RE: [EXTERNAL] Re: Can a BFD session change its source port to facilitate auto recovery

[Alexander Vainshtein <[e-mail]>]

Abhinav, Jeff and all,
FWIW I concur with Jeff.

In my experience, MH IP BFD sessions are typically used to monitor peering between iBGP neighbors, and when the MH IP BFD session goes down, BGP treats this as if its session has gone – and deletes the MH IP BFD session in question.

I.e., fast recovery of such a session will not happen until BGP would not re-create it.

Regards,
Sasha

From: Rtg-bfd <[e-mail]> On Behalf Of Jeff Tantsura
Sent: Thursday, March 23, 2023 8:27 AM
To: Abhinav Srivastava <[e-mail]>
Cc: [e-mail]
Subject: [EXTERNAL] Re: Can a BFD session change its source port to facilitate auto recovery

Abhinav,

Let’s clarify a couple of points.
What you are trying to do is to change entropy to change local hashing outcome, however for hashing to even be relevant there has to he either ECMP or LAG in the path to the destination otherwise shortest path will be he used regardless, so statistically, some of the flows between a given pair of end points (5 tuple) will be traversing the (partially)broken link, would you really like BFD to “pretend“ that everything is just fine?
Moreover, by far, in case of congestion  - most applications won’t change their ports but have their TX rate reduced.
There’s work done by Tom Herbert for IPv6/TCP (kernel patch upstreamed a few years ago)  - had beeb presented in RTGWG pre-Covid, that on RTO changes flow label value (that some might or might not include in hashing), which is strongly not recommended to be used outside of a tightly controlled homogenous  environment (think within DC).
Outside of what BFD spec tells us (don’t), the above should provide enough motivation not to do this.
Cheers,
Jeff


On Mar 23, 2023, at 05:44, Abhinav Srivastava <[e-mail]<mailto:[e-mail]>> wrote:

Multi-hop BFD would be the mechanism that detects the failure on the path it happens to be using for the session. I wasn't thinking of another mechanism.  Detection timer expiry would be the trigger for recovery which could be augmented with few other possible criteria like how long session hasn't been able to come back up or prolonged flapping.

Thanks
Abhinav

On Wed, 22 Mar, 2023, 3:05 pm Greg Mirsky, <[e-mail]<mailto:[e-mail]>> wrote:
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]<mailto:[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

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