STRUCTURE OF A ROUTER

1. STRUCTURE OF A ROUTER We represent a router as a black box that accepts incoming packets from one of the input ports (interfaces), uses a routing table to find the departing output port, and sends the packet from this output port. The topics discussed in this section include: Components

2. Router components Performs layer 1 and 2 functions: signal to bits, packet decapsulated from frame, error control performed on bits, buffers packets before going to the switching fabric This is where delay is incurred Performs layer 1 and 2 functions: bits to signal, packet encapsulated into frame, error control overhead added

3. Crossbar Switching Fabric Cross Point

4. A banyan switch Uses a binary string to route across the switch Example Given a packet came in on port 1 and needed to go out of port 6, the binary string of 110 will be used – explain this

5. Chapter 11 Unicast Routing Protocols (RIP, OSPF, BGP) (How the routers’ tables are filled in)

6. Before Starting Explain how a router uses a routing table when a packet arrives ?

7. Routing Protocols • At this stage, we understand how a router uses a routing table in making a next hop decision • However, what dictates HOW the routing tables are filled in ? • Tables are concerned about the next hop only • What’s responsible for looking across the entire path or route – what makes the decision of the best route ? • Routing Protocols and Algorithms are used • Routing protocols allow routers to share info with one another dynamically - as the Internet makes changes, the routing protocols allow routers to inform other routers • Routers communicate to their neighboring routers - gossip • Routing protocols implement the procedures for combining info received from other routers • Routing Algorithms – decision making analysis – the “brains” – using the info provided

8. Autonomous systems • Because the Internet is so large, one protocol cannot handle all of the updating of tables – create groups and networks and routers called Autonomous Systems • Routing within the autonomous system is called “interior routing” • Routing between the autonomous systems is called “exterior routing” NOTE: different interior routing protocols can be used for each autonomous systems HOWEVER, only one exterior routing protocol is used R1, R2, R3 and R4 use an interior and exterior routing protocol – all other routers only use an interior routing protocol

9. Tx Rx How does it work ? • A certain “cost” or “metric” is assigned each network • In figuring out the best route from Tx to Rx, the set of networks with the smallest sum is chosen • More generically, the set of networks best meeting the “metric’s” objective is chosen • If #hops was the metric, we would want to traverse the least number of networks in going from Tx to Rx • If max throughput was the metric, a fiber optic network would have a better metric than a coaxial network.

10. Explain routing using your street/highway analogy • Autonomous systems • Default routing

11. Popular Unicast Routing protocols • RIP – Routing Information Protocol – treats each network the same (assigns the same cost for each network) • OSPF – Open Shortest Path First protocol – assigns a cost for passing through a network based on the type of service required – routes through the network can have different cost – each router would have several tables • BGP – Border Gateway Protocol – is an exterior routing protocol that uses a policy that defines what paths should be chosen

12. RIP Algorithm Distance Vector Algorithm built from Bellman-Ford Algorithm Recall: each router sends message to it’s neighbor For the router receiving a RIP response 1st – add one to hop count for each destination advertised 2nd – repeat the following steps for each advertised destination 1. If destination is not in table add destination to table 2. Else if destination is in table 1. If next-hop field is the same replace entry in table with advertised one 2. Else next-hop different replace entry if advertised hop count is less

13. Explain RIP in Simple English

14. Example RIP Algorithm • Router receives RIP message for some router C • The RIP message list destination networks, corresponding hop count and next hop (not listed in diagram) • 1st step: increment hop count • Net1: no news, don’t change • Net2: same next hop, so replace 2 with 5 • Net 3: new router, so add • Net 6: different next hop, new hop count less, so replace • Net 8: different next hop, new hop count the same, don’t change • Net 9: different next hop, new hop count larger, do not change

15. Initial routing tables in a small autonomous system Initial tables are created from config file (and hop counts are set to 1) – next hop fields are empty initially because all networks are directly connected

16. Final routing tables for the previous figure For example, suppose packet hitting Router A first had a destination of Net 66 ?

17. RIP message format • Command – 8-bit field specifying the type of message: response (2) or request (1) • Version – 8-bit field specifying RIP version • Family – 16-bit specifying protocol family (TCP/IP=2) • Network Address – address of the destination network • Distance – 32-bit field defining the hop count from advertising router to destination network • NOTE: Request can be issued by a newly added router or by a router seeking certain info • NOTE: 2 response types: Solicited – response to request, Unsolicited – periodic updates • Gray fields repeated for each destination network

18. Example 1 What is the periodic response sent by router R1 in the figure below. Assume R1 knows about the whole autonomous system.

19. Solution R1 can advertise three networks 144.2.7.0, 144.2.9.0, and 144.2.12.0. The periodic response (update packet) is shown below

20. RIP timers • Periodic Timer – each router has timer set to 25-35 secs and when the timer counts down, an update message is sent • Expiration Timer – governs the validity of the next-hop – when router receives next-hop update, timer is set to 180 sec. If there is a problem and the router doesn’t receive it’s 30 sec update, the route info expires (invalid) after the 180 sec count down – then the hop count is set to 16 (infinity) • Garbage Collection Timer – once the route expires, this timer is set to 120 sec and counts downs – allows neighbors time to become aware of invalidity – after count down, info is purged

21. Problems with RIP • Slow Convergence – the time it takes a change in the Internet to propagate through the rest of the Internet – recall the periodic updates with neighbors. DEPENDING ON THE DATA RATE, millions or billions of bits could be sent in that time – therefore possibly lost nx15s

22. RIP Problem - Instability • RA and RB has hop counts 1 and 2, respectively for Net1 • Net1 goes down – RA can update fast due to direct connection – sets hop count to 16 • RA has to wait 30 sec to update RB (this is the problem) • In meanwhile, RB sends update to RA with hop count 2 for Net1 (incremented to 3) • Now when RA finally send the update to RB, it sends a hop count of 3 (incremented to 4) – RA thinks it’s another route to Net1 • This INSTABILITY (back-and-forth) continues until both set hop count to 16

23. Exam 3 Results & Grading Scale Average Score = 28 Standard Deviation= 14 • 65-51 A-grade (3 students) • 50-36 B-grade (3 students) • 35-21 C-grade (13 students) • 20-6 D-grade (12 students) • 5-0 F-grade (0 students) Lecture