Subject: [Int-dir] Intdir telechat review of draft-ietf-6man-spring-srv6-oam-10

From: Carlos Bernardos via Datatracker <[email-masked]>
Date: Fri, 28 May 2021 02:09:36 -0700

Reviewer: Carlos Bernardos
Review result: Ready with Nits

I think the document is well written and ready to move forward. I've
just inserted below (prepended with [CB]) some minor comments.

6man                                                              Z. Ali
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                           Cisco Systems
Expires: October 10, 2021                                  S. Matsushima
                                                                Softbank
                                                                D. Voyer
                                                             Bell Canada
                                                                 M. Chen
                                                                  Huawei
                                                           April 8, 2021

  Operations, Administration, and Maintenance (OAM) in Segment Routing
                  Networks with IPv6 Data plane (SRv6)
                   draft-ietf-6man-spring-srv6-oam-10

Abstract

   This document describes how the existing IPv6 mechanisms for ping and
   traceroute can be used in an SRv6 network.  The document also
   specifies the OAM flag in the Segment Routing Header (SRH) for
   performing controllable and predictable flow sampling from segment
   endpoints.  In addition, the document describes how a centralized
   monitoring system performs a path continuity check between any nodes
   within an SRv6 domain.

[CB] This is just a minor and personal comment (not intdir related): I'd
     prefer starting with a more generic overall summary of what the
     document does, and then the enumeration of the different parts. As
     written now, it seems like a plain enumeration of things, which also
     conveys the message of not being homogeneous.

Status of This Memo

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

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

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

   This Internet-Draft will expire on October 10, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Terminology and Reference Topology  . . . . . . . . . . .   4
   2.  OAM Mechanisms  . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  O-flag in Segment Routing Header  . . . . . . . . . . . .   5
       2.1.1.  O-flag Processing . . . . . . . . . . . . . . . . . .   6
     2.2.  OAM Operations  . . . . . . . . . . . . . . . . . . . . .   7
   3.  Illustrations . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Ping in SRv6 Networks . . . . . . . . . . . . . . . . . .   8
       3.1.1.  Classic Ping  . . . . . . . . . . . . . . . . . . . .   8
       3.1.2.  Pinging a SID . . . . . . . . . . . . . . . . . . . .  10
     3.2.  Traceroute  . . . . . . . . . . . . . . . . . . . . . . .  11
       3.2.1.  Classic Traceroute  . . . . . . . . . . . . . . . . .  11
       3.2.2.  Traceroute to a SID . . . . . . . . . . . . . . . . .  13
     3.3.  A Hybrid OAM Using O-flag . . . . . . . . . . . . . . . .  15
     3.4.  Monitoring of SRv6 Paths  . . . . . . . . . . . . . . . .  17
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .  18
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   As Segment Routing with IPv6 data plane (SRv6) [RFC8402] simply adds
   a new type of Routing Extension Header, existing IPv6 OAM mechanisms
   can be used in an SRv6 network.  This document describes how the
   existing IPv6 mechanisms for ping and traceroute can be used in an
   SRv6 network.  This includes illustrations of pinging an SRv6 SID to
   verify that the SID is reachable and is locally programmed at the
   target node.  This also includes illustrations for tracerouting to an

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   SRv6 SID for hop-by-hop fault localization as well as path tracing to
   a SID.

   The document also introduces enhancements for OAM mechanism for SRv6
   networks for performing controllable and predictable flow sampling
   from segment endpoints using, e.g., IP Flow Information Export
   (IPFIX) protocol [RFC7011].  Specifically, the document specifies the
   O-flag in SRH as a marking-bit in the user packets to trigger the
   telemetry data collection and export at the segment endpoints.

   The document also outlines how centralized OAM technique in [RFC8403]
   
[CB] Maybe "how the centralized OAM"?

   can be extended for SRv6 to perform a path continuity check between
   any nodes within an SRv6 domain.  Specifically, the document
   illustrates how a centralized monitoring system can monitor arbitrary
   SRv6 paths by creating the loopback probes that originates and
   terminates at the centralized monitoring system.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Abbreviations

   The following abbreviations are used in this document:

      SID: Segment ID.

      SL: Segments Left.

      SR: Segment Routing.

      SRH: Segment Routing Header [RFC8754].

      SRv6: Segment Routing with IPv6 Data plane.

      TC: Traffic Class.

      ICMPv6: ICMPv6 Specification [RFC4443].

      IS-IS: Intermediate System to Intermediate System

      OSPF: Open Shortest Path First protocol [RFC2328]

      IGP: Interior Gateway Protocols (e.g., OSPF, IS-IS).

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      BGP-LS: Border Gateway Protocol - Link State Extensions [RFC8571]

[CB] I'd add the RFC where the SR-related terms/acronyms are originally
     defined.

1.3.  Terminology and Reference Topology

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

   +--------------------------| N100 |---------------------------------+
   |                                                                   |
   |  ====== link1====== link3------ link5====== link9------   ======  |
      ||N1||------||N2||------| N3 |------||N4||------| N5 |---||N7||
      ||  ||------||  ||------|    |------||  ||------|    |---||  ||
      ====== link2====== link4------ link6======link10------   ======
         |            |                      |                   |
      ---+--          |       ------         |                 --+---
      |CE 1|          +-------| N6 |---------+                 |CE 2|
      ------            link7 |    | link8                     ------
                              ------

                           Figure 1 Reference Topology

   In the reference topology:

      Node k has a IPv6 loopback address 2001:db8::A:k::/128.

      Nodes N1, N2, N4 and N7 are SRv6-capable nodes.

      Nodes N3, N5 and N6 are IPv6 nodes that are not SRv6-capable.
      Such nodes are referred as classic IPv6 nodes.

      CE1 and CE2 are Customer Edge devices of any data plane capability
      (e.g., IPv4, IPv6, L2, etc.).

      A SID at node k with locator block 2001:db8:B::/48 and function F
      is represented by 2001:db8:B:k:F::.

      Node N100 is a controller.

      The IPv6 address of the nth Link between node i and j at the i
      side is represented as 2001:db8:i:j:in::, 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::.

      2001:db8:B:k:Cij:: is explicitly allocated as the END.X SID at
      node k towards neighbor node i via jth Link between node i and
      node k.  e.g., 2001:db8:B:2:C31:: represents END.X at N2 towards

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      N3 via link3 (the 1st link between N2 and N3).  Similarly,
      2001:db8:B:4:C52:: represents the END.X at N4 towards N5 via
      link10.  Please refer to [RFC8986] for description of END.X SID.

      A SID list is represented as <S1, S2, S3> where S1 is the first
      SID to visit, S2 is the second SID to visit and S3 is the last SID
      to visit along the SR path.

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

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

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

      *  Note the difference between the < > and () symbols: <S1, S2,
         S3> represents a SID list where S1 is the first SID and S3 is
         the last SID to traverse.  (S3, S2, S1; SL) represents the same
         SID list but encoded in the SRH format where the rightmost SID
         in the SRH is the first SID and the leftmost SID in the SRH is
         the last SID.  When referring to an SR policy in a high-level
         use-case, it is simpler to use the <S1, S2, S3> notation.  When
         referring to an illustration of the detailed packet behavior,
         the (S3, S2, S1; SL) notation is more convenient.

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

2.  OAM Mechanisms

   This section defines OAM enhancement for the SRv6 networks.

2.1.  O-flag in Segment Routing Header

   [RFC8754] describes the Segment Routing Header (SRH) and how SR
   capable nodes use it.  The SRH contains an 8-bit "Flags" field.

   This document defines the following bit in the SRH Flags field to
   carry the O-flag:

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

   Where:

      O-flag: OAM flag in the SRH Flags field defined in [RFC8754] .
[CB] Nit: \s\[RFC8754] .\[RFC8754]. 

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2.1.1.  O-flag Processing

   The O-flag in SRH is used as a marking-bit in the user packets to
   trigger the telemetry data collection and export at the segment
   endpoints.
[CB] Maybe this is obvious for someone expert in SRv6, but can only
     telemetry data collection be done at segment endpoints?

   This document does not specify the data elements that need to be
   exported and the associated configurations.  Similarly, this document
   does not define any formats for exporting the data elements.
   Nonetheless, without the loss of generality, this document assumes IP
   Flow Information Export (IPFIX) protocol [RFC7011] is used for
   exporting the traffic flow information from the network devices to a
   controller for monitoring and analytics.  Similarly, without the loss
   of generality, this document assumes requested information elements
   are configured by the management plane through data set templates
   (e.g., as in IPFIX [RFC7012]).

   Implementation of the O-flag is OPTIONAL.  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
   control plane protocol(s).

   When N receives a packet whose IPv6 DA is S and S is a local SID, the
   line S01 of the pseudo-code associated with the SID S, as defined in
   section 4.3.1.1 of [RFC8754], is appended as follows for the O-flag
   processing.

      S01.1. IF O-flag is set and local configuration permits
             O-flag processing {
                a. Make a copy of the packet.
                b. Send the copied packet, along with a timestamp
                to the OAM process for telemetry data collection
                and export.      ;; Ref1
             }
      Ref1: An implementation SHOULD copy and record the timestamp as
      soon as possible during packet processing. Timestamp or any other
      metadata is not
      carried in the packet forwarded to the next hop.
[CB] Weird line jump above.

   Please note that the O-flag processing happens before execution of
   regular processing of the local SID S.  Specifically, the line S01.1
   of the pseudo-code specified in this document is inserted between
   line S01 and S02 of the pseudo-code defined in section 4.3.1.1 of
   [RFC8754].

   Based on the requested information elements configured by the
   management plane through data set templates [RFC7012], the OAM

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   process exports the requested information elements.  The information
   elements include parts of the packet header and/or parts of the
   packet payload for flow identification.  The OAM process uses
   information elements defined in IPFIX [RFC7011] and PSAMP [RFC5476]
   for exporting the requested sections of the mirrored packets.

   If the telemetry data from the ultimate segment in the segment-list
   is required, a penultimate segment SHOULD NOT be a Penultimate
   Segment Pop (PSP) SID.  When the penultimate segment is a PSP SID,
   the SRH will be removed and the O-flag will not be processed at the
   ultimate segment.

   The processing node SHOULD rate-limit the number of packets punted to
   the OAM process to a configurable rate.  This is to avoid hitting any
   performance impact on the OAM and the telemetry collection processes.
   Failure in implementing the rate limit can lead to a denial-of-
   service attack, as detailed in Section 5.

   The OAM process MUST NOT process the copy of the packet or respond to
   any upper-layer header (like ICMP, UDP, etc.) payload to prevent
   multiple evaluations of the datagram.

   Specification of the OAM process or the external controller
   operations are beyond the scope of this document.  How to correlate
   the data collected from different nodes at an external controller is
   also outside the scope of the document.  Section 3 illustrates use of
   the O-flag for implementing a hybrid OAM mechanism, where the
   "hybrid" classification is based on RFC7799 [RFC7799].

2.2.  OAM Operations

   IPv6 OAM operations can be performed for any SRv6 SID whose behavior
   allows Upper Layer Header processing for an applicable OAM payload
   (e.g., ICMP, UDP).

   Ping to an SRv6 SID is used to verify that the SID is reachable and
   is locally programmed at the target node.  Traceroute to a SID is
   used for hop-by-hop fault localization as well as path tracing to a
   SID.  Section 3 illustrates the ICMPv6 based ping and the UDP based
   traceroute mechanisms for ping and traceroute to an SRv6 SID.
   Although this document only illustrates ICMPv6 ping and UDP based
   traceroute to an SRv6 SID, the procedures are equally applicable to
   other IPv6 OAM probing to an SRv6 SID (e.g., Bidirectional Forwarding
   Detection (BFD) [RFC5880], Seamless BFD (SBFD) [RFC7880], STAMP probe
   message processing [I-D.gandhi-spring-stamp-srpm], etc.).
   Specifically, as long as local configuration allows the Upper-layer
   Header processing of the applicable OAM payload for SRv6 SIDs, the

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   existing IPv6 OAM techniques can be used to target a probe to a
   (remote) SID.

   IPv6 OAM operations can be performed with the target SID in the IPv6
   destination address without SRH or with SRH where the target SID is
   the last segment.  In general, OAM operations to a target SID may not
   exercise all of its processing depending on its behavior definition.
   For example, ping to an END.X SID [RFC8986] only validates the SID is
   locally programmed at the target node and does not validate switching
   to the correct outgoing interface.  To exercise the behavior of a
   target SID, the OAM operation SHOULD construct the probe in a manner
   similar to a data packet that exercises the SID behavior, i.e. to
   include that SID as a transit SID in either an SRH or IPv6 DA of an
   outer IPv6 header or as appropriate based on the definition of the
   SID behavior.

3.  Illustrations

   This section shows how some of the existing IPv6 OAM mechanisms can
   be used in an SRv6 network.  It also illustrates an OAM mechanism for
   performing controllable and predictable flow sampling from segment
   endpoints.  How centralized OAM technique in [RFC8403] can be
   extended for SRv6 is also described in this Section.

3.1.  Ping in SRv6 Networks

   The following subsections outline some use cases of the ICMPv6 ping
   in the SRv6 networks.

3.1.1.  Classic Ping

   The existing mechanism to perform the reachability checks, 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 address 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 N2 and N3, which have
   IGP metric set to 100.  User issues a ping from node N1 to a loopback
   of node 5, via segment list <2001:db8:B:2:C31::, 2001:db8:B:4:C52::>.
   The SID behavior used in the example is End.X SID, as described in
   [RFC8986], but the procedure is equally applicable to any other
   (transit) SID type.

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   Figure 2 contains sample output for a ping request initiated at node
   N1 to the loopback address of node N5 via a segment list
   <2001:db8:B:2:C31::, 2001:db8:B:4:C52::>.

       > ping 2001:db8:A:5:: via segment-list 2001:db8:B:2:C31::,
              2001:db8:B:4:C52::

       Sending 5, 100-byte ICMPv6 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

   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
      (2001:db8:A:1::, 2001:db8:B:2:C31::) (2001:db8:A:5::,
      2001:db8:B:4:C52::, 2001:db8:B:2:C31::, SL=2, NH = ICMPv6)(ICMPv6
      Echo Request).

   o  Node N2, which is an SRv6-capable node, performs the standard SRH
      processing.  Specifically, it executes the END.X behavior
      (2001:db8:B:2:C31::) and forwards the packet on link3 to N3.

   o  Node N3, which is a classic IPv6 node, performs the standard IPv6
      processing.  Specifically, it forwards the echo request based on
      the DA 2001:db8:B:4:C52:: in the IPv6 header.

   o  Node N4, which is an SRv6-capable node, performs the standard SRH
      processing.  Specifically, it observes the END.X behavior
      (2001:db8:B:4:C52::) and forwards the packet on link10 towards N5.
      If 2001:db8:B:4:C52:: is a PSP SID, The penultimate node (Node N4)
      does not, should not and cannot differentiate between the data
      packets and OAM probes.  Specifically, if 2001:db8:B:4:C52:: is a
      PSP SID, node N4 executes the SID like any other data packet with
      DA = 2001:db8:B:4:C52:: and removes the SRH.

   o  The echo request packet at N5 arrives as an IPv6 packet with or
      without an SRH.  If N5 receives the packet with SRH, it skips SRH
      processing (SL=0).  In either case, Node N5 performs the standard

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      ICMPv6 processing on the echo request and responds with the echo
      reply message to N1.  The echo reply message is IP routed.

3.1.2.  Pinging a SID

   The classic ping described in the previous section applies equally to
   perform SID reachability check and to validate the SID is locally
   programmed at the target node.  This is explained using an example in
   the following.  The example uses ping to an END SID, as described in
   [RFC8986], but the procedure is equally applicable to ping any other
   SID behaviors.

   Consider the example where the user wants to ping a remote SID
   2001:db8:B:4::, via 2001:db8:B:2:C31::, from node N1.  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
      (2001:db8:A:1::, 2001:db8:B:2:C31::) (2001:db8:B:4::,
      2001:db8:B:2:C31::; SL=1; 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 behavior
      (2001:db8:B:2:C31::) on the echo request packet.  If
      2001:db8:B:2:C31:: is a PSP SID, node N4 executes the SID like any
      other data packet with DA = 2001:db8:B:2:C31:: and removes the
      SRH.

   o  Node N3, which is a classic IPv6 node, performs the standard IPv6
      processing.  Specifically, it forwards the echo request based on
      DA = 2001:db8:B:4:: in the IPv6 header.

   o  When node N4 receives the packet, it processes the target SID
      (2001:db8:B:4::).

   o  If the target SID (2001:db8:B:4::) is not locally instantiated,
      the packet is discarded

   o  If the target SID (2001:db8:B:4::) is locally instantiated, the
      node processes the upper layer header.  As part of the upper layer
      header processing node N4 respond to the ICMPv6 echo request
      message and responds with the echo reply message.  The echo reply
      message is IP routed.

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3.2.  Traceroute

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

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

3.2.1.  Classic Traceroute

   The existing mechanism to traceroute a remote IP address, 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 address
   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 N2 and N3,
   which have IGP metric set to 100.  User issues a traceroute from node
   N1 to a loopback of node 5, via segment list <2001:db8:B:2:C31::,
   2001:db8:B:4:C52::>.  The SID behavior used in the example is End.X
   SID, as described in [RFC8986], but the procedure is equally
   applicable to any other (transit) SID type.  Figure 3 contains sample
   output for the traceroute request.

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   > traceroute 2001:db8:A:5:: via segment-list 2001:db8:B:2:C31::,
                2001:db8:B:4:C52::

   Tracing the route to 2001:db8:A:5::
   1  2001:db8:1:2:21:: 0.512 msec 0.425 msec 0.374 msec
      DA: 2001:db8:B:2:C31::,
      SRH:(2001:db8:A:5::, 2001:db8:B:4:C52::, 2001:db8:B:2:C31::, SL=2)
   2  2001:db8:2:3:31:: 0.721 msec 0.810 msec 0.795 msec
      DA: 2001:db8:B:4:C52::,
      SRH:(2001:db8:A:5::, 2001:db8:B:4:C52::, 2001:db8:B:2:C31::, SL=1)
   3  2001:db8:3:4::41:: 0.921 msec 0.816 msec 0.759 msec
      DA: 2001:db8:B:4:C52::,
      SRH:(2001:db8:A:5::, 2001:db8:B:4:C52::, 2001:db8:B:2:C31::, SL=1)
   4  2001:db8:4:5::52:: 0.879 msec 0.916 msec 1.024 msec
      DA: 2001:db8:A:5::

      Figure 3 A sample traceroute output at an SRv6-capable node

   In the sample traceroute output, the information displayed at each
   hop is obtained using the contents of the "Time Exceeded" or
   "Destination Unreachable" ICMPv6 responses.  These ICMPv6 responses
   are IP routed.

   In the sample traceroute output, the information for link3 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 the first hop is returned
   by N2, which is an SRv6-capable node.  Just like for the second hop,
   the ingress node is able to display SR header contents for the first
   hop.

   There is no difference in processing of the traceroute probe at an
   IPv6 classic node and an SRv6-capable node.  Similarly, both IPv6
   classic and SRv6-capable nodes may use the address of the interface
   on which probe was received as the source address in the ICMPv6
   response.  ICMPv6 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

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   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 IP address of the interface on which the traceroute probe was
   received is useful.  This information can also be used to verify if
   SIDs 2001:db8:B:2:C31:: and 2001:db8:B:4:C52:: are executed correctly
   by N2 and N4, respectively.  Specifically, the information displayed
   for the second hop contains the incoming interface address
   2001:db8:2:3:31:: at N3.  This matches with the expected interface
   bound to END.X behavior 2001:db8:B:2:C31:: (link3).  Similarly, the
   information displayed for hop5 contains the incoming interface
   address 2001:db8:4:5::52:: at N5.  This matches with the expected
   interface bound to the END.X behavior 2001:db8:B:4:C52:: (link10).

3.2.2.  Traceroute to a SID

   The classic traceroute described in the previous section applies
   equally to traceroute a remote SID behavior, as explained using an
   example in the following.  The example uses traceroute to an END SID,
   as described in [RFC8986], but the procedure is equally applicable to
   tracerouting any other SID behaviors.

   Please note that traceroute to a SID is exemplified using UDP probes.
   However, the procedure is equally applicable to other implementations
   of traceroute mechanism.  The UDP encoded message to traceroute a SID
   uses the UDP ports assigned by IANA for "traceroute use".

   Consider the example where the user wants to traceroute a remote SID
   2001:db8:B:4::, via 2001:db8:B:2:C31::, from node N1.  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 (2001:db8:A:1::,
      2001:db8:B:2:C31::) (2001:db8:B:4::, 2001:db8:B:2:C31::; SL=1;
      NH=UDP)(Traceroute probe).

   o  When node N2 receives the packet with hop-count = 1, it processes
      the hop count expiry.  Specifically, the node N2 responses with
      the ICMPv6 message (Type: "Time Exceeded", Code: "Hop limit
      exceeded in transit").  The ICMPv6 response is IP routed.

   o  When Node N2 receives the packet with hop-count > 1, it performs
      the standard SRH processing.  Specifically, it executes the END.X
      behavior (2001:db8:B:2:C31::) on the traceroute probe.  If
      2001:db8:B:2:C31:: is a PSP SID, node N4 executes the SID like any
      other data packet with DA = 2001:db8:B:2:C31:: and removes the
      SRH.

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   o  When node N3, which is a classic IPv6 node, receives the packet
      with hop-count = 1, it processes the hop count expiry.
      Specifically, the node N3 responses with the ICMPv6 message (Type:
      "Time Exceeded", Code: "Hop limit exceeded in Transit").  The
      ICMPv6 response is IP routed.

   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
      2001:db8:B:4:: in the IPv6 header.

   o  When node N4 receives the packet with DA set to the local SID
      2001:db8:B:4::, it processes the END SID.

   o  If the target SID (2001:db8:B:4::) is not locally instantiated,
      the packet is discarded.

   o  If the target SID (2001:db8:B:4::) is locally instantiated, the
      node processes the upper layer header.  As part of the upper layer
      header processing node N4 responses with the ICMPv6 message (Type:
      Destination unreachable, Code: Port Unreachable).  The ICMPv6
      response is IP routed.

   Figure 4 displays a sample traceroute output for this example.

     > traceroute 2001:db8:B:4:C52:: via segment-list 2001:db8:B:2:C31::

     Tracing the route to SID 2001:db8:B:4:C52::
     1  2001:db8:1:2:21:: 0.512 msec 0.425 msec 0.374 msec
        DA: 2001:db8:B:2:C31::,
        SRH:(2001:db8:B:4:C52::, 2001:db8:B:2:C31::; SL=1)
     2  2001:db8:2:3:31:: 0.721 msec 0.810 msec 0.795 msec
        DA: 2001:db8:B:4:C52::,
        SRH:(2001:db8:B:4:C52::, 2001:db8:B:2:C31::; SL=0)
     3  2001:db8:3:4:41:: 0.921 msec 0.816 msec 0.759 msec
        DA: 2001:db8:B:4:C52::,
        SRH:(2001:db8:B:4:C52::, 2001:db8:B:2:C31::; SL=0)

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

[CB] Minor comment: since the sample outputs for both classic and traceroute to
     a SID shows 3-values of the RTT, I'd probably mention somewhere that the
     sample outputs assume sending 3x per hop-count, which is the typical
     implementation approach (but I'm not sure this is recommended/mandated).

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3.3.  A Hybrid OAM Using O-flag

   This section illustrates a hybrid OAM mechanism using the the O-flag.
   Without loss of the generality, the illustration assumes N100 is a
   centralized controller.

   The illustration is different than the In-situ OAM defined in [I.D-
   draft-ietf-ippm-ioam-data].  This is because In-situ OAM records
   operational and telemetry information in the packet as the packet
   traverses a path between two points in the network [I.D-draft-ietf-
   ippm-ioam-data].  The illustration in this subsection does not
   require the recording of OAM data in the packet.

   The illustration does not assume any formats for exporting the data
   elements or the data elements that need to be exported.

   Consider the example where the user wants to monitor sampled IPv4 VPN
   999 traffic going from CE1 to CE2 via a low latency SR policy P
   installed at Node N1.  To exercise a low latency path, the SR Policy
   P forces the packet via segments 2001:db8:B:2:C31:: and
   2001:db8:B:4:C52::.  The VPN SID at N7 associated with VPN 999 is
   2001:db8:B:7:DT999::.  2001:db8:B:7:DT999:: is a USP SID.  N1, N4,
   and N7 are capable of processing O-flag but N2 is not capable of
   processing O-flag.  N100 is the centralized controller capable of
   processing and correlating the copy of the packets sent from nodes
   N1, N4, and N7.  N100 is aware of O-flag processing capabilities.
   Controller N100 with the help from nodes N1, N4, N7 and implements a
   hybrid OAM mechanism using the O-flag as follows:

   o  A packet P1:(IPv4 header)(payload) is sent from CE1 to Node N1.

   o  Node N1 steers the packet P1 through the Policy P.  Based on a
      local configuration, Node N1 also implements logic to sample
      traffic steered through policy P for hybrid OAM purposes.
      Specification for the sampling logic is beyond the scope of this
      document.  Consider the case where packet P1 is classified as a
      packet to be monitored via the hybrid OAM.  Node N1 sets O-flag
      during encapsulation required by policy P.  As part of setting the
      O-flag, node N1 also sends a timestamped copy of the packet P1:
      (2001:db8:A:1::, 2001:db8:B:2:C31::) (2001:db8:B:7:DT999::,
      2001:db8:B:4:C52::, 2001:db8:B:2:C31::; SL=2; O-flag=1;
      NH=IPv4)(IPv4 header)(payload) to a local OAM process.  The local
      OAM process sends a full or partial copy of the packet P1 to the
      controller N100.  The OAM process includes the recorded timestamp,
      additional OAM information like incoming and outgoing interface,
      etc. along with any applicable metadata.  Node N1 forwards the
      original packet towards the next segment 2001:db8:B:2:C31::.

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   o  When node N2 receives the packet with O-flag set, it ignores the
      O-flag.  This is because node N2 is not capable of processing the
      O-flag.  Node N2 performs the standard SRv6 SID and SRH
      processing.  Specifically, it executes the END.X behavior
      (2001:db8:B:2:C31::) as described in [RFC8986] and forwards the
      packet P1 (2001:db8:A:1::, 2001:db8:B:4:C52::)
      (2001:db8:B:7:DT999::, 2001:db8:B:4:C52::, 2001:db8:B:2:C31::;
      SL=1; O-flag=1; NH=IPv4)(IPv4 header)(payload) over link 3 towards
      Node N3.

   o  When node N3, which is a classic IPv6 node, receives the packet P1
      , it performs the standard IPv6 processing.  Specifically, it
      forwards the packet P1 based on DA 2001:db8:B:4:C52:: in the IPv6
      header.
[CB] Nit: \s\P1 ,\P1,

   o  When node N4 receives the packet P1 (2001:db8:A:1::,
      2001:db8:B:4:C52::) (2001:db8:B:7:DT999::, 2001:db8:B:4:C52::,
      2001:db8:B:2:C31::; SL=1; O-flag=1; NH=IPv4)(IPv4
      header)(payload), it processes the O-flag.  As part of processing
      the O-flag, it sends a timestamped copy of the packet to a local
      OAM process.  The local OAM process sends a full or partial copy
      of the packet P1 to the controller N100.  The OAM process includes
      the recorded timestamp, additional OAM information like incoming
      and outgoing interface, etc. along with any applicable metadata.
      Node N4 performs the standard SRv6 SID and SRH processing on the
      original packet P1.  Specifically, it executes the END.X behavior
      (2001:db8:B:4:C52::) and forwards the packet P1 (2001:db8:A:1::,
      2001:db8:B:7:DT999::) (2001:db8:B:7:DT999::, 2001:db8:B:4:C52::,
      2001:db8:B:2:C31::; SL=0; O-flag=1; NH=IPv4)(IPv4 header)(payload)
      over link 10 towards Node N5.

   o  When node N5, which is a classic IPv6 node, receives the packet
      P1, it performs the standard IPv6 processing.  Specifically, it
      forwards the packet based on DA 2001:db8:B:7:DT999:: in the IPv6
      header.

   o  When node N7 receives the packet P1 (2001:db8:A:1::,
      2001:db8:B:7:DT999::) (2001:db8:B:7:DT999::, 2001:db8:B:4:C52::,
      2001:db8:B:2:C31::; SL=0; O-flag=1; NH=IPv4)(IPv4
      header)(payload), it processes the O-flag.  As part of processing
      the O-flag, it sends a timestamped copy of the packet to a local
      OAM process.  The local OAM process sends a full or partial copy
      of the packet P1 to the controller N100.  The OAM process includes
      the recorded timestamp, additional OAM information like incoming
      and outgoing interface, etc. along with any applicable metadata.
      Node N4 performs the standard SRv6 SID and SRH processing on the
      original packet P1.  Specifically, it executes the VPN SID

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      (2001:db8:B:7:DT999::) and based on lookup in table 100 forwards
      the packet P1 (IPv4 header)(payload) towards CE 2.

   o  The controller N100 processes and correlates the copy of the
      packets sent from nodes N1, N4 and N7 to find segment-by-segment
      delays and provide other hybrid OAM information related to packet
      P1.

   o  The process continues for any other sampled packets.

3.4.  Monitoring of SRv6 Paths

   In the recent past, network operators demonstrated interest in
   performing network OAM functions in a centralized manner.  [RFC8403]
   describes such a centralized OAM mechanism.  Specifically, the
   document describes a procedure that can be used to perform path
   continuity check between any nodes within an SR domain from a
   centralized monitoring system.  However, the document focuses on SR
   networks with MPLS data plane.  This document describes how the
   concept can be used to perform path monitoring in an SRv6 network
   from a centralized controller.

   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.  The controller leverages the visibility of the topology to
   monitor the paths between the various endpoints.

   The controller N100 advertises an END SID [RFC8986]
   2001:db8:B:100:1::. To monitor any arbitrary SRv6 paths, the
   controller can create a loopback probe that originates and terminates
   on Node N100.  To distinguish between a failure in the monitored path
   and loss of connectivity between the controller and the network, Node
   N100 runs a suitable mechanism to monitor its connectivity to the
   monitored network.

   The loopback probes are exemplified using an example where controller
   N100 needs to verify a segment list <2001:db8:B:2:C31::,
   2001:db8:B:4:C52::>:

   o  N100 generates an OAM packet (2001:db8:A:100::,
      2001:db8:B:2:C31::)(2001:db8:B:100:1::, 2001:db8:B:4:C52::,
      2001:db8:B:2:C31::, SL=2)(OAM Payload).  The controller routes the
      probe packet towards the first segment, which is
      2001:db8:B:2:C31::.

   o  Node N2 executes the END.X behavior (2001:db8:B:2:C31::) and
      forwards the packet (2001:db8:A:100::,

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      2001:db8:B:4:C52::)(2001:db8:B:100:1::, 2001:db8:B:4:C52::,
      2001:db8:B:2:C31::, SL=1)(OAM Payload) on link3 to N3.

   o  Node N3, which is a classic IPv6 node, performs the standard IPv6
      processing.  Specifically, it forwards the packet based on the DA
      2001:db8:B:4:C52:: in the IPv6 header.

   o  Node N4 executes the END.X behavior (2001:db8:B:4:C52::) and
      forwards the packet (2001:db8:A:100::,
      2001:db8:B:100:1::)(2001:db8:B:100:1::, 2001:db8:B:4:C52::,
      2001:db8:B:2:C31::, SL=0)(OAM Payload) on link10 to N5.

   o  Node N5, which is a classic IPv6 node, performs the standard IPv6
      processing.  Specifically, it forwards the packet based on the DA
      2001:db8:B:100:1:: in the IPv6 header.

   o  Node N100 executes the standard SRv6 END behavior.  It
      decapsulates the header and consume the probe for OAM processing.
      The information in the OAM payload is used to detect any missing
      probes, round trip delay, etc.

   The OAM payload type or the information carried in the OAM probe is a
   local implementation decision at the controller and is outside the
   scope of this document.

4.  Implementation Status

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

   See [I-D.matsushima-spring-srv6-deployment-status] for updated
   deployment and interoperability reports.

5.  Security Considerations

   This document does not define any new protocol extensions and relies
   on existing procedures defined for ICMPv6.

   [RFC8754] defines the notion of an SR domain and use of SRH within
   the SR domain.  The use of OAM procedures described in this document
   is restricted to an SR domain.  For example, similar to the SID
   manipulation, O-flag manipulation is not considered as a threat
   within the SR domain.  Procedures for securing an SR domain are
   defined the section 5.1 and section 7 of [RFC8754].

   As noted in section 7.1 of [RFC8754], compromised nodes within the SR
   domain may mount attacks.  The O-flag may be set by an attacking node
   attempting a denial-of-service attack on the OAM process at the
   segment endpoint node.  An implementation correctly implementing the

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   rate limiting in section 2.1.1 is not susceptible to that denial-of-
   service attack.  Additionally, SRH Flags are protected by the HMAC
   TLV, as described in Section 2.1.2.1 of [RFC8754].

   This document does not impose any additional security challenges to
   be considered beyond security threats described in [RFC4884],
   [RFC4443], [RFC0792], and [RFC8754].

6.  IANA Considerations

   This document requests that IANA allocate the following registrations
   in the "Segment Routing Header Flags" sub-registry for the "Internet
   Protocol Version 6 (IPv6) Parameters" registry maintained by IANA:

            +-------+------------------------------+---------------+
            | Bit   | Description                  | Reference     |
            +=======+==============================+===============+
            | 2     | O-flag                       | This document |
            +-------+------------------------------+---------------+

7.  Acknowledgements

   The authors would like to thank Joel M.  Halpern, Greg Mirsky, Bob
   Hinden, Loa Andersson, Gaurav Naik, Ketan Talaulikar and Haoyu Song
   for their review comments.

8.  Contributors

   The following people have contributed to this document:

      Robert Raszuk
      Bloomberg LP
      Email: [email-masked]

      John Leddy
      Individual
      Email: [email-masked]

      Gaurav Dawra
      LinkedIn
      Email: [email-masked]

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      Bart Peirens
      Proximus
      Email: [email-masked]

      Nagendra Kumar
      Cisco Systems, Inc.
      Email: [email-masked]

      Carlos Pignataro
      Cisco Systems, Inc.
      Email: [email-masked]

      Rakesh Gandhi
      Cisco Systems, Inc.
      Canada
      Email: [email-masked]

      Frank Brockners
      Cisco Systems, Inc.
      Germany
      Email: [email-masked]

      Darren Dukes
      Cisco Systems, Inc.
      Email: [email-masked]

      Cheng Li
      Huawei
      Email: [email-masked]

      Faisal Iqbal
      Individual
      Email: [email-masked]

9.  References

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9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

9.2.  Informative References

   [I-D.gandhi-spring-stamp-srpm]
              Gandhi, R., Filsfils, C., Voyer, D., Chen, M., and B.
              Janssens, "Performance Measurement Using Simple TWAMP
              (STAMP) for Segment Routing Networks", draft-gandhi-
              spring-stamp-srpm-04 (work in progress), January 2021.

   [I-D.ietf-ippm-ioam-data]
              Brockners, F., Bhandari, S., and T. Mizrahi, "Data Fields
              for In-situ OAM", draft-ietf-ippm-ioam-data-11 (work in
              progress), November 2020.

   [I-D.matsushima-spring-srv6-deployment-status]
              Matsushima, S., Filsfils, C., Ali, Z., Li, Z., and K.
              Rajaraman, "SRv6 Implementation and Deployment Status",
              draft-matsushima-spring-srv6-deployment-status-10 (work in
              progress), December 2020.

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

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

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

   [RFC5476]  Claise, B., Ed., Johnson, A., and J. Quittek, "Packet
              Sampling (PSAMP) Protocol Specifications", RFC 5476,
              DOI 10.17487/RFC5476, March 2009,
              <https://www.rfc-editor.org/info/rfc5476>.

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

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <https://www.rfc-editor.org/info/rfc5880>.

   [RFC7011]  Claise, B., Ed., Trammell, B., Ed., and P. Aitken,
              "Specification of the IP Flow Information Export (IPFIX)
              Protocol for the Exchange of Flow Information", STD 77,
              RFC 7011, DOI 10.17487/RFC7011, September 2013,
              <https://www.rfc-editor.org/info/rfc7011>.

   [RFC7012]  Claise, B., Ed. and B. Trammell, Ed., "Information Model
              for IP Flow Information Export (IPFIX)", RFC 7012,
              DOI 10.17487/RFC7012, September 2013,
              <https://www.rfc-editor.org/info/rfc7012>.

   [RFC7799]  Morton, A., "Active and Passive Metrics and Methods (with
              Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
              May 2016, <https://www.rfc-editor.org/info/rfc7799>.

   [RFC7880]  Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
              Pallagatti, "Seamless Bidirectional Forwarding Detection
              (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016,
              <https://www.rfc-editor.org/info/rfc7880>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

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Internet-Draft                  SRv6 OAM                      April 2021

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

   [RFC8571]  Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and
              C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of
              IGP Traffic Engineering Performance Metric Extensions",
              RFC 8571, DOI 10.17487/RFC8571, March 2019,
              <https://www.rfc-editor.org/info/rfc8571>.

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

Authors' Addresses

   Zafar Ali
   Cisco Systems

   Email: [email-masked]

   Clarence Filsfils
   Cisco Systems

   Email: [email-masked]

   Satoru Matsushima
   Softbank

   Email: [email-masked]

   Daniel Voyer
   Bell Canada

   Email: [email-masked]

   Mach Chen
   Huawei

   Email: [email-masked]

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