Network(3) -- The Network Core

The network core

The network core is the mesh of packet switches and links that interconnects the Internet’s end systems.

Packet Switching

End systems exchange messages. A message is sent from a source end system to a destination end system, the source breaks long messages into smaller chunks of data known as packets. Each packet travels through communication links and packet switches.

Two types: routers and link-layer switches.

If a source end system or a packet switch is sending a packet of L bits over a link with transmission rate R bits/sec, then the time to transmit the packet is L/R seconds.

Store and Forward Transmission

Store-and-forward transmission means that the packet switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link.

The time from the source begins to sent the packet until the destination has received the entire packet is 2L/R. Routers need to receive, store, and process the entire packet before forwarding.

Three packets need 4L/R seconds.

A path consisting of N links each of rate R(N-1 routers). The end-to-end delay is NL/R

Queuing Delays and Packet Loss

For each attched link, the packet switch has an output buffer(output queue). Packets suffer output buffer queuing delays. If the buffer is completely full, packet loss will occur.

Forwarding Tables and Routing Protocols

In the Internet, every end system has an address called an IP address. Each router has a forwarding table that maps destination addresses (or portions of the destination addresses) to that router’s outbound links.

How do forwarding tables get set? The Internet has a number of special routing protocols