Subject: LDP remote peer question

From: David Charlap <[email-masked]>
Date: Thu, 05 Apr 2001 17:15:03 -0400

I've a question regarding remote peers and the LDP protocol.

Remote peers are managed using unicast UDP messages for Hello and TCP
session.  These peers are not (I think) bound to an interface.  Which
means that if the local routing table should change, then the interface
associated with the remote peer may also change.

If per-platform label spaces are used, this is not a problem.

On routers with per-interface label spaces, however, this might be a
real problem.  Once the TCP socket moves to a different interface, all
of its labels must also move.  This may not be possible, since there may
be conflicting label assignments on the new interface.  But to release
all of the remote peer's labels before the move, and then re-request
them afterwards is a lot of overhead.  And it will cause service
interruptions during this reconvergence period.

Is this correct?  Is there a solution to this problem?

Obviously, the best way is to implement per-platform label spaces, but
this is may not be possible (or efficient) on all hardware platforms.

-- David

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From: "Philip Matthews" <[email-masked]>
Date: Wed, 11 Apr 2001 22:29:23 -0400

David:

I don't see how one can support targetted LDP sessions without using
a platform-wide label space, unless you can ensure that the packets
only arrive on one interface.

If you have hardware restrictions (for example, ATM hardware) that
begs for interface-specific labels, then one solution might be to reserve
a small portion of the range and allocate it in a platform-wide manner.

- Philip

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From: Giles Heron <[email-masked]>
Date: Wed, 18 Apr 2001 12:12:45 +0100

> only arrive on one interface.

How about turning off PHP for the "outer" label established hop-by-hop? 
That way packets associated with a targetted LDP session would arrive
with an explicit null label prepended to them.

Then you could associate the labels established through the targetted
session with a loopback interface.  Probably a moot point in that
interface-specific labels are mostly used whem there are hardware
restrictions...

> a small portion of the range and allocate it in a platform-wide manner.

Yes, that would work.  But I'm not sure you'd need to "reserve" the
label range?  It is a case of ensuring that the label mapping is present
on every interface that might receive a labelled packet.  In the
LDP/RSVP case (where the targetted LDP session is established between
LSRs at either side of the RSVP-TE domain) you could probably just
instantiate the mapping on all interfaces within the IGP area being used
for TE?

Giles

-- 
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Giles Heron      Principal Network Architect      Gone2 Inc.
ph: +44 7880 506185         "if you build it they will yawn"
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