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TCP/IP 49
• Hop limit: number of routers that can process a certain packet is limited
• Next header: defines the type of the first optional header
• Version field: This 4-bit field indicates the IP version number. For IPv6, this is value 6.
• Payload length field: this 16-bit number is an unsigned integer with which the number
of bytes in the IPv6 datagram is indicated that follows after the standard header with a
length of 40 bytes.
As the transport layer (TCP and UDP) and the data link layer (e.g. Ethernet) calculate protocols
on the Internet checksums, the designers of IPv6 have decided that the calculation of
checksums on the internet layer is no longer necessary.
3.3 Transmission Control Protocol (TCP)
3.3.1 Introduction
IP is a connectionless packet delivery service. TCP will have a difficult task. Using the unreliable
packet service of IP, it has to provide a reliable data delivery service to different
application programmes. For many applications it is essential that a transport system offers
reliability: the system has to guarantee that data are not lost, cannot be duplicated or do not
arrive in the right order.
3.3.2 End-to-end transport service
The TCP protocol is responsible for the correct sending of information over one or more networks.
The exchange form of TCP is known as connection oriented: a logical connection is
established, used and then stopped again. TCP is therefore a end-to-end protocol. Figure
3.10 shows why TCP is an end-to-end protocol. TCP sees IP as a mechanism with which TCP
can exchange data on a certain host with TCP on a remote host.
Figure 3.10: TCP if end-to-end transportprotocol
From the TCP point of view, the complete Internet is a communication system that can accept
and deliver messages without changing or interpreting the content.