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draft-partridge-e2e-ackspacing-00.txt
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End-To-End Research Group C. Partridge
Request for Comments: DRAFT BBN Technologies
Category: Informational July 1997
ACK Spacing for High Delay-Bandwidth Paths with Insufficient Buffering
<draft-partridge-e2e-ackspacing-00.txt>
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as ``work in progress.''
To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet- Drafts
Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
This document is a product of the End-To-End Research Group of the
Internet Research Task Force. Comments are solicited and should be
addressed to the author or the End-To-End Interest list (end2end-
interest@isi.edu).
This draft expires 29 January 1998.
1. Introduction
Suppose you want TCP implementations to be able to fill a 155 Mb/s
path. Further suppose that the path includes a satellite in a
geosynchronous orbit, so the round trip delay through the path is at
least 500 ms, and the delay-bandwidth product is 9.7 megabytes or
more.
If we further assume the TCP implementations support TCP Large
Windows and PAWS (many do), so they can manage 9.7 MB TCP window,
then we can be sure the TCP will eventually start sending at full
<draft-e2e-ackspacing-00.txt> [Page 1]
RFC DRAFT July 1997
path rate (unless the satellite channel is very lossy). But it may
take a long time to get the TCP up to full speed.
One (of several) possible causes of the delay is a shortage of
buffering in routers. To understand this particular problem,
consider the following idealized behavior of TCP during slow start.
During slow start, for every segment ACKed, the sender transmits two
new segments. In effect, this behavior means the sender is
transmitting at *twice* the data rate of the segments being ACKed.
And keep in mind the separation between ACKs represents (in an ideal
world) the rate segments can flow through the bottleneck router in
the path. So the sender is bursting data at twice the bottleneck
rate, and a queue must be forming during the burst. In the simplest
case, the queue is entirely at the bottleneck router, and at the end
of the burst, the queue is storing half the data in the burst. (Why
half? During the burst, we transmitted at twice the bottleneck rate.
Suppose it takes one time unit to send a segment on the bottlenecked
link. During the burst the bottleneck will receive two segments in
every time unit, but only be able to transmit one segment. The
result is a net of one new segment queued every time unit, for the
life of the burst.)
TCP will end the slow start phase in response to the first lost
datagram. Assuming good quality transmission links, the first lost
datagram will be lost because the bottleneck queue overflowed. We'd
like that loss to occur in the round-trip after the slow start
congestion window has reached the delay-bandwidth product. Now
consider the buffering required in the bottleneck link during the
next to last round trip. The sender will send an entire delay-
bandwidth worth of data in one-half a round-trip time (because it
sends at twice the channel rate). So for half the round-trip time,
the bottleneck router is in the mode of forwarding one segment while
receiving two. (For the second half of the round-trip, the router is
draining its queue). That means, to avoid losing any segments, the
router must have buffering equal to half the delay-bandwidth product,
or nearly 5 MB.
Most routers do not have anywhere near 5 MB of buffering for a single
link. Or, to express this problem another way, because routers do
not have this much buffering, the slow start stage will end
prematurely, when router buffering is exhausted. The consequence of
ending slow start prematurely is severe. At the end of slow start,
TCP goes into congestion avoidance, in which the window size is
increased much more slowly. So even though the channel is free,
because we did not have enough router buffering, we will transmit
slowly for a period of time (until the more conservative congestion
avoidance algorithm sends enough data to fill the channel).
<draft-e2e-ackspacing-00.txt> [Page 2]
RFC DRAFT July 1997
2. What to Do?
So how to get around the shortage of router buffering?
2.1 Cascading TCPs
One approach is to use cascading TCPs, in which we build a custom TCP
for the satellite (or bottleneck) link and insert it between the
sender's and receiver's TCPs, as shown below:
sender ---- Ground station -- satellite -- ground station -- receiver
|---------------| |------------------------| |---------------|
| loop 1 | | loop 2 | | loop 3 |
|---------------| |------------------------| |---------------|
This approach can work but is awkward. Among its limitations are:
the buffering problem remains (at points of bandwidth mismatches,
queues will form); the scheme violates end-to-end semantics of TCP
(the sender will get ACKs for data that has not and may never reach
the receiver); and it doesn't work with encryption (i.e. if data
above the IP layer is encrypted).
2.2 ACK Spacing
Another approach is to find some way to spread the bursts, either by
having the sender spread out the segments, or having the network
arrange for the ACKs to arrive at the sender with a two segment
spacing (or larger).
Changing the sender is feasible, although it requires very good
operating system timers. But it has the disadvantage that only
upgraded senders get the performance improvement.
Finding a way for the network to space the ACKs would allow TCP
senders to transmit at the right rate, without modification.
Furthermore, it can be done by a router. The router simply has to
snoop the returning TCP ACKs and spread them out. (Note that if the
transmissions are encrypted, in many scenarios the router can still
figure out which segments are likely TCP ACKs and spread them out).
There are some difficult issues with this approach. The most notable
ones are:
1. What algorithm to use to determine the proper ACK spacing.
<draft-e2e-ackspacing-00.txt> [Page 3]
RFC DRAFT July 1997
2. Related to (1), it may be necessary to known when a TCP is in
slow-start vs. congestion-avoidance, as the desired spacing
between ACKs is likely to be different in the two phases.
3. What to do about assymetric routes (if anything). If the ACKs
do not return through the ACK-spacing router, it may not be
possible to do ACK spacing.
Despite these challenges the approach has appeal. Changing software
in a few routers (particularly those at likely bottleneck links) on
high delay-bandwidth paths could give a performance boost to lots of
TCP connections.
Credit and Disclaimer
This memo presents thoughts from a discussion held at the recent
meeting of the End-To-End (E2E) Research Group. The particular idea
of ACK spacing was developed by during the meeting by Mark Handley
and Van Jacobson in response to an issue raised by the author, and
was inspired, in part by ideas to enhance wireless routers to improve
TCP performance [1].
The material presented is a half-baked suggestion and should not be
interpreted as an official recommendation of the Research Group.
References
1. H. Balakrishnan, V.N. Padmanabhan, S. Seshan and R.H. Katz, "A
Comparison of Mechanisms for Improving TCP Performance over Wireless
Links", Proc. ACM SIGCOMM '96, pp. 256-269.
<draft-e2e-ackspacing-00.txt> [Page 4]