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Re: Flow label - the problem




El 21/04/2005, a las 22:03, Iljitsch van Beijnum escribió:

On 21-apr-2005, at 20:53, marcelo bagnulo braun wrote:

In BGP and other routing protocols systems usually have a router id so another system can detect that it's talking to the same remote system over different links. We can do the same thing here. This value doesn't have to have any meaning to the remote system, the only requirement is that it's unique. A good choice for this would be one of the IP addresses of a host or maybe the interface identifier if that contains enough uniqueness.

but in this case, the real context tag used for demux would be the id, not the flow id value. so you would have to include it in every packet, back to where we start, i am afraid.. or i am missing something?

No, this id would be looked at when selecting the flow label for a new session. I.e.: when selecting a new flow label, the host would see if the flow label overlaps with one currently used, and then:


- overlaps with known non-shim session: allow if dest addr is different
- overlaps with known shim session: allow if shim id is different
- overlaps with session of unknown type: don't allow

There are probably other places wher ethe shim id would be useful too.


ok, i agree this could work AFAICT.

From the practical perspective, i guess that using the flow id may be a bit more trickier than an extension header, but it also has a clear practical benefit w.r.t. overhead.

My main doubt about using the flow id is w.r.t. alternatives usages of the flow id that may end up clashing with this shim usage. I mean, the flow id is located in the main IPv6 header, which makes it useful as a field to be used in a hop per hop basis, i.e. routers can used for forwarding and so on. OTOH, the shim protocol, and in particular the context tag used for demux, is clearly an end to end information i.e. it won't be used by routers, and there is no need to place it in the main IPv6 header.

So, my concern is that bits in the main IPv6 header are a precious scarce resource, and using them when it is not compelling (i.e. hop by hop processing of the field) seems to me a waste. Perhaps we should leave those for who really need them and don't have the choice of using an extension header.

Regards, marcelo

Iljitsch