Network Working Group S. Floyd

Network Working Group S. Floyd
Request for Comments: 4342 ICIR
Category: Standards Track E. Kohler
UCLA
J. Padhye
Microsoft Research
March 2006

Profile for Datagram Congestion Control Protocol (DCCP)
Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)

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
3, TCP-Friendly Rate Control (TFRC), in the Datagram Congestion
Control Protocol (DCCP). CCID 3 should be used by senders that want
a TCP-friendly sending rate, possibly with Explicit Congestion
Notification (ECN), while minimizing abrupt rate changes.

Table of Contents

1. Introduction ....................................................2
2. Conventions .....................................................3
3. Usage ...........................................................3
3.1. Relationship with TFRC .....................................4
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 ...........7
5.2. Response to Data Dropped and Slow Receiver .................8
5.3. Packet Sizes ...............................................9
6. Acknowledgements ................................................9
6.1. Loss Interval Definition ..................................10
6.1.1. Loss Interval Lengths ..............................12
6.2. Congestion Control on Acknowledgements ....................13

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6.3. Acknowledgements of Acknowledgements ......................13
6.4. Determining Quiescence ....................................14
7. Explicit Congestion Notification ...............................14
8. Options and Features ...........................................14
8.1. Window Counter Value ......................................15
8.2. Elapsed Time Options ......................................17
8.3. Receive Rate Option .......................................17
8.4. Send Loss Event Rate Feature ..............................18
8.5. Loss Event Rate Option ....................................18
8.6. Loss Intervals Option .....................................18
8.6.1. Option Details .....................................19
8.6.2. Example ............................................20
9. Verifying Congestion Control Compliance with ECN ...............22
9.1. Verifying the ECN Nonce Echo ..............................22
9.2. Verifying the Reported Loss Intervals and Loss
Event Rate ................................................23
10. Implementation Issues .........................................23
10.1. Timestamp Usage ..........................................23
10.2. Determining Loss Events at the Receiver ..................24
10.3. Sending Feedback Packets .................................25
11. Security Considerations .......................................27
12. IANA Considerations ...........................................28
12.1. Reset Codes ..............................................28
12.2. Option Types .............................................28
12.3. Feature Numbers ..........................................28
13. Thanks ........................................................29
A. Appendix: Possible Future Changes to CCID 3 ....................30
Normative References ..............................................31
Informative References ............................................31

List of Tables

Table 1: DCCP CCID 3 Options ......................................14
Table 2: DCCP CCID 3 Feature Numbers ..............................15

1. Introduction

This document contains the profile for Congestion Control Identifier
3, TCP-Friendly Rate Control (TFRC), 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.

TFRC is a receiver-based congestion control mechanism that provides a
TCP-friendly sending rate while minimizing the abrupt rate changes
characteristic of TCP or of TCP-like congestion control [RFC3448].
The sender's allowed sending rate is set in response to the loss

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event rate, which is typically reported by the receiver to the
sender. See Section 3 for more on application requirements.

2. Conventions

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].

All multi-byte numerical quantities in CCID 3, such as arguments to
options, are transmitted in network byte order (most significant byte
first).

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.

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.

This document uses a number of variables from [RFC3448], including
the following:

o X_recv: The receive rate in bytes per second. See [RFC3448],
Section 3.2.2.

o s: The packet size in bytes. See [RFC3448], Section 3.1.

o p: The loss event rate. See [RFC3448], Section 3.1.

3. Usage

CCID 3's TFRC congestion control is appropriate for flows that would
prefer to minimize abrupt changes in the sending rate, including
streaming media applications with small or moderate receiver
buffering before playback. TCP-like congestion control, such as that
of DCCP's CCID 2 [RFC4341], halves the sending rate in response to
each congestion event and thus cannot provide a relatively smooth
sending rate.

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As explained in [RFC3448], Section 1, the penalty of having smoother
throughput than TCP while competing fairly for bandwidth with TCP is
that the TFRC mechanism in CCID 3 responds slower to changes in
available bandwidth than do TCP or TCP-like mechanisms. Thus, CCID 3
should only be used for applications with a requirement for smooth
throughput. For applications that simply need to transfer as much
data as possible in as short a time as possible, we recommend using
TCP-like congestion control, such as CCID 2.

CCID 3 should also not be used by applications that change their
sending rate by varying the packet size, rather than by varying the
rate at which packets are sent. A new CCID will be required for
these applications.

3.1. Relationship with TFRC

The congestion control mechanisms described here follow the TFRC
mechanism standardized by the IETF [RFC3448]. Conforming CCID 3
implementations MAY track updates to the TCP throughput equation
directly, as updates are standardized in the IETF, rather than wait
for revisions of this document. However, conforming implementations
SHOULD wait for explicit updates to CCID 3 before implementing other
changes to TFRC congestion control.

3.2. Half-Connection Example

This example shows the typical progress of a half-connection using
CCID 3's TFRC Congestion Control, not including connection initiation
and termination. The example is informative, not normative.

1. The sender transmits DCCP-Data packets. Its sending rate is
governed by the allowed transmit rate as specified in [RFC3448],
Section 3.2. Each DCCP-Data packet has a sequence number and the
DCCP header's CCVal field contains the window counter value, which
is used by the receiver in determining when multiple losses belong
in a single loss event.

In the typical case of an ECN-capable half-connection, each DCCP-
Data and DCCP-DataAck packet is sent as ECN Capable, with either
the ECT(0) or the ECT(1) codepoint set. The use of the ECN Nonce
with TFRC is described in Section 9.

2. The receiver sends DCCP-Ack packets acknowledging the data packets
at least once per round-trip time, unless the sender is sending at
a rate of less than one packet per round-trip time, as indicated
by the TFRC specification ([RFC3448], Section 6). Each DCCP-Ack
packet uses a sequence number, identifies the most recent packet

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received from the sender, and includes feedback about the recent
loss intervals experienced by the receiver.

3. The sender continues sending DCCP-Data packets as controlled by
the allowed transmit rate. Upon receiving DCCP-Ack packets, the
sender updates its allowed transmit rate as specified in
[RFC3448], Section 4.3. This update is based on a loss event rate
calculated by the sender using the receiver's loss intervals
feedback. If it prefers, the sender can also use a loss event
rate calculated and reported by the receiver.

4. The sender estimates round-trip times and calculates a nofeedback
time, as specified in [RFC3448], Section 4.4. If no feedback is
received from the receiver in that time (at least four round-trip
times), the sender halves its sending rate.

4. Connection Establishment

The client initiates the connection by using mechanisms described in
the DCCP specification [RFC4340]. During or after CCID 3
negotiation, the client and/or server may want to negotiate the
values of the Send Ack Vector and Send Loss Event Rate features.

5. Congestion Control on Data Packets

CCID 3 uses the congestion control mechanisms of TFRC [RFC3448]. The
following discussion summarizes information from [RFC3448], which
should be considered normative except where specifically indicated
otherwise.

Loss Event Rate

The basic operation of CCID 3 centers around the calculation of a
loss event rate: the number of loss events as a fraction of the
number of packets transmitted, weighted over the last several loss
intervals. This loss event rate, a round-trip time estimate, and the
average packet size are plugged into the TCP throughput equation, as
specified in [RFC3448], Section 3.1. The result is a fair transmit
rate close to what a modern TCP would achieve in the same conditions.
CCID 3 senders are limited to this fair rate.

The loss event rate itself is calculated in CCID 3 using recent loss
interval lengths reported by the receiver. Loss intervals are
precisely defined in Section 6.1. In summary, a loss interval is up
to 1 RTT of possibly lost or ECN-marked data packets, followed by an
arbitrary number of non-dropped, non-marked data packets. Thus, long
loss intervals represent low congestion rates. The CCID 3 Loss

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Intervals option is used to report loss interval lengths; see Section
8.6.

Other Congestion Control Mechanisms

The sender starts in a slow-start phase, roughly doubling its allowed
sending rate each round-trip time. The slow-start phase is ended by
the receiver's report of a data packet drop or mark, after which the
sender uses the loss event rate to calculate its allowed sending
rate.

[RFC3448], Section 4, specifies an initial sending rate of one packet
per round-trip time (RTT) as follows: The sender initializes the
allowed sending rate to one packet per second. As soon as a feedback
packet is received from the receiver, the sender has a measurement of
the round-trip time and then sets the initial allowed sending rate to
one packet per RTT. However, while the initial TCP window used to be
one segment, [RFC2581] allows an initial TCP window of two segments,
and [RFC3390] allows an initial TCP window of three or four segments
(up to 4380 bytes). [RFC3390] gives an upper bound on the initial
window of min(4*MSS, max(2*MSS, 4380 bytes)).

Therefore, in contrast to [RFC3448], the initial CCID 3 sending rate
is allowed to be at least two packets per RTT, and at most four
packets per RTT, depending on the packet size. The initial rate is
only allowed to be three or four packets per RTT when, in terms of
segment size, that translates to at most 4380 bytes per RTT.

The sender's measurement of the round-trip time uses the Elapsed Time
and/or Timestamp Echo option contained in feedback packets, as
described in Section 8.2. The Elapsed Time option is required, while
the Timestamp Echo option is not. The sender maintains an average
round-trip time heavily weighted on the most recent measurements.

Each DCCP-Data packet contains a sequence number. Each DCCP-Data
packet also contains a window counter value, as described in Section
8.1. The window counter is generally incremented by one every
quarter round-trip time. The receiver uses it as a coarse-grained
timestamp to determine when a packet loss should be considered part
of an existing loss interval and when it must begin a new loss
interval.

Because TFRC is rate-based instead of window-based, and because
feedback packets can be dropped in the network, the sender needs some
mechanism for reducing its sending rate in the absence of positive
feedback from the receiver. As described in Section 6, the receiver
sends feedback packets roughly once per round-trip time. As
specified in [RFC3448], Section 4.3, the sender sets a nofeedback

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timer to at least four round-trip times, or to twice the interval
between data packets, whichever is larger. If the sender hasn't
received a feedback packet from the receiver when the nofeedback
timer expires, then the sender halves its allowed sending rate. The
allowed sending rate is never reduced below one packet per 64
seconds. Note that not all acknowledgements are considered feedback
packets, since feedback packets must contain valid Loss Intervals,
Elapsed Time, and Receive Rate options.

If the sender never receives a feedback packet from the receiver, and
as a consequence never gets to set the allowed sending rate to one
packet per RTT, then the sending rate is left at its initial rate of
one packet per second, with the nofeedback timer expiring after two
seconds. The allowed sending rate is halved each time the nofeedback
timer expires. Thus, if no feedback is received from the receiver,
the allowed sending rate is never above one packet per second and is
quickly reduced below one packet per second.

The feedback packets from the receiver contain a Receive Rate option
specifying the rate at which data packets arrived at the receiver
since the last feedback packet. The allowed sending rate can be at
most twice the rate received at the receiver in the last round-trip
time. This may be less than the nominal fair rate if, for example,
the application is sending less than its fair share.

5.1. Response to Idle and Application-Limited Periods

One consequence of the nofeedback timer is that the sender reduces
the allowed sending rate when the sender has been idle for a
significant period of time. In [RFC3448], Section 4.4, the allowed
sending rate is never reduced to fewer than two packets per round-
trip time as the result of an idle period. CCID 3 revises this to
take into account the larger initial windows allowed by [RFC3390]:
the allowed sending rate is never reduced to less than the [RFC3390]
initial sending rate as the result of an idle period. If the allowed
sending rate is less than the initial sending rate upon entry to the
idle period, then it will still be less than the initial sending rate
when the idle period is exited. However, if the allowed sending rate
is greater than or equal to the initial sending rate upon entry to
the idle period, then it should not be reduced below the initial
sending rate no matter how long the idle period lasts.

The sender's allowed sending rate is limited to at most twice the
receive rate reported by the receiver. Thus, after an application-
limited period, the sender can at most double its sending rate from
one round-trip time to the next, until it reaches the allowed sending
rate determined by the loss event rate.

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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. Its 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 3 senders respond to these options as described in
[RFC4340], with the following further clarifications.

o Drop Code 2 ("receive buffer drop"). The allowed sending rate is
reduced by one packet per RTT for each packet newly acknowledged
as Drop Code 2, except that it is never reduced below one packet
per RTT as a result of Drop Code 2.

o Adjusting the receive rate X_recv. A CCID 3 sender SHOULD also
respond to non-network-congestion events, such as those implied by
Data Dropped and Slow Receiver options, by adjusting X_recv, the
receive rate reported by the receiver in Receive Rate options (see
Section 8.3). The CCID 3 sender's allowed sending rate is limited
to at most twice the receive rate reported by the receiver via the
"min(..., 2*X_recv)" clause in TFRC's throughput calculations
([RFC3448], Section 4.3). When the sender receives one or more
Data Dropped and Slow Receiver options, the sender adjusts X_recv
as follows:

1. X_inrecv is equal to the Receive Rate in bytes per second
reported by the receiver in the most recent acknowledgement.

2. X_drop is set to the sending rate upper bound implied by Data
Dropped and Slow Receiver options. If the sender receives a
Slow Receiver option, which requests that the sender not
increase its sending rate for roughly a round-trip time
[RFC4340], then X_drop should be set to X_inrecv. Similarly,
if the sender receives a Data Dropped option indicating, for
example, that three packets were dropped with Drop Code 2, then
the upper bound on the sending rate will be decreased by at
most three packets per RTT, by the sender setting X_drop to

max(X_inrecv - 3*s/RTT, min(X_inrecv, s/RTT)).

Again, s is the packet size in bytes.

3. X_recv is then set to min(X_inrecv, X_drop/2).

As a result, the next round-trip time's sending rate will be
limited to at most 2*(X_drop/2) = X_drop. The effects of the Slow
Receiver and Data Dropped options on X_recv will mostly vanish by

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the round-trip time after that, which is appropriate for this
non-network-congestion feedback. This procedure MUST only be used
for those Drop Codes not related to corruption (see [RFC4340]).
Currently, this is limited to Drop Codes 0, 1, and 2.

5.3. Packet Sizes

CCID 3 is intended for applications that use a fixed packet size, and
that vary their sending rate in packets per second in response to
congestion. CCID 3 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. However,
some attention might be required for applications using CCID 3 that
vary their packet size not in response to congestion, but in response
to other application-level requirements.

The packet size s is used in the TCP throughput equation. A CCID 3
implementation MAY calculate s as the segment size averaged over
multiple round trip times -- for example, over the most recent four
loss intervals, for loss intervals as defined in Section 6.1.
Alternately, a CCID 3 implementation MAY use the Maximum Packet Size
to derive s. In this case, s is set to the Maximum Segment Size
(MSS), the maximum size in bytes for the data segment, not including
the default DCCP and IP packet headers. Each packet transmitted then
counts as one MSS, regardless of the actual segment size, and the TCP
throughput equation can be interpreted as specifying the sending rate
in packets per second.

CCID 3 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

The receiver sends a feedback packet to the sender roughly once per
round-trip time, if the sender is sending packets that frequently.
This rate is determined by the TFRC protocol as specified in
[RFC3448], Section 6.

Each feedback packet contains an Acknowledgement Number, which equals
the greatest valid sequence number received so far on this
connection. ("Greatest" is, of course, measured in circular sequence
space.) Each feedback packet also includes at least the following
options:

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1. An Elapsed Time and/or Timestamp Echo option specifying the amount
of time elapsed since the arrival at the receiver of the packet
whose sequence number appears in the Acknowledgement Number field.
These options are described in [RFC4340], Section 13.

2. A Receive Rate option, defined in Section 8.3, specifying the rate
at which data was received since the last DCCP-Ack was sent.

3. A Loss Intervals option, defined in Section 8.6, specifying the
most recent loss intervals experienced by the receiver. (The
definition of a loss interval is provided below.) From Loss
Intervals, the sender can easily calculate the loss event rate p
using the procedure described in [RFC3448], Section 5.4.

Acknowledgements not containing at least these three options are not
considered feedback packets.

The receiver MAY also include other options concerning the loss event
rate, including Loss Event Rate, which gives the loss event rate
calculated by the receiver (Section 8.5), and DCCP's generic Ack
Vector option, which reports the specific sequence numbers of any
lost or marked packets ([RFC4340], Section 11.4). Ack Vector is not
required by CCID 3's congestion control mechanisms: the Loss
Intervals option provides all the information needed to manage the
transmit rate and probabilistically verify receiver feedback.
However, Ack Vector may be useful for applications that need to
determine exactly which packets were lost. The receiver MAY also
include other acknowledgement-related options, such as DCCP's Data
Dropped option ([RFC4340], Section 11.7).

If the HC-Receiver is also sending data packets to the HC-Sender,
then it MAY piggyback acknowledgement information on those data
packets more frequently than TFRC's specified acknowledgement rate
allows.

6.1. Loss Interval Definition

As described in [RFC3448], Section 5.2, a loss interval begins with a
lost or ECN-marked data packet; continues with at most one round-trip
time's worth of packets that may or may not be lost or marked; and
completes with an arbitrarily long series of non-dropped, non-marked
data packets. For example, here is a single loss interval, assuming
that sequence numbers increase as you move right:

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Lossy Part
<= 1 RTT __________ Lossless Part __________
/ \/ \
*----*--*--*-------------------------------------
^ ^ ^ ^
losses or marks

Note that a loss interval's lossless part might be empty, as in the
first interval below:

Lossy Part Lossy Part
<= 1 RTT <= 1 RTT _____ Lossless Part _____
/ \/ \/ \
*----*--*--***--------*-*---------------------------
^ ^ ^ ^^^ ^ ^
\_ Int. 1 _/\_____________ Interval 2 _____________/

As in [RFC3448], Section 5.2, the length of the lossy part MUST be
less than or equal to 1 RTT. CCID 3 uses window counter values, not
receive times, to determine whether multiple packets occurred in the
same RTT and thus belong to the same loss event; see Section 10.2. A
loss interval whose lossy part lasts for more than 1 RTT, or whose
lossless part contains a dropped or marked data packet, is invalid.

A missing data packet doesn't begin a new loss interval until NDUPACK
packets have been seen after the "hole", where NDUPACK = 3. Thus, up
to NDUPACK of the most recent sequence numbers (including the
sequence numbers of any holes) might temporarily not be part of any
loss interval while the implementation waits to see whether a hole
will be filled. See [RFC3448], Section 5.1, and [RFC2581], Section
3.2, for further discussion of NDUPACK.

As specified by [RFC3448], Section 5, all loss intervals except the
first begin with a lost or marked data packet, and all loss intervals
are as long as possible, subject to the validity constraints above.

Lost and ECN-marked non-data packets may occur freely in the lossless
part of a loss interval. (Non-data packets consist of those packet
types that cannot carry application data; namely, DCCP-Ack, DCCP-
Close, DCCP-CloseReq, DCCP-Reset, DCCP-Sync, and DCCP-SyncAck.) In
the absence of better information, a receiver MUST conservatively
assume that every lost packet was a data packet and thus must occur
in some lossy part. DCCP's NDP Count option can help the receiver
determine whether a particular packet contained data; see [RFC4340],
Section 7.7.

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6.1.1. Loss Interval Lengths

[RFC3448] defines the TFRC congestion control mechanism in terms of a
one-way transfer of data, with data packets going from the sender to
the receiver and feedback packets going from the receiver back to the
sender. However, CCID 3 applies in a context of two half-
connections, with DCCP-Data and DCCP-DataAck packets from one half-
connection sharing sequence number space with DCCP-Ack packets from
the other half-connection. For the purposes of CCID 3 congestion
control, loss interval lengths should include data packets and should
exclude the acknowledgement packets from the reverse half-connection.
However, it is also useful to report the total number of packets in
each loss interval (for example, to facilitate ECN Nonce
verification).

CCID 3's Loss Intervals option thus reports three lengths for each
loss interval, the lengths of the lossy and lossless parts defined
above and a separate data length. First, the lossy and lossless
lengths are measured in sequence numbers. Together, they sum to the
interval's sequence length, which is the total number of packets the
sender transmitted during the interval. This is easily calculated in
DCCP as the greatest packet sequence number in the interval minus the
greatest packet sequence number in the preceding interval (or, if
there is no preceding interval, then the predecessor to the half-
connection's initial sequence number). The interval's data length,
however, is the number used in TFRC's loss event rate calculation, as
defined in [RFC3448], Section 5, and is calculated as follows.

For all loss intervals except the first, the data length equals the
sequence length minus the number of non-data packets the sender
transmitted during the loss interval, except that the minimum data
length is one packet. In the absence of better information, an
endpoint MUST conservatively assume that the loss interval contained
only data packets, in which case the data length equals the sequence
length. To achieve greater precision, the sender can calculate the
exact number of non-data packets in an interval by remembering which
sent packets contained data; the receiver can account for received
non-data packets by not including them in the data length, and for
packets that were not received, it may be able to discriminate
between lost data packets and lost non-data packets using DCCP's NDP
Count option.

The first loss interval's data length is undefined until the first
loss event. [RFC3448], Section 6.3.1 specifies how the first loss
interval's data length is calculated once the first loss event has
occurred; this calculation uses X_recv, the most recent receive rate,
as input. Until this first loss event, the loss event rate is zero,

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as is the data length reported for the interval in the Loss Intervals
option.

The first loss interval's data length might be less than, equal to,
or even greater than its sequence length. Any other loss interval's
data length must be less than or equal to its sequence length.

A sender MAY use the loss event rate or loss interval data lengths as
reported by the receiver, or it MAY recalculate loss event rate
and/or loss interval data lengths based on receiver feedback and
additional information. For example, assume the network drops a
DCCP-Ack packet with sequence number 50. The receiver might then
report a loss interval beginning at sequence number 50. If the
sender determined that this loss interval actually contained no lost
or ECN-marked data packets, then it might coalesce the loss interval
with the previous loss interval, resulting in a larger allowed
transmit rate.

6.2. Congestion Control on Acknowledgements

The rate and timing for generating acknowledgements is determined by
the TFRC algorithm ([RFC3448], Section 6). The sending rate for
acknowledgements is relatively low -- roughly once per round-trip
time -- so there is no need for explicit congestion control on
acknowledgements.

6.3. Acknowledgements of Acknowledgements

TFRC acknowledgements don't generally need to be reliable, so the
sender generally need not acknowledge the receiver's
acknowledgements. When Ack Vector or Data Dropped is used, however,
the sender, DCCP A, MUST occasionally acknowledge the receiver's
acknowledgements so that the receiver can free up Ack Vector or Data
Dropped state. When both half-connections are active, the necessary
acknowledgements will be contained in A's acknowledgements to B's
data. If the B-to-A half-connection goes quiescent, however, DCCP A
must send an acknowledgement proactively.

Thus, when Ack Vector or Data Dropped is used, an active sender MUST
acknowledge the receiver's acknowledgements approximately once per
round-trip time, within a factor of two or three, probably by sending
a DCCP-DataAck packet. No acknowledgement options are necessary,
just the Acknowledgement Number in the DCCP-DataAck header.

The sender MAY choose to acknowledge the receiver's acknowledgements
even if they do not contain Ack Vectors or Data Dropped options. For
instance, regular acknowledgements can shrink the size of the Loss
Intervals option. Unlike Ack Vector and Data Dropped, however, the

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Loss Intervals option is bounded in size (and receiver state), so
acks-of-acks are not required.

6.4. Determining Quiescence

This section describes how a CCID 3 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. (A
CCID 3 receiver has a rough measure of the round-trip time so that it
can pace its acknowledgements.) The receiver detects that the sender
has gone quiescent after T seconds have passed without receiving any
additional data from the sender.

7. Explicit Congestion Notification

CCID 3 supports Explicit Congestion Notification (ECN) [RFC3168]. In
the typical case of an ECN-capable half-connection (where the
receiver's ECN Incapable feature is set to zero), the sender will use
the ECN Nonce for its data packets, as specified in [RFC4340],
Section 12.2. Information about the ECN Nonce MUST be returned by
the receiver using the Loss Intervals option, and any Ack Vector
options MUST include the ECN Nonce Sum. The sender MAY maintain a
table with the ECN nonce sum for each packet and use this information
to probabilistically verify the ECN nonce sums returned in Loss
Intervals or Ack Vector options. Section 9 describes this further.

8. Options and Features

CCID 3 can make use of DCCP's Ack Vector, Timestamp, Timestamp Echo,
and Elapsed Time options, and its Send Ack Vector and ECN Incapable
features. In addition, the following CCID-specific options are
defined for use with CCID 3.

Option DCCP- Section
Type Length Meaning Data? Reference
----- ------ ------- ----- ---------
128-191 Reserved
192 6 Loss Event Rate N 8.5
193 variable Loss Intervals N 8.6
194 6 Receive Rate N 8.3
195-255 Reserved

Table 1: DCCP CCID 3 Options

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The "DCCP-Data?" column indicates that all currently defined CCID 3-
specific options MUST be ignored when they occur on DCCP-Data
packets.

The following CCID-specific feature is also defined.

Rec'n Initial Section
Number Meaning Rule Value Req'd Reference
------ ------- ----- ----- ----- ---------
128-191 Reserved
192 Send Loss Event Rate SP 0 N 8.4
193-255 Reserved

Table 2: DCCP CCID 3 Feature Numbers

The column meanings are described in [RFC4340], Table 4. "Rec'n
Rule" defines the feature's reconciliation rule, where "SP" means
server-priority. "Req'd" specifies whether every CCID 3
implementation MUST understand a feature; Send Loss Event Rate is
optional, in that it behaves like an extension ([RFC4340], Section
15).

8.1. Window Counter Value

The data sender stores a 4-bit window counter value in the DCCP
generic header's CCVal field on every data packet it sends. This
value is set to 0 at the beginning of the transmission and generally
increased by 1 every quarter of a round-trip time, as described in
[RFC3448], Section 3.2.1. Window counters use circular arithmetic
modulo 16 for all operations, including comparisons; see [RFC4340],
Section 3.1, for more information on circular arithmetic. For
reference, the DCCP generic header is as follows. (The diagram is
repeated from [RFC4340], Section 5.1, which also shows the generic
header with a 24-bit Sequence Number field.)

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Dest Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Offset | CCVal | CsCov | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Res | Type |1| Reserved | Sequence Number (high bits) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Sequence Number (low bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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The CCVal field has enough space to express 4 round-trip times at
quarter-RTT granularity. The sender MUST avoid wrapping CCVal on
adjacent packets, as might happen, for example, if two data-carrying
packets were sent 4 round-trip times apart with no packets
intervening. Therefore, the sender SHOULD use the following
algorithm for setting CCVal. The algorithm uses three variables:
"last_WC" holds the last window counter value sent, "last_WC_time" is
the time at which the first packet with window counter value
"last_WC" was sent, and "RTT" is the current round-trip time
estimate. last_WC is initialized to zero, and last_WC_time to the
time of the first packet sent. Before sending a new packet, proceed
like this:

Let quarter_RTTs = floor((current_time - last_WC_time) / (RTT/4)).
If quarter_RTTs > 0, then:
Set last_WC := (last_WC + min(quarter_RTTs, 5)) mod 16.
Set last_WC_time := current_time.
Set the packet header's CCVal field to last_WC.

When this algorithm is used, adjacent data-carrying packets' CCVal
counters never differ by more than five, modulo 16.

The window counter value may also change as feedback packets arrive.
In particular, after receiving an acknowledgement for a packet sent
with window counter WC, the sender SHOULD increase its window
counter, if necessary, so that subsequent packets have window counter
value at least (WC + 4) mod 16.

The CCVal counters are used by the receiver to determine whether
multiple losses belong to a single loss event, to determine the
interval to use for calculating the receive rate, and to determine
when to send feedback packets. None of these procedures require the
receiver to maintain an explicit estimate of the round-trip time.
However, implementors who wish to keep such an RTT estimate may do so
using CCVal. Let T(I) be the arrival time of the earliest valid
received packet with CCVal = I. (Of course, when the window counter
value wraps around to the same value mod 16, we must recalculate
T(I).) Let D = 2, 3, or 4 and say that T(K) and T(K+D) both exist
(packets were received with window counters K and K+D). Then the
value (T(K+D) - T(K)) * 4/D MAY serve as an estimate of the round-
trip time. Values of D = 4 SHOULD be preferred for RTT estimation.
Concretely, say that the following packets arrived:

Time: T1 T2 T3 T4 T5 T6 T7 T8 T9
------*---*---*-*----*------------*---*----*--*---->
CCVal: K-1 K-1 K K K+1 K+3 K+4 K+3 K+4

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Then T7 - T3, the difference between the receive times of the first
packet received with window counter K+4 and the first packet received
with window counter K, is a reasonable round-trip time estimate.
Because of the necessary constraint that measurements only come from
packet pairs whose CCVals differ by at most 4, this procedure does
not work when the inter-packet sending times are significantly
greater than the RTT, resulting in packet pairs whose CCVals differ
by 5. Explicit RTT measurement techniques, such as Timestamp and
Timestamp Echo, should be used in that case.

8.2. Elapsed Time Options

The data receiver MUST include an elapsed time value on every
required acknowledgement. This helps the sender distinguish between
network round-trip time, which it must include in its rate equations,
and delay at the receiver due to TFRC's infrequent acknowledgement
rate, which it need not include. The receiver MUST at least include
an Elapsed Time option on every feedback packet, but if at least one
recent data packet (i.e., a packet received after the previous DCCP-
Ack was sent) included a Timestamp option, then the receiver SHOULD
include the corresponding Timestamp Echo option, with Elapsed Time
value, as well. All of these option types are defined in the main
DCCP specification [RFC4340].

8.3. Receive Rate Option

+--------+--------+--------+--------+--------+--------+
|11000010|00000110| Receive Rate |
+--------+--------+--------+--------+--------+--------+
Type=194 Len=6

This option MUST be sent by the data receiver on all required
acknowledgements. Its four data bytes indicate the rate at which the
receiver has received data since it last sent an acknowledgement, in
bytes per second. To calculate this receive rate, the receiver sets
t to the larger of the estimated round-trip time and the time since
the last Receive Rate option was sent. (Received data packets'
window counters can be used to produce a suitable RTT estimate, as
described in Section 8.1.) The receive rate then equals the number
of data bytes received in the most recent t seconds, divided by t.

Receive Rate options MUST NOT be sent on DCCP-Data packets, and any
Receive Rate options on received DCCP-Data packets MUST be ignored.

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8.4. Send Loss Event Rate Feature

The Send Loss Event Rate feature lets CCID 3 endpoints negotiate
whether the receiver MUST provide Loss Event Rate options on its
acknowledgements. DCCP A sends a "Change R(Send Loss Event Rate, 1)"
option to ask DCCP B to send Loss Event Rate options as part of its
acknowledgement traffic.

Send Loss Event Rate has feature number 192 and is server-priority.
It takes one-byte Boolean values. DCCP B MUST send Loss Event Rate
options on its acknowledgements when Send Loss Event Rate/B is one,
although it MAY send Loss Event Rate options even when Send Loss
Event Rate/B is zero. Values of two or more are reserved. A CCID 3
half-connection starts with Send Loss Event Rate equal to zero.

8.5. Loss Event Rate Option

+--------+--------+--------+--------+--------+--------+
|11000000|00000110| Loss Event Rate |
+--------+--------+--------+--------+--------+--------+
Type=192 Len=6

The option value indicates the inverse of the loss event rate,
rounded UP, as calculated by the receiver. Its units are data
packets per loss interval. Thus, if the Loss Event Rate option value
is 100, then the loss event rate is 0.01 loss events per data packet
(and the average loss interval contains 100 data packets). When each
loss event has exactly one data packet loss, the loss event rate is
the same as the data packet drop rate.

See [RFC3448], Section 5, for a normative calculation of loss event
rate. Before any losses have occurred, when the loss event rate is
zero, the Loss Event Rate option value is set to
"11111111111111111111111111111111" in binary (or, equivalently, to
2^32 - 1). The loss event rate calculation uses loss interval data
lengths, as defined in Section 6.1.1.

Loss Event Rate options MUST NOT be sent on DCCP-Data packets, and
any Loss Event Rate options on received DCCP-Data packets MUST be
ignored.

8.6. Loss Intervals Option

+--------+--------+--------+--------...--------+--------+---
|11000001| Length | Skip | Loss Interval | More Loss
| | | Length | | Intervals...
+--------+--------+--------+--------...--------+--------+---
Type=193 9 bytes

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Each 9-byte Loss Interval contains three fields, as follows:

____________________ Loss Interval _____________________
/ \
+--------...-------+--------...--------+--------...--------+
| Lossless Length |E| Loss Length | Data Length |
+--------...-------+--------...--------+--------...--------+
3 bytes 3 bytes 3 bytes

The receiver reports its observed loss intervals using a Loss
Intervals option. Section 6.1 defines loss intervals. This option
MUST be sent by the data receiver on all required acknowledgements.
The option reports up to 28 loss intervals seen by the receiver,
although TFRC currently uses at most the latest 9 of these. This
lets the sender calculate a loss event rate and probabilistically
verify the receiver's ECN Nonce Echo.

The Loss Intervals option serves several purposes.

o The sender can use the Loss Intervals option to calculate the loss
event rate.

o Loss Intervals information is easily checked for consistency
against previous Loss Intervals options, and against any Loss
Event Rate calculated by the receiver.

o The sender can probabilistically verify the ECN Nonce Echo for
each Loss Interval, reducing the likelihood of misbehavior.

Loss Intervals options MUST NOT be sent on DCCP-Data packets, and any
Loss Intervals options on received DCCP-Data packets MUST be ignored.

8.6.1. Option Details

The Loss Intervals option contains information about one to 28
consecutive loss intervals, always including the most recent loss
interval. Intervals are listed in reverse chronological order.
Should more than 28 loss intervals need to be reported, then multiple
Loss Intervals options can be sent; the second option begins where
the first left off, and so forth. The options MUST contain
information about at least the most recent NINTERVAL + 1 = 9 loss
intervals unless (1) there have not yet been NINTERVAL + 1 loss
intervals, or (2) the receiver knows, because of the sender's
acknowledgements, that some previously transmitted loss interval
information has been received. In this second case, the receiver
need not send loss intervals that the sender already knows about,
except that it MUST transmit at least one loss interval regardless.

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The NINTERVAL parameter is equal to "n" as defined in [RFC3448],
Section 5.4.

Loss interval sequence numbers are delta encoded starting from the
Acknowledgement Number. Therefore, Loss Intervals options MUST NOT
be sent on packets without an Acknowledgement Number, and any Loss
Intervals options received on such packets MUST be ignored.

The first byte of option data is Skip Length, which indicates the
number of packets up to and including the Acknowledgement Number that
are not part of any Loss Interval. As discussed above, Skip Length
must be less than or equal to NDUPACK = 3. In a packet containing
multiple Loss Intervals options, the Skip Lengths of the second and
subsequent options MUST equal zero; such options with nonzero Skip
Lengths MUST be ignored.

Loss Interval structures follow Skip Length. Each Loss Interval
consists of a Lossless Length, a Loss Length, an ECN Nonce Echo (E),
and a Data Length.

Lossless Length, a 24-bit number, specifies the number of packets in
the loss interval's lossless part. Note again that this part may
contain lost or marked non-data packets.

Loss Length, a 23-bit number, specifies the number of packets in the
loss interval's lossy part. The sum of the Lossless Length and the
Loss Length equals the loss interval's sequence length. Receivers
SHOULD report the minimum valid Loss Length for each loss interval,
making the first and last sequence numbers in each lossy part
correspond to lost or marked data packets.

The ECN Nonce Echo, stored in the high-order bit of the 3-byte field
containing Loss Length, equals the one-bit sum (exclusive-or, or
parity) of data packet nonces received over the loss interval's
lossless part (which is Lossless Length packets long). If Lossless
Length is 0, the receiver is ECN Incapable, or the Lossless Length
contained no data packets, then the ECN Nonce Echo MUST be reported
as 0. Note that any ECN nonces on received non-data packets MUST NOT
contribute to the ECN Nonce Echo.

Finally, Data Length, a 24-bit number, specifies the loss interval's
data length, as defined in Section 6.1.1.

8.6.2. Example

Consider the following sequence of packets, where "-" represents a
safely delivered packet and "*" represents a lost or marked packet.

Floyd, et al. Standards Track [Page 20]

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Sequence
Numbers: 0 10 20 30 40 44
| | | | | |
----------*--------***-*--------*----------*-

Assuming that packet 43 was lost, not marked, this sequence might be
divided into loss intervals as follows:

0 10 20 30 40 44
| | | | | |
----------*--------***-*--------*----------*-
\________/\_______/\___________/\_________/
L0 L1 L2 L3

A Loss Intervals option sent on a packet with Acknowledgement Number
44 to acknowledge this set of loss intervals might contain the bytes
193,39,2, 0,0,10, 128,0,1, 0,0,10, 0,0,8, 0,0,5, 0,0,10, 0,0,8,
0,0,1, 0,0,8, 0,0,10, 128,0,0, 0,0,15. This option is interpreted as
follows.

193 The Loss Intervals option number.

39 The length of the option, including option type and length bytes.
This option contains information about (39 - 3)/9 = 4 loss
intervals.

2 The Skip Length is 2 packets. Thus, the most recent loss
interval, L3, ends immediately before sequence number 44 - 2 + 1
= 43.

0,0,10, 128,0,1, 0,0,10
These bytes define L3. L3 consists of a 10-packet lossless part
(0,0,10), preceded by a 1-packet lossy part. Continuing to
subtract, the lossless part begins with sequence number 43 - 10 =
33, and the lossy part begins with sequence number 33 - 1 = 32.
The ECN Nonce Echo for the lossless part (namely, packets 33
through 42, inclusive) equals 1. The interval's data length is
10, so the receiver believes that the interval contained exactly
one non-data packet.

0,0,8, 0,0,5, 0,0,10
This defines L2, whose lossless part begins with sequence number
32 - 8 = 24; whose lossy part begins with sequence number 24 - 5
= 19; whose ECN Nonce Echo (for packets [24,31]) equals 0; and
whose data length is 10.

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0,0,8, 0,0,1, 0,0,8
L1's lossless part begins with sequence number 11, its lossy part
begins with sequence number 10, its ECN Nonce Echo (for packets
[11,18]) equals 0, and its data length is 8.

0,0,10, 128,0,0, 0,0,15
L0's lossless part begins with sequence number 0, it has no lossy
part, its ECN Nonce Echo (for packets [0,9]) equals 1, and its
data length is 15. (This must be the first loss interval in the
connection; otherwise, a data length greater than the sequence
length would be invalid.)

9. Verifying Congestion Control Compliance with ECN

The sender can use Loss Intervals options' ECN Nonce Echoes (and
possibly any Ack Vectors' ECN Nonce Echoes) to probabilistically
verify that the receiver is correctly reporting all dropped or marked
packets. Even if ECN is not used (the receiver's ECN Incapable
feature is set to one), the sender could still check on the receiver
by occasionally not sending a packet, or sending a packet out-of-
order, to catch the receiver in an error in Loss Intervals or Ack
Vector information. This is not as robust or non-intrusive as the
verification provided by the ECN Nonce, however.

9.1. Verifying the ECN Nonce Echo

To verify the ECN Nonce Echo included with a Loss Intervals option,
the sender maintains a table with the ECN nonce sum for each data
packet. As defined in [RFC3540], the nonce sum for sequence number S
is the one-bit sum (exclusive-or, or parity) of data packet nonces
over the sequence number range [I,S], where I is the initial sequence
number. Let NonceSum(S) represent this nonce sum for sequence number
S, and define NonceSum(I - 1) as 0. Note that NonceSum does not
account for the nonces of non-data packets such as DCCP-Ack. Then
the Nonce Echo for an interval of packets with sequence numbers X to
Y, inclusive, should equal the following one-bit sum:

NonceSum(X - 1) + NonceSum(Y)

Since an ECN Nonce Echo is returned for the lossless part of each
Loss Interval, a misbehaving receiver -- meaning a receiver that
reports a lost or marked data packet as "received non-marked", to
avoid rate reductions -- has only a 50% chance of guessing the
correct Nonce Echo for each loss interval.

To verify the ECN Nonce Echo included with an Ack Vector option, the
sender maintains a table with the ECN nonce value sent for each
packet. The Ack Vector option explicitly says which packets were

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received non-marked; the sender just adds up the nonces for those
packets using a one-bit sum and compares the result to the Nonce Echo
encoded in the Ack Vector's option type. Again, a misbehaving
receiver has only a 50% chance of guessing an Ack Vector's correct
Nonce Echo. Alternatively, an Ack Vector's ECN Nonce Echo may also
be calculated from a table of ECN nonce sums, rather than from ECN
nonces. If the Ack Vector contains many long runs of non-marked,
non-dropped packets, the nonce sum-based calculation will probably be
faster than a straightforward nonce-based calculation.

Note that Ack Vector's ECN Nonce Echo is measured over both data
packets and non-data packets, while the Loss Intervals option reports
ECN Nonce Echoes for data packets only. Thus, different nonce sum
tables are required to verify the two options.

9.2. Verifying the Reported Loss Intervals and Loss Event Rate

Besides probabilistically verifying the ECN Nonce Echoes reported by
the receiver, the sender may also verify the loss intervals and any
loss event rate reported by the receiver, if it so desires.
Specifically, the Loss Intervals option explicitly reports the size
of each loss interval as seen by the receiver; the sender can verify
that the receiver is not falsely combining two loss events into one
reported Loss Interval by using saved window counter information.
The sender can also compare any Loss Event Rate option to the loss
event rate it calculates using the Loss Intervals option.

Note that in some cases the loss event rate calculated by the sender
could differ from an explicit Loss Event Rate option sent by the
receiver. In particular, when a number of successive packets are
dropped, the receiver does not know the sending times for these
packets and interprets these losses as a single loss event. In
contrast, if the sender has saved the sending times or window counter
information for these packets, then the sender can determine if these
losses constitute a single loss event or several successive loss
events. Thus, with its knowledge of the sending times of dropped
packets, the sender is able to make a more accurate calculation of
the loss event rate. These kinds of differences SHOULD NOT be
misinterpreted as attempted receiver misbehavior.

10. Implementation Issues

10.1. Timestamp Usage

CCID 3 data packets need not carry Timestamp options. The sender can
store the times at which recent packets were sent; the
Acknowledgement Number and Elapsed Time option contained on each
required acknowledgement then provide sufficient information to

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compute the round trip time. Alternatively, the sender MAY include
Timestamp options on some of its data packets. The receiver will
respond with Timestamp Echo options including Elapsed Times, allowing
the sender to calculate round-trip times without storing sent
packets' timestamps at all.

10.2. Determining Loss Events at the Receiver

The window counter is used by the receiver to determine whether
multiple lost packets belong to the same loss event. The sender
increases the window counter by one every quarter round-trip time.
This section describes in detail the procedure for using the window
counter to determine when two lost packets belong to the same loss
event.

[RFC3448], Section 3.2.1 specifies that each data packet contains a
timestamp and gives as an alternative implementation a "timestamp"
that is incremented every quarter of an RTT, as is the window counter
in CCID 3. However, [RFC3448], Section 5.2 on "Translation from Loss
History to Loss Events" is written in terms of timestamps, not in
terms of window counters. In this section, we give a procedure for
the translation from loss history to loss events that is explicitly
in terms of window counters.

To determine whether two lost packets with sequence numbers X and Y
belong to different loss events, the receiver proceeds as follows.
Assume Y > X in circular sequence space.

o Let X_prev be the greatest valid sequence number received with
X_prev < X.

o Let Y_prev be the greatest valid sequence number received with
Y_prev < Y.

o Given a sequence number N, let C(N) be the window counter value
associated with that packet.

o Packets X and Y belong to different loss events if there exists a
packet with sequence number S so that X_prev < S <= Y_prev, and
the distance from C(X_prev) to C(S) is greater than 4. (The
distance is the number D so that C(X_prev) + D = C(S) (mod
WCTRMAX), where WCTRMAX is the maximum value for the window
counter -- in our case, 16.)

That is, the receiver only considers losses X and Y as separate
loss events if there exists some packet S received between X and
Y, with the distance from C(X_prev) to C(S) greater than 4. This
complex calculation is necessary in order to handle the case where

Floyd, et al. Standards Track [Page 24]

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window counter space wrapped completely between X and Y. When
that space does not wrap, the receiver can simply check whether
the distance from C(X_prev) to C(Y_prev) is greater than 4; if so,
then X and Y belong to separate loss events.

Window counters can help the receiver disambiguate multiple losses
after a sudden decrease in the actual round-trip time. When the
sender receives an acknowledgement acknowledging a data packet with
window counter i, the sender increases its window counter, if
necessary, so that subsequent data packets are sent with window
counter values of at least i+4. This can help minimize errors where
the receiver incorrectly interprets multiple loss events as a single
loss event.

We note that if all of the packets between X and Y are lost in the
network, then X_prev and Y_prev are equal, and the series of
consecutive losses is treated by the receiver as a single loss event.
However, the sender will receive no DCCP-Ack packets during a period
of consecutive losses, and the sender will reduce its sending rate
accordingly.

As an alternative to the window counter, the sender could have sent
its estimate of the round-trip time to the receiver directly in a
round-trip time option; the receiver would use the sender's round-
trip time estimate to infer when multiple lost or marked packets
belong in the same loss event. In some respects, a round-trip time
option would give a more precise encoding of the sender's round-trip
time estimate than does the window counter. However, the window
counter conveys information about the relative *sending* times for
packets, while the receiver could only use the round-trip time option
to distinguish between the relative *receive* times (in the absence
of timestamps). That is, the window counter will give more robust
performance when there is a large variation in delay for packets sent
within a window of data. Slightly more speculatively, a round-trip
time option might possibly be used more easily by middleboxes
attempting to verify that a flow used conforming end-to-end
congestion control.

10.3. Sending Feedback Packets

[RFC3448], Sections 6.1 and 6.2 specify that the TFRC receiver must
send a feedback packet when a newly calculated loss event rate p is
greater than its previous value. CCID 3 follows this rule.

In addition, [RFC3448], Section 6.2, specifies that the receiver use
a feedback timer to decide when to send additional feedback packets.
If the feedback timer expires and data packets have been received
since the previous feedback was sent, then the receiver sends a

Floyd, et al. Standards Track [Page 25]

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feedback packet. When the feedback timer expires, the receiver
resets the timer to expire after R_m seconds, where R_m is the most
recent estimate of the round-trip time received from the sender.
CCID 3 receivers, however, generally use window counter values
instead of a feedback timer to determine when to send additional
feedback packets. This section describes how.

Whenever the receiver sends a feedback message, the receiver sets a
local variable last_counter to the greatest received value of the
window counter since the last feedback message was sent, if any data
packets have been received since the last feedback message was sent.
If the receiver receives a data packet with a window counter value
greater than or equal to last_counter + 4, then the receiver sends a
new feedback packet. ("Greater" and "greatest" are measured in
circular window counter space.)

This procedure ensures that when the sender is sending at a rate less
than one packet per round-trip time, the receiver sends a feedback
packet after each data packet. Similarly, this procedure ensures
that when the sender is sending several packets per round-trip time,
the receiver will send a feedback packet each time that a data packet
arrives with a window counter at least four greater than the window
counter when the last feedback packet was sent. Thus, the feedback
timer is not necessary when the window counter is used.

However, the feedback timer still could be useful in some rare cases
to prevent the sender from unnecessarily halving its sending rate.
In particular, one could construct scenarios where the use of the
feedback timer at the receiver would prevent the unnecessary
expiration of the nofeedback timer at the sender. Consider the case
below, in which a feedback packet is sent when a data packet arrives
with a window counter of K.

Window
Counters: K K+1 K+2 K+3 K+4 K+5 K+6 ... K+15 K+16 K+17 ...
| | | | | | | | | |
Data | | | | | | | | | |
Packets | | | | | | | | | |
Received: - - --- - ... - - -- - -- -- -
| | | | | |
| | | | | |
Events: 1: 2: 3: 4: 5: 6:
"A" "B" Timer "B"
sent sent received

1: Feedback message A is sent.
2: A feedback message would have been sent if feedback
timers had been used.

Floyd, et al. Standards Track [Page 26]

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3: Feedback message B is sent.
4: Sender's nofeedback timer expires.
5: Feedback message B is received at the sender.
6: Sender's nofeedback timer would have expired if feedback
timers had been used, and the feedback message at 2 had
been sent.

The receiver receives data after the feedback packet has been sent
but has received no data packets with a window counter between K+4
and K+14. A data packet with a window counter of K+4 or larger would
have triggered sending a new feedback packet, but no feedback packet
is sent until time 3.

The TFRC protocol specifies that after a feedback packet is received,
the sender sets a nofeedback timer to at least four times the round-
trip time estimate. If the sender doesn't receive any feedback
packets before the nofeedback timer expires, then the sender halves
its sending rate. In the figure, the sender receives feedback
message A (time 1) and then sets the nofeedback timer to expire
roughly four round-trip times later (time 4). The sender starts
sending again just before the nofeedback timer expires but doesn't
receive the resulting feedback message until after its expiration,
resulting in an unnecessary halving of the sending rate. If the
connection had used feedback timers, the receiver would have sent a
feedback message when the feedback timer expired at time 2, and the
halving of the sending rate would have been avoided.

For implementors who wish to implement a feedback timer for the data
receiver, we suggest estimating the round-trip time from the most
recent data packet, as described in Section 8.1. We note that this
procedure does not work when the inter-packet sending times are
greater than the RTT.

11. Security Considerations

Security considerations for DCCP have been discussed in [RFC4340],
and security considerations for TFRC have been discussed in
[RFC3448], Section 9. The security considerations for TFRC include
the need to protect against spoofed feedback and the need to protect
the congestion control mechanisms against incorrect information from
the receiver.

In this document, we have extensively discussed the mechanisms the
sender can use to verify the information sent by the receiver. When
ECN is used, the receiver returns ECN Nonce information to the
sender. When ECN is not used, then, as Section 9 shows, the sender
could still use various techniques that might catch the receiver in

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RFC 4342 DCCP CCID3 TFRC March 2006

an error in reporting congestion, but this is not as robust or non-
intrusive as the verification provided by the ECN Nonce.

12. IANA Considerations

This specification defines the value 3 in the DCCP CCID namespace
managed by IANA. This assignment is also mentioned in [RFC4340].

CCID 3 also introduces three sets of numbers whose values should be
allocated by IANA; namely, CCID 3-specific Reset Codes, option types,
and feature numbers. These ranges will prevent any future CCID 3-
specific allocations from polluting DCCP's corresponding global
namespaces; see [RFC4340], Section 10.3. However, we note that this
document makes no particular allocations from the Reset Code range,
except for experimental and testing use [RFC3692]. We refer to the
Standards Action policy outlined in [RFC2434].

12.1. Reset Codes

Each entry in the DCCP CCID 3 Reset Code registry contains a CCID 3-
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.

12.2. Option Types

Each entry in the DCCP CCID 3 option type registry contains a CCID
3-specific option type, which is a number in the range 128-255; the
name of the option, such as "Loss Intervals"; and a reference to the
RFC defining the option type. The registry is initially populated
using the values in Table 1, in Section 8. This document allocates
option types 192-194, and option types 184-190 and 248-254 are
permanently reserved for experimental and testing use. The remaining
option types -- 128-183, 191, 195-247, 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.

12.3. Feature Numbers

Each entry in the DCCP CCID 3 feature number registry contains a CCID
3-specific feature number, which is a number in the range 128-255;
the name of the feature, such as "Send Loss Event Rate"; and a
reference to the RFC defining the feature number. The registry is

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initially populated using the values in Table 2, in Section 8. This
document allocates feature number 192, and feature numbers 184-190
and 248-254 are permanently reserved for experimental and testing
use. The remaining feature numbers -- 128-183, 191, 193-247, 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.

13. Thanks

We thank Mark Handley for his help in defining CCID 3. We also thank
Mark Allman, Aaron Falk, Ladan Gharai, Sara Karlberg, Greg Minshall,
Arun Venkataramani, David Vos, Yufei Wang, Magnus Westerlund, and
members of the DCCP Working Group for feedback on versions of this
document.

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RFC 4342 DCCP CCID3 TFRC March 2006

A. Appendix: Possible Future Changes to CCID 3

There are a number of cases where the behavior of TFRC as specified
in [RFC3448] does not match the desires of possible users of DCCP.
These include the following:

1. The initial sending rate of at most four packets per RTT, as
specified in [RFC3390].

2. The receiver's sending of an acknowledgement for every data packet
received, when the receiver receives at a rate less than one
packet per round-trip time.

3. The sender's limitation of at most doubling the sending rate from
one round-trip time to the next (or, more specifically, of
limiting the sending rate to at most twice the reported receive
rate over the previous round-trip time).

4. The limitation of halving the allowed sending rate after an idle
period of four round-trip times (possibly down to the initial
sending rate of two to four packets per round-trip time).

5. The response function used in [RFC3448], Section 3.1, which does
not closely match the behavior of TCP in environments with high
packet drop rates [RFC3714].

One suggestion for higher initial sending rates is an initial sending
rate of up to eight small packets per RTT, when the total packet
size, including headers, is at most 4380 bytes. Because the packets
would be rate-paced out over a round-trip time, instead of sent
back-to-back as they would be in TCP, an initial sending rate of
eight small packets per RTT with TFRC-based congestion control would
be considerably milder than the impact of an initial window of eight
small packets sent back-to-back in TCP. As Section 5.1 describes,
the initial sending rate also serves as a lower bound for reductions
of the allowed sending rate during an idle period.

We note that with CCID 3, the sender is in slow-start in the
beginning and responds promptly to the report of a packet loss or
mark. However, in the absence of feedback from the receiver, the
sender can maintain its old sending rate for up to four round-trip
times. One possibility would be that for an initial window of eight
small packets, the initial nofeedback timer would be set to two
round-trip times instead of four, so that the sending rate would be
reduced after two round-trips without feedback.

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RFC 4342 DCCP CCID3 TFRC March 2006

Research and engineering will be needed to investigate the pros and
cons of modifying these limitations in order to allow larger initial
sending rates, to send fewer acknowledgements when the data sending
rate is low, to allow more abrupt changes in the sending rate, or to
allow a higher sending rate after an idle period.

Normative References

[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.

[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.

[RFC3448] Handley, M., Floyd, S., Padhye, J., and J. Widmer,
"TCP Friendly Rate Control (TFRC): Protocol
Specification", RFC 3448, January 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

[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust
Explicit Congestion Notification (ECN) Signaling with
Nonces", RFC 3540, June 2003.

Floyd, et al. Standards Track [Page 31]

RFC 4342 DCCP CCID3 TFRC March 2006

[RFC3714] Floyd, S. and J. Kempf, "IAB Concerns Regarding
Congestion Control for Voice Traffic in the Internet",
RFC 3714, March 2004.

[RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram
Congestion Control Protocol (DCCP) Congestion Control
ID 2: TCP-like Congestion Control", RFC 4341, March
2006.

[V03] Arun Venkataramani, August 2003. Citation for
acknowledgement purposes only.

Authors' Addresses

Sally Floyd
ICSI Center for Internet Research
1947 Center Street, Suite 600
Berkeley, CA 94704
USA

EMail: [email protected]

Eddie Kohler
4531C Boelter Hall
UCLA Computer Science Department
Los Angeles, CA 90095
USA

EMail: [email protected]

Jitendra Padhye
Microsoft Research
One Microsoft Way
Redmond, WA 98052
USA

EMail: [email protected]

Floyd, et al. Standards Track [Page 32]

RFC 4342 DCCP CCID3 TFRC March 2006

Full Copyright Statement

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contained in BCP 78, and except as set forth therein, the authors
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Floyd, et al. Standards Track [Page 33]