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

TCP, UDP

IP

Application

Physical

IP

Physical

IP

Physical

Router RouterHost Host

TCP, UDP

IP

Application

Physical

Protocol stack in the host

Protocol stack in the host

Protocol stack in the router

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

Used most often in Internet process-to-process communication, for example, email, 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, email 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).

User Datagram Protocol (UDP)

UDP is a connectionless, unreliable protocol that has no flow and error control.

No connection management. It solely provides a mechanism multiplexing data from

the application layer and demultiplexing at the receiver end by means of port numbers.

Use predominantly by protocols that do no require the strict service guarantees offered by TCP (e.g. real-time multimedia protocols).

It is also used for applications that provide flow and error control (e.g. LAN-services such as shared disk access)

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 Table 22.1 Well-known ports used by UDPWell-known ports used by UDP

Port 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 SNMP Simple Network Management Protocol (trap)

Figure 12.19 (b) UDP socket primitives: and their use.

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

TCP Header Fields

Figure 22.15 Control field

Table 22.2 Table 22.2 Well-known ports used by TCPWell-known ports used by TCPPort 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 Finger

  80 HTTP Hypertext Transfer Protocol

111 RPC 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

100 bytes of data numbered from 101 to

200, ack 801

acknowledge 801segment 2

acknowledge 901segment 3

acknowledge 201segment 2

acknowledge 101segment 1

100 bytes of data numbered from 701 to

800, ack 101

100 bytes of data numbered from 801 to

900, ack 201

The bytes of data being transferred in each connection are numbered by TCP. The numbering starts with a

randomly generated number.

NoteNote::

The value of the sequence number field in a segment defines the number of the first data byte contained in that

segment.

NoteNote::

Example 1Example 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?

SolutionSolution

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)

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.

NoteNote::

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.

time time

t1A sends a connection request

t2

t5

t4

t3

A sends connection request again

B receives connection request B establishes a connection and sends an acknowledgement

A B

A receives the acknowledgement and establishes a connection

A and B exchange data and eventually disconnect

B receives connection request B establishes a connection and sends an acknowledgement

Three-way Handshake Protocol for Connection Establishment

time time

t1

A sends a connection request with seq. no. x

t2

t5

t4

t3

A sends connection request again with seq. no. y

B sends acknowledgement y+1 and seq. no. z

A B

A receives the acknowledgement y+1 and sends acknowledgement z+1

B sends acknowledgement x+1 and seq. no. w

A does not send an acknowledgement and no connection is established

t6

The connection is established

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

SYN

SYN+ACK

ACK

DATA

Server does passive open

Accept connection requestSend acceptance

Start connection

Active openSend connectionrequest

The three-way handshake TCP segments are labeled with SYN. The length of data in the first two is 0

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.

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.

NoteNote::

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

If the cause of the lost segment is congestion, retransmission of the

segment does not remove the cause—it aggravates it.

NoteNote::

TCP assumes that the cause of a lost segment is due to congestion

in the network.

NoteNote::

Figure 23.8 Multiplicative decrease

Figure 12.10 TCP congestion control window procedure.

23.6 Techniques to Improve QoS23.6 Techniques to Improve QoS

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

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.

NoteNote::

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 QoS support mechanisms: (a) RSVP principles;

RSVP = Resource Reservation Protocol.A signalling protocol to help IP create a flow and improve QoS.Reserves resources in each router, for unicasting and multicasting traffic flows with guaranteed QoS.

DiffServ architecture.

DiffServ = Differentiated servicesA class-based QoS model.Flows are aggregated into service classes.The IP header Type-of-service fieldis replaced by a DiffServ field.

CR = core routerI/ER = ingress/egress routerMF = multifieldBA = behavior aggregatePHB = per-hop behavior

Figure 14.4 Example showing the sequence of messages exchanged for a local name resolution.

Service classes

CBR = Constant Bit RateVBR = Variable Bit RateABR = Available Bit RateUBR = Unspecified Bit Rate (best-effort)

Figure 23.28 Relationship of service classes to the total capacity