wdiff draft-ietf-tcpm-rfc3782-bis-00.txt draft-ietf-tcpm-rfc3782-bis-01.txt

Network Working Group T. Henderson
Internet-Draft Boeing
Obsoletes: 3782 (if approved) S. Floyd
Intended status: Standards Track ICSI
Expires: June 24, September 15, 2011 A. Gurtov
HIIT
Y. Nishida
WIDE Project
January 24,
March 14, 2011

The NewReno Modification to TCP's Fast Recovery Algorithm
draft-ietf-tcpm-rfc3782-bis-00.txt
draft-ietf-tcpm-rfc3782-bis-01.txt

Abstract

RFC 5681 [RFC5681] documents the following four intertwined TCP
congestion control algorithms: Slow Start, Congestion Avoidance, Fast
Retransmit, and Fast Recovery. RFC 5681 explicitly allows
certain modifications of these algorithms, including modifications
that use the TCP Selective Acknowledgement (SACK) option [RFC2883],
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. This document describes a specific
algorithm for responding to partial acknowledgments, referred to as
NewReno. This response to partial acknowledgments was first proposed
by Janey Hoe in [Hoe95].

The purpose of this revision from [RFC3782] is to make errata changes
and to adopt a proposal from Yoshifumi Nishida to slightly increase
the minimum window size after Fast Recovery from one to two segments,
to improve performance when the receiver uses delayed acknowledgments.

Status of this Memo

This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on June 24, September 15, 2011.

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1. Introduction

For the typical implementation of the TCP Fast Recovery algorithm
described in [RFC5681] (first implemented in the 1990 BSD Reno
release, and referred to as the Reno algorithm in [FF96]), the TCP
data sender only retransmits a packet after a retransmit timeout has
occurred, or after three duplicate acknowledgements acknowledgments have arrived
triggering the Fast Retransmit algorithm. A single retransmit
timeout might result in the retransmission of several data packets,
but each invocation of the Fast Retransmit algorithm in RFC 5681
leads to the retransmission of only a single data packet.

Problems can arise, therefore,

Two problems arise with Reno TCP when multiple packets are
dropped from packet losses occur
in a single window of data and the Fast Retransmit window. First, Reno will often take a timeout, as
has been documented in [Hoe95]. Second, even if a retransmission
timeout is avoided, multiple fast retransmits and Fast
Recovery algorithms are invoked. In this case, window reductions
can occur, as documented in [F94]. When multiple packet losses
occur, if the SACK option [RFC2883] is available, the TCP sender
has the information to make intelligent decisions about which packets
to retransmit and which packets not to retransmit during Fast
Recovery. This document applies only for 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.

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, acknowledgments, the sender
infers a packet loss, and retransmits the indicated packet. After
this, the data sender could receive additional duplicate
acknowledgements,
acknowledgments, as the data receiver acknowledges additional data
packets that were already in flight when the sender entered Fast
Retransmit.

In the case of multiple packets dropped from a single window of data,
the first new information available to the sender comes when the
sender receives an acknowledgement acknowledgment for the retransmitted packet (that
is, the packet retransmitted when Fast Retransmit was first
entered). If there is a single packet drop and no reordering, then the
acknowledgement
acknowledgment for this packet will acknowledge all of the packets
transmitted before Fast Retransmit was entered. However, if there
are multiple packet drops, then the acknowledgement acknowledgment for the
retransmitted packet will acknowledge some but not all of the packets
transmitted before the Fast Retransmit. We call this acknowledgement acknowledgment
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 5681 that
incorporates a response to partial acknowledgements acknowledgments 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 5681. 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 acknowledgments during Fast
Recovery. The version of NewReno in this document also draws on other
discussions of NewReno in the literature [LM97, Hen98].

We do not claim that the NewReno version of Fast Recovery described
here is an optimal modification of Fast Recovery for responding to
partial acknowledgements, acknowledgments, 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
[RFC2119]. This RFC indicates 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
SENDER MAXIMUM SEGMENT SIZE (SMSS), CONGESTION WINDOW (cwnd), and
FLIGHT SIZE (FlightSize) defined in [RFC5681]. FLIGHT SIZE is
defined as in [RFC5681] as follows:

FLIGHT SIZE:
The amount of data that has been sent but not yet cumulatively
acknowledged.

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
algorithms is given in [RFC5681]. This section specifies the basic
NewReno algorithm. Sections Section 4 through describes heuristics for processing
duplicate acknowledgments after a retransmission timeout. Sections
5 and 6 describe provide some optional
variants, guidance to implementors based on experience
with NewReno implementations. Several appendices provide more
background information and the motivations behind them, describe variations 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 applies to the Fast Recovery procedure that
begins when three duplicate ACKs are received and ends when either a
retransmission timeout occurs or an ACK arrives that acknowledges all
of the data up to and including the data that was outstanding when
the Fast Recovery procedure began.

The NewReno algorithm specified in this document differs from extends the
implementation in [RFC5681] in by introducing a variable specified as
"recover" whose initial value is the introduction of initial send sequence number.
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
"recover" is used below
in step 1, in the response to a partial or new
acknowledgement
acknowledgment 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 Acknowledgment 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 4 from [RFC5681]).

ssthresh = max (FlightSize / 2, 2*SMSS) (1)

In addition, record the highest sequence number transmitted in
the variable "recover", and go to Step 2.

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 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: acknowledgments:
If this ACK acknowledges all of the data up to and including
"recover", then the ACK acknowledges all the intermediate
segments sent between the original transmission of the lost
segment and the receipt of the third duplicate ACK. Set cwnd to
either (1) min (ssthresh, max(FlightSize, SMSS) + SMSS) or
(2) ssthresh, where ssthresh is the value set 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
is encouraged to take measures to avoid a possible burst of
data, in case the amount of data outstanding in the network is
much less than the new congestion window allows. A simple mechanism
is to limit the number of data packets that can be sent in response
to a single acknowledgement; this is known
as "maxburst_" in the NS simulator. acknowledgment. Exit the Fast Recovery procedure.

Partial acknowledgements: acknowledgments:
If this ACK does *not* acknowledge all of the data up to and
including "recover", then this is a partial ACK. In this case,
retransmit the first unacknowledged segment. Deflate the
congestion window by the amount of new data acknowledged by the
cumulative acknowledgement acknowledgment field. If the partial ACK
acknowledges at least one SMSS of new data, then add back SMSS
bytes to the congestion window. As in Step 3, this artificially
inflates the congestion window in order to reflect the additional
segment that has left the network. Send a new segment if
permitted by the new value of cwnd. This "partial window
deflation" attempts to ensure that, when Fast Recovery eventually
ends, approximately ssthresh amount of data will be outstanding
in the network. Do not exit the Fast Recovery procedure (i.e.,
if any duplicate ACKs subsequently arrive, execute Steps 3 and
4 above).

For the first partial ACK that arrives during Fast Recovery, also
reset the retransmit timer. Timer management is discussed in
more detail in Section 4.

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 Acknowledgment field
covers more than "recover". Because the acknowledgement acknowledgment field
contains the sequence number that the sender next expects to receive,
the acknowledgement acknowledgment "ack_number" covers more than "recover" when:

ack_number - 1 > recover;

i.e., at least one byte more of data is acknowledged beyond the
highest byte that was outstanding when Fast Retransmit was last
entered.

Note that in Step 5, the congestion window is deflated after a
partial acknowledgement acknowledgment is received. The congestion window was
likely to have been inflated considerably when the partial
acknowledgement
acknowledgment was received. In addition, depending on the original
pattern of packet losses, the partial acknowledgement acknowledgment might
acknowledge nearly a window of data. In this case, if the congestion
window was not deflated, the data sender might be able to send nearly
a window of data back-to-back.

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 acknowledgment
threshold, but assumes the threshold specified in the IETF standards;
the current standard is RFC 5681, which specifies a threshold of three
duplicate acknowledgements. acknowledgments.

As a final note, we would observe that in the absence of the SACK
option, the data sender is working from limited information. When
the issue of recovery from multiple dropped packets from a single
window of data is of particular importance, the best alternative
would be to use the SACK option.

4. Resetting the Retransmit Timer in Response to Partial
Acknowledgements

One possible variant to Handling Duplicate Acknowledgments After A Timeout

After each retransmit timeout, the response to partial acknowledgements
specified highest sequence number
transmitted so far is recorded in Section 3 concerns when to reset the retransmit timer variable "recover".
If, after a partial acknowledgement. The algorithm in Section 3, Step 5,
resets the retransmit timer only after timeout, the first partial ACK. In
this case, if a large number of TCP data sender retransmits three
consecutive packets were dropped from a window of
data, that have already been received by the TCP data sender's retransmit timer will ultimately expire,
and
receiver, then the TCP data sender will invoke Slow-Start. (This is illustrated
on page 12 of [F98].) We call receive three duplicate
acknowledgments that do not cover more than "recover". In this
case, the Impatient variant duplicate acknowledgments are not an indication of NewReno.
We note a new
instance of congestion. They are simply an indication that the Impatient variant
sender has unnecessarily retransmitted at least three packets.

However, when a retransmitted packet is itself dropped, the sender
can also receive three duplicate acknowledgments that do not cover
more than "recover". In this case, the sender would have been
better off if it had initiated Fast Retransmit. For a TCP that
implements the algorithm specified in Section 3 doesn't follow of this document, the
recommended algorithm
sender does not infer a packet drop from duplicate acknowledgments
in RFC 2988 of restarting this scenario. As always, the retransmit timer
after every packet transmission or retransmission (step 5.1 of
[RFC2988]).

In contrast, is the NewReno simulations backup
mechanism for inferring packet loss in [FF96] illustrate the
algorithm described above with the modification that the retransmit
timer is reset after each partial acknowledgement. We call this case.

There are several heuristics, based on timestamps or on the
Slow-but-Steady variant amount of NewReno. In this case, for a window with
a large number
advancement of packet drops, the TCP data sender retransmits at
most one packet per roundtrip time. (This behavior is illustrated in cumulative acknowledgment field, that allow the New-Reno TCP simulation of Figure 5
sender to distinguish, in [FF96], some cases, between three duplicate
acknowledgments following a retransmitted packet that was dropped,
and on page 11 three duplicate acknowledgments from the unnecessary
retransmission of
[F98]).

When N three packets have been dropped from [Gur03, GF04]. The TCP sender MAY use
such a window of data for heuristic to decide to invoke 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" Retransmit in some cases,
even when the NS simulator illustrate such a scenario, where the
Impatient variant performs better three duplicate acknowledgments do not cover more than
"recover".

For example, when three duplicate acknowledgments are caused by the Slow-but-Steady
variant. The Impatient variant can
unnecessary retransmission of three packets, this is likely to be particularly important for TCP
connections with large congestion windows, as illustrated
accompanied 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 cumulative acknowledgment field advancing by at
least four segments. Similarly, a heuristic based on timestamps uses
the Impatient variant. As an example,
this occurs fact that when only there is a small number of packets are dropped, hole in the RTO sequence space, the
timestamp echoed in the duplicate acknowledgment is sufficiently small the timestamp of
the most recent data packet that advanced the retransmit timer expires, and
performance would have been better without a retransmit timeout. The
tests "ns test-suite-newreno.tcl impatient2" cumulative
acknowledgment field [RFC1323]. If timestamps are used, 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
sender stores the
Impatient variant, by avoiding unnecessary retransmissions. This
could be timestamp 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 last acknowledged segment, then
the timestamp echoed by duplicate acknowledgments can also be more robust used to delay variation in
the network, where a delay spike might force the Impatient variant into
distinguish between a timeout retransmitted packet that was dropped and go-back-N recovery.

Neither
three duplicate acknowledgments from the unnecessary
retransmission of three packets.

4.1. ACK Heuristic

If the two variants discussed above are optimal. Our
recommendation ACK-based heuristic is for used, then following the Impatient variant, as specified in Section
3 of this document, because advancement of
the poor performance cumulative acknowledgment field, the sender stores the value 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 previous cumulative acknowledgment as does slow-start,
while resetting prev_highest_ack, and stores
the retransmit timers after each partial
acknowledgement, latest cumulative ACK as described in highest_ack. In addition, the section below. We note,
however, that there following
step 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 performed if Step 1 in Section 3 would be fails, before proceeding to retransmit
Step 1B.

1*) If the Cumulative Acknowledgment field didn't cover more than one packet
after each partial acknowledgement, and
"recover", check to reset see if the retransmit timer
after each retransmission. The algorithm specified in Section 3
retransmits a single packet after each partial acknowledgement. This congestion window is greater
than SMSS bytes and the difference between highest_ack and
prev_highest_ack is at most conservative alternative, 4*SMSS bytes. If true, duplicate
ACKs indicate a lost segment (proceed to Step 1A in that it is the least Section
3). Otherwise, duplicate ACKs likely
to result in an unnecessarily-retransmitted packet. A variant that
would recover faster from a unnecessary
retransmissions (proceed to Step 1B in Section 3).

The congestion window with many packet drops would be check serves to
effectively Slow-Start, retransmitting two packets protect against fast retransmit
immediately after each partial
acknowledgement. Such an approach would take less than N roundtrip
times to recover from N losses [Hoe96]. However, a retransmit timeout.

If several ACKs are lost, the sender can see a jump in the absence cumulative
ACK of
SACK, recovering as quickly as slow-start introduces more than three segments, and 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 heuristic can fail.
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 5681 recommends that the TCP sender responds to a partial ACK by deflating the
congestion window by the amount of new receiver should
send duplicate ACKs for every out-of-order data acknowledged, adding
back SMSS bytes packet, such as a
data packet received during Fast Recovery. The ACK heuristic is more
likely to fail if the partial ACK acknowledges at least SMSS bytes receiver does not follow this advice, because
then a smaller number of new data, and sending ACK losses are needed to produce a new segment if permitted by
sufficient jump in the new value
of cwnd. Thus, only one previously-sent packet cumulative ACK.

4.2. Timestamp Heuristic

If this heuristic is retransmitted in
response to each partial acknowledgement, but additional new packets
might be transmitted as well, depending on used, the amount sender stores the timestamp of new data
acknowledged by the partial acknowledgement.
last acknowledged segment. In contrast, addition, the
variant second paragraph of NewReno illustrated in [FF96] simply set the congestion
window to ssthresh when a partial acknowledgement was received. The
approach step
1 in [FF96] Section 3 is replaced as follows:

1**) If the Cumulative Acknowledgment field didn't cover more conservative, and does not attempt than
"recover", check to
accurately track see if the actual number of outstanding packets after a
partial acknowledgement is received. While either of these
approaches gives acceptable performance, echoed timestamp in the variant specified last
non-duplicate acknowledgment equals the
stored timestamp. If true, duplicate ACKs indicate a lost
segment (proceed to Step 1A in Section 3 recovers more smoothly when multiple packets are dropped 3). Otherwise, duplicate
ACKs likely result 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" unnecessary retransmissions (proceed
to 0; the behavior specified Step 1B in Section 3 is
achieved 3).

The timestamp heuristic works correctly, both when the receiver echoes
timestamps as specified by setting "partial_window_deflation_" to 1.)

6. Avoiding Multiple Fast Retransmits

This section describes [RFC1323], and by its revision attempts.
However, if the motivation for receiver arbitrarily echoes timestamps, the sender's state variable
"recover", and discusses possible heuristics for distinguishing
between heuristic
can fail. The heuristic can also fail if a retransmitted packet that timeout was dropped, spurious and three duplicate
acknowledgements
returning ACKs are not from retransmitted segments. This can be
prevented by detection algorithms such as [RFC3522].

5. Implementation Issues for the unnecessary retransmission of three
packets.

In Data Receiver

[RFC5681] specifies that "Out-of-order data segments SHOULD be
acknowledged immediately, in order to accelerate loss recovery."
Neal Cardwell has noted that some data receivers do not send an
immediate acknowledgment when they send a partial acknowledgment,
but instead wait first for their delayed acknowledgment timer to
expire [C98]. As [C98] notes, this severely limits the absence potential
benefit of NewReno by delaying the SACK option or timestamps, a duplicate
acknowledgement carries no information to identify receipt of the data packet or
packets partial
acknowledgment at the TCP data receiver that triggered that duplicate
acknowledgement. In this case, sender. Echoing RFC 5681, our
recommendation is that the TCP data sender is unable to
distinguish between receiver send an immediate
acknowledgment for an out-of-order segment, even when that
out-of-order segment fills a duplicate acknowledgement hole in the buffer.

6. Implementation Issues for the Data Sender

In Section 3, Step 5 above, it is noted that results from implementations should
take measures to avoid a
lost or delayed possible burst of data packet, and a duplicate acknowledgement that
results from when leaving Fast
Recovery, in case the sender's unnecessary retransmission amount of a new data packet that had already been received at the TCP data receiver. Because sender is eligible
to send due to the new value of
this, with the Retransmit and Fast Recovery algorithms in Reno TCP,
multiple segment losses from a single congestion window of data is large. This
can sometimes
result in unnecessary multiple Fast Retransmits (and multiple
reductions of arise during NewReno when ACKs are lost or treated as pure window
updates, thereby causing the congestion window) [F94].

With sender to underestimate the Fast Retransmit and Fast Recovery algorithms in Reno TCP, number of
new segments that can be sent during the performance problems caused by multiple Fast Retransmits are
relatively minor compared to recovery procedure.
Specifically, bursts can occur when the potential problems with Tahoe TCP,
which does not implement FlightSize is much less than
the new congestion window when exiting from Fast Recovery. Nevertheless, unnecessary
Fast Retransmits can occur with Reno TCP unless some explicit One
simple mechanism is added to avoid this, such as the use a burst of the "recover"
variable. (This modification data when leaving Fast Recovery
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, to limit the problem of multiple Fast
Retransmits from a single window number of data packets that can occur after be sent in response
to a Retransmit
Timeout (as single acknowledgment. (This is known as "maxburst_" in page 8 of [F98]) the ns
simulator.) Other possible mechanisms for avoiding bursts include
rate-based pacing, or in scenarios with reordering (as
in setting the validation test "./test-all-newreno newreno5_noBF" in
directory "tcl/test" of slow-start threshold to the NS simulator. This gives performance
similar
resultant congestion window and then resetting the congestion window
to that FlightSize. A recommendation on page 8 of [F03].) RFC 2582 also defined Careful
and Less Careful variants of the NewReno algorithm, and recommended general mechanism to avoid
excessively bursty sending patterns is outside the Careful variant.

The algorithm specified in Section 3 scope of this document corresponds
document.

An implementation may want to use a separate flag to record whether
or not it is presently in the Careful variant Fast Recovery procedure. The use of NewReno TCP from RFC 2582,
the value of the duplicate acknowledgment counter for this purpose is
not reliable because it can be reset upon window updates and eliminates
out-of-order acknowledgments.

When updating the
problem Cumulative Acknowledgment field outside of multiple
Fast Retransmits. This algorithm uses Recovery, the "recover" state variable "recover", whose initial value is may also need to be
updated in order to continue to permit possible entry into Fast
Recovery (Section 3, step 1). This issue arises when an update
of the initial send Cumulative Acknowledgment field results in a sequence
number. After each retransmit timeout,
wraparound that affects the highest sequence number
transmitted so far ordering between the Cumulative
Acknowledgment field and the "recover" state variable. Entry
into Fast Recovery is recorded in only possible when the variable "recover".

If, after a retransmit timeout, Cumulative
Acknowledgment field covers more than the "recover" state variable.

It is important for the TCP data sender retransmits three
consecutive packets that have already been to respond correctly to duplicate ACKs
received by the data
receiver, then when the TCP data sender will receive is no longer in Fast Recovery (e.g., because
of a Retransmit Timeout). The Limited Transmit procedure [RFC3042]
describes possible responses to the first and second duplicate
acknowledgments. When three or more duplicate
acknowledgements that do not acknowledgments are
received, the Cumulative Acknowledgment field doesn't cover more
than "recover". In this
case, the duplicate acknowledgements are not an indication of "recover", and 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 Fast Recovery is itself dropped, not invoked, it is
important that the sender
can also receive three duplicate acknowledgements that do not cover
more than "recover". In this case, execute the sender would have been
better off if it had initiated Fast Retransmit. For a TCP that
implements the algorithm specified Recovery steps (3) and
(4) in Section 3 of this document, 3. Otherwise, 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 could end up in a chain of
spurious timeouts. We mention this case.

There are only because several heuristics, based on timestamps or on NewReno
implementations had this bug, including the amount of
advancement of implementation in the cumulative acknowledgement field, NS
simulator.

It has been observed that allow the
sender to distinguish, in some cases, between three duplicate
acknowledgements following a retransmitted packet that was dropped,
and three duplicate acknowledgements from the unnecessary
retransmission of three packets [Gur03, GF04]. The TCP sender MAY use
such a heuristic to decide to invoke implementations enter 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 slow start
or congestion avoidance window updating algorithm immediately after
the
unnecessary retransmission of three packets, this cwnd is likely to be
accompanied set by the cumulative acknowledgement field advancing by at
least four segments. Similarly, equation found in (Section 3, step 5), even
without a heuristic based on timestamps uses new external event generating the fact cwnd change. Note that when there is a hole in the sequence space, the
timestamp echoed in the duplicate acknowledgement
after cwnd is set based on 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 procedure for exiting Fast Recovery
(Section 3, step 5), cwnd SHOULD NOT be used to
distinguish between updated until a retransmitted packet further
event occurs (e.g., arrival of an ack, or timeout) after this
adjustment.

7. Security Considerations

RFC 5681 discusses general security considerations concerning TCP
congestion control. This document describes a specific algorithm
that was dropped and
three duplicate acknowledgements from conforms with the unnecessary
retransmission congestion control requirements of three packets. The heuristics RFC 5681,
and so those considerations apply to this algorithm, too. There are illustrated in
no known additional security concerns for this specific algorithm.

8. IANA Considerations

This document has no actions for IANA.

9. Conclusions

This document specifies the NS simulator in NewReno Fast Retransmit and Fast Recovery
algorithms for TCP. This NewReno modification to TCP can even be
important for TCP implementations that support the validation test "./test-all-newreno".

6.1. ACK Heuristic

If SACK option,
because the ACK-based heuristic is used, then following SACK option can only be used for TCP connections when
both TCP end-nodes support the advancement SACK option. NewReno performs better
than Reno (RFC 5681) in a number of
the cumulative acknowledgement field, the sender stores the value scenarios discussed herein.

A number of options to the 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 basic algorithm presented in Section 3 fails, before proceeding are
also described in appendices to
Step 1B.

1*) If this document. These include the Cumulative Acknowledgement field didn't cover more than
"recover", check
handling of the retransmission timer (Appendix A), the response to see if
partial acknowledgments (Appendix B), and whether or not the congestion window sender
maintains a state variable called "recover" (Appendix C).
Our belief is greater
than SMSS bytes and that the difference differences between highest_ack and
prev_highest_ack is at most 4*SMSS bytes. If true, duplicate
ACKs indicate a lost segment (proceed to Step 1A in Section
3). Otherwise, duplicate ACKs likely result from unnecessary
retransmissions (proceed these variants of NewReno
are small compared to Step 1B in Section 3).

The congestion window check serves the differences between Reno and NewReno.
That is, the important thing is to protect against fast retransmit
immediately after implement NewReno instead of Reno,
for a retransmit timeout, similar TCP connection without SACK; it is less important exactly
which of the variants of NewReno is implemented.

10. Acknowledgments

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. Jeffrey
Hsu provided clarifications on the
"exitFastRetrans_" variable in NS. Examples handling of applying the ACK
heuristic are in validation tests "./test-all-newreno
newreno_rto_loss_ack" recover variable
that were applied to RFC 3782 as errata, and "./test-all-newreno newreno_rto_dup_ack" now are in
directory "tcl/test" Section 8
of this document. Yoshifumi Nishida contributed a modification
to the NS simulator.

If several ACKs are lost, fast recovery algorithm to account for the sender can see a jump case in which
flightsize is 0 when the cumulative
ACK of more than three segments, TCP sender leaves fast recovery, and the heuristic can fail. A
validation test for this scenario is "./test-all-newreno
newreno_rto_loss_ackf". RFC 5681 recommends that a
TCP receiver should
send duplicate ACKs for every out-of-order data packet, such as a
data packet received during Fast Recovery. The ACK heuristic is more
likely uses delayed acknowledgments. Alexander Zimmermann
provided several suggestions to fail if improve the receiver does not clarity of the document.

11. References

11.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.

[RFC5681] Allman, M., Paxson, V. and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.

11.2. Informative References

[C98] Cardwell, N., "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] Floyd, S., 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] Floyd, S., "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] Fall, K. 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] Floyd, S., "TCP and Successive Fast Retransmits", Technical
report, October 1994. URL
"ftp://ftp.ee.lbl.gov/papers/fastretrans.ps".

[GF04] Gurtov, A. and S. Floyd, "Resolving Acknowledgment Ambiguity
in non-SACK TCP", Next Generation Teletraffic and
Wired/Wireless Advanced Networking (NEW2AN'04), February
2004. URL "http://www.cs.helsinki.fi/u/gurtov/papers/
heuristics.html".

[Gur03] Gurtov, A., "[Tsvwg] resolving the problem of unnecessary fast
retransmits in go-back-N", email to the tsvwg mailing list, message
ID <3F25B467.9020609@cs.helsinki.fi>, July 28, 2003. URL
"http://www1.ietf.org/mail-archive/working-groups/tsvwg/current/msg04334.html".

[Hen98] Henderson, T., 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] Hoe, J., "Startup Dynamics of TCP's Congestion Control and
Avoidance Schemes", Master's Thesis, MIT, 1995.

[Hoe96] Hoe, J., "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] Lin, D. 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] Padhye, J. and S. Floyd, "Identifying the TCP Behavior of Web
Servers", June 2001, SIGCOMM 2001.

[RFC1323] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992.

[RFC2582] Floyd, S. and T. Henderson, "The NewReno Modification to
TCP's Fast Recovery Algorithm", RFC 2582, April 1999.

[RFC2883] Floyd, S., J. Mahdavi, M. Mathis, and M. Podolsky, "The
Selective Acknowledgment (SACK) Option for TCP, RFC 2883, July 2000.

[RFC3042] Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing TCP's
Loss Recovery Using Limited Transmit", RFC 3042, January 2001.

[RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for
TCP", RFC 3522, April 2003.

[RFC3782] Floyd, S., T. Henderson, and A. Gurtov, "The NewReno
Modification to TCP's Fast Recovery Algorithm", RFC 3782, April 2004.

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 advice, because
then the
Slow-but-Steady variant of NewReno. In this case, for a smaller window with
a large number of ACK losses are needed to produce 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
sufficient jump 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 cumulative ACK.

6.2. Timestamp Heuristic

If 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 heuristic occurs when only a small number of packets are dropped, the RTO
is used, sufficiently small that the sender stores 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 timestamp
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
last acknowledged segment. In addition, 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 second paragraph of step
1 Impatient variant, as specified in Section
3 is replaced as follows:

1**) If of this document, because of the Cumulative Acknowledgement field didn't cover poor performance of the
Slow-but-Steady variant for TCP connections with large congestion
windows.

One possibility for a more than
"recover", check to see if optimal algorithm would be one that
recovered from multiple packet drops as quickly as does slow-start,
while resetting the echoed timestamp retransmit timers after each partial
acknowledgment, as described in the last
non-duplicate acknowledgment equals the
stored timestamp. If true, duplicate ACKs indicate section below. We note,
however, that there is a lost
segment (proceed limitation to Step 1A the potential performance in Section 3). Otherwise, duplicate
ACKs likely result from unnecessary retransmissions (proceed
to Step 1B
this case in Section 3).

Examples the absence 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 SACK option.

Appendix B. Retransmissions after a Partial Acknowledgment

One possible variant to the receiver echoes timestamps as response to partial acknowledgments
specified by [RFC1323], in Section 3 would be to retransmit more than one packet
after each partial acknowledgment, and by
its revision attempts. However, if the receiver arbitrarily echoes
timestamps, to reset the heuristic can fail. retransmit timer
after each retransmission. The heuristic can also fail if algorithm specified in Section 3
retransmits 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

[RFC5681] specifies that "Out-of-order data segments SHOULD be
acknowledged immediately, single packet after each partial acknowledgment. This
is the most conservative alternative, in order to accelerate loss recovery."
Neal Cardwell has noted that some data receivers do not send it is the least likely
to result in an
immediate acknowledgement when they send 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,
but instead wait first for their delayed acknowledgement timer
acknowledgment. Such an approach would take less than N roundtrip
times to
expire [C98]. As [C98] notes, this severely limits recover from N losses [Hoe96]. However, in the potential
benefit absence of NewReno by delaying
SACK, recovering as quickly as slow-start introduces the receipt likelihood
of unnecessarily retransmitting packets, and this could significantly
complicate the partial
acknowledgement at recovery mechanisms.

We note that the data sender. Echoing response to partial acknowledgments specified in
Section 3 of this document and in RFC 5681, our
recommendation is that 2582 differs from the data receiver send an immediate
acknowledgement for an out-of-order segment, response
in [FF96], even when that
out-of-order segment fills a hole though both approaches only retransmit one packet in the buffer.

8. Implementation Issues for the Data Sender

In Section 3,
response to a partial acknowledgment. Step 5 above, it is noted of Section 3 specifies
that implementations should
take measures the TCP sender responds to avoid a possible burst of data when leaving Fast
Recovery, in case partial ACK by deflating the
congestion window by the amount of new data that acknowledged, adding
back SMSS bytes if the sender is eligible
to send due to partial ACK acknowledges at least SMSS bytes
of new data, and sending a new segment if permitted by the new value
of the congestion window cwnd. Thus, only one previously-sent packet is large. This
can arise during NewReno when ACKs are lost or treated as pure window
updates, thereby causing the sender retransmitted in
response to underestimate each partial acknowledgment, but additional new packets
might be transmitted as well, depending on the number amount of new segments that can be sent during data
acknowledged by the recovery procedure.
Specifically, bursts can occur when partial acknowledgment. In contrast, the FlightSize is much less than
variant of NewReno illustrated in [FF96] simply set the new congestion
window when exiting from Fast Recovery. One
simple mechanism to avoid a burst of data ssthresh when leaving Fast Recovery a partial acknowledgment was received. The
approach in [FF96] is more conservative, and does not attempt to limit
accurately track the actual number of data outstanding packets that can be sent in response
to after a single acknowledgment. (This
partial acknowledgment is known as "maxburst_" 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 ns
simulator.) Other possible mechanisms motivation for avoiding bursts include
rate-based pacing, or setting the slow-start threshold to sender's state
variable "recover".

In the
resultant congestion window and then resetting absence of the congestion window SACK option or timestamps, a duplicate
acknowledgment carries no information to FlightSize. A recommendation on identify the general mechanism to avoid
excessively bursty sending patterns is outside data packet or
packets at the scope of TCP data receiver that triggered that duplicate
acknowledgment. In this
document.

An implementation may want case, the TCP data sender is unable to use
distinguish between a separate flag to record whether duplicate acknowledgment that results from a
lost or not it is presently in delayed data packet, and a duplicate acknowledgment that
results from the Fast Recovery procedure. The use sender's unnecessary retransmission of a data packet
that had already been received at the value TCP data receiver. Because of
this, with the duplicate acknowledgment counter for this purpose is
not reliable because it can be reset upon window updates Retransmit and
out-of-order acknowledgments.

When updating the Cumulative Acknowledgement field outside of
Fast Recovery, the "recover" state variable may also need to be
updated in order to continue to permit possible entry into Fast Recovery (Section 3, step 1). This issue arises when an update
of the Cumulative Acknowledgement field results algorithms in Reno TCP,
multiple segment losses from a sequence
wraparound that affects single window of data can sometimes
result in unnecessary multiple Fast Retransmits (and multiple
reductions of the ordering between congestion window) [F94].

With the Cumulative
Acknowledgement field Fast Retransmit and the "recover" state variable. Entry
into Fast Recovery is only possible when the Cumulative
Acknowledgment field covers more than the "recover" state variable.

It is important for algorithms in Reno TCP,
the sender to respond correctly performance problems caused by multiple Fast Retransmits are
relatively minor compared to duplicate ACKs
received when the sender is no longer in potential problems with Tahoe TCP,
which does not implement Fast Recovery (e.g., because
of a Retransmit Timeout). The Limited Transmit procedure [RFC3042]
describes possible responses Recovery. Nevertheless, unnecessary
Fast Retransmits can occur with Reno TCP unless some explicit
mechanism is added to avoid this, such as the first and second duplicate
acknowledgements. When three or more duplicate acknowledgements are
received, use of the Cumulative Acknowledgement field doesn't cover more
than "recover", "recover"
variable. (This modification is called "bugfix" in [F98], and a new Fast Recovery is not invoked, it
illustrated on pages 7 and 9 of that document. Unnecessary Fast
Retransmits for Reno without "bugfix" is
important illustrated on page 6 of

[F98].)

Section 3 of [RFC2582] defined a default variant of NewReno TCP that
did not use the sender variable "recover", and did not execute check if duplicate
ACKs cover the variable "recover" before invoking Fast Recovery steps (3) and
(4) in Section 3. Otherwise, Retransmit.
With this default variant from RFC 2582, the sender could end up in problem of multiple Fast
Retransmits from a chain single window of
spurious timeouts. We mention this only because several NewReno
implementations had this bug, including the implementation data can occur after a Retransmit
Timeout (as in the NS
simulator. (This bug page 8 of [F98]) or in scenarios with reordering.
RFC 2582 also defined Careful and Less Careful variants of the NS simulator was fixed NewReno
algorithm, and recommended the Careful variant.

The algorithm specified in July 2003,
with Section 3 of this document corresponds to
the variable "exitFastRetrans_".)

It has been observed that some Careful variant of NewReno TCP implementations enter a slow start
or congestion avoidance window updating from RFC 2582, and eliminates the
problem of multiple Fast Retransmits. This algorithm immediately after uses the cwnd
variable "recover", whose initial value is set by the equation found 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 (Section 3, step 5), even
without a new external event generating
the cwnd change. Note NS simulator that
after cwnd reproduce behavior described above.

In Section 3, a simple mechanism is set based on described to limit the procedure for exiting Fast Recovery
(Section 3, step 5), cwnd SHOULD NOT number of
data packets that can be updated until sent in response to a further
event occurs (e.g., arrival of an ack, or timeout) after this
adjustment.

9. Simulations 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
acknowledgement.
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.

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 newreno_B" shows a single packet, combined simulation with a halving of the congestion window (shown on pages 4 and 6 of
[F03]). With same
scenario using the NewReno TCP, however, this reordering will also result algorithms described in this paper.

Regarding the unnecessary retransmission handling of an entire duplicate acknowledgments after a timeout,
the congestion window of data (shown
on page 5 of [F03]).

While Reno TCP performs better than NewReno TCP in check serves to protect against fast retransmit
immediately after a retransmit timeout, similar to the presence of
reordering, NewReno's superior performance
"exitFastRetrans_" variable in NS. Examples of applying the presence 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
multiple packet drops generally outweighs its less optimal
performance the NS simulator.
If several ACKs are lost, the sender can see a jump in the presence cumulative
ACK of reordering. (Sack TCP 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
preferred solution, with good performance timestamp heuristic (Section 4) are in both scenarios.) This
document recommends the Fast Retransmit
validation tests "./test-all-newreno newreno_rto_loss_tsh" 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
"./test-all-newreno newreno_rto_dup_tsh".

Section 6 described a problem involving possible spurious timeouts,
and Fast Recovery mechanisms are widely deployed mentions that this bug existed in TCP
implementations the NS simulator.
This bug in the Internet today, as documented NS simulator was fixed in [PF01]. For
example, July 2003,
with the variable "exitFastRetrans_".

Regarding the Slow-but-Steady and Impatient variants described
in Appendix A, The tests of TCP implementations "ns
test-suite-newreno.tcl impatient1" and "ns test-suite-newreno.tcl
slow1" in several thousand web servers the NS simulator illustrate a scenario in 2001 showed that which the
Impatient variant performs better than the Slow-but-Steady
variant. The Impatient variant can be particularly important for those TCP
connections where the web browser
was not SACK-capable, more web servers used with large congestion windows, as illustrated by the Fast Retransmit tests
"ns test-suite-newreno.tcl impatient4" 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 "ns test-suite-newreno.tcl
slow4" in the NewReno's Fast
Retransmit NS simulator. The tests
"ns test-suite-newreno.tcl impatient2" and Fast Recovery algorithms
"ns test-suite-newreno.tcl slow2" in RFC 2582 to Standards Track.

The main change the NS simulator illustrate
scenarios in this document relative to RFC 2582 is to specify which the Careful Slow-but-Steady variant of NewReno's Fast Retransmit and Fast Recovery
algorithms. outperforms the Impatient
variant. The base algorithm described in RFC 2582 did not attempt
to avoid unnecessary multiple Fast Retransmits that can occur after a
timeout (described tests "ns test-suite-newreno.tcl impatient3" and
"ns test-suite-newreno.tcl slow3" in more detail the NS simulator illustrate
scenarios in which the section above). However,
RFC 2582 also defined "Careful" and "Less Careful" Slow-but-Steady variants that avoid these unnecessary Fast Retransmits, and recommended the Careful
variant. This document specifies
retransmissions.

Appendix B describes different policies for partial window deflation.
The [FF96] behavior can be seen in the previously-named "Careful"
variant as NS
simulator by setting the basic version of NewReno. As described below, this
algorithm uses a variable "recover", whose initial value is the send
sequence number.

The algorithm "partial_window_deflation_" for
"Agent/TCP/Newreno" to 0; the behavior specified in Section 3 checks whether the
acknowledgement field is
achieved by setting "partial_window_deflation_" to 1.

Section 3 of a partial acknowledgement covers *more* than
"recover", as [RFC2582] defined in Section 3. Another possible a default variant would be
to simply require of NewReno TCP that
did not use the acknowledgement field covers *more than or
equal to* variable "recover", and did not check if duplicate
ACKs cover the variable "recover" before initiating another invoking Fast Retransmit. We
called
With this the Less Careful default variant in from 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 2582, the problem of multiple Fast
Retransmits from a single window of data sender
transmitted four packets can occur after a Retransmit
Timeout (as in page 8 of [F98]) or in scenarios with sequence numbers higher than "recover",
that the first packet was dropped reordering (as
An NS validation test "./test-all-newreno newreno5_noBF" 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
directory "tcl/test" of SACK, the TCP sender is
unable to distinguish between these two scenarios.

For NS simulator illustartes the Careful default
variant of Fast Retransmit, NewReno TCP that doesn't use the data sender would
have variable "recover";
this gives performance similar to wait for a retransmit timeout in the first scenario, but
would not have an unnecessary Fast Retransmit that on page 8 of [F03].

Appendix E. Comparisons between Reno and NewReno TCP

As we stated in the second
scenario. For introduction, we believe that the Less Careful variant to Fast Retransmit, NewReno
modification described in this document improves the performance of
the data
sender would Fast Retransmit as desired in the first scenario, and would
unnecessarily Fast Retransmit Recovery algorithms of Reno TCP in the second scenario. a
wide variety of scenarios. This document
only specifies the Careful variant has been discussed in Section 3. Unnecessary Fast
Retransmits with the Less Careful variant some depth in scenarios with
reordering

[FF96], which illustrates Reno TCP's poor performance when multiple
packets are illustrated in page 8 dropped from a window of [F03].

The document data and also specifies two heuristics 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 MAY
use to decide to invoke Fast Retransmit even when the receives three duplicate
acknowledgements do not cover more than "recover". These heuristics,
an ACK-based heuristic
acknowledgments. This will trigger the Fast Retransmit and a timestamp heuristic, are described Fast
Recovery algorithms. With Reno TCP or with Sack TCP, this will
result 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 unnecessary retransmission of a risk in single packet, combined
with a halving of the first logic which results congestion window (shown on pages 4 and 6 of
[F03]). With NewReno TCP, however, this reordering will also result
in performance
degradation. With the first logic, if FlightSize is zero, the result
will be 1 SMSS. This means unnecessary retransmission of an entire window of data (shown
on page 5 of [F03]).

While Reno TCP performs better than NewReno 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 the presence of
reordering, NewReno's superior performance in some situations.
A typical example is where sending window size during fast recovery is
small. In this case, the retransmitted presence of
multiple packet and new data packets can
be transmitted within a short interval. If all these packets
successfully arrive, drops generally outweighs its less optimal
performance in the receiver may generate a Full ACK that
acknowledges all outstanding data. Even if window size is not small,
loss presence of ACK packets or receive buffer shortage during fast recovery can
also increase reordering. (Sack TCP is the possibility to fall into this situation.

The proposed fix
preferred solution, with good performance 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)
has been applied.

13. Conclusions both scenarios.) This
document specifies recommends the NewReno Fast Retransmit and Fast Recovery algorithms for TCP. This
of NewReno modification to TCP can even be
important 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 support the SACK option,
because the SACK option can only be used for those TCP connections when
both TCP end-nodes support where the SACK option. web browser
was not SACK-capable, more web servers used the Fast Retransmit and
Fast Recovery algorithms of NewReno performs better than Reno (RFC 5681) in a number of scenarios discussed herein.

A number those of options Reno or Tahoe TCP
[PF01].

Appendix F. Changes Relative to the basic algorithm presented in Section 3 are
also described. These include the handling RFC 2582

The purpose of the retransmission
timer (Section 4), the response this document is to partial acknowledgments (Section
5), and the value of advance the congestion window when leaving NewReno's Fast
Retransmit and Fast Recovery
(section 3, step 5). Our belief algorithms in RFC 2582 to Standards Track.

The main change in this document relative to RFC 2582 is that the differences between
these variants of NewReno are small compared to specify
the differences
between Reno Careful variant of NewReno's Fast Retransmit and NewReno. That is, the important thing is Fast Recovery
algorithms. The base algorithm described in RFC 2582 did not attempt
to
implement NewReno instead of Reno, for avoid unnecessary multiple Fast Retransmits that can occur after a TCP connection without SACK;
it is less important exactly which of
timeout (described in more detail in the variants of NewReno is
implemented.

14. Security Considerations section above). However,
RFC 5681 discusses general security considerations concerning TCP
congestion control. 2582 also defined "Careful" and "Less Careful" variants that
avoid these unnecessary Fast Retransmits, and recommended the Careful
variant. This document describes specifies the previously-named "Careful"
variant as the basic version of NewReno. As described below, this
algorithm uses a specific variable "recover", whose initial value is the send
sequence number.

The algorithm
that conforms with specified in Section 3 checks whether the congestion control requirements
acknowledgment field of RFC 5681,
and so those considerations apply 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 algorithm, too. 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 known additional security concerns for this specific algorithm.

15. IANA Considerations

This document has no actions for IANA.

16. 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. Jeffrey
Hsu provided clarifications on more than "recover". One scenario would be that the handling of data sender
transmitted four packets with sequence numbers higher than "recover",
that the recover variable first packet was dropped in the network, and the following
three packets triggered three duplicate acknowledgments
acknowledging "recover". The second scenario would be that were applied to RFC 3782 as errata, the
sender unnecessarily retransmitted three packets below "recover", and now are in Section 8
that these three packets triggered three duplicate acknowledgments
acknowledging "recover". In the absence of this document. Yoshifumi Nishida contributed a modification SACK, the TCP sender is
unable to distinguish between these two scenarios.

For the fast recovery algorithm Careful variant of Fast Retransmit, the data sender would
have to account wait for a retransmit timeout in the case first scenario, but
would not have an unnecessary Fast Retransmit in which
flightsize is 0 when the TCP second
scenario. For the Less Careful variant to Fast Retransmit, the data
sender leaves fast recovery, would Fast Retransmit as desired in the first scenario, and would
unnecessarily Fast Retransmit in the
TCP receiver uses delayed acknowledgments.

17. References

17.1. Normative References

[RFC2119] Bradner, S., "Key words for use second scenario. This document
only specifies the Careful variant in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.

[RFC5681] Allman, M., Paxson, V. and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.

17.2. Informative References

[C98] Cardwell, N., "delayed ACKs for retransmitted packets: ouch!".
November 1998, Email to Section 3. Unnecessary Fast
Retransmits with 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] Floyd, S., Revisions to RFC 2001, "Presentation to Less Careful variant in scenarios with
reordering are illustrated in page 8 of [F03].

The document also specifies two heuristics that 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] Floyd, S., "Moving NewReno from Experimental TCP sender MAY
use to Proposed
Standard? Presentation decide 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] Fall, K. 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] Floyd, S., "TCP and Successive invoke Fast Retransmits", Technical
report, October 1994. URL
"ftp://ftp.ee.lbl.gov/papers/fastretrans.ps".

[GF04] Gurtov, A. Retransmit even when the three duplicate
acknowledgments do not cover more than "recover". These heuristics,
an ACK-based heuristic and S. Floyd, "Resolving Acknowledgment Ambiguity a timestamp heuristic, are described in non-SACK TCP", Next Generation Teletraffic
Sections 6.1 and
Wired/Wireless Advanced Networking (NEW2AN'04), February
2004. URL "http://www.cs.helsinki.fi/u/gurtov/papers/
heuristics.html".

[Gur03] Gurtov, A., "[Tsvwg] resolving 6.2 respectively.

Appendix G. Changes Relative to RFC 3782

In [RFC3782], the problem of unnecessary fast
retransmits in go-back-N", email cwnd after Full ACK reception will be set to
(1) min (ssthresh, FlightSize + SMSS) or (2) ssthresh. However,
there is a risk in the tsvwg mailing list, message
ID <3F25B467.9020609@cs.helsinki.fi>, July 28, 2003. URL
"http://www1.ietf.org/mail-archive/working-groups/tsvwg/current/msg04334.html".

[Hen98] Henderson, T., Re: NewReno and first logic which results in performance
degradation. With the 2001 Revision. September
1998. Email to first logic, if FlightSize is zero, the tcpimpl mailing list, Message ID
"Pine.BSI.3.95.980923224136.26134A-100000@raptor.CS.Berkeley.EDU",
archived result
will be 1 SMSS. This means TCP can transmit only 1 segment at "http://tcp-impl.lerc.nasa.gov/tcp-impl".

[Hoe95] Hoe, J., "Startup Dynamics of TCP's Congestion Control and
Avoidance Schemes", Master's Thesis, MIT, 1995.

[Hoe96] Hoe, J., "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] Lin, D. and R. Morris, "Dynamics of Random Early Detection",
SIGCOMM 97, September 1997. URL
"http://www.acm.org/sigcomm/sigcomm97/program.html".

[NS] this
moment, which can cause delay in ACK transmission at receiver due to
delayed ACK algorithm.

The Network Simulator (NS). URL "http://www.isi.edu/nsnam/ns/".

[PF01] Padhye, J. 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 S. Floyd, "Identifying new data packets can
be transmitted within a short interval. If all these packets
successfully arrive, the TCP Behavior receiver may generate a Full ACK that
acknowledges all outstanding data. Even if window size is not small,
loss of Web
Servers", June 2001, SIGCOMM 2001.

[RFC1323] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992.

[RFC2582] Floyd, S. and T. Henderson, "The NewReno Modification ACK packets or receive buffer shortage during fast recovery can
also increase the possibility to
TCP's Fast Recovery Algorithm", RFC 2582, April 1999.

[RFC2883] Floyd, S., J. Mahdavi, M. Mathis, and M. Podolsky, "The
Selective Acknowledgment (SACK) Option 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 TCP, RFC 2883, July 2000.

[RFC3042] Allman, M., Balakrishnan, H. RFC3782 (editorial clarification to Section 8
of RFC2582, which is now Section 6 of this document) has been applied.

Sections 4, 5, and S. Floyd, "Enhancing TCP's
Loss Recovery Using Limited Transmit", RFC 3042, January 2001.

[RFC3522] Ludwig, R. 9-11 of RFC2582 were relocated to appendices of
this document since they are non-normative and M. Meyer, "The Eifel Detection Algorithm for
TCP", RFC 3522, April 2003.

[RFC3782] Floyd, S., T. Henderson, provide background
information and A. Gurtov, "The NewReno
Modification references to TCP's Fast Recovery Algorithm", RFC 3782, April 2004. simulation results.

Appendix A. H. Document Revision History

To be removed upon publication

Authors' Addresses

Thomas R.

Tom Henderson
The Boeing Company
P.O. Box 3707
Seattle, WA 98124

EMail: thomas.r.henderson@boeing.com

Sally Floyd
International Computer Science Institute

Phone: +1 (510) 666-2989
EMail: floyd@acm.org
URL: http://www.icir.org/floyd/
Andrei Gurtov
HIIT
Helsinki Institute for Information Technology
Aalto University
P.O. Box 19800
Helsinki FIN-00076
FINLAND 19215
00076 Aalto
Finland

EMail: gurtov@hiit.fi

Yoshifumi Nishida
WIDE Project
Endo 5322
Fujisawa, Kanagawa 252-8520
Japan

Email: nishida@wide.ad.jp