The Linux Networking Architecture [Electronic resources] نسخه متنی

Figure 14-1. The structure of the global Internet.

Section 3.2).

Let's assume that, in the first case, the packet has not yet arrived in the destination computer. The router checks the destination address in the IP packet header and the information in the routing table to determine how the packet should be forwarded. Next, the packet travels from one router to the next until it eventually arrives in the destination computer. Chapter 16 discusses routing in IP networks.

14.1.2 The IP Packet Header

Figure 14-2 shows the format of an IP packet. The fields of the IP packet header have the properties described below.

Version: This field contains the version number of the Internet Protocol used. Including the version number provides a way to use several versions of the Internet Protocol. Currently, only versions v4 and v6 are defined. In general, the two versions are not specified in the Version field; they are identified by their protocol identifiers on the MAC layer (0x800 for IPv4, 0x86DD for IPv6see include/linux/if_ether.h)

IHL (Internet Header Length): This field contains the length of the packet header, because it can be longer than 20 bytes, if options are used. The length is stated in multiples of 32 bits. The smallest valid value is 5 (no options), and the highest value is 15 (corresponds to a header length of 60 bytes).

Codepoint: This field was originally called Type of Service. Its purpose was changed to Differentiated Services Codepoint in RFC 2474. This field shows the forwarding behavior used [NBBB98].

Total length: This value includes the entire length of the IP packet. Its 16-bit length makes the maximum size of an IP datagram 65,535 bytes. RFC 791 specifies that each IP-enabled computer should be capable of processing data packets with a size of 576 bytes. In general, however, it is possible to process packets with a bigger length. Otherwise, a packet has to be fragmented. (See Section 14.2.3.)

Fragment ID: The destination computer can use this identifier, together with the sender address, to reassemble fragments of IP datagrams to reconstruct the original datagrams. All fragments of an IP datagram have the same fragment ID, which is set by the sender.

Flags: An IP packet can include two flags (the third flag is currently not used): Don't Fragment (DF) and More Fragments (MF). MF is used for a fragmented packet. The DF bit means that a datagram must not be fragmented, even if this means that the packet cannot be transported any further. The MF bit shows whether more fragments follow this IP packet (i.e., the MF flag is set in all fragments of a datagram, except for the last fragment).

Fragment Offset: Section 14.3 describes all IP options.

14.1.3 Lifetime of an IP Data Packet

Faulty functions in the network can cause packets to circulate in the network rather than arriving at their destination address. These data packets consume valuable resources in the network, so they have to be destroyed by control mechanisms at some point in time.

The following method is used to destroy such packets: The TTL (Time To Live) field of the IP data header takes the number of routers (hops). This field is actually intended to specify the lifetime of a packet in seconds, but it is currently used to count the hops through the routers on the path. Each router reduces this value by 1, and the packet is rejected when the value 0 is reached. This prevents a packet that cannot be delivered from circulating forever. In addition, you can set a specific TTL value in the sender to limit the reach of a packet.

14.1.4 Addressing in the Internet

Three different addresses are used to reach a communication partner or an application in the Internet. These addresses identify a unique communication endpoint within the Internet and are often called sockets:

The IP address specifies a unique computer in the Internet. Each computer in an IP network has to have a unique Internet address. Section 14.1.5 explains the structure of this address format and the set of different classes.

The transport protocol ID specifies the transport protocol instance used (i.e., TCP, UDP, ICMP, etc.). The Internet Protocol uses this identifier to know which transport protocol is used.

The port number identifies a unique and specific application within the TCP and UDP transport protocols (multiplexing).

The following section discusses the first part of the sockets defined above, IP addresses and their structure. The chapters dealing with the transport layer introduce and describe the TCP and UDP protocols, which are the most important transport protocols today. These chapters also explain the meaning of port numbers.

14.1.5 IP Addresses and IP Address Classes

Each network device in the Internet or in other IP-based networks has its own unique IP address. Computers connected to several networks concurrently (multihomed hosts) have a separate address for each network connection. These addresses are assigned by the Internet Assigned Numbers Authority (IANA) and their national representatives (e.g., Reseau IP EuropeRIPE). Notice that these addresses are not assigned on an individual basis, but in blocks by so-called network classes. If somebody needs an IP address to connect a computer to the Internet, then he or she will obtain a network address and an entire range of addresses. For this reason, each range of network addresses is managed within those addresses themselves.

Accordingly, IP addresses are structured in a hierarchy: They are divided into a network part and a computer or host part. Figure 14-3 shows the classes and their different network and host parts.