Network Working Group M. Mathis
Request for Comments: 2018 J. Mahdavi
Category: Standards Track PSC
S. Floyd
LBNL
A. Romanow
Sun Microsystems
October 1996

TCP Selective Acknowledgment Options

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Abstract

TCP may eXPerience poor performance when multiple packets are lost
from one window of data. With the limited information available
from cumulative acknowledgments, a TCP sender can only learn about a
single lost packet per round trip time. An aggressive sender could
choose to retransmit packets early, but sUCh retransmitted segments
may have already been successfully received.

A Selective Acknowledgment (SACK) mechanism, combined with a
selective repeat retransmission policy, can help to overcome these
limitations. The receiving TCP sends back SACK packets to the sender
informing the sender of data that has been received. The sender can
then retransmit only the missing data segments.

This memo proposes an implementation of SACK and discusses its
performance and related issues.

Acknowledgements

Much of the text in this document is taken directly from RFC1072 "TCP
Extensions for Long-Delay Paths" by Bob Braden and Van Jacobson. The
authors would like to thank Kevin Fall (LBNL), Christian Huitema
(INRIA), Van Jacobson (LBNL), Greg Miller (MITRE), Greg Minshall
(Ipsilon), Lixia Zhang (XEROX PARC and UCLA), Dave Borman (BSDI),
Allison Mankin (ISI) and others for their review and constructive
comments.

1. Introduction

Multiple packet losses from a window of data can have a catastrophic
effect on TCP throughput. TCP [Postel81] uses a cumulative
acknowledgment scheme in which received segments that are not at the
left edge of the receive window are not acknowledged. This forces
the sender to either wait a roundtrip time to find out about each
lost packet, or to unnecessarily retransmit segments which have been
correctly received [Fall95]. With the cumulative acknowledgment
scheme, multiple dropped segments generally cause TCP to lose its
ACK-based clock, reducing overall throughput.

Selective Acknowledgment (SACK) is a strategy which corrects this
behavior in the face of multiple dropped segments. With selective
acknowledgments, the data receiver can inform the sender about all
segments that have arrived successfully, so the sender need
retransmit only the segments that have actually been lost.

Several transport protocols, including NETBLT [Clark87], XTP
[Strayer92], RDP [Velten84], NADIR [Huitema81], and VMTP [Cheriton88]
have used selective acknowledgment. There is some empirical evidence
in favor of selective acknowledgments -- simple experiments with RDP
have shown that disabling the selective acknowledgment facility
greatly increases the number of retransmitted segments over a lossy,
high-delay Internet path [Partridge87]. A recent simulation study by
Kevin Fall and Sally Floyd [Fall95], demonstrates the strength of TCP
with SACK over the non-SACK Tahoe and Reno TCP implementations.

RFC1072 [VJ88] describes one possible implementation of SACK options
for TCP. Unfortunately, it has never been deployed in the Internet,
as there was disagreement about how SACK options should be used in
conjunction with the TCP window shift option (initially described
RFC1072 and revised in [Jacobson92]).

We propose slight modifications to the SACK options as proposed in
RFC1072. Specifically, sending a selective acknowledgment for the
most recently received data reduces the need for long SACK options
[Keshav94, Mathis95]. In addition, the SACK option now carries full
32 bit sequence numbers. These two modifications represent the only
changes to the proposal in RFC1072. They make SACK easier to
implement and address concerns about robustness.

The selective acknowledgment extension uses two TCP options. The
first is an enabling option, "SACK-permitted", which may be sent in a
SYN segment to indicate that the SACK option can be used once the
connection is established. The other is the SACK option itself,
which may be sent over an established connection once permission has
been given by SACK-permitted.

The SACK option is to be included in a segment sent from a TCP that
is receiving data to the TCP that is sending that data; we will refer
to these TCP's as the data receiver and the data sender,
respectively. We will consider a particular simplex data flow; any
data flowing in the reverse direction over the same connection can be
treated independently.

2. Sack-Permitted Option

This two-byte option may be sent in a SYN by a TCP that has been
extended to receive (and presumably process) the SACK option once the
connection has opened. It MUST NOT be sent on non-SYN segments.

TCP Sack-Permitted Option:

Kind: 4

+---------+---------+
Kind=4 Length=2
+---------+---------+

3. Sack Option Format

The SACK option is to be used to convey extended acknowledgment
information from the receiver to the sender over an established TCP
connection.

TCP SACK Option:

Kind: 5

Length: Variable

+--------+--------+
Kind=5 Length
+--------+--------+--------+--------+
Left Edge of 1st Block
+--------+--------+--------+--------+
Right Edge of 1st Block
+--------+--------+--------+--------+

/ . . . /

+--------+--------+--------+--------+
Left Edge of nth Block
+--------+--------+--------+--------+
Right Edge of nth Block
+--------+--------+--------+--------+

The SACK option is to be sent by a data receiver to inform the data
sender of non-contiguous blocks of data that have been received and
queued. The data receiver awaits the receipt of data (perhaps by
means of retransmissions) to fill the gaps in sequence space between
received blocks. When missing segments are received, the data
receiver acknowledges the data normally by advancing the left window
edge in the Acknowledgement Number Field of the TCP header. The SACK
option does not change the meaning of the Acknowledgement Number
field.

This option contains a list of some of the blocks of contiguous
sequence space occupied by data that has been received and queued
within the window.

Each contiguous block of data queued at the data receiver is defined
in the SACK option by two 32-bit unsigned integers in network byte
order:

* Left Edge of Block

This is the first sequence number of this block.

* Right Edge of Block

This is the sequence number immediately following the last
sequence number of this block.

Each block represents received bytes of data that are contiguous and
isolated; that is, the bytes just below the block, (Left Edge of
Block - 1), and just above the block, (Right Edge of Block), have not
been received.

A SACK option that specifies n blocks will have a length of 8*n+2
bytes, so the 40 bytes available for TCP options can specify a
maximum of 4 blocks. It is expected that SACK will often be used in
conjunction with the Timestamp option used for RTTM [Jacobson92],
which takes an additional 10 bytes (plus two bytes of padding); thus
a maximum of 3 SACK blocks will be allowed in this case.

The SACK option is advisory, in that, while it notifies the data
sender that the data receiver has received the indicated segments,
the data receiver is permitted to later discard data which have been
reported in a SACK option. A discussion appears below in Section 8
of the consequences of advisory SACK, in particular that the data
receiver may renege, or drop already SACKed data.

4. Generating Sack Options: Data Receiver Behavior

If the data receiver has received a SACK-Permitted option on the SYN
for this connection, the data receiver MAY elect to generate SACK
options as described below. If the data receiver generates SACK
options under any circumstance, it SHOULD generate them under all
permitted circumstances. If the data receiver has not received a
SACK-Permitted option for a given connection, it MUST NOT send SACK
options on that connection.

If sent at all, SACK options SHOULD be included in all ACKs which do
not ACK the highest sequence number in the data receiver's queue. In
this situation the network has lost or mis-ordered data, such that
the receiver holds non-contiguous data in its queue. RFC1122,
Section 4.2.2.21, discusses the reasons for the receiver to send ACKs
in response to additional segments received in this state. The
receiver SHOULD send an ACK for every valid segment that arrives
containing new data, and each of these "duplicate" ACKs SHOULD bear a
SACK option.

If the data receiver chooses to send a SACK option, the following
rules apply:

* The first SACK block (i.e., the one immediately following the
kind and length fields in the option) MUST specify the contiguous
block of data containing the segment which triggered this ACK,
unless that segment advanced the Acknowledgment Number field in
the header. This assures that the ACK with the SACK option
reflects the most recent change in the data receiver's buffer
queue.

* The data receiver SHOULD include as many distinct SACK blocks as
possible in the SACK option. Note that the maximum available
option space may not be sufficient to report all blocks present in
the receiver's queue.

* The SACK option SHOULD be filled out by repeating the most
recently reported SACK blocks (based on first SACK blocks in
previous SACK options) that are not subsets of a SACK block
already included in the SACK option being constructed. This
assures that in normal operation, any segment remaining part of a
non-contiguous block of data held by the data receiver is reported
in at least three successive SACK options, even for large-window
TCP implementations [RFC1323]). After the first SACK block, the
following SACK blocks in the SACK option may be listed in
arbitrary order.

It is very important that the SACK option always reports the block
containing the most recently received segment, because this provides
the sender with the most up-to-date information about the state of
the network and the data receiver's queue.

5. Interpreting the Sack Option and Retransmission Strategy: Data
Sender Behavior

When receiving an ACK containing a SACK option, the data sender
SHOULD record the selective acknowledgment for future reference. The
data sender is assumed to have a retransmission queue that contains
the segments that have been transmitted but not yet acknowledged, in
sequence-number order. If the data sender performs re-packetization
before retransmission, the block boundaries in a SACK option that it
receives may not fall on boundaries of segments in the retransmission
queue; however, this does not pose a serious difficulty for the
sender.

One possible implementation of the sender's behavior is as follows.
Let us suppose that for each segment in the retransmission queue
there is a (new) flag bit "SACKed", to be used to indicate that this
particular segment has been reported in a SACK option.

When an acknowledgment segment arrives containing a SACK option, the
data sender will turn on the SACKed bits for segments that have been
selectively acknowledged. More specifically, for each block in the
SACK option, the data sender will turn on the SACKed flags for all
segments in the retransmission queue that are wholly contained within
that block. This requires straightforward sequence number
comparisons.

After the SACKed bit is turned on (as the result of processing a
received SACK option), the data sender will skip that segment during
any later retransmission. Any segment that has the SACKed bit turned
off and is less than the highest SACKed segment is available for
retransmission.

After a retransmit timeout the data sender SHOULD turn off all of the
SACKed bits, since the timeout might indicate that the data receiver
has reneged. The data sender MUST retransmit the segment at the left
edge of the window after a retransmit timeout, whether or not the
SACKed bit is on for that segment. A segment will not be dequeued
and its buffer freed until the left window edge is advanced over it.

5.1 Congestion Control Issues

This document does not attempt to specify in detail the congestion
control algorithms for implementations of TCP with SACK. However,
the congestion control algorithms present in the de facto standard
TCP implementations MUST be preserved [Stevens94]. In particular, to
preserve robustness in the presence of packets reordered by the
network, recovery is not triggered by a single ACK reporting out-of-
order packets at the receiver. Further, during recovery the data
sender limits the number of segments sent in response to each ACK.
Existing implementations limit the data sender to sending one segment
during Reno-style fast recovery, or to two segments during slow-start
[Jacobson88]. Other ASPects of congestion control, such as reducing
the congestion window in response to congestion, must similarly be
preserved.

The use of time-outs as a fall-back mechanism for detecting dropped
packets is unchanged by the SACK option. Because the data receiver
is allowed to discard SACKed data, when a retransmit timeout occurs
the data sender MUST ignore prior SACK information in determining
which data to retransmit.

Future research into congestion control algorithms may take advantage
of the additional information provided by SACK. One such area for
future research concerns modifications to TCP for a wireless or
satellite environment where packet loss is not necessarily an
indication of congestion.

6. Efficiency and Worst Case Behavior

If the return path carrying ACKs and SACK options were lossless, one
block per SACK option packet would always be sufficient. Every
segment arriving while the data receiver holds discontinuous data
would cause the data receiver to send an ACK with a SACK option
containing the one altered block in the receiver's queue. The data
sender is thus able to construct a precise replica of the receiver's
queue by taking the union of all the first SACK blocks.

Since the return path is not lossless, the SACK option is defined to
include more than one SACK block in a single packet. The redundant
blocks in the SACK option packet increase the robustness of SACK
delivery in the presence of lost ACKs. For a receiver that is also
using the time stamp option [Jacobson92], the SACK option has room to
include three SACK blocks. Thus each SACK block will generally be
repeated at least three times, if necessary, once in each of three
successive ACK packets. However, if all of the ACK packets reporting
a particular SACK block are dropped, then the sender might assume
that the data in that SACK block has not been received, and
unnecessarily retransmit those segments.

The deployment of other TCP options may reduce the number of
available SACK blocks to 2 or even to 1. This will reduce the
redundancy of SACK delivery in the presence of lost ACKs. Even so,
the exposure of TCP SACK in regard to the unnecessary retransmission
of packets is strictly less than the exposure of current
implementations of TCP. The worst-case conditions necessary for the
sender to needlessly retransmit data is discussed in more detail in a
separate document [Floyd96].

Older TCP implementations which do not have the SACK option will not
be unfairly disadvantaged when competing against SACK-capable TCPs.
This issue is discussed in more detail in [Floyd96].

7. Sack Option Examples

The following examples attempt to demonstrate the proper behavior of
SACK generation by the data receiver.

Assume the left window edge is 5000 and that the data transmitter
sends a burst of 8 segments, each containing 500 data bytes.

Case 1: The first 4 segments are received but the last 4 are
dropped.

The data receiver will return a normal TCP ACK segment
acknowledging sequence number 7000, with no SACK option.

Case 2: The first segment is dropped but the remaining 7 are
received.

Upon receiving each of the last seven packets, the data
receiver will return a TCP ACK segment that acknowledges
sequence number 5000 and contains a SACK option specifying
one block of queued data:

Triggering ACK Left Edge Right Edge
Segment

5000 (lost)
5500 5000 5500 6000
6000 5000 5500 6500
6500 5000 5500 7000
7000 5000 5500 7500
7500 5000 5500 8000
8000 5000 5500 8500
8500 5000 5500 9000

Case 3: The 2nd, 4th, 6th, and 8th (last) segments are
dropped.

The data receiver ACKs the first packet normally. The
third, fifth, and seventh packets trigger SACK options as
follows:

Triggering ACK First Block 2nd Block 3rd Block
Segment Left Right Left Right Left Right
Edge Edge Edge Edge Edge Edge

5000 5500
5500 (lost)
6000 5500 6000 6500
6500 (lost)
7000 5500 7000 7500 6000 6500
7500 (lost)
8000 5500 8000 8500 7000 7500 6000 6500
8500 (lost)

Suppose at this point, the 4th packet is received out of order.
(This could either be because the data was badly misordered in the
network, or because the 2nd packet was retransmitted and lost, and
then the 4th packet was retransmitted). At this point the data
receiver has only two SACK blocks to report. The data receiver
replies with the following Selective Acknowledgment:

Triggering ACK First Block 2nd Block 3rd Block
Segment Left Right Left Right Left Right
Edge Edge Edge Edge Edge Edge

6500 5500 6000 7500 8000 8500

Suppose at this point, the 2nd segment is received. The data
receiver then replies with the following Selective Acknowledgment:

Triggering ACK First Block 2nd Block 3rd Block
Segment Left Right Left Right Left Right
Edge Edge Edge Edge Edge Edge

5500 7500 8000 8500

8. Data Receiver Reneging

Note that the data receiver is permitted to discard data in its queue
that has not been acknowledged to the data sender, even if the data
has already been reported in a SACK option. Such discarding of
SACKed packets is discouraged, but may be used if the receiver runs
out of buffer space.

The data receiver MAY elect not to keep data which it has reported in
a SACK option. In this case, the receiver SACK generation is
additionally qualified:

* The first SACK block MUST reflect the newest segment. Even if
the newest segment is going to be discarded and the receiver has
already discarded adjacent segments, the first SACK block MUST
report, at a minimum, the left and right edges of the newest
segment.

* Except for the newest segment, all SACK blocks MUST NOT report
any old data which is no longer actually held by the receiver.

Since the data receiver may later discard data reported in a SACK
option, the sender MUST NOT discard data before it is acknowledged by
the Acknowledgment Number field in the TCP header.

9. Security Considerations

This document neither strengthens nor weakens TCP's current security
properties.

10. References

[Cheriton88] Cheriton, D., "VMTP: Versatile Message Transaction
Protocol", RFC1045, Stanford University, February 1988.

[Clark87] Clark, D., Lambert, M., and L. Zhang, "NETBLT: A Bulk Data
Transfer Protocol", RFC998, MIT, March 1987.

[Fall95] Fall, K. and Floyd, S., "Comparisons of Tahoe, Reno, and
Sack TCP", FTP://ftp.ee.lbl.gov/papers/sacks.ps.Z, December 1995.

[Floyd96] Floyd, S., "Issues of TCP with SACK",
ftp://ftp.ee.lbl.gov/papers/issues_sa.ps.Z, January 1996.

[Huitema81] Huitema, C., and Valet, I., An Experiment on High Speed
File Transfer using Satellite Links, 7th Data Communication
Symposium, Mexico, October 1981.

[Jacobson88] Jacobson, V., "Congestion Avoidance and Control",
Proceedings of SIGCOMM '88, Stanford, CA., August 1988.

[Jacobson88}, Jacobson, V. and R. Braden, "TCP Extensions for Long-
Delay Paths", RFC1072, October 1988.

[Jacobson92] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
for High Performance", RFC1323, May 1992.

[Keshav94] Keshav, presentation to the Internet End-to-End Research
Group, November 1994.

[Mathis95] Mathis, M., and Mahdavi, J., TCP Forward Acknowledgment
Option, presentation to the Internet End-to-End Research Group, June
1995.

[Partridge87] Partridge, C., "Private Communication", February 1987.

[Postel81] Postel, J., "Transmission Control Protocol - DARPA
Internet Program Protocol Specification", RFC793, DARPA, September
1981.

[Stevens94] Stevens, W., TCP/IP Illustrated, Volume 1: The Protocols,
Addison-Wesley, 1994.

[Strayer92] Strayer, T., Dempsey, B., and Weaver, A., XTP -- the
xpress transfer protocol. Addison-Wesley Publishing Company, 1992.

[Velten84] Velten, D., Hinden, R., and J. Sax, "Reliable Data
Protocol", RFC908, BBN, July 1984.

11. Authors' Addresses

Matt Mathis and Jamshid Mahdavi
Pittsburgh Supercomputing Center
4400 Fifth Ave
Pittsburgh, PA 15213
mathis@psc.edu
mahdavi@psc.edu

Sally Floyd
Lawrence Berkeley National Laboratory
One Cyclotron Road
Berkeley, CA 94720
floyd@ee.lbl.gov

Allyn Romanow
Sun Microsystems, Inc.
2550 Garcia Ave., MPK17-202
Mountain View, CA 94043
allyn@eng.sun.com