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

[Tom Herbert <[e-mail]>]

On Fri, Jan 26, 2018 at 5:35 PM, Sri Gundavelli (sgundave) <
[e-mail]> wrote:

> Hi Tom,
>
> > I am working on a comparison between ILA and SRv6 for the mobile
> user-plane.
>
> This is a good effort. I was wondering, about the key parameters that you
> will use for this comparison between ILA/ILNP/LISP/HICN etc. For example,
> ILA router the entries at the ILA router ( ID – L OC  - SIR Prefix), vs at
> the LISP mapping system. How do you compare the two, a cache/MAP query
> cost, vs a translation cost + local memory state for keeping that entry.
>
> Sri,

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.

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).


> 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.

Tom


> Sri
>
>
>
>
>
> From: dmm <[e-mail]> on behalf of Tom Herbert <
> [e-mail]>
> Date: Friday, January 26, 2018 at 9:13 AM
> To: "[e-mail]" <[e-mail]>, "[e-mail]" <[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
>
>
>
>
>