Network Working Group Y(J). Stein
Request for Comments: 5087 R. Shashoua
Category: Informational R. Insler
M. Anavi
RAD Data Communications
December 2007
Time Division Multiplexing over IP (TDMoIP)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
Time Division Multiplexing over IP (TDMoIP) is a structure-aware
method for transporting Time Division Multiplexed (TDM) signals using
pseudowires (PWs). Being structure-aware, TDMoIP is able to ensure
TDM structure integrity, and thus withstand network degradations
better than structure-agnostic transport. Structure-aware methods
can distinguish individual channels, enabling packet loss concealment
and bandwidth conservation. Accesibility of TDM signaling
facilitates mechanisms that exploit or manipulate signaling.
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RFC 5087 TDMoIP December 2007
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. TDM Structure and Structure-aware Transport . . . . . . . . . 4
3. TDMoIP Encapsulation . . . . . . . . . . . . . . . . . . . . . 6
4. Encapsulation Details for Specific PSNs . . . . . . . . . . . 9
4.1. UDP/IP . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. MPLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.3. L2TPv3 . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.4. Ethernet . . . . . . . . . . . . . . . . . . . . . . . . . 15
5. TDMoIP Payload Types . . . . . . . . . . . . . . . . . . . . . 17
5.1. AAL1 Format Payload . . . . . . . . . . . . . . . . . . . 18
5.2. AAL2 Format Payload . . . . . . . . . . . . . . . . . . . 19
5.3. HDLC Format Payload . . . . . . . . . . . . . . . . . . . 20
6. TDMoIP Defect Handling . . . . . . . . . . . . . . . . . . . . 21
7. Implementation Issues . . . . . . . . . . . . . . . . . . . . 24
7.1. Jitter and Packet Loss . . . . . . . . . . . . . . . . . . 24
7.2. Timing Recovery . . . . . . . . . . . . . . . . . . . . . 25
7.3. Congestion Control . . . . . . . . . . . . . . . . . . . . 26
8. Security Considerations . . . . . . . . . . . . . . . . . . . 27
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
10. Applicability Statement . . . . . . . . . . . . . . . . . . . 28
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
Appendix A. Sequence Number Processing (Informative) . . . . . . 30
Appendix B. AAL1 Review (Informative) . . . . . . . . . . . . . . 32
Appendix C. AAL2 Review (Informative) . . . . . . . . . . . . . . 36
Appendix D. Performance Monitoring Mechanisms (Informative) . . . 38
D.1. TDMoIP Connectivity Verification . . . . . . . . . . . . . 38
D.2. OAM Packet Format . . . . . . . . . . . . . . . . . . . . 39
Appendix E. Capabilities, Configuration and Statistics
(Informative) . . . . . . . . . . . . . . . . . . . . 42
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Normative References . . . . . . . . . . . . . . . . . . . . . . 45
Informative References . . . . . . . . . . . . . . . . . . . . . 47
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1. Introduction
Telephony traffic is conventionally carried over connection-oriented
synchronous or plesiochronous links (loosely called TDM circuits
herein). With the proliferation of Packet Switched Networks (PSNs),
transport of TDM services over PSN infrastructures has become
desirable. Emulation of TDM circuits over the PSN can be carried out
using pseudowires (PWs), as described in the PWE3 architecture
[RFC3985]. This emulation must maintain service quality of native
TDM; in particular voice quality, latency, timing, and signaling
features must be similar to those of existing TDM networks, as
described in the TDM PW requirements document [RFC4197].
Structure-Agnostic TDM over Packet (SAToP) [RFC4553] is a structure-
agnostic protocol for transporting TDM over PSNs. The present
document details TDM over IP (TDMoIP), a structure-aware method for
TDM transport. In contrast to SAToP, structure-aware methods such as
TDMoIP ensure the integrity of TDM structure and thus enable the PW
to better withstand network degradations. Individual multiplexed
channels become visible, enabling the use of per channel mechanisms
for packet loss concealment and bandwidth conservation. TDM
signaling also becomes accessible, facilitating mechanisms that
exploit or manipulate this signaling.
Despite its name, the TDMoIP(R) protocol herein described may operate
over several types of PSN, including UDP over IPv4 or IPv6, MPLS,
Layer 2 Tunneling Protocol version 3 (L2TPv3) over IP, and pure
Ethernet. Implementation specifics for particular PSNs are discussed
in Section 4. Although the protocol should be more generally called
TDMoPW and its specific implementations TDMoIP, TDMoMPLS, etc., we
retain the nomenclature TDMoIP for consistency with earlier usage.
The interworking function that connects between the TDM and PSN
worlds will be called a TDMoIP interworking function (IWF), and it
may be situated at the provider edge (PE) or at the customer edge
(CE). The IWF that encapsulates TDM and injects packets into the PSN
will be called the PSN-bound interworking function, while the IWF
that extracts TDM data from packets and generates traffic on a TDM
network will be called the TDM-bound interworking function. Emulated
TDM circuits are always point-to-point, bidirectional, and transport
TDM at the same rate in both directions.
As with all PWs, TDMoIP PWs may be manually configured or set up
using the PWE3 control protocol [RFC4447]. Extensions to the PWE3
control protocol required specifically for setup and maintenance of
TDMoIP pseudowires are described in [TDM-CONTROL].
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. TDM Structure and Structure-aware Transport
Although TDM circuits can be used to carry arbitrary bit-streams,
there are standardized methods for carrying constant-length blocks of
data called "structures". Familiar structures are the T1 or E1
frames [G704] of length 193 and 256 bits, respectively. By
concatenation of consecutive T1 or E1 frames we can build higher
level structures called superframes or multiframes. T3 and E3 frames
[G704][G751] are much larger than those of T1 and E1, and even larger
structures are used in the GSM Abis channel described in [TRAU]. TDM
structures contain TDM data plus structure overhead; for example, the
193-bit T1 frame contains a single bit of structure overhead and 24
bytes of data, while the 32-byte E1 frame contains a byte of overhead
and 31 data bytes.
Structured TDM circuits are frequently used to transport multiplexed
channels. A single byte in the TDM frame (called a timeslot) is
allocated to each channel. A frame of a channelized T1 carries 24
byte-sized channels, while an E1 frame consists of 31 channels.
Since TDM frames are sent 8000 times per second, a single byte-sized
channel carries 64 kbps.
TDM structures are universally delimited by placing an easily
detectable periodic bit pattern, called the Frame Alignment Signal
(FAS), in the structure overhead. The structure overhead may
additionally contain error monitoring and defect indications. We
will use the term "structured TDM" to refer to TDM with any level of
structure imposed by an FAS. Unstructured TDM signifies a bit stream
upon which no structure has been imposed, implying that all bits are
available for user data.
SAToP [RFC4553] is a structure-agnostic protocol for transporting TDM
using PWs. SAToP treats the TDM input as an arbitrary bit-stream,
completely disregarding any structure that may exist in the TDM bit-
stream. Hence, SAToP is ideal for transport of truly unstructured
TDM, but is also suitable for transport of structured TDM when there
is no need to protect structure integrity nor interpret or manipulate
individual channels during transport. In particular, SAToP is the
technique of choice for PSNs with negligible packet loss, and for
applications that do not require discrimination between channels nor
intervention in TDM signaling.
As described in [RFC4553], when a single SAToP packet is lost, an
"all ones" pattern is played out to the TDM interface. This pattern
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is interpreted by the TDM end equipment as an Alarm Indication Signal
(AIS), which, according to TDM standards [G826], immediately triggers
a "severely errored second" event. As such events are considered
highly undesirable, the suitability of SAToP is limited to extremely
reliable and underutilized PSNs.
When structure-aware TDM transport is employed, it is possible to
explicitly safeguard TDM structure during transport over the PSN,
thus making possible to effectively conceal packet loss events.
Structure-aware transport exploits at least some level of the TDM
structure to enhance robustness to packet loss or other PSN
shortcomings. Structure-aware TDM PWs are not required to transport
structure overhead across the PSN; in particular, the FAS MAY be
stripped by the PSN-bound IWF and MUST be regenerated by the TDM-
bound IWF. However, structure overhead MAY be transported over the
PSN, since it may contain information other than FAS.
In addition to guaranteeing maintenance of TDM synchronization,
structure-aware TDM transport can also distinguish individual
timeslots of channelized TDM, thus enabling sophisticated packet loss
concealment at the channel level. TDM signaling also becomes
visible, facilitating mechanisms that maintain or exploit this
information. Finally, by taking advantage of TDM signaling and/or
voice activity detection, structure-aware TDM transport makes
bandwidth conservation possible.
There are three conceptually distinct methods of ensuring TDM
structure integrity -- namely, structure-locking, structure-
indication, and structure-reassembly. Structure-locking requires
each packet to commence at the start of a TDM structure, and to
contain an entire structure or integral multiples thereof.
Structure-indication allows packets to contain arbitrary fragments of
basic structures, but employs pointers to indicate where each
structure commences. Structure-reassembly is only defined for
channelized TDM; the PSN-bound IWF extracts and buffers individual
channels, and the original structure is reassembled from the received
constituents by the TDM-bound IWF.
All three methods of TDM structure preservation have their
advantages. Structure-locking is described in [RFC5086], while the
present document specifies both structure-indication (see
Section 5.1) and structure-reassembly (see Section 5.2) approaches.
Structure-indication is used when channels may be allocated
statically, and/or when it is required to interwork with existing
circuit emulation systems (CES) based on AAL1. Structure-reassembly
is used when dynamic allocation of channels is desirable and/or when
it is required to interwork with existing loop emulation systems
(LES) based on AAL2.
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Operation, administration, and maintenance (OAM) mechanisms are vital
for proper TDM deployments. As aforementioned, structure-aware
mechanisms may refrain from transporting structure overhead across
the PSN, disrupting OAM functionality. It is beneficial to
distinguish between two OAM cases, the "trail terminated" and the
"trail extended" scenarios. A trail is defined to be the combination
of data and associated OAM information transfer. When the TDM trail
is terminated, OAM information such as error monitoring and defect
indications are not transported over the PSN, and the TDM networks
function as separate OAM domains. In the trail extended case, we
transfer the OAM information over the PSN (although not necessarily
in its native format). OAM will be discussed further in Section 6.
3. TDMoIP Encapsulation
The overall format of TDMoIP packets is shown in Figure 1.
+---------------------+
| PSN Headers |
+---------------------+
| TDMoIP Control Word |
+---------------------+
| Adapted Payload |
+---------------------+
Figure 1. Basic TDMoIP Packet Format
The PSN-specific headers are those of UDP/IP, L2TPv3/IP, MPLS or
layer 2 Ethernet, and contain all information necessary for
forwarding the packet from the PSN-bound IWF to the TDM-bound one.
The PSN is assumed to be reliable enough and of sufficient bandwidth
to enable transport of the required TDM data.
A TDMoIP IWF may simultaneously support multiple TDM PWs, and the
TDMoIP IWF MUST maintain context information for each TDM PW.
Distinct PWs are differentiated based on PW labels, which are carried
in the PSN-specific layers. Since TDM is inherently bidirectional,
the association of two PWs in opposite directions is required. The
PW labels of the two directions MAY take different values.
In addition to the aforementioned headers, an OPTIONAL 12-byte RTP
header may appear in order to enable explicit transfer of timing
information. This usage is a purely formal reuse of the header
format of [RFC3550]. RTP mechanisms, such as header extensions,
contributing source (CSRC) list, padding, RTP Control Protocol
(RTCP), RTP header compression, Secure RTP (SRTP), etc., are not
applicable.
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The RTP timestamp indicates the packet creation time in units of a
common clock available to both communicating TDMoIP IWFs. When no
common clock is available, or when the TDMoIP IWFs have sufficiently
accurate local clocks or can derive sufficiently accurate timing
without explicit timestamps, the RTP header SHOULD be omitted.
If RTP is used, the fixed RTP header described in [RFC3550] MUST
immediately follow the control word for all PSN types except UDP/IP,
for which it MUST precede the control word. The version number MUST
be set to 2, the P (padding), X (header extension), CC (CSRC count),
and M (marker) fields in the RTP header MUST be set to zero, and the
payload type (PT) values MUST be allocated from the range of dynamic
values. The RTP sequence number MUST be identical to the sequence
number in the TDMoIP control word (see below). The RTP timestamp
MUST be generated in accordance with the rules established in
[RFC3550]; the clock frequency MUST be an integer multiple of 8 kHz,
and MUST be chosen to enable timing recovery that conforms with the
appropriate standards (see Section 7.2).
The 32-bit control word MUST appear in every TDMoIP packet. Its
format, in conformity with [RFC4385], is depicted in Figure 2.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RES |L|R| M |RES| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. Structure of the TDMoIP Control Word
RES (4 bits) The first nibble of the control word MUST be set to
zero when the PSN is MPLS, in order to ensure that the packet does
not alias an IP packet when forwarding devices perform deep packet
inspection. For PSNs other than MPLS, the first nibble MAY be set
to zero; however, in earlier versions of TDMoIP this field
contained a format identifier that was optionally used to specify
the payload format.
L Local Failure (1 bit) The L flag is set when the IWF has detected
or has been informed of a TDM physical layer fault impacting the
TDM data being forwarded. In the "trail extended" OAM scenario
the L flag MUST be set when the IWF detects loss of signal, loss
of frame synchronization, or AIS. When the L flag is set the
contents of the packet may not be meaningful, and the payload MAY
be suppressed in order to conserve bandwidth. Once set, if the
TDM fault is rectified the L flag MUST be cleared. Use of the L
flag is further explained in Section 6.
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R Remote Failure (1 bit) The R flag is set when the IWF has detected
or has been informed, that TDM data is not being received from the
remote TDM network, indicating failure of the reverse direction of
the bidirectional connection. An IWF SHOULD generate TDM Remote
Defect Indicator (RDI) upon receipt of an R flag indication. In
the "trail extended" OAM scenario the R flag MUST be set when the
IWF detects RDI. Use of the R flag is further explained in
Section 6.
M Defect Modifier (2 bits) Use of the M field is optional; when
used, it supplements the meaning of the L flag.
When L is cleared (indicating valid TDM data) the M field is used
as follows:
0 0 indicates no local defect modification.
0 1 reserved.
1 0 reserved.
1 1 reserved.
When L is set (invalid TDM data) the M field is used as follows:
0 0 indicates a TDM defect that should trigger conditioning
or AIS generation by the TDM-bound IWF.
0 1 indicates idle TDM data that should not trigger any alarm.
If the payload has been suppressed then the preconfigured
idle code should be generated at egress.
1 0 indicates corrupted but potentially recoverable TDM data.
1 1 reserved.
Use of the M field is further explained in Section 6.
RES (2 bits) These bits are reserved and MUST be set to zero.
Length (6 bits) is used to indicate the length of the TDMoIP packet
(control word and payload), in case padding is employed to meet
minimum transmission unit requirements of the PSN. It MUST be
used if the total packet length (including PSN, optional RTP,
control word, and payload) is less than 64 bytes, and MUST be set
to zero when not used.
Sequence number (16 bits) The TDMoIP sequence number provides the
common PW sequencing function described in [RFC3985], and enables
detection of lost and misordered packets. The sequence number
space is a 16-bit, unsigned circular space; the initial value of
the sequence number SHOULD be random (unpredictable) for security
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purposes, and its value is incremented modulo 2^16 separately for
each PW. Pseudocode for a sequence number processing algorithm
that could be used by a TDM-bound IWF is provided in Appendix A.
In order to form the TDMoIP payload, the PSN-bound IWF extracts bytes
from the continuous TDM stream, filling each byte from its most
significant bit. The extracted bytes are then adapted using one of
two adaptation algorithms (see Section 5), and the resulting adapted
payload is placed into the packet.
4. Encapsulation Details for Specific PSNs
TDMoIP PWs may exploit various PSNs, including UDP/IP (both IPv4 and
IPv6), L2TPv3 over IP (with no intervening UDP), MPLS, and layer-2
Ethernet. In the following subsections, we depict the packet format
for these cases.
For MPLS PSNs, the format is aligned with those specified in [Y1413]
and [Y1414]. For UDP/IP PSNs, the format is aligned with those
specified in [Y1453] and [Y1452]. For transport over layer 2
Ethernet the format is aligned with [MEF8].
4.1. UDP/IP
ITU-T recommendation Y.1453 [Y1453] describes structure-agnostic and
structure-aware mechanisms for transporting TDM over IP networks.
Similarly, ITU-T recommendation Y.1452 [Y1452] defines structure-
reassembly mechanisms for this purpose. Although the terminology
used here differs slightly from that of the ITU, implementations of
TDMoIP for UDP/IP PSNs as described herein will interoperate with
implementations designed to comply with Y.1453 subclause 9.2.2 or
Y.1452 clause 10.
For UDP/IPv4, the headers as described in [RFC768] and [RFC791] are
prefixed to the TDMoIP data. The format is similar for UDP/IPv6,
except the IP header described in [RFC2460] is used. The TDMoIP
packet structure is depicted in Figure 3.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPVER | IHL | IP TOS | Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | Protocol | IP Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port Number | Destination Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Length | UDP Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt|RTV|P|X| CC |M| PT | RTP Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt| SSRC identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RES |L|R| M |RES| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Adapted Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3. TDMoIP Packet Format for UDP/IP
The first five rows are the IP header, the sixth and seventh rows are
the UDP header. Rows 8 through 10 are the optional RTP header. Row
11 is the TDMoIP control word.
IPVER (4 bits) is the IP version number, e.g., IPVER=4 for IPv4.
IHL (4 bits) is the length in 32-bit words of the IP header, IHL=5.
IP TOS (8 bits) is the IP type of service.
Total Length (16 bits) is the length in bytes of header and data.
Identification (16 bits) is the IP fragmentation identification
field.
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Flags (3 bits) are the IP control flags and MUST be set to 2 in
order to avoid fragmentation.
Fragment Offset (13 bits) indicates where in the datagram the
fragment belongs and is not used for TDMoIP.
Time to Live (8 bits) is the IP time to live field. Datagrams with
zero in this field are to be discarded.
Protocol (8 bits) MUST be set to 0x11 (17) to signify UDP.
IP Header Checksum (16 bits) is a checksum for the IP header.
Source IP Address (32 bits) is the IP address of the source.
Destination IP Address (32 bits) is the IP address of the
destination.
Source and Destination Port Numbers (16 bits each)
Either the source UDP port or destination UDP port MAY be used to
multiplex and demultiplex individual PWs between nodes.
Architecturally [RFC3985], this makes the UDP port act as the PW
Label. PW endpoints MUST agree upon use of either the source UDP
or destination UDP port as the PW Label.
UDP ports MUST be manually configured by both endpoints of the PW.
The configured source or destination port (one or the other, but
not both) together with both the source and destination IP
addresses uniquely identify the PW. When the source UDP port is
used as the PW label, the destination UDP port number MUST be set
to the IANA assigned value of 0x085E (2142). All UDP port values
that function as PW labels SHOULD be in the range of dynamically
allocated UDP port numbers (0xC000 through 0xFFFF).
While many UDP-based protocols are able to traverse middleboxes
without dire consequences, the use of UDP ports as PW labels makes
middlebox traversal more difficult. Hence, it is NOT RECOMMENDED
to use UDP-based PWs where port-translating middleboxes are
present between PW endpoints.
UDP Length (16 bits) is the length in bytes of UDP header and data.
UDP Checksum (16 bits) is the checksum of UDP/IP header and data.
If not computed it MUST be set to zero.
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4.2. MPLS
ITU-T recommendation Y.1413 [Y1413] describes structure-agnostic and
structure-aware mechanisms for transporting TDM over MPLS networks.
Similarly, ITU-T recommendation Y.1414 [Y1413] defines structure-
reassembly mechanisms for this purpose. Although the terminology
used here differs slightly from that of the ITU, implementations of
TDMoIP for MPLS PSNs as described herein will interoperate with
implementations designed to comply with Y.1413 subclause 9.2.2 or
Y.1414 clause 10.
The MPLS header as described in [RFC3032] is prefixed to the control
word and TDM payload. The packet structure is depicted in Figure 4.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tunnel Label | EXP |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW label | EXP |1| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RES |L|R| M |RES| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt|RTV|P|X| CC |M| PT | RTP Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt| SSRC identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Adapted Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4. TDMoIP Packet Format for MPLS
The first two rows depicted above are the MPLS header; the third is
the TDMoIP control word. Fields not previously described will now be
explained.
Tunnel Label (20 bits) is the MPLS label that identifies the MPLS
LSP used to tunnel the TDM packets through the MPLS network. The
label can be assigned either by manual provisioning or via an MPLS
control protocol. While transiting the MPLS network there may be
zero, one, or several tunnel label rows. For label stack usage
see [RFC3032].
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EXP (3 bits) experimental field, may be used to carry Diffserv
classification for tunnel labels.
S (1 bit) the stacking bit indicates MPLS stack bottom. S=0 for all
tunnel labels, and S=1 for the PW label.
TTL (8 bits) MPLS Time to live.
PW Label (20 bits) This label MUST be a valid MPLS label, and MAY be
configured or signaled.
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4.3. L2TPv3
The L2TPv3 header defined in [RFC3931] is prefixed to the TDMoIP
data. The packet structure is depicted in Figure 5.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPVER | IHL | IP TOS | Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | Protocol | IP Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session ID = PW label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cookie 1 (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cookie 2 (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RES |L|R| M |RES| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt|RTV|P|X| CC |M| PT | RTP Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt| SSRC identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Adapted Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5. TDMoIP Packet Format for L2TPv3
Rows 6 through 8 are the L2TPv3 header. Fields not previously
described will now be explained.
Protocol (8 bits) is the IP protocol field. It must be set to 0x73
(115), the user port number that has been assigned to L2TP by
IANA.
Session ID (32 bits) is the locally significant L2TP session
identifier, and contains the PW label. The value 0 is reserved.
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Cookie (32 or 64 bits) is an optional field that contains a randomly
selected value that can be used to validate association of the
received frame with the expected PW.
4.4. Ethernet
Metro Ethernet Forum Implementation Agreement 8 [MEF8] describes
structure-agnostic and structure-aware mechanisms for transporting
TDM over Ethernet networks. Implementations of structure-indicated
TDMoIP as described herein will interoperate with implementations
designed to comply with MEF 8 Section 6.3.3.
The TDMoIP payload is encapsulated in an Ethernet frame by prefixing
the Ethernet destination and source MAC addresses, optional VLAN
header, and Ethertype, and suffixing the four-byte frame check
sequence. TDMoIP implementations MUST be able to receive both
industry standard (DIX) Ethernet and IEEE 802.3 [IEEE802.3] frames
and SHOULD transmit Ethernet frames.
Ethernet encapsulation introduces restrictions on both minimum and
maximum packet size. Whenever the entire TDMoIP packet is less than
64 bytes, padding is introduced and the true length indicated by
using the Length field in the control word. In order to avoid
fragmentation, the TDMoIP packet MUST be restricted to the maximum
payload size. For example, the length of the Ethernet payload for a
UDP/IP encapsulation of AAL1 format payload with 30 PDUs per packet
is 1472 bytes, which falls below the maximal permitted payload size
of 1500 bytes.
Ethernet frames MAY be used for TDMoIP transport without intervening
IP or MPLS layers, however, an MPLS-style label MUST always be
present. In this four-byte header S=1, and all other non-label bits
are reserved (set to zero in the PSN-bound direction and ignored in
the TDM-bound direction). The Ethertype SHOULD be set to 0x88D8
(35032), the value allocated for this purpose by the IEEE, but MAY be
set to 0x8847 (34887), the Ethertype of MPLS. The overall frame
structure is as follows:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination MAC Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Destination MAC Address (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source MAC Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source MAC Address (cont) | VLAN Ethertype (opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|VLP|C| VLAN ID (opt) | Ethertype |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW label | RES |1| RES |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RES |L|R| M |RES| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt|RTV|P|X| CC |M| PT | RTP Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt| SSRC identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Adapted Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Frame Check Sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6. TDMoIP Packet Format for Ethernet
Rows 1 through 6 are the (DIX) Ethernet header; for 802.3 there may
be additional fields, depending on the value of the length field, see
[IEEE802.3]. Fields not previously described will now be explained.
Destination MAC Address (48 bits) is the globally unique address of
a single station that is to receive the packet. The format is
defined in [IEEE802.3].
Source MAC Address (48 bits) is the globally unique address of the
station that originated the packet. The format is defined in
[IEEE802.3].
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VLAN Ethertype (16 bits) 0x8100 in this position indicates that
optional VLAN tagging specified in [IEEE802.1Q] is employed, and
that the next two bytes contain the VLP, C, and VLAN ID fields.
VLAN tags may be stacked, in which case the two-byte field
following the VLAN ID is once again a VLAN Ethertype.
VLP (3 bits) is the VLAN priority, see [IEEE802.1Q].
C (1 bit) the "canonical format indicator" being set, indicates that
route descriptors appear; see [IEEE802.1Q].
VLAN ID (12 bits) the VLAN identifier uniquely identifies the VLAN
to which the frame belongs. If zero, only the VLP information is
meaningful. Values 1 and FFF are reserved. The other 4093 values
are valid VLAN identifiers.
Ethertype (16 bits) is the protocol identifier, as allocated by the
IEEE. The Ethertype SHOULD be set to 0x88D8 (35032), but MAY be
set to 0x8847 (34887).
PW Label (20 bits) This label MUST be manually configured. The
remainder of this row is formatted to resemble an MPLS label.
Frame Check Sequence (32 bits) is a Cyclic Redundancy Check (CRC)
error detection field, calculated per [IEEE802.3].
5. TDMoIP Payload Types
As discussed at the end of Section 3, TDMoIP transports real-time
streams by first extracting bytes from the stream, and then adapting
these bytes. TDMoIP offers two different adaptation algorithms, one
for constant-rate real-time traffic, and one for variable-rate real-
time traffic.
For unstructured TDM, or structured but unchannelized TDM, or
structured channelized TDM with all channels active all the time, a
constant-rate adaptation is needed. In such cases TDMoIP uses
structure-indication to emulate the native TDM circuit, and the
adaptation is known as "circuit emulation". However, for channelized
TDM wherein the individual channels (corresponding to "loops" in
telephony terminology) are frequently inactive, bandwidth may be
conserved by transporting only active channels. This results in
variable-rate real-time traffic, for which TDMoIP uses structure-
reassembly to emulate the individual loops, and the adaptation is
known as "loop emulation".
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TDMoIP uses constant-rate AAL1 [AAL1,CES] for circuit emulation,
while variable-rate AAL2 [AAL2] is employed for loop emulation. The
AAL1 mode MUST be used for structured transport of unchannelized data
and SHOULD be used for circuits with relatively constant usage. In
addition, AAL1 MUST be used when the TDM-bound IWF is required to
maintain a high timing accuracy (e.g., when its timing is further
distributed) and SHOULD be used when high reliability is required.
AAL2 SHOULD be used for channelized TDM when bandwidth needs to be
conserved, and MAY be used whenever usage of voice-carrying channels
is expected to be highly variable.
Additionally, a third mode is defined specifically for efficient
transport of High-Level Data Link Control (HDLC)-based Common Channel
Signaling (CCS) carried in TDM channels.
The AAL family of protocols is a natural choice for TDM emulation.
Although originally developed to adapt various types of application
data to the rigid format of ATM, the mechanisms are general solutions
to the problem of transporting constant or variable-rate real-time
streams over a packet network.
Since the AAL mechanisms are extensively deployed within and on the
edge of the public telephony system, they have been demonstrated to
reliably transfer voice-grade channels, data and telephony signaling.
These mechanisms are mature and well understood, and implementations
are readily available.
Finally, simplified service interworking with legacy networks is a
major design goal of TDMoIP. Re-use of AAL technologies simplifies
interworking with existing AAL1- and AAL2-based networks.
5.1. AAL1 Format Payload
For the prevalent cases of unchannelized TDM, or channelized TDM for
which the channel allocation is static, the payload can be
efficiently encoded using constant-rate AAL1 adaptation. The AAL1
format is described in [AAL1] and its use for circuit emulation over
ATM in [CES]. We briefly review highlights of AAL1 technology in
Appendix B. In this section we describe the use of AAL1 in the
context of TDMoIP.
+-------------+----------------+
|control word | AAL1 PDU |
+-------------+----------------+
Figure 7a. Single AAL1 PDU per TDMoIP Packet
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+-------------+----------------+ +----------------+
|control word | AAL1 PDU |---| AAL1 PDU |
+-------------+----------------+ +----------------+
Figure 7b. Multiple AAL1 PDUs per TDMoIP Packet
In AAL1 mode the TDMoIP payload consists of at least one, and perhaps
many, 48-byte "AAL1 PDUs", see Figures 7a and 7b. The number of PDUs
MUST be pre-configured and MUST be chosen such that the overall
packet size does not exceed the maximum allowed by the PSN (e.g., 30
for UDP/IP over Ethernet). The precise number of PDUs per packet is
typically chosen taking latency and bandwidth constraints into
account. Using a single PDU delivers minimal latency, but incurs the
highest overhead. All TDMoIP implementations MUST support between 1
and 8 PDUs per packet for E1 and T1 circuits, and between 5 and 15
PDUs per packet for E3 and T3 circuits.
AAL1 differentiates between unstructured and structured data
transfer, which correspond to structure-agnostic and structure-aware
transport. For structure-agnostic transport, AAL1 provides no
inherent advantage as compared to SAToP; however, there may be
scenarios for which its use is desirable. For example, when it is
necessary to interwork with an existing AAL1 ATM circuit emulation
system, or when clock recovery based on AAL1-specific mechanisms is
favored.
For structure-aware transport, [CES] defines two modes, structured
and structured with Channel Associated Signaling (CAS). Structured
AAL1 maintains TDM frame synchronization by embedding a pointer to
the beginning of the next frame in the AAL1 PDU header. Similarly,
structured AAL1 with CAS maintains TDM frame and multiframe
synchronization by embedding a pointer to the beginning of the next
multiframe. Furthermore, structured AAL1 with CAS contains a
substructure including the CAS signaling bits.
5.2. AAL2 Format Payload
Although AAL1 may be configured to transport fractional E1 or T1
circuits, the allocation of channels to be transported must be static
due to the fact that AAL1 transports constant-rate bit-streams. It
is often the case that not all the channels in a TDM circuit are
simultaneously active ("off-hook"), and activity status may be
determined by observation of the TDM signaling channel. Moreover,
even during active calls, about half the time is silence that can be
identified using voice activity detection (VAD). Using the variable-
rate AAL2 mode, we may dynamically allocate channels to be
transported, thus conserving bandwidth.
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The AAL2 format is described in [AAL2] and its use for loop emulation
over ATM is explained in [SSCS,LES]. We briefly review highlights of
AAL2 technology in Appendix C. In this section, we describe the use
of AAL2 in the context of TDMoIP.
+-------------+----------------+ +----------------+
|control word | AAL2 PDU |---| AAL2 PDU |
+-------------+----------------+ +----------------+
Figure 8. Concatenation of AAL2 PDUs in a TDMoIP Packet
In AAL2 mode the TDMoIP payload consists of one or more variable-
length "AAL2 PDUs", see Figure 8. Each AAL2 PDU contains 3 bytes of
overhead and between 1 and 64 bytes of payload. A packet may be
constructed by inserting PDUs corresponding to all active channels,
by appending PDUs ready at a certain time, or by any other means.
Hence, more than one PDU belonging to a single channel may appear in
a packet.
[RFC3985] denotes as Native Service Processing (NSP) functions all
processing of the TDM data before its use as payload. Since AAL2 is
inherently variable rate, arbitrary NSP functions MAY be performed
before the channel is placed in the AAL2 loop emulation payload.
These include testing for on-hook/off-hook status, voice activity
detection, speech compression, fax/modem/tone relay, etc.
All mechanisms described in [AAL2,SSCS,LES] may be used for TDMoIP.
In particular, channel identifier (CID) encoding and use of PAD
octets according to [AAL2], encoding formats defined in [SSCS], and
transport of CAS and CCS signaling as described in [LES] MAY all be
used in the PSN-bound direction, and MUST be supported in the TDM-
bound direction. The overlap functionality and AAL-CU timer and
related functionalities may not be required, and the STF (start
field) is NOT used. Computation of error detection codes -- namely,
the Header Error Check (HEC) in the AAL2 PDU header and the CRC in
the CAS packet -- is superfluous if an appropriate error detection
mechanism is provided by the PSN. In such cases, these fields MAY be
set to zero.
5.3. HDLC Format Payload
The motivation for handling HDLC in TDMoIP is to efficiently
transport common channel signaling (CCS) such as SS7 [SS7] or ISDN
PRI signaling [ISDN-PRI], embedded in the TDM stream. This mechanism
is not intended for general HDLC payloads, and assumes that the HDLC
messages are always shorter than the maximum packet size.
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The HDLC mode should only be used when the majority of the bandwidth
of the input HDLC stream is expected to be occupied by idle flags.
Otherwise, the CCS channel should be treated as an ordinary channel.
The HDLC format is intended to operate in port mode, transparently
passing all HDLC data and control messages over a separate PW. The
encapsulation is compatible with that of [RFC4618], however the
sequence number generation and processing SHOULD be performed
according to Section 3 above.
The PSN-bound IWF monitors flags until a frame is detected. The
contents of the frame are collected and the Frame Check Sequence
(FCS) tested. If the FCS is incorrect, the frame is discarded;
otherwise, the frame is sent after initial or final flags and FCS
have been discarded and zero removal has been performed. When a
TDMoIP-HDLC frame is received, its FCS is recalculated, and the
original HDLC frame reconstituted.
6. TDMoIP Defect Handling
Native TDM networks signify network faults by carrying indications of
forward defects (AIS) and reverse defects (RDI) in the TDM bit
stream. Structure-agnostic TDM transport transparently carries all
such indications; however, for structure-aware mechanisms where the
PSN-bound IWF may remove TDM structure overhead carrying defect
indications, explicit signaling of TDM defect conditions is required.
We saw in Section 3 that defects can be indicated by setting flags in
the control word. This insertion of defect reporting into the packet
rather than in a separate stream mimics the behavior of native TDM
OAM mechanisms that carry such indications as bit patterns embedded
in the TDM stream. The flags are designed to address the urgent
messaging, i.e., messages whose contents must not be significantly
delayed with respect to the TDM data that they potentially impact.
Mechanisms for slow OAM messaging are discussed in Appendix D.
+---+ +-----+ +------+ +-----+ +------+ +-----+ +---+
|TDM|->-| |->-|TDMoIP|->-| |->-|TDMoIP|->-| |->-|TDM|
| | |TDM 1| | | | PSN | | | |TDM 2| | |
|ES1|-<-| |-<-| IWF1 |-<-| |-<-| IWF2 |-<-| |-<-|ES2|
+---+ +-----+ +------+ +-----+ +------+ +-----+ +---+
Figure 9. Typical TDMoIP Network Configuration
The operation of TDMoIP defect handling is best understood by
considering the downstream TDM flow from TDM end system 1 (ES1)
through TDM network 1, through TDMoIP IWF 1 (IWF1), through the PSN,
through TDMoIP IWF 2 (IWF2), through TDM network 2, towards TDM end
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system 2 (ES2), as depicted in the figure. We wish not only to
detect defects in TDM network 1, the PSN, and TDM network 2, but to
localize such defects in order to raise alarms only in the
appropriate network.
In the "trail terminated" OAM scenario, only user data is exchanged
between TDM network 1 and TDM network 2. The IWF functions as a TDM
trail termination function, and defects detected in TDM network 1 are
not relayed to network 2, or vice versa.
In the "trail extended" OAM scenario, if there is a defect (e.g.,
loss of signal or loss of frame synchronization) anywhere in TDM
network 1 before the ultimate link, the following TDM node will
generate AIS downstream (towards TDMoIP IWF1). If a break occurs in
the ultimate link, the IWF itself will detect the loss of signal. In
either case, IWF1 having directly detected lack of validity of the
TDM signal, or having been informed of an earlier problem, raises the
local ("L") defect flag in the control word of the packets it sends
across the PSN. In this way the trail is extended to TDM network 2
across the PSN.
Unlike forward defect indications that are generated by all network
elements, reverse defect indications are only generated by trail
termination functions. In the trail terminated scenario, IWF1 serves
as a trail termination function for TDM network 1, and thus when IWF1
directly detects lack of validity of the TDM signal, or is informed
of an earlier problem, it MAY generate TDM RDI towards TDM ES1. In
the trail extended scenario IWF1 is not a trail termination, and
hence MUST NOT generate TDM RDI, but rather, as we have seen, sets
the L defect flag. As we shall see, this will cause the AIS
indication to reach ES2, which is the trail termination, and which
MAY generate TDM RDI.
When the L flag is set there are four possibilities for treatment of
payload content. The default is for IWF1 to fill the payload with
the appropriate amount of AIS (usually all-ones) data. If the AIS
has been generated before the IWF this can be accomplished by copying
the received TDM data; if the penultimate TDM link fails and the IWF
needs to generate the AIS itself. Alternatively, with structure-
aware transport of channelized TDM one SHOULD fill the payload with
"trunk conditioning"; this involves placing a preconfigured "out of
service" code in each individual channel (the "out of service" code
may differ between voice and data channels). Trunk conditioning MUST
be used when channels taken from several TDM PWs are combined by the
TDM-bound IWF into a single TDM circuit. The third possibility is to
suppress the payload altogether. Finally, if IWF1 believes that the
TDM defect is minor or correctable (e.g., loss of multiframe
synchronization, or initial phases of detection of incorrect frame
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sync), it MAY place the TDM data it has received into the payload
field, and specify in the defect modification field ("M") that the
TDM data is corrupted, but potentially recoverable.
When IWF2 receives a local defect indication without M field
modification, it forwards (or generates if the payload has been
suppressed) AIS or trunk conditioning towards ES2 (the choice between
AIS and conditioning being preconfigured). Thus AIS has been
properly delivered to ES2 emulating the TDM scenario from the TDM end
system's point of view. In addition, IWF2 receiving the L flag
uniquely specifies that the defect was in TDM network 1 and not in
TDM network 2, thus suppressing alarms in the correctly functioning
network.
If the M field indicates that the TDM has been marked as potentially
recoverable, then implementation specific algorithms (not herein
specified) may optionally be utilized to minimize the impact of
transient defects on the overall network performance. If the M field
indicates that the TDM is "idle", no alarms should be raised and IWF2
treats the payload contents as regular TDM data. If the payload has
been suppressed, trunk conditioning and not AIS MUST be generated by
IWF2.
The second case is when the defect is in TDM network 2. Such defects
cause AIS generation towards ES2, which may respond by sending TDM
RDI in the reverse direction. In the trail terminated scenario this
RDI is restricted to network 2. In the trail extended scenario, IWF2
upon observing this RDI inserted into valid TDM data, MUST indicate
this by setting the "R" flag in packets sent back across the PSN
towards IWF1. IWF1, upon receiving this indication, generates RDI
towards ES1, thus emulating a single conventional TDM network.
The final possibility is that of a unidirectional defect in the PSN.
In such a case, TDMoIP IWF1 sends packets toward IWF2, but these are
not received. IWF2 MUST inform the PSN's management system of this
problem, and furthermore generate TDM AIS towards ES2. ES2 may
respond with TDM RDI, and as before, in the trail extended scenario,
when IWF2 detects RDI it MUST raise the "R" flag indication. When
IWF1 receives packets with the "R" flag set it has been informed of a
reverse defect, and MUST generate TDM RDI towards ES1.
In all cases, if any of the above defects persist for a preconfigured
period (default value of 2.5 seconds) a service failure is declared.
Since TDM PWs are inherently bidirectional, a persistent defect in
either directional results in a bidirectional service failure. In
addition, if signaling is sent over a distinct PW as per Section 5.3,
both PWs are considered to have failed when persistent defects are
detected in either.
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When failure is declared the PW MUST be withdrawn, and both TDMoIP
IWFs commence sending AIS (and not trunk conditioning) to their
respective TDM networks. The IWFs then engage in connectivity
testing using native methods or TDMoIP OAM as described in Appendix D
until connectivity is restored.
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.
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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.
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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.
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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].
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RFC 5087 TDMoIP December 2007
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.
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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.
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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)
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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
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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.
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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.
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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,
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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.
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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.
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RFC 5087 TDMoIP December 2007
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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.
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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.
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RFC 5087 TDMoIP December 2007
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.
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RFC 5087 TDMoIP December 2007
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).
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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.
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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)
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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
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RFC 5087 TDMoIP December 2007
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.
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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)
Stein, et al. Informational [Page 45]
RFC 5087 TDMoIP December 2007
[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.
Stein, et al. Informational [Page 46]
RFC 5087 TDMoIP 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.
Stein, et al. Informational [Page 47]
RFC 5087 TDMoIP December 2007
[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.
Stein, et al. Informational [Page 48]
RFC 5087 TDMoIP December 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
Stein, et al. Informational [Page 49]
RFC 5087 TDMoIP December 2007
Full Copyright Statement
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Stein, et al. Informational [Page 50]
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