Internet Engineering Task Force G. Fairhurst
Internet-Draft T. Jones
Intended status: Standards Track University of Aberdeen
Expires: July 26, 2018 M. Tuexen
I. Ruengeler
Muenster University of Applied Sciences
January 22, 2018

Packetization Layer Path MTU Discovery for Datagram Transports
draft-ietf-tsvwg-datagram-plpmtud-00

Abstract

This document describes a robust method for Path MTU Discovery (PMTUD) for datagram Packetization layers. The method allows a Packetization layer (or a datagram application that uses it) to probe an network path with progressively larger packets to determine a maximum packet size. The document describes as an extension to RFC 1191 and RFC 8201, which specify ICMP-based Path MTU Discovery for IPv4 and IPv6. This provides functionally for datagram transports that is equivalent to the Packetization layer PMTUD specification for TCP, specified in RFC4821.

When published, this specification updates 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 https://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 July 26, 2018.

Copyright Notice

Copyright (c) 2018 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 (https://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 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

The IETF has specified datagram transport using UDP, SCTP, and DCCP, as well as protocols layered on top of these transports (e.g., SCTP/UDP, DCCP/UDP).

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]). The term PTB message is applied to both IPv4 ICMP Unreachable messages (type 3) that carry the error Fragmentation Needed (Type 3, Code 4) and ICMPv6 packet too big messages (Type 2). The sender adjusts the effective Path MTU (PMTU), based on reception of ICMP PTB messages to decrease the PMTU when a packet is sent with a size larger than the value reported as the Link MTU in the PTB message, 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 black-holed (all datagrams sent with this size are silently discarded). This could continue to 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 in some cases are not correctly processed by tunnel endpoints.

Another failure could result if a system not on the network path sends a PTB that attempts to force the sender to change the effective PMTU [RFC8201]. A sender can protect itself from reacting to such messages by utilising 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). Most routers implement RFC792 [RFC0792], which requires them to return only the first 64 bits of the IP payload of the packet, whereas RFC1812 [RFC1812] requires routers to return the full packet if possible.

Even when the PTB message includes sufficient bytes of the quoted packet, the network layer could lack sufficient context to perform verification, because this depends on information about the active transport flows at an endpoint node (e.g., the socket/address pairs being used, and other protocol header information).

The term Packetization Layer (PL) 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 above the transport. PTB verification is more straight forward at the PL or at a higher layer.

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 size of the successful probe.

PLPMTUD introduces flexibility in the implementation of PMTU discovery. At one extreme, it can be configured to only perform PTB black hole detection and 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 increased 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 probe packets, 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.

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). A method can utilise ICMP PTB messages when received messages are made available to the PL.

Finally, Section 5 specifies the method for a set of transports, and provides information to enables the implementation of PLPMTUD with other datagram transports and applications that use datagram transports.

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 transmits packets of a particular size with a previously known PMTU, but is unaware of a change to the path that resulted in 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 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 that is 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 can 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 can 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.
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.
Probe packet:
A datagram sent with a purposely chosen size (typically larger than the current Effective PMTU or MPS) to detect if messages of this size can be successfully sent along the end-to-end path.

3. Features required to provide Datagram PLPMTUD

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 nine 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 could be less than EMTU_R). Some applications also have a maximum transport protocol data unit (PDU) size, in which case there is 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). The method can use this 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. In IPv6, a probe packet is always sent without source fragmentation (as specified in section 5.4 of [RFC8201]).
  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 used by datagram PLPMTUD to reduce the size of a probe, but MUST NOT be used increase the effective PMTU ([RFC8201]).
  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 at the sending node is REQUIRED to pass this feedback to the sender-side 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 does not directly trigger a congestion control reaction [RFC4821].
  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 probe packets 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 MAY retransmit any data included in a lost probe packet without adjusting its congestion window [RFC4821].
  8. Cached effective PMTU: The sender MUST cache the effective PMTU value used by an instance of the PL 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 PMTU value could also be stored with the corresponding entry in the destination cache and used by other PL instances. 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. Section 5.2 of [RFC8201] provides guidance on the caching of PMTU information and also the relation to IPv6 flow labels.

In addition the following design principles are stated:

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, a datagram PL that needs to construct a probe packet has to either request an application to send a data block that is larger than that generated by an application, or to utilise padding functions to extend a 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, it may be required to send a probe packet with a size less than the size of the data block generated by an application. In this case, the PL could provide a way to fragment a datagram at the PL, or could instead utilise a control packet with padding.

A receiver needs to be able to distinguish an in-band data block from any added padding. This is needed to ensure that any added padding is not passed on to an application at the receiver.

This results in three possible 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 method requests the application to issue a data block of the desired probe size. If the application/transport needs protection from the loss of an unsuccessful probe packet, the application/transport needs then to 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).
Probing using appication data and padding 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 block in a datagram without the padding data).
Probing using padding data:
A probe packet that contains only control information together with any padding needed to inflate the packet to the size required for the probe. Since these probe packets do not carry an application-supplied data block,they do not typically require retransmission, although they do still consume network capacity and incur endpoint processing.

A datagram PLPMTUD MAY choose to use only one of these methods to simplify the implementation.

3.2. Validation of the current effective PMTU

The PL needs a method to determine when probe packets 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). When supported, this mechanism SHOULD 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 rely on an application protocol to detect this, or make use of an additional transport method such as UDP-Options [I-D.ietf-tsvwg-udp-options]. In addition, they might need to send reachability probes (e.g., periodically solicit a response from the destination) to determine whether the current effective PMTU is still supported by the network path.

Section Section 4 specifies this function for a set of IETF-specified protocols.

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.

A method can additionally utilise PTB messages to detect when the actual PMTU supported by a network path is less than the current size of datagrams (or probe messages) that are being sent.

4. Datagram Packetization Layer PMTUD

This section specifies Datagram PLPMTUD.

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.

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.

The PROBE_COUNT is initialised to zero when a probe packet is first sent with a particular size. Each time the probe_timer expires, the PROBE_COUNT is incremented, and a probe packet of the same size is retransmitted. The maximum number of retransmissions per probing size is configured (MAX_PROBES). If the value of the PROBE_COUNT reaches 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. Selecting the Size of a Probe Message

Path probing relies on generation of probe packets with a specific size. This section decribes how the algorithm selects the size of the next probe message.

XXX Details may be specified in later revisions of this document- the next list is for information XXX

There are serveral things to consider, these include:

4.3. Verification and use of PTB messages

XXX A decision on SHOULD/MUST needs to be made on how to verify messages XXX

This section describes processing for both IPv4 ICMP Unreachable messages (type 3) and ICMPv6 packet too big messages.

A node that receives a PTB message from a router or middlebox, SHOULD/MUST verify the PTB message. The node checks the protocol information in the quoted payload to verify that the message originated from the sending node. The node also checks that the reported MTU size is less than the size used by packet probes. PTB messages are discarded if they fail to pass these checks, or where there is insufficient ICMP payload to perform these checks. The checks are intended to provide protection from packets that originate from a node that is not on the network path or a node that attempts to report a larger MTU than the current probe size.

PTB messages that have been verified can be utilised by the DPLPMTUD algorithm. A method that utilises these PTB messages can improve performance compared to one that relies solely on probing.

XXX Specification needed of how to utilise the reported Link MTU size to generate a probe, and how to account for encapuslations that may be present at the point where the PTB message was generated. XXX

4.4. 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. This value MUST be larger than 1 second, and SHOULD be larger than 15 seconds. Guidance on selection of the timer value are provide in section 3.1.1 of the UDP Usage Guidelines [RFC8085].
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 PLPMTUD [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 application that needs to employ keep-alive messages to deliver useful service over UDP SHOULD NOT transmit them more frequently than once every 15 seconds and SHOULD use longer intervals when possible. DPLPMTUD ought to suspend reachability probes when no application data has been sent since the previous probe packet. Guidance on selection of the timer value are provide in section 3.1.1 of the UDP Usage Guidelines[RFC8085].

An implementation could implement the various timers using a single timer process.

4.5. Constants

The following constants are defined:

MAX_PROBES:
The maximum value of the PROBE_ERROR_COUNTER. The default value of MAX_PROBES is 10.
MIN_PMTU:
The smallest allowed probe packet size. This value is 1280 bytes, as specified in [RFC2460]. For IPv4, the minimum value is 68 bytes. (An IPv4 routed is required to be able to forward a datagram of 68 octets without further fragmentation. This is the combined size of an IPv4 header and the minimum fragment size of 8 octets.)
BASE_PMTU:
The BASE_PMTU is a considered a size that ought to work in most cases. The size is equal to or larger than the minimum permitted and smaller than the maximum allowed. In the case of IPv6, this value is 1280 bytes [RFC2460]. When using IPv4, a size of 1200 is RECOMMENDED.
MAX_PMTU:
The MAX_PMTU is the largest size of PMTU that is probed. This has to be less than or equal to the minimum of the local MTU of the outgoing interface and the destination effective MTU for receiving. An application or PL may reduce this when it knows there is no need to send packets above a specific size.

4.6. 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_Size is value returned by a verified PTB message indicating the local MTU size of a router along the path.

4.7. Selecting Probe Size

XXX There are serveral things to consider when selecting the size to probe, some of which may be specified in later revisions of this document XXX

Issues include:

4.8. 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.

                                      PROBE_TIMER expiry
                                   (PROBE_COUNT = MAX_PROBES)
                        +-------------+                +--------------+
                     =->| PROBE_START |--------------->|PROBE_DISABLED|
  PROBE_TIMER expiry |  +-------------+                +--------------+
 (PROBE_COUNT =      |     |       |  
         MAX_PROBES) -------       |  Connectivity confirmed
                                   v
                  ----------- +------------+ -- PROBE_TIMER expiry
MAX_PMTU acked or |           | PROBE_BASE |  | (PROBE_COUNT < 
PTB (>= BASE_PMTU)|    -----> +------------+ <-             MAX_PROBES)
  ----------------     |          /\   |  |
  |                    |           |   |  | 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)            or
                                Probe acked

Figure 1: State machine for Datagram PLPMTUD

XXX State machine to be updated for PTB messages - to probe for PTB size XXX

The following states are defined to reflect the probing process:

Appendix A contains an informative description of key events.

PROBE_START:
The PROBE_START 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 sent packet has been acknowledged 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 probing with datagram PLPMTUD. It is used to confirm whether the BASE_PMTU size is supported by the network path. On entry, the PROBED_SIZE is set 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 verified, 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. When used with an acknowledged PL (e.g., SCTP), DPLPMTUD SHOULD NOT continue to probe in this state.
PROBE_DISABLED:
The PROBE_DISABLED state indicates that connectivity could not be established. DPLPMTUD MUST NOT probe in this state.

5. Specification of Protocol-Specific Methods

This section specifies protocol-specific details for datagram PLPMTUD for IETF-specified transports.

5.1. DPLPMTUD for UDP and UDP-Lite

The current specifications of UDP [RFC0768] and UDP-LIte [RFC3828] do not define a method in the RFC-series that supports PLPMTUD. In particular, these transports do not provide the transport layer features needed to implement datagram PLPMTUD, and any support for Datagram PLPMTUD would therefore need to rely on higher-layer protocol features [RFC8085].

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.

XXX << Future versions of the spec 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. This token can be set to a value that is likely to be known only to the sender (and becomes known to nodes along the end-to-end path). The sender can then check the value returned in the response to provide additional protection from off-path insertion of data [RFC8085].

        +---------+--------+-----------------+
        | Kind=9  | Len=6  |     Token       |
        +---------+--------+-----------------+
          1 byte    1 byte       4 bytes       

Figure 2: UDP ECHOREQ Option Format

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 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       

Figure 3: UDP ECHORES Option Format

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 to the probe size. NOP padding is used to control the length of the probe packet.

A UDP Option carrying the ECHORES option is used to provide feedback when a 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 and that the reported LInk MTU is less than the current probe size. A verified PTB message MAY be used as input to the PLPMTUD algorithm.

5.2. DPLPMTUD for 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 using DATA chunks (with padding as required) as path probes.

XXX << Future versions of this specification 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.

To avoid 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 round trip time periods 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 a PTB message has been verified, the router Link MTU indicated in the PTB message SHOULD be used with the PLPMTUD algorithm, providing that the reported Link MTU is less than the current probe size.

5.2.2. DPLPMTUD for 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 providing that the reported LInk MTU is less than the current probe size.

5.2.3. DPLPMTUD for SCTP/DTLS

The Datagram Transport Layer Security (DTLS) encapsulation of SCTP is specified in [I-D.ietf-tsvwg-sctp-dtls-encaps]. It is used for data channels in WebRTC implementations.

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 message payload contains sufficient 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

Quick UDP Internet Connection (QUIC) is a UDP-based transport that provides reception feedback [I-D.ietf-quic-transport].

XXX << This section will be completed in a future revision of this ID >>

5.4. DPLPMTUD by Applications

Applications that use the Datagram API (e.g., applications built directly or indirectly on UDP) can implement DPLPMTUD. Some primitives used by DPLPMTUD might not be available via this interface (e.g., the ability to access the PMTU cache, or interpret received ICMP PTB messages).

In addition, it is important that PMTUD is not performed by multiple protocol layers.

XXX << 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.

XXX << If new UDP Options are specified in this document, a request to IANA will be included here.>>

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/MUST 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.

XXX Determine if parallel forwarding paths needs to be considered XXX

A node performing PLPMTUD could experience conflicting information about the size of supported probe packets. This could occur when there are multiple paths are concurrently in use and these exhibit a different PMTU. If not considered, this could result in data being blackholed when the effective PMTU is larger than the smallest PMTU across the current paths.

9. References

9.1. Normative References

[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.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, August 1980.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, DOI 10.17487/RFC0792, September 1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, DOI 10.17487/RFC1812, June 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, December 1998.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E. and G. Fairhurst, "The Lightweight User Datagram Protocol (UDP-Lite)", RFC 3828, DOI 10.17487/RFC3828, July 2004.
[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.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, September 2007.
[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.
[RFC8085] Eggert, L., Fairhurst, G. and G. Shepherd, "UDP Usage Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, March 2017.
[RFC8201] McCann, J., Deering, S., Mogul, J. and R. Hinden, "Path MTU Discovery for IP version 6", STD 87, RFC 8201, DOI 10.17487/RFC8201, July 2017.

9.2. Informative References

[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, November 1990.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, DOI 10.17487/RFC2923, September 2000.
[RFC4340] Kohler, E., Handley, M. and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, DOI 10.17487/RFC4340, March 2006.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007.
[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering ICMPv6 Messages in Firewalls", RFC 4890, DOI 10.17487/RFC4890, May 2007.

Appendix A. Event-driven state changes

This appendix contains an informative description of key events:

+--------------+                                    +----------------+
|  PROBE_START | --3------------------------------->| PROBE_DISABLED |
+--------------+ --4-----------\                    +----------------+
                                \
+--------------+                 \
| 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. Conition 3: Probe Timer expires and PROBE_COUNT = MAX_PROBEs. Condition 4: PROBE_ACK received.

Figure 4: State changes at the arrival of an acknowledgment

+--------------+                                    +----------------+
|  PROBE_START |----------------------------------->| PROBE_DISABLED |
+--------------+                                    +----------------+

+--------------+                                    +--------------+
| 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.

Figure 5: State changes at the expiration of the probe timer

Path Setup:
When a new path is initiated, the state is set to PROBE_START. 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.
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.
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.
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:

Individual draft -01:

Individual draft -02:

Working Group draft -00:

Authors' Addresses

Godred Fairhurst University of Aberdeen School of Engineering Fraser Noble Building Aberdeen, AB24 3U UK EMail: gorry@erg.abdn.ac.uk
Tom Jones University of Aberdeen School of Engineering Fraser Noble Building Aberdeen, AB24 3U UK EMail: tom@erg.abdn.ac.uk
Michael Tuexen Muenster University of Applied Sciences Stegerwaldstrasse 39 Stein fart, 48565 DE EMail: tuexen@fh-muenster.de
Irene Ruengeler Muenster University of Applied Sciences Stegerwaldstrasse 39 Stein fart, 48565 DE EMail: i.ruengeler@fh-muenster.de