Subject: Re: DLSw switch to switch protocol reliable transport alternatives

From: "jim (j.) cobban" <[email-masked]>
Date: Fri, 7 Apr 1995 09:30:00 -0400

In message "Re: DLSw switch to switch protocol reliable transport alternatives", 
you write:

>  I suspect that most
>  implementations will have difficulty dealing with the use of the second SAP,
>  since that would probably require the DLSw to inspect the XID as it passes
>  through.  Also since a second SAP is used without a prior circuit setup,
>  these UI frames will be handled as DATAFRAMEs rather than DGRMFRAMEs, which
>  would probably have a significant performance impact.
>
>Could you please explain a little bit more on this.
>thanks

If DLSw is the ONLY user of the LAN interface then there is no issue.  But
what if DLSw is present only for the purpose of optimising the handling of
SNA and NetBIOS traffic on the same interface where other protocols are being
handled by transparent or source-route bridging (as appropriate to the
interface).  In that case the interface must direct frames on an individual
basis to either DLSw or Bridging based upon the destination SAP.  That is
fine for conventional SNA, where you can pre-define which SAPs are used for
SNA traffic.  However since the HPR specification permits the individual
station to designate which SAP is to be used for HPR traffic, the XID must be
examined as it passes through to dynamically add the HPR SAP to the list of
SAPs which are being directed to DLSw.

The second point is that DLSw handles UI frames which arrive using the
MAC/SAP pair of an existing connection in an optimized manner by carrying
them as DATAFRAMEs with the existing circuit ids.  If we wish to handle the
HPR UI frames in an similarly efficient manner, it may be necessary to modify
the DLSw specification, so that a UI frame carried with the SAP which was
selected during HPR XID negotiation, is mapped over to a DGRMFRAME on the
already existing connection.  Otherwise the performance advantages of HPR may
be lost.

An alternative is to amend the HPR specification so that it does not use a
separate SAP pair.  In that case no modification of the DLSw specification is
required.