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Network Working Group                                     I. van Beijnum
Internet-Draft                                            IMDEA Networks
Intended status: Experimental                          February 24, 2008
Expires: August 27, 2008

                    Extensions for Multi-MTU Subnets

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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   This Internet-Draft will expire on August 27, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2008).


   In the early days of the internet, many different link types with
   many different maximum packet sizes were in use.  For point-to-point
   or point-to-multipoint links, there are still some other link types
   (PPP, ATM, Packet over SONET), but shared subnets are now almost
   exclusively implemented as ethernets.  Even though the relevant
   standards mandate a 1500 byte maximum packet size for ethernet, more
   and more ethernet equipment is capable of handling packets bigger
   than 1500 bytes.  However, since this capability isn't standardized,

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   it's seldom used today, despite the potential performance benefits of
   using larger packets.  This document specifies a mechanism for
   advertising a non-standard maximum packet size on a subnet.  It also
   specifies optional mechanisms to negotiate per-neighbor maximum
   packet sizes so that nodes on a shared subnet may use the maximum
   mutually supported packet size between them without being limited by
   nodes with smaller maximum sizes on the same subnet.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Disadvantages of larger packets  . . . . . . . . . . . . . . .  5
     3.1.  Delay and jitter . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Path MTU Discovery problems  . . . . . . . . . . . . . . .  6
     3.3.  Packet loss through bit errors . . . . . . . . . . . . . .  6
     3.4.  Undetected bit errors  . . . . . . . . . . . . . . . . . .  7
     3.5.  IEEE 802.3 compatibility . . . . . . . . . . . . . . . . .  8
     3.6.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  The protocol mechanisms  . . . . . . . . . . . . . . . . . . .  9
     4.1.  The multi-MTU router advertisement option  . . . . . . . .  9
     4.2.  General operation  . . . . . . . . . . . . . . . . . . . . 10
     4.3.  Determining the InterfaceMTU . . . . . . . . . . . . . . . 10
     4.4.  Changes to the RA MTU option semantics . . . . . . . . . . 11
     4.5.  The IPv6 neighbor discovery MTU option . . . . . . . . . . 11
     4.6.  The IPv6 neighbor discovery padding option . . . . . . . . 12
     4.7.  Use of the MTU and padding options . . . . . . . . . . . . 13
     4.8.  IPv4 ethernet jumbo ARP message  . . . . . . . . . . . . . 14
     4.9.  Probe considerations . . . . . . . . . . . . . . . . . . . 14
     4.10. Neighbor MTU garbage collection  . . . . . . . . . . . . . 15
   5.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 15
   6.  Security considerations  . . . . . . . . . . . . . . . . . . . 15
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 16
   Appendix A.  Document and discussion information . . . . . . . . . 16
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
   Intellectual Property and Copyright Statements . . . . . . . . . . 17

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1.  Introduction

   Some protocols inherently generate small packets.  Examples are VoIP,
   where it's necessary to send packets frequently before much data can
   be gathered to fill up the packet, and the DNS, where the queries are
   inherently small and the returned results also rarely fill up a full
   1500-byte packet.  However, most data that is transferred across the
   internet and private networks is at least several kilobytes in size
   (often much larger) and requires segmentation by TCP or another
   transport protocol.  These types of data transfer can benefit from
   larger packets in several ways:

   1.  A higher data-to-header ratio makes for fewer overhead bytes

   2.  Fewer packets means fewer per-packet operations on the source and
       destination hosts

   3.  Fewer packets also means fewer per-packet operations in routers
       and middleboxes

   4.  TCP performance increases with larger packet sizes

   Even though today, the capability to use larger packets (often called
   jumbo frames) is present in a lot of ethernet hardware, this
   capability isn't used because IP assumes a common MTU size for all
   nodes connected to a link or subnet.  In practice, this means that
   using a larger MTU requires manual configuration of the non-standard
   MTU size on all hosts and routers and possibly on switches.  Also,
   the MTU size for a subnet is limited to that of the least capable
   router, host or switch.

   In the future, when hosts support [RFC4821] in all relevant transport
   protocols, it will be possible to simply ignore MTU limitations by
   sending at the maximum locally supported size and determining the
   maximum packet size towards a correspondent from acknowledgements
   that come back for packets of different sizes.  However, [RFC4821]
   must be implemented in every transport protocol, and there is a
   significant probability for failures if hosts implementing [RFC4821]
   interact with hosts that don't implement this mechanism but do use a
   larger than standard MTU.

   This document provides for a set of mechanisms that allow the use of
   larger packets between nodes that support them which interacts well
   with both manually configured non-standard MTUs and expected future
   [RFC4821] operation with larger MTUs.  This is done using several new
   options and messages:

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   1.  An additional router advertisement Multi-MTU option to limit
       higher maximum packet sizes

   2.  A neighbor discovery option that allows nodes to inform their
       neighbors of the maximum packet size they support

   3.  A neighbor discovery option for padding messages to make them
       suitable for probing a neighbor's MTU and link-layer MTU

   4.  Padding for ARP messages to make them suitable for probing a
       neighbor's MTU and link-layer MTU limitations

   Only support of the Multi-MTU option is required to conform to to
   this specification, the neighbor discovery options and jumbo ARP are

2.  Terminology

   InterfaceMTU:  The maximum packet size considered usable on an
      interface, based on the physical MTU, the MTU and SPEED advertised
      by routers and administrative settings.

   MTU:  Maximum Transmission Unit.  This is the maximum IP packet size
      in bytes supported on a link, towards a neighbor (or towards a
      remote correspondent).  In some cases, the term MRU (Maximum
      Receive Unit) would be more appropriate, but for consistency, the
      term MTU is used throughout this document.

   NeighborMTU:  The maximum packet size that may be used towards a
      given on-link neighbor.

   Node:  A host or router running IPv4 and/or IPv6.

   Oversized packet:  A packet exceeding the Standard MTU size.

   PhysicalMTU:  The MTU reported by the driver for an interface when
      operating at a given link speed.

   Probe:  An ARP or neighbor solicitation packet of a specific
      (oversized) size sent for the purpose of determining whether a
      neighbor can successfully receive packets of this size sent by the
      local node.

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   SafeMTU:  Maximum packet size that is supported by all nodes an all
      link layer devices on a link.

   StandardMTU:  For IPv4: the MTU for a link type defined in the
      relevant IP-over-...  RFC.  For IPv6: the minimum of the MTU for a
      link type defined in the relevant IPv6-over-...  RFC and the value
      of the MTU option in router advertisements.

3.  Disadvantages of larger packets

   Although often desirable, the use of larger packets isn't universally
   advantageous for the following reasons:

   1.  Increased delay and jitter

   2.  Increased reliance on path MTU discovery

   3.  Increased packet loss through bit errors

   4.  Increased risk of undetected bit errors

3.1.  Delay and jitter

   An low-bandwidth links, the additional time it takes to transmit
   larger packets may lead to unacceptable delays.  For instance,
   transmitting a 9000-byte packet takes 7.23 milliseconds at 10 Mbps,
   while transmitting a 1500-byte packet takes only 1.23 ms.  Once
   transmission of a packet has started, additional traffic must wait
   for the transmission to finish, so a larger maximum packet size
   immediately leads to a higher worst-case head-of-line blocking delay,
   and thus, to a bigger difference between the best and worst cases
   (jitter).  The increase in average delay depends on the number of
   packets that are buffered, the average packet size and the queuing
   strategy in use.  Buffer sizes vary greatly between implementations,
   from only a few buffers in some switches and on low-speed interfaces
   on routers, to hundreds of megabytes of buffer space on 10 Gbps
   interfaces on some routers.

   If we assume that the delays involved with 1500-byte packets on 100
   Mbps ethernet are acceptable for most, if not all, applications, then
   the conclusion must be that 15000-byte packets on 1 Gbps ethernet
   should also be acceptable, as the delay is the same.  At 10 Gbps
   ethernet, much larger packet sizes could be accommodated without
   adverse impact on delay-sensitive applications.  At 100 Mbps, and
   certainly below that, larger packet sizes are probably not advisable.

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3.2.  Path MTU Discovery problems

   PMTUD issues arise when routers can't fragment packets in transit
   because the DF bit is set or because the packet is IPv6, but the
   packet is too large to be forwarded over the next link, and the
   resulting "packet too big" ICMP messages from the router don't make
   it back to the sending host.  If there is a PMTUD black hole, this
   will typically happen when there is an MTU bottleneck somewhere in
   the middle of the path.  If the MTU bottleneck is located at either
   end, the TCP MSS (maximum segment size) option makes sure that TCP
   packets conform to the smallest MTU in the path.  PMTUD problems are
   of course possible with non-TCP protocols, but this is rare in
   practice because non-TCP protocols are generally not capable of
   adjusting their packet size on the fly and therefore use more
   conservative packet sizes which won't trigger PMTUD issues.

   Taking the delay and jitter issues to heart, maximum packet sizes
   should be larger for faster links and smaller for slower links.  This
   means that in the majority of cases, the MTU bottleneck will tend to
   be at one of the ends of a path, rather than somewhere in the middle,
   as in today's internet, core of the network is quite fast, while
   users usually connect at lower speeds.

   A crucial difference between PMTUD problems that result from MTUs
   smaller than the standard 1500 bytes and PMTUD problems that result
   from MTUs larger than the standard 1500 bytes is that in the latter
   case, only a party that's actually using the non-standard MTU is
   affected.  This puts potential problems, the potential benefits and
   the ability to solve any resulting problems in the same place so it's
   always possible to revert to a 1500-byte MTU if PMTUD problems can't
   be resolved otherwise.

   Considering the above and the work that's going on in the IETF to
   resolve PMTUD issues as they exist today, means that increasing MTUs
   where desired doesn't involve undue risks.

3.3.  Packet loss through bit errors

   All transmission media are subject to bit errors.  In many cases, a
   bit error leads to a CRC failure, after which the packet is lost.  In
   other cases, packets are retransmitted a number of times, but if
   error conditions are severe, packets may still be lost because an
   error occurred at every try.  Using larger packets means that the
   chance of a packet being lost due to errors increases.  And when a
   packet is lost, more data has to be retransmitted.

   Both per-packet overhead and loss through errors reduce the amount of
   usable data transferred.  The optimum tradeoff is reached when both

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   types of loss are equal.  If we make the simplifying assumption that
   the relationship between the bit error rate of a medium and the
   resulting number of lost packets is linear with packet size for
   reasonable bit error rates, the optimum packet size is computed as

   packet size = sqrt( overhead bytes / bit error rate )

   According to this, the optimum packet size is one or more orders of
   magnitude larger than what's commonly used today.  For instance, the
   minimum BER for 1000BASE-T is 10^-10, which implies an optimum packet
   size of 312250 bytes with ethernet framing and IP overhead.

3.4.  Undetected bit errors

   Nearly all link layers employ some kind of checksum to detect bit
   errors so that packets with errors can be discarded.  In the case of
   ethernet, this is a frame check sequence in the form of a 32-bit CRC.
   Assuming a strong frame check sequence algorithm, this suggests that
   there is a 1 in 2^32 chance that a packet with one or more bit errors
   in it has the same CRC as the original packet, so the bit errors go
   undetected and data is corrupted.  However, according to [CRC] the
   CRC-32 that's used for FDDI and ethernet has the property that
   packets between 376 and 11454 bytes long (including) have a Hamming
   distance of 3.  (Smaller packets have a larger Hamming distance,
   larger packets a smaller Hamming distance.)  As a result, all errors
   where only a single bit is flipped or two bits are flipped, will be
   detected, because they can't result in the same CRC as the original
   packet.  The probability of a packet having undetected bit errors can
   be approximated as follows for a 32-bit CRC:

   PER = (PL * BER) ^ H / 2^32

   Where PER is the packet error rate, BER is the bit error rate, PL is
   the packet length in bits and H is the Hamming distance.  Another
   consideration is the impact of packet length on a multi-packet
   transmission of a given size.  This would be:

   TER = transmission length / PL * PER


   TER = transmission length / (PL ^ (H - 1) * BER ^ H) / 2^32

   Where TER is the transmission error rate.

   In the case of the ethernet FCS and a Hamming distance of 3 for a
   large range of packet sizes, this means that the risk of undetected

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   errors goes up with the square of the packet length, but goes down
   with the third power of the bit error rate.  This suggest that for a
   given acceptable risk of undetected errors, a maximum packet size can
   be calculated from the expected bit error rate.  It also suggests
   that given the low BER rates mandated for gigabit ethernet, packet
   sizes of up to 11454 bytes should be acceptable.

   Additionally, unlike properties such as the packet length, the frame
   check sequence can be made dependent on the physical media, so it
   should be possible to define a stronger FCS in future ethernet
   standards, or to negotiate a stronger FCS between two stations on a
   point-to-point ethernet link (i.e., a host and a switch or a router
   and a switch).

3.5.  IEEE 802.3 compatibility

   According to the IEEE 802.3 standard, the field following the
   ethernet addresses is a length field.  However, [RFC0894] uses this
   field as a type field.  Ambiguity is largely avoided by numbering
   type codes above 2048.  The mechanisms described in this memo only
   apply to the standard [RFC0894] and [RFC2464] encapsulation of IPv4
   and IPv6 in ethernet, not to possible encapsulations of IPv4 or IPv6
   in IEEE 802.3/IEEE 802.2 frames, so there is no change to the current
   use of the ethernet length/type field.

3.6.  Conclusion

   Larger packets aren't universally desirable.  The factors that factor
   into the decision to use larger packets include:

   o  A link's bit error rate

   o  The number of bits per symbol on a link and hence the likelihood
      of multiple bit errors in a single packet

   o  The strength of the frame check sequence

   o  The link speed

   o  The number of buffers

   o  Queuing strategy

   This means that choosing a good maximum packet size is, initially at
   least, the responsibility of hardware builders.  On top of that,
   robust mechanisms MUST be available to operators to further limit
   maximum packet sizes where appropriate.

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4.  The protocol mechanisms

   The new Multi-MTU router advertisement option lets IPv6 routers (and,
   if desired, devices that aren't IPv6 routers) inform hosts of the
   maximum packet sizes they should use, based on the link bandwidth of
   the host and whether the host supports probing for support of
   oversized packets.

4.1.  The multi-MTU router advertisement option

   Routers use this option to inform hosts on connected subnets about
   the maximum allowed MTU for three ranges of link speeds.

                          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      |    Length     | Pri |         Reserved        |
     |                             MAXMTU                            |
     |                            SLOWMTU                            |
     |                            SAFEMTU                            |

   Type: TBD

   Length: 1

   Pri:  Priority.  Values have the following meaning:

         000: Vendor default

         001: Local override of 000

         010: Site default

         011: Local override of 010

         100: Subnet default

         101: Local override of 100

         110: Per-node setting

         111: Local override of 110

      Vendors may only use priority 000 in default configurations.

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      Site-wide administrative settings may only use 000 and 010.
      Subnet-specific administrative settings may use 000, 010 or 110,
      but not 001, 011, 101 or 111.  Per-node configuration may use all

   Reserved:  Set to 0 on transmission, ignored on reception.

   MAXMTU:  The absolute maximum packets size allowed on a link.
      Packets larger than this size MUST NOT be sent.

   SLOWMTU:  The maximum packet size nodes operating at a link speed
      below 600 Mbps (Mbps = 1000000 bps) may use.

   SAFEMTU:  The maximum packet size supported by all nodes on a link,
      packets of this size can be sent without probing.

4.2.  General operation

   Hosts MUST recover the multi-MTU options from the router
   advertisements of at least the router they select as a default
   router, but it's encouraged (not required) to recover options from
   multiple routers.  The same option, or data constituting the same
   information, may be learned from other sources, such as local
   configuration and/or DHCPv6.

   When a node's interface speed changes, it MAY reinitiate negotiation
   of per-neighbor MTUs, but it SHOULD remain prepared to receive
   packets of the maximum size indicated to neighbors previously.

   Devices not acting as IPv6 routers that need to inform hosts on the
   local subnet of MTU limitations MAY send out a router advertisement
   with a Router Lifetime of 0 [RFC2461] and the pertinent information
   in a Multi MTU option.

   Routers and other systems generating router advertisements with a
   Multi-MTU option SHOULD NOT advertise a MAXMTU, SLOWMTU or SAFEMTU
   lower than the MTU defined in the relevant IP-over-... or
   IPv6-over-...  RFC.

   DISCUSSION: Is it appropriate that IPv4 and IPv6 use the same MTU?

4.3.  Determining the InterfaceMTU

   If the node supports probing and there is positive knowledge that the
   interface is currently operating at is at least 600 Mbps, the
   InterfaceMTU is set as follows:

   InterfaceMTU = max(StandardMTU, min(MAXMTU, PhysicalMTU))

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   If the node supports probing and the interface is operating at a
   speed below 600 Mbps, or the interface speed is unknown, the
   InterfaceMTU is set as follows:

   InterfaceMTU = max(StandardMTU, min(SLOWMTU, PhysicalMTU))

   If the node doesn't support probing and there is positive knowledge
   that the interface is currently operating at is at least 600 Mbps,
   the InterfaceMTU is set as follows:

   InterfaceMTU = max(StandardMTU, min(MAXMTU, SAFEMTU, PhysicalMTU))

   If none of the above rules apply, the InterfaceMTU is set as follows:

   InterfaceMTU = max(StandardMTU, min(SLOWMTU, SAFEMTU, PhysicalMTU))

   If InterfaceMTU is smaller than SAFEMTU, an error SHOULD be logged
   but operation SHOULD continue.

4.4.  Changes to the RA MTU option semantics

   If in addition to a Multi-MTU option, there is also an MTU option in
   a router advertisement, hosts MUST ignore the MTU option and use the
   value of the SAFEMTU field in the Multi-MTU option as the default MTU
   size on the interface.  However, it may be necessary to incorporate
   special case logic to allow for the use of larger packets than what
   the interface-wide MTU value that is set accordingly suggest.  For
   instance, if a node supports explicit probing as outlined below, or
   [RFC4821] probing for some transport protocols, the transport
   protocols in question may need to be aware of the possibility of
   using packets larger than the SAFEMTU.  For example TCP should
   probably advertise a maximum segment size based on the InterfaceMTU
   rather than on the SAFEMTU in the MSS option.

4.5.  The IPv6 neighbor discovery MTU option

   In order to be able to use the largest packet sizes under the widest
   range of circumstances, nodes SHOULD include a new MTU option in both
   neighbor solicitation and neighbor advertisement messages [RFC2461].

   The format of the neighbor discovery MTU option is as follows:

      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      |    Length     |R|T|  Transport flags  |  Res  |
     |                              MTU                              |

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   Type: TBD

   Length: 1

   R: Reply flag.  Set to 1 when the neighbor discovery packet is sent
      in reply to a neighbor discovery packet containing a padding
      option, otherwise set to 0.

   T: TCP-MSS-override flag.  If set to 1, the MTU field MAY overwrite
      the maximum segment size that was advertised earlier, in the TCP
      MSS option.  (Note that the MSS option advertises a value that
      doesn't include IP overhead; the MTU field is the size of an
      entire IP packet, including the IP header.)  If set to 0, the TCP
      MSS option MUST be honored even if it's smaller than the

   Transport flags:  Reserved for use with other transport protocols in
      the same way as the T flag.  Set to 0 on transmission, ignored
      when receiving.

   Res:  Set to 0 on transmission, ignored on reception.

   MTU:  If the R flag is 0: the maximum packet size in bytes that the
      node would like to receive.  The minimum valid value is 1280.
      However, the node MUST be prepared to receive packets up to the
      SAFEMTU size.  If the R flag is 1: the minimum of the maximum
      packet size that the node would like to receive (as with R=0) and
      the size of the packet that this packet is a reply to.

4.6.  The IPv6 neighbor discovery padding option

   The format of the neighbor discovery padding option is as follows:

      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      |    Length     |           Reserved            |
     |                            Padding                            |
     ~                                                               ~
     |                                                               |

   Type: TBD

   Length: see below.

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   Reserved: set to 0 on transmission, ignored on reception.

   Padding: 0 or more all-zero octets.

4.7.  Use of the MTU and padding options

   The MTU option is included in all neighbor advertisement and neighbor
   solicitation messages.

   Reception of a neighbor solicitation or a neighbor advertisement for
   a neighbor for which no per-neighbor MTU is known triggers, in
   addition to the normal response if it's a neighbor solicitation, the
   sending of an neighbor solicitation message with the MTU and padding
   options in it.  The size of this message is may vary between the IPv6
   StandardMTU size + 1 for the link and the minimum of the local MTU
   and the neighbor's MTU as advertised in the MTU option of the packet
   received.  See below for considerations about the packet sizes to
   choose.  The padding option is used to bring the neighbor
   solicitation message to this size.  The padding option MUST be the
   last option in the packet.

   There are two possible ways to determine the value of the length
   field in the padding option:

   1.  Set it to 0.  Since the option is in fact larger than 0, this
       means that nodes that don't implement the option will silently
       discard the packet.  Setting the length to 0 makes it possible to
       have packets with the padding option that aren't a multiple of 8
       bytes long.

   2.  If the intended packet length allows a valid value for the length
       field, the length field MAY be set to that value.  The node MAY
       reduce the size of the intended packet to accommodate the
       requirement that the size field is a multiple of 8 bytes.  I.e.,
       if the intended packet size is 4470 bytes with 40 and 24 bytes
       for the IPv4 and neighbor solicitation headers, respectively, the
       padding option would have to be 4406 bytes long, which can't be
       expressed in the length field.  The node may choose to use a
       packet size of 4464 instead, which results in a length field
       value of 550.  This of course means that subsequent data packets
       MUST be no larger than 4464 bytes.

   A neighbor solicitation message with the padding option is always
   sent in addition to a regular neighbor solicitation message, rather
   than in place of one.

   When a node receives a neighbor solicitation message with the padding
   option, it stops evaluating options when it reaches the padding

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   option and returns a regular neighbor advertisement message, which
   includes the MTU option with the R flag set to 1.  Whenever the
   neighbor advertisement is not the result of receiving a neighbor
   solicitation with a padding option, the R flag is set to 0.

   When a node receives a neighbor advertisement message, it must
   determine whether the message is in reaction to a locally sent
   neighbor solicitation with the padding option or not.  If the MTU
   option is included in the message received, an R flag of 1 indicates
   that it is indeed a reply.  If the message was a reply, the node sets
   the NeighborMTU to the size of the MTU field in the received neighbor
   discovery packet.

   If no reply is received after some time, either the neighbor is
   incapable of receiving packets of the size that was used, or a device
   operating at the link layer was incapable for forwarding the frame.
   (Incidental packet loss is also a possibility.)  In order to
   determine a workable MTU even in the presence of unknown limitations,
   a node may repeat sending a solicitation with the padding option.
   However, since presumably, some equipment may react badly to a large
   number of out-of-spec packets, it's important that nodes limit the
   number of oversized packets to destinations that aren't yet known to
   be capable of receiving them.  An upper limit would be to allow only
   5 unacknowledged oversized packets per 300 second period.

   Nodes that support probing MUST support reception of both types of
   probes, but MAY be limited to generating only one type.

4.8.  IPv4 ethernet jumbo ARP message

   Due to lack of neighbor discovery, with IPv4, it's necessary to use
   ARP to probe for non-standard MTU capabilities.  This is done by
   simply probing with an ARP packet padded to the desired size.  If a
   reply comes back, the neighbor supports the probed MTU size.

   MAXMTU, SLOWMTU and SAFEMTU parameters advertised by IPv6 routers
   MUST also be taken into account when probing and generating oversized
   IPv4 packets.

4.9.  Probe considerations

   In cases where the neighbor's MTU was advertised in an MTU option, it
   makes sense to try with this size.  If that probe fails or the
   neighbor's MTU is unknown, the best choice for a probe size would be
   the smallest possible non-standard MTU.  This could be the
   StandardMTU + 1, or a slightly larger value that represents the first
   larger size that is actually useful, such as 1508 or 1520 for
   ethernet.  Failure at this size wastes relatively little bandwidth

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   and indicates that further probes are unnecessary.  If this probe is
   successful, further choices for the probe size may be common MTU
   sizes such as 1508, 1530, 1536, 1546, 1998, 2000, 2018, 4464, 4470,
   8092, 8192, 9000, 9176, 9180, 9216, 17976, 64000 and 65280 bytes.  A
   useful heuristic would be to monitor all Multi-MTU options
   advertised, regardless of their priority, and use the values in those
   options as candidates for the largest supported packet size.

   There is no requirement that a node tries a number of probes of
   different sizes; only that before oversized packets are sent, a reply
   for a probe of that size or larger MUST have been received from the
   neighbor in question before packets larger than SAFEMTU are sent.  A
   simple strategy that would be to initially send just one probe sized
   at the InterfaceMTU size, and if unsuccessful, only send a second
   probe when a probe from the neighbor is received.  The second probe
   is made the same size as the neighbor's probe.

   Probes MUST be sent as unicast.

4.10.  Neighbor MTU garbage collection

   The MTU size for a neighbor is garbage collected along with a
   neighbor's link address in accordance with regular ARP and neighbor
   discovery timeouts.  Additionally, a neighbor's MTU size is reset to
   unknown after dead neighbor detection declares a neighbor "dead".

5.  IANA considerations

   IANA is requested to assign a router advertisement option number and
   two neighbor discovery options.  In addition, IANA is requested to
   start a registry for the transport flags.  There are 10 flags,
   numbered 18 to 27.  Each flag may be assigned to a transport protocol
   that communicates a maximum segment size in-band.  See the discussion
   of the T flag in section Section 4.5.

6.  Security considerations

   Generating false router advertisements and neighbor discovery packets
   with large MTUs may lead to a denial-of-serve condition, just like
   the advertisement of other false link parameters.

7.  References

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7.1.  Normative References

   [RFC0894]  Hornig, C., "Standard for the transmission of IP datagrams
              over Ethernet networks", STD 41, RFC 894, April 1984.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              December 1998.

   [RFC2462]  Thomson, S. and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 2462, December 1998.

   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", RFC 2464, December 1998.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

7.2.  Informative References

   [CRC]      Jain, R., "Error Characteristics of Fiber Distributed Data
              Interface (FDDI), IEEE Transactions on Communications",
              August 1990.

Appendix A.  Document and discussion information

   The latest version of this document will always be available at
   http://www.muada.com/drafts/.  Please direct questions and comments
   to the int-area mailinglist or directly to the author.

Author's Address

   Iljitsch van Beijnum
   IMDEA Networks
   Avda. del Mar Mediterraneo, 22
   Leganes, Madrid  28918

   Email: iljitsch@muada.com

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