Requirements for Edge-to-Edge Emulation of TDM Circuits over Packet Switching Networks (PSN)

This document specifies the particular requirements for edge-to-edge-emulation of circuits carrying time division multiplexed (TDM) digital signals over packet-switched networks. It is based on the common framework of PWE3 as defined in [PWE3-FW] and the considerations on protocol layering in PWE3 as discussed in [PWE3-LAYERS]. It makes references to requirements in [PWE3-REQ] where applicable and complements [PWE3-REQ] by defining additional requirements originating from specifics of TDM circuits. Circuits carrying TDM signals have been defined in such normative documents as [G.703], [G.704] and [T.107]. To support TDM traffic, which includes voice, data, and private leased line service, the network must emulate the circuit characteristics of a TDM network.

[PWE3-REQ] specifies common requirements for edge-to-edge-emulation of circuits of various types. However, these requirements, as well as references in [PWE3-FW] and [PWE3-LAYERS] do not fully cover all 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: 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. 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. 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. It is our intention that this document will be used in the PWE3 community discussions regarding specific proposals for edge-to-edge-emulation of TDM circuits.

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-FW], Section 1.4 are consistently used, usually without additional explanations. However: These terms are consistently used instead of 'outbound' and 'inbound' when describing traffic directions. These terms have been adopted in [PWE3-LAYERS]. This term is often used for describing the protocol operation with explicit references to CE-bound or PSN-bound direction of the IWF. Some terms and acronyms are commonly used in conjunction with the TDM services. In particular: 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?] 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.[## Ed-note##: more to be included here?] 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 [IPPM-IPDV]) are used.

Generic models that have been defined in [PWE3-FW] in Sections - 3.1 (Network Reference Model), - 3.2 (Maintenance Reference Model), - 3.4 (Protocol Stack Reference Model) and - 3.5 (Logical Protocol Layering 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 circuit-oriented payload type defined in Section 3.5.2.1 of [PWE3-FW].

The network synchronization reference model shown in Figure 1 below is copied, with some modifications, from one of the expired versions of [PWE3-FW]:

|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: Customer edge devices terminating TDM circuits to be emulated. Provider edge devices adapting these end services to PW. Provider core routers Physical interface terminating the TDM circuit. PSN-bound IWF of the PW 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: The clock used by CE1 for transmission of the TDM end circuit towards CE1. The clock recovered by PE1 from the incoming TDM end circuit. "A" and "B" always have the same frequency. The same as "A" and "B" respectively for CE2 and PE2 ("G" and "H" have the same frequency). Local oscillators available to PE1 and PE2 respectively. Clock used by PE2 to transmit the TDM end service circuit to CE2 (the recovered clock). Clock recovered by CE2 from the incoming TDM end service ("E and "F" have the same frequency). If it exists, it is the common network reference clock available to PE1 and PE2. The same as "E" and "F" respectively for PE1 and CE1 ("J" and "K" have the same frequency). 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:

The common network reference clock "I" is available to all the PE devices, and local oscillators "C" and "D" are locked to "I": One Synchronous Network: 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). Synchronous Carrier of Carriers: Clocks "A" and "G" are locked to the common reference clock "L" which is not locked to clock "I". More than one such clock is used by appropriate pairs of CE devices connected to the given pair of PE devices PE1 and PE2 (otherwise it could be used by PE1 and PE2 as their common network reference clock "I").

The asynchronous scenario is characterized by: No common network reference clock "I" is available to PE1 and PE2. More than one reference clock is used by CE devices connected to PE1 and PE2. 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.

[##Ed-note##: The section about network synchronization reference models is for further discussion.]

This document defines requirements for the payload and encapsulation layers for edge-to-edge emulation of TDM services with structured payload as well as unstructured 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. Applicability of these requirements to edge-to-edge-emulation of structured and unstructured SONET/SDH circuits is left for further study. The service-specific encapsulation layer for edge-to-edge emulation comprises the following TDM services over a PSN:

Transparent N*DS0, 1 <= N <= 31 as described in [G.704].This circuit can be accompanied by channel-associated or common channel CE application state signaling.

Unstructured E1 as described in [G.704]. Unstructured T1 (DS1) as described in [G.704]. Unstructured E3 as defined in [G.751]. Unstructured T3 (DS3) as described in [T.107].

The combination of encapsulation and payload layers for edge-to- edge-emulation of TDM circuits considered in this document should comply with the following common PW requirements defined in [PWE3-REQ]: Conveyance of Necessary L2/L1 Header Information: For unstructured circuits this functionality MAY be provided by the payload layer. For structured circuits, the necessary information MUST be provided by the encapsulation layer. Support of Multiplexing and Demultiplexing if supported by the native services: Relevant for N*DS0 circuits with or without signaling. For these circuits means that the combination of encapsulation and payload layers MUST provide for separate treatment of every DS0 sub-circuit. Intervention or transparent transfer of Control Messages of the Native Services depending on the particular scenario. Consideration of the PSN Tunnel Header Overhead (see also below). Detection and handling of PW faults. The list of faults is given in below. Clock Recovery (see also below). The following requirements listed in [PWE3-REQ] are not applicable to emulation of TDM services: Support of variable length PDUs, Segmentation and reassembly, Keep-alive, RFC 2914 conformance.

Structured circuits considered in this document belong to the generic 'Structured Bit-Stream Payload' type defined in [PWE3-LAYERS]. 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.

The combination of payload and encapsulation layers SHOULD comply with the general design principles of the Internet protocols as presented in [RFC1958], Section 3.

The emulation MUST support network interworking between end services of the same kind (structured, unstructured) and bit-rate. 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.

The encapsulation layer MUST provide synchronization services that are sufficient for: balancing of clock of ingress and egress end services regardless of the specific network synchronization scenario, keeping the jitter and wander of the clock of the egress service within the service-specific limits as defined by the appropriate normative references. 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).

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.

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 occasional loss of a single packet on the egress service, the encapsulation layer SHOULD: Allow independent interpretation of TDM data in each specific packet by the egress PE (see [RFC2736]. This requirement MAY be disregarded if the egress PE has to interpret structures that exceed the path MTU between the ingress and egress PEs. 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. Minimize possible effect of lost packets on recovery of the circuit clock by the egress PE depending on the actual network synchronization scheme deployed.

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: MUST be detected, SHOULD [##Ed-Note##: MAY?] 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.

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: Ability to support different signaling schemes with minimal impact on encapsulation of TDM data, Multiplexing of application-specific CE signals and data of the emulated service in the same PW, Synchronization (within the application-specific tolerance limits) between CE signals and data at the PW egress, Probabilistic recovery against possible occasional loss of packets in the PSN, 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.

The encapsulation layer SHOULD allow for an effective trade-off between the following requirements: 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. 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. The encapsulation layer SHOULD provide for saving the PSN bandwidth by not sending invalid TDM data across the PSN. The encapsulation layer MAY provide the ability to save the PSN bandwidth for the structured case by not sending TDM timeslots that are inactive.

In accordance with the PWE3 principles, the PWs do not exert any control over the underlying PSN. In particular, the encapsulation 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.

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.

Edge-to-edge emulation of TDM circuits may result in constant bit rate flows in the PSN. Such flows do not admit any meaningful congestion control.

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: Misconnection, or Stray Packets. Importance of this requirement stems from the customers' expectations based upon powerful means of misconnection detection in SONET/SDH networks. 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. Malformed packets. Loss of synchronization.

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.

[PWE3-REQ] draft-ietf-pwe3-requirements-02.txt XiPeng Xiao et al, Requirements for Pseudo Wire Emulation Edge-to- Edge (PWE3), Work in Progress, May 2002 [PWE3-FW] draft-ietf-pwe3-framework-01.txt Prayson Pate et al, Framework for Pseudo Wire Emulation Edge-to-Edge (PWE3), Work in progress, June 2002 [PWE3-LAYERS] draft-ietf-pwe3-protocol-layer-00.txt Stewart Bryant et al., Protocol Layering in PWE3, Work in Progress, May 2002 [IPPM-IPDV] draft-ietf-ippm-ipdv-08.txt C. Demichelis, P. Chimento, IP Packet Delay Variation Metric for IPPM, Work in progress, November 2001 [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 [G.703] ITU-T Recommendation G.703 (10/98) - Physical/electrical characteristics of hierarchical digital interfaces [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 [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.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.823 (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