Internet Draft Prayson Pate
Document: draft-pate-pwe3-framework-01.txt Overture Networks
Expires: January 13, 2002 XiPeng Xiao
Photuris Inc.
Tricci So
Caspian Networks
Craig White Kireeti Kompella
Level 3 Communications, LLC. Juniper Networks, Inc.
Andrew G. Malis Thomas K. Johnson
Vivace Networks Litchfield Communications
Framework for
Pseudo Wire Emulation Edge-to-Edge (PWE3)
draft-pate-pwe3-framework-01.txt
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Abstract
This document describes a framework for Pseudo Wires Emulation Edge-
to-Edge (PWE3). It discusses the emulation of circuits (such as T1,
E1, T3, E3 and SONET/SDH) and services (such as ATM and Frame Relay)
over packet switched networks (PSNs) using IP, L2TP 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 (2001). All Rights Reserved.
Internet Draft draft-pate-pwe3-framework-01 July 13, 2001
Table of Contents
1 Introduction ................................................. 3
2 Background and Motivation .................................... 5
3 Architecture of Pseudo Wires ................................. 8
4 Layer 1 (Circuit) Applications ............................... 15
5 Layer 2 (Packet/Cell) Applications ........................... 25
6 PW Maintenance ............................................... 36
7 Packet Switched Networks ..................................... 40
8 Acknowledgments .............................................. 43
9 References ................................................... 43
10 Security Considerations ..................................... 45
11 Authors' Addresses .......................................... 46
12 Full Copyright Section ...................................... 47
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1. Introduction
This document describes a framework for Pseudo Wires Emulation Edge-
to-Edge (PWE3). It discusses the emulation of circuits (such as T1,
E1, T3, E3 and SONET/SDH) and services (such as ATM and Frame Relay)
over packet switched networks (PSNs) using IP, L2TP 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.
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 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 signaling and alarm management.
ASes for each service will describe any shortfalls of the emulation's
faithfulness.
1.2. Goals of This Document
- Description of the motivation for creating PWs, and some background
on how they may be deployed.
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- Description of an architecture and terminology for PWs.
- Description of the relevant services that will be supported by PWs,
including any relevant service-specific considerations.
- Description of methods to ensure in-order final PDU delivery,
- Description of methods to perform clock recovery, as needed or
appropriate.
- Description of methods to perform edge-to-edge/inband signaling
functions across the PSN, as needed or appropriate.
- Description of the statistics and other network management
information needed for tunnel operation and management.
- Description of the security mechanisms to be used to protect the
control of the PW technology. The protection of the encapsulated
content (e.g., payload encryption) of the PW is outside of scope.
- Description of a mechanism to exchange encapsulation control
information at an administrative boundary of the PSN, including
security methods.
- Whenever possible, relevant requirements from existing IETF
documents and other sources will be incorporated by reference.
1.3. Non-Goals
The following are out of scope:
- 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 the underlying PSN.
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2. Background and Motivation
Many of today's service providers are struggling with the dilemma of
moving to an optical network based on IP and/or MPLS. How do they
realize the capital and operational benefits of a new packet-based
optical 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 circuits and/or 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
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.
Section 4 of [ANAVI] gives more background on why such emulation is
desirable:
The simplicity of TDMoIP translates into initial
expenditure and operational cost benefits. In addition, due
to its transparency TDMoIP can support mixed voice, data
and video services. It is transparent to both protocols and
signaling, irrespective of whether they are standards based
or proprietary with full timing support and the capability
of maintaining the integrity of framed and unframed DS1
formats.
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.
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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?
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.
Deployment of these examples range from limited (ATM CES) to fairly
common (FRF.5 interworking) to rapidly growing (VoIP).
2.2. 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.
2.3. Transition to a IP-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
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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 is based on TDM (time-division
multiplexing) technology. Voice, video, and data traffic has 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).
The second generation of converged network is 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 IP 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 IP-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.
To maximize the return on their assets and minimize their operational
costs, many service providers are looking to consolidate the delivery
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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
3.1. 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, MPLS or L2TP 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.
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.
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PSN Tunnel A PSN Tunnel is a tunnel inside which multiple
PWs can be nested so that they are transparent
to core network devices.
Pseudo Wire Domain A PW Domain (PWD) is a collection of instances
of PWs that are within the scope of a single
homogenous 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 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 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.
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
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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 particulars
of PWs for that service, as well as the degree
of faithfulness to that service.
Outbound The traffic direction where information from a
CE is adapted to a PW, and PW-PDUs are sent
into the PSN.
Inbound 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.
PE/PW Signaling PE/PW Signaling is signaling used by the PEs to
set up 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. This type of signaling is not within the
scope of PWE3.
<Editor's Note: The following figure is temporary. It is intended to
facilitate discussion of the preceding set of terms versus those used
in [MARTINI].>
[MARTINI] This Draft
----------------------------------------
MPLS Network PSN (includes MPLS)
Tunnel LSP PSN Tunnel
VC LSP PW
Edge LSR, R1, R2 PE
Figure 1: Comparison of Terms
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3.2. Reference Models
3.2.1. Network Reference Model
Figure 2 below shows the network reference model for PWs.
|<------- Pseudo Wire ------>|
| |
| |<-- PSN Tunnel -->| |
PW V V V V PW
End Service+----+ +----+ End Service
+-----+ | | PE1|==================| PE2| | +-----+
| |----------|............PW1.............|----------| |
| CE1 | | | | | | | | CE2 |
| |----------|............PW2.............|----------| |
+-----+ | | |==================| | | +-----+
^ +----+ +----+ | ^
| Provider Edge 1 Provider Edge 2 |
| |
|<-------------- Emulated Service ---------------->|
Customer Customer
Edge 1 Edge 2
Figure 2: PWE3 Network Reference Model
As shown, the PW provides an emulated service between the customer
edges (CEs). 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.
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3.2.2. Signaling Reference Model
Figure 3 below shows the signaling reference model for PWs.
|<------- CE (end-to-end) Signaling ------>|
| |
| |
| |<----- PW/PE Signaling ------>| |
| | | |
| | |<-- PSN Tunnel -->| | |
| | | Signaling | | |
| V V V V |
v +-----+ +-----+ v
+-----+ | PE1 |==================| PE2 | +-----+
| |-----|.............PW1..............|-----| |
| CE1 | | | | | | CE2 |
| |-----|.............PW2..............|-----| |
+-----+ | |==================| | +-----+
^ +-----+ +-----+ ^
| Provider Edge 1 Provider Edge 2 |
| |
|<----------- Emulated Service ----------->|
Customer Customer
Edge 1 Edge 2
Figure 3: PWE3 Signaling Reference Model
- 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 signaling is used between the PEs to set up 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.
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3.2.3. Protocol Stack Reference Model
Figure 4 below shows the protocol stack reference model for PWs. 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.
+-------------+ +-------------+
| Emulated | | Emulated |
| Service | | Service |
| (TDM, ATM, | Emulated Service | (TDM, ATM, |
| Ethernet, |<=================================>| Ethernet, |
|Frame Relay, | |Frame Relay, |
| etc. | | etc. |
+-------------+ Pseudo Wire +-------------+
|Encapsulation|<=================================>|Encapsulation|
+-------------+ +-------------+
| PSN | PSN Tunnel | PSN |
|IP/MPLS/L2TP |<=================================>|IP/MPLS/L2TP |
+-------------+ +-------------+
| Physical | | Physical |
+-----+-------+ +-----+-------+
| |
| IP/MPLS/L2TP Network |
| ____ ___ ____ |
| _/ \___/ \ _/ \__ |
| / \__/ \_ |
| / \ |
+=========/ |=====+
\ /
\ /
\ ___ ___ __ _/
\_/ \____/ \___/ \____/
Figure 4: PWE3 Protocol Stack Reference Model
3.3. 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 for further study.
2) The initial design of PWE3 is focused on a single homogenous
administrative PWD (e.g. PW over MPLS or PW over IP etc. ONLY).
Interworking between different PW types and the support of inter-
domain PWs are for further study.
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 AS shall be
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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. The switched emulated circuit type (e.g.
SVC/SVP) will 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, L2TP and MPLS. Other encapsulation approach 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), Frame Relay, ATM, TDM (e.g. DS1, DS3, E1, SONET/SDH
etc.) and MPLS.
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. No PW specific security mechanism will be
specified.
3.4. Suitable Applications for PWE3
<Editor's Note: This section will discuss the attributes that make an
application suitable (or not) for PWE3 emulation. This section is
currently under revision. >
When considering PWs as a means of providing a service, the following
questions regarding the application must be considered.
- Preservation of Order - Does the application require in-order
delivery of data? Emulation of an application that requires in-
order delivery over a PSN that does not guarantee such delivery may
be difficult.
- Preservation of Timing - Does the application require fine-grain
preservation of timing? If so, the adaptation may be complicated
by providing such timing where it is not normally available.
- Natural Delineation - What is the "natural" boundary for
delineation of data for encapsulation? (Note: For bit/byte-
oriented services, such as TDM emulation, this "natural"
delineation may not necessarily be the overriding consideration for
determining the best "chunk" for packetizing the service.)
- Packet Size - Are the encapsulated packets variable or fixed in
size?
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- Data Rate - Is the data rate presented at the interface fixed or
variable?
Figure 5 below shows a summary of the applications relevant to PWs,
along with a comparison of their attributes.
+------------+--------+--------+------------+--------+--------+
|Attribute ->|Preserve|Preserve| "Natural" | Packet | Data |
|Application | Order | Timing | Delineation| Size | Rate |
+------------+--------+--------+------------+--------+--------+
|T1/E1/T3/E3 | yes | yes |125 us frame| fixed | fixed |
+------------+--------+--------+------------+--------+--------+
|SONET/SDH | yes | yes |125 us frame| fixed | fixed |
+------------+--------+--------+------------+--------+--------+
|Frame Relay | yes | no | frame |variable|variable|
+------------+--------+--------+------------+--------+--------+
|ATM AAL1 | yes | yes | cell | fixed | fixed |
+------------+--------+--------+------------+--------+--------+
|ATM AAL2 | yes | yes | cell | fixed |variable|
+------------+--------+--------+------------+--------+--------+
|ATM AAL5 | yes | no |cell or PDU |variable|variable|
+------------+--------+--------+------------+--------+--------+
|Ethernet | yes | no | frame |variable|variable|
+------------+--------+--------+------------+--------+--------+
Figure 5: Summary of Applications and Attributes
4. Layer 1 (Circuit) Applications
For circuit applications the entire bit stream (or at least the
payload) needs to be recreated at the far end of the PW. As with ATM
CES, the physical layer coding is terminated and re-generated on the
far end. In addition, framing may be terminated and regenerated,
depending on the application.
4.1. Reference Model
Figure 6 below shows a pair of T1s being carried over a TDM/SONET
network. The node marked "M" is an M13 multiplexer, while the nodes
marked "S" are SONET Add-Drop Multiplexers (ADMs). Note that the
physical interface of the circuit may change without affecting the
circuit. For example, the T1s in Figure 6 below enter the network as
physical T1s but exit the network as Virtual Tributaries (VTs) in a
physical OC12.
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SONET/TDM Network
____ ___ ____
_/ \___/ \ _/ \__
+------+ Physical / \__/ \
|Site A| T1 / +---+ DS3 \ Hub Site
|T1 #1=|=================|\M/|-------------+-----+ \ OC12+------+
| | \ |/ \|=============|\ /| \----| |
+------+ /\ +---+-------------| \ / |========|=T1 #1|
/ | S | / | |
+------+ Physical/ +---+-------------| / \ |========|=T1 #2|
|Site B| T1 \ |\S/|=============|/ \| \----| |
|T1 #2=|=================|/ \|-------------+-----+ / +------+
| | \ +---+ OC3 __ /
+------+ \ __/ \ /
\ ___ ___ / \_/
\_/ \____/ \___/
Figure 6: T1/SONET Example Diagram
Figure 7 below shows the same pair of T1s being carried over a packet
network. Here the emulation is performed by the PEs marked "PE", and
the routers marked "R" carry the resulting packets. Note that the
PE, routing and/or SONET functions could be combined in the same
device.
SONET/TDM/Packet Network
____ ___ ____
_/ \___/ \ _/ \__
+------+ Physical /+-+ \__/ \_
|Site A| T1 / | | +---+ \ Hub Site
|T1 #1=|=============|P|=| R | +---+ +-+ +-----+ \ OC12+------+
| | \ |E| | |===| | | |=|\ /| \----| |
+------+ /\+-+ +---+ | | | | | \ / |========|=T1 #1|
/ | R |=|P| | S | / | |
+------+ Physical/ +-+ +---+ | | |E| | / \ |========|=T1 #2|
|Site B| T1 \ |P| | R |===| | | |=|/ \| \----| |
|T1 #2=|=============|E|=| | +---+ +-+ +-----+ / +------+
| | \ | | +---+ __ /
+------+ \ +-+ __/ \ /
\ ___ ___ / \_/
\_/ \____/ \___/
Figure 7: T1 Emulation Example Diagram
4.2. Operational Considerations
4.2.1. Transparency
Circuits such as T1/E1/T3/E3/SONET/SDH lines need a greater degree of
transparency than Layer 2 services. These circuits may be carrying
the services described in the section on Layer 2 services, but in the
Layer 1 scenario the higher layer application is irrelevant and is
ignored. In general, these are "bits in, bits out" applications.
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In this application a circuit or bit stream is encapsulated in fixed-
size frames that are sent at a fixed rate. The emulated stream must
be delivered in a reliable and predictable fashion to the far end.
Absolute delay and delay variation (also called jitter or wander)
must be minimized. Excess delay and delay variation may cause
problems with the application carried by the TDM/SONET CEs.
This encapsulation of TDM data must be transparent. The emulated
circuit could be carrying one or more types of data (ATM, Frame
Relay, TCP/IP, etc.), voice traffic, video or anything else. The
data is not interpreted; it is simply transported.
4.2.2. Structured Versus Unstructured Mode For TDM Circuits
As discussed in [ATMCES], emulation of a T1, E1 or other circuit can
be done in a structured (framed) mode or in an unstructured
(unframed) mode. This same distinction can be applied to higher rate
circuits such as DS3, E3, and SONET/SDH.
Unstructured mode generally involves collecting all bits received
from a physical port (including transport framing), and packing them
into packets for transport through the PSN. The fact that the
received bit stream contains a framed signal is more or less
irrelevant to the adaptation function.
Structured mode requires the use of a framer to identify and
terminate the incoming transport framing, and delineate logical TDM
channels within the TDM bit stream for emulation. In addition, TDM
framers are generally needed to detect maintenance signals such as
Alarm Indication Signal (AIS) and Remote Defect Indication (RDI).
Framers are also used to measure various performance parameters such
as Errored Seconds, Frame Errored Seconds, etc. Lastly, a framer is
needed to generate and terminate the Facility Data Link (FDL) as well
as the SONET/SDH Data Communications Channels (DCCs).
The capabilities described in the rest of this section (except for
LOS) are predicated on the presence of a framer.
4.2.3. Fractional T1/E1
A fractional T1 or E1 is composed of a number of concatenated DS0s
and is sometimes referred to as NxDS0. It may be emulated by
replicating the contents of the relevant DS0s at the other end of the
tunnel. The value of the other timeslots and/or framing are
irrelevant and are not transported in leased line application. Even
though the framing is not transported, a framer is still needed to
delineate the timeslots for encapsulation.
The SONET/SDH equivalent to Structured T1/E1 services are STS-1/Nc
and their SDH equivalents. For STS-1/Nc services a single SONET
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timeslot or a concatenation of multiple timeslots is used to carry a
single logical circuit. As with structured T1/E1 services, the
transport framing (i.e. SONET Section and Line Overhead) is
terminated, and only the relevant SONET timeslots are carried through
the packet network. A single physical SONET interface can be the
source of multiple STS-1/Nc services, each of which may be emulated
as an independent PWE3 service.
4.2.5. Loopbacks
It could be useful for a PE to process loopback messages as defined
in [T1.403]. This would allow for isolation of faults in a network.
It also facilitates the certification of equipment for operation in a
carrier's network.
There are also inband loopback commands that are used for voice
equipment. These loopback commands are triggered by patterns carried
in with the data itself. Voice is limited in the patterns it can
present, so it won't falsely mimic the inband loopback command.
These inband commands are falling out of favor due to their
incompatibility with data services. The inband pattern for the
loopback may inadvertently appear in a data stream due to its
arbitrary nature. A PE device that implements inband loopbacks must
have the capability to disable them.
4.2.6. Performance Processing
[T1.403] defines a Network Performance Report Message (NPRM) that
carry periodic reports on the performance of the link. It would be
useful for a PE to generate these messages, as they are frequently
used for surveillance and trouble-shooting.
4.2.7. LOS/LOF/AIS
Figure 8 shows an example for the generation of AIS and RAI.
<-- Upstream Downstream -->
LOS +-----+ AIS
------X----->|\ /|--------->
| \ / |
| / \ |
<------------|/ \|<---------
RAI/RDI+Data +-----+ Data
Figure 8: Generation of AIS and RAI/RDI
A TDM multiplexer, SONET ADM, switch or other line terminating
equipment (LTE) must respond to an LOS (Loss of Signal), LOF (Loss of
Frame) or AIS (Alarm Indication Signal) condition (traditionally
known as "red alarms") by generating AIS in the "downstream"
direction i.e. the same direction in which the LOS was detected. AIS
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is conveyed by sending a certain pattern in the data stream. It may
also send RAI (or RDI in SONET) in the "upstream" direction i.e. the
opposite direction from that where the LOS was detected. See section
9 of [T1.403] for more information on T1 AIS and RAI. See section
section 6.2.1 in [GR253] for more information on the SONET AIS-L and
RDI-L signals.
Bandwidth can be saved by suppressing the AIS signal in the emulated
stream and sending instead an indication in the control overhead.
This also applies to a received AIS signal. [MALIS] discusses the
propagation of AIS using the pointer bits in the TDM control word.
A device emulating TDM circuit must either replicate the AIS
indication or indicate this condition in the control overhead.
4.2.8. SONET/SDH STS Unequipped
The "STS Unequipped" indication may be treated in a fashion similar
to that for LOS/AIS. As discussed in [MALIS], bandwidth can be saved
by suppressing the payload in the emulated stream and sending instead
an indication in the control overhead.
4.3. Encapsulations
Encapsulation options for TDM services may be compared on the
following criteria.
- Timing - TDM services are very sensitive to timing and timing
variations ("jitter"). The encapsulation may need to provide
additional information (such as [RTP] timestamps) to help convey
timing across the PW.
- Line Signals - The encapsulation should provide a means to convey
signals such as AIS and line conditions such as LOF.
- The encapsulation should minimize overhead.
4.4. Timing
In the recent Ken Burns Jazz television series, it was said of Louis
Armstrong that he was very economical with his notes, but that the
timing of those notes was everything. The timing of the
reconstructed bit stream is similarly important. This section
describes the various approaches to this problem. A summary is also
provided at the end of the section.
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4.4.1. Reference Model
Consider the example network shown in Figure 9. In this network, CE1
and CE2 are connected by a PW provided by PE1, S1, S2 and PE2.
+---------------+ +---------------+
| PE1 | | PE2 | G
| | | | |
| | | | v
+---+ | +-+ +-+ +-+ | +--+ +--+ | +-+ +-+ +-+ | +---+
| | | |P| |D| |P| | | | | | | |P| |E| |P| | | |
| |<===|h|<:|e|<:|h|<:::| |<::| |<:::|h|<:|n|<=|h|<===| |
| | | |y| |c| |y| | | | | | | |y| |c| |y| | | |
| C | | +-+ +-+ +-+ | | | | | | +-+ +-+ +-+ | | C |
| E | | | |S1| |S2| | | | E |
| 1 | | +-+ +-+ +-+ | | | | | | +-+ +-+ +-+ | | 2 |
| | | |P| |E| |P| | | | | | | |P| |D| |P| | | |
| |===>|h|=>|n|:>|h|:::>| |::>| |:::>|h|:>|e|=>|h|===>| |
| | | |y| |c| |y| | | | | | | |y| |c| |y| | | |
+---+ | +-+ +-+ +-+ | +--+ +--+ | +-+ +-+ +-+ | +---+
^ ^ | | ^ ^ ^ | | | ^ ^ ^ | ^ ^
| | | |B | | | |<------+------>| |D | | | | | |
| A | +--+ +--+-C | | | +--+ +--+-E | F |
| +---------------+ +-+ +---------------+ |
| |I| |
| +-+ |
| | |
+-----------------------------+-----------------------------+
Where:
"CEn" is the TDM edge device
"PEn" is the PE adaptation device
"Sn" is a core switch
"A" - "I" are clocks
"=" is the T1 Bit Stream
":" is the switched connection
"Phy" is a physical interface
"Enc" is a PWE3 encapsulation device
"Dec" is a PWE3 decapsulation device
Figure 9: Timing Recovery Reference Diagram
For this application to work, CE2 needs to be clocked (by clock E) at
the same frequency as CE1 (which is being clocked by clock A). A
jitter correction buffer at PE2 can handle short-term differences
between these two clocks, but over time any absolute difference is
going to cause this buffer to overflow or underflow.
Bits are clocked into an encapsulation function in PE1 according to
clock B, which is recovered from the incoming data stream. Clocks A
and B will have the same frequency.
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The bits are packed into frames and clocked out according to clock C.
Clock C could be related to clock B, but in most cases these clocks
are completely independent.
The frames arrive at switch S1, and are clocked out according to the
internal oscillator on the output interface of switch S1. The frames
will depart at the same average rate at which they arrived, but the
instantaneous rate will be different on each side of S1. Note that
there could be short-term variations due to congestion, but S1 can't
experience long-term congestion with respect to the frames carrying
emulated data, or the service won't work.
The frames travel on to switch S2, which again forwards them. Note
that switch S2 introduces yet another clock, which it uses to
transmit the packets toward PE2. Again the average rate is
preserved, but the instantaneous rate will vary.
The frames arrive at PE2 and are clocked into the decapsulation
function when they arrive (using clock D). Note that clock D will
have the same average frequency as A and B, but will have short-term
variations. The bits are clocked out of the FIFO according to clock
E. Clock F at CE2 is recovered from the bit stream and therefore
runs at the same frequency as clock E.
4.4.2. Recreating the Timing
The big question is: where does clock E in Figure 9 come from? There
are 5 possibilities, and these are detailed in the following
sections.
1) Clock E is derived from an external source such as clock I or B
(indirectly via A) at CE1 and G (indirectly via H) at CE2. This
method is described in the "External Timing" section below.
2) Clock E could be derived from Clock I and the pointers. This
approach is described in the "SONET Pointer Justification" section
below.
3) Clock E is derived from the average rate of Clock D. This is the
"Adaptive Timing" scenario described in a subsequent section.
4) Clock E is derived from a combination of the local oscillator at
PE2 and received SRTS timestamps. The "Differential (SRTS)"
section below describes this approach.
5) Clock E is derived from inband RTP timestamps. This method is
discussed in the "RTP" section below.
4.4.2.1. External Timing
The simplest method for communicating timing from one end of a system
to the other is an external timing source, such as clock I in Figure
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9. This external timing source is normally a T1 or E1, but it could
be delivered via SONET or a GPS receiver. Its 8 KHz frame rate is
extracted and used to directly clock the reconstructed data streams,
or as an input to a phase-locked loop to synthesize the desired
clock. The drawback to this method is that a common external clock
is not commonly available in a data network or in a multi-carrier
network.
Note that clock I may actually be two separate clocks of a particular
accuracy or stratum. The difference in frequency will eventually
cause the FIFO to slip, but if the clock is of a high enough accuracy
then the slips will be very infrequent. For example, a stratum 1
clock is accurate to one part in 10**11 [G.811]. This gives a
frequency slip rate of 15.4**-6 bit-slip/sec:
slip rate = 1.544 Mbps * 10**-11
= 15.4**-6 bit-slip/sec
Taking the reciprocal yields 18 hours/bit-slip:
bit slip period = 1 / ( 15.4**-6 bit-slip/sec * 3600 s/h)
= 18 hours/bit-slip.
A typical multiplexer has a buffer that is two frames deep. Assuming
that it starts out centered, the expected time for a slip would be
almost 5 months:
frame slip period = 18 hours/bit-slip * 193 bits/frame
= 3474 hours
= 145 days
= 4.8 months
This slip rate could be higher or lower depending on the bit rate,
clock accuracy and the depth of the FIFO.
4.4.2.2. SONET Pointer Justification
SONET defines layers of pointers that allow for the multiplexing and
transmission of asynchronous signals. These pointers convey the
timing of the carried signal with respect to the timing of the
encapsulating signal. Each SONET ADM must manipulate these pointers
to preserve the timing. This method has the advantage of being well-
defined and understood.
One way to apply this method to a packet-based network would be to
ensure that all of the links on a given path are synchronous. This
would be difficult for Gigabit Ethernet or POS links.
Another way would be for each router to update the pointers as the
packet traversed the router. This would be compute intensive.
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The method defined in [MALIS] requires pointer manipulation only at
the end points. It does require an external clock as a reference for
the pointer adjustments.
4.4.2.3. Adaptive Timing
Adaptive timing is used when an external reference is not available.
[ATMCES] describes adaptive timing as follows:
The adaptive technique does not require a network-wide
synchronization signal to regenerate the input Service
clock at the output IWF. A variety of techniques could be
used to implement Adaptive clock recovery. For example,
the depth of the reassembly buffer in the output IWF could
be monitored:
1. When the buffer depth is too great or tends to increase
with time, the frequency of the Service clock could be
increased to cause the buffer to drain more quickly.
2. When the buffer contains fewer than the configured
number of bits, the Service clock could be slowed to
cause the buffer to drain less quickly. Wander may be
introduced by the Adaptive clock recovery technique if
there is a low-frequency component to the Cell Delay
Variation inserted by the ATM network carrying cells
from the input to output IWF.
Careful design is required to make adaptive timing work
well. The instantaneous buffer depth must be filtered to
extract the average frequency and to reject the jitter and
wander.
Adaptive timing is ideal for many network applications where there is
no external timing reference available (needed for SRTS), and where
the packet rate is decoupled from the line rate (as in a routed
network). Adaptive timing may not meet the requirements of [G.823],
[G.824] and other similar specifications.
4.4.2.4. Differential (SRTS)
[ATMCES] describes the SRTS (Synchronous Residual Time Stamp) method:
The SRTS technique measures the Service Interface input
clock frequency against a network-wide synchronization
signal that must be present in the IWF, and sends
difference signals, called Residual Time Stamps, in the
AAL1 header to the reassembly IWF. At the output IWF, the
differences can be combined with the network-wide
synchronization signal to re-create the input Service
Interface clock.
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The requirement for a network-wide signal is reasonable in a Telco or
SONET environment, where such clocks are commonly available. It may
be problematic in a packet network.
Here is the correspondence between the clocks in Figure 9
and.[I.363.1].
Description I.363.1 Figure 9
------------------------------------------
Service Clock Fs A, B
Network Clock Fn C
Derived Reference Fnx Based on C
[RTP] uses an absolute timestamp to play out the bits at the same
rate that they were received and packetized. RTCP (RTP Control
Protocol) provides a means to synchronize the transmit and receive
clocks.
Section 4 of [RTP] defines a timestamp that is either 32-bits or
64-bits in size:
Wallclock time (absolute time) is represented using the
timestamp format of the Network Time Protocol (NTP), which
is in seconds relative to 0h UTC on 1 January 1900 [NTP].
The full resolution NTP timestamp is a 64-bit unsigned
fixed-point number with the integer part in the first 32
bits and the fractional part in the last 32 bits. In some
fields where a more compact representation is appropriate,
only the middle 32 bits are used; that is, the low 16 bits
of the integer part and the high 16 bits of the fractional
part. The high 16 bits of the integer part must be
determined independently.
A 32-bit absolute time stamp with a 16-bit fractional part would give
a 15 us granularity (= 1/65535), which is too coarse for circuit
emulation. This means that the 64-bit timestamp must be used, with a
granularity of 23 ns.
The transmit timestamps are created according to clocks B and C at
PE1 and interpreted according to clocks D and E at PE2. These two
oscillators will vary by some amount, even if they are very accurate.
This drift means that RTCP, NTP or some other means must be used to
synchronize the clocks at each end.
4.4.3. Summary of Timing Recovery Methods
All of the previously described methods for timing recovery can be
made to work for Layer 1 circuit services. How then can we compare
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them? Here are some criteria:
- Is an external timing source required? This might be a direct
timing source as described in "External Timing", or it could be an
indirect source as with SRTS.
- Must the PE synthesize clocks? Synthesis of clocks generally
requires a Phase-Locked Loop (PLL) to create one clock from
another.
- Is the method provably correct? Some methods such as external
timing and SRTS can be proven to meet specifications. The
performance of others, such as adaptive timing, is more dependent
on particular implementations.
Figure 10 below shows a summary of the methods for timing recovery.
+-----------------------------+------+-----+-----+----+-----+
| Method->| Ext. |SONET|SRTS |RTP/|Adap-|
|Parameter |Source| Ptr | |RTCP|tive |
+-----------------------------+------+-----+-----+----+-----+
|External timing required? | yes | yes | yes | no | no |
|Clock synthesis? | no | yes | yes | yes| yes |
|Provably correct? | yes | yes | yes | ? | ? |
+-----------------------------+------+-----+-----+----+-----+
Figure 10: Summary of Timing Recovery Methods
5. Layer 2 (Packet/Cell) Applications
5.1. Layer 2 PW Reference Model
Figure 11 below shows the reference model for Layer 2 PWs. The Layer
2 emulated protocols/services include ATM VCC, ATM VPC, Frame Relay
DLCI, IEEE 802.1Q VLAN, IEEE 802.3x link, etc. The nodes marked "S"
are protocol-specific switches e.g. Frame Relay switches.
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Carrier A Carrier B
____ ___
Layer 2 _/ \___/ \ ___
+-------+ Link / \____ / \__ Layer 2
|Site A |---------/ +---+ \ / \ Link
| CE#1=|===============|\S/| || +-----+ \ +------+
| |--------\ |/ \|===============|\ /| \--|Site C|
+-------+ \ +---+ || | \ / |=====|=CE#3 |
/ || | S | / | |
+-------+ / +---+ || | / \ |=====|=CE#4 |
|Site B |--------\ |\S/|===============|/ \| \--| |
| CE#2=|===============|/ \| || +-----+ _/ +------+
| |---------\ +---+ / \__ /
+-------+ \ / \ _/
\ ___ ___ \ \_/
\_/ \___/ \___/
Figure 11: Layer 2 Interconnect Reference Model
Figure 12 below shows the reference model for PW emulation of Layer 2
services.
Carrier A Carrier B
____ ___
Layer 2 _/ \___/ \ ___
+-------+ Link /+-+ \____ / \__ Layer 2
|Site A |--------/ |P| +---+ +---+ \ / \ Link
| CE#1=|==========|W|=| R | | R | +-+ | | +-----+ \ +------+
| |-------\ |E| | |==| |=| |=|==|=|\ /| | |Site C|
+-------+ \ +-+ +---+ +---+ |P| | | | \ / |=|===|=CE#3 |
/\ |W| | | | F | | | |
+-------+ / +-+ +---+ +---+ |E| | | | / \ |=|===|=CE#4 |
|Site B |-------\ |P|=| R |==| R |=| |=|==|=|/ \| | | |
| CE#2=|==========|W| | | | | | | | | +-----+ | +------+
| |--------\ |E| +---+ +---+ +-+ | | |
+-------+ \+-+ / \_______/
\ /
\ _ __ /
\_/ \___/ \____/
Figure 12: Layer 2 PW Emulation
5.2. Ethernet
5.2.1. Reference Model Scope
PW carriage of Ethernet operates as point-to-point trunking in a non-
shared medium. The Ethernet interface can operate in a half-duplex
or full-duplex mode. Control functions such as IEEE 802.3 Carrier
Sense Multiple Access with Collision Detection (CSMA/CD)[802.3] and
IEEE 802.1D Spanning Tree [802.1D] are not applicable nor within the
scope of PWs. However, the PW shall conform to the service
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definitions as defined in IEEE 802.1P,Q [802.1Q], as required. Also,
it shall support all Ethernet framing i.e. Ethernet Frame and IEEE
802.3x including IEEE 802.3ac VLAN Tagging [802.3] as well as Jumbo
Frame.
5.2.2. Operational Considerations
5.2.2.1. Operational Modes
The design of the Ethernet PW must consider the support of the two
operational modes in this framework. Both modes shall be supported
for all Ethernet interfaces, i.e. from 10 Mbps to 10,000 Mbps, and
the design of the Ethernet PW functions shall be agnostic to the
Ethernet's link capacity. Both modes shall transparently support the
address resolution protocols, i.e. ARP, InverseARP, proxy ARP and
Ethernet-based control protocol (e.g. Generic Attribute Registration
Protocol (GARP), GARP VLAN Registration Protocol (GVRP) etc.).
- Opaque Trunking - In this mode, the ingress PE shall relay all of
the traffic from an Ethernet port into the PW.
- Transparent Trunking - This mode is particularly designed for
support of Virtual LANs (VLAN). VLAN types include Port-based
VLANs, MAC-Address-Based VLANs, IP-Based VLANs, 802.1Q Tag-based
VLANs and 802.10 Security-based VLANs.
The ingress PE may pay attention to the MAC header and other
relevant VLAN classification information based on the configuration
policy. The Ethernet PW shall carry multiple VLANs traffic and can
extend VLANs across the PWD. In the case when 802.1Q Tag-based
VLAN is configured, if the received frame is tagged with a NULL
VLAN_ID, it will be associated with the VLAN equal to the Port's
default VLAN. At frame transmission, all frames that are
associated with 802.1Q Tag-based VLAN shall be tagged except for
those assigned for the default VLAN.
The PE may provide translation of the VLAN_ID in order to
facilitate deployment. Note that this does not increase the
VLAN_ID space, so it has no effect on scalability.
5.2.2.2. Quality Of Service Support Considerations
The Ethernet AS shall describe the faithfulness of the PW with
respect to these attributes described in IEEE 802.1p [802.1Q].
- Service Availability - Service availability is measured as ratio
between times when MAC service is unavailable and when it is
available.
- Frame Loss - The MAC service does not provide guaranteed delivery
of service data units. However, the Ethernet PW system should
consider monitoring frame loss.
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- Frame Misorder - The MAC service does not permit reordering frames
within the same user-priority for a source and destination address
pair.
- Frame Duplication - The MAC service does not permit duplicating
frames.
- Transit Delay Performance - IEEE 802.1p [802.1Q] defines frame
transit delay is the elapsed time between an MA_UNITDATA.request
and corresponding MA_UNITDATA.indication on a successful transfer.
- Undetected Frame Error Rate - By using the Frame Checksum (FCS)
calculation for each frame, the undetected frame error rate should
be low.
- Maximum Service Data Unit Size - The maximum service data unit size
is dependent on the access media used. In general, it is the
lowest common denominator of the two adjacent Ethernet interface.
- Priority Tags and Traffic Classes - IEEE 802.1p defines eight
traffic classes (priority). The PRI bits on the VLAN fields should
be carried transparently over the PW. COS differentiation on the
PW level based on the received 802.1p bits is possible but is out-
of-scope.
5.3. Frame Relay
5.3.1. Reference Model
Frame Relay service offerings often have a different physical format
and speed at each end of the link. For example, a hub and spoke
deployment of Frame Relay might provide fractional T1 access at the
spokes and a clear channel T3 to the hub. The Virtual Circuits (VCs)
are aggregated by switches in the Frame Relay network. This is shown
in figure 13 below, where the Frame Relay switches are marked with an
"F".
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Wide Area Frame Relay Network
Carrier A Carrier B
____ ___
Fractional _/ \___/ \ ___
+------+ T1 / \____ / \__
|Site A|-----------/ +---+ \ / \ Hub Site
|VC #1=|=================|\F/| || +-----+ \ DS3+------+
| |----------\ |/ \|===============|\ /| \---| |
+------+ \ +---+ || | \ / |======|=VC #1|
/\ || | F | / | |
+------+ / +---+ || | / \ |======|=VC #2|
|Site B|----------\ |\F/|===============|/ \| \---| |
|VC #2=|=================|/ \| || +-----+ _/ +------+
| |-----------\ +---+ / \__ /
+------+ Trans- \ / \ _/
Atlantic E1 \ ___ ___ \ \_/
\_/ \___/ \___/
Figure 13: Frame Relay Example Model
Figure 14 shows an emulated network, where Carrier "A" is providing a
transparent Frame Relay emulation connection to Carrier "B".
Wide Area Frame Relay/Router Network
Carrier A Carrier B
____ ___
Fractional _/ \___/ \ ___
+------+ T1 /+-+ \____ / \__
|Site A|------------/ | | +---+ \ / \ Hub Site
|VC #1=|==============|P|=| R | +---+ +-+ || +-----+ \ DS3+------+
| |-----------\ |E| | |==| |=| |====|\ /| \---| |
+------+ \ +-+ +---+ | | | | || | \ / |======|=VC #1|
/ \ | R | |P| || | F | / | |
+------+ / +-+ +---+ | | |E| || | / \ |======|=VC #2|
|Site B|----------\ |P| | R |==| |=| |====|/ \| \---| |
|VC #2=|==============|E|=| | +---+ +-+ || +-----+ / +------+
| |-----------\ | | +---+ / \_ /
+------+ Trans- \ +-+ / \ _/
Atlantic E1 \___ ___ __ \ \__/
\_/ \___/ \__/
Figure 14: Frame Relay Emulation Example Diagram For Transparent
Emulation
Here the emulation is performed by the PEs marked "PE", and the
resulting packets are carried by the routers marked "R". In this
case, the emulated VCs can transparently carry the PVC status
signaling (if any) and need not perform any higher layer function.
Also, note that the emulation and routing functions could be combined
in the same device.
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If the Frame Relay switches are to be completely eliminated (as shown
in figure 15 below), then the emulation service must implement Frame
Relay PVC status signaling and/or connection signaling for SVCs. As
previously noted, the PE and routing functions could be combined in
the same device.
Wide Area Frame Relay/Router Network
Carrier A Carrier B
____ ___
Fractional _/ \___/ \ _____
+------+ T1 /+-+ \____ / \__
|Site A|----------/ | | +---+ \ / \ Hub Site
|VC #1=|============|P|=| R | +---+ +-+ || \ DS3 +------+
| |---------\ |E| | |==| |=| | || \----| |
+------+ \ +-+ +---+ | | | |====================|=VC #1|
/\ | R | |P| || / | |
+------+ / +-+ +---+ | | |E|====================|=VC #2|
|Site B|---------\ |P| | R |==| |=| | || \----| |
|VC #2=|============|E|=| | +---+ +-+ || _/ +------+
| |----------\ | | +---+ / \__ /
+------+ Trans- \+-+ / \ _/
Atlantic E1 \___ ___ _ \ \__/
\_/ \__/ \___/
Figure 15: Frame Relay Emulation Example Diagram For Non-Transparent
Emulation
5.3.2. Operational Considerations
Frame Relay provides a connection-oriented circuit-based carriage of
variable-sized frames. There are two types of virtual circuits
supported in Frame Relay: Permanent Virtual Circuits (PVCs) and
Switched Virtual Circuits (SVCs). The following sections describe
the considerations to support the operation of Frame Relay over the
PW.
5.3.2.1. Frame Sequence
The PW must deliver frames in the proper sequence.
In general, the maximum frame size for Frame Relay is 1600 bytes per
[FRF.1.2]. This can be made larger in some implementations. If the
MTU of the PW is less than (1600 bytes - size of PW headers), a
fragmentation and reassembly mechanism may be needed.
5.3.2.3. End-to-End Characteristics
[FRF.5] and [FRF.13] define a set of traffic parameters to support
Service Level Agreements (SLAs). The design of the PW may be
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required to preserve these end-to-end transport characteristics.
5.3.2.4. Connection Management and Congestion Control
Each Frame Relay header contains some connection management
information, including
- a command/response (CR) bit
- a discard eligibility (DE) bit
- a connection ID (DLCI)
- an address extension indicator (EA)
- Forward/Backward Congestion Notification (FECN/BECN). Figure 16
shows an example of how BECN and FECN are sent.
--Congestion->
+-----+ Data+FECN
------------>|\ /|--------->
| \ / |
| / \ |
<------------|/ \|<---------
BECN+Data +-----+ Data
Figure 16: Generation of BECN and FECN
All of this information is vital to the integrity of the Frame Relay
circuit. The Frame Relay PW AS must define a means to preserve such
information across the PWD.
5.3.2.5. Link Management Support
Frame Delay defines a set of link management functions for PVC Status
Management as specified in [T1.617D] and [Q.933A]. Link Management
runs on a dedicated PVC; therefore, its operation does not impact
actual user data. The management functions include:
- a heartbeat exchange that verifies that the link is operational
- a report regarding the status of one or more individual DLCIs
For some networks, such as the one shown in Figure 15, this link
management is the only means to verify the end-to-end integrity of
the Frame Relay virtual circuit. The PE may required to emulate such
functions. These functions will be transparent to the underlying
network.
Another important consideration is that there should be some
coordination between the PW's link status and the associated Frame
Relay VCs. For example, it might be necessary to tear down the VCs
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in the presence of a network fault.
5.3.2.6. DLCI Association
There are two scopes of DLCI addressing that have been defined by
ANSI and ITU-T: Local and Global DLCIs.
- Local DLCI addressing means that DLCI numbers are only significant
at one end of a Frame Relay virtual circuit.
- Global DLCI addressing is an extension to PVC status management
that allows a DLCI number to have universal significance. A global
DLCI identifies the same VC at both ends of the network.
In the case when the DLCI is locally significant, the management of
the PWD must provide a mechanism to coordinate the DLCIs at the two
ends of the PW. The association can be done via signaling or
configuration.
5.3.2.7. Multiplexing VCs over PWs
To preserve PW tunnel space and to enhance scalability of PWs, it
would be very valuable to allow one or more VCs to be multiplexed
onto the same PW. One scenario might be to associate an entire Frame
Relay logical interface to a PW. Another possibility is that the
assignment of VCs to the PW could be done via signaling or
management. In either of these cases, the DLCI for each frame would
need to be preserved across the PW.
If such multiplexing approach is used, the earlier discussion related
to the packet sequencing, end-to-end characteristics, SLA
preservation and link status management, shall be addressed with the
same considerations.
5.3.2.8. Signaling Transparency
Since Frame Relay supports SVCs, the PE may need to support signaling
interworking at the PWES. InverseARP frames should be passed on
without interpretation. In either case, these frames shall be
transparent to the underlying PSN.
5.3.2.9. Soft PVC Support
One type of connection service that is provided by Frame Relay
networks is called a Soft PVC (SPVC). A SPVC may be considered to be
composed of three parts: two peer-to-peer PVCs at each side of the
core, and a SVC between them.
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Figure 17 shows the SPVC interconnection example.
+---+ +---+ +---+ +---+ +---+
| E |========| C |:::::::| C |:::::::| C |======| E |
+---+ +---+ +---+ +---+ +---+
Where:
"E" is the edge switch
"C" is the core switch
"=" is the permanent connection
":" is the switched connection
Figure 17: Example of an SPVC Over a Frame Relay Network
The creation of the SPVC within the core is triggered by detecting
the existence of the PVCs at the edges. This detection is done
either by network management or by some proprietary signaling.
Now consider the case where the core switches are replaced by PEs as
shown in Figure 14. The SVC connection within the core is replaced
by the PW. The PVCs configuration which are maintained by the
ingress and the egress CEs of the PWD should remain the same. The
ingress and egress PEs shall maintain the SPVC behavior such that it
is transparent to the CEs.
5.4.1. Reference Model
As far as PWs are concerned, ATM is very similar to Frame Relay. We
will use the same reference network (Figure 13) for ATM as we did for
Frame Relay. Of course, the Frame Relay switches would be ATM
switches instead. Likewise, the PE networks shown in Figures 14 and
15 are applicable to ATM.
5.4.2. Operational Considerations
Like Frame Relay, ATM provides connection-oriented circuit-based
carriage of fixed-size cells. There are two types of virtual circuit
supported in ATM: PVC and SVC. In addition to virtual circuit
connections (VCCs), ATM also supports virtual path connections
(VPCs). There are also permanent virtual paths (PVPs) and switched
virtual paths (SVPs).
ATM carries data in fixed size (53 byte) frames called "cells".
Higher layer frames are adapted to these fixed size cells via ATM
Adaptation Layers (e.g. AAL1, AAL2 and AAL5) and SAR. Different
types of AALs have different cell header formats, and the cells may
contain signaling information.
The following sections describe the set of considerations for PW
support of ATM.
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5.4.2.1. Cell Sequence
The PW must deliver cells in the proper sequence.
5.4.2.2. End-to-end Transport Characteristics
The ITU-T [I.356] and the ATM Forum [TM4.0] each define a set of
traffic and QoS parameters. The AS for ATM PWs should specify how
the PW will maintain the end-to-end characteristics for such VCs.
The ITU-T [I.371] defines performance targets for managing ATM
traffic and congestion control. These targets may be used by some
service providers to define their ATM SLAs. The AS for ATM PWs
should specify how SLA transparency will be achieved.
5.4.2.4. Connection Management and Congestion Control
The ATM header contains some connection management and congestion
control information, as defined in [I.371]:
- Cell Loss priority (CLP)
- Connection Identifier (VPI/VCI)
- Payload Type Identifier (PTI) - distinguishes between an OAM
(Operations, Administration and Management) cell and a user cell
- Explicit Forward Congestion Indication (EFCI)
All of this information is vital to the integrity of the ATM circuit.
The ATM PW AS must define a means to preserve such information across
the PWD.
ATM OAM functions are defined in the ITU-T standard [I.610]. OAM
cells are used to provide functions like fault management,
performance management and continuity checks.
OAM is implemented differently in VCCs and VPCs. In the case of a
VCC, the OAM cell is sent along the same VC as the user cells. For a
VPC, the OAM cell is sent over a dedicated VC within the VPC. OAM
flows are also classified as end-to-end flows (covering the entire
virtual connection) or as segment flows (covering only parts of the
virtual connection).
The PE may emulate the end-to-end OAM flows by encapsulating the OAM
cells in a PW-PDU. A PE that supports the OAM function should
support coordination between the OAM behavior between the PE peers.
For example, an OAM AIS cell at one end can result in PW signaling
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that causes the PW link to go down at the far end. If the PE does
support OAM, the emulation of the OAM function shall be transparent
to the underlying network.
The Integrated Local Management Interface [ILMI] protocol facilitates
network deployment and management in several ways:
- ILMI allows an ATM node to determine the various features supported
by its neighbors (such as type of signaling, size of connection
space etc).
- ILMI allows for smoother administration of ATM addresses through
address registration.
- ILMI facilitates automatic configuration of private interfaces.
- ILMI supports procedures for detecting loss of connectivity through
periodic polling.
For some networks, such as the one shown in Figure 15, ILMI is the
only means to verify the end-to-end integrity of the ATM virtual
circuit. It may be desirable for the PE to emulate such functions.
If supported, these functions must be transparent to the underlying
network.
5.4.2.7. VC Associations
Each ATM connection identifier has local significance only. Local
significance means that each ATM connection identifier (VPI and/or
VCI) is only significant at a local ATM interface, and is independent
from the identifier at the other end of the link. The management of
the PWD must provide a mechanism to coordinate the identifiers at the
two ends of the PW. The association can be done via signaling or
configuration.
5.4.2.8. Multiplexing ATM VCs over PWs
See the discussion in the "Multiplexing VCs over PWs" sub-section of
the previous "Frame Relay" section of this document.
5.4.2.9. ATM Signaling Transparency
See the discussion in the "Signaling Transparency" sub-section of the
previous "Frame Relay" section of this document.
5.4.2.10. Soft PVC Support
See the discussion and figures in the "Soft PVC Support" sub-section
of the previous "Frame Relay" section of this document.
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5.4.2.11. Segmentation and Reassembly (SAR)
The bandwidth overhead of the ATM cell is about 10% (= 5 overhead
bytes out of 53 bytes total). For AAL5 traffic [I.363.5], it may be
more efficient in terms of bandwidth to carry the re-assembled AAL5
frames instead of the individual ATM cells. This would involve some
cost in terms of a SAR operation at each end of the PW. In some
cases, especially if OAM is required to be supported over the PW, the
PW may have no choice but to transport the ATM traffic in cell
format.
Whether the ATM traffic is transported in a frame or cell format, it
is the responsibility of the PE to emulate the OAM functions to the
adjacent ATM interface at each end.
6. PW Maintenance
6.1. 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.
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 AS for each protocol must describe the validation scheme to be
used.
6.2. PW-PDU Sequencing
One major consideration of PW design is 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, MPLS supports connection-oriented transport with a guarantee
of in-order delivery. A sequence number in the PW layer is not
needed when used with MPLS. 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).
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6.3. 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.
6.4. Security
Each AS 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.
6.5. Encapsulation Control
6.5.1. 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.
6.5.2. 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.
6.6. 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).
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- Service-specific interface counts.
These counters would be contained in a MIB, they should not replicate
existing MIB counters.
6.7. Traceroute
Tracing functionality is desirable for emulated circuits and
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.
6.8. Congestion Considerations
[RFC2914] describes how devices connected to the Internet should
handle congestion. The discussion of congestion with respect to PWE3
will be broken into two sections: CBR applications and VBR
applications.
6.8.1. VBR Applications
VBR applications include Ethernet, Frame Relay, and ATM (other than
AAL CES). During periods of congestion the PE may be able to take
action to communicate to the CE the need to slow down.
In the presence of congestion, the PE could perform several actions.
These are the same actions that a Frame Relay switch might take. If
available, a measure of the degree of congestion would be useful.
- While a service provide may define an SLA for a Frame Relay
Service, Frame Relay itself does not have a guarantee of delivery.
Given this fact, the PE could do nothing in the face of congestion.
The Frame Relay application at the CE would then have to detect
congestion and act appropriately.
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- Frame Relay defines BECN as an indication to a Frame Relay device
that traffic that it sent is experiencing congestion. See Figure
16 for an example of how BECN is sent. For mild congestion, the PE
could send BECN back to the CE. The CE could then reduce the
amount of traffic being sent. It is worth nothing that many Frame
Relay devices ignore BECN.
- The CE could also send FECN in the direction in which congestion is
occurring. See Figure 16 for an example of how FECN is sent.
- During congestion, the PE could discard all frames with DE set
- If the PE was aware of the CIR for the VCs, it could drop any
traffic in excess of CIR.
- For severe congestion the PE could take the interface down. If the
PE was generating the PVC status signaling then these messages
could be used to convey the problem to the CE. This approach is
not elegant and may not work well with existing Frame Relay
applications.
ATM has a forward notification of congestion (EFCI), but unlike Frame
Relay there is no backwards notification. This leaves the following
choices of action. These are the same actions that an ATM switch
might take.
- Do nothing and let the ATM application at the CE handle the
problem. This may work for some applications, but it will make it
difficult for service providers to guarantee a high QoS on the VC.
- If the PE was aware of the traffic parameters for the VCs, it could
drop any traffic that was out of profile.
- For severe congestion the PE could take the interface down. This
may be worse than doing nothing.
A PE providing a PW to an Ethernet CE could react to congestion in
one of the following ways.
- A PE could use Ethernet flow control during congestion by sending a
PAUSE frame as described in Annex 31B of [802.3].
- A PE could do nothing and let the Ethernet application at the CE
handle the problem.
- For severe congestion the PE could take the interface down. This
may be worse than doing nothing.
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6.8.2. CBR Applications
CBR applications include layer 1 applications such as emulated
TDM/SONET streams, as well as layer 2 applications such as ATM AAL1
CES. These applications present a constant load on the network at
all times. They cannot slow down; they are either running at full
speed, or they are impaired. If congestion causes an excessive
number of packets to be lost, the PE could take down the interface
and send AIS to the CE. There is probably not much point in doing
this if the PE is operating in an unstructured mode, as the framer in
the CE will probably declare a LOS condition anyway. A PE operating
in a structured (framed) mode would always send a clean frame pattern
to the CE, so it might be desirable to send AIS to notify the CE that
there are problems with the PW.
7. Packet Switched Networks
This section discusses various types of PSNs for providing the PW
transport. The areas of considerations are:
- Explicit Multi-protocol Encapsulation Identifier
- Transport Integrity
- Traffic Engineering Ability
- Session Multiplexing
- Flow and Congestion Control
- Packet Ordering
- Tunnel Maintenance
- Scalability
- Overhead
- QoS and Traffic Management
Below is a description of the aspects of the Internet Protocol [IP].
Explicit MP Encap ID No support for a full range of multi-service
protocols e.g. there is no protocol type
assigned for ATM or MPLS.
Transport Integrity IP has a checksum over the header but not
over the payload.
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Traffic Engineering The TOS bits may be used as DiffServ code
points.
Session Multiplexing No support for session multiplexing.
Packet Order No support for preservation of order.
Tunnel Maintenance Protocols such as L2TP may be used to
establish tunnels using IP packets.
Flow/Congestion Control No built-in flow control to manage
congestion. IP relies on the upper layer
protocol, e.g. TCP, to perform the
congestion management.
Scalability It would be hard to imagine a protocol more
scalable than IP.
Transport Overhead Minimum of 20-byte header.
QoS/Traffic Management No built-in QoS and traffic management.
However, one can apply DiffServ to select a
per-hop behavior for a class of traffic.
Layer 2 Tunneling (L2TP) [L2TP] provides a virtual extension of PPP
across an IP PSN.
Explicit MP Encap ID Supports any routed protocol, e.g. IP, IPX
and AppleTalk that is supported by PPP.
Transport Integrity Support a checksum for the entire
encapsulated frame.
Traffic Engineering No companion traffic engineering mechanism
to support L2TP tunnel establishment.
Session Multiplexing Supports two levels of session multiplexing
via the use of the "tunnel-id" and "session-
id" fields.
Packet Order By supporting the optional sequence number,
packet re-ordering can be done at the PWE
Tunnel Maintenance L2TP uses control messages to establish,
terminate and monitor the status of the
logical PPP sessions. These are independent
of the data messages. L2TP also provides an
optional keep-alive mechanism to detect non-
operational tunnel.
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Flow/Congestion Control L2TP defines flow and congestion control
mechanisms for the control traffic only; no
control for the data traffic. Even so, the
PE could apply value-added functions such as
admission control, policing and shaping of
the L2TP tunnel at the aggregate flows
level, e.g. DiffServ-TE.
Scalability Lack of label stacking ability.
Transport Overhead Minimum overhead is 44-byte (20-byte IP
header + 12-byte UDP header + 8-byte minimum
L2TP header + 4-byte PPP header) to support
L2TP encapsulation
QoS/Traffic Management No built-in QoS and traffic management.
However, one can apply IntServ or DiffServ
to select the preferred transport behavior
for the entire tunnel, i.e. one traffic
class per L2TP tunnel.
Multi-protocol Label Switching [MPLS] is designed to combine Layer 2
switching and Layer 3 routing technology to provide efficient packet
processing and forwarding over a variety of link layer and transport
technologies e.g. ATM, Frame Relay and SONET.
Explicit MP Encap ID No defined standard to identify the
encapsulated multi-protocol PDU.
Transport Integrity No checksum support.
Traffic Engineering Designed with many signaling, routing and
traffic management extensions to support
traffic engineering.
Session Multiplexing Supports session multiplexing via the MPLS
label and the EXP field.
Packet Order Connection-oriented transport with
guaranteed in-sequence delivery.
Tunnel Maintenance MPLS signaling provides the ability to
establish, terminate and monitor the status
of the LSP.
Flow and Congestion Control
MPLS-TE assumes external admission control,
policy and shaping mechanism to provide flow
and congestion control at the aggregate
traffic level.
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Scalability Label stacking facilitates scalability.
Transport Overhead Minimum overhead is 4-byte + any MPLS
extension to support multi-protocol
encapsulation and transport.
QoS/Traffic Management MPLS-TE supports QoS and traffic management.
8. Acknowledgments
This document was created by the PWE3 Framework design team.
9. References
9.1. IETF RFCs
[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.
[NTP] D. Mills, "Network Time Protocol Version 3", RFC1305, March
1992.
[MPLS] E. Rosen, "Multiprotocol Label Switching Architecture",
RFC3031, January 2001.
[IP] DARPA, "Internet Protocol", RFC791, September 1981.
9.2. IETF Drafts
[ANAVI] Anavi et al, "TDM over IP" draft-anavi-tdmoip-01.txt, work
in progress, February 2001.
[MALIS] Malis et al, "SONET/SDH Circuit Emulation Service Over MPLS
(CEM) Encapsulation" (draft-malis-sonet-ces-mpls-03.txt),
work in progress, February 2001.
[XIAO] Xiao et al, "Requirements for Pseudo Wire Emulation Edge-to-
Edge (PWE3)" (draft-pwe3-requirements-01.txt), work in
progress, July 2001.
[MARTINI] Martini et al, "Transport of Layer 2 Frames Over MPLS"
(draft-martini-l2circuit-trans-mpls-06.txt), work in
progress, May 2001.
[BONICA] Bonica et al, "Tracing Requirements for Generic Tunnels"
(draft-bonica-tunneltrace-01.txt), work in progress,
February 2001.
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[CALLON] Callon et al, "A Framework for Provider Provisioned Virtual
Private Networks" (draft-ietf-ppvpn-framework-00.txt), work
in progress, February 2001.
9.3. ATM Forum
[ATMCES] ATM Forum, "Circuit Emulation Service Interoperability
Specification Version 2.0" (af-vtoa-0078-000), January 1997.
[TM4.0] ATM Forum, "Traffic Management Specification Version 4.0",
(af-tm-0056.000), April, 1996.
[ILMI] ATM Forum, "Integrated Local Management Interface (ILMI)
Specification Version 4.0", (af-ilmi-0065.000), September,
1996.
9.4. Frame Relay Forum
[FRF.1.2] D. Sinicrope, "PVC User-to-Network Interface (UNI)
Implementation Agreement", Frame Relay Forum FRF.1.2, July
2000.
[FRF.5] O'Leary et al, "Frame Relay/ATM PVC Network Interworking
Implementation Agreement", Frame Relay Forum FRF.5, December
20, 1994.
[FRF.13] K. Rehbehn, "Service Level Definitions Implementation
Agreement", Frame Relay Forum FRF.13, August 4, 1998.
[Q.933A] ITU, "ISDN Signaling Specifications for Frame Mode Switched
and Permanent Virtual Connections Control and Status
Monitoring" ITU Recommendation Q.933, Annex A, Geneva, 1995.
[I.356] ITU, "B-ISDN ATM Layer Cell Transfer Performance", ITU
Recommendation I.356, To Be Published.
[I.363.1] ITU, "B-ISDN ATM Adaptation Layer specification: Type 1
AAL", Recommendation I.363.1, August, 1996.
[I.363.2] ITU, "B-ISDN ATM Adaptation Layer (AAL) type 2
specification", Recommendation I.363.2, To Be Published.
[I.363.5] ITU, "B-ISDN ATM Adaptation Layer specification: Type 5
AAL", Recommendation I.363.5, August, 1996.
[I.371] ITU, "Traffic Control and Congestion Control in B-ISDN" ITU
Recommendation I.371, To Be Published.
[I.610] ITU, "B-ISDN Operation and Maintenance Principles and
Functions", ITU Recommendation I.610, February, 1999.
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[G.811] ITU, "Timing Characteristics of Primary Reference Clocks",
ITU Recommendation G.811, September 1997.
[G.823] "The Control of Jitter and Wander Within Digital Networks
Which Are Based on the 2048 kbit/s Hierarchy", ITU
Recommendation G.823, March 2000.
[G.824] "The Control of Jitter and Wander Within Digital Networks
Which Are Based on the 1544 kbit/s Hierarchy", ITU
Recommendation G.824, March 2000.
[802.1D] IEEE, "ISO/IEC 15802-3:1998,(802.1D, 1998 Edition),
Information technology --Telecommunications and information
exchange between systems --IEEE standard for local and
metropolitan area networks --Common specifications--Media
access control (MAC) Bridges", June, 1998.
[802.1Q] ANSI/IEEE Standard 802.1Q, "IEEE Standards for Local and
Metropolitan Area Networks: Virtual Bridged Local Area
Networks", 1998 .
[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.
[T1.403] ANSI, "Network and Customer Installation Interfaces - DS1
Electrical Interfaces", T1.403-1999, May 24, 1999.
[T1.617D] ANSI, "Digital Subscriber System No. 1 DSS1 Signaling
Specification for Frame Relay Bearer Service", ANSI
T1.617-1991 (R1997), Annex D.
9.8. Telcordia
[GR253] Telcordia, "Synchronous Optical Network (SONET) Transport
Systems: Common Generic Criteria" (GR253CORE), Issue 3,
September 2000.
10. 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.
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11. Authors' Addresses
Prayson Pate
Overture Networks
P. O. Box 14864
RTP, NC, USA 27709
Email: prayson.pate@overturenetworks.com
XiPeng Xiao
Photuris, Inc.
2025 Stierlin Court
Mountain View, CA 94043
Email: xxiao@photuris.com
Tricci So
Caspian Networks
170 Baytech Dr.
San Jose, CA 95134
E-Mail: tso@caspiannetworks.com
Craig White
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: Craig.White@Level3.com
Kireeti Kompella
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
Email: kireeti@juniper.net
Andrew G. Malis
Vivace Networks, Inc.
2730 Orchard Parkway
San Jose, CA 95134
Email: Andy.Malis@vivacenetworks.com
Thomas K. Johnson
Litchfield Communications
76 Westbury Park Rd.
Watertown, CT 06795
Email: tom_johnson@litchfieldcomm.com
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12. Full Copyright Section
Copyright (C) The Internet Society (2000). 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
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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.
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