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

[Tom Herbert <[e-mail]>]

On Fri, Jan 26, 2018 at 11:59 AM, Uma Chunduri <[e-mail]>
wrote:

> Comments are spot-on.
>
>
>
> Can somebody tell 8200 update would be a possibility in future (w.r.t 6man
> consensus) i.e., EH insertion in the middle without re-encapsulating the
> SRH again.
>
> I presume the technical aspect for the 8200 mandate is the ability to
> fragment if needed at the insertion point. Anything else?  Please
> enlighten..
>
>
>
Uma,

There were several issues raised. I don't think fragmentation actually came
up as one of them, but that is good point. Intermediate nodes cannot
fragment packets in IPv6. Interestingly, the subject of fragmentation came
up yesterday on the 5gangIP list. There are providers that see
fragmentation happening because of GTP. Not all networks have converted to
us jumbo frames to preserve a 1500 MTU to end nodes and do encapsulation--
increasing packet size in intermediate nodes is still a problem.

Here is a summary of issues that were raised on 6man:

- The proposal attempted to carve out an exception to RFC8200 for just SR
and limited use to controlled domains. That entails many caveats an
assumptions that would need to be MUSTs.

- Encapsulation is recommended to allow EH insertion and several people
asked why not use it. It will work inasmuch as encapsulation within the
network already works.

- If a host is already using extension headers and the network tries to add
more, there is an ambiguity about which ones the.network is responsible
for. When packet leaves the domain, the EH that the network added needs to
be removed and it needs to be unambiguous which ones are to be removed.

- ICMP is a problem. If a host gets an ICMP error that contains extension
headers that it did not possibly send then that will be confused.
Presumably, ICMP errors will need to be stripped of EH before forwarding to
a source node.

- PTB is interesting. For instance, EH insertion could force PMTU to drop
below 576 minimum.

- If the added extension headers are causing packet drops this is a major
problem. The intermediate node that is inserting the EHs will never get
feedback that its actions are doing harm. An end host might detect packet
loss at the transport layer or might get an ICMP error (maybe something
like  draft-herbert-6man-icmp-limits-02
<https://tools.ietf.org/html/draft-herbert-6man-icmp-limits-02>), But, even
if it knows that inserted EHs are causing drops, there's is no action a
host can take to stop it. At least with encapsulation the tunnel ingress
might get and ICMP error about what it's doing.

I suppose the primary argument against encapsulation is that it's too much
overhead. But, I would point out that in the examples if only one sid is
required for mobility (address of destination)  in an IP/IP encapsulation
this would be the destination of the inner packet and SRH wouldn't be
needed. So encapsulation overhead = 40 bytes, SRH overhead = 20 bytes. I'm
not sure that difference justifies the complexity of EH insertion.

Tom



Hence, most significant issue has to be resolved perhaps would be the first
> item.
>
>
>
>
>
> BR,
>
> --
>
> Uma C.
>
>
>
> *From:* ila [mailto:[e-mail]] *On Behalf Of *Tom Herbert
> *Sent:* Friday, January 26, 2018 9:14 AM
> *To:* [e-mail]; [e-mail]
> *Subject:* [Ila] 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
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