Internet Engineering Task Force (IETF) D. Frost

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
Alcatel-Lucent
June 2014

MPLS Generic Associated Channel (G-ACh) Advertisement Protocol

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 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
http://www.rfc-editor.org/info/rfc7212.

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RFC 7212 MPLS G-ACh Advertisement Protocol June 2014

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.

Frost, et al. Standards Track [Page 2]

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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

Frost, et al. Standards Track [Page 3]

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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 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

Frost, et al. Standards Track [Page 4]

RFC 7212 MPLS G-ACh Advertisement Protocol June 2014

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
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
purpose.

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

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1.3. Requirements Language

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

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.

Frost, et al. Standards Track [Page 6]

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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
order.

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.

Frost, et al. Standards Track [Page 7]

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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].

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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.

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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).

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RFC 7212 MPLS G-ACh Advertisement Protocol June 2014

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
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=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
interface.

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

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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.

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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.

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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
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 (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
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.

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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
accordingly.

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
operator.

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
document.

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RFC 7212 MPLS G-ACh Advertisement Protocol June 2014

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
is RECOMMENDED.

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.

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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

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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
time.

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
provided.

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

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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

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RFC 7212 MPLS G-ACh Advertisement Protocol June 2014

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
are:

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.

Frost, et al. Standards Track [Page 20]

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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
follows:

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
document.

12. References

12.1. Normative References

[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.

[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.

Frost, et al. Standards Track [Page 21]

RFC 7212 MPLS G-ACh Advertisement Protocol June 2014

[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.

Frost, et al. Standards Track [Page 22]

RFC 7212 MPLS G-ACh Advertisement Protocol June 2014

[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: [email protected]

Stewart Bryant
Cisco Systems

EMail: [email protected]

Matthew Bocci
Alcatel-Lucent

EMail: [email protected]

Frost, et al. Standards Track [Page 23]