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rfc:rfc1191

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.

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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

  1. What layer or layers implement PMTU Discovery?
  1. Where is the PMTU information cached?
  1. How is stale PMTU information removed?
  1. 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

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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:
  1. The PMTU estimate is set to the MTU of the associated first

hop.

  1. 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.

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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.

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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

Mogul & Deering [page 14]

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:
  1. Specify that PMTU Discovery not be done on a given route.
  1. 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

Mogul & Deering [page 15]

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

Mogul & Deering [page 16]

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.

Mogul & Deering [page 17]

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

Mogul & Deering [page 19]

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