Download ECE 526

ECE 526 – Network
Processing Systems Design
Networking: protocols and packet format
Chapter 3: D. E. Comer
Fall 2008
Goals
• Review important protocols and packet formats
• Understand packet formats in different layers
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Outline
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•
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Internet reference model
Linker layer
Network layer
Transportation layer
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Internet 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 Protocols
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•
•
•
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Linker layer: Ethernet
Network layer: IP
Transport layer: TCP, UDP
We are not interested in
physical layer: CSMA …
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Data 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 Processing
• Machine access control address (MAC)
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─
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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 Protocol
• 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 Fields
• Which field changed between hops?
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IP 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 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 Forwarding
• Routing Table
─ Found in both hosts and routers
─ Stores destination, mask, next hop
• Example Routing Table
network destination
131.230.0.0
131.230.194.0
Default
netmask
gateway/next hop
255.255.0
131.230.193.66
255.255.255.0
131.230.194.66
131.230.195.
• Route lookup
─ Takes destination address as arguments
─ Finds next hop
─ Uses longest-prefix match
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IP Related Protocols
• Several other protocols support IP:
• We’ll cover routing tables with packet processing later
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Transport 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 Format
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Transport 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 Header
• Checksum, source and destination ports similar to UDP
• Sequence and Acknowledgement is byte count (not
packet)
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TCP 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 Encapsulation
• Protocols are encapsulated when sent out
• Example
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Address 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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Questions
• 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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Summary
• 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 Class
• Read Comer Chapter 4
─ Computer Architecture: traditional network processing systems
implementation
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