Re: Can a BFD session change its source port to facilitate auto recovery

[Greg Mirsky <[e-mail]>]

Hi Abhinav,
thank you for presenting an interesting scenario for a discussion. I have
several questions to better understand it:

   - How the network failure that triggers the recovery process is detected?
   - If the failure detection mechanism is not multi-hop BFD, what is the
   relationship between the detection intervals of heat mechanism and the
   multi-hop BFD session?

Regards,
Greg

On Wed, Mar 22, 2023 at 4:36 PM Abhinav Srivastava <[e-mail]>
wrote:

> Hi all,
>
>
>
> I needed clarification around whether source port can be changed for a BFD
> session in case of multi hop BFD.   The ability to change BFD source port
> when BFD session goes down helps BFD session to recover if its stuck on a
> network path where there is some intermittent but significant packet loss.
>
>
>
> In such cases, normally without BFD, end to end application traffic would
> eventually settle down on a good path as applications typically change
> source port after experiencing disconnection or failures.  But if BFD is
> being used to monitor some part of a path which is experiencing significant
> but not 100% packet loss, it will start causing next hop list of associated
> static route or the associated BGP sessions to start flapping forever, as
> BFD packets would be stuck to that partial lossy path forever (until BFD
> session is deleted and recreated by admin action).  This may also hinder
> the typical application recovery strategy of changing source port on
> failure.
>
>
>
> Ability to dynamically change BFD source port can help BFD recover in such
> cases.  Is this something that is allowed as per RFC?  The RFC5881, section
> 4 (for single hop) case states that –
>
> *“The source port MUST be in the range 49152 through 65535. The same UDP
> source port number MUST be used for all BFD Control packets associated with
> a particular session”*
>
>
>
> Thanks
>
> Abhinav
>