Chapter 4 OSI Transport Layer CCNA 1 1
- Slides: 76
Chapter 4 OSI Transport Layer CCNA 1 -1 Chapter 4
Note for Instructors • These presentations are the result of a collaboration among the instructors at St. Clair College in Windsor, Ontario. • Thanks must go out to Rick Graziani of Cabrillo College. His material and additional information was used as a reference in their creation. • If anyone finds any errors or omissions, please let me know at: • tdame@stclaircollege. ca. CCNA 1 -2 Chapter 4
OSI Transport Layer Roles of the Transport Layer CCNA 1 -3 Chapter 4
Purpose of the Transport Layer • The Layer 4 data stream is a logical connection between the endpoints of a network. • It provides transport services from a host to a destination. • This service is sometimes referred to as an end-to-end service. • Provides two protocols: • TCP – Transmission Control Protocol • UDP – User Datagram Protocol CCNA 1 -4 Chapter 4
Purpose of the Transport Layer • We will be focusing on the Layer that: • Segments the data. • Creates and inserts the header for either the TCP or the UDP protocol. CCNA 1 -5 Chapter 4
Purpose of the Transport Layer • Primary responsibilities: • Tracking the individual communications between applications on the source and destination hosts. • Segmenting the data and managing each piece. • Reassembling the segments into streams of application data. • Identifying the different applications. • Performing flow control between end users. • Enabling error recovery. • Initiating a session. CCNA 1 -6 Chapter 4
Segmentation and Reassembly • An Ethernet frame has a maximum frame size or Maximum Transmission Unit (MTU) of 1, 518 bytes. • When a larger message must be sent, the application data must be segmented into sections that will not exceed the maximum size. • The segment size must also take into account the encapsulation process that must take place before the frame can be transmitted. CCNA 1 -7 Chapter 4
Segmentation and Reassembly CCNA 1 -8 Chapter 4
Tracking Individual Conversations • Any host can have multiple applications running simultaneously. • Transport Layer maintains these separate data streams. • For example, it makes sure that Instant Messaging data does not appear on the E-mail application. CCNA 1 -9 Chapter 4
Segmenting Data Application Layer passes large amounts of data. Transport Layer breaks into smaller pieces called segments. CCNA 1 -10 Chapter 4
Segmenting Data Multiplexing allows multiple, concurrent applications. Segmentation allows multiplexing of the data from all sessions. CCNA 1 -11 Chapter 4
Reassembling Segments Segment numbers allow reassembly of the data in the proper order. CCNA 1 -12 Chapter 4
Reassembling Segments After reassembly, the port number is used to pass the data to the correct application. 25 80 25 CCNA 1 -13 When created, each segment is assigned a unique application identifier. (Port Number) Chapter 4
Flow Control Network hosts have limited resources. (Memory, Bandwidth) Regulating the amount that can be transmitted or received is flow control. CCNA 1 -14 Chapter 4
Error Recovery It is possible for a piece of data to become corrupted or lost. Transport layer protocols can request a retransmission. CCNA 1 -15 Chapter 4
Initiating a Session Transport layer provides for connection orientation. Allows the data to be closely managed. CCNA 1 -16 Chapter 4
Supporting Reliable Communication UDP TCP • Different applications have different requirements. • Different protocols have been developed to meet them. CCNA 1 -17 Chapter 4
OSI Transport Layer TCP and UDP CCNA 1 -18 Chapter 4
TCP and UDP • Two most common Transport Layer protocols CCNA 1 -19 Chapter 4
TCP and UDP CCNA 1 -20 Chapter 4
User Datagram Protocol (UDP) • Connectionless • “Best Effort” delivery • Low overhead CCNA 1 -21 Example Applications Domain Name System (DNS) Online Games Voice over IP (Vo. IP) Dynamic Host Configuration Protocol (DHCP) Trivial File Transfer Protocol (TFTP) Chapter 4
Transmission Control Protocol (TCP) • • Connection-oriented Reliable delivery Error checking Flow control CCNA 1 -22 Example Applications Hypertext Transfer Protocol (HTTP) File Transfer Protocol (FTP) Telnet Simple Message Transfer Protocol (SMTP) Chapter 4
OSI Transport Layer Port Addressing CCNA 1 -23 Chapter 4
Identifying the Conversations IP Packet Header • At the TCP/IP Internet Layer: • The IP Packet Header has a Protocol field that specifies whether the segment is TCP or UDP. CCNA 1 -24 Chapter 4
Identifying the Conversations IP Packet Header • When a packet is encapsulated at the Network Layer, it is coded to identify the source of the packet (TCP or UDP). CCNA 1 -25 Chapter 4
Identifying the Conversations IP Protocol = 6 Data sent to TCP _______ IP Protocol = 17 Data sent to UDP • When a packet is decapsulated at the destination, the code is used to send the packet to the proper protocol (TCP or UDP). CCNA 1 -26 Chapter 4
Identifying the Conversations • Both TCP and UDP use port numbers to pass information to the upper layers. • These ports are actually termed sockets. • A socket is simply the combination of the device’s IP address and the source/destination port for the data, separated by a colon. • e. g. 207. 134. 65. 2: 80 references an HTTP socket. CCNA 1 -27 Chapter 4
Identifying the Conversations CCNA 1 -28 Chapter 4
Port Addressing Types and Tools • Port numbers are managed and assigned by the Internet Assigned Number Authority (IANA). CCNA 1 -29 Chapter 4
Port Addressing Types and Tools • Some ports are reserved in both TCP and UDP, although applications might not be written to support them. • Three groupings of port types: • Well-known or Contact Ports (0 to 1023). • Registered Ports (1024 through 49151). • Private and/or Dynamic Ports (49152 through 65535). CCNA 1 -30 Chapter 4
Port Addressing Types and Tools • Well-Known Ports: • Reserved for common services and applications. 20 – FTP Data 25 – SMTP 21 – FTP Control 110 – POP 3 23 – Telnet CCNA 1 -31 194 – IRC 443 – HTTPS 69 – TFTP 520 – RIP Chapter 4
Port Addressing Types and Tools • Registered Ports: • Optional user processes and applications. 1863 – MSN Messenger 8008 – Alternate HTTP 8080 – Alternate HTTP CCNA 1 -32 1812 – RADIUS 2000 – Cisco Vo. IP 5004 – RTP 5060 – SIP (Vo. IP) Chapter 4
Port Addressing Types and Tools • Dynamic Ports: • Assigned to a user application at connect time. Dynamic port usage will become clearer as we move through the material. STAY TUNED! CCNA 1 -33 Chapter 4
Port Addressing Types and Tools • Using both TCP and UDP: • Some applications may use both TCP and UDP. • For example, the low overhead of UDP enables DNS to serve many client requests very quickly. • Sometimes, however, sending the requested information may require the reliability of TCP. In this case, the well known port number of 53 is used by both protocols with this service. CCNA 1 -34 Chapter 4
Port Addressing Types and Tools Source IP Source Port Destination IP Destination Port TCP/UDP Source Socket Destination Socket Connection State netstat –a –n command • Actually, when you open up a single web page, there are usually several TCP sessions created, not just one. CCNA 1 -35 Chapter 4
OSI Transport Layer TCP Communicating with Reliability CCNA 1 -36 Chapter 4
Transmission Control Protocol (TCP) • • Connection-oriented Reliable delivery Error checking Flow control CCNA 1 -37 Example Applications Hypertext Transfer Protocol (HTTP) File Transfer Protocol (FTP) Telnet Simple Message Transfer Protocol (SMTP) Chapter 4
Making Conversations Reliable • The key difference between TCP and UDP is reliability. • TCP uses connection-oriented sessions. • Before any data is exchanged, the Transport Layer initiates a connection to the destination. • This connection allows the tracking of the session. • Sequence Numbers • Acknowledgments • Creates the overhead of TCP. • Reliability is achieved by having fields in the TCP header that have specific functions. CCNA 1 -38 Chapter 4
Making Conversations Reliable TCP session that opened a connection. Usually a random value above 1023. The number of the last octet (byte) in the segment. CCNA 1 -39 Upper Layer application on the remote site. The number of the next octet (byte) expected by the receiver. Chapter 4
Making Conversations Reliable The length of the segment header in bytes. Individual 1 bit fields used in session management. Flow control. Error checking the header. CCNA 1 -40 If the Urgent FLAG is set, the location of the urgent data. Chapter 4
Making Conversations Reliable - FYI U R G • • • A C K P S H R S T S Y N F I N 6 Bits 0 = OFF 1 = ON URG – the Urgent Pointer Field is significant. ACK – the Acknowledgement Number field is significant PSH – push function RST – reset connection SYN – synchronize sequence numbers FIN – no more data from sender CCNA 1 -41 Chapter 4
TCP Server Processes Source = 49650 Destination = 80 • Server is listening on Port 80 for HTTP connections. • The client sets the destination port to 80 and uses a dynamic port as its source. CCNA 1 -42 Chapter 4
TCP Server Processes Source = 80 Destination = 49650 • Server replies with the web page. • Sets the source port to 80 and uses the client’s source port as the destination. CCNA 1 -43 Chapter 4
TCP Server Processes Source = 49650 Destination = 80 Source = 80 Destination = 49650 • Notice how the source and destination ports are used. CCNA 1 -44 Chapter 4
TCP Server Processes Source = 49650 Destination = 80 172. 16. 230. 5 Source = 49655 Destination = 80 Source = 80 Destination = 49650 207. 22. 146. 33 Source = 80 Destination = 49655 • What if there are two sessions to the same server? • The client uses another dynamic port as its source and the destination is still port 80. • Different source ports keep the sessions unique on the server. CCNA 1 -45 Chapter 4
TCP Server Processes Source = 49650 Destination = 80 172. 16. 230. 5 Source = 49655 Destination = 80 Source = 80 Destination = 49650 207. 22. 146. 33 Source = 80 Destination = 49655 • How does the Transport Layer keep them separate? • The socket (IP Address: Port) 172. 16. 230. 5: 49650 172. 16. 230. 5: 49655 CCNA 1 -46 207. 22. 146. 33: 49650 207. 22. 146. 33: 49655 Chapter 4
OSI Transport Layer TCP Connection Establishment and Termination CCNA 1 -47 Chapter 4
TCP Three-Way Handshake • For a connection to be established, the two end stations must synchronize on each other's initial sequence numbers (ISNs). • The ISN is the starting sequence number used when a TCP connection is established. CCNA 1 -48 Chapter 4
TCP Three-Way Handshake • Sequence numbers are used to track the order of segments and to ensure that no segments are lost in transmission. • The Flag fields are used to identify the type of segment. CCNA 1 -49 Chapter 4
TCP Three-Way Handshake I wish to connect and here’s my first sequence number. 1 CCNA 1 -50 Chapter 4
TCP Three-Way Handshake OK…. Here’s my first sequence number. I’ve added 1 to your ISN as an acknowledgement. I’ll expect that to be your next sequence number. 1 2 CCNA 1 -51 Chapter 4
TCP Three-Way Handshake OK…. Here’s the next segment with the sequence number you expect. I’ve added 1 to your ISN to acknowledge your segment. I’ll expect that to be your next sequence number. 1 2 3 CCNA 1 -52 Chapter 4
TCP Three-Way Handshake 1 2 3 • Only part of the TCP headers are displayed…. CCNA 1 -53 Chapter 4
TCP Session Termination • Four step process using the Flag and sequence number fields. CCNA 1 -54 Chapter 4
OSI Transport Layer TCP Acknowledgements and Windowing CCNA 1 -55 Chapter 4
TCP Acknowledgements and Windowing • Remember that the Transport Layer must reassemble the segments in the correct order. CCNA 1 -56 Chapter 4
TCP Acknowledgements and Windowing Used together to confirm the receipt of data CCNA 1 -57 Chapter 4
TCP Acknowledgements and Windowing Expectational Acknowledgement ACK = ISN + 1 CCNA 1 -58 Chapter 4
TCP Acknowledgements and Windowing • With a window size of 10, each segment carries only ten bytes of data and must be acknowledged before another segment is transmitted. • Window Size: • The amount of data that can be sent before requiring an acknowledgement. • Determined by the Window field in the header. CCNA 1 -59 Chapter 4
TCP Retransmission • A destination host service using TCP usually only acknowledges data for contiguous sequence bytes. • If one or more segments are missing, only the data in the segments that complete the stream are acknowledged. CCNA 1 -60 Chapter 4
TCP Flow Control CCNA 1 -61 Chapter 4
TCP Flow Control Amount of data that can be sent before an acknowledgement. The initial window size is determined during the three-way handshake. CCNA 1 -62 Chapter 4
TCP Dynamic Window Sizes • TCP Full-duplex Service: • Means data can be flowing in each direction, simultaneously. • Window sizes, sequence numbers and acknowledgment numbers are independent of each other’s data flow. • The receiver sends an acceptable window size to the sender during each segment transmission. • If too much is data being sent, the acceptable window size is reduced. • if more data can be handled, the acceptable window size is increased. • This is known as a Stop-and-Wait windowing protocol. CCNA 1 -63 Chapter 4
Dynamic Window Sizes Initial Window Size CCNA 1 -64 Chapter 4
OSI Transport Layer UDP Communicating with Low Overhead CCNA 1 -65 Chapter 4
User Datagram Protocol (UDP) • Connectionless • “Best Effort” delivery • Low overhead NO THREE-WAY HANDSHAKE CCNA 1 -66 Example Applications Domain Name System (DNS) Online Games Voice over IP (Vo. IP) Dynamic Host Configuration Protocol (DHCP) Trivial File Transfer Protocol (TFTP) Chapter 4
User Datagram Protocol (UDP) • Low Overhead: • Connectionless: • No connection establishment as with TCP. • Unreliable or “Best Effort” delivery: • No error checking • No flow control • No congestion control • No sequence numbers for ordered delivery CCNA 1 -67 Chapter 4
User Datagram Protocol (UDP) • UDP is said to be transaction-based. • When an application has data to send, it simply sends the data. • The UDP protocol data unit (PDU) is referred to as a datagram, although the terms segment and datagram are sometimes used interchangeably to describe a Transport layer PDU. CCNA 1 -68 Chapter 4
User Datagram Protocol (UDP) • If datagrams take multiple paths, they will sometimes arrive in the wrong order. • UDP does not sequence the datagrams as TCP does nor are there any acknowledgements. • Re-sequencing datagrams and handling missing data is up to the application. CCNA 1 -69 Chapter 4
User Datagram Protocol (UDP) IP Packet Header • When a packet is encapsulated at the Network Layer, it is coded to identify the source of the packet (TCP or UDP). CCNA 1 -70 Chapter 4
UDP Server Processes • Like TCP, UDP applications use Well-Known or Registered ports specifically set aside for UDP. • When UDP receives a datagram destined for one of these applications, it forwards the datagram based on its port number. CCNA 1 -71 Chapter 4
UDP Client Processes Source = 53478 Destination = 53 172. 16. 230. 5 Source = 53479 Destination = 53 Source = 53 Destination = 53478 207. 22. 146. 33 Source = 53 Destination = 53479 • Uses the same methods as TCP for keeping multiple conversations separate (Socket - IP Address: Port). 172. 16. 230. 5: 53478 172. 16. 230. 5: 53479 CCNA 1 -72 207. 22. 146. 33: 53478 207. 22. 146. 33: 53479 Chapter 4
Summary: Port Numbers Port Layer 4 Application Number Protocol Description 20 FTP TCP File Transfer Protocol – Data 21 FTP TCP File Transfer Protocol – Control Commands 23 TELNET TCP Terminal connection 25 SMTP TCP Simple Mail Transfer Protocol - Email 53 DNS UDP Domain Name System 67, 68 DHCP UDP Dynamic Host Configuration Protocol 69 TFTP UDP Trivial File Transfer Protocol 80 HTTP TCP Hypertext Transfer Protocol CCNA 1 -73 Chapter 4
Summary: TCP vs UDP • Transmission Control Protocol (TCP): • Connection-oriented • Reliable end-to-end delivery of messages • Error detection and recovery • Flow control • User Datagram Protocol (UDP): • Connectionless • Best-effort datagram delivery • Applications that do not require full TCP services CCNA 1 -74 Chapter 4
Summary: Applications • TCP: • File Transfer Protocol (FTP) • Telnet • Simple Mail Transfer Protocol (SMTP) • Post Office Protocol (POP 3) • Hypertext Transfer Protocol (HTTP) • UDP: • Trivial File Transfer Protocol (TFTP) • Domain Name System (DNS) • Simple Network Management Protocol (SNMP) • Dynamic Host Configuration Protocol (DHCP) CCNA 1 -75 Chapter 4
Am I Drivin’ too fast for ‘ya? CCNA 1 -76 Time to do some STUFF! Chapter 4
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