C SC 481.20 Lecture 2: Packet Switching and Internet Structure
major resource: Computer Networking (4th Edition), Kurose and Ross, Addison Wesley, 2008

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More on Packet Switching

Multiplexing occurs here too, only without reserved slots.

Since packets are sent randomly from various sources, this is called Statistical Multiplexing

Buffering occurs in router:

• input buffer contains incoming packets
• output buffer contains outgoing packets
• entire packet must be received before outputting, this is store-and-forward

Sources of delay in packet-switched network:

• transmission (a.k.a. store-and-forward) delay occurs at router (first arriving bit cannot be transmitted until last bit arrives) depends on transmission speed and message/packet length (a.k.a. transmission delay)
• processing delay occurs at router (time required for router to make routing decision)
• queuing delay occurs at router (packet waiting in output buffer for transmit line to become free)
• propagation delay occurs on links (time required for bit to travel from one router to next) depends on speed of energy through medium and length of link.

Delay Example:

Delay Example:

Delay Example:

Packet loss in a packet-switched network

• How can packets get "lost"?
• Well, what does a router do when its buffers are full and a new packet arrives?
• Internet router cannot control the timing of an incoming packet
• Routing is a Network Layer task, and the network layer on the Internet is IP (Internet Protocol)
• IP does not guarantee delivery, so routers are free to drop packets
• How can delivery be assured in this situation?

Routing Illustration: traceroute service

• Traceroute is a service that, well, traces the Internet route between a given source and destination
• Each router in the path is shown as one line of text output that includes performance data
• Can see delays on the network (round trip time from source to each router)
• Can see hierarchical nature of routing system
• Unix/Linux: traceroute command
• DOS prompt: tracert command
• On the web: www.traceroute.org and others
• Implementation uses TTL (time-to-live) field in IP header - is decremented at each router and error message returned if 0.
• First round go out with TTL=1, second round with TTL=2, etc

Internet Structure

The Hierarchical Internet: truly a network of networks

• Tier-1 ISPs, the international telecom companies and agencies, form the "backbone" of the Internet with fiber optic and wireless connections of very high speed, and connect to each other.
• Tier-2 ISPs, usually smaller organizations, tap into the backbone through one or more Tier-1's. They also connect to each other. They are customers of the Tier-1's.
• Tier-3 ISPs and local ISPs tap into Tier-2 and each other. They are the customers of the Tier-2's.
• Organizations tap into these.
• The hierarchy is illustrated through traceroute, as noted above

Layered Structure of Internet protocols and services architecture

• Layered protocols necessary to support heterogeneous networks
• Layering necessary to deal with complexity
• Layering can also muddy some issues (e.g. is there a layer dedicated to security? no!)
• We focus on the five-layer Internet protocol stack
  • Application Layer - protocols supporting networked application software (network edge)
  • Transport Layer - protocols supporting message delivery (network edge)
  • Network Layer - protocols supporting routing (network core)
  • Link Layer - protocols supporting link-to-link transmit (network core)
  • Physical Layer - transmitting bits
• Course will be largely structured around this stack, top-down