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Network Working Group                                     I. van Beijnum
Internet-Draft                                                Consultant
Expires: Febrary 29, 2008                                August 29, 2007


                 IPv6 Extensions for Multi-MTU Subnets
                    draft-van-beijnum-multi-mtu-01

Status of this Memo

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   This Internet-Draft will expire on Febrary 28, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).


Abstract

  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 almost exclusively
  implemented as ethernets. Even though the relevant standards madate a
  1500 octet maximum packet size for ethernet, more and more ethernet
  equipment is capable of handling packets bigger than 1500 octets.
  However, since this capability isn't standardized, it's seldom used
  today, despite the potential performance benefits of using larger


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  packets. This document specifies a mechanism 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.

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-octet 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 tends to increase 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 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.

  This document proposes to end this situation using several new
  options and messages:

  1. An additional router advertisement 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
     limitations



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

2 Terminology

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

  MTU:
      Maximum Transmission Unit. This is the maximum IP packet size in
      octets 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.

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

  Node:
    A host or router running IPv4 or IPv6.

  Oversized packet:
      A packet exceeding the size defined in the relevant
      IPv6-over-... or IP-over-... RFC.

  Physical MTU:
      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.

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





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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-octet packet takes 7.23 milliseconds at 10 Mbps,
  while transmitting a 1500-octet 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 as such, 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, but assuming 40 buffers
  (not uncommon) leads to the following results:

  Speed        500     1500     4500     9000    16384    65535

  10 Mbps    17.22    49.21   145.22   289.22   525.50  2098.34
  100 Mbps    1.72     4.92    14.52    28.92    52.55   209.83
  1 Gbps      0.17     0.49     1.45     2.89     5.26    20.98
  10 Gbps     0.02     0.05     0.15     0.29     0.52     2.01

  In milliseconds and counting 38 additional octets of ethernet
  overhead.

  If we assume that the delays involved with 1500-octet packets on 100
  Mbps ethernet are acceptable for most, if not all, applications, then
  the conclusion must be that 9000-octet packets on 1 Gbps ethernet
  should also be acceptable. At 10 Gbps ethernet, much larger packet
  sizes could be accommodated without adverse impact on delay-sensitive
  applications. Below 100 Mbps, larger packet sizes are probably not
  advisable.

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. 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 limited MTU.
  PMTUD problems are of course possible with non-TCP protocols, but this
  is rare in practice.

  Taking the delay and jitter issues to heart, maximum packet sizes
  should be larger for faster links. This means that in the majority of



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  cases, the MTU bottleneck will tend to be at one of the ends of a
  path, rather than somewhere in the middle.

  A crucial difference between PMTUD problems that result from MTUs
  smaller than the standard 1500 octets and PMTUD problems that result
  from MTUs larger than the standard 1500 octets is that in the latter
  case, only a party that's actually using the non-standard MTU is
  affected. This puts potential problems and potential benefits in the
  same place so it's always possible to revert to a 1500-octet 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
  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, the
  optimum packet size is computed as follows:

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

  For IPv6 in ethernet framing, with 14 octets of ethernet header, 40
  octets of IPv6 header, 20 octets of TCP header and 32 bits of ethernet
  CRC the total number of octets transmitted is 1538 while the useful
  data is 1440. (The preamble and inter frame gap are not relevant for
  error rate purposes.) 78 octets of overhead would result in a
  1518-octet frame length for a bit error rate of 10^-5.3.

  Note that the minimum BER for 1000BASE-T is 10^-10, which implies an
  optimum packet size of 312250 octets.

  In practice, it's better to err on the side of smaller packets and
  lower packet loss to avoid triggering TCP congestion mechanisms.
  However, it's obvious that current maximum packet sizes are far below
  the optimum size with respect to optimum throughput.



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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.
  The error detecting properties of the CRC are twofold: the minimum
  Hamming distance and the statistical unlikeliness of two packets
  resulting in the same CRC. Depending on the size of the packet, there
  is a minimum Hamming distance between two possible packets that result
  in the same CRC. For ethernet packets between 376 and 11454 octets
  long (including), the Hamming distance is 3 [CRC]. So all packets
  where transmission errors resulted in one or two flipped bits are
  detected. If 3 or more bits are flipped, most errors are caught
  because only in very few cases, the new bit pattern results in the
  same CRC as the old bit pattern. In theory, the chance of two
  packets having the same CRC-32 is 1 in 2^32, but this assumes the
  CRC is as strong as it possibly could be.

  It has been suggested that increasing packet lengths reduce the
  effectiveness of the CRC-32. For the statistical aspect of the CRC,
  this isn't true. Again, assuming a linear relationship between the
  likelihood of bit errors in a packet and the bit error rate, doubling
  the packet size means doubling the chance of a given number of bit
  errors in the packet. In turn, this doubles the chance of a packet
  with bit errors going undetected by the CRC. However, because the
  packet is twice as long, only half the number of packets is required
  to transmit any given amount of data. These aspects cancel each other
  out so the probability of a undetected errors occurring in any given
  data transfer doesn't vary with packet size when only considering the
  statistical properties of the CRC.

  Obviously, choosing a packet size that leads to a reduced Hamming
  distance greatly increases the risk of undetected bit errors. However,
  even choosing a larger packet size with a Hamming distance of 3 leads
  to a reduction in error detection strength. The likelihood of a packet
  having enough bit errors to satisfy a given Hamming distance (packet
  error rate) and then generate the same CRC is:

  PER = (packet length in bits * BER) ^ H / 2^32

  The likelihood of a packet with enough bit errors to meet the Hamming
  distance and then generate an identical CRC in a transmission of a
  certain number of bits is:

  TER = transmission length / packet length * PER

  In other words:

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


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  (Hence the irrelevance of the packet length for a Hamming distance of
  1.)

  For a 400 GB (approximately one hour) transmission over 1000BASE-T
  with a BER of 10^-10 and a 1518-octet ethernet frame length this
  means:

  TER = 3.44*10^12 * 12144 ^ 2 * 10^-10 ^ 3 / 2^32 = 1.18*10^-19

  For 11454-octet packets this becomes:

  TER = 3.44*10^12 * 91632 ^ 2 * 10^-10 ^ 3 / 2^32 = 6.73*10^-18

  Please note that this is 14 orders of magnitude better than the naive
  assumption of a Hamming distance of 1 suggests for standard 1518-octet
  ethernet frames:

  TER = 3.44*10^12 * 12144 ^ 0 * 10^-10 ^ 1 / 2^32 = 9.73*10^-4

  So the strength of the CRC, assuming a Hamming distance of 3, goes
  down with the square of the factor by which the packet length is
  increased. And it goes down with the third power of any increase of
  the bit error rate. However, this discussion is largely academic
  because of the assumption that bit errors happen in isolation. For
  instance, 1000BASE-T transmits two bits per symbol over four wire
  pairs, so bit errors are much more likely to (at least) happen in
  pairs rather than isolated.

  Also, it should be possible to implement stronger frame check
  sequences for newer versions of ethernet. Unlike the packet length,
  the FCS is something switches can change when interconnecting
  different types of ethernet without harming interoperability.

3.5 Conclusion

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

  - A link's bit error rate
  - The number of bits per symbol on a link and hence the likelihood of
    multiple bit errors in a single packet
  - The strength of the Frame Check Sequence
  - The link speed
  - The number of buffers
  - Queuing strategy

  This means that choosing a good maximum packet size is, initially at
  least, the responsibility of hardware vendors. On top of that, robust



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  mechanisms must be available to operators to further limit maximum
  packet sizes where appropriate.

4 The protocol mechanisms

  The basic idea is that nodes are free to negotiate larger MTUs with
  neighbors on a subnet. However, to avoid problems, probe packets
  are sent first before larger packets are used for actual traffic,
  and routers may inform hosts of MTU limitations that should be
  observed for three common ranges of link speeds. The rationale for
  having different MTU limitations for different link speeds is that
  it's common for devices operating at the link layer to support
  larger MTUs if they support and/or operate at higher link speeds.
  E.g., a LAN could consist of a gigabit ethernet switch with jumbo
  frame capabilities connected to a 10/100 Mbps ethernet switch which
  doesn't support jumbo frames. By limiting the use of oversized
  packets to nodes operating at 1000 Mbps, the 10/100 Mbps switch
  isn't exposed to oversized packets which would result in error
  conditions and use up unnecessary bandwidth. Additionally, it may
  be desireable to limit packet sizes at lower speeds even if a large
  MTU is supported for QoS purposes.

  Additionally, routers send out two flags. One is intended to signal
  hosts to be conservative in the number of probes they transmit to
  avoid triggering undesired behavior by link-layer devices seeing a
  large number of out-of-spec packets. The other flag suppresses
  probing for compatibility with the existing practice where all
  nodes on a subnet are administratively configured with a
  non-standard MTU.

  Probing consists of sending a large neighbor discovery or ARP
  packet to a neighbor. If the neighbor sends a reply, it managed to
  successfully receive the probe so the per-neighbor MTU for this
  neighbor can be set to the size of the probe packet and data
  packets of that size can now be sent.

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.











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                       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     |C|N|       Reserved      | Pri |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           MAXMTU1000                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                            MAXMTU100                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                            MAXMTU10                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  Type: TBD

  Length:
    1 or 2. A length of more than 2 indicates a future extension with
    additional fields and MUST NOT be treated as an error, the
    additional fields MUST be ignored.

  C:
    "Conservative" flag: when set, nodes should reduce the number of
    large packets sent by using a conservative timings and probing
    algorithms, if possible avoiding sending more than one
    unsuccessful probe per 60 seconds. When the flag is cleared,
    nodes may send send several oversized packets per second when
    probing.

  N:
    "No probe" flag: when set to 0, hosts MUST probe before using
    oversized packets towards a neighbor. When set to 1, hosts MUST
    NOT send probes and use the relevant MAXMTU field as their MTU.
    If MAXMTU is larger than the physical MTU, an error is logged.

  Reserved: 0 on transmission, ignored on reception.

  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.
      Site-wide administrative settings may only use 000 and 010.


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      Subnet-specific administrative settings may use 000, 010 or 110,
      but not 001, 011, 101 or 111.

  MAXMTU1000:
      The maximum packets size allowed on a link operating at a speed
      of 300 Mbps or more. Packets larger than this value SHOULD NOT
      be sent over the link in question. The MAXMTU1000 MUST be at
      least the MTU size specified in the relevant IPv6-over-... RFC.
      A value of 0 means that the MTU size is undefined and no
      maximum size is enforced for this link speed.

  MAXMTU100:
      The maximum packets size allowed on a link operating at a speed
      of 30 to 299 Mbps and links operating at an unknown speed if
      that speed can be 30 Mbps or higher. Packets larger than
      this value SHOULD NOT be sent over the link in question. The
      MAXMTU100 MUST be at least the MTU size specified in the
      relevant IPv6-over-... RFC. A value of 0 means that the MTU
      size is undefined and no maximum size is enforced for this link
      speed.

  MAXMTU10:
      The maximum packets size allowed on a link operating at a speed
      of less than 30 Mbps. Packets larger than this value SHOULD NOT
      be sent over the link in question. The MAXMTU10 MUST be at
      least the MTU size specified in the relevant IPv6-over-... RFC.
      A value of 0 means that the MTU size is undefined and no
      maximum size is enforced for this link speed.

  When MAXMTU1000, MAXMTU100 and MAXMTU10 all contain the same value,
  it is allowed to omit MAXMTU100 and MAXMTU10 so the option has a
  length of 1 (8 octets) rather than 2 (16 octets). The receiver of
  the option should treat the shorter option the same as a full lenth
  option where the three MAXMTU fields all contain the value from
  MAXMTU1000.

  Hosts are expected to 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. Hosts SHOULD use the MAXMTU value relevant for the link
  speed the interface is currently operating at from the option or
  equivalent information with the largest priority value. If the
  relevant MAXMTU field is unspecified (zero) in the option or
  information with the highest priority, the field from the option
  or information with the next highest priority is considered, and
  so on. If no information is available because no option or
  equivalent is available, or the relevant MAXMTU field never has a


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  non-zero value, the host SHOULD use its physical MTU as the
  MAXMTU.

  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.

4.2 Changes to the RA MTU option semantics

  Hosts are currently supposed to ignore an MTU of more than 1500 in
  the MTU option in router advertisements on ethernet links
  [RFC2464]. This makes it impossible to use an MTU larger than 1500
  octets for multicast packets. In order to lift this limitation,
  routers and hosts that implement multi-MTU subnets may advertise
  and accept, respectively, an MTU option with an MTU larger than
  1500. Hosts should use the minimum of the MAXMTU for their link
  speed and the MTU in the RA MTU option for the transmission of
  multicast packets.

  Note that advertising an MTU option larger than 1500 can only work on
  subnets where all the hosts implement multi-MTU subnets.

4.3 The IPv6 neighbor discovery MTU and padding options

  A node that implements the multi-MTU subnet capability SHOULD
  include an MTU option in both neighbor solicitation and neighbor
  advertisement messages [RFC2461]. A node MAY omit the option if the
  use of a larger MTU isn't desired at that time or if the MTU it would
  advertise is equal to or lower than the MTU that would otherwise be
  used. However, there is no requirement to omit the option depending on
  the value of the different MTU variables as the receiver must
  implement the logic required to determine which MTU to use anyway.

  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     |           Reserved            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                              MTU                              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  Type: TBD


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  Length: 1

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

  MTU:
      The maximum packet size in octets that the node is prepared to
      receive. The minimum valid value is 1280.

 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|          Reserved           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                            Padding                            |
  ~                                                               ~
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  Type: TBD

  Length: see below.

  R: reply flag.

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

  Padding: 0 or more all-zero octects.

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

  Reception of a neighbor solicitation or a neighbor advertisement
  triggers 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 wih
  the MTU and padding options in it. The size of this message is may
  vary between the IPv6-over-... size + 1 for the link and the
  minimum of the relevant MAXMTU, the physical 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:



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  1. Set it to 0. As the "length" field in options has a granularity
     of 8 octets and the behavior of nodes when they receive a
     neighbor solicitation packet which has a total length that
     doesn't match the length of the packet contents, an option
     length of 0 is used to make sure that hosts that don't
     understand the padding option will silently discard the packet.

  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 octets.
     I.e., if the intended packet size is 4470 octets with 40 and 24
     octets for the IPv4 and neighbor solicitation headers,
     respectively, the padding option would have to be 4406 octets
     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.

  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 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. In the absense of the MTU
  option the node must use heuristics relating to the timing of the
  messages it sent with and without the option, and the reception of
  the current message. If the message was a reply, the node sets the
  neighbor MTU to the size of the neighbor solicitation message that
  was replied to.

  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


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  nodes adjust their behavior in the presence of the C (conservative)
  flag in router advertisements.

  The above allows for two strategies in determining a neighbor's
  MTU: the node can depend on the presence of these mechanisms
  described in this document, including setting the padding option
  length field to 0, or it can try to interoperate with nodes that do
  have the capability of using larger packet sizes, but don't
  implement any of the mechanisms described. In that case, the
  padding option must conform to [RFC2461] and care must be taken to
  avoid overly aggressive probing of nodes that do not support larger
  packets.

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

4.4 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.

4.5 Probe considerations

  In cases where the neigbor'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
  IPv6-over-... RFC's MTU size + 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 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 octets.

  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, unless the N flag is set to 1. A
  simple strategy that would be appropriate when the C flag is set to
  1, but may also be used otherwise, would be to initially send just
  one probe sized at the local MTU value, 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.


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4.6 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 References

5.1 Normative References

   [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.

5.2 Informative References

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

6 Document and Author Information

  This document expires February, 2008. The latest version will always
  be available at http://www.muada.com/drafts/. Please direct questions
  and comments to the ipv6 or int area mailinglists or directly to the
  author:

    Iljitsch van Beijnum

    Email: iljitsch@muada.com

Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

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   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS


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   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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