Re: [lwip-users] local_ip and remote_ip

[David Empson]

Peter Nguyen wrote:
> I've been inspecting some lwip code, and to get IP addresses, i've noticed 
> two variables for storing ip addresses:
> local_ip, and remote_ip. I was wondering whether anyone is able to tell me 
> the difference between the two variables.

TCP connections operate by establishing a connection between a local device 
and a remote device. Each end of the connection is identified by an IP 
address and port number, and this information is carried in each packet 
transmitted or received for the connection. (All packets contain two IP 
addresses and two port numbers.)

UDP operates on the same principle, but without maintaining connections. A 
UDP packet is transmitted from a local IP address and port to a remote IP 
address and port (or vice versa).

LWIP has a "pcb" structure which maintains information about a connection 
(for TCP) or a sending/receiving port (UDP). This structure includes fields 
to keep track of the local and remote addresses, called local_ip and 
remote_ip. In some code, these addresses are copied into variables called 
local_ip and remote_ip (and the port numbers may also be copied into 
local_port and remote_port). Similar variables may be used in places where 
packets are being processed without reference to a pcb.

In general, local_ip is the address of LWIP (or one of LWIP's addresses, if 
it has multiple network interfaces), and remote_ip is the address of the 
other device involved in the same TCP connection or for a particular UDP 
packet.