Internet DRAFT - draft-ietf-pwe3-satop


     Network Working Group               A. Vainshtein (Axerra Networks)
                                    Y(J) Stein (RAD Data Communications)
     Internet Draft                                              Editors
     Expiration Date:                                                   
     August 2006                                                        
                                                           February 2006 
                Structure-Agnostic TDM over Packet (SAToP) 
 Status of this Memo 
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 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 
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 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 
 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 
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    Structure-Agnostic TDM over Packet                  February 2006 
 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................................................13 
   6.4. SAToP PW Performance Monitoring..............................13 
 7. QoS Issues.......................................................14 
 8. Congestion Control...............................................14 
 9. Security Considerations..........................................16 
 10. Applicability Statement.........................................16 
 11. IANA Considerations.............................................17 
 12. Disclaimer of Validity..........................................17 
 13. NORMATIVE REFERENCES............................................18 
 14. INFORMATIONAL REFERENCES........................................19 
 Annex A. Old Mode of SATOP Encapsulation over L2TPv3................20 
 ANNEX B. Parameters that must be agreed upon during the PW setup....21 
 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 [RFC4197] and suits applications where 
 the PEs have no need to interpret TDM data or to participate in the TDM 
 The SAToP solution presented in this document conforms to the PWE3 
 architecture described in [RFC3985] and satisfies both the relevant 
 general requirements put forward in [RFC3916] and specific requirements 
 for unstructured TDM signals presented in [RFC4197].  
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 As with all PWs, SAToP PWs may be manually configured or set up 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  
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",    
 document are to be interpreted as described in [RFC2119]. 
    2.1. Terminology 
 The following acronyms used in this document are defined in [RFC3985] 
 and [RFC4197]: 
 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 
 and hence  can only be handled by a structure-aware 
 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 

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 The generic models defined in Sections 4.1, 4.2 and 4.4 of [RFC3985] 
 fully apply to SAToP. 
 The native service addressed in this document is a special case of the 
 bit stream payload type defined in Section 3.3.3 of [RFC3985]. 
 The Network Synchronization reference model and deployment scenarios 
 for emulation of TDM services are described in [RFC4197], Section 4.3. 
 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 [T1.107], 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.  

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  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 [RFC3931] 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 [RFC3985]. 
 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] (see also [PWE3-FRAG]). 
    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 multiplexing, 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 multiplexing while making SAToP PWs 
 ECMP-safe for the MPLS PSN by providing for PW-IP packet discrimination 
 (see [RFC3985], Section 5.4.3) and facilitating seamless stitching of 
 L2TPv3-based and MPLS-based segments of SAToP PWs (see [PWE3-MS]). 

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  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 (multiplexing 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 Multiplexing 
  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 (multiplexing 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 

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  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 
      4. Signaling of faults detected at the PW egress to the PW 
 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 
 The use of Bits 0 to 3 is described in [PWE3-CW]. These bits MUST 
 be set to zero unless they are being used to indicate the start of an  
 Associated Channel Header (ACH). An ACH is needed if the state of the 
 SAToP PW is being monitored using Virtual Circuit Connectivity 
 Verification [PWE3-VCCV]. 
 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 
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     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. 
 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, see [PWE3-FRAG]) bits (6 to 9) - 
 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 as described in Section 5.1, and if the actual packet 
 size is inconsistent with this length, the packet MUST be considered to 
 be malformed. 
 The sequence number is used to provide the common PW sequencing 
 function as well as detection of lost packets. It MUST be generated in 
 accordance with the rules defined in Section 5.1 of [RFC3550], for the 
 RTP sequence number, i.e.: 
   o Its space is a 16-bit unsigned circular space 
   o Its initial value SHOULD be random (unpredictable). 
 It MUST be incremented with each SAToP data packet sent in the specific 
      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 
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 The sequence number MUST be the same as the sequence number in the 
 SAToP control word.  
 The RTP timestamps are used for carrying timing information over the 
 network. Their values are generated in accordance with the rules 
 established in [RFC3550].  
 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, i.e. incorrect interconnection of 
 attachment circuits. 
 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 [RFC3551]) 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 
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      o  Consecutive bits coming from the attachment circuit fill each 
          payload byte starting from most significant bit to least 
 All SAToP implementations MUST be capable of supporting the following 
 payload sizes: 
      o  E1 - 256 bytes 
      o  T1 - 192 bytes 
      o  E3 and T3 - 1024 bytes. 
      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 
 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 
 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. 
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 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 
 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-
    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 
 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 
 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. 
 Note: With SAToP, a valid sequence number can be always found in bits 
 16 - 31 of the first 32-bit word immediately following the multiplexing 
 header regardless of the specific PSN type, multiplexing method, usage 
 or non-usage of the RTP header etc. This approach simplifies 
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 implementations supporting multiple encapsulation types as well as 
 implementation of multi-segment (MS) PWs using different encapsulation 
 types in different segments.  
 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 
 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 transitions 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.  

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    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 
 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 
 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 
 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 MAY also be used for this purpose. Malformed in-
 order packets MUST be discarded by the CE-bound IWF and replacement 
 data generated as with 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 
 jitter buffer of the CE-bound IWF 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. 
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 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  
      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 employ 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   
 As explained in [RFC3985], the PSN carrying the PW may be subject to 
 congestion. SAToP PWs represent inelastic constant bit-rate (CBR) flows 
 and cannot respond to congestion in a TCP-friendly manner prescribed by 
 [RFC2914], although the percentage of total bandwidth they consume 
 remains constant.  
 Unless appropriate precautions are taken, undiminished demand of 
 bandwidth by SAToP PWs can contribute to network congestion that may 
 impact network control protocols.  
 Whenever possible, SAToP PWs SHOULD be carried across traffic-
 engineered PSNs that provide either bandwidth reservation and admission 
 control or forwarding prioritization and boundary traffic conditioning 

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 mechanisms. IntServ-enabled domains supporting Guaranteed Service (GS) 
 [RFC2212] and DiffServ-enabled domains [RFC2475] supporting Expedited 
 Forwarding (EF) [RFC3246] provide examples of such PSNs. Such 
 mechanisms will negate, to some degree, the effect of the SAToP PWs on 
 the neighboring streams. In order to facilitate boundary traffic 
 conditioning of SAToP traffic over IP PSNs, the SAToP IP packets SHOULD 
 NOT use the DiffServ Code Point (DSCP) value reserved for the Default 
 If SAToP PWs run over a PSN providing best-effort service, they SHOULD 
 monitor packet loss in order to detect "severe congestion". If such a 
 condition is detected, a SAToP PW SHOULD shut down bi-directionally for 
 some period of time as described in Section 6.5 of [RFC3985].  
 Note that: 
 1. The SAToP IWF can inherently provide packet loss measurement since 
     the expected rate of arrival of SAToP packets is fixed and known 
 2. The results of the SAToP packet loss measurement may not be a 
     reliable indication of presence or absence of severe congestion if 
     the PSN provides enhanced delivery, e.g.: 
     a) If SAToP traffic takes precedence over non-SAToP traffic, severe 
        congestion can develop without significant SAToP packet loss 
     b) If non-SAToP traffic takes precedence over SAToP traffic, SAToP 
        may experience substantial packet loss due to a short-term burst 
        of high-priority traffic 
 3. The TDM services emulated by the SAToP PWs have high availability 
     objectives (see [G.826]) that MUST be taken into account when 
     deciding on temporary shutdown of SAToP PWs. 
 This specification does not define the exact criteria for detecting 
 "severe congestion" using the SAToP packet loss rate or the specific 
 methods for bi-directional shutdown the SAToP PWs (when such severe 
 congestion has been detected) and their consequent re-start after a 
 suitable delay. This is left for further study. However, the following 
 considerations may be used as guidelines for implementing the SAToP 
 severe congestion shutdown mechanism: 
 1. SAToP Performance Monitoring techniques (see Section 6.4) provide 
     entry and exit criteria for the SAToP PW "Unavailable" state that 
     make it closely correlated with the "Unavailable" state of the 
     emulated TDM circuit as specified in [G.826]. Using the same 
     criteria for "severe congestion" detection may decrease the risk of 
     shutting down the SAToP PW while the emulated TDM circuit is still 
     considered available by the CE. 
 2. If the SAToP PW has been set up using either PWE3 control protocol 
     [PWE3-CONTROL] or L2TPv3 [RFC 3931], the regular PW teardown 
     procedures of these protocols SHOULD be used.  
 3. If one of the SAToP PW end points stops transmission of packets for 
     a sufficiently long period, its peer (observing 100% packet loss) 
     will necessarily detect "severe congestion" and also stop 
     transmission, thus achieving bi-directional PW shutdown. 
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 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.  
 SAToP PWs share susceptibility to a number of pseudowire-layer attacks,  
 and will use whatever mechanisms for confidentiality, integrity, and 
 authentication that are developed for general PWs. These methods are 
 beyond the scope of this document. 
 Although SAToP PWs MAY employ an RTP header when explicit transfer of 
 timing information is required, SRTP (see [RFC3711]) mechanisms are NOT 
 RECOMMENDED as a substitute for PW layer security. 
 Misconnection detection capabilities of SAToP increase its resilience 
 to misconfiguration and some types of DoS attacks. 
 Random initialization of sequence numbers, in both the control word and 
 the optional RTP header, makes known-plaintext attacks on encrypted 
 SAToP PWs more difficult. Encryption of PWs is beyond the scope of this 
 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.  
 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 

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 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-TDM-REQ)] and clock recovery techniques. The 
 quality of the TDM clock recovered by the SAToP IWF may be 
 implementation-specific. 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 
 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. 
 11. IANA Considerations 
 Allocation of PW Types for the corresponding SAToP PWs is defined in 
 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 
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    Structure-Agnostic TDM over Packet                  February 2006 
 specification can be obtained from the IETF on-line IPR repository  
 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 
 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 
 [RFC791] J. Postel (ed), Internet Protocol, RFC 791, IETF, 1981 
 [RFC2119] S.Bradner, Key Words in RFCs to Indicate Requirement Levels, 
 RFC 2119, 1997 
 [RFC2474] K. Nichols et al, Definition of the Differentiated Services 
 Field (DS Field) in the IPv4 and IPv6 Headers, RFC 2474, 19998 
 [RFC2475] S. Blake et al, An Architecture for Differentiated Services, 
 RFC 2475, 1998 
 [RFC2914] S. Floyd, Congestion Control Principles, RFC 2914, 2000 
 [RFC3086] K. Nichols, B. Carpenter, Definition of Differentiated 
 Services Per Domain Behaviors and Rules for their Specification, RFC 
 3086, 2001  
 [RFC3550] H. Schulzrinne et al, RTP: A Transport Protocol for Real-Time 
 Applications, RFC 3550, 2003 
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    Structure-Agnostic TDM over Packet                  February 2006 
 [G.702] ITU-T Recommendation G.702 (11/88) - Digital Hierarchy Bit 
 [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 
 [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 
 [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 
 [PWE3-CW] S. Bryant et al, PWE3 Control Word for use over an MPLS PSN, 
 Work in progress, October 2005, draft-ietf-pwe3-cw-06.txt 
 [PWE3-CONTROL] L. Martini et al, Pseudowire Setup and Maintenance using 
 LDP, Work in progress, June 2005, draft-ietf-pwe3-control-protocol-
 [PWE3-IANA] L. Martini, M. Townsley, IANA Allocations for pseudo Wire 
 Edge to Edge Emulation (PWE3), Work in progress, November 2005, draft-
 [RFC 3931] J. Lau, M.Townsley, I. Goyret, Layer Two Tunneling Protocol 
 - Version 3 (L2TPv3), RFC 3931, 2005 
 [RFC3916] XiPeng Xiao et al, Requirements for Pseudo Wire Emulation 
 Edge-to-Edge (PWE3), RFC 3916, 2004 
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    Structure-Agnostic TDM over Packet                  February 2006 
 [RFC4197] Maximilian Riegel, Requirements for Edge-to-Edge Emulation of 
 TDM Circuits over Packet Switching Networks (PSN), RFC 4197, 2005 
 [RFC3985] S. Bryant, P. Pate, PWE3 Architecture, RFC 3985, 2005 
 [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, November 2005, draft-
 [TDMoIP] Y. Stein, TDMoIP, Work in Progress, February 2005, draft-ietf-
 [PWE3-TDM-CONTROL] A. Vainshtein, Y. Stein, Control Protocol Extensions 
 for Setup of TDM Pseudowires, Work in Progress, July 2005, draft-ietf-
 [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-
 [PWE3-FRAG] A. Malis, M. Townsley, PWE3 Fragmentation and Reassembly, 
 Work in Progress, November 2005, draft-ietf-pwe3-fragmentation-10.txt 
 [RFC3551] H. Schulzrinne, S. Casner, RTP Profile for Audio and Video 
 Conferences with Minimal Control, RFC 3551, 2003 
 [RFC3711] M. Baugher et al, The Secure Real-time Transport Protocol 
 (SRTP), RFC 3711, 2004 
 [RFC2212] S. Shenker et al, Specification of Guaranteed Quality of 
 Service, RFC 2212, 1997 
 [RFC3246], B. Davie et al, An Expedited Forwarding PHB (Per-Hop 
 Behavior), RFC 3246, 2002 
 Previous versions of this specification defined a SAToP PW 
 encapsulation over L2TPv3, which differs from one, described in Section 
 4.3 and Diagram 2b. In these versions the RTP header, if used, precedes 
 the SAToP control word. 

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 Existing implementations of the old encapsulation mode MUST be 
 distinguished from the encapsulations conforming to this specification 
 via the SAToP PW setup. 
 The following parameters of the SAToP IWF MUST be agreed upon between 
 the peer IWFs during the PW setup. Such an agreement can be reached via 
 manual configuration or via one of the PW setup protocols: 
 1. Type of the Attachment Circuit (AC): 
     a) As mentioned in Section 3 above, SAToP supports the following AC 
        i)   E1 (2048 kbit/s) 
        ii)  T1 (1544 kbit/s) This service is also known as DS1 
        iii) E3(34368 kbit/s)  
     b) T3 (44736 kbit/s) This service is also known as DS3SAToP PWs 
        cannot be established between ACs of different types 
 2. Usage of octet-aligned mode for T1 
     a) This OPTIONAL mode of emulating T1 bit-streams with SAToP PWs is 
        described in Section 5.2 
     b) Both sides MUST agree on using this mode for a SAToP PW to be 
 3. Payload size, i.e. the amount of valid TDM data in a SAToP packet: 
     a) As mentioned in Section 5.1 above: 
        i)   The same payload size MUST be used in both directions of 
           the SAToP PW 
        ii)  The payload size cannot be changed once the PW has been set 
     b) In most cases any mutually agreed upon value can be used. 
        However, if octet-aligned T1 encapsulation mode is used, the 
        payload size MUST be an integral multiple of 25 and expresses 
        the amount  of valid TDM data  including padding  
 4. Usage of the RTP header in the encapsulation 
     a) Both sides MUST agree on using RTP header in the SAToP PW 
     b) In the case of a SAToP PW over L2TPv3 using the RTP header, both 
        sides MUST agree on usage of the "old mode" described in Annex A 
 5. RTP-dependent parameters. These following parameters MUST be agreed 
     upon if usage of the RTP header for the SAToP PW has been agreed 
     a) Timestamping mode (absolute or differential). This mode MAY be 
        different for the two directions of the PW, but the receiver and 
        transmitter MUST agree on the timestamping mode for each 
        direction of the PW 
     b) Timestamping clock frequency: 
        i)   The timestamping frequency MUST be a integral multiple of 
        ii)  The timestamping frequency MAY be different for the two 
           directions of the PW, but the receiver and transmitter MUST 
           agree on the timestamping mode for each direction of the PW 
     c) RTP Payload Type (PT) value.  
        Any dynamically assigned value can be used with SAToP PWs 
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     d) Synchronization Source (SSRC) value. The transmitter MUST agree 
        to send the SSRC value requested by the receiver. 
 Full Copyright Statement 
 Copyright (C) The Internet Society (2006).   
 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 
 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 
 Yaakov (Jonathan) Stein 
 RAD Data Communications 
 24 Raoul Wallenberg St., Bldg C 
 Tel Aviv 69719, Israel 

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