Unit 4: Transport protocols

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Presentation transcript:

Unit 4: Transport protocols

Describe the functions of TCP and UDP Transmission control protocol (TCP) is a network communication protocol designed to send data packets over the Internet. TCP is a transport layer protocol in the OSI layer and is used to create a connection between remote computers by transporting and ensuring the delivery of messages over supporting networks and the Internet.

For example, when a Web server sends anHTML file to a client, it uses the HTTP protocol to do so. The HTTP program layer asks the TCP layer to set up the connection and send the file.  The TCP stack divides the file into packets, numbers them and then forwards them individually to the IP layer for delivery. Although each packet in the transmission will have the same source and destination IP addresses, packets may be sent along multiple routes. The TCP program layer in the client computer waits until all of the packets have arrived, then acknowledges those it receives and asks for the retransmission on any it does not (based on missing packet numbers), then assembles them into a file and delivers the file to the receiving application.

Functions of TCP TCP is a connection-oriented protocol, which means a connection is established and maintained until the application programs at each end have finished exchanging messages. It determines how to break application data into packets that networks can deliver, sends packets to and accepts packets from the network layer, manages flow control, and—because it is meant to provide error-free data transmission—handles retransmission of dropped or garbled packets as well as acknowledgement of all packets that arrive. In the Open Systems Interconnection (OSI) communication model, TCP covers parts of Layer 4, the Transport Layer, and parts of Layer 5, the Session Layer.  

Transmission Control Protocol is one of the most used protocols in digital network communications and is part of the Internet protocol suite, commonly known as the TCP/IP suite.  TCP works in collaboration with Internet Protocol, which defines the logical location of the remote node, whereas TCP transports and ensures that the data is delivered to the correct destination.

User Datagram Protocol (UDP) is part of the Internet Protocol suite used by programs running on different computers on a network. UDP is used to send short messages called datagrams but overall, it is an unreliable, connectionless protocol. UDP is officially defined in RFC 768 and was formulated by David P. Reed.

Functions of UDP: User datagram protocol is an open systems interconnection (OSI) transport layer protocol for client- server network applications. UDP uses a simple transmission model but does not employ handshaking dialogs for reliability, ordering and data integrity. The protocol assumes that error-checking and correction is not required, thus avoiding processing at the network interface level.  UDP is widely used in video conferencing and real-time computer games. The protocol permits individual packets to be dropped and UDP packets to be received in a different order than that in which they were sent, allowing for better performance. 

Destination port number Datagram size Checksum UDP network traffic is organized in the form of datagrams, which comprise one message units. The first eight bytes of a datagram contain header information, while the remaining bytes contain message data. A UDP datagram header contains four fields of two bytes each: Source port number Destination port number Datagram size Checksum A checksum is a count of the number of bits in a transmission unit that is included with the unit so that the receiver can check to see whether the same number of bits arrived. If the counts match, it's assumed that the complete transmission was received.

Identify well-known ports In TCP/IP and UDP networks, a port is an endpoint to a logical connection and the way a client program specifies a specific server program on a computer in a network. The port number identifies what type of port it is. For example, port 80 is used for HTTP traffic. Some ports have numbers that are pre-assigned to them by the IANA(Internet Assigned Numbers Authority ), and these are called the "well-known ports" which are specified in RFC 1700.

List of Well-Known Ports Port numbers range from 0 to 65536, but only port numbers 0 to 1024 are reserved for privileged services and designated as well-known ports. This list of well-known port numbers specifies the port used by the server process as its contact port.

Port Number Description 1 TCP Port Service Multiplexer (TCPMUX) 5 Remote Job Entry (RJE) 7 ECHO 18 Message Send Protocol (MSP) 20 FTP -- Data 21 FTP -- Control 22 SSH Remote Login Protocol 23 Telnet 25 Simple Mail Transfer Protocol (SMTP) 29 MSG ICP 37 Time 42 Host Name Server (Nameserv) 43 WhoIs 49 Login Host Protocol (Login)

53 Domain Name System (DNS) 69 Trivial File Transfer Protocol (TFTP) 70 Gopher Services 79 Finger 80 HTTP 103 X.400 Standard 108 SNA Gateway Access Server 109 POP2 110 POP3 115 Simple File Transfer Protocol (SFTP) 118 SQL Services 119 Newsgroup (NNTP) 137 NetBIOS Name Service 139 NetBIOS Datagram Service

143 Interim Mail Access Protocol (IMAP) 150 NetBIOS Session Service 156 SQL Server 161 SNMP 179 Border Gateway Protocol (BGP) 190 Gateway Access Control Protocol (GACP) 194 Internet Relay Chat (IRC) 197 Directory Location Service (DLS) 389 Lightweight Directory Access Protocol (LDAP) 396 Novell Netware over IP 443 HTTPS

444 Simple Network Paging Protocol (SNPP) 445 Microsoft-DS 458 Apple QuickTime 546 DHCP Client 547 DHCP Server 563 SNEWS 569 MSN 1080 Socks

Use netstat and other port scanners to identify open connections and services. If you’re troubleshooting a service that you know is running normally the next step is to make sure it’s listening to the right network port. The netstat command shows the services listening to ports on a Linux server along with the details of any connections currently made to them. The connection details we look at during basic network daemon troubleshooting are the addresses the daemon is listening on (including the port number), the daemon’s PID (process identifier), and the program name. Of course, you need to run netstat on the server running the service. Remember that netstat is not affected by your firewall configuration.

In computing, netstat (network statistics) is a command-line tool that displays network connections for the Transmission Control Protocol (both incoming and outgoing), routing tables, and a number of network interface (network interface controller or software-defined network interface) and network protocol statistics.

sudo netstat -plnt Checking ports To list tcp ports that are being listened on, along with the name of each listener’s daemon and its PID, run: sudo netstat -plnt The following example shows netstat’s output for three common programs that are listening on three different sockets.

For example, if you only want to see TCP connections, use netstat --tcp. This shows a list of TCP connections to and from your machine. The following example shows connections to our machine on ports 993 (imaps), 143 (imap), 110 (pop3), 25 (smtp), and 22 (ssh).It also shows a connection from our machine to a remote machine on port 389 (ldap).

If you want to see what (TCP) ports your machine is listening on, use netstat --tcp --listening. Another useful flag to add to this is --programs which indicates which process is listening on the specified port. The following example shows a machine listening on ports 80 (www), 443 (https), 22 (ssh), and 25 (smtp);

The next example uses netstat --route to display the routing table The next example uses netstat --route to display the routing table. For most people, this will show one IP and and the gateway address but if you have more than one interface or have multiple IPs assigned to an interface, this command can help troubleshoot network routing problems.

Description Port scanner tool can be used to identify available services running on a server, it uses raw IP packets to find out what ports are open on a server or what Operating System is running or to check if a server has firewall enabled etc. The service can also detect uptime of a host if the host is running one of the known Operating Systems which the scanner can analyze to guess uptime.