Subject: [rohc] Replication packet type

From: "Lars-Erik Jonsson (EAB)" <[email-masked]>
Date: Wed, 14 Aug 2002 15:34:03 +0200

Hi all,

When doing the TCP profile, we have agreed to develop a context
replication mechanism to significantly reduce overhead for new
context establishments. This will require a new packet type and
it would be useful if that packet type was common for all profiles,
thus defined at the framework level, allowing replication of 
contexts running different profiles. 

In RFC 3095, we used almost all packet types, of which some will
have to be profile common and other profile specific. We left a
few types that could be considered as free to use by future
profiles, but we did not reserve any type(s) for future new
framework mechanisms. However, only the LLA profile has so far
made use of some "unused" types, and if we avoid collision with
those, we can define a new "Replication type" at the framework
level.

The packet type to use for this would then, according to my
investigations, be [11111001]. Are there any comments on using
that packet type for this purpose, and considering it a profile
common (framework) packet type?

Rgds,
/L-E

--------------------------------------
Lars-Erik Jonsson
IETF ROHC WG Chair
E-mail: [email-masked]
Phone: +46 920 20 21 07
Home: +46 920 999 57

My opinions are my personal opinions and should not be considered
as the opinions of my employer, if not explicitly stated. 

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---

From: "Price, Richard" <[email-masked]>
Date: Wed, 14 Aug 2002 15:34:35 +0100

Hi,

As far as I'm aware the bit pattern [11111001] is the last one not
currently used by the ROHC framework or by any of the existing
profiles. So to allow ourselves the opportunity to extend the ROHC
framework in the future, it would be prudent not to use the whole
bit pattern for a single packet type.

How about extending the bit pattern into the next octet, e.g:

11111001 0000XXXX   =   Context replication mechanism

11111001 0001
    :      :        =   Reserved for future framework packet types
11111001 1111

The exact number of reserved bit patterns can be adjusted depending
on how many are needed by the context replication mechanism. However
we should make sure to reserve at least one for future use!

Regards,

Richard

> -----Original Message-----
> From: Lars-Erik Jonsson (EAB) 
> [mailto:[email-masked]]
> Sent: Wednesday, August 14, 2002 2:34 PM
> To: [email-masked]'
> Subject: [rohc] Replication packet type
> 
> 
> Hi all,
>

---

From: "Lars-Erik Jonsson (EAB)" <[email-masked]>
Date: Wed, 14 Aug 2002 16:47:01 +0200

Yes, this is the only bit pattern left. However, I would not say we
really need to leave any bit patterns for future framework extensions.
I do not have any strong opinions about this, but we would get a smaller 
identifier if we just go for 11111001, and then we would basically
close the framework, which could even be seen as a good thing...=)

What do others think??

/L-E

---

From: "Price, Richard" <[email-masked]>
Date: Wed, 14 Aug 2002 16:59:26 +0100

> identifier if we just go for 11111001

Reserving some bit patterns for future use doesn't have to increase the
overhead of the context replication mechanism. The context replication
header will need to include a field that specifies the "source context"
(i.e. the context that's being replicated). If we specify this as a delta
offset from the "destination context" (i.e. the context that we're
currently updating) then the field can't take the value zero since we
can't replicate a context from itself. The "zero" bit pattern can then be
reserved for future framework packets at no extra cost :-)

> and then we would basically close the framework, which could even be
> seen as a good thing...=)

I think it's useful to keep at least one bit pattern in reserve, just
in case the ROHC framework needs to be upgraded in the future. It's
certainly more convenient than defining a new framework with a new PPP
protocol field value, particularly if we make sure only to upgrade the
framework in a backwards-compatible manner.

Regards,

Richard

---

From: "Lars-Erik Jonsson (EAB)" <[email-masked]>
Date: Thu, 15 Aug 2002 17:18:09 +0200

Carsten and others,

As previously concluded in this thread, let's then use the last
pattern for replication and make sure the replication packet
allows extending the meaning of that type for potential future
use.

/L-E

-----Original Message-----
From: Carsten Bormann
To: Lars-Erik Jonsson (EAB)
Cc: [email-masked]'
Sent: 2002-08-15 14:57
Subject: Re: [rohc] Replication packet type

> I do not think there will ever be a "need" to add things,

Hmm, famous last words.

Having missed out on the opportunity to provide extensibility in
protocol 
designs before, my experience tells me that something new is just around
the 
corner.  Something that we don't think about now, but will be painfully
obvious 
when it has come into view.

Maybe we didn't pay enough attention when we "spent" the bit patterns
available 
for their various uses in 3095.  Using up the last one does worry me.

Gruesse, Carsten