Internet Engineering Task Force Sally Floyd
INTERNET-DRAFT ICIR
Intended status: Experimental Eddie Kohler
Expires: 2 April 2008 UCLA
2 October 2007
Profile for Datagram Congestion Control Protocol (DCCP)
Congestion ID 4: TCP-Friendly Rate Control for Small Packets (TFRC-SP)
draft-ietf-dccp-ccid4-00.txt
Status of This Memo
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This Internet-Draft will expire on 2 April 2008.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document specifies an experimental profile for Congestion
Control Identifier 4, the Small-Packet variant of TCP-Friendly Rate
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Control (TFRC), in the Datagram Congestion Control Protocol (DCCP).
CCID 4 is for experimental use, and uses TFRC-SP [RFC4828], a variant
of TFRC designed for applications that send small packets. CCID 4 is
considered experimental because TFRC-SP is itself experimental, and
is not proposed for widespread deployment in the global Internet at
this time. The goal for TFRC-SP is to achieve roughly the same
bandwidth in 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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TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:
Changes from draft-floyd-dccp-ccid4-01.txt:
* Title changed to draft-ietf-dccp-ccid4-00.txt.
* Incorporated material from draft-kohler-dccp-ccid3-drops-01.txt.
* Added a reference to RFC3448bis.
* Added a sentence saying that this is Experimental because
TFRC-SP is Experimental.
* General editing in response to feedback from Gorry.
Changes from draft-floyd-dccp-ccid4-00.txt:
* Added a subsection describing calculation of the
average loss interval in TFRC-SP.
* Changed the assumed DCCP-Data header size from 12 bytes
to 16 bytes, for 48-bit sequence numbers. Feedback from
Ian McDonald.
* Added that the CCID4 sender can send two packets in a
burst, if limited by OS granularity. From Ian
McDonald.
* Added that the implementor may track Faster Restart
and implement it before an explicit update to the CCID4
RFC. From Ian McDonald.
* Added an example to Section 8.4 of when errors can
occur in using the Window Counter to detect loss
intervals of at most two round-trip times.
Changes from draft-floyd-ccid4-00.txt:
* Added the Dropped Packets option for reporting
the number of packets dropped in a loss interval.
* Added examples to Section 8.4 of the receiver incorrectly
inferring whether a loss interval was short or not.
END OF SECTION TO BE DELETED.
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Table of Contents
1. Introduction ....................................................6
2. Conventions .....................................................6
3. Usage ...........................................................7
3.1. Relationship with TFRC .....................................7
3.2. Example Half-Connection ....................................7
4. Connection Establishment ........................................8
5. Congestion Control on Data Packets ..............................8
5.1. Response to Idle and Application-limited Periods ..........10
5.2. Response to Data Dropped and Slow Receiver ................10
5.3. Packet Sizes ..............................................10
6. Acknowledgements ...............................................10
6.1. Loss Interval Definition ..................................10
6.2. Congestion Control on Acknowledgements ....................11
6.3. Acknowledgements of Acknowledgements ......................11
6.4. Quiescence ................................................11
7. Explicit Congestion Notification ...............................11
8. Options and Features ...........................................11
8.1. Window Counter Value ......................................12
8.2. Elapsed Time Options ......................................12
8.3. Receive Rate Option .......................................12
8.4. Send Loss Event Rate Feature ..............................12
8.5. Loss Event Rate Option ....................................13
8.6. Loss Intervals Option .....................................13
8.7. Dropped Packets Option ....................................13
8.7.1. Example ............................................15
8.8. Send Dropped Packets Feature ..............................16
9. Verifying Congestion Control Compliance With ECN ...............16
9.1. Verifying the ECN Nonce Echo ..............................16
9.2. Verifying the Reported Loss Intervals and Loss Event Rate
................................................................17
10. Implementation Issues .........................................17
10.1. Timestamp Usage ..........................................17
10.2. Determining Loss Events at the Receiver ..................17
10.3. Sending Feedback Packets .................................17
11. Security Considerations .......................................17
12. IANA Considerations ...........................................17
12.1. Reset Codes ..............................................17
12.2. Option Types .............................................18
12.3. Feature Numbers ..........................................18
13. Thanks ........................................................18
Normative References ..............................................18
Informative References ............................................19
Authors' Addresses ................................................19
Full Copyright Statement ..........................................20
Intellectual Property .............................................20
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List of Tables
Table 1: DCCP CCID 4 Options ......................................11
Table 2: DCCP CCID 4 Feature Numbers ..............................12
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1. Introduction
This document contains the profile for Congestion Control Identifier
4, TCP-Friendly Rate Control for Small Packets (TFRC-SP), in the
Datagram Congestion Control Protocol (DCCP) [RFC4340]. CCID 4
differs from CCID 3 in that CCID 4 uses TFRC-SP, the Small-Packet
variant of TFRC, while CCID 3 [RFC4342] uses standard TFRC [RFC3448].
This document assumes that the reader is familiar with [RFC4342],
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 [RFC4828],
and described for CCID 4 in Section 5 below.
o Minimum sending rate: TFRC-SP enforces a minimum interval of
10 milliseconds between data packets. This is specified for TFRC-
SP in Section 4.3 of [RFC4828], 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 [RFC4828]. The Dropped Packets option is thus mandated in
addition to CCID 3's Loss Intervals option, as specified in
Section 8.7 below. This section also describes the use of the
Dropped Packets option in calculating the loss event rate. 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 [RFC3448], 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 [RFC2119].
Additional terminology is described in Section 2 of [RFC4342].
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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 [RFC4828]. 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 [RFC4342] only in that the
allowed transmit rate is determined by [RFC4828] as well as by
[RFC3448].
1. The sender transmits DCCP-Data packets, where the sending rate is
governed by the allowed transmit rate as specified in [RFC4828].
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 single
loss event.
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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 [RFC3448]
(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 [RFC3448]
(Section 4.3) and [RFC4828]. 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 [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 connection establishment is as specified in Section 4 of
[RFC4342].
5. Congestion Control on Data Packets
CCID 4 uses the congestion control mechanisms of TFRC [RFC3448] and
TFRC-SP [RFC4828]. [RFC4828] MUST be considered normative except
where specifically indicated. If [RFC3448bis] is standardized in the
IETF as a revision of [RFC3448], then [RFC3448bis] MAY be implemented
with CCID4 without having to wait for an explicit update to this
document.
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 RFC 3448
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(Section 3.1) and [RFC4828].
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 [RFC4828]. The header size on data
packets is estimated as 36 bytes (20 bytes for the IP header, and 16
bytes for the DCCP-Data header with 48-bit sequence numbers). If the
DCCP sender is sending N-byte data packets, the allowed transmit rate
is reduced by N/(N+36). CCID 4 senders are limited to this fair
rate. The header size would be 32 bytes instead of 36 bytes when
24-bit sequence numbers were used in the DCCP-Data header.
As explained in Section 4.2 of [RFC4828], the actual header could be
larger or smaller than the assumed value, due to IP or DCCP options,
IPv6, IP tunnels, header compression, and the like. Because we are
only aiming at rough fairness, and at a rough incentive for
applications, the default use of a 32-byte or 36-byte header in the
calculations of the header bandwidth is sufficient for both IPv4 and
IPv6. LP 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 [RFC4342], with
the modification in [RFC4828] (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 3
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 [RFC4828].
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. The Dropped Packets option is used to report
K, the count of lost or marked data packets.
Unlike CCID 3, the CCID 4 sender enforces a minimum interval of 10 ms
between data packets, regardless of the allowed transmit rate. If
operating system scheduling granularity makes this impractical, up to
one additional packet MAY be sent per timeslice, providing that no
more than three packets are sent in any 30 ms interval.
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
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in [RFC4342].
5.1. Response to Idle and Application-limited Periods
This is described in Section 5.1 of [RFC4342]. If Faster Restart is
standardized in the IETF for TFRC [KFS07], then Faster Restart MAY be
implemented in CCID4 without having to wait for an explicit update to
this document.
5.2. Response to Data Dropped and Slow Receiver
This is described in Section 5.2 of [RFC4342].
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 [RFC4342] 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
[RFC4342], except as specified below. Section 6.1.1 of RFC 4342
specifies that for all loss intervals except the first one, the data
length equals the sequence length minus the number of non-data
packets the sender transmitted during the loss interval, with a
minimum data length of one packet. For TFRC-SP, for short loss
intervals of at most two round-trip times, the loss interval length
is computed not as the data length of the loss interval, but instead
as the data length divided by the number of dropped or marked data
packets.
Section 5.4 of RFC 4342 described when to use the most recent loss
interval in the calculation of the average loss interval. [RFC4828]
adds to this procedure the restriction that the most recent loss
interval is only used in the calculation of the average loss interval
if the most recent loss interval is greater than two round-trip
times. The pseudocode is given in Section 3 of [RFC4828].
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6.2. Congestion Control on Acknowledgements
The congestion control on acknowledgements is as specified in Section
6.2 of [RFC4342].
6.3. Acknowledgements of Acknowledgements
Procedures for the acknowledgement of acknowledgements are as
specified in Section 6.3 of [RFC4342].
6.4. Quiescence
The procedure for detecting that the sender has gone quiescent is as
specified in Section 6.4 of [RFC4342].
7. Explicit Congestion Notification
Procedures for the use of Explicit Congestion Notification (ECN) are
as specified in Section 7 of [RFC4342].
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. CCID 4 also imports the currently defined CCID 3-specific
options and features [RFC4342], augmented by the Dropped Packets
option and feature specified in this document. Each CCID 4-specific
option and feature contains the same data as the corresponding CCID 3
option or feature, and is interpreted in the same way, except as
specified elsewhere in this document.
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 variable Dropped Packets N 8.7
196-255 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 features are also
defined.
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Rec'n Initial Section
Number Meaning Rule Value Req'd Reference
------ ------- ----- ----- ----- ---------
128-191 Reserved
192 Send Loss Event Rate SP 0 N 8.4
193-194 Reserved
195 Send Dropped Packets SP 1 Y 8.8
196-255 Reserved
Table 2: DCCP CCID 4 Feature Numbers
More information is available in Section 8 of [RFC4342].
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 [RFC4342]. 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 [RFC4342] gives a procedure that implementors
may use if they wish to keep such an RTT estimate using CCVal.
8.2. Elapsed Time Options
The use of the Elapsed Time option is defined in Section 8.2 of
[RFC4342].
8.3. Receive Rate Option
The Receive Rate option is as specified in Section 8.3 of [RFC4342].
8.4. Send Loss Event Rate Feature
The Send Loss Event Rate feature is as defined in Section 8.4 of
[RFC4342].
See [RFC3448], Section 5 and [RFC4828], Section 4.4 for a normative
calculation of the loss event rate. Section 4.4 of [RFC4828]
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
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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. For
example, if the loss interval contained data packets with only two
Window Counter values, say, k and k+5, then the receiver could not
tell if the loss interval was at most two round-trip times long or
not. Similarly, if the sender sent data packets with Window Counter
values of 4, 8, 12, 0, 5, but the packets with Window Counter values
of 8, 12, and 0 were lost in the network, then the receiver would
only receive data packets with Window Counter values of 4 and 5, and
would incorrectly infer that the loss interval was at most two round-
trip times.
8.5. Loss Event Rate Option
The Loss Event Rate option is as specified in Section 8.5 of
[RFC4342].
See [RFC3448] (Section 5) and [RFC4828] for a normative calculation
of the loss event rate.
8.6. Loss Intervals Option
The Loss Intervals option is as specified in Section 8.6 of
[RFC4342].
8.7. Dropped Packets Option
This section describes the Dropped Packets option, a mechanism for
reporting the number of lost and marked packets per loss interval.
The Dropped Packets option is given in Table 1 above. CCID 4
receivers MUST always include Dropped Packets options on their
feedback packets, regardless of the value of the Send Dropped Packets
feature. If, nevertheless, a feedback packet does not include a
relevant Dropped Packets option, a CCID 4 sender MUST act as if the
relevant loss intervals' Drop Counts equal the corresponding Loss
Lengths, as specified below.
The core information reported by CCID 4 receivers is a list of recent
loss intervals, where 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. Loss intervals model the congestion behavior of TCP NewReno
senders, which reduce their sending rate at most once per window of
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data packets. Consequently, the number of packets lost in a loss
interval is not important for either TCP's or TFRC's congestion
response. CCID 3's Loss Intervals option reports the length of each
loss interval's lossy part, not the number of packets that were
actually lost or marked in that lossy part.
However, for short loss intervals the TFRC-SP sender needs to know
the number of packets lost or marked in a loss interval, over and
above the length of the loss interval in packets. The Dropped
Packets option, a CCID 4-specific option, reports this information.
Together with the existing Loss Intervals option, the Dropped Packets
option allows the CCID 4 sender to discover exactly how many packets
were dropped from each loss interval. The receiver reports the
number of lost or marked packets in its recently observed loss
intervals using the Dropped Packets option.
The Dropped Packets Option is specified as follows:
+--------+--------+-------...-------+--------+-------
|11000011| Length | Drop Count | More Drop Counts...
+--------+--------+-------...-------+--------+-------
Type=195 3 bytes
The Dropped Packets option contains information about one to 84
consecutive loss intervals, always including the most recent loss
interval. As with the Loss Intervals option, intervals are listed in
reverse chronological order. Should more than 84 loss intervals need
to be reported, multiple Dropped Packets options can be sent; the
second option begins where the first left off, and so forth.
One Drop Count is specified per loss interval. Drop Count is a
24-bit number that equals the number of packets lost or received ECN-
marked during the corresponding loss interval. By definition, this
number MUST NOT exceed the corresponding loss interval's Loss Length.
The Dropped Packets options MUST be sent in tandem with corresponding
Loss Intervals options. Consider a CCID 4 receiver that is reporting
Dropped Packets information. As specified in Section 8.6.1 of RFC
4342, the receiver sends the Loss Intervals option for all intervals
that have not been acknowledged by the sender. When this receiver
sends a feedback packet containing information about the N most
recent loss intervals (packaged in one or more Loss Intervals
options), it MUST include on the same feedback packet one or more
Dropped Packets options covering exactly those N loss intervals.
CCID 4 senders MUST ignore Drop Counts information for loss intervals
not covered by a Loss Intervals option on the same feedback packet.
Conversely, a CCID 4 sender might want to interpolate Drop Counts
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information for a loss interval not covered by any Dropped Packets
options; such a sender SHOULD use the corresponding loss interval's
Loss Length as its Drop Count.
Each loss interval's Drop Count MUST by definition be less than or
equal to its Loss Length. A Drop Count that exceeds the
corresponding Loss Length MUST be ignored.
8.7.1. Example
Consider the following sequence of packets, where "-" represents a
safely delivered packet and "*" represents a lost or marked packet.
This sequence is repeated from [RFC4342].
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; for interpretation of this
option, see [RFC4342]. A Dropped Packets option sent in tandem on
this packet would contain the bytes 195,14, 0,0,1, 0,0,4, 0,0,1,
0,0,0. This is interpreted as follows.
195 The Dropped Packets option number.
14 The length of the option, including option type and length bytes.
This option contains information about (14 - 2)/3 = 4 loss
intervals. Note that the two most recent sequence numbers are
not yet part of any loss interval -- the Loss Intervals option
includes them in its Skip Length -- and are thus not included in
the Dropped Packets option.
0,0,1
These bytes define the Drop Count for L3, which is 1. As
required, the Drop Count is less than or equal to L3's Loss
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Length, which is also 1.
0,0,4
The Drop Count for L2 is 4.
0,0,1
The Drop Count for L1 is 1.
0,0,0
Finally, the Drop Count for L0 is 0.
8.8. Send Dropped Packets Feature
The CCID 4-specific feature Send Dropped Packets governs the use of
the Dropped Packets option, and is given in Table 2 above.
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 4
implementation MUST understand a feature; Send Dropped Packets is
required in CCID 4.
The Send Dropped Packets feature lets CCID 4 endpoints negotiate
whether the receiver MUST provide Dropped Packets options on its
acknowledgements. DCCP A sends a "Change R(Send Dropped Packets, 1)"
option to ask DCCP B to send Dropped Packets options as part of its
acknowledgement traffic. In the current specification of CCID 4 in
this document, the Send Dropped Packets feature has an initial value
of 1, indicating that the receiver must send the Dropped Packets
options on its acknowledgements.
Send Dropped Packets has feature number 195 and is server-priority.
It takes one-byte Boolean values. DCCP B MUST send Dropped Packets
options on its acknowledgements when Send Dropped Packets/B is one,
although it MAY send Dropped Packets options even when Send Dropped
Packets/B is zero. Values of two or more are reserved. A CCID 4
half-connection starts with Send Dropped Packets equal to zero.
9. Verifying Congestion Control Compliance With ECN
Verifying congestion control compliance with ECN is as discussed in
Section 9 of [RFC4342].
9.1. Verifying the ECN Nonce Echo
Procedures for verifying the ECN Nonce Echo are as specified in
Section 9.1 of [RFC4342].
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9.2. Verifying the Reported Loss Intervals and Loss Event Rate
Section 9.2 of [RFC4342] 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
[RFC4342].
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 [RFC4342].
10.3. Sending Feedback Packets
The procedure for sending feedback packets is as described in Section
10.3 of [RFC4342].
11. Security Considerations
Security considerations include those discussed in Section 11 of
[RFC4342]. 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,
and feature numbers. 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 4 Reset Code registry contains a
CCID 4-specific Reset Code, which is a number in the range 128-255; a
Floyd, et al. Section 12.1. [Page 17]
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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 includes
the value 195 allocated for the Dropped Packets option. This
document allocates option types 192-195, and option types 184-190 and
248-254 are permanently reserved for experimental and testing use.
The remaining option types -- 128-183, 191, 196-247, 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
includes the value 195 allocated for the Send Dropped Packets
feature. This document allocates feature numbers 192 and 195, and
feature numbers 184-190 and 248-254 are permanently reserved for
experimental and testing use. The remaining feature numbers --
128-183, 191, 193-194, 196-247, 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
Normative References
[RFC2119] S. Bradner. Key Words For Use in RFCs to Indicate
Requirement Levels. RFC 2119.
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[RFC2434] T. Narten and H. Alvestrand. Guidelines for Writing
an IANA Considerations Section in RFCs. RFC 2434.
[RFC3448] M. Handley, S. Floyd, J. Padhye, and J. Widmer, TCP
Friendly Rate Control (TFRC): Protocol Specification,
RFC 3448, Proposed Standard, January 2003.
[RFC3692] T. Narten. Assigning Experimental and Testing Numbers
Considered Useful. RFC 3692.
[RFC4340] Kohler, E., Handley, M., and S. Floyd. Datagram
Congestion Control Protocol (DCCP), RFC 4340, March
2006.
[RFC4342] Floyd, S., Kohler, E., and J. Padhye. Profile for
Datagram Congestion Control Protocol (DCCP) Congestion
Control ID 3: TCP-Friendly Rate Control (TFRC), RFC
4342, March 2006.
[RFC4828] S. Floyd and E. Kohler. TCP Friendly Rate Control
(TFRC): the Small-Packet (SP) Variant. RFC 4828,
April 2007.
Informative References
[KFS07] Kohler, E., S. Floyd, and A. Sathiaseelan, Faster
Restart for TCP Friendly Rate Control (TFRC),
Internet-draft draft-ietf-dccp-tfrc-faster-
restart-04.txt, work-in-progress, September 2007.
[RFC3448bis] M. Handley, S. Floyd, J. Padhye, and J. Widmer, TCP
Friendly Rate Control (TFRC): Protocol Specification,
internet-draft draft-ietf-dccp-rfc3448bis-02.txt,
work-in-progress, July 2007.
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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Floyd, et al. [Page 20]