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Keywords: transport, layer, telephony, signaling
Network Working Group L. Ong
Request for Comments: 3286 Ciena Corporation
Category: Informational J. Yoakum
Nortel Networks
May 2002
An Introduction to the Stream Control Transmission Protocol (SCTP)
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document provides a high level introduction to the capabilities
supported by the Stream Control Transmission Protocol (SCTP). It is
intended as a guide for potential users of SCTP as a general purpose
transport protocol.
1. Introduction
The Stream Control Transmission Protocol (SCTP) is a new IP transport
protocol, existing at an equivalent level with UDP (User Datagram
Protocol) and TCP (Transmission Control Protocol), which provide
transport layer functions to many Internet applications. SCTP has
been approved by the IETF as a Proposed Standard [1]. The error
check algorithm has since been modified [2]. Future changes and
updates will be reflected in the IETF RFC index.
Like TCP, SCTP provides a reliable transport service, ensuring that
data is transported across the network without error and in sequence.
Like TCP, SCTP is a session-oriented mechanism, meaning that a
relationship is created between the endpoints of an SCTP association
prior to data being transmitted, and this relationship is maintained
until all data transmission has been successfully completed.
Unlike TCP, SCTP provides a number of functions that are critical for
telephony signaling transport, and at the same time can potentially
benefit other applications needing transport with additional
performance and reliability. The original framework for the SCTP
definition is described in [3].
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2. Basic SCTP Features
SCTP is a unicast protocol, and supports data exchange between
exactly 2 endpoints, although these may be represented by multiple IP
addresses.
SCTP provides reliable transmission, detecting when data is
discarded, reordered, duplicated or corrupted, and retransmitting
damaged data as necessary. SCTP transmission is full duplex.
SCTP is message oriented and supports framing of individual message
boundaries. In comparison, TCP is byte oriented and does not
preserve any implicit structure within a transmitted byte stream
without enhancement.
SCTP is rate adaptive similar to TCP, and will scale back data
transfer to the prevailing load conditions in the network. It is
designed to behave cooperatively with TCP sessions attempting to use
the same bandwidth.
3. SCTP Multi-Streaming Feature
The name Stream Control Transmission Protocol is derived from the
multi-streaming function provided by SCTP. This feature allows data
to be partitioned into multiple streams that have the property of
independently sequenced delivery, so that message loss in any one
stream will only initially affect delivery within that stream, and
not delivery in other streams.
In contrast, TCP assumes a single stream of data and ensures that
delivery of that stream takes place with byte sequence preservation.
While this is desirable for delivery of a file or record, it causes
additional delay when message loss or sequence error occurs within
the network. When this happens, TCP must delay delivery of data
until the correct sequencing is restored, either by receipt of an
out-of-sequence message, or by retransmission of a lost message.
For a number of applications, the characteristic of strict sequence
preservation is not truly necessary. In telephony signaling, it is
only necessary to maintain sequencing of messages that affect the
same resource (e.g., the same call, or the same channel). Other
messages are only loosely correlated and can be delivered without
having to maintain overall sequence integrity.
Another example of possible use of multi-streaming is the delivery of
multimedia documents, such as a web page, when done over a single
session. Since multimedia documents consist of objects of different
sizes and types, multi-streaming allows transport of these components
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to be partially ordered rather than strictly ordered, and may result
in improved user perception of transport.
At the same time, transport is done within a single SCTP association,
so that all streams are subjected to a common flow and congestion
control mechanism, reducing the overhead required at the transport
level.
SCTP accomplishes multi-streaming by creating independence between
data transmission and data delivery. In particular, each payload
DATA "chunk" in the protocol uses two sets of sequence numbers, a
Transmission Sequence Number that governs the transmission of
messages and the detection of message loss, and the Stream ID/Stream
Sequence Number pair, which is used to determine the sequence of
delivery of received data.
This independence of mechanisms allows the receiver to determine
immediately when a gap in the transmission sequence occurs (e.g., due
to message loss), and also whether or not messages received following
the gap are within an affected stream. If a message is received
within the affected stream, there will be a corresponding gap in the
Stream Sequence Number, while messages from other streams will not
show a gap. The receiver can therefore continue to deliver messages
to the unaffected streams while buffering messages in the affected
stream until retransmission occurs.
4. SCTP Multi-Homing Feature
Another core feature of SCTP is multi-homing, or the ability for a
single SCTP endpoint to support multiple IP addresses. The benefit
of multi-homing is potentially greater survivability of the session
in the presence of network failures. In a conventional single-homed
session, the failure of a local LAN access can isolate the end
system, while failures within the core network can cause temporary
unavailability of transport until the IP routing protocols can
reconverge around the point of failure. Using multi-homed SCTP,
redundant LANs can be used to reinforce the local access, while
various options are possible in the core network to reduce the
dependency of failures for different addresses. Use of addresses
with different prefixes can force routing to go through different
carriers, for example, route-pinning techniques or even redundant
core networks can also be used if there is control over the network
architecture and protocols.
In its current form, SCTP does not do load sharing, that is, multi-
homing is used for redundancy purposes only. A single address is
chosen as the "primary" address and is used as the destination for
all DATA chunks for normal transmission. Retransmitted DATA chunks
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use the alternate address(es) to improve the probability of reaching
the remote endpoint, while continued failure to send to the primary
address ultimately results in the decision to transmit all DATA
chunks to the alternate until heartbeats can reestablish the
reachability of the primary.
To support multi-homing, SCTP endpoints exchange lists of addresses
during initiation of the association. Each endpoint must be able to
receive messages from any of the addresses associated with the remote
endpoint; in practice, certain operating systems may utilize
available source addresses in round robin fashion, in which case
receipt of messages from different source addresses will be the
normal case. A single port number is used across the entire address
list at an endpoint for a specific session.
In order to reduce the potential for security issues, it is required
that some response messages be sent specifically to the source
address in the message that caused the response. For example, when
the server receives an INIT chunk from a client to initiate an SCTP
association, the server always sends the response INIT ACK chunk to
the source address that was in the IP header of the INIT.
5. Features of the SCTP Initiation Procedure
The SCTP Initiation Procedure relies on a 4-message sequence, where
DATA can be included on the 3rd and 4th messages of the sequence, as
these messages are sent when the association has already been
validated. A "cookie" mechanism has been incorporated into the
sequence to guard against some types of denial of service attacks.
5.1 Cookie Mechanism
The "cookie" mechanism guards specifically against a blind attacker
generating INIT chunks to try to overload the resources of an SCTP
server by causing it to use up memory and resources handling new INIT
requests. Rather than allocating memory for a Transmission Control
Block (TCB), the server instead creates a Cookie parameter with the
TCB information, together with a valid lifetime and a signature for
authentication, and sends this back in the INIT ACK. Since the INIT
ACK always goes back to the source address of the INIT, the blind
attacker will not get the Cookie. A valid SCTP client will get the
Cookie and return it in the COOKIE ECHO chunk, where the SCTP server
can validate the Cookie and use it to rebuild the TCB. Since the
server creates the Cookie, only it needs to know the format and
secret key, this is not exchanged with the client.
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Otherwise, the SCTP Initiation Procedure follows many TCP
conventions, so that the endpoints exchange receiver windows, initial
sequence numbers, etc. In addition to this, the endpoints may
exchange address lists as discussed above, and also mutually confirm
the number of streams to be opened on each side.
5.2 INIT Collision Resolution
Multi-homing adds to the potential that messages will be received out
of sequence or with different address pairs. This is a particular
concern during initiation of the association, where without
procedures for resolving the collision of messages, you may easily
end up with multiple parallel associations between the same
endpoints. To avoid this, SCTP incorporates a number of procedures
to resolve parallel initiation attempts into a single association.
6. SCTP DATA Exchange Features
DATA chunk exchange in SCTP follows TCP's Selective ACK procedure.
Receipt of DATA chunks is acknowledged by sending SACK chunks, which
indicate not only the cumulative Transmission Sequence Number (TSN)
range received, but also any non-cumulative TSNs, implying gaps in
the received TSN sequence. Following TCP procedures, SACKs are sent
using the "delayed ack" method, normally one SACK per every other
received packet, but with an upper limit on the delay between SACKs
and an increase to once per received packet when there are gaps
detected.
Flow and Congestion Control follow TCP algorithms. The advertised
receive window indicates buffer occupancy at the receiver, while a
per-path congestion window is maintained to manage the packets in
flight. Slow start, Congestion avoidance, Fast recovery and Fast
retransmit are incorporated into the procedures as described in RFC
2581, with the one change being that the endpoints must manage the
conversion between bytes sent and received and TSNs sent and
received, since TSN is per chunk rather than per byte.
The application can specify a lifetime for data to be transmitted, so
that if the lifetime has expired and the data has not yet been
transmitted, it can be discarded (e.g., time-sensitive signaling
messages). If the data has been transmitted, it must continue to be
delivered to avoid creating a hole in the TSN sequence.
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7. SCTP Shutdown Features
SCTP Shutdown uses a 3-message procedure to allow graceful shutdown,
where each endpoint has confirmation of the DATA chunks received by
the remote endpoint prior to completion of the shutdown. An Abort
procedure is also provided for error cases when an immediate shutdown
must take place.
Note that SCTP does not support the function of a "half-open"
connection as can occur in TCP, when one side indicates that it has
no more data to send, but the other side can continue to send data
indefinitely. SCTP assumes that once the shutdown procedure begins,
both sides will stop sending new data across the association, and
only need to clear up acknowledgements of previously sent data.
8. SCTP Message Format
The SCTP Message includes a common header plus one or more chunks,
which can be control or data. The common header has source and
destination port numbers to allow multiplexing of different SCTP
associations at the same address, a 32-bit verification tag that
guards against insertion of an out-of-date or false message into the
SCTP association, and a 32-bit checksum (this has been modified to
use the CRC-32c polynomial [2]) for error detection.
Each chunk includes chunk type, flag field, length and value.
Control chunks incorporate different flags and parameters depending
on the chunk type. DATA chunks in particular incorporate flags for
control of segmentation and reassembly, and parameters for the TSN,
Stream ID and Stream Sequence Number, and a Payload Protocol
Identifier.
The Payload Protocol ID has been included for future flexibility. It
is envisioned that the functions of protocol identification and port
number multiplexing will not be as closely linked in the future as
they are in current usage. Payload Protocol ID will allow the
protocol being carried by SCTP to be identified independent of the
port numbers being used.
The SCTP message format naturally allows support of bundling of
multiple DATA and control chunks in a single message, to improve
transport efficiency. Use of bundling is controllable by the
application, so that bundling of initial transmission can be
prohibited. Bundling will naturally occur on retransmission of DATA
chunks, to further reduce any chance of congestion.
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9. Error Handling
9.1 Retransmission
Retransmission of DATA chunks occurs from either (a) timeout of the
retransmission timer; or (b) receipt of SACKs indicating the DATA
chunk has not been received. To reduce the potential for congestion,
the rate of retransmission of DATA chunks is limited. The
retransmission timeout (RTO) is adjusted based on estimates of the
round trip delay and backs off exponentially as message loss
increases.
In an active association with fairly constant DATA transmission,
SACKs are more likely to cause retransmission than the timeout. To
reduce the chance of an unnecessary retransmission, a 4 SACK rule is
used, so that retransmission only occurs on receipt of the 4th SACK
that indicates that the chunk is missing. This is intended to avoid
retransmits due to normal occurrences such as packets received out of
sequence.
9.2 Path Failure
A count is maintained of the number of retransmissions to a
particular destination address without successful acknowledgement.
When this count exceeds a configured maximum, the address is declared
inactive, notification is given to the application, and the SCTP
begins to use an alternate address for the sending of DATA chunks.
Also, Heartbeat chunks are sent periodically to all idle destinations
(i.e., alternate addresses), and a counter is maintained on the
number of Heartbeats sent to an inactive destination without receipt
of a corresponding Heartbeat Ack. When this counter exceeds a
configured maximum, that destination address is also declared
inactive.
Heartbeats continue to be sent to inactive destination addresses
until an Ack is received, at which point the address can be made
active again. The rate of sending Heartbeats is tied to the RTO
estimation plus an additional delay parameter that allows Heartbeat
traffic to be tailored according to the needs of the user
application.
9.3 Endpoint Failure
A count is maintained across all destination addresses on the number
of retransmits or Heartbeats sent to the remote endpoint without a
successful Ack. When this exceeds a configured maximum, the endpoint
is declared unreachable, and the SCTP association is closed.
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10. API
The specification includes a model of the primitives exchanged
between the application and the SCTP layer, intended as informational
material rather than a formal API statement. A socket-based API is
being defined to simplify migration of TCP or UDP applications to the
use of SCTP.
11. Security Considerations
In addition to the verification tag and cookie mechanisms, SCTP
specifies the use of IPSec if strong security and integrity
protection is required. The SCTP specification does not itself
define any new security protocols or procedures.
Extensions to IPSec are under discussion to reduce the overhead
required to support multi-homing. Also, work is in progress on the
use of Transport Layer Security (TLS) over SCTP [4].
12. Extensions
The SCTP format allows new chunk types, flags and parameter fields to
be defined as extensions to the protocol. Any extensions must be
based on standard agreements within the IETF, as no vendor-specific
extensions are supported in the protocol.
Chunk Type values are organized into four ranges to allow extensions
to be made with a pre-defined procedure for responding if a new Chunk
Type is not recognized at the remote endpoint. Responses include:
whole packet discard; packet discard with reporting; ignoring the
chunk; ignoring with reporting. Similar pre-defined responses are
specified for unrecognized Parameter Type values.
Chunk Parameter Type values are in principle independent ranges for
each Chunk Type. In practice, the values defined in the SCTP
specification have been coordinated so that a particular parameter
type will have the same Chunk Parameter Type value across all Chunk
Types. Further experience will determine if this alignment needs to
be maintained or formalized.
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13. Informative References
[1] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer, H.,
Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson,
"Stream Control Transmission Protocol", RFC 2960, October 2000.
[2] Stewart, Sharp, et. al., "SCTP Checksum Change", Work in
Progress.
[3] Ong, L., Rytina, I., Garcia, M., Schwarzbauer, H., Coene, L.,
Lin, H., Juhasz, I., Holdrege, M. and C. Sharp, "Framework
Architecture for Signaling Transport", RFC 2719, October 1999.
[4] Jungmeier, Rescorla and Tuexen, "TLS Over SCTP", Work in
Progress.
14. Authors' Addresses
Lyndon Ong
Ciena Corporation
10480 Ridgeview Drive
Cupertino, CA 95014
EMail: lyong@ciena.com
John Yoakum
Emerging Opportunities
Nortel Networks
EMail: yoakum@nortelnetworks.com
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15. Full Copyright Statement
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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