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Versions: (draft-ali-spring-srv6-oam) 00 01 02 03 draft-ietf-6man-spring-srv6-oam

6man                                                              Z. Ali
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                           Cisco Systems
Expires: January 25, 2020                                  S. Matsushima
                                                                Softbank
                                                                D. Voyer
                                                             Bell Canada
                                                                 M. Chen
                                                                  Huawei
                                                           July 24, 2019


  Operations, Administration, and Maintenance (OAM) in Segment Routing
                  Networks with IPv6 Data plane (SRv6)
                   draft-ali-6man-spring-srv6-oam-03

Abstract

   This document defines building blocks for Operations, Administration,
   and Maintenance (OAM) in Segment Routing Networks with IPv6 Dataplane
   (SRv6).  The document also describes some SRv6 OAM mechanisms.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 25, 2020.







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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
     2.1.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Terminology and Reference Topology  . . . . . . . . . . .   3
   3.  OAM Building Blocks . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  O-flag in Segment Routing Header  . . . . . . . . . . . .   5
       3.1.1.  O-flag Processing . . . . . . . . . . . . . . . . . .   6
     3.2.  OAM Segments  . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  End.OP: OAM Endpoint with Punt  . . . . . . . . . . . . .   6
     3.4.  End.OTP: OAM Endpoint with Timestamp and Punt . . . . . .   7
     3.5.  SRH TLV . . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  OAM Mechanisms  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Ping  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
       4.1.1.  Classic Ping  . . . . . . . . . . . . . . . . . . . .   8
       4.1.2.  Pinging a SID Function  . . . . . . . . . . . . . . .   9
       4.1.3.  Error Reporting . . . . . . . . . . . . . . . . . . .  12
     4.2.  Traceroute  . . . . . . . . . . . . . . . . . . . . . . .  12
       4.2.1.  Classic Traceroute  . . . . . . . . . . . . . . . . .  13
       4.2.2.  Traceroute to a SID Function  . . . . . . . . . . . .  14
     4.3.  Monitoring of SRv6 Paths  . . . . . . . . . . . . . . . .  18
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  ICMPv6 type Numbers RegistrySEC . . . . . . . . . . . . .  19
     6.2.  SRv6 OAM Endpoint Types . . . . . . . . . . . . . . . . .  19
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  20
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22




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1.  Introduction

   This document defines building blocks for Operations, Administration,
   and Maintenance (OAM) in Segment Routing Networks with IPv6 Dataplane
   (SRv6).  The document also describes some SRv6 OAM mechanisms.

2.  Conventions Used in This Document

2.1.  Abbreviations

   The following abbreviations are used in this document:

      SID: Segment ID.

      SL: Segment Left.

      SR: Segment Routing.

      SRH: Segment Routing Header.

      SRv6: Segment Routing with IPv6 Data plane.

      TC: Traffic Class.

      ICMPv6: multi-part ICMPv6 messages [RFC4884].

2.2.  Terminology and Reference Topology

   This document uses the terminology defined in [I-D.ietf- spring-srv6-
   network-programming].  The readers are expected to be familiar with
   the same.

   Throughout the document, the following simple topology is used for
   illustration.

















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            +--------------------------| N100 |------------------------+
            |                                                          |
               ====== link1====== link3------ link5====== link9------
               ||N1||======||N2||======| N3 |======||N4||======| N5 |
               ||  ||------||  ||------|    |------||  ||------|    |
               ====== link2====== link4------ link6======link10------
                              |                      |
                              |       ------         |
                              +-------| N6 |---------+
                                link7 |    | link8
                                      ------

                            Figure 1 Reference Topology


   In the reference topology:

      Nodes N1, N2, and N4 are SRv6 capable nodes.

      Nodes N3, N5 and N6 are classic IPv6 nodes.

      Node N100 is a controller.

      Node k has a classic IPv6 loopback address A:k::/128.

      A SID at node k with locator block B and function F is represented
      by B:k:F::.

      The IPv6 address of the nth Link between node X and Y at the X
      side is represented as 2001:DB8:X:Y:Xn::, e.g., the IPv6 address
      of link6 (the 2nd link) between N3 and N4 at N3 in Figure 1 is
      2001:DB8:3:4:32::.  Similarly, the IPv6 address of link5 (the 1st
      link between N3 and N4) at node 3 is 2001:DB8:3:4:31::.

      B:k:Cij:: is explicitly allocated as the END.X function at node k
      towards neighbor node i via jth Link between node i and node j.
      e.g., B:2:C31:: represents END.X at N2 towards N3 via link3 (the
      1st link between N2 and N3).  Similarly, B:4:C52:: represents the
      END.X at N4 towards N5 via link10.

      A SID list 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 along the SR path.

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

      *  IPv6 header with source address SA, destination addresses DA
         and SRH as next-header



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

      *  Note the difference between the < > and () symbols: <S1, S2,
         S3> represents a SID list where S1 is the first SID and S3 is
         the last SID to traverse.  (S3, S2, S1; SL) represents the same
         SID list but encoded in the SRH format where the rightmost SID
         in the SRH is the first SID and the leftmost SID in the SRH is
         the last SID.  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 the detailed packet behavior,
         the (S3, S2, S1; SL) notation is more convenient.

      *  (payload) represents the the payload of the packet.

      SRH[SL] represents the SID pointed by the SL field in the first
      SRH.  In our example, SRH[2] represents S1, SRH[1] represents S2
      and SRH[0] represents S3.

3.  OAM Building Blocks

   This section defines the various building blocks for implementing OAM
   mechanisms in SRv6 networks.

3.1.  O-flag in Segment Routing Header

   [I-D.ietf-6man-segment-routing-header] describes the Segment Routing
   Header (SRH) and how SR capable nodes use it.  The SRH contains an
   8-bit "Flags" field [I-D.draft-ietf-6man-segment- routing-header].
   This document defines the following bit in the SRH.Flags to carry the
   O-flag:

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


   Where:

      O-flag: OAM flag.  When set, it indicates that this packet is an
      operations and management (OAM) packet.  This document defines the
      usage of the O-flag in the SRH.Flags.

   The document does not define any other flag in the SRH.Flags and
   meaning and processing of any other bit in SRH.Flags is outside of
   the scope of this document.





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3.1.1.  O-flag Processing

   Implementation of the O-flag is OPTIONAL.  A node MAY ignore
   SRH.Flags.O-flag.  It is also possible that a node is capable of
   supporting the O-bit but based on a local decision it MAY ignore it
   during processing on some local SIDs.  If a node does not support the
   O-flag, then upon reception it simply ignores it.  If a node supports
   the O-flag, it can optionally advertise its potential via node
   capability advertisement in IGP [I-D.ietf-isis-srv6- extensions] and
   BGP-LS [I-D.ietf-idr-bgpls-srv6-ext].

   The SRH.Flags.O-flag implements the "punt a timestamped copy and
   forward" behavior.

   When N receives a packet whose IPv6 DA is S and S is a local SID, N
   executes the following pseudo-code, before the execution of the local
   SID S.


     1. IF SRH.Flags.O-flag is one and local configuration permits THEN
            a. Make a copy of the packet.
            b. Send the copied packet, along with an accurate timestamp
               to the OAM process.      ;; Ref1
     Ref1: An implementation SHOULD copy and record the timestamp as soon as
     possible during packet processing. Timestamp is not carried in the packet
     forwarded to the next hop.


3.2.  OAM Segments

   OAM Segment IDs (SIDs) is another component of the SRv6 OAM building
   Blocks.  This document defines a couple of OAM SIDs.

3.3.  End.OP: OAM Endpoint with Punt

   Many scenarios require punting of SRv6 OAM packets at the desired
   nodes in the network.  The "OAM Endpoint with Punt" function (End.OP
   for short) represents a particular OAM function to implement the punt
   behavior for an OAM packet.  It is described using the pseudocode as
   follows:

   When N receives a packet destined to S and S is a local End.OP SID, N
   does:


        1.   Send the packet to the OAM process





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   Please note that in an SRH containing END.OP SID, it is RECOMMENDED
   to set the SRH.Flags.O-flag = 0.

3.4.  End.OTP: OAM Endpoint with Timestamp and Punt

   Scenarios demanding performance management of an SR policy/ path
   requires hardware timestamping before hardware punts the packet to
   the software for OAM processing.  The "OAM Endpoint with Timestamp
   and Punt" function (End.OTP for short) represents an OAM SID function
   to implement the timestamp and punt behavior for an OAM packet.  It
   is described using the pseudocode as follows:

   When N receives a packet destined to S and S is a local End.OTP SID,
   N does:


      1. Timestamp the packet ;; Ref1, Ref2

      2. Send the packet, along with an accurate timestamp, to the OAM process.

      Ref1: Timestamping SHOULD be done in hardware, as soon as possible
      during the packet processing.
      Ref2: An implementation should not generate further ICMP error during
      local SID S processing. If local SID S processing requires generation
      of an ICMP error, the error is generated by the local OAM process.

   Please note that in an SRH containing END.OTP SID, it is RECOMMENDED
   to set the SRH.Flags.O-flag = 0.

3.5.  SRH TLV

   [I-D.ietf-6man-segment-routing-header] defines TLVs of the Segment
   Routing Header.

   SRH TLV plays an important role in carrying OAM and Performance
   Management (PM) metadata.

4.  OAM Mechanisms

   This section describes how OAM mechanisms can be implemented using
   the OAM building blocks described in the previous section.
   Additional OAM mechanisms will be added in a future revision of the
   document.

   [RFC4443] describes Internet Control Message Protocol for IPv6
   (ICMPv6) that is used by IPv6 devices for network diagnostic and
   error reporting purposes.  As Segment Routing with IPv6 data plane
   (SRv6) simply adds a new type of Routing Extension Header, existing
   ICMPv6 ping mechanisms can be used in an SRv6 network.  This section



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   describes the applicability of ICMPv6 in the SRv6 network and how the
   existing ICMPv6 mechanisms can be used for providing OAM
   functionality.

   The document does not propose any changes to the standard ICMPv6
   [RFC4443], [RFC4884] or standard ICMPv4 [RFC792].

4.1.  Ping

   There is no hardware or software change required for ping operation
   at the classic IPv6 nodes in an SRv6 network.  That includes the
   classic IPv6 node with ingress, egress or transit roles.
   Furthermore, no protocol changes are required to the standard ICMPv6
   [RFC4443], [RFC4884] or standard ICMPv4 [RFC792].  In other words,
   existing ICMP ping mechanisms work seamlessly in the SRv6 networks.

   The following subsections outline some use cases of the ICMP ping in
   the SRv6 networks.

4.1.1.  Classic Ping

   The existing mechanism to ping a remote IP prefix, along the shortest
   path, continues to work without any modification.  The initiator may
   be an SRv6 node or a classic IPv6 node.  Similarly, the egress or
   transit may be an SRv6 capable node or a classic IPv6 node.

   If an SRv6 capable ingress node wants to ping an IPv6 prefix via an
   arbitrary segment list <S1, S2, S3>, it needs to initiate ICMPv6 ping
   with an SR header containing the SID list <S1, S2, S3>.  This is
   illustrated using the topology in Figure 1.  Assume all the links
   have IGP metric 10 except both links between node2 and node3, which
   have IGP metric set to 100.  User issues a ping from node N1 to a
   loopback of node 5, via segment list <B:2:C31, B:4:C52>.

   Figure 2 contains sample output for a ping request initiated at node
   N1 to the loopback address of node N5 via a segment list <B:2:C31,
   B:4:C52>.


       > ping A:5:: via segment-list B:2:C31, B:4:C52

       Sending 5, 100-byte ICMP Echos to B5::, timeout is 2 seconds:
       !!!!!
       Success rate is 100 percent (5/5), round-trip min/avg/max = 0.625
       /0.749/0.931 ms

               Figure 2 A sample ping output at an SRv6 capable node




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   All transit nodes process the echo request message like any other
   data packet carrying SR header and hence do not require any change.
   Similarly, the egress node (IPv6 classic or SRv6 capable) does not
   require any change to process the ICMPv6 echo request.  For example,
   in the ping example of Figure 2:

   o  Node N1 initiates an ICMPv6 ping packet with SRH as follows
      (A:1::, B:2:C31)(A:5::, B:4:C52, B:2:C31, SL=2, NH =
      ICMPv6)(ICMPv6 Echo Request).

   o  Node N2, which is an SRv6 capable node, performs the standard SRH
      processing.  Specifically, it executes the END.X function
      (B:2:C31) and forwards the packet on link3 to N3.

   o  Node N3, which is a classic IPv6 node, performs the standard IPv6
      processing.  Specifically, it forwards the echo request based on
      DA B:4:C52 in the IPv6 header.

   o  Node N4, which is an SRv6 capable node, performs the standard SRH
      processing.  Specifically, it observes the END.X function
      (B:4:C52) with PSP (Penultimate Segment POP) on the echo request
      packet and removes the SRH and forwards the packet across link10
      to N5.

   o  The echo request packet at N5 arrives as an IPv6 packet without an
      SRH.  Node N5, which is a classic IPv6 node, performs the standard
      IPv6/ ICMPv6 processing on the echo request and responds,
      accordingly.

4.1.2.  Pinging a SID Function

   The classic ping described in the previous section cannot be used to
   ping a remote SID function, as explained using an example in the
   following.

   Consider the case where the user wants to ping the remote SID
   function B:4:C52, via B:2:C31, from node N1.  Node N1 constructs the
   ping packet (A:1::, B:2:C31)(B:4:C52, B:2:C31, SL=1;
   NH=ICMPv6)(ICMPv6 Echo Request).  The ping fails because the node N4
   receives the ICMPv6 echo request with DA set to B:4:C52 but the next
   header is ICMPv6, instead of SRH.  To solve this problem, the
   initiator needs to mark the ICMPv6 echo request as an OAM packet.

   The OAM packets are identified either by setting the O-flag in SRH or
   by inserting the END.OP/ END.OTP SIDs at an appropriate place in the
   SRH.  The following illustration uses END.OTP SID but the procedures
   are equally applicable to the END.OP SID.




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   In an SRv6 network, the user can exercise two flavors of the ping:
   end-to-end ping or segment-by-segment ping, as outlined in the
   following subsection.

4.1.2.1.  End-to-end ping using END.OP/ END.OTP

   The end-to-end ping illustration uses the END.OTP SID but the
   procedures are equally applicable to the END.OP SID.

   Consider the same example where the user wants to ping a remote SID
   function B:4:C52, via B:2:C31, from node N1.  To force a punt of the
   ICMPv6 echo request at the node N4, node N1 inserts the END.OTP SID
   just before the target SID B:4:C52 in the SRH.  The ICMPv6 echo
   request is processed at the individual nodes along the path as
   follows:

   o  Node N1 initiates an ICMPv6 ping packet with SRH as follows
      (A:1::, B:2:C31)(B:4:C52, B:4:OTP, B:2:C31; SL=2;
      NH=ICMPv6)(ICMPv6 Echo Request).

   o  Node N2, which is an SRv6 capable node, performs the standard SRH
      processing.  Specifically, it executes the END.X function
      (B:2:C31) on the echo request packet.

   o  Node N3 receives the packet as follows (A:1::, B:4:OTP)(B:4:C52,
      B:4:OTP, B:2:C31 ; SL=1; NH=ICMPv6)(ICMPv6 Echo Request).  Node
      N3, which is a classic IPv6 node, performs the standard IPv6
      processing.  Specifically, it forwards the echo request based on
      DA B:4:OTP in the IPv6 header.

   o  When node N4 receives the packet (A:1::, B:4:OTP)(B:4:C52,
      B:4:OTP, B:2:C31 ; SL=1; NH=ICMPv6)(ICMPv6 Echo Request), it
      processes the END.OTP SID, as described in the pseudocode in
      Section 3.  The packet gets punted to the ICMPv6 process for
      processing.  The ICMPv6 process checks if the next SID in SRH (the
      target SID B:4:C52) is locally programmed.

   o  If the target SID is not locally programmed, N4 responses with the
      ICMPv6 message (Type: "SRv6 OAM (TBA)", Code: "SID not locally
      implemented (TBA)"); otherwise a success is returned.

4.1.2.2.  Segment-by-segment ping using O-flag (Proof of Transit)

   Consider the same example where the user wants to ping a remote SID
   function B:4:C52, via B:2:C31, from node N1.  However, in this ping,
   the node N1 wants to get a response from each segment node in the SRH
   as a "proof of transit".  In other words, in the segment-by-segment
   ping case, the node N1 expects a response from node N2 and node N4



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   for their respective local SID function.  When a response to O-bit is
   desired from the last SID in a SID-list, it is the responsibility of
   the ingress node to use USP as the last SID.  E.g., in this example,
   the target SID B:4:C52 is a USP SID.

   To force a punt of the ICMPv6 echo request at node N2 and node N4,
   node N1 sets the O-flag in SRH.  The ICMPv6 echo request is processed
   at the individual nodes along the path as follows:

   o  Node N1 initiates an ICMPv6 ping packet with SRH as follows
      (A:1::, B:2:C31)(B:4:C52, B:2:C31; SL=1, Flags.O=1;
      NH=ICMPv6)(ICMPv6 Echo Request).

   o  When node N2 receives the packet (A:1::, B:2:C31)(B:4:C52,
      B:2:C31; SL=1, Flags.O=1; NH=ICMPv6)(ICMPv6 Echo Request) packet,
      it processes the O-flag in SRH, as described in the pseudocode in
      Section 3.  A time-stamped copy of the packet gets punted to the
      ICMPv6 process for processing.  Node N2 continues to apply the
      B:2:C31 SID function on the original packet and forwards it,
      accordingly.  As B:4:C52 is a USP SID, N2 does not remove the SRH.
      The ICMPv6 process at node N2 checks if its local SID (B:2:C31) is
      locally programmed or not and responds to the ICMPv6 Echo Request.

   o  If the target SID is not locally programmed, N4 responses with the
      ICMPv6 message (Type: "SRv6 OAM (TBA)", Code: "SID not locally
      implemented (TBA)"); otherwise a success is returned.  Please note
      that, as mentioned in Section 3, if node N2 does not support the
      O-flag, it simply ignores it and process the local SID, B:2:C31.

   o  Node N3, which is a classic IPv6 node, performs the standard IPv6
      processing.  Specifically, it forwards the echo request based on
      DA B:4:C52 in the IPv6 header.

   o  When node N4 receives the packet (A:1::, B:4:C52)(B:4:C52,
      B:2:C31; SL=0, Flags.O=1; NH=ICMPv6)(ICMPv6 Echo Request), it
      processes the O-flag in SRH, as described in the pseudocode in
      Section 3.  A time-stamped copy of the packet gets punted to the
      ICMPv6 process for processing.  The ICMPv6 process at node N4
      checks if its local SID (B:2:C31) is locally programmed or not and
      responds to the ICMPv6 Echo Request.  If the target SID is not
      locally programmed, N4 responses with the ICMPv6 message (Type:
      "SRv6 OAM (TBA)", Code: "SID not locally implemented (TBA)");
      otherwise a success is returned.

   Support for O-flag is part of node capability advertisement.  That
   enables node N1 to know which segment nodes are capable of responding
   to the ICMPv6 echo request.  Node N1 processes the echo responses and
   presents data to the user, accordingly.



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   Please note that segment-by-segment ping can be used to address proof
   of transit use-case.

4.1.3.  Error Reporting

   Any IPv6 node can use ICMPv6 control messages to report packet
   processing errors to the host that originated the datagram packet.
   To name a few such scenarios:

   o  If the router receives an undeliverable IP datagram, or

   o  If the router receives a packet with a Hop Limit of zero, or

   o  If the router receives a packet such that if the router decrements
      the packet's Hop Limit it becomes zero, or

   o  If the router receives a packet with problem with a field in the
      IPv6 header or the extension headers such that it cannot complete
      processing the packet, or

   o  If the router cannot forward a packet because the packet is larger
      than the MTU of the outgoing link.

   In the scenarios listed above, the ICMPv6 response also contains the
   IP header, IP extension headers and leading payload octets of the
   "original datagram" to which the ICMPv6 message is a response.
   Specifically, the "Destination Unreachable Message", "Time Exceeded
   Message", "Packet Too Big Message" and "Parameter Problem Message"
   ICMPV6 messages can contain as much of the invoking packet as
   possible without the ICMPv6 packet exceeding the minimum IPv6 MTU
   [RFC4443], [RFC4884].  In an SRv6 network, the copy of the invoking
   packet contains the SR header.  The packet originator can use this
   information for diagnostic purposes.  For example, traceroute can use
   this information as detailed in the following subsection.

4.2.  Traceroute

   There is no hardware or software change required for traceroute
   operation at the classic IPv6 nodes in an SRv6 network.  That
   includes the classic IPv6 node with ingress, egress or transit roles.
   Furthermore, no protocol changes are required to the standard
   traceroute operations.  In other words, existing traceroute
   mechanisms work seamlessly in the SRv6 networks.

   The following subsections outline some use cases of the traceroute in
   the SRv6 networks.





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4.2.1.  Classic Traceroute

   The existing mechanism to traceroute a remote IP prefix, along the
   shortest path, continues to work without any modification.  The
   initiator may be an SRv6 node or a classic IPv6 node.  Similarly, the
   egress or transit may be an SRv6 node or a classic IPv6 node.

   If an SRv6 capable ingress node wants to traceroute to IPv6 prefix
   via an arbitrary segment list <S1, S2, S3>, it needs to initiate
   traceroute probe with an SR header containing the SID list <S1, S2,
   S3>.  That is illustrated using the topology in Figure 1.  Assume all
   the links have IGP metric 10 except both links between node2 and
   node3, which have IGP metric set to 100.  User issues a traceroute
   from node N1 to a loopback of node 5, via segment list <B:2:C31,
   B:4:C52>.  Figure 3 contains sample output for the traceroute
   request.


        > traceroute A:5:: via segment-list B:2:C31, B:4:C52

        Tracing the route to B5::
         1  2001:DB8:1:2:21:: 0.512 msec 0.425 msec 0.374 msec
            SRH: (A:5::, B:4:C52, B:2:C31, SL=2)
         2  2001:DB8:2:3:31:: 0.721 msec 0.810 msec 0.795 msec
            SRH: (A:5::, B:4:C52, B:2:C31, SL=1)
         3  2001:DB8:3:4::41:: 0.921 msec 0.816 msec 0.759 msec
            SRH: (A:5::, B:4:C52, B:2:C31, SL=1)
         4  2001:DB8:4:5::52:: 0.879 msec 0.916 msec 1.024 msec

        Figure 3 A sample traceroute output at an SRv6 capable node


   Please note that information for hop2 is returned by N3, which is a
   classic IPv6 node.  Nonetheless, the ingress node is able to display
   SR header contents as the packet travels through the IPv6 classic
   node.  This is because the "Time Exceeded Message" ICMPv6 message can
   contain as much of the invoking packet as possible without the ICMPv6
   packet exceeding the minimum IPv6 MTU [RFC4443].  The SR header is
   also included in these ICMPv6 messages initiated by the classic IPv6
   transit nodes that are not running SRv6 software.  Specifically, a
   node generating ICMPv6 message containing a copy of the invoking
   packet does not need to understand the extension header(s) in the
   invoking packet.

   The segment list information returned for hop1 is returned by N2,
   which is an SRv6 capable node.  Just like for hop2, the ingress node
   is able to display SR header contents for hop1.




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   There is no difference in processing of the traceroute probe at an
   IPv6 classic node and an SRv6 capable node.  Similarly, both IPv6
   classic and SRv6 capable nodes may use the address of the interface
   on which probe was received as the source address in the ICMPv6
   response.  ICMP extensions defined in [RFC5837] can be used to also
   display information about the IP interface through which the datagram
   would have been forwarded had it been forwardable, and the IP next
   hop to which the datagram would have been forwarded, the IP interface
   upon which a datagram arrived, the sub-IP component of an IP
   interface upon which a datagram arrived.

   The information about the IP address of the incoming interface on
   which the traceroute probe was received by the reporting node is very
   useful.  This information can also be used to verify if SID functions
   B:2:C31 and B:4:C52 are executed correctly by N2 and N4,
   respectively.  Specifically, the information displayed for hop2
   contains the incoming interface address 2001:DB8:2:3:31:: at N3.
   This matches with the expected interface bound to END.X function
   B:2:C31 (link3).  Similarly, the information displayed for hop5
   contains the incoming interface address 2001:DB8:4:5::52:: at N5.
   This matches with the expected interface bound to the END.X function
   B:4:C52 (link10).

4.2.2.  Traceroute to a SID Function

   The classic traceroute described in the previous section cannot be
   used to traceroute a remote SID function, as explained using an
   example in the following.

   Consider the case where the user wants to traceroute the remote SID
   function B:4:C52, via B:2:C31, from node N1.  The trace route fails
   at N4.  This is because the node N4 trace route probe where next
   header is UDP or ICMPv6, instead of SRH (even though the hop limit is
   set to 1).  To solve this problem, the initiator needs to mark the
   ICMPv6 echo request as an OAM packet.

   The OAM packets are identified either by setting the O-flag in SRH or
   by inserting the END.OP or END.OTP SID at an appropriate place in the
   SRH.

   In an SRv6 network, the user can exercise two flavors of the
   traceroute: hop-by-hop traceroute or overlay traceroute.

   o  In hop-by-hop traceroute, user gets responses from all nodes
      including classic IPv6 transit nodes, SRv6 capable transit nodes
      as well as SRv6 capable segment endpoints.  E.g., consider the
      example where the user wants to traceroute to a remote SID
      function B:4:C52, via B:2:C31, from node N1.  The traceroute



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      output will also display information about node3, which is a
      transit (underlay) node.

   o  The overlay traceroute, on the other hand, does not trace the
      underlay nodes.  In other words, the overlay traceroute only
      displays the nodes that acts as SRv6 segments along the route.
      I.e., in the example where the user wants to traceroute to a
      remote SID function B:4:C52, via B:2:C31, from node N1, the
      overlay traceroute would only display the traceroute information
      from node N2 and node N4; it will not display information from
      node 3.

4.2.2.1.  Hop-by-hop traceroute using END.OP/ END.OTP

   In this section, hop-by-hop traceroute to a SID function is
   exemplified using UDP probes.  However, the procedure is equally
   applicable to other implementation of traceroute mechanism.
   Furthermore, the illustration uses the END.OTP SID but the procedures
   are equally applicable to the END.OP SID.

   Consider the same example where the user wants to traceroute to a
   remote SID function B:4:C52, via B:2:C31, from node N1.  To force a
   punt of the traceroute probe only at the node N4, node N1 inserts the
   END.OTP SID just before the target SID B:4:C52 in the SRH.  The
   traceroute probe is processed at the individual nodes along the path
   as follows:

   o  Node N1 initiates a traceroute probe packet with a monotonically
      increasing value of hop count and SRH as follows (A:1::,
      B:2:C31)(B:4:C52, B:4:OTP, B:2:C31; SL=2; NH=UDP)(Traceroute
      probe).

   o  When node N2 receives the packet with hop-count = 1, it processes
      the hop count expiry.  Specifically, the node N2 responses with
      the ICMPv6 message (Type: "Time Exceeded", Code: "Time to Live
      exceeded in Transit").

   o  When Node N2 receives the packet with hop-count > 1, it performs
      the standard SRH processing.  Specifically, it executes the END.X
      function (B:2:C31) on the traceroute probe.

   o  When node N3, which is a classic IPv6 node, receives the packet
      (A:1::, B:4:OTP)(B:4:C52, B:4:OTP, B:2:C31 ; HC=1, SL=1;
      NH=UDP)(Traceroute probe) with hop-count = 1, it processes the hop
      count expiry.  Specifically, the node N3 responses with the ICMPv6
      message (Type: "Time Exceeded", Code: "Time to Live exceeded in
      Transit").




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   o  When node N3, which is a classic IPv6 node, receives the packet
      with hop-count > 1, it performs the standard IPv6 processing.
      Specifically, it forwards the traceroute probe based on DA B:4:OTP
      in the IPv6 header.

   o  When node N4 receives the packet (A:1::, B:4:OTP)(B:4:C52,
      B:4:OTP, B:2:C31 ; SL=1; HC=1, NH=UDP)(Traceroute probe), it
      processes the END.OTP SID, as described in the pseudocode in
      Section 3.  The packet gets punted to the traceroute process for
      processing.  The traceroute process checks if the next SID in SRH
      (the target SID B:4:C52) is locally programmed.  If the target SID
      B:4:C52 is locally programmed, node N4 responses with the ICMPv6
      message (Type: Destination unreachable, Code: Port Unreachable).
      If the target SID B:4:C52 is not a local SID, node N4 silently
      drops the traceroute probe.

   Figure 4 displays a sample traceroute output for this example.

     > traceroute srv6 B:4:C52 via segment-list B:2:C31

     Tracing the route to SID function B:4:C52
      1  2001:DB8:1:2:21 0.512 msec 0.425 msec 0.374 msec
         SRH: (B:4:C52, B:4:OTP, B:2:C31; SL=2)
      2  2001:DB8:2:3:31 0.721 msec 0.810 msec 0.795 msec
         SRH: (B:4:C52, B:4:OTP, B:2:C31; SL=1)
      3  2001:DB8:3:4::41 0.921 msec 0.816 msec 0.759 msec
         SRH: (B:4:C52, B:4:OTP, B:2:C31; SL=1)

        Figure 4 A sample output for hop-by-hop traceroute to a SID function



4.2.2.2.  Tracing SRv6 Overlay

   The overlay traceroute does not trace the underlay nodes, i.e., only
   displays the nodes that acts as SRv6 segments along the path.  This
   is achieved by setting the SRH.Flags.O bit.

   In this section, overlay traceroute to a SID function is exemplified
   using UDP probes.  However, the procedure is equally applicable to
   other implementation of traceroute mechanism.

   Consider the same example where the user wants to traceroute to a
   remote SID function B:4:C52, via B:2:C31, from node N1.

   o  Node N1 initiates a traceroute probe with SRH as follows (A:1::,
      B:2:C31)(B:4:C52, B:2:C31; HC=64, SL=1, Flags.O=1;
      NH=UDP)(Traceroute Probe).  Please note that the hop-count is set



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      to 64 to skip the underlay nodes from tracing.  The O-flag in SRH
      is set to make the overlay nodes (nodes processing the SRH)
      respond.

   o  When node N2 receives the packet (A:1::, B:2:C31)(B:4:C52,
      B:2:C31; SL=1, HC=64, Flags.O=1; NH=UDP)(Traceroute Probe), it
      processes the O-flag in SRH, as described in the pseudocode in
      Section 3.  A time-stamped copy of the packet gets punted to the
      traceroute process for processing.  Node N2 continues to apply the
      B:2:C31 SID function on the original packet and forwards it,
      accordingly.  The traceroute process at node N2 checks if its
      local SID (B:2:C31) is locally programmed.  If the SID is not
      locally programmed, it silently drops the packet.  Otherwise, it
      performs the egress check by looking at the SL value in SRH.

   o  As SL is not equal to zero (i.e., it's not egress node), node N2
      responses with the ICMPv6 message (Type: "SRv6 OAM (TBA)", Code:
      "O-flag punt at Transit (TBA)").  Please note that, as mentioned
      in Section 3, if node N2 does not support the O-flag, it simply
      ignores it and processes the local SID, B:2:C31.

   o  When node N3 receives the packet (A:1::, B:4:C52)(B:4:C52,
      B:2:C31; SL=0, HC=63, Flags.O=1; NH=UDP)(Traceroute Probe),
      performs the standard IPv6 processing.  Specifically, it forwards
      the traceroute probe based on DA B:4:C52 in the IPv6 header.
      Please note that there is no hop-count expiration at the transit
      nodes.

   o  When node N4 receives the packet (A:1::, B:4:C52)(B:4:C52,
      B:2:C31; SL=0, HC=62, Flags.O=1; NH=UDP)(Traceroute Probe), it
      processes the O-flag in SRH, as described in the pseudocode in
      Section 3.  A time-stamped copy of the packet gets punted to the
      traceroute process for processing.  The traceroute process at node
      N4 checks if its local SID (B:2:C31) is locally programmed.  If
      the SID is not locally programmed, it silently drops the packet.
      Otherwise, it performs the egress check by looking at the SL value
      in SRH.  As SL is equal to zero (i.e., N4 is the egress node),
      node N4 tries to consume the UDP probe.  As UDP probe is set to
      access an invalid port, the node N4 responses with the ICMPv6
      message (Type: Destination unreachable, Code: Port Unreachable)

   Figure 5 displays a sample overlay traceroute output for this
   example.  Please note that the underlay node N3 does not appear in
   the output.







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     Tracing the route to SID function B:4:C52
      1  2001:DB8:1:2:21:: 0.512 msec 0.425 msec 0.374 msec
         SRH: (B:4:C52, B:4:OTP, B:2:C31; SL=2)
      2  2001:DB8:3:4::41:: 0.921 msec 0.816 msec 0.759 msec
         SRH: (B:4:C52, B:4:OTP, B:2:C31; SL=1)

        Figure 5 A sample output for overlay traceroute to a SID function


4.3.  Monitoring of SRv6 Paths

   In the recent past, network operators are interested in performing
   network OAM functions in a centralized manner.  Various data models
   like YANG are available to collect data from the network and manage
   it from a centralized entity.

   SR technology enables a centralized OAM entity to perform path
   monitoring from centralized OAM entity without control plane
   intervention on monitored nodes.  [RFC 8403] describes such a
   centralized OAM mechanism.  Specifically, the draft describes a
   procedure that can be used to perform path continuity check between
   any nodes within an SR domain from a centralized monitoring system,
   with minimal or no control plane intervene on the nodes.  However,
   the draft focuses on SR networks with MPLS data plane.  The same
   concept applies to the SRv6 networks.  This document describes how
   the concept can be used to perform path monitoring in an SRv6
   network.  This document describes how the concept can be used to
   perform path monitoring in an SRv6 network as follows.

   In the above reference topology, N100 is the centralized monitoring
   system implementing an END function B:100:1::. In order to verify a
   segment list <B:2:C31, B:4:C52>, N100 generates a probe packet with
   SRH set to (B:100:1::, B:4:C52, B:2:C31, SL=2).  The controller
   routes the probe packet towards the first segment, which is B:2:C31.
   N2 performs the standard SRH processing and forward it over link3
   with the DA of IPv6 packet set to B:4:C52.  N4 also performs the
   normal SRH processing and forward it over link10 with the DA of IPv6
   packet set to B:100:1::. This makes the probe loops back to the
   centralized monitoring system.

   In the reference topology in Figure 1, N100 uses an IGP protocol like
   OSPF or ISIS to get the topology view within the IGP domain.  N100
   can also use BGP-LS to get the complete view of an inter-domain
   topology.  In other words, the controller leverages the visibility of
   the topology to monitor the paths between the various endpoints
   without control plane intervention required at the monitored nodes.





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5.  Security Considerations

   This document does not define any new protocol extensions and relies
   on existing procedures defined for ICMP.  This document does not
   impose any additional security challenges to be considered beyond
   security considerations described in [RFC4884], [RFC4443], [RFC792],
   RFCs that updates these RFCs, [I-D.ietf-6man-segment-routing-header]
   and [I-D.ietf-spring-srv6-network-programming].

6.  IANA Considerations

6.1.  ICMPv6 type Numbers RegistrySEC

   This document defines one ICMPv6 Message, a type that has been
   allocated from the "ICMPv6 'type' Numbers" registry of [RFC4443].
   Specifically, it requests to add the following to the "ICMPv6 Type
   Numbers" registry:

      TBA (suggested value: 162) SRv6 OAM Message.

   The document also requests the creation of a new IANA registry to the
   "ICMPv6 'Code' Fields" against the "ICMPv6 Type Numbers TBA - SRv6
   OAM Message" with the following codes:

       Code  Name                                     Reference
       --------------------------------------------------------
        0     No Error                                This document
        1     SID is not locally implemented          This document
        2     O-flag punt at Transit                  This document


6.2.  SRv6 OAM Endpoint Types

   This I-D requests to IANA to allocate, within the "SRv6 Endpoint
   Behaviors Registry" sub-registry belonging to the top-level "Segment-
   routing with IPv6 dataplane (SRv6) Parameters" registry [I-D.ietf-
   spring- srv6-network-programming], the following allocations:

           +------------------+-------------------+-----------+
           | Value (Suggested | Endpoint Behavior | Reference |
           | Value)           |                   |           |
           +------------------+-------------------+-----------+
           | TBA (40)         |        End.OP     | [This.ID] |
           | TBA (41)         |        End.OTP    | [This.ID] |
           +------------------+-------------------+-----------+






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7.  Acknowledgements

   The authors would like to thank Gaurav Naik for his review comments.

8.  Contributors

   The following people have contributed to this document:

      Robert Raszuk
      Bloomberg LP
      Email: robert@raszuk.net


      John Leddy
      Individual
      Email: john@leddy.net


      Gaurav Dawra
      LinkedIn
      Email: gdawra.ietf@gmail.com


      Bart Peirens
      Proximus
      Email: bart.peirens@proximus.com


      Nagendra Kumar
      Cisco Systems, Inc.
      Email: naikumar@cisco.com


      Carlos Pignataro
      Cisco Systems, Inc.
      Email: cpignata@cisco.com


      Rakesh Gandhi
      Cisco Systems, Inc.
      Canada
      Email: rgandhi@cisco.com









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      Frank Brockners
      Cisco Systems, Inc.
      Germany
      Email: fbrockne@cisco.com


      Darren Dukes
      Cisco Systems, Inc.
      Email: ddukes@cisco.com


      Cheng Li
      Huawei
      Email: chengli13@huawei.com


      Faisal Iqbal
      Individual
      Email: faisal.ietf@gmail.com


9.  References

9.1.  Normative References

   [I-D.ietf-6man-segment-routing-header]
              Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
              Matsushima, S., and d. daniel.voyer@bell.ca, "IPv6 Segment
              Routing Header (SRH)", draft-ietf-6man-segment-routing-
              header-21 (work in progress), June 2019.

   [I-D.ietf-spring-srv6-network-programming]
              Filsfils, C., Camarillo, P., Leddy, J.,
              daniel.voyer@bell.ca, d., Matsushima, S., and Z. Li, "SRv6
              Network Programming", draft-ietf-spring-srv6-network-
              programming-01 (work in progress), July 2019.

   [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>.

9.2.  Informative References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/info/rfc792>.




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   [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>.

   [RFC4884]  Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
              "Extended ICMP to Support Multi-Part Messages", RFC 4884,
              DOI 10.17487/RFC4884, April 2007,
              <https://www.rfc-editor.org/info/rfc4884>.

   [RFC5837]  Atlas, A., Ed., Bonica, R., Ed., Pignataro, C., Ed., Shen,
              N., and JR. Rivers, "Extending ICMP for Interface and
              Next-Hop Identification", RFC 5837, DOI 10.17487/RFC5837,
              April 2010, <https://www.rfc-editor.org/info/rfc5837>.

   [RFC8403]  Geib, R., Ed., Filsfils, C., Pignataro, C., Ed., and N.
              Kumar, "A Scalable and Topology-Aware MPLS Data-Plane
              Monitoring System", RFC 8403, DOI 10.17487/RFC8403, July
              2018, <https://www.rfc-editor.org/info/rfc8403>.

Authors' Addresses

   Zafar Ali
   Cisco Systems

   Email: zali@cisco.com


   Clarence Filsfils
   Cisco Systems

   Email: cfilsfil@cisco.com


   Satoru Matsushima
   Softbank

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


   Daniel Voyer
   Bell Canada

   Email: daniel.voyer@bell.ca






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   Mach Chen
   Huawei

   Email: mach.chen@huawei.com















































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