Re: [spring] Suggest some text //RE: Request to close the LC and move forward//RE: WGLC - draft-ietf-spring-srv6-network-programming

[Brian E Carpenter <[e-mail]>]

On 01-Mar-20 07:25, Robert Raszuk wrote:
> Hi John,
> 
> I respectfully will disagree with your assessment. 
> 
> Reason #1 - IPv6 can be encapsulated in IPv6 - RFC2473. This is base of SRv6 operation. If this would be deprecated, moved to historic or made illegal - games over. But if this is still legal then ultimate destination for a packet is what it listed in outer IPv6 header DA. That's pretty basic. Now what the destination in the header will do with the packet is completely different story. 
> 
> Reason #2 - "a node can’t be both the penultimate, and the ultimate, node." Of course it can. You are looking flat .. 

But I've been told by several people that this is not the use case. I believe
the little diagram I sent yesterday is the use case. And the trick in the description
of PSP is what I pointed out yesterday too: deleting the header when segments-left == 0
but the destination address is not yet set to the final one:

https://mailarchive.ietf.org/arch/msg/spring/n46VuroVGvRurIgGNLZxFbAHvIo/

The thing is, it can be coded and I fully believe there is running code. Also, answering
your question "what harm does it do?" I think the answer objectively is "none, unless
you want to use AH". Making a packet smaller on the last hop cannot break PMTUD.

So I think the text needs to admit the trick it's playing on RFC 8200. Then the IETF
can choose whether to let that trick pass.

   Brian


> If you look at different layers the same node is in fact acting in multiple roles - I can easily count 3 but with TI-LFA it could be  even more. 
> 
> The same node is ultimate destination for the outer header 
> in the same time 
> The same node is penultimate destination for SR path
> in the same time 
> The same node is just regular IPv6 transit from the perspective of the original non encapsulated packet 
> 
> Kind regards,
> R.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> On Sat, Feb 29, 2020 at 6:46 PM John Scudder <[e-mail] <mailto:[e-mail]>> wrote:
> 
>     Robert,
> 
>     I think your comment (emphasis added):
> 
>>     we are dealing here with an *encapsulated* packet which _has as its ultimate destination_ SR node X. That SR node X is to perform or not PSP.
> 
>     Is wrong. It contradicts everything else that’s been said in the hundreds of messages that have gone before, not to mention the plain language of draft-ietf-spring-srv6-network-programming-10. The word “penultimate” itself is enough to prove this: by definition a node can’t be both the penultimate, and the ultimate, node. It’s a contradiction in terms, like saying 0 equals 1. 
> 
>     Now, if node X were to remove the RH /and perform the decapsulation/ that would be a different story, but the whole point of PSP is that X removes the RH and then sends the encapsulated packet on to Y which performs the decapsulation. (This point was made in one of the other threads recently, but I’ve lost track of by whom and which thread.) As far as I can tell, this non-controversial behavior is described in 4.16.3 of the draft and referred to as “USD”.
> 
>     —John
> 
>>     On Feb 29, 2020, at 6:06 AM, Robert Raszuk <[e-mail] <mailto:[e-mail]>> wrote:
>>
>>     Dear Jinmei,
>>
>>     Even if RFC8200 section 4 text would say: 
>>
>>      "Extension headers cannot be added to a packet after it has left its source node and extension headers cannot be removed from a packet until it has arrived at its ultimate destination".
>>
>>     PSP would be still not be violating anything said in this sentence. Reason being is that we are dealing here with an *encapsulated* packet which has as its ultimate destination SR node X. That SR node X is to perform or not PSP. So it is still fully compliant with the specification. 
>>
>>     IMHO the only grey area as pointed by Brian is if RFC8200 section 4.4 really mandates you to look at segments_left before processing the packet or it is equally legal to look at that value after local processing occurs. Definition says: 
>>
>>
>>           Segments Left       8-bit unsigned integer.  Number of route
>>                               segments remaining, i.e., number of explicitly
>>                               listed intermediate nodes still to be visited
>>                               before reaching the final destination.
>>
>>     which to me really means that as long as you recognize given routing header type you can decrement this value and if zero remove it. 
>>
>>     Besides that is a minor detail - as NPG draft could be updated to say: 
>>
>>      S14.1.   If (Segments Left Before Local Decrement == 1) {
>>      S14.2.      Update the Next Header field in the preceding header to the
>>                     Next Header value of the SRH
>>      S14.3.      Decrease the IPv6 header Payload Length by the Hdr Ext Len
>>                     value of the SRH
>>      S14.4.      Remove the SRH from the IPv6 extension header chain
>>      S14.5.   }
>>
>>     Many thx,
>>     R. 
>>
>>     On Sat, Feb 29, 2020 at 2:28 AM 神明達哉 <[e-mail] <mailto:[e-mail]>> wrote:
>>
>>         At Fri, 28 Feb 2020 07:54:28 +0000,
>>         "Xiejingrong (Jingrong)" <[e-mail] <mailto:xiejingrong@huawei...com>> wrote:
>>
>>         > The design of PSP for the benefits of deployment is based on the understanding
>>         > that it does not violate section 4 of RFC8200. In case the RFC8200 text may be
>>         > modified in the future, the PSP may also need to change accordingly.
>>
>>         No, it violates Section 4 of RFC8200.  It's a pity that we have to
>>         discuss it at this level due to the poor editorial work then (I was
>>         also responsible for that as one of those reviewing the bis draft),
>>         but anyone who involved the discussion should know the intent of this
>>         text intended to say (borrowing from Ron's text) "Extension headers
>>         cannot be added to a packet after it has left the its source node and
>>         extension headers cannot be removed from a packet until it has arrived
>>         at its ultimate destination".  It might look "an attempt of blocking
>>         an innovation by a small group of vocal fundamentalists", but if you
>>         see the responses without a bias, you'd notice that even some of those
>>         who seem neutral about the underlying SRv6 matter interpret the text
>>         that way.
>>
>>         I'd also note that simply because PSP violates RFC8200 doesn't
>>         immediately mean it (PSP) "needs to change".  It can update RFC8200 with
>>         explaining why it's necessary and justified.  That's what I
>>         requested as you summarized:
>>
>>         > Jinmei: it should say it updates this part of RFC8200 and explain why it's justified.
>>
>>         And, since PSP at least wouldn't break PMTUD, I guess the update
>>         proposal will have much more chance to be accepted than a proposal
>>         including EH insertion.  On the other hand, pretending there's no
>>         violation will certainly trigger many appeals and objections at the
>>         IETF last call (I'll certainly object to it).  In the end, it can
>>         easily take much longer, or even fail, than formally claiming an
>>         update to RFC8200.
>>
>>         --
>>         JINMEI, Tatuya
>>
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