Network Working Group I. van Beijnum
Internet-Draft Consultant
Expires: December 29, 2007 June 29, 2007
IPv6 Extensions for Multi-MTU Subnets
draft-van-beijnum-multi-mtu-00
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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 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, 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-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 tends to increase with larger packet sizes
Even though today, the capability to use larger packets (often called
jumboframes) 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 IPv6
options and messages:
1. An additional router advertisement MTU option to limit higher
maximum packet sizes
2. A new switch advertisement message, similar to a router
advertisement message, so that switches can announce the maximum
packet size they support
3. A neighbor discovery option that allows nodes to inform their
neighbors of the maximum packet size they support
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4. A new ICMPv6 message for confirming that packets with an increased
maximum size can be transmitted and received successfully
Nodes running IPv6 may take advantage of these mechanisms to send
packets larger than the standard maximum size. Since IPv4 doesn't
support equivalent mechanisms, support for IPv4 requires additional
work that is best carried out after deployment experience with IPv6.
2 Terminology
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.
Advised MTU:
The MTU that is considered the best or safe choice at a given time
on a given link.
Allowed MTU:
The maximum MTU allowed administratively.
Local MTU:
The maximum packet size considered usable on a node, based on the
physical MTU, the allowed MTU and advised MTUs.
Neighbor MTU:
The maximum packet size that may be used towards a given on-link
neighbor.
Off-link MTU:
The maximum packet size that is appropriate for communicating with
off-link correspondents.
Physical MTU:
The MTU reported by the driver for an interface when operating at
a given link speed.
Tentative neighbor MTU:
The maximum packet size advertised by a neighbor.
3 Disadvantages of larger packets
Although often desirable, the use of larger packets isn't universally
advantageous for the following reasons:
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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 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 bytes of ethernet overhead.
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 9000-byte 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.
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Taking the delay and jitter issues to heart, maximum packet sizes
should be larger for faster 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.
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 and potential benefits 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
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 bytes / bit error rate)
For IPv6 in ethernet framing, with 14 bytes of ethernet header, 40
bytes of IPv6 header, 20 bytes of TCP header and 32 bits of ethernet
CRC the total number of bytes transmitted is 1538 while the useful
data is 1440. (The preamble and inter frame gap are not relevant for
error rate purposes.) 78 bytes of overhead would result in a 1518-byte
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 bytes.
In practice, it's better to err on the side of smaller packets and
lower packet loss to avoid triggering TCP congestion mechanisms.
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However, it's obvious that current maximum packet sizes are far below
the optimum size with respect to optimum throughput.
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 bytes 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
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In other words:
TER = transmission length / (packet length ^ (H - 1) * BER ^ H) / 2^32
(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-byte ethernet frame length this means:
TER = 3.44*10^12 * 12144 ^ 2 * 10^-10 ^ 3 / 2^32 = 1.18*10^-19
For 11454-byte 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-byte
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
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This means that choosing a good maximum packet size is, initially at
least, the responsibility of hardware vendors. On top of that, robust
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. However, to avoid problems, test packets are sent first
before larger packets are used for actual traffic, and routers and
switches may inform nodes of MTU limitations that are best observed
or are mandatory to observe.
4.1 The variable MTU router advertisement option
Routers use this option to inform hosts on connected subnets about the
maximum allowed MTU for a given link speed and the off-link MTU that
should be used towards off-link destinations.
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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Off-link MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Pri | Link speed |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Allowed MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD
Length: 2
Reserved: 0 on transmission, ignored on reception.
Off-link MTU:
This is the maximum packet size that a router can forward to other
links it connects to. Hosts SHOULD use a TCP MSS option based on
this value in all TCP sessions and limit packets sent to off-link
destinations to this maximum. The off-link MTU must be at least
1280. A value of 0 means the off-link MTU is undefined and hosts
should use their physical MTU in TCP MSS options and limit packets
sent to routers to the maximum MTU the router supports as
discovered through the neighbor discovery option.
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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.
Subnet-specific administrative settings may use 000, 010 or 110,
but not 001, 011, 101 or 111.
Link speed:
Minimum link speed the option may apply to. Values from 0 to 49151
indicate a link speed in megabits per second. Values from 49152 to
65535 are reserved for future use, but imply a link speed of more
than 49151 Mbps. Hosts MUST ignore all options with a link speed
value that's higher than the current link speed of the interface
the option is received over. For instance, if a host has an
interface that supports 10, 100 and 1000 Mbps ethernet which
currently operates at 100 Mbps, and the host receives options
with link speed values of 100 and 1000 over that interface, the
option with the link speed of 100 is processed and the option
with the link speed of 1000 is ignored.
Allowed MTU:
The maximum packets size allowed on a link. Packets larger than
this value MUST NOT be sent over the link in question. The allowed
MTU MUST be at least 1500. A value of 0 means that the allowed
MTU is undefined and no maximum MTU is enforced.
The number of variable MTU options in router advertisements is limited
to a maximum of 4.
Hosts are expected to recover the variable MTU options from the router
advertisements of at least the router they select as a default router,
but it's allowed (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. Host MUST only consider variable MTU options where the value
of the link speed field doesn't exceed that of the current link speed
of the associated interface. Any options (or equivalents) that satisfy
this condition are ordered by the priority, link speed and allowed MTU
fields, in that order. Hosts SHOULD copy the allowed MTU and off-link
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MTU information, if specified, from the option (or equivalent) with
the largest value for the concatenation of these three fields.
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 bytes for multicast
packets. In order to lift this limitation, routers and hosts that
implement variable MTU subnets may advertise and accept, respectively,
an MTU option with an MTU larger than 1500. Hosts should use the
minimum of the maximum feasible MTU 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 variable MTU subnets.
4.3 The switch MTU advertisement message
Switches and other layer 2 devices MAY advertise the maximum MTU they
support in an ICMPv6 [RFC2463] message sent to multicast address TBD.
The format of this ICMPv6 message is as follows:
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 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of MTUs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Switch identifier +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Link speed 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advised MTU 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Link speed 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advised MTU 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
| Reserved | Link speed N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advised MTU N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD (informational)
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Code: TBD
Checksum: see [RFC2463]
Number of MTUs:
Number of times the reserved/link speed/advised MTU fields are
repeated for different link speed values. The minimum is 1, the
maximum 4.
Switch identifier: a 64-bit value that is unique to the switch.
Reserved: 0 on transmission, ignored on reception.
Link speed:
Minimum link speed the option may apply to. Values from 0 to 49151
indicate a link speed in megabits per second. Values from 49152 to
65535 are reserved for future use, but imply a link speed of more
than 49151 Mbps. Hosts MUST ignore all options with a link speed
value that's lower than the current link speed of the interface
the option is received over. Note that this is the opposite
behavior of that specified for the link speed in the RA variable
MTU option.
Advised MTU:
The IPv6 MTU the switch supports on ports operating at the
indicated link speed. In the case of ethernet, the IPv6 MTU is the
maximum frame size after subtracting the size of the VLAN tag, the
14-byte Ethernet II header and the frame check sequence.
Switch MTU advertisements should be sent out at 5-minute intervals.
When a port transitions from an inactive or disconnected to an active
state, the interval MAY be reduced to 60 seconds, such that if it has
been 60 seconds or longer ago that the last switch MTU advertisement
was sent out, a switch MTU advertisement is sent out immediately.
If the switch doesn't otherwise implement IPv6, or the IPv6 protocol
is inactive, the IPv6 source address should be the unspecified
address. Since all the information in the message is thus known in
advance, the entire message, including the checksum, may be
pre-calculated without the need to implement IPv6 in the switch.
Host SHOULD monitor switch MTU advertisement messages, using the
switch identifier field to detect refreshes/duplicates, and retain all
switch MTU advertisements for 10 minutes. When the switch MTU
advertisement information changes (new advertisements, new information
in previously known advertisements, advertisements expire), hosts
SHOULD select the minimum advised MTU value where the associated link
speed is equal to or higher than the current link speed on the
associated interface. The thusly recovered advised MTU for the link is
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the minimum of the MTUs supported by all the switches for this
particular link speed if all switches implement the switch MTU
advertisement mechanism.
4.4 The neighbor discovery MTU option
A node that implements the variable 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
Length: 1
Reserved: set to 0 on transmission, ignored on reception.
MTU:
The maximum packet size the node is prepared to send and receive,
which is copied from the local MTU. The minimum valid value is
1280.
Reception of a neighbor solicitation or a neighbor advertisement
triggers the sending of an ICMPv6 MTU detection message.
The MTU detection message
Since it's possible that there are layer 2 devices that don't
implement the switch MTU advertisement message in the path between two
nodes, it's necessary to make that it is indeed possible to send and
receive packets larger than the standard MTU. This is what the ICMPv6
MTU detection message is for. It has the following format:
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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 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Padding |
...
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD (informational)
Code: TBD
Checksum: see [RFC2463]
R (reply requested): 0: no reply requested, 1: reply requested
Reserved: 0 on transmission, ignored on reception
Packet size:
Size of this packet, including IPv6 and other headers. A value of
0 indicates no padding is present and the size of the packet
shouldn't be considered.
Padding:
0 or more 0 bytes to bring the packet to the specified packet
size.
In order to avoid sending large numbers of packets that can't be
handled properly by switches or other layer 2 devices, after sending a
large MTU detection packet, no other maximum size MTU detection
packets may be transmitted on the same interface for 60 seconds or
until a large MTU detection packet has been received, whichever
happens first. In this context, "large" means larger than the standard
MTU size for the link type, i.e., 1500 bytes for ethernet.
When variable MTU subnet capability is detected for a neighbor by the
presence of an MTU option in a neighbor solicitation or neighbor
discovery message, an MTU detection message is constructed as follows:
R:
Set to 0 if the neighbor MTU is known and confirmed, set to 1
otherwise.
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Packet size:
Equal to the minimum of the local MTU and the (tentative) neighbor
MTU.
When an MTU detection packet is received, the size of the packet is
checked against the value in the packet size field to detect
truncation in transit. If the packet size and the packet size field
don't match, or if the packet size is smaller than 1280 bytes, the
message is silently discarded.
If the received message has the R flag set to 1, a reply is
constructed as follows:
R: 0
Packet size:
Equal to the minimum of the local MTU and the neighbor MTU.
The neighbor MTU overrules information in the TCP MSS option in TCP
sessions towards that neighbor. Neighbor MTU information expires along
with link addresses learned through neighbor discovery and upon dead
neighbor detection.
4.5 Determining the local MTU
The local MTU is the value communicated to neighbors. It is the
minimum of the physical MTU for an interface and the allowed MTU as
advertised by a router or learned through other means. The local MTU
may be further reduced by the reception of switch MTU advertisements.
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.
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6 Document and Author Information
This document expires December, 2007. 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).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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Van Beijnum Expires December 29, 2007 [Page 15]
Internet-Draft IPv6 Extensions for Multi-MTU Subnets June 2007
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