MTU The MTU is the maximum size of a single data unit (e.g., a frame) of digital communications. MTU sizes are inherent properties of physical network interfaces, normally measured in bytes. The MTU for Ethernet, for instance, is 1500 bytes. Some types of networks (like Token Ring) have larger MTUs, and some types have smaller MTUs, but the values are fixed for each physical technology. Higher-level network protocols like TCP/IP can be configured with a maximum packet size, a parameter independent of the physical layer MTU over which TCP/IP runs. Unfortunately, many network devices use the terms interchangeably. On both home broadband routers and Xbox Live enabled game consoles, for example, the parameter called MTU is in fact the maximum TCP packet size and not the physical MTU. In Microsoft Windows, the maximum packet size for protocols like TCP can be set in the Registry. If this value is set too low, streams of network traffic will be broken up into a relatively large number of small packets that adversely affects performance. Xbox Live, for example, requires the value of MTU (packet size) by at least 1365 bytes. If the maximum TCP packet size is set too high, it will exceed the network’s physical MTU and also degrade performance by requiring that each packet be subdivided into smaller ones (a process known as fragmentation). Microsoft Windows computers default to a maximum packet size of 1500 bytes for broadband connections and 576 bytes for dialup connections. Performance problems may also occur if the TCP “MTU” setting on the home broadband router differs from the setting on individual devices connected to it. MSS During session connection establishment, two peers, or hosts, engage in negotiations to determine the IP segment size of packets that they will exchange during their communication. The segment size is based on the MSS option (maximum segment size) value set in the TCP SYN (synchronize) packets that the peers exchange during session negotiation. The MSS field value to be used is largely determined by the maximum transmission unit (MTU) of the interfaces that the peers are directly connected to. About TCP and MSS The TCP protocol is designed to limit the size of segments of data to a maximum of number of bytes. The purpose for this is to constrain the need to fragment segments of data for transmission at the IP level. The TCP MSS specifies the maximum number of bytes that a TCP packet’s data field, or segment, can contain. It refers to the maximum amount of TCP data in a single IP datagram that the local system can accept and reassemble. A TCP packet includes data for headers as well as data contained in the segment. If the MSS value is set too low, the result is inefficient use of bandwidth; more packets are required to transmit the data. An MSS value that is set too high could result in an IP datagram that is too large to send and that must be fragmented. Typically a host bases its MSS value on its outgoing interface’s maximum transmission unit (MTU) size. The MTU is the maximum frame size along the path between peers. A packet is fragmented when it exceeds the MTU size. Because of variation of the MTU size of the interfaces of hosts in the path taken by TCP packets between two peers, some packets that are within the negotiated MSS size of the two peers might be fragmented but instead are dropped and an ICMP error message is sent to the source host of the packet. To diminish the likelihood of fragmentation and to protect against packet loss, you can decrease the TCP MSS. Addition information on MMS and VPNs |
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