Control Protocol Extensions for the Setup of Time-Division Multiplexing (TDM) Pseudowires in MPLS Networks
Extracted elements (25)
A 4-byte space is reserved for the CEP/TDM Bit-Rate parameter for compatibility with RFC 4842 (CEP). The bit-rate values (e.g., 24 for T1) are convenience codes and do not represent actual bit-rates in units of 64 kbps.
The PW Status TLV mechanism (RFC 4447) is deliberately avoided for operational TDM PWs because status is already carried in-band in the TDM PW control word (L, M, and R fields), and using both mechanisms simultaneously would create contention between control-plane and data-plane status indications.
Either PWId FEC (FEC 128) or Generalized PW FEC (FEC 129) MAY be used for TDM PW setup; when using the Generalized PW FEC for CESoPSN data/signaling PW pairs, no specific rules for matching the two FECs are defined and implementation-specific mechanisms MAY be used.
The TDMoIP AAL2 Options CID mapping bases field MUST NOT appear if AAL2 CID-to-timeslot mapping is statically configured or if AAL2 switching (Q.2630.1) is employed; maximum of 248 CIDs per PW and no more than 10 AAL2 PWs required per PE.
For CESoPSN PW pairs carrying TDM data and CE application signaling: if the PWId FEC is used, the data PW ID MUST be even and the signaling PW ID MUST be the next (odd) value. The two PWs MUST have the same PW type, setup method, and all interface parameters except the SP field.
For structure-aware emulation, the CEP/TDM Bit-Rate parameter MUST be set to N, where N is the number of DS0 channels in the attachment circuit. For structure-agnostic T1 basic emulation mode, it MUST be set to 24; for Octet-aligned T1, it MUST be set to 25.
If one of two associated CESoPSN PWs (data + signaling pair) has been established and the other fails to be established or fails after establishment, the established PW MUST be torn down.
PW type 0x0012 MUST also be used for the setup of structure-agnostic TDM PWs between a pair of J1 attachment circuits.
Setting the Control bit to zero MUST result in an LDP status of 'Illegal C-Bit'.
Specific LDP status codes from RFC 4446 MUST be used to indicate TDM PW setup failures: 'Incompatible bit-rate' for T1 encapsulation mode or timeslot mismatches, and 'CEP/TDM misconfiguration' for RTP header, timestamping clock, signaling, trunk framing, or AAL1 mode mismatches.
The CEP/TDM Payload Bytes parameter is OPTIONAL; if omitted, the default payload size defined for the corresponding service MUST be assumed. For CESoPSN PWs, the specified value P MUST be an integer multiple of N, where N is the number of timeslots.
The CEP/TDM Payload Bytes parameter MUST NOT be used for the setup of TDMoIP PWs (PW types 0x0016 and 0x0018).
The Control bit MUST always be set for TDM PWs since all TDM PW encapsulations always use a control word.
The Fragmentation Indicator MUST be omitted from the FEC of all TDM PWs except trunk-specific NxDS0 services with CAS using CESoPSN encapsulation, for which it MUST be present if the payload size differs from Nx(frames per trunk multiframe).
The R (RTP Header Usage) bit in TDM Options controls RTP header use: if cleared, Length MUST be 4; if set, Length MUST be 8 or 12. If a peer endpoint cannot meet the RTP requirement, the Label Mapping message MUST be rejected with the appropriate status code.
The TDMoIP AAL2 Options parameter MUST be present for TDMoIP AAL2 mode PWs (PW type 0x0018). The Encoding field MUST be zero when no native signal processing is performed (G.711), and both directions MUST match for PW setup to succeed.
The two endpoints MUST agree on the PW type, as both directions of the PW are required to be of the same type.
Usage of the PW Status TLV mechanism is NOT RECOMMENDED for TDM PWs while the IWF is operational to avoid contention with in-band data-plane status indications. However, if the IWF itself fails while the PWE3 control plane remains operational, a Status TLV with all fault bits set SHOULD be sent.
Usage or non-usage of the RTP header MUST match for the two directions making up the TDM PW. However, the Differential timestamping mode may be used in just one direction.
Eight PW types for TDM PWs are allocated in RFC 4446: structure-agnostic E1 (0x0011), T1/DS1 (0x0012), E3 (0x0013), T3/DS3 (0x0014), CESoPSN basic (0x0015), TDMoIP AAL1 (0x0016), CESoPSN with CAS (0x0017), and TDMoIP AAL2 (0x0018).
Seven interface parameters are defined for TDM PWs: CEP/TDM Payload Bytes (0x04, 4 bytes), CEP/TDM Bit-Rate (0x07, 6 bytes), Number of TDMoIP AAL1 Cells per Packet (0x0E, 4 bytes), TDMoIP AAL1 Mode (0x10, 4 bytes), TDMoIP AAL2 Options (0x11, 8+ bytes), Fragmentation Indicator (0x09, 4 bytes), and TDM Options (0x0B, 4/8/12 bytes).
Four new Interface Parameter Sub-TLV type assignments were made by IANA: TDM Options (0x0B), Number of TDMoIP AAL1 cells per packet (0x0E), TDMoIP AAL1 mode (0x10), and TDMoIP AAL2 Options (0x11). Most other required IANA assignments were already listed in RFC 4446.
This document does not introduce any additional security impact on pseudowires beyond that of basic LDP-based PW setup specified in RFC 4447.
The TDM Options interface parameter (Sub-TLV ID 0x0B) is a 4, 8, or 12 byte field containing: Parameter ID, Length, R (RTP Header Usage), D (Differential timestamping), F/X (reserved), SP (CESoPSN signaling), CAS (trunk framing), RSVD-1, PT (RTP Payload Type), RSVD-2, FREQ (timestamping clock frequency in 8 kHz units), and SSRC (RTP synchronization source ID).
The TDMoIP AAL2 Options parameter (Sub-TLV ID 0x11) is 8 bytes or larger, containing Type (0x11), Length, V (Voice Activity Detection, 2 bits), ENCODING (14 bits), Maximum Duration (32 bits), and optional CID mapping bases (1 byte per trunk).