Re: [lp-wan] [Last-Call] Tsvart last call review of draft-ietf-lpwan-coap-static-context-hc-12

[Benjamin Kaduk <[e-mail]>]

On Fri, Mar 13, 2020 at 11:48:18PM +0000, [e-mail] wrote:
> Hello all,
> 
> This is the response by the ietf-lpwan-ipv6-static-context-hc co-authors
> to the observation below by Joseph Touch.
> You are right that Section 10 of ietf-lpwan-ipv6-static-context-hc only
> shows how to compress IPv6/UDP packets that have no extensions.
> 
> However, the SCHC specification in Section 7.1 states
> "It is assumed that
>    there is a protocol parser alongside SCHC that is able to identify
>    all the fields encountered in the headers to be compressed, and to
>    label them with a Field ID."
> 
> Furthermore, Section 7.3 states
> "If any
>          header field of the packet being examined cannot be matched
>          with a Field Description with the correct FID, the Rule MUST be
>          disregarded.  If any Field Description in the Rule has a FID
>          that cannot be matched to one of the header fields of the
>          packet being examined, the Rule MUST be disregarded."
> 
> It follows that, if an IPv6 packet does contain the new UDP TLVs
> introduced by draft-tsvwg-udp-options, it will not match a Rule destined
> to compress the IPv6/UDP headers and that does not have these TLVs as part
> of the Field Descriptors.
> Therefore, it will no be compressed by that Rule and will not be
> decompressed with a wrong UDP Length at the receive end.
> 
> By contrast, if a Rule destined to compress the IPv6/UDP headers does have
> these UDP TLVs as part of the Field Descriptors, it is responsible for
> reconstructing the UDP Length properly. This can be achieved by means
> already provided by SCHC, such as sending the value over the wire or
> matching it to an expected constant, etc.
> 
> In conclusion, the SCHC compression mechanism, if properly implemented, is
> fully compatible with draft-tsvwg-udp-options.

This "if properly implemented" seems pretty critical.  Do we need to leave
a reminder that implementations have to know about UDP TLVs in order to
properly identify all fields in the headers to be compressed?

-Ben