Internet Draft Network Working Group A. Malis, K. Hsu Internet Draft Vivace Networks, Inc. Document: draft-malis-sonet-ces-mpls-00.txt A. Vernon Expiration Date: January 2001 BellSouth S. Vogelsang Laurel Networks, Inc. July 2000 SONET/SDH Circuit Emulation Service Over MPLS (CEM) 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]. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 1. Abstract This document describes a method for providing a SONET circuit emulation service 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]. 3. Introduction This document describes a method for transporting time division multiplexed (TDM) digital signals (TDM circuit emulation) 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 Malis, et al. Expires January 2001 [Page 1] SONET/SDH Circuit Emulation Over MPLS July 2000 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). 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. 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. Malis, et al. Expires January 2001 [Page 2] SONET/SDH Circuit Emulation Over MPLS July 2000 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. 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. 6. 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 TDM 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]. Malis, et al. Expires January 2001 [Page 3] SONET/SDH Circuit Emulation Over MPLS July 2000 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. 7. 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. 7.1 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 periodically slip back or advance in time through positive or Malis, et al. Expires January 2001 [Page 4] SONET/SDH Circuit Emulation Over MPLS July 2000 negative stuffing. The N(JE) and P(JE) bits are used to replay the stuff indicators and eliminate transport jitter. 7.2 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 [6]. 8. 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. A new value, TBD, will be added to the VC types 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 [5] is not used, as its functionality is included in the CEM encapsulation. Alternatively, a dedicated traffic engineered LSP may be used for each CEM circuit. 9. Open Issues Future revisions of this draft will discuss QoS requirements 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. 10. Security Considerations As with [5], this document does not affect the underlying security issues of MPLS. 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 Malis, et al. Expires January 2001 [Page 5] SONET/SDH Circuit Emulation Over MPLS July 2000 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-01.txt, work in progress, May 2000. 6 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 Phone: +1 408 383 7223 Email: Andy.Malis@vivacenetworks.com Ken Hsu Vivace Networks, Inc. 2730 Orchard Parkway San Jose, CA 95134 CA Phone: +1 408 432 7772 Email: Ken.Hsu@vivacenetworks.com Andrew Vernon BellSouth Room 40F82 BSC 675 West Peachtree St NE Atlanta, GA, 30375 Phone: +1 404 332 2101 Email: ajv@snt.bellsouth.com Steve Vogelsang Laurel Networks, Inc. 2706 Nicholson Rd. Sewickley, PA 15143 Phone: +1 724 933 7330 Email: sjv@laurelnetworks.com Malis, et al. Expires January 2001 [Page 6] SONET/SDH Circuit Emulation Over MPLS July 2000 Full Copyright Statement "Copyright (C) The Internet Society (2000). All Rights Reserved. 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