Internet Engineering Task Force (IETF) A. Bashandy, Ed.

Internet Engineering Task Force (IETF) A. Bashandy, Ed.
Request for Comments: 8660 Arrcus
Category: Standards Track C. Filsfils, Ed.
ISSN: 2070-1721 S. Previdi
Cisco Systems, Inc.
B. Decraene
S. Litkowski
Orange
R. Shakir
Google
December 2019

Segment Routing with the MPLS Data Plane

Abstract

Segment Routing (SR) leverages the source-routing paradigm. A node
steers a packet through a controlled set of instructions, called
segments, by prepending the packet with an SR header. In the MPLS
data plane, the SR header is instantiated through a label stack.
This document specifies the forwarding behavior to allow
instantiating SR over the MPLS data plane (SR-MPLS).

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8660.

Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction
1.1. Requirements Language
2. MPLS Instantiation of Segment Routing
2.1. Multiple Forwarding Behaviors for the Same Prefix
2.2. SID Representation in the MPLS Forwarding Plane
2.3. Segment Routing Global Block and Local Block
2.4. Mapping a SID Index to an MPLS Label
2.5. Incoming Label Collision
2.5.1. Tiebreaking Rules
2.5.2. Redistribution between Routing Protocol Instances
2.6. Effect of Incoming Label Collision on Outgoing Label
Programming
2.7. PUSH, CONTINUE, and NEXT
2.7.1. PUSH
2.7.2. CONTINUE
2.7.3. NEXT
2.8. MPLS Label Downloaded to the FIB for Global and Local SIDs
2.9. Active Segment
2.10. Forwarding Behavior for Global SIDs
2.10.1. Forwarding for PUSH and CONTINUE of Global SIDs
2.10.2. Forwarding for the NEXT Operation for Global SIDs
2.11. Forwarding Behavior for Local SIDs
2.11.1. Forwarding for the PUSH Operation on Local SIDs
2.11.2. Forwarding for the CONTINUE Operation for Local SIDs
2.11.3. Outgoing Label for the NEXT Operation for Local SIDs
3. IANA Considerations
4. Manageability Considerations
5. Security Considerations
6. References
6.1. Normative References
6.2. Informative References
Appendix A. Examples
A.1. IGP Segment Examples
A.2. Incoming Label Collision Examples
A.2.1. Example 1
A.2.2. Example 2
A.2.3. Example 3
A.2.4. Example 4
A.2.5. Example 5
A.2.6. Example 6
A.2.7. Example 7
A.2.8. Example 8
A.2.9. Example 9
A.2.10. Example 10
A.2.11. Example 11
A.2.12. Example 12
A.2.13. Example 13
A.2.14. Example 14
A.3. Examples for the Effect of Incoming Label Collision on an
Outgoing Label
A.3.1. Example 1
A.3.2. Example 2
Acknowledgements
Contributors
Authors' Addresses

1. Introduction

The Segment Routing architecture [RFC8402] can be directly applied to
the MPLS architecture with no change in the MPLS forwarding plane.
This document specifies forwarding-plane behavior to allow Segment
Routing to operate on top of the MPLS data plane (SR-MPLS). This
document does not address control-plane behavior. Control-plane
behavior is specified in other documents such as [RFC8665],
[RFC8666], and [RFC8667].

The Segment Routing problem statement is described in [RFC7855].

Coexistence of SR over the MPLS forwarding plane with LDP [RFC5036]
is specified in [RFC8661].

Policy routing and traffic engineering using Segment Routing can be
found in [ROUTING-POLICY].

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

2. MPLS Instantiation of Segment Routing

MPLS instantiation of Segment Routing fits in the MPLS architecture
as defined in [RFC3031] from both a control-plane and forwarding-
plane perspective:

* From a control-plane perspective, [RFC3031] does not mandate a
single signaling protocol. Segment Routing makes use of various
control-plane protocols such as link-state IGPs [RFC8665]
[RFC8666] [RFC8667]. The flooding mechanisms of link-state IGPs
fit very well with label stacking on the ingress. A future
control-layer protocol and/or policy/configuration can be used to
specify the label stack.

* From a forwarding-plane perspective, Segment Routing does not
require any change to the forwarding plane because Segment IDs
(SIDs) are instantiated as MPLS labels, and the Segment Routing
header is instantiated as a stack of MPLS labels.

We call the "MPLS Control Plane Client (MCC)" any control-plane
entity installing forwarding entries in the MPLS data plane. Local
configuration and policies applied on a router are examples of MCCs.

In order to have a node segment reach the node, a network operator
SHOULD configure at least one node segment per routing instance,
topology, or algorithm. Otherwise, the node is not reachable within
the routing instance, within the topology, or along the routing
algorithm, which restricts its ability to be used by an SR Policy and
as a Topology Independent Loop-Free Alternate (TI-LFA).

2.1. Multiple Forwarding Behaviors for the Same Prefix

The SR architecture does not prohibit having more than one SID for
the same prefix. In fact, by allowing multiple SIDs for the same
prefix, it is possible to have different forwarding behaviors (such
as different paths, different ECMP and Unequal-Cost Multipath (UCMP)
behaviors, etc.) for the same destination.

Instantiating Segment Routing over the MPLS forwarding plane fits
seamlessly with this principle. An operator may assign multiple MPLS
labels or indices to the same prefix and assign different forwarding
behaviors to each label/SID. The MCC in the network downloads
different MPLS labels/SIDs to the FIB for different forwarding
behaviors. The MCC at the entry of an SR domain or at any point in
the domain can choose to apply a particular forwarding behavior to a
particular packet by applying the PUSH action to that packet using
the corresponding SID.

2.2. SID Representation in the MPLS Forwarding Plane

When instantiating SR over the MPLS forwarding plane, a SID is
represented by an MPLS label or an index [RFC8402].

A global SID is a label, or an index that may be mapped to an MPLS
label within the Segment Routing Global Block (SRGB), of the node
that installs a global SID in its FIB and receives the labeled
packet. Section 2.4 specifies the procedure to map a global segment
represented by an index to an MPLS label within the SRGB.

The MCC MUST ensure that any label value corresponding to any SID it
installs in the forwarding plane follows the rules below:

* The label value MUST be unique within the router on which the MCC
is running, i.e., the label MUST only be used to represent the SID
and MUST NOT be used to represent more than one SID or for any
other forwarding purpose on the router.

* The label value MUST NOT come from the range of special-purpose
labels [RFC7274].

Labels allocated in this document are considered per-platform
downstream allocated labels [RFC3031].

2.3. Segment Routing Global Block and Local Block

The concepts of SRGB and global SID are explained in [RFC8402]. In
general, the SRGB need not be a contiguous range of labels.

For the rest of this document, the SRGB is specified by the list of
MPLS label ranges [Ll(1),Lh(1)], [Ll(2),Lh(2)],..., [Ll(k),Lh(k)]
where Ll(i) =< Lh(i).

The following rules apply to the list of MPLS ranges representing the
SRGB:

* The list of ranges comprising the SRGB MUST NOT overlap.

* Every range in the list of ranges specifying the SRGB MUST NOT
cover or overlap with a reserved label value or range [RFC7274],
respectively.

* If the SRGB of a node does not conform to the structure specified
in this section or to the previous two rules, the SRGB MUST be
completely ignored by all routers in the routing domain, and the
node MUST be treated as if it does not have an SRGB.

* The list of label ranges MUST only be used to instantiate global
SIDs into the MPLS forwarding plane.

A local segment MAY be allocated from the Segment Routing Local Block
(SRLB) [RFC8402] or from any unused label as long as it does not use
a special-purpose label. The SRLB consists of the range of local
labels reserved by the node for certain local segments. In a
controller-driven network, some controllers or applications MAY use
the control plane to discover the available set of Local SIDs on a
particular router [ROUTING-POLICY]. The rules applicable to the SRGB
are also applicable to the SRLB, except the SRGB MUST only be used to
instantiate global SIDs into the MPLS forwarding plane. The
recommended, minimum, or maximum size of the SRGB and/or SRLB is a
matter of future study.

2.4. Mapping a SID Index to an MPLS Label

This subsection specifies how the MPLS label value is calculated
given the index of a SID. The value of the index is determined by an
MCC such as IS-IS [RFC8667] or OSPF [RFC8665]. This section only
specifies how to map the index to an MPLS label. The calculated MPLS
label is downloaded to the FIB, sent out with a forwarded packet, or
both.

Consider a SID represented by the index "I". Consider an SRGB as
specified in Section 2.3. The total size of the SRGB, represented by
the variable "Size", is calculated according to the formula:

size = Lh(1)- Ll(1) + 1 + Lh(2)- Ll(2) + 1 + ... + Lh(k)- Ll(k) + 1

The following rules MUST be applied by the MCC when calculating the
MPLS label value corresponding to the SID index value "I".

0 =< I < size. If index "I" does not satisfy the previous
inequality, then the label cannot be calculated.

The label value corresponding to the SID index "I" is calculated
as follows:

j = 1 , temp = 0

While temp + Lh(j)- Ll(j) < I

temp = temp + Lh(j)- Ll(j) + 1

j = j+1

label = I - temp + Ll(j)

An example for how a router calculates labels and forwards traffic
based on the procedure described in this section can be found in
Appendix A.1.

2.5. Incoming Label Collision

The MPLS Architecture [RFC3031] defines the term Forwarding
Equivalence Class (FEC) as the set of packets with similar and/or
identical characteristics that are forwarded the same way and are
bound to the same MPLS incoming (local) label. In Segment Routing
MPLS, a local label serves as the SID for a given FEC.

We define SR FEC [RFC8402] as one of the following:

* (Prefix, Routing Instance, Topology, Algorithm) [RFC8402], where a
topology identifies a set of links with metrics. For the purpose
of incoming label collision resolution, the same Topology
numerical value SHOULD be used on all routers to identify the same
set of links with metrics. For MCCs where the "Topology" and/or
"Algorithm" fields are not defined, the numerical value of zero
MUST be used for these two fields. For the purpose of incoming
label collision resolution, a routing instance is identified by a
single incoming label downloader to the FIB. Two MCCs running on
the same router are considered different routing instances if the
only way the two instances know about each other's incoming labels
is through redistribution. The numerical value used to identify a
routing instance MAY be derived from other configuration or MAY be
explicitly configured. If it is derived from other configuration,
then the same numerical value SHOULD be derived from the same
configuration as long as the configuration survives router reload.
If the derived numerical value varies for the same configuration,
then an implementation SHOULD make the numerical value used to
identify a routing instance configurable.

* (next hop, outgoing interface), where the outgoing interface is
physical or virtual.

* (number of adjacencies, list of next hops, list of outgoing
interfaces IDs in ascending numerical order). This FEC represents
parallel adjacencies [RFC8402].

* (Endpoint, Color). This FEC represents an SR Policy [RFC8402].

* (Mirror SID). The Mirror SID (see [RFC8402], Section 5.1) is the
IP address advertised by the advertising node to identify the
Mirror SID. The IP address is encoded as specified in
Section 2.5.1.

This section covers the RECOMMENDED procedure for handling the
scenario where, because of an error/misconfiguration, more than one
SR FEC as defined in this section maps to the same incoming MPLS
label. Examples illustrating the behavior specified in this section
can be found in Appendix A.2.

An incoming label collision occurs if the SIDs of the set of FECs
{FEC1, FEC2, ..., FECk} map to the same incoming SR MPLS label "L1".

Suppose an anycast prefix is advertised with a Prefix-SID by some,
but not all, of the nodes that advertise that prefix. If the Prefix-
SID sub-TLVs result in mapping that anycast prefix to the same
incoming label, then the advertisement of the Prefix-SID by some, but
not all, of the advertising nodes MUST NOT be treated as a label
collision.

An implementation MUST NOT allow the MCCs belonging to the same
router to assign the same incoming label to more than one SR FEC.

The objective of the following steps is to deterministically install
in the MPLS Incoming Label Map, also known as label FIB, a single FEC
with the incoming label "L1". By "deterministically install", we
mean if the set of FECs {FEC1, FEC2,..., FECk} map to the same
incoming SR MPLS label "L1", then the steps below assign the same FEC
to the label "L1" irrespective of the order by which the mappings of
this set of FECs to the label "L1" are received. For example, first-
come, first-served tiebreaking is not allowed. The remaining FECs
may be installed in the IP FIB without an incoming label.

The procedure in this section relies completely on the local FEC and
label database within a given router.

The collision resolution procedure is as follows:

1. Given the SIDs of the set of FECs, {FEC1, FEC2,..., FECk} map to
the same MPLS label "L1".

2. Within an MCC, apply tiebreaking rules to select one FEC only,
and assign the label to it. The losing FECs are handled as if no
labels are attached to them. The losing FECs with algorithms
other than the shortest path first [RFC8402] are not installed in
the FIB.

a. If the same set of FECs are attached to the same label "L1",
then the tiebreaking rules MUST always select the same FEC
irrespective of the order in which the FECs and the label
"L1" are received. In other words, the tiebreaking rule MUST
be deterministic.

3. If there is still collision between the FECs belonging to
different MCCs, then reapply the tiebreaking rules to the
remaining FECs to select one FEC only, and assign the label to
that FEC.

4. Install the selected FEC into the IP FIB and its incoming label
into the label FIB.

5. The remaining FECs with the default algorithm (see the Prefix-SID
algorithm specification [RFC8402]) may be installed in the FIB
natively, such as pure IP entries in case of Prefix FEC, without
any incoming labels corresponding to their SIDs. The remaining
FECs with algorithms other than the shortest path first [RFC8402]
are not installed in the FIB.

2.5.1. Tiebreaking Rules

The default tiebreaking rules are specified as follows:

1. Determine the lowest administrative distance among the competing
FECs as defined in the section below. Then filter away all the
competing FECs with a higher administrative distance.

2. If more than one competing FEC remains after step 1, select the
smallest numerical FEC value. The numerical value of the FEC is
determined according to the FEC encoding described later in this
section.

These rules deterministically select which FEC to install in the MPLS
forwarding plane for the given incoming label.

This document defines the default tiebreaking rules that SHOULD be
implemented. An implementation MAY choose to support different
tiebreaking rules and MAY use one of these instead of the default
tiebreaking rules. To maximize MPLS forwarding consistency in case
of a SID configuration error, the network operator MUST deploy,
within an IGP flooding area, routers implementing the same
tiebreaking rules.

Each FEC is assigned an administrative distance. The FEC
administrative distance is encoded as an 8-bit value. The lower the
value, the better the administrative distance.

The default FEC administrative distance order starting from the
lowest value SHOULD be:

* Explicit SID assignment to a FEC that maps to a label outside the
SRGB irrespective of the owner MCC. An explicit SID assignment is
a static assignment of a label to a FEC such that the assignment
survives a router reboot.

- An example of explicit SID allocation is static assignment of a
specific label to an Adj-SID.

- An implementation of explicit SID assignment MUST guarantee
collision freeness on the same router.

* Dynamic SID assignment:

- All FEC types, except for the SR Policy, are ordered using the
default administrative distance defined by the implementation.

- The Binding SID [RFC8402] assigned to the SR Policy always has
a higher default administrative distance than the default
administrative distance of any other FEC type.

To maximize MPLS forwarding consistency, if the same FEC is
advertised in more than one protocol, a user MUST ensure that the
administrative distance preference between protocols is the same on
all routers of the IGP flooding domain. Note that this is not really
new as this already applies to IP forwarding.

The numerical sort across FECs SHOULD be performed as follows:

* Each FEC is assigned a FEC type encoded in 8 bits. The type
codepoints for each SR FEC defined at the beginning of this
section are as follows:

120: (Prefix, Routing Instance, Topology, Algorithm)

130: (next hop, outgoing interface)

140: Parallel Adjacency [RFC8402]

150: SR Policy [RFC8402]

160: Mirror SID [RFC8402]

The numerical values above are mentioned to guide implementation.
If other numerical values are used, then the numerical values must
maintain the same greater-than ordering of the numbers mentioned
here.

* The fields of each FEC are encoded as follows:

- All fields in all FECs are encoded in big endian order.

- The Routing Instance ID is represented by 16 bits. For routing
instances that are identified by less than 16 bits, encode the
Instance ID in the least significant bits while the most
significant bits are set to zero.

- The address family is represented by 8 bits, where IPv4 is
encoded as 100, and IPv6 is encoded as 110. These numerical
values are mentioned to guide implementations. If other
numerical values are used, then the numerical value of IPv4
MUST be less than the numerical value for IPv6.

- All addresses are represented in 128 bits as follows:

o The IPv6 address is encoded natively.

o The IPv4 address is encoded in the most significant bits,
and the remaining bits are set to zero.

- All prefixes are represented by (8 + 128) bits.

o A prefix is encoded in the most significant bits, and the
remaining bits are set to zero.

o The prefix length is encoded before the prefix in an 8-bit
field.

- The Topology ID is represented by 16 bits. For routing
instances that identify topologies using less than 16 bits,
encode the topology ID in the least significant bits while the
most significant bits are set to zero.

- The Algorithm is encoded in a 16-bit field.

- The Color ID is encoded using 32 bits.

* Choose the set of FECs of the smallest FEC type codepoint.

* Out of these FECs, choose the FECs with the smallest address
family codepoint.

* Encode the remaining set of FECs as follows:

- (Prefix, Routing Instance, Topology, Algorithm) is encoded as
(Prefix Length, Prefix, routing_instance_id, Topology, SR
Algorithm).

- (next hop, outgoing interface) is encoded as (next hop,
outgoing_interface_id).

- (number of adjacencies, list of next hops in ascending
numerical order, list of outgoing interface IDs in ascending
numerical order) is used to encode a parallel adjacency
[RFC8402].

- (Endpoint, Color) is encoded as (Endpoint_address, Color_id).

- (IP address) is the encoding for a Mirror SID FEC. The IP
address is encoded as described above in this section.

* Select the FEC with the smallest numerical value.

The numerical values mentioned in this section are for guidance only.
If other numerical values are used, then the other numerical values
MUST maintain the same numerical ordering among different SR FECs.

2.5.2. Redistribution between Routing Protocol Instances

The following rule SHOULD be applied when redistributing SIDs with
prefixes between routing protocol instances:

* If the SRGB of the receiving instance is the same as the SRGB of
the origin instance, then:

- the index is redistributed with the route.

* Else,

- the index is not redistributed and if the receiving instance
decides to advertise an index with the redistributed route, it
is the duty of the receiving instance to allocate a fresh index
relative to its own SRGB. Note that in this case, the
receiving instance MUST compute the local label it assigns to
the route according to Section 2.4 and install it in FIB.

It is outside the scope of this document to define local node
behaviors that would allow the mapping of the original index into a
new index in the receiving instance via the addition of an offset or
other policy means.

2.5.2.1. Illustration

A----IS-IS----B---OSPF----C-192.0.2.1/32 (20001)

Consider the simple topology above, where:

* A and B are in the IS-IS domain with SRGB = [16000-17000]

* B and C are in the OSPF domain with SRGB = [20000-21000]

* B redistributes 192.0.2.1/32 into the IS-IS domain

In this case, A learns 192.0.2.1/32 as an IP leaf connected to B,
which is usual for IP prefix redistribution

However, according to the redistribution rule above, B decides not to
advertise any index with 192.0.2.1/32 into IS-IS because the SRGB is
not the same.

2.5.2.2. Illustration 2

Consider the example in the illustration described in
Section 2.5.2.1.

When router B redistributes the prefix 192.0.2.1/32, router B decides
to allocate and advertise the same index 1 with the prefix
192.0.2.1/32.

Within the SRGB of the IS-IS domain, index 1 corresponds to the local
label 16001. Hence, according to the redistribution rule above,
router B programs the incoming label 16001 in its FIB to match
traffic arriving from the IS-IS domain destined to the prefix
192.0.2.1/32.

2.6. Effect of Incoming Label Collision on Outgoing Label Programming

When determining what outgoing label to use, the ingress node that
pushes new segments, and hence a stack of MPLS labels, MUST use, for
a given FEC, the label that has been selected by the node receiving
the packet with that label exposed as the top label. So in case of
incoming label collision on this receiving node, the ingress node
MUST resolve this collision by using this same "Incoming Label
Collision resolution procedure" and by using the data of the
receiving node.

In the general case, the ingress node may not have the exact same
data as the receiving node, so the result may be different. This is
under the responsibility of the network operator. But in a typical
case, e.g., where a centralized node or a distributed link-state IGP
is used, all nodes would have the same database. However, to
minimize the chance of misforwarding, a FEC that loses its incoming
label to the tiebreaking rules specified in Section 2.5 MUST NOT be
installed in FIB with an outgoing Segment Routing label based on the
SID corresponding to the lost incoming label.

Examples for the behavior specified in this section can be found in
Appendix A.3.

2.7. PUSH, CONTINUE, and NEXT

PUSH, NEXT, and CONTINUE are operations applied by the forwarding
plane. The specifications of these operations can be found in
[RFC8402]. This subsection specifies how to implement each of these
operations in the MPLS forwarding plane.

2.7.1. PUSH

As described in [RFC8402], PUSH corresponds to pushing one or more
labels on top of an incoming packet then sending it out of a
particular physical interface or virtual interface, such as a UDP
tunnel [RFC7510] or the Layer 2 Tunneling Protocol version 3 (L2TPv3)
[RFC4817], towards a particular next hop. When pushing labels onto a
packet's label stack, the Time-to-Live (TTL) field [RFC3032]
[RFC3443] and the Traffic Class (TC) field [RFC3032] [RFC5462] of
each label stack entry must, of course, be set. This document does
not specify any set of rules for setting these fields; that is a
matter of local policy. Sections 2.10 and 2.11 specify additional
details about forwarding behavior.

2.7.2. CONTINUE

As described in [RFC8402], the CONTINUE operation corresponds to
swapping the incoming label with an outgoing label. The value of the
outgoing label is calculated as specified in Sections 2.10 and 2.11.

2.7.3. NEXT

As described in [RFC8402], NEXT corresponds to popping the topmost
label. The action before and/or after the popping depends on the
instruction associated with the active SID on the received packet
prior to the popping. For example, suppose the active SID in the
received packet was an Adj-SID [RFC8402]; on receiving the packet,
the node applies the NEXT operation, which corresponds to popping the
topmost label, and then sends the packet out of the physical or
virtual interface (e.g., the UDP tunnel [RFC7510] or L2TPv3 tunnel
[RFC4817]) towards the next hop corresponding to the Adj-SID.

2.7.3.1. Mirror SID

If the active SID in the received packet was a Mirror SID (see
[RFC8402], Section 5.1) allocated by the receiving router, the
receiving router applies the NEXT operation, which corresponds to
popping the topmost label, and then performs a lookup using the
contents of the packet after popping the outermost label in the
mirrored forwarding table. The method by which the lookup is made,
and/or the actions applied to the packet after the lookup in the
mirror table, depends on the contents of the packet and the mirror
table. Note that the packet exposed after popping the topmost label
may or may not be an MPLS packet. A Mirror SID can be viewed as a
generalization of the context label in [RFC5331] because a Mirror SID
does not make any assumptions about the packet underneath the top
label.

2.8. MPLS Label Downloaded to the FIB for Global and Local SIDs

The label corresponding to the global SID "Si", which is represented
by the global index "I" and downloaded to the FIB, is used to match
packets whose active segment (and hence topmost label) is "Si". The
value of this label is calculated as specified in Section 2.4.

For Local SIDs, the MCC is responsible for downloading the correct
label value to the FIB. For example, an IGP with SR extensions
[RFC8667] [RFC8665] downloads the MPLS label corresponding to an Adj-
SID [RFC8402].

2.9. Active Segment

When instantiated in the MPLS domain, the active segment on a packet
corresponds to the topmost label and is calculated according to the
procedure specified in Sections 2.10 and 2.11. When arriving at a
node, the topmost label corresponding to the active SID matches the
MPLS label downloaded to the FIB as specified in Section 2.4.

2.10. Forwarding Behavior for Global SIDs

This section specifies the forwarding behavior, including the
calculation of outgoing labels, that corresponds to a global SID when
applying the PUSH, CONTINUE, and NEXT operations in the MPLS
forwarding plane.

This document covers the calculation of the outgoing label for the
top label only. The case where the outgoing label is not the top
label and is part of a stack of labels that instantiates a routing
policy or a traffic-engineering tunnel is outside the scope of this
document and may be covered in other documents such as
[ROUTING-POLICY].

2.10.1. Forwarding for PUSH and CONTINUE of Global SIDs

Suppose an MCC on router "R0" determines that, before sending the
packet towards a neighbor "N", the PUSH or CONTINUE operation is to
be applied to an incoming packet related to the global SID "Si". SID
"Si" is represented by the global index "I" and owned by the router
Ri. Neighbor "N" may be directly connected to "R0" through either a
physical or a virtual interface (e.g., UDP tunnel [RFC7510] or L2TPv3
tunnel [RFC4817]).

The method by which the MCC on router "R0" determines that the PUSH
or CONTINUE operation must be applied using the SID "Si" is beyond
the scope of this document. An example of a method to determine the
SID "Si" for the PUSH operation is the case where IS-IS [RFC8667]
receives the Prefix-SID "Si" sub-TLV advertised with the prefix "P/m"
in TLV 135, and the prefix "P/m" is the longest matching network
prefix for the incoming IPv4 packet.

For the CONTINUE operation, an example of a method used to determine
the SID "Si" is the case where IS-IS [RFC8667] receives the Prefix-
SID "Si" sub-TLV advertised with prefix "P" in TLV 135, and the top
label of the incoming packet matches the MPLS label in the FIB
corresponding to the SID "Si" on router "R0".

The forwarding behavior for PUSH and CONTINUE corresponding to the
SID "Si" is as follows:

* If neighbor "N" does not support SR or advertises an invalid SRGB
or a SRGB that is too small for the SID "Si", then:

- If it is possible to send the packet towards neighbor "N" using
standard MPLS forwarding behavior as specified in [RFC3031] and
[RFC3032], forward the packet. The method by which a router
decides whether it is possible to send the packet to "N" or not
is beyond the scope of this document. For example, the router
"R0" can use the downstream label determined by another MCC,
such as LDP [RFC5036], to send the packet.

- Else, if there are other usable next hops, use them to forward
the incoming packet. The method by which the router "R0"
decides on the possibility of using other next hops is beyond
the scope of this document. For example, the MCC on "R0" may
chose the send an IPv4 packet without pushing any label to
another next hop.

- Otherwise, drop the packet.

* Else,

- Calculate the outgoing label as specified in Section 2.4 using
the SRGB of neighbor "N".

- Determine the outgoing label stack

o If the operation is PUSH:

+ Push the calculated label according to the MPLS label
pushing rules specified in [RFC3032].

o Else,

+ swap the incoming label with the calculated label
according to the label-swapping rules in [RFC3031].

o Send the packet towards neighbor "N".

2.10.2. Forwarding for the NEXT Operation for Global SIDs

As specified in Section 2.7.3, the NEXT operation corresponds to
popping the topmost label. The forwarding behavior is as follows:

* Pop the topmost label

* Apply the instruction associated with the incoming label that has
been popped

The action on the packet after popping the topmost label depends on
the instruction associated with the incoming label as well as the
contents of the packet right underneath the top label that was
popped. Examples of the NEXT operation are described in Appendix A.1

2.11. Forwarding Behavior for Local SIDs

This section specifies the forwarding behavior for Local SIDs when SR
is instantiated over the MPLS forwarding plane.

2.11.1. Forwarding for the PUSH Operation on Local SIDs

Suppose an MCC on router "R0" determines that the PUSH operation is
to be applied to an incoming packet using the Local SID "Si" before
sending the packet towards neighbor "N", which is directly connected
to R0 through a physical or virtual interface such as a UDP tunnel
[RFC7510] or L2TPv3 tunnel [RFC4817].

An example of such a Local SID is an Adj-SID allocated and advertised
by IS-IS [RFC8667]. The method by which the MCC on "R0" determines
that the PUSH operation is to be applied to the incoming packet is
beyond the scope of this document. An example of such a method is
the backup path used to protect against a failure using TI-LFA
[FAST-REROUTE].

As mentioned in [RFC8402], a Local SID is specified by an MPLS label.
Hence, the PUSH operation for a Local SID is identical to the label
push operation using any MPLS label [RFC3031]. The forwarding action
after pushing the MPLS label corresponding to the Local SID is also
determined by the MCC. For example, if the PUSH operation was done
to forward a packet over a backup path calculated using TI-LFA, then
the forwarding action may be sending the packet to a certain neighbor
that will in turn continue to forward the packet along the backup
path.

2.11.2. Forwarding for the CONTINUE Operation for Local SIDs

A Local SID on router "R0" corresponds to a local label. In such a
scenario, the outgoing label towards next hop "N" is determined by
the MCC running on the router "R0", and the forwarding behavior for
the CONTINUE operation is identical to the swap operation on an MPLS
label [RFC3031].

2.11.3. Outgoing Label for the NEXT Operation for Local SIDs

The NEXT operation for Local SIDs is identical to the NEXT operation
for global SIDs as specified in Section 2.10.2.

3. IANA Considerations

This document has no IANA actions.

4. Manageability Considerations

This document describes the applicability of Segment Routing over the
MPLS data plane. Segment Routing does not introduce any change in
the MPLS data plane. Manageability considerations described in
[RFC8402] apply to the MPLS data plane when used with Segment
Routing. SR Operations, Administration, and Maintenance (OAM) use
cases for the MPLS data plane are defined in [RFC8403]. SR OAM
procedures for the MPLS data plane are defined in [RFC8287].

5. Security Considerations

This document does not introduce additional security requirements and
mechanisms other than the ones described in [RFC8402].

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031,
DOI 10.17487/RFC3031, January 2001,
<https://www.rfc-editor.org/info/rfc3031>.

[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
<https://www.rfc-editor.org/info/rfc3032>.

[RFC3443] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
in Multi-Protocol Label Switching (MPLS) Networks",
RFC 3443, DOI 10.17487/RFC3443, January 2003,
<https://www.rfc-editor.org/info/rfc3443>.

[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
Class" Field", RFC 5462, DOI 10.17487/RFC5462, February
2009, <https://www.rfc-editor.org/info/rfc5462>.

[RFC7274] Kompella, K., Andersson, L., and A. Farrel, "Allocating
and Retiring Special-Purpose MPLS Labels", RFC 7274,
DOI 10.17487/RFC7274, June 2014,
<https://www.rfc-editor.org/info/rfc7274>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.

6.2. Informative References

[FAST-REROUTE]
Litkowski, S., Bashandy, A., Filsfils, C., Decraene, B.,
Francois, P., Voyer, D., Clad, F., and P. Camarillo,
"Topology Independent Fast Reroute using Segment Routing",
Work in Progress, Internet-Draft, draft-ietf-rtgwg-
segment-routing-ti-lfa-01, 5 March 2019,
<https://tools.ietf.org/html/draft-ietf-rtgwg-segment-
routing-ti-lfa-01>.

[RFC4817] Townsley, M., Pignataro, C., Wainner, S., Seely, T., and
J. Young, "Encapsulation of MPLS over Layer 2 Tunneling
Protocol Version 3", RFC 4817, DOI 10.17487/RFC4817, March
2007, <https://www.rfc-editor.org/info/rfc4817>.

[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
October 2007, <https://www.rfc-editor.org/info/rfc5036>.

[RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
Label Assignment and Context-Specific Label Space",
RFC 5331, DOI 10.17487/RFC5331, August 2008,
<https://www.rfc-editor.org/info/rfc5331>.

[RFC7510] Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black,
"Encapsulating MPLS in UDP", RFC 7510,
DOI 10.17487/RFC7510, April 2015,
<https://www.rfc-editor.org/info/rfc7510>.

[RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
Litkowski, S., Horneffer, M., and R. Shakir, "Source
Packet Routing in Networking (SPRING) Problem Statement
and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
2016, <https://www.rfc-editor.org/info/rfc7855>.

[RFC8287] Kumar, N., Ed., Pignataro, C., Ed., Swallow, G., Akiya,
N., Kini, S., and M. Chen, "Label Switched Path (LSP)
Ping/Traceroute for Segment Routing (SR) IGP-Prefix and
IGP-Adjacency Segment Identifiers (SIDs) with MPLS Data
Planes", RFC 8287, DOI 10.17487/RFC8287, December 2017,
<https://www.rfc-editor.org/info/rfc8287>.

[RFC8403] Geib, R., Ed., Filsfils, C., Pignataro, C., Ed., and N.
Kumar, "A Scalable and Topology-Aware MPLS Data-Plane
Monitoring System", RFC 8403, DOI 10.17487/RFC8403, July
2018, <https://www.rfc-editor.org/info/rfc8403>.

[RFC8661] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., and S. Litkowski, "Segment Routing MPLS
Interworking with LDP", RFC 8661, DOI 10.17487/RFC8661,
December 2019, <https://www.rfc-editor.org/info/rfC8661>.

[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.

[RFC8666] Psenak, P., Ed. and S. Previdi, Ed., "OSPFv3 Extensions
for Segment Routing", RFC 8666, DOI 10.17487/RFC8666,
December 2019, <https://www.rfc-editor.org/info/rfc8666>.

[RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
Extensions for Segment Routing", RFC 8667,
DOI 10.17487/RFC8667, December 2019,
<https://www.rfc-editor.org/info/rfc8667>.

[ROUTING-POLICY]
Filsfils, C., Sivabalan, S., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", Work in
Progress, Internet-Draft, draft-ietf-spring-segment-
routing-policy-05, 17 November 2019,
<https://tools.ietf.org/html/draft-ietf-spring-segment-
routing-policy-05>.

Appendix A. Examples

A.1. IGP Segment Examples

Consider the network diagram of Figure 1 and the IP addresses and IGP
segment allocations of Figure 2. Assume that the network is running
IS-IS with SR extensions [RFC8667], and all links have the same
metric. The following examples can be constructed.

+--------+
/ \
R0-----R1-----R2----------R3-----R8
| \ / |
| +--R4--+ |
| |
+-----R5-----+

Figure 1: IGP Segments -- Illustration

+-----------------------------------------------------------+
| IP addresses allocated by the operator: |
| 192.0.2.1/32 as a loopback of R1 |
| 192.0.2.2/32 as a loopback of R2 |
| 192.0.2.3/32 as a loopback of R3 |
| 192.0.2.4/32 as a loopback of R4 |
| 192.0.2.5/32 as a loopback of R5 |
| 192.0.2.8/32 as a loopback of R8 |
| 198.51.100.9/32 as an anycast loopback of R4 |
| 198.51.100.9/32 as an anycast loopback of R5 |
| |
| SRGB defined by the operator as [1000,5000] |
| |
| Global IGP SID indices allocated by the operator: |
| 1 allocated to 192.0.2.1/32 |
| 2 allocated to 192.0.2.2/32 |
| 3 allocated to 192.0.2.3/32 |
| 4 allocated to 192.0.2.4/32 |
| 8 allocated to 192.0.2.8/32 |
| 1009 allocated to 198.51.100.9/32 |
| |
| Local IGP SID allocated dynamically by R2 |
| for its "north" adjacency to R3: 9001 |
| for its "east" adjacency to R3 : 9002 |
| for its "south" adjacency to R3: 9003 |
| for its only adjacency to R4 : 9004 |
| for its only adjacency to R1 : 9005 |
+-----------------------------------------------------------+

Figure 2: IGP Address and Segment Allocation -- Illustration

Suppose R1 wants to send IPv4 packet P1 to R8. In this case, R1
needs to apply the PUSH operation to the IPv4 packet.

Remember that the SID index "8" is a global IGP segment attached to
the IP prefix 192.0.2.8/32. Its semantic is global within the IGP
domain: any router forwards a packet received with active segment 8
to the next hop along the ECMP-aware shortest path to the related
prefix.

R2 is the next hop along the shortest path towards R8. By applying
the steps in Section 2.8, the outgoing label downloaded to R1's FIB
corresponding to the global SID index "8" is 1008 because the SRGB of
R2 = [1000,5000] as shown in Figure 2.

Because the packet is IPv4, R1 applies the PUSH operation using the
label value 1008 as specified in Section 2.10.1. The resulting MPLS
header will have the "S" bit [RFC3032] set because it is followed
directly by an IPv4 packet.

The packet arrives at router R2. Because top label 1008 corresponds
to the IGP SID index "8", which is the Prefix-SID attached to the
prefix 192.0.2.8/32 owned by Node R8, the instruction associated with
the SID is "forward the packet using one of the ECMP interfaces or
next hops along the shortest path(s) towards R8". Because R2 is not
the penultimate hop, R2 applies the CONTINUE operation to the packet
and sends it to R3 using one of the two links connected to R3 with
top label 1008 as specified in Section 2.10.1.

R3 receives the packet with top label 1008. Because top label 1008
corresponds to the IGP SID index "8", which is the Prefix-SID
attached to the prefix 192.0.2.8/32 owned by Node R8, the instruction
associated with the SID is "send the packet using one of the ECMP
interfaces and next hops along the shortest path towards R8".
Because R3 is the penultimate hop, we assume that R3 performs
penultimate hop popping, which corresponds to the NEXT operation; the
packet is then sent to R8. The NEXT operation results in popping the
outer label and sending the packet as a pure IPv4 packet to R8.

In conclusion, the path followed by P1 is R1-R2--R3-R8. The ECMP
awareness ensures that the traffic is load-shared between any ECMP
path; in this case, it's the two links between R2 and R3.

A.2. Incoming Label Collision Examples

This section outlines several examples to illustrate the handling of
label collision described in Section 2.5.

For the examples in this section, we assume that Node A has the
following:

* OSPF default admin distance for implementation=50

* IS-IS default admin distance for implementation=60

A.2.1. Example 1

The following example illustrates incoming label collision resolution
for the same FEC type using MCC administrative distance.

FEC1:

Node A receives an OSPF Prefix-SID Advertisement from Node B for
198.51.100.5/32 with index=5. Assuming that OSPF SRGB on Node A =
[1000,1999], the incoming label is 1005.

FEC2:

IS-IS on Node A receives a Prefix-SID Advertisement from Node C for
203.0.113.105/32 with index=5. Assuming that IS-IS SRGB on Node A =
[1000,1999], the incoming label is 1005.

FEC1 and FEC2 both use dynamic SID assignment. Since neither of the
FECs are of type 'SR Policy', we use the default admin distances of
50 and 60 to break the tie. So FEC1 wins.

A.2.2. Example 2

The following example Illustrates incoming label collision resolution
for different FEC types using the MCC administrative distance.

FEC1:

Node A receives an OSPF Prefix-SID Advertisement from Node B for
198.51.100.6/32 with index=6. Assuming that OSPF SRGB on Node A =
[1000,1999], the incoming label on Node A corresponding to
198.51.100.6/32 is 1006.

FEC2:

IS-IS on Node A assigns label 1006 to the globally significant Adj-
SID (i.e., when advertised, the L-Flag is clear in the Adj-SID sub-
TLV as described in [RFC8667]). Hence, the incoming label
corresponding to this Adj-SID is 1006. Assume Node A allocates this
Adj-SID dynamically, and it may differ across router reboots.

FEC1 and FEC2 both use dynamic SID assignment. Since neither of the
FECs are of type 'SR Policy', we use the default admin distances of
50 and 60 to break the tie. So FEC1 wins.

A.2.3. Example 3

The following example illustrates incoming label collision resolution
based on preferring static over dynamic SID assignment.

FEC1:

OSPF on Node A receives a Prefix-SID Advertisement from Node B for
198.51.100.7/32 with index=7. Assuming that the OSPF SRGB on Node A
= [1000,1999], the incoming label corresponding to 198.51.100.7/32 is
1007.

FEC2:

The operator on Node A configures IS-IS on Node A to assign label
1007 to the globally significant Adj-SID (i.e., when advertised, the
L-Flag is clear in the Adj-SID sub-TLV as described in [RFC8667]).

Node A assigns this Adj-SID explicitly via configuration, so the Adj-
SID survives router reboots.

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

A.2.4. Example 4

The following example illustrates incoming label collision resolution
using FEC type default administrative distance.

FEC1:

OSPF on Node A receives a Prefix-SID Advertisement from Node B for
198.51.100.8/32 with index=8. Assuming that OSPF SRGB on Node A =
[1000,1999], the incoming label corresponding to 198.51.100.8/32 is
1008.

FEC2:

Suppose the SR Policy Advertisement from the controller to Node A for
the policy identified by (Endpoint = 192.0.2.208, color = 100) that
consists of SID-List=<S1, S2> assigns the globally significant
Binding-SID label 1008.

From the point of view of Node A, FEC1 and FEC2 both use dynamic SID
assignment. Based on the default administrative distance outlined in
Section 2.5.1, the Binding SID has a higher administrative distance
than the Prefix-SID; hence, FEC1 wins.

A.2.5. Example 5

The following example illustrates incoming label collision resolution
based on FEC type preference.

FEC1:

IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.110/32 with index=10. Assuming that the IS-IS SRGB on Node
A = [1000,1999], the incoming label corresponding to 203.0.113.110/32
is 1010.

FEC2:

IS-IS on Node A assigns label 1010 to the globally significant Adj-
SID (i.e., when advertised, the L-Flag is clear in the Adj-SID sub-
TLV as described in [RFC8667]).

Node A allocates this Adj-SID dynamically, and it may differ across
router reboots. Hence, both 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 the FEC type codepoints. FEC1 has FEC
type codepoint=120, while FEC2 has FEC type codepoint=130.
Therefore, FEC1 wins.

A.2.6. Example 6

The following example illustrates incoming label collision resolution
based on address family preference.

FEC1:

IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.111/32 with index=11. Assuming that the IS-IS SRGB on Node
A = [1000,1999], the incoming label on Node A for 203.0.113.111/32 is
1011.

FEC2:

IS-IS on Node A receives a Prefix-SID Advertisement from Node C for
2001:DB8:1000::11/128 with index=11. Assuming that the IS-IS SRGB on
Node A = [1000,1999], the incoming label on Node A for
2001:DB8:1000::11/128 is 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 the FEC type codepoints. Both FECs have FEC type
codepoint=120. So we compare the address family. Since IPv4 is
preferred over IPv6, FEC1 wins.

A.2.7. Example 7

The following example illustrates incoming label collision resolution
based on prefix length.

FEC1:

IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.112/32 with index=12. Assuming that IS-IS SRGB on Node A =
[1000,1999], the incoming label for 203.0.113.112/32 on Node A is
1012.

FEC2:

IS-IS on Node A receives a Prefix-SID Advertisement from Node C for
203.0.113.128/30 with index=12. Assuming that the IS-IS SRGB on Node
A = [1000,1999], the incoming label for 203.0.113.128/30 on Node A is
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 the FEC type codepoints. Both FECs have FEC type
codepoint=120. So we compare the address family. Both are a part of
the IPv4 address family, so we compare the prefix length. FEC1 has
prefix length=32, and FEC2 has prefix length=30, so FEC2 wins.

A.2.8. Example 8

The following example illustrates incoming label collision resolution
based on the numerical value of the FECs.

FEC1:

IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.113/32 with index=13. Assuming that IS-IS SRGB on Node A =
[1000,1999], the incoming label for 203.0.113.113/32 on Node A is
1013.

FEC2:

IS-IS on Node A receives a Prefix-SID Advertisement from Node C for
203.0.113.213/32 with index=13. Assuming that IS-IS SRGB on Node A =
[1000,1999], the incoming label for 203.0.113.213/32 on Node A is
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 the FEC type codepoints. Both FECs have FEC type
codepoint=120. So we compare the address family. Both are a part of
the IPv4 address family, so we compare the prefix length. Prefix
lengths are the same, so we compare the prefix. FEC1 has the lower
prefix, so FEC1 wins.

A.2.9. Example 9

The following example illustrates incoming label collision resolution
based on the Routing Instance ID.

FEC1:

IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.114/32 with index=14. Assume that this IS-IS instance on
Node A has Routing Instance ID = 1000 and SRGB = [1000,1999]. Hence,
the incoming label for 203.0.113.114/32 on Node A is 1014.

FEC2:

IS-IS on Node A receives a Prefix-SID Advertisement from Node C for
203.0.113.114/32 with index=14. Assume that this is another instance
of IS-IS on Node A but Routing Instance ID = 2000 is different and
SRGB = [1000,1999] is the same. Hence, the incoming label for
203.0.113.114/32 on Node A is 1014.

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

A.2.10. Example 10

The following example illustrates incoming label collision resolution
based on the topology ID.

FEC1:

IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.115/32 with index=15. Assume that this IS-IS instance on
Node A has Routing Instance ID = 1000. Assume that the prefix
advertisement of 203.0.113.115/32 was received in the IS-IS Multi-
topology advertisement with ID = 50. If the IS-IS SRGB for this
routing instance on Node A = [1000,1999], then the incoming label of
203.0.113.115/32 for topology 50 on Node A is 1015.

FEC2:

IS-IS on Node A receives a Prefix-SID Advertisement from Node C for
203.0.113.115/32 with index=15. Assume that it has the same Routing
Instance ID = 1000, but 203.0.113.115/32 was advertised with IS-IS
Multi-topology ID = 40, which is different. If the IS-IS SRGB on
Node A = [1000,1999], then the incoming label of 203.0.113.115/32 for
topology 40 on Node A is also 1015.

Since these two FECs match all the way through the prefix length,
prefix, and Routing Instance ID, we compare the IS-IS Multi-topology
ID, so FEC2 wins.

A.2.11. Example 11

The following example illustrates incoming label collision for
resolution based on the algorithm ID.

FEC1:

IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.116/32 with index=16. Assume that IS-IS on Node A has
Routing Instance ID = 1000. Assume that Node B advertised
203.0.113.116/32 with IS-IS Multi-topology ID = 50 and SR algorithm =
0. Assume that the IS-IS SRGB on Node A = [1000,1999]. Hence, the
incoming label corresponding to this advertisement of
203.0.113.116/32 is 1016.

FEC2:

IS-IS on Node A receives a Prefix-SID Advertisement from Node C for
203.0.113.116/32 with index=16. Assume that it is the same IS-IS
instance on Node A with Routing Instance ID = 1000. Also assume that
Node C advertised 203.0.113.116/32 with IS-IS Multi-topology ID = 50
but with SR algorithm = 22. Since it is the same routing instance,
the SRGB on Node A = [1000,1999]. Hence, the incoming label
corresponding to this advertisement of 203.0.113.116/32 by Node C is
also 1016.

Since these two FECs match all the way through in terms of the prefix
length, prefix, Routing Instance ID, and Multi-topology ID, we
compare the SR algorithm IDs, so FEC1 wins.

A.2.12. Example 12

The following example illustrates incoming label collision resolution
based on the FEC numerical value, independent of how the SID is
assigned to the colliding FECs.

FEC1:

IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.117/32 with index=17. Assume that the IS-IS SRGB on Node A
= [1000,1999]; thus, the incoming label is 1017.

FEC2:

Suppose there is an IS-IS Mapping Server Advertisement (SID / Label
Binding TLV) from Node D that has range = 100 and prefix =
203.0.113.1/32. Suppose this Mapping Server Advertisement generates
100 mappings, one of which maps 203.0.113.17/32 to index=17.
Assuming that it is the same IS-IS instance, the SRGB = [1000,1999]
and hence the incoming label for 1017.

Even though FEC1 comes from a normal Prefix-SID Advertisement and
FEC2 is generated from a Mapping Server Advertisement, it is not used
as a tiebreaking parameter. Both FECs use dynamic SID assignment,
are from the same MCC, and have the same FEC type codepoint=120.
Their prefix lengths are the same as well. FEC2 wins based on its
lower numerical prefix value, since 203.0.113.17 is less than
203.0.113.117.

A.2.13. Example 13

The following example illustrates incoming label collision resolution
based on address family preference.

FEC1:

SR Policy Advertisement from the controller to Node A. Endpoint
address=2001:DB8:3000::100, color=100, SID-List=<S1, S2>, and the
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>, and the Binding-SID
label=1020.

The FEC tiebreakers match, and they have the same FEC type
codepoint=140. Thus, FEC2 wins based on the IPv4 address family
being preferred over IPv6.

A.2.14. Example 14

The following example illustrates incoming label resolution based on
the numerical value of the policy endpoint.

FEC1:

SR Policy Advertisement from the controller to Node A. Endpoint
address=192.0.2.70, color=100, SID-List=<S1, S2>, and Binding-SID
label=1021.

FEC2:

SR Policy Advertisement from the controller to Node A. Endpoint
address=192.0.2.71, color=100, SID-List=<S3, S4>, and Binding-SID
label=1021.

The FEC tiebreakers match, and they have the same address family.
Thus, FEC1 wins by having the lower numerical endpoint address value.

A.3. Examples for the Effect of Incoming Label Collision on an Outgoing
Label

This section presents examples to illustrate the effect of incoming
label collision on the selection of the outgoing label as described
in Section 2.6.

A.3.1. Example 1

The following example illustrates the effect of incoming label
resolution on the outgoing label.

FEC1:

IS-IS on Node A receives a Prefix-SID Advertisement from Node B for
203.0.113.122/32 with index=22. Assuming that the IS-IS SRGB on Node
A = [1000,1999], the corresponding incoming label is 1022.

FEC2:

IS-IS on Node A receives a Prefix-SID Advertisement from Node C for
203.0.113.222/32 with index=22. Assuming that the IS-IS SRGB on Node
A = [1000,1999], the corresponding incoming label is 1022.

FEC1 wins based on the 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 to use outgoing interface 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 imposition MPLS forwarding
entry corresponding to FEC1 for incoming prefix=203.0.113.122/32
pushing outgoing label N, and using outgoing interface I.

The rule in Section 2.6 means Node A MUST NOT install an ingress MPLS
forwarding entry corresponding to FEC2 (the losing FEC, which would
be for prefix 203.0.113.222/32).

A.3.2. Example 2

The following example illustrates the effect of incoming label
collision resolution on outgoing label programming on Node A.

FEC1:

SR Policy Advertisement from the controller to Node A. Endpoint
address=192.0.2.80, color=100, SID-List=<S1, S2>, and 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>, and 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 the outgoing interface
is determined by the SID-List for FEC1.

In this example, we assume that Node A receives two BGP/VPN routes:

* R1 with VPN label=V1, BGP next hop = 192.0.2.80, and color=100

* R2 with VPN label=V2, BGP next hop = 192.0.2.81, and color=100

We also assume that Node A has a BGP policy that matches color=100
and allows its usage as Service Level Agreement (SLA) steering
information. In this case, Node A will install a VPN route with
label stack = <S1,S2,V1> (corresponding to FEC1).

The rule described in Section 2.6 means that Node A MUST NOT install
a VPN route with label stack = <S3,S4,V1> (corresponding to FEC2.)

Acknowledgements

The authors would like to thank Les Ginsberg, Chris Bowers, Himanshu
Shah, Adrian Farrel, Alexander Vainshtein, Przemyslaw Krol, Darren
Dukes, Zafar Ali, and Martin Vigoureux for their valuable comments on
this document.

Contributors

The following contributors have substantially helped the definition
and editing of the content of this document:

Martin Horneffer
Deutsche Telekom
Email: [email protected]

Wim Henderickx
Nokia
Email: [email protected]

Jeff Tantsura
Email: [email protected]

Edward Crabbe
Email: [email protected]

Igor Milojevic
Email: [email protected]

Saku Ytti
Email: [email protected]

Authors' Addresses

Ahmed Bashandy (editor)
Arrcus

Email: [email protected]

Clarence Filsfils (editor)
Cisco Systems, Inc.
Brussels
Belgium

Email: [email protected]

Stefano Previdi
Cisco Systems, Inc.
Italy

Email: [email protected]

Bruno Decraene
Orange
France

Email: [email protected]

Stephane Litkowski
Orange
France

Email: [email protected]

Rob Shakir
Google
United States of America

Email: [email protected]