Re: [mpls] BFD Packet Flow

I suppose the source of confusion might be RFC 5884's choice of verbiage:

   On receipt of the LSP Ping Echo request message, the egress LSR MUST
   send a BFD Control packet to the ingress LSR...

This might give the impression that LSP Ping is just transmitting BFD control packets willy-nilly without any BFD context.  But it also says:

   A BFD session is bootstrapped using LSP Ping.

and

   A BFD session may be established for a FEC associated with an MPLS LSP.  

This seems unambiguous that a BFD session is being established, and the procedure for doing so is spelled out clearly in RFC 5880.  I believe the following to be unambiguous:

   State (Sta)

      Set to the value indicated by bfd.SessionState.

and

   bfd.SessionState

      ...This variable MUST be initialized to Down.

One might be tempted to start in Init state in order to save one packet during session establishment, but in doing so one would be breaking the semantics of the protocol, in particular the three-way handshake through session failure.

--Dave

On Aug 12, 2010, at 4:20 AM, RAKESH GUPTA wrote:

> hi,
> 
> I have query regarding BFD packet flow when LSP Ping is used for bootstrapping. 
> 
> As per my understanding from the RFC 5880 and RFC 5884 when LSP Ping is used for bootstrapping the remote end MUST reply with BFD control in response to the LSPPing.
> 
> The specification does not clearly specify about the Stat parameter value that should be returned in the response of LSPPing. Should it have the value Init or Down? Or it might be possible to return no stat information.
> 
> Can someone please share the exact packet flow that happens during BFD session set up with LSPPing in Bootstrap till the session comes into UP state?
> 
> thanks a lot in advance for ur help,
> rakesh
> 
> 
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