AW: [lwip-users] What is the different between IP_PROTO_UPPLITE a nd IP_PROTO_UDP(regular)????

[Zschocke, Florian]

Kelly Chan wrote on Freitag, 4. Juli 2003 16:09:

 
> I am a little confused as I don't understand the motivation
> behind two version of UDP IP Protocol: IP_PROTO_UDP &
> IP_PROTO_UDPLITE.  

UDP packets of the type IP_PROTO_UDP have a checksum calculated over the
complete UDP packet, i.e. the UDP header plus the payload. UDP packets of
the type IP_PROTO_UDPLITE have a checksum calculated ONLY over the UDP
header but not over the payload of the UDP packet. 

> Also, how does the receiver side know which version shall adopt
> when a UPD packet arrive (According to RFC792, the transmission
> protocol does not have version field)???  

UDPLITE is a different protocol type than UDP is. In the IPv4 header the
"protocol" field will have different values for UDP and UDPLITE: UDP is
protocol 17, UDPLITE is 170, just like the IP_PROTO_UDP(LITE) defines. For
both a checksum of 0 means no checksum at all.

> On top of that, the IP_PROTO_UDP has a flag with NO_CHECK_SUM
> but the codes still perform checksum??? 

Where, udp_send() in udp.c? No, a checksum is not calculated if
UDP_FLAGS_NOCHKSUM is set in the PCB.

Florian.