Re: Tp4


Bob Stine (stine@edn-vax.arpa)
24 Jan 86 18:57:23 GMT


One of the major differences between TCP and TP-4 is
in the support of passive opens. As related on p.16 of
RFC 942, TP-4 "passes a Call Indication to a user entity
whenever a Call Request is received." But, it is left
as ".. an implementation decision as to how TP-4 finds
and/or creates an appropriate user entity to give the
Call Indication...". In other words, there is "... no
Listen Service Primitive." This should make implementation
of server processes interesting.

Also, though TP-4 allows for the transmission of up to
16 octets of "expedited data," it has no equivalent of
the TCP 'push.' 'Push' is useful for supporting record
oriented or transaction oriented applications. Each
expedited data PDU of TP-4 requires a separate ack, which
could lead to high control traffic overhead if this option
is overused.

Another difference that could impact protocol developers is
the fact that TP-4 uses negotiated "segment sizes" (max
allowable TPDU sizes), rather than octets, as its unit of
space allocation. Inappropriate segment sizes could result
in over-allocation, and under utilization, of available
memory resources (e.g., tinygrams could be stored in blocks
of 512 bytes).

Many of the other differences between TCP and TP-4 have to
do with what TP-4 has not yet specified. In particular,
"... the use of addresses at different levels in the ISO
model has not yet been solidified ... addressing capabilities
similar to TCP's will EVENTUALLY be provided by TP-4 ..."
(p.15, RFC 942, emphasis added).



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