One document matched: draft-ietf-mpls-tp-gach-gal-02.xml
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<rfc category="std" docName="draft-ietf-mpls-tp-gach-gal-02" ipr="trust200811"
updates="3032, 4385, 5085">
<front>
<title abbrev="G-ACh and GAL">MPLS Generic Associated Channel
header</title>
<author fullname="Matthew Bocci" initials="M." role="editor"
surname="Bocci">
<organization>Alcatel-Lucent</organization>
<address>
<postal>
<street>Voyager Place, Shoppenhangers Road</street>
<city>Maidenhead</city>
<region>Berks</region>
<code>SL6 2PJ</code>
<country>UK</country>
</postal>
<email>matthew.bocci@alcatel-lucent.com</email>
</address>
</author>
<author fullname="Martin Vigoureux" initials="M." role="editor"
surname="Vigoureux">
<organization>Alcatel-Lucent</organization>
<address>
<postal>
<street>Route de Villejust</street>
<city>Nozay</city>
<region></region>
<code>91620</code>
<country>France</country>
</postal>
<email>martin.vigoureux@alcatel-lucent.com</email>
</address>
</author>
<author fullname="George Swallow" initials="G." surname="Swallow">
<organization>Cisco</organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<email>swallow@cisco.com</email>
</address>
</author>
<author fullname="David Ward" initials="D." surname="Ward">
<organization>Cisco</organization>
<address>
<email>dward@cisco.com</email>
</address>
</author>
<author fullname="Stewart Bryant" initials="S." surname="Bryant">
<organization>Cisco</organization>
<address>
<email>stbryant@cisco.com</email>
</address>
</author>
<author fullname="Rahul Aggarwal" initials="R." surname="Aggarwal">
<organization>Juniper Networks</organization>
<address>
<email>rahul@juniper.net</email>
</address>
</author>
<date day="23" month="February" year="2009" />
<area>Routing</area>
<workgroup>MPLS Working Group</workgroup>
<keyword>MPLS-TP</keyword>
<keyword>MPLS</keyword>
<keyword>OAM</keyword>
<keyword>GAL</keyword>
<keyword>G-ACh</keyword>
<abstract>
<t>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.</t>
</abstract>
<note title="Requirements Language">
<t>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 <xref
target="RFC2119">RFC 2119</xref>.</t>
</note>
</front>
<middle>
<section title="Introduction">
<t>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 <xref target="I-D.ietf-mpls-tp-framework"></xref>. Some of
these functions can be supported using existing tools such as Virtual
Circuit Connectivity Verification (VCCV) <xref target="RFC5085"></xref>,
Bidirectional Forwarding Detection for MPLS LSPs (BFD-MPLS)<xref
target="I-D.ietf-bfd-mpls"></xref>, LSP-Ping <xref
target="RFC4379"></xref>, or BFD-VCCV <xref
target="I-D.ietf-pwe3-vccv-bfd"></xref>. 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 <xref
target="I-D.ietf-mpls-tp-oam-requirements"></xref>. 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.</t>
<t>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 <xref target="RFC4385"></xref>,
may be used with additional code points to support additional MPLS
maintenance functions on the G-ACh.</t>
<t>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.</t>
<t>RFC 4379 <xref target="RFC4379"></xref> and BFD-MPLS <xref
target="I-D.ietf-bfd-mpls"></xref> 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 to use these mechanisms in
some MPLS applications, e.g. MPLS Transport Profile (MPLS-TP) <xref
target="I-D.ietf-mpls-tp-framework"></xref>, 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 <xref
target="RFC4379"></xref> and <xref target="I-D.ietf-bfd-mpls"></xref>
are not available. This mechanism MAY be used in addition to IP-based
mechanisms.</t>
<t>The GAL mechanism is defined to work together with the ACH for LSPs
and MPLS Sections.</t>
<t>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.</t>
<t>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.</t>
<section title="Contributing Authors">
<t>The editors gratefully acknowledge the contributions of Sami
Boutros, Italo Busi, Marc Lasserre, Lieven Levrau and Siva
Sivabalan</t>
</section>
<section title="Objectives">
<t>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.</t>
</section>
<section title="Scope">
<t>This document defines the encapsulation header for LSP, MPLS
Section and PW associated channel messages.</t>
<t>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.</t>
<t>This document does not deprecate existing MPLS and PW OAM
mechanisms.</t>
</section>
<section title="Terminology">
<t>ACH: Associated Channel Header</t>
<t>G-ACh: Generic Associated Channel</t>
<t>GAL: G-ACh Label</t>
<t>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.</t>
<t>The terms 'Section' and 'Concatenated Segment' are defined in <xref
target="I-D.ietf-mpls-tp-requirements"></xref>.</t>
</section>
</section>
<section title="Generic Associated Channel Header">
<t>VCCV <xref target="RFC5085"></xref> 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".</t>
<section title="Definition">
<t>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 <xref target="RFC4385"></xref> MUST be present in
the encapsulation of user packets when the ACH is used to realize the
associated control channel.</t>
<t>The CC Type 1 control channel header is depicted in figure
below:</t>
<t><figure anchor="G-ACH" title="Associated Channel Header">
<artwork><![CDATA[ 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t>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 <xref
target="RFC4385"></xref>. Bits 8 to 14 of the G-ACH are reserved and
MUST be set to 0 and ignored on reception.</t>
<t>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.</t>
</section>
<section title="Allocation of Channel Types">
<t>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.</t>
<t>Values for the Channel Type field currently used for VCCV are
specified elsewhere, e.g. in RFC 4446 <xref
target="RFC4446"></xref>and RFC 4385<xref target="RFC4385"></xref> .
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.</t>
<t>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 <xref
target="IANA"></xref> for further details.</t>
</section>
</section>
<section title="ACH TLVs ">
<t>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.</t>
<t>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 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.</t>
<t>If a channel type specification does not explicitly specify that ACH
TLVs MAY be used, then an ACH TLV Header MUST NOT be used.</t>
<section title="ACH TLV Payload Structure">
<t>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.</t>
<t>The following figure (<xref target="ACHContext"></xref>) shows the
structure of a G-ACh packet payload.</t>
<t><figure anchor="ACHContext" title="ACH TLV Payload Structure">
<artwork><![CDATA[+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACH TLV Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ zero or more ACH TLVs ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ Maintenance Message ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure></t>
</section>
<section title="ACH TLV Header">
<t>The ACH TLV Header defines the length of the set of ACH TLVs that
follow.</t>
<t><figure anchor="ACH-TLV-Head" title="ACH TLV Header">
<artwork><![CDATA[ 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure></t>
<t>The length field specifies the length in octets of the complete set
of TLVs including TLVs that follow the ACH TLV header. A length of
zero indicates that no ACH TLV follow this header.</t>
<t>The reserved field is for future use and MUST be set to zero on
transmission and ignored on reception.</t>
</section>
<section title="ACH TLV Object">
<t>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 <xref target="IANA"></xref> for further details.
Note that the Value field of ACH TLVs MAY contain sub-TLV objects.</t>
<t><figure anchor="ACHTLVFormat" title="ACH TLV Format">
<artwork><![CDATA[ 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 ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure></t>
</section>
</section>
<section title="Generalised Exception Mechanism">
<t>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 <xref
target="RFC3032"></xref> is assigned for this purpose. The value of the
label is to be allocated by IANA; this document suggests the value
13.</t>
<t>The GAL provides an alert based exception mechanism to:</t>
<t><list style="symbols">
<t>differentiate specific packets (e.g. maintenance messages) from
others, such as normal user-plane ones,</t>
<t>indicate that the ACH appears immediately after the bottom of the
label stack.</t>
</list>The GAL MUST only be used where both of these purposes
apply.</t>
<section title="Relationship with Existing MPLS OAM Alert Mechanisms">
<t>RFC 4379 <xref target="RFC4379"></xref> and BFD-MPLS <xref
target="I-D.ietf-bfd-mpls"></xref> 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.</t>
<t>These alert mechanisms SHOULD be used in non MPLS-TP environments,
although the mechanism defined in this document MAY also be used.</t>
</section>
<section title="GAL Applicability and Usage">
<t>The GAL MUST only be used with LSPs, concatenated segments of LSPs,
and with Sections.</t>
<t>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.</t>
<t>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.</t>
<section title="GAL Processing">
<t>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 <xref
target="I-D.ietf-mpls-cosfield-def"></xref>.</t>
<t>The Time-To-Live (TTL) field of the label stack entry that
contains the GAL follows the definition and processing rules
specified in <xref target="RFC3443"></xref>.</t>
<section title="MPLS Label Switched Paths and Segments">
<t>The following figure (<xref target="LSP-OAM"></xref>) 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.</t>
<t><figure anchor="LSP-OAM" title="MPLS-TP maintenance over a LSP">
<artwork><![CDATA[ +---+ +---+ +---+ +---+
| A |-------------| B |-------------| C |-------------| D |
+---+ +---+ +---+ +---+]]></artwork>
</figure></t>
<t>In this example, a G-ACh exists on an LSP that extends between
LERs A and D, via LSRs B and C. Only these nodes may insert,
extract or process packets on this G-ACh.</t>
<t>The following figure (<xref target="LSP-OAM-Message"></xref>)
depicts the format of a MPLS-TP maintenance message when used for
an LSP.</t>
<t><figure anchor="LSP-OAM-Message"
title="MPLS-TP maintenance message format for a LSP">
<artwork><![CDATA[ 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 ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure></t>
<t>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.</t>
<t>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.</t>
<t><list style="symbols">
<t>The TTL field of the GAL MUST be set to at least 1. The
exact value of the TTL is application specific.</t>
<t>The S bit of the GAL MUST be set according to its position
in the label stack.</t>
<t>The setting of the TC field is application specific.</t>
</list>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 label and GAL fields, SHOULD pass the whole
maintenance message to the appropriate processing entity.</t>
</section>
<section title="MPLS Section">
<t>The following figure (<xref target="Section-OAM"></xref>)
depicts an example of a MPLS Section.</t>
<t><figure anchor="Section-OAM"
title="Maintenance over an MPLS Section">
<artwork><![CDATA[ +---+ +---+
| A |-------------| Z |
+---+ +---+]]></artwork>
</figure></t>
<t>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.</t>
<t>The following figure (<xref
target="Section-OAM-Message"></xref>) 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.</t>
<t><figure anchor="Section-OAM-Message"
title="Maintenance message format for a MPLS Section">
<artwork><![CDATA[ 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 ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure></t>
<t>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.</t>
<t><list style="symbols">
<t>The TTL field of the GAL MUST be set to at least 1. The
exact value of the TTL is application specific.</t>
<t>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.</t>
<t>The setting of the TC field is application specific.</t>
</list></t>
<t>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.</t>
</section>
</section>
</section>
<section title="Relationship wth RFC 3429">
<t>RFC 3429 <xref target="RFC3429"></xref> describes the assignment of
one of the reserved label values, defined in RFC 3032 <xref
target="RFC3032"></xref>, 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.</t>
<t>Both this document and RFC 3429 <xref target="RFC3429"></xref>
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:</t>
<t><list style="symbols">
<t>Unlike the mechanisms described and referenced in RFC 3429
<xref target="RFC3429"></xref>, 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 <xref
target="RFC4928"></xref>.</t>
<t>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 <xref target="RFC3429"></xref>.</t>
<t>It has been reported that there are existing implementations
and running deployments using the 'OAM Alert Label' as described
in RFC 3429 <xref target="RFC3429"></xref>. 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.</t>
</list></t>
</section>
</section>
<section title="Compatability">
<t>Procedures for handling a packet received with an invalid incoming
label are specified in RFC 3031<xref target="RFC3031"></xref>.</t>
<t>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:</t>
<t><list style="symbols">
<t>It is not capable of processing packets on the Channel Type
indicated by the ACH of the received packet.</t>
<t>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.</t>
<t>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.</t>
<t>The ACH version is not recognised.</t>
</list>In addition, it MAY increment an error counter and MAY also
optionally issue a system and/or SNMP notification.</t>
</section>
<section title="Congestion Considerations">
<t>The congestion considerations detailed in RFC 5085 <xref
target="RFC5085"></xref> apply.</t>
</section>
<section title="Security Considerations">
<t>The security considerations for the associated control channel are
described in RFC 4385<xref target="RFC4385"> </xref>. Further security
considerations MUST be described in the relevant associated channel type
specification.</t>
<t>RFC 5085 <xref target="RFC5085"></xref> provides data plane related
security considerations. These also apply to a G-ACh, whether the alert
mechanism uses a GAL or only an ACH.</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>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.</t>
<t>Channel Types for the Associated Channel Header are allocated from
the IANA PW Associated Channel Type registry <xref
target="RFC4446"></xref>. The PW Associated Channel Type registry is
currently allocated based on the IETF consensus process, described in
<xref target="RFC5226"></xref>. 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.</t>
<t>However, a requirement has emerged (see <xref
target="I-D.ietf-mpls-tp-oam-requirements"></xref>) 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:</t>
<t>0 - 32751 : IETF Consensus</t>
<t>32752 - 32767 : Experimental</t>
<t>Code points in the experimental range MUST be used according to the
guidelines of RFC 3692 <xref target="RFC3692"></xref>. 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.</t>
<t>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:</t>
<t><figure anchor="PW-CT-registry" title="PW Channel Type registry">
<artwork><![CDATA[Registry:
Value Description TLV Follows Reference
----- ---------------------------- ----------- ---------
0x21 ACH carries an IPv4 packet No [RFC4385]
0x57 ACH carries an IPv6 packet No [RFC4385]]]></artwork>
</figure></t>
<t>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.</t>
<t><figure anchor="PW-ACH-TLV-registry" title="PW ACH TLV registry">
<artwork><![CDATA[Name Type Length Description Reference
(octets)]]></artwork>
</figure></t>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>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.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.5226"?>
<?rfc include="reference.RFC.3031"?>
<?rfc include="reference.RFC.3032"?>
<?rfc include="reference.RFC.3443"?>
<?rfc include="reference.RFC.4379"?>
<?rfc include="reference.RFC.4385"?>
<?rfc include="reference.RFC.4446"?>
<?rfc include="reference.RFC.4928"?>
<?rfc include="reference.RFC.5085"?>
<?rfc include="reference.RFC.3692"?>
<?rfc include="reference.I-D.ietf-bfd-mpls"?>
<?rfc include="reference.I-D.ietf-pwe3-vccv-bfd"?>
<?rfc include="reference.I-D.ietf-mpls-cosfield-def"?>
</references>
<references title="Informative References">
<?rfc include='reference.RFC.3429'?>
<?rfc include='reference.I-D.ietf-mpls-tp-requirements'?>
<?rfc include='reference.I-D.ietf-mpls-tp-oam-requirements'?>
<?rfc include='reference.I-D.ietf-mpls-tp-framework'?>
</references>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 11:07:21 |