FTP and TFTP / HTTP

FTP and TFTP
11.2.3 This page will describe the features of FTP and TFPT.


FTP is a reliable, connection-oriented service that uses TCP to transfer files between systems that support FTP. The main purpose of FTP is to transfer files from one computer to another by copying and moving files from servers to clients, and from clients to servers. When files are copied from a server, FTP first establishes a control connection between the client and the server. Then a second connection is established, which is a link between the computers through which the data is transferred. Data transfer can occur in ASCII mode or in binary mode. These modes determine the encoding used for data file, which in the OSI model is a presentation layer task. After the file transfer has ended, the data connection terminates automatically. When the entire session of copying and moving files is complete, the command link is closed when the user logs off and ends the session.

TFTP is a connectionless service that uses User Datagram Protocol (UDP). TFTP is used on the router to transfer configuration files and Cisco IOS images and to transfer files between systems that support TFTP. TFTP is designed to be small and easy to implement. Therefore, it lacks most of the features of FTP. TFTP can read or write files to or from a remote server but it cannot list directories and currently has no provisions for user authentication. It is useful in some LANs because it operates faster than FTP and in a stable environment it works reliably.

The next page will discuss HTTP.

HTTP
11.2.4 This page will describe the features of HTTP.


Hypertext Transfer Protocol (HTTP) works with the World Wide Web, which is the fastest growing and most used part of the Internet. One of the main reasons for the extraordinary growth of the Web is the ease with which it allows access to information. A Web browser is a client-server application, which means that it requires both a client and a server component in order to function. A Web browser presents data in multimedia formats on Web pages that use text, graphics, sound, and video. The Web pages are created with a format language called Hypertext Markup Language (HTML). HTML directs a Web browser on a particular Web page to produce the appearance of the page in a specific manner. In addition, HTML specifies locations for the placement of text, files, and objects that are to be transferred from the Web server to the Web browser.

Hyperlinks make the World Wide Web easy to navigate. A hyperlink is an object, word, phrase, or picture, on a Web page. When that hyperlink is clicked, it directs the browser to a new Web page. The Web page contains, often hidden within its HTML description, an address location known as a Uniform Resource Locator (URL).

In the URL http://www.cisco.com/edu/, the "http://" tells the browser which protocol to use. The second part, "www", is the hostname or name of a specific machine with a specific IP address. The last part, /edu/ identifies the specific folder location on the server that contains the default web page.

A Web browser usually opens to a starting or "home" page. The URL of the home page has already been stored in the configuration area of the Web browser and can be changed at any time. From the starting page, click on one of the Web page hyperlinks, or type a URL in the address bar of the browser. The Web browser examines the protocol to determine if it needs to open another program, and then determines the IP address of the Web server using DNS. Then the transport layer, network layer, data link layer, and physical layer work together to initiate a session with the Web server. The data that is transferred to the HTTP server contains the folder name of the Web page location. The data can also contain a specific file name for an HTML page. If no name is given, then the default name as specified in the configuration on the server is used.

The server responds to the request by sending to the Web client all of the text, audio, video, and graphic files specified in the HTML instructions. The client browser reassembles all the files to create a view of the Web page, and then terminates the session. If another page that is located on the same or a different server is clicked, the whole process begins again.

The next page will describe the protocol used to send e-mail.

Introduction to the TCP/IP application layer / DNS

Introduction to the TCP/IP application layer
11.2.1 This page will introduce some TCP/IP application layer protocols.


The session, presentation, and application layers of the OSI model are bundled into the application layer of the TCP/IP model. This means that representation, encoding, and dialog control are all handled in the TCP/IP application layer. This design ensures that the TCP/IP model provides maximum flexibility at the application layer for software developers.

The TCP/IP protocols that support file transfer, e-mail, and remote login are probably the most familiar to users of the Internet. These protocols include the following applications:

• DNS
• FTP
• HTTP
• SMTP
• SNMP
• Telnet

The next page will discuss DNS.

DNS
11.2.2 This page will describe DNS.


The Internet is built on a hierarchical addressing scheme. This scheme allows for routing to be based on classes of addresses rather than based on individual addresses. The problem this creates for the user is associating the correct address with the Internet site. It is very easy to forget an IP address to a particular site because there is nothing to associate the contents of the site with the address. Imagine the difficulty of remembering the IP addresses of tens, hundreds, or even thousands of Internet sites.

A domain naming system was developed in order to associate the contents of the site with the address of that site. The Domain Name System (DNS) is a system used on the Internet for translating names of domains and their publicly advertised network nodes into IP addresses. A domain is a group of computers that are associated by their geographical location or their business type. A domain name is a string of characters, number, or both. Usually a name or abbreviation that represents the numeric address of an Internet site will make up the domain name. There are more than 200 top-level domains on the Internet, examples of which include the following:

.us – United States
.uk – United Kingdom

There are also generic names, which examples include the following:

.edu – educational sites
.com – commercial sites
.gov – government sites
.org – non-profit sites
.net – network service

The next page will discuss FTP and TFTP.

UDP

UDP
11.1.8 This page will discuss UDP. UDP is the connectionless transport protocol in the TCP/IP protocol stack.


UDP is a simple protocol that exchanges datagrams without guaranteed delivery. It relies on higher-layer protocols to handle errors and retransmit data.

UDP does not use windows or ACKs. Reliability is provided by application layer protocols. UDP is designed for applications that do not need to put sequences of segments together.

The following protocols use UDP:

• TFTP
• SNMP
• DHCP
• DNS

The following are the definitions of the fields in the UDP segment:

• Source port – Number of the port that sends data
• Destination port – Number of the port that receives data
• Length – Number of bytes in header and data
• Checksum – Calculated checksum of the header and data fields
• Data – Upper-layer protocol data

The next page discusses port numbers used by both TCP and UDP.

TCP and UDP port numbers
11.1.9 This page examines port numbers.


Both TCP and UDP use port numbers to pass information to the upper layers. Port numbers are used to keep track of different conversations that cross the network at the same time.

Application software developers agree to use well-known port numbers that are issued by the Internet Assigned Numbers Authority (IANA). Any conversation bound for the FTP application uses the standard port numbers 20 and 21. Port 20 is used for the data portion and Port 21 is used for control. Conversations that do not involve an application with a well-known port number are assigned port numbers randomly from within a specific range above 1023. Some ports are reserved in both TCP and UDP. However, applications might not be written to support them. Port numbers have the following assigned ranges:

• Numbers below 1024 are considered well-known ports numbers.
• Numbers above 1024 are dynamically-assigned ports numbers.
• Registered port numbers are for vendor-specific applications. Most of these are above 1024.

End systems use port numbers to select the proper application. The source host dynamically assigns source port numbers. These numbers are always greater than 1023.

This page concludes this lesson. The next lesson will focus on the application layer. The first page provides an introduction.

Acknowledgment / TCP

Acknowledgment
11.1.6 This page will discuss acknowledgments and the sequence of segments.


Reliable delivery guarantees that a stream of data sent from one device is delivered through a data link to another device without duplication or data loss. Positive acknowledgment with retransmission is one technique that guarantees reliable delivery of data. Positive acknowledgment requires a recipient to communicate with the source and send back an ACK when the data is received. The sender keeps a record of each data packet, or TCP segment, that it sends and expects an ACK. The sender also starts a timer when it sends a segment and will retransmit a segment if the timer expires before an ACK arrives.

Figure shows a sender that transmits data packets 1, 2, and 3. The receiver acknowledges receipt of the packets with a request for packet 4. When the sender receives the ACK, it sends packets 4, 5, and 6. If packet 5 does not arrive at the destination, the receiver acknowledges with a request to resend packet 5. The sender resends packet 5 and then receives an ACK to continue with the transmission of packet 7.

TCP provides sequencing of segments with a forward reference acknowledgment. Each segment is numbered before transmission. At the destination, TCP reassembles the segments into a complete message. If a sequence number is missing in the series, that segment is retransmitted. Segments that are not acknowledged within a given time period will result in a retransmission.

The next page will describe TCP in more detail.

TCP
11.1.7 This page will discuss the protocols that use TCP and the fields included in a TCP segment.


TCP is a connection-oriented transport layer protocol that provides reliable full-duplex data transmission. TCP is part of the TCP/IP protocol stack. In a connection-oriented environment, a connection is established between both ends before the transfer of information can begin. TCP breaks messages into segments, reassembles them at the destination, and resends anything that is not received. TCP supplies a virtual circuit between end-user applications.

The following protocols use TCP:

• FTP
• HTTP
• SMTP
• Telnet

The following are the definitions of the fields in the TCP segment:

• Source port – Number of the port that sends data
• Destination port – Number of the port that receives data
• Sequence number – Number used to ensure the data arrives in the correct order
• Acknowledgment number – Next expected TCP octet
• HLEN – Number of 32-bit words in the header
• Reserved – Set to zero
• Code bits – Control functions, such as setup and termination of a session
• Window – Number of octets that the sender will accept
• Checksum – Calculated checksum of the header and data fields
• Urgent pointer – Indicates the end of the urgent data
• Option – One option currently defined, maximum TCP segment size
• Data – Upper-layer protocol data

The next page will define UDP.

Windowing

Windowing
11.1.5 This page will explain how windows are used to transmit data.


Data packets must be delivered to the recipient in the same order in which they were transmitted to have a reliable, connection-oriented data transfer. The protocol fails if any data packets are lost, damaged, duplicated, or received in a different order. An easy solution is to have a recipient acknowledge the receipt of each packet before the next packet is sent.

If a sender had to wait for an ACK after each packet was sent, throughput would be low. Therefore, most connection-oriented, reliable protocols allow multiple packets to be sent before an ACK is received. The time interval after the sender transmits a data packet and before the sender processes any ACKs is used to transmit more data. The number of data packets the sender can transmit before it receives an ACK is known as the window size, or window.

TCP uses expectational ACKs. This means that the ACK number refers to the next packet that is expected.

Windowing refers to the fact that the window size is negotiated dynamically in the TCP session. Windowing is a flow-control mechanism. Windowing requires the source device to receive an ACK from the destination after a certain amount of data is transmitted. The destination host reports a window size to the source host. This window specifies the number of packets that the destination host is prepared to receive. The first packet is the ACK.

With a window size of three, the source device can send three bytes to the destination. The source device must then wait for an ACK. If the destination receives the three bytes, it sends an acknowledgment to the source device, which can now transmit three more bytes. If the destination does not receive the three bytes, because of overflowing buffers, it does not send an acknowledgment. Because the source does not receive an acknowledgment, it knows that the bytes should be retransmitted, and that the transmission rate should be decreased.

In Figure , the sender sends three packets before it expects an ACK. If the receiver can handle only two packets, the window drops packet three, specifies three as the next packet, and indicates a new window size of two. The sender sends the next two packets, but still specifies a window size of three. This means that the sender will still expect a three-packet ACK from the receiver. The receiver replies with a request for packet five and again specifies a window size of two.

The next page describes the acknowledgment process.

Three-way handshake

Three-way handshake
11.1.4 This page will explain how TCP uses three-way handshakes for data transmission.


TCP is a connection-oriented protocol. TCP requires a connection to be established before data transfer begins. The two hosts must synchronize their initial sequence numbers to establish a connection. Synchronization occurs through an exchange of segments that carry a synchronize (SYN) control bit and the initial sequence numbers. This solution requires a mechanism that picks the initial sequence numbers and a handshake to exchange them.

The synchronization requires each side to send its own initial sequence number and to receive a confirmation of exchange in an acknowledgment (ACK) from the other side. Each side must receive the initial sequence number from the other side and respond with an ACK. The sequence is as follows:

1. The sending host (A) initiates a connection by sending a SYN packet to the receiving host (B) indicating its INS = X:

A - > B SYN, seq of A = X

2. B receives the packet, records that the seq of A = X, replies with an ACK of X + 1, and indicates that its INS = Y. The ACK of X + 1 means that host B has received all octets up to and including X and is expecting X + 1 next:

B - > A ACK, seq of A = X, SYN seq of B = Y, ACK = X + 1

3. A receives the packet from B, it knows that the seq of B = Y, and responds with an ACK of Y + 1, which finalizes the connection process:

A - > B ACK, seq of B = Y, ACK = Y + 1

This exchange is called the three-way handshake.

A three-way handshake is necessary because sequence numbers are not based on a global clock in the network and TCP protocols may use different mechanisms to choose the initial sequence numbers. The receiver of the first SYN would not know if the segment was delayed unless it kept track of the last sequence number used on the connection. If the receiver does not have this information, it must ask the sender to verify the SYN.

The next page will discuss the concept of windowing.

Flow control

Flow control
11.1.2 This page will describe how the transport layer provides flow control.


As the transport layer sends data segments, it tries to ensure that data is not lost. Data loss may occur if a host cannot process data as quickly as it arrives. The host is then forced to discard the data. Flow control ensures that a source host does not overflow the buffers in a destination host. To provide flow control, TCP allows the source and destination hosts to communicate. The two hosts then establish a data-transfer rate that is agreeable to both.

The next page will discuss data transport connections

Session establishment, maintenance, and termination
11.1.3 This page discusses transport functionality and how it is accomplished on a segment-by-segment basis.


Applications can send data segments on a first-come, first-served basis. The segments that arrive first will be taken care of first. These segments can be routed to the same or different destinations. Multiple applications can share the same transport connection in the OSI reference model. This is referred to as the multiplexing of upper-layer conversations. Numerous simultaneous upper-layer conversations can be multiplexed over a single connection.

One function of the transport layer is to establish a connection-oriented session between similar devices at the application layer. For data transfer to begin, the source and destination applications inform the operating systems that a connection will be initiated. One node initiates a connection that must be accepted by the other. Protocol software modules in the two operating systems exchange messages across the network to verify that the transfer is authorized and that both sides are ready.

The connection is established and the transfer of data begins after all synchronization has occurred. The two machines continue to communicate through their protocol software to verify that the data is received correctly.

Figure shows a typical connection between two systems. The first handshake requests synchronization. The second handshake acknowledge the initial synchronization request, as well as synchronizing connection parameters in the opposite direction. The third handshake segment is an acknowledgment used to inform the destination that both sides agree that a connection has been established. After the connection has been established, data transfer begins.

Congestion can occur for two reasons:

• First, a high-speed computer might generate traffic faster than a network can transfer it.

• Second, if many computers simultaneously need to send datagrams to a single destination, that destination can experience congestion, although no single source caused the problem.

When datagrams arrive too quickly for a host or gateway to process, they are temporarily stored in memory. If the traffic continues, the host or gateway eventually exhausts its memory and must discard additional datagrams that arrive.

Instead of allowing data to be lost, the TCP process on the receiving host can issue a “not ready” indicator to the sender. This indicator signals the sender to stop data transmission. When the receiver can handle additional data, it sends a “ready” transport indicator. When this indicator is received, the sender can resume the segment transmission.

At the end of data transfer, the source host sends a signal that indicates the end of the transmission. The destination host acknowledges the end of transmission and the connection is terminated.

The next page will define three-way handshakes.