ethernet checksum

[Christos Ricudis]

Giorgos D. Pallas wrote:
> Στο σχετικό RFC προβλέπεται checksum για κάθε ethernet πακέτο. Ο λόγος
> που δεν φαίνεται αυτό όταν γίνεται packet capture είναι ότι το
> υπολογιζει και το ενσωματώνει στα πακέτα η κάρτα δικτύου πριν τα
> μεταδώσει (και το ανάποδο όταν παραλαμβάνει πακέτα);
> 

(Opws synh8ws) mperdepses ta layers :)

H monada metadoshs sto data link layer, pou sthn periptwsh mas einai to 
ethernet, legetai frame.

To pio synh8ismeno frame format poy 8a deis panw se ena ethernet kalwdio 
exei ws ekshs [1] :

+-------------------+
|     Preamble      |
+-------------------+
| Dest. MAC address |
+-------------------+
| Src. MAC address  |
+-------------------+
| Frame length/type |
+-------------------+
|     Payload       |
+-------------------+
|    Frame Check    |
|     Sequence      |
+-------------------+

To preamble kai to Frame Check Sequence (dhladh to "checksum" poy anafereis 
parapanw) elegxontai synh8ws apo to hardware. An to FCS einai la8os, to 
frame ginetai drop PRIN perasei sto parapanw epipedo. Gi ayto kai den 
prokeitai na deis pote ena damaged ethernet frame, alla endexomenws na (mhn) 
deis lost ethernet frames.

H epilogh -e tou tcpdump, typwnei ta link-level header items POU PERNAEI o 
Ethernet driver sto parapanw layer. Ayta einai Dest/Src. MAC address, kai to 
frame type. To FCS den to blepeis pote. Etsi p.x. exoume :

14:32:05.323381 00:1a:92:d9:da:fc (oui Unknown) > Broadcast, ethertype ARP
Wra             Src MAC address        [2]        DST MAC      Frame type
                                                   Address

(0x0806), length 60: arp who-has diamond.itc.auth.gr tell alfa.itc.auth.gr
  Frame     Frame     Payload ------------------------------------>
  Type      Length
  in HEX


14:32:05.323552 00:18:f3:eb:a7:92 (oui Unknown) > 00:0b:45:b4:a5:c0
Wra             Src MAC address        [2]        Dst MAC Address

(oui Unknown), ethertype IPv4 (0x0800), length 87:
      [2]       Frame type      in hex   Frame length

komodino.itc.auth.gr.33320 > dns2.ccf.auth.gr.domain:  52588+ PTR? 
88.130.207.155.in-addr.arpa. (45)
Payload -------------------------------------------------------->




[1] Se OLH th sxetikh bibliografia, to ethernet frame einai ena orizontio 
parallhlogramo. Prepei na eimai o prwtos poy sxediazei ethernet frame
se ka8eto parallhlogramo. Copyright 2007. All rights Reserved. Patents 
pending. Tresspassers will be shot and then interrogated. No warranties 
included.

[2] Mh rwthsete, den prokeitai na apanthsw.

-- 
Christos Ricudis