Topics covered are Introduction, What’s Inside a Router?, The Internet Protocol (IP): Forwarding and Addressing in the Internet Routing Algorithms,Routing in the Internet, Broadcast and Multicast Routing

1. Vidya Vardhaka College of Engineering, Mysuru
Department of Computer Science & Engineering
Computer Networks (15CS52)
Module-3: Network Layer
Gururaj H L
Assistant Professor,
Dept. of CSE, VVCE, Mysuru
gururaj1711@vvce.ac.in
gururajhl.blogspot.com

2. Contents
1. Introduction
2. What’s Inside a Router?
3. The Internet Protocol (IP): Forwarding and Addressing in the
Internet
4. Routing Algorithms
5. Routing in the Internet
6. Broadcast and Multicast Routing
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3. Router Architecture
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4. Input and Output Processing
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5. • Transfer packet from input buffer to appropriate output buffer
• Switching rate: Rate at which packets can be transfer from inputs to
outputs
• often measured as multiple of input/output line rate
• N inputs: switching rate N times line rate desirable
• Three types of switching fabrics.
Switching via Memory
• Traditional computers with switching under direct control of CPU.
• Packet copied to system’s memory.
• Speed limited by memory bandwidth (2 bus crossings per datagram).
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7. Switching via Bus
• Datagram from input port memory
• To output port memory via a shared bus
• bus contention: Switching speed limited by bus bandwidth
• 32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers.
Switching via interconnection Network
• Overcome bus bandwidth limitations
• Banyan networks, crossbar, other interconnection nets initially developed to
connect processors in multiprocessor
• Advanced design: fragmenting datagram into fixed length cells, switch cells
through the fabric.
• Cisco 12000: switches 60 Gbps through the interconnection network
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8. Output port queueing
“Packet scheduler”
• QoS guarantees
• Active Queue Management
• RED (Random Early Detection)
• The buffering required
With N flows, buffering equal to
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RTT C.
N

9. HOL (Head Of Line)
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10. IPv6
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11. Transitioning from IPv4 to IPv6
Dual stack approach
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12. Tunneling
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13. A Brief Foray into IP Security
• Cryptographic agreement
• Encryption of IP datagram payloads
• Data integrity
• Origin authentication
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14. Routing Algorithms
• G = (N,E)
• Centralized & Decentralized
• Static & Dynamic
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15. The Link-State (LS) Routing Algorithm
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16. Least cost path and forwarding table for node u
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18. Consider the following network. With the indicated link costs, use
Dijkstra’s shortest-path algorithm to compute the shortest path from x to
all network nodes. Show how the algorithm works by computing a table
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19. The Distance-Vector (DV) Routing Algorithm
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20. Computation
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• Count to Infinity Problem
• Split horizon with poisoned reverse

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22. Consider the network shown below, and assume that each node initially
knows the costs to each of its neighbors. Consider the distance-vector
algorithm and show the distance table entries at node z.
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23. A Comparison of LS and DV Routing Algorithms
• Message complexity.
O(|N| |E|)
• Speed of convergence.
O(|N|2) Algorithm requiring O(|N| |E|)) Messages
DV also suffers from the count-to-infinity problem.
• Robustness.
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24. Hierarchical Routing
• All routers executing the same routing algorithms in the network
Scale
Administrative autonomy.
Autonomous System- A set of routers that are typically under the same
administrative control and runs with same routing algorithms.
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25. AS
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26. Routing in the Internet
• Intra-AS Routing in the Internet
RIP (Routing Information Protocol)
OSPF (Open Source Path First)
RIP (Routing Information Protocol)
• It follows the basic principles of Distance Vector Algorithm.
• The hop limit of RIP is 15.
• Each router in an AS forwards an advertisement messages to its
neighbor nodes in every 30 seconds.
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27. • Even after 30 seconds if the node doesn’t get reply and the time will be extended
up to 180 seconds. The nodes are considered as dead node if they wont receive
reply after 180 seconds and routing table will be updated.
• Routers sends RIP request and response messages to each other over UDP using
port number 520.
• The UDP segment is carried between routers in a standard IP datagram. The fact
that RIP uses a transport-layer protocol (UDP) on top of a network layer protocol
(IP) to implement network-layer functionality.
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28. Implementation of RIP as the routed daemon
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29. Intra-AS Routing in the Internet: OSPF
• OSPF is a link-state protocol that uses Flooding of link-state
information and a Dijkstra least-cost path algorithm.
• In OSPF, the router broadcasts the routing information to all the other
routers of a network in every 30 seconds whenever links state changes.
• Some of the advances embodied in OSPF include the following:
• Security-MD5.
• Multiple same-cost paths.
• Integrated support for unicast and multicast routing.
• Support for hierarchy within a single routing domain.
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30. Areas in AS
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31. Inter-AS Routing: BGP
BGP provides each AS a means to
1. Obtain subnet reachability information from neighboring ASs.
2. Propagate the reachability information to all routers internal to
the AS.
3. Determine “good” routes to subnets based on the reachability
information and on AS policy.
“I exist and I am here”
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32. • eBGP: obtain subnet reachability information from neighboring ASs.
• iBGP: propagate reachability information to all AS-internal routers
Using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1.
 1c can then use iBGP do distribute new prefix info to all routers in AS1
 1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP session
When router learns of new prefix, it creates entry for prefix in its forwarding table.
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33. Path Attributes
Two important attributes:
• AS-PATH: contains ASs through which prefix advertisement has passed: e.g.,
AS 67, AS 17
• NEXT-HOP: indicates specific internal-AS router to next-hop AS. (may be
multiple links from current AS to next-hop-AS)
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34. BGP Route Selection
• Router may learn about more than 1 route to destination AS, selects
route based on:
• Local preference value attribute: policy decision
• Shortest AS-PATH
• Closest NEXT-HOP router: hot potato routing
• Additional criteria
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35. BGP Messages
• BGP messages exchanged between peers over TCP connection
• BGP messages:
• OPEN: opens TCP connection to peer and authenticates sender
• UPDATE: advertises new path (or withdraws old)
• KEEPALIVE: keeps connection alive in absence of UPDATES; also ACKs
OPEN request
• NOTIFICATION: reports errors in previous msg; also used to close
connection
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36. • BGP message contains “routes”
• “route” is a prefix and attributes: AS-PATH, NEXT-HOP,…
• Example: route:
 Prefix:138.16.64/22 ; AS-PATH: AS3 AS131 ; NEXT-HOP: 201.44.13.125
• Router may receive multiple routes for same prefix
• Has to select one route
• Router selects route based on shortest AS-PATH
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37. BGP Routing Policy
• A,B,C are provider networks
• X,W,Y are customer (of provider networks)
• X is dual-homed: attached to two networks
• X does not want to route from B via X to C
• .. so X will not advertise to B a route to C
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38. Why Are There Different Inter-as And Intra-as Routing Protocols?
Policy:
• Inter-AS: admin wants control over how its traffic routed, who routes
through its net.
• Intra-AS: single admin, so no policy decisions needed.
Scale:
• Hierarchical routing saves table size, reduced update traffic.
Performance:
• Intra-AS: can focus on performance.
• Inter-AS: policy may dominate over performance.
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39. Broadcast and Multicast Routing
• Broadcast and Multicast Routing
• N-way unicast approach
• source duplication is inefficient
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40. • Flooding: when node receives broadcast packet, sends copy to all
neighbors
• problems: cycles & broadcast storm
• Controlled flooding: node only broadcasts packet if it has not
broadcast same packet before
• node keeps track of packet ids already broadcasted
• or reverse path forwarding (RPF): only forward packet if it arrived on shortest
path between node and source
• Spanning tree:
• no redundant packets received by any node
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41. • First construct a spanning tree
• Nodes then forward/make copies only along spanning tree
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42. • Center node
• Each node sends unicast join message to center node
• Message forwarded until it arrives at a node already belonging to spanning tree
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43. REFERENCES
Text Book:
[1]James F kurose, Keith W Ross “Computer Networking -A
top down approach” Sixth Edition, Pearson publication, 2017
[2] NPTEL videos on Computer Networks.
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44. THANK YOU
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