Packetization Layer Path MTU Discovery for Datagram Transports
draft-fairhurst-tsvwg-datagram-plpmtud-01
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
|
|
|---|---|---|---|
| Authors | Gorry Fairhurst , Tom Jones , Michael Tüxen , Irene Ruengeler | ||
| Last updated | 2017-10-30 | ||
| Replaced by | draft-ietf-tsvwg-datagram-plpmtud, RFC 8899 | ||
| RFC stream | (None) | ||
| Formats | |||
| Additional resources | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-fairhurst-tsvwg-datagram-plpmtud-01
Internet Engineering Task Force G. Fairhurst
Internet-Draft T. Jones
Intended status: Standards Track University of Aberdeen
Expires: May 01, 2018 M. Tuexen
I. Ruengeler
Muenster University of Applied Sciences
October 30, 2017
Packetization Layer Path MTU Discovery for Datagram Transports
draft-fairhurst-tsvwg-datagram-plpmtud-01.txt
Abstract
This document describes a robust method for Path MTU Discovery
(PMTUD) for datagram packetization layers. It allows these layers to
probe an Internet path with progressively larger packets to determine
a maximum packet size This method is described as an extension to RFC
1191 and RFC 8201, which specify ICMP-based Path MTU Discovery for IP
versions 4 and 6. The document provides functionally for datagram
transports that is equivalent to the packetization layer PMTUD
specification for TCP, specified in RFC4821.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 01, 2018.
Copyright Notice
Copyright (c) 2017 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 carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 1]
Internet-Draft Datagram PLPMTUD October 2017
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Features required to provide PLPMTUD at the Transport Layer . 6
3.1. PMTU Probe Packets . . . . . . . . . . . . . . . . . . . . 8
3.2. Validation of the current effective PMTU . . . . . . . . . 9
3.3. Reduction of the effective PMTU . . . . . . . . . . . . . 9
4. Datagram PLPMTUD . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Probing . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Timers . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Constants . . . . . . . . . . . . . . . . . . . . . . . . 11
4.4. Variables . . . . . . . . . . . . . . . . . . . . . . . . 11
4.5. State Machine . . . . . . . . . . . . . . . . . . . . . . 11
5. Specification of Protocol-Specific Methods . . . . . . . . . . 13
5.1. UDP and UDP-Lite . . . . . . . . . . . . . . . . . . . . . 13
5.1.1. UDP Options . . . . . . . . . . . . . . . . . . . . . 14
5.1.2. UDP Options required for PLPMTUD . . . . . . . . . . . 14
5.1.2.1. Echo Request Option . . . . . . . . . . . . . . . 14
5.1.2.2. Echo Response Option . . . . . . . . . . . . . . . 14
5.1.3. Sending UDP-Option Probe Packets . . . . . . . . . . . 14
5.1.4. Validating the Path with UDP Options . . . . . . . . . 15
5.1.5. Handling of PTB Messages by UDP . . . . . . . . . . . 15
5.2. SCTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.2.1. SCTP/IP4 and SCTP/IPv6 . . . . . . . . . . . . . . . . 15
5.2.1.1. Sending SCTP Probe Packets . . . . . . . . . . . . 15
5.2.1.2. Validating the Path with SCTP . . . . . . . . . . 16
5.2.1.3. PTB Message Handling by SCTP . . . . . . . . . . . 16
5.2.2. SCTP/UDP . . . . . . . . . . . . . . . . . . . . . . . 16
5.2.2.1. Sending SCTP/UDP Probe Packets . . . . . . . . . . 16
5.2.2.2. Validating the Path with SCTP/UDP . . . . . . . . 16
5.2.2.3. Handling of PTB Messages by SCTP/UDP . . . . . . . 16
5.2.3. SCTP/DTLS . . . . . . . . . . . . . . . . . . . . . . 16
5.2.3.1. Sending SCTP/DTLS Probe Packets . . . . . . . . . 17
5.2.3.2. Validating the Path with SCTP/DTLS . . . . . . . . 17
5.2.3.3. Handling of PTB Messages by SCTP/DTLS . . . . . . 17
5.3. Other IETF Transports . . . . . . . . . . . . . . . . . . 17
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . . 19
Appendix A. Event-driven state changes . . . . . . . . . . . . . . 19
Appendix B. Revision Notes . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 2]
Internet-Draft Datagram PLPMTUD October 2017
The IETF has specified datagram transport using UDP, SCTP, SCTP/UDP,
DCCP, and DCCP/UDP, as well as protocols layered on top of these
transports.
Classical Path Maximum Transmission Unit Discovery (PMTUD) can be
used with any transport that is able to process ICMP Packet Too Big
(PTB) messages (e.g., [RFC1191] and [RFC8201]). It adjusts the
effective Path MTU (PMTU), based on reception of ICMP Path too Big
(PTB) messages to decrease the PMTU when a packet is sent with a size
larger than the value supported along a path, and a method that from
time-to-time increases the packet size in attempt to discover an
increase in the supported PMTU.
However, Classical PMTUD is subject to protocol failures. One
failure arises when traffic using a packet size larger than the
actual supported PMTU is blackholed (silently discarded). This may
happen when ICMP PTB messages are not delivered back to the sender
for some reason [RFC2923]). For example, ICMP messages are
increasingly filtered by middleboxes (including Firewalls) [RFC4890],
and may not be correctly processed by tunnel endpoints.
Another failure could result if a system not on the path sends a PTB
that attempts to force the sender to change the effective PMTU
[RFC8201]. A sender could protect itself by using the quoted packet
within the PTB message payload to verify that the received PTB
message was generated in response to a packet that had actually been
sent. However, there are situations where a sender is unable to
provide this verification (e.g., when the PTB message does not
include sufficient information, often the case for IPv4; or where the
information corresponds to an encrypted packet). At the network layer
there also could be insufficient context to perform this
verification, which depends on information about the active transport
flows (e.g., the socket/address pairs being used, and other protocol
header information). This verification is more straight forward at a
the Packetization Layer (PL) or a higher layer.
The term Packetization Layer has been introduced to describe the
layer that is responsible for placing data blocks into the payload of
packets and selecting an appropriate maximum packet size. This
function is often performed by a transport protocol, but can also be
performed by other encapsulation methods working below the
application.
In contrast to PMTUD, Packetization Layer Path MTU Discovery
(PLPMTUD) [RFC4821] does not rely upon reception and verification of
PTB messages. It is therefore more robust than Classical PMTUD. This
has become the recommended approach for implementing PMTU discovery
with TCP. It uses a general strategy where the PL searches for an
appropriate PMTU by sending probe packets along the network path with
a progressively larger packet size. If a probe packet is
successfully delivered (as determined by the PL), then the effective
Path MTU is raised to the probe size.
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 3]
Internet-Draft Datagram PLPMTUD October 2017
PLPMTUD introduces flexibility in the implementation of PMTU
discovery. At one extreme, it can be configured to only perform PTB
black hole recovery to increase the robustness of Classical PMTUD, or
at the other extreme, all PTB processing can be disabled and PLPMTUD
can completely replace Classical PMTUD. PLPMTUD can also include
additional consistency checks without increasing the risk of
blackholing.
The UDP-Guidelines [RFC8085] state "an application SHOULD either use
the path MTU information provided by the IP layer or implement Path
MTU Discovery (PMTUD)", but does not provide a mechanism for
discovering the largest size of unfragmented datagram than can be
used on a path. PLPMTUD has not currently been specified for UDP,
while Section 10.2 of [RFC4821] recommends a PLPMTUD probing method
for SCTP that utilises heartbeat messages as packet probes, but does
not provide a complete specification. This document provides the
details to complete that specification. Similarly, the method
defined in this specification could be used with the Datagram
Congestion Control Protocol (DCCP) [RFC4340] requires implementations
to support Classical PMTUD and states that a DCCP sender "MUST
maintain the maximum packet size (MPS) allowed for each active DCCP
session". It also defines the current congestion control maximum
packet size (CCMPS) supported by a path. This recommends use of
PMTUD, and suggests use of control packets (DCCP-Sync) as path probe
packets, because they do not risk application data loss. The
document also contains information that enables the implementation of
PLPMTUD with other datagram transports
Section Section 4 of this document presents a set of algorithms for
datagram protocols to discover a maximum size for the effective PMTU
across a path. The methods described rely on features of the PL
Section 3 and apply to transport protocols over IPv4 and IPv6. It
does not require cooperation from the lower layers (except that they
are consistent about which packet sizes are acceptable). It can
utilise PTB messages when these are available.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Other terminology is directly copied from [RFC4821], and the
definitions in [RFC1122].
Black-Holed: When the sender is unaware that packets are not
delivered to the destination endpoint (e.g., when the sender is
unaware of a change in the path to one with a smaller PMTU).
Classical Path MTU Discovery: Classical PMTUD is a process described
in [RFC1191] and [RFC8201], in which nodes rely on PTB messages to
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 4]
Internet-Draft Datagram PLPMTUD October 2017
learn the largest size of unfragmented datagram than can be used
across a path.
Datagram: A datagram is a transport-layer protocol data unit,
transmitted in the payload of an IP packet.
Effective PMTU: The current estimated value for PMTU used by a
Packetization Layer.
EMTU_S: The Effective MTU for sending (EMTU_S) is defined in
[RFC1122] as "the maximum IP datagram size that may be sent, for a
particular combination of IP source and destination addresses...".
EMTU_R: The Effective MTU for receiving (EMTU_R) is designated in
[RFC1122] as "the largest datagram size that can be reassembled by
EMTU_R ("Effective MTU to receive")".
Link: A communication facility or medium over which nodes can
communicate at the link layer, i.e., a layer below the IP layer.
Examples are Ethernet LANs and Internet (or higher) layer and
tunnels.
Link MTU: The Maximum Transmission Unit (MTU) is the size in bytes of
the largest IP packet, including the IP header and payload, that
can be transmitted over a link. Note that this could more
properly be called the IP MTU, to be consistent with how other
standards organizations use the acronym MTU. This includes the IP
header, but excludes link layer headers and other framing that is
not part of IP or the IP payload. Other standards organizations
generally define link MTU to include the link layer headers.
MPS: The Maximum Packet Size (MPS), the largest size of application
data block that may be sent unfragmented across a path. In
PLPMTUD this quantity is derived from Effective PMTU by taking
into consideration the size of the application and lower protocol
layer headers, and may be limited by the application protocol.
Packet: An IP header plus the IP payload.
Packetization Layer (PL): The layer of the network stack that places
data into packets and performs transport protocol functions.
Path: The set of link and routers traversed by a packet between a
source node and a destination node.
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 5]
Internet-Draft Datagram PLPMTUD October 2017
Path MTU (PMTU): The minimum of the link MTU of all the links forming
a path between a source node and a destination node.
PLPMTUD: Packetization Layer Path MTU Discovery, the method described
in this document for datagram PLs, which is an extension to
Classical PMTU Discovery.
3. Features required to provide PLPMTUD at the Transport Layer
TCP PLPMTUD has been defined using standard TCP protocol mechanisms.
All of the requirements in [RFC4821] also apply to use of the
technique with a datagram PL. Unlike TCP, some datagram PLs require
additional mechanisms to implement PLPMTUD.
There are ten requirements for performing the datagram PLPMTUD method
described in this specification:
1. PMTU parameters: A PLPMTUD sender is REQUIRED to provide
information about the maximum size of packet that can be
transmitted by the sender on the local link (the Link MTU and MAY
utilize similar information about the receiver when this is
supplied (note this may be less than EMTU_R). Some applications
also have a maximum transport protocol data unit (PDU) size, in
which case there may be no benefit from probing for a size larger
than this (unless a transport allows multiplexing multiple
applications PDUs into the same datagram.)
2. Effective PMTU: A datagram application MUST be able to choose the
size of datagrams sent to the network, up to the effective PMTU,
or a smaller value (such as the MPS) derived from this. This
value is managed by the PMTUD method. The effective PMTU
(specified in Section 1 of [RFC1191]) is equivalent to the EMTU_S
(specified in [RFC1122]).
3. Probe packets: On request, a PLPMTUD sender is REQUIRED to be
able to transmit a packet larger than the current effective PMTU
(but always with a total size less than the link MTU), which the
method can use as a probe packet. In IPv4, a probe packet is
always sent with the Don't Fragment (DF) bit set and without
network layer endpoint fragmentation.
4. Processing PTB messages: A PLPMTUD sender MAY optionally utilize
PTB messages received from the network layer to help identify
when a path does not support the current size of packet probe.
Any received PTB message SHOULD/MUST be verified before it is
used to update the PMTU discovery information [RFC8201]. This
verification confirms that the PTB message was sent in response
to a packet originating by the sender, and needs to be performed
before the PMTU discovery method reacts to the PTB message. When
the router link MTU is indicated in the PTB message this MAY be
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 6]
Internet-Draft Datagram PLPMTUD October 2017
used by datagram PLPMTUD to reduce the size of a probe, but MUST
NOT be used increase the effective PMTU.
5. Reception feedback: The destination PL endpoint is REQUIRED to
provide a feedback method that indicates when a probe packet has
been received by the destination endpoint. The local PL endpoint
is REQUIRED to pass this feedback to the sender PLPMTUD method.
6. Probing and congestion control: The isolated loss of a probe
packet SHOULD NOT be treated as an indication of congestion and
its loss not directly trigger a congestion control reaction.
7. Probe loss recovery: If the data block carried by a probe message
needs to be sent reliably, the PL (or layers above) MUST arrange
retransmission/repair of any resulting loss. This method MUST be
robust in the case where paket probes are lost due to other
reasons (including link transmission error, congestion). The
PLPMTUD method treats isolated loss of a probe packet (with or
without an PTB message) as a potential indication of a PMTU limit
on the path. The PL is permitted to retransmit any data included
in a lost probe packet without adjusting its congestion window.
8. Cached effective PMTU: The sender MUST cache the effective PMTU
value between probes and needs also to consider the disruption
that could be incurred by an unsuccessful probe - both upon the
flow that incurs a probe loss, and other flows that experience
the effect of additional probe traffic.
9. Shared effective PMTU state: The specification of PLPMTUD
[RFC4821] states: "If PLPMTUD updates the MTU for a particular
path, all Packetization Layer sessions that share the path
representation (as described in Section 5.2 of [RFC4821]) SHOULD
be notified to make use of the new MTU and make the required
congestion control adjustments". Such methods need to robust to
the wide variety of underlying network forwarding behaviours.
Considerations about caching have been noted [RFC8201].
In addition the following design principles are stated:
o Suitable MPS: The PLPMTUD method SHOULD avoid forcing an
application to use an arbitrary small MPS (effective PMTU) for
transmission while the method is searching for the currently
supported PMTU. Datagram PLs do not necessarily support
fragmentation of PDUs larger than the PMTU. A reduced MPS can
adversely impact the performance of a datagram application.
o Path validation: The PLPMTUD method MUST be robust to path changes
that could have occurred since the path characteristics were last
confirmed.
o Datagram reordering: A method MUST be robust to the possibility
that a flow encounters reordering, or has the traffic (inlcuding
probe packets) is divided over more than one network path.
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 7]
Internet-Draft Datagram PLPMTUD October 2017
o When to probe: The PLPMTUD method SHOULD determine whether the
path capacity has increased since it last measured the path. This
determines when the path should again be probed.
3.1. PMTU Probe Packets
PMTU discovery relies upon the sender being able to generate probe
messages with a specific size. TCP is able to generate probe packets
by choosing to appropriately segment data being sent [RFC4821].
In contrast, datagram PLs either have to request an application to
send a data block with a specified size, or to utilise padding
functions to extend the datagram beyond the size of the application
data block. Protocols that permit exchange of control messages
(without an application data block) could alternatively prefer to
generate a probe packet by extending a control message with padding
data.
When the method fails to validate the PMTU for the path, the required
size of probe packet can need to be less than the size of the data
block generated by an application. In this case, the PL could
provide a wat to fragment a datagram at the PL, or could instead
utilise a control packet with padding.
A receiver needs to be able to be able to distinguish in-band data
from any added padding, and ensure that any added padding is not
passed to an application at the receiver.
This results in three ways that a sender can create a probe packet:
Probing using appication data: A probe packet that contains a data
block supplied by an application that matches the size required
for the probe. This requires a method to request the application
to issue a data block of the desired probe size. If the
application/transport needs protection from the loss of this probe
packet, the application/transport may perform transport-layer
retransmission/repair of the data block (e.g., by retransmission
after loss is detected or by duplicating the data block in a
datagram without the padding data).
Probing using appication data: A probe packet that contains a data
block supplied by an application that is combined with padding to
inflate the length of the datagram to the size required for the
probe. If the application/transport needs protection from the
loss of this probe packet, the application/transport may perform
transport-layer retransmission/repair of the data block (e.g., by
retransmission after loss is detected or by duplicating the data
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 8]
Internet-Draft Datagram PLPMTUD October 2017
block in a datagram without the padding data).
Probing using appication data: A probe packet that contains only
control information and padding to inflate the packet to the size
required for the probe. Since these probe packets do not carry
any application-supplied data block,they do not typically require
retransmission, although they do still consume network capacity.
3.2. Validation of the current effective PMTU
The PL needs a method to determine when packet probes have been
successfully received end-to-end across a network path.
Transport protocols can include end-to-end methods that detect and
report reception of specific datagrams that they send (e.g., DCCP and
SCTP provide keep-alive/heartbeat features). This can also be used by
PLPMTUD to acknowledge reception of a probe packet.
A PL that does not acknowledge data reception (e.g., UDP and UDP-
Lite) is unable to detect when the packets it sends are discarded
because their size is greater than the actual PMTUD. These PLs need
to either reply on application protocol to detect this, or use of an
additional transport method such as UDP-Options [I-D.ietf-tsvwg-udp-
options], and then need to send a reachability probe (e.g.,
periodically solicit a response) to determine if the current
effective PMTU is still supported by the network path.
PMTU discovery can also utilise PTB messages to detect when the
actual PMTU supported by a network path is less than the current size
of datagrams that are being sent.
3.3. Reduction of the effective PMTU
When the current effective PMTU is no longer supported by the network
path, the transport needs to detect this and reduce the effective
PMTU.
o A PL that sends a datagram larger than the actual PMTU that
includes no application data block, or one that does not attempt
to provide any retransmission, can send a new probe packet with an
updated probe size.
o A PL that wishes to resend the application data block, may need to
re-fragment the data block to a smaller datagram size. This could
utilise network-layer or PL fragmentation when these are
available.
4. Datagram PLPMTUD
This section specifies Datagram PLPMTUD.
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 9]
Internet-Draft Datagram PLPMTUD October 2017
The central idea of PLPMTU discovery is probing by a sender. Probe
packets of increasing size are sent to find out the maximum size of a
user message that is completely transferred across the network path
from the sender to the destination. If a PTB message is received
from a router or middlebox, this information ought to be verified and
SHOULD used. The PTB messages can improve performance compared to
one that relies solely on probing.
4.1. Probing
The PLPMTUD method utilises a timer to trigger the generation of
probe packets. The probe_timer is started each time a probe packet
is sent to the destination and is cancelled when receipt of the probe
packet is acknowledged. Each time the probe_timer expires, the
probe_error_counter is incremented, and the probe packet is
retransmitted. The counter is initialised to zero when a probe
packet is first sent with a particular size. The maximum number of
retransmissions per probing size is configured (MAX_PROBES). If the
value of the PROBE_COUNT exceeds MAX_PROBES, probing will be stopped
and the last successfully probed PMTU is set as the effective PMTU.
Once probing is completed, the sender continues to use the effective
PMTU until either a PTB message is received or the PMTU_RAISE_TIMER
expires. If the PL is unable to verify reachability to the
destination endpoint after probing has completed, the method uses a
REACHABILITY_TIMER to periodically repeat a probe packet for the
current effective PMTU size, while the PMTU_RAISE_TIMER is running.
If the resulting probe packet is not acknowledged (i.e. the
PROBE_TIMER expires), the method re-starts probing for the PMTU.
4.2. Timers
This method utilises three timers:
PROBE_TIMER: Configured to expire after a period longer than the
maximum time to receive an acknowledgment to a probe packet.
PMTU_RAISE_TIMER: Configured to the period a sender ought to continue
use the current effective PMTU, after which it re-commences
probing for a higher PMTU. This timer has a period of 600 secs, as
recommended by [RFC4821].
REACHABILITY_TIMER: Configured to the period a sender ought to wait
before confirming the current effective PMTU is still supported.
This is less than the PMTU_RAISE_TIMER.
An implementation could implement the various timers using a single
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 10]
Internet-Draft Datagram PLPMTUD October 2017
timer process.
4.3. Constants
The following constants are defined:
MAX_PROBES: The maximum value of the PROBE_ERROR_COUNTER.
MIN_PMTU: The smallest allowed probe packet size. This value is 1280
bytes as specified in [RFC2460].
BASE_PMTU: The BASE_PMTU is a considered a size that should work in
most cases. The size equal to or larger than the minimum
permitted and smaller than the maximum allowed. In the case of
IPv6, this value is 1280 bytes as specified in [RFC2460]. When
using IPv4, a size of 1200 is RECOMMENDED.
MAX_PMTU: This is the largest size of PMTU that is probed. It must
be less than or equal to the minimum of the local MTU of the
outgoing interface and the destination effective MTU for
receiving.
4.4. Variables
This method utilises a set of variables:
effective PMTU: The effective PMTU is the maximum size of datagram
that the method has currently determined can be supported along
the entire path.
PROBED_SIZE: The PROBED_SIZE is the size of the current probe packet.
This is a tentative value for the effective PMTU, which is
awaiting confirmation by an acknowledgment.
PROBE_COUNT: This is a count of the number of unsuccessful probe
packets that have been sent with size PROBED_SIZE. The value is
initialised to zero when a particular size of PROBED_SIZE is first
attempted.
PTB_SIZE: The PTB_Sizde is value returned by a verified PTB message
indicating the local MTU size of a router along the path.
4.5. State Machine
A state machine for Datagram PLPMTUD is depicted in Figure 1. If
multihoming is supported, a state machine is needed for each active
path.
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 11]
Internet-Draft Datagram PLPMTUD October 2017
+------------+
| PROBE_NONE |
+------------+
| Connectivity confirmed
v
---------- +------------+ -- PROBE_TIMER expiry
MAX_PMTU acked | | PROBE_BASE | | (PROBE_COUNT < MAX_PROBES)
PTB (>= BASE_PMTU)| -----> +------------+ <-
---------------- | /\ | |
| | | | | PTB
| PMTU_RAISE_TIMER| | | | (PTB_SIZE < BASE_PMTU)
| or reachability | | | | or
| (PROBE_COUNT | | | | PROBE_TIMER expiry
| = MAX_PROBES) | | | | (PROBE_COUNT = MAX_PROBES)
| ------------- | | \
| | PTB | | \
| | (< PROBED_SIZE)| | \
| | | | ---------------
| | | | |
| | | | Probe |
| | | | acked |
v | | v v
+------------+ +--------------+ Probe +-------------+
| PROBE_DONE |<-------------- | PROBE_SEARCH |<-------| PROBE_ERROR |
+------------+ MAX_PMTU acked +--------------+ acked +-------------+
/\ | or /\ |
| | PROBE_TIMER expiry | |
| |(PROBE_COUNT = MAX_PROBES) | |
| | | |
----- ------
Reachability probe acked PROBE_TIMER expiry
or PROBE_TIMER expiry (PROBE_COUNT < MAX_PROBES)
(PROBE_COUNT < MAX_PROBES)
The following states are defined to reflect the probing process.
PROBE_NONE: The PROBE_NONE state is the initial state before probing
has started. PLPMTUD is not performed in this state. The state
transitions to PROBE_BASE, when a path has been confirmed, i.e.
when a packet has arrived on this path. The effective PMTU is set
to the BASE_PMTU size. Probing ought to start immediately after
connection setup to prevent the loss of user data.
PROBE_BASE: The PROBE_BASE state is the starting point for datagram
PLPMTUD, and used to confirm whether the BASE_PMTU size is
supported by the network path. On entry, the PROBED_SIZE is set
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 12]
Internet-Draft Datagram PLPMTUD October 2017
to the BASE_PMTU size and the PROBE_COUNT is set to zero. A probe
packet is sent, and the PROBE_TIMER is started. The state is left
when the PROBE_COUNT reaches MAX_PROBES; a PTB message is
received, or a probe packet is acknowledged.
PROBE_SEARCH: The PROBE_SEARCH state is the main probing state. This
state is entered either when probing for the BASE_PMTU was
successful or when there is a successful reachability test in the
PROBE_ERROR state. On entry, the effective PMTU is set to the
last acknowledged PROBED_SIZE.
On the first probe packet for each probed size, the PROBE_COUNT is
set to zero. Each time a probe packet is acknowledged, the
effective PMTU is set to the PROBED_SIZE, and then the PROBED_SIZE
is increased. When a probe packet is not acknowledged within the
period of the PROBE_TIMER, the PROBE_COUNT is incremented and the
probe packet is retransmitted. The state is exited when the
PROBE_COUNT reaches MAX_PROBES; a PTB message is verified; or a
probe of size PMTU_MAX is acknowledged.
PROBE_ERROR: The PROBE_ERROR state represents the case where the
network path is not known to support an effective PMTU of at least
the BASE_PMTU size. It is entered when either a probe of size
BASE_PMTU has not been acknowledged or a verified PTB message
indicates a smaller link MTU than the BASE_PMTU. On entry, the
PROBE_COUNT is set to zero and the PROBED_SIZE is set to the
MIN_PMTU size, and the effective PMTU is reset to MIN_PMTU size.
In this state, a probe packet is sent, and the PROBE_TIMER is
started. The state transitions to the PROBE_SEARCH state when a
probe packet is acknowledged.
PROBE_DONE: The PROBE_DONE state indicates a successful end to a
probing phase. Datagram PLPMTUD remains in this state until
either the PMTU_RAISE_TIMER expires or a PTB message is verified.
When PLPMTUD uses an unacknowledged PL and is in the PROBE_DONE
state, a REACHABILITY_TIMER periodically resets the PROBE_COUNT
and schedules a probe packet with the size of the effective PMTU.
If the probe packet fails to be acknowledged after MAX_PROBES
attempts, the method enters the PROBE_BASE state. An acknowledged
PL SHOULD NOT continue to probe in this state.
Appendix Appendix A contains an informative description of key
events:
5. Specification of Protocol-Specific Methods
This section specifies protocol-specific details for datagram PLPMTUD
for IETF-specified transport protocols.
5.1. UDP and UDP-Lite
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 13]
Internet-Draft Datagram PLPMTUD October 2017
The current specifications of UDP and UDP-LIte [RFC3828] do not
define a method in the RFC-series that supports PLPMTUD. In
particular, these transport do not provide the transport layer
features needed to implement datagram PLPMTUD.
5.1.1. UDP Options
UDP-Options [I-D.ietf-tsvwg-udp-options] supply the additional
functionality required to implement datagram PLPMTUD. This enables
padding to be added to UDP datagrams and can be used to provide
feedback acknowledgement of received probe packets.
5.1.2. UDP Options required for PLPMTUD
This subsection proposes two new UDP-Options that add support for
requesting a datagram response be sent and to mark this datagram as a
response to a request.
<< We may define a parameter in an Option to indicate the EMTU_R to
the peer.>>
5.1.2.1. Echo Request Option
The Echo Request Option allows a sending endpoint to solicit a
response from a destination endpoint. The Echo Request carries a
four byte token set by the sender.
+---------+--------+-----------------+
| Kind=9 | Len=6 | Token |
+---------+--------+-----------------+
1 byte 1 byte 4 bytes
5.1.2.2. Echo Response Option
The Echo Response Option is generated by the PL in response to
reception of a previously received Echo Request. The Token field is
associates the response with the Token value carried in the most
recently-received Echo Request. The rate of generation of UDP
packets carrying an Echo Response Option MAY be rate-limited.
+---------+--------+-----------------+
| Kind=10 | Len=6 | Token |
+---------+--------+-----------------+
1 byte 1 byte 4 bytes
5.1.3. Sending UDP-Option Probe Packets
This method specifies a probe packet that does not carry an
application data block. The probe packet consists of a UDP datagram
header followed by a UDP Option containing the ECHOREQ option, which
is followed by NOP Options to pad the remainder of the datagram
payload. The NOP padding is used to control the length of the probe
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 14]
Internet-Draft Datagram PLPMTUD October 2017
packet.
A UDP Option carrying the ECHORES option is used to provide feedback
when the probe packet is received at the destination endpoint.
5.1.4. Validating the Path with UDP Options
Since UDP is an unacknowledged PL, a sender that does not have
higher-layer information confirming correct delivery of datagrams
SHOULD implement the REACHABILITY_TIMER to periodically send probe
packets while in the PROBE_DONE state.
5.1.5. Handling of PTB Messages by UDP
Normal ICMP verification MUST be performed as specified in Section
5.2 of [RFC8085]. This requires that the PL verifies each received
PTB messages to verify these are received in response to transmitted
traffic. A verified PTB message MAY be used as input to the PLPMTUD
algorithm.
5.2. SCTP
Section 10.2 of [RFC4821] specifies a recommended PLPMTUD probing
method for SCTP. It recommends the use of the PAD chunk, defined in
[RFC4820] to be attached to a minimum length HEARTBEAT chunk to build
a probe packet. This enables probing without affecting the transfer
of user messages and without interfering with congestion control.
This is preferred to the use of DATA chunks (with padding as
required) to serve as path probes.
<< We might define a parameter contained in the INIT and INIT ACK
chunk to indicate the MTU to the peer. However, multihoming makes
this a bit complex, so it might not be worth doing.>>
5.2.1. SCTP/IP4 and SCTP/IPv6
The base protocol is specified in [RFC4960].
5.2.1.1. Sending SCTP Probe Packets
Probe packets consist of an SCTP common header followed by a
HEARTBEAT chunk and a PAD chunk. The PAD chunk is used to control
the length of the probe packet. The HEARTBEAT chunk is used to
trigger the sending of a HEARTBEAT ACK chunk. The reception of the
HEARTBEAT ACK chunk acknowledges reception of a successful probe.
The HEARTBEAT chunk carries a Heartbeat Information parameter which
should include, besides the information suggested in [RFC4960], the
probing size, which is the MTU size the complete datagram will add up
to. The size of the PAD chunk is therefore computed by reducing the
probing size by the IPv4 or IPv6 header size, the SCTP common header,
the HEARTBEAT request and the PAD chunk header. The payload of the
PAD chunk contains arbitrary data.
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 15]
Internet-Draft Datagram PLPMTUD October 2017
To avoid the fragmentation of retransmitted data, probing starts
right after the handshake before data is sent. Assuming normal
behaviour (i.e., the PMTU is smaller than or equal to the interface
MTU), this process will take a few RTTs depending on the number of
PMTU sizes probed. The Heartbeat timer can be used to implement the
PROBE_TIMER.
5.2.1.2. Validating the Path with SCTP
Since SCTP provides an acknowledged PL, a sender does MUST NOT
implement the REACHABILITY_TIMER while in the PROBE_DONE state.
5.2.1.3. PTB Message Handling by SCTP
Normal ICMP verification MUST be performed as specified in Appendix C
of [RFC4960]. This requires that the first 8 bytes of the SCTP
common header are quoted in the payload of the PTB message , which
can be the case for ICMPv4 and is normally the case for ICMPv6. When
the verification is completed, the router Link MTU indicated in the
PTB message SHOULD be used with the PLPMTUD algorithm.
5.2.2. SCTP/UDP
The UDP encapsulation of SCTP is specified in [RFC6951].
5.2.2.1. Sending SCTP/UDP Probe Packets
Packet probing can be performed as specified in Section 5.2.1.1. The
maximum payload is reduced by 8 bytes, which has to be considered
when filling the PAD chunk.
5.2.2.2. Validating the Path with SCTP/UDP
Since SCTP provides an acknowledged PL, a sender does MUST NOT
implement the REACHABILITY_TIMER while in the PROBE_DONE state.
5.2.2.3. Handling of PTB Messages by SCTP/UDP
Normal ICMP verification MUST be performed for PTB messages as
specified in Appendix C of [RFC4960]. This requires that the first 8
bytes of the SCTP common header are contained in the PTB message,
which can be the case for ICMPv4 (but note the UDP header also
consumes a part of the quoted packet header) and is normally the case
for ICMPv6. When the verification is completed, the router Link MTU
size indicated in the PTB message SHOULD be used with the PLPMTUD
algorithm.
5.2.3. SCTP/DTLS
The DTLS encapsulation of SCTP is specified in [I-D.ietf-tsvwg-sctp-
dtls-encaps]. It is used for data channels in WebRTC
implementations.
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 16]
Internet-Draft Datagram PLPMTUD October 2017
5.2.3.1. Sending SCTP/DTLS Probe Packets
Packet probing can be done as specified in Section 5.2.1.1.
5.2.3.2. Validating the Path with SCTP/DTLS
Since SCTP provides an acknowledged PL, a sender does MUST NOT
implement the REACHABILITY_TIMER while in the PROBE_DONE state.
5.2.3.3. Handling of PTB Messages by SCTP/DTLS
It is not possible to perform normal ICMP verification as specified
in [RFC4960], since even if the ICMP contains enough information, the
reflected SCTP common header would be encrypted. Therefore it is not
possible to process PTB messages at the PL.
5.3. Other IETF Transports
QUIC is a UDP-based transport that provides reception feedback [I-D
.ietf-quic-transport].
<< This section will be completed in a future revision of this ID >>
6. Acknowledgements
This work was partially funded by the European Union's Horizon 2020
research and innovation programme under grant agreement No. 644334
(NEAT). The views expressed are solely those of the author(s).
7. IANA Considerations
This memo includes no request to IANA.
If there are no requirements for IANA, the section will be removed
during conversion into an RFC by the RFC Editor.
8. Security Considerations
The security considerations for the use of UDP and SCTP are provided
in the references RFCs. Security guidance for applications using UDP
is provided in the UDP-Guidelines [RFC8085].
PTB messages could potentially be used to cause a node to
inappropriately reduce the effective PMTU. A node supporting PLPMTUD
SHOULD appropriately verify the payload of PTB messages to ensure
these are received in response to transmitted traffic (i.e., a
reported error condition that corresponds to a datagram actually sent
by the path layer.
9. References
9.1. Normative References
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 17]
Internet-Draft Datagram PLPMTUD October 2017
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", Internet-Draft draft-ietf-quic-
transport-04, June 2017.
[I-D.ietf-tsvwg-sctp-dtls-encaps]
Tuexen, M., Stewart, R., Jesup, R. and S. Loreto, "DTLS
Encapsulation of SCTP Packets", Internet-Draft draft-ietf-
tsvwg-sctp-dtls-encaps-09, January 2015.
[I-D.ietf-tsvwg-udp-options]
Touch, J., "Transport Options for UDP", Internet-Draft
draft-ietf-tsvwg-udp-options-01, June 2017.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, DOI 10.17487/
RFC1122, October 1989, <http://www.rfc-editor.org/info/
rfc1122>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997, <http://www.rfc-editor.org/info/
rfc2119>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E.Ed.,
and G. Fairhurst, Ed., "The Lightweight User Datagram
Protocol (UDP-Lite)", RFC 3828, DOI 10.17487/RFC3828, July
2004, <http://www.rfc-editor.org/info/rfc3828>.
[RFC4820] Tuexen, M., Stewart, R. and P. Lei, "Padding Chunk and
Parameter for the Stream Control Transmission Protocol
(SCTP)", RFC 4820, DOI 10.17487/RFC4820, March 2007,
<https://www.rfc-editor.org/info/rfc4820>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007, <https://
www.rfc-editor.org/info/rfc4960>.
[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
Control Transmission Protocol (SCTP) Packets for End-Host
to End-Host Communication", RFC 6951, DOI 10.17487/
RFC6951, May 2013, <https://www.rfc-editor.org/info/
rfc6951>.
[RFC8085] Eggert, L., Fairhurst, G. and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <http://www.rfc-editor.org/info/rfc8085>.
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 18]
Internet-Draft Datagram PLPMTUD October 2017
[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>.
9.2. Informative References
[RFC1191] Mogul, J.C. and S.E. Deering, "Path MTU discovery", RFC
1191, DOI 10.17487/RFC1191, November 1990, <http://www
.rfc-editor.org/info/rfc1191>.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC
2923, DOI 10.17487/RFC2923, September 2000, <https://www
.rfc-editor.org/info/rfc2923>.
[RFC4340] Kohler, E., Handley, M. and S. Floyd, "Datagram Congestion
Control Protocol (DCCP)", RFC 4340, DOI 10.17487/RFC4340,
March 2006, <https://www.rfc-editor.org/info/rfc4340>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<http://www.rfc-editor.org/info/rfc4821>.
[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
ICMPv6 Messages in Firewalls", RFC 4890, DOI 10.17487/
RFC4890, May 2007, <http://www.rfc-editor.org/info/
rfc4890>.
Appendix A. Event-driven state changes
This appendix contains an informative description of key events:
Path Setup: When a new path is initiated, the state is set to
PROBE_NONE. As soon as the path is confirmed, the state changes to
PROBE_BASE and the probing mechanism for this path is started. A
probe packet with the size of the BASE_PMTU is sent.
Arrival of an Acknowledgment: Depending on the probing state, the
reaction differs according to Figure 4, which is just a
simplification of Figure 1 focusing on this event.
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 19]
Internet-Draft Datagram PLPMTUD October 2017
+--------------+
| PROBE_NONE | --------------
+--------------+ \
\
+--------------+ \
| PROBE_ERROR | --------------- \
+--------------+ \ \
\ \
+--------------+ \ \ +--------------+
| PROBE_BASE | --1---------- \ ------------> | PROBE_BASE |
+--------------+ --2----- \ \ +--------------+
\ \ \
+--------------+ \ \ ------------> +--------------+
| PROBE_SEARCH | --2--- \ -----------------> | PROBE_SEARCH |
+--------------+ --1---\----\---------------------> +--------------+
\ \
+--------------+ \ \ +--------------+
| PROBE_DONE | \ -------------------> | PROBE_DONE |
+--------------+ -----------------------> +--------------+
Condition 1: The maximum PMTU size has not yet been reached.
Condition 2: The maximum PMTU size has been reached.
Probing timeout: The PROBE_COUNT is initialised to zero each time the
value of PROBED_SIZE is changed. The PROBE_TIMER is started each
time a probe packet is sent. It is stopped when an acknowledgment
arrives that confirms delivery of a probe packet. If the probe
packet is not acknowledged before,the PROBE_TIMER expires, the
PROBE_ERROR_COUNTER is incremented. When the PROBE_COUNT equals
the value MAX_PROBES, the state is changed, otherwise a new probe
packet of the same size (PROBED_SIZE) is resent. The state
transitions are illustrated in Figure 5. This shows a
simplification of Figure 1 with a focus only on this event.
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 20]
Internet-Draft Datagram PLPMTUD October 2017
+--------------+
| PROBE_NONE |
+--------------+
+--------------+ +--------------+
| PROBE_ERROR | -----------------> | PROBE_ERROR |
+--------------+ / +--------------+
/
+--------------+ --2----------/ +--------------+
| PROBE_BASE | --1------------------------------> | PROBE_BASE |
+--------------+ +--------------+
+--------------+ +--------------+
| PROBE_SEARCH | --1------------------------------> | PROBE_SEARCH |
+--------------+ --2--------- +--------------+
\
+--------------+ \ +--------------+
| PROBE_DONE | -------------------> | PROBE_DONE |
+--------------+ +--------------+
Condition 1: The maximum number of probe packets has not been
reached. Condition 2: The maximum number of probe packets has been
reached.
PMTU raise timer timeout: The path through the network can change
over time. It impossible to discover whether a path change has
increased in the actual PMTU by exchanging packets less than or
equal to the effective PMTU. This requires PLPMTUD to periodically
send a probe packet to detect whether a larger PMTU is possible.
This probe packet is generated by the PMTU_RAISE_TIMER. When the
timer expires, probing is restarted with the BASE_PMTU and the
state is changed to PROBE_BASE.
Arrival of an ICMP message: The active probing of the path can be
supported by the arrival of PTB messages sent by routers or
middleboxes with a link MTU that is smaller than the probe packet
size. If the PTB message includes the router link MTU, three
cases can be distinguished:
1. The indicated link MTU in the PTB message is between the
already probed and effective MTU and the probe that triggered
the PTB message.
2. The indicated link MTU in the PTB message is smaller than the
effective PMTU.
3. The indicated link MTU in the PTB message is equal to the
BASE_PMTU.
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 21]
Internet-Draft Datagram PLPMTUD October 2017
In first case, the PROBE_BASE state transitions to the PROBE_ERROR
state. In the PROBE_SEARCH state, a new probe packet is sent with
the sized reported by the PTB message. Its result is handled
according to the former events.
The second case could be a result of a network re-configuration.
If the reported link MTU in the PTB message is greater than the
BASE_MTU, the probing starts again with a value of PROBE_BASE.
Otherwise, the method enters the state PROBE_ERROR.
In the third case, the maximum possible PMTU has been reached.
This is probed again, because there could be a link further along
the path with a still smaller MTU.
Note: Not all routers include the link MTU size when they send a
PTB message. If the PTB message does not indicate the link MTU,
the probe is handled in the same way as condition 2 of Figure 5.
Appendix B. Revision Notes
Note to RFC-Editor: please remove this entire section prior to
publication.
Individual draft -00:
o Comments and corrections are welcome directly to the authors or
via the IETF TSVWG working group mailing list.
o This update is proposed for WG comments.
Individual draft -01:
o Contains the first representation of the algorithm, showing the
states and timers
o The text describing when to set the effective PMTU has not yet
been verified by the authors
o The text describing how to handle a PTB message indicating a link
MTU larger than the probe has yet not been verified by the authors
o No text currently describes how to handle inconsistent results
from arbitrary re-routing along different parallel paths
o Some middleboxes lie about the MTU they report in PTB messages.
o Some constants and times do not yet have recommended values
o To determine security to off-path-attacks: We need to decide
whether a received PTB message SHOULD be verified or MUST be
verified?
o This update is proposed for WG comments.
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 22]
Internet-Draft Datagram PLPMTUD October 2017
Authors' Addresses
Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen, AB24 3UE
UK
Email: gorry@erg.abdn.ac.uk
Tom Jones
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen, AB24 3UE
UK
Email: tom@erg.abdn.ac.uk
Michael Tuexen
Muenster University of Applied Sciences
Stegerwaldstrasse 39
Steinfurt, 48565
DE
Email: tuexen@fh-muenster.de
Irene Ruengeler
Muenster University of Applied Sciences
Stegerwaldstrasse 39
Steinfurt, 48565
DE
Email: i.ruengeler@fh-muenster.de
Fairhurst, Jones, Tuexen &ExpireslMay 01, 2018 [Page 23]