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Versions: (draft-spring-srv6-oam) 00 01 02

SPRING Working Group                                              Z. Ali
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                                N. Kumar
Expires: August 30, 2018                                    C. Pignataro
                                                                F. Iqbal
                                                               R. Gandhi
                                                     Cisco Systems, Inc.
                                                                J. Leddy
                                                                 Comcast
                                                           S. Matsushima
                                                                SoftBank
                                                               R. Raszuk
                                                            Bloomberg LP
                                                              B. Peirens
                                                                Proximus
                                                                 G. Naik
                                                       Drexel University
                                                       February 26, 2018

     Operations, Administration, and Maintenance (OAM) in Segment
              Routing Networks with IPv6 Data plane (SRv6)
                   draft-ali-spring-srv6-oam-00.txt

Abstract

   This document describes mechanisms for Operations, Administration,
   and Maintenance (OAM) in Segment Routing with IPv6 data plane (SRv6)
   network.

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 . . . . . . . . . . . .  3
   3.  OAM Mechanisms . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Ping . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
       3.1.1.  Classic Ping . . . . . . . . . . . . . . . . . . . . .  5
       3.1.2.  Pinging SID Function . . . . . . . . . . . . . . . . .  6
         3.1.2.1.  End-to-end Ping Using END.OTP  . . . . . . . . . .  7
         3.1.2.2.  Segment-by-segment Ping Using O-bit (Proof of
                   Transit) . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Error Reporting  . . . . . . . . . . . . . . . . . . . . .  9
     3.3.  Traceroute . . . . . . . . . . . . . . . . . . . . . . . . 10
       3.3.1.  Classic Traceroute . . . . . . . . . . . . . . . . . . 10
       3.3.2.  Traceroute to a SID Function . . . . . . . . . . . . . 11
         3.3.2.1.  Hop-by-hop Traceroute Using END.OTP  . . . . . . . 12
         3.3.2.2.  Tracing SRv6 Overlay . . . . . . . . . . . . . . . 14
   4.  In-situ OAM Applicability  . . . . . . . . . . . . . . . . . . 15
   5.  Seamless BFD Applicability . . . . . . . . . . . . . . . . . . 16
   6.  Monitoring of SRv6 Paths . . . . . . . . . . . . . . . . . . . 16
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 18
   10.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20








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1.  Introduction

   This document describes mechanisms for Operations, Administrations,
   and Maintenance (OAM) in Segment Routing using IPv6 data plane (SRv6)
   networks.

   Additional 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.

   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

   In this document, the simple topology shown in Figure 1 is used for
   illustration.

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



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                    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.

   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>.

   SR policy is defined in Section 3 of
   [I-D.spring-segment-routing-policy].


3.  OAM Mechanisms

   This section describes how ping and traceroute mechanisms can be used
   in an SRv6 network.  Additional OAM mechanisms will be added in a
   future revision of the document.




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3.1.  Ping

   [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 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.

   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].

   There is no hardware or software change required for ping operation
   at the classic IPv6 nodes in an SRv6 network.  This 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 SRv6 networks.

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

3.1.1.  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 node N2 and node N3,
   which have IGP metric set to 100.  User issues a ping from node N1 to
   a loopback of node N5, via 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:



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   !!!!!
   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

   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
      (B1::,A2::C31)(B1::, A4::C52, A2::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
      (A2::C31) on the echo request packet.

   o  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.

   o  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 Popping) 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 a
      SRH.  Node N5, which is a classic IPv6 node, performs the standard
      IPv6/ICMPv6 processing on the echo request and responds,
      accordingly.

3.1.2.  Pinging 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 (as per
   [I-D.filsfils-spring-srv6-network-programming]).  To solve this



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   problem, the initiator needs to mark the ICMPv6 echo request as an
   OAM packet.

   The OAM packets are identified either by setting the O-bit in SRH
   [I-D.6man-segment-routing-header] or by inserting the SID Function
   END.OTP at an appropriate place in the SRH
   [I-D.filsfils-spring-srv6-network-programming].

   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.

3.1.2.1.  End-to-end Ping Using END.OTP

   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 SID function
   END.OTP 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:

   o  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).

   o  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.

   o  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.

   o  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
      SID Function END.OTP, as described in the pseudocode in
      [I-D.filsfils-spring-srv6-network-programming].  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.

   o  If the target SID is not locally programmed, N4 responses with the
      ICMPv6 message (Type: "SRv6 OAM (TBA1 by IANA)", Code: "SID not
      locally implemented (TBA2 by IANA)"); otherwise a success is
      returned.




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3.1.2.2.  Segment-by-segment Ping Using O-bit (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.

   To force a punt of the ICMPv6 echo request at node N2 and node N4,
   node N1 sets the O-bit in SRH [I-D.6man-segment-routing-header].  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
      (B1::0, A2::C31)(A4::C52, A2::C31; SL=1, Flags.O=1;
      NH=ICMPv6)(ICMPv6 Echo Request).

   o   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-bit in SRH, as described in the pseudo code in
      [I-D.filsfils-spring-srv6-network-programming].  A time-stamped
      copy of the packet is punted to the ICMPv6 process in control
      plane for processing.  Node N2 continues to apply the A2::C31 SID
      function on the original packet and forwards it, accordingly.  Due
      to SRH.Flags.O=1, Node N2 also disables the PSP behaviour, 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.

   o  If the target SID is not locally programmed, N4 responses with the
      ICMPv6 message (Type: "SRv6 OAM (TBA1 by IANA)", Code: "SID not
      locally implemented (TBA2 by IANA)"); otherwise a success is
      returned.  Note that, as mentioned in
      [I-D.filsfils-spring-srv6-network-programming], if node N2 does
      not support the O-bit, it simply ignores it and process the local
      SID, A2::C31.

   o  Node N3, which is a classic IPv6 node, performs standard IPv6
      processing.  Specifically, it forwards the echo request based on
      DA A4::C52 in the IPv6 header.

   o  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-bit in SRH, as described in the pseudo code in
      [I-D.filsfils-spring-srv6-network-programming].  A time-stamped
      copy of the packet is punted to the ICMPv6 process in control
      plane for processing.  The ICMPv6 process at node N4 checks if its



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      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 (TBA1 by IANA)", Code: "SID not
      locally implemented (TBA2 by IANA)"); otherwise a success is
      returned.

   Support for O-bit is part of node capability advertisement.  This
   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 the data to the user, accordingly.

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

3.2.  Error Reporting

   Any IPv6 node can use ICMPv6 control messages to report packet
   processing errors to the source 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.




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3.3.  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.

3.3.1.  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 node 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>.  This is illustrated using the topology in Figure 1.  Assume all
   the links have IGP metric 10 except both links between node N2 and
   node N3, which have IGP metric set to 100.  User issues a traceroute
   from node N1 to a loopback of node N5, 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



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   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
   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).

3.3.2.  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 as follows.

   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 (as per



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   [I-D.filsfils-spring-srv6-network-programming]).  To solve this
   problem, the initiator node needs to mark the traceroute probe as an
   OAM packet.

   The OAM packets are identified either by setting the O-bit in SRH
   [I-D.6man-segment-routing-header] or by inserting the SID Function
   END.OTP at an appropriate place in the SRH
   [I-D.filsfils-spring-srv6-network-programming].

   In SRv6 networks, 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 A4::C52, via A2::C31, from node N1.  The traceroute
      output will also display information about node N3, 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 A4::C52, via A2::C31, from node N1, the
      overlay traceroute would only display the traceroute information
      from node N2 and node N4 and will not display information from
      node N3.

3.3.2.1.  Hop-by-hop Traceroute Using 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.

   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
   SID Function END.OTP just before the target SID A4::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
      (B1::0,A2::C31)(A4::C52, A4::OTP, A2::C31; SL=2;
      NH=UDP)(Traceroute probe).

   o  When node N2 receives the packet with hop-count = 1, it processes



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      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 (A2::C31) on the traceroute probe.

   o  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").

   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 A4::OTP
      in the IPv6 header.

   o  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 SID Function END.OTP, as described in the pseudocode
      in  [I-D.filsfils-spring-srv6-network-programming].  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



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3.3.2.2.  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.

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

   o  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-bit in SRH, as described in the pseudocode in
      [I-D.filsfils-spring-srv6-network-programming].  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.

   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 (TBA1 by
      IANA)", Code: "O-bit punt at Transit (TBA3 by IANA)").  Note that,
      as mentioned in [I-D.filsfils-spring-srv6-network-programming], if
      node N2 does not support the O-bit, it simply ignores it and
      processes the local SID, A2::C31.

   o  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.

   o  When node N4 receives the packet (B1::0, A4::C52)(A4::C52,A2::C31;



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      SL=0, HC=62, Flags.O=1; NH=UDP)(Traceroute Probe), it processes
      the O-bit in SRH, as described in the pseudocode in
      [I-D.filsfils-spring-srv6-network-programming].  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

    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.  In-situ OAM Applicability

   [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.
   Furthermore, [I-D.brockners-inband-oam-data] defines information
   encoding for iOAM data.

   One of the application of iOAM is to perform inband traceroute.  In
   SRv6 network, iOAM traceroute feature can be used to trace the order
   set of segment ID executed by SRv6 nodes for packet forwarding along



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   the packet path.  This is achieved by recording the node details that
   the packet traversed in the packet header itself.

   Another important application of iOAM is to perform delay measurement
   in anycast server scenarios.  Anycast server deployment is commonly
   seen for redundancy and load balancing purpose.  In SRv6 network,
   iOAM can be used to collect the timestamp from different anycats
   servers to measure the delay induced by each server within the
   anycast cluster that helps to provide SLA constrained services.

   One of the other applications of iOAM is to provide the Proof of
   Transit (POT).  Among other features of iOAM, SRv6 networks can use
   the POT feature of iOAM to verify that all the function SIDs in SRH
   have been executed before the packet is delivered to the destination.
    It can also ensure that the order of execution of the SID function
   has been consistent with the SRH contents.

   More details on various applications of iOAM in SRv6 networks will be
   included in future versions of this document.

5.  Seamless BFD Applicability

   [RFC7880] defines Seamless BFD (S-BFD) architecture that simplifies
   BFD mechanism and enables it to perform path monitoring in a
   controlled and scalable manner.  [RFC7881] describes the procedure to
   perform continuity check using S-BFD in different environments
   including IPv6 networks.  Section 5.1 of [RFC7881] explains the
   SBFDInitiator specification and procedure to initiate S-BFD control
   packet in IP and MPLS network.  The specification described for
   IP-routed S-BFD control packet is also directly applicable to the
   SRv6 network.

   S-BFD has a fast bootstrapping capability.  Furthermore, in S-BFD,
   only the ingress is required to keep BFD states; the egress and
   transit node does not have any knowledge of the BFD session.  These
   attributes of S-BFD make it an excellent candidate for rapid failure
   detection in the SRv6 network.  More details on various S-BFD usage
   on the SRv6 network will be included in a future version.

6.  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.

   The SR technology enables a centralized OAM entity to perform path
   monitoring without control plane intervention on monitored nodes.



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   [I-D.ietf-spring-oam-usecase] describes such centralized OAM
   mechanism.  Specifically, it 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 intervention on the nodes.  However, the document
   focuses on SR networks with MPLS data plane.  The same concept is
   also applicable to the SRv6 networks.  This document describes how
   the concept can be used to perform path monitoring in an SRv6 network
   as follows.

   In the reference topology in Figure 1, N100 is the controller
   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 forwards it over link3 with the DA of
   IPv6 packet set to A4::C52.  N4 also performs the normal SRH
   processing and forwards it over link10 with the DA of IPv6 packet set
   to A100::.  This makes the probe packet loop back to the controller.

   In our 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.

7.  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.

8.  IANA Considerations

   This document requests IANA to allocate a new Type for ICMPv6 message
   for "SRv6 OAM".

9.  References

9.1.  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.

   [RFC7880]  C.Pignataro, D.Ward, N.Akiya, M.Bhatia, S.Pallagatti,
              "Seamless Bidirectional Forwarding Detection (S-BFD)", RFC
              7880, July 2016.

   [RFC7881]  C.Pignataro, D.Ward, N.Akiya, "Seamless Bidirectional
              Forwarding Detection (S-BFD) for IPv4, IPv6, and MPLS",
              RFC 7881 July 2016.

   [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.

9.2.  Informative References

   [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",



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              draft-filsfils-spring-segment-routing-policy, work in
              progress.

















































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10.  Acknowledgments

   To be added.


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



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


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


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




































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