The MPLS WG Archive

[George Swallow <swallow@cisco.com>]

Li -

> In section "3.2. Downstream Mapping" of Draft-ietf-mpls-lsp-ping-02.txt,I
> have some question concerning the handling of "Downstream Mapping".
> 
> In page 12,there is a paragraph as following:
> "The notion of 'downstream router' and 'downstream interface' should
>    be explained.  Consider an LSR X.  If a packet that was originated
>    with TTL n>1 arrived with outermost label L at LSR X, X must be able
>    to compute which LSRs could receive the packet if it was originated
>    with TTL=n+1, over which interface the request would arrive and what
>    label stack those LSRs would see.  (It is outside the scope of this
>    document to specify how this computation is done.)  The set of these
>    LSRs/interfaces are the downstream routers/interfaces (and their
>    corresponding labels) for X with respect to L.  Each pair of
>    downstream router and interface requires a separate Downstream
>    Mapping to be added to the reply, and is given a unique DS Index "
> 
> What I am confused is whether LSR X needs to compute all the downstream LSR
> until the TTL in the packet is decreased to 0,including those not directly
> connect to LSR X,or only needs to compute those directly downstream LSR of
> LSR X? If TTL n>1,then the packet can be received by LSR more than one hops
> away,then, what should be included in the Downstream mapping?
> 
> Can anyone clarify it to me?

The 'n' vs 'n+1' means one hop away.  So you need to figure out what
routers will see the packet if the ttl does not expire at you.  Mostly
this is just your immediate neighbors.  One exception that comes to
mind is on TE tunnels.  If your tunnel is set up to copy the TTL into
the tunnel label, then you want to report the router at the next hop
in the tunnel.  If, however you are hidding TTL (setting the TTL in
the tunnel label to some value that will not expire before the end of
the tunnel - usually 255) the you want to report the router at the
tail end of the TE tunnel.

...George

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George Swallow             Cisco Systems                  (978) 936-1398
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