Subject: Re: draft-ietf-pilc-asym-00.txt

From: Goran Johansson <[email-masked]>
Date: Thu, 14 Oct 1999 15:35:39 +0200

Hi!

We are working on an idea to remedy the effects of bandwidth asymmetry
using a version of TCP/IP header compression. Our algorithm should be
able to achieve effective ACK-sizes considerably smaller than one octet
and thus remove the effective bandwidth asymmetry in most realistic
systems.

We will implement this algorithm and hopefully we have
some quantitative information about its perfomance soon.

We implement a compressor at the queue where ACKs get congested
and a decompressor somewhere on the other side of the bottleneck link.
The compressor concatenates and compresses ACKs belonging to the
same TCP-connection. We add information about queue-arrival-time
of the concatenated ACKs. The decompressor then recreates the original
ACKs and sends them uplink with the same intervals as they arrived at
the bottleneck queue.

The key ingredient is the concatenation which makes the compression
really efficient. The effective ACK size scales as the inverse of the
number of concatenated ACKs. We believe it is possible to achieve an
effective ACK size of a fraction of an octet.

Standard header compression uses a differential coding scheme which
makes it vulnerable to packet losses. We code differences compared to
the SYN or SYN ACK header, stored as template in the decompressor,
so if we lose a compressed ACK packet we lose a number of ACKs
but nothing else. We also intend to compress the Timestamp and
SACK options.

You may see our method as a variant of ACK filtering and ACK
reconstruction, but we send enough information to fully recreate the
original number of ACKs and their temporal spacing, without making
educated guesses.

We believe that our method could be mentioned in the asymmetry-draft
and are open for discussions.

---------------------------------
Göran Johansson
Ericsson Mobile Data Design AB