP packets compared to RTP packets or
other unrelated packets. When the RTCP packet type field is compared
to the corresponding octet of the RTP header, this range corresponds
to the marker bit being 1 (which it usually is not in data packets)
and to the high bit of the standard payload type field being 1 (since
the static payload types are typically defined in the low half). This
range was also chosen to be some distance numerically from 0 and 255
since all-zeros and all-ones are common data patterns.
Since all compound RTCP packets must begin with SR or RR, these codes
were chosen as an even/odd pair to allow the RTCP validity check to
test the maximum number of bits with mask and value.
Other constants are assigned by IANA. Experimenters are encouraged to
register the numbers they need for experiments, and then unregister
those which prove to be unneeded.
11.2 SDES types
abbrev. name value
END end of SDES list 0
CNAME canonical name 1
NAME user name 2
EMAIL user's electronic mail address 3
PHONE user's phone number 4
LOC geographic user location 5
TOOL name of application or tool 6
NOTE notice about the source 7
PRIV private extensions 8
Other constants are assigned by IANA. Experimenters are encouraged to
register the numbers they need for experiments, and then unregister
those which prove to be unneeded.
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12. RTP Profiles and Payload Format Specifications
A complete specification of RTP for a particular application will
require one or more companion documents of two types described here:
profiles, and payload format specifications.
RTP may be used for a variety of applications with somewhat differing
requirements. The flexibility to adapt to those requirements is
provided by allowing multiple choices in the main protocol
specification, then selecting the appropriate choices or defining
extensions for a particular environment and class of applications in
a separate profile document. Typically an application will operate
under only one profile so there is no explicit indication of which
profile is in use. A profile for audio and video applications may be
found in the companion Internet-Draft draft-ietf-avt-profile for
The second type of companion document is a payload format
specification, which defines how a particular kind of payload data,
such as H.261 encoded video, should be carried in RTP. These
documents are typically titled "RTP Payload Format for XYZ
Audio/Video Encoding". Payload formats may be useful under multiple
profiles and may therefore be defined independently of any particular
profile. The profile documents are then responsible for assigning a
default mapping of that format to a payload type value if needed.
Within this specification, the following items have been identified
for possible definition within a profile, but this list is not meant
to be exhaustive:
RTP data header: The octet in the RTP data header that contains the
marker bit and payload type field may be redefined by a profile
to suit different requirements, for example with more or fewer
marker bits (Section 5.3).
Payload types: Assuming that a payload type field is included, the
profile will usually define a set of payload formats (e.g.,
media encodings) and a default static mapping of those formats
to payload type values. Some of the payload formats may be
defined by reference to separate payload format specifications.
For each payload type defined, the profile must specify the RTP
timestamp clock rate to be used (Section 5.1).
RTP data header additions: Additional fields may be appended to the
fixed RTP data header if some additional functionality is
required across the profile's class of applications independent
of payload type (Section 5.3).
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RTP data header extensions: The contents of the first 16 bits of the
RTP data header extension structure must be defined if use of
that mechanism is to be allowed under the profile for
implementation-specific extensions (Section 5.3.1).
RTCP packet types: New application-class-specific RTCP packet types
may be defined and registered with IANA.
RTCP report interval: A profile should specify that the values
suggested in Section 6.2 for the constants employed in the
calculation of the RTCP report interval will be used. Those are
the RTCP fraction of session bandwidth, the minimum report
interval, and the bandwidth split between senders and receivers.
A profile may specify alternate values if they have been
demonstrated to work in a scalable manner.
SR/RR extension: An extension section may be defined for the RTCP SR
and RR packets if there is additional information that should be
reported regularly about the sender or receivers (Section 6.3.3).
SDES use: The profile may specify the relative priorities for RTCP
SDES items to be transmitted or excluded entirely (Section
6.2.2); an alternate syntax or semantics for the CNAME item
(Section 6.4.1); the format of the LOC item (Section 6.4.5); the
semantics and use of the NOTE item (Section 6.4.7); or new SDES
item types to be registered with IANA.
Security: A profile may specify which security services and
algorithms should be offered by applications, and may provide
guidance as to their appropriate use (Section 9).
String-to-key mapping: A profile may specify how a user-provided
password or pass phrase is mapped into an encryption key.
Underlying protocol: Use of a particular underlying network or
transport layer protocol to carry RTP packets may be required.
Transport mapping: A mapping of RTP and RTCP to transport-level
addresses, e.g., UDP ports, other than the standard mapping
defined in Section 10 may be specified.
Encapsulation: An encapsulation of RTP packets may be defined to
allow multiple RTP data packets to be carried in one lower-layer
packet or to provide framing over underlying protocols that do
not already do so (Section 10).
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It is not expected that a new profile will be required for every
application. Within one application class, it would be better to
extend an existing profile rather than make a new one in order to
facilitate interoperation among the applications since each will
typically run under only one profile. Simple extensions such as the
definition of additional payload type values or RTCP packet types may
be accomplished by registering them through the Internet Assigned
Numbers Authority and publishing their descriptions in an addendum to
the profile or in a payload format specification.
Schulzrinne, et al Standards Track [Page 55]
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A. Algorithms
We provide examples of C code for aspects of RTP sender and receiver
algorithms. There may be other implementation methods that are faster
in particular operating environments or have other advantages. These
implementation notes are for informational purposes only and are
meant to clarify the RTP specification.
The following definitions are used for all examples; for clarity and
brevity, the structure definitions are only valid for 32-bit big-
endian (most significant octet first) architectures. Bit fields are
assumed to be packed tightly in big-endian bit order, with no
additional padding. Modifications would be required to construct a
portable implementation.
/*
* rtp.h -- RTP header file (RFC XXXX)
*/
#include <sys/types.h>
/*
* The type definitions below are valid for 32-bit architectures and
* may have to be adjusted for 16- or 64-bit architectures.
*/
typedef unsigned char u_int8;
typedef unsigned short u_int16;
typedef unsigned int u_int32;
typedef short int16;
/*
* Current protocol version.
*/
#define RTP_VERSION 2
#define RTP_SEQ_MOD (1<<16)
#define RTP_MAX_SDES 255 /* maximum text length for SDES */
typedef enum {
RTCP_SR = 200,
RTCP_RR = 201,
RTCP_SDES = 202,
RTCP_BYE = 203,
RTCP_APP = 204
} rtcp_type_t;
typedef enum {
RTCP_SDES_END = 0,
RTCP_SDES_CNAME = 1,
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RTCP_SDES_NAME = 2,
RTCP_SDES_EMAIL = 3,
RTCP_SDES_PHONE = 4,
RTCP_SDES_LOC = 5,
RTCP_SDES_TOOL = 6,
RTCP_SDES_NOTE = 7,
RTCP_SDES_PRIV = 8
} rtcp_sdes_type_t;
/*
* RTP data header
*/
typedef struct {
unsigned int version:2; /* protocol version */
unsigned int p:1; /* padding flag */
unsigned int x:1; /* header extension flag */
unsigned int cc:4; /* CSRC count */
unsigned int m:1; /* marker bit */
unsigned int pt:7; /* payload type */
u_int16 seq; /* sequence number */
u_int32 ts; /* timestamp */
u_int32 ssrc; /* synchronization source */
u_int32 csrc[1]; /* optional CSRC list */
} rtp_hdr_t;
/*
* RTCP common header word
*/
typedef struct {
unsigned int version:2; /* protocol version */
unsigned int p:1; /* padding flag */
unsigned int count:5; /* varies by packet type */
unsigned int pt:8; /* RTCP packet type */
u_int16 length; /* pkt len in words, w/o this word */
} rtcp_common_t;
/*
* Big-endian mask for version, padding bit and packet type pair
*/
#define RTCP_VALID_MASK (0xc000 | 0x2000 | 0xfe)
#define RTCP_VALID_VALUE ((RTP_VERSION << 14) | RTCP_SR)
/*
* Reception report block
*/
typedef struct {
u_int32 ssrc; /* data source being reported */
unsigned int fraction:8; /* fraction lost since last SR/RR */
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int lost:24; /* cumul. no. pkts lost (signed!) */
u_int32 last_seq; /* extended last seq. no. received */
u_int32 jitter; /* interarrival jitter */
u_int32 lsr; /* last SR packet from this source */
u_int32 dlsr; /* delay since last SR packet */
} rtcp_rr_t;
/*
* SDES item
*/
typedef struct {
u_int8 type; /* type of item (rtcp_sdes_type_t) */
u_int8 length; /* length of item (in octets) */
char data[1]; /* text, not null-terminated */
} rtcp_sdes_item_t;
/*
* One RTCP packet
*/
typedef struct {
rtcp_common_t common; /* common header */
union {
/* sender report (SR) */
struct {
u_int32 ssrc; /* sender generating this report */
u_int32 ntp_sec; /* NTP timestamp */
u_int32 ntp_frac;
u_int32 rtp_ts; /* RTP timestamp */
u_int32 psent; /* packets sent */
u_int32 osent; /* octets sent */
rtcp_rr_t rr[1]; /* variable-length list */
} sr;
/* reception report (RR) */
struct {
u_int32 ssrc; /* receiver generating this report */
rtcp_rr_t rr[1]; /* variable-length list */
} rr;
/* source description (SDES) */
struct rtcp_sdes {
u_int32 src; /* first SSRC/CSRC */
rtcp_sdes_item_t item[1]; /* list of SDES items */
} sdes;
/* BYE */
struct {
u_int32 src[1]; /* list of sources */
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/* can't express trailing text for reason */
} bye;
} r;
} rtcp_t;
typedef struct rtcp_sdes rtcp_sdes_t;
/*
* Per-source state information
*/
typedef struct {
u_int16 max_seq; /* highest seq. number seen */
u_int32 cycles; /* shifted count of seq. number cycles */
u_int32 base_seq; /* base seq number */
u_int32 bad_seq; /* last 'bad' seq number + 1 */
u_int32 probation; /* sequ. packets till source is valid */
u_int32 received; /* packets received */
u_int32 expected_prior; /* packet expected at last interval */
u_int32 received_prior; /* packet received at last interval */
u_int32 transit; /* relative trans time for prev pkt */
u_int32 jitter; /* estimated jitter */
/* ... */
} source;
A.1 RTP Data Header Validity Checks
An RTP receiver should check the validity of the RTP header on
incoming packets since they might be encrypted or might be from a
different application that happens to be misaddressed. Similarly, if
encryption is enabled, the header validity check is needed to verify
that incoming packets have been correctly decrypted, although a
failure of the header validity check (e.g., unknown payload type) may
not necessarily indicate decryption failure.
Only weak validity checks are possible on an RTP data packet from a
source that has not been heard before:
o RTP version field must equal 2.
o The payload type must be known, in particular it must not be
equal to SR or RR.
o If the P bit is set, then the last octet of the packet must
contain a valid octet count, in particular, less than the total
packet length minus the header size.
o The X bit must be zero if the profile does not specify that
the header extension mechanism may be used. Otherwise, the
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extension length field must be less than the total packet size
minus the fixed header length and padding.
o The length of the packet must be consistent with CC and
payload type (if payloads have a known length).
The last three checks are somewhat complex and not always possible,
leaving only the first two which total just a few bits. If the SSRC
identifier in the packet is one that has been received before, then
the packet is probably valid and checking if the sequence number is
in the expected range provides further validation. If the SSRC
identifier has not been seen before, then data packets carrying that
identifier may be considered invalid until a small number of them
arrive with consecutive sequence numbers.
The routine update_seq shown below ensures that a source is declared
valid only after MIN_SEQUENTIAL packets have been received in
sequence. It also validates the sequence number seq of a newly
received packet and updates the sequence state for the packet's
source in the structure to which s points.
When a new source is heard for the first time, that is, its SSRC
identifier is not in the table (see Section 8.2), and the per-source
state is allocated for it, s->probation should be set to the number
of sequential packets required before declaring a source valid
(parameter MIN_SEQUENTIAL ) and s->max_seq initialized to seq-1 s-
>probation marks the source as not yet valid so the state may be
discarded after a short timeout rather than a long one, as discussed
in Section 6.2.1.
After a source is considered valid, the sequence number is considered
valid if it is no more than MAX_DROPOUT ahead of s->max_seq nor more
than MAX_MISORDER behind. If the new sequence number is ahead of
max_seq modulo the RTP sequence number range (16 bits), but is
smaller than max_seq , it has wrapped around and the (shifted) count
of sequence number cycles is incremented. A value of one is returned
to indicate a valid sequence number.
Otherwise, the value zero is returned to indicate that the validation
failed, and the bad sequence number is stored. If the next packet
received carries the next higher sequence number, it is considered
the valid start of a new packet sequence presumably caused by an
extended dropout or a source restart. Since multiple complete
sequence number cycles may have been missed, the packet loss
statistics are reset.
Typical values for the parameters are shown, based on a maximum
misordering time of 2 seconds at 50 packets/second and a maximum
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dropout of 1 minute. The dropout parameter MAX_DROPOUT should be a
small fraction of the 16-bit sequence number space to give a
reasonable probability that new sequence numbers after a restart will
not fall in the acceptable range for sequence numbers from before the
restart.
void init_seq(source *s, u_int16 seq)
{
s->base_seq = seq - 1;
s->max_seq = seq;
s->bad_seq = RTP_SEQ_MOD + 1;
s->cycles = 0;
s->received = 0;
s->received_prior = 0;
s->expected_prior = 0;
/* other initialization */
}
int update_seq(source *s, u_int16 seq)
{
u_int16 udelta = seq - s->max_seq;
const int MAX_DROPOUT = 3000;
const int MAX_MISORDER = 100;
const int MIN_SEQUENTIAL = 2;
/*
* Source is not valid until MIN_SEQUENTIAL packets with
* sequential sequence numbers have been received.
*/
if (s->probation) {
/* packet is in sequence */
if (seq == s->max_seq + 1) {
s->probation--;
s->max_seq = seq;
if (s->probation == 0) {
init_seq(s, seq);
s->received++;
return 1;
}
} else {
s->probation = MIN_SEQUENTIAL - 1;
s->max_seq = seq;
}
return 0;
} else if (udelta < MAX_DROPOUT) {
/* in order, with permissible gap */
if (seq < s->max_seq) {
/*
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* Sequence number wrapped - count another 64K cycle.
*/
s->cycles += RTP_SEQ_MOD;
}
s->max_seq = seq;
} else if (udelta <= RTP_SEQ_MOD - MAX_MISORDER) {
/* the sequence number made a very large jump */
if (seq == s->bad_seq) {
/*
* Two sequential packets -- assume that the other side
* restarted without telling us so just re-sync
* (i.e., pretend this was the first packet).
*/
init_seq(s, seq);
}
else {
s->bad_seq = (seq + 1) & (RTP_SEQ_MOD-1);
return 0;
}
} else {
/* duplicate or reordered packet */
}
s->received++;
return 1;
}
The validity check can be made stronger requiring more than two
packets in sequence. The disadvantages are that a larger number of
initial packets will be discarded and that high packet loss rates
could prevent validation. However, because the RTCP header validation
is relatively strong, if an RTCP packet is received from a source
before the data packets, the count could be adjusted so that only two
packets are required in sequence. If initial data loss for a few
seconds can be tolerated, an application could choose to discard all
data packets from a source until a valid RTCP packet has been
received from that source.
Depending on the application and encoding, algorithms may exploit
additional knowledge about the payload format for further validation.
For payload types where the timestamp increment is the same for all
packets, the timestamp values can be predicted from the previous
packet received from the same source using the sequence number
difference (assuming no change in payload type).
A strong "fast-path" check is possible since with high probability
the first four octets in the header of a newly received RTP data
packet will be just the same as that of the previous packet from the
same SSRC except that the sequence number will have increased by one.
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Similarly, a single-entry cache may be used for faster SSRC lookups
in applications where data is typically received from one source at a
time.
A.2 RTCP Header Validity Checks
The following checks can be applied to RTCP packets.
o RTP version field must equal 2.
o The payload type field of the first RTCP packet in a compound
packet must be equal to SR or RR.
o The padding bit (P) should be zero for the first packet of a
compound RTCP packet because only the last should possibly need
padding.
o The length fields of the individual RTCP packets must total to
the overall length of the compound RTCP packet as received.
This is a fairly strong check.
The code fragment below performs all of these checks. The packet type
is not checked for subsequent packets since unknown packet types may
be present and should be ignored.
u_int32 len; /* length of compound RTCP packet in words */
rtcp_t *r; /* RTCP header */
rtcp_t *end; /* end of compound RTCP packet */
if ((*(u_int16 *)r & RTCP_VALID_MASK) != RTCP_VALID_VALUE) {
/* something wrong with packet format */
}
end = (rtcp_t *)((u_int32 *)r + len);
do r = (rtcp_t *)((u_int32 *)r + r->common.length + 1);
while (r < end && r->common.version == 2);
if (r != end) {
/* something wrong with packet format */
}
A.3 Determining the Number of RTP Packets Expected and Lost
In order to compute packet loss rates, the number of packets expected
and actually received from each source needs to be known, using per-
source state information defined in struct source referenced via
pointer s in the code below. The number of packets received is simply
the count of packets as they arrive, including any late or duplicate
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packets. The number of packets expected can be computed by the
receiver as the difference between the highest sequence number
received ( s->max_seq ) and the first sequence number received ( s-
>base_seq ). Since the sequence number is only 16 bits and will wrap
around, it is necessary to extend the highest sequence number with
the (shifted) count of sequence number wraparounds ( s->cycles ).
Both the received packet count and the count of cycles are maintained
the RTP header validity check routine in Appendix A.1.
extended_max = s->cycles + s->max_seq;
expected = extended_max - s->base_seq + 1;
The number of packets lost is defined to be the number of packets
expected less the number of packets actually received:
lost = expected - s->received;
Since this number is carried in 24 bits, it should be clamped at
0xffffff rather than wrap around to zero.
The fraction of packets lost during the last reporting interval
(since the previous SR or RR packet was sent) is calculated from
differences in the expected and received packet counts across the
interval, where expected_prior and received_prior are the values
saved when the previous reception report was generated:
expected_interval = expected - s->expected_prior;
s->expected_prior = expected;
received_interval = s->received - s->received_prior;
s->received_prior = s->received;
lost_interval = expected_interval - received_interval;
if (expected_interval == 0 || lost_interval <= 0) fraction = 0;
else fraction = (lost_interval << 8) / expected_interval;
The resulting fraction is an 8-bit fixed point number with the binary
point at the left edge.
A.4 Generating SDES RTCP Packets
This function builds one SDES chunk into buffer b composed of argc
items supplied in arrays type , value and length b
char *rtp_write_sdes(char *b, u_int32 src, int argc,
rtcp_sdes_type_t type[], char *value[],
int length[])
{
rtcp_sdes_t *s = (rtcp_sdes_t *)b;
rtcp_sdes_item_t *rsp;
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int i;
int len;
int pad;
/* SSRC header */
s->src = src;
rsp = &s->item[0];
/* SDES items */
for (i = 0; i < argc; i++) {
rsp->type = type[i];
len = length[i];
if (len > RTP_MAX_SDES) {
/* invalid length, may want to take other action */
len = RTP_MAX_SDES;
}
rsp->length = len;
memcpy(rsp->data, value[i], len);
rsp = (rtcp_sdes_item_t *)&rsp->data[len];
}
/* terminate with end marker and pad to next 4-octet boundary */
len = ((char *) rsp) - b;
pad = 4 - (len & 0x3);
b = (char *) rsp;
while (pad--) *b++ = RTCP_SDES_END;
return b;
}
A.5 Parsing RTCP SDES Packets
This function parses an SDES packet, calling functions find_member()
to find a pointer to the information for a session member given the
SSRC identifier and member_sdes() to store the new SDES information
for that member. This function expects a pointer to the header of the
RTCP packet.
void rtp_read_sdes(rtcp_t *r)
{
int count = r->common.count;
rtcp_sdes_t *sd = &r->r.sdes;
rtcp_sdes_item_t *rsp, *rspn;
rtcp_sdes_item_t *end = (rtcp_sdes_item_t *)
((u_int32 *)r + r->common.length + 1);
source *s;
while (--count >= 0) {
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rsp = &sd->item[0];
if (rsp >= end) break;
s = find_member(sd->src);
for (; rsp->type; rsp = rspn ) {
rspn = (rtcp_sdes_item_t *)((char*)rsp+rsp->length+2);
if (rspn >= end) {
rsp = rspn;
break;
}
member_sdes(s, rsp->type, rsp->data, rsp->length);
}
sd = (rtcp_sdes_t *)
((u_int32 *)sd + (((char *)rsp - (char *)sd) >> 2)+1);
}
if (count >= 0) {
/* invalid packet format */
}
}
A.6 Generating a Random 32-bit Identifier
The following subroutine generates a random 32-bit identifier using
the MD5 routines published in RFC 1321 [23]. The system routines may
not be present on all operating systems, but they should serve as
hints as to what kinds of information may be used. Other system calls
that may be appropriate include
o getdomainname() ,
o getwd() , or
o getrusage()
"Live" video or audio samples are also a good source of random
numbers, but care must be taken to avoid using a turned-off
microphone or blinded camera as a source [7].
Use of this or similar routine is suggested to generate the initial
seed for the random number generator producing the RTCP period (as
shown in Appendix A.7), to generate the initial values for the
sequence number and timestamp, and to generate SSRC values. Since
this routine is likely to be CPU-intensive, its direct use to
generate RTCP periods is inappropriate because predictability is not
an issue. Note that this routine produces the same result on repeated
calls until the value of the system clock changes unless different
values are supplied for the type argument.
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RFC 1889 RTP January 1996
/*
* Generate a random 32-bit quantity.
*/
#include <sys/types.h> /* u_long */
#include <sys/time.h> /* gettimeofday() */
#include <unistd.h> /* get..() */
#include <stdio.h> /* printf() */
#include <time.h> /* clock() */
#include <sys/utsname.h> /* uname() */
#include "global.h" /* from RFC 1321 */
#include "md5.h" /* from RFC 1321 */
#define MD_CTX MD5_CTX
#define MDInit MD5Init
#define MDUpdate MD5Update
#define MDFinal MD5Final
static u_long md_32(char *string, int length)
{
MD_CTX context;
union {
char c[16];
u_long x[4];
} digest;
u_long r;
int i;
MDInit (&context);
MDUpdate (&context, string, length);
MDFinal ((unsigned char *)&digest, &context);
r = 0;
for (i = 0; i < 3; i++) {
r ^= digest.x[i];
}
return r;
} /* md_32 */
/*
* Return random unsigned 32-bit quantity. Use 'type' argument if you
* need to generate several different values in close succession.
*/
u_int32 random32(int type)
{
struct {
int type;
struct timeval tv;
clock_t cpu;
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RFC 1889 RTP January 1996
pid_t pid;
u_long hid;
uid_t uid;
gid_t gid;
struct utsname name;
} s;
gettimeofday(&s.tv, 0);
uname(&s.name);
s.type = type;
s.cpu = clock();
s.pid = getpid();
s.hid = gethostid();
s.uid = getuid();
s.gid = getgid();
return md_32((char *)&s, sizeof(s));
} /* random32 */
A.7 Computing the RTCP Transmission Interval
The following function returns the time between transmissions of RTCP
packets, measured in seconds. It should be called after sending one
compound RTCP packet to calculate the delay until the next should be
sent. This function should also be called to calculate the delay
before sending the first RTCP packet upon startup rather than send
the packet immediately. This avoids any burst of RTCP packets if an
application is started at many sites simultaneously, for example as a
result of a session announcement.
The parameters have the following meaning:
rtcp_bw: The target RTCP bandwidth, i.e., the total bandwidth that
will be used for RTCP packets by all members of this session, in
octets per second. This should be 5% of the "session bandwidth"
parameter supplied to the application at startup.
senders: Number of active senders since sending last report, known
from construction of receiver reports for this RTCP packet.
Includes ourselves, if we also sent during this interval.
members: The estimated number of session members, including
ourselves. Incremented as we discover new session members from
the receipt of RTP or RTCP packets, and decremented as session
members leave (via RTCP BYE) or their state is timed out (30
minutes is recommended). On the first call, this parameter
should have the value 1.
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we_sent: Flag that is true if we have sent data during the last two
RTCP intervals. If the flag is true, the compound RTCP packet
just sent contained an SR packet.
packet_size: The size of the compound RTCP packet just sent, in
octets, including the network encapsulation (e.g., 28 octets for
UDP over IP).
avg_rtcp_size: Pointer to estimator for compound RTCP packet size;
initialized and updated by this function for the packet just
sent, and also updated by an identical line of code in the RTCP
receive routine for every RTCP packet received from other
participants in the session.
initial: Flag that is true for the first call upon startup to
calculate the time until the first report should be sent.
#include <math.h>
double rtcp_interval(int members,
int senders,
double rtcp_bw,
int we_sent,
int packet_size,
int *avg_rtcp_size,
int initial)
{
/*
* Minimum time between RTCP packets from this site (in seconds).
* This time prevents the reports from `clumping' when sessions
* are small and the law of large numbers isn't helping to smooth
* out the traffic. It also keeps the report interval from
* becoming ridiculously small during transient outages like a
* network partition.
*/
double const RTCP_MIN_TIME = 5.;
/*
* Fraction of the RTCP bandwidth to be shared among active
* senders. (This fraction was chosen so that in a typical
* session with one or two active senders, the computed report
* time would be roughly equal to the minimum report time so that
* we don't unnecessarily slow down receiver reports.) The
* receiver fraction must be 1 - the sender fraction.
*/
double const RTCP_SENDER_BW_FRACTION = 0.25;
double const RTCP_RCVR_BW_FRACTION = (1-RTCP_SENDER_BW_FRACTION);
/*
* Gain (smoothing constant) for the low-pass filter that
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* estimates the average RTCP packet size (see Cadzow reference).
*/
double const RTCP_SIZE_GAIN = (1./16.);
double t; /* interval */
double rtcp_min_time = RTCP_MIN_TIME;
int n; /* no. of members for computation */
/*
* Very first call at application start-up uses half the min
* delay for quicker notification while still allowing some time
* before reporting for randomization and to learn about other
* sources so the report interval will converge to the correct
* interval more quickly. The average RTCP size is initialized
* to 128 octets which is conservative (it assumes everyone else
* is generating SRs instead of RRs: 20 IP + 8 UDP + 52 SR + 48
* SDES CNAME).
*/
if (initial) {
rtcp_min_time /= 2;
*avg_rtcp_size = 128;
}
/*
* If there were active senders, give them at least a minimum
* share of the RTCP bandwidth. Otherwise all participants share
* the RTCP bandwidth equally.
*/
n = members;
if (senders > 0 && senders < members * RTCP_SENDER_BW_FRACTION) {
if (we_sent) {
rtcp_bw *= RTCP_SENDER_BW_FRACTION;
n = senders;
} else {
rtcp_bw *= RTCP_RCVR_BW_FRACTION;
n -= senders;
}
}
/*
* Update the average size estimate by the size of the report
* packet we just sent.
*/
*avg_rtcp_size += (packet_size - *avg_rtcp_size)*RTCP_SIZE_GAIN;
/*
* The effective number of sites times the average packet size is
* the total number of octets sent when each site sends a report.
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* Dividing this by the effective bandwidth gives the time
* interval over which those packets must be sent in order to
* meet the bandwidth target, with a minimum enforced. In that
* time interval we send one report so this time is also our
* average time between reports.
*/
t = (*avg_rtcp_size) * n / rtcp_bw;
if (t < rtcp_min_time) t = rtcp_min_time;
/*
* To avoid traffic bursts from unintended synchronization with
* other sites, we then pick our actual next report interval as a
* random number uniformly distributed between 0.5*t and 1.5*t.
*/
return t * (drand48() + 0.5);
}
A.8 Estimating the Interarrival Jitter
The code fragments below implement the algorithm given in Section
6.3.1 for calculating an estimate of the statistical variance of the
RTP data interarrival time to be inserted in the interarrival jitter
field of reception reports. The inputs are r->ts , the timestamp from
the incoming packet, and arrival , the current time in the same
units. Here s points to state for the source; s->transit holds the
relative transit time for the previous packet, and s->jitter holds
the estimated jitter. The jitter field of the reception report is
measured in timestamp units and expressed as an unsigned integer, but
the jitter estimate is kept in a floating point. As each data packet
arrives, the jitter estimate is updated:
int transit = arrival - r->ts;
int d = transit - s->transit;
s->transit = transit;
if (d < 0) d = -d;
s->jitter += (1./16.) * ((double)d - s->jitter);
When a reception report block (to which rr points) is generated for
this member, the current jitter estimate is returned:
rr->jitter = (u_int32) s->jitter;
Alternatively, the jitter estimate can be kept as an integer, but
scaled to reduce round-off error. The calculation is the same except
for the last line:
s->jitter += d - ((s->jitter + 8) >> 4);
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In this case, the estimate is sampled for the reception report as:
rr->jitter = s->jitter >> 4;
B. Security Considerations
RTP suffers from the same security liabilities as the underlying
protocols. For example, an impostor can fake source or destination
network addresses, or change the header or payload. Within RTCP, the
CNAME and NAME information may be used to impersonate another
participant. In addition, RTP may be sent via IP multicast, which
provides no direct means for a sender to know all the receivers of
the data sent and therefore no measure of privacy. Rightly or not,
users may be more sensitive to privacy concerns with audio and video
communication than they have been with more traditional forms of
network communication [24]. Therefore, the use of security mechanisms
with RTP is important. These mechanisms are discussed in Section 9.
RTP-level translators or mixers may be used to allow RTP traffic to
reach hosts behind firewalls. Appropriate firewall security
principles and practices, which are beyond the scope of this
document, should be followed in the design and installation of these
devices and in the admission of RTP applications for use behind the
firewall.
C. Authors' Addresses
Henning Schulzrinne
GMD Fokus
Hardenbergplatz 2
D-10623 Berlin
Germany
EMail: schulzrinne@fokus.gmd.de
Stephen L. Casner
Precept Software, Inc.
21580 Stevens Creek Boulevard, Suite 207
Cupertino, CA 95014
United States
EMail: casner@precept.com
Schulzrinne, et al Standards Track [Page 72]
RFC 1889 RTP January 1996
Ron Frederick
Xerox Palo Alto Research Center
3333 Coyote Hill Road
Palo Alto, CA 94304
United States
EMail: frederic@parc.xerox.com
Van Jacobson
MS 46a-1121
Lawrence Berkeley National Laboratory
Berkeley, CA 94720
United States
EMail: van@ee.lbl.gov
Acknowledgments
This memorandum is based on discussions within the IETF Audio/Video
Transport working group chaired by Stephen Casner. The current
protocol has its origins in the Network Voice Protocol and the Packet
Video Protocol (Danny Cohen and Randy Cole) and the protocol
implemented by the vat application (Van Jacobson and Steve McCanne).
Christian Huitema provided ideas for the random identifier generator.
D. Bibliography
[1] D. D. Clark and D. L. Tennenhouse, "Architectural considerations
for a new generation of protocols," in SIGCOMM Symposium on
Communications Architectures and Protocols , (Philadelphia,
Pennsylvania), pp. 200--208, IEEE, Sept. 1990. Computer
Communications Review, Vol. 20(4), Sept. 1990.
[2] H. Schulzrinne, "Issues in designing a transport protocol for
audio and video conferences and other multiparticipant real-time
applications", Work in Progress.
[3] D. E. Comer, Internetworking with TCP/IP , vol. 1. Englewood
Cliffs, New Jersey: Prentice Hall, 1991.
[4] Postel, J., "Internet Protocol", STD 5, RFC 791, USC/Information
Sciences Institute, September 1981.
[5] Mills, D., "Network Time Protocol Version 3", RFC 1305, UDEL,
March 1992.
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[6] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
USC/Information Sciences Institute, October 1994.
[7] Eastlake, D., Crocker, S., and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, DEC, Cybercash, MIT,
December 1994.
[8] J.-C. Bolot, T. Turletti, and I. Wakeman, "Scalable feedback
control for multicast video distribution in the internet," in
SIGCOMM Symposium on Communications Architectures and Protocols ,
(London, England), pp. 58--67, ACM, Aug. 1994.
[9] I. Busse, B. Deffner, and H. Schulzrinne, "Dynamic QoS control of
multimedia applications based on RTP," Computer Communications ,
Jan. 1996.
[10] S. Floyd and V. Jacobson, "The synchronization of periodic
routing messages," in SIGCOMM Symposium on Communications
Architectures and Protocols (D. P. Sidhu, ed.), (San Francisco,
California), pp. 33--44, ACM, Sept. 1993. also in [25].
[11] J. A. Cadzow, Foundations of digital signal processing and data
analysis New York, New York: Macmillan, 1987.
[12] International Standards Organization, "ISO/IEC DIS 10646-1:1993
information technology -- universal multiple-octet coded
character set (UCS) -- part I: Architecture and basic
multilingual plane," 1993.
[13] The Unicode Consortium, The Unicode Standard New York, New York:
Addison-Wesley, 1991.
[14] Mockapetris, P., "Domain Names - Concepts and Facilities", STD
13, RFC 1034, USC/Information Sciences Institute, November 1987.
[15] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC 1035, USC/Information Sciences
Institute, November 1987.
[16] Braden, R., "Requirements for Internet Hosts - Application and
Support", STD 3, RFC 1123, Internet Engineering Task Force,
October 1989.
[17] Rekhter, Y., Moskowitz, R., Karrenberg, D., and G. de Groot,
"Address Allocation for Private Internets", RFC 1597, T.J. Watson
Research Center, IBM Corp., Chrysler Corp., RIPE NCC, March 1994.
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[18] Lear, E., Fair, E., Crocker, D., and T. Kessler, "Network 10
Considered Harmful (Some Practices Shouldn't be Codified)", RFC
1627, Silicon Graphics, Inc., Apple Computer, Inc., Silicon
Graphics, Inc., July 1994.
[19] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, UDEL, August 1982.
[20] W. Feller, An Introduction to Probability Theory and its
Applications, Volume 1 , vol. 1. New York, New York: John Wiley
and Sons, third ed., 1968.
[21] Balenson, D., "Privacy Enhancement for Internet Electronic Mail:
Part III: Algorithms, Modes, and Identifiers", RFC 1423, TIS, IAB
IRTF PSRG, IETF PEM WG, February 1993.
[22] V. L. Voydock and S. T. Kent, "Security mechanisms in high-level
network protocols," ACM Computing Surveys , vol. 15, pp. 135--
171, June 1983.
[23] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, MIT
Laboratory for Computer Science and RSA Data Security, Inc.,
April 1992.
[24] S. Stubblebine, "Security services for multimedia conferencing,"
in 16th National Computer Security Conference , (Baltimore,
Maryland), pp. 391--395, Sept. 1993.
[25] S. Floyd and V. Jacobson, "The synchronization of periodic
routing messages," IEEE/ACM Transactions on Networking , vol. 2,
pp. 122-136, April 1994.
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