Internet Engineering Task Force (IETF) T. Henderson Request for Comments: 6582 Boeing Obsoletes: 3782 S. Floyd Category: Standards Track ICSI ISSN: 2070-1721 A. Gurtov University of Oulu Y. Nishida WIDE Project April 2012
The NewReno Modification to TCP's Fast Recovery Algorithm
RFC 5681 documents the following four intertwined TCP congestion control algorithms: slow start, congestion avoidance, fast retransmit, and fast recovery. RFC 5681 explicitly allows certain modifications of these algorithms, including modifications that use the TCP Selective Acknowledgment (SACK) option (RFC 2883), and modifications that respond to "partial acknowledgments" (ACKs that cover new data, but not all the data outstanding when loss was detected) in the absence of SACK. This document describes a specific algorithm for responding to partial acknowledgments, referred to as "NewReno". This response to partial acknowledgments was first proposed by Janey Hoe. This document obsoletes RFC 3782.
Status of This Memo
This is an Internet Standards Track document.
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For the typical implementation of the TCP fast recovery algorithm described in [RFC5681] (first implemented in the 1990 BSD Reno release, and referred to as the "Reno algorithm" in [FF96]), the TCP data sender only retransmits a packet after a retransmit timeout has occurred, or after three duplicate acknowledgments have arrived triggering the fast retransmit algorithm. A single retransmit timeout might result in the retransmission of several data packets, but each invocation of the fast retransmit algorithm in RFC 5681 leads to the retransmission of only a single data packet.
Two problems arise with Reno TCP when multiple packet losses occur in a single window. First, Reno will often take a timeout, as has been documented in [Hoe95]. Second, even if a retransmission timeout is avoided, multiple fast retransmits and window reductions can occur, as documented in [F94]. When multiple packet losses occur, if the SACK option [RFC2883] is available, the TCP sender has the information to make intelligent decisions about which packets to retransmit and which packets not to retransmit during fast recovery.
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This document applies to TCP connections that are unable to use the TCP Selective Acknowledgment (SACK) option, either because the option is not locally supported or because the TCP peer did not indicate a willingness to use SACK.
In the absence of SACK, there is little information available to the TCP sender in making retransmission decisions during fast recovery. From the three duplicate acknowledgments, the sender infers a packet loss, and retransmits the indicated packet. After this, the data sender could receive additional duplicate acknowledgments, as the data receiver acknowledges additional data packets that were already in flight when the sender entered fast retransmit.
In the case of multiple packets dropped from a single window of data, the first new information available to the sender comes when the sender receives an acknowledgment for the retransmitted packet (that is, the packet retransmitted when fast retransmit was first entered). If there is a single packet drop and no reordering, then the acknowledgment for this packet will acknowledge all of the packets transmitted before fast retransmit was entered. However, if there are multiple packet drops, then the acknowledgment for the retransmitted packet will acknowledge some but not all of the packets transmitted before the fast retransmit. We call this acknowledgment a partial acknowledgment.
Along with several other suggestions, [Hoe95] suggested that during fast recovery the TCP data sender respond to a partial acknowledgment by inferring that the next in-sequence packet has been lost and retransmitting that packet. This document describes a modification to the fast recovery algorithm in RFC 5681 that incorporates a response to partial acknowledgments received during fast recovery. We call this modified fast recovery algorithm NewReno, because it is a slight but significant variation of the behavior that has been historically referred to as Reno. This document does not discuss the other suggestions in [Hoe95] and [Hoe96], such as a change to the ssthresh parameter during slow start, or the proposal to send a new packet for every two duplicate acknowledgments during fast recovery. The version of NewReno in this document also draws on other discussions of NewReno in the literature [LM97] [Hen98].
We do not claim that the NewReno version of fast recovery described here is an optimal modification of fast recovery for responding to partial acknowledgments, for TCP connections that are unable to use SACK. Based on our experiences with the NewReno modification in the network simulator known as ns-2 [NS] and with numerous implementations of NewReno, we believe that this modification improves the performance of the fast retransmit and fast recovery
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algorithms in a wide variety of scenarios. Previous versions of this RFC [RFC2582] [RFC3782] provide simulation-based evidence of the possible performance gains.
The basic idea of these extensions to the fast retransmit and fast recovery algorithms described in Section 3.2 of [RFC5681] is as follows. The TCP sender can infer, from the arrival of duplicate acknowledgments, whether multiple losses in the same window of data have most likely occurred, and avoid taking a retransmit timeout or making multiple congestion window reductions due to such an event.
The NewReno modification applies to the fast recovery procedure that begins when three duplicate ACKs are received and ends when either a retransmission timeout occurs or an ACK arrives that acknowledges all of the data up to and including the data that was outstanding when the fast recovery procedure began.
The procedures specified in Section 3.2 of [RFC5681] are followed, with the modifications listed below. Note that this specification avoids the use of the key words defined in RFC 2119 [RFC2119], since it mainly provides sender-side implementation guidance for performance improvement, and does not affect interoperability.
1) Initialization of TCP protocol control block: When the TCP protocol control block is initialized, recover is set to the initial send sequence number.
2) Three duplicate ACKs: When the third duplicate ACK is received, the TCP sender first checks the value of recover to see if the Cumulative Acknowledgment field covers more than recover. If so, the value of recover is incremented to the value of the highest sequence number transmitted by the TCP so far. The TCP then enters fast retransmit (step 2 of Section 3.2 of [RFC5681]). If not, the TCP does not enter fast retransmit and does not reset ssthresh.
3) Response to newly acknowledged data: Step 6 of [RFC5681] specifies the response to the next ACK that acknowledges previously unacknowledged data. When an ACK arrives that acknowledges new data, this ACK could be the acknowledgment elicited by the initial retransmission from fast retransmit, or elicited by a later retransmission. There are two cases:
Full acknowledgments: If this ACK acknowledges all of the data up to and including recover, then the ACK acknowledges all the intermediate segments sent between the original transmission of the lost segment and the receipt of the third duplicate ACK. Set cwnd to either (1) min (ssthresh, max(FlightSize, SMSS) + SMSS) or (2) ssthresh, where ssthresh is the value set when fast retransmit was entered, and where FlightSize in (1) is the amount of data presently outstanding. This is termed "deflating" the window. If the second option is selected, the implementation is encouraged to take measures to avoid a possible burst of data, in case the amount of data outstanding in the network is much less than the new congestion window allows. A simple mechanism is to limit the number of data packets that can be sent in response to a single acknowledgment. Exit the fast recovery procedure.
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Partial acknowledgments: If this ACK does *not* acknowledge all of the data up to and including recover, then this is a partial ACK. In this case, retransmit the first unacknowledged segment. Deflate the congestion window by the amount of new data acknowledged by the Cumulative Acknowledgment field. If the partial ACK acknowledges at least one SMSS of new data, then add back SMSS bytes to the congestion window. This artificially inflates the congestion window in order to reflect the additional segment that has left the network. Send a new segment if permitted by the new value of cwnd. This "partial window deflation" attempts to ensure that, when fast recovery eventually ends, approximately ssthresh amount of data will be outstanding in the network. Do not exit the fast recovery procedure (i.e., if any duplicate ACKs subsequently arrive, execute step 4 of Section 3.2 of [RFC5681]).
For the first partial ACK that arrives during fast recovery, also reset the retransmit timer. Timer management is discussed in more detail in Section 4.
4) Retransmit timeouts: After a retransmit timeout, record the highest sequence number transmitted in the variable recover, and exit the fast recovery procedure if applicable.
Step 2 above specifies a check that the Cumulative Acknowledgment field covers more than recover. Because the acknowledgment field contains the sequence number that the sender next expects to receive, the acknowledgment "ack_number" covers more than recover when
ack_number - 1 > recover;
i.e., at least one byte more of data is acknowledged beyond the highest byte that was outstanding when fast retransmit was last entered.
Note that in step 3 above, the congestion window is deflated after a partial acknowledgment is received. The congestion window was likely to have been inflated considerably when the partial acknowledgment was received. In addition, depending on the original pattern of packet losses, the partial acknowledgment might acknowledge nearly a window of data. In this case, if the congestion window was not deflated, the data sender might be able to send nearly a window of data back-to-back.
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This document does not specify the sender's response to duplicate ACKs when the fast retransmit/fast recovery algorithm is not invoked. This is addressed in other documents, such as those describing the Limited Transmit procedure [RFC3042]. This document also does not address issues of adjusting the duplicate acknowledgment threshold, but assumes the threshold specified in the IETF standards; the current standard is [RFC5681], which specifies a threshold of three duplicate acknowledgments.
As a final note, we would observe that in the absence of the SACK option, the data sender is working from limited information. When the issue of recovery from multiple dropped packets from a single window of data is of particular importance, the best alternative would be to use the SACK option.
4. Handling Duplicate Acknowledgments after a Timeout
After each retransmit timeout, the highest sequence number transmitted so far is recorded in the variable recover. If, after a retransmit timeout, the TCP data sender retransmits three consecutive packets that have already been received by the data receiver, then the TCP data sender will receive three duplicate acknowledgments that do not cover more than recover. In this case, the duplicate acknowledgments are not an indication of a new instance of congestion. They are simply an indication that the sender has unnecessarily retransmitted at least three packets.
However, when a retransmitted packet is itself dropped, the sender can also receive three duplicate acknowledgments that do not cover more than recover. In this case, the sender would have been better off if it had initiated fast retransmit. For a TCP sender that implements the algorithm specified in Section 3.2 of this document, the sender does not infer a packet drop from duplicate acknowledgments in this scenario. As always, the retransmit timer is the backup mechanism for inferring packet loss in this case.
There are several heuristics, based on timestamps or on the amount of advancement of the Cumulative Acknowledgment field, that allow the sender to distinguish, in some cases, between three duplicate acknowledgments following a retransmitted packet that was dropped, and three duplicate acknowledgments from the unnecessary retransmission of three packets [Gur03] [GF04]. The TCP sender may use such a heuristic to decide to invoke a fast retransmit in some cases, even when the three duplicate acknowledgments do not cover more than recover.
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For example, when three duplicate acknowledgments are caused by the unnecessary retransmission of three packets, this is likely to be accompanied by the Cumulative Acknowledgment field advancing by at least four segments. Similarly, a heuristic based on timestamps uses the fact that when there is a hole in the sequence space, the timestamp echoed in the duplicate acknowledgment is the timestamp of the most recent data packet that advanced the Cumulative Acknowledgment field [RFC1323]. If timestamps are used, and the sender stores the timestamp of the last acknowledged segment, then the timestamp echoed by duplicate acknowledgments can be used to distinguish between a retransmitted packet that was dropped and three duplicate acknowledgments from the unnecessary retransmission of three packets.
If the ACK-based heuristic is used, then following the advancement of the Cumulative Acknowledgment field, the sender stores the value of the previous cumulative acknowledgment as prev_highest_ack, and stores the latest cumulative ACK as highest_ack. In addition, the following check is performed if, in step 2 of Section 3.2, the Cumulative Acknowledgment field does not cover more than recover.
2*) If the Cumulative Acknowledgment field didn't cover more than recover, check to see if the congestion window is greater than SMSS bytes and the difference between highest_ack and prev_highest_ack is at most 4*SMSS bytes. If true, duplicate ACKs indicate a lost segment (enter fast retransmit). Otherwise, duplicate ACKs likely result from unnecessary retransmissions (do not enter fast retransmit).
The congestion window check serves to protect against fast retransmit immediately after a retransmit timeout.
If several ACKs are lost, the sender can see a jump in the cumulative ACK of more than three segments, and the heuristic can fail. [RFC5681] recommends that a receiver should send duplicate ACKs for every out-of-order data packet, such as a data packet received during fast recovery. The ACK heuristic is more likely to fail if the receiver does not follow this advice, because then a smaller number of ACK losses are needed to produce a sufficient jump in the cumulative ACK.
If this heuristic is used, the sender stores the timestamp of the last acknowledged segment. In addition, the last sentence of step 2 in Section 3.2 of this document is replaced as follows:
2**) If the Cumulative Acknowledgment field didn't cover more than recover, check to see if the echoed timestamp in the last non-duplicate acknowledgment equals the stored timestamp. If true, duplicate ACKs indicate a lost segment (enter fast retransmit). Otherwise, duplicate ACKs likely result from unnecessary retransmissions (do not enter fast retransmit).
The timestamp heuristic works correctly, both when the receiver echoes timestamps, as specified by [RFC1323], and by its revision attempts. However, if the receiver arbitrarily echoes timestamps, the heuristic can fail. The heuristic can also fail if a timeout was spurious and returning ACKs are not from retransmitted segments. This can be prevented by detection algorithms such as the Eifel detection algorithm [RFC3522].
[RFC5681] specifies that "Out-of-order data segments SHOULD be acknowledged immediately, in order to accelerate loss recovery". Neal Cardwell has noted that some data receivers do not send an immediate acknowledgment when they send a partial acknowledgment, but instead wait first for their delayed acknowledgment timer to expire [C98]. As [C98] notes, this severely limits the potential benefit of NewReno by delaying the receipt of the partial acknowledgment at the data sender. Echoing [RFC5681], our recommendation is that the data receiver send an immediate acknowledgment for an out-of-order segment, even when that out-of-order segment fills a hole in the buffer.
In Section 3.2, step 3 above, it is noted that implementations should take measures to avoid a possible burst of data when leaving fast recovery, in case the amount of new data that the sender is eligible to send due to the new value of the congestion window is large. This can arise during NewReno when ACKs are lost or treated as pure window updates, thereby causing the sender to underestimate the number of new segments that can be sent during the recovery procedure. Specifically, bursts can occur when the FlightSize is much less than the new congestion window when exiting from fast recovery. One simple mechanism to avoid a burst of data when leaving fast recovery
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is to limit the number of data packets that can be sent in response to a single acknowledgment. (This is known as "maxburst_" in ns-2 [NS].) Other possible mechanisms for avoiding bursts include rate- based pacing, or setting the slow start threshold to the resultant congestion window and then resetting the congestion window to FlightSize. A recommendation on the general mechanism to avoid excessively bursty sending patterns is outside the scope of this document.
An implementation may want to use a separate flag to record whether or not it is presently in the fast recovery procedure. The use of the value of the duplicate acknowledgment counter for this purpose is not reliable, because it can be reset upon window updates and out-of- order acknowledgments.
When updating the Cumulative Acknowledgment field outside of fast recovery, the state variable recover may also need to be updated in order to continue to permit possible entry into fast recovery (Section 3.2, step 2). This issue arises when an update of the Cumulative Acknowledgment field results in a sequence wraparound that affects the ordering between the Cumulative Acknowledgment field and the state variable recover. Entry into fast recovery is only possible when the Cumulative Acknowledgment field covers more than the state variable recover.
It is important for the sender to respond correctly to duplicate ACKs received when the sender is no longer in fast recovery (e.g., because of a retransmit timeout). The Limited Transmit procedure [RFC3042] describes possible responses to the first and second duplicate acknowledgments. When three or more duplicate acknowledgments are received, the Cumulative Acknowledgment field doesn't cover more than recover, and a new fast recovery is not invoked, the sender should follow the guidance in Section 4. Otherwise, the sender could end up in a chain of spurious timeouts. We mention this only because several NewReno implementations had this bug, including the implementation in ns-2 [NS].
It has been observed that some TCP implementations enter a slow start or congestion avoidance window updating algorithm immediately after the cwnd is set by the equation found in Section 3.2, step 3, even without a new external event generating the cwnd change. Note that after cwnd is set based on the procedure for exiting fast recovery (Section 3.2, step 3), cwnd should not be updated until a further event occurs (e.g., arrival of an ack, or timeout) after this adjustment.
[RFC5681] discusses general security considerations concerning TCP congestion control. This document describes a specific algorithm that conforms with the congestion control requirements of [RFC5681], and so those considerations apply to this algorithm, too. There are no known additional security concerns for this specific algorithm.
This document specifies the NewReno fast retransmit and fast recovery algorithms for TCP. This NewReno modification to TCP can even be important for TCP implementations that support the SACK option, because the SACK option can only be used for TCP connections when both TCP end-nodes support the SACK option. NewReno performs better than Reno in a number of scenarios discussed in previous versions of this RFC ([RFC2582] [RFC3782]).
A number of options for the basic algorithms presented in Section 3 are also referenced in Appendix A of this document. These include the handling of the retransmission timer, the response to partial acknowledgments, and whether or not the sender must maintain a state variable called recover. Our belief is that the differences between these variants of NewReno are small compared to the differences between Reno and NewReno. That is, the important thing is to implement NewReno instead of Reno for a TCP connection without SACK; it is less important exactly which variant of NewReno is implemented.
Many thanks to Anil Agarwal, Mark Allman, Armando Caro, Jeffrey Hsu, Vern Paxson, Kacheong Poon, Keyur Shah, and Bernie Volz for detailed feedback on the precursor RFCs 2582 and 3782. Jeffrey Hsu provided clarifications on the handling of the variable recover; these clarifications were applied to RFC 3782 via an erratum and are incorporated into the text of Section 6 of this document. Yoshifumi Nishida contributed a modification to the fast recovery algorithm to account for the case in which FlightSize is 0 when the TCP sender leaves fast recovery and the TCP receiver uses delayed acknowledgments. Alexander Zimmermann provided several suggestions to improve the clarity of the document.
[GF04] Gurtov, A. and S. Floyd, "Resolving Acknowledgment Ambiguity in non-SACK TCP", NExt Generation Teletraffic and Wired/Wireless Advanced Networking (NEW2AN'04), February 2004. <http://www.cs.helsinki.fi/u/gurtov/ papers/heuristics.html>.
[Gur03] Gurtov, A., "[Tsvwg] resolving the problem of unnecessary fast retransmits in go-back-N", email to the tsvwg mailing list, July 28, 2003. <http://www.ietf.org/mail-archive/ web/tsvwg/current/msg04334.html>.
Previous versions of this RFC ([RFC2582] [RFC3782]) contained additional informative material on the following subjects, and may be consulted by readers who may want more information about possible variants to the algorithms and who may want references to specific [NS] simulations that provide NewReno test cases.
Section 4 of [RFC3782] discusses some alternative behaviors for resetting the retransmit timer after a partial acknowledgment.
Section 5 of [RFC3782] discusses some alternative behaviors for performing retransmission after a partial acknowledgment.
Section 6 of [RFC3782] describes more information about the motivation for the sender's state variable recover.
Section 9 of [RFC3782] introduces some NS simulation test suites for NewReno. In addition, references to simulation results can be found throughout [RFC3782].
In [RFC3782], the cwnd after Full ACK reception will be set to (1) min (ssthresh, FlightSize + SMSS) or (2) ssthresh. However, the first option carries a risk of performance degradation: With the first option, if FlightSize is zero, the result will be 1 SMSS. This means TCP can transmit only 1 segment at that moment, which can cause a delay in ACK transmission at the receiver due to a delayed ACK algorithm.
The FlightSize on Full ACK reception can be zero in some situations. A typical example is where the sending window size during fast recovery is small. In this case, the retransmitted packet and new data packets can be transmitted within a short interval. If all these packets successfully arrive, the receiver may generate a Full ACK that acknowledges all outstanding data. Even if the window size is not small, loss of ACK packets or a receive buffer shortage during fast recovery can also increase the possibility of falling into this situation.
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The proposed fix in this document, which sets cwnd to at least 2*SMSS if the implementation uses option 1 in the Full ACK case (Section 3.2, step 3, option 1), ensures that the sender TCP transmits at least two segments on Full ACK reception.
In addition, an erratum was reported for RFC 3782 (an editorial clarification to Section 8); this erratum has been addressed in Section 6 of this document.
Sections 4, 5, and 9-11 of [RFC3782] were removed, and instead Appendix A of this document was added to back-reference this informative material. A few references that have no citation in the main body of the document have been removed.
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Tom Henderson The Boeing Company
Sally Floyd International Computer Science Institute