Subject: [rohc] ROHC over PPP question

From: "Sarkar, Rajib" <[email-masked]>
Date: Mon, 18 Nov 2002 17:21:24 -0800

Hi,
    I have a question about ROHC over PPP profiles. There are two profiles
used 0x0003 and 0x0005, why do we need to use two profiles? The reaosn given
in the RFC is
To keep the PPP packets self-describing, in this
   document two new types for the PPP Data Link Layer Protocol Field are
   defined, one for small-CID ROHC packets and one for large-CID ROHC
   packets.  (This also avoids a problem that would occur if PPP were to
   negotiate which of the formats to use in each of IPCP and IPV6CP and
   the two negotiation processes were to arrive at different results.)
1. But the question is do we need to keep the packets self describing? we
include the CID information in the ROHC header, so we can always verify that
through ROHC header.
2. How it avoids the problem of two different negotiation process? Once we
negotiate should not we use only one CID space (either small or large)
Thanks
Rajib

---

From: "Lars-Erik Jonsson (EAB)" <[email-masked]>
Date: Tue, 19 Nov 2002 16:16:02 +0100

Rajib,
 
There seems to be some confusion about ROHC profiles and ROHC-over-PPP. ROHC-over-PPP defines a way to carry any ROHC packets over PPP, i.e. it is totally ROHC-PROFILE independent. As you know, the ROHC framework defines two kind of CID's large and small, and assumes that only one size will be used for ROHC over a specific channel. However, for ROHC-over-PPP, both sizes can be used simultaneously as the CID size is given by the PPP-Data-Link-Protocol-Field, i.e. two types have been reserved for ROHC (0x0003 and 0x005). You seem to be mixing these up with ROHC profile identifiers, but those are totally unrelated (they just have the same format, 16-bits). Note that only one CID space is used, i.e. a small-CID packet for CID 4 belongs to the same CID as a large-CID packet for CID 4.
 
This is what ROHC-over-PPP says about large and small CID packets:

-------------
   The ROHC specification [RFC3095] defines a single header format for
   all different types of compressed headers, with a variant for small
   CIDs and a variant for large CIDs.  Two PPP Data Link Layer Protocol
   Field values are specified below.
 
   ROHC small-CIDs
 
      The frame contains a ROHC packet with small CIDs as defined in
      [RFC3095].
 
      Value: 0003 (hex)
 
   ROHC large-CIDs
 
      The frame contains a ROHC packet with large CIDs as defined in
      [RFC3095].
 
      Value: 0005 (hex)
 
   Note that this implies that all CIDs within one ROHC packet MUST be
   of the same size as indicated by the Data Link Layer Protocol field,
   either small or large.  
-------------
 
Hope this helped!
 
BR
/L-E

-----Original Message-----
From: Sarkar, Rajib [mailto:[email-masked]]
Sent: den 19 november 2002 02:21
To: [email-masked]'
Subject: [rohc] ROHC over PPP question

Hi,
    I have a question about ROHC over PPP profiles. There are two profiles used 0x0003 and 0x0005, why do we need to use two profiles? The reaosn given in the RFC is
To keep the PPP packets self-describing, in this
   document two new types for the PPP Data Link Layer Protocol Field are
   defined, one for small-CID ROHC packets and one for large-CID ROHC
   packets.  (This also avoids a problem that would occur if PPP were to
   negotiate which of the formats to use in each of IPCP and IPV6CP and
   the two negotiation processes were to arrive at different results.)
1. But the question is do we need to keep the packets self describing? we include the CID information in the ROHC header, so we can always verify that through ROHC header.
2. How it avoids the problem of two different negotiation process? Once we negotiate should not we use only one CID space (either small or large)
Thanks
Rajib