Subject: [rohc] Regd STATIC NACK feeback logic and ecn_used control field (ROHC-TCP)

From: Karthik Balaguru <[email-masked]>
Date: Mon, 22 Feb 2010 14:10:33 +0530

Hi,

I have couple of queries regarding ROHC-TCP profile (RFC 4996).

1. In the case of ROHC TCP profile, the section 5.3.2 of
RFC 4996 seems to convey the below information for feedback logic -
"The decompressor MAY send positive feedback(ACKs) to
initially establish the feedback channel for a particular flow.
Either positive feedback(ACKs) or negative feedback(NACKs)
establishes this channel".

Does the term "negative feedback(NACKs)" in the above lines
highlighted from section 5.3.2 of RFC 4996 include "STATIC NACK" too ?

This query is due to scenarios that might send STATIC-NACK.
Consider a scenario of decompressor being in U Mode and NC State.
If there is a CRC failure or an invalid packet being received at the
decompressor, then we might need to send STATIC NACK to
establish the feedback channel assuming that the decompressor
has no valid context. But this seems to contradict with the
feedback channel establishment logic mentioned in section 5.3.2
of RFC 4996 as that section conveys that the feedback channel gets
established only during either positive feedback(ACKs) or negative
feedback(NACKs). So, kindly clarify whether the term "negative
feedback(NACKs)" in the lines from  section 5.3.2 of RFC 4996
include "STATIC NACK" too ?

2.  ECN is related with congestion notification. In RFC 4996(page 71),
 the Formal Notation for TCP header seems to convey that in the
encoding method for TCP (tcp_irreg_ip_ecn), the control field
'ecn_used' is evaluated for either 0 or 1.  But, the RFC 4996 does not
seem to convey when  & how this 'ecn_used'  control field would be
set as either 0 or 1 so that it can be used while TCP header encoding.
Kindly clarify, when & how the 'ecn_used' control field gets configured ?

Thx in advans,
Karthik Balaguru

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