Internet DRAFT - draft-ietf-6man-mtu-option
draft-ietf-6man-mtu-option
Network Working Group R. Hinden
Internet-Draft Check Point Software
Intended status: Experimental G. Fairhurst
Expires: 11 November 2022 University of Aberdeen
10 May 2022
IPv6 Minimum Path MTU Hop-by-Hop Option
draft-ietf-6man-mtu-option-15
Abstract
This document specifies a new IPv6 Hop-by-Hop option that is used to
record the minimum Path MTU along the forward path between a source
host to a destination host. The recorded value can then be
communicated back to the source using the return Path MTU field in
the option.
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
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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 11 November 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Example Operation . . . . . . . . . . . . . . . . . . . . 3
1.2. Use of the IPv6 Hop-by-Hop Options Header . . . . . . . . 4
2. Motivation and Problem Solved . . . . . . . . . . . . . . . . 5
3. Requirements Language . . . . . . . . . . . . . . . . . . . . 6
4. Applicability Statements . . . . . . . . . . . . . . . . . . 6
5. IPv6 Minimum Path MTU Hop-by-Hop Option . . . . . . . . . . . 6
6. Router, Host, and Transport Layer Behaviors . . . . . . . . . 8
6.1. Router Behavior . . . . . . . . . . . . . . . . . . . . . 8
6.2. Host Operating System Behavior . . . . . . . . . . . . . 8
6.3. Transport Layer Behavior . . . . . . . . . . . . . . . . 9
6.3.1. Including the Option in an Outgoing Packet . . . . . 10
6.3.2. Validation of the Packet that includes the Option . . 12
6.3.3. Receiving the Option . . . . . . . . . . . . . . . . 12
6.3.4. Using the Rtn-PMTU Field . . . . . . . . . . . . . . 13
6.3.5. Detecting Path Changes . . . . . . . . . . . . . . . 14
6.3.6. Detection of Dropping Packets that include the
Option . . . . . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8.1. Router Option Processing . . . . . . . . . . . . . . . . 15
8.2. Network Layer Host Processing . . . . . . . . . . . . . . 15
8.3. Validating use of the Option Data . . . . . . . . . . . . 16
8.4. Direct use of the Rtn-PMTU Value . . . . . . . . . . . . 16
8.5. Using the Rtn-PMTU Value as a Hint for Probing . . . . . 17
8.6. Impact of Middleboxes . . . . . . . . . . . . . . . . . . 17
9. Experiment Goals . . . . . . . . . . . . . . . . . . . . . . 17
10. Implementation Status . . . . . . . . . . . . . . . . . . . . 18
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
12. Change log [RFC Editor: Please remove] . . . . . . . . . . . 18
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
13.1. Normative References . . . . . . . . . . . . . . . . . . 21
13.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Examples of Usage . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
This document specifies a new IPv6 Hop-by-Hop (HBH) Option to record
the minimum Maximum Transmission Unit (MTU) along the forward path
between a source and a destination host. The source host creates a
packet with this option and initializes the Min-PMTU field with the
value of the MTU for the outbound link that will be used to forward
the packet towards the destination host.
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At each subsequent hop where the option is processed, the router
compares the value of the Min-PMTU Field in the option and the MTU of
its outgoing link. If the MTU of the link is less than the Min-PMTU,
it rewrites the value in the option data with the smaller value.
When the packet arrives at the destination host, the host can send
the value of the minimum reported MTU for the path back to the source
host using the Rtn-PMTU field in the option. The source host can
then use this value as input to the method that sets the Path MTU
(PMTU) used by upper layer protocols.
The IPv6 Minimum Path MTU Hop-by-Hop (MinPMTU HBH) Option is designed
to work with packet sizes that can be specified in the IPv6 header.
The maximum packet size that can be specified in an IPv6 header is
65,535 octets (2^^16).
This method has the potential to complete Path MTU discovery in a
single round trip time, even over paths that have successive links
each with a lower MTU.
The mechanism defined in this document is focused on Unicast, it does
not describe Multicast. That is left for future work.
1.1. Example Operation
The figure below illustrates the operation of the method. In this
case, the path between the source host and the destination host
comprises three links, the source has a link MTU of size MTU-S, the
link between routers R1 and R2 has an MTU of size 9000 bytes, and the
final link to the destination has an MTU of size MTU-D.
+--------+ +----+ +----+ +-------+
| | | | | | | |
| Sender +---------+ R1 +--------+ R2 +-------- + Dest. |
| | | | | | | |
+--------+ MTU-S +----+ 9000B +----+ MTU-D +-------+
Figure 1
Three scenarios are described:
* Scenario 1, considers all links to have an 9000 byte MTU and the
method is supported by both routers. The initial Min-PMTU is not
modified along the path, and therefore the PMTU is 9000 bytes.
* Scenario 2, considers the link between R2 and destination host
(MTU-D) to have an MTU of 1500 bytes. This is the smallest MTU,
router R2 updates the Min-PMTU to 1500 bytes and the method
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correctly updates the PMTU to 1500 bytes. Had there been another
smaller MTU at a link further along the path that also supports
the method, the lower MTU would also have been detected.
* Scenario 3, considers the case where the router preceding the
smallest link (R2) does not support the method, and the link to
the destination host (MTU-D) has an MTU of 1500 bytes. Therefore,
router R2 does not update the Min-PMTU to 1500 bytes. The method
then fails to detect the actual PMTU.
In Scenarios 2 and 3, a lower PMTU would also fail to be detected in
the case where PMTUD had been used and an ICMPv6 Packet Too Big (PTB)
message had not been delivered to the sender [RFC8201].
These scenarios are summarized in the table below. "H" in R1 and/or
R2 columns means the router understands the MinPMTU HBH option.
+-+-----+-----+----+----+----------+-----------------------+
| |MTU-S|MTU-D| R1 | R2 | Rec PMTU | Note |
+-+-----+-----+----+----+----------+-----------------------+
|1|9000B|9000B| H | H | 9000 B | Endpoints attempt to |
| | | | | | use a 9000 B PMTU. |
+-+-----+-----+----+----+----------+-----------------------+
|2|9000B|1500B| H | H | 1500 B | Endpoints attempt to |
| | | | | | | use a 1500 B PMTU. |
+-+-----+-----+----+----+----------+-----------------------+
|3|9000B|1500B| H | - | 9000 B | Endpoints attempt to |
| | | | | | | use a 9000 B PMTU, |
| | | | | | | but need to implement |
| | | | | | | a method to fall back |
| | | | | | | to discover and use a |
| | | | | | | 1500 B PMTU. |
+-+-----+-----+----+----+----------+-----------------------+
Figure 2
1.2. Use of the IPv6 Hop-by-Hop Options Header
IPv6 as specified in [RFC8200] allows nodes to optionally process the
Hop-by-Hop header. Specifically, from Section 4:
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* The Hop-by-Hop Options header is not inserted or deleted, but may
be examined or processed by any node along a packet's delivery
path, until the packet reaches the node (or each of the set of
nodes, in the case of multicast) identified in the Destination
Address field of the IPv6 header. The Hop-by-Hop Options header,
when present, must immediately follow the IPv6 header. Its
presence is indicated by the value zero in the Next Header field
of the IPv6 header.
* NOTE: While [RFC2460] required that all nodes must examine and
process the Hop-by-Hop Options header, it is now expected that
nodes along a packet's delivery path only examine and process the
Hop-by-Hop Options header if explicitly configured to do so.
The Hop-by-Hop Option defined in this document is designed to take
advantage of this property of how Hop-by-Hop options are processed.
Nodes that do not support this Option SHOULD ignore them. This can
mean that the Min-PMTU value does not account for all links along a
path.
2. Motivation and Problem Solved
The current state of Path MTU Discovery on the Internet is
problematic. The mechanisms defined in [RFC8201] are known to not
work well in all environments. It fails to work in various cases,
including when nodes in the middle of the network do not send ICMPv6
PTB messages, or rate-limited ICMPv6 messages, or do not have a
return path to the source host.
This results in many transport layer connections being configured to
use smaller packets (e.g., 1280 bytes) by default and makes it
difficult to take advantage of paths with a larger PMTU where they do
exist. Applications that send large packets are forced to use IPv6
Fragmentation [RFC8200], which can reduce the reliability of Internet
communication [RFC8900].
Encapsulations and network-layer tunnels further reduce the payload
size available for a transport protocol to use. Also, some use-cases
increase packet overhead, for example, Network Virtualization Using
Generic Routing Encapsulation (NVGRE) [RFC7637] encapsulates L2
packets in an outer IP header and does not allow IP Fragmentation.
Sending larger packets can improve host performance, e.g., avoiding
limits to packet processing by the packet rate. For example, the
packet per second rate required to reach wire speed on a 10G link
with 1280 byte packets is about 977K packets per second (pps), vs.
139K pps for 9000 byte packets.
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The purpose of this document is to improve the situation by defining
a mechanism that does not rely on reception of ICMPv6 Packet Too Big
messages from nodes in the middle of the network. Instead, this
provides information to the destination host about the minimum Path
MTU, and sends this information back to the source host. This is
expected to work better than the current RFC8201-based mechanisms.
A similar mechanism was proposed in 1988 for IPv4 in [RFC1063] by
Jeff Mogul, C. Kent, Craig Partridge, and Keith McCloghire. It was
later obsoleted in 1990 by [RFC1191], the current deployed approach
to Path MTU Discovery. In contrast, the method described in this
document uses the Hop-by-Hop option of IPv6. It does not replace
PMTUD [RFC8201], PLPPMTUD [RFC4821] or Datagram PLPMTUD [RFC8899],
but rather is designed to compliment these methods.
3. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
4. Applicability Statements
The Path MTU option is designed for environments where there is
control over the hosts and nodes that connect them, and where there
is more than one MTU size in use. For example, in Data Centers and
on paths between Data Centers, to allow hosts to better take
advantage of a path that is able to support a large PMTU.
The design of the option is sufficiently simple that it can be
executed on a router's fast path. A successful experiment depends on
both implementation by host and router vendors and deployment by
operators. The contained use-case of connections within and between
Data Centers could be a driver for deployment.
The method could also be useful in other environments, including the
general Internet, and offers advantage when this Hop-by-Hop Option is
supported on all paths. The method is more robust when used to probe
the path using packets that do not carry application data and when
also paired with a method such as Packetization Layer PMTUD [RFC4821]
or Datagram PLPMTUD [RFC8899].
5. IPv6 Minimum Path MTU Hop-by-Hop Option
The Minimum Path MTU Hop-by-Hop Option has the following format:
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Option Option Option
Type Data Len Data
+--------+--------+--------+--------+---------+-------+-+
|BBCTTTTT|00000100| Min-PMTU | Rtn-PMTU |R|
+--------+--------+--------+--------+---------+-------+-+
Option Type (see Section 4.2 of [RFC8200]):
BB 00 Skip over this option and continue processing.
C 1 Option data can change en route to the packet's final
destination.
TTTTT 10000 Option Type assigned from IANA [IANA-HBH].
Length: 4 The size of the value field in Option Data
field supports PMTU values from 0 to 65,534 octets, the
maximum size represented by the Path MTU option.
Min-PMTU: n 16-bits. The minimum MTU recorded along the path
in octets, reflecting the smallest link MTU that
the packet experienced along the path.
A value less than the IPv6 minimum link
MTU [RFC8200] MUST be ignored.
Rtn-PMTU: n 15-bits. The returned Path MTU field, carrying the 15
most significant bits of the latest received Min-PMTU
field for the forward path. The value zero means that
no Reported MTU is being returned.
R n 1-bit. R-Flag. Set by the source to signal that
the destination host should include the received
Rtn-PMTU field updated by the reported Min-PMTU value
when the destination host is to send a PMTU Option back
to the source host.
Figure 3
NOTE: The encoding of the final two octets (Rtn-PMTU and R-Flag)
could be implemented by a mask of the latest received Min-PMTU value
with 0xFFFE, discarding the right-most bit and then performing a
logical 'OR' with the R-Flag value of the sender. This encoding fits
in the minimum-sized Hop-by-Hop Option header.
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6. Router, Host, and Transport Layer Behaviors
6.1. Router Behavior
Routers that are not configured to support Hop-by-Hop Options are not
expected to examine or process the contents of this option [RFC8200].
Routers that support Hop-by-Hop Options, but are not configured to
support this option SHOULD skip over this option and continue to
processing the header [RFC8200].
Routers that support this option MUST compare the value of the Min-
PMTU field with the MTU configured for the outgoing link. If the MTU
of the outgoing link is less than the Min-PMTU, the router rewrites
the Min-PMTU in the Option to use the smaller value. (The router
processing is performed without checking the valid range of the Min-
PMTU or the Rtn-PMTU fields.)
A router MUST ignore and MUST NOT change the Rtn-PMTU field or the
R-Flag in the option.
6.2. Host Operating System Behavior
The PMTU entry associated with the destination in the host's
destination cache [RFC4861] SHOULD be updated after detecting a
change using the IPv6 Minimum Path MTU Hop-by-Hop Option. This
cached value can be used by other flows that share the host's
destination cache.
The value in the host destination cache SHOULD be used by PLPMTUD to
select an initial PMTU for a flow. The cached PMTU is only increased
by PLPMTUD when the Packetization Layer determines the path actually
supports a larger PMTU [RFC4821] [RFC8899].
When requested to send an IPv6 packet with the MinPMTU HBH option,
the source host includes the option in an outgoing packet. The
source host MUST fill the Min-PMTU field with the MTU configured for
the link over which it will send the packet on the next hop towards
the destination host.
When a host includes the option in a packet it sends, the host SHOULD
set the Rtn-PMTU field to the previously cached value of the received
Minimum Path MTU for the flow in the Rtn-PMTU field (see
Section 6.3.3). If this value is not set (for example, because there
is no cached reported Min-PMTU value), the Rtn-PMTU field value MUST
be set to zero.
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The source host MAY request the destination host to return the
reported Min-PMTU value by setting the R-Flag in the option of an
outgoing packet. The R-Flag SHOULD NOT be set when the MinPMTU HBH
Option was sent solely to provide requested feedback on the return
Path MTU to avoid each response generating another response.
The destination host controls when to send a packet with this option
in response to an R-flag, as well as which packets to include it in.
The destination host MAY limit the rate at which it sends these
packets.
A destination host only sets the R Flag if it wishes the source host
to also return the discovered PMTU value for the path from the
destination to the source.
The normal sequence of operation of the R-Flag using the terminology
from the diagram in Figure 1 is:
1. The source sends a probe to the destination. The sender sets the
R-Flag.
2. The destination responds by sending a probe including the
received Min-PMTU as the Rtn-PMTU. A destination that does not
wish to probe the return path sets the R-Flag to 0.
6.3. Transport Layer Behavior
This Hop-by-Hop option is intended to be used with a path MTU
discovery method.
PLPMTUD [RFC9000] uses probe packets for two distinct functions:
* Probe packets are used to confirm connectivity. Such probes can
be of any size up to the PLPMTU. These probe packets are sent to
solicit a response use the path to the remote node. These probe
packets can carry the Hop-by-Hop PMTU option, providing the final
size of the packet does not exceed the current PLPMTU. After
validating that the packet originates from the path (section
4.6.1), the PLPMTUD method can use the reported size from the Hop-
by-Hop option as the next search point when it resumes the search
algorithm. (This use resembles the use of the PTB_SIZE
information in section 4.6.2 of [RFC8899]
* A second use of probe packets is to explore if a path supports a
packet size greater than the current PLPMTU. If this probe packet
is successfully delivered (as determined by the source host), then
the PLPMTU is raised to the size of the successful probe. These
probe packets do not usually set the Path MTU Hop-by-Hop option.
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See section 1.2 of [RFC8899]. Section 4.1 of [RFC8899] also
describes ways that a Probe Packet can be constructed, depending
on whether the probe packets carry application data.
* The PMTU Hop-by-Hop Option Probe can be sent on packets that
include application data, but needs to be robust to potential loss
of the packet (i.e., with the possibility that retransmission
might be needed if the packet is lost).
* Using a PMTU Probe on packets that do not carry application data
will avoid the need for loss recovery if a router on the path
drops packets that set this option. (This avoids the transport
needing to retransmit a lost packet that includes this option.)
This is the normal default format for both uses of probes.
6.3.1. Including the Option in an Outgoing Packet
The upper layer protocol can request the MinPMTU HBH option to be
included in an outgoing IPv6 packet. A transport protocol (or upper
layer protocol) can include this option only on specific packets used
to test the path. This option does not need to be included in all
packets belonging to a flow.
NOTE: Including this option in a large packet (e.g., one larger than
the present PMTU) is not likely to be useful, since the large packet
would itself be dropped by any link along the path with a smaller
MTU, preventing the Min-PMTU information from reaching the
destination host.
Discussion:
* In the case of TCP, the option could be included in a packet that
carries a TCP segment sent after the connection is established. A
segment without data could be used, to avoid the need to
retransmit this data if the probe packet is lost. The discovered
value can be used to inform PLPMTUD [RFC4821].
NOTE: A TCP SYN can also negotiate the Maximum Segment Size (MSS),
which acts as an upper limit to the packet size that can be sent
by a TCP sender. If this option were to be included in a TCP SYN,
it could increase the probability that the SYN segment is lost
when routers on the path drop packets with this option (see
Section 6.3.6), which could have an unwanted impact on the result
of racing options [I-D.ietf-taps-arch] or feature negotiation.
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* The use with datagram transport protocols (e.g., UDP) is harder to
characterize because applications using datagram transports range
from very short-lived (low data-volume applications) exchanges, to
longer (bulk) exchanges of packets between the source and
destination hosts [RFC8085].
* Simple-exchange protocols (i.e., low data-volume applications
[RFC8085] that only send one or a few packets per transaction),
might assume that the PMTU is symmetrical. That is, the PMTU is
the same in both directions, or at least not smaller for the
return path. This optimization does not hold when the paths are
not symmetric.
* The MinPMTU HBH option can be used with ICMPv6 [RFC4443]. This
requires a response from the remote node and therefore is
restricted to use with ICMPv6 echo messages. The MinPMTU HBH
option could provide additional information about the PMTU that
might be supported by a path. This could be use as a diagnostic
tool to measure the PMTU of a path. As with other uses, the
actual supported PMTU is only confirmed after receiving a response
to a subsequent probe of the PMTU size.
* A datagram transport can utilise DPLPMTUD [RFC8899]. For example,
QUIC (see section 14.3 of [RFC9000]), can use DPLPMTUD to
determine whether the path to a destination will support a desired
maximum datagram size. When using the IPv6 MinPMTU HBH option,
the option could be added to an additional QUIC PMTU Probe that is
of minimal size (or one no larger than the currently supported
PMTU size). Once the return Path MTU value in the MinPMTU HBH
option has been learned, DPLPMTUD can be triggered to test for a
larger PLPMTU using an appropriately sized PLPMTU Probe Packet
(see section 5.3.1 of [RFC8899]).
* The use of this option with DNS and DNSSEC over UDP is expected to
work for paths where the PMTU is symmetric. The DNS server will
learn the PMTU from the DNS query messages. If the Rtn-PMTU value
is smaller, then a large DNSSEC response might be dropped and the
known problems with PMTUD will then occur. DNS and DNSSEC over
transport protocols that can carry the PMTU ought to work.
* This method also can be used with Anycast to discover the PMTU of
the path, but the use needs to be aware that the Anycast binding
might change.
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6.3.2. Validation of the Packet that includes the Option
An upper layer protocol (e.g., transport endpoint) using this option
needs to provide protection from data injection attacks by off-path
devices [RFC8085]. This requires a method to assure that the
information in the Option Data is provided by a node on the path.
This validates that the packet forms a part of an existing flow,
using context available at the upper layer. For example, a TCP
connection or UDP application that maintains the related state and
uses a randomized ephemeral port would provide this basic validation
to protect from off-path data injection, see Section 5.1 of
[RFC8085]. IPsec [RFC4301] and TLS [RFC8446] provide greater
assurance.
The upper layer discards any received packet when the packet
validation fails. When packet validation fails, the upper layer MUST
also discard the associated Option Data from the MinPMTU HBH option
without further processing.
6.3.3. Receiving the Option
For a connection-oriented upper layer protocol, caching of the
received Min-PMTU could be implemented by saving the value in the
connection context at the transport layer. A connection-less upper
layer (e.g., one using UDP), requires the upper layer protocol to
cache the value for each flow it uses.
A destination host that receives a MinPMTU HBH Option with the R-Flag
SHOULD include the MinPMTU HBH option in the next outgoing IPv6
packet for the corresponding flow.
A simple mechanism could only include this option (with the Rtn-PMTU
field set) the first time this option is received or when it notifies
a change in the Minimum Path MTU. This limits the number of packets
including the option packets that are sent. However, this does not
provide robustness to packet loss or recovery after a sender loses
state.
Discussion:
* Some upper layer protocols send packets less frequently than the
rate at which the host receives packets. This provides less
frequent feedback of the received Rtn-PMTU value. However, a host
always sends the most recent Rtn-PMTU value.
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6.3.4. Using the Rtn-PMTU Field
The Rtn-PMTU field provides an indication of the PMTU from on-path
routers. It does not necessarily reflect the actual PMTU between the
source and destination hosts. Care therefore needs to be exercised
in using the Rtn-PMTU value. Specifically:
* The actual PMTU can be lower than the Rtn-PMTU value because the
Min-PMTU field was not updated by a router on the path that did
not process the option.
* The actual PMTU may be lower than the Rtn-PMTU value because there
is a layer-2 device with a lower MTU.
* The actual PMTU may be larger than the Rtn-PMTU value because of a
corrupted, delayed or mis-ordered response. A source host MUST
ignore a Rtn-PMTU value larger than the MTU configured for the
outgoing link.
* The path might have changed between the time when the probe was
sent and when the Rtn-PMTU value received.
IPv6 requires that every link in the Internet have an MTU of 1280
octets or greater. A node MUST ignore a Rtn-PMTU value less than
1280 octets [RFC8200].
To avoid unintentional dropping of packets that exceed the actual
PMTU (e.g., Scenario 3 in Section 1.1), the source host can delay
increasing the PMTU until a probe packet with the size of the Rtn-
PMTU value has been successfully acknowledged by the upper layer,
confirming that the path supports the larger PMTU. This probing
increases robustness, but adds one additional path round trip time
before the PMTU is updated. This use resembles that of PTB messages
in section 4.6 of DPLPMTUD [RFC8899] (with the important difference
that a PTB message can only seek to lower the PMTU, whereas this
option could trigger a probe packet to seek to increase the PMTU.)
Section 5.2 of [RFC8201] provides guidance on the caching of PMTU
information and also the relation to IPv6 flow labels.
Implementations should consider the impact of Equal Cost Multipath
(ECMP) [RFC6438]. Specifically, whether a PMTU ought to be
maintained for each transport endpoint, or for each network address.
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6.3.5. Detecting Path Changes
Path characteristics can change and the actual PMTU could increase or
decrease over time. For instance, following a path change when
packets are forwarded over a link with a different MTU than that
previously used. To bound the delay in discovering an increase in
the actual PMTU, a host with a link MTU larger than the current PMTU
SHOULD periodically send the MinPMTU HBH Option with the R-bit set.
DPLPMTUD provides recommendations concerning how this could be
implemented (see Section 5.3 of [RFC8899]). Since the option
consumes less capacity than a full-sized probe packet, there can be
advantage in using this to detect a change in the path
characteristics.
6.3.6. Detection of Dropping Packets that include the Option
There is evidence that some middleboxes drop packets that include
Hop-by-Hop options. For example, a firewall might drop a packet that
carries an unknown extension header or option. This practice is
expected to decrease as an option becomes more widely used. It could
result in generation of an ICMPv6 message indicating the problem.
This could be used to (temporarily) suspend use of this option.
A middlebox that silently discards a packet with this option results
in dropping of any packet using the option. This dropping can be
avoided by appropriate configuration in a controlled environment,
such as within a data centre, but needs to be considered for Internet
usage. Section 6.2 recommends that this option is not used on
packets where loss might adversely impact performance.
7. IANA Considerations
IANA has assigned and registered an IPv6 Hop-by-Hop Option type with
Temporary status from the "Destination Options and Hop-by-Hop
Options" registry [IANA-HBH]. This assignment is shown in Section 5.
IANA is requested to update this registry to point to this document
and remove the Temporary status.
8. Security Considerations
This section discusses the security considerations. It first reviews
router option processing. It then reviews host processing when
receiving this option at the network layer. It then considers two
ways in which the Option Data can be processed, followed by two
approaches for using the Option Data. Finally, it discusses
middlebox implications related to use in the general Internet.
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8.1. Router Option Processing
This option shares the characteristics of all other IPv6 Hop-by-Hop
Options, in that if not supported at line rate it could be used to
degrade the performance of a router. This option, while simple, is
no different to other uses of IPv6 Hop-by-Hop options.
It is common for routers to ignore the Hop-by-Hop Option header or
drop packets containing a Hop-by-Hop Option header. Routers
implementing IPv6 according to [RFC8200] only examine and process the
Hop-by-Hop Options header if explicitly configured to do so.
8.2. Network Layer Host Processing
A malicious attacker can forge a packet directed at a host that
carries the MinPMTU HBH option. By design, the fields of this IP
option can be modified by the network.
For comparison, the ICMPv6 Packet Too Big message used in [RFC8201]
Path MTU Discovery, the source host has an inherent trust
relationship with the destination host including this option. This
trust relationship can be used to help verify the option. ICMPv6
Packet Too Big messages are sent from any router on the path to the
destination host, the source host has no prior knowledge of these
routers (except for the first hop router).
Reception of this packet will require processing as the network stack
parses the packet before the packet is delivered to the upper layer
protocol. This network layer option processing is normally completed
before any upper layer protocol delivery checks are performed.
The network layer does not normally have sufficient information to
validate that the packet carrying an option originated from the
destination (or an on-path node). It also does not typically have
sufficient context to demultiplex the packet to identify the related
transport flow. This can mean that any changes resulting from
reception of the option applies to all flows between a pair of
endpoints.
These considerations are no different to other uses of Hop-by-Hop
options, and this is the use case for PMTUD. The following section
describes a mitigation for this attack.
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8.3. Validating use of the Option Data
Transport protocols should be designed to provide protection from
data injection attacks by off-path devices and mechanisms should be
described in the Security Considerations for each transport
specification (see Section 5.1 of the UDP Guidelines [RFC8085]). For
example, a TCP or UDP application that maintains the related state
and uses a randomized ephemeral port would provide basic protection.
TLS [RFC8446] or IPsec [RFC4301] provide cryptographic
authentication. An upper layer protocol that validates each received
packet discards any packet when this validation fails. In this case,
the host MUST also discard the associated Option Data from the
MinPMTU HBH option without further processing (Section 6.3).
A network node on the path has visibility of all packets it forwards.
By observing the network packet payload, the node might be able to
construct a packet that might be validated by the destination host.
Such a node would also be able to drop or limit the flow in other
ways that could be potentially more disruptive. Authenticating the
packet, for example, using IPsec [RFC4301] or TLS [RFC8446] mitigates
this attack. Note that AH style authentication [RFC4302] while
authenticating the payload and outer IPv6 header, does not check Hop-
by-Hop options that change on route.
8.4. Direct use of the Rtn-PMTU Value
The simplest way to utilize the Rtn-PMTU value is to directly use
this to update the PMTU. This approach results in a set of security
issues when the option carries malicious data:
* A direct update of the PMTU using the Rtn-PMTU value could result
in an attacker inflating or reducing the size of the host PMTU for
the destination. Forcing a reduction in the PMTU can decrease the
efficiency of network use, might increase the number of packets/
fragments required to send the same volume of payload data, and
prevents sending an unfragmented datagram larger than the PMTU.
Increasing the PMTU can result in black-holing (see Section 1.1 of
[RFC8899]) when the source host sends packets larger than the
actual PMTU. This persists until the PMTU is next updated.
* The method can be used to solicit a response from the destination
host. A malicious attacker could forge a packet that causes the
destination to add the option to a packet sent to the source host.
A forged value of Rtn-PMTU in the Option Data might also impact
the remote endpoint, as described in the previous bullet. This
persists until a valid MinPMTU HBH option is received. This
attack could be mitigated by limiting the sending of the MinPMTU
HBH option in reply to incoming packets that carry the option.
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8.5. Using the Rtn-PMTU Value as a Hint for Probing
Another way to utilize the Rtn-PMTU value is to indirectly trigger a
probe to determine if the path supports a PMTU of size Rtn-PMTU.
This approach needs context for the flow, and hence assumes an upper
layer protocol that validates the packet that carries the option (see
Section 8.3). This is the case when used in combination with
DPLPMTUD [RFC8899]. A set of security considerations result when an
option carries malicious data:
* If the forged packet carries a validated option with a non-zero
Rtn-PMTU field, the upper layer protocol could utilize the
information in the Rtn-PMTU field. A Rtn-PMTU larger than the
current PMTU can trigger a probe for a new size.
* If the forged packet carries a non-zero Min-PMTU field, the upper
layer protocol would change the cached information about the path
from the source. The cached information at the destination host
will be overwritten when the host receives another packet that
includes a MinPMTU HBH option corresponding to the flow.
* Processing of the option could cause a destination host to add the
MinPMTU HBH option to a packet sent to the source host. This
option will carry a Rtn-PMTU value that could have been updated by
the forged packet. The impact of the source host receiving this
resembles that discussed previously.
8.6. Impact of Middleboxes
There is evidence that some middleboxes drop packets that include
Hop-by-Hop options. For example, a firewall might drop a packet that
carries an unknown extension header or option. This practice is
expected to decrease as the option becomes more widely used. Methods
to address this are discussed in Section 6.3.6.
When a forged packet causes a packet to be sent including the MinPMTU
HBH option, and the return path does not forward packets with this
option, the packet will be dropped Section 6.3.6. This attack is
mitigated by validating the option data before use and by limiting
the rate of responses generated. An upper layer could further
mitigate the impact by responding to an R-Flag by including the
option in a packet that does not carry application data.
9. Experiment Goals
This section describes the experimental goals of this specification.
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A successful deployment of the method depends upon several components
being implemented and deployed:
* Support in the sending node (see Section 6.2). This also requires
corresponding support in upper layer protocols (see Section 6.3).
* Router support in nodes (see Section 6.1). The IETF continues to
provide recommendations on the use of IPv6 Hop-by-Hop options, for
example Section 2.2.2 of [RFC9099]. This document does not update
the way router implementations configure support for Hop-by-Hop
options.
* Support in the receiving node (see Section 6.3.3).
Experience from deployment is an expected input to any decision to
progress this specification from Experimental to IETF Standards
Track. Appropriate inputs might include:
* Reports of implementation experience;
* Measurements of the number paths where the method can be used;
* Measurements showing the benefit realized or the implications of
using specific methods over specific paths.
10. Implementation Status
At the time this document was published there are two known
implementations of the Path MTU Hop-by-Hop option. These are:
* Wireshark dissector. This is shipping in production in Wireshark
version 3.2 [WIRESHARK].
* A prototype in the open source version of the FD.io Vector Packet
Processing (VPP) technology [VPP]. At the time this document was
published, the source code can be found [VPP_SRC].
11. Acknowledgments
Helpful comments were received from Tom Herbert, Tom Jones, Fred
Templin, Ole Troan, Tianran Zhou, Jen Linkova, Brian Carpenter, Peng
Shuping, Mark Smith, Fernando Gont, Michael Dougherty, Erik Kline,
and other members of the 6MAN working group.
12. Change log [RFC Editor: Please remove]
draft-ietf-6man-mtu-option-15, 2022-May-10
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* Correcting an editing mistake in Appendix A.
* Editorial Change.
draft-ietf-6man-mtu-option-14, 2022-April-15
* Area Director Reviews:
- Lars Eggert's Review: Fixed "nits".
- Eric Vyncke's Review: Added that this work is focused on
Unicast, removed Discussion from Section 6.1, revised text on
PLPMTUD probing, changed SHOULD to MUST in Section 6.3.4, and
fixed several NITs.
- Alvaro Retana's Review: Changed SHOULD language to more general
text in Section 6.1
- ARTART Review: Added new Appendix "Examples of Usage" with
diagrams showing examples of use.
- Zaheduzzaman Sarker's Review: Fixed some editorial issues, and
updated SHOULD language.
* Editorial Changes.
draft-ietf-6man-mtu-option-13, 2022-February-28
* Area Directorate Reviews:
- SECDIR Review: Fixed "nit".
- TSVART Review: Restructured Section 6 including making
Transport Behavior more prominent, added text about ICMPv6 to
Section 6.3.1, moved the text about prior work in RFC1063 to
Section 2.
- GENART Review: Added text to Section 1 that this option was
designed to work with packet sizes that can be specified in the
IPv6 Header.
* Editorial Changes.
draft-ietf-6man-mtu-option-12, 2022-January-26
* Clarified a few issues raised by AD review by Erik Kline AD
review.
draft-ietf-6man-mtu-option-11, 2021-September-30
* Clarifications and editorial changes to the Security
Considerations section based on early AD review by Erik Kline.
draft-ietf-6man-mtu-option-10, 2021-September-27
* Clarifications and editorial changes based on second chair review
by Ole Troan.
* Editorial changes.
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draft-ietf-6man-mtu-option-09, 2021-September-23
* Clarifications and editorial changes based on review by Michael
Dougherty.
draft-ietf-6man-mtu-option-08, 2021-September-7
* Clarifications and editorial changes based on chair review by Ole
Troan.
* Correction and clarifications based on review by Fernando Gont.
draft-ietf-6man-mtu-option-07, 2021-August-31
* Added Experiment Goals section.
* Added Implementation Status section.
* Updated the IANA Considerations section to point to this document
and remove Temporary status.
* Clarifications and editorial changes based on review by Mark
Smith.
draft-ietf-6man-mtu-option-06, 2021-August-7
* Transport usage of the mechanism clarified in response to feedback
and suggestions from Jen Linkova.
* Restructured Section 6 to improve readability.
* Editorial changes.
draft-ietf-6man-mtu-option-05, 2021-April-28
* Editorial changes.
draft-ietf-6man-mtu-option-04, 2020-Oct-23
* Fixes for typos.
draft-ietf-6man-mtu-option-03, 2020-Sept-14
* Rewrite to make text and terminology more consistent.
* Added the notion of validating the packet before use of the HBH
option data.
* Method aligned with the way common APIs send/receive HBH option
data.
* Added reference to DPLPMTUD and clarified upper layer usage.
* Completed security considerations section.
draft-ietf-6man-mtu-option-02, 2020-March-9
* Editorial changes to make text and terminology more consistent.
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* Added reference to DPLPMTUD.
draft-ietf-6man-mtu-option-01, 2019-September-13
* Changes to show IANA assigned code point.
* Editorial changes to make text and terminology more consistent.
* Added a reference to RFC8200 in Section 2 and a reference to
RFC6438 in Section 6.3.
draft-ietf-6man-mtu-option-00, 2019-August-9
* First 6man w.g. draft version.
* Changes to request IANA allocation of code point.
* Editorial changes.
draft-hinden-6man-mtu-option-02, 2019-July-5
* Changed option format to also include the Returned PMTU value and
Return flag and made related text changes in Section 6.2 to
describe this behavior.
* ICMPv6 Packet Too Big messages are no longer used for feedback to
the source host.
* Added to Acknowledgements Section that a similar mechanism was
proposed for IPv4 in 1988 in [RFC1063].
* Editorial changes.
draft-hinden-6man-mtu-option-01, 2019-March-05
* Changed requested status from Standards Track to Experimental to
allow use of experimental option type (11110) to allow for
experimentation. Removed request for IANA Option assignment.
* Added Section 2 "Motivation and Problem Solved" section to better
describe what the purpose of this document is.
* Added appendix describing planned experiments and how the results
will be measured.
* Editorial changes.
draft-hinden-6man-mtu-option-00, 2018-Oct-16
* Initial draft.
13. References
13.1. Normative References
[IANA-HBH] "Destination Options and Hop-by-Hop Options",
<https://www.iana.org/assignments/ipv6-parameters/
ipv6-parameters.xhtml#ipv6-parameters-2>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
13.2. Informative References
[I-D.ietf-taps-arch]
Pauly, T., Trammell, B., Brunstrom, A., Fairhurst, G., and
C. Perkins, "An Architecture for Transport Services", Work
in Progress, Internet-Draft, draft-ietf-taps-arch-12, 3
January 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-taps-arch-12>.
[RFC1063] Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP
MTU discovery options", RFC 1063, DOI 10.17487/RFC1063,
July 1988, <https://www.rfc-editor.org/info/rfc1063>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://www.rfc-editor.org/info/rfc4301>.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC4302, December 2005,
<https://www.rfc-editor.org/info/rfc4302>.
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[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
for Equal Cost Multipath Routing and Link Aggregation in
Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
<https://www.rfc-editor.org/info/rfc6438>.
[RFC7637] Garg, P., Ed. and Y. Wang, Ed., "NVGRE: Network
Virtualization Using Generic Routing Encapsulation",
RFC 7637, DOI 10.17487/RFC7637, September 2015,
<https://www.rfc-editor.org/info/rfc7637>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8899] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/info/rfc8899>.
[RFC8900] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and F. Gont, "IP Fragmentation Considered Fragile",
BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
<https://www.rfc-editor.org/info/rfc8900>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
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[RFC9099] Vyncke, É., Chittimaneni, K., Kaeo, M., and E. Rey,
"Operational Security Considerations for IPv6 Networks",
RFC 9099, DOI 10.17487/RFC9099, August 2021,
<https://www.rfc-editor.org/info/rfc9099>.
[VPP] "VPP/What is VPP?",
<https://wiki.fd.io/view/VPP/What_is_VPP%3F>.
[VPP_SRC] "VPP Source", <https://gerrit.fd.io/r/c/vpp/+/21948>.
[WIRESHARK]
"Wireshark Network Protocol Analyzer",
<https://www.wireshark.org>.
Appendix A. Examples of Usage
This section provides examples that illustrate a use of the MinPMTU
HBH option by a source using DPLPMTUD to discover the PLPMTU
supported by a path. They consider a path where the on-path router
has been configured with an outgoing MTU of d'. The source starts by
transmission of packets of size a, and then uses DPLPMTUD to seek to
increase the size in steps resulting in sizes of b,c,d,e, etc.,
(chosen by the search algorithm used by DPLPMTUD). The search
algorithm terminates with a PLPMTU that is at least d and is less
than or equal to d'.
The first example considers DPLPMTUD without using the MinPMTU HBH
option. In this case, DPLPMTUD searches using an increasing size of
probe packet. Probe packets of size (e) are sent, which are larger
than the actual PMTU. In this example, PTB messages are not received
from the routers and repeated unsuccessful probes result in the
search phase completing. Packets of data are never sent with a size
larger than the size of the last confirmed probe packet. ACKs of
data packets are not shown.
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----Packets of data size (a) ---------------------------->
----Probe size (b) -------------------------------------->
<---------------------------------- ACK of probe --------
----Packets of data size (b) ---------------------------->
----Probe size (c) -------------------------------------->
<---------------------------------- ACK of probe --------
----Packets of data size (c) ---------------------------->
----Probe size (d) -------------------------------------->
<---------------------------------- ACK of probe --------
----Packets of data size (d) ---------------------------->
<---------------------------------- ACK of probe --------
...
----Probe size (e) ------------X
X----ICMPv6 PTB (d') --|
----Packets of data size (d) ---------------------------->
----Probe size (e) ------------X (again)
X----ICMPv6 PTB (d') --|
----Packets of data size (d) -----------------------------
...
etc, until MaxProbes are unsuccessful and search phase completes.
----Packets of data size (d) ---------------------------->
Figure 4
The second example considers DPLPMTUD with the MinPMTU HBH option set
on a connectivity probe packet.
The IPv6 option is sent end-to-end, and the Min-PMTU is updated by a
router on the path to d', which is returned in a response that also
sets the MinPMTU HBH option. Upon receiving Rtn-PMTU value is
received, DPLPMTUD immediately sends a probe packet of the target
size (d'). If the probe packet is confirmed for the path, the PLPMTU
is updated, allowing the source to use data packets up to size d'.
(The search algorithm is allowed to continue to probe to see if the
path supports a larger size.) Packets of data are never sent with a
size larger than the last confirmed probe size, d'.
----Packets of data size (a) ---------------------------->
----Connectivity probe with MinPMTU-
+--updated to minPMTU=d'----->
<-----------------ACK with Rtn-PMTU=d'--------------------
----Packets of data size (a) ---------------------------->
----Probe size (d') ------------------------------------->
<---------------------------------- ACK of probe ---------
-----Packets of data size (d') -------------------------->
Search phase completes.
-----Packets of data size (d') -------------------------->
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Figure 5
The final example considers DPLPMTUD with the MinPMTU HBH option set
on a connectivity probe packet, but shows the effect when this
connectivity probe packet is dropped.
In this case, the packet with the MinPMTU HBH option is not received.
DPLPMTUD searches using probe packets of increasing size, increasing
the PLPMTU when the probes are confirmed. An ICMPv6 PTB message is
received when the probed size exceeds the actual PMTU, indicating a
PTB_SIZE of d'. DPLPMTUD immediately sends a probe packet of the
target size (d'). If the probe packet is confirmed for the path, the
PLPMTU is updated, allowing the source to use data packets up to size
d'. If the ICMPv6 PTB message is not received, the DPLPMTU will be
the last confirmed probe size, d.
----Packets of data size (a) ----------------------------->
----Connectivity probe with MinPMTU --------X
----Packets of data size (a) ----------------------------->
----Probe size (b) --------------------------------------->
<---------------------------------- ACK of probe --------
----Packets of data size (b) ----------------------------->
----Probe size (c) --------------------------------------->
<---------------------------------- ACK of probe --------
----Packets of data size (c) ----------------------------->
----Probe size (d) --------------------------------------->
<---------------------------------- ACK of probe --------
----Packets of data size (d) ----------------------------->
----Probe size (e) ----------X
<--ICMPv6 PTB PTB_SIZE(d') -|
----Packets of data size (d) ----------------------------->
----Probe size (d') using target set by PTB_SIZE --------->
<---------------------------------- ACK of probe --------
Search phase completes.
----Packets of data size (d') ---------------------------->
Figure 6
The number of probe rounds depends on the number of steps needed by
the search algorithm, and is typically larger for a larger PMTU.
Authors' Addresses
Robert M. Hinden
Check Point Software
959 Skyway Road
San Carlos, CA 94070
United States of America
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Email: bob.hinden@gmail.com
Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom
Email: gorry@erg.abdn.ac.uk
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