Networking: Network Layer Summer 2013 Cornell University 1 Today - - PowerPoint PPT Presentation

1

CS 4410 Operating Systems

Networking: Network Layer

Summer 2013 Cornell University

2

Today

• How packages are exchanged in a WAN?
• Network Layer
• IP
• Naming
• Subnetwork
• Forwarding
• Routing Algorithms

3

Protocol Stack

Application Link Physical Application Link Physical Computer A Computer B Transport Transport Network Network M M Ht M Ht Hn M Ht Hn Hl Message Segment Datagram Frame

4

WAN

• Usually, thousands of computers need to be

interconnected.

• The capabilities that LANs offer cannot support

larger networks.

• We need more services than the Link Layer
• ffers.
• Why?
• Clever Naming
• Efficient forwarding/routing of messages.

5

Network Layer

• Mission: Transfer messages from the source-computer to the destination-

computer.

• Attention: this is different from the mission of the Link Layer.
• Services:
• Forwarding / Routing
• Guaranteed delivery, bandwidth, etc
• Security
• Not all the protocols support these services.
• The Network Layer protocol depends on the kind of network we want to built:
• Virtual-circuit networks
• Datagram networks
• Necessary network device:
• Router: It knows where to forward the message.

6

Network Layer

• Virtual-circuit networks
• 3 phases
• Establish a virtual circuit.

–

The Network Layer finds the path from the source to the destination.

–

Reserve resources for the virtual circuit.

• Transfer data

–

Packets pass through the virtual circuit.

• Destroy virtual circuit.

–

Release resources.

• Disadvantages?
• Datagram networks
• Every packet has the destination

address and it is routed independently in the network.

• The router uses the destination

address to forward the packet towards the destination-computer.

7

IP

• Network Layer Protocol for the Internet:
• Internet Protocol
• For Datagram networks.
• IPv4, IPv6
• Datagram structure:

Version Header Length Type of service Length Identification Flags Fragment Offset Time to live Protocol Header Checksum Source IP Address (32-bit) Destination IP Address Options Data

8

Naming

• All the computers in the Internet have one or more IP addresses.
• For IPv4:
• 32 bits
• Dotted-decimal notation (Ex: 147.76.89.4)
• Contain information about the subnetwork in which a host belongs.

–

Example: For the address 140.251.27.18 we know that:

• It belongs to a host in Cornell, as Cornell gives addresses of the form

140.251.xxx.xxx. → subnetwork address 140.251.0.0/16, mask 255.255.0.0

• It belongs in host in Linguistics Department, as the addresses of this

department is 140.251.27.xxx. → subnetwork address 140.251.27.0/24, mask 255.255.255.0

• The number 18 distinguish this host from other hosts in the same

subnetwork of Linguistics.

• Assigned by a DHCP server in the subnetwork.
• Dynamic Host Configuration Protocol
• Every computer that is inserted in the subnetwork, communicated with the DHCP

server to obtain an IP address.

9

Subnetwork

223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.3.27 223.1.3.1 223.1.3.2 223.1.2.1 223.1.2.2

What happens when 223.1.1.1 wants to send a packet to 223.1.3.2 ?

10

Forwarding

• Each router has a routing table.
• The routing table is an array of triples (at least).
• Each tuple has:
• Subnetwork id, subnetwork mask, gateway
• So, the routing table of the previous example is:

223.1.1.0, 255.255.255.0, 0 223.1.3.0, 255.255.255.0, 1 223.1.2.0, 255.255.255.0, 2

• When a datagram is received, all the subnetwork masks are applied to its IP address (binary AND
• peration) to find the subnetwork in which the destination belongs.
• So, a datagram with destination to 223.1.3.2 is forwarded to gateway 1.
• Even though we know the destination IP, we do not know the MAC address of 223.1.3.2, in order the

packet to go from the gateway 1 to the destination.

• With the ARP protocol the router asks which node of one subnetwork has the needed IP.
• The destination replies with its MAC address.
• Finally, the packet can be sent to the destination.

11

Routing Algorithms

• How does the router construct its

routing table?

• The routing path should be the shortest

path from the source to the destination.

• General problem:
• Graph (V, E)
• V is the set of routers.
• E is the set of links between the

routers.

• Each edge has a cost related to

the distance.

• If the source is attached to the

router V1 and the destination to the router V2, what is the shortest path between V1 and V2? 2 1 2 3 1 1 2 5 3 V1 V2 5

12

Routing Algorithms

• Find shortest bath between all the possible pairs of nodes.
• Global routing algorithm
• The algorithm needs global knowledge (all links between routers and their costs)
• f the network as input.
• Dijkstra algorithm

–

Returns the shortest path between one node and all the other nodes.

• Decentralized routing algorithm
• No router has global knowledge about the cost of all links.
• Routers, iteratively, exchange information with their neighbors for shortest paths

between nodes.

• Distance vector algorithm

–

Returns the shortest path from all nodes to all nodes.

13

Network Layer

Computer A Application Link Physical Application Link Physical Computer B Transport Transport Network Network Link Physical Switch Link Physical Switch Link Physical Router Network

14

Network Layer

Switch Router

15

Today

• How packages are exchanged in a WAN?
• Network Layer
• IP
• Naming
• Subnetwork
• Forwarding
• Routing Algorithms