ITN Module 14: Transport Layer PDF

Module 14: Transport Layer Introduction to Networks v7.0 (ITN) 14.1 Transportation of Data © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 2 Transportation of Data Role of the Transport Layer The transport layer is: responsible for logica...

Module 14: Transport Layer Introduction to Networks v7.0 (ITN) 14.1 Transportation of Data © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 2 Transportation of Data Role of the Transport Layer The transport layer is: responsible for logical communications between applications running on different hosts. The link between the application layer and the lower layers that are responsible for network transmission. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 3 Transportation of Data Transport Layer Responsibilities The transport layer has the following responsibilities: Tracking individual conversations Segmenting data and reassembling segments Adds header information Identify, separate, and manage multiple conversations Uses segmentation and multiplexing to enable different communication conversations to be interleaved on the same network © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 4 Transportation of Data Transport Layer Protocols IP does not specify how the delivery or transportation of the packets takes place. Transport layer protocols specify how to transfer messages between hosts, and are responsible for managing reliability requirements of a conversation. The transport layer includes the TCP and UDP protocols. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 5 Transportation of Data Transmission Control Protocol TCP provides reliability and flow control. TCP basic operations: Number and track data segments transmitted to a specific host from a specific application Acknowledge received data Retransmit any unacknowledged data after a certain amount of time Sequence data that might arrive in wrong order Send data at an efficient rate that is acceptable by the receiver © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 6 Transportation of Data User Datagram Protocol (UDP) UDP provides the basic functions for delivering datagrams between the appropriate applications, with very little overhead and data checking. UDP is a connectionless protocol. UDP is known as a best-effort delivery protocol because there is no acknowledgment that the data is received at the destination. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 7 Transportation of Data The Right Transport Layer Protocol for the Right Application UDP is also used by request-and- reply applications where the data is minimal, and retransmission can be done quickly. If it is important that all the data arrives and that it can be processed in its proper sequence, TCP is used as the transport protocol. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 8 14.4 Port Numbers © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 9 Port Numbers Multiple Separate Communications TCP and UDP transport layer protocols use port numbers to manage multiple, simultaneous conversations. The source port number is associated with the originating application on the local host whereas the destination port number is associated with the destination application on the remote host. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 10 Port numbers Socket Pairs The source and destination ports are placed within the segment. The segments are then encapsulated within an IP packet. The combination of the source IP address and source port number, or the destination IP address and destination port number is known as a socket. Sockets enable multiple processes, running on a client, to distinguish themselves from each other, and multiple connections to a server process to be distinguished from each other. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 11 Port Numbers Port Number Groups Port Group Number Range Description These port numbers are reserved for common or popular services and applications such as web browsers, email clients, and remote access Well-known 0 to 1,023 clients. Ports Defined well-known ports for common server applications enables clients to easily identify the associated service required. These port numbers are assigned by IANA to a requesting entity to use with specific processes or applications. These processes are primarily individual applications that a user has Registered 1,024 to 49,151 chosen to install, rather than common applications that would receive a Ports well-known port number. For example, Cisco has registered port 1812 for its RADIUS server authentication process. These ports are also known as ephemeral ports. Private and/ The client’s OS usually assign port numbers dynamically when a or Dynamic 49,152 to 65,535 connection to a service is initiated. Ports The dynamic port is then used to identify the client application during communication. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 12 Port Numbers Port Number Groups (Cont.) Well-Known Port Numbers Port Number Protocol Application 20 TCP File Transfer Protocol (FTP) - Data 21 TCP File Transfer Protocol (FTP) - Control 22 TCP Secure Shell (SSH) 23 TCP Telnet 25 TCP Simple Mail Transfer Protocol (SMTP) 53 UDP, TCP Domain Name Service (DNS) 67 UDP Dynamic Host Configuration Protocol (DHCP) - Server 68 UDP Dynamic Host Configuration Protocol - Client 69 UDP Trivial File Transfer Protocol (TFTP) 80 TCP Hypertext Transfer Protocol (HTTP) 110 TCP Post Office Protocol version 3 (POP3) 143 TCP Internet Message Access Protocol (IMAP) 161 UDP Simple Network Management Protocol (SNMP) © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 13 443 TCP Hypertext Transfer Protocol Secure (HTTPS) Port Numbers The netstat Command Unexplained TCP connections can pose a major security threat. Netstat is an important tool to verify connections. C:\> netstat Active Connections Proto Local Address Foreign Address State TCP 192.168.1.124:3126 192.168.0.2:netbios-ssn ESTABLISHED TCP 192.168.1.124:3158 207.138.126.152:http ESTABLISHED TCP 192.168.1.124:3159 207.138.126.169:http ESTABLISHED TCP 192.168.1.124:3160 207.138.126.169:http ESTABLISHED TCP 192.168.1.124:3161 sc.msn.com:http ESTABLISHED TCP 192.168.1.124:3166 www.cisco.com:http ESTABLISHED © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 14