rfc1549.PPP in HDLC Framing
Network Working Group W. Simpson, Editor Request for Comments: 1549 Daydreamer Category: Standards Track December 1993 PPP in HDLC Framing
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited. Abstract
The Point-to-Point Protocol (PPP) [1] provides a standard method for
transporting multi-protocol datagrams over point-to-point links.
This document describes the use of HDLC for framing PPP encapsulated
packets. This document is the product of the Point-to-Point Protocol
Working Group of the Internet Engineering Task Force (IETF).
Comments should be submitted to the ietf-ppp@https://www.wendangku.net/doc/1818612996.html, mailing
list.
Table of Contents
1. Introduction (2)
1.1 Specification of Requirements (2)
1.2 Terminology (3)
2. Physical Layer Requirements (3)
3. The Data Link Layer (4)
3.1 Frame Format (5)
3.2 Modification of the Basic Frame (7)
4. Asynchronous HDLC (7)
5. Bit-synchronous HDLC (5)
6. Octet-synchronous HDLC (12)
APPENDIX A. Fast Frame Check Sequence (FCS) Implementation (13)
A.1 FCS Computation Method (13)
A.2 Fast FCS table generator (15)
SECURITY CONSIDERATIONS (16)
REFERENCES (17)
ACKNOWLEDGEMENTS (17)
CHAIR’S ADDRESS (18)
EDITOR’S ADDRESS (18)
Simpson [Page 1]
1. Introduction
This specification provides for framing over both bit-oriented and
octet-oriented synchronous links, and asynchronous links with 8 bits of data and no parity. These links MUST be full-duplex, but MAY be
either dedicated or circuit-switched. PPP uses HDLC as a basis for
the framing.
An escape mechanism is specified to allow control data such as
XON/XOFF to be transmitted transparently over the link, and to remove spurious control data which may be injected into the link by
intervening hardware and software.
Some protocols expect error free transmission, and either provide
error detection only on a conditional basis, or do not provide it at all. PPP uses the HDLC Frame Check Sequence for error detection.
This is commonly available in hardware implementations, and a
software implementation is provided.
1.1 Specification of Requirements
In this document, several words are used to signify the requirements of the specification. These words are often capitalized.
MUST
This word, or the adjective "required", means that the definition is an absolute requirement of the specification.
MUST NOT
This phrase means that the definition is an absolute prohibition
of the specification.
SHOULD
This word, or the adjective "recommended", means that there may
exist valid reasons in particular circumstances to ignore this
item, but the full implications must be understood and carefully
weighed before choosing a different course.
MAY
This word, or the adjective "optional", means that this item is
one of an allowed set of alternatives. An implementation which
does not include this option MUST be prepared to interoperate with another implementation which does include the option.
Simpson [Page 2]
1.2 Terminology
This document frequently uses the following terms:
datagram
The unit of transmission in the network layer (such as IP). A
datagram may be encapsulated in one or more packets passed to the data link layer.
frame
The unit of transmission at the data link layer. A frame may
include a header and/or a trailer, along with some number of units of data.
packet
The basic unit of encapsulation, which is passed across the
interface between the network layer and the data link layer. A
packet is usually mapped to a frame; the exceptions are when data link layer fragmentation is being performed, or when multiple
packets are incorporated into a single frame.
peer
The other end of the point-to-point link.
silently discard
This means the implementation discards the packet without further processing. The implementation SHOULD provide the capability of
logging the error, including the contents of the silently
discarded packet, and SHOULD record the event in a statistics
counter.
2. Physical Layer Requirements
PPP is capable of operating across most DTE/DCE interfaces (such as, EIA RS-232-C, EIA RS-422, EIA RS-423 and CCITT V.35). The only
absolute requirement imposed by PPP is the provision of a full-duplex circuit, either dedicated or circuit-switched, which can operate in
either an asynchronous (start/stop), bit-synchronous, or octet-
synchronous mode, transparent to PPP Data Link Layer frames.
Interface Format
PPP presents an octet interface to the physical layer. There is Simpson [Page 3]
no provision for sub-octets to be supplied or accepted.
PPP does not impose any restrictions regarding transmission rate,
other than that of the particular DTE/DCE interface.
Control Signals
PPP does not require the use of control signals, such as Request
To Send (RTS), Clear To Send (CTS), Data Carrier Detect (DCD), and Data Terminal Ready (DTR).
When available, using such signals can allow greater functionality and performance. In particular, such signals SHOULD be used to
signal the Up and Down events in the LCP Option Negotiation
Automaton [1]. When such signals are not available, the
implementation MUST signal the Up event to LCP upon
initialization, and SHOULD NOT signal the Down event.
Because signalling is not required, the physical layer MAY be
decoupled from the data link layer, hiding the transient details
of the physical transport. This has implications for mobility in cellular radio networks, and other rapidly switching links.
When moving from cell to cell within the same zone, an
implementation MAY choose to treat the entire zone as a single
link, even though transmission is switched among several
frequencies. The link is considered to be with the central
control unit for the zone, rather than the individual cell
transceivers. However, the link SHOULD re-establish its
configuration whenever the link is switched to a different
administration.
Due to the bursty nature of data traffic, some implementations
have choosen to disconnect the physical layer during periods of
inactivity, and reconnect when traffic resumes, without informing the data link layer. Robust implementations should avoid using
this trick over-zealously, since the price for decreased setup
latency is decreased security. Implementations SHOULD signal the Down event whenever "significant time" has elapsed since the link was disconnected. The value for "significant time" is a matter of considerable debate, and is based on the tariffs, call setup
times, and security concerns of the installation.
3. The Data Link Layer
PPP uses the principles, terminology, and frame structure of the
International Organization For Standardization’s (ISO) 3309-1979 Simpson [Page 4]
High-level Data Link Control (HDLC) frame structure [2], as modified by "Addendum 1: Start/stop transmission" [3], which specifies
modifications to allow HDLC use in asynchronous environments.
The PPP control procedures use the definitions and Control field
encodings standardized in ISO 4335-1979 [4] and ISO 4335-
1979/Addendum 1-1979 [5]. PPP framing is also consistent with CCITT Recommendation X.25 LAPB [6], and CCITT Recommendation Q.922 [7],
since those are also based on HDLC.
The purpose of this specification is not to document what is already standardized in ISO 3309. It is assumed that the reader is already
familiar with HDLC, or has access to a copy of [2] or [6]. Instead, this document attempts to give a concise summary and point out
specific options and features used by PPP.
To remain consistent with standard Internet practice, and avoid
confusion for people used to reading RFCs, all binary numbers in the following descriptions are in Most Significant Bit to Least
Significant Bit order, reading from left to right, unless otherwise
indicated. Note that this is contrary to standard ISO and CCITT
practice which orders bits as transmitted (network bit order). Keep this in mind when comparing this document with the international
standards documents.
3.1 Frame Format
A summary of the PPP HDLC frame structure is shown below. This
figure does not include start/stop bits (for asynchronous links), nor any bits or octets inserted for transparency. The fields are
transmitted from left to right.
+----------+----------+----------+
| Flag | Address | Control |
| 01111110 | 11111111 | 00000011 |
+----------+----------+----------+
+----------+-------------+---------+
| Protocol | Information | Padding |
| 16 bits | * | * |
+----------+-------------+---------+
+----------+----------+------------------+
| FCS | Flag | Inter-frame Fill |
| 16 bits | 01111110 | or next Address |
+----------+----------+------------------+
The Protocol, Information and Padding fields are described in the
Point-to-Point Protocol Encapsulation [1].
Simpson [Page 5]
Flag Sequence
The Flag Sequence indicates the beginning or end of a frame, and
always consists of the binary sequence 01111110 (hexadecimal
0x7e).
The Flag Sequence is a frame separator. Only one Flag Sequence is required between two frames. Two consecutive Flag Sequences
constitute an empty frame, which is ignored, and not counted as a FCS error.
Address Field
The Address field is a single octet and contains the binary
sequence 11111111 (hexadecimal 0xff), the All-Stations address.
PPP does not assign individual station addresses. The All-
Stations address MUST always be recognized and received. The use of other address lengths and values may be defined at a later
time, or by prior agreement. Frames with unrecognized Addresses
SHOULD be silently discarded.
Control Field
The Control field is a single octet and contains the binary
sequence 00000011 (hexadecimal 0x03), the Unnumbered Information
(UI) command with the P/F bit set to zero. The use of other
Control field values may be defined at a later time, or by prior
agreement. Frames with unrecognized Control field values SHOULD
be silently discarded.
Frame Check Sequence (FCS) Field
The Frame Check Sequence field is normally 16 bits (two octets).
The use of other FCS lengths may be defined at a later time, or by prior agreement. The FCS is transmitted with the coefficient of
the highest term first.
The FCS field is calculated over all bits of the Address, Control, Protocol, Information and Padding fields, not including any start and stop bits (asynchronous) nor any bits (synchronous) or octets (asynchronous or synchronous) inserted for transparency. This
also does not include the Flag Sequences nor the FCS field itself. Note: When octets are received which are flagged in the Async- Control-Character-Map, they are discarded before calculating
the FCS.
For more information on the specification of the FCS, see ISO Simpson [Page 6]
3309 [2] or CCITT X.25 [6].
The end of the Information and Padding fields is found by locating
the closing Flag Sequence and removing the Frame Check Sequence
field.
3.2. Modification of the Basic Frame
The Link Control Protocol can negotiate modifications to the basic
HDLC frame structure. However, modified frames will always be
clearly distinguishable from standard frames.
Address-and-Control-Field-Compression
When using the default HDLC framing, the Address and Control
fields contain the hexadecimal values 0xff and 0x03 respectively. On transmission, compressed Address and Control fields are formed by simply omitting them.
On reception, the Address and Control fields are decompressed by
examining the first two octets. If they contain the values 0xff
and 0x03, they are assumed to be the Address and Control fields.
If not, it is assumed that the fields were compressed and were not transmitted.
By definition, the first octet of a two octet Protocol field will never be 0xff (since it is not even). The Protocol field value
0x00ff is not allowed (reserved) to avoid ambiguity when
Protocol-Field-Compression is enabled and the first Information
field octet is 0x03.
When other Address or Control field values are in use, Address-
and-Control-Field-Compression MUST NOT be negotiated.
4. Asynchronous HDLC
This section summarizes the use of HDLC with 8-bit asynchronous
links.
Flag Sequence
The Flag Sequence indicates the beginning or end of a frame. The octet stream is examined on an octet-by-octet basis for the value 01111110 (hexadecimal 0x7e).
Simpson [Page 7]
Transparency
An octet stuffing procedure is used. The Control Escape octet is defined as binary 01111101 (hexadecimal 0x7d) where the bit
positions are numbered 87654321 (not 76543210, BEWARE).
Each end of the link maintains two Async-Control-Character-Maps.
The receiving ACCM is 32 bits, but the sending ACCM may be up to
256 bits. This results in four distinct ACCMs, two in each
direction of the link.
The default receiving ACCM is 0xffffffff. The default sending
ACCM is 0xffffffff, plus the Control Escape and Flag Sequence
characters themselves, plus whatever other outgoing characters are known to be intercepted.
After FCS computation, the transmitter examines the entire frame
between the two Flag Sequences. Each Flag Sequence, Control
Escape octet, and octet with value less than hexadecimal 0x20
which is flagged in the sending Async-Control-Character-Map, is
replaced by a two octet sequence consisting of the Control Escape octet and the original octet with bit 6 complemented (exclusive-
or’d with hexadecimal 0x20).
Prior to FCS computation, the receiver examines the entire frame
between the two Flag Sequences. Each octet with value less than
hexadecimal 0x20 is checked. If it is flagged in the receiving
Async-Control-Character-Map, it is simply removed (it may have
been inserted by intervening data communications equipment). For each Control Escape octet, that octet is also removed, but bit 6
of the following octet is complemented, unless it is the Flag
Sequence.
Note: The inclusion of all octets less than hexadecimal 0x20
allows all ASCII control characters [8] excluding DEL (Delete) to be transparently communicated through all known data
communications equipment.
The transmitter may also send octets with value in the range 0x40 through 0xff (except 0x5e) in Control Escape format. Since these octet values are not negotiable, this does not solve the problem
of receivers which cannot handle all non-control characters.
Also, since the technique does not affect the 8th bit, this does
not solve problems for communications links that can send only 7- bit characters.
A few examples may make this more clear. Packet data is
transmitted on the link as follows:
Simpson [Page 8]
0x7e is encoded as 0x7d, 0x5e. 0x7d is encoded as 0x7d, 0x5d. 0x01 is encoded as 0x7d, 0x21.
Some modems with software flow control may intercept outgoing DC1 and DC3 ignoring the 8th (parity) bit. This data would be
transmitted on the link as follows:
0x11 is encoded as 0x7d, 0x31. 0x13 is encoded as 0x7d, 0x33. 0x91 is encoded as 0x7d, 0xb1. 0x93 is encoded as 0x7d, 0xb3. Aborting a Transmission
On asynchronous links, frames may be aborted by transmitting a "0" stop bit where a "1" bit is expected (framing error) or by
transmitting a Control Escape octet followed immediately by a
closing Flag Sequence.
Time Fill
For asynchronous links, inter-octet and inter-frame time fill MUST be accomplished by transmitting continuous "1" bits (mark-hold
state).
Inter-frame time fill can be viewed as extended inter-octet time
fill. Doing so can save one octet for every frame, decreasing
delay and increasing bandwidth. This is possible since a Flag
Sequence may serve as both a frame close and a frame begin. After having received any frame, an idle receiver will always be in a
frame begin state.
Robust transmitters should avoid using this trick over-zealously, since the price for decreased delay is decreased reliability.
Noisy links may cause the receiver to receive garbage characters
and interpret them as part of an incoming frame. If the
transmitter does not send a new opening Flag Sequence before
sending the next frame, then that frame will be appended to the
noise characters causing an invalid frame (with high reliability). It is suggested that implementations will achieve the best results by always sending an opening Flag Sequence if the new frame is not back-to-back with the last. Transmitters SHOULD send an open Flag Sequence whenever "appreciable time" has elapsed after the prior
closing Flag Sequence. The maximum value for "appreciable time"
is likely to be no greater than the typing rate of a slow typist, say 1 second.
Encoding
All octets are transmitted with one start bit, eight bits of data, Simpson [Page 9]
and one stop bit. There is no provision for seven bit
asynchronous links.
5. Bit-synchronous HDLC
This section summarizes the use of HDLC with bit-synchronous links.
Flag Sequence
The Flag Sequence indicates the beginning or end of a frame, and
is used for frame synchronization. The bit stream is examined on a bit-by-bit basis for the binary sequence 01111110 (hexadecimal
0x7e).
The "shared zero mode" Flag Sequence "011111101111110" SHOULD NOT be used. When not avoidable, such an implementation MUST ensure
that the first Flag Sequence detected (the end of the frame) is
promptly communicated to the link layer. Use of the shared zero
mode hinders interoperability with synchronous-to-asynchronous
converters.
Transparency
The transmitter examines the entire frame between the two Flag
Sequences. A "0" bit is inserted after all sequences of five
contiguous "1" bits (including the last 5 bits of the FCS) to
ensure that a Flag Sequence is not simulated.
When receiving, any "0" bit that directly follows five contiguous "1" bits is discarded.
Since the Control Escape octet-stuffing method is not used, the
default receiving and sending Async-Control-Character-Maps are 0. There may be some use of synchronous-to-asynchronous converters
(some built into modems) in point-to-point links resulting in a
synchronous PPP implementation on one end of a link and an
asynchronous implementation on the other. It is the
responsibility of the converter to do all mapping conversions
during operation.
To enable this functionality, bit-synchronous PPP implementations MUST always respond to the Async-Control-Character-Map
Configuration Option with an LCP Configure-Ack. However,
acceptance of the Configuration Option does not imply that the
bit-synchronous implementation will do any octet mapping.
Instead, all such octet mapping will be performed by the
asynchronous-to-synchronous converter.
Simpson [Page 10]
Aborting a Transmission
A sequence of more than six "1" bits indicates an invalid frame,
which is ignored, and not counted as a FCS error.
Inter-frame Time Fill
For bit-synchronous links, the Flag Sequence SHOULD be transmitted during inter-frame time fill. There is no provision for inter-
octet time fill.
Mark idle (continuous ones) SHOULD NOT be used for inter-frame
ill. However, certain types of circuit-switched links require the use of mark idle, particularly those that calculate accounting
based on periods of bit activity. When mark idle is used on a
bit-synchronous link, the implementation MUST ensure at least 15
consecutive "1" bits between Flags during the idle period, and
that the Flag Sequence is always generated at the beginning of a
frame after an idle period.
Encoding
The definition of various encodings and scrambling is the
responsibility of the DTE/DCE equipment in use, and is outside the scope of this specification.
While PPP will operate without regard to the underlying
representation of the bit stream, lack of standards for
transmission will hinder interoperability as surely as lack of
data link standards. At speeds of 56 Kbps through 2.0 Mbps, NRZ
is currently most widely available, and on that basis is
recommended as a default.
When configuration of the encoding is allowed, NRZI is recommended as an alternative, because of its relative immunity to signal
inversion configuration errors, and instances when it MAY allow
connection without an expensive DSU/CSU. Unfortunately, NRZI
encoding obviates the (1 + x) factor of the 16-bit FCS, so that
one error in 2**15 goes undetected (instead of one in 2**16), and triple errors are not detected. Therefore, when NRZI is in use,
it is recommended that the 32-bit FCS be negotiated, which does
not include the (1 + x) factor.
At higher speeds of up to 45 Mbps, some implementors have chosen
the ANSI High Speed Synchronous Interface [HSSI]. While this
experience is currently limited, implementors are encouraged to
cooperate in choosing transmission encoding.
Simpson [Page 11]
6. Octet-synchronous HDLC
This section summarizes the use of HDLC with octet-synchronous links, such as SONET and optionally ISDN B or H channels.
Although the bit rate is synchronous, there is no bit-stuffing.
Instead, the octet-stuffing feature of 8-bit asynchronous HDLC is
used.
Flag Sequence
The Flag Sequence indicates the beginning or end of a frame. The octet stream is examined on an octet-by-octet basis for the value 01111110 (hexadecimal 0x7e).
Transparency
An octet stuffing procedure is used. The Control Escape octet is defined as binary 01111101 (hexadecimal 0x7d).
The octet stuffing procedure is described in "Asynchronous HDLC"
above.
The sending and receiving implementations need escape only the
Flag Sequence and Control Escape octets.
Considerations concerning the use of converters are described in
"Bit-synchronous HDLC" above.
Aborting a Transmission
Frames may be aborted by transmitting a Control Escape octet
followed immediately by a closing Flag Sequence. The preceding
frame is ignored, and not counted as a FCS error.
Inter-frame Time Fill
The Flag Sequence MUST be transmitted during inter-frame time
fill. There is no provision for inter-octet time fill.
Encoding
The definition of various encodings and scrambling is the
responsibility of the DTE/DCE equipment in use, and is outside the scope of this specification.
Simpson [Page 12]
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.
Simpson [Page 13]
/*
* u16 represents an unsigned 16-bit number. Adjust the typedef for
* your hardware.
*/
typedef unsigned short u16;
/*
* FCS lookup table as calculated by the table generator in section B.2 */
static u16 fcstab[256] = {
0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,
0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,
0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,
0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,
0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,
0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,
0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,
0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,
0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,
0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,
0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,
0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,
0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,
0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,
0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,
0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,
0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,
0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,
0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,
0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,
0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,
0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,
0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,
0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78
};
#define PPPINITFCS16 0xffff /* Initial FCS value */
#define PPPGOODFCS16 0xf0b8 /* Good final FCS value */
/*
Simpson [Page 14]
* Calculate a new fcs given the current fcs and the new data.
*/
u16 pppfcs16(fcs, cp, len)
register u16 fcs;
register unsigned char *cp;
register int len;
{
ASSERT(sizeof (u16) == 2);
ASSERT(((u16) -1) > 0);
while (len--)
fcs = (fcs >> 8) ^ fcstab[(fcs ^ *cp++) & 0xff];
return (fcs);
}
/*
* How to use the fcs
*/
tryfcs16(cp, len)
register unsigned char *cp;
register int len;
{
u16 trialfcs;
/* add on output */
trialfcs = pppfcs16( PPPINITFCS16, cp, len );
trialfcs ^= 0xffff; /* complement */
cp[len] = (trialfcs & 0x00ff); /* least significant byte first */
cp[len+1] = ((trialfcs >> 8) & 0x00ff);
/* check on input */
trialfcs = pppfcs16( PPPINITFCS16, cp, len + 2 );
if ( trialfcs == PPPGOODFCS16 )
printf("Good FCS0);
}
A.2. Fast FCS table generator
The following code creates the lookup table used to calculate the FCS. Simpson [Page 15]
/*
* Generate a FCS table for the HDLC FCS.
*
* Drew D. Perkins at Carnegie Mellon University.
*
* Code liberally borrowed from Mohsen Banan and D. Hugh Redelmeier.
*/
/*
* The HDLC polynomial: x**0 + x**5 + x**12 + x**16 (0x8408).
*/
#define P 0x8408
main()
{
register unsigned int b, v;
register int i;
printf("typedef unsigned short u16;0);
printf("static u16 fcstab[256] = {");
for (b = 0; ; ) {
if (b % 8 == 0)
printf("0);
v = b;
for (i = 8; i--; )
v = v & 1 ? (v >> 1) ^ P : v >> 1;
printf("0x%04x", v & 0xFFFF);
if (++b == 256)
break;
printf(",");
}
printf("0;0);
}
Security Considerations
As noted in the Physical Layer Requirements section, the link layer
might not be informed when the connected state of physical layer is
changed. This results in possible security lapses due to over-
reliance on the integrity and security of switching systems and
administrations. An insertion attack might be undetected. An
attacker which is able to spoof the same calling identity might be
able to avoid link authentication.
Simpson [Page 16]
References
[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",
RFC 1548, December 1993
[2] International Organization For Standardization, ISO Standard
3309-1979, "Data communication - High-level data link control
procedures - Frame structure", 1979.
[3] International Organization For Standardization, Proposed Draft
International Standard ISO 3309-1991/PDAD1, "Information
processing systems - Data communication - High-level data link
control procedures - Frame structure - Addendum 1: Start/stop
transmission", 1991.
[4] International Organization For Standardization, ISO Standard
4335-1979, "Data communication - High-level data link control
procedures - Elements of procedures", 1979.
[5] International Organization For Standardization, ISO Standard
4335-1979/Addendum 1, "Data communication - High-level data
link control procedures - Elements of procedures - Addendum 1", 1979.
[6] International Telecommunication Union, CCITT Recommendation
X.25, "Interface Between Data Terminal Equipment (DTE) and Data Circuit Terminating Equipment (DCE) for Terminals Operating in
the Packet Mode on Public Data Networks", CCITT Red Book,
Volume VIII, Fascicle VIII.3, Rec. X.25., October 1984.
[7] International Telegraph and Telephone Consultative Committee,
CCITT Recommendation Q.922, "ISDN Data Link Layer Specification for Frame Mode Bearer Services", April 1991.
[8] American National Standards Institute, ANSI X3.4-1977,
"American National Standard Code for Information Interchange",
1977.
[9] Perez, "Byte-wise CRC Calculations", IEEE Micro, June, 1983.
[10] Morse, G., "Calculating CRC’s by Bits and Bytes", Byte,
September 1986.
[11] LeVan, J., "A Fast CRC", Byte, November 1987.
Acknowledgments
This specification is based on previous RFCs, where many
Simpson [Page 17]
contributions have been acknowleged.
Additional implementation detail for this version was provided by
Fred Baker (ACC), Craig Fox (NSC), and Phil Karn (Qualcomm).
Special thanks to Morning Star Technologies for providing computing
resources and network access support for writing this specification. Chair’s Address
The working group can be contacted via the current chair:
Fred Baker
Advanced Computer Communications
315 Bollay Drive
Santa Barbara, California, 93111
EMail: fbaker@https://www.wendangku.net/doc/1818612996.html,
Editor’s Address
Questions about this memo can also be directed to:
William Allen Simpson
Daydreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
EMail: Bill.Simpson@https://www.wendangku.net/doc/1818612996.html,
Simpson [Page 18]
常用网络通信协议简介
常用网络通信协议简介 常用网络通信协议 物理层: DTE(Data Terminal Equipment):数据终端设备 DCE(Data Communications Equipment):数据电路端接设备 #窄宽接入: PSTN ( Public Switched Telephone Network )公共交换电话网络 ISDN(Integrated Services Digital Network)ISDN综合业务数字网 ISDN有6种信道: A信道 4khz模拟信道 B信道 64kbps用于语音数据、调整数据、数字传真 C信道 8kbps/16kbps的数字信道，用于传输低速数据 D信道 16kbps数字信道，用于传输用户接入信令 E信道 64kbps数字信道，用于传输内部信令 H信道 384kbps高速数据传输数字信道，用于图像、视频会议、快速传真等. B代表承载， D代表Delta. ISDN有3种标准化接入速率: 基本速率接口(BRI)由2个B信道，每个带宽64kbps和一个带宽16kbps的D信道组成。三个信道设计成2B+D。 主速率接口(PRI) - 由很多的B信道和一个带宽64Kbps的D信道组成，B信道的数量取决于不同的国家: 北美和日本: 23B+1D, 总位速率1.544 Mbit/s (T1) 欧洲，澳大利亚:30B+2D,总位速率2.048 Mbit/s (E1) FR(Frame Relay)帧中继
X.25 X.25网络是第一个面向连接的网络,也是第一个公共数据网络. #宽带接入: ADSL:(Asymmetric Digital Subscriber Line)非对称数字用户环路 HFC(Hybrid Fiber,Coaxial)光纤和同轴电缆相结合的混合网络 PLC:电力线通信技术 #传输网: SDH:(Synchronous Digital Hierarchy)同步数字体系 DWDM:密集型光波复用(DWDM:Dense Wavelength Division Multiplexing)是能组合一组光波长用一根光纤进行传送。这是一项用来在现有的光纤骨干网上提高带宽的激光技术。更确切地说，该技术是在一根指定的光纤中，多路复用单个光纤载波的紧密光谱间距，以便利用可以达到的传输性能(例如，达到最小程度的色散或者衰减)。 #无线/卫星: LMDS:(Local Multipoint Distribution Services)作区域多点传输服务。这是一种微波的宽带业务，工作在28GHz附近频段，在较近的距离双向传输话音、数据和图像等信息。 GPRS:(General Packet Radio Service)通用分组无线服务技术。 3G:(3rd-generation，3G)第三代移动通信技术 DBS:(Direct Broadcasting Satellite Service)直播卫星业务 VAST: 协议:RS-232、RS-449、X.21、V.35、ISDN、FDDI、IEEE802.3、IEEE802.4、IEEE802.5等。 RS-232:是个人计算机上的通讯接口之一，由电子工业协会(Electronic Industries
HDLC协议控制器的设计
目录 内容摘要 (1) 关键词 (1) Abｓｔracｔ (1) Keｙ Wｏrds (1) １.绪论……………………………………………………………………２ 1.１研究的意义 (2) 1.2本设计的主要功能………………………………………………２ 2．HDLC协议综述 (3) 2.１ HＤＬC协议的产生背景 (3) ２.2 HDLC协议的帧结构 (4) 2．3 HDＬC协议的规程分析 (7) 3.HDLC协议控制器的设计………………………………………………８ 3．1 HDLC协议控制器设计方案选择…………………………………８ ３.２ FPGＡ的设计原则 (9) 3.3 ＨDＬC协议控制器总框架………………………………………1０ 3.4 HDLC帧发送器的设计 (11) 3．5 HDＬＣ帧接收器的设计 (1) ５ 参考文献…………………………………………………………………１８ 致谢 (19) ［说明：在本页中,“目录”二字居中,宋体小二号，加黑， 其它统一由宋体小四号,不加黑排版打印、行间距为1.5］
内容摘要：HＤＬＣ（高级数据链路控制）协议是一种面向比特的链路控制规程，广泛的用作数据链路层的控制协议。论文在分析和研究HＤＬＣ协议的基础上,提出了一种基于ＦPGＡ（现场可编程门阵列)的HDLC协议控制器的设计。对HＤＬC协议控制器的功能进行划分，分别设计了标志位的检测和生成、插零和删零、FＣＳ的校验等控制模块。 采用VHＤL硬件描述语言在FPGA内部实现HDLC协议的各功能模块,本设计使用QuａrｔuｓII 9．1平台实现代码编写、综合、编译、仿真。对HDLC链路控制规程功能,帧控制和FCS校验功能进行了仿真实现。 关键词:HDLＣ；FPGA；帧收发器; Abstract：（宋体,小四号，加黑）××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××。(宋体,小四号，不加黑) Ｋｅy words:（宋体，小四号，加黑）×××□□×××□□×××□□(宋体,小四号,不加黑)
鸿鹄论坛_单选1-50
1.Refer to the exhibit. What will Router1 do when it receives the data frame shown? (Choose three.) 参考图示。位于会怎么做当它接收到数据帧显示吗?(选择三个)。 2.Refer to the exhibit. Which three statements correctly describe Network Device A? (Choose three.) 参考图示。这三个报表正确描述网络设备?(选择三 3.Which layer in the OSI reference model is responsible for determining the availability of the receiving program and checking to see if enough resources exist for that communication? 在OSI参考模型层负责确定接收的可用性 程序和检查是否足够的资源存在的沟通吗? 4.Which of the following describes the roles of devices in a WAN? (Choose three. 下列哪个描述的角色在广域网设备?(选择三个。 5.Refer to the exhibit, Host A pings interface S0/0 on router 3, what is the TTL value for that ping? 参考图示,主机ping路由器界面S0/0 3,萍的TTL值是什么? 6.A network administrator is verifying the configuration of a newly installed host by establishing an FTP connection to a remote server. What is the highest layer of the protocol stack that the nework administrator is using for this operation? 网络管理员验证的配置是一个新安装的主机通过建立FTP 连接到远程服务器。什么是最高的层的协议栈nework管理员吗 使用这个操作吗? 7.Refer to the exhibit. After HostA pings HostB, which entry will be in the ARP cache of HostA to support this transmission? 参考图示。玉簪属草本植物ping HostB之后,ARP缓存的条目将玉簪属草本植物,以支持这一点传播? 8.A network interface port has collision detection and carrier sensing enabled on a shared twisted pair network. From this statement, what is known about the network interface port? 网络接口端口碰撞检测和传感载体上启用共享双绞线网络。从这句话,知道网络接口的端口是什么? 9.A receiving host computes the checksum on a frame and determines that the frame is damaged.The frame
网络协议大全
网络协议大全 在网络的各层中存在着许多协议，它是定义通过网络进行通信的规则，接收方的发送方同层的协议必须一致，否则一方将无法识别另一方发出的信息，以这种规则规定双方完成信息在计算机之间的传送过程。下面就对网络协议规范作个概述。 ARP（Address Resolution Protocol）地址解析协议 它是用于映射计算机的物理地址和临时指定的网络地址。启动时它选择一个协议（网络层）地址，并检查这个地址是否已经有别的计算机使用，如果没有被使用，此结点被使用这个地址，如果此地址已经被别的计算机使用，正在使用此地址的计算机会通告这一信息，只有再选另一个地址了。 SNMP（Simple Network Management P）网络管理协议 它是TCP/IP协议中的一部份，它为本地和远端的网络设备管理提供了一个标准化途径，是分布式环境中的集中化管理的重要组成部份。 AppleShare protocol（AppleShare协议） 它是Apple机上的通信协议，它允许计算机从服务器上请求服务或者和服务器交换文件。AppleShare可以在TCP/IP协议或其它网络协议如IPX、AppleTalk上进行工作。使用它时，用户可以访问文件，应用程序，打印机和其它远程服务器上的资源。它可以和配置了AppleShare协议的任何服务器进行通信，Macintosh、Mac OS、Windows NT和Novell Netware都支持AppleShare协议。 AppleTalk协议 它是Macintosh计算机使用的主要网络协议。Windows NT服务器有专门为Macintosh服务，也能支持该协议。其允许Macintosh的用户共享存储在Windows NT文件夹的Mac-格式的文件，也可以使用和Windows NT连接的打印机。Windows NT共享文件夹以传统的Mac文件夹形式出现在Mac用户面前。Mac 文件名按需要被转换为FAT（8.3）格式和NTFS文件标准。支持MAc文件格式的DOS和Windows客户端能与Mac用户共享这些文件。 BGP4（Border Gateway Protocol Vertion 4）边界网关协议-版本4 它是用于在自治网络中网关主机（每个主机有自己的路由）之间交换路由信息的协议，它使管理
【网络工程】常见网络端口和网络协议
【网络工程】常见网络端口和网络协议 下面介绍的这些端口差不多上服务器默认的端口,因此认识这些服务器端口对我们学习，和故障排错时专门有关心的。 下面列出了这些服务所对应的端口。 ftp-data20/tcp#FTP, data ftp21/tcp#FTP. control smtp25/tcp mail#Simple Mail Tran sfer Protocol pop3110/tcp#Post Office Proto col - Version 3 domain53/udp#Domain Name Server tftp69/udp#Trivial File Trans fer http80/tcp www www-http#World Wide We b https443/tcp ms-sql-s1433/tcp#Microsoft-SQL-Ser ver ms-sql-m1434/udp#Microsoft-SQL-Monitor 终端服务3389/tcp [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Termi nal Server\Wds\rdpwd\Tds\tcp]下的PortNumber键值 同时还要修改 [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Termi nal Server\WinStations\RDP-Tcp]下的PortNumber键值
服务器端口数最大能够有65535个，然而实际上常用的端口才几十个，由此能够看出未定义的端口相当多。 从端口的性质来分，通常能够分为以下三类 （2）注册端口（Registered Ports）：端口号从1024到49151。它们松散地绑定于一些服务。也是讲有许多服务绑定于这些端口，这些端口同样用于许多其他目的。这些端口多数没有明确的定义服务对象，不同程序可按照实际需要自己定义，如后面要介绍的远程操纵软件和木马程序中都会有这些端口的定义的。记住这些常见的程序端口在木马程序的防护和查杀上是专门有必要的 （3）动态和/或私有端口（Dynamic and/or Private Ports）：端口号从49152到65535。理论上，不应为服务分配这些端口。实际上，有些较为专门的程序，专门是一些木马程序就专门喜爱用这些端口，因为这些端口常常不被引起注意，容易隐藏。 如果按照所提供的服务方式的不同，端口又可分为“TCP协议端口”和“UDP协议端口”两种。因为运算机之间相互通信一样采纳这两种通信协议。前面所介绍的“连接方式”是一种直截了当与接收方进行的连接，发送信息以后，能够确认信息是否到达，这种方式大多采纳TCP协议；另一种是不是直截了当与接收方进行连接，只管把信息放在网上发出去，而不管信息是否到达，也确实是前面所介绍的“无连接方式”。这种方式大多采纳UDP协议，IP协议也是一种无连接方式。对应使用以上这两种通信协议的服务所提供的端口，也就分为“TCP协议端口”和“UDP协议端口”。 另外还有些常见的端口： HTTP协议代理服务器常用端口号：80/8080/3128/8081/1080
CCNA 5.0路由和交换：网络简介 第5章 以太网(练习和答案)
第5章以太网 1下列有关MAC 地址的说法中哪一项正确？ MAC 地址由软件实施。 如果连接到WAN，网卡只需要MAC 地址。 前三个字节用于供应商分配的 OUI。 ISO负责MAC地址规范。 一个MAC地址包含6个字节。前3个字节用于供应商标识，最后3个字节必须在同一OUI中分配唯一的值。MAC地址在硬件中实施。网卡需要使用MAC地址来通过LAN通信。IEEE规范了MAC地址。 答案说明最高分值 correctness of response 2points for Option3 0points for any other option 2 2 下列哪项是争用访问方法的特征？ 它比受控访问方法处理更多开销。 它具有跟踪介质访问次序的机制。
它属于非确定性方法。 它在介质使用率高的情况下能够轻松扩展。 争用方法具有不确定性，没有受控访问方法中遇到的开销。因为设备不需要轮流访问介质，所以无需跟踪次序。争用方法在介质使用率高的情况下无法很好地扩展。 答案说明最高分值 correctness of response 2points for Option3 0points for any other option 2 3 下列哪两种说法正确描述了以太网标准中逻辑链路控制子层的特点或功能？（请选择两项。） 逻辑链路控制在软件中实现。 逻辑链路控制在IEEE802.3 标准中指定。 LLC 子层直接与网卡驱动程序软件进行交互。 数据链路层使用LLC 与协议簇的上层通信。 LLC子层负责定位和检索介质上和介质外的帧。 逻辑链路控制在软件中实施，能够让数据链路层与协议簇的上层协议通信。网卡驱动程序软件直接与网卡上的硬件交互，可在MAC子层和物理介质之间传输数据。逻辑链路控制在IEEE802.2标准中指定。IEEE802.3是定义不同以太网类型的一组标准。MAC（介质访问控制）子层负责定位和检索介质上和介质外的帧。
常用的网络协议有哪些
NAT：网络地址转换 Port Address Translation, 端口地址转换 局域网：LAN, Local Area Nerwork 网络服务提供商：Internet Server Provider 网络视频传输的服务质量（QoS） 而在Windows XP中，将安装光盘中的“VALUEADD\MSFT\NET\NETBEUI”目录下的“nbf.sys”文件拷贝到%SYSTEMROOT%\SYSTEM32\DRIVERS\目录中，再将“netnbf.inf”文件拷贝 到%SYSTEMROOT%\INF\目录中；这样在安装“协议”的时候，在选择窗口中就可以看到“NetBEUI 协议”了 常用的网络协议有哪些？ 作者：来源：发表时间：2007-11-09 浏览次数：大中小 ARP(Address Resolution Protocol)地址解析协议 它是用于映射计算机的物理地址和临时指定的网络地址。启动时它选择一个协议(网络层)地址，并检查这个地址是否已经有别的计算机使用，如果没有被使用，此结点被使用这个地址，如果此地址已经被别的计算机使用，正在使用此地址的计算机会通告这一信息，只有再选另一个地址了。 SNMP(Simple Network Management P)网络管理协议 它是TCP/IP协议中的一部份，它为本地和远端的网络设备管理提供了一个标准化途径，是分布式环境中的集中化管理的重要组成部份。 BGP4(Border Gateway Protocol Vertion 4)边界网关协议-版本4 它是用于在自治网络中网关主机(每个主机有自己的路由)之间交换路由信息的协议，它使管理员能够在已知的路由策略上配置路由加权，可以更方便地使用无级内部域名路由(CIDR)，它是一种在网络中可以容纳更多地址的机制，它比外部网关协议(EGP)更新。BGP4经常用于网关主机之间，主机中的路由表包括了已知路由的列表，可达的地址和路由加权，这样就可以在路由中选择最好的通路了。BGP在局域网中通信时使用内部BGP(IBGP)，因为IBGP不能很好工作。 DHCP(Dynamic Host Configuration Protocol)动态主机配置协议 它是在TCP/IP网络上使客户机获得配置信息的协议，它是基于BOOTP协议，并在BOOTP协议的基础上添加了自动分配可用网络地址等功能。这两个协议可以通过一些机制互操作。DHCP协议在安装TCP/IP协议和使用TCP/IP协议进行通迅时，必须配置IP地址、子网掩码、缺省网关三个参数，这三个参数可以手动配置，也可以使用DHCP自动配置。 FTP(File Transfer Protocol)文件传输协议 它是一个标准协议，是在计算机和网络之间交换文件的最简单的方法。象传送可显示文件的HTTP 和电子邮件的SMTP一样，FTP也是应用TCP/IP协议的应用协议标准。FTP通常用于将网页从创作者上传到服务器上供人使用，而从服务器上下传文件也是一种非常普遍的使用方式。作为用户，您可以用非常简单的DOS界面来使用FTP，也可以使用由第三方提供的图形界面的FTP来更新(删除，重命名，移动和复制)服务器上的文件。现在有许多服务器支持匿名登录，允许用户使用FTP和ANONYMOUS作为用户名进行登录，通常可使用任何口令或只按回车键。 HDLC(High-Level Data Link Control)高层数据链路协议
网络协议分析(免费下载)概要
实验报告 项目名称：网络协议分析工具的使用课程名称：计算机网络A 班级：计111 计111 姓名：葛一波叶博兴 学号：110776 110768 教师：张晓明 信息工程学院计算机系
一．实验目的 1.了解协议分析仪的使用方法和基本特点，掌握使用协议分析仪分析协议的方法。 2.了解Ping命令的工作过程； 3.了解FTP协议的工作过程。 二．实验前的准备 1.熟悉Ping命令，FTP协议； 2.了解协议分析仪的功能和工作原理； 3.了解Ethereal分析仪的使用方法； 4.阅读本实验的阅读文献； 三．实验内容 1.学习捕获选项的设置和使用。 2.使用Ethereal分析仪捕获一段Ping命令的数据流，并分析其工作过程。 3.登录ftp://https://www.wendangku.net/doc/1818612996.html,，并下载三个大小不同的文件（小于1KB、1KB—1MB、1MB 以上），使用Ethereal分析仪分析其工作过程。 4.设置显示过滤器，以显示所选部分的捕获数据。 5.保存捕获的数据，分别是TEXT文件和XML文件。 四．实验要求 1.完成上述实验内容； 2.记录捕获的关键数据，并分析协议工作过程。 3.上交实验报告和保存的实验数据。 Wireshark Wireshark（前称Ethereal）是一个网络封包分析软件。网络封包分析软件的功能是撷取网络封包，并尽可能显示出最为详细的网络封包资料。网络封包分析软件的功能可想像成 "电工技师使用电表来量测电流、电压、电阻" 的工作 - 只是将场景移植到网络上，并将电线替换成网络线。 在过去，网络封包分析软件是非常昂贵，或是专门属于营利用的软件。Ethereal的出现改变了这一切。在GNUGPL通用许可证的保障范围底下，使用者可以以免费的代价取得软件与其程式码，并拥有针对其源代码修改及客制化的权利。Ethereal是目前全世界最广泛的网络封包分析软件之一。 软件简介 Wireshark使用目的以下是一些使用Wireshark目的的例子： 网络管理员使用Wireshark来检测网络问题，网络安全工程师使用Wireshark来检查资讯安全相关问题，开发者使用Wireshark来为新的通讯协定除错，普通使用者使用Wireshark 来学习网络协定的相关知识当然，有的人也会“居心叵测”的用它来寻找一些敏感信息…… Wireshark不是入侵侦测软件（Intrusion DetectionSoftware,IDS）。对于网络上的异常流量行为，Wireshark不会产生警示或是任何提示。然而，仔细分析Wireshark撷取的封包能够帮助使用者对于网络行为有更清楚的了解。Wireshark不会对网络封包产生内容的修改，
鸿鹄论坛_CCNA中文版本题库(谷歌翻译,有错误自行纠正)
1、请参见图示。当收到下列帧将会怎么做？（选择三项。）ACF A. Router1的将剥离的源MAC地址，并将其与MAC地址0000.0c36.6965取代。 B. Router1的将剥离的源IP地址，并将其与IP地址192.168.40.1取代。 C.router1将剥离的目的MAC地址，并将其与MAC地址代替0000.0c07.4320。 D.router1将剥离的目的IP地址，并将其与192.168.40.1的IP地址替换。 E. Router1的将转发数据包输出接口的FastEthernet0/1。 F.router1将转发数据包输出接口的FastEthernet0/2。 2、请参见图示。哪三种说法正确描述了网络设备A？（选择三项。）BDE A.使用255.255.255.128子网掩码时，在各接口不需要的IP地址。 B.使用255.255.255.128子网掩码时，在各接口确实需要一个唯一的IP地址IP子网。 C.为255.255.255.0的子网掩码时，必须是一个二层设备的电脑进行通信彼此。 D.为255.255.255.0的子网掩码时，必须是一个三层设备的电脑进行通信彼此。 E.使用255.255.254.0的子网掩码时，每个接口并不需要一个IP地址。 3、OSI中那一层负责决定一个进程的可用性并查看是否有可用的资源分给该进程？E A.传输 B.网络
D.会话 E.应用 4、以下哪项描述了WAN设备的角色？（选择三项。） A.CSU/ DSU用于端接本地数字环路 B.调制解调器端接本地数字环路 C.一个CSU/ DSU终止模拟本地环路 D.调制解调器终止模拟本地环路 E.路由器通常被视为DTE设备 F.路由器通常被视为DCE设备 注：SU/DSU是用于连接终端和数字专线的设备，属于DCE设备 modem用于数字信号和模拟信号的转换，属于DCE设备 路由器一般是DTE设备 5、请参见图示，主机A ping接口S0/0的router3，什么是该ping的TTL值？ A. 253 B. 252 C. 255 D. 254 6、网络管理员可以通过建立一个FTP连接到远程服务器验证新安装的主机的配置。该联播网管理员利用此操作的协议堆栈的最高层是什么？ A.应用
我对简单网络协议的认识
我对简单网络协议的认识 网络协议的定义：为计算机网络中进行数据交换而建立的规则、标准或约定的集合。例如，网络中一个微机用户和一个大型主机的操作员进行通信，由于这两个数据终端所用字符集不同，因此操作员所输入的命令彼此不认识。为了能进行通信，规定每个终端都要将各自字符集中的字符先变换为标准字符集的字符后，才进入网络传送，到达目的终端之后，再变换为该终端字符集的字符。当然，对于不相容终端，除了需变换字符集字符外。其他特性，如显示格式、行长、行数、屏幕滚动方式等也需作相应的变换。 在网络的各层中存在着许多协议，它是定义通过网络进行通信的规则，接收方的发送方同层的协议必须一致，否则一方将无法识别另一方发出的信息，以这种规则规定双方完成信息在计算机之间的传送过程。下面就对网络协议规范作个概述。 ARP(Address Resolution Protocol)地址解析协议它是用于映射计算机的物理地址和临时指定的网络地址。启动时它选择一个协议(网络层)地址，并检查这个地址是否已经有别的计算机使用，如果没有被使用，此结点被使用这个地址，如果此地址已经被别的计算机使用，正在使用此地址的计算机会通告这一信息，只有再选另一个地址了。 SNMP(Simple Network Management P)网络管理协议它是TCP/IP协议中的一部份，它为本地和远端的网络设备管理提供了一个标准化途径，是分布式环境中的集中化管理的重要组成部份。 AppleShare protocol(AppleShare协议) 它是Apple机上的通信协议，它允许计算机从服务器上请求服务或者和服务器交换文件。AppleShare可以在TCP/IP协议或其它网络协议如IPX、AppleTalk上进行工作。使用它时，用户可以访问文件，应用程序，打印机和其它远程服务器上的资源。它可以和配置了AppleShare协议的任何服务器进行通信，Macintosh、Mac OS、Windows NT和Novell Netware都支持AppleShare协议。 AppleTalk协议它是Macintosh计算机使用的主要网络协议。Windows NT服务器有专门为Macintosh服务，也能支持该协议。其允许Macintosh的用户共享存储在 Windows NT文件夹的Mac-格式的文件，也可以使用和Windows NT
各种网络协议
Windows中常见的网络协议 1.TCP/IP协议 TCP/IP协议是协议中的老大，用得最多，只有TCP/IP协议允许与internet 进行完全连接。现今流行的网络软件和游戏大都支持TCP/IP协议。 2.IPX/SPX协议 IPX/SPX协议是Novell开发的专用于NetWare网络的协议，现在已经不光用于NetWare网络，大部分可以联机的游戏都支持IPX/SPX协议，例如星际、cs。虽然这些游戏都支持TCP/IP协议，但通过IPX/SPX协议更省事，不需要任何设置。IPX/SPX协议在局域网中的用途不大。它和TCP/IP协议的一个显著不同是它不使用ip地址，而是使用mac地址。 https://www.wendangku.net/doc/1818612996.html,BEUI协议 NetBEUI协议是有IBM开发的非路由协议，实际上是NetBIOS增强用户接口，是Windows 98前的操作系统的缺省协议，特别适用于在“网上邻居”传送数据，大大提高了在“网上邻居”查找电脑的速度。如果一台只装了TCP/IP协议的Windows 98电脑想加入到WINNT域，也必须安装NetBEUI协议。 4.Microsoft网络的文件和打印机共享 在局域网中设置了ip地址与子网掩码，网线也连接正常，但在“网上邻居”中别人就是看不到自己的电脑，估计多半是由于没有把本机的“Microsoft网络的文件和打印机共享”启用。 因为协议分为7层：应用层表示层会话层传输层网络层数据链路层物理层而这7层所使用的协议是不同的，所以你的问题基本是网络层的协议，而不是应用层的协议！ 下述参考： 网络层协议：包括：IP协议、ICMP协议、ARP协议、RARP协议。 传输层协议：TCP协议、UDP协议。 ICMP是（Internet Control Message Protocol）Internet控制报文协议。它是TCP/IP协议族的一个子协议，用于在IP主机、路由器之间传递控制消息。控制消息是指网络通不通、主机是否可达、路由是否可用等网络本身的消息。这些控制消息虽然并不传输用户数据，但是对于用户数据的传递起着重要的作用。 IP是英文Internet Protocol（网络之间互连的协议）的缩写，中文简称为“网协”，也就是为计算机网络相互连接进行通信而设计的协议。在因特网中，它是能使连接到网上的所有计算机网络实现相互通信的一套规则，规定了计算机在因
HDLC协议
什么是HDLC？HDLC是什么意思？ HDLC英文全称High level Data Link Control，高级数据链路控制，HDLC是一个在同步网上传输数据、面向位的数据链路层协议，它是个由1970年代IBM所提出的对称式资料连结控制(Synchronous Data Link Control,SDLC)所研发出来的ISO标准。 高级数据链路控制（HDLC）协议是基于的一种数据链路层协议，促进传送到下一层的数据在传输过程中能够准确地被接收（也就是差错释放中没有任何损失并且序列正确）。HDLC 的另一个重要功能是流量控制，换句话说，一旦接收端收到数据，便能立即进行传输。H DLC 具有两种不同的实现方式：高级数据链路控制正常响应模式即HDLC NRM（又称为SDLC）和 HDLC 链路访问过程平衡（LAPB）。其中第二种使用更为普遍。HDLC 是 X.25 栈的一部分。 HDLC 是面向比特的同步通信协议，主要为全双工点对点操作提供完整的数据透明度。它支持对等链路，表现在每个链路终端都不具有永久性管理站的功能。另一方面，HDLC NRM 具有一个永久基站以及一个或多个次站。 HDLC LAPB 是一种高效协议，为确保流量控制、差错监测和恢复它要求额外开销最小。如果数据在两个方向上（全双工）相互传输，数据帧本身就会传送所需的信息从而确保数据完整性。
帧窗口是用于在接收第一个帧已经正确收到的确认之前发送复帧。这就意味着在具有长“turn-around”时间滞后的情况下数据能够继续传送，而不需要停下来等待响应。例如在卫星通信中会发生这种情形。 通常，帧分为三种类型： 信息帧：在链路上传送数据，并封装OSI体系的高层； 管理帧：用于实现流量控制和差错恢复功能； 无编号帧：提供链路的初始化和终止操作。 协议结构 Flag ― 该字段值恒为 0x7E。 Address Field ― 定义发送帧的次站地址，或基站发送帧的目的地。该字段包括服务访问点（6比特）、命令/响应位（表示帧是否与节点发送的信息帧有关或帧是否被节点接收）、地址扩展位（通常设置为1字节长）。当设置错误时，表示一个附加字节。
鸿鹄论坛_HCNA(HCDA)-HNTD模拟试卷
华为认证网络工程师-网络技术与设备HCNA(HCDA)-HNTD模拟试卷 1.（题型：单选）当二层交换网络中出现冗余路径时，用什么方法可以阻止环路的产生，提高网络的可靠性（） A.生成树协议 B.水平分割 C.毒性逆转 D.最短路径树 2.（题型：单选）在VRP中，VLAN标记中TPID字段固定值为（） A.0x8100 B.0x0800 C.0x0806 D.0x9100 3.（题型：单选）ADSL Modem与电话线相连的接口是（） A.RJ11 B.RJ45 C.IEEE1394 D.S/V端子 4.（题型：判断）布放接地线时，保护地线的长度不应超过45m，且尽量短（）。 A.True B.False 5.（题型：多选）用Telnet方式登录路由器时，可以选择哪几种验证方式（） A.password验证 B.AAA本地验证 C.MD5密文验证 D.不验证 6.（题型：单选）某公司申请到一个C类IP地址段，但要连接6个的子公司，最大的一个子公司有26台计算机，每个子公司在一个网段中，则子网掩码应设为（） A.255.255.255.0 B.255.255.255.128 C.255.255.255.192 D.255.255.255.224 7.（题型：多选）以下关于S6500产品定位说法正确的有（） A.中型网络骨干交换机 B.大型网络的汇聚层交换机 C.高密度LAN的用户接入交换机 D.用户侧交换机
8.（题型：多选）SNMP消息包括（） A.Get-Request B.Get-Next-Request C.Get-Response D.Set-Request E.Trap 9.（题型：多选）以下关于以太网交换机交换方式的叙述中，哪些是正确的（） A.Store and Forward方式不检测帧错误 B.Cut-through方式，交换机收到一个帧的前64字节即开始转发该帧 C.Fragment-free方式检测帧的前64字节中的错误 D.Store and Forward方式丢弃总长度小于64字节的帧 10.（题型：多选）以下哪些是帧中继的参数（） A.CIR承诺信息速率 B.TTL生存时间 C.BE允许超出突发量 D.BC承诺突发量 11.（题型：多选）SNMP（简单网络管理协议）中，SNMP manager和SNMP agent之间通信使用的端口号是（） A.53 B.69 C.162 D.161 12.（题型：多选）STP协议中的网桥ID包含两个部分内容，分别是（） A.网桥的优先级 B.网桥的端口ID C.网桥的MAC地址 D.网桥的IP地址 13.（题型：单选）以下关于OSPF网络中路由自环描述正确的是（） A.OSPF区域内已消除路由自环 B.OSPF区域之间没有消除路由自环 C.OSPF自治系统内没有消除路由自环 D.OSPF自治系统外路由不存在自环 14.（题型：单选）以下关于生成树协议优缺点的描述不正确的是（） A.生成树协议能够管理冗余链路 B.生成树协议能够阻断冗余链路，防止环路的产生 C.生成树协议能够防止网络临时失去连通性 D.生成树协议能够使以太网交换机可以正常工作在存在物理环路的网络环境中
实验16 路由器接口HDLC协议封装配置(改写)
实验18路由器接口HDLC协议封装配置 【背景知识】 教材4.4.3内容。理解掌握如下知识点： （1）理解在广域网环境下采用串行方式进行通信，因此需要相应的串行通信协议，如HDLC、PPP、Frame-Relay等； （2）掌握串行通信时的DTE和DCE概念，理解主要区别在于DCE提供时钟信号而DTE只是接受时钟信号； （3）cisco路由器的串行端口上，HDLC是缺省配置，而且采用的是cisco的私有协议HDLC，而不是通用的HDLC标准。 （4）在实验室环境中，将两台路由器直接相连（称为背靠背连接）。虽然路由器本质上属于DTE，但在此环境下可以将其中一台路由器模拟为DCE。究竟哪台路由器是DCE，取决于具体的连线。确定为DCE的路由器串行端口，必须配置时钟信号。 【实验拓扑】 实验线路连接图8-21所示，实验时使用Cisco Packet Tracer5.2完成拓扑结构搭建。 图8-21 实验18线路连接图 【实验内容】 (1) 选择两台C2811 路由器，分别关闭电源后添加WIC-2T 模块，添加位置为插槽0/接口适配器0，如下图8.22 所示。开启电源之后使用Serial 电缆将两台路由器的Serial0/0/0接口进行连接，连接时使得C2811B 为DCE 端、C2811A 为DTE 端。 图8.22 WIC-2T 模块安装位置 【提示1】图8.22所示界面，可以单击某台路由器的图标，然后在弹出的框中选择“Physical”选项卡，接着在左侧一栏中选择WIC-2T，最后按住鼠标左键不变拖动到对应的适配器即可。 【提示2】在选择线缆时，用串行线旁边带时钟符号的线先连接C2811B，那么C2811B即为DCE 端，线另外一头所连接的路由器C2811A就是DTE；反之，亦成立。 (2) 参阅教材4.4.3 中内容，配置C2811A 接口Serial0/0/0 的IP 地址192.168.1.1/24 和二层协议封装为HDLC，配置C2811B 接口Serial0/0/0 的IP 地址192.168.1.2/24 和二层协
HDLC协议概述
HDLC协议概述 刘文龙（北京理工大学信息与电子学院）学号2120110886 摘要：不同企业和不同公司的产品越来越先进，单板也越来越复杂，单板与单板之间，与终端之间数据传输的容量与可靠性要求也越来越高，简单的通讯方式满足不了要求的。HDLC 链路控制协议是现在常见的同步协议，为使不了解它的人有一个初步的认识，本文对数据链路层的HDLC协议进行综述介绍，主要内容包括HDLC的发展数据链路控制协议，HDLC协议的主要内容、存在的技术标准以及HDLC的应用和发展前景等。并重点介绍了HDLC的基本概念及帧格式。如果想进一步了解，可以参考和查阅其他相关资料。 关键词：HDLC，数据链路层，帧格式，帧结构 一HDLC概述 1.1 HDLC的发展历史 高级数据链路控制（High-Level Data Link Control或简称HDLC），是一个在同步网上传输数据、面向比特的数据链路层协议，它是由国际标准化组织(ISO)根据IBM公司的SDLC(Synchronous Data Link Control)协议扩展开发而成的.其最大特点是不需要数据必须是规定字符集，对任何一种比特流，均可以实现透明的传输。1974年，IBM公司率先提出了面向比特的同步数据链路控制规程SDLC（S ynchronous Data Link Control）。 随后，ANSI和ISO均采纳并发展了SDLC，并分别提出了自己的标准： 1* ANSI的高级通信控制过程ADCCP（Advanced Data Control Procedure）， 2* ISO的高级数据链路控制规程HDLC（High-level Data Link Contl）。 从此，HDLC协议开始得到了人们的广泛关注，并开始应用于通信领域的各个方面。1.2 HDLC的特点 HDLC是面向比特的数据链路控制协议的典型代表，有着很大的优势： 1* HDLC协议不依赖于任何一种字符编码集； 2*数据报文可透明传输，用于实现透明传输的“0比特插入法”易于硬件实现； 3*全双工通信，有较高的数据链路传输效率； 4*所有帧采用CRC检验，对信息帧进行顺序编号，可防止漏收或重份，传输可靠性高； 5*传输控制功能与处理功能分离，具有较大灵活性。 由于以上特点，目前网络设计及整机内部通讯设计普遍使用HDLC数据链路控制协议。HDLC已经成为通信领域额不可缺少的一个重要协议。
鸿鹄论坛_H3CCE云计算题库
1、以下CAS软件版本升级的说法正确的是： A 在进行CAS软件版本升级前，需要做好业务数据的备份工作 B 断业务升级时，建议从虚拟机内部正常关闭虚拟机并暂停共享存储池 C 升级完成后不清除浏览器缓存会出现CAS页面访问异常的问题 D 只要在CVM主机执行升级操作，就可以同时完成CVM主机管理的CVK主机的升级操作 2、以下关于配置操作系统信息方式的说法正确的是： A 使用完全初始化方式配置操作系统信息时，要求虚拟机模板中安装了CAS Tools工具 B 部署的虚拟机需要加入AD域，则需要使用快速初始化方式配置操作系统信息 C 部署的虚拟机需要加入AD域，则需要使用完全初始化方式配置操作系统信息 D 使用快速初始化方式配置操作系统信息时，要求虚拟机模板中安装了CAS Tools工具 3、以下关于CVM管理功能说法正确的是 A 云业务基于CVM组件提供的虚拟化能力，将计算、存储和网络等物理资源抽象成按需提供的弹性虚 拟资源池 B 主机自动发现任务管理会自动搜索可添加的主机并将其添加到任务所属的集群下 C 操作日志详细记录所有操作员对CVM所做的操作，并提供下载功能 D 虚拟机视图管理功能使维护人员可以基于实际业务场景自定义虚拟机目录 4、关于用户自助平台（svv），下列说法正确的是 A 用户提出云主机申请后，需要组织管理员审批虚拟机电子流 B 云主机申请电子流审批通过后，系统会自动部署虚拟机 C 组织管理员可以通过虚拟机模板部署虚拟机并分配给组织用户 D 用户提出云主机申请后，便可以直接获取虚拟机使用权 5、某公司包含研发、市场两个部门，并相应分配了组织，如下说法正确的是 A 组织管理员只能审批一个组织内的虚拟机电子流 B 一个集群只能分配给某一个组织使用 C 研发组织中用户申请的虚拟机只占用研发组织的计算和存储资源 D 每个组织可以分配多个集群计算资源 6、H3Cloud云计算解决方案包含 A UIS服务器 B UIS存储 C UIS网络
华为(H12-211)中文题库
Exam A QUESTION 1 当指定接口运行在RIPv2组播方式时，以下说法正确的是（）（选择2个正确答案） A.只接收RIPv2组播报文 B.不接收RIPv1广播报文 C.接收RIPv1广播报文 D.接收RIPv1组播报文 Correct Answer: AB Section: (none) QUESTION 2 下面哪条命令是把PPP的认证方式设置为PAP？ A.ppp pap B.ppp chap C.ppp authentication-mode pap D.ppp authentication-mode chap Correct Answer: C Section: (none) QUESTION 3 某公司申请到一个C类IP地址段，但要分配给6个子公司，最大的一个子公司有26台计算机。不同的子公司必须在不同的网段，则子网掩码应该设置为（）。 A.255.255.255.0 B.255.255.255.128 C.255.255.255.192 D.255.255.255.224 Correct Answer: D Section: (none) QUESTION 4 下列关于eSight 的描述中，错误的是（）。 A.eSight可用于监控和管理企业网络 B.eSight只能管理华为的网络设备 C.eSight支持WLAN设备管理和热点覆盖的监控 D.eSight支持配置文件的备份和流量分析 Correct Answer: B Section: (none) QUESTION 5 客户访问FTP服务器时出错，检查发现服务器与客户端之间的连通性没有问题，则有可能是哪些服务器端口被堵塞而导致的问题？（选择2个答案） A.21 B.80 C.20 D.50649 Correct Answer: AC Section: (none)
- 鸿鹄论坛_整理后的832题库
- 鸿鹄论坛_H11-811 HCNA-UC模拟考试
- CCNA 5.0路由和交换：网络简介 第6章 网络层(练习和答案)
- CCNA 5.0路由和交换：网络简介 第5章 以太网(练习和答案)
- 华为(H12-211)中文题库
- CCNA 5.0路由和交换：网络简介 第2章 配置网络操作系统(习题)
- 鸿鹄论坛_HCNA-Storage试题
- CCNA 5.0路由和交换：网络简介 第4章 网络接入层(练习和答案)
- 鸿鹄论坛_CCNA中文版本题库(谷歌翻译,有错误自行纠正)
- CCNA必考EIGRP解析
- 鸿鹄论坛_思科锐捷三层交换机配置DHCP
- 鸿鹄论坛_闫辉CCNA_学习笔记10
- 鸿鹄论坛_单选1-50
- CISCOCCNA网络工程师题库
- 鸿鹄论坛_H3CCE云计算题库
- Linux全攻略--远程登录管理-Telnet与SSH
- 鸿鹄论坛_Outlook教程(自己编写超详细,附赠Foxmail)
- 鸿鹄论坛 HCNA Storage H 题库 v
- 鸿鹄论坛_HCNA-Storage (H13-611)题库 v4.0
- 鸿鹄论坛_OSPF MA和P2P网络类型的探究