Re: [tsvwg] OCS option in draft-ietf-tsvwg-udp-options-07

[Joe Touch <[e-mail]>]

Yes, when UDP CS=0 we don't cover length or ports. 

Length isn't an issue unless the reassembly checksum is also zero, in
which case we're doing the right thing - leaving it alone. 

As to ports when the frag checksum is NOT zero, I'm much less concerned
given how devices happily recompute that along packet paths for their
own convenience. 

Joe

On 2019-03-12 05:10, Raffaele Zullo wrote:

> On 2019-03-09 19:28, Joe Touch wrote: 
> 
>> 3. we have to deal with LITE/LITE+FRAG
>> we *can* use UDP CS=0, but that also may involve overriding the
>> user's desires here (i.e., the user API says "use UDP CS" and we're
>> not doing that)
>> which means we have a choice:
>> (d) override so that things work, i.e., force UDP CS=0 and then
>> (d.1) do OCS as a check on our stuff only
>> OR
>> (d.2) disallow the use of OCS (if CS=0, what's the point?)
>> OR
>> (d.3) use OCS=0 (this seems like a waste)
>> OR
>> (e) ignore the user setting of CS, then:
>> (e.1) always do OCS as a check
>> OR
>> (e.2) if UDP CS=0, omit OCS (save space), basically like d.2
>> OR
>> (e.3) if UDP CS=0, use OCS=0 (basically like e.2
>> 
>> If we do any of the (e) variants, we could either:
>> (f.1) require users to disable UDP CS to use OCS
>> OR
>> (f.2) ignore the user UDP CS setting
>> 
>> So which is it?
>> 
>> I don't like assuming user correct configuration (f.1).
>> 
>> So to me it's (f.2) and (d.1).
>> 
>> Thoughts?
> 
> Maybe I'm missing something but there could be
> two other problems when using CS=0 with LITE+FRAG
> 
> 1) We lose coverage of UDP Length
> If the UDP Length due to an error becomes 8 < UDP Length <= IPPayloadLength
> Options will probably become not parsable
> but part of the Options is delivered as UDP Payload to the user.
> 
> 2) We lose coverage of Source and Dest Ports
> If there are several UDP connections ongoing between two hosts (or two NATs)
> an error on Source/Dest Port can make the packet be delivered to the wrong connection.
> This issue is very limited since LITE+FRAG carries an identifier.
> 
> Raffaele Zullo