📄 rfc1662.txt
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Network Working Group W. Simpson, Editor
Request for Comments: 1662 Daydreamer
STD: 51 July 1994
Obsoletes: 1549
Category: Standards Track
PPP in HDLC-like Framing
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Point-to-Point Protocol (PPP) [1] provides a standard method for
transporting multi-protocol datagrams over point-to-point links.
This document describes the use of HDLC-like framing for PPP
encapsulated packets.
Table of Contents
1. Introduction .......................................... 1
1.1 Specification of Requirements ................... 2
1.2 Terminology ..................................... 2
2. Physical Layer Requirements ........................... 3
3. The Data Link Layer ................................... 4
3.1 Frame Format .................................... 5
3.2 Modification of the Basic Frame ................. 7
4. Octet-stuffed framing ................................. 8
4.1 Flag Sequence ................................... 8
4.2 Transparency .................................... 8
4.3 Invalid Frames .................................. 9
4.4 Time Fill ....................................... 9
4.4.1 Octet-synchronous ............................... 9
4.4.2 Asynchronous .................................... 9
4.5 Transmission Considerations ..................... 10
4.5.1 Octet-synchronous ............................... 10
4.5.2 Asynchronous .................................... 10
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RFC 1662 HDLC-like Framing July 1994
5. Bit-stuffed framing ................................... 11
5.1 Flag Sequence ................................... 11
5.2 Transparency .................................... 11
5.3 Invalid Frames .................................. 11
5.4 Time Fill ....................................... 11
5.5 Transmission Considerations ..................... 12
6. Asynchronous to Synchronous Conversion ................ 13
7. Additional LCP Configuration Options .................. 14
7.1 Async-Control-Character-Map (ACCM) .............. 14
APPENDICES ................................................... 17
A. Recommended LCP Options ............................... 17
B. Automatic Recognition of PPP Frames ................... 17
C. Fast Frame Check Sequence (FCS) Implementation ........ 18
C.1 FCS table generator ............................. 18
C.2 16-bit FCS Computation Method ................... 19
C.3 32-bit FCS Computation Method ................... 21
SECURITY CONSIDERATIONS ...................................... 24
REFERENCES ................................................... 24
ACKNOWLEDGEMENTS ............................................. 25
CHAIR'S ADDRESS .............................................. 25
EDITOR'S ADDRESS ............................................. 25
1. Introduction
This specification provides for framing over both bit-oriented and
octet-oriented synchronous links, and asynchronous links with 8 bits
of data and no parity. These links MUST be full-duplex, but MAY be
either dedicated or circuit-switched.
An escape mechanism is specified to allow control data such as
XON/XOFF to be transmitted transparently over the link, and to remove
spurious control data which may be injected into the link by
intervening hardware and software.
Some protocols expect error free transmission, and either provide
error detection only on a conditional basis, or do not provide it at
all. PPP uses the HDLC Frame Check Sequence for error detection.
This is commonly available in hardware implementations, and a
software implementation is provided.
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RFC 1662 HDLC-like Framing July 1994
1.1. Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST This word, or the adjective "required", means that the
definition is an absolute requirement of the specification.
MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.
SHOULD This word, or the adjective "recommended", means that there
may exist valid reasons in particular circumstances to
ignore this item, but the full implications must be
understood and carefully weighed before choosing a
different course.
MAY This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST be
prepared to interoperate with another implementation which
does include the option.
1.2. Terminology
This document frequently uses the following terms:
datagram The unit of transmission in the network layer (such as IP).
A datagram may be encapsulated in one or more packets
passed to the data link layer.
frame The unit of transmission at the data link layer. A frame
may include a header and/or a trailer, along with some
number of units of data.
packet The basic unit of encapsulation, which is passed across the
interface between the network layer and the data link
layer. A packet is usually mapped to a frame; the
exceptions are when data link layer fragmentation is being
performed, or when multiple packets are incorporated into a
single frame.
peer The other end of the point-to-point link.
silently discard
The implementation discards the packet without further
processing. The implementation SHOULD provide the
capability of logging the error, including the contents of
the silently discarded packet, and SHOULD record the event
in a statistics counter.
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RFC 1662 HDLC-like Framing July 1994
2. Physical Layer Requirements
PPP is capable of operating across most DTE/DCE interfaces (such as,
EIA RS-232-E, EIA RS-422, and CCITT V.35). The only absolute
requirement imposed by PPP is the provision of a full-duplex circuit,
either dedicated or circuit-switched, which can operate in either an
asynchronous (start/stop), bit-synchronous, or octet-synchronous
mode, transparent to PPP Data Link Layer frames.
Interface Format
PPP presents an octet interface to the physical layer. There is
no provision for sub-octets to be supplied or accepted.
Transmission Rate
PPP does not impose any restrictions regarding transmission rate,
other than that of the particular DTE/DCE interface.
Control Signals
PPP does not require the use of control signals, such as Request
To Send (RTS), Clear To Send (CTS), Data Carrier Detect (DCD), and
Data Terminal Ready (DTR).
When available, using such signals can allow greater functionality
and performance. In particular, such signals SHOULD be used to
signal the Up and Down events in the LCP Option Negotiation
Automaton [1]. When such signals are not available, the
implementation MUST signal the Up event to LCP upon
initialization, and SHOULD NOT signal the Down event.
Because signalling is not required, the physical layer MAY be
decoupled from the data link layer, hiding the transient details
of the physical transport. This has implications for mobility in
cellular radio networks, and other rapidly switching links.
When moving from cell to cell within the same zone, an
implementation MAY choose to treat the entire zone as a single
link, even though transmission is switched among several
frequencies. The link is considered to be with the central
control unit for the zone, rather than the individual cell
transceivers. However, the link SHOULD re-establish its
configuration whenever the link is switched to a different
administration.
Due to the bursty nature of data traffic, some implementations
have choosen to disconnect the physical layer during periods of
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RFC 1662 HDLC-like Framing July 1994
inactivity, and reconnect when traffic resumes, without informing
the data link layer. Robust implementations should avoid using
this trick over-zealously, since the price for decreased setup
latency is decreased security. Implementations SHOULD signal the
Down event whenever "significant time" has elapsed since the link
was disconnected. The value for "significant time" is a matter of
considerable debate, and is based on the tariffs, call setup
times, and security concerns of the installation.
3. The Data Link Layer
PPP uses the principles described in ISO 3309-1979 HDLC frame
structure, most recently the fourth edition 3309:1991 [2], which
specifies modifications to allow HDLC use in asynchronous
environments.
The PPP control procedures use the Control field encodings described
in ISO 4335-1979 HDLC elements of procedures, most recently the
fourth edition 4335:1991 [4].
This should not be construed to indicate that every feature of the
above recommendations are included in PPP. Each feature included
is explicitly described in the following sections.
To remain consistent with standard Internet practice, and avoid
confusion for people used to reading RFCs, all binary numbers in the
following descriptions are in Most Significant Bit to Least
Significant Bit order, reading from left to right, unless otherwise
indicated. Note that this is contrary to standard ISO and CCITT
practice which orders bits as transmitted (network bit order). Keep
this in mind when comparing this document with the international
standards documents.
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RFC 1662 HDLC-like Framing July 1994
3.1. Frame Format
A summary of the PPP HDLC-like frame structure is shown below. This
figure does not include bits inserted for synchronization (such as
start and stop bits for asynchronous links), nor any bits or octets
inserted for transparency. The fields are transmitted from left to
right.
+----------+----------+----------+
| Flag | Address | Control |
| 01111110 | 11111111 | 00000011 |
+----------+----------+----------+
+----------+-------------+---------+
| Protocol | Information | Padding |
| 8/16 bits| * | * |
+----------+-------------+---------+
+----------+----------+-----------------
| FCS | Flag | Inter-frame Fill
|16/32 bits| 01111110 | or next Address
+----------+----------+-----------------
The Protocol, Information and Padding fields are described in the
Point-to-Point Protocol Encapsulation [1].
Flag Sequence
Each frame begins and ends with a Flag Sequence, which is the
binary sequence 01111110 (hexadecimal 0x7e). All implementations
continuously check for this flag, which is used for frame
synchronization.
Only one Flag Sequence is required between two frames. Two
consecutive Flag Sequences constitute an empty frame, which is
silently discarded, and not counted as a FCS error.
Address Field
The Address field is a single octet, which contains the binary
sequence 11111111 (hexadecimal 0xff), the All-Stations address.
Individual station addresses are not assigned. The All-Stations
address MUST always be recognized and received.
The use of other address lengths and values may be defined at a
later time, or by prior agreement. Frames with unrecognized
Addresses SHOULD be silently discarded.
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RFC 1662 HDLC-like Framing July 1994
Control Field
The Control field is a single octet, which contains the binary
sequence 00000011 (hexadecimal 0x03), the Unnumbered Information
(UI) command with the Poll/Final (P/F) bit set to zero.
The use of other Control field values may be defined at a later
time, or by prior agreement. Frames with unrecognized Control
field values SHOULD be silently discarded.
Frame Check Sequence (FCS) Field
The Frame Check Sequence field defaults to 16 bits (two octets).
The FCS is transmitted least significant octet first, which
contains the coefficient of the highest term.
A 32-bit (four octet) FCS is also defined. Its use may be
negotiated as described in "PPP LCP Extensions" [5].
The use of other FCS lengths may be defined at a later time, or by
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