wdiff draft-van-beijnum-multi-mtu-00.txt draft-van-beijnum-multi-mtu-01.txt

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

IPv6 Extensions for Multi-MTU Subnets
draft-van-beijnum-multi-mtu-00
draft-van-beijnum-multi-mtu-01

Status of this Memo

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This Internet-Draft will expire on December 29, 2007. Febrary 28, 2008.

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 octet maximum packet size for ethernet, more and more ethernet
equipment is capable of handling packets bigger than 1500 bytes. octets.
However, since this capability isn't standardized, it's seldom used
today, despite the potential performance benefits of using larger
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
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
jumboframes)
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 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
4.

3. A new ICMPv6 message neighbor discovery option for confirming that packets with an increased
maximum size can be transmitted padding messages to make them
suitable for probing a neighbor's MTU and received successfully

Nodes running IPv6 may take advantage of these mechanisms link-layer MTU
limitations
4. Padding for ARP messages to send
packets larger than the standard maximum size. Since IPv4 doesn't
support equivalent mechanisms, support make them suitable for IPv4 requires additional
work that is best carried out after deployment experience with IPv6. 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
bytes
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.

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

Node:
A host or router running IPv4 or IPv6.

Oversized packet:
A packet exceeding the size that is appropriate for communicating with
off-link correspondents. 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.

Tentative

Probe:
An ARP or neighbor MTU:
The maximum solicitation packet of a specific (oversized)
size advertised by sent for the purpose of determining whether a neighbor. 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
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 9000-octet packet takes 7.23 milliseconds at 10 Mbps,
while transmitting a 1500-byte 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 bytes octets of ethernet
overhead.

If we assume that the delays involved with 1500-byte 1500-octet packets on 100
Mbps ethernet are acceptable for most, if not all, applications, then
the conclusion must be that 9000-byte 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
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 octets and PMTUD problems that result
from MTUs larger than the standard 1500 bytes 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-byte 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 bytes octets / bit error rate)

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

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 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
(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 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-byte 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-byte 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
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.
neighbors on a subnet. However, to avoid problems, test probe packets
are sent first before larger packets are used for actual traffic,
and routers and
switches may inform nodes hosts of MTU limitations that are best should be
observed
or 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 mandatory 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 observe. the size of the probe packet and data
packets of that size can now be sent.

4.1 The variable MTU multi-MTU router advertisement option

Routers use this option to inform hosts on connected subnets about the
maximum allowed MTU for a given three ranges of link speed and the off-link MTU that
should be used towards off-link destinations. 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 | |C|N| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Off-link MTU Pri |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved MAXMTU1000 | Pri
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link speed MAXMTU100 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Allowed MTU MAXMTU10 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type: TBD

Length:
1 or 2. A length of more than 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 indicates a TCP MSS option based on
this value in all TCP sessions future extension with
additional fields and limit packets sent to off-link
destinations to this maximum. The off-link MTU must MUST NOT be at least
1280. A value of 0 means treated as an error, the off-link MTU is undefined and hosts
additional fields MUST be ignored.

C:
"Conservative" flag: when set, nodes should use their physical MTU in TCP MSS options and limit 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 routers 0, hosts MUST probe before using
oversized packets towards a neighbor. When set to 1, hosts MUST
NOT send probes and use the maximum MTU the router supports relevant MAXMTU field as
discovered through their MTU.
If MAXMTU is larger than the neighbor discovery option. 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.

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

MAXMTU1000:
The maximum packets size allowed on a link speed in megabits per second. Values from 49152 to
65535 are reserved for future use, but imply operating at a link speed
of more 300 Mbps or more. Packets larger than 49151 Mbps. Hosts MUST ignore all options with a link speed this value that's higher than SHOULD NOT
be sent over the current link speed of in question. The MAXMTU1000 MUST be at
least the interface MTU size specified in the option is received over. For instance, if a host has an
interface relevant IPv6-over-... RFC.
A value of 0 means that supports 10, 100 and 1000 Mbps ethernet which
currently operates at 100 Mbps, and the host receives options
with 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 values
of 100 30 to 299 Mbps and 1000 over links operating at an unknown speed if
that interface, the
option with speed can be 30 Mbps or higher. Packets larger than
this value SHOULD NOT be sent over the link speed of 100 is processed and in question. The
MAXMTU100 MUST be at least the option
with MTU size specified in the link speed
relevant IPv6-over-... RFC. A value of 1000 0 means that the MTU
size is ignored.

Allowed MTU: undefined and no maximum size is enforced for this link
speed.

MAXMTU10:
The maximum packets size allowed on a link. link operating at a speed
of less than 30 Mbps. Packets larger than this value MUST SHOULD NOT
be sent over the link in question. The allowed
MTU MAXMTU10 MUST be at
least 1500. the MTU size specified in the relevant IPv6-over-... RFC.
A value of 0 means that the allowed MTU size is undefined and no
maximum MTU size is enforced.

The number of variable MTU options in router advertisements enforced for this link speed.

When MAXMTU1000, MAXMTU100 and MAXMTU10 all contain the same value,
it is limited allowed to omit MAXMTU100 and MAXMTU10 so the option has a maximum
length of 4. 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 variable MTU multi-MTU options from the router
advertisements of at least the router they select as a default router,
but it's allowed 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. Host MUST only consider variable MTU options where Hosts SHOULD use the MAXMTU value
of relevant for the link
speed field doesn't exceed that of the current link speed
of interface is currently operating at from the associated interface. Any options (or equivalents) that satisfy
this condition are ordered by option or
equivalent information with the priority, link speed and allowed MTU
fields, largest priority value. If the
relevant MAXMTU field is unspecified (zero) in that order. Hosts SHOULD copy the allowed MTU and off-link
MTU information, if specified, option or
information with the highest priority, the field from the option (or equivalent)
or information with the largest value for 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
non-zero value, the host SHOULD use its physical MTU as the concatenation
MAXMTU.

When a node's interface speed changes, it MAY reinitiate
negotiation of these three fields. 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 bytes
octets for multicast packets. In order to lift this limitation,
routers and hosts that implement variable MTU multi-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 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 variable MTU multi-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)
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
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 and padding options

A node that implements the variable MTU 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
Length: 1

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

MTU:
The maximum packet size in octets that the node is prepared to send and receive,
which is copied from the local MTU.
receive. 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 format of the ICMPv6 neighbor discovery MTU detection message option is for. It has the following format:

1 2 3 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 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Length |R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Padding |
...
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type: TBD (informational)

Code: TBD

Checksum:

Length: see [RFC2463]

R (reply requested): 0: no reply requested, 1: below.

R: reply requested flag.

Reserved: set to 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. reception.

Padding: 0 or more 0 bytes to bring the packet to the specified packet
size.

In order to avoid sending large numbers all-zero octects.

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

Reception of packets that can't be
handled properly by switches a neighbor solicitation or other layer 2 devices, after sending a
large MTU detection packet, neighbor advertisement
triggers for a neighbor for which no other maximum size per-neighbor MTU detection
packets may be transmitted on is known
triggers, in addition to the same interface for 60 seconds or
until normal response if it's a large neighbor
solicitation, the sending of an neighbor solicitation message wih
the MTU detection packet has been received, whichever
happens first. In and padding options in it. The size of this context, "large" means larger than message is may
vary between the standard
MTU IPv6-over-... size + 1 for the link type, i.e., 1500 bytes for ethernet.

When variable MTU subnet capability is detected for a neighbor by and the
presence
minimum of an the relevant MAXMTU, the physical MTU option and the neighbor's
MTU as advertised in a neighbor solicitation or neighbor
discovery message, an the MTU detection message option of the packet received. See
below for considerations about the packet sizes to choose. The
padding option is constructed as follows:

R:
Set used to 0 if bring the neighbor MTU is known and confirmed, set solicitation message
to 1
otherwise.

Packet size:
Equal this size. The padding option MUST be the last option in the
packet.

There are two possible ways to determine the minimum value of the local MTU length
field:

1. Set it to 0. As the "length" field in options has a granularity
of 8 octets and the (tentative) behavior of nodes when they receive a
neighbor
MTU.

When an MTU detection solicitation packet is received, which has a total length that
doesn't match the size length of the packet contents, an option
length of 0 is
checked against used to make sure that hosts that don't
understand the value in padding option will silently discard the packet.

2. If the intended packet size length allows a valid value for the
length field, the length field MAY be set to detect
truncation in transit. If that value. The
node MAY reduce the packet size and of the intended packet to accommodate
the requirement that the size field
don't match, or is a multiple of 8 octets.
I.e., if the intended packet size is smaller than 1280 bytes, 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 is silently discarded.

If with the received 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 has 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, 1.
Whenever the neighbor advertisement is not the result of receiving
a reply neighbor solicitation with a padding option, the R flag is
constructed as follows:

R: 0

Packet size:
Equal set to
0.

When a node receives a neighbor advertisement message, it must
determine whether the minimum 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 local 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.

The 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 overrules information even in the TCP MSS option 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 adjust their behavior in TCP
sessions towards 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 neighbor. Neighbor 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 information expires along capabilities. This is done by
simply probing with link addresses learned through neighbor discovery and upon dead an ARP packet padded to the desired size. If a
reply comes back, the neighbor detection. supports the probed MTU size.

4.5 Determining Probe considerations

In cases where the local neigbor's MTU

The local was advertised in an MTU is the value communicated option,
it makes sense to neighbors. It try with this size. If that probe fails or the
neighbor's MTU is unknown, the
minimum of best choice for a probe size would
be the physical 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 an interface 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 allowed probe size may
be common MTU sizes such as
advertised by 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 router 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 learned through other means. The local MTU 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 further reduced by used otherwise, would be to initially send just
one probe sized at the reception of switch local MTU advertisements. 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.

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 December, 2007. 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

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