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(Kapitel 23: Congestion control and QoS översiktligt.)
Kapitel 22: UDP och TCP. (Kapitel 23: Congestion control and QoS översiktligt.)
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PART V Transport Layer
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Position of transport layer
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Transport layer duties
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Chapters Chapter 22 Process-to-Process Delivery Chapter 23 Congestion Control and QoS
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Process-to-Process Delivery: UDP and TCP
Chapter 22 Process-to-Process Delivery: UDP and TCP
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The transport layer is responsible for process-to-process delivery.
Note: The transport layer is responsible for process-to-process delivery.
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Figure 22.1 Types of data deliveries
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Virtual Connection at the Transport Layer
Router Router Server Host Client Host TCP, UDP IP Application Physical TCP, UDP IP Application Physical IP Physical IP Physical Protocol stack in the host Protocol stack in the host Protocol stack in the router
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Client-Server Paradigm
Used most often in Internet process-to-process communication, for example, , web, file transfer, etc. The client process initiates the communication. The server process waits for the client to initiate communication, and responds by sending the information required. Example: Web server, server, ftp server, etc. A firewall often stops external clients from accessing internal servers, except certain web Opposite: Peer-to-peer communication, where a program can act both as client (taking initiative) and server (responding to other).
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Multiplexing and Demultiplexing
Sender processes Receiver processes Web MP3 Web MP3 TCP UDP TCP UDP IP IP IP datagrams IP datagrams
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UDP Port Numbers User Datagram Applications
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Note: UDP is a connectionless, unreliable protocol that has no flow and error control. It uses port numbers to multiplex data from the application layer.
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Table 22.1 Well-known ports used by UDP
Protocol Description 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 53 Nameserver Domain Name Service 67 Bootps Server port to download bootstrap information 68 Bootpc Client port to download bootstrap information 69 TFTP Trivial File Transfer Protocol 111 RPC Remote Procedure Call 123 NTP Network Time Protocol 161 SNMP Simple Network Management Protocol 162 Simple Network Management Protocol (trap)
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Figure 22.10 User datagram format
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Note: The calculation of checksum and its inclusion in the user datagram are optional.
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Note: UDP is a convenient transport-layer protocol for applications that provide flow and error control. It is also used by multimedia applications.
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22.3 TCP Port Numbers Services Sequence Numbers Segments Connection
Transition Diagram Flow and Error Control Silly Window Syndrome
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Table 22.2 Well-known ports used by TCP
Protocol Description 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
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Figure 22.7 Connection establishment
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Figure 22.8 Connection termination
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Connection-oriented vs. Conectionless
A connection-oriented service requires both sender and receiver to create a connection before any data is transferred TCP provides connection oriented service to the applications, allowing a byte stream to be delivered in order, allthough IP is a connectionless service. A connectionless service does not create a connection first but simply sends the data UDP provides connectionless service to the applications. UDP packets are called datagrams.
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Figure 22.11 Stream delivery
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Figure 22.12 Sending and receiving buffers
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Figure TCP segments
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Example: Connection-oriented Service
An analogy to the connection-oriented service is telephone conversation
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Example: Connectionless Service
An analogy to connectionless service is the delivery of the mail
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Data-link vs. Transport Layer
Data link layer Responsibile for reliability between two directly connected points Transport layer Resposibe for reliability over the internetwork Duties of the data-link layer Network 1 Network 3 Network 2 Internetwork Duties of the data-link layer Duties of the data-link layer Duties of the transport layer
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Reliable vs. Unreliable
Transport layer can offer Unreliable service (UDP) No guarantee that the packet will be delivered to the destination Useful especially for transmitting audio and video files where waiting for acknowledgement can be annoying for the user Reliable service (TCP) Connection establishment Connection maintenance Connection termination
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User Datagram Protocol (UDP)
No reliability or connection management! Serves solely as a labeling mechanism for demultiplexing at the receiver end Use predominantly by protocols that do no require the strict service guarantees offered by TCP (e.g. real-time multimedia protocols) Additional intelligence built at the application layer if needed
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Transmission Control protocol (TCP)
Provides a connection-oriented end-to-end (user-to-user) reliable byte stream service in both directions (full duplex) Divides a byte stream into a sequence of segments and sends them to the destination via IP Uses the destination port, source port to identify the application to which the segment is sent (multiplexing the sessions) Uses sliding window like scheme for flow control and congestion control
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Connection Management
Two way handshake protocol is not enough because of potential delays in either A’s request or B’s responce, as shown below. Possibility of confusion exists. A B A sends a connection request t1 A sends connection request again t2 B receives connection request B establishes a connection and sends an acknowledgement t3 A receives the acknowledgement and establishes a connection t4 A and B exchange data and eventually disconnect B receives connection request B establishes a connection and sends an acknowledgement t5 time time
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Three-way Handshake Protocol for Connection Establishment
A sends a connection request with seq. no. x t1 A sends connection request again with seq. no. y t2 t3 B sends acknowledgement y+1 and seq. no. z A receives the acknowledgement y+1 and sends acknowledgement z+1 t4 The connection is established t5 B sends acknowledgement x+1 and seq. no. w A does not send an acknowledgement and no connection is established t6 time time
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Connection Establishment and Termination
3-way handshake used for connection establishment Randomly chosen sequence number is conveyed to the other end Similar FIN, FIN+ACK exchange used for connection termination Server does passive open Accept connection request Send acceptance Start connection Active open Send connection request SYN SYN+ACK ACK DATA The three-way handshake TCP segments are labeled with SYN. The length of data in the first two is 0
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TCP’s Segments TCP treats data as a sequence of bytes to be divided and sent in segments. The size of the segment depends on the underlying physical network and on the number of bytes the sender is allowed to send (window size) Rather than numbering each segment, TCP stores the sequence number of the data byte in the segment The source and the destination each have separate sequence numbers The acknowledgement numbers are equal to the next expected sequence number
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Window Management in TCP
Sliding window scheme is used with variable window The window can change depending on the traffic in the network (TCP provides congestion control) The size of the window is expressed in bytes instead of packets The window size depends on the receiver’s capabilites and the congestion in the network
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TCP Sliding Window segment 1 100 bytes of data numbered from 1 to 100
100 bytes of data numbered from 701 to 800, ack 101 acknowledge 801 segment 2 acknowledge 101 segment 1 100 bytes of data numbered from 101 to 200, ack 801 acknowledge 901 segment 3 100 bytes of data numbered from 801 to 900, ack 201 acknowledge 201 segment 2
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Example 1 Imagine a TCP connection is transferring a file of 6000 bytes. The first byte is numbered What are the sequence numbers for each segment if data are sent in five segments with the first four segments carrying 1000 bytes and the last segment carrying 2000 bytes? Solution The following shows the sequence number for each segment: Segment 1 ==> sequence number: 10,010 (range: 10,010 to 11,009) Segment 2 ==> sequence number: 11,010 (range: 11,010 to 12,009) Segment 3 ==> sequence number: 12,010 (range: 12,010 to 13,009) Segment 4 ==> sequence number: 13,010 (range: 13,010 to 14,009) Segment 5 ==> sequence number: 14,010 (range: 14,010 to 16,009)
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Note: The bytes of data being transferred in each connection are numbered by TCP. The numbering starts with a randomly generated number.
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Note: The value of the sequence number field in a segment defines the number of the first data byte contained in that segment.
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Note: The value of the acknowledgment field in a segment defines the number of the next byte a party expects to receive. The acknowledgment number is cumulative.
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Figure 22.14 TCP segment format
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Figure Control field
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Table 22.3 Description of flags in the control field
URG The value of the urgent pointer field is valid. ACK The value of the acknowledgment field is valid. PSH Push the data. RST The connection must be reset. SYN Synchronize sequence numbers during connection. FIN Terminate the connection.
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Figure 22.16 Three-step connection establishment
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Figure 22.17 Four-step connection termination
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Table 22.4 States for TCP State Description CLOSED
There is no connection. LISTEN The server is waiting for calls from the client. SYN-SENT A connection request is sent; waiting for acknowledgment. SYN-RCVD A connection request is received. ESTABLISHED Connection is established. FIN-WAIT-1 The application has requested the closing of the connection. FIN-WAIT-2 The other side has accepted the closing of the connection. TIME-WAIT Waiting for retransmitted segments to die. CLOSE-WAIT The server is waiting for the application to close. LAST-ACK The server is waiting for the last acknowledgment.
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Figure 22.18 State transition diagram
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Note: A sliding window is used to make transmission more efficient as well as to control the flow of data so that the destination does not become overwhelmed with data. TCP’s sliding windows are byte-oriented.
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Figure Sender buffer
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Figure 22.20 Receiver window
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Figure 22.21 Sender buffer and sender window
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Figure 22.22 Sliding the sender window
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Figure 22.23 Expanding the sender window
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Figure 22.24 Shrinking the sender window
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Note: In TCP, the sender window size is totally controlled by the receiver window value (the number of empty locations in the receiver buffer). However, the actual window size can be smaller if there is congestion in the network.
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Figure Lost segment
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Figure 22.26 Lost acknowledgment
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Congestion Control and Quality of Service
Chapter 23 Congestion Control and Quality of Service
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Data Traffic Traffic Descriptor Traffic Profiles
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Figure 23.1 Traffic descriptors
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Figure 23.2 Constant-bit-rate traffic
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Figure 23.3 Variable-bit-rate traffic
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Figure Bursty traffic
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Congestion Network Performance
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Figure 23.5 Incoming packet
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Figure 23.6 Packet delay and network load
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Figure 23.7 Throughput versus network load
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Congestion Control Open Loop Open Loop Closed Loop
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23.4 Two Examples Congestion Control in TCP
Congestion Control in Frame Relay
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Note: TCP assumes that the cause of a lost segment is due to congestion in the network.
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Note: If the cause of the lost segment is congestion, retransmission of the segment does not remove the cause—it aggravates it.
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Figure 23.8 Multiplicative decrease
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Quality of Service Flow Characteristics Flow Classes
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Figure 23.12 Flow characteristics
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Figure 23.24 Traffic conditioner
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23.6 Techniques to Improve QoS
Scheduling Traffic Shaping Resource Reservation Admission Control
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Figure FIFO queue
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Figure 23.14 Priority queuing
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Figure 23.15 Weighted fair queuing
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Figure Leaky bucket
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Figure 23.17 Leaky bucket implementation
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Note: A leaky bucket algorithm shapes bursty traffic into fixed-rate traffic by averaging the data rate. It may drop the packets if the bucket is full.
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Figure Token bucket
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The token bucket allows bursty traffic at a regulated maximum rate.
Note: The token bucket allows bursty traffic at a regulated maximum rate.
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