Re: [Idr] WG Adoption - draft-li-idr-sr-policy-path-mtu-03.txt - 2 Week WG adoption call (3/30 - 4/13)

Aijun Wang <> Wed, 08 April 2020 16:10 UTC

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Date: Thu, 09 Apr 2020 00:07:56 +0800
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Cc: "Ketan Talaulikar (ketant)" <>, Susan Hares <>, IDR List <>, SPRING WG <>
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To: "Chengli (Cheng Li)" <>
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Subject: Re: [Idr] WG Adoption - draft-li-idr-sr-policy-path-mtu-03.txt - 2 Week WG adoption call (3/30 - 4/13)
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Hi, Cheng:
I am considering the following scenarios:
When the size of some packets of one flow are small and do not exceed the PMTU value, they are encapsulated upon the corresponding SR policy and will be transferred according to the appointed path as expected. 
But how about other larger packets within the same flow? Will they be dropped or transferred via other non-SR path? If so, will the SR influence the flow’s jitter?

PMTU for the path is one useful information but I think current draft has not elaborate how to use it in different situations. 

Without SR, we can tell the customer the MTU of the network. But with SR, the network MTU will be changed based on the path length. Considering this, maybe the PMTU that detected by the host will be more convincible?

Aijun Wang
China Telecom

> On Apr 8, 2020, at 17:07, Chengli (Cheng Li) <> wrote:
> Hi Ketan,
> Many thanks for your comments, and sorry for my delay, please see my reply inline.
> Thanks,
> Cheng
> From: Idr [] On Behalf Of Ketan Talaulikar (ketant)
> Sent: Friday, April 3, 2020 4:18 PM
> To: Susan Hares <>; 'IDR List' <>
> Cc: SPRING WG <>
> Subject: Re: [Idr] WG Adoption - draft-li-idr-sr-policy-path-mtu-03.txt - 2 Week WG adoption call (3/30 - 4/13)
> Hello,
> I have a few questions for the authors of this draft and some discussion points for the WG.
> 1)      What is precisely the definition of this “path MTU” for an SR Policy? I am guessing that it includes all the labels/SIDs that are used for the SR path?
> [Cheng] Yes, The Path MTU describes the largest size of the packet, including the overhead of Labels/SIDs/IPv6 header/SRH and others. When encapsulating a packet, the length of the payload(inner IP packet or something else) should be less than the PMTU minus the overhead of SID List/SRH/ FRR overhead and Binding SID overhead, but it is a implementation choice.
> 2)      While defines “path MTU” for RSVP-TE LSPs, it does describe the procedures for the same for handling IP packets/payloads on the headend. It does not cover the scenarios where the incoming packets may be themselves labelled.
> [Cheng]Well, I may misunderstand the text in, but I think it covers the scenarios where the incoming packets may be themselves labelled. I may be wrong.
>    “
>    The following algorithm applies to all unlabeled IP datagrams and to
>    any labeled packets which the node knows to be IP datagrams, to which
>    labels need to be added before forwarding.  For labeled packets the
>    bottom of stack is found, the IP header examined.
>    Using the terminology defined in [5], an LSR MUST execute the
>    following algorithm:
>    1. Let N be the number of bytes in the label stack (i.e, 4 times the
>       number of label stack entries) including labels to be added by
>       this node.
>        “
> 3)      Shouldn’t the concept of “path MTU” for SR Policies and its’ applicability and operations be first defined in a (Spring WG?) document before we introduce its signalling aspects in protocols like BGP? Note that such a document would bring in requirements and guidelines for how the value is going to be computed and it’s usage for different steering mechanisms over SR Policies.
> [Cheng] This is a really small but useful and straight forward extensions, it might not need to write a draft to describe the requirement instead of adding text in the current SR policy architecture draft or it current document.  
> 4)      Finally, specific to the proposed encoding here, would this “path MTU” not be more suitable on the CP level since each SL may have different size label stack and different paths and one does not know which SL would be picked for a particular flow? So may be the lowest value computed for all SLs is what gets applied to the packets at the CP (i.e. SR Policy) level?
> [Cheng] You are correct. The PMTU is defined for SID List. When we talk about Path MTU for SR policy, it CAN be the lowest value of the PMTU of SID lists. But do we need this value? If we need, then the node can compute it based on PMTUs.
> Thanks,
> Ketan
> From: Idr <> On Behalf Of Susan Hares
> Sent: 30 March 2020 18:06
> To: 'IDR List' <>
> Subject: [Idr] WG Adoption - draft-li-idr-sr-policy-path-mtu-03.txt - 2 Week WG adoption call (3/30 - 4/13)
> This begins a 2 week WG adoption call for draft-li-idr-sr-policy-path-mtu-03.txt
> You can view this draft at:
> This draft distributes path maximum transmission unit for the
> SR policy via BGP. 
> Any discussion regarding on whether one desires
> SR Policy should be clearly distinguished from the
> Technical discussions on the mechanisms to pass SR policy MTU.
> The questions for the people to discuss on this draft are:
> 1) Is there a need for this mechanism in networks using
>         MPLS-SR or SR-V6 and SR policy?
> 2) Are there any error handling issues besides what is being
>      Taken care of in RFC7752bis-03.txt
> 3) Do you think this draft is ready to be adopted?
>      In this category, please list any concerns you have
>      regarding adoption.  This category can include
>      general concerns about BGP-LS, MPLS-SR,
>     SR-V6, and SR-Policy.   
> Cheers, Sue Hares  
> _______________________________________________
> Idr mailing list