Network Working Group M. Mathis
Request for Comments: 4821 J. Heffner
Category: Standards Track PSC
March 2007 Packetization Layer Path MTU Discovery
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The IETF Trust (2007).
This document describes a robust method for Path MTU Discovery
(PMTUD) that relies on TCP or some other Packetization Layer to probe
an Internet path with progressively larger packets. This method is
described as an extension to RFC 1191 and RFC 1981, which specify
ICMP-based Path MTU Discovery for IP versions 4 and 6, respectively.
This document describes a method for Packetization Layer Path MTU
Discovery (PLPMTUD), which is an extension to existing Path MTU
Discovery methods described in [RFC1191] and [RFC1981]. In the
absence of ICMP messages, the proper MTU is determined by starting
with small packets and probing with successively larger packets. The
bulk of the algorithm is implemented above IP, in the transport layer
(e.g., TCP) or other "Packetization Protocol" that is responsible for
determining packet boundaries.
This document does not update RFC 1191 or RFC 1981; however, since it
supports correct operation without ICMP, it implicitly relaxes some
of the requirements for the algorithms specified in those documents.
The methods described in this document rely on features of existing
protocols. They apply to many transport protocols over IPv4 and
IPv6. They do not require cooperation from the lower layers (except
that they are consistent about which packet sizes are acceptable) or
from peers. As the methods apply only to senders, variants in
implementations will not cause interoperability problems.
For sake of clarity, we uniformly prefer TCP and IPv6 terminology.
In the terminology section, we also present the analogous IPv4 terms
and concepts for the IPv6 terminology. In a few situations, we
describe specific details that are different between IPv4 and IPv6.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document is a product of the Path MTU Discovery (PMTUD) working
group of the IETF and draws heavily on RFC 1191 and RFC 1981 for
terminology, ideas, and some of the text.
Packetization Layer Path MTU Discovery (PLPMTUD) is a method for TCP
or other Packetization Protocols to dynamically discover the MTU of a
path by probing with progressively larger packets. It is most
efficient when used in conjunction with the ICMP-based Path MTU
Discovery mechanism as specified in RFC 1191 and RFC 1981, but
resolves many of the robustness problems of the classical techniques
since it does not depend on the delivery of ICMP messages.
This method is applicable to TCP and other transport- or application-
level protocols that are responsible for choosing packet boundaries
(e.g., segment sizes) and have an acknowledgment structure that
delivers to the sender accurate and timely indications of which
packets were lost.
The general strategy is for the Packetization Layer to find an
appropriate Path MTU by probing the path with progressively larger
packets. If a probe packet is successfully delivered, then the
effective Path MTU is raised to the probe size.
The isolated loss of a probe packet (with or without an ICMP Packet
Too Big message) is treated as an indication of an MTU limit, and not
as a congestion indicator. In this case alone, the Packetization
Protocol is permitted to retransmit any missing data without
adjusting the congestion window.
If there is a timeout or additional packets are lost during the
probing process, the probe is considered to be inconclusive (e.g.,
the lost probe does not necessarily indicate that the probe exceeded
the Path MTU). Furthermore, the losses are treated like any other
congestion indication: window or rate adjustments are mandatory per
the relevant congestion control standards [RFC2914]. Probing can
resume after a delay that is determined by the nature of the detected
PLPMTUD uses a searching technique to find the Path MTU. Each
conclusive probe narrows the MTU search range, either by raising the
lower limit on a successful probe or lowering the upper limit on a
failed probe, converging toward the true Path MTU. For most
transport layers, the search should be stopped once the range is
narrow enough that the benefit of a larger effective Path MTU is
smaller than the search overhead of finding it.
The most likely (and least serious) probe failure is due to the link
experiencing congestion-related losses while probing. In this case,
it is appropriate to retry a probe of the same size as soon as the
Packetization Layer has fully adapted to the congestion and recovered
from the losses. In other cases, additional losses or timeouts
indicate problems with the link or Packetization Layer. In these
situations, it is desirable to use longer delays depending on the
severity of the error.
An optional verification process can be used to detect situations
where raising the MTU raises the packet loss rate. For example, if a
link is striped across multiple physical channels with inconsistent
MTUs, it is possible that a probe will be delivered even if it is too
large for some of the physical channels. In such cases, raising the
Path MTU to the probe size can cause severe packet loss and abysmal
performance. After raising the MTU, the new MTU size can be verified
by monitoring the loss rate.
Packetization Layer PMTUD (PLPMTUD) introduces some flexibility in
the implementation of classical Path MTU Discovery. It can be
configured to perform just ICMP black hole recovery to increase the
robustness of classical Path MTU Discovery, or at the other extreme,
all ICMP processing can be disabled and PLPMTUD can completely
replace classical Path MTU Discovery.
Classical Path MTU Discovery is subject to protocol failures
(connection hangs) if ICMP Packet Too Big (PTB) messages are not
delivered or processed for some reason [RFC2923]. With PLPMTUD,
classical Path MTU Discovery can be modified to include additional
consistency checks without increasing the risk of connection hangs
due to spurious failures of the additional checks. Such changes to
classical Path MTU Discovery are beyond the scope of this document.
In the limiting case, all ICMP PTB messages might be unconditionally
ignored, and PLPMTUD can be used as the sole method to discover the
Path MTU. In this configuration, PLPMTUD parallels congestion
control. An end-to-end transport protocol adjusts properties of the
data stream (window size or packet size) while using packet losses to
deduce the appropriateness of the adjustments. This technique seems
to be more philosophically consistent with the end-to-end principle
of the Internet than relying on ICMP messages containing transcribed
headers of multiple protocol layers.
Most of the difficulty in implementing PLPMTUD arises because it
needs to be implemented in several different places within a single
node. In general, each Packetization Protocol needs to have its own
implementation of PLPMTUD. Furthermore, the natural mechanism to
share Path MTU information between concurrent or subsequent
connections is a path information cache in the IP layer. The various
Packetization Protocols need to have the means to access and update
the shared cache in the IP layer. This memo describes PLPMTUD in
terms of its primary subsystems without fully describing how they are
assembled into a complete implementation.
The vast majority of the implementation details described in this
document are recommendations based on experiences with earlier
versions of Path MTU Discovery. These recommendations are motivated
by a desire to maximize robustness of PLPMTUD in the presence of less
than ideal network conditions as they exist in the field.
This document does not contain a complete description of an
implementation. It only sketches details that do not affect
interoperability with other implementations and have strong
externally imposed optimality criteria (e.g., the MTU searching and
caching heuristics). Other details are explicitly included because
there is an obvious alternative implementation that doesn't work well
in some (possibly subtle) case.
Section 3 provides a complete glossary of terms.
Section 4 describes the details of PLPMTUD that affect
interoperability with other standards or Internet protocols.
Section 5 describes how to partition PLPMTUD into layers, and how to
manage the path information cache in the IP layer.
Section 6 describes the general Packetization Layer properties and
features needed to implement PLPMTUD.
Section 7 describes how to use probes to search for the Path MTU.
Section 8 recommends using IPv4 fragmentation in a configuration that
mimics IPv6 functionality, to minimize future problems migrating to
Section 9 describes a programming interface for implementing PLPMTUD
in applications that choose their own packet boundaries and for tools
to be able to diagnose path problems that interfere with Path MTU
Section 10 discusses implementation details for specific protocols,
We use the following terms in this document:
IP: Either IPv4 [RFC0791] or IPv6 [RFC2460].
Node: A device that implements IP.
Upper layer: A protocol layer immediately above IP. Examples are
transport protocols such as TCP and UDP, control protocols such as
ICMP, routing protocols such as OSPF, and Internet or lower-layer
protocols being "tunneled" over (i.e., encapsulated in) IP such as
IPX, AppleTalk, or IP itself.
Link: A communication facility or medium over which nodes can
communicate at the link layer, i.e., the layer immediately below
IP. Examples are Ethernets (simple or bridged); PPP links; X.25,
Frame Relay, or Asynchronous Transfer Mode (ATM) networks; and
Internet (or higher) layer "tunnels", such as tunnels over IPv4 or
IPv6. Occasionally we use the slightly more general term "lower
layer" for this concept.
Interface: A node's attachment to a link.
Address: An IP layer identifier for an interface or a set of
Packet: An IP header plus payload.
MTU: Maximum Transmission Unit, the size in bytes of the largest IP
packet, including the IP header and payload, that can be
transmitted on a link or path. Note that this could more properly
be called the IP MTU, to be consistent with how other standards
organizations use the acronym MTU.
Link MTU: The Maximum Transmission Unit, i.e., maximum IP packet
size in bytes, that can be conveyed in one piece over a link. Be
aware that this definition is different from the definition used
by other standards organizations.
For IETF documents, link MTU is uniformly defined as the IP MTU
over the link. This includes the IP header, but excludes link
layer headers and other framing that is not part of IP or the IP
Be aware that other standards organizations generally define link
MTU to include the link layer headers.
Path: The set of links traversed by a packet between a source node
and a destination node.
Path MTU, or PMTU: The minimum link MTU of all the links in a path
between a source node and a destination node.
Classical Path MTU Discovery: Process described in RFC 1191 and RFC
1981, in which nodes rely on ICMP Packet Too Big (PTB) messages to
learn the MTU of a path.
Packetization Layer: The layer of the network stack that segments
data into packets.
Effective PMTU: The current estimated value for PMTU used by a
Packetization Layer for segmentation.
PLPMTUD: Packetization Layer Path MTU Discovery, the method
described in this document, which is an extension to classical
PTB (Packet Too Big) message: An ICMP message reporting that an IP
packet is too large to forward. This is the IPv6 term that
corresponds to the IPv4 ICMP "Fragmentation Needed and DF Set"
Flow: A context in which MTU Discovery algorithms can be invoked.
This is naturally an instance of a Packetization Protocol, for
example, one side of a TCP connection.
MSS: The TCP Maximum Segment Size [RFC0793], the maximum payload
size available to the TCP layer. This is typically the Path MTU
minus the size of the IP and TCP headers.
Probe packet: A packet that is being used to test a path for a
Probe size: The size of a packet being used to probe for a larger
MTU, including IP headers.
Probe gap: The payload data that will be lost and need to be
retransmitted if the probe is not delivered.
Leading window: Any unacknowledged data in a flow at the time a
probe is sent.
Trailing window: Any data in a flow sent after a probe, but before
the probe is acknowledged.
Search strategy: The heuristics used to choose successive probe
sizes to converge on the proper Path MTU, as described in
Full-stop timeout: A timeout where none of the packets transmitted
after some event are acknowledged by the receiver, including any
retransmissions. This is taken as an indication of some failure
condition in the network, such as a routing change onto a link
with a smaller MTU. This is described in more detail in
All links MUST enforce their MTU: links that might non-
deterministically deliver packets that are larger than their rated
MTU MUST consistently discard such packets.
In the distant past, there were a small number of network devices
that did not enforce MTU, but could not reliably deliver oversized
packets. For example, some early bit-wise Ethernet repeaters would
forward arbitrarily sized packets, but could not do so reliably due
to finite hardware data clock stability. This is the only
requirement that PLPMTUD places on lower layers. It is important
that this requirement be explicit to forestall the future
standardization or deployment of technologies that might be
incompatible with PLPMTUD.
All hosts SHOULD use IPv4 fragmentation in a mode that mimics IPv6
functionality. All fragmentation SHOULD be done on the host, and all
IPv4 packets, including fragments, SHOULD have the DF bit set such
that they will not be fragmented (again) in the network. See
The requirements below only apply to those implementations that
To use PLPMTUD, a Packetization Layer MUST have a loss reporting
mechanism that provides the sender with timely and accurate
indications of which packets were lost in the network.
Normal congestion control algorithms MUST remain in effect under all
conditions except when only an isolated probe packet is detected as
lost. In this case alone, the normal congestion (window or data
rate) reduction SHOULD be suppressed. If any other data loss is
detected, standard congestion control MUST take place.
Suppressed congestion control MUST be rate limited such that it
occurs less frequently than the worst-case loss rate for TCP
congestion control at a comparable data rate over the same path
(i.e., less than the "TCP-friendly" loss rate [tcp-friendly]). This
SHOULD be enforced by requiring a minimum headway between a
suppressed congestion adjustment (due to a failed probe) and the next
attempted probe, which is equal to one round-trip time for each
packet permitted by the congestion window. This is discussed further
in Section 7.6.2.
Whenever the MTU is raised, the congestion state variables MUST be
rescaled so as not to raise the window size in bytes (or data rate in
bytes per seconds).
Whenever the MTU is reduced (e.g., when processing ICMP PTB
messages), the congestion state variable SHOULD be rescaled so as not
to raise the window size in packets.
If PLPMTUD updates the MTU for a particular path, all Packetization
Layer sessions that share the path representation (as described in
Section 5.2) SHOULD be notified to make use of the new MTU and make
the required congestion control adjustments.
All implementations MUST include mechanisms for applications to
selectively transmit packets larger than the current effective Path
MTU, but smaller than the first-hop link MTU. This is necessary to
implement PLPMTUD using a connectionless protocol within an
application and to implement diagnostic tools that do not rely on the
operating system's implementation of Path MTU Discovery. See
Section 9 for further discussion.
Implementations MAY use different heuristics to select the initial
effective Path MTU for each protocol. Connectionless protocols and
protocols that do not support PLPMTUD SHOULD have their own default
value for the initial effective Path MTU, which can be set to a more
conservative (smaller) value than the initial value used by TCP and
other protocols that are well suited to PLPMTUD. There SHOULD be
per-protocol and per-route limits on the initial effective Path MTU
(eff_pmtu) and the upper searching limit (search_high). See
Section 7.2 for further discussion.
Packetization Layer Path MTU Discovery is most easily implemented by
splitting its functions between layers. The IP layer is the best
place to keep shared state, collect the ICMP messages, track IP
header sizes, and manage MTU information provided by the link layer
interfaces. However, the procedures that PLPMTUD uses for probing
and verification of the Path MTU are very tightly coupled to features
of the Packetization Layers, such as data recovery and congestion
control state machines.
Note that this layering approach is a direct extension of the advice
in the current PMTUD specifications in RFC 1191 and RFC 1981.
5.1. Accounting for Header Sizes
The way in which PLPMTUD operates across multiple layers requires a
mechanism for accounting header sizes at all layers between IP and
the Packetization Layer (inclusive). When transmitting non-probe
packets, it is sufficient for the Packetization Layer to ensure an
upper bound on final IP packet size, so as not to exceed the current
effective Path MTU. All Packetization Layers participating in
classical Path MTU Discovery have this requirement already. When
conducting a probe, the Packetization Layer MUST determine the probe
packet's final size including IP headers. This requirement is
specific to PLPMTUD, and satisfying it may require additional inter-
layer communication in existing implementations.
5.2. Storing PMTU Information
This memo uses the concept of a "flow" to define the scope of the
Path MTU Discovery algorithms. For many implementations, a flow
would naturally correspond to an instance of each protocol (i.e.,
each connection or session). In such implementations, the algorithms
described in this document are performed within each session for each
protocol. The observed PMTU (eff_pmtu in Section 7.1) MAY be shared
between different flows with a common path representation.
Alternatively, PLPMTUD could be implemented such that its complete
state is associated with the path representations. Such an
implementation could use multiple connections or sessions for each
probe sequence. This approach is likely to converge much more
quickly in some environments, such as where an application uses many
small connections, each of which is too short to complete the Path
MTU Discovery process.
Within a single implementation, different protocols can use either of
these two approaches. Due to protocol specific differences in
constraints on generating probes (Section 6.2) and the MTU searching
algorithm (Section 7.3), it may not be feasible for different
Packetization Layer protocols to share PLPMTUD state. This suggests
that it may be possible for some protocols to share probing state,
but other protocols can only share observed PMTU. In this case, the
different protocols will have different PMTU convergence properties.
The IP layer SHOULD be used to store the cached PMTU value and other
shared state such as MTU values reported by ICMP PTB messages.
Ideally, this shared state should be associated with a specific path
traversed by packets exchanged between the source and destination
nodes. However, in most cases a node will not have enough
information to completely and accurately identify such a path.
Rather, a node must associate a PMTU value with some local
representation of a path. It is left to the implementation to select
the local representation of a path.
An implementation MAY use the destination address as the local
representation of a path. The PMTU value associated with a
destination would be the minimum PMTU learned across the set of all
paths in use to that destination. The set of paths in use to a
particular destination is expected to be small, in many cases
consisting of a single path. This approach will result in the use of
optimally sized packets on a per-destination basis, and integrates
nicely with the conceptual model of a host as described in [RFC2461]:
a PMTU value could be stored with the corresponding entry in the
destination cache. Since Network Address Translators (NATs) and
other forms of middle boxes may exhibit differing PMTUs
simultaneously at a single IP address, the minimum value SHOULD be
Network or subnet numbers MUST NOT be used as representations of a
path, because there is not a general mechanism to determine the
network mask at the remote host.
For source-routed packets (i.e., packets containing an IPv6 routing
header, or IPv4 Loose Source and Record Route (LSRR) or Strict Source
and Record Route (SSRR) options), the source route MAY further
qualify the local representation of a path. An implementation MAY
use source route information in the local representation of a path.
If IPv6 flows are in use, an implementation MAY use the 3-tuple of
the Flow label and the source and destination addresses
[RFC2460][RFC3697] as the local representation of a path. Such an
approach could theoretically result in the use of optimally sized
packets on a per-flow basis, providing finer granularity than MTU
values maintained on a per-destination basis.
5.3. Accounting for IPsec
This document does not take a stance on the placement of IP Security
(IPsec) [RFC2401], which logically sits between IP and the
Packetization Layer. A PLPMTUD implementation can treat IPsec either
as part of IP or as part of the Packetization Layer, as long as the
accounting is consistent within the implementation. If IPsec is
treated as part of the IP layer, then each security association to a
remote node may need to be treated as a separate path. If IPsec is
treated as part of the Packetization Layer, the IPsec header size
MUST be included in the Packetization Layer's header size
In the case of a multicast destination address, copies of a packet
may traverse many different paths to reach many different nodes. The
local representation of the "path" to a multicast destination must in
fact represent a potentially large set of paths.
Minimally, an implementation MAY maintain a single MTU value to be
used for all multicast packets originated from the node. This MTU
SHOULD be sufficiently small that it is expected to be less than the
Path MTU of all paths comprising the multicast tree. If a Path MTU
of less than the configured multicast MTU is learned via unicast
means, the multicast MTU MAY be reduced to this value. This approach
is likely to result in the use of smaller packets than is necessary
for many paths.
If the application using multicast gets complete delivery reports
(unlikely since this requirement has poor scaling properties),
PLPMTUD MAY be implemented in multicast protocols such that the
smallest path MTU learned across a group becomes the effective MTU
for that group.
6. Common Packetization Properties
This section describes general Packetization Layer properties and
characteristics needed to implement PLPMTUD. It also describes some
implementation issues that are common to all Packetization Layers.
6.1. Mechanism to Detect Loss
It is important that the Packetization Layer has a timely and robust
mechanism for detecting and reporting losses. PLPMTUD makes MTU
adjustments on the basis of detected losses. Any delays or
inaccuracy in loss notification is likely to result in incorrect MTU
decisions or slow convergence. It is important that the mechanism
can robustly distinguish between the isolated loss of just a probe
and other losses in the probe's leading and trailing windows.
It is best if Packetization Protocols use an explicit loss detection
mechanism such as a Selective Acknowledgment (SACK) scoreboard
[RFC3517] or ACK Vector [RFC4340] to distinguish real losses from
reordered data, although implicit mechanisms such as TCP Reno style
duplicate acknowledgments counting are sufficient.
PLPMTUD can also be implemented in protocols that rely on timeouts as
their primary mechanism for loss recovery; however, timeouts SHOULD
NOT be used as the primary mechanism for loss indication unless there
are no other alternatives.
6.2. Generating Probes
There are several possible ways to alter Packetization Layers to
generate probes. The different techniques incur different overheads
in three areas: difficulty in generating the probe packet (in terms
of Packetization Layer implementation complexity and extra data
motion), possible additional network capacity consumed by the probes,
and the overhead of recovering from failed probes (both network and
Some protocols might be extended to allow arbitrary padding with
dummy data. This greatly simplifies the implementation because the
probing can be performed without participation from higher layers and
if the probe fails, the missing data (the "probe gap") is ensured to
fit within the current MTU when it is retransmitted. This is
probably the most appropriate method for protocols that support
arbitrary length options or multiplexing within the protocol itself.
Many Packetization Layer protocols can carry pure control messages
(without any data from higher protocol layers), which can be padded
to arbitrary lengths. For example, the SCTP PAD chunk can be used in
this manner (see Section 10.2). This approach has the advantage that
nothing needs to be retransmitted if the probe is lost.
These techniques do not work for TCP, because there is not a separate
length field or other mechanism to differentiate between padding and
real payload data. With TCP the only approach is to send additional
payload data in an over-sized segment. There are at least two
variants of this approach, discussed in Section 10.1.
In a few cases, there may be no reasonable mechanisms to generate
probes within the Packetization Layer protocol itself. As a last
resort, it may be possible to rely on an adjunct protocol, such as
ICMP ECHO ("ping"), to send probe packets. See Section 10.3 for
further discussion of this approach.
7. The Probing Method
This section describes the details of the MTU probing method,
including how to send probes and process error indications necessary
to search for the Path MTU.
7.1. Packet Size Ranges
This document describes the probing method using three state
search_low: The smallest useful probe size, minus one. The network
is expected to be able to deliver packets of size search_low.
search_high: The greatest useful probe size. Packets of size
search_high are expected to be too large for the network to
eff_pmtu: The effective PMTU for this flow. This is the largest
non-probe packet permitted by PLPMTUD for the path.
search_low eff_pmtu search_high
| | |
non-probe size range
probe size range
When transmitting non-probes, the Packetization Layer SHOULD create
packets of a size less than or equal to eff_pmtu.
When transmitting probes, the Packetization Layer MUST select a probe
size that is larger than search_low and smaller than or equal to
When probing upward, eff_pmtu always equals search_low. In other
states, such as initial conditions, after ICMP PTB message processing
or following PLPMTUD on another flow sharing the same path
representation, eff_pmtu may be different from search_low. Normally,
eff_pmtu will be greater than or equal to search_low and less than
search_high. It is generally expected but not required that probe
size will be greater than eff_pmtu.
For initial conditions when there is no information about the path,
eff_pmtu may be greater than search_low. The initial value of
search_low SHOULD be conservatively low, but performance may be
better if eff_pmtu starts at a higher, less conservative, value. See
If eff_pmtu is larger than search_low, it is explicitly permitted to
send non-probe packets larger than search_low. When such a packet is
acknowledged, it is effectively an "implicit probe" and search_low
SHOULD be raised to the size of the acknowledged packet. However, if
an "implicit probe" is lost, it MUST NOT be treated as a probe
failure as a true probe would be. If eff_pmtu is too large, this
condition will only be detected with ICMP PTB messages or black hole
discovery (see Section 7.7).
7.2. Selecting Initial Values
The initial value for search_high SHOULD be the largest possible
packet that might be supported by the flow. This may be limited by
the local interface MTU, by an explicit protocol mechanism such as
the TCP MSS option, or by an intrinsic limit such as the size of a
protocol length field. In addition, the initial value for
search_high MAY be limited by a configuration option to prevent
probing above some maximum size. Search_high is likely to be the
same as the initial Path MTU as computed by the classical Path MTU
It is RECOMMENDED that search_low be initially set to an MTU size
that is likely to work over a very wide range of environments. Given
today's technologies, a value of 1024 bytes is probably safe enough.
The initial value for search_low SHOULD be configurable.
Properly functioning Path MTU Discovery is critical to the robust and
efficient operation of the Internet. Any major change (as described
in this document) has the potential to be very disruptive if it
causes any unexpected changes in protocol behaviors. The selection
of the initial value for eff_pmtu determines to what extent a PLPMTUD
implementation's behavior resembles classical PMTUD in cases where
the classical method is sufficient.
A conservative configuration would be to set eff_pmtu to search_high,
and rely on ICMP PTB messages to set the eff_pmtu down as
appropriate. In this configuration, classical PMTUD is fully
functional and PLPMTUD is only invoked to recover from ICMP black
holes through the procedure described in Section 7.7.
In some cases, where it is known that classical PMTUD is likely to
fail (for example, if ICMP PTB messages are administratively disabled
for security reasons), using a small initial eff_pmtu will avoid the
costly timeouts required for black hole detection. The trade-off is
that using a smaller than necessary initial eff_pmtu might cause
Note that the initial eff_pmtu can be any value in the range
search_low to search_high. An initial eff_pmtu of 1400 bytes might
be a good compromise because it would be safe for nearly all tunnels
over all common networking gear, and yet close to the optimal MTU for
the majority of paths in the Internet today. This might be improved
by using some statistics of other recent flows: for example, the
initial eff_pmtu for a flow might be set to the median of the probe
size for all recent successful probes.
Since the cost of PLPMTUD is dominated by the protocol specific
overheads of generating and processing probes, it is probably
desirable for each protocol to have its own heuristics to select the
initial eff_pmtu. It is especially important that connectionless
protocols and other protocols that may not receive clear indications
of ICMP black holes use conservative (smaller) initial values for
eff_pmtu, as described in Section 10.3.
There SHOULD be per-protocol and per-route configuration options to
override initial values for eff_pmtu and other PLPMTUD state
7.3. Selecting Probe Size
The probe may have a size anywhere in the "probe size range"
described above. However, a number of factors affect the selection
of an appropriate size. A simple strategy might be to do a binary
search halving the probe size range with each probe. However, for
some protocols, such as TCP, failed probes are more expensive than
successful ones, since data in a failed probe will need to be
retransmitted. For such protocols, a strategy that raises the probe
size in smaller increments might have lower overhead. For many
protocols, both at and above the Packetization Layer, the benefit of
increasing MTU sizes may follow a step function such that it is not
advantageous to probe within certain regions at all.
As an optimization, it may be appropriate to probe at certain common
or expected MTU sizes, for example, 1500 bytes for standard Ethernet,
or 1500 bytes minus header sizes for tunnel protocols.
Some protocols may use other mechanisms to choose the probe sizes.
For example, protocols that have certain natural data block sizes
might simply assemble messages from a number of blocks until the
total size is smaller than search_high, and if possible larger than
Each Packetization Layer MUST determine when probing has converged,
that is, when the probe size range is small enough that further
probing is no longer worth its cost. When probing has converged, a
timer SHOULD be set. When the timer expires, search_high should be
reset to its initial value (described above) so that probing can
resume. Thus, if the path changes, increasing the Path MTU, then the
flow will eventually take advantage of it. The value for this timer
MUST NOT be less than 5 minutes and is recommended to be 10 minutes,
per RFC 1981.
7.4. Probing Preconditions
Before sending a probe, the flow MUST meet at least the following
o It has no outstanding probes or losses.
o If the last probe failed or was inconclusive, then the probe
timeout has expired (see Section 7.6.2).
o The available window is greater than the probe size.
o For a protocol using in-band data for probing, enough data is
available to send the probe.
In addition, the timely loss detection algorithms in most protocols
have pre-conditions that SHOULD be satisfied before sending a probe.
For example, TCP Fast Retransmit is not robust unless there are
sufficient segments following a probe; that is, the sender SHOULD
have enough data queued and sufficient receiver window to send the
probe plus at least Tcprexmtthresh [RFC2760] additional segments.
This restriction may inhibit probing in some protocol states, such as
too close to the end of a connection, or when the window is too
Protocols MAY delay sending non-probes in order to accumulate enough
data to meet the pre-conditions for probing. The delayed sending
algorithm SHOULD use some self-scaling technique to appropriately
limit the time that the data is delayed. For example, the returning
ACKs can be used to prevent the window from falling by more than the
amount of data needed for the probe.
7.5. Conducting a Probe
Once a probe size in the appropriate range has been selected, and the
above preconditions have been met, the Packetization Layer MAY
conduct a probe. To do so, it creates a probe packet such that its
size, including the outermost IP headers, is equal to the probe size.
After sending the probe it awaits a response, which will have one of
the following results:
Success: The probe is acknowledged as having been received by the
Failure: A protocol mechanism indicates that the probe was lost, but
no packets in the leading or trailing window were lost.
Timeout failure: A protocol mechanism indicates that the probe was
lost, and no packets in the leading window were lost, but is
unable to determine whether any packets in the trailing window
were lost. For example, loss is detected by a timeout, and
go-back-n retransmission is used.
Inconclusive: The probe was lost in addition to other packets in the
leading or trailing windows.
7.6. Response to Probe Results
When a probe has completed, the result SHOULD be processed as
follows, categorized by the probe's result type.
7.6.1. Probe Success
When the probe is delivered, it is an indication that the Path MTU is
at least as large as the probe size. Set search_low to the probe
size. If the probe size is larger than the eff_pmtu, raise eff_pmtu
to the probe size. The probe size might be smaller than the eff_pmtu
if the flow has not been using the full MTU of the path because it is
subject to some other limitation, such as available data in an
Note that if a flow's packets are routed via multiple paths, or over
a path with a non-deterministic MTU, delivery of a single probe
packet does not indicate that all packets of that size will be
delivered. To be robust in such a case, the Packetization Layer
SHOULD conduct MTU verification as described in Section 7.8.
7.6.2. Probe Failure
When only the probe is lost, it is treated as an indication that the
Path MTU is smaller than the probe size. In this case alone, the
loss SHOULD NOT be interpreted as congestion signal.
In the absence of other indications, set search_high to the probe
size minus one. The eff_pmtu might be larger than the probe size if
the flow has not been using the full MTU of the path because it is
subject to some other limitation, such as available data in an
interactive session. If eff_pmtu is larger than the probe size,
eff_pmtu MUST be reduced to no larger than search_high, and SHOULD be
reduced to search_low, as the eff_pmtu has been determined to be
invalid, similar to after a full-stop timeout (see Section 7.7).
If an ICMP PTB message is received matching the probe packet, then
search_high and eff_pmtu MAY be set from the MTU value indicated in
the message. Note that the ICMP message may be received either
before or after the protocol loss indication.
A probe failure event is the one situation under which the
Packetization Layer SHOULD ignore loss as a congestion signal.
Because there is some small risk that suppressing congestion control
might have unanticipated consequences (even for one isolated loss),
it is REQUIRED that probe failure events be less frequent than the
normal period for losses under standard congestion control.
Specifically, after a probe failure event and suppressed congestion
control, PLPMTUD MUST NOT probe again until an interval that is
larger than the expected interval between congestion control events.
See Section 4 for details. The simplest estimate of the interval to
the next congestion event is the same number of round trips as the
current congestion window in packets.
7.6.3. Probe Timeout Failure
If the loss was detected with a timeout and repaired with go-back-n
retransmission, then congestion window reduction will be necessary.
The relatively high price of a failed probe in this case may merit a
longer time interval until the next probe. A time interval that is
five times the non-timeout failure case (Section 7.6.2) is
7.6.4. Probe Inconclusive
The presence of other losses near the loss of the probe may indicate
that the probe was lost due to congestion rather than due to an MTU
limitation. In this case, the state variables eff_pmtu, search_low,
and search_high SHOULD NOT be updated, and the same-sized probe
SHOULD be attempted again as soon as the probing preconditions are
met (i.e., once the packetization layer has no outstanding
unrecovered losses). At this point, it is particularly appropriate
to re-probe since the flow's congestion window will be at its lowest
point, minimizing the probability of congestive losses.
7.7. Full-Stop Timeout
Under all conditions, a full-stop timeout (also known as a
"persistent timeout" in other documents) SHOULD be taken as an
indication of some significantly disruptive event in the network,
such as a router failure or a routing change to a path with a smaller
MTU. For TCP, this occurs when the R1 timeout threshold described by
If there is a full-stop timeout and there was not an ICMP message
indicating a reason (PTB, Net unreachable, etc., or the ICMP message
was ignored for some reason), the RECOMMENDED first recovery action
is to treat this as a detected ICMP black hole as defined in
The response to a detected black hole depends on the current values
for search_low and eff_pmtu. If eff_pmtu is larger than search_low,
set eff_pmtu to search_low. Otherwise, set both eff_pmtu and
search_low to the initial value for search_low. Upon additional
successive timeouts, search_low and eff_pmtu SHOULD be halved, with a
lower bound of 68 bytes for IPv4 and 1280 bytes for IPv6. Even lower
lower bounds MAY be permitted to support limited operation over links
with MTUs that are smaller than permitted by the IP specifications.
7.8. MTU Verification
It is possible for a flow to simultaneously traverse multiple paths,
but an implementation will only be able to keep a single path
representation for the flow. If the paths have different MTUs,
storing the minimum MTU of all paths in the flow's path
representation will result in correct behavior. If ICMP PTB messages
are delivered, then classical PMTUD will work correctly in this
If ICMP delivery fails, breaking classical PMTUD, the connection will
rely solely on PLPMTUD. In this case, PLPMTUD may fail as well since
it assumes a flow traverses a path with a single MTU. A probe with a
size greater than the minimum but smaller than the maximum of the
Path MTUs may be successful. However, upon raising the flow's
effective PMTU, the loss rate will significantly increase. The flow
may still make progress, but the resultant loss rate is likely to be
unacceptable. For example, when using two-way round-robin striping,
50% of full-sized packets would be dropped.
Striping in this manner is often operationally undesirable for other
reasons (e.g., due to packet reordering) and is usually avoided by
hashing each flow to a single path. However, to increase robustness,
an implementation SHOULD implement some form of MTU verification,
such that if increasing eff_pmtu results in a sharp increase in loss
rate, it will fall back to using a lower MTU.
A RECOMMENDED strategy would be to save the value of eff_pmtu before
raising it. Then, if loss rate rises above a threshold for a period
of time (e.g., loss rate is higher than 10% over multiple
retransmission timeout (RTO) intervals), then the new MTU is
considered incorrect. The saved value of eff_pmtu SHOULD be
restored, and search_high reduced in the same manner as in a probe
failure. PLPMTUD implementations SHOULD implement MTU verification.
8. Host Fragmentation
Packetization Layers SHOULD avoid sending messages that will require
fragmentation [Kent87] [frag-errors]. However, entirely preventing
fragmentation is not always possible. Some Packetization Layers,
such as a UDP application outside the kernel, may be unable to change
the size of messages it sends, resulting in datagram sizes that
exceed the Path MTU.
IPv4 permitted such applications to send packets without the DF bit
set. Oversized packets without the DF bit set would be fragmented in
the network or sending host when they encountered a link with an MTU
smaller than the packet. In some case, packets could be fragmented
more than once if there were cascaded links with progressively
smaller MTUs. This approach is NOT RECOMMENDED.
It is RECOMMENDED that IPv4 implementations use a strategy that
mimics IPv6 functionality. When an application sends datagrams that
are larger than the effective Path MTU, they SHOULD be fragmented to
the Path MTU in the host IP layer even if they are smaller than the
MTU of the first link, directly attached to the host. The DF bit
SHOULD be set on the fragments, so they will not be fragmented again
in the network. This technique will minimize the likelihood that
applications will rely on IPv4 fragmentation in a way that cannot be
implemented in IPv6. At least one major operating system already
uses this strategy. Section 9 describes some exceptions to this rule
when the application is sending oversized packets for probing or
Since protocols that do not implement PLPMTUD are still subject to
problems due to ICMP black holes, it may be desirable to limit to
these protocols to "safe" MTUs likely to work on any path (e.g., 1280
bytes). Allow any protocol implementing PLPMTUD to operate over the
full range supported by the lower layer.
Note that IP fragmentation divides data into packets, so it is
minimally a Packetization Layer. However, it does not have a
mechanism to detect lost packets, so it cannot support a native
implementation of PLPMTUD. Fragmentation-based PLPMTUD requires an
adjunct protocol as described in Section 10.3.
9. Application Probing
All implementations MUST include a mechanism where applications using
connectionless protocols can send their own probes. This is
necessary to implement PLPMTUD in an application protocol as
described in Section 10.4 or to implement diagnostic tools for
debugging problems with PMTUD. There MUST be a mechanism that
permits an application to send datagrams that are larger than
eff_pmtu, the operating systems estimate of the Path MTU, without
being fragmented. If these are IPv4 packets, they MUST have the DF
At this time, most operating systems support two modes for sending
datagrams: one that silently fragments packets that are too large,
and another that rejects packets that are too large. Neither of
these modes is suitable for implementing PLPMTUD in an application or
diagnosing problems with Path MTU Discovery. A third mode is
REQUIRED where the datagram is sent even if it is larger than the
current estimate of the Path MTU.
Implementing PLPMTUD in an application also requires a mechanism
where the application can inform the operating system about the
outcome of the probe as described in Section 7.6, or directly update
search_low, search_high, and eff_pmtu, described in Section 7.1.
Diagnostic applications are useful for finding PMTUD problems, such
as those that might be caused by a defective router that returns ICMP
PTB messages with incorrect size information. Such problems can be
most quickly located with a tool that can send probes of any
specified size, and collect and display all returned ICMP PTB
10. Specific Packetization Layers
All Packetization Layer protocols must consider all of the issues
discussed in Section 6. For many protocols, it is straightforward to
address these issues. This section discusses specific details for
implementing PLPMTUD with a couple of protocols. It is hoped that
the descriptions here will be sufficient illustration for
implementers to adapt to additional protocols.
10.1. Probing Method Using TCP
TCP has no mechanism to distinguish in-band data from padding.
Therefore, TCP must generate probes by appropriately segmenting data.
There are two approaches to segmentation: overlapping and non-
In the non-overlapping method, data is segmented such that the probe
and any subsequent segments contain no overlapping data. If the
probe is lost, the "probe gap" will be a full probe size minus
headers. Data in the probe gap will need to be retransmitted with
multiple smaller segments.
TCP sequence number
i <--------> (probe)
. (probe lost)
<----> (probe gap retransmitted)
An alternate approach is to send subsequent data overlapping the
probe such that the probe gap is equal in length to the current MSS.
In the case of a successful probe, this has added overhead in that it
will send some data twice, but it will have to retransmit only one
segment after a lost probe. When a probe succeeds, there will likely
be some duplicate acknowledgments generated due to the duplicate data
sent. It is important that these duplicate acknowledgments not
trigger Fast Retransmit. As such, an implementation using this
approach SHOULD limit the probe size to three times the current MSS
(causing at most 2 duplicate acknowledgments), or appropriately
adjust its duplicate acknowledgment threshold for data immediately
after a successful probe.
TCP sequence number
i <--------> (probe)
. (probe lost)
<----> (probe gap retransmitted)
The choice of which segmentation method to use should be based on
what is simplest and most efficient for a given TCP implementation.
10.2. Probing Method Using SCTP
In the Stream Control Transmission Protocol (SCTP) [RFC2960], the
application writes messages to SCTP, which divides the data into
smaller "chunks" suitable for transmission through the network. Each
chunk is assigned a Transmission Sequence Number (TSN). Once a TSN
has been transmitted, SCTP cannot change the chunk size. SCTP multi-
path support normally requires SCTP to choose a chunk size such that
its messages to fit the smallest PMTU of all paths. Although not
required, implementations may bundle multiple data chunks together to
make larger IP packets to send on paths with a larger PMTU. Note
that SCTP must independently probe the PMTU on each path to the peer.
The RECOMMENDED method for generating probes is to add a chunk
consisting only of padding to an SCTP message. The PAD chunk defined
in [RFC4820] SHOULD be attached to a minimum length HEARTBEAT (HB)
chunk to build a probe packet. This method is fully compatible with
all current SCTP implementations.
SCTP MAY also probe with a method similar to TCP's described above,
using inline data. Using such a method has the advantage that
successful probes have no additional overhead; however, failed probes
will require retransmission of data, which may impact flow
10.3. Probing Method for IP Fragmentation
There are a few protocols and applications that normally send large
datagrams and rely on IP fragmentation to deliver them. It has been
known for a long time that this has some undesirable consequences
[Kent87]. More recently, it has come to light that IPv4
fragmentation is not sufficiently robust for general use in today's
Internet. The 16-bit IP identification field is not large enough to
prevent frequent mis-associated IP fragments, and the TCP and UDP
checksums are insufficient to prevent the resulting corrupted data
from being delivered to higher protocol layers [frag-errors].
As mentioned in Section 8, datagram protocols (such as UDP) might
rely on IP fragmentation as a Packetization Layer. However, using IP
fragmentation to implement PLPMTUD is problematic because the IP
layer has no mechanism to determine whether the packets are
ultimately delivered to the far node, without direct participation by
To support IP fragmentation as a Packetization Layer under an
unmodified application, an implementation SHOULD rely on the Path MTU
sharing described in Section 5.2 plus an adjunct protocol to probe
the Path MTU. There are a number of protocols that might be used for
the purpose, such as ICMP ECHO and ECHO REPLY, or "traceroute" style
UDP datagrams that trigger ICMP messages. Use of ICMP ECHO and ECHO
REPLY will probe both forward and return paths, so the sender will
only be able to take advantage of the minimum of the two. Other
methods that probe only the forward path are preferred if available.
All of these approaches have a number of potential robustness
problems. The most likely failures are due to losses unrelated to
MTU (e.g., nodes that discard some protocol types). These non-MTU-
related losses can prevent PLPMTUD from raising the MTU, forcing IP
fragmentation to use a smaller MTU than necessary. Since these
failures are not likely to cause interoperability problems they are
However, other more serious failure modes do exist, such as might be
caused by middle boxes or upper-layer routers that choose different
paths for different protocol types or sessions. In such
environments, adjunct protocols may legitimately experience a
different Path MTU than the primary protocol. If the adjunct
protocol finds a larger MTU than the primary protocol, PLPMTUD may
select an MTU that is not usable by the primary protocol. Although
this is a potentially serious problem, this sort of situation is
likely to be viewed as incorrect by a large number of observers, and
thus there will be strong motivation to correct it.
Since connectionless protocols might not keep enough state to
effectively diagnose MTU black holes, it would be more robust to err
on the side of using too small of an initial MTU (e.g., 1 kByte or
less) prior to probing a path to measure the MTU. For this reason,
implementations that use IP fragmentation SHOULD use an initial
eff_pmtu, which is selected as described in Section 7.2, except using
a separate global control for the default initial eff_mtu for
Connectionless protocols also introduce an additional problem with
maintaining the path information cache: there are no events
corresponding to connection establishment and tear-down to use to
manage the cache itself. A natural approach would be to keep an
immutable cache entry for the "default path", which has a eff_pmtu
that is fixed at the initial value for connectionless protocols. The
adjunct Path MTU Discovery protocol would be invoked once the number
of fragmented datagrams to any particular destination reaches some
configurable threshold (e.g., 5 datagrams). A new path cache entry
would be created when the adjunct protocol updates eff_pmtu, and
deleted on the basis of a timer or a Least Recently Used cache
10.4. Probing Method Using Applications
The disadvantages of relying on IP fragmentation and an adjunct
protocol to perform Path MTU Discovery can be overcome by
implementing Path MTU Discovery within the application itself, using
the application's own protocol. The application must have some
suitable method for generating probes and have an accurate and timely
mechanism to determine whether the probes were lost.
Ideally, the application protocol includes a lightweight echo
function that confirms message delivery, plus a mechanism for padding
the messages out to the desired probe size, such that the padding is
not echoed. This combination (akin to the SCTP HB plus PAD) is
RECOMMENDED because an application can separately measure the MTU of
each direction on a path with asymmetrical MTUs.
For protocols that cannot implement PLPMTUD with "echo plus pad",
there are often alternate methods for generating probes. For
example, the protocol may have a variable length echo that
effectively measures minimum MTU of both the forward and return
path's, or there may be a way to add padding to regular messages
carrying real application data. There may also be alternate ways to
segment application data to generate probes, or as a last resort, it
may be feasible to extend the protocol with new message types
specifically to support MTU discovery.
Note that if it is necessary to add new message types to support
PLPMTUD, the most general approach is to add ECHO and PAD messages,
which permit the greatest possible latitude in how an application-
specific implementation of PLPMTUD interacts with other applications
and protocols on the same end system.
All application probing techniques require the ability to send
messages that are larger than the current eff_pmtu described in
11. Security Considerations
Under all conditions, the PLPMTUD procedures described in this
document are at least as secure as the current standard Path MTU
Discovery procedures described in RFC 1191 and RFC 1981.
Since PLPMTUD is designed for robust operation without any ICMP or
other messages from the network, it can be configured to ignore all
ICMP messages, either globally or on a per-application basis. In
such a configuration, it cannot be attacked unless the attacker can
identify and cause probe packets to be lost. Attacking PLPMTUD
reduces performance, but not as much as attacking congestion control
by causing arbitrary packets to be lost. Such an attacker might do
far more damage by completely disrupting specific protocols, such as
Since packetization protocols may share state with each other, if one
packetization protocol (particularly an application) were hostile to
other protocols on the same host, it could harm performance in the
other protocols by reducing the effective MTU. If a packetization
protocol is untrusted, it should not be allowed to write to shared
12.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery",
RFC 1191, November 1990.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU
Discovery for IP version 6", RFC 1981, August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol,
Version 6 (IPv6) Specification", RFC 2460,
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC3697] Rajahalme, J., Conta, A., Carpenter, B., and S.
Deering, "IPv6 Flow Label Specification", RFC 3697,
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L., and V. Paxson, "Stream Control
Transmission Protocol", RFC 2960, October 2000.
[RFC4820] Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk
and Parameter for the Stream Control Transmission
Protocol (SCTP)", RFC 4820, March 2007.
12.2. Informative References
[RFC2760] Allman, M., Dawkins, S., Glover, D., Griner, J.,
Tran, D., Henderson, T., Heidemann, J., Touch, J.,
Kruse, H., Ostermann, S., Scott, K., and J. Semke,
"Ongoing TCP Research Related to Satellites",
RFC 2760, February 2000.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, September 2000.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for
the Internet Protocol", RFC 2401, November 1998.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, September 2000.
[RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461,
[RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
Conservative Selective Acknowledgment (SACK)-based
Loss Recovery Algorithm for TCP", RFC 3517,
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
[Kent87] Kent, C. and J. Mogul, "Fragmentation considered
harmful", Proc. SIGCOMM '87 vol. 17, No. 5,
[tcp-friendly] Mahdavi, J. and S. Floyd, "TCP-Friendly Unicast Rate-
Based Flow Control", Technical note sent to the
end2end-interest mailing list , January 1997, <http:/
[frag-errors] Heffner, J., "IPv4 Reassembly Errors at High Data
Rates", Work in Progress, December 2007.
Appendix A. Acknowledgments
Many ideas and even some of the text come directly from RFC 1191 and
Many people made significant contributions to this document,
including: Randall Stewart for SCTP text, Michael Richardson for
material from an earlier ID on tunnels that ignore DF, Stanislav
Shalunov for the idea that pure PLPMTUD parallels congestion control,
and Matt Zekauskas for maintaining focus during the meetings. Thanks
to the early implementors: Kevin Lahey, John Heffner, and Rao Shoaib,
who provided concrete feedback on weaknesses in earlier versions.
Thanks also to all of the people who made constructive comments in
the working group meetings and on the mailing list. We are sure we
have missed many deserving people.
Matt Mathis and John Heffner are supported in this work by a grant
from Cisco Systems, Inc.
Pittsburgh Supercomputing Center
4400 Fifth Avenue
Pittsburgh, PA 15213
John W. Heffner
Pittsburgh Supercomputing Center
4400 Fifth Avenue
Pittsburgh, PA 15213
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