CCNA1_Mod14_Transport_Layer v1.pdf

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. The...

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. The transport layer has no knowledge of the underlying network and hosts details. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 1 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 2 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 3 Transmission Control Protocol (TCP) IP is not responsible for guaranteeing delivery. On the other hand, TCP provides reliability and flow control. TCP basic operations: Number and track data segments transmitted to 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 4 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. Best-effort delivery describes a network service in which the network does not provide any special features that recover lost or corrupted packets. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 5 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. UDP reassembles the received datagrams in the order they were received. 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 6 TCP Features  Establishes a Session - TCP is a connection-oriented protocol that negotiates and establishes a permanent connection (or session) between source and destination devices prior to forwarding any traffic.  Ensures Reliable Delivery - For many reasons, it is possible for a segment to become corrupted or lost completely, as it is transmitted over the network. TCP ensures that each segment that is sent by the source arrives at the destination.  Provides Same-Order Delivery - Because networks may provide multiple routes that can have different transmission rates, data can arrive in the wrong order.  Supports Flow Control - Network hosts have limited resources (i.e., memory and processing power). When TCP is aware that these resources are overtaxed (overloaded), it can request that the sending application reduce the rate of data flow. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 7 UDP Features UDP features include the following: Data is reconstructed in the order that it is received. Any segments that are lost are not resent. There is no session establishment. The sending is not informed about resource availability. UDP relies on (depend on) application layer protocols for error detection. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 8 Applications that use UDP  Live video and multimedia applications - These applications can tolerate some data loss but require little or no delay. Examples include VoIP and live streaming video.  Simple request and reply applications - Applications with simple transactions where a host sends a request and may or may not receive a reply. Examples include DNS and DHCP.  Applications that handle reliability themselves - Unidirectional communications where flow control, error detection, acknowledgments, and error recovery is not required, or can be handled by the application. Examples include SNMP and TFTP. 1. Voice over Internet Protocol (VoIP) make voice calls using Internet instead of phone line 2. Domain Name System (DNS) translate hostnames to IP addresses 3. Dynamic Host Configuration Protocol (DHCP) distribution of IP addresses 4. Simple Network Management Protocol (SNMP) managed devices on IP networks 5. Trivial File Transfer Protocol (TFTP) allows a client to get a file from or put a file onto a remote host © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 9 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 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 Number Groups Numbe Port Group r Description Range 0 These port numbers are reserved for common or popular services and Well-known Ports to 1,023 applications such as web browsers, email clients, and remote access clients. These port numbers are assigned by IANA to a requesting entity to use with specific processes or applications. This can be used when a user has chosen 1,024 to install, rather than common applications that would receive a well- Registered Ports to 49,151 known port number. For example, Cisco has registered port 1812 for its RADIUS server authentication process. These ports are also known as ephemeral ports (short-lived). Private and/or 49,152 to The client’s OS usually assign port numbers dynamically when a Dynamic Ports 65,535 connection to a service is initiated. 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 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) 443 TCP Hypertext Transfer Protocol Secure (HTTPS)© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 13 TCP Server Processes Each application process running on a server is configured to use a port number. An individual server cannot have two services assigned to the same port number within the same transport layer services. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 14 TCP Three-Way Handshake Functions of the Three-Way Handshake: 1. It establishes that the destination device is present on the network. 2. It verifies that the destination device has an active service and is accepting requests on the destination port number that the initiating client intends to use. 3. It informs the destination device that the source client intends to establish a communication session on that port number. After the communication is completed the sessions are closed, and the connection is terminated. The connection and session mechanisms enable TCP reliability function. 1.The client first sends a Synchronization packet (SYN). 2.If the server accepts, it responds with a Synchronization Acknowledgment (SYN-ACK) to let the client know it's open and ready for communication. 3.The client responds with an Acknowledgment (ACK). Now the session begins and the socket is created. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 15 TCP Reliability- Guaranteed and Ordered Delivery TCP can also help maintain the flow of packets so that devices do not become overloaded. There may be times when TCP segments do not arrive at their destination or arrive out of order. All the data must be received and the data in these segments must be reassembled into the original order. Sequence numbers are assigned in the header of each packet to achieve this goal. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 16 TCP Reliability – Data Loss and Retransmission  No matter how well designed a network is, data loss occasionally occurs.  TCP provides methods of managing these segment losses.  Among these is a mechanism to retransmit segments for unacknowledged data. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 17 TCP Reliability – Data Loss and Retransmission  Host operating systems today typically employ an optional TCP feature called selective acknowledgment (SACK), negotiated during the three-way handshake.  If both hosts support SACK, the receiver can explicitly acknowledge which segments (bytes) were received including any discontinuous segments. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 18 UDP Low Overhead versus Reliability UDP does not establish a connection. UDP provides low overhead data transport because it has a small datagram header and no network management traffic. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 19 UDP Datagram Reassembly UDP does not track sequence numbers the way TCP does. UDP has no way to reorder the datagrams into their transmission order. UDP simply reassembles the data in the order that it was received and forwards it to the application. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 20