What is MTU and how it works?

A maximum transmission unit (MTU) is the largest packet or frame size, specified in octets (eight-bit bytes) that can be sent in a packet- or frame-based network such as the internet. This ensures the IP layer of the destination host knows it should reassemble the packets into the original datagram.

How do I find my path MTU?

You can simulate the Path MTU Discovery process using the ICMP Ping command with additional flags. Perform a Ping to the target destination address. It can be the local gateway, a server, or a remote IP address.

How ICMP is being used for MTU Path Discovery?

“fragmentation needed and DF set”. This ICMP message is supposed to be delivered to the originating host, which in turn should adjust the MTU setting for that particular connection. This mechanism is called Path MTU Discovery. The most common problems are caused by misconfigured firewalls that drop ICMP packets.

What protocol is essential for the Path MTU Discovery?

RFC 1191 defines path MTU discovery, a simple process through which a host can detect a path MTU smaller than its interface MTU. Two components are key to this process: the Don’t Fragment (DF) bit of the IP header, and a subcode of the ICMP Destination Unreachable message, Fragmentation Needed.

How do you test MTU?

One of the easy and most accurate ways to test for optimum MTU is to do a simple DO S P ing test. You will simply send out ping requests and progressively lower your packet size until the packet no longer needs to be fragmented.

What is MTU fragment?

IP fragmentation is an Internet Protocol (IP) process that breaks packets into smaller pieces (fragments), so that the resulting pieces can pass through a link with a smaller maximum transmission unit (MTU) than the original packet size. The fragments are reassembled by the receiving host.

What is MTU black hole?

A black-hole hop is a device that fails to send the “fragmentation needed and DF set” message, which will confuse the socket and cause some applications to run slowly, or even fail. A packet-loss condition can cause an MTU underestimate.

What is black hole router detection?

This feature detects a connection failure caused by black hole routers and helps to recover such connections. Whenever a router receives a datagram with a Don’t Fragment (DF) bit set in its header and the packet size is greater than the next MTU, the router cannot forward the packet.