Network Working Group S. Bryant, Ed.
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
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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
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(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.
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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.
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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.
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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.
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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.
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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
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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
some type of compression, such as silence suppression or voice
compression.
The generic cell payload service will normally need sequence number
support and may also need real-time support. The generic cell
payload service would not normally require fragmentation.
The Encapsulation Layer may apply some form of compression to some of
these sub-types (e.g., idle cells may be suppressed).
In some instances, the cells to be incorporated in the payload may be
selected by filtering them from the stream of cells presented on the
wire. For example, an ATM PWE3 service may select cells based on
their VCI or VPI fields. This is a forwarder function, and the
selection would therefore be made before the packet was presented to
the PW Encapsulation Layer.
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3.3.3. Bit Stream
A bit stream payload is created by capturing, transporting, and
replaying the bit pattern on the emulated wire, without taking
advantage of any structure that, on inspection, may be visible within
the relayed traffic (i.e., the internal structure has no effect on
the fragmentation into packets).
In some instances it is possible to apply suppression to bit streams.
For example, E1 and T1 send "all-ones" to indicate failure. This
condition can be detected without any knowledge of the structure of
the bit stream, and transmission of packetized can be data
suppressed.
This service will require sequencing and real-time support.
3.3.4. Structured Bit Stream
A structured bit stream payload is created by using some knowledge of
the underlying structure of the bit stream to capture, transport, and
replay the bit pattern on the emulated wire.
Two important points distinguish structured and unstructured bit
streams:
o Some parts of the original bit stream may be stripped in the
PSN-bound direction by an NSP block. For example, in
Structured SONET the section and line overhead (and possibly
more) may be stripped. A framer is required to enable such
stripping. It is also required for frame/payload alignment for
fractional T1/E1 applications.
o The PW must preserve the structure across the PSN so that the
CE-bound NSP block can insert it correctly into the
reconstructed unstructured bit stream. The stripped
information (such as SONET pointer justifications) may appear
in the encapsulation layer to facilitate this reconstitution.
As an option, the Encapsulation Layer may also perform silence/idle
suppression or similar compression on a structured bit stream.
Structured bit streams are distinguished from cells in that the
structures may be too long to be carried in a single packet. Note
that "short" structures are indistinguishable from cells and may
benefit from the use of methods described in section 3.3.2.
This service requires sequencing and real-time support.
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3.3.5. Principle of Minimum Intervention
To minimize the scope of information, and to improve the efficiency
of data flow through the Encapsulation Layer, the payload should be
transported as received, with as few modifications as possible
[RFC1958].
This minimum intervention approach decouples payload development from
PW development and requires fewer translations at the NSP in a system
with similar CE interfaces at each end. It also prevents unwanted
side effects due to subtle misrepresentation of the payload in the
intermediate format.
An approach that does intervene can be more wire efficient in some
cases and may result in fewer translations at the NSP whereby the CE
interfaces are of different types. Any intermediate format
effectively becomes a new framing type, requiring documentation and
assured interoperability. This increases the amount of work for
handling the protocol that the intermediate format carries and is
undesirable.
4. Architecture of Pseudo Wires
This section describes the PWE3 architectural model.
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4.1. Network Reference Model
Figure 2 illustrates the network reference model for point-to-point
PWs.
|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudo Wire ------>| |
| | | |
| | |<-- PSN Tunnel -->| | |
| V V V V |
V AC +----+ +----+ AC V
+-----+ | | PE1|==================| PE2| | +-----+
| |----------|............PW1.............|----------| |
| CE1 | | | | | | | | CE2 |
| |----------|............PW2.............|----------| |
+-----+ ^ | | |==================| | | ^ +-----+
^ | +----+ +----+ | | ^
| | Provider Edge 1 Provider Edge 2 | |
| | | |
Customer | | Customer
Edge 1 | | Edge 2
| |
| |
Native service Native service
Figure 2. PWE3 Network Reference Model
The two PEs (PE1 and PE2) have to provide one or more PWs on behalf
of their client CEs (CE1 and CE2) to enable the client CEs to
communicate over the PSN. A PSN tunnel is established to provide a
data path for the PW. The PW traffic is invisible to the core
network, and the core network is transparent to the CEs. Native data
units (bits, cells, or packets) arrive via the AC, are encapsulated
in a PW-PDU, and are carried across the underlying network via the
PSN tunnel. The PEs perform the necessary encapsulation and
decapsulation of PW-PDUs and handle any other functions required by
the PW service, such as sequencing or timing.
4.2. PWE3 Pre-processing
Some applications have to perform operations on the native data units
received from the CE (including both payload and signaling traffic)
before they are transmitted across the PW by the PE. Examples
include Ethernet bridging, SONET cross-connect, translation of
locally-significant identifiers such as VCI/VPI, or translation to
another service type. These operations could be carried out in
external equipment, and the processed data could be sent to the PE
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over one or more physical interfaces. In most cases, could be in
undertaking these operations within the PE provides cost and
operational benefits. Processed data is then presented to the PW via
a virtual interface within the PE. These pre-processing operations
are included in the PWE3 reference model to provide a common
reference point, but the detailed description of these operations is
outside the scope of the PW definition given here.
PW
End Service
|
|<------- Pseudo Wire ------>|
| |
| |<-- PSN Tunnel -->| |
V V V V PW
+-----+----+ +----+ End Service
+-----+ |PREP | PE1|==================| PE2| | +-----+
| | | |............PW1.............|----------| |
| CE1 |----| | | | | | | CE2 |
| | ^ | |............PW2.............|----------| |
+-----+ | | | |==================| | | ^ +-----+
| +-----+----+ +----+ | |
| ^ | |
| | | |
| |<------- Emulated Service ------->| |
| | |
| Virtual physical |
| termination |
| ^ |
CE1 native | CE2 native
service | service
|
CE2 native
service
Figure 3. Pre-processing within the PWE3 Network Reference Model
Figure 3 shows the interworking of one PE with pre-processing (PREP),
and a second without this functionality. This reference point
emphasizes that the functional interface between PREP and the PW is
that represented by a physical interface carrying the service. This
effectively defines the necessary inter-working specification.
The operation of a system in which both PEs include PREP
functionality is also supported.
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The required pre-processing can be divided into two components:
o Forwarder (FWRD)
o Native Service Processing (NSP)
4.2.1. Forwarders
Some applications have to forward payload elements selectively from
one or more ACs to one or more PWs. In such cases, there will also be
a need to perform the inverse function on PWE3-PDUs received by a PE
from the PSN. This is the function of the forwarder.
The forwarder selects the PW based on, for example, the incoming AC,
the contents of the payload, or some statically and/or dynamically
configured forwarding information.
+----------------------------------------+
| PE Device |
+----------------------------------------+
Single | | |
AC | | Single | PW Instance
<------>o Forwarder + PW Instance X<===========>
| | |
+----------------------------------------+
Figure 4a. Simple Point-to-Point Service
+----------------------------------------+
| PE Device |
+----------------------------------------+
Multiple| | Single | PW Instance
AC | + PW Instance X<===========>
<------>o | |
| |----------------------|
<------>o | Single | PW Instance
| Forwarder + PW Instance X<===========>
<------>o | |
| |----------------------|
<------>o | Single | PW Instance
| + PW Instance X<===========>
<------>o | |
+----------------------------------------+
Figure 4b. Multiple AC to Multiple PW Forwarding
Figure 4a shows a simple forwarder that performs some type of
filtering operation. Because the forwarder has a single input and a
single output interface, filtering is the only type of forwarding
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operation that applies. Figure 4b shows a more general forwarding
situation where payloads are extracted from one or more ACs and
directed to one or more PWs. In this case filtering, direction, and
combination operations may be performed on the payloads. For
example, if the AC were Frame Relay, the forwarder might perform
Frame Relay switching and the PW instances might be the inter-switch
links.
4.2.2. Native Service Processing
Some applications required some form of data or address translation,
or some other operation requiring knowledge of the semantics of the
payload. This is the function of the Native Service Processor (NSP).
The use of the NSP approach simplifies the design of the PW by
restricting a PW to homogeneous operation. NSP is included in the
reference model to provide a defined interface to this functionality.
The specification of the various types of NSP is outside the scope of
PWE3.
+----------------------------------------+
| PE Device |
Multiple+----------------------------------------+
AC | | | Single | PW Instance
<------>o NSP # + PW Instance X<===========>
| | | |
|------| |----------------------|
| | | Single | PW Instance
<------>o NSP #Forwarder + PW Instance X<===========>
| | | |
|------| |----------------------|
| | | Single | PW Instance
<------>o NSP # + PW Instance X<===========>
| | | |
+----------------------------------------+
Figure 5. NSP in a Multiple AC to Multiple PW Forwarding PE
Figure 5 illustrates the relationship between NSP, forwarder, and PWs
in a PE. The NSP function may apply any transformation operation
(modification, injection, etc.) on the payloads as they pass between
the physical interface to the CE and the virtual interface to the
forwarder. These transformation operations will, of course, be
limited to those that have been implemented in the data path, and
that are enabled by the PE configuration. A PE device may contain
more than one forwarder.
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This model also supports the operation of a system in which the NSP
functionality includes terminating the data-link, and the application
of Network Layer processing to the payload.
4.3. Maintenance Reference Model
Figure 6 illustrates the maintenance reference model for PWs.
|<------- CE (end-to-end) Signaling ------>|
| |<---- PW/PE Maintenance ----->| |
| | |<-- PSN Tunnel -->| | |
| | | Signaling | | |
| V V (out of scope) V V |
v +-----+ +-----+ v
+-----+ | PE1 |==================| PE2 | +-----+
| |-----|.............PW1..............|-----| |
| CE1 | | | | | | CE2 |
| |-----|.............PW2..............|-----| |
+-----+ | |==================| | +-----+
+-----+ +-----+
Customer Provider Provider Customer
Edge 1 Edge 1 Edge 2 Edge 2
Figure 6. PWE3 Maintenance Reference Model
The following signaling mechanisms are required:
o The CE (end-to-end) signaling is between the CEs. This
signaling could be Frame Relay PVC status signaling, ATM SVC
signaling, TDM CAS signaling, etc.
o The PW/PE Maintenance is used between the PEs (or NSPs) to set
up, maintain, and tear down PWs, including any required
coordination of parameters.
o The PSN Tunnel signaling controls the PW multiplexing and some
elements of the underlying PSN. Examples are L2TP control
protocol, MPLS LDP, and RSVP-TE. The definition of the
information that PWE3 needs signaled is within the scope of
PWE3, but the signaling protocol itself is not.
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4.4. Protocol Stack Reference Model
Figure 7 illustrates the protocol stack reference model for PWs.
+-----------------+ +-----------------+
|Emulated Service | |Emulated Service |
|(e.g., TDM, ATM) |<==== Emulated Service ===>|(e.g., TDM, ATM) |
+-----------------+ +-----------------+
| Payload | | Payload |
| Encapsulation |<====== Pseudo Wire ======>| Encapsulation |
+-----------------+ +-----------------+
|PW Demultiplexer | |PW Demultiplexer |
| PSN Tunnel, |<======= PSN Tunnel ======>| PSN Tunnel, |
| PSN & Physical | | PSN & Physical |
| Layers | | Layers |
+-------+---------+ ___________ +---------+-------+
| / \ |
+===============/ PSN \===============+
\ /
\_____________/
Figure 7. PWE3 Protocol Stack Reference Model
The PW provides the CE with an emulated physical or virtual
connection to its peer at the far end. Native service PDUs from the
CE are passed through an Encapsulation Layer at the sending PE and
then sent over the PSN. The receiving PE removes the encapsulation
and restores the payload to its native format for transmission to the
destination CE.
4.5. Pre-processing Extension to Protocol Stack Reference Model
Figure 8 illustrates how the protocol stack reference model is
extended to include the provision of pre-processing (forwarding and
NSP). This shows the placement of the physical interface relative to
the CE.
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/======================================\
H Forwarder H<----Pre-processing
H----------------======================/
H Native Service H | |
H Processing H | |
\================/ | |
| | | Emulated |
| Service | | Service |
| Interface | | (TDM, ATM, |
| (TDM, ATM, | | Ethernet, |<== Emulated Service ==
| Ethernet, | | Frame Relay, |
| Frame Relay, | | etc.) |
| etc.) | +-----------------+
| | | Payload |
| | | Encapsulation |<=== Pseudo Wire ======
| | +-----------------+
| | |PW Demultiplexer |
| | | PSN Tunnel, |
| | | PSN & Physical |<=== PSN Tunnel =======
| | | Headers |
+----------------+ +-----------------+
| Physical | | Physical |
+-------+--------+ +-------+---------+
| |
| |
| |
| |
| |
| |
To CE <---+ +---> To PSN
Figure 8. Protocol Stack Reference Model with Pre-processing
5. PW Encapsulation
The PW Encapsulation Layer provides the necessary infrastructure to
adapt the specific payload type being transported over the PW to the
PW Demultiplexer Layer used to carry the PW over the PSN.
The PW Encapsulation Layer consists of three sub-layers:
o Payload Convergence
o Timing
o Sequencing
The PW Encapsulation sub-layering and its context with the protocol
stack are shown in Figure 9.
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+---------------------------+
| Payload |
/===========================\ <------ Encapsulation
H Payload Convergence H Layer
H---------------------------H
H Timing H
H---------------------------H
H Sequencing H
\===========================/
| PW Demultiplexer |
+---------------------------+
| PSN Convergence |
+---------------------------+
| PSN |
+---------------------------+
| Data-Link |
+---------------------------+
| Physical |
+---------------------------+
Figure 9. PWE3 Encapsulation Layer in Context
The Payload Convergence sub-layer is highly tailored to the specific
payload type. However grouping a number of target payload types into
a generic class, and then providing a single convergence sub-layer
type common to the group, reduces the number of payload convergence
sub-layer types. This decreases implementation complexity. The
provision of per-packet signaling and other out-of-band information
(other than sequencing or timing) is undertaken by this layer.
The Timing and Sequencing Layers provide generic services to the
Payload Convergence Layer for all payload types that require them.
5.1. Payload Convergence Layer
5.1.1. Encapsulation
The primary task of the Payload Convergence Layer is the
encapsulation of the payload in PW-PDUs. The native data units to be
encapsulated may contain an L2 header or L1 overhead. This is
service specific. The Payload Convergence header carries the
additional information needed to replay the native data units at the
CE-bound physical interface. The PW Demultiplexer header is not
considered part of the PW header.
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Not all the additional information needed to replay the native data
units have to be carried in the PW header of the PW PDUs. Some
information (e.g., service type of a PW) may be stored as state
information at the destination PE during PW set up.
5.1.2. PWE3 Channel Types
The PW Encapsulation Layer and its associated signaling require one
or more of the following types of channels from its underlying PW
Demultiplexer and PSN Layers (channel type 1 plus one or more of
channel types 2 through 4):
1. A reliable control channel for signaling line events, status
indications, and, in exceptional cases, CE-CE events that must be
translated and sent reliably between PEs. PWE3 may need this type
of control channel to provide faithful emulation of complex data-
link protocols.
2. A high-priority, unreliable, sequenced channel. A typical use is
for CE-to-CE signaling. "High priority" may simply be indicated
via the DSCP bits for IP or the EXP bits for MPLS, giving the
packet priority during transit. This channel type could also use
a bit in the tunnel header itself to indicate that packets
received at the PE should be processed with higher priority
[RFC2474].
3. A sequenced channel for data traffic that is sensitive to packet
reordering (one classification for use could be for any non-IP
traffic).
4. An unsequenced channel for data traffic insensitive to packet
order.
The data channels (2, 3, and 4 above) should be carried "in band"
with one another to as much of a degree as is reasonably possible on
a PSN.
Where end-to-end connectivity may be disrupted by address translation
[RFC3022], access-control lists, firewalls, etc., the control channel
may be able to pass traffic and setup the PW, while the PW data
traffic is blocked by one or more of these mechanisms. In these
cases unless the control channel is also carried "in band", the
signaling to set up the PW will not confirm the existence of an end-
to-end data path. In some cases there is a need to synchronize CE
events with the data carried over a PW. This is especially the case
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with TDM circuits (e.g., the on-hook/off-hook events in PSTN switches
might be carried over a reliable control channel whereas the
associated bit stream is carried over a sequenced data channel).
PWE3 channel types that are not needed by the supported PWs need not
be included in such an implementation.
5.1.3. Quality of Service Considerations
Where possible, it is desirable to employ mechanisms to provide PW
Quality of Service (QoS) support over PSNs.
5.2. Payload-Independent PW Encapsulation Layers
Two PWE3 Encapsulation sub-layers provide common services to all
payload types: Sequencing and Timing. These services are optional
and are only used if a particular PW instance needs them. If the
service is not needed, the associated header may be omitted in order
to conserve processing and network resources.
Sometimes a specific payload type will require transport with or
without sequence and/or real-time support. For example, an invariant
of Frame Relay transport is the preservation of packet order. Some
Frame Relay applications expect delivery in order and may not cope
with reordering of the frames. However, where the Frame Relay
service is itself only being used to carry IP, it may be desirable to
relax this constraint to reduce per-packet processing cost.
The guiding principle is that, when possible, an existing IETF
protocol should be used to provide these services. When a suitable
protocol is not available, the existing protocol should be extended
or modified to meet the PWE3 requirements, thereby making that
protocol available for other IETF uses. In the particular case of
timing, more than one general method may be necessary to provide for
the full scope of payload timing requirements.
5.2.1. Sequencing
The sequencing function provides three services: frame ordering,
frame duplication detection, and frame loss detection. These
services allow the emulation of the invariant properties of a
physical wire. Support for sequencing depends on the payload type
and may be omitted if it is not needed.
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The size of the sequence-number space depends on the speed of the
emulated service, and on the maximum time of the transient conditions
in the PSN. A sequence number space greater than 2^16 may therefore
be needed to prevent the sequence number space from wrapping during
the transient.
5.2.1.1. Frame Ordering
When packets carrying the PW-PDUs traverse a PSN, they may arrive out
of order at the destination PE. For some services, the frames
(control frames, data frames, or both) must be delivered in order.
For these services, some mechanism must be provided for ensuring in-
order delivery. Providing a sequence number in the sequence sub-
layer header for each packet is one possible approach.
Alternatively, it can be noted that sequencing is a subset of the
problem of delivering timed packets, and that a single combined
mechanism such as [RFC3550] may be employed.
There are two possible misordering strategies:
o Drop misordered PW PDUs.
o Try to sort PW PDUs into the correct order.
The choice of strategy will depend on
o how critical the loss of packets is to the operation of the PW
(e.g., the acceptable bit error rate),
o the speeds of the PW and PSN,
o the acceptable delay (as delay must be introduced to reorder),
and
o the expected incidence of misordering.
5.2.1.2. Frame Duplication Detection
In rare cases, packets traversing a PW may be duplicated by the
underlying PSN. For some services, frame duplication is not
acceptable. For these services, some mechanism must be provided to
ensure that duplicated frames will not be delivered to the
destination CE. The mechanism may be the same as that used to ensure
in-order frame delivery.
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5.2.1.3. Frame Loss Detection
A destination PE can determine whether a frame has been lost by
tracking the sequence numbers of the PW PDUs received.
In some instances, if a PW PDU fails to arrive within a certain time,
a destination PE will have to presume that it is lost. If a PW-PDU
that has been processed as lost subsequently arrives, the destination
PE must discard it.
5.2.2. Timing
A number of native services have timing expectations based on the
characteristics of the networks they were designed to travel over.
The emulated service may have to duplicate these network
characteristics as closely as possible: e.g., in delivering native
traffic with bitrate, jitter, wander, and delay characteristics
similar to those received at the sending PE.
In such cases, the receiving PE has to play out the native traffic as
it was received at the sending PE. This relies on timing information
either sent between the two PEs, or in some cases received from an
external reference.
Therefore, Timing Sub-layer must support two timing functions: clock
recovery and timed payload delivery. A particular payload type may
require either or both of these services.
5.2.2.1. Clock Recovery
Clock recovery is the extraction of output transmission bit timing
information from the delivered packet stream, and it requires a
suitable mechanism. A physical wire carries the timing information
natively, but extracting timing from a highly jittered source, such
as packet stream, is a relatively complex task. Therefore, it is
desirable that an existing real-time protocol such as [RFC3550] be
used for this purpose, unless it can be shown that this is unsuitable
or unnecessary for a particular payload type.
5.2.2.2. Timed Delivery
Timed delivery is the delivery of non-contiguous PW PDUs to the PW
output interface with a constant phase relative to the input
interface. The timing of the delivery may be relative to a clock
derived from the packet stream received over the PSN clock recovery,
or to an external clock.
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5.3. Fragmentation
Ideally, a payload would be relayed across the PW as a single unit.
However, there will be cases where the combined size of the payload
and its associated PWE3 and PSN headers will exceed the PSN path MTU.
When a packet size exceeds the MTU of a given network, fragmentation
and reassembly have to be performed for the packet to be delivered.
Since fragmentation and reassembly generally consume considerable
network resources, as compared to simply switching a packet in its
entirety, the need for fragmentation and reassembly throughout a
network should be reduced or eliminated to the extent possible. Of
particular concern for fragmentation and reassembly are aggregation
points where large numbers of PWs are processed (e.g., at the PE).
Ideally, the equipment originating the traffic sent over the PW will
have adaptive measures in place (e.g., [RFC1191], [RFC1981]) that
ensure that packets needing to be fragmented are not sent. When this
fails, the point closest to the sending host with fragmentation and
reassembly capabilities should attempt to reduce the size of packets
to satisfy the PSN MTU. Thus, in the reference model for PWE3
(Figure 3), fragmentation should first be performed at the CE if
possible. Only if the CE cannot adhere to an acceptable MTU size for
the PW should the PE attempt its own fragmentation method.
In cases where MTU management fails to limit the payload to a size
suitable for transmission of the PW, the PE may fall back to either a
generic PW fragmentation method or, if available, the fragmentation
service of the underlying PSN.
It is acceptable for a PE implementation not to support
fragmentation. A PE that does not will drop packets that exceed the
PSN MTU, and the management plane of the encapsulating PE may be
notified.
If the length of a L2/L1 frame, restored from a PW PDU, exceeds the
MTU of the destination AC, it must be dropped. In this case, the
management plane of the destination PE may be notified.
5.4. Instantiation of the Protocol Layers
This document does not address the detailed mapping of the Protocol
Layering model to existing or future IETF standards. The
instantiation of the logical Protocol Layering model is shown in
Figure 9.
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5.4.1. PWE3 over an IP PSN
The protocol definition of PWE3 over an IP PSN should employ existing
IETF protocols where possible.
+---------------------+ +-------------------------+
| Payload |------------->| Raw payload if possible |
/=====================\ +-------------------------+
H Payload Convergence H-----------+->| Flags, seq #, etc. |
H---------------------H / +-------------------------+
H Timing H---------/--->| RTP |
H---------------------H / +-------------+ |
H Sequencing H----one of | |
\=====================/ \ | +-----------+
| PW Demultiplexer |---------+--->| L2TP, MPLS, etc. |
+---------------------+ +-------------------------+
| PSN Convergence |------------->| Not needed |
+---------------------+ +-------------------------+
| PSN |------------->| IP |
+---------------------+ +-------------------------+
| Data-Link |------------->| Data-link |
+---------------------+ +-------------------------+
| Physical |------------->| Physical |
+---------------------+ +-------------------------+
Figure 10. PWE3 over an IP PSN
Figure 10 shows the protocol layering for PWE3 over an IP PSN. As a
rule, the payload should be carried as received from the NSP, with
the Payload Convergence Layer provided when needed. However, in
certain circumstances it may be justifiable to transmit the payload
in some processed form. The reasons for this must be documented in
the Encapsulation Layer definition for that payload type.
Where appropriate, explicit timing is provided by RTP [RFC3550],
which, when used, also provides a sequencing service. When the PSN
is UDP/IP, the RTP header follows the UDP header and precedes the PW
control field. For all other cases the RTP header follows the PW
control header.
The encapsulation layer may additionally carry a sequence number.
Sequencing is to be provided either by RTP or by the PW encapsulation
layer, but not by both.
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PW Demultiplexing is provided by the PW label, which may take the
form specified in a number of IETF protocols; e.g., an MPLS label
[MPLSIP], an L2TP session ID [RFC3931], or a UDP port number
[RFC768]. When PWs are carried over IP, the PSN Convergence Layer
will not be needed.
As a special case, if the PW Demultiplexer is an MPLS label, the
protocol architecture of section 5.4.2 can be used instead of the
protocol architecture of this section.
5.4.2. PWE3 over an MPLS PSN
The MPLS ethos places importance on wire efficiency. By using a
control word, some components of the PWE3 protocol layers can be
compressed to increase this efficiency.
+---------------------+
| Payload |
/=====================\
H Payload Convergence H--+
H---------------------H | +--------------------------------+
H Timing H--------->| RTP |
H---------------------H | +--------------------------------+
H Sequencing H--+------>| Flags, Frag, Len, Seq #, etc |
\=====================/ | +--------------------------------+
| PW Demultiplexer |--------->| PW Label |
+---------------------+ | +--------------------------------+
| PSN Convergence |--+ +--->| Outer Label or MPLS-in-IP encap|
+---------------------+ | +--------------------------------+
| PSN |-----+
+---------------------+
| Data-Link |
+---------------------+
| Physical |
+---------------------+
Figure 11. PWE3 over an MPLS PSN Using a Control Word
Figure 11 shows the protocol layering for PWE3 over an MPLS PSN. An
inner MPLS label is used to provide the PW demultiplexing function.
A control word is used to carry most of the information needed by the
PWE3 Encapsulation Layer and the PSN Convergence Layer in a compact
format. The flags in the control word provide the necessary payload
convergence. A sequence field provides support for both in-order
payload delivery and a PSN fragmentation service within the PSN
Convergence Layer (supported by a fragmentation control method).
Ethernet pads all frames to a minimum size of 64 bytes. The MPLS
header does not include a length indicator. Therefore, to allow PWE3
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to be carried in MPLS to pass correctly over an Ethernet data-link, a
length correction field is needed in the control word. As with an IP
PSN, where appropriate, timing is provided by RTP [RFC3550].
In some networks, it may be necessary to carry PWE3 over MPLS over
IP. In these circumstances, the PW is encapsulated for carriage over
MPLS as described in this section, and then a method of carrying MPLS
over an IP PSN (such as GRE [RFC2784], [RFC2890]) is applied to the
resultant PW-PDU.
5.4.3. PW-IP Packet Discrimination
For MPLS PSNs, there is an additional constraint on the PW packet
format. Some label switched routers detect IP packets based on the
initial four bits of the packet content. To facilitate proper
functioning, these bits in PW packets must not be the same as an IP
version number in current use.
6. PW Demultiplexer Layer and PSN Requirements
PWE3 places three service requirements on the protocol layers used to
carry it across the PSN:
o Multiplexing
o Fragmentation
o Length and Delivery
6.1. Multiplexing
The purpose of the PW Demultiplexer Layer is to allow multiple PWs to
be carried in a single tunnel. This minimizes complexity and
conserves resources.
Some types of native service are capable of grouping multiple
circuits into a "trunk"; e.g., multiple ATM VCs in a VP, multiple
Ethernet VLANs on a physical media, or multiple DS0 services within a
T1 or E1. A PW may interconnect two end-trunks. That trunk would
have a single multiplexing identifier.
When a MPLS label is used as a PW Demultiplexer, setting of the TTL
value [RFC3032] in the PW label is application specific.
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6.2. Fragmentation
If the PSN provides a fragmentation and reassembly service of
adequate performance, it may be used to obtain an effective MTU that
is large enough to transport the PW PDUs. See section 5.3 for a full
discussion of the PW fragmentation issues.
6.3. Length and Delivery
PDU delivery to the egress PE is the function of the PSN Layer.
If the underlying PSN does not provide all the information necessary
to determine the length of a PW-PDU, the Encapsulation Layer must
provide it.
6.4. PW-PDU Validation
It is a common practice to use an error detection mechanism such as a
CRC or similar mechanism to ensure end-to-end integrity of frames.
The PW service-specific mechanisms must define whether the packet's
checksum shall be preserved across the PW or be removed from PE-bound
PDUs and then be recalculated for insertion in CE-bound data.
The former approach saves work, whereas the latter saves bandwidth.
For a given implementation, the choice may be dictated by hardware
restrictions, which may not allow the preservation of the checksum.
For protocols such as ATM and FR, the scope of the checksum is
restricted to a single link. This is because the circuit identifiers
(e.g., FR DLCI or ATM VPI/VCI) only have local significance and are
changed on each hop or span. If the circuit identifier (and thus
checksum) were going to change as part of the PW emulation, it would
be more efficient to strip and recalculate the checksum.
The service-specific document for each protocol must describe the
validation scheme to be used.
6.5. Congestion Considerations
The PSN carrying the PW may be subject to congestion. The congestion
characteristics will vary with the PSN type, the network architecture
and configuration, and the loading of the PSN.
If the traffic carried over the PW is known to be TCP friendly (by,
for example, packet inspection), packet discard in the PSN will
trigger the necessary reduction in offered load, and no additional
congestion avoidance action is necessary.
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If the PW is operating over a PSN that provides enhanced delivery,
the PEs should monitor packet loss to ensure that the requested
service is actually being delivered. If it is not, then the PE
should assume that the PSN is providing a best-effort service and
should use the best-effort service congestion avoidance measures
described below.
If best-effort service is being used and the traffic is not known to
be TCP friendly, the PEs should monitor packet loss to ensure that
the loss rate is within acceptable parameters. Packet loss is
considered acceptable if a TCP flow across the same network path and
experiencing the same network conditions would achieve an average
throughput, measured on a reasonable timescale, not less than that
which the PW flow is achieving. This condition can be satisfied by
implementing a rate-limiting measure in the NSP, or by shutting down
one or more PWs. The choice of which approach to use depends upon
the type of traffic being carried. Where congestion is avoided by
shutting down a PW, a suitable mechanism must be provided to prevent
it from immediately returning to service and causing a series of
congestion pulses.
The comparison to TCP cannot be specified exactly but is intended as
an "order-of-magnitude" comparison in timescale and throughput. The
timescale on which TCP throughput is measured is the round-trip time
of the connection. In essence, this requirement states that it is
not acceptable to deploy an application (using PWE3 or any other
transport protocol) on the best-effort Internet, which consumes
bandwidth arbitrarily and does not compete fairly with TCP within an
order of magnitude. One method of determining an acceptable PW
bandwidth is described in [RFC3448].
7. Control Plane
This section describes PWE3 control plane services.
7.1. Setup or Teardown of Pseudo Wires
A PW must be set up before an emulated service can be established and
must be torn down when an emulated service is no longer needed.
Setup or teardown of a PW can be triggered by an operator command,
from the management plane of a PE, by signaling set-up or teardown of
an AC (e.g., an ATM SVC), or by an auto-discovery mechanism.
During the setup process, the PEs have to exchange information (e.g.,
learn each other's capabilities). The tunnel signaling protocol may
be extended to provide mechanisms that enable the PEs to exchange all
necessary information on behalf of the PW.
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Manual configuration of PWs can be considered a special kind of
signaling and is allowed.
7.2. Status Monitoring
Some native services have mechanisms for status monitoring. For
example, ATM supports OAM for this purpose. For these services, the
corresponding emulated services must specify how to perform status
monitoring.
7.3. Notification of Pseudo Wire Status Changes
7.3.1. Pseudo Wire Up/Down Notification
If a native service requires bi-directional connectivity, the
corresponding emulated service can only be signaled as being up when
the PW and PSN tunnels (if used), are functional in both directions.
Because the two CEs of an emulated service are not adjacent, a
failure may occur at a place so that one or both physical links
between the CEs and PEs remain up. For example, in Figure 2, if the
physical link between CE1 and PE1 fails, the physical link between
CE2 and PE2 will not be affected and will remain up. Unless CE2 is
notified about the remote failure, it will continue to send traffic
over the emulated service to CE1. Such traffic will be discarded at
PE1. Some native services have failure notification so that when the
services fail, both CEs will be notified. For these native services,
the corresponding PWE3 service must provide a failure notification
mechanism.
Similarly, if a native service has notification mechanisms so that
all the affected services will change status from "Down" to "Up" when
a network failure is fixed, the corresponding emulated service must
provide a similar mechanism for doing so.
These mechanisms may already be built into the tunneling protocol.
For example, the L2TP control protocol [RFC2661] [RFC3931] has this
capability, and LDP has the ability to withdraw the corresponding
MPLS label.
7.3.2. Misconnection and Payload Type Mismatch
With PWE3, misconnection and payload type mismatch can occur.
Misconnection can breach the integrity of the system. Payload
mismatch can disrupt the customer network. In both instances, there
are security and operational concerns.
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The services of the underlying tunneling mechanism and its associated
control protocol can be used to mitigate this. As part of the PW
setup, a PW-TYPE identifier is exchanged. This is then used by the
forwarder and the NSP to verify the compatibility of the ACs.
7.3.3. Packet Loss, Corruption, and Out-of-Order Delivery
A PW can incur packet loss, corruption, and out-of-order delivery on
the PSN path between the PEs. This can affect the working condition
of an emulated service. For some payload types, packet loss,
corruption, and out-of-order delivery can be mapped either to a bit
error burst, or to loss of carrier on the PW. If a native service
has some mechanism to deal with bit error, the corresponding PWE3
service should provide a similar mechanism.
7.3.4. Other Status Notification
A PWE3 approach may provide a mechanism for other status
notifications, if any are needed.
7.3.5. Collective Status Notification
The status of a group of emulated services may be affected
identically by a single network incident. For example, when the
physical link (or sub-network) between a CE and a PE fails, all the
emulated services that go through that link (or sub-network) will
fail. It is likely that a group of emulated services all terminate
at a remote CE. There may also be multiple such CEs affected by the
failure. Therefore, it is desirable that a single notification
message be used to notify failure of the whole group of emulated
services.
A PWE3 approach may provide a mechanism for notifying status changes
of a group of emulated circuits. One possible method is to associate
each emulated service with a group ID when the PW for that emulated
service is set up. Multiple emulated services can then be grouped by
associating them with the same group ID. In status notification,
this group ID can be used to refer all the emulated services in that
group. The group ID mechanism should be a mechanism provided by the
underlying tunnel signaling protocol.
7.4. Keep-Alive
If a native service has a keep-alive mechanism, the corresponding
emulated service must provide a mechanism to propagate it across the
PW. Transparently transporting keep-alive messages over the PW would
follow the principle of minimum intervention. However, to reproduce
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RFC 3985 PWE3 Architecture March 2005
the semantics of the native mechanism accurately, some PWs may
require an alternative approach, such as piggy-backing on the PW
signaling mechanism.
7.5. Handling Control Messages of the Native Services
Some native services use control messages for circuit maintenance.
These control messages may be in-band (e.g., Ethernet flow control,
ATM performance management, or TDM tone signaling) or out-of-band,
(e.g., the signaling VC of an ATM VP, or TDM CCS signaling).
Given the principle of minimum intervention, it is desirable that the
PEs participate as little as possible in the signaling and
maintenance of the native services. This principle should not,
however, override the need to emulate the native service
satisfactorily.
If control messages are passed through, it may be desirable to send
them by using either a higher priority or a reliable channel provided
by the PW Demultiplexer layer. See Section 5.1.2, PWE3 Channel
Types.
8. Management and Monitoring
This section describes the management and monitoring architecture for
PWE3.
8.1. Status and Statistics
The PE should report the status of the interface and tabulate
statistics that help monitor the state of the network and help
measure service-level agreements (SLAs). Typical counters include
the following:
o Counts of PW-PDUs sent and received, with and without errors.
o Counts of sequenced PW-PDUs lost.
o Counts of service PDUs sent and received over the PSN, with and
without errors (non-TDM).
o Service-specific interface counts.
o One-way delay and delay variation.
These counters would be contained in a PW-specific MIB, and they
should not replicate existing MIB counters.
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8.2. PW SNMP MIB Architecture
This section describes the general architecture for SNMP MIBs used to
manage PW services and the underlying PSN. The intent here is to
provide a clear picture of how all the pertinent MIBs fit together to
form a cohesive management framework for deploying PWE3 services.
Note that the names of MIB modules used below are suggestions and do
not necessarily require that the actual modules used to realize the
components in the architecture be named exactly so.
8.2.1. MIB Layering
The SNMP MIBs created for PWE3 should fit the architecture shown in
Figure 12. The architecture provides a layered modular model into
which any supported emulated service can be connected to any
supported PSN type. This model fosters reuse of as much
functionality as possible. For instance, the emulated service layer
MIB modules do not redefine the existing emulated service MIB module;
rather, they only associate it with the pseudo wires used to carry
the emulated service over the configured PSN. In this way, the PWE3
MIB architecture follows the overall PWE3 architecture.
The architecture does allow for the joining of unsupported emulated
service or PSN types by simply defining additional MIB modules to
associate new types with existing ones. These new modules can
subsequently be standardized. Note that there is a separate MIB
module for each emulated service, as well as one for each underlying
PSN. These MIB modules may be used in various combinations as
needed.
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RFC 3985 PWE3 Architecture March 2005
Native
Service MIBs ... ... ...
| | |
+-----------+ +-----------+ +-----------+
Service | CEP | | Ethernet | | ATM |
Layer |Service MIB| |Service MIB| ... |Service MIB|
+-----------+ +-----------+ +-----------+
\ | /
\ | /
- - - - - - - - - - - - \ - - - | - - - - / - - - - - - -
\ | /
+-------------------------------------------+
Generic PW | Generic PW MIBs |
Layer +-------------------------------------------+
/ \
- - - - - - - - - - - - / - - - - - - - - \ - - - - - - -
/ \
/ \
+--------------+ +----------------+
PSN VC |L2TP VC MIB(s)| | MPLS VC MIB(s) |
Layer +--------------+ +----------------+
| |
Native +-----------+ +-----------+
PSN |L2TP MIB(s)| |MPLS MIB(s)|
MIBs +-----------+ +-----------+
Figure 12. MIB Module Layering Relationship
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RFC 3985 PWE3 Architecture March 2005
Figure 13 shows an example for a SONET PW carried over MPLS Traffic
Engineering Tunnel and an LDP-signaled LSP.
+-----------------+
| SONET MIB | RFC3592
+-----------------+
|
+------------------------------+
Service | Circuit Emulation Service MIB|
Layer +------------------------------+
- - - - - - - - - - - - - | - - - - - - - - - - - - -
+-----------------+
Generic PW | Generic PW MIB |
Layer +-----------------+
- - - - - - - - - - - - - | - - - - - - - - - - - - -
+-----------------+
PSN VC | MPLS VC MIBs |
Layer +-----------------+
| |
+-----------------+ +------------------+
| MPLS-TE-STD-MIB | | MPLS-LSR-STD-MIB |
+-----------------+ +------------------+
Figure 13. SONET PW over MPLS PSN Service-Specific Example
8.2.2. Service Layer MIB Modules
This conceptual layer in the model contains MIB modules used to
represent the relationship between emulated PWE3 services such as
Ethernet, ATM, or Frame Relay and the pseudo-wire used to carry that
service across the PSN. This layer contains corresponding MIB
modules used to mate or adapt those emulated services to the generic
pseudo-wire representation these are represented in the "Generic PW
MIB" functional block in Figure 13 above. This working group should
not produce any MIB modules for managing the general service; rather,
it should produce just those modules used to interface or adapt the
emulated service onto the PWE3 management framework as shown above.
For example, the standard SONET-MIB [RFC3592] is designed and
maintained by another working group. The SONET-MIB is designed to
manage the native service without PW emulation. However, the PWE3
working group is chartered to produce standards that show how to
emulate existing technologies such as SONET/SDH over pseudo-wires
rather than reinvent those modules.
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RFC 3985 PWE3 Architecture March 2005
8.2.3. Generic PW MIB Modules
The middle layer in the architecture is referred to as the Generic PW
Layer. MIBs in this layer are responsible for providing pseudo-wire
specific counters and service models used for monitoring and
configuration of PWE3 services over any supported PSN service. That
is, this layer provides a general model of PWE3 abstraction for
management purposes. This MIB is used to interconnect the MIB
modules residing in the Service Layer to the PSN VC Layer MIBs (see
section 8.2.4).
8.2.4. PSN VC Layer MIB Modules
The third layer in the PWE3 management architecture is referred to as
the PSN VC Layer. It is composed of MIBs that are specifically
designed to associate pseudo-wires onto those underlying PSN
transport technologies that carry the pseudo-wire payloads across the
PSN. In general, this means that the MIB module provides a mapping
between the emulated service that is mapped to the pseudo-wire via
the Service Layer and the Generic PW MIB Layer onto the native PSN
service. For example, in the case of MPLS, for example, it is
required that the general VC service be mapped into MPLS LSPs via the
MPLS-LSR-STD-MIB [RFC3813] or Traffic-Engineered (TE) Tunnels via the
MPLS-TE-STD-MIB [RFC3812]. In addition, the MPLS-LDP-STD-MIB
[RFC3815] may be used to reveal the MPLS labels that are distributed
over the MPLS PSN in order to maintain the PW service. As with the
native service MIB modules described earlier, the MIB modules used to
manage the native PSN services are produced by other working groups
that design and specify the native PSN services. These MIBs should
contain the appropriate mechanisms for monitoring and configuring the
PSN service that the emulated PWE3 service will function correctly.
8.3. Connection Verification and Traceroute
A connection verification mechanism should be supported by PWs.
Connection verification and other alarm mechanisms can alert the
operator that a PW has lost its remote connection. The opaque nature
of a PW means that it is not possible to specify a generic connection
verification or traceroute mechanism that passes this status to the
CEs over the PW. If connection verification status of the PW is
needed by the CE, it must be mapped to the native connection status
method.
For troubleshooting purposes, it is sometimes desirable to know the
exact functional path of a PW between PEs. This is provided by the
traceroute service of the underlying PSN. The opaque nature of the
PW means that this traceroute information is only available within
the provider network; e.g., at the PEs.
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9. IANA Considerations
IANA considerations will be identified in the PWE3 documents that
define the PWE3 encapsulation, control, and management protocols.
10. Security Considerations
PWE3 provides no means of protecting the integrity, confidentiality,
or delivery of the native data units. The use of PWE3 can therefore
expose a particular environment to additional security threats.
Assumptions that might be appropriate when all communicating systems
are interconnected via a point-to-point or circuit-switched network
may no longer hold when they are interconnected with an emulated wire
carried over some types of PSN. It is outside the scope of this
specification to fully analyze and review the risks of PWE3,
particularly as these risks will depend on the PSN. An example
should make the concern clear. A number of IETF standards employ
relatively weak security mechanisms when communicating nodes are
expected to be connected to the same local area network. The Virtual
Router Redundancy Protocol [RFC3768] is one instance. The relatively
weak security mechanisms represent a greater vulnerability in an
emulated Ethernet connected via a PW.
Exploitation of vulnerabilities from within the PSN may be directed
to the PW Tunnel end point so that PW Demultiplexer and PSN tunnel
services are disrupted. Controlling PSN access to the PW Tunnel end
point is one way to protect against this. By restricting PW Tunnel
end point access to legitimate remote PE sources of traffic, the PE
may reject traffic that would interfere with the PW Demultiplexing
and PSN tunnel services.
Protection mechanisms must also address the spoofing of tunneled PW
data. The validation of traffic addressed to the PW Demultiplexer
end-point is paramount in ensuring integrity of PW encapsulation.
Security protocols such as IPSec [RFC2401] may be used by the PW
Demultiplexer Layer in order provide authentication and data
integrity of the data between the PW Demultiplexer End-points.
IPSec may provide authentication, integrity, and confidentiality, of
data transferred between two PEs. It cannot provide the equivalent
services to the native service.
Based on the type of data being transferred, the PW may indicate to
the PW Demultiplexer Layer that enhanced security services are
required. The PW Demultiplexer Layer may define multiple protection
profiles based on the requirements of the PW emulated service. CE-
to-CE signaling and control events emulated by the PW and some data
types may require additional protection mechanisms. Alternatively,
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RFC 3985 PWE3 Architecture March 2005
the PW Demultiplexer Layer may use peer authentication for every PSN
packet to prevent spoofed native data units from being sent to the
destination CE.
The unlimited transformation capability of the NSP may be perceived
as a security risk. In practice the type of operation that the NSP
may perform will be limited to those that have been implemented in
the data path. A PE designed and managed to best current practice
will have controls in place that protect and validate its
configuration, and these will be sufficient to ensure that the NSP
behaves as expected.
11. Acknowledgements
We thank Sasha Vainshtein for his work on Native Service Processing
and advice on bit stream over PW services and Thomas K. Johnson for
his work on the background and motivation for PWs.
We also thank Ron Bonica, Stephen Casner, Durai Chinnaiah, Jayakumar
Jayakumar, Ghassem Koleyni, Danny McPherson, Eric Rosen, John
Rutemiller, Scott Wainner, and David Zelig for their comments and
contributions.
12. References
12.1. Normative References
[RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two
Tunneling Protocol - Version 3 (L2TPv3), RFC 3931, March
2005.
[RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474, December
1998.
[RFC3592] Tesink, K., "Definitions of Managed Objects for the
Synchronous Optical Network/Synchronous Digital Hierarchy
(SONET/SDH) Interface Type", RFC 3592, September 2003.
Bryant & Pate Standards Track [Page 38]
RFC 3985 PWE3 Architecture March 2005
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
RFC 2661, August 1999.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
March 2000.
[RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE",
RFC 2890, September 2000.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
12.2. Informative References
[DVB] EN 300 744 Digital Video Broadcasting (DVB); Framing
structure, channel coding and modulation for digital
terrestrial television (DVB-T), European
Telecommunications Standards Institute (ETSI).
[RFC3815] Cucchiara, J., Sjostrand, H., and J. Luciani,
"Definitions of Managed Objects for the Multiprotocol
Label Switching (MPLS), Label Distribution Protocol
(LDP)", RFC 3815, June 2004.
[RFC3813] Srinivasan, C., Viswanathan, A., and T. Nadeau,
"Multiprotocol Label Switching (MPLS) Label Switching
Router (LSR) Management Information Base (MIB)", RFC
3813, June 2004.
[MPLSIP] Rosen et al, "Encapsulating MPLS in IP or Generic Routing
Encapsulation (GRE)", Work in Progress, March 2004.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1958] Carpenter, B., "Architectural Principles of the
Internet", RFC 1958, June 1996.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU
Discovery for IP version 6", RFC 1981, August 1996.
Bryant & Pate Standards Track [Page 39]
RFC 3985 PWE3 Architecture March 2005
[RFC2022] Armitage, G., "Support for Multicast over UNI 3.0/3.1
based ATM Networks", RFC 2022, November 1996.
[RFC3768] Hinden, R., "Virtual Router Redundancy Protocol (VRRP)",
RFC 3768, April 2004.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
2001.
[RFC3448] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 3448, January 2003.
[RFC3812] Srinivasan, C., Viswanathan, A., and T. Nadeau,
"Multiprotocol Label Switching (MPLS) Traffic Engineering
(TE) Management Information Base (MIB)", RFC 3812, June
2004.
[RFC3916] Xiao, X., McPherson, D., and P. Pate, Eds, "Requirements
for Pseudo-Wire Emulation Edge-to-Edge (PWE3)", RFC 3916,
September 2004.
13. Co-Authors
The following are co-authors of this document:
Thomas K. Johnson
Litchfield Communications
Kireeti Kompella
Juniper Networks, Inc.
Andrew G. Malis
Tellabs
Thomas D. Nadeau
Cisco Systems
Tricci So
Caspian Networks
W. Mark Townsley
Cisco Systems
Bryant & Pate Standards Track [Page 40]
RFC 3985 PWE3 Architecture March 2005
Craig White
Level 3 Communications, LLC.
Lloyd Wood
Cisco Systems
14. Editors' Addresses
Stewart Bryant
Cisco Systems
250, Longwater
Green Park
Reading, RG2 6GB,
United Kingdom
EMail:
[email protected]
Prayson Pate
Overture Networks, Inc.
507 Airport Boulevard
Morrisville, NC, USA 27560
EMail:
[email protected]
Bryant & Pate Standards Track [Page 41]
RFC 3985 PWE3 Architecture March 2005
Full Copyright Statement
Copyright (C) The Internet Society (2005).
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contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
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Acknowledgement
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Bryant & Pate Standards Track [Page 42]
