[Ila] Fwd: [DMM] Questions about SRv6 mobile user-plane

["Sri Gundavelli (sgundave)" <[e-mail]>]

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From: "Sri Gundavelli (sgundave)" <[e-mail]<mailto:[e-mail]>>
Date: January 29, 2018 at 7:25:47 AM PST
To: <[e-mail]<mailto:[e-mail]>>
Subject: Re: [DMM] Questions about SRv6 mobile user-plane

HI Tom,

Thanks for your response. Please see inline.



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https://tools.ietf.org/html/draft-herbert-ila-motivation-00 provides some comparisons between ILA and ILNP, encapsulations, SR, and transport layer mechanisms that can achieve some effects in mobility.

The choice of mapping system is critical. The mapping of identifier, or equivalently virtual to physical address mapping, seems to be a common problem in mobility and networking virtualization. As you mentioned, LISP defines a query method to populate a mapping cache. I assume this problem needs to be tackled in SR for mobile user-plane but I'm not sure what solution is preferred after reading the draft.

[Sri] There are multiple approaches on how we manage this mapping state. Obviously, ILA is one approach, but there are few other approaches as well that we need to review.



ILA partitions the problem into a two level hierarchy: ILA routers and IL forwarding nodes. This is somewhat analogous to core IP routers and nodes running neighbor discovery.  ILA routers contain all the (possibly sharded) mappings. They are authoritative. Forwarding nodes are located close to user devices and maintain a working set  cache of entries driven by user activity. If a packet doesn't hit the cache it's forwarded to a router that will do the ILA transformation. If the cache is hit, the packet can be transformed at the forwarding node to eliminate triangular routing. Caches can be populated by pull or push models. ILAMP (the ILA mapping protocol) supports both of these, but my current preference for scalability and mitigating DOS attacks on the cache is to use secure redirects sent by ILA routers  (analogous to ICMP redirects).


[Sri] When I last reviewed the ILA I-D, I do not seem to remember reading about the cache state, ILMP. or about how the mapping gets to the ILA routers. Looks like the spec is evolving as we speak. With ILAMP type control protocol for cache management, I see more similarities to LISP.




On a different note, just curious if SID prefix can ever have topological relevance and can be used for routing. In other words, can you ever route a packet without translating  the SIR prefix of the destination address with the locator? Can SID prefix be used as a locator in some special cases?

Yes, the SIR prefix is routable to forward to an ILA router. This is necessary for the redirect mechanism I describe above. I suppose this could be contorted to make the SIR address be a home address like in MobileIP and locators are COAs (if my use of MobileIP terminology is correct). There also might be nodes in the network, as well as external nodes that don't do go through a cache to their packets need to hit an ILA router to get forwarded to the location of mobile nodes. An upshot of that is that edge routers might need to perform transformations (SIR to ILA) at high rates so the mechanism needs to be very efficient and amenable to HW implementation.


[Sri] This is precisely what I was thinking.

I get that SIR prefix takes the packet awards the ILA domain and some ILA router in the path can apply the mapping. I was thinking there may not be a good reason to have more than one or two SIR prefixes for each ILA domain. As long as the SIR prefix can take the packet from a non-ILA domain (internet) to ILA domain, then the edge router can apply the mapping. But, that also implies the edge routers will have to have too much of mapping state. Now, if we have many SIR prefixes and associate a SIR prefix for each PGW/UPF, that state can be distributed and keep the edge routers stateless, but it also brings anchoring back into the picture. In one simplest mode, as you say, HNP (home network prefix) can be a SIR and the PGW/SGW or  (LMA/MAG) can do the translation of SIR - ILA, without the need for tunneling.

So, in your mind how many SIR prefixes will be used in a typical T1 operator domain? Also, how can we quantify the state that ILA introduces in different parts of the network?


Regards
Sri







From: dmm <[e-mail]<mailto:[e-mail]>> on behalf of Tom Herbert <[e-mail]<mailto:[e-mail]>>
Date: Friday, January 26, 2018 at 9:13 AM
To: "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>
Subject: [DMM] Questions about SRv6 mobile user-plane

Hello,

I am working on a comparison between ILA and SRv6 for the mobile user-plane. I have some questions/comments about SRv6 and particularly on the example use cases that were depicted in the slides that were presented in IETF100:

https://datatracker.ietf.org/meeting/100/materials/slides-100-dmm-srv6-for-mobile-user-plane/

- It's clear from the depicted use cases that extension header insertion is being done by intermediate nodes, but extension header insertion is currently prohibited by RFC8200. There was an I-D posted on 6man to allow this for SR, but that was met with pushback. Is there going to be followup to resolve this?

- For the uplink use cases, this seems to be more like using SR to source route to an egress router. In other words, it's not strictly related to mobility. Is there some connection to mobility that I'm missing?

- The size or number of SR headers in the uplink cases seems to be larger than necessary (IMO minimizing these is important since each additional sid is ~1% overhead of standard MTU). In this first scenario sid[1]=A2::1 and DA=A2::1-- this seems to be redundant information. Also this depicts a second SR being inserted, but the first one should no longer be relevant. Why not just discard the first one and save the overhead? In the second scenario, DA is changing from A2::1 to A3::1<https://maps.google.com/?q=A3::1&entry=gmail&source=g>, but AFAICT that was not done per the SR processing. What is the operation that happened here? (it's actaully looks like an ILA transfomation).

- Considering the points above, could this have been done in the following manner to minimize overhead? A1 creates one SRH with one sid and makes DA=A2. A2 makes DA=A3. At A3 SR is processed, DA is restored to Internet address, and EH is removed.

- For downlink this does see to be relevant to mobility. But I have the same question, wouldn't it be less overhead to only use one SRH and one sid? i.e. A3 creates an SRH with just one sid that is the S:: (identifier in identifier/locator speak) and set DA to A2, and then A2 sets DA to A1, A1 restores original packet for delivery.

- One possible typo. In the last use case slide SA=S:: and DA=D::, I believe these should be swapped?

Thanks,
Tom