The MPLS WG Archive

[Bob Thomas <rhthomas@cisco.com>]

Amar,

> Common Hello parameters TLV
> 
>       0                   1                   2                   3
>       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
>      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>      |0|0| Common Hello Parms(0x0400)|      Length                   |
>      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>      |      Hold Time                |T|R| Reserved                  |
>      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> 
> 
> [1] Why do we need R bits explicitly in Common Hello parameters TLV. ?
>           R- A value of 1 reuest the receiver to send periodic Targeted
> Hellos to the source of this Hello. a value of 0 makes no request.
> 
> As far as I understand, we have to send Hello message(even targetted
> hello) periodically, if it is configured to send Targetted Hello. How
> does this R bit help ???

Please see Section 2.4.2, Extended Discovery Mechanism, specifically the
second item:

  "Extended Discovery differs from Basic Discovery in the following
   ways:

     - A Targeted Hello is sent to a specific address rather than to the
       "all routers" group multicast address for the outgoing interface.

     - Unlike Basic Discovery, which is symmetric, Extended Discovery is
       asymmetric.

       One LSR initiates Extended Discovery with another targeted LSR,
       and the targeted LSR decides whether to respond to or ignore the
       Targeted Hello.  A targeted LSR that chooses to respond does so
       by periodically sending Targeted Hellos to the initiating LSR."


Consider 2 LSRs: LSR I and LSR T, and assume that:

* LSR I is configured to initiate a targeted session with LSR T;

* LSR T is configured to:
 - Not initiate targeted sessions; and
 - Respond to targeted sessions initiated by LSR I.

For example, LSR I may be the head end of a TE tunnel, T may be the
tail end, and the situation may require that I learn labels from T via
LDP.

In this situation, LSR I would set the R bit to 1 in its Hellos and T
would set the R bit to 0 in Hellos sent in response.

If (when) LSR I no longer needs the LDP session, it stops sending
Hellos to T.  Although T stops receiving Hellos from I it would
continue sending them until its discovery hello hold timer expired.
LSR I would not respond because it is no longer an initiator for a
targeted session with LSR T (and because the R bit in the Hellos is 0).
The hold timer on T would expire since it is no longer being
refreshed, causing T to stop sending Hellos to LSR I.

Bob