RFC 5087
Time Division Multiplexing over IP (TDMoIP)
7. Implementation Issues
General requirements for transport of TDM over pseudo-wires are detailed in [RFC4197]. In the following subsections we review additional aspects essential to successful TDMoIP implementation.7.1. Jitter and Packet Loss
In order to compensate for packet delay variation that exists in any PSN, a jitter buffer MUST be provided. A jitter buffer is a block of memory into which the data from the PSN is written at its variable arrival rate, and data is read out and sent to the destination TDM equipment at a constant rate. Use of a jitter buffer partially hides the fact that a PSN has been traversed rather than a conventional synchronous TDM network, except for the additional latency. Customary practice is to operate with the jitter buffer approximately half full, thus minimizing the probability of its overflow or underflow. Hence, the additional delay equals half the jitter buffer size. The length of the jitter buffer SHOULD be configurable and MAY be dynamic (i.e., grow and shrink in length according to the statistics of the Packet Delay Variation (PDV)). In order to handle (infrequent) packet loss and misordering, a packet sequence integrity mechanism MUST be provided. This mechanism MUST track the serial numbers of arriving packets and MUST take appropriate action when anomalies are detected. When lost packet(s) are detected, the mechanism MUST output filler data in order to retain TDM timing. Packets arriving in incorrect order SHOULD be reordered. Lost packet processing SHOULD ensure that proper FAS is sent to the TDM network. An example sequence number processing algorithm is provided in Appendix A. While the insertion of arbitrary filler data may be sufficient to maintain the TDM timing, for telephony traffic it may lead to audio gaps or artifacts that result in choppy, annoying or even unintelligible audio. An implementation MAY blindly insert a preconfigured constant value in place of any lost samples, and this value SHOULD be chosen to minimize the perceptual effect. Alternatively one MAY replay the previously received packet. When computational resources are available, implementations SHOULD conceal the packet loss event by properly estimating missing sample values in such fashion as to minimize the perceptual error.
7.2. Timing Recovery
TDM networks are inherently synchronous; somewhere in the network there will always be at least one extremely accurate primary reference clock, with long-term accuracy of one part in 1E-11. This node provides reference timing to secondary nodes with somewhat lower accuracy, and these in turn distribute timing information further. This hierarchy of time synchronization is essential for the proper functioning of the network as a whole; for details see [G823][G824]. Packets in PSNs reach their destination with delay that has a random component, known as packet delay variation (PDV). When emulating TDM on a PSN, extracting data from the jitter buffer at a constant rate overcomes much of the high frequency component of this randomness ("jitter"). The rate at which we extract data from the jitter buffer is determined by the destination clock, and were this to be precisely matched to the source clock proper timing would be maintained. Unfortunately, the source clock information is not disseminated through a PSN, and the destination clock frequency will only nominally equal the source clock frequency, leading to low frequency ("wander") timing inaccuracies. In broadest terms, there are four methods of overcoming this difficulty. In the first and second methods timing information is provided by some means independent of the PSN. This timing may be provided to the TDM end systems (method 1) or to the IWFs (method 2). In a third method, a common clock is assumed available to both IWFs, and the relationship between the TDM source clock and this clock is encoded in the packet. This encoding may take the form of RTP timestamps or may utilize the synchronous residual timestamp (SRTS) bits in the AAL1 overhead. In the final method (adaptive clock recovery) the timing must be deduced solely based on the packet arrival times. Example scenarios are detailed in [RFC4197] and in [Y1413]. Adaptive clock recovery utilizes only observable characteristics of the packets arriving from the PSN, such as the precise time of arrival of the packet at the TDM-bound IWF, or the fill-level of the jitter buffer as a function of time. Due to the packet delay variation in the PSN, filtering processes that combat the statistical nature of the observable characteristics must be employed. Frequency Locked Loops (FLL) and Phase Locked Loops (PLL) are well suited for this task.
Whatever timing recovery mechanism is employed, the output of the TDM-bound IWF MUST conform to the jitter and wander specifications of TDM traffic interfaces, as defined in [G823][G824]. For some applications, more stringent jitter and wander tolerances MAY be imposed.7.3. Congestion Control
As explained in [RFC3985], the underlying PSN may be subject to congestion. Unless appropriate precautions are taken, undiminished demand of bandwidth by TDMoIP can contribute to network congestion that may impact network control protocols. The AAL1 mode of TDMoIP is an inelastic constant bit-rate (CBR) flow and cannot respond to congestion in a TCP-friendly manner prescribed by [RFC2914], although the percentage of total bandwidth they consume remains constant. The AAL2 mode of TDMoIP is variable bit-rate (VBR), and it is often possible to reduce the bandwidth consumed by employing mechanisms that are beyond the scope of this document. Whenever possible, TDMoIP SHOULD be carried across traffic- engineered PSNs that provide either bandwidth reservation and admission control or forwarding prioritization and boundary traffic conditioning 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 TDMoIP on neighboring streams. In order to facilitate boundary traffic conditioning of TDMoIP traffic over IP PSNs, the TDMoIP packets SHOULD NOT use the Diffserv Code Point (DSCP) value reserved for the Default Per-Hop Behavior (PHB) [RFC2474]. When TDMoIP is run over a PSN providing best-effort service, packet loss SHOULD be monitored in order to detect congestion. If congestion is detected and bandwidth reduction is possible, then such reduction SHOULD be enacted. If bandwidth reduction is not possible, then the TDMoIP PW SHOULD shut down bi-directionally for some period of time as described in Section 6.5 of [RFC3985]. Note that: 1. In AAL1 mode TDMoIP can inherently provide packet loss measurement since the expected rate of packet arrival is fixed and known.
2. The results of the packet loss measurement may not be a
reliable indication of presence or absence of severe congestion if
the PSN provides enhanced delivery. For example, if TDMoIP
traffic takes precedence over other traffic, severe congestion may
not significantly affect TDMoIP packet loss.
3. The TDM services emulated by TDMoIP have high availability
objectives (see [G826]) that MUST be taken into account when
deciding on temporary shutdown.
This specification does not define exact criteria for detecting
severe congestion or specific methods for TDMoIP shutdown or
subsequent re-start. However, the following considerations may be
used as guidelines for implementing the shutdown mechanism:
1. If the TDMoIP PW has been set up using the PWE3 control
protocol [RFC4447], the regular PW teardown procedures of these
protocols SHOULD be used.
2. If one of the TDMoIP IWFs 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.
TDMoIP does not provide mechanisms to ensure timely delivery or
provide other quality-of-service guarantees; hence it is required
that the lower-layer services do so. Layer 2 priority can be
bestowed upon a TDMoIP stream by using the VLAN priority field, MPLS
priority can be provided by using EXP bits, and layer 3 priority is
controllable by using TOS. Switches and routers which the TDMoIP
stream must traverse should be configured to respect these
priorities.
8. Security Considerations
TDMoIP does not enhance or detract from the security performance of
the underlying PSN, rather it relies upon the PSN's mechanisms for
encryption, integrity, and authentication whenever required. The
level of security provided may be less than that of a native TDM
service.
When the PSN is MPLS, PW-specific security mechanisms MAY be
required, while for IP-based PSNs, IPsec [RFC4301] MAY be used.
TDMoIP using L2TPv3 is subject to the security considerations
discussed in Section 8 of [RFC3931].
TDMoIP shares susceptibility to a number of pseudowire-layer attacks (see [RFC3985]) and implementations SHOULD use whatever mechanisms for confidentiality, integrity, and authentication are developed for general PWs. These methods are beyond the scope of this document. Random initialization of sequence numbers, in both the control word and the optional RTP header, makes known-plaintext attacks on encrypted TDMoIP more difficult. Encryption of PWs is beyond the scope of this document. PW labels SHOULD be selected in an unpredictable manner rather than sequentially or otherwise in order to deter session hijacking. When using L2TPv3, a cryptographically random [RFC4086] Cookie SHOULD be used to protect against off-path packet insertion attacks, and a 64- bit Cookie is RECOMMENDED for protection against brute-force, blind, insertion attacks. Although TDMoIP MAY employ an RTP header when explicit transfer of timing information is required, SRTP (see [RFC3711]) mechanisms are not applicable.9. IANA Considerations
For MPLS PSNs, PW Types for TDMoIP PWs are allocated in [RFC4446]. For UDP/IP PSNs, when the source port is used as PW label, the destination port number MUST be set to 0x085E (2142), the user port number assigned by IANA to TDMoIP.10. Applicability Statement
It must be recognized that the emulation provided by TDMoIP may be imperfect, and the service may differ from the native TDM circuit in the following ways. The end-to-end delay of a TDM circuit emulated using TDMoIP may exceed that of a native TDM circuit. When using adaptive clock recovery, the timing performance of the emulated TDM circuit depends on characteristics of the PSN, and thus may be inferior to that of a native TDM circuit. If the TDM structure overhead is not transported over the PSN, then non-FAS data in the overhead will be lost.
When packets are lost in the PSN, TDMoIP mechanisms ensure that frame synchronization will be maintained. When packet loss events are properly concealed, the effect on telephony channels will be perceptually minimized. However, the bit error rate will be degraded as compared to the native service. Data in inactive channels is not transported in AAL2 mode, and thus this data will differ from that of the native service. Native TDM connections are point-to-point, while PSNs are shared infrastructures. Hence, the level of security of the emulated service may be less than that of the native service.11. Acknowledgments
The authors would like to thank Hugo Silberman, Shimon HaLevy, Tuvia Segal, and Eitan Schwartz of RAD Data Communications for their invaluable contributions to the technology described herein.
Appendix A. Sequence Number Processing (Informative)
The sequence number field in the control word enables detection of lost and misordered packets. Here we give pseudocode for an example algorithm in order to clarify the issues involved. These issues are implementation specific and no single explanation can capture all the possibilities. In order to simplify the description, modulo arithmetic is consistently used in lieu of ad-hoc treatment of the cyclicity. All differences between indexes are explicitly converted to the range [-2^15 ... +2^15 - 1] to ensure that simple checking of the difference's sign correctly predicts the packet arrival order. Furthermore, we introduce the notion of a playout buffer in order to unambiguously define packet lateness. When a packet arrives after previously having been assumed lost, the TDM-bound IWF may discard it, and continue to treat it as lost. Alternatively, if the filler data that had been inserted in its place has not yet been played out, the option remains to insert the true data into the playout buffer. Of course, the filler data may be generated upon initial detection of a missing packet or upon playout. This description is stated in terms of a packet-oriented playout buffer rather than a TDM byte oriented one; however, this is not a true requirement for re-ordering implementations since the latter could be used along with pointers to packet commencement points. Having introduced the playout buffer we explicitly treat over-run and under-run of this buffer. Over-run occurs when packets arrive so quickly that they can not be stored for playout. This is usually an indication of gross timing inaccuracy or misconfiguration, and we can do little but discard such early packets. Under-run is usually a sign of network starvation, resulting from congestion or network failure. The external variables used by the pseudocode are: received: sequence number of packet received played: sequence number of the packet being played out (Note 1) over-run: is the playout buffer full? (Note 3) under-run: has the playout buffer been exhausted? (Note 3) The internal variables used by the pseudocode are: expected: sequence number we expect to receive next D: difference between expected and received (Note 2) L: difference between sequence numbers of packet being played out and that just received (Notes 1 and 2)
In addition, the algorithm requires one parameter:
R: maximum lateness for a packet to be recoverable (Note 1).
Note 1: this is only required for the optional re-ordering
Note 2: this number is always in the range -2^15 ... +2^15 - 1
Note 3: the playout buffer is emptied by the TDM playout process,
which runs asynchronously to the packet arrival processing,
and which is not herein specified
Sequence Number Processing Algorithm
Upon receipt of a packet
if received = expected
{ treat packet as in-order }
if not over-run then
place packet contents into playout buffer
else
discard packet contents
set expected = (received + 1) mod 2^16
else
calculate D = ( (expected-received) mod 2^16 ) - 2^15
if D > 0 then
{ packets expected, expected+1, ... received-1 are lost }
while not over-run
place filler (all-ones or interpolation) into playout buffer
if not over-run then
place packet contents into playout buffer
else
discard packet contents
set expected = (received + 1) mod 2^16
else { late packet arrived }
declare "received" to be a late packet
do NOT update "expected"
either
discard packet
or
if not under-run then
calculate L = ( (played-received) mod 2^16 ) - 2^15
if 0 < L <= R then
replace data from packet previously marked as lost
else
discard packet
Note: by choosing R=0 we always discard the late packet
Appendix B. AAL1 Review (Informative)
The first byte of the 48-byte AAL1 PDU always contains an error- protected 3-bit sequence number. 1 2 3 4 5 6 7 8 +-+-+-+-+-+-+-+-+----------------------- |C| SN | CRC |P| 47 bytes of payload +-+-+-+-+-+-+-+-+----------------------- C (1 bit) convergence sublayer indication, its use here is limited to indication of the existence of a pointer (see below); C=0 means no pointer, C=1 means a pointer is present. SN (3 bits) The AAL1 sequence number increments from PDU to PDU. CRC (3 bits) is a 3-bit error cyclic redundancy code on C and SN. P (1 bit) even byte parity. As can be readily inferred, incrementing the sequence number forms an eight-PDU sequence number cycle, the importance of which will become clear shortly. The structure of the remaining 47 bytes in the AAL1 PDU depends on the PDU type, of which there are three, corresponding to the three types of AAL1 circuit emulation service defined in [CES]. These are known as unstructured circuit emulation, structured circuit emulation, and structured circuit emulation with CAS. The simplest PDU is the unstructured one, which is used for transparent transfer of whole circuits (T1,E1,T3,E3). Although AAL1 provides no inherent advantage as compared to SAToP for unstructured transport, in certain cases AAL1 may be required or desirable. For example, when it is necessary to interwork with an existing AAL1- based network, or when clock recovery based on AAL1-specific mechanisms is favored. For unstructured AAL1, the 47 bytes after the sequence number byte contain the full 376 bits from the TDM bit stream. No frame synchronization is supplied or implied, and framing is the sole responsibility of the end-user equipment. Hence, the unstructured mode can be used to carry data, and for circuits with nonstandard frame synchronization. For the T1 case the raw frame consists of 193 bits, and hence 1 183/193 T1 frames fit into each AAL1 PDU. The E1 frame consists of 256 bits, and so 1 15/32 E1 frames fit into each PDU.
When the TDM circuit is channelized according to [G704], and in particular when it is desired to fractional E1 or T1, it is advantageous to use one of the structured AAL1 circuit emulation services. Structured AAL1 views the data not merely as a bit stream, but as a bundle of channels. Furthermore, when CAS signaling is used it can be formatted so that it can be readily detected and manipulated. In the structured circuit emulation mode without CAS, N bytes from the N channels to be transported are first arranged in order of channel number. Thus if channels 2, 3, 5, 7 and 11 are to be transported, the corresponding five bytes are placed in the PDU immediately after the sequence number byte. This placement is repeated until all 47 bytes in the PDU are filled. byte 1 2 3 4 5 6 7 8 9 10 --- 41 42 43 44 45 46 47 channel 2 3 5 7 11 2 3 5 7 11 --- 2 3 5 7 11 2 3 The next PDU commences where the present PDU left off. byte 1 2 3 4 5 6 7 8 9 10 --- 41 42 43 44 45 46 47 channel 5 7 11 2 3 5 7 11 2 3 --- 5 7 11 2 3 5 7 And so forth. The set of channels 2,3,5,7,11 is the basic structure and the point where one structure ends and the next commences is the structure boundary. The problem with this arrangement is the lack of explicit indication of the byte identities. As can be seen in the above example, each AAL1 PDU starts with a different channel, so a single lost packet will result in misidentifying channels from that point onwards, without possibility of recovery. The solution to this deficiency is the periodic introduction of a pointer to the next structure boundary. This pointer need not be used too frequently, as the channel identifications are uniquely inferable unless packets are lost. The particular method used in AAL1 is to insert a pointer once every sequence number cycle of eight PDUs. The pointer is seven bits and protected by an even parity MSB (most significant bit), and so occupies a single byte. Since seven bits are sufficient to represent offsets larger than 47, we can limit the placement of the pointer byte to PDUs with even sequence numbers. Unlike most AAL1 PDUs that contain 47 TDM bytes, PDUs that contain a pointer (P-format PDUs) have the following format.
0 1
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-----------------------
|C| SN | CRC |P|E| pointer | 46 bytes of payload
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-----------------------
where
C (1 bit) convergence sublayer indication, C=1 for P-format PDUs.
SN (3 bits) is an even AAL1 sequence number.
CRC (3 bits) is a 3-bit error cyclic redundancy code on C and SN.
P (1 bit) even byte parity LSB (least significant bit) for sequence
number byte.
E (1 bit) even byte parity MSB for pointer byte.
pointer (7 bits) pointer to next structure boundary.
Since P-format PDUs have 46 bytes of payload and the next PDU has 47
bytes, viewed as a single entity the pointer needs to indicate one of
93 bytes. If P=0 it is understood that the structure commences with
the following byte (i.e., the first byte in the payload belongs to
the lowest numbered channel). P=93 means that the last byte of the
second PDU is the final byte of the structure, and the following PDU
commences with a new structure. The special value P=127 indicates
that there is no structure boundary to be indicated (needed when
extremely large structures are being transported).
The P-format PDU is always placed at the first possible position in
the sequence number cycle that a structure boundary occurs, and can
only occur once per cycle.
The only difference between the structured circuit emulation format
and structured circuit emulation with CAS is the definition of the
structure. Whereas in structured circuit emulation the structure is
composed of the N channels, in structured circuit emulation with CAS
the structure encompasses the superframe consisting of multiple
repetitions of the N channels and then the CAS signaling bits. The
CAS bits are tightly packed into bytes and the final byte is padded
with zeros if required.
For example, for E1 circuits the CAS signaling bits are updated once
per superframe of 16 frames. Hence, the structure for N*64 derived
from an E1 with CAS signaling consists of 16 repetitions of N bytes,
followed by N sets of the four ABCD bits, and finally four zero bits
if N is odd. For example, the structure for channels 2,3 and 5 will
be as follows:
2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5
2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 [ABCD2 ABCD3] [ABCD5 0000]
Similarly for T1 ESF circuits the superframe is 24 frames, and the
structure consists of 24 repetitions of N bytes, followed by the ABCD
bits as before. For the T1 case the signaling bits will in general
appear twice, in their regular (bit-robbed) positions and at the end
of the structure.
Appendix C. AAL2 Review (Informative)
The basic AAL2 PDU is: | Byte 1 | Byte 2 | Byte 3 | 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+------------ | CID | LI | UUI | HEC | PAYLOAD +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+------------ CID (8 bits) channel identifier is an identifier that must be unique for the PW. The values 0-7 are reserved for special purposes, (and if interworking with VoDSL is required, so are values 8 through 15 as specified in [LES]), thus leaving 248 (240) CIDs per PW. The mapping of CID values to channels MAY be manually configured manually or signaled. LI (6 bits) length indicator is one less than the length of the payload in bytes. Note that the payload is limited to 64 bytes. UUI (5 bits) user-to-user indication is the higher layer (application) identifier and counter. For voice data, the UUI will always be in the range 0-15, and SHOULD be incremented modulo 16 each time a channel buffer is sent. The receiver MAY monitor this sequence. UUI is set to 24 for CAS signaling packets. HEC (5 bits) the header error control Payload - voice A block of length indicated by LI of voice samples are placed as- is into the AAL2 packet. Payload - CAS signaling For CAS signaling the payload is formatted as an AAL2 "fully protected" (type 3) packet (see [AAL2]) in order to ensure error protection. The signaling is sent with the same CID as the corresponding voice channel. Signaling MUST be sent whenever the state of the ABCD bits changes, and SHOULD be sent with triple redundancy, i.e., sent three times spaced 5 milliseconds apart. In addition, the entire set of the signaling bits SHOULD be sent periodically to ensure reliability.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RED| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RES | ABCD | type | CRC
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
CRC (cont) |
+-+-+-+-+-+-+-+-+
RED (2 bits) is the triple redundancy counter. For the first packet
it takes the value 00, for the second 01 and for the third 10.
RED=11 means non-redundant information, and is used when triple
redundancy is not employed, and for periodic refresh messages.
Timestamp (14 bits) The timestamp is optional and in particular is
not needed if RTP is employed. If not used, the timestamp MUST be
set to zero. When used with triple redundancy, it MUST be the
same for all three redundant transmissions.
RES (4 bits) is reserved and MUST be set to zero.
ABCD (4 bits) are the CAS signaling bits.
type (6 bits) for CAS signaling this is 000011.
CRC-10 (10 bits) is a 10-bit CRC error detection code.
Appendix D. Performance Monitoring Mechanisms (Informative)
PWs require OAM mechanisms to monitor performance measures that impact the emulated service. Performance measures, such as packet loss ratio and packet delay variation, may be used to set various parameters and thresholds; for TDMoIP PWs adaptive timing recovery and packet loss concealment algorithms may benefit from such information. In addition, OAM mechanisms may be used to collect statistics relating to the underlying PSN [RFC2330], and its suitability for carrying TDM services. TDMoIP IWFs may benefit from knowledge of PSN performance metrics, such as round trip time (RTT), packet delay variation (PDV) and packet loss ratio (PLR). These measurements are conventionally performed by a separate flow of packets designed for this purpose, e.g., ICMP packets [RFC792] or MPLS LSP ping packets [RFC4379] with multiple timestamps. For AAL1 mode, TDMoIP sends packets across the PSN at a constant rate, and hence no additional OAM flow is required for measurement of PDV or PLR. However, separate OAM flows are required for RTT measurement, for AAL2 mode PWs, for measurement of parameters at setup, for monitoring of inactive backup PWs, and for low-rate monitoring of PSNs after PWs have been withdrawn due to service failures. If the underlying PSN has appropriate maintenance mechanisms that provide connectivity verification, RTT, PDV, and PLR measurements that correlate well with those of the PW, then these mechanisms SHOULD be used. If such mechanisms are not available, either of two similar OAM signaling mechanisms may be used. The first is internal to the PW and based on inband VCCV [RFC5085], and the second is defined only for UDP/IP PSNs, and is based on a separate PW. The latter is particularly efficient for a large number of fate-sharing TDM PWs.D.1. TDMoIP Connectivity Verification
In most conventional IP applications a server sends some finite amount of information over the network after explicit request from a client. With TDMoIP PWs the PSN-bound IWF could send a continuous stream of packets towards the destination without knowing whether the TDM-bound IWF is ready to accept them. For layer-2 networks, this may lead to flooding of the PSN with stray packets. This problem may occur when a TDMoIP IWF is first brought up, when the TDM-bound IWF fails or is disconnected from the PSN, or the PW is broken. After an aging time the destination IWF becomes unknown, and intermediate switches may flood the network with the TDMoIP packets in an attempt to find a new path.
The solution to this problem is to significantly reduce the number of TDMoIP packets transmitted per second when PW failure is detected, and to return to full rate only when the PW is available. The detection of failure and restoration is made possible by the periodic exchange of one-way connectivity-verification messages. Connectivity is tested by periodically sending OAM messages from the source IWF to the destination IWF, and having the destination reply to each message. The connectivity verification mechanism SHOULD be used during setup and configuration. Without OAM signaling, one must ensure that the destination IWF is ready to receive packets before starting to send them. Since TDMoIP IWFs operate full-duplex, both would need to be set up and properly configured simultaneously if flooding is to be avoided. When using connectivity verification, a configured IWF may wait until it detects its peer before transmitting at full rate. In addition, configuration errors may be readily discovered by using the service specific field of the OAM PW packets. In addition to one-way connectivity, OAM signaling mechanisms can be used to request and report on various PSN metrics, such as one-way delay, round trip delay, packet delay variation, etc. They may also be used for remote diagnostics, and for unsolicited reporting of potential problems (e.g., dying gasp messages).D.2. OAM Packet Format
When using inband performance monitoring, additional packets are sent using the same PW label. These packets are identified by having their first nibble equal to 0001, and must be separated from TDM data packets before further processing of the control word. When using a separate OAM PW, all OAM messages MUST use the PW label preconfigured to indicate OAM. All PSN layer parameters MUST remain those of the PW being monitored. The format of an inband OAM PW message packet for UDP/IP PSNs is based on [RFC2679]. The PSN-specific layers are identical to those defined in Section 4.1 with the PW label set to the value preconfigured or assigned for PW OAM.
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-specific layers (with preconfigured PW label) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0|L|R| M |RES| Length | OAM Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OAM Msg Type | OAM Msg Code | Service specific information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forward PW label | Reverse PW label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Transmit Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Receive Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Transmit Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
L, R, and M are identical to those of the PW being tested.
Length is the length in bytes of the OAM message packet.
OAM Sequence Number (16 bits) is used to uniquely identify the
message. Its value is unrelated to the sequence number of the
TDMoIP data packets for the PW in question. It is incremented in
query messages, and replicated without change in replies.
OAM Msg Type (8 bits) indicates the function of the message. At
present the following are defined:
0 for one-way connectivity query message
8 for one-way connectivity reply message.
OAM Msg Code (8 bits) is used to carry information related to the
message, and its interpretation depends on the message type. For
type 0 (connectivity query) messages the following codes are
defined:
0 validate connection.
1 do not validate connection
for type 8 (connectivity reply) messages the available codes are:
0 acknowledge valid query
1 invalid query (configuration mismatch).
Service specific information (16 bits) is a field that can be used
to exchange configuration information between IWFs. If it is not
used, this field MUST contain zero. Its interpretation depends on
the payload type. At present, the following is defined for AAL1
payloads.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of TSs | Number of SFs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Number of TSs (8 bits) is the number of channels being transported,
e.g., 24 for full T1.
Number of SFs (8 bits) is the number of 48-byte AAL1 PDUs per
packet, e.g., 8 when packing 8 PDUs per packet.
Forward PW label (16 bits) is the PW label used for TDMoIP traffic
from the source to destination IWF.
Reverse PW label (16 bits) is the PW label used for TDMoIP traffic
from the destination to source IWF.
Source Transmit Timestamp (32 bits) represents the time the PSN-
bound IWF transmitted the query message. This field and the
following ones only appear if delay is being measured. All time
units are derived from a clock of preconfigured frequency, the
default being 100 microseconds.
Destination Receive Timestamp (32 bits) represents the time the
destination IWF received the query message.
Destination Transmit Timestamp (32 bits) represents the time the
destination IWF transmitted the reply message.
Appendix E. Capabilities, Configuration and Statistics (Informative)
Every TDMoIP IWF will support some number of physical TDM connections, certain types of PSN, and some subset of the modes defined above. The following capabilities SHOULD be able to be queried by the management system: AAL1 capable AAL2 capable (and AAL2 parameters, e.g., support for VAD and compression) HDLC capable Supported PSN types (UDP/IPv4, UDP/IPv6, L2TPv3/IPv4, L2TPv3/IPv6, MPLS, Ethernet) OAM support (none, separate PW, VCCV) and capabilities (CV, delay measurement, etc.) maximum packet size supported. For every TDM PW the following parameters MUST be provisioned or signaled: PW label (for UDP and Ethernet the label MUST be manually configured) TDM type (E1, T1, E3, T3, fractional E1, fractional T1) for fractional links: number of timeslots TDMoIP mode (AAL1, AAL2, HDLC) for AAL1 mode: AAL1 type (unstructured, structured, structured with CAS) number of AAL1 PDUs per packet for AAL2 mode: CID mapping creation time of full minicell (units of 125 microsecond)
size of jitter buffer (in 32-bit words)
clock recovery method (local, loop-back timing, adaptive, common
clock)
use of RTP (if used: frequency of common clock, PT and SSRC
values).
During operation, the following statistics and impairment indications
SHOULD be collected for each TDM PW, and can be queried by the
management system.
average round-trip delay
packet delay variation (maximum delay - minimum delay)
number of potentially lost packets
indication of misordered packets (successfully reordered or
dropped)
for AAL1 mode PWs:
indication of malformed PDUs (incorrect CRC, bad C, P or E)
indication of cells with pointer mismatch
number of seconds with jitter buffer over-run events
number of seconds with jitter buffer under-run events
for AAL2 mode PWs:
number of malformed minicells (incorrect HEC)
indication of misordered minicells (unexpected UUI)
indication of stray minicells (CID unknown, illegal UUI)
indication of mis-sized minicells (unexpected LI)
for each CID: number of seconds with jitter buffer over-run
events
for HDLC mode PWs:
number of discarded frames from TDM (e.g., CRC error, illegal
packet size)
number of seconds with jitter buffer over-run events.
During operation, the following statistics MAY be collected for each
TDM PW.
number of packets sent to PSN
number of packets received from PSN
number of seconds during which packets were received with L flag
set
number of seconds during which packets were received with R flag
set.
References
Normative References
[AAL1] ITU-T Recommendation I.363.1 (08/96) - B-ISDN ATM Adaptation Layer (AAL) specification: Type 1 [AAL2] ITU-T Recommendation I.363.2 (11/00) - B-ISDN ATM Adaptation Layer (AAL) specification: Type 2 [CES] ATM forum specification atm-vtoa-0078 (CES 2.0) Circuit Emulation Service Interoperability Specification Ver. 2.0 [G704] ITU-T Recommendation G.704 (10/98) - Synchronous frame structures used at 1544, 6312, 2048, 8448 and 44736 kbit/s hierarchical levels [G751] 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 [G823] ITU-T Recommendation G.823 (03/00) - The control of jitter and wander within digital networks which are based on the 2048 Kbit/s hierarchy [G824] ITU-T Recommendation G.824 (03/00) - The control of jitter and wander within digital networks which are based on the 1544 Kbit/s hierarchy [G826] ITU-T Recommendation G.826 (12/02) - End-to-end error performance parameters and objectives for international, constant bit-rate digital paths and connections [IEEE802.1Q] IEEE 802.1Q, IEEE Standards for Local and Metropolitan Area Networks -- Virtual Bridged Local Area Networks (2003) [IEEE802.3] IEEE 802.3, IEEE Standard Local and Metropolitan Area Networks - Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications (2002)
[LES] ATM forum specification atm-vmoa-0145 (LES) Voice and Multimedia over ATM - Loop Emulation Service Using AAL2 [MEF8] Metro Ethernet Forum, "Implementation Agreement for the Emulation of PDH Circuits over Metro Ethernet Networks", October 2004. [RFC768] Postel, J., "User Datagram Protocol (UDP)", STD 6, RFC 768, August 1980. [RFC791] Postel, J., "Internet Protocol (IP)", STD 5, RFC 791, September 1981. [RFC2119] Bradner, S., "Key Words in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack Encoding", RFC 3032, January 2001. [RFC3931] Lau, J., Townsley, M., Goyret, I., "Layer Two Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and Jacobson, V., "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006. [RFC4447] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G. Heron, "Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)", RFC 4447, April 2006. [RFC4553] Vainshtein A., and Stein YJ., "Structure-Agnostic TDM over Packet (SAToP)", RFC 4553, June 2006. [RFC4618] Martini L., Rosen E., Heron G., and Malis A., "Encapsulation Methods for Transport of PPP/High-Level Data Link Control (HDLC) over MPLS Networks", RFC 4618, September 2006. [RFC5085] Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire Virtual Circuit Connectivity Verification: A Control Channel for Pseudowires", RFC 5085, December 2007.
[SSCS] ITU-T Recommendation I.366.2 (11/00) - AAL type 2 service specific convergence sublayer for narrow-band services. [Y1413] ITU-T Recommendation Y.1413 (03/04) - TDM-MPLS network interworking - User plane interworking [Y1414] ITU-T Recommendation Y.1414 (07/04) - Voice services - MPLS network interworking. [Y1452] ITU-T Recommendation Y.1452 (03/06) - Voice trunking over IP networks. [Y1453] ITU-T Recommendation Y.1453 (03/06) - TDM-IP interworking - User plane interworking.Informative References
[ISDN-PRI] ITU-T Recommendation Q.931 (05/98) - ISDN user-network interface layer 3 specification for basic call control. [RFC792] Postel J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. [RFC2212] Shenker, S., Partridge, C., and R. Guerin, "Specification of Guaranteed Quality of Service", RFC 2212, September 1997. [RFC2330] Paxson, V., Almes, G., Mahdavi, J., Mathis M., "Framework for IP Performance Metrics", RFC 2330, May 1998. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998. [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., and W. Weiss, "An Architecture for Differentiated Service", RFC 2475, December 1998. [RFC2679] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way Delay Metric for IPPM", RFC 2679, September 1999.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC 2914, September 2000. [RFC3246] Davie, B., Charny, A., Bennet, J.C., Benson, K., Le Boudec, J., Courtney, W., Davari, S., Firoiu, V., and D. Stiliadis, "An Expedited Forwarding PHB (Per-Hop Behavior)", RFC 3246, March 2002. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge- to-Edge (PWE3) Architecture", RFC 3985, March 2005. [RFC4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. [RFC4197] Riegel, M., "Requirements for Edge-to-Edge Emulation of Time Division Multiplexed (TDM) Circuits over Packet Switching Networks", RFC 4197, October 2005. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. [RFC4379] Kompella, K. and Swallow, G., "Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures", RFC 4379, February 2006. [RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson, "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN", RFC 4385, February 2006. [RFC5086] Vainshtein, A., Ed., Sasson, I., Metz, E., Frost, T., and P. Pate, "Structure-Aware Time Division Multiplexed (TDM) Circuit Emulation Service over Packet Switched Network (CESoPSN)", RFC 5086, December 2007. [SS7] ITU-T Recommendation Q.700 (03/93) - Introduction to CCITT Signalling System No. 7. [TDM-CONTROL] Vainshtein, A. and Y(J) Stein, "Control Protocol Extensions for Setup of TDM Pseudowires in MPLS Networks", Work in Progress, November 2007.
[TRAU] GSM 08.60 (10/01) - Digital cellular telecommunications system (Phase 2+); Inband control of remote transcoders and rate adaptors for Enhanced Full Rate (EFR) and full rate traffic channels.Authors' Addresses
Yaakov (Jonathan) Stein RAD Data Communications 24 Raoul Wallenberg St., Bldg C Tel Aviv 69719 ISRAEL Phone: +972 3 645-5389 EMail: yaakov_s@rad.com Ronen Shashoua RAD Data Communications 24 Raoul Wallenberg St., Bldg C Tel Aviv 69719 ISRAEL Phone: +972 3 645-5447 EMail: ronen_s@rad.com Ron Insler RAD Data Communications 24 Raoul Wallenberg St., Bldg C Tel Aviv 69719 ISRAEL Phone: +972 3 645-5445 EMail: ron_i@rad.com Motty (Mordechai) Anavi RAD Data Communications 900 Corporate Drive Mahwah, NJ 07430 USA Phone: +1 201 529-1100 Ext. 213 EMail: motty@radusa.com
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