Re: [mpls] Requesting WG LC for draft-ietf-mpls-rsvp-shared-labels

[Alexander Okonnikov <[e-mail]>]

Hi Pavan,

I'm sorry for delay with answer.

If ingress uses regular Path MTU discovery mechanism, it could produce value of MTU lower than actual one. This is because ingress doesn't know MTU per each hop. Let's consider case with four routers: R1 - R2 - R3 - R4. MTU for R1-R2 link is 2000, MTU for R2-R3 is 1600 and MTU for R3-R4 is 2000. By virtue of regular Path MTU discovery mechanism R1 will derive from FLOWSPEC that path MTU is 1600. As soon as R1 doesn't know how many labels in the stack will be on the lowest MTU hop, it can only set LSP MTU to most conservative value (1600 - 4 - 4 = 1592, provided that R4 has advertised implicit null label). In fact actual path MTU is 1596 (1600 - 4 on R2-R3 hop). Of course, it could be acceptable, but calculated LSP MTU as more lower than actual as longer LSP path. For correct path MTU discovery ingress needs to know MTU per each hop.

Thank you.

> 6 сент. 2018 г., в 18:27, Vishnu Pavan Beeram <[e-mail]> написал(а):
> 
> Alexander, Hi!
>  
> I apologize for the delayed response.
>  
> This draft does not propose any changes to the standard RSVP MTU signaling procedures (Int Serv object specific signaling procedures). After the initial signaling sequence is complete, an ingress implementation (RFC3209) would typically take the path MTU learnt via signaling, run it through some local logic and then arrive at an MTU value that can be assigned to the LSP. This local logic typically involves deducting the number of bytes in the label stack used for the LSP from the path MTU learnt via signaling. The ingress implementation supporting this draft will rely on the Resv RRO to accurately determine the max-number of labels pushed along the path of the LSP (note that with delegation, downstream hops can impose label stacks) and account for it in the local logic used to arrive at the MTU value assigned to the LSP.
>  
> I hope this addresses your question.
>  
> Regards,
> -Pavan
> 
> On Tue, Aug 7, 2018 at 12:22 PM Alexander Okonnikov <[e-mail] <mailto:[e-mail]>> wrote:
> Hi Pavan,
> 
> 
> Regular RSVP-TE LSPs use standard RSVP path MTU discovery mechanism. That one cannot be used "as is" for approach described in the draft, and the draft doesn't address path MTU identification. Is it to be considered?
> 
> Thank you.
> 
> 26.07.2018 06:07, Vishnu Pavan Beeram пишет:
>> Chairs, Hi!
>> 
>> As mentioned (in our presentation) in last week's WG session, we believe that the draft is sufficiently baked and ready to progress to the next stage. We would like to formally request this draft to be considered for WG LC.
>> 
>> Regards,
>> - Pavan (on behalf of the authors)
>> 
>> 
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