Re: [Tsv-art] Tsvart early review of draft-ietf-trill-over-ip-10 - ECN & DSCP considerations

[Joe Touch <[e-mail]>]

Hi, Wes (et al.),


On 7/12/2017 12:31 PM, Wesley Eddy wrote:
> On 7/1/2017 2:03 PM, Donald Eastlake wrote:
>> On Sat, Jul 1, 2017 at 10:53 AM, Black, David <[e-mail]>
>> wrote:
>>> Also, as noted earlier in this discussion, RFC 7657 explicitly
>>> discourages use of multiple DSCPs in a single TCP connection.  That
>>> needs to be reflected in the TCP encapsulation text in the
>>> trill-over-ip draft - in particular, the current text in Section 4.3
>>> on mapping to DSCPs from TRILL priority and DEI does not appear to
>>> be consistent with RFC 7657 for TCP-based encapsulation.
>> I'm surprised it only "discourages" rather than "prohibits"...
>>
>>
>
> RFC 1122 section 4.2.4.2 might be of interest.
>
> It says that TOS "SHOULD" be able to change during TCP connection
> lifetime.  The example given there is an SMTP connection whose nature
> might change during its lifetime.
>
> I'm not saying whether or not this is really still a sensible concern,
> just pointing it out.
>
> The change from TOS to DSCP terminology and semantics didn't make any
> change here as far as I can tell.

Agreed, but only on that point.

My interpretation of the relevant RFC1122 text is that it's OK for a TCP
session to "shift" from one TOS (or DSCP) to another when the nature of
its transfer changes. I would expect those changes to be infrequent and
atypical.

I would not expect a single TCP connection to issue individual segments
with TOS (or DSCP) values that change nearly every segment, which would
be the consequence of copying those values from the payload of
encapsulated traffic. Furthermore, it would be impossible to determine
the appropriate DSCP values when a segment contains all or part of more
than one encapsulated packet with different values.

Joe