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

Abhinav Srivastava <absrivas@gmail.com> Wed, 22 March 2023 21:35 UTC

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From: Abhinav Srivastava <absrivas@gmail.com>
Date: Wed, 22 Mar 2023 14:35:02 -0700
Message-ID: <CAL9v8R0Nv6VrdU3gdk8uJ+_K_tehGLXQSor_gBeYq90Tf4WL=Q@mail.gmail.com>
Subject: Can a BFD session change its source port to facilitate auto recovery
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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