Subject: Revised PIM-SM draft

From: Francis Arts <[email-masked]>
Date: Sun, 24 Dec 2000 11:45:22 +0100

Hello,

Could someone share his / her point of view on the parameters (DSCP,
TTL, ...) that should be used in the outer IP header of a PIM-SM
register encapsulated packet? Please find more detailed questions
included below.

Thank you.

Kind regards,

    Francis Arts.

-------------------

        *******************************
        *Terminology used in this mail*
        *******************************

PIM-SM tunnel ingress point
---------------------------

Further on in this mail the term 'PIM-SM tunnel ingress point' refers to
the source DR or the PMBR encapsulating the multicast IP packet in a PIM
register message.

Outer IP header
---------------

The 'outer IP header' is the header of the (unicast) IP packet having
the PIM-SM register message as its payload. The 'outer IP header' is
sometimes also called the 'outer header' for short.

        ************
        *Questions *
        ************

Note:
--> Next I have also included some information I found in RFC 2003 on IP
over IP tunnels. I am not sure whether or not it is relevant to compare
IP over IP and PIM-SM tunnels. Please be the judge on this.

1) TOS byte in tunnel ingress point.

How should the PIM-SM tunnel ingress point determine which TOS byte to
use in the outer IP header:
** Should the TOS byte be copied from the multicast IP packet into the
outer IP header
OR
** should a fixed value be used for the TOS byte (e.g. always BE),
irrespective of the value of the TOS byte in the multicast IP packet
OR
** ...?

Note: RFC 2003 indicates that for IP over IP tunnels the IP/IP tunnel
ingress point copies the TOS byte from the multicast IP packet in the
outer header.

2) TOS byte in RP.

How should the TOS byte be handled in the RP:
** Is the TOS byte copied from the outer IP header in the inner
(multicast) IP header
OR
** Is the TOS byte of the outer IP header ignored and the TOS byte value
as is in the multicast IP header used
OR
** ...?

Note: RFC 2003 on IP over IP tunnels is not clear on the behavior of IP
over IP tunnels in this case.

3) TTL in tunnel ingress point.

How should the PIM-SM tunnel ingress point determine which TTL value to
use in the outer IP header:
** Is the TTL (after decrementation) copied from the multicast IP packet
into the outer IP header
OR
** is a fixed value used for the TTL in the outer IP header, large
enough to reach the RP
OR
** ...?

The big question here seems to be whether or not the PIM-SM tunnel
should be considered as 1 hop between the source DR / PMBR and the RP.

Notes:
--> RFC 2003 indicates that for IP over IP tunnels the TTL is set to a
value that is large enough to reach the tunnel egress point.
--> However, the draft-ietf-mpls-label-encaps-08.txt indicates that for
MPLS the TTL must be copied from the IP header to the MPLS SHIM header.

4) TTL in RP.

How should the TTL be handled in the RP:
** Is the TTL copied from the outer IP header in the header of the
multicast IP packet
OR
** Is the TTL of the outer IP header ignored and the TTL value as is in
the multicast IP header used
OR
** ...?

The big question here seems to be whether or not the PIM-SM tunnel
should be considered as 1 hop between the source DR / PMBR and the RP.

Notes:
--> RFC 2003 indicates that for IP over IP tunnels the TTL of the inner
IP header as is, is used.
--> However, the draft-ietf-mpls-label-encaps-08.txt indicates that for
MPLS the TTL should be copied from the MPLS SHIM header to the IP
header.

5) DF bit.

How should the DF bit in the outer IP header be handled in the tunnel
ingress point:
** Should the DF bit in the outer IP header always be set to 'don't
fragment'.
OR
** Should the DF bit in the outer IP header be set to the same value as
the DF bit in the header of the multicast IP packet
OR
** ...?

Notes:
--> RFC 2003 indicates that if the "Don't Fragment" bit is set in the
inner IP header, it MUST be set in the outer IP header; if the "Don't
Fragment" bit is not set in the inner IP header, it MAY be set in the
outer IP header.

6) IP options.

How should IP options be handled in the tunnel ingress point:
** Are there any IP options that have to (...) be copied from the
multicast IP header to the outer IP header?
** Are there any IP options that MAY be copied from the multicast IP
header to the outer IP header?
** Are there any IP options that are not present in the header of the
multicast IP packet, that MAY be added to the outer IP header in the
PIM-SM tunnel ingress point?

Note:
RFC 2003 states the following:
"Any options present in the inner IP header are in general NOT copied to
the outer IP header.  However, new options specific to the tunnel path
MAY be added.  In particular, any supported types of security options of
the inner IP header MAY affect the choice of security options for the
outer header.  It is not expected that there be a one-to-one mapping of
such options to the options or security headers selected for the
tunnel."