Socket programming at low level

I am unable to understand or grasp rather; what it means to program at a lower layer in socket programming. I am used to working with tcp/udp/file system sockets. These are all wrapped around their own protocol specifications ... which as i understand would make it working at the a开发者_JS百科pplication layer in the stack.

In the project i am on , i have seen some files which are "named" LinkLayer, TransportLayer... but i don't see any more calls other than standard socket calls....send /recv/ seletct...

Does the fact you are setting a socket options mean you are programming at a lower level ? Is it just restricted to it? Or are there other API's which grant you access at the representation in kernel ?

Typically this refers to using SOCK_RAW sockets, which requires you to assemble your own packet headers, calculate checksums, etc. You still use send/recv/etc. but now you are responsible for making sure every bit is in the right place.

You can use SOCK_RAW sockets to implement protocols other than TCP or UDP, or to do things with the Internet protocols that higher-level interfaces don't accommodate (like tweaking the TTL of your packets to implement something like traceroute).

This usually means working on a lower OSI-Layer, for example, not directly sending TCP-streams or UDP-packets, but crafting own IP or even Ethernet packets or other low-layer protocols which would - in normal case - be handled by the operating system.

This can be done done via specific socket options which enable you to receive or send data on any layer, even layer 2 (Data Link).