Security - Network data packet-tested and (checksum) Calculationnetwork

Network data packet-tested and (checksum) Calculation

In the transmission network to ensure that data packets are transmitted with an effective post-mortem right field, IP, arp, such as every tcp

Data has its own validity of the post-mortem and.
- Tested the calculation is not complicated. The corresponding data of all data packets as a byte array
(A, b, c, d, e), they were divided into a group of 16 bit
([Ab], [cd], [e0])
And calculated:
[Ab]
[Cd]
[E0]
——–
[Xy]
Calculated using binary cycle, the highest lowest Add to rounding,
If the calculation of the [xy] are all for a bit (ie 1111 1111)-tested through.

If [cd]-tested and it has already completed all other byte, to how the [cd]

(Checksum) values? We first filled with 0 [cd], and then calculated [xy], right [cd] should include

Calculated by the [xy] for the anti-bit results. This will ensure that the entire data packet through the validity of the post-mortem.

Actual procedures, because the machines are now 32, so modifications to the algorithms used:
Byte-by-byte "Normal" Swapped
Order Order

Byte 0 / 1: 00 01 0001 0100
Byte 2 / 3: f2 03 f203 03f2
Byte 4 / 5: f4 f5 f4f5 f5f4
Byte 6 / 7: f6 f7 f6f7 f7f6
— — —– —–
Sum1: 2dc 1f0 2ddf0 1f2dc

Dc f0 ddf0 f2dc
Carrys: 1 2 2 1
— - —- —-
Sum2: dd f2 ddf2 f2dd

Final Swap: dd f2 ddf2 ddf2
——————————
Byte 0/1/2/3: 0001f203 010003f2 03f20100
Byte 4/5/6/7: f4f5f6f7 f5f4f7f6 f7f6f5f4
——– ——– ——–
Sum1: 0f4f7e8fa 0f6f4fbe8 0fbe8f6f4

Carries: 0 0 0

Top half: f4f7 f6f4 fbe8
Bottom half: e8fa fbe8 f6f4
—– —– —–
Sum2: 1ddf1 1f2dc 1f2dc

Ddf1 f2dc f2dc
Carrys: 1 1 1
—- —- —-
Sum3: ddf2 f2dd f2dd

Final Swap: ddf2 ddf2 ddf2

Can be seen for 32 to the checksum, and then the low and high eight eight together, and ultimately sought

Checksum is the same.
Is a section of the C + + algorithm:
In 6
(
/ * Compute Internet Checksum for "count" bytes
* Beginning at location "addr."
* /
Register long sum = 0;

While (count> 1) (
/ * This is the inner loop * /
Sum + = * (unsigned short) addr + +;
Count -= 2;
)

/ * Add left-over byte, if any * /
If (count> 0)
Sum + = * (unsigned char *) addr;

/ * Fold 32-bit sum to 16 bits * /
While (sum>> 16)
Sum = (sum & 0xffff) + (sum>> 16);

Checksum = ~ sum;
)

References: RFC1071 Website: http://www.faqs.org/rfcs/rfc1071.html

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