INTERNET-DRAFT                         Robert Pohlmann
Expire in six months                      October 1997


     HTTP Traffic over Satellite
<draft-pohlmann-http-traffic-satellite-00.txt>


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Abstract

Internet use over satellites (LEO, MEO and GEO) has not
received enough attention.  Satellites have an inherent
broadcasting ability useful for transmitting Internet
traffic.  They can send information to many locations
simultaneously depending on the coverage beam of the
satellite.  The issue of Internet use over satellites needs
to be addressed so that future implementations of networking
protocols can be used with this medium.  The vast majority
of Internet traffic is TCP traffic resulting from the HTTP
protocol so this issue will be addressed here.  It is hoped
that this facilitates the transition from the use of
terrestial networks, to networks incorporating satellite
links.

Acknowledgements

The TCP algorithms that are mentioned were developed by Van
Jacobsen.  An excellent explanation of their use is
described in [1].


.  The Issues

Several issues arise when using satellites (LEO, MEO and
GEO) for information transmission.  The delay, or latency,
of a satellite link (sender to receiver) is greater than a
terrestrial communications link.  This varies depending on
the type of satellite.  LEO satellites typically have a
latency on the order of 100 milliseconds and GEO satellites
have a latency of approximately 250 milliseconds.  The data
traveling over a satellite will also travel over terrestrial
links in addition to the satellite link.  Consequently the
latency from sender to receiver can grow to as much as 400
msec when using a GEO satellite.  These latency values raise
questions.  What effect will the latency have on HTTP
traffic, and am I using the communications link effectively?

The other issue in a satellite link is that errors may be
bursty rather than random and so lost packets may be an
indication of discarding due to errors rather than
congestion.  Although there are powerful error-correcting
codes used on a satellite link, the distribution of errors
in packets that have traversed a satellite link are not as
uniformly distributed as errors seen on terrestrial
connections such as fiber-optic cable.  This can cause
problems for networks using technology where the error
correction mechanisms are not meant to deal with multiple
errored bits in a cell.  ATM is one such technology.  In
this case it may be helpful to reduce the bit error rate of
the link in order to reduce the probability of grouping
errored bits.

.  Consequences for HTTP Traffic

TCP's receive buffer or receive window is very important for
good performance of TCP.  A rule of thumb is that the
optimum receive buffer size is the bandwidth-delay product
of the link.  The Internet user would like to make efficient
use of bandwidth by using a TCP receive buffer approximately
equal to the bandwidth-delay product.  Assuming an
individual is downloading a web page using a 56.6 kbps
modem, the bandwidth they would use is around 7 kbytes/sec.
Assuming an end to end delay of 1.0 second, an optimum
receive window would be 7 kbytes.  Since most PC's use
receive windows of 8 kbytes or slightly more, they make
efficient use of the connection.  Doubling the speed of the
connection means that a window size of 14 kbytes would be
needed.  This would mean the user cannot increase their
window size enough to make efficient use of the TCP
connection.

RFC1323 [2], specifies "large windows", which are windows
greater than TCP's maximum theoretical window of 64 kbytes
and practical window of 32 kbytes.  Large windows increase
the theoretical maximum window size of TCP to 2^32 bytes.
Unfortunately, the adoption and use of TCP with RFC1323
support is not widespread even though its importance is
increasing.  Newer versions of HTTP state that HTTP will use
a single TCP connection rather than multiple connections as
in HTTP/1.0.  With all of the HTTP data flowing through a
single connection, it is critical for TCP to efficiently use
the channel bandwidth.  TCP is making efficient use of the
bandwidth when it is in congestion avoidance mode.
Congestion avoidance mode is where the importance of the
receive window is greatest since TCP is trying to have many
segments "in route" between the sender and receiver.  A
smaller than optimum window means that all of the available
bandwidth is not being used.  Recent work [3] shows that the
penalty suffered for using "smaller than optimum" window
sizes increases with increasing file size and increasing
latency.  Consequently, the importance of large windows is
increasing.

When TCP is in slow-start mode it is not making efficient
use of the channel's bandwidth.  TCP is initially in slow
start mode and falls back into slow-start if no data is sent
in a period approximately equal to the round trip time of a
segment.  HTTP/1.1, which sends data through a single TCP
connection rather than multiple TCP connections, will reduce
the chances of TCP falling back into slow-start.  When slow
start begins, TCP starts the sender's congestion window at
one segment.  This lets TCP determine the bandwidth that is
available to it.  One technique that is being investigated
for using TCP over satellites is to start the congestion
window at a value greater than one segment. This would mean
that TCP starts out using a larger percentage of the
available bandwidth.  Since the available bandwidth will
usually allow sending many segments per window, it is overly
conservative for TCP to start out sending a single segment.
Other techniques are implemented in the routers in a
network.  One such protocol is called RED [4] and it will
help in congested networks.  RED will allow TCP to prepare
itself for increasing congestion in a controlled manner.  By
discarding a few TCP segments as congestion begins, TCP will
stop increasing its congestion window.  This will prevent
the loss of many segments that could force TCP to severely
reduce its congestion window.  By preventing a large
reduction in the congestion window, the available bandwidth
can be better used.
.  References
     [1]  W. R. Stevens, "TCP Slow Start, Congestion
Avoidance, Fast Retransmit, and Fast Recovery Algorithms"
RFC 2001, Jan 1997.
      [2]  V. Jacobson, R. Braden, and D. Borman, "TCP
Extensions for High Performance" RFC 1323, May 1992.

     []  Y Zhang, D DeLucia, B Ryu, S Dao, "Satellite
Communications in the Global Internet: Issues, Pitfalls, and
Potential",
http://www.isoc.org/isoc/whatis/conferences/inet/97/proceedi
ngs/F5/F5-1.HTM

     []  S. Floyd and V. Jacobson. "Random early dectection
gateways for congestion avoidance", IEEE/ACM Transactions on
Networking, 1(4):397-413, Aug 1993

Author's Address:

     Robert Pohlmann
     1761 Business Center Drive
     Reston, VA 20190

     Phone: (703) 438-7805

     Email: pohlmann@sed.stel.com