Re: [lwip-users] Max Datagram Size

[David Vos]

Frédéric,

In RFC 879, it says "HOSTS MUST NOT SEND DATAGRAMS LARGER THAN 576
OCTETS UNLESS THEY HAVE SPECIFIC KNOWLEDGE THAT THE DESTINATION HOST
IS PREPARED TO ACCEPT LARGER DATAGRAMS."

However, since you say you are sending/receiving on an Ethernet
network, you can assume that you can use the full Ethernet MTU.

Another RFC maximum size would be around 64KB, the maximum length of
an IP packet.

If you are using TCP, the MSS option allows the two endpoints to
communicate their local MTUs, which makes efficient use of the
network.  I did an isolated experiment recently with a Linux box and
an unknown box on the Internet and discovered that they negotiated to
using around 1500 byte packets, and there was no IP fragmentation.
Therefore, the Internet core in this case was able to handle packets
of that size.  However this was only a single test, and was not
comprehensive.

I hope this helps,
-David Vos


On 8/1/07, Frédéric BERNON <address@hidden> wrote:
> Hi group,
>
> Is someone can tell me if there is any RFC restricting the size of a datagram 
> in broadcast and in multicast on a ethernet network (other than the MTU)?
>
> I have in mind 512 bytes for broadcast datagrams, but I'm not sure if it's a 
> "RFC requirement", or a "known good practice"...
>
> ====================================
> Frédéric BERNON
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