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IETF RFC 5718
An In-Band Data Communication Network For the MPLS Transport Profile
Last modified on Thursday, January 7th, 2010
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Internet Engineering Task Force (IETF) D. Beller
Request for Comments: 5718 Alcatel-Lucent
Category: Standards Track A. Farrel
ISSN: 2070-1721 Old Dog Consulting
January 2010
An In-Band Data Communication Network For the MPLS Transport Profile
 Abstract
The Generic Associated Channel (G-ACh) has been defined as a
generalization of the pseudowire (PW) associated control channel to
enable the realization of a control/communication channel that is
associated with Multiprotocol Label Switching (MPLS) Label Switched
Paths (LSPs), MPLS PWs, MPLS LSP segments, and MPLS sections between
adjacent MPLS-capable devices.
The MPLS Transport Profile (MPLS-TP) is a profile of the MPLS
architecture that identifies elements of the MPLS toolkit that may be
combined to build a carrier-grade packet transport network based on
MPLS packet switching technology.
This document describes how the G-ACh may be used to provide the
infrastructure that forms part of the Management Communication
Network (MCN) and a Signaling Communication Network (SCN).
Collectively, the MCN and SCN may be referred to as the Data
Communication Network (DCN). This document explains how MCN and SCN
messages are encapsulated, carried on the G-ACh, and demultiplexed
for delivery to the management or signaling/routing control plane
components on an MPLS-TP node.
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/RFC 5718.
Beller & Farrel Standards Track PAGE 1 
RFC 5718 DCN for MPLS Transport Profile January 2010
Copyright Notice
Copyright (c) 2010 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.
1. Introduction
The associated channel header (ACH) is specified in [RFC 4385]. It is
a packet header format for use on pseudowires (PWs) in order to
identify packets used for Operations, Administration, and Maintenance
(OAM) and similar functions.
The use of the ACH is generalized in [RFC 5586] and can be applied on
any Multiprotocol Label Switching (MPLS) Label Switching Path (LSP).
This is referred to as the Generic Associated Channel (G-ACh) and is
intended to create a control/management communication channel
associated with the LSP that can be used to carry packets used for
OAM and similar functions (e.g., control/management plane messages).
The purpose of a packet carried on the G-ACh is indicated by the
value carried by the Channel Type field of the ACH and a registry of
values is maintained by IANA ([RFC 4446] and [RFC 4385]). The ACH is
referred to in this document as the G-ACh header.
The MPLS transport profile (MPLS-TP) is described in [MPLS-TP] and in
[RFC 5654]. MPLS-TP is the application of MPLS to construct a packet
transport network. It constitutes a profile of MPLS that enables
operational models typical in transport networks, which includes
additional OAM, survivability, and other maintenance functions not
previously supported by MPLS.
Label Switching Routers (LSRs) in MPLS networks may be operated using
management protocols or control plane protocols. Messaging in these
protocols is normally achieved using IP packets exchanged over IP-
capable interfaces. However, some nodes in MPLS-TP networks may be
constructed without support for direct IP encapsulation on their
line-side interfaces and without access to an out-of-fiber data
Beller & Farrel Standards Track PAGE 2
RFC 5718 DCN for MPLS Transport Profile January 2010
communication network. In order that such nodes can communicate
using management plane or control plane protocols, channels must be
provided, and the only available mechanism is to use an MPLS label.
The G-ACh provides a suitable mechanism for this purpose, and this
document defines processes and procedures to allow the G-ACh to be
used to build a Management Communication Network (MCN) and a
Signaling Communication Network (SCN), together known as the Data
Communication Network (DCN) [G.7712].
It should be noted that the use of the G-ACh to provide connectivity
for the DCN is intended for use only where the MPLS-TP network is not
capable of encapsulating or delivering native DCN messages.
1.1. Requirements
The requirements presented in this section are based on those
communicated to the IETF by the ITU-T.
1. A packet-encapsulation mechanism must be provided to support the
transport of MCN and SCN packets over the G-ACh.
2. The G-ACh carrying the MCN and SCN packets shall support the
following application scenarios:
a. The G-ACh interconnects two adjacent MPLS-TP nodes (used when
the server layer does not provide a Management Communication
Channel (MCC) or a Signalling Communication Channel (SCC)).
b. The G-ACh is carried by an MPLS-TP tunnel that traverses
another operator's domain (the carrier's carrier scenario).
3. The G-ACh shall provide two independent channels: an MCC to build
the MCN and an SCC to build the SCN. The G-ACh packet header
shall indicate whether the packet is an MCC or an SCC packet in
order to forward it to the management or control plane application
for processing. This facilitates easy demultiplexing of control
and management traffic from the DCN, and enables separate or
overlapping address spaces and duplicate protocol instances in the
management and control planes.
4. The channel-separation mechanism shall not preclude the use of
separate rate limiters and traffic-shaping functions for each
channel (MCC and SCC), ensuring that the flows do not exceed their
assigned traffic profiles. The rate limiters and traffic shapers
are outside the scope of the MCC and SCC definitions.
Beller & Farrel Standards Track PAGE 3
RFC 5718 DCN for MPLS Transport Profile January 2010
5. The G-ACh that carries the MCC and SCC shall be capable of
carrying different OSI layer 3 (network layer) PDUs. These shall
include IPv4, IPv6, and OSI PDUs. The G-ACh header of the MCC/SCC
packet shall indicate which layer 3 PDU is contained in the
payload field of the packet such that the packet can be delivered
to the related layer 3 process within the management and control
plane application, respectively, for further processing.
6. The G-ACh is not required to provide specific security mechanisms.
However, the management or control plane protocols that operate
over the MCC or SCC are required to provide adequate security
mechanisms in order to not be susceptible to security attacks.
1.2. Conventions Used in This Document
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 RFC 2119
[RFC 2119].
2. Procedures
Figure 1 depicts the format of an MCC/SCC packet that is sent on
the G-ACh. The Channel Type field indicates the function of the
ACH message and so, to send an MCC/SCC packet on the G-ACh, the
MCC/SCC message is prepended with an ACH with the Channel Type set
to indicate that the message is an MCC or SCC message. The ACH
MUST NOT include the ACH TLV Header [RFC 5586], meaning that no ACH
TLVs can be included in the message. A two-byte Protocol
Identifier (PID) field indicates the protocol type of the payload
DCN message.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1|Version| Reserved | Channel Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PID | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| MCC/SCC Message |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: G-ACh MCC/SCC Packet
o The Channel Type field determines whether the message is an MCC or
an SCC message. See Section 5 for the codepoint assignments.
Beller & Farrel Standards Track PAGE 4
RFC 5718 DCN for MPLS Transport Profile January 2010
o The presence of the PID field is deduced from the Channel Type
value indicating MCC or SCC. The field contains an identifier of
the payload protocol using the PPP protocol identifiers ([RFC 1661],
[RFC 3818]).
When the G-ACh sender receives an MCC message that is to be sent over
the MCC, the sender creates the G-ACh header, sets the Channel Type
field to MCC, fills in the PID to indicate the MCC layer 3 PDU type,
and prepends the MCC message with the G-ACh header. The same
procedure is applied when a control plane message is to be sent over
the SCC. In this case, the sender sets the Channel Type field to
SCC.
If the G-ACh is associated with an MPLS section, the Generic
Associated Channel Label (GAL) is added to the message as defined in
[RFC 5586]. The Time to Live (TTL) field MUST be set to 1, and the
S-bit of the GAL MUST be set to 1.
If the G-ACh is associated with an LSP, the GAL is added to the
packet and the LSP label is pushed on top of the GAL as defined in
[RFC 5586]. The TTL field of the GAL MUST be set to 1, and the S-bit
of the GAL MUST be set to 1.
Note that packet processing for DCN packets in the G-ACh is, in
common with all G-ACh MPLS packets, subject to the normal processing
of the Traffic Class (TC) field of the MPLS header. This could be
used to enable prioritization of different DCN packets.
The DCN channel MUST NOT be used to transport user traffic and SHALL
only be used to carry management or control plane messages.
Procedures that ensure this (such as deep packet inspection) are
outside the scope of this specification.
When a receiver has received a packet on the G-ACh with the ACH
Channel Type set to MCC or SCC, it SHALL look at the PID field. If
the PID value is known by the receiver, it delivers the MCC/SCC
message to the appropriate processing entity. If the PID value is
unknown, the receiver SHALL silently discard the received packet, MAY
increment a counter that records discarded or errored messages, and
MAY log an event.
It must be noted that according to [RFC 5586], a receiver MUST NOT
forward a GAL packet based on the GAL label as is normally the case
for MPLS packets. If the GAL appears at the bottom of the label
stack, it MUST be processed as described in the previous paragraph.
Beller & Farrel Standards Track PAGE 5
RFC 5718 DCN for MPLS Transport Profile January 2010
Note that there is no requirement for MPLS-TP devices to support IP
or OSI forwarding in the fast (forwarding) path. Thus, if a message
is received on the MCC or SCC and is not targeted to an address of
the receiving MPLS-TP node, the packet might not be forwarded in the
fast path. A node MAY apply layer 3 forwarding procedures in the
slow or fast path and MAY discard or reject the message using the
layer 3 protocol if it is unable to forward it. Thus, protocols
making use of the DCN should make no assumptions about the forwarding
capabilities unless they are determined a priori or through the use
of a routing protocol. Furthermore, it is important that user data
(i.e., data traffic) is not routed through the DCN, as this would
potentially cause the traffic to be lost or delayed and might
significantly congest the DCN.
2.1. Pseudowire Setup
Provider Edge nodes (PEs) may wish to set up PWs using a signaling
protocol that uses remote adjacencies (such as LDP [RFC 5036]). In
the absence of an IP-based control plane network, these PEs MUST
first set up an LSP tunnel across the MPLS-TP network. This tunnel
can be used both to carry the PW once it has been set up and to
provide a G-ACh-based DCN for control plane communications between
the PEs.
3. Applicability
The DCN is intended to provide connectivity between management
stations and network nodes, and between pairs of network nodes, for
the purpose of exchanging management plane and control plane
messages.
Appendix A of [NM-REQ] describes how Control Channels (CCh) that are
the links in an MPLS-TP DCN can be out-of-fiber and out-of-band, in-
fiber and out-of-band, or in-band with respect to the user data
carried by the MPLS-TP network. That appendix also explains how the
DCN can be constructed from a mix of different types of links and how
routing and forwarding can be used within the DCN to facilitate
multi-hop delivery of management and control plane messages.
The G-ACh used as described in this document allows the creation of a
"data channel associated CCh" (type 6 in Appendix A of [NM-REQ]) and
an "in-band CCh" (type 7 in Appendix A of [NM-REQ]). In the former
case, the G-ACh is associated with an MPLS-TP section. In the latter
case, the G-ACh is associated with an MPLS-TP LSP or PW and may span
one or more hops in the MPLS-TP network.
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RFC 5718 DCN for MPLS Transport Profile January 2010
There is no need to create a CCh for every LSP between a pair of
MPLS-TP nodes. Indeed, where the nodes are physically adjacent, the
G-ACh associated with the MPLS-TP section would normally be used.
Where nodes are virtually adjacent (that is, connected by LSP
tunnels), one or two of the LSPs might be selected to provide the CCh
and a back-up CCh.
4. Security Considerations
The G-ACh provides a virtual link between MPLS-TP nodes and might be
used to induce many forms of security attack. The MPLS data plane
does not include any security mechanisms of its own; therefore, it is
important that protocols using the DCN apply their own security.
Protocols that operate over the MCN or SCN are REQUIRED to include
adequate security mechanisms, and implementations MUST allow
operators to configure the use of those mechanisms.
5. IANA Considerations
Channel Types for the Generic Associated Channel are allocated from
the IANA PW Associated Channel Type registry defined in [RFC 4446] and
updated by [RFC 5586].
IANA has allocated two further Channel Types as follows:
0x0001 Management Communication Channel (MCC)
0x0002 Signaling Communication Channel (SCC)
6. References
6.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
Use over an MPLS PSN", RFC 4385, February 2006.
[RFC 4446] Martini, L., "IANA Allocations for Pseudowire Edge to Edge
Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
[RFC 5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
"MPLS Generic Associated Channel", RFC 5586, June 2009.
6.2. Informative References
[G.7712] ITU-T Recommendation G.7712, "Architecture and
specification of data communication network", June 2008.
Beller & Farrel Standards Track PAGE 7
RFC 5718 DCN for MPLS Transport Profile January 2010
[MPLS-TP] Bocci, M., Bryant, S., Frost, D., and L. Levrau, "A
Framework for MPLS in Transport Networks", Work in
Progress, October 2009.
[NM-REQ] Lam, K. and S. Mansfield, "MPLS TP Network Management
Requirements", Work in Progress, October 2009.
[RFC 1661] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, July 1994.
[RFC 3818] Schryver, V., "IANA Considerations for the Point-to-Point
Protocol (PPP)", BCP 88, RFC 3818, June 2004.
[RFC 5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, October 2007.
[RFC 5654] Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,
Sprecher, N., and S. Ueno, "Requirements of an MPLS
Transport Profile", RFC 5654, September 2009.
7. Acknowledgements
The editors wish to thank Pietro Grandi, Martin Vigoureux, Kam Lam,
Ben Niven-Jenkins, Francesco Fondelli, Walter Rothkegel, Shahram
Davari, Liu Guoman, and Alexander Vainshtein for their contribution
to this document, and the MEAD team for thorough review.
Study Group 15 of the ITU-T provided the basis for the requirements
text in Section 1.1.
Authors' Addresses
Dieter Beller
Alcatel-Lucent Germany
EMail: dieter.beller@alcatel-lucent.com
Adrian Farrel
Old Dog Consulting
EMail: adrian@olddog.co.uk
Beller & Farrel Standards Track PAGE 8
An In-Band Data Communication Network For the MPLS Transport Profile
RFC TOTAL SIZE: 18997 bytes
PUBLICATION DATE: Thursday, January 7th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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