20CS2007 Computer Communication Networks

20CS2007
Computer Communication
Networks
Module 6
Transport and Application Protocols Browsing E-commerce website, Remote
access to server from workstation, Chat application. Process-to-process
delivery, UDP, TCP, Mobile Transport Protocols, Congestion control, DNS,
TELNET, HTTP.
Dr.A.Kathirvel, Professor,
DCSE, KITS
kathirvel@karunya.edu

23-1 PROCESS-TO-PROCESS DELIVERY
The transport layer is responsible for process-to-
process delivery—the delivery of a packet, part of a
message, from one process to another. Two processes
communicate in a client/server relationship, as we will
see later.
Client/Server Paradigm
Multiplexing and Demultiplexing
Connectionless Versus Connection-Oriented Service
Reliable Versus Unreliable
Three Protocols
Topics discussed in this section:
2

The transport layer is responsible for
process-to-process delivery.
Note
3
Figure 23.1 Types of data deliveries

Figure 23.2 Port numbers
Figure 23.4 IANA ranges
4

Figure 23.3 IP addresses versus port numbers
5
Figure 23.6 Multiplexing and demultiplexing
Figure 23.5 Socket address

Figure 23.8 Position of UDP, TCP, and SCTP in TCP/IP suite
7

23-2 USER DATAGRAM PROTOCOL (UDP)
The User Datagram Protocol (UDP) is called a
connectionless, unreliable transport protocol. 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 for UDP
User Datagram
Checksum
UDP Operation
Use of UDP
8

Table 23.1 Well-known ports used with UDP
9

In UNIX, the well-known ports are stored in a file called
/etc/services. Each line in this file gives the name of the server and
the well-known port number. We can use the grep utility to extract
the line corresponding to the desired application. The following
shows the port for FTP. Note that FTP can use port 21 with either
UDP or TCP.
Example 23.1
10
SNMP uses two port numbers 161&162, each for a different purpose.

Figure 23.9 User datagram format
UDP length
= IP length – IP header’s length
Note
11

Figure 23.10 Pseudoheader for checksum calculation
12

Figure shows the checksum calculation for a very small user datagram
with only 7 bytes of data. Because the number of bytes of data is odd,
padding is added for checksum calculation. The pseudoheader as well
as the padding will be dropped when the user datagram is delivered to
IP.
Example 23.2
13
Figure Checksum calculation of a simple UDP user datagram

23-3 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 mechanisms
at the transport level.
TCP Services
TCP Features
Segment
A TCP Connection
Flow Control
Error Control
15

Table 23.2 Well-known ports used by TCP
16

Figure 23.13 Stream delivery
17
The bytes of data being transferred in each
connection are numbered by TCP. The numbering
starts with a randomly generated number.
Note

Figure 23.14 Sending and receiving buffers
18

The following shows the sequence number for
each segment:
Example 23.3
20
The value in the sequence number field
of a segment defines the number of the
first data byte contained in that segment.
Note

Figure 23.16 TCP segment format
The value of the acknowledgment field in a segment
defines the number of the next byte a party expects
to receive.The acknowledgment no. is cumulative.
Note
21

Figure 23.18 Connection establishment using three-way handshaking
Note
A SYN segment cannot carry data,
but it consumes one seq. no.
A SYN+ACK segment cannot carry
data, but does consume one seq. no.
An ACK segment, if carrying no data,
consumes no seq. no.
23

Figure 23.19 Data transfer
24
The FIN segment consumes
one sequence number if it
does not carry data.
Note
The FIN + ACK segment
consumes one sequence
number if it does not carry
data.

Figure 23.20 Connection termination using three-way handshaking
25

Figure 23.21 Half-close Figure 23.22 Sliding window
A sliding window is used to make
transmission more efficient as
well as to control the flow of data
so that the destination does not
become overwhelmed with data.
TCP sliding windows are byte-
oriented.
Note
26

Solution
The value of rwnd = 5000 − 1000 = 4000. Host B can receive only
4000 bytes of data before overflowing its buffer. Host B advertises this
value in its next segment to A.
What is the value of the receiver window (rwnd) for host A if
the receiver, host B, has a buffer size of 5000 bytes and 1000
bytes of received and unprocessed data?
Example 23.4
27
Example 23.5 : What is the size of the window for host A if the
value of rwnd is 3000 bytes and the value of cwnd is 3500 bytes?
Solution
The size of the window is the smaller of rwnd and cwnd, which is
3000 bytes.

reality this value is thousands of bytes). The receiver has sent an
acknowledgment number of 200 with an rwnd of 9 bytes (in reality this
value is thousands of bytes). The size of the sender window is the
minimum of rwnd and cwnd, or 9 bytes. Bytes 200 to 202 are sent, but
not acknowledged. Bytes 203 to 208 can be sent without worrying
about acknowledgment. Bytes 209 and above cannot be sent.
Example 23.6
Figure shows an unrealistic example of a sliding window. The
sender has sent bytes up to 202. We assume that cwnd is 20 (in
28

Some points about TCP sliding windows:
❏ The size of the window is the lesser of rwnd and
cwnd.
❏ The source does not have to send a full window’s
worth of data.
❏ The window can be opened or closed by the
receiver, but should not be shrunk.
❏ The destination can send an acknowledgment at
any time as long as it does not result in a shrinking
window.
❏ The receiver can temporarily shut down the
window; the sender, however, can always send a
segment of 1 byte after the window is shut down.
Note
29

ACK segments do not consume sequence numbers
and are not acknowledged.
Note
In modern implementations, a retransmission occurs if the
retransmission timer expires or three duplicate ACK
segments have arrived.
No retransmission timer is set for an ACK segment.
Data may arrive out of order and be temporarily stored by
the receiving TCP, but TCP guarantees that no out-of-order
segment is delivered to the process.
30

Figure 23.24 Normal operation
31

Figure 23.26 Fast retransmission
The receiver TCP
delivers only
ordered data to
the process.
Note
33

23-4 SCTP
Stream Control Transmission Protocol (SCTP) is a
new reliable, message-oriented transport layer
protocol. SCTP, however, is mostly designed for
Internet applications that have recently been
introduced. These new applications need a more
sophisticated service than TCP can provide.
SCTP Services and Features
Packet Format
An SCTPAssociation
Flow Control and Error Control
34

Table 23.4 Some SCTP applications
SCTP is a message-oriented, reliable protocol that
combines the best features of UDP and TCP.
Note
35

Figure 23.27 Multiple-stream concept
An association in SCTP can involve
multiple streams.
Note
36

Figure 23.28 Multihoming concept
SCTP association allows multiple IP
addresses for each end.
Note
37

In SCTP, a data chunk is numbered using a TSN.
Note
38
To distinguish between different streams, SCTP uses an SI.
To distinguish between different data chunks belonging to
the same stream, SCTP uses SSNs.
TCP has segments; SCTP has packets.

Figure 23.29 Comparison between a TCP segment and an SCTP packet
39
In SCTP, control information and data
information are carried in separate chunks.
Note

Figure 23.30 Packet, data chunks, and streams
Data chunks are identified by three items: TSN, SI, and SSN.
TSN is a cumulative number identifying the association; SI
defines the stream; SSN defines the chunk in a stream.
Note
40

Figure 23.31 SCTP packet format
In SCTP, acknowledgment numbers are used to acknowledge only data
chunks; control chunks are ack. by other control chunks if necessary.
Note
In an SCTP packet, control chunks come before data chunks.
41

Figure 23.32 General header Figure 23.33 Four-way handshaking
42

A connection in SCTP is called an association.
Note
No other chunk is allowed in a packet carrying an INIT or
INIT ACK chunk. A COOKIE ECHO or a COOKIE ACK chunk
can carry data chunks.
In SCTP, only DATA chunks consume TSNs; DATA chunks
are the only chunks that are acknowledged.
44

Figure 23.34 Simple data transfer
The
acknowledgment
in SCTP defines
the cumulative
TSN, the TSN of
the last data
chunk received in
order.
Note
45

Figure 23.35 Association termination
46

Figure 23.36 Flow control, receiver site
Figure 23.37 Flow control, sender site
47

Figure 23.38 Flow control scenario
48

Figure 23.39 Error control, receiver site
49

Figure 23.40 Error control, sender site
50

What Is Congestion?
 Congestion occurs when the number of
packets being transmitted through the
network approaches the packet handling
capacity of the network
 Congestion control aims to keep number
of packets below level at which
performance falls off dramatically
 Data network is a network of queues
 Generally 80% utilization is critical
 Finite queues mean data may be lost

Effects of Congestion
 Packets arriving are stored at input buffers
 Routing decision made
 Packet moves to output buffer
 Packets queued for output transmitted as fast as
possible
 Statistical time division multiplexing
 If packets arrive to fast to be routed, or to be output,
buffers will fill
 Can discard packets
 Can use flow control
 Can propagate congestion through network

Practical Performance
 Ideal assumes infinite buffers and no
overhead
 Buffers are finite
 Overheads occur in exchanging congestion
control messages

Effects of
Congestion -
No Control

Mechanisms for
Congestion Control

Backpressure
 If node becomes congested it can slow down or halt flow
of packets from other nodes
 May mean that other nodes have to apply control on
incoming packet rates
 Propagates back to source
 Can restrict to logical connections generating most traffic
 Used in connection oriented that allow hop by hop
congestion control (e.g. X.25)
 Not used in ATM nor frame relay
 Only recently developed for IP

Choke Packet
 Control packet
 Generated at congested 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
 Rather crude mechanism

Implicit Congestion Signaling
 Transmission delay may increase with
congestion
 Packet may be discarded
 Source can detect these as implicit
indications of congestion
 Useful on connectionless (datagram)
networks
 e.g. IP based
 (TCP includes congestion and flow control - see
chapter 17)

Explicit Congestion Signaling
 Network alerts end systems of increasing
congestion
 End systems take steps to reduce offered
load
 Backwards
 Congestion avoidance in opposite direction to
packet required
 Forwards
 Congestion avoidance in same direction as
packet required

Categories of Explicit Signaling
 Binary
 A bit set in a packet indicates congestion
 Credit based
 Indicates how many packets source may send
 Common for end to end flow control
 Rate based
 Supply explicit data rate limit
 e.g. ATM

Traffic Management
 Fairness
 Quality of service
 May want different treatment for different
connections
 Reservations
 e.g. ATM
 Traffic contract between user and network

Congestion Control in Packet
Switched Networks
 Send control packet to some or all source
nodes
 Requires additional traffic during congestion
 Rely on routing information
 May react too quickly
 End to end probe packets
 Adds to overhead
 Add congestion info to packets as they
cross nodes
 Either backwards or forwards

Frame Relay
Congestion Control
 Minimize discards
 Maintain agreed QoS
 Minimize probability of one end user monoply
 Simple to implement
 Little overhead on network or user
 Create minimal additional traffic
 Distribute resources fairly
 Limit spread of congestion
 Operate effectively regardless of traffic flow
 Minimum impact on other systems
 Minimize variance in QoS

Techniques
 Discard strategy
 Congestion avoidance
 Explicit signaling
 Congestion recovery
 Implicit signaling mechanism

Traffic Rate Management
 Must discard frames to cope with
congestion
 Arbitrarily, no regard for source
 No reward for restraint so end systems
transmit as fast as possible
 Committed information rate (CIR)
 Data in excess of this liable to discard
 Not guaranteed
 Aggregate CIR should not exceed physical data
rate
 Committed burst size

Relationship
Among
Congestion
Parameters

Explicit Signaling
 Network alerts end systems of growing
congestion
 Backward explicit congestion notification
 Forward explicit congestion notification
 Frame handler monitors its queues
 May notify some or all logical connections
 User response
 Reduce rate

ATM Traffic Management
 High speed, small cell size, limited
overhead bits
 Still evolving
 Requirements
 Majority of traffic not amenable to flow
control
 Feedback slow due to reduced transmission
time compared with propagation delay
 Wide range of application demands
 Different traffic patterns
 Different network services

Latency/Speed Effects
 ATM 150Mbps
 ~2.8x10-6 seconds to insert single cell
 Time to traverse network depends on propagation delay,
switching delay
 Assume propagation at two-thirds speed of light
 If source and destination on opposite sides of USA,
propagation time ~ 48x10-3 seconds
 Given implicit congestion control, by the time dropped
cell notification has reached source, 7.2x106 bits have
been transmitted
 So, this is not a good strategy for ATM

Cell Delay Variation
 For ATM voice/video, data is a stream of
cells
 Delay across network must be short
 Rate of delivery must be constant
 There will always be some variation in
transit
 Delay cell delivery to application so that
constant bit rate can be maintained to
application

Network Contribution to
 Packet switched networks
 Queuing delays
 Routing decision time
 Frame relay
 As above but to lesser extent
 ATM
 Less than frame relay
 ATM protocol designed to minimize processing overheads at
switches
 ATM switches have very high throughput
 Only noticeable delay is from congestion
 Must not accept load that causes congestion

At The UNI
 Application produces data at fixed rate
 Processing at three layers of ATM causes
delay
 Interleaving cells from different connections
 Operation and maintenance cell interleaving
 If using synchronous digital hierarchy frames,
these are inserted at physical layer
 Can not predict these delays

Origins of Cell Delay Variation

Traffic and Congestion
Control Framework
 ATM layer traffic and congestion control
should support QoS classes for all
foreseeable network services
 Should not rely on AAL protocols that are
network specific, nor higher level
application specific protocols
 Should minimize network and end to end
system complexity

Timings Considered
 Cell insertion time
 Round trip propagation time
 Connection duration
 Long term
 Determine whether a given new
connection can be accommodated
 Agree performance parameters with
subscriber

Traffic Management and
Congestion Control Techniques
 Resource management using virtual paths
 Connection admission control
 Usage parameter control
 Selective cell discard
 Traffic shaping

Resource Management Using
Virtual Paths
 Separate traffic flow according to service
characteristics
 User to user application
 User to network application
 Network to network application
 Concern with:
 Cell loss ratio
 Cell transfer delay
 Cell delay variation

Configuration of
VCCs and VPCs

Allocating VCCs within VPC
 All VCCs within VPC should experience
similar network performance
 Options for allocation:
 Aggregate peak demand
 Statistical multiplexing

Connection Admission Control
 First line of defense
 User specifies traffic characteristics for
new connection (VCC or VPC) by selecting
a QoS
 Network accepts connection only if it can
meet the demand
 Traffic contract
 Peak cell rate
 Cell delay variation
 Sustainable cell rate

Usage Parameter Control
 Monitor connection to ensure traffic
cinforms to contract
 Protection of network resources from
overload by one connection
 Done on VCC and VPC
 Peak cell rate and cell delay variation
 Sustainable cell rate and burst tolerance
 Discard cells that do not conform to traffic
contract
 Called traffic policing

Traffic Shaping
 Smooth out traffic flow and reduce cell
clumping
 Token bucket

Token Bucket for
Traffic Shaping

GFR Traffic Management
 Guaranteed frame rate is as simple as UBR
from end system viewpoint
 Places modest requirements on ATM
network elements
 End system does no policing or shaping of
traffic
 May transmit at line rate of ATM adaptor
 No guarantee of delivery
 Higher layer (e.g. TCP) must do congestion
control
 User can reserve capacity for each VC

Frame Recognition
 GFR recognizes frames as well as cells
 When congested, network discards whole
frame rather than individual cells
 All cells of a frame have same CLP bit
setting
 CLP=1 AAL5 frames are lower priority
 Best efforts
 CLP=0 frames minimum guaranteed
capacity

GFR Contract Parameters
 Peak cell rate (PCR)
 Minimum cell rate (MCR)
 Maximum burst size (MBS)
 Maximum frame size (MFS)
 Cell delay variation tolerance (CDVT)

Mechanisms for Supporting Rate
Guarantees (1)
 Tagging and policing
 Discriminate between frames that conform to contract and those
that don’t
 Set CLP=1 on all cells in frame if not
 Gives lower priority
 Maybe done by network or source
 Network may discard CLP=1 cells
 Policing
 Buffer management
 Treatment of buffered cells
 Congestion indicated by high buffer occupancy
 Discard tagged cells
 Including ones already in buffer to make room
 To be fair, per VC buffering
 Cell discard based on queue-specific thresholds

Mechanisms for Supporting Rate
Guarantees (2)
 Scheduling
 Give preferential treatment to untagged cells
 Separate queues for each VC
 Make per-VC scheduling decisions
 Enables control of outgoing rate of VCs
 VCs get fair capacity allocation
 Still meet contract

Conformance Definition
 UPC
 Monitors each active VC
 Ensure traffic conforms to contract
 Tag or discard nonconforming cells
 Frame conforms if all cells conform
 Cell conforms if:
 Rate of cells within contract
 All cells in frame have same CLP
 Frame satisfies MFS parameter (check for last cell
in frame or cell count < MFS)

QoS Eligibility Test
 Two stage filtering process
 Frame tested for conformance to contract
 If not, may discard
 If not discarded, tag
 Sets upper bound
 Penalize cells above upper bound
 Implementations expected to attempt delivery of
tagged cells
 Determine frames eligible for QoS guarantees
 Under GFR contract for VC
 Lower bound on traffic

GFR VC Frame Categories
 Nonconforming frame
 Cells of this frame will be tagged or discarded
 Conforming but ineligible frames
 Cells will receive a best-effort service
 Conforming and eligible frames
 Cells will receive a guarantee of delivery

DNS : Domain Name System
 To map a hostname to an IP-address and vice
versa.
Example:
www.acad.bg 194.141.0.9
 Each hostname may has zero or more corresponding IP-
addresses.
 Each IP-address may correspond to zero or more
hostnames.
98

DNS
 Host name to address mappings were maintained by the Network
Information Center (NIC) in a single file (HOSTS.TXT) which was
transferred to all hosts using FTP (about 25 years ago).
Disadvantages:
 Ineffective – Bandwidth consumed in distributing a new version by
this scheme is proportional to the square of the number of hosts in the
network.
 The network population was also changing in character.
 The applications on the Internet were getting more sophisticated and
creating a need for general purpose name service.
99

DNS design goals
The primary goal is a consistent name space which will be
used for referring to resources.
The database which holds the names must be distributed.
The costs of implementing such a facility dictate that it be
generally useful, and not restricted to a single application.
Use the same name space with different protocol families
or management.
Name server transactions must be independent of the
communications system that carries them.
The system should be useful across a wide spectrum of
host capabilities. Both personal computers and large
timeshared hosts (mainframes) should be able to use the
system, though perhaps in different ways. 10
0

DNS Elements: 3 main components
1. The DOMAIN NAME SPACE and RESOURCE RECORDS,
which are specifications for a tree structured name space and data
associated with the names.
2. NAME SERVERS are server programs which hold information
about the domain tree's structure and set information.
3. RESOLVERS are programs that extract information from name
servers in response to client requests.
These three components roughly correspond to the three layers or
views of the DNS:
 From the user's point of view, the domain system is accessed through
a simple procedure.
 From the resolver's point of view, the domain system is composed of
an unknown number of name servers.
 From a name server's point of view, the domain system consists of
separate sets of local information called zones.
10
1

Name space specifications and terminology
 The domain name space is a tree structure composed by nodes.
 Each node has a label, which is zero to 63 octets in length.
 The domain name of a node is the list of the labels on the path from
the node to the root of the tree.
 The name of the root node is empty string (zero length) – “”.
 Internally, programs that manipulate domain names should represent
them as sequences of labels, where each label is a length octet
followed by an octet string.
 Domain names are case-insensitive using the ASCII codes with high
order zero bit.
 When a user types a domain name, the length of each label is omitted
and the labels are separated by dots ("."). Since a complete domain
name ends with the root label, this leads to a printed form which ends
in a dot.
10
2

Example Name Space
(a graphical tree view)
com edu bg eu info
ucla
physics
yahoo
joe
acad
amigo val
see-
grid
www
kevin
mary
Top Level Domains (TLD)
One Root domain (“”)
test
sigma
book
mail
Delegation
2nd Level Domains
Up to 127 levels
3
rd Level Domains
ICANN, IANA
10
3

Top Level Domains
 Generic domains (gTLD): com, org, net, int (all around the world);
edu, gov, mil (located in USA only); aero, biz, coop, info,
museum, name, pro(new domains,since 2000)
 Country code domains (ccTLD): bg, uk, fr, it, ...
 Special infrastructure domain: arpa
 Loopback domains: localhost, localdomain
 And the domain eu for European Union!
Resource
Records
 The distributed database which holds the
names consists of several Resource Records.
 Resource record - Set of resource information
associated with a particular name.
10
4

DNS server
A computer with software which:
 Holds and maintains specified part of the distributed database of
resource records.
 Responds to the name queries.
 Exchanges the zone information with other DNS servers.
These tasks are described in the DNS protocol (RFC 1035) located on
the Application Layer of the “TCP/IP reference model”.
Delegation:
 ICANN organization delegates root DNS servers.
 The root servers delegate TLD DNS servers.
 A TLD DNS server delegates Second Level DNS server etc. 10
5

The root DNS servers
At the heart of the DNS are 13 special computers, called root
servers. They are coordinated by ICANN and are distributed
around the world.
A.ROOT-SERVERS.NET. 198.41.0.4
B.ROOT-SERVERS.NET. 192.228.79.201
C.ROOT-SERVERS.NET. 192.33.4.12
D.ROOT-SERVERS.NET. 128.8.10.90
E.ROOT-SERVERS.NET. 192.203.230.10
F.ROOT-SERVERS.NET. 192.5.5.241
G.ROOT-SERVERS.NET. 192.112.36.4
H.ROOT-SERVERS.NET. 128.63.2.53
I.ROOT-SERVERS.NET. 192.36.148.17
J.ROOT-SERVERS.NET. 192.58.128.30
K.ROOT-SERVERS.NET. 193.0.14.129
L.ROOT-SERVERS.NET. 199.7.83.42
M.ROOT-SERVERS.NET. 202.12.27.33
10
6

K.ROOT-SERVERS.NET
(operated by RIPE NCC)
Source: http://k.root-servers.org 10
7

DNS servers
Primary DNS server for a given domain – holds resource records for the zone.
The zone configuration is written by the system administrator.
Secondary DNS server for a given domain – it exists for the case when the
Primary DNS server is failed or overloaded. The Secondary server
downloads the zone information from the Primary periodically or by
notification.
 Every domain must have one Primary DNS server and at least one
Secondary.
The Primary and Secondary DNS servers are also called authoritative for
their domain and non-authoritative for the rest of domains in the world.
E.g. the authoritative servers always hold up to date zone information for
the domain.
 Caching only DNS servers – they cache DNS queries and answers for
speed-up the service. These servers are not authoritative for any domain.
Most of the authoritative DNS servers are caching too. 10
8

DNS query flow
1. A DNS client forms the query and sends it according the OS configuration.
2. The local resolver opens a special file called hosts and looks in case the
answer is already written here. If not then next:
3. The client calls one of the OS configured DNS servers.
4. If the DNS server knows (or already cached) the answer, then returns it.
5. If the DNS server doesn’t know the answer, it sends a recursive query
through the DNS hierarchy, starting from the root domain. The recursive
query consists of several sub-queries to the corresponding DNS servers.
6. The DNS servers returns the result – the answer or error message.
10
9

Reverse DNS query
 The reverse query uses the special infrastructure domain in-
addr.arpa
 The IP is written before this special domain in reverse order of
numbers separated with dots.
 Example: 194.141.0.0 corresponds to 0.0.141.194.in-
addr.arpa
 The reverse query starts from a root DNS server and follows the
delegation path.
 Example in Unix/Linux:dig 0.0.141.194.in-addr.arpa +trace
 (It traces and shows the delegation path from the root name
servers down to SOA DNS server)
11
0

DNS protocol details
 The protocol uses two transport protocols from the lower layer:
 UDP on port 53 and
 TCP on port 53.
 The queries and answers are transferred using the unreliable protocol
UDP for speed. But it is preferred because the OS configured DNS
servers are near, usually in the same LAN.
 The zone transfers between the DNS servers are performed through
TCP, because it must be reliable.
 Each resource record consists of the following data fields
( in text readable format! ): owner type class TTL RDATA
11
1

Resource Records
The data fields
owner - A string which is the domain name where the RR is found.
type - which is an encoded 16 bit value that specifies the type of the resource in this
resource record. Types refer to abstract resources. Each abstract resource has memo
name:
A - a host address
CNAME - identifies the canonical name of an alias
MX - identifies a mail exchange for the domain
PTR - a pointer to another part of the domain name space etc.
class - which is an encoded 16 bit value which identifies a protocol family or instance
of a protocol. This memo uses the following classes:
IN - the Internet system or CH - the Chaos system
TTL - which is the time to live of the RR. This field is a 32 bit integer in units of
seconds, an is primarily used by resolvers when they cache RRs. The TTL describes
how long a RR can be cached before it should be discarded.
RDATA - which is the type and sometimes class dependent data which describes the
resource.
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2

Resource Records
Example
(in text readable format!)
acad.bg. IN A 194.141.0.9
acad.bg. IN MX 10 mx-a.acad.bg.
IN MX 20 mx-b.acad.bg.
IN NS amigo.acad.bg.
IN NS unicom.acad.bg.
localhost IN A 127.0.0.1
poseidon IN A 194.141.0.1
backbone IN A 194.141.252.2
iris IN A 194.141.0.2
amigo IN AAAA 2001:4b58:acad::3
unicom IN AAAA 2001:4b58:acad::9
bis-21++ IN CNAME unicom.acad.bg.
3 IN PTR amigo.acad.bg.
...
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3

Message in binary format
(RR means Resource Record)
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4

DNS message data fields
 The IDENTIFICATION field is set by the client and returned by the server.
 The 16-bit PARAMETER consists of:
 0-th bit field: 0 means the message is a query,1 means it's a response.
 1-4 bit fields - OPCODE:
 0000 - is a normal value (Standard query).
 0001 - an inverse query.
 0010 - the server status request.
 5-th bit field - Authoritative answer. The name server is authoritative for the
domain in the question section.
 6-th bit field is set if message truncated. With UDP this means that the total size
of the reply exceeded 512 bytes, and only the first 512 bytes the of the reply
were returned.
 7-th bit field - Recursion Desired. This bit can be set in a query and is then
returned in the response.
 8-th bit field - Recursion Available.
 9-11 -th bits field has to be 0.
 12-15 -th bits field - Return Code. 0- no error, 3- name error.
115
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5

DNS message data fields
 The fields labeled NUMBER OF ... give each a count of entries in the
corresponding sections in the message.
 The QUESTION SECTION contains queries for which answers are
desired. The client fills in only the question section; the server returns
the question and answers with its response. Each question has Query
Domain Name followed by Query Type and Query Class fields (as
depicted in the next slides)
 ANSWER,AUTHORITY,ADDITIONAL INFORMATION sections
consist of a set of resource records that describe domain names and
mappings. Each resource record describes one name (as depicted in
the next slides)
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6

The QUESTION section format in the DNS message
The DOMAIN NAME has variable length.
Clients fill in the questions; servers return them along with the answers.
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7

The RESOURCE RECORD binary format returned by the DNS
servers
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8

26-4 TELNET
It is impossible to have a client/server pair for each type of
service we need; the number of servers soon becomes
intractable. The idea is not scalable. The solution is to have a
specific client/server program for a set of common scenarios, but
to have some generic client/server programs for the rest.
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9

Trivial FTP (TFTP)
A very simple file transfer protocol, with the functionality of a very basic
form of FTP. Defined in 1980.
TFTP uses UDP port 69 (not TCP!)
An ex. of TFTP usage for remote boot through BOOTP protocol in a LAN:
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0

FTP pros and cons
 Advantages:
 Simple implementation
 Universal application
 Wide used and standartized
 Disadvantages:
 Clear text passwords, unencrypted data.
 Multiple TCP/IP connections are used = > Firewalls problems.
 Hard to filter active mode FTP traffic on the client side by using a
firewall.
 It is possible to abuse the protocol's built-in proxy features to tell a server
to send data to an arbitrary port of a third computer;
 High latency.
 No integrity check on the receiver side.
 No date/timestamp attribute transfer.
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1

26.4.1 Local versus Remote Logging
We first discuss the concept of local and remote logging as
shown in Figure 26.23.
Figure 26.23: Local versus remote logging
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2

Figure 26.24: Concept of NVT
Table 26.11: Examples of interface commands
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3

26.26.2 HyperText Transfer Protocol
The HyperText Transfer Protocol (HTTP) is used to
define how the client-server programs can be written
to retrieve web pages from the Web. An HTTP client
sends a request; an HTTP server returns a response.
The server uses the port number 80; the client uses a
temporary port number. HTTP uses the services of
TCP, which, as discussed before, is a connection-
oriented and reliable protocol.
12

The World Wide Web - HTTP
 Hypertext Transfer Protocol (HTTP) is a
communications protocol used to transfer or convey
information on intranets and the World Wide Web. Its
original purpose was to provide a way to publish and
retrieve hypertext pages, mainly scientific paper. Created in
CERN, Geneva.
 Development of HTTP was coordinated by the W3C
(World Wide Web Consortium) and the IETF (Internet
Engineering Task Force), culminating in the publication of a
series of RFCs, most notably
RFC 2616 (June 1999), which defines HTTP/1.1, the
current version.
 HTTP is a request/response protocol between a client and
a server. It works on the well known TCP port 80.
12

Figure 26.3 shows an example of a nonpersistent connection. The client needs to
access a file that contains one link to an image. The text file and image are located
on the same server. Here we need two connections. For each connection, TCP
requires at least three handshake messages to establish the connection, but the
request can be sent with the third one. After the connection is established, the object
can be transferred. After receiving an object, another three handshake messages are
needed to terminate the connection.
Example 26.3
12
Figure 26.3: Example 26.3

Figure 26.4 shows the same scenario as in Example 26.3, but using a
persistent connection. Only one connection establishment and
connection termination is used, but the request for the image is sent
separately.
Example 26.4
12
Figure 26.4: Example 26.4

Figure 26.5: Formats of the request and response messages
12

Table 26.2: Request Header Names
13

Table 26.3: Response Header Names
13

This example retrieves a document (see Figure 26.6). We use the GET method to
retrieve an image with the path /usr/bin/image26. The request line shows the method
(GET), the URL, and the HTTP version (26.1). The header has two lines that show that
the client can accept images in the GIF or JPEG format. The request does not have a
body. The response message contains the status line and four lines of header. The
header lines define the date, server, content encoding (MIME version, which will be
described in electronic mail), and length of the document. The body of the document
follows the header..
Example 26.5
13
Figure 26.6:
Example 26.5

In this example, the client wants to send a web page to be posted on the server.
We use the PUT method. The request line shows the method (PUT), URL, and
HTTP version (26.1). There are four lines of headers. The request body
contains the web page to be posted. The response message contains the status
line and four lines of headers. The created document, which is a CGI
document, is included as the body (see Figure 26.7).
Example 26.6
Figure 26.7:
Example 26.6
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3

Figure 26.8 shows a scenario in which an electronic store
can benefit from the use of cookies. Assume a shopper
wants to buy a toy from an electronic store named BestToys.
The shopper browser (client) sends a request to the BestToys
server. The server creates an empty shopping cart (a list) for
the client and assigns an ID to the cart (for example, 12343).
The server then sends a response message, which contains
the images of all toys available, with a link under each toy
that selects the toy if it is being clicked. This response
message also includes the Set-Cookie header line whose
value is 12343. The client displays the images and stores the
cookie value in a file named BestToys.
Example 26.8
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5

Figure 26.9 shows an example of a use of a proxy server in a local network, such as
the network on a campus or in a company. The proxy server is installed in the local
network. When an HTTP request is created by any of the clients (browsers), the
request is first directed to the proxy server If the proxy server already has the
corresponding web page, it sends the response to the client. Otherwise, the proxy
server acts as a client and sends the request to the web server in the Internet. When
the response is returned, the proxy server makes a copy and stores it in its cache
before sending it to the requesting client.
Example 26.9
13
Figure 26.9: Example
of a proxy server

HTML – HyperText Markup Language
 In 1980, physicist Tim Berners-Lee, who was an
independent contractor at CERN, proposed and
prototyped ENQUIRE, a hypertext system for CERN
researchers to use to share documents.
 In 1989, Berners-Lee and CERN data systems
engineer Robert Cailliau each submitted separate
proposals for an Internet-based hypertext system
providing similar functionality.
 In 1990, they collaborated on a joint proposal, the
World Wide Web (W3) project, which was accepted
by CERN.
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HTML
A selection of common HTML tags. some can have additional
parameters (attributes).
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20CS2007 Computer Communication Networks

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