draft-ietf-6man-rfc1981bis-08.txt   rfc8201.txt 
Network Working Group J. McCann Internet Engineering Task Force (IETF) J. McCann
Internet-Draft Digital Equipment Corporation Request for Comments: 8201 Digital Equipment Corporation
Obsoletes: 1981 (if approved) S. Deering STD: 87 S. Deering
Intended status: Standards Track Retired Obsoletes: 1981 Retired
Expires: November 28, 2017 J. Mogul Category: Standards Track J. Mogul
Digital Equipment Corporation ISSN: 2070-1721 Digital Equipment Corporation
R. Hinden, Ed. R. Hinden, Ed.
Check Point Software Check Point Software
May 27, 2017 July 2017
Path MTU Discovery for IP version 6 Path MTU Discovery for IP version 6
draft-ietf-6man-rfc1981bis-08
Abstract Abstract
This document describes Path MTU Discovery for IP version 6. It is This document describes Path MTU Discovery (PMTUD) for IP version 6.
largely derived from RFC 1191, which describes Path MTU Discovery for It is largely derived from RFC 1191, which describes Path MTU
IP version 4. It obsoletes RFC1981. Discovery for IP version 4. It obsoletes RFC 1981.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on November 28, 2017. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8201.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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skipping to change at page 2, line 22 skipping to change at page 3, line 7
modifications of such material outside the IETF Standards Process. modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling Without obtaining an adequate license from the person(s) controlling
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outside the IETF Standards Process, and derivative works of it may outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other it for publication as an RFC or to translate it into languages other
than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 5 3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6
4. Protocol Requirements . . . . . . . . . . . . . . . . . . . . 6 4. Protocol Requirements . . . . . . . . . . . . . . . . . . . . 7
5. Implementation Issues . . . . . . . . . . . . . . . . . . . . 7 5. Implementation Issues . . . . . . . . . . . . . . . . . . . . 8
5.1. Layering . . . . . . . . . . . . . . . . . . . . . . . . 7 5.1. Layering . . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Storing PMTU information . . . . . . . . . . . . . . . . 8 5.2. Storing PMTU Information . . . . . . . . . . . . . . . . 9
5.3. Purging stale PMTU information . . . . . . . . . . . . . 10 5.3. Purging Stale PMTU Information . . . . . . . . . . . . . 11
5.4. Packetization layer actions . . . . . . . . . . . . . . . 11 5.4. Packetization Layer Actions . . . . . . . . . . . . . . . 12
5.5. Issues for other transport protocols . . . . . . . . . . 12 5.5. Issues for Other Transport Protocols . . . . . . . . . . 13
5.6. Management interface . . . . . . . . . . . . . . . . . . 12 5.6. Management Interface . . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 8.1. Normative References . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . 14 8.2. Informative References . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . 14 Appendix A. Comparison to RFC 1191 . . . . . . . . . . . . . . . 17
Appendix A. Comparison to RFC 1191 . . . . . . . . . . . . . . . 15 Appendix B. Changes Since RFC 1981 . . . . . . . . . . . . . . . 17
Appendix B. Changes Since RFC 1981 . . . . . . . . . . . . . . . 16 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 19
B.1. Change History Since RFC1981 . . . . . . . . . . . . . . 17 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction 1. Introduction
When one IPv6 node has a large amount of data to send to another When one IPv6 node has a large amount of data to send to another
node, the data is transmitted in a series of IPv6 packets. These node, the data is transmitted in a series of IPv6 packets. These
packets can have a size less than or equal to the Path MTU (PMTU). packets can have a size less than or equal to the Path MTU (PMTU).
Alternatively, they can be larger packets that are fragmented into a Alternatively, they can be larger packets that are fragmented into a
series of fragments each with a size less than or equal to the PMTU. series of fragments each with a size less than or equal to the PMTU.
It is usually preferable that these packets be of the largest size It is usually preferable that these packets be of the largest size
that can successfully traverse the path from the source node to the that can successfully traverse the path from the source node to the
destination node without the need for IPv6 fragmentation. This destination node without the need for IPv6 fragmentation. This
packet size is referred to as the Path MTU, and it is equal to the packet size is referred to as the Path MTU, and it is equal to the
minimum link MTU of all the links in a path. This document defines a minimum link MTU of all the links in a path. This document defines a
standard mechanism for a node to discover the PMTU of an arbitrary standard mechanism for a node to discover the PMTU of an arbitrary
path. path.
IPv6 nodes should implement Path MTU Discovery in order to discover IPv6 nodes should implement Path MTU Discovery in order to discover
and take advantage of paths with PMTU greater than the IPv6 minimum and take advantage of paths with PMTU greater than the IPv6 minimum
link MTU [I-D.ietf-6man-rfc2460bis]. A minimal IPv6 implementation link MTU [RFC8200]. A minimal IPv6 implementation (e.g., in a boot
(e.g., in a boot ROM) may choose to omit implementation of Path MTU ROM) may choose to omit implementation of Path MTU Discovery.
Discovery.
Nodes not implementing Path MTU Discovery must use the IPv6 minimum Nodes not implementing Path MTU Discovery must use the IPv6 minimum
link MTU defined in [I-D.ietf-6man-rfc2460bis] as the maximum packet link MTU defined in [RFC8200] as the maximum packet size. In most
size. In most cases, this will result in the use of smaller packets cases, this will result in the use of smaller packets than necessary,
than necessary, because most paths have a PMTU greater than the IPv6 because most paths have a PMTU greater than the IPv6 minimum link
minimum link MTU. A node sending packets much smaller than the Path MTU. A node sending packets much smaller than the Path MTU allows is
MTU allows is wasting network resources and probably getting wasting network resources and probably getting suboptimal throughput.
suboptimal throughput.
Nodes implementing Path MTU Discovery and sending packets larger than Nodes implementing Path MTU Discovery and sending packets larger than
the IPv6 minimum link MTU are susceptible to problematic connectivity the IPv6 minimum link MTU are susceptible to problematic connectivity
if ICMPv6 [ICMPv6] messages are blocked or not transmitted. For if ICMPv6 [ICMPv6] messages are blocked or not transmitted. For
example, this will result in connections that complete the TCP three- example, this will result in connections that complete the TCP three-
way handshake correctly but then hang when data is transferred. This way handshake correctly but then hang when data is transferred. This
state is referred to as a black hole connection [RFC2923]. Path MTU state is referred to as a black-hole connection [RFC2923]. Path MTU
Discovery relies on ICMPv6 Packet Too Big (PTB) to determine the MTU Discovery relies on ICMPv6 Packet Too Big (PTB) to determine the MTU
of the path. of the path.
An extension to Path MTU Discovery defined in this document can be An extension to Path MTU Discovery defined in this document can be
found in [RFC4821]. RFC4821 defines a method for Packetization Layer found in [RFC4821]. RFC 4821 defines a method for Packetization
Path MTU Discovery (PLPMTUD) designed for use over paths where Layer Path MTU Discovery (PLPMTUD) designed for use over paths where
delivery of ICMPv6 messages to a host is not assured. delivery of ICMPv6 messages to a host is not assured.
Note: This document is an update to [RFC1981] that was published Note: This document is an update to [RFC1981] that was published
prior to [RFC2119] being published. Consequently although RFC1981 prior to [RFC2119] being published. Consequently, although RFC 1981
used the "should/must" style language in upper and lower case, this used the "should/must" style language in upper and lower case, this
document does not cite the RFC2119 definitions and only uses lower document does not cite the RFC 2119 definitions and only uses lower
case for these words. case for these words.
2. Terminology 2. Terminology
node a device that implements IPv6. node a device that implements IPv6.
router a node that forwards IPv6 packets not explicitly router a node that forwards IPv6 packets not explicitly
addressed to itself. addressed to itself.
host any node that is not a router. host any node that is not a router.
upper layer a protocol layer immediately above IPv6. upper layer a protocol layer immediately above IPv6.
Examples are transport protocols such as TCP and Examples are transport protocols such as TCP and
UDP, control protocols such as ICMPv6, routing UDP, control protocols such as ICMPv6, routing
protocols such as OSPF, and internet or lower- protocols such as OSPF, and internet-layer or
layer protocols being "tunneled" over (i.e., lower-layer protocols being "tunneled" over
encapsulated in) IPv6 such as IPX, AppleTalk, or (i.e., encapsulated in) IPv6 such as Internetwork
IPv6 itself. Packet Exchange (IPX), AppleTalk, or IPv6 itself.
link a communication facility or medium over which link a communication facility or medium over which
nodes can communicate at the link layer, i.e., nodes can communicate at the link layer, i.e.,
the layer immediately below IPv6. Examples are the layer immediately below IPv6. Examples are
Ethernets (simple or bridged); PPP links; X.25, Ethernets (simple or bridged); PPP links; X.25,
Frame Relay, or ATM networks; and internet (or Frame Relay, or ATM networks; and internet-layer
higher) layer "tunnels", such as tunnels over or higher-layer "tunnels", such as tunnels over
IPv4 or IPv6 itself. IPv4 or IPv6 itself.
interface a node's attachment to a link. interface a node's attachment to a link.
address an IPv6-layer identifier for an interface or a address an IPv6-layer identifier for an interface or a
set of interfaces. set of interfaces.
packet an IPv6 header plus payload. The packet can have packet an IPv6 header plus payload. The packet can have
a size less than or equal to the PMTU. a size less than or equal to the PMTU.
Alternatively, this can be a larger packet that Alternatively, this can be a larger packet that
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link MTU the maximum transmission unit, i.e., maximum link MTU the maximum transmission unit, i.e., maximum
packet size in octets, that can be conveyed in packet size in octets, that can be conveyed in
one piece over a link. one piece over a link.
path the set of links traversed by a packet between a path the set of links traversed by a packet between a
source node and a destination node. source node and a destination node.
path MTU the minimum link MTU of all the links in a path path MTU the minimum link MTU of all the links in a path
between a source node and a destination node. between a source node and a destination node.
PMTU path MTU PMTU path MTU.
Path MTU Discovery process by which a node learns the PMTU of a path Path MTU Discovery the process by which a node learns the PMTU of a
path.
EMTU_S Effective MTU for sending, used by upper layer EMTU_S Effective MTU for sending; used by upper-layer
protocols to limit the size of IP packets they protocols to limit the size of IP packets they
queue for sending [RFC6691] [RFC1122]. queue for sending [RFC6691] [RFC1122].
EMTU_R Effective MTU for receiving, the largest packet EMTU_R Effective MTU for receiving; the largest packet
that can be reassembled at the receiver that can be reassembled at the receiver
[RFC1122]. [RFC1122].
flow a sequence of packets sent from a particular flow a sequence of packets sent from a particular
source to a particular (unicast or multicast) source to a particular (unicast or multicast)
destination for which the source desires special destination for which the source desires special
handling by the intervening routers. handling by the intervening routers.
flow id a combination of a source address and a non-zero flow id a combination of a source address and a non-zero
flow label. flow label.
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This memo describes a technique to dynamically discover the PMTU of a This memo describes a technique to dynamically discover the PMTU of a
path. The basic idea is that a source node initially assumes that path. The basic idea is that a source node initially assumes that
the PMTU of a path is the (known) MTU of the first hop in the path. the PMTU of a path is the (known) MTU of the first hop in the path.
If any of the packets sent on that path are too large to be forwarded If any of the packets sent on that path are too large to be forwarded
by some node along the path, that node will discard them and return by some node along the path, that node will discard them and return
ICMPv6 Packet Too Big messages. Upon receipt of such a message, the ICMPv6 Packet Too Big messages. Upon receipt of such a message, the
source node reduces its assumed PMTU for the path based on the MTU of source node reduces its assumed PMTU for the path based on the MTU of
the constricting hop as reported in the Packet Too Big message. The the constricting hop as reported in the Packet Too Big message. The
decreased PMTU causes the source to send smaller packets or change decreased PMTU causes the source to send smaller packets or change
EMTU_S to cause upper layer to reduce the size of IP packets it EMTU_S to cause the upper layer to reduce the size of IP packets it
sends. sends.
The Path MTU Discovery process ends when the source node's estimate The Path MTU Discovery process ends when the source node's estimate
of the PMTU is less than or equal to the actual PMTU. Note that of the PMTU is less than or equal to the actual PMTU. Note that
several iterations of the packet-sent/Packet-Too-Big-message-received several iterations of the packet-sent/Packet-Too-Big-message-received
cycle may occur before the Path MTU Discovery process ends, as there cycle may occur before the Path MTU Discovery process ends, as there
may be links with smaller MTUs further along the path. may be links with smaller MTUs further along the path.
Alternatively, the node may elect to end the discovery process by Alternatively, the node may elect to end the discovery process by
ceasing to send packets larger than the IPv6 minimum link MTU. ceasing to send packets larger than the IPv6 minimum link MTU.
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destinations. In the case of a multicast destination, copies of a destinations. In the case of a multicast destination, copies of a
packet may traverse many different paths to many different nodes. packet may traverse many different paths to many different nodes.
Each path may have a different PMTU, and a single multicast packet Each path may have a different PMTU, and a single multicast packet
may result in multiple Packet Too Big messages, each reporting a may result in multiple Packet Too Big messages, each reporting a
different next-hop MTU. The minimum PMTU value across the set of different next-hop MTU. The minimum PMTU value across the set of
paths in use determines the size of subsequent packets sent to the paths in use determines the size of subsequent packets sent to the
multicast destination. multicast destination.
Note that Path MTU Discovery must be performed even in cases where a Note that Path MTU Discovery must be performed even in cases where a
node "thinks" a destination is attached to the same link as itself, node "thinks" a destination is attached to the same link as itself,
it might have a PMTU lower than the link MTU. In a situation such as as it might have a PMTU lower than the link MTU. In a situation such
when a neighboring router acts as proxy [ND] for some destination, as when a neighboring router acts as proxy [ND] for some destination,
the destination can appear to be directly connected but it is in fact the destination can appear to be directly connected, but it is in
more than one hop away. fact more than one hop away.
4. Protocol Requirements 4. Protocol Requirements
As discussed in Section 1, IPv6 nodes are not required to implement As discussed in Section 1, IPv6 nodes are not required to implement
Path MTU Discovery. The requirements in this section apply only to Path MTU Discovery. The requirements in this section apply only to
those implementations that include Path MTU Discovery. those implementations that include Path MTU Discovery.
Nodes should appropriately validate the payload of ICMPv6 PTB Nodes should appropriately validate the payload of ICMPv6 PTB
messages to ensure these are received in response to transmitted messages to ensure these are received in response to transmitted
traffic (i.e., a reported error condition that corresponds to an IPv6 traffic (i.e., a reported error condition that corresponds to an IPv6
packet actually sent by the application) per [ICMPv6]. packet actually sent by the application) per [ICMPv6].
If a node receives a Packet Too Big message reporting a next-hop MTU If a node receives a Packet Too Big message reporting a next-hop MTU
that is less than the IPv6 minimum link MTU, it must discard it. A that is less than the IPv6 minimum link MTU, it must discard it. A
node must not reduce its estimate of the Path MTU below the IPv6 node must not reduce its estimate of the Path MTU below the IPv6
minimum link MTU on receipt of an Packet Too Big message. minimum link MTU on receipt of a Packet Too Big message.
When a node receives a Packet Too Big message, it must reduce its When a node receives a Packet Too Big message, it must reduce its
estimate of the PMTU for the relevant path, based on the value of the estimate of the PMTU for the relevant path, based on the value of the
MTU field in the message. The precise behavior of a node in this MTU field in the message. The precise behavior of a node in this
circumstance is not specified, since different applications may have circumstance is not specified, since different applications may have
different requirements, and since different implementation different requirements, and since different implementation
architectures may favor different strategies. architectures may favor different strategies.
After receiving a Packet Too Big message, a node must attempt to After receiving a Packet Too Big message, a node must attempt to
avoid eliciting more such messages in the near future. The node must avoid eliciting more such messages in the near future. The node must
reduce the size of the packets it is sending along the path. Using a reduce the size of the packets it is sending along the path. Using a
PMTU estimate larger than the IPv6 minimum link MTU may continue to PMTU estimate larger than the IPv6 minimum link MTU may continue to
elicit Packet Too Big messages. Because each of these messages (and elicit Packet Too Big messages. Because each of these messages (and
the dropped packets they respond to) consume network resources, Nodes the dropped packets they respond to) consume network resources, nodes
using Path MTU Discovery must detect decreases in PMTU as fast as using Path MTU Discovery must detect decreases in PMTU as fast as
possible. possible.
Nodes may detect increases in PMTU, but because doing so requires Nodes may detect increases in PMTU, but because doing so requires
sending packets larger than the current estimated PMTU, and because sending packets larger than the current estimated PMTU, and because
the likelihood is that the PMTU will not have increased, this must be the likelihood is that the PMTU will not have increased, this must be
done at infrequent intervals. An attempt to detect an increase (by done at infrequent intervals. An attempt to detect an increase (by
sending a packet larger than the current estimate) must not be done sending a packet larger than the current estimate) must not be done
less than 5 minutes after a Packet Too Big message has been received less than 5 minutes after a Packet Too Big message has been received
for the given path. The recommended setting for this timer is twice for the given path. The recommended setting for this timer is twice
its minimum value (10 minutes). its minimum value (10 minutes).
A node must not increase its estimate of the Path MTU in response to A node must not increase its estimate of the Path MTU in response to
the contents of a Packet Too Big message. A message purporting to the contents of a Packet Too Big message. A message purporting to
announce an increase in the Path MTU might be a stale packet that has announce an increase in the Path MTU might be a stale packet that has
been floating around in the network, a false packet injected as part been floating around in the network, a false packet injected as part
of a denial-of-service attack, or the result of having multiple paths of a denial-of-service (DoS) attack, or the result of having multiple
to the destination, each with a different PMTU. paths to the destination, each with a different PMTU.
5. Implementation Issues 5. Implementation Issues
This section discusses a number of issues related to the This section discusses a number of issues related to the
implementation of Path MTU Discovery. This is not a specification, implementation of Path MTU Discovery. This is not a specification,
but rather a set of notes provided as an aid for implementers. but rather a set of notes provided as an aid for implementers.
The issues include: The issues include:
- What layer or layers implement Path MTU Discovery? - What layer or layers implement Path MTU Discovery?
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5.1. Layering 5.1. Layering
In the IP architecture, the choice of what size packet to send is In the IP architecture, the choice of what size packet to send is
made by a protocol at a layer above IP. This memo refers to such a made by a protocol at a layer above IP. This memo refers to such a
protocol as a "packetization protocol". Packetization protocols are protocol as a "packetization protocol". Packetization protocols are
usually transport protocols (for example, TCP) but can also be usually transport protocols (for example, TCP) but can also be
higher-layer protocols (for example, protocols built on top of UDP). higher-layer protocols (for example, protocols built on top of UDP).
Implementing Path MTU Discovery in the packetization layers Implementing Path MTU Discovery in the packetization layers
simplifies some of the inter-layer issues, but has several drawbacks: simplifies some of the inter-layer issues but has several drawbacks:
the implementation may have to be redone for each packetization the implementation may have to be redone for each packetization
protocol, it becomes hard to share PMTU information between different protocol, it becomes hard to share PMTU information between different
packetization layers, and the connection-oriented state maintained by packetization layers, and the connection-oriented state maintained by
some packetization layers may not easily extend to save PMTU some packetization layers may not easily extend to save PMTU
information for long periods. information for long periods.
It is therefore suggested that the IP layer store PMTU information It is therefore suggested that the IP layer store PMTU information
and that the ICMPv6 layer process received Packet Too Big messages. and that the ICMPv6 layer process received Packet Too Big messages.
The packetization layers may respond to changes in the PMTU by The packetization layers may respond to changes in the PMTU by
changing the size of the messages they send. To support this changing the size of the messages they send. To support this
layering, packetization layers require a way to learn of changes in layering, packetization layers require a way to learn of changes in
the value of MMS_S, the "maximum send transport-message size" the value of MMS_S, the "maximum send transport-message size"
[RFC1122]. [RFC1122].
MMS_S is a transport message size calculated by subtracting the size MMS_S is a transport message size calculated by subtracting the size
of the IPv6 header (including IPv6 extension headers) from the of the IPv6 header (including IPv6 extension headers) from the
largest IP packet that can be sent, EMTU_S. MMS_S is limited by a largest IP packet that can be sent, EMTU_S. MMS_S is limited by a
combination of factors, including the PMTU, support for packet combination of factors, including the PMTU, support for packet
fragmentation and reassembly, and the packet reassembly limit (see fragmentation and reassembly, and the packet reassembly limit (see
[I-D.ietf-6man-rfc2460bis] section "Fragment Header"). When source "Fragment Header", Section 4.5 of [RFC8200]). When source
fragmentation is available, EMTU_S is set to EMTU_R, as indicated by fragmentation is available, EMTU_S is set to EMTU_R, as indicated by
the receiver using an upper layer protocol or based on protocol the receiver using an upper-layer protocol or based on protocol
requirements (1500 octets for IPv6). When a message larger than PMTU requirements (1500 octets for IPv6). When a message larger than PMTU
is to be transmitted, the source creates fragments, each limited by is to be transmitted, the source creates fragments, each limited by
PMTU. When source fragmentation is not desired, EMTU_S is set to PMTU. When source fragmentation is not desired, EMTU_S is set to
PMTU, and the upper layer protocol is expected to either perform its PMTU, and the upper-layer protocol is expected to either perform its
own fragmentation and reassembly or otherwise limit the size of its own fragmentation and reassembly or otherwise limit the size of its
messages accordingly. messages accordingly.
However, packetization layers are encouraged to avoid sending However, packetization layers are encouraged to avoid sending
messages that will require source fragmentation (for the case against messages that will require source fragmentation (for the case against
fragmentation, see [FRAG]). fragmentation, see [FRAG]).
5.2. Storing PMTU information 5.2. Storing PMTU Information
Ideally, a PMTU value should be associated with a specific path Ideally, a PMTU value should be associated with a specific path
traversed by packets exchanged between the source and destination traversed by packets exchanged between the source and destination
nodes. However, in most cases a node will not have enough nodes. However, in most cases a node will not have enough
information to completely and accurately identify such a path. information to completely and accurately identify such a path.
Rather, a node must associate a PMTU value with some local Rather, a node must associate a PMTU value with some local
representation of a path. It is left to the implementation to select representation of a path. It is left to the implementation to select
the local representation of a path. For nodes with multiple the local representation of a path. For nodes with multiple
interfaces, Path MTU information should be maintained for each IPv6 interfaces, Path MTU information should be maintained for each IPv6
link. link.
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In the case of a multicast destination address, copies of a packet In the case of a multicast destination address, copies of a packet
may traverse many different paths to reach many different nodes. The may traverse many different paths to reach many different nodes. The
local representation of the "path" to a multicast destination must local representation of the "path" to a multicast destination must
represent a potentially large set of paths. represent a potentially large set of paths.
Minimally, an implementation could maintain a single PMTU value to be Minimally, an implementation could maintain a single PMTU value to be
used for all packets originated from the node. This PMTU value would used for all packets originated from the node. This PMTU value would
be the minimum PMTU learned across the set of all paths in use by the be the minimum PMTU learned across the set of all paths in use by the
node. This approach is likely to result in the use of smaller node. This approach is likely to result in the use of smaller
packets than is necessary for many paths. In the case of multipath packets than is necessary for many paths. In the case of multipath
routing (e.g., Equal Cost Multipath Routing (ECMP) ), a set of paths routing (e.g., Equal-Cost Multipath Routing (ECMP)), a set of paths
can exist even for a single source and destination pair. can exist even for a single source and destination pair.
An implementation could use the destination address as the local An implementation could use the destination address as the local
representation of a path. The PMTU value associated with a representation of a path. The PMTU value associated with a
destination would be the minimum PMTU learned across the set of all destination would be the minimum PMTU learned across the set of all
paths in use to that destination. This approach will result in the paths in use to that destination. This approach will result in the
use of optimally sized packets on a per-destination basis. This use of optimally sized packets on a per-destination basis. This
approach integrates nicely with the conceptual model of a host as approach integrates nicely with the conceptual model of a host as
described in [ND]: a PMTU value could be stored with the described in [ND]: a PMTU value could be stored with the
corresponding entry in the destination cache. corresponding entry in the destination cache.
If flows [I-D.ietf-6man-rfc2460bis] are in use, an implementation If flows [RFC8200] are in use, an implementation could use the flow
could use the flow id as the local representation of a path. Packets id as the local representation of a path. Packets sent to a
sent to a particular destination but belonging to different flows may particular destination but belonging to different flows may use
use different paths, as with ECMP, in which the choice of path might different paths, as with ECMP, in which the choice of path might
depending on the flow id. This approach might result in the use of depend on the flow id. This approach might result in the use of
optimally sized packets on a per-flow basis, providing finer optimally sized packets on a per-flow basis, providing finer
granularity than PMTU values maintained on a per-destination basis. granularity than PMTU values maintained on a per-destination basis.
For source routed packets (i.e. packets containing an IPv6 Routing For source-routed packets (i.e. packets containing an IPv6 Routing
header [I-D.ietf-6man-rfc2460bis]), the source route may further header [RFC8200]), the source route may further qualify the local
qualify the local representation of a path. representation of a path.
Initially, the PMTU value for a path is assumed to be the (known) MTU Initially, the PMTU value for a path is assumed to be the (known) MTU
of the first-hop link. of the first-hop link.
When a Packet Too Big message is received, the node determines which When a Packet Too Big message is received, the node determines which
path the message applies to based on the contents of the Packet Too path the message applies to based on the contents of the Packet Too
Big message. For example, if the destination address is used as the Big message. For example, if the destination address is used as the
local representation of a path, the destination address from the local representation of a path, the destination address from the
original packet would be used to determine which path the message original packet would be used to determine which path the message
applies to. applies to.
skipping to change at page 10, line 36 skipping to change at page 11, line 46
recover from the dropped packets. recover from the dropped packets.
It is important to understand that the notification of the It is important to understand that the notification of the
packetization layer instances using the path about the change in the packetization layer instances using the path about the change in the
PMTU is distinct from the notification of a specific instance that a PMTU is distinct from the notification of a specific instance that a
packet has been dropped. The latter should be done as soon as packet has been dropped. The latter should be done as soon as
practical (i.e., asynchronously from the point of view of the practical (i.e., asynchronously from the point of view of the
packetization layer instance), while the former may be delayed until packetization layer instance), while the former may be delayed until
a packetization layer instance wants to create a packet. a packetization layer instance wants to create a packet.
5.3. Purging stale PMTU information 5.3. Purging Stale PMTU Information
Internetwork topology is dynamic; routes change over time. While the Internetwork topology is dynamic; routes change over time. While the
local representation of a path may remain constant, the actual local representation of a path may remain constant, the actual
path(s) in use may change. Thus, PMTU information cached by a node path(s) in use may change. Thus, PMTU information cached by a node
can become stale. can become stale.
If the stale PMTU value is too large, this will be discovered almost If the stale PMTU value is too large, this will be discovered almost
immediately once a large enough packet is sent on the path. No such immediately once a large enough packet is sent on the path. No such
mechanism exists for realizing that a stale PMTU value is too small, mechanism exists for realizing that a stale PMTU value is too small,
so an implementation should "age" cached values. When a PMTU value so an implementation should "age" cached values. When a PMTU value
has not been decreased for a while (on the order of 10 minutes), it has not been decreased for a while (on the order of 10 minutes), it
should probe to find if a larger PMTU is supported. should probe to find if a larger PMTU is supported.
Note: an implementation should provide a means for changing the Note: an implementation should provide a means for changing the
timeout duration, including setting it to "infinity". For timeout duration, including setting it to "infinity". For
example, nodes attached to a link with a large MTU which is then example, nodes attached to a link with a large MTU that is then
attached to the rest of the Internet via a link with a small MTU attached to the rest of the Internet via a link with a small MTU
are never going to discover a new non-local PMTU, so they should are never going to discover a new non-local PMTU, so they should
not have to put up with dropped packets every 10 minutes. not have to put up with dropped packets every 10 minutes.
5.4. Packetization layer actions 5.4. Packetization Layer Actions
A packetization layer (e.g., TCP) must use the PMTU for the path(s) A packetization layer (e.g., TCP) must use the PMTU for the path(s)
in use by a connection; it should not send segments that would result in use by a connection; it should not send segments that would result
in packets larger than the PMTU, except to probe during PMTU in packets larger than the PMTU, except to probe during PMTU
discovery (this probe packet must not be fragmented to the PMTU). A Discovery (this probe packet must not be fragmented to the PMTU). A
simple implementation could ask the IP layer for this value each time simple implementation could ask the IP layer for this value each time
it created a new segment, but this could be inefficient. An it created a new segment, but this could be inefficient. An
implementation typically caches other values derived from the PMTU. implementation typically caches other values derived from the PMTU.
It may be simpler to receive asynchronous notification when the PMTU It may be simpler to receive asynchronous notification when the PMTU
changes, so that these variables may be also updated. changes, so that these variables may be also updated.
A TCP implementation must also store the Maximum Segment Size (MSS) A TCP implementation must also store the Maximum Segment Size (MSS)
value received from its peer, which represents the EMTU_R, the value received from its peer, which represents the EMTU_R, the
largest packet that can be reassembled by the receiver, and must not largest packet that can be reassembled by the receiver, and must not
send any segment larger than this MSS, regardless of the PMTU. send any segment larger than this MSS, regardless of the PMTU.
The value sent in the TCP MSS option is independent of the PMTU; it The value sent in the TCP MSS option is independent of the PMTU; it
is determined by the receiver reassembly limit EMTU_R. This MSS is determined by the receiver reassembly limit EMTU_R. This MSS
option value is used by the other end of the connection, which may be option value is used by the other end of the connection, which may be
using an unrelated PMTU value. See [I-D.ietf-6man-rfc2460bis] using an unrelated PMTU value. See Section 5, "Packet Size Issues",
sections "Packet Size Issues" and "Maximum Upper-Layer Payload Size" and Section 8.3, "Maximum Upper-Layer Payload Size", of [RFC8200] for
for information on selecting a value for the TCP MSS option. information on selecting a value for the TCP MSS option.
Reception of a Packet Too Big message implies that a packet was Reception of a Packet Too Big message implies that a packet was
dropped by the node that sent the ICMPv6 message. A reliable upper dropped by the node that sent the ICMPv6 message. A reliable upper-
layer protocol will detect this loss by its own means, and recover it layer protocol will detect this loss by its own means, and recover it
by its normal retransmission methods. The retransmission could by its normal retransmission methods. The retransmission could
result in delay, depending on the loss detection method used by the result in delay, depending on the loss detection method used by the
upper layer protocol. If the Path MTU Discovery process requires upper-layer protocol. If the Path MTU Discovery process requires
several steps to find the PMTU of the full path, this could finally several steps to find the PMTU of the full path, this could finally
delay the retransmission by many round-trip times. delay the retransmission by many round-trip times.
Alternatively, the retransmission could be done in immediate response Alternatively, the retransmission could be done in immediate response
to a notification that the Path MTU was decreased, but only for the to a notification that the Path MTU was decreased, but only for the
specific connection specified by the Packet Too Big message, but only specific connection specified by the Packet Too Big message. The
based on the message and connection. The packet size used in the packet size used in the retransmission should be no larger than the
retransmission should be no larger than the new PMTU. new PMTU.
Note: A packetization layer that determines a probe packet is Note: A packetization layer that determines a probe packet is lost
lost, needs to adapt the segment size of the retransmission. needs to adapt the segment size of the retransmission. Using the
Using the reported size in the last Packet Too Big message, reported size in the last Packet Too Big message, however, can
however, can lead to further losses as there might be smaller PMTU lead to further losses as there might be smaller PMTU limits at
limits at the routers further along the path. This would lead to the routers further along the path. This would lead to loss of
loss of all retransmitted segments and therefore cause unnecessary all retransmitted segments and therefore cause unnecessary
congestion as well as additional packets to be sent each time a congestion as well as additional packets to be sent each time a
new router announces a smaller MTU. Any packetization layer that new router announces a smaller MTU. Any packetization layer that
uses retransmission is therefore also responsible for congestion uses retransmission is therefore also responsible for congestion
control of its retransmissions [RFC8085]. control of its retransmissions [RFC8085].
A loss caused by a PMTU probe indicated by the reception of a Packet A loss caused by a PMTU probe indicated by the reception of a Packet
Too Big message must not be considered as a congestion notification Too Big message must not be considered as a congestion notification,
and hence the congestion window may not change. and hence the congestion window may not change.
5.5. Issues for other transport protocols 5.5. Issues for Other Transport Protocols
Some transport protocols are not allowed to repacketize when doing a Some transport protocols are not allowed to repacketize when doing a
retransmission. That is, once an attempt is made to transmit a retransmission. That is, once an attempt is made to transmit a
segment of a certain size, the transport cannot split the contents of segment of a certain size, the transport cannot split the contents of
the segment into smaller segments for retransmission. In such a the segment into smaller segments for retransmission. In such a
case, the original segment can be fragmented by the IP layer during case, the original segment can be fragmented by the IP layer during
retransmission. Subsequent segments, when transmitted for the first retransmission. Subsequent segments, when transmitted for the first
time, should be no larger than allowed by the Path MTU. time, should be no larger than allowed by the Path MTU.
Path MTU Discovery for IPv4 [RFC1191] used NFS as an example of a Path MTU Discovery for IPv4 [RFC1191] used NFS as an example of a
UDP-based application that benefits from PMTU discovery. Since then UDP-based application that benefits from PMTU Discovery. Since then,
[RFC7530], states the supported transport layer between NFS and IP [RFC7530] states that the supported transport layer between NFS and
must be an IETF standardized transport protocol that is specified to IP must be an IETF standardized transport protocol that is specified
avoid network congestion; such transports include TCP, Stream Control to avoid network congestion; such transports include TCP, Stream
Transmission Protocol (SCTP) [RFC4960], and the Datagram Congestion Control Transmission Protocol (SCTP) [RFC4960], and the Datagram
Control Protocol (DCCP) [RFC4340]. In this case, the transport is Congestion Control Protocol (DCCP) [RFC4340]. In this case, the
responsible for ensuring that transmitted segments (except probes) transport is responsible for ensuring that transmitted segments
conform to the the Path MTU, including supporting PMTU discovery (except probes) conform to the Path MTU, including supporting PMTU
probe transmissions as needed. Discovery probe transmissions as needed.
5.6. Management interface 5.6. Management Interface
It is suggested that an implementation provide a way for a system It is suggested that an implementation provides a way for a system
utility program to: utility program to:
- Specify that Path MTU Discovery not be done on a given path. - Specify that Path MTU Discovery not be done on a given path.
- Change the PMTU value associated with a given path. - Change the PMTU value associated with a given path.
The former can be accomplished by associating a flag with the path; The former can be accomplished by associating a flag with the path;
when a packet is sent on a path with this flag set, the IP layer does when a packet is sent on a path with this flag set, the IP layer does
not send packets larger than the IPv6 minimum link MTU. not send packets larger than the IPv6 minimum link MTU.
These features might be used to work around an anomalous situation, These features might be used to work around an anomalous situation or
or by a routing protocol implementation that is able to obtain Path by a routing protocol implementation that is able to obtain Path MTU
MTU values. values.
The implementation should also provide a way to change the timeout The implementation should also provide a way to change the timeout
period for aging stale PMTU information. period for aging stale PMTU information.
6. Security Considerations 6. Security Considerations
This Path MTU Discovery mechanism makes possible two denial-of- This Path MTU Discovery mechanism makes possible two DoS attacks,
service attacks, both based on a malicious party sending false Packet both based on a malicious party sending false Packet Too Big messages
Too Big messages to a node. to a node.
In the first attack, the false message indicates a PMTU much In the first attack, the false message indicates a PMTU much
smaller than reality. In response, the victim node should never smaller than reality. In response, the victim node should never
set its PMTU estimate below the IPv6 minimum link MTU. A sender set its PMTU estimate below the IPv6 minimum link MTU. A sender
that falsely reduces to this MTU would observe suboptimal that falsely reduces to this MTU would observe suboptimal
performance. performance.
In the second attack, the false message indicates a PMTU larger In the second attack, the false message indicates a PMTU larger
than reality. If believed, this could cause temporary blockage as than reality. If believed, this could cause temporary blockage as
the victim sends packets that will be dropped by some router. the victim sends packets that will be dropped by some router.
Within one round-trip time, the node would discover its mistake Within one round-trip time, the node would discover its mistake
(receiving Packet Too Big messages from that router), but frequent (receiving Packet Too Big messages from that router), but frequent
repetition of this attack could cause lots of packets to be repetition of this attack could cause lots of packets to be
dropped. A node, however, must not raise its estimate of the PMTU dropped. A node, however, must not raise its estimate of the PMTU
based on a Packet Too Big message, so should not be vulnerable to based on a Packet Too Big message, so it should not be vulnerable
this attack. to this attack.
Both of these attacks can cause a black hole connection, that is, the Both of these attacks can cause a black-hole connection, that is, the
TCP three-way handshake completes correctly but the connection hangs TCP three-way handshake completes correctly but the connection hangs
when data is transfered. when data is transferred.
A malicious party could also cause problems if it could stop a victim A malicious party could also cause problems if it could stop a victim
from receiving legitimate Packet Too Big messages, but in this case from receiving legitimate Packet Too Big messages, but in this case
there are simpler denial-of-service attacks available. there are simpler DoS attacks available.
If ICMPv6 filtering prevents reception of ICMPv6 Packet Too Big If ICMPv6 filtering prevents reception of ICMPv6 Packet Too Big
messages, the source will not learn the actual path MTU. messages, the source will not learn the actual path MTU.
Packetization Layer Path MTU Discovery [RFC4821] does not rely upon "Packetization Layer Path MTU Discovery" [RFC4821] does not rely upon
network support for ICMPv6 messages and is therefore considered more network support for ICMPv6 messages and is therefore considered more
robust than standard PMTUD. It is not susceptible to "black holed" robust than standard PMTUD. It is not susceptible to "black-holed"
connections caused by filtering of ICMPv6 message. See [RFC4890] for connections caused by the filtering of ICMPv6 messages. See
recommendations regarding filtering ICMPv6 messages. [RFC4890] for recommendations regarding filtering ICMPv6 messages.
7. Acknowledgements
We would like to acknowledge the authors of and contributors to
[RFC1191], from which the majority of this document was derived. We
would also like to acknowledge the members of the IPng working group
for their careful review and constructive criticisms.
We would also like to acknowledge the contributors to this update of
"Path MTU Discovery for IP version 6". This includes members of the
6MAN w.g., area directorate reviewers, the IESG, and especially to
Joe Touch and Gorry Fairhurst.
8. IANA Considerations
This document does not have any IANA actions 7. IANA Considerations
9. References This document does not require any IANA actions.
9.1. Normative References 8. References
[I-D.ietf-6man-rfc2460bis] 8.1. Normative References
Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", draft-ietf-6man-rfc2460bis-13 (work
in progress), May 2017.
[ICMPv6] Conta, A., Deering, S., and M. Gupta, Ed., "Internet [ICMPv6] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", RFC 4443, DOI Protocol Version 6 (IPv6) Specification", STD 89,
10.17487/RFC4443, March 2006, RFC 4443, DOI 10.17487/RFC4443, March 2006,
<http://www.rfc-editor.org/info/rfc4443>. <http://www.rfc-editor.org/info/rfc4443>.
9.2. Informative References [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<http://www.rfc-editor.org/info/rfc8200>.
8.2. Informative References
[FRAG] Kent, C. and J. Mogul, "Fragmentation Considered Harmful", [FRAG] Kent, C. and J. Mogul, "Fragmentation Considered Harmful",
In Proc. SIGCOMM '87 Workshop on Frontiers in Computer In Proc. SIGCOMM '87 Workshop on Frontiers in Computer
Communications Technology , August 1987. Communications Technology, DOI 10.1145/55483.55524, August
1987.
[ND] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, [ND] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007, DOI 10.17487/RFC4861, September 2007,
<http://www.rfc-editor.org/info/rfc4861>. <http://www.rfc-editor.org/info/rfc4861>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, DOI 10.17487/ Communication Layers", STD 3, RFC 1122,
RFC1122, October 1989, DOI 10.17487/RFC1122, October 1989,
<http://www.rfc-editor.org/info/rfc1122>. <http://www.rfc-editor.org/info/rfc1122>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990, DOI 10.17487/RFC1191, November 1990,
<http://www.rfc-editor.org/info/rfc1191>. <http://www.rfc-editor.org/info/rfc1191>.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery [RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
1996, <http://www.rfc-editor.org/info/rfc1981>. 1996, <http://www.rfc-editor.org/info/rfc1981>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ Requirement Levels", BCP 14, RFC 2119,
RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
2923, DOI 10.17487/RFC2923, September 2000, RFC 2923, DOI 10.17487/RFC2923, September 2000,
<http://www.rfc-editor.org/info/rfc2923>. <http://www.rfc-editor.org/info/rfc2923>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, DOI Congestion Control Protocol (DCCP)", RFC 4340,
10.17487/RFC4340, March 2006, DOI 10.17487/RFC4340, March 2006,
<http://www.rfc-editor.org/info/rfc4340>. <http://www.rfc-editor.org/info/rfc4340>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<http://www.rfc-editor.org/info/rfc4821>. <http://www.rfc-editor.org/info/rfc4821>.
[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering [RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
ICMPv6 Messages in Firewalls", RFC 4890, DOI 10.17487/ ICMPv6 Messages in Firewalls", RFC 4890,
RFC4890, May 2007, DOI 10.17487/RFC4890, May 2007,
<http://www.rfc-editor.org/info/rfc4890>. <http://www.rfc-editor.org/info/rfc4890>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007, RFC 4960, DOI 10.17487/RFC4960, September 2007,
<http://www.rfc-editor.org/info/rfc4960>. <http://www.rfc-editor.org/info/rfc4960>.
[RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS)", [RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS)",
RFC 6691, DOI 10.17487/RFC6691, July 2012, RFC 6691, DOI 10.17487/RFC6691, July 2012,
<http://www.rfc-editor.org/info/rfc6691>. <http://www.rfc-editor.org/info/rfc6691>.
[RFC7530] Haynes, T., Ed. and D. Noveck, Ed., "Network File System [RFC7530] Haynes, T., Ed. and D. Noveck, Ed., "Network File System
(NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530, (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
March 2015, <http://www.rfc-editor.org/info/rfc7530>. March 2015, <http://www.rfc-editor.org/info/rfc7530>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <http://www.rfc-editor.org/info/rfc8085>. March 2017, <http://www.rfc-editor.org/info/rfc8085>.
Appendix A. Comparison to RFC 1191 Appendix A. Comparison to RFC 1191
This document is based in large part on RFC 1191, which describes RFC 1981 (obsoleted by this document) was based in large part on RFC
Path MTU Discovery for IPv4. Certain portions of RFC 1191 were not 1191, which describes Path MTU Discovery for IPv4. Certain portions
needed in this document: of RFC 1191 were not needed in RFC 1981:
router specification Packet Too Big messages and corresponding router specification Packet Too Big messages and corresponding
router behavior are defined in [ICMPv6] router behavior are defined in [ICMPv6]
Don't Fragment bit there is no DF bit in IPv6 packets Don't Fragment bit there is no DF bit in IPv6 packets
TCP MSS discussion selecting a value to send in the TCP MSS option TCP MSS discussion selecting a value to send in the TCP MSS option
is discussed in [I-D.ietf-6man-rfc2460bis] is discussed in [RFC8200]
old-style messages all Packet Too Big messages report the MTU of old-style messages all Packet Too Big messages report the MTU of
the constricting link the constricting link
MTU plateau tables not needed because there are no old-style MTU plateau tables not needed because there are no old-style
messages messages
Appendix B. Changes Since RFC 1981 Appendix B. Changes Since RFC 1981
This document is based on RFC1981 has the following changes from This document is based on RFC 1981 and has the following changes from
RFC1981: RFC 1981:
o Clarified Section 1 "Introduction" that the purpose of PMTUD is to o Clarified in Section 1, "Introduction", that the purpose of PMTUD
reduce the need for IPv6 fragmentation. is to reduce the need for IPv6 fragmentation.
o Added text to Section 1 "Introduction" about the effects on PMTUD o Added text to Section 1, "Introduction", about the effects on
when ICMPv6 messages are blocked. PMTUD when ICMPv6 messages are blocked.
o Added Note to Introduction that document that this document o Added a "Note" to the introduction to document that this
doesn't cite RFC2119 and only uses lower case "should/must" specification doesn't cite RFC 2119 and only uses lower case
language. Changed all upper case "should/must" to lower case. "should/must" language. Changed all upper case "should/must" to
lower case.
o Added a short summary to the Section 1 "Introduction" of o Added a short summary to Section 1, "Introduction", about PLPMTUD
Packetization Layer Path MTU Discovery ((PLPMTUD) and a reference and a reference to RFC 4821 that defines it.
to RFC4821 that defines it.
o Aligned text in Section 2 "Terminology" to match current o Aligned text in Section 2, "Terminology", to match current
packetization layer terminology. packetization layer terminology.
o Added clarification in Section 4 "Protocol Requirements" that o Added clarification in Section 4, "Protocol Requirements", that
nodes should validate the payload of ICMP PTB message per RFC4443, nodes should validate the payload of ICMP PTB messages per RFC
and that nodes should detect decreases in PMTU as fast as 4443, and that nodes should detect decreases in PMTU as fast as
possible. possible.
o Remove Note from Section 4 "Protocol Requirements" about a Packet o Removed a "Note" from Section 4, "Protocol Requirements", about a
Too Big message reporting a next-hop MTU that is less than the Packet Too Big message reporting a next-hop MTU that is less than
IPv6 minimum link MTU because this was removed from the IPv6 minimum link MTU because this was removed from [RFC8200].
[I-D.ietf-6man-rfc2460bis].
o Added clarification in Section 5.2 "Storing PMTU information" to o Added clarification in Section 5.2, "Storing PMTU Information", to
discard an ICMPv6 Packet Too Big message if it contains a MTU less discard an ICMPv6 Packet Too Big message if it contains an MTU
than the IPv6 minimum link MTU. less than the IPv6 minimum link MTU.
o Added clarification Section 5.2 "Storing PMTU information" that o Added clarification in Section 5.2, "Storing PMTU Information",
nodes with multiple interface, Path MTU information should be that for nodes with multiple interfaces, Path MTU information
stored for each link. should be stored for each link.
o Removed text in Section 5.2 "Storing PMTU information" about the o Removed text in Section 5.2, "Storing PMTU Information", about
RH0 routing header because it was deprecated by RFC5095. Routing Header type 0 (RH0) because it was deprecated by RFC 5095.
o Removed text about obsolete security classification from o Removed text about obsolete security classification from
Section 5.2 "Storing PMTU information". Section 5.2, "Storing PMTU Information".
o Changed title of Section 5.4 to "Packetization Layer actions" and o Changed the title of Section 5.4 to "Packetization Layer Actions"
changed to text in the first paragraph to to generalize this and changed the text in the first paragraph to generalize this
section to cover all packetization layers, not just TCP. section to cover all packetization layers, not just TCP.
o Clarified text in Section 5.4 "Packetization Layer actions" to use o Clarified text in Section 5.4, "Packetization Layer Actions", to
normal packetization layer retransmission methods. use normal packetization layer retransmission methods.
o Removed text in Section 5.4 "Packetization Layer actions" that o Removed text in Section 5.4, "Packetization Layer Actions", that
described 4.2 BSD because it is obsolete, and removed reference to described 4.2 BSD because it is obsolete, and removed reference to
TP4. TP4.
o Updated text in Section 5.5 "Issues for other transport protocols" o Updated text in Section 5.5, "Issues for Other Transport
about NFS including adding a current reference to NFS and removing Protocols", about NFS, including adding a current reference to NFS
obsolete text. and removing obsolete text.
o Added paragraph to Section 6 "Security Considerations" about black o Added a paragraph to Section 6, "Security Considerations", about
hole connections if PTB messages are not received, and comparison black-hole connections if PTB messages are not received and
to PLPMTD. comparison to PLPMTUD.
o Updated Section 7 "Acknowledgements". o Updated "Acknowledgements".
o Editorial Changes. o Editorial Changes.
B.1. Change History Since RFC1981 Acknowledgements
NOTE TO RFC EDITOR: Please remove this subsection prior to RFC
Publication
This section describes change history made in each Internet Draft
that went into producing this version. The numbers identify the
Internet-Draft version in which the change was made.
Working Group Internet Drafts
08) Based on IESG comments, cleaned up text in Section 5.3
regarding suggested action when PMTU value has not been
decreased recently.
08) Revision of Note in Section 5.4 to make text clearer.
08) Updated Section 7 "Acknowledgements".
08) Editorial Changes.
07) Changes from the IESG Discuss comments from IESG reviews.
The changes include:
o Added Note to Introduction that document that this
document doesn't cite RFC2119 and only uses lower case
"should/must" language. Changed all upper case "should/
must" to lower case.
o Added references for EMTU_S and EMTU_R.
o Added clarification to Section 4 "Protocol Requirements"
that nodes should detect decreases in PMTU as fast as
possible.
o Added clarification Section 5.2 "Storing PMTU information"
that nodes with multiple interface, Path MTU information
should be stored for each link.
o Removed text in Section 5.2 about Retransmission because
it was unneeded.
o Removed text in Section 5.3 about Retransmission because
it was unneeded.
o Rewrote text in Section 5.4 "Packetization Layer actions"
regarding reception to make it clearer.
o Rewrote the text at the end of Section 5.4 to remove
unnecessary details and clarify not change congestion
window.
o Added references in Section 5.5 for SCTP and added DCCP
(and reference) the list of examples.
o Added paragraph to Section 5.5 "Security Considerations"
about black hole connections if PTB messages are not
received, and comparison to PLPMTD.
07) Editorial changes.
06) Revised Appendix B "Changes since RFC1981" to have a summary
of changes since RFC1981 and a separate subsection with a
change history of each Internet Draft. This subsection will
be removed when the RFC is published.
06) Editorial changes based on comments received after publishing
the -05 draft.
05) Changes based on IETF last call reviews by Gorry Fairhurst,
Joe Touch, Susan Hares, Stewart Bryant, Rifaat Shekh-Yusef,
and Donald Eastlake. This includes includes:
o Clarify that the purpose of PMTUD is to reduce the need
for IPv6 Fragmentation.
o Added text to Introduction about effects on PMTUD when
ICMPv6 messages are blocked.
o Clarified in Section 4. that nodes should validate the
payload of ICMPv6 PTB messages per RFC4443.
o Removed text in Section 5.2 about the number of paths to a
destination.
o Changed title of Section 5.4 to "Packetization layer
actions".
o Clarified first paragraph in Section 5.4 to to cover all
packetization layers, not just TCP.
o Clarified text in Section 5.4 to use normal retransmission
methods.
o Add clarification to Note in Section 5.4 about
retransmissions.
o Removed text in Section 5.4 that described 4.2BSD as it is
now obsolete.
o Removed reference to TP4 in Section 5.5.
o Updated text in Section 5.5 about NFS including adding a
current reference to NFS and removing obsolete text.
o Revised text in Section 6 to clarify first attack
response.
o Added new text in Section 6 to clarify the effect of
ICMPv6 filtering on PMTUD.
o Aligned terminology for the packetization layer
terminology.
o Editorial changes.
04) Changes based on AD Evaluation including removing details
about RFC4821 algorithm in Section 1, remove text about
decrementing hop limit from Section 3, and removed text about
obsolete security classifications from Section 5.2.
04) Editorial changes and clarification in Section 5.2 based on
IP Directorate review by Donald Eastlake
03) Remove text in Section 5.3 regarding RH0 since it was
deprecated by RFC5095
02) Clarified in Section 3 that ICMPv6 Packet Too Big should be
sent even if the node doesn't decrement the hop limit
01) Revised the text about PLPMTUD to use the word "path".
01) Editorial changes.
00) Added text to discard an ICMPv6 Packet Too Big message
containing an MTU less than the IPv6 minimum link MTU.
00) Revision of text regarding RFC4821.
00) Added R. Hinden as Editor to facilitate ID submission.
00) Editorial changes.
Individual Internet Drafts
01) Remove Note about a Packet Too Big message reporting a next-
hop MTU that is less than the IPv6 minimum link MTU. This
was removed from [I-D.ietf-6man-rfc2460bis].
01) Include a link to RFC4821 along with a short summary of what
it does.
01) Assigned references to informative and normative.
01) Editorial changes. We would like to acknowledge the authors of and contributors to
[RFC1191], from which the majority of this document was derived. We
would also like to acknowledge the members of the IPng Working Group
for their careful review and constructive criticisms.
00) Establish a baseline from RFC1981. The only intended changes We would also like to acknowledge the contributors to this update of
are formatting (XML is slightly different from .nroff), "Path MTU Discovery for IP Version 6". This includes members of the
differences between an RFC and Internet Draft, fixing a few 6MAN Working Group, area directorate reviewers, the IESG, and
ID Nits, updating references, and updates to the authors especially Joe Touch and Gorry Fairhurst.
information. There should not be any content changes to the
specification.
Authors' Addresses Authors' Addresses
Jack McCann Jack McCann
Digital Equipment Corporation Digital Equipment Corporation
Stephen E. Deering Stephen E. Deering
Retired Retired
Vancouver, British Columbia Vancouver, British Columbia
Canada Canada
Jeffrey Mogul Jeffrey Mogul
Digital Equipment Corporation Digital Equipment Corporation
Robert M. Hinden (editor) Robert M. Hinden (editor)
Check Point Software Check Point Software
959 Skyway Road 959 Skyway Road
San Carlos, CA 94070 San Carlos, CA 94070
USA United States of America
Email: bob.hinden@gmail.com Email: bob.hinden@gmail.com
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