Re: [tsvwg] I-D Action: draft-ietf-tsvwg-udp-options-13.txt

[Joseph Touch <[e-mail]>]

Hi, Mike,

> On Jul 31, 2021, at 4:28 PM, C. M. Heard <[e-mail]> wrote:
> 
> On Wed, Jul 7, 2021 at 10:25 PM Joseph Touch wrote:
> Hi, Mike,
> 
>> On Jun 26, 2021, at 10:13 PM, C. M. Heard wrote:
>> 
>> On Sat, Jun 19, 2021 at 12:39 AM Joseph Touch wrote:
>> Here’s a summary - I tried to catch both everything Mike provided feedback on as well as what I have proposed as a way forward.
>> 
>> This is much appreciated, thank you. My item-by-item responses follow.
> 
> 
>> OCS
>> Uses the standard 2-byte prefix (all but NOP and EOL do)
>> Added discussion of RFC 6935 regarding exception to requiring the UDP checksum and thus OCS
>> Allow OCS’s checksum to be precomputed, but still check in the order options occur
>> Occurs over the entire surplus area (doesn’t stop at EOL)
>> OK on most points, but I strongly object to the following when OCS is mandatory (as is the case when UDP CS <> 0):
>> 
>>    Note that a receiver can compute the OCS checksum before processing
>>    any UDP options, but that computation would assume OCS is used and
>>    would not be verified until the OCS option is interpreted.
>> 
>> If I perform the computation before processing any options, and OCS is mandatory, it would be foolish of me to waste any more effort seeking the OCS option in the TLV chain. I already know that the option area is invalid,
> 
> How exactly do you know this? If OCS is mandatory, you need to find it (seek it in the TLV chain) in order to know whether the value you compute differs from the value in the option.
> 
> This last statement is not true, and I think it's important to clarify that fact.
> 
> If OCS is mandatory (because UDP CS<>0), then I can determine whether the options area has an INVALID checksum by computing the 16-bit Internet checksum over the options area conceptually prepended by a 2-byte options area pseudo-header containing the options area length. If the result is not a 1s complement zero, then I KNOW that the option checksum is not valid. Full stop. Whether that is because the OCS TLV is absent or because it is present and has the wrong value makes no difference; the option checksum is not valid, and I do not need to parse a single TLV to determine that. I can immediately stop option processing and discard the contents of the option area.

OK - I see what you’re getting at now. We can adjust the text accordingly.

Joe