Subject: Re: PIM spec

From: Francis Arts <[email-masked]>
Date: Wed, 11 Apr 2001 19:16:39 +0200

Hello Bill,

After our discussion at the 50th IETF meeting in Minneapolis, I have reread
the PIM spec. p.29 of the latest PIM spec (dated March 2) states the
following:

> Conceptually, the Register Tunnel is an interface with a smaller MTU
> than the underlying IP interface towards the RP.  IP fragmentation on
> packets forwarded on the Register Tunnel is performed based upon this
> smaller MTU.  The encapsulating DR may perform Path-MTU Discovery to
> the RP to determine the effective MTU of the tunnel.

You explained me that the smaller MTU takes both the outer IP header and the
PIM register header into account. So the order of actions will be:
     1st step:
              Fragment the multicast IP packet taking the
              outer IP header and PIM register header
              overhead into account.
     2nd step:
              Stick the outer IP header and the PIM register
              header in front of each fragment.

Question:
          Could we add as a clarification the following explicit
          statement to the text:

                       "The smaller MTU takes both the outer IP header
                       and the PIM register header overhead into account.
                       So if a multicast IP packet is fragmented, the outer
                       IP header and PIM register header are present in
                       every fragment."

What is your opinion?

Kind regards,

    Francis.