Re: Serial TCP/IP - all the same?


William Westfield (BILLW@MATHOM.CISCO.COM)
Mon 22 Aug 88 13:29:06-PDT


There are many different serial encapsulations used by TCP/IP over
serial lines. To start with, it depends on whether the line is
synchronous or asynchronous. The protocol used on async lines (SLIP)
is pretty standard, and has its own RFC (RFC1055). On synchronous lines,
things are more complicated - for example, our (cisco Systems) products
support about 5 different encapsulations. Two of these are X.25 and
DDN X.25, which are pretty much standardized. An ordinary X.25 protocol
analyzer is useful for debugging these. We also support LAPB (level 2
of X.25) with and without a "packet type" field, and a sort of bare HDLC
protocol (which is not any sort of standard, but can be documented).
Other vendors are rumored to use DEC's DDCMP protocols, and/or various
non-standard protocols, some of which are considered proprietary.

There is currently an Internet Engineering Task Force who is
investigating the specification of a standard for Point-to-Point
serial links, but this is likely to take a while, and it is still
likely that a vendor would like to talk their own protocol to their
own boxes.

I don't know of any serial analyzers that support ANY of the TCP/IP
formats used on serial lines - even the standard ones...

Bill Westfield
cisco Systems.
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