draft-shepard-tcp-4-packets-3-buff-00.txt Tim Shepard
Internet Draft Craig Partridge
BBN Technologies
July 1997
This Internet Draft expires February 4, 1998.
When TCP Starts Up With Four Packets Into Only Three Buffers
Status of this Memo
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This memo provides information for the Internet community. This memo
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Background and Abstract
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] and in [2] 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
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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.
This memo is to document that in the case of a 9600 bps modem at the
edges 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
with a four-packet start when compared with a normal slow start
(which starts with one packet).
Scenario and experimental setup
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 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. The third link is a 9600
bps link with a 150 ms one-way delay. 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 alogorithms (tahoe, reno, sack, fack) and the
conclusion reported here is independent of which TCP algorithm is
used (in general, we believe). The "tahoe" module will be used to
illustrate what happens in this memo. 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.)
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Observations are made of all packets and acknowledgements crossing
the 100 Mbps no-delay link. (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.
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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.
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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.
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 (To be submitted as an Internet Draft).
Authors' Addresses:
Tim Shepard, Craig Partridge
BBN Technologies
10 Moulton Street
Cambridge, MA 02138
shep@bbn.com, craig@bbn.com
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