The Internet Protocol (IP)

The Internet Protocol (IP)

The Internet Protocol (IP): Forwarding and Addressing in the Internet


Our discussion of network-layer addressing and forwarding thus far has been without reference to any particular computer network. In this section, we'll turn our attention to how addressing and forwarding are done in the Internet. We'll see that Internet addressing and forwarding are important components of the Internet Protocol (IP). There are two versions of IP in use today. We'll first study the widely deployed IP protocol version 4, which is generally referred to simply as IPv4. We'll study IP version 6, which has been proposed to replace IPv4, at the end of this section.

But before beginning our foray into IP, let's take a step back and consider the components that make up the Internet's network layer. As shown in Figure 1, the Internet's network layer has three major components. The first component is the IP protocol, the topic of this section. The second major component is the routing component, which determines the path a datagram follows from source to destination. We mentioned earlier that routing protocols compute the forwarding tables that are used to forward packets through the network. We'll study the Internet's routing protocols in "Routing in the Internet". The Final component of the network layer is a facility to report errors in datagrams and respond to requests for certain network-layer information.

A look inside the Internets network layer

Well cover the Internets network-layer error- and information-reporting protocol, the Internet Control Message Protocol (ICMP), in "Internet Control Message Protocol (ICMP)".

Datagram Format


Remember that a network-layer packet is referred to as a datagram. We begin our study of IP with an overview of the syntax and semantics of the IPv4 datagram. You might be thinking that nothing could be drier than the syntax and semantics of a packets bits. However, the datagram plays a vital role in the Internet - every networking student and professional needs to see it, absorb it, and master it. The IPv4 datagram format is shown in Figure 2. The key fields in the IPv4 datagram are the following:

●  Version number. These 4 bits specify the IP protocol version of the datagram. By looking at the version number, the router can determine how to interpret the remainder of the IP datagram. Different versions of IP use different datagram formats. The datagram format for the current version of IP, IPv4, is shown in Figure 2. The datagram format for the new version of IP (IPv6) is discussed at the end of this section.

●  Header length. Because an IPv4 datagram can contain a variable number of options (which are included in the IPv4 datagram header), these 4 bits are needed

IPv4 datagram format

to determine where in the IP datagram the data actually begins. Most IP datagrams do not contain options, so the typical IP datagram has a 20-byte header.

●  Type of service. The type of service (TOS) bits were included in the IPv4 header to allow different types of IP datagrams (for instance, datagrams particularly requiring low delay, high throughput, or reliability) to be distinguished from each other. For instance, it might be useful to distinguish red-time datagrams (such as those used by an IP telephony application) from non-real-time traffic (for example, FTP). The specific level of service to be provided is a policy issue determined by the routers administrator. Well explore the topic of differentiated service in detail in "Multimedia Networking".

●  Datagram length. This is the total length of the IP datagram (header plus data), measured in bytes. Since this field is 16 bits long, the theoretical maximum size of the IP datagram is 65,535 bytes. However, datagrams are rarely larger than 1,500 bytes.

●  Identifier, flags, fragmentation offset. These three fields have to do with so-called IP fragmentation, a topic we will consider in depth shortly.  Interestingly, the new version of lP, IPv6, does not allow for fragmentation at routers.

●  Time-to-live. The time-to-live (TTL) field is included to ensure that datagrams do not circulate forever (due to, for example, a long-lived routing loop) in the network. This field is decremented by one each time the datagram is processed by a router. If the TTL field reaches 0, the datagram must be dropped.

●  Protocol. This field is used only when an IP datagram reaches its final destination. The value of this field indicates the specific transport-layer protocol to which the data portion of this IP datagram should be passed. For example, a value of 6 indicates that the data portion is passed to TCP, while a value of 17 indicates that the data is passed to UDP. For a list of all possible values, see [IANA Protocol Numbers 2009]. Note that the protocol number in the IP datagram has a role that is analogous to the role of the port number field in the transport-layer segment. The protocol number is the glue that binds the network and transport layers together, whereas the port number is the glue that binds the transport and application layers together. We'll see in "The Link Layer and Local Area Networks" that the link-layer frame also has a special field that binds the link layer to the network layer.

●  Header checksum. The header checksum aids a router in detecting bit errors in a received IP datagram. The header checksum is computed by treating each 2 bytes in the header as a number and summing these numbers using 1s complement arithmetic. As discussed in "Connectionless Transport: UDP", the 1s complement of this sum, known as the Internet checksum, is stored in the checksum field. A router computes the header checksum for each received IP datagram and detects an error condition if the checksum carried in the datagram header does not equal the computed checksum. Routers typically discard datagrams for which an error has been detected. Note that the checksum must be recomputed and stored again at each router, as the TTL field, and possibly the options field as well, may change. A question often asked at this point is, why does TCP/IP perform error checking at both the transport and network layers? There are several reasons for this repetition. First, note that only the IP header is checksummed at the IP layer, while the TCP/UDP checksum is computed over the entire TCP/UDP segment. Second, TCP/UDP and IP do not necessarily both have to belong to the same protocol stack. TCP can, in principle, run over a different protocol (for example, ATM) and IP can carry data that will not be passed to TCP/UDP.

●  Source and destination IP addresses. When a source creates a datagram, it inserts its IP address into the source IP address field and inserts the address of the ultimate destination into the destination IP address field. Often the source host determines the destination address via a DNS lookup, as discussed in "Application Layer". We'll discuss IP addressing in detail in "IPv4 Addressing".

●  Options. The options fields allow an IP header to be extended. Header options were meant to be used rarely - hence the decision to save overhead by not including the information in options fields in every datagram header. However, the mere existence of options does complicate matters - since datagram headers can be of variable length, one cannot determine a priori where the data field will start. Also, since some datagrams may require options processing and others may not, the amount of time needed to process an IP datagram at a router can vary greatly, These considerations become particularly important for IP processing in high-performance routers and hosts. For these reasons and others, IP options were dropped in the IPv6 header, as discussed in "IPv6".       

●  Data (payload). Finally, we come to the last and most important field - the raison d etre for the datagram in the first place! In most circumstances, the data field of the IP datagram contains he transport-layer segment (TCP or UDP) to be delivered to the destination. However, the data field can carry other types of data, such as ICMP messages (discussed in "Internet Control Message Protocol (ICMP)").

Note that an IP datagram has a total of 20 bytes of header (assuming no options). If the datagram carries a TCP segment, then each (nonfragmented) datagram carries a total of 40 bytes of header (20 bytes of IP header plus 20 bytes of TCP header) along with the application-layer message.


Tags

internet addressing, internet forwarding, internet protocol, routing protocols, forwarding tables

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