Network Working Group                                         S. Previdi
Internet-Draft                                                Individual
Intended status: Standards Track                        C. Filsfils, Ed.
Expires: September 29, October 21, 2018                            Cisco Systems, Inc.
                                                                J. Leddy
                                                                 Comcast
                                                           S. Matsushima
                                                                Softbank
                                                           D. Voyer, Ed.
                                                             Bell Canada
                                                          March 28,
                                                          April 19, 2018

                   IPv6 Segment Routing Header (SRH)
               draft-ietf-6man-segment-routing-header-11
               draft-ietf-6man-segment-routing-header-12

Abstract

   Segment Routing (SR) allows a node to steer a packet through a
   controlled set of instructions, called segments, by prepending an SR
   header to the packet.  A segment can represent any instruction,
   topological or service-based.  SR allows a node to enforce steer 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 document
   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
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   This Internet-Draft will expire on September 29, October 21, 2018.

Copyright Notice

   Copyright (c) 2018 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  . . . . . . . . . . . . . . . . . . . . . . . .   4   3
     2.1.  Data Planes supporting Segment Routing  . . . . . . . . .   4
     2.2.  SRv6 Segment  . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Segment Routing (SR) Domain . . . . . . . . . . . . . . .   5
       2.3.1.  SR Domain in a Service Provider Network . . . . . . .   6
       2.3.2.  SR Domain in a Overlay Network  . . . . . . . . . . .   7
   3.  Segment Routing Extension Header (SRH)  . . . . . . . . . . .   8   5
     3.1.  SRH TLVs  . . . . . . . . . . . . . . . . . . . . . . . .  10   7
       3.1.1.  Ingress Node TLV  . . . . . . . . . . . . . . . . . .  11
       3.1.2.  Egress Node TLV . . . . . . . . . . . . . . . . . . .  12
       3.1.3.  Opaque Container TLV  . . . . . . . . . . . . . . . .  13
       3.1.4.  Padding TLV . TLVs  . . . . . . . . . . . . . . . . . . . .  13
       3.1.5.   8
       3.1.2.  HMAC TLV  . . . . . . . . . . . . . . . . . . . . . .  14
       3.1.6.  NSH Carrier TLV . . . . . . . . . . . . . .  10
     3.2.  SRH Processing  . . . . .  15
     3.2.  SRH and RFC8200 behavior . . . . . . . . . . . . . . . .  15  11
   4.  SRH Functions . . . . . . . . . . . . . . . . . . . . . . . .  16  11
     4.1.  Endpoint Function (End) . . . . . . . . . . . . . . . . .  16  11
     4.2.  End.X: Endpoint with Layer-3 cross-connect  . . . . . . .  17  12
   5.  SR Nodes  . . . . . . . . . . . . . . . . . . . . . . . . . .  18  13
     5.1.  Source SR Node  . . . . . . . . . . . . . . . . . . . . .  18  13
     5.2.  Transit Node  . . . . . . . . . . . . . . . . . . . . . .  19  14
     5.3.  SR Segment Endpoint Node  . . . . . . . . . . . . . . . .  20  14
     5.4.  Load Balancing and ECMP . . . . . . . . . . . . . . . . .  14
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  20  15
     6.1.  Threat model  . . . . . . . . . . . . . . . . . . . . . .  21  15
       6.1.1.  Source routing threats  . . . . . . . . . . . . . . .  21  15
       6.1.2.  Applicability of RFC 5095 to SRH  . . . . . . . . . .  21  16
       6.1.3.  Service stealing threat . . . . . . . . . . . . . . .  22  17
       6.1.4.  Topology disclosure . . . . . . . . . . . . . . . . .  22  17
       6.1.5.  ICMP Generation . . . . . . . . . . . . . . . . . . .  22  17
     6.2.  Security fields in SRH  . . . . . . . . . . . . . . . . .  23  18
       6.2.1.  Selecting a hash algorithm  . . . . . . . . . . . . .  24  19
       6.2.2.  Performance impact of HMAC  . . . . . . . . . . . . .  24  19
       6.2.3.  Pre-shared key management . . . . . . . . . . . . . .  25  20
     6.3.  Deployment Models . . . . . . . . . . . . . . . . . . . .  26  20
       6.3.1.  Nodes within the SR domain  . . . . . . . . . . . . .  26  20
       6.3.2.  Nodes outside of the SR domain  . . . . . . . . . . .  26  21
       6.3.3.  SR path exposure  . . . . . . . . . . . . . . . . . .  27  21
       6.3.4.  Impact of BCP-38  . . . . . . . . . . . . . . . . . .  27  22
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27  22
     7.1.  Segment Routing Header Flags Register . . . . . . . . . .  22
     7.2.  Segment Routing Header TLVs Register  . . . . . . . . . .  28  23
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .  28  23
     8.1.  Linux . . . . . . . . . . . . . . . . . . . . . . . . . .  28  23
     8.2.  Cisco Systems . . . . . . . . . . . . . . . . . . . . . .  28  23
     8.3.  FD.io . . . . . . . . . . . . . . . . . . . . . . . . . .  29  24
     8.4.  Barefoot  . . . . . . . . . . . . . . . . . . . . . . . .  29  24
     8.5.  Juniper . . . . . . . . . . . . . . . . . . . . . . . . .  29  24
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  29  24
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  30  24
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30  25
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  30  25
     11.2.  Informative References . . . . . . . . . . . . . . . . .  31  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33  28

1.  Segment Routing Documents

   Segment Routing terminology is defined in
   [I-D.ietf-spring-segment-routing].

   The network programming paradigm
   [I-D.filsfils-spring-srv6-network-programming] defines additional
   functions associated to an SRv6 SID. a Segment Routing for IPv6 (SRv6) Segment ID
   (SID).

   Segment Routing use cases are described in [RFC7855] and
   [I-D.ietf-spring-ipv6-use-cases]. [RFC8354].

   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 an SR header to the
   packet.  A segment can represent any instruction, topological or
   service-based.  SR allows a node to enforce steer 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
   [I-D.ietf-spring-segment-routing] is inherited inherits from the ones proposed by
   [RFC1940] and [RFC8200].  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])
   (SRMPLS)([I-D.ietf-spring-segment-routing-mpls]) and IPv6. IPv6 (SRv6).
   This document defines its instantiation over the IPv6 data-plane
   based on the use-
   cases use-cases defined in [I-D.ietf-spring-ipv6-use-cases]. [RFC8354].

   This document defines a new type of Routing Header (originally
   defined in [RFC8200]) 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 segments
   are known and advertised are outside the scope of this document.

2.2.  SRv6 Segment

   An SRv6 Segment is a 128-bit value.  "SRv6 SID" or simply "SID" are
   often used as a shorter reference for "SRv6 Segment".

   An SRv6-capable node N maintains a "My Local SID Table".  This table
   contains all the local SRv6 segments explicitly instantiated at node
   N.  N is the parent node for these SID's. SIDs.

   A local SID of N could be an IPv6 address of a local interface of N
   but it does not have to. to be.  Most often, a local SID is not an
   address of a local interface of N.  The My Local SID Table is thus
   not populated by default with all the addresses of a node.

   An illustration is provided in
   [I-D.filsfils-spring-srv6-network-programming].

   Every SRv6 local SID instantiated has a specific instruction bounded
   to it.

   This information is stored in the "My My Local SID Table". Table.  The "My My Local
   SID Table" Table has three main purposes:

   o  Define which local SID's SIDs are explicitly instantiated.

   o  Specify which instruction is bound to each of the instantiated
      SID's.
      SIDs.

   o  Store the parameters associated to such instruction (i.e.  OIF,
      NextHop,...).

   A Segment ID (SID) in the destination address of an IPv6 header, is
   routed through an IPv6 network as an IPv6 address.  SIDs, or the
   prefix(es) covering SIDs in the My Local SID Table, and their
   reachability may be distributed by means outside the scope of this
   document.  For example, [RFC5308] or [RFC5340] may be used to
   advertise a prefix covering the My Local SID Table.

   A node may receive a packet with an SRv6 SID in the DA without an
   SRH.  In such case the packet should still be processed by the
   Segment Routing engine.

2.3.  Segment Routing (SR) Domain

   We define the concept of the

   The Segment Routing Domain (SR Domain) is defined as the set of nodes
   participating into in 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:

   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 network).

   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.

   It is assumed in this document that the SRH is added to the packet by its source, consistently consistent with the
   source routing model defined in [RFC8200].  For example:

   o  At the node originating the packet (host, server).

   o  At the ingress node of an SR domain where the ingress node
      receives an IPv6 a packet and encapsulates it into in an outer IPv6 header
      followed by a Segment Routing header.

2.3.1.  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,

3.  Segment Routing is used within the operator networks and across the ASes
   boundaries (all being under the control Extension Header (SRH)

   A new type 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 Routing Header (originally defined in
   [RFC7855], [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 [RFC8200]) is
   defined: 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 5) 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.

2.3.2.  SR Domain in a Overlay Network

   The following figure illustrates an SR domain consisting of an
   overlay network over multiple operator's networks.

       (--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, as
   specified in [RFC8200], MUST NOT inspect the routing extension header
   since they 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 never see a
   destination address belonging to their infrastructure.  These SR-
   enabled overlay packets will thus never be filtered by the transit
   operators.

   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.

3.  Segment Routing Extension Header (SRH)

   A new type of the Routing Header (originally defined in [RFC8200]) 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 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Last Entry   |     Flags     |              Tag              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Segment List[0] (128 bits IPv6 address)            |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
                                  ...
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Segment List[n] (128 bits IPv6 address)            |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                                                             //
    //         Optional Type Length Value objects (variable)       //
    //                                                             //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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 (suggested value: 4).

   o  Segments Left.  Defined in [RFC8200], it contains the index, in
      the Segment List, of the next segment to inspect.  Segments Left
      is decremented at each segment.

   o  Last Entry: contains the index, in the Segment List, of the last
      element of the Segment List.

   o  Flags: 8 bits of flags.  Following flags are defined:

          0 1 2 3 4 5 6 7
         +-+-+-+-+-+-+-+-+
         |U|P|O|A|H|  U  |
         +-+-+-+-+-+-+-+-+

         U: Unused and for future use.  SHOULD be unset on transmission
         and MUST be ignored on receipt.

         P-flag: Protected flag.  Set when the packet has been rerouted
         through FRR mechanism by an SR endpoint node.

         O-flag: OAM flag.  When set, it indicates that this packet is
         an operations and management (OAM) packet.

         A-flag: Alert flag.  If present, it means important Type Length
         Value (TLV) objects are present.  See Section 3.1 for details
         on TLVs objects.

         H-flag: HMAC flag.  If set, the HMAC TLV is present and is
         encoded as the last TLV of the SRH.  In other words, the last
         36 octets of the SRH represent the HMAC information.  See
         Section 3.1.5 for details on the HMAC TLV.

   o  Tag: tag a packet as part of a class or group of packets, e.g.,
      packets sharing the same set of properties.

   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, the second element contains the penultimate segment of the
      path and so on.

   o  Type Length Value (TLV) are described in Section 3.1.

3.1.  SRH TLVs

   This section defines TLVs of the Segment Routing Header.

   Type Length Value (TLV) contain optional information that may be used
   by the node identified in the DA of the packet.  It Segment Routing Header (SRH) which has a new Routing
   Type, (suggested value 4) to be noted
   that the information carried in the TLVs is not intended to be used
   by the routing layer.  Typically, TLVs carry information that is
   consumed assigned by other components (e.g.: OAM) than the routing function.

   Each TLV has its own length, format and semantic.  The code-point
   allocated (by IANA) to each TLV defines both the format and the
   semantic of the information carried in the TLV.  Multiple TLVs may be
   encoded in the same SRH. IANA.

   The "Length" field of the TLV Segment Routing Header (SRH) is primarily used to skip the TLV while
   inspecting the SRH in case the node doesn't support or recognize the
   TLV codepoint.  The "Length" defines the TLV length in octets and not
   including the "Type" and "Length" fields.

   The following TLVs are defined in this document:

      Ingress Node TLV

      Egress Node TLV

      Opaque TLV

      Padding TLV

      HMAC TLV

      NSH Carrier TLV

   Additional TLVs may be defined in the future.

3.1.1.  Ingress Node TLV

   The Ingress Node TLV is optional and identifies the node this packet
   traversed when entered the SR domain.  The Ingress Node TLV has
   following format: 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  |    Length Segments Left |   RESERVED
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Last Entry   |     Flags     |              Tag              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                 Ingress Node (16 octets)            Segment List[0] (128 bits IPv6 address)            |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
                                  ...
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Segment List[n] (128 bits IPv6 address)            |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                                                             //
    //         Optional Type Length Value objects (variable)       //
    //                                                             //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Next Header: Defined in [RFC8200]

   o  Hdr Ext Len: Defined in [RFC8200]

   o  Routing Type: TBD, to be assigned by IANA (suggested value 1). value: 4).

   o  Length: 18.  Segments Left: Defined in [RFC8200].  See Section 3.2 for detailed
      usage during SRH processing.

   o  RESERVED:  Last Entry: contains the index (zero based), in the Segment List,
      of the last element of the Segment List.

   o  Flags: 8 bits. bits of flags.  Following flags are defined:

          0 1 2 3 4 5 6 7
         +-+-+-+-+-+-+-+-+
         |    U  |H|  U  |
         +-+-+-+-+-+-+-+-+

         U: Unused and for future use.  SHOULD be unset 0 on transmission and
         MUST be ignored on receipt.

         H-flag: HMAC flag.  If set, the HMAC TLV is present and is
         encoded as the last TLV of the SRH.  In other words, the last
         36 octets of the SRH represent the HMAC information.  See
         Section 3.1.2 for details on the HMAC TLV.

   o  Flags: 8 bits.  No flags are defined in this document.  Tag: tag a packet as part of a class or group of packets, e.g.,
      packets sharing the same set of properties.  When tag is not used
      at source it SHOULD be set to 0 zero on transmission and transmission.  When tag is
      not used during SRH Processing it MUST be ignored on receipt. ignored.  The allocation
      and use of tag is outside the scope of this document.

   o  Ingress Node:  Segment List[n]: 128 bits.  Defines bit IPv6 addresses representing the node where nth
      segment in the packet Segment List.  The Segment List is
      expected to enter encoded starting
      from the SR domain.  In last segment of the encapsulation case path.  I.e., the first element of the
      segment list (Segment List [0]) contains the last segment of the
      path, the second element contains the penultimate segment of the
      path and so on.

   o  Type Length Value (TLV) are described in Section 2.3.1, this information corresponds to the SA 3.1.

3.1.  SRH TLVs

   This section defines TLVs of the encapsulating header.

3.1.2.  Egress Node TLV

   The Egress Node TLV is optional and identifies the node this packet
   is expected to traverse when exiting the SR domain.  The Egress Node
   TLV has following format:

    0                   1                   2                   3 Segment Routing Header.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-----------------------
   |     Type      |    Length     |   RESERVED    |     Flags     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                  Egress Node (16 octets)                      |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o Variable length data
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-----------------------

   Type: to be assigned by IANA (suggested value 2).

   o  Length: 18.

   o  RESERVED: An 8 bits.  SHOULD be unset on transmission and bit value.  Unrecognized Types MUST be ignored on receipt.

   o  Flags:

   Length: The length of the Variable length data.  It is RECOMMENDED
   that the total length of new TLVs be multiple of 8 bits.  No flags are defined bytes to avoid the
   use of Padding TLVs.

   Variable length data: Length bytes of data that is specific to the
   Type.

   Type Length Value (TLV) contain optional information that may be used
   by the node identified in this document.  SHOULD the DA of the packet.  The information
   carried in the TLVs is not intended to be
      set used by the routing layer.
   Typically, TLVs carry information that is consumed by other
   components (e.g.: OAM) than the routing function.

   Each TLV has its own length, format and semantic.  The code-point
   allocated (by IANA) to each TLV Type defines both the format and the
   semantic of the information carried in the TLV.  Multiple TLVs may be
   encoded in the same SRH.

   TLVs may change en route at each segment.  To identify when a TLV
   type may change en route the most significant bit of the Type has the
   following significance:

      0: TLV data does not change en route

      1: TLV data does change en route

   Identifying which TLVs change en route, without having to 0 on transmission understand
   the Type, is required for Authentication Header Integrity Check Value
   (ICV) computation.  Any TLV that changes en route is considered
   mutable for the purpose of ICV computation, the Type Length and MUST be
   Variable Length Data is ignored on receipt.

   o  Egress Node: 128 bits.  Defines for the node where purpose of ICV Computation as
   defined in [RFC4302].

   The "Length" field of the packet TLV is
      expected used to exit skip the SR domain.  In TLV while
   inspecting the encapsulation case
      described SRH in Section 2.3.1, this information corresponds to case the
      last segment of node doesn't support or recognize the SRH in
   Type.  The "Length" defines the encapsulating header.

3.1.3.  Opaque Container TLV length in octets, not including
   the "Type" and "Length" fields.

   The Opaque Container following TLVs are defined in this document:

      Padding TLV is optional

      HMAC TLV

   Additional TLVs may be defined in the future.

3.1.1.  Padding TLVs

   There are two types of padding TLVs, pad0 and has padN, the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7
   applies to both:

      Padding TLVs are optional and more than one Padding TLV MUST NOT
      appear in the SRH.

      The Padding TLVs are used to align the SRH total length on the 8 9
      octet boundary.

      When present, a single Pad0 or PadN TLV MUST appear as the last
      TLV before the HMAC TLV (if HMAC TLV is present).

      When present, a PadN TLV MUST have a length from 0 1 2 3 4 to 5 6 7 8 9 in order
      to align the SRH total length on a 8-octet boundary.

      Padding TLVs are ignored by a node processing the SRH TLV, even if
      more than one is present.

      Padding TLVs are ignored during ICV calculation.

3.1.1.1.  PAD0

     0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +-+-+-+-+-+-+-+-+
     |     Type      |    Length     |   RESERVED    |     Flags     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |             Opaque Container (16 octets)                      |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o
     +-+-+-+-+-+-+-+-+

      Type: to be assigned by IANA (suggested (Suggested value 3).

   o  Length: 18.

   o  RESERVED: 8 bits.  SHOULD be unset on transmission and MUST be
      ignored on receipt.

   o  Flags: 8 bits.  No flags are defined in this document.  SHOULD be
      set to 0 on transmission and 128)

   A single Pad0 TLV MUST be ignored on receipt.

   o  Opaque Container: 128 bits of opaque data not relevant for the
      routing layer.  Typically, this information is consumed by used when a non-
      routing component single byte of the node receiving the packet (i.e.: the node
      in the DA).

3.1.4.  Padding TLV

   The Padding TLV padding is optional and with the purpose
   required.  If more than one byte of aligning the SRH
   on padding is required a 8 octet boundary.  The Padding Pad0 TLV has
   MUST NOT be used, the following format: PadN TLV MUST be used.

3.1.1.2.  PADN

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |      Padding (variable)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                    Padding (variable)                       //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: to be assigned by IANA (suggested value 4).

   o  Length: 0 to 7

   o  Padding: from 0 to 7 octets of padding.  Padding bits have no
      semantic.  They SHOULD be set to 0 on transmission and MUST be
      ignored on receipt.

   The following applies to the Padding TLV:

   o  Padding TLV is optional and MAY only appear once in the SRH.

   o  The Padding TLV is used in order to align the SRH total length on
      the 8 octet boundary.

   o  When present, the Padding TLV MUST appear as the last TLV before
      the HMAC TLV (if HMAC TLV is present).

   o  When present, the Padding TLV MUST have a length from (variable)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                    Padding (variable)                       //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: to be assigned by IANA (suggested value 129).

      Length: 0 to 7 in
      order 5

      Padding: Length octets of padding.  Padding bits have no
      semantics.  They SHOULD be set to align the SRH total lenght 0 on a 8-octet boundary.

   o  When a router inspecting the SRH encounters the Padding TLV, it transmission and MUST assume that no other be
      ignored on receipt.

   The PadN TLV (other MUST be used when more than the HMAC) follow the
      Padding TLV.

3.1.5. one byte of padding is
   required.

3.1.2.  HMAC TLV

   HMAC TLV is optional and contains the HMAC information.  The HMAC TLV
   has the following format:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |     Length    |          RESERVED             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      HMAC Key ID (4 octets)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                              //
   |                      HMAC (32 octets)                        //
   |                                                              //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: to be assigned by IANA (suggested value 5).

   o  Length: 38.

   o  RESERVED: 2 octets.  SHOULD be unset on transmission and MUST be
      ignored on receipt.

   o  HMAC Key ID: 4 octets.

   o  HMAC: 32 octets.

   o  HMAC and HMAC Key ID usage is described in Section 6

   The Following applies to the HMAC TLV:

   o  When present, the HMAC TLV MUST be encoded as the last TLV of the
      SRH.

   o  If the HMAC TLV is present, the SRH H-Flag (Figure 4) 2) MUST be set.

   o  When the H-flag is set in the SRH, the router inspecting the SRH
      MUST find the HMAC TLV in the last 38 octets of the SRH.

3.1.6.  NSH Carrier TLV

   The NSH Carrier TLV is a container used in order to carry TLVs that
   have been defined in [RFC8300].  The NSH Carrier TLV has the
   following format:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |     Length    |     Flags     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //            NSH Carried Object                               //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   where:

   o  Type: to be assigned by IANA (suggested value 6).

   o  Length: the total length of the TLV.

   o  Flags: 8 bits.  No flags are defined in this document.  SHOULD be
      set to 0 on transmission and MUST be ignored on receipt.

   o  NSH Carried Object: the content of the TLV which consists of the
      NSH data as defined in [RFC8300].

3.2.  SRH and RFC8200 behavior

   The
      Nodes processing SRH being a new type of the Routing Header, it also has the same
   properties: SHOULD process the HMAC TLV only appear once in when the packet.

      Only
      H-Flag is set, and local policy.

   o  When the router whose address H-flag is set in the DA field of SRH, the packet
      header router inspecting the SRH
      MUST inspect find the SRH.

   Therefore, Segment Routing HMAC TLV in IPv6 networks implies that the segment
   identifier (i.e.: last 38 octets of the SRH.

3.2.  SRH Processing

   Extension header processing is as defined in RFC8200 for packets
   destined to a local IPv6 address.  If SRH is present, it is processed
   as follows (DA represents the destination address of in the segment) is moved into IPv6
   header):

   1. IF the DA of the packet. is in My Local SID Table
   2.   The DA of the packet changes at each segment termination/completion
   and therefore function associated with the final DA of the packet MUST be encoded is processed.
           (Examples are as described in section 4).
   3. ELSE
   4.   Treat the last
   segment of the path. extension header as unrecognized
           and process as defined in RFC8200 section 4.4.

4.  SRH Functions

   Segment Routing architecture, as defined in
   [I-D.ietf-spring-segment-routing] defines a segment as an instruction
   or, more generally, a set of instructions (function).

   In this section we review two functions that may be associated to a
   segment:

   o  End: the endpoint (End) function is the base of the source routing
      paradigm.  It consists of updating the DA with the next segment
      and forward the packet accordingly.

   o  End.X: The endpoint layer-3 cross-connect function.

   More

   Other functions are may be defined in
   [I-D.filsfils-spring-srv6-network-programming]. other documents.

4.1.  Endpoint Function (End)

   The Endpoint function (End) is the most basic function.

   When the endpoint node receives a packet destined to DA "S" and S is
   an entry in the MyLocalSID table My Local SID Table and the function associated with S
   in that entry is "End" then the endpoint does:

   1. IF SegmentsLeft > 0 THEN
   2.    decrement SL
   3.    update the IPv6 DA with SRH[SL]
   4.    FIB lookup on updated DA
   5.    forward accordingly to the matched entry
   6.  ELSE IF SID is assigned to an interface
   7.    proceed to process the next header
   8.  ELSE
   9.    drop the packet
   It has to be noted that:

   o  The End function performs the FIB lookup in the forwarding table
      of the ingress interface.

   o  If the FIB lookup matches a multicast state, then the related
      Reverse Path Forwarding (RPF) check must be considered successful.

   o  An SRv6-capable node MUST include the FlowLabel of the IPv6 header
      in its hash computation for ECMP load-balancing.

   o  By default, a local SID bound to the End function does not allow
      the decapsulation of an outer header.  As a consequence, an End
      SID cannot be the last SID of an SRH and cannot be the DA of a
      packet without SRH.

   o  If the decapsulation is desired, then another function must be
      bound to the SID (e.g., End.DX6 defined in
      [I-D.filsfils-spring-srv6-network-programming]).  This prevents
      any unintentional decapsulation by the segment endpoint node.  The
      details of the advertisement of a SID load-balancing as described in the control plane are
      outside the scope of this document (e.g.,
      [I-D.previdi-idr-segment-routing-te-policy],
      [I-D.dawra-bgp-srv6-vpn] and [I-D.bashandy-isis-srv6-extensions].
      Section 5.4.

4.2.  End.X: Endpoint with Layer-3 cross-connect

   When the endpoint node receives a packet destined to DA "S" and S is
   an entry in the MyLocalSID table My Local SID Table and the function associated with S
   in that entry is "End.X" then the endpoint does:

   1. IF SegmentsLeft > 0 THEN
   2.    decrement SL
   3.    update the IPv6 DA with SRH[SL]
   4.    forward to layer-3 adjacency bound to the SID "S"
   5.  ELSE
   6.    drop the packet

   It has to be noted that:

   o  If an array of layer-3 adjacencies is bound to the End.X SID, then one
      entry of the array is selected based on a hash of the packet's
      header (at least SA, DA, Flow Label). as described in Section 5.4

   o  An End.X function does not allow decaps.  An End.X SID cannot be
      the last SID of an SRH and cannot be the DA of a packet without
      SRH.

   o  The End.X function is required to express any traffic-engineering
      policy. an explicit path through
      an IP network.

   o  The End.X function is the SRv6 instantiation of a Adjacency SID as
      defined in [I-D.ietf-spring-segment-routing].

   o  Note that with SR-MPLS ([I-D.ietf-spring-segment-routing-mpls]),
      an AdjSID is typically preceded by a PrefixSID.  This is unlikely
      in SRv6 as SRv6, most likely an End.X SID is globally routed address of N.

   o  If a node N has 30 outgoing interfaces to 30 neighbors, usually
      the operator would explicitly instantiate 30 End.X SID's SIDs at N: one
      per layer-3 adjacency to a neighbor.  Potentially, more End.X
      could be explicitly defined (groups of layer-3 adjacencies to the
      same neighbor or to different neighbors).

   o  If node N has a bundle outgoing interface I to a neighbor Q made
      of 10 member links, N may allocate up to 11 End.X local SID's SIDs for
      that bundle: one for the bundle itself and then up to one for each
      member link.  This is the equivalent of the L2-Link Adj SID in SR-
      MPLS ([I-D.ietf-isis-l2bundles]).

5.  SR Nodes

   There are different types of nodes that may be involved in segment
   routing networks: source nodes that originate packets with an SRH,
   transit nodes that forward packets having an SRH and segment endpoint
   nodes that MUST process the SRH.

5.1.  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.

      An SR domain ingress router encapsulating a received IPv6 packet
      into in an
      outer IPv6 header followed by an 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 as specified in
      [RFC8200].

      Routing Type field is set as TBD (to be allocated by IANA,
      suggested value 4).

      The DA of the packet is set with the value of the first segment.
      ).

      The first element of the segment list is the last segment (the
      final DA of the packet).  The second element is the penultimate
      segment and so on.

      In other words, the Segment List is encoded in the reverse order
      of the path.

      The Segments Left field is set to n-1 where n is the number of
      elements in the Segment List.

      The Last_Entry field is set to n-1 where n is the number of
      elements in the Segment List.

      The packet is sent out towards the packet's DA (the first
      segment).

      HMAC TLV may be set according to Section 6.

      If the segment list contains a single segment and there is no need
      for information in flag or TLV, then the SRH MAY be omitted.

      The packet is sent towards the packet's DA (the first segment).

5.2.  Transit Node

   As specified in [RFC8200], 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 2.3.2, 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

   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 extension header in it.

5.3.  SR Segment Endpoint Node

   The SR segment endpoint node is the node whose MyLocalSID table My Local SID
   Table contains an entry for the DA of the packet.

   The segment endpoint node executes the function bound to the SID as
   per the matched MyLocalSID My Local SID entry (Section 4).

5.4.  Load Balancing and ECMP

   Within an SR domain, an SR source node encapsulates a packet in an
   outer IPv6 header for transport to an endpoint.  The SR source node
   MUST impose a Flow Label computed based on the inner packet.  The
   computation of the flow label is as recommended in [RFC6438] for the
   sending TEP.

   At any transit node within an SR domain, the flow label MUST be used
   as defined in [RFC6438] to calculate the ECMP hash toward the
   destination address.

   At an SR segment endpoint node, the flow label MUST be used as
   defined in [RFC6438] to calculate any ECMP hash used to forward the
   processed packet to the next segment.

6.  Security Considerations

   This section analyzes the security threat model, the security issues
   and proposed solutions related to the new Segment Routing Header.

   The Segment Routing Header (SRH) is simply another type of the
   routing header as described in RFC 8200 [RFC8200] and is:

   o  Added by an SR edge router when entering the segment routing
      domain or by the originating host itself.  The source host can
      even be outside the SR domain;

   o  inspected and acted upon when reaching the destination address of
      the IP header per RFC 8200 [RFC8200].

   Per RFC8200 [RFC8200], 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 the SRH.  Routers whose one
   interface IPv6 address equals the destination address field of the
   IPv6 packet MUST parse the SRH and, if supported and if the local
   configuration allows it, MUST act accordingly to the SRH content.

   As specified in RFC8200 [RFC8200], the default behavior of a non SR-
   capable SR-capable
   router upon receipt of an IPv6 packet with SRH destined to an address
   of its, is to:

   o  ignore the SRH completely if the Segment Left field is 0 and
      proceed to process the next header in the IPv6 packet;

   o  discard the IPv6 packet if Segment Left field is greater than 0,
      it MAY send a Parameter Problem ICMP message back to the Source
      Address.

6.1.  Threat model

6.1.1.  Source routing threats

   Using an SRH is similar to 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).

6.1.2.  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] (RPL) also creates a new Routing Header type for a specific
   application confined in a single network.

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

   o  nodes within the SR 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, man-in-the-middle
      attacks, more server DoS by dropping packets, and so on.

   o  nodes outside of the SR 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 wrong
      behaviors.

   The main use case for SR consists of the single administrative domain
   where only trusted nodes with SR enabled and configured participate
   in SR: this is the same model as in RFC6554 [RFC6554].  All non-
   trusted 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 nodes 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 nodes 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.

6.1.3.  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'.

6.1.4.  Topology disclosure

   The SRH may also contains IPv6 addresses of some intermediate SR-
   nodes 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.

6.1.5.  ICMP Generation

   Per section Section 4.4 of RFC8200 [RFC8200], 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
      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 not set to 0) destined to a node
   not supporting SRH.  Per RFC8200 [RFC8200], the destination node could
   generate an ICMP message, 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.

6.2.  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 the SRH are instantiated by the HMAC
   TLV, containing:

   o  HMAC Key-id, 32 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 identified by HMAC Key-id and of the text
   which consists of the concatenation of:

   o  the source IPv6 address;

   o  Last Entry field;

   o  an octet of bit flags;

   o  HMAC Key-id;

   o  all addresses in the Segment List.

   The purpose of the HMAC TLV 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 TLV 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.  The HMAC Key-id field is opaque, i.e., it
   has neither syntax nor 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.  Having an HMAC Key-id
   field 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.  It could also allow for interoperation among
   different SR domains if allowed by local policy and assuming a
   collision-free HMAC Key Id allocation.

   When a specific SRH is linked to a time-related service (such as
   turbo-QoS for a 1-hour period) where the DA, Segment ID (SID) are
   identical, 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.

6.2.1.  Selecting a hash algorithm

   The HMAC field in the HMAC TLV is 256 bit 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.

6.2.2.  Performance impact of HMAC

   While adding an 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 is used only when SRH is added 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 an HMAC field, hence no
      performance impact.

   o  when present, the HMAC field MUST only be checked and validated 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.

   o  Last point, hash functions such as SHA-2 have been optimized 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.

6.2.3.  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> for the currently active and future
      keys.

   o  different algorithms: 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 6.2.1)

   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.

6.3.  Deployment Models

6.3.1.  Nodes within the SR domain

   An SR domain is defined as a set of interconnected routers where all
   routers at the perimeter are configured to add and act on SRH.  Some
   routers inside the SR domain can also act on SRH or simply forward
   IPv6 packets.

   The routers inside an SR domain can be trusted to generate SRH and to
   process SRH received on interfaces that are part of the SR domain.
   These nodes MUST drop all SRH packets received on an interface that
   is not part of the SR domain and containing an SRH whose HMAC field
   cannot be validated by local policies.  This includes obviously
   packet with an 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.

6.3.2.  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 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 an
   HMAC TLV which is computed correctly (see Section 6.2).

   When an outside node sends a packet with an SRH and towards an SR
   domain ingress node, the packet MUST contain the HMAC TLV (with a
   Key-id and HMAC fields) and the the destination address MUST be an
   address of an SR domain ingress node .

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

   If the validation is successful, then the packet is simply forwarded
   as usual for an 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 TLV 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.

6.3.3.  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 MUST
   assume that the internal topology and addressing is known by the
   attacker.  A simple traceroute will also give the same information
   (with even more information as all intermediate nodes between SID
   will also be exposed).  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).

   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.

6.3.4.  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 an 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.

7.  IANA Considerations

   This document makes the following registrations in the Internet
   Protocol Version 6 (IPv6) Parameters "Routing Type" registry
   maintained by IANA:

   Suggested            Description             Reference
     Value
   ----------------------------------------------------------
      4         Segment Routing Header (SRH)    This document

   This document request IANA to create and maintain a new Registry:
   "Segment Routing Header TLVs"

7.1. TLVs"

7.1.  Segment Routing Header Flags Register

   This document requests the creation of a new IANA managed registry to
   identify SRH Flags Bits.  The registration procedure is "Expert
   Review" as defined in [RFC8126].  Suggested registry name is "Segment
   Routing Header Flags".  Flags is 8 bits, the following bits are
   defined in this document:

   Suggested      Description               Reference
     Bit
   -----------------------------------------------------
      4           HMAC                      This document

7.2.  Segment Routing Header TLVs Register

   This document requests the creation of a new IANA managed registry to
   identify SRH TLVs.  The registration procedure is "Expert Review" as
   defined in [RFC8126].  Suggested registry name is "Segment Routing
   Header TLVs".  A TLV is identified through an unsigned 8 bit
   codepoint value.  The following codepoints are defined in this
   document:

   Suggested      Description               Reference
     Value
   -----------------------------------------------------
      1         Ingress Node TLV          This document
      2         Egress Node  TLV          This document
      3         Opaque Container TLV      This document
      4         Padding
      5           HMAC TLV                  This document
      5         HMAC
      128         Pad0 TLV                  This document
      6         NSH Carrier
      129         PadN TLV                  This document

8.  Implementation Status

   This section is to be removed prior to publishing as an RFC.

8.1.  Linux

   Name: Linux Kernel v4.14

   Status: Production

   Implementation: adds SRH, performs END and END.X processing, supports
   HMAC TLV

   Details: https://irtf.org/anrw/2017/anrw17-final3.pdf and
   [I-D.filsfils-spring-srv6-interop]

8.2.  Cisco Systems

   Name: IOS XR and IOS XE

   Status: Pre-production

   Implementation: adds SRH, performs END and END.X processing, no TLV
   processing
   Details: [I-D.filsfils-spring-srv6-interop]

8.3.  FD.io

   Name: VPP/Segment Routing for IPv6

   Status: Production

   Implementation: adds SRH, performs END and END.X processing, no TLV
   processing

   Details: https://wiki.fd.io/view/VPP/Segment_Routing_for_IPv6 and
   [I-D.filsfils-spring-srv6-interop]

8.4.  Barefoot

   Name: Barefoot Networks Tofino NPU

   Status: Prototype

   Implementation: performs END and END.X processing, no TLV processing

   Details: [I-D.filsfils-spring-srv6-interop]

8.5.  Juniper

   Name: Juniper Networks Trio and vTrio NPU's

   Status: Prototype & Experimental

   Implementation: SRH insertion mode, Process SID where SID is an
   interface address, no TLV processing

9.  Contributors

   Kamran Raza, Darren Dukes, Brian Field, Daniel Bernier, Ida Leung,
   Jen Linkova, Ebben Aries, Tomoya Kosugi, Eric Vyncke, David Lebrun,
   Dirk Steinberg, Robert Raszuk, Dave Barach, John Brzozowski, Pierre
   Francois, Nagendra Kumar, Mark Townsley, Christian Martin, Roberta
   Maglione, James Connolly, Aloys Augustin contributed to the content
   of this document.

10.  Acknowledgements

   The authors would like to thank Ole Troan, Bob Hinden, Fred Baker,
   Brian Carpenter, Alexandru Petrescu, Punit Kumar Jaiswal, and David
   Lebrun for their comments to this document.

11.  References

11.1.  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>.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-15 (work
              in progress), January 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://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,
              <https://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, <https://www.rfc-editor.org/info/rfc6407>.

   [RFC7855]  Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
              Litkowski, S., Horneffer, M., and R. Shakir, "Source
              Packet Routing in Networking (SPRING) Problem Statement
              and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
              2016, <https://www.rfc-editor.org/info/rfc7855>.

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

   [RFC8300]  Quinn, P., Ed., Elzur, U.,

   [RFC8354]  Brzozowski, J., Leddy, J., Filsfils, C., Maglione, R.,
              Ed., and C. Pignataro, Ed.,
              "Network Service Header (NSH)", M. Townsley, "Use Cases for IPv6 Source Packet
              Routing in Networking (SPRING)", RFC 8300, 8354,
              DOI 10.17487/RFC8300, January 10.17487/RFC8354, March 2018,
              <https://www.rfc-editor.org/info/rfc8300>.
              <https://www.rfc-editor.org/info/rfc8354>.

11.2.  Informative References

   [I-D.bashandy-isis-srv6-extensions]
              Ginsberg, L., Bashandy, A., Filsfils, C., Decraene, B.,
              and Z. Hu, "IS-IS Extensions to Support Routing over IPv6
              Dataplane", draft-bashandy-isis-srv6-extensions-02 (work
              in progress), March 2018.

   [I-D.dawra-bgp-srv6-vpn]
              (Unknown), (., Dawra, G., Filsfils, C., Dukes, D.,
              Brissette, P., Camarillo, P., Leddy, J.,
              daniel.voyer@bell.ca, d., daniel.bernier@bell.ca, d.,
              Steinberg, D., Raszuk, R., Decraene, B., and S.
              Matsushima, "BGP Signaling of IPv6-Segment-Routing-based
              VPN Networks", draft-dawra-bgp-srv6-vpn-00 (work in
              progress), March 2017.

   [I-D.filsfils-spring-srv6-interop]
              Filsfils, C., Clad, F., Camarillo, P., Abdelsalam, A.,
              Salsano, S., Bonaventure, O., Horn, J., and J. Liste,
              "SRv6 interoperability report", draft-filsfils-spring-
              srv6-interop-00 (work in progress), March 2018.

   [I-D.filsfils-spring-srv6-network-programming]
              Filsfils, C., Li, Z., Leddy, J., daniel.voyer@bell.ca, d.,
              daniel.bernier@bell.ca, d., Steinberg, D., Raszuk, R.,
              Matsushima, S., Lebrun, D., Decraene, B., Peirens, B.,
              Salsano, S., Naik, G., Elmalky, H., Jonnalagadda, P., and
              M. Sharif, "SRv6 Network Programming", draft-filsfils-
              spring-srv6-network-programming-04 (work in progress),
              March 2018.

   [I-D.ietf-isis-l2bundles]
              Ginsberg, L., Bashandy, A., Filsfils, C., Nanduri, M., and
              E. Aries, "Advertising L2 Bundle Member Link Attributes in
              IS-IS", draft-ietf-isis-l2bundles-07 (work in progress),
              May 2017.

   [I-D.ietf-isis-segment-routing-extensions]
              Previdi, S., Ginsberg, L., 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-15 (work in progress), December
              2017.

   [I-D.ietf-ospf-ospfv3-segment-routing-extensions]
              Psenak, P., Filsfils, C., Previdi, S., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3
              Extensions for Segment Routing", draft-ietf-ospf-ospfv3-
              segment-routing-extensions-11 (work in progress), January
              2018.

   [I-D.ietf-spring-ipv6-use-cases]
              Brzozowski, J., Leddy, J., Filsfils, C., Maglione, R., and
              M. Townsley, "IPv6 SPRING Use Cases", draft-ietf-spring-
              ipv6-use-cases-12 (work in progress), December 2017.

   [I-D.ietf-spring-resiliency-use-cases]
              Filsfils, C., Previdi, S., Decraene, B., and R. Shakir,
              "Resiliency use cases in SPRING networks", draft-ietf-
              spring-resiliency-use-cases-12 (work in progress),
              December 2017.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-15 (work
              in progress), January 2018.

   [I-D.ietf-spring-segment-routing-mpls]
              Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing with MPLS
              data plane", draft-ietf-spring-segment-routing-mpls-12 draft-ietf-spring-segment-routing-mpls-13
              (work in progress), February April 2018.

   [I-D.previdi-idr-segment-routing-te-policy]
              Previdi, S., Filsfils, C., Mattes, P., Rosen, E., and S.
              Lin, "Advertising Segment Routing Policies in BGP", draft-
              previdi-idr-segment-routing-te-policy-07 (work in
              progress), June 2017.

   [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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc2104>.

   [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, <https://www.rfc-editor.org/info/rfc2827>.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,
              <https://www.rfc-editor.org/info/rfc4302>.

   [RFC4942]  Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/
              Co-existence Security Considerations", RFC 4942,
              DOI 10.17487/RFC4942, September 2007,
              <https://www.rfc-editor.org/info/rfc4942>.

   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
              DOI 10.17487/RFC5308, October 2008,
              <https://www.rfc-editor.org/info/rfc5308>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
              <https://www.rfc-editor.org/info/rfc6438>.

   [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,
              <https://www.rfc-editor.org/info/rfc6554>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

Authors' Addresses

   Stefano Previdi
   Individual
   Italy

   Email: stefano@previdi.net

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

   Email: cfilsfil@cisco.com

   John Leddy
   Comcast
   4100 East Dry Creek Road
   Centennial, CO  80122
   US

   Email: John_Leddy@comcast.com

   Satoru Matsushima
   Softbank

   Email: satoru.matsushima@g.softbank.co.jp

   Daniel Voyer (editor)
   Bell Canada

   Email: daniel.voyer@bell.ca