One document matched: draft-ietf-pwe3-cesopsn-00.txt
Network Working Group A. Vainshtein - Editor (Axerra Networks)
Internet Draft I. Sasson (Axerra Networks)
A. Sadovski (Axerra Networks)
Expiration Date: E. Metz (TNO Telecom)
July 2004 T. Frost (Zarlink Semiconductor)
P. Pate (Overture Networks)
January 2004
Structure-aware TDM Circuit Emulation Service over Packet Switched
Network (CESoPSN)
draft-ietf-pwe3-cesopsn-00.txt
Status of this Memo
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provisions of section 10 of RFC 2026.
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Abstract
This document describes a method for encapsulating structured (NxDS0)
TDM signals as pseudo-wires over packet-switching networks (PSN). In
this regard, it complements similar work for structure-agnostic
emulation of TDM bit-streams [PWE3-SAToP].
TABLE OF CONTENTS
1. Introduction......................................................2
2. Terminology and Reference Models..................................2
2.1. Terminology...................................................2
2.2. Reference Models..............................................4
2.3. Requirements and Design Constraint............................4
3. Emulated Services.................................................5
4. CESoPSN Encapsulation Layer.......................................6
4.1. CESoPSN Packet Format.........................................6
4.2. PSN and Multiplexing Layer Headers............................7
4.3. CESoPSN Control Word..........................................7
4.4. Usage of the RTP header.......................................9
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5. CESoPSN Payload Layer............................................10
5.1. Common Payload Format Considerations.........................10
5.2. Basic NxDS0 Services.........................................10
5.3. Basic NxDS0 Services with Signaling..........................11
5.4. Trunk-Specific NxDS0 Services with CAS.......................13
6. CESoPSN Operation................................................15
6.1. Common Considerations........................................15
6.2. IWF operation................................................15
6.2.1. PSN-bound Direction......................................15
6.2.2. CE-bound Direction.......................................15
6.3. CESoPSN Defects..............................................17
6.4. CESoPSN PW Performance Monitoring............................17
7. QoS Issues.......................................................18
8. Congestion Control (RFC 2914) Conformance........................18
9. FFS Issues.......................................................18
10. Security Considerations.........................................19
11. Applicability Statement.........................................19
12. IANA Considerations.............................................20
13. Intellectual Property Disclaimer................................20
ANNEX A. A COMMON CE APPLICATION STATE SIGNALING MECHANISM..........24
Annex B. Reference PE Architecture for Emulation of NxDS0 SERvices..26
1. Introduction
This document describes a method for structure-aware encapsulation
structured of NxDS0 TDM signals as pseudo-wires over packet-switching
networks (PSN). In this regard, it complements similar work for for
structure-agnostic emulation of TDM bit-streams [PWE3-SAToP].
Ability to emulate NxDS0 circuits provides for saving PSN bandwidth,
supports DS0-level grooming and distributed cross-connect applications.
It also enhances resilience of CE devices to effects of loss of packets
in the PSN and, in conjunction with purely TDM mappings, can be also
used for carrying unstructured T1 bit-streams.
The CESoPSN solution presented in this document fits the PWE3
architecture described in [PWE3-ARCH], satisfies the general
requirements put forward in [PWE3-REQ] and specific requirements for
structured TDM emulation put forward in [PWE3-TDM-REQ].
2. Terminology and Reference Models
2.1. Terminology
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].
The terms defined in [PWE3-ARCH], Section 1.4 are consistently used
without additional explanations.
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This document uses some terms and acronyms that are commonly used in
conjunction with the TDM services. In particular:
o Loss of Signal (LOS) is a common term denoting a condition
where a valid TDM signal cannot be extracted from the
physical layer of the trunk. Actual criteria for detecting
and clearing LOS are described in [G.775]
o Frame Alignment Signal (FAS) is a common term denoting a
special periodic pattern that is used to impose TDM
structures on E1, T1, E3 and T3 circuits. Actual FAS
patterns are described in [G.704] (E1, T1 and T3) and
[G.751] (E3)
o Out of Frame Synchronization (OOF) is a common term
denoting the state of the receiver of a TDM signal when it
failed to find valid FAS. Actual criteria for declaring and
clearing OOF are described in [G.706]. Handling of this
condition includes invalidation of the TDM data
o Alarm Indication Signal (AIS) is a common term denoting a
special bit pattern in the TDM bit stream that indicates
presence of an upstream circuit outage. Actual criteria for
declaring and clearing the AIS condition in a TDM stream
are defined in [G.775]
o Remote Alarm Indication (RAI) and Remote Defect Indication
(RDI) are common terms (often used as synonyms) denoting a
special pattern in the framing of a TDM service that is
sent back by the receiver that experiences an AIS
condition. This condition cannot be detected while a LOS,
OOF or AIS condition is detected. Specific rules for
encoding this pattern in the TDM framing are discussed in
[G.775]
o Channel-Associated Signaling (CAS) is a common term
describing one of the methods of exchanging signals between
telephony applications. CAS is based on allocation of up to
4 constant-rate synchronous bit-streams for each Voice-
carrying DS0 channel in an E1 or T1 trunk. (These bit-
streams are commonly denoted A, B, C and D.) The actual
methods of carrying the sets of these bit streams
isochronously in an E1 or T1 trunk and establishing
association between a specific DS0 channel with an
appropriate set of these bit streams are described in
[G.704].
Note: CAS can be interpreted in two different ways. A "synchronous"
interpretation treats it as a set of synchronous bit-streams, while a
"signaling" interpretation treats it as a method to encode signals
reflecting change of state of telephony applications based upon
generation and detection of certain stable bit patterns in the CAS-
related bit-streams. The most commonly used patterns include "stable
ones" and "stable zeroes"; (i.e., two states per bit-stream); in some
cases they are augmented by a "stable alternate pattern" (providing the
3rd state of the bit-stream). The combination of these patterns allows
encoding of up to 16 different telephony application states. Most
modern E1 and T1 framers support both approaches by providing:
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1. For the synchronous approach - dedicated pins that allow
extraction/insertion of the relevant constant-rate bit-streams into
appropriate positions in the E1 or T1 trunk
2. For the signaling approach:
a) Dedicated memory-mapped registers which allow reading the actual
stabilized CAS bits values and writing the desired combination
of CAS bits values to be inserted into the resulting E1 or T1
trunk
b) Generation of interrupts when a de-bounced change of CAS bits
has been detected.
Note: Another method of exchanging signals between telephony
applications is called Common Channel Signaling (CCS). This method is
not considered in this document in detail.
2.2. Reference Models
Generic models that have been defined in Sections 4.1, 4.2 and 4.4 of
[PWE3-ARCH] are fully applicable for the purposes of this document
without any modifications.
The Network Synchronization reference model and deployment scenarios
for emulation of TDM services have been described in [PWE3-TDM-REQ],
Section 4.2.
Structured services considered in this document represent special cases
of the structured bit stream payload type defined in Section 3.3.4 of
[PWE3-ARCH]. In each specific case the basic service structures that
are preserved by a CESoPSN PW are explicitly specified (see Section 3
below).
In accordance with the principle of minimum intervention ([PWE3-ARCH],
Section 3.3.5) the TDM payload is encapsulated without any changes.
2.3. Requirements and Design Constraint
The protocol defined in this document has been designed in order to
satisfy the requirements presented in [PWE3-REQ] and [PWE3-TDM-REQ].
The CESoPSN protocol has been designed in order to meet the following
design constrains:
1. Fixed amount of TDM data per packet: All the packets belonging to a
given CESoPSN PW MUST carry the same amount of TDM data. This
approach simplifies compensation of a lost PW packet with a packet
carrying exactly the same amount of "replacement" TDM data
2. Fixed end-to-end delay: CESoPSN implementations SHOULD provide the
same end-to-end delay between any given pair of PEs regardless of
the bit-rate of the emulated service.
3. Packetization latency range:
a) All the implementations of CESoPSN SHOULD support packetization
latencies in the range 1 to 5 milliseconds
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b) CESoPSN implementations that support configurable packetization
latency MUST allow configuration of this parameter with the
granularity which is a multiple of 125 microseconds
3. Emulated Services
Structured TDM services are usually carried within appropriate physical
trunks, and PEs providing their emulation include appropriate Native
Service Processing (NSP) blocks commonly referred to as Framers.
The NSPs may also act as digital cross-connects, creating structured
TDM services from multiple synchronous trunks. As a consequence, the
service may contain more timeslots that could be carried over any
single trunk, or may not originate from any single trunk/
In accordance with [PWE3-TDM-REQ], structured services considered in
this specification are NxDS0 with and without CAS. The reference PE
architecture supporting these services is described in Annex B.
[ATM-CES] provides the following taxonomy of structured TDM services:
1. Basic NxDS0 service:
a) The structure ("frame") associated with this service that MUST
be preserved in edge-to-edge emulation is an array of N octets,
where the all the octets belong to the same frame of the "trunk"
E1 or T1 circuit (in case of a service created by the NSP acting
as a digital cross-connect from several synchronous E1 or T1
trunks, all the octets belong to the frame defined by the common
frame clock pulse of these services), and i-th octet contains
the data of the i-th DS0 channel (timeslot) in the bundle. The
circuit generates 8000 frames per second
b) This service can be optionally extended to support CAS by
employing the "signaling" interpretation of CAS and carrying CE
application signals in dedicated signaling packets
c) Implementations MUST support N <=31 and MAY optionally support
larger values of N
2. "Trunk-specific" NxDS0 service with CAS. The definition of these
services employs "synchronous" interpretation of CAS, and the
structures that must be preserved by the PW are trunk multiframes
sub-divided into trunk frames. Signaling information is carried
appended to TDM data in the "signaling sub-structures" defines in
[ATM-CES]. Since the number and bit rates of the CAS bit-streams
depend on the specific framing method used with an E1 or T1 trunk,
the following services are considered:
a) E1-NxDS0 service with CAS, 1 <= n <= 30
b) T1/ESF-NxDS0 service with CAS, 1 <= n <= 24
c) T1/SF-NxDS0 service with CAS, 1 <= n <= 24.
Note: For T1 trunks using SF format (12 frames per multiframe), CESoPSN
preserves the structure comprising two consecutive trunk multiframes
subdivided into 24 trunk frames. This consideration is aligned with
[ATM-CES].
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4. CESoPSN Encapsulation Layer
4.1. CESoPSN Packet Format
The CESoPSN header MUST contain the CESoPSN Control Word (4 bytes) and
MAY also contain a fixed RTP header [RFC3550]. If the RTP header is
included in the CESoPSN header, it MUST immediately precede the CESoPSN
control word in case of an IPv4 or IPv6 PSN, and MUST immediately
follow it in the case of an MPLS PSN (see Fig. 2a and Fig. 2b below).
Note: Such an arrangement complies with the traditional usage of RTP
for the IPv4/IPv6 PSN while making CESoPSN PWs ECMP-safe for the MPLS
PSN (see [PWE3-ARCH], Section 5.4.4).
Note: Both UDP and L2TPv3 can provide the multiplexing mechanisms for
CESoPSN PWs over an IPv4/IPv6 PSN.
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 multiplexing layer headers |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| OPTIONAL |
+-- --+
| |
+-- --+
| Fixed RTP Header (see [RFC3550]) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| CESoPSN Control Word |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Packetized TDM data (Payload) |
| ... |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2a. CESoPSN Packet Format for an IPv4/IPv6 PSN
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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 |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| CESoPSN Control Word |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| OPTIONAL |
+-- --+
| |
+-- --+
| Fixed RTP Header (see [RFC3550]) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Packetized TDM data (Payload) |
| ... |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2b. CESoPSN Packet Format for an MPLS PSN
4.2. PSN and Multiplexing Layer Headers
The total size of a CESoPSN packet for a specific PW MUST NOT exceed
path MTU between the pair of PEs terminating this PW. CESoPSN
implementations working with IPv4 PSN MUST set the "Don't Fragment"
flag in IP headers of the packets they generate.
4.3. CESoPSN Control Word
The structure of the CESoPSN 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| M |FRG| LEN | Sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3. Structure of the CESoPSN Control Word
Bits 0 to 3 MUST be set to 0 as described in [PWE3-ARCH], Section 5.4.4
L - if set, indicates some abnormal condition of the
attachment circuit.
M - a 2-bit modifier field. In case of L cleared allows
discrimination of signaling packets, carrying RDI of the
attachment circuit across the PSN etc. In case of L set
only the '00' value is currently defined, other values are
reserved for future extensions. L and M bits can be
treated as a 3-bit code point space that is described in
detail in Table 1 below
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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.
|=================================================================|
| L | M | Code Point Interpretation |
|===|=====|=======================================================|
| 0 | 00 | CESoPSN data packet - normal situation. All CESoPSN |
| | | implementations MUST recognize this code point. |
| | | Payload MUST be played out "as received". |
|---|-----|-------------------------------------------------------|
| 0 | 01 | Reserved for future extensions. |
|---|-----|-------------------------------------------------------|
| 0 | 10 | CESoPSN data packet, RDI condition of the AC. All |
| | | CESoPSN implementations MUST support this codepoint: |
| | I payload MUST be played out "as received", and, if |
| | | so configured, the receiving CESoPSN IWF instance |
| | | SHOULD be able to command the NSP to force the RDI |
| | | condition on the outgoing TDM trunk. |
|---|-----|-------------------------------------------------------|
| 0 | 11 | Reserved for CESoPSN signaling packets. |
|---|-----|-------------------------------------------------------|
| 1 | 00 | TDM data is invalid, payload MAY be omitted. All |
| | | implementations MUST recognize this code point and |
| | | insert appropriate amount of the configured "idle |
| | | code" in the outgoing attachment circuit. In addition,|
| | | if so configured, the receiving CESoPSN IWF instance |
| | | SHOULD be able to force the AIS condition on the |
| | | outgoing TDM trunk. |
|---|-----|-------------------------------------------------------|
| 1 | 01 | Reserved for future extensions |
|---|-----|-------------------------------------------------------|
| 1 | 10 | Reserved for future extensions |
|---|-----|-------------------------------------------------------|
| 1 | 11 | Reserved for future extensions |
|=================================================================|
Table 1. Interpretation of bits L and M in the CESoPSN CW.
Note: Implementations that do not support the reserved code points MUST
silently discard the corresponding packets upon reception.
LEN (bits (10 to 15) MAY be used to carry the length of the CESoPSN
packet (defined as the size of the CESoPSN header + the payload size)
if it is less than 64 bytes, and MUST be set to zero otherwise.
Sequence number is used to provide the common PW sequencing function as
well as detection of lost packets. It MUST be generated in accordance
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with the rules defined in [RFC3550], Section 5 for the RTP sequence
number.
4.4. Usage of the RTP header
When fixed RTP header (see [RFC3550], Section 5.1) is used with
CESoPSN, its fields are used in the following way:
1. V (version) is always set to 2
2. P (padding), X (header extension), CC (CSRC count) and M (marker)
are always set to 0
3. PT (payload type) is used as following:
a) 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
b) The PE at the PW ingress MUST set the PT field in the RTP header
to the allocated value
c) The PE at the PW egress MAY use the received value to detect
malformed packets
4. Sequence number in the RTP header MUST be equal to the sequence
number in the CESoPSN CW
5. Timestamps are used for carrying timing information over the
network:
a) Their values are generated in accordance with the rules
established in [RFC3550]
b) Frequency of the clock used for generating timestamps MUST be an
integer multiple of 8 kHz. All implementations of CESoPSN MUST
support the 8 kHz clock. Other frequencies that are integer
multiples of 8 kHz MAY be used if both sides agree to that
c) Possible modes of timestamp generation are discussed below
6. The SSRC (synchronization source) value in the RTP header MAY be
used for detection of misconnections.
The RTP header in CESoPSN can be used in conjunction with at least the
following modes of timestamp generation:
1. Absolute mode: the ingress PE sets timestamps using the clock
recovered from the incoming TDM circuit. As a consequence, the
timestamps are closely correlated with the sequence numbers. All
CESoPSN implementations MUST support this mode
2. Differential mode: PE devices connected by the PW have access to
the same high-quality synchronization source, and this
synchronization source is used for timestamp generation. As a
consequence, the second derivative of the timestamp series
represents the difference between the common timing source and the
clock of the incoming TDM circuit. Support of this mode is
OPTIONAL.
Usage of other timestamp generation modes is left for further study.
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5. CESoPSN Payload Layer
5.1. Common Payload Format Considerations
All the services considered in this document are treated as sequences
of "basic structures" (see Section 3 above). The payload of a CESoPSN
packet always consists of a fixed number of octets filled, octet by
octet, with the data contained in the corresponding consequent basic
structures preserving octet alignment between these structures and the
packet payload boundaries in accordance with the following rules:
1. The order of the payload octets corresponds to their order on the
TDM AC.
2. Consecutive bits coming from the TDM AC fill each payload octet
starting from its most significant bit to the least significant
one.
3. All the CESoPSN packets MUST carry the same amount of valid TDM
data in both directions of the PW. In other words, the time that is
required to fill a CESoPSN packet with the TDM data must be
constant. The PE devices terminating a CESoPSN PW MUST agree on the
number of TDM payload octets in the PW packets for both directions
of the PW at the time of the PW setup.
Notes:
1. CESoPSN packets MAY omit invalid TDM data in order to save the PSN
BW. If the CESoPSN packet payload is omitted, the L bit in the
CESoPSN control word MUST be set
2. CESoPSN PWs MAY carry CE signaling information either in separate
packets or appended to packets carrying valid TDM data. If
signaling information and valid TDM data are carried in the same
CESoPSN packet, the amount of the former does not affect the amount
of the latter.
5.2. Basic NxDS0 Services
As mentioned above, the basic structure preserved across the PSN for
this service consists of n octets filled with the data of the
corresponding DS0 channels belonging to the same frame of the
originating trunk(s), and the service generates 8000 such structures
per second.
CESoPSN MUST use alignment of the basic structures with the packet
payload boundaries in order to carry the structures across the PSN.
This means that:
1. The amount of TDM data in a CESoPSN packet MUST be either an
integer multiple or an integer divisor of the basic structure size
2. The first structure in the packet SHOULD start immediately at the
beginning of the packet payload
This mode of operation complies with the recommendation in [PWE3-ARCH]
to use similar encapsulations for structured bit stream and cell
generic payload types.
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Packetization latency, number of timeslots and payload size are linked
by the following obvious relationship:
L = 8*n*D
where:
o D is packetization latency, milliseconds
o L is packet payload size, octets
o n is number of DS0 channels.
CESoPSN implementations supporting NxDS0 services MUST support the
following set of configurable packetization latency values:
o For n >= 5: 1millisecond (with the corresponding
packet payload size of 8*n bytes)
o For 1 <= n <= 4: 5 milliseconds (with the
corresponding packet payload size of 40*n octets).
Usage of any other packetization latency (packet payload size) that is
compatible with the restrictions given in Section 6.2.1 above is
OPTIONAL.
5.3. Basic NxDS0 Services with Signaling
Implementations that have chosen to extend the basic NxDS0 to support
CE application state signaling carry encoded CE application state
signals in separate signaling packets. Signaling packets MUST use their
own sequence numbers in the control word. If RTP header is used in the
data packets, it MUST be also used in the signaling packets with the
following restrictions:
1. An additional RTP payload type (from the range of dynamically
allocated types) MUST be allocated for the signaling packets.
2. In addition, the signaling packets MUST use their own SSRC value
(different from that used for the TDM data packets).
Format of the CESoPSN signaling packets over IPv4/IPv6 and MPLS PSN is
shown in Fig. 4a and 4b below respectively.
Regardless of the specific PSN, L and M bits in the CESoPSN control
word MUST be set to "010".
Detailed protocol to be used with the signaling packets is described in
Annex A.
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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 multiplexing layer headers |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Fixed |
+-- --+
| RTP |
+-- --+
| Header (see [RFC3550]) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| CESoPSN Control Word |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Encoded CE application state entry for the DS0 channel #1 |
+-- --+
| ... |
+-- --+
| Encoded CE application state entry for the DS0 channel #n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4a. CESoPSN Signaling Packet Format over an IPv4/IPv6 PSN
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 |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| CESoPSN Control Word |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Fixed |
+-- --+
| RTP |
+-- --+
| Header (see [RFC3550]) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Encoded CE application state entry for the DS0 channel #1 |
+-- --+
| ... |
+-- --+
| Encoded CE application state entry for the DS0 channel #n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4b. CESoPSN Signaling Packet Format over an MPLS PSN
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5.4. Trunk-Specific NxDS0 Services with CAS
As mentioned above, the structure preserved by CESoPSN for this group
of services is the trunk multiframe sub-divided into the trunk frames,
and signaling information is carried appended to the TDM data using the
signaling substructures defined it [ATM-CES]. These substructures
comprise N consecutive nibbles, so that the i-th nibble carries CAS
bits for the i-th DS0 channel, and are padded with a dummy nibble for
odd values of N.
CESoPSN implementations supporting trunk-specific NxDS0 services with
CAS MUST NOT carry more TDM data per packet than is contained in a
single trunk multiframe.
All CESoPSN implementations supporting trunk-specific NxDS0 with CAS
MUST support the default mode where a single CESoPSN packet carries
exactly the amount of TDM data contained in exactly one trunk
multiframe and appended with the signaling sub-structure. The TDM data
is aligned with the packet payload. In this case:
1. Packetization latency is:
a) 2 milliseconds for E1 NxDS0
b) 3 milliseconds for T1 NxDS0
2. The packet payload size is:
a) 16*n + floor((n+1)/2) for E1-NxDS0
b) 24*n + floor((n+1)/2) for T1/ESF-NxDS0 and T1/SF-
NxDS0
3. The packet payload format coincides with the "superframe
structure with signaling" defined in [ATM-CES].
In order to provide lower packetization latency, CESoPSN
implementations for trunk-specific NxDS0 with CAS SHOULD support
fragmentation of multiframe structures between multiple CESoPSN
packets. In this case:
1. The FRG bits MUST be used to indicate first, intermediate and last
fragment of a multiframe as described in [PWE3-FRAG]
2. The amount of the TDM data per CESoPSN packet must be constant.
3. Each multiframe fragment MUST comprise an integer multiple of the
trunk frames
4. The signaling substructure MUST be appended to the last fragment of
each multiframe.
Format of CESoPSN packets carrying trunk-specific NxDS0 service with
CAS that do and do not contain signaling substructures is shown in Fig.
5(a) and (b) respectively. In these figures the number of the trunk
frames per multiframe fragment ("m") MUST be an integer divisor of the
number of frames per trunk multiframe.
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0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
--- +-+-+-+-+-+-+-+-+ --- +-+-+-+-+-+-+-+-+
| Timeslot 1 | | Timeslot 1 |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| Timeslot 2 | | Timeslot 2 |
Frame #1 | ... | Frame #1 | ... |
| Timeslot N | | Timeslot N |
--- +-+-+-+-+-+-+-+-+ --- +-+-+-+-+-+-+-+-+
| Timeslot 1 | | Timeslot 1 |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| Timeslot 2 | Frame #2 | Timeslot 2 |
Frame #2 | ... | | ... |
| Timeslot N | | Timeslot N |
--- +-+-+-+-+-+-+-+-+ --- +-+-+-+-+-+-+-+-+
... | ... | | ... |
--- +-+-+-+-+-+-+-+-+ --- +-+-+-+-+-+-+-+-+
| Timeslot 1 | | Timeslot 1 |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| Timeslot 2 | | Timeslot 2 |
Frame #m | ... | Frame #m | ... |
| Timeslot N | | Timeslot N |
--- +-+-+-+-+-+-+-+-+ --- +-+-+-+-+-+-+-+-+
Nibbles 1,2 |A B C D|A B C D|
+-+-+-+-+-+-+-+-+
Nibbles 3,4 |A B C D|A B C D|
+-+-+-+-+-+-+-+-+
Nibble n |A B C D| (pad) |
(odd) & pad +-+-+-+-+-+-+-+-+
(a) The packet with (b) The packet without
the signaling structure the signaling structure
(the last fragment of (not the last fragment
the multiframe) of the multiframe)
Figure 5. The CESoPSN Packet Payload Format for
Trunk-Specific NxDS0 with CAS
Notes:
1. In case of T1-NxDS0 with CAS, the signaling bits are
carried in the TDM data as well as in the signaling
substructure. However, the receiver MUST use the CAS bits
as carried in the signaling substructures
2. It is possible to emulate trunk-specific NxDS0 services
with CAS by just carrying the trunk multiframe structures
over the PSN (and, in case of an E1 trunk, NxDS0, including
timeslot 16 in the end service). Such an approach would be
fully consistent with the Principle of Minimum
Intervention. Its applicability is left for further study
3. In case of trunk-specific NxDS0 with CAS originating in a
T1-SF trunk, each nibble of the signaling substructure
contains A and B bits from two consecutive trunk
multiframes as described in [ATM-CES].
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6. CESoPSN Operation
6.1. Common Considerations
Edge-to-edge emulation of a TDM service using CESoPSN is only possible
when the two PW attachment circuits are of the same type (basic NxDS0
or one of the trunk-specific NxDS0 with CAS) and bit rate. The service
type and bit rate are 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 CESoPSN 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 CESoPSN headers and, for trunk-specific NxDS0 with CAS,
signaling substructures are appended to the packets carrying the last
fragment of a multiframe.
CESoPSN, 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 CESoPSN IWF SHOULD include a jitter buffer where payload
of the received CESoPSN 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 CESoPSN 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.
The CE-bound CESoPSN 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 CESoPSN data packets marked with the L bit
set SHOULD be replaced by the equivalent amount of some locally
configured "idle" bit pattern even if it has not been omitted. If
necessary, the signaling substructure of such a packet SHOULD be
replaced with the substructure carrying a (locally known) "Channel
Idle" code for all channels. In addition, the trunk carrying the local
attachment circuit MAY be forced to transmit an AIS bit pattern.
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If the data packets received have the A bit set, the trunk carrying the
local attachment circuit MAY be forced to transmit an RAI bit pattern.
Note: If the pair of IWFs at the two ends of the PW has been configured
to force the TDM trunks carrying their ACs to transmit AIS upon
reception of data packets with the L bit set and to transmit RAI upon
reception of data packets with the A bit set, this PW provides a BW-
saving emulation of a fractional E1 or T1 service between the pair of
CE devices.
The payload of each lost CESoPSN data 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 CESoPSN implementations MUST support
generation of the locally configurable "idle" 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 locally configurable "idle" pattern to its TDM
attachment circuit.
Once the PW has been set up, the CE-bound IWF begins to receive CESoPSN
packets and to store their payload in the jitter buffer but continues
to play out the locally configurable "idle" 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 CESoPSN packets or until a
preconfigured intermediate state timer expires.
Once the preconfigured amount of the TDM data has been received, the
CE-bound CESoPSN IWF enters its normal operation state where it
continues to receive CESoPSN 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 CESoPSN 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:
o The locally configurable "idle" pattern SHOULD be played out to the
TDM attachment circuit
o The local PSN-bound CESoPSN IWF SHOULD mark every packet it
transmits with the R bit set.
The CE-bound CESoPSN IWF leaves this state and transits to the
normal one once a preconfigured number of consecutive CESoPSN
packets have been received.
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6.3. CESoPSN Defects
In addition to the packet loss state of the CE-bound CESoPSN 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 MAT also be used for this purpose. Malformed in-
order packets MUST be discarded by the CE-bound IWF and replacement
data generated as for 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 CE
bound IWF's jitter buffer cannot accommodate newly arrived CESoPSN
packets.
Remote packet loss is indicated by reception of packets with their R
bit set.
6.4. CESoPSN 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 CESoPSN PWs
enhances their maintenance capabilities.
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Collection of the CESoPSN PW performance monitoring parameters is
OPTIONAL, and if implemented, is only performed after the CE-bound IWF
has exited its intermediate state.
CESoPSN defines error events, errored blocks and defects as follows:
o A CESoPSN error event is defined as insertion of a single
replacement packet into the jitter buffer (replacement of
payload of CESoPSN packets with the L bit set is not
considered as insertion of a replacement packet)
o A CESoPSN 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
CESoPSN error event
o A CESoPSN defect is defined as the packet loss state of the
CE-bound CESoPSN IWF.
The CESoPSN PW PM parameters (Errored, Severely Errored and Unavailable
Seconds) are derived from these definitions in accordance with [G.826].
7. QoS Issues
If the PSN providing connectivity between PE devices is Diffserv-
enabled and provides a PDB [RFC3086] that guarantees low-jitter and
low-loss, the CESoPSN 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).
8. Congestion Control (RFC 2914) Conformance
CESoPSN PWs represent a special case of PWs carrying constant bit rate
(CBR) services across the PSN. These services, by definition, cannot
behave in a TCP-friendly manner prescribed by [RFC2914] under
congestion while retaining any value for the user.
CESoPSN will use the generic PWE3 approach for handling congestion in
PWs carrying CBR services when such an approach has been specified.
9. FFS Issues
Note: This section will be removed from the final revision of the
document.
The following issues will be addressed in the next revisions of this
document:
o Applicability of trunk multiframe-aligned methods for
carrying trunk-specific NxDS0 with CAS
o Effect of timestamp resolution on quality of clock recovery
in Differential mode
o Extension of techniques for forcing states on outgoing ACs
to emulation of other services.
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10. Security Considerations
This document does not affect the underlying security issues of
specific PSN.
In addition, it defines misconnection detection capabilities of
CESoPSN. These capabilities increase resilience of CESoPSN to
misconfiguration and some types of DoS attacks.
11. Applicability Statement
CESoPSN is an encapsulation layer intended for carrying circuits NxDS0
services with or without CAS over PSN.
CESoPSN allows, within reasonable limits, to emulate end-to-end delay
properties of TDM networks. In particular, in most cases the edge-to-
edge delay introduced by CESoPSN PWs does not depend upon the type and
bit-rate of the emulated service.
CESoPSN fully complies with the principle of minimal intervention
minimizing overhead and computational power required for encapsulation.
CESoPSN can be used in conjunction with various clock recovery
techniques and does not presume availability of a global synchronous
clock at the ends of a PW. However, if the global synchronous clock is
available at both ends of a CESoPSN PW, using RTP and differential mode
of timestamp generation improves the quality of the recovered clock.
CESoPSN allows carrying CE application state signaling that requires
synchronization with data in-band in separate signaling packets. A
special combination of flags in the CESoPSN control word is used to
distinguish between data and signaling packets, while the Timestamp
field in the RTP headers is used for synchronization. This makes
CESoPSN extendable to support different types of CE signaling without
affecting the data path in the PE devices.
CESoPSN also allows emulation of NxDS0 services with CAS carrying the
signaling information appended to (some of) the packets carrying TDM
data.
CESoPSN allows the PSN bandwidth conservation by carrying only AIS
and/or Idle Code indications instead of data.
CESoPSN allows deployment of BW-saving Fractional point-to-point E1/T1
applications. These applications can be described like following:
o The pair of CE devices operates as if they were connected
by an emulated E1 or T1 circuit. In particular they react
to AIS and RAI states of their local ACs in the standard
way
o The PSN carries only an NxDS0 service where N is the number
of actually used timeslots in the circuit connecting the
pair of CE devices thus saving the BW.
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Being a constant bit rate (CBR) service, CESoPSN cannot provide TCP-
friendly behavior under network congestion. If the service encounters
congestion, it should be temporarily shut down.
CESoPSN allows collection of TDM-like faults and performance monitoring
parameters hence emulating 'classic' carrier services of TDM circuits
(e.g., SONET/SDH). Similarity with these services is increased by the
CESoPSN ability to carry 'far end error' indications.
CESoPSN 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.
CESoPSN provides for detection of lost packets and allows using various
techniques for generation of "replacement packets".
CESoPSN carries indications of outages of incoming attachment circuit
across the PSN thus providing for effective fault isolation.
Faithfulness of a CESoPSN PW may be increased if the carrying PSN is
Diffserv-enabled and implements a PDB that guarantees low loss and low
jitter.
CESoPSN does not provide any mechanisms for protection against PSN
outages. As a consequence, resilience of the emulated service to such
outages is defined by the PSN behavior. On the other hand:
o The jitter buffer and packets' reordering mechanisms
associated with CESoPSN increase resilience of the emulated
service to fast PSN rerouting events
o Remote indication of lost packets is carried backward
across the PSN from the receiver (that has detected loss of
packets) to transmitter. Such an indication MAY be used as
a trigger for activation of proprietary service-specific
protection mechanisms.
12. IANA Considerations
This specification requires assignment of new PW Types for CESoPSN PWs
listed in Section 4.1.
13. Intellectual Property Disclaimer
This document is being submitted for use in IETF standards discussions.
Axerra Networks, Inc. has filed one or more patent applications
relating to the CESoPSN technology outlined in this document. Axerra
Networks, Inc. will grant free unlimited licenses for use of this
technology to the users who will register and sign up at the Axerra web
site.
ACKNOWLEDGEMENTS
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We express deep gratitude to Stephen Casner who reviewed this document
in detail, corrected some serious errors and provided many valuable
inputs.
The present version of the text of the QoS section has been suggested
by Kathleen Nichols.
We thank Maximilian Riegel, Sim Narasimha, Tom Johnson, Ron Cohen and
Yaron Raz for valuable feedbacks.
We thank Alik Shimelmits for many fruitful discussions.
MANDATORY REFERENCES
[RFC1122] R. Braden (ed.), Requirements for Internet Hosts --
Communication Layers, RFC 1122, IETF, 1989
[RFC2119] S.Bradner, Key Words in RFCs to Indicate Requirement Levels,
RFC 2119, IETF, 1997
[RFC2833] H. Schulzrinne, S. Petrack, RTP Payload for DTMF Digits,
Telephony Tones and Telephony Signals. RFC 2833, IETF, 2000
[RFC2914] S. Floyd, Congestion Control Principles, RFC 2914, IETF, 2000
[RFC3086] K. Nichols, B. Carpenter, Definition of Differentiated
Services Per Domain Behaviors and Rules for their Specification, RFC
3086, IETF, 2001
[RFC3095] C. Bormann (Ed.), RObust Header Compression (ROHC): Framework
and four profiles: RTP, UDP, ESP, and uncompressed, RFC 3095, IETF,
2001
[RFC3550] H. Schulzrinne et al, RTP: A Transport Protocol for Real-Time
Applications, RFC 1889, IETF, 2003
[RTP-TYPES] RTP PARAMETERS, http://www.iana.org/assignments/rtp-
parameters
[G.114] ITU-T Recommendation G.114 (05/2000) - International telephone
connections and circuits - Recommendations on the transmission quality
for an entire international telephone connection. One-way transmission
time
[G.702] ITU-T Recommendation G.702 (11/88) - Digital Hierarchy Bit
Rates
[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
Vainshtein et al. Expires July 2004 [Page 21]
Structured TDM Circuit Emulation Service over PSN January 2004
[G.706] ITU-T Recommendation G.706 (04/91) - Frame Alignment and Cyclic
Redundancy Check (CRC) Procedures Relating to Basic Frame Structured
Defined in Recommendation G.704
[G.751] ITU-T Recommendation G.751 (xx/93) - 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.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
[ATM-CES] The ATM Forum Technical Committee. Circuit Emulation Service
Interoperability Specification version 2.0 af-vtoa-0078.000, January
1997.
INFORMATIONAL REFERENCES
[PWE3-REQ] XiPeng Xiao et al, Requirements for Pseudo Wire Emulation
Edge-to-Edge (PWE3), Work in Progress, December 2003, draft-ietf-pwe3-
requirements-08.txt
[PWE3-TDM-REQ] Maximilian Riegel et al, Requirements for Edge-to-Edge
Emulation of TDM Circuits over Packet Switching Networks (PSN), Work in
Progress, January 2004, draft-ietf-pwe3-tdm-requirements-04.txt
[PWE3-ARCH] S. Bryant, P. Pate et al, PWE3 Architecture, Work in
progress, October 2003, draft-ietf-pwe3-arch-06.txt
[PWE3-SONET] A. Malis et al, SONET/SDH Circuit Emulation over Packet
(CEP), Work in progress, October 2003, draft-ietf-pwe3-sonet-03.txt
[PWE3-IANA] L. Martini, M. Townsley, IANA Allocations for pseudo Wire
Edge to Edge Emulation (PWE3), Work in progress, October 2003, draft-
ietf-pwe3-iana-allocation-02.txt
[PWE3-FRAG] A. Malis, M. Townsley, PWE3 Fragmentation and Reassembly,
Work in Progress, December 203, draft-ietf-pwe3-fragmentation-04.txt
[PWE3-SATOP] A. Vainshtein, Y. Stein, Structure-Agnostic TDM over
Packet (SAToP), Work in Progress, December 2004, draft-ietf-pwe3-satop-
01.txt
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Authors' Addresses
Alexander ("Sasha") Vainshtein
Axerra Networks
24 Raoul Wallenberg St.,
Tel Aviv 69719, Israel
email: sasha@axerra.com
Israel Sasson
Axerra Networks
24 Raoul Wallenberg St.,
Tel Aviv 69719, Israel
email: israel@axerra.com
Akiva Sadovski
Axerra Networks
24 Raoul Wallenberg St.
Tel Aviv 69719, Israel
email: akiva@axerra.com
Eduard Metz
Thrupoint
Paasheuvelweg 16,
email: eduard.metz@hetnet.nl
Tim Frost
Zarlink Semiconductor
Tamerton Road, Roborough, Plymouth, PL6 7BQ, UK
email: tim.frost@zarlink.com
Prayson Pate
Overture Networks
507 Airport Boulevard
Building 111 Morrisville, North Carolina, 27560
Email: prayson.pate@overturenetworks.com
Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved. This
document and translations of it may be copied and furnished to others,
and derivative works that comment on or otherwise explain it or assist
in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing the
copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of developing
Internet standards in which case the procedures for copyrights defined
in the Internet Standards process must be followed, or as required to
translate it into languages other than English.
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The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS 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.
ANNEX A. A COMMON CE APPLICATION STATE SIGNALING MECHANISM
A PW that requires conveyance of CE application state signals that must
be synchronized with data carries encoded CE applications in special
signaling packets using:
o An additional PT value allocated for this purpose from
the range of unused values (see [IANA]). This value
MUST be different from one allocated for the TDM data
packets for the same PW
o An additional SSRC value that MUST be different from
one used for the data packets in order to allow a
separate numbering sequence for the signaling packets
o A sequence numbering scheme that does not depend on
one used for the data packets. This allows re-use of
common sequence numbers-based mechanisms (like
reordering and detection of lost packets) for the data
packets for all types of circuits
Handling of loss of signaling packets is not required; as a
consequence, detection of loss of these packets is not required either.
The RTP header of the signaling packets is used in the following way:
1. V (version) is always set to 2
2. P (padding) MAY be used in accordance with the
application-specific CE state encoding rules
3. X (header extension) is always set to 0
4. CC (CSRC count) is always set to 0
5. M (marker) is set to 1 to for "urgent" signaling
packets. The CE application state carried in these
packets will be conveyed to the CE at the egress of
the PW immediately, without any re-synchronization
with the data. State carried in "normal" signaling
packets will be conveyed to the CE at the PW egress
after re-synchronization with the TDM data
6. PT (payload type) is used to distinguish between
packets carrying the packetized TDM data and
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signaling packets. In accordance with that, CESoPSN
PWs using the CE application state signaling
mechanism MUST:
a) Allocate an additional PT value from the
range of dynamic values (see [RTP-TYPES])
for its signaling packets. The allocation is
done during the PW setup and MUST be the
same for both PW directions
b) The PE at the PW ingress MUST set the PT
value in the RTP header of signaling packets
to the allocated value
c) The PE at the PW egress MUST use this value
to distinguish between TDM data and
signaling packets.
7. The SSRC (synchronization source) value in the RTP
header of signaling packets MUST be different from
that used by the data packets
8. Sequence number is generated and processed in
accordance with the rules established in [RFC3550].
There should be no connection between the sequence
numbers used by the data and signaling packets
9. Timestamps are used for re-synchronization between
TDM data and CE application state signals at the PW
egress:
a) Their values are generated in accordance
with the rules established in [RFC3550]
b) Frequency of the clock used for generating
timestamps MUST be a multiple of 8 KHz and
SHOULD be the same as that used for the data
packets
10. Each PE terminating the PW SHOULD send RTCP sender
reports (see RFC3550], Section 6.3.1) for the clock
sources used for generation of timestamps of both
TDM data and signaling packets to its peer:
a) These packets MAY be limited only to the
header and 'Sender Info' sections
b) The PE receiving these packets SHOULD use
the information contained in the 'Sender
Info' in order to map (approximately)
timestamps received in the signaling packets
to these received in the data packets.
Signaling packets are generated by the ingress PE in accordance with
the following logic (adapted from [RFC2833]):
1. The CESoPSN signaling packet with the same information
is sent 3 times at an interval of 5 ms under one of
the following conditions:
a) The CESoPSN PW has been set up. These packets
MUST be marked as "urgent"
b) A change in the CE application state has been
detected. If another change of the CE
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application state has been detected during the
15 ms period, this process continues
c) Loss of packets defect has been cleared. These
packets SHOULD be marked as "urgent"
d) Remote Loss of Packets indication has been
cleared (after previously being set) These
packets SHOULD be marked as "urgent"
2. Otherwise, the CESoPSN signaling packet with the
current CAS state information is sent every 5 seconds.
These rules allow fast probabilistic recovery after loss of a single
signaling packet as well as deterministic (but, possibly, slow)
recovery following PW setup and PSN outages.
Encoding of CE application state for various common applications will
be considered in separate documents.
ANNEX B. REFERENCE PE ARCHITECTURE FOR EMULATION OF NXDS0 SERVICES
Structured TDM services do not exist as physical circuits. They are
always carried within appropriate physical attachment circuits (AC),
and the PE providing their emulation always includes a Native
Processing Block (NSP) commonly referred to as Framer. As a
consequence, the architecture of a PE device providing edge-to-edge
emulation for these services includes the Framer and Forwarder blocks.
In case of NxDS0 services (the only type of structured services
considered in this document), the AC is either an E1 or a T1 trunk, and
bundles of NxDS0 are cut out of it using one of the framing methods
described in [G.704].
In addition to detecting the FAS and imposing associated structure on
the "trunk" AC, E1 and T1 framers commonly support some additional
functionality including:
1. Detection of special states of the incoming AC (e.g.,
AIS, OOF or RAI)
2. Forcing special states (e.g., AIS and RAI) on the
outgoing AC upon an explicit request
3. Extraction and insertion of CE application signals
that may accompany specific DS0 channel(s).
The resulting PE architecture for NxDS0 services is shown in Fig. B.1
below. In this diagram:
1. In the PSN-bound direction:
a) The Framer:
i) Detects frame alignment signal (FAS)
and splits the incoming ACs into separate
DS0 channels
ii) Detects special AC states
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iii) If necessary, extracts CE application
signals accompanying each of the separate
DS0 services
b) The Forwarder:
i) Creates one or more NxDS0 bundles
ii) Sends the data received in each such
bundle to the PSN-bound direction of a
respective CESoPSN IWF instance
iii) If necessary, sends the current CE
application state data of the DS0
services in the bundle to the PSN-bound
direction of the respective CESoPSN IWF
instance
iv) If necessary sends the AC state
indications to the PSN-bound directions
of all the CESoPSN instances associated
with the given AC
c) Each PSN-bound PW IWF instance encapsulates the
received data, application state signal and the
AC state into PW PDUs and sends the resulting
packets to the PSN
2. In the CE-bound direction:
i) Each CE-bound instance of the CESoPSN
IWF receives the PW PDUs from the PSN,
extracts the TDM data, AC state and CE
application state signals and sends them
b) The Forwarder sends the TDM data, application
state signals and, if necessary, a single
command representing the desired AC state, to
the Framer
c) The Framer accepts all the data of one or more
NxDS0 bundles possibly accompanied by the
associated CE application state and commands
referring to the desired AC state, and
generates a single AC accordingly with correct
FAS.
Notes: This model is asymmetric:
o AC state indication can be forwarded from the framer
to multiple instances of the CESoPSN IWF
o No more than one CESoPSN IWF instance should forward
AC state-affecting commands to the framer.
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Structured TDM Circuit Emulation Service over PSN January 2004
+------------------------------------------+
| PE Device |
+------------------------------------------+
| | Forwarder | |
| |---------------------| |
| | | |
| +<-- AC State---->- | |
| | | | |
| | | | |
E1 or T1 | | | | |
AC | | | | |
<=======>| |-----------------+---|--------------|
| | | | At most one |
| | |-->+ PW IWF |
| | | instance im- |
... | +<---NxDS0 TDM Data-->+ posing state | PW Instance
| F | | on the X<===========>
| +<---CE App State --->+ outgoing AC |
E1 or T1 | R | | |
AC | +<--AC Command -------+ |
<=======>o A |---------------------|--------------|
| | ... | ... | ...
| M |-----------------+---|--------------|
| | | | Zero, one or |
| E | |-->+ more PW IWF |
| | | instances
| R +<---NxDS0 TDM Data-->+ that do not | PW Instance
| | | impose state X<===========>
| +<---CE App State --->+ on the outgo-|
| | | ing AC |
+------------------------------------------+
Figure B.1. Reference PE Architecture for NxDS0 Services
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