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

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: X o = (,,,…, ). 2. Send updates of components of X n to neighbors 3. Calculate: X n+1 = F(X n ) 4. If X n+1  X n then go to (2) 5. Stop

7 Bellman-Ford Algorithm Example: Calculate from R8 R5R5 R3R3 R7R7 R8R8 R6R6 R4R4 R2R2 R1R R3R3 R2R2 R5R5 R7R7 R4R4 R6R6 R8R8 R1R nd step

8 Bellman-Ford Algorithm Result: R8R8 R6R6 R4R4 R2R2 R1R1 R3R3 R5R R7R7 R3R3 R5R5 R7R7 R8R8 R6R6 R4R4 R2R2 R1R rd step

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 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 Link State (cont.)

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 numberHost number Class B address Subnet mask ( ) Subnetted address Network numberHost IDSubnet ID

18 Subnet Example Forwarding table at router R1 Subnet # Subnet Mask Next Hop interface interface R2 111….1.0xxx….x Net host Subnet Subnet mask: Subnet number: H1 R Subnet mask: Subnet number: R2 H Subnet mask: Subnet number: H3

19 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)

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

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

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  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 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 : 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)

25 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

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

27 Why not minimize “AS hop count”? Regional ISP1 Regional ISP2 Regional ISP3 Cust2 Cust3 Cust1 National ISP1 National ISP2 YESNO

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

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

30 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

31 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 /16 AS Path = Don’t Accept!

32 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

33 Tunneling

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

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