Introduction to Computer Networks

1. Introduction to Computer Networks Routing University of Ilam By: Dr. Mozafar Bag-Mohammadi

2. Routing Process • Question? How to populate the lookup table? • Primary solutions: • Build the lookup table Manually? Is it practical? The answer is no. • Flooding- Broadcast to all node except the one we have received the packet. • Waste the bandwidth • Does not scale well.

3. Overview • Network as a Graph • Problem: Find lowest cost, or shortest, path between two nodes • The process is distributed and this makes it complicated, i.e, it may create loop. • Factors • static: topology • dynamic: load

4. Distance Vector • Each node maintains a set of triples (Destination, Cost, NextHop) • Exchange updates with directly connected neighbors • periodically ( on the order of several seconds) • whenever its table changes (called triggered update) • Each update is a list of pairs: (Destination, Cost) • Update local table if receive a “better” route • smaller cost • came from next-hop • Refresh existing routes; delete if they time out

5. Example Destination Cost NextHop A 1 A C 1 C D 2 C E 2 A F 2 A G 3 A • Distance of other nodes from Node B. • The cost between two nodes has been assumed 1. • All nodes keep a routing table from themselves.

6. The Bellman-Ford Algorithm • Bellman-Ford algorithm solve the distance Vector problem in general case. • 1. Set: Xo = ( , , ,…, ). • 2. Send updates of components of Xn to neighbors • 3. Calculate: Xn+1 = F(Xn) • 4. If Xn+1  Xn then go to (2) • 5. Stop

7. 1 1 4 R1 R2 R4 R6 2 3 2 2 R7 3 R5 2 R3 4 R8 2 4 1 1 R4 R6 R1 R2 3 2 2 3 2 2 R7 3 R5 2nd step 2 4 R8 4 R3 Bellman-Ford Algorithm Example: Calculate from R8

8. 6 4 6 2 1 1 4 R1 R2 R4 R6 3 2 3 2 2 2 3rd step R7 3 4 R5 2 4 R3 R8 5 4 5 2 1 1 R1 R2 R4 R6 2 3 2 Result: 2 3 R7 4 R5 2 R3 R8 4 Bellman-Ford Algorithm

9. Node Failure • F detects that link to G has failed • F sets distance to G to infinity and sends update to A • A sets distance to G to infinity since it uses F to reach G • A receives periodic update from C with 2-hop path to G • A sets distance to G to 3 and sends update to F • F decides it can reach G in 4 hops via A

10. Routing Loops • link from A to E fails • A advertises distance of infinity to E • B and C advertise a distance of 2 to E • B decides it can reach E in 3 hops; advertises this to A • A decides it can read E in 4 hops; advertises this to C • C decides that it can reach E in 5 hops…

11. The count-to-infinity problem

12. Loop-Breaking Heuristics • Set infinity to a reasonably small number. For instance, RIP sets to 16 • Split horizon: Don’t announce the distance to the node the distance has been gotten from. • Split horizon with poison reverse: Instead of not announcing the distance put negative numbers.

13. Link State • Strategy • send to all nodes (not just neighbors) information about directly connected links (not entire routing table) • Link State Packet (LSP) • id of the node that created the LSP • cost of the link to each directly connected neighbor • sequence number (SEQNO) • time-to-live (TTL) for this packet

14. Link State (cont.) • Reliable flooding • store most recent LSP from each node • forward LSP to all nodes but one that sent it • generate new LSP periodically • increment SEQNO • start SEQNO at 0 when reboot • decrement TTL of each stored LSP • discard when TTL=0

15. Route Calculation • Dijkstra’s shortest path algorithm • Let • N denotes set of nodes in the graph • l (i, j) denotes non-negative cost (weight) for edge (i, j) • s denotes this node • M denotes the set of nodes incorporated so far • C(n) denotes cost of the path from s to node n M = {s} for each n in N - {s} C(n) = l(s, n) while (N != M) M = M union {w} such that C(w) is the minimum for all w in (N - M) for each n in (N - M) C(n) = MIN(C(n), C (w) + l(w, n ))

16. Shortest Path Routing: Dijkstra Algorithm

17. Subnetting • Add another level to address/routing hierarchy: subnet • Subnet masks define variable partition of host part • Subnets visible only within site Network number Host number Class B address 0000000000000000 1111111111111111111 Subnet mask (255.255.0.0) Network number Subnet ID Host ID Subnetted address

18. Net host Subnet mask: 255.255.255.128. Subnet number: 128.96.34.0 111….1.0xxx….x 128.96.34.15 128.96.34.1 H1 R1 Subnet mask: 255.255.255.128 128.96.34.130 Subnet number: 128.96.34.128 128.96.34.139 128.96.34.129 H2 R2 H3 128.96.33.1 128.96.33.14 Subnet mask: 255.255.255.0 Subnet number: 128.96.33.0 Subnet Example Subnet Forwarding table at router R1 Subnet # Subnet Mask Next Hop 128.96.34.0 255.255.255.128 interface 0 128.96.34.128 255.255.255.128 interface 1 128.96.33.0 255.255.255.0 R2

19. Supernetting • Assign block of contiguous network numbers to nearby networks • Called CIDR: Classless Inter-Domain Routing • Represent blocks with a single pair (first_network_address, count) • Restrict block sizes to powers of 2 • Use a bit mask (CIDR mask) to identify block size • All routers must understand CIDR addressing

20. Route Propagation • Know a smarter router • hosts know local router • local routers know site routers • site routers know core router • core routers know everything • Autonomous System (AS) • corresponds to an administrative domain • examples: University, company, backbone network • assign each AS a 16-bit number • Two-level route propagation hierarchy • interior gateway protocol (each AS selects its own) • exterior gateway protocol (Internet-wide standard)

21. IGP EGP EGP IGP IGP EGP Architecture of Routing Protocols Interior Gateway Protocols (IGP) : inside autonomous systems Exterior Gateway Protocols (EGP) : between autonomous systems AS 701 UUNet OSPF, IS-IS, RIP, EIGRP, ... BGP Metric Based Policy Based AT&T Common Backbone AT&T Research AS 6431 AS 7018

22. Interior Gateway Protocols • RIP: Route Information Protocol • developed for XNS • distributed with Unix • distance-vector algorithm • based on hop-count • OSPF: Open Shortest Path First • recent Internet standard • uses link-state algorithm • supports load balancing • supports authentication

23. EGP: Exterior Gateway Protocol • Overview • designed for tree-structured Internet • concerned with reachability, not optimal routes • Protocol messages • neighbor acquisition: one router requests that another be its peer; peers exchange reachability information • neighbor reachability: one router periodically tests if the another is still reachable; exchange HELLO/ACK messages; uses a k-out-of-n rule • routing updates: peers periodically exchange their routing tables (distance-vector)

24. The Most Common Routing Protocols BGP RIP Cisco proprietary UDP OSPF IS-IS TCP EIGRP IP (and ICMP) Routing protocols exchange network reachability information between routers.

25. BGP-4 • BGP = Border Gateway Protocol • Is a Policy-Based routing protocol • Is the de facto EGP of today’s global Internet • Relatively simple protocol, but configuration is complex and the entire world can see, and be impacted by, your mistakes. • 1989 : BGP-1 [RFC 1105] • Replacement for EGP (1984, RFC 904) • 1990 : BGP-2 [RFC 1163] • 1991 : BGP-3 [RFC 1267] • 1995 : BGP-4 [RFC 1771] • Support for Classless Interdomain Routing (CIDR)

26. BGP-4: Border Gateway Protocol • AS Types • stub AS: has a single connection to one other AS • carries local traffic only • multihomed AS: has connections to more than one AS • refuses to carry transit traffic • transit AS: has connections to more than one AS • carries both transit and local traffic • Each AS has: • one or more border routers • one BGP speaker that advertises: • local networks • other reachable networks (transit AS only) • gives path information

27. YES NO Policy-Based vs. Distance-Based Routing? Host 1 Cust1 Minimizing “hop count” can violate commercial relationships that constrain inter- domain routing. ISP1 ISP3 Host 2 ISP2 Cust3 Cust2

28. YES NO Why not minimize “AS hop count”? National ISP1 National ISP2 Regional ISP3 Regional ISP2 Regional ISP1 Cust2 Cust3 Cust3

29. AS1 AS2 BGP Operations Simplified Establish Peering on TCP port 179 BGP Peers Exchange All Routes While connection is ALIVE exchange route UPDATE messages Exchange Incremental Updates

30. Two Types of BGP Neighbor Relationships • External Neighbor (eBGP) in a different Autonomous Systems • Internal Neighbor (iBGP) in the same Autonomous System AS1 eBGP iBGP Physical Connection AS2 Logical (TCP) Connection

31. Four Types of BGP Messages • Open : Establish a peering session. • Keep Alive : Handshake at regular intervals. • Notification : Shuts down a peering session. • Update : Announcing new routes or withdrawing previously announced routes. announcement = Network prefix + attributes

32. AS Path Attribute (cont.) BGP at AS YYY will never accept a route whose AS Path contains YYY. This avoids interdomain routing loops. AS702 UUnet 10.22.0.0/16 AS Path = 1 333 702 877 Don’t Accept!

33. Local Preference Attribute Used only in iBGP to prefer a point of exit Frank’s Upstream Provider AS 4 13.13.0.0/16 AS Path = 4 1 Loc pref = 80 13.13.0.0/16 AS Path = 3 1 Loc pref = 90 Frank’s Internet Barn Frank’s Local Competition AS 3 13.13.0.0/16 AS Path = 2 1 Loc pref = 100 Frank’s Customer AS 2 Customer of Frank’s Customer Higher Local Preference Values are more preferred AS 1 13.13.0.0/16

34. IP Version 6 • Features • 128-bit addresses (classless) • multicast • real-time service • authentication and security • autoconfiguration • end-to-end fragmentation • protocol extensions • Header • 40-byte “base” header • extension headers (fixed order, mostly fixed length) • fragmentation • source routing • authentication and security • other options

35. Tunneling

36. Routing for Mobile Hosts 1- finding location of the mobile host 2- hand-off 3- security

37. Routing for Mobile Hosts (2) Packet routing for mobile users.