Network Working Group Andrew G. Malis
Internet Draft Ken Hsu
Expiration Date: May 2001 Vivace Networks, Inc.
Steve Vogelsang
John Shirron
Laurel Networks, Inc.
Luca Martini
Level 3 Communications, LLC.
November 2000
SONET/SDH Circuit Emulation Service Over MPLS (CEM) Encapsulation
draft-malis-sonet-ces-mpls-01.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026 [1].
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1. Abstract
This document describes a method for encapsulating SONET/SDH signals
for transport across an MPLS network.
2. Conventions used in this document
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 RFC 2119 [2].
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3. Introduction
This document describes a method for encapsulating time division
multiplexed (TDM) digital signals (TDM circuit emulation) for
transmission over a packet-oriented MPLS network. The transmission
system for circuit-oriented TDM signals is the Synchronous Optical
Network (SONET)[3]/Synchronous Digital Hierarchy (SDH) [4]. To
support TDM traffic, which includes voice, data, and private leased
line service, the MPLS network must emulate the circuit
characteristics of SONET/SDH payloads. MPLS labels and a new
circuit emulation header are used to encapsulate TDM signals and
provide the Circuit Emulation Service over MPLS (CEM).
This document is closely related to references [5], which describes
the control protocol methods used to signal the usage of CEM, and
[6], which describes a related method of encapsulating Layer 2
frames over MPLS and which shares the same signaling.
4. Scope
This document describes how to provide CEM for the following digital
signals:
1. SONET STS-1 synchronous payload envelope (SPE)/SDH VC-3
2. STS-Nc SPE (N = 3, 12, or 48)/SDH VC-4, VC-4-4c, VC-4-16c
Other SONET/SDH signals, such as virtual tributary (VT) structured
sub-rate mapping, are not explicitly discussed in this document;
however, it can be extended in the future to support VT services.
OC-192c SPE/VC-4-64c are also not included at this point, since most
MPLS networks use OC-192c or slower trunks, and thus would not have
sufficient capacity. As trunk capacities increase in the future,
the scope of this document can be accordingly extended.
5. CEM Encapsulation Format
A TDM data stream is segmented into packets and encapsulated in MPLS
packets. Each packet has one or more MPLS labels, followed by a 32-
bit CEM header to associate the packet with the TDM stream.
The outside label is used to identify the MPLS LSP used to tunnel
the TDM packets through the MPLS network (the tunnel LSP). The
interior label is used to multiplex multiple TDM connections within
the same tunnel. This is similar to the label stack usage defined
in [5] and [6].
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The 32-bit TDM header has the following format:
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Bytes | Struct Pointer |N|P| Seq num | BIP-4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. TDM Header Format
The above fields are defined as follows:
Payload Bytes(N): the number of TDM payload bytes contained in this
packet, from 48 to 1,023 bytes. All of the packets in a given CES
stream have the same number of payload bytes. Note that there is a
possibility that the packet size may exceed the SPE size in the case
of an STS-1 SPE, which could cause two pointers to be needed in the
CEM header, since the payload may contain two J1 bytes for
consecutive SPEs. For this reason, the number of payload bytes must
be less than 783 for STS-1 SPEs.
Structure Pointer: The pointer points to the J1 byte in the payload
area. The value is from 0 to 1,022, where 0 means the first byte
after the TDM header. The pointer is set to 0x3FF (1,023) if a
packet does not carry the J1 byte. See [3] and [4] for more
information on the J1 byte and the structure pointer.
The N and P bits: See Section 7 below for their definition.
Seq Num: This is a packet sequence number, which continuously
cycles from 0 to 63. It begins at 0 when a TDM LSP is created.
BIP-4: The bit interleaved even parity is over the first 28 header
bits.
6. Clocking Mode
It is necessary to be able to regenerate the input service clock at
the output interface. Two clocking modes are supported: synchronous
and asynchronous.
6.1 Synchronous
When synchronous SONET timing is available at both ends of the
circuit, the N(JE) and P(JE) bits are set for negative or positive
justification events. The event is carried in five consecutive
packets at the transmitter. The receiver plays out the event when
three out of five packets with NJE/PJE bit set are received. If both
bits are set, then path AIS event has occurred. If there is a
frequency offset between the frame rate of the transport overhead
and that of the STS SPE, then the alignment of the SPE shall
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periodically slip back or advance in time through positive or
negative stuffing. The N(JE) and P(JE) bits are used to replay the
stuff indicators and eliminate transport jitter.
6.2 Asynchronous
If synchronous timing is not available, the N and P bits are not
used for frequency justification and adaptive methods are used to
recover the timing. The N and P bits are only checked for the
occurrence of a path AIS event. An example adaptive method can be
found in Section 3.4.2 of [7].
7. CEM LSP Signaling
For maximum network scaling, CEM LSP signaling may be performed
using the LDP Extended Discovery mechanism as described in [5].
MPLS traffic tunnels may be dedicated to CEM, or shared with other
MPLS-based services. The value 8008 is used for the VC Type in the
VC FEC Element defined in [5] in order to signify that the LSP being
signaled is to carry CEM. Note that the sequencing control word in
[6] is not used, as its functionality is included in the CEM
encapsulation.
Alternatively, static label assignment may be used, or a dedicated
traffic engineered LSP may be used for each CEM circuit.
8. Open Issues
Future revisions of this draft will discuss QoS requirements and
mechanisms for CEM, methods to provide (or simulate) bi-directional
LSPs (perhaps using the Group ID from [5]), signaling for the number
of payload bytes, and sending additional end-to-end alarm
information in addition to AIS.
9. Security Considerations
As with [5], this document does not affect the underlying security
issues of MPLS.
10. Intellectual Property Disclaimer
This document is being submitted for use in IETF standards
discussions. Vivace Networks, Inc. has filed one or more patent
applications relating to the CEM technology outlined in this
document. Vivace Networks, Inc. will grant free unlimited licenses
for use of this technology.
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11. References
[1] Bradner, S., "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
[3] American National Standards Institute, "Synchronous Optical
Network (SONET) - Basic Description including Multiplex
Structure, Rates and Formats," ANSI T1.105-1995.
[4] ITU Recommendation G.707, "Network Node Interface For The
Synchronous Digital Hierarchy", 1996.
[5] Martini et al, "Transport of Layer 2 Frames Over MPLS", draft-
martini-l2circuit-trans-mpls-04.txt, work in progress, November
2000.
[6] Martini et al, "Encapsulation Methods for Transport of Layer 2
Frames Over MPLS", draft-martini-l2circuit-encap-mpls-00.txt,
work in progress, November 2000.
[7] ATM Forum, "Circuit Emulation Service Interoperability
Specification Version 2.0", af-vtoa-0078.000, January 1997.
12. Acknowledgments
The authors would like to thank Mitri Halabi and Bob Colvin, both of
Vivace Networks, for their comments and suggestions.
13. Authors' Addresses
Andrew G. Malis
Vivace Networks, Inc.
2730 Orchard Parkway
San Jose, CA 95134
Email: Andy.Malis@vivacenetworks.com
Ken Hsu
Vivace Networks, Inc.
2730 Orchard Parkway
San Jose, CA 95134
Email: Ken.Hsu@vivacenetworks.com
Steve Vogelsang
Laurel Networks, Inc.
2706 Nicholson Rd.
Sewickley, PA 15143
Email: sjv@laurelnetworks.com
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John Shirron
Laurel Networks, Inc.
2607 Nicholson Rd.
Sewickley, PA 15143
Email: jshirron@laurelnetworks.com
Luca Martini
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO 80021
Email: luca@level3.net
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Appendix A. SONET/SDH Rates and Formats
For simplicity, the discussion in this section uses SONET
terminology, but it applies equally to SDH as well. SDH-equivalent
terminology is shown in the tables.
The basic SONET modular signal is the synchronous transport signal-
level 1 (STS-1). A number of STS-1s may be multiplexed into higher-
level signals denoted as STS-N, with N synchronous payload envelopes
(SPEs). The optical counterpart of the STS-N is the Optical Carrier-
level N, or OC-N. Table 1 lists standard SONET line rates discussed
in this document.
OC Level OC-1 OC-3 OC-12 OC-48 OC-192
SDH Term - STM-1 STM-4 STM-16 STM-64
Line Rate(Mb/s) 51.840 155.520 622.080 2,488.320 9,953.280
Table 1. Standard SONET Line Rates
Each SONET frame is 125 ´s and consists of nine rows. An STS-N frame
has nine rows and N*90 columns. Of the N*90 columns, the first N*3
columns are transport overhead and the other N*87 columns are SPEs.
A number of STS-1s may also be linked together to form a super-rate
signal with only one SPE. The optical super-rate signal is denoted
as OC-Nc, which has a higher payload capacity than OC-N.
The first 9-byte column of each SPE is the path overhead (POH) and
the remaining columns form the payload capacity with fixed stuff
(STS-Nc only). The fixed stuff, which is purely overhead, is N/3-1
columns for STS-Nc. Thus, STS-1 and STS-3c do not have any fixed
stuff, STS-12c has three columns of fixed stuff, and so on.
The POH of an STS-1 or STS-Nc is always nine bytes in nine rows. The
payload capacity of an STS-1 is 86 columns (774 bytes) per frame.
The payload capacity of an STS-Nc is (N*87)û(N/3) columns per frame.
Thus, the payload capacity of an STS-3c is (3*87 û 1)*9 = 2,340
bytes per frame. As another example, the payload capacity of an STS-
192c is 149,760 bytes, which is exactly 64 times larger than the
STS-3c.
There are 8,000 SONET frames per second. Therefore, the SPE size,
(POH plus payload capacity) of an STS-1 is 783*8*8,000 = 50.112
Mb/s. The SPE size of a concatenated STS-3c is 2,349 bytes per frame
or 150.336 Mb/s. The payload capacity of an STS-192c is 149,760
bytes per frame, which is equivalent to 9,584.640 Mb/s. Table 2
lists the SPE and payload rates supported.
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SONET STS Level STS-1 STS-3c STS-12c STS-48c STS-192c
SDH VC Level - VC-4 VC-4-4c VC-4-16c VC-4-64c
Payload Size(Bytes) 774 2,340 9,360 37,440 149,760
Payload Rate(Mb/s) 49.536 149.760 599.040 2,396.160 9,584.640
SPE Size(Bytes) 783 2,349 9,396 37,584 150,336
SPE Rate(Mb/s) 50.112 150.336 601.344 2,405.376 9,621.504
Table 2. Payload Size and Rate
To support circuit emulation, the entire SPE of a SONET STS or SDH
VC level is encapsulated into packets, using the encapsulation
defined in the next section, for carriage across MPLS networks.
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