Re: [Idr] WG Adoption for draft-zhu-idr-bgp-ls-path-mtu (11/1/2020 to 11/16/2020)

Susan Hares <> Thu, 12 November 2020 18:50 UTC

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From: Susan Hares <>
To: 'Jeff Tantsura' <>, "'Stephane Litkowski (slitkows)'" <>,, "'Acee Lindem (acee)'" <>
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Subject: Re: [Idr] WG Adoption for draft-zhu-idr-bgp-ls-path-mtu (11/1/2020 to 11/16/2020)
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Jeff and Authors of draft-zhu-idr-bgp-ls-path-mtu: 


I do believe the authors agreed to renaming the draft.   


Does the name:   draft-xxx-idr-bgp-ls-link-mtu work for 

the authors and interested IDR participants. 


As the end of 12 days of the 14 day WG LC, this draft appears 

to have general consensus from the WG as a useful draft. 


I plan to allow 2 more days of comment, but at this point 

it appears the WG wishes to adopt this draft.  


Here’s my understanding of the best way forward: 


If LSR adopts a version of the draft, IDR will allow the 

LSR WG to be the main source as long as cross-working 

review occurs so the BGP-only function can be reviewed. 


Please continue to comment on the draft and 

the planned progression. 


Cheers,  Sue 


From: Jeff Tantsura [] 
Sent: Thursday, November 12, 2020 12:53 PM
To: Susan Hares; Stephane Litkowski (slitkows);; Acee Lindem (acee)
Subject: Re: [Idr] WG Adoption for draft-zhu-idr-bgp-ls-path-mtu (11/1/2020 to 11/16/2020)


+1 to everything Acee said




On Nov 10, 2020, 1:01 PM -0800, Acee Lindem (acee) <>, wrote:

Speaking as an IDR WG member:


The name of the draft is wrong – the extension is for a Link MTU and not a path MTU.


Speaking as LSR Chair:


We could this in LSR as there is currently no MTU advertisement in the LSAs for OSPFv2 and OSPFv3. Implementations already make use of this information as it is used in the OSPF DBD packets and for LSA packing. Of course, we’d require a more accurate draft name and title.





From: Idr <> on behalf of Susan Hares <>
Date: Monday, November 9, 2020 at 4:20 PM
To: "'Stephane Litkowski (slitkows)'" <>, IDR List <>
Subject: Re: [Idr] WG Adoption for draft-zhu-idr-bgp-ls-path-mtu (11/1/2020 to 11/16/2020)




My second message to this thread asked a few questions about the technology.   


This information can be more than IGP information.   If SR segments statically defined (static or direct interfaces) tunnels and pass the endpoints via BGP tunnel-encaps draft with SR Policy tunnel type, this can just be BGP.


I’ll keep this WG adoption call going until we can be sure if:  1) it something LSR wants to standardize, and 2) whether there is a BGP only case.   It is clear to me that standardizing MTU for a SR segments with stacked tunnel segments passed by BGP was useful.


The authors should be the ones to propose this in LSR.       


Cheers,  Sue


From: Idr [] On Behalf Of Stephane Litkowski (slitkows)
Sent: Monday, November 9, 2020 4:28 AM
To: Susan Hares;
Subject: Re: [Idr] WG Adoption for draft-zhu-idr-bgp-ls-path-mtu (11/1/2020 to 11/16/2020)


Hi Sue,


> The purpose behind this mechanism is to reduce administrative work rather than to reduce the review on drafts.


That’s exactly my point. If we don’t do OSPF extension now and in the same draft, we leave a gap that will require a new draft for a very very small extension. Just adds process overhead for nothing…






From: Susan Hares <>
Sent: lundi 9 novembre 2020 10:10
To: Stephane Litkowski (slitkows) <>;
Subject: RE: [Idr] WG Adoption for draft-zhu-idr-bgp-ls-path-mtu (11/1/2020 to 11/16/2020)




I want to pick up on your email from two points:


1)  Why not do everything in LSR? 

<WG-chair hat>

If the feature comes with interest in doing all 3 (ISIS, OSPF, and BGP-LS data gathering), then the authors may select to do everything in LSR rather than have 2 or 3 drafts to maintain.


This is optional and the mechanism may not fit every draft.   The drafts may also start out adopted and vetted in LSR and IDR.    The purpose behind this mechanism is to reduce administrative work rather than to reduce the review on drafts.


</wg-chair hat off>



2) TRILL implementations of IS-IS has some MTU subTLV - 


If you are interested in whether this has been implemented in TRILL, you might want to check with Donald Eastlake.   My vague and foggy recollection is that had some implementations or came from pre-TRILL implementations.



Cheers, Susan Hares




From: Stephane Litkowski (slitkows) []
Sent: Wednesday, November 4, 2020 3:03 AM
To: Susan Hares;
Subject: RE: [Idr] WG Adoption for draft-zhu-idr-bgp-ls-path-mtu (11/1/2020 to 11/16/2020)




“a) Are there ways to pass IGP link MTUs in

the IGPs?  If so, is this needed in BGP-LS”


This is a valid point, most of the time BGP-LS is feeded by IGP LSDBs (of course there are other ways too). While I see that IS-IS has some MTU subTLV coming from TRILL RFC7176 (possibly never been implemented), I don’t see anything for OSPF (I’m not an OSPF expert, so I may have missed it).

Shouldn’t this be checked and validated with LSR WG before adopting ?






From: Idr <> On Behalf Of Susan Hares
Sent: lundi 2 novembre 2020 06:04
Subject: [Idr] WG Adoption for draft-zhu-idr-bgp-ls-path-mtu (11/1/2020 to 11/16/2020)


This begins a 2 week WG adoption call for

draft-zhu-idr-bgp-ls-path-mtu-04.txt (11/1 – 11/16/2020).


The authors should send in an IPR statement for this draft

by 11/5 so the WG can include the IPR status in their decision.


You can access the draft at:


Since this draft is reference by an existing IDR draft

I’ve included a bit of background below to help you place  

this draft into the larger context of the SR additions to BGP-LS

and the draft-ietf-idr-tunnel-encaps-19.txt.


This draft does continue BGP-LS additions.  if you 

are opposed to any BGP-LS additions rather than 

this specific addition, please make that clear in your 

comment in this discussion.   


The authors requested a WG adoption at IETF 108. 

The IDR co-chairs thank the authors for their patience.   

This draft has been delayed by process of having a

new document shepherd (Sue Hares) come up to speed

on draft-ietf-idr-tunnel-encapsulation.


Cheers, Sue




Segment Routing technology creates SR tunnels that are

directly overlaid on MPLS or SRv6.  While existing MPLS technology

(LDP and RSV-TE) provides mechanisms to negotiate path MTU

based on individual link MTU limits, the Segment Routing (SR)

on BGP-LS Link Attribute does not pass information on

MTU size per link.   


draft-ietf-idr-sr-policy-path-mtu-02.txt sends PATH MTU

information in the tunnel-encapsulation attribute for the tunnel type  

SR-Policy that handles segment routing (SR) paths.       

However, it lacks the information to create a reasonable

Path size since the BGP-LS Link Attribute does distribute

this information.


The draft proposes adding a new sub-TLV for MTU size

to the BGP-LS Link Attribute TLV, and

draft-ietf-idr-sr-policy-path-mtu-02.txt mentions this

draft as one possible way to distribute the per link



Questions for the authors might be:

a) Are there ways to pass IGP link MTUs in

the IGPs?  If so, is this needed in BGP-LS


b) What other mechanisms pass link MTU?   








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