Re: [Pals] [Int-area] L2TP sequencing: Commonly disabled for IP data? Or always?

["Andrew G. Malis" <[e-mail]>]

Mark,

The original question was, how many (if any) of these L2TPv2 and v3 use
cases use sequencing, especially when carrying IP?

Cheers,
Andy


On Wed, Jun 9, 2021 at 6:32 PM Mark Townsley <[e-mail]> wrote:

> Hi folks,
>
> In addition to the DSL arena, L2TP is still in use with host-based VPN
> clients as it is embedded in Apple, Android, and Windows based operating
> systems (new and old). Despite most VPN solutions preferring their own
> client software that must be installed on hosts to operate, there are still
> admins that appreciate not having to ask their employees to download an app
> for the VPN to work - in which case PPTP and L2TP with transport-mode IPsec
> are your most widely available options.
>
> Regarding L2TPv3 replacing L2TP: L2TPv2 (RFC 2661) was PPP only. L2TPv3
> generalized L2TP to support other L2 (including MPLS, but I don’t want to
> argue what layer MPLS operates within here). There was never a strong push
> to replace L2TPv2 used in DSL, Dialup and host VPN software with L2TPv3
> (there was one use case for an important L2TP operator that wanted to carry
> PPPoE over L2TPv3 in DSL, but that was overcome by RFC3817 which achieved
> the same goal without altering the dataplane). Ironically, I would expect
> to see very little PPP over L2TPv3 in the wild, though obviously it is
> possible.
>
> In the cable broadband world, the DOCSIS DEPI “Remote PHY” specification
> (similar I suppose to the split BNG spec Joel is referring to) standardized
> on L2TPv3 and is in active use.
>
> I also know of at least one vendor that uses Ethernet over L2TPv3 for some
> wifi backhaul use cases.
>
> There could be more, this is just what I am personally aware of off the
> top of my head. Even I am surprised to see how much L2TP is still out there
> once you start really looking around ;-)
>
> Best Regards,
>
> - Mark
>
>
>
>
> > On Jun 9, 2021, at 6:10 AM, Joel Halpern Direct <
> [e-mail]> wrote:
> >
> > BNGs are still a big busienss.  And BNG resale /emote control uses L2TP
> in many cases.  The BBF has been working on (and published a first version
> of) protocol for control of split BNG.  L2TP is commonly used for these use
> cases.
> >
> > Yours,
> > Joel
> >
> > On 6/9/2021 7:50 AM, Stewart Bryant wrote:
> >> Which applications still use it Joel?
> >> Stewart
> >>> On 9 Jun 2021, at 12:42, Joel M. Halpern <[e-mail]> wrote:
> >>>
> >>> There is plenty of L2TP still in use.
> >>> Yours,
> >>> Joel
> >>>
> >>> On 6/9/2021 6:23 AM, Stewart Bryant wrote:
> >>>> Sequence number checking in the forwarder is always a problem because
> it is stateful so I doubt that many high-scale or high-speed forwarders
> ever did this.
> >>>> I think there is an undisclosed assumption that go up enough levels
> and its IP so sequence number checking in the transport network (as opposed
> to the transport layer) is not really needed.
> >>>> I doubt that there is much L2TP still out there. It was in its prime
> with dialup modems. L2TPv3 which was intended to replace it became niche
> with, as Andy says, operators who did not want MPLS. Much of what L2TPv3
> was intended for was actually done with PW over MPLS with some replacement
> with by Mac in Mac for cost reasons.
> >>>> If Carlos does not know the answer, Mark T would be my next port of
> call.
> >>>> Stewart
> >>>>> On 8 Jun 2021, at 22:41, Andrew G. Malis <[e-mail] <mailto:
> [e-mail]>> wrote:
> >>>>>
> >>>>> 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 <
> [e-mail] <mailto:
> [e-mail]>> 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)
> >>>>>
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