Which two protocols use UDP as a transport protocol by default?（）A、TelnetB、ICMPC、DHCPD、RIP

参考答案：B, C

参考答案：D, F

正确答案：AB
A,B 解析：Explanation:
DHCP uses UDP as its transport protocol. DHCP messages from a client to a server are sent to the DHCP server port (UDP port 67), and DHCP messages from a server to a client are sent to the DHCP client port (UDP port 68).The client broadcasts a DHCPDISCOVER message on its local physical subnet. The DHCPDISCOVER message may include options that suggest values for the network address and lease duration. BOOTP relay agents may pass the message on to DHCP servers not on the same physical subnet.

Each server may respond with a DHCPOFFER message that includes an available network address in the "ipaddr" field (and other configuration parameters in DHCP options). Servers need not reserve the offered network address, although the protocol will work more efficiently if the server avoids allocating the offered network address to another client. The server unicasts the DHCPOFFER message to the client (using the DHCP/BOOTP relay agent if necessary) if possible, or may broadcast the message to a broadcast address (preferably 255.255.255.255) on the client's subnet.

Incorrect Answers:
C. DHCP messages are broadcast to the "all hosts" address. IP multicast addresses are not used.

D. UDP is used, not TCP.

E. Since DHCP is used so that a client can obtain an IP address, a layer two destination address must be used, as the layer 3 IP address does not yet exist on the client for the return traffic.

F. DHCP, along with nearly every other type of traffic, requires the use of a transport layer protocol.

参考答案：A

参考答案：C, E, F

正确答案：E
解析：Explanation:TCPandUDPmustuseportnumberstocommunicatewiththeupperlayers.Portnumberskeeptrackofdifferentconversationscrossingthenetworksimultaneously.Originating-sourceportnumbersaredynamicallyassignedbythesourcehost,whichwillbesomenumberstartingat1024.1023andbelowaredefinedinRFC1700,whichdiscusseswhatiscalledwell-knownportnumbers.Virtualcircuitsthatdonotuseanapplicationwithawell-knownportnumberareassignedportnumbersrandomlychosenfromwithinaspecificrangeinstead.TheseportnumbersidentifythesourceanddestinationhostintheTCPsegment.OnlyDNSusesbothUDPandTCPport53.