IP HEADER CHECKSUM CALCULATOR CODE

The former is useful for universal applications and the latter is for maintaining signaling and routing applications tightly coupled with the host stack. Since the OP states that the use case is just for 'dealing with the wire format' and the package description of x/net/ipv4 says 'IP-level socket options', I think the objectives are different (even though the implementation in CL 112817 may work for any layer tapping on Linux). The next two bytes (B1E6) correspond to the IP header checksum of the packet, we'll calculate this 'manually. The next byte (06) correspond to the IP protocol field, which is set to 6, so the packet contains a TCP segment on it's payload. if you want MTU for QinQ over physical Ethernet links, just add QinQ, as your physical link. Protocol overhead values here are just what they add to the frame. for TCP over IPv6 encapsulated into GRE over IPv4, add IPv4, GRE, IPv6, and TCP. Otherwise, applications may face funny troubles like 'ipv4.PaserHeader parses IPv4 datagrams captured in link-layers on BSD variants incorrectly.' The next byte (40) correspond to the Time To Live field (TTL), which actually is 64 (4x161 + 0x160 64). If you want to calculate TCP MSS, add the underlying protocol and TCP after tunnel protocols. As long as the kernels tweak IPv4 datagrams when they process incoming datagrams, applications must be sure whether the datagrams pass through the IPv4 protocol stack or not on the kernels, unfortunately. X means a protocol instance that is capable of tunneling/stacking with IPv4. In CL 112817, it looks to me that calculating a wire-format IPv4 header checksum field using the output of 'without calibration' may generate a wrong value on the kernels mentioned above. So I'd prefer making a new wire-format friendly API instead. I still don't think it's worth the effort of abstraction (or adding API just work for Linux and IPv4 datagrams captured in link layers.) I still don't want to mix the existing host-stack friendly IP-level socket option API, x/net/ packages, with wire-format friendly API for avoiding unnecessary confusion. In addition, the tweaks/requirements change in context, for example, X over IPv4, IPv4 over X and IPv4 header in ICMP error notification may need different tweaks/requirements. On IPv4 datagram reception via raw IP socket, some kernels tweak a few fields of the received IPv4 header, and on the transmission, some kernels require a few wire-format incompatible fields of IPv4 header, ignore the existing checksum field and recalculate and set the checksum field.

IP HEADER CHECKSUM CALCULATOR CODE

It sounds like some operating systems separately require user-space code to send the kernel a different checksum over IPv4 headers in certain contexts I think I didn't use the word 'system checksum.' Please let me know the place where I used the word for correction for avoiding unnecessary confusion.

Two checksums (a system checksum vs a wire checksum?) as says.

Ip header checksum calculator