Network Working Group S. Previdi, Ed.
Internet-Draft C. Filsfils
Intended status: Standards Track Cisco Systems, Inc.
Expires: April 4, 2016 B. Field
Comcast
I. Leung
Rogers Communications
J. Linkova
Google
E. Aries
Facebook
T. Kosugi
NTT
E. Vyncke
Cisco Systems, Inc.
D. Lebrun
Universite Catholique de Louvain
October 2, 2015
IPv6 Segment Routing Header (SRH)
draft-previdi-6man-segment-routing-header-08
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.
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Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 4, 2016.
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Copyright (c) 2015 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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Table of Contents
1. Segment Routing Documents . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Data Planes supporting Segment Routing . . . . . . . . . 4
2.2. Segment Routing (SR) Domain . . . . . . . . . . . . . . . 4
2.2.1. SR Domain in a Service Provider Network . . . . . . . 5
2.2.2. SR Domain in a Overlay Network . . . . . . . . . . . 6
2.3. Illustration . . . . . . . . . . . . . . . . . . . . . . 8
3. IPv6 Instantiation of Segment Routing . . . . . . . . . . . . 10
3.1. Segment Identifiers (SIDs) . . . . . . . . . . . . . . . 10
3.1.1. Node-SID . . . . . . . . . . . . . . . . . . . . . . 10
3.1.2. Adjacency-SID . . . . . . . . . . . . . . . . . . . . 11
3.2. Segment Routing Extension Header (SRH) . . . . . . . . . 11
3.2.1. SRH and RFC2460 behavior . . . . . . . . . . . . . . 14
4. SRH Procedures . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. Segment Routing Node Functions . . . . . . . . . . . . . 15
4.1.1. Source SR Node . . . . . . . . . . . . . . . . . . . 16
4.1.2. SR Domain Ingress Node . . . . . . . . . . . . . . . 17
4.1.3. Transit Node . . . . . . . . . . . . . . . . . . . . 17
4.1.4. SR Segment Endpoint Node . . . . . . . . . . . . . . 17
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5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
5.1. Threat model . . . . . . . . . . . . . . . . . . . . . . 19
5.1.1. Source routing threats . . . . . . . . . . . . . . . 19
5.1.2. Applicability of RFC 5095 to SRH . . . . . . . . . . 19
5.1.3. Service stealing threat . . . . . . . . . . . . . . . 20
5.1.4. Topology disclosure . . . . . . . . . . . . . . . . . 20
5.1.5. ICMP Generation . . . . . . . . . . . . . . . . . . . 20
5.2. Security fields in SRH . . . . . . . . . . . . . . . . . 21
5.2.1. Selecting a hash algorithm . . . . . . . . . . . . . 22
5.2.2. Performance impact of HMAC . . . . . . . . . . . . . 22
5.2.3. Pre-shared key management . . . . . . . . . . . . . . 23
5.3. Deployment Models . . . . . . . . . . . . . . . . . . . . 23
5.3.1. Nodes within the SR domain . . . . . . . . . . . . . 23
5.3.2. Nodes outside of the SR domain . . . . . . . . . . . 24
5.3.3. SR path exposure . . . . . . . . . . . . . . . . . . 24
5.3.4. Impact of BCP-38 . . . . . . . . . . . . . . . . . . 25
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
7. Manageability Considerations . . . . . . . . . . . . . . . . 25
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 25
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
10.1. Normative References . . . . . . . . . . . . . . . . . . 26
10.2. Informative References . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Segment Routing Documents
Segment Routing terminology is defined in
[I-D.ietf-spring-segment-routing].
Segment Routing use cases are described in
[I-D.ietf-spring-problem-statement] and
[I-D.ietf-spring-ipv6-use-cases].
Segment Routing protocol extensions are defined in
[I-D.ietf-isis-segment-routing-extensions], and
[I-D.ietf-ospf-ospfv3-segment-routing-extensions].
2. Introduction
Segment Routing (SR), defined in [I-D.ietf-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,
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Locators, etc. The list of segment forming the path is called the
Segment List and is encoded in the packet header.
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.ietf-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.
2.1. Data Planes supporting Segment Routing
Segment Routing (SR), can be instantiated over MPLS
([I-D.ietf-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].
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.ietf-spring-segment-routing]. Mechanisms through which segment
are known and advertised are outside the scope of this document.
A segment is materialized by an IPv6 address. A segment identifies a
topological instruction or a service instruction. A segment can be
either:
o global: a global segment represents an instruction supported by
all nodes in the SR domain and it is instantiated through an IPv6
address globally known in the SR domain.
o local: a local segment represents an instruction supported only by
the node who originates it and it is instantiated through an IPv6
address that is known only by the local node.
2.2. Segment Routing (SR) Domain
We define the concept of the Segment Routing Domain (SR Domain) as
the set of nodes participating into the source based routing model.
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).
A non-exhaustive list of examples of SR Domains is:
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o The network of an operator, service provider, content provider,
enterprise including nodes, links and Autonomous Systems.
o A set of nodes connected as an overlay over one or more transit
providers. The overlay nodes exchange SR-enabled traffic with
segments belonging solely to the overlay routers (the SR domain).
None of the segments in the SR-enabled packets exchanged by the
overlay belong to the transit networks
The source based routing model through its instantiation of the
Segment Routing Header (SRH) defined in this document equally applies
to all the above examples.
While the source routing model defined in [RFC2460] doesn't mandate
which node is allowed to insert (or modify) the SRH, it is assumed in
this document that the SRH is inserted in the packet by its source.
For example:
o At the node originating the packet (host, server).
o At the ingress node of a SR domain where the ingress node receives
an IPv6 packet and encapsulates it into an outer IPv6 header
followed by a Segment Routing header.
2.2.1. SR Domain in a Service Provider Network
The following figure illustrates an SR domain consisting of an
operator's network infrastructure.
(-------------------------- Operator 1 -----------------------)
( )
( (-----AS 1-----) (-------AS 2-------) (----AS 3-------) )
( ( ) ( ) ( ) )
A1--(--(--11---13--14-)--(-21---22---23--24-)--(-31---32---34--)--)--Z1
( ( /|\ /|\ /| ) ( |\ /|\ /|\ /| ) ( |\ /|\ /| \ ) )
A2--(--(/ | \/ | \/ | ) ( | \/ | \/ | \/ | ) ( | \/ | \/ | \)--)--Z2
( ( | /\ | /\ | ) ( | /\ | /\ | /\ | ) ( | /\ | /\ | ) )
( ( |/ \|/ \| ) ( |/ \|/ \|/ \| ) ( |/ \|/ \| ) )
A3--(--(--15---17--18-)--(-25---26---27--28-)--(-35---36---38--)--)--Z3
( ( ) ( ) ( ) )
( (--------------) (------------------) (---------------) )
( )
(-------------------------------------------------------------)
Figure 1: Service Provider SR Domain
Figure 1 describes an operator network including several ASes and
delivering connectivity between endpoints. In this scenario, Segment
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Routing is used within the operator networks and across the ASes
boundaries (all being under the control of the same operator). In
this case segment routing can be used in order to address use cases
such as end-to-end traffic engineering, fast re-route, egress peer
engineering, data-center traffic engineering as described in
[I-D.ietf-spring-problem-statement], [I-D.ietf-spring-ipv6-use-cases]
and [I-D.ietf-spring-resiliency-use-cases].
Typically, an IPv6 packet received at ingress (i.e.: from outside the
SR domain), is classified according to network operator policies and
such classification results into an outer header with an SRH applied
to the incoming packet. The SRH contains the list of segment
representing the path the packet must take inside the SR domain.
Thus, the SA of the packet is the ingress node, the DA (due to SRH
procedures described in Section 4) is set as the first segment of the
path and the last segment of the path is the egress node of the SR
domain.
The path may include intra-AS as well as inter-AS segments. It has
to be noted that all nodes within the SR domain are under control of
the same administration. When the packet reaches the egress point of
the SR domain, the outer header and its SRH are removed so that the
destination of the packet is unaware of the SR domain the packet has
traversed.
The outer header with the SRH is no different from any other
tunneling encapsulation mechanism and allows a network operator to
implement traffic engineering mechanisms so to efficiently steer
traffic across his infrastructure.
2.2.2. SR Domain in a Overlay Network
The following figure illustrates an SR domain consisting of an
overlay network over multiple operator's networks.
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(--Operator 1---) (-----Operator 2-----) (--Operator 3---)
( ) ( ) ( )
A1--(--11---13--14--)--(--21---22---23--24--)--(-31---32---34--)--C1
( /|\ /|\ /| ) ( |\ /|\ /|\ /| ) ( |\ /|\ /| \ )
A2--(/ | \/ | \/ | ) ( | \/ | \/ | \/ | ) ( | \/ | \/ | \)--C2
( | /\ | /\ | ) ( | /\ | /\ | /\ | ) ( | /\ | /\ | )
( |/ \|/ \| ) ( |/ \|/ \|/ \| ) ( |/ \|/ \| )
A3--(--15---17--18--)--(--25---26---27--28--)--(-35---36---38--)--C3
( ) ( | | | ) ( )
(---------------) (--|----|---------|--) (---------------)
| | |
B1 B2 B3
Figure 2: Overlay SR Domain
Figure 2 describes an overlay consisting of nodes connected to three
different network operators and forming a single overlay network
where Segment routing packets are exchanged.
The overlay consists of nodes A1, A2, A3, B1, B2, B3, C1, C2 and C3.
These nodes are connected to their respective network operator and
form an overlay network.
Each node may originate packets with an SRH which contains, in the
segment list of the SRH or in the DA, segments identifying other
overlay nodes. This implies that packets with an SRH may traverse
operator's networks but, obviously, these SRHs cannot contain an
address/segment of the transit operators 1, 2 and 3. The SRH
originated by the overlay can only contain address/segment under the
administration of the overlay (e.g. address/segments supported by A1,
A2, A3, B1, B2, B3, C1,C2 or C3).
In this model, the operator network nodes are transit nodes and,
according to [RFC2460], MUST NOT inspect the routing extension header
since there are not the DA of the packet.
It is a common practice in operators networks to filter out, at
ingress, any packet whose DA is the address of an internal node and
it is also possible that an operator would filter out any packet
destined to an internal address and having an extension header in it.
This common practice does not impact the SR-enabled traffic between
the overlay nodes as the intermediate transit networks do never see a
destination address belonging to their infrastructure. These SR-
enabled overlay packets will thus never be filtered by the transit
operators.
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In all cases, transit packets (i.e.: packets whose DA is outside the
domain of the operator's network) will be forwarded accordingly
without introducing any security concern in the operator's network.
This is similar to tunneled packets.
2.3. Illustration
In the context of Figure 3 we illustrate an example of how segment
routing an be used within a SR domain in order to engineer traffic.
Let's assume that the SR domain is configured as a single AS and the
IGP (OSPF or IS-IS) is configured using the same cost on every link.
Let's also 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:
o The local service S offered by node B must be applied to packet P.
o The links AB and CE cannot be used to transport the packet P.
o 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.
o Per-flow State for packet P should only be created at the ingress
edge router.
o The operator can forbid, for security reasons, anyone outside the
operator domain to exploit its intra-domain SR capabilities.
S
I---A---B---C---E
\ | / \ /
\ | / F
\|/
D
Figure 3: An illustration of SR properties
All these properties may be realized by instructing the ingress SR
edge router I to create a SRH with the list of segments the packet
must traverse: D, B, S, F, E. Therefore, the ingress router I
creates an outer header where:
o the SA is the IPv6 address of I
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o the final destination of the packet is the SR egress node E
however, D being the first segment of the path, the DA is set to D
IPv6 address.
o the SRH is inserted with the segment list consisting of following
IPv6 addresses: D, B, S, F, E
The SRH contains a source route encoded as a list of segments (D, B,
S, F, E). The ingress and egress nodes are identified in the packet
respectively by the SA and the last segment of the segment list.
The packet P reaches the ingress SR node I. Node I pushes the newly
created outer header and SRH with the Segment List as illustrated
above (D, B, S, F, E)
D is the IPv6 address of node D and it is recognized by all nodes in
the SR domain as the forwarding instruction "forward to D according
to D route in the IPv6 routing table". The routing table being built
through IGPs (OSPF or IS-IS) it is equivalent to say "forward
according to shortest path to D".
Once at D, the next segment is inspected and executed (segment B).
B 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 next segment is executed (segment S).
S is an instruction only recognized by node B which causes the packet
to receive service S.
Once the service S is applied, the next segment is executed (segment
F) which causes the packet to be forwarded along the shortest path to
F.
Once at F, the next segment is executed (segment E).
E 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 being the destination of the packet, removes the outer header and
the SRH. Then, it inspects the inner packet header and forwards the
packet accordingly.
All of the requirements are met:
o 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,
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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.
o Second, the service S supported by B has been applied on packet P.
o Third, any node along the packet path is able to identify the
service and topological journey of the packet within the SR domain
by inspecting the SRH and SA/DA fields of the packet header.
o 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. Intermediate
nodes only hold states related to the global node segments and
their local 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 segments.
o Fifth, the SR header (and its outer 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 (e.g.
configuring ACL that deny any packet with a DA towards its
infrastructure segment).
3. IPv6 Instantiation of Segment Routing
3.1. Segment Identifiers (SIDs)
Segment Routing, as described in [I-D.ietf-spring-segment-routing],
defines Node-SID and Adjacency-SID. When SR is used over IPv6 data-
plane the following applies.
3.1.1. Node-SID
The Node-SID identifies a node. With SR-IPv6 the Node-SID is an IPv6
address 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.
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3.1.2. Adjacency-SID
Adjacency-SIDs can be either globally scoped IPv6 addresses or IPv6
addresses known locally by the node but not advertised in any control
plane (in other words an Adjacency-SID may well be any 128-bit
identifier). Obviously, in the latter case, the scope of the
Adjacency-SID is local to the router and any packet with the a such
Adjacency-SID would need first to reach the node through the node's
Segment Identifier (i.e.: Node-SID) prior for the node to process the
Adjacency-SID. In other words, two segments (SIDs) would then be
required: the first is the node's Node-SID that brings the packet to
the node and the second is the Adjacency-SID that will make the node
to forward the packet through the interface the Adjacency-SID is
allocated to.
In the SR architecture defined in [I-D.ietf-spring-segment-routing] a
node may advertise one (or more) Adj-SIDs allocated to the same
interface as well as a node can advertise the same Adj-SID for
multiple interfaces. Use cases of Adj-SID advertisements are
described in [I-D.ietf-spring-segment-routing]The semantic of the
Adj-SID is:
Send out the packet to the interface this Adj-SID is allocated to.
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.ietf-ospf-ospfv3-segment-routing-extensions]. The distinction
between local and global significance of the Adj-SID is given in the
encoding of the Adj-SID advertisement.
3.2. 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.
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 | Segments Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First Segment | Flags | HMAC Key ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
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| Segment List[0] (128 bits ipv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
...
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[n] (128 bits ipv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Policy List[0] (optional) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Policy List[1] (optional) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Policy List[2] (optional) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Policy List[3] (optional) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| |
| HMAC (256 bits) |
| (optional) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
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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 (suggested value: 4).
o Segments Left. Defined in [RFC2460], it contains the index, in
the Segment List, of the next segment to inspect. Segments Left
is decremented at each segment.
o First Segment: contains the index, in the Segment List, of the
first segment of the path which is in fact the last element of the
Segment List.
o Flags: 16 bits of flags. Following flags are defined:
1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|P|R|R| Policy Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
C-flag: Clean-up flag. Set when the SRH has to be removed from
the packet when packet reaches the last segment.
P-flag: Protected flag. Set when the packet has been rerouted
through FRR mechanism by a SR endpoint node.
R-flags. Reserved and for future use.
Policy Flags. Define the type of the IPv6 addresses encoded
into the Policy List (see below). The following have been
defined:
Bits 4-6: determine the type of the first element after the
segment list.
Bits 7-9: determine the type of the second element.
Bits 10-12: determine the type of the third element.
Bits 13-15: determine the type of the fourth element.
The following values are used for the type:
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0x0: Not present. If value is set to 0x0, it means the
element represented by these bits is not present.
0x1: SR Ingress.
0x2: SR Egress.
0x3: Original Source Address.
0x4 to 0x7: currently unused and SHOULD be ignored on
reception.
o HMAC Key ID and HMAC field, and their use are defined in
Section 5.
o Segment List[n]: 128 bit IPv6 addresses representing the nth
segment in the Segment List. The Segment List is encoded starting
from the last segment of the path. I.e., the first element of the
segment list (Segment List [0]) contains the last segment of the
path while the last segment of the Segment List (Segment List[n])
contains the first segment of the path. The index contained in
"Segments Left" identifies the current active segment.
o Policy List. Optional addresses representing specific nodes in
the SR path such as:
SR Ingress: a 128 bit generic identifier representing the
ingress in the SR domain (i.e.: it needs not to be a valid IPv6
address).
SR Egress: a 128 bit generic identifier representing the egress
in the SR domain (i.e.: it needs not to be a valid IPv6
address).
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.
3.2.1. 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.
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Only the router whose address is in the DA field of the packet
header MUST inspect the SRH.
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 2.3, 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.
4. SRH Procedures
In this section we describe the different procedures on the SRH.
4.1. Segment Routing Node Functions
SR packets are forwarded to segments endpoints (i.e.: the segment
endpoint is the node representing the segment and whose address is in
the segment list and in the DA of the packet when traveling in the
segment). The segment endpoint, when receiving a SR packet destined
to itself, does:
o Inspect the SRH.
o Determine the next active segment.
o Update the Segments Left field (or, if requested, remove the SRH
from the packet).
o Update the DA.
o Forward the packet to the next segment.
The procedures applied to the SRH are related to the node function.
Following nodes functions are defined:
Source SR Node.
SR Domain Ingress Node.
Transit Node.
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SR Endpoint Node.
4.1.1. Source SR Node
A Source SR Node can be any node originating an IPv6 packet with its
IPv6 and Segment Routing Headers. This include either:
A host originating an IPv6 packet
A SR domain ingress router encapsulating a received IPv6 packet
into an outer IPv6 header followed by a SRH
The mechanism through which a Segment List is derived is outside of
the scope of this document. As an example, the Segment List may be
obtained through:
Local path computation.
Local configuration.
Interaction with a centralized controller delivering the path.
Any other mechanism.
The following are the steps of the creation of the SRH:
Next Header and Hdr Ext Len fields are set according to [RFC2460].
Routing Type field is set as TBD (SRH).
The Segment List is built with the FIRST segment of the path
encoded in the LAST element of the Segment List. Subsequent
segments are encoded on top of the first segment. Finally, the
LAST segment of the path is encoded in the FIRST element of the
Segment List. In other words, the Segment List is encoded in the
reverse order of the path.
The original DA of the packet is encoded as the last segment of
the path (encoded in the first element of the Segment List).
The DA of the packet is set with the value of the first segment
(found in the last element of the segment list).
The Segments Left field is set to n-1 where n is the number of
elements in the Segment List.
The First Segment field is set to n-1 where n is the number of
elements in the Segment List.
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The packet is sent out towards the first segment (i.e.:
represented in the packet DA).
HMAC and HMAC Key ID may be set according to Section 5.
4.1.2. SR Domain Ingress Node
The SR Domain Ingress Node is the node where ingress policies are
applied and where the packet path (and processing) is determined.
After policies are applied and packet classification is done, the
result may be instantiated into a Segment List representing the path
the packet should take. In such case, the SR Domain Ingress Node
instantiate a new outer IPv6 header to which the SRH is appended
(with the computed Segment List). The procedures for the creation
and insertion of the new SRH are described in Section 4.1.1.
4.1.3. Transit Node
According to [RFC2460], the only node who is allowed to inspect the
Routing Extension Header (and therefore the SRH), is the node
corresponding to the DA of the packet. Any other transit node MUST
NOT inspect the underneath routing header and MUST forward the packet
towards the DA and according to the IPv6 routing table.
In the example case described in Section 2.2.2, when SR capable nodes
are connected through an overlay spanning multiple third-party
infrastructure, it is safe to send SRH packets (i.e.: packet having a
Segment Routing Header) between each other overlay/SR-capable nodes
as long as the segment list does not include any of the transit
provider nodes. In addition, as a generic security measure, any
service provider will block any packet destined to one of its
internal routers, especially if these packets have an extended header
in it.
4.1.4. 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 Segments Left > 0 THEN
decrement Segments Left
update DA with Segment List[Segments Left]
3. IF Segments Left == 0 THEN
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
5. Security Considerations
This section analyzes the security threat model, the security issues
and mitigation techniques of SRH.
SRH is simply another type of the routing header as described in RFC
2460 [RFC2460] and is:
o added to a new outer IP header by the ingress router when entering
the SR domain or by the originating node itself. The source host
can be outside the SR domain;
o inspected and acted upon when reaching the destination address of
the IP header per RFC 2460 [RFC2460].
Per RFC2460 [RFC2460], routers on the path that simply forward an
IPv6 packet (i.e. the IPv6 destination address is none of theirs)
will never inspect and process the content of any routing header
(including SRH). Routers whose one interface IPv6 address equals the
destination address field of the IPv6 packet MUST to parse the SRH
and, if supported and if the local configuration allows it, MUST act
accordingly to the SRH content.
According to RFC2460 [RFC2460], non SR-capable (or non SR-configured)
router upon receipt of an IPv6 packet with SRH destined to an address
of its:
o must ignore the SRH completely if the Segment Left field is 0 and
proceed to process the next header in the IPv6 packet;
o must discard the IPv6 packet if Segment Left field is greater than
0 and send a Parameter Problem ICMP message back to the Source
Address.
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5.1. Threat model
5.1.1. Source routing threats
Using a SRH is a specific case of loose source routing, therefore it
has some well-known security issues as described in RFC4942 [RFC4942]
section 2.1.1 and RFC5095 [RFC5095]:
o amplification attacks: where a packet could be forged in such a
way to cause looping among a set of SR-enabled routers causing
unnecessary traffic, hence a Denial of Service (DoS) against
bandwidth;
o reflection attack: where a hacker could force an intermediate node
to appear as the immediate attacker, hence hiding the real
attacker from naive forensic;
o bypass attack: where an intermediate node could be used as a
stepping stone (for example in a De-Militarized Zone) to attack
another host (for example in the datacenter or any back-end
server).
5.1.2. Applicability of RFC 5095 to SRH
First of all, the reader must remember this specific part of section
1 of RFC5095 [RFC5095], "A side effect is that this also eliminates
benign RH0 use-cases; however, such applications may be facilitated
by future Routing Header specifications.". In short, it is not
forbidden to create new secure type of Routing Header; for example,
RFC 6554 (RPL) [RFC6554] also creates a new Routing Header type for a
specific application confined in a single network.
The main use case for SR consists of the single administrative domain
(or cooperating administrative domains) where only trusted nodes with
SR enabled and explicitely configured participate in SR: this is the
same model as in RFC6554 [RFC6554]. All non-trusted nodes do not
participate as either SR processing is not enabled by default or
because they only process SRH from nodes within their domain.
Moreover, all SR routers SHOULD ignore SRH created by outsiders based
on topology information (received on a peering or internal interface)
or on presence and validity of the HMAC field. Therefore, if
intermediate SR routers ONLY act on valid and authorized SRH (such as
within a single administrative domain), then there is no security
threat similar to RH-0. Hence, the RFC 5095 [RFC5095] attacks are
not applicable.
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5.1.3. Service stealing threat
Segment routing is used for added value services, there is also a
need to prevent non-participating nodes to use those services; this
is called 'service stealing prevention'.
5.1.4. Topology disclosure
The SRH may also contains IPv6 addresses of some intermediate SR
routers in the path towards the destination, this obviously reveals
those addresses to the potentially hostile attackers if those
attackers are able to intercept packets containing SRH. On the other
hand, if the attacker can do a traceroute whose probes will be
forwarded along the SR path, then there is little learned by
intercepting the SRH itself. The clean-bit of SRH can help by
removing the SRH before forwarding the packet to potentially a non-
trusted part of the network; if the attacker can force the generation
of an ICMP message during the transit in the SR domain, then the ICMP
will probably contain the SRH header (totally or partially) depending
on the ICMP-generating router behavior.
5.1.5. ICMP Generation
Per section 4.4 of RFC2460 [RFC2460], when destination nodes (i.e.
where the destination address is one of theirs) receive a Routing
Header with unsupported Routing Type, the required behavior is:
o If Segments Left is zero, the node must ignore the Routing header
and proceed to process the next header in the packet.
o If Segments Left is non-zero, the node must discard the packet and
SHOULD send an ICMP Parameter Problem, Code 0, message to the
packet's Source Address, pointing to the unrecognized Routing
Type.
This required behavior could be used by an attacker to force the
generation of ICMP message by any node. The attacker could send
packets with SRH (with Segment Left different than 0) destined to a
node not supporting SRH. Per RFC2460 [RFC2460], the destination node
must then generate an ICMP message per RFC 2460, causing a local CPU
utilization and if the source of the offending packet with SRH was
spoofed could lead to a reflection attack without any amplification.
It must be noted that this is a required behavior for any unsupported
Routing Type and not limited to SRH packets. So, it is not specific
to SRH and the usual rate limiting for ICMP generation is required
anyway for any IPv6 implementation and has been implemented and
deployed for many years.
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5.2. Security fields in SRH
This section summarizes the use of specific fields in the SRH. They
are based on a key-hashed message authentication code (HMAC).
The security-related fields in SRH are:
o HMAC Key-id, 8 bits wide;
o HMAC, 256 bits wide (optional, exists only if HMAC Key-id is not
0).
The HMAC field is the output of the HMAC computation (per RFC 2104
[RFC2104]) using a pre-shared key and hashing algorithm identified by
HMAC Key-id and of the text which consists of the concatenation of:
o the source IPv6 address;
o First Segment field;
o an octet whose bit-0 is the clean-up bit flag and others are 0;
o HMAC Key-id;
o all addresses in the Segment List.
The purpose of the HMAC field is to verify the validity, the
integrity and the authorization of the SRH itself. If an outsider of
the SR domain does not have access to a current pre-shared secret,
then it cannot compute the right HMAC field and the first SR router
on the path processing the SRH and configured to check the validity
of the HMAC will simply reject the packet.
The HMAC field is located at the end of the SRH simply because only
the router on the ingress of the SR domain needs to process it, then
all other SR nodes can ignore it (based on local policy) because they
trust the upstream router. This is to speed up forwarding operations
because SR routers which do not validate the SRH do not need to parse
the SRH until the end.
The HMAC Key-id field allows for the simultaneous existence of
several hash algorithms (SHA-256, SHA3-256 ... or future ones) as
well as pre-shared keys. This allows for pre-shared key roll-over
when two pre-shared keys are supported for a while when all SR nodes
converged to a fresher pre-shared key. The HMAC Key-id field is
opaque, i.e., it has neither syntax not semantic except as an index
to the right combination of pre-shared key and hash algorithm and
except that a value of 0 means that there is no HMAC field. It could
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also allow for interoperation among different SR domains if allowed
by local policy and assuming a collision-free Key Id allocation which
is out of scope of this memo.
When a specific SRH is linked to a time-related service (such as
turbo-QoS for a 1-hour period), then it is important to refresh the
shared-secret frequently as the HMAC validity period expires only
when the HMAC Key-id and its associated shared-secret expires.
5.2.1. Selecting a hash algorithm
The HMAC field in the SRH is 256 bits wide. Therefore, the HMAC MUST
be based on a hash function whose output is at least 256 bits. If
the output of the hash function is 256, then this output is simply
inserted in the HMAC field. If the output of the hash function is
larger than 256 bits, then the output value is truncated to 256 by
taking the least-significant 256 bits and inserting them in the HMAC
field.
SRH implementations can support multiple hash functions but MUST
implement SHA-2 [FIPS180-4] in its SHA-256 variant.
NOTE: SHA-1 is currently used by some early implementations used for
quick interoperations testing, the 160-bit hash value must then be
right-hand padded with 96 bits set to 0. The authors understand that
this is not secure but is ok for limited tests.
5.2.2. Performance impact of HMAC
While adding a HMAC to each and every SR packet increases the
security, it has a performance impact. Nevertheless, it must be
noted that:
o the HMAC field SHOULD be used only when SRH is inserted by a
device (such as a home set-up box) which is outside of the segment
routing domain. If the SRH is added by a router in the trusted
segment routing domain, then, there is no need for a HMAC field,
hence no performance impact.
o when present, the HMAC field MUST be checked and validated only by
the first router of the segment routing domain, this router is
named 'validating SR router'. Downstream routers may not inspect
the HMAC field.
o this validating router can also have a cache of <IPv6 header +
SRH, HMAC field value> to improve the performance. It is not the
same use case as in IPsec where HMAC value was unique per packet,
in SRH, the HMAC value is unique per flow.
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o Last point, hash functions such as SHA-2 have been optmized for
security and performance and there are multiple implementations
with good performance.
With the above points in mind, the performance impact of using HMAC
is minimized.
5.2.3. Pre-shared key management
The field HMAC Key-id allows for:
o key roll-over: when there is a need to change the key (the hash
pre-shared secret), then multiple pre-shared keys can be used
simultaneously. The validating routing can have a table of <HMAC
Key-id, pre-shared secret, hash algorithm> for the currently
active and future keys.
o different algorithm: by extending the previous table to <HMAC Key-
id, hash function, pre-shared secret>, the validating router can
also support simultaneously several hash algorithms (see section
Section 5.2.1)
The pre-shared secret distribution can be done:
o in the configuration of the validating routers, either by static
configuration or any SDN oriented approach;
o dynamically using a trusted key distribution such as [RFC6407]
The intent of this document is NOT to define yet-another-key-
distribution-protocol.
5.3. Deployment Models
5.3.1. Nodes within the SR domain
The routers inside a SR domain can be trusted to generate the outer
IP header and the SRH and to process SRH received on interfaces that
are part of the SR domain. These nodes MUST drop all SRH packets
received on any interface that is not part of the SR domain and
containing a SRH whose HMAC field cannot be validated by local
policies. This includes obviously packet with a SRH generated by a
non-cooperative SR domain.
If the validation fails, then these packets MUST be dropped, ICMP
error messages (parameter problem) SHOULD be generated (but rate
limited) and SHOULD be logged.
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5.3.2. Nodes outside of the SR domain
Nodes outside of the SR domain cannot be trusted for physical
security; hence, they need to obtain by some trusted means (outside
of the scope of this document) a complete SRH for each new connection
(i.e. new destination address). The received SRH MUST include a HMAC
Key-id and HMAC field which has been computed correctly (see
Section 5.2).
When a outside the SR domain sends a packet with a SRH and towards a
SR domain ingress node, the packet MUST contain the HMAC Key-id and
HMAC field and the the destination address MUST be an address of a SR
domain ingress node .
The ingress SR router, i.e., the router with an interface address
equals to the destination address, MUST verify the HMAC field with
respect to the HMAC Key-id.
If the validation is successful, then the packet is simply forwarded
as usual for a SR packet. As long as the packet travels within the
SR domain, no further HMAC check needs to be done. Subsequent
routers in the SR domain MAY verify the HMAC field when they process
the SRH (i.e. when they are the destination).
If the validation fails, then this packet MUST be dropped, an ICMP
error message (parameter problem) SHOULD be generated (but rate
limited) and SHOULD be logged.
5.3.3. SR path exposure
As the intermediate SR nodes addresses appears in the SRH, if this
SRH is visible to an outsider then he/she could reuse this knowledge
to launch an attack on the intermediate SR nodes or get some insider
knowledge on the topology. This is especially applicable when the
path between the source node and the first SR domain ingress router
is on the public Internet.
The first remark is to state that 'security by obscurity' is never
enough; in other words, the security policy of the SR domain SHOULD
assume that the internal topology and addressing is known by the
attacker.
IPsec Encapsulating Security Payload [RFC4303] cannot be use to
protect the SRH as per RFC4303 the ESP header must appear after any
routing header (including SRH).
When the SRH is not generated by the actual source node but by an SR
domain ingress router, it is added after a new outer IP header, this
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means that a normal traceroute will not reveal the routers in the SR
domain (pretty much like in a MPLS network) and that if ICMP are
generated by routers in the SR domain they will be sent to the
ingress router of the SR domain without revealing anything to the
outside of the SR domain.
To prevent a user to leverage the gained knowledge by intercepting
SRH, it it recommended to apply an infrastructure Access Control List
(iACL) at the edge of the SR domain. This iACL will drop all packets
from outside the SR-domain whose destination is any address of any
router inside the domain. This security policy should be tuned for
local operations.
5.3.4. Impact of BCP-38
BCP-38 [RFC2827], also known as "Network Ingress Filtering", checks
whether the source address of packets received on an interface is
valid for this interface. The use of loose source routing such as
SRH forces packets to follow a path which differs from the expected
routing. Therefore, if BCP-38 was implemented in all routers inside
the SR domain, then SR packets could be received by an interface
which is not expected one and the packets could be dropped.
As a SR domain is usually a subset of one administrative domain, and
as BCP-38 is only deployed at the ingress routers of this
administrative domain and as packets arriving at those ingress
routers have been normally forwarded using the normal routing
information, then there is no reason why this ingress router should
drop the SRH packet based on BCP-38. Routers inside the domain
commonly do not apply BCP-38; so, this is not a problem.
6. IANA Considerations
TBD but should at least require a new type for routing header
7. Manageability Considerations
TBD should we talk about traceroute? about SRH in ICMP replies?
8. Contributors
The authors would like to thank Dave Barach, John Leddy, John
Brzozowski, Pierre Francois, Nagendra Kumar, Mark Townsley, Christian
Martin, Roberta Maglione, James Connolly, Aloys Augustin and Fred
Baker for their contribution to this document.
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9. Acknowledgements
TBD
10. References
10.1. Normative References
[FIPS180-4]
National Institute of Standards and Technology, "FIPS
180-4 Secure Hash Standard (SHS)", March 2012,
<http://csrc.nist.gov/publications/fips/fips180-4/
fips-180-4.pdf>.
[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>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<http://www.rfc-editor.org/info/rfc4303>.
[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>.
[RFC6407] Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
of Interpretation", RFC 6407, DOI 10.17487/RFC6407,
October 2011, <http://www.rfc-editor.org/info/rfc6407>.
10.2. Informative References
[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-05 (work in progress), June 2015.
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[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-03 (work in progress), June
2015.
[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-05 (work in progress), September 2015.
[I-D.ietf-spring-problem-statement]
Previdi, S., Filsfils, C., Decraene, B., Litkowski, S.,
Horneffer, M., and R. Shakir, "SPRING Problem Statement
and Requirements", draft-ietf-spring-problem-statement-04
(work in progress), April 2015.
[I-D.ietf-spring-resiliency-use-cases]
Francois, P., Filsfils, C., Decraene, B., and R. Shakir,
"Use-cases for Resiliency in SPRING", draft-ietf-spring-
resiliency-use-cases-01 (work in progress), March 2015.
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
and r. rjs@rob.sh, "Segment Routing Architecture", draft-
ietf-spring-segment-routing-05 (work in progress),
September 2015.
[I-D.ietf-spring-segment-routing-mpls]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., Horneffer, M., Shakir, R., Tantsura, J.,
and E. Crabbe, "Segment Routing with MPLS data plane",
draft-ietf-spring-segment-routing-mpls-01 (work in
progress), May 2015.
[RFC1940] Estrin, D., Li, T., Rekhter, Y., Varadhan, K., and D.
Zappala, "Source Demand Routing: Packet Format and
Forwarding Specification (Version 1)", RFC 1940,
DOI 10.17487/RFC1940, May 1996,
<http://www.rfc-editor.org/info/rfc1940>.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<http://www.rfc-editor.org/info/rfc2104>.
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[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <http://www.rfc-editor.org/info/rfc2827>.
[RFC4942] Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/
Co-existence Security Considerations", RFC 4942,
DOI 10.17487/RFC4942, September 2007,
<http://www.rfc-editor.org/info/rfc4942>.
[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554,
DOI 10.17487/RFC6554, March 2012,
<http://www.rfc-editor.org/info/rfc6554>.
Authors' Addresses
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
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Ida Leung
Rogers Communications
8200 Dixie Road
Brampton, ON L6T 0C1
CA
Email: Ida.Leung@rci.rogers.com
Jen Linkova
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: furry@google.com
Ebben Aries
Facebook
US
Email: exa@fb.com
Tomoya Kosugi
NTT
3-9-11, Midori-Cho Musashino-Shi,
Tokyo 180-8585
JP
Email: kosugi.tomoya@lab.ntt.co.jp
Eric Vyncke
Cisco Systems, Inc.
De Kleetlaann 6A
Diegem 1831
Belgium
Email: evyncke@cisco.com
Previdi, et al. Expires April 4, 2016 [Page 29]
Internet-Draft IPv6 Segment Routing Header (SRH) October 2015
David Lebrun
Universite Catholique de Louvain
Place Ste Barbe, 2
Louvain-la-Neuve, 1348
Belgium
Email: david.lebrun@uclouvain.be
Previdi, et al. Expires April 4, 2016 [Page 30]
