PWE3 Y(J) Stein
Internet-Draft R. Shashoua
Expires: November 29, 2004 R. Insler
M. Anavi
RAD Data Communications
May 31, 2004
TDM over IP
draft-ietf-pwe3-tdmoip-01.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes methods for structure-aware transport of TDM
traffic over packet switched networks using pseudowires.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. TDMoIP Encapsulation . . . . . . . . . . . . . . . . . . . . 5
3. Encapsulation Details for Specific PSNs . . . . . . . . . . 8
3.1 UDP/IP . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2 MPLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3 L2TPv3 . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4 Ethernet . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4. TDMoIP Payload types . . . . . . . . . . . . . . . . . . . . 14
4.1 AAL1 Format Payload . . . . . . . . . . . . . . . . . . . . 15
4.2 AAL2 Format Payload . . . . . . . . . . . . . . . . . . . . 16
4.3 HDLC Format Payload . . . . . . . . . . . . . . . . . . . . 17
5. TDMoIP Defect Handling . . . . . . . . . . . . . . . . . . . 18
6. Implementation Issues . . . . . . . . . . . . . . . . . . . 20
6.1 Jitter and Packet Loss . . . . . . . . . . . . . . . . . . . 20
6.2 Timing Recovery . . . . . . . . . . . . . . . . . . . . . . 21
6.3 Quality of Service . . . . . . . . . . . . . . . . . . . . . 22
7. Security Considerations . . . . . . . . . . . . . . . . . . 23
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . 23
9. Trademarks . . . . . . . . . . . . . . . . . . . . . . . . . 23
10. IPR Disclosure Acknowledgement . . . . . . . . . . . . . . . 23
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
11.1 Normative References . . . . . . . . . . . . . . . . . . . . 24
11.2 Informative References . . . . . . . . . . . . . . . . . . . 25
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 26
A. Sequence Number Processing . . . . . . . . . . . . . . . . . 27
B. AAL1 Review . . . . . . . . . . . . . . . . . . . . . . . . 29
C. AAL2 Review . . . . . . . . . . . . . . . . . . . . . . . . 33
D. TDMoIP OAM Mechanisms . . . . . . . . . . . . . . . . . . . 35
D.1 TDMoIP Connectivity Verification Messages . . . . . . . . . 35
D.2 Performance Measurements . . . . . . . . . . . . . . . . . . 36
D.3 OAM Packet Format . . . . . . . . . . . . . . . . . . . . . 36
Full Copyright Statement . . . . . . . . . . . . . . . . . . 39
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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),
integration of TDM services into a unified PSN infrastructure has
become desirable. Such integration requires emulation of TDM
circuits within the PSN, a function that can be carried out using
pseudowires (PWs), as described in the PWE3 architecture [PWE-ARCH].
This emulation must ensure QoS and voice quality similar to those of
existing TDM networks as well as preserving signaling features, as
described in the TDM PW requirements [TDM-REQ].
The interworking function that connects between the TDM and PSN
worlds will be called a TDMoIP gateway (GW), and it may be situated
at the provider edge (PE) or at the customer edge (CE). The TDM
gateway that encapsulates TDM and injects packets into the PSN will
be called the PSN-bound gateway, while the gateway that extracts TDM
data from packets and generates traffic on a TDM network will be
called the TDM-bound gateway. Emulated TDM circuits are always
point-to-point, bidirectional, and transport the same TDM rate in
both directions.
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 [G.704] of length 193 and 256 bits, respectively. T3 and E3
frames [G.704,G.751] 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.
TDM circuits are often used to transport multiplexed 64 kbps
channels. A frame of a channelized T1 carries 24 byte-sized
channels, while an E1 frame consists of 32 channels. By
concatenation of consecutive T1 or E1 frames we can build higher
level structures called superframes or multiframes.
TDM structures are universally delimited placing an easily detectable
periodic bit pattern, called the Frame Alignment Signal (FAS), in the
structure overhead. We will use the term "structured TDM" to refer
to TDM with any level of structure imposed by an FAS. Unstructured
TDM means that no structure has been imposed, so that all bits in the
bit stream are available for user data.
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SAToP [SAToP] is a structure-agnostic protocol for transporting TDM
over PWs. SAToP treats the TDM as a 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, and
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.
When it is required or desirable to explicitly safeguard TDM
structure during transport over the PSN, structure-aware TDM
transport must be employed. 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 might not
transport structure overhead across the PSN; in particular, the FAS
MAY be stripped by the PSN-bound GW and MUST be regenerated by the
TDM-bound GW. However, structure overhead MAY be transported over
the PSN, since in addition to FAS it can contain maintenance
information.
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 meaningful for channelized TDM; the PSN-
bound GW extracts and buffers the individual channels, and the
original structure is reassembled from the received constituents by
the TDM-bound GW.
All three methods of TDM structure preservation have their
advantages. Structure-locking is described in [CESoPSN], while the
present document specifies both structure-indication (see Section
4.1) and structure-reassembly (see Section 4.2) 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.
Despite its name, the TDMoIP(R) protocol herein described tolerates
several types of PSN, including UDP over IPv4 or IPv6, MPLS, L2TPv3
over IP, or pure Ethernet. Implementation specifics for particular
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PSNs are discussed in Section 3. Although the protocol should be
more generally called TDMoPW and its specific implementations TDMoIP,
TDMoMPLS, etc. We will use the nomenclature TDMoIP for reasons of
consistency with earlier usage.
2. TDMoIP Encapsulation
The overall format of TDMoIP packets is shown in the following
figure.
+---------------------+
| PSN Headers |
+---------------------+
| TDMoIP Control Word |
+---------------------+
| Adapted Payload |
+---------------------+
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 GW 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 gateway may simultaneously support multiple TDM PWs, and the
TDMoIP gateway 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. In
general the PW labels of these PWs will take different values.
In addition to the aforementioned headers, an OPTIONAL 12-byte RTP
header may appear in order to provide a mechanism for explicit
transfer of timing information in the packet. If RTP is used, the
fixed RTP header described in [RTP], MUST immediately precede the
control word for UDP/IP, and MUST immediately follow it for all other
cases. The P (padding), X (header extension), CC (CSRC count), and M
(marker) fields in the RTP header MUST be set to zero, and the 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 generated in
accordance with the rules established in [RTP]; the clock frequency
should be an integer multiple of 8 kHz, and MUST be chosen to enable
timing recovery that conforms with the appropriate standards (see
Section 6.2). When the TDMoIP gateways have sufficiently accurate
local clocks or can derive sufficiently accurate timing without
explicit timestamps, the RTP header SHOULD be omitted.
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The 32-bit control word MUST appear in every TDMoIP packet. Its
format is depicted in the following figure.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|FORMID |L|R| M |RES| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FORMID Format identifier (4 bits) is an OPTIONAL field that MAY be
used to specify the payload format. When it is not used it MUST
be set to zero by the PSN-bound GW and ignored by the TDM-bound
GW. It SHOULD NOT be used when PSN forwarding mechanisms identify
PWs based on the initial four bits of their packets being zero.
The following values are presently defined (none of which alias an
IP packet):
1100 AAL1 unstructured
1101 AAL1 structured
1110 AAL1 structured with CAS
1001 AAL2
1111 HDLC mode
The payload format for each of these cases will be described in
Section 4.
L Local TDM Failure (1 bit) The L flag being set indicates that the
PSN-bound GW has detected or has been informed of a TDM physical
layer fault impacting the TDM data being forwarded. The L flag
MUST be set when the GW 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 5.
R Remote PSN Failure (1 bit) The R flag being set indicates that the
PSN-bound GW is not receiving packets from the PSN, indicating
failure of the reverse direction of the bidirectional connection.
The R flag MUST be set after a preconfigured number of consecutive
packets are not received, and MUST be cleared once packets are
once again received. The TDMoIP gateway MAY be configured to
generate TDM RDI upon receipt of an R flag indication. The R flag
can be used to signal congestion or other PSN faults, and may
trigger fall-back mechanisms for congestion avoidance. Use of the
R flag is further explained in Section 5.
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Defect Modifier (2 bits) Use of the M field is optional, and when
used 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 reports receipt of TDM RDI at PSN-bound GW.
1 1 reserved.
When L is set (indicating 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 gateway.
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 5.
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 [PWE-ARCH], 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
purposes, and its value is incremented modulo 2^16 separately for
PW. Pseudocode for a sequence number processing algorithm that
could be used by a TDM-bound GW is provided in Appendix A.
In order to form the TDMoIP payload, the PSN-bound GW 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 4), and the resulting adapted
payload is placed into the packet.
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3. Encapsulation Details for Specific PSNs
3.1 UDP/IP
The UDP/IP header as described in [UDP] and [IP] is prefixed to the
TDMoIP data. The TDMoIP packet structure is as follows:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VER | PW label | Destination Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Length | UDP Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt|RTV|P|X| CC |M| PT | RTP Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt| SSRC identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|FORMID |L|R| M |RES| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Adapted Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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. for IPv4 IPVER=4.
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.
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Identification (16 bits) is the IP fragmentation identification
field.
Flags (3 bits) are the IP control flags and MUST be set to Flags=010
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 11 hex = 17 dec 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.
VER (3 bits) is the TDMoIP version number. The original version
(VER=000) was experimental and should no longer be used.
Presently VER=001 when RTP is not used, and VER=011 when RTP is
used.
PW label (13 bits) This field is usually dedicated to the Source
Port Number, but here identifies the unique data stream emanating
from a given TDM circuit and sharing a common destination. This
nonstandard use of a UDP port number is similar to RTP/RTCP's use
of port numbers to uniquely identify sessions, and to allocation
of arbitrary UDP port numbers for VoIP sessions. Placing the PW
label in the UDP header rather than the application area
facilitates implementation. The value 0 is reserved; a
preconfigured (default 1FFF hex = 8191 dec) label value is used
for PW-layer OAM messages (see Appendix D); other PW labels in the
range 1-8191 are available for use.
Destination Port Number (16 bits) MUST be set to 0x085E (2142), the
user port number that has been assigned to TDMoIP by IANA.
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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3.2 MPLS
ITU-T recommendation Y.1413 [Y1413] describes structure-agnostic and
structure-aware mechanisms for transporting TDM over MPLS networks.
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.
The MPLS header as described in [MPLS] is prefixed to the control
word and TDM payload. The packet structure is as follows:
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 |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|FORMID |L|R| M |RES| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt|RTV|P|X| CC |M| PT | RTP Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt| SSRC identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Adapted Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
[MPLS].
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.
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TTL (8 bits) MPLS Time to live. Should be set to 2 for the PW label.
PW Label (20 bits) The value 0 is reserved; a preconfigured (default
FFFFF hex = 1048575 dec) label value is used for PW-layer OAM
messages (see Appendix D); other PW labels are available for use.
3.3 L2TPv3
L2TPv3 may be used directly over IP. The L2TPv3 header defined in
[L2TPv3] is prefixed to the TDMoIP data. The packet structure is as
follows:
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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|FORMID |L|R| RES | Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt|RTV|P|X| CC |M| PT | RTP Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt| SSRC identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Adapted Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Rows 6 through 8 are the L2TPv3 header. Fields not previously
described will now be explained.
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Protocol the IP protocol field must be set to 73 hex = 115 dec, 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; a
preconfigured (default FFFFFFFF hex) label value is used for PW-
layer OAM messages (see Appendix D); other PW labels are available
for use.
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.
3.4 Ethernet
The TDMoIP packet described in the previous subsections will
frequently be further encapsulated in an Ethernet frame by prefixing
the Ethernet preamble, 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, zero 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 1460 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. In this case an MPLS-style label will always be
present, but if VLAN tags are sufficient to identify the PW the MPLS
label MUST be set to one. The Ethertype SHOULD be set to the value
allocated for CESoETH, but MAY be set to the Ethertype of MPLS. The
packet 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|FORMID |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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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].
VLAN Ethertype (16 bits) a 8100 hex in this position indicates that
optional VLAN tagging according to [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.
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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 4193 values
are valid VLAN identifiers.
Ethertype (16 bits) is the protocol identifier, as allocated by the
IEEE.
Frame Check Sequence (32 bits) is a CRC error detection field,
calculated per [IEEE802.3].
4. TDMoIP Payload types
As discussed at the end of Section 2, 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.
Since native TDM is always constant bit-rate, why is a variable rate
adaptation needed? For unstructured TDM, or structured but
unchannelized TDM, of structured channelized TDM with all channels
active all the time, there is indeed no need. In such cases TDMoIP
uses structure-indication to emulate the native TDM circuit,
utilizing an adaptation known as circuit emulation. However,
individual "local loops" are frequently "on-hook" and thus inactive,
and bandwidth may be conserved by transporting only channels
corresponding to active loops. This results in variable rate real-
time traffic, for which TDMoIP uses structure-reassembly to emulate
the individual loops, utilizing an adaptation known as loop
emulation.
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 GW 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
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defined specifically for efficient transport of HDLC-based CCS
signaling 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.
4.1 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 |
+-------------+----------------+
Single AAL1 PDU per TDMoIP packet
+-------------+----------------+ +----------------+
|control word | AAL1 PDU |---| AAL1 PDU |
+-------------+----------------+ +----------------+
Multiple AAL1 PDUs per TDMoIP packet
In AAL1 mode the TDMoIP payload consists of between one and thirty
48-byte "AAL1 PDUs". The number of PDUs must be pre-configured and
typically chosen according to latency and bandwidth constraints.
Using a single PDU reduces latency to a minimum, but incurs the
highest overhead, while using, for example, eight PDUs reduces the
overhead percentage while increasing the latency by a factor of
eight. All TDMoIP implementations MUST support between 1 and 8 PDUs
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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 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.
4.2 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 by observation of the TDM
signaling channel activity status may be determined. 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.
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 |
+-------------+----------------+ +----------------+
Concatenation of AAL2 PDUs in a TDMoIP packet
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In AAL2 mode the TDMoIP payload consists of one or more variable-
length "AAL2 PDUs". 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.
[PWE-ARCH] 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, CID encoding according to [AAL2], encoding formats
defined in [SSCS], and transport of CAS and CCS signaling as
described in [LES] may all be used. The overlap functionality and
AAL-CU timer and related functionalities are not required, and the
STF field is NOT used. Computation of error detection codes, namely
the HEC in the AAL2 PDU header and the CRC in the CAS packet, MAY be
omitted, if an appropriate error detection mechanism is provided by
the PSN. In such cases these fields MUST be set to zero.
4.3 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.
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 PSN-bound GW monitors flags until a frame is detected. The
contents of the frame are collected and the 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 an TDMoIP- HDLC frame is received
its FCS is recalculated, and the original HDLC frame reconstituted.
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5. 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 transports
all such indications; however, structure-aware mechanisms for which
the PSN-bound GW removes TDM structure overhead will require explicit
signaling of TDM defect conditions. Interworking of TDM defect
indications with native PSN OAM mechanisms, as well as translation of
PSN OAM defect states to the appropriate TDM ones, is a subject for
further study.
We saw in Section 2 that TDM and PSN 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|
|end| |TDM 1| | GW | | PSN | | GW | |TDM 2| |end|
| 1 |-<-| |-<-| 1 |-<-| |-<- | 2 |-<-| |-<-| 2 |
+---+ +-----+ +------+ +-----+ +------+ +-----+ +---+
Example TDMoIP network configuration
To understand the operation of TDMoIP defect handling, let us
consider the downstream TDM flow from TDM end equipment 1 through TDM
network 1, through the PSN, through TDM network 2, towards TDM end
equipment 2, as depicted in the figure. We wish not only to detect
defects in TDM network 1, the PSN, or TDM network 2, but to localize
the defect in order to raise alarms only in the appropriate network.
If there is a defect (e.g. loss of signal or loss of
synchronization) anywhere in TDM network 1 before the last link, the
following TDM node will generate AIS downstream (towards TDMoIP GW 1)
and RDI upstream (towards TDM end equipment 1). If the failure is in
the link, the GW itself will detect the loss of signal, and must
generate RDI upstream. In either case, GW 1 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.
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When the "L" flag is set there are four possibilities for treatment
of payload content. The default is for GW 1 to fill the payload with
the appropriate amount of AIS (usually all-ones) data. If the AIS
has been generated before the GW this can be accomplished by copying
the received TDM data; if the penultimate TDM link fails and the GW
needs to generate the AIS itself. Alternatively, structure-aware
transport of channelized TDM MAY 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 GW into a single TDM circuit. The third possibility is to
conserve bandwidth by suppressing the payload altogether. Finally,
if GW 1 believes that the TDM defect is minor or correctable (e.g.
loss of multiframe synchronization, or initial phases of detection of
incorrect frame sync), it MAY place the TDM data it has received into
the payload field, and specify in the defect modification field (ôMö)
that the TDM data is corrupted, but potentially recoverable.
When TDMoIP GW 2 receives a local defect indication without ôMö-field
modification, it forwards (or generates if the payload has been
suppressed) AIS or trunk conditioning towards TDM end equipment 2
(the choice between AIS and conditioning being preconfigured). Thus
AIS has been properly delivered to end equipment 2 emulating the TDM
scenario from the TDM end equipment point of view. In addition, GW 2
receiving the ôLö indication 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 specified
here) 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 GW 2
treats the payload contents as regular TDM data. If the payload has
been suppressed, trunk conditioning and not AIS MUST be generated by
GW 2.
The next possibility is that of a unidirectional defect in the PSN.
In such a case TDMoIP GW 1 sends packets toward GW 2, but these are
not received. Once again GW 2 MAY generate AIS towards end equipment
2, but it also sets the remote defect "R" flag on packets in the
opposite direction. When GW 1 receives the "R" flag indication, it
has been informed of a reverse defect, and as we shall see shortly
this uniquely defines that the defect lies in the PSN.
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The final case is when the defect is in TDM network 2. Such defects
cause AIS generation towards TDM end equipment 2, which responds by
setting the TDM RDI in the reverse direction. When GW 2 observes
this RDI inserted into valid TDM data, it MAY indicate this by
setting the RDI value of the ôMö field of valid packets sent back
across the PSN towards GW 1. GW 1, upon receiving this indication,
generates RDI towards end equipment 1, thus emulating the
conventional TDM network. The distinction between PSN and TDM
network 2 defects is maintained by differentiation between "R" and
"M" indications.
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 4.3,
both PWs are considered to have failed when persistent defects are
detected in either.
When failure is declared the PW MUST be withdrawn, and both TDMoIP
GWs commence sending AIS (and not trunk conditioning) to their
respective TDM networks. The GWs then engage in connectivity testing
using TDMoIP OAM as described in Appendix D until connectivity is
restored.
6. Implementation Issues
General requirements for transport of TDM over pseudo-wires are
detailed in [TDM-REQ]. In the following subsections we review
additional aspects essential to successful TDMoIP implementation.
6.1 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 PDV).
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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 packets in the jitter buffer and MUST
take appropriate action when anomalies are detected. When missing
packet(s) are detected the mechanism MUST output filler packet(s) in
order to retain TDM timing. Packets with incorrect serial numbers or
other detectable header errors MAY be discarded. Packets arriving in
incorrect order SHOULD be swapped. Filler packets 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 packets may be sufficient to
maintain the TDM timing, for voice traffic it may lead to gaps or
artifacts that result in choppy, annoying or even unintelligible
speech, see [TDM-PLC]. An implementation MAY blindly insert a
preconfigured constant value in place of any lost speech samples, and
this value SHOULD be chosen to minimize the perceptual effect.
Alternatively one MAY replay the previously received packet. Since a
TDMoIP packet is usually declared lost following the reception of the
next packet, when computational resources are available,
implementations SHOULD conceal the packet loss event by properly
estimating the missing speech sample values.
6.2 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 10E-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 three methods of overcoming this
difficulty. In the first method timing information is provided by
some means independent of the PSN. This timing may be provided to
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the TDM end equipment or to the GWs. In a second method a common
clock is assumed available to both gateways, and the relationship
between the TDM source clock and this clock is encoded in the packet.
This encoding may be take the form of RTP timestamps or may utilizing
the 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 [TDM-REQ] 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 GW, or the fill-level of the
jitter buffer as a function of time. Due to the packet delay
variation in the PSN, filtering processes that combat the statistical
nature of the observable characteristics must be employed. Frequency
Locked Loops (FLL) and Phase Locked Loops (PLL) are well suited for
this task.
Whatever timing recovery mechanism is employed, the output of the
TDM-bound GW 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
required.
6.3 Quality of Service
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.
If the PSN is Diffserv-enabled then an EF-PHB (expedited forwarding)
class based PDB SHOULD be used, in order to provide a low latency and
minimal jitter service. It is suggested that the transport LSP be
somewhat overprovisioned.
If the MPLS network is Intserv enabled, then GS (Guaranteed Service)
with the appropriate bandwidth reservation SHOULD be used in order to
provide a bandwidth BW guarantee equal or greater than that of the
aggregate TDM traffic. The delay introduced by the MPLS network
SHOULD be measured prior to traffic flow, to ensure its compliance
with latency requirements.
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7. 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.
TDMoIP does not provide protection against malicious users utilizing
snooping or packet injection during setup or operation. However,
random initialization of sequence numbers makes known-plaintext
attacks on link encryption methods more difficult.
PW labels SHOULD be selected in an unpredictable manner rather than
sequentially or otherwise in order to deter session hijacking. When
using L2TPv3, randomly selected cookies MAY be used to validate
circuit origin. Sequence numbers SHOULD be randomly initialized in
order to increase the difficulty of decrypting based on packet
headers.
8. IANA Considerations
When used with UDP/IP the destination port number MUST be set to
0x085E (2142), the user port number which has been assigned by IANA
to TDMoIP.
The format identifiers (FORMID) will need to be standardized.
9. Trademarks
TDMoIP is a registered trademark of RAD Data Communications. RAD
Data Communications grants the IETF a perpetual license to reproduce
this trademark solely in connection with the reproduction,
distribution or publication of this contribution and derivative works
thereof, in accordance with RFC 3667.
10. IPR Disclosure Acknowledgement
By submitting this Internet-Draft, we certify that any applicable
patent or other IPR claims of which we are aware have been disclosed,
and any of which we become aware will be disclosed, in accordance
with RFC 3668.
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11. References
11.1 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
[CONNECT] RFC 2678 IPPM Metrics for Measuring Connectivity
[DELAY] RFC 2679 A One-way Delay Metric for IPPM
[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
[IEEE802.1Q] IEEE 802.1Q, IEEE Standards for Local and Metropolitan
Area Networks ù 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)
[IPPM] RFC 2330 Framework for IP Performance Metrics
[IPv4] RFC 791 (STD0005) Internet Protocol (IP)
[LES] ATM forum specification atm-vmoa-0145 (LES) Voice and
Multimedia over ATM - Loop Emulation Service Using AAL2
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[L2TPv3] draft-ietf-l2tpext-l2tp-base-10.txt (08/03) Layer Two
Tunneling Protocol (L2TPv3), J. Lau et al., work in progress
[MPLS] RFC 3032 MPLS Label Stack encoding
[RTP] RFC 3550 RTP: Transport Protocol for Real-Time Applications
[SAToP] draft-ietf-pwe3-satop-00.txt (09/03) Structure-Agnostic TDM
over Packet (SAToP), A. Vainshtein and Y. Stein, work in
progress
[SSCS] ITU-T Recommendation I.366.2 (11/00) AAL type 2 service
specific convergence sublayer for narrow-band services
[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
[UDP] RFC 768 (STD0006) User Datagram Protocol (UDP)
[Y1413] ITU-T Recommendation Y.1413 (03/04) TDM-MPLS network
interworking - User plane interworking
11.2 Informative References
[CESoPSN] draft-ietf-cesopsn-00.txt (01/04), TDM Circuit Emulation
Service over Packet Switched Network, A. Vainshtein et al, work
in progress
[ICMP] RFC 792 Internet Control Message Protocol. (09/81)
[ISDN-PRI] ITU-T Recommendation Q.931 (05/98) ISDN user-network
interface layer 3 specification for basic call control
[PWE3-ARCH] draft-ietf pwe3-arch-07.txt (3/04), PWE3 Architecture,
Stewart Bryant et al, work in progress
[SS7] ITU-T Recommendation Q.700 (03/93) Introduction to CCITT
Signalling System No. 7
[TDM-PLC] draft-stein-pwe3-tdm-packetloss-01.txt (10/03), The Effect
of Packet Loss on Voice Quality for TDM over Pseudowires, Y(J)
Stein and I. Druker, work in progress
[TDM-REQ] draft-ietf-pwe3-tdm-requirements-04.txt (1/04),
Requirements for Edge-to-Edge Emulation of TDM Circuits over
Packet Switching Networks, M. Riegel et al., work in progress
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12. Acknowledgments
The authors would like to thank Hugo Silberman, Shimon HaLevy, Tuvia
Segal, and Eitan Schwartz of RAD Data Communications for their
valuable contributions to the technology described herein.
Authors' Addresses
Yaakov (Jonathan) Stein
RAD Data Communications
24 Raoul Wallenberg St., Bldg C
Tel Aviv 69719
ISRAEL
Phone: +972 3 645-5389
EMail: yaakov_s@rad.com
Ronen Shashoua
RAD Data Communications
24 Raoul Wallenberg St., Bldg C
Tel Aviv 69719
ISRAEL
Phone: +972 3 645-5447
EMail: ronen_s@rad.com
Ron Insler
RAD Data Communications
24 Raoul Wallenberg St., Bldg C
Tel Aviv 69719
ISRAEL
Phone: +972 3 645-5445
EMail: ron_i@rad.com
Motty (Mordechai) Anavi
RAD Data Communications
900 Corporate Drive
Mahwah, NJ 07430
USA
Phone: +1 201 529-1100 Ext. 213
EMail: motty@radusa.com
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Appendix A. Sequence Number Processing
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
[û2^15 ... +2^15 û 1] to ensure that simple checking of the
differenceÆs 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
having previously having been assumed lost, the TDM-bound GW 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 sequence numbers (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 of packet 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
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
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
calculate L = ( (played-received) mod 2^16 ) - 2^15
if 0 < L <= R
replace packet previously marked as lost with actual data
else
discard packet
Note: by choosing R=0 we always discard the late packet
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Appendix B. AAL1 Review
The first byte of the 48-byte AAL1 PDU always contains an error-
protected three-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 this byte can only take on eight different
values, and 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 namely 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 5/8 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 taken;
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, 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 number. 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 for sequence number byte.
E (1 bit) even byte parity MSB for pointer byte.
pointer (7 bits) pointer to next structure boundary.
Since P-format PDUs have 46 bytes of payload and the next PDU has 47
bytes, viewed as a single entity the pointer needs to indicate one of
93 bytes. If P=0 it is understood that the structure commences with
the following byte (i.e. the first byte in the payload belongs to
the lowest numbered channel). P=93 means that the last byte of the
second PDU is the final byte of the structure, and the following PDU
commences with a new structure. The special value P=127 indicates
that there is no structure boundary to be indicated (needed when
extremely large structures are being transported).
The P-format PDU is always placed at the first possible position in
the sequence number cycle that a structure boundary occurs, and can
only occur once per cycle.
The only difference between the structured circuit emulation format
and structured circuit emulation with CAS is the definition of the
structure. Whereas in structured circuit emulation the structure is
composed of the N channels, in structured circuit emulation with CAS
the structure encompasses the superframe consisting of multiple
repetitions of the N channels and then the CAS signaling bits. The
CAS bits are tightly packed into bytes and the final byte is padded
with zeros if required.
For example, for E1 circuits the CAS signaling bits are updated once
per superframe of 16 frames. Hence the structure for N*64 derived
from an E1 with CAS signaling consists of 16 repetitions of N bytes,
followed by N sets of the four ABCD bits, and finally four zero bits
if N is odd. For example, the structure for channels 2,3 and 5 will
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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
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 below 8 are reserved and so there are 248
possible channels. The mapping of CID values to channels is
beyond the scope of the TDMoIP protocol and must be configured
manually or via network management.
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
a type 3 packet (in the notation of [AAL2]) in order to ensure
error protection. The signaling is sent with the same CID as the
corresponding voice channel. Signaling is sent whenever the state
of the ABCD bits changes, and is sent with triple redundancy, i.e.
sent three times spaced 5 milliseconds apart. In addition, the
entire set of the signaling bits is sent periodically to ensure
reliability.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RED| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RES | ABCD | type | CRC
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
CRC (cont) |
+-+-+-+-+-+-+-+-+
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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 for periodic
refresh of the CAS information.
Timestamp (14 bits) The timestamp is 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. TDMoIP OAM Mechanisms
PWs require OAM mechanisms to detect network defects and monitor
performance measures that impact the emulated service. When defects
are detected they need to be localized and alarms may need to be
raised to alert network maintenance personnel. 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 [IPPM], and its suitability
for carrying TDM services.
TDMoIP GWs 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 [ICMP] 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, the ICMP-like
procedures [ICMP] detailed below SHOULD be followed. This TDMoIP OAM
message flow occupies a separate PW in the same tunnel as the TDMoIP
PW(s).
All TDMoIP OAM messages herein described MUST use the PW label
preconfigured to indicate OAM (the default value is the highest label
available). All PSN layer parameters (for example, IP addresses,
TOS, EXP bits, and VLAN ID) MUST remain those of the PW being
investigated. The format of OAM messages is given in Appendix D.3.
D.1 TDMoIP Connectivity Verification Messages
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 GW could send a continuous
stream of packets towards the destination without knowing whether the
TDM-bound GW is ready to accept them. For layer-2 networks this may
lead to flooding of the PSN with stray packets.
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This problem may occur when a TDMoIP GW is first brought up, when the
TDM-bound GW fails or is disconnected from the PSN, or the PW is
broken. After an aging time the destination gateway disappears from
the routing tables, and intermediate switches may flood the network
with the TDMoIP packets in an attempt to find a new path.
The solution to this problem is to significantly reduce the number of
TDMoIP packets transmitted per second when PW failure is detected,
and to return to full rate only when the PW is available. The
detection of failure and restoration is made possible by the periodic
exchange of one-way connectivity-verification messages, as defined in
[CONNECT].
Connectivity is tested by periodically sending OAM messages from the
source gateway to the destination gateway, 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 gateway is ready to receive
packets before starting to send them. Since TDMoIP gateways operate
full-duplex, both would need to be set up and properly configured
simultaneously if flooding is to be avoided. By using connectivity
verification a configured gateway waits until it can detect its
destination before transmitting at full rate. In addition, errors in
configuration can be readily discovered by using the service specific
field.
D.2 Performance Measurements
In addition to one way connectivity, the OAM signaling mechanism can
be used to request and report on various PSN metrics, such as one way
delay, round trip delay, packet delay variation, etc. It can also be
used for remote diagnostics, and for unsolicited reporting of
potential problems (e.g. dying gasp messages).
D.3 OAM Packet Format
The format of an OAM message packet is depicted in the following
figure. Note that PSN-specific layers are identical to those used to
carry the TDMoIP data, with the exception that the PW label MUST be
set to the preconfigured value instead of the usual PW identifier.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PSN-specific layers (with preconfigured PW label) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|FORMID |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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FORMID, L, R, and M are identical to those of the PW being tested.
Length is the length in bytes of the OAM message packet.
OAM Sequence Number (16 bits) is used to uniquely identify the
message. Its value is unrelated to the sequence number of the
TDMoIP data packets for the PW in question. It is incremented in
query messages, and replicated without change in replies.
OAM Msg Type (8 bits) indicates the function of the message. At
present the following are defined:
0 for one way connectivity query message
8 for one way connectivity reply message.
OAM Msg Code (8 bits) is used to carry information related to the
message, and its interpretation depends on the message type. For
type 0 (connectivity query) messages the following codes are
defined:
0 validate connection.
1 do not validate connection
for type 8 (connectivity reply) messages the available codes are:
0 acknowledge valid query
1 invalid query (configuration mismatch).
Service specific information (16 bits) is a field that can be used to
exchange configuration information between gateways. If it is not
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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 gateway.
Reverse PW label (16 bits) is the PW label used for TDMoIP traffic
from the destination to source gateway.
Source Transmit Timestamp (32 bits) represents the time the PSN-bound
GW 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 gateway received the query message.
Destination Transmit Timestamp (32 bits) represents the time the
destination gateway transmitted the reply message.
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Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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