Connected: An Internet Encyclopedia
Header Compression (CPPP/CSLIP)
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Header Compression (CPPP/CSLIP)
The Internet protocols introduce heavy overhead for some common
types of network traffic. Probably the most extreme case is an
interactive session with a user typing on a keyboard. With each
keystroke, a packet is sent to the remote host and a reply is returned.
Even though only a single byte of data is being sent, the addition
of a TCP header (20 bytes) and an IP header (another 20 bytes)
expand the resulting packet to at least 41 bytes. This represents
4000% overhead!
These problems are not apparent on Ethernet media, which enforces
a 64-byte minimum size on all packets at the Data Link Layer.
Other media may also be fast enough to disguise the problem,
but dial-up modems can't tolerate such inefficiencies.
RFC 1144
documents Van Jacobson compression, a popular technique
for compressing TCP/IP headers.
Van Jacobson compression can reduce packet header overhead from
4000% to 300%, enough to yield acceptable interactive performance
at 2400 bps.
This is often used in conjunction
with SLIP or PPP, and referred to as CSLIP or CPPP.
- Header, not data compression. Van Jacobson compression
makes no attempt to compress the data portion of the packet.
- End-to-end checksum. The orignal TCP checksum is always preserved,
transmitted unmodified, and used to check the validity of the reconstructed
packet. A sound case (the so-called end-to-end argument) can
be made on the inherent robustness of this approach. Of course, the
robustness is only as good as the checksum algorithm itself.
Connected: An Internet Encyclopedia
Header Compression (CPPP/CSLIP)