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School of Information Technologies

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1 School of Information Technologies
Transport Protocols Lesson 12 NETS2150/2850 School of Information Technologies

2 Lesson Outcome understand principles behind transport layer services:
multiplexing/demultiplexing reliable data transfer flow control congestion control learn about transport layer protocols in the Internet: UDP: connectionless transport TCP: connection-oriented transport

3 Position of transport layer

4 Transport layer duties

5 Transport services and protocols
application transport network data link physical logical end-end transport provide logical communication between app processes running on different hosts transport protocols run in end systems (not in ISs) send side: breaks app messages into segments, passes to network layer rcv side: reassembles segments into messages, passes to app. layer

6 Transport vs. Network Layer
network layer: logical communication between hosts transport layer: logical communication between processes relies on, enhances, network layer services

7 Types of data deliveries

8 TCP & UDP Transmission Control Protocol User Datagram Protocol (UDP)
Connection oriented RFC 793 User Datagram Protocol (UDP) Connectionless RFC 768 Stream Control Transmission Protocol (SCTP) – for VoIP SCTP Some protocols in the TCP/IP protocol suite

9 IP addresses & port numbers
IP addresses versus port numbers

10 Well-known ports Port Protocol Description 7 Echo
   7 Echo Echoes a received datagram back to the sender     9 Discard Discards any datagram that is received   11 Users Active users   13 Daytime Returns the date and the time   17 Quote Returns a quote of the day   19 Chargen Returns a string of characters   20 FTP, Data File Transfer Protocol (data connection)   21 FTP, Control File Transfer Protocol (control connection)   23 TELNET Terminal Network   25 SMTP Simple Mail Transfer Protocol   53 DNS Domain Name Server   67 BOOTP Bootstrap Protocol   79 Finger   80 HTTP Hypertext Transfer Protocol 111 RPC Remote Procedure Call 161 SNMP Simple Network Management Protocol

11 Internet transport-layer protocols
application transport network data link physical reliable, in-order delivery (TCP) congestion control flow control connection setup unreliable, unordered delivery: UDP extension of “best-effort” IP network data link physical network data link physical network data link physical logical end-end transport network data link physical network data link physical application transport network data link physical

12 Issues in a Simple Transport Protocol
Multiplexing and Addressing Flow Control Connection establishment and termination

13 Socket Address Process-to-process delivery needs two IDs at each end:
- IP address - Port number

14 Multiplexing/Demultiplexing
gathering data from multiple sockets, enveloping data with header (later used for demultiplexing) Multiplexing at send host: delivering received segments to correct socket/process Demultiplexing at rcv host:

15 How Demultiplexing Works
source port # dest port # 32 bits application data (message) other header fields TCP/UDP segment format host receives IP packets each packet has source IP address, destination IP address each packet carries 1 transport-layer segment each segment has source, destination port number host uses IP addresses & port numbers to direct segment to appropriate socket

16 Connectionless demultiplexing
UDP socket identified by two-tuple: (dest IP address, dest port number) When host receives UDP segment: checks destination port number in segment directs UDP segment to socket with that port number IP packets with different source IP addresses and/or source port numbers directed to same socket

17 Connectionless demux (cont)
Eg.: UDP P2 P1 P1 P3 SP: 6428 DP: 9157 SP: 6428 DP: 5775 SP: 9157 SP: 5775 DP: 6428 client IP: A DP: 6428 Client IP:B server IP: C SP provides “return address”

18 Connection-oriented demux
TCP socket identified by 4-tuple: source IP address source port number dest IP address dest port number recv host uses all four values to direct segment to appropriate socket Server host may support many simultaneous TCP sockets: each socket identified by its own 4-tuple E.g.: Web servers have different sockets for each connecting client

19 Connection-oriented demux (cont)
Eg.: TCP P2 P3 P4 P1 P1 SP: 80 DP: 9157 SP: 80 DP: 5775 SP: 9157 SP: 5775 DP: 80 client IP: A DP: 80 Client IP:B server IP: C

20 TCP Flow Control receive side of TCP connection has a receive buffer:
sender won’t overflow receiver’s buffer by transmitting too much, too fast flow control receive side of TCP connection has a receive buffer: speed-matching service: matching the send rate to the receiving app’s drain rate app process may be slow at reading from buffer J. Kurose and Ross, Computer Networking

21 Connection Establishment and Termination
Allow each end to know the other exists Negotiation of optional parameters by mutual agreement (3-way handshake) Triggers allocation of transport entity resources Only in TCP

22 UDP: User Datagram Protocol [RFC 768]
“bare bones” Internet transport protocol Best-effort service, UDP segments may be: lost delivered out of order to app connectionless: no handshaking between UDP sender, receiver each UDP segment handled independently of others Why is there a UDP? fast: no connection establishment (which can add delay) simple: no connection state at sender, receiver small segment header no congestion control: UDP can blast away as fast as desired

23 UDP (2) often used for streaming multimedia apps loss tolerant other UDP uses DNS (domain name resolution) SNMP (network management) Need to perform application-specific error recovery! 32 bits source port # dest port # Length, in octets, including header length checksum Application data (message) UDP segment format


25 TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581 point-to-point:
one sender, one receiver reliable, in-order send & receive buffers full duplex data: bi-directional data flow in same connection Specifies maximum segment size (MSS) connection-oriented: handshaking (exchange of control msgs) init’s sender, receiver state before data exchange flow controlled: sender will not overwhelm receiver

26 TCP segment structure source port # dest port # application data
32 bits application data (variable length) sequence number acknowledgement number Receive window Urg data pnter checksum F S R P A U head len not used Options (variable length) URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP) J. Kurose and Ross, Computer Networking

27 simple Telnet scenario
TCP seq. #’s and ACKs Seq. #’s: byte stream “number” of first octet in segment’s data ACKs: seq # of next octet expected from other side cumulative ACK time Host A Host B Seq=42, ACK=79, data = ‘C’ Seq=79, ACK=43, data = ‘C’ Seq=43, ACK=80 User types ‘C’ host ACKs receipt of echoed receipt of ‘C’, echoes back ‘C’ simple Telnet scenario

28 TCP Connection Management
Three way handshake: Step 1: client host sends TCP SYN segment to server specifies initial seq # no data Step 2: server host receives SYN, replies with SYNACK segment server allocates buffers specifies server initial seq. # Step 3: client receives SYNACK, replies with ACK segment, which may contain data Recall: TCP sender, receiver establish “connection” before exchanging data segments initialize TCP variables: seq. #s buffers, flow control info (e.g. RcvWindow)

29 TCP Connection Management (cont.)
Closing a connection: Step 1: client end system sends TCP FIN control segment to server Step 2: server receives FIN, replies with ACK. Closes connection, sends FIN. Step 3: client receives FIN, replies with ACK. Step 4: server, receives ACK. Connection closed. client FIN server ACK close closed timed wait

30 TCP Connection Management (cont.)
Could be combined (3-way) Setup Termination

31 Principles of Congestion Control
informally: “too many sources sending too much data too fast for network to handle” different from flow control!! manifestations: lost packets (buffer overflow at routers) long delays (queueing in router buffers) a top-10 problem!

32 TCP congestion control
There is no explicit feedback from network Congestion inferred from end-system observed loss, delay When this happens, TCP reduces its window size Using CWnd and RcvWnd More details in NETS3303/3603!

33 Summary Transport layer establishes the process-to-process communication TCP uses reliable, connection oriented approach Used by FTP, HTTP, Telnet apps UDP uses unreliable, connectionless approach Used by DNS, SNMP and streaming apps like Realplayer and MS Windows Media Player Stallings 20.1, 20.2 and 204.

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