Subject: Rtgdir early review of draft-bashandy-rtgwg-segment-routing-ti-lfa-00

From: Stewart Bryant <[email-masked]>
Date: Wed, 31 May 2017 11:35:41 -0700

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 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 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.

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.

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.

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.

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.

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.

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.

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.

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.

   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?

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.

Also you are considering Adj segments, and presumably they were there
for a reason, but you do not discuss that.

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.

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.

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