The Transmission Control Protocol (TCP) Application Services (Telnet, FTP, , WWW) Reliable Stream Transport (TCP) Connectionless Packet Delivery Service (IP) Unreliable Transport Service (UDP)

The Transmission Control Protocol (TCP) TCP is a protocol that specifies: –How to distinguish among multiple destinations on a given machine –How to initiate and terminate a stream transfer –Format of the data and acknowledgments that two computers exchange to achieve a reliable transfer –Procedures the computers use to ensure that the data arrives correctly

Distinguishing Among Multiple Destinations Like UDP, TCP uses protocol ports to identify the ultimate destination within a machine Unlike UDP, TCP defines a connection as the fundamental abstraction for data transfer Connection: –Virtual circuit –Identified by a pair of endpoints

Initiating Stream Transfers Virtual Circuit - simulate a circuit switched network over a packet-switched network –Sender and receiver interact before transfer begins to set up the connection –Once the connection is established a stream transfer can begin –During the transfer, protocol software on the two machines communicate to ensure that data is delivered correctly –Upon completion of the transfer the connection is closed

Initiating Stream Transfers (cont) An endpoint for a TCP connection is defined by a (host, port) pair –Host = the IP address of for a host –Port = a TCP port on that host A TCP connection is defined by a pair of endpoints: –Port 1037 on and port 76 on viper.cs.virginia.edu: ( , 1037) and ( , 76)

Initiating Stream Transfers (cont) Multiple connections to same host: –Port 1037 on and port 76 on viper.cs.virginia.edu: ( , 1037) and ( , 76) –Port 355 on falcon.cs.jmu.edu and port 801 on viper.cs.virginia.edu: ( , 355) and ( , 801)

Initiating Stream Transfers (cont) Multiple connections to same port: –Port 1037 on and port 444 on viper.cs.virginia.edu: ( , 1037) and ( , 444) –Port 355 on falcon.cs.jmu.edu and port 444 on viper.cs.virginia.edu: ( , 355) and ( , 444) No ambiguity - connection identified by both endpoints

Initiating Stream Transfers (cont) Both endpoints must agree to participate: –Passive open - one application program contacts its O.S. and indicates that it will accept an incoming connection –Active open - application program at other end contacts its O.S. and requests a connection Both TCP software modules cooperate to establish a connection Application programs can transmit data (TCP provides reliability)

TCP Data Stream Format Data from application programs is a sequence of octets (with no type associated) TCP divides data into segments for transmission Usually, each segment travels across the internet in a single IP datagram

Review: Unreliable Packet Delivery Service Benefits: –Fast –Cost-effective Drawbacks: –Packet loss, corruption, delay, duplication, out-of-order delivery –Sender might transmit faster than receiver can receive –Dictates a set packet size

The Need for a Reliable Stream Delivery Service Application-level programmers don’t want: –To have to provide reliability in each application –To be constrained by fixed-size packets Application-level programmers do want: –Reliability –Unstructured, stream-oriented service –Full duplex virtual circuit connection –Buffered transfer

Providing Reliability with Acknowledgments and Retransmissions SenderNetworkReceiver Receive ACK 2 Send packet 1 Send ACK 1 Receive packet 1 Send packet 2 Receive ACK 1 Receive packet 2 Send ACK 2

Packet Loss SenderNetworkReceiver Send packet 1 Receive ACK 1 Receive packet 1 Send ACK 1 (Timeout)

ACK Loss SenderNetworkReceiver Send packet 1 Receive ACK 1 (Timeout) Send ACK 1 Receive packet 1 Send ACK 1 (Discard)

ACK Delayed SenderNetworkReceiver Send packet 1 Receive ACK 1 (Timeout) Send packet 1 (Discard) Send ACK 1 Receive packet 1 Send ACK 1 (Discard)

A Problem A simple positive acknowledgment protocol wastes a substantial amount of network bandwidth because it must delay sending a new packet until it receives an acknowledgment for the previous packet.

A Solution SenderNetworkReceiver ACK1 ACK2 ACK3

Sliding Windows All packets inside the window can be transmitted When an acknowledgment for packet 1 is received the window slides to the right and allows the transmission of packet 9: window

Sliding Windows (cont) Receiver must keep a sliding window as well: After packet 1 is received and acknowledged: window

Sliding Windows (cont) Conceptually partitions the packets into three classes: –Transmitted, received, and acknowledged packets (outside and left of sliding window) –Packets being transmitted (inside the window) –Packets waiting to be transmitted (outside and right of sliding window)

Sliding Windows (cont) Only unacknowledged packets are retransmitted –Keep separate timer for each packet Performance : –Depends on the window size and the speed of the underlying network –Note: sliding window protocol with window size = 1 is the same as the simple positive acknowledgment protocol we saw earlier

Sliding Windows: Performance Tuning the sliding window protocol –Increase window size so that sender is transmitting packets as fast as the network can carry them –Eliminates network idle time –Increases throughput (as compared to the simple positive acknowledgment protocol)

Ideal Sliding Window Performance SenderNetworkReceiver

TCP - Efficient Transmission and Flow Control Uses a specialized sliding window mechanism Multiple segments can be sent before an acknowledgment arrives –Efficient transmission Allows receiver to restrict transmission until it has sufficient buffer space to accommodate more data –Flow control

TCP Sliding Windows Operates at the octet level Octets of the data stream are numbered sequentially Octets 1&2 have been sent and acknowledged Octets 3-6 sent but not acknowledged Octets 7&8 can be sent

TCP Sliding Windows Four per (full duplex) connection: –A sends to B –A receives from B –B sends to A –B receives from A

TCP Sliding Windows - Flow Control Each window can vary in size over time –Each ACK contains a window advisement –Specifies how many additional octets of data the receiver is willing to accept –Sender increases or decreases sending window sized based on the receiver’s advise Provides end-to-end flow control

TCP Sliding Windows - Flow Control Example Sender transmits 3 octets Receives an ACK 3 with window advisement = 3 Sender transmits 1 octet

TCP Sliding Windows - Flow Control Example (cont) Receives an ACK 4 with window advisement = 7 Sender transmits 4 octets Receives an ACK 8 with window advisement =

Summary TCP is a transport protocol that specifies: –How to distinguish among multiple destinations on a given machine –How to initiate and terminate a stream transfer –Format of the data and acknowledgments that two computers exchange to achieve a reliable transfer –Procedures the computers use to ensure that the data arrives correctly