The Path MTU is the Maximum Transmission Unit (MTU) supported by a network path. It is the minimum of the MTUs of the links (segments) that make up the path. Larger Path MTUs generally allow for more efficient data transfers, because source and destination hosts, as well as the switching devices (routers) along the network path have to process fewer packets. However, it should be noted that modern high-speed network adapters have mechanisms such as LSO (Large Send Offload) and Interrupt Coalescence that diminish the influence of MTUs on performance. Furthermore, routers are typically dimensioned to sustain very high packet loads (so that they can resist denial-of-service attacks) so the packet rates caused by high-speed transfers is not normally an issue for today's high-speed networks.
The prevalent Path MTU on the Internet is now 1500 bytes, the Ethernet MTU. There are some initiatives to support larger MTUs (JumboMTU) in networks, in particular on research networks. But their usability is hampered by last-mile issues and lack of robustness of RFC 1191 Path MTU Discovery.
RFC 1191 describes a method for a sender to detect the Path MTU to a given receiver. (RFC 1981 describes the equivalent for IPv6.) The method works as follows:
This method is widely implemented, but is not robust in today's Internet because it relies on ICMP packets sent by routers along the path. Such packets are often suppressed either at the router that should generate them (to protect its resources) or on the way back to the source, because of firewalls and other packet filters or rate limitations. These problems are described in RFC 2923. When packets are lost due to MTU issues without any ICMP "Too Big" message, this is sometimes called a (MTU) black hole. Some operating systems have added heuristics to detect such black holes and work around them. Workarounds can include lowering the MTU estimate or disabling PMTUD for certain destinations.
An IETF Working Group (pmtud) was chartered to define a new mechanism for Path MTU Discovery to solve these issues. This process resulted in RFC 4821, Packetization Layer Path MTU Discovery ("PLPMTUD"), which was published in March 2007. This scheme requires cooperation from a network layer above IP, namely the layer that performs "packetization". This could be TCP, but could also be a layer above UDP, let's say an RPC or file transfer protocol. PLPMTUD does not require ICMP messages. The sending packetization layer starts with small packets, and probes progressively larger sizes. When there's an indication that a larger packet was successfully transmitted to the destination (presumably because some sort of ACK was received), the Path MTU estimate is raised accordingly.
When a large packet was lost, this might have been due to an MTU limitation, but it might also be due to other causes, such as congestion or a transmission error - or maybe it's just the ACK that was lost! PLPMTUD recommends that the first time this happens, the sending packetization layer should assume an MTU issue, and try smaller packets. An isolated incident need not be interpreted as an indication of congestion.
An implementation of the new scheme for the Linux kernel was integrated into version 2.6.17. It is controlled by a "sysctl" value that can be observed and set through /proc/sys/net/ipv4/tcp_mtu_probing . Possible values are:
0 : Don't perform PLPMTUD1 : Perform PLPMTUD only after detecting a "blackhole" in old-style PMTUD2 : Always perform PLPMTUD, and use the value of tcp_base_mss as the initial MSS.A user-space implementation over UDP is included in the VFER bulk transfer tool.
RFC 8899, published in September 2020, updates RFC 4821 by extending its use to datagram-oriented transports.
Recently, the IETF 6man Working Group has adopted an Internet-Draft (see reference below) proposing an IPv6 hop-by-hop option that would allow discovering the Path MTU with assistance from routers. Note that this still requires a mechanism to "echo" the discovered PMTU back to the sender. (Some people are skeptical about the chances that this option will see successful deployment, for example Geoff Huston in his August 2021 IETF 111 report.
-- HankNussbacher - 2005-07-03
– SimonLeinen - 2006-07-19 - 2021-09-04