Subject: [6lowpan] HC Contexts - How Many?

From: Jonathan Hui <[email-masked]>
Date: Tue, 23 Sep 2008 09:43:10 -0700

In moving the HC draft from -00 to -01, we added support for multiple  
compression contexts so that more than one prefix (other than link- 
local) can be compressed.

The obvious question now is how many contexts are enough? It is clear  
that two contexts are useful (one for the 6LoWPAN and another for the  
common destination that all nodes may be sending to). But should we  
add more?

Some thoughts:
- Fewer contexts allow the HC encoding to use fewer bits in  
identifying the context in use.
- Specifying support for X contexts requires nodes to allocate enough  
memory to maintain X contexts (ip addresses, timers, etc.).
- Additionally, we cannot simply say that we will change X to some  
larger Y at some future date since the nodes that only support X will  
not be able to support Y > X contexts.
- One argument for supporting more contexts is that it allows network  
renumbering while allowing all nodes to communicate with compressed  
addresses. My thought, however, is that renumbering of 6LoWPAN  
networks should be rare and when they do occur it is okay to incur  
some extra cost and communicate with full addresses during that time.

So my thought is to keep the number of supported contexts small (2).  
What do other people think?

--
Jonathan Hui

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From: "Stephen Dawson-Haggerty" <[email-masked]>
Date: Tue, 23 Sep 2008 11:38:40 -0700

I would definitly be in favor of fewer contexts and agree with your
reasons.  I liked the scheme in HC-00 where you could always use CP or link
local.  That seems to cover the common case very well.

Steve

On Tue, Sep 23, 2008 at 9:43 AM, Jonathan Hui <[email-masked]> wrote:

> compression contexts so that more than one prefix (other than link-local)
> can be compressed.
>
> The obvious question now is how many contexts are enough? It is clear that
> two contexts are useful (one for the 6LoWPAN and another for the common
> destination that all nodes may be sending to). But should we add more?
>
> Some thoughts:
> - Fewer contexts allow the HC encoding to use fewer bits in identifying the
> context in use.
> - Additionally, we cannot simply say that we will change X to some larger Y
> at some future date since the nodes that only support X will not be able to
> support Y > X contexts.
> addresses. My thought, however, is that renumbering of 6LoWPAN networks
> should be rare and when they do occur it is okay to incur some extra cost
> and communicate with full addresses during that time.
>
> So my thought is to keep the number of supported contexts small (2). What
> do other people think?
>
> --
> Jonathan Hui
>
>
>

---

From: Zach Shelby <[email-masked]>
Date: Tue, 23 Sep 2008 23:19:42 +0300

Hi,

For me, 2 contexts is enough, and I agree with the reasoning behind it 
from my experience as well.

- Zach

Jonathan Hui wrote:
> compression contexts so that more than one prefix (other than 
> link-local) can be compressed.
> 
> common destination that all nodes may be sending to). But should we add 
> more?
> 
> Some thoughts:
> - Fewer contexts allow the HC encoding to use fewer bits in identifying 
> the context in use.
> should be rare and when they do occur it is okay to incur some extra 
> cost and communicate with full addresses during that time.
> 

-- 

Zach Shelby
Head of Research
Sensinode Ltd.
Kidekuja 2
88610 Vuokatti
FINLAND

mobile: +358 40 7796297

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---

From: "Samita Chakrabarti" <[email-masked]>
Date: Wed, 24 Sep 2008 15:06:50 -0700

Hi Jonathan,

2 contexts looks good to me as well for simplicity, less memory usage.

Thanks,
-Samita

On Tue, Sep 23, 2008 at 9:43 AM, Jonathan Hui <[email-masked]> wrote:
> - Specifying support for X contexts requires nodes to allocate enough memory
> to maintain X contexts (ip addresses, timers, etc.).
> So my thought is to keep the number of supported contexts small (2). What do
> other people think?
>
> --
> Jonathan Hui
>
>
>

---

From: "Julien Abeille (jabeille)" <[email-masked]>
Date: Thu, 25 Sep 2008 10:33:24 +0200

Hi all,

- I guess using one context only means we plan to use one prefix most of
the time (in Ras...). Is this very limitative? Are there usual scenarios
using several?
- Regarding implementation, contexts will probably come with Ras and be
bound to prefixes. If you store these prefixes anyway in some prefix
list, the context can be bound to the prefix list structures, hence the
memory overhead is just one byte per new context.
- The bit overhead in the HC encoding is 1, which seems acceptable,
there are still 3 reserved bits.

Hence, in my opinion, if using one bit only creates issues, we should
not hesitate to use two.

Cheers,
Julien

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On
Behalf Of Samita Chakrabarti
Sent: jeudi 25 septembre 2008 00:07
To: Jonathan Hui
Cc: 6lowpan
Subject: Re: [6lowpan] HC Contexts - How Many?

Hi Jonathan,

Thanks,
-Samita

add more?