Download - Internetworking 2
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EECB 423 V.11
Internetworking 2
Datagram EncapsulationTransmission
Fragmentation and Reassembly
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EECB 423 V.12
The Internet Transmission
Source host Forms datagram Includes destination address Sends to nearest router
Intermediate routers Forward datagram to next router
Final router Delivers to destination host
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EECB 423 V.13
Datagram Transmission
Datagram sent across conventional network From source host and router Between intermediate routers From final router to destination host
Network hardware does not recognize Datagram format IP addresses
Encapsulation needed
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EECB 423 V.14
Illustration of IP Encapsulation
Entire datagram treated like data Frame type identifies contents as IP datagram Frame destination address gives next hop
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EECB 423 V.15
Frame and Datagram Destination Address
Frame address Hardware (MAC) address Next hop
Datagram address IP address Ultimate destination
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EECB 423 V.16
Frame Address For Encapsulated Datagram
A datagram is encapsulated in a frame for transmission across a physical network.
The destination address in the frame is the address of the next hop to which the datagram should be sent; the address is obtained by translating the IP address of the next hop to an equivalent hardware address.
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EECB 423 V.17
Frames versus Datagrams
Datagram survives entire trip across Internet Frame only survives one hop
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EECB 423 V.18
Illustration Of Frame HeadersUsed For Datagram Transmission
Each hop extracts datagram and discards frame
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EECB 423 V.19
Maximum Frame Size
Each network technology imposes maximum frame size Called Maximum Transmission Unit
(MTU) MTUs differ
Internet Can contain heterogeneous technologies Must accommodate multiple MTUs
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EECB 423 V.110
Illustration Of How Two MTUsCause A Problem For IP
Host 1 Creates datagram for Host 2 Chooses datagram size of 1500 octets Transmits datagram across network 1
Router R Receives datagram over network 1 Must send datagram over network 2 Employs fragmentation
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EECB 423 V.111
Datagram Fragmentation
Performed by routers Needed when datagram larger than MTU
of network Divides datagram into pieces called
fragments Each fragment has datagram header Fragments sent separately Ultimate destination reassembles
fragments
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EECB 423 V.112
Illustration Of Datagram Fragmentation
Each fragment has IP datagram header Header fields
Identify original datagram Indicate where fragment fits
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EECB 423 V.113
Example Of Reassembly
Host H1 generates 1500-octet datagram Router R1 fragments Router R2 transmits fragments Host H2 reassembles
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EECB 423 V.114
Fragmenting A Fragment
Needed when fragment too large for network MTU
Arbitrary subfragmentation possible Router divides fragments into smaller pieces All fragments at same ‘‘level’’
Offset given with respect to original datagram
Destination cannot distinguish subfragments
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EECB 423 V.115
Fragment Loss
Receiver Collects incoming fragments Reassembles when all fragments arrive Does not know identity of router that
did fragmentation Cannot request missing pieces
Consequence: Loss of one fragment means entire datagram lost
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EECB 423 V.116
Summary Internet transmission paradigm
Source host Zero or more routers Destination host
Datagram encapsulated in network frame for transmission
Network hardware has maximum payload size Called MTU Datagram must be smaller than hardware
MTU
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EECB 423 V.117
Summary
Datagram fragmentation Accommodates multiple MTUs Performed by router Divides datagram into pieces Ultimate destination reassembles
Fragments can be fragmented Multiple levels possible All offsets at one level Loss of any fragment means loss of entire
datagram