CS 453 Computer Networks Lecture 18 Introduction to Layer 3 Network Layer

So far, our communications has assumed that everything is point to point… We have a message at A and we send it to B We have a message at A and we send it to B We had, in layers 1 and 2, channels that gave us a “connection” between A and B

Network Layer Granted, we discussed a lot of media and protocols to make these connections… …and we had hubs and switches in the middle… But the intervening media (and hubs and switches) to took care of path… And made it seem like a “wire”

Network Layer Well, as you probably have guessed, its usually not that simple Once we push our networking out past a LAN we have a new realm of complications Internetworking LANs or even stations is the domain of the network layer

Network Layer Stations connect to networks, networks connect to other network, which connect to other networks,…. Networks take on complex topologies Fundamentally, the job of the network layer is to take messages from the transport layer and create packets for the data link layer It must do this in the context of the topology of the network

Network Layer From: Data Link Layer Network Layer Transport Layer Application Layer

Network Layer From:

Network Layer Network layer services Independent of router technology Independent of router technology Network topology, technology should be transparent to the Transport layer Network topology, technology should be transparent to the Transport layer Addressing scheme should be uniform Addressing scheme should be uniform Two classes Connectionless Service Connectionless Service Connection-oriented Service Connection-oriented Service

Network Layer Connectionless Service Strongly advocated by the Internet community Strongly advocated by the Internet community Underlying principle of the Internet Underlying principle of the Internet That the Network layer should That the Network layer should Send Packets Receive Packets And that’s about it

Network Layer Connectionless Service Packets are fed into the network Packets are fed into the network Network must route packets to destination Network must route packets to destination Packets are routed to destination independently Packets are routed to destination independently Packets transmitted in connectionless context are often referred to as datagrams Packets transmitted in connectionless context are often referred to as datagrams A connectionless network or portion thereof, might be called a datagram subnet A connectionless network or portion thereof, might be called a datagram subnet

Network Layer Connectionless Service Think of a datagram like a postal letter Think of a datagram like a postal letter You drop it in the mailbox and it arrives at its destination You drop it in the mailbox and it arrives at its destination It could have taken any of several destinations to get there,… It could have taken any of several destinations to get there,… But figuring that out is not your problem But figuring that out is not your problem Imagine sending several letters to the same destination Imagine sending several letters to the same destination They all arrive, but… They all arrive, but… They may have taken different routes to get there They may have taken different routes to get there

Network Layer Connectionless Service Imagine that the transport layer (a process) in host H1 has a message to send to host H2, and,… Imagine that the transport layer (a process) in host H1 has a message to send to host H2, and,… … it is a big message – 4 times the max packet size … it is a big message – 4 times the max packet size The network layer takes the message and breaks into 4 packets, and sends it… The network layer takes the message and breaks into 4 packets, and sends it…

Network Layer Connectionless Service From: Tanenbaum, 2003, pg. 346

Network Layer Connectionless Service Note: each packet can take a different route Note: each packet can take a different route Route decisions are made by the routers… Route decisions are made by the routers… On packet by packet basis On packet by packet basis Route decisions are made based on route tables located in each route Route decisions are made based on route tables located in each route Route table does not tell exact how to get to the destination, but which direction (which router port, or connection link) Route table does not tell exact how to get to the destination, but which direction (which router port, or connection link)

Network Layer Connectionless Service From: Tanenbaum, 2003, pg. 346 Each router has its own table Each table entry says for a given destination address – send the packet out on this port So, in router A, a packet with a destination of F is sent to port C (like to router C).. At least initially Router C gets the packet, see that it is destined for F sends it out on port E Notice that router A’s route table changes so that packets headed for F get sent to B (rather than C)

Network Layer Connectionless Service Routers can change their route tables Routers can change their route tables Routers’s job to maximize the flow of packets Routers’s job to maximize the flow of packets May change route tables because May change route tables because Link down Congested link … Router decisions and route table updates are Routing Algorithms Routing Algorithms - complex topic We will look at Routing Algorithms in more detail

Network Layer Connectionless Service By the way, this is the way IP routing works By the way, this is the way IP routing works

Network Layer Connection-Oriented Service Route through network established before packets flow Route through network established before packets flow Fixed route Fixed route Once defined all packets flow through the same route Once defined all packets flow through the same route Referred to as a Virtual Circuit Referred to as a Virtual Circuit When a sending host wants to send a packet… When a sending host wants to send a packet… The network first sends a setup packet The network first sends a setup packet The setup packet finds a path and … The setup packet finds a path and … Make forwarding table entries along the path Make forwarding table entries along the path

Network Layer Connection-Oriented Service Virtual Circuit has a connection ID Virtual Circuit has a connection ID Once a packet is assigned a connection ID (a label) it no longer needs a destination address Once a packet is assigned a connection ID (a label) it no longer needs a destination address Packet follows the virtual circuit, … Packet follows the virtual circuit, … …not routed by destination address …not routed by destination address Sort of like a phone call – once you are talking the other person, you don’t need their phone number to continue the conversation Sort of like a phone call – once you are talking the other person, you don’t need their phone number to continue the conversation When the packet stream is finished, the connection is released, and …. When the packet stream is finished, the connection is released, and …. The virtual circuit is torn-down The virtual circuit is torn-down

Network Layer Connection-Oriented Service From: Tanenbaum (2003), pg 348

Network Layer Connection-Oriented Service Once the circuit is know the virtual circuit label is added to the packet Once the circuit is know the virtual circuit label is added to the packet Virtual circuit labels are per hop Virtual circuit labels are per hop …that is, the label in a packet can be changed by the routers along the route …that is, the label in a packet can be changed by the routers along the route

Network Layer Connection-Oriented Service In the previous diagram, H1, then H3 initiate the flow of packets. In the previous diagram, H1, then H3 initiate the flow of packets. H1 establishes VC 1, so it VC label is 1 H1 establishes VC 1, so it VC label is 1 H3 starts a flow of packets, H3 starts a flow of packets, …and since it is coorinating with H1, it assigns the VC with label 1… …and since it is coorinating with H1, it assigns the VC with label 1… But Router A recognizes that VC 1 belongs to another packet flow,…so But Router A recognizes that VC 1 belongs to another packet flow,…so It renames the VC as 2, It renames the VC as 2, From A to the destination then that VC is VC 2… From A to the destination then that VC is VC 2…

Network Layer Connection-Oriented Service So, at A the router removes the VC label from the connection Id field of the packet header, and… So, at A the router removes the VC label from the connection Id field of the packet header, and… …replaces it the new VC number (2, in this example) …replaces it the new VC number (2, in this example) Replacing a label in route is called “label switch” Replacing a label in route is called “label switch” We will talk about this more later We will talk about this more later

Network Layer

So, how do they compare Connectless vs. Connection-Oriented Service Connectless vs. Connection-Oriented Service Datagrams vs. Virtual Circuits Datagrams vs. Virtual Circuits Datagrams must carry full source and destination addresses VC packets only carry the VC Label Datagram addresses must be parse and translated at each hop VC packet label simply used as a lookup

Network Layer So, how do they compare Connectionless service requires no setup Connectionless service requires no setup Send a datagram and the routers will help it find its way No pretransmission overhead Overhead per packet Connection-Oriented Services requires the VC to be setup before packet flow Connection-Oriented Services requires the VC to be setup before packet flow Setup packet sent first to establish VC For short transactions – a packet or two – connection-oriented service very expensive

Network Layer So, how do they compare Resilency to failure - router failure, line failure Connectionless – good, basically doesn’t care Connectionless – good, basically doesn’t care Not a problem – routers find a different route Connection Oriented – big problem Connection Oriented – big problem VC torn down, packet flow lost New VC must be established be transmission restarted

Network Layer So, how do they compare Quality of Service Connectionless Service Connectionless Service None, difficult, maybe Connectionless – best available Connection-Oriented Service Connection-Oriented Service Good, easy to establish Resources can be reserved when VC is setup

Network Layer So, how do they compare Traffic load, congestion handling Connectionless Service Connectionless Service Not much, sort of Connection-Oriented Connection-Oriented Easy, reliable Reserve resources (bandwidth) at VC setup