> On Aug 15, 2017:11:20 AM, at 11:20 AM, Kireeti Kompella <kire...@juniper.net> > wrote: > > As a co-author, I can say that the intent was that the LSP ping reply be > sent, but the BFD discriminator be optional. Not sending an LSP ping reply > could lead to the LSP being torn down. > > The basic idea here is to use LSP ping to bootstrap a bfd session. But the > semantics of LSP ping don't change; not sending a reply indicates that the > LSP is down. > > Kireeti
That is exactly my recollection of how we designed (and implemented) this too. —Tom > >> On Aug 11, 2017, at 10:39, Jeffrey Haas <jh...@pfrc.org> wrote: >> >> [Note that I have adjusted the addresses in the headers to try to catch the >> RFC authors' current accounts.] >> >> >> The 5884 interop issue keeps bubbling up. Balaji submitted an errata, which >> provides us with a good place to start technical discussion. >> >> Please note I also spent some time off-list discussing this errata with >> Balaji. >> >> >>> On Thu, Aug 10, 2017 at 10:35:50PM -0700, RFC Errata System wrote: >>> Section: 6 >>> >>> Original Text >>> ------------- >>> The egress LSR MAY respond with an LSP Ping Echo >>> reply message that carries the local discriminator assigned by it for >>> the BFD session. >>> >>> Corrected Text >>> -------------- >>> The egress LSR MUST respond with an LSP Ping Echo reply message that >>> MAY carry the local discriminator assigned by it for the BFD session. >>> >>> >>> Notes >>> ----- >>> It is not clear from the original text which of the following is optional: >>> - The egress MUST send a reply, but the discriminator in the reply is >>> optional >>> - The reply itself is optional >>> >>> Technically, the reply cannot be optional, because the egress needs to >>> report LSP-Ping verification status to the ingress. >>> >>> The proposed text recommends to include BFD discriminator in the reply. >>> This was the intent of the original text. >> >> My opinion follows: >> >> In section 6 - >> >> : On receipt of the LSP Ping Echo request message, the egress LSR MUST >> : send a BFD Control packet to the ingress LSR, if the validation of >> : the FEC in the LSP Ping Echo request message succeeds. This BFD >> : Control packet MUST set the Your Discriminator field to the >> : discriminator received from the ingress LSR in the LSP Ping Echo >> : request message. The egress LSR MAY respond with an LSP Ping Echo >> : reply message that carries the local discriminator assigned by it for >> : the BFD session. The local discriminator assigned by the egress LSR >> : MUST be used as the My Discriminator field in the BFD session packets >> : sent by the egress LSR. >> >> In the text above, I consider it quite clear that the receipt of the BFD >> packet contains sufficient state to bring up the BFD session. The receipt >> of the same Discriminator in the LSP Ping Echo Reply is optional. >> >> This makes sense partially because the reply may be dropped and we want the >> BFD session to come up as fast as possible. >> >> The point of contention appears to be what to do if we *never* get such >> replies. It's worth pointing out additional text in RFC 5884, section 3.2. >> >> : Hence, BFD is used in conjunction with LSP Ping for MPLS LSP fault >> : detection: >> : >> : i) LSP Ping is used for bootstrapping the BFD session as described >> : later in this document. >> : >> : ii) BFD is used to exchange fault detection (i.e., BFD session) >> : packets at the required detection interval. >> : >> : iii) LSP Ping is used to periodically verify the control plane >> : against the data plane by ensuring that the LSP is mapped to >> : the same FEC, at the egress, as the ingress. >> >> iii above reminds us that the LSP may be torn down because LSP Ping fails. >> Thus, it seems problematic that we do not get a reply ever. >> >> However, with the BFD session in the Up state, we have information proving >> that the LSP is up. Thus we have contradictory intent. >> >> --- >> >> My opinion is that the MAY in the RFC 5884 procedures is intended to have >> the BFD session come up by the most expedient means. I do not believe the >> likely intent was to say "don't send Echo Reply". Among other things, that >> seems contrary to the intent of the general LSP Ping procedures. >> >> Having given my personal observations, we now get to the business of the >> Working Group: Debating intent and related text. >> >> -- Jeff