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How MTU and MSS Affect Networks?

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  • Difficulty Level : Easy
  • Last Updated : 06 Mar, 2022
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TCP is one of the important protocols of the network protocol suite. TCP is a connection-oriented and reliable data transmission protocol. TCP uses MTU (Maximum Transmission Unit) for identifying the maximum size of any data packet and MSS (maximum segment size) for specifying the maximum size of packets that can be sent over a network. In this article, we will discuss how large or small MTU and MSS’s size can affect networks.

Maximum Transmission Unit: 

Maximum transmission unit (MTU) is a measurement in computer networking that represents the largest data packet that a networked device will accept. Data packets that are larger than the MTU are chopped up into smaller pieces to fit through. This process is called fragmentation and it is done by the Network layer. Routers will drop any packet that exceeds the MTU if fragmentation is not permitted.

The limit at the Network layer is derived directly from the data link layer. Frames are used in Data link Layer, and each frame has a maximum size.

The Ethernet standard, for example, specifies a maximum frame size of 1518 bytes. Ethernet headers are 18 bytes long, giving the packet 1500 bytes to work with. As a result, the MTU of the packet is 1500 bytes.

Maximum Transmission Unit

Maximum Transmission Unit

The network’s MTU size can have a significant impact on performance.

Larger MTU size effect on Network:

Advantages:

  • More data can be put into fewer packets with a bigger MTU, allowing for faster and more efficient transmission.
  • Lower overhead is associated with larger MTU.
  • For the same amount of data, a greater MTU requires the processing of fewer packets.
  • The larger packets lower the amount of processing that the operating system has to do.

Disadvantages:

  • Large packets take up more bandwidth on a slow link than smaller packets, causing subsequent packets to be delayed longer and increasing network delay.
  • Increasing MTU could require costly hardware changes.
  • The increased MTU size will not help if the workload only sends small messages.

Smaller MTU size effect on Network:

Advantages:

  • when the MTU is smaller the network latency is reduced.
  • A smaller MTU size improves Ethernet’s CRC-32 algorithm, Which is used for error detection and correction of data.

Disadvantages:

  • More bandwidth is getting wasted on headers.
  • The Workload of the Operating System increases.

Maximum Segment Size: 

The MSS (maximum segment size) specifies the maximum size of packets that can be sent over a network. It is a parameter in the TCP header’s options field. Several headers are appended to packets, each of which contains information on the source and destination of the packet. The non-header section of a packet, commonly known as the payload, is measured by MSS.

Note: The value is the maximum amount of data that a segment can store, excluding TCP headers. As a result, if the MSS is 100, the actual maximum segment size could be 120 (for a standard TCP header) or even larger.

The Value of MSS is derived from the value of MTU. Consider the following scenario: you have 2460 bytes of data to send. If the MTU is 1500 and the IP header + TCP header is 40 bytes, the first IP packet can only send 1460 bytes of data. In the second IP packet, the remaining 1000 bytes will be delivered. As a result, if MSS = 1000, the MTU must be at least 1040.

Maximum Segment Size

Maximum Segment Size

Effect of MSS on the Network:

Advantages:

  • Network latency can be reduced by reducing the maximum TCP packet size.
  • MSS is generally determined during the TCP three-way handshake. The MSS isn’t negotiated for each packet, but rather for the entire TCP session.
  • The operating system can be used to set or alter the maximum size.

Disadvantages:

  • MSS is generally determined during the TCP three-way handshake. If MSS is set too low, network traffic streams are broken up into a large number of little packets, which has a negative impact on performance.
  • Data packets that exceed the maximum MSS over the network are simply discarded.
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