More on TCP/IP Module A Copyright 2001 Prentice Hall

More on TCP/IP

Module A

Copyright 2001 Prentice Hall

A Closer Look at HTTP, TCP, IP, and PPP

Chapter 3

Copyright 2001 Prentice Hall

3

A Closer Look at

HTTP at the Application Layer

TCP (and UDP) at the Transport Layer

IP at the Internet Layer

PPP at the Data Link Layer

A Closer Look at HTTP

HyperText Transfer Protocol

5

HTTP: A Closer Look

World Wide Web Standards

– HTML (HyperText Markup Language) governs the structure of the HTML webpage

– HTTP (HyperText Transfer Protocol) governs the format and timing of requests and responses between the browser and the webserver application program

Browser

WebserverProgramHTML

HTTP

6

HTTP: A Closer Look

Sample HTTP Request Line– All text, often only a single line– Keyword<sp>absolute path<sp>HTTP/

version– Keyword (Method) is GET for Retrievals

• GET must be capitalized

– Absolute path is URL without http://hostname

GET /report1/home.htm HTTP/1.1

Absolute Path

7

HTTP: A Closer Look

Sample HTTP Request Header

– Although the book only discusses the request-line, if only the absolute path is give, the request line is supposed to be followed by a header line naming the host.

– So if the host is puka.hawaii.edu, the request-line and header would be (where CRLF is carriage return followed by line feed):

GET /report1/home.htm HTTP/1.1[CRLF]

Host:puka.hawaii.edu

New

8

HTTP: A Closer Look Sample HTTP Response Message

– Header is multiple lines of text– Each ends with [CRLF]

HTTP/1.1 200 OK[CRLF]Date: Tuesday, 20-JAN-1999 18:32:15 GMT[CRLF]Server: name of server software[CRLF]MIME-version: 1.0[CRLF]Content-type: text/plain[CRLF][CRLF]File to be downloaded.

9


– 200 is a code for a successful retrieval, followed by the humanly readable code, OK


10


– There are other codes, indicating errors, such as 404

– Followed by a humanly readable code, such as, host not found

HTTP/1.1 404 Host not Found[CRLF]

11


– MIME-version and Content-type tell the type of file being retrieved

– For HTML document, text/plain


12

HTTP: A Closer Look HTML Pages Contain Text

– To be displayed HTML Pages Contain Tags

– Some tags say “place a graphic here”, “place a JAVA applet in a box here”, etc.

– For instance, the tag <img src=“big.jpg”> says “Get file big.jpg and place it here in the HTML document”

– Graphics, Java programs, etc. are separate files

13

HTTP: A Closer Look

Downloading a “page” may require several downloads– One for the HTML document– One each for the other files it calls for

HTML

Big.jpg

App.java

WebserverUserPC

File Downloads

2

3

1

14

HTTP: A Closer Look

Each download requires a separate HTTP request-response cycle!– So downloading complex webpages

may require many HTTP request-response cycles

HTML

Big.jpg

App.java

WebserverUserPC

HTTPRequest-Response

Cycle 1

2

3

15HTTP: A Closer Look How Many HTTP messages will be created

to download the three files?– 3 HTTP request-response cycles– 2 HTTP messages per cycle (request and

response)– 6 HTTP messages total

HTML

Big.jpg

App.java

WebserverUserPC

HTTP Request-ResponseCycle 1

2

3

16

HTTP: A Closer Look HTTP is Unreliable

– There is no error detection and correction for transmission errors

– This helps makes HTTP simple, allowing browsers and webserver application programs to be simple and inexpensive

– So Unreliability can be good!

17

HTTP: A Closer Look HTTP is Unreliable

– As we will see later, TCP at the transport layer is reliable, offering error detection and correction

– TCP gives HTTP clean data, so there is no need for HTTP to do error checking

– Unreliability saves processing time on the source and destination hosts, reducing host costs

– Unreliability makes browsers and webservers easier to write, reducing their costs

18

HTTP: A Closer Look

HTTP is Connectionless

– Connection-oriented services establish a connection, maintain it for the duration of a communication, and then break the connection

– For instance, in a telephone call, people say hello (agreeing to talk), note if the line goes dead, and agree to break the connection (we don’t simply hang up on the other person)

19

HTTP: A Closer Look


– Connectionless services merely send the message without a connection. Like sending a letter

– HTTP is connectionless. Just send requests, get responses. No opens, maintenance, or closes

BrowserWebserverApplication

HTTPRequest

20

HTTP: A Closer Look


– Connectionless operation is simple because of the lack of opens, maintenance, and closes

– This makes HTTP simple, allowing browsers and webservers to be built easily

BrowserWebserverApplication

HTTPRequest

TCP: A Closer Look

Transmission Control Protocol

22

TCP: A Closer Look Browser does not send HTTP Requests

directly to the webserver application– The application layer programs are not

physically connected– Browser sends HTTP Request to the

user PC’s transport layer process for delivery

Browser

TransportProcess

HTTP Request

23

TCP: A Closer Look

Transport layer process stores the HTTP Request Temporarily

– Does NOT merely add a TCP header and then pass the TCP segment down to the Internet layer process as noted in Chapter 2

Browser

TransportProcess

TransportProcess

Request

24

TCP: A Closer Look

User PC transport process opens a connection to the webserver transport layer process– This connection can be used to send

several TCP segments to handle a several HTTP request-response cycles

Browser

TransportProcess

TransportProcess

25

TCP: A Closer Look TCP has a Flags Field

– Six one-bit flags

Source Port # (16) Destination Port # (16)

Sequence Number (32 bits)

Acknowledgement Number (32 bits)

Hdr Len(4) Flags (6) Window Size (16)

Options (if any) PAD

Bit 0 Bit 31

Reserved (6)

TCP Checksum (16) Urgent Pointer (16)

TCP Segment

Data Field

26

TCP/IP: A Closer Look

Three Flags are Widely Used– SYN bit set to 1 in a TCP segment to

request a connection– ACK bit set to 1 in a segment to

acknowledge a received TCP segment– FIN bit set to 1 in a segment to inform of

a connection closure

Hdr Len(4) Flags (6) Window Size (16)Reserved (6)

27

TCP: A Closer Look Opening a connection takes 3 Segments

– First TCP segment has SYN flag set. Requests connection to webserver transport process

User PCTransportProcess

WebserverTransportProcess

SYN

28


– Second TCP segment asks to open a connection and also acknowledges the first SYN message (ACK flag set too)



SYN

SYN, ACK

29


– Third TCP segment acknowledges the second

– All segments are acknowledged except for pure ACKs like the third segment



SYN

SYN, ACK

ACK

30TCP: A Closer Look Next, user PC sends the HTTP Request

– Sends HTTP Request in data field of a TCP segment

– Will receive an ACK TCP segment to acknowledge receipt



TCP segment containingHTTP Request

ACK

31

Delivering the HTTP Response Transport layer process on the webserver

receives the TCP segment delivering the HTTP Request– The transport process on the webserver

passes the HTTP Request in the TCP segment data field to the webserver application program

WebserverApplication

TransportProcess

HTTP Request

32

Delivering the HTTP Response Webserver application creates the HTTP

Response message– Webserver application passes the HTTP

Response message to the webserver transport layer process for delivery to the user PC transport layer process

WebserverApplication

TransportProcess

HTTP Response

33

Delivering the HTTP Response Webserver Transport Process Delivers the

Response– User PC transport process sends an

acknowledgement



TCP segment containingHTTP Response

ACK

34

TCP: A Closer Look Multiple HTTP Request-Response Cycles can be

Handled with a Single TCP Connection in HTTP/1.1 and later versions– Request message must have Connection:

Keep-Alive

Each will have the following:– TCP segment carrying the HTTP request– Acknowledgement of this segment– TCP segment carrying the HTTP response– Acknowledgement of response segment

35TCP: A Closer Look Closing the Connection takes 4 TCP

Segments– Initiated by the browser when it

downloads the last file



FIN

FIN

ACK

ACK

36

TCP: A Closer Look Refinement

– If you respond quickly to a letter, often include acknowledgement of letter you received in your response letter

– TCP segments can send new information and acknowledge a segment received

• Ex. SYN, ACK at start of connection

– If response will take time, however, send a pure ACK message

37

TCP Connections: Recap For EACH Page Download…

– 3 TCP segments to open the connection

– 4 TCP segments per file downloaded

• More if file downloaded is long, because may take multiple segments to download

– 4 TCP segments to close the connection

– A very chatty protocol

38

TCP: Error Handling TCP is reliable--it does error correction

– Each TCP segment has a 32-bit sequence number that increases with each TCP segment a transport process sends



Bit 0 Bit 31TCP Segment

39

TCP: Error Handling TCP is reliable--it does error correction

– ACKs acknowledge a specific TCP segment in the sequence

– If a TCP segment is not acknowledged, the other side retransmits it after awhile

– This adds TCP segments beyond the ones we have seen

40

TCP: Error Handling TCP is reliable--it Places TCP Segments in

Order

– TCP segments are encapsulated in IP packets

– IP does not guarantee that packets will arrive in order

– TCP can place TCP segments in order based on their sequence numbers

41

TCP: Error Handling TCP Reliability in Perspective

– Provides clean application data to application program

– If data link layer frame or IP packet is lost at lower layers, receiving transport process will not acknowledge a segment

– The sending transport process will resend automatically

– So TCP protects against errors at lower layers as well as transport layer errors

42TCP: Why Connections? Opens, closes, and ACKs create overhead,

so undesirable

Why do it?– Allows sequence numbers, so that

errors can be handled easily

Overall, TCP is a high-overhead, highly reliable transport layer protocol that catches any errors at lower layers, giving “clean” service to the application programs

43

TCP: Refinements

Fragmentation

– Transport process must fragment long application messages (break them into several pieces) and transmits them in several TCP segments

– So when large files are downloaded in webservice, many more TCP segments will be generated than we discussed above

Mod A

App Frag 2 TCP-H App Frag 1 TCP-H

44

TCP: Refinements

Flow Control

– TCP implements flow control, so that one transport process will not overload its peer by transmitting too quickly

Mod A



Data

ACKCommand toLimit Speed

45

User Datagram Protocol (UDP) UDP

– User Datagram Protocol– Alternative to TCP at the transport layer– Lightweight

• No connections (connectionless)

• No error correction (unreliable)

• Reduces work source and destination host must do

46

User Datagram Protocol (UDP) UDP

– Good for voice, for which the delay of error correction would be unacceptable

– Good for applications for which the cost of an occasional message is small compared to the cost of connection-oriented service

More on TCP Acknowledgements

Sequence Number Field

Initial Sequence Number

Acknowledgement Number Field

48

TCP

TCP is Reliable

– IP packets carrying TCP segments may arrive out of order

– TCP must put the TCP segments in order

3 4 2 15

49

TCP

TCP is Reliable

– Each correct TCP segment is acknowledged by the receiver

SourceTransportProcess

SourceTransportProcess

DestinationTransportProcess

DestinationTransportProcess

TCP SegmentTCP Segment

ACKACK

50

TCP Segment Each TCP segment sent by a side must

have a sequence number

– Simplest: 1,2,3,4,5,6,7

– To detect lost or out-of-sequence messages

– TCP uses a more complex approach

11 44 22 55

3?

51

TCP Sequence Numbers

TCP header has a 32-bit sequence number field to identify a TCP segment’s position in a series of segments






Reserved (6)


Data Field

52


Initial Sequence Number is randomly selected by the sender; Say, 79

Sent in the sequence number field of the first TCP segment

79

TCP Data Field

TCP Header

Sequence Number Fieldwith Initial Sequence Number (79)

53


Data octets in data fields of all segments in a connection are viewed as a long string

TCP Segment 1 79

TCP Segment 2 808182

TCP Segment 3 8384

3 Octets in Data Field

2 Octets in Data Field

ISN

54


Supervisory segments, which contain a header but no data, are treated as carrying a single octet of data

TCP seg 1 898899

TCP seg 2 900

TCP seg 3 901902…

Supervisory segment

Carries data

Carries data

55


Sequence number field gets the value of the first octet in the data field

TCP 1 79

TCP 2 808182

TCP 3 8384

80 is SeqNum Field Value



56

TCP Acknowledgements

Acknowledgement must indicate which TCP segment is being acknowledged

SourceTCP

Process

SourceTCP

Process

DestinationTCP

Process

DestinationTCP

Process

TCP SegmentTCP Segment

ACKACK

57

TCP Acknowledgements

TCP header contains a 32-bit Acknowledgement Number field to designate the TCP segment being acknowledged






Reserved (6)


Data Field

58

TCP Acknowledgment Numbers

Acknowledgement Number field contains the next byte expected--the last byte of the segment being acknowledged, plus one

TCP 1 79

TCP 2 808182

TCP 3 8384

83 is AckNum Field Value



59

TCP Acknowledgement Number

Quiz: A TCP segment contains the following data octets– 567, 568, 569, 570, 571, 572, 573, 574

What will be in the sequence number field of the TCP segment delivering the data?

What will be in the acknowledgement number field of the TCP segment acknowledging the TCP segment that delivers these octets?

60

TCP Flow Control

Flow Control– One TCP process transmits too fast– Other TCP process is overwhelmed– Receiver must control transmission rate– This is flow control

TCP Process TCP Process

Too MuchData

Flow Control Message

61

TCP Flow Control

A TCP segment has a Window Size field– Used in acknowledgements






Reserved (6)


Data Field

62

TCP Flow Control

A TCP segment has a Window Size field– Tell how many more octets the sender

can send beyond the segment being acknowledged

TCP Process TCP Process

Data

Acknowledgement with Window Size Field

63TCP Flow Control Example

– TCP segment contained octets 45-89– Acknowledgement number for TCP

segment acknowledging the segment is 90

– If Window Size field value is 50, then– Sender may send through octet 140 (90

plus 50)– Must stop before 50 unless the window

is extended in another acknowledgement

64

TCP Flow Control

Each Acknowledgement extends the window of octets that may be sent– Called a sliding window protocol

1-44 45-79 80-419 420-630

400May send through 480

1-44 45-79 80-419 420-630

500May send through 920

65

TCP Fragmentation

TCP Segments have maximum data field sizes– (Size limit details are discussed later)– What if an application layer message is

too large?

TCP HeaderTCP Data Field Max

Application Layer Message

66

TCP Fragmentation

Application layer message must be fragmented– Broken into several pieces– Delivered in separate TCP segments


App Frag 1 App Frag 2 App Frag 3

67

TCP Fragmentation

Note that, in TCP fragmentation, the TCP segment is NOT fragmented– The application layer message is

fragmented


App Frag 1 App Frag 2 App Frag 3

68

TCP Fragmentation Transport layer process on the source

host does the fragmentation– Application layer on the source host is

not involved– Transparent to the application layer

Application

Transport

Internet

Application Message

TCP Segment TCP Segment

69

TCP Fragmentation Transport layer process on the destination

host does the reassembly– Application layer on the destination host

is not involved; Gets original application layer message

Application

Transport

Internet

Application Message

TCP Segment TCP Segment

70

TCP Fragmentation

What is the maximum TCP data field size?– Complex

Maximum Segment Size (MSS)– Maximum size of a TCP segment’s data

field– NOT maximum size of the segment as

its name would suggest!!!

71

TCP Fragmentation

MSS Default is 536 octets

– Minimum IP packet size any network must support is 576 octets

• Larger IP packets MAY be fragmented

– IP and TCP headers are 20 octets each if there are no options

– This gives the default MSS of 536

– Smaller if there are options in the IP or TCP header

72

TCP Fragmentation

MSS Default is 536 octets

– Suppose the application layer process is 1,000 octets long

– Two TCP segments will be needed to send the data

– The first can send the first 536 octets

– The second can carry the remaining 464 octets of the application layer message

73

TCP Fragmentation

Each side MAY announce a larger MSS

– An option usually used in the initial SYN message it sends to the other

– If announces MSS of 2,048, this many octets of data may be sent in each TCP segments

– 536 is only the default—the value to use if no other value is specified by the other side

More on Internet Layer Processes

75

Mask Operations

Masks were introduced in Chapter 3

IP addresses alone do not tell you the size of their network or subnet parts

Network Mask– Has 1s in the network part– Has 0s in the remaining bits

Subnet Mask– Has 1s in the network plus subnet parts– Has 0s in the remaining bits

76

Multiplexing

IP packets can carry different things in their data fields

– TCP segments

– UDP datagrams

– ICMP supervisory messages (later)

– RIP messages (later)

IP Data Field IP Header

77

Multiplexing

We say that IP can multiplex (mix) different types of traffic in a stream of IP packets

UDP IP-H TCP IP-H UDP IP-H ICMP IP-H

Stream of Arriving or Outgoing IP Packets

Single IP PacketCarrying UDP Datagram

78

Multiplexing IP process must pass contents of arriving

IP packets to the correct process for subsequent handling

IP

TCP UDP

ICMPUDP IP-H

IP ProcessArrivingPackets

79

Multiplexing IP process must also accept messages

from multiple processes and multiplex them on an outgoing stream

IP

TCP UDP

ICMPUDPIP-H

IP ProcessOutgoingPackets

Documents

More on TCP/IP Module A Copyright 2001 Prentice Hall