Network Working Group S. Floyd Request for Comments: 4341 ICIR Category: Standards Track E. Kohler UCLA March 2006 Profile for Datagram Congestion Control Protocol (DCCP) Congestion Control ID 2: TCP-like Congestion Control Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2006).
AbstractThis document contains the profile for Congestion Control Identifier 2 (CCID 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. 1. Introduction ....................................................2 2. Conventions and Notation ........................................2 3. Usage ...........................................................3 3.1. Relationship with TCP ......................................3 3.2. Half-Connection Example ....................................4 4. Connection Establishment ........................................5 5. Congestion Control on Data Packets ..............................5 5.1. Response to Idle and Application-Limited Periods ...........8 5.2. Response to Data Dropped and Slow Receiver .................8 5.3. Packet Size ................................................8 6. Acknowledgements ................................................9 6.1. Congestion Control on Acknowledgements .....................9 6.1.1. Detecting Lost and Marked Acknowledgements .........10
6.1.2. Changing Ack Ratio .................................10 6.2. Acknowledgements of Acknowledgements ......................11 6.2.1. Determining Quiescence .............................12 7. Explicit Congestion Notification ...............................12 8. Options and Features ...........................................12 9. Security Considerations ........................................13 10. IANA Considerations ...........................................13 10.1. Reset Codes ..............................................13 10.2. Option Types .............................................13 10.3. Feature Numbers ..........................................14 11. Thanks ........................................................14 A. Appendix: Derivation of Ack Ratio Decrease ....................15 B. Appendix: Cost of Loss Inference Mistakes to Ack Ratio ........15 Normative References ..............................................17 Informative References ............................................18 RFC4340]. DCCP uses Congestion Control Identifiers, or CCIDs, to specify the congestion control mechanism in use on a half-connection. The TCP-like Congestion Control CCID sends data using a close variant of TCP's congestion control mechanisms, incorporating a variant of selective acknowledgements (SACK) [RFC2018, RFC3517]. CCID 2 is suitable for senders who can adapt to the abrupt changes in congestion window typical of TCP's Additive Increase Multiplicative Decrease (AIMD) congestion control, 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. RFC2119]. A DCCP half-connection consists of the application data sent by one endpoint and the corresponding acknowledgements sent by the other endpoint. The terms "HC-Sender" and "HC-Receiver" denote the endpoints sending application data and acknowledgements, respectively. Since CCIDs apply at the level of half-connections, we abbreviate HC-Sender to "sender" and HC-Receiver to "receiver" in this document. See [RFC4340] for more discussion.
For simplicity, we say that senders send DCCP-Data packets and receivers send DCCP-Ack packets. Both of these categories are meant to include DCCP-DataAck packets. The phrases "ECN-marked" and "marked" refer to packets marked ECN Congestion Experienced unless otherwise noted. RFC4342], which is appropriate for flows that would prefer to minimize abrupt changes in the sending rate. 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 halving 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 its being the dominant transport protocol in the Internet, some applications might prefer the more well-known dynamics of TCP-like congestion control over those of newer congestion control mechanisms, which haven't yet met the test of widespread Internet deployment. RFC793], [RFC2581], [RFC3465], and [RFC3517]. TCP congestion control continues to evolve, but CCID 2 implementations SHOULD wait for explicit updates to CCID 2 rather than track TCP's evolution directly.
Differences between CCID 2 and straight TCP congestion control include the following: o CCID 2 applies congestion control to acknowledgements, a mechanism not currently standardized for use in TCP. o DCCP is a datagram protocol, so several parameters whose units are specified in bytes in TCP, such as the congestion window cwnd, have units of packets in DCCP. o As an unreliable protocol, DCCP never retransmits a packet, so congestion control mechanisms that distinguish retransmissions from new packets have been redesigned for the DCCP context. RFC3540]. 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 a DCCP-Ack packet is that of the received packet with the highest sequence number; it is not a TCP-like cumulative acknowledgement. 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. Because DCCP-Ack packets use sequence numbers, the sender has some information about 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 at least one DCCP-DataAck packet per congestion window. 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 determines 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. RFC3517], since the Ack Vector provides all the information that might be transmitted in SACK options. A CCID 2 data sender maintains three integer parameters measured in packets.
1. The congestion window "cwnd", which equals the maximum number of data packets allowed in the network at any time. ("Data packet" means any DCCP packet that contains user data: DCCP-Data, DCCP- DataAck, and occasionally DCCP-Request and DCCP-Response.) 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 packets outstanding in the network. These parameters are manipulated, and their initial values determined, according to SACK-based TCP's behavior, except that they are measured in packets, not bytes. The rest of this section provides more specific guidance. The sender MAY send a data packet when pipe < cwnd but MUST NOT send a data packet when pipe >= cwnd. Every data packet sent increases pipe by 1. The sender reduces pipe as it infers that data packets have left the network, either by being received or by being dropped. In particular: 1. Acked data packets. The sender reduces pipe by 1 for each data packet newly acknowledged as received (Ack Vector State 0 or State 1) by some DCCP-Ack. 2. Dropped data packets. The sender reduces pipe by 1 for each data packet it can infer as lost due to the DCCP equivalent of TCP's "duplicate acknowledgements". This depends on the NUMDUPACK parameter, the number of duplicate acknowledgements needed to infer a loss. The NUMDUPACK parameter is set to three, as is currently the case in TCP. A packet P is inferred to be lost, rather than delayed, when at least NUMDUPACK packets transmitted after P have been acknowledged as received (Ack Vector State 0 or 1) by the receiver. Note that the acknowledged packets following the hole may be DCCP-Acks or other non-data packets. 3. Transmit timeouts. Finally, the sender needs transmit timeouts, handled like TCP's retransmission timeouts, in case an entire window of packets is 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 [RFC2988]. (If more than one measurement per round-trip time was used for these calculations, then the weights of the averagers would have to be adjusted to ensure that the average round-trip time is effectively derived from measurements
over multiple round-trip times.) 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 transmit timeout occurs, the sender sets pipe to zero. The adjustments to cwnd and ssthresh are described below. The sender MUST NOT decrement pipe more than once per data packet. True duplicate acknowledgements, for example, MUST NOT affect pipe. The sender also MUST NOT decrement pipe again upon receiving acknowledgement of a packet previously inferred as lost. 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 cause CCID 2 to reduce its congestion window. A congestion event contains at least one lost or marked packet. As in TCP, two losses or marks are considered part of a single congestion event when the second packet was sent before the loss or mark of the first packet was detected. As an approximation, a sender can consider two losses or marks to be part of a single congestion event when the packets were sent within one RTT estimate of one another, using an RTT estimate current at the time the packets were sent. 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 cwnd is never less than one and ssthresh is never less than two. When cwnd < ssthresh, meaning that the sender is in slow-start, the congestion window is increased by one packet for every two newly acknowledged data packets with Ack Vector State 0 (not ECN-marked), up to a maximum of Ack Ratio/2 packets per acknowledgement. This is a modified form of Appropriate Byte Counting [RFC3465] that is consistent with TCP's current standard (which does not include byte counting), but allows CCID 2 to increase as aggressively as TCP when CCID 2's Ack Ratio is greater than the default value of two. 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 at most four packets for new connections, following the rules from [RFC3390]; the ssthresh parameter is initialized to an arbitrarily high value. Senders MAY use a form of rate-based pacing when sending multiple data packets liberated by a single ack packet, rather than sending all liberated data packets in a single burst.
RFC2581] recommends that the TCP sender SHOULD slow-start after an idle period, where an idle period is defined as a period exceeding the timeout interval. [RFC2861], currently Experimental, suggests a slightly more moderate mechanism where the congestion window is halved for every round-trip time that the sender has remained idle. There are currently no standards governing TCP's use of the congestion window during an application-limited period. In particular, it is possible for TCP's congestion window to grow quite large during a long uncongested period when the sender is application limited, sending at a low rate. [RFC2861] essentially suggests that TCP's congestion window not be increased during application-limited periods when the congestion window is not being fully utilized. RFC4340], 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. o Exiting slow start. The sender MUST exit slow start whenever it receives a relevant Data Dropped or Slow Receiver option.
CCID 2 maintains a congestion window in packets and does not increase the congestion window in response to a decrease in the packet size. 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. (Preliminary simulations indicate that applications may not be able to increase their overall transfer rates this way, so it is not clear that this manipulation will occur in practice [V03].) RFC4340], Section 11.4.2. Any Data Dropped options SHOULD likewise cover the receiver's entire Acknowledgement Window. CCID 2 senders use DCCP's Ack Ratio feature to influence the rate at which receivers generate DCCP-Ack packets, thus controlling reverse- path congestion. This differs from TCP, which presently has no congestion control for pure acknowledgement traffic. CCID 2's reverse-path congestion control does not try to be TCP friendly; it just tries to avoid congestion collapse, and to be somewhat better than TCP is in the presence of a high packet loss or mark rate on the reverse path. The default Ack Ratio is two, and CCID 2 with this Ack Ratio behaves like TCP with delayed acks. [RFC4340], Section 11.3, describes the Ack Ratio in more detail, including its relationship to acknowledgement pacing and DCCP-DataAck packets. This document's Section 6.1.1 describes how a CCID 2 sender detects lost or marked acknowledgements, and Section 6.1.2 describes how it changes the Ack Ratio.
marked DCCP-Ack), the sender halves the acknowledgement rate by doubling Ack Ratio; for every RTT containing no DCCP-Ack congestion event, it additively increases the acknowledgement rate through gradual decreases in Ack Ratio. Appendix B discusses the costs of mistaking data packet loss for non-data packet loss.) A receiver that implements its own acknowledgement congestion control independent of Ack Ratio SHOULD NOT reduce its DCCP-Ack acknowledgement rate due to losses or marks on its data packets. Appendix A for the derivation.) Changes in Ack Ratio are signalled through feature negotiation; see [RFC4340], Section 11.3.
For a constant congestion window, this gives an Ack sending rate that is roughly TCP friendly. Of course, cwnd usually varies over time; the dynamics will be rather complex, but roughly TCP friendly. We recommend that the sender use the most recent value of cwnd when determining whether to decrease Ack Ratio by 1. The sender need not keep 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. The sender SHOULD NOT attempt to renegotiate the Ack Ratio more than once per round-trip time. Additionally, it MAY enforce a minimum Ack Ratio of two, or it MAY set Ack Ratio to one for half-connections with persistent congestion windows of 1 or 2 packets. Putting it all together, the receiver always sends at least one acknowledgement per window of data when cwnd = 1, and at least two acknowledgements 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, all the ack packets are dropped, and then the sender falls back on an exponentially backed-off timeout, 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.
RFC4340], Section 11.1, for general information on quiescence. 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. If it does not, it should use a default RTT of 0.2 seconds, as described in [RFC4340], Section 3.4.) Once the sender acknowledges the receiver's Ack Vectors and the sender has not sent additional data for at least T seconds, the receiver can infer that the sender is quiescent. More precisely, the receiver infers that the sender has gone quiescent when at least T seconds have passed without receiving any data from the sender, and when the sender has acknowledged receiver Ack Vectors covering all data packets received at the receiver. RFC3168]. The sender will use the ECN Nonce for data packets, and the receiver will echo those nonces in its Ack Vectors, as specified in [RFC4340], Section 12.2. Information about marked packets is also 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 [RFC3540] and returns it 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 may also be used for its acknowledgements. In this case we do not make 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.
RFC4340], and security considerations for TCP have been discussed in [RFC2581]. [RFC2581] discusses ways in which 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. RFC4340]. CCID 2 also introduces three sets of numbers whose values should be allocated by IANA; namely, CCID 2-specific Reset Codes, option types, and feature numbers. These ranges will prevent any future CCID 2-specific allocations from polluting DCCP's corresponding global namespaces; see [RFC4340], Section 10.3. However, this document makes no particular allocations from any range, except for experimental and testing use [RFC3692]. We refer to the Standards Action policy outlined in [RFC2434].
Section 6.1.2. The congestion avoidance phase of TCP halves the cwnd for every window with congestion. Similarly, CCID 2 doubles Ack Ratio for every window with congestion on the return path, roughly halving the DCCP-Ack sending rate. The congestion avoidance phase of TCP increases cwnd by one MSS for every congestion-free window. When this congestion avoidance behavior is applied to acknowledgement 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 Ack Ratio, since it is an integer. Instead, we must decrease 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, rough TCP-friendliness for the ack traffic can be accomplished by setting K to 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. Since we can only reduce R by increments of one, we find K so that, after K congestion-free windows, W/R + K would equal W/(R-1). (W/R) + K = W/(R-1), so K = W/(R-1) - W/R = W/(R^2 - R). Section 6.1.1, the sender often cannot determine whether lost packets carried data. This hinders its ability to separate non-data loss events from other loss events. In the absence of better information, the sender assumes, for the purpose of Ack Ratio calculation, that all lost packets were non-data packets. This may overestimate the non-data loss event rate, which can lead to a too-high Ack Ratio, and thus to a too-slow acknowledgement rate. All acknowledgement information will still get through -- DCCP
acknowledgements are reliable -- but acknowledgement information will arrive in a burstier fashion. Absent some form of rate-based pacing, this could lead to increased burstiness for the sender's data traffic. There are several cases when the problem of an overly-high Ack Ratio, and the resulting increased burstiness of the data traffic, will not arise. In particular, call the receiver DCCP B and the sender DCCP A: o The problem won't arise unless DCCP B is sending a significant amount of data itself. When the B-to-A half-connection is quiescent or low rate, most packets sent by DCCP B will, in fact, be pure acknowledgements, and DCCP A's estimate of the DCCP-Ack loss rate will be reasonably accurate. o The problem won't arise if DCCP B habitually piggybacks acknowledgement information on its data packets. The piggybacked acknowledgements are not limited by Ack Ratio, so they can arrive frequently enough to prevent burstiness. o The problem won't arise if DCCP A's sending rate is low, since burstiness isn't a problem at low rates. o The problem won't arise if DCCP B's sending rate is high relative to DCCP A's sending rate, since the B-to-A loss rate must be low to support DCCP B's sending rate. This bounds the Ack Ratio to reasonable values even when DCCP A labels every loss as a DCCP- Ack loss. o The problem won't arise if DCCP B sends NDP Count options when appropriate (the Send NDP Count/B feature is true). Then the sender can use the receiver's NDP Count options to detect, in most cases, whether lost packets were data packets or DCCP-Acks. o Finally, the problem won't arise if DCCP A rate-paces its data packets. This leaves the case when DCCP B is sending roughly the same amount of data packets and non-data packets, without NDP Count options, and with all acknowledgement information in DCCP-Ack packets. We now quantify the potential cost, in terms of a too-large Ack Ratio, due to the sender's misclassifying data packet losses as DCCP-Ack losses. For simplicity, we assume an environment of large-scale statistical multiplexing where the packet drop rate is independent of the sending rate of any individual connection.
Assume that when DCCP A correctly counts non-data losses, Ack Ratio is set so that B-to-A data and acknowledgement traffic both have a sending rate of D packets per second. Then when DCCP A incorrectly counts data losses as non-data losses, the sending rate for the B-to-A data traffic is still D pps, but the reduced sending rate for the B-to-A acknowledgement traffic is f*D pps, with f < 1. Let the packet loss rate be p. The sender incorrectly estimates the non-data loss rate as (pD+pfD)/fD, or, equivalently, as p(1 + 1/f). Because the congestion control mechanism for acknowledgement traffic is roughly TCP friendly, and therefore the non-data sending rate and the data sending rate both grow as 1/sqrt(x) for x the packet drop rate, we have fD/D = sqrt(p)/sqrt(p(1 + 1/f)), so f^2 = 1/(1 + 1/f). Solving, we get f = 0.62. If the sender incorrectly counts lost data packets as non-data in this scenario, the acknowledgement rate is decreased by a factor of 0.62. This would result in a moderate increase in burstiness for the A-to-B data traffic, which could be mitigated by sending NDP Count options or piggybacked acknowledgements, or by rate-pacing out the data. [RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP Selective Acknowledgement Options", RFC 2018, October 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion Control", RFC 2581, April 1999. [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission Timer", RFC 2988, November 2000.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001. [RFC3390] Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's Initial Window", RFC 3390, October 2002. [RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A Conservative Selective Acknowledgment (SACK)-based Loss Recovery Algorithm for TCP", RFC 3517, April 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. [RFC2861] Handley, M., Padhye, J., and S. Floyd, "TCP Congestion Window Validation", RFC 2861, June 2000. [RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte Counting (ABC)", RFC 3465, February 2003. [RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit Congestion Notification (ECN) Signaling with Nonces", RFC 3540, June 2003. [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. [V03] Arun Venkataramani, August 2003. Citation for acknowledgement purposes only.
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