Technological Prerequisites

1
Technological Prerequisites

• George Macri
• ltgmacri_at_homemail.comgt
• ROMTELECOM S.A.
• Romania
• 5th Network Technologies Workshop
• .

2
Technological Prerequisites

• Internetworks
• Internet Protocols
• Internet Addresses
• Routing
• Subneting
• CIDR

3
What internetworks are

• Start with lots of little networks
• Many different types
• ethernet, dedicated leased lines, dialup, ATM,
  Frame Relay, FDDI
• Each type has its own idea of addressing and
  protocols
• Want to connect them all together and provide a
  unified view of the whole lot

4
The unifying effect of the network layer

• Define a protocol that works in the same way with
  any underlying network
• Call it the network layer
• routers operate at the network layer
• There are defined ways of using
• protocol over ethernet, ATM, FDDI
• protocol over serial lines (PPP)
• protocol over almost anything

5
The 7 Layer OSI Model
6
Protocol Stacks

• Layers

Applications
TCP / UDP
Transport layer
IP
Network layer
atm
x.25
hdlc
ethernet
token ring
dialup
frame relay
7
Layer Functions
Mail, Web etc.
Application
Presentation
Session
Transport
TCP
End to end reliability
Forwarding best-effort
IP
Network
Data Link
Packet delivery
Physical
Raw signal
8
ISO seven layer model

• 1 Physical layer
• moves bits using voltage, current, light, etc.
• 2 Data Link layer
• bundles bits into frames and moves frames between
  hosts on the same link

9
ISO seven layer model

• 3 Network layer (e.g. IP)
• Makes routing decisions
• uses destination address in packet
• Forwards packet hop by hop
• encapsulates network layer packet inside data
  link layer frame
• different framing on different underlying network
  types
• Unreliable
• Single address space for the entire internetwork

10
ISO seven layer model

• 4 Transport layer (e.g. TCP)
• end to end transport of datagrams
• encapsulates datagrams in network layer packets
• adds reliability by detecting and retransmitting
  lost packets
• uses acknowledgements and sequence numbers to
  keep track

11
ISO seven layer model

• 5 Session layer
• not used in the TCP/IP network model
• 6 Presentation layer
• not used in the TCP/IP network model
• 7 Application layer
• Uses the underlying layers to carry out work

12
Layer interaction
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Network
Network
Link
Link
Link
Link
Physical
Physical
Physical
13
INTERNET PROTOCOLS

• Internet protocols
• can be used for communications between
  heterogeneous systems
• can be used for communications between systems
  connected in a LAN
• can be used for communications between systems
  connected in a WAN
• can be used for communications between a set of
  interconnected networks
• Documents called RFCs (Requests For Comments),
  which are reviewed and analyzed by the IETF
  community improvements, additions and
  refinements of protocols are published in new
  RFCs (see ftp//ftp.rs.internic.net.,
  ftp//ftp.ripe.net/).
• Looking at all RFCs, you can see the history of
  the development of Internet protocols, people and
  companies that have contributed to this
• TCP and IP are the best known of the Internet
  protocols and very often the term TCP/IP refers
  to the whole family of protocols.

14
TCP/IP Model
Message Segment Datagram Frame Bit
5 4 3 2 1
15
TCP/IP is a 5 Layered model

• Layers 1 and 2 are not actually defined by TCP/IP
  , as TCP/IP was defined to be independent of
  physical media .

16

• Layer 3 is the Internet Protocol (IP) layerThis
  provides a basic datagram service
• ICMP (Internet Control Message Protocol) is
  normally provided in this layerICMP reports
  problems in transmission of datagrams
• ARP (Adress Resolution Protocol)
• RARP (Reverse Address Resolution Protocol)

17

• In layer 4 are 2 possible protocols TCP
  (Transport Control Protocol) and UDP (User
  Datagram Protocol) .
• TCP provides a reliable service with error
  correction and flow control .The cost of
  providing a reliable service is more overhead in
  connection setup and closedown, processing power
  for correcting errors and data transmission, but
  some applications need reliability irrespective
  of cost.
• UDP just extends IPs connectionless datagram
  service to applications that do not require
  reliability .UDP datagrams can be sent to a
  network without the overhead of creating and
  maintaining a connection

18

• Layer 5 is the Application layerThis layer
  provides services suitable for the different
  types of application that might wish to use the
  network .It does not provide the application
  itself .For example SMTP , FTP , Telnet ...

19
TCP/IP
20
Internet Protocols
NFS RPC
FTP RFC 959
SNMP
RIP RFC 1058
Routing protocols BGP OSPF IGRP EIGRP
Telnet RFC 854
SMTP RFC 821
DNS RFC 1035
ICMP RFC 792
TCP RFC 793
UDP RFC 768
IP RFC 791
X.25
ARP RFC 826
PPP
HDLC
SLIP
LAPB
Ethernet/IEEE 802.3
LAN
Public telephone network
21
SMTP mail exchange as an example

• There is a protocol for mail that defines a set
  of commands and messages that one machine sends
  to the other, for example, a conversation between
  machines linkguide.ici.ro and mail.iob.ro
• Linkguide HELO linkguide.ici.ro
• Mail.iob.ro 250 mail.iob.ro - HELO
  Linkguide.ici.ro
• Linkguide MAIL Fromltgmacri_at_linkguide.ici.rogt
• Mail.iob.ro 250 MAIL accepted
• Linkguide RCPT Toltmihai_at_mail.iob.rogt
• Mail.iob.ro 250 Recipient accepted
• Linkguide DATA
• Mail.iob.ro 354 Start mail input end with
  ltCTRLgt,ltCRLFgt
• Linkguide Date Sat, 26 Jul 96 142334 02
• Linkguide From gmacri_at_linkguide.ici.ro
• Linkguide To mihai_at_mail.iob.ro
• Linkguide Subject helo
• Linkguide text of the message
• Linkguide .
• Mail.iob.ro 250 OK
• Linkguide QUIT
• Mail.iob.ro 221 mail.iob.ro Service closing
  transmission channel
• The protocol assumes that we have a reliable
  way of command and message communication

22
TCP/IP Architecture Terms
Host B
router
IP
eth drv
t.r. drv
Ethernet Driver
23
Encapsulation

• Lower layers add headers (and sometimes trailers)
  to data from higher layers

Data
Application
Data
Header
Transport
Data
Header
Header
Internet
Data
Header
Header
Header
Network Access
24
IP Addresses

• Purpose
• Basic Structure
• Network mask
• Special addresses

25
Purpose of an IP address

• Unique Identification of
• SourceSometimes used for security or
  policy-based filtering of data
• DestinationSo the networks know where to send
  the data
• Network Independent Format
• IP over anything

26
Basic Structure of an IP Address

• 32 bit / 4 byte number(e.g. 204.152.8.1)
• Decimal Representation
• Binary Representation

204
152
8
1
11001100
10011000 00001000 00000001
27
Address Structure Revisited

• Hierarchical Division in IP Address
• Network Part (Prefix)
• describes which physical network
• Host Part (Host Address)
• describes which host on that network
• Boundary can be anywhere
• not necessarily at a multiple of 8 bits

1
205 . 154 . 8
11001101 10011010 00001000 00000001
Network
Host
28
Network Masks

• Define which bits are used to describe the
  Network Part
• Different Representations
• decimal dot notation 255.255.248.0
• number of network bits /19
• Binary AND of 32 bit IP address with 32 bit
  netmask yields network part of address

29
Subnetting

• One class address (either B or C) space could be
  too large for a given organization, or for a
  certain site of the organization.
• Subnetting divides a single network address into
  many subnet addresses, so that each subnetwork
  can have its own unique address.
• A subnet is defined by applying a bit mask (the
  subnet mask) to the IP address.
• If a bit is 1 in the mask, the equivalent bit in
  the address is interpreted as a network bit.
• If a bit in the mask is 0, the bit belongs to the
  host part of the address.
• Ex mask to divide the 193.226.2.0 address into 4
  subnets
• 11111111 11111111 11111111
  11000000

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Example Prefixes

• 137.158.128.0/17 (netmask 255.255.128.0)
• 198.134.0.0/16 (netmask 255.255.0.0)
• 205.37.193.128/26 (netmask 255.255.255.192)

11111111 11111111 1 0000000 00000000
10001001 10011110 1 0000000 00000000
11111111 11111111 00000000 00000000
11000110 10000110 00000000 00000000
11111111 11111111 11111111 11 000000
11001101 00100101 11000111 10 000000
31
Old-Style Classes of Address

• Different classes used to represent different
  sizes of network (small, medium, large)
• Class A networks x.0.0.0 - 16.777.215 host
  addresses
• 8 bits network, 24 bits host (/8, 255.0.0.0)
• First byte in range x1-127
• Class B networks x.y.0.0 - 65.536 host addresses
  
• 16 bits network, 16 bits host (/16 ,255.255.0.0)
• First byte in range x128-191 y0-254
• Class C networks x.y.z.0 - 256 host address
• 24 bits network, 8 bits host (/24, 255.255.255.0)
• First byte in range x192-223 y,z0-254

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IP Address Structure - Class-full

Address format 32 bits
Network address
Host address
Class A network8 bits
0
Class B network16 bits
1
0
Class C network24 bits
1
1
0
Class D (multicast)
1
1
1
0
Class E (reserved)
1
1
1
1
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Special Addresses

• All 0s in host part Represents Network
• e.g. 193.0.0.0/24
• e.g. 138.37.128.0/17
• All 1s in host part Broadcast
• e.g. 137.156.255.255 (137.156.0.0/16)
• e.g. 134.132.100.255 (134.132.100.0/24)
• e.g. 190.0.127.255 (190.0.0.0/17)
• 127.0.0.0/8 Loopback address (127.0.0.1)
• 0.0.0.0 Various special purposes

34
TCP/IP Basics Physical Datalink
35
The Physical and Datalink layer

• Ethernet
• IEEE and ISO
• Token Ring
• FDDI
• SLIP
• PPP
• ISDN

36
Ehernet

• Network access protocol
• The medium for communication between two machines
  directly connected can be coax, twisted cable,
  telephone link, radio link, satellite link, etc.
  The lowest layer of protocols provides functions
  that manage the data transmission specific to a
  certain physical medium.
• Classes of links
• Point to point
• Broadcast
• Non-broadcast multi-access
• Ethernet/IEEE 802.3 is a coaxial based bus
  cabling system developed by Digital Equipment
  Corporation, Intel, Xerox (DIX)
• Ethernet was the technological basis for the IEEE
  802.3 specification
• Both of them specify the CSMA/CD (Carrier Sense
  Multiple Access with Collision Detection), also
  referred as listen while talk (LWT)
• Both are broadcast networks

37
Ethernet Topologies
Fiber concentrator
10/100/1000 Base F
Transceivers
38
The Ethernet frame

• This Ethernet frame encapsulates the TCP/IP
  protocol and is responsible for transporting it
  across the cabling system to layer 2 of the
  destination device , whether its a Router ,
  Gateway or end node .

39
MAC addressing

• The ethernet frame uses addresses referred to as
  MAC (Medium Access Control)
• MAC addresses identify the specific network cards
• These are 48 bits long
• Each network card has a unique address configured
  by its manufacturer

40

• The LAN card will accept only 3 types of MAC
  address .
• Unicast - Frames with destination to the exact
  MAC address .
• Broadcast - Has all 48 bits set to binary 1 (or
  Hex FF FF FF FF FF FF) .This type of frame is
  used when the sender does not know the
  destination MAC address it tries to communicate ,
  so we broadcast to all .
• Multicast - Addressing to groups of LAN cards
  that are related in some way .The LAN cards have
  to be configured to know they are part of a
  multicast group .

41
The type field

• The Type field identifies different protocols .
• A computer running multiple protocols can easily
  differentiate between them , and path the
  contents to the relevant layer .
• TCP/IP Generally uses 3 Ethernet types registered
  in IEEE .

42
CRC - Cyclic Redundancy Check

• At the end of the frame is a CRC .
• This is a 32 bit value that is calculated from
  all the bits of the Ethernet frame and its
  contents , but ignoring the preamble and the CRC
  itself .
• The remote node does the same calculation and
  compares the CRC .If the value is different ,
  the LAN card will not pass the Frame to the
  network layer .

43
The service provided by Ethernet

• The medium access mechanism used by Ethernet is
  CSMA/CD (Carrier Sense Multiple Access with
  Collision Detection) .
• This allows nodes on the network to manage shared
  access to the cable , but it restricts the length
  of the cabling , and the number of nodes that use
  it .
• They are not specific to Protocol , therefore for
  TCP/IP .

44
Ethernet Packet size

• Minimum packet size - 64 octets
• Maximum packet size - 1518 octets
• The sizes above include all the frame apart from
  the preamble .
• Because of the frame header fields , the CRC and
  the overhead of the IP and TCP or UDP higher
  layer protocols , the amount left for useful
  application data is less then 1518 .

45

• To give an example The Ethernet frame overhead
  consists of 18 octets and the higher layer
  protocols often need 40 octets .That leaves 1460
  (1518-40-181460) octets for application data .

46
IEEE and ISO systems

• IEEE 802.3 uses CSMA/CD .
• IEEE 802.4 uses a token mechanism on a bus .
• IEEE 802.5 and FDDI (IS9314) use a token passing
  mechanism on a ring .

47
LLC (Logical Link Layer)

• For LANs , layer 2 is split to 2 sublayers .
• The lower is MAC and above we have the LLC ,
  which has the standard number IEEE 802.2 .
• One of the major functions of LLC is to
  differentiate between the different types of
  network layer protocols , in a similar way to the
  type field of Ethernet .

48
Ethernet
49
Token Ring
50
FDDI
51
Encapsulation

• The type field specifies the upper-layer protocol
  to receive the data after Ethernet processing is
  complete
• The CRC (Cyclic Redundancy check) is created by
  the sender and recalculated by the receiver
• The frame length (header, data, and CRC) 64-1518
  bytes

Application
Application
Data
TCP
TCP
T
Data
T
Data
IP
Data
I
T
I
T
Data
IP
E
I
T
Data
Ethernet
E
I
T
Data
C
C
Ethernet
Ethernet
52
The IEEE 802.3 frame

• The IEEE 802.3 frame has the same general format
  as DIX Ethernet (Ethernet_II) frame .
• The Type field in Ethernet DIX is the Length
  field in IEEE 802.3
• THE FCS (Frame Check Sequence) is instead of CRC
• As there is no Type field , it is not possible to
  detect which network layer protocol is carried in
  the MAC layerThe MAC frame consists of only
  addresses , length and FCS.It is the function of
  LLC to separate the different network layer
  protocols .

53
IEEE 802.3 frame
46-1500 Octets
54
Bridging TCP/IP

• Bridging between IEEE LANs is often promoted as
  transparent to any protocol above the MAC layer
  .This will bring expectations that there are no
  particular problems with TCP/IP .
• There are 4 issues that need consideration
• The length field for the 802.3 bus.
• Encapsulation on bus networks.
• The maximum frame sizes.
• The representation of MAC addresses.

55
Length fields

• The IEEE 802.3 CSMA/CD network has a length field
  immediately before the LLC .Other IEEE networks
  do not .
• Bridging will at least involve changing the
  content of the frame and recalculating the FCS
  .This action will be totally transparent to the
  network planners .

56
Frame size

• For TCP/IP , the transmitted frame size is
  determined by the Maximum Transfer Unit (MTU) set
  in the driver software for the LAN interface .
• It is possible on most TCP/IP implementations to
  modify the MTU to match the number of data octets
  carried by the Link Layer protocol .Setting the
  MTUs of each interface on a Token Ring to 1492
  will prevent its frames from being to large for
  bridging to IEEE 802.3 .This reduction will
  limit Token Ring efficiency .

57
Representation of MAC addresses

• The IEEE 802.1 committee defined how LANs should
  represent 48 bit MAC addresses as a bit stream on
  the cable .IEEE 802.3 and 802.5 committee chose
  to represent these addresses higher in the
  protocol .
• IEEE 802.3 and 802.5 represent differently the
  MAC address .
• Bridges now have to be wise and not only reverse
  the address but also to calculate the FCS .

58
Example of vendor-dependant Ethernet addresses

• Prefix Manufacturer
• 00000C Cisco
• 000095 Proteon
• 0000A2 Wellfleet
• 0000C0 Western Digital
• 00AA00 Intel
• 02608C 3Comm
• 080009 Hewlett-Packard
• 080010 ATT
• 08000B Unisys
• 080020 Sun
• 08002B DEC
• 080046 Sony
• 08005A IBM
• AA0003 DEC
• AA0004 DEC

59
TCP/IP Basics Serial Connections
60
SLIP - Serial Line Internet Protocol

• In some situations , it is advantageous to use
  asynchronous Serial lines to carry TCP/IP
  protocols , either by
• Dialup modems
• Modems on private wires
• through an asynchronous network
• Direct connection between 2 computers

61
SLIP functionality
Asynchronous connections V.24/RS232C
Direct connection
Modem link
LAN
PCs with SLIP
Host
Dialup modem link
62
SLIP frame format

• SLIP defines 2 special characters
• SLIP END - 0xC0
• SLIP ESC - 0xDB
• Datagrams sent using SLIP are framed SLIP END
  characters .

63
SLIP frame format
64
PPP - Point to Point Protocol

• PPP came to overcome a number of limitations of
  SLIP .
• PPP has been designed to operate over both
  asynchronous (start/stop) connections , and bit
  oriented synchronous systems .

65

• PPP provides more then just a simple connection
  between hosts .It also defines several
  management and testing functions to deal with
  line quality , option negotiation and the setup
  of IP addresses .

66
The service provided by PPP

• PPP provides a Point to Point connection between
  2 TCP/IP systems for the transfer of IP datagrams
  .
• PPP can operate over virtually any serial link
  interface .
• The only limitation is that it requires a full
  duplex connection .

67

• It does not need serial interface control signals
  , but the standard recommends it for performance
  improvements .
• There is no restriction for the speed used for
  PPP .

68
The PPP frame

• The address field is all 1s.
• The control octet contains the value 0x03.
• The protocol field defines the protocol carried
  by this frame
• Link Control Protocol - 0xC021
• Network Control Protocol - 0x8021
• Internet Protocol - 0x0021

69

• PPP can multiplex data from many sources, which
  makes it practical for high speed connections
  between bridges or routers.

70
TCP/IP Basics Network Layer
71
Why do we need IP protocol layer?

• Although the services provided by TCP protocol
  are needed by many applications, there are still
  some kind of applications that dont need them
• However, there are some services that every
  application needs.
• The services that every application needs are put
  together into the IP protocol layer
• IP protocol provides the basic service for the
  transmission of a datagram from one machine to
  another machine which do not need to be connected
  directly
• As a result, TCP calls on the services of IP
• Like TCP, IP protocol layer can be viewed as a
  library of routines that TCP calls on, but which
  is also available to applications that dont use
  TCP

72
IP - Internet Protocol

• IP is described as a connectionless datagram
  service .
• Datagrams are packets of information that can be
  destined for one , many or all stations (unique ,
  multicast or broadcast) - provide addressing.
• There is no requirement for the intended
  recipient/s to acknowledge whether the datagram
  was received (no flow control, no end-to-end data
  reliability).
• As IP is connectionless , no specific route is
  defined between 2 communicating nodes , so
  datagrams traveling can travel through different
  routes and reach destination in a different order
  (no sequencing and allow for fragmentation).
• One of the major roles of IP layer is to make it
  unnecessary for higher layer protocols to
  understand anything about the physical
  capabilities of the media supporting them .Note
  This is important for application developers
  writing programs on top of the transport layer
  with no variations because of the different kind
  of media used .

73
The IP Architecture
Message Segment Datagram Frame Bit
5 4 3 2 1
( )
1
0800
( )
( )
( )
8035
0806
74
Encapsulation

• Both the header and data of the IP datagram
  become the datalink frame of whichever network
  they happen to be on.This is called encapsulation
  .
• Protocol number identifies the protocol in the
  layer above IP to which the data is passed
  (/etc/protocols)
• 0 IP pseudo protocol number
• 1 ICMP
• 6 TCP
• 17 UDP

75
Fragmentation and Reassemble

• IEEE 802.3 and Ethernet systems have maximum data
  sizes of 1492 and 1500 octets respectively .IEEE
  802.5 frames is not defined , but in practice it
  is usually no greater then 8192 octets .
• This size limit seen by IP is known as the
  Maximum Transfer Unit (MTU) .
• The MTU can be adjusted for each interface , but
  its not necessary unless bridging different LAN
  technologies .

76
IP datagram Format
77

• Version - 4 bitsVersion of the IP
  protocolCurrent version is 4
• Internet Header Length - 4 bitsFor easy finding
  of beginning of data .Normally the value is 5
  indicated no options are used .
• Type Of Service - 8 bitsThe first of 3 bits are
  used to indicate 1 of 8 levels of priority .Some
  Routers Ignore these flags .

• Total length - 16 bitsThe total length of the IP
  datagramThe size of data is computed from the
  total length field and IHL .
• Identification - 16 bitsThis is an integer value
  used to help identify all fragments of a datagram
  .This field is unique for each new datagram .

78

• Flags - 3 bitsThe 2 low order bits are used as
  flags to control fragmentation .The low order
  bit , if 0 , indicates the last fragment of a
  datagram - MF (More Flag) .The middle bit is
  used to indicate that the datagram should not be
  fragmented - DF (Do not Fragment) .
• Fragment Offset - 13 bitsUsed in a fragmented
  datagram to indicate the position that the
  fragment occupies .

• Time To Live (TTL) - 8 bitsThis prevents
  datagrams to get routed in a loop .If its set
  to 0 , a router should discard the datagram .The
  recommended value is 32 , but it can be set to a
  maximum of 255 too .
• Protocol - 8 bitsThe transport layer protocol
  carried by this datagram .It tells the IP layer
  where to path the datagram .17 - UDP6 - TCP1 -
  ICMP

79

• Header checksum - 16 bitsIt protects only the
  header and not the data .The reason is because
  the checksum must be recalculated every time it
  passes through a router .Other parameters change
  too .
• Source IP address - 32 bits
• Destination IP address - 32 bits
• Data variableThis includes the headers of higher
  layer protocols and users data .

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Routing IP Datagrams
Target
81
IP Routing
Subnet
Default Gateway
Direct Connection

• local host
• default gateway

• local host
• same subnet
• next-hop

SubNet

• local host
• same subnet
• default gateway

82
IP algorithm

• 1. Search the routing table for an entry that
  matches the complete destination IP address
  (network ID or host ID). If found, send the
  packet to the indicated next-hop router or to the
  directly connected interface. (second interface
  or ppp)
• 2. Search the routing table for an entry that
  matches just the destination network ID. If
  found, send the packet to the indicated next-hop
  router or to the directly connected interface.
  (local networks)
• 3. Search the routing table for an entry labeled
  default. If found, send the packet to the
  indicated next-hop router

83
ARP - Address Resolution Protocol

• If we wish to connect to a remote computer we
  must know its IP address , but we do not need to
  know its MAC address .
• ARP was invented for this reason .It relates
  IPs to MAC addresses only on media that supports
  broadcasts .
• Each node maintains a cache called the ARP cache
  , which holds a table of IPs against MAC
  addresses .

84
How ARP works

• When IP is requested to send a datagram to
  another IP address , it first looks in the ARP
  cache to find the corresponding MAC address .If
  there is no entry it then attempts to look for it
  using ARP .
• In order to do this ARP sends an ARP request
  datagram to all LAN cards using a broadcast
  address .

85

• ARP uses its own Ethernet type 0x0806 for these
  requests , so they are passed to the ARP software
  in all nodes within the broadcast area .
• All cards on a network read this request datagram
  and any that discover a match between their IP
  and the requested IP reply with an ARP response .
• If a response is received , the answer is entered
  to the ARP cache for future use .If none is
  received , the request is repeated .
• ARP datagrams are not passed through routers , as
  a router operates at the IP layer and will not
  relay MAC broadcast traffic .This makes routers
  a good buffer between broadcast domains and
  prevent flooding networks .

86
ARP commands

• arp command can be used to display the content of
  the ARP table
• Formats
• arp -a ! displays all the entries in the ARP
  table
• arp lthostnamegt ! displays the entry for
  lthostnamegt specified
• arp -d lthostnamegt ! deletes an entry for
  lthostnamegt
• arp -s lthastnamegt ltether-addressgt ! adds a new
  entry

87
RARP - Reverse ARP

• RARP is intended for use with devices that cannot
  store their IP address , usually diskless
  workstations.
• RARP , like ARP , operates directly over the
  datalink layer and has an Ethernet type 0x8035 .
• Nodes acting as RARP servers that find a match
  for the MAC address in their RARP tables will
  reply with the corresponding IP address in a RARP
  response .

88

• This system requires that at least one server is
  present and that the server has a table defining
  which IP addresses should be used by each MAC
  address .

89
ICMP - Internet Control Message Protocol

• Even though IP is a datagram service and there is
  no delivery guarantee , ICMP is provided within
  IP and can generate error messages regarding
  datagram delivery .
• ICMP uses IP datagrams to carry its messages back
  and forth between relevant nodes .

90

• ICMP error messages are generated by a node
  recognizing there is a transmission problem and
  they are sent back to the originating address of
  the datagram that caused the problem .

91
(No Transcript)
92
General format of ICMP message

• Type (8) specifies the type of ICMP message
• Code (8) used to specify parameters of the
  message that can be encoded in a few bits
• Checksum (16) checksum of the entire ICMP
  message
• Parameters (32) used to specify more lengthy
  parameters
• Information (variable)provides additional
  information related to the message
• ECHO and ECHO REPLY - mechanism for testing if
  communication is possible between two entities. A
  host can send the ICMP ECHO message to see if a
  remote IP is up and operational. When a system
  receives an echo message, it send the same packet
  back to the source host in an ICMP ECHO REPLY
  message. The ping command uses this message.
• A TIME EXCEEDED message is sent by a gateway if
  the ttl value of a datagram expires (becomes
  zero). This facility is used by the traceroute
  command.

Type (8 bits)
Code (8 bits)
Checksum (16 bits)
Parameters (32 bits)
Information (variable)
93
Type field
Message Type

• 0
• 3
• 4
• 5
• 8
• 11
• 12
• 13
• 14
• 15
• 16
• 17
• 18

Echo reply Destination unreachable Source
quench Redirect Echo request Time exceeded for
datagram Parameter problem on datagram Time stamp
request Time stamp reply Information
request Information reply Address mask
request Address mask response
94
The ping command

• ping
• it is a simple function, extremely useful for
  testing the network connection
• it allows the network administrator to determine
  whether further testing should be directed toward
  the network (the lower layers) or the application
  (the upper layers)
• if ping shows that packets can travel to the
  destination system and back, the problem is
  probably in the upper layers
• If packets cant make the round-trip, lower
  protocol layers are probably at fault
• Basic format
• ping lthostgt ltpacketsizegt ltcountgt
• lthostgt The host name or IP address of the remote
  host being testyed.
• ltpacketsizegt Defines the size in bytes of the
  test packets. This field is only required if the
  count field is going to be used. Default packet
  size is 56 bytes.
• ltcountgt The number of packets to be sent in the
  test. Default number is usually 5.

95
ping example

• Examples
• ping ftp.ripe.net
• info.ripe.net is alive
• ping -s ftp.ripe.net 100 10
• PING info.ripe.net 100 data bytes
• 108 bytes from info.ripe.net (39.13.5.97)
  icmp_seq0. time1070. ms
• 108 bytes from info.ripe.net (39.13.5.97)
  icmp_seq1. time990. ms
• 108 bytes from info.ripe.net (39.13.5.97)
  icmp_seq2. time990. ms
• 108 bytes from info.ripe.net (39.13.5.97)
  icmp_seq3. time990. ms
• 108 bytes from info.ripe.net (39.13.5.97)
  icmp_seq4. time990. ms
• 108 bytes from info.ripe.net (39.13.5.97)
  icmp_seq5. time990. ms
• 108 bytes from info.ripe.net (39.13.5.97)
  icmp_seq6. time990. ms
• 108 bytes from info.ripe.net (39.13.5.97)
  icmp_seq7. time980. ms
• ----info.ripe.net PING Statistics----
• 8 packets transmitted, 8 packets received, 0
  packet loss
• round-trip (ms) min/avg/max 980/998/1070

96
traceroute - Tracing routes

• is the program that can help the network
  administrator locate the problem when something
  is down between the local host and a remote
  destination
• traces the route of UDP packets from the local
  host to a remote host
• prints the name (if it can be determined) and IP
  address of each gateway along the route to the
  remote host
• uses two techniques small ttl values and
  invalid port number

97
traceroute - Tracing routes

• Operation
• traceroute sends out 3 UDP packets with ttl value
  set to one
• the first gateway decrement ttl and gets the
  value zero.
• The first gateway will send back to the source
  host an ICMP TIME EXCEEDED message as error
  message
• traceroute displays one line of output for each
  gateway from which it receives an ICMP TIME
  EXCEEDED message
• traceroute will then increment by one the ttl
  value and sends again 3 UDP packets
• the flow of packets tracing to a host three hops
  away is illustrated below
• When the destination host receives a packet from
  traceroute, it returns back an ICMP Unreachable
  Port message. This happens because traceroute
  intentionally uses an invalid port number (33434)
  to force this error.
• When traceroute receives the Unreachable Port
  message, it knows that it has reached the
  destination host, and it terminates the trace.
• In this way, traceroute is able to develop a list
  of the gateways, starting at one hop away and
  increasing one hop at a time, until the remote
  host is reached.

98
traceroute example

• traceroute ftp.ripe.net
• traceroute to info.ripe.net (39.13.5.97), 30 hops
  max, 40 byte packets
• 1 agsici1.ici.ro (192.162.16.25) 20 ms 10 ms
  0 ms
• 2 Vienna-EBS1.Ebone.NET (192.121.159.97) 870
  ms 870 ms 870 ms
• 3 Paris-EBS2.Ebone.net (192.121.156.17) 900 ms
  890 ms 890 ms
• 4 Stockholm-ebs.ebone.net (192.121.154.21) 920
  ms 930 ms 960 ms
• 5 Amsterdam-ebs.Ebone.NET (192.121.155.13) 970
  ms 990 ms 970 ms
• 6 Amsterdam.ripe.net (193.0.15.130) 1000 ms
  970 ms 970 ms
• 7 info.ripe.net (39.13.5.97) 1040 ms 970 ms
  990 ms

99
Flow of traceroute packets
ping program
First router
Second router
Third router
ttl1
decrements ttl to 0 return error TIME EXCEEDED
decrements ttl to 1 forward
ttl2
decrements ttl to 0 return error TIME EXCEEDED
ttl3
decrements ttl to 2 forward
decrements ttl to 1 forward
received at destination port unreachable
Return error port unreachable
100

• ICMP has its own IP protocol number (1) so the
  IP layer knows when it receives them.
• Even though ICMP uses the IP layer, it is
  considered as being within IP, because it does
  not necessarily provide any service to the layers
  above.

101
ICMP types 0 and 8 - echo

• The most common ICMP messages used for
  diagnostics are type 0 and 8.
• These are generated by Ping.Ping sends ICMP type
  8 datagrams to a node and expects an ICMP type 0
  reply, returning the data sent in the request.

102
ICMP echo datagram (0 or 8)
103
Note

• How can Ping generate ICMP echo requests if ICMP
  does not provide a service to Ping ?
• A Ping implementation does not use ICMP to
  generate the request.It merely mimics what ICMP
  would do as a program that operates over the IP
  layer.Ping generates an IP datagram with a data
  field that equates to ICMP echo request (protocol
  number 1 and the first octet of data is 8 - ICMP
  echo request).It then adds the rest of the
  fields including the data pattern that it expects
  to be echoed.

104
ICMP type 3 - destination unreachable

• If a router is unable to deliver a datagram, it
  can return the destination unreachable ICMP
  datagram to indicate why.
• The code field is used to identify the cause of
  failure.
• The values in the code field help to pinpoint the
  reason for the datagram failure to arrive its
  destination.

105
ICMP type 3 - Destination Unreachable
106
Code value
Meaning

• 0 Network unreachable
• 1 Host unreachable
• 2 Protocol unreachable
• 3 Port unreachable
• 4 Fragmentation needed and
  the do not fragment bit
  set
• 5 Source route failed

107

• If a router is unable to deliver a datagram , it
  can return the destination unreachable ICMP
  datagram to indicate why .
• Network unreachable - The network specified in
  the IP address cannot be found .
• The IP address and routing tables should be
  checked .
• This error message is only generated by a router
  .
• We can find where the error occurred , from the
  source address in IP header that carried the ICMP
  message .
• Host unreachable - The datagram reached the
  router which is directly connected to the
  destination network, but failed to communicate
  with the host.This message is generated by a
  router only .

108

• Protocol unreachable - The datagram reached the
  destination host , but the particular protocol
  carried in the datagram is not available .
• Port unreachable - A host sends the message that
  the particular application layer service is not
  available .
• Fragmentation needed and the do not fragment bit
  set - Normally comes from a router , indicating
  that it needs to fragment the datagram , but is
  instructed not to by the do not fragment (DF) bit
  in the flags field of the IP header .This fault
  is uncommon , DF is normally used on diskless
  workstations booting via TFTP .
• TFTP has only 512 octets of user data .
• Check MTU size .

109

• Source route failed - If we specified a route and
  the datagram failed to complete the route , we
  will get this error .The point of failure will
  be the router that generated the ICMP message .

110
ICMP type 4 , code 0 - Source Quench

• The format of the datagram is the same as
  destination unreachable , but with a type of 4
  and a code of 0 .
• Source quench gives a router or a host the
  ability to request that a source of datagrams
  will slow down .
• Source quench will occur if a node is running low
  on buffer resources and is unable to process
  datagrams quickly enough .

111
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112
ICMP type 5 - route change request

• It is used only by routers .
• A router that knows that it is not the optimum
  router for a particular destination , uses the
  relevant field of a route change request to
  suggest a more suitable router .

113
ICMP type 11 - time exceeded for datagram

• The format is the same as destination unreachable
  .
• It can be sent in 2 situations
• From a router - Indicating that the TTL in the IP
  header has been decremented to 0 .It indicates
  that the original Time To Live was not suitable
  to the number of hops needed .
• From a node - An attempt to recreate the original
  datagram by reassembly of fragments failed .The
  code value is 1 .

114
ICMP type 12 - Parameter problem message

• Indicates that a wrong argument has been used
  with an option field in the IP header .It can
  also indicate an error in the implementation of
  IP .
• Its sent only if the datagram has been discarded
  .
• The pointer field indicates the position of the
  octet position of the suspect field .

115
ICMP types 13,14 - Time stamp request reply

• This message is used to obtain the time from a
  clock in a distant machine .
• It is rarely used today .

116
ICMP types 15,16 - information request

• This message is used to obtain the network number
  of the requesting host if its unknown .
• It can be used in dial in systems using SLIP, as
  a method for allocating the appropriate network
  addresses for each end of the link .

117
ICMP types 17,18 - Address mask request

• Used to allow a node to discover the subnet mask
  of the network it is connected to .
• The node can send the request to a known address
  or to broadcast .

118
Transport Protocol Ports
The address of an application within a host

• Port 0 - Special use
• Ports 1 - 255 - Well-known ports
• Ports 256 - 1023 - Reserved ports
• Ports 1024 - 4999 - Dynamic client ports
• Ports 5000 - 65,535 - Fixed server ports

119
User Datagram Protocol

• Connectionless delivery service
• Uses the IP layer service
• Does not add reliability to the IP protocol
• Enables distinguishing among multiple
  destinations within a host computer

End point
120
UDP Protocol Header Format

• Fragmentation
• What if the packet size is larger then 1500?
• It is divided to 1500xN frames.
• fragmentation flags are set

121
Flow using Datagrams (UDP)
Server
Client
socket()
socket()
bind()
sendto()/recvfrom()
sendto()/recvfrom()
closesocket()
closesocket()
122
Transmission Control Protocol

• Connection based communication
• Uses the IP layer service
• Provides reliable service
• Enables distinguishing among multiple
  destinations within a host computer

123
TCP - Transmission Control Protocol

• TCP is the protocol layer responsible for making
  sure that the commands and messages are
  transmitted reliably from one application program
  running on a machine to another one on the other
  machine
• A message is transmitted and then a positive
  acknowledgement is being waited for
• If the positive acknowledgement does not arrive
  in a certain period of time, the message is
  retransmitted
• Messages are numbered in sequence so that no one
  is being lost or duplicated
• Messages are delivered at the destination in the
  same order they were sent by the source
• If the text of a mail is too large, the TCP
  protocol will split it into several fragments
  called datagrams and it makes sure that all the
  datagrams arrive correctly at the other end where
  they are reassembled into the original message
• The TCP protocol layer provides all the functions
  that are needed for many applications and it is
  better to put them together on a separate
  protocol rather than being part of each
  application
• TCP can be viewed as forming a library of
  routines that many applications can use when they
  need reliable network communication with an
  application on another computer
• TCP provides also flow control and congestion
  control

124
TCP Protocol Format
Source Port
Destination Port
Sequence Number
Acknowledgment Number
Offset Reserv Flags(6)
Window (16 bits)
Checksum (16)
Urgent Pointer
Options(If any)
Padding
Data (variable
length)
0 4 10
16 24
31
125
Establishing and closing TCP Connections
FIN
SYN
time
ACK
SYNACK
FIN
ACK
ACK
Close
Open
Three-way handshake
126
Sliding Windows
segment 1
ack1
time
segment 2
ack2
Positive acknowledgment with retransmission
Sliding window transmission
127
Application Addresses Sockets

• On a network server, normally several application
  programs are running at the same time FTP
  server, telnet server, mail server, www server,
  gopher server, etc.
• TCP must know to which program to deliver the
  received message
• If you want to connect to the FTP server it is
  not enough to know the IP address of the server,
  you have to specify that you want to talk to the
  FTP server program
• This is done by having the well-known sockets
  - TCP ports - (see the file /etc/services on a
  UNIX machine)
• In a file server session, e.g., two different
  applications are involved FTP server and FTP
  client
• The client program gets commands from the user
  and passes them to the FTP server program
• There is no need for the client FTP program to
  use a well know socket number, because nobody is
  trying to find it, as opposed to the FTP server
  program which have to have a well-known socket
  number, so that people can open connections to it
  and start sending commands
• The client FTP program asks the network software
  to assign it a port number that is guarantee to
  be unique, for example 1236 if that number was
  free
• A connection is identified by four numbers
• connection 1 192.162.16.2, 1236 193.230.3.120,
  21
• connection 2 192.162.16.2, 1237 193.230.3.120,
  21
• Two connections are different if at least one
  number is different

128
Application Addresses Sockets
Socket IP address port
Message
Segment
Datagram
Frame
129
Well-known TCP ports
21 - FTP server 23 - telnet server 25 - SMTP
mail server 53 - domain nameserver 109 - POP2
server 110 - POP3 server
130
Flow using Streams (TCP)
Server
Client
socket()
bind()
socket()
listen()
connect()
accept()
send()/recv()
send()/recv()
closesocket()
closesocket()
131
ROUTING

• The source and the destination hosts are on the
  same LAN
• There is no decisions for routing
• The packet is transmitted on the cable (coax,
  twisted cable, optical fiber)
• Every computer connected to the LAN will receive
  it.
• That computer which finds that the destination
  Ethernet address in the header is equal to his
  Ethernet address will get the message, the others
  will discard it.
• Note that the address of each computer on the LAN
  begins with the same network number
• Routing table for host A

132
Example of complex configuration
A .1
G .4
ec0
eth0
.4 .1
Routing tables net
gw int. M 193.230.5 none
eth0 193.230.6.2
sl0 193.230.4 193.230.5.1 eth0
193.230.3 193.230.5.1 eth0 192.162.16
193.230.5.1 eth0 default
193.230.6.2 sl0 I 193.230.5 none
eth0 193.230.4.1
sl0 193.230.3 193.230.4.1 sl0
192.162.16 193.230.4.1 sl0 default
193.230.5.5 eth0 H 193.230.3 none
ec0 193,230.4.2
sl0 192.162.16 193.230.1 ec0
default 193.230.4.2 sl0 A 192.162.16
none eth0 default
192.162.16.4 eth0
D
193.230.3.
eth0
ec0
ec1
H
.2 .1
192.162.16.
sl0
193.230.4.
sl0
.2 .1
J .2
K .3
L .4
I
eth0
193.230.5.
.5 .1
M
sl0
193.230.6.
backbone network with Internet connectivity
sl0
.2
N
133
Routing table initialization and updating

• Initialization of routing table
• Normally at startup time by executing script
  command files
• Static routes
• route add ltnetwork-addressgt ltgw-addressgt
  ltmetricgt
• route add 192.162.16.0 192.162.16.4 1
• route add 193.230.3.0 192.162.16.4 1
• route add default 192.162.16.4 1
• netstat -rn displays the routing table on a UNIX
  machine
• Static routes have the disadvantage that they do
  not adapt to the changes in the network topology
• Dinamic routing protocols are run to update the
  routing table so that they reflect the changes in
  topology
• Router classes
• dedicated routers - special purpose equipment
• Cisco, Wellfleet, Proteon, Telebit
• cheap router sollution - public domain software
  for PCs
• ka9q, PCROUTE, Linux, Free BSD, etc.

134
Routing protocols

• Types of routing protocols
• Interior Gateway Protocol (IGP) RIP, IGRP, OSPF,
  Hello
• Exterior routing Protocol (EGP) BGP, EGP

AS1
AS2
EGP
IGP
IGP
135
Autonomous System Number

• An Autonomous System Number (AS) is a set of
  routers under a single technical administration,
  using an interior gateway protocol and an
  exterior gateway protocol to route packets to
  other ASs.
• An AS is a connected group of IP networks run by
  one or more network operators which has a single
  and defined routing policy.
• AS number is a 16 bit number (65535 unique AS
  numbers).
• It is a finite amount of address space.
• Sometimes, the term AS is misunderstood and used
  for grouping together a set of prefixes which
  belong under the same administrative umbrella.
• AS number are assigned by RIPE in Europe

136
Example for routing
static
IGRP
IGRP
National Network
IGRP
IGRP
BGP4
BGP4
EUROPANET
EBONE
Access to Internet
137
CIDR - Classless Inter-Domain Routing
Internet
customers
Internet Service Provider
193.230.3.0
193.230.0.0
193.230.1.0
193.230.02.0
host
network
Class-full representation
00000000
00000000
11000001
193.230.0.0
11100110
00000000
11100110
11000001
00000001
193.230.1.0
00000000
00000010
11100110
11000001
193.230.2.0
1110010
00000011
00000000
11000001
193.230.3.0
Host
Prefix
Classless representation
138
Example of CIDR configuration (supernetting)

• Using BGP4 routing protocol, all the 4 C class
  addresses (193.230.0.0, 193.230.1.0, 193.230.2.0,
  193.230.3.0) can be advertised like one entry in
  the routing table
• router bgp 3233
• agregate-address 193.230.0.0 255.255.252.0
  summary-only
• neighbor 192,121,159,97 remote-as 1755
• neighbor 193.226.27.86 remote-as 2614
• Using BGP4 routing protocols, all the 256 C
  addresses of the block 193.230.0.0 -
  193.230.255.255 can be advertised like one entry
  in the routing table
• router bgp 3233
• agregate-address 193.230.0.0 255.255.0.0
  summary-only
• neighbor 192,121,159,97 remote-as 1755
• neighbor 193.226.27.86 remote-as 2614

139
IPng Features/Functionality

• Expanded Address Space
• Autoconfiguration
• Real-time/Multimedia support
• Integrated Security support
• IPv4 IPv6 Transition Strategy

140
IP Version 6 - So whats really changed ?!

• Address space
• quadrupled to 16 bytes
• Fixed Length
• (optional headers daisy-chained)
• No Check sum
• (Done by Link Layer)
• No hop-by-hop
• segmentation
• (Path MTU discovery)
• Flow Label/Priority
• (Integrated QoS support)

IPv4 Header
Total Length
IHL
Type of Service
Version
Identification
Flags
Fragment Offset
Protocol
Time to Live
Header Checksum
Source Address
Destination Address
Padding
Options
IPv6 Header
Priority
Flow Label
Version
Payload Length
Next Header
Hop Limit
Source Address
Destination Address
141
IPv6 Autoconfiguration

• Stateless
• Host autonomously configures
• its own address
• Link Local Addressing

(single subnet scope, formed from reserved prefix
and link layer address)

• Stateful
• DHCPng
• Addressing Lifetime
• Facilitates graceful renumbering
• Addresses defined as valid, deprecated or invalid

142
IPv6 Real Time/Premium Services support

• Flow based, defines flow label and priority
• Can be combined with Source Routing header
  options
• Integration with Tag Switching/MPLS

(Reference/Draft RFC- draft-baker-flow-label-00.
txt)
143
IPv6 Security

• IPSec Architecture
• Export restrictions recently relaxed
• Authentication - MD5 based
• Confidentiality - DES
• Encrypt entire datagram or IP payload
• Retain existing use of (packet filtering based)
  firewalls

144
IPv6 Transition Strategy - Approaches

• Networks - Tunneling

• More efficient than building new IPv6 topology

145
IPv6 Tunneling

• Configured tunnels - manual point-2-point links
• Automatic tunnels - via IPv4 compatible IPv6
  addresses
• (96
  bits of zeros prefix - 000000/96)

Driver
IPv6
IPv4
IPv4 Backbone
IPv6
IPv4
Driver
IPv6
IPv6
IPv6

• Instrumental in building existing 6-Bone
  (http//www.6bone.net)

• Network Address Translation IPv4 IPv6

146
IPv6 Routing

• Hierarchy is key
• Test address space allocation available- (RFC
  1897)

• Existing routing protocols extensions for IPv6
• RIPv6 - Same destination/mask/metric
  information as RIPv2
• Multiprotocol BGP4 - Currently Draft
• Integrated IS-IS - 20 byte NSAP
  support