Flow label versus Extension header

[marcelo bagnulo braun <marcelo@it.uc3m.es>]

Hi,

continuing with the topic initiated by Pekka, i guess that the next 
item to discuss is whether is better to carry the context tag in the 
flow id or in a new Extension header.

As has been pointed out in the previous thread, the difficulties with 
flow id are:
- There is the possibility of running out of flow ids values and that 
at some point, it is not possible to accept a locator being added to an 
established context because it would turn impossible to demultiplex. 
the probability of this happening is quite low though, imho
- The locator set must be known before hand, since the information used 
for demux is src, dst and flow id in the general case. Probably, in 
most cases using only the flow id is good enough if we follow the rules 
of assigning the flow id suggested by Pekka, but this cannot be assured 
in the general case. This may be problem in the case that new locators 
are available and that all the previous locators are no longer 
available (The situation where this could happen is when mobility comes 
into play) ( An additional question here is whether we need to assign a 
flow  id value per direction in the communication, or in other words, 
if there is a single flow id for both directions of the communication, 
then who picks up the flow id? what if the flow id is already being 
used by the other host in another context?)
- We are overloading a field that is supposed to be used for something 
else, i guess, so perhaps we may end up with incompatible usages??

The dificulties with the Extension header is clear, i guess:
- overhead. However, it must be noted that this overhead only occurs 
when there have been a rehoming event, i.e. when no rehoming has 
occurred, no need for the extension header in data packets, since there 
is no need to demultiplex.
the benefit of this approach is that is cleaner and that is probably 
simpler (since we can use a 128 bit context tag, making sure that is 
unique)

Any additional considerations?

Regards, marcelo