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Ipv 4 Pk t Processing

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Ipv 4 Pk t Processing
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  IP Packet Processing Transmission of a frame over Ethernet The IP packet is placed in an Ethernet frames as follows: IP Broadcast/Multicast Address: TheIP destination address is checkedto see if the system should also receive a copy of the packet. This happensif this is an IP network broadcast address (or a multicast address is usedthat matches one of the registered IP multicast filters set by the IPreceiver). If a copy is required, it is sent to the loopback interface. Thisdirectly delivers the packet to the IP input routine. the srcinal packetcontinues to be processed1. IP Unicast Address: TheIP destination address is checked to see if theaddress is the unicast (source) IP address of the sending system. Suchpackets are sent directly to the loopback interface (i.e. never reach thephysical Ethernet interface)2. Next Hop IP Address: The sender then determines the next hop address -that is the IP address of the next Intermediate System/End System toreceive the packet. Once this address is known, the  Address ResolutionProtocol (arp) is used to find the appropriate MAC address to be used inthe Ethernet frame. This is a two stage process: (i) the arp cache isconsulted, to see if the MAC address is already known, in which case thecorrect address is added and the packet queued for transmission. (ii) If the3. IP Packet Processinghttp://www.erg.abdn.ac.uk/~gorry/eg3567/inet-p...1 of 410/23/2014 03:25 AM  MAC address is not in the arp cache, the arp protocol is used to requestthe address, and the packet is queued until an appropriate response (ortimeout) occurs. MTU: Thesize of the packet is checked against the MTU of the link onwhich it is to be sent. (Note the MTU of the loopback interface may bedifferent to that of Ethernet). If required, IP fragmentation is performed, oran ICMP error message is returned, which may trigger Path MTU Discovery at the sending End Host.4. Encapsulation:  The Ethernet frame is completed, by inserting theDestination, Source and Ethernet Type fields. When Tags are used, theappropriate 802.1pQ Tag is inserted following the MAC header (thePriority field in the Tag may be set based on the IP DSCP value).5. Transmit: The frame is transmitted using the MAC procedure forEthernet.6. Reception of a frame from Ethernet The following summary shows the processing performed by an end system in anIP network . It is assumed that the system is connected to an Ethernet network. The received frames are processed as follows: MAC Protocol: The Ethernet controller in the network interface card verifies that the frame is:Not less than the minimum frame length not greater than themaximum length (1500 B)Contains a valid CRC at the endDoes not contain a residue (i.e. extra bits which do not form a byte)1. MAC Address: The frame is then filtered based on the MAC destination2. IP Packet Processinghttp://www.erg.abdn.ac.uk/~gorry/eg3567/inet-p...2 of 410/23/2014 03:25 AM  address and accepted only if:It is a broadcast frame (i.e. all bits of the destination address field areset to 1)It is a multicast frame to a registered MAC group addressIt is a unicast frame to the node's own MAC address Or the interface is acting in promiscuous mode (i.e. as a bridge) MAC SAP: The frame is then demultiplexed based on the specified MACpacket type (SAP)Frames carrying an IEEE 802.1pQ Tag will have their  Virtual LAN information checked and processed, before skipping the Tag field andreading the following EtherType field.It is passed to the appropriate protocol layer (e.g. LLC,  ARP, IP) Frames carrying Packets destined for IP have a type field of 0x0800and those for arp have a value 0x0806.3. IP Check: The IP packet header is checked, including:By checking the protocol type =4 (i.e. current version of IP)By verifying the header checksumBy checking the header packet length4. IP Address: The destination IP network address is then checked:If it matches an IP address of the node then it is acceptedIf it is network broadcast packet to the node's network it is acceptedIf it is a multicast packet to an IP multicast address which is in usethen it is acceptedIf it is none of these, it is forwarded using the routing table (if possible) or discarded5. IP Fragmentation: Packets for the node are then checked concerningwhether reassembly is required:The fragmentation offset value and more flags are inspectedFragments are placed in a buffer until other fragments are received tocomplete the packet.6. IP SAP: The IP protocol field (SAP) is checked:The SAP field identifies the transport protocol (e.g. 1 = ICMP; 6 =TCP; 17= UDP) The complete packet is passed to the appropriate transport layerprotocol.7. See also: IPIP FragmentationSome decodes of IP Packet HeadersIntroduction to RoutersOperation of a Router IP Packet Processinghttp://www.erg.abdn.ac.uk/~gorry/eg3567/inet-p...3 of 410/23/2014 03:25 AM  Gorry Fairhurst - Date: 11/02/2006  EG3557 IP Packet Processinghttp://www.erg.abdn.ac.uk/~gorry/eg3567/inet-p...4 of 410/23/2014 03:25 AM
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