draft-ietf-tcpm-rfc3782-bis-00.txt   draft-ietf-tcpm-rfc3782-bis-01.txt 
Network Working Group T. Henderson Network Working Group T. Henderson
Internet-Draft Boeing Internet-Draft Boeing
Obsoletes: 3782 (if approved) S. Floyd Obsoletes: 3782 (if approved) S. Floyd
Intended status: Standards Track ICSI Intended status: Standards Track ICSI
Expires: June 24, 2011 A. Gurtov Expires: September 15, 2011 A. Gurtov
HIIT HIIT
Y. Nishida Y. Nishida
WIDE Project WIDE Project
January 24, 2011 March 14, 2011
The NewReno Modification to TCP's Fast Recovery Algorithm The NewReno Modification to TCP's Fast Recovery Algorithm
draft-ietf-tcpm-rfc3782-bis-00.txt draft-ietf-tcpm-rfc3782-bis-01.txt
Abstract Abstract
RFC 5681 [RFC5681] documents the following four intertwined TCP RFC 5681 [RFC5681] documents the following four intertwined TCP
congestion control algorithms: Slow Start, Congestion Avoidance, Fast congestion control algorithms: Slow Start, Congestion Avoidance, Fast
Retransmit, and Fast Recovery. RFC 5681 explicitly allows Retransmit, and Fast Recovery. RFC 5681 explicitly allows
certain modifications of these algorithms, including modifications certain modifications of these algorithms, including modifications
that use the TCP Selective Acknowledgement (SACK) option [RFC2883], that use the TCP Selective Acknowledgement (SACK) option [RFC2883],
and modifications that respond to "partial acknowledgments" (ACKs and modifications that respond to "partial acknowledgments" (ACKs
which cover new data, but not all the data outstanding when loss was which cover new data, but not all the data outstanding when loss was
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 24, 2011. This Internet-Draft will expire on September 15, 2011.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as Copyright (c) 2011 IETF Trust and the persons identified as
the document authors. All rights reserved. the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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not be created outside the IETF Standards Process, except to format not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other it for publication as an RFC or to translate it into languages other
than English. than English.
1. Introduction 1. Introduction
For the typical implementation of the TCP Fast Recovery algorithm For the typical implementation of the TCP Fast Recovery algorithm
described in [RFC5681] (first implemented in the 1990 BSD Reno described in [RFC5681] (first implemented in the 1990 BSD Reno
release, and referred to as the Reno algorithm in [FF96]), the TCP release, and referred to as the Reno algorithm in [FF96]), the TCP
data sender only retransmits a packet after a retransmit timeout has data sender only retransmits a packet after a retransmit timeout has
occurred, or after three duplicate acknowledgements have arrived occurred, or after three duplicate acknowledgments have arrived
triggering the Fast Retransmit algorithm. A single retransmit triggering the Fast Retransmit algorithm. A single retransmit
timeout might result in the retransmission of several data packets, timeout might result in the retransmission of several data packets,
but each invocation of the Fast Retransmit algorithm in RFC 5681 but each invocation of the Fast Retransmit algorithm in RFC 5681
leads to the retransmission of only a single data packet. leads to the retransmission of only a single data packet.
Problems can arise, therefore, when multiple packets are Two problems arise with Reno TCP when multiple packet losses occur
dropped from a single window of data and the Fast Retransmit and Fast in a single window. First, Reno will often take a timeout, as
Recovery algorithms are invoked. In this case, if the SACK option is has been documented in [Hoe95]. Second, even if a retransmission
available, the TCP sender has the information to make intelligent timeout is avoided, multiple fast retransmits and window reductions
decisions about which packets to retransmit and which packets not to can occur, as documented in [F94]. When multiple packet losses
retransmit during Fast Recovery. This document applies only for TCP occur, if the SACK option [RFC2883] is available, the TCP sender
connections that are unable to use the TCP Selective Acknowledgement has the information to make intelligent decisions about which packets
(SACK) option, either because the option is not locally supported or to retransmit and which packets not to retransmit during Fast
Recovery. This document applies to 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. because the TCP peer did not indicate a willingness to use SACK.
In the absence of SACK, there is little information available to the In the absence of SACK, there is little information available to the
TCP sender in making retransmission decisions during Fast TCP sender in making retransmission decisions during Fast
Recovery. From the three duplicate acknowledgements, the sender Recovery. From the three duplicate acknowledgments, the sender
infers a packet loss, and retransmits the indicated packet. After infers a packet loss, and retransmits the indicated packet. After
this, the data sender could receive additional duplicate this, the data sender could receive additional duplicate
acknowledgements, as the data receiver acknowledges additional data acknowledgments, as the data receiver acknowledges additional data
packets that were already in flight when the sender entered Fast packets that were already in flight when the sender entered Fast
Retransmit. Retransmit.
In the case of multiple packets dropped from a single window of data, In the case of multiple packets dropped from a single window of data,
the first new information available to the sender comes when the the first new information available to the sender comes when the
sender receives an acknowledgement for the retransmitted packet (that sender receives an acknowledgment for the retransmitted packet (that
is, the packet retransmitted when Fast Retransmit was first is, the packet retransmitted when Fast Retransmit was first
entered). If there is a single packet drop and no reordering, then the entered). If there is a single packet drop and no reordering, then the
acknowledgement for this packet will acknowledge all of the packets acknowledgment for this packet will acknowledge all of the packets
transmitted before Fast Retransmit was entered. However, if there transmitted before Fast Retransmit was entered. However, if there
are multiple packet drops, then the acknowledgement for the are multiple packet drops, then the acknowledgment for the
retransmitted packet will acknowledge some but not all of the packets retransmitted packet will acknowledge some but not all of the packets
transmitted before the Fast Retransmit. We call this acknowledgement transmitted before the Fast Retransmit. We call this acknowledgment
a partial acknowledgment. a partial acknowledgment.
Along with several other suggestions, [Hoe95] suggested that during Along with several other suggestions, [Hoe95] suggested that during
Fast Recovery the TCP data sender responds to a partial Fast Recovery the TCP data sender responds to a partial
acknowledgment by inferring that the next in-sequence packet has been acknowledgment by inferring that the next in-sequence packet has been
lost, and retransmitting that packet. This document describes a lost, and retransmitting that packet. This document describes a
modification to the Fast Recovery algorithm in RFC 5681 that modification to the Fast Recovery algorithm in RFC 5681 that
incorporates a response to partial acknowledgements received during incorporates a response to partial acknowledgments received during
Fast Recovery. We call this modified Fast Recovery algorithm Fast Recovery. We call this modified Fast Recovery algorithm
NewReno, because it is a slight but significant variation of the NewReno, because it is a slight but significant variation of the
basic Reno algorithm in RFC 5681. This document does not discuss the basic Reno algorithm in RFC 5681. This document does not discuss the
other suggestions in [Hoe95] and [Hoe96], such as a change to 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 ssthresh parameter during Slow-Start, or the proposal to send a new
packet for every two duplicate acknowledgements during Fast packet for every two duplicate acknowledgments during Fast
Recovery. The version of NewReno in this document also draws on other Recovery. The version of NewReno in this document also draws on other
discussions of NewReno in the literature [LM97, Hen98]. discussions of NewReno in the literature [LM97, Hen98].
We do not claim that the NewReno version of Fast Recovery described We do not claim that the NewReno version of Fast Recovery described
here is an optimal modification of Fast Recovery for responding to here is an optimal modification of Fast Recovery for responding to
partial acknowledgements, for TCP connections that are unable to use partial acknowledgments, for TCP connections that are unable to use
SACK. Based on our experiences with the NewReno modification in the SACK. Based on our experiences with the NewReno modification in the
NS simulator [NS] and with numerous implementations of NewReno, we NS simulator [NS] and with numerous implementations of NewReno, we
believe that this modification improves the performance of the Fast believe that this modification improves the performance of the Fast
Retransmit and Fast Recovery algorithms in a wide variety of Retransmit and Fast Recovery algorithms in a wide variety of
scenarios. scenarios.
2. Terminology and Definitions 2. Terminology and Definitions
In this document, the key words "MUST", "MUST NOT", "REQUIRED", In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
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SENDER MAXIMUM SEGMENT SIZE (SMSS), CONGESTION WINDOW (cwnd), and SENDER MAXIMUM SEGMENT SIZE (SMSS), CONGESTION WINDOW (cwnd), and
FLIGHT SIZE (FlightSize) defined in [RFC5681]. FLIGHT SIZE is FLIGHT SIZE (FlightSize) defined in [RFC5681]. FLIGHT SIZE is
defined as in [RFC5681] as follows: defined as in [RFC5681] as follows:
FLIGHT SIZE: FLIGHT SIZE:
The amount of data that has been sent but not yet cumulatively The amount of data that has been sent but not yet cumulatively
acknowledged. acknowledged.
3. The Fast Retransmit and Fast Recovery Algorithms in NewReno 3. The Fast Retransmit and Fast Recovery Algorithms in NewReno
The basic idea of these extensions to the Fast Retransmit and
Fast Recovery algorithms described in [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 standard implementation of the Fast Retransmit and Fast Recovery The standard implementation of the Fast Retransmit and Fast Recovery
algorithms is given in [RFC5681]. This section specifies the basic algorithms is given in [RFC5681]. This section specifies the basic
NewReno algorithm. Sections 4 through 6 describe some optional NewReno algorithm. Section 4 describes heuristics for processing
variants, and the motivations behind them, that an implementor may duplicate acknowledgments after a retransmission timeout. Sections
want to consider when tuning performance for certain network 5 and 6 provide some guidance to implementors based on experience
scenarios. Sections 7 and 8 provide some guidance to implementors with NewReno implementations. Several appendices provide more
based on experience with NewReno implementations. background information and describe variations that an implementor
may want to consider when tuning performance for certain network
scenarios.
The NewReno modification concerns the Fast Recovery procedure that The NewReno modification applies to the Fast Recovery procedure that
begins when three duplicate ACKs are received and ends when either a begins when three duplicate ACKs are received and ends when either a
retransmission timeout occurs or an ACK arrives that acknowledges all retransmission timeout occurs or an ACK arrives that acknowledges all
of the data up to and including the data that was outstanding when of the data up to and including the data that was outstanding when
the Fast Recovery procedure began. the Fast Recovery procedure began.
The NewReno algorithm specified in this document differs from the The NewReno algorithm specified in this document extends the
implementation in [RFC5681] in the introduction of the variable implementation in [RFC5681] by introducing a variable specified as
"recover" in step 1, in the response to a partial or new "recover" whose initial value is the initial send sequence number.
acknowledgement in step 5, and in modifications to step 1 and the This new variable is used by the sender to record the send sequence
number that must be acknowledged before the Fast Recovery
procedure is declared to be over. This variable is used below
in step 1, in the response to a partial or new
acknowledgment in step 5, and in modifications to step 1 and the
addition of step 6 for avoiding multiple Fast Retransmits caused by addition of step 6 for avoiding multiple Fast Retransmits caused by
the retransmission of packets already received by the receiver. 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: 1) Three duplicate ACKs:
When the third duplicate ACK is received and the sender is not When the third duplicate ACK is received and the sender is not
already in the Fast Recovery procedure, check to see if the already in the Fast Recovery procedure, check to see if the
Cumulative Acknowledgement field covers more than Cumulative Acknowledgment field covers more than
"recover". If so, go to Step 1A. Otherwise, go to Step 1B. "recover". If so, go to Step 1A. Otherwise, go to Step 1B.
1A) Invoking Fast Retransmit: 1A) Invoking Fast Retransmit:
If so, then set ssthresh to no more than the value given in If so, then set ssthresh to no more than the value given in
equation 1 below. (This is equation 4 from [RFC5681]). equation 1 below. (This is equation 4 from [RFC5681]).
ssthresh = max (FlightSize / 2, 2*SMSS) (1) ssthresh = max (FlightSize / 2, 2*SMSS) (1)
In addition, record the highest sequence number transmitted in In addition, record the highest sequence number transmitted in
the variable "recover", and go to Step 2. the variable "recover", and go to Step 2.
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that has left the network. that has left the network.
4) Fast Recovery, continued: 4) Fast Recovery, continued:
Transmit a segment, if allowed by the new value of cwnd and the Transmit a segment, if allowed by the new value of cwnd and the
receiver's advertised window. receiver's advertised window.
5) When an ACK arrives that acknowledges new data, this ACK could be 5) When an ACK arrives that acknowledges new data, this ACK could be
the acknowledgment elicited by the retransmission from step 2, or the acknowledgment elicited by the retransmission from step 2, or
elicited by a later retransmission. elicited by a later retransmission.
Full acknowledgements: Full acknowledgments:
If this ACK acknowledges all of the data up to and including If this ACK acknowledges all of the data up to and including
"recover", then the ACK acknowledges all the intermediate "recover", then the ACK acknowledges all the intermediate
segments sent between the original transmission of the lost segments sent between the original transmission of the lost
segment and the receipt of the third duplicate ACK. Set cwnd to segment and the receipt of the third duplicate ACK. Set cwnd to
either (1) min (ssthresh, max(FlightSize, SMSS) + SMSS) or either (1) min (ssthresh, max(FlightSize, SMSS) + SMSS) or
(2) ssthresh, where ssthresh is the value set in step 1; this is (2) ssthresh, where ssthresh is the value set in step 1; this is
termed "deflating" the window. (We note that "FlightSize" in step 1 termed "deflating" the window. (We note that "FlightSize" in step 1
referred to the amount of data outstanding in step 1, when Fast referred to the amount of data outstanding in step 1, when Fast
Recovery was entered, while "FlightSize" in step 5 refers to the Recovery was entered, while "FlightSize" in step 5 refers to the
amount of data outstanding in step 5, when Fast Recovery is amount of data outstanding in step 5, when Fast Recovery is
exited.) If the second option is selected, the implementation exited.) If the second option is selected, the implementation
is encouraged to take measures to avoid a possible burst of is encouraged to take measures to avoid a possible burst of
data, in case the amount of data outstanding in the network is data, in case the amount of data outstanding in the network is
much less than the new congestion window allows. A simple mechanism much less than the new congestion window allows. A simple mechanism
is to limit the number of data packets that can be is to limit the number of data packets that can be sent in response
sent in response to a single acknowledgement; this is known to a single acknowledgment. Exit the Fast Recovery procedure.
as "maxburst_" in the NS simulator. Exit the Fast Recovery
procedure.
Partial acknowledgements: Partial acknowledgments:
If this ACK does *not* acknowledge all of the data up to and If this ACK does *not* acknowledge all of the data up to and
including "recover", then this is a partial ACK. In this case, including "recover", then this is a partial ACK. In this case,
retransmit the first unacknowledged segment. Deflate the retransmit the first unacknowledged segment. Deflate the
congestion window by the amount of new data acknowledged by the congestion window by the amount of new data acknowledged by the
cumulative acknowledgement field. If the partial ACK cumulative acknowledgment field. If the partial ACK
acknowledges at least one SMSS of new data, then add back SMSS acknowledges at least one SMSS of new data, then add back SMSS
bytes to the congestion window. As in Step 3, this artificially bytes to the congestion window. As in Step 3, this artificially
inflates the congestion window in order to reflect the additional inflates the congestion window in order to reflect the additional
segment that has left the network. Send a new segment if segment that has left the network. Send a new segment if
permitted by the new value of cwnd. This "partial window permitted by the new value of cwnd. This "partial window
deflation" attempts to ensure that, when Fast Recovery eventually deflation" attempts to ensure that, when Fast Recovery eventually
ends, approximately ssthresh amount of data will be outstanding ends, approximately ssthresh amount of data will be outstanding
in the network. Do not exit the Fast Recovery procedure (i.e., in the network. Do not exit the Fast Recovery procedure (i.e.,
if any duplicate ACKs subsequently arrive, execute Steps 3 and if any duplicate ACKs subsequently arrive, execute Steps 3 and
4 above). 4 above).
For the first partial ACK that arrives during Fast Recovery, also For the first partial ACK that arrives during Fast Recovery, also
reset the retransmit timer. Timer management is discussed in reset the retransmit timer. Timer management is discussed in
more detail in Section 4. more detail in Section 4.
6) Retransmit timeouts: 6) Retransmit timeouts:
After a retransmit timeout, record the highest sequence number After a retransmit timeout, record the highest sequence number
transmitted in the variable "recover" and exit the Fast transmitted in the variable "recover" and exit the Fast
Recovery procedure if applicable. Recovery procedure if applicable.
Step 1 specifies a check that the Cumulative Acknowledgement field Step 1 specifies a check that the Cumulative Acknowledgment field
covers more than "recover". Because the acknowledgement field covers more than "recover". Because the acknowledgment field
contains the sequence number that the sender next expects to receive, contains the sequence number that the sender next expects to receive,
the acknowledgement "ack_number" covers more than "recover" when: the acknowledgment "ack_number" covers more than "recover" when:
ack_number - 1 > recover; ack_number - 1 > recover;
i.e., at least one byte more of data is acknowledged beyond the i.e., at least one byte more of data is acknowledged beyond the
highest byte that was outstanding when Fast Retransmit was last highest byte that was outstanding when Fast Retransmit was last
entered. entered.
Note that in Step 5, the congestion window is deflated after a Note that in Step 5, the congestion window is deflated after a
partial acknowledgement is received. The congestion window was partial acknowledgment is received. The congestion window was
likely to have been inflated considerably when the partial likely to have been inflated considerably when the partial
acknowledgement was received. In addition, depending on the original acknowledgment was received. In addition, depending on the original
pattern of packet losses, the partial acknowledgement might pattern of packet losses, the partial acknowledgment might
acknowledge nearly a window of data. In this case, if the congestion 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 window was not deflated, the data sender might be able to send nearly
a window of data back-to-back. a window of data back-to-back.
This document does not specify the sender's response to duplicate This document does not specify the sender's response to duplicate
ACKs when the Fast Retransmit/Fast Recovery algorithm is not ACKs when the Fast Retransmit/Fast Recovery algorithm is not
invoked. This is addressed in other documents, such as those invoked. This is addressed in other documents, such as those
describing the Limited Transmit procedure [RFC3042]. This document describing the Limited Transmit procedure [RFC3042]. This document
also does not address issues of adjusting the duplicate acknowledgement also does not address issues of adjusting the duplicate acknowledgment
threshold, but assumes the threshold specified in the IETF standards; threshold, but assumes the threshold specified in the IETF standards;
the current standard is RFC 5681, which specifies a threshold of three the current standard is RFC 5681, which specifies a threshold of three
duplicate acknowledgements. duplicate acknowledgments.
As a final note, we would observe that in the absence of the SACK As a final note, we would observe that in the absence of the SACK
option, the data sender is working from limited information. When option, the data sender is working from limited information. When
the issue of recovery from multiple dropped packets from a single the issue of recovery from multiple dropped packets from a single
window of data is of particular importance, the best alternative window of data is of particular importance, the best alternative
would be to use the SACK option. would be to use the SACK option.
4. Resetting the Retransmit Timer in Response to Partial 4. Handling Duplicate Acknowledgments After A Timeout
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 (step 5.1 of
[RFC2988]).
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 when 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
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, adding
back SMSS bytes if the partial ACK acknowledges at least SMSS bytes
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
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; 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 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 [RFC2582] 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.
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 After each retransmit timeout, the highest sequence number
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". transmitted so far is recorded in the variable "recover".
If, after a retransmit timeout, the TCP data sender retransmits three If, after a retransmit timeout, the TCP data sender retransmits three
consecutive packets that have already been received by the data consecutive packets that have already been received by the data
receiver, then the TCP data sender will receive three duplicate receiver, then the TCP data sender will receive three duplicate
acknowledgements that do not cover more than "recover". In this acknowledgments that do not cover more than "recover". In this
case, the duplicate acknowledgements are not an indication of a new case, the duplicate acknowledgments are not an indication of a new
instance of congestion. They are simply an indication that the instance of congestion. They are simply an indication that the
sender has unnecessarily retransmitted at least three packets. sender has unnecessarily retransmitted at least three packets.
However, when a retransmitted packet is itself dropped, the sender However, when a retransmitted packet is itself dropped, the sender
can also receive three duplicate acknowledgements that do not cover can also receive three duplicate acknowledgments that do not cover
more than "recover". In this case, the sender would have been more than "recover". In this case, the sender would have been
better off if it had initiated Fast Retransmit. For a TCP that better off if it had initiated Fast Retransmit. For a TCP that
implements the algorithm specified in Section 3 of this document, the implements the algorithm specified in Section 3 of this document, the
sender does not infer a packet drop from duplicate acknowledgements sender does not infer a packet drop from duplicate acknowledgments
in this scenario. As always, the retransmit timer is the backup in this scenario. As always, the retransmit timer is the backup
mechanism for inferring packet loss in this case. mechanism for inferring packet loss in this case.
There are several heuristics, based on timestamps or on the amount of There are several heuristics, based on timestamps or on the amount of
advancement of the cumulative acknowledgement field, that allow the advancement of the cumulative acknowledgment field, that allow the
sender to distinguish, in some cases, between three duplicate sender to distinguish, in some cases, between three duplicate
acknowledgements following a retransmitted packet that was dropped, acknowledgments following a retransmitted packet that was dropped,
and three duplicate acknowledgements from the unnecessary and three duplicate acknowledgments from the unnecessary
retransmission of three packets [Gur03, GF04]. The TCP sender MAY use retransmission of three packets [Gur03, GF04]. The TCP sender MAY use
such a heuristic to decide to invoke a Fast Retransmit in some cases, such a heuristic to decide to invoke a Fast Retransmit in some cases,
even when the three duplicate acknowledgements do not cover more than even when the three duplicate acknowledgments do not cover more than
"recover". "recover".
For example, when three duplicate acknowledgements are caused by the For example, when three duplicate acknowledgments are caused by the
unnecessary retransmission of three packets, this is likely to be unnecessary retransmission of three packets, this is likely to be
accompanied by the cumulative acknowledgement field advancing by at accompanied by the cumulative acknowledgment field advancing by at
least four segments. Similarly, a heuristic based on timestamps uses least four segments. Similarly, a heuristic based on timestamps uses
the fact that when there is a hole in the sequence space, the the fact that when there is a hole in the sequence space, the
timestamp echoed in the duplicate acknowledgement is the timestamp of timestamp echoed in the duplicate acknowledgment is the timestamp of
the most recent data packet that advanced the cumulative the most recent data packet that advanced the cumulative
acknowledgement field [RFC1323]. If timestamps are used, and the acknowledgment field [RFC1323]. If timestamps are used, and the
sender stores the timestamp of the last acknowledged segment, then sender stores the timestamp of the last acknowledged segment, then
the timestamp echoed by duplicate acknowledgements can be used to the timestamp echoed by duplicate acknowledgments can be used to
distinguish between a retransmitted packet that was dropped and distinguish between a retransmitted packet that was dropped and
three duplicate acknowledgements from the unnecessary three duplicate acknowledgments from the unnecessary
retransmission of three packets. The heuristics are illustrated in retransmission of three packets.
the NS simulator in the validation test "./test-all-newreno".
6.1. ACK Heuristic 4.1. ACK Heuristic
If the ACK-based heuristic is used, then following the advancement of If the ACK-based heuristic is used, then following the advancement of
the cumulative acknowledgement field, the sender stores the value of the cumulative acknowledgment field, the sender stores the value of
the previous cumulative acknowledgement as prev_highest_ack, and stores the previous cumulative acknowledgment as prev_highest_ack, and stores
the latest cumulative ACK as highest_ack. In addition, the following 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 is performed if Step 1 in Section 3 fails, before proceeding to
Step 1B. Step 1B.
1*) If the Cumulative Acknowledgement field didn't cover more than 1*) If the Cumulative Acknowledgment field didn't cover more than
"recover", check to see if the congestion window is greater "recover", check to see if the congestion window is greater
than SMSS bytes and the difference between highest_ack and than SMSS bytes and the difference between highest_ack and
prev_highest_ack is at most 4*SMSS bytes. If true, duplicate prev_highest_ack is at most 4*SMSS bytes. If true, duplicate
ACKs indicate a lost segment (proceed to Step 1A in Section ACKs indicate a lost segment (proceed to Step 1A in Section
3). Otherwise, duplicate ACKs likely result from unnecessary 3). Otherwise, duplicate ACKs likely result from unnecessary
retransmissions (proceed to Step 1B in Section 3). retransmissions (proceed to Step 1B in Section 3).
The congestion window check serves to protect against fast retransmit The congestion window check serves to protect against fast retransmit
immediately after a retransmit timeout, similar to the immediately after a retransmit timeout.
"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 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 ACK of more than three segments, and the heuristic can fail.
validation test for this scenario is "./test-all-newreno RFC 5681 recommends that a receiver should
newreno_rto_loss_ackf". RFC 5681 recommends that a receiver should
send duplicate ACKs for every out-of-order data packet, such as a send duplicate ACKs for every out-of-order data packet, such as a
data packet received during Fast Recovery. The ACK heuristic is more data packet received during Fast Recovery. The ACK heuristic is more
likely to fail if the receiver does not follow this advice, because likely to fail if the receiver does not follow this advice, because
then a smaller number of ACK losses are needed to produce a then a smaller number of ACK losses are needed to produce a
sufficient jump in the cumulative ACK. sufficient jump in the cumulative ACK.
6.2. Timestamp Heuristic 4.2. Timestamp Heuristic
If this heuristic is used, the sender stores the timestamp of the If this heuristic is used, the sender stores the timestamp of the
last acknowledged segment. In addition, the second paragraph of step last acknowledged segment. In addition, the second paragraph of step
1 in Section 3 is replaced as follows: 1 in Section 3 is replaced as follows:
1**) If the Cumulative Acknowledgement field didn't cover more than 1**) If the Cumulative Acknowledgment field didn't cover more than
"recover", check to see if the echoed timestamp in the last "recover", check to see if the echoed timestamp in the last
non-duplicate acknowledgment equals the non-duplicate acknowledgment equals the
stored timestamp. If true, duplicate ACKs indicate a lost stored timestamp. If true, duplicate ACKs indicate a lost
segment (proceed to Step 1A in Section 3). Otherwise, duplicate segment (proceed to Step 1A in Section 3). Otherwise, duplicate
ACKs likely result from unnecessary retransmissions (proceed ACKs likely result from unnecessary retransmissions (proceed
to Step 1B in Section 3). to Step 1B in Section 3).
Examples of applying the timestamp heuristic are in validation tests The timestamp heuristic works correctly, both when the receiver echoes
"./test-all-newreno newreno_rto_loss_tsh" and "./test-all-newreno timestamps as specified by [RFC1323], and by its revision attempts.
newreno_rto_dup_tsh". The timestamp heuristic works correctly, both However, if the receiver arbitrarily echoes timestamps, the heuristic
when the receiver echoes timestamps as specified by [RFC1323], and by can fail. The heuristic can also fail if a timeout was spurious and
its revision attempts. However, if the receiver arbitrarily echoes returning ACKs are not from retransmitted segments. This can be
timestamps, the heuristic can fail. The heuristic can also fail if a prevented by detection algorithms such as [RFC3522].
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 5. Implementation Issues for the Data Receiver
[RFC5681] specifies that "Out-of-order data segments SHOULD be [RFC5681] specifies that "Out-of-order data segments SHOULD be
acknowledged immediately, in order to accelerate loss recovery." acknowledged immediately, in order to accelerate loss recovery."
Neal Cardwell has noted that some data receivers do not send an Neal Cardwell has noted that some data receivers do not send an
immediate acknowledgement when they send a partial acknowledgment, immediate acknowledgment when they send a partial acknowledgment,
but instead wait first for their delayed acknowledgement timer to but instead wait first for their delayed acknowledgment timer to
expire [C98]. As [C98] notes, this severely limits the potential expire [C98]. As [C98] notes, this severely limits the potential
benefit of NewReno by delaying the receipt of the partial benefit of NewReno by delaying the receipt of the partial
acknowledgement at the data sender. Echoing RFC 5681, our acknowledgment at the data sender. Echoing RFC 5681, our
recommendation is that the data receiver send an immediate recommendation is that the data receiver send an immediate
acknowledgement for an out-of-order segment, even when that acknowledgment for an out-of-order segment, even when that
out-of-order segment fills a hole in the buffer. out-of-order segment fills a hole in the buffer.
8. Implementation Issues for the Data Sender 6. Implementation Issues for the Data Sender
In Section 3, Step 5 above, it is noted that implementations should In Section 3, Step 5 above, it is noted that implementations should
take measures to avoid a possible burst of data when leaving Fast 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 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 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 can arise during NewReno when ACKs are lost or treated as pure window
updates, thereby causing the sender to underestimate the number of updates, thereby causing the sender to underestimate the number of
new segments that can be sent during the recovery procedure. new segments that can be sent during the recovery procedure.
Specifically, bursts can occur when the FlightSize is much less than Specifically, bursts can occur when the FlightSize is much less than
the new congestion window when exiting from Fast Recovery. One the new congestion window when exiting from Fast Recovery. One
skipping to change at page 15, line 7 skipping to change at page 12, line 5
to FlightSize. A recommendation on the general mechanism to avoid to FlightSize. A recommendation on the general mechanism to avoid
excessively bursty sending patterns is outside the scope of this excessively bursty sending patterns is outside the scope of this
document. document.
An implementation may want to use a separate flag to record whether 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 or not it is presently in the Fast Recovery procedure. The use of
the value of the duplicate acknowledgment counter for this purpose is the value of the duplicate acknowledgment counter for this purpose is
not reliable because it can be reset upon window updates and not reliable because it can be reset upon window updates and
out-of-order acknowledgments. out-of-order acknowledgments.
When updating the Cumulative Acknowledgement field outside of When updating the Cumulative Acknowledgment field outside of
Fast Recovery, the "recover" state variable may also need to be Fast Recovery, the "recover" state variable may also need to be
updated in order to continue to permit possible entry into Fast updated in order to continue to permit possible entry into Fast
Recovery (Section 3, step 1). This issue arises when an update Recovery (Section 3, step 1). This issue arises when an update
of the Cumulative Acknowledgement field results in a sequence of the Cumulative Acknowledgment field results in a sequence
wraparound that affects the ordering between the Cumulative wraparound that affects the ordering between the Cumulative
Acknowledgement field and the "recover" state variable. Entry Acknowledgment field and the "recover" state variable. Entry
into Fast Recovery is only possible when the Cumulative into Fast Recovery is only possible when the Cumulative
Acknowledgment field covers more than the "recover" state variable. Acknowledgment field covers more than the "recover" state variable.
It is important for the sender to respond correctly to duplicate ACKs 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 received when the sender is no longer in Fast Recovery (e.g., because
of a Retransmit Timeout). The Limited Transmit procedure [RFC3042] of a Retransmit Timeout). The Limited Transmit procedure [RFC3042]
describes possible responses to the first and second duplicate describes possible responses to the first and second duplicate
acknowledgements. When three or more duplicate acknowledgements are acknowledgments. When three or more duplicate acknowledgments are
received, the Cumulative Acknowledgement field doesn't cover more received, the Cumulative Acknowledgment field doesn't cover more
than "recover", and a new Fast Recovery is not invoked, it is than "recover", and a new Fast Recovery is not invoked, it is
important that the sender not execute the Fast Recovery steps (3) and 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 (4) in Section 3. Otherwise, the sender could end up in a chain of
spurious timeouts. We mention this only because several NewReno spurious timeouts. We mention this only because several NewReno
implementations had this bug, including the implementation in the NS implementations had this bug, including the implementation in the NS
simulator. (This bug in the NS simulator was fixed in July 2003, simulator.
with the variable "exitFastRetrans_".)
It has been observed that some TCP implementations enter a slow start It has been observed that some TCP implementations enter a slow start
or congestion avoidance window updating algorithm immediately after or congestion avoidance window updating algorithm immediately after
the cwnd is set by the equation found in (Section 3, step 5), even the cwnd is set by the equation found in (Section 3, step 5), even
without a new external event generating the cwnd change. Note that without a new external event generating the cwnd change. Note that
after cwnd is set based on the procedure for exiting Fast Recovery after cwnd is set based on the procedure for exiting Fast Recovery
(Section 3, step 5), cwnd SHOULD NOT be updated until a further (Section 3, step 5), cwnd SHOULD NOT be updated until a further
event occurs (e.g., arrival of an ack, or timeout) after this event occurs (e.g., arrival of an ack, or timeout) after this
adjustment. adjustment.
9. Simulations 7. Security Considerations
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
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 so 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 Standards Track.
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.
The algorithm specified in Section 3 checks whether the
acknowledgement field of a partial acknowledgement covers *more* than
"recover", as defined in Section 3. Another possible variant would be
to simply require that the acknowledgement field covers *more than or
equal to* "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 is
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. Changes Relative to RFC 3782
In [RFC3782], the cwnd after Full ACK reception will be set to
(1) min (ssthresh, FlightSize + SMSS) or (2) ssthresh. However,
there is a risk in the first logic which results in performance
degradation. With the first logic, if FlightSize is zero, the result
will be 1 SMSS. This means TCP can transmit only 1 segment at this
moment, which can cause delay in ACK transmission at receiver due to
delayed ACK algorithm.
The FlightSize on Full ACK reception can be zero in some situations. RFC 5681 discusses general security considerations concerning TCP
A typical example is where sending window size during fast recovery is congestion control. This document describes a specific algorithm
small. In this case, the retransmitted packet and new data packets can that conforms with the congestion control requirements of RFC 5681,
be transmitted within a short interval. If all these packets and so those considerations apply to this algorithm, too. There are
successfully arrive, the receiver may generate a Full ACK that no known additional security concerns for this specific algorithm.
acknowledges all outstanding data. Even if window size is not small,
loss of ACK packets or receive buffer shortage during fast recovery can
also increase the possibility to fall into this situation.
The proposed fix in this document ensures that sender TCP transmits at 8. IANA Considerations
least two segments on Full ACK reception.
In addition, errata for RFC3782 (editorial clarification to Section 8) This document has no actions for IANA.
has been applied.
13. Conclusions 9. Conclusions
This document specifies the NewReno Fast Retransmit and Fast Recovery This document specifies the NewReno Fast Retransmit and Fast Recovery
algorithms for TCP. This NewReno modification to TCP can even be algorithms for TCP. This NewReno modification to TCP can even be
important for TCP implementations that support the SACK option, important for TCP implementations that support the SACK option,
because the SACK option can only be used for TCP connections when because the SACK option can only be used for TCP connections when
both TCP end-nodes support the SACK option. NewReno performs better both TCP end-nodes support the SACK option. NewReno performs better
than Reno (RFC 5681) in a number of scenarios discussed herein. than Reno (RFC 5681) in a number of scenarios discussed herein.
A number of options to the basic algorithm presented in Section 3 are A number of options to the basic algorithm presented in Section 3 are
also described. These include the handling of the retransmission also described in appendices to this document. These include the
timer (Section 4), the response to partial acknowledgments (Section handling of the retransmission timer (Appendix A), the response to
5), and the value of the congestion window when leaving Fast Recovery partial acknowledgments (Appendix B), and whether or not the sender
(section 3, step 5). Our belief is that the differences between maintains a state variable called "recover" (Appendix C).
these variants of NewReno are small compared to the differences Our belief is that the differences between these variants of NewReno
between Reno and NewReno. That is, the important thing is to are small compared to the differences between Reno and NewReno.
implement NewReno instead of Reno, for a TCP connection without SACK; That is, the important thing is to implement NewReno instead of Reno,
it is less important exactly which of the variants of NewReno is for a TCP connection without SACK; it is less important exactly
implemented. which of the variants of NewReno is implemented.
14. Security Considerations
RFC 5681 discusses general security considerations concerning TCP
congestion control. This document describes a specific algorithm
that conforms with the congestion control requirements of RFC 5681,
and so those considerations apply to this algorithm, too. There are
no known additional security concerns for this specific algorithm.
15. IANA Considerations
This document has no actions for IANA.
16. Acknowledgements 10. Acknowledgments
Many thanks to Anil Agarwal, Mark Allman, Armando Caro, Jeffrey Hsu, Many thanks to Anil Agarwal, Mark Allman, Armando Caro, Jeffrey Hsu,
Vern Paxson, Kacheong Poon, Keyur Shah, and Bernie Volz for detailed Vern Paxson, Kacheong Poon, Keyur Shah, and Bernie Volz for detailed
feedback on this document or on its precursor, RFC 2582. Jeffrey feedback on this document or on its precursor, RFC 2582. Jeffrey
Hsu provided clarifications on the handling of the recover variable Hsu provided clarifications on the handling of the recover variable
that were applied to RFC 3782 as errata, and now are in Section 8 that were applied to RFC 3782 as errata, and now are in Section 8
of this document. Yoshifumi Nishida contributed a modification of this document. Yoshifumi Nishida contributed a modification
to the fast recovery algorithm to account for the case in which to the fast recovery algorithm to account for the case in which
flightsize is 0 when the TCP sender leaves fast recovery, and the flightsize is 0 when the TCP sender leaves fast recovery, and the
TCP receiver uses delayed acknowledgments. TCP receiver uses delayed acknowledgments. Alexander Zimmermann
provided several suggestions to improve the clarity of the document.
17. References 11. References
17.1. Normative References 11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000. Timer", RFC 2988, November 2000.
[RFC5681] Allman, M., Paxson, V. and E. Blanton, "TCP Congestion [RFC5681] Allman, M., Paxson, V. and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009. Control", RFC 5681, September 2009.
17.2. Informative References 11.2. Informative References
[C98] Cardwell, N., "delayed ACKs for retransmitted packets: ouch!". [C98] Cardwell, N., "delayed ACKs for retransmitted packets: ouch!".
November 1998, Email to the tcpimpl mailing list, Message-ID November 1998, Email to the tcpimpl mailing list, Message-ID
"Pine.LNX.4.02A.9811021421340.26785-100000@sake.cs.washington.edu", "Pine.LNX.4.02A.9811021421340.26785-100000@sake.cs.washington.edu",
archived at "http://tcp-impl.lerc.nasa.gov/tcp-impl". archived at "http://tcp-impl.lerc.nasa.gov/tcp-impl".
[F98] Floyd, S., Revisions to RFC 2001, "Presentation to the TCPIMPL [F98] Floyd, S., Revisions to RFC 2001, "Presentation to the TCPIMPL
Working Group", August 1998. URLs 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.ps" and
"ftp://ftp.ee.lbl.gov/talks/sf-tcpimpl-aug98.pdf". "ftp://ftp.ee.lbl.gov/talks/sf-tcpimpl-aug98.pdf".
skipping to change at page 21, line 8 skipping to change at page 15, line 24
[RFC3042] Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing TCP's [RFC3042] Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing TCP's
Loss Recovery Using Limited Transmit", RFC 3042, January 2001. Loss Recovery Using Limited Transmit", RFC 3042, January 2001.
[RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for [RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for
TCP", RFC 3522, April 2003. TCP", RFC 3522, April 2003.
[RFC3782] Floyd, S., T. Henderson, and A. Gurtov, "The NewReno [RFC3782] Floyd, S., T. Henderson, and A. Gurtov, "The NewReno
Modification to TCP's Fast Recovery Algorithm", RFC 3782, April 2004. Modification to TCP's Fast Recovery Algorithm", RFC 3782, April 2004.
Appendix A. Document Revision History Appendix A. Resetting the Retransmit Timer in Response to Partial
Acknowledgments
One possible variant to the response to partial acknowledgments
specified in Section 3 concerns when to reset the retransmit timer
after a partial acknowledgment. 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 (step 5.1 of
[RFC2988]).
In contrast, the NewReno simulations in [FF96] illustrate the
algorithm described above with the modification that the retransmit
timer is reset after each partial acknowledgment. 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 Impatient variant can be particularly important for TCP
connections with large congestion windows.
One can also construct scenarios where the Slow-but-Steady variant
gives better performance than the Impatient variant. As an example,
this occurs when 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 Slow-but-Steady variant can also achieve higher goodput than the
Impatient variant, by avoiding unnecessary retransmissions. This
could be of special interest for cellular links, where every
transmission costs battery power and money. 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
acknowledgment, 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.
Appendix B. Retransmissions after a Partial Acknowledgment
One possible variant to the response to partial acknowledgments
specified in Section 3 would be to retransmit more than one packet
after each partial acknowledgment, and to reset the retransmit timer
after each retransmission. The algorithm specified in Section 3
retransmits a single packet after each partial acknowledgment. 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
acknowledgment. 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 acknowledgments 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 acknowledgment. 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, adding
back SMSS bytes if the partial ACK acknowledges at least SMSS bytes
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 acknowledgment, but additional new packets
might be transmitted as well, depending on the amount of new data
acknowledged by the partial acknowledgment. In contrast, the
variant of NewReno illustrated in [FF96] simply set the congestion
window to ssthresh when a partial acknowledgment 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 acknowledgment 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.
Appendix C. Avoiding Multiple Fast Retransmits
This appendix describes the motivation for the sender's state
variable "recover".
In the absence of the SACK option or timestamps, a duplicate
acknowledgment carries no information to identify the data packet or
packets at the TCP data receiver that triggered that duplicate
acknowledgment. In this case, the TCP data sender is unable to
distinguish between a duplicate acknowledgment that results from a
lost or delayed data packet, and a duplicate acknowledgment 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 [RFC2582] 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.
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.
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".
Appendix D. Simulations
This section provides pointers to simulation scripts available in
the NS simulator that reproduce behavior described above.
In Section 3, a simple mechanism is described 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.
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
acknowledgment. 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.
Regarding the handling of duplicate acknowledgments after a timeout,
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 (Section 4) 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".
Examples of applying the timestamp heuristic (Section 4) are in
validation tests "./test-all-newreno newreno_rto_loss_tsh" and
"./test-all-newreno newreno_rto_dup_tsh".
Section 6 described a problem involving possible spurious timeouts,
and mentions that this bug existed in the NS simulator.
This bug in the NS simulator was fixed in July 2003,
with the variable "exitFastRetrans_".
Regarding the Slow-but-Steady and Impatient variants described
in Appendix A, The tests "ns
test-suite-newreno.tcl impatient1" and "ns test-suite-newreno.tcl
slow1" in the NS simulator illustrate a scenario in which 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. The tests
"ns test-suite-newreno.tcl impatient2" and
"ns test-suite-newreno.tcl slow2" in the NS simulator illustrate
scenarios in which the Slow-but-Steady variant outperforms the Impatient
variant. The tests "ns test-suite-newreno.tcl impatient3" and
"ns test-suite-newreno.tcl slow3" in the NS simulator illustrate
scenarios in which the Slow-but-Steady variants avoid unnecessary
retransmissions.
Appendix B describes different policies for partial window deflation.
The [FF96] behavior can be seen in the NS
simulator by setting the variable "partial_window_deflation_" for
"Agent/TCP/Newreno" to 0; the behavior specified in Section 3 is
achieved by setting "partial_window_deflation_" to 1.
Section 3 of [RFC2582] 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.
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
An NS validation test "./test-all-newreno newreno5_noBF" in
directory "tcl/test" of the NS simulator illustartes the default
variant of NewReno TCP that doesn't use the variable "recover";
this gives performance similar to that on page 8 of [F03].
Appendix E. 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
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 so that the TCP sender receives three duplicate
acknowledgments. 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].
Appendix F. 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 Standards Track.
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.
The algorithm specified in Section 3 checks whether the
acknowledgment field of a partial acknowledgment covers *more* than
"recover", as defined in Section 3. Another possible variant would be
to simply require that the acknowledgment field covers *more than or
equal to* "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 acknowledgments 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 acknowledgments
acknowledging "recover". The second scenario would be that the
sender unnecessarily retransmitted three packets below "recover", and
that these three packets triggered three duplicate acknowledgments
acknowledging "recover". In the absence of SACK, the TCP sender is
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
acknowledgments 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.
Appendix G. Changes Relative to RFC 3782
In [RFC3782], the cwnd after Full ACK reception will be set to
(1) min (ssthresh, FlightSize + SMSS) or (2) ssthresh. However,
there is a risk in the first logic which results in performance
degradation. With the first logic, if FlightSize is zero, the result
will be 1 SMSS. This means TCP can transmit only 1 segment at this
moment, which can cause delay in ACK transmission at receiver due to
delayed ACK algorithm.
The FlightSize on Full ACK reception can be zero in some situations.
A typical example is where 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 window size is not small,
loss of ACK packets or receive buffer shortage during fast recovery can
also increase the possibility to fall into this situation.
The proposed fix in this document ensures that sender TCP transmits at
least two segments on Full ACK reception.
In addition, errata for RFC3782 (editorial clarification to Section 8
of RFC2582, which is now Section 6 of this document) has been applied.
Sections 4, 5, and 9-11 of RFC2582 were relocated to appendices of
this document since they are non-normative and provide background
information and references to simulation results.
Appendix H. Document Revision History
To be removed upon publication To be removed upon publication
+----------+--------------------------------------------------+ +----------+--------------------------------------------------+
| Revision | Comments | | Revision | Comments |
+----------+--------------------------------------------------+ +----------+--------------------------------------------------+
| draft-00 | RFC3782 errata applied, and changes applied from | | draft-00 | RFC3782 errata applied, and changes applied from |
| | draft-nishida-newreno-modification-02 | | | draft-nishida-newreno-modification-02 |
+----------+--------------------------------------------------+ +----------+--------------------------------------------------+
| draft-01 | Non-normative sections moved to appendices, |
| | editorial clarifications applied as suggested |
| | by Alexander Zimmermann. |
+----------+--------------------------------------------------+
Authors' Addresses Authors' Addresses
Thomas R. Henderson Tom Henderson
The Boeing Company The Boeing Company
P.O. Box 3707
Seattle, WA 98124
EMail: thomas.r.henderson@boeing.com EMail: thomas.r.henderson@boeing.com
Sally Floyd Sally Floyd
International Computer Science Institute International Computer Science Institute
Phone: +1 (510) 666-2989 Phone: +1 (510) 666-2989
EMail: floyd@acm.org EMail: floyd@acm.org
URL: http://www.icir.org/floyd/ URL: http://www.icir.org/floyd/
Andrei Gurtov Andrei Gurtov
HIIT HIIT
Helsinki Institute for Information Technology Helsinki Institute for Information Technology
Aalto University P.O. Box 19215
P.O. Box 19800 00076 Aalto
Helsinki FIN-00076 Finland
FINLAND
EMail: gurtov@hiit.fi EMail: gurtov@hiit.fi
Yoshifumi Nishida Yoshifumi Nishida
WIDE Project WIDE Project
Endo 5322 Endo 5322
Fujisawa, Kanagawa 252-8520 Fujisawa, Kanagawa 252-8520
Japan Japan
Email: nishida@wide.ad.jp Email: nishida@wide.ad.jp
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