c++ - Understanding the TCP checksum function -


i believe tcp checksum function following:

  1. break pseudoheader , tcp segment header , data 2 byte blocks.
  2. add 1 byte padding of 0s end of last block if it's not 2 bytes long, make 2 bytes.
  3. take one's complement of sum tcp checksum.

sounds simple enough. hence wrote own generic checksum function:

#include <inttypes.h> #include <arpa/inet.h>  uint16_t checksum(uint16_t * data, int size) {     uint16_t sum = 0;      int = 0, length = size / 2;      while (i < length) sum += data[i++];      if (size % 2) sum += data[i] & 0xff00;      return htons(~sum); }

however other people have written checksum functions seem more complicated. example:

uint16_t checksum(uint16_t * addr, int len) {     int nleft = len;     int sum = 0;      uint16_t * w = addr;     uint16_t answer = 0;      while (nleft > 1) {         sum += *w++;         nleft -= sizeof(uint16_t);     }      if (nleft == 1) {         *(uint8_t *) (&answer) = *(uint8_t *) w;         sum += answer;     }      sum = (sum >> 16) + (sum & 0xffff);     sum += (sum >> 16);     answer = ~sum;     return (answer); }

i have few questions regarding code:

  1. what statement *(uint8_t *) (&answer) = *(uint8_t *) w; do?

  2. why take sum as:

    sum = (sum >> 16) + (sum & 0xffff); sum += (sum >> 16);

  3. did way calculate tcp checksum change?

i don't see why sum = (sum >> 16) + (sum & 0xffff). consider sum 0xabcd:

0xabcd >> 16    == 0x0000  0xabcd & 0xffff == 0xabcd  0x0000 + 0xabcd == 0xabcd

it seems redundant step. same goes next statement sum += (sum >> 16).

the checksum function appears big-endian processors only.

the first while loop optimized speed.

the &answer trick loads last byte (if there odd number of bytes) high byte of answer, leaving low byte zero, similar code data[i] & 0xff00. way works this

1) take address of answer      (&answer) 2) convert byte pointer  (uint8_t *)   2a) on big endian processor first byte of 16-bit quantity high byte 3) overwrite high byte last byte of data

the checksum supposed computed carries added in. it's assumed here code running on machine int 32-bits. therefore, (sum & 0xffff) 16-bit checksum, , (sum >> 16) carry bits (if any) need added in. hence, line

sum = (sum >> 16) + (sum & 0xffff);

adjusts sum include carries. however, line of code generate carry bit. next line sum += (sum >> 16) adds carry (if any) checksum.

finally, take ones-complement of answer. note htons not used since whole function implicitly assumes running on big endian processor.


Comments

Post a Comment

Popular posts from this blog

get url and add instance to a model with prefilled foreign key :django admin -

Image
i have page in each contest shown in row. each row contains contest id , link other model names have foreign key contest. news model: class news(models.model): title = models.charfield(max_length=50, null=true) description = models.textfield() time = models.timefield( default= datetime.datetime.now().time()) contest = models.foreignkey(contest) i want use admin adding , changing these models. want when user clicks on 'news' has foreign key contest id=1, shows list of news value foreign key. use {% instance in instances %} <tr class="{% cycle 'row1' 'row2' %}"> <td>{{ instance.title }}</td> <td>{{ instance.starttime }}</td> <td>{{ instance.timespan }}</td> <td>{{ instance.date }}</td> <td><a href="/adm...
Read more

android - Keyboard hides my half of edit-text and button below it even in scroll view -

i having trouble keyboard covers edit-text , half of button in scroll view. here's layout , please tell me how should resolve problem. many questions on topic none of them s working me. <?xml version="1.0" encoding="utf-8"?> <linearlayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:background="@drawable/bg" android:layout_height="match_parent"> <scrollview android:layout_width="match_parent" android:fillviewport="true" android:layout_height="match_parent"> <relativelayout android:layout_width="match_parent" android:layout_height="wrap_content" android:orientation="vertical"> <linearlayout android:layout_width="match_parent" android:layout_height="w...
Read more

css - Make div keyboard-scrollable in jQuery Mobile? -

from can tell, although jquery-mobile-powered pages can contain divs overflow set scroll or auto, , these divs can scrolled one-screen bar or mouse wheel, cannot scrolled using arrow keys, page-up/page-down, or home/end. instead, official "page" div (with data-role="page") absorbs input. perhaps other divs can't acquire focus, i'm not sure. is there way around this? edit: jsfiddle of simple example: https://jsfiddle.net/qogz0shx/ <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.4/jquery.min.js"></script> <script src="https://ajax.googleapis.com/ajax/libs/jquerymobile/1.4.5/jquery.mobile.min.js"> </script> <link rel="stylesheet" href="https://code.jquery.com/mobile/1.4.5/jquery.mobile-1.4.5.css"/> <style> #outer { overflow:scroll; height: 50vh; width: 50vw; } #inner { height: 500vh; width: 500vw; } ...
Read more