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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?
Prefix Match
• Router forwards packet based on prefix match
• If a tie, then the longest prefix match is used
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
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
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