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Bronze

pseudo header in udp

Hi  every body.

I was little confused about the usage of " pseudo header" in udp.  Below is the link which explains the use of " pseudo  header"  in udp.

http://http://www.tcpipguide.com/free/t_UDPMessageFormat-2.htm

against it?

I am still at loss  as to what  purpose this header serves. According to the link,this " pseuo header protects against the accidental delivery of message to wrong destination. 

The question is how could a message end up at  wrong destination? how does pseudo header protect against it?

thanks and have a great day.

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Blue

Re: pseudo header in udp

sarahr202 wrote:

Hi  every body.

I was little confused about the usage of " pseudo header" in udp.  Below is the link which explains the use of " pseudo  header"  in udp.

http://http://www.tcpipguide.com/free/t_UDPMessageFormat-2.htm

against it?

I am still at loss  as to what  purpose this header serves. According to the link,this " pseuo header protects against the accidental delivery of message to wrong destination. 

The question is how could a message end up at  wrong destination? how does pseudo header protect against it?

thanks and have a great day.

Sarah

UDP is fire and forget ie. there is no acknowledgement from the receiving machine that it has received the packet and there is no initial connection setup as you get with TCP. When the protocol was originally designed one of the concerns was that the packet could actually end up at the wrong destination ie. it only takes a bit to be flipped  by an intermediate device for this to happen. TCP has inbuilt checks because of the 3 way handshake etc. but UDP does not.

So the pseudo header was used. By including the IP addresses/port numbers in the checksum only the correct receiving host would have the same combination of IP addresses and port numbers and so could compute the same checksum. If the checksum was incorrect then the receiving host would simply drop the packet.

Jon

2 REPLIES
Hall of Fame Super Blue

Re: pseudo header in udp

sarahr202 wrote:

Hi  every body.

I was little confused about the usage of " pseudo header" in udp.  Below is the link which explains the use of " pseudo  header"  in udp.

http://http://www.tcpipguide.com/free/t_UDPMessageFormat-2.htm

against it?

I am still at loss  as to what  purpose this header serves. According to the link,this " pseuo header protects against the accidental delivery of message to wrong destination. 

The question is how could a message end up at  wrong destination? how does pseudo header protect against it?

thanks and have a great day.

Sarah

UDP is fire and forget ie. there is no acknowledgement from the receiving machine that it has received the packet and there is no initial connection setup as you get with TCP. When the protocol was originally designed one of the concerns was that the packet could actually end up at the wrong destination ie. it only takes a bit to be flipped  by an intermediate device for this to happen. TCP has inbuilt checks because of the 3 way handshake etc. but UDP does not.

So the pseudo header was used. By including the IP addresses/port numbers in the checksum only the correct receiving host would have the same combination of IP addresses and port numbers and so could compute the same checksum. If the checksum was incorrect then the receiving host would simply drop the packet.

Jon

Bronze

Re: pseudo header in udp

thanks Jon.

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