Re: [spring] WG Last Call for draft-ietf-spring-segment-routing-mpls-13

SPRING WG,

I generally support publication of
draft-ietf-spring-segment-routing-mpls. However, I think
that the text in sections 2.5 and 2.6 (on incoming label collisions)
needs some work before publication. This text was added to
the draft a few months ago, and has not gotten much review
from the WG as a whole. The review and proposed text below
focuses on these sections.

As I understand the current text of the draft, the general
approach to resolving incoming label collisions seems
well-reasoned and complete.  However, it is possible that
my interpretation of these tie-breaking rules is
not what the authors intended.

I'd like to propose the examples below to be included
in the draft to help clarify the tie-breaking rules
for incoming label collisions described in section 2.5.
I have highlighted several cases in these examples,
where I think the rules in section 2.5 need
to be clarified in order to unambiguously determine
the winning FEC in an example.

It may also be the case that the authors or other
WG participants will disagree with the interpretation of the
rules used to choose a winning FEC in some of these
examples.  In that case, we should discuss
what is the correct interpretation, and clarify the
text in the draft to make the correct interpretation
clear.

Incoming label collision examples
=========

Node A
OSPF default admin distance for implementation=50
ISIS default admin distance for implementation=60

=========
Example incoming label conflict for label=1005 on node A

FEC1)
OSPF prefix sid advertisement from node B for 198.51.100.5/32 with index=5
OSPF SRGB on node A = [1000,1999]
Incoming label=1005

FEC2)
ISIS prefix sid advertisement from node C for 203.0.113.105/32 with index=5
ISIS SRGB on node A = [1000,1999]
Incoming label=1005

FEC1 and FEC2 both use dynamic SID assignment.  Since neither of
the FEC types is SR Policy, we use the default admin distances of 50
and 60 to break the tie.  So FEC1 wins.

=========
Example incoming label conflict for label=1006 on node A

FEC1)
OSPF prefix sid advertisement from node B for 198.51.100.6/32 with index=6
OSPF SRGB on node A = [1000,1999]
Incoming label=1006

FEC2)
ISIS adjacency sid advertisement from node A with label=1006
Incoming label=1006
Node A allocates this adjacency SID dynamically,
and it may differ across router reboots.

FEC1 and FEC2 both use dynamic SID assignment.  Since neither of
the FEC types is SR Policy, we use the default admin distances of 50
and 60 to break the tie.  So FEC1 wins.

=========
Example incoming label conflict for label=1007 on node A

FEC1)
OSPF prefix sid advertisement from node B for 198.51.100.7/32 with index=7
OSPF SRGB on node A = [1000,1999]
Incoming label=1007

FEC2)
ISIS adjacency sid advertisement from node A with label=1007
Incoming label=1007
Node A assigns this adjacency SID explicitly via configuration,
so the adjacency SID survives router reboots.

FEC1 uses dynamic SID assignment, while FEC2 uses explicit SID
assignment. So FEC2 wins.

=========
Example incoming label conflict for label=1008 on node A

FEC1)
OSPF prefix sid advertisement from node B for 198.51.100.8/32 with index=8
OSPF SRGB on node A = [1000,1999]
Incoming label=1008

FEC2)
SR Policy advertisement from controller to node A
Endpoint = 192.0.2.208, color = 100, SID-List = <S1, S2>
Binding-SID label = 1008

FEC1 and FEC2 both use dynamic SID assignment.
Since one of the FEC types is SR Policy, default admin
distance is not used to break the tie.
/* The text in Section 2.5.1 needs to be clarified to specify
whether SR Policy always loses or always wins in this case. */

=========
Example incoming label conflict for label=1009 on node A

FEC1)
OSPF adjacency sid advertisement by node A with label=1009
Incoming label=1009
Node A assigns this adjacency SID explicitly via configuration,
so the adjacency SID survives router reboots.

FEC2)
ISIS adjacency sid advertisement by node A with label=1009
Incoming label=1009
Node A assigns this adjacency SID explicitly via configuration,
so the adjacency SID survives router reboots.

FEC1 and FEC2 both use explicit SID assignment.  This kind of
incoming label collision should never occur, since an
implement of explicit SID assignment MUST guarantee
collision freeness on the same router.

========
Example incoming label conflict for label=1010 on node A

FEC1)
ISIS prefix sid advertisement from node B for 203.0.113.110/32 with index=10
ISIS SRGB on node A = [1000,1999]
Incoming label=1010

FEC2)
ISIS adjacency sid advertisement by node A with label=1010
Incoming label=1010
Node A allocates this adjacency SID dynamically,
and it may differ across router reboots.

FEC1 and FEC2 both use dynamic SID assignment. Since both FECs
are from the same MCC, they have the same default admin distance.
So we compare FEC type code-point.  FEC1 has FEC type
code-point=120, while FEC2 has FEC type code-point=130.
Therefore, FEC1 wins.

=========
Example incoming label conflict for label=1011 on node A

FEC1)
ISIS prefix sid advertisement from node B for 203.0.113.111/32 with index=11
ISIS SRGB on node A = [1000,1999]
Incoming label=1011

FEC2)
ISIS prefix sid advertisement from node C for 2001:DB8:1000::11/128 with
index=11
ISIS SRGB on node A = [1000,1999]
Incoming label=1011

FEC1 and FEC2 both use dynamic SID assignment. Since both FECs
are from the same MCC, they have the same default admin distance.
So we compare FEC type code-point.  Both FECs have FEC type
code-point=120. So we compare address family.  Since IPv4 is
preferred over IPv6, FEC1 wins.

=========
Example incoming label conflict for label=1012 on node A

FEC1)
ISIS prefix sid advertisement from node B for 203.0.113.112/32 with index=12
ISIS SRGB on node A = [1000,1999]
Incoming label=1012

FEC2)
ISIS prefix sid advertisement from node C for 203.0.113.128/30 with index=12
ISIS SRGB on node A = [1000,1999]
Incoming label=1012

FEC1 and FEC2 both use dynamic SID assignment. Since both FECs
are from the same MCC, they have the same default admin distance.
So we compare FEC type code-point.  Both FECs have FEC type
code-point=120. So we compare address family.  Both are IPv4 address
family, so we compare prefix length.  FEC1 has prefix length=32,
and FEC2 has prefix length=30, so FEC2 wins.

=========
Example incoming label conflict for label=1013 on node A

FEC1)
ISIS prefix sid advertisement from node B for 203.0.113.113/32 with index=13
ISIS SRGB on node A = [1000,1999]
Incoming label=1013

FEC2)
ISIS prefix sid advertisement from node C for 203.0.113.213/32 with index=13
ISIS SRGB on node A = [1000,1999]
Incoming label=1013

FEC1 and FEC2 both use dynamic SID assignment. Since both FECs
are from the same MCC, they have the same default admin distance.
So we compare FEC type code-point.  Both FECs have FEC type
code-point=120. So we compare address family.  Both are IPv4 address
family, so we compare prefix length.  Prefix lengths are the same,
so we compare prefix.  FEC1 has the lower prefix, so FEC1 wins.

=========
Example incoming label conflict for label=1014 on node A

FEC1)
ISIS prefix sid advertisement from node B for 203.0.113.114/32 with index=14
Routing Instance ID = 1000
ISIS SRGB on node A = [1000,1999]
Incoming label=1014

FEC2)
ISIS prefix sid advertisement from node C for 203.0.113.114/32 with index=14
Routing Instance ID = 2000
ISIS SRGB on node A = [1000,1999]
Incoming label=1014

These two FECs match all the way through the prefix length and prefix.
So Routing Instance ID breaks the tie, with FEC1 winning.

=========
Example incoming label conflict for label=1015 on node A

FEC1)
ISIS prefix sid advertisement from node B for 203.0.113.115/32 with index=15
Routing Instance ID = 1000
ISIS Multi-topology ID = 50
ISIS SRGB on node A = [1000,1999]
Incoming label=1015

FEC2)
ISIS prefix sid advertisement from node C for 203.0.113.115/32 with index=15
Routing Instance ID = 1000
ISIS Multi-topology ID = 40
ISIS SRGB on node A = [1000,1999]
Incoming label=1015

These two FECs match all the way through the prefix length, prefix, and
Routing Instance ID.  We compare ISIS Multi-topology ID, so FEC2 wins.

/* There appears to be a typo in section 2.5.1, with two different
orderings shown for a prefix-based FEC:
Prefix, Routing Instance, Topology, Algorithm
and
(Prefix Length, Prefix, SR Algorithm, routing_instance_id, Topology)
This needs to be corrected. */

=========
Example incoming label conflict for label=1016 on node A

FEC1)
ISIS prefix sid advertisement from node B for 203.0.113.116/32 with index=16
Routing Instance ID = 1000
ISIS Multi-topology ID = 50
SR algorithm = 0
ISIS SRGB on node A = [1000,1999]
Incoming label=1016

FEC2)
ISIS prefix sid advertisement from node C for 203.0.113.116/32 with index=16
Routing Instance ID = 1000
ISIS Multi-topology ID = 50
SR algorithm = 22
ISIS SRGB on node A = [1000,1999]
Incoming label=1016

These two FECs match all the way through the prefix length, prefix, and
Routing Instance ID, and Multi-topology ID. We compare SR algorithm ID, so
FEC1 wins.

=========
Example incoming label conflict for label=1017 on node A

FEC1)
ISIS prefix sid advertisement from node B for 203.0.113.117/32 with index=17
ISIS SRGB on node A = [1000,1999]
Incoming label=1017

FEC2)
ISIS mapping server advertisement (SID/Label Binding TLV) from node D:
Range=100, Prefix = 203.0.113.1/32
This mapping server advertisment generates 100 mappings, one of which
maps 203.0.113.17/32 to index=17.
ISIS SRGB on node A = [1000,1999]
Incoming label=1017

The fact that FEC1 comes from a normal prefix sid advertisement and
FEC2 is generated from a mapping server advertisement is not
used as a tie-breaking parameter. Both FECs use dynamic SID assignment,
are from the same MCC, have the same FEC type code-point=120. Their prefix
lengths are the same as well.  FEC2 wins based on lower numerical prefix
value,
since 203.0.113.17 is less than 203.0.113.117.

=========
Example incoming label conflict for label=1018 on node A

FEC1)
ISIS IPv4 adjacency sid advertisement from node A with label=1018
corresponding to next-hop interface address=192.0.2.100, outgoing interface
ID=5
Incoming label=1018
Node A allocates this adjacency SID dynamically,
and it may differ across router reboots.

FEC2)
ISIS IPv6 adjacency sid advertisement from node A with label=1018
corresponding to 2001:DB8:2000::100/128, outgoing interface ID=6.
Incoming label=1018
Node A allocates this adjacency SID dynamically,
and it may differ across router reboots.

Both FECs use dynamic SID assignment, are from the same MCC,
and have the same FEC type code-point=130.  FEC1 wins
because IPv4 address family is preferred over IPv6.

=========
Example incoming label conflict for label=1019 on node A

FEC1)
ISIS IPv4 adjacency sid advertisement from node A with label=1019
corresponding to next-hop interface address=192.0.2.220, outgoing interface
ID=7
Incoming label=1019
Node A allocates this adjacency SID dynamically,
and it may differ across router reboots.

FEC2)
ISIS IPv4 adjacency sid advertisement from node A with label=1019
corresponding to next-hop interface address=192.0.2.230, outgoing interface
ID=8
Incoming label=1019
Node A allocates this adjacency SID dynamically,
and it may differ across router reboots.

Both FECs use dynamic SID assignment, are from the same MCC,
and have the same FEC type code-point=130. Both FECs have to
same address family.  FEC1 wins based on having the lowest next-hop
interface address.

/* It is not clear how to construct an example that
would result in using the outgoing interface ID as a tie-breaker.
It would be useful to understand why this is and clarify
it in the text. */
=========
Example incoming label conflict for label=1020 on node A

FEC1)
SR Policy advertisement from controller to node A
Endpoint address=2001:DB8:3000::100, color=100, SID-List=<S1, S2>
Binding-SID label=1020

FEC2)
SR Policy advertisement from controller to node A
Endpoint address=192.0.2.60, color=100, SID-List=<S3, S4>
Binding-SID label=1020

The FECs match through the tie-breaks up to and including
having the same FEC type code-point=140.
FEC2 wins based on IPv4 address family being preferred
over IPv6.

=========
Example incoming label conflict for label=1021 on node A

FEC1)
SR Policy advertisement from controller to node A
Endpoint address=192.0.2.70, color=100, SID-List=<S1, S2>
Binding-SID label=1021

FEC2)
SR Policy advertisement from controller to node A
Endpoint address=192.0.2.71, color=100, SID-List=<S3, S4>
Binding-SID label=1021

The FECs match through the tie-breaks up to and including
having the same address family. FEC1 wins by having the
lower numerical endpoint address value.

=========

I'd like to propose the examples below to be included
in the draft to help clarify section 2.6
(currently entitled "Outgoing Label Collision").

Examples of the Effect Incoming Label Collision on Outgoing Label
Programming
====================================

Example of effect of incoming label collision for label=1022
on outgoing label programming on node A

FEC1)
ISIS prefix sid advertisement from node B for 203.0.113.122/32 with index=22
ISIS SRGB on node A = [1000,1999]
Incoming label=1022

FEC2)
ISIS prefix sid advertisement from node C for 203.0.113.222/32 with index=22
ISIS SRGB on node A = [1000,1999]
Incoming label=1022

FEC1 wins based on lowest numerical prefix value.  This means that node A
installs a transit MPLS forwarding entry to SWAP incoming label=1022, with
outgoing label N,
and use outgoing interace I. N is determined by the index associated with
FEC1 (index=22) and
the SRGB advertised by the next-hop node on the shortest path to reach
203.0.113.122/32.

Node A will generally also install an ingress MPLS forwarding entry
corresponding to FEC1 for
incoming prefix=203.0.113.122/32 pushing outgoing label N, and using
outgoing interace I.

The rule in section 2.6 means that node A MUST NOT install an ingress MPLS
forwarding entry
corresponding to FEC2 ( which would be for incoming prefix=203.0.113.222/32).

========

Example of effect of incoming label collision for label=1023
on outgoing label programming on node A

FEC1)
SR Policy advertisement from controller to node A
Endpoint address=192.0.2.80, color=100, SID-List=<S1, S2>
Binding-SID label=1023

FEC2)
SR Policy advertisement from controller to node A
Endpoint address=192.0.2.81, color=100, SID-List=<S3, S4>
Binding-SID label=1023

FEC1 wins by having the lower numerical endpoint address value. This means
that node A
installs a transit MPLS forwarding entry to SWAP incoming label=1023, with
outgoing labels
and outgoing interface determined by the SID-List for FEC1.
Node A will generally also install an ingress MPLS forwarding entry
corresponding to FEC1 for
incoming prefix=192.0.2.80/32 pushing outgoing labels and using the
outgoing interface
determined by the SID-List for FEC1.

The rule in section 2.6 means that node A MUST NOT install an ingress MPLS
forwarding entry
corresponding to FEC2 ( which would be for incoming prefix=192.0.2.81/32).

========

General comment:

section 2.6 title:
existing:
Outgoing Label Collision:
proposed:
Effect of Incoming Label Collision on Outgoing Label Programming :
--------------------------------------

Thanks,
Chris

On Thu, May 24, 2018 at 12:14 PM, <bruno.decraene@orange.com> wrote:

> Hello Working Group,
>
>
>
> This email starts a Working Group Last Call on draft-ietf-spring-segment-routing-mpls-13 [1] which is considered mature and ready for a final working group review.
>
>
>
> Please read this document if you haven't read the most recent version yet, and send your comments to the list, no later than *June 08*.
>
>
>
> As a reminder, this document had already passed a WGLC more than a year ago on version -06 [2], had been sent to the AD but then returned to the WG.
>
> Since then, the document has significantly changed, so please read it again. In particular, it now includes the resolution in case of incoming label collision. Hence it killed draft-ietf-spring-conflict-resolution.
>
>
>
> Both co-chairs co-author this document, hence:
>
> - Shraddha has agreed to be the document shepherd.. Thank you Shraddha.
>
> - Martin, our AD, has agreed to evaluate the WG consensus.
>
>
>
> Thank you,
>
> Bruno, Rob
>
>
>
> [1] https://tools.ietf.org/html/draft-ietf-spring-segment-routing-mpls-13
>
> [2] https://mailarchive.ietf.org/arch/msg/spring/STiYsQJWuVXA1C9hK4BiUnyMu7Y
>
>
>
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