Network Layer

Moving datagrams

How do it know?

• Tom-Tom

Terms

• Forwarding: involves the transfer of a packet from an incoming link to an outgoing link within a single router.

• Routing: involves all of a network’s routers whose collective interactions determine the path (route) that packets take.

• Forwarding Table: a router forwards a packet by examining the value of a field in the arriving packet’s header, and then using this value to index into the router’s forwarding table

• Routing Algorithm: determines the values that are inserted into the router’s forwarding table

Best-Effort Service

• No guarantee of delivery

• No guarantee of delay

• No guarantee of in-order packet delivery

• No guarantee of minimum jitter

• So, is this any way to run a network?

Datagram Network

Prefix Match

• Router forwards packet based on prefix match

• If a tie, then the longest prefix match is used

What’s a Router?

This is where Cisco got theirStart in 1984

Input Port

• For many routers, this is where the determination of output port is made

Do this at line speed

Processing Speed

• OC-48 link runs at 2.5Gbps; packets 256 bytes long. Lookup speed is 1 Mlps

• Content Addressable Memory (CAM): allows 32-bit IP address to be presented to CAM, which returns the table value for that address in essentially zero time

Ternary CAMs have “I don’t care” option

Switching Fabric

• Memory: (traditional computers) with input and output ports

• Bus: Input ports transfer a packet directly to the output port over a shared bus; only one packet at a time on the bus; limited switch speed to bus speed

• Crossbar: 2n buses; can still collide at bus to output port

Output Ports

Datagram Format

• Version number: Version 4 or version 6• Header length: where the data actually begins;

HL=20B, usually• Datagram length: total (header plus data); rarely

longer than 1500 B• Time-to-live: so datagrams do not circulate

forever; decremented at each router• Protocol field: TCP or UDP (6 or 17)• Header checksum• Source and Destination IP addresses

Packet Fragmentation

• Different link-layer protocols can have different Maximum Transmission Units (MTU)

• Ethernet frames carry up to 1500 bytes

• Wide-area links as little as 576 bytes

IP Addressing

• IP addresses are for interfaces (NIC card); router can have multiple IP addresses

• IP address is 32-bits long

• Written in dotted-decimal notation

• Classes:– Class A 1-126 (first bit is zero)– Class B 128-191 (first bit is one)– Class C 192-223 (first two bits are ones)

CIDR

• Classless Interdomain Routing: a.b.c.d/x; where x is the network prefix (name)

• Subnet mask: 255.255.255.0 for class C

• Classful addressing is too “chunky” to work anymore (too many requests for IP addresses)

• Typically get a (block) of addresses from Internet Service Provider

Address Aggregation

Special Addresses

• Network address: all zeros in host part of address

• Broadcast address: all ones in host part of address; will broadcast to subnet, routers generally drop these

ICANN

• Internet Corporation for Assigned Names and Numbers

• Controls IP address space,

• Manages DNS root servers

• Controls domain names and disputes

DHCP

• Dynamic Host Configuration Protocol

• Stradles the boundary between network and link layers

• Automatically assigns IP address and can optionally provide additional information

• Will be back to this next session

Network Address Translation

• Private address spaces– 10.0.0.0 – 10.255.255.255– 172.16.0.0 – 172.31.255.255– 192.168.0.0 – 192.168.255.255

• For SOHO and small business use• “Real” IP address to outside world, private

within network• Interferes with P2P and VoIP, but there

are solutions

Internet Control Message Protocol

• ICMP, used by hosts and routers to communicate network-layer information

• Architecturally lies just above IP (sent in IP datagram)

• Ping sends an ICMP type 8 code 0 message to the specified host who returns a type 0 code 0 reply.

• Tracert is implemented in ICMP messages

IP v 6

• From 32 bits to 128

• Everything (and then some) can have an IP address

• Slow to take hold

Routing Algorithms

• Global routing algorithm computes the least cost path using complete, global knowledge about the network

• Decentralized routing algorithm calculation of the least-cost path is carried out in an iterative, distributed manner.

• Static routing algorithms change very slowly over time

• Dynamic routing algorithms change as the network traffic loads or topology change

Distance-Vector Algorithm

• Iterative; process continues on until no more information is exchanged

• Asynchronous; does not require all of the nodes to operate in unison

• Distributed; each node receives some information from one or more of its directly attached neighbors, calculation, distribution of results

• Only talks to its immediate neighbors but has information about additional paths

• It just stops

Link-State Algorithm

• Loops n times where n is number of nodes in network

• Requires link costs to be known

• Will compute shortest paths from the source node to every other node in the network

• When you compare LS and DV there is no clear winner – both are used…

Autonomous Systems

• Organize routers into ASs – typically under same administrative control

• One router will have added task of forwarding packets out: gateway router

Intra-AS Routing

• Used to determine how routing is performed within an autonomous system

• Routing Information Protocol (RIP) and Open Shortest Path First (OSPF)

RIP

• One of the earliest and still in widespread use today

• Distance-Vector protocol; uses hop count as a cost metric; each hop = 1

• Maximum cost is limited to 15

• Routing updates sent approx. 30 seconds; up to 25 destinations

OSPF

• Open means open source

• Administrator sets link “costs”

• Link-State protocol

• Router broadcasts routing information to all other routers in the AS

• OSPF messages contained in IP datagrams with upperlayer number 89

Border Gateway Protocol

• BGP v4 is the de facto standard inter-AS routing protocol

• Obtain subnet reachability information from neighboring ASs

• Propagate the reachability information to all routers internal to the AS

• Determine “good” routes• Destinations are CDIRized prefixes

(subnets)