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RFC 7054

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Addressing Requirements and Design Considerations for Per-Interface Maintenance Entity Group Intermediate Points (MIPs)


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Internet Engineering Task Force (IETF)                         A. Farrel
Request for Comments: 7054                              Juniper Networks
Category: Informational                                          H. Endo
ISSN: 2070-1721                                            Hitachi, Ltd.
                                                               R. Winter
                                                                Y. Koike
                                                                 M. Paul
                                                        Deutsche Telekom
                                                           November 2013

         Addressing Requirements and Design Considerations for
   Per-Interface Maintenance Entity Group Intermediate Points (MIPs)


   The framework for Operations, Administration and Maintenance (OAM)
   within the MPLS Transport Profile (MPLS-TP) describes how the
   Maintenance Entity Group Intermediate Points (MIPs) may be situated
   within network nodes at incoming and outgoing interfaces.

   This document elaborates on important considerations for internal MIP
   addressing.  More precisely, it describes important restrictions for
   any mechanism that specifies a way of forming OAM messages so that
   they can be targeted at MIPs on either incoming or outgoing
   interfaces and forwarded correctly through the forwarding engine.
   Furthermore, the document includes considerations for node
   implementations where there is no distinction between the incoming
   and outgoing MIP.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Page 2 
Copyright Notice

   Copyright (c) 2013 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
   ( 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
   2. Terminology .....................................................3
   3. Summary of the Problem Statement ................................3
   4. Requirements and Design Considerations for Internal-MIP
      Addressing ......................................................6
   5. Security Considerations ........................................10
   6. Acknowledgments ................................................10
   7. References .....................................................10
      7.1. Normative References ......................................10
      7.2. Informative References ....................................11

1.  Introduction

   The framework for Operations, Administration and Maintenance (OAM)
   within the MPLS Transport Profile (MPLS-TP)(the MPLS-TP OAM
   Framework, [RFC6371]) distinguishes two configurations for the
   Maintenance Entity Group Intermediate Points (MIPs) on a node.  It
   defines per-node MIPs and per-interface MIPs, where a per-node MIP is
   a single MIP per node in an unspecified location within the node and
   per-interface MIPs are two (or more) MIPs per node on each side of
   the forwarding engine.

   In-band OAM messages are sent using the Generic Associated Channel
   (G-ACh) [RFC5586].  OAM messages for the transit points of
   pseudowires (PWs) or Label Switched Paths (LSPs) are delivered using
   the expiration of the MPLS shim header Time-to-Live (TTL) field.  OAM
   messages for the end points of PWs and LSPs are simply delivered as

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   OAM messages delivered to end points or transit points are
   distinguished from other (data) packets so that they can be processed
   as OAM.  In LSPs, the mechanism used is the presence of the Generic
   Associated Channel Label (GAL) in the Label Stack Entry (LSE) under
   the top LSE [RFC5586].  In PWs, the mechanism used is the presence of
   the PW Associated Channel Header (PWACH) [RFC4385] or the presence of
   a GAL [RFC6423].

   If multiple MIPs are present on a single node, these mechanisms alone
   provide no way to address one particular MIP out of the set of MIPs.
   A mechanism that addresses this shortcoming has to obey a few
   important design considerations, which are discussed in this

2.  Terminology

   In this document, we use the term in-MIP (incoming MIP) to refer to
   the MIP that processes OAM messages before they pass through the
   forwarding engine of a node.  An out-MIP (outgoing MIP) processes OAM
   messages after they have passed the forwarding engine of the node.
   The two together are referred to as internal MIPs.  The term
   "forwarding engine" is used as defined in [RFC6371].

   Note that the acronym "OAM" is used in conformance with [RFC6291].

3.  Summary of the Problem Statement

   Figure 1 shows an abstract functional representation of an MPLS-TP
   node.  It is decomposed as an incoming interface (labeled "In"), a
   forwarding engine (FW), and an outgoing interface (labeled "Out").
   As per the discussion in [RFC6371], MIPs may be placed in each of the
   functional interface components.

                           |-----              -----|
                           | MIP |            | MIP |
                           |     |    ----    |     |
                    ----->-| In  |->-| FW |->-| Out |->----
                           | i/f |    ----    | i/f |
                           |-----              -----|

      Figure 1: Abstract Functional Representation of an MPLS-TP Node

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   Several distinct OAM functions are required within this architectural
   model for both PWs and LSPs, such as:

   o  Connectivity Verification (CV) between a Maintenance Entity Group
      End Point (MEP) and a MIP

   o  traceroute over an MPLS-TP LSP and/or an MPLS-TP PW containing

   o  data-plane loopback configuration at a MIP

   o  diagnostic tests

   The MIPs in these OAM functions may also be the MIPs at the incoming
   or outgoing interfaces.

   Per-interface MIPs have the advantage that they enable a more
   accurate localization and identification of faults and diagnostic
   tests.  In particular, the identification of whether a problem is
   located between nodes or on a particular node and where on that node
   is greatly enhanced.  For obvious reasons, it is important to narrow
   down the cause of a fault quickly to initiate a timely and well-
   directed maintenance action to resume normal network operation.

   The following two figures illustrate the fundamental difference
   between using per-node and per-interface MEPs and MIPs for OAM.
   Figure 2 depicts OAM using per-node MIPs and MEPs.  For reasons of
   exposition, we pick a location for the MIPs on the nodes but the
   standard does not mandate the exact location for the per-node model.
   In the figure, a bidirectional LSP is depicted where in the forward
   (FWD) direction MEP1, MIP1, and MEP2 are located on the ingress
   interface.  In the backward (BWD) direction MEP1', MIP1' and MEP2'
   are located on the egress interface, i.e., the same interfaces.  S1
   in the figure denotes the segment from PE1 In to P1 In and S2 denotes
   the segment from PE1 In to P2 In.  Figure 3, on the other hand, shows
   the same basic network, but per-interface maintenance points are
   configured for OAM operations.  Note that these figures are merely
   examples.  It is important to note that per-interface MEPs or per-
   interface MIPs must logically be placed at a point before (for
   in-MIP) or after (for out-MIP) passing the forwarding engine as
   defined in [RFC6371].  All traffic associated with the MEP/MIP must
   pass through or be terminated at that point.

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         Customer|           Operator's Administrative     | Customer
         Domain  |           Domain                        | Domain
         ------> |<--------------------------------------->| <------
           CE1   |   T-PE/PE1      S-PE/P1        T-PE/PE2 |   CE2
                 |  <-------->    <-------->    <--------> |
          +---+  | +-+ +-+ +-+   +-+ +-+ +-+   +-+ +-+ +-+ |  +---+
          |   |  | | | | | | |   | | | | | |   | | | | | | |  |   |
          |   |  | | | | | | |   | | | | | |   | | | | | | |  |   |
          +---+  | +-+ +-+ +-+   +-+ +-+ +-+   +-+ +-+ +-+ |  +---+
                 | In  FW  Out   In  FW  Out   In  FW  Out |
                 |                                         |
      FWD PW/LSP |  o-------------------------- >          |
                 |  V-------------*-------------V          |
                 | MEP1          MIP1          MEP2        |
      BWD PW/LSP |  <---------------------------o          |
                 |  V-------------*-------------V          |
                 |         MEP1'        MIP1'         MEP2'|

        Figure 2: Example of OAM Relying on Per-Node MIPs and MEPs

   To illustrate the difference between these two modes of operation, we
   use fault detection as an example.  Consider the case where the
   client traffic between CE1 and CE2 experiences a fault.  Also assume
   that an on-demand CV test between PE1 and PE2 was successful.  The
   scenario in Figure 2 therefore leaves the forwarding engine (FW) of
   PE2, the out-going interface of PE2, the transmission line between
   PE2 and CE2, or CE2 itself as a potential location of the fault as
   on-demand CV can only be performed on segment S2.  Note that in this
   scenario, the PWs or LSPs are to be understood as two examples (not
   one), i.e., the figures do not show the layer structure of PWs and

   The per-interface model in Figure 3 allows more fine-grained OAM
   operations to be performed.  At first, CV on segment S'4 and in
   addition CV on segment S'5 can help to rule out, for example, the
   forwarding engine of PE2.  This is of course only a single example,
   and other OAM functions and scenarios are trivially conceivable.  The
   basic message is that with the per-interface OAM model, an operator
   can configure smaller segments on a transport path to which OAM
   operations apply.  This enables a more fine-grained scoping of OAM
   operations, such as fault localization and performance monitoring,
   which gives operators better information to deal with adverse
   networking conditions.

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         Customer|          Operator's Administrative      |Customer
         Domain  |          Domain                         |Domain
           CE1   |   T-PE/PE1      S-PE/P1       T-PE/PE2  |   CE2
                 |  <-------->    <-------->    <--------> |
          +---+  | +-+ +-+ +-+   +-+ +-+ +-+   +-+ +-+ +-+ |  +---+
          |   |  | | | | | | |   | | | | | |   | | | | | | |  |   |
          |   |  | | | | | | |   | | | | | |   | | | | | | |  |   |
          +---+  | +-+ +-+ +-+   +-+ +-+ +-+   +-+ +-+ +-+ |  +---+
                 | In  FW  Out   In  FW  Out   In  FW  Out |
                 |                                         |
      FWD PW/LSP |  o----------------------------------->  |
                 |  V-------*------*------*-----*-------V  |
                 | MEP1    MIP1   MIP2   MIP3  MIP4    MEP2|
                 |                                         |
      BWD PW/LSP |  <-----------------------------------o  |
                 | MEP1'   MIP1'  MIP2'  MIP3' MIP4'  MEP2'|

      Figure 3: Example of OAM Relying on Per-Interface MIPs and MEPs

4.  Requirements and Design Considerations for Internal-MIP Addressing

   OAM messages for transit points of PWs or LSPs are delivered using
   the expiration of the TTL field in the top LSE of the MPLS packet
   header.  OAM messages for the end points of PWs and LSPs are simply
   delivered as normal.  These messages are distinguished from other
   (data) packets so that they can be processed as OAM.  In LSPs, the
   mechanism used is the presence of the GAL in the LSE under the top
   LSE [RFC5586].  In PWs, the mechanism used is the presence of the PW
   Associated Channel Header [RFC4385] or the presence of a GAL
   [RFC6423].  In addition, two sets of identifiers exist that can be
   used to address MIPs, which are defined in [RFC6370] and [RFC6923]

   Any solution for sending OAM messages to the in- and out-MIPs must
   fit within these existing models of handling OAM.

   Additionally, many MPLS-TP nodes are implemented in a way that all
   queuing and the forwarding function are performed at the incoming
   interface.  The abstract functional representation of such a node is
   shown in Figure 4.  As shown in the figure, the outgoing interfaces
   are minimal and for this reason it may not be possible to include

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   MIP functions on those interfaces.  This is the case for existing
   deployed implementations in particular.

   Any solution that attempts to send OAM messages to the outgoing
   interface of an MPLS-TP node must not cause any problems when such
   implementations are present (such as leaking OAM packets with a TTL
   of 0).
                         |------------         |
                         | MIP        |        |
                         |      ----  |        |
                  ----->-| In  | FW | |-->-Out-|->---
                         | i/f  ----  |    i/f |
                         |------------         |

              Figure 4: Abstract Functional Representation of
                        Some Existing MPLS-TP Nodes

   OAM must operate on MPLS-TP nodes that are branch points on point-to-
   multipoint (P2MP) trees.  This means that it must be possible to
   target individual outgoing MIPs as well as all outgoing MIPs in the
   abstract functional representation shown in Figure 5, and to handle
   the P2MP node implementations as shown in Figure 6 without causing
                       |                     -----|
                       |                    | MIP |
                       |                 ->-|     |->----
                       |                |   | Out |
                       |                |   | i/f |
                       |                |    -----|
                       |-----           |    -----|
                       | MIP |    ----  |   | MIP |
                       |     |   |    |-    |     |
                ----->-| In  |->-| FW |--->-| Out |->----
                       | i/f |   |    |-    | i/f |
                       |-----     ----  |    -----|
                       |                |    -----|
                       |                |   | MIP |
                       |                |   |     |
                       |                 ->-| Out |->----
                       |                    | i/f |
                       |                     -----|

            Figure 5: Abstract Functional Representation of an
                       MPLS-TP Node Supporting P2MP

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                         |               ->-Out-|->----
                         |              |   i/f |
                         |------------  |       |
                         |            | |       |
                         | MIP  ----  | |       |
                         |     |    | |-        |
                  ----->-| In  | FW | |--->-Out-|->----
                         | i/f |    | |-    i/f |
                         |      ----  | |       |
                         |            | |       |
                         |------------  |       |
                         |              |   Out |
                         |               ->-i/f-|->----

              Figure 6: Abstract Functional Representation of
                Some Existing MPLS-TP Nodes Supporting P2MP

   In summary, the solution for OAM message delivery must behave as

   o  Delivery of OAM messages to the correct MPLS-TP node.

   o  Delivery of OAM instructions to the correct MIP within an MPLS-TP

   o  Forwarding of OAM packets exactly as data packets.

   o  Packet inspection at the incoming and outgoing interfaces must be

   The first and second bullet points are obvious.  However, the third
   bullet point is also vital.  To illustrate the importance, a rejected
   solution is depicted in Figure 7.  In the figure, all data and non-
   local OAM is handled as normal.  Local OAM is intercepted at the
   incoming interface and delivered to the MIP at the incoming
   interface.  If the OAM is intended for the incoming MIP, it is
   handled there with no issue.  If the OAM is intended for the outgoing
   MIP, it is forwarded to that MIP using some internal messaging system
   that is implementation specific.

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                          |-----              -----|
         local OAM ----->-| MIP |----->------| MIP |
                          |     |    ----    |     |
              data =====>=| In  |=>=| FW |=>=| Out |=>==== data
     non-local OAM ~~~~~>~| i/f |~>~|    |~>~| i/f |~>~~~~ non-local OAM
                          |-----     ----     -----|

             Figure 7: OAM Control Message Delivery Bypassing
                           the Forwarding Engine

   This solution is fully functional for the incoming MIP.  It also
   supports control of data loopback for the outgoing MIP and can
   adequately perform some OAM features such as interface identity
   reporting at the outgoing MIP.

   However, because the OAM message is not forwarded through the
   forwarding engine, this solution cannot correctly perform OAM
   loopback, connectivity verification, LSP tracing, or performance

   The last bullet point is also an important requirement for any
   solution to the internal-MIP addressing problem.  Since OAM packets
   that target an out-MIP need to be sent through the forwarding engine
   and treated exactly as regular data packets, the determination of
   whether to forward the packet or process it at the incoming MIP needs
   to be fast; therefore, the processing overhead must be kept to a
   minimum.  In addition, there are a few OAM procedures that operate at
   line rate such as OAM loopback.  This adds to the requirement of
   minimal processing overhead for both the in-MIP and out-MIP.

   Most of the above superficially appears to be an implementation
   matter local to an individual node; however, the format of the
   message needs to be standardized so that:

   o  A MEP can correctly target the outgoing MIP of a specific MPLS-TP

   o  A node can correctly filter out any OAM messages that were
      intended for its upstream neighbor's outgoing MIP, but which were
      not handled there because the upstream neighbor is an
      implementation as shown in Figures 4 and 6.

   Note that the last bullet point describes a safety net in case OAM
   messages leak beyond their intended scope, but implementations should
   take care that messages do not leak so that the safety net does not
   need to be used.

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5.  Security Considerations

   OAM security is discussed in [RFC6371] and security aspects specific
   to MPLS-TP in general are outlined in [RFC6941].

   OAM can provide useful information for detecting and tracing security

   OAM can also be used to illicitly gather information or for denial-
   of-service attacks and other types of attack.  Implementations
   therefore are required to offer security mechanisms for OAM.
   Deployments are therefore strongly advised to follow the security
   advice provided in RFCs 6371 and 6941.

   Mixing of per-node and per-interface OAM on a single node is not
   advised as OAM message leakage could be the result.

6.  Acknowledgments

   The authors gratefully acknowledge the substantial contributions of
   Zhenlong Cui.  We would also like to thank Eric Gray, Sami Boutros,
   and Shahram Davari for interesting input to this document through

7.  References

7.1.  Normative References

   [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
              "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
              Use over an MPLS PSN", RFC 4385, February 2006.

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586, June 2009.

   [RFC6370]  Bocci, M., Swallow, G., and E. Gray, "MPLS Transport
              Profile (MPLS-TP) Identifiers", RFC 6370, September 2011.

   [RFC6371]  Busi, I., Ed., and D. Allan, Ed., "Operations,
              Administration, and Maintenance Framework for MPLS-Based
              Transport Networks", RFC 6371, September 2011.

   [RFC6423]  Li, H., Martini, L., He, J., and F. Huang, "Using the
              Generic Associated Channel Label for Pseudowire in the
              MPLS Transport Profile (MPLS-TP)", RFC 6423, November

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   [RFC6923]  Winter, R., Gray, E., van Helvoort, H., and M. Betts,
              "MPLS Transport Profile (MPLS-TP) Identifiers Following
              ITU-T Conventions", RFC 6923, May 2013.

7.2.  Informative References

   [RFC6291]  Andersson, L., van Helvoort, H., Bonica, R., Romascanu,
              D., and S. Mansfield, "Guidelines for the Use of the "OAM"
              Acronym in the IETF", BCP 161, RFC 6291, June 2011.

   [RFC6941]  Fang, L., Ed., Niven-Jenkins, B., Ed., Mansfield, S., Ed.,
              and R. Graveman, Ed., "MPLS Transport Profile (MPLS-TP)
              Security Framework", RFC 6941, April 2013.

Authors' Addresses

   Adrian Farrel
   Juniper Networks


   Hideki Endo
   Hitachi, Ltd.


   Rolf Winter


   Yoshinori Koike


   Manuel Paul
   Deutsche Telekom