Ethernet – How does data encapsulation work in the TCP/IP stack

ethernetipv6udp

As a UDP segment traverses down the stack, each layer adds a header.

What occurs for this to happen? Does the kernel do a bunch stuff then add the header?

I am particularly interested in the transition from the network to data-link layer on an Ethernet system.

Is ARP performed and then the cache searched in order to provide the next hope information?

Best Answer

Each OS can do this differently. It is up to the OS designers how this specifically happens in an OS.

RFC 826, An Ethernet Address Resolution Protocol -- or -- Converting Network Protocol Addresses to 48.bit Ethernet Address for Transmission on Ethernet Hardware gives you a general outline of what is supposed to happen, but an OS could skip the whole table creation and use ARP requests for every packet.

Packet Generation:

As a packet is sent down through the network layers, routing determines the protocol address of the next hop for the packet and on which piece of hardware it expects to find the station with the immediate target protocol address. In the case of the 10Mbit Ethernet, address resolution is needed and some lower layer (probably the hardware driver) must consult the Address Resolution module (perhaps implemented in the Ethernet support module) to convert the <protocol type, target protocol address> pair to a 48.bit Ethernet address. The Address Resolution module tries to find this pair in a table. If it finds the pair, it gives the corresponding 48.bit Ethernet address back to the caller (hardware driver) which then transmits the packet. If it does not, it probably informs the caller that it is throwing the packet away (on the assumption the packet will be retransmitted by a higher network layer), and generates an Ethernet packet with a type field of ether_type$ADDRESS_RESOLUTION. The Address Resolution module then sets the ar$hrd field to ares_hrd$Ethernet, ar$pro to the protocol type that is being resolved, ar$hln to 6 (the number of bytes in a 48.bit Ethernet address), ar$pln to the length of an address in that protocol, ar$op to ares_op$REQUEST, ar$sha with the 48.bit ethernet address of itself, ar$spa with the protocol address of itself, and ar$tpa with the protocol address of the machine that is trying to be accessed. It does not set ar$tha to anything in particular, because it is this value that it is trying to determine. It could set ar$tha to the broadcast address for the hardware (all ones in the case of the 10Mbit Ethernet) if that makes it convenient for some aspect of the implementation. It then causes this packet to be broadcast to all stations on the Ethernet cable originally determined by the routing mechanism.

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