The Network Layer

Basic concepts

Objective: Provide a packet delivery system for the transport layer, using the point-to-point links of the data link layer.
Give up the assumption that every node is directly connected to every other node.

Instead, nodes are connected indirectly through a subnet of routers.



Routers use store-and-forward packet switching.

• Data is split into shorter "packets".
• Packets are sent from source to destination, and reassembled at the destination.

Two fundamental router functions:

• Forwarding -- For each packet read on an input port, determine which output port should it be written to. (local)
• Routing -- Determine the end-to-end path along which to send a packet from host A to host B. (global)

Design Issues

• Class of service
• Connectionless vs connection-oriented
• Level of service
• Router design
• What's inside a router?
• Routing algorithms
• Congestion control algorithms
• Addressing

Connectionless vs. connection-oriented service

Two methods of packet switching::

1. Datagram (connectionless)
• Each packet is routed independently of all others.
• Each router has a routing table, mapping addresses to output ports. Q: How is the routing table populated? Q: There are lots of addresses. Answers determined by a routing algorithm and the addressing structure.
• example:  IP
• Advantage:  More adaptable to failures
• Disadvantage:  Routing has to be recalculated for each packet.
2. Virtual circuit  (connection oriented)
• Connection is set up between sender and receiver
• The connection is essentially a path/route with an identifier.
Issue: how to assign unique identifiers? Answer: Don't. The VC can have a different id on each leg between routers. This is called label-switching.
• Every switch along the way knows about the virtual circuit
• Once established, all packets follow the same route.
• In order delivery
• examples:  X.25, ATM
• SVC -- Switched virtual circuit
• PVC -- Permanent virtual circuit

Tradeoffs:

• VC requires setup time and protocols. All routers on the path implementing the VC must communicate to set up the VC.
• Datagram is stateless; better to survive router crashes, since router memory does not contain state infomration about connections
• VC is better able to provide quality of service guarantees and congestion control, since resources can be preallocated along the route.

Note:  TCP/IP is a connection-oriented transport layer on top of a connectionless network layer.  UDP/IP is a connectionless transport layer over a connectionless network layer.

Levels of service

• Guaranteed delivery
• Guaranteed delivery with bounded delay
• In-order delivery
• Guaranteed minimum bandwidth (data rate)
• Guaranteed minimum jitter
• Best-effort service (nothing is guaranteed)
• Internet:
• best effort
• no guaranteed bandwidth
• no guarantee against lost packets
• in-order delivery not guaranteed
• ATM (connection-oriented) provides two service levels:
• CBR (Constant bit rate):
• guaranteed constant bandwidth
• no lost packets
• in-order delivery
• ABR (Available bit rate):
• guaranteed minimum bandwidth
• no guarantee against lost packets
• in-order delivery guaranteed

Question: How can/should IP evolve to support real-time services like multimedia?

Router design

What's inside a router?

Basic functions: forwarding and routing




Structure:

• Input ports
• Output ports
• Switching fabric
• Memory
• Bus
• Crossbar
• Routing processor

Queueing

• Packets may need to be queued at input ports or output ports. Why?
• Input: Packets may arrive faster than the switching network can process them.
• Output: Several packets arrive at the same time on different input ports, all addressed to the same desitination.