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Versions: (draft-fbb-mpls-gach-adv) 00 01 02 03 04 05 06 07 08 RFC 7212

MPLS                                                       D. Frost, Ed.
Internet-Draft                                            S. Bryant, Ed.
Intended status: Standards Track                           Cisco Systems
Expires: July 30, 2012                                     M. Bocci, Ed.
                                                          Alcatel-Lucent
                                                        January 27, 2012


     MPLS Generic Associated Channel (G-ACh) Advertisement Protocol
                      draft-ietf-mpls-gach-adv-00

Abstract

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

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

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

   This Internet-Draft will expire on July 30, 2012.

Copyright Notice

   Copyright (c) 2012 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



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


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
   2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Message Format . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  G-ACh Advertisement Protocol TLVs  . . . . . . . . . . . . . .  8
     4.1.  Identifier TLVs  . . . . . . . . . . . . . . . . . . . . .  8
     4.2.  GAP Request TLV  . . . . . . . . . . . . . . . . . . . . .  8
     4.3.  GAP Flush TLV  . . . . . . . . . . . . . . . . . . . . . .  9
     4.4.  GAP Suppress TLV . . . . . . . . . . . . . . . . . . . . .  9
     4.5.  GAP Authentication TLV . . . . . . . . . . . . . . . . . .  9
   5.  Operation  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  G-ACh Advertisement Message Transmission . . . . . . . . . 10
     5.2.  G-ACh Advertisement Message Reception  . . . . . . . . . . 11
   6.  Message Authentication . . . . . . . . . . . . . . . . . . . . 11
     6.1.  Authentication Key Identifiers . . . . . . . . . . . . . . 11
     6.2.  Authentication Process . . . . . . . . . . . . . . . . . . 12
     6.3.  Hash Computation . . . . . . . . . . . . . . . . . . . . . 13
   7.  Link-Layer Considerations  . . . . . . . . . . . . . . . . . . 14
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     10.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17













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1.  Introduction

   The MPLS Generic Associated Channel (G-ACh) is defined and described
   in [RFC5586].  It provides an auxiliary logical data channel
   associated with an MPLS Label Switched Path (LSP), a pseudowire, or a
   section (link) over which a variety of protocols may flow.  A primary
   purpose of the G-ACh and the protocols it supports is to provide
   Operations, Administration, and Maintenance (OAM) capabilities
   associated with the underlying 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.

   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 is created to allow GAP data elements to
   be defined as needed.

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, which 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
   inconsistencies.

   In MPLS networks, the G-ACh provides a convenient link-layer-agnostic



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   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.

   An important special case arises in networks based on the MPLS
   Transport Profile (MPLS-TP) [RFC5921] that do not also support IP:
   without IP, protocols for determining 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.
   The G-ACh advertisement protocol can be used to discover the Ethernet
   MAC addresses of MPLS nodes lacking IP capability
   [I-D.fbb-mpls-tp-ethernet-addressing].

   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
   LSR   Label Switching Router
   OAM   Operations, Administration, and Maintenance

1.3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


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



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   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 is
   created to identify specific applications.

   Although one GAP message can contain data for several applications,
   the receiver maintains the data associated with each application
   separately.  This enables the sender to transmit a targeted update
   that refreshes the data for a subset of applications without
   affecting the data of other applications.

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

      Application A: A-TLV1, A-TLV2, A-TLV3

      Application B: B-TLV1

      Application C: C-TLV1, C-TLV2

   A second message might then be sent containing:

      Application B: B-TLV1, B-TLV2

   Upon receiving the second message, the receiver flushes the old data
   for Application B and replaces it with the new data.  The data
   associated with Applications A and C from the first message is
   retained.  In other words, the GAP update granularity is per-
   application, not per-message or per-TLV-object.

   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.


3.  Message Format

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




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

   For this Channel Type, 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
   order.

   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 TLV
   objects for the application it describes.

   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)               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            GAP Message Format

   The meanings of the fields are:

      Version: Protocol version, currently set to 0

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

      Message Identifier: Unique identifier of this message

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

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



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        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                           ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                                                               .
       .                                                               .

                      Application Data Block Element

   In this format, the Application ID identifies the application this
   element describes; an IANA registry has been created to track the
   values for this field.  Any two ADB elements in the same ADB SHALL
   have distinct Application IDs.  The Element Length field specifies
   the total length in octets of this block element.  The Lifetime field
   specifies how long, in seconds, the receiver should retain the data
   in this message.

   The remainder of the Application Data Block element consists of a
   sequence of TLV objects, which are of the form:

        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                             ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                             TLV Object Format

   The Type field identifies the TLV Object; an IANA registry has been
   created to track the values for this field, which are defined on a
   per-application basis.  The Length field specifies the length in
   octets of the Value field.

   It is permissible for the sequence of TLV objects in an ADB element
   to be empty.  This is useful in conjunction with setting the Lifetime
   to zero in order to instruct the receiver to flush all data
   associated with this application.




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   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.  When an ADB element for the GAP is
   present in a GAP message, it MUST precede other elements.

4.1.  Identifier TLVs

   The following TLV objects are defined for purposes of conveying
   identification information associated with the transmitting device
   and the data channel:

   o  Interface Identifier TLV

   o  LSP Identifier TLV

   o  Pseudowire Path Identifier TLV

   The Value portion of these identifier objects follows the format of
   the respective identifier as defined in [RFC6370].

   The LSP and Pseudowire Path Identifiers SHOULD be present in GAP
   messages transmitted over LSPs and pseudowires, respectively, and
   MUST NOT be present for other data channel types.  The Interface
   Identifier SHOULD be present in GAP messages transmitted over data-
   link sections.

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:











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        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             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |        Application ID 1       |        Application ID 2       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |        Application ID N-1     |        Application ID N       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          GAP Request TLV Format

4.3.  GAP Flush TLV

   This object is an instruction to the receiver to flush the GAP data
   for all applications.  It is a null object, i.e. its Length is set to
   zero.  Note that data for a specific application can be flushed by
   sending an update for the application with the Lifetime 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.

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.  The object format is
   the same as the GAP Request TLV object.  If the Length is set to
   zero, suppression of all GAP messages is requested; otherwise
   suppression of only those updates pertaining to applications listed
   in the Value field is requested.

   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:







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        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             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Reserved             |            Key ID             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                    Authentication Data                        ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       GAP Authentication TLV Format

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


5.  Operation

5.1.  G-ACh Advertisement Message Transmission

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

   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 uniquely identifies this message and is set at
   the sender's discretion.  The Timestamp field SHALL be set to the
   time at which this message is transmitted.

   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.

   Lifetimes SHOULD be set in such a way that at least three updates
   will be sent prior to Lifetime expiration.  For example, if updates
   are sent at least every 60 seconds, a Lifetime of 185 seconds may be
   used.

   In some cases additional reliability may be desired for the delivery
   of a GAP message.  When this is the case, the RECOMMENDED procedure
   is to send three instances of the message in succession, separated by
   a delay appropriate to the application.  This procedure SHOULD be
   used, if at all, only for messages that are in some sense



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   'exceptional'; for example when sending a flush instruction following
   device reset.

5.2.  G-ACh Advertisement Message Reception

   Upon receiving a G-ACh Advertisement Protocol message containing data
   for a set of applications, the receiver MUST discard any earlier data
   retained for each application in the set, and SHOULD retain the new
   data associated with each application in the set by this message for
   the number of seconds specified by the Lifetime field, or until a
   newer message describing the application is received.

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


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, 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 hash and compares the result with the hash value in the message;
   if the hash values are not equal, the message is discarded.

   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.



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   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 are possible: HMAC-SHA-1, HMAC-SHA-
      224, HMAC-SHA- 256, HMAC-SHA-384, and HMAC-SHA-512.

   o  Authentication Keystring: A secret string that forms the basis for
      the crytographic key used by the Authentication Algorithm.

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 hash for
   the message as described below, and fills the Authentication Data
   field of the GAP Authentication TLV with the hash value.  The message
   is then sent.

   When the message is received, the receiver computes a hash for it as
   described below.  The receiver compares its computed value to the
   hash value received in the Authentication Data field.  If the two
   hash values 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 accepts or rejects the
   message accordingly.

   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.




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6.3.  Hash Computation

   In the algorithm description below, the following nomenclature, which
   is consistent with [FIPS-198], is used:

   Symbol         Definition
   -------------- ------------------------------------------------------
   H              The specific hash algorithm, e.g.  SHA-256
   K              The Authentication Keystring
   Ko             The cryptographic key used with the hash algorithm
   B              The block size of H, measured in octets rather than in
                  bits.  Note that B is the internal block size, not the
                  hash size.  This is equal to 64 for SHA-1 and SHA-256,
                  and to 128 for SHA-384 and SHA-512.
   L              The length of the hash, measured in octets rather than
                  in bits
   XOR            The exclusive-or operation
   Opad           The hexadecimal value 0x5c repeated B times
   Ipad           The hexadecimal value 0x36 repeated B times
   Apad           hexadecimal value 0x878FE1F3 repeated (L/4) times

   1.  Preparation of the Key

          In this application, Ko is always L octets long.

          If the Authentication Keystring (K) is L octets long, then Ko
          is equal to K. If the Authentication Keystring (K) is more
          than L octets long, then Ko is set to H(K).  If the
          Authentication Keystring (K) is less than L octets long, then
          Ko is set to the Authentication Keystring (K) with zeros
          appended to the end of the Authentication Keystring (K) such
          that Ko is L octets long.

   2.  First Hash

          First, the Authentication Data field is filled with the value
          Apad.

          Then, a first hash, also known as the inner hash, is computed
          as follows:

             First-Hash = H(Ko XOR Ipad || (GAP Message))

          Here the GAP Message is the portion of the packet that follows
          the Associated Channel Header.






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   3.  Second Hash

          Then a second hash, also known as the outer hash, is computed
          as follows:

             Second-Hash = H(Ko XOR Opad || First-Hash)

   4.  Result

          The resulting second hash becomes the authentication data that
          is sent in the Authentication Data field of the GAP
          Authentication TLV.  The length of the Authentication Data
          field is always identical to the message digest size of the
          specific hash function H that is being used.

          This also means that the use of hash functions with larger
          output sizes will increase the size of the GAP message as
          transmitted on the wire.


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.  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.




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   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 are specified in this document that 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.


9.  IANA Considerations

   This document requests that IANA allocate an entry in the Pseudowire
   Associated Channel Types registry [RFC5586] for the G-ACh
   Advertisement Protocol, as follows:

   Value Description                  TLV Follows Reference
   ----- ---------------------------- ----------- ------------
   (TBD) G-ACh Advertisement Protocol No          (this draft)

   This document also requests that IANA create a new registry, "G-ACh
   Advertisement Protocol Applications and Data Types", with fields and
   initial allocations as follows:

   Application Description         Type Name            Type   Reference
   ID                                                   ID
   ----------- ------------------- -------------------- ------ ---------
   0x0000      G-ACh Advertisement GAP Request          0      (this
               Protocol                                        draft)
                                   GAP Flush            1      (this
                                                               draft)
                                   GAP Suppress         2      (this
                                                               draft)
                                   GAP Authentication   3      (this
                                                               draft)

   The allocation policy for this registry is IETF Review.


10.  References

10.1.  Normative References

   [FIPS-198]
              US National Institute of Standards and Technology, "The
              Keyed-Hash Message Authentication Code (HMAC)", FIPS PUB
              198, March 2002.



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   [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.

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

10.2.  Informative References

   [I-D.fbb-mpls-tp-ethernet-addressing]
              Frost, D., Bryant, S., and M. Bocci, "MPLS-TP Next-Hop
              Ethernet Addressing",
              draft-fbb-mpls-tp-ethernet-addressing-01 (work in
              progress), November 2011.

   [LLDP]     IEEE, "Station and Media Access Control Connectivity
              Discovery (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.




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

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


Authors' Addresses

   Dan Frost (editor)
   Cisco Systems

   Email: danfrost@cisco.com


   Stewart Bryant (editor)
   Cisco Systems

   Email: stbryant@cisco.com


   Matthew Bocci (editor)
   Alcatel-Lucent

   Email: matthew.bocci@alcatel-lucent.com

























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