This document summarizes key topics in network layer design from Chapter 5 of Tanenbaum's computer networks textbook, including store-and-forward packet switching, connectionless and connection-oriented services, routing algorithms like flooding and link-state routing, congestion control, and quality of service techniques. It provides figures and explanations for many routing and congestion control concepts.

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The Network Layer

2. 2/21/2023 Tanenbaum Chapter 5 Network 2
Network Layer Design Isues
• Store-and-Forward Packet Switching
• Services Provided to the Transport Layer
• Implementation of Connectionless Service
• Implementation of Connection-Oriented Service
• Comparison of Virtual-Circuit and Datagram Subnets

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Store-and-Forward Packet Switching
The environment of the network layer protocols.
fig 5-1

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Implementation of Connectionless Service
Routing within a diagram subnet.

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Implementation of Connection-Oriented Service
Routing within a virtual-circuit subnet.

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Comparison of Virtual-Circuit and
Datagram Subnets
5-4

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Routing Algorithms
• The Optimality Principle
• Shortest Path Routing
• Flooding
• Distance Vector Routing
• Link State Routing
• Hierarchical Routing
• Broadcast Routing
• Multicast Routing
• Routing for Mobile Hosts
• Routing in Ad Hoc Networks

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Routing Algorithms (2)
Conflict between fairness and optimality.

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The Optimality Principle
(a) A subnet. (b) A sink tree for router B.

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Shortest Path Routing
The first 5 steps used in computing the shortest path from A to D.
The arrows indicate the working node.

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Flooding
Dijkstra's algorithm to compute the shortest path through a graph.
5-8 top

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Flooding (2)
Dijkstra's algorithm to compute the shortest path through a graph.
5-8
bottom

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Distance Vector Routing
(a) A subnet. (b) Input from A, I, H, K, and the new
routing table for J.

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Distance Vector Routing (2)
The count-to-infinity problem.

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Link State Routing
Each router must do the following:
1. Discover its neighbors, learn their network address.
2. Measure the delay or cost to each of its neighbors.
3. Construct a packet telling all it has just learned.
4. Send this packet to all other routers.
5. Compute the shortest path to every other router.

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Learning about the Neighbors
(a) Nine routers and a LAN. (b) A graph model of (a).

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Measuring Line Cost
A subnet in which the East and West parts are connected by two lines.

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Building Link State Packets
(a) A subnet. (b) The link state packets for this subnet.

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Distributing the Link State Packets
The packet buffer for router B in the previous slide (Fig. 5-13).

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Oscillation in Shortest Path Routing
Example 1.
Consider the 16-node network shown in Fig.
5.25, where node 16 is the only destination.
Let the traffic input (in data units/sec) at each
node i = 1, ... , 7, 9, ... ,15 be one unit and let
the traffic input of node 8 be  > 0, where  is
very small. Assume that the length of link (i, j)
is
dij = Fij
where Fij is the arrival rate at the link
counting input and relayed traffic. Suppose
that all nodes compute their shortest path to
the destination every T seconds using as link
lengths the arrival rates Fij during the
preceding T seconds, and route all their traffic
along that path for the next T seconds.
Assume that we start with nodes 1 through 7
routing clockwise and nodes 8 through 15
routing counterclockwise. This is a rather
good routing, balancing the traffic input
between the two directions.
From “Data Network” by
Bertsekas/Gallager

21. 2/21/2023 Tanenbaum Chapter 5 Network 21
Oscillation in Shortest Path Routing
Figure 5.26 Oscillations in a ring network for link lengths dij equal to the link
arrival rates Fij. Each node sends one unit of input traffic to the destination
except for the middle node 8, which sends  > 0, where  < is very small. The
numbers next to the links are the link rates in each of the two directions. As an
example of the shortest path calculations, at the first iteration the middle node
8 computes the length of the clockwise path as 28(= 0 + 1 + 2 + ... + 7), and
the length of the counterclockwise path as 28+8 (=  + 1+e+2+e+...+7+e), and
switches its traffic to the shortest (clockwise) path at the 2nd routing. The
corresponding numbers for node 9 are 28 and 28 + 7 , so node 9 also
switches its traffic to the clockwise path. All other nodes find that the path used
at the first routing is shortest, and therefore they do not switch their traffic to
the other path.

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Oscillation in Shortest Path Routing
Figure 5.26 shows the link rates corresponding to the initial
and subsequent shortest path routings. Thus, after three
shortest path updates, the algorithm is locked into an
oscillatory pattern whereby all traffic swings from the
clockwise to the counterclockwise direction and back at
alternate updates. This is certainly the worst type of routing
performance that could occur.

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Hierarchical Routing
Hierarchical routing.

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Broadcast Routing
Reverse path forwarding. (a) A subnet. (b) a Sink tree. (c) The
tree built by reverse path forwarding.

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Multicast Routing
(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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Routing for Mobile Hosts
A WAN to which LANs, MANs, and wireless cells are attached.

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

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Routing in Ad Hoc Networks
Possibilities when the routers are mobile:
1. Military vehicles on battlefield.
– No infrastructure.
2. A fleet of ships at sea.
– All moving all the time
3. Emergency works at earthquake .
– The infrastructure destroyed.
4. A gathering of people with notebook computers.
– In an area lacking 802.11.

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Route Discovery
a) (a) Range of A's broadcast.
b) (b) After B and D have received A's broadcast.
c) (c) After C, F, and G have received A's broadcast.
d) (d) After E, H, and I have received A's broadcast.
Shaded nodes are new recipients. Arrows show possible reverse routes.

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Route Discovery (2)
Format of a ROUTE REQUEST packet.

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Route Discovery (3)
Format of a ROUTE REPLY packet.

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Route Maintenance
(a) D's routing table before G goes down.
(b) The graph after G has gone down.

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Node Lookup in Peer-to-Peer Networks
(a) A set of 32 node identifiers arranged in a circle. The shaded ones
correspond to actual machines. The arcs show the fingers from
nodes 1, 4, and 12. The labels on the arcs are the table indices.
(b) Examples of the finger tables.

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Congestion Control Algorithms
• General Principles of Congestion Control
• Congestion Prevention Policies
• Congestion Control in Virtual-Circuit Subnets
• Congestion Control in Datagram Subnets
• Load Shedding
• Jitter Control

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Congestion
When too much traffic is offered, congestion sets in and
performance degrades sharply.

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General Principles of Congestion Control
1. Monitor the system .
– detect when and where congestion occurs.
2. Pass information to where action can be taken.
3. Adjust system operation to correct the problem.

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Congestion Prevention Policies
Policies that affect congestion.
5-26

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Congestion Control in Virtual-Circuit
Subnets
(a) A congested subnet. (b) A redrawn subnet, eliminates
congestion and a virtual circuit from A to B.

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Hop-by-Hop
Choke Packets
(a) A choke packet that affects
only the source.
(b) A choke packet that affects
each hop it passes through.

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Jitter Control
(a) High jitter. (b) Low jitter.

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Quality of Service
• Requirements
• Techniques for Achieving Good Quality of Service
• Integrated Services
• Differentiated Services
• Label Switching and MPLS

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Requirements
How stringent the quality-of-service requirements are.
5-30

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Buffering
Smoothing the output stream by buffering packets.

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The Leaky Bucket Algorithm
(a) A leaky bucket with water. (b) a leaky bucket with packets.

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The Leaky
Bucket
Algorithm
(a) Input to a leaky bucket.
(b) Output from a leaky
bucket. Output from a token
bucket with capacities of (c)
250 KB, (d) 500 KB, (e)
750 KB, (f) Output from a
500KB token bucket feeding
a 10-MB/sec leaky bucket.

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The Token Bucket Algorithm
(a) Before. (b) After.
5-34

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Admission Control
An example of flow specification.
5-34

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Packet Scheduling
(a) A router with five packets queued for line O.
(b) Finishing times for the five packets.

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RSVP-The ReSerVation Protocol
(a) A network, (b) The multicast spanning tree for host 1.
(c) The multicast spanning tree for host 2.

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RSVP-The ReSerVation Protocol (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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Expedited Forwarding
Expedited packets experience a traffic-free network.

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Assured Forwarding
A possible implementation of the data flow for assured forwarding.

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

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Internetworking
• How Networks Differ
• How Networks Can Be Connected
• Concatenated Virtual Circuits
• Connectionless Internetworking
• Tunneling
• Internetwork Routing
• Fragmentation

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Connecting Networks
A collection of interconnected networks.

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How Networks Differ
Some of the many ways networks can differ.
5-43

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How Networks Can Be Connected
(a) Two Ethernets connected by a switch.
(b) Two Ethernets connected by routers.

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Concatenated Virtual Circuits
Internetworking using concatenated virtual circuits.

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Connectionless Internetworking
A connectionless internet.

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Tunneling
Tunneling a packet from Paris to London.

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Tunneling (2)
Tunneling a car from France to England.

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Internetwork Routing
(a) An internetwork. (b) A graph of the internetwork.

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Fragmentation
(a) Transparent fragmentation. (b) Nontransparent fragmentation.

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Fragmentation (2)
Fragmentation when the elementary data size is 1 byte.
(a) Original packet, containing 10 data bytes.
(b) Fragments after passing through a network with maximum
packet size of 8 payload bytes plus header.
(c) Fragments after passing through a size 5 gateway.

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The Network Layer in the Internet
• The IP Protocol
• IP Addresses
• Internet Control Protocols
• OSPF – The Interior Gateway Routing Protocol
• BGP – The Exterior Gateway Routing Protocol
• Internet Multicasting
• Mobile IP
• IPv6

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Design Principles for Internet
1. Make sure it works.
2. Keep it simple.
3. Make clear choices.
4. Exploit modularity.
5. Expect heterogeneity.
6. Avoid static options and parameters.
7. Look for a good design; it need not be perfect.
8. Be strict when sending and tolerant when receiving.
9. Think about scalability.
10. Consider performance and cost.

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Collection of Subnetworks
The Internet is an interconnected collection of many networks.

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The IP Protocol
The IPv4 (Internet Protocol) header.

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The IP Protocol (2)
Some of the IP options.
5-54

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IP Addresses
IP address formats.

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IP Addresses (2)
Special IP addresses.

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Subnets
A campus network consisting of LANs for various departments.

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Subnets (2)
A class B network subnetted into 64 subnets.

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CDR – Classless InterDomain Routing
A set of IP address assignments.
5-59

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NAT – Network Address Translation
Placement and operation of a NAT box.

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Internet Control Message Protocol
The principal ICMP message types.
5-61

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ARP– The Address Resolution Protocol
Three interconnected /24 networks: two Ethernets and an FDDI ring.

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Dynamic Host Configuration Protocol
Operation of DHCP.

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OSPF – The Interior Gateway Routing
Protocol
(a) An autonomous system. (b) A graph representation of (a).

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OSPF (2)
The relation between ASes, backbones, and areas in OSPF.

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OSPF (3)
The five types of OSPF messeges.
5-66

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BGP – The Exterior Gateway Routing
Protocol
(a) A set of BGP routers. (b) Information sent to F.

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The Main IPv6 Header
The IPv6 fixed header (required).

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Extension Headers
IPv6 extension headers.
5-69

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Extension Headers (2)
The hop-by-hop extension header for large datagrams (jumbograms).

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Extension Headers (3)
The extension header for routing.