RE: [rohc] FW: ESP and header compression (ROHC)

["Lars-Erik Jonsson (EAB)" <Lars-Erik.Jonsson@epl.ericsson.se>]

Hi all,

A few notes:
1) ROHC is applied on a per-link basis, e.g. over PPP
2) Based on the above point, there are ROHC compression profiles
   defined for IP/UDP, IP/UDP/RTP, IP/ESP (and soon IP-only)

Compressing the network layer header is most important to gain
anything, but that can only be done on a per-link basis. Header
compression is thus applied per-link to compress network and
transport layer headers (and by heuristics also the application
layer RTP header). It is also simpler to do compression per-link, 
as one can optimize for certain assumed characteristics (such as
in-order delivery). Further, it makes most sense as header
compression is an optimization for "narrow links".

Therefore, I can not see why/how one could do "ROHC over ESP".

BR
/Lars-Erik




> -----Original Message-----
> From: Yaron Sheffer [mailto:yaronf@gmx.net]
> Sent: den 13 april 2003 21:26
> To: IPSec List; rohc@ietf.org
> Subject: [rohc] FW: ESP and header compression (ROHC)
> 
> 
> (please reply to both lists)
> 
> Below is my question to Steve Kent (author of the new rev of ESP, and
> co-author of the original RFC) and his reply. I understand 
> that IPCOMP is
> inferior to ROHC for RTP streams, and I'd like to hear other opinions
> regarding the usefulness of an "ROHC" indicator in ESP.
> 
> This might certaily add complexity to IPSec, but if you make it
> non-negotiable and non-mandatory, it cannot be too terrible.
> 
> Thanks,
> 	Yaron
> 
> -----Original Message-----
> From: Stephen Kent [mailto:kent@bbn.com]
> Sent: Thursday, April 10, 2003 12:06 AM
> To: Yaron Sheffer
> Cc: Sara Bitan; kent@bbn.com
> Subject: Re: ESP and header compression (ROHC)
> 
> 
> At 11:25 PM +0200 4/9/03, Yaron Sheffer wrote:
> >Hi Steve,
> >
> >I have lately looked at issues with IPSec encryption of RTP 
> streams (I am
> >aware of SRTP but I think we will want RTP over IPSec for 
> some time to
> >come). A major issue is packet overhead. You can use Robust Header
> >Compression (ROHC) on the external IP+ESP headers - this is 
> defined by the
> >ROHC RFC. But if you want to header-compress the RTP packets 
> before it is
> >tunneled in ESP (IP+UDP+RTP headers), you cannot do it 
> because there's no
> >way to detect ROHC packets in the ESP header. I'd expect ESP 
> to contain a
> >marker for ROHC packets, similarly to PPP. Has this option 
> been considered
> >for the new version of ESP?
> >
> >Thanks,
> >	Yaron
> 
> No, the WG has not considered that option. The WG has been striving
> to make IPsec simpler and thus adding support for ROHC is contrary to
> that theme.  For example, ROHC would have be be implemented within
> IPsec, after the SA lookup was performed, and ROHC decompression
> would have to be implemented in IPsec at the receiver, since the
> receiver has to check the headers against the SAD. IPsec already
> supports IPCOMP as a compression method for whole packets, not just
> headers, and thus it might be hard to persuade the WG to add ROHC
> support too.
> 
> But, that's just my impression. You can always raise the 
> question on the
> list.
> 
> Steve
> 
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