Internet Engineering Task Force Sally Floyd INTERNET-DRAFT ICIR draft-floyd-dccp-ccid4-00.txt Eddie Kohler Expires: 21 April 2007 UCLA 21 October 2006 Profile for Datagram Congestion Control Protocol (DCCP) Congestion ID 4: TCP-Friendly Rate Control for Small Packets (TFRC-SP) Status of This Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on 21 April 2007. 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 variant of TFRC designed for applications that send small packets. 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 Floyd, et al. [Page 1]
INTERNET-DRAFT Expires: 21 April 2007 October 2006 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. Floyd, et al. [Page 2]
INTERNET-DRAFT Expires: 21 April 2007 October 2006 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 6.2. Congestion Control on Acknowledgements .....................9 6.3. Acknowledgements of Acknowledgements .......................9 6.4. Quiescence .................................................9 7. Explicit Congestion Notification ................................9 8. Options and Features ............................................9 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 8.7. Dropped Packets Option ....................................11 8.8. Send Dropped Packets Feature ..............................12 9. Verifying Congestion Control Compliance With ECN ...............12 9.1. Verifying the ECN Nonce Echo ..............................12 9.2. Verifying the Reported Loss Intervals and Loss Event Rate ................................................................12 10. Implementation Issues .........................................12 10.1. Timestamp Usage ..........................................12 10.2. Determining Loss Events at the Receiver ..................12 10.3. Sending Feedback Packets .................................12 11. Security Considerations .......................................12 12. IANA Considerations ...........................................13 12.1. Reset Codes ..............................................13 12.2. Option Types .............................................13 12.3. Feature Numbers ..........................................13 13. Thanks ........................................................14 Normative References ..............................................14 Informative References ............................................14 Authors' Addresses ................................................14 Full Copyright Statement ..........................................15 Intellectual Property .............................................15 Floyd, et al. [Page 3]
INTERNET-DRAFT Expires: 21 April 2007 October 2006 List of Tables Table 1: DCCP CCID 4 Options ......................................10 Table 2: DCCP CCID 4 Feature Numbers ..............................10 Floyd, et al. [Page 4]
INTERNET-DRAFT Expires: 21 April 2007 October 2006 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 [TFRC-SP], 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 [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 CCID 3 Dropped Packets option [CCID3-DP] is thus mandated above and beyond to CCID 3's Loss Intervals option, as specified in 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]. Floyd, et al. Section 2. [Page 5]
INTERNET-DRAFT Expires: 21 April 2007 October 2006 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 [RFC4342] only in that the allowed transmit rate is determined by [TFRC-SP] 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 [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 single loss event. Floyd, et al. Section 3.2. [Page 6]
INTERNET-DRAFT Expires: 21 April 2007 October 2006 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 [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 [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 [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 [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 RFC 3448 (Section 3.1) and [TFRC-SP]. Floyd, et al. Section 5. [Page 7]
INTERNET-DRAFT Expires: 21 April 2007 October 2006 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 [RFC4342], 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 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 [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. The CCID 3 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. 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 [RFC4342]. 5.1. Response to Idle and Application-limited Periods This is described in Section 5.1 of [RFC4342]. 5.2. Response to Data Dropped and Slow Receiver This is described in Section 5.2 of [RFC4342]. Floyd, et al. Section 5.2. [Page 8]
INTERNET-DRAFT Expires: 21 April 2007 October 2006 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]. 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 options and features [CCID3-DP]. Each CCID 4-specific option and feature contains the same data as the corresponding CCID 3 option or Floyd, et al. Section 8. [Page 9]
INTERNET-DRAFT Expires: 21 April 2007 October 2006 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. 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 0 N 196-255 Reserved Table 2: DCCP CCID 4 Feature Numbers More information is available in Section 8 of [RFC4342] and in [CCID3-DP]. 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. Floyd, et al. Section 8.1. [Page 10]
INTERNET-DRAFT Expires: 21 April 2007 October 2006 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 [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 [RFC4342]. See [RFC3448] (Section 5) and [TFRC-SP] for a normative calculation of 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 The Dropped Packets option is as specified in [CCID3-DP]. CCID 4 receivers MUST always include Dropped Packets options on their feedback packets, regardless of the value of the Send Dropped Packets Floyd, et al. Section 8.7. [Page 11]
INTERNET-DRAFT Expires: 21 April 2007 October 2006 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 in [CCID3-DP]. 8.8. Send Dropped Packets Feature The Send Dropped Packets feature is as specified in [CCID3-DP]. 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. Security Considerations Security considerations include those discussed in Section 11 of [RFC4342]. There are no new security considerations introduced by Floyd, et al. Section 11. [Page 12]
INTERNET-DRAFT Expires: 21 April 2007 October 2006 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 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 192-195, 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 document allocates feature numbers 192 and 195, and feature numbers Floyd, et al. Section 12.3. [Page 13]
INTERNET-DRAFT Expires: 21 April 2007 October 2006 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. [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. [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. [CCID3-DP] Kohler, E., Datagram Congestion Control Protocol (DCCP) Congestion Control ID 3 Dropped Packets Option, Internet-draft draft-kohler-dccp-ccid3-drops-00.txt, August 2006. URL "http://www.read.cs.ucla.edu/dccp/". Informative References Authors' Addresses Sally Floyd <floyd@icir.org> ICSI Center for Internet Research Floyd, et al. [Page 14]
INTERNET-DRAFT Expires: 21 April 2007 October 2006 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 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. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. Floyd, et al. [Page 15]
