Re: [mpls] [PWE3] ELI as a reserved label

[Shane Amante <[e-mail]>]

Hi Curtis,

I had some time to think about this some more on the plane ride home.  See below.

On Jul 30, 2010, at 14:02 GMT+02:00, Curtis Villamizar wrote:
> In message <[e-mail]>
> Shane Amante writes:
>> 
>> Curtis,
>> 
>> On Jul 30, 2010, at 11:06 GMT+02:00, Curtis Villamizar wrote:
>>> The Entropy Label Indicator (ELI) is called for in
>>> draft-kompella-mpls-entropy-label-01.txt
>>> 
>>> After brief discussion with Stewart and with Kireeti and Lucy Yong,
>>> there were three uses of ELI that would benefit from making the ELI a
>>> reserved label.  It may be premature to make any such request and the
>>> remaining reserved label number space is sparse, so this will be
>>> floated as a separate draft, and put on hold awaiting advancement of
>>> the drafts that depend on it.
>>> 
>>> The three benefits of making ELI a reserved label are in:
>>> 
>>> 1.  Extending any use of entropy-label to P2MP (where there are
>>>     multiple egress and the possibility of each egress requiring a
>>>     separate entropy label value is problematic for the ingress).
>>> 
>>> 2.  Extending any use of entropy-label to any arbitrary position in
>>>     the label stack such that in an aggregation LSP with aggregates
>>>     both MPLS-TP and MPLS LSP, load split on the MPLS LSPs is
>>>     simplified and not impacted by limiting label stack depth for
>>>     the MPLS-TP LSPs.  This is when label stack hash is limited to
>>>     support MPLS-TP as proposed in
>>>     draft-villamizar-mpls-tp-multipath-00.txt .
>> 
>> I agree there are benefits to a reserved label for the above two
>> applications.  Most importantly, a receiving node would have pretty
>> clearly semantics that what [immediately] follows a reserved ELI is
>> the entropy-label, regardless of where the ELI is in the MPLS label
>> stack and without needing to be in the signaling path to know what is
>> the ELI value.
> 
> "Now don't be sad ... 'cause two out of three aint bad".  [meatloaf]
> 
> Its good to hear that you consider these to be useful.
> 
> Note that an ingress LSR still needs to determine that egress LSR
> understands ELI to do anything.  To support MPLS-TP in MPLS the
> ingress needs to veryify that all LSR on the path support MPLS in a
> way that provides a compliant path for a contained TP LSP, but that is
> out of scope of this discussion.

I need to revise my statements wrt entropy-labels and P2MP LSP's.  After thinking about this some more, I no longer believe it makes sense to use a reserved ELI value for P2MP LSP's.  More specifically, for a given P2MP LSP, all receivers /MUST/ agree to receive entropy labels, because it would be silly to (for example) expect core LSR's to strip-off the ELI and an entropy-label mid-flight toward receivers that don't understand (or, can't receive) entropy-labels.  The ramifications of this are that if a head-end detects any receivers of a P2MP LSP that can't receive entropy-labels, then the head-end LER/LSR must either: a) re-signal the whole P2MP LSP toward all receivers so that the head-end will not send any entropy-labels at all on the P2MP LSP; or, b) signal 2 separate P2MP LSP's: one for entropy-label capable receivers and a second for non-entropy-label capable receivers.  I don't know that we want to mandate one of those two behaviors over the other, because SP's may make separate choices depending on whether they want to avoid extra replication in the network at the expense of not being able to do flow-based load-balancing, i.e.: Option (a), or don't mind the extra replication of (potentially) 2 parallel P2MP trees on the same links in order to attain some amount of flow-based load-balancing of the P2MP traffic.

So, in summary, I no longer see that draft-kompella-mpls-entropy-label has a requirement for reserved labels for either P2P or P2MP LSP's (scenario 1 in your original e-mail).  Furthermore, I still maintain that discussion of LFC and use of the MPLS TC field for indication of large vs. small flows is orthogonal to draft-kompella-mpls-entropy-label (scenario 3 in your original e-mail).  I haven't read draft-villamizar-mpls-tp-multipath-00 (scenario 2 in your original e-mail) so I can't speak to whether or not there is a requirement for a reserved label there.

Anyway, perhaps this will save you some time in that you don't need to write a whole new draft as you originally proposed and, instead, may only need to look at modifying your original draft.  :-)

On a separate note, we intend to update draft-kompella-mpls-entropy-label shortly to cover P2MP LSP's (RSVP & mLDP), plus add details surrounding labelled BGP and RSVP P2P LSP's so that we have a more thorough treatment of all use cases to hopefully make all of this more clear.  (In fact, I got a good start on this text on this on the plane ride home :-).

-shane