Subject: preemption in RSVP-TE

From: "Parmar, Pankaj N" <[email-masked]>
Date: Wed, 30 May 2001 09:50:46 -0700

I have a doubt regarding preemption, as explained in the draft, specifically
the following statement:

"if the requested bandwidth is less than the used bandwidth then the
processing is complete. If the requested bandwidth is available, but is in
use by lower priority sessions, then lower priority sessions (beginning with
lower priority) MAY be pre-empted to free the necessary bandwidth."

It is not clear when the session gets preempted. It seems preemption could
to be done during PATH message processing, but then there is a issue here.
If a lower priority session gets preempted at a given LSR, and gets
forwarded downstream, it is possible that another downstream LSR may reject
the PATH message and send a PATH_ERR upstream. The issue is, a lower
priority session got preempted for no reason. On the other hand, if
preemption is done during a RESV message, we are more certain that we really
want to preempt a lower priority session.

Experts, please comment.

Thanks,

-Pankaj

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From: Eric Gray <[email-masked]>
Date: Wed, 30 May 2001 13:34:09 -0400

Pankaj,

    Assuming you are referring to text in section 4.7.3 ("Procedures
applying to both C-Types") of draft-ietf-mpls-rsvp-lsp-tunnel-08,
at least part of the confusion stems from a typo in your quote.  The
correct word is 'unused' (as in 'unused bandwidth').  In other words,
no preemption is needed where there is already sufficient 'unused'
bandwidth.

    Admittedly, a part of the confusion may be caused by a somewhat
arbitrary distinction between 'available' and 'unused'.  It helps to
read 'requested bandwidth is available' as 'requested bandwidth is
within the capacity of links which are usable for this session'.

--
Eric Gray