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Network Working Group                                       K. MurakamiRequest for Comments: 2171                                  M. MaruyamaCategory: Informational                                NTT Laboratories                                                              June 1997MAPOS - Multiple Access Protocol over SONET/SDH  Version 1Status 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. Introduction1.1 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 19971.2 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 configurationMurakami & Maruyama          Informational                      [Page 2]

RFC 2171                         MAPOS                         June 1997           +------+                                +---------------+           | 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]

RFC 2171                         MAPOS                         June 1997   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.Murakami & Maruyama          Informational                      [Page 4]

RFC 2171                         MAPOS                         June 19973. Data Link Layer3.1 HDLC Frame Format   MAPOS uses the same HDLC-like framing as used in PPP-over-SONET,   described inRFC-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.Murakami & Maruyama          Informational                      [Page 5]

RFC 2171                         MAPOS                         June 1997             +-------------+-+             | | | | | | | | |             | | 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.Murakami & Maruyama          Informational                      [Page 6]

RFC 2171                         MAPOS                         June 1997     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.Murakami & Maruyama          Informational                      [Page 7]

RFC 2171                         MAPOS                         June 1997   [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.netMurakami & Maruyama          Informational                      [Page 8]

RFC 2171                         MAPOS                         June 1997APPENDIX 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.Murakami & Maruyama          Informational                      [Page 9]