Subject: MTU Discovery and fragmentation for non-RSVP LSPs?

From: [email-masked]
Date: Fri, 10 Nov 2000 13:04:56 -0500

Hi,

Section 3.6 of draft-ietf-mpls-label-encaps-08.txt includes the
following text:

 If it is not possible to forward an ICMP message from within an MPLS
 "tunnel" to a packet's source address, but the network configuration
 makes it possible for the LSR at the transmitting end of the tunnel
 to receive packets that must go through the tunnel, but are too large
 to pass through the tunnel unfragmented, then:

  - The LSR at the transmitting end of the tunnel MUST be able to
    determine the MTU of the tunnel as a whole.  It MAY do this by
    sending packets through the tunnel to the tunnel's receiving
    endpoint, and performing Path MTU Discovery with those packets.

My questions are:

1. It seems to me that it is often not possible to forward an
ICMP mesage from within an MPLS tunnel to a packet's source
address, and that the LSR may receive packets which are too
large to go through the tunnel unfragmented. Consequently,
the LSR MUST be able to determine the MTU of the tunnel.

Is this correct?

2. Section 2.6 of draft-ietf-mpls-rsvp-lsp-tunnel-07.txt defines
the mechanism whereby RSVP identifies the MTU for an LSP. This
draft also requires that the LSR perform fragmentation processing
for packets (labeled or unlabeled) being forwarded into the LSP
by that LSR.

Do CR-LDP or LDP provide any mechanism to identify the
MTU for an LSP?

3. The encapsulation draft indicates that the LSR may perform
Path MTU Discovery by sending packets through the tunnel.
Is there a more complete description of this procedure?

Thank you in advance for your assistance.