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工作学习 / 专业技术讨论 / 请教网络高手一个超级难题：
-holdon(again);
2005-12-6
{1181}
(#2646245@0)
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另外一些测试结果：不管MRU,MTU MSS为何值，router里的route table总是正确的。gateway 正确指向公司的一台Cisco 3640 设备
发现
ping 3640.ip -l 1432 成功
ping 3640.ip -l 1433 time out.
但是ping其它任何公司的机器都是
ping ip -l 1430 成功
ping ip -l 1431 time out
ping www.yahoo.com 也是1430成功，1431失败。
但是使用Sympatico时，好像
ping www.yahoo.com -l 2000 都可以成功。
-holdon(again);
2005-12-6
{364}
(#2646286@0)
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顶
-holdon(again);
2005-12-6
(#2646481@0)
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.
-canadiantire(轮胎-M.I.N.K.);
2005-12-6
(#2646520@0)
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找到一篇很好的文章：
http://info.connect.com.au/docs/permconnections/adsl-customer-info.pdf
在它的提示下，发现MSS最大值是1418. 设成1419就不工作了。
-holdon(again);
2005-12-6
(#2646531@0)
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下面的一段话是要点：
-holdon(again);
2005-12-6
{1820}
(#2646538@0)
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高手帮忙看看阿：用tcpdump 在web server 纪录了一下 MSS=1432 时dsl client 的一个http request:
-holdon(again);
2005-12-6
{1887}
(#2647203@0)
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哎。这么多高手，CISCO认证一堆，就没人发表一点意见。
-holdon(again);
2005-12-7
(#2648431@0)
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Ha, you already got the answer. The MSS caculation is the
MTU - size(TCP_IP_HEAD) - size(other_head)Try to capture some tcp packet on your network, you will find there 40 bytes for TCP + IP head. You must using some wired tcp head in your tcp/ip header occupied other 120 bytes.
-bugfree(BugFree);
2005-12-7
{178}
(#2648475@0)
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多谢多谢。到没有120字节那么多。最后确定MSS最大工作值是1418.我是早就可以上网了，但是还有少量客户上不了阿。
-holdon(again);
2005-12-7
{725}
(#2648673@0)
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let me have the access to your devices, 让我进去看看。
-coolj(酷不起来);
2005-12-7
(#2648756@0)
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多谢。还得先找我们公司cisco guy看，如果不行，他就该被踢出去了，那时再找你吧，还可以给你挣点零花。
-holdon(again);
2005-12-7
(#2648880@0)
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there are 2 things you may want to look at it. one is the icmp filter, the common problem is that people often blocks the icmp unreachable packets on the perimeter routers,
-coolj(酷不起来);
2005-12-7
(#2648918@0)
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it causes the router to drop the packet when the mtu size exceeds it's buffer.
second, configure the router to clear the DF bit.
-coolj(酷不起来);
2005-12-7
(#2648921@0)
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Yes, this sounds like a good idea, it's also suggested in the pdf document.
-holdon(again);
2005-12-7
(#2648926@0)
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another thing is to have your client hack the system's registry, manually edit the mtu size but it would be pain in the neck.
-coolj(酷不起来);
2005-12-7
(#2648934@0)
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That's what I was doing yesterday. I tried to explain to one customer what's MTU and why we need to change it (she is beyond our average customer) and here was her response: don't talk chinese to me.
-holdon(again);
2005-12-7
(#2648960@0)
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kao, 还是她很！
-coolj(酷不起来);
2005-12-7
(#2649095@0)
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U could go to CSDN.NET to post ur question, there are always some expert there glad to answer the questions
-binghongcha76(一只大猫);
2005-12-7
(#2648893@0)
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谢谢。我一般去experts-exchange,以前的问题一般都有答案。这次没人理我。有空去看看csdn.
-holdon(again);
2005-12-7
(#2648912@0)