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Network Working Group                                        K. MurakamiRequest for Comments: 2175                                   M. MaruyamaCategory: Informational                                 NTT Laboratories                                                               June 1997MAPOS 16 - Multiple Access Protocol overSONET/SDH with 16 Bit AddressingStatus 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 MAPOS 16, a multiple access protocol for   transmission of network-protocol datagrams, encapsulated in HDLC   frames with 16 bit addressing, over SONET/SDH.  The primary   difference with MAPOS version 1 is that it has 16 bit address field   that offers significant wide address space. 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 16, Multiple Access   Protocol over SONET/SDH with 16 Bit Addressing, for transmitting   network-protocol datagrams over SONET/SDH.  The primary difference   with MAPOS version 1 is that it has 16 bit address field that offers   significant wide address space. It first describes the major   differences between MAPOS and MAPOS 16 briefly. Then, it explains the   header format and its 16 bit address format.1. Introduction   MAPOS is a multiple access protocol for transmission of High-level   Datalink Control (HDLC) frames over the Synchronous Optical Network /   Synchronous Digital Hierarchy(SONET/SDH)[1][2][3][4]. It provides   multiple access capability to SONET/SDH, an inherently point-to-point   link.   MAPOS version 1[5] focuses on the frame format compatibility with the   conventional PPP[6], resulting with its narrow 8 bit address field.   In contrast to MAPOS version 1, MAPOS 16 has a 16 bit address space.Murakami & Maruyama          Informational                      [Page 1]

RFC 2175                        MAPOS 16                       June 1997   In this document, header format and its 16 bit format are described.   It also explains that 16 bit addressing has minimal influence on the   conventional MAPOS protocol family such as Node-Switch Protocol[7]   and Switch-Switch Protocol[8].2. MAPOS 16 Frame Format   Like MAPOS version 1, MAPOS 16 framing is based on the HDLC-like   framing used in PPP-over-SONET/SDH, described inRFC-1662[6].   However, the address field is extended to 16 bits, and the control   field in the MAPOS version 1 is omitted for maintain the 32bit   alignment of the header.   Figure 2 shows the MAPOS 16 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       | Protocol |           | 01111110 |        16bits       |  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 16 bits wide.     The LSB of the first byte indicates the continuation of this field,     and must always be 0. The LSB of the second byte indicates the end     of this field, and must always be 1.  The MSB of the first byte isMurakami & Maruyama          Informational                      [Page 2]

RFC 2175                        MAPOS 16                       June 1997     used to indicate if the frame is a unicast or multicast frame.  The     MSB of 0 means unicast, with the remaining thirteen bits indicating     the destination node address including two E/A bits. MSB of 1 means     multicast, with the remaining thirteen bits indicating the group     address.  The address 0xFEFF means that the frame is a broadcast     frame. The address (0x0001) is reserved to identify the control     processor inside a switch.  Frames with an invalid address should     be silently discarded.             +-------------+-+-------------+-+             | | | | | | | | | | | | | | | | |             | | node addr |0|  node addr  |1|             +-+-----------+-+-------------+-+              ^             ^               ^              |             |               |              |             |               +------- EA bit (always 1)              |             +------- EA bit (always 0)              1 : broadcast, multicast              0 : unicast                        Figure 3 Address format     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'' [9] and ``MAPOS     Version 1 Assigned Numbers'' [10].     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, 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.Murakami & Maruyama          Informational                      [Page 3]

RFC 2175                        MAPOS 16                       June 1997     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.     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 16 uses the same octet stuffing procedure[6] as MAPOS version   1[5]. 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. Influence on MAPOS ARP, UNARP, NSP, and SSP   Since all of the MAPOS protocol family, ARP[11], UNARP[11], NSP[7],   and SSP[8], use 32-bit address field for 8-bit MAPOS address, the   16-bit addressing has no influence on them.  Each protocol only have   to place a 16 bit address in the least significant place in their 32   bit address fields as follows;   (1) MAPOS ARP and UNARP    16-bit addresses are placed in the least significant places of the    32-bit sender and target HDLC addresses.   (2) NSP    In address assignment packet, a 16-bit address is placed in the    least significant bytes of the 32-bit address field. The rest of the    field is padded with zeros.   (3) SSP    In route entry of an SSP packet, the 16-bit MAPOS address is placed    in the least significant bytes of the 32-bit address field.Murakami & Maruyama          Informational                      [Page 4]

RFC 2175                        MAPOS 16                       June 19975. Mapping IP Multicast Address to MAPOS 16 Address   When transmitting IP multicast[11], the thirteen bits of the HDLC   address must contain the lowest-order thirteen bits of the IP   multicast group address.  Exceptions arise when these thirteen bits   are either all zeros or all ones, in which case the address 1111 1110   1111 1101 should be used.6. 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, " MAPOS - Multiple Access Protocol        over SONET/SDH version 1",RFC2171, June, 1997.   [6]  Simpson, W., editor, "PPP in HDLC-like Framing,"RFC1662, July        1994.   [7]  Murakami, K. and M. Maruyama, "A MAPOS version 1 Extension -        Node Switch Protocol,"RFC2173, June, 1997.   [8]  Murakami, K. and M. Maruyama, "A MAPOS version 1 Extension -        Switch Switch Protocol,"RFC2174, June, 1997.   [9]  Reynolds, J. and J. Postel, "ASSIGNED NUMBERS,"RFC1700, Oct.        1994.   [10] Maruyama, M. and K. Murakami, "MAPOS Version 1 Assigned        Numbers,"RFC2172, June, 1997.   [11] Murakami, K. and M. Maruyama, "IPv4 over MAPOS Version 1,"RFC2176, June, 1997.Murakami & Maruyama          Informational                      [Page 5]

RFC 2175                        MAPOS 16                       June 1997Acknowledgements   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 6]