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Versions: 00 draft-ali-spring-srv6-oam

SPRING Working Group                                             Z. Ali
Internet Draft                                              C. Filsfils
Intended status: Standards Track                               N. Kumar
Expires: June 23, 2018                                     C. Pignataro
                                                               F. Iqbal
                                                    Cisco Systems, Inc.
                                                                J. Leddy
                                                                 Comcast
                                                           S. Matsushima
                                                                SoftBank
                                                               R. Raszuk
                                                            Bloomberg LP
                                                              B. Peirens
                                                                Proximus
                                                                 G. Naik
                                                       Drexel University
                                                      December 23, 2017



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


Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on June 23, 2018.




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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (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.

Abstract

   This document outlines various use-cases for Operations,
   Administration, and Maintenance (OAM) in Segment Routing with the
   IPv6 data plane (SRv6) network. It also specifies solutions to
   address the SRv6 OAM requirements.

Table of Contents

   1. Introduction...................................................3
      1.1. Terminology and Reference Topology........................3
   2. Use-cases......................................................4
      2.1. Connectivity Verification.................................5
      2.2. Monitoring a Specific Flow................................5
      2.3. Monitoring all ECMP/ UCMP Paths...........................5
      2.4. Traceroute................................................6
      2.5. Proof of Transit..........................................6
      2.6. Anycast Server selection..................................7
      2.7. Detecting Path Divergence.................................7
      2.8. Fault Isolation...........................................7
      2.9. Connectivity Verification from arbitrary node.............7
   3. OAM Mechanisms.................................................8
      3.1. Ping......................................................8
         3.1.1. Classic Ping.........................................8
         3.1.2. Pinging a SID Function...............................9
            3.1.2.1. End-to-end ping using EDN.OTP..................10
            3.1.2.2. Segment-by-segment ping using O-bit (Proof of
            Transit)................................................11
      3.2. Error Reporting..........................................12
      3.3. Traceroute...............................................12
         3.3.1. Classic Traceroute..................................13
         3.3.2. Traceroute to a SID Function........................14


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            3.3.2.1. Hop-by-hop traceroute using END.OTP............15
            3.3.2.2. Tracing SRv6 Overlay...........................16
      3.4. In-situ OAM..............................................18
      3.5. Seamless BFD Applicability...............................19
      3.6. Connectivity Verification from arbitrary SR node.........19
   4. Security Considerations.......................................20
   5. IANA Considerations...........................................20
   6. References....................................................20
      6.1. Normative References.....................................20
      6.2. Informative References...................................21
   7. Acknowledgments...............................................21

1. Introduction

   This document outlines various SRv6 OAM use-cases. It also describes
   OAM mechanisms that can be used to address SRv6 OAM requirements.

   Additional OAM use-cases and mechanisms will be added in a future
   revision of the document.

1.1. 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
                                   ------


                                 Reference Topology

   In the reference topology:

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


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   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 99:X:Y::Xn. e.g., the IPv6 address of link6 (the
   2nd link) between N3 and N4 at N3 in Figure 1 is 99:3:4:32.
   Similarly, the IPv6 address of link5 (the 1st link between N3 and
   N4) at node 3 is 99:3:4::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.

   SRH is the abbreviation for the Segment Routing Header.

   SL is the abbreviation for the Segment Left.

   SID is the abbreviation for the Segment ID.

   <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.

   ECMP is the abbreviation for the Equal Cost Multi-Path.

   UCMP is the abbreviation for the Unequal Cost Multi-Path.

2. Use-cases

   This section outlines some for the basic OAM use-cases in an SRv6
   network. Additional use-cases will be added in a future revision of
   the document.






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2.1. Connectivity Verification

   One of the basic OAM use-cases for any network is the capability to
   perform path monitoring between different end points over any
   possible shortest path without any path preference. Such essential
   path monitoring helps to monitor the path availability and the
   liveliness of the remote end point.

   The shortest path monitoring can be done continuously or can be
   triggered on demand basis using an external event like a script or a
   CLI trigger. It may be required to perform the connectivity
   verification in the order of milliseconds, or at a slower pace.

   In the reference topology in Figure 1, N1 can send OAM probe packet
   destined to loopback address of N5 (B5::) to monitor the path
   liveliness between N1 and N5. N1 optionally may include any relevant
   segment list in SRH. N1 is not concerned about which route is taken
   by the probe between N1 and N5 as long as N1 receives the response
   back from N5. All transit nodes treat the probe packet as like other
   data packet and forward it based on the Destination Address (DA). N5
   looks into the payload of probe packet and respond back to the
   source address of the probe packet (N1).

2.2. Monitoring a Specific Flow

   The network OAM needs to have the ability to monitor a particular
   path from the available ECMP paths. For example, in the reference
   topology in figure 1, there are many ECMP paths between N1 and N5.
   However, the service provider may like to monitor a flow that
   follows [N1]-<link1>-[N2]-<link7>-[N6]-<link8>-[N4]-<link9>-[N5].

   The flow monitoring can be done continuously or can be triggered on
   demand basis. It may be required to perform the connectivity
   verification in the order of milliseconds, or at a slower pace.

2.3. Monitoring all ECMP/ UCMP Paths

   In any network, it is common to see multiple ECMP paths between end
   points that are used for load balancing or redundancy. While
   monitoring, the shortest path helps to monitor the path and
   liveliness of remote node, it may not be sufficient to detect any
   failure in one of the ECMP paths. In our reference topology in
   figure 1, N6 has 2 ECMP paths to reach N5 as below:

   N6--<link8>--N4--<link9>--N5

   N6--<link8>--N4--<link10>--N5


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   If the probe packet from N6 to N5 uses link10, it may not detect any
   failure on link9. It is critical and beneficial to discover and
   monitor all ECMP/ UCMP paths. Monitoring of all ECMP/ UCMP paths can
   be done by probing the candidate paths from end-to-end or by each
   node by monitoring its data plane.

2.4. Traceroute

   It is essential to trace the path between different end points for
   troubleshooting and fault localization purpose. In the SRv6 network,
   depending on the forwarding instruction encoded in SRH, a packet may
   traverse over zero or more SRv6 transit nodes which in turn are
   connected through transit IPv6 nodes. For example, the best effort
   traffic may traverse the shortest path between Ingress and egress
   nodes while an SLA constrained traffic may follow a specific path
   that involves one or more transit SRv6 nodes.

   In either of these cases, traceroute functionality allows an
   operator to discover the set of SRv6 and/or IPv6 nodes along the
   path between different end points. Multipath being inevitable in any
   network, it is also essential to identify the exact path (among the
   available equal cost multi paths) that a particular flow or packet
   is traversing.



2.5. Proof of Transit

   Various scenarios require the packet to be steered over a particular
   links or nodes. For example:

   -    Voice traffic in a SLA constrained network needs to traverse a
   low latency path between endpoints which may not be the shortest
   path, i.e. the voice traffic needs to be traffic engineered and
   steered over the specified segment list that satisfies the SLA
   constraint.

   -    In a service chaining environment, the traffic may need to
   traverse over an ordered list of service functions.

   In these scenarios, the SRH contains the list of SID functions that
   the packet should execute before reaching the destination. It is
   possible, due to an error, that the packet may reach the destination
   without visiting all the segments in the segment list. It is,
   therefore, important to have the ability to verify that all the
   function SIDs have been executed correctly before the packet is
   delivered to the destination. It is also important to ensure that


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   the order of execution of the SID function has been consistent with
   the SRH contents.

2.6. Anycast Server selection

   For application redundancy and load sharing purpose, it is prevalent
   to see anycast deployment where the service address will assign to
   different application servers spanned across the network. While
   traditionally this type of deployment model was used to terminate
   the client session to the nearest server, the recent capability of
   collecting network and application telemetry along with the traffic
   steering characteristics of SRv6 allows an operator to leverage the
   knowledge to choose the right server and path based on not just the
   shortest path, but also based on other performance metrics.

   It is therefore essential to have the ability to monitor the anycast
   server performance and detect any deviation and take corrective
   actions.

2.7. Detecting Path Divergence

   Path divergence occurs when network traffic diverges from the
   expected path that packet was supposed to take. Path divergence may
   result in congestion, delay, or breakage of strict SLAs promised to
   customers. It is, therefore, important to exercise mechanisms that
   can detect path divergence in the SRv6 network.

2.8. Fault Isolation

   In the cases where a monitoring technique discovers an issue, it is
   required to have the ability to pinpoint the failure location. The
   fault isolation mechanisms are required to help service providers
   troubleshoot failure in an SRv6 network.

2.9. Connectivity Verification from arbitrary node

   In the recent past, network operators are interested in performing
   network operations, administration, and maintenance configuration in
   a centralized manner. In this use-case, one of the requirements is
   to implement OAM functionality like connectivity verification
   between different SRv6 end points in a centralized manner by
   triggering it from any arbitrary node. The other requirement in this
   use-case is to perform the connectivity verification between end
   points without any control plane intervention at the monitored or
   other transit nodes.




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   Additional OAM use-cases will be included in a future revision of
   the document.

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.

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 to address many use-cases outlined in Section 2.

   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. 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.

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


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   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 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
                    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:

   - 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.

3.1.2. 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;


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   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.draft-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-bit in SRH or
   by inserting the END.OTP SID at an appropriate place in the SRH [I-
   D.draft-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 EDN.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 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  [I-
     D.draft-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. 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.





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


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   Support for O-bit 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.

3.2. 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.

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.


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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 may be an SRv6 node or a classic IPv6 node.

   If an SRv6 capable ingress node wants to traceroute to IPv6 prefix
   via an arbitrary segment list <S1, S2, S3>, it needs to initiate
   traceroute probe with an SR header containing the SID list <S1, S2,
   S3>. That is illustrated using the topology in Figure 1. Assume all
   the links have IGP metric 10 except both links between node2 and
   node3, which have IGP metric set to 100. User issues a traceroute
   from node N1 to a loopback of node 5, via segment list <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  99:1:2::21 0.512 msec 0.425 msec 0.374 msec
       SRH: (B5::, A4::C52, A2::C31, SL=2)

    2  99:2:3::31 0.721 msec 0.810 msec 0.795 msec
       SRH: (B5::, A4::C52, A2::C31, SL=1)

    3  99:3:4::41 0.921 msec 0.816 msec 0.759 msec
       SRH: (B5::, A4::C52, A2::C31, SL=1)

    4  99:4:5::52 0.879 msec 0.916 msec 1.024 msec

                 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.




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   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 99:2:3::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 99:4:5::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 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-bit in SRH or
   by inserting the END.OTP SID at an appropriate place in the SRH [I-
   D.draft-filsfils-spring-srv6-network-programming].




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   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 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.

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 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



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     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.
   - 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  [I-
     D.draft-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  99:1:2::21 0.512 msec 0.425 msec 0.374 msec
       SRH: (A4::C52, A4::OTP, A2::C31; SL=2)

    2  99:2:3::31 0.721 msec 0.810 msec 0.795 msec
       SRH: (A4::C52, A4::OTP, A2::C31; SL=1)

    3  99:3:4::41 0.921 msec 0.816 msec 0.759 msec
       SRH: (A4::C52, A4::OTP, A2::C31; SL=1)

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



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.



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   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 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.
   - 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.draft-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. 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-bit punt at Transit (TBA)"). Please note that, as mentioned in
     [I-D.draft-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.
   - 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-bit in SRH, as described in the pseudocode in [I-
     D.draft-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).




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   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  99:1:2::21 0.512 msec 0.425 msec 0.374 msec
       SRH: (A4::C52, A4::OTP, A2::C31; SL=2)

    2  99:3:4::41 0.921 msec 0.816 msec 0.759 msec
       SRH: (A4::C52, A4::OTP, A2::C31; SL=1)

              A sample output for overlay traceroute to a SID function



3.4. In-situ OAM

   [I-D.draft-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
   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 constrainted services.




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   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.

3.5. 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.

3.6. Connectivity Verification from arbitrary SR node

   In the recent past, network operators are interested in performing
   network operations, administration, and maintenance configuration 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



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   describes how the concept can be used to perform path monitoring in
   an SRv6 network.

   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 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.

4. 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.

5. IANA Considerations

   This document does not define any new protocol or any extension to
   an existing protocol.

6. References

6.1. Normative References

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

   [RFC4443] Internet Control Message Protocol (ICMPv6) for the
             Internet Protocol Version 6 (IPv6) Specification. A.
             Conta, S. Deering, M. Gupta, Ed. March 2006.

   [RFC792] Internet Control Message Protocol. J. Postel. September
             1981.


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

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

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

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

6.2. Informative References

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

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

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

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

7. 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

   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

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

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




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