RFC1662 - PPP in HDLC-like Framing

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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

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.

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.

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

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.

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.

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
prior agreement.

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.

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 the
Appendices.

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 standard
HDLC-like frame structure. However, modified frames will always be
clearly distinguishable from standard frames.

Address-and-Control-Field-Compression

When using the standard HDLC-like framing, the Address and Control
fields contain the hexadecimal values 0xff and 0x03 respectively.
When other Address or Control field values are in use, Address-
and-Control-Field-Compression MUST NOT be negotiated.

On transmission, compressed Address and Control fields are simply
omitted.

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.

4. Octet-stuffed framing

This chapter summarizes the use of HDLC-like framing with 8-bit
asynchronous and octet-synchronous links.

4.1. 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).

4.2. Transparency

An octet stuffing procedure is used. The Control Escape octet is
defined as binary 01111101 (hexadecimal 0x7d), most significant bit
first.

As a minimum, sending implementations MUST escape the Flag Sequence
and Control Escape octets.

After FCS computation, the transmitter examines the entire frame
between the two Flag Sequences. Each Flag Sequence, Control Escape
octet, and any octet which is flagged in the sending Async-Control-
Character-Map (ACCM), is replaced by a two octet sequence consisting
of the Control Escape octet followed by the original octet
exclusive-or'd with hexadecimal 0x20.

This is bit 5 complemented, where the bit positions are numbered
76543210 (the 6th bit as used in ISO numbered 87654321 -- BEWARE
when comparing documents).

Receiving implementations MUST correctly process all Control Escape
sequences.

On reception, prior to FCS computation, each octet with value less
than hexadecimal 0x20 is checked. If it is flagged in the receiving
ACCM, it is simply removed (it may have been inserted by intervening
data communications equipment). Each Control Escape octet is also
removed, and the following octet is exclusive-or'd with hexadecimal
0x20, unless it is the Flag Sequence (which aborts a frame).

A few examples may make this more clear. Escaped data is transmitted
on the link as follows:

0x7e is encoded as 0x7d, 0x5e. (Flag Sequence)
0x7d is encoded as 0x7d, 0x5d. (Control Escape)
0x03 is encoded as 0x7d, 0x23. (ETX)

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. (XON)
0x13 is encoded as 0x7d, 0x33. (XOFF)
0x91 is encoded as 0x7d, 0xb1. (XON with parity set)
0x93 is encoded as 0x7d, 0xb3. (XOFF with parity set)

4.3. Invalid Frames

Frames which are too short (less than 4 octets when using the 16-bit
FCS), or which end with a Control Escape octet followed immediately
by a closing Flag Sequence, or in which octet-framing is violated (by
transmitting a "0" stop bit where a "1" bit is expected), are
silently discarded, and not counted as a FCS error.

4.4. Time Fill

4.4.1. Octet-synchronous

There is no provision for inter-octet time fill.

The Flag Sequence MUST be transmitted during inter-frame time fill.

4.4.2. Asynchronous

Inter-octet 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 end 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, about
1 second.

4.5. Transmission Considerations

4.5.1. Octet-synchronous

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.

4.5.2. Asynchronous

All octets are transmitted least significant bit first, with one
start bit, eight bits of data, and one stop bit. There is no
provision for seven bit asynchronous links.

5. Bit-stuffed framing

This chapter summarizes the use of HDLC-like framing with bit-
synchronous links.

5.1. 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 bit-synchronous to asynchronous and bit-
synchronous to octet-synchronous converters.

5.2. Transparency

After FCS computation, 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.

On reception, prior to FCS computation, any "0" bit that directly
follows five contiguous "1" bits is discarded.

5.3. Invalid Frames

Frames which are too short (less than 4 octets when using the 16-bit
FCS), or which end with a sequence of more than six "1" bits, are
silently discarded, and not counted as a FCS error.

5.4. Time Fill

There is no provision for inter-octet time fill.

The Flag Sequence SHOULD be transmitted during inter-frame time fill.
However, certain types of circuit-switched links require the use of

mark idle (continuous ones), 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.

This differs from practice in ISO 3309, which allows 7 to 14 bit
mark idle.

5.5. Transmission Considerations

All octets are transmitted least significant bit first.

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
exacerbates the missing x1 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 includes the x1
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.

6. Asynchronous to Synchronous Conversion

There may be some use of asynchronous-to-synchronous converters (some
built into modems and cellular interfaces), resulting in an
asynchronous PPP implementation on one end of a link and a
synchronous implementation on the other. It is the responsibility of
the converter to do all stuffing conversions during operation.

To enable this functionality, synchronous PPP implementations MUST
always respond to the Async-Control-Character-Map Configuration
Option with the LCP Configure-Ack. However, acceptance of the
Configuration Option does not imply that the synchronous
implementation will do any ACCM mapping. Instead, all such octet
mapping will be performed by the asynchronous-to-synchronous
converter.

7. Additional LCP Configuration Options

The Configuration Option format and basic options are already defined
for LCP [1].

Up-to-date values of the LCP Option Type field are specified in the
most recent "Assigned Numbers" RFC[10]. This document concerns the
following values:

2 Async-Control-Character-Map

7.1. Async-Control-Character-Map (ACCM)

Description

This Configuration Option provides a method to negotiate the use
of control character transparency on asynchronous links.

Each end of the asynchronous 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.

For asynchronous links, 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 flagged (by prior configuration) as likely
to be intercepted.

For other types of links, the default value is 0, since there is
no need for mapping.

The default inclusion of all octets less than hexadecimal 0x20
allows all ASCII control characters [6] excluding DEL (Delete)
to be transparently communicated through all known data
communications equipment.

The transmitter MAY also send octets with values 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.

Note that this specification differs in detail from later
amendments, such as 3309:1991/Amendment 2 [3]. However, such
"extended transparency" is applied only by "prior agreement".
Use of the transparency methods in this specification
constitute a prior agreement with respect to PPP.

For compatibility with 3309:1991/Amendment 2, the transmitter
MAY escape DEL and ACCM equivalents with the 8th (most
significant) bit set. No change is required in the receiving
algorithm.

Following ACCM negotiation, the transmitter SHOULD cease
escaping DEL.

However, it is rarely necessary to map all control characters, and
often it is unnecessary to map any control characters. The
Configuration Option is used to inform the peer which control
characters MUST remain mapped when the peer sends them.

The peer MAY still send any other octets in mapped format, if it
is necessary because of constraints known to the peer. The peer
SHOULD Configure-Nak with the logical union of the sets of mapped
octets, so that when such octets are spuriously introduced they
can be ignored on receipt.

A summary of the Async-Control-Character-Map Configuration Option
format is shown below. The fields are transmitted from left to
right.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | ACCM
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ACCM (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

2

Length

6

ACCM

The ACCM field is four octets, and indicates the set of control
characters to be mapped. The map is sent most significant octet
first.

Each numbered bit corresponds to the octet of the same value. If
the bit is cleared to zero, then that octet need not be mapped.
If the bit is set to one, then that octet MUST remain mapped. For
example, if bit 19 is set to zero, then the ASCII control
character 19 (DC3, Control-S) MAY be sent in the clear.

Note: The least significant bit of the least significant octet
(the final octet transmitted) is numbered bit 0, and would map
to the ASCII control character NUL.

A. Recommended LCP Options

The following Configurations Options are recommended:

High Speed links

Magic Number
Link Quality Monitoring
No Address and Control Field Compression
No Protocol Field Compression

Low Speed or Asynchronous links

Async Control Character Map
Magic Number
Address and Control Field Compression
Protocol Field Compression

B. Automatic Recognition of PPP Frames

It is sometimes desirable to detect PPP frames, for example during a
login sequence. The following octet sequences all begin valid PPP
LCP frames:

7e ff 03 c0 21
7e ff 7d 23 c0 21
7e 7d df 7d 23 c0 21

Note that the first two forms are not a valid username for Unix.
However, only the third form generates a correctly checksummed PPP
frame, whenever 03 and ff are taken as the control characters ETX and
DEL without regard to parity (they are correct for an even parity
link) and discarded.

Many implementations deal with this by putting the interface into
packet mode when one of the above username patterns are detected
during login, without examining the initial PPP checksum. The
initial incoming PPP frame is discarded, but a Configure-Request is
sent immediately.

C. 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.

C.1. FCS table generator

The following code creates the lookup table used to calculate the
FCS-16.

/*
* Generate a FCS-16 table.
*
* Drew D. Perkins at Carnegie Mellon University.
*
* Code liberally borrowed from Mohsen Banan and D. Hugh Redelmeier.
*/

/*
* The FCS-16 generator polynomial: x**0 + x**5 + x**12 + x**16.
*/
#define P 0x8408

main()
{
register unsigned int b, v;
register int i;

printf("typedef unsigned short u16;\n");
printf("static u16 fcstab[256] = {");
for (b = 0; ; ) {
if (b % 8 == 0)
printf("\n");

v = b;
for (i = 8; i--; )

v = v & 1 ? (v >> 1) ^ P : v >> 1;

printf("\t0x%04x", v & 0xFFFF);
if (++b == 256)
break;
printf(",");
}
printf("\n};\n");
}

C.2. 16-bit 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 [7], [8], and [9].

/*
* 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.
*/
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 */

/*
* 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 FCS\n");
}

C.3. 32-bit FCS Computation Method

The following code provides a table lookup computation for
calculating the 32-bit Frame Check Sequence as data arrives at the
interface.

/*
* The FCS-32 generator polynomial: x**0 + x**1 + x**2 + x**4 + x**5
* + x**7 + x**8 + x**10 + x**11 + x**12 + x**16
* + x**22 + x**23 + x**26 + x**32.
*/

/*
* u32 represents an unsigned 32-bit number. Adjust the typedef for
* your hardware.
*/
typedef unsigned long u32;

static u32 fcstab_32[256] =
{
0x00000000, 0x77073096, 0xee0e612c, 0x990951ba,
0x076dc419, 0x706af48f, 0xe963a535, 0x9e6495a3,
0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988,
0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91,
0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec,
0x14015c4f, 0x63066cd9, 0xfa0f3d63, 0x8d080df5,
0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172,
0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940,
0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116,
0x21b4f4b5, 0x56b3c423, 0xcfba9599, 0xb8bda50f,
0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d,
0x76dc4190, 0x01db7106, 0x98d220bc, 0xefd5102a,
0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818,
0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,

0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e,
0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457,
0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c,
0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb,
0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0,
0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9,
0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086,
0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4,
0x59b33d17, 0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad,
0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a,
0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683,
0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe,
0xf762575d, 0x806567cb, 0x196c3671, 0x6e6b06e7,
0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc,
0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252,
0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60,
0xdf60efc3, 0xa867df55, 0x316e8eef, 0x4669be79,
0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f,
0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04,
0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a,
0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38,
0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21,
0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e,
0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45,
0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2,
0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db,
0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0,
0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6,
0xbad03605, 0xcdd70693, 0x54de5729, 0x23d967bf,
0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94,
0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d
};

#define PPPINITFCS32 0xffffffff /* Initial FCS value */
#define PPPGOODFCS32 0xdebb20e3 /* Good final FCS value */

/*
* Calculate a new FCS given the current FCS and the new data.
*/
u32 pppfcs32(fcs, cp, len)
register u32 fcs;
register unsigned char *cp;
register int len;
{
ASSERT(sizeof (u32) == 4);
ASSERT(((u32) -1) > 0);
while (len--)
fcs = (((fcs) >> 8) ^ fcstab_32[((fcs) ^ (*cp++)) & 0xff]);

return (fcs);
}

/*
* How to use the fcs
*/
tryfcs32(cp, len)
register unsigned char *cp;
register int len;
{
u32 trialfcs;

/* add on output */
trialfcs = pppfcs32( PPPINITFCS32, cp, len );
trialfcs ^= 0xffffffff; /* complement */
cp[len] = (trialfcs & 0x00ff); /* least significant byte first */
cp[len+1] = ((trialfcs >>= 8) & 0x00ff);
cp[len+2] = ((trialfcs >>= 8) & 0x00ff);
cp[len+3] = ((trialfcs >> 8) & 0x00ff);

/* check on input */
trialfcs = pppfcs32( PPPINITFCS32, cp, len + 4 );
if ( trialfcs == PPPGOODFCS32 )
printf("Good FCS\n");
}

Security Considerations

As noted in the Physical Layer Requirements section, the link layer
might not be informed when the connected state of the physical layer
has 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.

References

[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",
STD 50, RFC1661, Daydreamer, July 1994.

[2] ISO/IEC 3309:1991(E), "Information Technology -
Telecommunications and information exchange between systems -
High-level data link control (HDLC) procedures - Frame
structure", International Organization For Standardization,
Fourth edition 1991-06-01.

[3] ISO/IEC 3309:1991/Amd.2:1992(E), "Information Technology -
Telecommunications and information exchange between systems -
High-level data link control (HDLC) procedures - Frame
structure - Amendment 2: Extended transparency options for
start/stop transmission", International Organization For
Standardization, 1992-01-15.

[4] ISO/IEC 4335:1991(E), "Information Technology -
Telecommunications and information exchange between systems -
High-level data link control (HDLC) procedures - Elements of
procedures", International Organization For Standardization,
Fourth edition 1991-09-15.

[5] Simpson, W., Editor, "PPP LCP Extensions", RFC1570,
Daydreamer, January 1994.

[6] ANSI X3.4-1977, "American National Standard Code for
Information Interchange", American National Standards
Institute, 1977.

[7] Perez, "Byte-wise CRC Calculations", IEEE Micro, June 1983.

[8] Morse, G., "Calculating CRC's by Bits and Bytes", Byte,
September 1986.

[9] LeVan, J., "A Fast CRC", Byte, November 1987.

[10] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
1340, USC/Information Sciences Institute, July 1992.

Acknowledgements

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@merit.edu mailing list.

This specification is based on previous RFCs, where many
contributions have been acknowleged.

The 32-bit FCS example code was provided by Karl Fox (Morning Star
Technologies).

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 93117

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

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