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5-LEC- 5.pptxTransport Layer. Transport Layer Protocols
- 1. Computer Networks LEC #2 CommunicatingOver the Network Computer Networks LEC #5 Transport Layer
- 2. Lecture Outlines Transport Layer. Transport Layer Protocols
- 3. Transport services andprotocols Provide logical communication between application processes running on different hosts. While Network layer communicate between hosts mobile network home network enterprise network national or global ISP local or regional ISP datacenter network content provider network applicatio n transport network data link physical applicatio n transport network data link physical Transport protocols actions in end systems: sender: breaks application messages into segments, passes to network layer receiver: reassembles segments into messages, passes to application layer Two transport protocols available TCP, UDP
- 4. physical link network (IP) application physical link network (IP) application transport TransportLayer Actions Sender: app. msg Application-layer passes message to the transport layer Transport layer determines segment header fields values Transport layer creates segment passes segment to IP transport Th Th app. msg S R
- 5. physical link network (IP) application physical link network (IP) application transport transport Receiver: app.msg extracts application-layer message checks header values Transport layer receives segment from network layer Th app. msg demultiplexes message up to application via socket Transport Layer Actions S R
- 6. UDP segment header sourceport # dest port # 32 bits application data (payload) UDP segment format length checksum length, in bytes of UDP segment, including header data to/from application layer •Source port: It is a 16 bits field that specifies the port number of the sender. •Destination port: It is a 16 bits field that specifies the port number of the receiver. •Checksum: 16 bits are used for a checksum to check if the UDP / TCP header is OK or not. • UDP (User Datagram Protocol) is a connectionless protocol that does not setup a connection…just sends data and it is unreliable, Low overhead and No data-recovery features.
- 7. checksum sender: treat contentsof UDP segment (including UDP header fields and IP addresses) as sequence of 16-bit integers checksum: addition (one’s complement sum) of segment content checksum value put into UDP checksum field receiver: compute checksum of received segment check if computed checksum equals checksum field value: • not equal - error detected • equal - no error detected. But maybe errors Goal: detect errors (i.e., flipped bits) in transmitted segment
- 8. checksum: an example example:add two 16-bit integers sum checksum Note: when adding numbers, a carryout from the most significant bit needs to be added to the result 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 wraparound 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
- 9. checksum: weak protection! example:add two 16-bit integers sum checksum 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 wraparound 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1 0 1 1 0 Even though numbers have changed (bit flips), no change in checksum! Video- Adding in binary system
- 10. 10 TCP (Transmission ControlProtocol) is a reliable transport protocol as it establishes a connection before sending any data and everything that it sends is acknowledged by the receiver. Application Data 32 bits TCP
- 11. TCP 11 Application Data 32 bits •Sequencenumber: the sequence number is a 32 bits field that Specifies the sequence number of the segment. •Acknowledgment number: this 32 bits field is used by the receiver to request the next TCP segment. This value will be the sequence number incremented by 1. The receiver will use this sequence number and sends back an acknowledgment. •DO: It is the 4 bits data offset field, also known as the header length. It indicates the length of the TCP header so that we know where the actual data begins. •RSV: these are 3 bits for the reserved field. They are unused and are always set to 0.
- 12. 12 Application Data 32 bits •Window:the 16 bits window field specifies how many bytes the receiver is willing to receive. •Urgent pointer: these 16 bits are used when the URG bit has been set, the urgent pointer is used to indicate where the urgent data ends. •Options: this field is optional and can be anywhere between 0 and 320 bits. •Flags: there are 9 bits, for control. They used to establish connections, send data and terminate connections (URG, ACK, PSH, RST, FIN, SYN): URG: It is set to 1 if URGENT pointer is in use, which indicates start of urgent data. ACK: It is set to 1 to indicate that the acknowledgement number is valid. TCP
- 13. 13 Application Data 32 bits PSH: It is the push function. This tells an application that the data should be transmitted immediately. SYN: we use this for the initial three-way handshake and it’s used to set the initial sequence number. FIN: this finish bit is used to end the TCP connection. RST: This resets the connection, when you receive this, you have to terminate the connection right away. This is only used when there are unrecoverable errors and it’s not a normal way to finish the TCP connection. TCP
- 14. TCP sequence numbers,ACKs simple TCP scenario User types‘C’ Seq=42, ACK=79, data = ‘C’ host ACKs receipt of echoed ‘C’ host ACKs receipt of‘C’, echoes back ‘C’ Host B Host A Seq=79, ACK=43, data = ‘C’ Seq=43, ACK=80
- 15. TCP Sender (simplified) event:data received from application create segment with seq # seq # is byte-stream number of first data byte in segment start timer if not already running • think of timer as for oldest unACKed segment • expiration interval: TimeOutInterval event: timeout retransmit segment that caused timeout restart timer event: ACK received if ACK acknowledges previously unACKed segments • update what is known to be ACKed • start timer if there are still unACKed segments
- 16. Congestion: informally: “too manysources sending too much data too fast for network to handle” Manifestations مظاهر : • long delays (queueing in router buffers) • packet loss (buffer overflow at routers) different from flow control! Transport Layer Services congestion control: too many senders, sending too fast flow control: one sender too fast for one receiver congestion control
- 17. Flow Control Network hostshave limited resources. (Memory, Bandwidth) Regulating the amount that can be transmitted or received is flow control. Buffering (congestion Avoidance ) 1) If PC-1 sends data to other PC-2, and 2) The PC-2 buffer is full then 3) PC-2 sends STOP message to PC-1 and 4) After PC-2 finishes from its processing. It again sends GO ON message to PC-1 to send the rest of data Transport Layer Services
- 18. Flow Control Windowing 1.In case of sender sends 3 packets (window =3 while the receiver can not receive more than window size 2) 2. The receiver receives two packets only while the third one will lost. And ask the sender to update the window size to 2 (Ack3). So 3. Sender updates the window size and resent packet 3 again Transport Layer Services
- 19. Error Recovery It ispossible for a piece of data to become corrupted or lost. Transport layer protocols can request a retransmission. Transport Layer Services
- 20. Reassembling Segments When created,each segment is assigned a unique application identifier. (Port Number) After reassembly, the port number is used to pass the data to the correct application. 25 25 80 Transport Layer Services
- 21. Multiplexing 21 Multiplexing schemeshave been developed to share lines among many signals. It is a combination of information streams from multiple sources for transmission over a shared medium. Multiplexor: is a device that implements the concept. De-multiplexing: is a separation of a combination back into separate information streams. De-multiplxor: is a device that implements the concept. N - Senders Transport Layer Services
- 22. transport physical link network transport application physical link network transport application physical link network HTTP server client HTTP msg Multiplexingand demultiplexing Scenario Transport Layer Services
- 23. transport physical link network transport application physical link network transport application physical link network HTTP server client HTTP msg Ht HTTP msg TransportLayer Services Multiplexing and demultiplexing Scenario
- 24. transport physical link network transport application physical link network transport application physical link network HTTP server client HTTP msg Ht HTTP msg Ht Hn HTTP msg TransportLayer Services Multiplexing and demultiplexing Scenario
- 25. transport physical link network transport application physical link network transport application physical link network HTTP server client HTTP msg Ht Hn TransportLayer Services Multiplexing and demultiplexing Scenario
- 26. transport physical link network transport application physical link network transport application physical link network HTTP server client HTTP msg Ht HTTP msg HTTP msg Ht Hn TransportLayer Services Multiplexing and demultiplexing Scenario
- 27. transport physical link network transport application physical link network transport application physical link network HTTP server client HTTP msg Ht HTTP msg HTTP msg Ht TransportLayer Services Multiplexing and demultiplexing Scenario
- 28. transport physical link network transport application physical link network transport application physical link network client HTTP msg Ht HTTP msg HTTP msg Q: howdid transport layer know to deliver message to Firefox browser process rather then Netflix process or Skype process? Transport Layer Services
- 29. 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.
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- 31. Port Addressing Typesand Tools • Port numbers are managed and assigned by the Internet Assigned Number Authority (IANA). 21 – FTP Control 23 – Telnet 25 – SMTP 194 – IRC 110 – POP3 20 – FTP Data 443 – HTTPS 520 – RIP 69 – TFTP • Well-Known Ports: •Reserved for common services and applications.
- 32. • Registered Ports: •Itis used with a certain protocol or application. 1863 – MSN Messenger 8080 – Alternate HTTP 2000 – Cisco VoIP 5004 – RTP 5060 – SIP (VoIP) Port Addressing Types and Tools • Private Ports: • It is used by any computer application program to communicate with any other application program.
- 33. Example • What ifthere 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. Source = 49650 Destination = 80 Source = 49655 Destination = 80 Source = 80 Destination = 49650 Source = 80 Destination = 49655 172.16.230.5 207.22.146.33
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Editor's Notes
- #15 The key thing to note here is that the ACK number (43) on the B-to-A segment is one more than the sequence number (42) on the A-toB segment that triggered that ACK Similarly, the ACK number (80) on the last A-to-B segment is one more than the sequence number (79) on the B-to-A segment that triggered that ACK
- #16 Given these details of TCP sequence numbers, acks, and timers, we can now describe the big picture view of how the TCP sender and receiver operate You can check out FSMs in book; let’s just give an English text description here and let’s start with the sender.