Network Working Group                                   C. Filsfils, Ed.
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                              S. Previdi
Expires: April 25, August 8, 2019                                           Huawei
                                                                J. Leddy
                                                              Individual
                                                           S. Matsushima
                                                                Softbank
                                                           D. Voyer, Ed.
                                                             Bell Canada
                                                        October 22, 2018
                                                        February 4, 2019

                   IPv6 Segment Routing Header (SRH)
               draft-ietf-6man-segment-routing-header-15
               draft-ietf-6man-segment-routing-header-16

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 April 25, August 8, 2019.

Copyright Notice

   Copyright (c) 2018 2019 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  . . . . . . . . . . . . . . . . . . . . . .   7   8
   3.  SR Nodes  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.1.  Source SR Node  . . . . . . . . . . . . . . . . . . . . .  10
     3.2.  Transit Node  . . . . . . . . . . . . . . . . . . . . . .  11
     3.3.  SR Segment Endpoint Node  . . . . . . . . . . . . . . . .  11
   4.  Packet Processing . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Source SR Node  . . . . . . . . . . . . . . . . . . . . .  11
       4.1.1.  Reduced SRH . . . . . . . . . . . . . . . . . . . . .  12
     4.2.  Transit Node  . . . . . . . . . . . . . . . . . . . . . .  12
     4.3.  SR Segment Endpoint Node  . . . . . . . . . . . . . . . .  12
       4.3.1.  FIB Entry Is Locally Instantiated SRv6 END SID  . . .  12
       4.3.2.  FIB Entry is a Local Interface  . . . . . . . . . . .  14
       4.3.3.  FIB Entry Is A Non-Local Route  . . . . . . . . . . .  15
       4.3.4.  FIB Entry Is A No Match . . . . . . . . . . . . . . .  15
       4.3.5.  Load Balancing and ECMP .
   5.  Intra SR Domain Deployment Model  . . . . . . . . . . . . . .  15
   5.  Illustrations . . . .
     5.1.  Securing the SR Domain  . . . . . . . . . . . . . . . . .  15
     5.2.  SR Domain as a single system with delegation among
           components  . . .  15
     5.1.  Abstract Representation of an SRH . . . . . . . . . . . .  15
     5.2.  Example Topology . . . . . . . .  16
     5.3.  MTU Considerations  . . . . . . . . . . . .  16
     5.3.  Source SR Node . . . . . . .  17
     5.4.  AH ICV  . . . . . . . . . . . . . .  17
       5.3.1.  Intra SR Domain Packet . . . . . . . . . . .  17
     5.5.  ESP ICV . . . .  17
       5.3.2.  Transit Packet Through SR Domain . . . . . . . . . .  17
     5.4.  Transit Node . . . . . . . . . . .  17
     5.6.  ICMP Error Processing . . . . . . . . . . .  18
     5.5.  SR Segment Endpoint Node . . . . . . .  17
     5.7.  Load Balancing and ECMP . . . . . . . . .  18
   6.  Deployment Models . . . . . . . .  18
     5.8.  Other Deployments . . . . . . . . . . . . . .  18
     6.1.  Nodes Within the SR domain . . . . . .  18
   6.  Illustrations . . . . . . . . .  18
     6.2.  Nodes Outside the SR Domain . . . . . . . . . . . . . . .  18
       6.2.1.  SR Source Nodes Not Directly Connected
     6.1.  Abstract Representation of an SRH . . . . . . .  19

   7.  Security Considerations . . . . .  18
     6.2.  Example Topology  . . . . . . . . . . . . . .  20
     7.1.  Source Routing Attacks . . . . . .  19
     6.3.  Source SR Node  . . . . . . . . . . .  21
     7.2.  Service Theft . . . . . . . . . .  20
       6.3.1.  Intra SR Domain Packet  . . . . . . . . . . . .  21
     7.3.  Topology Disclosure . . .  20
       6.3.2.  Inter SR Domain Packet - Transit  . . . . . . . . . .  20
       6.3.3.  Inter SR Domain Packet - Internal to External . . . .  21
     6.4.  Transit Node  . .  22
     7.4.  ICMP Generation . . . . . . . . . . . . . . . . . . . .  21
     6.5.  SR Segment Endpoint Node  .  22
   8.  IANA Considerations . . . . . . . . . . . . . . .  21
     6.6.  Delegation of Function with HMAC Verification . . . . . .  23
     8.1.  Segment Routing Header Flags Register  21
       6.6.1.  SID List Verification . . . . . . . . . .  23
     8.2.  Segment Routing Header TLVs Register . . . . . .  21
   7.  Security Considerations . . . .  23
   9.  Implementation Status . . . . . . . . . . . . . . .  22
     7.1.  Source Routing Attacks  . . . . .  23
     9.1.  Linux . . . . . . . . . . . .  23
     7.2.  Service Theft . . . . . . . . . . . . . .  24
     9.2.  Cisco Systems . . . . . . . .  23
     7.3.  Topology Disclosure . . . . . . . . . . . . . .  24
     9.3.  FD.io . . . . .  23
     7.4.  ICMP Generation . . . . . . . . . . . . . . . . . . . . .  24
     9.4.  Barefoot
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
     8.1.  Segment Routing Header Flags Register . . .  24
     9.5.  Juniper . . . . . . .  24
     8.2.  Segment Routing Header TLVs Register  . . . . . . . . . .  24
   9.  Implementation Status . . . . . . . .  24 . . . . . . . . . . . .  25
     9.1.  Linux . . . . . . . . . . . . . . . . . . . . . . . . . .  25
     9.2.  Cisco Systems . . . . . . . . . . . . . . . . . . . . . .  25
     9.3.  FD.io . . . . . . . . . . . . . . . . . . . . . . . . . .  25
     9.4.  Barefoot  . . . . . . . . . . . . . . . . . . . . . . . .  25
     9.5.  Juniper . . . . . . . . . . . . . . . . . . . . . . . . .  26
     9.6.  Huawei  . . . . . . . . . . . . . . . . . . . . . . . . .  25  26
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  25  26
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  25  26
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25  26
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  25  26
     12.2.  Informative References . . . . . . . . . . . . . . . . .  26  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27  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 [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
   [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

   The keyed Hashed Message Authentication Code (HMAC) TLV is OPTIONAL
   and 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 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 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 Generation and Verification

   Local policy determines when to check for an HMAC and potentially
   provides an alternate composition of Text, and 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, HMAC Key ID, or other packet fields.

   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

   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 the Segment List (variable octets)

   The HMAC digest is truncated to 32 octets and placed in the HMAC
   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.

   If

   If HMAC verification is successful, the packet is forwarded to the
   next segment.

   If HMAC verification fails, an ICMP error message (parameter problem,
   error code 0, pointing to the HMAC TLV) SHOULD be generated (but rate
   limited) and SHOULD be logged.

2.1.2.3.

2.1.2.2.  HMAC Pre-Shared Key Algorithm

   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 identifier of the 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 HMAC TLV verification node the Key ID uniquely identifies the
   pre-shared key and HMAC algorithm.

   At the HMAC TLV generating node the Key ID and destination address
   uniquely identify the pre-shared key and HMAC algorithm.  Utilizing
   the destination address with the Key ID allows for overlapping key
   IDs amongst different HMAC verification nodes.  The Text for the HMAC
   computation is set to the IPv6 header fields and SRH fields as they
   would appear at the verification node, not necessarily the same as
   the source node sending a packet with the HMAC TLV.

   Pre-shared key roll-over is supported by having two key IDs in use
   while the HMAC TLV generating node and verifying node converge to 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 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 trusted key distribution protocol such as
      [RFC6407]

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
   the destination address of the IPv6 header, transit nodes that
   forward packets destined to a remote segment, and SR segment endpoint
   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 that originates an 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 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

   S01. When an SRH is processed {
   S02.   If Segments Left is equal to zero {
   S03.     Proceed to process the next header in the packet,
            whose type is identified by the Next Header field in
            the Routing header.
   S04.   }
   S05.   Else {
   S06.     If local policy requires TLV processing {
   S07.       Perform TLV processing (see TLV Processing)
   S08.     }
   S09.     max_last_entry  =  ( Hdr Ext Len /  2 ) - 1
   S10.     If  ((Last Entry > max_last_entry) or
   S11.          (Segments Left is greater than (Last Entry+1)) {
   S12.       Send an ICMP Parameter Problem, Code 0, message to
              the Source Address, pointing to the Segments Left
              field, and discard the packet.
   S13.     }
   S14.     Else {
   S15.       Decrement Segments Left by 1.
   S16.       Copy Segment List[Segments Left] from the SRH to the
              destination address of the IPv6 header.
   S17.       If the IPv6 Hop Limit is less than or equal to 1 {
   S18.         Send an ICMP Time Exceeded -- Hop Limit Exceeded in
                Transit message to the Source Address and discard
                the packet.
   S19.       }
   S20.       Else {
   S21.         Decrement the Hop Limit by 1
   S22.         Resubmit the packet to the IPv6 module for transmission
                to the new destination.
   S23.       }
   S24.     }
   S25.   }
   S26. }

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

   When processing the Upper-layer header of a packet matching a FIB
   entry locally instantiated as an SRv6 END SID.

   IF (Upper-layer Header is IPv4 or IPv6) and local policy permits {
     Perform IPv6 decapsulation
     Resubmit the decapsulated packet to the IPv4 or IPv6 module
   }
   ELSE {
     Send an ICMP 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

5.  Intra SR Domain Deployment Model

   The use of the SIDs exclusively within the SR Domain and ECMP

   Within an solely for
   packets of the SR domain, Domain is an important deployment model.

   This enables the SR source node encapsulates Domain to act as a packet in an
   outer IPv6 header for transport single routing system.

   This section covers:

   o  securing the SR Domain from external attempt to an endpoint.  The use its SIDs

   o  SR source node
   MUST impose Domain as a flow label computed based on the inner packet.  The
   computation single system with delegation between components

   o  handling packets 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 Domain

5.1.  Securing the ECMP hash toward SR Domain

   Nodes outside the
   destination address.  If flow label is SR Domain are not used, trusted: they cannot directly use
   the transit node may
   hash all packets between a pair SID's 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 domain.  This section provides illustrations is enforced by two levels of SRv6 access
   control lists:

   1.  Any packet processing at entering the SR
   source, transit Domain and destined to a SID within
       the SR segment endpoint nodes.

5.1.  Abstract Representation Domain is dropped.  This may be realized with the
       following logic, other methods with equivalent outcome are
       considered compliant:

       *  allocate all the SID's from a block S/s

       *  configure each external interface of each edge node of the
          domain with an SRH

   For inbound infrastructure access list (IACL) which
          drops any incoming packet with a node k, its IPv6 destination address is represented in S/s

       *  Failure to implement this method of ingress filtering exposes
          the SR Domain to source routing attacks as Ak, its SRv6 SID described and
          referenced in [RFC5095]

   2.  The distributed protection in #1 is
   represented as Sk.

   IPv6 headers are represented as complemented with per node
       protection, dropping packets to SIDs from source addresses
       outside the tuple SR Domain.  This may be realized with the following
       logic, other methods with equivalent outcome are considered
       compliant:

       *  assign all interface addresses from prefix A/a

       *  at node k, all SIDs local to k are assigned from prefix Sk/sk

       *  configure each internal interface of (source, destination).
   For example, a each SR node k in the SR
          Domain with an inbound IACL which drops any incoming packet
          with source address A1 and a destination address
   A2 in Sk/sk if the source address is represented
          not in A/a.

5.2.  SR Domain as (A1,A2).  The payload a single system with delegation among components

   All intra SR Domain packets are of the packet SR Domain.  The IP v6 header
   is omitted.

   An originated by a node of the SR Policy Domain, and is destined to a list node
   of segments.  A list of segments is
   represented as <S1,S2,S3> where S1 is the first SID to visit, S2 is SR Domain.

   All inter domain packets are encapsulated for the second SID to visit and S3 is part of the last SID to visit.

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

   o  Source Address
   journey that is SA, Destination Addresses within the SR Domain.  The outer IPv6 header is DA,
   originated by a node of the SR Domain, and next-header is SRH.

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

   o  Note the difference between destined to a node of
   the <> and () symbols.  <S1, S2, S3>
      represents SR Domain.

   As a SID list where consequence, any packet within the leftmost segment SR Domain is of the first
      segment.  Whereas, (S3, S2, S1; SL) represents the same SID list
      but encoded SR
   Domain.

   The SR Domain is a system in which the operator may want to
   distribute or delegate different operations of the outer most header
   to different nodes within the system.

   An operator of an SR domain may choose to delegate SRH Segment List format where addition to a
   host node within the leftmost
      segment is SR domain, and validation of the last segment.  When referring contents of any
   SRH to a more trusted router or switch attached to the host.
   Consider a top of rack switch (T) connected to host (H) via interface
   (I).  H receives an SR policy in SRH (SRH1) with a
      high-level use-case, computed HMAC via some SDN
   method outside the scope of this document.  H classifies traffic it is simpler
   sources and adds SRH1 to use traffic requiring a specific SLA.  T is
   configured with an IACL on I requiring verification of the <S1, S2, S3>
      notation.  When referring SRH for
   any packet destined to an illustration the SID block 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 (S/s).  T
   checks and 4 are SR Domain edge routers

   o  5, 6, verifies that SRH1 is valid, contains an HMAC TLV and 7 are all
   verifies the HMAC.

5.3.  MTU Considerations

   Within the SR Domain routers

   o  8 and 9 Domain, well known mitigation techniques are hosts
   RECOMMENDED, such as deploying a greater MTU value within the SR
   Domain
   o  1 than at the ingress edges.

5.4.  AH ICV

   Within the domain, an SR source node which includes SRH and 2 are hosts outside AH
   extension headers can predict the content of the SRH and calculate
   the ICV at the SR Domain

5.3.  Source source node, ensuring it can be confirmed at the
   destination.

5.5.  ESP ICV

   Within the domain, an SR Node

5.3.1.  Intra source node may include SRH and ESP
   extension headers.  Only the data following the ESP header is
   included in ICV computation.  SRH precedes ESP.

5.6.  ICMP Error Processing

   ICMP error packets generated within the SR Domain Packet

   When host 8 sends a packet are sent to host 9 via an SR Policy <S7,A9> source
   nodes within the SR Domain.  The invoking packet is

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

5.3.1.1.  Reduced Variant

   When host 8 sends in the ICMP error
   message may contain an SRH.  Since the destination address of a
   packet to host 9 via with an SR Policy <S7,A9> and SRH changes as each segment is processed, it
   wants to use a reduced SRH, may not
   be the packet is

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

5.3.2.  Transit Packet Through SR Domain

   When host 1 sends a destination used by the socket or application that generated
   the invoking packet.

   For the source of an invoking packet to host 2, process the packet is

   P3: (A1,A2) ICMP error
   message, the correct destination address must be determined.  The SR Domain ingress router 3 receives P3 and steers it
   following logic is used to SR Domain
   egress router 4 via an SR Policy <S7, S4>.  Router 3 encapsulates determine the
   received destination address for use
   by protocol error handlers.

   o  Walk all extension headers of the invoking IPv6 packet P3 in an outer to the
      routing extension header with an SRH.  The packet is

   P4: (A3, S7)(S4, S7; SL=1)(A1, A2) preceding the upper layer header.

      *  If routing header is type 4 (SRH)

         +  Use the SR Policy contains only one 0th segment (the egress router 4), in the
   ingress Router 3 encapsulates P3 into segment list as the destination
            address of the invoking packet.

   ICMP errors are then processed by upper layer transports as defined
   in [RFC4443].

   For IP packets encapsulated in an outer header (A3, S4).  The
   packet IPv6 header, ICMP error
   handling is

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

5.3.2.1.  Reduced Variant

   The SR Domain ingress router 3 receives P3 as defined in [RFC2473].

5.7.  Load Balancing and steers it to SR Domain
   egress router 4 via an ECMP

   For any inter domain packet, the SR Policy <S7, S4>.  If router 3 wants to use Source node MUST impose a reduced SRH, Router 3 encapsulates the received packet P3 in an
   outer header with a reduced SRH. flow
   label computed based on the inner packet.  The packet computation of the
   flow label is

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

5.4.  Transit Node

   Nodes 5 acts as transit nodes recommended in [RFC6438] for packet P1, and sends packet

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

   on the interface toward sending Tunnel End
   Point.

   At any transit node 7.

5.5. within an SR Segment Endpoint Node

   Node 7 receives packet P1 and, using domain, the logic flow label MUST be used
   as defined in section 4.3.1,
   sends packet

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

   on [RFC6438] to calculate the interface ECMP hash toward router 6.

6.  Deployment Models

6.1.  Nodes Within the
   destination address.  If flow label is not used, the transit node may
   hash all packets between a pair of SR domain

   SR Source Nodes within Edge nodes to the same link.

   At an SR Domain are trusted segment endpoint node, the flow label MUST be used as
   defined in [RFC6438] to generate IPv6
   packets calculate any ECMP hash used to forward the
   processed packet to the next segment.

5.8.  Other Deployments

   Other deployment models and their implications on security, MTU,
   HMAC, ICMP error processing and interaction with SRH. other extension
   headers are outside the scope of this document.

6.  Illustrations

   This section provides illustrations of SRv6 packet processing at SR
   source, transit and SR segment endpoint nodes receiving packets on
   interface that are part nodes.

6.1.  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 SR Domain may process any packet
   destined to tuple of (source, destination).
   For example, a local segment, containing an SRH.

   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 Node connected 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

6.2.  Example Topology

   The following topology is used in examples below:

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

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

           + * * * * * * *  SR Domain via a secure tunnel, e.g.
   IPSec tunnel mode [RFC4303] or Ethernet pseudowire [RFC4448], may be
   considered trusted  * * * * * * * +

                                 Figure 3

   o  3 and directly connected.  Some types of tunnels may
   result in additional processing overhead that should be considered in
   a deployment.

6.2.  Nodes Outside the SR Domain

   Nodes outside the SR Domain cannot be trusted. 4 are SR Domain Ingress edge routers SHOULD discard packets destined to SIDs within the SR Domain
   (regardless of the presence of an SRH) to avoid attacks on the SR
   Domain as described and referenced in [RFC5095].  As an additional
   layer of protection, SR Segment Endpoint nodes SHOULD discard packets
   destined to local SIDs from source addresses not part of the SR
   Domain.

   For example, using the example topology from section

   o  5, 6, and 7 are all SIDs in
   the SR Domain (SIDS S1-S9) are assigned within a single IPv6 prefix,
   Prefix-S.  All SIDs assigned to a node k routers

   o  8 and 9 are assigned hosts within a single
   IPv6 prefix Prefix-Sk, all addresses permitted to source packets
   destined to SIDs in the SR Domain

   o  1 and 2 are assigned within a single IPv6
   prefix Prefix-A.

   An Infrastructure Access List (IACL), applied to hosts outside the external
   interfaces of SR Domain ingress nodes 3 and 4, that discards packets
   destined to a SID covered by Prefix-S
   o  The SR domain is used to discard packets
   destined to SIDs within secured as per Section 5.1 and no external packet
      can enter the SR Domain.

   An IACL, applied to each interface of SR Segment Endpoint Nodes k,
   that discards packets destined to a SID covered by Prefix-Sk domain with a
   source destination address not covered by Prefix-A.

   Failure equal to implement a method segment
      of ingress filtering, as defined above,
   exposes the domain.

6.3.  Source SR domain Node

6.3.1.  Intra SR Domain Packet

   When host 8 sends a packet to source routing attacks from nodes outside host 9 via an SR Policy <S7,A9> the
   packet is

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

6.3.1.1.  Reduced Variant

   When host 8 sends a packet to host 9 via an SR Domain, as described Policy <S7,A9> and referenced in [RFC5095].

6.2.1.  SR Source Nodes Not Directly Connected

   Nodes outside it
   wants to use a reduced SRH, the packet is

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

6.3.2.  Inter SR Domain may request, by some trusted means
   outside the scope of this document, Packet - Transit

   When host 1 sends a complete SRH including an HMAC
   TLV which is computed correctly for packet to host 2, the SRH. packet is

   P3: (A1,A2)

   The SR Domain ingress routers permit traffic destined router 3 receives P3 and steers it to select SIDs SR Domain
   egress router 4 via an SR Policy <S7, S4>.  Router 3 encapsulates the
   received packet P3 in an outer header with
   local policy requiring HMAC TLV processing for those select SIDs,
   i.e. those SIDs provide a gateway to an SRH.  The packet is

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

   If the SR Domain for a set of Policy contains only one segment lists.

   If HMAC verification is successful, (the egress router 4), the
   ingress Router 3 encapsulates P3 into an outer header (A3, S4).  The
   packet is forwarded

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

6.3.2.1.  Reduced Variant

   The SR Domain ingress router 3 receives P3 and steers it to the
   next segment.  Within the SR Domain no further HMAC check need be
   performed.

   If HMAC verification fails,
   egress router 4 via an ICMP error message (parameter problem,
   error code 0, pointing SR Policy <S7, S4>.  If router 3 wants to use
   a reduced SRH, Router 3 encapsulates the HMAC TLV) SHOULD be generated (but rate
   limited) and SHOULD be logged.

   For example, extending the topology defined received packet P3 in Figure 3, consider
   node 3 offering access to an
   outer header with a premium SLA service to node 20.  Node 20 reduced SRH.  The packet is a trusted

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

6.3.3.  Inter SR Source not directly connected Domain Packet - Internal to External

   When host 8 sends a packet to host 1, the packet is encapsulated for
   the portion of its journey within the SR Domain.

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

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

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

   In order  From 8 to access 3 the SLA service, node 20 must be able
   packet is

   P7: (A8,S3)(A8,A1)

   In the opposite direction, the packet generated from 1 to access
   segments 8 is

   P8: (A1,A8)

   At node 3 P8 is encapsulated for the portion of its journey within
   the SR Domain.  To provide a secure entry point domain.  Resulting in

   P9: (A3,S8)(A1,A8)

   At node 9 the outer IPv6 header is removed by S6 processing then
   processed again when received by A8.

6.4.  Transit Node

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

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

   on the SLA service, SIDs interface toward node 7.

6.5.  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.

6.6.  Delegation of Function with local policy requiring HMAC verification
   at node k are defined as Hk and assigned from Verification

   This section describes how a function may be delegated within the SR
   Domain to non SR source nodes.  In the following sections consider a
   host 8 connected to a prefix Prefix-H.
   Prefix-H is disjoint with Prefix-S and Prefix-A defined earlier.

   Prefix-H is not part top of rack 5.

6.6.1.  SID List Verification

   An operator may prefer to add the IACLs applied SRH at source 8, while 5 verifies
   the external facing
   interfaces of node 3 and 4, allowing external nodes access to it. SID H3 list is a SID covered by Prefix-H valid.

   For illustration purpose, an SDN controller provides 8 an SRH
   terminating at node 3.

   Node 20 requests the premium SLA service to node 2 and is provided a
   pre-computed SRH 9, with segment list <S5,S7,S6,A9>, and HMAC TLV
   computed for the SRH.  The HMAC key is shared with destination address H3. 5, node 8 does not
   know the key.  Node 20 sends a packet 5 is configured with destination addresses set an IACL applied to H2, SRH and the
   interface connected to 8, requiring HMAC TLV are as provided verification for any packet
   destined to S/s.

   Node 8 originates packets with the premium SLA service. received SRH with HMAC TLV.

   P15:(A8,S5)(A9,S6,S7,S5;SL=3;HMAC)

   Node 3 5 receives the packet and verifies the HMAC as defined in
   section 4.3, forwarding for the SRH, then forwards the
   packet to the next segment in the segment
   list or dropping it based on the HMAC result.

   P16:(A8,S7)(A9,S6,S7,S5;SL=2;HMAC)

   Node 6 receives

   P17:(A8,S6)(A9,S6,S7,S5;SL=1;HMAC)

   Node 9 receives

   P18:(A8,A9)(A9,S6,S7,S5;SL=0;HMAC)

   This use of an HMAC is particularly valuable within an enterprise
   based SR Domain to authenticate a host which is using SRv6 segment
   routing as documented in [SRN].  In that example, the HMAC is used to
   validate a source node is using a permitted segment list.

7.  Security Considerations

   This section reviews security considerations related to the SRH,
   given the SRH processing and deployment models discussed in this
   document.

   As describe in Section 6, 5, it is necessary to filter packets ingress
   to the SR Domain destined to segments within the 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 SR Domain,
   destined to SIDs in the SR Domain.  ACLs are easily supported for
   small numbers of prefixes, making summarization important, and when
   the prefixes requiring filtering is kept to a seldom changing set.

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

   SR Source Nodes not directly connected to the SR Domain may access
   specific sets of segments within the SR Domain when secured with the
   SRH HMAC TLV.  The SRH HMAC TLV provides a means of verifying the
   validity of ingress packets SRH, limiting access to the segments in prefixes, making summarization important, and when
   the SR Domain prefixes requiring filtering is kept to only those a seldom changing set.

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

7.1.  Source Routing Attacks

   [RFC5095] deprecates the Type 0 Routing header due to a 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 for use within an SR
   domain protected by ingress filtering via IACLs, and by
   cryptographically authenticated SR source nodes not directly
   connected to the SR Domain; IACLs; such attacks cannot
   be mounted from outside an SR Domain.  As specified in this document,
   SR Domain ingress edge nodes drop packets entering the SR Domain
   destined to segments within the SR Domain.

   Aditionally,

   Additionally, this document specifies the use of IACL on SR Segment
   Endpoint nodes within the SR Domain to limit the source addresses
   permitted to send packets 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 by a node not authorized to use the service.

   Service theft is not a concern within the SR Domain as all SR Source
   nodes and SR segment endpoint nodes within the domain are able to
   utilizing
   utilize the services of the Domain.  If a node outside the SR Domain
   learns of segments or a topological service within the SR domain,
   IACL filtering denies access to those segments.

   Nodes outside the SR Domain, capable of intercepting packets from SR
   Source nodes not directly connected to the SR Domain utilizing the
   SRH HMAC, may steel the outer IP header SRH and HMAC TLV.  If such an
   attacker is capable of spoofing the source address of the original
   sender it may use the IP header and HMAC to access services of the SR
   Domain intended for the original SR Source node.

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

   However, as described in Section 6.2.1, there exist use cases where
   the risk of service threat is of minimum concern and the HMAC TLV is
   used primarily to validate that the source is permitted to use the
   segment list in the SRH.

7.3.  Topology Disclosure

   The SRH may contains SIDs of some intermediate SR-nodes in the path
   towards the destination, this reveals those addresses to attackers if
   they are able to intercept packets containing SRH.

   This is applicable within an SR Domain but the disclosure is less
   relevant as an attacker has other means of learning topology.

   For an SR Source node not directly connected to the SR Domain this
   disclosure is applicable.  While the segments within the SR domain
   disclosed in SRH are protected by ingress filtering, they may be
   learned by an attacker external to the SR Domain.

   As described in Section 6.2.1, there exist use cases where the risk
   of topology disclosure is of minimum concern when the HMAC TLV is
   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 in back-to-back packets.  An implementation that
   correctly follows Section 2.4 of [RFC4443] would be protected by the
   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"

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

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

9.  Implementation Status

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

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]

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]

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]

9.4.  Barefoot

   Name: Barefoot Networks Tofino NPU

   Status: Prototype
   Implementation: performs END processing, no TLV processing

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

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

9.6.  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.

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.

12.  References

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>.

   [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>.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
              December 1998, <https://www.rfc-editor.org/info/rfc2473>.

   [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>.

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

   [I-D.filsfils-spring-srv6-network-programming]
              Filsfils, C., Camarillo, P., Leddy, J.,
              daniel.voyer@bell.ca, d., Matsushima, S., and Z. Li, "SRv6
              Network Programming", draft-filsfils-spring-srv6-network-
              programming-05
              programming-06 (work in progress), July October 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 draft-ietf-spring-segment-routing-mpls-18
              (work in progress), June December 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>.

   [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>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 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>.

   [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
   Huawei
   Italy

   Email: stefano@previdi.net

   John Leddy
   Individual
   US

   Email: john@leddy.net

   Satoru Matsushima
   Softbank

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

   Daniel Voyer (editor)
   Bell Canada

   Email: daniel.voyer@bell.ca