Multimedie och kommunikationssystem Frelsning 11 Kapitel 12 Transportprotokoll
- Slides: 57
Multimedie- och kommunikationssystem Föreläsning 11 Kapitel 12: Transportprotokoll. Flödesstyrning och trafikstocknings hantering. Sliding window.
Transport layer duties
Figure 22. 1 Types of data deliveries
Virtual Connection at the Transport Layer Host Router Application TCP, UDP IP Physical Protocol stack in the host Host TCP, UDP IP IP Physical Protocol stack in the router IP Physical Protocol stack in the host
Figure 22. 5 Socket address A socket is a data flow between two processes that is identified by its socket address pair, i. e. a unique combination of: Transport protocol (UDP or TCP). Source IP address and port number. Destination IP address and port number.
Figure 22. 7 Connection establishment
Figure 22. 8 Connection termination
Client-Server Paradigm q Used most often in Internet process to process communication, for example, email, web, file transfer, etc. q The client process initiates the communication. q The server process waits for the client to initiate communication, and responds by sending the information required. Example: Web server, email server, ftp server, etc. q A firewall often stops external clients from accessing internal servers, except certain web q Opposite: Peer to peer communication, where a program can act both as client (taking initiative) and server (responding to other).
User Datagram Protocol (UDP) q UDP is a connectionless, unreliable protocol that has no flow and error control. q No connection management. q It solely provides a mechanism multiplexing data from the application layer and demultiplexing at the receiver end by means of port numbers. q Use predominantly by protocols that do no require the strict service guarantees offered by TCP (e. g. real time multimedia protocols). q It is also used for applications that provide flow and error control (e. g. LAN services such as shared disk access) q Additional intelligence built at the application layer if needed.
Figure 12. 20 (a) UDP header fields The calculation of checksum and its inclusion in the user datagram are optional.
Table 22. 1 Well-known ports used by UDP Port Protocol Description 7 Echo 9 Discard 11 Users 13 Daytime 17 Quote 19 Chargen 53 Nameserver 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 SNMP Simple Network Management Protocol (trap) Echoes a received datagram back to the sender Discards any datagram that is received Active users Returns the date and the time Returns a quote of the day Returns a string of characters Domain Name Service
Figure 12. 19 (b) UDP socket primitives: and their use.
Transmission Control protocol (TCP) q Provides a connection oriented end to end (user to user) reliable byte stream service in both directions (full duplex) q Divides a byte stream into a sequence of segments and sends them to the destination via IP q Uses the destination port, source port to identify the application to which the segment is sent (multiplexing the sessions) q Uses sliding window like scheme for flow control and congestion control
TCP Header Fields
Figure 22. 15 Control field
Table 22. 2 Well-known ports used by TCP Port Protocol Description 7 Echo 9 Discard 11 Users 13 Daytime 17 Quote 19 Chargen 20 FTP, Data 21 FTP, Control 23 TELNET 25 SMTP 53 DNS 67 BOOTP 79 Finger 80 HTTP Hypertext Transfer Protocol 111 RPC Echoes a received datagram back to the sender Discards any datagram that is received Active users Returns the date and the time Returns a quote of the day Returns a string of characters File Transfer Protocol (data connection) File Transfer Protocol (control connection) Terminal Network Simple Mail Transfer Protocol Domain Name Server Bootstrap Protocol Remote Procedure Call
Figure 22. 11 Stream delivery
Figure 22. 12 Sending and receiving buffers
Figure 22. 13 TCP segments
TCP Sliding Window segment 1 100 bytes of data numbered from 1 to 100 acknowledge 801 segment 2 100 bytes of data numbered from 701 to 800, ack 101 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
Note: The bytes of data being transferred in each connection are numbered by TCP. The numbering starts with a randomly generated number.
Note: The value of the sequence number field in a segment defines the number of the first data byte contained in that segment.
Example 1 Imagine a TCP connection is transferring a file of 6000 bytes. The first byte is numbered 10010. 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)
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.
Connection Management q 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 t 1 connection request A sends connection t 2 request again A receives the acknowledgement and establishes a t 4 connection t 3 A and B exchange data and eventually disconnect time B receives connection request B establishes a connection and sends an acknowledgement B receives connection t 5 request B establishes a connection and sends an acknowledgement time
Three-way Handshake Protocol for Connection Establishment A A sends a connection request with seq. no. x t 1 A sends connection request t 2 again with seq. no. y A receives the acknowledgement y+1 and t 4 sends acknowledgement z+1 A does not send an acknowledgement and no connection is established B t 3 B sends acknowledgement y+1 and seq. no. z The connection is established t 5 B sends acknowledgement x+1 and seq. no. w t 6 time
Connection Establishment and Termination Active open SYN Send connection request SYN+ACK Server does passive open Accept connection request Send acceptance ACK DATA Start connection The three-way handshake TCP segments are labeled with SYN. The length of data in the first two is 0 q 3 way handshake used for connection establishment q Randomly chosen sequence number is conveyed to the other end q Similar FIN, FIN+ACK exchange used for connection termination
Figure 22. 16 Three-step connection establishment 1) Request. 2) Acknowledgement of the request. 3) Acknowledgement of the acknowledgement.
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
Figure 12. 9 TCP sliding window.
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.
Figure 12. 11 TCP congestion window adjustments: (a) on receipt of duplicate ACKs;
Figure 12. 11 TCP congestion window adjustments (b) on expiry of a retransmission timer.
Figure 23. 6 Packet delay and network load
Figure 23. 7 Throughput versus network load
Note: If the cause of the lost segment is congestion, retransmission of the segment does not remove the cause—it aggravates it.
Note: TCP assumes that the cause of a lost segment is due to congestion in the network.
Figure 23. 8 Multiplicative decrease
Figure 12. 10 TCP congestion control window procedure.
23. 6 Techniques to Improve Qo. S Scheduling Traffic Shaping Resource Reservation Admission Control
Figure 23. 12 Flow characteristics
Figure 23. 13 FIFO queue
Figure 23. 14 Priority queuing
Figure 23. 15 Weighted fair queuing
Figure 23. 16 Leaky bucket
Figure 23. 17 Leaky bucket implementation
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.
Figure 12. 21 Real-time transport protocol (RTP) (a) usage A protocol for real time traffic; used in conjunction with UDP.
Figure 12. 21 Real-time transport protocol (RTP) (b) packet format.
Figure 12. 22 Real-time transport control protocol (RTCP) usage. A companion protocol to RTP with messages that control the flow and quality of data and allow the recipient to send feedback to the source or sources.
Figure 23. 5 Incoming packet
Figure 9. 28 Qo. S support mechanisms: (a) RSVP principles; RSVP = Resource Reservation Protocol. A signalling protocol to help IP create a flow and improve Qo. S. Reserves resources in each router, for unicasting and multicasting traffic flows with guaranteed Qo. S.
Diff. Serv architecture. Diff. Serv = Differentiated services A class based Qo. S model. Flows are aggregated into service classes. The IP header Type of service field is replaced by a Diff. Serv field. CR = core router I/ER = ingress/egress router MF = multifield BA = behavior aggregate PHB = per hop behavior
Figure 14. 4 Example showing the sequence of messages exchanged for a local name resolution.
Service classes CBR = Constant Bit Rate VBR = Variable Bit Rate ABR = Available Bit Rate UBR = Unspecified Bit Rate (best effort)
Figure 23. 28 Relationship of service classes to the total capacity