The MPLS WG Archive

["Andrew R Frame" <andrew@foundrynet.com>]

The spec is not contradicting itself, the last paragraph is defining an
alternative behavior. If you would prefer for the make the LSP look like one
hop to tools like traceroute, or if you are trying to solve the situation
given by your example below, implementing the alternative behavior might be
a good idea.

-andrew

-----Original Message-----
From: owner-mpls@UU.NET [mailto:owner-mpls@UU.NET]On Behalf Of Patrick Lee
Sent: Wednesday, January 31, 2001 7:49 AM
To: mpls@UU.NET
Subject: [Fwd: Comment for rfc3032 on TTL process]


To whom it may concern,
On the draft for MPLS label stack encoding, section 2.4.3 has mentioned
about the processing of MPLS TTL field.

The last paragraph state that:
   It is recognized that there may be situations where a network
   administration prefers to decrement the IPv4 TTL by one as it
   traverses an MPLS domain, instead of decrementing the IPv4 TTL by the
   number of LSP hops within the domain.

That means IP packet with small TTL value can also go through the MPLS
domain since the MPLS domain is treated as 1 hop for IP forwarding.

But for the second paragraph on section 2.4.3 state that:
   When an IP packet is first labeled, the TTL field of the label stack b
   entry MUST BE set to the value of the IP TTL field.  (If the IP TTL
   field needs to be decremented, as part of the IP processing, it is
   assumed that this has already been done.)

That means the first label stack entry's TTL value MUST be initialized to
the IP header's TTL value.

Imagine that an IP packet with TTL value 3 go into a LSP with 6 LSP hops.
Although the network can view the MPLS domain as 1 IP forwarding hop, the
packet can not go through that 1 hop.

It seems to be NOT CONSISTENT for both paragraph.

Regards,
Patrick Lee


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