On Wed, Mar 14, 2001 at 05:03:10PM +0000, David Malone wrote:
> I'm trying to figure out what #define I should use for the largest
> UDP datagram. The header files don't seem to define a constant for
> this. The correct size would seem to be 65535 'cos there are two
> bytes in the UDP header for the size.
>
> IP_MAXPACKET and IPV6_MAXPACKET are available and have the right
> values, but these don't really seem to be the correct thing to use
> 'cos they don't take headers or the possibility Jumbo payload into
> account.
>
> I wanted to commit something for:
>
> http://www.FreeBSD.org/cgi/query-pr.cgi?pr=25050
>
> but I'm not convinced that the patch is spot on. I could determine
> the data size and malloc memory dynamically I guess.
>
The maximum size of IPv4 UDP datagram is 65535-28=65507:
IP_MAXPACKET - sizeof(struct ip) - sizeof(struct udp)
provided that the send/receive buffers were set appropriately:
: tcpdump: listening on lo0
: 127.0.0.1.49431 > 127.0.0.1.1: udp 65507 (frag 7691:16360@0+)
: 127.0.0.1 > 127.0.0.1: (frag 7691:16360@16360+)
: 127.0.0.1 > 127.0.0.1: (frag 7691:16360@32720+)
: 127.0.0.1 > 127.0.0.1: (frag 7691:16360@49080+)
: 127.0.0.1 > 127.0.0.1: (frag 7691:75@65440)
--
Ruslan Ermilov Oracle Developer/DBA,
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