Ethernet packet size python

[Grant Edwards]

On 2009-01-13, Steve Holden <steve at holdenweb.com> wrote:
> K-man wrote:
>>
>> I am sending data using the socket interface in python, but I
>> want to know how big the ethernet packet size is (in bytes).
>> I didn't really see a way using the socket library of how to
>> do this.  Any suggestions?
>
> There is no way to know what size Ethernet packets will result
> from specific traffic.

Unless he's using a low level API such as AF_PACKET/SOCK_RAW.
If that's the case then the packets will be exactly as big as
he makes them. And he wouldn't be asking this question. :)

> Or do you want to know the MTU size (largest possible Ethernet
> packet size)? This shouldn't really matter, since large TCP
> messages will be split into a sequence of IP datagrams, and
> large IP datagrams will be automatically fragmented and then
> reassembled at the other end.
>
> Is there a specific reason this is important to you?

If it's just idle curiosity, then wireshark or tcpdump can show
one exactly what's going on on the wire.  A reading of 
http://en.wikipedia.org/wiki/Transmission_Control_Protocol
should also allow one to predict pretty accurately what's going
to happen when you call send().

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