MPLS D. Frost, Ed.
Internet-Draft S. Bryant, Ed.
Intended status: Standards Track Cisco Systems
Expires: September 13, 2010 M. Bocci, Ed.
Alcatel-Lucent
March 12, 2010
MPLS Transport Profile Data Plane Architecture
draft-ietf-mpls-tp-data-plane-01
Abstract
The Multiprotocol Label Switching (MPLS) Transport Profile (MPLS-TP)
is the set of MPLS protocol functions applicable to the construction
and operation of packet-switched transport networks. This document
specifies the subset of these functions that comprises the MPLS-TP
data plane: the architectural layer concerned with the encapsulation
and forwarding of packets within an MPLS-TP network.
This document is a product of a joint Internet Engineering Task Force
(IETF) / International Telecommunication Union Telecommunication
Standardization Sector (ITU-T) effort to include an MPLS Transport
Profile within the IETF MPLS and PWE3 architectures to support the
capabilities and functionalities of a packet transport network.
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 [RFC2119].
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
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http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on September 13, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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described in the BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. MPLS-TP Packet Encapsulation and Forwarding . . . . . . . . . 5
3. MPLS-TP Transport Entities . . . . . . . . . . . . . . . . . . 5
3.1. Label Switched Paths . . . . . . . . . . . . . . . . . . . 6
3.1.1. LSP Packet Encapsulation and Forwarding . . . . . . . 6
3.1.2. LSP Payloads . . . . . . . . . . . . . . . . . . . . . 6
3.1.3. LSP Types . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Sections . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3. Pseudowires . . . . . . . . . . . . . . . . . . . . . . . 9
4. MPLS-TP Generic Associated Channel . . . . . . . . . . . . . . 9
5. Server Layer Considerations . . . . . . . . . . . . . . . . . 10
5.1. Ethernet Media . . . . . . . . . . . . . . . . . . . . . . 10
5.1.1. Point-to-Point Links . . . . . . . . . . . . . . . . . 10
5.1.2. Multipoint Links . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
The MPLS Transport Profile (MPLS-TP) [I-D.ietf-mpls-tp-framework] is
the set of protocol functions that meet the requirements [RFC5654]
for the application of MPLS to the construction and operation of
packet-switched transport networks. Packet transport networks are
defined and described in [I-D.ietf-mpls-tp-framework].
This document specifies the subset of protocol functions that
comprises the MPLS-TP data plane: the architectural layer concerned
with the encapsulation and forwarding of packets within an MPLS-TP
network.
This document is a product of a joint Internet Engineering Task Force
(IETF) / International Telecommunication Union Telecommunication
Standardization Sector (ITU-T) effort to include an MPLS Transport
Profile within the IETF MPLS and PWE3 architectures to support the
capabilities and functionalities of a packet transport network.
1.1. Scope
This document has the following purposes:
o To identify the data-plane functions within the MPLS Transport
Profile;
o To indicate which of these data-plane functions an MPLS-TP
implementation is required to support.
Note that the MPLS-TP functions discussed in this document are
considered OPTIONAL unless stated otherwise.
1.2. Terminology
Term Definition
------- ------------------------------------------
G-ACh Generic Associated Channel
GAL G-ACh Label
LSP Label Switched Path
LSR Label Switching Router
MAC Media Access Control
MPLS-TP MPLS Transport Profile
OAM Operations, Administration and Maintenance
PW Pseudowire
QoS Quality of Service
TTL Time To Live
Additional definitions and terminology can be found in
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[I-D.ietf-mpls-tp-framework] and [RFC5654].
2. MPLS-TP Packet Encapsulation and Forwarding
This document defines the encapsulation and forwarding functions
applicable to packets traversing an MPLS-TP Label Switched Path
(LSP), Pseudowire (PW), or Section (see Section 3 for the definitions
of these transport entities). Encapsulation and forwarding functions
for packets outside an MPLS-TP LSP, PW, or Section, and mechanisms
for delivering packets to or from MPLS-TP LSPs, PWs, and Sections,
are outside the scope of this document.
MPLS-TP packet encapsulation and forwarding operates according to the
MPLS data-plane architecture described in [RFC3031] and [RFC3032],
and the data-plane architectures for Single-Segment Pseudowires
[RFC3985], Multi-Segment Pseudowires [RFC5659], and Point-to-
Multipoint Pseudowires [I-D.ietf-pwe3-p2mp-pw-requirements], except
as noted otherwise in this document.
MPLS-TP forwarding is based on the label that identifies an LSP or
PW. The label value specifies the processing operation to be
performed by the next hop at that level of encapsulation. A swap of
this label is an atomic operation in which the contents of the packet
after the swapped label are opaque to the forwarder. The only event
that interrupts a swap operation is Time To Live (TTL) expiry.
Further processing to determine the context of a packet occurs when a
swap operation is interrupted in this manner, when a pop operation
exposes a specific reserved label, or when the packet is received
with the Generic Associated Channel Label (GAL) (see Section 4) at
the top of the stack. Otherwise the packet is forwarded according to
the procedures in [RFC3032].
3. MPLS-TP Transport Entities
The MPLS Transport Profile includes the following data-plane
transport entities:
o Label Switched Paths (LSPs)
o Sections
o Pseudowires (PWs)
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3.1. Label Switched Paths
MPLS-TP LSPs are ordinary MPLS LSPs as defined in [RFC3031] except as
specifically noted otherwise in this document.
3.1.1. LSP Packet Encapsulation and Forwarding
Encapsulation and forwarding of packets traversing MPLS-TP LSPs MUST
follow standard MPLS packet encapsulation and forwarding as defined
in [RFC3031] and [RFC3032], except as explicitly stated otherwise in
this document.
Data-plane support for Internet Protocol (IP) packet encapsulation,
addressing, and forwarding is OPTIONAL.
Data-plane Quality of Service capabilities are included in the
MPLS-TP in the form of the MPLS Differentiated Services (DiffServ)
architecture [RFC3270]. Both E-LSP and L-LSP MPLS DiffServ modes are
included. The Traffic Class field (formerly the EXP field) of an
MPLS label follows the definition of [RFC5462] and [RFC3270] and MUST
be processed according to the rules specified in those documents.
The Pipe and Short Pipe DiffServ tunneling and TTL processing models
described in [RFC3270] and [RFC3443] are included in the MPLS-TP.
The Uniform model is outside the scope of the MPLS-TP.
Per-platform, per-interface or other context-specific label space
[RFC5331] MAY be used for MPLS-TP LSPs. Downstream [RFC3031] or
upstream [RFC5331] label allocation schemes MAY be used for MPLS-TP
LSPs. Note that the requirements of a particular LSP type may
dictate which label spaces or allocation schemes it can use.
Per-packet Equal-Cost Multi-Path (ECMP) load-balancing is outside the
scope of the MPLS-TP.
Penultimate Hop Popping (PHP) MUST be disabled by default on MPLS-TP
LSPs.
3.1.2. LSP Payloads
The MPLS-TP includes support for the following LSP payload types:
o Network-layer protocol packets
o Pseudowire packets
The rules for processing LSP payloads that are network-layer protocol
packets SHALL be as specified in [RFC3032].
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The rules for processing LSP payloads that are pseudowire packets
SHALL be as specified in [RFC3985] and the attendant standards
defined by the IETF Pseudowire Emulation Edge-to-Edge (PWE3) Working
Group.
Note that the payload of an MPLS-TP LSP may be a packet type that
itself contains one or more MPLS labels. This is true, for instance,
when the payload is a pseudowire or another MPLS-TP LSP. From the
data-plane perspective, however, an MPLS-TP packet is an MPLS packet
as specified in [RFC3032], and so in particular has precisely one
label stack, and one label in the stack with its S (Bottom of Stack)
bit set to 1.
3.1.3. LSP Types
The MPLS-TP includes the following LSP types:
o Point-to-point unidirectional
o Point-to-point associated bidirectional
o Point-to-point co-routed bidirectional
o Point-to-multipoint unidirectional
Point-to-point unidirectional LSPs are supported by the basic MPLS
architecture [RFC3031] and are REQUIRED to function in the same
manner in the MPLS-TP data plane except as explicitly stated
otherwise in this document.
A point-to-point associated bidirectional LSP between LSRs A and B
consists of two unidirectional point-to-point LSPs, one from A to B
and the other from B to A, which are regarded as a pair providing a
single logical bidirectional transport path. The nodes A and B are
REQUIRED to be aware of this pairing relationship, but other nodes
need not be.
A point-to-point co-routed bidirectional LSP is a point-to-point
associated bidirectional LSP with the additional constraint that its
two unidirectional component LSPs follow the same path in the
network. This means that if one of the component LSPs follows the
path through the nodes N0, ..., Nk, originating on N0 and terminating
on Nk, then the path of the other component LSP is Nk, ..., N0, and
that at each node an ingress interface of one component LSP is an
egress interface of the other. In addition, each node along the path
is REQUIRED to be aware of the pairing relationship between the
component LSPs.
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A point-to-multipoint unidirectional LSP functions in the same manner
in the data plane, with respect to basic label processing and packet-
switching operations, as a point-to-point unidirectional LSP, with
one difference: an LSR may have more than one (egress interface,
outgoing label) pair associated with the LSP, and any packet it
transmits on the LSP is transmitted out all associated egress
interfaces. Point-to-multipoint LSPs are described in [RFC4875] and
[RFC5332].
3.2. Sections
Two MPLS-TP LSRs are considered to be topologically adjacent at a
particular layer n >= 0 of the MPLS-TP LSP hierarchy if there exists
a link between them at the next lowest network layer. Such a link,
if it exists, will be either an MPLS-TP LSP (if n > 0) or a data-link
provided by the underlying server layer network (if n = 0), and is
referred to as an MPLS-TP Section at layer n of the MPLS-TP LSP
hierarchy. Thus, the links traversed by a layer n+1 MPLS-TP LSP are
layer n MPLS-TP sections. Such an LSP is referred to as a client of
the section layer, and the section layer as the server layer with
respect to its clients.
Note that the MPLS label stack associated with an MPLS-TP section at
layer n consists of n labels, in the absence of stack optimisation
mechanisms such as PHP. Note also that in order for two LSRs to
exchange MPLS-TP control packets over a section, an additional label,
the G-ACh Label (GAL) (see Section 4) must appear at the bottom of
the label stack.
An MPLS-TP section may provide one or more of the following types of
service to its client layer:
o Point-to-point bidirectional
o Point-to-point unidirectional
o Point-to-multipoint unidirectional
The manner in which a section provides such a service is outside the
scope of the MPLS-TP.
Note that an LSP of any of the types listed in Section 3.1.3 may
serve as a section for a client-layer transport entity as long as it
supports the type of service the client requires.
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3.3. Pseudowires
The data-plane architectures for Single-Segment Pseudowires
[RFC3985], Multi-Segment Pseudowires [RFC5659], and Point-to-
Multipoint Pseudowires [I-D.ietf-pwe3-p2mp-pw-requirements], and the
associated data-plane pseudowire protocol functions, as defined by
the IETF Pseudowire Emulation Edge-to-Edge (PWE3) Working Group, are
included in the MPLS-TP.
This document specifies no modifications or extensions to pseudowire
data-plane architectures or protocols.
4. MPLS-TP Generic Associated Channel
The MPLS Generic Associated Channel (G-ACh) mechanism is specified in
[RFC5586] and included in the MPLS-TP. The G-ACh provides an
auxiliary logical data channel associated with MPLS-TP Sections,
LSPs, and PWs in the data plane. The primary purpose of the G-ACh in
the context of MPLS-TP is to support control, management, and OAM
traffic associated with MPLS-TP transport entities. The G-ACh MUST
NOT be used to transport client layer network traffic in MPLS-TP
networks.
For pseudowires, the G-ACh uses the first four bits of the PW control
word to provide the initial discrimination between data packets and
packets belonging to the associated channel, as described in
[RFC4385]. When this first nibble of a packet, immediately following
the label at the bottom of stack, has a value of '1', then this
packet belongs to a G-ACh. The first 32 bits following the bottom of
stack label then have a defined format called an Associated Channel
Header (ACH), which further defines the content of the packet. The
ACH is therefore both a demultiplexer for G-ACh traffic on the PW,
and a discriminator for the type of G-ACh traffic.
When the the control message is carried over a section or an LSP,
rather than over a PW, it is necessary to provide an indication in
the packet that the payload is something other than a client data
packet. This is achieved by including a reserved label with a value
of 13 in the label stack. This reserved label is referred to as the
G-ACh Label (GAL), and is defined in [RFC5586]. When a GAL is found,
it indicates that the payload begins with an ACH. The GAL is thus a
demultiplexer for G-ACh traffic on the section or the LSP, and the
ACH is a discriminator for the type of traffic carried on the G-ACh.
Note however that MPLS-TP forwarding follows the normal MPLS model,
and that a GAL is invisible to an LSR unless it is the top label in
the label stack. The only other circumstance under which the label
stack may be inspected for a GAL is when the TTL has expired. Any
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MPLS-TP component that intentionally performs this inspection must
assume that it is asynchronous with respect to the forwarding of
other packets. All operations on the label stack are in accordance
with [RFC3031] and [RFC3032].
5. Server Layer Considerations
This section discusses considerations for support of the MPLS-TP data
plane by server layer technologies and media.
In general, the MPLS-TP network has no awareness of the internals of
the server layer of which it is a client, requiring only that the
server layer be capable of delivering the type of service required by
the MPLS-TP transport entities that make use of it. Note that what
appears to be a single server layer link to the MPLS-TP network may
be a complicated construct underneath, such as an LSP or a collection
of underlying links operating as a bundle. Special care may be
needed in network design and operation when such constructs are used
as a server layer for MPLS-TP.
5.1. Ethernet Media
5.1.1. Point-to-Point Links
When two MPLS-TP nodes are connected by a point-to-point Ethernet
link, the question arises as to what destination Ethernet Media
Access Control (MAC) address should be specified in Ethernet frames
transmitted to the peer node over the link. The problem of
determining this address does not arise in IP/MPLS networks because
of the presence of the Ethernet Address Resolution Protocol (ARP)
[RFC0826] or IP version 6 Neighbor Discovery protocol [RFC4861],
which allow the unicast MAC address of the peer device to be learned
dynamically.
If existing mechanisms are available in an MPLS-TP network to
determine the destination unicast MAC addresses of peer nodes - for
example if the network also happens to be an IP/MPLS network - such
mechanisms SHOULD be used. The remainder of this section discusses
the available options when this is not the case.
One possibility is for each node to be statically configured with the
MAC address of its peer. Static MAC address configuration MAY be
used in an MPLS-TP network, but can present an administrative burden
and lead to operational problems. For example, replacement of an
Ethernet interface to resolve a hardware fault when this approach is
used requires that the peer node be manually reconfigured with the
new MAC address. This is especially problematic if the peer is
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operated by another provider.
Another possibility is to use the Ethernet broadcast address, but
this may lead to excessive frame distribution and processing at the
Ethernet layer. Broadcast traffic may also be treated specially by
some devices and this may not be desirable for MPLS-TP data frames.
The preferred approach is therefore to use as the destination MAC
address an Ethernet multicast address reserved for MPLS-TP for use
over point-to-point links. The address allocated for this purpose by
the Internet Assigned Numbers Authority (IANA) is 01-00-5E-XX-XX-XX.
An MPLS-TP implementation MUST process Ethernet frames received over
a point-to-point link with this destination MAC address by default.
Note that this approach is applicable only when the attached Ethernet
link is known to be point-to-point. If a link is not known to be
point-to-point, the reserved MAC address noted above MUST NOT be
used.
A further alternative is to adapt or introduce a protocol mechanism
for learning the Ethernet unicast MAC addresses of MPLS-TP peers that
are not also IP peers. This topic is for further study.
5.1.2. Multipoint Links
When a multipoint Ethernet link serves as a section for a point-to-
multipoint MPLS-TP LSP, and multicast destination MAC addressing at
the Ethernet layer is used for the LSP, the addressing and
encapsulation procedures specified in [RFC5332] SHALL be used.
When a multipoint Ethernet link - that is, a link which is not known
to be point-to-point - serves as a section for a point-to-point
MPLS-TP LSP, unicast destination MAC addresses must be used for
Ethernet frames carrying packets of the LSP. Note that according to
the discussion in the previous section, this implies the use of
either static MAC address configuration or a protocol that enables
peer MAC address discovery.
6. Security Considerations
This document serves primarily to specify which aspects of existing
MPLS data-plane functionality apply to MPLS-TP. As such it
introduces no new security considerations in itself, but the security
considerations documented in the specifications to which it refers
apply as well to MPLS-TP.
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7. IANA Considerations
The authors request that IANA allocate an Ethernet Multicast Address
from the Ethernet Multicast Addresses table in the ethernet-numbers
registry for use by MPLS-TP LSRs over point-to-point links as
described in Section 5.1.1. The entry should specify an address of
the form 01-00-5E-XX-XX-XX, a Type Field of 8847/8848, and a usage
"MPLS-TP point-to-point (this draft)".
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC5654] Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N.,
and S. Ueno, "Requirements of an MPLS Transport Profile",
RFC 5654, September 2009.
[RFC5586] Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
Associated Channel", RFC 5586, June 2009.
[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
Protocol Label Switching (MPLS) Support of Differentiated
Services", RFC 3270, May 2002.
[RFC3443] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
in Multi-Protocol Label Switching (MPLS) Networks",
RFC 3443, January 2003.
[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
Class" Field", RFC 5462, February 2009.
[RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
Label Assignment and Context-Specific Label Space",
RFC 5331, August 2008.
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[RFC4875] Aggarwal, R., Papadimitriou, D., and S. Yasukawa,
"Extensions to Resource Reservation Protocol - Traffic
Engineering (RSVP-TE) for Point-to-Multipoint TE Label
Switched Paths (LSPs)", RFC 4875, May 2007.
[RFC5332] Eckert, T., Rosen, E., Aggarwal, R., and Y. Rekhter, "MPLS
Multicast Encapsulations", RFC 5332, August 2008.
[RFC4385] 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.
8.2. Informative References
[I-D.ietf-mpls-tp-framework]
Bocci, M., Bryant, S., Frost, D., Levrau, L., and L.
Berger, "A Framework for MPLS in Transport Networks",
draft-ietf-mpls-tp-framework-10 (work in progress),
February 2010.
[I-D.ietf-pwe3-p2mp-pw-requirements]
Heron, G., Wang, L., Aggarwal, R., Vigoureux, M., Bocci,
M., Jin, L., JOUNAY, F., Niger, P., Kamite, Y., DeLord,
S., and L. Martini, "Requirements for Point-to-Multipoint
Pseudowire", draft-ietf-pwe3-p2mp-pw-requirements-02 (work
in progress), January 2010.
[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC5659] Bocci, M. and S. Bryant, "An Architecture for Multi-
Segment Pseudowire Emulation Edge-to-Edge", RFC 5659,
October 2009.
[RFC0826] Plummer, D., "Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet
address for transmission on Ethernet hardware", STD 37,
RFC 826, November 1982.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
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Authors' Addresses
Dan Frost (editor)
Cisco Systems
Email: danfrost@cisco.com
Stewart Bryant (editor)
Cisco Systems
Email: stbryant@cisco.com
Matthew Bocci (editor)
Alcatel-Lucent
Email: matthew.bocci@alcatel-lucent.com
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