Subject: Flow Label consistency question

From: Ran Liebermann <[email-masked]>
Date: Thu, 21 Apr 2005 19:19:27 +0300

Hi,

I've been reading a few drafts and RFCs in this matter and I couldn't
find an explanation why the Flow Label field has to read the
destination with the same value the source sent it.

Wouldn't it introduce a whole lot of new capabilities (without as many
limitations) if the Flow Label field had only local significance?

I've read drafts that describe local significance uses for the Flow
Label field, but still they state that the destination has to receive
this field as it was sent by the source. This ofcourse, if we plan to
change this field hop-by-hop, requires some sort of synchronization
between the first-hop-router after the source and penultimate router
before the packet reaches it's destination.

I'd appreaciate if someone can explain this matter a little bit.

Thanks,
--
Ran.

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From: Brian E Carpenter <[email-masked]>
Date: Fri, 22 Apr 2005 11:37:38 +0200

Ran,

You probably need to go through the mail archive and meeting minutes
from the period when RFC 3697 was being developed to find all the
arguments. That was most of 2002 and 2003. But I think the simplest
form of the argument is that it is intended to allow rapid
classification of a packet as belonging to a specific e2e flow,
regardless of where that classification happens - and the only way
to do that is an e2e label. If you allow mutability of the label,
you break that property. If you want a mutable local field for
QoS, use diffserv. If you want a mutable local field for switching,
use MPLS.

It is true (see my comments on draft-chakravorty-bcc-flowlabel-00.txt)
that this property allows certain use cases and makes life diffcult
for others. But the converse would also be true.

    Brian