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C More aggressive compression Connected: An Internet Encyclopedia
C More aggressive compression

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C More aggressive compression

C More aggressive compression

As noted in sec. 3.2.2, easily detected patterns exist in the stream of compressed headers, indicating that more compression could be done. Would this be worthwhile?

The average compressed datagram has only seven bits of header./50/ The framing must be at least one bit (to encode the `type') and will probably be more like two to three bytes. In most interesting cases there will be at least one byte of data. Finally, the end-to-end check---the TCP checksum---must be passed through unmodified./51/

The framing, data and checksum will remain even if the header is completely compressed out so the change in average packet size is, at best, four bytes down to three bytes and one bit --- roughly a 25% improvement in delay./52/ While this may seem significant, on a 2400 bps line it means that typing echo response takes 25 rather than 29 ms. At the present stage of human evolution, this difference is not detectable.

However, the author sheepishly admits to perverting this compression scheme for a very special case data-acquisition problem: We had an instrument and control package floating at 200KV, communicating with ground level via a telemetry system. For many reasons (multiplexed communication, pipelining, error recovery, availability of well tested implementations, etc.), it was convenient to talk to the package using TCP/IP. However, since the primary use of the telemetry link was data acquisition, it was designed with an uplink channel capacity <0.5% the downlink's. To meet application delay budgets, data packets were 100 bytes and, since TCP acks every other packet, the relative uplink bandwidth for acks is a/200 where `a' is the total size of ack packets. Using the scheme in this paper, the smallest ack is four bytes which would imply an uplink bandwidth 2% of the downlink. This wasn't possible so we used the scheme described in footnote 15: If the first bit of the frame was one, it meant `same compressed header as last time'. Otherwise the next two bits gave one of the types described in sec. 3.2. Since the link had excellent forward error correction and traffic made only a single hop, the TCP checksum was compressed out (blush!) of the `same header' packet types/53/ so the total header size for these packets was one bit. Over several months of operation, more than 99% of the 40 byte TCP/IP headers were compressed down to one bit./54/


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