Computer network protocol to determine the maximum size of packets to transmit
Path MTU Discovery
PMTUD
) is a standardized technique in
computer networking
for determining the
maximum transmission unit (MTU)
size on the network path between two Internet Protocol (IP) hosts, usually with the goal of avoiding
IP fragmentation
. PMTUD was originally intended for routers in
Internet Protocol Version 4
(IPv4).
However, all modern operating systems use it on endpoints. In
IPv6
, this function has been explicitly delegated to the end points of a communications session.
As an extension to the standard path MTU discovery, a technique called
Packetization Layer Path MTU Discovery
works without support from
ICMP
For IPv4 packets, Path MTU Discovery works by setting the
Don't Fragment
(DF) flag bit in the IP headers of outgoing packets. Then, any device along the path whose MTU is smaller than the packet will drop it, and send back an
Internet Control Message Protocol
(ICMP)
Fragmentation Needed
(Type 3, Code 4) message containing its MTU, allowing the source host to reduce its path MTU appropriately. The process is repeated until the MTU is small enough to traverse the entire path without fragmentation.
As IPv6 routers do not
fragment
packets, there is no
Don't Fragment
option in the
IPv6 header
. For IPv6, Path MTU Discovery works by initially assuming the path MTU is the same as the MTU on the
link layer
interface where the traffic originates. Then, similar to IPv4, any device along the path whose MTU is smaller than the packet will drop the packet and send back an
ICMPv6
Packet Too Big
(Type 2) message containing its MTU, allowing the source host to reduce its path MTU appropriately. The process is repeated until the MTU is small enough to traverse the entire path without fragmentation.
If the path MTU changes after the connection is set up and becomes lower than the previously determined path MTU, the first large packet will cause an ICMP error and the new, lower path MTU will be found. If the path changes and the new path MTU is larger, the source will not learn about the increase, because all routers along the new path will be capable of relaying all packets that the source sends using the originally determined, lower path MTU.
Many network security devices block all ICMP messages for perceived security benefits, including the errors that are necessary for the proper operation of PMTUD. This can result in connections that complete the
TCP three-way handshake
correctly but then hang when attempting to transfer data. This state is referred to as a
black hole connection
Some implementations of PMTUD attempt to circumvent this problem by inferring that large payload packets have been dropped due to MTU rather than link congestion. One such scheme is standardized under RFC 8899,
Datagram Packetization Layer Path MTU Discovery
(DPLPMTUD).
Upon loss of connectivity, DPLPMTUD utilizes
probe packets
of controlled sizes to probe the MTU of the path. Acknowledgement of a probe packet indicates that the path MTU is at least the size of that packet. Usage of DPLPMTUD is standardized in
QUIC
However, in order for transport layer protocols to operate most efficiently, ICMP
Unreachable
messages (type 3) should still be permitted.
Some routers, including the Linux kernel
and Cisco,
10
provide an option to reduce the
maximum segment size
(MSS) advertised in the TCP handshake as a workaround. This is known as
MSS clamping
Another problem occurs when network administrators don't properly update the MTU between two adjacent layer 3 hops if the link between these hops is composed of multiple layer 2 segments with switches between them. Usually the MTU on the outgoing L3 interface is taken from the first L2 segment. But, if the second or further segment has a lower MTU, the switch that is between will silently drop the packet without reporting back any ICMP (because only layer 3 hops can generate ICMP "packet too big"). In this case, admins should update the MTU for each outgoing L3 interface to the minimum MTU of the layer 2 segments used until the next L3 hop.
citation needed