RFC2175 - MAPOS 16 - Multiple Access Protocol over SONET/SDH with 16 Bit Address

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　　Network Working Group K. Murakami
Request for Comments: 2175 M. Maruyama
Category: Informational NTT Laboratories
June 1997

MAPOS 16 - Multiple Access Protocol over
SONET/SDH with 16 Bit Addressing

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 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.

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 in RFC-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 is

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.

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.

5. 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.

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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