Re: Flow label versus Extension header

[Erik Nordmark <erik.nordmark@sun.com>]

marcelo bagnulo braun wrote:

> Any additional considerations?

I think there are some considerations related to when and by whom the 
value is allocated.
The flow label value would be allocated by the sender before sending the 
first packet to the peer.

A context tag carried separately doesn't need to be allocated until the 
context is established. This allows the receiver of the packet (carrying 
the tag) to do the allocation (and pass it to the peer as part of the 
context establishment).
The receiver-based allocation has the benefit that the receiver can make 
sure that the tag (either by itself or in combination with the 
destination locator) will uniquely identify the context even if the 
receivers set of locators change in the future.

One additional thing we haven't discussed much yet is that there can be 
a different flow label or context tag for each direction, and that each 
end needs to be able to uniquely find the shim state based on both the 
flow label/tag and locators the *peer sent* (for normal reception), and 
the flow label and locators *it sent* itself (to be able to demux an 
ICMP error to the right context).
I haven't figured out how this can be handled, nor whether the flow 
label (and sender-based allocation) or a context tag (with potential for 
receiver or sender-based allocation) makes any difference.

   Erik