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+Network Working Group K. Murakami
+Request for Comments: 2171 M. Maruyama
+Category: Informational NTT Laboratories
+ June 1997
+
+ MAPOS - Multiple Access Protocol over SONET/SDH Version 1
+
+Status of this Memo
+
+ This memo provides information for the Internet community. This memo
+ does not specify an Internet standard of any kind. Distribution of
+ this memo is unlimited.
+
+Authors' Note
+
+ This memo documents a multiple access protocol for transmission of
+ network-protocol datagrams, encapsulated in High-Level Data Link
+ Control (HDLC) frames, over SONET/SDH. This document is NOT the
+ product of an IETF working group nor is it a standards track
+ document. It has not necessarily benefited from the widespread and
+ in depth community review that standards track documents receive.
+
+Abstract
+
+ This document describes the protocol MAPOS, Multiple Access Protocol
+ over SONET/SDH, for transmitting network-protocol datagrams over
+ SONET/SDH. It focuses on the core protocol -- other documents listed
+ in the bibliography may be referenced in conjunction with this
+ document to provide support and services for protocols at higher
+ layers.
+
+1. Introduction
+
+1.1 SONET/SDH
+
+ The Synchronous Optical Network/Synchronous Digital Hierarchy
+ (SONET/SDH) [1][2][3][4] family of ITU-T standard protocols are
+ designed to provide common, simple, and flexible interface for
+ broadband optical fiber transmission systems. It enables direct
+ octet-synchronous multiplexing of lower rate tributaries.
+ SONET/SDH-compliant transmission systems are widely deployed by
+ telephone carriers world wide.
+
+ This document defines the MAPOS protocol -- a method for transmitting
+ HDLC frames over SONET/SDH. The protocol provides multiple access
+ capability to SONET/SDH, an inherently point-to-point link. This
+ enables construction of seamless networking environment using
+ SONET/SDH as transmission media for both LAN and WAN.
+
+
+
+Murakami & Maruyama Informational [Page 1]
+
+RFC 2171 MAPOS June 1997
+
+
+1.2 Possible Configurations
+
+ The MAPOS protocol provides multiple access, broadcast / multicast-
+ capable switched LAN environment using SONET/SDH lines as
+ transmission media. Possible configurations of MAPOS system are
+ shown in the following diagrams. In (a), two end nodes are connected
+ to each other. Figure (b) shows a star-topology "SONET-LAN" where
+ multiple end nodes are connected to an HDLC frame switch. The frame
+ switch forwards packets between nodes and provides multiple access
+ capability. In (c), multiple frame switches are linked together,
+ creating a switching cluster.
+
+
+ +------+ +------+
+ | Node +--------------------------------+ Node |
+ +------+ +------+
+
+ (a) Point-to-Point configuration
+
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+Murakami & Maruyama Informational [Page 2]
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+RFC 2171 MAPOS June 1997
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+
+ +------+ +---------------+
+ | Node +--------------------------------+ |
+ +------+ | |
+ | |
+ +------+ | |
+ | Node +--------------------------------+ |
+ +------+ | |
+ | Frame Switch |
+ +------+ | |
+ | Node +--------------------------------+ |
+ +------+ | |
+ | |
+ +------+ | |
+ | Node +--------------------------------+ |
+ +------+ +---------------+
+
+ (b) Point-to-Multipoint configuration
+
+
+ +--------+ +--------+
+ | Frame +----------------------+ Frame |
+ | Switch +--------+ +--------+ Switch |
+ +--+-----+ +-+----+-+ +--------+
+ | | Frame | +--------+
+ +--+-----+ | Switch | +--------+ | Frame |
+ | Frame | +-----+--+ | Frame +------+ Switch |
+ | Switch | +---------+ Switch | ++-------+
+ +-------++ +--------+ |
+ |________________________________________|
+
+ (c) Switching cluster configuration
+
+ Figure 1. Possible configurations
+
+ Each port on a switch has an unique identifier within the switch. A
+ node connected to a switch port must inherit the address of the port.
+ That is, the node address is equal to the port identifier and is
+ unique within the switch.
+
+ In a switch cluster, a node address is subnetted. The high-order
+ bits, the part where the corresponding bits in the "subnet mask" are
+ 1, indicate the switch address. The remaining low-order bits
+ indicate the unique node address within the switch. The two fields
+ form an unique address for a given node.
+
+ In either case, the address may be configured manually into a node
+ interface, or automatically by the address assignment mechanism
+ described in [5].
+
+
+
+Murakami & Maruyama Informational [Page 3]
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+RFC 2171 MAPOS June 1997
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+
+ Note that any two components may be connected either directly, or via
+ a long-haul SONET/SDH leased line.
+
+1.3 Packet Transmission
+
+ The protocol is connection-less -- when a node wish to communicate
+ with some other node, it simply fills-in the destination address of
+ an HDLC frame, places it in one or more SONET/SDH payloads, and sends
+ it over a SONET/SDH link.
+
+ The switch forwards the frame to its destination based on the
+ destination address. In a switch cluster, the frame may be forwarded
+ by multiple switches and is eventually delivered to the specified
+ node. Broadcast and multicast are also supported. Frames with an
+ invalid destination address are silently discarded.
+
+ Like ethernet, the multiple access capability is provided by a switch
+ or a switch cluster. Since MAPOS is a link layer protocol, it is
+ independent of the upper layer protocols. That is, it can support any
+ network layer protocols such as IP. MAPOS IPv4 support is described
+ in [6].
+
+2. Physical Layer
+
+ This protocol treats the underlying end-to-end SONET/SDH transmission
+ link as if it was a plain, transparent channel. It sends HDLC frames
+ in SONET/SDH payloads, and expects them to arrive at the other end
+ unaltered.
+
+ Each node and switch should terminate SONET/SDH overhead such as
+ section overhead, line overhead, and path overhead according to the
+ specification of SONET/SDH. Unfortunately, SONET and SDH overhead
+ interpretations are not identical. In addition, some SONET/SDH
+ implementations utilize some overhead bytes in proprietary manner.
+
+ The detail of the interpretation is beyond the scope of this
+ document. Appendix A describes some of the most significant
+ differences among SONET, SDH, and their implementations that often
+ causes interoperability problems. Implementors of SONET/SDH
+ interfaces are strongly encouraged to be aware of such differences,
+ and provide workaround options in their products.
+
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+Murakami & Maruyama Informational [Page 4]
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+RFC 2171 MAPOS June 1997
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+3. Data Link Layer
+
+3.1 HDLC Frame Format
+
+ MAPOS uses the same HDLC-like framing as used in PPP-over-SONET,
+ described in RFC-1662[7]. Figure 2 shows the frame format. Logical
+ Link Control (LLC), and Sublayer/Sub-Network Access Protocol (SNAP)
+ are not used. It does not include the bytes for transparency. The
+ fields are transmitted from left to right.
+
+ +----------+----------+----------+----------+
+ | | | | |
+ | Flag | Address | Control | Protocol |
+ | 01111110 | 8bits | 00000011 | 16 bits |
+ +----------+----------+----------+----------+
+ +-------------+------------+----------+-----------
+ | | | | Inter-frame
+ | Information | FCS | Flag | fill or next
+ | | 16/32 bits | 01111110 | address
+ +-------------+------------+----------+------------
+
+ Figure 2. Frame format
+
+ Flag Sequence
+
+ Flag sequence is used for frame synchronization. Each frame begins
+ and ends with a flag sequence 01111110 (0x7E). If a frame
+ immediately follows another, one flag sequence may be treated as
+ the end of the preceding frame and the beginning of the immediately
+ following frame. When the line is idle, the flag sequence is to be
+ transmitted continuously on the line.
+
+ Address
+
+ The address field contains the destination HDLC address. A frame
+ is forwarded by a switch based on this field. It is 8 bits wide.
+ The LSB indicates the end of this field, and must always be 1. The
+ MSB is used to indicate if the frame is a unicast or a multicast
+ frame. The MSB of 0 means unicast, with the remaining six bits
+ indicating the destination node address. MSB of 1 means multicast,
+ with the remaining six bits indicating the group address. The
+ address 11111111 (0xFF) means that the frame is a broadcast frame.
+ The address 00000001 (0x01) is reserved to identify the control
+ processor inside a switch. Frames with an invalid address should
+ be silently discarded.
+
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+Murakami & Maruyama Informational [Page 5]
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+RFC 2171 MAPOS June 1997
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+ +-------------+-+
+ | | | | | | | | |
+ | | node addr |1|
+ +-+-----------+-+
+ ^ ^
+ | |
+ | +------- EA bit (always 1)
+ |
+ 1 : broadcast, multicast
+ 0 : unicast
+
+ Figure 3 Address format
+
+ Control
+
+ The control field contains single octet 00000011 (0x03) which, in
+ HDLC nomenclature, means that the frame is an Unnumbered
+ Information (UI) with the Poll/Final (P/F) bit set to zero. Frames
+ with any other control field values should be silently discarded.
+
+ Protocol
+
+ The protocol field indicates the protocol to which the datagram
+ encapsulated in the information field belongs. It conforms to the
+ ISO 3309 extension mechanism, and the value for this field may be
+ obtained from the most recent "Assigned Numbers" [8] and "MAPOS
+ Version 1 Assigned Numbers" [9].
+
+ Information
+
+ The information field contains the datagram for the protocol
+ specified in the protocol field. The length of this field may
+ vary, but shall not exceed 65,280 (64K - 256) octets.
+
+ Frame Check Sequence (FCS)
+
+ By default, the frame check sequence (FCS) field is 16-bits long.
+ Optionally, 32 bit FCS may be used instead. The FCS is calculated
+ over all bits of the address, control, protocol, and information
+ fields prior to escape conversions. The least significant octet of
+ the result is transmitted first as it contains the coefficient of
+ the highest term.
+
+ Inter-frame fill
+
+ A sending station must continuously transmit the flag sequence as
+ inter-frame fill after the FCS field. The inter-frame flag
+ sequences must be silently discarded by the receiving station.
+
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+Murakami & Maruyama Informational [Page 6]
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+RFC 2171 MAPOS June 1997
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+ When an under-run occurs during DMA in the sending station, it must
+ abort the frame transfer and continuously send the flag sequence to
+ indicate the error.
+
+3.2 Octet-Synchronous Framing
+
+ MAPOS uses an octet stuffing procedure because it treats SONET/SDH as
+ a byte-oriented synchronous link. Since SONET/SDH provides
+ transparency, Async-Control-Character-Map (ACCM) is not used. HDLC
+ frames are mapped into the SONET/SDH payload as follows.
+
+ Each HDLC frame is separated from another frame by one or more flag
+ sequence, 01111110 (0x7E). An escape sequence is defined to escape
+ the flag sequence and itself. Prior to sending the frame, but after
+ the FCS computation, every occurrence of 01111110 (0x7E) other than
+ the flags is to be converted to the sequence 01111101 01011110 (0x7D
+ 0x5E), and the sequence 01111101 (0x7D) is to be converted to the
+ sequence 01111101 01011101 (0x7D 0x5D). Upon receiving a frame, this
+ conversion must be reversed prior to FCS computation.
+
+4. Further Reading
+
+ To fully utilize MAPOS protocol, it is useful to reference other
+ documents[5][6][9][10] in conjunction with this document.
+
+5. Security Considerations
+
+ Security issues are not discussed in this memo.
+
+References
+
+ [1] CCITT Recommendation G.707: Synchronous Digital Hierarchy Bit
+ Rates (1990).
+
+ [2] CCITT Recommendation G.708: Network Node Interface for
+ Synchronous Digital Hierarchy (1990).
+
+ [3] CCITT Recommendation G.709: Synchronous Multiplexing Structure
+ (1990).
+
+ [4] American National Standard for Telecommunications - Digital
+ Hierarchy - Optical Interface Rates and Formats Specification,
+ ANSI T1.105-1991.
+
+ [5] Murakami, K. and M. Maruyama, "A MAPOS version 1 Extension -
+ Node Switch Protocol," RFC2173, June, 1997.
+
+
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+Murakami & Maruyama Informational [Page 7]
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+RFC 2171 MAPOS June 1997
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+ [6] Murakami, K. and M. Maruyama, "IPv4 over MAPOS Version 1,"
+ RFC2176, June, 1997.
+
+ [7] Simpson, W., editor, "PPP in HDLC-like Framing," RFC1662, July
+ 1994.
+
+ [8] IANA, "IANA-Assignments,"
+ http://www.iana.org/iana/assignments.html
+
+ [9] Maruyama, M. and K. Murakami, "MAPOS Version 1 Assigned
+ Numbers," RFC2172, June 1997.
+
+ [10] Murakami, K. and M. Maruyama, "A MAPOS version 1 Extension -
+ Switch Switch Protocol," RFC2174, June, 1997.
+
+Acknowledgements
+
+ The authors would like to acknowledge the contributions and
+ thoughtful suggestions of John P. Mullaney, Clark Bremer, Masayuki
+ Kobayashi, Paul Francis, Toshiaki Yoshida, and Takahiro Sajima.
+
+Author's Address
+
+ Ken Murakami
+ NTT Software Laboratories
+ 3-9-11, Midori-cho
+ Musashino-shi
+ Tokyo-180, Japan
+ E-mail: murakami@ntt-20.ecl.net
+
+ Mitsuru Maruyama
+ NTT Software Laboratories
+ 3-9-11, Midori-cho
+ Musashino-shi
+ Tokyo-180, Japan
+ E-mail: mitsuru@ntt-20.ecl.net
+
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+Murakami & Maruyama Informational [Page 8]
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+RFC 2171 MAPOS June 1997
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+APPENDIX A. Differences among SONET, SDH, and their Implementations
+
+ This section briefly describes the major differences among SONET
+ which is an ANSI standard, SDH, an ITU-T standard, and their
+ implementations.
+
+ AU pointer (H1, H2, H3)
+
+ The AU pointer consists of bytes H1, H2, and H3. The bits 5 and 6
+ of the H1 byte are called "SS bits," and are used to indicate the
+ offset into the payload where the beginning of a SPE is located.
+ (Note that "SPE" is a SONET term -- SDH calls it "VC.") In the
+ case of OC-3c, SONET sets the SS bits of the second and the third
+ H1 bytes to 0, whereas SDH sets them to 10 for AU-4, and 01 for
+ AU-31. Although the SS bits may be ignored at the receiving
+ station, some transmission systems discards SONET/SDH frames with
+ SS bits that it doesn't expect -- the sending station should be
+ aware of this, and include a configuration option to handle it.
+
+ Z1 and Z2
+
+ The Z bytes are reserved in SONET/SDH. Some transmission systems,
+ however, use them in a proprietary manner. SONET uses Z1 for Line
+ Error Monitoring. NTT, a carrier in Japan, utilized Z1 for
+ Automatic Protection Switching (APS.)
+
+ DCC Bytes
+
+ The D bytes are called the Data Communication channel (DCC), and
+ are defined for maintenance and operations. However, some carriers
+ and vendors use them in a proprietary manner. For example, NTT's
+ STM-1 UNI uses the D4, D5, and D6 bytes to transfer section and
+ path maintenance information.
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