Khác biệt giữa các bản “Thành viên:Nhiêu Lộc/Sandbox”

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While the driver test measures you skill behind the wheel, <p>
==={{visible anchor|Header}}===
it's important to remember that the test <p>
The IPv4 packet header consists of 14 fields, of which 13 are required. The 14th field is optional (red background in table) and aptly named: options. The fields in the header are packed with the most significant byte first ([[Endianness|big endian]]), and for the diagram and discussion, the most significant bits are considered to come first ([[bit numbering#MSB 0 bit numbering|MSB 0 bit numbering]]). The most significant bit is numbered 0, so the version field is actually found in the four most significant bits of the first byte, for example.
actually begins before you start your engine. <p>
A pretest must be succefully completed <p>
{| class="wikitable" style="text-align:center"
before the driver can enter the open road. <p>
If the isn't in proper working order <p>
! style="width:4%;"|bit offset
or the driver fails to have proper paperwork, <p>
! colspan="4" style="width:12%;"| 0–3
the test is over. <p>
! colspan="4" style="width:12%;"| 4–7
Here's what a pre-test looks like. <p>
! colspan="6" style="width:24%;"| 8–13
Hi there. <p>
! colspan="2" style="width:6%;"| 14-15
Hi. <p>
! colspan="3" style="width:9%;"| 16–18
I'm Examiner Glenn.I'll be conducting your test today. <p>
! colspan="13" style="width:39%;"| 19–31
How are you doing? <p>
Fine, thank you. <p>
! 0
Good. May I see your appointment slip, your ID and your insurance, please. <p>
| colspan="4"| Version
That's my appointment slip … my ID and my proof of insurance. <p>
| colspan="4"| Internet Header Length
Thank you. <p>
| colspan="6"| [[Differentiated Services Code Point]]
Okay, I see that you do have a valid insurance card for this car. <p>
| colspan="2"| [[Explicit Congestion Notification]]
You can go ahead and put those away now. <p>
| colspan="16"| Total Length
Thank you. <p>
Is the engine shut off? <p>
! 32
It is. <p>
| colspan="16"| Identification
Very good. How about your parking brake? <p>
| colspan="3"| Flags
Could you release that and reset it, please? <p>
| colspan="13"| Fragment Offset
Okay, the next thing I'd like you to do is <p>
demonstrade your hand signal, please. <p>
! 64
First, I'd like you to demonstrate to slowing or stopping. <p>
| colspan="8"| [[Time to Live#IP packets|Time to Live]]
Now, second, right turn. <p>
| colspan="8"| [[List of IP protocol numbers|Protocol]]
And left turn, thank you. <p>
| colspan="16"| [[Header checksum]]
Next, I'd you to demonstrate your ability <p>
to stop the vehicle. <p>
! 96
Okay. <p>
| colspan="32"| Source IP Address
Please your foot over the gas pedal. <p>
Don't press it, but when I say, "Stop" <p>
! 128
step on the brake as fast as you can. <p>
| colspan="32"| Destination IP Address
Okay. <p>
Stop. Very good. <p>
! 160
Next, I'd like you to start the vehicle. <p>
| colspan="32" style="background:#fbb;"| Options ( if Header Length > 5 )
Now, keep your foot on the brake pedal. <p>
Push the brake. Keep your foot on the brake. <p>
! 160<br>or<br>192+
I see the left one is good. The right one's good. <p>
| colspan="32"| &nbsp;<br />Data<br />&nbsp;
And the center one's good. <p>
Next, we're going to check your electric turn signals. <p>
First left. <p>
; Version : The first header field in an IP [[packet (information technology)|packet]] is the four-bit version field. For IPv4, this has a value of 4 (hence the name IPv4).
Thank you. And right. <p>
And the tabs are good. <p>
; Internet Header Length (IHL) : The second field (4 bits) is the Internet Header Length (IHL), which is the number of 32-bit [[Word (data type)|words]] in the header. Since an IPv4 header may contain a variable number of options, this field specifies the size of the header (this also coincides with the offset to the data). The minimum value for this field is 5 (RFC 791), which is a length of 5×32 = 160 bits = 20 bytes. Being a 4-bit value, the maximum length is 15 words (15×32 bits) or 480 bits = 60 bytes.
Okay, right turn signal. <p>
Left turn signal. <p>
; Differentiated Services Code Point (DSCP)
Good, we're ready to go. <p>
:Originally defined as the [[Type of Service]] field, this field is now defined by RFC 2474 for [[Differentiated services]] (DiffServ). New technologies are emerging that require real-time data streaming and therefore make use of the DSCP field. An example is [[Voice over IP]] (VoIP), which is used for interactive data voice exchange.
Having arrived with the correct documents, <p>
an understanding of hand signals <p>
; Explicit Congestion Notification (ECN) : This field is defined in RFC 3168 and allows end-to-end notification of [[network congestion]] without dropping packets. ECN is an optional feature that is only used when both endpoints support it and are willing to use it. It is only effective when supported by the underlying network.
and a car in proper working order, <p>
this driver has successfully completed the pre-test <p>
; Total Length : This 16-bit field defines the entire datagram size, including header and data, in bytes. The minimum-length datagram is 20 bytes (20-byte header + 0 bytes data) and the maximum is 65,535 bytes — the maximum value of a 16-bit word. The largest datagram that any host is required to be able to handle is 576 bytes, but most modern hosts handle much larger packets. Sometimes [[subnetwork]]s impose further restrictions on the size, in which case datagrams must be fragmented. Fragmentation is handled in either the host or packet switch in IPv4.
and moves on to the open road for the next phrase <p>
of her driver test. <p>
; Identification : This field is an identification field and is primarily used for uniquely identifying fragments of an original IP datagram. Some experimental work has suggested using the ID field for other purposes, such as for adding packet-tracing information to help trace datagrams with spoofed source addresses.<ref>
{{cite web | last=Savage | first=Stefan | title=Practical network support for IP traceback | url= | accessdate=2010-09-06}}</ref>
In this video we're going to demonstrate <p>
the backing maneuver. <p>
; Flags : A three-bit field follows and is used to control or identify fragments. They are (in order, from high order to low order):
Back you vehicle around the corner <p>
:* bit 0: Reserved; must be zero.<ref group=note>As an [[April Fools']] joke, proposed for use in RFC 3514 as the "[[Evil bit]]".</ref>
staying as close to the curb as you can. <p>
:* bit 1: Don't Fragment (DF)
Then, safely continue backing in a straight line <p>
:* bit 2: More Fragments (MF)
until directed to stop. <p>
:If the DF flag is set, and fragmentation is required to route the packet, then the packet is dropped. This can be used when sending packets to a host that does not have sufficient resources to handle fragmentation. It can also be used for [[Path MTU Discovery]], either automatically by the host IP software, or manually using diagnostic tools such as [[ping]] or [[traceroute]].
Thic test measure skills needed for <p>
:For unfragmented packets, the MF flag is cleared. For fragmented packets, all fragments except the last have the MF flag set. The last fragment has a non-zero Fragment Offset field, differentiating it from an unfragmented packet.
backing out of a driveway or alley. <p>
It requires proper vision tecniques; <p>
; Fragment Offset : The fragment offset field, measured in units of eight-byte blocks, is 13 bits long and specifies the offset of a particular fragment relative to the beginning of the original unfragmented IP datagram. The first fragment has an offset of zero. This allows a maximum offset of (2<sup>13</sup> &ndash; 1) &times; 8 = 65,528 bytes, which would exceed the maximum IP packet length of 65,535 bytes with the header length included (65,528 + 20 = 65,548 bytes).
primarily, looking out the back window when backing. <p>
You want to look where you driving, <p>
; Time To Live (TTL) : An eight-bit [[time to live]] field helps prevent datagrams from persisting (e.g. going in circles) on an internet. This field limits a datagram's lifetime. It is specified in seconds, but time intervals less than 1 second are rounded up to 1. In practice, the field has become a [[hop count]]—when the datagram arrives at a [[router (computing)|router]], the router decrements the TTL field by one. When the TTL field hits zero, the router discards the packet and typically sends a [[ICMP Time Exceeded]] message to the sender.
not just in the mirror or to the front. <p>
Remember to stop before the crosswalk area. <p>
: The program [[traceroute]] uses these ICMP Time Exceeded messages to print the routers used by packets to go from the source to the destination.
Making sure that there aren't any pedestrians <p>
or other traffic in that zone. <p>
; Protocol : This field defines the protocol used in the data portion of the IP datagram. The [[Internet Assigned Numbers Authority]] maintains a [[list of IP protocol numbers]] which was originally defined in RFC 790.
Once you know the area is clear, <p>
continue backing. <p>
; Header Checksum : {{Main|Header checksum}} The 16-bit [[checksum]] field is used for error-checking of the header. When a packet arrives at a router, the router calculates the checksum of the header and compares it to the checksum field. If the values do not match, the router discards the packet. Errors in the data field must be handled by the encapsulated protocol. Both [[User Datagram Protocol|UDP]] and [[Transmission Control Protocol|TCP]] have checksum fields.
Troungout the backing manuever, <p>
: When a packet arrives at a router, the router decreases the TTL field. Consequently, the router must calculate a new checksum. RFC 1071 defines the checksum calculation:
stay witin 18-inches of the curb. <p>
:: ''The checksum field is the 16-bit [[Ones' complement|one's complement]] of the one's complement sum of all 16-bit words in the header. For purposes of computing the checksum, the value of the checksum field is zero.''
That will help you stay out of the way of other traffic. <p>
: For example, use Hex 4500003044224000800600008c7c19acae241e2b (20 bytes IP header):
Being around of what's going on around you <p>
::4500 + 0030 + 4422 + 4000 + 8006 + 0000 + 8c7c + 19ac + ae24 + 1e2b = 2BBCF
on all directions is extremly important. <p>
::2 + BBCF = BBD1 = 1011101111010001, the 1'S of sum = 0100010000101110 = 442E
Just like a quarterback with great field vision, <p>
: To validate a header's checksum the same algorithm may be used - the checksum of a header which contains a correct checksum field is a word containing all zeros (value 0):
you want to be aware of everything that's going on <p>
::2BBCF + 442E = 2FFFD. 2 + FFFD = FFFF. the 1'S of FFFF = 0.
all around you. <p>
The difference, of course, is that driving <p>
; Source address
is not a game. <p>
: This field is the IPv4 [[IPv4 address|address]] of the sender of the packet. Note that this address may be changed in transit by a [[network address translation]] device.
A single mistake made behind the wheel <p>
can be deadly. <p>
; Destination address
It really can't be stressed enough. <p>
: This field is the IPv4 [[IPv4 address|address]] of the receiver of the packet. As with the source address, this may be changed in transit by a [[network address translation]] device.
When you're moving backwards, <p>
You want to be looking backwards, too. <p>
; Options
Making sure that nothing <p>
: The options field is not often used. Note that the value in the IHL field must include enough extra 32-bit words to hold all the options (plus any padding needed to ensure that the header contains an integral number of 32-bit words). The list of options may be terminated with an EOL ([[End of Options List]], 0x00) option; this is only necessary if the end of the options would not otherwise coincide with the end of the header. The possible options that can be put in the header are as follows:
and no one is in you way. <p>
{| class="wikitable"
Once you've come to a stop, <p>
you ca prepaire to re-enter the roadway. <p>
! Field !! Size (bits) !! Description
Check your side view mirror <p>
and turn to look behind you <p>
| Copied || 1 || Set to 1 if the options need to be copied into all fragments of a fragmented packet.
so that you can see if there's any oncoming <p>
traffic approaching from behind. <p>
| Option Class || 2 || A general options category. 0 is for "''control''" options, and 2 is for "''debugging and measurement''". 1, and 3 are reserved.
Once the way is clear, <p>
you can safe pull forward and onto the street. <p>
| Option Number || 5 || Specifies an option.
Make it a point to follow the proper steps <p>
of doing the backing maneuver <p>
| Option Length || 8 || Indicates the size of the entire option (including this field). This field may not exist for simple options.
and they'll become habit forming. <p>
That will make you a better driver <p>
| Option Data || Variable || Option-specific data. This field may not exist for simple options.
and will help keep you and those around you safe. <p>
*Note: If the header length is greater than 5, i.e. it is from 6 to 15, it means that the options field is present and must be considered.
When driving, it's crucial to always be aware <p>
*Note: Copied, Option Class, and Option Number are sometimes referred to as a single eight-bit field - the ''Option Type''.
of what's going on around you <p>
: The following two options are discouraged because they create security concerns: [[Loose Source and Record Route]] (LSRR) and [[Strict Source and Record Route]] (SSRR). Many routers block packets containing these options.{{Citation needed|date=November 2008}}
in every direction. <p>
And that's certainly true when parallel parking. <p>
Do you see the brown car parked up ahead? <p>
Yes. <p>
Imagine another vehicle parked <p>
one-and-a-half car lengths behind it. <p>
Parallel between the tow vehicles. <p>
The driver pulls along side the SUV <p>
she'll park behind. <p>
Then, executes three moves as she backs up. <p>
She begins by turning the steering wheel <p>
to the right. <p>
Then back to the left to straighten-out the car <p>
and pull-up evenly along side the curb. <p>
Next, she pulls forward and parks. <p>
It only takes her three moves to park, <p>
limiting the potential impact on other drivers. <p>
Here, the driver has pulled up alongside <p>
the SUV, once again. <p>
Btu before backing up, <p>
shee sees a car approaching form behind. <p>
She waits for it to pass, <p>
then begin to parallel park. <p>
Notice that she's not just looking at her mirrors. <p>
She's turned, looking directly into <p>
her direction of travel. <p>
To be parked legally, <p>
she needs to have both right tires <p>
within twelve inches of the curb. <p>
Parking further away from the curb <p>
presens a traffic hazzard. <p>
But you careful not to run your tires up <p>
into the curb, either. <p>
That can damage them. <p>
Once you've safely parked, <p>
the drive test continues. <p>
Re-enter traffic. <p>
After all, you'll need to safely drive away <p>
fron that parking spot. <p>
Notice that our driver looks out her window <p>
before re-entering. <p>
Then, as she begins to re-enter, <p>
she looks a second time <p>
make king sure there's no traffic <p>
coming up from behind. <p>
She's successfully completed <p>
the parallel parking test <p>
and back on the road <p>
of her driver test. <p>
On the road, every move a driver makes <p>
or doesn't make <p>
can significantly impact <p>
any one in the area. <p>
A lane change or turn <p>
might seem simple <p>
But when drivers aren't aware <p>
of what's going on <p>
all around their car, <p>
the result can be catastrophic. <p>
At the next corner, turn right. <p>
Before turning, look twice <p>
and make sure the way is clear. <p>
Turn into the near lane, <p>
being careful not to swing wide <p>
as that can impact other drivers. <p>
When safe to do so, move to the left lane. <p>
When changing lanes, <p>
it's extremely important <p>
to check your mirror <p>
and look directly into your blind spots. <p>
When safe to do so, move to the right lane. <p>
You want to make distinct head checks, <p>
not just casual glances. <p>
This way, you know the path is clear <p>
before you begin to change lanes. <p>
After all, the last thing you want to đo <p>
in moving to the path of an unexpected <p>
car, van, bus <p>
or semi. <p>
Bottom line whether your turning, <p>
changing lanes or starting or stopping your car <p>
you need to be aware of everything. <p>
You might think of these areas as zones. <p>
The areas in front, behind <p>
and on both sides of you. <p>
Be aware of what's happening <p>
in the zones at all times <p>
and you'll be a better driver. <p>