Point-toPoint Routing in the Internet in Java

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Point-toPoint Routing in the Internet
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number destination next of hops to interface network router destination 22311 22312 22313 1 1 1 1 2 3
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Figure 44-7: Routing table in router In Figure 44-7, note that the entries in the "next router" column are all empty This is because all of the networks (22311 , 22312 , and 22313 ) are each directly attached to the router, that is, there is no need to go through an intermediate router to get to the destination host However, if host A and host E were separated by two routers, then within the routing table of the first router along the path from A to B, the appropriate row would indicate 2 hops to the destination and would specify the IP address of the second router along the path The first router would then forward the datagram to the second router, using the link layer protocol that connects the two routers The second router then forwards the datagram to the destination host, using the link layer protocol that connects the second router to the destination host You may recall from 1 that we said that routing a datagram in the Internet is similar to a person driving a car and asking gas station attendants at each intersection along the way how to get to the ultimate destination It should now be clear why this an appropriate analogy for routing in the Internet As a datagram travels from source to destination, it visits a series of routers At each router in the series, it stops and asks the router how to get to its ultimate destination Unless the router is on the same LAN as the ultimate destination, the routing table essentially says to the datagram: "I don't know exactly how to get to the ultimate destination, put I do know that the ultimate destination is in the direction of the link (analogous to a road) connected to interface 3" The datagram then sets out on the link connected to interface 3, arrives at a new router, and again asks for new directions From this discussion we see that the routing tables in the routers play a central role in routing datagrams through the Internet But how are these routing tables configured and maintained for large networks with mulitple paths between sources and destinations (such as in the Internet) Clearly, these routing tables should be configured so that the datagrams follow good (if not optimal) routes from source to destination As you probably guessed, routing algorithms - like those studied in Section 42 have the job of configuring and maintaining the routing tables Furthermore, as discussed in Section 43, the Internet is partitioned into autonomous systems (ASs): intra-AS routing algorithms independently configure the routing tables within the autonomous systems; inter-AS routing algorithms have the job configuring routing tables so that datagrams can pass through multiple autonomous systems We will discuss the Internet's intra-AS and inter-AS routing algorithms in Section 45 But before moving on to routing algorithms, we cover three more important topics for the IP protocol, namely, the datagram format, datagram fragmentation, and the Internet Control Message Protocol (ICMP)
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443 Datagram Format
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The IPv4 datagram format is shown in Figure 44-8
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Point-toPoint Routing in the Internet
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Figure 44-8: IPv4 datagram format The key fields in the IPv4 datagram are the following:
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Version Number: These 4 bits specify the IP protocol version of the datagram By looking at the version number, the router can then 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 44-8 The datagram format for the "new" version of IP (IPv6) is discussed in Section 47 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 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 TOS: The type of service (TOS) bits were included in the IPv4 header to allow different "types" of IP datagrams to be distinguished from each other, presumably so that they could be handled differently in times of overload When the network is overloaded, for example, it would be useful to be able to distinguish network control datagrams (eg, see the ICMP discussion in Section 445) from datagrams carrying data (eg, HTTP messages) It would also be useful to distinguish real-time datagrams (eg, used by an IP telephony application) from non-real-time traffic (eg, FTP) More recently, one major routing vendor (Cisco) interprets the first three ToS bits as defining differential levels of service that can be provided by the router The specific level of service to be provided is a policy issue determined by the router's administrator We shall explore the topic of differentiated service in detail in 6 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 to 65,535 bytes However, datagrams are rarely greater than 1500 bytes, and are often limited in size to 576 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 IP, IPv6, simply does not allow for fragmentation Time-to-live: The time-to-live (TTL) field is included to insure that datagrams do not circulate forever (due to, for example, a long lived router 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 only used when an IP datagram reaches its final destination The value of this field indicates the transport-layer protocol at the destination to which the data portion of this IP datagram will 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 listing of all possible numbers, see [RFC 1700] Note that the the protocol number in the IP datagram has a role
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