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How does Path MTU Discovery work?
For IPv4 packets, Path MTU Discovery works by setting the Don’t Fragment (DF) flag bit in the IP headers of outgoing packets. Conversely, if PMTUD finds that the path allows a larger MTU than is possible on the lower link, the OS will periodically reprobe to see if the path has changed and now allows larger packets.
What is TCP Path MTU Discovery?
Path MTU discovery (PMTUD) was developed as a solution to the problem of finding the optimal TCP packet length. PMTUD is an optimization (detailed in RFC 1191) wherein a TCP connection attempts to send the longest packets that will not be fragmented along the path from source to destination.
How ICMP is being used for MTU Path Discovery?
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. In theory, it’s great but unfortunately many things can go wrong when delivering ICMP packets.
Is MTU negotiated?
The largest data payload that can be transferred over this protocol is negotiated during the 3-way UDP handshake process, called MTU negotiation. The size of the Internet Protocol (IP) or MAC layer headers and other underlying network headers is not a part of this negotiation.
Why is MTU required?
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.
What are the properties of Path MTU Discovery?
The Path MTU Discovery (PMTUD) relies on the following properties of the IP and ICMP protocols: The sending host can indicate that its IP datagrams shall not be fragmented by setting the Don’t Fragment (DF) bit in each outgoing datagram.
What should the path MTU be for PMTUD?
A robust method for PMTUD that relies on TCP or another protocol to probe the path with progressively larger packets has been standardized in RFC 4821. A workaround used by some routers is to change the maximum segment size (MSS) of all TCP connections passing through links with MTU lower than the Ethernet default of 1500.
How does path MTU affect the host application?
The effects of the computed path MTU on host applications depends on the transport protocol they are using. TCP sessions use PMTUD transparently, as the TCP protocol models a continuous stream that is not sensitive to packet boundaries. PMTUD is usually built into the TCP stack and can be disabled.
How to do MTU discovery in ipspace.net?
This is usually done with a setsockopt call setting the IP_DONTFRAGMENT option (Windows) or IP_MTU_DISCOVER option (Linux). As soon as the UDP application has indicated it wants to use the PMTUD, all the outgoing datagrams are sent with the DF bit set, potentially resulting in dropped UDP packets.