Re: Can of worms revisited


CERF@A.ISI.EDU
23 Sep 1987 13:18-EDT


Public data nets in the U.S. and in other countries, utilizing
X.25 and X.75, have chosen an accounting fiction, the "segment"
as a way to deal with variance in actual and in maximum packet
sizes.

Exchange tariffs and user tariffs are based on the number of
segments sent or received by a particular pair of parties.
Typically the bill is accumulated against the originator of the
virtual circuit although reverse charging is permitted; usually
only domestically as international reverse charge calls are
sometimes hard to collect for.

The public network administration announces the segment size (in
octets) and the charges are for the number of octets sent on a
given virtual circuit. If a packet contains less than one
segment's worth or an integral number plus a fraction of
segments, the accounting is rounded up to the nearest segment for
that packet.

Between administrations that normally charge for service using
DIFFERENT segment sizes, there is an agreement at the X.75
interconnect on an interexchange segment size for purposes of
calculating balances due between each administration. Charges to
users are up to each domestic public data net - some stick with
their standard domestic segment size and some use the smaller of
the domestic and the size for going to the target administration.
This assumes, of course, that the target administration's network
is "one hop" from the originating network. There are cases of
transit nets, and these require 3-way negotiations to determine
how much of the user charges will be shared by each
administration.

In the case of datagram systems, similar segment size accounting
methods can be used - binding the charges to a particular user
rather than to a host might be the easier path since datagrams
often don't have user level identifying information in them.l

Vint Cerf



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