Network Working Group A. Vainshtein (Axerra Networks) Y(J) Stein (RAD Data Communications) Internet Draft Editors Expiration Date: March 2006 September 2005 Structure-Agnostic TDM over Packet (SAToP) draft-ietf-pwe3-satop-03.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document describes a pseudowire encapsulation for TDM (T1, E1, T3, E3) bit-streams that disregards any structure that may be imposed on these streams, in particular the structure imposed by the standard TDM framing. Co-Authors The following are co-authors of this document: Motty Anavi RAD Data Communications Tim Frost Zarlink Semiconductors Eduard Metz TNO Telecom Prayson Pate Overture Networks Akiva Sadovski Israel Sasson Axerra Networks Ronen Shashoua RAD Data Communications Vainshtein & Stein Standards Track [Page 1] Structure-Agnostic TDM over Packet September 2005 TABLE OF CONTENTS 1. Introduction......................................................2 2. Terminology and Reference Models..................................3 2.1. Terminology...................................................3 2.2. Reference Models..............................................3 3. Emulated Services.................................................4 4. SAToP Encapsulation Layer.........................................4 4.1. SAToP Packet Format...........................................4 4.2. PSN and Multiplexing Layer Headers............................5 4.3. SAToP Header..................................................5 4.3.1. Usage and Structure of the Control Word...................7 4.3.2. Usage of RTP Header.......................................8 5. SAToP Payload Layer...............................................9 5.1. General Payloads..............................................9 5.2. Octet-aligned T1.............................................10 6. SAToP Operation..................................................11 6.1. Common Considerations........................................11 6.2. IWF operation................................................11 6.2.1. PSN-bound Direction......................................11 6.2.2. CE-bound Direction.......................................11 6.3. SAToP Defects................................................12 6.4. SAToP PW Performance Monitoring..............................13 7. QoS Issues.......................................................14 8. Congestion Control...............................................14 9. Security Considerations..........................................14 10. Applicability Statement.........................................14 11. IANA Considerations.............................................16 12. Disclaimer of Validity..........................................16 13. NORMATIVE REFERENCES............................................16 14. INFORMATIONAL REFERENCES........................................18 Annex A. Old Mode of SATOP Encapsulation over L2TPv3................18 1. Introduction This document describes a method for encapsulating TDM bit-streams (T1, E1, T3, E3) as pseudo-wires over packet-switching networks (PSN). It addresses only structure-agnostic transport, i.e., the protocol completely disregards any structure that may possibly be imposed on these signals, in particular the structure imposed by standard TDM framing [G.704]. This emulation is referred to as "emulation of unstructured TDM circuits" in [PWE3-TDM-REQ] and suits applications where the PEs have no need to interpret TDM data or to participate in the TDM signaling. The SAToP solution presented in this document conforms to the PWE3 architecture described in [PWE3-ARCH] and satisfies both the relevant general requirements put forward in [PWE3-REQ] and specific requirements for unstructured TDM signals presented in [PWE3-TDM-REQ]. Vainshtein & Stein Expires March 2006 [Page 2] Structure-Agnostic TDM over Packet September 2005 As for all PWs, SAToP PWs may be manually configured or setup using the PWE3 control protocol. Extensions to the PWE3 control protocol required for setup and maintenance of SAToP pseudo-wires and allocations of code points used for this purpose are described in separate documents ([PWE3-TDM-CONTROL] and [PWE3-IANA] respectively). 2. Terminology and Reference Models 2.1. Terminology "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 following acronyms used in this document are defined in [PWE3-ARCH] and [PWE3-TDM-REQ]: ATM Asynchronous Transfer Mode CE Customer Edge CES Circuit Emulation Service NSP Native Service Processing PE Provider Edge PDH Plesiochronous Digital Hierarchy PW Pseudo-Wire SDH Synchronous Digital Hierarchy SONET Synchronous Optical Network TDM Time Domain Multiplexing In addition, the following TDM-specific terms are needed: o Loss of Signal (LOS) - a condition of the TDM attachment circuit wherein the incoming signal cannot be detected. Criteria for entering and leaving the LOS condition can be found in [G.775] o Alarm Indication Signal (AIS) - a special bit pattern (e.g. as described in [G.775]) in the TDM bit stream that indicates presence of an upstream circuit outage. For E1, T1 and E3 circuits the AIS pattern is a sequence of binary "1" values of appropriate duration (the "all ones" pattern) and hence it can be detected and generated by structure-agnostic means. The T3 AIS pattern requires T3 framing (see [G.704], Section 2.5.3.6.1) and hence can only be handled by a structure-aware NSP. We also use the term Interworking Function (IWF) to describe the functional block that segments and encapsulates TDM into SAToP packets and in the reverse direction decapsulates SAToP packets and reconstitutes TDM. 2.2. Reference Models The generic models defined in Sections 4.1, 4.2 and 4.4 of [PWE3-ARCH] fully apply to SAToP. Vainshtein & Stein Expires March 2006 [Page 3] Structure-Agnostic TDM over Packet September 2005 The native service addressed in this document is a special case of the bit stream payload type defined in Section 3.3.3 of [PWE3-ARCH]. The Network Synchronization reference model and deployment scenarios for emulation of TDM services are described in [PWE3-TDM-REQ], Section 4.2. 3. Emulated Services This specification describes edge-to-edge emulation of the following TDM services described in [G.702]: 1. E1 (2048 kbit/s) 2. T1 (1544 kbit/s) This service is also known as DS1 3. E3 (34368 kbit/s) 4. T3 (44736 kbit/s) This service is also known as DS3. The protocol used for emulation of these services does not depend on the method in which attachment circuits are delivered to the PEs. For example, a T1 attachment circuit is treated in the same way regardless of whether it is delivered to the PE on copper [G.703], multiplexed in a T3 circuit [T.107], mapped into a virtual tributary of a SONET/SDH circuit [G.707] or carried over an ATM network using unstructured ATM Circuit Emulation Service (CES) [ATM-CES]. Termination of any specific "carrier layers" used between the PE and CE is performed by an appropriate NSP. 4. SAToP Encapsulation Layer 4.1. SAToP Packet Format The basic format of SAToP packets is shown in Fig. 1 below. Vainshtein & Stein Expires March 2006 [Page 4] Structure-Agnostic TDM over Packet September 2005 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | | PSN and multiplexing layer headers | | ... | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | ... | +-- --+ | SAToP Encapsulation Header | +-- --+ | ... | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | Packetized TDM data (Payload) | | ... | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1. Basic SAToP Packet Format 4.2. PSN and Multiplexing Layer Headers Both UDP and L2TPv3 can provide the multiplexing mechanisms for SAToP PWs over an IPv4/IPv6 PSN. The PW label provides the multiplexing mechanism over an MPLS PSN as described in Section 5.4.2 of [PWE3- ARCH]. The total size of a SAToP packet for a specific PW MUST NOT exceed path MTU between the pair of PEs terminating this PW. SAToP implementations using IPv4 PSN MUST mark the IPv4 datagrams they generate as "Don't Fragment" [RFC791]. 4.3. SAToP Header The SAToP header MUST contain the SAToP Control Word (4 bytes) and MAY also contain a fixed RTP header [RFC3550]. If the RTP header is included in the SAToP header, it MUST immediately follow the SAToP control word in all cases except UDP demultiplexing, where it MUST precede it (see Fig. 2a, Fig. 2b and Fig. 2c below). Note: Such an arrangement complies with the traditional usage of RTP for the IPv4/IPv6 PSN with UDP demultiplexing while making SAToP PWs ECMP-safe for the MPLS PSN by providing for PW-IP packet discrimination(see [PWE3-ARCH], Section 5.4.3) and facilitating smoothless stitching of L2TPv3-based and MPLS-based segments of SAToP PWs (see [PWE3-MS]). Vainshtein & Stein Expires March 2006 [Page 5] Structure-Agnostic TDM over Packet September 2005 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | | IPv4/IPv6 and UDP (demultiplexing layer) headers | | ... | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | OPTIONAL | +-- --+ | | +-- --+ | Fixed RTP Header (see [RFC3550]) | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | SAToP Control Word | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | Packetized TDM data (Payload) | | ... | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2a. SAToP Packet Format for an IPv4/IPv6 PSN with UDP DeMultiplexing 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | | IPv4/IPv6 and L2TPv3 (demultiplexing layer) headers | | ... | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | SAToP Control Word | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | OPTIONAL | +-- --+ | | +-- --+ | Fixed RTP Header (see [RFC3550]) | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | Packetized TDM data (Payload) | | ... | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2b. SAToP Packet Format for an IPv4/IPv6 PSN with L2TPv3 DeMultiplexing Vainshtein & Stein Expires March 2006 [Page 6] Structure-Agnostic TDM over Packet September 2005 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | | MPLS Label Stack | | ... | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | SAToP Control Word | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | OPTIONAL | +-- --+ | | +-- --+ | Fixed RTP Header (see [RFC3550]) | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | Packetized TDM data (Payload) | | ... | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2c. SAToP Packet Format for an MPLS PSN 4.3.1. Usage and Structure of the Control Word Usage of the SAToP control word allows: 1. Detection of packet loss or mis-ordering 2. Differentiation between the PSN and attachment circuit problems as causes for the outage of the emulated service 3. PSN bandwidth conservation by not transferring invalid data (AIS) 4. Signaling of faults detected at the PW egress to the PW ingress. The structure of the SAToP Control Word is shown in Fig. 3 below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0|0|0|L|R|RSV|FRG| LEN | Sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3. Structure of the SAToP Control Word Bits 0 to 3 MUST be set to 0 as described in [PWE3-ARCH], Section 5.4.4 L - if set, indicates that TDM data carried in the payload is invalid due an attachment circuit fault. When the L bit is set the payload MAY be omitted in order to conserve bandwidth. The CE-bound IWF MUST play out an appropriate amount of filler data regardless of the payload size. Once set, if the fault is rectified the L bit MUST be cleared. Vainshtein & Stein Expires March 2006 [Page 7] Structure-Agnostic TDM over Packet September 2005 Note: This document does not specify which TDM fault conditions are treated as invalidating the data carried in the SAToP packets. Possible examples include, but are not limited to LOS and AIS. R - if set by the PSN-bound IWF, indicates that its local CE-bound IWF is in the packet loss state, i.e. has lost a preconfigured number of consecutive packets. The R bit MUST be cleared by the PSN-bound IWF once its local CE-bound IWF has exited the packet loss state, i.e. has received a preconfigured number of consecutive packets. RSV (reserved) and FRG (fragmentation) bits (6 to 10) - MUST be set to 0 by the PSN-bound IWF and MUST be ignored by the CE-bound IWF. LEN (bits (10 to 15) MAY be used to carry the length of the SAToP packet (defined as the size of the SAToP header + the payload size) if it is less than 64 bytes, and MUST be set to zero otherwise. When the LEN field is set to 0, the preconfigured size of the SAToP packet payload MUST be assumed, and if the actual packet size is inconsistent with this length, the packet MUST be considered to be malformed. Sequence number provides the common PW sequencing function and allows detection of lost packets. It MUST be generated in accordance with the rules defined in [RFC3550], Section 5 for the RTP sequence number. VCCV packets (see [PWE3-VCCV]) in SAToP PWs MUST be discriminated by setting the first nibble of the control word to '0001' regardless of the PSN type and demultiplexing layer. 4.3.2. Usage of RTP Header When RTP is used, SAToP requires the fields of the fixed RTP header (see [RFC3550], Section 5.1) with P (padding), X (header extension), CC (CSRC count), and M fields (marker) to be set to zero. The PT (payload type) field is used as following: 1. One PT value MUST be allocated from the range of dynamic values (see [RTP-TYPES]) for each direction of the PW. The same PT value MAY be reused for both directions of the PW and also reused between different PWs 2. The PSN-bound IWF MUST set the PT field in the RTP header to the allocated value 3. The CE-bound IWF MAY use the received value to detect malformed packets The sequence number field MAY be used to provide the common PW sequencing function as well as detection of lost packets. It MUST be generated in accordance with the rules established in [RFC3550] and MUST be the same as the sequence number in the SAToP control word. Timestamps are used for carrying timing information over the network. Their values are generated in accordance with the rules established in [RFC3550]. Vainshtein & Stein Expires March 2006 [Page 8] Structure-Agnostic TDM over Packet September 2005 The frequency of the clock used for generating timestamps MUST be an integer multiple of 8 kHz. All implementations of SAToP MUST support the 8 kHz clock. Other multiples of 8 kHz MAY be used. The SSRC (synchronization source) value in the RTP header MAY be used for detection of misconnections. Timestamp generation MAY be used in the following modes: 1. Absolute mode: the PSN-bound IWF sets timestamps using the clock recovered from the incoming TDM attachment circuit. As a consequence, the timestamps are closely correlated with the sequence numbers. All SAToP implementations that support usage of the RTP header MUST support this mode. 2. Differential mode: Both IWFs have access to a common high- quality timing source, and this source is used for timestamp generation. Support of this mode is OPTIONAL. Usage of the fixed RTP header in a SAToP PW and all the options associated with its usage (the time-stamping clock frequency, the time- stamping mode, selected PT and SSRC values) MUST be agreed upon between the two SAToP IWFs at the PW setup as described in [PWE3-TDM-CONTROL]. Other, RTP-specific, methods (e.g., see [RFC 3551]) MUST NOT be used. 5. SAToP Payload Layer 5.1. General Payloads In order to facilitate handling of packet loss in the PSN, all packets belonging to a given SAToP PW are REQUIRED to carry a fixed number of bytes filled with TDM data received from the attachment circuit. The packet payload size MUST be defined during the PW setup, MUST be the same for both directions of the PW and MUST remain unchanged for the lifetime of the PW. The CE-bound and PSN-bound IWFs MUST agree on SAToP packet payload size at the PW setup (default payload size values defined below guarantee that such an agreement is always possible). The SAToP packet payload size can be exchanged over the PWE3 control protocol ([PWE3-TDM- CONTROL]) by using the CEP/TDM Payload Bytes sub-TLV of the Interface Parameters TLV([PWE3-IANA]). SAToP uses the following ordering for packetization of the TDM data: o The order of the payload bytes corresponds to their order on the attachment circuit o Consecutive bits coming from the attachment circuit fill each payload byte starting from most significant bit to least significant. All SAToP implementations MUST be capable of supporting the following payload sizes: o E1 - 256 bytes Vainshtein & Stein Expires March 2006 [Page 9] Structure-Agnostic TDM over Packet September 2005 o T1 - 192 bytes o E3 and T3 - 1024 bytes. Notes: 1. Whatever the selected payload size, SAToP does not assume alignment to any underlying structure imposed by TDM framing (byte, frame or multiframe alignment). 2. When the L bit in the SAToP control word is set, SAToP packets MAY omit invalid TDM data in order to conserve PSN bandwidth. 3. Payload sizes that are multiples of 47 bytes MAY be used in conjunction with unstructured ATM-CES [ATM-CES]. 5.2. Octet-aligned T1 An unstructured T1 attachment circuit is sometimes provided already padded to an integer number of bytes, as described in Annex B of [G.802]. This occurs when the T1 is de-mapped from a SONET/SDH virtual tributary/container, or when it is deframed by a dual-mode E1/T1 framer. In order to facilitate operation in such cases, SAToP defines a special "octet-aligned T1" transport mode. When operating in this mode, the SAToP payload consists of a number of 25-byte subframes, each subframe carrying 193 bits of TDM data and 7 bits of padding. This mode is depicted in Fig. 4 below. | 1 | 2 | ... | 25 | |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7| ... |0 1 2 3 4 5 6 7| |=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | TDM Data | padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ................................. | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TDM Data | padding | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ Figure 4. SAToP Payload Format for Octet-Aligned T1 Transport Notes: 1. No alignment with the framing structure that may be imposed on the T1 bit-stream is implied. 2. An additional advantage of the octet-aligned T1 transport mode is ability to select the SAToP packetization latency as an arbitrary integer multiple of 125 microseconds. Support of the octet-aligned T1 transport mode is OPTIONAL. An octet- aligned T1 SAToP PW is not interoperable with a T1 SAToP PW that carries a non-aligned bit-stream, as described in the previous section. Implementations supporting octet-aligned T1 transport mode MUST be capable of supporting a payload size of 200 bytes (i.e., a payload of Vainshtein & Stein Expires March 2006 [Page 10] Structure-Agnostic TDM over Packet September 2005 eight 25-byte subframes) corresponding to precisely 1 millisecond of TDM data. 6. SAToP Operation 6.1. Common Considerations Edge-to-edge emulation of a TDM service using SAToP is only possible when the two PW attachment circuits are of the same type (T1, E1, T3, E3). The service type is exchanged at PW setup as described in [PWE3- CONTROL]. 6.2. IWF operation 6.2.1. PSN-bound Direction Once the PW is set up, the PSN-bound SAToP IWF operates as follows: TDM data is packetized using the configured number of payload bytes per packet. Sequence numbers, flags, and timestamps (if the RTP header is used) are inserted in the SAToP headers. SAToP, multiplexing layer and PSN headers are prepended to the packetized service data. The resulting packets are transmitted over the PSN. 6.2.2. CE-bound Direction The CE-bound SAToP IWF SHOULD include a jitter buffer where the payload of the received SAToP packets is stored prior to play-out to the local TDM attachment circuit. The size of this buffer SHOULD be locally configurable to allow accommodation to the PSN-specific packet delay variation. The CE-bound SAToP IWF SHOULD use the sequence number in the control word for detection of lost and mis-ordered packets. If the RTP header is used, the RTP sequence numbers MAY be used for the same purposes. The CE-bound SAToP IWF MAY re-order mis-ordered packets. Mis-ordered packets that cannot be reordered MUST be discarded and treated as lost. The payload of the received SAToP packets marked with the L bit set SHOULD be replaced by the equivalent amount of the "all ones" pattern even if it has not been omitted. The payload of each lost SAToP packet MUST be replaced with the equivalent amount of the replacement data. The contents of the replacement data are implementation-specific and MAY be locally Vainshtein & Stein Expires March 2006 [Page 11] Structure-Agnostic TDM over Packet September 2005 configurable. By default, all SAToP implementations MUST support generation of the "all ones" pattern as the replacement data. Before a PW has been set up and after a PW has been torn down, the IWF MUST play out the "all ones" pattern to its TDM attachment circuit. Once the PW has been set up, the CE-bound IWF begins to receive SAToP packets and to store their payload in the jitter buffer but continues to play out the "all ones" pattern to its TDM attachment circuit. This intermediate state persists until a preconfigured amount of TDM data (usually half of the jitter buffer) has been received in consecutive SAToP packets or until a preconfigured intermediate state timer (started when the PW setup is completed) expires. Once the preconfigured amount of the TDM data has been received, the CE-bound SAToP IWF enters its normal operation state where it continues to receive SAToP packets and to store their payload in the jitter buffer while playing out the contents of the jitter buffer in accordance with the required clock. In this state the CE-bound IWF performs clock recovery, MAY monitor PW defects, and MAY collect PW performance monitoring data. If the CE-bound SAToP IWF detects loss of a preconfigured number of consecutive packets or if the intermediate state timer expires before the required amount of TDM data has been received, it enters its packet loss state. While in this state, the local PSN-bound SAToP IWF SHOULD mark every packet it transmits with the R bit set. The CE-bound SAToP IWF leaves this state and transits to the normal one once a preconfigured number of consecutive valid SAToP packets have been received. (Successfully re-ordered packets contribute to the count of consecutive packets.) The CE-bound SAToP IWF MUST provide an indication of TDM data validity to the CE. This can be done by transporting or by generating the native AIS indication. As mentioned above, T3 AIS cannot be detected or generated by structure-agnostic means and hence a structure-aware NSP MUST be used when generating a valid AIS pattern. 6.3. SAToP Defects In addition to the packet loss state of the CE-bound SAToP IWF defined above, it MAY detect the following defects: o Stray packets o Malformed packets o Excessive packet loss rate o Buffer overrun o Remote packet loss. Corresponding to each defect is a defect state of the IWF, a detection criterion that triggers transition from the normal operation state to the appropriate defect state, and an alarm that MAY be reported to the Vainshtein & Stein Expires March 2006 [Page 12] Structure-Agnostic TDM over Packet September 2005 management system and thereafter cleared. Alarms are only reported when the defect state persists for a preconfigured amount of time (typically 2.5 seconds) and MUST be cleared after the corresponding defect is undetected for a second preconfigured amount of time (typically 10 seconds). The trigger and release times for the various alarms may be independent. Stray packets MAY be detected by the PSN and multiplexing layers. When RTP is used, the SSRC field in the RTP header MAY be used for this purpose as well. Stray packets MUST be discarded by the CE-bound IWF and their detection MUST NOT affect mechanisms for detection of packet loss. Malformed packets are detected by mismatch between the expected packet size (taking the value of the L bit into account) and the actual packet size inferred from the PSN and multiplexing layers. When RTP is used, lack of correspondence between the PT value and that allocated for this direction of the PW MAT also be used for this purpose. Malformed in- order packets MUST be discarded by the CE-bound IWF and replacement data generated as for lost packets. Excessive packet loss rate is detected by computing the average packet loss rate over a configurable amount of times and comparing it with a preconfigured threshold. Buffer overrun is detected in the normal operation state when the CE bound IWF's jitter buffer cannot accommodate newly arrived SAToP packets. Remote packet loss is indicated by reception of packets with their R bit set. 6.4. SAToP PW Performance Monitoring Performance monitoring (PM) parameters are routinely collected for TDM services and provide an important maintenance mechanism in TDM networks. Ability to collect compatible PM parameters for SAToP PWs enhances their maintenance capabilities. Collection of the SAToP PW performance monitoring parameters is OPTIONAL, and if implemented, is only performed after the CE-bound IWF has exited its intermediate state. SAToP defines error events, errored blocks and defects as follows: o A SAToP error event is defined as insertion of a single replacement packet into the jitter buffer (replacement of payload of SAToP packets with the L bit set is not considered as insertion of a replacement packet) o A SAToP errored data block is defined as a block of data played out to the TDM attachment circuit and of size defined in accordance with the [G.826] rules for the corresponding TDM service that has experienced at least one SAToP error event Vainshtein & Stein Expires March 2006 [Page 13] Structure-Agnostic TDM over Packet September 2005 o A SAToP defect is defined as the packet loss state of the CE- bound SAToP IWF. The SAToP PW PM parameters (Errored, Severely Errored and Unavailable Seconds) are derived from these definitions in accordance with [G.826]. 7. QoS Issues SAToP SHOULD exploit existing QoS capabilities of the underlying PSN. If the PSN providing connectivity between PE devices is Diffserv- enabled and provides a PDB [RFC3086] that guarantees low-jitter and low-loss, the SAToP PW SHOULD use this PDB in compliance with the admission and allocation rules the PSN has put in place for that PDB (e.g., marking packets as directed by the PSN). If the PSN is Intserv-enabled, then GS (Guaranteed Service) [RFC 2212] with the appropriate bandwidth reservation SHOULD be used in order to provide a bandwidth guarantee equal or greater than that of the aggregate TDM traffic. 8. Congestion Control SAToP PWs represent a special case of PWs carrying constant bit rate (CBR) services across the PSN. These services cannot behave in a TCP- friendly manner prescribed by [RFC2914] under congestion. SAToP will use the generic PWE3 approach for handling congestion in PWs carrying CBR services when such an approach has been specified. 9. Security Considerations SAToP does not enhance or detract from the security performance of the underlying PSN, rather it relies upon the PSN mechanisms for encryption, integrity, and authentication whenever required. Misconnection detection capabilities of SAToP increase its resilience to misconfiguration and some types of DoS attacks. Random initialization of sequence numbers defined in [RFC3550] makes known-plaintext attacks on encryption more difficult. 10. Applicability Statement SAToP is an encapsulation layer intended for carrying TDM circuits (E1/T1/E3/T3) over PSN in a structure-agnostic fashion. SAToP fully complies with the principle of minimal intervention, thus minimizing overhead and computational power required for encapsulation. Vainshtein & Stein Expires March 2006 [Page 14] Structure-Agnostic TDM over Packet September 2005 SAToP provides sequencing and synchronization functions needed for emulation of TDM bit-streams, including detection of lost or mis- ordered packets and appropriate compensation. TDM bit-streams carried over SAToP PWs may experience delays exceeding those typical of native TDM networks. These delays include the SAToP packetization delay, edge-to-edge delay of the underlying PSN and the delay added by the jitter buffer. It is recommended to estimate both delay and delay variation prior to setup of a SAToP PW. SAToP carries TDM streams over PSN in their entirety including any TDM signaling contained within the data. Consequently the emulated TDM services are sensitive to the PSN packet loss. Appropriate generation of replacement data can be used to prevent shutting down the CE TDM interface due to occasional packet loss. Other effects of packet loss on this interface (e.g., errored blocks) cannot be prevented. Note: Structure-aware TDM emulation (see [CESoPSN] or [TDMoIP]) completely hides effects of the PSN packet loss on the CE TDM interface (because framing and CRCs are generated locally) and allows usage of application-specific packet loss concealment methods to minimize effects on the applications using the emulated TDM service. SAToP can be used in conjunction with various network synchronization scenarios (see [PWE3-RDM-REQ)] and clock recovery techniques. The quality of the TDM clock recovered by the SAToP IWF may be implementation-specific. If a common. The quality may be improved by using RTP if a common clock is available at both ends of the SAToP PW. SAToP provides for effective fault isolation by carrying the local attachment circuit failure indications. The option not to carry invalid TDM data enables PSN bandwidth conservation. SAToP allows collection of TDM-like faults and performance monitoring parameters hence emulating 'classic' carrier services of TDM. SAToP provides for a carrier-independent ability to detect misconnections and malformed packets. This feature increases resilience of the emulated service to misconfiguration and DoS attacks. Being a constant bit rate (CBR) service, SAToP cannot provide TCP- friendly behavior under network congestion. Faithfulness of a SAToP PW may be increased by exploiting QoS features of the underlying PSN. SAToP does not provide any mechanisms for protection against PSN outages, and hence its resilience to such outages is limited. However, lost-packet replacement and packet reordering mechanisms increase resilience of the emulated service to fast PSN rerouting events. Vainshtein & Stein Expires March 2006 [Page 15] Structure-Agnostic TDM over Packet September 2005 11. IANA Considerations Allocation of PW Types for the corresponding SAToP PWs is defined in [PWE3-IANA]. 12. Disclaimer of Validity The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. ACKNOWLEDGEMENTS We acknowledge the work of Gil Biran and Hugo Silberman who implemented TDM transport over IP in 1998. We would like to thank Alik Shimelmits for many productive discussions and Ron Insler for his assistance in deploying TDM over PSN. We express deep gratitude to Stephen Casner who has reviewed in detail one of the predecessors of this document and provided valuable feedback regarding various aspects of RTP usage, and to Kathleen Nichols who has provided the current text of the QoS section considering Diffserv- enabled PSN. We thank William Bartholomay, Robert Biksner, Stewart Bryant, Rao Cherukuri, Ron Cohen, Alex Conta, Shahram Davari, Tom Johnson, Sim Narasimha, Yaron Raz, and Maximilian Riegel for their valuable feedback. 13. NORMATIVE REFERENCES [RFC791] J. Postel (ed), Internet Protocol, RFC 791, IETF, 1981 Vainshtein & Stein Expires March 2006 [Page 16] Structure-Agnostic TDM over Packet September 2005 [RFC1122] R. Braden (ed.), Requirements for Internet Hosts -- Communication Layers, RFC 1122, IETF, 1989 [RFC2119] S.Bradner, Key Words in RFCs to Indicate Requirement Levels, RFC 2119, IETF, 1997 [RFC2112] S. Shenker et al, Specification of Guaranteed Quality of Service, IETF, RFC 2212, 1997 [RFC2914] S. Floyd, Congestion Control Principles, RFC 2914, IETF, 2000 [RFC3086] K. Nichols, B. Carpenter, Definition of Differentiated Services Per Domain Behaviors and Rules for their Specification, RFC 3086, IETF, 2001 [RFC3550] H. Schulzrinne et al, RTP: A Transport Protocol for Real-Time Applications, RFC 3550, IETF, 2003 [RTP-TYPES] RTP PARAMETERS, http://www.iana.org/assignments/rtp- parameters [G.702] ITU-T Recommendation G.702 (11/88) - Digital Hierarchy Bit Rates [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.707] ITU-T Recommendation G.707 (03/96) - 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 34368 kbit/s and the Fourth Order Bit Rate of 139264 kbit/s and Using Positive Justification [G.775] ITU-T Recommendation G.775 (10/98) - Loss of Signal (LOS), Alarm Indication Signal (AIS) and Remote Defect Indication (RDI) Defect Detection and Clearance Criteria for PDH Signals [G.802] ITU-T Recommendation G.802 (11/88) - Interworking between Networks Based on Different Digital Hierarchies and Speech Encoding Laws [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 [T1.107] American National Standard for Telecommunications - Digital Hierarchy - Format Specifications, ANSI T1.107-1988 Vainshtein & Stein Expires March 2006 [Page 17] Structure-Agnostic TDM over Packet September 2005 14. INFORMATIONAL REFERENCES [PWE3-REQ] XiPeng Xiao et al, Requirements for Pseudo Wire Emulation Edge-to-Edge (PWE3), IETF RFC 3916, 2004 [PWE3-TDM-REQ] Maximilian Riegel, Requirements for Edge-to-Edge Emulation of TDM Circuits over Packet Switching Networks (PSN), Work in Progress, April 2005, draft-ietf-pwe3-tdm-requirements-08.txt [PWE3-ARCH] S. Bryant, P. Pate, PWE3 Architecture, IETF RFC 3985, 2005 [PWE3-CONTROL] L. Martini et al, Pseudowire Setup and Maintenance using LDP, Work in progress, June 2005, draft-ietf-pwe3-control-protocol- 17.txt [PWE3-IANA] L. Martini, M. Townsley, IANA Allocations for pseudo Wire Edge to Edge Emulation (PWE3), Work in progress, September 2005, draft- ietf-pwe3-iana-allocation-12.txt [ATM-CES] ATM forum specification af-vtoa-0078 (CES 2.0) Circuit Emulation Service Interoperability Specification Ver. 2.0 [CESoPSN] A.Vainshtein et al, TDM Circuit Emulation Service over Packet Switched Network (CESoPSN), Work in Progress, July 2005, draft-ietf- pwe3-cesopsn-03.txt [TDMoIP] Y. Stein, TDMoIP, Work in Progress, February 2005, draft-ietf- pwe3-tdmoip-03.txt [PWE3-TDM-CONTROL] A. Vainshtein, Y. Stein, Control Protocol Extensions for Setup of TDM Pseudowires, Work in Progress, July 2005, draft-ietf- pwe3-tdm-control-protocol-extensi-00.txt [PWE3-MS] L. Martini et al, Segmented Pseudo Wire, Work in Progress, July 2005, draft-ietf-pwe3-segmented-pw-00.txt [PWE3-VCCV] T. Nadeau, R. Aggarwal, Pseudo Wire Virtual Circuit Connectivity, Work in Progress, August 2005, draft-ietf-pwe3-vccv- 05.txt [RFC3551] H. Schulzrinne, S. Casner, RTP Profile for Audio and Video Conferences with Minimal Control, RFC 3551, IETF, 2003 ANNEX A. OLD MODE OF SATOP ENCAPSULATION OVER L2TPV3 Previous versions of this specification defined a SAToP PW encapsulation over L2TPv3 which differs from one described in Section Vainshtein & Stein Expires March 2006 [Page 18] Structure-Agnostic TDM over Packet September 2005 4.3 and Diagram 2b. In these versions the RTP header, if used, precedes the SAToP control word. Existing implementations of the old encapsulation mode MUST be distinguished from the encapsulations conforming to this specification via the SAToP PW setup. Full Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Editors' Addresses Alexander ("Sasha") Vainshtein Axerra Networks 24 Raoul Wallenberg St., Tel Aviv 69719, Israel email: sasha@axerra.com Yaakov (Jonathan) Stein RAD Data Communications 24 Raoul Wallenberg St., Bldg C Tel Aviv 69719, Israel Email: yaakov_s@rad.com Vainshtein & Stein Expires March 2006 [Page 19]