From: Lavanya Srivatsa <lavanya.srivatsa@aricent.com>
To: rtg-bfd <rtg-bfd@ietf.org>, "dkatz@juniper.net" <dkatz@juniper.net>
Date: Thu, 26 Aug 2010 16:39:06 +0530
Thread-Topic: [mpls] BFD Packet Flow
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Cc: "mpls@ietf.org" <mpls@ietf.org>,
 "gupta_rakesh62@yahoo.co.in" <gupta_rakesh62@yahoo.co.in>
Subject: Re: [mpls] BFD Packet Flow
Precedence: list

Dave,

Thanks for the earlier response about the so-called 4-way handshake. I got =
what you are trying to say...

I had a question on the below though:
>> In some sense the question is meaningless;  the active and passive roles=
 are really a way of describing the way the protocol behaves in a larger co=
ntext.  In the >> LSP context, LSR-Z is by definition in the active role be=
cause it must send the first BFD packet.

If so, does this mean that BFD-MPLS is applicable only in the Active-Active=
 scenario i.e. both LSR-A and LSR-Z in Active roles? The reason I am saying=
 this is one would expect LSR-A to be playing the Active role because LSR-A=
 is the **head-end of the forward unidirectional path** that is being monit=
ored in case of an unidirectional MPLS path. Now if LSR-Z is also required =
to be in Active state because it sends the first BFD packet, then this woul=
d imply that only Active-Active scenario is applicable for BFD-MPLS, right?

The BFD-MPLS RFC also goes on to say that the node receiving a LSP Ping req=
uest MUST send a BFD control packet and MAY send an LSP Ping response.
Does this imply the Active-Passive scenario for BFD-MPLS (i.e. LSR-A in Act=
ive role and LSR-Z in Passive role)?

In the sense, if LSR-Z is Active, upon receiving an LSP Ping request, LSR-Z=
 will respond with a BFD control packet.
If LSR-Z is Passive, upon receiving an LSP Ping request, LSR-Z will *not* s=
end a BFD control packet but will respond with an LSP Ping response (carryi=
ng LSR-Z's discriminator) instead.

Is my understanding correct? If so, the text in the RFC seems to slightly m=
islead with the "MAY" clause for the LSP Ping response.

- Lavanya


Date: Sat, 21 Aug 2010 12:45:09 -0700
From: Dave Katz <dkatz@juniper.net>
Subject: Re: [mpls] BFD Packet Flow
To: "Weingarten, Yaacov (NSN - IL/Hod HaSharon)"
        <yaacov.weingarten@nsn.com>
Cc: mpls@ietf.org, rtg-bfd@ietf.org, RAKESH GUPTA
        <gupta_rakesh62@yahoo.co.in>
Message-ID: <6F1E904F-CF1B-49B2-B8B7-088EA91CC661@juniper.net>
Content-Type: text/plain; charset=3D"windows-1252"


On Aug 19, 2010, at 2:08 AM, Weingarten, Yaacov (NSN - IL/Hod HaSharon) wro=
te:

> Hi,
>
> Just to verify that I understand the flow of messages and the state of th=
e BFD Session ? is the following flow correct ?
>
> Assume a LSP between LSR-A and LSR-Z ?
> 1. Both LSRs initialize the BFD Session and set the state to Down
> 2. LSR-A (the ingress LSR) sends a LSP-Ping (with the Discriminator TLV) =
to LSR-Z ? both sessions still in Down state.
> 3. LSR-Z receives the LSP-Ping, and (assuming that LSR-Z is in Active mod=
e) sends a BFD Control Packet with its own Discriminator, the Discriminator=
 received from the LSP-Ping, and State=3DDown to LSR-A, both session still =
in Down State.
> 4. LSR-A receives the BFD Control and switches to Init state (as per RFC5=
880), and sends a BFD Control Packet with State=3DInit to LSR-Z
> 5. LSR-Z receives the BFD Control and switches to Up state, and sends a B=
FD Control Packet with State=3DUp to LSR-A
> 6. LSR-A receives the BFD Control and switches to UP state
>
> Additional question ? regarding step 3 above ? must LSR-Z be in Active mo=
de? What happens if LSR-Z is in Passive mode and LSR-A is in Active mode?

In some sense the question is meaningless;  the active and passive roles ar=
e really a way of describing the way the protocol behaves in a larger conte=
xt.  In the LSP context, LSR-Z is by definition in the active role because =
it must send the first BFD packet.

--Dave

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