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Network Working Group S. Floyd
Request for Comments: 4341 ICIR
Category: Standards Track E. Kohler
UCLA
March 2006
Profile for Datagram Congestion Control Protocol (DCCP)
Congestion Control ID 2: TCP-like Congestion Control
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.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document contains the profile for Congestion Control Identifier
2 (CCID 2), TCP-like Congestion Control, in the Datagram Congestion
Control Protocol (DCCP). CCID 2 should be used by senders who would
like to take advantage of the available bandwidth in an environment
with rapidly changing conditions, and who are able to adapt to the
abrupt changes in the congestion window typical of TCP's Additive
Increase Multiplicative Decrease (AIMD) congestion control.
Table of Contents
1. Introduction ....................................................2
2. Conventions and Notation ........................................2
3. Usage ...........................................................3
3.1. Relationship with TCP ......................................3
3.2. Half-Connection Example ....................................4
4. Connection Establishment ........................................5
5. Congestion Control on Data Packets ..............................5
5.1. Response to Idle and Application-Limited Periods ...........8
5.2. Response to Data Dropped and Slow Receiver .................8
5.3. Packet Size ................................................8
6. Acknowledgements ................................................9
6.1. Congestion Control on Acknowledgements .....................9
6.1.1. Detecting Lost and Marked Acknowledgements .........10
Floyd & Kohler Standards Track [Page 1]
RFC 4341 DCCP CCID2 March 2006
6.1.2. Changing Ack Ratio .................................10
6.2. Acknowledgements of Acknowledgements ......................11
6.2.1. Determining Quiescence .............................12
7. Explicit Congestion Notification ...............................12
8. Options and Features ...........................................12
9. Security Considerations ........................................13
10. IANA Considerations ...........................................13
10.1. Reset Codes ..............................................13
10.2. Option Types .............................................13
10.3. Feature Numbers ..........................................14
11. Thanks ........................................................14
A. Appendix: Derivation of Ack Ratio Decrease ....................15
B. Appendix: Cost of Loss Inference Mistakes to Ack Ratio ........15
Normative References ..............................................17
Informative References ............................................18
1. Introduction
This document contains the profile for Congestion Control Identifier
2 (CCID 2), TCP-like Congestion Control, in the Datagram Congestion
Control Protocol (DCCP) [RFC4340]. DCCP uses Congestion Control
Identifiers, or CCIDs, to specify the congestion control mechanism in
use on a half-connection.
The TCP-like Congestion Control CCID sends data using a close variant
of TCP's congestion control mechanisms, incorporating a variant of
selective acknowledgements (SACK) [RFC2018, RFC3517]. CCID 2 is
suitable for senders who can adapt to the abrupt changes in
congestion window typical of TCP's Additive Increase Multiplicative
Decrease (AIMD) congestion control, and particularly useful for
senders who would like to take advantage of the available bandwidth
in an environment with rapidly changing conditions. See Section 3
for more on application requirements.
2. Conventions and Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
A DCCP half-connection consists of the application data sent by one
endpoint and the corresponding acknowledgements sent by the other
endpoint. The terms "HC-Sender" and "HC-Receiver" denote the
endpoints sending application data and acknowledgements,
respectively. Since CCIDs apply at the level of half-connections, we
abbreviate HC-Sender to "sender" and HC-Receiver to "receiver" in
this document. See [RFC4340] for more discussion.
Floyd & Kohler Standards Track [Page 2]
RFC 4341 DCCP CCID2 March 2006
For simplicity, we say that senders send DCCP-Data packets and
receivers send DCCP-Ack packets. Both of these categories are meant
to include DCCP-DataAck packets.
The phrases "ECN-marked" and "marked" refer to packets marked ECN
Congestion Experienced unless otherwise noted.
3. Usage
CCID 2, TCP-like Congestion Control, is appropriate for DCCP flows
that would like to receive as much bandwidth as possible over the
long term, consistent with the use of end-to-end congestion control.
CCID 2 flows must also tolerate the large sending rate variations
characteristic of AIMD congestion control, including halving of the
congestion window in response to a congestion event.
Applications that simply need to transfer as much data as possible in
as short a time as possible should use CCID 2. This contrasts with
CCID 3, TCP-Friendly Rate Control (TFRC) [RFC4342], which is
appropriate for flows that would prefer to minimize abrupt changes in
the sending rate. For example, CCID 2 is recommended over CCID 3 for
streaming media applications that buffer a considerable amount of
data at the application receiver before playback time, insulating the
application somewhat from abrupt changes in the sending rate. Such
applications could easily choose DCCP's CCID 2 over TCP itself,
possibly adding some form of selective reliability at the application
layer. CCID 2 is also recommended over CCID 3 for applications where
halving the sending rate in response to congestion is not likely to
interfere with application-level performance.
An additional advantage of CCID 2 is that its TCP-like congestion
control mechanisms are reasonably well understood, with traffic
dynamics quite similar to those of TCP. While the network research
community is still learning about the dynamics of TCP after 15 years
of its being the dominant transport protocol in the Internet, some
applications might prefer the more well-known dynamics of TCP-like
congestion control over those of newer congestion control mechanisms,
which haven't yet met the test of widespread Internet deployment.
3.1. Relationship with TCP
The congestion control mechanisms described here closely follow
mechanisms standardized by the IETF for use in SACK-based TCP, and we
rely partially on existing TCP documentation, such as [RFC793],
[RFC2581], [RFC3465], and [RFC3517]. TCP congestion control
continues to evolve, but CCID 2 implementations SHOULD wait for
explicit updates to CCID 2 rather than track TCP's evolution
directly.
Floyd & Kohler Standards Track [Page 3]
RFC 4341 DCCP CCID2 March 2006
Differences between CCID 2 and straight TCP congestion control
include the following:
o CCID 2 applies congestion control to acknowledgements, a mechanism
not currently standardized for use in TCP.
o DCCP is a datagram protocol, so several parameters whose units are
specified in bytes in TCP, such as the congestion window cwnd,
have units of packets in DCCP.
o As an unreliable protocol, DCCP never retransmits a packet, so
congestion control mechanisms that distinguish retransmissions
from new packets have been redesigned for the DCCP context.
3.2. Half-Connection Example
This example shows the typical progress of a half-connection using
CCID 2's TCP-like Congestion Control, not including connection
initiation and termination. The example is informative, not
normative.
1. The sender sends DCCP-Data packets, where the number of packets
sent is governed by a congestion window, cwnd, as in TCP. Each
DCCP-Data packet uses a sequence number. The sender also sends an
Ack Ratio feature option specifying the number of data packets to
be covered by an Ack packet from the receiver; Ack Ratio defaults
to two. The DCCP header's CCVal field is set to zero.
Assuming that the half-connection is Explicit Congestion
Notification (ECN) capable (the ECN Incapable feature is zero, the
default), each DCCP-Data packet is sent as ECN Capable with either
the ECT(0) or the ECT(1) codepoint set, as described in [RFC3540].
2. The receiver sends a DCCP-Ack packet acknowledging the data
packets for every Ack Ratio data packets transmitted by the
sender. Each DCCP-Ack packet uses a sequence number and contains
an Ack Vector. The sequence number acknowledged in a DCCP-Ack
packet is that of the received packet with the highest sequence
number; it is not a TCP-like cumulative acknowledgement.
The receiver returns the sum of received ECN Nonces via Ack Vector
options, allowing the sender to probabilistically verify that the
receiver is not misbehaving. DCCP-Ack packets from the receiver
are also sent as ECN Capable, since the sender will control the
acknowledgement rate in a roughly TCP-friendly way using the Ack
Ratio feature. There is little need for the receiver to verify
the nonces of its DCCP-Ack packets, since the sender cannot get
significant benefit from misreporting the ack mark rate.
Floyd & Kohler Standards Track [Page 4]
RFC 4341 DCCP CCID2 March 2006
3. The sender continues sending DCCP-Data packets as controlled by
the congestion window. Upon receiving DCCP-Ack packets, the
sender examines their Ack Vectors to learn about marked or dropped
data packets and adjusts its congestion window accordingly.
Because this is unreliable transfer, the sender does not
retransmit dropped packets.
4. Because DCCP-Ack packets use sequence numbers, the sender has some
information about lost or marked DCCP-Ack packets. The sender
responds to lost or marked DCCP-Ack packets by modifying the Ack
Ratio sent to the receiver.
5. The sender acknowledges the receiver's acknowledgements at least
once per congestion window. If both half-connections are active,
the sender's acknowledgement of the receiver's acknowledgements is
included in the sender's acknowledgement of the receiver's data
packets. If the reverse-path half-connection is quiescent, the
sender sends at least one DCCP-DataAck packet per congestion
window.
6. The sender estimates round-trip times, either through keeping
track of acknowledgement round-trip times as TCP does or through
explicit Timestamp options, and calculates a TimeOut (TO) value
much as the RTO (Retransmit Timeout) is calculated in TCP. The TO
determines when a new DCCP-Data packet can be transmitted when the
sender has been limited by the congestion window and no feedback
has been received from the receiver.
4. Connection Establishment
Use of the Ack Vector is MANDATORY on CCID 2 half-connections, so the
sender MUST send a "Change R(Send Ack Vector, 1)" option to the
receiver as part of connection establishment. The sender SHOULD NOT
send data until it has received the corresponding "Confirm L(Send Ack
Vector, 1)" from the receiver, except that it MAY send data on DCCP-
Request packets.
5. Congestion Control on Data Packets
CCID 2's congestion control mechanisms are based on those for SACK-
based TCP [RFC3517], since the Ack Vector provides all the
information that might be transmitted in SACK options.
A CCID 2 data sender maintains three integer parameters measured in
packets.
Floyd & Kohler Standards Track [Page 5]
RFC 4341 DCCP CCID2 March 2006
1. The congestion window "cwnd", which equals the maximum number of
data packets allowed in the network at any time. ("Data packet"
means any DCCP packet that contains user data: DCCP-Data, DCCP-
DataAck, and occasionally DCCP-Request and DCCP-Response.)
2. The slow-start threshold "ssthresh", which controls adjustments to
cwnd.
3. The pipe value "pipe", which is the sender's estimate of the
number of data packets outstanding in the network.
These parameters are manipulated, and their initial values
determined, according to SACK-based TCP's behavior, except that they
are measured in packets, not bytes. The rest of this section
provides more specific guidance.
The sender MAY send a data packet when pipe < cwnd but MUST NOT send
a data packet when pipe >= cwnd. Every data packet sent increases
pipe by 1.
The sender reduces pipe as it infers that data packets have left the
network, either by being received or by being dropped. In
particular:
1. Acked data packets. The sender reduces pipe by 1 for each data
packet newly acknowledged as received (Ack Vector State 0 or State
1) by some DCCP-Ack.
2. Dropped data packets. The sender reduces pipe by 1 for each data
packet it can infer as lost due to the DCCP equivalent of TCP's
"duplicate acknowledgements". This depends on the NUMDUPACK
parameter, the number of duplicate acknowledgements needed to
infer a loss. The NUMDUPACK parameter is set to three, as is
currently the case in TCP. A packet P is inferred to be lost,
rather than delayed, when at least NUMDUPACK packets transmitted
after P have been acknowledged as received (Ack Vector State 0 or
1) by the receiver. Note that the acknowledged packets following
the hole may be DCCP-Acks or other non-data packets.
3. Transmit timeouts. Finally, the sender needs transmit timeouts,
handled like TCP's retransmission timeouts, in case an entire
window of packets is lost. The sender estimates the round-trip
time at most once per window of data and uses the TCP algorithms
for maintaining the average round-trip time, mean deviation, and
timeout value [RFC2988]. (If more than one measurement per
round-trip time was used for these calculations, then the weights
of the averagers would have to be adjusted to ensure that the
average round-trip time is effectively derived from measurements
Floyd & Kohler Standards Track [Page 6]
RFC 4341 DCCP CCID2 March 2006
over multiple round-trip times.) Because DCCP does not retransmit
data, DCCP does not require TCP's recommended minimum timeout of
one second. The exponential backoff of the timer is exactly as in
TCP. When a transmit timeout occurs, the sender sets pipe to
zero. The adjustments to cwnd and ssthresh are described below.
The sender MUST NOT decrement pipe more than once per data packet.
True duplicate acknowledgements, for example, MUST NOT affect pipe.
The sender also MUST NOT decrement pipe again upon receiving
acknowledgement of a packet previously inferred as lost.
Furthermore, the sender MUST NOT decrement pipe for non-data packets,
such as DCCP-Acks, even though the Ack Vector will contain
information about them.
Congestion events cause CCID 2 to reduce its congestion window. A
congestion event contains at least one lost or marked packet. As in
TCP, two losses or marks are considered part of a single congestion
event when the second packet was sent before the loss or mark of the
first packet was detected. As an approximation, a sender can
consider two losses or marks to be part of a single congestion event
when the packets were sent within one RTT estimate of one another,
using an RTT estimate current at the time the packets were sent. For
each congestion event, either indicated explicitly as an Ack Vector
State 1 (ECN-marked) acknowledgement or inferred via "duplicate
acknowledgements", cwnd is halved, then ssthresh is set to the new
cwnd. Cwnd is never reduced below one packet. After a timeout, the
slow-start threshold is set to cwnd/2, then cwnd is set to one
packet. When halved, cwnd and ssthresh have their values rounded
down, except that cwnd is never less than one and ssthresh is never
less than two.
When cwnd < ssthresh, meaning that the sender is in slow-start, the
congestion window is increased by one packet for every two newly
acknowledged data packets with Ack Vector State 0 (not ECN-marked),
up to a maximum of Ack Ratio/2 packets per acknowledgement. This is
a modified form of Appropriate Byte Counting [RFC3465] that is
consistent with TCP's current standard (which does not include byte
counting), but allows CCID 2 to increase as aggressively as TCP when
CCID 2's Ack Ratio is greater than the default value of two. When
cwnd >= ssthresh, the congestion window is increased by one packet
for every window of data acknowledged without lost or marked packets.
The cwnd parameter is initialized to at most four packets for new
connections, following the rules from [RFC3390]; the ssthresh
parameter is initialized to an arbitrarily high value.
Senders MAY use a form of rate-based pacing when sending multiple
data packets liberated by a single ack packet, rather than sending
all liberated data packets in a single burst.
Floyd & Kohler Standards Track [Page 7]
RFC 4341 DCCP CCID2 March 2006
5.1. Response to Idle and Application-Limited Periods
CCID 2 is designed to follow TCP's congestion control mechanisms to
the extent possible, but TCP does not have complete standardization
for its congestion control response to idle periods (when no data
packets are sent) or to application-limited periods (when the sending
rate is less than that allowed by cwnd). This section is a brief
guide to the standards for TCP in this area.
For idle periods, [RFC2581] recommends that the TCP sender SHOULD
slow-start after an idle period, where an idle period is defined as a
period exceeding the timeout interval. [RFC2861], currently
Experimental, suggests a slightly more moderate mechanism where the
congestion window is halved for every round-trip time that the sender
has remained idle.
There are currently no standards governing TCP's use of the
congestion window during an application-limited period. In
particular, it is possible for TCP's congestion window to grow quite
large during a long uncongested period when the sender is application
limited, sending at a low rate. [RFC2861] essentially suggests that
TCP's congestion window not be increased during application-limited
periods when the congestion window is not being fully utilized.
5.2. Response to Data Dropped and Slow Receiver
DCCP's Data Dropped option lets a receiver declare that a packet was
dropped at the end host before delivery to the application -- for
instance, because of corruption or receive buffer overflow. DCCP's
Slow Receiver option lets a receiver declare that it is having
trouble keeping up with the sender's packets, although nothing has
yet been dropped. CCID 2 senders respond to these options as
described in [RFC4340], with the following further clarifications.
o Drop Code 2 ("receive buffer drop"). The congestion window "cwnd"
is reduced by one for each packet newly acknowledged as Drop Code
2, except that it is never reduced below one.
o Exiting slow start. The sender MUST exit slow start whenever it
receives a relevant Data Dropped or Slow Receiver option.
5.3. Packet Size
CCID 2 is optimized for applications that generally use a fixed
packet size and vary their sending rate in packets per second in
response to congestion. CCID 2 is not appropriate for applications
that require a fixed interval of time between packets and vary their
packet size instead of their packet rate in response to congestion.
Floyd & Kohler Standards Track [Page 8]
RFC 4341 DCCP CCID2 March 2006
CCID 2 maintains a congestion window in packets and does not increase
the congestion window in response to a decrease in the packet size.
However, some attention might be required for applications using CCID
2 that vary their packet size not in response to congestion, but in
response to other application-level requirements.
CCID 2 implementations MAY check for applications that appear to be
manipulating the packet size inappropriately. For example, an
application might send small packets for a while, building up a fast
rate, then switch to large packets to take advantage of the fast
rate. (Preliminary simulations indicate that applications may not be
able to increase their overall transfer rates this way, so it is not
clear that this manipulation will occur in practice [V03].)
6. Acknowledgements
CCID 2 acknowledgements are generally paced by the sender's data
packets. Each required acknowledgement MUST contain Ack Vector
options that declare exactly which packets arrived and whether those
packets were ECN-marked. Acknowledgement data in the Ack Vector
options SHOULD generally cover the receiver's entire Acknowledgement
Window; see [RFC4340], Section 11.4.2. Any Data Dropped options
SHOULD likewise cover the receiver's entire Acknowledgement Window.
CCID 2 senders use DCCP's Ack Ratio feature to influence the rate at
which receivers generate DCCP-Ack packets, thus controlling reverse-
path congestion. This differs from TCP, which presently has no
congestion control for pure acknowledgement traffic. CCID 2's
reverse-path congestion control does not try to be TCP friendly; it
just tries to avoid congestion collapse, and to be somewhat better
than TCP is in the presence of a high packet loss or mark rate on the
reverse path. The default Ack Ratio is two, and CCID 2 with this Ack
Ratio behaves like TCP with delayed acks. [RFC4340], Section 11.3,
describes the Ack Ratio in more detail, including its relationship to
acknowledgement pacing and DCCP-DataAck packets. This document's
Section 6.1.1 describes how a CCID 2 sender detects lost or marked
acknowledgements, and Section 6.1.2 describes how it changes the Ack
Ratio.
6.1. Congestion Control on Acknowledgements
When Ack Ratio is R, the receiver sends one DCCP-Ack packet per R
data packets, more or less. Since the sender sends cwnd data packets
per round-trip time, the acknowledgement rate equals cwnd/R DCCP-Acks
per round-trip time. The sender keeps the acknowledgement rate
roughly TCP friendly by monitoring the acknowledgement stream for
lost and marked DCCP-Ack packets and modifying R accordingly. For
every RTT containing a DCCP-Ack congestion event (that is, a lost or
Floyd & Kohler Standards Track [Page 9]
RFC 4341 DCCP CCID2 March 2006
marked DCCP-Ack), the sender halves the acknowledgement rate by
doubling Ack Ratio; for every RTT containing no DCCP-Ack congestion
event, it additively increases the acknowledgement rate through
gradual decreases in Ack Ratio.
6.1.1. Detecting Lost and Marked Acknowledgements
All packets from the receiver contain sequence numbers, so the sender
can detect both losses and marks on the receiver's packets. The
sender infers receiver packet loss in the same way that it infers
losses of its data packets: a packet from the receiver is considered
lost after at least NUMDUPACK packets with greater sequence numbers
have been received.
DCCP-Ack packets are generally small, so they might impose less load
on congested network links than DCCP-Data and DCCP-DataAck packets.
For this reason, Ack Ratio depends on losses and marks on the
receiver's non-data packets, not on aggregate losses and marks on all
of the receiver's packets. The non-data packet category consists of
those packet types that cannot carry application data: DCCP-Ack,
DCCP-Close, DCCP-CloseReq, DCCP-Reset, DCCP-Sync, and DCCP-SyncAck.
The sender can easily distinguish non-data marks from other marks.
This is harder for losses, though, since the sender can't always know
whether a lost packet carried data. Unless it has better
information, the sender SHOULD assume, for the purpose of Ack Ratio
calculation, that every lost packet was a non-data packet. Better
information is available via DCCP's NDP Count option, if necessary.
(Appendix B discusses the costs of mistaking data packet loss for
non-data packet loss.)
A receiver that implements its own acknowledgement congestion control
independent of Ack Ratio SHOULD NOT reduce its DCCP-Ack
acknowledgement rate due to losses or marks on its data packets.
6.1.2. Changing Ack Ratio
Ack Ratio always meets three constraints: (1) Ack Ratio is an
integer. (2) Ack Ratio does not exceed cwnd/2, rounded up, except
that Ack Ratio 2 is always acceptable. (3) Ack Ratio is two or more
for a congestion window of four or more packets.
The sender changes Ack Ratio within those constraints as follows.
For each congestion window of data with lost or marked DCCP-Ack
packets, Ack Ratio is doubled; and for each cwnd/(R^2 - R)
consecutive congestion windows of data with no lost or marked DCCP-
Ack packets, Ack Ratio is decreased by 1. (See Appendix A for the
derivation.) Changes in Ack Ratio are signalled through feature
negotiation; see [RFC4340], Section 11.3.
Floyd & Kohler Standards Track [Page 10]
RFC 4341 DCCP CCID2 March 2006
For a constant congestion window, this gives an Ack sending rate that
is roughly TCP friendly. Of course, cwnd usually varies over time;
the dynamics will be rather complex, but roughly TCP friendly. We
recommend that the sender use the most recent value of cwnd when
determining whether to decrease Ack Ratio by 1.
The sender need not keep Ack Ratio completely up to date. For
instance, it MAY rate-limit Ack Ratio renegotiations to once every
four or five round-trip times, or to once every second or two. The
sender SHOULD NOT attempt to renegotiate the Ack Ratio more than once
per round-trip time. Additionally, it MAY enforce a minimum Ack
Ratio of two, or it MAY set Ack Ratio to one for half-connections
with persistent congestion windows of 1 or 2 packets.
Putting it all together, the receiver always sends at least one
acknowledgement per window of data when cwnd = 1, and at least two
acknowledgements per window of data otherwise. Thus, the receiver
could be sending two ack packets per window of data even in the face
of very heavy congestion on the reverse path. We would note,
however, that if congestion is sufficiently heavy, all the ack
packets are dropped, and then the sender falls back on an
exponentially backed-off timeout, as in TCP. Thus, if congestion is
sufficiently heavy on the reverse path, then the sender reduces its
sending rate on the forward path, which reduces the rate on the
reverse path as well.
6.2. Acknowledgements of Acknowledgements
An active sender DCCP A MUST occasionally acknowledge its peer DCCP
B's acknowledgements so that DCCP B can free up Ack Vector state.
When both half-connections are active, A's acknowledgements of B's
acknowledgements are automatically contained in A's acknowledgements
of B's data. If the B-to-A half-connection is quiescent, however,
DCCP A must occasionally send acknowledgements proactively, such as
by sending a DCCP-DataAck packet that includes an Acknowledgement
Number in the header.
An active sender SHOULD acknowledge the receiver's acknowledgements
at least once per congestion window. Of course, the sender's
application might fall silent. This is no problem; when neither side
is sending data, a sender can wait arbitrarily long before sending an
ack.
Floyd & Kohler Standards Track [Page 11]
RFC 4341 DCCP CCID2 March 2006
6.2.1. Determining Quiescence
This section describes how a CCID 2 receiver determines that the
corresponding sender is not sending any data and therefore has gone
quiescent. See [RFC4340], Section 11.1, for general information on
quiescence.
Let T equal the greater of 0.2 seconds and two round-trip times.
(The receiver may know the round-trip time in its role as the sender
for the other half-connection. If it does not, it should use a
default RTT of 0.2 seconds, as described in [RFC4340], Section 3.4.)
Once the sender acknowledges the receiver's Ack Vectors and the
sender has not sent additional data for at least T seconds, the
receiver can infer that the sender is quiescent. More precisely, the
receiver infers that the sender has gone quiescent when at least T
seconds have passed without receiving any data from the sender, and
when the sender has acknowledged receiver Ack Vectors covering all
data packets received at the receiver.
7. Explicit Congestion Notification
CCID 2 supports Explicit Congestion Notification (ECN) [RFC3168].
The sender will use the ECN Nonce for data packets, and the receiver
will echo those nonces in its Ack Vectors, as specified in [RFC4340],
Section 12.2. Information about marked packets is also returned in
the Ack Vector. Because the information in the Ack Vector is
reliably transferred, DCCP does not need the TCP flags of ECN-Echo
and Congestion Window Reduced.
For unmarked data packets, the receiver computes the ECN Nonce Echo
as in [RFC3540] and returns it as part of its Ack Vector options.
The sender SHOULD check these ECN Nonce Echoes against the expected
values, thus protecting against the accidental or malicious
concealment of marked packets.
Because CCID 2 acknowledgements are congestion controlled, ECN may
also be used for its acknowledgements. In this case we do not make
use of the ECN Nonce, because it would not be easy to provide
protection against the concealment of marked ack packets by the
sender, and because the sender does not have much motivation for
lying about the mark rate on acknowledgements.
8. Options and Features
DCCP's Ack Vector option, and its ECN Capable, Ack Ratio, and Send
Ack Vector features, are relevant for CCID 2.
Floyd & Kohler Standards Track [Page 12]
RFC 4341 DCCP CCID2 March 2006
9. Security Considerations
Security considerations for DCCP have been discussed in [RFC4340],
and security considerations for TCP have been discussed in [RFC2581].
[RFC2581] discusses ways in which an attacker could impair the
performance of a TCP connection by dropping packets, or by forging
extra duplicate acknowledgements or acknowledgements for new data.
We are not aware of any new security considerations created by this
document in its use of TCP-like congestion control.
10. IANA Considerations
This specification defines the value 2 in the DCCP CCID namespace
managed by IANA. This assignment is also mentioned in [RFC4340].
CCID 2 also introduces three sets of numbers whose values should be
allocated by IANA; namely, CCID 2-specific Reset Codes, option types,
and feature numbers. These ranges will prevent any future CCID
2-specific allocations from polluting DCCP's corresponding global
namespaces; see [RFC4340], Section 10.3. However, this document
makes no particular allocations from any range, except for
experimental and testing use [RFC3692]. We refer to the Standards
Action policy outlined in [RFC2434].
10.1. Reset Codes
Each entry in the DCCP CCID 2 Reset Code registry contains a CCID
2-specific Reset Code, which is a number in the range 128-255; a
short description of the Reset Code; and a reference to the RFC
defining the Reset Code. Reset Codes 184-190 and 248-254 are
permanently reserved for experimental and testing use. The remaining
Reset Codes -- 128-183, 191-247, and 255 -- are currently reserved
and should be allocated with the Standards Action policy, which
requires IESG review and approval and standards-track IETF RFC
publication.
10.2. Option Types
Each entry in the DCCP CCID 2 option type registry contains a CCID
2-specific option type, which is a number in the range 128-255; the
name of the option; and a reference to the RFC defining the option
type. Option types 184-190 and 248-254 are permanently reserved for
experimental and testing use. The remaining option types -- 128-183,
191-247, and 255 -- are currently reserved and should be allocated
with the Standards Action policy, which requires IESG review and
approval and standards-track IETF RFC publication.
Floyd & Kohler Standards Track [Page 13]
RFC 4341 DCCP CCID2 March 2006
10.3. Feature Numbers
Each entry in the DCCP CCID 2 feature number registry contains a CCID
2-specific feature number, which is a number in the range 128-255;
the name of the feature; and a reference to the RFC defining the
feature number. Feature numbers 184-190 and 248-254 are permanently
reserved for experimental and testing use. The remaining feature
numbers -- 128-183, 191-247, and 255 -- are currently reserved and
should be allocated with the Standards Action policy, which requires
IESG review and approval and standards-track IETF RFC publication.
11. Thanks
We thank Mark Handley and Jitendra Padhye for their help in defining
CCID 2. We also thank Mark Allman, Aaron Falk, Nils-Erik Mattsson,
Greg Minshall, Arun Venkataramani, Magnus Westerlund, and members of
the DCCP Working Group for feedback on this document.
Floyd & Kohler Standards Track [Page 14]
RFC 4341 DCCP CCID2 March 2006
A. Appendix: Derivation of Ack Ratio Decrease
This section justifies the algorithm for increasing and decreasing
the Ack Ratio given in Section 6.1.2.
The congestion avoidance phase of TCP halves the cwnd for every
window with congestion. Similarly, CCID 2 doubles Ack Ratio for
every window with congestion on the return path, roughly halving the
DCCP-Ack sending rate.
The congestion avoidance phase of TCP increases cwnd by one MSS for
every congestion-free window. When this congestion avoidance
behavior is applied to acknowledgement traffic, this would correspond
to increasing the number of DCCP-Ack packets per window by one after
every congestion-free window of DCCP-Ack packets. We cannot achieve
this exactly using Ack Ratio, since it is an integer. Instead, we
must decrease Ack Ratio by one after K windows have been sent without
a congestion event on the reverse path, where K is chosen so that the
long-term number of DCCP-Ack packets per congestion window is roughly
TCP friendly, following AIMD congestion control.
In CCID 2, rough TCP-friendliness for the ack traffic can be
accomplished by setting K to cwnd/(R^2 - R), where R is the current
Ack Ratio.
This result was calculated as follows:
R = Ack Ratio = # data packets / ack packets, and
W = Congestion Window = # data packets / window, so
W/R = # ack packets / window.
Requirement: Increase W/R by 1 per congestion-free window. Since
we can only reduce R by increments of one, we find K so that,
after K congestion-free windows, W/R + K would equal W/(R-1).
(W/R) + K = W/(R-1), so
K = W/(R-1) - W/R = W/(R^2 - R).
B. Appendix: Cost of Loss Inference Mistakes to Ack Ratio
As discussed in Section 6.1.1, the sender often cannot determine
whether lost packets carried data. This hinders its ability to
separate non-data loss events from other loss events. In the absence
of better information, the sender assumes, for the purpose of Ack
Ratio calculation, that all lost packets were non-data packets. This
may overestimate the non-data loss event rate, which can lead to a
too-high Ack Ratio, and thus to a too-slow acknowledgement rate. All
acknowledgement information will still get through -- DCCP
Floyd & Kohler Standards Track [Page 15]
RFC 4341 DCCP CCID2 March 2006
acknowledgements are reliable -- but acknowledgement information will
arrive in a burstier fashion. Absent some form of rate-based pacing,
this could lead to increased burstiness for the sender's data
traffic.
There are several cases when the problem of an overly-high Ack Ratio,
and the resulting increased burstiness of the data traffic, will not
arise. In particular, call the receiver DCCP B and the sender DCCP
A:
o The problem won't arise unless DCCP B is sending a significant
amount of data itself. When the B-to-A half-connection is
quiescent or low rate, most packets sent by DCCP B will, in fact,
be pure acknowledgements, and DCCP A's estimate of the DCCP-Ack
loss rate will be reasonably accurate.
o The problem won't arise if DCCP B habitually piggybacks
acknowledgement information on its data packets. The piggybacked
acknowledgements are not limited by Ack Ratio, so they can arrive
frequently enough to prevent burstiness.
o The problem won't arise if DCCP A's sending rate is low, since
burstiness isn't a problem at low rates.
o The problem won't arise if DCCP B's sending rate is high relative
to DCCP A's sending rate, since the B-to-A loss rate must be low
to support DCCP B's sending rate. This bounds the Ack Ratio to
reasonable values even when DCCP A labels every loss as a DCCP-
Ack loss.
o The problem won't arise if DCCP B sends NDP Count options when
appropriate (the Send NDP Count/B feature is true). Then the
sender can use the receiver's NDP Count options to detect, in most
cases, whether lost packets were data packets or DCCP-Acks.
o Finally, the problem won't arise if DCCP A rate-paces its data
packets.
This leaves the case when DCCP B is sending roughly the same amount
of data packets and non-data packets, without NDP Count options, and
with all acknowledgement information in DCCP-Ack packets. We now
quantify the potential cost, in terms of a too-large Ack Ratio, due
to the sender's misclassifying data packet losses as DCCP-Ack losses.
For simplicity, we assume an environment of large-scale statistical
multiplexing where the packet drop rate is independent of the sending
rate of any individual connection.
Floyd & Kohler Standards Track [Page 16]
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Assume that when DCCP A correctly counts non-data losses, Ack Ratio
is set so that B-to-A data and acknowledgement traffic both have a
sending rate of D packets per second. Then when DCCP A incorrectly
counts data losses as non-data losses, the sending rate for the
B-to-A data traffic is still D pps, but the reduced sending rate for
the B-to-A acknowledgement traffic is f*D pps, with f < 1. Let the
packet loss rate be p. The sender incorrectly estimates the non-data
loss rate as (pD+pfD)/fD, or, equivalently, as p(1 + 1/f). Because
the congestion control mechanism for acknowledgement traffic is
roughly TCP friendly, and therefore the non-data sending rate and the
data sending rate both grow as 1/sqrt(x) for x the packet drop rate,
we have
fD/D = sqrt(p)/sqrt(p(1 + 1/f)),
so
f^2 = 1/(1 + 1/f).
Solving, we get f = 0.62. If the sender incorrectly counts lost data
packets as non-data in this scenario, the acknowledgement rate is
decreased by a factor of 0.62. This would result in a moderate
increase in burstiness for the A-to-B data traffic, which could be
mitigated by sending NDP Count options or piggybacked
acknowledgements, or by rate-pacing out the data.
Normative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow,
"TCP Selective Acknowledgement Options", RFC 2018,
October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
2434, October 1998.
[RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP
Congestion Control", RFC 2581, April 1999.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's
Retransmission Timer", RFC 2988, November 2000.
Floyd & Kohler Standards Track [Page 17]
RFC 4341 DCCP CCID2 March 2006
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The
Addition of Explicit Congestion Notification (ECN) to
IP", RFC 3168, September 2001.
[RFC3390] Allman, M., Floyd, S., and C. Partridge, "Increasing
TCP's Initial Window", RFC 3390, October 2002.
[RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
Conservative Selective Acknowledgment (SACK)-based
Loss Recovery Algorithm for TCP", RFC 3517, April
2003.
[RFC3692] Narten, T., "Assigning Experimental and Testing
Numbers Considered Useful", BCP 82, RFC 3692, January
2004.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March
2006.
Informative References
[RFC2861] Handley, M., Padhye, J., and S. Floyd, "TCP Congestion
Window Validation", RFC 2861, June 2000.
[RFC3465] Allman, M., "TCP Congestion Control with Appropriate
Byte Counting (ABC)", RFC 3465, February 2003.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust
Explicit Congestion Notification (ECN) Signaling with
Nonces", RFC 3540, June 2003.
[RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for
Datagram Congestion Control Protocol (DCCP) Congestion
Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC
4342, March 2006.
[V03] Arun Venkataramani, August 2003. Citation for
acknowledgement purposes only.
Floyd & Kohler Standards Track [Page 18]
RFC 4341 DCCP CCID2 March 2006
Authors' Addresses
Sally Floyd
ICSI Center for Internet Research
1947 Center Street, Suite 600
Berkeley, CA 94704
USA
EMail: floyd@icir.org
Eddie Kohler
4531C Boelter Hall
UCLA Computer Science Department
Los Angeles, CA 90095
USA
EMail: kohler@cs.ucla.edu
Floyd & Kohler Standards Track [Page 19]
RFC 4341 DCCP CCID2 March 2006
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Floyd & Kohler Standards Track [Page 20]