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: October 11, 2009 S. Bryant
Cisco
R. Aggarwal
Juniper Networks
April 9, 2009
MPLS Generic Associated Channel
draft-ietf-mpls-tp-gach-gal-03
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Abstract
This document generalizes 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].
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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. Generalized 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 with RFC 3429 . . . . . . . . . . . . . . . . 13
5. Compatibility . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Congestion Considerations . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
10.2. Informative References . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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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 pseudowire (PW) and a Label
Switched Path (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 [14].
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 [15]. 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 traffic,
and MUST also NOT depend on dynamic control plane functions.
VCCV [2] 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
generalizes the applicability 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 applicability of the ACH 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. The label used for that purpose is one
of the MPLS reserved labels and is referred to as the GAL (G-ACh
Label). The GAL mechanism is defined to work together with the ACH
for LSPs and MPLS Sections.
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, for
example, on Time To Live (TTL) expiration and/or on the use of an IP
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destination address in the range of 127/8 or 0:0:0:0:0:FFFF:
127.0.0.0/104, respectively for IPv4 and IPv6. These mechanisms are
the default mechanisms for identifying MPLS OAM packets when
encapsulated in an IP header. However it may not always be possible
to use these mechanisms in some MPLS applications e.g., MPLS
Transport Profile (MPLS-TP) [14], 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
used in [4] and [3] are not available. Yet, this mechanism MAY be
used in addition to IP-based mechanisms.
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), Signaling 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 normalizes 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 Sections, LSPs,
and PWs associated control 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.
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1.4. Terminology
ACH: Associated Channel Header
G-ACh: Generic Associated Channel
GAL: G-ACh Label
G-ACh packet: Any packet containing a message belonging to a protocol
that is carried on a PW, LSP or MPLS Section associated control
channel. Examples include maintenance protocols such as OAM
functions, signaling communications or management communications.
The terms 'Section' and 'Concatenated Segment' are defined in [16].
2. Generic Associated Channel Header
VCCV [2] defines three 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 ACH, previously limited to PWs, is here generalized 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 ACH used by CC Type 1 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
15 of the ACH are reserved and MUST be set to 0 and ignored on
reception. Bits 16 to 31 are used to encode the possible Channel
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Types.
Note that VCCV [2] also includes mechanisms for negotiating the
Control Channel and Connectivity Verification (i.e., OAM function)
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 indicates the specific 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 [7], 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 generalized associated control channel it
is necessary to include one or more ACH TLVs to provide additional
context information to the G-ACh packet. One use of these ACH TLVs
might be to identify the source and/or intended destination of the
associated channel 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 G-ACh 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 G-ACh message, an ACH TLV Header MUST follow
the ACH. If no ACH TLVs are required in a specific associated
channel packet, but the Channel Type nevertheless defines that ACH
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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 an ACH Channel Type specification does not explicitly specify that
ACH TLVs MAY be used, then the 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 ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ G-ACh 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
of TLVs including sub-TLVs that follow the ACH TLV header. A length
of zero indicates that no ACH TLV follow this header. Note that no
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padding is required for the set of ACH TLVs.
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-TLVs. Note that no padding
is required for individual TLVs or sub-TLVs.
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. Generalized Exception Mechanism
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. 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 (i.e., G-ACh packets) from others,
such as 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 these purposes apply.
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4.1. Relationship with Existing MPLS OAM Alert Mechanisms
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, for
example, on Time To Live (TTL) expiration and/or on the use of an IP
destination address in the range of 127/8 or 0:0:0:0:0:FFFF:
127.0.0.0/104, respectively for IPv4 and IPv6.
These mechanisms are the default mechanisms for identifying MPLS OAM
packets when encapsulated in an IP header 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, for
example those using entropy labels [17], this document places no
restrictions on where the GAL may appear within the label stack.
Where the GAL is at the bottom of the label stack (i.e. S bit set to
1) then it MUST always be followed by an ACH.
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 (LSE) containing the GAL follows the definition and
processing rules specified and referenced in [9].
The Time-To-Live (TTL) field of the LSE 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.
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+---+ +---+ +---+ +---+
| A |-------------| B |-------------| C |-------------| D |
+---+ +---+ +---+ +---+
Figure 5: Maintenance over a LSP
In this example, a G-ACh exists on the LSP that extends between LERs
A and D, via LSRs B and C. Only A and D may initiate the generation
of G-ACh packets. A, B, C and D may also originate and process G-ACh
packets.
The following figure (Figure 6) depicts the format of a MPLS-TP G-ACh
packet 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ G-ACh Message ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: G-ACh packet format for a LSP
Note that it is possible that the LSP may be tunneled in another LSP
(e.g., if a MPLS Tunnel exists between B and C), and as such other
LSEs may be present in the label stack.
To send a maintenance message on the LSP associated control channel,
the LER (A) generates a G-ACh message, to which it MAY prepend an ACH
TLV Header and appropriate ACH TLVs, adds an ACH to which it, pushes
a GAL LSE and finally the LSP Label LSE.
o The TTL field of the GAL LSE MUST be set to at least 1. The exact
value of the TTL is application specific. See Section 4.2.1 for
definition and processing rules.
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o The S bit of the GAL MUST be set according to its position in the
label stack (see Section 4.2).
o The setting of the TC field of the GAL is application specific.
See Section 4.2.1 for definition and processing rules.
The G-ACh message, the ACH or the GAL SHOULD NOT be modified towards
the targeted destination. Upon reception of the labeled packet, the
targeted destination, after having checked both the LSP Label and GAL
LSEs fields, SHOULD pass the whole packet to the appropriate
processing entity.
4.2.1.2. MPLS Section
The following figure (Figure 7) depicts an example of an 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 G-ACh packet
when used for an 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.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GAL | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACH TLV Header (if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ Zero or more ACH TLVs ~
~ (if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ G-ACh message ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: G-ACh packet format for an MPLS Section
To send a G-ACh message on a control channel associated to the
Section, the head-end LSR (A) of the Section generates a G-ACh
message, to which it MAY prepend an ACH TLV Header and appropriate
ACH TLVs, adds an ACH to which it pushes a GAL LSE.
o The TTL field of the GAL MUST be set to at least 1. The exact
value of the TTL is application specific. See Section 4.2.1 for
definition and processing rules.
o The S bit of the GAL MUST be set according to its position in the
label stack. (see Section 4.2).
o The setting of the TC field of the GAL is application specific.
See Section 4.2.1 for definition and processing rules.
The G-ACh message, the ACH and the GAL SHOULD NOT be modified towards
the tail-end LSR (Z). Upon reception of the G-ACh packet, the tail-
end LSR (Z), after having checked the GAL LSE fields, SHOULD pass the
whole packet to the appropriate processing entity.
4.3. Relationship with 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
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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],
G-ACh messages will not reside immediately after the GAL but
instead behind the ACH, which itself resides after the bottom of
the label stack.
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 that no further OAM extensions based on 'OAM Alert
Label' (Label 14) usage be specified or developed.
5. Compatibility
Procedures for handling a packet received with an invalid incoming
label are specified in RFC 3031[11].
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 The packet is received on an Experimental Channel Type that is
locally disabled.
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 recognized.
In addition, it MAY increment an error counter and MAY also issue a
system and/or SNMP notification.
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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 in the "Multiprotocol Label
Switching Architecture (MPLS) Label Values" registry, 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 [12]. 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 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 [15]) 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
32760 - 32767 : Experimental
Code points in the experimental range MUST be used according to the
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guidelines of RFC 3692 [13]. Functions using experimental G-ACh code
points 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
Header 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: ACH TLV registry
9. Acknowledgments
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
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Internet-Draft G-ACh and GAL April 2009
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, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
Class" Field", RFC 5462, February 2009.
[10] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing in
Multi-Protocol Label Switching (MPLS) Networks", RFC 3443,
January 2003.
[11] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label
Switching Architecture", RFC 3031, January 2001.
[12] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
[13] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692, January 2004.
10.2. Informative References
[14] 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.
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[15] Vigoureux, M., Ward, D., and M. Betts, "Requirements for OAM in
MPLS Transport Networks",
draft-ietf-mpls-tp-oam-requirements-01 (work in progress),
March 2009.
[16] Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N., and
S. Ueno, "MPLS-TP Requirements",
draft-ietf-mpls-tp-requirements-06 (work in progress),
April 2009.
[17] Kompella, K. and S. Amante, "The Use of Entropy Labels in MPLS
Forwarding", draft-kompella-mpls-entropy-label-00 (work in
progress), July 2008.
[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
Martin Vigoureux (editor)
Alcatel-Lucent
Route de Villejust
Nozay, 91620
France
Email: martin.vigoureux@alcatel-lucent.com
George Swallow
Cisco
Email: swallow@cisco.com
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David Ward
Cisco
Email: dward@cisco.com
Stewart Bryant
Cisco
Email: stbryant@cisco.com
Rahul Aggarwal
Juniper Networks
Email: rahul@juniper.net
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