Pseudo-Wire Edge-to-Edge (PWE3) Working Group Stewart Bryant
Internet Draft Cisco Systems
Document: <draft-ietf-pwe3-arch-04.txt>
Expires: December 2003 Prayson Pate
Overture Networks, Inc.
Editors
June 2003
PWE3 Architecture
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of section 10 of RFC2026.
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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, specifies the
various protocol elements and their functions.
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Co-Authors
The following are co-authors of this document:
Thomas K. Johnson Litchfield Communications
Kireeti Kompella Juniper Networks, Inc.
Andrew G. Malis Vivace Networks
Thomas D. Nadeau Cisco Systems
Tricci So Caspian Networks
W. Mark Townsley Cisco Systems
Craig White Level 3 Communications, LLC.
Lloyd Wood Cisco Systems
XiPeng Xiao Redback Networks
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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Table of Contents
1. Introduction............................................. 5
1.1 Pseudo Wire Definition............................... 5
1.2 PW Service Functionality............................. 6
1.3 Non-Goals of this document........................... 6
1.4 Terminology.......................................... 6
2. PWE3 Applicability....................................... 9
3. Protocol Layering Model.................................. 9
3.1 Protocol Layers...................................... 9
3.2 Domain of PWE3....................................... 11
3.3 Payload Types........................................ 11
4. Architecture of Pseudo-wires............................. 14
4.1 Network Reference Model.............................. 14
4.2 PWE3 Pre-processing.................................. 15
4.3 Maintenance Reference Model.......................... 19
4.4 Protocol Stack Reference Model....................... 19
4.5 Pre-processing Extension to Protocol Stack Reference
Model................................................ 20
5. PW Encapsulation......................................... 21
5.1 Payload Convergence Layer............................ 22
5.2 Payload-independent PW Encapsulation Layers.......... 24
5.3 Fragmentation........................................ 27
5.4 Instantiation of the Protocol Layers................. 27
6. PW Demultiplexer Layer and PSN Requirements.............. 32
6.1 Multiplexing......................................... 32
6.2 Fragmentation........................................ 32
6.3 Length and Delivery.................................. 32
6.4 PW-PDU Validation.................................... 33
6.5 Congestion Considerations............................ 33
7. Control Plane............................................ 34
7.1 Set-up or Teardown of Pseudo-Wires................... 34
7.2 Status Monitoring.................................... 34
7.3 Notification of Pseudo-wire Status Changes........... 35
7.4 Keep-alive........................................... 36
7.5 Handling Control Messages of the Native Services..... 36
8. Management and Monitoring................................. 37
8.1 Status and Statistics................................ 37
8.2 PW SNMP MIB Architecture............................. 37
8.3 Connection Verification and Traceroute................ 41
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9. IANA considerations...................................... 41
10. Security Considerations................................. 41
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1. Introduction
This document describes an architecture for Pseudo Wire Emulation
Edge-to-Edge (PWE3) in support of [XIAO]. 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, specifies the various protocol elements and their
functions.
1.1 Pseudo Wire Definition
PWE3 is a mechanism that emulates the essential attributes of a
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 a IP path or MPLS tunnel. In some cases it is necessary
to perform other operation 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 a Customer Edge Equipment (CE), the PW is
characterised 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 hence 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.
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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
set-up 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, while multicast services like MARS [RFC2022] that
are implemented on top of the medium are out of scope.
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 circuit or virtual circuit attaching
(AC) a CE to a PE.
CE-bound The traffic direction where 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 CEs
control plane. It may be desirable or
even necessary for the PE to participate
in or monitor this signaling in order
to effectively emulate the service.
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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 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. Fragmentation MAY be
performed in order to allow sending of packets
of a larger size than the network MTU which
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, ATMF, etc.)
Packet Switched Within the context of PWE3, this is a
Network (PSN) network using IP or MPLS as the mechanism
for packet forwarding.
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.
PE-bound The traffic direction where 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
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CE Signaling in order to effectively manage
the PW.
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.
PW End Service The interface between a PE and a CE. This
(PWES) can be a physical interface like a T1 or
Ethernet, or a virtual interface like a VC
or VLAN.
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.
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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 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 be such that it is not possible to provide
the required service guarantees.
3. Protocol Layering Model
The PWE3 protocol-layering model is intended to minimise 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 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.
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+---------------------------+
| Payload |
+---------------------------+
| Encapsulation | <==== MAY be null
+---------------------------+
| PW Demultiplexer |
+---------------------------+
| PSN Convergence | <==== MAY be null
+---------------------------+
| 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 information is needed beyond that in the payload itself, this
layer is empty.
This layer also provides support for real-time processing, and
sequencing, if needed.
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. This layer
therefore 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.
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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 unit:
o Packet
o Cell
o Bit-stream
o Structured bit-stream
Within these generic types there are specific service types. For
example:
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 presented to the PE on
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 as 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 needs
to be applied. This may, for example, be the case when a user is
providing the service and attaching to the service provider via
Ethernet, or where nested pseudo-wires are involved. Fragmentation is
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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, using a suitable MAC bridge filter. This
is an FWRD 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 an FWRD 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 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 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 minimise 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 which does intervene can be more wire-efficient in some
cases and may result in fewer translations at the NSP where 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 the intermediate format carries, and is
undesirable.
4. Architecture of Pseudo-wires
This section describes the PWE3 architectural model.
4.1 Network Reference Model
Figure 2 illustrates the network reference model for point-to-point
PWs.
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|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudo Wire ------>| |
| | | |
| | |<-- PSN Tunnel -->| | |
| PW End V V V V PW End |
V Service +----+ +----+ Service 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) need 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) presented to the PW End Service (PWES)
are encapsulated in a PW-PDU and carried across the underlying
network via the PSN tunnel. The PEs perform the necessary
encapsulation and decapsulation of PW-PDUs, as well as handling any
other functions required by the PW service, such as sequencing or
timing. A PE MAY provide multiple PWESs.
4.2 PWE3 Pre-processing
In some applications, there is a need 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 sent to the
PE over one or more physical interfaces. In most cases, there are
cost and operational benefits in undertaking these operations within
the PE. This processed data is then presented to the PW via a
virtual interface within the PE.
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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 inter-working of one PE with pre-processing
(PREP), and a second without this functionality. This is a useful
reference point because it emphasises 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.
The required pre-processing can be divided into two components:
o Forwarder (FWRD)
o Native Service Processing (NSP)
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4.2.1. Forwarders
In some applications there is the need to selectively forward payload
elements from one of more ACs to one or more PWs. In such cases there
will also be the need to perform the inverse function on PWE3-PDUs
received by a PE from the PSN. This is the function of the FWRD.
The FWRD 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 | | |
PWES | | Single | PW Instance
<------>o Forwarder + PW Instance X<===========>
| | |
+----------------------------------------+
Figure 4a: Simple point-to-point service
+----------------------------------------+
| PE Device |
+----------------------------------------+
Multiple| | Single | PW Instance
PWES | + PW Instance X<===========>
<------>o | |
| |----------------------|
<------>o | Single | PW Instance
| Forwarder + PW Instance X<===========>
<------>o | |
| |----------------------|
<------>o | Single | PW Instance
| + PW Instance X<===========>
<------>o | |
+----------------------------------------+
Figure 4b: Multiple PWES to Multiple PW Forwarding
Figure 4a shows a simple FWRD that performs some type of filtering
operation. Because the FWRD has a single input and a single output
interface, filtering is the only type of forwarding operation that
applies. Figure 4b shows a more general forwarding situation where
payloads are extracted from one or more PWESs and directed to one or
more PWs, including, in this instance, a multipoint PW. In this case
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both filtering and direction operations MAY be performed on the
payloads.
4.2.2. Native Service Processing
In some applications some form of data or address translation, or
other operation requiring knowledge of the semantics of the payload,
will be required. 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+----------------------------------------+
PWES | | | 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 PWEs to Multiple
PW Forwarding PE
Figure 5 illustrates the relationship between NSP, FWRD 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
FWRD. A PE device MAY contain more than one FWRD.
This model also supports the operation of a system in which the NSP
functionality includes terminating the data-link, and applying
Network Layer processing to the payload is also supported.
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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 to be signaled is within the scope
of PWE3, but the signaling protocol itself is not.
4.4 Protocol Stack Reference Model
Figure 7 illustrates the protocol stack reference model for PWs.
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+-----------------+ +-----------------+
|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 that is used to carry the PW over the PSN.
The PW Encapsulation Layer consists of three sub-layers:
o Payload Convergence
o Timing
o Sequencing
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The PW Encapsulation sub-layering and its context with the protocol
stack are shown, in Figure 9.
+---------------------------+
| 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, but, by grouping a number of target payload types into
a generic class, and then providing a single convergence sub-layer
type common to the group, we achieve a reduction in 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 Layer and the Sequencing Layer 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 a 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 as
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part of the PW header.
Not all the additional information needed to replay the native data
units need 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:
1. A reliable control channel for signaling line events, status
indications, and, in some exceptional cases, CE-CE events
that must be translated and sent reliably between PEs.
For example, this capability is needed in [PPPoL2TP]
(PPP negotiation has to be split between the two ends of the
tunnel). PWE3 may also need this type of control channel to
provide faithful emulation of complex data-link protocols.
plus one or more data channels with the following characteristics:
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 un-sequenced 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., there exists the
possibility that the control channel may be able to pass traffic and
set-up the PW, but the PW data traffic is blocked by one or more of
these mechanisms. In these cases unless the control channel is also
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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 with TDM circuits
(e.g., the on-hook/off-hook events in PSTN switches might be carried
over a reliable control channel, whilst 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 needed by a particular PW instance. If the
service is not needed, the associated header MAY be omitted in order
to conserve processing and network resources.
There will be instances where a specific payload type will be
required to be transported 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
in-order delivery, 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 that constraint in return for
reduced per-packet processing cost.
The guiding principle is that, where possible, an existing IETF
protocol SHOULD be used to provide these services. Where 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
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omitted if not needed.
The size of the sequence-number space depends on the speed of the
emulated service, and 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 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 control and data frames) MUST
be delivered in order. For such 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 to out-of-sequence detection. 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 [RFC1889] 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 (since delay must be introduced to
reorder)
o The incidence of expected 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 such 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 the mechanism 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 received PW PDUs.
In some instances, a destination PE will have to presume that a PW
PDU is lost if it fails to arrive within a certain time. 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 that they were designed to travel
over, and it can be necessary for the emulated service to duplicate
these network characteristics as closely as possible, e.g. in
delivering native traffic with bit-rate, jitter, wander and delay
characteristics similar to those received at the sending PE.
In such cases, it is necessary for the receiving PE to play out the
native traffic as it was received at the sending PE. This relies on
either timing information sent between the two PEs, or in some case
timing information received from an external reference.
The Timing Sub-layer must therefore 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 requires a suitable
mechanism. A physical wire carries the timing information natively,
but it is a relatively complex task to extract timing from a highly
jittered source such as packet stream. It is therefore desirable
that an existing real-time protocol such as [RFC1889] 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 with reference to an external clock.
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5.3 Fragmentation
A payload would ideally 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 exceeds the PSN path MTU.
When a packet size exceeds the MTU of a given network, fragmentation
and reassembly have to be performed in order for the packet to be
delivered. Since fragmentation and reassembly generally consume a
considerable network resources as compared to simply switching a
packet in its entirety, efforts SHOULD be made to reduce or eliminate
the need for fragmentation and reassembly throughout a network 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 being sent over the PW
will be configured to have adaptive measures (e.g. [RFC1191],
[RFC1981]) in place that ensure that packets that need 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 at all possible. If and 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 support fragmentation 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 PWES, 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-----------+->| As Needed |
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. (It is accepted
that there MAY sometimes be good reason not to follow this rule, but
the exceptional circumstances need to be documented in the
Encapsulation Layer definition for that payload type).
Where appropriate, timing is provided by RTP [RFC1889], which when
used also provides a sequencing service. PW Demultiplexing may be
provided by a number of existing IETF tunnel protocols. Some of
these tunnel protocols provide an optional sequencing service.
(Sequencing is provided either by RTP, or by the PW Demultiplexer
Layer, but not both). A PSN Convergence Layer is not needed, because
all the tunnel protocols shown above are designed to operate directly
over an IP PSN.
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.
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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--+------>| Flags, Frag, Len, Seq #, etc |
H---------------------H | +--------------------------------+
H Timing H--------->| RTP |
H---------------------H | +--------------------------------+
H Sequencing H--+ | MPLS Payload Type Ident |
\=====================/ | +--------------------------------+
| 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 (supported by a fragmentation control method) a
PSN fragmentation service within the PSN Convergence Layer. Ethernet
pads all frames to a minimum size of 64 bytes. The MPLS header does
not include a length indicator. Therefore to allow PWE3 to be carried
in MPLS to correctly pass over an Ethernet data-link, a length
correction field is needed in the control word. Where the design of
the control word would alias an IP packet, an MPLS Payload Type
Identifier should be interposed between the PW label and the control
word (see 5.4.4). As with an IP PSN, where appropriate, timing is
provided by RTP [RFC1889].
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
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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 over MPLS Generic Control Word
To allow accurate packet inspection in an MPLS PSN, and/or to operate
correctly over MPLS PSNs that have deployed equal-cost multiple-path
load-balancing (ECMP), a PW packet MUST NOT alias an IP packet. IP
packets are carried in MPLS label stacks without any protocol
identifier. Historic values of the IP version number [RFC791]
[RFC1881] are therefore used to distinguish between IP and non-IP
MPLS payloads.
To disambiguate the PW from an IP flow the PW SHOULD employ either
the generic PW control word shown in Figure 12, or an MPLS payload
type identifier. Note that an MPLS payload with bits 0..3 = 4 is an
IPv4 packet and an MPLS payload with bits 0..3 = 6 is an IPv6 packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Specified by PW Encapsulation |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Generic PW Control Word
The PW set-up protocol determines whether a PW uses a control word.
When a control word is used, it SHOULD have the following preferred
form:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Flags |FRG| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: MPLS Preferred Control Word
The meaning of the fields of the MPLS Preferred Control Word (Figure
13) is as follows:
Flags (bits 4 to 7):
These bits are available for per payload signaling. Their
definition is encapsulation specific.
FRG (bits 8 and 9):
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These bits are used when fragmenting a PW payload. Their use
is defined in [FRAG]. When the PW is of a type that will
never need payload fragmentation, these bits may be used as
general purpose flags.
Length (bits 10 to 15):
The length field is used to determine the size of a PW
payload that might have been padded to the minimum Ethernet
MAC frame size during its transit across the PSN. If the
MPLS payload (defined as the CW + the PW payload + any
additional PW headers is less than 46 bytes, the length MUST
be set to the length of the MPLS payload. If the MPLS
payload is between 46 bytes and 63 bytes the implementation
MAY either set to the length to the length of the MPLS
payload, or it MAY set it to 0. If the length of the MPLS
payload is greater than 63 bytes the length MUST be set to 0.
Sequence number (Bit 16 to 31):
If the sequence number is not used, it is set to zero by
the sender and ignored by the receiver. Otherwise it
specifies the sequence number of a packet. A circular list
of sequence numbers is used. A sequence number takes a value
from 1 to 65535 (2**16-1). If the payload is an OAM packet
the sequence number MAY be used to mark the position in the
sequence, in which case it has the same value as the last
data PDU sent. The use of the sequence number is optional
for OAM payloads.
5.4.4. MPLS Payload Identifier
If technical considerations result in a PW control word that may
alias an IP packet, the control word SHOULD be preceeded by an MPLS
payload type identifier.
The MPLS payload type is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1| reserved = 0 | PPP DLL Protocol Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As defined by PPP DLL protocol definition |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: MPLS Payload Type Identifier
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PPP DLL Protocol Number [16:31]:
These numbers are assigned by IANA.
Bits 4 to 15 inclusive are reserved for future use and must be zero.
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, however in a
strict point to point application the TTL SHOULD be set to 2.
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.
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6.4 PW-PDU Validation
It is a common practice to use an error detection mechanism such as a
CRC or similar mechanism to assure 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 re-calculated for insertion in CE-bound data.
The former approach saves work, while 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) have only local significance and are
changed on each hop or span. If the circuit identifier (and thus
checksum) were going to change as a part of the PW emulation, it
would be more efficient to strip and re-calculate 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.
Where 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.
If the PW is operating over a PSN that provides enhanced delivery,
the PEs SHOULD monitor packet loss to ensure that the service that
was requested 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 packet 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, that is not less than
the PW flow is achieving. This condition can be satisfied by
implementing a rate-limiting measure in the NSP, or by shutting down
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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 immediately returning to service, 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 [TFRC].
7. Control Plane
This section describes PWE3 control plane services.
7.1 Set-up 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.
Set up or teardown of a PW can be triggered by an operator command,
from the management plane of a PE, by signaling (i.e., set-up or
teardown) of a PWES, e.g., an ATM SVC, or by an auto-discovery
mechanism.
During the set-up process, the PEs need to exchange some information
(e.g. learn each other's capabilities). The tunnel signaling
protocol MAY be extended to provide mechanisms to enable the PEs to
exchange all necessary information on behalf of the PW.
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 such services, the
corresponding emulated services MUST specify how to perform status
monitoring.
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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 associated PWs, and PSN tunnels if any, are functional in both
directions.
Because the two CEs of an emulated service are not adjacent, a
failure may occur at a place such 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 such native services,
the corresponding PWE3 service MUST provide a failure notification
mechanism.
Similarly, if a native service has notification mechanisms so that
when a network failure is fixed, all the affected services will
change status from "Down" to "Up", 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] [L2TPv3] 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. If a
misconnection occurs it can breach the integrity of the system. If a
payload mismatch occurs it can disrupt the customer network. In both
instances, there are security and operational concerns.
The services of the underlying tunneling mechanism, and its
associated control protocol, can be used to mitigate this. As part
of the PW set-up a PW-TYPE identifier is exchanged. This is then used
by the FWRD and NSP to verify the compatibility of the PWESs.
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 impact the working condition
of an emulated service. For some payload types, packet loss,
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corruption, and out-of-order delivery can be mapped to either a bit
error burst, or 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
notification, if any are needed.
7.3.5. Collective Status Notification
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 there exists a group of emulated services that 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 some 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, that 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 this across
the PW. An approach following the principle of minimum intervention
would be to transparently transport keep-alive messages over the PW.
However, to accurately reproduce the semantics of the native
mechanism, 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 they MAY be
out-of-band, e.g. the signaling VC of an ATM VP, or TDM CCS
signaling.
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From 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 satisfactorily emulate the native
service.
If control messages are passed through, it may be desirable to send
them using either a higher priority or a reliable channel provided by
the PW Demultiplexer layer. See 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 to help
with measurement of service level agreements (SLAs). Typical counters
include:
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.
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 of the pertinent MIBs fit together
to form a cohesive management framework for deploying PWE3 services.
8.2.1. MIB Layering
The SNMP MIBs created for PWE3 should fit the architecture shown in
Figure 15.
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+-----------+ +-----------+ +-----------+
Service | CEM | | Ethernet | | ATM |
Layer |Service MIB| |Service MIB| ... |Service MIB|
+-----------+ +-----------+ +-----------+
\ | /
\ | /
- - - - - - - - - - - - \ - - - | - - - - / - - - - - - -
\ | /
+-------------------------------------------+
Generic PW | Generic PW MIBs |
Layer +-------------------------------------------+
/ \
- - - - - - - - - - - - / - - - - - - - - \ - - - - - - -
/ \
/ \
+-----------+ +-----------+
PSN VC |L2TP VC MIB| |MPLS VC MIB|
Layer +-----------+ +-----------+
| |
- - - - - - - - - | - - - - - - - - - - - - - - - | - - -
| |
+-----------+ +-----------+
PSN |L2TP MIB(s)| |MPLS MIB(s)|
Layer +-----------+ +-----------+
Figure 15: Relationship of SNMP MIBs
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Figure 16 shows an example for a SONET PW carried over MPLS.
+-----------------+
| SONET MIB | RFC2558
+-----------------+
|
+-----------------+
Service |SONET Service MIB| pw-cem-mib
Layer +-----------------+
- - - - - - - - - - | - - - - - - - - - - - - - - -
+-----------------+
Generic PW | Generic PW MIBS | pw-tc-mib
Layer +-----------------+ pw-mib
- - - - - - - - - - | - - - - - - - - - - - - - - -
+-----------------+
PSN VC | MPLS VC MIBS | pw-mpls-mib
Layer +-----------------+
- - - - - - - - - - | - - - - - - - - - - - - - - -
+-----------------+
PSN | MPLS MIBs | mpls-te-mib
Layer +-----------------+ mpls-lsr-mib
Figure 16: Service-specific Example for MIBs
Note that there is a separate MIB for each emulated service as well
as one for each underlying PSN. These MIBs MAY be used in various
combinations as needed.
8.2.2. Service Layer MIBs
The first layer is referred to as the Service Layer. It contains
MIBs for PWE3 services such as Ethernet, ATM, circuits and Frame
Relay. This layer contains those corresponding MIBs used to mate or
adapt those emulated services to the underlying services. This
working group should not produce any MIBs for managing the general
service; rather, it should produce just those MIBs that are used to
interface or adapt the emulated service onto the PWE3 management
framework. For example, the standard SONET MIB [SONETMIB] is
designed and maintained by another working group. Also, the SONET MIB
is designed to manage the native service without PW emulation. Since
the PWE3 working group is chartered to produce the corresponding
adaptation MIB, in this case, it would produce the PW-CEM-MIB
[PWMPLSMIB] that would be used to adapt SONET services to the
underlying PSN that carries the PWE3 service.
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8.2.3. Generic PW MIBs
The second layer is referred to as the Generic PW Layer. This layer
is composed of two MIBs: the PWE-TC-MIB [PWTCMIB] and the PWE-MIB
[PWMIB]. These MIBs are responsible for providing general PWE3
counters and service models used for monitoring and configuration of
PWE3 services over any supported PSN service. That is, this MIB
provides a general model of PWE3 abstraction for management purposes.
This MIB is used to interconnect the Service Layer MIBs to the PSN VC
Layer MIBs. The latter will be described in the next section. This
layer also provides the PW-TC-MIB [PWTCMIB]. This MIB contains
common SMI textual conventions [RFC1902] that MAY be used by any PW
MIB.
8.2.4. PSN VC Layer MIBs
The third layer in the PWE3 management architecture is referred to as
the PSN VC layer. This layer is comprised of MIBs that are
specifically designed to interface general PWE3 services (VCs) onto
those underlying PSN services. In general this means that the MIB
provides a means with which an operator can map the PW service onto
the native PSN service. For example, in the case of MPLS, it is
required that the general VC service be layered onto MPLS LSPs or
Traffic Engineered (TE) Tunnels [RFC3031]. In this case, the PW-
MPLS-MIB [PWMPLSMIB] was created to adapt the general PWE3 circuit
services onto MPLS. Like the Service Layer described above the PWE3
working group should produce these MIBs.
8.2.5. PSN Layer MIBs
The fourth and final layer in the PWE3 management architecture is
referred to as the PSN layer. This layer is comprised of those MIBs
that control the PSN service-specific services. For example, in the
case of the MPLS [RFC3031] PSN service, the MPLS-LSR-MIB [LSRMIB] and
the MPLS-TE-MIB [TEMIB] are used to interface the general PWE3 VC
services onto native MPLS LSPs and/or TE tunnels to carry the
emulated services. In addition, the MPLS-LDP-MIB [LDPMIB] MAY be
used to reveal the MPLS labels that are distributed over the MPLS PSN
in order to maintain the PW service. The MIBs in this layer 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 such that the emulated
PWE3 service will function correctly.
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8.3 Connection Verification and Traceroute
A connection verification mechanism should be supported by PWs.
Connection verification as well as 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.
9. IANA considerations
The control word PID bits need to be assigned by IANA.
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 using 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 [RFC2338] 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
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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 to maintain the integrity of the PW by
authenticating data between the PW Demultiplexer End-points.
IPSec MAY provide authentication, integrity, non-repudiation, 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,
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.
Acknowledgments
We thank: Sasha Vainshtein for his work on Native Service Processing
and advice on bit-stream over PW services. 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.
References
Internet-drafts are works in progress available from
<http://www.ietf.org/internet-drafts/>
[DVB] EN 300 744 Digital Video Broadcasting (DVB); Framing
structure, channel coding and modulation for digital
Bryant and Pate. Standards Track [Page 42]
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terrestrial television (DVB-T), European
Telecommunications Standards Institute (ETSI)
[FRAG] Malis and Townsley, "PWE3 Fragmentation and
Reassembly", <draft-ietf-pwe3-fragmentation-00.txt>,
work in progress, October 2002.
[LDPMIB] Cucchiara, J., Sjostrand, H., and Luciani, J.,
"Definitions of Managed Objects for the Multiprotocol
Label Switching, Label Distribution Protocol (LDP)",
<draft-ietf-mpls-ldp-mib-09.txt>, work in progress,
October 2002.
[LSRMIB] Srinivasan et al, "MPLS Label Switch Router Management
Information Base Using SMIv2",
<draft-ietf-mpls-lsr-mib-09.txt>, work in progress,
October 2002.
[L2TPv3] Layer Two Tunneling Protocol (Version 3)'L2TPv3', J Lau,
et. al. <draft-ietf-l2tpext-l2tp-base-05.txt>, work
in progress, January 2003.
[PPPoL2TP] PPP Tunneling Using Layer Two Tunneling Protocol,
J Lau et al. <draft-ietf-l2tpext-l2tp-ppp-02.txt>,
work in progress, June 2002.
[PWMIB] Zelig et al, "Pseudo Wire (PW) Management Information
Base Using SMIv2", <draft-ietf-pwe3-pw-mib-00.txt>,
work in progress, June 2002.
[PWTCMIB] Nadeau et al, "Definitions for Textual Conventions and
OBJECT-IDENTITIES for Pseudo-Wires Management"
<draft-ietf-pwe3-pw-tc-mib-00.txt>, work in progress,
June 2002.
[PWMPLSMIB] Danenberg et al, "SONET/SDH Circuit Emulation Service
Over MPLS (CEM) Management Information Base Using
SMIv2", <draft-ietf-pwe3-cep-mib-01.txt>, work in
progress, October 2002.
[RFC791] RFC-791: DARPA Internet Program, Protocol Specification,
ISI, September 1981.
[RFC1191] RFC-1191: Path MTU discovery. J.C. Mogul, S.E. Deering.
[RFC1883] RFC-1883: Internet Protocol, Version 6 (IPv6),
S. Deering, et al, December 1995
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[RFC1889] RFC-1889: RTP: A Transport Protocol for Real-Time
Applications. H. Schulzrinne et. al.
[RFC1902] RFC-1902: Structure of Management Information for
Version 2 of the Simple Network Management Protocol
(SNMPv2), Case et al, January 1996.
[RFC1958] RFC-1958: Architectural Principles of the Internet,
B. Carpenter et al.
[RFC1981] RFC-1981: Path MTU Discovery for IP version 6. J. McCann,
S. Deering, J. Mogul.
[RFC2022] RFC-2022: Support for Multicast over UNI 3.0/3.1 based
ATM Networks, G. Armitage.
[RFC2119] RFC-2119, BCP-14: Key words for use in RFCs to Indicate
Requirement Levels, S. Bradner.
[RFC2338] RFC-2338: Virtual Router Redundancy Protocol,
S. Knight, M. Shand et. al.
[RFC2401] RFC-2401: Security Architecture for the Internet
Protocol. S. Kent, R. Atkinson.
[RFC2474] RFC-2474: Definition of the Differentiated Services
Field (DS Field) in the IPv4 and IPv6 Headers,
K. Nichols, et. al.
[RFC2661] RFC-2661: Layer Two Tunneling Protocol "L2TP".
W. Townsley, et. al.
[RFC2784] RFC-2784: Generic Routing Encapsulation (GRE).
D. Farinacci et al.
[RFC2890] RFC-2890: Key and Sequence Number Extensions to GRE.
G. Dommety.
[RFC3022] RFC-3022: Traditional IP Network Address Translator
(Traditional NAT). P Srisuresh et al.
[RFC3031] RFC3031: Multiprotocol Label Switching Architecture,
E. Rosen, January 2001.
[RFC3032] RFC3032: MPLS Label Stack Encoding, E. Rosen,
January 2001.
[SONETMIB] K. Tesink, "Definitions of Managed Objects for the
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SONET/SDH Interface Type", RFC2558, March 1999.
[TEMIB] Srinivasan et al, "Traffic Engineering Management
Information Base Using SMIv2",
<draft-ietf-mpls-te-mib-09.txt>, work in progress,
November 2002.
[TFRC] M. Handley et al, "TCP Friendly Rate Control (TFRC):
Protocol Specification" <draft-ietf-tsvwg-tfrc-05.txt>,
work in progress, October 2002.
[VPLS] M. Lasserre, "Virtual Private LAN Services over MPLS",
<draft-lasserre-vkompella-ppvpn-vpls-03.txt>, work in
progress, January 2003.
[XIAO] Xiao et al, "Requirements for Pseudo-Wire Emulation
Edge-to-Edge (PWE3)",
(draft-ietf-pwe3-requirements-04.txt), X Xiao et al.
work in progress, December 2002.
Editors' Addresses
Stewart Bryant
Cisco Systems,
4, The Square,
Stockley Park,
Uxbridge UB11 1BL,
United Kingdom. Email: stbryant@cisco.com
Prayson Pate
Overture Networks, Inc.
507 Airport Boulevard
Morrisville, NC, USA 27560 Email: prayson.pate@overturenetworks.com
Full copyright statement
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Bryant and Pate. Standards Track [Page 45]
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