On 2010-02-22 16:15, Sheng Jiang wrote:
>>> This may seem a bit unexpected, but after working on
>>> draft-carpenter-flow-ecmp (just updated) and working with
>> my student
>>> Qinwen Hu on some aspects of the flow label, it seemed like
>> time for
>>> another look at the flow label standard, and Sheng Jiang
>> was having similar thoughts.
>>> We'd like to discuss this in Anaheim if possible.
>>>
>> I've had a read through this draft and support what it is proposing.
>>
>> In regarding the following processing rules:
>>
>>
>> o Considering packets outbound from the Flow Label
>> Domain, if MSB =
>> 0, a boundary router MUST NOT change the flow label.
>> If MSB = 1,
>> it MUST set all 20 bits of the flow label to zero, so that the
>> locally defined behaviour is not exported from the domain.
>> o Considering packets inbound to the Flow Label Domain,
>> if MSB = 0,
>> a boundary router MUST NOT change the flow label. If an inbound
>> packet has MSB = 1, it has originated from a source not
>> following
>> the current specification. This is considered to be an
>> extremely
>> unlikely case, and the boundary router MUST set all 20
>> bits of the
>> flow label to zero, as the choice least likely to cause unwanted
>> behaviour. (Note that this means the rules for inbound and
>> outbound packets at the boundary router are identical.)
>>
>> one thought I had would be that there could be a use case where e.g.
>> when a packet leaves a flow domain and has it's MSB=1, and
>> enters another flow domain e.g. crossing the boundary between
>> enterprise network and an ISP, the flow label could be
>> changed to a MSB=1 value of local significance to the second
>> flow domain. This would be somewhat similar to how ISPs can
>> provide specific BGP community values for it's customers to
>> set to influence routing within the ISP's AS. If there is no
>> specified MSB=1 value for the second flow domain, then the
>> flow label must be set to all zeros.
>
> Yes, the second flow domain should be also able to use the local defined
> behaviors. In my opinion, when the packet leaves a flow domain, the egress
> router does not sure whether there will be another flow domain or not (the
> second flow domain may be appear to be several hops away). So, it should
> leave the MSB=1 to keep the possibility. The rest 19 bits may be reset to
> all 0 depends or untouched depending on whether the egress router wants to
> prevent the exporting of local defined behaviours. Actually, I don't think
> it is necessary to do so. The leaking of locally defined behaviours is not
> harmful.
>
> Cheers,
>
> Sheng
After some thought, I believe that is correct. (I don't know if we'll have
time to update the draft before the cutoff, since I have some travel coming
up.)
Brian
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