The Routing & the IP network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical The network layer moves transport layer segments from host to host in the network, to deliver them to their destination. This layer involves each and every host and router in the network. application transport network data link physical application transport network data link physical

Network layer functions r transport packet from sending to receiving hosts r network layer protocols in every host, router three important functions: r path determination: route taken by packets from source to destination - routing algorithms r switching: move packets from router’s input to appropriate router output r call setup: some network architectures require router call setup along path before data flows (connection oriented networks) network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical application transport network data link physical

Datagram networks: the Internet model r no call setup at network layer r routers: do not maintain state for the end-to-end connections m no network-level concept of a “connection” r packets are typically routed using only destination host ID which is carried in the packet m packets between same source-destination pair may take different paths application transport network data link physical application transport network data link physical 1. Send data 2. Receive data

Routing Graph abstraction for routing algorithms: r graph nodes are routers r graph edges are physical links m link cost: delay, distance, # of hops, rate structure or congestion level = $$ m Other costs?? Goal: determine a “good” path (sequence of routers) thru the network from the source to the destination Routing protocol A E D CB F r “good” path: m typically means minimum cost path m other definitions also possible

Hierarchical Routing scale: with 55 million+ destination hosts: r can’t store all destinations in routing tables! r routing table exchange would swamp links! administrative autonomy r internet = network of networks r each network admin may want to control routing in its own network Our routing study thus far – an idealization r all routers are identical r the network is “flat” … not true in practice Why?

Hierarchical Architecture of the Internet

Hierarchical Routing r aggregate routers into regions, called “autonomous systems” (AS) r routers in same AS run same routing protocol m “intra-AS” routing protocol m routers in different AS can run different intra- AS routing protocol r special routers in AS r run intra-AS routing protocol with all other routers in AS r also responsible for routing to destinations outside AS m run inter-AS routing protocol with other gateway routers gateway routers

Internet AS Hierarchy Inter-AS border (exterior gateway) routers Intra-AS interior (gateway) routers

Internet inter-AS routing: BGP r BGP (Border Gateway Protocol): the de facto standard r Path Vector protocol: m similar to Distance Vector protocol m each Border Gateway broadcasts to neighbors (peers) the entire path (I.e, sequence of ASs) to destination

Intra-AS and Inter-AS routing Gateways: perform inter-AS routing amongst themselves perform intra-AS routers with other routers in their AS inter-AS, intra-AS routing in gateway A.c network layer data link layer physical layer a b b a a C A B d A.a A.c C.b B.a c b c

Intra-AS and Inter-AS routing Host h2 a b b a a C A B d c A.a A.c C.b B.a c b Host h1 Intra-AS routing within AS A Inter-AS routing between A and B Intra-AS routing within AS B r We’ll examine specific inter-AS and intra-AS Internet routing protocols shortly

IP Addressing: introduction r IP address: 32-bit identifier for host or router interface r interface: connection between host or router and the physical link m routers typically have multiple interfaces m hosts typically have only one m IP addresses are associated with the interface, not the host or the router = dotted-decimal notation:

IP Addressing r IP address: m network part (high order bits) m host part (low order bits) r What’s a network ? ( from the IP address perspective) m device interfaces with the same network part of their IP address m hosts can physically reach each other without an intervening router Example: network consisting of 3 IP networks (for IP addresses starting with 223, the first 24 bits are the network address – more later) LAN

IP Addresses to to to to bits Given the notion of a “network”, let’s look closer at IP addresses: “classful” addressing - What is the address space size (number of hosts) for each class? 0 network 10 network host (16 bits) 110 networkhost (8 bits) 1110 multicast address (28 bits) A B C D class host (24 bits) 2 7 = 127 networks 2 24 = 16.8 million+ hosts 2 14 = 16,384 networks 2 16 = 65,536 hosts 2 21 = 2 million+ networks 2 8 = 256 hosts 2 28 = million+ hosts

Map of the Internet

r The minimum & maximum values of the range: r r E … EF.FF.FF.FF 16 r The first part of the abbreviation is the common byte(s) in the range r The second part of the abbreviation is the number of bits, which are common for the all members of the range Abbreviated Format of the Address Ranges 224/

The private address ranges r Used locally m Never used in the Internet m Gateways do not forward the packets addressed to private addresses r The network, which uses the private address range can be connected to the Internet by the NAT (Network Address Translation)

IP addressing: CIDR r classful addressing: m inefficient use of address space, address space exhaustion m e.g., class B network is allocated enough addresses for 65K hosts, even if only 2K hosts exist in that network r CIDR: Classless InterDomain Routing m network portion of address of arbitrary length m address format: a.b.c.d/x, where x is # bits in the network portion of an address network part host part /23

IP addresses: how to get one? Hosts (host portion): r hard-coded by system admin in a file r DHCP: Dynamic Host Configuration Protocol: dynamically get address (RFC 2131): “plug-and-play” m host broadcasts “DHCP discover” msg m DHCP server responds with “DHCP offer” msg m host requests IP address: “DHCP request” msg m DHCP server sends address: “DHCP ack” msg

Why different Intra- and Inter-AS routing ? Policy: r Inter-AS: admin wants control over how its traffic is routed, who routes through its net. r Intra-AS: single admin, so no policy decisions needed Scale: r hierarchical routing saves table size, reduces update traffic Performance: r Intra-AS: can focus on performance r Inter-AS: policy may dominate over performance

Intra-AS Routing r Also known as Interior Gateway Protocols (IGP) r Most common IGPs: m RIP: Routing Information Protocol (legacy) m OSPF: Open Shortest Path First (common) m EIGRP: Enhanced Interior Gateway Routing Protocol (proprietary – Cisco Systems)

Distance-vector routing algorithm (DVR) r The different names of the background mathematical algorithm: m Backward search algorithm m Bellman-Ford algorithm r Goal: search the smallest delay paths for the traffic r For this reason in each router a table is created, which contains: m The interface to the smallest delay path to every node m The estimated delay of each path r This table is called distance vector

The link state tables of an example network

Routing tables (Step 1) Modified entry Unmodified entry Distance vector Bellman-Ford algorithm

Step 1 Shortest paths in the routing table of the router A Bellman-Ford algorithm

Routing tables (Step 2) Routing tables resulted from the previous step Bellman-Ford algorithm

Step 2 Bellman-Ford algorithm Shortest paths in the routing table of the router A

Routing tables (Step 3) Bellman-Ford algorithm Routing tables resulted from the previous step

Routing tables (Step 4) Bellman-Ford algorithm Routing tables resulted from the previous step

Step 3-4 Bellman-Ford algorithm Shortest paths in the routing table of the router A

Routing tables (Step 5) Bellman-Ford algorithm Routing tables resulted from the previous step

Step 5 and the final result Bellman-Ford algorithm Shortest paths in the routing table of the router A

Link-state routing algorithm r The different names of the background mathematical algorithm: m Forward search m Dijkstra algorithm m Shortest path (SP) r The SP is the optimal path, however, this is not obviously the geometrically shortest path r Other factor, which can be taken into account: m Number of routers in the path m delay m cost m Average traffic m Reliability of the links in a certain path

Dijkstra’s algorithm r Dijkstra's algorithm, named after its inventor the Dutch computer scientist Edsger Dijkstra, solves a shortest path problem for a directed and connected graph G(V,E) which has nonnegative (>=0) edge weights r Dijkstra's algorithm is known to be a good algorithm to find a shortest path

The method… r Finds the shortest path between a source node and the rest r Finds routes between nodes by cost precedence r Assumes every cost is a positive number r Supports directed or bidirectional communication Dijkstra’s algorithm

Initialisation – Example network Dijkstra’s algorithm

Step 1 D Least cost new node: D Dijkstra’s algorithm

Step 2 Dijkstra’s algorithm B Least cost new node (with smaller IP address) : B

Step 3 Dijkstra’s algorithm E Least cost new node: E

Step 4 Dijkstra’s algorithm C Least cost new node: C

G Least cost new node: G Step 5 Dijkstra’s algorithm

Step 6 (final result) Dijkstra’s algorithm

Second example Step 1 Dijkstra’s algorithm

Second example Step 2

Dijkstra’s algorithm Second example Step 3

Dijkstra’s algorithm Second example Step 4

Dijkstra’s algorithm Second example Step 5

Dijkstra’s algorithm Second example Step 6

Dijkstra’s algorithm Second example Step 7

Dijkstra’s algorithm Second example Step 8

Dijkstra’s algorithm Second example Step 9

Dijkstra’s algorithm Second example Step 10

Dijkstra’s algorithm Second example Step 11

Dijkstra’s algorithm Second example Step 12

Differences between the forward and the backward search algorithms r Forward search (Dijkstra algorithm) m It increases the scope of the search in each step with including new node r Backward search (Bellman-Ford algorithm) m It increases the scope of the search in each step with including new hop

Comparison of the distance-vector and the link-state algorithms r Distance vector: m Each router sends distance-vector, but to its neighbours m The distance-vector contains the estimated distance to all other nodes m Older method m Problem of the ”count-to-infinity” due to the fact, that the bad news are distributed too slowly r Link-state: m Each router sends link-state distance-vector to all others m The link-state distance-vector contains the distance to the neighbours, only m The distance value to the neighbour (called link-state) is accurate m Recent method