Network Working Group I. van Beijnum
Internet-Draft Institute IMDEA Networks
Intended status: Experimental October 27, 2014
Expires: April 30, 2015
Extensions for Multi-MTU Subnets
draft-van-beijnum-multi-mtu-04
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 multipoint subnets are now almost
exclusively implemented as Ethernets. Even though the relevant
standards mandate a 1500 byte maximum packet size for Ethernet, more
and more Ethernet equipment is capable of handling packets bigger
than 1500 bytes. However, since this capability isn't standardized,
it is seldom used today, despite the potential performance benefits
of using larger packets. This document specifies mechanisms to
negotiate per-neighbor maximum packet sizes so that nodes on a
multipoint subnet may use the maximum mutually supported packet size
between them without being limited by nodes with smaller maximum
sizes on the same subnet.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 30, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Overview of operation . . . . . . . . . . . . . . . . . . . . 5
5. The ND NODEMTU option . . . . . . . . . . . . . . . . . . . . 6
6. The MTUTEST packet format . . . . . . . . . . . . . . . . . . 6
7. Changes to the RA MTU option semantics . . . . . . . . . . . 8
8. The TCP MSS option . . . . . . . . . . . . . . . . . . . . . 9
9. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Initialization . . . . . . . . . . . . . . . . . . . . . 9
9.2. Probing . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.3. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 14
9.4. Neighbor MTU garbage collection . . . . . . . . . . . . . 15
10. IANA considerations . . . . . . . . . . . . . . . . . . . . . 15
11. Security considerations . . . . . . . . . . . . . . . . . . . 16
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
13.1. Normative References . . . . . . . . . . . . . . . . . . 16
13.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Document and discussion information . . . . . . . . 17
Appendix B. Advantages and disadvantages of larger packets . . . 17
B.1. Delay and jitter . . . . . . . . . . . . . . . . . . . . 18
B.2. Path MTU Discovery problems . . . . . . . . . . . . . . . 18
B.3. Packet loss through bit errors . . . . . . . . . . . . . 19
B.4. Undetected bit errors . . . . . . . . . . . . . . . . . . 19
B.5. Interaction TCP congestion control . . . . . . . . . . . 20
B.6. IEEE 802.3 compatibility . . . . . . . . . . . . . . . . 21
B.7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
Some protocols inherently generate small packets. Examples are VoIP,
where it is 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 often do not fill up a
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full 1500-byte packet. However, most data that is transferred across
the internet and private networks is part of long-lived sessons and
requires segmentation by a transport protocol, which is almost always
TCP. These types of data transfers 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 for the source
and destination hosts
3. Fewer packets also means fewer per-packet operations in routers
and middleboxes
4. TCP performance increases with larger packet sizes
Even though today, the capability to use larger packets (often called
jumboframes) is present in a lot of Ethernet hardware, this
capability typically isn't used because IP assumes a common MTU size
for all nodes connected to a link or subnet. In practice, this means
that using a larger MTU requires manual configuration of the non-
standard MTU size on all hosts and routers and possibly on layer 2
switches connected to a subnet. Also, the MTU size for a subnet is
limited to that of the least capable router, host or switch.
Perhaps in the future, when hosts support packetization layer path
MTU discovery ([RFC4821], "Packetization Layer Path MTU Discovery")
in all relevant transport protocols, it will be possible to simply
ignore MTU limitations by sending at the maximum locally supported
size and determining the maximum packet size towards a correspondent
from acknowledgements that come back for packets of different sizes.
However, [RFC4821] must be implemented in every transport protocol,
and problems arise in the case where hosts implementing [RFC4821]
interact with hosts that don't implement this mechanism, but do use a
larger than standard MTU.
This document provides for a set of mechanisms that allow the use of
larger packets between nodes that support them which interacts well
with both manually configured non-standard MTUs and expected future
[RFC4821] operation with larger MTUs. This is done using a new IPv6
Neighbor Discovery option and a new UDP-based protocol for exchanging
MTU information and testing whether jumboframes can be transmitted
successfully.
Appendix B discusses several potential issues with larger packets,
such as head-of-line blocking delays, path MTU discovery black holes
and the strength of the CRC32 with increasing packet sizes.
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2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Note that this specification is not standards track, and as such,
can't overrule existing specifications. Whenever [RFC2119] language
is used, this must be interpreted within the context of this
specification: while the specification as a whole is optional and
non-standard, whenever it is implemented, such an implementation can
only function properly when all MUSTs are observed.
3. Terminology
Advertised MTU: The MTU size announced by a node to other nodes on
the local subnet.
Confirmed MTU: The largest packet size successfully received from
the neighbor or the largest packet size sent to the neighbor for
which an acknowledgment was received; whichever size is greater.
Confirmed Time: When a packet the size of the confirmed MTU was last
received or acknowledged.
Local MTU: The MTU configured on an interface. By default, this is
the largest MTU size supported by the hardware, but the Local MTU
may be lowered administratively or automatically based on policy.
(For instance, the MTU may be set to the Standard MTU if the link
speed is below 1000 Mbps.)
MRU: Maximum Receive Unit. The size of the largest IP packet that
can be received on an interface. This document doesn't use the
term MRU, and assumes that the MRU is equal to the MTU.
MTU: Maximum Transfer Unit. The size of the largest IP packet that
can be transmitted on an interface, considering hardware (and
administrative) limitations.
Neighbor: Another node on a connected subnet. Neighbors are
identified by the combination of a link address and an IP version.
The MTU may be set to different values for IPv4 and IPv6
administratively, but it is assumed that if a node has multiple
IPv4 or IPv6 addresses, the MTU for each set of addresses is the
same.
Neighbor MTU: The currently used MTU towards a neighboring node on a
subnet. The Neighbor MTU reflects the current best understanding
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of the maximum packet size that can successfully be transmitted
towards that neighbor.
Safe MTU: The maximum packet size that is assumed to work without
testing. Defaults to the Standard MTU, but may be set to a
subnet-wide higher or lower value administratively, or to a lower
value using the MTU option in IPv6 Router Advertisements.
Standard MTU: The MTU specified in the relevant IPv4-over-... or
IPv6-over-... document, which is 1500 for Ethernet ([RFC0894] and
[RFC2464]).
4. Overview of operation
The mechanisms described in this document come into play when a node
is connected to a subnet using an interface that supports an MTU size
larger than the standard MTU size for that link type.
For each remote node connected to such a subnet, the local node
maintains a neighbor MTU setting. The length of packets transmitted
to a neighbor is always limited to the neighbor MTU size.
When a node starts communicating with another node on the same
subnet, it follows the following procedure:
1. Initialization: the neighbor MTU is set to local maximum MTU for
the interface used to reach the neighbor.
2. Discovery: learning the other node's MTU.
3. Probing: determining the maximum packet size that can
successfully be transmitted to and and received from the other
node, considering the (unknown) maximum packet size supported by
the layer 2 infrastructure.
4. Monitoring: making sure that when large packets are transmitted,
they are not silently discarded, for instance as the result of a
layer 2 reconfiguration.
During the discovery and probing stages, the neighbor MTU is adjusted
as new information becomes available. The monitoring stage is
ongoing. If during the monitoring stage it is determined that large
packets aren't successfully exchanged with the neighboring node, the
neighbor MTU is set to the safe MTU and the node returns to the
testing stage.
Unless administrative configuration or policy specifies otherwise,
the link, IPv4 and IPv6 MTU sizes are set to the maximum supported by
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the hardware. This means that when TCP sessions are created, they
carry a maximum segment size (MSS) option that reflects the larger-
than-standard MTU.
5. The ND NODEMTU option
All MTU values are 32-bit unsigned integers in network byte order.
All other values are also unsigned and in network byte order .
The MTU size and two flags are exchanged as an IPv6 Neighbor
Discovery option. The new option, as well as the MTU value it
avertises, are named "NODEMTU".
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NodeMTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ HintMTU (optional) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD
Length: 1 or 2
Reserved: Set to 0 on transmission, ignored when received.
NodeMTU The maximum packet size the node wishes to receive on this
interface.
HintMTU The maximum packet size the node believes it can
successfully receive on this interface at this time. If the
HintMTU is equal to the NodeMTU or no value for HintMTU is known,
this field may be omitted and the Length field is set to 1. If
the HintMTU field is present, the Length field is set to 2.
When a node's interface speed changes, it MAY advertise a new MTU,
but it SHOULD remain prepared to receive packets of the maximum size
advertised to neighbors previously (if the old maximum size is larger
than the newly advertised one).
6. The MTUTEST packet format
The packets used to test whether large packets can be transmitted
successfully and communicate status are sent using UDP ([RFC0768]).
Their format is as follows:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 'M' | 'T' | 'U' | 'T' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|B| Reserved | Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NodeMTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HintMTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Padding |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source port (UDP): For outgoing requests: an ephemeral port number.
For replies: 1022. (16 bits.)
Destination port (UDP): For outgoing requests: 1022. For replies:
the source port used in the request being replied to. (16 bits.)
Length (UDP): for IPv4 and IPv6 packets smaller than or equal to
65575 bytes, the length of the UDP segment. For IPv6 packets
larger than 65575 bytes, 0 (as per [RFC2675]). (16 bits.)
Checksum (UDP): the UDP checksum. (16 bits.)
R: reply request flag. If set to 0, no reply is sent. If set to 1,
the receiver is asked to send a reply. (1 bit.)
MTUT: The value corresponding to the ASCII string "MTUT", used to
differentiate MTUTEST packets from other UDP packets that use port
1022. Packets with a value other than "MTUT" at the beginning of
the UDP payload MUST be ignored. (32 bits.)
B: big reply request flag. If set to 0, replies are not padded. If
set to 1, replies are padded to be the same size as the request.
(1 bit.)
Reserved: set to 0 on transmission, ignored on reception. (6 bits.)
Nonce: a hard-to-guess value. (24 bits.)
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NodeMTU: The maximum packet size that the sender is prepared to
receive at this time. (32 bits.)
HintMTU: The maximum packet size that the sender believes it can
successfully receive at this time. (32 bits.)
Padding: Filled with 0 or more all-zero bytes on transmission,
ignored on reception.
In addition to the fields listed above, the following IP and link
layer fields are taken into consideration:
Source link-layer address: On transmission: set automatically by the
networking stack. On reception: used to identify a neighbor.
IP version: On transmission: set automatically by the networking
stack. On reception: used to identify a neighbor. (The IP
version may also be identified implicitly through the API without
directly observing the version field.)
Time To Live / Hop Limit: On transmission: set to 255. On
reception: if 255, the packet is processed. If other than 255,
the packet is silently discarded. (To enforce that the protocol
is only used within a local subnet.)
Source IP address: On transmission, for requests: set to the address
the node intends to use to communicate with the neighbor. For
replies: set to the destination IP address in the request being
replied to. On reception: used to identify a neighbor.
Destination IP address: On transmission, for requests: set to the
address the node intends to use to communicate with the neighbor.
For replies: set to the source IP address in the request being
replied to.
7. Changes to the RA MTU option semantics
Section 6.3.4 of [RFC4861] specifies:
"If the MTU option is present, hosts SHOULD copy the option's value
into LinkMTU so long as the value is greater than or equal to the
minimum link MTU and does not exceed the maximum LinkMTU value
specified in the link-type-specific document"
This document changes the handling of the Router Advertisement MTU
option such that it may also be used by routers to tell hosts that
they SHOULD use an MTU larger than the LinkMTU and update their
SafeMTU value. If multiple routers advertise different MTUs that are
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higher or lower than the standard MTU, behavior is undefined. MTU
options containing the standard MTU SHOULD be ignored.
The ability to advertise a larger-than-standard MTU must be used with
extreme care by nework administrators, as advertising an MTU size
that exceeds the capabilities of routers or the layer 2
infrastructure will lead to reachability problems.
If the advertised larger-than-standard MTU is ignored or not
supported by some hosts connected to the subnet, TCP will presumably
still work because the MSS option ([RFC0793]) limits the size of
transmitted TCP segments to what the receiver suports. However, non-
TCP protocols that use large packets will likely fail. The most
prominent example of this is DNS over UDP with EDNS0 when requesting
large records, such as those used for DNSSEC ([RFC6891]).
8. The TCP MSS option
Hosts SHOULD advertise the maximum MTU size they are prepared to use
on a link in the TCP MSS value, even during times when probing has
failed: should larger neighbor MTUs be established later, it will not
be possible to adjust the MSS for ongoing sessions.
9. Operation
9.1. Initialization
When an interface is activated, an appropriate local MTU is
determined, based on hardware limitations and admnistrative settings.
Additionally, a policy may be in place to constrain packet sizes when
operating at lower bandwidths, to avoid excessive delays as queues of
large packets build up and cause significant head-of-line blocking
for subsequent time-sensitive packts. Also, layer 2 devices
operating at lower interface speeds are less likely to support non-
standard MTUs.
In the absense of operational experience, this document RECOMMENDS
limiting the use of larger than standard MTUs to interfaces operating
at 400 Mbps or faster; and if a larger MTU is used for interfaces
operating at lower speeds, a "mini jumbo" size of 2000 bytes or less
is used for Ethernets.
For IPv4, the local MTU is limited to 65535 bytes. For IPv6, if
[RFC2675] jumbograms are not supported, the local MTU is limited to
65575 bytes. These limits apply even if the interface hardware
supports a larger MTU. IPv6 nodes that implement [RFC2675]
jumbograms MAY use MTU sizes larger than 65575 bytes.
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When the interface speed changes, the local MTU MAY be changed to
reflect the new speed. However, the node SHOULD remain prepared to
receive packets of the size of a previously advertised MTU.
The local MTU MAY be different for IPv4 and IPv6. The local MTU is
the size used to calculate the value of the TCP MSS option. The
HintMTU is set to undefined.
When sending Neighbor Solicitations and Neighbor Advertisements, a
node includes its local MTU in the NodeMTU field of the NODEMTU
option. If the size of the HintMTU is known, it is also included.
When a node starts communicating with a new IPv4 neighbor (i.e., ARP
[RFC0826] messages are exchanged), the node sends a MTUTEST packet to
the new neighbor and sets the neighbor MTU to the safe MTU. The
MTUTEST packet has the local MTU in the NodeMTU field. If a hint MTU
is known, it is included in the HintMTU field. The R and B flags are
set to 0. No padding is included.
9.2. Probing
Upon reception of a Neighor Solicitation or a Neighbor Advertisement
with the NODEMTU option or an MTUTEST packet, the node determines if
the packet is received from a known neighbor IP address and a known
neighor link layer address. If the values match the values stored
for a known neighbor, no action occurs.
If the values match the values for a known link layer address and IP
version, but an unknown IP address, the IP address is added to the
list of IP addresses for the neighbor in question and the known
neighbor MTU for the neighbor is applied to the new address.
If the NodeMTU matches the NodeMTU previously sent by a known
neighbor but the HintMTU as a different non-zero value, the HintMTU
is updated.
If the HintMTU sent by a known neighbor is 0, the neighbor MTU is set
to the safe MTU, the HintMTU for the neighbor is set to unknown and
the probing procedure is started.
If the combination of link layer address and IP version is unknown,
the neighbor MTU is set to the safe MTU, the HintMTU is set to the
HintMTU value in the packet and the probing procedure is started.
Before starting the probing procedure, a node compares its link layer
address to the neighbor's link layer address. If the node's link
layer address is numerically larger than the neighbor's link layer
address, the node applies a waiting period before starting the
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probing procedure. The waiting period SHOULD be at least 250
milliseconds and at most 1 second.
The probing procedure is as follows:
Neighbor.ConfirmedTime = UNDEFINED
if LocalMTU > Neighbor.AdvertisedMTU
let Max = Neighbor.AdvertisedMTU
else
let Max = LocalMTU
# test with maximum supported packet size first
# and finish probing upon success
test (Max)
if Success:
Neighbor.MTU = Max
return
# maximum size doesn't work, now find
# what does work
# assumption: 256 works for IPv4, 1280 for IPv6
let WorksNo = Max
if IPv6:
let Neighbor.ConfirmedMTU = 1280
if IPv4:
let Neighbor.ConfirmedMTU = 256
# test with the hinted size
# if successful, this becomes the minimum for further tests
# if unsuccessful, this becomes the maximum
test (HintMTU)
if Success:
let Neighbor.ConfirmedMTU = HintMTU
else
let WorksNo = HintMTU
# test the smallest usable size larger than
# the standard MTU (if that size is still
# in the range to be tested) so we avoid wasting
# time probing non-jumbo-capable nodes
if (StandardMTU + 8 > Neighbor.ConfirmedMTU and StandardMTU + 8 < WorksNo)
test (StandardMTU + 8)
if Success:
let Neighbor.ConfirmedMTU = StandardMTU + 8
else
let WorksNo = StandardMTU + 8
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# to establish an upper bound quickly,
# test (320, 640, 1280, ) 2560, 5120, 10240, 20480, 40960, ...
let Current = 320
while (Current < WorksNo)
if (Current > Neighbor.ConfirmedMTU)
test (Current)
if Success:
let Neighbor.ConfirmedMTU = Current
else
let WorksNo = Current
let Current = Current * 2
# we have now established that
# WorksNo =< Neighbor.ConfirmedMTU * 2
# further testing is based on a list of hints.
# there SHOULD be a mechanism for administrators
# to add hints.
#
# hint sources:
# 576: common PPP low delay
# 1492: PPP over Ethernet [RFC2516]
# 2304: 802.11
# [CATALYST]:
# 9216, 8092, 1600, 1998, 2000, 1546, 1530, 17976, 2018
# sizes observed by the author:
# 576, 4070, 9000, 16384, 64000
let Hints = 576, 1492, 1530, 1998, 2304, 4070, 8092, 9000, 16384, 32000, 64000
foreach Size in Hints
if Size > Neighbor.ConfirmedMTU and Size < WorksNo
test (Size)
if Success:
let Neighbor.ConfirmedMTU = Size
else
let WorksNo = Size
# finished testing, maximum working packet isze
# is now known to within about a factor 1.5,
# depending on the number of hints
if Neighbor.ConfirmedTime <> UNDEFINED
# we got at least one probe back, use discovered MTU
Neighbor.MTU = Neighbor.ConfirmedMTU
else
# we never got any probes back, neighbor probably does
# not implement MTUTEST protocol, so we use the safe MTU
Neighbor.MTU = SafeMTU
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# done!
return
# sending probes
function test (Size)
# wait 20 milliseconds between sending probes
let MsecSinceProbe = now () - ProbeTime
if (MsecSinceProbe < 20)
sleep (20 - MsecSinceProbe)
# create probe, request reply (but not a big one)
let Probe.TTL = 255
let Probe.ReplyFlag = 1
Let Probe.BigFlag = 0
Let Nonce = rand ()
Let Probe.Nonce = Nonce
let Probe.NodeMTU = LocalMTU
let Probe.HintMTU = HintMTU
let Probe.Padding = pad (Size - sizeof (Probe))
send (Probe)
let ProbeTime = now ()
# wait 2000 milliseconds for reply
# (this also avoids sending packets that are too large more
# than once every two seconds)
let Success = receive (Reply, 2000)
if not Success
return false
if not (Reply.TTL = 255 and Reply.Nonce = Nonce
and Reply.LinkAddress = Neighbor.LinkAddress)
return false
# valid reply received
# note that Neighbor.MTU is not updated yet,
# this happens after probing has finished
Neighbor.ConfirmedMTU = Reply.NodeMTU
Neighbor.ConfirmedTime = now ()
Neighbor.HintMTU = Reply.HintMTU;
if HintMTU < Size
HintMTU = Size
return true
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Note that if at any time an unsolicited packet arrives from the
neighbor and the confirmed MTU of that neighbor is smaller than the
size of the packet received, the confirmed MTU for the neighbor is
set to the size of the received packet. If this happens during the
probing procedure, this may lead to fewer outgoing probes being sent.
The packets that update the confirmed MTU do not have to be MTUTEST
protocol packets.
There are no retransmissions. Both nodes run the probing procedure,
so there are two opportunities to succeed. However, if both fail to
determine the maximum packet size that can be used because of lost
packets, the hosts will have to use a smaller packet size.
It is assumed that the maximum packet size that A can send to B is
the same as the maximum packet size that B can send to A. As such,
the reception of a large packet is treated the same as receiving an
acknowledgment for a sent large packet.
9.3. Monitoring
Once a working neighbor MTU is found, large packets can be exchanged.
Presumably, this situation will persist indefinitely. However, it is
possible that the network is reconfigured and then no longer supports
the MTU used between two nodes. The aim of the monitoring phase is
to detect this when it happens and establish a working MTU value
before sessions time out.
For each neighbor (as defined by a unique combination of link layer
address and IP version) with a neighbor MTU larger than the safe MTU,
the ability to successfully send or receive large packets is
monitored. In the monitoring phase, a node tracks whether it sends
any packets larger than the safe MTU to a neighbor and whether it
receives either acknowledgments for those packets, or it receives
packets of length neighbor MTU from that neighbor. (So acknowledged
outgoing packets don't have to be the maximum size supported to/from
the neighbor, but incoming packets do.)
The ability to track acknowledgment of non-MTUTEST packets is not
required. However, it is expected that hosts will be able to do this
for TCP packets because the TCP state is readily available.
Monitoring is happens in intervals. This document RECOMMENDS that
this interval is between 25 and 35 seconds for hosts and between 35
and 45 seconds for routers. At the end of each monitoring interval,
if acknowledgments or large packets were received, everything is fine
and the neighbor confirmed time is updated.
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At the end of a monitoring interval, if no large packets were sent,
everything is fine and nothing happens.
At the end of a monitoring interval, if large packets were sent, but
no acknowledgments or incoming maximum size packets were seen, there
may have been a network reconfiguration that has made it impossible
for large packets to be transmitted successfully between the two
nodes. To determine whether this is the case, the node sends an
MTUTEST packet with lenght neighbor MTU. The R flag is set to 1 and
the B flag SHOULD be set to 0. A random nonce and the local MTU and
the hint MTU are included.
The node waits 2 seconds for a reply. If there is no reply, the
probe is retransmitted and the node waits 4 seconds for a reply. If
after 4 seconds there is still no reply, the node sets the hint MTU
to 0 and reinitializes all of the neighbor's MTU-related information
to initial values. Most notably, this means that the neighbor MTU is
set to the safe MTU.
If the node sets is own hint MTU to 0 or receives a hint MTU of 0
from a neighbor using an ND or MTUTEST packet, the node MAY start
sending probes to other neighbors before the monitoring interval
expires. However, nodes SHOULD limit the number of probes for all
neighbors combined to no more than one every two seconds. If a node
has many neighbors and sending probes at one every two seconds would
take too long, it MAY reset the neighbor MTUs of all of its neighbors
to the safe MTU without sending probes if at least two neighbors
appear to be affected by a reduction of the maximum working packet
size.
9.4. 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".
10. IANA considerations
IANA is requested to assign a neighbor discovery option type value.
[TO BE REMOVED: This registration should take place at the following
location: http://www.iana.org/assignments/icmpv6-parameters
UDP port 1022 is used in accordance with [RFC4727]. Presumably,
unlike an ND option type value, a UDP port would be relatively easy
to change when experimentation makes way for production deployment.
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11. Security considerations
Generating false neighbor discovery and MTUTEST packets with large
MTUs may lead to a denial-of-serve condition, just like the
advertisement of other false link parameters. Requests are large and
replies typically short to avoid the MTUTEST protocol being used as
an amplification vector. The nonce is used together with the
ephemeral UDP port number to make sure that malicious nodes cannot
generate a reply to a request in the blind.
12. Acknowledgements
This document benefited from feedback by Dave Thaler, Jari Arkko, Joe
Touch and others.
13. References
13.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC0826] Plummer, D., "Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet
address for transmission on Ethernet hardware", STD 37,
RFC 826, November 1982.
[RFC0894] Hornig, C., "Standard for the transmission of IP datagrams
over Ethernet networks", STD 41, RFC 894, April 1984.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, December 1998.
[RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, August 1999.
[RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
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[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
13.2. Informative References
[RFC2516] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D.,
and R. Wheeler, "A Method for Transmitting PPP Over
Ethernet (PPPoE)", RFC 2516, February 1999.
[IEEE.802-3.1985]
Institute of Electrical and Electronics Engineers,
"Carrier Sense Multiple Access with Collision Detection
(CSMA/CD) Access Method and Physical Layer
Specifications", IEEE Standard 802.3, 1985.
[CRC] Jain, R., "Error Characteristics of Fiber Distributed Data
Interface (FDDI), IEEE Transactions on Communications",
August 1990.
[CATALYST]
Cisco, "Jumbo/Giant Frame Support on Catalyst Switches
Configuration Example",
<http://www.cisco.com/c/en/us/support/docs/switches/
catalyst-6000-series-switches/24048-148.html>.
Appendix A. Document and discussion information
The latest version of this document will always be available at
http://www.muada.com/drafts/. Please direct questions and comments to
the int-area mailinglist or directly to the author.
Appendix B. Advantages and 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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B.1. Delay and jitter
An low-bandwidth links, the additional time it takes to transmit
larger packets may lead to unacceptable delays. For instance,
transmitting a 9000-byte packet takes 7.23 milliseconds at 10 Mbps,
while transmitting a 1500-byte packet takes only 1.23 ms. Once
transmission of a packet has started, additional traffic must wait
for the transmission to finish, so a larger maximum packet size
immediately leads to a higher worst-case head-of-line blocking delay,
and thus, to a bigger difference between the best and worst cases
(jitter). The increase in average delay depends on the number of
packets that are buffered, the average packet size and the queuing
strategy in use. Buffer sizes vary greatly between implementations,
from only a few buffers in some switches and on low-speed interfaces
in routers, to hundreds of megabytes of buffer space on 10 Gbps
interfaces in some routers.
If we assume that the delays involved with 1500-byte packets on 100
Mbps Ethernet are acceptable for most, if not all, applications, then
the conclusion must be that 15000-byte packets on 1 Gbps Ethernet
should also be acceptable, as the delay is the same. At 10 Gbps
Ethernet, much larger packet sizes could be accommodated without
adverse impact on delay-sensitive applications. At below 100 Mbps,
larger packet sizes are probably not advisable.
B.2. Path MTU Discovery problems
PMTUD issues arise when routers can't fragment packets in transit
because the DF bit is set or because the packet is IPv6, but the
packet is too large to be forwarded over the next link, and the
resulting "packet too big" ICMP messages from the router don't make
it back to the sending host. If there is a PMTUD black hole, this
will typically happen when there is an MTU bottleneck somewhere in
the middle of the path. If the MTU bottleneck is located at either
end, the TCP MSS (maximum segment size) option makes sure that TCP
packets conform to the smallest MTU in the path. PMTUD problems are
of course possible with non-TCP protocols, but this is rare in
practice because non-TCP protocols are generally not capable of
adjusting their packet size on the fly and therefore use more
conservative packet sizes which won't trigger PMTUD issues.
Taking the delay and jitter issues to heart, maximum packet sizes
should be larger for faster links and smaller for slower links. This
means that in the majority of cases, the MTU bottleneck will tend to
be at, or close to, one of the ends of a path, rather than somewhere
in the middle, as in today's internet, the core of the network is
quite fast, while users usually connect to the core at lower speeds.
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A crucial difference between PMTUD problems that result from MTUs
smaller than the de facto standard 1500 bytes and PMTUD problems that
result from MTUs larger than 1500 bytes is that in the latter case,
only the party that's actually using the non-standard MTU is
affected. This puts potential problems, the potential benefits and
the ability to solve any resulting problems in the same place: 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, increasing MTUs where
desired doesn't seem to involve undue risks.
B.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 for
reasonable bit error rates, the optimum packet size is computed as
follows:
packet size = sqrt( overhead bytes / bit error rate )
According to this, the optimum packet size is one or more orders of
magnitude larger than what's commonly used today. For instance, the
maximum BER for 1000BASE-T is 10^-10, which implies an optimum packet
size of 312250 bytes with Ethernet framing and IP overhead.
B.4. Undetected bit errors
Nearly all link layers employ some kind of checksum to detect bit
errors so that packets with errors can be discarded. In the case of
Ethernet, this is a frame check sequence in the form of a 32-bit CRC.
Assuming a strong frame check sequence algorithm, a 32-bit checksum
suggests that there is a 1 in 2^32 chance that a packet with one or
more bit errors in it has the same checksum as the original packet,
so the bit errors go undetected and data is corrupted. However,
according to [CRC] the CRC-32 that's used for FDDI and Ethernet has
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the property that packets between 376 and 11454 bytes long
(including) have a Hamming distance of 3. (Smaller packets have a
larger Hamming distance, larger packets a smaller Hamming distance.)
As a result, all errors where only a single bit is flipped or two
bits are flipped, will be detected, because they can't result in the
same CRC as the original packet. The probability of a packet having
undetected bit errors can be approximated as follows for a 32-bit
CRC:
PER = (PL * BER) ^ H / 2^32
Where PER is the packet error rate, BER is the bit error rate, PL is
the packet length in bits and H is the Hamming distance. Another
consideration is the impact of packet length on a multi-packet
transmission of a given size. This would be:
TER = transmission length / PL * PER
So
TER = transmission length / (PL ^ (H - 1) * BER ^ H) / 2^32
Where TER is the transmission error rate.
In the case of the Ethernet FCS and a Hamming distance of 3 for a
large range of packet sizes, this means that the risk of undetected
errors goes up with the square of the packet length, but goes down
with the third power of the bit error rate. This suggest that for a
given acceptable risk of undetected errors, a maximum packet size can
be calculated from the expected bit error rate. It also suggests
that given the low BER rates mandated for Gigabit Ethernet, packet
sizes of up to 11454 bytes should be acceptable.
Additionally, unlike properties such as the packet length, the frame
check sequence can be made dependent on the physical media, so it
should be possible to define a stronger FCS in future Ethernet
standards, or to negotiate a stronger FCS between two stations on a
point-to-point Ethernet link (i.e., a host and a switch or a router
and a switch).
B.5. Interaction TCP congestion control
TCP performance is based on the inverse of the square of the packet
loss probability. Using larger and thus fewer packets is therefore a
competitative advantage. Larger packets increase burstiness, which
can be problematic in some circumstances. Larger packets also allow
TCP to ramp up its transmission speed faster, which is helpful on
fast links, where large packets will be more common. In general, it
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would seem advantageous for an individual user to use larger packets,
but under some circumstances, users using smaller packets may be put
at a slight disadvantage.
B.6. IEEE 802.3 compatibility
According to the IEEE 802.3 standard ([IEEE.802-3.1985]), the field
following the Ethernet addresses is a length field. However,
[RFC0894] uses this field as a type field. Ambiguity is largely
avoided by numbering type codes above 2048. The mechanisms described
in this memo only apply to the standard [RFC0894] and [RFC2464]
encapsulation of IPv4 and IPv6 in Ethernet, not to possible
encapsulations of IPv4 or IPv6 in IEEE 802.3/IEEE 802.2 frames, so
there is no change to the current use of the Ethernet length/type
field.
B.7. Conclusion
Larger packets aren't universally desirable. The factors that factor
into the decision to use larger packets include:
o A link's bit error rate
o The number of bits per symbol on a link and hence the likelihood
of multiple bit errors in a single packet
o The strength of the frame check sequence
o The link speed
o The number of buffers
o Queuing strategy
o Number of sessions on shared links and paths
This means that choosing a good maximum packet size is, initially at
least, the responsibility of hardware builders. A conservative
approach may be called for, but even under conservative assumptions,
9000-byte jumboframes on Gigabit Ethernet links seem reasonable.
Author's Address
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Iljitsch van Beijnum
Institute IMDEA Networks
Avda. del Mar Mediterraneo, 22
Leganes, Madrid 28918
Spain
Email: iljitsch@muada.com
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