Internet Engineering Task Force Sally Floyd INTERNET-DRAFT ICIR draft-ietf-dccp-ccid2-05.txt Eddie Kohler Expires: August 2004 UCLA 16 February 2004 Profile for DCCP Congestion Control ID 2: TCP-like Congestion Control Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of [RFC 2026]. 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 Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document contains the profile for Congestion Control Identifier 2, TCP-like Congestion Control, in the Datagram Congestion Control Protocol (DCCP). CCID 2 should be used by senders who would like to take advantage of the available bandwidth in an environment with rapidly changing conditions, and who are able to adapt to the abrupt changes in the congestion window typical of TCP's Additive Increase Multiplicative Decrease (AIMD) congestion control. Floyd/Kohler [Page 1] INTERNET-DRAFT Expires: August 2004 February 2004 TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION: Changes from draft-ietf-dccp-ccid3-03.txt: * Disallow direct tracking of TCP standards. Changes from draft-ietf-dccp-ccid2-02.txt: * Added to the section on application requirements. * Changed the default Ack Ratio to be two, as recommended for TCP. * Added a paragraph about packet sizes. Changes from draft-ietf-dccp-ccid2-01.txt: * Added "Security Considerations" and "IANA Considerations" sections. * Refer explicitly to SACK-based TCP, and flesh out Section 3 ("Congestion Control on Data Packets"). * When cwnd < ssthresh, increase cwnd by one per newly acknowledged packet up to some limit, in line with TCP Appropriate Byte Counting. * Refined definition of quiescence. Changes from draft-ietf-dccp-ccid2-00.txt: * Said that the Acknowledgement Number reports the largest sequence number, not the most recent packet, for consistency with draft-ietf- dccp-spec. * Added notes about ECN nonces for acknowledgements, and about dealing with piggybacked acknowledgements. Floyd/Kohler [Page 2] INTERNET-DRAFT Expires: August 2004 February 2004 Table of Contents 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Example Half-Connection. . . . . . . . . . . . . . . . . 5 3.2. Updates. . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Connection Establishment. . . . . . . . . . . . . . . . . . . 7 5. Congestion Control on Data Packets. . . . . . . . . . . . . . 7 5.1. Response to Data Dropped . . . . . . . . . . . . . . . . 9 5.2. Packet Size. . . . . . . . . . . . . . . . . . . . . . . 9 6. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . 9 6.1. Congestion Control on Acknowledgements . . . . . . . . . 10 6.1.1. Sending Acknowledgements. . . . . . . . . . . . . . 10 6.1.2. Setting Ack Ratio . . . . . . . . . . . . . . . . . 11 6.1.3. Derivation of Ack Ratio Decrease. . . . . . . . . . 12 6.2. Quiescence . . . . . . . . . . . . . . . . . . . . . . . 12 6.3. Acknowledgements of Acknowledgements . . . . . . . . . . 13 7. Explicit Congestion Notification. . . . . . . . . . . . . . . 13 8. Options and Features. . . . . . . . . . . . . . . . . . . . . 14 9. Security Considerations . . . . . . . . . . . . . . . . . . . 14 10. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 14 11. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Normative References . . . . . . . . . . . . . . . . . . . . . . 14 Informative References . . . . . . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 Intellectual Property Notice . . . . . . . . . . . . . . . . . . 15 Floyd/Kohler [Page 3] INTERNET-DRAFT Expires: August 2004 February 2004 1. Introduction This document contains the profile for Congestion Control Identifier 2, TCP-like Congestion Control, in the Datagram Congestion Control Protocol (DCCP) [DCCP]. DCCP uses Congestion Control Identifiers, or CCIDs, to specify the congestion control mechanism in use on a half- connection. (A half-connection consists of data packets sent from DCCP A to DCCP B, plus acknowledgements sent from DCCP B to DCCP A. DCCP A is the HC-Sender, and DCCP B the HC-Receiver, for this half- connection. In this document, we abbreviate HC-Sender and HC- Receiver as "sender" and "receiver", respectively. These terms are defined more fully in [DCCP].) The TCP-like Congestion Control CCID sends data using a close variant of TCP's congestion control mechanisms, particularly selective-acknowledgement (SACK) based TCP's congestion control mechanisms [RFC 3517]. It is suitable for senders who can adapt to the abrupt changes in congestion window typical of AIMD (Additive Increase Multiplicative Decrease) congestion control in TCP, and particularly useful for senders who would like to take advantage of the available bandwidth in an environment with rapidly changing conditions. See Section 3 for more on application requirements. 2. Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC 2119]. For simplicity, we refer to DCCP-Data packets sent by the sender, and DCCP-Ack packets sent by the receiver. Both of these categories are meant to include DCCP-DataAck packets. 3. Usage TCP-like Congestion Control is intended to provide congestion control for applications that do not require fully reliable data transmission, or that desire to implement reliability on top of DCCP. It is appropriate for flows that would like to receive as much bandwidth as possible over the long term, consistent with the use of end-to-end congestion control, and that are willing to undergo halving of the congestion window in response to a congestion event. Whereas CCID 3, TCP-Friendly Rate Control (TFRC) Congestion Control [CCID 3 PROFILE], is appropriate for flows that would prefer to minimize abrupt changes in the sending rate, CCID 2 is recommended for applications that simply need to transfer as much data as Floyd/Kohler Section 3. [Page 4] INTERNET-DRAFT Expires: August 2004 February 2004 possible in as short a time. For example, CCID 2 is recommended over CCID 3 for streaming media applications that buffer a considerable amount of data at the application receiver before playback time, insulating the application somewhat from abrupt changes in the sending rate. Such applications could easily choose DCCP's CCID 2 over TCP itself, possibly adding some form of selective reliability at the application layer. CCID 2 is also recommended over CCID 3 for applications where the halving of the sending rate in response to congestion is not likely to interfere with application-level performance. An additional advantage of CCID 2 is that its TCP-like congestion control mechanisms are reasonably well-understood, with traffic dynamics quite similar to those of TCP. While the network research community is still learning about the dynamics of TCP after 15 years of TCP congestion control as the dominant transport protocol in the Internet, some applications might prefer the more well-known dynamics of TCP-like congestion control over that of newer congestion control mechanisms that have not yet met the test of widespread deployment in the Internet. 3.1. Example Half-Connection This example shows the typical progress of a half-connection using TCP-like Congestion Control specified by CCID 2, not including connection initiation and termination. Again, the "sender" is the HC-Sender, and the "receiver" is the HC-Receiver. The example is informative, not normative. (1) The sender sends DCCP-Data packets, where the number of packets sent is governed by a congestion window, cwnd, as in TCP. Each DCCP-Data packet uses a sequence number. The sender also sends an Ack Ratio feature option specifying the number of data packets to be covered by an Ack packet from the receiver. Assuming that the half-connection is Explicit Congestion Notification (ECN) capable (the ECN Capable feature is turned on---the default), each DCCP-Data packet is sent as ECN-Capable with either the ECT(0) or the ECT(1) codepoint set, as described in [RFC 3540]. (2) The receiver sends a DCCP-Ack packet acknowledging the data packets for every Ack Ratio data packets transmitted by the sender. Each DCCP-Ack packet uses a sequence number and contains an Ack Vector. The sequence number acknowledged in DCCP-Ack packets is that of the received packet with the highest sequence number, rather than a TCP-like cumulative acknowledgement. Floyd/Kohler Section 3.1. [Page 5] INTERNET-DRAFT Expires: August 2004 February 2004 If the half-connection is ECN capable, the receiver returns the sum of received ECN Nonces via Ack Vector options, allowing the sender to probabilistically verify that the receiver is not misbehaving. DCCP-Ack packets from the receiver are also sent as ECN-Capable, since the sender will control the acknowledgement rate in a roughly TCP-friendly way using the Ack Ratio feature. There is little need for the receiver to verify the nonces of its DCCP-Ack packets, since the sender cannot get significant benefit from misreporting the ack mark rate. (3) The sender continues sending DCCP-Data packets as controlled by the congestion window. Upon receiving DCCP-Ack packets, the sender examines their Ack Vectors to learn about marked or dropped data packets, and adjusts its congestion window accordingly. Because this is unreliable transfer, the sender does not retransmit dropped packets. (4) DCCP-Ack packets use sequence numbers, so the sender has direct information about the fraction of lost or marked DCCP-Ack packets. The sender responds to lost or marked DCCP-Ack packets by modifying the Ack Ratio sent to the receiver. (5) The sender acknowledges the receiver's acknowledgements at least once per congestion window. If both half-connections are active, the sender's acknowledgement of the receiver's acknowledgements is included in the sender's acknowledgement of the receiver's data packets. If the reverse-path half- connection is quiescent, the sender sends a DCCP-DataAck packet that includes an Acknowledgement Number in the header. (6) The sender estimates round-trip times, either through keeping track of acknowledgement round-trip times as TCP does or through explicit Timestamp options, and calculates a TimeOut (TO) value much as the RTO (Retransmit Timeout) is calculated in TCP. The TO is used to determine when a new DCCP-Data packet can be transmitted when the sender has been limited by the congestion window and no feedback has been received from the receiver. 3.2. Updates The congestion control mechanisms described here closely follow mechanisms standardized by the IETF for use in SACK-based TCP, and we rely partially on existing TCP documentation, such as [RFC 793], [RFC 3465], and [RFC 3517]. TCP congestion control continues to evolve, but conformant CCID 2 implementations SHOULD wait for explicit updates to CCID 2, rather than tracking TCP's evolution directly. The differences between CCID 2 and straight TCP include: CCID 2 defines an additional mechanism not currently standardized Floyd/Kohler Section 3.2. [Page 6] INTERNET-DRAFT Expires: August 2004 February 2004 for use in TCP, namely congestion control on acknowledgements as achieved by the Ack Ratio. DCCP is a datagram protocol, so several parameters whose units are bytes in TCP, such as the congestion window cwnd, have units of packets in DCCP. Unreliability also leads to differences from TCP: DCCP never retransmits a packet, so congestion control mechanisms that distinguish retransmissions from new packets need rethinking in the DCCP context. 4. Connection Establishment Use of the Ack Vector is MANDATORY on CCID 2 half-connections, so the sender MUST send a "Change R(Use Ack Vector, 1)" option to the receiver as part of connection establishment. The sender SHOULD NOT send data until it has received the corresponding "Confirm L(Use Ack Vector, 1)" from the receiver, except for possible data included on the initial DCCP-Request packet. CCID 2 requires only generic feedback, namely Ack Vector. Therefore, CCID 2 MAY masquerade as CCID 1 as long as the receiver's Use Ack Vector feature is set to 1. 5. Congestion Control on Data Packets CCID 2's congestion control mechanisms are based on those for SACK- based TCP [RFC 3517], since the Ack Vector provides all the information that might be transmitted in SACK options. A CCID 2 data sender maintains three integer parameters. All of their units are packets, not bytes; for example, CCID 2 expresses its window in terms of how many packets may be sent. (1) The congestion window "cwnd", which equals the maximum number of data-carrying packets allowed in the network at any time. ("Data-carrying packet" means any DCCP packet that contains user data: DCCP-Data, DCCP-DataAck, and occasionally DCCP-Request, DCCP-Response, and DCCP-Move.) (2) The slow-start threshold "ssthresh", which controls adjustments to cwnd. (3) The pipe value "pipe", which is the sender's estimate of the number of data-carrying packets outstanding in the network. These parameters are manipulated, and their initial values determined, according to SACK-based TCP's behavior. The rest of this section provides more specific guidance. Floyd/Kohler Section 5. [Page 7] INTERNET-DRAFT Expires: August 2004 February 2004 The sender MAY send a data-carrying packet only when pipe < cwnd. In particular, it MUST NOT send a data-carrying packet when pipe >= cwnd. Every data-carrying packet sent increases pipe by 1. The sender reduces pipe as it infers that data-carrying packets have left the network, either by being received or by being dropped. In particular: (1) The sender reduces pipe by 1 for each packet newly-acknowledged as received (Ack Vector State 0 or State 1) by some DCCP-Ack. (2) The sender reduces pipe by 1 for each packet it can infer as lost due to the DCCP equivalent of TCP's "duplicate acknowledgements". This depends on TCP's NUMDUPACK parameter, the number of duplicate acknowledgements TCP needs to infer a loss, which currently equals 3. A packet P is inferred to be lost, rather than delayed, when at least NUMDUPACK packets after P have been acknowledged as received (Ack Vector State 0 or 1) by the receiver. Note that these acknowledgements are not duplicates, and that the acknowledged packets might include DCCP-Ack packets. (3) Finally, the sender needs "retransmit" timeouts, handled like TCP's retransmission timeouts, in case an entire window of packets are lost. The sender estimates the round-trip time at most once per window of data, and uses the TCP algorithms for maintaining the average round-trip time, mean deviation, and timeout value. Because DCCP does not retransmit data, DCCP does not require TCP's recommended minimum timeout of one second. The exponential backoff of the timer is exactly as in TCP. When a "retransmit" timeout occurs, the sender sets pipe to 0. The sender MUST NOT decrement pipe more than once for any given packet. True duplicate acknowledgements, for example, MUST not affect pipe. Furthermore, the sender MUST NOT decrement pipe for non-data packets, such as DCCP-Acks, even though the Ack Vector will contain information about them. Congestion events, namely one or more packets lost or marked from a window of data, cause CCID 2 to reduce its congestion window. For each congestion event, either indicated explicitly as an Ack Vector State 1 (ECN-marked) acknowledgement or inferred via "duplicate acknowledgements", cwnd is halved, then ssthresh is set to the new cwnd. Cwnd is never reduced below one packet. After a timeout, the slow-start threshold is set to cwnd/2, then cwnd is set to one packet. When halved, cwnd and ssthresh have their values rounded down, except that neither parameter is ever less than one. Floyd/Kohler Section 5. [Page 8] INTERNET-DRAFT Expires: August 2004 February 2004 When cwnd < ssthresh, meaning that the sender is in slow-start, the congestion window is increased by one packet for every newly acknowledged (with Ack Vector State 0 or 1) data-carrying packet, up to a maximum of Ack Ratio packets per acknowledgement. This differs from TCP's historical behavior, which (in DCCP terms) would increase cwnd by one per DCCP-Ack received, not by one per packet newly acknowledged by some DCCP-Ack; but it is in line with TCP's behavior with appropriate byte counting [RFC 3465]. When cwnd >= ssthresh, the congestion window is increased by one packet for every window of data acknowledged without lost or marked packets. The cwnd parameter is initialized to four for new connections [RFC 3390]; the ssthresh parameter is initialized to an arbitrarily high value. 5.1. Response to Data Dropped CCID 2 senders respond to packets acknowledged as Data Dropped as described in [DCCP], with the following further clarifications. o Drop Code 2 ("receive buffer drop"). The congestion window "cwnd" is reduced by one for each packet newly acknowledged as Drop Code 2, except that it is never reduced below one. 5.2. Packet Size CCID 2 is intended for applications that use a fixed packet size, and that vary their sending rate in packets per second in response to congestion. CCID 2 is not appropriate for applications that require a fixed interval of time between packets, and vary their packet size instead of their packet rate in response to congestion. However, some attention might be required for applications using CCID 2 that vary their packet size not in response to congestion, but in response to other application-level requirements. CCID 2 implementations MAY check for applications that appear to be manipulating the packet size inappropriately. For example, an application might send small packets for a while, building up a fast rate, then switch to large packets to take advantage of the fast rate. However, preliminary simulations indicate that applications may not be able to increase their overall transfer rates this way, so it is not clear this manipulation will occur in practice. 6. Acknowledgements This section describes how the receiver reports acknowledgement information back to the sender. DCCP-Ack packets from the receiver MUST include Ack Vector options, as well as an Acknowledgement Number acknowledging the packet with the largest valid sequence number received from the sender. Acknowledgement data in the Ack Floyd/Kohler Section 6. [Page 9] INTERNET-DRAFT Expires: August 2004 February 2004 Vector options SHOULD generally cover the receiver's entire Acknowledgement Window, as described in [DCCP]. 6.1. Congestion Control on Acknowledgements The acknowledgement subflow is loosely congestion-controlled by an Ack Ratio specified by the sender. The receiver sends approximately (cwnd / Ack Ratio) acknowledgement packets for each congestion window of data packets. When the acknowledgement stream is congested, the sender will increase the receiver's Ack Ratio, limiting its acknowledgement rate. This differs from TCP, which presently has no congestion control for pure acknowledgement traffic. In the absence of congestion on the ack stream, CCID 2 acknowledgements will be sent in roughly the same way as TCP acknowledgements. For instance, the Ack Ratio will be set to 2, leading to behavior like TCP's delayed acks. When the ack stream is congested, CCID 2 does not try to be TCP-friendly, but just tries to avoid congestion collapse, and to be somewhat better than TCP in explicitly reducing the ack sending rate in the presence of a high packet loss or marking rate on the return path. 6.1.1. Sending Acknowledgements A CCID 2 receiver SHOULD send one acknowledgement for every Ack Ratio data packets it receives. This is only a rough guideline, however. We intend CCID 2's acknowledgement behavior to resemble TCP's when there is no ack- stream congestion, and to be somewhat more conservative when there is ack-stream congestion; following this intent is more important than implementing Ack Ratio precisely. Suggested variations from strict Ack Ratio compliance include: (1) If the HC-Receiver, DCCP B, is not quiescent---it is actively sending data---then its acknowledgements may be piggybacked on its data packets. It is acceptable in this case to send more piggybacked acknowledgements than the Ack Ratio would suggest. If the data packets are too big to carry acknowledgement information, or the data sending rate is too low, then DCCP B SHOULD send some pure acknowledgements as well as piggybacked data-plus-acknowledgement packets, to maintain the rate of one acknowledgement per Ack Ratio received data packets. (2) The receiver SHOULD implement an algorithm like TCP's delayed acknowledgements, whereby every data packet is acknowledged within at most T seconds of its receipt, regardless of Ack Floyd/Kohler Section 6.1.1. [Page 10] INTERNET-DRAFT Expires: August 2004 February 2004 Ratio. The delayed-ack timeout T SHOULD be set as for TCP---to 200 milliseconds, for example. 6.1.2. Setting Ack Ratio There are three guidelines for setting Ack Ratio. First, it is always an integer. Second, it should not exceed half the congestion window, rounded up (except that Ack Ratio 2 is always acceptable). Third, it should be two or larger for a congestion window of four or more packets. DCCP-Ack packets from the receiver contain sequence numbers, so the sender can infer when DCCP-Ack packets are lost. The sender considers a DCCP-Ack packet lost if at least NUMDUPACK packets with higher sequence numbers have been received from the receiver. (Again, NUMDUPACK equals 3.) If DCCP-Ack packets from the receiver are marked in the network, the sender sees these marks directly. DCCP responds to congestion events on the return path by modifying the Ack Ratio, loosely emulating TCP. For each congestion window of data with lost or marked DCCP-Ack packets, the Ack Ratio is doubled, subject to the constraints noted above. Similarly, if the Ack Ratio is R, then for each (cwnd/(R^2 - R)) congestion windows of data with no lost or marked DCCP-Ack packets, the Ack Ratio is decreased by 1, again subject to the constraints on the Ack Ratio. See the section below for the derivation. For a constant congestion window, this gives an Ack sending rate that is roughly TCP-friendly. The sender need not keep the receiver's Ack Ratio completely up to date. For instance, it MAY rate-limit Ack Ratio renegotiations to once every four or five round-trip times, or to once every second or two. Additionally, it MAY bound Ack Ratio below by two, or it MAY set Ack Ratio to one for half-connections with persistent congestion windows of 1 or 2 packets. Since the sending rate for acknowledgement packets changes as a function of both the Ack Ratio and the congestion window, the dynamics will be rather complex, and this Ack congestion control mechanism is intended only to be very roughly TCP-friendly. As a result of the constraints given earlier in this section, the receiver always sends at least one ack packet for a congestion window of one packet, and the receiver always sends at least two ack packets per window of data otherwise. Thus, the receiver could be sending two ack packets per window of data even in the face of very heavy congestion on the reverse path. We would note, however, that if congestion is sufficiently heavy that all of the ack packets are dropped, then the sender falls back on a timeout, and the Floyd/Kohler Section 6.1.2. [Page 11] INTERNET-DRAFT Expires: August 2004 February 2004 exponential backoff of the timer, as in TCP. Thus, if congestion is sufficiently heavy on the reverse path, then the sender reduces its sending rate on the forward path, which reduces the rate on the reverse path as well. The sender sets the receiver's Ack Ratio by sending "Change L(Ack Ratio)" options, either on its data packets or on separate acknowledgements. 6.1.3. Derivation of Ack Ratio Decrease The congestion avoidance phase of TCP increases cwnd by one MSS for every congestion-free window. Applying this congestion avoidance behavior to the ack traffic, this would correspond to increasing the number of DCCP-Ack packets per window by one after every congestion- free window of DCCP-Ack packets. We cannot achieve this exactly using the Ack Ratio, since the Ack Ratio is an integer. Instead, we must decrease the Ack Ratio by one after K windows have been sent without a congestion event on the reverse path, where K is chosen so that the long-term number of DCCP-Ack packets per congestion window is roughly TCP-friendly, following AIMD congestion control. In CCID 2, K = (cwnd/(R^2 - R)), where R is the current Ack Ratio. This result was calculated as follows: R = Ack Ratio = # data packets / ack packets, and W = Congestion Window = # data packets / window, so W/R = # ack packets / window. Requirement: Increase W/R by 1 per congestion-free window. But can only reduce R by increments of one. Therefore, find K so that, after K congestion-free windows, the adjusted W/R would equal W/(R-1). (W/R) + K = W/(R-1), so K = W/(R-1) - W/R = W/(R^2 - R). 6.2. Quiescence This section refers to quiescence in the DCCP sense (see section 8.1 of [DCCP]): How does a CCID 2 receiver determine that the corresponding sender is not sending any data? Let T equal the greater of 0.2 seconds and two round-trip times. (The receiver may know the round-trip time in its role as the sender for the other half-connection; or if it does not, it SHOULD use an Floyd/Kohler Section 6.2. [Page 12] INTERNET-DRAFT Expires: August 2004 February 2004 estimated RTT of 0.2 seconds.) The receiver detects that the sender has gone quiescent when at least T seconds have passed without receiving any additional data from the sender, and the sender has acknowledged receiver Ack Vectors that covered all data packets sent. That is, once the sender acknowledges the receiver's Ack Vectors and the sender has not sent additional data for at least T, the receiver can determine that the sender is quiescent. 6.3. Acknowledgements of Acknowledgements The sender, DCCP A, must occasionally acknowledge the receiver's acknowledgements, so that the receiver can free up Ack Vector state. To let the receiver free Ack Vector state, DCCP A must occasionally acknowledge that it has received one of DCCP B's acknowledgements. When both half-connections are active, this information is automatically contained in A's acknowledgements to B's data. If the B-to-A half-connection goes quiescent, however, DCCP A must do it proactively. In particular, an active sender MUST occasionally acknowledge the receiver's acknowledgements. No acknowledgement options are necessary; an Acknowledgement Number, such as that included on a DCCP-DataAck packet, suffices. The sender SHOULD acknowledge approximately one of the receiver's acknowledgements per congestion window. Of course, the sender's application might fall silent. This is no problem; when neither side is sending data, a sender can wait arbitrarily long before sending an ack. 7. Explicit Congestion Notification Explicit Congestion Notification (ECN) [RFC 3168] may be used with CCID 2. If ECN is used, then the ECN Nonce will automatically be used for the data packets, following the specification for the ECN Nonce in TCP in [RFC 3540]. For the data subflow, the sender sets either the ECT(0) or ECT(1) codepoint on DCCP-Data packets. Information about marked packets is returned in the Ack Vector. Because the information in the Ack Vector is reliably transferred, DCCP does not need the TCP flags of ECN-Echo and Congestion Window Reduced. For unmarked data packets, the receiver computes the ECN Nonce Echo as in [RFC 3540], and returns the ECN Nonce Echo as part of its Ack Vector options. The sender SHOULD check these ECN Nonce Echoes against the expected values, thus protecting against the accidental or malicious concealment of marked packets. Because CCID 2 acknowledgements are congestion-controlled, ECN can also be used for its DCCP-Ack packets. In this case we do not make Floyd/Kohler Section 7. [Page 13] INTERNET-DRAFT Expires: August 2004 February 2004 use of the ECN Nonce, because it would not be easy to provide protection against the concealment of marked ack packets by the sender, and because the sender does not have much motivation for lying about the mark rate on acknowledgements. 8. Options and Features DCCP's Ack Vector option and its Ack Ratio, Use Ack Vector, and ECN Capable features are relevant for CCID 2. 9. Security Considerations Security considerations for DCCP have been discussed in [DCCP], and security considerations for TCP have been discussed in [RFC 2581]. [RFC 2581] discusses ways that an attacker could impair the performance of a TCP connection by dropping packets, or by forging extra duplicate acknowledgements or acknowledgements for new data. We are not aware of any new security considerations created by this document in its use of TCP-like congestion control. 10. IANA Considerations There are no new IANA considerations created in this document. 11. Thanks We thank Mark Handley and Jitendra Padhye for their help in defining CCID 2. We also thank Greg Minshall and Arun Venkataramani for feedback on this document. Normative References [DCCP] E. Kohler, M. Handley, and S. Floyd. Datagram Congestion Control Protocol, draft-ietf-dccp-spec-06.txt, work in progress, February 2004. [RFC 793] J. Postel, editor. Transmission Control Protocol. RFC 793. [RFC 2026] S. Bradner. The Internet Standards Process -- Revision 3. RFC 2026. [RFC 2119] S. Bradner. Key Words For Use in RFCs to Indicate Requirement Levels. RFC 2119. [RFC 2581] M. Allman, V. Paxson, and W. Stevens. TCP Congestion Control. RFC 2581. Floyd/Kohler [Page 14] INTERNET-DRAFT Expires: August 2004 February 2004 [RFC 3168] K.K. Ramakrishnan, S. Floyd, and D. Black. The Addition of Explicit Congestion Notification (ECN) to IP. RFC 3168. [RFC 3390] M. Allman, S. Floyd, and C. Partridge. Increasing TCP's Initial Window. RFC 3390. [RFC 3465] M. Allman. TCP Congestion Control with Appropriate Byte Counting (ABC). RFC 3465. [RFC 3517] E. Blanton, M. Allman, K. Fall, and L. Wang. A Conservative Selective Acknowledgment (SACK)-based Loss Recovery Algorithm for TCP. RFC 3517. [RFC 3540] N. Spring, D. Wetherall, and D. Ely. Robust Explicit Congestion Notification (ECN) Signaling with Nonces. RFC 3540. Informative References [CCID 3 PROFILE] S. Floyd, E. Kohler, and J. Padhye. Profile for DCCP Congestion Control ID 3: TFRC Congestion Control. draft- ietf-dccp-ccid3-05.txt, work in progress, February 2004. Authors' Addresses Sally Floyd ICSI Center for Internet Research 1947 Center Street, Suite 600 Berkeley, CA 94704. Eddie Kohler 4531C Boelter Hall UCLA Computer Science Department Los Angeles, CA 90095 USA Intellectual Property Notice The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication 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 Floyd/Kohler [Page 15] INTERNET-DRAFT Expires: August 2004 February 2004 proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. Full Copyright Statement Copyright (C) The Internet Society (2004). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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