TCP/IP Application Model

1. TCP/IP Application Model Application TCP UDP IP Network Protocols Protocol Stack

2. UDP Connectionless Transport Protocol UDP extends the host-host delivery service of the underlying network into a process-process communication service. • Provides a multiplexing/demultiplexing service to allow multiple processes on each host to share the network. • Ensures correctness of the message by use of a checksum. • Adds no other functionality beyond “best effort” delivery service of the underlying network.

3. 0 16 31 UDP Header Format Destination Port Source Port Checksum Length Data

4. Determining the UDP Checksum PseudoHeader UDP Length Protocol Pad = 00000000 Source IP Address Destination IP Address Source Port Destination Port Length Checksum Data UDP Datagram

5. Application process Application process Application process UDP Ports Queues Packets demultiplexed Packets arrive

6. TCP a Reliable Byte Stream Protocol TCP (Transport Control Protocol) is a connection oriented protocol that guarantees the reliable, in-order delivery of a stream of bytes. • TCP is full-duplex (it supports a pair of byte streams flowing in opposite directions.) • It provides for flow control which allows a receiver to limit the rate at which a sender can transmit. • It implements a congestion control mechanism which throttles the sender from overloading the network. • Like UDP, it provides for multiplexing/demultiplexing of packets

7. TCP Packet Format For reliable, in order delivery For flow control For process demultiplexing Other fields Destination Port Source Port Sequence Number (byte count) Acknowledgement 0 Flags Advertised Window HdrLen Checksum Urgent Pointer Options (variable length) Data

8. SrcPort=21 DstPort=13 ScrPort=13 DstPort=21 ScrPort=13 DestPort=21 SEQ = 1234 SEQ = 6789 SEQ = 1235 ACK = xxxx ACK = 1235 ACK = 6790 5 5 0 0 SYN ACK 1024 1024 5 0 SA 2048 Checksum NIL Checksum NIL Checksum NIL Receiver sends acceptance Initiator prepares ACK Initiator sends ACK The Three-way Handshake Initiator Receiver ScrPort=13 DstPort=21 ScrPort=21 DstPort=13 ScrPort=13 DestPort=21 SEQ = 1235 SEQ = 1234 SEQ = 6789 ACK = 6790 ACK = xxxx ACK = 1235 5 5 0 0 SYN ACK 1024 1024 5 0 SA 2048 Checksum NIL NIL Checksum Checksum NIL Receiver prepares acceptance Initiator sends request to Receiver

9. Demonstration of TCP Byte Transfer TCP Demonstration This applet demonstrates an initial 3-way handshake to establish a TCP connection, then illustrates the use of the advertised window to regulate the exchange of bytes between the two hosts. Current limitations: Buttons are not installed, the processing time is taken to be about 25 seconds for easier viewing.

10. Host B’s State CLOSED active open/SYN passive open close LISTEN LISTEN LISTEN SYN / SYN+ACK Send/SYN SYN_RCVD SYN_SENT SYN / SYN+ACK ACK Close / FIN SYN+ACK / ACK ESTABLISHED FIN / ACK Close / FIN CLOSE_WAIT FIN_WAIT_1 FIN / ACK Close / FIN ACK LAST_ACK CLOSING FIN_WAIT_2 ACK ACK ACK+FIN / ACK TIME_WAIT CLOSED FIN / ACK Host A’s State A Sends SYN TCP Connection Closed Connected A Initiates Close LISTEN SYN_RCVD SYN_RCVD SYN_SENT SYN_SENT ESTABLISHED ESTABLISHED CLOSE_WAIT CLOSE_WAIT FIN_WAIT_1 FIN_WAIT_1 LAST_ACK LAST_ACK FIN_WAIT_2 FIN_WAIT_2 TIME_WAIT TIME_WAIT CLOSED CLOSED

11. Port: An abstraction for locating services Each service is associated with a given port number on a given machine. When you ask for a particular port, you are requesting the service associated with that port number. There are 2 ** 16 available numbers, but the numbers 1 – 1024 are reserved for “well-known” services and should not be used for user initiated services. Port number 7 is used for the echo server and Port number 13 is used for the time and date service It is up to the client to know which port number the desired service is running on.

12. Flow Control • Data is sent over a TCP (virtual) connection as a byte stream • An ACK contains the number of the last byte received and an Advertised Window specifying the number of additional bytes that can be sent. • A sender will set a timer after each TCP packet is sent, and if an ACK is not returned before the timer times out, it will resend that packet. • Problem – on an Internet where the distance to the receiver and the route to this destination are not known a priori, to what value should the timer be set?

13. Flow Control • Determining Timeout value • The timeout value must be determined by sampling the the RTT (round trip time) periodically • The Jacobson-Karels algorithm • Let Estimated RTT be the running average of the measurements Don’t count retransmissions in the sample RTT Diff = SampleRTT - EstimatedRTT EstimatedRTT = EstimatedRTT + δ* Diff where 0 < δ < 1 Deviation = Deviation + δ * (Diff - Deviation) TimeOut = μ* EstimatedRTT + φ * Deviation where by experience μ = 1 and φ = 4

14. Previous AW = 4096 Bytes Last byte sent = 3072 Previous ACK =1025 Byte # 3608 New AW = 1560 Bytes Last ACK = 2049 Use of the AW in Flow Control ACK # + AW size means sender can send AW-size bytes starting with last byte ACKed Additional bytes permitted = 536 Start: (1) Sender has permission to send 4096 bytes starting with byte # 1025 (2) Sender sends 2 packets of 1024 bytes each (3) ACK of first packet is returned with AW = 1560 bytes (reduced from previous) (4) Sender can transmit an additional 536 bytes before having to receive new ACK

15. Congestion Control Flow Control is the restriction placed upon data rate of the sender by the receiver. It is accomplished by using the ACK and AW (advertised window) Congestion Control is implemented by routers along the path that are experiencing congestion. They simply drop an IP packet when they exceed a certain buffer threshold. When a sender does not receive an ACK, or receives duplicate ACKs (indicating that a packet did not get through), it has to reduce its Congestion Window size. The effective window size is the min(AW, Congestion Window size). The Congestion Window is regulated by a sender based upon the feedback it receives from its previous transmissions.

16. Congestion Control The sender “experimentally” tries to determine the maximum achievable throughput on the network. Example below illustrates slow start with multiplicative decrease and additive increase. (Each packet is MSS bytes) packets

17. Congestion Control There are a number of techniques that can be used to “smooth out” this sawtooth pattern a bit. Some of these variations are named after cities in Nevada. TCP-Vegas: The sender periodically measures the RTT and calculates the number of bytes sent between the sending of the first segment and the receipt of the ACK. This information is used to determine a maximum and an effective Congestion Window size and the CW is raised or lowered accordingly. TCP-Vegas tries to minimize the number of times that the CW must be abruptly lowered.

18. The Future? TCP was designed in an era where performance was limited by bandwidth (throughput) With the availability Gb/s and greater data rates, network performance is limited by latency. How does TCP adapt to these new circumstances?