View Source: MTU - Waikato Linux Users Group

Edit PageHistory Diff Info LikePages
[Acronym] for __M__aximum __T__ransmittion __U__nit.

The size of the largest packet that can be sent over a link.  For [Ethernet] and [PPP] this is usually 1500. The minimum [MTU] you should ever use is 576. The maximum [MTU] for [IPv4] is 65535, used in some gigabit [Ethernet] configurations. Since tunneling protocols such as [PPP] require a few bytes of overhead per packets, the [MTU] on their link is usually slightly lower than 1500. Many [ADSL] MoDem~s use 1470 here.

Each packet requires overhead: it needs a header containing addresses, port numbers, [TTL], flags, etc. Therefore, larger packets equal better use of the bandwidth. If the [MTU] is smaller than necessary, more packets than necessary will be sent. However, with an [MTU] set high, if packets have to be routed through a link with a smaller [MTU], they'll need to be fragmented, ie broken into smaller packets. F.ex, when sending 1500 byte [Ethernet] packets through a 1470 byte [MTU] [PPP] link, the end of every packet will be broken into a tiny 30-byte fragment.

The [TCP/IP] stacks in [Linux] and most other recent OperatingSystem~s use a clever system called __path [MTU] discovery__ to address this problem. The [MTU] for a new connection is set low, but gradually increases, while the packets are sent with the __don't fragment__ flag set. When the [MTU] grows beyond the smallest [MTU] of any link anywhere in the route, the responsible gateway will refuse to route the packets because they're too large to send them without fragmenting them but it's not allowed to do that. Instead, it will notify the source host with a __Need to fragment, but don't fragment is set__ [ICMP] message. At this point the sending host knows the maximum fragmentation-free [MTU] size it can use on this connection, which is the most bandwidth-efficient size for packets.

Unfortunately, some particularly clueless SysAdmin~s ignorantly block any and all [ICMP] packets from passing through their gateways without fully understanding why [ICMP] is an important part of [IP], and/or without thinking of the consequences. Banks seem to be a major transgressor here.

Another cause of MTU-related brokenness is routers that need to send ICMP packets but are using private non-routable IP addresses.


See also:
* Some common (and not so common) [MTU sizes | http://www-12.lotus.com/ldd/doc/domino_notes/5.0/readme.nsf/0/f397306f052d9ea3852567740049a10a?OpenDocument]
12 pages link to MTU:
Last edited on Saturday, January 8, 2011 8:01:50 pm by "WiFiMan"
Edit PageHistory Diff Info LikePages