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IETF RFC 1549
PPP in HDLC Framing
Last modified on Saturday, December 4th, 1993
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Network Working Group W. Simpson, Editor
Request for Comments: 1549 Daydreamer
Category: Standards Track December 1993
PPP in HDLC 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 for framing PPP encapsulated
packets. This document is the product of the Point-to-Point Protocol
Working Group of the Internet Engineering Task Force (IETF).
Comments should be submitted to the ietf-ppp@ucdavis.edu mailing
list.
Table of Contents
1. Introduction ..................................................2
1.1 Specification of Requirements .................................2
1.2 Terminology ...................................................3
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. Asynchronous HDLC .............................................7
5. Bit-synchronous HDLC ..........................................5
6. Octet-synchronous HDLC ........................................12
APPENDIX A. Fast Frame Check Sequence (FCS) Implementation .........13
A.1 FCS Computation Method ........................................13
A.2 Fast FCS table generator ......................................15
SECURITY CONSIDERATIONS ............................................16
REFERENCES .........................................................17
ACKNOWLEDGEMENTS ...................................................17
CHAIR'S ADDRESS ....................................................18
EDITOR'S ADDRESS ...................................................18
Simpson PAGE 1 
RFC 1549 HDLC Framing Decvember 1993
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. PPP uses HDLC as a basis for
the framing.
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.
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.
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RFC 1549 HDLC Framing Decvember 1993
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
This means 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.
2. Physical Layer Requirements
PPP is capable of operating across most DTE/DCE interfaces (such as,
EIA RS-232-C, EIA RS-422, EIA RS-423 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
Simpson PAGE 3
RFC 1549 HDLC Framing Decvember 1993
no provision for sub-octets to be supplied or accepted.
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
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, terminology, and frame structure of the
International Organization For Standardization's (ISO) 3309-1979
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RFC 1549 HDLC Framing Decvember 1993
High-level Data Link Control (HDLC) frame structure [2], as modified
by "Addendum 1: Start/stop transmission" [3], which specifies
modifications to allow HDLC use in asynchronous environments.
The PPP control procedures use the definitions and Control field
encodings standardized in ISO 4335-1979 [4] and ISO 4335-
1979/Addendum 1-1979 [5]. PPP framing is also consistent with CCITT
Recommendation X.25 LAPB [6], and CCITT Recommendation Q.922 [7],
since those are also based on HDLC.
The purpose of this specification is not to document what is already
standardized in ISO 3309. It is assumed that the reader is already
familiar with HDLC, or has access to a copy of [2] or [6]. Instead,
this document attempts to give a concise summary and point out
specific options and features used by PPP.
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.
3.1 Frame Format
A summary of the PPP HDLC frame structure is shown below. This
figure does not include start/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 |
| 16 bits | * | * |
+----------+-------------+---------+
+----------+----------+------------------+
| FCS | Flag | Inter-frame Fill |
| 16 bits | 01111110 | or next Address |
+----------+----------+------------------+
The Protocol, Information and Padding fields are described in the
Point-to-Point Protocol Encapsulation [1].
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RFC 1549 HDLC Framing Decvember 1993
Flag Sequence
The Flag Sequence indicates the beginning or end of a frame, and
always consists of the binary sequence 01111110 (hexadecimal
0x7e).
The Flag Sequence is a frame separator. Only one Flag Sequence is
required between two frames. Two consecutive Flag Sequences
constitute an empty frame, which is ignored, and not counted as a
FCS error.
Address Field
The Address field is a single octet and contains the binary
sequence 11111111 (hexadecimal 0xff), the All-Stations address.
PPP does not assign individual station addresses. 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.
Control Field
The Control field is a single octet and contains the binary
sequence 00000011 (hexadecimal 0x03), the Unnumbered Information
(UI) command with the 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 is normally 16 bits (two octets).
The use of other FCS lengths may be defined at a later time, or by
prior agreement. The FCS is transmitted with the coefficient of
the highest term first.
The FCS field is calculated over all bits of the Address, Control,
Protocol, Information and Padding fields, not including any start
and stop bits (asynchronous) nor any bits (synchronous) or octets
(asynchronous or synchronous) inserted for transparency. This
also does not include the Flag Sequences nor the FCS field itself.
Note: When octets are received which are flagged in the Async-
Control-Character-Map, they are discarded before calculating
the FCS.
For more information on the specification of the FCS, see ISO
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RFC 1549 HDLC Framing Decvember 1993
3309 [2] or CCITT X.25 [6].
The end of the Information and Padding fields is found by locating
the closing Flag Sequence and removing the Frame Check Sequence
field.
3.2. Modification of the Basic Frame
The Link Control Protocol can negotiate modifications to the basic
HDLC frame structure. However, modified frames will always be
clearly distinguishable from standard frames.
Address-and-Control-Field-Compression
When using the default HDLC framing, the Address and Control
fields contain the hexadecimal values 0xff and 0x03 respectively.
On transmission, compressed Address and Control fields are formed
by simply omitting them.
On reception, the Address and Control fields are decompressed by
examining the first two octets. If they contain the values 0xff
and 0x03, they are assumed to be the Address and Control fields.
If not, it is assumed that the fields were compressed and were not
transmitted.
By definition, the first octet of a two octet Protocol field will
never be 0xff (since it is not even). The Protocol field value
0x00ff is not allowed (reserved) to avoid ambiguity when
Protocol-Field-Compression is enabled and the first Information
field octet is 0x03.
When other Address or Control field values are in use, Address-
and-Control-Field-Compression MUST NOT be negotiated.
4. Asynchronous HDLC
This section summarizes the use of HDLC with 8-bit asynchronous
links.
Flag Sequence
The Flag Sequence indicates the beginning or end of a frame. The
octet stream is examined on an octet-by-octet basis for the value
01111110 (hexadecimal 0x7e).
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RFC 1549 HDLC Framing Decvember 1993
Transparency
An octet stuffing procedure is used. The Control Escape octet is
defined as binary 01111101 (hexadecimal 0x7d) where the bit
positions are numbered 87654321 (not 76543210, BEWARE).
Each end of the link maintains two Async-Control-Character-Maps.
The receiving ACCM is 32 bits, but the sending ACCM may be up to
256 bits. This results in four distinct ACCMs, two in each
direction of the link.
The default receiving ACCM is 0xffffffff. The default sending
ACCM is 0xffffffff, plus the Control Escape and Flag Sequence
characters themselves, plus whatever other outgoing characters are
known to be intercepted.
After FCS computation, the transmitter examines the entire frame
between the two Flag Sequences. Each Flag Sequence, Control
Escape octet, and octet with value less than hexadecimal 0x20
which is flagged in the sending Async-Control-Character-Map, is
replaced by a two octet sequence consisting of the Control Escape
octet and the original octet with bit 6 complemented (exclusive-
or'd with hexadecimal 0x20).
Prior to FCS computation, the receiver examines the entire frame
between the two Flag Sequences. Each octet with value less than
hexadecimal 0x20 is checked. If it is flagged in the receiving
Async-Control-Character-Map, it is simply removed (it may have
been inserted by intervening data communications equipment). For
each Control Escape octet, that octet is also removed, but bit 6
of the following octet is complemented, unless it is the Flag
Sequence.
Note: The inclusion of all octets less than hexadecimal 0x20
allows all ASCII control characters [8] excluding DEL (Delete)
to be transparently communicated through all known data
communications equipment.
The transmitter may also send octets with value in the range 0x40
through 0xff (except 0x5e) in Control Escape format. Since these
octet values are not negotiable, this does not solve the problem
of receivers which cannot handle all non-control characters.
Also, since the technique does not affect the 8th bit, this does
not solve problems for communications links that can send only 7-
bit characters.
A few examples may make this more clear. Packet data is
transmitted on the link as follows:
Simpson PAGE 8
RFC 1549 HDLC Framing Decvember 1993
0x7e is encoded as 0x7d, 0x5e. 0x7d is encoded as 0x7d, 0x5d.
0x01 is encoded as 0x7d, 0x21.
Some modems with software flow control may intercept outgoing DC1
and DC3 ignoring the 8th (parity) bit. This data would be
transmitted on the link as follows:
0x11 is encoded as 0x7d, 0x31. 0x13 is encoded as 0x7d, 0x33.
0x91 is encoded as 0x7d, 0xb1. 0x93 is encoded as 0x7d, 0xb3.
Aborting a Transmission
On asynchronous links, frames may be aborted by transmitting a "0"
stop bit where a "1" bit is expected (framing error) or by
transmitting a Control Escape octet followed immediately by a
closing Flag Sequence.
Time Fill
For asynchronous links, inter-octet and inter-frame time fill MUST
be accomplished by transmitting continuous "1" bits (mark-hold
state).
Inter-frame time fill can be viewed as extended inter-octet time
fill. Doing so can save one octet for every frame, decreasing
delay and increasing bandwidth. This is possible since a Flag
Sequence may serve as both a frame close and a frame begin. After
having received any frame, an idle receiver will always be in a
frame begin state.
Robust transmitters should avoid using this trick over-zealously,
since the price for decreased delay is decreased reliability.
Noisy links may cause the receiver to receive garbage characters
and interpret them as part of an incoming frame. If the
transmitter does not send a new opening Flag Sequence before
sending the next frame, then that frame will be appended to the
noise characters causing an invalid frame (with high reliability).
It is suggested that implementations will achieve the best results
by always sending an opening Flag Sequence if the new frame is not
back-to-back with the last. Transmitters SHOULD send an open Flag
Sequence whenever "appreciable time" has elapsed after the prior
closing Flag Sequence. The maximum value for "appreciable time"
is likely to be no greater than the typing rate of a slow typist,
say 1 second.
Encoding
All octets are transmitted with one start bit, eight bits of data,
Simpson PAGE 9
RFC 1549 HDLC Framing Decvember 1993
and one stop bit. There is no provision for seven bit
asynchronous links.
5. Bit-synchronous HDLC
This section summarizes the use of HDLC with bit-synchronous links.
Flag Sequence
The Flag Sequence indicates the beginning or end of a frame, and
is used for frame synchronization. The bit stream is examined on
a bit-by-bit basis for the binary sequence 01111110 (hexadecimal
0x7e).
The "shared zero mode" Flag Sequence "011111101111110" SHOULD NOT
be used. When not avoidable, such an implementation MUST ensure
that the first Flag Sequence detected (the end of the frame) is
promptly communicated to the link layer. Use of the shared zero
mode hinders interoperability with synchronous-to-asynchronous
converters.
Transparency
The transmitter examines the entire frame between the two Flag
Sequences. A "0" bit is inserted after all sequences of five
contiguous "1" bits (including the last 5 bits of the FCS) to
ensure that a Flag Sequence is not simulated.
When receiving, any "0" bit that directly follows five contiguous
"1" bits is discarded.
Since the Control Escape octet-stuffing method is not used, the
default receiving and sending Async-Control-Character-Maps are 0.
There may be some use of synchronous-to-asynchronous converters
(some built into modems) in point-to-point links resulting in a
synchronous PPP implementation on one end of a link and an
asynchronous implementation on the other. It is the
responsibility of the converter to do all mapping conversions
during operation.
To enable this functionality, bit-synchronous PPP implementations
MUST always respond to the Async-Control-Character-Map
Configuration Option with an LCP Configure-Ack. However,
acceptance of the Configuration Option does not imply that the
bit-synchronous implementation will do any octet mapping.
Instead, all such octet mapping will be performed by the
asynchronous-to-synchronous converter.
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RFC 1549 HDLC Framing Decvember 1993
Aborting a Transmission
A sequence of more than six "1" bits indicates an invalid frame,
which is ignored, and not counted as a FCS error.
Inter-frame Time Fill
For bit-synchronous links, the Flag Sequence SHOULD be transmitted
during inter-frame time fill. There is no provision for inter-
octet time fill.
Mark idle (continuous ones) SHOULD NOT be used for inter-frame
ill. However, certain types of circuit-switched links require the
use of mark idle, particularly those that calculate accounting
based on periods of bit activity. When mark idle is used on a
bit-synchronous link, the implementation MUST ensure at least 15
consecutive "1" bits between Flags during the idle period, and
that the Flag Sequence is always generated at the beginning of a
frame after an idle period.
Encoding
The definition of various encodings and scrambling is the
responsibility of the DTE/DCE equipment in use, and is outside the
scope of this specification.
While PPP will operate without regard to the underlying
representation of the bit stream, lack of standards for
transmission will hinder interoperability as surely as lack of
data link standards. At speeds of 56 Kbps through 2.0 Mbps, NRZ
is currently most widely available, and on that basis is
recommended as a default.
When configuration of the encoding is allowed, NRZI is recommended
as an alternative, because of its relative immunity to signal
inversion configuration errors, and instances when it MAY allow
connection without an expensive DSU/CSU. Unfortunately, NRZI
encoding obviates the (1 + x) factor of the 16-bit FCS, so that
one error in 2**15 goes undetected (instead of one in 2**16), and
triple errors are not detected. Therefore, when NRZI is in use,
it is recommended that the 32-bit FCS be negotiated, which does
not include the (1 + x) factor.
At higher speeds of up to 45 Mbps, some implementors have chosen
the ANSI High Speed Synchronous Interface [HSSI]. While this
experience is currently limited, implementors are encouraged to
cooperate in choosing transmission encoding.
Simpson PAGE 11
RFC 1549 HDLC Framing Decvember 1993
6. Octet-synchronous HDLC
This section summarizes the use of HDLC with octet-synchronous links,
such as SONET and optionally ISDN B or H channels.
Although the bit rate is synchronous, there is no bit-stuffing.
Instead, the octet-stuffing feature of 8-bit asynchronous HDLC is
used.
Flag Sequence
The Flag Sequence indicates the beginning or end of a frame. The
octet stream is examined on an octet-by-octet basis for the value
01111110 (hexadecimal 0x7e).
Transparency
An octet stuffing procedure is used. The Control Escape octet is
defined as binary 01111101 (hexadecimal 0x7d).
The octet stuffing procedure is described in "Asynchronous HDLC"
above.
The sending and receiving implementations need escape only the
Flag Sequence and Control Escape octets.
Considerations concerning the use of converters are described in
"Bit-synchronous HDLC" above.
Aborting a Transmission
Frames may be aborted by transmitting a Control Escape octet
followed immediately by a closing Flag Sequence. The preceding
frame is ignored, and not counted as a FCS error.
Inter-frame Time Fill
The Flag Sequence MUST be transmitted during inter-frame time
fill. There is no provision for inter-octet time fill.
Encoding
The definition of various encodings and scrambling is the
responsibility of the DTE/DCE equipment in use, and is outside the
scope of this specification.
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RFC 1549 HDLC Framing Decvember 1993
A. Fast Frame Check Sequence (FCS) Implementation
The FCS was originally designed with hardware implementations in
mind. A serial bit stream is transmitted on the wire, the FCS is
calculated over the serial data as it goes out, and the complement of
the resulting FCS is appended to the serial stream, followed by the
Flag Sequence.
The receiver has no way of determining that it has finished
calculating the received FCS until it detects the Flag Sequence.
Therefore, the FCS was designed so that a particular pattern results
when the FCS operation passes over the complemented FCS. A good
frame is indicated by this "good FCS" value.
A.1 FCS Computation Method
The following code provides a table lookup computation for
calculating the Frame Check Sequence as data arrives at the
interface. This implementation is based on [9], [10], and [11]. The
table is created by the code in section B.2.
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RFC 1549 HDLC Framing Decvember 1993
/*
* u16 represents an unsigned 16-bit number. Adjust the typedef for
* your hardware.
*/
typedef unsigned short u16;
/*
* FCS lookup table as calculated by the table generator in section B.2
*/
static u16 fcstab[256] = {
0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,
0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,
0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,
0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,
0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,
0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,
0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,
0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,
0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,
0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,
0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,
0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,
0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,
0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,
0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,
0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,
0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,
0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,
0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,
0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,
0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,
0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,
0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,
0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78
};
#define PPPINITFCS16 0xffff /* Initial FCS value */
#define PPPGOODFCS16 0xf0b8 /* Good final FCS value */
/*
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RFC 1549 HDLC Framing Decvember 1993
* Calculate a new fcs given the current fcs and the new data.
*/
u16 pppfcs16(fcs, cp, len)
register u16 fcs;
register unsigned char *cp;
register int len;
{
ASSERT(sizeof (u16) == 2);
ASSERT(((u16) -1) > 0);
while (len--)
fcs = (fcs >> 8) ^ fcstab[(fcs ^ *cp++) & 0xff];
return (fcs);
}
/*
* How to use the fcs
*/
tryfcs16(cp, len)
register unsigned char *cp;
register int len;
{
u16 trialfcs;
/* add on output */
trialfcs = pppfcs16( PPPINITFCS16, cp, len );
trialfcs ^= 0xffff; /* complement */
cp[len] = (trialfcs & 0x00ff); /* least significant byte first */
cp[len+1] = ((trialfcs >> 8) & 0x00ff);
/* check on input */
trialfcs = pppfcs16( PPPINITFCS16, cp, len + 2 );
if ( trialfcs == PPPGOODFCS16 )
printf("Good FCS0);
}
A.2. Fast FCS table generator
The following code creates the lookup table used to calculate the FCS.
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RFC 1549 HDLC Framing Decvember 1993
/*
* Generate a FCS table for the HDLC FCS.
*
* Drew D. Perkins at Carnegie Mellon University.
*
* Code liberally borrowed from Mohsen Banan and D. Hugh Redelmeier.
*/
/*
* The HDLC polynomial: x**0 + x**5 + x**12 + x**16 (0x8408).
*/
#define P 0x8408
main()
{
register unsigned int b, v;
register int i;
printf("typedef unsigned short u16;0);
printf("static u16 fcstab[256] = {");
for (b = 0; ; ) {
if (b % 8 == 0)
printf("0);
v = b;
for (i = 8; i--; )
v = v & 1 ? (v >> 1) ^ P : v >> 1;
printf("0x%04x", v & 0xFFFF);
if (++b == 256)
break;
printf(",");
}
printf("0;0);
}
Security Considerations
As noted in the Physical Layer Requirements section, the link layer
might not be informed when the connected state of physical layer is
changed. This results in possible security lapses due to over-
reliance on the integrity and security of switching systems and
administrations. An insertion attack might be undetected. An
attacker which is able to spoof the same calling identity might be
able to avoid link authentication.
Simpson PAGE 16
RFC 1549 HDLC Framing Decvember 1993
References
[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",
RFC 1548, December 1993
[2] International Organization For Standardization, ISO Standard
3309-1979, "Data communication - High-level data link control
procedures - Frame structure", 1979.
[3] International Organization For Standardization, Proposed Draft
International Standard ISO 3309-1991/PDAD1, "Information
processing systems - Data communication - High-level data link
control procedures - Frame structure - Addendum 1: Start/stop
transmission", 1991.
[4] International Organization For Standardization, ISO Standard
4335-1979, "Data communication - High-level data link control
procedures - Elements of procedures", 1979.
[5] International Organization For Standardization, ISO Standard
4335-1979/Addendum 1, "Data communication - High-level data
link control procedures - Elements of procedures - Addendum 1",
1979.
[6] International Telecommunication Union, CCITT Recommendation
X.25, "Interface Between Data Terminal Equipment (DTE) and Data
Circuit Terminating Equipment (DCE) for Terminals Operating in
the Packet Mode on Public Data Networks", CCITT Red Book,
Volume VIII, Fascicle VIII.3, Rec. X.25., October 1984.
[7] International Telegraph and Telephone Consultative Committee,
CCITT Recommendation Q.922, "ISDN Data Link Layer Specification
for Frame Mode Bearer Services", April 1991.
[8] American National Standards Institute, ANSI X3.4-1977,
"American National Standard Code for Information Interchange",
1977.
[9] Perez, "Byte-wise CRC Calculations", IEEE Micro, June, 1983.
[10] Morse, G., "Calculating CRC's by Bits and Bytes", Byte,
September 1986.
[11] LeVan, J., "A Fast CRC", Byte, November 1987.
Acknowledgments
This specification is based on previous RFCs, where many
Simpson PAGE 17
RFC 1549 HDLC Framing Decvember 1993
contributions have been acknowleged.
Additional implementation detail for this version was provided by
Fred Baker (ACC), Craig Fox (NSC), and Phil Karn (Qualcomm).
Special thanks to Morning Star Technologies for providing computing
resources and network access support for writing this specification.
Chair's Address
The working group can be contacted via the current chair:
Fred Baker
Advanced Computer Communications
315 Bollay Drive
Santa Barbara, California, 93111
EMail: fbaker@acc.com
Editor's Address
Questions about this memo can also be directed to:
William Allen Simpson
Daydreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
EMail: Bill.Simpson@um.cc.umich.edu
Simpson PAGE 18
PPP in HDLC Framing
RFC TOTAL SIZE: 36352 bytes
PUBLICATION DATE: Saturday, December 4th, 1993
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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