Chapter 3 Review of Protocols And Packet Formats

Outline Protocol Layering Ethernet IP TCP / IP ARP

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

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

Layer 1 And 2 (Physical And Network Interface) Two protocols are important Ethernet ATM We will concentrate on Ethernet

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

Ethernet Frame Processing Dest. Address Source Address Frame Type 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 Data In Frame 6 2 46 - 1500 Header Payload

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)

Ethernet Addressing (2 / 2) Broadcast address is all 1s Single bit determines whether remaining addresses are unicast or multicast multicast bit x x x x x x x m x x x x x x x x

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)

IP OPTION (MAY BE OMITTED) IP Datagram Format Format of each packet sent across Internet Fixed-size fields make parsing efficient 4 8 16 19 24 31 VERS HLEN SERVICE TOTAL LENGTH ID FLAGS F.OFFSET TTL TYPE HDR CHECKSUM SOURCE DESTINATION IP OPTION (MAY BE OMITTED) PADDING BEGINNING OF PAYLOAD

IP Datagram Fields Field Meaning VERS Version number of IP being used (4) HLEN Header length measured in 32-bit units SERVICE Level of service desired TOTAL LENGTH Datagram length in octets including header ID Unique value for this datagram FLAGS Bits to control fragmentation F. OFFSET Position of fragment in original datagram TTL Time to live (hop countdown) TYPE Contents of payload area HDR CHECKSUM One’s-complement checksum over header SOURCE IP address of original sender DESTINATION IP address of ultimate destination IP OPTIONS Special handling parameters PADDING To make options a 32-bit multiple

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

Next-Hop Forwarding Routing table Route lookup 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

UDP Datagram Format 16 31 SOURCE PORT DESTINATION PORT MESSAGE LENGTH 16 31 SOURCE PORT DESTINATION PORT MESSAGE LENGTH CHECKSUM BEGINNING OF PAYLOAD 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

OPTIONS (MAY BE OMITTED) TCP Segment Format Sent end-to-end Fixed-size fields make parsing efficient 4 10 16 24 31 SOURCE PORT DESTINATION PORT SEQUENCE ACKNOWLEDGEMENT HLEN NOT USED CODE BITS WINDOW CHECKSUM URGENT PRT OPTIONS (MAY BE OMITTED) PADDING BEGINNING OF PAYLOAD

TCP Segment Fields Field Meaning SOURCE PORT ID of sending application DESTINATION PORT ID of receiving application SEQUENCE Sequence number for data in payload ACKNOWLEDGEMENT Acknowledgement of data received HLEN Header length measured in 32-bit units NOT USED Currently unassigned CODE BITS URGENT, ACK, PUSH, RESET, SYN, FIN WINDOW Receiver’s buffer size for additional data CHECKSUM One’s-complement checksum over entire segment URGENT PTR Pointer to urgent data in segment OPTIONS Special handling PADDING To make options a 32-bit multiple

Destination IP Address Destination IP Address 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.

Encapsulation And Transmission Field in each header specifies type of encapsulated packet UDP HEADER UDP PAYLOAD IP HEADER IP PAYLOAD ETHERNET HEADER ETHERNET PAYLOAD

Address Resolution Protocol (ARP) 8 16 24 31 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) Format when ARP used with Ethernet and IP Each Ethernet address is six octets Each IP address is four octets

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