Subject: [rohc] ESP NULL again (padding)

From: Kristofer Sandlund <[email-masked]>
Date: Tue, 16 Sep 2003 13:43:38 +0200

Hi all,

I have one more issue with the ESP NULL header in the list compression that I 
feel is something that could be added in the upcoming implementer's guide update.

 From 5.8.4.3 (ESP NULL header):

 >      Padding: The padding octets are assumed to be as defined in [ESP],
 >          i.e., to take the values 1, 2, ..., k, where k = Pad Length.
 >          If the padding in the static context has this pattern, padding
 >          in compressed headers is assumed to have this pattern as well
 >          and is removed.  If padding in the static context does not
 >          have this pattern, the padding is not removed.

Which seems to suggest that the ESP padding should be added to the static 
context, and this is the only part of the spec that seems to mention this.
It seems a bit strange, since an ESP NULL list item is sent as part of the 
dynamic chain (do I update the static part of the context because of a change in 
a header that is sent as part of the dynamic chain?)... Does this suggest that 
the 'ESP NULL trailer' should be considered part of the static chain? This seems 
really wierd to me.
Also, why do I need to store the padding itself in the context? It should be 
sufficient with a flag saying if the padding is compressible or not, since the 
pad length is always sent and can take on different values for each packet.

Shouldn't it be expressed more like:

"When an uncompressed ESP NULL item is received, a flag MUST be set in the 
dynamic context of the decompressor indicating whether the ESP padding will be 
removed or not in compressed ESP NULL items. If this flag is set, padding is 
assumed to take the values 1, 2, ..., k, where k = Pad Length. When the flag is 
not set, the padding is always sent as-is."

Does this make more sense?

Rgds,
     Kristofer Sandlund, Effnet AB

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From: "Lars-Erik Jonsson (LU/EAB)" <[email-masked]>
Date: Thu, 18 Sep 2003 11:21:19 +0200

> My concern was mostly the fact that the text implicitly tells 
> me to update the 
> "static context" with something from the dynamic chain. 
> That's a bit ugly, but I 
> guess that does not affect interoperability, so you probably 
> don't need to 
> update the impl.guide for that.

I agree with you that the text is weird, but this is not the only
place where 3095 just states "too much" of what is really about
implementation. Therefore, I also agree with you to not address
it in the impl. guide.

/L-E