[spring] WG LC for draft-ietf-spring-segment-routing

Stewart Bryant <stewart.bryant@gmail.com> Tue, 29 November 2016 19:22 UTC

From: Stewart Bryant <stewart.bryant@gmail.com>
To: draft-ietf-spring-segment-routing@ietf.org, spring@ietf.org, spring-chairs@tools.ietf.org
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Subject: [spring] WG LC for draft-ietf-spring-segment-routing
Precedence: list

The following are my comments on this text in response to the WGLC.
A lot of comments are embedded in the draft text below.

However I have some major overarching comments. Although this is called
an architecture it seems to be rather more of a description of how
a large number of other documents combine to produce an overall
specification for SR. Certainly for an architecture the number
of forward references to detailed solutions for a description of the
concept is quite extraordinary.

So embedded is the contents of some of these referenced documents
that I do not think that it safe to publish this text other than
synchronously with some of those documents. This is absolutely the case
for the dataplane definitions, especially for IPv6, but seems
likely to apply to other references. The further implication of
the constant dependence on other documents is that many of them
are really normative rather than informative references, making
this document a hostage to their fate.

It is far more conventional in an architecture to set out the general
description and state the invariants, and put the detail into
specific protocol documents, but to have the architecture as a
standalone text. In other words to set things out so that
the reader understands how components fit together, what the subtleties
are and what the constraints on the components are, but leave the
component design decisions to the component designers.

Clearly I think this draft needs significant work before it is
ready for submission to the IESG for publication.

- Stewart

Network Working Group C. Filsfils, Ed.
Internet-Draft S. Previdi, Ed.
Intended status: Standards Track Cisco Systems, Inc.
Expires: May 23, 2017 B. Decraene
S. Litkowski
Orange
R. Shakir
Google
November 19, 2016

Segment Routing Architecture
draft-ietf-spring-segment-routing-10

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 local semantic to an SR node or
global within an SR domain. SR allows to enforce a flow through any
topological path and service chain while maintaining per-flow state
only at the ingress node to the SR domain.

SB> Since you mention service chains here, we really should be having
SB> a wider discussion about whether SR and SFC are really the same
SB> technology.

Segment Routing can be directly applied to the MPLS architecture with
no change on the forwarding plane.

SB> Applied to or implemented using MPLS?

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 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 of the packet. The next active segment is
indicated by a pointer in the new routing header.

SB> You really cannot say this until the v6 design goes to RFC, although
SB> I do not see why this needs to be stated.
SB> What I did not see in here is a proper comparision of the consequences
SB> of the stack vs list and pointer approach. The consequences of the
SB> difefrence between these two approaches may be far reaching in the long
SB> term and lead to biforcation of the architecture, something we should
SB> think about carefully up front.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Companion Documents . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Link-State IGP Segments . . . . . . . . . . . . . . . . . . . 7
3.1. IGP Segment, IGP SID . . . . . . . . . . . . . . . . . . 7
3.2. IGP-Prefix Segment, Prefix-SID . . . . . . . . . . . . . 7
3.2.1. Prefix-SID Algorithm . . . . . . . . . . . . . . . . 7
3.2.2. MPLS Dataplane . . . . . . . . . . . . . . . . . . . 9
3.2.3. IPv6 Dataplane . . . . . . . . . . . . . . . . . . . 10
3.3. IGP-Node Segment, Node-SID . . . . . . . . . . . . . . . 10
3.4. IGP-Anycast Segment, Anycast SID . . . . . . . . . . . . 11
3.5. IGP-Adjacency Segment, Adj-SID . . . . . . . . . . . . . 14
3.5.1. Parallel Adjacencies . . . . . . . . . . . . . . . . 15
3.5.2. LAN Adjacency Segments . . . . . . . . . . . . . . . 16
3.6. Binding Segment . . . . . . . . . . . . . . . . . . . . . 16
3.6.1. Mapping Server . . . . . . . . . . . . . . . . . . . 16
3.6.2. Tunnel Headend . . . . . . . . . . . . . . . . . . . 17
3.7. Inter-Area Considerations . . . . . . . . . . . . . . . . 17
4. BGP Peering Segments . . . . . . . . . . . . . . . . . . . . 18

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5. IGP Mirroring Context Segment . . . . . . . . . . . . . . . 19
6. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
8.1. MPLS Data Plane . . . . . . . . . . . . . . . . . . . . . 20
8.2. IPv6 Data Plane . . . . . . . . . . . . . . . . . . . . . 21
9. Manageability Considerations . . . . . . . . . . . . . . . . 22
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 24
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
12.1. Normative References . . . . . . . . . . . . . . . . . . 25
12.2. Informative References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29

1. Introduction

With Segment Routing (SR), 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

SB> It really is a pity that we did not use the more descriptive term instructions
SB> which would have help people understand what they are. I wonder if it is
SB> too late to change?
SB> Service based what?

local semantic to an SR node or global within an SR domain. SR
allows to enforce a flow through any path and service chain while
maintaining per-flow state only at the ingress node of the SR domain.

SB> I wonder if we should be pulling together SR and SFC into
SB> a common architecture, since they seem to have converged?

Segment Routing can be directly applied to the MPLS architecture
([RFC3031]) with no change on the forwarding plane. A segment is
encoded as an MPLS label. An ordered list of segments is encoded as
a stack of labels. The active segment is on the top of the stack. A
completed segment is popped off the stack. The addition of a segment
is performed with a push.

SB> All true, but we are designing a solution for both MPLS and IP.
SB> Shouldn't this text be establishing the architectural princples
SB> first before getting down in the weeds of the MPLS solution?
SB>

SB> IP and MPLS took different approaches so at this level we need to
SB> be discussing the principles, and establish the properties of
SB> the list, which again are radically different, and then let the
SB> solutions drafts describe the instantiation of the list.

In the Segment Routing MPLS instantiation, a segment could be of
several types:

o an IGP segment,

o a BGP Peering segments,

o an LDP LSP segment,

o an RSVP-TE LSP segment,

o a BGP LSP segment.

SB> All true, but right down in the weeds. What about the functional
SB> equivalents in IP?

The first two (IGP and BGP Peering segments) types of segments are
defined in this document. The use of the last three types of
segments is illustrated in [I-D.ietf-spring-segment-routing-mpls].

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Segment Routing can be applied to the IPv6 architecture ([RFC2460]),
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 of the packet. Upon completion
of a segment, a pointer in the new routing header is incremented and
indicates the next segment.

SB> Again this is down in the weeds considering that we are in an architecture
SB> document and also proposes the detail of a solution that may or may
SB> not be finalized.

Numerous use-cases illustrate the benefits of source routing either
for FRR, OAM or Traffic Engineering reasons.

SB> This needs a reference.

This document defines a set of instructions (called segments) that
are required to fulfill the described use-cases. These segments can
either be used in isolation (one single segment defines the source
route of the packet) or in combination (these segments are part of an
ordered list of segments that define the source route of the packet).

1.1. Companion Documents

This document defines the SR architecture, its routing model, the
IGP-based segments, the BGP-based segments and the service segments.

Use cases are described in [RFC7855],
[I-D.ietf-spring-segment-routing-central-epe],
[I-D.ietf-spring-segment-routing-msdc],
[I-D.filsfils-spring-large-scale-interconnect],
[I-D.ietf-spring-ipv6-use-cases],
[I-D.ietf-spring-resiliency-use-cases], [I-D.ietf-spring-oam-usecase]
and [I-D.ietf-spring-sr-oam-requirement].

SB> It would be helpful to the reader to indicate the contents, so that
SB> if this just becomes a set of RFC numbers they had some better its
SB> what the documents are about.
SB>
SB> It would also be useful to get an understanding from the AD
SB> as to which of the use case documents will be published, merged
SB> become part of a wiki etc given recent policy statements from the IESG.

Segment Routing for MPLS dataplane is documented in
[I-D.ietf-spring-segment-routing-mpls].

Segment Routing for IPv6 dataplane is documented in
[I-D.ietf-6man-segment-routing-header].

IGP protocol extensions for Segment Routing are described in
[I-D.ietf-isis-segment-routing-extensions],
[I-D.ietf-ospf-segment-routing-extensions] and
[I-D.ietf-ospf-ospfv3-segment-routing-extensions] referred in this
document as "IGP SR extensions documents".

The FRR solution for SR is documented in
[I-D.francois-rtgwg-segment-routing-ti-lfa].

The PCEP protocol extensions for Segment Routing are defined in
[I-D.ietf-pce-segment-routing].

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The interaction between SR/MPLS with other MPLS Signaling planes is
documented in [I-D.ietf-spring-segment-routing-ldp-interop].

2. Terminology

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. Examples of SIDs are: a MPLS label, an
index value in a MPLS label space, an IPv6 address. Other types of
SIDs can be defined in the future.

SB> Definition by example is not a definition.

Segment List: ordered list of SID's encoding the topological and
service source route of the packet.

SB> Isn't it an ordered list of SID encoding the ordered set of
SB> instructions to be applies to the packet as it traverses the
SB> SR domain?

It is a stack of labels in the
MPLS architecture. It is an ordered list of IPv6 addresses in the
IPv6 architecture.

SB> Again this a architecture it should not go down in those weeds.

Segment Routing Domain (SR Domain): the set of nodes participating
into the source based routing model.
SB> Surely is is the set of nodes that form an SR Instance having a
SB> common view of the mapping of SID to instruction definition

These nodes may be connected to
the same physical infrastructure (e.g.: a Service Provider's network)
as well as nodes remotely connected to each other (e.g.: an
enterprise VPN or an overlay). Note that a SR domain may also be
confined within an IGP instance, in which case it is named SR-IGP
Domain.

Active segment: the segment that MUST be used by the receiving router
to process the packet. In the MPLS dataplane is the top label. In
the IPv6 dataplane is the destination address of a packet having the
Segment Routing Header as defined in
[I-D.ietf-6man-segment-routing-header].

SB> I am surprised that you don't need to define POP or Remove

PUSH: the insertion of a segment at the head of the Segment list.

SB> This works for a stack model, but I am not sure it works for
SB> a list model where you really do an insert.

NEXT: the active segment is completed, the next segment becomes
active.

CONTINUE: the active segment is not completed and hence remains
active. The CONTINUE instruction is implemented as the SWAP
instruction in the MPLS dataplane. In IPv6, this is the plain IPv6
forwarding action of a regular IPv6 packet according to its
Destination Address.

SB> Again I worry about definition by example.

SR Global Block (SRGB): local property of an SR node. In the MPLS
architecture, SRGB is the set of local labels reserved for global
segments. Using the same SRGB on all nodes within the SR domain ease
operations and troubleshooting and is expected to be a deployment

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guideline. In the IPv6 architecture, the equivalent of the SRGB is
in fact the set of addresses used as global segments. Since there
are no restrictions on which IPv6 address can be used, the concept of
the SRGB includes all IPv6 global address space used within the SR
domain.

SB> I worry about whether this is an architectural concept of a
SB> specific dataplane concept, or an implementation concept. Since
SB> the IPv6 design moved from a set of short instructions to full
SB> IPv6 addresses, this does not look like an architectural construct.

Global Segment: the related instruction is supported by all the SR-
capable nodes in the domain.

SB> instruction or identifier. Isn't the point about this that any node
SB> knows how to execute its view of the instruction, and indeed
SB> it is possible that the mapping at some nodes (for example forward)
SB> may be different from the mapping at another node (for example
SB> receive, or deliver to attached firewall)

In the MPLS architecture, a Global
Segment has a globally-unique index. The related local label at a
given node N is found by adding the globally-unique index to the SRGB
of node N. In the IPv6 architecture, a global segment is a globally-
unique IPv6 address.

SB> Again this muddles architecture and mapping to an instantiation
SB> of that architecture.
SB> nit s/has a globally-unique/ is a globally-unique/
SB> However this begs the question of the scope of global. Certainly
SB> in MPLS it is restricted to the SR-Domain, and even then it may
SB> only be a sub-set of it.

Local Segment: the related instruction is supported only by the node
originating it.

SB> Again I think it is the mapping of the instruction identifier to
SB> the instruction rather than the instruction.

In the MPLS architecture, this is a local label
outside the SRGB. In the IPv6 architecture, this can be any IPv6
address whose reachability is not advertised in any routing protocol
(hence, the segment is known only by the local node).

SB> Wait a moment the instruction is understood by the imposing node(s)
SB> and the executing node

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.

SB> I don't think it's a name. Isn't it simply a segment that is advertised
SB> by an IGP? Of course that takes us back to the scoping definition, since
SB> all nodes receive the IGP information.

IGP-prefix Segment, Prefix-SID: an IGP-Prefix Segment is an IGP
segment attached to an IGP prefix.

SB> What does attached mean here?

An IGP-Prefix Segment is global
(unless explicitly advertised otherwise) within the SR IGP instance/
topology and identifies an instruction to forward the packet along
the path computed using the routing algorithm specified in the
algorithm field, in the topology and the IGP instance where it is
advertised.

SB> More precisely isn't it an instruction to forward a packet
SB> along the path computed for a specified prefix?

The Prefix-SID is the SID of the IGP-Prefix Segment.
SB> I think that this should be a separate definition.

IGP-Anycast: an IGP-Anycast Segment is an IGP-prefix segment which
does not identify a specific router, but a set of routers. The terms
"Anycast Segment" or "Anycast-SID" are often used as an abbreviation.

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

SB> What are the semantics of a non local adjacency segment?

IGP-Node: an IGP-Node Segment is an IGP-Prefix Segment which
identifies a specific router (e.g. a loopback). The terms "Node
Segment" or Node-SID" are often used as an abbreviation.

SR Tunnel: a list of segments to be pushed on the packets directed on
the tunnel. The list of segments can be specified explicitly or
implicitly via a set of abstract constraints (latency, affinity,
SRLG, ...). In the latter case, a constraint-based path computation

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is used to determine the list of segments associated with the tunnel.
The computation can be local or delegated to a PCE server. An SR
tunnel can be configured by the operator, provisioned via netconf or
provisioned via PCEP. An SR tunnel can be used for traffic-
engineering, OAM or FRR reasons.

SB> So where does tunnel fit into that definition? Isn't the point
SB> about a tunnel that it is a type of virtual link that constrains
SB> a packet to a path other than the natural path that would be
SB> inferred from its native address?

Segment List Depth: the number of segments of an SR tunnel. The
entity instantiating an SR Tunnel at a node N should be able to
discover the depth insertion capability of the node N. The PCEP
discovery capability is described in [I-D.ietf-pce-segment-routing].

SB> Isn't that just one way that such a size might be discovered?

3. Link-State IGP Segments

Within a link-state IGP 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 as they enable the expression of any
topological path throughout the IGP domain. Such a topological path
is either expressed as a single IGP segment or a list of multiple IGP
segments.

SB> I am not sure that topological path is a well known term. A quick check
SB> in google only found the term is one paper. Do you simply mean path?

3.1. IGP Segment, IGP SID

The terms "IGP Segment" and "IGP SID" are the generic names for a
segment attached to a piece of information advertised by a link-state
IGP, e.g. an IGP prefix or an IGP adjacency.

3.2. IGP-Prefix 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/IGP domain.

The required IGP protocol extensions are defined in IGP SR extensions
documents.

3.2.1. Prefix-SID Algorithm

The IGP protocol extensions for Segment Routing define the Prefix-SID
advertisement which includes a set of flags and the algorithm field.
The algorithm field has the purpose of associating a given Prefix-SID
to a routing algorithm.

In the context of an instance and a topology, multiple Prefix-SID's
MAY be allocated to the same IGP Prefix as long as the algorithm
value is different in each one.

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Multiple instances and topologies are defined in IS-IS and OSPF in:
[RFC5120], [RFC6822], [RFC6549] and [RFC4915].

Initially, two "algorithms" have been defined:

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

SB> If a node is going to apply local policy, doesn't there need to be a
SB> comment about loop avoidance, and also possibly cleaning up the
SB> SR header if local policy is to send the packet out of the domain?
SB> I worry about what this means when this is applied to a SID
SB> other than the final SID specifying the path.

o "Strict Shortest Path": This algorithm mandates that the packet is
forwarded according to ECMP-aware SPF algorithm and instruct any
router in the path to ignore any possible local policy overriding
SPF decision. The SID advertised with "Strict Shortest Path"
algorithm ensures that the path the packet is going to take is the
expected, and not altered, SPF path.

An IGP-Prefix Segment identifies the path, to the related prefix,
along the path computed as per the algorithm field.

A packet injected anywhere within the SR/IGP domain with an active
Prefix-SID will be forwarded along path computed by the algorithm
expressed in the algorithm field.

The ingress node of an SR domain validates that the path to a prefix,
advertised with a given algorithm, includes nodes all supporting the
advertised algorithm. As a consequence, if a node on the path does
not support algorithm X, the IGP-Prefix segment will be interrupted
and will drop packet on that node. It's the responsibility of the
ingress node using a segment to check that all downstream nodes
support the algorithm of the segment.

A router MUST NOT forward any SR traffic associated with the SR
algorithm to the adjacent router, if the adjacent router has not
advertised support for such SR algorithm.

It has to be noted that Fast Reroute (FRR) mechanisms, such as the
one described in [I-D.francois-rtgwg-segment-routing-ti-lfa], that
are based on post-convergence SPF, are still compliant to the Strict-
SPF algorithm definition.

Details of the two defined algorithms are defined in
[I-D.ietf-isis-segment-routing-extensions],
[I-D.ietf-ospf-segment-routing-extensions] and
[I-D.ietf-ospf-ospfv3-segment-routing-extensions].

SB> I am not convinced that the statements on IPFRR belong in the
SB> architecture, surely they belong in the IPFRR document together
SB> a declaration of architectural conformance?

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3.2.2. MPLS Dataplane

SB> I am not convinced that this is architecture, more implementation
SB> in a specific dataplane. It is not particularly critical in the case of
SB> MPLS as we pretty much know what it looks like. I remain to be convinced
SB> about IP. The problem is that if the dataplane design changes, it may
SB> invalidate the architecture. Best practise is to be invariant to the
SB> implementation when there are multiple possible data planes.

When SR is used over the MPLS dataplane:

o the IGP signaling extension for IGP-Prefix segment includes the
P-Flag ([I-D.ietf-isis-segment-routing-extensions]) or the NP-Flag
([I-D.ietf-ospf-segment-routing-extensions]). A Node N
advertising a Prefix-SID SID-R for its attached prefix R unset the
P-Flag (or NP-Flag) in order to instruct its connected neighbors
to perform the NEXT operation while processing SID-R. This
behavior is equivalent to Penultimate Hop Popping in MPLS. When
the flag is unset, the neighbors of N MUST perform the NEXT
operation while processing SID-R. When the flag is set, the
neighbors of N MUST perform the CONTINUE operation while
processing SID-R.

SB> That is really down in the weeds, and I am not sure it belongs here.
SB> surely you need to specify the requirement on the solution, not the
SB> solution itself in this document. Alternatively, if it does belong here
SB> it needs a more complete description here.

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

o While SR allows to attach a local segment to an IGP prefix (using
the L-Flag),
SB> what is an L-flag?

we specifically assume that when the terms "IGP-
Prefix Segment" and "Prefix-SID" are used, the segment is global
(the SID is allocated from the SRGB or as an index). This is
consistent with all the described 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 a Prefix-SID having 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 warning for
misconfiguration.

o If a node N advertises Prefix-SID SID-R for a prefix R that is
attached to N, N MUST either clear the P-Flag in the advertisement
of SID-R, or else maintain the following FIB entry:

SB> Where did the P-Flag come from?

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:

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

SB> This is quite confusing. Don't these sorts of operations apply to other sorts of
SB> SID, such as nodal SIDs? Why are these called out in detail but not others?

SB> You talk about ECMP in nodal, doesn't that also apply here?

3.2.3. IPv6 Dataplane

When SR is used over the IPv6 dataplane:

o The Prefix-SID is the prefix itself. No additional identifier is
needed for Segment Routing over IPv6.

o Any address belonging to any of the node's prefixes can be used as
Prefix-SIDs.

o An operator may want to explicitly indicate which of the node's
prefixes can be used as Prefix-SIDs through the setting of a flag
(e.g.: using the IGP prefix attribute defined in [RFC7794]) in the
routing protocol used for advertising the prefix.

o A global SID is instantiated through any globally advertised IPv6
address.

o A local SID is instantiated through a local IPv6 prefix not being
advertised and therefore known only by the local node.

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

Regardless Segment Routing, any remote IPv6 node will maintain a
plain IPv6 FIB entry for any prefix, no matter if they represent a
segment or not.

3.3. IGP-Node Segment, Node-SID

An IGP Node Segment is a an IGP Prefix Segment which identifies a
specific router (e.g. a loopback). The terms "Node Segment" or
"Node-SID" are often used as an abbreviation. The IGP SR extensions
define a flag that identifies Node-SIDs.

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A "Node Segment" or "Node-SID" is fundamental to SR. From anywhere
in the network, it enforces the ECMP-aware shortest-path forwarding
of the packet towards the related node.

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

3.4. IGP-Anycast Segment, Anycast SID

An IGP-Anycast Segment is an IGP-prefix segment which does not
identify a specific router, but a set of routers. The terms "Anycast
Segment" or "Anycast-SID" are often used as an abbreviation.

An "Anycast Segment" or "Anycast SID" enforces the ECMP-aware
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 MUST advertise the same prefix
with the same SID value.

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+--------------+
| 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 |
+--------------+

Transit device groups

The figure above describes 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 100.

Similarly, group B consists of devices {B1, B2, B3 and B4} and are
all provisioned with the anycast address 192.0.2.1/32, anycast SID
200. In the above network topology, each PE device is connected to
two routers in 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 of the
group in the stack.

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

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Obviously, the value of the SID following the anycast SID MUST be
understood by all nodes advertising the same anycast segment.

+-------------------------+
| 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)|
| |
+-------------------------+

Transit paths via anycast group A

Considering a MPLS deployment, in the above topology, if device PE1
(or PE2) requires to send a packet to the device PE3 (or PE4) it
needs to encapsulate the packet in a MPLS payload with the following
stack of labels.

SB> AS an MPLS payload?

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

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

While the first label is easy to compute, in this case since there
are more than one topologically nearest devices (A1 and A2), unless
A1 and A2 allocated the same label value to the same prefix,
determining the second label is impossible. 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.

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To ease the use of anycast segment in a short term, it is recommended
to configure the same SRGB 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 anycast segment without configuring the same SRGB on nodes
belonging to the same device group may lead to misrouting (in a MPLS
VPN deployment, some traffic may leak between VPNs).

SB> So is this an architectural statement that mixed vendor anycast
SB> does not work? In which case I wonder if it should be in the
SB> architecture at all.

3.5. IGP-Adjacency Segment, Adj-SID

An IGP-Adjacency Segment is an IGP segment attached to a
unidirectional adjacency or a set of unidirectional adjacencies. By
default, an IGP-Adjacency Segment is local to the node which
advertises it. However, an Adjacency Segment can be global if
advertised by the IGP as such. The SID of the IGP-Adjacency Segment
is called the Adj-SID.

SB> I think that there is some confusion about the meaning of global
SB> in this draft. Earlier on the term implied that global meant that
SB> any node would know how to execute the instruction, here it
SB> seems to imply that it is global if the value is known globally.

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]).

SB> Aren't Adjacency segments a concept in their own right with the
SB> IGP just being one way of learning them? In which case shouldn't they
SB> be introduced and explored in their own right first?

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.

SB> Ah, I think some clarification is needed earlier in the text.
SB> You have two types of ADJ-SID, the original one which was
SB> a local label attached to a node so it only had meaning in
SB> conjunction with the node identifier, and this new one which
SB> is a full identity in it's own right. I think that needs to be
SB> more clearly expressed, together with some discussion on scaling.
SB>
SB> This causes me to wonder why there is no overall discussion on the
SB> scaling properties and issues, since that is very much an
SB> an architectural concern.

If the
Adj-SID identifies a set of adjacencies, then the node N load-
balances 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).

SB> Doesn't it also use labels from the global label table which
SB> is itself of a limited size?

An "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.

SB> This is surely a fundamental point that should be earlier in the
SB> discussion.

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The encodings of the Adj-SID include the B-flag. When set, the Adj-
SID refers to an adjacency that is eligible for protection (e.g.:
using IPFRR or MPLS-FRR).

SB> Where did the B-flag come from?

The encodings of the Adj-SID include the L-flag. When set, the Adj-
SID has local significance. By default the L-flag is set.

A node SHOULD allocate one Adj-SIDs for each of its adjacencies.
SB> This needs further discussion - for example why .. and is this
SB> local or global?

A node MAY allocate multiple Adj-SIDs to the same adjacency. An
example is where the adjacency is established over a bundle
interface. Each bundle member MAY have its own Adj-SID.

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

SB> I am wondering is Adj is the right term here. In routing
SB> an adjacency is a neighbouring node, but I think we are
SB> actually talking here about Link-SIDs and Link-Bundle SIDs.

Adjacency suppression MUST NOT be performed by the IGP.

SB> Why/why not?

A node MUST install a FIB entry for any Adj-SID of value V attached
to data-link L:

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

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

SB> nit: by the SPF

3.5.1. Parallel Adjacencies

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

SB> So we need to be clearer that an Adj-SID can be a Link, a Link Bundle or a link Group.

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

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

When parallel adjacencies are used and associated to the same Adj-
SID, and in order to optimize the load balancing function, a "weight"
factor can be associated to the Adj-SID advertised with each
adjacency. The weight tells the ingress (or a SDN/orchestration
system) about the loadbalancing factor over the parallel adjacencies.
As shown in Figure 1, A and B are connected through two parallel
adjacencies

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

Figure 1: 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 loadbalance the
traffic across the parallel links (link-1 and link-2) according to a
1:2 ratio.

SB> What happens about flow order when you use this construct?

The weight value is advertised with the Adj-SID as defined in IGP SR
extensions documents.

3.5.2. 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 other 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 each neighbor in the LAN. These extensions are
defined in IGP SR extensions documents.

SB> This should really be in the form "will need to be provided"

3.6. Binding Segment

SB> I have read this section several times, and it is really not clear.
SB> Nor is it clear that this is part of SR as opposed to a general
SB> MPLS feature.

3.6.1. Mapping Server

A Remote-Binding SID S advertised by the mapping server M for remote
prefix R attached to non-SR-capable node N signals the same
information as if N had advertised S as a Prefix-SID. Further
details are described in the SR/LDP interworking procedures
([I-D.ietf-spring-segment-routing-ldp-interop].

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The segment allocation and SRGB Maintenance rules are the same as
those defined for Prefix-SID.

3.6.2. Tunnel Headend

The segment allocation and SRGB Maintenance rules are the same as
those defined for Adj-SID. A tunnel attached to a head-end H acts as
an adjacency attached to H.

Note: an alternative consists of representing tunnels as forwarding-
adjacencies ( [RFC4206]). In such case, the tunnel is presented to
the routing area as a routing adjacency and is considered as such by
all area routers. The Remote-Binding SID is preferred as it allows
to advertise the presence of a tunnel without influencing the LSDB
and the SPF computation.

3.7. Inter-Area Considerations

In the following example diagram we assume an IGP deployed using
areas and where SR has been deployed.

! !
! !
B------C-----F----G-----K
/ | | |
S---A/ | | |
\ | | |
\D------I----------J-----L----Z (192.0.2.1/32, Node-SID: 150)
! !
Area-1 ! Backbone ! Area 2
! area !

Figure 2: Inter-Area Topology Example

In area 2, node Z allocates Node-SID 150 to his local prefix
192.0.2.1/32. ABRs G and J will propagate the prefix 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
192.0.2.1/32 with Prefix-SID 150 and advertised by nodes C and I.

It therefore results that a Prefix-SID remains attached to its
related IGP Prefix through the inter-area process.

When node S sends traffic to 192.0.2.1/32, it pushes Node-SID(150) as
active segment and forward it to A.

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When 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.

When an ABR propagates a prefix from one area to another it MUST set
the R-Flag.

SB> As far as I can see these flags are not properly defined in this architecture document.
SB> What is really needed is a section on routing protocol indicators.

4. BGP Peering Segments

In the context of BGP Egress Peer Engineering (EPE), as described in
[I-D.ietf-spring-segment-routing-central-epe], an EPE enabled Egress
PE 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 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
minimum, a BGP Peering Engineering policy applied at an ingress PE
involves two segments: the Node SID of the chosen egress PE and then
the BGP Peering Segment for the chosen egress PE peer or peering
interface.

Hereafter, we will define three types of BGP peering segments/SID's:
PeerNodeSID, PeerAdjSID and PeerSetSID.

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

* SR header 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, its semantics is:

* SR header 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, its semantics is:

* SR header operation: NEXT.

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* Next-Hop: loadbalance 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 will be defined in a separate document.

5. IGP Mirroring Context Segment

It is beneficial for an IGP node to be able 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" segment be 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 segment is
called "Mirror Context Segment" and identified by the Mirror SID.

The Mirror SID is advertised using the Binding Segment defined in SR
IGP protocol extensions ( [I-D.ietf-isis-segment-routing-extensions],
[I-D.ietf-ospf-segment-routing-extensions] and
[I-D.ietf-ospf-ospfv3-segment-routing-extensions]).

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. B, when receiving the traffic with the Mirror SID as the
active segment, uses that segment and process 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.

SB> A reference to BIER might be apropriate since that is the
SB> conceptually similar.

7. IANA Considerations

This document does not require any action from IANA.

8. Security Considerations

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

SB> Isn't it applicable to any forwarding plane in which an ordered
SB> list of instructions can be imposed on a packet, at least from
SB> an architectural perspective.

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Segment Routing adds some meta-data on the packet, with the list of
forwarding path elements (e.g.: nodes, links, services, etc.) that
the packet must traverse.

SB> Earlier they were instructions, or segments, and it was an ordered list.
SB> I am trying to figure out if you traverse a service. Either way
SB> I am struck by the difference between the description here and at
SB> the front of the document.

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.

SB> I am not sure what that adds. The important point is to consider the
SB> vulnerabilities and it is not clear whether BS is an increased vulnerability
SB> if not it is unclear what it adds to the analysis.

8.1. MPLS Data Plane

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

SB> Well not quite. One characteristic of MPLS was that the behaviour
SB> of a label was only known to its peers. If a packet mislanded at
SB> a node the behaviour was thus completely unpredictable and thus
SB> had to exploit. MPLS-SR reduces that unpredictability and thus
SB> add potential exploits that do not exist in the original MPLS design.

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
term 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
meta-data is equivalent between RSVP-TE and SR.

SB> One of the differences is that RSVP actively maintains the path.
SB> Is there a danger of stale paths being left in an SR network
SB> and subsequently exploited?

When a source routed path is expressed with a list of segments
additional meta-data 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.

SB> The difference is that an actor could construct an explicit path
SB> in a way that was not possible in regular MPLS. In both cases
SB> they need to get the packet inside the network, but once inside the
SB> network they could construct various types of amplification attack
SB> that are not possible in classic MPLS

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 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
install labels, only if these labels have been previously advertised
by a trusted source. The received information is validated using
existing control plane protocols providing authentication and

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security mechanisms. Segment routing does not define any additional
security mechanism in existing control plane protocols.

Segment Routing does not introduce signaling between the source and
the mid points 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/instruction.

For the MPLS data plane, there are no new requirement as the existing
MPLS architecture already allow such source routing by stacking
multiple labels.

SB> I think the concern is whether SR make it easier to construct an attack
SB> given how widely know the labels are in the network compared to
SB> classic MPLS?

And for security protection, [RFC4381] section 2.4
and [RFC5920] section 8.2 already calls for the filtering of MPLS
packets on trust boundaries.

8.2. IPv6 Data Plane

When applied to the IPv6 data plane, Segment Routing does introduce
the Segment Routing Header (SRH,
[I-D.ietf-6man-segment-routing-header]) which is a type of Routing
Extension header as defined in [RFC2460].

The SRH adds some meta-data on 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).

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.

SB> As I understand it there is current debate as to whether adding
SB> a header to a packet is allowed in the IPv6 architecture.

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.

SB> The worry of course is that the information is so widely known
SB> in the network that any rogue node can leak this.

In the general case, an SR IPv6 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.

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In addition, SR domain boundary routers, by default, MUST apply data
plane filters so to only accept packets whose DA and SRH (if any)
contain addresses previously advertised as SIDs.

SB> I am wondering how deep the dpi needs to be here? Also don't you need
SB> to forbid any packet with an SRH from entering the network?

There are a number of security concerns with source routing at the
IPv6 data plane [RFC5095]. The new IPv6-based segment routing header
defined in [I-D.ietf-6man-segment-routing-header] and its associated
security measures address these concerns.

SB> You can only really say that when that draft is an RFC.

The IPv6 Segment Routing
Header is defined in a way that blind attacks are never possible,
i.e., attackers will be unable to send source routed packets that get
successfully processed, without being part of the negations for
setting up the source routes or being able to eavesdrop legitimate
source routed packets. In some networks this base level security may
be complemented with other mechanisms, such as packet filtering,
cryptographic security, etc.

SB> I am surprised that there are no dataplane invariant aspects to
SB> the security, and that there are no separate control plane discussion,
SB> particularly as you are introducing a new control plane to MPLS.

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,

SB> Is this true for Binding SID?

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
and hence the number of elements that a transit node has to manage is
smaller.

SR OAM use cases and requirements for the MPLS data plane are defined
in [I-D.ietf-spring-oam-usecase] and
[I-D.ietf-spring-sr-oam-requirement]. OAM procedures for the MPLS
data plane are defined in [I-D.ietf-mpls-spring-lsp-ping].

SR routers receive advertisement 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 so 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 globally-unique index or address. The
management of the allocation of such index or address by the operator
is critical for the network behavior to avoid situations like mis-
routing. 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.

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An operator may implement tools in order to audit the network and
ensure the good allocation of indexes, SIDs or IP addresses.
Conflict detection between SIDs, including Mapping Server binding
SIDs, and their resolution are addressed in
[I-D.ietf-spring-conflict-resolution].

SR with the MPLS data plane, can be gracefully introduced in an
existing LDP [RFC5036] network. This is described in
[I-D.ietf-spring-segment-routing-ldp-interop]. SR and LDP may also
inter-work. In this case, the introduction of mapping-server may
introduce some additional manageability considerations that are
discussed in [I-D.ietf-spring-segment-routing-ldp-interop].

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

A YANG data model [RFC6020] for segment routing configuration and
operations has been defined in [I-D.ietf-spring-sr-yang].

When Segment Routing 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.

In the SR over IPv6 data plane context, the allocation of SIDs
results into the allocation of IPv6 addresses. Therefore,
management, troubleshooting, monitoring functions are the same as the
one used for IPv6 addresses.

The control of a source routed path of an IPv6 packet having an SRH
SHOULD be implemented through the inspection of the packet header and
more precisely its DA and segment list (in the SRH). 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 SR over the IPv6 data plane, the source routed path
is encoded in the SRH as described in
[I-D.ietf-6man-segment-routing-header]. The SR IPv6 source routed
path is instantiated into the SRH as a list of IPv6 address where the
active segment is in the Destination Address (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 it belongs
to. Similar to the context of SR over MPLS data plane, an

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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. 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
Alcatel-Lucent
Email: wim.henderickx@alcatel-lucent.com

Jeff Tantsura
Ericsson
Email: Jeff.Tantsura@ericsson.com

Edward Crabbe
Individual
Email: edward.crabbe@gmail.com

Igor Milojevic
Email: milojevicigor@gmail.com

Saku Ytti
TDC
Email: saku@ytti.fi

11. Acknowledgements

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

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12. References

12.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.

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

[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,
<http://www.rfc-editor.org/info/rfc4206>.

12.2. Informative References

SB> It is unclear to me whether or not many of these references are truely
SB> informative. It seems that in many cases the architectural description
SB> is so scant that the reader cannot fully understand elements of the
SB> the architecture without reading some of these references, and that
SB> makes them normative.

[I-D.filsfils-spring-large-scale-interconnect]
Filsfils, C., Cai, D., Previdi, S., Henderickx, W.,
Cooper, D., Ferguson, F., Laberge, T., Lin, S., Decraene,
B., Jalil, L., jefftant@gmail.com, j., and R. Shakir,
"Interconnecting Millions Of Endpoints With Segment
Routing", draft-filsfils-spring-large-scale-
interconnect-04 (work in progress), October 2016.

[I-D.francois-rtgwg-segment-routing-ti-lfa]
Francois, P., Bashandy, A., and C. Filsfils, "Abstract",
draft-francois-rtgwg-segment-routing-ti-lfa-02 (work in
progress), November 2016.

[I-D.ietf-6man-segment-routing-header]
Previdi, S., Filsfils, C., Field, B., Leung, I., Linkova,
J., Aries, E., Kosugi, T., Vyncke, E., and D. Lebrun,
"IPv6 Segment Routing Header (SRH)", draft-ietf-6man-
segment-routing-header-02 (work in progress), September
2016.

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[I-D.ietf-isis-segment-routing-extensions]
Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
Litkowski, S., Decraene, B., and j. jefftant@gmail.com,
"IS-IS Extensions for Segment Routing", draft-ietf-isis-
segment-routing-extensions-09 (work in progress), October
2016.

[I-D.ietf-mpls-spring-lsp-ping]
Kumar, N., Swallow, G., Pignataro, C., Akiya, N., Kini,
S., Gredler, H., and M. Chen, "Label Switched Path (LSP)
Ping/Trace for Segment Routing Networks Using MPLS
Dataplane", draft-ietf-mpls-spring-lsp-ping-01 (work in
progress), October 2016.

[I-D.ietf-ospf-ospfv3-segment-routing-extensions]
Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3
Extensions for Segment Routing", draft-ietf-ospf-ospfv3-
segment-routing-extensions-07 (work in progress), October
2016.

[I-D.ietf-ospf-segment-routing-extensions]
Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", draft-ietf-ospf-segment-
routing-extensions-10 (work in progress), October 2016.

[I-D.ietf-pce-segment-routing]
Sivabalan, S., Medved, J., Filsfils, C., Crabbe, E.,
Raszuk, R., Lopez, V., Tantsura, J., Henderickx, W., and
J. Hardwick, "PCEP Extensions for Segment Routing", draft-
ietf-pce-segment-routing-08 (work in progress), October
2016.

[I-D.ietf-spring-conflict-resolution]
Ginsberg, L., Psenak, P., Previdi, S., and M. Pilka,
"Segment Routing Conflict Resolution", draft-ietf-spring-
conflict-resolution-02 (work in progress), October 2016.

[I-D.ietf-spring-ipv6-use-cases]
Brzozowski, J., Leddy, J., Townsley, W., Filsfils, C., and
R. Maglione, "IPv6 SPRING Use Cases", draft-ietf-spring-
ipv6-use-cases-07 (work in progress), July 2016.

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[I-D.ietf-spring-oam-usecase]
Geib, R., Filsfils, C., Pignataro, C., and N. Kumar, "A
Scalable and Topology-Aware MPLS Dataplane Monitoring
System", draft-ietf-spring-oam-usecase-04 (work in
progress), October 2016.

[I-D.ietf-spring-resiliency-use-cases]
Filsfils, C., Previdi, S., Decraene, B., and R. Shakir,
"Resiliency use cases in SPRING networks", draft-ietf-
spring-resiliency-use-cases-08 (work in progress), October
2016.

[I-D.ietf-spring-segment-routing-central-epe]
Filsfils, C., Previdi, S., Aries, E., Ginsburg, D., and D.
Afanasiev, "Segment Routing Centralized BGP Peer
Engineering", draft-ietf-spring-segment-routing-central-
epe-02 (work in progress), September 2016.

[I-D.ietf-spring-segment-routing-ldp-interop]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., and
S. Litkowski, "Segment Routing interworking with LDP",
draft-ietf-spring-segment-routing-ldp-interop-04 (work in
progress), July 2016.

[I-D.ietf-spring-segment-routing-mpls]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., Horneffer, M., Shakir, R.,
jefftant@gmail.com, j., and E. Crabbe, "Segment Routing
with MPLS data plane", draft-ietf-spring-segment-routing-
mpls-05 (work in progress), July 2016.

[I-D.ietf-spring-segment-routing-msdc]
Filsfils, C., Previdi, S., Mitchell, J., Aries, E., and P.
Lapukhov, "BGP-Prefix Segment in large-scale data
centers", draft-ietf-spring-segment-routing-msdc-02 (work
in progress), October 2016.

[I-D.ietf-spring-sr-oam-requirement]
Kumar, N., Pignataro, C., Akiya, N., Geib, R., Mirsky, G.,
and S. Litkowski, "OAM Requirements for Segment Routing
Network", draft-ietf-spring-sr-oam-requirement-02 (work in
progress), July 2016.

[I-D.ietf-spring-sr-yang]
Litkowski, S., Qu, Y., Sarkar, P., and J. Tantsura, "YANG
Data Model for Segment Routing", draft-ietf-spring-sr-
yang-05 (work in progress), October 2016.

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[RFC4381] Behringer, M., "Analysis of the Security of BGP/MPLS IP
Virtual Private Networks (VPNs)", RFC 4381,
DOI 10.17487/RFC4381, February 2006,
<http://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,
<http://www.rfc-editor.org/info/rfc4915>.

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

[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,
<http://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,
<http://www.rfc-editor.org/info/rfc5120>.

[RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
<http://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,
<http://www.rfc-editor.org/info/rfc6020>.

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

[RFC6822] Previdi, S., Ed., Ginsberg, L., Shand, M., Roy, A., and D.
Ward, "IS-IS Multi-Instance", RFC 6822,
DOI 10.17487/RFC6822, December 2012,
<http://www.rfc-editor.org/info/rfc6822>.

[RFC7794] Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794,
March 2016, <http://www.rfc-editor.org/info/rfc7794>.

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[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, <http://www.rfc-editor.org/info/rfc7855>.

Authors' Addresses

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

Email: cfilsfil@cisco.com

Stefano Previdi (editor)
Cisco Systems, Inc.
Via Del Serafico, 200
Rome 00142
Italy

Email: sprevidi@cisco.com

Bruno Decraene
Orange
FR

Email: bruno.decraene@orange.com

Stephane Litkowski
Orange
FR

Email: stephane.litkowski@orange.com

Rob Shakir
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043

Email: robjs@google.com

Filsfils, et al. Expires May

[spring] WG LC for draft-ietf-spring-segment-rout… Stewart Bryant
Re: [spring] WG LC for draft-ietf-spring-segment-… Stefano Previdi (sprevidi)
Re: [spring] WG LC for draft-ietf-spring-segment-… Stewart Bryant
Re: [spring] WG LC for draft-ietf-spring-segment-… Stefano Previdi (sprevidi)
Re: [spring] WG LC for draft-ietf-spring-segment-… Stefano Previdi (sprevidi)