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

Mark Townsley <> Wed, 09 June 2021 22:32 UTC

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From: Mark Townsley <>
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Date: Wed, 9 Jun 2021 15:32:24 -0700
Cc: Joel Halpern <>, "Andrew G. Malis" <>, Carlos Pignataro <>, Ignacio Goyret <>, intarea IETF list <>, Derek Fawcus <>,, Joel Halpern Direct <>
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To: Stewart Bryant <>
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Subject: Re: [Pals] [Int-area] L2TP sequencing: Commonly disabled for IP data? Or always?
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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 <> 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 <> 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 < <>> 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 < <>> 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
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