Re: [spring] C-SIDs and upper layer checksums (draft-ietf-spring-srv6-srh-compression)

[Joel Halpern <[e-mail]>]

The concern with regard to the text that the chairs are asking about is 
not about intermediate nodes verifying the checksum.  The text does not 
talk aabout that, so we are not asking about that.

But, the text in 8200 specifies how the originating node is to compute 
the upper layer checksum.  It doesn't say "do whatever you need to do to 
make the destination come out right".  It provides specific 
instructions.  Yes, it is understandable that those instructions do not 
cover the compressed container cases.  Which is why the compression 
document specifies changes to those procedures.

Thus, we need to ask 6man how they want to handle the change in the 
instructions in 8200.

the question we are asking SPRING is whether there is any clarification 
people want to the text in the compression draft before we send the 
question over to 6man.

Yours,

Joel

On 4/3/2024 5:15 PM, Robert Raszuk wrote:
> Hi Joel,
>
> My interpretation of text from RFC8200 is that it allows discrepancy 
> between the header and the upper layer checksum as long as final 
> packet's destination sees the correct one.
>
> The last condition is met.
>
> So I see no issue.
>
> Sure RFC8200 does not talk about SRH nor cSIDs, but provides a hint on 
> how to handle such future situations.
>
> With that being said I would like to still understand what real 
> problem are we hitting here ...
>
> Kind regards,
> Robert
>
> On Wed, Apr 3, 2024 at 11:09 PM Joel Halpern <[e-mail]> wrote:
>
>     There are two cases covered in section 6.5 of the compression
>     draft that appear to be at variance with secton 8.1 of RFC 8200.
>
>     First, if the final destination in the routing header is a
>     compressed container, then the ultimate destination address will
>     not be the same as the final destination shown in the routing header.
>
>     Second, if a uSID container is used as the destination address and
>     no SRH is present, then in addition to the above problem there is
>     no routing header to trigger the behavior described.
>
>     Yours,
>
>     Joel
>
>     On 4/3/2024 4:22 PM, Robert Raszuk wrote:
>>     Hi Alvaro,
>>
>>         Section 6.5 of draft-ietf-spring-srv6-srh-compression
>>         describes the
>>         behavior when an originating node inside an SRv6 domain creates a
>>         packet with a C-SID as the final destination. _This
>>         description differs
>>         from the text in Section 8.1 of RFC8200._
>>
>>
>>     I would like you to clarify the above statement - specifically of
>>     the last sentence.
>>
>>     Reason for this that after looking at both drafts I find section
>>     6.5 of the subject draft to be exactly in line with RFC8200
>>     section 8.1 especially with the paragraf which says:
>>
>>     *         If the IPv6 packet contains a Routing header, the
>>     Destination
>>              Address used in the pseudo-header is that of the final
>>              destination.  At the originating node, that address will
>>     be in
>>              the last element of the Routing header; at the recipient(s),
>>              that address will be in the Destination Address field of the
>>              IPv6 header.
>>     *
>>
>>     So before we dive into solutions (as Andrew has already provided
>>     a few of) I think we should first agree on what precise problem
>>     are we solving here ?
>>
>>     Many thx,
>>     Robert
>>
>>     PS. As a side note I spoke with my hardware folks - just to check
>>     if validation of upper-layer checksum is even an option for
>>     transit nodes. The answer is NO as most data plane hardware can
>>     read at most 256 bytes of packets. So unless there is some
>>     specialized hardware processing up to 9K packets in hardware at
>>     line rates this entire discussion about checksum violations,
>>     fears of firing appeals is just smoke.
>>