2. 计算机
  3. The TTL field was originally designed to hold a time stamp．which was decremented by each visited router．The datagram Was_______

The TTL field was originally designed to hold a time stamp．which was decremented by each visited router．The datagram Was_______

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问题 The TTL field was originally designed to hold a time stamp．which was decremented by  each visited router．The datagram Was________________ when the value became zero．However，for this  scheme，all the machines must have synchronized clocks and must know how long it takes  for a datagram to go from one machine to another．Today,this field is used mostly to  control the________________  number of hops(reuters)visited by the datagram．When a source host  sends the datagram，it________________  a number in this field．Each router that processes the datagram  decrements this number by 1．Ifthis value，after being decremented，is zero，the router discards  the datagram．This field is needed because routing tables in the Internet can become corrupted． A datagram may travel between two or more reuters for a long time without ever getting  delivered to the ________________．This field limits the________________ of a datagram．
选项 A、lifetime
B、moving time
C、receiving time
D、transmitting time
答案A
解析     TTL字段最初设计用于保存时间戳，每次访问路由器时间戳则递减。当时间戳的值变为零时，数据报就被丢弃。然而这个方案要求所有的机器都必须与时钟同步，并且必须知道数据报从一台机器到另一台机器需要多长时间。现在，这个字段主要用于控制数据报访问的最大跳(路由器)数。当源主机发送数据报时，它在此字段中存储一个数字。处理这个数据报的每一个路由器都将这个数字递减1。如果该值在递减后为零，路由器则将数据报丢弃。此字段是必需的，因为Internet中路由表可能会被损坏，数据报就会在两个或多个路由器之间长时间传输，而不传送到目的主机。此字段限制了数据报的生存时间。
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