Network Working Group T. Shepard
Request for Comments: 2416 C. Partridge
Category: Informational BBN Technologies
September 1998

When TCP Starts Up With Four Packets Into Only Three Buffers

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Abstract

This memo is to document a simple eXPeriment. The experiment showed
that in the case of a TCP receiver behind a 9600 bps modem link at
the edge of a fast Internet where there are only 3 buffers before the
modem (and the fourth packet of a four-packet start will surely be
dropped), no significant degradation in performance is experienced by
a TCP sending with a four-packet start when compared with a normal
slow start (which starts with just one packet).

Background

Sally Floyd has proposed that TCPs start their initial slow start by
sending as many as four packets (instead of the usual one packet) as
a means of getting TCP up-to-speed faster. (Slow starts instigated
due to timeouts would still start with just one packet.) Starting
with more than one packet might redUCe the start-up latency over
long-fat pipes by two round-trip times. This proposal is documented
further in [1], [2], and in [3] and we assume the reader is familiar
with the details of this proposal.

On the end2end-interest mailing list, concern was raised that in the
(allegedly common) case where a slow modem is served by a router
which only allocates three buffers per modem (one buffer being
transmitted while two packets are waiting), that starting with four
packets would not be good because the fourth packet is sure to be
dropped.

Vern Paxson replied with the comment (among other things) that the
four-packet start is no worse than what happens after two round trip
times in normal slow start, hence no new problem is introduced by
starting with as many as four packets. If there is a problem with a
four-packet start, then the problem already exists in a normal slow-
start startup after two round trip times when the slow-start
algorithm will release into the net four closely spaced packets.

The experiment reported here confirmed Vern Paxson's reasoning.

Scenario and experimental setup

+--------+ 100 Mbps +---+ 1.5 Mbps +---+ 9600 bps +----------+
source +------------+ R +-------------+ R +--------------+ receiver
+--------+ no delay +---+ 25 ms delay +---+ 150 ms delay +----------+



(we spy here) (this router has only 3 buffers
to hold packets going into the
9600 bps link)

The scenario studied and simulated consists of three links between
the source and sink. The first link is a 100 Mbps link with no
delay. It connects the sender to a router. (It was included to have
a means of logging the returning ACKs at the time they would be seen
by the sender.) The second link is a 1.5 Mbps link with a 25 ms
one-way delay. (This link was included to roughly model traversing
an un-congested, intra-continental piece of the terrestrial
Internet.) The third link is a 9600 bps link with a 150 ms one-way
delay. It connects the edge of the net to a receiver which is behind
the 9600 bps link.

The queue limits for the queues at each end of the first two links
were set to 100 (a value sufficiently large that this limit was never
a factor). The queue limits at each end of the 9600 bps link were
set to 3 packets (which can hold at most two packets while one is
being sent).

Version 1.2a2 of the the NS simulator (available from LBL) was used
to simulate both one-packet and four-packet starts for each of the
available TCP algorithms (tahoe, reno, sack, fack) and the conclusion
reported here is independent of which TCP algorithm is used (in
general, we believe). In this memo, the "tahoe" module will be used
to illustrate what happens. In the 4-packet start cases, the
"window-init" variable was set to 4, and the TCP implementations were
modified to use the value of the window-init variable only on

connection start, but to set cwnd to 1 on other instances of a slow-
start. (The tcp.cc module as shipped with ns-1.2a2 would use the
window-init value in all cases.)

The packets in simulation are 1024 bytes long for purposes of
determining the time it takes to transmit them through the links.
(The TCP modules included with the LBL NS simulator do not simulate
the TCP sequence number mechanisms. They use just packet numbers.)

Observations are made of all packets and acknowledgements crossing
the 100 Mbps no-delay link, near the sender. (All descriptions below
are from this point of view.)

What happens with normal slow start

At time 0.0 packet number 1 is sent.

At time 1.222 an ack is received covering packet number 1, and
packets 2 and 3 are sent.

At time 2.444 an ack is received covering packet number 2, and
packets 4 and 5 are sent.

At time 3.278 an ack is received covering packet number 3, and
packets 6 and 7 are sent.

At time 4.111 an ack is received covering packet number 4, and
packets 8 and 9 are sent.

At time 4.944 an ack is received covering packet number 5, and
packets 10 and 11 are sent.

At time 5.778 an ack is received covering packet number 6, and
packets 12 and 13 are sent.

At time 6.111 a duplicate ack is recieved (covering packet number 6).

At time 7.444 another duplicate ack is received (covering packet
number 6).

At time 8.278 a third duplicate ack is received (covering packet
number 6) and packet number 7 is retransmitted.

(And the trace continues...)

What happens with a four-packet start

At time 0.0, packets 1, 2, 3, and 4 are sent.

At time 1.222 an ack is received covering packet number 1, and
packets 5 and 6 are sent.

At time 2.055 an ack is received covering packet number 2, and
packets 7 and 8 are sent.

At time 2.889 an ack is received covering packet number 3, and
packets 9 and 10 are sent.

At time 3.722 a duplicate ack is received (covering packet number 3).

At time 4.555 another duplicate ack is received (covering packet
number 3).

At time 5.389 a third duplicate ack is received (covering packet
number 3) and packet number 4 is retransmitted.

(And the trace continues...)

Discussion

At the point left off in the two traces above, the two different
systems are in almost identical states. The two traces from that
point on are almost the same, modulo a shift in time of (8.278 -
5.389) = 2.889 seconds and a shift of three packets. If the normal
TCP (with the one-packet start) will deliver packet N at time T, then
the TCP with the four-packet start will deliver packet N - 3 at time
T - 2.889 (seconds).

Note that the time to send three 1024-byte TCP segments through a
9600 bps modem is 2.66 seconds. So at what time does the four-
packet-start TCP deliver packet N? At time T - 2.889 + 2.66 = T -
0.229 in most cases, and in some cases earlier, in some cases later,
because different packets (by number) experience loss in the two
traces.

Thus the four-packet-start TCP is in some sense 0.229 seconds (or
about one fifth of a packet) ahead of where the one-packet-start TCP
would be. (This is due to the extra time the modem sits idle while
waiting for the dally timer to go off in the receiver in the case of
the one-packet-start TCP.)

The states of the two systems are not exactly identical. They differ
slightly in the round-trip-time estimators because the behavior at
the start is not identical. (The observed round trip times may differ
by a small amount due to dally timers and due to that the one-packet
start experiences more round trip times before the first loss.) In
the cases where a retransmit timer did later go off, the additional

difference in timing was much smaller than the 0.229 second
difference discribed above.

Conclusion

In this particular case, the four-packet start is not harmful.

Non-conclusions, opinions, and future work

A four-packet start would be very helpful in situations where a
long-delay link is involved (as it would reduce transfer times for
moderately-sized transfers by as much as two round-trip times). But
it remains (in the authors' opinions at this time) an open question
whether or not the four-packet start would be safe for the network.

It would be nice to see if this result could be duplicated with real
TCPs, real modems, and real three-buffer limits.

Security Considerations

This document discusses a simulation study of the effects of a
proposed change to TCP. Consequently, there are no security
considerations directly related to the document. There are also no
known security considerations associated with the proposed change.

References

1. S. Floyd, Increasing TCP's Initial Window (January 29, 1997).
URL FTP://ftp.ee.lbl.gov/papers/draft.jan29.

2. S. Floyd and M. Allman, Increasing TCP's Initial Window (July,
1997). URL http://gigahertz.lerc.nasa.gov/~mallman/share/draft-
ss.txt

3. Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
Initial Window", RFC2414, September 1998.

Authors' Addresses

Tim Shepard
BBN Technologies
10 Moulton Street
Cambridge, MA 02138

EMail: shep@alum.mit.edu

Craig Partridge
BBN Technologies
10 Moulton Street
Cambridge, MA 02138

EMail: craig@bbn.com

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