Network and Transport Layers

1
Network and Transport Layers

• After studying this Chapter you should
• Become familiar with
• packetizing and linking to the application layer
• four standard transport/network protocols
• network addressing
• different types of routing
• Understand how TCP/IP works

2
Introduction

• Transport Network Layer Protocols
• TCP/IP, IPX/SPX, X.25, SNA
• Transport Layer Functions
• Interacting with Application Layer
• Packetizing
• End-to-en delivery of application layer messages
• Network Layer Functions
• Addressing
• Routing

3
Introduction

• Transport and Network layers
• Responsible for moving
  messages from end-to-end
  in a network
• Closely tied together
• TCP/IP most commonly used
  protocol
• Used in Internet
• Compatible with a variety of Application Layer
  protocols as well as with many Data Link Layer
  protocols

Application Layer
Transport Layer
Network Layer
Data Link Layer
4
Introduction - Transport layer
Application Layer

• Responsible for end-to-end delivery
• of messages
• Sets up virtual circuits (when needed)
• Responsible for segmentation and
• reassembly
• Breaking the message into several smaller pieces
  at the sending end
• Reconstructing the original message into a single
  whole at the receiving end
• Interacts with Application Layer

Transport Layer
Network Layer
5
Introduction Network Layer

• Responsible for addressing and routing
• of messages
• Selects the best path from computer to
• computer until the message reaches
  
  
  destination
• Performs encapsulation on sending end
• Adds network layer header to message segments and
  passes the message on to the Data Link Layer
• Performs decapsulation on receiving end
• Removes the network layer header at receiving end
  and passes them up to the transport layer
• Both the sender and receiver have to agree on the
  rules or protocols that govern how their network
  layers will communicate with each other.

Transport Layer
Network Layer
Data Link Layer
6
Transport/Network Layer Protocols

• there are many transport and network protocols
• they all perform the same or similar functions
• they are incompatible with each other
• vendors now provide software with multiprotocol
  stacks

7
Protocols

• The four most commonly used protocols are
• TCP/IP
• IPX/SPX
• X.25
• SNA

8
TCP/IP

• Transmission Control Protocol / Internet Protocol
  
• The oldest networking standard (DoD, 1974)
• The most popular protocol suite (70)
• TCP/IP allows reasonable efficient and error-free
  transmission.
• Is a combination of two protocols
• TCP - Transmission Control Protocol
• IP - Internet Protocol

9
Transmission Control Protocol

• TCP performs the packetizing function
• breaking data into smaller packets
• numbering packets
• ensuring reliable delivery of packets
• ordering packets at the destination

10
Internet Protocol

• IP performs network routing and addressing
  functions
• IPv4 - 32-bit address - 192-bit header (24
  bytes)
• IPv6 - 128-bit address - 320-bit header (40
  bytes)
• Both versions have a variable length data field
• Max size depends on the data link layer protocol.
• e.g., Ethernets max message size is 1,492 bytes,
  so max size of TCP message field
• 1492 24 24 1444 bytes

TCP header
IPv4 header
11
TCP Packet
1 Source port number 16 bits 2 Destination port
number 16 bits 3 Sequence number 32
bits 4 ACK number 32 bits 5 Header length 4
bits 6 Unused 6 bits 7 Flags 6
bits 8 Flow control 16 bits 9 CRC 16 16
bits 10 Urgent pointer 16 bits 11 Options
16 bits
12
IP Packet version
IP4
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1 Version number 4 bits 2 Header length 4
bits 3 Type of Service 8 bits 4 Total length 16
bits 5 Identifiers 16 bits 6 Flags 3
bits 7 Packet offset 13 bits 8 Hop limit 8 bits
9 Protocol 8 bits 10 CRC 16 16 bits 11 Source
address 32 bits 12 Destination Address 32
bits 13 Options varies 14 User
data varies 15 Priority 4 bits 16 Flow name 8
bits 17 Next header
IP6
1
15
16
4
17
14
11 (128 bits)
8
12 (128 bits)
13
IPX/SPX

• Internetwork Packet Exchange / Sequenced Packet
  Exchange
• Based on a routing protocol developed by XeroX
  (mid 70s)
• Primary network protocol used by Novell up until
  they released version 5 of Netware

14
IPX/SPX

• Is a combination of two protocols
• SPX - Sequenced Packet Exchange
• breaking the data into smaller packets
• numbering them
• ensuring each packet is reliably delivered
• putting them in proper order at the destination
• IPX - Internetwork Packet Exchange
• routing
• addressing
• Similar to TCP/IP

15
IPX Packet
User Data
1 Checksum 2 bytes 2 Length 2 byte 3
Control 1 byte 4 Type 1 byte 5
Destination address 6 bytes 6 Destination
network address 4 bytes 7 Destination socket 2
bytes 8 Source address 6 bytes 9 Source
network address 4 bytes 10 Source socket 2 bytes
IPX packet
16
SPX Packet
User Data
SPX packet
1 Control 1 byte 2 Type 1 byte 3 Source
ID 2 bytes 4 Destination ID 2 bytes 5
Sequence number 2 bytes 6 ACK number 2 bytes 7
Allocation number 2 bytes
17
X.25

• Developed by ITU-T for use in WANs
• Mature, global standard used by many
  international organizations
• Seldom used in North America except by
  organizations with WANs having non-North American
  connections

18
X.25

• Is a combination of two protocols
• X.3 - responsible for packetizing
• PLP - Packet Layer Protocol
• routing
• addressing
• 128 bytes of data recommended but up to 1024
  bytes supported
• PLP is usually combined with Data Link Protocol
  LAP-B

19
SNA

• Systems Network Architecture
• Developed by IBM in 1974, IBM proprietary
  non-industry standard protocol
• Transmission Control layer performs packetizing
• Path Control layer performs routing and
  addressing
• Requires special equipment to translate between
  LANs and mainframes
• Uses SDLC as its data link layer protocol
• Likely to disappear over time
• IBM now offers TCP/IP on its networks

20
SNA - 7 Layer Model
Application Layer
Presentation Layer
Data Flow Layer
Transmission Control
Path Control
Data Link Control
Physical Layer
21
Transport Layer Functions

• Linking to the Application layer
• Packetizing and Reassembly
• Establishing connections (virtual)
• Connection-Oriented routing
• Connectionless routing
• Quality of Service (QoS)

22
Linking to the Application Layer

• TCP may serve several Application Layer protocols
  at the same time
• Problem Which application layer program to send
  a message to?
• Solution Port numbers located in TCP header
  fields 2-byte each (source, destination)
• Each type of application has a unique port
  address
• Standard port address
• port 80 - Web server
• port 21 - FTP
• port 23 - Telnet
• port 25 - SMTP

23
Application Layer Services
Figure 5.5
24
Packetization Reassembly

• breaking large data messages into smaller packets
  for transmission through the network
• size is dependent of data link layer protocol
• default size without protocol is 536 bits
• size can be negotiated between sender and
  receiver
• numbering packets (sequencing) when needed
• ensuring reliable delivery of every packet
• delivered one at a time or held until all have
  arrived at the destination
• reassembling and ordering packets at the
  destination

25
Connection-Oriented Routing

• Sets up a virtual circuit between sender and
  receiver
• Network layer decides which route the packets
  will be travelling
• transport layer sends a special packet called a
  SYN
• Network layer sends the packets sequentially (all
  packets travel the same path, from source to
  destination)
• packet deliveries are acknowledged
• used by HTTP, SMTP, FTP
• Virtual circuit appears to the application
  software to use point-to-point circuit-switching
• actually uses store and forward switching
• High overhead - open/close of circuit

26
Setting up Virtual Connections
SYN
Requests a virtual circuit (TCP connection) and
negotiates packet size with B
SYN
SYN
Data 1
Data 2
Sends data packets one by one (in order) using
continuous ARQ (sliding window)
ACK 2
Data 3
Data 4
FIN
Closes virtual circuit
not busy
27
Connectionless Routing

• Each packet of a large transmission is treated
  separately and makes its own way through the
  network without a virtual circuit
• Packets may travel different routes and at
  different speeds through the network
• Sequence number must be added to each packet by
  the Network layer
• Network layer at receivers side must reassemble
  packet in sequence

28
Connectionless vs. Connection-Oriented Routing

• TCP/IP,IPX/SPX, X.25, and SNA can all operate as
  connection-oriented or connectionless.
• When connection-oriented routing is needed, both
  the transport layer protocol and network protocol
  are used. The transport layer protocol
  establishes the virtual circuit and the network
  layer routes the messages.
• In TCP/IP when connectionless routing is desired,
  only IP is needed, and the TCP packet is replaced
  with a User Datagram Protocol (UDP) packet.

29
UDP - User Datagram Protocol

• Protocol used for connectionless routing in
  TCP/IP suite (no acks, no flow control)
• Uses only a small packet header
• Only 8 bytes containing only 4 fields
• Source port
• Destination port
• Message length
• Header checksum
• Commonly used for control messages that are
  usually small, such as DNS, DHCP, RIP and SNMP.

30
Quality of Service (QoS) Routing

• A special kind of connection-oriented dynamic
  routing
• Packets are assigned different priorities
• depending on the type of packet sent
• different classes of service are defined to
  determine the priority
• Transport layer specifies the class of service
  when requesting virtual circuit
• Each path designed to support different service
  classes or priorities
• Real-time applications - highest
• A graphical file for a Web page - a lower
  priority
• E-mail - lowest (can wait a long time before
  delivery)

31
QoS - Quality of Service

• QoS parameters
• Availability, Reliability, Timeliness
• Timeliness - timely delivery of packets
• Packets be delivered within a certain period of
  time (to produce a smooth, continuous output
• Required by some applications, especially real
  time applications (e.g., voice and video frames)
• (e-mail doesnt require this)
• QoS Protocols
• RSVP
• RTSP
• RTP

32
Protocols Supporting QoS

• Asynchronous Transfer Mode (ATM)
• A high-speed data link layer protocol
• TCP/IP protocol suite
• Resource Reservation Protocol (RSVP)
• Sets up virtual circuits for general
  purpose real-time applications
• Real-Time Streaming Protocol (RTSP)
• Sets up virtual circuits for audio-video
  applications
• Real-Time Transport Protocol (RTP)
• Used after a virtual connection setup by RSVP or
  RTSP
• Adds a sequence number and a timestamp for
  helping applications to synchronize delivery
• Uses UDP (because of its small header) as
  transport

RTSP
RSVP
RTP
UDP
IP
33
Transport and Network Protocols - Summary
Routing and Addressing
Packetizing
TCP
IP
TCP/IP
IPX
SPX
IPX/SPX
X.25
X.3
PLP
Transmission Control Layer
SNA
Path Control Layer
34
Network Layer Functions

• Addressing
• Each equipment on the path between source and
  destination must have an address
• Internet Addresses
• Assignment of addresses
• Translation between network layer addresses and
  other addresses (address resolution)
• Routing
• Process of deciding what path a packet must take
  to reach destination
• Routing protocols

35
Addressing
Key Concept Each computer has several addresses,
each used by a different layer.
Example Address
Example Software
Address
Application Layer
Web Browser
www.cob.niu.edu
Network Layer
IP
131.156.120.128
Data Link Layer
00-0C-00-F5-03-5A
Ethernet
36
Assignment of Addresses

• Application Layer address (URL)
• For servers only (clients dont need it)
• Assigned by network managers and placed in
  configuration files.
• Some servers may have several application layer
  addresses
• Network Layer Address (IP address)
• Assigned by network managers, or by programs such
  as DHCP, and placed in configuration files
• Every network on the Internet is assigned a range
  of possible IP addresses for use on its network
• Data Link Layer Address (MAC or Physical address)
• Unique hardware addresses placed on network
  interface cards by their manufacturers ( based on
  a standardized scheme)
• Servers have permanent addresses, clients usually
  do not

37
ICANN

• Internet Corporation for Assigned Names and
  Numbers (ICANN)
• manages the assignment of application layer and
  network layer name space or addresses
• sets the rules by which new domain names are
  created and IP address numbers are assigned
• manages a set of Internet domains (.com, .org,
  .net)
• authorizes private companies to become domain
  name registrars
• approves request for application layer addresses
  and assigns IP numbers for those request
• organizations can use any registered company for
  the specific domain for a fee

38
Addressing
Dotted Decimal Notation
IPv4 address is four bytes long (strings of 32
binary bits)
Address Class
0
7 8
31
Host number
Network number
1.0.0.0 to 126.0.0.0 2 Billion
A
0
16 million user addresses
15 16
31
0
1
128.1.0.0 to 191.254.0.0 1 Billion
B
Network number
Host number
0
1
65,000 user addresses
0
1
31
23 24
192.0.1.0 to 223.255.254.0 536Million
Network number
Host number
C
0
1
1
254 addresses
Assigned by ICANN
39
IPv6 Addressing

• Need
• IPv4 uses 4 byte addresses
• Total of three billion possible addresses
• IP addresses often assigned in (large) groups
• Giving out many numbers at a time
• ? IPv4 address space has been used up quickly
  
• e.g., Indiana University uses a Class B IP
  address space (65,000 addresses many more than
  needed)
• IPv6 uses 16 byte addresses
• 3.2 x 1038 addresses, a very large number
• Little chance this address space will ever be
  used up

40
Subnets

• Each organization assigns IP addresses it has
  received to specific computers on its networks
• IP addresses are assigned so that all computers
  on the same LAN have similar addresses (those
  with the same prefix)
• Each of these LANs is known as a TCP/IP subnet
• Any portion of the IP address can be designated
  as a subnet using a subnet mask

41
Subnet Addressing
Figure 5.7
42
Subnets

• Group of computers on the same LAN with IP
  numbers with the same prefix
• Assigned addresses that are 8 bits in length
• For example
• Subnet 128.192.56.x
• Computers in Business (x is between 0 254)
• Subnet 128.192.55.x
• Computers in CS department
• Assigned addresses could be more or less than
  eight bits in length
• For example If 7 bits used for a subnet
• Subnet 1 128.192.56.1-128
• Subnet 2 128.192.56.129-254

43
Subnet Addressing

• Subnet masks are used to make it easier to
  separate the network part of the address from the
  host part.
• Suppose that the first two bytes are the subnet
  indicator with addresses of the form 131.156.x.x
• Then, 131.156.29.156 and 131.156.34.215 would be
  on the same subnet.
• The subnet mask would be 255.255.0.0, which
  corresponds to 11111111.11111111.00000000.00000000
  , where 1 indicates that the position is part of
  the specific subnet (network) and a 0 indicates
  that it is not (it is the host id).

44
Subnet Addressing

• Example 2
• Partial bytes can also be used as subnets.
• For example, consider the subnet mask
  255.255.255.128, which is 11111111.11111111.111111
  11.10000000.
• Here, all computers with the same first three
  bytes and last byte from either 0-127 or 128 to
  254 would be on the same subnet.

45
Providing Addresses

• There are two ways of providing addresses to
  networked computers.
• Static addressing
• Dynamic addressing

46
Static Addressing

• Each computer is given an address through a
  configuration file
• Stored on individual computers
• Problems
• moves, changes, adds and deletes
• individuals could change their own IP address
• network renumbered
• Most companies do not have a good way of tracking
  the addresses

47
Dynamic Addressing

• Server supplies a network layer address
  automatically
• each time user logs in or connects to the network
• for a specific lease period
• eliminates permanent addresses to clients
• when a computer is moved to another location it
  is automatically assigned a new IP address
• Makes efficient use of IP address space
• For example a small ISP with several thousands
  subscribers Might only need to assign 500 IP
  addresses to clients at any one time

48
Dynamic Addressing Programs

• Bootstrap Protocol (bootp)
• Dynamic Host Control Protocol (DHCP)
• Different approaches, but the same basic
  operation
• Software installed on the client instructs the
  client to contact the server using data link
  layer addresses (MAC address)
• Broadcasts a message asking the server to assign
  the client a unique network layer address (IP
  address)
• Server runs corresponding software that sends the
  client its network address and subnet mask
• server maintains a IP address pool for
  distribution
• assigns the same network layer address to the
  client each time the client requests it (bootp)
• lease the network address from the next available
  on a list of authorized addresses for as long as
  the client is connected or for a specified amount
  of time (DHCP)

49
Address Resolution

• The process of
• translating an application layer address (server
  name) to a network address (server name
  resolution)
• translating the network layer address to a data
  link layer address (data link layer address
  resolution)

50
Server Name Resolution

• accomplished by the use of Domain Name Service
  (DNS)
• computers called name servers provide these DNS
  services
• address data base includes server names and
  their corresponding IP address
• Large organizations maintain their own name
  servers
• When a domain name is registered, the IP address
  of the DNS server must be provided to the
  registrar for all URLS in this domain

51
University of North Texas
DNS Server sol.acs.unt.edu 129.120.220.42
DNS Response
DNS Request
Client computer
LAN
DNS Request
DNS Response
Root DNS Server for .EDU domain
Internet
DNS Request
Northern Illinois University
DNS Server netmgr.cso.niu.edu 131.156.1.11
LAN
DNS Response
Figure 5.8 modified
52
MAC Address Resolution

• broadcast message is sent to all computers in its
  subnet to identify the MAC address of the next
  node that the packet must be forwarded to
• if your IP address is xxx.yyy.zzz.ttt, please
  send your data link layer address
• uses Address Resolution Protocol (ARP)
• Broadcast an ARP message to all nodes on a LAN
  asking which node has a certain IP address
• Host with that IP address then responds by
  sending back its MAC address
• Store this MAC address in its address table
• Send the message to the destination node

53
Network Routing

• The process of determining the route or path a
  message will take through the network from the
  sender to the receiver
• centralized
• decentralized
• static routing
• dynamic routing
• broadcast Or multicast routing
• connectionless
• connection-oriented routing

54
Routing Tables

• Routing Tables
• Used to make routing decisions
• Shows which path to send packets on
  to reach a given destination
• Kept by computers making routing decisions
• Routers
• Special purpose devices used to handle
  routing decisions on the Internet
• Maintain their own routing tables

55
Route and Route Table
Computer B Destination Next
Computer Route A A C C D A E E
F E G C
C
B
G
A
F
D
Each node has its own routing table
E
56
Internet Routes
NIU Destination Route U NorthTexas
Texas Oxford Europe U of Toronto Canada U of
Singapore Asia UC Stanford West Coast Other
Other
57
Types of Routing

• Centralized routing
• Decentralized routing
• static
• dynamic
• Other types
• broadcast routing
• multicast routing

58
Centralized Routing

• All routing decisions are made by one computer
• Used on small, mainframe-based networks
• star topologies
• mesh topologies
• Routing tables located on each computer
• central computer sends updated tables as needed
• routing table tells the device where to send
  messages
• Simplicity - no wasted resources
• Hardware failures or changing conditions cause
  table to be out of sync

59
Decentralized Routing

• Each of the following types of routing fall under
  the heading of decentralized routing
• Each device makes its own routing decisions with
  the use of a formal routing protocol
• Routing protocols are self-adjusting
• can automatically adapt to changes in the network
  configuration
• Drawbacks
• slows down the network with status messages
• requires more processing by each computer

60
Static Decentralized Routing

• Routing table developed by the network manager or
  some type of committee
• initial table sent to each computer which then
  updates the routing table as needed
• reroutes as needed with down or removed circuits
• updated when new devices announce their presence
• used in relatively static networks that have few
  routing options

61
Dynamic Decentralized Routing (Adaptive)

• Routing messages over the fastest route
• used when there are multiple routes in the
  network
• improves network performance by selecting the
  fastest route to avoid bottlenecks or busy
  circuits
• initial table developed by network manager
• dynamically updated with changing conditions by
  the devices themselves
• monitors message transmission time or each
  device reports how busy it is to avoid
  bottlenecks
• Disadvantages
• requires more processing by each computer
• wastes network capacity

62
Dynamic Routing Algorithms

• Distance vector
• the number of hops along a route
• exchange information with the neighboring
  computers every few minutes
• Link state
• the number of hops along a route
• the speed of the circuits on the route
• how busy the route is
• exchanges information with other routing devices
  every 15-30 minutes
• tries to determine the fastest/cheapest route
• converges reliable routing information more
  quickly

63
Routing Protocols

• Used to exchange info among nodes for building
  and maintaining routing tables
• Autonomous System (AS)
• A network operated by an organization (e.g.,
  Indiana U.)
• Protocols classified based on autonomous systems
• Types of Routing Protocols
• Interior routing protocols (RIP, OSPF, EIGRP,
  ICMP)
• Operate within a network (autonomous system)
• Provide detailed info about each node and paths
• Exterior routing protocols (BGP)
• Operate between networks (autonomous systems)

64
Routing Protocols

• Border Gateway Protocol
• Internet Control Message Protocol
• Routing Information Protocol
• Open Shortest Path First
• Enhanced Interior Gateway Routing Protocol

65
Routing Protocols

• BGP (Border Gateway Protocol)
• exchanges information between autonomous systems
  about the condition of the Internet
• dynamic distance vector exterior routing protocol
• provides routing on only selected or preferred
  routes
• complex, hard to administer, privacy concerns
• ICMP (Internet Control Message Protocol)
• simple, interior routing protocol used with the
  Internet
• reports routing errors but is limited in the
  ability to update
• RIP (Routing Information Protocol)
• dynamic distance vector interior routing protocol
• counts the number of devices on each route
• selects the route with the least number of devices

66
Routing Protocols

• OSPF (Open Shortest Path First)
• link state interior routing protocol used on the
  Internet
• counts number of computers, network traffic,
  network error rates to select the best route
• only sends updates to routing devices/ no
  broadcasting
• preferred TCP/IP, but also used by IPX/SPX
• EIGRP (Enhanced Interior Gateway Routing
  Protocol)
• link state interior routing protocol developed by
  CISCO
• uses route transmission capacity, delay,
  reliability and load to select best route
• stores multiple routing tables
• SAP (Service Advertisement Protocol)
• Netware servers send SAP advertisements
• Novells broadcast protocol

67
Internet Routing using BGP, OSPF and RIP
RIP
68
Broadcast Routing

• Sends the message to all computers on the network
• Only computer with correct address processes the
  message
• Used only in bus networks
• Wastes network bandwidth

69
Multicasting

• Casting
• Unicast message one computer ? another computer
• Multicast message one computer ? a group of
  computers (e.g., videoconference)
• Internet Group Management Protocol (IGMP)
• Provides a way for a computer to report its
  multicast group membership to adjacent routers
• A special IP address assigned to identify the
  group (class D IP address)
• Routing node sets MAC address to a matching MAC
  address
• When multicast session ends, IGMP sends a message
  to the organizing computer( or router) to remove
  multicast group

70
Requirements for TCP/IP

• This information should either come from an
  internal configuration file or a bootp or DHCP
  server
• IP address
• Subnet mask
• IP address of DNS server
• IP address of router, gateway or switch

71
Known Address, Same Subnet

• A Client (128.192.98.130) requests a Web page
  from a server (www1.anyorg.com) and knows the
  servers IP and MAC address
• Prepare HTTP packet and send it to TCP
• Place HTTP packet into a TCP packet and sent it
  to IP
• Place TCP packet into an IP packet, add
  destination IP address, 128.192.98.53
• Use its subnet mask to see that the destination
  is on the same subnet as itself
• Add servers Ethernet address into its
  destination address field, and send the frame to
  the Web server

72
HTTP response to client

• Server
• Receives the Ethernet frame, performs error
  checking and send back an ACK
• Process incoming frame successively up the layers
  (data link, network, transport and application)
  until the HTTP request emerges
• Process HTTP request and sends back an HTTP
  response (with requested Web page)
• Process outgoing HTTP response successively down
  the layers until an Ethernet frame is created
• Send Ethernet frame to the client
• Operations (performed by the client)
• Receive Ethernet frame and process it
  successively up the layers until the HTTP
  response emerges at browser

73
Known Address, Different Subnet

• Similar to known address, same subnet
• Differences
• Use subnet mask to determine that the destination
  is NOT on the same subnet
• Send outgoing frames to the local subnets GW
• Local gateway operations
• Receive the frame and remove the Ethernet header
• Determine the next node (via Router Table)
• Make a new frame and send it to the destination
  GW
• Destination gateway operations
• Remove the header, determine the destination (by
  destination IP address)
• Place the IP packet in a new Ethernet frame and
  send it to its final destination.

74
Unknown Address

• Operations (by the host)
• Determine the destination IP address
• Send a UDP packet to the local DNS server
• Local DNS server knows the destination hosts IP
  address
• Sends a DNS response back to the sending host
• Local DNS server does not know the destination IP
  address
• Send a second UDP packet to the next highest DNS
  host, and so on, until the destination hosts IP
  address is determined
• Same as previous

75
TCP Connections

• Before any data packet is sent, a connection is
  established
• Use SYN packet to establish connection
• Use FIN packet to close the connection
• Handling of HTTP packets
• Old version
• a separate TCP connection for each HTTP Request
• New version
• Open a connection when a request (first HTTPP
  Request) send to the server
• Leave the connection open for all subsequent HTTP
  requests to the same server
• Close the connection when the session ends

76
TCP/IP and Layers

• Host Computers
• Packets move through all layers
• Gateways, Routers
• Packet moves from Physical layer to Data Link
  Layer through the network Layer
• At each stop along the way
• Ethernet packets is removed and a new one is
  created for the next node
• IP and above packets never change in transit
  (created by the original sender and destroyed by
  the final receiver)

77
Message Moves Through Layers
Figure 5.16
78
Implications for Management

• Most organizations moving toward a single
  standard, TCP/IP
• Decreased cost of buying and maintaining network
  equipment
• Decreased cost of training networking staff
• Telephone companies (having large non-TCP/IP
  networks) moving toward TCP/IP
• Significant financial implications for telcos
• Significant financial implications of networking
  equipment manufacturers