wdiff rfc8402v1.txt rfc8402.txt

Internet Engineering Task Force (IETF) C. Filsfils, Ed.
Request for Comments: 8402 S. Previdi, Ed.
Category: Standards Track Cisco Systems, Inc.
ISSN: 2070-1721 L. Ginsberg
ISSN: 2070-1721 Cisco Systems, Inc Inc.
B. Decraene
S. Litkowski
Orange
R. Shakir
Google, Inc.
May
July 2018

Segment Routing Architecture

Abstract

Segment Routing (SR) leverages the source routing paradigm. A node
steers a packet through an ordered list of instructions, called
"segments". A segment can represent any instruction, topological or
service based. A segment can have a semantic local to an SR node or
global within an SR domain. SR provides a mechanism that allows to enforce a
flow through any to be restricted to a specific topological path path, while
maintaining per-flow state only at the ingress
nodes node(s) to the SR
domain.

Segment Routing

SR can be directly applied to the MPLS architecture with no change on to
the forwarding plane. A segment is encoded as an MPLS label. An
ordered list of segments is encoded as a stack of labels. The
segment to process is on the top of the stack. Upon completion of a
segment, the related label is popped from the stack.

Segment Routing

SR can be applied to the IPv6 architecture, with a new type of
routing header. A segment is encoded as an IPv6 address. An ordered
list of segments is encoded as an ordered list of IPv6 addresses in
the routing header. The active segment is indicated by the
Destination Address (DA) of the packet. The next active segment is
indicated by a pointer in the new routing header.

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/rfc8402.

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
carefully, as they describe your rights and restrictions with respect
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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 6
3. Link-State IGP Segments . . . . . . . . . . . . . . . . . . . 8 9
3.1. IGP-Prefix Segment, Prefix-SID Segment (Prefix-SID) . . . . . . . . . . . . . 8 9
3.1.1. Prefix-SID Algorithm . . . . . . . . . . . . . . . . 8 9
3.1.2. SR-MPLS . . . . . . . . . . . . . . . . . . . . . . . 9 10
3.1.3. SRv6 . . . . . . . . . . . . . . . . . . . . . . . . 11 12
3.2. IGP-Node Segment, Node-SID Segment (Node-SID) . . . . . . . . . . . . . . . 12 13
3.3. IGP-Anycast Segment, Anycast SID Segment (Anycast-SID) . . . . . . . . . . . . 12 13
3.3.1. Anycast SID Anycast-SID in SR-MPLS . . . . . . . . . . . . . . . 12 13
3.4. IGP-Adjacency Segment, Adj-SID Segment (Adj-SID) . . . . . . . . . . . . . 15
3.4.1. Parallel Adjacencies . . . . . . . . . . . . . . . . 16 17
3.4.2. LAN Adjacency Segments . . . . . . . . . . . . . . . 17 18
3.5. Inter-Area Considerations . . . . . . . . . . . . . . . . 18
4. BGP Peering Segments . . . . . . . . . . . . . . . . . . . . . . . . 19
4.1. BGP Prefix BGP-Prefix Segment . . . . . . . . . . . . . . . . . . . 19
4.2. BGP Peering Segments . . . . . . . . . . . . . . . . . . 19 20
5. Binding Segment . . . . . . . . . . . . . . . . . . . . . . . 20 21
5.1. IGP Mirroring Context Segment . . . . . . . . . . . . . . 20 21
6. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . 21 22
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 22
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21 22
8.1. SR-MPLS . . . . . . . . . . . . . . . . . . . . . . . . . 21 22
8.2. SRv6 . . . . . . . . . . . . . . . . . . . . . . . . . . 23 24
8.3. Congestion Control . . . . . . . . . . . . . . . . . . . 24 25
9. Manageability Considerations . . . . . . . . . . . . . . . . 24 25
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 25 26
10.1. Normative References . . . . . . . . . . . . . . . . . . 25 26
10.2. Informative References . . . . . . . . . . . . . . . . . 26 27
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 29 30
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 29 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30 32

1. Introduction

Segment Routing (SR) leverages the source routing paradigm. A node
steers a packet through an SR Policy instantiated as an ordered list
of instructions called "segments". A segment can represent any
instruction, topological or service based. A segment can have a
semantic local to an SR node or global within an SR domain. SR
supports per-flow explicit routing while maintaining per-flow state
only at the ingress nodes to the SR domain.

A segment is often referred to by its Segment Identifier (SID).

A segment may be associated with a topological instruction. A
topological local segment may instruct a node to forward the packet
via a specific outgoing interface. A topological global segment may
instruct an SR domain to forward the packet via a specific path to a
destination. Different segments may exist for the same destination,
each with different path objectives (e.g., which metric is minimized,
what constraints are specified).

A segment may be associated with a service instruction (e.g., the
packet should be processed by a container or Virtual Machine (VM)
associated with the segment). A segment may be associated with a QoS
treatment (e.g., shape the packets received with this segment at x
Mbps).

The SR architecture supports any type of instruction associated with
a segment.

The SR architecture supports any type of control-plane: control plane: distributed,
centralized, or hybrid.

In a distributed scenario, the segments are allocated and signaled by
IS-IS or OSPF or BGP. A node individually decides to steer packets
on a source-routed policy an SR Policy (e.g., pre-computed local protection [RFC8355]). A
node individually computes the source-routed policy. SR Policy.

In a centralized scenario, the segments are allocated and
instantiated by an SR controller. The SR controller decides which
nodes need to steer which packets on which source-routed policies.
The SR controller computes the source-routed policies. The SR
architecture does not restrict how the controller programs the
network. Likely options are Network Configuration Protocol
(NETCONF), Path Computation Element Communication Protocol (PCEP),
and BGP. The SR architecture does not restrict the number of SR
controllers. Specifically, multiple SR controllers may program the
same SR domain. The SR architecture allows these SR controllers to
discover which SID's SIDs are instantiated at which nodes and which sets of
local (SRLB) and global (SRGB) labels are available at which node.

A hybrid scenario complements a base distributed control-plane control plane with a
centralized controller. For example, when the destination is outside
the IGP domain, the SR controller may compute a source-routed policy an SR Policy on behalf
of an IGP node. The SR architecture does not restrict how the nodes
that are part of the distributed control-plane control plane interact with the SR
controller. Likely options are PCEP and BGP.

Hosts MAY be part of an SR Domain. domain. A centralized controller can
inform hosts about policies either by pushing these policies to hosts
or by responding to requests from hosts.

The SR architecture can be instantiated on various dataplanes. data planes. This
document introduces two dataplane data-plane instantiations of SR: SR over MPLS
(SR-MPLS) and SR over IPv6 (SRv6).

Segment Routing

SR can be directly applied to the MPLS architecture with no change on to
the forwarding plane [SR-MPLS-DP]. [SR-MPLS]. A segment is encoded as an MPLS
label. An SR Policy is instantiated as a stack of labels. The
segment to process (the active segment) is on the top of the stack.
Upon completion of a segment, the related label is popped from the
stack.

Segment Routing

SR can be applied to the IPv6 architecture with a new type of routing
header called the SR Header (SRH) [IPv6-SRH]. An instruction is
associated with a segment and encoded as an IPv6 address. An SRv6
segment is also called an SRv6 SID. An SR Policy is instantiated as
an ordered list of SRv6 SID's SIDs in the routing header. The active
segment is indicated by the DA Destination Address (DA) of the packet.
The next active segment is indicated by the SegmentsLeft (SL) pointer
in the SRH. When an SRv6 SID is completed, the SL is decremented and
the next segment is copied to the DA. When a packet is steered on an
SR policy, Policy, the related SRH is added to the packet.

In the context of an IGP-based distributed control-plane, control plane, two
topological segments are defined: the IGP adjacency IGP-Adjacency segment and the
IGP prefix
IGP-Prefix segment.

In the context of a BGP-based distributed control-plane, control plane, two
topological segments are defined: the BGP peering segment and the BGP
prefix
BGP-Prefix segment.

The headend of an SR Policy binds a SID (called a Binding segment or
BSID) to its policy. When the headend receives a packet with active
segment matching the BSID of a local SR Policy, the headend steers
the packet into the associated SR Policy.

This document defines the IGP, BGP, and Binding segments for the SR-
MPLS
SR-MPLS and SRv6 dataplanes. data planes.

Note: This document defines the architecture for Segment Routing,
including definitions of basic objects and functions and a
description of the overall design. It does NOT define the means of
implementing the architecture -- that is contained in numerous
referenced documents, some of which are mentioned in this document as
a convenience to the reader.

2. Terminology

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.

SR-MPLS: the instantiation of SR on the MPLS dataplane data plane.

SRv6: the instantiation of SR on the IPv6 dataplane. data plane.

Segment: an instruction a node executes on the incoming packet (e.g.,
forward packet according to shortest path to destination, or, forward
packet through a specific interface, or, deliver the packet to a
given application/service instance).

SID: a segment identifier. Note that the term SID is commonly used
in place of the term Segment, "Segment", though this is technically imprecise
as it overlooks any necessary translation.

SR-MPLS SID: an MPLS label or an index value into an MPLS label space
explicitly associated with the segment.

SRv6 SID: an IPv6 address explicitly associated with the segment.

Segment Routing Domain domain (SR Domain): domain): the set of nodes participating in
the source-based routing model. These nodes may be connected to the
same physical infrastructure (e.g., a Service Provider's network).
They may as well be remotely connected to each other (e.g., an
enterprise VPN or an overlay). If multiple protocol instances are
deployed, the SR domain most commonly includes all of the protocol
instances in a network. However, some deployments may wish to
subdivide the network into multiple SR domains, each of which
includes one or more protocol instances. It is expected that all
nodes in an SR Domain domain are managed by the same administrative entity.

Active Segment: the segment that is used by the receiving router to
process the packet. In the MPLS dataplane, data plane, it is the top label. In
the IPv6 dataplane, data plane, it is the Destination Address destination address [IPv6-SRH].

PUSH: the instruction operation consisting of the insertion of a segment at the
top of the segment list. In SR-MPLS, the top of the segment list is
the topmost (outer) label of the label stack. In SRv6, the top of
the segment list is represented by the first segment in the Segment
Routing Header as defined in [IPv6-SRH].

NEXT: when the active segment is completed, NEXT is the instruction operation
consisting of the inspection of the next segment. The next segment
becomes active. In SR-MPLS, NEXT is implemented as a POP of the top
label. In SRv6, NEXT is implemented as the copy of the next segment
from the SRH to the Destination Address destination address of the IPv6 header.

CONTINUE: the active segment is not completed; hence, it remains
active. In SR-MPLS, the CONTINUE instruction operation is implemented as a SWAP
of the top label [RFC3031]. In SRv6, this is the plain IPv6
forwarding action of a regular IPv6 packet according to its
Destination Address.
destination address.

SR Global Block (SRGB): the set of global segments in the SR Domain. domain.
If a node participates in multiple SR domains, there is one SRGB for
each SR domain. In SR-MPLS, SRGB is a local property of a node and
identifies the set of local labels reserved for global segments. In
SR-MPLS, using identical SRGBs on all nodes within the SR Domain domain is
strongly recommended. Doing so eases operations and troubleshooting
as the same label represents the same global segment at each node.
In SRv6, the SRGB is the set of global SRv6 SIDs in the SR Domain. domain.

SR Local Block (SRLB): local property of an SR node. If a node
participates in multiple SR domains, there is one SRLB for each SR
domain. In SR-MPLS, SRLB is a set of local labels reserved for local
segments. In SRv6, SRLB is a set of local IPv6 addresses reserved
for local SRv6 SID's. SIDs. In a controller-driven network, some
controllers or applications may use the control plane to discover the
available set of local segments.

Global Segment: a segment that is part of the SRGB of the domain.
The instruction associated to with the segment is defined at the SR Domain
domain level. A topological shortest-path segment to a given
destination within an SR domain is a typical example of a global
segment.

Local Segment: In SR-MPLS, this is a local label outside the SRGB.
It may be part of the explicitly advertised SRLB. In SRv6, this can
be any IPv6 address, i.e., the address may be part of the SRGB, but
used such that it has local significance. The instruction associated
to
with the segment is defined at the node level.

IGP Segment: the generic name for a segment attached to a piece of
information advertised by a link-state IGP, e.g., an IGP prefix or an
IGP adjacency.

IGP-Prefix Segment: an IGP-Prefix Segment segment is an IGP Segment segment
representing an IGP prefix. When an IGP-Prefix Segment segment is global
within the SR IGP instance/topology, it identifies an instruction to
forward the packet along the path computed using the routing
algorithm specified in the algorithm field, in the topology, and in
the IGP instance where it is advertised. Also referred to as "Prefix
Segment".

Prefix SID: "prefix
segment".

Prefix-SID: the SID of the IGP-Prefix Segment. segment.

IGP-Anycast Segment: an IGP-Anycast Segment segment is an IGP-Prefix Segment segment
that identifies an anycast prefix advertised by a set of routers.

Anycast-SID: the SID of the IGP-Anycast Segment. segment.

IGP-Adjacency Segment: an IGP-Adjacency Segment segment is an IGP Segment segment
attached to a unidirectional adjacency or a set of unidirectional
adjacencies. By default, an IGP-Adjacency Segment segment is local (unless
explicitly advertised otherwise) to the node that advertises it.
Also referred to as "Adjacency Segment". "Adj-SID".

Adj-SID: the SID of the IGP-Adjacency Segment. segment.

IGP-Node Segment: an IGP-Node Segment segment is an IGP-Prefix Segment segment that
identifies a specific router (e.g., a loopback). Also referred to as
"Node Segment".

Node-SID: the SID of the IGP-Node Segment. segment.

SR Policy: an ordered list of segments. The headend of an SR Policy
steers packets onto the SR policy. Policy. The list of segments can be
specified explicitly in SR-MPLS as a stack of labels and in SRv6 as
an ordered list of SRv6 SID's. SIDs. Alternatively, the list of segments is
computed based on a destination and a set of optimization objective
and constraints (e.g., latency, affinity, SRLG, etc.). The
computation can be local or delegated to a PCE server. An SR policy Policy
can be configured by the operator, provisioned via NETCONF [RFC6241]
or provisioned via PCEP [RFC5440]. An SR policy Policy can be used for
traffic-engineering,
Traffic Engineering (TE), Operations, Administration, and Maintenance
(OAM), or Fast Reroute (FRR) reasons.

Segment List Depth: the number of segments of an SR policy. Policy. The
entity instantiating an SR Policy at a node N should be able to
discover the depth-insertion capability of the node N. For example,
the PCEP SR capability advertisement described in [PCEP-SR-EXT] is
one means of discovering this capability.

Forwarding Information Base (FIB): the forwarding table of a node

3. Link-State IGP Segments

Within an SR domain, an SR-capable IGP node advertises segments for
its attached prefixes and adjacencies. These segments are called
"IGP segments" or "IGP SIDs". They play a key role in Segment
Routing and use-cases use cases as they enable the expression of any path
throughout the SR domain. Such a path is either expressed as a
single IGP segment or a list of multiple IGP segments.

Advertisement of IGP segments requires extensions in link-state IGP
protocols. These extensions are defined in [ISIS-SR-EXT],
[OSPF-SR-EXT], and [OSPFv3-SR-EXT].

3.1. IGP-Prefix Segment, Prefix-SID Segment (Prefix-SID)

An IGP-Prefix segment is an IGP segment attached to an IGP prefix.
An IGP-Prefix segment is global (unless explicitly advertised
otherwise) within the SR domain. The context for an IGP-Prefix
segment includes the prefix, topology, and algorithm. Multiple SIDs
MAY be allocated to the same prefix so long as the tuple <prefix,
topology, algorithm> is unique.

Multiple instances and topologies are defined in IS-IS and OSPF in:
[RFC5120], [RFC8202], [RFC6549], and [RFC4915].

3.1.1. Prefix-SID Algorithm

Segment Routing supports the use of multiple routing algorithms, i.e,
different constraint-based shortest-path calculations can be
supported. An algorithm identifier is included as part of a Prefix-
SID advertisement. Specification of how an algorithm-specific path
calculation is done is required in the document defining the
algorithm.

This document defines two algorithms:

o "Shortest Path": Shortest Path First: this algorithm is the default behavior. The
packet is forwarded along the well-known well known ECMP-aware Shortest Path
First (SPF) algorithm employed by the IGPs. However, it is
explicitly allowed for a midpoint to implement another forwarding
based on local policy. The "Shortest Path" Shortest Path First algorithm is, in
fact, the default and current behavior of most of the networks
where local policies may override the SPF decision.

o "Strict Strict Shortest Path (Strict-SPF)": First (Strict-SPF): This algorithm mandates
that the packet be forwarded according to the ECMP-aware SPF
algorithm and instructs any router in the path to ignore any
possible local policy overriding the SPF decision. The SID
advertised with the Strict-SPF algorithm ensures that the path the
packet is going to take is the expected, and not altered, SPF
path. Note that Fast Reroute (FRR) [RFC5714] mechanisms are still
compliant with the Strict Shortest Path. Path First algorithm. In other
words, a packet received with a Strict-SPF SID may be rerouted
through an FRR mechanism. Strict-
SPF Strict-SPF uses the same topology used
by "Shortest Path". the Shortest Path First algorithm. Obviously, nodes that do
not support Strict-SPF will not install forwarding entries for
this algorithm. Restricting the topology only to those nodes that
support this algorithm will not produce the desired forwarding
paths since the desired behavior is to follow the path calculated
by "Shortest Path". the Shortest Path First algorithm. Therefore, a source SR node
MUST NOT use a source-routing policy an SR Policy containing a strict SPF segment if the
path crosses a node not supporting the Strict-SPF algorithm.

An IGP-Prefix Segment segment identifies the path, to the related prefix,
computed as per the associated algorithm. A packet injected anywhere
within the SR domain with an active Prefix-SID is expected to be
forwarded along a path computed using the specified algorithm. For
this to be possible, a fully connected topology of routers supporting
the specified algorithm is required.

3.1.2. SR-MPLS

When SR is used over the MPLS dataplane, data plane, SIDs are an MPLS label or
an index into an MPLS label space (either SRGB or SRLB).

Where possible, it is recommended that identical SRGBs be configured
on all nodes in an SR Domain. domain. This simplifies troubleshooting as the
same label will be associated with the same prefix on all nodes. In
addition, it simplifies support for anycast as detailed in
Section 3.3.

The following behaviors are associated with SR operating over the
MPLS dataplane: data plane:

o The IGP signaling extension for IGP-Prefix segment includes a flag
to indicate whether directly connected neighbors of the node on
which the prefix is attached should perform the NEXT operation or
the CONTINUE operation when processing the SID. This behavior is
equivalent to Penultimate Hop Popping (NEXT) or Ultimate Hop
Popping (CONTINUE) in MPLS.

o A Prefix-SID is allocated in the form of an MPLS label (or an
index in the SRGB) according to a process similar to IP address
allocation. Typically, the Prefix-SID is allocated by policy by
the operator (or Network Management System (NMS)), and the SID
very rarely changes.

o While SR allows to attach a local segment to be attached to an IGP prefix, it is
specifically assumed that when
where the terms terminology "IGP-Prefix Segment" and segment" or "Prefix-SID" are is
used, the segment is assumed to be global (the (i.e., the SID is allocated
defined from the SRGB or as an index into the advertised SRGB). This is consistent with all
the described use-cases use cases that require global segments attached to
IGP prefixes.

o The allocation process MUST NOT allocate the same Prefix-SID to
different IP prefixes.

o If a node learns of a Prefix-SID that has a value that falls
outside the locally configured SRGB range, then the node MUST NOT
use the Prefix-SID and SHOULD issue an error log reporting a
misconfiguration.

o If a node N advertises Prefix-SID SID-R for a prefix R that is
attached to N, and if N and specifies CONTINUE as the operation to be
performed by directly connected neighbors, then N MUST maintain
the following FIB entry:

Incoming Active Segment: SID-R
Ingress Operation: NEXT
Egress interface: NULL

o A remote node M MUST maintain the following FIB entry for any
learned Prefix-SID SID-R attached to IP prefix R:

Incoming Active Segment: SID-R
Ingress Operation:
If the next-hop of R is the originator of R
and is M has been instructed to remove the active segment: NEXT
Else: CONTINUE
Egress interface: the interface interface(s) towards the next-hop along the
path computed using the algorithm advertised with
the SID toward prefix R.

As Prefix-SIDs are specific to a given algorithm, if traffic
associated with an algorithm arrives at a node that does not support
that algorithm, the traffic will be dropped as there will be no
forwarding entry matching the incoming label.

3.1.3. SRv6

When SR is used over the IPv6 dataplane: data plane:

o A Prefix-SID is an IPv6 address.

o An operator MUST explicitly instantiate an SRv6 SID. IPv6 node
addresses are not SRv6 SIDs by default.

A node N advertising an IPv6 address R usable as a segment identifier
MUST maintain the following FIB entry:

Incoming Active Segment: R
Ingress Operation: NEXT
Egress interface: NULL

Note that forwarding to R does not require an entry in the FIBs of
all other routers for R. Forwarding can be, and most often will be,
achieved by a shorter mask prefix that covers R.

Independent of SR support, any remote IPv6 node will maintain a plain
IPv6 FIB entry for any prefix, no matter if the prefix represents a
segment or not. This allows forwarding of packets to the node that
owns the SID even by nodes that do not support SR.

Support of multiple algorithms applies to SRv6. Since algorithm-
specific SIDs are simply IPv6 addresses, algorithm-specific
forwarding entries can be achieved by assigning algorithm-specific
subnets to the (set of) algorithm specific SIDs that a node
allocates.

Nodes that do not support a given algorithm may still have a FIB
entry covering an algorithm-specific address even though an
algorithm-specific path has not been calculated by that node. This
is mitigated by the fact that nodes that do not support a given
algorithm will not be included in the topology associated with that
algorithm-specific SPF; therefore, traffic using the algorithm-
specific destination will normally not flow via the excluded node.
If such traffic were to arrive and be forwarded by such a node, it
will still progress towards the destination node. The nexthop next-hop will
be either a node that supports the algorithm -- in which case, the
packet will be forwarded along algorithm-specific paths (or be
dropped if none are available) -- or a node that does NOT support the
algorithm -- in which case, the packet will continue to be forwarded
along Algorithm 0 paths towards the destination node.

3.2. IGP-Node Segment, Node-SID Segment (Node-SID)

An IGP Node-SID MUST NOT be associated with a prefix that is owned by
more than one router within the same routing domain.

3.3. IGP-Anycast Segment, Anycast SID Segment (Anycast-SID)

An "Anycast Segment" Anycast segment or "Anycast SID" Anycast-SID enforces the ECMP-aware
shortest-path shortest-
path forwarding towards the closest node of the anycast set. This is
useful to express macro-engineering policies or protection
mechanisms.

An IGP-Anycast segment MUST NOT reference a particular node.

Within an anycast group, all routers in an SR domain MUST advertise
the same prefix with the same SID value.

3.3.1. Anycast SID Anycast-SID in SR-MPLS

+--------------+
| Group A |
|192.0.2.10/32 |
| SID:100 |
| |
+-----------A1---A3----------+
| | | \ / | | |
SID:10 | | | / | | | SID:30
203.0.113.1/32 | | | / \ | | | 203.0.113.3/32
PE1------R1----------A2---A4---------R3------PE3
\ /| | | |\ /
\ / | +--------------+ | \ /
\ / | | \ /
/ | | /
/ \ | | / \
/ \ | +--------------+ | / \
/ \| | | |/ \
PE2------R2----------B1---B3---------R4------PE4
203.0.113.2/32 | | | \ / | | | 203.0.113.4/32
SID:20 | | | / | | | SID:40
| | | / \ | | |
+-----------B2---B4----------+
| |
| Group B |
| 192.0.2.1/32 |
| SID:200 |
+--------------+

Figure 1: Transit device groups Device Groups
The Figure 1 illustrates a network example with two groups of transit
devices. Group A consists of devices {A1, A2, A3, and A4}. They are
all provisioned with the anycast address 192.0.2.10/32 and the
anycast SID
Anycast-SID 100.

Similarly, Group B consists of devices {B1, B2, B3, and B4}, and they
are all provisioned with the anycast address 192.0.2.1/32 and the
anycast SID
Anycast-SID 200. In the above network topology, each Provide Edge
(PE) device has a path to each of the groups: A and B.

PE1 can choose a particular transit device group when sending traffic
to PE3 or PE4. This will be done by pushing the anycast SID Anycast-SID of the
group in the stack.

Processing the anycast, and subsequent segments, requires special
care.

+-------------------------+
| Group A |
| 192.0.2.10/32 |
| SID:100 |
|-------------------------|
| |
| SRGB: SRGB: |
SID:10 |(1000-2000) (3000-4000)| SID:30
PE1---+ +-------A1-------------A3-------+ +---PE3
\ / | | \ / | | \ /
\ / | | +-----+ / | | \ /
SRGB: \ / | | \ / | | \ / SRGB:
(7000-8000) R1 | | \ | | R3 (6000-7000)
/ \ | | / \ | | / \
/ \ | | +-----+ \ | | / \
/ \ | | / \ | | / \
PE2---+ +-------A2-------------A4-------+ +---PE4
SID:20 | SRGB: SRGB: | SID:40
|(2000-3000) (4000-5000)|
| |
+-------------------------+

Figure 2: Transit paths Paths via anycast group Anycast Group A
Considering an MPLS deployment, in the above topology, if device PE1
(or PE2) requires the sending of a packet to the device PE3 (or PE4),
it needs to encapsulate the packet in an MPLS payload with the
following stack of labels.

o Label allocated by R1 for anycast SID Anycast-SID 100 (outer label).

o Label allocated by the nearest router in Group A for SID 30 (for
destination PE3).

While

In this case, the first label is easy to compute, in this case, since compute. However, because
there
are is more than one device that is topologically nearest devices (A1 and
A2), determining the second label is impossible unless A1 and A2
allocated the same label value to the same prefix,
determining the second label is impossible. prefix. Devices A1 and A2
may be devices from different hardware vendors. If both don't
allocate the same label value for SID 30, it is impossible to use the
anycast Group A as a transit anycast group towards PE3. Hence, PE1
(or PE2) cannot compute an appropriate label stack to steer the
packet exclusively through the Group A devices. Same holds true for
devices PE3 and PE4 when trying to send a packet to PE1 or PE2.

To ease the use of an anycast segment, it is recommended to configure
identical SRGBs on all nodes of a particular anycast group. Using
this method, as mentioned above, computation of the label following
the anycast segment is straightforward.

Using an anycast segment without configuring identical SRGBs on all
nodes belonging to the same device anycast group may lead to misrouting (in
an MPLS VPN deployment, some traffic may leak between VPNs).

3.4. IGP-Adjacency Segment, Adj-SID Segment (Adj-SID)

The adjacency is formed by the local node (i.e., the node advertising
the adjacency in the IGP) and the remote node (i.e., the other end of
the adjacency). The local node MUST be an IGP node. The remote node
may be an adjacent IGP neighbor or a non-adjacent neighbor (e.g., a
forwarding adjacency, [RFC4206]).

A packet injected anywhere within the SR domain with a segment list
{SN, SNL} where SN is the Node-SID of node N and SNL is an Adj-SID
attached by node N to its adjacency over link L will be forwarded
along the shortest path to N and then be switched by N, without any
IP shortest-path consideration, towards link L. If the Adj-SID
identifies a set of adjacencies, then the node N load-balances the
traffic among the various members of the set.

Similarly, when using a global Adj-SID, a packet injected anywhere
within the SR domain with a segment list {SNL}, where SNL is a global
Adj-SID attached by node N to its adjacency over link L, will be
forwarded along the shortest path to N and then be switched by N,
without any IP shortest-path consideration, towards link L. If the
Adj-SID identifies a set of adjacencies, then the node N does load-
balance the traffic among the various members of the set. The use of
global Adj-SID allows to reduce the size of the segment list when
expressing a path at the cost of additional state (i.e., the global
Adj-SID will be inserted by all routers within the area in their
forwarding table).

An "IGP Adjacency Segment" "IGP-Adjacency segment" or "Adj-SID" enforces the switching of the
packet from a node towards a defined interface or set of interfaces.
This is key to theoretically prove that any path can be expressed as
a list of segments.

The encodings of the Adj-SID include a set of flags supporting the
following functionalities:

o Eligible for Protection (e.g., using IPFRR or MPLS-FRR).
Protection allows that in the event the interface(s) associated
with the Adj-SID are down, that the packet can still be forwarded
via an alternate path. The use of protection is clearly a policy-
based decision; that is, for a given policy protection may or may
not be desirable.

o Indication whether the Adj-SID has local or global scope. Default
scope SHOULD be local.

o Indication whether the Adj-SID is persistent across control plane
restarts. Persistence is a key attribute in ensuring that an SR
Policy does not temporarily result in misforwarding due to
reassignment of an Adj-SID.

A weight (as described below) is also associated with the Adj-SID
advertisement.

A node SHOULD allocate one Adj-SID for each of its adjacencies.

A node MAY allocate multiple Adj-SIDs for the same adjacency. An
example is to support an Adj-SID that is eligible for protection and
an Adj-SID that is NOT eligible for protection.

A node MAY associate the same Adj-SID to multiple adjacencies.

In order to be able to advertise in the IGP all the Adj-SIDs
representing the IGP adjacencies between two nodes, parallel
adjacency suppression MUST NOT be performed by the IGP.

When a node binds an Adj-SID V to a local data-link L, the node MUST
install the following FIB entry:

Incoming Active Segment: V
Ingress Operation: NEXT
Egress Interface: L

The Adj-SID implies, from the router advertising it, the forwarding
of the packet through the adjacency or adjacencies identified by the
Adj-SID, regardless of its IGP/SPF cost. In other words, the use of
adjacency segments overrides the routing decision made by the SPF
algorithm.

3.4.1. Parallel Adjacencies

Adj-SIDs can be used in order to represent a set of parallel
interfaces between two adjacent routers.

A node MUST install a FIB entry for any locally originated adjacency
segment (Adj-SID) Adj-SID of
value W attached to a set of links B with:

Incoming Active Segment: W
Ingress Operation: NEXT
Egress interface: interfaces: load-balance between any data-link within set B

When parallel adjacencies are used and associated to with the same Adj-
SID, and, in order to optimize the load-balancing function, a
"weight" factor can be associated to with the Adj-SID advertised with
each adjacency. The weight tells the ingress (or an SDN/orchestration SDN/
orchestration system) about the load-balancing factor over the
parallel adjacencies. As shown in Figure 3, A and B are connected
through two parallel adjacencies

link-1

Link-1
+--------+
| |
S---A B---C
| |
+--------+
link-2
Link-2

Figure 3: Parallel Links and Adj-SIDs
Node A advertises following Adj-SIDs and weights:

o Link-1: Adj-SID 1000, weight: 1

o Link-2: Adj-SID 1000, weight: 2

Node S receives the advertisements of the parallel adjacencies and
understands that by using Adj-SID 1000 node A will load-balance the
traffic across the parallel links (link-1 (Link-1 and link-2) Link-2) according to a
1:2 ratio i.e., twice as many packets will flow over Link-2 as
compared to Link-1.

3.4.2. LAN Adjacency Segments

In LAN subnetworks, link-state protocols define the concept of
Designated Router (DR, in OSPF) or Designated Intermediate System
(DIS, in IS-IS) that conduct flooding in broadcast subnetworks and
that describe the LAN topology in a special routing update (OSPF
Type2 LSA or IS-IS Pseudonode LSP).

The difficulty with LANs is that each router only advertises its
connectivity to the DR/DIS and not to each of the individual nodes in
the LAN. Therefore, additional protocol mechanisms (IS-IS and OSPF)
are necessary in order for each router in the LAN to advertise an
Adj-SID associated to with each neighbor in the LAN.

3.5. Inter-Area Considerations

In the following example diagram, it is assumed that the all areas
are part of a single SR Domain. domain.

The Figure 4 assumes the IPv6 control plane with the MPLS dataplane. data plane.

! !
! !
B------C-----F----G-----K
/ | | |
S---A/ | | |
\ | | |
\D------I----------J-----L----Z (2001:DB8::2:1/128, Node-SID 150)
! !
Area-1
Area 1 ! Backbone ! Area 2
! area !

Figure 4: Inter-Area Topology Example
In Area 2, node Z allocates Node-SID 150 to his local IPv6 prefix
2001:DB8::2:1/128.

Area Border Routers (ABRs) G and J will propagate the prefix and its
SIDs into the backbone area by creating a new instance of the prefix
according to normal inter-area/level IGP propagation rules.

Nodes C and I will apply the same behavior when leaking prefixes from
the backbone area down to area 1. Therefore, node S will see prefix
2001:DB8::2:1/128 with Prefix-SID 150 and advertised by nodes C and
I.

Therefore, the result is that a Prefix-SID remains attached to its
related IGP Prefix prefix through the inter-area process, which is the
expected behavior in a single SR Domain. domain.

When node S sends traffic to 2001:DB8::2:1/128, it pushes Node-
SID(150) as an active segment and forwards it to A.

When a packet arrives at ABR I (or C), the ABR forwards the packet
according to the active segment (Node-SID(150)). Forwarding
continues across area borders, using the same Node-SID(150) until the
packet reaches its destination.

4. BGP Peering Segments

BGP segments may be allocated and distributed by BGP.

4.1. BGP Prefix BGP-Prefix Segment

A BGP-Prefix segment is a BGP segment attached to a BGP prefix.

A BGP-Prefix segment is global (unless explicitly advertised
otherwise) within the SR domain.

The BGP Prefix SID BGP-Prefix segment is the BGP equivalent to the IGP Prefix Segment. IGP-Prefix
segment.

A likely use-case use case for the BGP Prefix Segment BGP-Prefix segment is an IGP-free hyper-
scale spine-leaf topology where connectivity is learned solely via
BGP [RFC7938]

4.2. BGP Peering Segments

In the context of BGP Egress Peer Engineering (EPE), as described in
[SR-CENTRAL-EPE], an EPE-enabled Egress PE egress node MAY advertise segments
corresponding to its attached peers. These segments are called BGP
peering segments or BGP peering SIDs. They enable the expression of
source-routed inter-domain paths.

An ingress border router of an Autonomous System (AS) may compose a
list of segments to steer a flow along a selected path within the AS
towards a selected egress border router C of the AS and through a
specific peer. At a minimum, a BGP peering engineering policy
applied at an ingress PE node involves two segments: the Node SID Node-SID of the
chosen egress PE node and the BGP peering segment for the chosen egress PE
node peer or peering interface.

Three types of BGP peering segments/SIDs are defined: PeerNode SID,
PeerAdj SID, and PeerSet SID.

o PeerNode SID: a BGP PeerNode segment/SID is a local segment. At
the BGP node advertising it, its semantics are:

* SRH SR operation: NEXT.

* Next-Hop: the connected peering node to which the segment is
related.

o PeerAdj SID: a BGP PeerAdj segment/SID is a local segment. At the
BGP node advertising it, the semantics are:

* SRH SR operation: NEXT.

* Next-Hop: the peer connected through the interface to which the
segment is related.

o PeerSet SID: a BGP PeerSet segment/SID is a local segment. At the
BGP node advertising it, the semantics are:

* SRH SR operation: NEXT.

* Next-Hop: load-balance across any connected interface to any
peer in the related group.

A peer set could be all the connected peers from the same AS or a
subset of these. A group could also span across AS. The group
definition is a policy set by the operator.

The BGP extensions necessary in order to signal these BGP peering
segments are defined in [BGPLS-SR-EPE].

5. Binding Segment

In order to provide greater scalability, network opacity, and service
independence, SR utilizes a Binding SID (BSID). The BSID is bound to
an SR policy, Policy, instantiation of which may involve a list of SIDs. Any
packets received with an active segment equal to BSID are steered
onto the bound SR Policy.

A BSID may be either a local or a global SID. If local, a BSID
SHOULD be allocated from the SRLB. If global, a BSID MUST be
allocated from the SRGB.

Use of a BSID allows the instantiation of the policy (the SID list)
to be stored only on the node or nodes that need to impose the
policy. Direction of traffic to a node supporting the policy then
only requires imposition of the BSID. If the policy changes, this
also means that only the nodes imposing the policy need to be
updated. Users of the policy are not impacted.

5.1. IGP Mirroring Context Segment

One use case for a Binding Segment segment is to provide support for an IGP
node to advertise its ability to process traffic originally destined
to another IGP node, called the "mirrored node" and identified by an
IP address or a Node-SID, provided that a "Mirroring Context" Mirroring Context segment
is inserted in the segment list prior to any service segment local to
the mirrored node.

When a given node B wants to provide egress node A protection, it
advertises a segment identifying node's A context. Such a segment is
called "Mirror "Mirroring Context Segment" segment" and is identified by the Mirror
SID.

The Mirror SID is advertised using the binding Binding segment defined in SR
IGP protocol extensions [ISIS-SR-EXT].

In the event of a failure, a Point of Local Repair (PLR) diverting
traffic from A to B does a PUSH of the Mirror SID on the protected
traffic. When receiving the traffic with the Mirror SID as the
active segment, B uses that segment and processes underlying segments
in the context of A.

6. Multicast

Segment Routing is defined for unicast. The application of the
source-route concept to Multicast is not in the scope of this
document.

7. IANA Considerations

This document has no IANA actions.

8. Security Considerations

Segment Routing is applicable to both MPLS and IPv6 data planes.

SR adds some metadata (instructions) to the packet, with the list of
forwarding path elements (e.g., nodes, links, services, etc.) that
the packet must traverse. It has to be noted that the complete
source-routed path may be represented by a single segment. This is
the case of the Binding SID.

By default, SR operates within a trusted domain. Traffic MUST be
filtered at the domain boundaries.

The use of best practices to reduce the risk of tampering within the
trusted domain is important. Such practices are discussed in
[RFC4381] and are applicable to both SR-MPLS and SRv6.

8.1. SR-MPLS

When applied to the MPLS data plane, SR does not introduce any new
behavior or any change in the way the MPLS data plane works.
Therefore, from a security standpoint, this document does not define
any additional mechanism in the MPLS data plane.

SR allows the expression of a source-routed path using a single
segment (the Binding SID). Compared to RSVP-TE, which also provides
explicit routing capability, there are no fundamental differences in
terms of information provided. Both RSVP-TE and Segment Routing may
express a source-routed path using a single segment.

When a path is expressed using a single label, the syntax of the
metadata is equivalent between RSVP-TE [RFC3209] and SR.

When a source-routed path is expressed with a list of segments,
additional metadata is added to the packet consisting of the source-
routed path the packet must follow expressed as a segment list.

When a path is expressed using a label stack, if one has access to
the meaning (i.e., the Forwarding Equivalence Class) of the labels,
one has the knowledge of the explicit path. For the MPLS data plane,
as no data-plane modification is required, there is no fundamental
change of capability. Yet, the occurrence of label stacking will
increase.

SR domain boundary routers MUST filter any external traffic destined
to a label associated with a segment within the trusted domain. This
includes labels within the SRGB of the trusted domain, labels within
the SRLB of the specific boundary router, and labels outside either
of these blocks. External traffic is any traffic received from an
interface connected to a node outside the domain of trust.

From a network protection standpoint, there is an assumed trust model
such that any node imposing a label stack on a packet is assumed to
be allowed to do so. This is a significant change compared to plain
IP offering shortest path routing, but it is not fundamentally
different compared to existing techniques providing explicit routing
capability such as RSVP-TE. By default, the explicit routing
information MUST NOT be leaked through the boundaries of the
administered domain. Segment Routing extensions that have been
defined in various protocols, leverage the security mechanisms of
these protocols such as encryption, authentication, filtering, etc.

In the general case, a segment-routing-capable router accepts and
installs labels only if the labels have been previously advertised by
a trusted source. The received information is validated using
existing control-plane protocols providing authentication and
security mechanisms. Segment Routing does not define any additional
security mechanism in existing control-plane protocols.

SR does not introduce signaling between the source and the midpoints
of a source-routed path. With SR, the source-routed path is computed
using SIDs previously advertised in the IP control plane. Therefore,
in addition to filtering and controlled advertisement of SIDs at the
boundaries of the SR domain, filtering in the data plane is also
required. Filtering MUST be performed on the forwarding plane at the
boundaries of the SR domain and may require looking at multiple
labels/instructions.

For the MPLS data plane, there are no new requirements as the
existing MPLS architecture already allows such source routing by
stacking multiple labels. And, for security protection, [RFC4381]
and [RFC5920] already call for the filtering of MPLS packets on trust
boundaries.

8.2. SRv6

When applied to the IPv6 data plane, Segment Routing does introduce
the Segment Routing Header (SRH, [IPv6-SRH]) which is a type of
Routing Extension header as defined in [RFC8200].

The SRH adds some metadata to the IPv6 packet, with the list of
forwarding path elements (e.g., nodes, links, services, etc.) that
the packet must traverse and that are represented by IPv6 addresses.
A complete source-routed path may be encoded in the packet using a
single segment (single IPv6 address).

SR domain boundary routers MUST filter any external traffic destined
to an address within the SRGB of the trusted domain or the SRLB of
the specific boundary router. External traffic is any traffic
received from an interface connected to a node outside the domain of
trust.

From a network-protection standpoint, there is an assumed trust model
such that any node adding an SRH to the packet is assumed to be
allowed to do so. Therefore, by default, the explicit routing
information MUST NOT be leaked through the boundaries of the
administered domain. Segment Routing extensions that have been
defined in various protocols, leverage the security mechanisms of
these protocols such as encryption, authentication, filtering, etc.

In the general case, an SR IPv6 SRv6 router accepts and install segments
identifiers (in the form of IPv6 addresses), only if these SIDs are
advertised by a trusted source. The received information is
validated using existing control-plane protocols providing
authentication and security mechanisms. Segment Routing does not
define any additional security mechanism in existing control-plane
protocols.

Problems that may arise when the above behaviors are not implemented
or when the assumed trust model is violated (e.g., through a security
breach) include:

o Malicious looping

o Evasion of access controls

o Hiding the source of DoS attacks

Security concerns with SR at the IPv6 data plane are more completely
discussed in [RFC5095]. The new IPv6-based Segment Routing Header is
defined in [IPv6-SRH]. This document also discusses the above
security concerns.

8.3. Congestion Control

SR does not introduce new requirements for congestion control. By
default, traffic delivery is assumed to be best effort. Congestion
control may be implemented at endpoints. Where SR policies are in
use, bandwidth allocation may be managed by monitoring incoming
traffic associated with the binding SID identifying the SR policy. Policy.
Other solutions such as presented in [RFC8084] may be applicable.

9. Manageability Considerations

In SR-enabled networks, the path the packet takes is encoded in the
header. As the path is not signaled through a protocol, OAM
mechanisms are necessary in order for the network operator to
validate the effectiveness of a path as well as to check and monitor
its liveness and performance. However, it has to be noted that SR
allows to reduce substantially the number of states in transit nodes;
hence, the number of elements that a transit node has to manage is
smaller.

SR OAM use cases for the MPLS data plane are defined in [RFC8403].
SR OAM procedures for the MPLS data plane are defined in [RFC8287].

SR routers receive advertisements of SIDs (index, label, or IPv6
address) from the different routing protocols being extended for SR.
Each of these protocols have monitoring and troubleshooting
mechanisms to provide operation and management functions for IP
addresses that must be extended in order to include troubleshooting
and monitoring functions of the SID.

SR architecture introduces the usage of global segments. Each global
segment MUST be bound to a unique index or address within an SR
domain. The management of the allocation of such an index or address
by the operator is critical for the network behavior to avoid
situations like misrouting. In addition to the allocation policy/
tooling that the operator will have in place, an implementation
SHOULD protect the network in case of conflict detection by providing
a deterministic resolution approach.

When a path is expressed using a label stack, the occurrence of label
stacking will increase. A node may want to signal, in the control
plane, its ability in terms of size of the label stack it can
support.

A YANG data model [RFC6020] for SR configuration and operations has
been defined in [SR-YANG].

When SR is applied to the IPv6 data plane, segments are identified
through IPv6 addresses. The allocation, management, and
troubleshooting of segment identifiers is no different than the
existing mechanisms applied to the allocation and management of IPv6
addresses.

The DA of the packet gives the active segment address. The segment
list in the SRH gives the entire path of the packet. The validation
of the source-routed path is done through inspection of DA and SRH
present in the packet header matched to the equivalent routing table
entries.

In the context of the SRv6 data plane, the source-routed path is
encoded in the SRH as described in [IPv6-SRH]. The SR IPv6 SRv6 source-
routed path is instantiated into the SRH as a list of IPv6 addresses
where the active segment is in the DA field of the IPv6 packet
header. Typically, by inspecting, in any node, the packet header, it
is possible to derive the source-routed path to which it belongs.
Similar to the context of the SR-MPLS data plane, an implementation
may originate path control and monitoring packets where the source-
routed path is inserted in the SRH and where each segment of the path
inserts in the packet the relevant data in order to measure the end-
to-end path and performance.

10. References

10.1. 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>.

[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>.

[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.

10.2. Informative References

[BGPLS-SR-EPE]
Previdi, S., Filsfils, C., Patel, K., Ray, S., and J.
Dong, "BGP-LS extensions for Segment Routing BGP Egress
Peer Engineering", Work in Progress, draft-ietf-idr-bgpls-
segment-routing-epe-15, March 2018.

[IPv6-SRH]
Previdi, S.,
Filsfils, C., Ed., Previdi, S., Leddy, J., Matsushima, S.,
and
d. daniel.voyer@bell.ca, D. Voyer, Ed., "IPv6 Segment Routing Header (SRH)",
Work in Progress, draft-ietf-6man-segment-routing-
header-12, April
header-14, June 2018.

[ISIS-SR-EXT]
Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
Bashandy, A., Gredler, H., Litkowski, S., Decraene, B.,
and J. Tantsura, "IS-IS Extensions for Segment Routing",
Work in Progress,
draft-ietf-isis-segment-routing-extensions-16, April draft-ietf-isis-segment-routing-
extensions-19, July 2018.

[OSPF-SR-EXT]
Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", Work in Progress, draft-
ietf-ospf-segment-routing-extensions-25,
draft-ietf-ospf-segment-routing-extensions-25, April 2018.

[OSPFv3-SR-EXT]
Psenak, P., Ed., Filsfils, C., Previdi, S., Ed., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3
Extensions for Segment Routing", Work in Progress, draft-
ietf-ospf-ospfv3-segment-routing-extensions-12, April
draft-ietf-ospf-ospfv3-segment-routing-extensions-13, May
2018.

[PCEP-SR-EXT]
Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "PCEP Extensions for Segment Routing",
Work in Progress, draft-ietf-pce-segment-routing-11,
November 2017. draft-ietf-pce-segment-routing-12, June
2018.

[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.

[RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
Hierarchy with Generalized Multi-Protocol Label Switching
(GMPLS) Traffic Engineering (TE)", RFC 4206,
DOI 10.17487/RFC4206, October 2005,
<https://www.rfc-editor.org/info/rfc4206>.

[RFC4381] Behringer, M., "Analysis of the Security of BGP/MPLS IP
Virtual Private Networks (VPNs)", RFC 4381,
DOI 10.17487/RFC4381, February 2006,
<https://www.rfc-editor.org/info/rfc4381>.

[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, DOI 10.17487/RFC4915, June 2007,
<https://www.rfc-editor.org/info/rfc4915>.

[RFC5095] Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
of Type 0 Routing Headers in IPv6", RFC 5095,
DOI 10.17487/RFC5095, December 2007,
<https://www.rfc-editor.org/info/rfc5095>.

[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC5120, February 2008,
<https://www.rfc-editor.org/info/rfc5120>.

[RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC5440, March 2009,
<https://www.rfc-editor.org/info/rfc5440>.

[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, DOI 10.17487/RFC5714, January 2010,
<https://www.rfc-editor.org/info/rfc5714>.

[RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
<https://www.rfc-editor.org/info/rfc5920>.

[RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)", RFC 6020,
DOI 10.17487/RFC6020, October 2010,
<https://www.rfc-editor.org/info/rfc6020>.

[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.

[RFC6549] Lindem, A., Roy, A., and S. Mirtorabi, "OSPFv2 Multi-
Instance Extensions", RFC 6549, DOI 10.17487/RFC6549,
March 2012, <https://www.rfc-editor.org/info/rfc6549>.

[RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016,
<https://www.rfc-editor.org/info/rfc7938>.

[RFC8084] Fairhurst, G., "Network Transport Circuit Breakers",
BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
<https://www.rfc-editor.org/info/rfc8084>.

[RFC8202] Ginsberg, L., Previdi, S., and W. Henderickx, "IS-IS
Multi-Instance", RFC 8202, DOI 10.17487/RFC8202, June
2017, <https://www.rfc-editor.org/info/rfc8202>.

[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>.

[RFC8355] Filsfils, C., Ed., Previdi, S., Ed., Decraene, B., and R.
Shakir, "Resiliency Use Cases in Source Packet Routing in
Networking (SPRING) Networks", RFC 8355,
DOI 10.17487/RFC8355, March 2018,
<https://www.rfc-editor.org/info/rfc8355>.

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

[SR-CENTRAL-EPE]
Filsfils, C., Previdi, S., Dawra, G., Aries, E., and D.
Afanasiev, "Segment Routing Centralized BGP Egress Peer
Engineering", Work in Progress, draft-ietf-spring-segment-
routing-central-epe-10, December 2017.

[SR-MPLS-DP]

[SR-MPLS] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with MPLS data plane", Work in Progress, draft-ietf-spring-segment-
routing-mpls-13, April
draft-ietf-spring-segment-routing-mpls-14, June 2018.

[SR-YANG] Litkowski, S., Qu, Y., Sarkar, P., and J. Tantsura, "YANG
Data Model for Segment Routing", Work in Progress, draft-
ietf-spring-sr-yang-08, December 2017.
draft-ietf-spring-sr-yang-09, June 2018.

Acknowledgements

We would like to thank Dave Ward, Peter Psenak, Dan Frost, Stewart
Bryant, Pierre Francois, Thomas Telkamp, Ruediger Geib, Hannes
Gredler, Pushpasis Sarkar, Eric Rosen, Chris Bowers, and Alvaro
Retana for their comments and review of this document.

Contributors

The following people have substantially contributed to the definition
of the Segment Routing architecture and to the editing of this
document:

Ahmed Bashandy
Cisco Systems, Inc.
Email: bashandy@cisco.com

Martin Horneffer
Deutsche Telekom
Email: Martin.Horneffer@telekom.de

Wim Henderickx
Nokia
Email: wim.henderickx@nokia.com

Jeff Tantsura
Email: jefftant@gmail.com

Edward Crabbe
Email: edward.crabbe@gmail.com

Igor Milojevic
Email: milojevicigor@gmail.com

Saku Ytti
TDC
Email: saku@ytti.fi

Authors' Addresses

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

Email: cfilsfil@cisco.com

Stefano Previdi (editor)
Cisco Systems, Inc.
Italy

Email: stefano@previdi.net

Les Ginsberg
Cisco Systems, Inc Inc.

Email: ginsberg@cisco.com

Bruno Decraene
Orange
FR

Email: bruno.decraene@orange.com

Stephane Litkowski
Orange
France

Email: stephane.litkowski@orange.com

Rob Shakir
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America

Email: robjs@google.com