COEN_233_M5 - Network Layer(Part II Anant Jalnapurkar...

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Network Layer (Part II) Anant Jalnapurkar
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TCP/IP Protocol Suite ADDRESS MAPPING The delivery of a packet to a host or a router requires two levels of addressing: logical and physical . We need to be able to map a logical address to its corresponding physical address and vice versa. These can be done using either static or dynamic mapping.
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TCP/IP Protocol Suite Address Resolution Protocol Anytime a host or a router has an IP datagram to send to another host or router, it has the logical (IP) address of the receiver. But the IP datagram must be encapsulated in a frame to be able to pass through the physical network. This means that the sender needs the physical address of the receiver. A mapping corresponds a logical address to a physical address. ARP accepts a logical address from the IP protocol, maps the address to the corresponding physical address and pass it to the data link layer.
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TCP/IP Protocol Suite Position of ARP in TCP/IP protocol suite
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TCP/IP Protocol Suite ARP Operation ARP Cache Timeout
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ARP Packet Format HardwareType: type of physical network (e.g., Ethernet) ProtocolType: type of higher layer protocol (e.g., IP) HLEN & PLEN: length of physical and protocol addresses Operation: request or response Source/Target Physical/Protocol addresses
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TCP/IP Protocol Suite Encapsulation of ARP packet Data Preamble and SFD Destination address Source address Type CRC 8 bytes 6 bytes 6 bytes 2 bytes 4 bytes Type : 0x0806
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TCP/IP Protocol Suite 8 An ARP request is L2 broadcast; an ARP reply is unicast. Note
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TCP/IP Protocol Suite Four cases using ARP
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TCP/IP Protocol Suite A host with IP address 130.23.43.20 and physical address B2:34:55:10:22:10 has a packet to send to another host with IP address 130.23.43.25 and physical address A4:6E:F4:59:83:AB. The two hosts are on the same Ethernet network. Show the ARP request and reply packets encapsulated in Ethernet frames. Solution Figure 8.6 shows the ARP request and reply packets. Note that the ARP data field in this case is 28 bytes, and that the individual addresses do not fit in the 4-byte boundary. That is why we do not show the regular 4-byte boundaries for these addresses. Also note that the IP addresses are shown in hexadecimal. Example 8.1
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TCP/IP Protocol Suite Example 8.1
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TCP/IP Protocol Suite Error Reporting: ICMP v4 The IP protocol has no error-reporting or error correcting mechanism. What happens if something goes wrong? What happens if a router must discard a datagram because it cannot find a router to the final destination, or because the time-to-live field has a zero value? These are examples of situations where an error has occurred and the IP protocol has no built-in mechanism to notify the original host. Internet Control Message Protocol (ICMP) is used for error reporting and building debug tools.
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TCP/IP Protocol Suite Figure 9.1 Position of ICMP in the network layer
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TCP/IP Protocol Suite ICMP encapsulation
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TCP/IP Protocol Suite ICMP MESSAGES ICMP messages are divided into two broad categories: error-reporting messages and query messages. The error-reporting messages report
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