Re: [Detnet] Magnus Westerlund's Discuss on draft-ietf-detnet-mpls-11: (with DISCUSS)

[Magnus Westerlund <[e-mail]>]

Hi,

Have thought about the discuss a bit more and think I have come to kernel of the
issue related to bounded latency in this context.

So the POF must be able to buffer for a certain period to be able to reorder.
The Detnet very prominently says it can provide bounded latency and I think the
only shortcoming is the consideration of the POF and how take that into account.

So https://datatracker.ietf.org/doc/draft-ietf-detnet-data-plane-framework/:

3.5.2.2.  Path Differential Delay

   In the preceding example, proper working of duplicate elimination and
   reordering of packets are dependent on the number of out-of-order
   packets that can be buffered and the delay difference of arriving
   packets.  DetNet uses flow-specific requirements (e.g., maximum
   number of out-of-order packets, maximum latency of the flow) for
   configuration of POF-related buffers.  If the differential delay
   between paths is excessively large or there is excessive mis-ordering
   of the packets, then packets may be dropped instead of being
   reordered.  Likewise, PEF uses the sequence number to identify
   duplicate packets, and large differential delays combined with high
   numbers of packets may exceed the ability of the PEF to work
   properly.

I think this makes it clear that there are a requirement on the POF also for
MPLS to handle these aspects. So the main question from my perspective for the
WG to consider I think is if there is a solution where the POF can be configured
with a maximum latency incurred by the POF or if there is need for additional
meta data (timestamp) in the MPLS encapsulation to be able to provide this
function. 

Assuming that links and forwarding and other packet buffers can be bounded then
the reordering should be possible to be simply configured with a maximum latency
in reordering so that packet that arrives are released within that time frame to
achieve bound latency. However, I am not certain that I have considered all
aspects of inter packet arrival and its impact also on the actual reordering
window. 

Thus, resolving this aspect of the discuss may be as simple as mandating a
configuration of the maximum latency incurred by the POF so that the latency of
the MPLS network in dealing with a particular S-Label can be bounded. 

Cheers

Magnus Westerlund 


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