DCCN Network Layer congestion control TCP

DATA COMMUNICATION
AND
Network Layer; Congestion Control; TCP, UDP
AND
COMPUTER NETWORKS
B.Tech CSE V Semester
For educational purpose only; Source: Forouzan & Tanenbaum

Unit 3
Network layer:
Design Issues: store-and-forward,
Services to transport layer - Connection less and
Connection oriented services
Connection oriented services
Routing Algorithms:
The optimality principle, shortest path routing,
Flooding,
Distance vector and Link state,
Multicast Routings.

Unit 4
Congestion Control:
Principles, congestion prevention policies,
congestion control in virtual circuits and
datagram subnets, load shedding, jitter control.
datagram subnets, load shedding, jitter control.
Internetworking:
Tunneling, Internet work routing,
Fragmentation.
The IP protocol, IP address,
Gateway routing protocols: OSPF, BGP.

Unit 5
Transport Layer:
UDP, TCP- service model, protocol, segment
header,
connection management, Transmission Policy.
connection management, Transmission Policy.
Application Layer:
The DNS Name Space,
Resource Records, Name Servers.

Network Layer: Design Issues
A
Physical Physical
Data link
Data link
R1 R3 R4 B
Network
Network
Source Destination Data
D Header
H
Legend
Source: Forouzan
Datagram
D3 H3
Datagram
D3 H3

Network Layer: Design Issues
Internetworking: process of connecting different networks by
using networking devices such as routers, gateways etc.,
Packetizing: encapsulate packets received from upper
layer protocols
Addressing: identify each device uniquely to allow
global communication
Routing: determine optimal route for sending a packet
from one host to another
Fragmenting: decapsulate packets from one and
encapsulate them for another network
Source: Forouzan

Network layer at the source
Packetizer
encapsulate packet from upper layer
Add universal source and destination address
Processing Module
verify whether destination address is host address.
If so routing is not needed
Routing Module
find interface from which packet must be sent
Fragmentation Module
Breaking packets into smaller pieces(fragments) such that
resulting pieces can pass through a link
Source: Forouzan

Network layer at a router
Processing Module
Checks if the packet has reached its destination or
needs to be forwarded
Routing Module
finds the interface from which packet must be sent
8 Source: Forouzan

Network layer at the
destination
9 Source: Forouzan

Store and Forward Packet Switching
Source: Tanenbaum

Services Provided to Transport Layer
 Designing goals
 Independent of subnet technology
 Transport layer shielded from number, type, and
 Transport layer shielded from number, type, and
topology of subnets
 Uniform network address numbering
 Two Types of Services
 Connectionless
 Connection-oriented

Implementation of Connectionless Service
Source: Tanenbaum

Implementation of Connection-Oriented Service
Routing within a virtual-circuit network
Source: Tanenbaum

Comparison of Virtual-Circuit
and Datagram Networks

Routing
 Routing is the process of forwarding of a packet in
a network so that it reaches its intended destination.
 The main goals of routing are:
 The main goals of routing are:
 Correctness
 low overhead
 robustness
 stability
 fairness

Routing Algorithms
•The Optimality Principle
•Shortest Path Routing
•Flooding
•Flooding
•Distance Vector Routing
•Link State Routing
•Hierarchical Routing
•Broadcast Routing
•Multicast Routing

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.

Hierarchical Routing
Source: Tanenbaum

Broadcast Routing
 Sending a packet to all destinations simultaneously is
called broadcasting.
Ex: When a company needs to update version.
 Ex: When a company needs to update version.
 Make ‘n’ copies for every user and then broadcast
packet, to each destination:
 Drawback: Uses lots of bandwidth and source needs to have
complete list of all destinations

Broadcast Routing
 2. when router receives a packet that is to be broadcasted,
it simply floods those packets out of all interfaces.
 Drawback: This method is easy on router's CPU but may cause
 Drawback: This method is easy on router's CPU but may cause
the problem of duplicate packets received from peer routers.
 Reverse path forwarding is a technique, in which router
knows in advance about its predecessor from where it
should receive broadcast.
 Advantage: Detects and discards duplicates.

Multicast Routing
 In broadcast routing, packets are sent to all nodes even if
they do not want it. But in Multicast routing, the data is sent
to only nodes which wants to receive the packets.
to only nodes which wants to receive the packets.
 Multicast routing uses spanning tree to avoid looping.
 Multicast routing also uses reverse path Forwarding
technique, to detect and discard duplicates and loops.

Link State Routing
Following 5 steps are followed to implement LSR.
1. Learning about the Neighbors
2. Measuring Line Cost.
2. Measuring Line Cost.
3. Building Link State Packets.
4. Distributing the Link State Packets.
5. Computing the New Routes.

Step 1:Learning about the Neighbors
 Upon router booting, discover who the neighbours are, using
HELLO packet.
 Receiving packet router replies back who am i
Receiving packet router replies back who am i
Source: Tanenbaum

Step 2: Measuring Line Cost
 Line cost or delay is measured by sending ECHO message
and measure return time.
 For better results, the above step can be repeated several
 For better results, the above step can be repeated several
times and average is considered
 Waiting time in router queue can be added to include line
traffic load

Step 3: Building Link State Packets:
 Packet containing:
 Identity of sender
 Sequence number
 age
List of neighbours
 When to build the link
state packets?
 Periodically
 when significant events
 List of neighbours
when significant events
occur
Source: Tanenbaum

Step 4: Distributing Link State Packets:
 Distributing link state packets reliably
 Arrival time for packets different
 How to keep consistent routing tables?
 Flooding is used
 each packet contains seq no. that is incremented for each new packet sent
 Routers keep track of source router, seq
 When new LSP arrives, it is checked against LSP already seen:
 new forwarded on all lines except the one it arrived on; Duplicate, discarded
 Packet with seq no lower than highest one, rejected

Distributing the Link State Packets
Source: Tanenbaum

Step 5: Computing new routes:
 With a full set of link state packets, a router can:
 Construct the entire subnet graph
 Run Dijkstra’s algorithm to compute the shortest
path to each destination
path to each destination
 Problems for large subnets
 Memory to store data
 Compute time for developing these tables.

Adaptive and Non adaptive Routing
 Adaptive
 Routing is based on current
measurements of traffic
 Non Adaptive
 Routing is computed in advance
 N/w Admin manually enters
measurements of traffic
 Routers exchange updates and
router table information.
 Router may select a new route for
each packet.
 Ex: DVR, LSR
 N/w Admin manually enters
routing paths into router.
 Once the path to dest has been
selected, router sends all
packets along the route
 Ex: Flooding, SPR

Distance Vector Vs. Link State Routing
 Distance Vector
 Sends entire routing table
 Slow convergence
 Link State
 Sends only link state info
 Fast convergence
 Susceptible to routing loops
 Does not know network topology
 Simple to configure
 Periodic updates (30/60 sec)
 Ex: RIP; BGP
 Less susceptible to routing loops
 Knows entire n/w topology
 Hard to configure
 Updates are triggered not
periodic
 Ex: OSPF; IS-IS

Single-Source Shortest Path Problem
The problem of finding shortest paths from a source
vertex v to all other vertices in the graph.

Dijkstra's algorithm
Dijkstra's algorithm - is a solution to the single-source
shortest path problem in graph theory.
Works on both directed and undirected graphs. However,
all edges must have nonnegative weights.
all edges must have nonnegative weights.
Input: Weighted graph G={E,V} and source vertex v V,
such that all edge weights are nonnegative
Output: Lengths of shortest paths (or the shortest paths
themselves) from a given source vertex v V to all other
vertices

Dijkstra's algorithm - Pseudocode
dist[s] ←0 (distance to source vertex is zero)
for all v V–{s}
do dist[v] ←∞ (set all other distances to infinity)
S← (S, the set of visited vertices is initially empty)
Q←V (Q, the queue initially contains all
vertices)
while Q ≠ (while the queue is not empty)
while Q ≠ (while the queue is not empty)
do u ← mindistance(Q,dist) (select the element of Q with the min.
distance)
S←S {u} (add u to list of visited vertices)
for all v neighbors[u]
do if dist[v] > dist[u] + w(u, v) (if new shortest path found)
then d[v] ←d[u] + w(u, v) (set new value of shortest path)
return dist

Dijkstra(Graph, source):
for each vertex v in Graph.Vertices:
dist[v] ← INFINITY
prev[v] ← UNDEFINED
add v to Q
dist[source] ← 0
while Q is not empty:
u ← vertex in Q with min dist[u]
remove u from Q
for each neighbor v of u still in Q:
alt ← dist[u] + Graph.Edges(u, v)
if alt < dist[v]:
dist[v] ← alt
prev[v] ← u

DCCN Network Layer congestion control TCP

SPR: Example 2
B
C
2
7
3
3
2
2
A D
E
G
F
H
3
2
2
3
2
4
2
6
1

Solution: SPR Example 2
A
B C
D(10,H)
2
7
3
(6,E)
2
(9,B)
(4,B) 2
3
(2,A)
A D(10,H)
E F
2
(6,E)
4
2
6
1
(4,B)
G(5,E) H(8,F)
2
2

Explanation: SPR Example 2
A
B
C
D
E F
2
7
3
2
2
3
2
4
2
6
1
G H
2
4
6

Distance Vector Routing
 c(x,v) = cost for direct link from x to v
 Dx(y) = estimate of least cost from x to y
 Node x maintains its neighbors’ distance vectors
For each neighbor v, x maintains D = [D (y): y є N ]
56
 For each neighbor v, x maintains Dv = [Dv(y): y є N ]
 Each node v periodically sends Dv to its neighbors
 And neighbors update their own distance vectors
Dx(y) ← minv{c(x,v) + Dv(y)} for each node y N
 Over time, the distance vector Dx converges

Distance Vector Routing: Example1
Y
2 1
X Z
2 1
7

Distance Vector Routing: Example 2
Y
1
X
Z
4
1
50

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

Flooding
 Flooding adapts the technique in which every
incoming packet is sent on every outgoing line
except the one on which it arrived.
except the one on which it arrived.
 One problem with this method is that packets may
go in a loop. As a result of this a node may receive
several copies of a particular packet which is
undesirable.

Flooding
 Some techniques adapted to overcome these
problems are as follows:
 Sequence Numbers
 Sequence Numbers
 Hop Count
 Spanning Tree
 A flooding attack occupies the host memory buffer,
making it impossible to make new connections,
resulting in a denial of service.

Flooding: Advantages
 Simple to setup and implement, since a router may know only
its neighbours.
 Robust ie., Even in case of malfunctioning of a large number
Robust ie., Even in case of malfunctioning of a large number
routers, the packets find a way to reach the destination.
 All nodes which are directly or indirectly connected are visited.
So, there are no chances for any node to be left out. This is a
main criteria in case of broadcast messages.
 The shortest path is always chosen by flooding

Flooding: Disadvantages
 Network congestion
 Wastage of network resources
Security risks
 Security risks
 Difficulty in network troubleshooting

Flooding
 Every incoming packet is sent out on every outgoing
line except for the input line
 Problem
 Problem
 Large number of packets are generated
 Solutions
 Hop counter
 Avoiding duplicates
 Selective flooding

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Flooding - Conclusion
 Optimal
 Shortest path is always chosen
 No other algorithm can produce a shorter delay
Robust
 Robust
 Not practical in most applications
 Useful in some applications
 Military application: robustness
 Distributed database applications
 Concurrent update
 Wireless networks

Leaky Bucket Algorithm (congestion control)
 The leaky bucket is an algorithm related to
congestion control, based on an analogy of how a
bucket with a constant leak will overflow if either
bucket with a constant leak will overflow if either
the average rate at which water is poured in
exceeds the rate at which the bucket leaks or if
more water than the capacity of the bucket is
poured in all at once

No matter the rate at which water enters the bucket, the outflow is at a constant rate
once the bucket is full, any additional water entering it spills over the sides and is lost
Leaky bucket
Source: Tanenbaum

 Consider that, each network interface has a leaky bucket.
 Now, when the sender wants to transmit packets, the packets are thrown into the
bucket. These packets get accumulated in the bucket present at the network
interface.
 If the bucket is full, the packets are discarded by the buckets and are lost.
Leaky bucket
 This bucket will leak at a constant rate. This means that the packets will be
transmitted to the network at a constant rate. This constant rate is known as the
Leak Rate or the Average Rate.
 In this way, bursty traffic is converted into smooth, fixed traffic by the leaky
bucket.
 Queuing and releasing the packets at different intervals help in reducing network
congestion and increasing overall performance.

 Consider leaky bucket capacity = 5, fixed data flow
rate = 2, packet sizes = 5, 4, 3
Leaky bucket Example

bucket capacity = 5, fixed data flow rate
= 2, packet sizes = 5, 4, 3

Leaky bucket Example 2
 consider Bucket size capacity = 1000 bytes, leaky
bucket rate (fixed data flow rate) = 500
bytes/second.
bytes/second.
 Packet sizes [600,900,800,1000,1900]
 Solve using leaky bucket

 Enter The Bucket Size capacity: 1000
 Enter fixed data flow rate : 500
 Enter no. of seconds to simulate: 5
 Enter The Size Of The Packet Entering At 1sec: 600
Enter The Size Of The Packet Entering At 1sec: 600

Unit 4: Congestion Control & Internetworking
 Congestion control
principles
 Congestion control
 Internetworking:
 Concatenated VCs
 Connection less
Congestion control
prevention policies
 Congestion control in VC
and datagram subnets
 Load shedding
 Jitter control
 Connection less
internetworking
 Tunneling,
Fragmentation, IP
 IP Address, Gateway
Routing protocols

Congestion?
Congestion refers to a network state, where a node or router or link
carries so much of data that it may degrade the performance of network.
Too many packets in (a part of) the subnet.
May occur if the load on the network – the number of packets sent to the
Packet flow
Input Queue
Input Queue
Output Queue
network – is greater than the capacity of the network – the number of
packets a network can handle.
The network layer and transport layer share the responsibility of
handling congestion.
Congestion control refers to mechanisms and techniques to control the
congestion and keep the load below the capacity.
Packet flow
Input Queue

Congestion?
Congestion refers to a network state, where a node or router or link
carries so much of data that it may degrade the performance of network.
Too many packets in (a part of) the subnet.
Packet flow
network – is greater than the capacity of the network – the number of
packets a network can handle.
The network layer and transport layer share the responsibility of
handling congestion.
Congestion control refers to mechanisms and techniques to control the
Packet flow

84
Congestion
 On packet arrival:
 Packet is put at the end of input queue
 Processing module of the router removes the packet from front of queue and make
routing decisions using routing table.
 Packet is put into respective output queue and waits its turn to be sent.
 If rate of packet arrival > packet processing rate, Input queue size will increase.
If rate of processing > rate of departure, output queue increases
 If rate of processing > rate of departure, output queue increases

Congestion
Destination
1.5-Mbps T1 link
Router
Source
2
Source
1
100-Mbps FDDI
10-Mbps Ethernet

Flow Control Vs. Congestion Control
 Flow control: slow down the sender if the sender sends the data at a
faster rate than the receiving capacity of the receiver.
 Flow control relates to traffic between two machines, while congestion
Flow control relates to traffic between two machines, while congestion
control is more global. Flow control makes sure that fast sender cannot
continually transmit data faster than the receiver is able to absorb it.
 Congestion control has to make sure that subnet is able to carry the
offered traffic and is global issue.
 Congestion control is a mechanism and techniques to control the

Causes of Congestion
 limited resources
 insufficient memory
 insufficient memory
 Slow processors
 Low bandwidth
 Mismatch in updates of parts of system

Congestion Control
 Congestion control refers to techniques and mechanisms
that can either prevent congestion, before it happens,
or remove congestion, after it has happened.
or remove congestion, after it has happened.
 In general, congestion control mechanisms can be
divided into two broad categories:
 open-loop congestion control (prevention) and
 closed-loop congestion control (removal).

Congestion control: Principles
 makes an attempt to prevent
congestion from occurring
 Make sure the problem does not occur
 Tools
 Decide when to accept traffic
 Monitor: where and when congestion?
 % packets discarded
 average queue length
 number of packets that time out
 average packet delay
open loop (Prevention) closed loop (Removal)
 Decide when to accept traffic
 Decide when to discard packets and
which ones
 Make scheduling decisions in the
subnet
 Two types: Source based & destination
based
They make decision without considering
current state of network
average packet delay
Rising number indicates growing congestion
 Pass collected info to places where actions can
be taken = source of traffic
 Adjust system operation
 Increase resources: bandwidth
 Decrease load: deny, degrade service

Congestion: Prevention Policies
 open loop solutions: Minimize congestion, they try to achieve there goals by using
appropriate policies at various levels
Layer Policies
Transport  Retransmission policy
 Acknowledgement policy
 Flow control policy
 Timeout determination ( transit time over the network is hard to predict)
91
 Timeout determination ( transit time over the network is hard to predict)
Network  Virtual circuits vs datagrams in subnet( many cog. Control algo work only with VC)
 Packet queueing and service policy ( 1 Q / input/output line and round robin)
 Packet discard policy
 Routing algorithm ( spreading traffic over all lines)
 Packet lifetime management
Data link  Retransmission policy( Go back N will put heavy load than Selective Reject)
 Acknowledgement policy( piggyback onto reverse traffic )
 Flow control policy ( small window reduce traffic and thus congestion )

 Several techniques can be employed for congestion
control. These include:
 Warning bit
 Choke packets
 Choke packets
 Load shedding
 Random early discard
 Traffic shaping
 The first 3 deal with congestion detection and recovery.
The last 2 deal with congestion avoidance.

Congestion Control: Virtual circuit subnets
 Admission control is a validation process in
communication systems where a check is performed
before a connection is established to see if current
resources are sufficient for the proposed connection
No new virtual circuits are setup when congestion is
93
 No new virtual circuits are setup when congestion is
signalled
 Route new virtual circuits around problem areas.

Congestion Control: VC contd..
Assume there is no congestion, despite huge resource
allocation, leads to wastage of resources
Balanced reservation of resources
Negotiation when virtual circuit is set up
 Negotiation when virtual circuit is set up
 About kind of traffic + service desired
 Resource reservation in subnet
Line capacity
Buffers in routers

Congestion Control in Datagram
Subnets: Warning Bit
A warning bit is sent back in the ack to the source in the case
congestion.
Every router on the path can set the warning bit.
f
a
au
u )
1
( 

 f
a
au
u old
new )
1
( 


Where a indicates how fast the router forgets recent history
f indicates line utilization
‘u’ value ranges from 0.0 to 1.0
If u is above a threshold, a warning state is reached.

96
Warning Bit
 A special bit in the packet header is set by the
router to warn the source when congestion is
detected.
 The bit is copied and piggy-backed on the ACK and
sent to the sender.
sent to the sender.
 The sender monitors the number of ACK packets it
receives with the warning bit set and adjusts its
transmission rate accordingly.

97
Choke Packets
 Used in both VC and datagram subnets
• Control packet
 Generated at congested node
Sent to source node
 Sent to source node
 e.g. ICMP source quench
 From router or destination
 Source cuts back until no more source quench message
 Sent for every discarded packet, or anticipated

98
Choke Packet
 Source, upon receiving a choke packet
 Reduces traffic by a percentage after receiving choke packet
 After time interval expired, listens
 If choke packet received then
goto the step of reducing traffic
goto the step of reducing traffic
else increase traffic
 Typically
 First choke packet causes data rate reduced to 50%, then 25%, …
 Traffic is increased in smaller increments

Hop-by-Hop Choke Packets
 Choke packet takes too long to get back to source
 Choke packet affect each hop along the path
 The goal is to address congestion quickly at the point of
greatest need  propagate the “relief” back to the
greatest need  propagate the “relief” back to the
source
 This generates greater need for buffers at the router
 Required to reduce output
 Meanwhile the input continues full blast until the choke
packet propagates to the next hop

Load Shedding
 When routers are being inundated by packets that they can not handle, they just
throw them away
 Dropping packets randomly may not reduce congestion.
 Select the right packets to drop is very important
 For file transfer, old packet is worth more than a new one
 For file transfer, old packet is worth more than a new one
 For multimedia, a new packet is more important than an old one
 Senders must mark packets in priority classes to indicate how important they are
 A full frame is more important than a difference frame in compressed video
transmission
 The routers can first drop packets from the lowest class, then the next lowest class,
and so on.

102
Random Early Discard (RED)
 This is a proactive approach in which the router discards one
or more packets before the buffer becomes completely full.
 Each time a packet arrives, the RED algorithm computes the
average queue length, avg.
average queue length, avg.
 Congestion is minimal, when avg < lower threshold
 Congestion is severe, If avg >upper threshold, packet is
discarded.
 Onset of congestion, If avg is between the two thresholds

Jitter Control
The variation (i.e., standard deviation) in the packet arrival times is called jitter.
High jitter, for example, having some packets taking 20 msec and others taking 30 msec to arrive will give an
uneven quality to the sound or movie.
When a packet arrives at a router, the router checks to see how much the packet is behind or
ahead of its schedule.
This information is stored in the packet and updated at each hop.
If the packet is ahead of schedule, it is held just long enough to get it back on schedule.
If it is behind schedule, the router tries to get it out the door quickly.

• Traffic shaping is a technique for regulating the
average rate and burstiness of a flow of data that
enters the network.
enters the network.
 Traffic shaping controls the rate at which packets
are sent. Used in ATM and Integrated Services
networks.
 At connection set-up time, the sender and carrier
negotiate a traffic pattern (shape).

UDP
 The User Datagram Protocol (UDP) is called a
connectionless, unreliable transport protocol.
 It does not add anything to the services of IP except
 It does not add anything to the services of IP except
to provide process-to-process communication
instead of host-to-host communication.

Well-known ports used with UDP

TCP
 TCP is a connection-oriented protocol; it creates a
virtual connection between two TCPs to send data.
 In addition, TCP uses flow and error control
 In addition, TCP uses flow and error control
mechanisms at the transport level.

TCP: Transmission Control Protocol
 provides reliable end to end byte stream over unreliable intern/w.
 TCP was designed to dynamically adapt to properties of the
internetwork and to be robust in the face of many kinds of failures.
internetwork and to be robust in the face of many kinds of failures.
 TCP service is obtained by both the sender and receiver creating
end points, called sockets.
 Each socket has a socket number (address) consisting of the IP
address of the host and a 16-bit number local to that host, called a
port. A port is the TCP name for a TSAP.

TCP Connection Establishment
(a) TCP connection establishment in the normal case.
(b) Simultaneous connection establishment on both sides.

Connection establishment using three-way handshaking

TCP Connection Release
 Each simplex connection is released independently of its sibling.
 To release a connection, either party can send a TCP segment with
FIN bit set, which means that it has no more data to transmit.
 When the FIN is acknowledged, that direction is shut down for new data.
 Data may continue to flow indefinitely in the other direction, however.
When both directions have been shut down, the connection is released.
 Normally, four TCP segments are needed to release a connection, one FIN
and one ACK for each direction. However, it is possible for the first ACK and
second FIN to be contained in same segment, reducing total count to three.

TCP Connection management: States for TCP
TCP Connection management: States for TCP
State Description
CLOSED There is no connection.
LISTEN The server is waiting for calls from the client.
SYN-SENT A connection request is sent; waiting for acknowledgment.
SYN-RCVD A connection request is received.
ESTABLISHED Connection is established.
FIN-WAIT-1 The application has requested the closing of the connection.
FIN-WAIT-2 The other side has accepted the closing of the connection.
TIME-WAIT Waiting for retransmitted segments to die.
CLOSE-WAIT The server is waiting for the application to close.
LAST-ACK The server is waiting for the last acknowledgment.

TCP Connection Management
Recall: TCP sender, receiver establish
“connection” before exchanging
data segments
 initialize TCP variables:
 seq. #s
buffers, flow control info
Three way handshake:
Step 1: client host sends TCP SYN segment to
server
 specifies initial seq #
no data
 buffers, flow control info
 client: connection initiator
Socket clientSocket = new
Socket("hostname","port
number");
 server: contacted by client
Socket connectionSocket =
welcomeSocket.accept();
 no data
Step 2: server host receives SYN, replies with
SYNACK segment
 server allocates buffers
 specifies server initial seq. #
Step 3: client receives SYNACK, replies with
ACK segment, which may contain data

TCP Connection Management (cont.)
Closing a connection:
client closes socket:
clientSocket.close();
client server
close
close
3-117
Step 1: client end system sends
TCP FIN control segment to
server
Step 2: server receives FIN,
replies with ACK. Closes
connection, sends FIN.
close
closed
timed
wait

TCP Connection Management (cont.)
Step 3: client receives FIN, replies
with ACK.
 Enters “timed wait” - will
respond with ACK to
client server
closing
closing
3-118
respond with ACK to
received FINs
Step 4: server, receives ACK.
Connection closed.
Note: with small modification, can
handle simultaneous FINs.
closing
closed
timed
wait
closed

TCP Connection Management (cont)
TCP server
lifecycle
TCP client
lifecycle

DNS: Domain Name System
 DNS is a service that translates the domain name into IP
addresses.
 Generic Domain - It defines the category of the domain.
 Generic Domain - It defines the category of the domain.
For example - (.com- commercial), (.org - non-profit
organization), (.edu - educational).
 Country Domain - It categorizes according to the
country. For example - (.in - India), (.uk - United
Kingdom).

The DNS Name Space (1)
A portion of the Internet domain name space.

The DNS Name Space (2)
Generic top-level domains

DNS: Resource Records
 Resource records are used to store data about domain names and IP
addresses.
 A DNS zone database is made up of a collection of resource records.
 Each resource record specifies information about a particular object.
 A record - The record that holds the IP address of a domain.
 AAAA record - The record that contains the IPv6 address for a
domain
 MX record - Directs mail to an email serve
 NS record - Stores the name server for a DNS entry

 A record - The record that holds the IP address of a
domain.
 AAAA record - The record that contains the IPv6
 AAAA record - The record that contains the IPv6
address for a domain
 MX record - Directs mail to an email serve
 NS record - Stores the name server for a DNS entry

Domain Resource Records (1)
The principal DNS resource record types

Domain Resource Records (2)
A portion of a possible DNS database for cs.vu.nl.

Name Servers (1)
Part of the DNS name space divided into zones
(which are circled).

Name Servers
Example of a resolver looking up a remote name in
10 steps.

DCCN Network Layer congestion control TCP

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