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Internet Engineering Task Force M. Allman
INTERNET-DRAFT ICSI
File: draft-ietf-tcpm-rto-consider-03.txt April 15, 2016
Intended Status: Best Current Practice
Expires: October 15, 2016
Retransmission Timeout Considerations
Status of this Memo
This document may not be modified, and derivative works of it may
not be created, except to format it for publication as an RFC or to
translate it into languages other than English.
This Internet-Draft is submitted in full conformance with the
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Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
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Abstract
Each implementation of a retransmission timeout mechanism represents
a balance between correctness and timeliness and therefore no
implementation suits all situations. This document provides
high-level requirements for retransmission timeout schemes
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appropriate for general use in the Internet. Within the
requirements, implementations have latitude to define particulars
that best address each situation.
Terminology
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 BCP 14, RFC 2119
[RFC2119].
1 Introduction
Despite our best intentions and most robust mechanisms, reliability
in networking ultimately requires a timeout and re-try mechanism.
Often there are more timely and precise mechanisms than a timeout
for repairing loss (e.g., TCP's fast retransmit [RFC5681], NewReno
[RFC6582] or selective acknowledgment scheme [RFC2018,RFC6675])
which require information exchange between components in the system.
Such communication cannot be guaranteed. Alternatively, information
coding---e.g., FEC---can allow the recipient to recover from some
amount of lost information without use of a retransmission. This
latter provides probabilistic reliability. Finally, negative
acknowledgment schemes exist that do not depend on continuous
feedback to trigger retransmissions (e.g., [RFC3940]). However,
regardless of these useful alternatives, the only thing we can truly
depend on is the passage of time and therefore our ultimate backstop
to ensuring reliability is a timeout. (Note: There is a case when
we cannot count on the passage of time, but in this case we believe
repairing loss will be a moot point and hence we do not further
consider this case in this document.)
Various protocols have defined their own timeout mechanisms (e.g.,
TCP [RFC6298], SCTP [RFC4960], SIP [RFC3261]). Ideally, if we know
a segment will be lost before reaching the destination, a second
copy of it would be sent immediately after the first transmission.
However, in reality the specifics of retransmission timeouts often
represent a particular tradeoff between correctness and
responsiveness [AP99]. In other words we want to simultaneously:
- Wait long enough to ensure the decision to retransmit is
correct.
- Bound the delay we impose on applications before
retransmitting.
However, serving both of these goals is difficult as they pull in
opposite directions. I.e., towards either (a) withholding needed
retransmissions too long to ensure the retransmissions are truly
needed or (b) not waiting long enough to help application
responsiveness and sending spurious retransmissions. Given this
fundamental tradeoff [AP99], we have found that even though the
retransmission timeout (RTO) procedures are standardized,
implementations often add their own subtle imprint on the specifics
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of the process to tilt the tradeoff between correctness and
responsiveness in some particular way.
At this point we recognize that often these specific tweaks are not
crucial for network safety. Hence, in this document we outline the
high-level requirements that are crucial for any retransmission
timeout scheme to follow. The intent is to then allow
implementations to instantiate mechanisms that best realize their
specific goals within this framework. These specific mechanisms
could be standardized by the IETF or ad-hoc, but as long as they
adhere to the requirements given in this document they would be
considered consistent with the standards.
Finally, we note the requirements in this document are applicable to
any protocol that uses a retransmission timeout mechanism. The
examples and discussion are framed in terms of TCP, however, that is
an artifact of where much of our experience with RTOs comes from and
should not be read as narrowing the scope of the requirements.
2 Scope
This document offers high-level requirements based on experience
with retransmission timer algorithms. However, this document
explicitly does not update or obsolete currently standardized
algorithms nor limit future standardization of specific RTO
mechanisms. Specifically:
(a) RTO mechanisms that are currently standardized are not updated
or obsoleted by this document. This holds even in cases where
the existing specification differs from the requirements in this
document (e.g., [RFC3261] uses a smaller initial RTO than this
document specifies). Existing standard specifications enjoy
their own consensus which this document does not change.
(b) Future standardization efforts that specify RTO mechanisms
SHOULD follow the requirements in this document. This follows
the definition of "SHOULD" [RFC2119] and is explicitly not a
"MUST". That is, the requirements in this document hold unless
the community has consensus that specific deviations in a
particular context are warranted.
(c) RTO mechanisms that are not standardized but adhere to the
requirements in the following section are deemed consistent with
the standards. This includes RTO mechanisms that are deviations
from a specific standardized algorithm, but are still within the
requirements below.
More colloquially we note that each RTO implementation can be placed
into one of the following four categories:
- The implementation precisely follows a standard RTO mechanism
(e.g., [RFC6298]), as well as adhering to the requirements in this
document.
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This document represents no change for this situation as such an
implementation is clearly standards compliant.
- The implementation does not precisely follow a standard RTO
mechanism and does not adhere to the requirements in this
document.
This document makes no change to this situation as such an
implementation is clearly not standards compliant.
- The implementation precisely follows a standard RTO mechanism
(e.g., [RFC3261]), but does not precisely adhere to the
requirements in this document.
This document represents no change for this situation as such an
implementation is considered standards compliant by virtue of
precisely implementing a standard mechanism that has community
consensus as a reasonable approach. That is, this document's
stance is to not limit the community's ability to make exceptions
to the requirements herein for particular cases.
- The implementation does not precisely follow a standard RTO
mechanism, yet does adhere to the requirements in this document.
This document represents a change for these implementations and
considers them to be consistent with the standards by virtue of
following the requirements herein that provide for an RTO safe for
operation in the Internet.
In other words, the requirements in this document can be viewed as
specifying the default properties of an RTO mechanism.
Specifications can more concretely nail down specifics within these
defaults or work outside the defaults as necessary. However,
implementations that fall within the defaults do not require
explicit specifications to be considered consistent with the
standards.
3 Requirements
We now list the requirements that SHOULD apply when designing
retransmission timeout (RTO) mechanisms.
(1) In the absence of any knowledge about the latency of a path, the
RTO MUST be conservatively set to no less than 1 second.
This requirement ensures two important aspects of the RTO.
First, when transmitting into an unknown network,
retransmissions will not be sent before an ACK would reasonably
be expected to arrive and hence possibly waste scarce network
resources. Second, as noted below, sometimes retransmissions
can lead to ambiguities in assessing the latency of a network
path. Therefore, it is especially important for the first
latency sample to be free of ambiguities such that there is a
baseline for the remainder of the communication.
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The specific constant (1 second) comes from the analysis of
Internet RTTs found in Appendix A of [RFC6298].
(2) We specify three requirements that pertain to the sampling of
the latency across a path.
Often measuring the latency is framed as assessing the
round-trip time (RTT)---e.g., in TCP's RTO computation
specification [RFC6298]. This is somewhat mis-leading as the
latency is better framed as the "feedback time" (FT). In other
words, it is not simply a network property, but the length of
time before a sender should reasonably expect a response to a
query.
For instance, consider a DNS request from a client to a
resolver. When the request can be served from the resolver's
cache the FT likely well approximates the network RTT between
the client and resolver. However, on a cache miss the resolver
will have to request the needed information from authoritative
DNS servers, which will non-trivially increase the FT and
therefore the FT between the client and resolver does not well
match the network-based RTT between the two hosts.
(a) In steady state the RTO MUST be set based on recent
observations of both the FT and the variance of the FT.
In other words, the RTO should be based on a reasonable
amount of time that the sender should wait for an
acknowledgment of the data before retransmitting the given
data.
(b) FT observations MUST be taken regularly.
The exact definition of "regularly" is deliberately left
vague. TCP takes a FT sample roughly once per RTT, or if
using the timestamp option [RFC7323] on each acknowledgment
arrival. [AP99] shows that both these approaches result in
roughly equivalent performance for the RTO estimator.
Additionally, [AP99] shows that taking only a single FT
sample per TCP connection is suboptimal and hence the
requirement that the FT be sampled continuously throughout
the lifetime of a connection. For the purpose of this
requirement, we state that FT samples SHOULD be taken at
least once per RTT or as frequently as data is exchanged and
ACKed if that happens less frequently than every RTT.
However, we also recognize that it may not always be
practical to take a FT sample this often in all cases.
Hence, this once-per-RTT sampling requirement is explicitly
a "SHOULD" and not a "MUST".
(c) FT samples used in the computation of the RTO MUST NOT be
ambiguous.
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Assume two copies of some segment X are transmitted at times
t0 and t1 and then segment X is acknowledged at time t2. In
some cases, it is not clear which copy of X triggered the
ACK and hence the actual FT is either t2-t1 or t2-t0, but
which is a mystery. Therefore, in this situation an
implementation MUST use Karn's algorithm [KP87,RFC6298] and
use neither version of the FT sample and hence not update
the RTO.
There are cases where two copies of some data are
transmitted in a way whereby the sender can tell which is
being acknowledged by an incoming ACK. E.g., TCP's
timestamp option [RFC7323] allows for segments to be
uniquely identified and hence avoid the ambiguity. In such
cases there is no ambiguity and the resulting samples can
update the RTO.
(3) Each time the RTO fires and causes a retransmission the value of
the RTO MUST be exponentially backed off such that the next
firing requires a longer interval. The backoff may be removed
after the successful transmission of non-retransmitted data.
A maximum value MAY be placed on the RTO provided it is at least
60 seconds (a la [RFC6298]).
This ensures network safety.
(4) Retransmission timeouts MUST be taken as indications of
congestion in the network and the sending rate adapted using a
standard mechanism (e.g., TCP collapses the congestion window to
one segment [RFC5681]).
This ensures network safety.
An exception is made to this rule if an IETF standardized
mechanism is used to determine that a particular loss is due to
a non-congestion event (e.g., packet corruption). In such a
case a congestion control action is not required. Additionally,
RTO-triggered congestion control actions may be reversed when a
standard mechanism determines that the cause of the loss was not
congestion after all.
4 Discussion
We note that research has shown the tension between the
responsiveness and correctness of retransmission timeouts seems to
be a fundamental tradeoff [AP99]. That is, making the RTO more
aggressive (e.g., via changing TCP's EWMA gains, lowering the
minimum RTO, etc.) can reduce the time spent waiting on needed
retransmissions. However, at the same time, such aggressiveness
leads to more needless retransmissions. Therefore, being as
aggressive as the requirements given in the previous section allow
in any particular situation may not be the best course of action
because an RTO expiration carries a requirement to slow down.
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While the tradeoff between responsiveness and correctness seems
fundamental, the tradeoff can be made less relevant if the sender
can detect and recover from spurious RTOs. Several mechanisms have
been proposed for this purpose, such as Eifel [RFC3522], F-RTO
[RFC5682] and DSACK [RFC2883,RFC3708]. Using such mechanisms may
allow a data originator to tip towards being more responsive without
incurring (as much of) the attendant costs of needless retransmits.
Also, note, that in addition to the experiments discussed in [AP99],
the Linux TCP implementation has been using various non-standard RTO
mechanisms for many years seemingly without large scale problems
(e.g., using different EWMA gains). Further, a number of
implementations use minimum RTOs that are less than the 1 second
specified in [RFC6298]. While the implication of these deviations
from the standard may be more spurious retransmits (per [AP99]), we
are aware of no large scale problems caused by this change to the
minimum RTO.
Finally, we note that while allowing implementations to be more
aggressive may in fact increase the number of needless
retransmissions the above requirements fail safe in that they insist
on exponential backoff of the RTO and a transmission rate reduction.
Therefore, allowing implementers latitude in their instantiations of
an RTO mechanism does not somehow open the flood gates to aggressive
behavior. Since there is a downside to being aggressive the
incentives for proper behavior are retained in the mechanism.
5 Security Considerations
This document does not alter the security properties of
retransmission timeout mechanisms. See [RFC6298] for a discussion
of these within the context of TCP.
Acknowledgments
This document benefits from years of discussions with Ethan Blanton,
Sally Floyd, Jana Iyengar, Shawn Ostermann, Vern Paxson, and the
members of the TCPM and TCP-IMPL working groups. Ran Atkinson,
Yuchung Cheng, Jonathan Looney and Michael Scharf provided useful
comments on a previous version of this draft.
Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Informative References
[AP99] Allman, M., V. Paxson, "On Estimating End-to-End Network Path
Properties", Proceedings of the ACM SIGCOMM Technical Symposium,
September 1999.
[KP87] Karn, P. and C. Partridge, "Improving Round-Trip Time
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Estimates in Reliable Transport Protocols", SIGCOMM 87.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC2883] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
Extension to the Selective Acknowledgement (SACK) Option for
TCP", RFC 2883, July 2000.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. Schooler,
"SIP: Session Initiation Protocol", RFC 3261, June 2002.
[RFC3522] Ludwig, R., M. Meyer, "The Eifel Detection Algorithm for
TCP", RFC 3522, april 2003.
[RFC3708] Blanton, E., M. Allman, "Using TCP Duplicate Selective
Acknowledgement (DSACKs) and Stream Control Transmission
Protocol (SCTP) Duplicate Transmission Sequence Numbers (TSNs)
to Detect Spurious Retransmissions", RFC 3708, February 2004.
[RFC3940] Adamson, B., C. Bormann, M. Handley, J. Macker,
"Negative-acknowledgment (NACK)-Oriented Reliable Multicast
(NORM) Protocol", November 2004, RFC 3940.
[RFC4960] Stweart, R., "Stream Control Transmission Protocol", RFC
4960, September 2007.
[RFC5682] Sarolahti, P., M. Kojo, K. Yamamoto, M. Hata, "Forward
RTO-Recovery (F-RTO): An Algorithm for Detecting Spurious
Retransmission Timeouts with TCP", RFC 5682, September 2009.
[RFC6298] Paxson, V., M. Allman, H.K. Chu, M. Sargent, "Computing
TCP's Retransmission Timer", June 2011, RFC 6298.
[RFC6582] Henderson, T., S. Floyd, A. Gurtov, Y. Nishida, "The
NewReno Modification to TCP's Fast Recovery Algorithm", April
2012, RFC 6582.
[RFC6675] Blanton, E., M. Allman, L. Wang, I. Jarvinen, M. Kojo,
Y. Nishida, "A Conservative Loss Recovery Algorithm Based on
Selective Acknowledgment (SACK) for TCP", August 2012, RFC 6675.
[RFC7323] Borman D., B. Braden, V. Jacobson, R. Scheffenegger, "TCP
Extensions for High Performance", September 2014, RFC 7323.
Authors' Addresses
Mark Allman
International Computer Science Institute
1947 Center St. Suite 600
Berkeley, CA 94704
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EMail: mallman@icir.org
http://www.icir.org/mallman
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