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

[Ahmed Bashandy <[e-mail]>]

Thanks a lot for the examples.

Version 15 of the document incorporates all of the examples in the 
appendix (all examples have been moved to appendix), except the examples 
where
- Incoming label=1009
- Incoming label=1018
- Incoming label=1019
- Binding-SID label=1023

See my comments at "#Ahmed" under each of these 4 examples


I have a response for your comment right after the examples that use  
"Incoming label=1008" and "Incoming label=1015"


Ahmed



On 6/8/18 11:14 AM, Chris Bowers wrote:
> 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 
> <http://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 
> <http://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 
> <http://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 
> <http://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 
> <http://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. */
#Ahmed:
Section 2.5.1 in page 9 clearly says
      The default FEC administrative distance order starting from the
      lowest value SHOULD be
and then lists the FECs
Hence the list specifies the default admin distance starting from the
"lowest"
The Binding SID appears in the 2nd sub-bullet of the second
bullet. Hence the binding SID of a policy receives the maximal default
admin distance. Hence FEC1 should win
I adapted this example so that it shows the the default admin distance
of a binding SID is always higher than the default admin distances of
other FECs

>
> =========
> 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.
#Ahmed
The example is not clear. If the adjacency SIDs for ISIS and OSPF are
out of the same interface towards the same neighbor, then there is no
collision
If they are out of different interfaces and/or towards different
neighbors, then this is an example of a faulty
implementation. Implementation must not allow multiple MCCs on the
same box to assign the same local label to two different FECs

Although assigning the same label to more than one FEC is a problem from 
the MPLS point of view, in version 15,  I added a  paragraph in page 8 
to prohibit that for completeness
>
> ========
> Example incoming label conflict for label=1010 on node A
>
> FEC1)
> ISIS prefix sid advertisement from node B for 203.0.113.110/32 
> <http://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 
> <http://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 
> <http://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 
> <http://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 
> <http://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 
> <http://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 
> <http://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 
> <http://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 
> <http://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 
> <http://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. */
#Ahmed:
This is not a mistake. This bullet illustrates how to encode a
prefix and hence it separates the prefix into  its two components:
"prefix length" and "prefix"
>
> =========
> Example incoming label conflict for label=1016 on node A
>
> FEC1)
> ISIS prefix sid advertisement from node B for 203.0.113.116/32 
> <http://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 
> <http://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 
> <http://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 <http://203.0.113.1/32>
> This mapping server advertisment generates 100 mappings, one of which
> maps 203.0.113.17/32 <http://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.

#Ahmed:
This is another example of a faulty implementation because the
implementation of ISIS must not allow allocating the same local label
to two different adj-SIDs on the same node A

>
> =========
> 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. */
#Ahmed:
This is a third example of a faulty implementation because the
implementation of ISIS must not allow allocating the same local label
to two different adj-SIDs on the same box
> =========
> 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 
> <http://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 
> <http://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 <http://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 <http://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 <http://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 <http://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 <http://192.0.2.81/32>).
#Ahmed
It seems like there is a misunderstanding here. A policy is identified 
by a color and an IP address. What gets installed in the FIB is the MPLS 
label representing the binding SID. A packet arriving with the top label 
equal to the binding SID gets forwarded into the policy. The endpoint 
and color are unique identifier of the policy on the box
The IP address is the address of an endpoint, NOT a prefix. So the 
instantiation or advertisement of a policy does not result in installing 
any prefixes in FIB
>
> ========
>
> 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, <[e-mail] 
> <mailto:[e-mail]>> 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
>     <https://tools.ietf.org/html/draft-ietf-spring-segment-routing-mpls-13>
>
>     [2]
>     https://mailarchive.ietf.org/arch/msg/spring/STiYsQJWuVXA1C9hK4BiUnyMu7Y
>     <https://mailarchive.ietf.org/arch/msg/spring/STiYsQJWuVXA1C9hK4BiUnyMu7Y>
>
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