One document matched: draft-ietf-mpls-tp-gach-gal-05.xml
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<rfc category="std" docName="draft-ietf-mpls-tp-gach-gal-05" ipr="trust200811"
updates="3032, 4385, 5085">
<front>
<title abbrev="G-ACh and GAL">MPLS Generic Associated Channel</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="16" month="May" 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 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.</t>
</abstract>
</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 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 <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 traffic, and MUST also NOT depend on dynamic control plane
functions.</t>
<t>VCCV <xref target="RFC5085"></xref> 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 <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 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.</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, for
example, on Time To Live (TTL) expiration and/or on the use of an IP
destination address in the range of 127.0.0.0/8 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) <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 used in <xref target="RFC4379"></xref> and
<xref target="I-D.ietf-bfd-mpls"></xref> are not available. Yet, this
mechanism MAY be used in addition to IP-based mechanisms.</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), Signaling Communication Channel (SCC) and Management
Communication Channel (MCC) messages.</t>
<t>Also note that the GAL and ACH are applicable to MPLS and PWs in
general. This document specifies general mechanism and uses MPLS-TP as
an example application. The application of the GAL and ACH to other
specific MPLS uses is outside the scope of this document.</t>
<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 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
label stack.</t>
</section>
<section title="Scope">
<t>This document defines the encapsulation header for Sections, LSPs,
and PWs associated control 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="Requirements Language and Terminology">
<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>
<t>This document uses the following additional terminology:</t>
<t>ACH: Associated Channel Header</t>
<t>G-ACh: Generic Associated Channel</t>
<t>GAL: G-ACh Label</t>
<t>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.</t>
<t>The terms 'Section' and 'Concatenated Segment' are defined in <xref
target="I-D.ietf-mpls-tp-requirements"></xref> as follows (note that
the terms 'Section' and 'Section Layer Network' are synonymous):</t>
<t>Concatenated Segment: A serial-compound link connection as defined
in <xref target="G805"></xref>. A concatenated segment is a contiguous
part of an LSP or multi-segment PW that comprises a set of segments
and their interconnecting nodes in sequence.</t>
<t>Section Layer Network: A section is a server layer (which may be
MPLS-TP or a different technology) which provides for encapsulation
and OAM of a client layer network. A section layer may provide for
aggregation of multiple MPLS-TP clients. Note that G.805 <xref
target="G805"></xref> defines the section layer as one of the two
layer networks in a transmission media layer network. The other layer
network is the physical media layer network.</t>
</section>
</section>
<section title="Generic Associated Channel Header">
<t>VCCV <xref target="RFC5085"></xref> 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".</t>
<section title="Definition">
<t>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 <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 ACH used by CC Type 1 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 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 Types.</t>
<t>Note that VCCV <xref target="RFC5085"></xref> 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.</t>
<t>The G-ACh MUST NOT be used to transport user traffic.</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 indicates the specific 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 <xref target="RFC4446"></xref>, 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 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.</t>
<t>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 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 an ACH Channel Type specification does not explicitly specify that
ACH TLVs MAY be used, then the 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.</t>
<t>The following figure (<xref target="ACHContext"></xref>) shows the
structure of a G-ACh packet payload.</t>
<t><figure anchor="ACHContext" title="G-ACh Packet Payload">
<artwork><![CDATA[+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACH TLV Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ zero or more ACH TLVs ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ G-ACh 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 sub-TLVs that follow the ACH TLV header. A length of
zero indicates that no ACH TLV follow this header. Note that no
padding is required for the set of ACH TLVs.</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>The structure of ACH TLVs that MAY follow an ACH TLV Header is
defined and described in this section.</t>
<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-TLVs. Note that
no padding is required for individual TLVs or sub-TLVs.</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="Generalized Exception Mechanism">
<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. 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.</t>
<t>The GAL provides an alert based exception mechanism to:</t>
<t><list style="symbols">
<t>differentiate specific packets (i.e., G-ACh packets) from others,
such as 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 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> 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 127.0.0.0/8 or
0:0:0:0:0:FFFF:127.0.0.0/104, respectively for IPv4 and IPv6.</t>
<t>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.</t>
</section>
<section title="GAL Applicability and Usage">
<t>In MPLS-TP, the GAL MUST be used with packets on a G-ACh on LSPs,
Concatenated Segments of LSPs, and with Sections, and MUST NOT be used
with PWs. It 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 or its use with PWs. 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.</t>
<t>The GAL MUST NOT appear in the label stack when transporting normal
user-plane packets. Furthermore, when present, the GAL MUST NOT appear
more than once in the label stack.</t>
<t>A receiving LSR, LER or PE MUST NOT forward a G-ACh packet to
another node based on the GAL label.</t>
<section title="GAL Processing">
<t>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 <xref
target="RFC5462"></xref>.</t>
<t>The Time-To-Live (TTL) field of the LSE 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="Maintenance over a LSP">
<artwork><![CDATA[ +---+ +---+ +---+ +---+
| A |-------------| B |-------------| C |-------------| D |
+---+ +---+ +---+ +---+]]></artwork>
</figure></t>
<t>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 new
G-ACh packets. A, B, C and D may process and respond to G-ACh
packets.</t>
<t>The following figure (<xref target="LSP-OAM-Message"></xref>)
depicts the format of an MPLS-TP G-ACh packet when used for an
LSP.</t>
<t><figure anchor="LSP-OAM-Message"
title="G-ACh packet 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ G-ACh Message ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure></t>
<t>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.</t>
<t>To send a G-ACh 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. It then adds an ACH, onto
which it pushes a GAL LSE. Finally, the LSP Label LSE is pushed
onto the resulting packet.</t>
<t><list style="symbols">
<t>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.</t>
<t>The S bit of the GAL MUST be set according to its position
in the label stack (see Section 4.2).</t>
<t>The setting of the TC field of the GAL is application
specific. See Section 4.2.1 for definition and processing
rules.</t>
</list>LSRs MUST NOT modify the G-ACh message, the ACH or the
GAL towards the targeted destination.</t>
<t><list style="hanging">
<t hangText="Note:">This is because once a G-ACh packet has
been sent on an LSP, no node has visibility of it unless the
LSP label TTL expires or the GAL is exposed when the LSP label
is popped. If this is at the targeted destination, for example
indicated by an address in an ACH TLV, then processing can
proceed as specified below. If this is not the targeted
destination, but the node has agreed to process packets on
that ACH channel, then the processing applied to the packet is
out of scope of this docuemnt. However, the ACH type MUST be
maintained if the packet is forwarded unmodified to another
node.</t>
</list></t>
<t>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.</t>
</section>
<section title="MPLS Section">
<t>The following figure (<xref target="Section-OAM"></xref>)
depicts an example of an 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
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.</t>
<t><figure anchor="Section-OAM-Message"
title="G-ACh packet format for an 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ G-ACh message ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure></t>
<t>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. Next the LSR adds an ACH. Finally it pushes a GAL
LSE.</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. See Section
4.2.1 for definition and processing rules.</t>
<t>The S bit of the GAL MUST be set according to its position
in the label stack. (see Section 4.2).</t>
<t>The setting of the TC field of the GAL is application
specific. See Section 4.2.1 for definition and processing
rules.</t>
</list></t>
<t>Intermediate nodes of the MPLSsection MUST NOT modify the G-ACh
message, the ACH and the GAL 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.</t>
</section>
</section>
</section>
<section title="Relationship with 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>, 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.</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 that no further OAM
extensions based on 'OAM Alert Label' (Label 14) usage be
specified or developed.</t>
</list></t>
</section>
</section>
<section anchor="Section-Compatibility" title="Compatibility">
<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>The packet is received on an Experimental Channel Type that is
locally disabled.</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 0001b.</t>
<t>The ACH version is not recognized.</t>
</list>In addition, the LER, LSR or PE MAY increment an error counter
and MAY also 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="Contributing Authors">
<t>The editors gratefully acknowledge the contributions of Sami Boutros,
Italo Busi, Marc Lasserre, Lieven Levrau and Siva Sivabalan</t>
</section>
<section anchor="Acknowledgments" title="Acknowledgments">
<t>The authors would like to thank Malcolm Betts, ITU-T Study Group 15,
and 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 the MPLS
Transport Profile.</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 in the "Multiprotocol Label Switching
Architecture (MPLS) Label Values" registry, and suggests this value to
be 13.</t>
<t><list style="hanging">
<t hangText="Note to RFC Editor:">The above text "and suggests this
value to be 13" needs to be replaced with "with a value of 13." when
the RFC is published and IANA has allocated the value.</t>
</list></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 (termed "IETF
Review" 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>32760 - 32767 : Experimental</t>
<t>Code points in the experimental range MUST be used according to the
guidelines of RFC 3692 <xref target="RFC3692"></xref>. 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.</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><vspace blankLines="6" />IANA is requested create a new registry
called the Associated Channel Header TLV Registry. The allocation policy
for this registry is IETF review. This registry MUST record the
following information. There are no initial entries.</t>
<t><figure anchor="ACH-TLV-registry" title="ACH TLV registry">
<artwork><![CDATA[Name Type Length Description Reference
(octets)]]></artwork>
</figure></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.4385"?>
<?rfc include="reference.RFC.4446"?>
<?rfc include="reference.RFC.5085"?>
<?rfc include="reference.RFC.3692"?>
<?rfc include="reference.RFC.5462"?>
</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'?>
<?rfc include='reference.RFC.4379'?>
<?rfc include='reference.I-D.ietf-bfd-mpls'?>
<?rfc include='reference.I-D.ietf-pwe3-vccv-bfd'?>
<reference anchor="G805" target="">
<front>
<title>Generic Functional Architecture of Transport Networks</title>
<author fullname="International Telecommunication Union"
surname="International Telecommunication Union"></author>
<date month="March" year="2000" />
</front>
<seriesInfo name="ITU-T" value="G.805" />
</reference>
</references>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 16:43:47 |