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<hr><p align="left"><small>发信人: scircle (yuanyuan), 信区: UNP <br>
标 题: Re: BOOTP的问题 <br>
发信站: UNIX编程 (2001年08月20日18:43:13 星期一), 站内信件 <br>
<br>
看了RFC1541(Dynamic Host Configuration Protocol),4.4.1的 <br>
第四段讲解了为什么sender's ip address is 0.0.0.0 <br>
The client collects DHCPOFFER messages over a period of time, selects <br>
one DHCPOFFER message from the (possibly many) incoming DHCPOFFER <br>
messages (e.g., the first DHCPOFFER message or the DHCPOFFER message <br>
from the previously used server) and extracts the server address from <br>
the 'server identifier' option in the DHCPOFFER message. The time <br>
over which the client collects messages and the mechanism used to <br>
select one DHCPOFFER are implementation dependent. The client may <br>
~~~~~~~~~~~~~~~~~~~~~~~~(1) <br>
perform a check on the suggested address to ensure that the address <br>
is not already in use. For example, if the client is on a network <br>
that supports ARP, the client may issue an ARP request for the <br>
suggested request. When broadcasting an ARP request for the <br>
suggested address, the client must fill in its own hardware address <br>
as the sender's hardware address, and 0 as the sender's IP address, <br>
to avoid confusing ARP caches in other hosts on the same subnet. If <br>
~~~~~~~~~~ (2) <br>
the network address appears to be in use, the client sends a <br>
DHCPDECLINE message to the server and waits for another DHCPOFFER. As <br>
the client does not have a valid network address, the client must <br>
broadcast the DHCPDECLINE message. <br>
我的理解是： <br>
TCP/IP v1 p219开头说client立即发送arp请求，开始的sender's ip <br>
address is 0.0.0.0 .原因见(2) <br>
发送两次，可能是与系统实现相关.见(1) <br>
第三次，sender's ip address is proeus,再发送一次arp请求， <br>
是为了： <br>
The client SHOULD broadcast an ARP reply to announce the <br>
client's new IP address and clear an outdated ARP cache <br>
entries in hosts on the client's subnet. <br>
总之，先发送0.0.0.0的那个arp request,是为了avoid confusing ARP <br>
caches in other hosts on the same subnet(因为此时client并不清楚它 <br>
的ip address是否被别的host占用了).再发送proeus的arp request,此时 <br>
通知相同子网的主机刷新它们的ARP Caches. <br>
【 在 scz (小四) 的大作中提到: 】 <br>
: 这种因为历史原因留下来的实现根本无从考究的。 <br>
: 那里仅仅是举了一个实例，不见得所有涉及BOOTP的都要那样 <br>
: 做。发两次，可能考虑丢包，可能其他什么原因，这些都和BOOTP <br>
: 无关紧要了。实在不放心了，找个现代FreeBSD的相关代码看实现。 <br>
: 免费ARP请求的重要目的在于通知大家刷新自己的ARP CACHE <br>
: 前面两个ARP确认没有其他IP对应这个IP地址，然后发免费ARP <br>
: 固定大家的ARP Cache，我觉得没有什么很奇怪的地方啊。 <br>
: 【 在 scircle (yuanyuan) 的大作中提到: 】 <br>
: : 你说的Q1解释我也知道，它是个gratuitous ARP request(section <br>
: : 4.7).我的意思是为什么还有Line 3和Line4,Lin5不就直接解决问题 <br>
: .................（以下省略） <br>
-- <br>
※ 来源:·UNIX编程 www.tiaozhan.com/unixbbs/·[FROM: 202.108.211.32] <br>
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