Re: [L2tpext] draft-galtzur-l2tpext-gr-01.txt and draft-ietf-l2tpext-failover-02.txt

[Vipin Jain <[e-mail]>]

My comments:

I think both drafts are complete (I am attaching
draft-ietf-l2tpext-failover-03.txt for everyone's review).

However, the main trade-off seems to be the simplicity vs. scalability, as
identified as goals in in Appendix A of draft-ietf-l2tpext-failover-03.txt.

Just wondering: Why the mechanism described in
draft-ietf-l2tpext-failover-03.txt would not work (and scale) for L2TPv3 as
well?

thanks,
-- vipin


--- "W. Mark Townsley" <[e-mail]> wrote:
> 
> I would like to try and offer a level set here for continuing the discussion.
> 
> I see that these two drafts are trying to solve similar problems, though
> perhaps 
> optimized differently. Fundamentally, both intend to address the
> restablishment 
> of control connection and session state for L2TP during a failure scenario.
> That 
> may mean failover from an active to standby RP, or the restart of an RP while
> 
> continuing to forward packets.
> 
> For the most part, either mechanism could be made to work for L2TPv3 or
> L2TPv2. 
> I think there are obvious benefits if we can have a single mechanism for
> both.
> 
> The only fundamental difference I see between the two protocols themselves
> which 
> might affect the design of the associated restart method is that in L2TPv3
> the 
> Control Connection ID is *not* carried in the L2TP Data Message Header, and
> in 
> L2TPv2 the analogous Tunnel ID *is* carried in the L2TP Data Message Header. 
> Thus, v3 has the advantage over v2 in that it can restart with a different 
> Control Connection ID without affecting the forwarding plane. This difference
> is 
> tangible, but would hopefully not prohibit convergence on a common mechanism.
> 
> I believe the more fundamental reason we see two divergent solutions is that 
> each is being targeted to different deployment environments.
> 
> draft-galtzur-l2tpext-gr-01.txt is targeted at the current L2TPv3 space,
> while 
> draft-ietf-l2tpext-failover-02.txt is targeted at the more mature RFC2661
> L2TPv2 
> space. As such,
> 
> draft-galtzur-l2tpext-gr-01.txt is optimized for an environment which is:
> 
> - Provisioned in a relatively static manner
> - Has a fairly small number of sessions per LCCE
> - Largely addresses restart of a single RP while continuing to forward
> traffic 
> on a separate dataplane
> 
> draft-ietf-l2tpext-failover-02.txt is optimized for an environment which is:
> 
> - Provisioned in a largely dynamic manner
> - Has a large number of sessions per LCCE
> - Addresses restart of an RP, or failover to a secondary RP or node, where
> some 
> amount of session and control connection state may be checkpointed (e.g.,
> beyond 
> that necessary for forwarding alone), and where traffic flow may be
> interrupted 
> during failover.
> 
> One might argue that, over time, the deployment environment of L2TPv3 will
> look 
> more and more like that of L2TPv2. e.g., higher density, more dynamic 
> provisioning via auto-discovery or NMS systems, etc. Thus, some of the
> scaling 
> benefits of draft-ietf-l2tpext-failover-02.txt could become more relevant as
> well.
> 
> That said, I don't believe the draft-ietf-l2tpext-failover-02.txt has all the
> 
> kinks worked out in it. draft-galtzur-l2tpext-gr-01.txt has the advantage of 
> being quite simple, is relatively complete, and requires very little (if any)
> 
> state checkpointing beyond that which could be gleaned from the still-active 
> forwarding plane which is assumed to be present. It's perfectly adequate for 
> cases where you aren't concerned about how fast the sessions recover (and if
> you 
> have a forwarding plane that is still passing packets for fairly static 
> sessions, why should you care?).
> 
> Authors, please let me know if I have not articulated the situation
> accurately.
> 
> Next step is to try and come up with a single method (or very close to a
> single 
> method) that operates with both versions of L2TP.
> 
> Thanks,
> 
> - Mark
> 
> 
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