#readwise # Maximum Transmission Unit - Wikipedia  ## Metadata - Author: [[wikipedia.org]] - Full Title: Maximum Transmission Unit - Wikipedia - URL: https://en.wikipedia.org/wiki/Maximum_transmission_unit ## Summary The maximum transmission unit (MTU) is the largest size of data that can be sent in one piece over a network. It affects network performance, with larger MTUs reducing overhead and smaller ones potentially lowering delays. Different networks have different MTU sizes, and they may need to be adjusted based on specific capabilities. Path MTU Discovery helps find the maximum packet size that can travel through a network without being broken into smaller pieces. ## Highlights In [computer networking](https://en.wikipedia.org/wiki/Computer_networking), the maximum transmission unit (MTU) is the size of the largest [protocol data unit](https://en.wikipedia.org/wiki/Protocol_data_unit) (PDU) that can be communicated in a single [network layer](https://en.wikipedia.org/wiki/Network_layer) transaction. The MTU relates to, but is not identical to the maximum [frame](https://en.wikipedia.org/wiki/Frame_(networking)) size that can be transported on the [data link layer](https://en.wikipedia.org/wiki/Data_link_layer), e.g., [Ethernet frame](https://en.wikipedia.org/wiki/Ethernet_frame). Larger MTU is associated with reduced [overhead](https://en.wikipedia.org/wiki/Overhead_(computing)). Smaller MTU values can reduce [network delay](https://en.wikipedia.org/wiki/Network_delay). In many cases, MTU is dependent on underlying network capabilities and must be adjusted manually or automatically so as to not exceed these capabilities. ([View Highlight](https://read.readwise.io/read/01jhpzy1rk0pjhf0jwvqjdt1n4)) --- Underlying [data link](https://en.wikipedia.org/wiki/Data_link_layer) and [physical layers](https://en.wikipedia.org/wiki/Physical_layer) usually add overhead to the network layer data to be transported, so for a given maximum frame size of a medium, one needs to subtract the amount of overhead to calculate that medium's MTU. For example, with Ethernet, the maximum [frame](https://en.wikipedia.org/wiki/Ethernet_frame#Structure) size is 1518 bytes, 18 bytes of which are overhead ([header](https://en.wikipedia.org/wiki/Ethernet_frame#Header) and [frame check sequence](https://en.wikipedia.org/wiki/Frame_check_sequence)), resulting in an MTU of 1500 bytes. ([View Highlight](https://read.readwise.io/read/01jhpzz9femgmby8yhvn582ez6)) --- The [[Internet Protocol Suite - Wikipedia|Internet protocol suite]] was designed to work over many different networking technologies, each of which may use packets of different sizes. While a host will know the MTU of its own interface and possibly that of its peers (from initial handshakes), it will not initially know the lowest MTU in a chain of links to other peers. Another potential problem is that higher-level protocols may create packets larger than even the local link supports. ([View Highlight](https://read.readwise.io/read/01jhq001ye90v0zaerhj96pzyn)) --- The Internet Protocol requires that hosts must be able to process IP datagrams of at least 576 bytes (for IPv4) or 1280 bytes (for IPv6). However, this does not preclude [[Link Layer - Wikipedia|link layers]] with an MTU smaller than this minimum MTU from conveying IP data. For example, according to IPv6's specification, if a particular link layer cannot deliver an IP datagram of 1280 bytes in a single frame, then the link layer must provide its own fragmentation and reassembly mechanism, separate from the IP fragmentation mechanism, to ensure that a 1280-byte IP datagram can be delivered, intact, to the IP layer. ([View Highlight](https://read.readwise.io/read/01jhq021ywsptjtw5zaxq6jrgq)) ^83iij3 --- *Path MTU Discovery* is a technique for determining the path MTU between two IP hosts, defined for both [[IPv4 - Wikipedia|IPv4]] and [[IPv6 - Wikipedia|IPv6]]. It works by sending packets with the DF (don't fragment) option in the IP header set. Any device along the path whose MTU is smaller than the packet will drop such packets and send back an [ICMP Destination Unreachable (Datagram Too Big)](https://en.wikipedia.org/wiki/ICMP_Destination_Unreachable) message which indicates its MTU. This information allows the source host to reduce its assumed path MTU appropriately. The process repeats until the MTU becomes small enough to traverse the entire path without fragmentation. ([View Highlight](https://read.readwise.io/read/01jhq04aqjxst258567n80gxzr)) ^x8kvge ---