Request for Comments: 3985 Cisco Systems
Category: Informational P. Pate, Ed.
Overture Networks, Inc.
March 2005
Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes an architecture for Pseudo Wire Emulation
Edge-to-Edge (PWE3). It discusses the emulation of services such as
Frame Relay, ATM, Ethernet, TDM, and SONET/SDH over packet switched
networks (PSNs) using IP or MPLS. It presents the architectural
framework for pseudo wires (PWs), defines terminology, and specifies
the various protocol elements and their functions.
Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Pseudo Wire Definition. . . . . . . . . . . . . . . . . 2
1.2. PW Service Functionality. . . . . . . . . . . . . . . . 3
1.3. Non-Goals of This Document. . . . . . . . . . . . . . . 4
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . 4
2. PWE3 Applicability. . . . . . . . . . . . . . . . . . . . . . 6
3. Protocol Layering Model . . . . . . . . . . . . . . . . . . . 6
3.1. Protocol Layers . . . . . . . . . . . . . . . . . . . . 7
3.2. Domain of PWE3. . . . . . . . . . . . . . . . . . . . . 8
3.3. Payload Types . . . . . . . . . . . . . . . . . . . . . 8
4. Architecture of Pseudo Wires. . . . . . . . . . . . . . . . . 11
4.1. Network Reference Model . . . . . . . . . . . . . . . . 12
4.2. PWE3 Pre-processing . . . . . . . . . . . . . . . . . . 12
4.3. Maintenance Reference Model . . . . . . . . . . . . . . 16
4.4. Protocol Stack Reference Model. . . . . . . . . . . . . 17
4.5. Pre-processing Extension to Protocol Stack Reference
Model . . . . . . . . . . . . . . . . . . . . . . . . . 17
5. PW Encapsulation. . . . . . . . . . . . . . . . . . . . . . . 18
5.1. Payload Convergence Layer . . . . . . . . . . . . . . . 19
5.2. Payload-independent PW Encapsulation Layers . . . . . . 21
5.3. Fragmentation . . . . . . . . . . . . . . . . . . . . . 24
5.4. Instantiation of the Protocol Layers. . . . . . . . . . 24
6. PW Demultiplexer Layer and PSN Requirements . . . . . . . . . 27
6.1. Multiplexing. . . . . . . . . . . . . . . . . . . . . . 27
6.2. Fragmentation . . . . . . . . . . . . . . . . . . . . . 28
6.3. Length and Delivery . . . . . . . . . . . . . . . . . . 28
6.4. PW-PDU Validation . . . . . . . . . . . . . . . . . . . 28
6.5. Congestion Considerations . . . . . . . . . . . . . . . 28
7. Control Plane . . . . . . . . . . . . . . . . . . . . . . . . 29
7.1. Set-up or Teardown of Pseudo Wires. . . . . . . . . . . 29
7.2. Status Monitoring . . . . . . . . . . . . . . . . . . . 30
7.3. Notification of Pseudo Wire Status Changes. . . . . . . 30
7.4. Keep-alive. . . . . . . . . . . . . . . . . . . . . . . 31
7.5. Handling Control Messages of the Native Services. . . . 32
8. Management and Monitoring . . . . . . . . . . . . . . . . . . 32
8.1. Status and Statistics . . . . . . . . . . . . . . . . . 32
8.2. PW SNMP MIB Architecture. . . . . . . . . . . . . . . . 33
8.3. Connection Verification and Traceroute. . . . . . . . . 36
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
10. Security Considerations . . . . . . . . . . . . . . . . . . . 37
11. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . 38
12. References. . . . . . . . . . . . . . . . . . . . . . . . . . 38
12.1. Normative References . . . . . . . . . . . . . . . . . 38
12.2. Informative References . . . . . . . . . . . . . . . . 39
13. Co-Authors. . . . . . . . . . . . . . . . . . . . . . . . . . 40
14. Editors’ Addresses. . . . . . . . . . . . . . . . . . . . . . 41
Full Copyright Statement. . . . . . . . . . . . . . . . . . . 42
1. Introduction
This document describes an architecture for Pseudo Wire Emulation
Edge-to-Edge (PWE3) in support of [RFC3916]. It discusses the
emulation of services such as Frame Relay, ATM, Ethernet, TDM, and
SONET/SDH over packet switched networks (PSNs) using IP or MPLS. It
presents the architectural framework for pseudo wires (PWs), defines
terminology, and specifies the various protocol elements and their
functions.
1.1. Pseudo Wire Definition
PWE3 is a mechanism that emulates the essential attributes of a
telecommunications service (such as a T1 leased line or Frame Relay)
over a PSN. PWE3 is intended to provide only the minimum necessary
functionality to emulate the wire with the required degree of
faithfulness for the given service definition. Any required
switching functionality is the responsibility of a forwarder function
(FWRD). Any translation or other operation needing knowledge of the
payload semantics is carried out by native service processing (NSP)
elements. The functional definition of any FWRD or NSP elements is
outside the scope of PWE3.
The required functions of PWs include encapsulating service-specific
bit streams, cells, or PDUs arriving at an ingress port and carrying
them across an IP path or MPLS tunnel. In some cases it is necessary
to perform other operations such as managing their timing and order,
to emulate the behavior and characteristics of the service to the
required degree of faithfulness.
From the perspective of Customer Edge Equipment (CE), the PW is
characterized as an unshared link or circuit of the chosen service.
In some cases, there may be deficiencies in the PW emulation that
impact the traffic carried over a PW and therefore limit the
applicability of this technology. These limitations must be fully
described in the appropriate service-specific documentation.
For each service type, there will be one default mode of operation
that all PEs offering that service type must support. However,
optional modes may be defined to improve the faithfulness of the
emulated service, if it can be clearly demonstrated that the
additional complexity associated with the optional mode is offset by
the value it offers to PW users.
1.2. PW Service Functionality
PWs provide the following functions in order to emulate the behavior
and characteristics of the native service.
o Encapsulation of service-specific PDUs or circuit data arriving
at the PE-bound port (logical or physical).
o Carriage of the encapsulated data across a PSN tunnel.
o Establishment of the PW, including the exchange and/or
distribution of the PW identifiers used by the PSN tunnel
endpoints.
o Managing the signaling, timing, order, or other aspects of the
service at the boundaries of the PW.
o Service-specific status and alarm management.
1.3. Non-Goals of This Document
The following are non-goals for this document:
o The on-the-wire specification of PW encapsulations.
o The detailed definition of the protocols involved in PW setup
and maintenance.
The following are outside the scope of PWE3:
o Any multicast service not native to the emulated medium. Thus,
Ethernet transmission to a "multicast" IEEE-48 address is in
scope, but multicast services such as MARS [RFC2022] that are
implemented on top of the medium are not.
o Methods to signal or control the underlying PSN.
1.4. Terminology
This document uses the following definitions of terms. These terms
are illustrated in context in Figure 2.
Attachment Circuit The physical or virtual circuit attaching
(AC) a CE to a PE. An attachment Circuit may be, for
example, a Frame Relay DLCI, an ATM VPI/VCI, an
Ethernet port, a VLAN, a PPP connection on a
physical interface, a PPP session from an L2TP
tunnel, or an MPLS LSP. If both physical and
virtual ACs are of the same technology (e.g.,
both ATM, both Ethernet, both Frame Relay), the
PW is said to provide "homogeneous transport";
otherwise, it is said to provide "heterogeneous
transport".
CE-bound The traffic direction in which PW-PDUs are
received on a PW via the PSN, processed, and
then sent to the destination CE.
CE Signaling Messages sent and received by the CE’s control
plane. It may be desirable or even necessary
for the PE to participate in or to monitor this
signaling in order to emulate the service
effectively.
Control Word (CW) A four-octet header used in some encapsulations
to carry per-packet information when the PSN is
MPLS.
Customer Edge (CE) A device where one end of a service originates
and/or terminates. The CE is not aware that it
is using an emulated service rather than a
native service.
Forwarder (FWRD) A PE subsystem that selects the PW to use in
order to transmit a payload received on an AC.
Fragmentation The action of dividing a single PDU into
multiple PDUs before transmission with the
intent of the original PDU being reassembled
elsewhere in the network. Packets may undergo
fragmentation if they are larger than the MTU of
the network they will traverse.
Maximum Transmission The packet size (excluding data link header)
unit (MTU) that an interface can transmit without needing
to fragment.
Native Service Processing of the data received by the PE
Processing (NSP) from the CE before presentation to the PW for
transmission across the core, or processing of
the data received from a PW by a PE before it is
output on the AC. NSP functionality is defined
by standards bodies other than the IETF, such as
ITU-T,ANSI, or ATMF.)
Packet Switched Within the context of PWE3, this is a
Network (PSN) network using IP or MPLS as the mechanism for
packet forwarding.
PE-Bound The traffic direction in which information from
a CE is adapted to a PW, and PW-PDUs are sent
into the PSN.
PE/PW Maintenance Used by the PEs to set up, maintain, and tear
down the PW. It may be coupled with CE
Signaling in order to manage the PW effectively.
Protocol Data The unit of data output to, or received
Unit (PDU) from, the network by a protocol layer.
Provider Edge (PE) A device that provides PWE3 to a CE.
Pseudo Wire (PW) A mechanism that carries the essential elements
of an emulated service from one PE to one or
more other PEs over a PSN.
Pseudo Wire A mechanism that emulates the essential
Emulation Edge to attributes of service (such as a T1 leased
Edge (PWE3) line or Frame Relay) over a PSN.
Pseudo Wire PDU A PDU sent on the PW that contains all of
(PW-PDU) the data and control information necessary to
emulate the desired service.
PSN Tunnel A tunnel across a PSN, inside which one or more
PWs can be carried.
PSN Tunnel Used to set up, maintain, and tear down the
Signaling underlying PSN tunnel.
PW Demultiplexer Data-plane method of identifying a PW
terminating at a PE.
Time Domain Time Division Multiplexing. Frequently used
Multiplexing (TDM) to refer to the synchronous bit streams at rates
defined by G.702.
Tunnel A method of transparently carrying information
over a network.
2. PWE3 Applicability
The PSN carrying a PW will subject payload packets to loss, delay,
delay variation, and re-ordering. During a network transient there
may be a sustained period of impaired service. The applicability of
PWE3 to a particular service depends on the sensitivity of that
service (or the CE implementation) to these effects, and on the
ability of the adaptation layer to mask them. Some services, such as
IP over FR over PWE3, may prove quite resilient to IP and MPLS PSN
characteristics. Other services, such as the interconnection of PBX
systems via PWE3, will require more careful consideration of the PSN
and adaptation layer characteristics. In some instances, traffic
engineering of the underlying PSN will be required, and in some cases
the constraints may make the required service guarantees impossible
to provide.
3. Protocol Layering Model
The PWE3 protocol-layering model is intended to minimize the
differences between PWs operating over different PSN types. The
design of the protocol-layering model has the goals of making each PW
definition independent of the underlying PSN, and of maximizing the
reuse of IETF protocol definitions and their implementations.
3.1. Protocol Layers
The logical protocol-layering model required to support a PW is shown
in Figure 1.
+---------------------------+
| Payload |
+---------------------------+
| Encapsulation | <==== may be empty
+---------------------------+
| PW Demultiplexer |
+---------------------------+
| PSN Convergence | <==== may be empty
+---------------------------+
| PSN |
+---------------------------+
| Data-Link |
+---------------------------+
| Physical |
+---------------------------+
Figure 1. Logical Protocol Layering Model
The payload is transported over the Encapsulation Layer. The
Encapsulation Layer carries any information, not already present
within the payload itself, that is needed by the PW CE-bound PE
interface to send the payload to the CE via the physical interface.
If no further information is needed in the payload itself, this layer
is empty.
The Encapsulation Layer also provides support for real-time
processing, and if needed for sequencing.
The PW Demultiplexer layer provides the ability to deliver multiple
PWs over a single PSN tunnel. The PW demultiplexer value used to
identify the PW in the data plane may be unique per PE, but this is
not a PWE3 requirement. It must, however, be unique per tunnel
endpoint. If it is necessary to identify a particular tunnel, then
that is the responsibility of the PSN layer.
The PSN Convergence layer provides the enhancements needed to make
the PSN conform to the assumed PSN service requirement. Therefore,
this layer provides a consistent interface to the PW, making the PW
independent of the PSN type. If the PSN already meets the service
requirements, this layer is empty.
The PSN header, MAC/Data-Link, and Physical Layer definitions are
outside the scope of this document. The PSN can be IPv4, IPv6, or
MPLS.
3.2. Domain of PWE3
PWE3 defines the Encapsulation Layer, the method of carrying various
payload types, and the interface to the PW Demultiplexer Layer. It
is expected that the other layers will be provided by tunneling
methods such as L2TP or MPLS over the PSN.
3.3. Payload Types
The payload is classified into the following generic types of native
data units:
o Packet
o Cell
o Bit stream
o Structured bit stream
Within these generic types there are specific service types:
Generic Payload Type PW Service
-------------------- ----------
Packet Ethernet (all types), HDLC framing,
Frame Relay, ATM AAL5 PDU.
Cell ATM.
Bit stream Unstructured E1, T1, E3, T3.
Structured bit stream SONET/SDH (e.g., SPE, VT, NxDS0).
3.3.1. Packet Payload
A packet payload is a variable-size data unit delivered to the PE via
the AC. A packet payload may be large compared to the PSN MTU. The
delineation of the packet boundaries is encapsulation specific. HDLC
or Ethernet PDUs can be considered examples of packet payloads.
Typically, a packet will be stripped of transmission overhead such as
HDLC flags and stuffing bits before transmission over the PW.
A packet payload would normally be relayed across the PW as a single
unit. However, there will be cases where the combined size of the
packet payload and its associated PWE3 and PSN headers exceeds the
PSN path MTU. In these cases, some fragmentation methodology has to
be applied. This may, for example, be the case when a user provides
the service and attaches to the service provider via Ethernet, or
when nested pseudo-wires are involved. Fragmentation is discussed in
more detail in section 5.3.
A packet payload may need sequencing and real-time support.
In some situations, the packet payload may be selected from the
packets presented on the emulated wire on the basis of some sub-
multiplexing technique. For example, one or more Frame Relay PDUs
may be selected for transport over a particular pseudo wire based on
the Frame Relay Data-Link Connection Identifier (DLCI), or, in the
case of Ethernet payloads, by using a suitable MAC bridge filter.
This is a forwarder function, and this selection would therefore be
made before the packet was presented to the PW Encapsulation Layer.
3.3.2. Cell Payload
A cell payload is created by capturing, transporting, and replaying
groups of octets presented on the wire in a fixed-size format. The
delineation of the group of bits that comprise the cell is specific
to the encapsulation type. Two common examples of cell payloads are
ATM 53-octet cells, and the larger 188-octet MPEG Transport Stream
packets [DVB].
To reduce per-PSN packet overhead, multiple cells may be concatenated
into a single payload. The Encapsulation Layer may consider the
payload complete on the expiry of a timer, after a fixed number of
cells have been received or when a significant cell (e.g., an ATM OAM
cell) has been received. The benefit of concatenating multiple PDUs
should be weighed against a possible increase in packet delay
variation and the larger penalty incurred by packet loss. In some
cases, it may be appropriate for the Encapsulation Layer to perform