tyt05fi.htm

tcpip 协议学习电子书籍 第一次上传东西

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</TD></TABLE></CENTER><P>When a message is to be routed using the fewest-hops approach, the table of distances is consulted, and the route with the fewest number of hops is selected. The message is then routed to the gateway that is closest to the destination network. When intermediate gateways receive the message, they perform the same type of table lookup and forward to the next gateway on the route.

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<P>There are several problems with the fewest-hops approach. If the tables of the gateways through which a message travels to its destination have different route information, it is conceivable that a message that left the source machine on the shortest route could end up following a more circuitous path because of differing tables in the intervening gateways. The fewest-hops method also doesn't account for transfer speed, line failures, or other factors that could affect the overall time to travel to the destination; it is merely concerned with the shortest apparent distance, assuming that all connections are equal. To accommodate these factors, another routing method must be used.

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<A ID="E69E73" NAME="E69E73"></A>

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<FONT SIZE=4 COLOR="#FF0000"><B>Type of Service Routing</B></FONT></CENTER></H4>

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<P>This type of routing depends on the type of routing service available from gateway to gateway. This is called <I>type of service</I> (TOS) routing. It is also more formally called <I>quality of service</I> (QOS) by OSI. TOS includes consideration for the speed and reliability of connections, as well as security and route-specific factors.

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<P>To effect TOS routing, most systems use dynamic updating of tables that reflect traffic and link conditions. They also take into account current queue lengths at each gateway, because the fastest theoretical route might not matter if the message is backlogged in a queue. This information is obtained through the frequent transfer of status messages between gateways, especially when conditions deteriorate.

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<P>Dynamic updating of tables can have a disadvantage in that if tables are updated too frequently, a message might circulate through a section of the internetwork without proper routing to its destination, or proceed through a long and convoluted path. For this reason, dynamic updating occurs at regular but not too frequent intervals. To prevent stray datagrams from circulating on the internetwork too long, the Time to Live information in the IP message header is important.

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<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>The IP header's Time to Live (TTL) field is very important to dynamic gateway routing protocols, which is why it is a mandatory field. Without it, datagrams could circulate throughout the network indefinitely.</NOTE>

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<P>The dynamic nature of TOS routing can sometimes cause a message's fragments to be routed in different ways to a destination. For example, if a long message of 10 datagrams is being sent by one route, but the routing tables are changed during transmission to reflect a backlog, the remainder of the datagrams might be sent via an alternate route. This doesn't matter, of course, because the receiving machine reassembles the message in the proper order as the datagrams are received.

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<FONT SIZE=4 COLOR="#FF0000"><B>Updating Gateway Routing Information</B></FONT></CENTER></H4>

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<P>A somewhat simplified example of a dynamic update is useful at this stage. The exact communications protocols between gateways are examined in more detail later today.

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<P>Assume that two autonomous networks are connected to each other at two locations, as shown in Figure 5.3, with connections to different autonomous networks at other locations. The A&#150;C connection and the B&#150;D connection can both be used for routing from within the networks, depending on which is the optimal path. Gateway C has a copy of gateway A's routing table, and vice versa. Gateways B and D each have copies of the other's routing tables, as well. These copies are transmitted at intervals so the gateways can maintain an up-to-date picture of the connections available through the other gateway. The gateways use EGP to send the messages. (They would use GGP if they were core gateways.)

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<P><B><A HREF="05tyt03.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/05tyt03.gif">Figure 5.3. Two interconnected networks.</A></B>

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<P>Suppose that a gateway link within one of the networks was broken due to a machine or connection failure, such as that between gateway C and machine X in Figure 5.3. Gateway C would find out about the problem through an IGP message and update its routing table to reflect the break, usually by putting the largest legal value for routing length in that entry. (Remember that IGP is a general term for any internal network protocol for gateway communications, such as RIP or HELLO.) Gateway C transfers its new copy of the routing table to gateway A.

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<P>Routing a message to machine Y would now be impossible through the C&#150;X connection. However, because gateway A has the routing information from C, and it exchanges routing information with gateway B, which also exchanges with gateway D, any message passing through either D or B for machine Y could be rerouted up through gateway A, then C, and finally to Y. An EGP message between B&#150;D and A&#150;C would indicate that the new route costs less than the maximum value assigned going through C&#150;X (which is broken), so the round-about transfer through the four gateways can be used.

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<P>EGP messages between gateways are usually sent whenever a connection problem exists and the routing information is set to its maximum (worst) value, or when a better connection alternative has been discovered for some reason. This can be because of an update from a remote gateway's routing table, or the addition of new connections, machines, or networks to the system. Whichever happens, an EGP message informs all the connected gateways of the changes.

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<A ID="E68E52" NAME="E68E52"></A>

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<FONT SIZE=5 COLOR="#FF0000"><B>The IGP and EGPGateway Protocols</B></FONT></CENTER></H3>

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<P>Gateways need to know what is happening to the rest of the network in order to route datagrams properly and efficiently. This includes not only routing information but also the characteristics of subnetworks. For example, if one gateway is particularly slow but is the only access method to a subnetwork, other gateways on the network can tailor the traffic to suit.

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<P>A GGP is used to exchange routing information between devices. It is important not to confuse routing information, which contains addresses, topology, and details on routing delays, with the algorithms used to make routing information. Usually the routing algorithms are fixed within a gateway and not modified. Of course, as the routing information changes, the algorithm adapts the chosen routes to reflect the new information.

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<P>GGPs are primarily for autonomous (self-complete) networks. An autonomous system uses gateways that are connected in one large network, such as one might find in a large corporation. Two kinds of gateways must be considered in an autonomous network. The gateways between smaller subnetworks help tie the small systems into the larger corporate network, but the gateways for each subnetwork are usually under the control of one system (usually in the IS department). These gateways are considered autonomous because the connections between gateways are constant and seldom change. These gateways communicate through an IGP.

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<P>Large internetworks like the Internet are not as static as corporate systems. Gateways can change constantly as the subsidiary networks make changes, and the communications routes between gateways are more subject to change, too. For widely spread companies, there might be gateways spread throughout the country (or the world) that are all part of the same corporate network but use the Internet to communicate. The communications between these gateways are slightly different than when they are all physically connected together. These gateways communicate through an EGP.

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<P>There are fewer rules governing IGPs than EGPs simply because the IGP can handle custom-developed applications and protocols within its local network. When the Internet is used for gateway-to-gateway communications, the messages must conform to the internetwork standards. Also, when connecting two subnetworks, it is possible to send only one message to the subnetwork gateway through EGP, which can then be duplicated, modified, and propagated to all gateways on the internal system using IGP. EGP has formalized rules governing its use.

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<A ID="E68E53" NAME="E68E53"></A>

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<FONT SIZE=5 COLOR="#FF0000"><B>Gateway-to-Gateway Protocol (GGP)</B></FONT></CENTER></H3>

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<P>GGP is used for communications between core gateways. A recent improvement of the protocol, called SPREAD, is starting to be used but is not yet as common as GGP. Even if GGP is phased out in favor of SPREAD, it is a useful illustration of gateway-to-gateway protocols.

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<P>GGP is a vector-distance protocol, meaning that messages tend to specify a destination (vector) and the distance to that destination. Vector-distance protocols are also called Bellman-Ford protocols, after the researchers who first published the idea. For a vector-distance protocol to be effective, a gateway must have complete information about all the gateways on the internetwork; otherwise, computing a distance with a fewest-hops type of protocol cannot succeed.

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<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>You might recall from earlier today that core gateways have complete information about all other core gateways, so a vector-distance protocol works. Non-core gateways don't have a complete internetwork map, so GGP-type messages are not useful.</NOTE>

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<P>A gateway establishes its connections to other gateways by sending out messages, waiting for replies, and then building a table. This is initially accomplished when a gateway is installed and has no routing information at all. This aspect of communications is not defined within GGP but relies on network-specific messages. Once the initial table has been defined, GGP is used for all messages.

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<P>Connectivity with another gateway on the Internet is determined using the K-out-of-N method. In this procedure, a gateway sends an echo message to another gateway and waits for a reply. It repeats this every fifteen seconds. According to the Internet standards, if the gateway does not receive three (K) replies out of four (N) requests, the other gateway is considered down, or unusable, and routing messages are not sent to that gateway. This process can be repeated at regular intervals.

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<P>If a down gateway becomes active again, the Internet standards require two out of four echo messages to be acknowledged. This is called J-out-of-M, where J is two and M is four. The Internet-assigned values for J, K, M, and N can be changed for autonomous networks, but the standard defines the values for use on the Internet itself.

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<P>Each message between gateways has a sequence number that is incremented with each transmitted message. Each gateway tracks its own sequence number for sending to every other gateway it is connected to, as well as the incoming sequence numbers from that gateway. They are not necessarily the same, because more messages might flow one way than the other, although usually each message should have an acknowledgment or reply of some type.

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<P>Sequence numbers have important meanings for the messages and are not just for the sake of keeping an incremental count of the traffic volume. When a gateway receives a message from another gateway, it compares the sequence number in that message to the last received sequence number in its internal tables. If the latest message has a higher sequence number than the last message received, the gateway accepts the message and updates its sequence number to the latest received value. If the number was less than the last received sequence number, the message is considered old and is ignored, with an error message containing the just-received message sent back. This process is shown in Figure 5.4.

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<P><B><A HREF="05tyt04.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/05tyt04.gif">Figure 5.4. Processing sequence numbers in </B><B>GGP.</A></B>

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<P>The receiving gateway acknowledges the received message by sending a return message that contains the sequence number of the just-received message. The other gateway compares that number with the number of its last sent message, and if they are the same, the gateway knows that the message was properly received. If the numbers do not match, the gateway knows an error occurred and transmits the message again.

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<P>When a message is ignored by the recipient gateway, the sending gateway receives a message with the sequence number of the ignored message. It can then determine which messages were skipped and adjust itself accordingly, resending messages that need to be sent.

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<P>The GGP message format is shown in Figure 5.5. After it is constructed, it is encapsulated into an IP datagram that includes source and target addresses. The first field is a message type, which is set to a value of 12 for routing information. The sequence number was discussed earlier and provides an incremental counter for each message. The Update field is set to a value of 0 unless the sending gateway wants a routing update for the provided destination address, in which case it is set to a value of 1. The Number of Distances field holds the number of groups of addresses contained in the current message.

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<P><B><A HREF="05tyt05.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/05tyt05.gif">Figure 5.5. The GGP message format.</A></B>

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<P>For each distance group in the message, a distance value and the number of networks that can be reached at that distance are provided, followed by all the network address identifications. According to the GGP standard, not all the distances need to be reported, but the more information supplied, the more useful the message is to each gateway.

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<P>GGP does not deal with full Internet addresses specifically, so the host portion of the address does not necessarily have to be included in the address, although the network address is always provided. This can result in different lengths of addresses in the identification field (8, 16, or 24 bits, depending on the type of address).

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<P>Three other formats are used with GGP messages, as shown in Figure 5.6. The acknowledgment message uses the Type field to indicate whether the message is a positive acknowledgment (type is set to 2) or a negative acknowledgment (type is set to 10) . The sequence number, as mentioned earlier today, is used to identify the message to which the acknowledgment applies.

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<P><B><A HREF="05tyt06.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/05tyt06.gif">Figure 5.6. Other GGP message formats.</A></B>

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<P>The echo request and echo reply formats are passed between gateways to inform the gateways of status changes and to ensure the gateway is up. An echo request has the Type field set to the value 8, whereas an echo reply has the Type field set to a value of 0. Because the address of the sending gateway is embedded in the IP header, it is not duplicated in the GGP message. The remaining 24 bits of the message are unused.

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<P>The network interface status message is used by a gateway to ensure that it is able to send and receive messages properly. This type of message can be sent to the originating gateway itself, with the type field set to a value of 9 and the IP address in the header set to the network interface's address.

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<A ID="E68E54" NAME="E68E54"></A>

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<FONT SIZE=5 COLOR="#FF0000"><B>The External Gateway Protocol (EGP)</B></FONT></CENTER></H3>

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<P>As mentioned earlier, an EGP is used to transfer information between non-core neighboring gateways. Non-core gateways contain complete details about their immediate neighbors and the machines attached to them, but they lack information about the rest of the network. Core gateways know about all the other core gateways but often lack the details of the machines beyond a gateway.

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<P>EGP is usually restricted to information within the gateway's autonomous system. This prevents too much information from passing through the networks, especially when most of the information that relates to external autonomous systems would be unusable to another gateway. EGP therefore imposes restrictions on the gateways about the machines EGP passes routing information about.

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<A ID="E69E75" NAME="E69E75"></A>

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<FONT SIZE=4 COLOR="#FF0000"><B>Neighbors and EGP</B></FONT></CENTER></H4>

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<P>Because EGP was developed to enable remote systems to exchange routing information and status messages, the protocol is heavily based in requests or commands followed by replies. The four EGP commands and their possible responses are shown in Table 5.2.

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<FONT COLOR="#000080"><B>Table 5.2. EGP commands.</B></FONT></CENTER>

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<CENTER><TABLE  BORDERCOLOR=#000040 BORDER=1 CELLSPACING=2 CELLPADDING=3 >

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<P><B><I>Command Name</I></B>

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<P><B><I>Command </I></B><B><I>Description</I></B>

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<P><B><I>Response Name</I></B>

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<P><B><I>Response </I></B><B><I>Description</I></B>

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<P>Request

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<P>Request that a neighbor become a gateway

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<P>Confirm/Refuse

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<P>Agree or refuse the request

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<P>Cease

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<P>Request the termination of a neighbor

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<P>Cease-Ack

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<P>Agree to termination

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<P>Hello

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<P>Request confirmation of routing to neighbor (neighbor reachability)

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<P>IHU

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<P>Confirms the routing

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<P>Poll

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<P>Request that the neighbor provide network information (network reachability)

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<P>Update

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<P>Provides network information</FONT>

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<P>To understand Table 5.2 properly, you must understand the concept of <I>neighbor</I> to an internetwork. Gateways are neighbors if they share the same subnetwork. They might be gateways to the same network (such as the Internet) or work with different networks. When the two want to exchange information, they must first establish communications between each other; the two gateways are essentially agreeing to exchange routing information. This process is called <I>neighbor acquisition.</I>

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<P>Neighbor doesn't mean the networks have to be next to each other. They are connected by a gateway, but the networks can be on different continents. The term neighbor has to do with connections, not geography.

<P>The process of becoming neighbors is formal, because one gateway might not want to become a neighbor at that particular time (for any number of reasons, but usually because the gateway is busy). It begins with a Request, which is followed by either an acceptance (Confirm) or refusal (Refuse) from the second machine. If the two gateways are neighbors, either can break the relationship with a Cease message.

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<P>After two gateways become neighbors, they assure each other that they are still in contact by occasionally sending a Hello message, to which the second gateway responds with an IHU (I Heard You) message as soon as possible. These Hello/IHU messages can be sent at any time. With several gateways involved on a network, the number of Hello messages can become appreciable as the gateways continue to remain in touch. This process is called <I>neighbor reachability.</I>

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<P>The other message pair sent by EGP is network reachability, in which case one gateway sends a Poll message and expects an Update message in response. The response contains a list of networks that can be reached through that gateway, with a number representing the number of hops that must be made to reach the networks. By assembling the Update messages from different neighbors, a gateway can decide the best route to send a datagram.

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<P>Finally, an error message is returned whenever the gateway cannot understand an incoming EGP message.

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<A ID="E69E76" NAME="E69E76"></A>

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<FONT SIZE=4 COLOR="#FF0000"><B>EGP Messages</B></FONT></CENTER></H4>