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From: tonyg@retix.retix.COM (Tony Goulding)
Newsgroups: comp.dcom.lans
Subject: Re: What services does X.25 provide?
Message-ID: <472@retix.retix.COM>
Date: 20 Sep 89 17:16:58 GMT
References: <796@maxim.erbe.se> <3279@wasatch.utah.edu> <522@wet.UUCP> <1167@virtech.UUCP>
Organization: Retix, Santa Monica CA
Lines: 29
In-reply-to: dennis@virtech.UUCP's message of 15 Sep 89 23:20:24 GMT

In article <1167@virtech.UUCP> dennis@virtech.UUCP (Dennis P. Bednar) writes:

> This is correct. X.25 is specified as a protocol between a DTE
> (packet switching host) and a DCE (IMP or interface message
> processor a.k.a. packet switching node). While X.25 level
> 2 (the lowest protocol layer) has reliability built into it
> (checksums, acks, retransmissions, sequenced packets), the
> reliablility is only between the host and the IMP. There is no
> guarantee of end-to-end reliablity between two hosts:

I  was under  the    impression  that,  depending  upon  the   network
configuration, the acknowledge can originate either from the local IMP
or  the remote   IMP.   As  such,  this  does  not provide  end-to-end
reliability.  But,  if  the  X.25 'D' bit   is  set to 1, end   to end
reliability is guaranteed,  as the packet  ack will then originate not
from one  of  the IMP's,  but from the   remote DTE.

In an OSI  stack, network  end-to-end reliability is  not assumed. So,
effectively, the D bit is set to 0.   A class 0 transport will provide
implicit  flow control (sequence   numbering),  class  2 will  provide
negotiated flow control (sequencing & credit).  Both these classes can
therefore detect unreliability  (out  of sequence packets)  and   will
disconnect.  They will  not recover.  Class  4 transport also provides
sequencing  and credit info,  and  will  reset  and retransmit  if  it
discovers a sequencing problem.  However, even class  4 will  give  up
after a while,  and so to  guarantee end-to-end reliability, a Session
layer would be used.

Tony.