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Network Working Group J. Mogul
Request for Comments: 1191 DECWRL
Obsoletes: RFC 1063 S. Deering
Stanford University
November 1990
Path MTU Discovery
Status of this Memo
This RFC specifies a protocol on the IAB Standards Track for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "IAB
Official Protocol Standards" for the standardization state and status
of this protocol. Distribution of this memo is unlimited.
Table of Contents
Status of this Memo 1
Abstract 2
Acknowledgements 2
1. Introduction 2
2. Protocol overview 3
3. Host specification 4
3.1. TCP MSS Option 5
4. Router specification 6
5. Host processing of old-style messages 7
6. Host implementation 8
6.1. Layering 9
6.2. Storing PMTU information 10
6.3. Purging stale PMTU information 11
6.4. TCP layer actions 13
6.5. Issues for other transport protocols 14
6.6. Management interface 15
7. Likely values for Path MTUs 15
7.1. A better way to detect PMTU increases 16
8. Security considerations 18
References 18
Authors' Addresses 19
List of Tables
Table 7-1: Common MTUs in the Internet 17
Mogul & Deering [page 1]
RFC 1191 Path MTU Discovery November 1990
Abstract
This memo describes a technique for dynamically discovering the
maximum transmission unit (MTU) of an arbitrary internet path. It
specifies a small change to the way routers generate one type of ICMP
message. For a path that passes through a router that has not been
so changed, this technique might not discover the correct Path MTU,
but it will always choose a Path MTU as accurate as, and in many
cases more accurate than, the Path MTU that would be chosen by
current practice.
Acknowledgements
This proposal is a product of the IETF MTU Discovery Working Group.
The mechanism proposed here was first suggested by Geof Cooper [2],
who in two short paragraphs set out all the basic ideas that took the
Working Group months to reinvent.
1. Introduction
When one IP host has a large amount of data to send to another host,
the data is transmitted as a series of IP datagrams. It is usually
preferable that these datagrams be of the largest size that does not
require fragmentation anywhere along the path from the source to the
destination. (For the case against fragmentation, see [5].) This
datagram size is referred to as the Path MTU (PMTU), and it is equal
to the minimum of the MTUs of each hop in the path. A shortcoming of
the current Internet protocol suite is the lack of a standard
mechanism for a host to discover the PMTU of an arbitrary path.
Note: The Path MTU is what in [1] is called the "Effective MTU
for sending" (EMTU_S). A PMTU is associated with a path,
which is a particular combination of IP source and destination
address and perhaps a Type-of-service (TOS).
The current practice [1] is to use the lesser of 576 and the
first-hop MTU as the PMTU for any destination that is not connected
to the same network or subnet as the source. In many cases, this
results in the use of smaller datagrams than necessary, because many
paths have a PMTU greater than 576. A host sending datagrams much
smaller than the Path MTU allows is wasting Internet resources and
probably getting suboptimal throughput. Furthermore, current
practice does not prevent fragmentation in all cases, since there are
some paths whose PMTU is less than 576.
Mogul & Deering [page 2]
RFC 1191 Path MTU Discovery November 1990
It is expected that future routing protocols will be able to provide
accurate PMTU information within a routing area, although perhaps not
across multi-level routing hierarchies. It is not clear how soon
that will be ubiquitously available, so for the next several years
the Internet needs a simple mechanism that discovers PMTUs without
wasting resources and that works before all hosts and routers are
modified.
2. Protocol overview
In this memo, we describe a technique for using the Don't Fragment
(DF) bit in the IP header to dynamically discover the PMTU of a path.
The basic idea is that a source host initially assumes that the PMTU
of a path is the (known) MTU of its first hop, and sends all
datagrams on that path with the DF bit set. If any of the datagrams
are too large to be forwarded without fragmentation by some router
along the path, that router will discard them and return ICMP
Destination Unreachable messages with a code meaning "fragmentation
needed and DF set" [7]. Upon receipt of such a message (henceforth
called a "Datagram Too Big" message), the source host reduces its
assumed PMTU for the path.
The PMTU discovery process ends when the host's estimate of the PMTU
is low enough that its datagrams can be delivered without
fragmentation. Or, the host may elect to end the discovery process
by ceasing to set the DF bit in the datagram headers; it may do so,
for example, because it is willing to have datagrams fragmented in
some circumstances. Normally, the host continues to set DF in all
datagrams, so that if the route changes and the new PMTU is lower, it
will be discovered.
Unfortunately, the Datagram Too Big message, as currently specified,
does not report the MTU of the hop for which the rejected datagram
was too big, so the source host cannot tell exactly how much to
reduce its assumed PMTU. To remedy this, we propose that a currently
unused header field in the Datagram Too Big message be used to report
the MTU of the constricting hop. This is the only change specified
for routers in support of PMTU Discovery.
The PMTU of a path may change over time, due to changes in the
routing topology. Reductions of the PMTU are detected by Datagram
Too Big messages, except on paths for which the host has stopped
setting the DF bit. To detect increases in a path's PMTU, a host
periodically increases its assumed PMTU (and if it had stopped,
resumes setting the DF bit). This will almost always result in
datagrams being discarded and Datagram Too Big messages being
Mogul & Deering [page 3]
RFC 1191 Path MTU Discovery November 1990
generated, because in most cases the PMTU of the path will not have
changed, so it should be done infrequently.
Since this mechanism essentially guarantees that host will not
receive any fragments from a peer doing PMTU Discovery, it may aid in
interoperating with certain hosts that (improperly) are unable to
reassemble fragmented datagrams.
3. Host specification
When a host receives a Datagram Too Big message, it MUST reduce its
estimate of the PMTU for the relevant path, based on the value of the
Next-Hop MTU field in the message (see section 4). We do not specify
the precise behavior of a host in this circumstance, since different
applications may have different requirements, and since different
implementation architectures may favor different strategies.
We do require that after receiving a Datagram Too Big message, a host
MUST attempt to avoid eliciting more such messages in the near
future. The host may either reduce the size of the datagrams it is
sending along the path, or cease setting the Don't Fragment bit in
the headers of those datagrams. Clearly, the former strategy may
continue to elicit Datagram Too Big messages for a while, but since
each of these messages (and the dropped datagrams they respond to)
consume Internet resources, the host MUST force the PMTU Discovery
process to converge.
Hosts using PMTU Discovery MUST detect decreases in Path MTU as fast
as possible. Hosts MAY detect increases in Path MTU, but because
doing so requires sending datagrams larger than the current estimated
PMTU, and because the likelihood is that the PMTU will not have
increased, this MUST be done at infrequent intervals. An attempt to
detect an increase (by sending a datagram larger than the current
estimate) MUST NOT be done less than 5 minutes after a Datagram Too
Big message has been received for the given destination, or less than
1 minute after a previous, successful attempted increase. We
recommend setting these timers at twice their minimum values (10
minutes and 2 minutes, respectively).
Hosts MUST be able to deal with Datagram Too Big messages that do not
include the next-hop MTU, since it is not feasible to upgrade all the
routers in the Internet in any finite time. A Datagram Too Big
message from an unmodified router can be recognized by the presence
of a zero in the (newly-defined) Next-Hop MTU field. (This is
required by the ICMP specification [7], which says that "unused"
fields must be zero.) In section 5, we discuss possible strategies
Mogul & Deering [page 4]
RFC 1191 Path MTU Discovery November 1990
for a host to follow in response to an old-style Datagram Too Big
message (one sent by an unmodified router).
A host MUST never reduce its estimate of the Path MTU below 68
octets.
A host MUST not increase its estimate of the Path MTU in response to
the contents of a Datagram Too Big message. A message purporting to
announce an increase in the Path MTU might be a stale datagram that
has been floating around in the Internet, a false packet injected as
part of a denial-of-service attack, or the result of having multiple
paths to the destination.
3.1. TCP MSS Option
A host doing PMTU Discovery must obey the rule that it not send IP
datagrams larger than 576 octets unless it has permission from the
receiver. For TCP connections, this means that a host must not send
datagrams larger than 40 octets plus the Maximum Segment Size (MSS)
sent by its peer.
Note: The TCP MSS is defined to be the relevant IP datagram
size minus 40 [9]. The default of 576 octets for the maximum
IP datagram size yields a default of 536 octets for the TCP
MSS.
Section 4.2.2.6 of "Requirements for Internet Hosts -- Communication
Layers" [1] says:
Some TCP implementations send an MSS option only if the
destination host is on a non-connected network. However, in
general the TCP layer may not have the appropriate information
to make this decision, so it is preferable to leave to the IP
layer the task of determining a suitable MTU for the Internet
path.
Actually, many TCP implementations always send an MSS option, but set
the value to 536 if the destination is non-local. This behavior was
correct when the Internet was full of hosts that did not follow the
rule that datagrams larger than 576 octets should not be sent to
non-local destinations. Now that most hosts do follow this rule, it
is unnecessary to limit the value in the TCP MSS option to 536 for
non-local peers.
Moreover, doing this prevents PMTU Discovery from discovering PMTUs
larger than 576, so hosts SHOULD no longer lower the value they send
Mogul & Deering [page 5]
RFC 1191 Path MTU Discovery November 1990
in the MSS option. The MSS option should be 40 octets less than the
size of the largest datagram the host is able to reassemble (MMS_R,
as defined in [1]); in many cases, this will be the architectural
limit of 65495 (65535 - 40) octets. A host MAY send an MSS value
derived from the MTU of its connected network (the maximum MTU over
its connected networks, for a multi-homed host); this should not
cause problems for PMTU Discovery, and may dissuade a broken peer
from sending enormous datagrams.
Note: At the moment, we see no reason to send an MSS greater
than the maximum MTU of the connected networks, and we
recommend that hosts do not use 65495. It is quite possible
that some IP implementations have sign-bit bugs that would be
tickled by unnecessary use of such a large MSS.
4. Router specification
When a router is unable to forward a datagram because it exceeds the
MTU of the next-hop network and its Don't Fragment bit is set, the
router is required to return an ICMP Destination Unreachable message
to the source of the datagram, with the Code indicating
"fragmentation needed and DF set". To support the Path MTU Discovery
technique specified in this memo, the router MUST include the MTU of
that next-hop network in the low-order 16 bits of the ICMP header
field that is labelled "unused" in the ICMP specification [7]. The
high-order 16 bits remain unused, and MUST be set to zero. Thus, the
message has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 | Code = 4 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unused = 0 | Next-Hop MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internet Header + 64 bits of Original Datagram Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value carried in the Next-Hop MTU field is:
The size in octets of the largest datagram that could be
forwarded, along the path of the original datagram, without
being fragmented at this router. The size includes the IP
header and IP data, and does not include any lower-level
headers.
Mogul & Deering [page 6]
RFC 1191 Path MTU Discovery November 1990
This field will never contain a value less than 68, since every
router "must be able to forward a datagram of 68 octets without
fragmentation" [8].
5. Host processing of old-style messages
In this section we outline several possible strategies for a host to
follow upon receiving a Datagram Too Big message from an unmodified
router (i.e., one where the Next-Hop MTU field is zero). This
section is not part of the protocol specification.
The simplest thing for a host to do in response to such a message is
to assume that the PMTU is the minimum of its currently-assumed PMTU
and 576, and to stop setting the DF bit in datagrams sent on that
path. Thus, the host falls back to the same PMTU as it would choose
under current practice (see section 3.3.3 of "Requirements for
Internet Hosts -- Communication Layers" [1]). This strategy has the
advantage that it terminates quickly, and does no worse than existing
practice. It fails, however, to avoid fragmentation in some cases,
and to make the most efficient utilization of the internetwork in
other cases.
More sophisticated strategies involve "searching" for an accurate
PMTU estimate, by continuing to send datagrams with the DF bit while
varying their sizes. A good search strategy is one that obtains an
accurate estimate of the Path MTU without causing many packets to be
lost in the process.
Several possible strategies apply algorithmic functions to the
previous PMTU estimate to generate a new estimate. For example, one
could multiply the old estimate by a constant (say, 0.75). We do NOT
recommend this; it either converges far too slowly, or it
substantially underestimates the true PMTU.
A more sophisticated approach is to do a binary search on the packet
size. This converges somewhat faster, although it still takes 4 or 5
steps to converge from an FDDI MTU to an Ethernet MTU. A serious
disadvantage is that it requires a complex implementation in order to
recognize when a datagram has made it to the other end (indicating
that the current estimate is too low). We also do not recommend this
strategy.
One strategy that appears to work quite well starts from the
observation that there are, in practice, relatively few MTU values in
use in the Internet. Thus, rather than blindly searching through
arbitrarily chosen values, we can search only the ones that are
Mogul & Deering [page 7]
RFC 1191 Path MTU Discovery November 1990
likely to appear. Moreover, since designers tend to chose MTUs in
similar ways, it is possible to collect groups of similar MTU values
and use the lowest value in the group as our search "plateau". (It
is clearly better to underestimate an MTU by a few per cent than to
overestimate it by one octet.)
In section 7, we describe how we arrived at a table of representative
MTU plateaus for use in PMTU estimation. With this table,
convergence is as good as binary search in the worst case, and is far
better in common cases (for example, it takes only two round-trip
times to go from an FDDI MTU to an Ethernet MTU). Since the plateaus
lie near powers of two, if an MTU is not represented in this table,
the algorithm will not underestimate it by more than a factor of 2.
Any search strategy must have some "memory" of previous estimates in
order to chose the next one. One approach is to use the
currently-cached estimate of the Path MTU, but in fact there is
better information available in the Datagram Too Big message itself.
All ICMP Destination Unreachable messages, including this one,
contain the IP header of the original datagram, which contains the
Total Length of the datagram that was too big to be forwarded without
fragmentation. Since this Total Length may be less than the current
PMTU estimate, but is nonetheless larger than the actual PMTU, it may
be a good input to the method for choosing the next PMTU estimate.
Note: routers based on implementations derived from 4.2BSD
Unix send an incorrect value for the Total Length of the
original IP datagram. The value sent by these routers is the
sum of the original Total Length and the original Header
Length (expressed in octets). Since it is impossible for the
host receiving such a Datagram Too Big message to know if it
sent by one of these routers, the host must be conservative
and assume that it is. If the Total Length field returned is
not less than the current PMTU estimate, it must be reduced by
4 times the value of the returned Header Length field.
The strategy we recommend, then, is to use as the next PMTU estimate
the greatest plateau value that is less than the returned Total
Length field (corrected, if necessary, according to the Note above).
6. Host implementation
In this section we discuss how PMTU Discovery is implemented in host
software. This is not a specification, but rather a set of
suggestions.
The issues include:
Mogul & Deering [page 8]
RFC 1191 Path MTU Discovery November 1990
- What layer or layers implement PMTU Discovery?
- Where is the PMTU information cached?
- How is stale PMTU information removed?
- What must transport and higher layers do?
6.1. Layering
In the IP architecture, the choice of what size datagram to send is
made by a protocol at a layer above IP. We refer to such a protocol
as a "packetization protocol". Packetization protocols are usually
transport protocols (for example, TCP) but can also be higher-layer
protocols (for example, protocols built on top of UDP).
Implementing PMTU Discovery in the packetization layers simplifies
some of the inter-layer issues, but has several drawbacks: the
implementation may have to be redone for each packetization protocol,
it becomes hard to share PMTU information between different
packetization layers, and the connection-oriented state maintained by
some packetization layers may not easily extend to save PMTU
information for long periods.
We therefore believe that the IP layer should store PMTU information
and that the ICMP layer should process received Datagram Too Big
messages. The packetization layers must still be able to respond to
changes in the Path MTU, by changing the size of the datagrams they
send, and must also be able to specify that datagrams are sent with
the DF bit set. We do not want the IP layer to simply set the DF bit
in every packet, since it is possible that a packetization layer,
perhaps a UDP application outside the kernel, is unable to change its
datagram size. Protocols involving intentional fragmentation, while
inelegant, are sometimes successful (NFS being the primary example),
and we do not want to break such protocols.
To support this layering, packetization layers require an extension
of the IP service interface defined in [1]:
A way to learn of changes in the value of MMS_S, the "maximum
send transport-message size", which is derived from the Path
MTU by subtracting the minimum IP header size.
Mogul & Deering [page 9]
RFC 1191 Path MTU Discovery November 1990
6.2. Storing PMTU information
In general, the IP layer should associate each PMTU value that it has
learned with a specific path. A path is identified by a source
address, a destination address and an IP type-of-service. (Some
implementations do not record the source address of paths; this is
acceptable for single-homed hosts, which have only one possible
source address.)
Note: Some paths may be further distinguished by different
security classifications. The details of such classifications
are beyond the scope of this memo.
The obvious place to store this association is as a field in the
routing table entries. A host will not have a route for every
possible destination, but it should be able to cache a per-host route
for every active destination. (This requirement is already imposed
by the need to process ICMP Redirect messages.)
When the first packet is sent to a host for which no per-host route
exists, a route is chosen either from the set of per-network routes,
or from the set of default routes. The PMTU fields in these route
entries should be initialized to be the MTU of the associated
first-hop data link, and must never be changed by the PMTU Discovery
process. (PMTU Discovery only creates or changes entries for
per-host routes). Until a Datagram Too Big message is received, the
PMTU associated with the initially-chosen route is presumed to be
accurate.
When a Datagram Too Big message is received, the ICMP layer
determines a new estimate for the Path MTU (either from a non-zero
Next-Hop MTU value in the packet, or using the method described in
section 5). If a per-host route for this path does not exist, then
one is created (almost as if a per-host ICMP Redirect is being
processed; the new route uses the same first-hop router as the
current route). If the PMTU estimate associated with the per-host
route is higher than the new estimate, then the value in the routing
entry is changed.
The packetization layers must be notified about decreases in the
PMTU. Any packetization layer instance (for example, a TCP
connection) that is actively using the path must be notified if the
PMTU estimate is decreased.
Note: even if the Datagram Too Big message contains an
Original Datagram Header that refers to a UDP packet, the TCP
layer must be notified if any of its connections use the given
Mogul & Deering [page 10]
RFC 1191 Path MTU Discovery November 1990
path.
Also, the instance that sent the datagram that elicited the Datagram
Too Big message should be notified that its datagram has been
dropped, even if the PMTU estimate has not changed, so that it may
retransmit the dropped datagram.
Note: The notification mechanism can be analogous to the
mechanism used to provide notification of an ICMP Source
Quench message. In some implementations (such as
4.2BSD-derived systems), the existing notification mechanism
is not able to identify the specific connection involved, and
so an additional mechanism is necessary.
Alternatively, an implementation can avoid the use of an
asynchronous notification mechanism for PMTU decreases by
postponing notification until the next attempt to send a
datagram larger than the PMTU estimate. In this approach,
when an attempt is made to SEND a datagram with the DF bit
set, and the datagram is larger than the PMTU estimate, the
SEND function should fail and return a suitable error
indication. This approach may be more suitable to a
connectionless packetization layer (such as one using UDP),
which (in some implementations) may be hard to "notify" from
the ICMP layer. In this case, the normal timeout-based
retransmission mechanisms would be used to recover from the
dropped datagrams.
It is important to understand that the notification of the
packetization layer instances using the path about the change in the
PMTU is distinct from the notification of a specific instance that a
packet has been dropped. The latter should be done as soon as
practical (i.e., asynchronously from the point of view of the
packetization layer instance), while the former may be delayed until
a packetization layer instance wants to create a packet.
Retransmission should be done for only for those packets that are
known to be dropped, as indicated by a Datagram Too Big message.
6.3. Purging stale PMTU information
Internetwork topology is dynamic; routes change over time. The PMTU
discovered for a given destination may be wrong if a new route comes
into use. Thus, PMTU information cached by a host can become stale.
Because a host using PMTU Discovery always sets the DF bit, if the
stale PMTU value is too large, this will be discovered almost
Mogul & Deering [page 11]
RFC 1191 Path MTU Discovery November 1990
immediately once a datagram is sent to the given destination. No
such mechanism exists for realizing that a stale PMTU value is too
small, so an implementation should "age" cached values. When a PMTU
value has not been decreased for a while (on the order of 10
minutes), the PMTU estimate should be set to the first-hop data-link
MTU, and the packetization layers should be notified of the change.
This will cause the complete PMTU Discovery process to take place
again.
Note: an implementation should provide a means for changing
the timeout duration, including setting it to "infinity". For
example, hosts attached to an FDDI network which is then
attached to the rest of the Internet via a slow serial line
are never going to discover a new non-local PMTU, so they
should not have to put up with dropped datagrams every 10
minutes.
An upper layer MUST not retransmit datagrams in response to an
increase in the PMTU estimate, since this increase never comes in
response to an indication of a dropped datagram.
One approach to implementing PMTU aging is to add a timestamp field
to the routing table entry. This field is initialized to a
"reserved" value, indicating that the PMTU has never been changed.
Whenever the PMTU is decreased in response to a Datagram Too Big
message, the timestamp is set to the current time.
Once a minute, a timer-driven procedure runs through the routing
table, and for each entry whose timestamp is not "reserved" and is
older than the timeout interval:
- The PMTU estimate is set to the MTU of the associated first
hop.
- Packetization layers using this route are notified of the
increase.
PMTU estimates may disappear from the routing table if the per-host
routes are removed; this can happen in response to an ICMP Redirect
message, or because certain routing-table daemons delete old routes
after several minutes. Also, on a multi-homed host a topology change
may result in the use of a different source interface. When this
happens, if the packetization layer is not notified then it may
continue to use a cached PMTU value that is now too small. One
solution is to notify the packetization layer of a possible PMTU
change whenever a Redirect message causes a route change, and
whenever a route is simply deleted from the routing table.
Mogul & Deering [page 12]
RFC 1191 Path MTU Discovery November 1990
Note: a more sophisticated method for detecting PMTU increases
is described in section 7.1.
6.4. TCP layer actions
The TCP layer must track the PMTU for the destination of a
connection; it should not send datagrams that would be larger than
this. A simple implementation could ask the IP layer for this value
(using the GET_MAXSIZES interface described in [1]) each time it
created a new segment, but this could be inefficient. Moreover, TCP
implementations that follow the "slow-start" congestion-avoidance
algorithm [4] typically calculate and cache several other values
derived from the PMTU. It may be simpler to receive asynchronous
notification when the PMTU changes, so that these variables may be
updated.
A TCP implementation must also store the MSS value received from its
peer (which defaults to 536), and not send any segment larger than
this MSS, regardless of the PMTU. In 4.xBSD-derived implementations,
this requires adding an additional field to the TCP state record.
Finally, when a Datagram Too Big message is received, it implies that
a datagram was dropped by the router that sent the ICMP message. It
is sufficient to treat this as any other dropped segment, and wait
until the retransmission timer expires to cause retransmission of the
segment. If the PMTU Discovery process requires several steps to
estimate the right PMTU, this could delay the connection by many
round-trip times.
Alternatively, the retransmission could be done in immediate response
to a notification that the Path MTU has changed, but only for the
specific connection specified by the Datagram Too Big message. The
datagram size used in the retransmission should, of course, be no
larger than the new PMTU.
Note: One MUST not retransmit in response to every Datagram
Too Big message, since a burst of several oversized segments
will give rise to several such messages and hence several
retransmissions of the same data. If the new estimated PMTU
is still wrong, the process repeats, and there is an
exponential growth in the number of superfluous segments sent!
This means that the TCP layer must be able to recognize when a
Datagram Too Big notification actually decreases the PMTU that
it has already used to send a datagram on the given
connection, and should ignore any other notifications.
Mogul & Deering [page 13]
RFC 1191 Path MTU Discovery November 1990
Modern TCP implementations incorporate "congestion advoidance" and
"slow-start" algorithms to improve performance [4]. Unlike a
retransmission caused by a TCP retransmission timeout, a
retransmission caused by a Datagram Too Big message should not change
the congestion window. It should, however, trigger the slow-start
mechanism (i.e., only one segment should be retransmitted until
acknowledgements begin to arrive again).
TCP performance can be reduced if the sender's maximum window size is
not an exact multiple of the segment size in use (this is not the
congestion window size, which is always a multiple of the segment
size). In many system (such as those derived from 4.2BSD), the
segment size is often set to 1024 octets, and the maximum window size
(the "send space") is usually a multiple of 1024 octets, so the
proper relationship holds by default. If PMTU Discovery is used,
however, the segment size may not be a submultiple of the send space,
and it may change during a connection; this means that the TCP layer
may need to change the transmission window size when PMTU Discovery
changes the PMTU value. The maximum window size should be set to the
greatest multiple of the segment size (PMTU - 40) that is less than
or equal to the sender's buffer space size.
PMTU Discovery does not affect the value sent in the TCP MSS option,
because that value is used by the other end of the connection, which
may be using an unrelated PMTU value.
6.5. Issues for other transport protocols
Some transport protocols (such as ISO TP4 [3]) are not allowed to
repacketize when doing a retransmission. That is, once an attempt is
made to transmit a datagram of a certain size, its contents cannot be
split into smaller datagrams for retransmission. In such a case, the
original datagram should be retransmitted without the DF bit set,
allowing it to be fragmented as necessary to reach its destination.
Subsequent datagrams, when transmitted for the first time, should be
no larger than allowed by the Path MTU, and should have the DF bit
set.
The Sun Network File System (NFS) uses a Remote Procedure Call (RPC)
protocol [11] that, in many cases, sends datagrams that must be
fragmented even for the first-hop link. This might improve
performance in certain cases, but it is known to cause reliability
and performance problems, especially when the client and server are
separated by routers.
We recommend that NFS implementations use PMTU Discovery whenever
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RFC 1191 Path MTU Discovery November 1990
routers are involved. Most NFS implementations allow the RPC
datagram size to be changed at mount-time (indirectly, by changing
the effective file system block size), but might require some
modification to support changes later on.
Also, since a single NFS operation cannot be split across several UDP
datagrams, certain operations (primarily, those operating on file
names and directories) require a minimum datagram size that may be
larger than the PMTU. NFS implementations should not reduce the
datagram size below this threshold, even if PMTU Discovery suggests a
lower value. (Of course, in this case datagrams should not be sent
with DF set.)
6.6. Management interface
We suggest that an implementation provide a way for a system utility
program to:
- Specify that PMTU Discovery not be done on a given route.
- Change the PMTU value associated with a given route.
The former can be accomplished by associating a flag with the routing
entry; when a packet is sent via a route with this flag set, the IP
layer leaves the DF bit clear no matter what the upper layer
requests.
These features might be used to work around an anomalous situation,
or by a routing protocol implementation that is able to obtain Path
MTU values.
The implementation should also provide a way to change the timeout
period for aging stale PMTU information.
7. Likely values for Path MTUs
The algorithm recommended in section 5 for "searching" the space of
Path MTUs is based on a table of values that severely restricts the
search space. We describe here a table of MTU values that, as of
this writing, represents all major data-link technologies in use in
the Internet.
In table 7-1, data links are listed in order of decreasing MTU, and
grouped so that each set of similar MTUs is associated with a
"plateau" equal to the lowest MTU in the group. (The table also
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RFC 1191 Path MTU Discovery November 1990
includes some entries not currently associated with a data link, and
gives references where available). Where a plateau represents more
than one MTU, the table shows the maximum inaccuracy associated with
the plateau, as a percentage.
We do not expect that the values in the table, especially for higher
MTU levels, are going to be valid forever. The values given here are
an implementation suggestion, NOT a specification or requirement.
Implementors should use up-to-date references to pick a set of
plateaus; it is important that the table not contain too many entries
or the process of searching for a PMTU might waste Internet
resources. Implementors should also make it convenient for customers
without source code to update the table values in their systems (for
example, the table in a BSD-derived Unix kernel could be changed
using a new "ioctl" command).
Note: It might be a good idea to add a few table entries for
values equal to small powers of 2 plus 40 (for the IP and TCP
headers), where no similar values exist, since this seems to
be a reasonably non-arbitrary way of choosing arbitrary
values.
The table might also contain entries for values slightly less
than large powers of 2, in case MTUs are defined near those
values (it is better in this case for the table entries to be
low than to be high, or else the next lowest plateau may be
chosen instead).
7.1. A better way to detect PMTU increases
Section 6.3 suggests detecting increases in the PMTU value by
periodically increasing the PTMU estimate to the first-hop MTU.
Since it is likely that this process will simply "rediscover" the
current PTMU estimate, at the cost of several dropped datagrams, it
should not be done often.
A better approach is to periodically increase the PMTU estimate to
the next-highest value in the plateau table (or the first-hop MTU, if
that is smaller). If the increased estimate is wrong, at most one
round-trip time is wasted before the correct value is rediscovered.
If the increased estimate is still too low, a higher estimate will be
attempted somewhat later.
Because it may take several such periods to discover a significant
increase in the PMTU, we recommend that a short timeout period should
be used after the estimate is increased, and a longer timeout be used
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RFC 1191 Path MTU Discovery November 1990
Plateau MTU Comments Reference
------ --- -------- ---------
65535 Official maximum MTU RFC 791
65535 Hyperchannel RFC 1044
65535
32000 Just in case
17914 16Mb IBM Token Ring ref. [6]
17914
8166 IEEE 802.4 RFC 1042
8166
4464 IEEE 802.5 (4Mb max) RFC 1042
4352 FDDI (Revised) RFC 1188
4352 (1%)
2048 Wideband Network RFC 907
2002 IEEE 802.5 (4Mb recommended) RFC 1042
2002 (2%)
1536 Exp. Ethernet Nets RFC 895
1500 Ethernet Networks RFC 894
1500 Point-to-Point (default) RFC 1134
1492 IEEE 802.3 RFC 1042
1492 (3%)
1006 SLIP RFC 1055
1006 ARPANET BBN 1822
1006
576 X.25 Networks RFC 877
544 DEC IP Portal ref. [10]
512 NETBIOS RFC 1088
508 IEEE 802/Source-Rt Bridge RFC 1042
508 ARCNET RFC 1051
508 (13%)
296 Point-to-Point (low delay) RFC 1144
296
68 Official minimum MTU RFC 791
Table 7-1: Common MTUs in the Internet
after the PTMU estimate is decreased because of a Datagram Too Big
message. For example, after the PTMU estimate is decreased, the
timeout should be set to 10 minutes; once this timer expires and a
larger MTU is attempted, the timeout can be set to a much smaller
value (say, 2 minutes). In no case should the timeout be shorter
than the estimated round-trip time, if this is known.
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RFC 1191 Path MTU Discovery November 1990
8. Security considerations
This Path MTU Discovery mechanism makes possible two denial-of-
service attacks, both based on a malicious party sending false
Datagram Too Big messages to an Internet host.
In the first attack, the false message indicates a PMTU much smaller
than reality. This should not entirely stop data flow, since the
victim host should never set its PMTU estimate below the absolute
minimum, but at 8 octets of IP data per datagram, progress could be
slow.
In the other attack, the false message indicates a PMTU greater than
reality. If believed, this could cause temporary blockage as the
victim sends datagrams that will be dropped by some router. Within
one round-trip time, the host would discover its mistake (receiving
Datagram Too Big messages from that router), but frequent repetition
of this attack could cause lots of datagrams to be dropped. A host,
however, should never raise its estimate of the PMTU based on a
Datagram Too Big message, so should not be vulnerable to this attack.
A malicious party could also cause problems if it could stop a victim
from receiving legitimate Datagram Too Big messages, but in this case
there are simpler denial-of-service attacks available.
References
[1] R. Braden, ed. Requirements for Internet Hosts -- Communication
Layers. RFC 1122, SRI Network Information Center, October, 1989.
[2] Geof Cooper. IP Datagram Sizes. Electronic distribution of the
TCP-IP Discussion Group, Message-ID
<8705240517.AA01407@apolling.imagen.uucp>.
[3] ISO. ISO Transport Protocol Specification: ISO DP 8073. RFC 905,
SRI Network Information Center, April, 1984.
[4] Van Jacobson. Congestion Avoidance and Control. In Proc. SIGCOMM
'88 Symposium on Communications Architectures and Protocols, pages
314-329. Stanford, CA, August, 1988.
[5] C. Kent and J. Mogul. Fragmentation Considered Harmful. In Proc.
SIGCOMM '87 Workshop on Frontiers in Computer Communications
Technology. August, 1987.
[6] Drew Daniel Perkins. Private Communication.
Mogul & Deering [page 18]
RFC 1191 Path MTU Discovery November 1990
[7] J. Postel. Internet Control Message Protocol. RFC 792, SRI
Network Information Center, September, 1981.
[8] J. Postel. Internet Protocol. RFC 791, SRI Network Information
Center, September, 1981.
[9] J. Postel. The TCP Maximum Segment Size and Related Topics. RFC
879, SRI Network Information Center, November, 1983.
[10] Michael Reilly. Private Communication.
[11] Sun Microsystems, Inc. RPC: Remote Procedure Call Protocol. RFC
1057, SRI Network Information Center, June, 1988.
Authors' Addresses
Jeffrey Mogul
Digital Equipment Corporation Western Research Laboratory
100 Hamilton Avenue
Palo Alto, CA 94301
Phone: (415) 853-6643
EMail: mogul@decwrl.dec.com
Steve Deering
Xerox Palo Alto Research Center
3333 Coyote Hill Road
Palo Alto, CA 94304
Phone: (415) 494-4839
EMail: deering@xerox.com
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