1 chapter 21 internetworking part 2 (datagram encapsulation, transmission, fragmentation,...
TRANSCRIPT
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Chapter 21Chapter 21
Internetworking
Part 2(Datagram Encapsulation, Transmission,
Fragmentation, Reassembly)
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Internet Transmission Paradigm
(General Case)
Internet Transmission Paradigm
(General Case)Source host
Forms datagramIncludes destination addressSends to nearest router
Intermediate routersForward datagram to next router
Final routerDelivers to destination host
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Datagram TransmissionDatagram Transmission
Datagram sent across conventional networkFrom source host and routerBetween intermediate routersFrom final router to destination host
Network hardware does not recognizeDatagram formatIP addresses
Encapsulation needed
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Illustration of IP Encapsulation
Illustration of IP Encapsulation
Entire datagram treated like dataFrame type identifies contents as IP
datagramFrame destination address gives next hop
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Frame and Datagram Destination AddressesFrame and Datagram Destination Addresses
Frame addressHardware (MAC) addressNext hop
Datagram addressIP addressUltimate destination
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Frame Address For Encapsulated Datagram
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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Frames and DatagramsFrames and Datagrams
Datagram survives entire trip across InternetFrame only survives one hop
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Illustration of Frame Headers
Used for Datagram Transmission
Illustration of Frame Headers
Used for Datagram Transmission
Each hop extracts datagram and discards frame
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Maximum Frame SizeMaximum Frame Size
Each network technology imposes maximum frame size
Called Maximum Transmission Unit (MTU)MTUs differ
InternetCan contain heterogeneous technologiesMust accommodate multiple MTUs
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Illustration of How Two MTUs
Cause a Problem for IP
Illustration of How Two MTUs
Cause a Problem for IPHost 1
Creates datagram for Host 2Chooses datagram size of 1500 octetsTransmits datagram across network 1
Router RReceives datagram over network 1Must send datagram over network 2Employs fragmentation
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Datagram FragmentationDatagram Fragmentation
Performed by routersNeeded when datagram larger than MTU of
networkDivides datagram into pieces called fragmentsEach fragment has datagram headerFragments sent separatelyUltimate destination reassembles fragments
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Illustration of Datagram Fragmentation
Illustration of Datagram Fragmentation
Each fragment has IP datagram headerHeader fields
Identify original datagramIndicate where fragment fits
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Datagram header – fields for fragmentsDatagram header – fields for fragments
Flags1 – reserved (0), 2 – Do not fragment, 3 – More Fragments
Fragment offset
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Example of ReassemblyExample of Reassembly
Host H1 generates 1500-octet datagram
Router R1 fragments
Router R2 transmits fragments
Host H2 reassembles
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Multiple Fragmenting Points
Multiple Fragmenting Points
Let MTUs along internet path be15001500100015005761500
Result: fragmentation can occur twice
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Fragmenting a FragmentFragmenting a Fragment
Needed when fragment too large for network MTUArbitrary subfragmentation possibleRouter divides fragments into smaller piecesAll fragments at same “level”
Offset given with respect to original datagramDestination cannot distinguish subfragments
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Fragment LossFragment Loss
ReceiverCollects incoming fragmentsReassembles when all fragments arriveDoes not know identity of router that did
fragmentationCannot request missing pieces
Consequence: loss of one fragment means entire datagram lost
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SummarySummary
Internet transmission paradigmSource hostZero or more routersDestination host
Datagram encapsulated in network frame for transmission
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Summary (continued)Summary (continued)
Network hardware has maximum payload size
Called MTUDatagram must be smaller than hardware MTU
Internet can have multiple MTUs
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Summary (continued)Summary (continued)
Datagram fragmentationAccommodates multiple MTUsPerformed by routerDivides datagram into piecesUltimate destination reassembles
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Summary (continued)Summary (continued)
Fragments can be fragmentedMultiple levels possibleAll offsets at one level
Loss of any fragment means loss of entire datagram