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, <[email protected]
<mailto:[email protected]>> 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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