单项选择题 Traditional network layer packet forwarding relies on the information provided by network layer ______ protocols, or static routing, to make an independent forwarding decision at each ______ within the network. The forwarding decision is based solely on the destination ______ IP address. All packets for the same destination follow the same path across the network, if no other equal-cost ______ exist. Whenever a router has two equal-cost paths toward a destination, the packets toward the destination might take one or both of them, resulting in some degree of load sharing. Enhanced Interior Gateway Routing Protocol (EIGRP) also supports non-equal-cost ______ sharing although the default behavior of this protocol is equal-cost. You must configure EIGRP variance for non-equal-cost load balancing.
The traditional way of allocating a single channel, such as a telephone think, among multiple competing users is to chop up its ______ by using one of the multiplexing schemesr, such as FDM. If there are N users, the bandwidth is divided into N equal-sized portions, with each user being assigned one portion. Since each user has private frequency ______, there is now no interference among users. When there is only a small and constant number of users, each of which has a steady stream or heavy load of ______ this division is a simple and efficient allocation mechanism. A wireless example is FM radio stations. Each station gets a portion of the FM band and uses it most of the time to broadcast its signal. However when the number of senders is large and varying or the traffic is ______, FDM presents some problems. If the spectrum is cut up into N regions and fewer than N users are currently interested in communicating, a large piece of valuable spectrum will be wasted. And if more than N users want to communicate, some of them will be denied ______ for lack of bandwidth, even if some of the users who have been assigned a frequency band ever transmit or receive anything.
CDMA for cellular systems can be described as follows. As with FDMA, each cell is allocated a frequency ______, which is split into two parts: half for reverse (mobile unit to base station) and half for ______ (base station to mobile unit). For full-duplex ______, a mobile unit uses both reverse and forward channels. Transmission is in the form of direct-sequence spread ______ which uses a chipping code to increase the data rate of the transmission, resulting in an increased signal bandwidth. Multiple access is provided by assigning ______ chipping codes to multiple users, so that the receiver can recover the transmission of an individual unit from multiple transmissions.
Traditional IP packet forwarding analyzes the ______ IP address contained in the network layer header of each packet as the packet travels from its source to its final destination. A router analyzes the destination IP address independently at each hop in the network. Dynamic ______ protocols or static configuration builds the database needed to analyze the destination IP address (the routing table). The process of implementing traditional IP routing also is called hop-by-hop destination-based ______ routing. Although successful, and obviously widely deployed, certain restrictions, which have been realized for some time, exist for this method of packet forwarding that diminish its ______. New techniques are therefore required to address and expand the functionality of an IP-based network infrastructure. This first chapter concentrate on identifying these restrictions and presents a new architecture, known as multipleprotocol ______ switching, that provides solutions to some of these restrictions.
The de facto standard application program interface (API) for TCP/IP applications is the "sockets" interface. Although this API was developed for ______ in the early 1980s. it has also been implemented on a wide variety of non-Unix systems. TCP/IP ______ written using the sockets API have in the past enjoyed a high degree of portability and we would like the same ______ with IPv6 applications. But changes are required to the sockets API to support IPv6 and this memo describes these changes. These include a new socket address structure to carry IPv6 ______, new address conversion functions, and some new socket options. These extensions are designed to provide Access to the basic IPv6 features required by TCP and UDP applications, including multicasting, while introducing a minimum of change into the system and providing complete ______ for existing IPv4 applications.
The TCP protocolis a ______ layer protocol. Each connection connects two TCPs that may be just one physical network apart or located on opposite sides of the globe. In other words, each connection creates a ______ with a length that may be totally different from another path created by another connection. This means that TCP cannot use the same retransmission time for all connections. Selecting a fixed retransnussion time for all connections can result in serious consequences. If the retransmission time does not allow enough time for a ______ to reach the destination and an acknowledgment to reach the source, it can result in retransmission of segments that are still on the way. Conversely, if the retransmission time is longer than necessary for a short path, it may result in delay for the application programs. Even for one single connection, the retransmission time should not be fixed. A connection may be able to send segments and receive ______ faster during nontraffic period than during congested periods. TCP uses the dynamic retransmission time, a transmission time is different for each connection and which may be changed during the same connection. Retransmission time can be made ______ by basing it on the round-trip time (RTT). Several formulas are used for this purpose.
Let us now see how randomization is done when a collision occurs. After a ______, time is divided into discrete slots whose length is equal to the worst-case round-trip propagation time on the ether (2t). To accommodate the longest path allowed by Ethernet, the slot tome has been set to 512 bit times, or 51.2μsec. After the first collision, each station waits either 0 or 1 ______ times before trying again. If two stations collide and each one picks the same random number, they will collide again. After the second collision, each one picks either 0, 1, 2, or 3 at random and waits that number of slot times. If a third collision occurs (the probability of this happening is 0.25), then the next time the number of slots to wait is chosen at ______ from the interval 0 to 23-1. In general, after i collisions, a random number between 0 and 2i-1 is chosen, and that number of slots is skipped. However, after ten collisions have been reached, the randomization ______ is frozen at a maximum of 1023 slots. After 16 collisions, the controller throws in the towel and reports failure back to the computer. Further recovery is up to ______ layers.
Border Gateway Protocol (BGP) is inter-autonomous system ______ protocol. BGP is based on a routing method called path vector routing. Distance vector routing is not a good candidate for inter-autonomous system routing because there are occasions on which the route with the smallest ______ count is not the preferred route. For example, we may not want a packet through an autonomous system that is not secure even though it is shortest route. Also, distance vector routing is unstable due to the fact that the routers announce only the number of hop counts to the destination without defining the path that leads to that ______. A router that receives a distance vector advertisement packet may be fooled if the shortest path is actually calculated through the receiving router itself. Link ______ routing is also not a good candidate for inner-autonomous system routing because an internet is usually too big for this routing method. To use link state routing for the whole internet would require each router to have a huge link state database. It would also take a long time for each router to calculate its routing ______ using the Dijkstra algorism.
A transport layer protocol usually has several responsibilties. One is to create a process-to-process communication UDP uses ______ numbers to accomplish this. Another responsibility is to provide control mechanisms at the transport level UDP does this task at a very minimal level. There is no flow control mechanism and there is no ______ for received packet. UDP, however, does provide error control to some extent. If UDP detects an error in the received packet, it will silently drop it. The transport layer also provides a connection mechanism for the processes. The ______ must be able to send streams of data to the transport layer. It is the responsibility of the transport layer at ______ station to make the connection with the receiver chop the stream into transportable units, number them, and send them one by one. it is the responsibility of the transport layer at the receiving end to wait until all the different units belonging to the same process have arrived, check and pass those that are ______ free, and deliver them to the receiving process as a stream.
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