ECE 526 – Network ECE 526 – Network Processing Systems Processing Systems

DesignDesignNetworking: protocols and packet

formatChapter 3: D. E. Comer

Fall 2008

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GoalsGoals

• Review important protocols and packet formats

• Understand packet formats in different layers

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OutlineOutline

• Internet reference model• Linker layer• Network layer• Transportation layer

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Internet Reference ModelInternet Reference Model

• Five layer Internet reference model

• Multiple protocol can occur at each layer

• Question: example protocols in each layer?

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Example Network ProtocolsExample Network Protocols

• Linker layer: Ethernet• Network layer: IP• Transport layer: TCP,

UDP• We are not interested in

physical layer: CSMA …

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Data Link LayerData Link Layer• Providing communication between adjacent

systems─ Point-to-point or shared media communication─ Specification of media access (e.g., CSMA)

• Three types of communication─ Unicast: frame destined for single station─ Multicast: frame destined for a subset of stations─ Broadcast: frame destined for all stations in broadcast

domain

• Two protocols are important─ Ethernet─ ATM

• We will concentrate on Ethernet

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Ethernet Frame ProcessingEthernet Frame Processing

• Machine access control address (MAC)─ Hardware address─ 48-bit─ Globally unique─ Not assigned to computer but Network Interface Card (NIC)

• 16-bit frame type: indicating the next layer protocol─ 0800 (IP), 0806 (ARP)

• Additionally: 64-bit preamble, 32-bit CRC trailer

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Network Layer: Internet Network Layer: Internet ProtocolProtocol

• Internet Protocol (IP) functionality─ Defines basic delivery service throughout the Internet─ Provides end-to-end connectivity

• Processing by IP routers• Internet packet header:

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IP Datagram FieldsIP Datagram Fields

• Which field changed between hops?

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IP AddressingIP Addressing• 32-bit Internet address assigned to each

computer• Virtual, hardware independent value• Prefix identifies network; suffix identifies host

─ Hosts within a subnet have same address prefix─ Example: ECE 131.230.194.*, SIUC 131.230.*─ Network address mask to specify boundary between

prefix and suffix

• Who assigns the prefix?─ Internet Assigned Numbers Authority (IANA) sign it to

Regional Internet Registries (RIR)

• Are IP addresses globally unique?─ No, network address translator allow private subnets─ Private subnet address space: 192.168.0.0/16,

172.16.0.0/12 and 10.*

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Addressing QuestionAddressing Question• What is the difference between Ethernet and IP

address?─ Software (reconfigurable) vs. hardware (hard coded)─ Globally unique or not─ IP: prefix and suffix

• Why need another address (IP), if Ethernet address is globally unique?─ mobility moving the hosts or routers from one network

to another─ easiness to construct a subnet: prefix and suffix

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IP ForwardingIP Forwarding• Routing Table

─ Found in both hosts and routers─ Stores destination, mask, next hop

• Example Routing Table network destination netmask gateway/next hop

131.230.0.0 255.255.0 131.230.193.66 131.230.194.0 255.255.255.0

131.230.194.66 Default 131.230.195.

• Route lookup─ Takes destination address as arguments─ Finds next hop─ Uses longest-prefix match

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IP Related ProtocolsIP Related Protocols• Several other protocols support IP:

• We’ll cover routing tables with packet processing later

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Transport LayerTransport Layer• End-to-end protocols for communication between

applications─ Transport layer is not changed inside the network

• How are applications identified?─ Port number used for de-multiplexing

• Two transport layer protocols─ UDP (user datagram protocol)─ TCP (transport control protocol)

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UDP Datagram FormatUDP Datagram Format

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Transport Control ProtocolTransport Control Protocol• UDP shortcomings

─ Unreliable (packet loss)─ Packet reordering─ No congestion control─ No flow control

• TCP addresses these problems─ Acknowledgements and retransmission timers─ Sequence numbers─ Congestion and flow control windows

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TCP HeaderTCP Header• Checksum, source and destination ports similar to

UDP• Sequence and Acknowledgement is byte count

(not packet)

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TCP QuestionsTCP Questions• How is a connection between two hosts

established?─ Three-way handshake

• How is reliability ensured when packet is lost?─ Packet is not acknowledged, timeout occurs,

retransmission.

• How is congestion control achieved?─ Congestion window is continually increased to use

available bandwidth─ Congestion window is reduced when packet loss occurs

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Protocol EncapsulationProtocol Encapsulation• Protocols are encapsulated when sent out• Example

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Address Resolution ProtocolAddress Resolution Protocol• Used to map IP address of a computer to an

Ethernet address• Used only within one network• ARP header

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QuestionsQuestions• Why not one protocol

instead of many ones?─ different applications

need a different functionalities

─ different protocols solve different problems

─ easy to debug and design: concentrate one layer only

─ many protocols is connected by common protocol (IP)

• Any cons for layered structure?

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SummarySummary• Protocols standards

─ Defines semantics of computer communication─ Specify packet formats─ Addressing, forwarding

• Internet protocols are divided into fiver conceptual layers─ Layer 2: ethernet, ARP─ Layer 3: IP─ Layer 4: TCP, UDP

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For Next ClassFor Next Class

• Read Comer Chapter 4─ Computer Architecture: traditional network processing

systems implementation