An IP packet is simply a the data you want transferred encapsulated in a frame or packet. The packet is typically broken up into smaller chunks to be transmitted over a T1 line. It’s kind of like writing a letter, you could take one line at a time and mail it in separate envelopes, if you lose one envelope then no big deal you could probably understand the message or at least decipher it after you received the rest.
The IP packet can be a rather complex entity with header information such as IP address sent from, destination IP address, header checksums, Identification field, protocol used and many others. Below is pictorial view of a packet. The data itself may also have sending and destination addresses, checksums, flags, etc so deciphering a packet can be like peeling an onion, revealing layer after layer.
Version – static 4 bit value indicating which version of IP is being used.
IHL IP Header Length – 4 bit value indicating the number of 32-bit words that make up the header.
Type of Service – 8 bit value that is used tell the network how to treat the IP packet. These bits are generally used to indicate the Quality of Service (QoS) for the IP Packet.
Packet Length – 16 bit value indicating the size of the IP Packet in terms of bytes. This gives a maximum packet size of 65536 bytes.
Identification – 16 bit field used for reassembling the packet at the destination.
Flags – 3 bits indicating to network equipment if the IP packet can be further fragmented or not and if the packet is the last fragment or not of a larger transfer.
Fragment offset – 13 bit value used in the reassembly process at the destination.
Time to Live – 8 bit value telling the network how long an IP packet can exist in a network before it is destroyed.
Protocol – 8 bit value used to indicate the type of protocol being used (TCP, UDP, etc).
Header checksum – 16 bit 1’s compliment value designed to indicate errors in the header only. Every node in the network has to check and re-insert a new checksum as the header changes at every node (TTL value is decremented)
Source address – 32 bit value representing the IP address of the sender of the IP packet.
Destination address – 32 bit value representing the IP address of the packets final destination.
Options – variable bit field representing options 🙂 The actual options requested are not optional in that there are well defined options that can be requested, what is optional is whether to request theses options or not.
Padding -Variable size bit field. These bits are used to ensure a 32 bit boundary for the header is achieved.
Next time we’ll highlight the main difference between a data transfer and a VoIP call over a network.