RFC2001 - TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recove

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　　Network Working Group W. Stevens
Request for Comments: 2001 NOAO
Category: Standards Track January 1997

TCP Slow Start, Congestion Avoidance,
Fast Retransmit, and Fast Recovery Algorithms

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

Modern implementations of TCP contain four intertwined algorithms
that have never been fully documented as Internet standards: slow
start, congestion avoidance, fast retransmit, and fast recovery. [2]
and [3] provide some details on these algorithms, [4] provides
examples of the algorithms in action, and [5] provides the source
code for the 4.4BSD implementation. RFC1122 requires that a TCP
must implement slow start and congestion avoidance (Section 4.2.2.15
of [1]), citing [2] as the reference, but fast retransmit and fast
recovery were implemented after RFC1122. The purpose of this
document is to document these four algorithms for the Internet.

Acknowledgments

Much of this memo is taken from "TCP/IP Illustrated, Volume 1: The
Protocols" by W. Richard Stevens (Addison-Wesley, 1994) and "TCP/IP
Illustrated, Volume 2: The Implementation" by Gary R. Wright and W.
Richard Stevens (Addison-Wesley, 1995). This material is used with
the permission of Addison-Wesley. The four algorithms that are
described were developed by Van Jacobson.

1. Slow Start

Old TCPs would start a connection with the sender injecting multiple
segments into the network, up to the window size advertised by the
receiver. While this is OK when the two hosts are on the same LAN,
if there are routers and slower links between the sender and the
receiver, problems can arise. Some intermediate router must queue
the packets, and it's possible for that router to run out of space.
[2] shows how this naive approach can reduce the throughput of a TCP
connection drastically.

The algorithm to avoid this is called slow start. It operates by
observing that the rate at which new packets should be injected into
the network is the rate at which the acknowledgments are returned by
the other end.

Slow start adds another window to the sender's TCP: the congestion
window, called "cwnd". When a new connection is established with a
host on another network, the congestion window is initialized to one
segment (i.e., the segment size announced by the other end, or the
default, typically 536 or 512). Each time an ACK is received, the
congestion window is increased by one segment. The sender can
transmit up to the minimum of the congestion window and the
advertised window. The congestion window is flow control imposed by
the sender, while the advertised window is flow control imposed by
the receiver. The former is based on the sender's assessment of
perceived network congestion; the latter is related to the amount of
available buffer space at the receiver for this connection.

The sender starts by transmitting one segment and waiting for its
ACK. When that ACK is received, the congestion window is incremented
from one to two, and two segments can be sent. When each of those
two segments is acknowledged, the congestion window is increased to
four. This provides an exponential growth, although it is not
exactly exponential because the receiver may delay its ACKs,
typically sending one ACK for every two segments that it receives.

At some point the capacity of the internet can be reached, and an
intermediate router will start discarding packets. This tells the
sender that its congestion window has gotten too large.

Early implementations performed slow start only if the other end was
on a different network. Current implementations always perform slow
start.

2. Congestion Avoidance

Congestion can occur when data arrives on a big pipe (a fast LAN) and
gets sent out a smaller pipe (a slower WAN). Congestion can also
occur when multiple input streams arrive at a router whose output
capacity is less than the sum of the inputs. Congestion avoidance is
a way to deal with lost packets. It is described in [2].

The assumption of the algorithm is that packet loss caused by damage
is very small (much less than 1%), therefore the loss of a packet
signals congestion somewhere in the network between the source and
destination. There are two indications of packet loss: a timeout
occurring and the receipt of duplicate ACKs.

Congestion avoidance and slow start are independent algorithms with
different objectives. But when congestion occurs TCP must slow down
its transmission rate of packets into the network, and then invoke
slow start to get things going again. In practice they are
implemented together.

Congestion avoidance and slow start require that two variables be
maintained for each connection: a congestion window, cwnd, and a slow
start threshold size, ssthresh. The combined algorithm operates as
follows:

1. Initialization for a given connection sets cwnd to one segment
and ssthresh to 65535 bytes.

2. The TCP output routine never sends more than the minimum of cwnd
and the receiver's advertised window.

3. When congestion occurs (indicated by a timeout or the reception
of duplicate ACKs), one-half of the current window size (the
minimum of cwnd and the receiver's advertised window, but at
least two segments) is saved in ssthresh. Additionally, if the
congestion is indicated by a timeout, cwnd is set to one segment
(i.e., slow start).

4. When new data is acknowledged by the other end, increase cwnd,
but the way it increases depends on whether TCP is performing
slow start or congestion avoidance.

If cwnd is less than or equal to ssthresh, TCP is in slow start;
otherwise TCP is performing congestion avoidance. Slow start
continues until TCP is halfway to where it was when congestion
occurred (since it recorded half of the window size that caused
the problem in step 2), and then congestion avoidance takes over.

Slow start has cwnd begin at one segment, and be incremented by
one segment every time an ACK is received. As mentioned earlier,
this opens the window exponentially: send one segment, then two,
then four, and so on. Congestion avoidance dictates that cwnd be
incremented by segsize*segsize/cwnd each time an ACK is received,
where segsize is the segment size and cwnd is maintained in bytes.
This is a linear growth of cwnd, compared to slow start's
exponential growth. The increase in cwnd should be at most one
segment each round-trip time (regardless how many ACKs are
received in that RTT), whereas slow start increments cwnd by the
number of ACKs received in a round-trip time.

Many implementations incorrectly add a small fraction of the segment
size (typically the segment size divided by 8) during congestion
avoidance. This is wrong and should not be emulated in future
releases.

3. Fast Retransmit

Modifications to the congestion avoidance algorithm were proposed in
1990 [3]. Before describing the change, realize that TCP may
generate an immediate acknowledgment (a duplicate ACK) when an out-
of-order segment is received (Section 4.2.2.21 of [1], with a note
that one reason for doing so was for the experimental fast-
retransmit algorithm). This duplicate ACK should not be delayed.
The purpose of this duplicate ACK is to let the other end know that a
segment was received out of order, and to tell it what sequence
number is expected.

Since TCP does not know whether a duplicate ACK is caused by a lost
segment or just a reordering of segments, it waits for a small number
of duplicate ACKs to be received. It is assumed that if there is
just a reordering of the segments, there will be only one or two
duplicate ACKs before the reordered segment is processed, which will
then generate a new ACK. If three or more duplicate ACKs are
received in a row, it is a strong indication that a segment has been
lost. TCP then performs a retransmission of what appears to be the
missing segment, without waiting for a retransmission timer to
expire.

4. Fast Recovery

After fast retransmit sends what appears to be the missing segment,
congestion avoidance, but not slow start is performed. This is the
fast recovery algorithm. It is an improvement that allows high
throughput under moderate congestion, especially for large windows.

The reason for not performing slow start in this case is that the
receipt of the duplicate ACKs tells TCP more than just a packet has
been lost. Since the receiver can only generate the duplicate ACK
when another segment is received, that segment has left the network
and is in the receiver's buffer. That is, there is still data
flowing between the two ends, and TCP does not want to reduce the
flow abruptly by going into slow start.

The fast retransmit and fast recovery algorithms are usually
implemented together as follows.

1. When the third duplicate ACK in a row is received, set ssthresh
to one-half the current congestion window, cwnd, but no less
than two segments. Retransmit the missing segment. Set cwnd to
ssthresh plus 3 times the segment size. This inflates the
congestion window by the number of segments that have left the
network and which the other end has cached (3).

2. Each time another duplicate ACK arrives, increment cwnd by the
segment size. This inflates the congestion window for the
additional segment that has left the network. Transmit a
packet, if allowed by the new value of cwnd.

3. When the next ACK arrives that acknowledges new data, set cwnd
to ssthresh (the value set in step 1). This ACK should be the
acknowledgment of the retransmission from step 1, one round-trip
time after the retransmission. Additionally, this ACK should
acknowledge all the intermediate segments sent between the lost
packet and the receipt of the first duplicate ACK. This step is
congestion avoidance, since TCP is down to one-half the rate it
was at when the packet was lost.

The fast retransmit algorithm first appeared in the 4.3BSD Tahoe
release, and it was followed by slow start. The fast recovery
algorithm appeared in the 4.3BSD Reno release.

5. Security Considerations

Security considerations are not discussed in this memo.

6. References

[1] B. Braden, ed., "Requirements for Internet Hosts --
Communication Layers," RFC1122, Oct. 1989.

[2] V. Jacobson, "Congestion Avoidance and Control," Computer
Communication Review, vol. 18, no. 4, pp. 314-329, Aug. 1988.
ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z.

[3] V. Jacobson, "Modified TCP Congestion Avoidance Algorithm,"
end2end-interest mailing list, April 30, 1990.
ftp://ftp.isi.edu/end2end/end2end-interest-1990.mail.

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

[5] G. R. Wright, W. R. Stevens, "TCP/IP Illustrated, Volume 2:
The Implementation", Addison-Wesley, 1995.

Author's Address:

W. Richard Stevens
1202 E. Paseo del Zorro
Tucson, AZ 85718

Phone: 520-297-9416

EMail: rstevens@noao.edu
Home Page: http://www.noao.edu/~rstevens

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