CODEWITHSUNDEEP
network-programmingheaderudppacketpayload
user23
user23

Reputation: 425

UDP payload length and packet transmission

I have data 1245 MB for trasmission via UDP over IPv4. For calculation of expected number of packet transmission from A to B then B relay to C, if the data transmitted in blocks of size 320bytes (i.e; payload = 320bytes), and header is 20 bytes, do we minus 20 from 320 or add in?

For instance, 
1245MB = 1305477120 bytes
Total UDP Payload = 320 - 20 or 320 + 20?

Upvotes: 2

Views: 11150

Answers (2)

Joel Cunningham
Joel Cunningham

Reputation: 671

For calculating the number of packets you don't need to take into account the size of the transport or network layer headers. You specified a payload size of 320 bytes, which is well within the maximum size of a UDP payload without fragmentation.

Each time you call send() or sendto(), this will create a datagram (packet), so the math is simply dividing the total size by your 320 byte chunks:

1305477120 / 320 = 4079616 packets

As a side point, if you were to make your UDP payload larger, that would reduce the total number of packets. On a lot of networks, the MTU is 1500 bytes, so you can send:

1500 bytes - IP header (20 bytes) - UDP header (8) bytes = 1472 bytes for payload

As a second side point, if your UDP payload is too big, i.e. payload + IP/UDP headers exceeds the MTU, then your single call to send() would result in multiple IP fragment packets.

Upvotes: 1

user207421
user207421

Reputation: 310840

The packet consists of:

  • the IP header (20 bytes)
  • the UDP header (8 bytes)
  • your payload (320 bytes).

Total: 348 bytes.

Upvotes: 3

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