How does a packet travel from one computer to another over the Internet based on OSI model

Question

I am familiar with the basic OSI model but I always get confused how does a packet travel from one machine to another over the Internet and what OSI layers do come into picture?

Answer 1

Routers use the IP layer (layer 3) and switches use the data-link layer (layer 2). Layer 1 is the physical 1s and 0s that go over a wire, Layer 2 is the data-link layer, which is protocols like Ethernet and Point-To-Point Protocol (PPP), which carries information between adjacent nodes about MAC address from and to and allows for error detection and retransmission. Layer 3 is the IP layer, which carries information about where in the whole network the packet is from and to, not just the current hop.

The transmission would go like this:

Machine A wants to send a packet to Machine B. Machine A knows Machine B's IP address, so it places that in the layer 3 packet. Machine A needs to place the MAC Address of the next hop in the layer 2 packet, however. If it does not know, then it will send something called an ARP request (Address Resolution Protocol, read here: http://www.tildefrugal.net/tech/arp.php ) to the network, with the destination IP. One of a few things will happen here:

1. The IP is local. The machine with that IP will reply back to the sender with its MAC address.
2. The IP is non-local. The gateway router will detect this and send its MAC address.
3. The IP is non-local and Machine A's default gateway and subnet mask are set. Using this information Machine A can determine the non-locality of the IP address and send it to the router's MAC address (ARPing if not known yet).

(If Machine A found this out earlier, it will be in the ARP cache and Machine A will just use that.) Now that the MAC address is sent, the packet can be transferred (the physical layer 1 performing the actual transfer of data on the wire). The next stop will be the switch. The switch knows which outbound port the MAC address listed as the layer 2 destination is on, because it tracks every MAC address it's seen a packet come from and which port it came on - if it does not know, then it will flood it out every single port, guaranteeing it'll arrive.

As such, the packet arrives at the router. The cool thing about the IP model is that it divides every single IP address in the network/world into a hierarchy - Subnets by definition cannot overlap subnets partially, they either wholly contain them or are wholly contained by them. So as long as subnets follow this hierarchy, the router can unambiguously determine where each of the 4 billion possible IP addresses are on the network just by looking at what subnet the IP will fall under in its table! The packet is then sent out that port.

As the packet travels through interconnected ISPs' routers, backbone infrastructure and so on, it arrives at Machine B's router, where the opposite process happens - router B sees that its destined for Machine B and sends it inbound. (Similarly, Router B will have to use a process like ARP to find Machine B's MAC address if not known.) The rest should be trivial from here.

good references:

https://web.archive.org/web/20120129120350/http://www.tildefrugal.net/tech/arp.php

http://en.wikipedia.org/wiki/Data_link_layer

http://en.wikipedia.org/wiki/Network_switch

http://en.wikipedia.org/wiki/Network_layer

http://en.wikipedia.org/wiki/Routing

http://en.wikipedia.org/wiki/Router_(computing)

http://en.wikipedia.org/wiki/Address_Resolution_Protocol

Answer 2

The only thing that can travel over a copper wire are pulses of electricity. The binary number 1 is represented by a pulse of electricity or no pulse of electricity for 0. Just keep in mind that real data of any kind cannot be sent over copper wire, fibre optic, or through the air ...only a representation of the data which has previously been converted to a 1 or a 0 and then is reconverted back at the receiving end.

Answer 3

Network layer protocol supervises the transmission of packets from a source machine to a destination. Data is broken down into packets, or datagrams, up to 64 kb long before it is transmitted, with a stamp of destination IP address, and forwarded to the network gateway. A gateway can be router to interconnect networks.