Network Working Group S. Previdi, Ed.
Internet-Draft C. Filsfils
Intended status: Standards Track Cisco Systems, Inc.
Expires: January 4, 2015 B. Field
Comcast
I. Leung
Rogers Communications
July 3, 2014
IPv6 Segment Routing Header (SRH)
draft-previdi-6man-segment-routing-header-02
Abstract
Segment Routing (SR) allows a node to steer a packet through a
controlled set of instructions, called segments, by prepending a SR
header to the packet. A segment can represent any instruction,
topological or service-based. SR allows to enforce a flow through
any path (topological, or application/service based) while
maintaining per-flow state only at the ingress node to the SR domain.
Segment Routing can be applied to the IPv6 data plane with the
addition of a new type of Routing Extension Header. This draft
describes the Segment Routing Extension Header Type and how it is
used by SR capable nodes.
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on January 4, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License.
Table of Contents
1. Structure of this document . . . . . . . . . . . . . . . . . 3
2. Segment Routing Documents . . . . . . . . . . . . . . . . . . 3
3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Data Planes supporting Segment Routing . . . . . . . . . 4
3.2. Illustration . . . . . . . . . . . . . . . . . . . . . . 4
4. Abstract Routing Model . . . . . . . . . . . . . . . . . . . 7
4.1. Segment Routing Global Block (SRGB) . . . . . . . . . . . 8
4.2. Traffic Engineering with SR . . . . . . . . . . . . . . . 9
4.3. Segment Routing Database . . . . . . . . . . . . . . . . 10
5. IPv6 Instantiation of Segment Routing . . . . . . . . . . . . 10
5.1. Segment Identifiers (SIDs) and SRGB . . . . . . . . . . . 10
5.1.1. Node-SID . . . . . . . . . . . . . . . . . . . . . . 11
5.1.2. Adjacency-SID . . . . . . . . . . . . . . . . . . . . 11
5.2. Segment Routing Extension Header (SRH) . . . . . . . . . 11
5.2.1. SRH and RFC2460 behavior . . . . . . . . . . . . . . 14
5.2.2. SRH Optimization . . . . . . . . . . . . . . . . . . 15
6. SRH Procedures . . . . . . . . . . . . . . . . . . . . . . . 16
6.1. Segment Routing Operations . . . . . . . . . . . . . . . 16
6.2. Segment Routing Node Functions . . . . . . . . . . . . . 16
6.2.1. Ingress SR Node . . . . . . . . . . . . . . . . . . . 17
6.2.2. Transit Non-SR Capable Node . . . . . . . . . . . . . 18
6.2.3. SR Intra Segment Transit Node . . . . . . . . . . . . 18
6.2.4. SR Segment Endpoint Node . . . . . . . . . . . . . . 18
6.3. FRR Flag Settings . . . . . . . . . . . . . . . . . . . . 19
7. SR and Tunneling . . . . . . . . . . . . . . . . . . . . . . 19
8. Example Use Case . . . . . . . . . . . . . . . . . . . . . . 19
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10. Manageability Considerations . . . . . . . . . . . . . . . . 22
11. Security Considerations . . . . . . . . . . . . . . . . . . . 22
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12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 22
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
14.1. Normative References . . . . . . . . . . . . . . . . . . 22
14.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Structure of this document
Section 3 gives an introduction on SR for IPv6 networks.
Section 4 describes the Segment Routing abstract model.
Section 5 defines the Segment Routing Header (SRH) allowing
instantiation of SR over IPv6 dataplane.
Section 6 details the procedures of the Segment Routing Header.
2. Segment Routing Documents
Segment Routing terminology is defined in
[I-D.filsfils-spring-segment-routing].
Segment Routing use cases are described in
[I-D.filsfils-spring-segment-routing-use-cases].
Segment Routing IPv6 use cases are described in
[I-D.ietf-spring-ipv6-use-cases].
Segment Routing protocol extensions are defined in
[I-D.ietf-isis-segment-routing-extensions], and
[I-D.psenak-ospf-segment-routing-ospfv3-extension].
The security mechanisms of the Segment Routing Header (SRH) are
described in [I-D.vyncke-6man-segment-routing-security].
3. Introduction
Segment Routing (SR), defined in
[I-D.filsfils-spring-segment-routing], allows a node to steer a
packet through a controlled set of instructions, called segments, by
prepending a SR header to the packet. A segment can represent any
instruction, topological or service-based. SR allows to enforce a
flow through any path (topological or service/application based)
while maintaining per-flow state only at the ingress node to the SR
domain. Segments can be derived from different components: IGP, BGP,
Services, Contexts, Locators, etc. The list of segment forming the
path is called the Segment List and is encoded in the packet header.
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SR allows the use of strict and loose source based routing paradigms
without requiring any additional signaling protocols in the
infrastructure hence delivering an excellent scalability property.
The source based routing model described in
[I-D.filsfils-spring-segment-routing] is inherited from the ones
proposed by [RFC1940] and [RFC2460]. The source based routing model
offers the support for explicit routing capability.
3.1. Data Planes supporting Segment Routing
Segment Routing (SR), can be instantiated over MPLS
([I-D.filsfils-spring-segment-routing-mpls]) and IPv6. This document
defines its instantiation over the IPv6 data-plane based on the use-
cases defined in [I-D.ietf-spring-ipv6-use-cases].
Segment Routing for IPv6 (SR-IPv6) is required in networks where MPLS
data-plane is not used or, when combined with SR-MPLS, in networks
where MPLS is used in the core and IPv6 is used at the edge (home
networks, datacenters).
This document defines a new type of Routing Header (originally
defined in [RFC2460]) called the Segment Routing Header (SRH) in
order to convey the Segment List in the packet header as defined in
[I-D.filsfils-spring-segment-routing]. Mechanisms through which
segment are known and advertised are outside the scope of this
document.
3.2. Illustration
In the context of Figure 1 where all the links have the same IGP
cost, let us assume that a packet P enters the SR domain at an
ingress edge router I and that the operator requests the following
requirements for packet P:
The local service S offered by node B must be applied to packet P.
The links AB and CE cannot be used to transport the packet P.
Any node N along the journey of the packet should be able to
determine where the packet P entered the SR domain and where it
will exit. The intermediate node should be able to determine the
paths from the ingress edge router to itself, and from itself to
the egress edge router.
Per-flow State for packet P should only be created at the ingress
edge router.
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The operator can forbid, for security reasons, anyone outside the
operator domain to exploit its intra-domain SR capabilities.
I---A---B---C---E
\ | / \ /
\ | / F
\|/
D
Figure 1: An illustration of SR properties
All these properties may be realized by instructing the ingress SR
edge router I to push the following abstract SR header on the packet
P.
+---------------------------------------------------------------+
| | |
| Abstract SR Header | |
| | |
| {SD, SB, SS, SF, SE}, Ptr, SI, SE | Transported |
| ^ | | Packet |
| | | | P |
| +---------------------+ | |
| | |
+---------------------------------------------------------------+
Figure 2: Packet P at node I
The abstract SR header contains a source route encoded as a list of
segments {SD, SB, SS, SF, SE}, a pointer (Ptr) and the identification
of the ingress and egress SR edge routers (segments SI and SE).
A segment identifies a topological instruction or a service
instruction. A segment can either be global or local. The
instruction associated with a global segment is recognized and
executed by any SR-capable node in the domain. The instruction
associated with a local segment is only supported by the specific
node that originates it.
Let us assume some IGP (i.e.: ISIS and OSPF) extensions to define a
"Node Segment" as a global instruction within the IGP domain to
forward a packet along the shortest path to the specified node. Let
us further assume that within the SR domain illustrated in Figure 1,
segments SI, SD, SB, SE and SF respectively identify IGP node
segments to I, D, B, E and F.
Let us assume that node B identifies its local service S with local
segment SS.
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With all of this in mind, let us describe the journey of the packet
P.
The packet P reaches the ingress SR edge router. I pushes the SR
header illustrated in Figure 2 and sets the pointer to the first
segment of the list (SD).
SD is an instruction recognized by all the nodes in the SR domain
which causes the packet to be forwarded along the shortest path to D.
Once at D, the pointer is incremented and the next segment is
executed (SB).
SB is an instruction recognized by all the nodes in the SR domain
which causes the packet to be forwarded along the shortest path to B.
Once at B, the pointer is incremented and the next segment is
executed (SS).
SS is an instruction only recognized by node B which causes the
packet to receive service S.
Once the service applied, the next segment is executed (SF) which
causes the packet to be forwarded along the shortest path to F.
Once at F, the pointer is incremented and the next segment is
executed (SE).
SE is an instruction recognized by all the nodes in the SR domain
which causes the packet to be forwarded along the shortest path to E.
E then removes the SR header and the packet continues its journey
outside the SR domain.
All of the requirements are met.
First, the packet P has not used links AB and CE: the shortest-path
from I to D is I-A-D, the shortest-path from D to B is D-B, the
shortest-path from B to F is B-C-F and the shortest-path from F to E
is F-E, hence the packet path through the SR domain is I-A-D-B-C-F-E
and the links AB and CE have been avoided.
Second, the service S supported by B has been applied on packet P.
Third, any node along the packet path is able to identify the service
and topological journey of the packet within the SR domain. For
example, node C receives the packet illustrated in Figure 3 and hence
is able to infer where the packet entered the SR domain (SI), how it
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got up to itself {SD, SB, SS, SE}, where it will exit the SR domain
(SE) and how it will do so {SF, SE}.
+---------------------------------------------------------------+
| | |
| SR Header | |
| | |
| {SD, SB, SS, SF, SE}, Ptr, SI, SE | Transported |
| ^ | | Packet |
| | | | P |
| +--------+ | |
| | |
+---------------------------------------------------------------+
Figure 3: Packet P at node C
Fourth, only node I maintains per-flow state for packet P. The
entire program of topological and service instructions to be executed
by the SR domain on packet P is encoded by the ingress edge router I
in the SR header in the form of a list of segments where each segment
identifies a specific instruction. No further per-flow state is
required along the packet path. The per-flow state is in the SR
header and travels with the packet. Intermediate nodes only hold
states related to the IGP global node segments and the local IGP
adjacency segments. These segments are not per-flow specific and
hence scale very well. Typically, an intermediate node would
maintain in the order of 100's to 1000's global node segments and in
the order of 10's to 100 of local adjacency segments. Typically the
SR IGP forwarding table is expected to be much less than 10000
entries.
Fifth, the SR header is inserted at the entrance to the domain and
removed at the exit of the operator domain. For security reasons,
the operator can forbid anyone outside its domain to use its intra-
domain SR capability.
4. Abstract Routing Model
At the entrance of the SR domain, the ingress SR edge router pushes
the SR header on top of the packet. At the exit of the SR domain,
the egress SR edge router removes the SR header.
The abstract SR header contains an ordered list of segments, a
pointer identifying the next segment to process and the
identifications of the ingress and egress SR edge routers on the path
of this packet. The pointer identifies the segment that MUST be used
by the receiving router to process the packet. This segment is
called the active segment.
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A property of SR is that the entire source route of the packet,
including the identity of the ingress and egress edge routers is
always available with the packet. This allows for interesting
accounting and service applications.
We define three SR-header operations:
"PUSH": an SR header is pushed on an IP packet, or additional
segments are added at the head of the segment list. The pointer
is moved to the first entry of the added segments.
"NEXT": the active segment is completed, the pointer is moved to
the next segment in the list.
"CONTINUE": the active segment is not completed, the pointer is
left unchanged.
In the future, other SR-header management operations may be defined.
As the packet travels through the SR domain, the pointer is
incremented through the ordered list of segments and the source route
encoded by the SR ingress edge node is executed.
A node processes an incoming packet according to the instruction
associated with the active segment.
Any instruction might be associated with a segment: for example, an
intra-domain topological strict or loose forwarding instruction, a
service instruction, etc.
At minimum, a segment instruction must define two elements: the
identity of the next-hop to forward the packet to (this could be the
same node or a context within the node) and which SR-header
management operation to execute.
Each segment is known in the network through a Segment Identifier
(SID). The terms "segment" and "SID" are interchangeable.
4.1. Segment Routing Global Block (SRGB)
In the SR abstract model, a segment is identified by a Segment
Routing Identifier (SID). The SR abstract model doesn't mandate a
specific format for the SID (IPv6 address or other formats).
In Segment Routing IPv6 the SID is an IPv6 address. Therefore, the
SRGB is materialized by the global IPv6 address space which
represents the set of IPv6 routable addresses in the SR domain. The
following rules apply:
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o Each node of the SR domain MUST be configured with the Segment
Routing Global Block (SRGB).
o All global segments must be allocated from the SRGB. Any SR
capable node MUST be able to process any global segment advertised
by any other node within the SR domain.
o Any segment outside the SRGB has a local significance and is
called a "local segment". An SR-capable node MUST be able to
process the local segments it originates. An SR-capable node MUST
NOT support the instruction associated with a local segment
originated by a remote node.
4.2. Traffic Engineering with SR
An SR Traffic Engineering policy is composed of two elements: a flow
classification and a segment-list to prepend on the packets of the
flow.
In SR, this per-flow state only exists at the ingress edge node where
the policy is defined and the SR header is pushed.
It is outside the scope of the document to define the process that
leads to the instantiation at a node N of an SR Traffic Engineering
policy.
[I-D.filsfils-spring-segment-routing-use-cases] illustrates various
alternatives:
N is deriving this policy automatically (e.g. FRR).
N is provisioned explicitly by the operator.
N is provisioned by a controller or server (e.g.: SDN Controller).
N is provisioned by the operator with a high-level policy which is
mapped into a path thanks to a local CSPF-based computation (e.g.
affinity/SRLG exclusion).
N could also be provisioned by other means.
[I-D.filsfils-spring-segment-routing-use-cases] explains why the
majority of use-cases require very short segment-lists, hence
minimizing the performance impact, if any, of inserting and
transporting the segment list.
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A SDN controller, which desires to instantiate at node N an SR
Traffic Engineering policy, collects the SR capability of node N such
as to ensure that the policy meets its capability.
4.3. Segment Routing Database
The Segment routing Database (SRDB) is a set of entries where each
entry is identified by a SID. The instruction associated with each
entry at least defines the identity of the next-hop to which the
packet should be forwarded and what operation should be performed on
the SR header (PUSH, CONTINUE, NEXT).
+---------+-----------+---------------------------------+
| Segment | Next-Hop | SR Header operation |
+---------+-----------+---------------------------------+
| Sk | M | CONTINUE |
| Sj | N | NEXT |
| Sl | NAT Srvc | NEXT |
| Sm | FW srvc | NEXT |
| Sn | Q | NEXT |
| etc. | etc. | etc. |
+---------+-----------+---------------------------------+
Figure 4: SR Database
Each SR-capable node maintains its local SRDB. SRDB entries can
either derive from local policy or from protocol segment
advertisement.
5. IPv6 Instantiation of Segment Routing
5.1. Segment Identifiers (SIDs) and SRGB
Segment Routing, as described in
[I-D.filsfils-spring-segment-routing], defines Node-SID and
Adjacency-SID. When SR is used over IPv6 data-plane the following
applies.
The SRGB is the global IPv6 address space which represents the set of
IPv6 routable addresses in the SR domain.
Node SIDs are IPv6 addresses part of the SRGB (i.e.: routable
addresses). Adjacency-SIDs are IPv6 addresses which may not be part
of the global IPv6 address space.
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5.1.1. Node-SID
The Node-SID identifies a node. With SR-IPv6 the Node-SID is an IPv6
prefix that the operator configured on the node and that is used as
the node identifier. Typically, in case of a router, this is the
IPv6 address of the node loopback interface. Therefore, SR-IPv6 does
not require any additional SID advertisement for the Node Segment.
The Node-SID is in fact the IPv6 address of the node.
5.1.2. Adjacency-SID
In the SR architecture defined in
[I-D.filsfils-spring-segment-routing] the Adjacency-SID (or Adj-SID)
identifies a given interface and may be local or global (depending on
how it is advertised). A node may advertise one (or more) Adj-SIDs
allocated to a given interface so to force the forwarding of the
packet (when received with that particular Adj-SID) into the
interface regardless the routing entry for the packet destination.
The semantic of the Adj-SID is:
Send out the packet to the interface this prefix is allocated to.
When SR is applied to IPv6, any SID is in a global IPv6 address and
therefore, an Adj-SID has a global significance (i.e.: the IPv6
address representing the SID is a global address). In other words, a
node that advertises the Adj-SID in the form of a global IPv6 address
representing the link/adjacency the packet has to be forwarded to,
will apply to the Adj-SID a global significance.
Advertisement of Adj-SID may be done using multiple mechanisms among
which the ones described in ISIS and OSPF protocol extensions:
[I-D.ietf-isis-segment-routing-extensions] and
[I-D.psenak-ospf-segment-routing-ospfv3-extension]. The distinction
between local and global significance of the Adj-SID is given in the
encoding of the Adj-SID advertisement.
5.2. Segment Routing Extension Header (SRH)
A new type of the Routing Header (originally defined in [RFC2460]) is
defined: the Segment Routing Header (SRH) which has a new Routing
Type, (suggested value 4) to be assigned by IANA.
As an example, if an explicit path is to be constructed across a core
network running ISIS or OSPF, the segment list will contain SIDs
representing the nodes across the path (loose or strict) which,
usually, are the IPv6 loopback interface address of each node. If
the path is across service or application entities, the segment list
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contains the IPv6 addresses of these services or application
instances.
The Segment Routing Header (SRH) is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | Routing Type | Next Segment |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Last Segment | Flags | HMAC Key ID | Policy List Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[0] (128 bits ipv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
...
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[n] (128 bits ipv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Policy List[0] (128 bits ipv6 address) |
| (optional) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Policy List[1] (128 bits ipv6 address) |
| (optional) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Policy List[2] (128 bits ipv6 address) |
| (optional) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| |
| HMAC (256 bits) |
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| (optional) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
o Next Header: 8-bit selector. Identifies the type of header
immediately following the SRH.
o Hdr Ext Len: 8-bit unsigned integer, is the length of the SRH
header in 8-octet units, not including the first 8 octets.
o Routing Type: TBD, to be assigned by IANA.
o Next Segment (originally defined as "Segments Left" in [RFC2460]):
offset (in multiple of 8 octets not including the first 8 octets)
of the next active segment (according to terminology defined in
[I-D.filsfils-spring-segment-routing]) in the SRH. Note that this
differs from the semantic defined in the Routing Header
specification ([RFC2460] defines it as "Segments Left").
Therefore, in the Segment Routing context, the "Segments Left"
field is renamed as "Next Segment".
o Last Segment: offset (in multiple of 8 octets not including the
first 8 octets) of the last segment of the path in the SRH.
o Flags: 4 bits of flags. Two flags are defined:
0 1 2 3
+-+-+-+-+
|C|P| | |
+-+-+-+-+
Bit-0: Clean-up Bit. Set when the SRH has to be removed from
the packet when packet reaches the last segment.
Bit-1: Protected Bit. Set when the packet has been rerouted
through FRR mechanism by a SR endpoint node. See Section 6.3
for more details.
o HMAC Key ID and HMAC field, and their use are defined in
[I-D.vyncke-6man-segment-routing-security].
o Policy List flags. Define the type of the IPv6 addresses encoded
into the Policy List (see below). The following have been
defined:
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Bits 0-2: determine the type of the first element after the
segment list.
Bits 3-5: determine the type of the second element.
Bits 6-8: determine the type of the third element.
Bits 9-11: determine the type of the fourth element.
The following values are used for the type:
0x0: Not present. If value is set to 0x0, it means the element
represented by these bits is not present.
0x1: Ingress SR PE address.
0x2: Egress SR PE address.
0x3: Original Source Address.
o Segment List[n]: 128 bit IPv6 addresses representing the nth
segment of the path.
o Policy List. Optional addresses representing specific nodes in
the SR path such as:
Ingress SR PE: IPv6 address representing the SR node which has
imposed the SRH (SR domain ingress).
Egress SR PE: IPv6 address representing the egress SR domain
node.
Original Source Address: IPv6 address originally present in the
SA field of the packet.
The segments in the Policy List are encoded after the segment list
and they are optional. If none are in the SRH, all bits of the
Policy List Flags MUST be set to 0x0.
5.2.1. SRH and RFC2460 behavior
The SRH being a new type of the Routing Header, it also has the same
properties:
SHOULD only appear once in the packet.
Only the router whose address is in the DA field of the packet
header MUST inspect the SRH.
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Therefore, Segment Routing in IPv6 networks implies that the segment
identifier (i.e.: the IPv6 address of the segment) is moved into the
DA of the packet.
The DA of the packet changes at each segment termination/completion
and therefore the original DA of the packet MUST be encoded as the
last segment of the path.
As illustrated in Section 3.2, nodes that are within the path of a
segment will forward packets based on the DA of the packet without
inspecting the SRH. This ensures full interoperability between SR-
capable and non-SR-capable nodes.
5.2.2. SRH Optimization
In order to optimize the way the SRH and, more precisely, the Segment
List is processed by SR nodes, it is desirable that most of the
necessary information of the SL is placed at the top of the list so
to avoid reading the whole content of the SRH prior to make
forwarding decisions.
With this in mind, when the SRH is created and the segment list is
inserted, the order of the segments in the segment list is as
follows:
o The Next Segment field points to the next segment to be examined
(offset within the SRH).
o The first segment being encoded in the DA by the ingress node, it
doesn't need to sit in the first position of the list.
o Hence, the first element of the segment list is the second segment
of the path so that, when the packet reaches the end of the first
segment, the node inspecting the SRH will find the second segment
at the beginning of the segment list.
o The other segments of the path are encoded sequentially after the
second segment.
o The last segment of the path is the original DA address.
o The last segment in the Segment List is used to encode the first
segment. This segment will never be inspected anyway (at least
not for forwarding purposes).
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6. SRH Procedures
In this section we describe the different procedures on the SRH.
6.1. Segment Routing Operations
When Segment Routing is instantiated over the IPv6 data plane the
following applies:
o The segment list is encoded in the SRH.
o The active segment is in the destination address of the packet.
o The Segment Routing CONTINUE operation (as described in
[I-D.filsfils-spring-segment-routing]) is implemented as a
regular/plain IPv6 operation consisting of DA based forwarding.
o The NEXT operation is implemented through the update of the DA
with the value represented by the Next Segment field in the SRH.
o The PUSH operation is implemented through the insertion of the SRH
or the insertion of additional segments in the SRH segment list.
6.2. Segment Routing Node Functions
SR packets are forwarded to segments endpoints (i.e.: nodes whose
address is in the DA field of the packet). The segment endpoint,
when receiving a SR packet destined to itself, does:
o Inspect the SRH.
o Determine the next segment.
o Update the SRH (or, if requested, remove the SRH from the packet).
o Update the DA.
o Send the packet to the next segment.
The procedures applied to the SRH are related to the node function.
Following nodes functions are defined:
Ingress SR Node.
Transit Non-SR Node.
Transit SR Intra Segment Node.
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SR Endpoint Node.
6.2.1. Ingress SR Node
Ingress Node can be a router at the edge of the SR domain or a SR-
capable host. The ingress SR node may obtain the segment list by
either:
Local path computation.
Local configuration.
Interaction with an SDN controller delivering the path as a
complete SRH.
Any other mechanism (mechanisms through which the path is acquired
are outside the scope of this document).
When creating the SRH (either at ingress node or in the SDN
controller) the following is done:
Next Header and Hdr Ext Len fields are set according to [RFC2460].
Routing Type field is set as TBD (SRH).
The DA of the packet is set with the address of the FIRST segment
of the path.
Next Segment field contains the offset of the SECOND segment of
the path which is encoded in the FIRST position of the segment
list. The segment list is encoded as follows:
The first element of the list contains the second segment (as
stated above).
All subsequent segments are encoded following the second
segment.
The original DA of the packet is encoded as the last segment of
the path (which is NOT the last segment of the segment list).
The last segment of the segment list is the FIRST segment of
the path.
Last Segment field contains the offset of the last segment of the
path (i.e.: the original DA of the packet).
The packet is sent out to the first segment.
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6.2.1.1. Security at Ingress
The procedures related to the Segment Routing security are detailed
in [I-D.vyncke-6man-segment-routing-security].
In the case where the SR domain boundaries are not under control of
the network operator (e.g.: when the SR domain edge is in a home
network), it is important to authenticate and validate the content of
any SRH being received by the network operator. In such case, the
security procedure described in
[I-D.vyncke-6man-segment-routing-security] is to be used.
The ingress node (e.g.: the host in the home network) requests the
SRH from a control system (e.g.: an SDN controller) which delivers
the SRH with its HMAC signature on it.
Then, the home network host can send out SR packets (with an SRH on
it) that will be validated at the ingress of the network operator
infrastructure.
The ingress node of the network operator infrastructure, is
configured in order to validate the incoming SRH HMACs in order to
allow only packets having correct SRH according to their SA/DA
addresses.
6.2.2. Transit Non-SR Capable Node
SR is interoperable with plain IPv6 forwarding. Any non SR-capable
node will forward SR packets solely based on the DA. There's no SRH
inspection. This ensures full interoperability between SR and non-SR
nodes.
6.2.3. SR Intra Segment Transit Node
Only the node whose address is in DA inspects and processes the SRH
(according to [RFC2460]). An intra segment transit node is not in
the DA and its forwarding is based on DA and its SR-IPv6 FIB.
6.2.4. SR Segment Endpoint Node
The SR segment endpoint node is the node whose address is in the DA.
The segment endpoint node inspects the SRH and does:
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1. IF DA = myself (segment endpoint)
2. IF Next Segment <> Last Segment THEN
update DA with Next Segment
increment Next Segment
3. ELSE IF Last Segment <> DA THEN
update DA with Next Segment
IF Clean-up bit is set THEN remove the SRH
4. ELSE give the packet to next PID (application)
End of processing.
5. Forward the packet out
6.3. FRR Flag Settings
A node supporting SR and doing Fast Reroute (as described in
[I-D.filsfils-spring-segment-routing-use-cases], when rerouting
packets through FRR mechanisms, SHOULD inspect the rerouted packet
header and look for the SRH. If the SRH is present, the rerouting
node SHOULD set the Protected bit on all rerouted packets.
7. SR and Tunneling
Encapsulation can be realized in two different ways with SR-IPv6:
Outer encapsulation.
SRH with SA/DA original addresses.
Outer encapsulation tunneling is the traditional method where an
additional IPv6 header is prepended to the packet. The original IPv6
header being encapsulated, everything is preserved and the packet is
switched/routed according to the outer header (that could contain a
SRH).
SRH allows encoding both original SA and DA and therefore, hence an
operator may decide to change the SA/DA at ingress and restore them
at egress. This can be achieved without outer encapsulation, by
changing SA/DA and encoding the original values in the Segment List
(the last segment of the path being the original DA) and in the
Policy List (original SA).
8. Example Use Case
A more detailed description of use cases are available in
[I-D.ietf-spring-ipv6-use-cases]. In this section, a simple SR-IPv6
example is illustrated.
In the topology described in Figure 6 it is assumed an end-to-end SR
deployment. Therefore SR is supported by all nodes from A to J.
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Home Network | Backbone | Datacenter
| |
| +---+ +---+ +---+ | +---+ |
+---|---| C |---| D |---| E |---|---| I |---|
| | +---+ +---+ +---+ | +---+ |
| | | | | | | | +---+
+---+ +---+ | | | | | | |--| X |
| A |---| B | | +---+ +---+ +---+ | +---+ | +---+
+---+ +---+ | | F |---| G |---| H |---|---| J |---|
| +---+ +---+ +---+ | +---+ |
| |
| +-----------+
| SDN |
| Orch/Ctlr |
+-----------+
Figure 6: Sample SR topology
The following workflow applies to packets sent by host A and destined
to server X.
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. Host A sends a request for a path to server X to the SDN
controller or orchestration system.
. The SDN controller/orchestrator builds a SRH with:
. Segment List: C, F, J, X
. HMAC
that satisfies the requirements expressed in the request
by host A and based on policies applicable to host A.
. Host A receives the SRH and insert it into the packet.
The packet has now:
. SA: A
. DA: C
. SRH with
. SL: F,J,X,C
. PL: C (ingress), J (egress)
Note that X is the last segment and C is the
first segment (encoded at the end of the SL).
. When packet arrives in C (first segment), C does:
. Validate the HMAC of the SRH.
. Update the DA with the next segment (found in SRH):
DA is set to F.
. Forward the packet to F.
. Packet arrives in F which inspects the SRH and find the
next segment:
. DA is set to J.
. Packet travels across G and H nodes which do plain
IPv6 forwarding based on DA. No inspection of SRH needs
to be done in these nodes. However, any SR capable node
is allowed to set the Protected bit in case of FRR
protection.
. Packet arrives in J where two options are available
depending on the settings of the cleanup bit set in the
SRH:
. If the cleanup bit is set, then node J will strip out
the SRH from the packet, set the DA as X and send
the packet out.
. If the clean-up bit is not set, the DA is set to X
and the packet is sent out with the SRH.
The packet arrives in the server that may or may not support SR. The
return traffic, from server to host, may be sent using the same
procedures.
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9. IANA Considerations
TBD
10. Manageability Considerations
TBD
11. Security Considerations
Security mechanisms applied to Segment Routing over IPv6 networks are
detailed in [I-D.vyncke-6man-segment-routing-security].
12. Contributors
TBD.
13. Acknowledgements
The authors would like to thank John Leddy, John Brzozowski, Pierre
Francois, Nagendra Kumar, Mark Townsley, Christian Martin, Roberta
Maglione, James Connolly and David Lebrun for their contribution to
this document.
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
14.2. Informative References
[I-D.filsfils-spring-segment-routing]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
"Segment Routing Architecture", draft-filsfils-spring-
segment-routing-03 (work in progress), June 2014.
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[I-D.filsfils-spring-segment-routing-mpls]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
"Segment Routing with MPLS data plane", draft-filsfils-
spring-segment-routing-mpls-02 (work in progress), June
2014.
[I-D.filsfils-spring-segment-routing-use-cases]
Filsfils, C., Francois, P., Previdi, S., Decraene, B.,
Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
Ytti, S., Henderickx, W., Tantsura, J., Kini, S., and E.
Crabbe, "Segment Routing Use Cases", draft-filsfils-
spring-segment-routing-use-cases-00 (work in progress),
March 2014.
[I-D.ietf-isis-segment-routing-extensions]
Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS
Extensions for Segment Routing", draft-ietf-isis-segment-
routing-extensions-02 (work in progress), June 2014.
[I-D.ietf-spring-ipv6-use-cases]
Brzozowski, J., Leddy, J., Leung, I., Previdi, S.,
Townsley, W., Martin, C., Filsfils, C., and R. Maglione,
"IPv6 SPRING Use Cases", draft-ietf-spring-ipv6-use-
cases-00 (work in progress), May 2014.
[I-D.psenak-ospf-segment-routing-ospfv3-extension]
Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
Shakir, R., and W. Henderickx, "OSPFv3 Extensions for
Segment Routing", draft-psenak-ospf-segment-routing-
ospfv3-extension-01 (work in progress), February 2014.
[I-D.vyncke-6man-segment-routing-security]
Vyncke, E. and S. Previdi, "IPv6 Segment Routing Header
(SRH) Security Considerations", July 2014.
[RFC1940] Estrin, D., Li, T., Rekhter, Y., Varadhan, K., and D.
Zappala, "Source Demand Routing: Packet Format and
Forwarding Specification (Version 1)", RFC 1940, May 1996.
Authors' Addresses
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Stefano Previdi (editor)
Cisco Systems, Inc.
Via Del Serafico, 200
Rome 00142
Italy
Email: sprevidi@cisco.com
Clarence Filsfils
Cisco Systems, Inc.
Brussels
BE
Email: cfilsfil@cisco.com
Brian Field
Comcast
4100 East Dry Creek Road
Centennial, CO 80122
US
Email: Brian_Field@cable.comcast.com
Ida Leung
Rogers Communications
8200 Dixie Road
Brampton, ON L6T 0C1
CA
Email: Ida.Leung@rci.rogers.com
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