RE: Request for RTGWG Working Group adoption for draft-bashandy-rtgwg-segment-routing-ti-lfa

Chris Bowers <> Mon, 28 May 2018 23:30 UTC

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From: Chris Bowers <>
To: Jeff Tantsura <>, Ahmed Bashandy <>, "" <>, Stewart Bryant <>
CC: "" <>, "" <>, "" <>, "" <>, "" <>, "" <>, "" <>, "" <>
Subject: RE: Request for RTGWG Working Group adoption for draft-bashandy-rtgwg-segment-routing-ti-lfa
Thread-Topic: Request for RTGWG Working Group adoption for draft-bashandy-rtgwg-segment-routing-ti-lfa
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Several participants in the WG (including Stewart) have provided feedback requesting that the draft address particular issues.
The response you have provided to this feedback is that these issues are out-of-scope for the draft, so they will not be addressed.
I have seen no change in the text of the draft to incorporate this feedback.

It is up to the working group to decide what is the scope of a document it works on.  I do not think that it would be productive
to conduct a poll for WG adoption until the scope of the draft is broadened to address the feedback that has already been provided.


From: Jeff Tantsura <>
Sent: Monday, May 28, 2018 4:27 PM
To: Ahmed Bashandy <>om>;; Stewart Bryant <>
Subject: Re: Request for RTGWG Working Group adoption for draft-bashandy-rtgwg-segment-routing-ti-lfa

Hi Ahmed,

I’m awaiting Stewart’s response.

From: Ahmed Bashandy <<>>
Date: Monday, May 28, 2018 at 13:59
To: Jeff Tantsura <<>>, rtgwg-chairs <<>>
Cc: <<>>, <<>>, <<>>, <<>>, <<>>, <<>>, <<>>, RTGWG <<>>
Subject: Re: Request for RTGWG Working Group adoption for draft-bashandy-rtgwg-segment-routing-ti-lfa

Hi Jeff

All comments have been addressed as shown in the email below

Can we initiate the WG adoption


On 5/19/18 12:20 PM, Ahmed Bashandy wrote:
Hi Jeff

These comments are already addressed with the exception of the minor comment about section 5.3.1 and 5.3.2. But for the convenience of everyone, I will respond to each specific comment here

See "#Ahmed" below



> Reviewer: Stewart Bryant

> Review result: Has Issues


> These review comments were incorrectly posted against the uloop draft,

> apologies for any confustion.


> I have been asked to perform an early review of this document on

> behalf of the Routing Directorate.


> Summary:


> A document on this subject is something that the WG should publish,

> but I think that there are number of issues that the WG need to

> discuss and reach consensus on before deciding whether or not they

> should adopt this draft as a starting point for that work.



> Major Issues:


> Before I get into the substance I am surprised that there are no IPR

> disclosures. In an earlier and related work

> (draft-francois-segment-routing-ti-lfa-00) there were three IPR

> disclosures.


The IPR link is<>

If there is anything else for us to do regarding IPR I will be more than happy to take care of


> The work has four basic components, the concept of resolving the

> problem of P and Q being non-adjacent, the use of SR to solve the

> non-adjacency, the use of the post convergence path following failure

> and the applicability of these techniques to an SR network. The first

> and second points seem of utility in non-SR networks, and so I am

> surprised that they are not called out as such, in the first case

> perhaps with consideration to strategically places RSVP tunnels, or

> binding segments.

The draft already mentions that the work builds on top of existing FRR work. For example

the second statement of the abstract already says

  builds on proven IP-FRR concepts being

  LFAs, remote LFAs (RLFA), and remote LFAs with directed forwarding


The statement about the possibility of using RSVP is clearly outside the scope of document as mentioned in first paragraph of the introduction.


> The issue of mapping repair path to the post convergence path to the

> something that has always concerned me in this concept. It is true

> that traffic that always passes through the PLR will experience the

> properties the authors describe, but not all traffic will pass through

> the PLR post convergence. The post failure path will be topology

> dependent, and may take a different path from the point of ingress.


The fourth paragraph in the introduction clearly mentions that we are protecting the traffic passing through the PLR.


> I am also concerned that the authors do not discuss the need for loop

> free convergence, since although traffic going through the repair path

> will be loop-free, traffic arriving at the PLR might not be. Consider

> for example a topology fragment that looks like a clock with a router

> at each minute. Traffic enters at 9 o'clock, leave at 3 o'clock and

> goes via 12 o'clock and 12 o'clock fails.  The routers 9..12 will

> re-converge at different times and this may give rise to the

> micro-looping of traffic trying to get to the PLR. A summary of the

> problem and a pointer to the companion draft may be sufficient.


The last statement in the first paragraph in the introduction refers the reader to the uloop avoidance draft which handles non-local failures


> Finally on the basic concept it would be good to state up from whether

> the proposal is constrained solely to SR networks, or whether the

> authors believe that the concept is of wider applicability. It see no

> reason why it would be constrained to only work on SR networks.


As it is quite clear from the title of the draft as well as many statements inside it, the scope of document is restricted to segment routing.


> There is no discussion of multiple failures, nor as far as I can see

> of failures that are worse than anticipated. This is an important

> point that needs to be established early. Some methods, (MRT)

> intrinsically address multiple failures, others (NV) intrinsically

> exclude them. Simple LFA needs a supervisor to quickly abandon all

> hope when they occur.


As specified in the 3rd paragraph of the introduction the scope of the document is limited to single link, single node, and single local SRLG failure.


> In an SR network the paths used are not the shortest paths, they are a

> collection of shortest paths, so there needs to be some discussion on

> the interaction between the SR paths and repair paths to consider

> whether it is unconditionally safe against forwarding loops. It would

> presumably be so if the authors borrowed the concept of repair

> addresses rather than normal forwarding addresses from not-via, but I

> don't think they have done this.


Again the second statement of the 1st paragraph of the Introduction says

  By relying on segment routing this document provides

         a local repair mechanism for standard IGP shortest path

So the scope of the document is quite clear


> There should also be some discussion on the original path constraints

> that are applicable to the repair. Presumably the ingress node

> constrained the traffic to go though failed node F for a reason. If

> the repair is unconstrained that reason could be violated, but this is

> not discussed in the text.


Same response as the response to the previous comment. The scope is standard IGP shortest paths



> In the Security section you say:


>     The behavior described in this document is internal functionality

>     to a router that result in the ability to guarantee an upper bound

>     on the time taken to restore traffic flow upon the failure of a

>     directly connected link or node. As such no additional security

>     risk is introduced by using the mechanisms proposed in this

>     document.



> SB> I am not sure that the above is correct. There may be a security

> reason

> SB> why a packet was steered along a path which breaks when you use

> this

> SB> technique.


The security consideration section has been modified to to indicate that

the traffic is being steered over the post convergence path and hence there

is no security risk because this is the path that the operator intended to use

after the failure through the metrics configured on the links. In fact by expediting

rerouting the traffic over the intended post convergence path without waiting

for IGP reconvergence, we have introduced a minor security enhancement by reducing

misforwarding and/or traffic drop


> In the conclusion you say:


>     The

>     mechanism is able to calculate the backup path irrespective of the

>     topology as long as the topology is sufficiently redundant.



> SB> That is certainly true in classic. I am not sure this is

> universally

> SB> true under SR which includes the use of non-shortest path and

> SB> binding segments.


Again the document is restricted to IGP shortest path as mentioned in the introduction

> Minor issues:


>     For each destination in the network, TI-LFA prepares a data-plane

>     switch-over to be activated upon detection of the failure of a

>     link used to reach the destination.


> SB> To make the scaling clearer to the reader, I think you need

> SB> to make it clear that for each protected link, you determine

> SB> the repair needed to reach every destination reachable over that

> SB> link. You sort of say that, but it's a bit hidden.


I do not understand the difference between the text in the draft and the

text that you are proposing. We think that our text is quite clear

>     We provide the TI-LFA approach that achieves guaranteed coverage

>     against link, node, and local SRLG failure, in any IGP network,

>     relying on the flexibility of SR.


> SB> Should that be any SINGLE link.... failure?


As mentioned above few times above, the introduction clearly mentions *single*

> In the text (and the text that follows)


>     To do so, S applies a "NEXT" operation on Adj(S-F) and then two

>     consecutive "PUSH" operations: first it pushes a node segment for

> F,

>     and then it pushes a protection list allowing to reach F while

>     bypassing S-F.


> You need to reference the SR operations.


This paragraph is in Section 5.2.1. The latest version refers to the SR draft


> Also you are considering Adj segments, and presumably they were there

> for a reason, but you do not discuss that.


Section 5.2 discusses protecting adjacency segments


> In 5.3.1 and 5.3.2 you have a list of conditions, but do not make it

> clear whether any or all must be true.



The intention is for all of the conditions to be true. I will make it clear in the next version

> Nits


> 1. Introduction


>     Segment Routing aims at supporting services with tight SLA

>     guarantees [1]. This document provides a local repair mechanism

>     relying on SR-capable of restoring end-to-end connectivity in the

>     case of a sudden failure of a network component.


> SB> Grammar needs a little work in the last sentence.


Addressed in the latest version of the document

> In Fig 1, I assume that the blobs are network fragments.


> In the conclusion you say:

>     This document proposes a mechanism that is able to pre-calculate a

>     backup path for every primary path so as to be able to protect

>     against the failure of a directly connected link or node.

> SB> you need to add SRLG


Addressed in the latest version of the draft

On 5/10/18 9:40 AM, Jeff Tantsura wrote:

Hi Ahmed,

We would like you to address the comments from Early Review and get OK from Stewart, before progressing the document<>

Please let us know when this could be done.



On 4/25/18, 02:17, "Ahmed Bashandy" <><> wrote:


    We would like to request the WG adoption of


    The draft has been stable for a long while and the IPR declaration has

    been recorded

    The latest version addresses all comments and the draft has been

    presented in IETF-96 and IETF-99