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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