Network Working Group Max Riegel
Internet-Draft Siemens AG
Expires: August 9, 2003 Editor
February 8, 2003
Requirements for Edge-to-Edge Emulation of TDM Circuits over Packet
Switching Networks (PSN)
draft-ietf-pwe3-tdm-requirements-00.txt
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Abstract
This document specifies the particular requirements for
edge-to-edge-emulation of circuits carrying time division multiplexed
digital signals of the PDH as well as the SONET/SDH hierarchy over
packet-switched networks. It is based on the common architecture for
Pseudo Wire Emulation Edge-to-Edge (PWE3) as defined in [PWE3-ARCH].
It makes references to requirements in [PWE3-REQ] where applicable
and complements [PWE3-REQ] by defining requirements originating from
specifics of TDM circuits.
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Co-Authors
The following are co-authors of this document:
Sasha Vainshtein Axerra Networks
Yaakov Stein RAD Data Communication
Prayson Pate Overture Networks, Inc.
Tom Johnson Litchfield Communication, Inc.
Ron Cohen Lycium Networks
Tim Frost Zarlink Semiconductor
Changes from the last revision:
The last revision of this document was filed under:
draft-riegel-pwe3-tdm-requirements-01.txt
- editorial corrections
- updated references
- streamlined use of the term "TDM" throughout the document. "TDM"
now refers to circuits of both the PDH as well as the SONET/SDH
hierarchy.
- Chapter 7.3.1: replacement of 'occasional loss of a single packet'
by 'lost packets'
- Editor's-Note in section 7.8 (Congestion Control).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 TDM circuits . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.1 Structured TDM circuits . . . . . . . . . . . . . . . . . . 4
1.1.2 Unstructured TDM circuits . . . . . . . . . . . . . . . . . 5
1.2 SONET/SDH circuits . . . . . . . . . . . . . . . . . . . . . 5
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Reference Models . . . . . . . . . . . . . . . . . . . . . . 7
4.1 Generic PWE3 Models . . . . . . . . . . . . . . . . . . . . 8
4.2 Timing Synchronization . . . . . . . . . . . . . . . . . . . 8
4.2.1 Clock Recovery . . . . . . . . . . . . . . . . . . . . . . . 8
4.2.2 Timed delivery . . . . . . . . . . . . . . . . . . . . . . . 8
4.3 Network Synchronization Reference Model . . . . . . . . . . 9
4.3.1 Synchronous Network Scenarios . . . . . . . . . . . . . . . 11
4.3.2 Relative Network Scenario . . . . . . . . . . . . . . . . . 12
4.3.3 Adaptive Network Scenario . . . . . . . . . . . . . . . . . 13
5. Emulated Services . . . . . . . . . . . . . . . . . . . . . 14
5.1 Unstructured TDM Circuits . . . . . . . . . . . . . . . . . 14
5.2 Structured TDM Circuits . . . . . . . . . . . . . . . . . . 15
5.3 SONET/SDH Circuits . . . . . . . . . . . . . . . . . . . . . 15
6. Generic Requirements . . . . . . . . . . . . . . . . . . . . 15
6.1 Relevant Common PW Requirements . . . . . . . . . . . . . . 15
6.2 Common Circuit Payload Requirements . . . . . . . . . . . . 16
6.3 General Design Issues . . . . . . . . . . . . . . . . . . . 16
7. Service-Specific Requirements . . . . . . . . . . . . . . . 17
7.1 Interworking . . . . . . . . . . . . . . . . . . . . . . . . 17
7.2 Network Synchronization . . . . . . . . . . . . . . . . . . 17
7.3 Robustness . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.3.1 Packet loss . . . . . . . . . . . . . . . . . . . . . . . . 17
7.3.2 Out-of-order delivery . . . . . . . . . . . . . . . . . . . 18
7.4 CE Signaling . . . . . . . . . . . . . . . . . . . . . . . . 18
7.5 PSN bandwidth utilization . . . . . . . . . . . . . . . . . 19
7.6 Packet Delay Variation . . . . . . . . . . . . . . . . . . . 19
7.7 Compatibility with the Existing PSN Infrastructure . . . . . 20
7.8 Congestion Control . . . . . . . . . . . . . . . . . . . . . 20
7.9 Fault Detection and Handling . . . . . . . . . . . . . . . . 20
7.10 Performance Monitoring . . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . 21
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . 25
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1. Introduction
This document specifies the particular requirements for
edge-to-edge-emulation of circuits carrying time division multiplexed
digital signals of the PDH as well as the SONET/SDH hierarchy over
packet-switched networks. It is based on the common architecture for
Pseudo Wire Emulation Edge-to-Edge (PWE3) as defined in [PWE3-ARCH].
It makes references to requirements in [PWE3-REQ] where applicable
and complements [PWE3-REQ] by defining requirements originating from
specifics of TDM circuits.
1.1 TDM circuits
The term "TDM" will be used in this documents as general descriptor
of the synchronous bit streams belonging to either the PDH or the
SONET/SDH hierarchies.
The bit rates traditionally used in various regions of the world are
detailed in the normative reference [G.702]. For example, in North
America the T1 bit stream of 1.544 Mbps and the T3 bit stream of
44.736 Mbps are mandated, while in Europe the E1 bit stream of 2.048
Mbps and the E3 bit stream of 34.368 Mbps are utilized.
Although TDM can be used to carry unstructured bit streams at the
rates defined in [G.702], there is a standardized method of carrying
bit streams in larger units each containing the same amount of bits.
These units are called frames, and the transport mode is denoted
"framed TDM".
Related to the sampling frequency of voice traffic, there are always
8000 such frames per second, hence the T1 frame consists of 193 bits
and the E1 frame of 256 bits. The number of bits in a frame is called
the frame size.
Framed TDM is using some bits in the bit stream to identify the
boundaries of the frames (e.g. 1 framing bit per T1 frame, a sequence
of 8 framing bits per E1 frame). The details of how these framing
bits are generated and used are elucidated in [G.704], [G.751] and
[G.752]. Unframed TDM has all bits available for payload.
Framed TDM is often used to multiplex multiple voice channels each
consisting of 8000 8bit-samples per second in a sequence of timeslots
recurring in each frame. This multiplexing is called "channelized
TDM" and introduces additional structure.
1.1.1 Structured TDM circuits
The term "structured TDM" is used in this document to refer to both
'channelized TDM' as well as 'framed TDM' whenever framing and
eventually channelization exist and are deemed significant for the
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transport of TDM over PWs.
1.1.2 Unstructured TDM circuits
A TDM stream is denoted "unstructured" when it is unframed, or when
it is framed or even channelized, but the framing and channelization
structure are deemed inconsequential from the transport point of
view. In such cases all structural overhead is transparently
transported by the PW along with the payload data, and the
encapsulation method employed provides no mechanisms for its location
or utilization.
1.2 SONET/SDH circuits
The term SONET refers to the North American Synchronous Optical
NETwork as specified by [GR-253] [Ed-Note###: add T.105a here???].
The Synchronous Digital Hierarchy (SDH) is the international
equivalent and enhancement of SONET and is specified by [G.707].
Although terminology between the two technologies is different, both
have the concept of a Nx783 byte payload container repeated every
125us. This payload is referred to for SONET as an STS-1 SPE and may
be concatenated into higher bandwidth circuits (e.g. STS-Nc) or
sub-divided into lower bandwidth circuits (Virtual Tributaries). The
higher bandwidth concatenated circuits can be used to carry anything
from IP Packets to ATM cells to Digital Video Signals. Individual
STS-1 SPEs are frequently used to carry individual DS3 or E3 TDM
circuits. When the 783 byte containers are sub-divided for lower
rate payloads, they are frequently used to carry individual T1 or E1
TDM circuits.
Both SONET and SDH include a substantial amount of transport overhead
that is used for performance monitoring, fault isolation, and other
maintenance functions along different types of optical or electrical
spans. In addition, the payload area includes dedicated overhead for
end-to-end performance monitoring, fault isolation, and maintenance
for the service being carried. If the main payload area is
sub-divided into lower rate circuits (such as T1/E1), additional
overhead is included for end-to-end monitoring of the individual T1/
E1 circuits. A key feature of STS-1/Nc and VT service emulation is
the carriage of the Path or VT maintenance overhead through the PSN.
This requirements document discusses the requirements for emulation
of the SONET/SDH services. These services include end-to-end
emulation of the core 783 byte payload (e.g. STS-1 SPE), emulation of
concatenated payloads (e.g. STS-Nc SPE), as well as emulation of a
variety of sub-STS-1 rate circuits jointly referred to as Virtual
Tributaries (VT).
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2. Motivation
[PWE3-REQ] specifies common requirements for edge-to-edge-emulation
of circuits of various types. However, these requirements, as well as
references in [PWE3-ARCH] do not cover specifics of PWs carrying TDM
circuits.
The need for a specific document complementing [PWE3-REQ] with regard
to edge-to-edge-emulation of TDM circuits arises from following
causes:
o Specifics of the TDM circuits,
e.g.:
* the need for balance between the clock of ingress and egress
end services in each direction of the PW,
* the need to maintain jitter and wander of the clock of the
egress end service within the limits imposed by the appropriate
normative documents in spite of the packet delay variation
produced by the PSN.
o Specifics of applications using (native and emulated) TDM
circuits,
e.g. voice applications:
* put special emphasis on minimization of one-way delay,
* are relatively tolerant to errors in data.
Other applications might have different specifics.
e.g. transport of signaling information:
* is relatively tolerant to one-way delay,
* is sensitive to errors in transmitted data.
o Specifics of the customers' expectations regarding end-to-end
behavior of services that contain emulated TDM circuits,
e.g., experience with carrying such services over SONET/SDH
networks increases the need for:
* isolation of problems introduced by the PSN from those
occurring beyond the PSN bounds,
* higher sensitivity to misconnection, etc.
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3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The terms defined in [PWE3-ARCH], Section 1.4 are consistently used.
However some terms and acronyms are specific in conjunction with the
TDM services. In particular:
CAS (Channel-Associated Signaling)
It is one of several signaling techniques used by the telephony
applications to convey various states of these applications (e.g.,
off-hook and on-hook). CAS uses a certain, circuit-specific
multiframe structure that is imposed on the TDM bit stream and a
predefined association between the relative timeslot (= channel)
number within this stream and position of certain bits within this
multiframe structure. In the case of E1 there are four 500 bit/s
channels for each timeslot used to distinguish and signal
application states (see [G.704] for details).
CAS is also used in conjunction with D4 and ESF formats of T1
using "robbed bits". In case of D4 this results in 2 channels of
333.(3) bit/s, and in case of ESF - 4 such channels.
[## Ed-note##: more details to be included here?]
CCS (Common Channel Signaling)
This is an alternative to the CAS method of signaling used by the
telephony applications. E.g., for SS7 Common Channel Signaling is
described in [Q.700] and references therein.
SDH (Synchronous Digital Hierarchy)
SONET (Synchronous Optical NETwork)
SPE (Synchronous Payload Envelope)
STS-n (Synchonous Transport Signal n (SONET))
VT (Virtual Tributary (SONET))
VC-n (Virtual Container N (SDH))
For the TDM network we use the terms "jitter" and "wander" as defined
in [G.823] and [G.824], while for the PSN measures from IETF IPPM
(like packet delay variation - see [RFC3393]) are used.
4. Reference Models
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4.1 Generic PWE3 Models
Generic models that have been defined in [PWE3-ARCH] in Sections
- 4.1 (Network Reference Model),
- 4.2 (PWE3 Preprocessing),
- 4.3 (Maintenance Reference Model),
- 4.4 (Protocol Stack Reference Model) and
- 4.5 (Pre-processing Extension to Protocol Stack Reference Model).
They are fully applicable for the purposes of this document without
any modifications.
All the services considered in this document represent special cases
of the generic bit-stream and structured bit-stream payload type
defined in Section 3.3 of [PWE3-ARCH].
4.2 Timing Synchronization
Timing synchronization of emulated TDM services comprises
Clock recovery,
Timed delivery (delay), and
Frame recovery.
The availability of a common clock at the ends of PW is not presumed.
However, without a common clock the fidelity of the recovered TDM
timing will be dependent on the packet delay variation behavior of
the underlying PSN and the robustness of the applied timing recovery
algorithms.
4.2.1 Clock Recovery
Clock recovery is the extraction of the transmission bit timing
information out of the delivered packet stream. Extraction of this
information from a highly jittered source such as a packet stream is
quite a complicated task.
4.2.2 Timed delivery
Timed delivery is the delivery of non-contiguous PW PDUs to the PW
output interface with a constant delay (phase shift) relative to the
input interface. The delay of the delivery may be relative to a clock
derived from the packet stream via clock recovery, or via an external
clock.
4.2.2.1 Frame Recovery
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Frame recovery is the process to detect the frame boundaries. It
starts with the hunting process in the out-of-alignment state and
provides the frame alignment reacquisition in the correct-alignment
state.
Frame recovery provides access to signaling and maintenance
information embedded in the framing bits and allows for advanced
functions to cope with transmission errors and to enhance bandwidth
utilization in the underlying PSN.
4.3 Network Synchronization Reference Model
A generic network synchronization reference model shown in Figure 1
below:
+---------------+ +---------------+
| PE1 | | PE2 |
K | +--+ | | +--+ | G
| | | J| | | | H| | |
v | v | | | v | | 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 | | | |<------+------>| | | | | |
| A | +--+ +--+ | | | +--+-E | F |
| +---------------+ +-+ +---------------+ |
| ^ |I| ^ |
| | +-+ | |
| C D |
+-----------------------------L-----------------------------+
Figure 1: Timing Recovery Reference Diagram
The following notations are used in Figure 1:
CE1, CE2
Customer edge devices terminating TDM circuits to be emulated.
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PE1, PE2
Provider edge devices adapting these end services to PW.
S1, S2
Provider core routers
Phy
Physical interface terminating the TDM circuit.
Enc
PSN-bound IWF of the PW
Dec
CE-bound IWF of the PW. It contains a compensation buffer (also
known as the "jitter buffer") of limited size.
"==>"
TDM end service circuits
"::>"
PW providing edge-to-edge-emulation for the TDM circuit.
The characters "A" - "L" are denoting various clocks:
"A"
The clock used by CE1 for transmission of the TDM end circuit
towards CE1.
"B"
The clock recovered by PE1 from the incoming TDM end circuit. "A"
and "B" always have the same frequency.
"G", "H"
The same as "A" and "B" respectively for CE2 and PE2 ("G" and "H"
have the same frequency).
"C", "D"
Local oscillators available to PE1 and PE2 respectively.
"E"
Clock used by PE2 to transmit the TDM end service circuit to CE2
(the recovered clock).
"F"
Clock recovered by CE2 from the incoming TDM end service ("E and
"F" have the same frequency).
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"I"
If it exists, it is the common network reference clock available
to PE1 and PE2.
"J", "K"
The same as "E" and "F" respectively for PE1 and CE1 ("J" and "K"
have the same frequency).
"L"
If it exists, it is the common reference clock of CE1 and CE2.
Note that different pairs of CE devices may use different common
reference clocks.
One of the objectives of edge-to-edge-emulation of a TDM circuit is
balance between clocks "B" and "E" (i.e., these clocks MUST have the
same frequency). This objective may be achieved by different means
depending on the actual network synchronization scheme deployed.
The following groups of the network synchronization deployment
scenarios can be considered:
4.3.1 Synchronous Network Scenarios
Depending on which part of the network is synchronized by a common
clock there are two scenarios:
o PE Synchronized Network:
The common network reference clock "I" is available to all the PE
devices, and local oscillators "C" and "D" are locked to "I":
* Clocks "E" and "J" are the same as "D" and "C" respectively.
* Clocks "A" and "G" are the same as "K" and "F" respectively
(i.e., CE1 and CE2 use the so-called loop timing).
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+-----+ +-----+
+-----+ | |- - -|=================|- - -| | +-----+
| /-- |<---------|............PW1..............|<---------| <-\ |
|| CE | | | PE1 | | PE2 | | |CE2 ||
| \-> |--------->|............PW2..............|--------->| --/ |
+-----+ | |- - -|=================|- - -| | +-----+
+-----+ +-----+
^ ^
|C |D
+-----------+-----------+
|
+-+
|I|
+-+
Figure 2: PE synchronized scenario
o CE Synchronized Network:
The common network reference clock "L" is available to all the CE
devices, and local oscillators "A" and "G" are locked to "L":
* Clocks "E" and "J" are the same as "G" and "A" respectively
(i.e., PE1 and PE2 use the so-called loop timing).
+-----+ +-----+
+-----+ | |- - -|=================|- - -| | +-----+
| |<---------|............PW1..............|<---------| |
| CE1 | | | PE1 | | PE2 | | | CE2 |
| |--------->|............PW2..............|--------->| |
+-----+ | |- - -|=================|- - -| | +-----+
^ +-----+ +-----+ ^
|A G|
+----------------------------+------------------------------+
|
+-+
|L|
+-+
Figure 3: CE synchronized scenario
No timing information has to be transferred in these cases.
4.3.2 Relative Network Scenario
In this case each CE uses its own transmission clock source that must
be carried across the PSN and recovered by the remote PE,
respectively. The common PE clock "I" can be used as reference for
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this purpose.
The common network reference clock "I" is available to all the PE
devices, and local oscillators "C" and "D" are locked to "I":
o Clocks "A" and "G" are generated locally without reference to a
common clock.
o Clocks "E" and "J" are generated in reference to a common clock
available at all PE devices.
In a slight modification of this scenario, one (but not both!) of the
CE devices may use its receive clock as its transmission clock (i.e.
use the so-called loop timing).
|G
+-----+ +-----+ v
+-----+ | |- - -|=================|- - -| | +-----+
| |<---------|............PW1..............|<---------| |
| CE1 | | | PE1 | | PE2 | | | CE2 |
| |--------->|............PW2..............|--------->| |
+-----+ | |- - -|=================|- - -| | +-----+
^ +-----+<-------+------->+-----+
|A |
+-+
|I|
+-+
Figure 3: Relative network scenario
Timing information may be transferred in this case.
4.3.3 Adaptive Network Scenario
The asynchronous scenario is characterized by:
o No common network reference clock "I" is available to PE1 and PE2.
o No common reference clock "L" is available to CE1 and CE2.
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|J |G
v |
+-----+ +-----+ v
+-----+ | |- - -|=================|- - -| | +-----+
| |<---------|............PW1..............|<---------| |
| CE1 | | | PE1 | | PE2 | | | CE2 |
| |--------->|............PW2..............|--------->| |
+-----+ | |- - -|=================|- - -| | +-----+
^ +-----+ +-----+
| ^
A| E|
Figure 4: Asynchronous Scenario
Asynchronous Carrier of Carriers scenario clearly represents the
worst case for achieving the goal of balancing clocks "A" and "E".
Note that one of the means available for achieving this goal is the
compensation buffer in the CE-bound IWF, and the balance between
clocks "A" and "E" must be exact over the period required for
replaying out of this buffer.
Timing information must be transferred in this case.
5. Emulated Services
This document defines requirements for the payload and encapsulation
layers for edge-to-edge emulation of TDM services with bit-stream
payload as well as structured bit-stream payload.
Wherever possible, the requirements specified in this document SHOULD
be satisfied by appropriate arrangements of the encapsulation layer
only. The (rare) cases when the requirements apply to both the
encapsulation and payload layers (or even only to the payload layer
only) will be explicitly noted.
The service-specific encapsulation layer for edge-to-edge emulation
comprises the following services over a PSN:
5.1 Unstructured TDM Circuits
o Unstructured E1 as described in [G.704].
o Unstructured T1 (DS1) as described in [G.704].
o Unstructured E3 as defined in [G.751].
o Unstructured T3 (DS3) as described in [T.107].
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5.2 Structured TDM Circuits
o Structured E1/T1 with or without CAS as described in [G.704]
o NxDS0 with or without CAS
5.3 SONET/SDH Circuits
o SONET STS-1 synchronous payload envelope (SPE)/SDH VC-3
o SONET STS-Nc SPE (N = 3, 12, 48, 192) / SDH VC-4, VC-4-4c,
VC-4-16c, VC-4-64c
o SONET VT-N (N = 1.5, 2, 3, 6) / SDH VC-11, VC-12, VC-2
o SONET Nx VT-N / SDH Nx VC-11/VC-12/VC-2/VC-3
6. Generic Requirements
6.1 Relevant Common PW Requirements
The combination of encapsulation and payload layers for edge-to-
edge-emulation considered in this document should comply with the
following common PW requirements defined in [PWE3-REQ]:
1. Conveyance of Necessary Header Information:
1. For unstructured circuits this functionality MAY be provided
by the payload layer.
2. For structured circuits, the necessary information MUST be
provided by the encapsulation layer.
2. Support of Multiplexing and Demultiplexing if supported by the
native services:
1. Relevant for Nx DS0 circuits with or without signaling and Nx
VT-x in a single STS-1 or VC-4.
2. For these circuits means that the combination of
encapsulation and payload layers MUST provide for separate
treatment of every sub-circuit.
3. Enough information SHOULD be provided by the pseudo wire to
allow multiplexing and demultiplexing by the NSP. Reduction
of the complexity of the PW emulation by using NSP circuitry
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for multiplexing and demultiplexing MAY be the favorite
solution.
3. Intervention or transparent transfer of Control Messages of the
Native Services depending on the particular scenario.
4. Consideration of the PSN Tunnel Header Overhead (see also Section
7.5 below).
5. Detection and handling of PW faults. The list of faults is given
in Section 7.9 below.
The following requirements listed in [PWE3-REQ] are not applicable to
emulation of TDM services:
o Support of variable length PDUs,
o Segmentation and reassembly,
o Keep-alive,
[###Editor's note: When dealing with congestion control by
switching down the pseudo wires this might be changed.###]
o RFC 2914 conformance.
6.2 Common Circuit Payload Requirements
Structured circuits considered in this document belong to the generic
'Structured Bit-Stream Payload' type defined in [PWE3-ARCH].
Unstructured circuits considered in this document belong to the
generic Bit-Stream Payload type.
Accordingly, the encapsulation layer MUST provide the common
Sequencing service and SHOULD provide Timing information
(Synchronization services).
Note: The encapsulation layer for the (Structured) Bit-Stream Payload
circuits MAY NOT provide the length service.
6.3 General Design Issues
The combination of payload and encapsulation layers SHOULD comply
with the general design principles of the Internet protocols as
presented in [RFC1958], Section 3 and [PWE3-ARCH].
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7. Service-Specific Requirements
7.1 Interworking
1. The emulation MUST support network interworking between end
services of the same type (see Section 5) and, wherever
appropriate, bit-rate.
2. The encapsulation layer SHOULD remain unaffected by specific
characteristics of connection between the end services and PE
devices at the two ends of the PW.
7.2 Network Synchronization
1. The encapsulation layer MUST provide synchronization services
that are sufficient for:
1. balancing of clock of ingress and egress end services
regardless of the specific network synchronization scenario,
2. keeping the jitter and wander of the clock of the egress
service within the service-specific limits as defined by the
appropriate normative references.
2. If the same high-quality synchronization source is available to
all the PE devices in the given domain, the encapsulation layer
SHOULD be able to offer additional benefits (e.g., facilitate
better reconstruction of the native service clock).
7.3 Robustness
The robustness of the emulated service does not only depend upon
means applied to the edge-to-edge-emulation but also upon proper
implementation of the procedures of the native TDM service.
7.3.1 Packet loss
Edge-to-edge-emulation of TDM circuits MAY assume very low
probability of packet loss between ingress and egress PE. In
particular, no retransmission mechanisms are required.
In order to minimize effect of lost packets on the egress service,
the encapsulation layer SHOULD:
1. Allow independent interpretation of TDM data in each specific
packet by the egress PE (see [RFC2736]. This requirement MAY be
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disregarded if the egress PE has to interpret structures that
exceed the path MTU between the ingress and egress PEs.
2. Allow reliable detection of lost packets (See next section). In
particular, it should allow prediction (within reasonable limits)
of the arrival time of the next PW packet and detection of lost
packets that takes such a prediction into account.
3. Minimize possible effect of lost packets on recovery of the
circuit clock by the egress PE depending on the actual network
synchronization scheme deployed.
4. In case of unstructured emulation, facilitate increased
resilience of CEs against lost packets by allowing the egress PE
to substitute appropriate data.
7.3.2 Out-of-order delivery
The encapsulation layer MUST provide the necessary mechanisms that
guarantee ordered delivery of packets carrying the TDM data over the
PSN. Packets that have arrived out-of-order:
1. MUST be detected,
2. SHOULD be reordered if not judged to be too late or too early for
playout.
Out-of-order packets that cannot be reordered MUST be treated as
lost.
7.4 CE Signaling
Unstructured TDM circuits do not usually require any special
mechanisms for carrying CE signals as these would be carried as part
of the emulated service.
Some CE applications using structured TDM circuits (e.g., telephony)
require specific signaling that conveys changes of state of these
applications relative to the TDM data.
The encapsulation layer SHOULD support signaling of state of CE
applications for the relevant circuits providing for:
1. Ability to support different signaling schemes with minimal
impact on encapsulation of TDM data,
2. Multiplexing of application-specific CE signals and data of the
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emulated service in the same PW,
3. Synchronization (within the application-specific tolerance
limits) between CE signals and data at the PW egress,
4. Probabilistic recovery against possible occasional loss of
packets in the PSN,
5. Deterministic recovery of the CE application state after PW setup
and network outages.
CE signaling that is used for maintenance purposes (loopback
commands, performance monitoring data retrieval, etc.) SHOULD be
dealt within the scope of the generic PWE3 maintenance protocol.
7.5 PSN bandwidth utilization
1. The encapsulation layer SHOULD allow for an effective trade-off
between the following requirements:
1. Effective PSN bandwidth utilization. Assuming that the size
of encapsulation layer header does not depend on the size of
its payload, increase in the packet payload size results in
increased efficiency.
2. Low edge-to-edge latency. Low end-to-end latency is the
common requirement for Voice applications over TDM services.
Packetization latency is one of the components comprising
edge- to-edge latency and decreases with the packet payload
size.
The compensation buffer used by the CE-bound IWF increases
latency to the emulated circuit. Additional delay introduced by
this buffer SHOULD NOT exceed the packet delay variation observed
in the PSN.
2. The encapsulation layer SHOULD provide for saving the PSN
bandwidth by not sending corrupted TDM data across the PSN.
3. The encapsulation layer MAY provide the ability to save the PSN
bandwidth for the structured case by not sending sub-circuits
that are inactive.
7.6 Packet Delay Variation
In accordance with the PWE3 principles, the PWs do not exert any
control over the underlying PSN. In particular, the encapsulation
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layer for edge-to-edge-emulation of TDM circuits does neither affect
one-way delay of packets from ingress to egress PE, nor its
variation.
The encapsulation layer SHOULD provide for ability to compensate for
the packet delay variation without affecting jitter and wander of the
egress end service clock.
The encapsulation layer MAY provide for run-time adaptation of delay
introduced by the jitter buffer if the packet delay variation varies
with time. Such an adaptation MAY introduce low level of errors
(within the limits tolerated by the application) but SHOULD NOT
introduce additional wander of the egress end service clock.
7.7 Compatibility with the Existing PSN Infrastructure
The combination of encapsulation and PSN tunnel layers used for
edge-to-edge emulation of TDM circuits SHOULD be compatible with the
existing PSN infrastructures. In particular, compatibility with the
mechanisms of header compression over links where capacity is at a
premium SHOULD be provided.
7.8 Congestion Control
Edge-to-edge emulation of TDM circuits may result in constant bit
rate flows in the PSN. When transfered over the Internet congestion
control MUST be provided by appropriate means. It MUST be avoided
that all pseudo wires in the congested network are switched down
simultaneously or are reestablished again simultaneously to avoid
unstable behaviour of the network.
[###Editor's note: This section needs to be expanded. ###]
7.9 Fault Detection and Handling
The encapsulation layer for edge-to-edge emulation of TDM services
SHOULD, separately or in conjunction with the lower layers of the
PWE3 stack, provide for detection, handling and reporting of the
following defects:
1. Misconnection, or Stray Packets. Importance of this requirement
stems from the customers' expectations based upon powerful means
of misconnection detection in SONET/SDH networks.
2. Loss of packets. Importance of this requirement stems from the
providers' need to distinguish between various causes of the
end-to-end outage of the emulated service.
3. Malformed packets.
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4. Loss of synchronization.
7.10 Performance Monitoring
The encapsulation layer for edge-to-edge emulation of TDM services
SHOULD provide for collection of performance monitoring (PM) data
that is compatible with the parameters defined for 'classic', TDM-
based carriers of these services. The applicability of [G.826] is
left for further study.
8. Security Considerations
The security considerations listed in [PWE3-REQ] fully apply also to
emulation of TDM circuits.
9. References
[PWE3-REQ] draft-ietf-pwe3-requirements-04.txt XiPeng Xiao et al,
Requirements for Pseudo Wire Emulation Edge-to- Edge (PWE3), Work in
Progress, December 2002
[PWE3-ARCH] draft-ietf-pwe3-arch-02.txt Stewart Bryant et al, PWE3
Architecture, Work in progress, February 2002
[RFC1958] B. Carpenter (ed.). Architectural Principles of the
Internet, RFC 1958, IETF, 1996
[RFC2119] S.Bradner, Key Words in RFCs to Indicate Requirement
Levels, RFC 2119, IETF, 1997
[RFC2736] M. Handley, C. Perkins, Guidelines for Writers of RTP
Payload Format Specifications, RFC 2736, IETF, 1999
[RFC3393] C. Demichelis, P. Chimento, IP Packet Delay Variation
Metric for IPPM, RFC 3393, IETF, 2002
[GR253] Telecordia Technologies, "Synchronous Optical Network (SONET)
Transport Systems: Common Generic Criteria", GR-253-CORE, Issue 3,
(09/00)
[G.702] ITU-T Recommendation G.702 (11/88) - Digital hierarchy bit
rates
[G.704] ITU-T Recommendation G.704 (10/98) - Synchronous frame
structures used at 1544, 6312, 2048, 8448 and 44 736 Kbit/s
hierarchical levels
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[G.707] ITU-T Recommendation G.707 (10/00) - Network node interface
for the synchronous digital hierarchy (SDH)
[G.751] ITU-T Recommendation G.751 (11/88) - Digital multiplex
equipments operating at the third order bit rate of 34 368 Kbit/s and
the fourth order bit rate of 139 264 Kbit/s and using positive
justification
[G.752] ITU-T Recommendation G.752 (11/88) - Characteristics of
digital multiplex equipments based on a second order bit rate of 6312
kbit/s and using positive justification
[G.823] ITU-T Recommendation G.823 (03/00) - The control of jitter
and wander within digital networks which are based on the 2048 kbit/s
hierarchy
[G.824] ITU-T Recommendation G.824 (03/00) - The control of jitter
and wander within digital networks which are based on the 1544 kbit/s
hierarchy
[G.826] ITU-T Recommendation G.826 (02/99) - Error performance
parameters and objectives for international, constant bit rate
digital paths at or above the primary rate
[Q.700] ITU-T Recommendation Q.700 (03/93) - Introduction to CCITT
Signalling System No. 7
[T1.107] ANSI T1.107 - 1995. Digital Hierarchy - Format Specification
Authors' Addresses
Maximilian Riegel
Siemens AG
Hofmannstr. 51
Munich 81359
Germany
Phone: +49-89-722-49557
EMail: maximilian.riegel@siemens.com
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Alexander (Sasha) Vainshtein
Axerra Networks
24 Raoul Wallenberg St.
Tel Aviv 69719
Israel
Phone: +972-3-7569993
EMail: sasha@axerra.com
Yaakov (Jonathan) Stein
RAD Data Communications
24 Raoul Wallenberg St., Bldg. C
Tel Aviv 69719
Israel
Phone: +972-3-645-5389
EMail: yaakov_s@rad.co.il
Prayson Pate
Overture Networks, Inc.
P.O. Box 14864
RTP, NC 27709
USA
EMail: prayson.pate@overturenetworks.com
Tom Johnson
Litchfield Communications, Inc.
27 Princeton Rd.
Princeton Center West
Watertown, CT 06795
USA
EMail: tom_johnson@litchfieldcomm.com
Ron Cohen
Lycium Networks
9 Hamanofim St.
Herzelia, NC 46733
Israel
Phone: +972-9-971-7794
EMail: ronc@lyciumnetworks.com
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Tim Frost
Zarlink Semiconductor
Tamerton Road
Roborough, Plymouth PL6 7BQ
UK
EMail: tim.frost@zarlink.com
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