Network Working Group R. Hinden
Internet-Draft Check Point Software
Intended status: Experimental G. Fairhurst
Expires: January 6, 2020 University of Aberdeen
July 5, 2019
IPv6 Minimum Path MTU Hop-by-Hop Option
draft-hinden-6man-mtu-option-02
Abstract
This document specifies a new Hop-by-Hop IPv6 option that is used to
record the minimum Path MTU along the forward path between a source
to a destination host. This collects a minimum recorded MTU along
the path to the destination. The value can then be communicated back
to the source using the return Path MTU field in the option.
This Hop-by-Hop option is intended to be used in environments like
Data Centers and on paths between Data Centers, to allow them to
better take advantage of paths able to support a large Path MTU.
Status of This Memo
This Internet-Draft is submitted to IETF 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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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 6, 2020.
Copyright Notice
Copyright (c) 2019 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Motivation and Problem Solved . . . . . . . . . . . . . . . . 4
3. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
4. Applicability Statements . . . . . . . . . . . . . . . . . . 5
5. IPv6 Minimum Path MTU Hop-by-Hop Option . . . . . . . . . . . 5
6. Router, Host, and Transport Behaviors . . . . . . . . . . . . 6
6.1. Router Behaviour . . . . . . . . . . . . . . . . . . . . 6
6.2. Host Behavior . . . . . . . . . . . . . . . . . . . . . . 7
6.3. Transport Behavior . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
10. Change log [RFC Editor: Please remove] . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . 12
11.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Planned Experiments . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
This draft proposes a new Hop-by-Hop Option to be used to record the
minimum MTU along the forward path between the source and destination
nodes. The source node creates a packet with this Hop-by-Hop Option
and fills the Reported PMTU Field in the option with the value of the
MTU for the outbound link that will be used to forward the packet
towards the destination.
At each subsequent hop where the option is processed, the router
compares the value of the Reported PMTU in the option and the MTU of
its outgoing link. If the MTU of the outgoing link is less than the
Reported PMTU specified in the option, it rewrites the value in the
Option Data with the smaller value. When the packet arrives at the
Destination node, the Destination node can send the minimum reported
PMTU value back to the Source Node using the Return PMTU field in the
option.
The figure below can be used to illustrate the operation of the
method. In this case, the path between the Sender and Destination
nodes comprises three links, the sender has a link MTU of size MTU-S,
the link between routers R1 and R2 has an MTU of size 8 KBytes, and
the final link to the destination has an MTU of size MTU-D.
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+--------+ +----+ +----+ +-------+
| | | | | | | |
| Sender +---------+ R1 +--------+ R2 +-------- + Dest. |
| | | | | | | |
+--------+ MTU-S +----+ 9000B +----+ MTU-D +-------+
The scenarios are described:
Scenario 1, considers all links to have an 9000 Byte MTU and the
method is supported by both routers.
Scenario 2, considers the destination link to have an MTU of 1500
Byte. This is the smallest MTU, router R2 resets the reported PMTU
to 1500 Byte and this is detected by the method. Had there been
another smaller MTU at a link further along the path that supports
the method, the lower PMTU would also have been detected.
Scenario 3, considers the case where the router preceding the
smallest link does not support the method, and the method then fails
to detect the actual PMTU. These scenarios are summarized in the
table below. This scenario would also arise if the PTB message was
not delivered to the sender.
+-+-----+-----+----+----+----------+-----------------------+
| |MTU-S|MTU-D| R1 | R2 | Rec PMTU | Note |
+-+-----+-----+----+----+----------+-----------------------+
|1|9000B|9000B| H | H | 9000 B | Endpoints attempt to |
| | | | | | use an 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 an 9000 B PMTU, |
| | | | | | | but need to implement |
| | | | | | | a method to fall back |
| | | | | | | use a 1500 B PMTU. |
+-+-----+-----+----+----+----------+-----------------------+
IPv6 as specified in [RFC8200] allows nodes to optionally process
Hop-by-Hop headers. Specifically from Section 4:
o 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
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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.
o 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 value returned in the response message 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 problems with the mechanisms defined in [RFC8201]
are known to not work well in all environments. Nodes in the middle
of the network may not send ICMP Packet Too Big messages or they are
rate limited to the point of not making them a useful mechanism.
This results in many connection defaulting to 1280 octets and makes
it very difficult to take advantage of links with larger MTU where
they exist. Applications that need to send large packets over UDP
are forced to use IPv6 Fragmentation.
Transport encapsulations and network-layer tunnels reduce the PMTU
available for a transport to use. For example, Network
Virtualization Using Generic Routing Encapsulation (NVGRE) [RFC7637]
encapsulates L2 packets in an outer IP header and does not allow IP
Fragmentation.
The use of 10G Ethernet will not achieve it's potential because the
packet per second rate will exceed what most nodes can send to
achieve multi-gigabit rates if the packet size limited to 1280
octets. For example, the packet per second rate required to reach
wire speed on a 10G Ethernet link with 1280 octet packets is about
977K packets per second (pps), vs. 139K pps for 9,000 octet packets.
A significant difference.
The purpose of the this draft is to improve the situation by defining
a mechanism that does not rely on nodes in the middle of the network
to send ICMPv6 Packet Too Big messages, instead it provides the
destination host information on the minimum Path MTU and it can send
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this information back to the source host. This is expected to work
better than the current RFC8201 based mechanisms.
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
This Hop-by-Hop Option header is intended to be used in environments
such as Data Centers and on paths between Data Centers, to allow them
to better take advantage of a path that is able to support a large
PMTU. For example, it helps inform a sender that the path includes
links that have a MTU of 9,000 Bytes. This has many performance
advantages compared to the current practice of limiting packets to
1280 Bytes.
The design of the option is sufficiently simple that it could be
executed on a router's fast path. To create critical mass for this
to happen will have to be a strong pull from router vendors
customers. This could be the case for connections within and between
Data Centers.
The method could also be useful in other environments, including the
general Internet.
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:
BB 00 Skip over this option and continue processing.
C 1 Option data can change en route to the packet's final
destination.
TTTT 11110 Experimental Option Type from [IANA-HBH].
Length: 4 The size of the each value field in Option Data
field supports Path MTU values from 0 to 65,535 octets.
Min-PMTU: n 16-bits. The minimum PMTU in octets, reflecting the
smallest link MTU that the packet experienced across
the path. This is called the Reported PMTU. A value
less than the IPv6 minimum link MTU [RFC8200]
should be ignored.
Rtn-PMTU: n 15-bits. The returned mimimum PMTU, carrying the 15
most significant bits of the latest received Min-PMTU
field. 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 should include the received
Reported PMTU in Rtn-PMTU field.
NOTE: The encoding of the final two octets (Rtn-PMTU and R-Flag)
could be implemented by a mask of the latest received Min-MTU value
with 0xFFFE, discarding the right-most bit and then performing a
logical 'OR' with the R-Flag value of the sender.
6. Router, Host, and Transport Behaviors
6.1. Router Behaviour
Routers that do not support Hop-by-Hop options SHOULD ignore this
option and SHOULD forward the packet.
Routers that support Hop-by-Hop Options, but do not recognize this
option SHOULD ignore the option and SHOULD forward the packet.
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Routers that recognize this option SHOULD compare the Reported PMTU
in the Min-PMTU field and the MTU configured for the outgoing link.
If the MTU of the outgoing link is less than the Reported PMTU, the
router rewrites the Reported PMTU in the Option to use the smaller
value.
The router MUST ignore and not change the Rtn-PMTU field and R-Flag
in the option.
Discussion:
o The design of this Hop-by-Hop Option makes it feasible to be
implemented within the fast path of a router, because the required
processing is simple.
6.2. Host Behavior
The source host that supports this option SHOULD create a packet with
this Hop-by-Hop Option and fill the Min-PMTU field of the option with
the MTU of configured for the link over which it will send the packet
on the next hop towards the destination.
The source host may request that the destination host return the
received minimum MTU value by setting the R-Flag in the option. This
will cause the destination host to include a PMTU option in an
outgoing packet.
Discussion:
o This option does not need to be sent in all packets belonging to a
flow. A transport protocol (or packetization layer) can set this
option only on specific packets used to test the path.
o In the case of TCP, the option could be included in packets
carrying a SYN segment as part of the connection set up, or can
periodically be sent in packets carrying other segments.
Including this packet in a SYN could increase the probability that
SYN segment is lost, when routers on the path drop packets with
this option. Including this option in a large packet is not
likely to be useful, since the large packet might itself also be
dropped by a link along the path with a smaller MTU, preventing
the Reported PMTU information from reaching the Destination node.
o 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 nodes [RFC8085].
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o For applications that use Anycast, this option should be included
in all packets as the actual destination will vary due to the
nature of Anycast.
o Simple-exchange protocols (i.e low data-volume applications
[RFC8085] that only send one or a few packets per transaction,
could be optimized by assuming that the Path MTU is symmetrical,
that is where the Path MTU is the same in both directions, or at
least not smaller in the return path. This optimisation does not
hold when the paths are not symmetric.
o The use of this option with DNS and DNSSEC over UDP ought to work
as long as the paths are symmetric. The DNS server will learn the
Path MTU from the DNS query messages. If the return Path MTU is
smaller, then the large DNSSEC response may be dropped and the
known problems with PMTUD will occur. DNS and DNSSEC over
transport protocols that can carry the Path MTU should work.
The Source Host can request the destination host to send a packet
carrying the PMTU Option using the R-Flag.
A Destination Host SHOULD respond to each packet received with the
R-Flag set, by setting the PMTU Option in the next packet that it
sends to the Source Host by the same upper layer protocol instance.
The upper layer protocol MAY generate a packet when any of these
conditions is met when the R Flag is set in the PMTU Option and
either:
o It is the first Reported PMTU value it has received from the
Source.
o The Reported PMTU value is lower than previously received.
The R-Flag SHOULD NOT be set when the PMTU Option was sent solely to
carry the feedback of a Reported PMTU.
The PMTU Option sent back to the source SHOULD contain the outgoing
link MTU in Min-PMTU field and SHOULD set the last Received PMTU in
the Rtn-PMTU field. If these values are not present the field MUST
be set to zero.
For a connection-oriented upper layer protocol, this could be
implemented by saving the value of the last received option within
the connection context. This last received value is then used to set
the return Path MTU field for all packets belonging to this flow that
carry the IPv6 Minimum Path MTU Hop-by-Hop Option.
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A connection-less protocol, e.g., based on UDP, requires the
application to be updated to cache the Received PMTU value, and to
ensure that this corresponding value is used to set the last Received
PMTU in the Rtn-PMTU field of any PMTU Option that it sends.
NOTE: The Rtn-PMTU value is specific to the instance of the upper
layer protocol (i.e. matching the IPv6 flow ID, port-fields in UDP or
the SPI in IPsec, etc), not the protocol itself, because network
devices can make forwarding decisions that impact the PMTU based on
the presence and values of these upper layer fields, and therefore
these fields need to correspond to those of the packets for the flow
received by the Destination Host set to ensure feedback is provided
to the corresponding Source Host.
NOTE: An upper layer protocol that send packets from the Destination
Host towards the Source Host less frequently than the Destination
Host receives packets from the Source Host, provides less frequent
feedback of the received Min-PMTU value. However, it will always
needs to send the most recent value.
Discussion:
o A simple mechanism could only send an MTU Option with the Rtn-PMTU
field filled in the first time this option is received or when the
Received PMTU is reduced. This is good because it limits the
number sent, but there is no provision for retransmission of the
PMTU Option fails to reach the sender, or the sender looses state.
o The Reported PMTU value could increase or decrease over time. For
instance, it would increase when the path changes and the packets
become then forwarded over a link with a MTU larger than the link
previously used.
6.3. Transport Behavior
A transport endpoint using this option needs to use a method to
verify the information provided by this option.
The Received PMTU does not necessarily reflect the actual PMTU
between the sender and destination. Care therefore needs to be
exercised in using this value at the sender. Specifically:
o If the Received PMTU value returned by the Destination is the same
as the initial Reported PMTU value, there could still be a router
or layer 2 device on the path that does not support this PMTU.
The usable PMTU therefore needs to be confirmed.
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o If the Received PMTU value returned by the Destination is smaller
than the initial Reported PMTU value, this is an indication that
there is at least one router in the path with a smaller MTU.
There could still be another router or layer 2 device on the path
that does not support this MTU.
o If the Received PMTU value returned by the Destination is larger
than the initial Reported PMTU value, this may be a corrupted,
delayed or mis-ordered response, and SHOULD be ignored.
A sender needs to discriminate between the Received PMTU value in a
PTB message generated in response to a Hop-by-Hop option requesting
this, and a PTB message received from a router on the path.
A PMTUD or PLPMTUD method could use the Received PMTU value as an
initial target size to probe the path. This can significantly
decrease the number of probe attempts (and hence time taken) to
arrive at a workable PMTU. It has the potential to complete
discovery of the correct value in a single Round Trip Time (RTT),
even over paths that may have successive links configured with lower
MTUs.
Since the method can delay notification of an increase in the actual
PMTU, a sender with a link MTU larger than the current PMTU SHOULD
periodically probe for a PMTU value that is larger than the Received
PMTU value. This specification does not define an interval for the
time between probes.
Since the option consumes less capacity than an a full probe packet,
there may be advantage in using this to detect a change in the path
characteristics.
NOTE: Further details to be included in next version.
NOTE: A future version of the document will consider more the impact
of Equal Cost Multipath (ECMP). Specifically, whether a Received
PMTU value should be maintained by the method for each transport
endpoint, or for each network address, and how these are best used by
methods such as PLPMTUD or DPLPMTUD.
7. IANA Considerations
No IANA assignments are requested. Document uses experimental option
from [IANA-HBH].
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8. Security Considerations
The method has no way to protect the destination from off-path attack
using this option in packets that do not originate from the source.
This attack could be used to inflate or reduce the size of the
reported PMTU. Mechanisms to provide this protection can be provided
at a higher layer (e.g., the transport packetization layer using
PLPMTUD or DPLPMTUD), where more information is available about the
size of packet that has successfully traversed a path.
The method solicits a response from the destination, which should be
used to generate a response to the IPv6 node originating the option
packet. A malicious attacker could generate a packet to the
destination for a previously inactive flow or one that advertises a
change in the size of the MTU for an active flow. This would create
additional work at the destination, and could induce creation of
state when a new flow is created. It could potentially result in
additional traffic on the return path to the sender, which could be
mitigated by limiting the rate at which responses are generated.
A sender MUST check the quoted packet within the PTB message to
validate that the message is in response to a packet that was
originated by the sender. This is intended to provide protection
against off-path insertion of ICMP PTB messages by an attacker trying
to disrupt the service. Messages that fail this check MAY be logged,
but the information they contain MUST be discarded.
TBD
9. Acknowledgments
A somewhat similar mechanism was proposed for IPv4 in 1988 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.
Helpful comments were received from Tom Herbert, Tom Jones, Fred
Templin, Ole Troan, [Your name here], and other members of the 6MAN
working group.
10. Change log [RFC Editor: Please remove]
draft-hinden-6man-mtu-option-02, 2019-July-5
o Changed option format to also include the Returned MTU value and
Return flag and made related text changes in Section 6.2 to
describe this behaviour.
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o ICMP Packet Too Big messages are no longer used for feedback to
the Source host.
o Added to Acknowledgements Section that a similar mechanism was
proposed for IPv4 in 1988 in [RFC1063].
o Editorial changes.
draft-hinden-6man-mtu-option-01, 2019-March-05
o 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.
o Added Section 2 "Motivation and Problem Solved" section to better
describe what the purpose of this document is.
o Added Appendix A describing planned experiments and how the
results will be measured.
o Editorial changes.
draft-hinden-6man-mtu-option-00, 2018-Oct-16
o Initial draft.
11. References
11.1. Normative References
[IANA-HBH]
"Destination Options and Hop-by-Hop Options",
<https://www.iana.org/assignments/ipv6-parameters/
ipv6-parameters.xhtml#ipv6-parameters-2>.
[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>.
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11.2. Informative References
[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>.
[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>.
Appendix A. Planned Experiments
TBD
This section will describe a set of experiments planned for the use
of the option defined in this document. There are many aspects of
the design that require experimental data or experience to evaluate
this experimental specification.
This includes experiments to understand the pathology of packets sent
with the specified option to determine the likelihood that they are
lost within specific types of network segment.
This includes consideration of the cost and alternatives for
providing the feedback required by the mechanism and how to
effectively limit the rate of transmission.
This includes consideration of the potential for integration in
frameworks such as that offered by DPLPMTUD.
There are also security-related topics to be understood as described
in the Security Considerations (Section 8).
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Authors' Addresses
Robert M. Hinden
Check Point Software
959 Skyway Road
San Carlos, CA 94070
USA
Email: bob.hinden@gmail.com
Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
UK
Email: gorry@erg.abdn.ac.uk
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