Ports for clients

Ports for clients

10.2.3 This page will discuss source ports, which are set by clients.

Whenever a client connects to a service on a server, a source and destination port must be specified. TCP and UDP segments contain fields for source and destination ports. Destination ports, or ports for services, are normally defined using the well-known ports. Source ports set by the client are determined dynamically.

In general, a client determines the source port by randomly assigning a number above 1023. For example, a client that attempts to communicate with a Web server will use TCP and assign the destination port as 80 and the source port as 1045. When the packet arrives at the server, it moves up to the transport layer and eventually to the HTTP service, which operates at port 80. The HTTP server responds to the clients request with a segment that uses port 80 as the source and 1045 as the destination. Clients and servers use ports to distinguish which process each segment is associated with.

The next page will teach students about the three categories of port numbers.

Ports for services

Ports for services
10.2.2 Services running on hosts must have a port number assigned to them so communication can occur. A remote host attempting to connect to a service expects that service to use specific transport layer protocols and ports. Some ports, which are defined in RFC 1700, are known as the well-known ports. These ports are reserved in both TCP and UDP. 

These well-known ports define applications that run above the transport layer protocols. For example, a server that runs FTP will use ports 20 and 21 to forward TCP connections from clients to its FTP application. This allows the server to determine which service a client requests. TCP and UDP use port numbers to determine the correct service to which requests are forwarded.

The next page will discuss ports in greater detail.

Multiple conversations between hosts

Multiple conversations between hosts
10.2.1 At any given moment, thousands of packets that provide hundreds of different services travel through a modern network. Many servers use a multitude of services and this causes unique problems for the addressing of packets. If a server is running both SMTP and HTTP, it uses the destination port field to determine what service the source is requesting. The source cannot construct a packet destined for just the server IP address because the destination would not know what service was being requested. A port number must be associated with the conversation between hosts to ensure that the packet reaches the appropriate service on the server. If a server could not distinguish between different conversations, a client could not send an e-mail and browse a Web page at the same time. A method for transport layer conversations to be separated must be used.

Hosts running TCP/IP associate ports at the transport layer with certain applications. Port numbers are used to keep track of different conversations that cross the network at the same time. Port numbers are needed for a host to communicate with a server that uses multiple services. Both TCP and UDP use port or socket numbers to pass information to the upper layers.

Application software developers have agreed to use the well-known port numbers that are defined in RFC1700. Any conversation bound for the FTP application uses the standard port number 21. Conversations that do not involve applications with well-known port numbers are assigned port numbers that have been randomly selected from within a specific range. These port numbers are used as source and destination addresses in the TCP segment. 

Port numbers have the following assigned ranges:

• The Well Known Ports are those from 0 through 1023
• The Registered Ports are those from 1024 through 49151
• The Dynamic and/or Private Ports are those from 49152 through 65535

Systems initiating communication requests use port numbers to select proper applications. Source port numbers for these requests are dynamically assigned by the originating host, and are usually a number larger than 1023. Port numbers in the range of 0-1023 are considered public port numbers and are controlled by the Internet Assigned Numbers Authority (IANA).

Post office box numbers are a good analogy for port numbers. A piece of mail may be sent to a zip code, city, and P.O. box. The zip code and city direct mail to the correct general mail facility while the P.O. box ensures the item is delivered to the one individual to whom the mail is addressed. Similarly, the IP address gets the packet to the correct server, but the TCP or UDP port number guarantees the packet is passed to the correct application.

The next page will discuss well-known ports.

UDP operation

UDP operation
10.1.7 This page will explain the similarities and differences between TCP and UDP.

The TCP/IP protocol stack contains many different protocols, each designed to perform a certain task. IP provides Layer 3 connectionless transport through an internetwork. TCP enables connection-oriented, reliable transmission of packets at Layer 4 of the OSI model. UDP provides connectionless, unreliable transmission of packets at Layer 4 of the OSI model.

Both TCP and UDP use IP as their Layer 3 protocol. In addition, TCP and UDP are used by various application layer protocols. TCP provides services for applications such as FTP, HTTP, SMTP, and DNS. UDP is the transport layer protocol used by DNS, TFTP, SNMP, and DHCP. 

TCP must be used when applications need to guarantee that a packet arrives intact, in sequence, and unduplicated. The overhead necessary to ensure delivery of a packet is sometimes a problem with TCP. Not all applications need to guarantee delivery of the data packet, so they use the faster, connectionless delivery mechanism afforded by UDP. The UDP protocol standard is described in RFC 768.

UDP does not use windowing or ACKs so application layer protocols must provide error detection. 

The Source Port field is an optional field used only if information needs to return to the sending host. When a destination router receives a routing update, the source router is not requesting anything so nothing needs to return to the source. There is no exchange of information or data. The Destination Port field specifies the application to which UDP needs to pass the protocol. A DNS request from a host to a DNS server would have a Destination Port field of 53, the UDP port number for DNS. The Length field identifies the number of octets in the UDP segment. The UDP checksum is optional but should be used to ensure that the data has not been damaged during transmission. For transport across the network, UDP is encapsulated within the IP packet.

Once a UDP segment arrives at the destination IP address, a mechanism must exist which allows the receiving host to determine the exact destination application. Destination ports are used for this purpose. If a host is running both TFTP and DNS services, it must be able to determine what service the arriving UDP segments need. The Destination Port field in the UDP header determines the application to which a UDP segment will be delivered.

This page concludes this lesson. The next lesson will provide an overview of transport layer ports. The first page describes multiple conversations between hosts

Positive acknowledgments

Positive acknowledgments

10.1.6 Acknowledgment is a common step in the synchronization process, which includes sliding windows and data sequencing. In a TCP segment, the sequence number field is followed by the Acknowledgment Number field. This field is where tracking of transmitted and received bytes are indicated.

One problem with the IP protocol is that there is no verification method to determine if data segments reach their destination. So data segments may be constantly forwarded with no knowledge as to whether or not they were actually received. TCP uses positive acknowledgment and retransmission (PAR) to control data flow and confirm data delivery.

Many protocols use PAR to provide reliability. With PAR, the source sends a packet, starts a timer, and waits for an acknowledgment before it sends the next packet in the session. If the timer expires before the source receives an acknowledgment, the source retransmits the packet and resets the timer. The acknowledgment is provided by the value of Acknowledgment Number and the ACK flag set in the TCP header. TCP uses expectational acknowledgment in which the Acknowledgment Number value refers to the next octet that is expected as part of the TCP session.

Windowing is a flow control mechanism that requires the source device to receive an acknowledgment from the destination after a specific amount of data bytes has been transmitted. With a window size of three, the source device can send three octets to the destination. It must then wait for an acknowledgment of these bytes. If the destination receives the three octets, it sends an acknowledgment to the source device, which can then transmit three more octets. If the destination does not receive the three octets, it does not send an acknowledgment. This may be caused by overflowing buffers or packets lost in transit. Since the source does not receive an acknowledgment, it knows that the octets should be retransmitted and that the window size should be reduced. This window size reduction provides the receiving host less bytes to process from its buffers before more data arrives. This effectively slows the communication between hosts to provide more reliability between the hosts.

The Lab Activity will teach students how to enable and monitor multiple host sessions. The Interactive Media Activity will help students become more familiar with windows.

The next page will explain how UDP works. 

Sequencing numbers

Sequencing numbers 

10.1.5 TCP breaks data into segments. After the synchronization process occurs and the window size has been established, the data segments are transported from the sender to the receiver. The data segments must be reassembled after all the data is received. There is no guarantee that the data will arrive in the order it was transmitted. TCP applies sequence numbers to the data segments that are transmitted so that the receiver can reassemble the bytes in their original order. This way, if TCP segments arrive out of order, the segments will still be assembled correctly.

These sequencing numbers also act as reference numbers so that the receiver will know if it has received all of the data. They also identify the missing data pieces to the sender so it can retransmit the missing data. This offers increased efficiency since the sender only needs to resend the missing segments instead of the entire set of data.

Each TCP segment is numbered before transmission. 

The sequence number portion comes after the destination port in the segment format. At the receiving station, TCP uses the sequence numbers to reassemble the segments into a complete message. If a sequence number is missing in the series, that segment is retransmitted.

The next page discusses positive acknowledgments.

Windowing and window size

Windowing and window size 
10.1.4 The amount of data that needs to be transmitted is often too large to be sent in a single data segment. In this case, the data must be broken into smaller pieces to allow for proper data transmission. TCP is responsible for breaking data into segments. This can be compared to the way that small children are fed. Their food is cut into smaller pieces that their mouths can accommodate. Additionally, a device may not be able to receive data as quickly as the source can send it. The device may be busy with other tasks or the sender may be a more robust device.

Once the data is segmented, it must be transmitted to the destination device. One of the services provided by TCP is flow control, which regulates how much data is sent during a given transmission period. The process of flow control is known as windowing.

Window size determines the amount of data that can be transmitted at one time before the destination responds with an acknowledgment. After a host transmits the window-sized number of bytes, the host must receive an acknowledgment that the data has been received before it can send any more data. For example, if the window size is 1, each byte must be acknowledged before the next byte is sent. 

TCP utilizes windowing to dynamically determine transmission size. Devices negotiate a window size to allow a specific number of bytes to be transmitted before an acknowledgment. 

This process of dynamically varying the window size increases reliability. The window size can be varied based upon acknowledgments.

The Interactive Media Activity will help students understand the concept of windowing.

The next page describes TCP sequence numbers.