Pseudo-Wire Edge-to-Edge (PWE3) Working Group Stewart Bryant
Internet Draft Lloyd Wood
Document: <draft-ietf-pwe3-protocol-layer-00.txt> Mark Townsley
Expires: November 2002 Cisco Systems Ltd
Danny McPherson
TCB
May 2002
Protocol Layering in PWE3
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt The list of
Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This draft proposes a unified protocol layering approach for pseudo-
wire emulation edge-to-edge (PWE3). It adopts the principle that PWE3
should be a single transport type operating over a common packet-
switched network (PSN) service model using, wherever possible,
existing IETF protocols. The draft defines the protocol layering
model for pseudo-wires, guidelines for the design of a specific
encapsulation type, and the service requirements on the underlying
PSN tunneling mechanism.
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Table of Contents
1. Introduction............................................. 3
2. Terminology.............................................. 3
3. Protocol Layering Model.................................. 5
3.1 Protocol Layers...................................... 5
3.2 Domain of PWE3....................................... 7
3.3 Payload Types........................................ 7
4. Architecture of Pseudo-wires............................. 10
4.1 Network Reference Model.............................. 10
4.2 PWE3 Pre-processing.................................. 11
4.3 Maintenance Reference Model.......................... 15
4.4 Protocol Stack Reference Model....................... 15
4.5 Pre-processing Extension to Protocol Stack Reference
Model................................................ 16
5. PW Encapsulation......................................... 17
5.1 Payload Convergence Layer............................ 18
5.2 Payload-independent PW Encapsulation Layers.......... 20
5.3 Fragmentation........................................ 22
5.4 Instantiation of the Protocol Layers................. 23
6. PW Demultiplexer Layer and PSN Requirements.............. 25
6.1 Multiplexing......................................... 26
6.2 Fragmentation........................................ 26
6.3 Length and Delivery.................................. 26
7. Control Plane............................................ 26
7.1 Set-up or Teardown of Pseudo-Wires................... 26
7.2 Status Monitoring.................................... 27
7.3 Notification of Pseudo-wire Status Changes........... 27
7.4 Keep-alive........................................... 29
7.5 Handling Control Messages of the Native Services..... 29
8. IANA considerations...................................... 29
9. Security Considerations.................................. 29
9.1 PW Tunnel End-Point and PW Demultiplexer Security.... 30
9.2 Validation of PW Encapsulation....................... 30
9.3 End-to-End Security.................................. 30
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1. Introduction
This document presents a unified protocol layering approach for
pseudo-wire emulation edge-to-edge (PWE3). Wherever possible,
existing IETF protocols [RFC-1958] are used. PWE3 is intended to
provide only the necessary and sufficient functionality to emulate
the wire with the required degree of faithfulness for the given
service definition. A pseudo-wire (PW) may be point-to-point,
multipoint-to-point or point-to-multipoint. Any required switching
functionality, is the responsibility of a forwarder function. Any
translation or other operation needing a knowledge of the payload
semantics is carried out by native service processing (NSP) elements.
The functional definition of any forwarder of NSP elements is outside
the scope of PWE3.
This document defines the protocol layering model for pseudo-wires
(PW), guidelines for the design of a pseudowire encapsulations, and
the service requirements on the underlying PSN tunneling mechanism.
2. Terminology
This document uses the following definition 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.
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.
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Forwarder A PE sub-system that determines which PW
a payload received on an AC must be sent over.
Fragmentation When a packet MTU is greater than that
supported by the PSN, either the PSN
packet or the payload is fragmented into
smaller data units which are transmitted
separately and reassembled elsewhere in the
network.
Inter-working Interactions between networks, between end
systems, or between parts thereof, with the
aim of providing a functional entity
capable of supporting an end-to-end
communication.
Inter-working A function that facilitates inter-working
Function (IWF) between two dissimilar services. NSP may
perform the IWF function.
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.
Packet Switched A network using IP or MPLS as the mechanism
Network (PSN) 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
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
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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 PW terminating
at a PE.
Tunnel A method of transparently carrying information
over a network.
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 thus 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 empty
+---------------------------+
| PW Demultiplexer |
+---------------------------+
| PSN Convergence | <==== May be empty
+---------------------------+
| PSN |
+---------------------------+
| MAC/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.
If needed, this layer also provides support for real-time processing,
and also 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. 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 any PSN type
defined by the IETF. These are currently IPv4, IPv6 and 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 Bit-stream
o Structured bit-stream
o Cell
o Packet
Within these generic types there are specific service types. For
example:
Generic Payload Type PW Service
-------------------- ----------
Bit-stream SONET, TDM (e.g. DS1, DS3, E1).
Structured bit-stream SONET, TDM.
Cell ATM.
Packet Ethernet (all types), HDLC,
frame-relay, ATM AAL5 PDU.
3.3.1. 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. The Encapsulation Layer submits an identical
number of bits for transport in each PW-PDU.
This service will require sequencing and real-time support.
3.3.2. Structured bit-stream
A 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.
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Two important points distinguish structured and unstructured bit-
streams:
o Some part of the original (unstructured) bit stream are
stripped by, for example, the PSN-bound direction of the
NSP block. For example, in Structured SONET the section
and line overhead (and, possibly, more) may be stripped.
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 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 (i.e.
structured SONET). Note that "short" structures are
indistinguishable from cells and may benefit from the use of cell
encapsulations.
This service will require sequencing and real-time support.
3.3.3. Cell Payload
A cell payload is created by capturing, transporting and replaying
groups of bits 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
53-octet cells carrying ATM AAL2, and the larger 188-octet MPEG
Transport Stream packets [ETSI].
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, or after a fixed number of
cells have been received. The benefit of concatenating multiple PDUs
should be weighed against the resulting increase in jitter and the
larger penalty incurred by packet loss. In some cases, it may be
appropriate for the Encapsulation Layer to perform a silence
suppression or a similar 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.
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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 NSP function, and the selection
would therefore be made before the packet was presented to the PW
Encapsulation Layer.
3.3.4. 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 this case some fragmentation methodology needs to
be applied. This is likely to be the case when a user is providing
the service and attaching to the service provider via an Ethernet, or
where nested pseudo-wires are involved. Fragmentation is discussed in
more detail in Section 5.
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, on the basis of the VLAN identifier. This
is an NSP function, and this selection would therefore be made before
the packet was presented to the PW Encapsulation Layer.
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 [RFC-
1958].
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 any
unwanted side-effects due to subtle mis-representation of the payload
in the intermediate format.
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An intervention approach can be more wire-efficient in some cases and
may result in fewer translations at the NSP where the the CE
interfaces are of different types.
The intermediate format is effectively a new framing type.
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.
|<-------------- 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 at the PW End Service (PWES)
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are encapsulated in a PW-PDU and carried across the underlying
network via the PSN tunnel. The PEs perform the necesssary
encapsulation and decapsulation of PW-PDUs, as well as handling any
other functions required by the PW service, such as sequencing or
timing.
There are situations in which a particular packet payload needs to be
multicast so that it is received by a number of CEs. This is useful
when using PWs as part of a "virtual LAN" service (see, e.g.,
[VPLS]). This can be achieved by replicating the payload and
transmitting the replicas on PWs, but it may also be useful to have a
type of PW which is inherently point-to-multipoint. In that case,
the PW would need to be carried through a point-to-multipoint PSN
tunnel, employing a multicast mechanism provided by the PSN.
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
signalling 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.
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.
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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 Forwarding (FWD)
o Native Service Processing (NSP)
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
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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 Forwarder
(FWD).
The forwarder selects the PW based on for example: the incoming AC,
the contents of the payload, or some statically or dynamically
configured forwarding information.
+----------------------------------------+
| PE Device |
+----------------------------------------+
Single | | |
PWES | | Single | PW Instance
<------>o Forwarder + PW IWF X<===========>
| | Instance |
+----------------------------------------+
Figure 4a: Simple point-to-point service
+----------------------------------------+
| PE Device |
+----------------------------------------+
Multiple| | Single | PW Instance
PWES | + PW IWF X<===========>
<------>o | Instance |
| |----------------------|
<------>o | Single | PW Instance
| + PW IWF X<===========>
<------>o | Instance |
| Forwarder |----------------------|
<------>o | Single | PW Instance
| + PW IWF X<===========>
<------>o | Instance |
| |----------------------| Multipoint
| | Multipoint | PW Instance
| + PW IWF X<===========>
| | Instance |
+----------------------------------------+
Figure 4b: Multiple PWEs to Multiple PW Forwarding
Figure 4a shows a simple forwarder that performs some type of
filtering operation. 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.
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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 IWF X<===========>
| | | Instance |
|------| |----------------------|
| | | Single | PW Instance
<------>o NSP # + PW IWF X<===========>
| | | Instance |
|------|Forwarder |----------------------|
| | | Single | PW Instance
<------>o NSP # + PW IWF X<===========>
| | | Instance |
|------| |----------------------| Multipoint
| | | Multipoint | PW Instance
<------>o NSP # + PW IWF X<===========>
| | | Instance |
+----------------------------------------+
Figure 5: NSP in a Multiple PWEs to Multiple
PW Forwarding PE
Figure 5 illustrates the relationship between NSP, Forwarding 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. A PE device may contain more than one Forwarder.
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, 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. This type of signaling is not within the
scope of PWE3.
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 data units 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 (Fowarding and
NSP). This shows the ideal 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 |
+---------------------------+
| MAC/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 signalling 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, when required.
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 or may not contain L2 or L1 header information.
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 part of the PW header.
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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 channel 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
which 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 DSCP/EXP bits for 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.
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-path data traffic is blocked by one or
more of these mechanisms. In these cases unless the control channel
is also carried "in band" the signalling to set-up the PW will not
confirms the existence of an end-to-end data path.
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In some cases there is a need to synchronize some CE events with the
data carried over a PW. This is especially the case with TDM
circuits (e.g., on-hook/off-hook events in PSTN switches).
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. Specification of a QoS
design common to all PW Service types needs further investigation.
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 invariant properties of a physical wire to be emulated.
Support for sequencing depends on the payload type, and may be
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
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the PSN. A sequence number space greater than approximately 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 [RTP] 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 or may not be the same as the
mechanism used to ensure in-order frame delivery.
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.
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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 the same jitter, bit-rate and timing
characteristics as it was sent.
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 phase-
locking mechanism. A physical wire provides this naturally, but it
is a relatively complex task to extract this from a highly jittered
source such as packet stream. It is therefore desirable that an
existing real-time protocol such as [RTP] 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 via clock recovery, or via an external
clock.
5.3 Fragmentation
A payload would normally 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 exceeds the MTU of a given network, fragmentation and
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reassembly may have to be performed in order for the packet to be
delivered. Since fragmentation and reassembly generally consume a
large amount of network resource as compared to simply switching a
packet in its entirety, efforts should be made to reduce or eliminate
the need for fragmentation and reassembly as much as possible
throughout a network. Of particular concern for fragmentation and
reassembly are aggregation points where large numbers of pseudowires
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 such that it never sends a packet which must be
fragmented. When this fails, the point closest to the sending host
with fragmentation and reassembly capabilities should attempt to
reduce the size of packets further into the network. 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 methods. Further, if an adequate general
fragmentation method exists (e.g. as part of the native protocol
being carried by the PW, or the PSN itself) then this should be
employed when possible.
Examining fragmentation mechanisms for PWE3 is a current work item.
Further study may establish the need for a common PWE3 specific
method.
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.
5.4.1. PWE3 over an IP PSN
The protocol definition of PWE3 over an IP PSN therefore should
employ existing IETF protocols where possible.
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+---------------------+ +-------------------------+
| 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, GRE, IPSec, MPLS |
+---------------------+ +-------------------------+
| PSN Convergence |------------->| Not needed |
+---------------------+ +-------------------------+
| PSN |------------->| IP |
+---------------------+ +-------------------------+
| MAC/Data-link |------------->| MAC/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, 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 label is MPLS, 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 wire efficiency.
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+---------------------+
| Payload |
/=====================\
H Payload Convergence H-----------------+
H---------------------H |
H Timing H |
H---------------------H |
H Sequencing H------------------------------------+
\=====================/ | |
| PW Demultiplexer |---+ | |
+---------------------+ | | |
| PSN Convergence |-----------------------+----+----+ |
+---------------------+ | | | | | |
| PSN |-+ | v v v v v
+---------------------+ | | +--------------------------------+
| MAC/Data-link | | | | Flags |Frag| Len |Seq # |
+---------------------+ | | +--------------------------------+
| Physical | | +->| Inner Label |
+---------------------+ | +--------------------------------+
+--->| Outer Label or MPLS-in-IP encap|
+--------------------------------+
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 fragmentation control bits) a PSN
fragmentation service within the PSN Convergence Layer. To allow
PWE3 carried in MPLS to correctly pass over an Ethernet data-link, a
length correction field is needed in the control word.
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 standard method of carrying
MPLS over an IP PSN is applied to the resultant PW-PDU.
6. PW Demultiplexer Layer and PSN Requirements
PWE3 places three service requirements on the protocol layers used to
carry it across the PSN:
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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 in a port, or multiple DS0 services within a T1 or E1.
A PW may interconnect two end-trunks. That trunk would have a single
multiplexing value.
6.2 Fragmentation
Fragmentation is discussed in Section 5.3.
If the PSN provides a fragmentation service of adequate performance,
that mechanism may be used by the PE to fragment and reassemble PW
PDUs which exceed the PSN MTU. This fragmentation service is
transparent to the PW Encapsulation Layer.
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 will
provide it.
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 a CLI command, from
the management plane of a PE, by signaling (i.e., set-up or teardown)
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of a PWES, e.g., an ATM SVC, or by an auto-discovery mechanism e.g.
[BGPAUTO].
During the set-up process, the PEs need to exchange some information
(e.g. learn each others' capabilities). The tunneling control
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 explicitly 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.
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 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 has this capability and LDP
has the ability to withdraw the corresponding MPLS label.
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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 concerns.
The services of the underlying tunneling mechanism, and its
associated control protocol, can be used to mitigate this.
This area needs further study.
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,
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.
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.
This should be a mechanism provided by the underlying tunneling
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protocol.
7.4 Keep-alive
If a native service has a keep-alive mechanism, the corresponding
emulated service needs to use 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 signalling mechanism.
7.5 Handling Control Messages of the Native Services
Some native services use control messages for maintaining the
circuits. These control messages may be in-band, e.g. Ethernet flow
control or ATM performance management, or out-of-band, e.g. the
signaling VC of an ATM VP.
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 a reliable channel provided by the PW Demultiplexer layer.
See Bearer Channel Types.
8. IANA considerations
There are no IANA considerations for this document.
9. Security Considerations
PWE3 provides no means of protecting the contents or delivery of the
native data units. PWE3 may, however, leverage security mechanisms
provided by the PW Demultiplexer or PSN Layers, such as IPSec
[RFC2401]. This section addresses the PWE3 vulnerabilities, and the
mechanisms available to protect the emulated native services.
The PW Tunnel End-Point, PW demultiplexing mechanism, and the
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payloads of the native service are all vulnerable to attack.
The security aspects of PWE3 need further study.
9.1 PW Tunnel End-Point and PW Demultiplexer Security
Protection mechanisms must be considered for the PW Tunnel end-point
and PW Demultiplexer mechanism in order to avoid denial-of-service
attacks upon the native service, and to prevent spoofing of the
native data units. 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 may 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.
9.2 Validation of PW Encapsulation
Protection mechanisms must 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 PE. 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.
9.3 End-to-End Security
Protection of the PW encapsulated data stream between PEs should not
be considered equivalent to end-to-end security, because the CE-PE
interface and the PE processing element remain unprotected. PW
service emulation does not preclude the application of additional
security mechanisms, such as IPSec, that are implemented end-to-end.
Likewise, end-to-end security mechanisms applied in the native
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service do not protect the PW demultiplexing and PSN tunnel services
provided by the PE for PW encapsulation.
Acknowledgments
We thank Sasha Vainshtein for his work on Native Service Processing
and advice on bit-stream over PW services. We thank Scott Wainner
Stephen Casner, Andy Malis and Eric Rosen for their comments and
contributions.
References
Internet-drafts are works in progress available from
<http://www.ietf.org/internet-drafts/>
[BGPAUTO] Using BGP as an Auto-Discovery Mechanism for Network-based
VPNs. Ould-Brahim et al.
<draft-ietf-ppvpn-bgpvpn-auto-02.txt>, work in progress.
[ETSI] EN 300 744 Digital Video Broadcasting (DVB); Framing
structure, channel coding and modulation for digital
terrestrial television (DVB-T), European Telecommunications
Standards Institute (ETSI)
[PPPoL2TP] PPP Tunneling Using Layer Two Tunneling Protocol,
J Lau et al. <draft-ietf-l2tpext-l2tp-ppp-01.txt>,
work in progress.
[RFC1191] RFC-1191: Path MTU discovery. J.C. Mogul, S.E. Deering.
[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.
[RFC2401] RFC-2401: Security Architecture for the Internet Protocol.
S. Kent, R. Atkinson.
[RFC3022] RFC-3022: Traditional IP Network Address Translator
(Traditional NAT). P Srisuresh et al.
[RTP] RFC-1889: A Transport Protocol for Real-Time Applications, H.
Schulzrinne et al.
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Authors' Addresses
Stewart Bryant
Cisco Systems,
4, The Square,
Stockley Park,
Uxbridge UB11 1BL,
United Kingdom. Email: stbryant@cisco.com
Danny McPherson
TCB. Email: danny@tcb.net
W. Mark Townsley
Cisco Systems,
7025 Kit Creek Road,
PO Box 14987,
Research Triangle Park,
NC 27709 Email: mark@townsley.net
Lloyd Wood
Cisco Systems,
4, The Square,
Stockley Park,
Uxbridge UB11 1BL,
United Kingdom. Email: lwood@cisco.com
Full copyright statement
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or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
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English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
Bryant, et al. Informational [Page 32]
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This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Bryant, et al. Informational [Page 33]
