Path: utzoo!attcan!uunet!wuarchive!cs.utexas.edu!hellgate.utah.edu!basset.utah.edu!haas
From: haas%basset.utah.edu@cs.utah.edu (Walt Haas)
Newsgroups: comp.dcom.lans
Subject: Re: What services does X.25 provide?
Keywords: x.25, services, login, e-mail, file transfer, IPC
Message-ID: <1989Sep22.193558.21399@hellgate.utah.edu>
Date: 23 Sep 89 01:35:58 GMT
References: <796@maxim.erbe.se> <3279@wasatch.utah.edu> <522@wet.UUCP> <1167@virtech.UUCP>
Organization: University of Utah CS Dept
Lines: 25

In article <1167@virtech.UUCP> dennis@virtech.UUCP (Dennis P. Bednar) writes:
> While X.25 level 2 (the lowest protocol layer) has reliability built into it
>...reliablility is only between the host and the IMP[sic]
correct
> There is no guarantee of end-to-end reliablity between two hosts
>...
>In other words, when the local DTE on the left sends
>a level 3 DATA packet and receives a level 3 RR acknowledgement
>packet from the local IMP (on the left side of the above
>diagram), it only means that the local IMP has acked the
>packet, but not necessarily that the remove DTE in the
>right side of the figure has even received it.

Incorrect - if you set the D bit in the data packet, the RR has
end-to-end signifcance.  Paragraph 4.3.3 of CCITT Recommendation X.25:

4.3.3 Delivery confirmation bit

The setting of the Delivery Confirmation bit (D bit) is used to indicate
whether or not the DTE wises to receive an end-to-en acknowledgement of
delivery, for data it is transmitting, by means of the packet receive
sequence number P(R).


-- Walt