Re: Question about luasocket-2.0-alpha and udp packet size.

[Philipp Janda <siffiejoe@...>]

Stewart, Joseph schröbte:
> Hello Diego,
>  
> First let me say THANKS for your luasocket implementations over the years.
> They've been very useful to me.
>  
> Recently I began using luasocket-2.0-alpha to send/receive udp packets. I
> noticed in the documentation and code that you've set the max send/receive
> size to 576, but there's no real explanation as why.
>  
> Since I need to send/receive udp data ~1500 bytes, I just modified the
> UDP_DATAGRAMSIZE in udp.h to 1500 and everything seems to be okay.
>  
> Can you shed some illumination on why 576 was chosen as the buffer size and
> what I might break by changing it?

From http://www.faqs.org/rfcs/rfc791.html (IP Specification):
...
Total Length:  16 bits

    Total Length is the length of the datagram, measured in octets,
    including internet header and data.  This field allows the length of
    a datagram to be up to 65,535 octets.  Such long datagrams are
    impractical for most hosts and networks.  All hosts must be prepared
    to accept datagrams of up to 576 octets (whether they arrive whole
    or in fragments).  It is recommended that hosts only send datagrams
    larger than 576 octets if they have assurance that the destination
    is prepared to accept the larger datagrams.

    The number 576 is selected to allow a reasonable sized data block to
    be transmitted in addition to the required header information.  For
    example, this size allows a data block of 512 octets plus 64 header
    octets to fit in a datagram.  The maximal internet header is 60
    octets, and a typical internet header is 20 octets, allowing a
    margin for headers of higher level protocols.
...


And from 
http://www.thinkingsecure.com/docs/TCPIP-Illustrated-1/ip_inter.htm#3_2
...
Although it's possible to send a 65535-byte IP datagram, most link 
layers will fragment this. Furthermore, a host is not required to 
receive a datagram larger than 576 bytes. TCP divides the user's data 
into pieces, so this limit normally doesn't affect TCP. With UDP we'll 
encounter numerous applications in later chapters (RIP, TFTP, BOOTP, the 
DNS, and SNMP) that limit themselves to 512 bytes of user data, to stay 
below this 576-byte limit. Realistically, however, most implementations 
today (especially those that support the Network File System, NFS) allow 
for just over 8192-byte IP datagrams.
...

>  
> Thanks again!
>  
> -joe

Philipp