Internet Engineering Task Force S. Floyd
INTERNET DRAFT ICSI
draft-ietf-tsvwg-newreno-01.txt T. Henderson
Boeing
A. Gurtov
U. Helsinki
September 2003
The NewReno Modification to TCP's Fast Recovery Algorithm
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
RFC 2581 [RFC2581] documents the following four intertwined TCP
congestion control algorithms: Slow Start, Congestion Avoidance, Fast
Retransmit, and Fast Recovery. RFC 2581 [RFC2581] explicitly allows
certain modifications of these algorithms, including modifications
that use the TCP Selective Acknowledgement (SACK) option [RFC2018],
and modifications that respond to "partial acknowledgments" (ACKs
which cover new data, but not all the data outstanding when loss was
detected) in the absence of SACK. The NewReno mechanism uses an
algorithm for responding to partial acknowledgments that was first
proposed by Janey Hoe in [Hoe95].
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RFC 2582 [RFC2582] specified the NewReno mechanisms as Experimental
in 1999. This document is a small revision of RFC 2582 intended to
advance the NewReno mechanisms to Proposed Standard. RFC 2581 notes
that the Fast Retransmit/Fast Recovery algorithm specified in that
document does not recover very efficiently from multiple losses in a
single flight of packets, and that RFC 2582 contains one set of
modifications to address this problem.
NOTE TO THE RFC EDITOR: PLEASE REMOVE THIS SECTION UPON PUBLICATION.
Changes from draft-ietf-tsvwg-newreno-00.txt:
* In Section 8, added a cautionary note about using the duplicate
acknowledgment counter as a flag for whether Fast Recovery is in
effect.
* In Section 8, added a note about pulling along "recover" with
"snd_una" when Fast Recovery is not in effect.
* Added a discussion in Section 6 about heuristics for distinguishing
between a retransmitted packet that was dropped, and three duplicate
acknowledgements simply from the unnecessary retransmission of three
packets.
* Added more text and examples for comparing the Impatient and the
Slow-but-Steady variants.
* In Section 8, added a cautionary note saying that when the sender
is not in Fast Retransmit, the sender should not use the Fast
Recovery response to multiple duplicate acknowledgements.
Changes from draft-floyd-newreno-00.txt:
* In Section 8 on "Implementation issues for the data sender",
mentioned alternate methods for limiting bursts when exiting Fast
Recovery.
* Changed draft from draft-floyd-newreno to draft-ietf-tsvwg-newreno
Changes from RFC 2582:
* Rephrasing and rearrangements of the text.
* RFC 2582 described the Careful and Less Careful variants of
NewReno, along with a default version that was neither Careful nor
Less Careful, and recommended the Careful variant. This document
only specifies the Careful version.
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* RFC 2582 used two separate variables, "send_high" and "recover",
and this document has merged them into a single variable "recover".
* Added sections on "Comparisons between Reno and NewReno TCP", and
on "Changes relative to RFC 2582". The section on "Comparisons
between Reno and NewReno TCP" includes a discussion of the one area
where NewReno is known to perform worse than Reno or SACK, and that
is in the response to reordering.
* Moved all of the discussions of the Impatient and Slow-but-Steady
variants to one place, and recommended the Impatient variant (as in
the default version in RFC 2582).
* Added a section on Implementation issues for the data sender,
mentioning maxburst_.
* Added a paragraph about differences between RFC 2582 and [FF96].
END OF NOTE TO RFC EDITOR
1. Introduction
For the typical implementation of the TCP Fast Recovery algorithm
described in [RFC2581] (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 acknowledgements 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 2581
leads to the retransmission of only a single data packet.
Problems can arise, therefore, when multiple packets have been
dropped from a single window of data and the Fast Retransmit and Fast
Recovery algorithms are invoked. In this case, if the SACK option 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. This document applies only for TCP
connections that are unable to use the TCP Selective Acknowledgement
(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 acknowledgements, the sender infers a packet
loss, and retransmits the indicated packet. After this, the data
sender could receive additional duplicate acknowledgements, as the
data receiver acknowledges additional data packets that were already
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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 acknowledgement for the retransmitted packet (that
is, the packet retransmitted when Fast Retransmit was first entered).
If there had been a single packet drop and no reordering, then the
acknowledgement for this packet will acknowledge all of the packets
transmitted before Fast Retransmit was entered. However, when there
were multiple packet drops, then the acknowledgement for the
retransmitted packet will acknowledge some but not all of the packets
transmitted before the Fast Retransmit. We call this acknowledgement
a partial acknowledgment.
Along with several other suggestions, [Hoe95] suggested that during
Fast Recovery the TCP data sender responds 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 2581 that
incorporates a response to partial acknowledgements received during
Fast Recovery. We call this modified Fast Recovery algorithm
NewReno, because it is a slight but significant variation of the
basic Reno algorithm in RFC 2581. 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 acknowledgements during Fast Recovery.
The version of NewReno in this document also draws on other
discussions of NewReno in the literature [LM97].
We do not claim that the NewReno version of Fast Recovery described
here is an optimal modification of Fast Recovery for responding to
partial acknowledgements, for TCP connections that are unable to use
SACK. Based on our experiences with the NewReno modification in the
NS simulator [NS] and with numerous implementations of NewReno, we
believe that this modification improves the performance of the Fast
Retransmit and Fast Recovery algorithms in a wide variety of
scenarios.
2. Terminology and Definitions
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
and indicate requirement levels for compliant TCP implementations
implementing the NewReno Fast Retransmit and Fast Recovery algorithms
described in this document.
This document assumes that the reader is familiar with the terms
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SENDER MAXIMUM SEGMENT SIZE (SMSS), CONGESTION WINDOW (cwnd), and
FLIGHT SIZE (FlightSize) defined in [RFC2581]. FLIGHT SIZE is
defined as in [RFC2581] as follows:
FLIGHT SIZE:
The amount of data that has been sent but not yet acknowledged.
3. The Fast Retransmit and Fast Recovery Algorithms in NewReno
The standard implementation of the Fast Retransmit and Fast Recovery
algorithms is given in [RFC2581]. This section specifies the basic
NewReno algorithm. Sections 4 through 6 describe some optional
variants, and the motivations behind them, that an implementor may
want to consider when tuning performance for certain network
scenarios. Sections 7 and 8 provide some guidance to implementors
based on experience with NewReno implementations.
The NewReno modification concerns 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 NewReno algorithm specified in this document differs from the
implementation in [RFC2581] in the introduction of the variable
"recover" in step 1, in the response to a partial or new
acknowledgement in step 5, and in modifications to step 1 and the
addition of step 6 for avoiding multiple Fast Retransmits caused by
the retransmission of packets already received by the receiver.
The algorithm specified in this document uses a variable "recover",
whose initial value is the initial send sequence number.
1) Three duplicate ACKs:
When the third duplicate ACK is received and the sender is not
already in the Fast Recovery procedure, check to see if the
Cumulative Acknowledgement field covers more than "recover".
If so, go to Step 1A. Otherwise, go to Step 1B.
1A) Invoking Fast Retransmit:
If so, then set ssthresh to no more than the value given in
equation 1 below. (This is equation 3 from [RFC2581]).
ssthresh = max (FlightSize / 2, 2*SMSS) (1)
In addition, record the highest sequence number transmitted in
the variable "recover", and go to Step 2.
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1B) Not invoking Fast Retransmit:
Do not enter the Fast Retransmit and Fast Recovery procedure.
In particular, do not change ssthresh, do not go to Step 2 to
retransmit the "lost" segment, and do not execute Step 3 upon
subsequent duplicate ACKs.
2) Entering Fast Retransmit:
Retransmit the lost segment and set cwnd to ssthresh plus 3*SMSS.
This artificially "inflates" the congestion window by the number
of segments (three) that have left the network and which the
receiver has buffered.
3) Fast Recovery:
For each additional duplicate ACK received while in Fast
Recovery, increment cwnd by SMSS. This artificially inflates
the congestion window in order to reflect the additional segment
that has left the network.
4) Fast Recovery, continued:
Transmit a segment, if allowed by the new value of cwnd and the
receiver's advertised window.
5) When an ACK arrives that acknowledges new data, this ACK could be
the acknowledgment elicited by the retransmission from step 2, or
elicited by a later retransmission.
Full acknowledgements:
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, FlightSize + SMSS); or (2) ssthresh,
where ssthresh is the value set in step 1; this is termed
"deflating" the window. (We note that "FlightSize" in step 1
referred to the amount of data outstanding in step 1, when Fast
Recovery was entered, while "FlightSize" in step 5 refers to the
amount of data outstanding in step 5, when Fast Recovery is
exited.) If the second option is selected, the implementation
should take measures to avoid a possible burst of data, in case
the amount of data outstanding in the network was 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 acknowledgement. (This is known as "maxburst_" in the NS
simulator). Exit the Fast Recovery procedure.
Partial acknowledgements:
If this ACK does *not* acknowledge all of the data up to and
including "recover", then this is a partial ACK. In this case,
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retransmit the first unacknowledged segment. Deflate the
congestion window by the amount of new data acknowledged, then
add back one SMSS (if the partial ACK acknowledges at least one
SMSS of new data) and 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 Steps 3 and 4 above).
For the first partial ACK that arrives during Fast Recovery, also
reset the retransmit timer.
6) 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 1 specifies a check that the Cumulative Acknowledgement field
covers more than "recover". Because the acknowledgement field
contains the sequence number that the sender next expects to receive,
the acknowledgement "ack_number" covers more than "recover" when:
ack_number - one > recover.
Note that in Step 5, the congestion window is deflated after a
partial acknowledgement is received. The congestion window was
likely to have been inflated considerably when the partial
acknowledgement was received. In addition, depending on the original
pattern of packet losses, the partial acknowledgement 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.
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 acknowledgement threshold,
but assumes the threshold of three duplicate acknowledgements
currently specified in RFC 2581.
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.
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4. Resetting the Retransmit Timer in Response to Partial
Acknowledgements.
One possible variant to the response to partial acknowledgements
specified in Section 3 concerns when to reset the retransmit timer
after a partial acknowledgement. The algorithm in Section 3, Step 5,
resets the retransmit timer only after the first partial ACK. In
this case, if a large number of packets were dropped from a window of
data, the TCP data sender's retransmit timer will ultimately expire,
and the TCP data sender will invoke Slow-Start. (This is illustrated
on page 12 of [F98].) We call this the Impatient variant of NewReno.
We note that the Impatient variant in Section 3 doesn't follow the
recommended algorithm in RFC 2988 of restarting the retransmit timer
after every packet transmission or retransmission [RFC2988, Step
5.1].
In contrast, the NewReno simulations in [FF96] illustrate the
algorithm described above with the modification that the retransmit
timer is reset after each partial acknowledgement. We call this the
Slow-but-Steady variant of NewReno. In this case, for a window with
a large number of packet drops, the TCP data sender retransmits at
most one packet per roundtrip time. (This behavior is illustrated in
the New-Reno TCP simulation of Figure 5 in [FF96], and on page 11 of
[F98].
When N packets have been dropped from a window of data for a large
value of N, the Slow-but-Steady variant can remain in Fast Recovery
for N round-trip times, retransmitting one more dropped packet each
round-trip time; for these scenarios, the Impatient variant gives a
faster recovery and better performance. The tests "ns test-suite-
newreno.tcl impatient1" and "ns test-suite-newreno.tcl slow1" in the
NS simulator illustrate such a scenario, where the Impatient variant
performs better than the Slow-but-Steady variant. The Impatient
variant can be particularly important for TCP connections with large
congestion windows, as illustrated by the tests "ns test-suite-
newreno.tcl impatient4" and "ns test-suite-newreno.tcl slow4" in the
NS simulator.
One can also construct scenarios where the Slow-but-Steady variant
gives better performance than the Impatient variant. As an example,
this occurs then only a small number of packets are dropped, the RTO
is sufficiently small that the retransmit timer expires, and
performance would have been better without a retransmit timeout. The
tests "ns test-suite-newreno.tcl impatient2" and "ns test-suite-
newreno.tcl slow2" in the NS simulator illustrate such a scenario.
The Slow-but-Steady variant can also achieve higher goodput than the
Impatient variant, by avoiding unnecessary retransmissions. This
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could be of special interest for cellular links, where every
transmission costs battery power and money. The tests "ns test-
suite-newreno.tcl impatient3" and "ns test-suite-newreno.tcl slow3"
in the NS simulator illustrate such a scenario. The Slow-but-Steady
variant can also be more robust to delay variation in the network,
where a delay spike might force the Impatient variant into a timeout
and go-back-N recovery.
Neither of the two variants discussed above are optimal. Our
recommendation is for the Impatient variant, as specified in Section
3 of this document, because of the poor performance of the Slow-but-
Steady variant for TCP connections with large congestion windows.
One possibility for a more optimal algorithm would be one that
recovered from multiple packet drops as quickly as does slow-start,
while resetting the retransmit timers after each partial
acknowledgement, as described in the section below. We note,
however, that there is a limitation to the potential performance in
this case in the absence of the SACK option.
5. Retransmissions after a Partial Acknowledgement
One possible variant to the response to partial acknowledgements
specified in Section 3 would be to retransmit more than one packet
after each partial acknowledgement, and to reset the retransmit timer
after each retransmission. The algorithm specified in Section 3
retransmits a single packet after each partial acknowledgement. This
is the most conservative alternative, in that it is the least likely
to result in an unnecessarily-retransmitted packet. A variant that
would recover faster from a window with many packet drops would be to
effectively Slow-Start, retransmitting two packets after each partial
acknowledgement. Such an approach would take less than N roundtrip
times to recover from N losses [Hoe96]. However, in the absence of
SACK, recovering as quickly as slow-start introduces the likelihood
of unnecessarily retransmitting packets, and this could significantly
complicate the recovery mechanisms.
We note that the response to partial acknowledgements specified in
Section 3 of this document and in RFC 2582 differs from the response
in [FF96], even though both approaches only retransmit one packet in
response to a partial acknowledgement. Step 5 of Section 3 specifies
that the TCP sender responds to a partial ACK by deflating the
congestion window by the amount of new data acknowledged, then adding
back one SMSS if the partial ACK acknowledges at least one SMSS of
new data, and sending a new segment if permitted by the new value of
cwnd. Thus, only one previously-sent packet is retransmitted in
response to each partial acknowledgement, but additional new packets
might be transmitted as well, depending on the amount of new data
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acknowledged by the partial acknowledgement. In contrast, the
variant of NewReno illustrated in [FF96] simply set the congestion
window to ssthresh when a partial acknowledgement was received. The
approach in [FF96] is more conservative, and does not attempt to
accurately track the actual number of outstanding packets after a
partial acknowledgement is received. While either of these
approaches gives acceptable performance, the variant specified in
Section 3 recovers more smoothly when multiple packets are dropped
from a window of data. (The [FF96] behavior can be seen in the NS
simulator by setting the variable "partial_window_deflation_" for
"Agent/TCP/Newreno" to 0, and the behavior specified in Section 3 is
achieved by setting "partial_window_deflation_" to 1.)
6. Avoiding Multiple Fast Retransmits
This section describes the motivation for the sender's state variable
"recover", and discusses possible heuristics for distinguishing
between a retransmitted packet that was dropped, and three duplicate
acknowledgements simply from the unnecessary retransmission of three
packets.
In the absence of the SACK option or timestamps, a duplicate
acknowledgement carries no information to identify the data packet or
packets at the TCP data receiver that triggered that duplicate
acknowledgement. In this case, the TCP data sender is unable to
distinguish between a duplicate acknowledgement that results from a
lost or delayed data packet, and a duplicate acknowledgement that
results from the sender's unnecessary retransmission of a data packet
that had already been received at the TCP data receiver. Because of
this, with the Retransmit and Fast Recovery algorithms in Reno TCP,
multiple segment losses from a single window of data can sometimes
result in unnecessary multiple Fast Retransmits (and multiple
reductions of the congestion window) [F94].
With the Fast Retransmit and Fast Recovery algorithms in Reno TCP,
the performance problems caused by multiple Fast Retransmits are
relatively minor compared to the potential problems with Tahoe TCP,
which does not implement Fast Recovery. Nevertheless, unnecessary
Fast Retransmits can occur with Reno TCP unless some explicit
mechanism is added to avoid this, such as the use of the "recover"
variable. (This modification is called "bugfix" in [F98], and is
illustrated on pages 7 and 9 of that document. Unnecessary Fast
Retransmits for Reno without "bugfix" is illustrated on page 6 of
[F98].)
Section 3 of RFC 2582 defined a default variant of NewReno TCP that
did not use the variable "recover", and did not check if duplicate
ACKs cover the variable "recover" before invoking Fast Retransmit.
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With this default variant from RFC 2582, the problem of multiple Fast
Retransmits from a single window of data can occur after a Retransmit
Timeout (as in page 8 of [F98]) or in scenarios with reordering (as
in the validation test "./test-all-newreno newreno5_noBF" in
directory "tcl/test" of the NS simulator. This gives performance
similar to that on page 8 of [F03].) RFC 2582 also defined Careful
and Less Careful variants of the NewReno algorithm, and recommended
the Careful variant.
The algorithm specified in Section 3 of this document corresponds to
the Careful variant of NewReno TCP from RFC 2582, and eliminates the
problem of multiple Fast Retransmits. This algorithm uses the
variable "recover", whose initial value is the initial send sequence
number. 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
acknowledgements that do not cover more than "recover". In this
case, the duplicate acknowledgements 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 acknowledgements that do not cover
more than "recover", and in this case the sender would have been
better off if it had initiated Fast Retransmit. For a TCP that
implements the algorithm specified in Section 3 of this document, the
sender does not infer a packet drop from duplicate acknowledgements
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 acknowledgement field, that allow the
sender to distinguish in some cases between three duplicate
acknowledgements following a retransmitted packet that was dropped,
and three duplicate acknowledgements simply from the unnecessary
retransmission of three packets [Gur03]. The TCP sender MAY use such
a heuristic to decide to invoke a Fast Retransmit in some cases even
when the three duplicate acknowledgements do not cover more than
"recover".
For example, when three duplicate acknowledgements are caused by the
unnecessary retransmission of three packets, this is likely to be
accompanied by the cumulative acknowledgement 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
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timestamp echoed in the duplicate acknowledgement is the timestamp of
the most recent data packet that advanced the cumulative
acknowledgement field [RFC1323]. If timestamps are used, and the
sender stores the timestamp of the last acknowledged segment, then
the timestamp echoed by duplicate acknowledgements can be used to
distinguish between a retransmitted packet that was dropped, and
three duplicate acknowledgements simply from the unnecessary
retransmission of three packets. The heuristics are illustrated in
the NS simulator in the validation test "./test-all-newreno".
6.1. ACK Heuristic.
If the ACK-based heuristic is used, then following the advancement of
the cumulative acknowledgement field, the sender stores the value of
previous cumulative acknowledgement as prev_highest_ack and stores
the latest cumulative ACK as highest_ack. In addition, the following
step is performed if Step 1 in Section 3 fails, before proceeding to
Step 1B.
1*) If the Cumulative Acknowledgement field didn't cover more than
"recover", check to see if the congestion window is greater
than one SMSS and the difference between highest_ack and
prev_highest_ack is at most four SMSS. If true, duplicate
ACKs indicate a lost segment (proceed to Step 1A in Section
3). Otherwise, duplicate ACKs likely result from unnecessary
retransmissions (proceed to Step 1B in Section 3).
The congestion window check serves to protect against fast retransmit
immediately after a retransmit timeout, similar to the
"exitFastRetrans_" variable in NS. Examples of applying the ACK
heuristic are in validation tests "./test-all-newreno
newreno_rto_loss_ack" and "./test-all-newreno newreno_rto_dup_ack" in
directory "tcl/test" of the NS simulator.
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. A
validation test for this scenario is "./test-all-newreno
newreno_rto_loss_ackf". The ACK heuristic is more likely to fail if
the receiver uses delayed ACKs, because then a smaller number of ACK
losses are needed to produce a sufficient jump in the cumulative ACK.
6.2. Timestamp Heuristic.
If this heuristic is used, the sender stores the timestamp of the
last acknowledged segment. In addition, the second paragraph of step
1 in Section 3 is replaced as follows:
1**) If the Cumulative Acknowledgement field didn't cover more than
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"recover", check to see if the echoed timestamp equals the
stored timestamp. If true, duplicate ACKs indicate a lost
segment (proceed to Step 1A in Section 3). Otherwise, duplicate
ACKs likely result from unnecessary retransmissions (proceed
to Step 1B in Section 3).
Examples of applying the timestamp heuristic are in validation tests
"./test-all-newreno newreno_rto_loss_tsh" and "./test-all-newreno
newreno_rto_dup_tsh". The timestamp heuristic works correctly both
when the receiver echoes timestamps as specified by [RFC1323] or by
its revision attempts. However, if the receiver arbitrarily echos
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
[RFC3522].
7. Implementation Issues for the Data Receiver
[RFC2581] 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 acknowledgement when they send a partial acknowledgment,
but instead wait first for their delayed acknowledgement timer to
expire [C98]. As [C98] notes, this severely limits the potential
benefit from NewReno by delaying the receipt of the partial
acknowledgement at the data sender. Our recommendation is that the
data receiver send an immediate acknowledgement for an out-of-order
segment, even when that out-of-order segment fills a hole in the
buffer.
8. Implementation Issues for the Data Sender
In Section 3, Step 5 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
is to limit the number of data packets that can be sent in response
to a single acknowledgment. (This is known as "maxburst_" in the ns
simulator.) 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
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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 as such a flag is
not reliable because it can be reset upon window updates and out-of-
order acknowledgments.
When not in Fast Recovery, the value of the state variable "recover"
should be pulled along with the value of the state variable for
acknowledgments (typically, "snd_una") so that, when large amounts of
data has been sent and acked, the sequence space does not wrap and
falsely indicate that Fast Recovery should not be entered (Section 3,
step 1, last paragraph).
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
acknowledgements. When three or more duplicate acknowledgements are
received, the Cumulative Acknowledgement field doesn't cover more
than "recover", and a new Fast Recovery is not invoked, it is
important that the sender not execute the Fast Recovery steps (3) and
(4) in Section 3. 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 the NS
simulator. (This bug in the NS simulator was fixed in July 2003,
with the variable "exitFastRetrans_".)
9. Simulations
Simulations with NewReno are illustrated with the validation test
"tcl/test/test-all-newreno" in the NS simulator. The command
"../../ns test-suite-newreno.tcl reno" shows a simulation with Reno
TCP, illustrating the data sender's lack of response to a partial
acknowledgement. In contrast, the command "../../ns test-suite-
newreno.tcl newreno_B" shows a simulation with the same scenario
using the NewReno algorithms described in this paper.
10. Comparisons between Reno and NewReno TCP
As we stated in the introduction, we believe that the NewReno
modification described in this document improves the performance of
the Fast Retransmit and Fast Recovery algorithms of Reno TCP in a
wide variety of scenarios. This has been discussed in some depth in
[FF96], which illustrates Reno TCP's poor performance when multiple
packets are dropped from a window of data and also illustrates
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NewReno TCP's good performance in that scenario.
We do, however, know of one scenario where Reno TCP gives better
performance than NewReno TCP, that we describe here for the sake of
completeness. Consider a scenario with no packet loss, but with
sufficient reordering that the TCP sender receives three duplicate
acknowledgements. This will trigger the Fast Retransmit and Fast
Recovery algorithms. With Reno TCP or with Sack TCP, this will
result in the unnecessary retransmission of a single packet, combined
with a halving of the congestion window (shown on pages 4 and 6 of
[F03]). With NewReno TCP, however, this reordering will also result
in the unnecessary retransmission of an entire window of data (shown
on page 5 of [F03]).
While Reno TCP performs better than NewReno TCP in the presence of
reordering, NewReno's superior performance in the presence of
multiple packet drops generally outweighs its less optimal
performance in the presence of reordering. (Sack TCP is the
preferred solution, with good performance in both scenarios.) This
document recommends the Fast Retransmit and Fast Recovery algorithms
of NewReno TCP instead of those of Reno TCP for those TCP connections
that do not support SACK. We would also note that NewReno's Fast
Retransmit and Fast Recovery mechanisms are widely deployed in TCP
implementations in the Internet today, as documented in [PF01]. For
example, tests of TCP implementations in several thousand web servers
in 2001 showed that for those TCP connections where the web browser
was not SACK-capable, more web servers used the Fast Retransmit and
Fast Recovery algorithms of NewReno than those of Reno or Tahoe TCP
[PF01].
11. Changes Relative to RFC 2582
The purpose of this document is to advance the NewReno's Fast
Retransmit and Fast Recovery algorithms in RFC 2582 to Proposed
Standard.
The main change in this document relative to RFC 2582 is to specify
the Careful variant of NewReno's Fast Retransmit and Fast Recovery
algorithms. The base algorithm described in RFC 2582 did not attempt
to avoid unnecessary multiple Fast Retransmits that can occur after a
timeout (described in more detail in the section above). However,
RFC 2582 also defined "Careful" and "Less Careful" variants that
avoid these unnecessary Fast Retransmits, and recommended the Careful
variant. This document specifies the previously-named "Careful"
variant as the basic version of NewReno. As described below, this
algorithm uses a variable "recover", whose initial value is the send
sequence number.
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The algorithm specified in Section 3 checks whether the
acknowledgement field of a partial acknowledgement covers *more* than
"recover". Another possible variant would be to require simply that
the acknowledgement field *cover* "recover" before initiating another
Fast Retransmit. We called this the Less Careful variant in RFC
2582.
There are two separate scenarios in which the TCP sender could
receive three duplicate acknowledgements acknowledging "recover" but
no more than "recover". One scenario would be that the data sender
transmitted four packets with sequence numbers higher than "recover",
that the first packet was dropped in the network, and the following
three packets triggered three duplicate acknowledgements
acknowledging "recover". The second scenario would be that the
sender unnecessarily retransmitted three packets below "recover", and
that these three packets triggered three duplicate acknowledgements
acknowledging "recover". In the absence of SACK, the TCP sender in
unable to distinguish between these two scenarios.
For the Careful variant of Fast Retransmit, the data sender would
have to wait for a retransmit timeout in the first scenario, but
would not have an unnecessary Fast Retransmit in the second scenario.
For the Less Careful variant to Fast Retransmit, the data sender
would Fast Retransmit as desired in the first scenario, and would
unnecessarily Fast Retransmit in the second scenario. This document
only specifies the Careful variant in Section 3. Unnecessary Fast
Retransmits with the Less Careful variant in scenarios with
reordering are illustrated in page 8 of [F03].
The document also specifies two heuristics that the TCP sender MAY
use to decide to invoke Fast Retransmit even when the three duplicate
acknowledgements do not cover more than "recover". These heuristics,
an ACK-based heuristic and a timestamp heuristic, are described in
Sections 6.1 and 6.2 respectively.
12. Conclusions
This document specifies the NewReno Fast Retransmit and Fast Recovery
algorithms for TCP. This NewReno modification to TCP can be
important even 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 (RFC 2581) in a number of scenarios discussed herein.
A number of options to the basic algorithm presented in Section 3 are
also described. These include the handling of the retransmission
timer (Section 4), the response to partial acknowledgments (Section
5), and the value of the congestion window when leaving Fast Recovery
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(section 3, step 5). 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 of the variants of NewReno is
implemented.
13. Acknowledgements
Many thanks to Anil Agarwal, Mark Allman, Armando Caro, Jeffrey Hsu,
Vern Paxson, Kacheong Poon, Keyur Shah, and Bernie Volz for detailed
feedback on this document or on its precursor RFC 2582.
Floyd et al. [Page 17]
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14. References
Normative References
[RFC2018] M. Mathis, J. Mahdavi, S. Floyd, A. Romanow, "TCP Selective
Acknowledgement Options", RFC 2018, October 1996.
[RFC2581] W. Stevens, M. Allman, and V. Paxson, "TCP Congestion
Control", RFC 2581, April 1999.
[RFC2582] S. Floyd and T. Henderson, The NewReno Modification to
TCP's Fast Recovery Algorithm, RFC 2582, April 1999.
[RFC2988] V. Paxson and M. Allman, Computing TCP's Retransmission
Timer, RFC 2988, November 2000.
[RFC3042] M. Allman, H. Balakrishnan, and S. Floyd, Enhancing TCP's
Loss Recovery Using Limited Transmit, RFC 3042, January 2001.
Informative References
[C98] N. Cardwell, "delayed ACKs for retransmitted packets: ouch!".
November 1998, Email to the tcpimpl mailing list, Message-ID
"Pine.LNX.4.02A.9811021421340.26785-100000@sake.cs.washington.edu",
archived at "http://tcp-impl.lerc.nasa.gov/tcp-impl".
[F98] S. Floyd, Revisions to RFC 2001, "Presentation to the TCPIMPL
Working Group", August 1998. URLs "ftp://ftp.ee.lbl.gov/talks/sf-
tcpimpl-aug98.ps" and "ftp://ftp.ee.lbl.gov/talks/sf-tcpimpl-
aug98.pdf".
[F03] S. Floyd, "Moving NewReno from Experimental to Proposed
Standard? Presentation to the TSVWG Working Group", March 2003.
URLs "http://www.icir.org/floyd/talks/newreno-Mar03.ps" and
"http://www.icir.org/floyd/talks/newreno-Mar03.pdf".
[FF96] K. Fall and S. Floyd, "Simulation-based Comparisons of Tahoe,
Reno and SACK TCP", Computer Communication Review, July 1996. URL
"ftp://ftp.ee.lbl.gov/papers/sacks.ps.Z".
[F94] S. Floyd, "TCP and Successive Fast Retransmits", Technical
report, October 1994. URL
"ftp://ftp.ee.lbl.gov/papers/fastretrans.ps".
[Gur03] A. Gurtov, "[Tsvwg] resolving the problem of unnecessary fast
retransmits in go-back-N", email to the tsvwg mailing list, message
ID <3F25B467.9020609@cs.helsinki.fi>, July 28, 2003. URL
"http://www1.ietf.org/mail-archive/working-
Floyd et al. [Page 18]
draft-ietf-tsvwg-newreno September 2003
groups/tsvwg/current/msg04334.html".
[Hen98] T. Henderson, Re: NewReno and the 2001 Revision. September
1998. Email to the tcpimpl mailing list, Message ID
"Pine.BSI.3.95.980923224136.26134A-100000@raptor.CS.Berkeley.EDU",
archived at "http://tcp-impl.lerc.nasa.gov/tcp-impl".
[Hoe95] J. Hoe, "Startup Dynamics of TCP's Congestion Control and
Avoidance Schemes", Master's Thesis, MIT, 1995. URL "http://ana-
www.lcs.mit.edu/anaweb/ps-papers/hoe-thesis.ps".
[Hoe96] J. Hoe, "Improving the Start-up Behavior of a Congestion
Control Scheme for TCP", ACM SIGCOMM, August 1996. URL
"http://www.acm.org/sigcomm/sigcomm96/program.html".
[LM97] D. Lin and R. Morris, "Dynamics of Random Early Detection",
SIGCOMM 97, September 1997. URL
"http://www.acm.org/sigcomm/sigcomm97/program.html".
[NS] The Network Simulator (NS). URL "http://www.isi.edu/nsnam/ns/".
[PF01] J. Padhye and S. Floyd, "Identifying the TCP Behavior of Web
Servers", June 2001, SIGCOMM 2001.
[RFC1323] V. Jacobson, R. Braden, and D. Borman, "TCP Extensions for
High Performance,", RFC 1323, May 1992.
[RFC3522] R. Ludwig and M. Meyer, The Eifel Detection Algorithm for
TCP, RFC 3522, April 2003.
15. Security Considerations
RFC 2581 discusses general security considerations concerning TCP
congestion control. This document describes a specific algorithm
that conforms with the congestion control requirements of RFC 2581,
and so those considerations apply to this algorithm, too. There are
no known additional security concerns for this specific algorithm.
AUTHORS' ADDRESSES
Sally Floyd
International Computer Science Institute
Phone: +1 (510) 666-2989
Email: floyd@acm.org
URL: http://www.icir.org/floyd/
Floyd et al. [Page 19]
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Tom Henderson
The Boeing Company
Email: thomas.r.henderson@boeing.com
Andrei Gurtov
U. Helsinki
Email: gurtov@cs.helsinki.fi
Floyd et al. [Page 20]
