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 their Start 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 A1-126 (first bit is zero) –Class B (first bit is one) –Class C (first two bits are ones)

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