1 chapter 5 protocol underlying http web protocols and practice

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Chapter 5

Protocol Underlying

HTTP

Web Protocols and Practice

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Topics


PROTOCOLS UNDERLYING HTTP

Protocol Definition Domain Name System Application-Layer Protocols Internet Protocol Transmission Control Protocol

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Protocol Definition


A protocol defines both the syntax and semantics of the message exchanged between senders and receivers.

The protocol suite for the Internet consists of four main layers:

Link layer» Handles the hardware details of interfacing with the

physical communication medium, such as Ethernet, Asynchronous Transfer Mode (ATM), or Synchronous Optical Network (SONET).


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Protocol Definition


Network layer» Handles the delivery of individual packets of data

through the network. A network-layer protocol is implemented in routers and the end hosts.

Transport layer» Coordinates the communication between hosts on

behalf of the application layer. In practice, a transport layer protocol is typically implemented in the operating system of the end host.

Application layer» Handles the details of specific applications. In

practice, an application-layer protocol is typically implemented as part of the application software, such as a Web browser or Web server.


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Protocol Definition


Figure 5.1 illustrates layering of protocols.


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Protocol Definition


DNS Telnet FTP SMTP NNTP HTTP

UDP TCP

IP

ATM SONET Ethernet

Application layer

Transport layer

Network layer

Link layer

Figure 5.1. Layering of protocols


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Protocol Definition


The three main protocols involved in the transfer of HTTP messages are:

Internet Protocol (IP)» Is a network-layer protocol that coordinates the

delivery of individual packets from one host to another, based on the IP address of the destination host.

Transmission Control Protocol (TCP)» Is a transport-layer protocol that coordinates the

transmission of IP packets in order to provide the abstraction of a reliable, bidirectional connection between two communicating applications.

» Some applications use User Datagram Protocol (UDP).


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Protocol Definition


Domain Name System (DNS)» Is an application-layer protocol that controls the

translation of hostnames into IP addresses, and vice versa.


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Domain Name System


Domain Name System Definition DNS Resolver DNS Architecture DNS Protocol DNS Queries and the Web


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Domain Name System Definition


The Domain Name System (DNS) coordinates the translation of hostnames to IP addresses and IP addresses into hostnames.

Machines on the Internet have hostnames because:

Remembering a hostname is much easier than remembering an IP address.

The IP address associated with a hostname may change over time.


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DNS Resolver


A software library that is linked with the Internet applications is named resolver.

A DNS resolver performs two main functions: Gethostbyname()

» The function converts a hostname to an IP address.

Gethostbyaddr()» The function converts an IP address to a hostname.

The resolver interacts with one or more DNS servers to perform these functions on behalf of the application.


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DNS Architecture


In the early days, a single master file listed the IP addresses associated with each hostname.

Now DNS is a distributed database that consists of a hierarchical set of name servers, each responsible for a portion of the domain names and address space.

The DNS architecture reflects the hierarchy of hostnames and IP addresses.

(Figure5.10)


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DNS Architecture


ac

camwest east

bar

www ftp

com edu org ac uk zw

users

57

34

56

12

In-addr

arpa

Unnamed root

Top-level domains

Second-level domains

Generic domains

Country domains

www.west.bar.com ftp.east.bar.com user.cam.ac.uk

Figure 5.10. DNS hierarchy


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DNS Architecture


The top level includes the three-character generic or organizational domains and two-character country domains.

The top level domains are handled by a collection of root servers.

The hierarchy of domain names does not correspond to the hierarchical structure of IP addresses.

Efficient mapping of IP addressing to hostnames requires a separate hierarchy based on IP addresses.


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DNS Protocol


The DNS protocol governs communication between a DNS client and a DNS server.

A DNS client sends a query for information (e.g. ,the IP address associated with a particular hostname) to a DNS server, and the DNS server returns a response with the requested information (e.g., the IP address).


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DNS Protocol


DNS queries can be recursive or iterative. A recursive query requests that the receiving DNS

server resolve the entire query itself. An iterative query requests that the receiving DNS

server respond directly to the DNS client with the IP address of the next DNS server in the DNS hierarchy. Root servers handle only iterative queries.

(Figure 5.11)


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DNS Protocol


Web browser

DNS resolver

1 10

Client host

DNS cache

Local DNS server

DNS query2

DNS response

9

Local area network

3

4

5

67

8

Root server

Top-level domain server

Second-level domain server

Figure 5.11. DNS resolver and local DNS server


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DNS Protocol


Figure 5.11 shows that for a recursive query: The resolver is invoked by a system call from the

application (step 1). Then the resolver sends a DNS query to the local

DNS server (step 2). Then the resolver waits for the reply (step 9). The resolver provides the IP address to the

application (step 10).


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DNS Protocol


Figure 5.11 shows that for an iterative query: The resolver is invoked by a system call from the

application (step 1). Then the resolver sends a DNS query to the local

DNS server (step 2). The local DNS server sends a query to the root

DNS server (step 3). The local DNS server learns the names and IP

addresses of the DNS servers for the zone at the next level (step 4).


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DNS Protocol


Then the local DNS server can send a query to the next DNS server in the chain (steps 5,6,7,8)

Ultimately, the local DNS server responds to the resolver (step 9).

The resolver provides the IP address to the application (step 10).


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DNS Protocol


DNS servers employ caching To reduce the latency in responding to queries To reduce the amount of DNS traffic in the

Internet

DNS primarily uses UDP for sending queries and responses, although TCP may also be used.


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DNS Queries and the Web


A Web client performs a gethostbyname() query before establishing a transport connection to the Web server. In some cases, the client may not need to perform a DNS lookup:

Request directed to a proxy Request satisfied by the client cache Using the result of the previous query

Although the Web client needs to learn the IP address of the Web server, the Web server knows the IP address of the client when receiving a request because the client’s IP address is included the header of each IP packet.


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DNS Queries and the Web


The mapping of the Web client’s IP address to a hostname is controlled by the DNS servers at the Web client institution.

Mapping the client’s IP address into a hostname often incurs significant delay.

In addition, the DNS queries consume resources at the Web server.


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Application-Layer Protocols


Application-Layer Protocols Definition Telnet Protocol File Transfer Protocol Simple Mail Transfer Protocol Network News Transfer Protocol Properties of Application-Layer Protocols


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Application-Layer Protocols Definition


Applications execute on end hosts and communicate via application-level protocols.

An application-layer protocol defines both the syntax and the semantics of the messages exchanged between the end systems.

Four key internet applications are: Telnet File transfer E-mail Network news


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Telnet Protocol


Telnet permits a user to connect to an account on a remote machine.

A client program running on the user’s machine communicates using the Telnet protocol with a server program running on the remote machine.

The Telnet client program performs two important functions:

Interacting with the user terminal on the local host Exchanging messages with the Telnet server


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File Transfer Protocol


FTP allows a user to copy files to and from a remote machine.

The client program sends commands to the server program to coordinate the copying of files between the two machines on behalf of the user.

FTP uses separate TCP connections for control and data.


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Simple Mail Transfer Protocol


SMTP supports the transfer of e-mail. SMTP is used to send an e-mail message from a

local mail server to a remote mail server. SMTP is used to send an e-mail message from

the user’s mail agent to the local mail server. The separation of functionality between the user

agent and the mail server is valuable: The mail agent provides rich features for a single user. The mail server provides reliable service for multiple users.

FTP and SMTP are text oriented and command based.


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The communication between the two servers starts with a greeting message from the remote mail server. Then the local mail server issues commands to transfer the e-mail message.

A typical exchange involves separate commands to

Identify the local mail server Identify the sender of the e-mail message Identify each recipient of the e-mail message Send the actual e-mail message


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In contrast to FTP, SMTP uses a single TCP connection for both

The command reply exchanges The transfer of the e-mail message

In addition to transferring the message between mail servers, delivering an e-mail message requires two additional steps involving the mail agent:

The transfer of the message to the local mail server The reception of the message from the remote mail server


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Network News Transfer Protocol


NNTP supports the transfer of articles associated with electronic news groups.

A user agent uses NNTP to communicate with a local news server, which uses NNTP to communicate with a central repository of news article.

The key idea is to store the messages in a central database instead of having a separate copy in each subscriber’s mailbox.


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The database consists of a collection of newsgroups, each associated with an ordered list of messages.

An article includes header lines such as: E-mail address of the person who posted the

article Subject matter of the article Date/time when the article was generated Number of lines of text in the article Unique message identifier for the article List of newsgroups receiving with the article


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NNTP coordinates the transfer of messages between the local news server and the central repository.

NNTP may also be used between the user agent and the local news server.


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Properties of Application-Layer Protocols


Telnet, FTP, SMTP, and NNTP have important similarities and differences, as follows:

Command/reply» Telnet clients and servers send commands in binary

format.» FTP, SMTP, NNTP commands are text-based and

are sent by the client. The commands have a well-defined, fixed format, and the server responds with a three-digit reply code and an optional text message.

Data types » Telnet, FTP, SMTP, and NNTP transmit textual data

in the standard U.S. 7-bit ASCII format.» FTP also supports the transfer of data in binary form.


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Transport» All four protocols rely on a reliable transport protocol,

typically TCP.» Telnet, SMTP, and NNTP use a single TCP

connection for transmitting commands/replies and data.

» FTP uses separate connections for control and data.

Directionality » FTP and NNTP can transfer data in both directions-

copying data from the client and retrieving files from the server.

» SMTP is used to transmit e-mail messages from the client to the server.


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Statefulness » Under all four protocols, the server retains information

about the session with the client.


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Internet Protocol


The Internet protocol (IP) is the network-level protocol underlying the Internet, a collection of interconnected networks spanning the globe.

IP provides a framework for sending individual packets. In traveling from the sending host to the receiving host, a packet traverses a collection of routers that communicate via IP.

(Figure 5.2)


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Internet Protocol


HTTP

TCP

IP

Ethernet

interface

HTTP

TCP

IP

Ethernet

interface

IP

Ethernet

interface

SONET

interface

IP

Ethernet

interface

SONET

interface

Web client Web server

EthernetEthernet SONET link

HTTP message

TCP segmentRoute

rRoute

rIP packet IP packet IP packet

Figure 5.2. Protocols involved in transferring HTTP messages


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Internet Protocol


The routers in the Internet treat each packet independently and do not need to retain state across successive packets.

A sequence of IP packets traveling from one host to another may not traverse the same path through the network.

Packets may be lost, corrupted, or delivered out of order.

The model of the Internet is referred to as packet switching.


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Internet Protocol


Internet hosts are identified by numerical addresses (IP addresses).

An IP address can be divided into a network part and a host part.

Once the packet reaches the destination network, the host portion of the address is used to direct the packet to the appropriate destination machine.

IP addresses are allocated in five classes.

(As discussed before in socket programming)


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Internet Protocol


Each IP packet has a header. The fields of the IP header are set by operating

system on the sending machine and are important for successful communication between the sender and receiver:

Version number (4 bits) Header length (4 bits) Type of service (8 bits) Total length (16 bits) Identification (16 bits) IP flags (3 bits)


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Internet Protocol


Fragment offset (13 bits) Time-to-live (8 bits) Protocol (8 bits) Header checksum (16 bits) Source IP address (32 bits) Destination IP address (32 bits) IP options (variable length)

(Figure 5.4)


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Internet Protocol

Ver#hdr

len

Type of service

total length

identificationflagsfragment offset


time to liveprotocolHeader checksum

Figure 5.4. Format of an IP packet

Source IP address

destination IP address

Options (0 or more)

data

0 4 8 16 20 32

IP h

ead

er20

b

ytes


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Transmission Control Protocol


Fallowing topics will be discussed: Transmission Control Protocol Definition Opening and Closing a TCP Connection Sliding-Window Flow Control Retransmission of Lost Packets


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Transmission Control Protocol Definition


The Transmission Control Protocol (TCP) coordinates the transmission of data between a pair of applications.

Applications communicate by reading from and writing to a socket that presents data as an ordered, reliable stream of bytes.

The TCP sender divides data into segments and transmits each segment in an IP packet along with a TCP header.


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Transmission Control Protocol Definition


The TCP header includes information necessary to coordinate the ordered, reliable delivery of segments.

The sending and receiving applications should be allowed to assume that they communicate over a channel that provides an ordered, reliable byte stream. IP does not provide this service. Instead, this abstraction is provided by TCP.


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Opening and Closing a TCP Definition


The SYN, ACK, FIN, and RST flags in the TCP header are used in opening and closing a TCP connection.

Establishing a TCP connection between two applications, A and B, involves a three-way handshake.

» SYN from A to B» SYN-ACK from B to A» ACK from A to B (Figure 5.5)


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Opening and Closing a TCP Connection


Termination a TCP connection between two applications, A and B, involves a four-way handshake.

» FIN from B to A» ACK from A to B» FIN from A to B» ACK from B to A (Figure 5.5)


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Opening and Closing a TCP Connection


AC

K

DA

TA

SY

N

AC

K

AC

K

FIN

SY

N-A

CK

AC

K

DA

TA

DA

TA

FIN

AC

K

Figure 5.5. Timeline of a TCP connection

A

B


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Sliding-Window Flow Control


The TCP sender limits the transmission of data to avoid overflowing the buffer space at the receiver for two reasons:

The sender should not transmit more data than the receiver can store in its buffers.

The sender should not transmit data more quickly than the network can handle.

Each TCP sender limits the number of unacknowledged bytes in the network, using sliding-window flow control.


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Sliding-Window Flow Control


To avoid overflow of the buffer at the receiver, packets from B to A include the receiver window in the TCP header.


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Retransmission of Lost Packets


The retransmission of lost packets plays a crucial role in how TCP provides reliable delivery of a stream of bytes.

The sender infers that a packet has been lost in two ways:

A retransmission timeout (RTO) Duplicate acknowledgement


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Selecting an appropriate value for the RTO is a delicate process:

Setting RTO too low results in a false alarm, and the sender unnecessarily transmits a packet that was not actually lost.

Setting RTO too high postpones the detection of a lost packet, resulting in unnecessary delay in retransmitting the packet.

The right value for RTO depends on: The distance between the sender and receiver The network congestion


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The time between transmission of a packet and receipt of the acknowledgement is called Round Trip Time (RTT).

The RTO is set to the average RTT plus an additive factor.

In some cases, the sender can infer that a packet has been lost without waiting for the retransmission timer to expire.


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