MTU – An Important Overview For 2021

Introduction 

The full form of MTU is the Maximum Transmission Unit referred to as MTU. The term Maximum Transmission Unit in operating and networking systems is the data packet’s largest size that can be transmitted in a network connection as a single entity. The size of the Maximum Transmission Unit dictates the amount of data that can be transmitted in bytes over and is specified in bytes. Larger Maximum Transmission Units can relay more data reducing the overheads while smaller Maximum Transmission Units can be relayed faster due to the small size reducing network delays. Maximum Transmission Unit size is thus adjusted to a default value of 1500 B in Ethernet standards.

1. Characteristics
2. Working of Maximum Transmission Unit
3. Applications
4. What is path MTU discovery?
5. MSS vs MTU

1) Characteristics

• The Maximum Transmission Unit size and amount of data transferred are in direct proportion.
• Maximum Transmission Unit size is based on network administration specifications.
• When the Maximum Transmission Unit size is greater than the router capacity, it is re-sent causing delays.
• The Maximum Transmission Unit is the network’s optimal packet size.

2) Working of Maximum Transmission Unit 

Let us assume the Internet’s TCP (Transmission Control Protocol) Maximum Transmission Unit size specified is 750 B which can be transmitted from source to destination. If the packet size is greater than 750 B, it is broken down into smaller packets with no packet larger than the MTU’s optimal size in a fragmentation operation. At the client destination, these smaller packets are reassembled. When the packet size is close to the MTU value it is transmitted as a single frame. Very small packets cause inefficiency in the MTU network and result in delays.  

3) Applications 

The maximum transmission unit has applications like

• Determining the TCP’s optimal packet size transmitted over the internet.
• In the Ethernet protocol, the Maximum Transmission Unit is also referred to as PDU (Protocol Data Unit).

4) What is path MTU discovery? 

PMTUD or Path Maximum Transmission Unit Discovery is the process of MTU discovery of all routers, switches and devices in the networking path. If a Computer-A with Server-A uses PMTUD, then it identifies the Maximum Transmission Unit on Router-B and adjusts the Maximum Transmission Unit to avoid fragmentation. PMTU works differently when connected to devices using IPv6 or IPv4. IPv4 allows fragmentation and includes in the IP header a Don’t Fragment flag. PMTUD sends test packets on the network path with this Don’t Fragment flag. I, it will send an ICMP message along with its Maximum Transmission Unit for the source device to adjust the Maximum Transmission Unit.

The source will adjust the Maximum Transmission Unit and send a new test packet repeating this process till the packet travels along the entire path without being dropped. IPv6, does not allow fragmentation and here the PMTUD works similarly but without the Don’t Fragment option in the header and no flag being sent. Routers on IPv6 will drop packets that are too large with ICMP and Maximum Transmission Unit messages being sent out. The PMTUD will keep reducing the packet size until the packet traverses the entire network path

5) MSS vs MTU 

MTU and MSS are important terms in the networking world. Based on the MTU of the receiving port data packets are either dropped or fragmentation in the network layer occurs at the layers of the IP (OSI layer 3) with a mark to ensure the destination IP layer knows and reassembles data packets in keeping with the original datagram. Adjusting the TCP’s MSS (maximum segment size) before reaching the data link layer can avoid fragmentation in servers that do not support fragmentation. In GRE tunnels the TCP MSS value is set to an optimum of less than 1436 though the default size is 1460. MSS announcement or negotiation is sent in a 3-way handshake.

Conclusion

 Having studied the maximum Transmission Unit, its characteristics, working, applications, and the Path Maximum Transmission Unit discovery process the terms MTU vs MSS can be easily differentiated from MSS. Maximum Transmission Unit means the largest size of the data packets possible to be transmitted between two points in a network whereas the MSS announcement is the largest segment size possible excluding IP and TCP headers that can be received in a single segment. If the Maximum Transmission Unit is exceeded the packet is dropped or fragmented. Whereas when MSS is exceeded the packet is always dropped.

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