linux下MTU值的设定

beyond027

我经过实验发现把MTU值改为***好像最快

faint

有什么真实的证据哦？

jiaozhu

在我们寝室
ADSL 也有这种问题 因为大家都是windows设置都是1450
不知道为什么，其他值都不行！

faint

不懂M$下adsl默认的MTU是多少？在Debian下是1500的，把MTU改小些就ok了.

txkss

最大传输单元我觉得修改没有必要  就要在网络上通过最小的MTU时  情况就不让你掌握了

yongjian

"In computer networking, the term Maximum Transmission Unit (MTU) refers to the size (in bytes) of the largest packet that a given layer of a communications protocol can pass onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). The MTU may be fixed by standards (as is the case with Ethernet) or decided at connect time (as is usually the case with point-to-point serial links). A higher MTU brings higher bandwidth efficiency. However large packets can block up a slow interface for some time, increasing the lag on other packets. For example a 1500 byte packet, the largest allowed by Ethernet (and hence most of the Internet), would block up a 14.4k modem for about one second." -- From Wikipedia.org
改变MTU的值会加快或减慢网络速度还没有看到实际的测试结果，理论上包越大速度应该越快，如果所有的网络设备都能承受的话。但实际情况是很多路由器都设置了比1500小的MTU值来防止网络堵塞

churchmouse

今天看到你们这些讨论真是受益匪浅,学习了.

lijun

mtu关键是要和对端匹配，当然越大越好

ChaosConst

Post by faint;711405
这个好像在RFC2923里面提到。共享上网的，如果是用iptables的，可以通过设置规则来把MTU值强行减小:
解决的方法是：
1. 让icmp包通过
2. 在中间某个地方修改数据包中的MSS，强制改小.

呵呵，linux的netfilter/iptables中有个TCPMSS target就是专门干这个的.

#man iptables

TCPMSS
       This  target  allows to alter the MSS value of TCP SYN packets, to con-
       trol the maximum size for that connection (usually limiting it to  your
       outgoing  interface's MTU minus 40).  Of course, it can only be used in
       conjunction with -p tcp.
       This target is used to overcome criminally braindead  ISPs  or  servers
       which  block  ICMP  Fragmentation Needed packets.  The symptoms of this
       problem are that everything works fine from your Linux firewall/router,
       but machines behind it can never exchange large packets:
        1) Web browsers connect, then hang with no data received.
        2) Small mail works fine, but large emails hang.
        3) ssh works fine, but scp hangs after initial handshaking.
       Workaround:  activate  this option and add a rule to your firewall con-
       figuration like:
        iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
                    -j TCPMSS --clamp-mss-to-pmtu

       --set-mss value
              Explicitly set MSS option to specified value.

       --clamp-mss-to-pmtu
              Automatically clamp MSS value to (path_MTU - 40).

       These options are mutually exclusive.

我配置一台机器的pptp-vpn并做端口转发遇到mtu问题，在iptables里面设置set-mtu成功了，多谢！！