The Network Layer

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Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011

Chapter 5 The Network Layer

Routing Algorithms, Congestion Control

Algorithms, QoS

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Network Layer Design Issues

• Store-and-forward packet switching

• Services provided to transport layer

• Implementation of connectionless service

• Implementation of connection-oriented service

• Comparison of virtual-circuit and datagram

networks

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Store-and-Forward Packet Switching

The environment of the network layer protocols.

ISP’s equipment

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Services Provided to the Transport Layer

1. Services independent of router technology.

2. Transport layer shielded from number, type, topology of routers.

3. Network addresses available to transport layer use uniform numbering plan

– even across LANs and WANs

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Implementation of Connectionless Service

Routing within a datagram network

ISP’s equipment

A’s table (initially) A’s table (later) C’s Table E’s Table

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Implementation of

Connection-Oriented Service

Routing within a virtual-circuit network

ISP’s equipment

A’s table C’s Table E’s Table

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Comparison of Virtual-Circuit and Datagram Networks

Comparison of datagram and virtual-circuit networks

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Routing Algorithms

• Routing is a process of transferring the packets from source m/c to destination machine, while routing algorithms (RA) are s/w responsible for deciding which outgoing line an incoming packet should be transmitted on.

• The purpose of RA is to decide which route is to be followed by a packet on the basis of following parameters:

• Availability of channels (paths/links/routes)

• Link transmission delay

• Traffic intensity, and

• Capacity (bandwidth) of the link

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Routing Algorithms

• On the basis of nature of information used in the algorithm, Routing Algorithms are categorized into two classes:

• Non-Adaptive: These do not base their routing decisions on the measurements/estimates of current traffic or topology, instead the choice of route to get from node ‘I’ to node ‘j’ is computed in advance or off-line, and hence also called as ‘Static Routing’ or ‘Pre-determined Routing’.

• Adaptive: It attempts to change their routing decisions to reflect changes in topology and the current traffic.

• On the basis of scope of information used in the algorithm, Routing Algorithms are classified into three types:

• Centralized Routing: The global algorithm uses information collected from the entire subnet in an attempt to make optimal decision.

• Isolated Routing: The local algorithm runs separately on each IMP and uses information only available there e.g., queue length.

• Distributed Routing: These use information available locally as well as

information available at their neighbors.

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Routing Algorithms

Properties of the R.A. (SCRSFO)

• Simplicity: The algorithm should not use very complex features.

• Correctness: The algorithm should clearly say about the start and end of the route searching.

• Robustness: The algorithms should either be capable of correcting smaller mistakes or displaying message to the user to correct errors (Detection and Corrections)

• Stability: In the event of failure of one or few IMPs, the total system should not be crashed or down.

• Fairness: Nothing should be ambiguous, everything should be stated clearly and the algorithms should not lead to congestion.

• Optimality: The algorithms should ensure to minimize the mean packet delay

time as well as maximize the total throughput of the network (maximum number

if message transmission with minimum delay).

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Routing Algorithms (1)

• Optimality principle

• Shortest path algorithm

• Flooding

• Distance vector routing

• Link state routing

• Routing in ad hoc networks

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Routing Algorithms (2)

• Broadcast routing

• Multicast routing

• Anycast routing

• Routing for mobile hosts

• Routing in ad hoc networks

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Fairness vs. Efficiency

Network with a conflict between fairness and efficiency.

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The Optimality Principle

(a) A network. (b) A sink tree for router B.

It states that if router J is on the optimal path from router I to router K, then the optimal path from J to K also falls along the same route.

The goal of routing algorithms is to discover and use the sink trees for all routers.

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Shortest Path Algorithm (1)

This algorithm finds the shortest path between any two given nodes on the basis of any of the following (metrics):

• Number of hops

• Geographical distance

• Mean queuing delay

In general, the labels of the arcs can be function of distance, bandwidth, average traffic, communication cost, mean queuing delay or transmission delay. Many algorithms may be designed using these parameters.

One such algorithms is designed by Dijkstra (1959) to

determine the shortest path between two nodes.

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Shortest Path Algorithm (1)

Steps of Dijkstra’s SPA algorithm to determine the shortest path between two nodes:

1. Initially, no path is known. So all the nodes are labeled as at an infinite distance from source node.

2. As the algorithm proceeds, the labels of the nodes changes accordingly reflecting a better path from the given source to the given sink.

3. Start from a node, and examine all adjacent node(s) to it. If the

sum of labels of nodes and distance from working node to the

node being examined is less than the label on that node, then we

have a shortest path, and the node is re-labeled. In a similar

fashion, all the adjacent nodes to the working node are inspected

and the tentative labels are changed. If possible the entire graph

is searched for tentatively labeled nodes with the smallest value,

the node is made the permanent node. With the progress of the

algorithm, all permanent nodes are encircled, so the shortest path

could be reconstructed.

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Shortest Path Algorithm (1)

The first five steps used in computing the shortest path from A to D. The arrows indicate the working node

L(B) = min(∞, 0+2)= 2 L(G) = min(∞, 0+6)= 6

L(C) = min(∞, 2+7)= 9 L(E) = min(∞, 2+2)= 4

L(F) = min(∞, 4+2)= 6 L(G) = min(∞, 4+1)= 5

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Shortest Path Algorithm (2)

Dijkstra’s algorithm to compute the shortest path through a graph.

. . .

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Shortest Path Algorithm (3)

Dijkstra’s algorithm to compute the shortest path through a graph.

. . .

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Shortest Path Algorithm (4)

Dijkstra’s algorithm to compute the shortest path through a graph.

. . .

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Shortest Path Algorithm

Disadvantages:

• Total traffic is routed via the calculated single path, which may lead to congestion.

• Sometimes, there exists some more paths that are equally good, but packets can’t be routed through these paths to reduce congestion.

Solution:

To overcome these problems, a new algorithm

called Multipath Algorithm, was designed by

Evan (1975).

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Multipath Algorithm

Each IMP maintains a table, with one row for each possible destination IMPs. Each row gives the best, the second best, and the third best outgoing line for that destination with a relative weight.

Refer to the routing table for node j. If node j receives a packet for node A, it uses the row labeled A, and IMP at node j will generate a number between 0 and 0.99, if the generated number is less than 0.63, then line A will be selected, if the number lies between 0.63 and 0.83, then second choice will be selected, otherwise the third choice will be selected for routing the packet.

Advantage:

MPA distributes the traffic uniformly, thereby avoiding the

congestion.

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MPA

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Other Routing Algorithms

a) Baran’s Hot Potato Algorithm (1964): Isolated routing, also called as Shortest Queue Algorithm. As soon as a node receives a packet, it tries to get rid of it by putting/forwarding it to the line having shortest queue.

b) Flooding: Each incoming packet is forwarded to every outgoing line, thereby ensuring shortest path and shortest transmission delay. But, operative for very low traffic condition.

c) Centralized Routing using RCC: All IMPs in the network

periodically sends information to Routing Control Centre

(RCC), regarding their queue length, delay offered, list of

neighbors that are up, etc. Based on these global

information, RCC computes the routing table and

distributes to all IMPs. It was used in TYMNET in 1971.

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(a) A network.

(b) Input from A, I, H, K, and the new routing table for J.

Distance Vector Routing

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1. The Count-to-Infinity Problems

2. Delay metric was queue length, it did not take line bandwidth into account,

when choosing routes. Initially all the lines were 50 kbps, and hence no

problem, but later some were upgraded to 230 kbps, and others to 1.544 Mbps.

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Link State Routing (IS-IS, OSPF)

1. Discover neighbors, learn network addresses (Hello).

2. Set/Measure distance/cost metric to each neighbor (Echo).

3. Construct packet telling all it has learned.

4. Send packet to, receive packets from other routers (trickiest part, flooding(to check flooding, packet seq. no. and age are used)).

5. Compute shortest path to every other router (Dijkstra’s

Algorithm).

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Learning about the Neighbors (1)

Nine routers and a broadcast LAN.

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Learning about the Neighbors (2)

A graph model of previous slide.

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Building Link State Packets

(a) A network. (b) The link state packets for this network.

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Distributing the Link State Packets

The packet buffer for router B in previous slide

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Hierarchical Routing

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Hierarchical Routing

Hierarchical routing.

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Broadcast Routing

Reverse path forwarding. (a) A network. (b) A sink tree.

(c) The tree built by reverse path forwarding.

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Multicast Routing (1)

(a) A network. (b) A spanning tree for the leftmost router. (c) A

multicast tree for group 1. (d) A multicast tree for group 2.

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Multicast Routing (2)

(a) Core-based tree for group 1.

(b) Sending to group 1.

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Anycast Routing

(a) Anycast routes to group 1.

(b) Topology seen by the routing protocol.

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Routing for Mobile Hosts

Packet routing for mobile hosts

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Routing for Mobile Hosts

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Routing in Ad Hoc Networks

(a) Range of A’s broadcast.

(b) After B and D receive it.

(c) After C, F, and G receive it.

(d) After E, H, and I receive it.

The shaded nodes are new recipients. The dashed lines show

possible reverse routes.

The solid lines show the

discovered route.

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Congestion Control Algorithms (1)

• Approaches to congestion control

• Traffic-aware routing

• Admission control

• Traffic throttling

• Load shedding

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Congestion Control Algorithms (2)

When too much traffic is offered, congestion sets in and

performance degrades sharply.

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Approaches to Congestion Control

Timescales of approaches to congestion control

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Traffic-Aware Routing

A network in which the East and West parts

are connected by two links.

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Traffic Throttling (1)

(a) A congested network. (b) The portion of the network that is

not congested. A virtual circuit from A to B is also shown.

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Traffic Throttling (2)

Explicit congestion notification

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Load Shedding (1)

A choke packet that affects only the source..

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Load Shedding (2)

A choke packet that affects each hop it passes through.

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Quality of Service

• Application requirements

• Traffic shaping

• Packet scheduling

• Admission control

• Integrated services

• Differentiated services

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Application Requirements (1)

How stringent the quality-of-service requirements are.

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Categories of QoS and Examples

1. Constant bit rate

• Telephony

2. Real-time variable bit rate

• Compressed videoconferencing

3. Non-real-time variable bit rate

• Watching a movie on demand

4. Available bit rate

• File transfer

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Traffic Shaping (1)

(a) Shaping packets. (b) A leaky bucket. (c) A token bucket

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Traffic Shaping (2)

(a) Traffic from a host. Output shaped by a token bucket of rate

200 Mbps and capacity (b) 9600 KB, (c) 0 KB.

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Traffic Shaping (3)

Token bucket level for shaping with rate 200 Mbps and capacity

(d) 16000 KB, (e) 9600 KB, and (f) 0KB..

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Packet Scheduling (1)

Kinds of resources can potentially be reserved for different flows:

1. Bandwidth.

2. Buffer space.

3. CPU cycles.

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Packet Scheduling (2)

Round-robin Fair Queuing

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Packet Scheduling (3)

(a) Weighted Fair Queueing.

(b) Finishing times for the packets.

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Admission Control (1)

An example flow specification

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Admission Control (2)

Bandwidth and delay guarantees with token buckets and WFQ.

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Integrated Services (1)

(a) A network. (b) The multicast spanning tree for host 1.

(c) The multicast spanning tree for host 2.

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Integrated Services (2)

(a) Host 3 requests a channel to host 1. (b) Host 3 then requests a second channel, to host 2.

(c) Host 5 requests a channel to host 1.

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Differentiated Services (1)

Expedited packets experience a traffic-free network

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Differentiated Services (2)

A possible implementation of assured forwarding

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Internetworking

• How networks differ

• How networks can be connected

• Tunneling

• Internetwork routing

• Packet fragmentation

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How Networks Differ

Some of the many ways networks can differ

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How Networks Can Be Connected

(a) A packet crossing different networks.

(b) Network and link layer protocol processing.

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Tunneling (1)

Tunneling a packet from Paris to London.

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Tunneling (2)

Tunneling a car from France to England

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Packet Fragmentation (1)

Packet size issues:

1. Hardware

2. Operating system 3. Protocols

4. Compliance with (inter)national standard.

5. Reduce error-induced retransmissions

6. Prevent packet occupying channel too long.

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Packet Fragmentation (2)

(a) Transparent fragmentation.

(b) Nontransparent fragmentation

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Packet Fragmentation (3)

Fragmentation when the elementary data size is 1 byte.

(a) Original packet, containing 10 data bytes.

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Packet Fragmentation (4)

Fragmentation when the elementary data size is 1 byte (b) Fragments after passing through a network

with maximum packet size of 8 payload bytes plus header.

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Packet Fragmentation (5)

Fragmentation when the elementary data size is 1 byte

(c) Fragments after passing through a size 5 gateway.

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Packet Fragmentation (6)

Path MTU Discovery

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The Network Layer Principles (1)

1. Make sure it works 2. Keep it simple

3. Make clear choices 4. Exploit modularity

5. Expect heterogeneity

. . .

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The Network Layer Principles (2)

. . .

6. Avoid static options and parameters 7. Look for good design (not perfect) 8. Strict sending, tolerant receiving 9. Think about scalability

10. Consider performance and cost

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The Network Layer in the Internet (1)

• The IP Version 4 Protocol

• IP Addresses

• IP Version 6

• Internet Control Protocols

• Label Switching and MPLS

• OSPF—An Interior Gateway Routing Protocol

• BGP—The Exterior Gateway Routing Protocol

• Internet Multicasting

• Mobile IP

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The Network Layer in the Internet (2)

The Internet is an interconnected collection of many networks.

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The IP Version 4 Protocol (1)

The IPv4 (Internet Protocol) header.

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The IP Version 4 Protocol (2)

Some of the IP options.

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IP Addresses (1)

An IP prefix.

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IP Addresses (2)

Splitting an IP prefix into separate networks with subnetting.

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IP Addresses (3)

A set of IP address assignments

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IP Addresses (4)

Aggregation of IP prefixes

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IP Addresses (5)

Longest matching prefix routing at the New York router.

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IP Addresses (6)

IP address formats

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IP Addresses (7)

Special IP addresses

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IP Addresses (8)

Placement and operation of a NAT box.

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IP Version 6 Goals

• Support billions of hosts

• Reduce routing table size

• Simplify protocol

• Better security

• Attention to type of service

• Aid multicasting

• Roaming host without changing address

• Allow future protocol evolution

• Permit coexistence of old, new protocols . . .

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IP Version 6 (1)

The IPv6 fixed header (required).

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IP Version 6 (2)

IPv6 extension headers

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IP Version 6 (3)

The hop-by-hop extension header for

large datagrams (jumbograms).

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IP Version 6 (4)

The extension header for routing.

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Internet Control Protocols (1)

The principal ICMP message types.

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Internet Control Protocols (2)

Two switched Ethernet LANs joined by a router

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Label Switching and MPLS (1)

Transmitting a TCP segment using IP, MPLS, and PPP.

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Label Switching and MPLS (2)

Forwarding an IP packet through an MPLS network

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OSPF—An Interior Gateway Routing Protocol (1)

An autonomous system

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OSPF—An Interior Gateway Routing Protocol (2)

A graph representation of the previous slide.

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OSPF—An Interior Gateway Routing Protocol (3)

The relation between ASes, backbones, and areas in OSPF.

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OSPF—An Interior Gateway Routing Protocol (4)

The five types of OSPF messages

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BGP—The Exterior Gateway Routing Protocol (1)

Examples of routing constraints:

1. No commercial traffic for educat. network

2. Never put Iraq on route starting at Pentagon 3. Choose cheaper network

4. Choose better performing network

5. Don’t go from Apple to Google to Apple

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BGP—The Exterior Gateway Routing Protocol (2)

Routing policies between four Autonomous Systems

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BGP—The Exterior Gateway Routing Protocol (3)

Propagation of BGP route advertisements

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Mobile IP

Goals

1. Mobile host use home IP address anywhere.

2. No software changes to fixed hosts

3. No changes to router software, tables

4. Packets for mobile hosts – restrict detours

5. No overhead for mobile host at home.

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