MPLS Working Group M. Bocci, Ed.
Internet-Draft M. Vigoureux, Ed.
Updates: 3032, 4385, 5085 Alcatel-Lucent
(if approved) G. Swallow
Intended status: Standards Track D. Ward
Expires: August 27, 2009 S. Bryant
Cisco
R. Aggarwal
Juniper Networks
February 23, 2009
MPLS Generic Associated Channel header
draft-ietf-mpls-tp-gach-gal-02
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on August 27, 2009.
Copyright Notice
Copyright (c) 2009 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
Bocci, et al. Expires August 27, 2009 [Page 1]
Internet-Draft G-ACh and GAL February 2009
to this document.
Abstract
This document generalises the applicability of the pseudowire (PW)
Associated Channel Header (ACH), enabling the realization of a
control channel associated to MPLS Label Switched Paths (LSPs) and
MPLS Sections in addition to MPLS pseudowires. In order to identify
the presence of this Associated Channel Header in the label stack,
this document also assigns one of the reserved MPLS label values to
the Generic Associated channel Label (GAL), to be used as a label
based exception mechanism.
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 RFC 2119 [1].
Bocci, et al. Expires August 27, 2009 [Page 2]
Internet-Draft G-ACh and GAL February 2009
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Contributing Authors . . . . . . . . . . . . . . . . . . . 5
1.2. Objectives . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. Generic Associated Channel Header . . . . . . . . . . . . . . 6
2.1. Definition . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Allocation of Channel Types . . . . . . . . . . . . . . . 7
3. ACH TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. ACH TLV Payload Structure . . . . . . . . . . . . . . . . 8
3.2. ACH TLV Header . . . . . . . . . . . . . . . . . . . . . . 8
3.3. ACH TLV Object . . . . . . . . . . . . . . . . . . . . . . 9
4. Generalised Exception Mechanism . . . . . . . . . . . . . . . 9
4.1. Relationship with Existing MPLS OAM Alert Mechanisms . . . 10
4.2. GAL Applicability and Usage . . . . . . . . . . . . . . . 10
4.2.1. GAL Processing . . . . . . . . . . . . . . . . . . . . 10
4.3. Relationship wth RFC 3429 . . . . . . . . . . . . . . . . 13
5. Compatability . . . . . . . . . . . . . . . . . . . . . . . . 13
6. Congestion Considerations . . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
10.2. Informative References . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Bocci, et al. Expires August 27, 2009 [Page 3]
Internet-Draft G-ACh and GAL February 2009
1. Introduction
There is a need for Operations, Administration and Maintenance (OAM)
mechanisms that can be used for fault detection, diagnostics,
maintenance and other functions on a PW and an LSP. These functions
can be used between any two Label Edge Routers (LERs) / Label
Switching Router (LSRs) or Terminating Provider Edge routers (T-PEs)
/ Switching Provider Edge routers (S-PEs) along the path of an LSP or
PW respectively [15]. Some of these functions can be supported using
existing tools such as Virtual Circuit Connectivity Verification
(VCCV) [2], Bidirectional Forwarding Detection for MPLS LSPs (BFD-
MPLS)[3], LSP-Ping [4], or BFD-VCCV [5]. However, a requirement has
been indicated to augment this set of maintenance functions, in
particular when MPLS networks are used for packet transport services
and transport network operations [16]. Examples of these functions
include performance monitoring, automatic protection switching, and
support for management and signaling communication channels. These
tools MUST be applicable to, and function in essentially the same
manner (from an operational point of view) on MPLS PWs, MPLS LSPs and
MPLS Sections. They MUST also operate in-band on the PW or LSP such
that they do not depend on Packet Switched Network (PSN) routing or
on user data traffic, and MUST also not depend on dynamic control
plane functions.
VCCV can use an Associated Channel Header (ACH) to provide a PW-
associated control channel between a PW's end points, over which OAM
and other control messages can be exchanged. This document
generalises the use of the ACH to enable the same associated control
channel mechanism to be used for Sections, LSPs and PWs. The
associated control channel thus generalized is known as the Generic
Associated Channel (G-ACh). The ACH, specified in RFC 4385 [6], may
be used with additional code points to support additional MPLS
maintenance functions on the G-ACh.
Generalizing the associated control channel mechanism to LSPs and
Sections also requires a method to identify that a packet contains an
ACH followed by a non-service payload. Therefore, this document also
defines a label based exception mechanism that serves to inform an
LSR (or LER) that a packet it receives on an LSP or Section belongs
to an associated control channel for that LSP or Section.
RFC 4379 [4] and BFD-MPLS [3] define alert mechanisms that enable an
MPLS LSR to identify and process MPLS OAM packets when these are
encapsulated in an IP header. These alert mechanisms are based on
MPLS or PW label Time to Live (TTL) expiration and/or on the use of
an IP destination address in the range 127/8. These mechanisms are
the default mechanisms for identifying MPLS OAM packets when
encapsulated in an IP header. However it may not always be possible
Bocci, et al. Expires August 27, 2009 [Page 4]
Internet-Draft G-ACh and GAL February 2009
to use these mechanisms in some MPLS applications, e.g. MPLS
Transport Profile (MPLS-TP) [15], particularly when IP based
demultiplexing cannot be used. This document defines a mechanism
that is RECOMMENDED for identifying and encapsulating MPLS OAM and
other maintenance messages when IP based mechanisms such as those in
[4] and [3] are not available. This mechanism MAY be used in
addition to IP-based mechanisms.
The GAL mechanism is defined to work together with the ACH for LSPs
and MPLS Sections.
Note that, in this document, maintenance functions and packets should
be understood in the broad sense. That is, a set of maintenance and
management mechanisms that include OAM, Automatic Protection
Switching (APS), Signalling Communication Channel (SCC) and
Management Communication Channel (MCC) messages.
Also note that the GAL and ACH are applicable to MPLS in general.
Their applicability to specific applications of MPLS is outside the
scope of this document.
1.1. Contributing Authors
The editors gratefully acknowledge the contributions of Sami Boutros,
Italo Busi, Marc Lasserre, Lieven Levrau and Siva Sivabalan
1.2. Objectives
This document defines a mechanism that provides a solution to the
extended maintenance needs of emerging applications for MPLS. It
creates a generic control channel mechanism that may be applied to
MPLS LSPs and Sections, while maintaining compatibility with the PW
associated channel. It also normalises the use of the ACH for PWs in
a transport context, and defines a label based exception mechanism to
alert LERs/LSRs of the presence of an ACH after the bottom of the
stack.
1.3. Scope
This document defines the encapsulation header for LSP, MPLS Section
and PW associated channel messages.
It does not define how associated control channel capabilities are
signaled or negotiated between LERs/LSRs or PEs, or the operation of
various OAM functions.
This document does not deprecate existing MPLS and PW OAM mechanisms.
Bocci, et al. Expires August 27, 2009 [Page 5]
Internet-Draft G-ACh and GAL February 2009
1.4. Terminology
ACH: Associated Channel Header
G-ACh: Generic Associated Channel
GAL: G-ACh Label
Maintenance packet: Any packet containing a message belonging to a
maintenance protocol that is carried on a PW, LSP or MPLS Section
associated control channel. Examples of such maintenance protocols
include OAM functions, signaling communications or management
communications.
The terms 'Section' and 'Concatenated Segment' are defined in [17].
2. Generic Associated Channel Header
VCCV [2] defines three MPLS Control Channel (CC) Types that may be
used to exchange OAM messages through a PW: CC Type 1 uses an ACH and
is referred to as "In-band VCCV"; CC Type 2 uses the MPLS Router
Alert Label to indicate VCCV packets and is referred to as "Out of
Band VCCV"; CC Type 3 uses the TTL to force the packet to be
processed by the targeted router control plane and is referred to as
"MPLS PW Label with TTL == 1".
2.1. Definition
The use of the CC Type 1, previously limited to PWs, is here extended
to also apply to LSPs and to Sections. Note that for PWs, the PWE3
control word [6] MUST be present in the encapsulation of user packets
when the ACH is used to realize the associated control channel.
The CC Type 1 control channel header is depicted in figure below:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Associated Channel Header
In the above figure, the first nibble is set to 0001b to indicate a
control channel associated with a PW, an LSP or a Section. The
Version field is set to 0, as specified in RFC 4385 [6]. Bits 8 to
14 of the G-ACH are reserved and MUST be set to 0 and ignored on
Bocci, et al. Expires August 27, 2009 [Page 6]
Internet-Draft G-ACh and GAL February 2009
reception.
Note that VCCV also includes mechanisms for negotiating the Control
Channel and Connectivity Verification (i.e. OAM functions) Types
between PEs. It is anticipated that similar mechanisms will be
applied to LSPs. Such application will require further
specification. However, such specification is beyond the scope of
this document.
2.2. Allocation of Channel Types
The Channel Type field indicates the type of message carried on the
associated control channel e.g. IPv4 or IPv6 if IP demultiplexing is
used for messages sent on the associated control channel, or OAM or
other maintenance function if IP demultiplexing is not used. For
associated control channel packets where IP is not used as the
multiplexer, the Channel Type SHOULD indicate the specific
maintenance protocol carried in the associated control channel.
Values for the Channel Type field currently used for VCCV are
specified elsewhere, e.g. in RFC 4446 [7]and RFC 4385[6] .
Additional Channel Type values and the associated maintenance
functionality will be defined in other documents. Each document
specifying a protocol solution relying on the ACH MUST also specify
the applicable Channel Type field value.
Note that these values are allocated from the PW Associated Channel
Type registry, but this document modifies the existing policy to
accommodate a level of experimentation. See Section 8 for further
details.
3. ACH TLVs
In some applications of the "In-band VCCV" associated control channel
it is necessary to include one or more ACH TLVs to provide additional
context information to the maintenance packet. One use of these ACH
TLVs might be to identify the source and/or intended destination of
the associated control channel maintenance message. However, the use
of this construct is not limited to providing addressing information
nor is the applicability restricted to transport network
applications.
If the maintenance message MAY be preceded by one or more ACH TLVs,
then this MUST be explicitly specified in the definition of an ACH
Channel Type. If the ACH Channel Type definition does state that one
or more ACH TLVs MAY precede the maintenance message, an ACH TLV
Header MUST follow the ACH. If no ACH TLVs are required in a
Bocci, et al. Expires August 27, 2009 [Page 7]
Internet-Draft G-ACh and GAL February 2009
specific associated control channel packet, but the Channel Type
nevertheless defines that ACH TLVs MAY be used, an ACH TLV Header
MUST be present but with a length field set to zero to indicate that
no ACH TLV follow this header.
If a channel type specification does not explicitly specify that ACH
TLVs MAY be used, then an ACH TLV Header MUST NOT be used.
3.1. ACH TLV Payload Structure
This section defines and describes the structure of an ACH payload
when an ACH TLV Header is present. The structure of ACH TLVs that
MAY follow an ACH TLV Header is defined and described in the
following sections.
The following figure (Figure 2) shows the structure of a G-ACh packet
payload.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACH TLV Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ zero or more ACH TLVs ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ Maintenance Message ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: ACH TLV Payload Structure
3.2. ACH TLV Header
The ACH TLV Header defines the length of the set of ACH TLVs that
follow.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: ACH TLV Header
The length field specifies the length in octets of the complete set
Bocci, et al. Expires August 27, 2009 [Page 8]
Internet-Draft G-ACh and GAL February 2009
of TLVs including TLVs that follow the ACH TLV header. A length of
zero indicates that no ACH TLV follow this header.
The reserved field is for future use and MUST be set to zero on
transmission and ignored on reception.
3.3. ACH TLV Object
An ACH TLV consists of a 16-bit Type field, followed by a 16-bit
Length field which specifies the number of octets of the Value field
which follows the Length field. This 32-bit word is followed by zero
or more octets of Value information. The format and semantics of the
value information are defined by the TLV Type as recorded in the TLV
Type registry. See Section 8 for further details. Note that the
Value field of ACH TLVs MAY contain sub-TLV objects.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ Value ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: ACH TLV Format
4. Generalised Exception Mechanism
Generalizing the associated channel mechanism to LSPs and Sections
also requires a method to identify that a packet contains an ACH
followed by a non-service payload. This document specifies that a
label is used for that purpose and calls this special label the G-ACh
Label (GAL). One of the reserved label values defined in RFC 3032
[8] is assigned for this purpose. The value of the label is to be
allocated by IANA; this document suggests the value 13.
The GAL provides an alert based exception mechanism to:
o differentiate specific packets (e.g. maintenance messages) from
others, such as normal user-plane ones,
o indicate that the ACH appears immediately after the bottom of the
label stack.
The GAL MUST only be used where both of these purposes apply.
Bocci, et al. Expires August 27, 2009 [Page 9]
Internet-Draft G-ACh and GAL February 2009
4.1. Relationship with Existing MPLS OAM Alert Mechanisms
RFC 4379 [4] and BFD-MPLS [3] have defined alert mechanisms that
enable a MPLS LSR to identify and process MPLS OAM packets when the
OAM packets are encapsulated in an IP header. These alert mechanisms
are based on TTL expiration and/or use an IP destination address in
the range 127/8.
These alert mechanisms SHOULD be used in non MPLS-TP environments,
although the mechanism defined in this document MAY also be used.
4.2. GAL Applicability and Usage
The GAL MUST only be used with LSPs, concatenated segments of LSPs,
and with Sections.
In MPLS-TP, the GAL MUST always be at the bottom of the label stack
(i.e. S bit set to 1). However, in other MPLS environments, this
document places no restrictions on where the GAL may appear within
the label stack.
The GAL MUST NOT appear in the label stack when transporting normal
user-plane packets. Furthermore, when present, the GAL MUST only
appear once in the label stack.
4.2.1. GAL Processing
The Traffic Class (TC) field (formerly known as the EXP field) of the
label stack entry containing the GAL follows the definition and
processing rules specified and referenced in [9].
The Time-To-Live (TTL) field of the label stack entry that contains
the GAL follows the definition and processing rules specified in
[10].
4.2.1.1. MPLS Label Switched Paths and Segments
The following figure (Figure 5) depicts two LERs (A and D) and two
LSRs (B and C) for a given LSP which is established from A to D and
switched in B and C.
+---+ +---+ +---+ +---+
| A |-------------| B |-------------| C |-------------| D |
+---+ +---+ +---+ +---+
Figure 5: MPLS-TP maintenance over a LSP
In this example, a G-ACh exists on an LSP that extends between LERs A
Bocci, et al. Expires August 27, 2009 [Page 10]
Internet-Draft G-ACh and GAL February 2009
and D, via LSRs B and C. Only these nodes may insert, extract or
process packets on this G-ACh.
The following figure (Figure 6) depicts the format of a MPLS-TP
maintenance message when used for an LSP.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSP Label | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GAL | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACH TLV Header (if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ Zero or more ACH TLVs ~
~ (if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ Maintenance Message ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: MPLS-TP maintenance message format for a LSP
Note that it is possible that the LSP may be tunnelled in another LSP
(e.g. if a MPLS Tunnel exists between B and C), and as such other
labels may be present in the label stack.
To send a maintenance message on the LSP associated control channel,
the LER (A) generates a maintenance message, to which it MAY
prepended an ACH TLV header and appropriate ACH TLVs, and with a ACH
to which it pushes a GAL and finally the LSP label.
o The TTL field of the GAL MUST be set to at least 1. The exact
value of the TTL is application specific.
o The S bit of the GAL MUST be set according to its position in the
label stack.
o The setting of the TC field is application specific.
The maintenance message, the ACH or the GAL SHOULD NOT be modified
towards the targeted destination. Upon reception of the labelled
packet, the targeted destination, after having checked both the LSP
Bocci, et al. Expires August 27, 2009 [Page 11]
Internet-Draft G-ACh and GAL February 2009
label and GAL fields, SHOULD pass the whole maintenance message to
the appropriate processing entity.
4.2.1.2. MPLS Section
The following figure (Figure 7) depicts an example of a MPLS Section.
+---+ +---+
| A |-------------| Z |
+---+ +---+
Figure 7: Maintenance over an MPLS Section
With regard to the MPLS Section, a G-ACh exists between A and Z. Only
A and Z can insert, extract or process packets on this G-ACh.
The following figure (Figure 8) depicts the format of a maintenance
message when used for a MPLS Section. The GAL MAY provide the
exception mechanism for a control channel in its own right without
being associated with a specific LSP, thus providing maintenance
related communications across a specific link interconnecting two
LSRs. In this case, the GAL is the only label in the stack.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GAL | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACH TLV Header (if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ Zero or more ACH TLVs ~
~ (if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ Maintenance Message ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Maintenance message format for a MPLS Section
To send a maintenance message on a control channel associated to the
Section, the head-end LSR (A) of the Section generates a maintenance
message, to which it MAY prepend an ACH TLV Header and appropriate
ACH TLVs, and with a ACH to which it pushes a GAL.
Bocci, et al. Expires August 27, 2009 [Page 12]
Internet-Draft G-ACh and GAL February 2009
o The TTL field of the GAL MUST be set to at least 1. The exact
value of the TTL is application specific.
o The S bit of the GAL MUST be set according to its position in the
label stack. For MPLS Sections, the S bit MUST be set to 1.
o The setting of the TC field is application specific.
The maintenance message, the ACH and the GAL SHOULD NOT be modified
towards the tail-end LSR (Z). Upon reception of the labelled packet,
the tail-end LSR (Z), after having checked the GAL fields, SHOULD
pass the whole packet to the appropriate processing entity.
4.3. Relationship wth RFC 3429
RFC 3429 [18] describes the assignment of one of the reserved label
values, defined in RFC 3032 [8], to the 'OAM Alert Label' that is
used by user-plane MPLS OAM functions for the identification of MPLS
OAM packets. The value of 14 is used for that purpose.
Both this document and RFC 3429 [18] therefore describe the
assignment of reserved label values for similar purposes. The
rationale for the assignment of a new reserved label can be
summarized as follows:
o Unlike the mechanisms described and referenced in RFC 3429 [18],
MPLS-TP maintenance messages will not reside immediately after the
GAL but instead behind the ACH, which itself resides after the
bottom of the label stack. This ensures that OAM, using the
G-ACh, complies with RFC 4928 [11].
o The set of maintenance functions potentially operated in the
context of the G-ACh is wider than the set of OAM functions
referenced in RFC 3429 [18].
o It has been reported that there are existing implementations and
running deployments using the 'OAM Alert Label' as described in
RFC 3429 [18]. It is therefore not possible to modify the 'OAM
Alert Label' allocation, purpose or usage. Nevertheless, it is
RECOMMENDED by this document that no further OAM extensions based
on 'OAM Alert Label' (Label 14) usage be specified or developed.
5. Compatability
Procedures for handling a packet received with an invalid incoming
label are specified in RFC 3031[12].
Bocci, et al. Expires August 27, 2009 [Page 13]
Internet-Draft G-ACh and GAL February 2009
An LER, LSR or PE MUST discard received associated channel packets on
which all of the MPLS or PW labels have been popped if any one of the
following conditions is true:
o It is not capable of processing packets on the Channel Type
indicated by the ACH of the received packet.
o It has not, through means outside the scope of this document,
indicated to the sending LSR, LER or PE that it will process
associated channel packets on the Channel Type indicated by the
ACH of the received packet.
o If the ACH was indicated by the presence of a GAL, and the first
nibble of the ACH of the received packet is not 0b0001.
o The ACH version is not recognised.
In addition, it MAY increment an error counter and MAY also
optionally issue a system and/or SNMP notification.
6. Congestion Considerations
The congestion considerations detailed in RFC 5085 [2] apply.
7. Security Considerations
The security considerations for the associated control channel are
described in RFC 4385[6]. Further security considerations MUST be
described in the relevant associated channel type specification.
RFC 5085 [2] provides data plane related security considerations.
These also apply to a G-ACh, whether the alert mechanism uses a GAL
or only an ACH.
8. IANA Considerations
This document requests that IANA allocates a label value, to the GAL,
from the pool of reserved labels, and suggests this value to be 13.
Channel Types for the Associated Channel Header are allocated from
the IANA PW Associated Channel Type registry [7]. The PW Associated
Channel Type registry is currently allocated based on the IETF
consensus process, described in [13]. This allocation process was
chosen based on the consensus reached in the PWE3 working group that
pseudowire associated channel mechanisms should be reviewed by the
Bocci, et al. Expires August 27, 2009 [Page 14]
Internet-Draft G-ACh and GAL February 2009
IETF and only those that are consistent with the PWE3 architecture
and requirements should be allocated a code point.
However, a requirement has emerged (see [16]) to allow for
optimizations or extensions to OAM and other control protocols
running in an associated channel to be experimented without resorting
to the IETF standards process, by supporting experimental code
points. This would prevent code points used for such functions from
being used from the range allocated through the IETF standards and
thus protects an installed base of equipment from potential
inadvertent overloading of code points. In order to support this
requirement, this document requests that the code point allocation
scheme for the PW Associated Channel Type be changed as follows:
0 - 32751 : IETF Consensus
32752 - 32767 : Experimental
Code points in the experimental range MUST be used according to the
guidelines of RFC 3692 [14]. Experimental OAM functions MUST be
disabled by default. The Channel Type value used for a given
experimental OAM function MUST be configurable, and care MUST be
taken to ensure that different OAM functions that are not inter-
operable are configured to use different Channel Type values.
The PW Associated Channel Type registry needs to be updated to
include a column indicating whether the ACH is followed by a ACH TLV
header (Yes/No). There are two ACH Channel Type code-points
currently assigned and in both cases no ACH TLV header is used. Thus
the new format of the PW Channel Type registry is:
Registry:
Value Description TLV Follows Reference
----- ---------------------------- ----------- ---------
0x21 ACH carries an IPv4 packet No [RFC4385]
0x57 ACH carries an IPv6 packet No [RFC4385]
Figure 9: PW Channel Type registry
IANA is requested create a new registry called the Associated Channel
TLV Registry. The allocation policy for this registry is IETF
consensus. This registry MUST record the following information.
There are no initial entries.
Name Type Length Description Reference
(octets)
Figure 10: PW ACH TLV registry
Bocci, et al. Expires August 27, 2009 [Page 15]
Internet-Draft G-ACh and GAL February 2009
9. Acknowledgements
The authors would like to thank all members of the teams (the Joint
Working Team, the MPLS Interoperability Design Team in IETF and the
MPLS-TP Ad-Hoc Team in ITU-T) involved in the definition and
specification of MPLS Transport Profile.
10. References
10.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit
Connectivity Verification (VCCV): A Control Channel for
Pseudowires", RFC 5085, December 2007.
[3] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, "BFD
For MPLS LSPs", draft-ietf-bfd-mpls-07 (work in progress),
June 2008.
[4] Kompella, K. and G. Swallow, "Detecting Multi-Protocol Label
Switched (MPLS) Data Plane Failures", RFC 4379, February 2006.
[5] Nadeau, T. and C. Pignataro, "Bidirectional Forwarding
Detection (BFD) for the Pseudowire Virtual Circuit
Connectivity Verification (VCCV)", draft-ietf-pwe3-vccv-bfd-03
(work in progress), February 2009.
[6] 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.
[7] Martini, L., "IANA Allocations for Pseudowire Edge to Edge
Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
[8] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci,
D., Li, T., and A. Conta, "MPLS Label Stack Encoding",
RFC 3032, January 2001.
[9] Andersson, L. and R. Asati, "Multi-Protocol Label Switching
(MPLS) label stack entry: "EXP" field renamed to "Traffic
Class" field", draft-ietf-mpls-cosfield-def-08 (work in
progress), December 2008.
[10] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing in
Bocci, et al. Expires August 27, 2009 [Page 16]
Internet-Draft G-ACh and GAL February 2009
Multi-Protocol Label Switching (MPLS) Networks", RFC 3443,
January 2003.
[11] Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal Cost
Multipath Treatment in MPLS Networks", BCP 128, RFC 4928,
June 2007.
[12] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label
Switching Architecture", RFC 3031, January 2001.
[13] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
[14] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692, January 2004.
10.2. Informative References
[15] Bocci, M., Bryant, S., and L. Levrau, "A Framework for MPLS in
Transport Networks", draft-ietf-mpls-tp-framework-00 (work in
progress), November 2008.
[16] Vigoureux, M., Ward, D., and M. Betts, "Requirements for OAM in
MPLS Transport Networks",
draft-ietf-mpls-tp-oam-requirements-00 (work in progress),
December 2008.
[17] Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N., and
S. Ueno, "MPLS-TP Requirements",
draft-ietf-mpls-tp-requirements-04 (work in progress),
February 2009.
[18] Ohta, H., "Assignment of the 'OAM Alert Label' for
Multiprotocol Label Switching Architecture (MPLS) Operation and
Maintenance (OAM) Functions", RFC 3429, November 2002.
Authors' Addresses
Matthew Bocci (editor)
Alcatel-Lucent
Voyager Place, Shoppenhangers Road
Maidenhead, Berks SL6 2PJ
UK
Email: matthew.bocci@alcatel-lucent.com
Bocci, et al. Expires August 27, 2009 [Page 17]
Internet-Draft G-ACh and GAL February 2009
Martin Vigoureux (editor)
Alcatel-Lucent
Route de Villejust
Nozay, 91620
France
Email: martin.vigoureux@alcatel-lucent.com
George Swallow
Cisco
Email: swallow@cisco.com
David Ward
Cisco
Email: dward@cisco.com
Stewart Bryant
Cisco
Email: stbryant@cisco.com
Rahul Aggarwal
Juniper Networks
Email: rahul@juniper.net
Bocci, et al. Expires August 27, 2009 [Page 18]
