IP Datagram

[ananda_id]

Hi,

This is a silly question but I failed to find the answer. What is the minimum size of IP payload?

Thanks,

Ananda

[Richard Burts]

Ananda

The answer to your question is somewhat dependent on the media over which you will send the frame. We typically think about this kind of question in terms of Ethernet which does define minimum frame size. And on Ethernet with minimum frame size of 64, you can subtract out the bytes for IP header and the bytes for Ethernet framing and think that you will find the minimum payload size. But in Ethernet if the payload is not big enough the frame will be padded to achieve the minimum size. And other media may not have minimum frame size defined. So the minimum payload of an IP frame is 1 byte.

HTH

Rick

[ananda_id]

Rick,

Thx for the answer.

Yes I've found some related article that say minimum ethernet payload are 46 bytes. The ethernet payload itself is 20 bytes ip packet header + data payload. Is the minimum data payload of the layer 3 packet also 1 byte? Can it have 0 byte size for payload?

Thank,

Ananda

[Richard Burts]

Ananda

If the payload of the IP datagram were 0 bytes I am not sure why we would send any frame. If there is no payload why would the datagram be transmitted?

At some layers of the protocol stack there might be a need to transmit a protocol acknowledgement which can be transmitted without any payload. But this is not a factor with the IP datagram.

HTH

Rick

[ananda_id]

Rick,

Actually that is what I try to find out. Minimum MTU possible. Can I set 20 bytes for MTU? Just for ip header without any payload?

Thanks,

Ananda

[JORGE RODRIGUEZ]

Amanda,

I found this link , this is a brakedown of an ip datagram (IPv4) based on ethernet media MTU max size of 1500 bytes,

but as Rick posted it, minimun size for ip datagram is 64.

check this link, if you add the size(bytes) colum it will leave the last three fields as variants, the last field at the bottom is the data payload which can be bigining with 64 bytes being the minimun size, or 128, or 256, 512 and so on to come up with a total of 1500 bytes for ethernet max MTU size.

http://www.tcpipguide.com/free/t_IPDatagramGeneralFormat.htm

ip packet header format, size is 20 bytes

http://www.erg.abdn.ac.uk/users/gorry/course/inet-pages/ip-packet.html

now the question to you is why do you want to change the MTU to 20 bytes without any data payload ? I am curious of what you are trying to do.

rgds

Jorge

[ananda_id]

Jorge,

The link that u gave me still can't answer the question. Can I send 20 bytes IP Packet header only without any payload?

Well, first I only try to help my friend with his thesis. But I found this question rather interesting so I share with all netpros.

Thanks,

Ananda

[JORGE RODRIGUEZ]

you can't change the mtu to 20 becasue the interface would not allow it, MTU minimun is 64 on any given interface on a router.

Jorge