The MPLS WG Archive

[lidefeng <lidefeng@huawei.com>]

In section "5.3. Procedures at the ingress LSR" of
draft-ietf-mpls-lsp-ping-02.txt,it states that"To test an LSP that carries
non-IP traffic, before injecting ICMP and MPLS ping messages into the LSP,
the IPv4 Explicit NULL label should be prepended to such messages. The
ingress and egress LSR's must follow the procedures defined in
[LABEL-STACK]."

While in RFC 3032(section 2.1. Encoding the Label Stack),the definition of
IPv4 Explicit NULL label is as following:

           i. A value of 0 represents the "IPv4 Explicit NULL Label".
              This label value is only legal at the bottom of the label
              stack.  It indicates that the label stack must be popped,
              and the forwarding of the packet must then be based on the
              IPv4 header.

It is explicitly stated that " This label value is only legal at the bottom
of the label  stack." ,while in  draft-ietf-mpls-lsp-ping-02.txt,it states
that this "IPv4 Explicit NULL label should be prepended to such messages",Is
it inconsistent with each other?

Another question is that why shouls this IPv4 Explicit NULL label should be
prepended to such messages?

TIA

Defeng Li

Huawei technologies