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

Susan Hares <> Mon, 09 November 2020 09:09 UTC

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From: Susan Hares <>
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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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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


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?