As per Relevance of the word implementation, we have this rfc below:
Network Working Group W. Simpson,
Request for Comments: 1549
Category: Standards Track December 1993
PPP in HDLC
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
The Point-to-Point Protocol (PPP) [1] provides a standard method
transporting multi-protocol datagrams over point-to-point links
This document describes the use of HDLC for framing PPP
packets. This document is the product of the Point-to-Point
Working Group of the Internet Engineering Task Force (IETF).
Comments should be submitted to the ietf-ppp@ucdavis.edu
list
Table of
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.
This specification provides for framing over both bit-oriented
octet-oriented synchronous links, and asynchronous links with 8
of data and no parity. These links MUST be full-duplex, but MAY
either dedicated or circuit-switched. PPP uses HDLC as a basis
the framing
An escape mechanism is specified to allow control data such
XON/XOFF to be transmitted transparently over the link, and to
spurious control data which may be injected into the link
intervening hardware and software
Some protocols expect error free transmission, and either
error detection only on a conditional basis, or do not provide it
all. PPP uses the HDLC Frame Check Sequence for error detection
This is commonly available in hardware implementations, and
software implementation is provided
1.1 Specification of
In this document, several words are used to signify the
of the specification. These words are often capitalized
This word, or the adjective "required", means that the
is an absolute requirement of the specification
MUST
This phrase means that the definition is an absolute
of the specification
This word, or the adjective "recommended", means that there
exist valid reasons in particular circumstances to ignore
item, but the full implications must be understood and
weighed before choosing a different course
This word, or the adjective "optional", means that this item
one of an allowed set of alternatives. An implementation
does not include this option MUST be prepared to interoperate
another implementation which does include the option
Simpson [Page 2]
RFC 1549 HDLC Framing Decvember 1993
1.2
This document frequently uses the following terms
The unit of transmission in the network layer (such as IP).
datagram may be encapsulated in one or more packets passed to
data link layer
The unit of transmission at the data link layer. A frame
include a header and/or a trailer, along with some number of
of data
The basic unit of encapsulation, which is passed across
interface between the network layer and the data link layer.
packet is usually mapped to a frame; the exceptions are when
link layer fragmentation is being performed, or when
packets are incorporated into a single frame
The other end of the point-to-point link
silently
This means the implementation discards the packet without
processing. The implementation SHOULD provide the capability
logging the error, including the contents of the
discarded packet, and SHOULD record the event in a
counter
2. Physical Layer
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
absolute requirement imposed by PPP is the provision of a full-
circuit, either dedicated or circuit-switched, which can operate
either an asynchronous (start/stop), bit-synchronous, or octet
synchronous mode, transparent to PPP Data Link Layer frames
Interface
PPP presents an octet interface to the physical layer. There
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
PPP does not require the use of control signals, such as
To Send (RTS), Clear To Send (CTS), Data Carrier Detect (DCD),
Data Terminal Ready (DTR).
When available, using such signals can allow greater
and performance. In particular, such signals SHOULD be used
signal the Up and Down events in the LCP Option
Automaton [1]. When such signals are not available,
implementation MUST signal the Up event to LCP
initialization, and SHOULD NOT signal the Down event
Because signalling is not required, the physical layer MAY
decoupled from the data link layer, hiding the transient
of the physical transport. This has implications for mobility
cellular radio networks, and other rapidly switching links
When moving from cell to cell within the same zone,
implementation MAY choose to treat the entire zone as a
link, even though transmission is switched among
frequencies. The link is considered to be with the
control unit for the zone, rather than the individual
transceivers. However, the link SHOULD re-establish
configuration whenever the link is switched to a
administration
Due to the bursty nature of data traffic, some
have choosen to disconnect the physical layer during periods
inactivity, and reconnect when traffic resumes, without
the data link layer. Robust implementations should avoid
this trick over-zealously, since the price for decreased
latency is decreased security. Implementations SHOULD signal
Down event whenever "significant time" has elapsed since the
was disconnected. The value for "significant time" is a matter
considerable debate, and is based on the tariffs, call
times, and security concerns of the installation
3. The Data Link
PPP uses the principles, terminology, and frame structure of
International Organization For Standardization's (ISO) 3309-1979
Simpson [Page 4]
RFC 1549 HDLC Framing Decvember 1993
High-level Data Link Control (HDLC) frame structure [2], as
by "Addendum 1: Start/stop transmission" [3], which
modifications to allow HDLC use in asynchronous environments
The PPP control procedures use the definitions and Control
encodings standardized in ISO 4335-1979 [4] and ISO 4335-
1979/Addendum 1-1979 [5]. PPP framing is also consistent with
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
standardized in ISO 3309. It is assumed that the reader is
familiar with HDLC, or has access to a copy of [2] or [6]. Instead
this document attempts to give a concise summary and point
specific options and features used by PPP
To remain consistent with standard Internet practice, and
confusion for people used to reading RFCs, all binary numbers in
following descriptions are in Most Significant Bit to
Significant Bit order, reading from left to right, unless
indicated. Note that this is contrary to standard ISO and
practice which orders bits as transmitted (network bit order).
this in mind when comparing this document with the
standards documents
3.1 Frame
A summary of the PPP HDLC frame structure is shown below.
figure does not include start/stop bits (for asynchronous links),
any bits or octets inserted for transparency. The fields
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
Point-to-Point Protocol Encapsulation [1].
Simpson [Page 5]
RFC 1549 HDLC Framing Decvember 1993
Flag
The Flag Sequence indicates the beginning or end of a frame,
always consists of the binary sequence 01111110 (
0x7e).
The Flag Sequence is a frame separator. Only one Flag Sequence
required between two frames. Two consecutive Flag
constitute an empty frame, which is ignored, and not counted as
FCS error
Address
The Address field is a single octet and contains the
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
of other address lengths and values may be defined at a
time, or by prior agreement. Frames with unrecognized
SHOULD be silently discarded
Control
The Control field is a single octet and contains the
sequence 00000011 (hexadecimal 0x03), the Unnumbered
(UI) command with the P/F bit set to zero. The use of
Control field values may be defined at a later time, or by
agreement. Frames with unrecognized Control field values
be silently discarded
Frame Check Sequence (FCS)
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
prior agreement. The FCS is transmitted with the coefficient
the highest term first
The FCS field is calculated over all bits of the Address, Control
Protocol, Information and Padding fields, not including any
and stop bits (asynchronous) nor any bits (synchronous) or
(asynchronous or synchronous) inserted for transparency.
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
the FCS
For more information on the specification of the FCS, see
Simpson [Page 6]
RFC 1549 HDLC Framing Decvember 1993
3309 [2] or CCITT X.25 [6].
The end of the Information and Padding fields is found by
the closing Flag Sequence and removing the Frame Check
field
3.2. Modification of the Basic
The Link Control Protocol can negotiate modifications to the
HDLC frame structure. However, modified frames will always
clearly distinguishable from standard frames
Address-and-Control-Field-
When using the default HDLC framing, the Address and
fields contain the hexadecimal values 0xff and 0x03 respectively
On transmission, compressed Address and Control fields are
by simply omitting them
On reception, the Address and Control fields are decompressed
examining the first two octets. If they contain the values 0
and 0x03, they are assumed to be the Address and Control fields
If not, it is assumed that the fields were compressed and were
transmitted
By definition, the first octet of a two octet Protocol field
never be 0xff (since it is not even). The Protocol field
0x00ff is not allowed (reserved) to avoid ambiguity
Protocol-Field-Compression is enabled and the first
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
This section summarizes the use of HDLC with 8-bit
links
Flag
The Flag Sequence indicates the beginning or end of a frame.
octet stream is examined on an octet-by-octet basis for the
01111110 (hexadecimal 0x7e).
Simpson [Page 7]
RFC 1549 HDLC Framing Decvember 1993
An octet stuffing procedure is used. The Control Escape octet
defined as binary 01111101 (hexadecimal 0x7d) where the
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
256 bits. This results in four distinct ACCMs, two in
direction of the link
The default receiving ACCM is 0xffffffff. The default
ACCM is 0xffffffff, plus the Control Escape and Flag
characters themselves, plus whatever other outgoing characters
known to be intercepted
After FCS computation, the transmitter examines the entire
between the two Flag Sequences. Each Flag Sequence,
Escape octet, and octet with value less than hexadecimal 0x20
which is flagged in the sending Async-Control-Character-Map,
replaced by a two octet sequence consisting of the Control
octet and the original octet with bit 6 complemented (exclusive
or'd with hexadecimal 0x20).
Prior to FCS computation, the receiver examines the entire
between the two Flag Sequences. Each octet with value less
hexadecimal 0x20 is checked. If it is flagged in the
Async-Control-Character-Map, it is simply removed (it may
been inserted by intervening data communications equipment).
each Control Escape octet, that octet is also removed, but bit 6
of the following octet is complemented, unless it is the
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
communications equipment
The transmitter may also send octets with value in the range 0x40
through 0xff (except 0x5e) in Control Escape format. Since
octet values are not negotiable, this does not solve the
of receivers which cannot handle all non-control characters
Also, since the technique does not affect the 8th bit, this
not solve problems for communications links that can send only 7-
bit characters
A few examples may make this more clear. Packet data
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 DC
and DC3 ignoring the 8th (parity) bit. This data would
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
On asynchronous links, frames may be aborted by transmitting a "0"
stop bit where a "1" bit is expected (framing error) or
transmitting a Control Escape octet followed immediately by
closing Flag Sequence
Time
For asynchronous links, inter-octet and inter-frame time fill
be accomplished by transmitting continuous "1" bits (mark-
state).
Inter-frame time fill can be viewed as extended inter-octet
fill. Doing so can save one octet for every frame,
delay and increasing bandwidth. This is possible since a
Sequence may serve as both a frame close and a frame begin.
having received any frame, an idle receiver will always be in
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
and interpret them as part of an incoming frame. If
transmitter does not send a new opening Flag Sequence
sending the next frame, then that frame will be appended to
noise characters causing an invalid frame (with high reliability).
It is suggested that implementations will achieve the best
by always sending an opening Flag Sequence if the new frame is
back-to-back with the last. Transmitters SHOULD send an open
Sequence whenever "appreciable time" has elapsed after the
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
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
asynchronous links
5. Bit-synchronous
This section summarizes the use of HDLC with bit-synchronous links
Flag
The Flag Sequence indicates the beginning or end of a frame,
is used for frame synchronization. The bit stream is examined
a bit-by-bit basis for the binary sequence 01111110 (
0x7e).
The "shared zero mode" Flag Sequence "011111101111110" SHOULD
be used. When not avoidable, such an implementation MUST
that the first Flag Sequence detected (the end of the frame)
promptly communicated to the link layer. Use of the shared
mode hinders interoperability with synchronous-to-
converters
The transmitter examines the entire frame between the two
Sequences. A "0" bit is inserted after all sequences of
contiguous "1" bits (including the last 5 bits of the FCS)
ensure that a Flag Sequence is not simulated
When receiving, any "0" bit that directly follows five
"1" bits is discarded
Since the Control Escape octet-stuffing method is not used,
default receiving and sending Async-Control-Character-Maps are 0.
There may be some use of synchronous-to-asynchronous
(some built into modems) in point-to-point links resulting in
synchronous PPP implementation on one end of a link and
asynchronous implementation on the other. It is
responsibility of the converter to do all mapping
during operation
To enable this functionality, bit-synchronous PPP
MUST always respond to the Async-Control-Character-
Configuration Option with an LCP Configure-Ack. However
acceptance of the Configuration Option does not imply that
bit-synchronous implementation will do any octet mapping
Instead, all such octet mapping will be performed by
asynchronous-to-synchronous converter
Simpson [Page 10]
RFC 1549 HDLC Framing Decvember 1993
Aborting a
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
For bit-synchronous links, the Flag Sequence SHOULD be
during inter-frame time fill. There is no provision for inter
octet time fill
Mark idle (continuous ones) SHOULD NOT be used for inter-
ill. However, certain types of circuit-switched links require
use of mark idle, particularly those that calculate
based on periods of bit activity. When mark idle is used on
bit-synchronous link, the implementation MUST ensure at least 15
consecutive "1" bits between Flags during the idle period,
that the Flag Sequence is always generated at the beginning of
frame after an idle period
The definition of various encodings and scrambling is
responsibility of the DTE/DCE equipment in use, and is outside
scope of this specification
While PPP will operate without regard to the
representation of the bit stream, lack of standards
transmission will hinder interoperability as surely as lack
data link standards. At speeds of 56 Kbps through 2.0 Mbps,
is currently most widely available, and on that basis
recommended as a default
When configuration of the encoding is allowed, NRZI is
as an alternative, because of its relative immunity to
inversion configuration errors, and instances when it MAY
connection without an expensive DSU/CSU. Unfortunately,
encoding obviates the (1 + x) factor of the 16-bit FCS, so
one error in 2**15 goes undetected (instead of one in 2**16),
triple errors are not detected. Therefore, when NRZI is in use
it is recommended that the 32-bit FCS be negotiated, which
not include the (1 + x) factor
At higher speeds of up to 45 Mbps, some implementors have
the ANSI High Speed Synchronous Interface [HSSI]. While
experience is currently limited, implementors are encouraged
cooperate in choosing transmission encoding
Simpson [Page 11]
RFC 1549 HDLC Framing Decvember 1993
6. Octet-synchronous
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
used
Flag
The Flag Sequence indicates the beginning or end of a frame.
octet stream is examined on an octet-by-octet basis for the
01111110 (hexadecimal 0x7e).
An octet stuffing procedure is used. The Control Escape octet
defined as binary 01111101 (hexadecimal 0x7d).
The octet stuffing procedure is described in "Asynchronous HDLC
above
The sending and receiving implementations need escape only
Flag Sequence and Control Escape octets
Considerations concerning the use of converters are described
"Bit-synchronous HDLC" above
Aborting a
Frames may be aborted by transmitting a Control Escape
followed immediately by a closing Flag Sequence. The
frame is ignored, and not counted as a FCS error
Inter-frame Time
The Flag Sequence MUST be transmitted during inter-frame
fill. There is no provision for inter-octet time fill
The definition of various encodings and scrambling is
responsibility of the DTE/DCE equipment in use, and is outside
scope of this specification
Simpson [Page 12]
RFC 1549 HDLC Framing Decvember 1993
A. Fast Frame Check Sequence (FCS)
The FCS was originally designed with hardware implementations
mind. A serial bit stream is transmitted on the wire, the FCS
calculated over the serial data as it goes out, and the complement
the resulting FCS is appended to the serial stream, followed by
Flag Sequence
The receiver has no way of determining that it has
calculating the received FCS until it detects the Flag Sequence
Therefore, the FCS was designed so that a particular pattern
when the FCS operation passes over the complemented FCS. A
frame is indicated by this "good FCS" value
A.1 FCS Computation
The following code provides a table lookup computation
calculating the Frame Check Sequence as data arrives at
interface. This implementation is based on [9], [10], and [11].
table is created by the code in section B.2.
Simpson [Page 13]
RFC 1549 HDLC Framing Decvember 1993
/*
* u16 represents an unsigned 16-bit number. Adjust the typedef
* 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 */
/*
Simpson [Page 14]
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
*/
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
The following code creates the lookup table used to calculate the FCS
Simpson [Page 15]
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
As noted in the Physical Layer Requirements section, the link
might not be informed when the connected state of physical layer
changed. This results in possible security lapses due to over
reliance on the integrity and security of switching systems
administrations. An insertion attack might be undetected.
attacker which is able to spoof the same calling identity might
able to avoid link authentication
Simpson [Page 16]
RFC 1549 HDLC Framing Decvember 1993
[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",
RFC 1548, December 1993
[2] International Organization For Standardization, ISO
3309-1979, "Data communication - High-level data link
procedures - Frame structure", 1979.
[3] International Organization For Standardization, Proposed
International Standard ISO 3309-1991/PDAD1, "
processing systems - Data communication - High-level data
control procedures - Frame structure - Addendum 1: Start/
transmission", 1991.
[4] International Organization For Standardization, ISO
4335-1979, "Data communication - High-level data link
procedures - Elements of procedures", 1979.
[5] International Organization For Standardization, ISO
4335-1979/Addendum 1, "Data communication - High-level
link control procedures - Elements of procedures - Addendum 1",
1979.
[6] International Telecommunication Union, CCITT
X.25, "Interface Between Data Terminal Equipment (DTE) and
Circuit Terminating Equipment (DCE) for Terminals Operating
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
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.
This specification is based on previous RFCs, where
Simpson [Page 17]
RFC 1549 HDLC Framing Decvember 1993
contributions have been acknowleged
Additional implementation detail for this version was provided
Fred Baker (ACC), Craig Fox (NSC), and Phil Karn (Qualcomm).
Special thanks to Morning Star Technologies for providing
resources and network access support for writing this specification
Chair's
The working group can be contacted via the current chair
Fred
Advanced Computer
315 Bollay
Santa Barbara, California, 93111
EMail: fbaker@acc.
Editor's
Questions about this memo can also be directed to
William Allen
Computer Systems Consulting
1384
Madison Heights, Michigan 48071
EMail: Bill.Simpson@um.cc.umich.
Simpson [Page 18]
if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
RFC documents can be found at I.E.T.F.
Relevance System Copyright © 2002 Spectrum WorldResearch
other technical nosh by ServerMasters Corporation
collaboration of BobX
