IP Receive Packet Processing

Reception of a frame over Ethernet

The received frames are processed as follows:

1. 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 the maximum length (1500 B)
   • Contains a valid CRC at the end
   • Does not contain a residue (i.e. extra bits which do not form a byte)
2. MAC Address: The frame is then filtered based on the MAC destination address and accepted only if:
   • It is a broadcast frame (i.e. all bits of the destination address field are set to 1)
   • It is a multicast frame to a registered MAC group address
   • It is a unicast frame to the node's own MAC address
   • Or the interface is acting in promiscuous mode (i.e. as a bridge)
3. MAC SAP: The frame is then demultiplexed based on the specified MAC packet type (SAP)
   • Frames carrying an IEEE 802.1pQ Tag will have their Virtual LAN information checked and processed, before skipping the Tag field and reading 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 0x0800 and those for arp have a value 0x0806.
4. 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 checksum
   • By checking the header packet length
5. IP Address: The destination IP network address is then checked:
   • If it matches an IP address of the node then it is accepted
   • If it is network broadcast packet to the node's network it is accepted
   • If it is a multicast packet to an IP multicast address which is in use then it is accepted
   • If it is none of these, it is forwarded using the routing table (if possible) or discarded
6. IP Fragmentation: Packets for the node are then checked concerning whether reassembly is required:
   • The fragmentation offset value and more flags are inspected
   • Fragments are placed in a buffer until other fragments are received to complete the packet.
7. IP protocol: The IP protocol field (SAP) is checked:
   • The protocol field identifies the transport protocol (e.g. 1 = ICMP; 6 = TCP; 17= UDP)
   • The complete packet is passed to the appropriate transport layer protocol.

The following overview shows the demuxing performed by an end system in an IP system connected to an Ethernet network.

See also:

IP

IP Send Processing

Some decodes of IP Packet Headers

Introduction to Routers

Operation of a Router