Subject: Flowspec > Tspec

From: Ron Cohen <[email-masked]>
Date: Tue, 11 Jan 2000 10:41:10 -0800

Hi,

There is an issue that confuse me in RFC 2205:

Section 1.3:

"Shared reservations, created by WF and SE styles, are appropriate for 
those multicast applications in which multiple data sources are unlikely to 
transmit simultaneously.  Packetized audio is an example of an application 
suitable for shared reservations; since a limited number of people talk at 
once, each receiver might issue a WF or SE reservation request for twice 
the bandwidth required for one sender (to allow some over-speaking)."

while in section:

Section 2.2:
...
"3.  (Re, Resv_Te) and Path_Te are passed to traffic control. Traffic 
control will compute the effective flowspec as the "minimum" of Path_Te and 
Resv_Te, in a service-dependent manner."

The first section seems to suggest that you can ask to reserve twice the 
rate specified in the TSPEC, while the second section seems to suggest that 
it is not possible, as the actual reservation will be limited by the TSPEC. 

What is the right interpretation?

Thanks
Ron

---

From: John Wroclawski <[email-masked]>
Date: Tue, 11 Jan 2000 17:21:54 -0500

>What is the right interpretation?

Ron,

The first. This is because the TSPEC's in all sender PATH messages 
for a session are summed before being used to bound the maximum 
reservation as described in the text quoted in your note. The effect 
is to bound the reservation to the minimum of 1) the sum of the 
senders' offered traffic, or 2) the requested reservation level.

This is done because at some point in every path the RESV message 
will have propagated upstream far enough to reach, say, a point where 
there is only one remaining upstream sender. If that sender is 
connected to the net by a link that is adequate for the sender, but 
not larger, than a request for twice or three or N times the sender's 
rate might fail completely. On the other hand, a request for the 
sender's rate can be honored, and does not change the semantics of SE 
or WF because there are no other senders sharing that particular 
link. So the reservation request is adjusted downwards accordingly.

John