Internet DRAFT - draft-ietf-pppext-framing
draft-ietf-pppext-framing
Network Working Group W A Simpson
Internet Draft Daydreamer
expires in six months July 1993
PPP HDLC Framing
Status of this Memo
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.
Distribution of this memo is unlimited.
This document is an Internet Draft. Internet Drafts are working
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Abstract
The Point-to-Point Protocol (PPP) [1] provides a method for
transmitting multi-protocol datagrams over point-to-point links.
This document describes the use of HDLC for framing PPP encapsulated
datagrams.
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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 should 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:
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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.
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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
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.
Encoding
PPP does not require any particular synchronous encoding, such as
FM, NRZ, or NRZI. The use 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 octet stream, lack of standards for
transmission will hinder interoperability as surely as lack of
data link standards. At speeds of up to 2.0 Mbps, NRZ is
currently most widely available, and on that basis is recommended
as a default.
When some 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 speeds of up to 45 Mbps, some implementors have chosen the ANSI
High Speed Synchronous Interface as the underlying transport.
[Anybody have any restrictions or references?]
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Control Signals
PPP does not require the use of modem 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.
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3. The Data Link Layer
PPP uses the principles, terminology, and frame structure of the
International Organization For Standardization's (ISO) High-level
Data Link Control (HDLC) procedures (ISO 3309-1979 [2]), as modified
by ISO 3309:1984/PDAD1 "Addendum 1: Start/stop transmission" [3].
ISO 3309-1979 specifies the HDLC frame structure for use in
synchronous environments. ISO 3309:1984/PDAD1 specifies proposed
modifications to ISO 3309-1979 to allow its 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]. The PPP frame structure 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 paper attempts to give a concise summary and point out specific
options and features used by PPP. Since "Addendum 1: Start/stop
transmission", is not yet standardized and widely available, it is
summarized in a following section.
To remain consistent with standard Internet practice, and avoid
confusion for people used to reading RFCs, all binary numbers in the
following descriptions are in Most Significant Bit to Least
Significant Bit order, reading from left to right, unless otherwise
indicated. Note that this is contrary to standard ISO and CCITT
practice which orders bits as transmitted (network bit order). Keep
this in mind when comparing this document with the international
standards documents.
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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].
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, and reported through the normal
network management facility.
Control Field
The Control field is a single octet and contains the binary
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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) inserted for transparency. This also does not
include the Flag Sequences or 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 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.
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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.
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4. Bit-synchronous HDLC
This section summarizes the considerations for interoperability of
ISO 3309-1979, as applied in the Point-to-Point Protocol to 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.
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 an LCP Configure-
Request specifying 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 character mapping, since bit-
synchronous equipment uses bit-stuffing rather than character-
stuffing. Instead, all such character mapping will be performed
by the asynchronous-to-synchronous converter.
Aborting a Transmission
A sequence of more than six "1" bits indicates an invalid frame,
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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
time fill. 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.
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5. Asynchronous HDLC
This section summarizes the modifications to ISO 3309-1979 proposed
in ISO 3309:1984/PDAD1, as applied in the Point-to-Point Protocol.
These modifications allow HDLC to be used with 8-bit asynchronous
links.
Transmission Considerations
All octets are transmitted with one start bit, eight bits of data,
and one stop bit. There is no provision in either PPP or ISO
3309:1984/PDAD1 for seven 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).
Transparency
On asynchronous links, a character 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.
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.
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Note: The inclusion of all octets less than hexadecimal 0x20
allows all ASCII control characters [8] excluding DEL (Delete)
to be transparently communicated through almost 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:
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
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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 transmit 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).
Transmitters should avoid this by transmitting an open Flag
Sequence whenever "appreciable time" has elapsed since the prior
closing Flag Sequence. 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. The
maximum value for "appreciable time" is likely to be no greater
than the typing rate of a slow typist, say 1 second.
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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.
/*
* 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,
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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 */
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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");
}
A.2. Fast FCS table generator
The following code creates the lookup table used to calculate the
FCS.
/*
* 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;\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)
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break;
printf(",");
}
printf("\n};\n");
}
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Security Considerations
Security issues are not discussed in this memo.
References
[1] Simpson, W. A., "The Point-to-Point Protocol (PPP)", work in
progress.
[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 3309:1983/PDAD1, "Information
processing systems - Data communication - High-level data link
control procedures - Frame structure - Addendum 1: Start/stop
transmission", 1984.
[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.
Simpson expires in six months [Page 18]
DRAFT PPP Framing July 1993
Acknowledgments
This specification is based on previous RFCs, where many
contributions have been acknowleged.
Additional implementation detail for this version was provided by
Fred Baker (ACC), and Craig Fox (NSC).
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
Author's Address
Questions about this memo can also be directed to:
William Allen Simpson
Daydreamer
Computer Systems Consulting Services
P O Box 6205
East Lansing, MI 48826-6205
EMail: Bill.Simpson@um.cc.umich.edu
Simpson expires in six months [Page 19]
DRAFT PPP Framing July 1993
Table of Contents
1. Introduction .......................................... 1
1.1 Specification of Requirements ................... 1
1.2 Terminology ..................................... 1
2. Physical Layer Requirements ........................... 3
3. The Data Link Layer ................................... 5
3.1 Frame Format .................................... 6
3.2 Modification of the Basic Frame ................. 8
4. Bit-synchronous HDLC .................................. 9
5. Asynchronous HDLC ..................................... 11
APPENDICES ................................................... 14
A. Fast Frame Check Sequence (FCS) Implementation ........ 14
A.1 FCS Computation Method .......................... 14
A.2 Fast FCS table generator ........................ 16
SECURITY CONSIDERATIONS ...................................... 18
REFERENCES ................................................... 18
ACKNOWLEDGEMENTS ............................................. 19
CHAIR'S ADDRESS .............................................. 19
AUTHOR'S ADDRESS ............................................. 19
