IP Transport Mechanisms

TCP and User Datagram Protocol (UDP) have different characteristics that various applications can use. If reliability is more important than delay, for instance, you can use TCP/IP to guarantee packet delivery. UDP/IP does not utilize packet re-transmissions, however. This can lower reliability, but in some cases a late retransmission is of no use.

To compare various transport layer protocols, you must first understand what makes up an IP packet. Figure 7-4 shows the fields of the IP packet.

Figure 7-4. IP Packet Fields

Figure 7-4. IP Packet Fields

IP packet fields are defined as follows:

• Version—indicates whether IPv4 or IPv6 is being used.

• IP header length (IHL)—Indicates the datagram header length in 32-bit words.

• Type of service—Specifies how a particular upper-layer protocol wants the current datagram to be handled. You can assign packets various quality of service (QoS) levels based on this field.

• Total length—Specifies the length of the entire IP packet, including data and header, in bytes.

• Identification—Contains an integer that identifies the current datagram. This field is used to help piece together datagram fragments.

• Flags—A 3-bit field of which the low-order 2 bits control fragmentation. The high-order bit in this field is not used. One bit specifies whether you can fragment the packet; the second bit specifies whether the packet is the last fragment in a series of fragmented packets.

• Time To Live—Maintains a counter that gradually decrements down to zero, at which point the datagram is discarded. This keeps packets from looping endlessly.

• Protocol—Indicates which upper-layer protocol receives incoming packets after IP processing is complete.

• Header checksum—Verifies that the header is not corrupted.

• Source address—The sending address.

• Destination address—The address to receive the datagram.

• Options—Enables IP to support various options, such as security.

• Data—Contains application data as well as upper-layer protocol information.

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