Internet DRAFT - draft-manhoudt-pwe3-tsop
draft-manhoudt-pwe3-tsop
INTERNET-DRAFT G. Manhoudt
Intended Status: Proposed Standard AimValley
Expires: July 21, 2016
S. Roullot
Alcatel-Lucent
K.Y. Wong
Alcatel-Lucent
H.L. Kuo
Chunghwa Telecom
January 18, 2016
Transparent SDH/SONET over Packet
draft-manhoudt-pwe3-tsop-08
Abstract
This document describes the Transparent SDH/SONET over Packet (TSoP)
mechanism to encapsulate Synchronous Digital Hierarchy (SDH) or
Synchronous Optical NETwork (SONET) bit-streams in a packet format,
suitable for Pseudowire (PW) transport over a packet switched network
(PSN). The key property of the TSoP method is that it transports
the SDH/SONET client signal in its entirety through the PW, i.e., no
use is made of any specific characteristic of the SONET/SDH signal
format, other than its bit rate. The TSoP transparency includes
transporting the timing properties of the SDH/SONET client signal.
This ensures a maximum of transparency and a minimum of complexity,
both in implementation and during operation.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
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Copyright and License Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology and Conventions . . . . . . . . . . . . . . . . . 5
2.1. Conventions Used in This Document . . . . . . . . . . . . 5
2.2. Acronyms and Terms . . . . . . . . . . . . . . . . . . . . 5
3. Emulated STM-N Services . . . . . . . . . . . . . . . . . . . 6
3.1. PSN-bound Direction . . . . . . . . . . . . . . . . . . . 9
3.2. CE-bound Direction . . . . . . . . . . . . . . . . . . . . 9
4. TSoP Encapsulation Layer . . . . . . . . . . . . . . . . . . . 11
4.1. TSoP Packet Format . . . . . . . . . . . . . . . . . . . . 11
4.2. PSN/PW Headers . . . . . . . . . . . . . . . . . . . . . . 11
4.2.1 Transport over an MPLS(-TP) PSN . . . . . . . . . . . . 11
4.3. TSoP Encapsulation Headers . . . . . . . . . . . . . . . . 12
4.3.1. Location and Order of TSoP Encapsulation Headers . . . 12
4.3.2. Usage and Structure of the TSoP Control Word . . . . . 12
4.3.3. Usage of the RTP Header . . . . . . . . . . . . . . . 14
5. TSoP Payload Field . . . . . . . . . . . . . . . . . . . . . . 16
6. TSoP Operation . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1. Common Considerations . . . . . . . . . . . . . . . . . . 16
6.2. IWF Operation . . . . . . . . . . . . . . . . . . . . . . 17
6.2.1. PSN-Bound Direction . . . . . . . . . . . . . . . . . 17
6.2.2. CE-Bound Direction . . . . . . . . . . . . . . . . . . 17
6.3. TSoP Defects . . . . . . . . . . . . . . . . . . . . . . . 19
6.4. TSoP Performance Monitoring . . . . . . . . . . . . . . . 20
7. Quality of Service (QoS) Issues . . . . . . . . . . . . . . . 22
8. Congestion Control . . . . . . . . . . . . . . . . . . . . . . 22
9. Security Considerations . . . . . . . . . . . . . . . . . . . 23
10. Applicability Statements . . . . . . . . . . . . . . . . . . . 24
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
13.1. Normative References . . . . . . . . . . . . . . . . . . . 27
13.2. Informative References . . . . . . . . . . . . . . . . . . 27
Appendix A. Parameter Configuration for TSoP PW Set-up . . . . . . 31
Appendix B. Buffer Configuration in the CE-bound IWF . . . . . . . 32
Appendix C. Synchronization Considerations for the CE-bound IWF . 34
C.1. Layer 1 Synchronized PEs . . . . . . . . . . . . . . . . . 36
C.2. Synchronous CEs . . . . . . . . . . . . . . . . . . . . . . 37
C.3. Pleisiochronous CEs . . . . . . . . . . . . . . . . . . . . 37
Appendix D. Effect of G-AIS Insertion on a Downstream SDH Node . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40
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1. Introduction
This document describes the Transparent SDH/SONET over Packet (TSoP)
method for encapsulating SDH or SONET signals with bit rates of
51.84 Mbit/s or N * 155.52 Mbit/s (where N = 1, 4, 16 or 64) for
Pseudowire (PW) transport over a packet switched network (PSN), using
circuit emulation techniques.
The selected approach for this encapsulation scheme avoids using any
particular signal characteristics of the SDH/SONET signal, other than
its bit rate. This approach closely follows the SAToP method
described in [RFC4553] for PW transport of E1, DS1, E3 or DS3 over a
PSN.
An alternative to the TSoP method for STM-N transport over PW is
known as CEP (Circuit Emulation over Packet) and is described in
[RFC4842]. The key difference between the CEP approach and the TSoP
approach is that within CEP an incoming STM-N is terminated and
demultiplexed to its constituent VCs (Virtual Containers).
Subsequently, each VC is individually circuit emulated and
encapsulated into a PW and transported over the PSN to potentially
different destinations, where they are reassembled into (newly
constructed) STM-N signals again. The TSoP approach, on the other
hand, is to encapsulate the entire STM-N in a single circuit
emulating Pseudowire and transport it to a single destination over
the PSN. The essential difference between both methods is that CEP
offers more routing flexibility and better bandwidth efficiency than
TSoP at the cost of the loss of transparency (overhead, timing,
scrambling) at the STM-N layer and at the cost of added complexity
associated with the inclusion of what in essence is an SDH/SONET VC
cross-connect function in the PEs.
Within the context of this document, there is no difference between
SONET [GR-253] signals, often denoted as OC-M, and SDH [G.707]
signals, usually denoted as STM-N. For ease of reading, this document
will only refer to STM-N, but any statement about an STM-N signal
should be understood to apply equally to the equivalent OC-M signal,
unless it is specifically mentioned otherwise. The equivalency can
be described by the following relations between N and M: If N = 0
then M = 1 and if N >= 1 then M = 3 * N.
The TSoP solution presented in this document conforms to the PWE3
architecture described in [RFC3985] and satisfies the relevant
general requirements put forward in [RFC3916].
As with all PWs, TSoP PWs may be manually configured or set up using
a suitably expanded version of the PWE3 control protocol [RFC4447].
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2. Terminology and Conventions
2.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
2.2. Acronyms and Terms
The following acronyms used in this document are defined in [RFC3985]
and [RFC4197]:
AC Attachment Circuit
CE Customer Edge
PE Provider Edge
PREP Pre-Processing
PSN Packet Switched Network
PW Pseudowire
SDH Synchronous Digital Hierarchy
SONET Synchronous Optical Network
In addition, the following specific terms are used in this document:
G-AIS Generic Alarm Indication Signal - A specific bit pattern
that replaces the normal STM-N signal in the case of certain
failure scenarios. The G-AIS pattern [G.709] is constructed
by continuously repeating the 2047 bit pseudo random bit
sequence based on the generating polynomial 1 + x^9 + x^11
according to [O.150].
IWF Interworking Function - A functional block that segments and
encapsulates a constant bit-rate signal into PW packets and
that in the reverse direction decapsulates PW packets and
reconstitutes the constant bit-rate signal.
LOF Loss Of Frame - A condition of an STM-N signal in which the
frame pattern cannot be detected. Criteria for raising and
clearing a LOF condition can be found in [G.783].
LOPS Loss of Packet State - A defect that indicates that the PE
at the receiving end of a TSoP carrying PW experiences an
interruption in the stream of received TSoP packets. See
[RFC5604]
LOS Loss Of Signal - A condition of the STM-N attachment circuit
in which the incoming signal has an insufficient energy
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level for reliable reception. Criteria for raising and
clearing a LOS condition can be found in [G.783].
NIM Non-Intrusive Monitor - A circuit that monitors a signal in
a certain direction of transmission, without changing the
binary content of it. A NIM can be used for Fault
Management and Performance Monitoring purposes
PDV Packet Delay Variation - A (statistical) measure that
describes the distribution of the variation in transit times
of packets in a certain flow between two reference points in
the network. See [G.8260]
SF Signal Fail - A control signal, that exists internally in a
system, to convey the failed state of an incoming signal,
from a server layer process to the adjacent client layer
process. See [G.783]
3. Emulated STM-N Services
A TSoP emulated STM-N service over a Pseudowire makes use of a
bi-directional point-to-point connection over the PSN between two
TSoP-IWF blocks, located in the PE nodes that terminate the PW that
interconnects them, as shown in figure 1. The TSoP-IWF blocks each
consist of two half-functions, a PSN-bound IWF and a CE-bound IWF,
one for each direction of transmission. As the name implies, the
PSN-bound part of the TSoP-IWF performs the conversion of an STM-N
bitstream to a packet flow, suitable for transport over the PSN and
the CE-bound part of the TSoP-IWF performs the inverse operation.
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|<------------------ Emulated Service ----------------->|
| |
| |<-------------- Pseudowire ------------->| |
| AC | | AC |
|<---->| |<----- PSN ----->| |<---->|
| | | | | |
| . PE1 . . PE2 . |
. +-----------+ +-----------+ .
+---+ | | | | +---+
| |----->| PSN-bound |====> . . . ====>| CE-bound |----->| |
| C | | IWF | | IWF | | C |
| E | | _ _ _ | | _ _ _ | | E |
| | | | | | | |
| 1 | | | | | | 2 |
| |<-----| CE-bound |<==== . . . <====| PSN-bound |<-----| |
+---+ | | IWF | | IWF | | +---+
| +-----------+ +-----------+ |
| TSoP-IWF TSoP-IWF |
native native
STM-N STM-N
Figure 1. Overview of STM-N emulated service architecture
The following list provides the STM-N services, as specified in
[G.707] and [GR-253], that can be supported by a TSoP PW:
1. STM-0 or OC-1 (51.84 Mbit/s)
2. STM-1 or OC-3 (155.52 Mbit/s)
3. STM-4 or OC-12 (622.08 Mbit/s)
4. STM-16 or OC-48 (2488.32 Mbit/s)
5. STM-64 or OC-192 (9953.28 Mbit/s)
The TSoP protocol used for emulation of STM-N services does not
depend on the method in which the STM-N is delivered to the PE. For
example, an STM-1 attachment circuit is treated in the same way
regardless of whether it is a copper [G.703] or a fiber optic [G.957]
link.
Also, in case the STM-N is carried in an OTN signal [G.709], the
functionality in the TSoP-IWF operates in the same way, but a PWE3
Preprocessing (PREP) functional block will be present between the AC
and the PE to perform the OTN (de)multiplexing functions.
The TSoP-IWF function in figure 1 is further broken down in
functional blocks in figure 2. These individual functional blocks
are described in the next two sections.
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AC TSoP-IWF PSN
------>|<------------------------------------------------->|<--------
+---------------------------------------------------+
| +-------+ |
| +-------+ SF | PSN- | CE-side_ |
| | |----->| bound | defect +--------+ |
STM-N | | STM-N | +->| NIM |------------>| | |
=========>| Rx | | +-------+ | | |
| | |===0=======================>| PSN- | | Packet
| +-------+ +--------+ | bound |===========>
| | Subst. | | IWF | |
| | Signal |===========>| | |
| | Gen. | +-->| | |
| +--------+ | +--------+ |
| | |
| PSN-bound direction | |
- - - -|- - - - - - - - - - - - - - - - - -|- - - - - - - -|- - - - -
| CE-bound direction | |
| | |
| +--------+ | PSN-side_ |
| | G-AIS | | defect |
| | Gen. |====+ | |
| +--------+ | | |
| | | +--------+ |
| +--------+ | +---| | |
| +-------+ | |<===+ | | |
| | | | STM-N/ | No_Packet | CE- | |
<=========| STM-N |<=====| G-AIS |<-----------| bound |<===========
STM-N | | Tx | | Switch | | IWF | | Packet
| | | +-->| |<========0==| | |
| +-------+ | +--------+ | | | |
| | +----------+ | +--------+ |
| | | optional | | |
| +------| CE-bound |<---+ |
| | NIM | |
| +----------+ |
+---------------------------------------------------+
Figure 2. TSoP functional block diagram
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3.1. PSN-bound Direction
In the PSN-bound direction the STM-N signal is received from the CE
via an AC by the STM-N Rx function. This function recovers the
optical or electrical signal and converts it to a suitable internal
format. In addition, it detects the LOS condition and it asserts the
SF signal whenever this is the case. The STM-N Rx block is
equivalent to the OSn_TT_Sk & OSn/RSn_A_Sk (in the case of an optical
STM-N) or the ESn_TT_Sk & ESn/RSn_A_Sk (in the case of an electrical
STM-N interface) function pairs defined in [G.783].
The PSN-bound IWF segments the STM-N ingress bitstream, which it
receives from the STM-N Rx function, in blocks of equal length. Each
block of bits is supplied with the appropriate TSoP Encapsulation
Headers and then delivered to the PSN Multiplexing layer to add the
required headers for transport over the PSN.
The PSN-bound NIM function controls the state of the CE-side_defect
signal. It will assert this signal in case the SF signal is asserted
or in case another defect is detected in the incoming STM-N signal.
The inclusion of other defects than LOS in the CE-side_defect signal
is OPTIONAL.
When the CE-side_defect signal is asserted, the PSN-bound IWF will
set the corresponding flag (L-bit) in the overhead of the affected
packets. Packets in which the L-bit is set MUST have a substitution
payload (created by the Substitution Signal Generator function) of
the same length as the regular TSoP payload. This substitution
payload is RECOMMENDED to be the G-AIS pattern or a fixed "all ones"
pattern.
Lastly, when the PSN-side_defect state is asserted, the PSN-bound IWF
will set the corresponding flag (R-bit) in the overhead of all
packets that are transmitted while this signal is in the asserted
state.
3.2. CE-bound Direction
In the CE-bound direction, the CE-bound IWF receives the PW packets
from the PSN and strips off the PSN, PW, and TSoP encapsulation
headers and writes the payload data in a buffer. The output data
stream towards the CE is created by playing out this buffer with a
suitable clock signal. The thus reconstructed STM-N signal is
forwarded to the STM-N/G-AIS Switch function.
The No_Packet signal is asserted by the CE-bound IWF in case the
internal packet buffer empties due to lack of input packets from the
PSN or in case a packet is missing or invalid.
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The PSN-side_defect signal is asserted by the CE-bound IWF in case
the LOPS condition is detected by the CE-bound IWF (see section
6.2.2). The state of this signal controls the value of the R-bit in
the overhead of the packets returned towards the far-end TSoP-IWF.
The G-AIS Generator generates a G-AIS signal at the nominal frequency
of the recovered STM-N signal, +/- 20 ppm.
The STM-N/G-AIS Switch normally takes its input from the CE-bound IWF
and forwards the recovered STM-N signal towards the STM-N Tx
function, but during the time that the No_Packet signal is asserted,
it will select the G-AIS Generator as its active input and forward a
G-AIS signal towards the STM-N Tx function.
The CE-bound NIM function is an OPTIONAL function that can be used to
detect additional defects in the recovered CE-bound STM-N signal.
The presence of such defects (e.g. STM-N LOF) MAY be used as an
additional reason for the STM-N/G-AIS Switch function to select the
G-AIS signal as its active input.
Lastly, the STM-N Tx function converts the internal signal that is
output by the STM-N/G-AIS Switch block into a regular STM-N signal
towards the CE via the AC. The STM-N Tx block is equivalent to the
OSn_TT_So & OSn/RSn_A_So (in the case of an optical STM-N) or the
ESn_TT_So & ESn/RSn_A_So (in the case of an electrical STM-N
interface) function pairs defined in [G.783].
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4. TSoP Encapsulation Layer
4.1. TSoP Packet Format
The general format of TSoP packets during transport over the PSN is
shown in Figure 3.
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/PW Headers |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| ... |
| TSoP Encapsulation Headers |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
| ... |
| |
| TSoP Payload Field |
| |
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3. Generic TSoP Packet Format
4.2. PSN/PW Headers
A TSoP PW can be transported over different types of PSNs based on
different switching technology. Below the transmission over MPLS is
described, but other methods are not precluded. The selected method
will determine the format of the PSN/PW Headers part and influence
the order of the fields in the TSoP Encapsulation Headers part.
4.2.1 Transport over an MPLS(-TP) PSN
In case a TSoP PW is forwarded over an MPLS(-TP) PSN, a standard
"bottom of stack" PW label as shown in figure 4 is prepended before
the TSoP Encapsulation Headers. Subsequently, one or more MPLS(-TP)
labels need to be pushed according to the standard MPLS transport
methods outlined in [RFC3031] and [RFC3032].
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4. PW Label (S = 1)
4.3. TSoP Encapsulation Headers
4.3.1. Location and Order of TSoP Encapsulation Headers
The TSoP Encapsulation Headers MUST contain the TSoP Control Word
(figure 6) and MUST contain a Minimum length RTP Header [RFC3550]
(figure 7). The TSoP Encapsulation Headers must immediately follow
the PSN/PW header, as shown in figure 5.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
| PSN/PW Headers |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| TSoP Control Word |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
+-- --+
| Minimum length RTP Header (see [RFC3550]) |
+-- --+
| |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| ... |
| STM-N data (Payload) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5. General TSoP Packet Format
4.3.2. Usage and Structure of the TSoP Control Word
The purpose of the TSoP control word is to allow:
1. Detection of packet loss or misordering
2. Differentiation between PSN and attachment circuit problems as a
cause for outage of the emulated service
3. Signaling of faults detected at the PW egress to the PW ingress
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The structure of the TSoP Control Word is in accordance with the
general PW Control Word format specified in [RFC4385]. The TSoP CW
format is shown in Figure 6 below. This TSoP Control Word MUST be
present in each TSoP PW packet.
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 6. Structure of the TSoP Control Word
The use of Bits 0 to 3 is described in [RFC4385]. 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 TSoP PW is being monitored using Virtual Circuit Connectivity
Verification [RFC5085] or in case OAM functionality according to
[RFC6371] is added.
L (bit 4) - If this bit is set, it indicates that the STM-N data
ingressing in the PSN-bound IWF is currently experiencing a fault
condition. Once set, if the fault is rectified, the L-bit MUST
be cleared. For each frame that is transmitted with L-bit = 1,
the PSN-bound IWF MUST insert such an amount of substitution data
in the TSoP payload field that the TSoP frame length, as it is
during normal operation, is maintained. The CE-bound IWF MUST
play out an amount of G-AIS data corresponding to the original
TSoP Payload Field for each received packet with the L-bit set.
Note: This document does not prescribe exactly which STM-N fault
conditions are to be treated as invalidating the payload carried
in the TSoP packets. An example of such a fault condition would
be LOS.
R (bit 5) - If this bit is set by the PSN-bound IWF, it indicates
that its local CE-bound IWF is in the LOPS state, i.e., it 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 LOPS state, i.e., has received a preconfigured
number of consecutive packets. See also section 6.2.2.
RSV (bits 6 to 7) - This field MUST be set to 0 by the PSN-bound IWF
and MUST be ignored by the CE-bound IWF. RSV is reserved.
FRG (bits 8 to 9) - This field MUST be set to 0 by the PSN-bound IWF
and MUST be ignored by the CE-bound IWF. FRG is fragmentation;
see [RFC4623].
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LEN (bits 10 to 15) - This field MAY be used to carry the length of
the TSoP packet (defined as the length of the TSoP Encapsulation
Header + TSoP Payload Field) if it is less than 64 octets, and
MUST be set to zero otherwise. When the LEN field is set to 0,
the preconfigured size of the TSoP packet payload MUST be assumed
to be as described in Section 5, and if the actual packet size is
inconsistent with this length, the packet MUST be considered
malformed.
Sequence number (bits 16 to 31) - This field is used to enable 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:
o Its space is a 16-bit unsigned circular space
o Its initial value SHOULD be random (unpredictable).
It MUST be incremented with each TSoP data packet sent in the
specific PW.
4.3.3. Usage of the RTP Header
A minimum length RTP Header as specified in [RFC3550] MUST be
included in the TSoP Encapsulation Header. The reason for mandating
the insertion of an RTP Header by the PSN-bound IWF is that it is
expected that in most cases the CE-bound IWF will need to use the
contained timestamps to be able to recover a clock signal of
sufficient quality. By avoiding to make the presence of RTP Headers
subject to configuration, the design of the CE-bound IWF can be
simplified and another potential source of errors during
commissioning is eliminated.
The RTP Header fields in the list below (see also figure 7) MUST have
the following specific values:
V (version) = 2
P (padding) = 0
X (header extension) = 0
CC (CSRC count) = 0
M (marker) = 0
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V |P|X| CC |M| PT | Sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7. Structure of the RTP Header field
The PT (payload type) field is used as follows:
1. One PT value SHOULD be allocated from the range of dynamic values
(see [RTP-TYPE]) 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.
The sequence number MUST be the same as the sequence number in the
TSoP control word.
The RTP timestamps are used for carrying timing information over the
network. Their values MUST be generated in accordance with the rules
established in [RFC3550].
A TSoP implementation MUST support RTP timestamping at the PW ingress
with a nominal clock frequency of 25 MHz. This is also the default
value. Other clock frequencies MAY be supported to generate the RTP
Timestamps. Selection of the applicable clock frequency is done
during commissioning of the PW that carries the emulated STM-N
service.
The SSRC (synchronization source) value in the RTP header MAY be used
for detection of misconnections, i.e., incorrect interconnection of
attachment circuits. In case this option is not used, this field
should contain an all zero pattern.
The usage of the options associated with the RTP Header (the
timestamping clock frequency, selected PT and SSRC values) MUST be
aligned between the two TSoP IWFs during Pseudowire commissioning.
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5. TSoP Payload Field
In order to facilitate handling of packet loss in the PSN, all
packets belonging to a given TSoP PW are REQUIRED to carry a fixed
number of octets in its TSoP Payload Field.
The TSoP Payload Field length MUST be defined during PW
commissioning, MUST be the same for both directions of the PW, and
MUST remain unchanged for the lifetime of the PW.
All TSoP implementations MUST be capable of supporting the following
TSoP Payload Field length:
o STM-N (for N = 0, 1, 4, 16 and 64) - 810 octets
Notes:
1. Whatever the selected payload size, TSoP does not assume
alignment to any underlying structure imposed by STM-N framing
(octet, frame, or multiframe alignment). The STM-N signal
remains scrambled through the TSoP encapsulation and
decapsulation processes.
2. With a payload size of 810 octets, the STM-N emulated service
over the PSN will have a nominal packet rate of 8000 packets/s
when N = 0 and a nominal packet rate of 24000 * N packets/s for
N >= 1.
TSoP uses the following ordering for packetization of the STM-N data:
o The order of the payload octets corresponds to their order on
the attachment circuit.
o Consecutive bits coming from the attachment circuit fill each
payload octet starting from most significant bit to least
significant.
6. TSoP Operation
6.1. Common Considerations
Edge-to-edge emulation of an STM-N service using TSoP is only
possible when the two PW attachment circuits are of the same type,
i.e., both are STM-N with equal N.
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6.2. IWF Operation
6.2.1. PSN-Bound Direction
Once the PW is commissioned, the PSN-bound TSoP IWF operates as
follows:
The ingressing STM-N bit-stream is segmented, such that each segment
contains the configured number of payload octets per packet. This
forms the TSoP Payload Field. The STM-N bit-stream MUST NOT be
descrambled before segmentation and packetization for PW transport.
Subsequently, the TSoP Encapsulation Headers are prepended according
to the rules in section 4.3.
Lastly, the PSN/PW Headers are added to the packetized service data,
and, depending on the applicable layer 1 technology, additional
overhead is added. The resulting packets are transmitted over the
PSN.
6.2.2. CE-Bound Direction
Once the PW is commissioned, the CE-bound TSoP IWF operates as
follows:
Each time a valid TSoP packet is received from the PSN, its sequence
number is checked to determine its relative position in the stream of
received packets. Packets that are received out-of-order MAY be
reordered. Next, the data in the fixed length TSoP payload field of
each packet is written into a (jitter) buffer in the order indicated
by its sequence number. In case data is missing due to a lost packet
or a packet that could not be re-ordered, an equivalent amount of
dummy data (G-AIS pattern) is substituted.
Subsequently, the STM-N stream towards the CE is reconstructed by
playing out the buffer content with a clock that is reconstructed to
have the same average frequency as the STM-N clock at the PW ingress.
In addition, this clock signal must have such properties that the
following requirements can be met:
o A reconstructed SDH-type STM-N signal delivered to an
Attachment Circuit MUST meet [G.825] and [G.823] jitter and
wander requirements (for synchronization interfaces), or,
o A reconstructed SONET-type OC-M signal delivered to an
Attachment Circuit MUST meet [GR-253] jitter and wander
requirements.
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The size of the buffer in the CE-bound TSoP IWF SHOULD be
configurable to allow accommodation to the PSN specific packet delay
variation (see appendix B).
The CE-bound TSoP IWF MUST use the sequence number in either the TSoP
Control Word or in the RTP header for detection of lost and
misordered packets. The use of the sequence number in the TSoP
Control Word for this purpose is RECOMMENDED.
The CE-bound TSoP IWF MAY reorder misordered packets. Misordered
packets that can not be reordered MUST be discarded and treated the
same way as lost packets.
The payload of received TSoP packets marked with the L-bit set MUST
be replaced by the equivalent number of bits from the G-AIS pattern.
Likewise, the payload of each lost or malformed (see section 6.3)
TSoP packet MUST be replaced with the equivalent number of bits from
the G-AIS pattern.
Before a TSoP PW has been commissioned and after a TSoP PW has been
decommissioned, the IWF MUST play out the G-AIS pattern to its STM-N
attachment circuit.
Once a TSoP PW has been commissioned, the CE-bound IWF begins to
receive TSoP packets and to store their payload in the buffer, but
continues to play out the G-AIS pattern to its STM-N attachment
circuit. This intermediate state persists until a preconfigured
degree of filling (for example half of the CE-bound IWF buffer) has
been reached by writing consecutive TSoP packets or until a
preconfigured intermediate state timer (started when the TSoP
commissioning is complete) expires. See appendix B for
considerations regarding the configuration of the initial degree of
filling of this buffer.
Each time an STM-N signal is replaced by a G-AIS signal at the same
nominal bitrate, this signal may start at an arbitrary point in its
repeating 2047-bit sequence. Once the starting point is selected,
the G-AIS signal is sent uninterrupted until the condition that
invoked it has been removed. The frequency of the clock that is used
to generate this G-AIS signal MUST have an accuracy that is better
than +/- 20 ppm relative to the nominal STM-N frequency. Appendix D
describes the effect of G-AIS insertion on downstream SDH equipment.
Once the preconfigured amount of the STM-N data has been received,
the CE-bound TSoP IWF enters its normal operational state where it
continues to receive TSoP packets and to store their payload in the
buffer while playing out the contents of the jitter buffer in
accordance with the required clock. In this state, the CE-bound IWF
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performs clock recovery, MAY monitor PW defects, and MAY collect PW
performance monitoring data.
The CE-bound IWF enters the LOPS defect state in case it detects the
loss of a preconfigured number of consecutive packets or if the
intermediate state timer expires before the required amount of STM-N
data has been received. While in this state, the local PSN-bound
TSoP IWF SHOULD mark every packet it transmits with the R-bit set.
The CE-bound IWF leaves the LOPS defect state and transits to the
normal state once a preconfigured number of consecutive valid TSoP
packets have been received (successfully reordered packets contribute
to the count of consecutive packets).
The RTP timestamps inserted in each TSoP packet at the PW ingress
allow operation in differential mode, provided that both PW ingress
and PW egress IWFs have a local clock that is traceable to a common
timing source.
The use of adaptive mode clocking mode, i.e., recovering the STM-N
clock in the CE-bound IWF by essentially averaging the arrival times
of the TSoP packets from the PSN without using RTP information, is
not recommended for TSoP-based circuit emulation. Appendix C
provides some considerations regarding the implementation and
configuration of the CE-bound IWF.
6.3. TSoP Defects
In addition to the LOPS state defined above, the CE-bound TSoP IWF
MAY detect the following defects:
o Stray packets
o Malformed packets
o Excessive packet loss rate
o Buffer overrun
o Buffer underrun
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 PW demultiplexing
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layers. 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 and the actual packet size inferred from the PSN and PW
demultiplexing layers (taking the value of the L-bit into account).
Differences between the received PT value and the PT value 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 time and comparing it
with preconfigured raise and clear thresholds.
Buffer overrun is detected in normal operational state when the
(jitter) buffer of the CE-bound IWF cannot accommodate newly arrived
TSoP packets.
Buffer underrun can detected in normal operational state when the
(jitter) buffer of the CE-bound IWF has insufficient data to maintain
playing out the STM-N signal towards the CE at the recovered clock
rate. In this situation G-AIS MUST be substituted until the buffer
fill has reached its preconfigured degree of filling again.
Remote packet loss is indicated by reception of packets with their
R-bit set.
6.4. TSoP Performance Monitoring
Performance monitoring (PM) parameters are routinely collected for
STM-N services and provide an important maintenance mechanism in SDH
networks. However, STM-N level PM data provides the information over
the performance of the end-to-end STM-N connection, which may extend
well beyond the part in which it is carried over a TSoP Pseudowire.
It may be important to be able to measure the performance of a TSoP
Pseudowire section, which forms a part of the STM-N end-to-end
connection, in isolation. For that reason a set of packet level
counters are specified that can be used to assess the performance of
the TSoP Pseudowire section. Collection of the TSoP PW performance
monitoring data is OPTIONAL and, if implemented, is only performed
after the CE-bound IWF has exited its intermediate state.
The following counters are defined:
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ENCAP_TXTOTAL_PKTS (counter size: 32 bits) - The total number of
TSoP packets that is transmitted towards the PSN by the
PSN-bound IWF function. This includes packets with the L-bit
set.
DECAP_RXTOTAL_PKTS (counter size: 32 bits) - The total number of
TSoP packets that is received from the PSN by the CE-bound IWF
function. This includes malformed packets, out-of-order
packets and packets with the L-bit set.
DECAP_REORDERED_PKTS (counter size: 32 bits) - The number of out-
of-order TSoP packets that is received from the PSN by the
CE-bound IWF, based on the received sequence numbers, for which
the ordering could be corrected by the CE-bound IWF.
DECAP_MISSING_PKTS (counter size: 32 bits) - The number of TSoP
packets that did not arrive at the CE-bound IWF from the PSN,
based on the received sequence numbers.
DECAP_MALFORMED_PKTS (counter size: 32 bits) - The number of TSoP
packets that is received from the PSN by the CE-bound IWF
function which contains one of the following RTP related
errors: TSoP Payload Field length mismatch, PT-value mismatch
(if checked) and/or SSRC mismatch (if checked).
DECAP_OUTOFORDER_PKTS (counter size: 32 bits) - The number of out-
of-order TSoP packets that is received from the PSN by the
CE-bound IWF, based on the received sequence numbers, for which
the ordering could not be corrected by the CE-bound IWF.
DECAP_OVERRUN_BITS (counter size: 64 bits) - The number of bits of
TSoP Payload that is received from the PSN but dropped by the
CE-bound IWF due to the fact that the (jitter) buffer has
insufficient capacity available to store the complete TSoP
Payload Field content.
DECAP_UNDERRUN_BITS (counter size: 64 bits) - The number of bits
that is not played out towards the CE by the CE-bound IWF
because the (jitter) buffer is empty at the moment they need to
be played out.
DECAP_PLAYEDOUT_PKTS (counter size: 32 bits) - The number of
packets that has been successfully played out towards the CE by
the CE-bound IWF containing valid STM-N payload, including the
packets that have been received with the L-bit set, containing
substituted data. Packets which are lost in transmission over
the PSN or packets which are (partially) discarded by the
CE-bound IWF due to some error condition are not counted.
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Note that packets with the L-bit set are considered normal data from
the perspective of TSoP Pseudowire Performance Monitoring, since in
such cases the location of the fault is in the STM-N path, before it
ingresses the PSN-bound IWF, so outside the scope of the TSoP PW.
7. Quality of Service (QoS) Issues
TSoP 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 TSoP 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) [RFC2212]
with the appropriate bandwidth reservation SHOULD be used in order to
provide a bandwidth guarantee equal or greater than that of the
encapsulated STM-N traffic.
8. Congestion Control
As explained in [RFC3985], the PSN carrying the PW may be subject to
congestion. TSoP PWs represent inelastic constant bit-rate 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 TSoP PWs can contribute to network congestion that may
impact network control protocols.
Whenever possible, TSoP PWs SHOULD be carried across traffic-
engineered PSNs that provide either bandwidth reservation and
admission control or forwarding prioritization and boundary traffic
conditioning mechanisms. IntServ-enabled domains supporting
Guaranteed Service (GS) [RFC2212] and DiffServ-enabled domains
[RFC2475] supporting Expedited Forwarding (EF) [RFC3246] provide
examples of such PSNs. Such mechanisms will negate, to some degree,
the effect of the TSoP PWs on the neighboring streams.
If TSoP 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 TSoP PW SHOULD shut down bi-directionally
for some period of time as described in Section 6.5 of [RFC3985].
Note that:
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1. The TSoP IWF can inherently provide packet loss measurement since
the expected rate of arrival of TSoP packets is fixed and known
2. The results of the TSoP packet loss measurement may not be a
reliable indication of presence or absence of severe congestion if
the PSN provides enhanced delivery. For example:
a) If TSoP traffic takes precedence over non-TSoP traffic, severe
congestion can develop without significant TSoP packet loss.
b) If non-TSoP traffic takes precedence over TSoP traffic, TSoP
may experience substantial packet loss due to a short-term
burst of high-priority traffic.
3. The availability objectives for the digital paths that are
supported by an STM-N signal (see [G.827]) MUST be taken into
account when deciding on temporary shutdown of TSoP PWs.
This specification does not define the exact criteria for detecting
"severe congestion" using the TSoP packet loss rate or the specific
methods for bi-directional shutdown the TSoP PWs (when such severe
congestion has been detected) and their subsequent re-start after a
suitable delay. This is left for further study. However, the
following considerations may be used as guidelines for implementing
the TSoP severe congestion shutdown mechanism:
1. If the TSoP PW has been set up using either PWE3 control protocol
[RFC4447], the regular PW teardown procedures of these protocols
SHOULD be used.
2. If one of the TSoP 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.
9. Security Considerations
TSoP 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.
TSoP PWs share susceptibility to a number of Pseudowire layer attacks
and will use whatever mechanisms for confidentiality, integrity, and
authentication are developed for general PWs. These methods are
beyond the scope of this document.
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Although TSoP PWs MUST employ an RTP header to achieve an explicit
transfer of timing information, SRTP (see [RFC3711]) mechanisms are
NOT RECOMMENDED as a substitute for PW layer security.
Misconnection detection capabilities of TSoP increase its resilience
to misconfiguration.
Random initialization of sequence numbers, in both the control word
and the optional RTP header, makes known plaintext attacks on
encrypted TSoP PWs more difficult. Encryption of PWs is beyond the
scope of this document.
10. Applicability Statements
TSoP is an encapsulation layer intended for carrying SDH STM-N
circuits over the PSN in a structure-agnostic and fully transparent
fashion.
TSoP fully complies with the principle of minimal intervention,
minimizing overhead and computational power required for
encapsulation.
TSoP provides sequencing and synchronization functions needed for
emulation of STM-N bit-streams, including detection of lost or
misordered packets and perform the appropriate compensation.
Furthermore, explicit timing information is provided by the presence
of an RTP timestamp in each TSoP packet.
STM-N bit-streams carried over TSoP PWs may experience delays
exceeding those typical of native SDH networks. These delays include
the TSoP 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 TSoP PW.
See appendix B for more information on jitter buffer configuration.
TSoP carries STM-N streams over PSN in their entirety, including any
control plane data contained within the data. Consequently, the
emulated STM-N services are sensitive to the PSN packet loss.
Appropriate generation of replacement data can be used to prevent LOF
defects and declaration of severely errored seconds (SES) due to
occasional packet loss. Other effects of packet loss on this
interface (e.g., errored blocks) cannot be prevented. See appendix D
for more information.
TSoP provides for effective fault isolation by forwarding the local
attachment circuit failure indications to the remote attachment
circuit.
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TSoP provides for a carrier independent ability to detect
misconnections and malformed packets via the PT and SSRC fields in
the RTP Header. This feature increases resilience of the emulated
service to misconfiguration.
Being a constant bit rate (CBR) service, TSoP cannot provide TCP
friendly behavior under network congestion.
Faithfulness of a TSoP PW may be increased by exploiting QoS features
of the underlying PSN.
TSoP 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.
A key requirement for TSoP to achieve transparent transport of the
timing information of an STM-N signal, is that the recovered STM-N
signal meets all relevant SDH and SONET jitter/wander requirements
(see section 6.2.2). It will depend on the synchronization situation
of the PSN whether or not a given CE-bound TSoP implementation can
meet this requirement. In appendix C a number of network
synchronization situations are listed, in which it is possible to
meet this requirement with a reasonable CE-bound IWF design. In
other network synchronization scenarios, the application of TSoP is
not generically recommended.
11. IANA Considerations
IANA is requested to assign a new MPLS Pseudowire (PW) type for the
following TSoP encapsulated services:
PW type Description Reference
-------- ---------------- ----------
0x0020 STM-0 or OC-1 RFC XXXX
0x0021 STM-1 or OC-3 RFC XXXX
0x0022 STM-4 or OC-12 RFC XXXX
0x0023 STM-16 or OC-48 RFC XXXX
0x0024 STM-64 or OC-192 RFC XXXX
The above value is suggested as the next available value and has been
reserved for this purpose by IANA.
RFC Editor: Please replace RFC XXXX above with the RFC number of this
document and remove this note.
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12. Acknowledgements
The authors of this document are much indebted to the authors of
[RFC4553]. This latter RFC has been used as a template and example
for the current document. Moreover, many paragraphs and sentences
have been copied from this RFC without alteration or with only slight
modification into the current document.
Furthermore, we thank Zhu Hao, Jeff Towne, Willem van den Bosch,
Peter Roberts and Matthew Bocci for their valuable feedback.
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13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[G.707] ITU-T Recommendation G.707/Y.1322 (01/2007) - Network node
interface for the synchronous digital hierarchy (SDH)
[G.783] ITU-T Recommendation G.783 (03/2006) - Characteristics of
synchronous digital hierarchy (SDH) equipment functional
blocks
[O.150] ITU-T Recommendation O.150 (05/1996) - General
requirements for instrumentation for performance
measurement on digital transmission equipment
[G.823] ITU-T Recommendation G.823 (03/2000) - The control of
jitter and wander within digital networks which are based
on the 2048 kbit/s hierarchy
[G.825] ITU-T Recommendation G.825 (03/2000) - The control of
jitter and wander within digital networks which are based
on the synchronous digital hierarchy (SDH)
[GR-253] Telcordia GR-253-CORE - Synchronous Optical Network
(SONET) Transport Systems: Common Generic Criteria, Issue
5, October 2009
13.2. Informative References
[RFC2212] Shenker, S., Partridge, C., and R. Guerin, "Specification
of Guaranteed Quality of Service", RFC 2212, September
1997.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, September 2000.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
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Label Switching Architecture", RFC 3031, January 2001.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC3086] Nichols, K. and B. Carpenter, "Definition of
Differentiated Services Per Domain Behaviors and Rules for
their Specification", RFC 3086, April 2001.
[RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
J., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, March 2002.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC3916] Xiao, X., Ed., McPherson, D., Ed., and P. Pate, Ed.,
"Requirements for Pseudo-Wire Emulation Edge-to-Edge
(PWE3)", RFC 3916, September 2004.
[RFC3985] Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation
Edge-to-Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC4197] Riegel, M., Ed., "Requirements for Edge-to-Edge Emulation
of Time Division Multiplexed (TDM) Circuits over Packet
Switching Networks", RFC 4197, October 2005.
[RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
Use over an MPLS PSN", RFC 4385, February 2006.
[RFC4447] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
G. Heron, "Pseudowire Setup and Maintenance Using the
Label Distribution Protocol (LDP)", RFC 4447, April 2006.
[RFC4553] Vainshtein, A., Ed., and YJ. Stein, Ed., "Structure-
Agnostic Time Division Multiplexing (TDM) over Packet
(SAToP)", RFC 4553, June 2006.
[RFC4623] Malis, A. and M. Townsley, "Pseudowire Emulation Edge-to-
Edge (PWE3) Fragmentation and Reassembly", RFC 4623,
August 2006.
[RFC4842] Malis, A., Pate, P., Cohen, R., Ed., and D. Zelig,
"Synchronous Optical Network/Synchronous Digital Hierarchy
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(SONET/SDH) Circuit Emulation over Packet (CEP)",
RFC 4842, April 2007.
[RFC5085] Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire
Virtual Circuit Connectivity Verification (VCCV): A
Control Channel for Pseudowires", RFC 5085, December 2007.
[RFC5604] Nicklass, O., "Managed Objects for Time Division
Multiplexing (TDM) over Packet Switched Networks (PSNs)",
RFC 5604, July 2009.
[RFC6371] Busi, I., Ed., and D. Allan, Ed., "Operations,
Administration, and Maintenance Framework for MPLS-Based
Transport Networks", RFC 6371, September 2011.
[G.703] ITU-T Recommendation G.703 (11/2001) - Physical/electrical
characteristics of hierarchical digital interfaces
[G.709] ITU-T Recommendation G.709/Y.1331 (12/2009) - Interfaces
for the Optical Transport Network (OTN)
[G.781] ITU-T Recommendation G.781 (09/2008) - Synchronization
layer functions
[G.811] ITU-T Recommendation G.811 (09/1997) - Timing
characteristics of primary reference clocks
[G.813] ITU-T Recommendation G.813 (03/2003) - Timing
characteristics of SDH equipment slave clocks (SEC)
[G.827] ITU-T Recommendation G.827 (09/2003) - Availability
performance parameters and objectives for end-to-end
international constant bit-rate digital paths
[G.828] ITU-T Recommendation G.828 (03/2000) - Error performance
parameters and objectives for international, constant bit
rate synchronous digital paths
[G.957] ITU-T Recommendation G.957 (06/1999) - Optical interfaces
for equipments and systems relating to the synchronous
digital hierarchy
[G.8260] ITU-T Recommendation G.8260 (02/2012) - Definitions and
terminology for synchronization in packet networks
[G.8262] ITU-T Recommendation G.8262/Y.1362 (07/2010) - Timing
characteristics of a synchronous Ethernet equipment slave
clock
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[RTP-TYPE] RTP PARAMETERS, <http://www.iana.org/assignments/rtp-
parameters>
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Appendix A. Parameter Configuration for TSoP PW Set-up
The following parameters of the TSoP 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 set-up protocols:
1. Type of attachment circuit, i.e., the value of N of the STM-N
signal, which corresponds to a bit rate as mentioned in section 3.
2. Payload size, i.e., the (constant) number of octets that is
transmitted in the TSoP Payload Field of each TSoP packet. The
default value is 810 octets.
3. Timestamping clock frequency: 25 MHz (default) or an alternative
value.
4. The configurability of the following parameters (see
section 6.2.2) governing the behavior of the CE-bound IWF buffer
is optional:
a) The maximum amount of payload data that may be stored in the
CE-bound IWF payload buffer
b) The desired degree of filling of the CE-bound IWF buffer in
steady state (see appendix B)
c) The "intermediate state" timer, i.e., the maximum amount of
time that the CE-bound IWF waits before it starts to play out
data towards the CE
5. The content of the following RTP header fields must be provided by
the user:
a) The 7-bit RTP Payload Type (PT) value; any value can be
assigned to be used with TSoP PWs. Default is an all zero
pattern.
b) The 32-bit Synchronization Source (SSRC) value. Default is an
all zero pattern.
6. The number of TSoP packets that must be missed consecutively
before the CE-bound IWF enters the LOPS defect state (default: 10)
and the number of TSoP packets that must be received consecutively
to clear the LOPS defect state (default: 2). See section 4.3.2
and [RFC5604]
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7. To support the optional excessive packet loss event by the
CE-bound IWF, the following parameters must be configured:
a) The length of the observation period for detecting excessive
packet loss. Default value is 10 s.
b) The minimum number of lost packets that is to be detected in
the observation interval before an excessive packet loss alarm
is raised. Default value is 30% of the expected packets.
c) The maximum number of lost packets that is to be detected in
the observation interval to clear an excessive packet loss
alarm. Default value is 1% of the expected packets.
Appendix B. Buffer Configuration in the CE-bound IWF
The buffer in the CE-bound IWF (often called the "jitter buffer") is
used to compensate the differences in transit time that each bit of
the STM-N signal experiences between the moment it ingresses the
PSN-bound IWF and the moment it ingresses the CE-bound IWF. There
are two mechanisms that cause the transit times of individual bits to
be different:
1. The packetization delay (Tpkt(n)) in the PSN-bound IWF: After
arrival in the PSN-bound IWF, STM-N bit #n has to wait until
sufficient bits have been received to fill the complete Payload
Field of a TSoP packet. Clearly if two STM-N bits end up in the
same TSoP packet, the bit that arrives earlier has to wait longer
than the bit that arrives later. The (variable part of the)
packetization delay, Tpkt(n), varies between zero and the time
between the transmission of two subsequent TSoP packets.
2. The packet delay variation (Tpdv(n)), i.e. the difference in
transit time that the TSoP packet containing bit #n experiences
relative to some reference (minimum) transit time, due to the
presence of non-empty shapers and queues (or any other cause for
variable delay) on its path through the PSN.
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----- direction of transmission ----->
+------+ +-------------+ +-------------+ +------+
| Clk1 | | Clk2 | | Clk3 | | Clk4 |
|------| |-------------| |-------------| |------|
| | | | | | | |
| | | +---------+ | +---------+ | +---------+ | | |
| SDH1 |----|-| Tpkt(n) |=|==| Tpdv(n) |==|=| Tjbf(n) |-|----| SDH2 |
| | | +---------+ | +---------+ | +---------+ | | |
| | |packetization| packet | jitter | | |
| | | delay | delay | buffer | | |
+------+ +-------------+ variation +-------------+ +------+
CE1 PE1 PE2 CE2
|----| |===============| |----|
AC1 PSN AC2
(STM-N) (STM-N)
+-----------------------------------+
| Clk1 is the system clock of CE1 |
| Clk2 is the system clock of PE1 |
| Clk3 is the system clock of PE2 |
| Clk4 is the system clock of CE2 |
+-----------------------------------+
Figure 8. Delay components in a uni-directional TSoP
Pseudowire (from PE1 to PE2).
Figure 8 schematically shows three contributors to the over-all
transit delay of STM-N bits between ingressing PE1 and egressing PE2:
The paketization delay in PE1 and the packet delay variation over the
PSN, as mentioned above, which cause delay differences between the
bits and the jitter buffer delay in PE2, Tjbf(n), which is intended
to equalize these differences.
When the CE-bound IWF in PE2 starts up, it writes the bits in the
TSoP Payload Field of incoming packets into the jitter buffer until a
preconfigured degree of filling is reached. From this moment onward,
bits are played out from this buffer under control of an STM-N clock
(derived from Clk3) that is locally synthesized in PE2. A necessary
condition to avoid overflow or underflow of the jitter buffer is that
this clock must have the same average frequency as the clock in PE1
(derived from Clk2) that governs the encapsulation process.
The dwelling time of an individual bit in the jitter buffer, Tjbf(n),
is determined by the actual delay time that this bit experienced in
transiting through PE1 and the PSN. Bits traveling fast reside long
in the jitter buffer, while slow bits reside only a short while in
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the buffer, such that the total time is equal for all bits.
For a given bit #n, this behavior can be expressed by the relation
Tpkt(n) + Tpdv(n) + Tjbf(n) = constant (within the allowed jitter and
wander limits of an STM-N signal). Since especially Tpdv(n) can vary
significantly, the initial degree of filling of the jitter buffer,
Tjbf(0), should be configured large enough to allow lowering the
buffer fill when at a later time bits arrive that happen to be
slower. This compensating effect of the jitter buffer fill on the
overall transit time requires the frequency of Clk3, and so the
frequency of the outgoing STM-N signal, to change slow compared to
the PDV variations.
Note that the Tjbf(n) term adds to the total delay that an STM-N bit
experiences crossing the TSoP PW section. For this reason the
initial degree of filling of the jitter buffer should not be
configured larger than necessary, i.e. it is a compromise between the
delay permitted by the application and the probability of buffer
underrun due to large PDV excursions. Depending on the requirements
for a particular STM-N client signal, a small probability of buffer
underrun may be acceptable in order to meet the delay specifications.
See appendix D for a description of the effects of jitter buffer
underrun on CE2.
Apart from the initial fill level of the jitter buffer, its total
size can also be configurable (see section 6.2.2). The fill level
increases when a number of faster packets arrives and the buffer
needs sufficient "headroom" to avoid overflow under such conditions.
However, it is not necessary to reserve more "headroom" than is
needed to accommodate the fastest packets, corresponding to the
minimum delay of the PW.
Appendix C. Synchronization Considerations for the CE-bound IWF
In section 6.2.2 the requirements for reconstruction of the STM-N
client signal from the incoming TSoP packets in the CE-bound IWF
function are given, without reference to the quality of the CE-bound
IWF system clock or the method of STM-N clock recovery. This is done
on purpose, to avoid as much as possible restrictions on the
implementation, as these factors depend on the (network)
synchronization situation. This appendix provides information on
this dependency.
Figure 9 shows the reference network that is used to analyze the
dependency of the CE-bound IWF requirements on the synchronization
situation in the network. Since network synchronization operates
uni-directional, only the corresponding direction of transmission is
depicted. The return path will never be used at the same time as a
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synchronization reference signal: If Ref4 is derived from AC2, the
S1-byte of the returning STM-N will contain the message "Don't Use
for Synchronization".
----- direction of transmission ----->
Ref2 Ref3
| |
Ref1 | | Ref4
| V V |
| +--------+ +--------+ |
V | Clk2 | | Clk3 | V
+------+ |--------| |--------| +------+
| Clk1 | | STM-N /| | TSoP /| | Clk4 |
|------| | / | | -PW / | |------|
| | | / | | / | | |
| SDH1 |--------->| / |============>| / |--------->| SDH2 |
| | STM-N | / | TSoP | / | STM-N | |
| | | / | Packets | / | | |
+------+ | / TSoP | | / | +------+
CE1 |/ -PW | |/ STM-N | CE2
| +--------+ +--------+ |
| PE1 PE2 |
| | | | | |
|-- AC1 -->| |==== PSN ===>| |-- AC2 -->|
| | | |
| |--------- Pseudowire --------->| |
Up- | | Down-
stream |------------- STM-N (multiplex) section ------------>| stream
+---------------------------------------------------+
| Clk1 is the system clock of CE1, locked to Ref1 |
| Clk2 is the system clock of PE1, locked to Ref2 |
| Clk3 is the system clock of PE2, locked to Ref3 |
| Clk4 is the system clock of CE2, locked to Ref4 |
+---------------------------------------------------+
Figure 9. Reference network for analysis of TSoP
synchronization requirements
The intention of the requirements in section 6.2.2 is to ascertain
that the reconstruction of the STM-N client signal in PE2 is
sufficiently faithful that not only its binary content is recovered
but also its timing properties are maintained. This latter aspect
implies that the recovered STM-N signal can still be used as a
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synchronization reference signal in the downstream STM-N network as
is required per [G.825] and [GR-253] and that normal processing rules
(see [G.781]) can be applied to the S1-byte.
The requirements regarding maintaining STM-N timing properties of the
reconstructed STM-N are based on two rules:
1. The reconstructed STM-N signal at the TSoP PW egress must have the
same average frequency as the STM-N signal at the TSoP PW ingress.
Since all TSoP packets carry the same number of payload bits and a
sequence numbering mechanism is applied to the TSoP packets, the
TSoP decapsulation function can regenerate an STM-N signal that
has, averaged over time, the same number of bits, as long as the
jitter buffer does not overrun or underrun.
2. The jitter and wander properties of the recovered STM-N signal
must meet the applicable SDH/SONET standards.
The sub-sections below provide the synchronization situations in
the network in which this requirement can be met with a reasonable
design of the CE-bound IWF. The implication is that for other
network synchronization situations such is, in general, not
possible.
The considerations in this appendix are applicable during "normal"
operation. As soon as the input STM-N signal to PE1 is lost or the
TSoP PW itself is failing, PE2 forwards a G-AIS signal towards CE2 at
a frequency that may deviate 20 ppm from the nominal value. A
sustained G-AIS pattern will cause a Loss of Frame condition in CE2,
which will consequently stop reading the contained S1 information and
look for an alternative synchronization reference signal or revert to
hold-over mode.
C.1. Layer 1 Synchronized PEs
The PEs that terminate the TSoP PW operate synchronously in case Ref2
and Ref3 are traceable over the physical layer to the same clock.
In such cases the timing characteristics of the STM-N signal can be
transferred over the PW by applying differential mode, i.e. use the
RTP timestamps in the CE-bound IWF to determine the rate at which the
STM-N bits need to be played out. This will ensure that the average
frequency is maintained over the PW section.
To satisfy the STM-N wander requirements, Clk3 must filter the phase
noise on Ref3 down to the levels specified in [G.825] or [GR-253].
In case the phase noise on Ref3 stays below the network limit
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specified for Synchronous Ethernet [G.8262], it is sufficient that
Clk3 meets the jitter transfer specifications of [G.8262] to achieve
this goal.
C.2. Synchronous CEs
The CEs that terminate the STM-N multiplex section operate
synchronously in case Ref1 and Ref4 are traceable over the physical
layer to the same clock.
In such cases, Ref2 can be derived from Clk1, via AC1 and Ref3 can be
derived from Clk4 via AC2. The timing characteristics of the STM-N
signal can be transferred over the PW by applying differential mode,
i.e. use the RTP timestamps in the CE-bound IWF to determine the rate
at which the STM-N bits need to be played out. This will ensure that
the average frequency is maintained over the PW section.
To satisfy the STM-N wander requirements, Clk3 must filter the phase
noise on Ref3 down to the levels specified in [G.825] or [GR-253].
In case the phase noise on Ref3 stays below the network limit
specified for STM-N [G.825], it is sufficient if Clk3 meets the
jitter transfer specifications of [G.813] to achieve this goal.
C.3. Pleisiochronous CEs
In case Clk1 and Clk4 are both traceable to different [G.811] type
clocks, the STM-N link operates pleisiochronously, i.e. the long term
frequency difference between both clocks is less than 2E-11.
In such cases, Ref2 can be derived from Clk1, via AC1 and Ref3 can be
derived from Clk4 via AC2. Again, differential mode clock recovery
is assumed in the CE-bound IWF. The very small frequency difference
will cause a re-center action of the jitter buffer in PE2, each time
it overflows or underflows. The time interval between such events
depends on the depth of the buffer and the actual frequency
difference between Ref1 and Ref4.
As an example, if the frequency difference is 2E-11 (this represents
the worst case) and if an overflow or underflow event shifts the
buffer fill by 125 microseconds, a re-center event happens once every
72 days. In the downstream SDH node, a re-center event causes
1 errored second (ES) in all VC paths that are carried over this STM-
N signal (see appendix D). A level of one ES per 72 days is
negligible compared to the ES limits formulated in [G.828] for
VC-paths.
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Appendix D. Effect of G-AIS Insertion on a Downstream SDH Node
There are a number of network events that force the CE-bound IWF to
replace the content of the Payload Field of a TSoP packet by the same
number of bits from the local G-AIS pattern generator (see figure 2).
This is the case when:
a) A TSoP packet is lost, or,
b) A TSoP packet arrives out-of-order and its position can't be
restored, or,
c) A TSoP packet arrives with its L-bit set.
In case the STM-N payload of K consecutive TSoP packets is replaced
by G-AIS, the effect is that parity violations are detected in the
downstream SDH Node (CE) for a period T = K / (N * 8000) seconds. As
long as K is small, this will typically lead to counting 1 Errored
Second (ES) event (or occasionally 2 in case the event straddles two
adjacent 1 second monitoring periods) for the STM-N signal itself and
all contained VC-path layers. As long the number of ES that is
induced in a given VC-path by the effects above, remains small
compared to the applicable limit defined in [G.828], this effect of
G-AIS substitution can be tolerated.
A second consequence of G-AIS substitution is that the G-AIS bits may
overwrite the STM-N alignment word (A1 and A2 bytes) in the recovered
STM-N signal. This A1-A2 alignment word repeats every 0.125 ms. The
length of the period T determines the number of framing patterns that
is affected by the inserted G-AIS bits.
Each time an STM-N framing word is changed by G-AIS bits overwriting
it, this will cause a framing anomaly in the downstream SDH Node.
Such anomalies have no effect on the STM-N framer as long as a next
A1-A2 word is correct and is still in its expected location. Only in
case the G-AIS streak lasts for a 3 ms or longer period, these
persistent anomalies will cause a LOF defect (see [G.783]) to be
raised and consequently the declaration of a Severely Errored Second
(SES). Note that the limits for SES events are much stricter than
for ES event (see [G.828]). On the other hand, the probability of
losing 3 ms worth of TSoP packets (e.g. 72 consecutive TSoP packets
for STM-1/OC-3) is much lower than losing one or a few packets in a
row.
In case the jitter buffer is re-centered, for instance due to a
buffer overrun or underrun, the position of the A1-A2 framing pattern
will in general shift once by an amount of time that is not an
integral multiple of 0.125 ms. Also such an event will not cause a
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LOF defect in the downstream STM-N node, because an STM-N framer that
conforms to [G.783] must be able to detect the out-of-frame condition
within 0.625 ms and find the new frame position within 0.250 ms, so
the entire operation is completed well within 3 ms (the minimum time
to declare a LOF defect).
Note that in case of buffer underrun, G-AIS is transmitted while the
CE-bound IWF waits for the buffer to reach its configured initial
fill level. This waiting time has to be added to the STM-N out-of-
frame detection time and frame recovery time mentioned above, to
assess the overall impact on the downstream SDH node. This implies
that if the initial fill level is configured somewhat larger than
2 ms (actually, 3 - 0.625 - 0.250 = 2.125 ms), an underrun event can
trigger a LOF defect and consequently a SES event in the downstream
SDH network. So while a larger jitter buffer diminishes the
probability of an underrun event, the consequences of such an event
are more severe once this ~2 ms threshold is crossed. This must be
weighed carefully.
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Authors' Addresses
Gert Manhoudt
AimValley B.V.
Utrechtseweg 38
1213 TV Hilversum
The Netherlands
E-mail: gmanhoudt@aimvalley.nl
Stephan Roullot
Alcatel-Lucent
600 March Road
Kanata, Ontario, K2K 2E6
Canada
E-mail: stephan.roullot@alcatel-lucent.com
Kin Yee Wong
Alcatel-Lucent
600 March Road
Kanata, Ontario, K2K 2E6
Canada
E-mail: kin-yee.wong@alcatel-lucent.com
Kuo, Huei-Long
Chunghwa Telecom Co. Ltd.
No.21-3, Sec. 1, Xinyi Rd.
Zhongzheng Dist., Taipei City 100
Taiwan
E-mail: hlkuo@cht.com.tw
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