One document matched: draft-floyd-ccid4-00.txt
Internet Engineering Task Force Sally Floyd
INTERNET-DRAFT ICIR
draft-floyd-ccid4-00.txt Eddie Kohler
Expires: 18 December 2006 UCLA
18 June 2006
Profile for DCCP Congestion Control ID 4:
the Small-Packet Variant of
TFRC Congestion Control
Status of this Memo
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Abstract
This document contains the profile for Congestion Control Identifier
4, the Small-Packet variant of TCP-Friendly Rate Control (TFRC), in
the Datagram Congestion Control Protocol (DCCP). CCID 4 is for
experimental use, and uses TFRC-SP [TFRC-SP], a Small-Packet (SP)
variant of TFRC designed for applications that send small packets.
The goal for TFRC-SP is to achieve roughly the same bandwidth in
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bits per second (bps) as a TCP flow using packets of up to 1500
bytes but experiencing the same level of congestion. CCID 4 is for
experimental use for senders that send small packets and would like
a TCP-friendly sending rate, possibly with Explicit Congestion
Notification (ECN), while minimizing abrupt rate changes.
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Table of Contents
1. Introduction ...................................................5
2. Conventions ....................................................5
3. Usage ..........................................................6
3.1. Relationship with TFRC ....................................6
3.2. Example Half-Connection ...................................6
4. Connection Establishment .......................................7
5. Congestion Control on Data Packets .............................7
5.1. Response to Idle and Application-limited Periods ..........8
5.2. Response to Data Dropped and Slow Receiver ................8
5.3. Packet Sizes ..............................................9
6. Acknowledgements ...............................................9
6.1. Loss Interval Definition ..................................9
6.1.1. Loss Interval Lengths ..............................9
6.2. Congestion Control on Acknowledgements ....................9
6.3. Acknowledgements of Acknowledgements ......................9
6.4. Quiescence ................................................9
7. Explicit Congestion Notification ...............................9
8. Options and Features ..........................................10
8.1. Window Counter Value .....................................10
8.2. Elapsed Time Options .....................................11
8.3. Receive Rate Option ......................................11
8.4. Send Loss Event Rate Feature .............................11
8.5. Loss Event Rate Option ...................................11
8.6. Loss Intervals Option ....................................11
8.6.1. Option Details ....................................12
9. Verifying Congestion Control Compliance With ECN ..............13
9.1. Verifying the ECN Nonce Echo .............................13
9.2. Verifying the Reported Loss Intervals and Loss Event Rate
...............................................................13
10. Implementation Issues ........................................13
10.1. Timestamp Usage .........................................13
10.2. Determining Loss Events at the Receiver .................13
10.3. Sending Feedback Packets ................................13
11. Security Considerations ......................................13
12. IANA Considerations ..........................................13
12.1. Reset Codes .............................................14
12.2. Option Types ............................................14
12.3. Feature Numbers .........................................14
13. Thanks .......................................................15
Normative References .............................................15
Informative References ...........................................15
Authors' Addresses ...............................................15
Full Copyright Statement .........................................16
Intellectual Property ............................................16
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List of Tables
Table 1: DCCP CCID 4 Options .....................................10
Table 2: DCCP CCID 4 Feature Numbers .............................10
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1. Introduction
This document contains the profile for Congestion Control Identifier
4, the Small-Packet variant of TCP-friendly rate control (TFRC), in
the Datagram Congestion Control Protocol (DCCP) [RFC 4340]. CCID 4
differs from CCID 3 in that CCID 4 uses TFRC-PS, while CCID 3 [RFC
4342] uses standard TFRC [RFC 3448]. This document assumes that the
reader is familiar with [RFC 4342], instead of repeating from that
document unnecessarily.
CCID 4 differs from CCID 3 only in the following respects:
o Header size: For TFRC-SP, the allowed transmit rate in bytes per
second is reduced by a factor that accounts for packet header
size. This is specified for TFRC-SP in Section 4.2 of [TFRC-SP],
and described for CCID 4 in Section 5 below.
o Minimum sending rate: TFRC-SP enforces a minimum interval of 10
ms. between data packets. This is specified for TFRC-SP in
Section 4.3 of [TFRC-SP], and described for CCID 4 in Section 5
below.
o Loss rates for short loss intervals: For short lost intervals of
at most two round-trip times, the loss rate is computed by
counting the actual number of packets lost or marked. For such a
short loss interval with N data packets, including K lost or
marked data packets, the loss interval length is calculated as
N/K, instead of as N. This is specified for TFRC-SP in Section
4.4 of [TFRC-SP]. The addition of a Dropped Packets field to
CCID 4's Loss Intervals Option is specified in 8.6 below, and its
use in calculating the loss event rate is specified in 8.6 below.
The computation of the loss rate by the receiver for the Loss
Event Rate option is described for CCID 4 in Section 8.4 below.
o The nominal segment size: In TFRC-SP, the nominal segment size
used by the TCP throughput equation is set to 1460 bytes. This
is specified for TFRC-SP in Section 4.5 of RFC 3448, and
described for CCID 4 in Section 5 below.
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 [RFC 2119].
Additional terminology is described in Section 2 of [RFC 4342].
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3. Usage
Like CCID 3, CCID 4's 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.
CCID 4 is designed to be used either by applications that use a
small fixed segment size, or by applications that change their
sending rate by varying the segment size. If CCID 4 is used by an
application that varies its segment size in response to changes in
the allowed sending rate in bps, we note that CCID 4 doesn't dictate
the segment size to be used by the application; this is done by the
application itself. The CCID 4 sender determines the allowed
sending rate in bps, in response to on-going feedback from the
CCID 4 receiver, and the application can use information about the
current allowed sending rate to decide whether to change the current
segment size.
We note that in some environments there will be a feedback loop,
with changes in the packet size or in the sending rate in bps
affecting congestion along the path, therefore affecting the allowed
sending rate in the future.
3.1. Relationship with TFRC
The congestion control mechanisms described here follow the TFRC-SP
mechanism specified in [TFRC-SP]. As with CCID 3, conformant CCID 4
implementations MAY track updates to the TCP throughput equation
directly, as updates are standardized in the IETF, rather than
waiting for revisions of this document. However, conformant
implementations SHOULD wait for explicit updates to CCID 4 before
implementing other changes to TFRC congestion control.
3.2. Example Half-Connection
This example shows the typical progress of a half-connection using
CCID 4's TFRC Congestion Control, not including connection
initiation and termination. The example is informative, not
normative. This example differs from that for CCID 3 in [RFC 4342]
only in that the allowed transmit rate is determined by [TFRC-SP] as
well as by [RFC 3448].
1. The sender transmits DCCP-Data packets, where the sending rate
is governed by the allowed transmit rate as specified in [TFRC-
SP]. Each DCCP-Data packet has a sequence number, and the DCCP
header's CCVal field contains the window counter value, used by
the receiver in determining when multiple losses belong in a
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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 at least once per round-trip
time acknowledging the data packets, unless the sender is
sending at a rate of less than one packet per round-trip time,
as indicated by the TFRC specification [RFC 3448] (Section 6).
Each DCCP-Ack packet uses a sequence number, identifies the most
recent packet 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 [RFC
3448] (Section 4.3) and [TFRC-SP]. This update is based upon a
loss event rate calculated by the sender, based on 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 [RFC 3448] (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 connection establishment is as specified in Section 4 of [RFC
4342].
5. Congestion Control on Data Packets
CCID 4 uses the congestion control mechanisms of TFRC [RFC 3448] and
TFRC-SP [TFRC-SP]. [TFRC-SP] should be considered normative except
where specifically indicated.
Loss Event Rate
As with CCID 3, the basic operation of CCID 4 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. For CCID 4, this loss event rate, a
round-trip time estimate, and a nominal packet size of 1460 bytes
are plugged into the TCP throughput equation, as specified in
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RFC 3448 (Section 3.1) and [TFRC-SP].
Because CCID 4 is intended for applications that send small packets,
the allowed transmit rate derived from the TCP throughput equation
is reduced by a factor that accounts for packet header size, as
specified in Section 4.2 of [TFRC-SP]. The header size on data
packets is estimated as 32 bytes (20 bytes for the IP header, and 12
bytes for the DCCP-Data header with 24-bit sequence numbers). If
the DCCP sender is sending N-byte data packets, the allowed transmit
rate is reduced by N/(N+32). CCID 4 senders are limited to this
fair rate.
The loss event rate itself is calculated in CCID 4 using recent loss
interval lengths reported by the receiver. Loss intervals are
precisely defined in Section 6.1 of [RFC 4342], with the
modification in [TFRC-SP] (Section 3) for loss intervals of at most
two round-trip times. 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. The CCID 4 Loss
Intervals option is used to report loss interval lengths; see
Section 8.6.
For loss intervals of at most two round-trip times, CCID 4
calculates the loss event rate for that interval by counting the
number of packets lost or marked, as described in Section 4.4 of
[TFRC-SP]. Thus, for such a short loss interval with N data
packets, including K lost or marked data packets, the loss interval
length is calculated as N/K, instead as N.
Unlike CCID 3, the CCID 4 sender enforces a minimum interval of 10
ms. between data packets, regardless of the allowed transmit rate.
Other Congestion Control Mechanisms
The other congestion control mechanisms such as slow-start, feedback
packets, and the like are exactly as in CCID 3, and are described in
the subsection on "Other Congestion Control Mechanisms" of Section 5
in [RFC 4342].
5.1. Response to Idle and Application-limited Periods
This is described in Section 5.1 of [RFC 4342].
5.2. Response to Data Dropped and Slow Receiver
This is described in Section 5.2 of [RFC 4342].
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5.3. Packet Sizes
CCID 4 is intended for applications that use a fixed small segment
size, or that vary their segment size in response to congestion.
The CCID 4 sender uses a segment size of 1460 bytes in the TCP
throughput equation. This gives the CCID 4 sender roughly the same
sending rate in bytes per second as a TFRC flow using 1460-byte
segments but experiencing the same packet drop rate.
6. Acknowledgements
The acknowledgements are as specified in Section 6 of [RFC 4342]
with the exception of the Loss Interval lengths specified below.
6.1. Loss Interval Definition
The loss interval definition is as defined in Section 6.1 of [RFC
4342].
6.1.1. Loss Interval Lengths
The Loss Intervals option specified for CCID 3 in [RFC 4342] reports
three lengths for each loss interval, the lengths of the lossy and
lossless parts, and a separate data length; the data length is used
in TFRC's loss event rate calculation. The Loss Intervals option
specified in this document for CCID 4 includes an additional Dropped
Packets field, described below in Section 8.6.
6.2. Congestion Control on Acknowledgements
The congestion control on acknowledgements is as specified in
Section 6.2 of [RFC 4342].
6.3. Acknowledgements of Acknowledgements
Procedures for the acknowledgement of acknowledgements are as
specified in Section 6.3 of [RFC 4342].
6.4. Quiescence
The procedure for detecting that the sender has gone quiescent is as
specified in Section 6.4 of [RFC 4342].
7. Explicit Congestion Notification
Procedures for the use of Explicit Congestion Notification (ECN) are
as specified in Section 7 of [RFC 4342].
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8. Options and Features
CCID 4 can make use of DCCP's Ack Vector, Timestamp, Timestamp Echo,
and Elapsed Time options, and its Send Ack Vector and ECN Incapable
features. As with CCID 3, the following CCID-specific options
defined for use with CCID 4.
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 4 Options
The "DCCP-Data?" column indicates that all currently defined
CCID 4-specific options MUST be ignored when they occur on DCCP-Data
packets.
As with CCID 3, 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 4 Feature Numbers
More information is available in Section 8 of [RFC 4342].
8.1. Window Counter Value
The use of the Window Counter Value in the DCCP generic header's
CCVal field is as specified in Section 8.1 of [RFC 4342].
In addition to their use described in CCID 3, the CCVal counters are
used by the receiver in CCID 4 to determine when the length of a
loss interval is at most two round-trip times. None of these
procedures require the receiver to maintain an explicit estimate of
the round-trip time. However, Section 8.1 of [RFC 4342] gives a
procedure that implementors may use if they wish to keep such an RTT
estimate using CCVal.
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8.2. Elapsed Time Options
The use of the Elapsed Time option is defined in Section 8.2 of [RFC
4342].
8.3. Receive Rate Option
The Receive Rate Option is as specified in Section 8.3 of [RFC
4342].
8.4. Send Loss Event Rate Feature
The Send Loss Event Rate feature is as defined in Section 8.4 of
[RFC 4342].
See [RFC 3448], Section 5 and [TFRC-SP], Section 4.4 for a normative
calculation of the loss event rate. Section 4.4 of [TFRC-SP]
modifies the calculation of the loss interval size for loss
intervals of at most two round-trip times.
If the CCID 4 receiver is using the Loss Event Rate option, the
receiver needs to be able to determine if a loss interval is short,
of at most two round-trip times. The receiver can heuristically
detect a short loss interval by using the Window Counter in arriving
data packets. The sender increases the Window Counter by 1 every
quarter of a round-trip time, with the caveat that the Window
Counter is never increased by more than five, modulo 16, from one
data packet to the next. Using the Window Counter to detect loss
intervals of at most two round-trip times could result in some false
positives, with some longer loss intervals incorrectly identified as
short ones.
8.5. Loss Event Rate Option
The Loss Event Rate Option is as specified in Section 8.5 of [RFC
4342].
See [RFC 3448] (Section 5) and [TFRC-SP] for a normative calculation
of loss event rate.
8.6. Loss Intervals Option
In CCID 3, each Loss Interval reported in the Loss Intervals Option
includes a Lossless Length, Loss Length, and Data Length. The Data
Length is used in the calculation of the loss event rate, and the
Lossless Length is used for the ECN Nonce Echo. In CCID 4, each
Loss Interval includes an additional 3-byte field, the Dropped
Packets field.
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+--------+--------+--------+--------...--------+--------+---
|11000001| Length | Skip | Loss Interval | More Loss
| | | Length | | Intervals...
+--------+--------+--------+--------...--------+--------+---
Type=193 12 bytes
For CCID 4, each 12-byte Loss Interval contains four fields, as
follows:
____________________ Loss Interval _________________________
/ \
+-------...-------+------...------+-----...-----+------...-----+
| Lossless Length |E| Loss Length | Data Length | Dropped Pkts |
+-------...-------+------...------+-----...-----+------...-----+
3 bytes 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 21 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, as described in
Section 8.6 of [RFC 4342].
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 details for the use of the Loss Intervals Option are as
described in Section 8.6.1 of [RFC 4342], with the exception of the
added field for Dropped Packets.
Dropped Packets: Dropped Packets, a 24-bit number, specifies the
number of dropped or marked packets in the loss interval. For Loss
Intervals of at most two round-trip times, the Dropped Packets field
MUST report the receiver's estimate of the number of dropped or
marked data packets in that loss interval. For Loss Intervals
greater than two round-trip times, the Dropped Packets field MAY
instead be set to zero.
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9. Verifying Congestion Control Compliance With ECN
Verifying congestion control compliance with ECN is as discussed in
Section 9 of [RFC 4342].
9.1. Verifying the ECN Nonce Echo
Procedures for verifying the ECN Nonce Echo are as specified in
Section 9.1 of [RFC 4342].
9.2. Verifying the Reported Loss Intervals and Loss Event Rate
Section 9.2 of [RFC 4342] discusses the sender's possible
verification of loss intervals and loss event rate information
reported by the receiver.
10. Implementation Issues
10.1. Timestamp Usage
The use of the Timestamp option is as discussed in Section 10.1 of
[RFC 4342].
10.2. Determining Loss Events at the Receiver
The use of the window counter by the receiver to determine if
multiple lost packets belong to the same loss event is as described
in Section 10.2 of [RFC 4342].
10.3. Sending Feedback Packets
The procedure for sending feedback packets is as described in
Section 10.3 of [RFC 4342].
11. Security Considerations
Security considerations include those discussed in Section 11 of
[RFC 4342]. There are no new security considerations introduced by
CCID 4.
12. IANA Considerations
This specification defines the value 4 in the DCCP CCID namespace
managed by IANA.
CCID 4 also uses three sets of numbers whose values should be
allocated by IANA, namely CCID 4-specific Reset Codes, option types,
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and feature numbers. This document makes no particular allocations
from the Reset Code range, except for experimental and testing use
[RFC 3692]. We refer to the Standards Action policy outlined in
[RFC 2434].
12.1. Reset Codes
Each entry in the DCCP CCID 4 Reset Code registry contains a
CCID 4-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 4 option type registry contains a
CCID 4-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 4 feature number registry contains a
CCID 4-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 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.
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13. Thanks
Normative References
[RFC 2119] S. Bradner. Key Words For Use in RFCs to Indicate
Requirement Levels. RFC 2119.
[RFC 2434] T. Narten and H. Alvestrand. Guidelines for Writing
an IANA Considerations Section in RFCs. RFC 2434.
[RFC 3448] M. Handley, S. Floyd, J. Padhye, and J. Widmer, TCP
Friendly Rate Control (TFRC): Protocol
Specification, RFC 3448, Proposed Standard, January
2003.
[RFC 3692] T. Narten. Assigning Experimental and Testing
Numbers Considered Useful. RFC 3692.
[RFC 4340] Kohler, E., Handley, M., and S. Floyd. Datagram
Congestion Control Protocol (DCCP), RFC 4340, March
2006.
[RFC 4342] 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.
[TFRC-SP] S. Floyd and E. Kohler. TCP Friendly Rate Control
(TFRC): the Small-Packet (SP) Variant. Internet-
draft draft-ietf-dccp-tfrc-voip-05.txt, March 2005.
Informative References
Authors' Addresses
Sally Floyd <floyd@icir.org>
ICSI Center for Internet Research
1947 Center Street, Suite 600
Berkeley, CA 94704
USA
Eddie Kohler <kohler@cs.ucla.edu>
4531C Boelter Hall
UCLA Computer Science Department
Los Angeles, CA 90095
USA
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Full Copyright Statement
Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
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