module a panko and panko business data networks and telecommunications, 8 th edition © 2011 pearson...
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Module A
Panko and PankoBusiness Data Networks and Telecommunications, 8th Edition© 2011 Pearson Education, Inc. Publishing as Prentice Hall
This module presents additional material about TCP/IP standards.
Most of the material in this module can be read after Chapter 2, but some of it is designed to be covered after Chapter 10.
The material in this module is not designed to be read front-to-back like a regular chapter, although it can be covered this way.
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IP packets can carry different things in their data fields.◦ TCP segments
◦ UDP datagrams
◦ ICMP supervisory messages (later)
◦ RIP messages (later)
4
IP Data Field IP Header
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We say that IP can multiplex (mix) different types of traffic in a stream of IP packets.
5
UDP IP-H TCP IP-H UDP IP-H ICMP IP-H
Stream of Arriving or Outgoing IP Packets
Single IP PacketCarrying UDP Datagram
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The IP process must pass contents of arriving IP packets to the correct process for subsequent handling.
6
IP
TCP UDP
ICMPUDP IP-H
IP ProcessArrivingPackets
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IP process must also accept messages from multiple processes and multiplex them on an outgoing stream.
7
IP
TCP UDP
ICMPUDPIP-H
IP ProcessOutgoingPackets
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Need a way for receiving IP process to know what is in the data field
◦ So it can pass the contents to the appropriate process
8
IP Data Field IP Header
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IP Header has an 8-bit Protocol field.
◦ Identifies the contents of the data field
1=ICMP, 8=TCP, 17=UDP, and so on
9
Total Length in Bytes (16)
Time to Live (8)
Version(4)
Hdr Len(4) TOS (8)
Indication (16 bits) Flags (3) Fragment Offset (13)
Source IP Address
Destination IP Address
Header Checksum (16)Protocol (8)
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Other Messages Have Analogous Fields◦ Identify contents of data field
TCP and UDP◦ Have Port number fields◦ Identify the application process (80=HTTP)
10
Source Port # (16) Destination Port # (16)
Sequence Number (32 bits)
Acknowledgement Number (32 bits)
Hdr Len(4) Flags (6) Window Size (16)Reserved (6)
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Other Messages Have Analogous Fields◦ Identify contents of data field
PPP◦ Protocol field identifies contents of
information field as IP, IPX, a supervisory message, and so on.
11
Flag Addr Ctrl Prot Info CRC Flag
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TCP is Reliable.
◦ IP packets carrying TCP segments may arrive out of order.
◦ TCP must put the TCP segments in order.
13
3 4 2 15
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TCP is Reliable.
◦ Each correct TCP segment is acknowledged by the receiver.
14
SourceTransportProcess
SourceTransportProcess
DestinationTransportProcess
DestinationTransportProcess
TCP SegmentTCP Segment
ACKACK
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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
15
11 44 22 55
3?
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TCP header has a 32-bit sequence number field.
16
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
Reserved (6)
TCP Checksum (16) Urgent Pointer (16)
Data Field
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Initial Sequence Number is randomly selected by the sender; say, 79.
Sent in the sequence number field of the first TCP segment.
17
79
TCP Data Field
TCP Header
Sequence Number Fieldwith Initial Sequence Number (79)
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Data octets in data fields of all segments in a connection are viewed as a long string.
TCP Segment 1 79 TCP Segment 2 80
8182
TCP Segment 3 8384
18
3 Octets in Data Field
2 Octets in Data Field
ISN
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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 3 901
902…
19
Supervisory Segment
Carries Data
Carries Data
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Sequence number field gets the value of the first octet in the data field.
TCP 1 79 TCP 2 80
8182
TCP 3 8384
20
80 is SeqNum Field Value
83 is SeqNum Field Value
79 is SeqNum Field Value
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Acknowledgement must indicate which TCP segment is being acknowledged.
21
SourceTCP
Process
SourceTCP
Process
DestinationTCP
Process
DestinationTCP
Process
TCP SegmentTCP Segment
ACKACK
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TCP header contains a 32-bit Acknowledgement Number field to designate the TCP segment being acknowledged.
22
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
Reserved (6)
TCP Checksum (16) Urgent Pointer (16)
Data Field
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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
23
83 is AckNum Field Value
85 is AckNum Field Value
80 is AckNum Field Value
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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?
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Flow Control
◦ One TCP process transmits too fast.
◦ Other TCP process is overwhelmed.
◦ Receiver must control transmission rate.
◦ This is flow control.
25
TCP Process TCP Process
Too MuchData
Flow Control Message
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A TCP segment has a Window Size field.◦ Used in acknowledgements
26
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
Reserved (6)
TCP Checksum (16) Urgent Pointer (16)
Data Field
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A TCP segment has a Window Size field.◦ Tell how many more octets the sender can send
beyond the segment being acknowledged
27
TCP Process TCP Process
Data
Acknowledgement with Window Size Field
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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
◦ Must then stop unless the window has been extended in another acknowledgement
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Each Acknowledgement extends the window of octets that may be sent.◦ Called a sliding window protocol
29
1–44 45–79 80–419 420–630
400May send through 480
1–44 45–79 80–419 420–630
500May send through 920
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TCP Segments have maximum data field sizes.◦ (Size limit details are discussed later.)◦ What if an application layer message is too large?
30
TCP HeaderTCP Data Field Max
Application Layer Message
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Application layer message must be fragmented.◦ Broken into several pieces◦ Delivered in separate TCP segments
31
TCP HeaderTCP Data Field Max
App Frag 1 App Frag 2 App Frag 3
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Note that, in TCP fragmentation, the TCP segment is not fragmented.◦ The application layer message is fragmented.
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TCP HeaderTCP Data Field Max
App Frag 1 App Frag 2 App Frag 3
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Transport layer process on the source host does the fragmentation.◦ Application layer on the source host is not
involved
◦ Transparent to the application layer
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Application
Transport
Internet
Application Message
TCP Segment TCP Segment
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Transport layer process on the destination host does the reassembly.◦ Application layer on the destination host is
not involved; gets original application layer message
34
Application
Transport
Internet
Application Message
TCP Segment TCP Segment
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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!!!
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MSS Default is 536 octets.
◦ Maximum 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.
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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.
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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 segment.
◦ 536 is only the default—the value to use if no other value is specified by the other side.
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Masks were introduced in Chapter 9. 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
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Based on Logical AND◦ Both must be true for the result to be true
Example◦ 1010101010 Data
◦ 1111100000 Mask
◦ 1010100000 Result
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Based on Logical AND◦ If mask bit is 1, get back original data◦ If mask bit is 0, bet back zero
Example◦ 1010101010 Data
◦ 1111100000 Mask
◦ 1010100000 Result
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IP packet arrives at a router◦ Router sees destination IP address◦ 11111111 01000000 10101010 00000000
Compares to each router forwarding table row◦ Address Part in First Entry
◦ 11111111 01000000 00000000 00000000
◦ Mask in First Entry
◦ 11111111 11100000 00000000 00000000
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Mask the IP destination Address◦ 11111111 01000000 10101010 00000000 (IP address)◦ 11111111 11100000 00000000 00000000 (mask)◦ 11111111 01000000 00000000 00000000 (result)
Compare Result with First Entry Address part◦ 11111111 01000000 00000000 00000000 (address part)◦ 11111111 01000000 00000000 00000000 (result)
The Entry is a Match!
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Recap◦ Read destination IP address of incoming IP packet.
◦ For each entry in the router forwarding table
Read the mask (prefix).
Mask the incoming IP address.
Compare the result with the entry’s IP address part.
Do they match or not?
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Simple for Computers
◦ Computers have circuitry AND two numbers.
◦ Computers have circuitry to COMPARE two numbers to see if they are equal or not.
◦ Very computer-friendly, so used on routers.
Difficult for people, unfortunately
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The dominant version of the Internet Protocol is Version 4 (v4).◦ Earlier versions were not implemented
The emerging version is Version 6 (v6).◦ V5 was defined but not implemented
◦ Informally called IPng (Next Generation)
IPv6 is already defined.◦ Continuing improvements in V4 may delay its
adoption
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IPv6 raises the size of the Internet address field from 32 bits to 128 bits.
◦ We are running out of IP V4 addresses.
◦ V6 will solve the problem.
◦ But current work-arounds are delaying the need for IPv6 addresses—mostly Network Address Translation.
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Improved Security
◦ But, through IPsec, v4 is being upgraded in security as well
Improved Quality of Service (QoS)
◦ But, under IETF Differentiated Services (diffserv) initiative, IPv4 is being upgraded in this area as well
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Extension Headers◦ IPv4 headers are complex.
◦ IPv6 basic header is simple.
◦ IPv6 uses extension headers for options.
51
Basic Header
Extension Header 1
Extension Header 2
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Extension Headers◦ Basic header has 8-bit Next Header field◦ Identifies first extension header or says
that payload follows
52
Basic Header
Extension Header 1
Extension Header 2
NH
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Extension Headers◦ Each extension header also has 8-bit Next
Header field
◦ Identifies next extension header or says that payload follows
53
Basic Header
Extension Header 1
Extension Header 2
NH
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Extension Headers◦ Next header field is an elegant way to allow
options◦ Easy to add new extension headers for new
needs
54
Basic Header
Extension Header 1
Extension Header 2
NH
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Maximum Transmission Unit (MTU)◦ Largest IP packet a network will accept◦ Arriving IP packet may be larger
56
IP PacketIP Packet
MTU
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If IP packet is longer than the MTU, the router breaks packet into smaller packets.◦ Called IP fragments◦ Fragments are still IP packets◦ Earlier in Mod A, fragmentation in TCP
57
IP PacketIP Packet 22 11
IP PacketsFragmentation
MTU
33
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What is Fragmented?◦ Only the original data field
◦ New headers are created
58
IP PacketIP Packet 22 11
IP PacketsFragmentation
MTU
33
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What Does the Fragmentation?◦ The router◦ Not the subnet
59
IP PacketIP Packet 22 11
IP PacketsFragmentation
MTU
33
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Original packet may be fragmented multiple times along its route.
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DestinationHost
InternetProcess
SourceHost
InternetProcess
Fragmentation
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Internet layer process on destination host defragments, restoring the original packet.
IP defragmentation only occurs once.
61
DestinationHost
InternetProcess
Defragmentation
SourceHost
InternetProcess
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More Fragments field (1 bit)◦ 1 if more fragments
◦ 0 if not
◦ Source host internet process sets to 0
◦ If router fragments, sets More Fragments field in last fragment to 0
◦ In all other fragments, sets to 1
62
0 0 1 1
Original IP Packet Fragments
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IP packet has a 16-bit Identification field.
63
Total Length in Bytes (16)
Time to Live (8)
Options (if any)
Version(4)
Hdr Len(4) TOS (8)
Identification(16 bits) Flags (3) Fragment Offset (13)
Source IP Address
Destination IP Address
Header Checksum (16)Protocol (8)
PAD
Data Field
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IP packet has a 16-bit Identification field.
◦ Source host internet process places a number in the Identification field.
◦ Different for each original (non-fragmented) IP packet.
64
Total Length in Bytes (16)
Time to Live (8)
Version(4)
Hdr Len(4) TOS (8)
Identification(16 bits) Flags (3) Fragment Offset (13)
Header Checksum (16)Protocol (8)
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IP packet has a 16-bit Identification field.
◦ If router fragments a packet, it places the original Identification field value in the Identification field of each fragment.
65
47 47 47 47
Original IP Packet Fragments
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Purpose
◦ Allows receiving host’s internet layer process to know what fragments belong to each original packet
◦ Works even if an IP packet is fragmented several times
66
47 47 47 47
Original IP Packet Fragments
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Fragment offset field (13 bits) is used to reorder fragments with the same Identification field.
Contains the data field’s starting point (in octets) from the start of the data field in the original IP packet.
67
Total Length in Bytes (16)Version
(4)Hdr Len
(4) TOS (8)
Identification (16 bits) Flags (3) Fragment Offset (13)
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Receiving host’s internet layer process assembles fragments in order of increasing fragment offset field value.
This works even if fragments arrive out of order!
It works even if fragmentation occurs multiple times.
68
0212730
Fragment Offset Field
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IP Fragmentation
◦ Data field of a large IP packet is fragmented.
◦ The fragments are sent into a series of smaller IP packets fitting a network’s MTU.
◦ Fragmentation is done by routers.
◦ Fragmentation may be done multiple times along the route.
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IP Defragmentation
◦ Defragmentation (reassembly) is done once, by destination host’s internet layer process.
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All IP packets resulting from the fragmentation of the same original IP packet have the same Identification field value.
Destination host internet process orders all IP packets from the same original on the basis of their Fragment Offset field values.
More Fragments field tells whether there are no more fragments coming.
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Why Dynamic Routing Protocols?◦ Each router acts independently, based on
information in its router forwarding table.
◦ Dynamic routing protocols allow routers to share information in their router forwarding tables.
73
RouterForwardingTable Data
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Routing Information Protocol (RIP) is the simplest dynamic routing protocol.◦ Each router broadcasts its entire routing table
frequently.
◦ Broadcasting makes RIP unsuitable for large networks.
74
RoutingTable
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RIP is the simplest dynamic routing protocol.◦ Broadcasts go to hosts as well as to routers.
◦ RIP interrupts hosts frequently, slowing them down; unsuitable for large networks.
75
RoutingTable
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RIP is limited.◦ RIP routing table has a field to indicate the
number of router hops to a distant host.
◦ The RIP maximum is 15 hops.
◦ Farther networks are ignored.
◦ Unsuitable for very large networks.
76
Hop Hop
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Is a Distance Vector Protocol◦ “New York” starts, announces itself with a RIP
broadcast.
◦ “Chicago” learns that New York is one hop away.
◦ Passes this on in its broadcasts.
77
New York Chicago Dallas
1 hop
NY is 1
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Learning Routing Information◦ “Dallas” receives broadcast from Chicago.
◦ Already knows “Chicago” is one hop from Dallas.
◦ So New York must be two hops from Dallas.
◦ Places this information in its routing table.
78
New York Chicago Dallas
1 hop 1 hop
NY is 1
NY is 2
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Slow Convergence
◦ Convergence is getting correct routing tables after a failure in a router or link.
◦ RIP converges very slowly.
◦ May take minutes.
◦ During that time, many packets may be lost.
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Encapsulation
◦ Carried in data field of UDP datagram Port number is 520
◦ UDP is unreliable, so RIP messages do not always get through.
◦ A single lost RIP message usually does little or no harm.
80
UDPHeader
UDP Data FieldRIP Message
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Link State Protocol◦ Link is a connection between two routers.
◦ OSPF routing table stores more information about each link than just its hop count: cost, reliability, and so on.
◦ Allows OSPF routers to optimize routing based on these variables.
81
Link
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Network is Divided into Areas.◦ Each area has a designated router
82
AreaDesignated
Router
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When a router senses a link state change◦ Sends this information to the designated router
83
AreaDesignated
Router
Notice ofLink State Change
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Designed router notifies all routers◦ Within its area
84
AreaDesignated
Router
Notice ofLink State Change
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Efficient◦ Only routers are informed (not hosts).
◦ Usually only updates are transmitted, not whole tables.
85
AreaDesignated
Router
Notice ofLink State Change
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Fast Convergence
◦ When a failure occurs, a router transmits the notice to the designated router.
◦ Designated router send the information back out to other routers immediately.
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Encapsulation
◦ Carried in data field of IP packet Protocol value is 89
◦ IP is unreliable, so OSPF messages do not always get through.
◦ A single lost OSPF message usually does little or no harm.
87
IPHeader
IP Data FieldOSPF Message
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Within a network you control, it is your choice.◦ Your network is an autonomous system.
◦ Select RIP or OSPF based on your needs.
◦ Interior routing protocol.
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RIP is fine for small networks.◦ Easy to implement
◦ 15 hops is not a problem
◦ Broadcasting, interrupting hosts are not too important
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OSPF is scalable.
◦ Works with networks of any size
◦ Management complexities are worth the cost in large networks
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To connect different autonomous systems◦ Must standardize cross-system routing
information exchanges
◦ BGP is most popular today
◦ Gateway is the old name for router
◦ Exterior routing protocol
91
AutonomousSystem
AutonomousSystemBGP
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Distance vector approach◦ Number of hops to a distant system is stored in
the router forwarding table
Normally only sends updates
92
AutonomousSystem
AutonomousSystemBGP
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Encapsulation◦ BGP uses TCP for delivery
◦ Reliable
◦ TCP is only for one-to-one connections
◦ If a border router connects to multiple external routers, must establish a TCP and BGP connection to each
93
AutonomousSystem
AutonomousSystemBGP
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Each host and router on a subnet needs a data link layer address to specify its address on the subnet.◦ This address appears in the data link layer
frame sent on a subnet.
◦ For instance, 48-bit 802.3 MAC layer frame addresses for LANs.
95
Subnet DADL Frame for Subnet
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Each host and router also needs an IP address at the internet layer to designate its position in the overall Internet.
96
Subnet
Subnet
Subnet128.171.17.13
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IP address◦ To guide delivery to destination host across the
Internet (across multiple networks)
Subnet Address◦ To guide delivery between two hosts, two
routers, and a host and router within a single LAN, Frame Relay network, and so on
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In a company, each person has a company-wide ID number (like IP address).
In a company, each person also has a local office number in a building.
Paychecks are made out to ID numbers. For delivery, also need to know office
number.
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Problem
◦ Router knows that destination host is on its subnet based on the IP address of an arriving packet.
◦ Does not know the destination host’s subnet address, so cannot deliver the packet across the subnet.
99
Subnet128.171.17.13
Subnet Address?
Destination Host
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Router creates an ARP Request message to be sent to all hosts on the subnet.
◦ Address resolution protocol message asks “Who has IP address 128.171.17.13?”
◦ Passes ARP request to data link layer process for delivery.
100
Subnet
ARP Request
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Data link process of router broadcasts the ARP Request message to all hosts on the subnet.
◦ On a LAN, MAC address of 48 ones tells all stations to pay attention to the frame.
101
Subnet
ARP Request
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Host with IP address 128.171.17.13 responds.◦ Internet process creates an ARP Response
message.◦ Contains the destination host’s subnet address
(48-bit MAC address on a LAN).
102
Subnet
ARP Response
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Router delivers the IP packet to the destination host.◦ Places the IP packet in the subnet frame
◦ Puts the destination host’s subnet address in the destination address field of the frame
103
Subnet
Deliver IP Packetwithin a Subnet Frame
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ARP Requests and Responses are sent between the internet layer processes on the router and the destination host.
104
InternetProcess
Router
InternetProcess
Destination HostARP
Request
ARPResponse
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However, the data link processes deliver these ARP packets.◦ Router broadcasts the ARP Request.◦ Destination host sends ARP Response to the
subnet source address found in the broadcast frame.
105
InternetProcess
Router
InternetProcess
Destination Host
Broadcast ARP Request
Direct ARP Response
Data LinkProcess
Data LinkProcess
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How large is the network part in an IP address?
Today we use network masks to tell. Originally, IP had address classes with fixed
numbers of bits in the network part.◦ Class A: 8 bits (24 bits in local part)◦ Class B: 16 bits (16 bits in local part)◦ Class C: 24 bits (8 bits in local part)
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All Class A IP addresses begin with 0. 7 remaining bits in network part.
◦ Only 128 possible Class A networks.
24 bits in local part.◦ Over 16 million hosts per Class A network!
All Class A network parts are assigned or reserved.
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All Class B IP address begin with 10 (1st zero in 2nd position).
14 remaining bits in network part◦ Over 16,000 possible Class B networks
16 bits in local part◦ Over 65,000 possible hosts
A good trade-off between number of networks and hosts per network
Most have been assigned
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All Class C IP address begin with 110 (1st zero in 3d position).
21 more bits in network part◦ Over 2 million possible Class C networks!
8 bits in local part◦ Only 256 possible hosts per Class C network!
Unpopular, because large firms must have several
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All Class D IP address begin with 1110. Used for multicasting, not defining
networks.
◦ Sending message to group of hosts
◦ Not just to one (unicasting)
◦ Not ALL hosts (broadcasting)
◦ Say, to send a videoconference stream to a group of receivers
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All hosts in a multicast group listen for this multicast address as well as for their specific own host IP address.
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Packets toMulticast Address
Not in GroupReject
In GroupAccept
In GroupAccept
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Traditionally, unicasting and broadcasting◦ Unicasting: send to one host
◦ Broadcasting: send to ALL hosts
Multicasting◦ Send to SOME hosts
◦ 500 stations viewing a video course
◦ 50 computers getting software upgrades
◦ Standards exist and are improving
◦ Not widely used yet
113© 2011 Pearson Education, Inc. Publishing as Prentice Hall
Do not need to send an IP packet to each host◦ Single packets go out
◦ Only multiplied when necessary
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SinglePacket
MultiplePackets
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IP addresses are associated with fixed physical locations.
Mobile IP is needed for notebooks, other portable equipment.
Computer still gets a permanent IP address. When travels, also gets a temporary IP
address at its location. This is linked dynamically to its permanent
IP address.
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mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. Printed in the United States of America.
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