TCP

1. TCP CST 415 CST 415 - Computer Networks

2. Topics • Definitions • Motivation • Properties CST 415 - Computer Networks

3. Definitions • TCP – Transmission Control Protocol IP provides unreliable packet delivery. • Send it and forget it TCP sits on top of IP and adds the functionality to turn a unreliable packet delivery into reliable packet delivery. Reliable Delivery – The packet is guaranteed to get to the recipient. CST 415 - Computer Networks

4. Definitions • TCP – Transmission Control Protocol TCP is a stream oriented transport service. • On the application layer, information is transferred as a stream of bits. • The TCP layer breaks the information into octets and groups them into packets. • The sender sends a stream of octets. • The receiver receives the stream of octets in the same order the sender sent them. CST 415 - Computer Networks

5. Motivation for TCP With IP as a transport mechanism: • Packets can be lost in transit across a network. • Fragmented packets may arrive out of order. • Pieces of a fragmented packet may be lost. • Duplicate packets may be delivered. CST 415 - Computer Networks

6. Motivation for TCP If IP were our only method for delivering packets over a network, application programmers would have to manage packet transmission. • Build an ACK/NAK protocol. CST 415 - Computer Networks

7. TCP The definition of TCP provides for an efficient protocol to avoid the necessity of application programs providing their own “ack/nack” protocol. CST 415 - Computer Networks

8. TCP – Properties • Stream Oriented – Sender sends a stream of octets. Receiver receives those octets in the order the sender sent them. • Virtual Circuit Based – Even though the data transmission is connectionless (e.g. over IP), the sender and receiver perceive they are communicating over a circuit (analogous to a phone call). CST 415 - Computer Networks

9. TCP – Properties • Buffered Transfer – Underlying protocol software will choose an optimal transmission size. TCP will buffer data until it gets a packet the size it wishes to send. • To avoid buffering, data must be “pushed” through the stream. • Unstructured Stream – Hey, let’s face it…bits is bits. • Full Duplex Communications – From the application point of view, sending and receiving of information can be done concurrently. CST 415 - Computer Networks

10. Reliability Simple Ack Protocol A Full Success Scenario (nothing lost) CST 415 - Computer Networks

11. Reliability Simple Ack Protocol Packet Lost Scenario CST 415 - Computer Networks

12. Reliability • Sliding window protocol • Set up a window for packet “group” transmission. • Move the window forward as the sequence of packets gets “acked” by the receiver. • Allow the receiver to “ack” groups of packets. • Packets not “acked” will be retransmitted. CST 415 - Computer Networks

13. Reliability – SWP Example CST 415 - Computer Networks

14. Reliability – SWP Example CST 415 - Computer Networks

15. Reliability – SWP Example CST 415 - Computer Networks

16. Reliability – SWP Example CST 415 - Computer Networks

17. Reliability – SWP Example CST 415 - Computer Networks

18. Reliability – SWP Example CST 415 - Computer Networks

19. Reliability – SWP Example CST 415 - Computer Networks

20. Reliability – SWP Example CST 415 - Computer Networks

21. Reliability – SWP Example CST 415 - Computer Networks

22. Reliability • The protocol can work with different Ack/Nack schemes. • Go back N : This is what was shown above. • Assume everything up to the last acknowledged packet is good. Resend everything after. • Selective Nack : Allow the receiver to request retransmission of selected packets. • Timer Retransmit : Set a timer for each sent packet. • If a packet timer expires, resend the packet. CST 415 - Computer Networks

23. TCP – Sliding Window • TCP uses a modification of the sliding window protocol. • Window size is adjusted “on the fly” • This allows the receiver to “throttle” down the sender. • Receiver send a window size along with each ack. • This forces the sender to either • Increase window size – send faster please • Decrease window size – slow down transmission please CST 415 - Computer Networks

24. TCP – Sliding Window • TCP uses a modification of the sliding window protocol. • The window is on a “per byte” granularity. • Ack messages are sent for individual bytes. • The Ack message sends along the sequence number of the next byte to send. • The next byte to send will be the next byte of the longest contiguous sequence of bytes received so far. CST 415 - Computer Networks

25. TCP – Sliding Window • Retransmission is based on a timer. • When TCP sends a segment of information, it will start a timer for that segment. • If the sender does not receive an “ack” containing a sequence number covering the sent segment before the timer expires, the sender will resend the expired segment. • To accommodate varying message latency times, TCP performs continuous monitoring of “send-ack” pairs and adjusts it’s timers dynamically. CST 415 - Computer Networks

26. TCP – Ports/Connections/Endpoints • Just like UDP, TCP uses ports as logical addressing. • Provides associations between peer applications. • Applications are addressed via port and IP address. • This provides a unique address for a specific application running on a specific hardware device. • The combination is known as an “endpoint”. • Unlike UDP, TCP is connection based. • TCP requires a logical division of labor in applications. • TCP must operate in a client/server mode. • Server actively waits for a connection on a specific TCP port. • Clients will connect to that port to begin communication. CST 415 - Computer Networks

27. TCP – Ports/Connections/Endpoints TCP lives in the transport layer along with UDP. Therefore TCP messages will be carried as data in an IP packet. CST 415 - Computer Networks

28. TCP – Ports/Connections/Endpoints Server State Machine CST 415 - Computer Networks

29. TCP – Ports/Connections/Endpoints • Open Port – The server opens the logical TCP port. • Listen – The server listens to the port, waiting for any incoming connection requests. • Spawn Connection Handler – Upon receiving a connection request, the server will spawn off a thread to handle the connection. • At this stage, the server may get a private TCP port and assign it to the handler thread. • Communication will be performed over this private port from within the handler thread. • Confirm Connection – The server sends a confirmation message to the client. • In this confirmation message, the server will send the private port # to be used for further communication. CST 415 - Computer Networks

30. TCP – Ports/Connections/Endpoints • Handle Server Thread – Given the new private logical port: • Wait for activity on the logical port. • Send/Receive protocol is controlled at the application layer by application logic. • This logic is built into the server application handler thread. • Handle Send – Send a message to the peer endpoint. • Handle Receive – Receive a message from the peer endpoint and process it. CST 415 - Computer Networks

31. TCP – Ports/Connections/Endpoints Note: For a TCP server to operate properly, the server must be capable of handling interaction from multiple clients. CST 415 - Computer Networks

32. TCP – Ports/Connections/Endpoints Client State Machine CST 415 - Computer Networks

33. TCP – Ports/Connections/Endpoints • Connection Request – The client requests a connection given the server IP address and TCP port number. • Connection Confirm – The server responds with a confirmation message passing a private logical port on which further communication will be performed. • Handle Client Thread – Given the new private logical port: • Wait for activity on the logical port. • Send/Receive protocol is controlled at the application layer by application logic. • This logic is built into the client/server peer applications. • Handle Send – Send a message to the peer endpoint. • Handle Receive – Receive a message from the peer endpoint and process it. CST 415 - Computer Networks