Network Working Group                                   C. Filsfils, Ed.
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                              S. Previdi
Expires: December 30, 2018                                    Individual April 25, 2019                                           Huawei
                                                                J. Leddy
                                                                 Comcast
                                                              Individual
                                                           S. Matsushima
                                                                Softbank
                                                           D. Voyer, Ed.
                                                             Bell Canada
                                                           June 28,
                                                        October 22, 2018

                   IPv6 Segment Routing Header (SRH)
               draft-ietf-6man-segment-routing-header-14
               draft-ietf-6man-segment-routing-header-15

Abstract

   Segment Routing can be applied to the IPv6 data plane using a new
   type of Routing Extension Header.  This document describes the
   Segment Routing Extension Header and how it is used by Segment
   Routing capable nodes.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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 December 30, 2018. April 25, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Segment Routing Extension Header  . . . . . . . . . . . . . .   4
     2.1.  SRH TLVs  . . . . . . . . . . . . . . . . . . . . . . . .   5
       2.1.1.  Padding TLVs  . . . . . . . . . . . . . . . . . . . .   6
       2.1.2.  HMAC TLV  . . . . . . . . . . . . . . . . . . . . . .   8   7
   3.  SR Nodes  . . . . . . . . . . . . . . . . . . . . . . . . . .   9  10
     3.1.  Source SR Node  . . . . . . . . . . . . . . . . . . . . .   9  10
     3.2.  Transit Node  . . . . . . . . . . . . . . . . . . . . . .   9  11
     3.3.  SR Segment Endpoint Node  . . . . . . . . . . . . . . . .   9  11
   4.  Packet Processing . . . . . . . . . . . . . . . . . . . . . .   9  11
     4.1.  Source SR Node  . . . . . . . . . . . . . . . . . . . . .   9  11
       4.1.1.  Reduced SRH . . . . . . . . . . . . . . . . . . . . .  10  12
     4.2.  Transit Node  . . . . . . . . . . . . . . . . . . . . . .  10  12
     4.3.  SR Segment Endpoint Node  . . . . . . . . . . . . . . . .  11  12
       4.3.1.  FIB Entry Is Locally Instantiated SRv6 END SID  . . .  11  12
       4.3.2.  FIB Entry is a Local Interface  . . . . . . . . . . .  13  14
       4.3.3.  FIB Entry Is A Non-Local Route  . . . . . . . . . . .  13  15
       4.3.4.  FIB Entry Is A No Match . . . . . . . . . . . . . . .  13  15
       4.3.5.  Load Balancing and ECMP . . . . . . . . . . . . . . .  13  15
   5.  Illustrations . . . . . . . . . . . . . . . . . . . . . . . .  14  15
     5.1.  Abstract Representation of an SRH . . . . . . . . . . . .  14  15
     5.2.  Example Topology  . . . . . . . . . . . . . . . . . . . .  15  16
     5.3.  Source SR Node  . . . . . . . . . . . . . . . . . . . . .  15  17
       5.3.1.  Intra SR Domain Packet  . . . . . . . . . . . . . . .  15  17
       5.3.2.  Transit Packet Through SR Domain  . . . . . . . . . .  16  17
     5.4.  Transit Node  . . . . . . . . . . . . . . . . . . . . . .  16  18
     5.5.  SR Segment Endpoint Node  . . . . . . . . . . . . . . . .  16  18
   6.  Security Considerations . . . . . . . . . . . . . .  Deployment Models . . . . .  17
     6.1.  Threat model . . . . . . . . . . . . . . . . .  18
     6.1.  Nodes Within the SR domain  . . . . .  17
       6.1.1.  Source routing threats . . . . . . . . . .  18
     6.2.  Nodes Outside the SR Domain . . . . .  17
       6.1.2.  Applicability of RFC 5095 to SRH . . . . . . . . . .  18
       6.1.3.  Service stealing threat . . .
       6.2.1.  SR Source Nodes Not Directly Connected  . . . . . . .  19

   7.  Security Considerations . . . . .  19
       6.1.4.  Topology disclosure . . . . . . . . . . . . . .  20
     7.1.  Source Routing Attacks  . . .  19
       6.1.5.  ICMP Generation . . . . . . . . . . . . . .  21
     7.2.  Service Theft . . . . .  19
     6.2.  Security fields in SRH . . . . . . . . . . . . . . . . .  19
       6.2.1.  Selecting a hash algorithm  21
     7.3.  Topology Disclosure . . . . . . . . . . . . .  21
       6.2.2.  Performance impact of HMAC . . . . . .  22
     7.4.  ICMP Generation . . . . . . .  21
       6.2.3.  Pre-shared key management . . . . . . . . . . . . . .  22
     6.3.  Deployment Models .
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . .  22
       6.3.1.  Nodes within the SR domain . .  23
     8.1.  Segment Routing Header Flags Register . . . . . . . . . .  23
     8.2.  Segment Routing Header TLVs Register  .  22
       6.3.2.  Nodes outside of the SR domain . . . . . . . . .  23
   9.  Implementation Status . .  22
       6.3.3.  SR path exposure . . . . . . . . . . . . . . . . . .  23
       6.3.4.  Impact of BCP-38  .
     9.1.  Linux . . . . . . . . . . . . . . . . .  23
   7.  IANA Considerations . . . . . . . . .  24
     9.2.  Cisco Systems . . . . . . . . . . . .  24
     7.1.  Segment Routing Header Flags Register . . . . . . . . . .  24
     7.2.  Segment Routing Header TLVs Register  . . . .
     9.3.  FD.io . . . . . .  24
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .  25
     8.1.  Linux  24
     9.4.  Barefoot  . . . . . . . . . . . . . . . . . . . . . . . .  24
     9.5.  Juniper . .  25
     8.2.  Cisco Systems . . . . . . . . . . . . . . . . . . . . . .  25
     8.3.  FD.io .  24
     9.6.  Huawei  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     8.4.  Barefoot
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  25
     8.5.  Juniper . . . . . .
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . .  26
   9.  Contributors . . .  25
   12. References  . . . . . . . . . . . . . . . . . . . . .  26
   10. Acknowledgements . . . .  25
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  26
   11.  25
     12.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . .  26
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     11.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   Segment Routing can be applied to the IPv6 data plane using a new
   type of Routing Extension Header (SRH).  This document describes the
   Segment Routing Extension Header and how it is used by Segment
   Routing capable nodes.

   The Segment Routing Architecture [I-D.ietf-spring-segment-routing] [RFC8402] describes Segment Routing
   and its instantiation in two data planes MPLS and IPv6.

   SR with the MPLS data plane is defined in
   [I-D.ietf-spring-segment-routing-mpls].

   SR with the IPv6 data plane is defined in
   [I-D.filsfils-spring-srv6-network-programming].

   The encoding of MPLS labels and label stacking are defined in
   [RFC3032].

   The encoding of IPv6 segments in the Segment Routing Extension Header
   is defined in this document.

   Terminology used within this document is defined in detail in
   [I-D.ietf-spring-segment-routing].
   [RFC8402].  Specifically, these terms: Segment Routing, SR Domain,
   SRv6, Segment ID (SID), SRv6 SID, Active Segment, and SR Policy.

2.  Segment Routing Extension Header

   Routing Headers are defined in [RFC8200].  The Segment Routing Header
   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: Defined in [RFC8200]

   o  Hdr Ext Len: Defined in [RFC8200]

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

   o  Segments Left: Defined in [RFC8200]

   o  Last Entry: contains the index (zero based), 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 U U U U U U U|
         +-+-+-+-+-+-+-+-+

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

   o  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 MUST be set to zero on transmission.  When tag is not
      used during SRH Processing it SHOULD be ignored.  The allocation
      and use of tag is outside the scope of this document.

   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 SR Policy.  I.e., the first element
      of the segment list (Segment List [0]) contains the last segment
      of the SR Policy, the second element contains the penultimate
      segment of the SR Policy and so on.

   o  Type Length Value (TLV) are described in Section 2.1.

2.1.  SRH TLVs

   This section defines TLVs of the Segment Routing Header.

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-----------------------
   |     Type      |    Length     | Variable length data
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-----------------------

   Type: An 8 bit value.  Unrecognized Types MUST be ignored on receipt.

   Length: The length of the Variable length data.  It is RECOMMENDED
   that the total length of new TLVs be multiple of 8 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 the Destination Address (DA) of the packet.

   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 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
   Variable Length Data is ignored for the purpose of ICV Computation as
   defined in [RFC4302].

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

   The following TLVs are defined in this document:

      Padding TLV

      HMAC TLV

   Additional TLVs may be defined in the future.

2.1.1.  Padding TLVs

   There are two types of padding TLVs, pad0 and padN, the following
   applies to both:

      Padding TLVs are used to pad the TLVs to a multiple of 8 octets.

      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
      octet boundary.

      When present, a single Pad0 or PadN TLV MUST appear as the last
      TLV.

      When present, a PadN TLV MUST have a length from 0 to 5 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.

2.1.1.1.  PAD0

     0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |     Type      |
     +-+-+-+-+-+-+-+-+

      Type: to be assigned by IANA (Suggested value 128)

   A single Pad0 TLV MUST be used when a single byte of padding is
   required.  If more than one byte of padding is required a Pad0 TLV
   MUST NOT be used, the PadN TLV MUST be used.

2.1.1.2.  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)                       //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

      Length: 0 to 5

      Padding: Length octets of padding.  Padding bits have no
      semantics.  They MUST be set to 0 on transmission and ignored on
      receipt.

   The PadN TLV MUST be used when more than one byte of padding is
   required.

2.1.2.  HMAC TLV

   HMAC

   The keyed Hashed Message Authentication Code (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.  MUST be 0 on transmission and ignored on
      receipt.

   o  HMAC Key ID: A 4 octets. octet opaque number which uniquely identifies the
      pre-shared key and algorithm used to generate the HMAC.  If 0, the
      HMAC is not included.

   o  HMAC: 32 octets.

   o octets of keyed HMAC, not present if Key ID is 0.

   The HMAC TLV is used to verify the source of a packet is permitted to
   use the current segment in the destination address of the packet, and
   ensure the segment list is not modified in transit.

2.1.2.1.  HMAC generation

   The HMAC field is the output of the HMAC computation as defined in
   [RFC2104], using:

   o  key: the pre-shared key identified by HMAC Key ID usage is described

   o  HMAC algorithm: identified by the HMAC Key ID

   o  Text: a concatenation of the following fields from the IPv6 header
      and the SRH, as it would be received at the node verifying the
      HMAC:

      *  IPv6 header: source address (16 octets)

      *  IPv6 header: destination address (16 octets)
      *  SRH: Segments Left (1 octet)

      *  SRH: Last Entry (1 octet)

      *  SRH: Flags (1 octet)

      *  SRH: HMAC Key-id (4 octets)

      *  SRH: all addresses in Section 6 the Segment List (variable octets)

   The Following applies HMAC digest is truncated to 32 octets and placed in the HMAC TLV:

   o
   field of the HMAC TLV.

   For HMAC algorithms producing digests less than 32 octets, the digest
   is placed in the lowest order octets of the HMAC field.  Remaining
   octets MUST be set to zero.

2.1.2.2.  HMAC Verification

   Local policy determines when to check for an HMAC and potentially a
   requirement on where the HMAC TLV must appear (e.g.  first TLV).
   This local policy is outside the scope of this document.  It may be
   based on the active segment at an SR Segment endpoint node, the
   result of an ACL that considers incoming interface, or other packet
   fields.

3.  SR Nodes

   There are different types of nodes that may be involved in segment
   routing networks: source SR nodes originate packets with a segment in

   If HMAC verification is successful, the destination address of packet is forwarded to the IPv6 header, transit nodes that
   forward packets destined
   next segment.

   If HMAC verification fails, an ICMP error message (parameter problem,
   error code 0, pointing to a remote segment, the HMAC TLV) SHOULD be generated (but rate
   limited) and SR segment endpoint
   nodes that process a local segment in SHOULD be logged.

2.1.2.3.  HMAC Pre-Shared Key Algorithm

   The HMAC Key ID field allows for the destination address simultaneous existence of an
   IPv6 header.

3.1.  Source SR Node

   A Source SR Node
   several hash algorithms (SHA-256, SHA3-256 ... or future ones) as
   well as pre-shared keys.

   The HMAC Key ID field is any node that originates opaque, i.e., it has neither syntax nor
   semantic except as an IPv6 packet with a
   segment (i.e.  SRv6 SID) in identifier of the destination address right combination of pre-
   shared key and hash algorithm, and except that a value of 0 means
   that there is no HMAC field.

   At the IPv6
   header.  The packet leaving HMAC TLV verification node the source SR Node may or may not contain
   an SRH.  This includes either:

      A host originating an IPv6 packet.

      An SR domain ingress router encapsulating a received packet in an
      outer IPv6 header, followed by an optional SRH.

   The mechanism through which a segment in Key ID uniquely identifies the
   pre-shared key and HMAC algorithm.

   At the HMAC TLV generating node the Key ID and destination address of
   uniquely identify the IPv6 header pre-shared key and HMAC algorithm.  Utilizing
   the Segment List in the SRH, is derived is
   outside destination address with the scope of this document.

3.2.  Transit Node

   A transit node Key ID allows for overlapping key
   IDs amongst different HMAC verification nodes.  The Text for the HMAC
   computation is any node forwarding an set to the IPv6 packet where header fields and SRH fields as they
   would appear at the
   destination address of that packet is verification node, not locally configured necessarily the same as a
   segment nor a local interface.  A transit
   the source node sending a packet with the HMAC TLV.

   Pre-shared key roll-over is not required supported by having two key IDs in use
   while the HMAC TLV generating node and verifying node converge to be
   capable a
   new key.

   SRH implementations can support multiple hash functions but MUST
   implement SHA-2 [FIPS180-4] in its SHA-256 variant.

   The selection of processing pre-shared key and algorithm, and their distribution
   is outside the scope of this document, some options may include:

   o  in the configuration of the HMAC generating or verifying nodes,
      either by static configuration or any SDN oriented approach

   o  dynamically using a segment nor SRH.

3.3. trusted key distribution protocol such as
      [RFC6407]

3.  SR Segment Endpoint Node

   A Nodes

   There are different types of nodes that may be involved in segment
   routing networks: source SR nodes originate packets with a segment endpoint node is any node receiving an IPv6 packet where in
   the destination address of the IPv6 header, transit nodes that packet is locally configured as
   forward packets destined to a
   segment or local interface.

4.  Packet Processing

   This section describes SRv6 packet processing at the SR source,
   Transit remote segment, and SR segment endpoint nodes.

4.1.
   nodes that process a local segment in the destination address of an
   IPv6 header.

3.1.  Source SR Node

   A Source SR Node is any node steers a packet into that originates an SR Policy.  If the SR Policy IPv6 packet with a
   segment (i.e.  SRv6 SID) in the destination address of the IPv6
   header.  The packet leaving the source SR Node may or may not contain
   an SRH.  This includes either:

      A host originating an IPv6 packet.

      An SR domain ingress router encapsulating a received packet in an
      outer IPv6 header, followed by an optional SRH.

   The mechanism through which a segment in the destination address of
   the IPv6 header and the Segment List in the SRH, is derived is
   outside the scope of this document.

3.2.  Transit Node

   A transit node is any node forwarding an IPv6 packet where the
   destination address of that packet is not locally configured as a
   segment nor a local interface.  A transit node is not required to be
   capable of processing a segment nor SRH.

3.3.  SR Segment Endpoint Node

   A SR segment endpoint node is any node receiving an IPv6 packet where
   the destination address of that packet is locally configured as a
   segment or local interface.

4.  Packet Processing

   This section describes SRv6 packet processing at the SR source,
   Transit and SR segment endpoint nodes.

4.1.  Source SR Node

   A Source node steers a packet into an SR Policy.  If the SR Policy
   results in a segment list containing a single segment, and there is
   no need to add information to SRH flag or TLV, the DA is set to the
   single segment list entry and the SRH MAY be omitted.

   When needed, the SRH is created as follows:

      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 SRH Segment List is the ultimate segment.
      The second element is the penultimate segment and so on.

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

      The Last Entry field is set to n-1 where n is the number of
      elements in the SR Policy.

      HMAC TLV may be set according to Section 6. 7.

      The packet is forwarded toward the packet's Destination Address
      (the first segment).

4.1.1.  Reduced SRH

   When a source does not require the entire SID list to be preserved in
   the SRH, a reduced SRH may be used.

   A reduced SRH does not contain the first segment of the related SR
   Policy (the first segment is the one already in the DA of the IPv6
   header), and the Last Entry field is set to n-2 where n is the number
   of elements in the SR Policy.

4.2.  Transit Node

   As specified in [RFC8200], the only node 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
   toward the DA according to its IPv6 routing table.

   When a SID is in the destination address of an IPv6 header of a
   packet, it's routed through an IPv6 network as an IPv6 address.
   SIDs, or the prefix(es) covering SIDs, 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 SIDs on a node.

4.3.  SR Segment Endpoint Node

   Without constraining the details of an implementation, the SR segment
   endpoint node creates Forwarding Information Base (FIB) entries for
   its local SIDs.

   When an SRv6-capable node receives an IPv6 packet, it performs a
   longest-prefix-match lookup on the packets destination address.  This
   lookup can return any of the following:

       A FIB entry that represents a locally instantiated SRv6 SID
       A FIB entry that represents a local interface, not locally
                                     instantiated as an SRv6 SID
       A FIB entry that represents a non-local route
       No Match

4.3.1.  FIB Entry Is Locally Instantiated SRv6 END SID

   This document, and section, defines a single SRv6 SID called END.
   Future documents may define additional SRv6 SIDs.  In which case, the
   entire content of this section will be defined in that document.

   If the FIB entry represents a locally instantiated SRv6 SID, process
   the next header of the IPv6 header as defined in section 4 of
   [RFC8200]

   The following sections describe the actions to take while processing
   next header fields.

4.3.1.1.  SRH Processing

   When an SRH is processed {
     If Segments Left is equal to zero {
       Proceed to process the next header in the packet, whose type
       is identified by the Next Header field in the Routing header.
     }
     Else {
       If local policy requires TLV processing {
         Perform TLV processing (see TLV Processing)
       }
       max_last_entry  =  ( Hdr Ext Len /  2 ) - 1

       If  ((Last Entry > max_last_entry) or
            (Segments Left is greater than (Last Entry+1)) {
         Send an ICMP Parameter Problem, Code 0, message to the
         Source Address, pointing to the Segments Left field, and
         discard the packet.
       }
       Else {
         Decrement Segments Left by 1.
         Copy Segment List[Segments Left] from the SRH to the
         destination address of the IPv6 header.
         If the IPv6 Hop Limit is less than or equal to 1 {
           Send an ICMP Time Exceeded -- Hop Limit Exceeded in
           Transit message to the Source Address and discard
           the packet.
         }
         Else {
           Decrement the Hop Limit by 1
           Resubmit the packet to the IPv6 module for transmission
           to the new destination.
         }
       }
     }
   }

4.3.1.1.1.  TLV Processing

   Local policy determines how TLV's are to be processed when the Active
   Segment is a local END SID.  The definition of local policy is
   outside the scope of this document.

   For illustration purpose only, two example local policies that may be
   associated with an END SID are provided below.

   Example 1:
   For any packet received from interface I2
     Skip TLV processing

   Example 2:
   For any packet received from interface I1
     If first TLV is HMAC {
       Process the HMAC TLV
     }
     Else {
       Discard the packet
     }

4.3.1.2.  Upper-layer Header or No Next Header

   Send an ICMP destination unreachable parameter problem message to the Source Address and
   discard the packet.  Error code (TBD by IANA) "SR Upper-layer Header
   Error", pointer set to the offset of the upper-layer header.

   A unique error code allows an SR Source node to recognize an error in
   SID processing at an endpoint.

4.3.2.  FIB Entry is a Local Interface

   If the FIB entry represents a local interface, not locally
   instantiated as an SRv6 SID, the SRH is processed as follows:

      If Segments Left is zero, the node must ignore the Routing header
      and proceed to process the next header in the packet, whose type
      is identified by the Next Header field in the Routing Header.

      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.

4.3.3.  FIB Entry Is A Non-Local Route

   Processing is not changed by this document.

4.3.4.  FIB Entry Is A No Match

   Processing is not changed by this document.

4.3.5.  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 Tunnel End Point.

   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.  If flow label is not used, the transit node may
   hash all packets between a pair of SR Edge nodes to the same link.

   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.

5.  Illustrations

   This section provides illustrations of SRv6 packet processing at SR
   source, transit and SR segment endpoint nodes.

5.1.  Abstract Representation of an SRH

   For a node k, its IPv6 address is represented as Ak, its SRv6 SID is
   represented as Sk.

   IPv6 headers are represented as the tuple of (source, destination).
   For example, a packet with source address A1 and destination address
   A2 is represented as (A1,A2).  The payload of the packet is omitted.

   An SR Policy is a list of segments.  A list of segments is
   represented as <S1,S2,S3> where S1 is the first SID to visit, S2 is
   the second SID to visit and S3 is the last SID to visit.

   (SA,DA) (S3, S2, S1; SL) represents an IPv6 packet with:

   o  Source Address is SA, Destination Addresses is DA, and next-header
      is SRH.

   o  SRH with SID list <S1, S2, S3> with SegmentsLeft = SL.

   o  Note the difference between the <> and () symbols.  <S1, S2, S3>
      represents a SID list where the leftmost segment is the first
      segment.  Whereas, (S3, S2, S1; SL) represents the same SID list
      but encoded in the SRH Segment List format where the leftmost
      segment is the last segment.  When referring to an SR policy in a
      high-level use-case, it is simpler to use the <S1, S2, S3>
      notation.  When referring to an illustration of detailed behavior,
      the (S3, S2, S1; SL) notation is more convenient.

   At its SR Policy headend, the Segment List <S1,S2,S3> results in SRH
   (S3,S2,S1; SL=2) represented fully as:

       Segments Left=2
       Last Entry=2
       Flags=0
       Tag=0
       Segment List[0]=S3
       Segment List[1]=S2
       Segment List[2]=S1

5.2.  Example Topology

   The following topology is used in examples below:

           + * * * * * * * * * * * * * * * * * * * * +

           *         [8]                [9]          *
                      |                  |
           *          |                  |           *
   [1]----[3]--------[5]----------------[6]---------[4]---[2]
           *          |                  |           *
                      |                  |
           *          |                  |           *
                      +--------[7]-------+
           *                                         *

           + * * * * * * *  SR Domain  * * * * * * * +

                                 Figure 3

   o  3 and 4 are SR Domain edge routers

   o  5, 6, and 7 are all SR Domain routers

   o  8 and 9 are hosts within the SR Domain
   o  1 and 2 are hosts outside the SR Domain

5.3.  Source SR Node

5.3.1.  Intra SR Domain Packet

   When host 8 sends a packet to host 9 via an SR Policy <S7,A9> the
   packet is

   P1: (A8,S7)(A9,S7; SL=1)

5.3.1.1.  Reduced Variant

   When host 8 sends a packet to host 9 via an SR Policy <S7,A9> and it
   wants to use a reduced SRH, the packet is

   P2: (A8,S7)(A9; SL=1)

5.3.2.  Transit Packet Through SR Domain

   When host 1 sends a packet to host 2, the packet is

   P3: (A1,A2)

   The SR Domain ingress router 3 receives P3 and steers it to SR Domain
   egress router 4 via an SR Policy <S7, S4>.  Router 3 encapsulates the
   received packet P3 in an outer header with an SRH.  The packet is

   P4: (A3, S7)(S4, S7; SL=1)(A1, A2)

   If the SR Policy contains only one segment (the egress router 4), the
   ingress Router 3 encapsulates P3 into an outer header (A3, S4).  The
   packet is

   P5: (A3, S4)(A1, A2)

5.3.2.1.  Reduced Variant

   The SR Domain ingress router 3 receives P3 and steers it to SR Domain
   egress router 4 via an SR Policy <S7, S4>.  If router 3 wants to use
   a reduced SRH, Router 3 encapsulates the received packet P3 in an
   outer header with a reduced SRH.  The packet is

   P6: (A3, S7)(S4; SL=1)(A1, A2)

5.4.  Transit Node

   Nodes 5 acts as transit nodes for packet P1, and sends packet

   P1: (A8,S7)(A9,S7;SL=1)

   on the interface toward node 7.

5.5.  SR Segment Endpoint Node

   Node 7 receives packet P1 and, using the logic in section 4.3.1,
   sends packet

   P7: (A8,A9)(A9,S7; SL=0)

   on the interface toward router 6.

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 [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 [RFC8200].

   Per [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 FIB
   contains a locally instantiated SRv6 SID equal to the destination
   address field of the IPv6 packet MUST parse the SRH if present, and
   if supported and if the local configuration allows it, MUST act
   accordingly to the SRH content.

   As specified in [RFC8200], the default behavior of a non 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] 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 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], "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,
   [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].  All non-trusted nodes
   do not participate as either SR processing is not enabled by default
   or because they only process SRH from nodes within their domain.

   Moreover, all SR 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 [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 SIDs 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
   Policy, then there is little learned by intercepting the SRH itself.

6.1.5.  ICMP Generation

   Per Section 4.4 of [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 7 receives packet P1 and, using the logic in section 4.3.1,
   sends packet and
      send an ICMP Parameter Problem, Code 0, message to

   P7: (A8,A9)(A9,S7; SL=0)

   on the packet's
      Source Address, pointing to interface toward router 6.

6.  Deployment Models

6.1.  Nodes Within the unrecognized Routing Type.

   This required behavior could be used by SR domain

   SR Source Nodes within an attacker SR Domain are trusted to force the
   generation of ICMP message by any node.  The attacker could send generate IPv6
   packets with SRH (with Segment Left not set to 0) destined to a node
   not supporting SRH.  Per [RFC8200], the destination node could
   generate an ICMP message, causing a local CPU utilization and if the
   source  SR segment endpoint nodes receiving packets on
   interface that are part of the offending SR Domain may process any packet with SRH was spoofed could lead
   destined 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 local segment, containing an SRH.

   A SR Source Node connected to SRH and the usual rate limiting for ICMP generation is required
   anyway for any IPv6 implementation and has been implemented SR Domain via a secure tunnel, e.g.
   IPSec tunnel mode [RFC4303] or Ethernet pseudowire [RFC4448], may be
   considered trusted and
   deployed for many years.

6.2.  Security fields in SRH

   This section summarizes the use directly connected.  Some types of specific fields tunnels may
   result in the SRH.  They
   are based on a key-hashed message authentication code (HMAC).

   The security-related fields additional processing overhead that should be considered in
   a deployment.

6.2.  Nodes Outside the SRH are instantiated by SR Domain

   Nodes outside 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 SR Domain cannot be trusted.  SR Domain Ingress
   routers SHOULD discard packets destined to SIDs within the SR Domain
   (regardless of the output presence of an SRH) to avoid attacks on the HMAC computation (per [RFC2104])
   using a pre-shared key identified by HMAC Key-id SR
   Domain as described and referenced in [RFC5095].  As an additional
   layer of the text
   which consists protection, SR Segment Endpoint nodes SHOULD discard packets
   destined to local SIDs from source addresses not part of the concatenation of:

   o SR
   Domain.

   For example, using the source example topology from section 5, all SIDs in
   the SR Domain (SIDS S1-S9) are assigned within a single IPv6 address;

   o  Last Entry field;

   o  an octet of bit flags;

   o  HMAC Key-id;

   o prefix,
   Prefix-S.  All SIDs assigned to a node k are assigned within a single
   IPv6 prefix Prefix-Sk, all addresses permitted to source packets
   destined to SIDs in the Segment List.

   The purpose of the HMAC TLV is SR Domain are assigned within a single IPv6
   prefix Prefix-A.

   An Infrastructure Access List (IACL), applied to verify the validity, the integrity external
   interfaces of SR Domain ingress nodes 3 and 4, that discards packets
   destined to a SID covered by Prefix-S is used to discard packets
   destined to SIDs within the authorization SR Domain.

   An IACL, applied to each interface of the SRH itself.  If an outsider SR Segment Endpoint Nodes k,
   that discards packets destined to a SID covered by Prefix-Sk with a
   source address not covered by Prefix-A.

   Failure to implement a method of ingress filtering, as defined above,
   exposes the SR domain does not have access to a current pre-shared secret, then it
   cannot compute source routing attacks from nodes outside
   the right HMAC field SR Domain, as described and referenced in [RFC5095].

6.2.1.  SR Source Nodes Not Directly Connected

   Nodes outside the first SR router on Domain may request, by some trusted means
   outside the
   path scope of this document, a complete SRH including an HMAC
   TLV which is computed correctly for the SRH.

   SR Domain ingress routers permit traffic destined to select SIDs with
   local policy requiring HMAC TLV processing the SRH and configured for those select SIDs,
   i.e. those SIDs provide a gateway to check the validity SR Domain for a set of the
   HMAC will simply reject the packet.

   The
   segment lists.

   If HMAC TLV verification 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 successful, the upstream router.  This packet is forwarded to speed up forwarding operations
   because SR routers which do not validate the SRH do not need to parse the SRH until
   next segment.  Within the end.

   The SR Domain no further 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 check need be
   performed.

   If HMAC Key-id field is opaque, i.e., it
   has neither syntax nor semantic except as verification fails, an index ICMP error message (parameter problem,
   error code 0, pointing to the right
   combination of pre-shared key and hash algorithm HMAC TLV) SHOULD be generated (but rate
   limited) and except that SHOULD be logged.

   For example, extending the topology defined in Figure 3, consider
   node 3 offering access to a
   value of 0 means that there premium SLA service to node 20.  Node 20
   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 trusted SR Source not directly connected to the SR Domain.

                + * * * * * * * * * * * * * * * * * * * * +

                *         [8]                [9]          *
                           |                  |
                *          |                  |           *
   [20]--[11]--[3]--------[5]----------------[6]---------[4]---[2]
                *          |                  |           *
                           |                  |
                *          |                  |           *
                           +--------[7]-------+
                *                                         *

                   + * * * * * * *  SR nodes converged Domain  * * * * * * * +

   In order to access the SLA service, node 20 must be able to access
   segments within the SR Domain.  To provide a fresher
   pre-shared key.  It could also allow secure entry point for interoperation among
   different SR domains if allowed by
   the SLA service, SIDs with local policy and assuming a
   collision-free requiring 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) verification
   at node k are
   identical, then it is important to refresh the shared-secret
   frequently defined as the HMAC validity period expires only when the HMAC
   Key-id Hk and its associated shared-secret expires.

6.2.1.  Selecting assigned from a hash algorithm

   The HMAC field in the HMAC TLV prefix Prefix-H.
   Prefix-H is 256 bit wide.  Therefore, the HMAC
   MUST be based on a hash function whose output disjoint with Prefix-S and Prefix-A defined earlier.

   Prefix-H is at least 256 bits.
   If the output not part of the hash function is 256, then this output is simply
   inserted in the HMAC field.  If IACLs applied at the output external facing
   interfaces of the hash function is
   larger than 256 bits, then the output value is truncated node 3 and 4, allowing external nodes access to 256 it.

   SID H3 is a SID covered by
   taking Prefix-H at node 3.

   Node 20 requests the least-significant 256 bits premium SLA service to node 2 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 provided a
   pre-computed SRH and HMAC with 96 bits destination address H3.

   Node 20 sends a packet with destination addresses 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 H2, SRH and
   HMAC to each TLV are as provided for the premium SLA service.

   Node 3 receives the packet and every SR verifies the HMAC as defined in
   section 4.3, forwarding the packet increases to the
   security, it has a performance impact.  Nevertheless, next segment in the segment
   list or dropping it must be
   noted that:

   o based on the HMAC field is used only when SRH result.

   This use of an HMAC is added by a device (such as particularly valuable within an enterprise
   based SR Domain to authenticate a home set-top box) host which is outside of the using SRv6 segment
   routing
      domain.  If as documented in [SRN].  In that example, the SRH HMAC is added by used to
   validate a router in the trusted segment
      routing domain, then, there source node is no need for an HMAC field, hence no
      performance impact.

   o  when present, the HMAC field need only be checked and validated by using a permitted segment list.

7.  Security Considerations

   This section reviews security considerations related to the first router of SRH,
   given the segment routing domain, SRH processing and deployment models discussed in this router
   document.

   As describe in Section 6, it is
      named 'validating SR router'.

   o  this validating necessary to filter packets ingress
   to the SR router can also have a cache of <IPv6 header +
      SRH, HMAC field value> Domain destined to improve segments within the performance.  It SR Domain.  This
   ingress filtering is via an IACL at SR Domain ingress border nodes.
   Additional protection is applied via an IACL at each SR Segment
   Endpoint node, filtering packets not from within the
      same use case as in IPsec where HMAC value was unique per packet, SR Domain,
   destined to SIDs in SRH, the HMAC value is unique per flow.

   o  hash functions such as SHA-2 have been optimized SR Domain.  ACLs are easily supported for security and
      performance
   small numbers of prefixes, making summarization important, and there are multiple implementations with good
      performance.

   With when
   the above points prefixes requiring filtering is kept to a seldom changing set.

   Additionally, ingress filtering of IPv6 source addresses as
   recommended in mind, BCP38 SHOULD be used.

   SR Source Nodes not directly connected to the performance impact SR Domain may access
   specific sets of using HMAC
   is minimized.

6.2.3.  Pre-shared key management

   The field HMAC Key-id allows for:

   o  key roll-over: segments within the SR Domain when there is a need to change secured with the key (the hash
      pre-shared secret), then multiple pre-shared keys can be used
      simultaneously.
   SRH HMAC TLV.  The validating SR router can have SRH HMAC TLV provides a table means of
      <HMAC Key-id, pre-shared secret> for the currently active and
      future keys.

   o  different algorithms: by extending verifying the previous table
   validity of ingress packets SRH, limiting access to <HMAC
      Key-id, hash function, pre-shared secret>, the validating SR
      router can also support several hash algorithms (see section
      Section 6.2.1)

   The pre-shared secret distribution can be done:

   o segments in
   the configuration of the validating SR routers, either by
      static configuration or any SDN oriented approach;

   o  dynamically using Domain to only those source nodes with permission.

7.1.  Source Routing Attacks

   [RFC5095] deprecates the Type 0 Routing header due to a trusted key distribution such as [RFC6407]

   The intent number of
   significant attacks that are referenced in that document.  Such
   attacks include bypassing filtering devices, reaching otherwise
   unreachable Internet systems, network topology discovery, bandwidth
   exhaustion, and defeating anycast.

   Because this document specifies that the SRH is NOT to define yet-another-key-
   distribution-protocol.

6.3.  Deployment Models

6.3.1.  Nodes for use within the an SR
   domain protected by ingress filtering via IACLs, and by
   cryptographically authenticated SR Source Nodes within source nodes not directly
   connected to the SR Domain; such attacks cannot be mounted from
   outside an SR Domain are trusted to generate IPv6
   packets with SRH. Domain.  As specified in this document, SR segment endpoint Domain
   ingress edge nodes receiving drop packets on
   interface that are part of entering the SR Domain may process any packet destined to a local segment, containing an SRH.

6.3.2.  Nodes outside of
   segments within the SR domain

   Nodes outside Domain.

   Aditionally, this document specifies the use of IACL on SR Domain cannot be trusted. Segment
   Endpoint nodes within the SR Domain Ingress
   routers SHOULD discard to limit the source addresses
   permitted to send packets destined to a SID in the SR Domain.

   Such attacks may, however, be mounted from within the SR Domain, from
   nodes permitted to source traffic to SIDs in the domain.  As such,
   these attacks and other known attacks on an IP network (e.g.  DOS/
   DDOS, topology discovery, man-in-the-middle, traffic interception/
   siphoning), can occur from compromised nodes within an SR Domain.

7.2.  Service Theft

   Service theft is defined as the use of a service offered by the SR
   Domain
   (regardless of the presence of an SRH) by a node not authorized to avoid attacks on use the SR
   Domain.  This service.

   Service theft is accomplished via infrastructure Access Lists (iACLs)
   applied on domain ingress nodes.  However not a concern within the SR Domain may be
   extended to as all SR Source
   nodes outside of it via use and SR segment endpoint nodes within the domain are able to
   utilizing the services of the SRH HMAC.

   Nodes Domain.  If a node outside the SR
   Domain may request, by some trusted means
   outside the scope learns of this document, segments or a complete SRH including an HMAC
   TLV which is computed correctly for topological service within the SRH (see Section 6.2). SR Domain ingress routers permit traffic destined
   domain, IACL filtering denies access to select SIDs with
   local policy requiring HMAC TLV processing for those select SIDs,
   i.e. these SIDs provide a gateway to segments.

   Nodes outside the SR Domain for a set Domain, capable of
   segment lists.

   If HMAC validation is successful, the packet is forwarded intercepting packets from SR
   Source nodes not directly connected to the next
   segment.  Within the SR Domain no further HMAC check need be
   performed.  However, other segments in utilizing the SR domain MAY verify the
   HMAC TLV when
   SRH HMAC, may steel the outer IP header SRH is processed, dependent on local policy.

   If HMAC validation fails an ICMP error message (parameter problem)
   SHOULD be generated (but rate limited) and SHOULD be logged.

6.3.3.  SR path exposure

   The SRH contains a Segment List. HMAC TLV.  If such an observer outside the SR
   Domain
   attacker is able to inspect capable of spoofing the SRH, they source address of the original
   sender it may use the segments in the
   Segment List IP header and HMAC to launch an access services of the SR
   Domain intended for the original SR Source node.

   Frequent rekeying of the HMAC TLV helps mitigate against this attack on
   but cannot prevent it.

   However, as described in Section 6.2.1, there exist use cases where
   the SR Domain or obtain knowledge risk of the topology within the SR Domain.  When the SR Source node service threat is
   outside the SR Domain of minimum concern and the packet traverses the public internet HMAC TLV is
   used primarily to validate that the SR Domain ingress router it source is likely that others will have
   access permitted to use the Segment List
   segment list in the SRH.

   IPSec Encapsulating Security Payload (ESP), [RFC4303], cannot be use
   to protect the

7.3.  Topology Disclosure

   The SRH as may contains SIDs of some intermediate SR-nodes in the ESP header must appear after path
   towards the routing
   header (including SRH).

   Exposure of segments and TLV content destination, this reveals those addresses to observers outside the SR
   Domain should be considered in any deployment.  There attackers if
   they are two methods able to limit exposure, and attacks on segments intercept packets containing SRH.

   This is applicable within the an SR Domain from
   outside the SR Domain:

      Limit but the number disclosure is less
   relevant as an attacker has other means of segments and the TLV data exposed in SRH from
      nodes outside the learning topology.

   For an SR Domain.

      Restrict which SIDs may accept traffic from outside Source node not directly connected to the SR Domain
      to only those enforcing HMAC verification by using iACLs (as
      described in Section 6.3.2).

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
   disclosure is applicable.  While the expected
   routing.  Therefore, if BCP-38 was implemented in all routers inside segments within the SR domain, SR packets could domain
   disclosed in SRH are protected by ingress filtering, they may be received
   learned by an interface which is
   not the expected one, and attacker external to the packets could be dropped. SR Domain.

   As BCP-38 is only deployed at described in Section 6.2.1, there exist use cases where the ingress routers risk
   of an
   administrative domain, and as Packets arriving at those ingress
   routers have been forwarded using topology disclosure is of minimum concern when the routing information, then there HMAC TLV is no reason why this ingress router should drop
   used primarily to validate that the source is permitted to use the
   segment list in the SRH.

7.4.  ICMP Generation

   The generation of ICMPv6 error messages may be used to attempt
   denial-of-service attacks by sending an error-causing destination
   address or SRH packet based
   on BCP-38.  Routers inside in back-to-back packets.  An implementation that
   correctly follows Section 2.4 of [RFC4443] would be protected by the domain commonly do not apply BCP-38;
   so, this is not a problem.

7.
   ICMPv6 rate-limiting mechanism.

8.  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.

8.1.  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.

8.2.  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
   -----------------------------------------------------
      5           HMAC TLV                  This document
      128         Pad0 TLV                  This document
      129         PadN TLV                  This document

8.

9.  Implementation Status

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

8.1.

9.1.  Linux

   Name: Linux Kernel v4.14

   Status: Production

   Implementation: adds SRH, performs END processing, supports HMAC TLV

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

8.2.

9.2.  Cisco Systems

   Name: IOS XR and IOS XE

   Status: Pre-production

   Implementation: adds SRH, performs END processing, no TLV processing

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

8.3.

9.3.  FD.io

   Name: VPP/Segment Routing for IPv6

   Status: Production

   Implementation: adds SRH, performs END processing, no TLV processing

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

8.4.

9.4.  Barefoot

   Name: Barefoot Networks Tofino NPU

   Status: Prototype

   Implementation: performs END processing, no TLV processing

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

8.5.

9.5.  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.

9.6.  Huawei

   Name: Huawei Systems VRP Platform

   Status: Production

   Implementation: adds SRH, performs END processing, no TLV processing

10.  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.

11.  Acknowledgements

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

11.

12.  References

11.1.

12.1.  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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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>.

11.2.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

12.2.  Informative References

   [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
              srv6-interop-01 (work in progress), March September 2018.

   [I-D.filsfils-spring-srv6-network-programming]
              Filsfils, C., Li, Z., Camarillo, P., 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, Z. Li, "SRv6
              Network Programming", draft-filsfils-
              spring-srv6-network-programming-04 draft-filsfils-spring-srv6-network-
              programming-05 (work in progress),
              March July 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-14
              (work in progress), June 2018.

   [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>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <https://www.rfc-editor.org/info/rfc3032>.

   [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,

   [RFC4303]  Kent, S., and P. Savola, "IPv6 Transition/
              Co-existence "IP Encapsulating Security Considerations", Payload (ESP)",
              RFC 4942, 4303, DOI 10.17487/RFC4942, September 2007,
              <https://www.rfc-editor.org/info/rfc4942>. 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC4448]  Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron,
              "Encapsulation Methods for Transport of Ethernet over MPLS
              Networks", RFC 4448, DOI 10.17487/RFC4448, April 2006,
              <https://www.rfc-editor.org/info/rfc4448>.

   [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>.

   [SRN]      and , "Software Resolved Networks: Rethinking Enterprise
              Networks with IPv6 Segment Routing", 2018,
              <https://inl.info.ucl.ac.be/system/files/
              sosr18-final15-embedfonts.pdf>.

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

   Email: cfilsfil@cisco.com

   Stefano Previdi
   Individual
   Huawei
   Italy

   Email: stefano@previdi.net

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

   Email: John_Leddy@comcast.com john@leddy.net

   Satoru Matsushima
   Softbank

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

   Daniel Voyer (editor)
   Bell Canada

   Email: daniel.voyer@bell.ca