Abstract

This document specifies a 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 (RFC 4828), 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 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.

Status of This Memo

This memo defines an Experimental Protocol for the Internet community. It does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited.

Copyright Notice

Copyright © 2009 IETF Trust and the persons identified as the document authors. All rights reserved.

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This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.

Table of Contents

   1. Introduction ....................................................3
   2. Conventions .....................................................4
   3. Usage ...........................................................4
      3.1. Relationship with TFRC and TFRC-SP .........................5
      3.2. Example Half-Connection ....................................5
   4. Connection Establishment ........................................6
   5. Congestion Control on Data Packets ..............................6
      5.1. Response to Idle and Application-Limited Periods ...........7
      5.2. Response to Data Dropped and Slow Receiver .................7
      5.3. Packet Sizes ...............................................7
   6. Acknowledgements ................................................8
      6.1. Loss Interval Definition ...................................8
      6.2. Congestion Control on Acknowledgements .....................8
      6.3. Acknowledgements of Acknowledgements .......................8
      6.4. Quiescence .................................................8
   7. Explicit Congestion Notification ................................8
   8. Options and Features ............................................9
      8.1. Window Counter Value ......................................10
      8.2. Elapsed Time Options ......................................10
      8.3. Receive Rate Option .......................................10
      8.4. Send Loss Event Rate Feature ..............................10
      8.5. Loss Event Rate Option ....................................11
      8.6. Loss Intervals Option .....................................11
      8.7. Dropped Packets Option ....................................11
           8.7.1. Example ............................................13
   9. Verifying Congestion Control Compliance with ECN ...............14
      9.1. Verifying the ECN Nonce Echo ..............................14
      9.2. Verifying the Reported Loss Intervals and Loss
           Event Rate ................................................14
   10. Implementation Issues .........................................14
      10.1. Timestamp Usage ..........................................14
      10.2. Determining Loss Events at the Receiver ..................15
      10.3. Sending Feedback Packets .................................15
   11. Design Considerations .........................................15
      11.1. The Field Size in the Loss Intervals Option ..............15
      11.2. The Field Size in the Dropped Packets Option .............16
   12. Experimental Status of This Document ..........................17
   13. Security Considerations .......................................17
   14. IANA Considerations ...........................................17
      14.1. Reset Codes ..............................................17
      14.2. Option Types .............................................17
      14.3. Feature Numbers ..........................................18
   15. Thanks ........................................................18
   16. References ....................................................18
      16.1. Normative References .....................................18
      16.2. Informative References ...................................19

List of Tables

   Table 1: DCCP CCID 4 Options .......................................9
   Table 2: DCCP CCID 4 Feature Numbers ...............................9

1. Introduction

This document specifies an experimental 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 is a modified version of Congestion Control Identifier 3, CCID 3, which has been specified in [RFC4342]. This document assumes that the reader is familiar with CCID 3, instead of repeating from that document unnecessarily.

CCID 3 uses TCP-Friendly Rate Control (TFRC), which is now specified in RFC 5348 [RFC5348]. CCID 4 differs from CCID 3 in that CCID 4 uses TFRC-SP [RFC4828], an experimental, small-packet variant of TFRC. The original specification of TFRC, RFC 3448 [RFC3448], has been obsoleted by RFC 5348. The CCID 3 and TFRC-SP documents both predate RFC 5348 and refer instead to RFC 3448 for the specification of TFRC. However, this document assumes that RFC 5348 will be used instead of RFC 3448 for the specification of TFRC.

CCID 4 differs from CCID 3 only in the following respects:

• 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.

• Maximum 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.

• Loss rates for short loss intervals: For short loss intervals of at most two round-trip times (RTTs), 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]. If the sender is computing the loss event rate, the Dropped Packets option specified in Section 8.7 is required, in addition to the default CCID 3's Loss Intervals option. Section 8.7 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.

• 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 [RFC4828], 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].

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 and TFRC-SP

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. This document is based on CCID 3 [RFC4342], TFRC, and TFRC-SP. For TFRC, RFC 3448 [RFC3448] has been obsoleted by RFC 5348 [RFC5348].

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 one respect; with CCID 4, the allowed transmit rate is determined by [RFC4828] as well as by [RFC5348].

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.

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 [RFC5348] (Section 6). Each DCCP-Ack packet uses a sequence number, identifying 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 [RFC5348] (Section 4.3) and [RFC4828]. This update is based upon a loss event rate calculated by the sender, based on the receiver's loss-interval 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 [RFC5348] (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 [RFC5348] and TFRC-SP [RFC4828]. [RFC4828] MUST 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 RFC 5348 (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 IPv4 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.

If the sender is calculating the loss event rate itself, the loss event rate can be calculated 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 and feedback packets are exactly as in CCID 3, and are described in the subsection on "Other Congestion Control Mechanisms" of Section 5 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 short loss intervals of at most two round-trip times, TFRC-SP computes the loss interval length as the data length divided by the number of dropped or marked data packets (rather than as the data length of the loss interval).

Section 5.4 of RFC 4342 describes 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].

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 specified in this document. Each CCID4-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 (or in a subsequent IETF standards-track RFC that updates or obsoletes this specification).

                Option                        DCCP-   Section
       Type     Length     Meaning            Data?  Reference
       -----    ------     -------            -----  ---------
      128-183              Unassigned
      184-190              Reserved for
                            experimental
                            and testing use
        191                Unassigned
        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
      196-247              Unassigned
      248-254              Reserved for
                            experimental
                            and testing use
        255                Unassigned

Table 1: DCCP CCID 4 Options

The "DCCP-Data?" column indicates that all currently defined CCID4- specific options MUST be ignored when they occur on DCCP-Data packets.

As with CCID 3, the following CCID-specific features are also defined.

      Number   Meaning                  Rule   Value  Req'd Reference
      ------   -------                  -----  -----  ----- ---------
      128-183  Unassigned
      184-190  Reserved for experimental
                and testing use
        191    Unassigned
        192    Send Loss Event Rate      SP      0      N      8.4
      193-247  Unassigned
      248-254  Reserved for experimental
                and testing use
        255    Unassigned

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 [RFC5348], 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 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 [RFC5348] (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. By reporting both the Loss Intervals and Dropped Packets options on the feedback packets, the receiver gives the sender sufficient information to calculate the loss event rate, or to verify the calculation of the reported loss event rate, if the sender so desires.

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 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 computing the loss event rate for periods that include 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 CCID4-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

Figure 1: Dropped Packets Option

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.

CCID 4 receivers MUST report Dropped Packets options with every feedback packet. Any packet containing a Loss Intervals option MUST also contain a Dropped Packets option covering the same loss intervals. If a feedback packet does not include a relevant Dropped Packets option, and the CCID 4 sender is computing the loss event rate itself, the sender MUST treat the relevant loss intervals' Drop Counts as equal to the corresponding Loss Lengths, as specified below.

Consider a CCID 4 receiver. 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), then the receiver includes 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 information for a loss interval not covered by any Dropped Packets options; such a sender MUST 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 treated as equal to the Loss Length.

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
                |         |         |         |         |   |
                ----------*--------***-*--------*----------*-
      
      Figure 2:  Sequence of Delivered (-) and Lost (*) Packets

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
      
      Figure 3:  Loss Intervals for the Packet Sequence
   
   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
            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.

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].

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. Design Considerations

This section discusses design considerations for the field sizes in the Loss Intervals and Dropped Packets options.

11.1. The Field Size in the Loss Intervals Option

Section 8.6 of RFC 4342 specifies a Loss Intervals option with three fields for each loss interval, for reporting the Lossless Length, Loss Length, and Data Length. Each field is specified to be three bytes. Section 8.6 of this document specifies that CCID 4 use the same Loss Intervals option as CCID 3, with the same field sizes. This has the significant advantage of minimizing the implementation differences between CCID 3 and CCID 4. However, it has been suggested that CCID 4 *could* use a Loss Intervals option with smaller field sizes, since a CCID 4 sender enforces a minimum interval of 10 ms between data packets. This section explains the reason for CCID 4 to use the same Loss Intervals option as specified for CCID 3.

The Lossless Length field reports the number of packets in the loss intervals' lossless part, and the Loss Length field reports the number of packets in the loss interval's lossy part. The Data Length field reports the number of packets in the loss interval's data length (excluding non-data packets). A two-byte Data Length field can report a data length of 65,536 packets, corresponding to a loss event rate of 0.00002; this is enough to give the CCID 4 sender an allowed sending rate of roughly 250 packets per RTT, which is enough for a connection with a round-trip time of at most 2.5 seconds. For a CCID 4 connection with a larger round-trip time, the three-byte Lossless Length and Data Length fields would be needed.

For the Loss Length field in the Loss Intervals option, reporting the number of packets in the one-RTT lossy part of the loss interval, a one-byte field would not be sufficient for a CCID 4 connection with a long RTT (three seconds or longer). For the Loss Length field, a two-byte field should be sufficient for CCID 4. However, our judgement is that the advantages of using the same Loss Intervals option as in CCID 3 outweigh any advantages of using a CCID 4 Loss Intervals option that uses eight bytes instead of nine bytes for reporting the fields for each loss interval.

11.2. The Field Size in the Dropped Packets Option

Section 8.7 specifies the Dropped Packets option for reporting the number of lost or marked packets per loss interval, allocating three bytes for the drop count field for each loss interval reported. The three-byte field is partly for simplicity, to give the same field size as the fields in the Loss Intervals option specified in RFC 4342. It has been suggested that CCID 4 *could* use a smaller field size for the Dropped Packets option. This section discusses the issue of the size of the drop count field in the Dropped Packets option.

It is not necessary to specify a three-byte field for the Dropped Packets option. A one-byte field would allow a reported drop count of 255, and a two-byte field would allow a reported drop count of 65,535. A two-byte field would clearly be sufficient for the drop count field for CCID 4.

In fact, a one-byte field would *probably* be adequate for reporting the drop count for a loss interval in a CCID 4 connection. Because a CCID 4 sender enforces a minimum interval of 10 ms between data packets, a sender would need a round-trip time of over 2.55 seconds to have more than 255 packets lost or marked in a single loss interval; round-trip times of greater than three seconds are not unusual for some flows traversing satellite links. The drop count field is used in CCID 4 to compute the actual loss rate for short loss intervals, rather than using the loss event rate that is used for longer loss intervals. If a loss interval of at most two round- trip times included N packets sent, with more than 255 of those packets lost or marked, a drop count field of one byte would allow a drop count of at most 255 to be reported, resulting in a computed loss rate for that interval of 255/N. This loss rate might be less than the actual loss rate, but it is significantly higher than the loss event rate of 1/N, and should be sufficient to prevent a steady- state condition of a CCID 4 connection with multiple packets dropped each round-trip time. Thus, a one-byte field would probably be adequate for reporting the drop count for a loss interval in a CCID 4 connection. However, at the moment this document specifies a three- byte field, for consistency with the field size in the Loss Intervals option.

12. Experimental Status of This Document

TFRC-SP is a congestion control mechanism defined in RFC 4828. Section 10 of [RFC4828] describes why TFRC-SP is currently specified as experimental and why it is not intended for widespread deployment at this time in the global Internet. Since TFRC-SP is Experimental, CCID 4 is therefore also considered experimental. If the IETF publishes a Standards-Track RFC that changes the status of TFRC-SP, then CCID 4 should then be updated to reflect the change of status.

13. Security Considerations

Security considerations include those discussed in Section 11 of [RFC4342]. There are no new security considerations introduced by CCID 4.

14. IANA Considerations

This specification defines the value 4 in the DCCP CCID namespace managed by IANA. This is a permanent codepoint, as is needed for experimentation across the Internet using different codebases.

CCID 4 also uses three sets of numbers whose values have been allocated by IANA, namely CCID4-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 [RFC5226].

14.1. Reset Codes

Each entry in the DCCP CCID 4 Reset Code registry contains a CCID4- 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.

14.2. Option Types

Each entry in the DCCP CCID 4 option type registry contains a CCID4- 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.

14.3. Feature Numbers

Each entry in the DCCP CCID 4 feature number registry contains a CCID4-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.

15. Thanks

We thank Gorry Fairhurst, Alfred Hoenes, Ian McDonald, Gerrit Renker, and Leandro Sales for feedback on this document.

16. References

16.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [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.
   
   [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]  Floyd, S. and E. Kohler, "TCP Friendly Rate Control
              (TFRC): The Small-Packet (SP) Variant", RFC 4828, April
              2007.
   
   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.
   
   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification", RFC
              5348, September 2008.

16.2. Informative References

   [KFS07]    Kohler, E., Floyd, S., and A. Sathiaseelan, "Faster
              Restart for TCP Friendly Rate Control (TFRC)", Work in
              Progress, July 2008.

Authors' Addresses

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

   EMail:  floyd@icir.org

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

   EMail: kohler@cs.ucla.edu