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Versions: 00 01 02 03 04 05 RFC 4553
Network Working Group A. Vainshtein (Axerra Networks)
Y(J) Stein (RAD Data Communications)
Internet Draft Editors
Expiration Date:
August 2006
February 2006
Structure-Agnostic TDM over Packet (SAToP)
draft-ietf-pwe3-satop-05.txt
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Abstract
This document describes a pseudowire encapsulation for TDM (T1, E1, T3,
E3) bit-streams that disregards any structure that may be imposed on
these streams, in particular the structure imposed by the standard TDM
framing.
Co-Authors
The following are co-authors of this document:
Motty Anavi RAD Data Communications
Tim Frost Zarlink Semiconductors
Eduard Metz TNO Telecom
Prayson Pate Overture Networks
Akiva Sadovski
Israel Sasson Axerra Networks
Ronen Shashoua RAD Data Communications
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TABLE OF CONTENTS
1. Introduction......................................................2
2. Terminology and Reference Models..................................3
2.1. Terminology...................................................3
2.2. Reference Models..............................................3
3. Emulated Services.................................................4
4. SAToP Encapsulation Layer.........................................4
4.1. SAToP Packet Format...........................................4
4.2. PSN and Multiplexing Layer Headers............................5
4.3. SAToP Header..................................................5
4.3.1. Usage and Structure of the Control Word...................7
4.3.2. Usage of RTP Header.......................................8
5. SAToP Payload Layer...............................................9
5.1. General Payloads..............................................9
5.2. Octet-aligned T1.............................................10
6. SAToP Operation..................................................11
6.1. Common Considerations........................................11
6.2. IWF operation................................................11
6.2.1. PSN-bound Direction......................................11
6.2.2. CE-bound Direction.......................................11
6.3. SAToP Defects................................................13
6.4. SAToP PW Performance Monitoring..............................13
7. QoS Issues.......................................................14
8. Congestion Control...............................................14
9. Security Considerations..........................................16
10. Applicability Statement.........................................16
11. IANA Considerations.............................................17
12. Disclaimer of Validity..........................................17
13. NORMATIVE REFERENCES............................................18
14. INFORMATIONAL REFERENCES........................................19
Annex A. Old Mode of SATOP Encapsulation over L2TPv3................20
ANNEX B. Parameters that must be agreed upon during the PW setup....21
1. Introduction
This document describes a method for encapsulating TDM bit-streams (T1,
E1, T3, E3) as pseudo-wires over packet-switching networks (PSN). It
addresses only structure-agnostic transport, i.e., the protocol
completely disregards any structure that may possibly be imposed on
these signals, in particular the structure imposed by standard TDM
framing [G.704]. This emulation is referred to as "emulation of
unstructured TDM circuits" in [RFC4197] and suits applications where
the PEs have no need to interpret TDM data or to participate in the TDM
signaling.
The SAToP solution presented in this document conforms to the PWE3
architecture described in [RFC3985] and satisfies both the relevant
general requirements put forward in [RFC3916] and specific requirements
for unstructured TDM signals presented in [RFC4197].
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As with all PWs, SAToP PWs may be manually configured or set up using
the PWE3 control protocol. Extensions to the PWE3 control protocol
required for setup and maintenance of SAToP pseudo-wires and
allocations of code points used for this purpose are described in
separate documents ([PWE3-TDM-CONTROL] and [PWE3-IANA] respectively).
2. Terminology and Reference Models
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2.1. Terminology
The following acronyms used in this document are defined in [RFC3985]
and [RFC4197]:
ATM Asynchronous Transfer Mode
CE Customer Edge
CES Circuit Emulation Service
NSP Native Service Processing
PE Provider Edge
PDH Plesiochronous Digital Hierarchy
PW Pseudo-Wire
SDH Synchronous Digital Hierarchy
SONET Synchronous Optical Network
TDM Time Domain Multiplexing
In addition, the following TDM-specific terms are needed:
o Loss of Signal (LOS) - a condition of the TDM attachment
circuit wherein the incoming signal cannot be detected.
Criteria for entering and leaving the LOS condition can be
found in [G.775]
o Alarm Indication Signal (AIS) - a special bit pattern (e.g. as
described in [G.775]) in the TDM bit stream that indicates
presence of an upstream circuit outage. For E1, T1 and E3
circuits the AIS pattern is a sequence of binary "1" values of
appropriate duration (the "all ones" pattern) and hence it can
be detected and generated by structure-agnostic means. The T3
AIS pattern requires T3 framing (see [G.704], Section
2.5.3.6.1) and hence can only be handled by a structure-aware
NSP.
We also use the term Interworking Function (IWF) to describe the
functional block that segments and encapsulates TDM into SAToP packets
and in the reverse direction decapsulates SAToP packets and
reconstitutes TDM.
2.2. Reference Models
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The generic models defined in Sections 4.1, 4.2 and 4.4 of [RFC3985]
fully apply to SAToP.
The native service addressed in this document is a special case of the
bit stream payload type defined in Section 3.3.3 of [RFC3985].
The Network Synchronization reference model and deployment scenarios
for emulation of TDM services are described in [RFC4197], Section 4.3.
3. Emulated Services
This specification describes edge-to-edge emulation of the following
TDM services described in [G.702]:
1. E1 (2048 kbit/s)
2. T1 (1544 kbit/s) This service is also known as DS1
3. E3 (34368 kbit/s)
4. T3 (44736 kbit/s) This service is also known as DS3.
The protocol used for emulation of these services does not depend on
the method in which attachment circuits are delivered to the PEs. For
example, a T1 attachment circuit is treated in the same way regardless
of whether it is delivered to the PE on copper [G.703], multiplexed in
a T3 circuit [T1.107], mapped into a virtual tributary of a SONET/SDH
circuit [G.707] or carried over an ATM network using unstructured ATM
Circuit Emulation Service (CES) [ATM-CES]. Termination of any specific
"carrier layers" used between the PE and CE is performed by an
appropriate NSP.
4. SAToP Encapsulation Layer
4.1. SAToP Packet Format
The basic format of SAToP packets is shown in Fig. 1 below.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
| PSN and multiplexing layer headers |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| ... |
+-- --+
| SAToP Encapsulation Header |
+-- --+
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Packetized TDM data (Payload) |
| ... |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. Basic SAToP Packet Format
4.2. PSN and Multiplexing Layer Headers
Both UDP and L2TPv3 [RFC3931] can provide the multiplexing mechanisms
for SAToP PWs over an IPv4/IPv6 PSN. The PW label provides the
multiplexing mechanism over an MPLS PSN as described in Section 5.4.2
of [RFC3985].
The total size of a SAToP packet for a specific PW MUST NOT exceed path
MTU between the pair of PEs terminating this PW. SAToP implementations
using IPv4 PSN MUST mark the IPv4 datagrams they generate as "Don't
Fragment" [RFC791] (see also [PWE3-FRAG]).
4.3. SAToP Header
The SAToP header MUST contain the SAToP Control Word (4 bytes) and MAY
also contain a fixed RTP header [RFC3550]. If the RTP header is
included in the SAToP header, it MUST immediately follow the SAToP
control word in all cases except UDP multiplexing, where it
MUST precede it (see Fig. 2a, Fig. 2b and Fig. 2c below).
Note: Such an arrangement complies with the traditional usage of RTP
for the IPv4/IPv6 PSN with UDP multiplexing while making SAToP PWs
ECMP-safe for the MPLS PSN by providing for PW-IP packet discrimination
(see [RFC3985], Section 5.4.3) and facilitating seamless stitching of
L2TPv3-based and MPLS-based segments of SAToP PWs (see [PWE3-MS]).
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
| IPv4/IPv6 and UDP (multiplexing layer) headers |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| OPTIONAL |
+-- --+
| |
+-- --+
| Fixed RTP Header (see [RFC3550]) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SAToP Control Word |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Packetized TDM data (Payload) |
| ... |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2a. SAToP Packet Format for an IPv4/IPv6 PSN with
UDP Multiplexing
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
| IPv4/IPv6 and L2TPv3 (multiplexing layer) headers |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SAToP Control Word |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| OPTIONAL |
+-- --+
| |
+-- --+
| Fixed RTP Header (see [RFC3550]) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Packetized TDM data (Payload) |
| ... |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2b. SAToP Packet Format for an IPv4/IPv6 PSN with
L2TPv3 Demultiplexing
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
| MPLS Label Stack |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SAToP Control Word |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| OPTIONAL |
+-- --+
| |
+-- --+
| Fixed RTP Header (see [RFC3550]) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Packetized TDM data (Payload) |
| ... |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2c. SAToP Packet Format for an MPLS PSN
4.3.1. Usage and Structure of the Control Word
Usage of the SAToP control word allows:
1. Detection of packet loss or mis-ordering
2. Differentiation between the PSN and attachment circuit
problems as causes for the outage of the emulated service
3. PSN bandwidth conservation by not transferring invalid data
(AIS)
4. Signaling of faults detected at the PW egress to the PW
ingress.
The structure of the SAToP Control Word is shown in Fig. 3 below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|L|R|RSV|FRG| LEN | Sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3. Structure of the SAToP Control Word
The use of Bits 0 to 3 is described in [PWE3-CW]. These bits MUST
be set to zero unless they are being used to indicate the start of an
Associated Channel Header (ACH). An ACH is needed if the state of the
SAToP PW is being monitored using Virtual Circuit Connectivity
Verification [PWE3-VCCV].
L - if set, indicates that TDM data carried in the payload is invalid
due an attachment circuit fault. When the L bit is set the payload
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MAY be omitted in order to conserve bandwidth. The CE-bound IWF
MUST play out an appropriate amount of filler data regardless of
the payload size. Once set, if the fault is rectified the L bit
MUST be cleared.
Note: This document does not specify which TDM fault conditions are
treated as invalidating the data carried in the SAToP packets. Possible
examples include, but are not limited to LOS and AIS.
R - if set by the PSN-bound IWF, indicates that its local CE-bound IWF
is in the packet loss state, i.e. has lost a preconfigured number
of consecutive packets. The R bit MUST be cleared by the PSN-bound
IWF once its local CE-bound IWF has exited the packet loss state,
i.e. has received a preconfigured number of consecutive packets.
RSV (reserved) and FRG (fragmentation, see [PWE3-FRAG]) bits (6 to 9) -
MUST be set to 0 by the PSN-bound IWF and MUST be ignored by the CE-
bound IWF.
LEN (bits 10 to 15) MAY be used to carry the length of the SAToP packet
(defined as the size of the SAToP header + the payload size) if it is
less than 64 bytes, and MUST be set to zero otherwise. When the LEN
field is set to 0, the preconfigured size of the SAToP packet payload
MUST be assumed as described in Section 5.1, and if the actual packet
size is inconsistent with this length, the packet MUST be considered to
be malformed.
The sequence number is used to provide the common PW sequencing
function as well as detection of lost packets. It MUST be generated in
accordance with the rules defined in Section 5.1 of [RFC3550], for the
RTP sequence number, i.e.:
o Its space is a 16-bit unsigned circular space
o Its initial value SHOULD be random (unpredictable).
It MUST be incremented with each SAToP data packet sent in the specific
PW.
4.3.2. Usage of RTP Header
When RTP is used, SAToP requires the fields of the fixed RTP header
(see [RFC3550], Section 5.1) with P (padding), X (header extension), CC
(CSRC count), and M fields (marker) to be set to zero.
The PT (payload type) field is used as following:
1. One PT value MUST be allocated from the range of dynamic
values (see [RTP-TYPES]) for each direction of the PW. The
same PT value MAY be reused for both directions of the PW and
also reused between different PWs
2. The PSN-bound IWF MUST set the PT field in the RTP header to
the allocated value
3. The CE-bound IWF MAY use the received value to detect
malformed packets
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The sequence number MUST be the same as the sequence number in the
SAToP control word.
The RTP timestamps are used for carrying timing information over the
network. Their values are generated in accordance with the rules
established in [RFC3550].
The frequency of the clock used for generating timestamps MUST be an
integer multiple of 8 kHz. All implementations of SAToP MUST support
the 8 kHz clock. Other multiples of 8 kHz MAY be used.
The SSRC (synchronization source) value in the RTP header MAY be used
for detection of misconnections, i.e. incorrect interconnection of
attachment circuits.
Timestamp generation MAY be used in the following modes:
1. Absolute mode: the PSN-bound IWF sets timestamps using the
clock recovered from the incoming TDM attachment circuit. As a
consequence, the timestamps are closely correlated with the
sequence numbers. All SAToP implementations that support usage
of the RTP header MUST support this mode.
2. Differential mode: Both IWFs have access to a common high-
quality timing source, and this source is used for timestamp
generation. Support of this mode is OPTIONAL.
Usage of the fixed RTP header in a SAToP PW and all the options
associated with its usage (the time-stamping clock frequency, the time-
stamping mode, selected PT and SSRC values) MUST be agreed upon between
the two SAToP IWFs at the PW setup as described in [PWE3-TDM-CONTROL].
Other, RTP-specific, methods (e.g., see [RFC3551]) MUST NOT be used.
5. SAToP Payload Layer
5.1. General Payloads
In order to facilitate handling of packet loss in the PSN, all packets
belonging to a given SAToP PW are REQUIRED to carry a fixed number of
bytes filled with TDM data received from the attachment circuit. The
packet payload size MUST be defined during the PW setup, MUST be the
same for both directions of the PW and MUST remain unchanged for the
lifetime of the PW.
The CE-bound and PSN-bound IWFs MUST agree on SAToP packet payload size
at the PW setup (default payload size values defined below guarantee
that such an agreement is always possible). The SAToP packet payload
size can be exchanged over the PWE3 control protocol ([PWE3-TDM-
CONTROL]) by using the CEP/TDM Payload Bytes sub-TLV of the Interface
Parameters TLV([PWE3-IANA]).
SAToP uses the following ordering for packetization of the TDM data:
o The order of the payload bytes corresponds to their order on
the attachment circuit
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o Consecutive bits coming from the attachment circuit fill each
payload byte starting from most significant bit to least
significant.
All SAToP implementations MUST be capable of supporting the following
payload sizes:
o E1 - 256 bytes
o T1 - 192 bytes
o E3 and T3 - 1024 bytes.
Notes:
1. Whatever the selected payload size, SAToP does not assume
alignment to any underlying structure imposed by TDM framing
(byte, frame or multiframe alignment).
2. When the L bit in the SAToP control word is set, SAToP packets
MAY omit invalid TDM data in order to conserve PSN bandwidth.
3. Payload sizes that are multiples of 47 bytes MAY be used in
conjunction with unstructured ATM-CES [ATM-CES].
5.2. Octet-aligned T1
An unstructured T1 attachment circuit is sometimes provided already
padded to an integer number of bytes, as described in Annex B of
[G.802]. This occurs when the T1 is de-mapped from a SONET/SDH virtual
tributary/container, or when it is deframed by a dual-mode E1/T1
framer.
In order to facilitate operation in such cases, SAToP defines a special
"octet-aligned T1" transport mode. When operating in this mode, the
SAToP payload consists of a number of 25-byte subframes, each subframe
carrying 193 bits of TDM data and 7 bits of padding. This mode is
depicted in Fig. 4 below.
| 1 | 2 | ... | 25 |
|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7| ... |0 1 2 3 4 5 6 7|
|=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| TDM Data | padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ................................. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TDM Data | padding |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
Figure 4. SAToP Payload Format for Octet-Aligned T1 Transport
Notes:
1. No alignment with the framing structure that may be imposed on the
T1 bit-stream is implied.
2. An additional advantage of the octet-aligned T1 transport mode is
ability to select the SAToP packetization latency as an arbitrary
integer multiple of 125 microseconds.
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Support of the octet-aligned T1 transport mode is OPTIONAL. An octet-
aligned T1 SAToP PW is not interoperable with a T1 SAToP PW that
carries a non-aligned bit-stream, as described in the previous section.
Implementations supporting octet-aligned T1 transport mode MUST be
capable of supporting a payload size of 200 bytes (i.e., a payload of
eight 25-byte subframes) corresponding to precisely 1 millisecond of
TDM data.
6. SAToP Operation
6.1. Common Considerations
Edge-to-edge emulation of a TDM service using SAToP is only possible
when the two PW attachment circuits are of the same type (T1, E1, T3,
E3). The service type is exchanged at PW setup as described in [PWE3-
CONTROL].
6.2. IWF operation
6.2.1. PSN-bound Direction
Once the PW is set up, the PSN-bound SAToP IWF operates as follows:
TDM data is packetized using the configured number of payload bytes per
packet.
Sequence numbers, flags, and timestamps (if the RTP header is used) are
inserted in the SAToP headers.
SAToP, multiplexing layer and PSN headers are prepended to the
packetized service data.
The resulting packets are transmitted over the PSN.
6.2.2. CE-bound Direction
The CE-bound SAToP IWF SHOULD include a jitter buffer where the payload
of the received SAToP packets is stored prior to play-out to the local
TDM attachment circuit. The size of this buffer SHOULD be locally
configurable to allow accommodation to the PSN-specific packet delay
variation.
The CE-bound SAToP IWF SHOULD use the sequence number in the control
word for detection of lost and mis-ordered packets. If the RTP header
is used, the RTP sequence numbers MAY be used for the same purposes.
Note: With SAToP, a valid sequence number can be always found in bits
16 - 31 of the first 32-bit word immediately following the multiplexing
header regardless of the specific PSN type, multiplexing method, usage
or non-usage of the RTP header etc. This approach simplifies
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implementations supporting multiple encapsulation types as well as
implementation of multi-segment (MS) PWs using different encapsulation
types in different segments.
The CE-bound SAToP IWF MAY re-order mis-ordered packets. Mis-ordered
packets that cannot be reordered MUST be discarded and treated as lost.
The payload of the received SAToP packets marked with the L bit set
SHOULD be replaced by the equivalent amount of the "all ones" pattern
even if it has not been omitted.
The payload of each lost SAToP packet MUST be replaced with the
equivalent amount of the replacement data. The contents of the
replacement data are implementation-specific and MAY be locally
configurable. By default, all SAToP implementations MUST support
generation of the "all ones" pattern as the replacement data.
Before a PW has been set up and after a PW has been torn down, the IWF
MUST play out the "all ones" pattern to its TDM attachment circuit.
Once the PW has been set up, the CE-bound IWF begins to receive SAToP
packets and to store their payload in the jitter buffer but continues
to play out the "all ones" pattern to its TDM attachment circuit. This
intermediate state persists until a preconfigured amount of TDM data
(usually half of the jitter buffer) has been received in consecutive
SAToP packets or until a preconfigured intermediate state timer
(started when the PW setup is completed) expires.
Once the preconfigured amount of the TDM data has been received, the
CE-bound SAToP IWF enters its normal operation state where it continues
to receive SAToP packets and to store their payload in the jitter
buffer while playing out the contents of the jitter buffer in
accordance with the required clock. In this state the CE-bound IWF
performs clock recovery, MAY monitor PW defects, and MAY collect PW
performance monitoring data.
If the CE-bound SAToP IWF detects loss of a preconfigured number of
consecutive packets or if the intermediate state timer expires before
the required amount of TDM data has been received, it enters its packet
loss state. While in this state, the local PSN-bound SAToP IWF SHOULD
mark every packet it transmits with the R bit set. The CE-bound SAToP
IWF leaves this state and transitions to the normal one once a
preconfigured number of consecutive valid SAToP packets have been
received. (Successfully re-ordered packets contribute to the count of
consecutive packets.)
The CE-bound SAToP IWF MUST provide an indication of TDM data validity
to the CE. This can be done by transporting or by generating the native
AIS indication. As mentioned above, T3 AIS cannot be detected or
generated by structure-agnostic means and hence a structure-aware NSP
MUST be used when generating a valid AIS pattern.
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6.3. SAToP Defects
In addition to the packet loss state of the CE-bound SAToP IWF defined
above, it MAY detect the following defects:
o Stray packets
o Malformed packets
o Excessive packet loss rate
o Buffer overrun
o Remote packet loss.
Corresponding to each defect is a defect state of the IWF, a detection
criterion that triggers transition from the normal operation state to
the appropriate defect state, and an alarm that MAY be reported to the
management system and thereafter cleared. Alarms are only reported when
the defect state persists for a preconfigured amount of time (typically
2.5 seconds) and MUST be cleared after the corresponding defect is
undetected for a second preconfigured amount of time (typically 10
seconds). The trigger and release times for the various alarms may be
independent.
Stray packets MAY be detected by the PSN and multiplexing layers. When
RTP is used, the SSRC field in the RTP header MAY be used for this
purpose as well. Stray packets MUST be discarded by the CE-bound IWF
and their detection MUST NOT affect mechanisms for detection of packet
loss.
Malformed packets are detected by mismatch between the expected packet
size (taking the value of the L bit into account) and the actual packet
size inferred from the PSN and multiplexing layers. When RTP is used,
lack of correspondence between the PT value and that allocated for this
direction of the PW MAY also be used for this purpose. Malformed in-
order packets MUST be discarded by the CE-bound IWF and replacement
data generated as with lost packets.
Excessive packet loss rate is detected by computing the average packet
loss rate over a configurable amount of times and comparing it with a
preconfigured threshold.
Buffer overrun is detected in the normal operation state when the
jitter buffer of the CE-bound IWF cannot accommodate newly arrived
SAToP packets.
Remote packet loss is indicated by reception of packets with their R
bit set.
6.4. SAToP PW Performance Monitoring
Performance monitoring (PM) parameters are routinely collected for TDM
services and provide an important maintenance mechanism in TDM
networks. Ability to collect compatible PM parameters for SAToP PWs
enhances their maintenance capabilities.
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Collection of the SAToP PW performance monitoring parameters is
OPTIONAL, and if implemented, is only performed after the CE-bound IWF
has exited its intermediate state.
SAToP defines error events, errored blocks and defects as follows:
o A SAToP error event is defined as insertion of a single
replacement packet into the jitter buffer (replacement of
payload of SAToP packets with the L bit set is not considered
as insertion of a replacement packet)
o A SAToP errored data block is defined as a block of data
played out to the TDM attachment circuit and of size defined
in accordance with the [G.826] rules for the corresponding TDM
service that has experienced at least one SAToP error event
o A SAToP defect is defined as the packet loss state of the CE-
bound SAToP IWF.
The SAToP PW PM parameters (Errored, Severely Errored and Unavailable
Seconds) are derived from these definitions in accordance with [G.826].
7. QoS Issues
SAToP SHOULD employ existing QoS capabilities of the underlying PSN.
If the PSN providing connectivity between PE devices is Diffserv-
enabled and provides a PDB [RFC3086] that guarantees low-jitter and
low-loss, the SAToP PW SHOULD use this PDB in compliance with the
admission and allocation rules the PSN has put in place for that PDB
(e.g., marking packets as directed by the PSN).
If the PSN is Intserv-enabled, then GS (Guaranteed Service) [RFC 2212]
with the appropriate bandwidth reservation SHOULD be used in order to
provide a bandwidth guarantee equal or greater than that of the
aggregate TDM traffic.
8. Congestion Control
As explained in [RFC3985], the PSN carrying the PW may be subject to
congestion. SAToP PWs represent inelastic constant bit-rate (CBR) flows
and cannot respond to congestion in a TCP-friendly manner prescribed by
[RFC2914], although the percentage of total bandwidth they consume
remains constant.
Unless appropriate precautions are taken, undiminished demand of
bandwidth by SAToP PWs can contribute to network congestion that may
impact network control protocols.
Whenever possible, SAToP PWs SHOULD be carried across traffic-
engineered PSNs that provide either bandwidth reservation and admission
control or forwarding prioritization and boundary traffic conditioning
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mechanisms. IntServ-enabled domains supporting Guaranteed Service (GS)
[RFC2212] and DiffServ-enabled domains [RFC2475] supporting Expedited
Forwarding (EF) [RFC3246] provide examples of such PSNs. Such
mechanisms will negate, to some degree, the effect of the SAToP PWs on
the neighboring streams. In order to facilitate boundary traffic
conditioning of SAToP traffic over IP PSNs, the SAToP IP packets SHOULD
NOT use the DiffServ Code Point (DSCP) value reserved for the Default
PHB[RFC2474].
If SAToP PWs run over a PSN providing best-effort service, they SHOULD
monitor packet loss in order to detect "severe congestion". If such a
condition is detected, a SAToP PW SHOULD shut down bi-directionally for
some period of time as described in Section 6.5 of [RFC3985].
Note that:
1. The SAToP IWF can inherently provide packet loss measurement since
the expected rate of arrival of SAToP packets is fixed and known
2. The results of the SAToP packet loss measurement may not be a
reliable indication of presence or absence of severe congestion if
the PSN provides enhanced delivery, e.g.:
a) If SAToP traffic takes precedence over non-SAToP traffic, severe
congestion can develop without significant SAToP packet loss
b) If non-SAToP traffic takes precedence over SAToP traffic, SAToP
may experience substantial packet loss due to a short-term burst
of high-priority traffic
3. The TDM services emulated by the SAToP PWs have high availability
objectives (see [G.826]) that MUST be taken into account when
deciding on temporary shutdown of SAToP PWs.
This specification does not define the exact criteria for detecting
"severe congestion" using the SAToP packet loss rate or the specific
methods for bi-directional shutdown the SAToP PWs (when such severe
congestion has been detected) and their consequent re-start after a
suitable delay. This is left for further study. However, the following
considerations may be used as guidelines for implementing the SAToP
severe congestion shutdown mechanism:
1. SAToP Performance Monitoring techniques (see Section 6.4) provide
entry and exit criteria for the SAToP PW "Unavailable" state that
make it closely correlated with the "Unavailable" state of the
emulated TDM circuit as specified in [G.826]. Using the same
criteria for "severe congestion" detection may decrease the risk of
shutting down the SAToP PW while the emulated TDM circuit is still
considered available by the CE.
2. If the SAToP PW has been set up using either PWE3 control protocol
[PWE3-CONTROL] or L2TPv3 [RFC 3931], the regular PW teardown
procedures of these protocols SHOULD be used.
3. If one of the SAToP PW end points stops transmission of packets for
a sufficiently long period, its peer (observing 100% packet loss)
will necessarily detect "severe congestion" and also stop
transmission, thus achieving bi-directional PW shutdown.
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9. Security Considerations
SAToP does not enhance or detract from the security performance of the
underlying PSN, rather it relies upon the PSN mechanisms for
encryption, integrity, and authentication whenever required.
SAToP PWs share susceptibility to a number of pseudowire-layer attacks,
and will use whatever mechanisms for confidentiality, integrity, and
authentication that are developed for general PWs. These methods are
beyond the scope of this document.
Although SAToP PWs MAY employ an RTP header when explicit transfer of
timing information is required, SRTP (see [RFC3711]) mechanisms are NOT
RECOMMENDED as a substitute for PW layer security.
Misconnection detection capabilities of SAToP increase its resilience
to misconfiguration and some types of DoS attacks.
Random initialization of sequence numbers, in both the control word and
the optional RTP header, makes known-plaintext attacks on encrypted
SAToP PWs more difficult. Encryption of PWs is beyond the scope of this
document.
10. Applicability Statement
SAToP is an encapsulation layer intended for carrying TDM circuits
(E1/T1/E3/T3) over PSN in a structure-agnostic fashion.
SAToP fully complies with the principle of minimal intervention, thus
minimizing overhead and computational power required for encapsulation.
SAToP provides sequencing and synchronization functions needed for
emulation of TDM bit-streams, including detection of lost or mis-
ordered packets and appropriate compensation.
TDM bit-streams carried over SAToP PWs may experience delays exceeding
those typical of native TDM networks. These delays include the SAToP
packetization delay, edge-to-edge delay of the underlying PSN and the
delay added by the jitter buffer. It is recommended to estimate both
delay and delay variation prior to setup of a SAToP PW.
SAToP carries TDM streams over PSN in their entirety including any TDM
signaling contained within the data. Consequently the emulated TDM
services are sensitive to the PSN packet loss. Appropriate generation
of replacement data can be used to prevent shutting down the CE TDM
interface due to occasional packet loss. Other effects of packet loss
on this interface (e.g., errored blocks) cannot be prevented.
Note: Structure-aware TDM emulation (see [CESoPSN] or [TDMoIP])
completely hides effects of the PSN packet loss on the CE TDM interface
(because framing and CRCs are generated locally) and allows usage of
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application-specific packet loss concealment methods to minimize
effects on the applications using the emulated TDM service.
SAToP can be used in conjunction with various network synchronization
scenarios (see [PWE3-TDM-REQ)] and clock recovery techniques. The
quality of the TDM clock recovered by the SAToP IWF may be
implementation-specific. The quality may be improved by using RTP if a
common clock is available at both ends of the SAToP PW.
SAToP provides for effective fault isolation by carrying the local
attachment circuit failure indications.
The option not to carry invalid TDM data enables PSN bandwidth
conservation.
SAToP allows collection of TDM-like faults and performance monitoring
parameters hence emulating 'classic' carrier services of TDM.
SAToP provides for a carrier-independent ability to detect
misconnections and malformed packets. This feature increases resilience
of the emulated service to misconfiguration and DoS attacks.
Being a constant bit rate (CBR) service, SAToP cannot provide TCP-
friendly behavior under network congestion.
Faithfulness of a SAToP PW may be increased by exploiting QoS features
of the underlying PSN.
SAToP does not provide any mechanisms for protection against PSN
outages, and hence its resilience to such outages is limited. However,
lost-packet replacement and packet reordering mechanisms increase
resilience of the emulated service to fast PSN rerouting events.
11. IANA Considerations
Allocation of PW Types for the corresponding SAToP PWs is defined in
[PWE3-IANA].
12. Disclaimer of Validity
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology
described in this document or the extent to which any license
under such rights might or might not be available; nor does it
represent that it has made any independent effort to identify any
such rights. Information on the procedures with respect to rights
in RFC documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
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specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention
any copyrights, patents or patent applications, or other
proprietary rights that may cover technology that may be required
to implement this standard. Please address the information to the
IETF at ietf-ipr@ietf.org.
ACKNOWLEDGEMENTS
We acknowledge the work of Gil Biran and Hugo Silberman who implemented
TDM transport over IP in 1998.
We would like to thank Alik Shimelmits for many productive discussions
and Ron Insler for his assistance in deploying TDM over PSN.
We express deep gratitude to Stephen Casner who has reviewed in detail
one of the predecessors of this document and provided valuable feedback
regarding various aspects of RTP usage, and to Kathleen Nichols who has
provided the current text of the QoS section considering Diffserv-
enabled PSN.
We thank William Bartholomay, Robert Biksner, Stewart Bryant, Rao
Cherukuri, Ron Cohen, Alex Conta, Shahram Davari, Tom Johnson, Sim
Narasimha, Yaron Raz, and Maximilian Riegel for their valuable
feedback.
13. NORMATIVE REFERENCES
[RFC791] J. Postel (ed), Internet Protocol, RFC 791, IETF, 1981
[RFC2119] S.Bradner, Key Words in RFCs to Indicate Requirement Levels,
RFC 2119, 1997
[RFC2474] K. Nichols et al, Definition of the Differentiated Services
Field (DS Field) in the IPv4 and IPv6 Headers, RFC 2474, 19998
[RFC2475] S. Blake et al, An Architecture for Differentiated Services,
RFC 2475, 1998
[RFC2914] S. Floyd, Congestion Control Principles, RFC 2914, 2000
[RFC3086] K. Nichols, B. Carpenter, Definition of Differentiated
Services Per Domain Behaviors and Rules for their Specification, RFC
3086, 2001
[RFC3550] H. Schulzrinne et al, RTP: A Transport Protocol for Real-Time
Applications, RFC 3550, 2003
[RTP-TYPES] RTP PARAMETERS, http://www.iana.org/assignments/rtp-
parameters
Vainshtein & Stein Expires August 2006 [Page 18]
Structure-Agnostic TDM over Packet February 2006
[G.702] ITU-T Recommendation G.702 (11/88) - Digital Hierarchy Bit
Rates
[G.703] ITU-T Recommendation G.703 (10/98) - Physical/Electrical
Characteristics of Hierarchical Digital Interfaces
[G.704] ITU-T Recommendation G.704 (10/98) - Synchronous frame
structures used at 1544, 6312, 2048, 8448 and 44 736 Kbit/s
hierarchical levels
[G.707] ITU-T Recommendation G.707 (03/96) - Network Node Interface for
the Synchronous Digital Hierarchy (SDH)
[G.751] ITU-T Recommendation G.751 (11/88) - Digital Multiplex
Equipments Operating at the Third Order Bit Rate of 34368 kbit/s and
the Fourth Order Bit Rate of 139264 kbit/s and Using Positive
Justification
[G.775] ITU-T Recommendation G.775 (10/98) - Loss of Signal (LOS),
Alarm Indication Signal (AIS) and Remote Defect Indication (RDI) Defect
Detection and Clearance Criteria for PDH Signals
[G.802] ITU-T Recommendation G.802 (11/88) - Interworking between
Networks Based on Different Digital Hierarchies and Speech Encoding
Laws
[G.826] ITU-T Recommendation G.826 (02/99) - Error performance
parameters and objectives for international, constant bit rate digital
paths at or above the primary rate
[T1.107] American National Standard for Telecommunications - Digital
Hierarchy - Format Specifications, ANSI T1.107-1988
[PWE3-CW] S. Bryant et al, PWE3 Control Word for use over an MPLS PSN,
Work in progress, October 2005, draft-ietf-pwe3-cw-06.txt
[PWE3-CONTROL] L. Martini et al, Pseudowire Setup and Maintenance using
LDP, Work in progress, June 2005, draft-ietf-pwe3-control-protocol-
17.txt
[PWE3-IANA] L. Martini, M. Townsley, IANA Allocations for pseudo Wire
Edge to Edge Emulation (PWE3), Work in progress, November 2005, draft-
ietf-pwe3-iana-allocation-15.txt
[RFC 3931] J. Lau, M.Townsley, I. Goyret, Layer Two Tunneling Protocol
- Version 3 (L2TPv3), RFC 3931, 2005
14. INFORMATIONAL REFERENCES
[RFC3916] XiPeng Xiao et al, Requirements for Pseudo Wire Emulation
Edge-to-Edge (PWE3), RFC 3916, 2004
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Structure-Agnostic TDM over Packet February 2006
[RFC4197] Maximilian Riegel, Requirements for Edge-to-Edge Emulation of
TDM Circuits over Packet Switching Networks (PSN), RFC 4197, 2005
[RFC3985] S. Bryant, P. Pate, PWE3 Architecture, RFC 3985, 2005
[ATM-CES] ATM forum specification af-vtoa-0078 (CES 2.0)
Circuit Emulation Service Interoperability Specification Ver. 2.0
[CESoPSN] A.Vainshtein et al, TDM Circuit Emulation Service over Packet
Switched Network (CESoPSN), Work in Progress, November 2005, draft-
ietf-pwe3-cesopsn-06.txt
[TDMoIP] Y. Stein, TDMoIP, Work in Progress, February 2005, draft-ietf-
pwe3-tdmoip-03.txt
[PWE3-TDM-CONTROL] A. Vainshtein, Y. Stein, Control Protocol Extensions
for Setup of TDM Pseudowires, Work in Progress, July 2005, draft-ietf-
pwe3-tdm-control-protocol-extensi-00.txt
[PWE3-MS] L. Martini et al, Segmented Pseudo Wire, Work in Progress,
July 2005, draft-ietf-pwe3-segmented-pw-00.txt
[PWE3-VCCV] T. Nadeau, R. Aggarwal, Pseudo Wire Virtual Circuit
Connectivity, Work in Progress, August 2005, draft-ietf-pwe3-vccv-
05.txt
[PWE3-FRAG] A. Malis, M. Townsley, PWE3 Fragmentation and Reassembly,
Work in Progress, November 2005, draft-ietf-pwe3-fragmentation-10.txt
[RFC3551] H. Schulzrinne, S. Casner, RTP Profile for Audio and Video
Conferences with Minimal Control, RFC 3551, 2003
[RFC3711] M. Baugher et al, The Secure Real-time Transport Protocol
(SRTP), RFC 3711, 2004
[RFC2212] S. Shenker et al, Specification of Guaranteed Quality of
Service, RFC 2212, 1997
[RFC3246], B. Davie et al, An Expedited Forwarding PHB (Per-Hop
Behavior), RFC 3246, 2002
ANNEX A. OLD MODE OF SATOP ENCAPSULATION OVER L2TPV3
Previous versions of this specification defined a SAToP PW
encapsulation over L2TPv3, which differs from one, described in Section
4.3 and Diagram 2b. In these versions the RTP header, if used, precedes
the SAToP control word.
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Existing implementations of the old encapsulation mode MUST be
distinguished from the encapsulations conforming to this specification
via the SAToP PW setup.
ANNEX B. PARAMETERS THAT MUST BE AGREED UPON DURING THE PW SETUP
The following parameters of the SAToP IWF MUST be agreed upon between
the peer IWFs during the PW setup. Such an agreement can be reached via
manual configuration or via one of the PW setup protocols:
1. Type of the Attachment Circuit (AC):
a) As mentioned in Section 3 above, SAToP supports the following AC
types:
i) E1 (2048 kbit/s)
ii) T1 (1544 kbit/s) This service is also known as DS1
iii) E3(34368 kbit/s)
b) T3 (44736 kbit/s) This service is also known as DS3SAToP PWs
cannot be established between ACs of different types
2. Usage of octet-aligned mode for T1
a) This OPTIONAL mode of emulating T1 bit-streams with SAToP PWs is
described in Section 5.2
b) Both sides MUST agree on using this mode for a SAToP PW to be
operational
3. Payload size, i.e. the amount of valid TDM data in a SAToP packet:
a) As mentioned in Section 5.1 above:
i) The same payload size MUST be used in both directions of
the SAToP PW
ii) The payload size cannot be changed once the PW has been set
up
b) In most cases any mutually agreed upon value can be used.
However, if octet-aligned T1 encapsulation mode is used, the
payload size MUST be an integral multiple of 25 and expresses
the amount of valid TDM data including padding
4. Usage of the RTP header in the encapsulation
a) Both sides MUST agree on using RTP header in the SAToP PW
b) In the case of a SAToP PW over L2TPv3 using the RTP header, both
sides MUST agree on usage of the "old mode" described in Annex A
above
5. RTP-dependent parameters. These following parameters MUST be agreed
upon if usage of the RTP header for the SAToP PW has been agreed
upon:
a) Timestamping mode (absolute or differential). This mode MAY be
different for the two directions of the PW, but the receiver and
transmitter MUST agree on the timestamping mode for each
direction of the PW
b) Timestamping clock frequency:
i) The timestamping frequency MUST be a integral multiple of
8kHz
ii) The timestamping frequency MAY be different for the two
directions of the PW, but the receiver and transmitter MUST
agree on the timestamping mode for each direction of the PW
c) RTP Payload Type (PT) value.
Any dynamically assigned value can be used with SAToP PWs
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d) Synchronization Source (SSRC) value. The transmitter MUST agree
to send the SSRC value requested by the receiver.
Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"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.
Editors' Addresses
Alexander ("Sasha") Vainshtein
Axerra Networks
24 Raoul Wallenberg St.,
Tel Aviv 69719, Israel
email: sasha@axerra.com
Yaakov (Jonathan) Stein
RAD Data Communications
24 Raoul Wallenberg St., Bldg C
Tel Aviv 69719, Israel
Email: yaakov_s@rad.com
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