Re: [trill] [RTG-DIR] RTG-DIR QA review for draft-ietf-trill-multilevel-single-nickname

[Alexander Vainshtein <[e-mail]>]

Donald,
Lots of thanks for a very detailed response!

Lots of thanks for very important information about the actual and expected scale of TRILL deployments as well as for presenting some of the factors (line active-active TRILL operation) that affect the consumption of the nickname space. It addresses my question about the reason to go for multi-level IS-IS at all. Flat IGP configuration (both IS-IS and OSPF) are very popular in IP/MPLS deployments due to LDP  (and, now, IP/LDP FRR techniques), so this information was important to me in order to understand that the multi-level TRILL drafts solve a real problem. I would suggest adding this information to the multi-level TRILL draft.

However, I am still not sure if the Single Nickname draft (one I have been reviewing)  represents an attempt to solve a real problem. My understanding so far has been that it follows the Aggregate Nicknames approach in the multi-level TRILL draft, but eliminates the need to assign nicknames to L1 areas. I do not see if, even with the scale you have mentioned) this could be a serious issue (e.g., contribute significantly to depletion of the nickname space). Do I  miss something here?

The swapping vs. re-write issue I have discussed with Mingui is a pure case of terminology. AsI have said, I consider it as closed.

As for the metadata issue  - I am perfectly ready to follow the guidance of ADs and WG chairs. especially since this policy has recently undergone serious changes.

Two additional questions - for the sake of my curiosity:

  1.  Can possibly you explain what has happened to draft that proposed using BGP with TRILL?
  2.  Did anybody consider combining IPFRR techniques with TRILL?

Regards, and lots of thanks in advance,
Sasha










________________________________________
From: Donald Eastlake <[e-mail]>
Sent: Thursday, May 26, 2016 12:03 AM
To: Alexander Vainshtein
Cc: Mingui Zhang; Zhangxian (Xian); [e-mail]; [e-mail]; Susan Hares; [e-mail]; Jonathan Hardwick; [e-mail]
Subject: Re: [RTG-DIR] RTG-DIR QA review for draft-ietf-trill-multilevel-single-nickname

Hi Alexander,

Thanks from me also for your review.  I'd like to chime in with a few
thoughts:

On Wed, May 25, 2016 at 8:23 AM, Alexander Vainshtein
<[e-mail]> wrote:
>
> Hi Mingui,
> I will try to summarize our agreements and disagreements.
>
> 1.       The scale of TRILL deployments:
>
> a. I have not seen any specific numbers or references to any
> specific topologies that could really make single-level TRILL not
> scalable enough - neither in the TRILL drafts I've read nor in our
> discussions so far

I am puzzled as to why you think there would be some specific numeric
hard boundary, other than number space or memory space exhaustion,
beyond which you cannot scale. Can you point to a documentation of
such a thing for IP use of IS-IS? Surely it depends on how much
computer power the routers have, link stability, and numerous other
factors.

Just looking at convergence time and approximating it as the amount of
computation required at a typical router in the TRILL campus, it is on
the order of N*(log N) for computation of least cost routes where
there are N routers in a single level campus while it is on the order
of (sqrt N)*(log N) for multi-level, in both cases assuming optimized
calculations. The largest TRILL campuses I am aware of are on the
order of 3,000 routers. So one would expect that converting such a
campus to multi-level TRILL would reduce convergence time by
approximately a factor of 50. Furthermore, one would expect the rate
of failures within each Level 1 area in the multi-level case to be
approximately proportional to the number of links/routers and thus
also fall by one and a half orders of magnitude. Do you claim these
improvements would never be valuable?

There are many other scaling factors such as the size of the link
state database, etc. I believe the informational
draft-ietf-trill-rbridge-multi-level gives a good summary.

> b. Regarding your reference to multiple interconnected TRILL-based
> campuses in your last email: I do not think that TRILL is an
> alternative to or competes with Internet.

I agree that TRILL does not compete with the Internet.  :-)

I believe this facet of the discussion was in connection with the
possibility of TRILL nickname space exhaustion. Consider the following
factors:

1) TRILL supports active-active connection of end stations at the
TRILL edge. Using the techniques in RFC 7781 (Pseudo-Nickname for
Active-Active Access) consumes TRILL nicknames for active-active edge
groups.

2) Assuming, for the moment, you are using multi-level with unique
nicknames, the nickname allocation mechanism will waste many nicknames
due to hierarchical assignment, the same way power-of-two sized IP
subnets waste IP addresses.

3) There is a desire to interconnect TRILL campuses that are under
joint or cooperative management with limited control plane coupling so
as to limit error propagation, etc. There are various possible ways to
do this but most of them assume non-conflicting nicknames (or
non-conflicting level 2 nicknames if the campuses are multi-level).

I admit that even taking the largest existing TRILL campuses I know
about and adding extensive active-active end station support at the
edge and multi-level with unique nicknames that are hierarchically
allocated, you would still probably not exhaust the TRILL nickname
space. But you could be getting close to that hard limit. This seems
like enough reason to me to be advancing a standard where Level 1
areas are aggregated (whether by a single nickname or set of border
router nicknames) to, for all practical purposes, eliminate the
nickname space restriction.

> c. I have also noticed that, once upon a time (4 years ago) there
> was an attempt to use BGP with TRILL. I wonder why this draft has
> been left to expire because, from my POV, BGP is greatly preferable
> to multi-level IS-IS when it comes to scalability issues.
>
> 2. Nickname Re-write vs Nickname Swapping: Looks like a clear case
> of misunderstanding between us, probably due to the fact that I am
> not a TRILL expert:
>
> a.       I have used the term "swapping" in the same way it is used
> in MPLS (e.g., see RFC 3031 discussing label swapping). In other
> words, from my POV "nickname swapping" and "nickname re-write" were
> synonyms.
>
> b.      It seems that some yet to be standardized extension of TRILL
> considers some dedicated nickname swapping mechanism that carries
> new nicknames in some extension of the TRILL header.  In this
> parlance "nickname re-write" and "nickname swapping" are different.

Right. The possibility was discussed some time ago of expanding the
TRILL header so that, for TRILL Data packets going between different
Level 1 Areas, there could be, in effect, two ingress nicknames (an
ingress RBridge nickname and an ingress Area nickname) and two egress
nicknames (an egress RBridge nickname and an egress Area nickname).
Appropriate swapping would occur at border routers to avoid changes in
fast path logic at all non-border routers. Within Level 2, the Area
nicknames would be in the existing header slots that are routed on,
etc. However, as far as I can recall, no specification was ever been
produced for this "nickname swapping" although it is mentioned in the
informational multi-level draft.

> c.       I think that we now safely consider this discussion issue
> as closed.
>
> 3.       Metadata:
>
> a.       I fully agree that we should hear from other RTG-DIR
> members on what exactly (if at all) should be specified to clarify
> the relationship between the Single Nickname draft and RFC 6325

I would note that the IESG policy on "updates" has become very strict
recently. It used to be a judgment call. Now, when a Standards Track
RFC merely extends another such RFC so you could implement an instance
of the earlier standard as specified without reference to or violating
the subsequent specification, the IESG will generally prohibit you
from claiming that the subsequent specification "updates" the first.
(I do not particularly agree with this policy change myself.)

> b.      I can only add that, AFAIK, the Multi-Level TRILL draft,
> being positioned as an Informational, can neither update nor
> obsolete RFC 6325 (which is Standards Track). So there is no issue
> with its metadata being empty.
>
> There is one issue in my original review that we have not discussed
> at all - namely, the behavior implied by the Single Nickname draft
> when a new border RBridge is added to a certain area.

I agree that what needs to be done when border RBridges are
added/deleted needs to be clear in the draft.

Thanks,
Donald
===============================
Donald E. Eastlake 3rd   [phone] (cell)
155 Beaver Street, Milford, MA 01757 USA
[e-mail]

> Regards,
> Sasha
>
> Office: [phone]
> Cell:      [phone]
> Email:   [e-mail]
>
> From: Mingui Zhang [mailto:[e-mail]]
> Sent: Wednesday, May 25, 2016 6:07 AM
> To: Alexander Vainshtein
> Cc: '[e-mail]'; Zhangxian (Xian); [e-mail]; [e-mail]; Susan Hares; [e-mail]; Jonathan Hardwick
> Subject: RE: [RTG-DIR] RTG-DIR QA review for draft-ietf-trill-multilevel-single-nickname
>
> Hi Sasha,
>
> Thanks for the comments.
>
>
> [[Sasha]] Do you (or the authors of the multi-level TRILL draft) consider deployment scenarios with more than 64K RBridges in a single TRILL campus? Is this a realistic scenario?
> [Mingui] We can also doubt whether a domain with more that 2^32 IP routers is a realistic scenario. The fact is that a single campus is usually not allowed to use up the entire 64K namespace. Please consider the scenario that lots of TRILL campuses are to be interconnected.
>
>
> [[Sasha]] In other words, your draft explicitly states that the area border RBridges modify the nicknames in the TRILL header of a packet that crosses the Level 2 domain. How is this different from swapping (save from the name of the operation)?
> [Mingui] As I said, there is no "swap nickname field" conception in the draft.  Yes, the border RBridge needs to modify the nickname but it does not have to modify it through the "swapping" operation. Instead, the border RBridge "replaces" the nickname in the TRILL data packets with its own nickname (rather than a nickname in the "swap nickname field" provided by the originating RBridge). Why authors prefer the replacing operation than the swapping operation? Because the swapping operation requires a new TRILL header (two additional 16-bit fields) which has not been standardized yet.
>
> As for the "Updates" metadata, let's see if people on the RTG-DIR list would give directions.
>
> Best regards,
> Mingui
> From: Alexander Vainshtein [mailto:[e-mail]]
> Sent: Tuesday, May 24, 2016 6:45 PM
> To: Mingui Zhang
> Cc: '[e-mail]'; Zhangxian (Xian); [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; Susan Hares; [e-mail]<mailto:[e-mail]>; Jonathan Hardwick
> Subject: RE: [RTG-DIR] RTG-DIR QA review for draft-ietf-trill-multilevel-single-nickname
>
> Mingui hi!
> Lots of thanks for a prompt response.
>
> A few short comments inline below.
>
> Regards,
> Sasha
>
> Office: [phone]
> Cell:      [phone]
> Email:   [e-mail]<mailto:[e-mail]>
>
> From: Mingui Zhang [mailto:[e-mail]]
> Sent: Tuesday, May 24, 2016 11:23 AM
> To: Alexander Vainshtein
> Cc: '[e-mail]'; Zhangxian (Xian); [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; Susan Hares; [e-mail]<mailto:[e-mail]>; Jonathan Hardwick
> Subject: RE: [RTG-DIR] RTG-DIR QA review for draft-ietf-trill-multilevel-single-nickname
>
> Hi Sasha,
>
> Thanks for your comments. Please see responses inline below.
>
> Thanks,
> Mingui
>
> From: Alexander Vainshtein [mailto:[e-mail]]
> Sent: Monday, May 23, 2016 6:13 PM
> To: Mingui Zhang
> Cc: '[e-mail]'; Zhangxian (Xian); [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; Susan Hares ([e-mail]<mailto:[e-mail]>); [e-mail]<mailto:[e-mail]>; Jonathan Hardwick ([e-mail]<mailto:[e-mail]>)
> Subject: RE: [RTG-DIR] RTG-DIR QA review for draft-ietf-trill-multilevel-single-nickname
>
>
> Mingui hi!
>
> Lots of thanks for a prompt response to some of the issues I've raised in the review.
>
>
>
> Please see some comments to you responses inline below.
>
>
>
> Regards,
>
> Sasha
>
>
>
> Office: [phone]
>
> Cell:      [phone]
>
> Email:   [e-mail]<mailto:[e-mail]>
>
>
>
> -----Original Message-----
> From: rtg-dir [mailto:[e-mail]] On Behalf Of Mingui Zhang
> Sent: Monday, May 23, 2016 12:31 PM
> To: Alexander Vainshtein; Jonathan Hardwick ([e-mail]<mailto:[e-mail]>)
> Cc: '[e-mail]'; Zhangxian (Xian); [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; Susan Hares ([e-mail]<mailto:[e-mail]>); [e-mail]<mailto:[e-mail]>
> Subject: Re: [RTG-DIR] RTG-DIR QA review for draft-ietf-trill-multilevel-single-nickname
>
>
>
> Hi Alexander,
>
>
>
> Thanks for the review!
>
>
>
> The multilevel conception itself is abstract and not easily understandable.
>
> [[Sasha]] Do you refer to the multi-level IS-IS in general or multi-level TRILL specifically? I am asking because I believe am reasonably well aware of the multi-level architecture of IS-IS as used for IP routing. It is somewhat different from that of OSPF, but I would not call it "abstract and not easily understandable".  And there are quite a few excellent introductions to the subject (again in the context of IP routing). However, I am definitely not a TRILL expert, and have stated that in the review.
>
>
>
> [Mingui] Yes, multi-level arch of IS-IS has already been well understood. However, the extending TRILL to multi-levels brings new challenges. As stated in the informational draft, one issue is on processing the TRILL switch nicknames and the other issue is on handling multi-destination packet distribution trees. In order not to make the specifications "abstract", the draft carefully designed two walking-through examples in Section 3. If the examples were understood, it would be non-abstract as well. ;-)
>
>
>
> However, it was really interesting in designing such a solution. Appreciate the review and the time on relevant documents to figure out the whole scheme.
>
>
>
> > ?  Nor provides any explanations about the reasons that make
>
> > single-level IS-IS used by TRILL less scalable that single-level IS-IS
>
> > when it is used for distributing IP reachability
>
>
>
> The reason comes from the fact that the length of a nickname is different from an IP address.
>
> [[Sasha]] I must admit that I do not understand the connection. By this logic, IS-IS for CLNS and IPv6 should be much more scalable than IS-IS for IPv4, but I have never seen such claims before. Could you please elaborate? Could somebody on the RTG-DIR list to comment on that?
>
>
>
> [Mingui] For a single-level IS-IS instance, the length of the address determines the name space. In the informational draft, Section 1.1 TRILL Scalability Issues, the following statement is relevant
>
> "   5. the limit of the number of TRILL switches, due to the 16-bit nickname space,"
>
> [[Sasha]] Do you (or the authors of the multi-level TRILL draft) consider deployment scenarios with more than 64K RBridges in a single TRILL campus? Is this a realistic scenario?
>
>
>
>
>
> I think this could be addressed in the updated version of the draft: https://datatracker.ietf.org/doc/draft-ietf-trill-rbridge-multilevel/?include_text=1.
>
>
>
> > *         The draft positions itself as an alternative to the Aggregate
>
> > Nicknames approach while, from my POV, it is just provides additional
>
> > details on one of the possible flavors of this approach
>
>
>
> The WG used to discuss several ways to address the "Aggregate Nickname" approach.
>
> [[Sasha]] I do not follow the TRILL WG mailing list, so I am not aware of any discussions that have been hold there. I am only speaking about what I could find in the two drafts mentioned in my review.
>
>
>
> [Mingui] Actually, the informational draft had included the information of those alternatives as well. Please see "Section 2.2.2.2 Swap Nickname Field Aggregated Nicknames" and read the words about "pseudonode" of Section 2.5