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RFC 7212 - MPLS Generic Associated Channel (G-ACh) Advertisement

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Internet Engineering Task Force (IETF)                          D. Frost
Request for Comments: 7212                                      Blue Sun
Category: Standards Track                                      S. Bryant
ISSN: 2070-1721                                            Cisco Systems
                                                                M. Bocci
                                                               June 2014

     MPLS Generic Associated Channel (G-ACh) Advertisement Protocol


   The MPLS Generic Associated Channel (G-ACh) provides an auxiliary
   logical data channel associated with a Label Switched Path (LSP), a
   pseudowire, or a section (link) over which a variety of protocols may
   flow.  These protocols are commonly used to provide Operations,
   Administration, and Maintenance (OAM) mechanisms associated with the
   primary data channel.  This document specifies simple procedures by
   which an endpoint of an LSP, pseudowire, or section may inform the
   other endpoints of its capabilities and configuration parameters, or
   other application-specific information.  This information may then be
   used by the receiver to validate or adjust its local configuration,
   and by the network operator for diagnostic purposes.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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

Copyright Notice

   Copyright (c) 2014 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Requirements Language . . . . . . . . . . . . . . . . . .   6
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Message Format  . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  GAP Message Format  . . . . . . . . . . . . . . . . . . .   8
     3.2.  Applications Data Block . . . . . . . . . . . . . . . . .   9
     3.3.  TLV Object Format . . . . . . . . . . . . . . . . . . . .  10
   4.  G-ACh Advertisement Protocol TLVs . . . . . . . . . . . . . .  10
     4.1.  Source Address TLV  . . . . . . . . . . . . . . . . . . .  11
     4.2.  GAP Request TLV . . . . . . . . . . . . . . . . . . . . .  11
     4.3.  GAP Flush TLV . . . . . . . . . . . . . . . . . . . . . .  12
     4.4.  GAP Suppress TLV  . . . . . . . . . . . . . . . . . . . .  13
     4.5.  GAP Authentication TLV  . . . . . . . . . . . . . . . . .  14
   5.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Message Transmission  . . . . . . . . . . . . . . . . . .  14
     5.2.  Message Reception . . . . . . . . . . . . . . . . . . . .  15
   6.  Message Authentication  . . . . . . . . . . . . . . . . . . .  16
     6.1.  Authentication Key Identifiers  . . . . . . . . . . . . .  16
     6.2.  Authentication Process  . . . . . . . . . . . . . . . . .  17
     6.3.  MAC Computation . . . . . . . . . . . . . . . . . . . . .  18
   7.  Link-Layer Considerations . . . . . . . . . . . . . . . . . .  18
   8.  Manageability Considerations  . . . . . . . . . . . . . . . .  19
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
     10.1.  Associated Channel Type Allocation . . . . . . . . . . .  20
     10.2.  Allocation of Address Family Numbers . . . . . . . . . .  20
     10.3.  Creation of G-ACh Advertisement Protocol Application
            Registry . . . . . . . . . . . . . . . . . . . . . . . .  20
     10.4.  Creation of G-ACh Advertisement Protocol TLV Registry  .  21
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     12.2.  Informative References . . . . . . . . . . . . . . . . .  22

1.  Introduction

   The MPLS Generic Associated Channel (G-ACh) is defined and described
   in [RFC5586].  It provides an auxiliary logical data channel over
   which a variety of protocols may flow.  Each such data channel is
   associated with an MPLS Label Switched Path (LSP), a pseudowire, or a
   section (link).  An important use of the G-ACh and the protocols it
   supports is to provide Operations, Administration, and Maintenance
   (OAM) [RFC6291] capabilities for the associated LSP, pseudowire, or
   section.  Examples of such capabilities include Pseudowire Virtual
   Circuit Connectivity Verification (VCCV) [RFC5085]; Bidirectional
   Forwarding Detection (BFD) for MPLS [RFC5884]; and MPLS packet loss,
   delay, and throughput measurement [RFC6374]; as well as OAM functions
   developed for the MPLS Transport Profile (MPLS-TP) [RFC5921].

   This document specifies procedures for an MPLS Label Switching Router
   (LSR) to advertise its capabilities and configuration parameters, or
   other application-specific information, to its peers over LSPs,
   pseudowires, and sections.  Receivers can then make use of this
   information to validate or adjust their own configurations, and
   network operators can make use of it to diagnose faults and
   configuration inconsistencies between endpoints.  Note that in this
   document the term "application" refers to an application that uses
   the protocol defined herein (and hence operates over the G-ACh), and
   it should not be confused with an end-user application.

   The main principle guiding the design of the MPLS G-ACh Advertisement
   Protocol (GAP) is simplicity.  The protocol provides a one-way method
   of distributing information about the sender.  How this information
   is used by a given receiver is a local matter.  The data elements
   distributed by the GAP are application specific and, except for those
   associated with the GAP itself, are outside the scope of this
   document.  An IANA registry has been created to allow GAP
   applications to be defined as needed.

   The assignment of application identifiers and associated GAP
   parameters for protocols other than the GAP itself is outside the
   scope of this document.  Such assignments can be made in subsequent
   documents according to the IANA considerations specified here.

1.1.  Motivation

   It is frequently useful in a network for a node to have general
   information about its adjacent nodes, i.e., those nodes to which it
   has links.  At a minimum, this allows a human operator or management
   application with access to the node to determine which adjacent nodes
   this node can see; this is helpful when troubleshooting connectivity
   problems.  A typical example of an "adjacency awareness protocol" is

   the Link Layer Discovery Protocol [LLDP], which can provide various
   pieces of information about adjacent nodes in Ethernet networks, such
   as system name, basic functional capabilities, link speed/duplex
   settings, and maximum supported frame size.  Such data is useful both
   for human diagnostics and for automated detection of configuration

   In MPLS networks, the G-ACh provides a convenient link-layer-agnostic
   means for communication between LSRs that are adjacent at the link
   layer.  The G-ACh advertisement protocol presented in this document
   thus allows LSRs to exchange information of a similar sort to that
   supported by LLDP for Ethernet links.  The GAP, however, does not
   depend on the specific link-layer protocol in use, and it can be used
   to advertise information on behalf of any MPLS application.

   In networks based on the MPLS Transport Profile (MPLS-TP) [RFC5921]
   that do not also support IP, the normal protocols used to determine
   the Ethernet address of an adjacent MPLS node, such as the Address
   Resolution Protocol [RFC0826] and IP version 6 Neighbor Discovery
   [RFC4861], are not available.  One possible use of the G-ACh
   advertisement protocol is to discover the Ethernet media access
   control addresses of MPLS-TP nodes lacking IP capability [RFC7213].
   However, where it is anticipated that the only data that needs to be
   exchanged between LSRs over an Ethernet link are their Ethernet
   addresses, then the operator may instead choose to use LLDP for that

   The applicability of the G-ACh advertisement protocol is not limited
   to link-layer adjacency, either in terms of message distribution or
   message content.  The G-ACh exists for any MPLS LSP or pseudowire, so
   GAP messages can be exchanged with remote LSP or pseudowire
   endpoints.  The content of GAP messages is extensible in a simple
   manner and can include any kind of information that might be useful
   to MPLS LSRs connected by links, LSPs, or pseudowires.  For example,
   in networks that rely on the G-ACh for OAM functions, GAP messages
   might be used to inform adjacent LSRs of a node's OAM capabilities
   and configuration parameters.

1.2.  Terminology

   Term  Definition
   ----- -------------------------------------------
   G-ACh Generic Associated Channel
   GAL   G-ACh Label
   GAP   G-ACh Advertisement Protocol
   LSP   Label Switched Path
   OAM   Operations, Administration, and Maintenance

1.3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

2.  Overview

   The G-ACh Advertisement Protocol has a simple one-way mode of
   operation: a device configured to send information for a particular
   data channel (MPLS LSP, pseudowire, or section) transmits GAP
   messages over the G-ACh associated with the data channel.  The
   payload of a GAP message is a collection of Type-Length-Value (TLV)
   objects, organized on a per-application basis.  An IANA registry has
   been created to identify specific applications.  Application TLV
   objects primarily contain static data that the receiver is meant to
   retain for a period of time, but they may also represent metadata or
   special processing instructions.

   Each GAP message can contain data for several applications.  A sender
   may transmit a targeted update that refreshes the data for a subset
   of applications without affecting the data of other applications sent
   in a previous message.  GAP messages are processed in the order in
   which they are received.

   For example, a GAP message might be sent containing the following

      Application A: A-TLV4, A-TLV15, A-TLV9

      Application B: B-TLV1, B-TLV3

      Application C: C-TLV6,

   where the TLVx refers to an example GAP TLV.

   A second message might then be sent containing:

      Application B: B-TLV7, B-TLV3

   Upon receiving the second message, the receiver retains B-TLV1 from
   the first message and adds B-TLV7 to its B-database.  How it handles
   the new B-TLV3 depends on the rules B has specified for this object
   type; this object could replace the old one or be combined with it in
   some way.  The second message has no effect on the databases
   maintained by the receiver for Applications A and C.

   The rate at which GAP messages are transmitted is at the discretion
   of the sender and may fluctuate over time as well as differ per
   application.  Each message contains, for each application it
   describes, a lifetime that informs the receiver how long to wait
   before discarding the data for that application.

   The GAP itself provides no fragmentation and reassembly mechanisms.
   In the event that an application wishes to send larger chunks of data
   via GAP messages than fall within the limits of packet size, it is
   the responsibility of the application to fragment its data
   accordingly.  It is the responsibility of the application and the
   network operator to ensure that the use of the GAP does not congest
   the link to the peer.

   The GAP is designed to run over a unidirectional channel.  However,
   where the channel is bidirectional, communication may be optimized
   through the use of a number of messages defined for transmission from
   the receiver back to the sender.  These are optimizations and are not
   required for protocol operation.

3.  Message Format

   An Associated Channel Header (ACH) Channel Type has been allocated
   for the GAP as follows:

   Protocol                           Channel Type
   ---------------------------------- ------------
   G-ACh Advertisement Protocol       0x0059

   For this Channel Type, as noted in [RFC7026], the ACH SHALL NOT be
   followed by the ACH TLV Header defined in [RFC5586].

   Fields in this document shown as Reserved or Resv are reserved for
   future specification and MUST be set to zero.  All integer values for
   fields defined in this document SHALL be encoded in network byte

   A GAP message consists of a fixed header followed by a GAP payload.
   The payload of a GAP message is an Application Data Block (ADB)
   consisting of one or more block elements.  Each block element
   contains an application identifier, a lifetime, and a series of zero
   or more TLV objects for the application it describes.

   Malformed GAP messages MUST be discarded by the receiver, although an
   error MAY be logged.  If the error is logged remotely, a suitable
   form of rate limiting SHOULD be used to prevent excessive logging
   messages being transmitted over the network.

   Implementations of this protocol version MUST set reserved fields in
   the message formats that follow to all zero bits when sending and
   ignore any value when receiving messages.

3.1.  GAP Message Format

   The following figure shows the format of a G-ACh Advertisement
   Protocol message, which follows the Associated Channel Header (ACH):

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |Version|       Reserved        |        Message Length         |
       |                        Message Identifier                     |
       |                           Timestamp                           |
       |                                                               |
       ~                    Application Data Block (ADB)               ~

                       Figure 1: GAP Message Format

   The meanings of the fields are:

      Version (4 bits): Protocol version.  This is set to zero.

      Reserved (12 bits): MUST be sent as zero.

      Message Length (16 bits): Size in octets of this message, i.e., of
      the portion of the packet following the Associated Channel Header.

      Message Identifier (MI) (32 bits): Unique identifier of this
      message.  For disambiguation, a sender MUST NOT reuse an MI over a
      given channel until it is confident that all ADBs associated with
      it have been expired by the receiver.  The sole purpose of this
      field is duplicate detection in the event of a message burst
      (Section 5.1).

      Timestamp: 64-bit Network Time Protocol (NTP) transmit timestamp,
      as specified in Section 6 of [RFC5905].

3.2.  Applications Data Block

   An ADB consists of one or more elements of the following format:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |        Application ID         |        Element Length         |
       |            Lifetime           |           Reserved            |
       ~                          TLV Object                           ~
       ~                          TLV Object                           ~
       .                                                               .
       .                                                               .
       .                                                               .

                 Figure 2: Application Data Block Element

      Application ID (16 bits): Identifies the application this element
      describes; an IANA registry has been created to track the values
      for this field.  More than one block element with the same
      Application ID may be present in the same ADB, and block elements
      with different Application IDs may also be present in the same
      ADB.  The protocol rules for the mechanism, including what ADB
      elements are present and which TLVs are contained in an ADB
      element, are to be defined in the document that specifies the
      application-specific usage.

      Element Length (16 bits): Specifies the total length in octets of
      this block element (including the Application ID and Element
      Length fields).

      Lifetime field (16 bits): Specifies how long, in seconds, the
      receiver should retain the data in this message (i.e., it
      specifies the lifetime of the static data carried in the TLV set
      of this ADB).  For TLVs not carrying static data, the Lifetime is
      of no significance.  The sender of a GAP message indicates this by
      setting the Lifetime field to zero.  If the Lifetime is zero, TLVs
      in this ADB are processed by the receiver, and the data associated
      with these TLV types is immediately marked as expired.  If the ADB
      contains no TLVs, the receiver expires all data associated with
      TLVs previously sent to this application.

   The remainder of the Application Data Block element consists of a
   sequence of zero or more TLV objects that use the format defined in
   Section 3.3.

   The scope of an ADB element is an application instance attached to a
   specific channel between a specific source-destination pair, and the

   Lifetime field specifies the lifetime of the ADB element data in that
   specific context.

3.3.  TLV Object Format

   GAP TLV objects use the following format:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |      Type     |    Reserved   |            Length             |
       ~                             Value                             ~

                        Figure 3: TLV Object Format

      Type (8 bits): Identifies the TLV Object and is scoped to a
      specific application; each application creates an IANA registry to
      track its Type values.

      Reserved (8 bits): MUST be sent as zero.

      Length (16 bits): The length in octets of the Value field.  The
      Value field need not be padded to provide alignment.

   GAP messages do not contain a checksum.  If validation of message
   integrity is desired, the authentication procedures in Section 6
   should be used.

4.  G-ACh Advertisement Protocol TLVs

   The GAP supports several TLV objects related to its own operation via
   the Application ID 0x0000.  These objects represent metadata and
   processing instructions rather than static data that is meant to be
   retained.  When an ADB element for the GAP is present in a GAP
   message, it MUST precede other elements.  This is particularly
   important for the correct operation of the Flush message
   (Section 4.3).

   Any application using the GAP inherits the ability to use facilities
   provided by Application 0x0000.

   Application 0x0000 GAP messages MUST be processed in the order in
   which they are received.

4.1.  Source Address TLV

   The Source Address object identifies the sending device and possibly
   the transmitting interface and the channel; it has the following

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |     Type=0    |    Reserved   |            Length             |
       |      Reserved (16 bits)       |    Address Family (16 bits)   |
       ~                            Address                            ~

                    Figure 4: Source Address TLV Format

   The Address Family field indicates the type of the address; it SHALL
   be set to one of the assigned values in the IANA "Address Family
   Numbers" registry.

   In IP networks, a Source Address SHOULD be included in GAP messages
   and set to an IP address of the sending device; when the channel is a
   link, this address SHOULD be an address of the transmitting

   In non-IP MPLS-TP networks, a Source Address SHOULD be included in
   GAP messages and set to the endpoint identifier of the channel.  The
   formats of these channel identifiers SHALL be as given in Sections
   3.5.1, 3.5.2, and 3.5.3 of [RFC6428] (excluding the initial Type and
   Length fields shown in those sections).  IANA has allocated Address
   Family Numbers for these identifiers; see Section 10.2.

   On multipoint channels, a Source Address TLV is REQUIRED.

4.2.  GAP Request TLV

   This object is a request by the sender for the receiver to transmit
   an immediate unicast GAP update to the sender.  If the Length field
   is zero, this signifies that an update for all applications is

   requested.  Otherwise, the Value field specifies the applications for
   which an update is requested, in the form of a sequence of
   Application IDs:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |     Type=1    |    Reserved   |            Length             |
       |        Application ID 1       |        Application ID 2       |
       .                                                               .
       .                                                               .
       .                                                               .
       |        Application ID N-1     |        Application ID N       |

                     Figure 5: GAP Request TLV Format

   The intent of this TLV is to request the immediate transmission of
   data following a local event such as a restart, rather than waiting
   for a periodic update.  Applications need to determine what
   information is meaningful to send in response to such a request.  The
   inclusion of an Application ID in a Request TLV does not guarantee
   that the response will provide information for that application.  The
   responder may also include information for applications not included
   in the request.  A receiver SHOULD discard GAP Request messages that
   arrive at a rate in excess of that which is considered reasonable for
   the application.

   For an Application ID 0x0000 GAP Request, it is meaningful to respond
   with the Source Address.

   This TLV is considered to be part of the GAP and thus does not need
   to be retained.  The reception of the TLV may however be recorded for
   management purposes.

4.3.  GAP Flush TLV

   This object is an instruction to the receiver to flush the GAP data
   for all applications associated with this (sender, channel) pair.  It
   is a null object, i.e., its Length is set to zero.

   The GAP Flush instruction does not apply to data contained in the
   message carrying the GAP Flush TLV object itself.  Any application
   data contained in the same message SHALL be processed and retained by
   the receiver as usual.

   The Flush TLV type is 2.

   This TLV is considered to be part of the GAP and thus does not need
   to be retained.  The reception of the TLV may however be recorded for
   management purposes.

4.4.  GAP Suppress TLV

   This object is a request to the receiver to cease sending GAP updates
   to the transmitter over the current channel for the specified
   duration.  Duration is a 16-bit non-negative integer in units of
   seconds.  The receiver MAY accept and act on the request, MAY ignore
   the request, or MAY resume transmissions at any time according to
   implementation or configuration choices, and depending on local
   pragmatics.  The format of this object is as follows:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |     Type=3    |    Reserved   |            Length             |
       |           Duration            |        Application ID 1       |
       .                                                               .
       .                                                               .
       .                                                               .
       |        Application ID N-1     |        Application ID N       |

                     Figure 6: GAP Suppress TLV Format

   If the Length is set to 2, i.e., if the list of Application IDs is
   empty, then suppression of all GAP messages is requested; otherwise,
   suppression of only those updates pertaining to the listed
   applications is requested.  A duration of zero cancels any existing
   suppress requests for the listed applications.

   This object makes sense only for point-to-point channels or when the
   sender is receiving unicast GAP updates.

4.5.  GAP Authentication TLV

   This object is used to provide authentication and integrity
   validation for a GAP message.  It has the following format:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |     Type=4    |    Reserved   |            Length             |
       |          Reserved             |            Key ID             |
       ~                    Authentication Data                        ~

                  Figure 7: GAP Authentication TLV Format

   The data and procedures associated with this object are explained in
   Section 6.

5.  Operation

5.1.  Message Transmission

   G-ACh Advertisement Protocol message transmission SHALL operate on a
   per-data-channel basis and be configurable by the operator

   Because GAP message transmission may be active for many logical
   channels on the same physical interface, message transmission timers
   SHOULD be randomized across the channels supported by a given
   interface so as to reduce the likelihood of large synchronized
   message bursts.

   The Message Identifier (MI) uniquely identifies this message and its
   value is set at the sender's discretion.  It MUST NOT be assumed to
   be a sequence number.  The scope of an MI is a channel between a
   specific source-destination pair.

   The Timestamp field SHALL be set to the time at which this message is

   The Lifetime field of each Application Data Block element SHALL be
   set to the number of seconds the receiver is advised to retain the
   data associated with this message and application.

   When the transmitter wishes the data previously sent in an ADB
   element to persist, then it must refresh the ADB element by sending
   another update.  Refresh times SHOULD be set in such a way that at
   least three updates will be sent prior to Lifetime expiration.  For
   example, if the Lifetime is set to 210 seconds, then updates should
   be sent at least once every 60 seconds.

   A sender may signal that previously sent data SHOULD be marked as
   expired by setting the ADB element lifetime to zero as previously
   described in Section 3.

   In some cases, an application may desire additional reliability for
   the delivery of some of its data.  When this is the case, the
   transmitter MAY send several (for example, three) instances of the
   message in succession, separated by a delay appropriate to, or
   specified by, the application.  For example, this procedure might be
   invoked when sending a Flush instruction following device reset.  The
   expectation is that the receiver will detect duplicate messages using
   the MI.

5.2.  Message Reception

   G-ACh Advertisement Protocol message reception SHALL operate on a
   per-data-channel basis and be configurable by the operator

   Upon receiving a G-ACh Advertisement Protocol message that contains
   data for some application X, the receiver determines whether it can
   interpret X-data.  If it cannot, then the receiver MAY retain this
   data for the number of seconds specified by the Lifetime field;
   although it cannot parse this data, it may still be of use to the

   If the receiver can interpret X-data, then it processes the data
   objects accordingly, retaining the data associated with those that
   represent static data for the number of seconds specified by the
   Lifetime field.  If the Lifetime is zero, such data is immediately
   marked as expired, and, if no TLVs are specified, all data associated
   with previously received TLVs is marked as expired (Section 3).  If
   one of the received TLV objects has the same Type as a previously
   received TLV, then the data from the new object SHALL replace the
   data associated with that Type unless the X specification dictates a
   different behavior.

   The received data is made available to local applications that
   require it and are locally authorized to view it.  The method for
   doing this is local to the receiver and outside the scope of this

   The receiver MAY make use of the application data contained in a GAP
   message to perform some level of auto-configuration, for example, if
   the application is an OAM protocol.  The application SHOULD, however,
   take care to prevent cases of oscillation resulting from each
   endpoint attempting to adjust its configuration to match the other.
   Any such auto-configuration based on GAP information MUST be disabled
   by default.

   The MI may be used to detect and discard duplicate messages.

6.  Message Authentication

   The GAP provides a means of authenticating messages and ensuring
   their integrity.  This is accomplished by attaching a GAP
   Authentication TLV and including, in the Authentication Data field,
   the output of a cryptographic hash function (known as a Message
   Authentication Code (MAC)), the input to which is the message
   together with a secret key known only to the sender and receiver.
   Upon receipt of the message, the receiver computes the same MAC and
   compares the result with the MAC in the message; if the MACs are not
   equal, the message is discarded.  Use of GAP message authentication

   The remainder of this section gives the details of this procedure,
   which is based on the procedures for generic cryptographic
   authentication for the Intermediate System to Intermediate System
   (IS-IS) routing protocol as described in [RFC5310].

6.1.  Authentication Key Identifiers

   An Authentication Key Identifier (Key ID) is a 16-bit tag shared by
   the sender and receiver that identifies a set of authentication
   parameters.  These parameters are not sent over the wire; they are
   assumed to be associated, on each node, with the Key ID by external
   means, such as via explicit operator configuration or a separate key-
   exchange protocol.  Multiple Key IDs may be active on the sending and
   receiving nodes simultaneously, in which case the sender locally
   selects a Key ID from this set to use in an outbound message.  This
   capability facilitates key migration in the network.

   The parameters associated with a Key ID are:

   o  Authentication Algorithm: This signifies the authentication
      algorithm to use to generate or interpret authentication data.  At
      present, the following values MAY be supported: HMAC-SHA-1, HMAC-
      SHA-256.  HMAC-SHA-1 MUST be supported.

   o  Authentication Keystring: A secret octet string that forms the
      basis for the cryptographic key used by the Authentication
      Algorithm.  It SHOULD NOT be a human-memorable string.
      Implementations MUST be able to use random binary values of the
      appropriate length as a keystring.

   Implementers SHOULD consider the use of [RFC7210] for key management.
   If used, authenticated information sent over the GAP MUST only
   considered valid if it was sent during the Keying and Authentication
   for Routing Protocols (KARP) interval between SendLifetimeStart and
   SendLifeTimeEnd.  However, if the GAP TLV used to send it expires
   before the KARP SendLifetimeStart, then information is never used; if
   it expires before KARP SendNotAfter, the key becomes invalid on
   expiry of the GAP TLV.

   At the time of this writing, mechanisms for dynamic key management in
   the absence of IP are not available.  Key management in such
   environments therefore needs to take place via the equipment
   management system or some other out-of-band service.  The MPLS layer
   in a network is normally isolated from direct access by users and
   thus is a relatively protected environment.  Therefore, key turnover
   is expected to be a relatively infrequent event.

6.2.  Authentication Process

   The authentication process for GAP messages is straightforward.
   First, a Key ID is associated on both the sending and receiving nodes
   with a set of authentication parameters.  Following this, when the
   sender generates a GAP message, it sets the Key ID field of the GAP
   Authentication TLV accordingly.  (The length of the Authentication
   Data field is also known at this point because it is a function of
   the Authentication Algorithm.)  The sender then computes a MAC for
   the message as described in Section 6.3 and fills the Authentication
   Data field of the GAP Authentication TLV with the MAC, overwriting
   the zeros used in computation.  The message is then sent.

   When the message is received, the receiver computes a MAC for it as
   described below, again setting the Authentication Data field of the
   GAP Authentication TLV to all zeros before computing the MAC.  The
   receiver compares its computed MAC to the MAC received in the
   Authentication Data field.  If the two MACs are equal, authentication
   of the message is considered to have succeeded; otherwise, it is
   considered to have failed.

   This process suffices to ensure the authenticity and integrity of
   messages but is still vulnerable to a replay attack, in which a third
   party captures a message and sends it on to the receiver at some
   later time.  The GAP message header contains a Timestamp field, which

   can be used to protect against replay attacks.  To achieve this
   protection, the receiver checks that the time recorded in the
   Timestamp field of a received and authenticated GAP message
   corresponds to the current time, within a reasonable tolerance that
   allows for message propagation delay, and it accepts or rejects the
   message accordingly.  Clock corrections SHOULD be monotonic to avoid
   replay attacks, unless operator intervention overrides the monotonic
   configuration setting to achieve a faster convergence with current

   If the clocks of the sender and receiver are not synchronized with
   one another, then the receiver must perform the replay check against
   its best estimate of the current time according to the sender's
   clock.  The timestamps that appear in GAP messages can be used to
   infer the approximate clock offsets of senders, and, while this does
   not yield high-precision clock synchronization, it suffices for
   purposes of the replay check with an appropriately chosen tolerance.

6.3.  MAC Computation

   The HMAC procedure described in [RFC2104] is used to compute the MAC.

   The Authentication Data field of the GAP Authentication TLV is set to
   all zeros.  The MAC is then computed over the entire GAP message as
   shown in Figure 1.

   Where there is less data than is needed for the MAC computation, a
   value of zero MUST be used.

   The length of the Authentication Data field is always less than or
   equal to the message digest size of the specific hash function that
   is being used.  However, the implementer needs to consider that
   although MAC truncation decreases the size of the message, it results
   in a corresponding reduction in the strength of the assurance

   MAC truncation is NOT RECOMMENDED.

7.  Link-Layer Considerations

   When the GAP is used to support device discovery on a data link, GAP
   messages must be sent in such a way that they can be received by
   other listeners on the link without the sender first knowing the
   link-layer addresses of the listeners.  In short, they must be
   multicast.  Considerations for multicast MPLS encapsulation are
   discussed in [RFC5332].  For example, Section 8 of [RFC5332]
   describes how destination Ethernet MAC addresses are selected for
   multicast MPLS packets.  Since a GAP packet transmitted over a data

   link contains just one label, the G-ACh Label (GAL) with label value
   13, the correct destination Ethernet address for frames carrying GAP
   packets intended for device discovery, according to these selection
   procedures, is 01-00-5e-80-00-0d.

8.  Manageability Considerations

   The data sent and received by this protocol MUST be made accessible
   for inspection by network operators, and where local configuration is
   updated by the received information, it MUST be clear why the
   configured value has been changed.  This allows the operator to
   determine the operational parameters currently in use and to
   understand when local configuration has been superseded by inbound
   parameters received from its peer.

   In the event of a system restart, any GAP application data and peer
   state data that has been retained as a consequence of prior
   advertisements from GAP peers MUST be discarded; this prevents
   incorrect operation on the basis of stale data.

   All GAP applications MUST be disabled by default and need to be
   enabled by the operator if required.

9.  Security Considerations

   G-ACh Advertisement Protocol messages contain information about the
   sending device and its configuration, which is sent in cleartext over
   the wire.  If an unauthorized third party gains access to the MPLS
   data plane or the lower network layers between the sender and
   receiver, it can observe this information.  In general, however, the
   information contained in GAP messages is no more sensitive than that
   contained in other protocol messages, such as routing updates, which
   are commonly sent in cleartext.  No attempt is therefore made to
   guarantee confidentiality of GAP messages.  Therefore, the GAP MUST
   NOT be used to send TLVs in cleartext where the value concerned
   requires confidentiality, for example, GAP or application TLVs
   containing 'bare' cryptographic keying material.  Applications that
   require confidentiality will need to implement a suitable
   confidentiality method.

   A more significant potential threat is the transmission of GAP
   messages by unauthorized sources, or the unauthorized manipulation of
   messages in transit; this can disrupt the information receivers hold
   about legitimate senders.  To protect against this threat, message
   authentication procedures (specified in Section 6) enable receivers
   to ensure the authenticity and integrity of GAP messages.  These

   procedures include the means to protect against replay attacks in
   which a third party captures a legitimate message and "replays" it to
   a receiver at some later time.

10.  IANA Considerations

10.1.  Associated Channel Type Allocation

   IANA has allocated an entry in the "MPLS Generalized Associated
   Channel (G-ACh) Types (including Pseudowire Associated Channel
   Types)" registry for the "G-ACh Advertisement Protocol", as follows:

   Value  Description                  Reference
   ------ ---------------------------- ---------
   0x0059 G-ACh Advertisement Protocol This RFC

   The reader should note that the "TLV Follows" column in the registry
   has been deleted [RFC7026].

10.2.  Allocation of Address Family Numbers

   IANA has allocated three entries from the Standards Track range in
   the "Address Family Numbers" registry for MPLS-TP Section, LSP, and
   Pseudowire endpoint identifiers, per Section 4.1.  The allocations

   Number Description                            Reference
   ------ -------------------------------------- ---------
   26     MPLS-TP Section Endpoint Identifier    This RFC
   27     MPLS-TP LSP Endpoint Identifier        This RFC
   28     MPLS-TP Pseudowire Endpoint Identifier This RFC

10.3.  Creation of G-ACh Advertisement Protocol Application Registry

   IANA has created a new registry, "G-ACh Advertisement Protocol
   Application Registry" in the "Generic Associated Channel (G-ACh)
   Parameters" registry, with fields and initial allocations as follows:

   Application ID Description                  Reference
   -------------- ---------------------------- ---------
   0x0000         G-ACh Advertisement Protocol This RFC

   The range of the Application ID field is 0x0000 - 0xFFFF.

   The allocation policy for this registry is IETF Review.

10.4.  Creation of G-ACh Advertisement Protocol TLV Registry

   IANA has created a new registry, "G-ACh Advertisement Protocol: GAP
   TLV Objects (Application ID 0)" in the "Generic Associated Channel
   (G-ACh) Parameters" registry, with fields and initial allocations as

   Type Name          Type ID Reference
   ------------------ ------- ---------
   Source Address     0       This RFC
   GAP Request        1       This RFC
   GAP Flush          2       This RFC
   GAP Suppress       3       This RFC
   GAP Authentication 4       This RFC

   The range of the Type ID field is 0 - 255.

   The allocation policy for this registry is IETF Review.

11.  Acknowledgements

   We thank Adrian Farrel for his valuable review comments on this

12.  References

12.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5332]  Eckert, T., Rosen, E., Aggarwal, R., and Y. Rekhter, "MPLS
              Multicast Encapsulations", RFC 5332, August 2008.

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

   [RFC5905]  Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
              Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, June 2010.

   [RFC6428]  Allan, D., Swallow Ed. , G., and J. Drake Ed. , "Proactive
              Connectivity Verification, Continuity Check, and Remote
              Defect Indication for the MPLS Transport Profile", RFC
              6428, November 2011.

   [RFC7210]  Housley, R., Polk, T., Hartman, S., and D. Zhang,
              "Database of Long-Lived Symmetric Cryptographic Keys", RFC
              7210, April 2014.

12.2.  Informative References

   [LLDP]     IEEE, "Station and Media Access Control Connectivity
              Discovery", IEEE 802.1AB, September 2009.

   [RFC0826]  Plummer, D., "Ethernet Address Resolution Protocol: Or
              converting network protocol addresses to 48.bit Ethernet
              address for transmission on Ethernet hardware", STD 37,
              RFC 826, November 1982.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC5085]  Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit
              Connectivity Verification (VCCV): A Control Channel for
              Pseudowires", RFC 5085, December 2007.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, February 2009.

   [RFC5884]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
              "Bidirectional Forwarding Detection (BFD) for MPLS Label
              Switched Paths (LSPs)", RFC 5884, June 2010.

   [RFC5921]  Bocci, M., Bryant, S., Frost, D., Levrau, L., and L.
              Berger, "A Framework for MPLS in Transport Networks", RFC
              5921, July 2010.

   [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.

   [RFC6374]  Frost, D. and S. Bryant, "Packet Loss and Delay
              Measurement for MPLS Networks", RFC 6374, September 2011.

   [RFC7026]  Farrel, A. and S. Bryant, "Retiring TLVs from the
              Associated Channel Header of the MPLS Generic Associated
              Channel", RFC 7026, September 2013.

   [RFC7213]  Frost, D., Bryant, S., and M. Bocci, "MPLS-TP Next-Hop
              Ethernet Addressing", RFC 7213, June 2014.

Authors' Addresses

   Dan Frost
   Blue Sun

   EMail: frost@mm.st

   Stewart Bryant
   Cisco Systems

   EMail: stbryant@cisco.com

   Matthew Bocci

   EMail: matthew.bocci@alcatel-lucent.com


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