Transport Layer：UDP&TCP (Commonalities) End-to-End Communication How a process is identified on Internet: Each process communicating via TCP or UDP is assigned an host-unique 16-bit binary called “port number” ； A port number is either TCP- or UDP-type； A process is uniquely identified by an IP address and its port number. Port numbers in range of 1-1024 are assigned by IANA。

Transport Layer：UDP(1) UDP is end-to-end and connectionless. UDP is NOT reliable per se; UDP does NOT do flow-adjustment; UDP CAN(!) go in broadcast or multicast way( in contrast TCP CANNOT do).

Transport layer：UDP(2)

Transport layer：UDP(3)

Transport layer：UDP(4)

Transport Layer：TCP(1) TCP is end-to-end and connection-oriented; TCP is bi-directional; TCP is reliable: receiver is guaranteed to get exactly whatever is outgoing from the sender; TCPP does flow-adjustment; TCP can support neither broadcast nor multicast(!).

Transport Layer：TCP(2) What sort of reliability issues on IP packets transmitting over Internet？ Packet Loss( because of, e.g., link/gateway saturation、media interference、gateway overloading、interface fault, etc)； Packet Disorder( packets in the same stream arrive at destination host in different order from that of sender) ； Flow-mismatch( Lower-speed receiver cannot catch up with a higher-speed sender, as a result, some packets are dropped).

Transport Layer：TCP(3-1) How TCP resolve those reliability issues？ Rule#1: sequence number TCP regards whatever is in transmission (Email/file data, Web-page, voice, video, etc.) as the ordered byte sequence (called stream). Each byte is assigned an unique sequence number( a 32-bit binary ) which is contiguous with the byte’s position in the stream.

Transport Layer：TCP(3-2) How TCP resolve those reliability issues？ Rule#2: Acknowledgement/resending 1)Everytime TCP sending a block of bytes( a segment), the sender must wait until explicitly acknowledged by its receiver. 2) Everytime TCP receives a block of bytes, the receiver must explicitly acknowledge the sender( by acknowledge number which value is sequence number of the first byte received + the number of received bytes or equivalently 1+sequence number of the last received byte)； 3) If the sender is NOT acknowledged after a period(1-2ms), it must resend the segment again, repeat the procedure until explicitly acknowledged.

Transport Layer：TCP(3-3) How TCP resolve those reliability issues？ Rule#3 Flow Adjustment TCP explicitly repots the receiving buffer size via parameter Windowsize and does flow adjustment via Window algorithm.

Transport Layer： TCP(4) TCP’s connection-oriented communication procedure: phase#1：open connection phase#2：(bi-directional) data transmission phase#3：close connection

Transport Layer： TCP(5-1)

Transport Layer： TCP(5-2)

Transport Layer： TCP(6) What is TCP connection？ A TCP connection is just a group of parameters to indicate peer status under which communication is completely controlled； Which parameters? Sequence number of the currently transmitted bytes, acknowledgement number, current windowsize, etc. On internet each connection is uniquely identified by a 4-tupe: source host’s IP address, source port #, destination host’s IP address, destination port #.

Transport Layer： TCP(7) How is a TCP connection opened (3-way handshaking)? TCP client TCP server SYN=1，Seq# = m SYN=1，Seq#=n, ACK=1, Ack#= m+1 ACK=1，Ack#=n+1 m=base seq# at client side，n=base seq# at server side

m=base seq# at client side TCP(8-1) How is a TCP connection closed? FIN=1，Seq#=M ACK=1，Ack#= M+1 M=acknowledgment number right before the connection-closure； Any side (client or server) can issue a closing-request.

Transport Layer: TCP（8-2） TCP connection-closing is unidirectional(!) Proc.A Proc.B FIN=1，Seq#=M ACK=1，Ack#= M+1 Stream In this example: stream from A to B is closed, in contrast, stream from B to A can still go(!).

Transport Layer：TCP(9-1) How does TCP transmit data: Interactive Mode Proc. A Proc. B 100 bytes ready (new)250B ready Seq#=M, TCP Seg. Size =100 ACK=1，Ack#= M+100 Seq#=M+100, TCP Seg. size=250 ACK=1，Ack#= M+350 Real-world Example: Telnet, etc.

Transfer layer：TCP(9-2) Merge Data Transmission and Acknowledgement Proc#A Proc#B 100B ready Curr. Seq#=M Seq#=M, TCP Seg.size=100 Curr. Seq#=N 500B Ready Seq#=N, TCP Seg.size=500 ACK=1，ACK#= M+100 ACK=1，Seq#= N+500 Each TCP Seg. can carry both data and acknowledgment at the same time

Transfer Layer：TCP(9-3) Pls. Speculate: 1 How does the receiver know the exact number of bytes in a TCP payload? TCP段的载荷长度=IP分组的载荷长度-TCP首长度 What does the TCP Ack#. mean? 若确认号=N, 则接收方A是在向原数据发送方B报告: A期待B下次从第N号字节开始传输数据(记住:TCP把每个字节都编号了!)

Transfer Layer：TCP(9-4) Exercise (to be continued) Proc. A Proc. B SYN=1,序列号= 6999 Base Seq#.=6999 Open Conn. 400B ready Base Seq#=1999 500B Ready SYN=1, Seq#= 1999 , ACK=1，Ack#= 7000 ACK=1，Ack#= 2000 Seq#= 2000, 500B ACK=1, Ack#=2500 Seq#= 7000, 400B ACK=1, Ack#=7400

Transfer Layer：TCP(9-5) (continued) What happens if the receiver cannot receive the whole segment at one time? Proc.A Proc.B Seq#= 7400 1000B 1000B ready Know 400B remained Only 70B accepted Only 600B accepted Accept all 270B ready ACK=1，Ack#= 8000 Seq#= 8000, 400B ACK=1, Ack#= 8400 Seq#= 2500, 270B ACK=1, Ack#=2570

Transfer Layer：TCP(9-6) How does TCP deal with packet loss (I)? Proc.A Proc.B Seq#= 4096, 1000B 1000B ready Start timer timeout Resend/restart timer Clear timer Timeout point Seq#= 4096, 1000B ACK=1, Ack# = 5096 Timeout period is exponentially increased, Why?

Transfer Layer：TCP(9-7) How does TCP deal with packet loss (II)? Proc.A Proc.B Seq# = 90000, 100B 1000B ready start timer timeout Timeout point ACK=1, Ack# = ? (resend) seq# = 90000, 100B ACK=1, Ack# = ? (resend) seq# = 90000, 100B

Transfer Layer：TCP(9-8) Merge the data transmit Proc.A Proc.B 100B： A’s curr. Seq# = M Wait a moment Another 250B ready (Merged bytes)：Seq# =M+100, 250B ACK=1，Ack# = M+100 The sender tries to merge the bytes as far as possible. Example: Telnet

Transfer Layer：TCP(9-9) Pls. reasonably guess how to merge acknowledgments for multi-segments? Proc.A Proc.B A’s curr. Seq#=M Seq# =M, 100B Wait a moment Merge Seq# =M+100, 450B ACK=1，Ack# = M+550 特点: 接收方每次等待1ms或更短，目的是尽量合并确认. 例子: Telnet

Transfer Layer：TCP(9-10) Batch Mode Proc A Proc B Cur. Seq#=M Bold size data ready Ack. time Seg. Size=1000B：each seg.’s seq#=？ ACK=1，Ack# = ？ ACK=1，Ack# =？ Feature: 1) Sender sends lots of (e.g., 4) segments before waiting for the acknowledgements. 2) Receiver acknowledges segments in batch, e.g., acknowledges for every receiving two segments. Example: HTTP carrying image、video, etc.

Transfer Layer：TCP(9-11) Slow Start Proc. A Proc. B Curr. Seq#=M Data ready Ack. time 1000B each seg.：each seg.’s seq# =？ ACK=1，Ack#= ？ ACK=1，Ack# =？

Transfer Layer：TCP(9-12) Example & exercise#1 Proc.A Proc.B Curr. Seq#=78910 Ack. time 1000B each seg. size：each seg.’s seq# =？ ACK=1，Ack# = ？ Question: If the second seg. Is lost，how does TCP behave? Whatever the receiver does, seg.#2、seg.#3、seg.#4 ALL have to be resent ！

Transfer Layer：TCP(9-13) Example & exercise#2 Proc A Proc B Curr.seq#=78910 Ack. time 1000B in each seg.：each seg.’s seq# =？ ACK=1，Ack# = ？ Question: The second segment arrives late，how can the receiver know there is a gap in the upcoming stream? how does TCP behave？

Transfer Layer：TCP(9-14) Example for Flow Adjustment Proc A Proc B 当前接收缓空间大小为X字节 当前接收缓空间饱和 接收区有Y字节空间可用 当前序列号=78910 大量数据就绪 A停止发送任何数据 用下一个待发送字节试探B A恢复传输 每个段1000字节：每段的序列号=？ ACK=1，确认号#，窗口参数=X ACK=1，确认号#，窗口参数=0 序列号#，段长度=1 ACK=1，确认号#，窗口参数=Y>0

Transfer Layer：TCP(9-15) 成批传输情况下如何进行流量调节：例子及练习3-2 B 进程A 当前接收缓空间大小为X字节 当前接收缓空间饱和 接收区有Y字节空间可用 当前序列号=78910 大量数据就绪 A停止发送任何数据 A被动等待 恢复传输 每个段1000字节：每段的序列号=？ ACK=1，确认号#，窗口参数=X ACK=1，确认号#，窗口参数=0 序列号#，段长度=1 ACK=1，确认号#，窗口参数=Y>0