Sirak Kaewjamnong Computer Network Tech and Security Routing Concept Sirak Kaewjamnong Computer Network Tech and Security
Agenda Basic concepts Routing components Classes of routing protocol Internet routing protocol
What’s Routing Routing - path finding from one end to the other Routing occurs at layer 3 Bridging occurs at layer 2 Physical link Data link layer Network layer Transport layer Session layer Presentation layer Application layer Physical link Data link layer Network layer Transport layer Session layer Presentation layer Application layer Physical link Data link layer Network layer Network B Network A
IP Routing IP performs: search for a matching host address search for a matching network address search for a default entry Routing done by IP router, when it searches the routing table and decide which interface to end a packet out. incoming which interface ?
Routing Tables Routing is carried out in a router by consulting routing table. No unique format for routing tables, typically table contains: address of a destination IP address of next hop router network interface to be used subnet mask for the this interface distance to the destination
Routing Component Three important routing elements : algorithm database protocol Algorithm : can be differentiate based on several key characteristics Database : table in routers or routing table Protocol: the way information for routing to be gathered and distributed
Routing Algorithm Design goals Optimality - compute the best route Simplicity/low overhead - efficient with a minimum software and utilization overhead Robustness/stability- perform correctly in the face of unusual circumstances Rapid convergence- responds quickly when the network changes Flexibility- accurate adapt to a variety of network
Routing Protocols Routing protocol : protocol to exchange of information between routers about the current state of the network Routing protocol jobs create routing table entries keep routing table up-to-date compute the best choice for the next hop router
Routing Metrics How do we decide that one route is better than another? Solution : using a metric as a measurement to compare routes Metrics may be distance, throughput, delay, error rate, and cost. Today, IP supports Delay, Throughput, Reliability and Cost (DTRC)
Hop Count A hop is defined as a passage through one router 2 hops
Routing Algorithm Types Static V.S. Dynamic Source routing V.S. Hop-by-hop Centralize V.S. Distributed Distance vector V.S. Link state
Routing Algorithm: Static Route Manually configuration routing table Can’t react dynamically to network change such as router’s crash Work well with small network or simple topology Unix hosts use command route to add an entry point to point connection route to this way only, no need for update
Routing Algorithm: Static Technique Flooding Every incoming packet is sent out every outgoing Retransmit on all outgoing at each node Simple technique, require no network information Generate vast numbers of duplicate packet flooding incoming
Routing Algorithm: Dynamic Route Network protocol adjusts automatically for topology or traffic changes Unix hosts run routing daemon routed or gated
Routing Algorithm: Dynamic Route operation Routing protocol maintains and distributes routing information Routing Table Routing Table Routing Protocol Routing Protocol Update Routing Information
Routing Algorithm: Source Routing Source will determine the entire route Routers only act as store-forward devices Hop-by-hop Routers determine the path based on theirs own calculation
Routing Algorithm: Distance Vector Distance means routing metric Vector means destination Flood routing table only to its neighbors RIP is an example Also known as Bellmann-Ford algorithm or Ford-Fulkerson algorithm
Distance Vector Algorithm Using hop count as a metric Each router periodically sends a copy of its routing table to neighbors send <network X, hopcount Y> R1 R2 R3 W X Z Y routing table W 0 X 0 Y 1 Z 2 routing table W 1 X 0 Y 0 Z 1 routing table W 2 X 1 Y 0 Z 0
Distance Vector Routing Update Step by step from router to router Slow convergence recompute R1’s routing table recompute R2’s routing table recompute R3’s routing table topology change R1 R2 R3 R2 sends out the updated table R3 sends out the updated table
Distance Vector: Broadcast (I) The first round I, 1 hop J, 1 hop R1 J, 1 hop K, 1 hop M, 1 hop N, 1 hop J I I, 1 hop K, 1 hop L, 1 hop R3 K R2 N, 1 hop O, 1 hop N L M R5 L, 1 hop M, 1 hop O 1 hop O R4
Distance Vector: Broadcast (II) The second round I, 1 hop J, 1 hop K, 2 hops L, 2 hops M, 2 hops N, 2 hops J, 1 hop K, 1 hop M, 1 hop N, 1 hop I, 2 hops L, 2 hops O, 2 hops R1 N, 1 hop O, 1 hop J, 2 hops K, 2 hops M, 1 hop L, 2 hops J I R3 K R2 N I, 1 hop K, 1 hop L, 1 hop J, 2 hops M, 2 hops N, 2 hops O, 2 hops L M R5 O R4 L, 1 hop M, 1 hop O, 1 hop I, 2 hops K, 2 hops J, 2 hops N, 2 hops
Distance Vector: Broadcast (III) The third round I, 1 hop J, 1 hop K, 2 hops L, 2 hops M, 2 hops N, 2 hops O, 3 hops J, 1 hop K, 1 hop M, 1 hop N, 1 hop I, 2 hops L, 2 hops O, 2 hops R1 N, 1 hop O, 1 hop J, 2 hops K, 2 hops M, 1 hop L, 2 hops I, 3 hops J I R3 K R2 N I, 1 hop K, 1 hop L, 1 hop J, 2 hops M, 2 hops N, 2 hops O, 2 hops L M R5 O R4 L, 1 hop M, 1 hop O, 1 hop I, 2 hops K, 2 hops J, 2 hops N, 2 hops
Distance Vector: Crashed Recovery New complete route of R1 R1 J I R3 R2 K R1 routing table hop via 1 N/A 2 R2 2 R3 3 R5 L N net I J K L M N O M R5 O net I J K L M N O hop via 1 N/A 2 R2 3 R2 4 R2 R4
Count to Infinity R2 does not hear any thing from R3 R1 says : don’t worry, I can reach R3 in 2 hops, R2 update hop count to 3 R1 sees R2’s update, then update itself to 4 and so on…… R3 crashed R1 I R2 J R3 2 1 initial 2 3 1st round 4 3 2nd round 4 5 3rd round hop count to R3
Split Horizon Solve by set distance “16” as infinity No destination can be more than 15 hops away from any other Distance to X is not reported on the line that packet for X are sent Actually, it reports with infinity R1 I R2 J R3 R3 crashed to R3 to R3
Dijkstra’s Shortest Path First Algorithm Routers send out update messages whenever the state of a link changes. Hence the name: “Link State” algorithm. Each router calculates lowest cost path to all others, starting from itself. At each step of the algorithm, router adds the next shortest (i.e. lowest-cost) path to the tree. Finds spanning tree routed on source router.
Open Shortest Path First (OSPF) RIP limited in large internets OSPF preferred interior routing protocol for TCP/IP based internets Link state routing used
Routing Algorithm: Link State Flood routing information to all nodes Each router finds who is up and flood this information to the entire routers Use the link state to build a shortest path map to everybody OSPF is an example Also known as Shortest Path First (SPF) algorithm
Flooding Packet sent by source router to every neighbor Incoming packet resent to all outgoing links except source link Duplicate packets already transmitted are discarded Prevent incessant retransmission All possible routes tried so packet will get through if route exists Highly robust At least one packet follows minimum delay route Reach all routers quickly All nodes connected to source are visited All routers get information to build routing table High traffic load
Link State Overview Using cost as a metric Exchange its connection and cost to its neighbors Each router compute the set of optimum path to all destination (Shortest Path First) W X Y Z link state W 0 X 0 link state X 0 Y 0 link state Y 0 Z 0
Link State Concept Each router initially begins with directly connected network Determine full knowledge of distant routers and theirs connection R1 exchange link state packets R2 R4 build topological database Routing Table R3 compute SPF update routing table
Link State Routing Update Send information to other routers Fast convergence R1 R4 topology change R2 R3
OSPF Overview Router maintains descriptions of state of local links Transmits updated state information to all routers it knows about Router receiving update must acknowledge Lots of traffic generated Each router maintains database Directed graph
Link Costs Cost of each hop in each direction is called routing metric OSPF provides flexible metric scheme based on type of service (TOS) Normal (TOS) 0 Minimize monetary cost (TOS 2) Maximize reliability (TOS 4) Maximize throughput (TOS 8) Minimize delay (TOS 16)
Areas Make large internets more manageable Configure as backbone and multiple areas Area – Collection of contiguous networks and hosts plus routers connected to any included network Backbone – contiguous collection of networks not contained in any area, their attached routers and routers belonging to multiple areas
Operation of Areas Each are runs a separate copy of the link state algorithm Topological database and graph of just that area Link state information broadcast to other routers in area Reduces traffic Intra-area routing relies solely on local link state information
Comparison Distance Vector Link State pass a copy of pass links state update whole routing table add metric from calculate the shortest path router to router to other routers frequent periodic update: event updated: fast slow convergence convergence
Internet Routing Architecture Autonomous System IGP EGP/BGP IGP EGP/BGP IGP Autonomous System BGP4 BGP4 Autonomous System IGP IGP BGP4 Autonomous System EGP/BGP EGP/BGP EGP/BGP EGP/BGP IGP IGP IGP IGP Autonomous System
Routing in the Internet The Internet uses hierarchical routing The Internet is split into Autonomous Systems (AS’s) Within an AS, the administrator chooses an Interior Gateway Protocol (IGP) Examples of IGPs: RIP (rfc 1058), OSPF (rfc 1247). Between AS’s, the Internet uses an Exterior Gateway Protocol AS’s today use the Border Gateway Protocol, BGP-4 (rfc 1771)
Autonomous System AS is a collectionof LANs and WANs and the interconnectting routers which under the control of one management authority The same AS runs the same Interior Gateway Protocol Why setting up AS? - establish a direct link to each other rather than route through the core Internet How to select AS? - register and get the AS number from IAB
Gateway Protocol Interior gateway protocol Exterior gateway protocol exchange routing information between routers within a single AS RIP, RIP II, OSPF Exterior gateway protocol collect network-reachablity information for the AS EGP, BGP
Interior Routing Protocols RIP Uses distributed Bellman-Ford algorithm. Updates sent every 30 seconds. No authentication. Originally in BSD UNIX. OSPF Link-state updates sent (using flooding) as and when required. Every router runs Dijkstra’s algorithm. Authenticated updates. Autonomous system may be partitioned into “areas”.
Exterior Routing Protocols Problems: Topology: The Internet is a complex mesh of different AS’s with very little structure. Autonomy of AS’s: Each AS defines link costs in different ways, so not possible to find lowest cost paths. Trust: Some AS’s can’t trust others to advertise good routes (e.g. two competing backbone providers), or to protect the privacy of their traffic (e.g. two warring nations). Policies: Different AS’s have different objectives (e.g. route over fewest hops; use one provider rather than another).
Border Gateway Protocol (BGP-4) BGP is not a link-state or distance-vector routing protocol. BGP advertises complete paths (a list of AS’s). Example of path advertisement: “The network 171.64/16 can be reached via the path {AS1, AS5, AS13}”. Paths with loops are detected locally and ignored. Local policies pick the preferred path among options. When a link/router fails, the path is “withdrawn”.
References http://www.cisco.com/en/US/products/hw/routers/ps274/index.html http://www.cisco.com/en/US/products/hw/routers/ps274/products_data_sheet09186a008010fba1.html Computer Networks with Internet Technology By William Stallings Chapter 11 Interior Routing Protocols