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Versions: (draft-spring-srv6-oam) 00 01

SPRING Working Group                                              Z. Ali
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                                N. Kumar
Expires: January 1, 2019                                    C. Pignataro
                                                                F. Iqbal
                                                               R. Gandhi
                                                     Cisco Systems, Inc.
                                                                J. Leddy
                                                                 Comcast
                                                           S. Matsushima
                                                                SoftBank
                                                               R. Raszuk
                                                            Bloomberg LP
                                                                D. Voyer
                                                             Bell Canada
                                                                G. Dawra
                                                                LinkedIn
                                                              B. Peirens
                                                                Proximus
                                                                 M. Chen
                                                                  Huawei
                                                                 G. Naik
                                                       Drexel University
                                                            July 2, 2018

     Operations, Administration, and Maintenance (OAM) in Segment
              Routing Networks with IPv6 Data plane (SRv6)
                   draft-ali-spring-srv6-oam-01.txt

Abstract

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

Status of This Memo

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

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

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


Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the



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   document authors.  All rights reserved.

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

     Table of Contents

     1. Introduction......................................................3
     2. Conventions Used in This Document.................................3
           2.1. Abbreviations.............................................3
           2.2. Terminology and Reference Topology........................4
     3. OAM Building Blocks...............................................5
           3.1. O-flag in Segment Routing Header..........................5
              3.1.1. O-flag Processing....................................6
              3.1.2. Disabling Penultimate Segment Pop (PSP)..............7
           3.2. OAM Segments..............................................7


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              3.2.1. End.OP: OAM Endpoint with Punt.......................7
              3.2.2. End.OTP: OAM Endpoint with Timestamp and Punt........8
     4. OAM Mechanisms....................................................8
           4.1. Ping......................................................9
              4.1.1. Classic Ping.........................................9
              4.1.2. Pinging a SID Function..............................10
                 4.1.2.1. End-to-end ping using END.OP/ END.OTP..........11
                 4.1.2.2. Segment-by-segment ping using O-flag (Proof of
                 Transit)................................................11
           4.2. Error Reporting..........................................13
           4.3. Traceroute...............................................13
              4.3.1. Classic Traceroute..................................13
              4.3.2. Traceroute to a SID Function........................15
                 4.3.2.1. Hop-by-hop traceroute using END.OP/ END.OTP....16
                 4.3.2.2. Tracing SRv6 Overlay...........................17
           4.4. In-situ OAM..............................................19
           4.5. Monitoring of SRv6 Paths.................................19
     5. Security Considerations..........................................20
     6. IANA Considerations..............................................20
           6.1. Segment Routing Header Flags Register....................20
           6.2. ICMPv6 type Numbers Registry.............................20
     7. References.......................................................21
           7.1. Normative References.....................................21
           7.2. Informative References...................................22
     8. Acknowledgments..................................................22

     1. Introduction

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

     Additional OAM mechanisms will be added in a future revision of the
     document.

     2. Conventions Used in This Document

     2.1. Abbreviations


        ECMP: Equal Cost Multi-Path.

        SID: Segment ID.



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        SL: Segment Left.

        SR: Segment Routing.

        SRH: Segment Routing Header.

        SRv6: Segment Routing with IPv6 Data plane.

        TC: Traffic Class.

        UCMP: Unequal Cost Multi-Path.

     2.2. Terminology and Reference Topology

     This document uses the terminology defined in [I-D.draft-filsfils-
     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.

           +--------------------------| 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 100 is a controller.



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     Node Nk has a classic IPv6 loopback address Bk::/128

     Node Nk has Ak::/48 for its local SID space from which Local SIDs
     are explicitly allocated.

     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::.

     Ak::0 is explicitly allocated as the END function at Node k.

     Ak::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., A2::C31 represents END.X at N2 towards N3 via link3 (the 1st
     link between N2 and N3). Similarly, A4::C52 represents the END.X at
     N4 towards N5 via link10.

     <S1, S2, S3> represents a SID list where S1 is the first SID and S3
     is the last SID. (S3, S2, S1; SL) represents the same SID list but
     encoded in the SRH format where the rightmost SID (S1) in the SRH is
     the first SID and the leftmost SID (S3) in the SRH is the last SID.

     (SA, DA) (S3, S2, S1; SL) represents an IPv6 packet, SA is the IPv6
     Source Address, DA the IPv6 Destination Address, (S3, S2, S1; SL) is
     the SRH header that includes the SID list <S1, S2, S3>.

     3. OAM Building Blocks

     This section defines the various building blocks that can be used to
     implement OAM mechanisms in SRv6 networks. The following section
     describes some SRv6 OAM mechanisms that can be implemented using
     these building blocks.

     3.1. O-flag in Segment Routing Header

     [I-D. draft-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|         |
              +-+-+-+-+-+-+-+-+


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     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.

     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.bashandy-isis-srv6-
     extensions] and BGP-LS [I-D.dawra-idr-bgpls-srv6-ext].

     The SRH.Flags.O-flag implements the "punt a timestamped copy and
     forward" behavior. To avoid the head of the line processing of the
     packet, some implementation may implement the "forward and punt a
     timestamped copy" behavior, instead. In order to implement "punt a
     timestamped copy and forward" or "forward and punt a timestamped
     copy" behavior, the following instructions are inserted at the
     beginning or the end of the pseudo-code for all SID Functions,
     respectively.

     When N receives a packet whose IPv6 DA is S and S is a local SID, N executes the
     following the pseudo-code, either before or after the execution of the local SID
     S.
       1. IF SRH.Flags.O-flag is True and SRH.Flags.O-flag is locally
          supported for S THEN
            a. Timestamp a local copy of the packet. ;; Ref1
            b. Punt the time-stamped copy of the packet to CPU for processing in
               software (slow-path).      ;; Ref2
     Ref1: Timestamping is done in hardware, as soon as possible during
     the packet processing. As timestamping is done on a copy of the
     packet which is locally punted, timestamp value can be carried in
     the local packet (punt) header.
     Ref1: Hardware (microcode) just punts the packet. There is no
     requirement for the hardware to manipulate any TLV in SRH (or


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     elsewhere). Software (slow path) implements the required OAM
     mechanism. Timestamp is not carried in the packet forwarded to the
     next hop.
     3.1.2. Disabling Penultimate Segment Pop (PSP)

     Penultimate Segment Pop (PSP) needs to be disabled when SRH.Flags.O-
     flag is set. If a node supports SRH.Flags.O-flag, it adds the
     following check after executing the instruction 'update the IPv6 DA
     with SRH[SL]' during processing of a local SID as described in [I-
     D.draft-filsfils-spring-srv6-network-programming]:


      1.   IF updated SL = 0 & PSP is TRUE and SRH.Flags.O-bit is False
      2.      pop the top SRH              ;; Ref1

        Ref1: PSP behavior is disabled when SRH.Flags.O-flag is set.

     3.2. OAM Segments

     OAM Segment IDs (SIDs) is another components of the building blocks
     needed to implement SRv6 OAM mechanisms. This document defines a
     couple of OAM SIDs. Additional SIDs will be added in the later
     version of the document.

     3.2.1. 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.   Punt the packet to CPU for SW processing (slow-path)  ;; Ref1

     Ref1: Hardware (microcode) only punts the packet.  There is no
     requirement for the hardware to manipulate any TLV in the SRH (or
     elsewhere).  Software (slow path) implements the required OAM
     mechanisms.

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


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     3.2.2. 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

      2.   Punt the packet to CPU for SW processing (slow-path)  ;; Ref2

        Ref1: Timestamping is done in hardware, as soon as possible
     during the packet processing. As timestamping is done on a copy of
     the packet which is locally punted, timestamp value can be carried
     in the local packet (punt) header.

        Ref2: Hardware (microcode) only punts the packet.  There is no
     requirement for the hardware to manipulate any TLV in the SRH (or
     elsewhere).  Software (slow path) implements the required OAM
     mechanisms.


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

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



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     Throughout this document, unless otherwise specified, the acronym
     ICMPv6 refers to multi-part ICMPv6 messages [RFC4884]. 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 <A2::C31, A4::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 <A2::C31,
     A4::C52>.

     > ping B5:: via segment-list A2::C31, A4::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:

        - Node N1 initiates an ICMPv6 ping packet with SRH as follows
          (B1::, A2::C31)(B1::, A4::C52, A2::C31, SL=2, NH:
          ICMPv6)(ICMPv6 Echo Request).
        - Node N2, which is an SRv6 capable node, performs the standard
          SRH processing. Specifically, it executes the END.X function
          (A2::C31) on the echo request packet.
        - Node N3, which is a classic IPv6 node, performs the standard
          IPv6 processing. Specifically, it forwards the echo request
          based on DA A4::C52 in the IPv6 header.
        - Node N4, which is an SRv6 capable node, performs the standard
          SRH processing. Specifically, it observes the END.X function
          (A4::C52) with PSP (Penultimate Segment POP) on the echo
          request packet and removes the SRH and forwards the packet
          across link10 to N5.
        - The echo request packet at N5 arrives as an IPv6 packet without
          a 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 A4::C52, via A2::C31, from node N1. Node N1 constructs the
     ping packet (B1::0, A2::C31)( A4::C52, A2::C31, SL=1;
     NH=ICMPv6)(ICMPv6 Echo Request). When the node N4 receives the
     ICMPv6 echo request with DA set to A4::C52 and next header set to
     ICMPv6, it silently drops it (see [I-D.filsfils-spring-srv6-
     network-programming] for details). 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.

     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 A4::C52 , via A2::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 A4::C52 in the SRH.
          The ICMPv6 echo request is processed at the individual nodes
          along the path as follows:

        - Node N1 initiates an ICMPv6 ping packet with SRH as follows
          (B1::0, A2::C31)(A4::C52, A4::OTP, A2::C31; SL=2;
          NH=ICMPv6)(ICMPv6 Echo Request).
        - Node N2, which is an SRv6 capable node, performs the standard
          SRH processing. Specifically, it executes the END.X function
          (A2::C31) on the echo request packet.
        - Node N3 receives the packet as follows (B1::0,
          A4::OTP)(A4::C52, A4::OTP, A2::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 A4::OTP in the IPv6 header.
        - When node N4 receives the packet (B1::0, A4::OTP)(A4::C52,
          A4::OTP, A2::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 A4::C52) is locally programmed.
        - 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 A4::C52, via A2::C31, from node N1. However, in this ping,
     the node N1 wants to get a response from each segment node in the
     SRH. In other words, in the segment-by-segment ping case, the node
     N1 expects a response from node N2 and node N4 for their respective
     local SID function.



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     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:

        - Node N1 initiates an ICMPv6 ping packet with SRH as follows
          (B1::0, A2::C31)(A4::C52, A2::C31; SL=1, Flags.O=1;
          NH=ICMPv6)(ICMPv6 Echo Request).
        - When node N2 receives the packet (B1::0, A2::C31)(A4::C52,
          A2::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 A2::C31 SID function on the original packet and
          forwards it, accordingly. As SRH.Flags.O=1, Node N2 also
          disables the PSP flavour, i.e., does not remove the SRH. The
          ICMPv6 process at node N2 checks if its local SID (A2::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.
          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, A2::C31.
        - Node N3, which is a classic IPv6 node, performs the standard
          IPv6 processing. Specifically, it forwards the echo request
          based on DA A4::C52 in the IPv6 header.
        - When node N4 receives the packet (B1::0, A4::C52)(A4::C52,
          A2::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 (A2::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.

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




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     4.2. 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:

        - If the router receives an undeliverable IP datagram, or
        - If the router receives a packet with a Hop Limit of zero, or
        - If the router receives a packet such that if the router
          decrements the packet's Hop Limit it becomes zero, or
        - 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
        - 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.

     4.3. 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.

     4.3.1. 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.



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     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 <A2::C31,
     A4::C52>. Figure 3 contains sample output for the traceroute
     request.

     > traceroute B5:: via segment-list A2::C31, A4::C52

     Tracing the route to B5::

      1  2001:DB8:1:2:21:: 0.512 msec 0.425 msec 0.374 msec
         SRH: (B5::, A4::C52, A2::C31, SL=2)

      2  2001:DB8:2:3:31:: 0.721 msec 0.810 msec 0.795 msec
         SRH: (B5::, A4::C52, A2::C31, SL=1)

      3  2001:DB8:3:4::41:: 0.921 msec 0.816 msec 0.759 msec
         SRH: (B5::, A4::C52, A2::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.

     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 use the address of the interface on
     which probe was received as the source address in the ICMPv6


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     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 A2::C31 and A4::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
     A2::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
     A4::C52 (link10).

     4.3.2. 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 A4::C52, via A2::C31, from node N1. Node N1 constructs the
     traceroute packet (B1::0, A2::C31, HC=1)( A4::C52, A2::C31, SL=1;
     NH=UDP)(traceroute probe). Even though Hop Count of the packet is
     set to 1, when the node N4 receives the traceroute probe with DA set
     to A4::C52 and next header set to UDP, it silently drops it (see [I-
     D.draft-filsfils-spring-srv6-network-programming] for details). To
     solve this problem, the initiator needs to mark the traceroute probe
     as an OAM packet.

     The OAM packets are identified either by setting the O-flag in SRH
     or by inserting the 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.

        - 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 A4::C52 , via A2::C31, from node N1. The traceroute



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          output will also display information about node3, which is a
          transit (underlay) node.
        - 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 A4::C52 , via A2::C31, from node N1, the
          overlay traceroute would only display the traceroute
          information from node N2 and node N2 and will not display
          information from node 3.

     4.3.2.1. 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 A4::C52 , via A2::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 A4::C52 in the SRH. The
     traceroute probe is processed at the individual nodes along the path
     as follows.

        - Node N1 initiates a traceroute probe packet with a
          monotonically increasing value of hop count and SRH as follows
          (B1::0, A2::C31)(A4::C52, A4::OTP, A2::C31; SL=2;
          NH=UDP)(Traceroute probe).
        - 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").
        - When Node N2 receives the packet with hop-count > 1, it
          performs the standard SRH processing. Specifically, it executes
          the END.X function (A2::C31) on the traceroute probe.
        - When node N3, which is a classic IPv6 node, receives the packet
          (B1::0, A4::OTP)(A4::C52, A4::OTP, A2::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").
        - 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
          A4::OTP in the IPv6 header.


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        - When node N4 receives the packet (B1::0, A4::OTP)(A4::C52,
          A4::OTP, A2::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 A4::C52) is locally programmed. If the
          target SID A4::C52 is locally programmed, node N4 responses
          with the ICMPv6 message (Type: Destination unreachable, Code:
          Port Unreachable). If the target SID A4::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 A4::C52 via segment-list A2::C31

     Tracing the route to SID function A4::C52

      1  2001:DB8:1:2:21 0.512 msec 0.425 msec 0.374 msec
         SRH: (A4::C52, A4::OTP, A2::C31; SL=2)

      2  2001:DB8:2:3:31 0.721 msec 0.810 msec 0.795 msec
         SRH: (A4::C52, A4::OTP, A2::C31; SL=1)

      3  2001:DB8:3:4::41 0.921 msec 0.816 msec 0.759 msec
         SRH: (A4::C52, A4::OTP, A2::C31; SL=1)

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



     4.3.2.2. 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 A4::C52 , via A2::C31, from node N1.

        - Node N1 initiates a traceroute probe with SRH as follows
          (B1::0, A2::C31)(A4::C52, A2::C31; HC=64, SL=1, Flags.O=1;
          NH=UDP)(Traceroute Probe). Please note that the hop-count is


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          set 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.
        - When node N2 receives the packet (B1::0, A2::C31)(A4::C52,
          A2::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 A2::C31 SID function on the original packet and forwards
          it, accordingly. As SRH.Flags.O=1, Node N2 also disables the
          PSP flavor, i.e., does not remove the SRH. The traceroute
          process at node N2 checks if its local SID (A2::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 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, A2::C31.
        - When node N3 receives the packet (B1::0, A4::C52)(A4::C52,
          A2::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 A4::C52 in the IPv6
          header. Please note that there is no hop-count expiration at
          the transit nodes.
        - When node N4 receives the packet (B1::0, A4::C52)(A4::C52,
          A2::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 (A2::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.

     > traceroute srv6 A4::C52 via segment-list A2::C31

     Tracing the route to SID function A4::C52


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      1  2001:DB8:1:2:21:: 0.512 msec 0.425 msec 0.374 msec
         SRH: (A4::C52, A4::OTP, A2::C31; SL=2)

      2  2001:DB8:3:4::41:: 0.921 msec 0.816 msec 0.759 msec
         SRH: (A4::C52, A4::OTP, A2::C31; SL=1)

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



     4.4. In-situ OAM

     [I-D.brockners-inband-oam-requirements] describes motivation
     and requirements for In-situ OAM (iOAM). iOAM records operational
     and telemetry information in the data packet while the packet
     traverses the network of telemetry domain. iOAM complements out-of-
     band probe based OAM mechanisms such ICMP ping and traceroute by
     directly encoding tracing and the other kind of telemetry
     information to the regular data traffic.

     [I-D.brockners-inband-oam-transport] describes transport mechanisms
     for iOAM data including IPv6 and Segment Routing traffic. To address
     iOAM requirements in an SRv6 network, the draft describes iOAM TLV
     in SRH and its usage.


     4.5. 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. [I.D-draft-ietf-spring-oam-usecase]
     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.


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     In the above reference topology, N100 is the centralized monitoring
     system implementing an END function A100::. In order to verify a
     segment list <A2::C31, A4::C52>, N100 generates a probe packet with
     SRH set to (A100::, A4::C52, A2::C31, SL=2). The controller routes
     the probe packet towards the first segment, which is A2::C31. N2
     performs the standard SRH processing and forward it over link3 with
     the DA of IPv6 packet set to A4::C52. N4 also performs the normal
     SRH processing and forward it over link10 with the DA of IPv6 packet
     set to A100::. 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.

     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]
     and RFCs that updates these RFCs.

     6. IANA Considerations

     6.1. 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".  SRH.Flags is an 8 bits field; the
     following bit is defined in this document:

        Suggested Bit      Description               Reference

        -----------------------------------------------------

           2               OAM                       This document

     6.2.   ICMPv6 type Numbers Registry

     This document defines one ICMPv6 Message, a type that has been
     allocated from the "ICMPv6 'type' Numbers" registry of [RFC4443].



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     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


     6.3. SRv6 OAM Endpoint Types

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



                +-------------+-----+-------------------+-----------+
                | Value (Suggested | Endpoint function | Reference |
                | Value)           |                   |           |
                +------------------+-------------------+-----------+
                | TBA (25)         |        End.OTP    | [This.ID] |
                | TBA (30)         |        End.OTP    | [This.ID] |
                +------------------+-------------------+-----------+


     7. References

     7.1. Normative References

   [RFC792]   J. Postel, "Internet Control Message Protocol", RFC 792,
              September 1981.

   [RFC4443]  A. Conta, S. Deering, M. Gupta, Ed., "Internet Control



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              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4884]  R. Bonica, D. Gan, D. Tappan, C. Pignataro, "Extended ICMP
              to Support Multi-Part Messages", RFC 4884, April 2007.

   [RFC5837]  A. Atlas, Ed., R. Bonica, Ed., C. Pignataro, Ed., N. Shen,
              JR. Rivers, "Extending ICMP for Interface and Next-Hop
              Identification", RFC 5837, April 2010.

   [I-D.filsfils-spring-srv6-network-programming]  C. Filsfils, et al.,
              "SRv6 Network Programming",
              draft-filsfils-spring-srv6-network-programming, work in
              progress.

   [I-D.6man-segment-routing-header]  Previdi, S., Filsfils, et al,
              "IPv6 Segment Routing Header (SRH)",
              draft-ietf-6man-segment-routing-header, work in progress.

     7.2. Informative References

   [I-D.bashandy-isis-srv6-extensions] IS-IS Extensions to Support Routing
              over IPv6 Dataplane. L. Ginsberg, P. Psenak, C. Filsfils,
              A. Bashandy, B. Decraene, Z. Hu,
              draft-bashandy-isis-srv6-extensions, work in progress.

   [I-D.dawra-idr-bgpls-srv6-ext] G. Dawra, C. Filsfils, K. Talaulikar,
              et al., BGP Link State extensions for IPv6 Segment Routing
              (SRv6), draft-dawra-idr-bgpls-srv6-ext, work in progress.

   [I-D.ietf-spring-oam-usecase]  A Scalable and Topology-Aware MPLS
              Dataplane Monitoring System. R. Geib, C. Filsfils, C.
              Pignataro, N. Kumar, draft-ietf-spring-oam-usecase, work
              in progress.

   [I-D.brockners-inband-oam-data]  F. Brockners, et al., "Data Formats
              for In-situ OAM", draft-brockners-inband-oam-data, work in
              progress.

   [I-D.brockners-inband-oam-transport]  F.Brockners, at al.,
              "Encapsulations for In-situ OAM Data",
              draft-brockners-inband-oam-transport, work in progress.

   [I-D.brockners-inband-oam-requirements]  F.Brockners, et al.,
              "Requirements for In-situ OAM",
              draft-brockners-inband-oam-requirements, work in progress.

   [I-D.spring-segment-routing-policy]  Filsfils, C., et al., "Segment
              Routing Policy for Traffic Engineering",
              draft-filsfils-spring-segment-routing-policy, work in
              progress.

     8. Acknowledgments

     To be added.








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Authors' Addresses

   Clarence Filsfils
   Cisco Systems, Inc.
   Email: cfilsfil@cisco.com


   Zafar Ali
   Cisco Systems, Inc.
   Email: zali@cisco.com


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


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


   Faisal Iqbal
   Cisco Systems, Inc.
   Email: faiqbal@cisco.com


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


   John Leddy
   Comcast
   Email: John_Leddy@cable.comcast.com


   Robert Raszuk
   Bloomberg LP
   731 Lexington Ave
   New York City, NY10022, USA
   Email: robert@raszuk.net


   Satoru Matsushima
   SoftBank
   Japan
   Email: satoru.matsushima@g.softbank.co.jp

   Daniel Voyer
   Bell Canada
   Email: daniel.voyer@bell.ca



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

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

   Mach Chen
   Huawei
   Email: mach.chen@huawei.com

   Gaurav Naik
   Drexel University
   United States of America
   Email: gn@drexel.edu






























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