9

The maximum size is 1500 Bytes. Is there a lower limit? 1? 64? Must it be a multiple of two? Is there somewhere I can look to learn more?

Improve this question
7

Yes, IPv4 implementations require a minimum MTU of 68 bytes per RFC 791.

See Peter's answer for more detail.

Improve this answer
4
  • 2
    You contradict yourself. The quote says "up to 576", not "at least 576".
    – Nobody
    Jun 5 '15 at 21:22
  • 1
    Edited answer for clarity. Devices can receive packets with smaller MTU's, but per the RFC, 576 bytes was the right size to use. Most OSs enforce a number around 576, it varies slightly - so there's really no reason to send smaller than necessary.
    – Jordan Head
    Jun 7 '15 at 13:22
  • 1
    This answer is wrong, it's confusing datagram size with MTU.
    – Peter Green
    Mar 7 '16 at 13:11
  • Yup, you're right - edited my answer. Thanks for catching that @PeterGreen.
    – Jordan Head
    Mar 7 '16 at 21:18
8

The minimum MTU for IPv4 is 68 bytes. Specifically from https://tools.ietf.org/html/rfc791

Every internet module must be able to forward a datagram of 68 octets without further fragmentation. This is because an internet header may be up to 60 octets, and the minimum fragment is 8 octets.

However such a low MTU would be extremely inefficient.

IPv6 sets a much higher minimum of 1280 bytes and requires links that can't support that MTU to provide a link-specific fragmentation and reassembly layer. From https://tools.ietf.org/html/rfc2460

IPv6 requires that every link in the internet have an MTU of 1280 octets or greater. On any link that cannot convey a 1280-octet packet in one piece, link-specific fragmentation and reassembly must be provided at a layer below IPv6.

Presumablly the motivation is to stop low MTU links from driving inefficiency across the network. I assume 1280 bytes was chosen to allow for a packet to be wrapped in some encapsulation/tunneling crap and still fit in a standard ethernet frame.

Improve this answer
3
  • So are there some reasons in a real-world IPv4 networks to set MTU to 68 or e.g. 200 or something very small relative to the usual 1500?
    – red0ct
    Sep 22 '20 at 15:15
  • IPv4 was designed in an era when connections were much slower. If you are running over a 1200 baud modem then a 1500 byte packet will block the link for over a second, this is generally not desirable.
    – Peter Green
    Sep 22 '20 at 15:21
  • Thanks! In other words MTU value is determined by the physical transmission medium (which in turn is determined by the devices). So if there are some slow devices with corresponding link-layer protocol between them - there could be some small MTU value. Isn't it? But I'm not sure that this is the case in modern IP-networks.
    – red0ct
    Sep 22 '20 at 16:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.