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Chapter 3Review of Protocols And Packet Formats
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Outline
Protocol Layering Ethernet IP TCP / IP ARP
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Protocol Layering (1 / 2)
OSI 7-layers Internet 5-layer
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
Application
Transport
Internet
Network Interface
Physical
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Protocol Layering (2 / 2)
Multiple protocols can occur at each layer The resulting software is known as a stack Outgoing data must pass down through all layers of
the stack between the application and the physical network
Incoming data must pass up through all layers of the stack
We will learn that optimizing communication among layers is an important aspect of building high-speed networks system
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Layer 1 And 2 (Physical And Network Interface)
Two protocols are important– Ethernet– ATM
We will concentrate on Ethernet
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Ethernet
Layer 1 standards specify details such as the voltage used on copper wires– Carrier Sense Multiple Access with Collision
Detection (CSMA / CD)
Layer 2 standards specify details such as the format of frames and addresses
Variable-size frames
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Ethernet Frame Processing
Dedicated physical layer hardware– Checks and removes preamble and CRC on input– Computes and appends CRC and preamble on output
Layer 2 systems use source, destination and (possibly) type fields
6 6 2 46 - 1500
Dest.Address
SourceAddress
FrameType Data In Frame
Header Payload
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Ethernet Addressing (1 / 2)
48-bit addressing Unique address assigned to each station
(NIC) Destination address in each packet can
specify delivery to– A single computer (unicast)– All computers in broadcast domain (broadcast)– Some computers in broadcast domain (multicast)
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Ethernet Addressing (2 / 2)
Broadcast address is all 1s Single bit determines whether remaining
addresses are unicast or multicast
x x x x x x x m
x x x x x x x x
x x x x x x x x
x x x x x x x x
x x x x x x x x
x x x x x x x x
multicast bit
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Internet Protocols Of Interest
Layer 2– Address Resolution Protocol (ARP)
Layer 3– Internet Protocol (IP)
Layer 4– User Datagram Protocol (UDP)– Transmission Control Protocol (TCP)
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IP Datagram Format
Format of each packet sent across Internet Fixed-size fields make parsing efficient
VERS HLEN SERVICE TOTAL LENGTH
ID FLAGS F.OFFSET
TTL TYPE HDR CHECKSUM
SOURCE
DESTINATION
IP OPTION (MAY BE OMITTED) PADDING
BEGINNING OF PAYLOAD
0 4 8 16 19 24 31
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IP Datagram Fields
Field MeaningVERS Version number of IP being used (4)HLEN Header length measured in 32-bit unitsSERVICE Level of service desiredTOTAL LENGTH Datagram length in octets including headerID Unique value for this datagramFLAGS Bits to control fragmentationF. OFFSET Position of fragment in original datagramTTL Time to live (hop countdown)TYPE Contents of payload areaHDR CHECKSUM One’s-complement checksum over headerSOURCE IP address of original senderDESTINATION IP address of ultimate destinationIP OPTIONS Special handling parametersPADDING To make options a 32-bit multiple
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IP addressing
32-bit Internet address assigned to each computer
Virtual, hardware independent value Prefix identifies network; suffix identifies host Network systems use address mask to
specify boundary between prefix and suffix The address remain unchanged as the
datagram passes across the Internet
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Next-Hop Forwarding
Routing table– Found in both hosts and routers– Stores ( destination, mask, next_hop ) tuples
Route lookup– Takes destination address as argument– Finds next hop– Uses longest-prefix match
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UDP Datagram Format
Field Meaning
SOURCE PORT ID of sending application
DESTINATION PORT ID of receiving application
MESSAGE LENGTH Length of datagram including the header
CHECKSUM One’s-complement checksum over entire datagram
SOURCE PORT DESTINATION PORT
MESSAGE LENGTH CHECKSUM
BEGINNING OF PAYLOAD
0 16 31
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TCP Segment Format
Sent end-to-end Fixed-size fields make parsing efficient
SOURCE PORT DESTINATION PORT
SEQUENCE
ACKNOWLEDGEMENT
HLEN NOT USED CODE BITS WINDOW
CHECKSUM URGENT PRT
OPTIONS (MAY BE OMITTED) PADDING
BEGINNING OF PAYLOAD
0 4 10 16 24 31
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TCP Segment Fields
Field MeaningSOURCE PORT ID of sending applicationDESTINATION PORT ID of receiving applicationSEQUENCE Sequence number for data in payloadACKNOWLEDGEMENT Acknowledgement of data receivedHLEN Header length measured in 32-bit unitsNOT USED Currently unassignedCODE BITS URGENT, ACK, PUSH, RESET, SYN, FINWINDOW Receiver’s buffer size for additional dataCHECKSUM One’s-complement checksum over entire segmentURGENT PTR Pointer to urgent data in segmentOPTIONS Special handlingPADDING To make options a 32-bit multiple
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Protocol Port Numbers
16-bits values knows as protocol port numbers Each UDP user datagram and each TCP segment carries the
port numbers :– SOURCE PORT– DESTINATION PORT
The port spaces of the two protocols are completely independent
UDP demultiplex packets (each application)
TCP demultiplex packets (each application)
Destination IP Address Destination Port NO.
Destination IP Address Destination Port NO.
Source IP Address Source Port NO.
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Encapsulation And Transmission
Field in each header specifies type of encapsulated packet
UDP HEADER UDP PAYLOAD
IP HEADER IP PAYLOAD
ETHERNET HEADER ETHERNET PAYLOAD
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Address Resolution Protocol (ARP)
Format when ARP used with Ethernet and IP Each Ethernet address is six octets Each IP address is four octets
ETHERNET ADDRESS TYPE (1) IP ADDRESS TYPE (0800)
ETH ADDRLEN(6) IP ADDRLEN(4) OPERATION
SENDER’S ETH ADDR (first 4 octets)
SENDER’S ETH ADDR (last 2 octets) SENDER’S IP ADDR (first 2 octets)
SENDER’S IP ADDR (last 2 octets) TARGET’S ETH ADDR (first 2 octets)
TARGET’S ETH ADDR (last 4 octets)
TARGET’S IP ADDR (all 4 octets)
0 8 16 24 31
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QUESTION?
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