One document matched: draft-van-beijnum-multi-mtu-01.txt
Differences from draft-van-beijnum-multi-mtu-00.txt
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
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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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Internet-Draft IPv6 Extensions for Multi-MTU Subnets August 2007
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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