HTTP Overview

1
HTTP Overview

• HTTP - protocol used by servers and clients along
  with others (TCP, IP, URL) to exchange data
• HTTP - simple request/response "conversation
• HTTP/1.0 - TCP connection made for each webpage
  item
• HTTP/1.1 - persistent TCP connections
• HTTP is Stateless - server doesn't remember
  previous requests
• Must use cookies or server's API to maintain
  state
• HTTP uses MIME for data typing and type
  negotiation

2
Multiple HTTP Requests/Responses

• An HTML document may require many HTTP Requests
  and Responses between client and server
• Client (browser) creates initial HTTP-Request-PDU
  for an HTML document
• Server finds document and returns it with
  HTTP-Response-PDU
• Browser reads HTML document sees ltimg
  srcmy.pnggt and sends HTTP-Request-PDU for
  my.png
• Server finds my.png, creates HTTP-Response-PDU
  and returns them
• HTTP/1.0 would require 2 TCP connections.
• HTTP/1.1 needs only 1 TCP connection.

3
HTTP-Request PDU from Client

• Using HTTP, clients make requests of servers for
  resources. The HTTP-Request-PDU consists of
• Client request line request method, URI, HTTP
  protocol versionExample GET /simple.html
  HTTP/1.1
• Header fields metainformation about the
  requestExample Accept image/gif, image/jpeg,
  /
• Blank line separates Header fields from Entity
  body
• Entity body data (e.g., form data) passed to
  server from client if POST method used. GET
  method requests contain no data in Entity body.
  Example nameBobSweeney21

4
Client HTTP Request Methods

• Request Method - determines what to do with
  request. Examples
• The GET Method - it retrieves information from
  server, and Entity Body is empty.
• To pass information to the server it must be
  included in URL (info parameters follow a ?)
• The POST Method allows server to receive data
  from client, usually HTML form data, in Entity
  body
• The HEAD Method like GET except server returns
  only header section. Verification.
• The PUT Method used (along with DELETE method)
  to publish documents to Web server from a client

5
HTTP-Response PDU from Server

• Using HTTP, servers respond to client requests.
  The HTTP-Response-PDU consists of
• Server response status lineprotocol version,
  status code, reason phraseExample HTTP/1.1 200
  OK
• Header fields metainformation about the
  responseExample Server Apache/1.3.29 (Unix)
• Blank line separates Header fields from Entity
  body
• Entity body data (e.g., an html document) sent
  to client. ExampleltHTMLgtltTITLEgt
  Welcomelt/TITLEgt...lt/HTMLgt

6
HTTP-Response Status codes examples

• The Server response status line contains a status
  code and a reason phrase. Here are some common
  ones
• 1XX Informational request received and is being
  processed
• 100 Continue
• 2XX Success client request successful
• 200 OK (most common)
• 204 No Content (Entity body is empty)
• 3XX Redirection
• 301 Moved Permanently (URL no longer valid,
  server returns new URL
• 302 Found (Moved Temporarily) (resource is in
  new location)
• 304 Not Modified (if GET If-Modified-Since
  request)

7
HTTP-Response Status codes

• 4XX Client Errors
• 400 Bad Request (server doesnt understand
  request)
• 401 Unauthorized (information is restricted
  retry with proper authentication)
• 403 Forbidden (client requested data it didnt
  have permission to access)
• 404 Not Found (resource not on server, URL does
  not exist or URL spelling error)
• 5XX Server Errors
• 500 Internal Service Error (unexpected problem
  on server)

8
HTTP Headers

• HTTP Headers - metainformation about an HTTP
  request or responseClient Request Headernames
  (explanation)
• Accept (MIME types browser prepared to accept,
  /)
• Cookie (returns cookie information to server)
• User-Agent (kind of browser making request)
• If-Modified-Since (used for conditional GET
  request)
• Server Response Headernames
• Server (type of server providing the response)
• Set-Cookie (sends cookie information to
  browser)
• Content-Length (bytes of data to be sent to
  client)
• Last-Modified (date/time most recently changed)
• Content-Type (MIME data type being returned to
  client)
• Content-Encoding (how browser is to decode
  document)

9
Internet Protocol Suite

• Requests for Comments (RFCs) define standards for
  TCP, IP, and other Internet Protocol Suite
  protocols
• TCP is RFC793
• IP is RFC791

10
TCP-PDU

• Transmission Control Protocol - Protocol Data
  Unit (TCP-PDU) functions include
• connection management e.g., 3 way handshake
• stream data transfer TCP groups bytes from
  application layer into segments and passes to IP
  for delivery
• reliable end-to-end packet delivery TCP
  sequences bytes with a forwarding acknowledgement
  number indicating next byte expected.
  Unacknowledged bytes are resent
• flow control TCP indicates the highest sequence
  number it can receive without overflowing its
  buffers
• full-duplex TCP can send and receive at the
  same time
• TCP-PDU header consists of a group of fields
  (strings of bits), each field has a specific
  purpose - see Appendix E in text, page 558

11
Connect-oriented v. Connectionless

• TCP is connection-oriented - requires a channel
  to be established before any messages transmitted
• UDP (User Datagram Protocol) is connectionless
  protocol, messages can be sent without
  establishing a connection
• UDP performs similar function to TCP at Transport
  layer but is faster but less reliable than TCP
• UDP used for broadcasts
• see Appendix E in text, page 558

12
TCP Three-Way Handshake

• TCP - reliable protocol but also uses many PDUs
  because of positive acknowledgement
• simple HTTP-Request generates 11 TCP-PDUs
• TCP establishes connection between Host A Host
  B
• A sends a request to open a connection to B (SYN
  flag field),
• B returns an acknowledgement (ACK SYN SYN) to A
• A acknowledges the acknowledgement back to B (ACK
  SYN) and a (virtual) connection is established.
• Three-Way Handshake
• Sequence numbers are synchronized during
  handshake (assure communications between A and B
  are assembled in the proper order, and no missing
  packets exist)
• A similar conversation occurs when closing TCP
  connections involving exchange of FIN and ACK

13
TCP-PDU header fields

• Window Size - used for flow control
• sets the number of bytes (windows size) ok to
  transmit beyond last acknowledgement
• Sequence - identifies each TCP-PDU
• Acknowledgement - specifies the TCP-PDU being
  acknowledged
• if ACK not received, TCP-PDU is resent
• Port - number to identify which application the
  incoming communication stream must be delivered

14
Ports

• Used by TCP and UDP to communicate with
  Application layer programs
• Certain port numbers have been established for
  certain applications by IANA
• Well Known Port numbers between 0 and 1,023
  need super user authority to use reserved for
  certain applications
• Registered Ports between 1,024 and 49,151can
  be used by applications or programs executed by
  users
• Dynamic/Private Ports between 49152 and 65,535
  for dynamically assigned use by applications

15
Network Layer

• Several key protocols handle logical addressing
  at this layer including
• Internet Protocol (IP) - provides a
  connectionless delivery service
• Address Resolution Protocol (ARP) resolves IP
  (logical) address to MAC (physical) address.
• A host needing MAC address sends broadcast to all
  hosts on subnet and only host with that MAC
  responds a globally unique 6 byte address
  assigned to network interface card (NIC) by
  manufacturer and stored in PROM

16
Internet Protocol (IP) Functions

• Routing provides connetionless, best-effort
  delivery of packets through an internetwork (a
  collection of networks, e.g., the Internet)
• Fragmentation dividing packets into fragments
  and reassembling them to support data links in an
  internetwork with different MTU (maximum
  transmission unit) sizes.
• Addressing provides unique number to identify
  each node on each subnet

17
IP-PDU header fields

• see Appendix E in text, page 557
• version field - current version is IPv4 (no
  versions 1-3)
• next version is IPv6
• total length - maximum bytes is 65,535 (216) but
  most are lt1500 bytes
• time to live - number of seconds packet can exist
• Each router reduces number by one, at 0 packet is
  discarded
• protocol transport layer protocol at
  destination where data portion of packet is
  passed (TCP or UDP)
• checksum - checks errors in IP header only, not
  reliable
• packets with bad headers are discarded

18
IP-PDU header fields

• source and destination IP address fields
• see IP Addresses slide for more information
• identification, flags, and fragment offset if an
  IP Packet will not fit in a single Data Link
  layer frame, it must be fragmented for
  transmission
• Ethernet/PPP frames hold a maximum of 1500 bytes
  however many WAN links use 576 byte frames

19
IP Addresses

• IPv4 uses 32 bits in address fields (source and
  destination)
• these numbers are organized into classes (see
  next slide)
• most owners of IP classes use only a fraction of
  their addresses
• dotted decimal notation - consist of four octets
  of numbers between 0 and 255
• octet each group of 8 bits, 28 256
• octets are separated by dots

20
Network Classes (the Network Part) Reference
only, not on exam
Note look for first zero in binary address to
determine class
21
IP Address Components

• IP address has 2 parts network part and local
  part
• Network part - sometimes called the netid
  (prefix)
• identifies the subnet
• Local part - sometimes called the hostid (suffix)
• Subnet Mask delineates netid and hostid used
  to further divide a network into subnets
  Example 255.255.255.0
• On TCP/IP networks, subnets are defined as all
  devices whose IP addresses have the same netid

22
Special IP Addresses

• 127.xxx.xxx.xxx
• Local loopback address. The value of the last 3
  bytes are ignored. The datagram with this IP
  address is never transmitted over the network.
• xxx.255.255.255 or xxx.xxx.255.255 or
  xxx.xxx.xxx.255
• Directed broadcast address. The datagram with
  this IP address is received by all the hosts in
  the specified network. The x represents the
  network ID (netid) bits.

23
IPv6 - IP Version 6

• In 1994, the IETF defined Ipv6 as next IP
  protocol in response to IPv4 Problems
• Address Demand and Depletion
• IPv6 uses 128 bit address fields, 2128 is about
  1040 (each square meter of the earths surface to
  have 1024 addresses)
• IPv4 uses 32 bit address fields, 232 is about
  4.1 billion
• Packet flow - primarily for streaming, IPv6 adds
• Flow Label field added to control the routing of
  special packets
• Priority field used to identify high priority
  services (e.g., email - low, telnet - high)
• Security - IPv4 contains no security fields, IPv6
  adds
• Security field - provides for packet encryption
  using DES
• Authentication field - provides for packet
  authentication

24
IPv6 Compatibility with IPv4

• To solve the problem of how to use both address
  methods at the same time IPv6 puts an IPv4
  address into the IPv6 structure.
• A special address format called an IPv4 mapped
  address has 80 zero bits, followed by 16 one bits
  followed by an IPv4 address. An IPv6 router will
  know that the address is an IPv4 address and
  route it according to the IPv4 rules.
• Tunneling - If an IPv6 address encounters an IPv4
  router then the address space of the IPv4 router
  is insufficient to move the address and data.
• The IPv6 packet is fragmented into several IPv4
  packets and routed through the IPv4 system.
• At some point a router that is capable of both
  methods will reassemble the IPv4 tunneled packets
  back into an IPv6 packet and it is on its way.