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Introduction Previous chapter:

IP datagram format

Routing

This chapter:

IP datagram transmission

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Datagram Transmission IP software selects the next hop to

Transmit the datagram across the

physical network Network hardware does not understand

datagram format or internet addressing

Hardware understands frames Frames may differ from network to

network

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EncapsulationFigure 21.1 - An IP datagram encapsulated in ahardware frame. The entire datagram resides in theframe data area. In practice, the frame format used

with some technologies includes a frame trailer as wellas frame header.

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Encapsulation How does a receiver know whether the

data area in an incoming frame contains

an IP datagramor other data?

Answer: The sender and receiver agreeon the value used in the frame type

field.Aspecial value is reserved for IP.

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Encapsulation In addition to placing a datagram in the data

area of a frame, encapsulation requires the

sender to supply the physical address of thenext computer

Address binding: computes the appropriatehardware address (ARP - chapter 19)

The binding translatesthe IP addressof thenext hop intoan equivalent hardwareaddress.

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SummaryA datagram is encapsulated in a frame

The destination address in the frame isthe address of the next hop

The destination of the next hop isobtained by translating the IP address

of the next hop to an equivalenthardware address.

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Transmission Across An

Internet

Figure 21.2 - An IP datagram as it appears at each step during a tripacross an internet. Whenever it travels across a physical network, thedatagram is encapsulated in a frame appropriate to the network.

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Transmission Across An

Internet Encapsulation applies to one transmission at

a time

After the sender selects a next hop, thesender encapsulates the datagram in a frameand transmits the result across the physicalnetwork to the next hop.

When the frame receives the next hop, thereceiving software removes the IP datagramand discards the frame.

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Transmission Across An

Internet Frame headers do not accumulate during a

trip through the internet.

Before a datagram is transmitted across agiven network, the datagram is encapsulated

When the frame arrives at the next hop, thedatagram is removed from the incomingframe, before being routed and encapsulatedin an outgoing frame.

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MTU, Datagram Size, and

Encapsulation MTU is the Maximum Transmission Unitthe maximum amount of data that a

frame can carry. In an Internet that connects

heterogeneous networks, MTU

restrictions can cause a problem

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MTU, Datagram Size, and

Encapsulation

Figure 21.3 - An example of a router that connects two networks withdifferent MTU values. A frame that travels across the network 1 cancontain 1500 octets of data, while a frame that travels across network 2

can contain at most 1000 octets of data.

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MTU, Datagram Size, and

EncapsulationAn IP router uses a technique known as

fragmentationto solve the problem of

heterogeneous MTUs. When a datagram is larger than the

MTU, the router divides the datagram

into smaller pieces called fragments. Each fragment is sent separately.

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MTU, Datagram Size, and

Encapsulation A bit in the FLAGS fieldindicate whether a

datagram is a fragment or a complete

datagram. A fragment has the same format as the other

datagrams.

The FRAGMENT OFFSETfield in the header ofa fragment specifies where in the originaldatagram the fragment belongs.

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MTU, Datagram Size, and

Encapsulation

Figure 21.4 - An IP datagram divided into threefragments. Each fragment carries some data from theoriginal datagram, and has an IP header similar to the

original datagram.

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SummaryA datagram cannot be larger than the

MTU of a network over which it is sent.

A router divides the datagram intosmaller pieces called fragments.

Each fragment uses the IP datagram

format, but carries only parts of thedata.

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Reassembly The process of creating a copy of the original

datagram from fragments is calledreassembly.

All fragments have the same destinationaddress as the original datagram.

The fragment that carries the final piece of

data has an additional bit set in the header. A receiver performing reassembly can tell

whether all fragments have arrivedsuccessfully.

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Reassembly

Figure 21.5 - An example internet in which hosts can generate datagramsthat require fragmentation. Once a datagram has been fragmented, thefragments are forwarded to the final destination, which reassembles them.

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Fragment Loss Recall that IP does not guarantee datagram

delivery Some fragments may be delayed or lost Datagrams with lost fragments cannot be

reassembled Fragments may be saved temporarily. IP specifies a maximum time to hold

fragments. After a timer expires, saved fragments are

discarded.

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Fragmenting a Fragment If a fragment reaches another network that

has a smaller MTU, it is possible to further

fragment a fragment. IP does not distinguish between original

fragments and sub fragments.

The advantage of making all fragments thesame is that a receiver can performreassembly without first reassembling subfragments.

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Summary An IP datagram is encapsulated in a network

frame for transmission across a hardware

network. To encapsulate a datagram, the sender

places the entire datagram in the data area ofa network frame.

Each network technology defines themaximum amountof data (MTU)accepted.

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Summary When a router receives a datagram that

is larger than the network MTU, the

router divides the datagram intofragments.

Each fragment travels to the ultimate

destination, which is responsible forreassembling fragments into theoriginal datagram.