One document matched: draft-ietf-pwe3-framework-01.txt
Differences from draft-ietf-pwe3-framework-00.txt
Internet Draft Prayson Pate, Editor
Document: draft-ietf-pwe3-framework-01.txt Overture Networks, Inc.
XiPeng Xiao
Redback Networks
Craig White Tricci So
Level 3 Communications, LLC. Caspian Networks
Kireeti Kompella Andrew G. Malis
Juniper Networks, Inc. Vivace Networks
Thomas K. Johnson Thomas D. Nadeau
Litchfield Communications Stewart Bryant
Cisco Systems
Framework for Pseudo Wire Emulation Edge-to-Edge (PWE3)
draft-ietf-pwe3-framework-01.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026. 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 document describes a framework for Pseudo Wire Emulation
Edge-to-Edge (PWE3). It discusses the emulation of services (such
Frame Relay, ATM, Ethernet T1, E1, T3, E3 and SONET/SDH) over packet
switched networks (PSNs) using IP or MPLS. It presents an
architectural framework for pseudo wires (PWs), defines terminology,
specifies the various protocol elements and their functions,
overviews some of the services that will be supported and discusses
how PWs fit into the broader context of protocols.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
Table of Contents
1 Introduction ................................................. 3
1.1 What Are Pseudo Wires? .................................... 3
1.2 Goals of This Document ..................................... 4
1.3 Non-Goals .................................................. 4
1.4 Terminology ................................................ 4
2 Background and Motivation .................................... 7
2.1 Current Network Architecture ............................... 7
2.2 PWE3 as a Path to Convergence .............................. 9
2.3 Suitable Applications for PWE3 ............................. 10
2.4 Summary .................................................... 10
3 Architecture of Pseudo Wires ................................. 10
3.1 Network Reference Model .................................... 11
3.2 Maintenance Reference Model ................................ 11
3.3 Maintenance Architecture ................................... 12
3.4 Protocol Stack Reference Model ............................. 12
3.5 Logical Protocol Layering Model ............................ 13
3.6 Architecture Assumptions ................................... 16
4 Design Considerations ........................................ 16
4.1 PW-PDU Validation .......................................... 16
4.2 PW-PDU Sequencing .......................................... 17
4.3 Session Multiplexing ....................................... 17
4.4 Security ................................................... 17
4.5 Encapsulation Control ...................................... 17
4.6 Statistics ................................................. 18
4.7 Traceroute ................................................. 18
4.8 Congestion Considerations .................................. 19
4.9 PW SNMP MIB Architecture ................................... 19
5 Acknowledgments .............................................. 21
6 References ................................................... 21
7 Security Considerations ...................................... 23
8 IANA Considerations .......................................... 23
9 Authors' Addresses ........................................... 23
10 Full Copyright Section ...................................... 24
Pate et al. Expires December 2002 [Page 2]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
1. Introduction
This document describes a framework for Pseudo Wire Emulation
Edge-to-Edge (PWE3). It discusses the emulation of services (such
Frame Relay, ATM, Ethernet T1, E1, T3, E3 and SONET/SDH) over packet
switched networks (PSNs) using IP or MPLS. It presents an
architectural framework for pseudo wires (PWs), defines terminology,
specifies the various protocol elements and their functions,
overviews the services supported and discusses how PWs fit into the
broader context of protocols. See [XIAO] for the requirements for
PWs.
1.1. What Are Pseudo Wires?
1.1.1. 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. The
required functions of PWs include encapsulating service-specific
bit-streams or PDUs arriving at an ingress port, and carrying them
across a path or tunnel, managing their timing and order, and any
other operations required to emulate the behavior and characteristics
of the service as faithfully as possible.
From the customer perspective, the PW is perceived as an unshared
link or circuit of the chosen service. However, there may be
deficiencies that impede some applications from being carried on a
PW. These limitations should be fully described in the appropriate
service-specific Encapsulation, Emulation and Maintenance Documents
(EEMDs) and Applicability Statements (ASes).
1.1.2. Functions
PWs provide the following functions in order to emulate the behavior
and characteristics of the desired service.
- Encapsulation of service-specific PDUs or circuit data arriving at
an ingress port (logical or physical).
- Carrying the encapsulated data across a tunnel.
- Managing the signaling, timing, order or other aspects of the
service at the boundaries of the PW.
- Service-specific status and alarm management.
EEMDs and/or ASes for each service will describe any shortfalls of
the emulation's faithfulness.
Pate et al. Expires December 2002 [Page 3]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
1.2. Goals of This Document
- Description of the motivation for creating PWs, and some background
on how they may be deployed.
- Description of an architecture and terminology for PWs.
- Description of the statistics and other network management
information needed for tunnel operation and management.
- Whenever possible, relevant requirements from existing IETF
documents and other sources will be incorporated by reference.
1.3. Non-Goals
The following are non-goals for this document:
- The detailed specification of the bits and bytes of the
encapsulations of the various services. This description is
contained in an EEMD and/or AS.
- The detailed definition of the protocols involved in PW setup and
maintenance.
The following are outside the scope of PWE3:
- Discussion of the protection of the encapsulated content of the PW.
- 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 that are implemented on top of
the medium are out of scope.
- Methods to signal or control the underlying PSN.
1.4. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALLNOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119. Below are
the definitions for the terms used throughout the document.
Packet Switched Network
A Packet Switched Network (PSN) is a network using
IP or MPLS as the unit of switching.
Pseudo Wire Emulation Edge to Edge
Pseudo Wire Emulation Edge to Edge (PWE3) is a
mechanism that emulates the essential attributes of
a service (such as a T1 leased line or Frame Relay)
over a PSN.
Pate et al. Expires December 2002 [Page 4]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
Customer Edge A Customer Edge (CE) is a device where one end of an
emulated service originates and terminates. The CE
is not aware that it is using an emulated service
rather than a "real" service.
Provider Edge A Provider Edge (PE) is a device that provides PWE3
to a CE.
Pseudo Wire A Pseudo Wire (PW) is a connection between two PEs
carried over a PSN. The PE provides the adaptation
between the CE and the PW.
PW End Service A Pseudo Wire End Service (PWES) is the interface
between a PE and a CE. This can be a physical
interface like a T1 or Ethernet, or a virtual
interface like a VC or VLAN.
Pseudo Wire PDU A Pseudo Wire PDU is a PDU sent on the PW that
contains all of the data and control information
necessary to provide the desired service.
PSN Tunnel A PSN Tunnel is a tunnel inside which multiple PWs
can be nested so that they are transparent to core
network devices.
PW Domain A PW Domain (PWD) is a collection of instances of
PWs that are within the scope of a single
homogeneous administrative domain (e.g. PW over MPLS
network or PW over IP network etc.).
Path-oriented PW A Path-oriented PW is a PW for which the network
devices of the underlying PSN must maintain path
state information.
Non-path-oriented PW
A Non-path-oriented PW is a PW for which the network
devices of the underlying PSN need not maintain path
state information.
Interworking Interworking is used to express 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. The interactions required to provide
a functional entity rely on functions and on the
means to select these functions.
Interworking Function
An Interworking Function (IWF) is a functional
entity that facilitates interworking between two
dissimilar networks (e.g., ATM & MPLS, ATM & L2TP,
etc.). A PE performs the IWF function.
Pate et al. Expires December 2002 [Page 5]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
Service Interworking
In Service Interworking, the IWF (Interworking
Function) between two dissimilar protocols (e.g.,
ATM & MPLS, Frame Relay & ATM, ATM & IP, ATM & L2TP,
etc.) terminates the protocol used in one network
and translates (i.e. maps) its Protocol Control
Information (PCI) to the PCI of the protocol used in
other network for User, Control and Management Plane
functions to the extent possible. In general, since
not all functions may be supported in one or other
of the networks, the translation of PCI may be
partial or non-existent. However, this should not
result in any loss of user data since the payload is
not affected by PCI conversion at the service
interworking IWF.
Network Interworking
In Network Interworking, the PCI (Protocol Control
Information) of the protocol and the payload
information used in two similar networks are
transferred transparently by an IWF of the PE across
the PSN. Typically the IWF of the PE encapsulates
the information which is transmitted by means of an
adaptation function and transfers it transparently
to the other network.
Applicability Statement
Each PW service will have an Applicability Statement
(AS) that describes the applicability of PWs for
that service.
Encapsulation, Emulation and Maintenance Documents
Each PW service will have an Encapsulation,
Emulation and Maintenance Document (EEMDs) that
described the particulars of PWs for that service,
as well as the degree of faithfulness to that
service.
PSN Bound The traffic direction where information from a CE is
adapted to a PW, and PW-PDUs are sent into the PSN.
CE Bound The traffic direction where PW-PDUs are received on
a PW from the PSN, re-converted back in the emulated
service, and sent out to a CE.
CE Signaling CE (end-to-end) Signaling refers to messages sent
and received by the CEs. It may be desirable or
even necessary for the PE to participate in or
monitor this signaling in order to effectively
emulate the service.
Pate et al. Expires December 2002 [Page 6]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
PE/PW Maintenance
PE/PW Maintenance is 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.
PSN Tunnel Signaling
PSN Tunnel Signaling is used to set up, maintain and
remove the underlying PSN tunnel. An example would
be LDP in MPLS for maintaining LSPs.
2. Background and Motivation
Why is anyone interested in PWs? This section gives some background
on where networks are today and why they are changing. It also talks
about the motivation to provide converged networks while continuing
to support existing services. Finally it discusses how PWs can be a
solution for this dilemma.
2.1. Current Network Architecture
2.1.1. Multiple Networks
For any given service provider delivering multiple services, the
current "network" usually consists of parallel or "overlay" networks.
Each of these networks implements a specific service, such as voice,
Frame Relay, Internet access, etc. This is quite expensive, both in
terms of capital expense as well as in operational costs.
Furthermore, the presence of multiple networks complicates planning.
Service providers wind up asking themselves these questions:
- Which of my networks do I build out?
- How many fibers do I need for each network?
- How do I efficiently manage multiple networks?
A converged network helps service providers answer these questions in
a consistent and economical fashion.
2.1.2. Convergence Today
There are some examples of convergence in today's network:
- Frame Relay is frequently carried over ATM networks using [FRF.5]
interworking.
- T1, E1 and T3 circuits are sometimes carried over ATM networks
using [ATMCES].
- Voice is carried over ATM (using AAL2), Frame Relay (using FRF.11
VoFR), IP (using VoIP) and MPLS (using VoMPLS) networks.
Pate et al. Expires December 2002 [Page 7]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
Deployment of these examples range from limited (ATM CES) to fairly
common (FRF.5 interworking) to rapidly growing (VoIP).
2.1.3. The Emerging Converged Network
Many service providers are finding that the new IP-based and
MPLS-based switching systems are much less costly to acquire, deploy
and maintain than the systems that they replace. The new systems
take advantage of advances in technology in these ways:
- The newer systems leverage mass production of ASICs and optical
interfaces to reduce capital expense.
- The bulk of the traffic in the network today originates from packet
sources. Packet switches can economically switch and deliver this
traffic natively.
- Variable-length switches have lower system costs than ATM due to
simpler switching mechanisms as well as elimination of segmentation
and reassembly (SAR) at the edges of the network.
- Deployment of services is simpler due to the connectionless nature
of IP services or the rapid provisioning of MPLS applications.
2.1.4. Transition to a Packet-Optimized Converged Network
The greatest assets for many service providers are the physical
communications links that they own. The time and costs associated
with acquiring the necessary rights of way, getting the required
governmental approvals, and physically installing the cabling over a
variety of terrains and obstacles represents a significant asset that
is difficult to replace. Their greatest on-going costs are the
operational expenses associated with maintaining and operating their
networks. In order to maximize the return on their assets and
minimize their operating costs, service providers often look to
consolidate the delivery of multiple service types onto a single
networking technology.
The first generation converged network was based on TDM
(time-division multiplexing) technology. Voice, video, and data
traffic have been carried successfully across TDM/DACS-based networks
for decades. TDM technology has some significant drawbacks as a
converged networking technology. Operational costs for TDM networks
remain relatively high because the provisioning of end-to-end TDM
circuits is typically a tedious and labor-intensive task. In
addition, TDM switching does not make the best use of the
communications links. This is because fixed assignment of timeslots
does not allow for the statistical multiplexing of bursty data
traffic (i.e. temporarily unused bandwidth on one timeslot cannot be
dynamically re-allocated to another service).
Pate et al. Expires December 2002 [Page 8]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
The second generation of converged network was based on ATM
technology. Today many service providers convert voice, video, and
data traffic into fixed-length cells for carriage across ATM-based
networks. ATM improves upon TDM technology by providing the ability
to statistically multiplex different types of traffic onto
communications links. In addition, ATM SPVC technology is often used
to automatically provision end-to-end services, providing an
additional advantage over traditional TDM networks. However, ATM has
several significant drawbacks. One of the most frequently cited
problems with ATM is the so-called cell-tax, which refers to the 5
bytes out of 53 used as an ATM cell header. Another significant
problem with ATM is the AAL5 SAR, which becomes extremely difficult
to implement above 1 Gbps. There are also issues with the long-term
scalability of ATM, especially as a switching layer beneath IP.
As packet traffic takes up a larger and larger portion of the
available network bandwidth, it becomes increasingly useful to
optimize public networks for the Internet Protocol. However, many
service providers are confronting several obstacles in engineering
packet-optimized networks. Although Internet traffic is the fastest
growing traffic segment, it does not generate the highest revenue per
bit. For example, Frame Relay traffic currently generates a higher
revenue per bit than do native IP services. Private line TDM
services still generate even more revenue per bit than does Frame
Relay. In addition, there is a tremendous amount of legacy equipment
deployed within public networks that does not communicate using the
Internet Protocol. Service providers continue to utilize non-IP
equipment to deploy a variety of services, and see a need to
interconnect this legacy equipment over their IP-optimized core
networks.
2.2. PWE3 as a Path to Convergence
How do service providers realize the capital and operational benefits
of a new packet-based infrastructure, while leveraging the existing
base of SONET (Synchronous Optical Network) gear, and while also
protecting the large revenue stream associated with this equipment?
How do they move from mature Frame Relay or ATM networks, while still
being able to provide these lucrative services?
One possibility is the emulation of circuits or services via PWs.
Circuit emulation over ATM and interworking of Frame Relay and ATM
have already been standardized. Emulation allows existing services
to be carried across the new infrastructure, and thus enables the
interworking of disparate networks. [ATMCES] provides some insight
into the requirements for such a service:
There is a user demand for carrying certain types of
constant bit rate (CBR) or "circuit" traffic over
Asynchronous Transfer Mode (ATM) networks. As ATM is
essentially a packet- rather than circuit-oriented
transmission technology, it must emulate circuit
Pate et al. Expires December 2002 [Page 9]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
characteristics in order to provide good support for CBR
traffic.
A critical attribute of a Circuit Emulation Service (CES)
is that the performance realized over ATM should be
comparable to that experienced with the current PDH/SDH
technology.
Implemented correctly, PWE3 can provide a means for supporting
today's services over a new network.
2.3. Suitable Applications for PWE3
What makes an application suitable (or not) for PWE3 emulation? When
considering PWs as a means of providing an application, the following
questions must be considered:
- Is the application sufficiently deployed to warrant emulation?
- Is there interest on the part of service providers in providing an
emulation for the given application?
- Is there interest on the part of equipment manufacturers in
providing products for the emulation of a given application?
- Are the complexities and limitations of providing an emulation
worth the savings in capital and operational expenses?
If the answer to all four questions is "yes", then the application is
likely to be a good candidate for PWE3. Otherwise, there may not be
sufficient overlap between the customers, service providers,
equipment manufacturers and technology to warrant providing such an
emulation.
2.4. Summary
To maximize the return on their assets and minimize their operational
costs, many service providers are looking to consolidate the delivery
of multiple service offerings and traffic types onto a single
IP-optimized network.
In order to create this next-generation converged network, standard
methods must be developed to emulate existing telecommunications
formats such as Ethernet, Frame Relay, ATM, and TDM over IP-optimized
core networks. This document describes a framework accomplishing
this goal.
3. Architecture of Pseudo Wires
Pate et al. Expires December 2002 [Page 10]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
3.1. Network Reference Model
Figure 1 below shows the network reference model for PWs. As shown,
the PW provides an emulated service between the customer edges (CEs).
|<------- Pseudo Wire ------>|
| |<-- PSN Tunnel -->| |
PW V V V V PW
End Service +----+ +----+ End Service
+-----+ | | PE1|==================| PE2| | +-----+
| |----------|............PW1.............|----------| |
| CE1 | | | | | | | | CE2 |
| |----------|............PW2.............|----------| |
+-----+ | | |==================| | | +-----+
Customer^ +----+ +----+ | ^Customer
Edge 1 | Provider Edge 1 Provider Edge 2 | Edge 1
|<-------------- Emulated Service ---------------->|
Figure 1: PWE3 Network Reference Model
Any bits or packets presented at the PW End Service (PWES) are
encapsulated in a PW-PDU and carried across the underlying network.
The PEs perform the encapsulation, decapsulation, order management,
timing and any other functions required by the service. In some
cases the PWES can be treated as a virtual interfaces into a further
processing (like switching or bridging) of the original service
before the physical connection to the CE. Examples include Ethernet
bridging, SONET cross-connect, translation of locally-significant
identifiers such as VCI/VPI, etc. to other service type, etc. The
underlying PSN is not involved in any of these service-specific
operations.
3.2. Maintenance Reference Model
Figure 2 below shows 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 2: PWE3 Maintenance Reference Model
Pate et al. Expires December 2002 [Page 11]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
- The CE (end-to-end) signaling is between the CEs. This signaling
includes Frame Relay PVC status signaling, ATM SVC signaling, etc.
- The PW/PE Maintenance is used between the PEs to set up, maintain
and tear down PWs, including any required coordination of
parameters between the two ends.
- The PSN Tunnel signaling controls the underlying PSN. An example
would be LDP in MPLS for maintaining LSPs. This type of signaling
is not within the scope of PWE3.
3.3. Maintenance Architecture
<Editor's Note: Luca Martini has asked that a section be added
describing the architecture of the maintenance protocol(s). This
section will contain that architecture. Some possible topics are
listed below.>
3.3.1. PW Setup
How are PWs set up?
3.3.2. PW Integrity
How do we ensure the sanity or connectivity of the PW over the PSN?
3.3.3. Service Maintenance
How does CE maintenance behavior e.g. FR LMI, ATM AIS/RDI, etc.
affect the PW?
3.4. Protocol Stack Reference Model
Figure 3 below shows the protocol stack reference model for PWs.
+----------------+ +----------------+
|Emulated Service| Emulated Service |Emulated Service|
|(TDM, ATM, etc).|<===========================>|(TDM, ATM, etc.)|
+----------------+ Pseudo Wire +----------------+
| Encapsulation |<===========================>| Encapsulation |
+----------------+ PSN Tunnel +----------------+
| IP/MPLS |<===========================>| IP/MPLS |
+----------------+ +----------------+
| Physical | | Physical |
+-------+--------+ _____________ +--------+-------+
| / \ |
+===============/ PSN \===============+
\ /
\_____________/
Figure 3: PWE3 Protocol Stack Reference Model
Pate et al. Expires December 2002 [Page 12]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
The PW provides the CE with what appears to be a connection to its
peer at the far end. Bits or PDUs from the CE are passed through an
encapsulation layer.
3.5. Logical Protocol Layering Model
3.5.1. Protocol Layers
The logical protocol-layering model needed to support a PW is shown
in Figure 4 below.
+---------------------------+
| Payload |
+---------------------------+
| Encapsulation Layer |
+---------------------------+
| Multiplexing Layer |
+---------------------------+
| PSN Header |
+---------------------------+
| MAC/Datalink |
+---------------------------+
| Physical |
+---------------------------+
Figure 4: Logical Protocol Layering Model
The logical protocol-layering model shown in Figure 4 above minimizes
the differences between the PWs operating over different PSN types.
The payload is transported over the Encapsulation Layer that carries
any information that is not available in the payload itself and which
is needed by the CE Bound interface to send the reconstructed service
to the CE. If needed, this layer also provides support for real-time
processing, sequencing and indication of length.
The Multiplexing Layer provides the ability to deliver multiple PWs
over a single PSN tunnel.
The PSN header, MAC/datalink and physical layer definitions are
outside the scope of this framework.
3.5.2. Instantiation of the Protocol Layers
The instantiation of the logical protocol-layering model of Figure 4
is shown in Figure 5 below.
Where possible, the components shown below use existing IETF
standards and common work in progress. Otherwise, the goal was to
call for the design of components that have the wider application
within the IETF.
Pate et al. Expires December 2002 [Page 13]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
+=========================================+ - - - - - - - - - -
| Payload (circuit/cell/packet) | Payload
+=========================================+ - - - - - - - - - -
| Encapsulation-specific information |
| Sequencing (optional) | Encapsulation
| Length (payload/PSN-specific) | Layer
| Timing Information (payload-specific) |
+============+============================+ - - - - - - - - - -
| L2TPv3 | MPLS shim | Multiplexing Layer
+------------+-------+--------------------+ - - - - - - - - - -
| IPv4/v6 | MPLS | PSN Header
+====================+====================+ - - - - - - - - - -
| MAC/Data-link |
+=========================================+
| Physical |
+=========================================+
Figure 5: Instantiated Protocol Layering
3.5.2.1. Payload
The payload is generically classified into three types: circuit, cell
and packet. Within these generic types there will be specific service
types. For example, the generic packet type includes Frame Relay and
Ethernet. The design of the encapsulation layer, and the choice
between transporting the payload in a native or intermediate format,
will be defined in the service-specific EEMD and/or AS.
3.5.2.1.1. Circuit Payload
A circuit payload is a sampled set of bits relayed across the PW.
This service will need sequencing and real-time support.
3.5.2.1.2. Cell Payload
A cell payload is one, or more, ATM cells relayed across the PW. This
service will need sequence number support, and may also need
real-time support. The payload may consist of a single cell or a
cluster of cells. The cells to be incorporated in the payload may be
selected by filtering on VCI/VPI. In the case of a trunked interface
the payload may be considered complete on the expiry of a timer or
when a fixed number of cells have been received or both.
3.5.2.1.3. Packet Payload
A packet payload would normally be relayed across the PW as a single
unit. A packet payload may need sequencing or sequencing and
real-time support. A packet payload may additionally require
fragmentation
Pate et al. Expires December 2002 [Page 14]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
3.5.2.2. Sequencing
Support for sequencing depends on the payload type, and may be
omitted if not needed. For example, Frame Relay always requires the
preservation of packet order. However, where the Frame Relay service
is only being used to carry IP, it may be desirable to relax that
constraint in return for reduced per-packet processing cost.
[RTP] or encapsulation-specific sequence numbers may be used for
sequencing.
3.5.2.3. Timing
A suitable real-time protocol is RTP [RTP]. This is an extensible
protocol designed to be extended to carry new payload types over a
transport layer running over an IP network. It includes a control
protocol for managing the timing service. RTP is not currently
defined for operation over L2TP. A short document describing the
correct method of multiplexing the RTP control and data over LT2Pv3
is required.
3.5.2.4. Multiplexing Layer
Suitable protocols for use as a multiplexing layer are L2TPv3 [LAU]
and MPLS [MPLS].
- The updated definition of L2TP in [LAU] is specified to operate
directly over a PSN, and in the limiting case the L2TP data
encapsulation reduces to a four-octet opaque data field. L2TPv3 is
specified for operation in both manually configured and negotiated
mode. The associated control protocol is extensible and may be
used to signal PW-specific configuration or run-time information.
- MPLS may also be used in the multiplexing layer. In this case, an
MPLS tag is used as a "shim" to identify the particular PW of
interest.
3.5.2.5. PSN Layer
The three PSN types within the scope of the IETF are IPv4, IPv6 and
MPLS. IPv4 and IPv6 both provide the necessary switching, length and
fragmentation services needed to support all IETF specified transport
protocols. L2TPv3 is specified to run directly over IPv4 and IPv6.
When the PSN is IPv4 or IPv6 no PSN convergence layer is needed.
MPLS provides switching service, but no length or fragmentation
service. When MPLS is used as the PSN, the encapsulation must provide
length and fragmentation services, if needed.
Pate et al. Expires December 2002 [Page 15]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
3.6. Architecture Assumptions
1) The current design is focused on a point-to-point and same-to-same
service interface at both end of the PW. Only network
interworking will be performed at the edge or the PW. Support for
service interworking is out of scope.
2) The initial design of PWE3 is focused on a single homogeneous
administrative PWD (e.g. PW over MPLS or PW over IP etc. ONLY).
Support for interworking between different PW types is for further
study, as is the support of inter-domain PWs.
3) The design of PW will not perfectly emulate the characteristics of
the native service. It will be dependent on both the emulated
service, as well as on the network implementation. An EEMD and AS
shall be created for each service to describe the degree of
faithfulness of a PW to the native service.
4) Only the permanent emulated circuit type (e.g. PVC/PVP) is
considered initially. Support for the switched emulated circuit
type (e.g. SVC/SVP) is be for further study.
5) The creation and placement of the PSN tunnel to support the PW is
not within the scope.
6) The current PW encapsulation approach considerations are focused
on IPv4, IPv6 and MPLS. Support for other encapsulation
approaches is for further study.
7) Current PW service applications are focused on Ethernet (i.e.
Ethernet II (DIX), 802.3 "raw", Ethernet 802.2, Ethernet SNAP,
802.3ac VLAN, 802.1Q), Frame Relay, ATM and TDM (e.g. DS1, DS3,
E1, SONET/SDH etc.).
8) Within the single administrative PWD, the design of the PW assumes
the inheritance of the security mechanism that has been applied to
the emulated services. The PW also inherits any security features
from the PSN e.g. IPsec for an IP PSN. No PW-specific security
mechanism will be specified.
4. Design Considerations
4.1. PW-PDU Validation
It is a common practice to use a checksum, CRC or FCS 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 PWD or be removed at the ingress PE and then be re-calculated at
the egress PE. The former approach saves work, while the later saves
bandwidth.
Pate et al. Expires December 2002 [Page 16]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
For protocols like ATM and Frame Relay, the checksum is only
applicable to a single link. This is because the circuit identifiers
(e.g. Frame Relay DLCI or ATM VPI/VCI) have only local significance
and are changed on each hop or span. If the circuit identifier (and
thus checksum) is going to change as a part of the PW emulation, it
would be more efficient to strip and re-calculate the checksum.
Other PDU headers (e.g. UDP in IP) do not change during transit. It
would make sense to preserve these types of checksums.
The EEMD for each protocol must describe the validation scheme to be
used.
4.2. PW-PDU Sequencing
One major consideration of PW design is how to ensure in-sequence
delivery of packets, if needed. The design of the PW for each
protocol must consider the support of the PSN for in-order delivery
as well as the requirements of the particular application. For
example, IP is connectionless and does not guarantee in-order
delivery. When using IP, a PW sequence number may be needed for some
applications (such as TDM).
4.3. Session Multiplexing
One way to facilitate scaling is to increase the number of PWs per
underlying tunnel. There are two ways to achieve this:
- For a service like Relay or ATM, all of the VCs on a given port
could be lumped together. VCs would not be distinguishable within
the PWD.
- Service SDUs could be distinguished within a PW-PDU by port,
channel or VC identifiers. This approach would allow for switching
or grooming in the PWD.
4.4. Security
Each EEMD must specify a means to protect the control of the PWE and
the PE/PW signaling. The security-related protection of the
encapsulated content of the PW is outside of scope.
4.5. Encapsulation Control
4.5.1. Scalability
Different service types may be required between CEs. Support of
multiple services implies that a range of PWD label spaces may be
needed. If the PWD spans a PSN supporting traffic engineering, then
the ability to supporting label stacking would be desirable.
Pate et al. Expires December 2002 [Page 17]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
4.5.2. Service Integration
It may be desirable to design a PW to transport a variety of services
which have different transport characteristics. To achieve this
integration it may be useful to allow the service requirements to be
mapped to the tunneling label in such a way that the PWD can apply
the appropriate service and transport management to the PW.
4.6. Statistics
The PE can tabulate statistics that help monitor the state of the
network, and to help with measurement of SLAs. Typical counters
include:
- Counts of PW-PDUs sent and received, with and without errors.
- Counts of PW-PDUs lost (TDM only).
- Counts of service PDUs sent and received, with and without errors
(non-TDM).
- Service-specific interface counts.
These counters would be contained in a PW-specific MIB, and they
should not replicate existing MIB counters.
4.7. Traceroute
Tracing functionality is desirable for emulated services, because it
allows verification and remediation of the operation and
configuration of the forwarding plane. [BONICA] describes the
requirements for a generic route tracing application. Applicability
of these requirements to PWE3 is an interesting problem, as many of
the emulated services have no equivalent function. In general, there
is not a way to trace the forwarding plane of an TDM or Frame Relay
PVC. ATM does provide an option in the loopback OAM cell to return
each intermediate hop (see [I.610]).
There needs to be a mechanism through which upper layers can ask
emulated services to reveal their internal forwarding details. A
common mechanism for all emulated services over a particular PSN may
be possible. For example, if MPLS is the PSN, the path for a VC LSP
could be revealed via the signaling from the underlying TE tunnel
LSP, or perhaps via the proposed MPLS OAM. However, when we are
trying to trace the entire emulated service, starting from the CE
(e.g. an ATM VCC), then a uniform approach probably will not work and
different approaches would be required for different emulated
services.
Pate et al. Expires December 2002 [Page 18]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
4.8. 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.
Each PW EEMD and/or AS will have to specify whether it needs an
appropriate mechanism for operating in the presence of this
congestion, including methods of mapping between its native
congestion reporting and avoidance mechanisms, and those provided by
the PW.
4.9. 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
provides a clear picture of how all of the pertinent MIBs fit
together to form a cohesive management framework for deploying PWE3
services.
4.9.1. MIB Layering
The SNMP MIBs created for PWE3 should fit the architecture shown in
Figure 6.
+-----------+ +-----------+ +-----------+
Service | CEM | | Ethernet | | ATM |
Layer |Service MIB| |Service MIB| ... |Service MIB|
+-----------+ +-----------+ +-----------+
\ | /
\ | /
- - - - - - - - - - - - \ - - - | - - - - / - - - - - - -
\ | /
+-------------------------------------------+
Generic PW | Generic PW MIBs |
Layer +-------------------------------------------+
/ | \
- - - - - - - - - - - - / - - - | - - - - \ - - - - - - -
/ | \
/ | \
+-----------+ +-----------+ +-----------+
PSN VC |GRE VC MIB | |L2TP VC MIB| |MPLS VC MIB|
Layer +-----------+ +-----------+ +-----------+
| | |
- - - - - - - - - | - - - - - - | - - - - - - - - | - - -
| | |
+-----------+ +-----------+ +-----------+
PSN |GRE MIB(s) | |L2TP MIB(s)| |MPLS MIB(s)|
Layer +-----------+ +-----------+ +-----------+
Figure 6: Relationship of SNMP MIBs
Pate et al. Expires December 2002 [Page 19]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
Figure 7 shows an example for a TDM 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 7: 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.
4.9.2. 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.
4.9.3. 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
Pate et al. Expires December 2002 [Page 20]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
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 [SMIv2] that may be used by any PW MIB.
4.9.4. 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 [MPLS]. 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.
4.9.5. 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 [MPLS] 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.
5. Acknowledgments
This document was created by the PWE3 Framework design team.
Valuable input was also provided by John Rutemiller, David Zelig,
Durai Chinnaiah, Jayakumar Jayakumar, Ron Bonica, Ghassem Koleyni,
and Eric Rosen.
6. References
[L2TP] W.M. Townsley, A. Valencia, A. Rubens, G. Singh Pall, G.
Zorn, B. Palter, "Layer Two Tunneling Protocol (L2TP)", RFC
2661, August 1999.
[RTP] H. Schulzrinne et al, "RTP: A Transport Protocol for
Real-Time Applications", RFC1889, January 1996.
Pate et al. Expires December 2002 [Page 21]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
[MPLS] E. Rosen, "Multiprotocol Label Switching Architecture",
RFC3031, January 2001.
[IP] DARPA, "Internet Protocol", RFC791, September 1981.
[CONGEST] S. Floyd, "Congestion Control Principles", RFC2914,
September 2000.
[SONETMIB] K. Tesink, "Definitions of Managed Objects for the SONET/SDH
Interface Type", RFC2558, March 1999.
[SMIv2] Case et al, "Structure of Management Information for Version
2 of the Simple Network Management Protocol (SNMPv2)",
RFC1902, January 1996.
[MALIS] Malis et al, "SONET/SDH Circuit Emulation over Packet (CEP)"
(draft-malis-pwe3-sonet-01.txt), work in progress, November
2001.
[XIAO] Xiao et al, "Requirements for Pseudo Wire Emulation
Edge-to-Edge (PWE3)" (draft-pwe3-requirements-02.txt), work
in progress, November 2001.
[BONICA] Bonica et al, "Tracing Requirements for Generic Tunnels"
(draft-bonica-tunneltrace-02.txt), work in progress,
November 2001.
[LAU] J Lau et al, Layer Two Tunneling Protocol "L2TP",
(draft-ietf-l2tpext-l2tp-base-02.txt) work in progress,
March 2002.
[PWMIB] Zelig et al, "Pseudo Wire (PW) Management Information Base
Using SMIv2", (draft-zelig-pw-mib-02.txt), work in progress,
February 2002.
[PWTCMIB] Nadeau et al, "Definitions for Textual Conventions and
OBJECT-IDENTITIES for Pseudo-Wires Management"
(draft-nadeau-pw-tc-mib-02.txt), work in progress, February
2002.
[PWMPLSMIB] Danenberg et al, "SONET/SDH Circuit Emulation Service Over
MPLS (CEM) Management Information Base Using SMIv2",
(draft-danenberg-pw-cem-mib-01.txt), work in progress,
November 2001.
[LSRMIB] Srinivasan et al, "MPLS Label Switch Router Management
Information Base Using SMIv2",
draft-ietf-mpls-lsr-mib-08.txt, work in progress, January
2002.
[TEMIB] Srinivasan et al, "Traffic Engineering Management
Information Base Using SMIv2",
Pate et al. Expires December 2002 [Page 22]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
<draft-ietf-mpls-te-mib-05.txt>, work in progress, January
2002.
[LDP-MIB] 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-08.txt>, work in progress, August
2001.
[ATMCES] ATM Forum, "Circuit Emulation Service Interoperability
Specification Version 2.0" (af-vtoa-0078-000), January 1997.
[FRF.5] O'Leary et al, "Frame Relay/ATM PVC Network Interworking
Implementation Agreement", Frame Relay Forum FRF.5, December
20, 1994. ITU Recommendation Q.933, Annex A, Geneva, 1995.
[I.363.1] ITU, "B-ISDN ATM Adaptation Layer specification: Type 1
AAL", Recommendation I.363.1, August, 1996.
[I.610] ITU, "B-ISDN Operation and Maintenance Principles and
Functions", ITU Recommendation I.610, February, 1999.
[802.3] IEEE, "ISO/IEC 8802-3: 2000 (E), Information
technology--Telecommunications and information exchange
between systems --Local and metropolitan area networks
--Specific requirements --Part 3: Carrier Sense Multiple
Access with Collision Detection (CSMA/CD) Access Method and
Physical Layer Specifications", 2000.
7. Security Considerations
It may be desirable to define methods for ensuring security during
exchange of encapsulation control information at an administrative
boundary of the PSN.
8. IANA Considerations
There are no IANA considerations for this document.
9. Authors' Addresses
Prayson Pate XiPeng Xiao
Overture Networks, Inc. Redback Networks
P. O. Box 14864 300 Holger Way
RTP, NC, USA 27709 San Jose, CA 95134
prayson.pate@overturenetworks.com xipeng@redback.com
Tricci So Craig White
Caspian Networks Level 3 Communications, LLC.
170 Baytech Dr. 1025 Eldorado Blvd.
San Jose, CA 95134 Broomfield, CO, 80021
tso@caspiannetworks.com Craig.White@Level3.com
Pate et al. Expires December 2002 [Page 23]
Internet Draft draft-ietf-pwe3-framework-01 June, 2002
Kireeti Kompella Andrew G. Malis
Juniper Networks, Inc. Vivace Networks, Inc.
1194 N. Mathilda Ave. 2730 Orchard Parkway
Sunnyvale, CA 94089 San Jose, CA 95134
kireeti@juniper.net Andy.Malis@vivacenetworks.com
Thomas K. Johnson Thomas D. Nadeau
Litchfield Communications Cisco Systems, Inc.
76 Westbury Park Rd. 250 Apollo Drive
Watertown, CT 06795 Chelmsford, MA 01824
tom_johnson@litchfieldcomm.com tnadeau@cisco.com
Stewart Bryant
Cisco Systems Ltd
4, The Square
Stockley Park
Uxbridge UB11 1BL UK
stbryant@cisco.com
10. Full Copyright Section
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Pate et al. Expires December 2002 [Page 24]
| PAFTECH AB 2003-2026 | 2026-04-21 11:59:10 |