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I have read from E. Comer - Internetworking with TCP/IP 4th edition that "The purpose of using a pseudo-header is to verify that the UDP datagram has reached its correct destination. The key to understanding the pseudo-header lies in realizing that the correct destination consists of a specific machine and a specific protocol port within that machine. The UDP header itself specifies only the protocol port number. Thus, to verify the destination, UDP on the sending machine computes a checksum that covers the destination IP address as well as the UDP datagram. The pseudo-header is not transmitted with the UDP datagram, nor is it included in the length."

But I asked one question from that understands the UDP datagram is transmitted with pseudo header. I don't understand the exact concepts.

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  • a udp pseudoheader consists of udp header + IP addresses from IP header. Every UDP packet is transmitted inside an IP packet which has IP header. Thus all information in pseudoheader is transmitted. Just not all inside UDP header. And there is no point on having IP addresses in a packet twice.
    – Effie
    Sep 12 '21 at 17:10
  • @effenok when packets reaches in receiver side in transport layer the layer3 header will be omitted. So how could we calculate checksum with pseudo IP header in layer4?
    – Alok Maity
    Sep 16 '21 at 11:43
  • you have to implement your protocol stack on end systems so that this is possible.
    – Effie
    Sep 16 '21 at 11:45
  • @effenok protocol stack available for every 7 layer?
    – Alok Maity
    Sep 16 '21 at 11:48
  • usually, you have protocol stack up to layer 4 implemented by the operating system. the rest is part of application.
    – Effie
    Sep 16 '21 at 11:49
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The "pseudo header" is a way to include vital information from the IP header in the UDP checksum. You could also call it "virtual header".

Basically, the UDP checksum is calculated on the UDP datagram plus selected fields from the IP header. The UDP handler is provided with sufficient information from the underlying network layer to calculate the checksum that way. Actually sending that pseudo header along with the UDP datagram would be completely redundant.

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  • But Wikipedia saying pseudo header going to with udp header for checksum calculation.
    – Alok Maity
    Sep 12 '21 at 9:47
  • Yes, it's used for checksum calculation. But no, it isn't transmitted along with the datagram (just the information is, elsewhere).
    – Zac67 ♦
    Sep 12 '21 at 9:52
  • but during transmission what will contained by 16 bit checksum? Only checksum value?
    – Alok Maity
    Sep 16 '21 at 11:39
  • The checksum is calculated over the payload and the pseudo header.
    – Zac67 ♦
    Sep 16 '21 at 12:34
  • when packets reaches in receiver side in transport layer the layer3 header will be omitted. So how could we calculate checksum with pseudo IP header in layer4?
    – Alok Maity
    Sep 16 '21 at 12:36

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