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

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Hello Benjamin,

See my comment to your point, inlined.

Dominique 

Le 17/03/20 22:13, « Benjamin Kaduk » <[e-mail]> a écrit :

>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?

Indeed, the protocol parser and the SCHC rules need to know about the UDP
TLVs if one wants to compress them.
But the same is true of all the other fields. I don't think this one
warrants a special notice.
What I insist on is that, if an implementation does not know of the UDP
TLVs, it will not reconstruct an erroneous UDP Length, even for a packet
that contains these TLVs, assuming that the protocol parser checks the UDP
and IPv6 lengths for consistency. See my example in a separate mail.

>
>-Ben


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