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Versions: 00 01 02 03 04 05 06 07 08 draft-ietf-6man-segment-routing-header

Network Working Group                                    S. Previdi, Ed.
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: September 6, 2014                                      B. Field
                                                                 Comcast
                                                                I. Leung
                                                   Rogers Communications
                                                           March 5, 2014


                   IPv6 Segment Routing Header (SRH)
              draft-previdi-6man-segment-routing-header-00

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.

   The Segment Routing architecture 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 September 6, 2014.

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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



































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Table of Contents

   1.  Structure of this document . . . . . . . . . . . . . . . . . .  4
   2.  Segment Routing Documents  . . . . . . . . . . . . . . . . . .  4
   3.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Data Planes supporting Segment Routing . . . . . . . . . .  5
     3.2.  Illustrative Example . . . . . . . . . . . . . . . . . . .  5
   4.  IPv6 Instantiation of Segment Routing  . . . . . . . . . . . .  6
     4.1.  Segment Routing Extension Header (SRH) . . . . . . . . . .  6
       4.1.1.  SRH and RFC2460 behavior . . . . . . . . . . . . . . .  9
       4.1.2.  SRH Optimization . . . . . . . . . . . . . . . . . . . 10
     4.2.  Segment Identifiers (SIDs) . . . . . . . . . . . . . . . . 10
       4.2.1.  Node-SID . . . . . . . . . . . . . . . . . . . . . . . 10
       4.2.2.  Adjacency-SID  . . . . . . . . . . . . . . . . . . . . 11
   5.  SRH Procedures . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.1.  Segment Routing Operations . . . . . . . . . . . . . . . . 11
     5.2.  Segment Routing Node Functions . . . . . . . . . . . . . . 11
       5.2.1.  Ingress SR Node  . . . . . . . . . . . . . . . . . . . 12
       5.2.2.  Transit Non-SR Capable Node  . . . . . . . . . . . . . 13
       5.2.3.  SR Intra Segment Transit Node  . . . . . . . . . . . . 13
       5.2.4.  SR Segment Endpoint Node . . . . . . . . . . . . . . . 14
     5.3.  FRR Flag Settings  . . . . . . . . . . . . . . . . . . . . 14
   6.  SR-IPv6 Security . . . . . . . . . . . . . . . . . . . . . . . 14
     6.1.  Threat model . . . . . . . . . . . . . . . . . . . . . . . 15
     6.2.  Applicability of RFC 5095 to SRH . . . . . . . . . . . . . 15
     6.3.  Security fields in SRH . . . . . . . . . . . . . . . . . . 16
     6.4.  Nodes within the SR domain . . . . . . . . . . . . . . . . 17
     6.5.  Nodes outside of the SR domain . . . . . . . . . . . . . . 17
   7.  SR and Tunneling . . . . . . . . . . . . . . . . . . . . . . . 17
   8.  Example Use Case . . . . . . . . . . . . . . . . . . . . . . . 18
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   10. Manageability Considerations . . . . . . . . . . . . . . . . . 20
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 20
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 20
     14.2. Informative References . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22












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1.  Structure of this document

   Section 3 gives an introduction on SR for IPv6 networks.

   Section 4 defines the Segment Routing Header (SRH) allowing
   instantiation of SR over IPv6 dataplane.

   Section 5 details the procedures of the Segment Routing Header.

   Section 6 describes the security aspect of SR-IPv6.


2.  Segment Routing Documents

   Segment Routing is described in [I-D.filsfils-rtgwg-segment-routing].

   Segment Routing use cases are described in
   [I-D.filsfils-rtgwg-segment-routing-use-cases].

   Segment Routing IPv6 use cases are described in
   [draft-martin-spring-segment-routing-ipv6-use-cases-00].

   Segment Routing protocol extensions are defined in
   [I-D.previdi-isis-segment-routing-extensions], and
   [I-D.psenak-ospf-segment-routing-ospfv3-extension].

   The terminology is used in this document has been defined in
   [I-D.filsfils-rtgwg-segment-routing].


3.  Introduction

   Segment Routing (SR), defined in
   [I-D.filsfils-rtgwg-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.

   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



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   [I-D.filsfils-rtgwg-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), defined in
   [I-D.filsfils-rtgwg-segment-routing], can be instantiated over MPLS
   ([I-D.filsfils-spring-segment-routing-mpls]) and IPv6.  This document
   defines its instantiation over the IPv6 data-plane.

   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-rtgwg-segment-routing].  Mechanisms through which
   segment are known and advertised are outside the scope of this
   document.

3.2.  Illustrative Example

   Typically, the domain ingress node obtains the path it has to use for
   a given packet flow through either local configuration, local
   computation or through an interaction with an external server such as
   an SDN controller.

   The output of the above is a segment list: a list of IPv6 addresses
   (each representing a segment) that is encoded in the SRH.  The
   Segment List represents the path of the packet.

   The ingress node encodes the first segment into the Destination
   Address of the IPv6 header and the packet is forwarded towards the
   first segment endpoint.

   Each segment endpoint inspects the SRH, updates the DA (with the next
   segment) and forwards the packet towards the next segment.

   The SRH MAY be removed from the packet prior to send it to its
   original destination.

   When traveling within a segment, a packet may traverse non-SR-capable
   nodes.  These nodes will forward the packet based on its DA
   regardless the content of the SRH that, in their case, will be
   silently ignored as mandated by [RFC2460].  Therefore,



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   interoperability between SR-capable and non-SR-capable nodes being
   ensured, a gradual deployment of SR in existing networks is possible.
   The details of the procedures of SR-IPv6 are described in Section 5.


4.  IPv6 Instantiation of Segment Routing

   When Segment Routing is applied to IPv6, segments are encoded as 128-
   bit IPv6 addresses.  This implies that, in the IPv6 instantiation of
   SR, the SID values are in fact the prefixes advertised in the IPv6
   control-plane.  Hence there's no need to advertise any additional
   specific identifier (other than IPv6 prefix) for the purpose of SR.
   This simplifies the introduction of IPv6 Segment Routing in existing
   protocols (i.e.: IS-IS, OSPF and BGP).

4.1.  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, to be assigned by IANA.

   According to [I-D.filsfils-rtgwg-segment-routing], each segment is
   represented by a Segment Identifier (SID).  When SR is used over IPv6
   networks, the SID is an IPv6 address (or prefix) as learned by IGP,
   BGP or other protocols.

   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
   contains the IPv6 addresses of these services or application
   instances.

   The Segment Routing Header (SRH) is defined as follows:
















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     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)                         |
    |                        (optional)                             |
    |                                                               |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   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-rtgwg-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:

         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 5.3
         for more details.

   o  HMAC Key ID and HMAC field are defined in Section 6.

   o  Policy List flags.  Define the type of the IPv6 addresses encoded
      into the Policy List (see below).  The following have been
      defined:

         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 9-1: 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: 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.

4.1.1.  SRH and RFC2460 behavior

   The SRH being a new type of the Routing Header, it also has the same
   properties:

      Can only appear once in the packet.

      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 3.2, nodes that are within the path of a
   segment will forward packets based on the DA of the packet without



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   inspecting the SRH.  This ensures full interoperability between SR-
   capable and non-SR-capable nodes.

4.1.2.  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).

4.2.  Segment Identifiers (SIDs)

   The Segment Routing architecture described in
   [I-D.filsfils-rtgwg-segment-routing], defines Node-SID and Adjacency-
   SID.  When SR is used over IPv6 data-plane the following applies.

4.2.1.  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.  The SID is in fact the



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   IPv6 address of the node.

4.2.2.  Adjacency-SID

   The Adjacency-SID identifies a given interface.  In the SR
   architecture 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 same is defined
   for SR-IPv6: a node may advertise a given IPv6 prefix which is
   associated to the SR semantic of "send out the packet to the
   interface this prefix is allocated to".  Here also, the SID is in
   fact the IPv6 prefix.


5.  SRH Procedures

   In this section we describe the different procedures on the SRH.

5.1.  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-rtgwg-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.

5.2.  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.




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   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.

      SR Endpoint Node.

5.2.1.  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 obtain the segment list by either:

      Local path computation.

      Interaction with an SDN controller delivering the path as a
      complete SRH.

   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.





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         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.

5.2.1.1.  Security at Ingress

   The procedures related to the Segment Routing security are detailed
   in Section 6.

   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 Section 6 is to be used.

   The ingress node (e.g.: the host in the home network) requests the
   SRH to 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.

5.2.2.  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.

5.2.3.  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.




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5.2.4.  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:

           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

5.3.  FRR Flag Settings

   A node supporting SR and doing Fast Reroute (as described in
   [I-D.filsfils-rtgwg-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.


6.  SR-IPv6 Security

   This section analyses the security threat model as well as the
   security issues and proposed solutions related to the new routing
   header for segment routing (a.k.a. segment routing header SRH).

   The segment routing header is simply another version of the routing
   header as described in [RFC2460] and is:

   o  inserted when entering the segment routing domain which could be
      done by a node or by a router;

   o  read and acted upon when reaching the destination of the IP
      header.

   Routers on the path that simply forward an IPv6 packet (i.e. the IPv6
   destination address is not one of theirs) will never read and process
   the SRH.  Routers whose one interface IPv6 address equals the
   destination address field of the SRH will have to parse the SRH and,
   if supported and if the local configuration allows it, will act on
   the SRH.





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6.1.  Threat model

   Using a routing extension header which is basically source routing
   has some well-known security issues as described in [RFC4942] section
   2.1.1 and [RFC5095]:

   o  amplification attacks: where a packet could be forged in such a
      way to cause looping among a set of intermediate routers causing
      unnecessary traffic, hence a denial of service 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 DMZ) to attack another host (for
      example in the datacenter or any back-end server.

   These security issues did lead to obsoleting the routing header type
   0 with [RFC5095] because:

   o  it was assumed to be acted upon by default by each and every
      router on the Internet;

   o  it contained multiple segments in the payload.

   Therefore, if intermediate nodes ONLY act on valid and authorized
   SRH, then there is no security threat similar to RH-0.  But as SR 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'.

6.2.  Applicability of RFC 5095 to SRH

   In the segment routing architecture described in
   [I-D.filsfils-rtgwg-segment-routing], there are basically two kinds
   of nodes (routers and hosts):

   o  nodes within the segment routing domain, which is within one
      single administrative domain, i.e., where all nodes are trusted
      anyway else the damage caused by those nodes could be worse than
      amplification attacks: traffic interception and man-in-the-middle
      attacks, more server DoS by dropping packets, and so on.

   o  Nodes outside of the segment routing domain, which is outside of
      the administrative segment routing domain hence they cannot be
      trusted because there is no physical security for those nodes,
      i.e., they can be replaced by hostile nodes or can be coerced in



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      wrong behaviors.

6.3.  Security fields in SRH

   The security-related fields in SRH are:

   o  HMAC Key-id, 8bits wide, if HMAC key-id is null, then there is no
      HMAC field;

   o  HMAC, 256 bits wide.

   The HMAC field is the output of the hash of the concatenation of:

   o  the source IPv6 address;

   o  last segment, clean-up bit flag, HMAC key id, all addresses in the
      Segment List;

   o  a pre-shared secret between SR nodes in the SR domain (routers,
      controllers, ...).

   The purpose of the HMAC field is to verify the validity and
   authorization of the SRH itself.  If an outsider of the SR domain
   does not have access to the pre-shared secret, then it cannot compute
   the right HMAC field and the first SR router on the path processing
   the SRH and configure 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, then all
   other SR nodes can ignore it (based on local policy).  This is to
   speed up forwarding operations because some hardware platforms can
   only parse in hardware so many bytes.

   The HMAC Key-id field allows for the simultaneous existence of
   several hash algorithms (SHA-128, SHA-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 is opaque,
   i.e., it has no syntax except as an index to the right combination of
   pre-shared key and hash algorithm.  It also allows for interoperation
   among different SR domains if allowed by local policy.

   How HMAC key-id and pre-shared secret are synchronized between
   participating nodes in the SR domain is outside of the scope of this
   document ([RFC6407] GDOI could be a basis).





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6.4.  Nodes within the SR domain

   Those nodes can be trusted to generate and to process SRH received on
   interfaces that are part of the SR domain (AS or set of ASs under
   common administration where SR is enabled).  These nodes MUST drop
   all packets received on an 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.

6.5.  Nodes outside of the SR domain

   Nodes outside of the SR domain cannot be trusted for physical
   security; hence, they need to request by some means (outside of the
   scope of this document) a complete SRH for each new connection.  The
   SRH MUST include a HMAC key-id and HMAC field which is computed
   correctly (see Section 6.3).

   When an outside node sends a packet with an SRH and towards a SR
   ingress node, the packet MUST contain the HMAC field and the SR
   ingress node MUST be in the segment list of the SRH.

   The ingress SR router, i.e., the router with an interface address
   equals to the destination address, MUST verify the HMAC field.

   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 is done.  Subsequent routers in the
   SR domain MAY verify the HMAC field.

   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.


7.  SR and Tunneling

   Encapsulation can be realized in two different ways with SR-IPv6:

      Outer encapsulation.







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      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
   [draft-martin-spring-segment-routing-ipv6-use-cases-00].  In this
   section, a simple SR-IPv6 example is illustrated.

   In the topology described in Figure 2 it is assumed an end-to-end SR
   deployment.  Therefore SR is supported by all nodes from A to J.


    Home Network |          Backbone         |    Datacenter
                 |                           |
                 |   +---+   +---+   +---+   |   +---+   |
             +---|---| C |---| D |---| E |---|---| I |---|
             |   |   +---+   +---+   +---+   |   +---+   |
             |   |     |       |       |     |     |     |  +---+
   +---+   +---+ |     |       |       |     |     |     |--| X |
   | A |---| B | |   +---+   +---+   +---+   |   +---+   |  +---+
   +---+   +---+ |   | F |---| G |---| H |---|---| J |---|
                 |   +---+   +---+   +---+   |   +---+   |
                 |                           |
                 |        +-----------+
                          |    SDN    |
                          | Orch/Ctlr |
                          +-----------+

                       Figure 2: 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 Section 6.


12.  Contributors

   Eric Vynke contributed to this document through the writings of
   Section 6.


13.  Acknowledgements

   The authors would like to thank John Leddy, John Brzozowski, Mark
   Townsley, Christian Martin, Roberta Maglione and James Connolly 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.

   [RFC5095]  Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
              of Type 0 Routing Headers in IPv6", RFC 5095,
              December 2007.

   [RFC6407]  Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
              of Interpretation", RFC 6407, October 2011.






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14.2.  Informative References

   [I-D.filsfils-rtgwg-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-rtgwg-segment-routing-01 (work in
              progress), October 2013.

   [I-D.filsfils-rtgwg-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-rtgwg-segment-routing-use-cases-02 (work in
              progress), October 2013.

   [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-00 (work in
              progress), October 2013.

   [I-D.previdi-isis-segment-routing-extensions]
              Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
              Litkowski, S., and J. Tantsura, "IS-IS Extensions for
              Segment Routing",
              draft-previdi-isis-segment-routing-extensions-05 (work in
              progress), February 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.

   [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.

   [RFC4942]  Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/
              Co-existence Security Considerations", RFC 4942,
              September 2007.




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   [draft-martin-spring-segment-routing-ipv6-use-cases-00]
              Brzozowski, J., Leddy, J., Leung, I., Previdi, S.,
              Townsley, M., Martin, C., Filsfils, C., and R. Maglione,
              "IPv6 Segment Routing Use Cases", March 2014.


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


   Ida Leung
   Rogers Communications
   8200 Dixie Road
   Brampton, ON  L6T 0C1
   CA

   Email: Ida.Leung@rci.rogers.com










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