4-4 Network Layer.pptx

8/13/2019 4-4 Network Layer.pptx

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Network Layer (Part 4)

Computer Networks

Tutun JuhanaTelecommunication EngineeringSchool of Electrical Engineering & InformaticsInstitut Teknologi Bandung

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INTERNET PROTOCOLVERSION 4 (IPV4)


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This is why we needTotal Length field


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FRAGMENTATION

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Maximum Transfer Unit (MTU)

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Maximum IP datagram length is 65,535bytes

We might have to divide the datagram tomake it possible to pass through differentphysical networks This is calledfragmentation.

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When a datagram is fragmented, each fragmenthas its own header with most of the fieldsrepeated, but some changed

A fragmented datagram may itself be

fragmented if it encounters a network with aneven smaller MTU (a datagram can befragmented several times before it reaches thefinal destination)

A datagram can be fragmented by the sourcehost or any router in the path

The reassembly of the datagram, however, is

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When a datagram is fragmented, required partsof the header must be copied by all fragments

The option field may or may not be copied The host or router that fragments a datagrammust change the values of three fields: flags,fragmentation offset, and total length (The rest of

the fields must be copied) The value of the checksum must be recalculated

regardless of fragmentation.

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Fields Related to Fragmentation

Identification Flags

Fragmentation offset

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Identification

16 bit Identifies a datagram originating from the source host The combination of the identification and source IP

address must uniquely define a datagram as it leaves

the source host. When a datagram is fragmented, the value in the

identification field is copied into all fragments allfragments have the same identification number, which is

also the same as the original datagram The identification number helps the destination in

reassembling the datagram all fragments having thesame identification value should be assembled into onedatagram

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Flags

D (Do not fragment) 1 : machine must not fragment the datagram 0 : datagram can be fragmented if necessary

M (More fragments) 1 : datagram is not the last datagram 0 : This is the last or only fragment

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not used


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Fragmentation offset

This 13-bit field shows the relative position of thisfragment with respect to the whole datagram. Measured in units of 8 bytes

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identification

The last fragment

the offset field isalways relativeto the originaldatagram


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Final destination host can reassemble theoriginal datagram from the fragments received (ifnone of them is lost) using the following strategy:a. The first fragment has an offset field value of zero.

b. Divide the length of the first fragment by 8. Thesecond fragment has an offset value equal to thatresult.

c. Divide the total length of the first and second

fragment by 8. The third fragment has an offsetvalue equal to that result.

d. Continue the process. The last fragment has a morebit value of 0.

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OPTIONS

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Are not required for a datagram But all IP software implementations must be

able to handle options if they are present inthe header.

Can be used for network testing anddebugging.

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Format

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type-length-value (TLV)


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Option Types

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do not require the length or the data fields

require the length or the data fields


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No-Operation Option

1-byte option used as a filler betweenoptions

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End-of-Option Option

1-byte option used for padding at the end of the optionfield. It can only be used as the last option Only one end-of-

option option can be used After this option, the

receiver looks for the payload data If more than 1 byteis needed to align the option field, some no-operationoptions must be used, followed by an end-of-optionoption

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Record-Route Option is used to record the Internet routers that handle

the datagram It can list up to nine router IP addresses

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Pointer field is an offset integer field containing the bytenumber of the first empty entry It points to the firstavailable entry.


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Strict-Source-Route Option

used by the source to predetermine aroute for the datagram as it travels throughthe Internet

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Loose-Source-Route Option

similar to the strict source route, but it is morerelaxed Each router in the list must be visited,but the datagram can visit other routers as well.

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Timestamp A timestamp option is used to record the time of

datagram processing by a router The time is expressed in milliseconds from midnight,

Universal Time.

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CHECKSUM

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Checksum Calculation at the Sender

The packet is divided into k sections, eachof n bits.

All sections are added together usingones complement arithmetic.

The final result is complemented to makethe checksum.

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Checksum in the IP Packet

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4-4 Network Layer.pptx

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