Bob, In addition to the cases listed by Derek, L2TPv3 can also carry non-IP pseudowire data, such as Ethernet frames (see RFC 4719 for example). Even though 4719 says that sequencing is optional, I would certainly recommend it :-).
But I guess that's really not what you were asking about, since you specifically mentioned IP data. But it is a case where you would probably see sequencing in use. Back in the day, Sprint made good use of Ethernet over L2TPv3, as they were in the anti-MPLS camp at the time. But that's water over the bridge, and I really don't know if this solution continues to be in active use. Mark Townsley might know. Cheers, Andy On Sat, Jun 5, 2021 at 10:07 AM Derek Fawcus < [email protected]> wrote: > On Fri, Jun 04, 2021 at 03:13:15PM +0100, Bob Briscoe wrote: > > The L2TP RFC says sequencing /can/ be disabled for IP data, but it > > doesn't say SHOULD or MUST. Is it possible that some operators enable > > L2TP sequencing for IP data? And if so, do you know why they would? > > Also, are you aware of any other types of tunnel that might try to keep > > IP data packets in sequence? > > How many intermediate headers are you considering between L2TP and where > a carried IP header may exist? > > Maybe I'm getting the wrong end of the stick, but surely this engages > the text from section 5.4 of RFC 2661: > > "For example, if the PPP session being tunneled is not > utilizing any stateful compression or encryption protocols and is > only carrying IP (as determined by the PPP NCPs that are > established), then the LNS might decide to disable sequencing as IP > is tolerant to datagram loss and reordering." > > This would then suggest if L2TP is carrying PPP, the PPP session is not > multi-link, and is making use of compression (including one of the > versions of IP header compression) in some form for IP packets, then > reordering will impact the ability to decompress. > > So such an L2TP data session may well make use of sequence numbers to > prevent reordering. > > I guess similarly in L2TPv3 when the PW is for PPP, and possibly also > the fragmentation scheme in RFC 4623 which requires sequence numbers; > and such PWE3 links could ultimately be carrying IP packets. > > > DF > > (not an operator) > > _______________________________________________ > Int-area mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/int-area >
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