Packetization Layer Path MTU Discovery for Datagram Transports University of Aberdeen
School of Engineering Fraser Noble Building Aberdeen AB24 3UE UK gorry@erg.abdn.ac.uk
University of Aberdeen
School of Engineering Fraser Noble Building Aberdeen AB24 3UE UK tom@erg.abdn.ac.uk
Muenster University of Applied Sciences
Stegerwaldstrasse 39 48565 Steinfurt DE tuexen@fh-muenster.de
Muenster University of Applied Sciences
Stegerwaldstrasse 39 48565 Steinfurt DE i.ruengeler@fh-muenster.de
Muenster University of Applied Sciences
Stegerwaldstrasse 39 48565 Steinfurt DE timo.voelker@fh-muenster.de
Transport Internet Engineering Task Force UDP SCTP Transport PMTUD PLPMTUD This document describes a robust method for Path MTU Discovery (PMTUD) for datagram Packetization Layers (PLs). It describes an extension to RFC 1191 and RFC 8201, which specifies ICMP-based Path MTU Discovery for IPv4 and IPv6. The method allows a PL, or a datagram application that uses a PL, to discover whether a network path can support the current size of datagram. This can be used to detect and reduce the message size when a sender encounters a network black hole (where packets are discarded). The method can probe a network path with progressively larger packets to discover whether the maximum packet size can be increased. This allows a sender to determine an appropriate packet size, providing functionally for datagram transports that is equivalent to the Packetization Layer PMTUD specification for TCP, specified in RFC 4821. The document also provides implementation notes for incorporating Datagram PMTUD into IETF datagram transports or applications that use datagram transports. When published, this specification updates RFC 4821.
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 . 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 7 December 2019.
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Table of Contents
  • .  Introduction
    • .  Classical Path MTU Discovery
    • .  Packetization Layer Path MTU Discovery
    • .  Path MTU Discovery for Datagram Services
  • .  Terminology
  • .  Features Required to Provide Datagram PLPMTUD
  • .  DPLPMTUD Mechanisms
    • .  PLPMTU Probe Packets
    • .  Confirmation of Probed Packet Size
    • .  Detection of Unsupported PLPMTU Size, aka Black Hole Detection
    • .  Response to PTB Messages
      • .  Validation of PTB Messages
      • .  Use of PTB Messages
  • .  Datagram Packetization Layer PMTUD
    • .  DPLPMTUD Components
      • .  Timers
      • .  Constants
      • .  Variables
      • .  Overview of DPLPMTUD Phases
    • .  State Machine
    • .  Search to Increase the PLPMTU
      • .  Probing for a larger PLPMTU
      • .  Selection of Probe Sizes
      • .  Resilience to Inconsistent Path Information
    • .  Robustness to Inconsistent Paths
  • .  Specification of Protocol-Specific Methods
    • .  Application support for DPLPMTUD with UDP or UDP-Lite
      • .  Application Request
      • .  Application Response
      • .  Sending Application Probe Packets
      • .  Validating the Path
      • .  Handling of PTB Messages
    • .  DPLPMTUD for SCTP
      • .  SCTP/IPv4 and SCTP/IPv6
      • .  DPLPMTUD for SCTP/UDP
      • .  DPLPMTUD for SCTP/DTLS
    • .  DPLPMTUD for QUIC
      • .  Sending QUIC Probe Packets
      • .  Validating the Path with QUIC
      • .  Handling of PTB Messages by QUIC
    • .  DPLPMTUD for UDP-Options
  • .  Acknowledgements
  • .  IANA Considerations
  • .  Security Considerations
  • . References
    • .  Normative References
    • .  Informative References
  • .  Revision Notes
  •   Authors' Addresses
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, QUIC/UDP), and direct datagram transport over the IP network layer. This document describes a robust method for Path MTU Discovery (PMTUD) that may be used with these transport protocols (or the applications that use their transport service) to discover an appropriate size of packet to use across an Internet path.
Classical Path MTU Discovery 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., and ). In this document, 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) . When a sender receives a PTB message, it reduces the effective MTU to 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. The packets sent with a size larger than the current effective PMTU are known as probe packets. Packets not intended as probe packets are either fragmented to the current effective PMTU, or the attempt to send fails with an error code. Applications are sometimes provided with a primitive to let them read the Maximum Packet Size (MPS), derived from the current effective PMTU. Classical PMTUD is subject to protocol failures. One failure arises when traffic using a packet size larger than the actual PMTU is black-holed (all datagrams sent with this size, or larger, are discarded). This could arise when the PTB messages are not delivered back to the sender for some reason (see for example ). Examples where PTB messages are not delivered include:
  • The generation of ICMP messages is usually rate limited. This could result in no PTB messages being generated to the sender (see section 2.4 of )
  • ICMP messages can be filtered by middleboxes (including firewalls) . A stateful firewall could be configured with a policy to block incoming ICMP messages, which would prevent reception of PTB messages to a sending endpoint behind this firewall.
  • When the router issuing the ICMP message drops a tunneled packet, the resulting ICMP message will be directed to the tunnel ingress. This tunnel endpoint is responsible for forwarding the ICMP message and also processing the quoted packet within the payload field to remove the effect of the tunnel, and return a correctly formatted ICMP message to the sender . Failure to do this prevents the PTB message reaching the original sender.
  • Asymmetry in forwarding can result in there being no return route to the original sender, which would prevent an ICMP message being delivered to the sender. This issue can also arise when policy-based routing is used, Equal Cost Multipath (ECMP) routing is used, or a middlebox acts as an application load balancer. An example is where the path towards the server is chosen by ECMP routing depending on bytes in the IP payload. In this case, when a packet sent by the server encounters a problem after the ECMP router, then any resulting ICMP message needs to also be directed by the ECMP router towards the original sender.
  • There are additional cases where the next hop destination fails to receive a packet because of its size. This could be due to misconfiguration of the layer 2 path between nodes, for instance the MTU configured in a layer 2 switch, or misconfiguration of the Maximum Receive Unit (MRU). If the packet is dropped by the link, this will not cause a PTB message to be sent to the original sender.
Another failure could result if a node that is not on the network path sends a PTB message that attempts to force a sender to change the effective PMTU . A sender can protect itself from reacting to such messages by utilising the quoted packet within a PTB message payload to validate that the received PTB message was generated in response to a packet that had actually originated from the sender. However, there are situations where a sender would be unable to provide this validation. Examples where validation of the PTB message is not possible include:
  • When a router issuing the ICMP message implements RFC792 , it is only required to include the first 64 bits of the IP payload of the packet within the quoted payload. There could be insufficient bytes remaining for the sender to interpret the quoted transport information. Note: The recommendation in RFC1812 is that IPv4 routers return a quoted packet with as much of the original datagram as possible without the length of the ICMP datagram exceeding 576 bytes. IPv6 routers include as much of the invoking packet as possible without the ICMPv6 packet exceeding 1280 bytes .
  • The use of tunnels/encryption can reduce the size of the quoted packet returned to the original source address, increasing the risk that there could be insufficient bytes remaining for the sender to interpret the quoted transport information.
  • Even when the PTB message includes sufficient bytes of the quoted packet, the network layer could lack sufficient context to validate the message, because validation 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).
  • When a packet is encapsulated/tunneled over an encrypted transport, the tunnel/encapsulation ingress might have insufficient context, or computational power, to reconstruct the transport header that would be needed to perform validation.
Packetization Layer Path MTU Discovery The term Packetization Layer (PL) has been introduced to describe the layer that is responsible for placing data blocks into the payload of IP packets and selecting an appropriate MPS. This function is often performed by a transport protocol, but can also be performed by other encapsulation methods working above the transport layer. In contrast to PMTUD, Packetization Layer Path MTU Discovery (PLPMTUD) does not rely upon reception and validation 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 sends probe packets to search for the largest size of unfragmented datagram that can be sent over a network path. Probe packets are sent with a progressively larger packet size. If a probe packet is successfully delivered (as determined by the PL), then the PLPMTU is raised to the size of the successful probe. If no response is received to a probe packet, the method reduces the probe size. The result of probing with the PLPMTU is used to set the application MPS. PLPMTUD introduces flexibility in the implementation of PMTU discovery. At one extreme, it can be configured to only perform ICMP 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 that data is lost when probing to discover the path MTU. For example, information available at the PL, or higher layers, enables received PTB messages to be validated before being utilized.
Path MTU Discovery for Datagram Services of this document presents a set of algorithms for datagram protocols to discover the largest size of unfragmented datagram that can be sent over a network path. The method described relies on features of the PL described in and applies to transport protocols operating over IPv4 and IPv6. It does not require cooperation from the lower layers, although it can utilize PTB messages when these received messages are made available to the PL. The UDP Usage Guidelines 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 that can be used on a network path. Prior to this document, PLPMTUD had not been specified for UDP. Section 10.2 of recommends a PLPMTUD probing method for the Stream Control Transport Protocol (SCTP). SCTP utilizes probe packets consisting of a minimal sized HEARTBEAT chunk bundled with a PAD chunk as defined in , but RFC4821 does not provide a complete specification. The present document provides the details to complete that specification. The Datagram Congestion Control Protocol (DCCP) requires implementations to support Classical PMTUD and states that a DCCP sender "MUST maintain the MPS allowed for each active DCCP session". It also defines the current congestion control MPS (CCMPS) supported by a network 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 method defined in this specification could be used with DCCP. specifies the method for a set of transports, and provides information to enable the implementation of PLPMTUD with other datagram transports and applications that use datagram transports.
Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. Other terminology is directly copied from , and the definitions in .
Actual PMTU:
The Actual PMTU is the PMTU of a network path between a sender PL and a destination PL, which the DPLPMTUD algorithm seeks to determine.
Black Hole:
A Black Hole is encountered when a sender is unaware that packets are not being delivered to the destination end point. Two types of Black Hole are relevant to DPLPMTUD:
Packet Black Hole:
Packets encounter a Packet Black Hole when packets are not delivered to the destination endpoint (e.g., when the sender transmits packets of a particular size with a previously known effective PMTU and they are discarded by the network).
ICMP Black Hole
An ICMP Black Hole is encountered when the sender is unaware that packets are not delivered to the destination endpoint because PTB messages are not received by the originating PL sender.
Black holed :
Traffic is black-holed when the sender is unaware that packets are not being delivered. This could be due to a Packet Black Hole or an ICMP Black Hole.
Classical Path MTU Discovery:
Classical PMTUD is a process described in and , in which nodes rely on PTB messages to learn the largest size of unfragmented datagram that can be used across a network path.
Datagram:
A datagram is a transport-layer protocol data unit, transmitted in the payload of an IP packet.
Effective PMTU:
The Effective PMTU is the current estimated value for PMTU that is used by a PMTUD. This is equivalent to the PLPMTU derived by PLPMTUD.
EMTU_S:
The Effective MTU for sending (EMTU_S) is defined in 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 as the largest datagram size that can be reassembled by EMTU_R (Effective MTU to receive).
Link:
A Link is 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 Link 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. 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 the link MTU to include the link layer headers.
MAX_PMTU:
The MAX_PMTU is the largest size of PLPMTU that DPLPMTUD will attempt to use.
MPS:
The Maximum Packet Size (MPS) is the largest size of application data block that can be sent across a network path by a PL. In DPLPMTUD this quantity is derived from the PLPMTU by taking into consideration the size of the lower protocol layer headers. Probe packets generated by DPLPMTUD can have a size larger than the MPS.
MIN_PMTU:
The MIN_PMTU is the smallest size of PLPMTU that DPLPMTUD will attempt to use.
Packet:
A Packet is the IP header plus the IP payload.
Packetization Layer (PL):
The Packetization Layer (PL) is the layer of the network stack that places data into packets and performs transport protocol functions.
Path:
The Path is the set of links and routers traversed by a packet between a source node and a destination node by a particular flow.
Path MTU (PMTU):
The Path MTU (PMTU) is the minimum of the Link MTU of all the links forming a network path between a source node and a destination node.
PTB_SIZE:
The PTB_SIZE is a value reported in a validated PTB message that indicates next hop link MTU of a router along the path.
PLPMTU:
The Packetization Layer PMTU is an estimate of the actual PMTU provided by the DPLPMTUD algorithm.
PLPMTUD:
Packetization Layer Path MTU Discovery (PLPMTUD), the method described in this document for datagram PLs, which is an extension to Classical PMTU Discovery.
Probe packet:
A probe packet is a datagram sent with a purposely chosen size (typically the current PLPMTU or larger) to detect if packets of this size can be successfully sent end-to-end across the network path.
Features Required to Provide Datagram PLPMTUD TCP PLPMTUD has been defined using standard TCP protocol mechanisms. All of the requirements in also apply to the use of the technique with a datagram PL. Unlike TCP, some datagram PLs require additional mechanisms to implement PLPMTUD. There are eight requirements for performing the datagram PLPMTUD method described in this specification:
  1. PMTU parameters: A DPLPMTUD sender is RECOMMENDED to provide information about the maximum size of packet that can be transmitted by the sender on the local link (the local Link MTU). It MAY utilize similar information about the receiver when this is supplied (note this could be less than EMTU_R). This avoids implementations trying to send probe packets that can not be transmitted by the local link. Too high of a value could reduce the efficiency of the search algorithm. 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. PLPMTU: A datagram application using a PL not supporting fragmentation is REQUIRED to be able to choose the size of datagrams sent to the network, up to the PLPMTU, or a smaller value (such as the MPS) derived from this. This value is managed by the DPLPMTUD method. The PLPMTU (specified as the effective PMTU in Section 1 of ) is equivalent to the EMTU_S (specified in ).
  3. Probe packets: On request, a DPLPMTUD sender is REQUIRED to be able to transmit a packet larger than the PLMPMTU. This is used to send a probe packet. In IPv4, a probe packet MUST be sent with the Don't Fragment (DF) bit set in the IP header, and without network layer endpoint fragmentation. In IPv6, a probe packet is always sent without source fragmentation (as specified in section 5.4 of ).
  4. Processing PTB messages: A DPLPMTUD sender MAY optionally utilize PTB messages received from the network layer to help identify when a network path does not support the current size of probe packet. Any received PTB message MUST be validated before it is used to update the PLPMTU discovery information . This validation confirms that the PTB message was sent in response to a packet originating by the sender, and needs to be performed before the PLPMTU discovery method reacts to the PTB message. A PTB message MUST NOT be used to increase the PLPMTU .
  5. Reception feedback: The destination PL endpoint is REQUIRED to provide a feedback method that indicates to the DPLPMTUD sender when a probe packet has been received by the destination PL endpoint. The mechanism needs to be robust to the possibility that packets could be significantly delayed along a network path. The local PL endpoint at the sending node is REQUIRED to pass this feedback to the sender DPLPMTUD method.
  6. Probe loss recovery: It is RECOMMENDED to use probe packets that do not carry any user data. Most datagram transports permit this. If a probe packet contains user data requiring retransmission in case of loss, the PL (or layers above) are REQUIRED to arrange any retransmission/repair of any resulting loss. DPLPMTUD is REQUIRED to be robust in the case where probe packets are lost due to other reasons (including link transmission error, congestion).
  7. Probing and congestion control: The DPLPMTUD sender treats isolated loss of a probe packet (with or without a corresponding PTB message) as a potential indication of a PMTU limit for the path. Loss of a probe packet SHOULD NOT be treated as an indication of congestion and the loss SHOULD NOT directly trigger a congestion control reaction .
  8. Shared PLPMTU state: The PLPMTU value could also be stored with the corresponding entry in the destination cache and used by other PL instances. The specification of PLPMTUD 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 ) SHOULD be notified to make use of the new MTU". Such methods MUST be robust to the wide variety of underlying network forwarding behaviors, PLPMTU adjustments based on shared PLPMTU values should be incorporated in the search algorithms. Section 5.2 of provides guidance on the caching of PMTU information and also the relation to IPv6 flow labels.
In addition, the following principles are stated for design of a DPLPMTUD method:
  • MPS: A method is REQUIRED to signal an appropriate MPS to the higher layer using the PL. The value of the MPS can change following a change to the path. It is RECOMMENDED that methods avoid forcing an application to use an arbitrary small MPS (PLPMTU) for transmission while the method is searching for the currently supported PLPMTU. Datagram PLs do not necessarily support fragmentation of PDUs larger than the PLPMTU. A reduced MPS can adversely impact the performance of a datagram application.
  • Path validation: It is RECOMMENDED that methods are robust to path changes that could have occurred since the path characteristics were last confirmed, and to the possibility of inconsistent path information being received.
  • Datagram reordering: A method is REQUIRED to be robust to the possibility that a flow encounters reordering, or the traffic (including probe packets) is divided over more than one network path.
  • When to probe: It is RECOMMENDED that methods determine whether the path has changed since it last measured the path. This can help determine when to probe the path again.
DPLPMTUD Mechanisms This section lists the protocol mechanisms used in this specification.
PLPMTU Probe Packets The DPLPMTUD method relies upon the PL sender being able to generate probe packets with a specific size. TCP is able to generate these probe packets by choosing to appropriately segment data being sent . 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 utilize 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. 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 listed in order of preference:
Probing using padding data:
A probe packet that contains only control information together with any padding, which is needed to be inflated to the size required for the probe packet. 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.
Probing using application 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 packet. If the application/transport needs protection from the loss of this probe packet, the application/transport could 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 application data:
A probe packet that contains a data block supplied by an application that matches the size required for the probe packet. 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).
A PL that uses a probe packet carrying an application data block, could need to retransmit this application data block if the probe fails. This could need the PL to re-fragment the data block to a smaller packet size that is expected to traverse the end-to-end path (which could utilize endpoint network-layer or PL fragmentation when these are available). DPLPMTUD MAY choose to use only one of these methods to simplify the implementation. Probe messages sent by a PL MUST contain enough information to uniquely identify the probe within Maximum Segment Lifetime, while being robust to reordering and replay of probe response and PTB messages.
Confirmation of Probed Packet Size The PL needs a method to determine (confirm) 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 DPLPMTUD to acknowledge reception of a probe packet. A PL that does not acknowledge data reception (e.g., UDP and UDP-Lite) is unable itself to detect when the packets that it sends are discarded because their size is greater than the actual PMTU. These PLs need to either rely on an application protocol to detect this loss, or make use of an additional transport method such as UDP-Options . specifies this function for a set of IETF-specified protocols.
Detection of Unsupported PLPMTU Size, aka Black Hole Detection A PL sender needs to reduce the PLPMTU when it discovers the actual PMTU supported by a network path is less than the PLPMTU. This can be triggered when a validated PTB message is received, or by another event that indicates the network path no longer sustains the current packet size, such as a loss report from the PL, or repeated lack of response to probe packets sent to confirm the PLPMTU. Detection is followed by a reduction of the PLPMTU. This is performed by sending packet probes of size PLPMTU to verify that a network path still supports the last acknowledged PLPMTU size. There are two alternative mechanism:
  • A PL can rely upon a mechanism implemented within the PL to detect excessive loss of data sent with a specific packet size and then conclude that this excessive loss could be a result of an invalid PMTU (as in PLPMTUD for TCP ).
  • A PL can use the DPLPMTUD probing mechanism to periodically generate probe packets of the size of the current PLPMTU (e.g., using the confirmation timer ). A timer tracks whether acknowledgments are received. Successive loss of probes is an indication that the current path no longer supports the PLPMTU (e.g., when the number of probe packets sent without receiving an acknowledgement, PROBE_COUNT, becomes greater than MAX_PROBES).
A PL MAY inhibit sending probe packets when no application data has been sent since the previous probe packet. A PL preferring to use an up-to-data PLPMTU once user data is sent again, MAY choose to continue PLPMTU discovery for each path. However, this may result in additional packets being sent. When the method detects the current PLPMTU is not supported, DPLPMTUD sets a lower MPS. The PL then confirms that the updated PLPMTU can be successfully used across the path. The PL could need 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 use a control packet as the packet probe.
Response to PTB Messages This method requires the DPLPMTUD sender to validate any received PTB message before using the PTB information. The response to a PTB message depends on the PTB_SIZE indicated in the PTB message, the state of the PLPMTUD state machine, and the IP protocol being used. first describes validation for both IPv4 ICMP Unreachable messages (type 3) and ICMPv6 packet too big messages, both of which are referred to as PTB messages in this document.
Validation of PTB Messages This section specifies utilization of PTB messages.
  • A simple implementation MAY ignore received PTB messages and in this case the PLPMTU is not updated when a PTB message is received.
  • An implementation that supports PTB messages MUST validate messages before they are further processed.
A PL that receives a PTB message from a router or middlebox, performs ICMP validation as specified in Section 5.2 of . Because DPLPMTUD operates at the PL, the PL needs to check that each received PTB message is received in response to a packet transmitted by the endpoint PL performing DPLPMTUD. The PL MUST check the protocol information in the quoted packet carried in an ICMP PTB message payload to validate the message originated from the sending node. This validation includes determining that the combination of the IP addresses, the protocol, the source port and destination port match those returned in the quoted packet - this is also necessary for the PTB message to be passed to the corresponding PL. The validation SHOULD utilize information that it is not simple for an off-path attacker to determine . For example, by checking the value of a protocol header field known only to the two PL endpoints. A datagram application that uses well-known source and destination ports ought to also rely on other information to complete this validation. These checks are intended to provide protection from packets that originate from a node that is not on the network path. A PTB message that does not complete the validation MUST NOT be further utilized by the DPLPMTUD method. PTB messages that have been validated MAY be utilized by the DPLPMTUD algorithm, but MUST NOT be used directly to set the PLPMTU. A method that utilizes these PTB messages can improve the speed at the which the algorithm detects an appropriate PLPMTU, compared to one that relies solely on probing. describes this processing.
Use of PTB Messages A set of checks are intended to provide protection from a router that reports an unexpected PTB_SIZE. The PL also needs to check that the indicated PTB_SIZE is less than the size used by probe packets and larger than minimum size accepted. This section provides a summary of how PTB messages can be utilized. This processing depends on the PTB_SIZE and the current value of a set of variables:
MIN_PMTU < PTB_SIZE < BASE_PMTU
  • A robust PL MAY enter an error state (see ) for an IPv4 path when the PTB_SIZE reported in the PTB message is larger than or equal to 68 bytes and when this is less than the BASE_PMTU.
  • A robust PL MAY enter an error state (see ) for an IPv6 path when the PTB_SIZE reported in the PTB message is larger than or equal to 1280 bytes and when this is less than the BASE_PMTU.
PTB_SIZE = PLPMTU
  • Completes the search for a larger PLPMTU.
PTB_SIZE > PROBED_SIZE
  • Inconsistent network signal.
  • PTB message ought to be discarded without further processing (e. g. PLPMTU not modified).
  • The information could be utilized as an input to trigger enabling a resilience mode.
BASE_PMTU <= PTB_SIZE < PLPMTU
  • Black Hole Detection is triggered and the PLPMTU ought to be set to BASE_PMTU.
  • The PL could use the PTB_SIZE reported in the PTB message to initialize a search algorithm.
PLPMTU < PTB_SIZE < PROBED_SIZE
  • The PLPMTU continues to be valid, but the last PROBED_SIZE searched was larger than the actual PMTU.
  • The PLPMTU is not updated.
  • The PL can use the reported PTB_SIZE from the PTB message as the next search point when it resumes the search algorithm.
xxx Author Note: Do we want to specify how to handle PTB Message with PTB_SIZE = 0? xxx
Datagram Packetization Layer PMTUD This section specifies Datagram PLPMTUD (DPLPMTUD). The method can be introduced at various points (as indicated with * in the figure below) in the IP protocol stack to discover the PLPMTU so that an application can utilize an appropriate MPS for the current network path. DPLPMTUD SHOULD NOT be used by an application if it is already used in a lower layer.
Examples where DPLPMTUD can be implemented a l i t i P D e p c I C e Q C P O S P t a n * * T * N r c U U U t e * r I w o n A p o D k f +----------------------+ | Application* | +-+-------+----+----+--+ | | | | +---+--+ +--+--+ | +-+---+ | QUIC*| |UDPO*| | |SCTP*| +---+--+ +--+--+ | +--+--+ | | | | | +-------+--+ | | | | | | | +-+-+--+ | | UDP | | +---+--+ | | | +--------------+-----+-+ | Network Interface | +----------------------+
The central idea of DPLPMTUD is probing by a sender. Probe packets are sent to find the maximum size of a user message that can be completely transferred across the network path from the sender to the destination. The folloowing sections identify the components needed for implementation, provides an overvoew of the phases of operation, and specifies the state machine and search algorithm.
DPLPMTUD Components This section describes the timers, constants, and variables of DPLPMTUD.
Timers The method utilizes up to three timers:
PROBE_TIMER:
The PROBE_TIMER is configured to expire after a period longer than the maximum time to receive an acknowledgment to a probe packet. This value MUST NOT be smaller than 1 second, and SHOULD be larger than 15 seconds. Guidance on selection of the timer value are provided in section 3.1.1 of the UDP Usage Guidelines . If the PL has a path Round Trip Time (RTT) estimate and timely acknowledgements the PROBE_TIMER can be derived from the PL RTT estimate.
PMTU_RAISE_TIMER:
The PMTU_RAISE_TIMER is configured to the period a sender will continue to use the current PLPMTU, after which it re-enters the Search phase. This timer has a period of 600 secs, as recommended by PLPMTUD . DPLPMTUD MAY inhibit sending probe packets when no application data has been sent since the previous probe packet. A PL preferring to use an up-to-data PMTU once user data is sent again, can choose to continue PMTU discovery for each path. However, this may result in sending additional packets.
CONFIRMATION_TIMER:
When an acknowledged PL is used, this timer MUST NOT be used. For other PLs, the CONFIRMATION_TIMER is configured to the period a PL sender waits before confirming the current PLPMTU is still supported. This is less than the PMTU_RAISE_TIMER and used to decrease the PLPMTU (e.g., when a black hole is encountered). Confirmation needs to be frequent enough when data is flowing that the sending PL does not black hole extensive amounts of traffic. Guidance on selection of the timer value are provided in section 3.1.1 of the UDP Usage Guidelines . DPLPMTUD MAY inhibit sending probe packets when no application data has been sent since the previous probe packet. A PL preferring to use an up-to-data PMTU once user data is sent again, can choose to continue PMTU discovery for each path. However, this may result in sending additional packets.
An implementation could implement the various timers using a single timer.
Constants The following constants are defined:
MAX_PROBES:
The MAX_PROBES is the maximum value of the PROBE_COUNT counter (see ). The default value of MAX_PROBES is 10.
MIN_PMTU:
The MIN_PMTU is the smallest allowed probe packet size. For IPv6, this value is 1280 bytes, as specified in . For IPv4, the minimum value is 68 bytes. Note: An IPv4 router is required to be able to forward a datagram of 68 bytes without further fragmentation. This is the combined size of an IPv4 header and the minimum fragment size of 8 bytes. In addition, receivers are required to be able to reassemble fragmented datagrams at least up to 576 bytes, as stated in section 3.3.3 of .
MAX_PMTU:
The MAX_PMTU is the largest size of PLPMTU. This has to be less than or equal to the minimum of the local MTU of the outgoing interface and the destination PMTU for receiving. An application, or PL, MAY reduce the MAX_PMTU when there is no need to send packets larger than a specific size.
BASE_PMTU:
The BASE_PMTU is a configured size expected to work for most paths. The size is equal to or larger than the MIN_PMTU and smaller than the MAX_PMTU. In the case of IPv6, this value is 1280 bytes . When using IPv4, a size of 1200 bytes is RECOMMENDED.
Variables This method utilizes a set of variables:
PROBED_SIZE:
The PROBED_SIZE is the size of the current probe packet. This is a tentative value for the PLPMTU, which is awaiting confirmation by an acknowledgment.
PROBE_COUNT:
The PROBE_COUNT is a count of the number of unsuccessful probe packets that have been sent with a size of PROBED_SIZE. The value is initialized to zero when a particular size of PROBED_SIZE is first attempted.
The figure below illustrates the relationship between the packet size constants and variables at a point of time when the DPLPMTUD algorithm performs path probing to increase the size of the PLPMTU. A probe packet has been sent of size PROBED_SIZE. Once this is acknowledged, the PLPMTU will raise to PROBED_SIZE allowing the DPLPMTUD algorithm to further increase PROBED_SIZE towards the actual PMTU.
Relationships between packet size constants and variables M M _ M c t U M R Z E _ E E _ T M N U D I M A _ T U U a O M S X S T u l P B P A P T P U B A P T P I P L MIN_PMTU MAX_PMTU <--------------------------------------------------> | | | | v | | v BASE_PMTU | v Actual PMTU | PROBED_SIZE v PLPMTU
Overview of DPLPMTUD Phases This section provides a high-level informative view of the DPLPMTUD method, by describing the movement of the method through several phases of operation. More detail is available in the state machine .
DPLPMTUD Phases e c c o e f n i x e p a n i d e n s t y E c B r t o e l d P a o t s e e e A c l r U i C o C n d e i C S t o y f S i B n c p S t y E e S a M i t e o o e t o P r d r l v a r o i r P t a U o n t i r i T B f m p n a M r S A U n i T v a i t e n n v E c h o m e c P d S m o t r i n E M c a r s m i m f t f d e A n c e c a e a m C m n h d i _ M P n l e r B i _ o o e s t o r a l r r e i t n T c h L T _ a R h f n i c g i m d U i +------+ +------->| Base |----------------+ Connectivity | +------+ | or BASE_PMTU | | | confirmation failed | | v | | Connectivity +-------+ | | and BASE_PMTU | Error | | | confirmed +-------+ | | | | v | Consistent connectivity PLPMTU | +--------+ | and BASE_PMTU confirmation | | Search |<--------------+ confirmed failed | +--------+ | ^ | | | | | Raise | | Search | timer | | algorithm | expired | | completed | | | | | v | +-----------------+ +---| Search Complete | +-----------------+
BASE_PMTU Confirmation Phase
  • The BASE_PMTU Confirmation Phase confirms connectivity to the remote peer. This phase is implicit for a connection-oriented PL (where it can be performed in a PL connection handshake). A connectionless PL needs to send an acknowledged probe packet to confirm that the remote peer is reachable.
  • The sender also confirms that BASE_PMTU is supported across the network path.
  • A PL that does not wish to support a path with a PLPMTU less than BASE_PMTU can simplify the phase into a single step by performing the connectivity checks with a probe of the BASE_PMTU size.
  • Once confirmed, DPLPMTUD enters the Search Phase. If this phase fails to confirm, DPLPMTUD enters the Error Phase.
Search Phase
  • The Search Phase utilizes a search algorithm to send probe packets to seek to increase the PLPMTU.
  • The algorithm concludes when it has found a suitable PLPMTU, by entering the Search Complete Phase.
  • A PL could respond to PTB messages using the PTB to advance or terminate the search, see .
  • Black Hole Detection can also terminate the search by entering the BASE_PMTU Confirmation phase.
Search Complete Phase
  • The Search Complete Phase is entered when the PLPMTU is supported across the network path.
  • A PL can use a CONFIRMATION_TIMER to periodically repeat a probe packet for the current PLPMTU size. If the sender is unable to confirm reachability (e.g., if the CONFIRMATION_TIMER expires) or the PL signals a lack of reachability, DPLPMTUD enters the BASE_PMTU Confirmation phase.
  • The PMTU_RAISE_TIMER is used to periodically resume the search phase to discover if the PLPMTU can be raised.
  • Black Hole Detection or receipt of a validated PTB message ) can cause the sender to enter the BASE_PMTU Confirmation Phase.
Error Phase
  • The Error Phase is entered when there is conflicting or invalid PLPMTU information for the path (e.g. a failure to support the BASE_PMTU) that cause DPLPMTUD to be unable to progress and the PLPMTU is lowered
  • DPLPMTUD remains in the Error Phase until a consistent view of the path can be discovered and it has also been confirmed that the path supports the BASE_PMTU (or DPLPMTUD is suspended).
  • Note: MIN_PMTU may be identical to BASE_PMTU, simplifying the actions in this phase.
An implementation that only reduces the PLPMTU to a suitable size would be sufficient to ensure reliable operation, but can be very inefficient when the actual PMTU changes or when the method (for whatever reason) makes a suboptimal choice for the PLPMTU. A full implementation of DPLPMTUD provides an algorithm enabling the DPLPMTUD sender to increase the PLPMTU following a change in the characteristics of the path, such as when a link is reconfigured with a larger MTU, or when there is a change in the set of links traversed by an end-to-end flow (e.g., after a routing or path fail-over decision).
State Machine A state machine for DPLPMTUD is depicted in . If multipath or multihoming is supported, a state machine is needed for each path. Note: Some state changes are not shown to simplify the diagram.
State machine for Datagram PLPMTUD I L R A E X s l C C U : o E : T I P e T D U E S M s c R P T o R r o B _ C P P E b s c t S E R E o P P O r M M E P T e P Z R P P E c T e R I R a e P U R e M E R G = B L R B c M c t U l O L O l d L n B M M A : O S I k _ M U O p N P _ t _ B M H A R i P T = E O E : P R B t R c I E t _ I x _ U k i O B P R E r r a B Z E e : E e r o r P A c A E _ M P o r e N < e e S E P P U O o P S E N k R _ e n x T i E C U r U p T e E P C U : O R T E t v C o p X E Z l r p b a S r L S S = T _ e l c B T x e E R E r E a B M C X B e T B S h O y : T M y T Z D o h o S r c n B P T < I E v I L _ T _ O i a P _ U d d B a L a a _ B P L A C k n P T R O _ E P r O M x E P B < < I O _ P b i e S N T P B O o B R P O t E i d < i p P B B M i E M A I B t P c t R e S P k T _ B o P T I S e P A P _ O s y E R d i P O R R _ r T o n A d N B B P a t T b I y O M y d d k H S O U R M i T x T i A t M _ R P i M E y B T X N P M P E y o O X M e X N : i i I U P R T E t P B _ I e y o _ S T B S d < n r T A e F P A I Z T p d e R r c D B I o N e E _ O T M T _ c B E T o _ E A c x r : A < Z S O e < L _ P r e R B I r P B c C U e _ S M O S : f o _ | | | Start | PL indicates loss | | of connectivity v v +---------------+ +---------------+ | DISABLED | | ERROR | +---------------+ PROBE_TIMER expiry: +---------------+ | PL indicates PROBE_COUNT = MAX_PROBES or ^ | | connectivity PTB: PTB_SIZE < BASE_PMTU | | +--------------------+ +---------------+ | | | | v | BASE_PMTU Probe | +---------------+ acked | | BASE |----------------------+ +---------------+ | Black hole detected or ^ | ^ ^ Black hole detected or | PTB: PTB_SIZE < PLPMTU | | | | PTB: PTB_SIZE < PLPMTU | +--------------------+ | | +--------------------+ | | +----+ | | | PROBE_TIMER expiry: | | | PROBE_COUNT < MAX_PROBES | | | | | | PMTU_RAISE_TIMER expiry | | | +-----------------------------------------+ | | | | | | | | | v | v +---------------+ +---------------+ |SEARCH_COMPLETE| | SEARCHING | +---------------+ +---------------+ | ^ ^ | | ^ | | | | | | | | +-----------------------------------------+ | | | | MAX_PMTU Probe acked or PROBE_TIMER | | | | expiry: PROBE_COUNT = MAX_PROBES or | | +----+ PTB: PTB_SIZE = PLPMTU +----+ CONFIRMATION_TIMER expiry: PROBE_TIMER expiry: PROBE_COUNT < MAX_PROBES or PROBE_COUNT < MAX_PROBES or PLPMTU Probe acked Probe acked or PTB: PLPMTU < PTB_SIZE < PROBED_SIZE
The following states are defined:
DISABLED:
The DISABLED state is the initial state before probing has started. It is also entered from any other state, when the PL indicates loss of connectivity. This state is left, once the PL indicates connectivity to the remote PL.
BASE:
The BASE state is used to confirm that the BASE_PMTU size is supported by the network path and is designed to allow an application to continue working when there are transient reductions in the actual PMTU. It also seeks to avoid long periods where traffic is black holed while searching for a larger PLPMTU. On entry, the PROBED_SIZE is set to the BASE_PMTU size and the PROBE_COUNT is set to zero. Each time a probe packet is sent, the PROBE_TIMER is started. The state is exited when the probe packet is acknowledged, and the PL sender enters the SEARCHING state. The state is also left when the PROBE_COUNT reaches MAX_PROBES or a received PTB message is validated. This causes the PL sender to enter the ERROR state.
SEARCHING:
The SEARCHING state is the main probing state. This state is entered when probing for the BASE_PMTU was successful. The PROBE_COUNT is set to zero when the first probe packet is sent for each probe size. Each time a probe packet is acknowledged, the PLPMTU is set to the PROBED_SIZE, and then the PROBED_SIZE is increased using the search algorithm. When a probe packet is sent and 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 received PTB message is validated, a probe of size MAX_PMTU is acknowledged, or a black hole is detected.
SEARCH_COMPLETE:
The SEARCH_COMPLETE state indicates a successful end to the SEARCHING state. DPLPMTUD remains in this state until either the PMTU_RAISE_TIMER expires, a received PTB message is validated, or a black hole is detected. When DPLPMTUD uses an unacknowledged PL and is in the SEARCH_COMPLETE state, a CONFIRMATION_TIMER periodically resets the PROBE_COUNT and schedules a probe packet with the size of the PLPMTU. If the probe packet fails to be acknowledged after MAX_PROBES attempts, the method enters the BASE state. When used with an acknowledged PL (e.g., SCTP), DPLPMTUD SHOULD NOT continue to generate PLPMTU probes in this state.
ERROR:
The ERROR state represents the case where either the network path is not known to support a PLPMTU of at least the BASE_PMTU size or when there is contradictory information about the network path that would otherwise result in excessive variation in the MPS signalled to the higher layer. The state implements a method to mitigate oscillation in the state-event engine. It signals a conservative value of the MPS to the higher layer by the PL. The state is exited when packet probes no longer detect the error or when the PL indicates that connectivity has been lost. Implementations are permitted to enable endpoint fragmentation if the DPLPMTUD is unable to validate MIN_PMTU within PROBE_COUNT probes. If DPLPMTUD is unable to validate MIN_PMTU the implementation should transition to the DISABLED state.
Search to Increase the PLPMTU This section describes the algorithms used by DPLPMTUD to search for a larger PLPMTU.
Probing for a larger PLPMTU Implementations use a search algorithm across the search range to determine whether a larger PLPMTU can be supported across a network path. The method discovers the search range by confirming the minimum PLPMTU and then using the probe method to select a PROBED_SIZE less than or equal to MAX_PMTU. MAX_PMTU is the minimum of the local MTU and EMTU_R (learned from the remote endpoint). The MAX_PMTU MAY be reduced by an application that sets a maximum to the size of datagrams it will send. The PROBE_COUNT is initialized to zero when a probe packet is first sent with a particular size. A timer is used by the search algorithm to trigger the sending of probe packets of size PROBED_SIZE, larger than the PLPMTU. Each probe packet successfully sent to the remote peer is confirmed by acknowledgement at the PL, see . Each time a probe packet is sent to the destination, the PROBE_TIMER is started. The timer is canceled when the PL receives acknowledgment that the probe packet has been successfully sent across the path . This confirms that the PROBED_SIZE is supported, and the PROBED_SIZE value is then assigned to the PLPMTU. The search algorithm can continue to send subsequent probe packets of an increasing size. If the timer expires before a probe packet is acknowledged, the probe has failed to confirm the PROBED_SIZE. Each time the PROBE_TIMER expires, the PROBE_COUNT is incremented, the PROBE_TIMER is reinitialized, and a probe packet of the same size is retransmitted (the replicated probe improve the resilience to loss). The maximum number of retransmissions for a particular size is configured (MAX_PROBES). If the value of the PROBE_COUNT reaches MAX_PROBES, probing will stop, and the PL sender enters the SEARCH_COMPLETE state.
Selection of Probe Sizes The search algorithm needs to determine a minimum useful gain in PLPMTU. It would not be constructive for a PL sender to attempt to probe for all sizes. This would incur unnecessary load on the path and has the undesirable effect of slowing the time to reach a more optimal MPS. Implementations SHOULD select the set of probe packet sizes to maximize the gain in PLPMTU from each search step. Implementations could optimize the search procedure by selecting step sizes from a table of common PMTU sizes. When selecting the appropriate next size to search, an implementor ought to also consider that there can be common sizes of MPS that applications seek to use, and their could be common sizes of MTU used within the network.
Resilience to Inconsistent Path Information A decision to increase the PLPMTU needs to be resilient to the possibility that information learned about the network path is inconsistent. A path is inconsistent, when, for example, probe packets are lost due to other reasons (i. e. not packet size) or due to frequent path changes. Frequent path changes could occur by unexpected "flapping" - where some packets from a flow pass along one path, but other packets follow a different path with different properties. A PL sender is able to detect inconsistency from the sequence of PLPMTU probes that it sends or the sequence of PTB messages that it receives. When inconsistent path information is detected, a PL sender could use an alternate search mode that clamps the offered MPS to a smaller value for a period of time. This avoids unnecessary loss of packets due to MTU limitation.
Robustness to Inconsistent Paths Some paths could be unable to sustain packets of the BASE_PMTU size. To be robust to these paths an implementation could implement the Error State. This allows fallback to a smaller than desired PLPMTU, rather than suffer connectivity failure. This could utilize methods such as endpoint IP fragmentation to enable the PL sender to communicate using packets smaller than the BASE_PMTU.
Specification of Protocol-Specific Methods This section specifies protocol-specific details for datagram PLPMTUD for IETF-specified transports. The first subsection provides guidance on how to implement the DPLPMTUD method as a part of an application using UDP or UDP-Lite. The guidance also applies to other datagram services that do not include a specific transport protocol (such as a tunnel encapsulation). The following subsections describe how DPLPMTUD can be implemented as a part of the transport service, allowing applications using the service to benefit from discovery of the PLPMTU without themselves needing to implement this method.
Application support for DPLPMTUD with UDP or UDP-Lite The current specifications of UDP and UDP-Lite do not define a method in the RFC-series that supports PLPMTUD. In particular, the UDP transport does not provide the transport layer features needed to implement datagram PLPMTUD. The DPLPMTUD method can be implemented as a part of an application built directly or indirectly on UDP or UDP-Lite, but relies on higher-layer protocol features to implement the method . Some primitives used by DPLPMTUD might not be available via the Datagram API (e.g., the ability to access the PLPMTU cache, or interpret received PTB messages). In addition, it is desirable that PMTU discovery is not performed by multiple protocol layers. An application SHOULD avoid using DPLPMTUD when the underlying transport system provides this capability. To use common method for managing the PLPMTU has benefits, both in the ability to share state between different processes and opportunities to coordinate probing.
Application Request An application needs an application-layer protocol mechanism (such as a message acknowledgement method) that solicits a response from a destination endpoint. The method SHOULD allow the sender to check the value returned in the response to provide additional protection from off-path insertion of data , suitable methods include a parameter known only to the two endpoints, such as a session ID or initialized sequence number.
Application Response An application needs an application-layer protocol mechanism to communicate the response from the destination endpoint. This response may indicate successful reception of the probe across the path, but could also indicate that some (or all packets) have failed to reach the destination.
Sending Application Probe Packets A probe packet that may carry an application data block, but the successful transmission of this data is at risk when used for probing. Some applications may prefer to use a probe packet that does not carry an application data block to avoid disruption to data transfer.
Validating the Path An application that does not have other higher-layer information confirming correct delivery of datagrams SHOULD implement the CONFIRMATION_TIMER to periodically send probe packets while in the SEARCH_COMPLETE state.
Handling of PTB Messages An application that is able and wishes to receive PTB messages MUST perform ICMP validation as specified in Section 5.2 of . This requires that the application to check each received PTB messages to validate it is received in response to transmitted traffic and that the reported PTB_SIZE is less than the current probed size (see ). A validated PTB message MAY be used as input to the DPLPMTUD algorithm, but MUST NOT be used directly to set the PLPMTU.
DPLPMTUD for SCTP Section 10.2 of specifies a recommended PLPMTUD probing method for SCTP. It recommends the use of the PAD chunk, defined in 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 Author Note: Future versions of this document might define a parameter contained in the INIT and INIT ACK chunk to indicate the remote peer MTU to the local peer. However, multihoming makes this a bit complex, so it might not be worth doing. XXX
SCTP/IPv4 and SCTP/IPv6 The base protocol is specified in . This provides an acknowledged PL. A sender can therefore enter the BASE state as soon as connectivity has been confirmed.
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 , the probe size, which is the size of the complete datagram. 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 PL handshake, before data is sent. Assuming this behavior (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.
Validating the Path with SCTP Since SCTP provides an acknowledged PL, a sender MUST NOT implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.
PTB Message Handling by SCTP Normal ICMP validation MUST be performed as specified in Appendix C of . 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 validated, the PTB_SIZE reported in the PTB message SHOULD be used with the DPLPMTUD algorithm, providing that the reported PTB_SIZE is less than the current probe size (see ).
DPLPMTUD for SCTP/UDP The UDP encapsulation of SCTP is specified in .
Sending SCTP/UDP Probe Packets Packet probing can be performed as specified in . The maximum payload is reduced by 8 bytes, which has to be considered when filling the PAD chunk.
Validating the Path with SCTP/UDP Since SCTP provides an acknowledged PL, a sender MUST NOT implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.
Handling of PTB Messages by SCTP/UDP ICMP validation MUST be performed for PTB messages as specified in Appendix C of . 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 validation is completed, the PTB_SIZE indicated in the PTB message SHOULD be used with the DPLPMTUD providing that the reported PTB_SIZE is less than the current probe size.
DPLPMTUD for SCTP/DTLS The Datagram Transport Layer Security (DTLS) encapsulation of SCTP is specified in . It is used for data channels in WebRTC implementations.
Sending SCTP/DTLS Probe Packets Packet probing can be done as specified in .
Validating the Path with SCTP/DTLS Since SCTP provides an acknowledged PL, a sender MUST NOT implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.
Handling of PTB Messages by SCTP/DTLS It is not possible to perform ICMP validation as specified in , 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.
DPLPMTUD for QUIC Quick UDP Internet Connection (QUIC) is a UDP-based transport that provides reception feedback. The UDP payload includes the QUIC packet header, protected payload, and any authentication fields. QUIC depends on a PMTU of at least 1280 bytes. Section 14.1 of describes the path considerations when sending QUIC packets. It recommends the use of PADDING frames to build the probe packet. Pure probe-only packets are constructed with PADDING frames and PING frames to create a padding only packet that will elicit an acknowledgement. Such padding only packets enable probing without affecting the transfer of other QUIC frames. The recommendation for QUIC endpoints implementing DPLPMTUD is that a MPS is maintained for each combination of local and remote IP addresses . If a QUIC endpoint determines that the PMTU between any pair of local and remote IP addresses has fallen below an acceptable MPS, it needs to immediately cease sending QUIC packets on the affected path. This could result in termination of the connection if an alternative path cannot be found .
Sending QUIC Probe Packets A probe packet consists of a QUIC Header and a payload containing PADDING Frames and a PING Frame. PADDING Frames are a single octet (0x00) and several of these can be used to create a probe packet of size PROBED_SIZE. QUIC provides an acknowledged PL, a sender can therefore enter the BASE state as soon as connectivity has been confirmed. The current specification of QUIC sets the following:
  • BASE_PMTU: 1200. A QUIC sender needs to pad initial packets to 1200 bytes to confirm the path can support packets of a useful size.
  • MIN_PMTU: 1200 bytes. A QUIC sender that determines the PMTU has fallen below 1200 bytes MUST immediately stop sending on the affected path.
Validating the Path with QUIC QUIC provides an acknowledged PL. A sender therefore MUST NOT implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.
Handling of PTB Messages by QUIC QUIC operates over the UDP transport, and the guidelines on ICMP validation as specified in Section 5.2 of therefore apply. In addition to UDP Port validation QUIC can validate an ICMP message by looking for valid Connection IDs in the quoted packet.
DPLPMTUD for UDP-Options UDP Options provides a way to extend UDP to provide new transport mechanisms. Support for using DPLPMTUD with UDP-Options is defined in the UDP-Options specification .
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).
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.
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 Usage Guidelines , specifically the generation of probe packets is regarded as a "Low Data-Volume Application", described in section 3.1.3 of this document. This recommends that sender limits generation of probe packets to an average rate lower than one probe per 3 seconds. A PL sender needs to ensure that the method used to confirm reception of probe packets offers protection from off-path attackers injecting packets into the path. This protection if provided in IETF-defined protocols (e.g., TCP, SCTP) using a randomly-initialized sequence number. A description of one way to do this when using UDP is provided in section 5.1 of ). There are cases where ICMP Packet Too Big (PTB) messages are not delivered due to policy, configuration or equipment design (see ), this method therefore does not rely upon PTB messages being received, but is able to utilize these when they are received by the sender. PTB messages could potentially be used to cause a node to inappropriately reduce the PLPMTU. A node supporting DPLPMTUD MUST therefore appropriately validate 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, see ). An on-path attacker, able to create a PTB message could forge PTB messages that include a valid quoted IP packet. Such an attack could be used to drive down the PLPMTU. There are two ways this method can be mitigated against such attacks: First, by ensuring that a PL sender never reduces the PLPMTU below the base size, solely in response to receiving a PTB message. This is achieved by first entering the BASE state when such a message is received. Second, the design does not require processing of PTB messages, a PL sender could therefore suspend processing of PTB messages (e.g., in a robustness mode after detecting that subsequent probes actually confirm that a size larger than the PTB_SIZE is supported by a path). Parallel forwarding paths SHOULD be considered. identifies the need for robustness in the method when the path information may be inconsistent. A node performing DPLPMTUD 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 black holed when the PLPMTU is larger than the smallest PMTU across the current paths.
References Normative References QUIC: A UDP-Based Multiplexed and Secure Transport This document defines the core of the QUIC transport protocol. Accompanying documents describe QUIC's loss detection and congestion control and the use of TLS for key negotiation. Note to Readers Discussion of this draft takes place on the QUIC working group mailing list (quic@ietf.org), which is archived at <https://mailarchive.ietf.org/arch/search/?email_list=quic>. Working Group information can be found at <https://github.com/ quicwg>; source code and issues list for this draft can be found at <https://github.com/quicwg/base-drafts/labels/-transport>. User Datagram Protocol Path MTU discovery This memo describes a technique for dynamically discovering the maximum transmission unit (MTU) of an arbitrary internet path. It specifies a small change to the way routers generate one type of ICMP message. For a path that passes through a router that has not been so changed, this technique might not discover the correct Path MTU, but it will always choose a Path MTU as accurate as, and in many cases more accurate than, the Path MTU that would be chosen by current practice. [STANDARDS-TRACK] Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Internet Protocol, Version 6 (IPv6) Specification This document specifies version 6 of the Internet Protocol (IPv6), also sometimes referred to as IP Next Generation or IPng. [STANDARDS-TRACK] The Lightweight User Datagram Protocol (UDP-Lite) This document describes the Lightweight User Datagram Protocol (UDP-Lite), which is similar to the User Datagram Protocol (UDP) (RFC 768), but can also serve applications in error-prone network environments that prefer to have partially damaged payloads delivered rather than discarded. If this feature is not used, UDP-Lite is semantically identical to UDP. [STANDARDS-TRACK] Padding Chunk and Parameter for the Stream Control Transmission Protocol (SCTP) This document defines a padding chunk and a padding parameter and describes the required receiver side procedures. The padding chunk is used to pad a Stream Control Transmission Protocol (SCTP) packet to an arbitrary size. The padding parameter is used to pad an SCTP INIT chunk to an arbitrary size. [STANDARDS-TRACK] Stream Control Transmission Protocol This document obsoletes RFC 2960 and RFC 3309. It describes the Stream Control Transmission Protocol (SCTP). SCTP is designed to transport Public Switched Telephone Network (PSTN) signaling messages over IP networks, but is capable of broader applications. SCTP is a reliable transport protocol operating on top of a connectionless packet network such as IP. It offers the following services to its users: -- acknowledged error-free non-duplicated transfer of user data, -- data fragmentation to conform to discovered path MTU size, -- sequenced delivery of user messages within multiple streams, with an option for order-of-arrival delivery of individual user messages, -- optional bundling of multiple user messages into a single SCTP packet, and -- network-level fault tolerance through supporting of multi-homing at either or both ends of an association. The design of SCTP includes appropriate congestion avoidance behavior and resistance to flooding and masquerade attacks. [STANDARDS-TRACK] UDP Encapsulation of Stream Control Transmission Protocol (SCTP) Packets for End-Host to End-Host Communication This document describes a simple method of encapsulating Stream Control Transmission Protocol (SCTP) packets into UDP packets and its limitations. This allows the usage of SCTP in networks with legacy NATs that do not support SCTP. It can also be used to implement SCTP on hosts without directly accessing the IP layer, for example, implementing it as part of the application without requiring special privileges. Please note that this document only describes the functionality required within an SCTP stack to add on UDP encapsulation, providing only those mechanisms for two end-hosts to communicate with each other over UDP ports. In particular, it does not provide mechanisms to determine whether UDP encapsulation is being used by the peer, nor the mechanisms for determining which remote UDP port number can be used. These functions are out of scope for this document. This document covers only end-hosts and not tunneling (egress or ingress) endpoints. UDP Usage Guidelines The User Datagram Protocol (UDP) provides a minimal message-passing transport that has no inherent congestion control mechanisms. This document provides guidelines on the use of UDP for the designers of applications, tunnels, and other protocols that use UDP. Congestion control guidelines are a primary focus, but the document also provides guidance on other topics, including message sizes, reliability, checksums, middlebox traversal, the use of Explicit Congestion Notification (ECN), Differentiated Services Code Points (DSCPs), and ports. Because congestion control is critical to the stable operation of the Internet, applications and other protocols that choose to use UDP as an Internet transport must employ mechanisms to prevent congestion collapse and to establish some degree of fairness with concurrent traffic. They may also need to implement additional mechanisms, depending on how they use UDP. Some guidance is also applicable to the design of other protocols (e.g., protocols layered directly on IP or via IP-based tunnels), especially when these protocols do not themselves provide congestion control. This document obsoletes RFC 5405 and adds guidelines for multicast UDP usage. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Path MTU Discovery for IP version 6 This document describes Path MTU Discovery (PMTUD) for IP version 6. It is largely derived from RFC 1191, which describes Path MTU Discovery for IP version 4. It obsoletes RFC 1981. Datagram Transport Layer Security (DTLS) Encapsulation of SCTP Packets The Stream Control Transmission Protocol (SCTP) is a transport protocol originally defined to run on top of the network protocols IPv4 or IPv6. This document specifies how SCTP can be used on top of the Datagram Transport Layer Security (DTLS) protocol. Using the encapsulation method described in this document, SCTP is unaware of the protocols being used below DTLS; hence, explicit IP addresses cannot be used in the SCTP control chunks. As a consequence, the SCTP associations carried over DTLS can only be single-homed. Informative References IP Tunnels in the Internet Architecture This document discusses the role of IP tunnels in the Internet architecture. An IP tunnel transits IP datagrams as payloads in non- link layer protocols. This document explains the relationship of IP tunnels to existing protocol layers and the challenges in supporting IP tunneling, based on the equivalence of tunnels to links. The implications of this document are used to derive recommendations that update MTU and fragment issues in RFC 4459. Transport Options for UDP Transport protocols are extended through the use of transport header options. This document experimentally extends UDP by indicating the location, syntax, and semantics for UDP transport layer options. Internet Control Message Protocol Requirements for Internet Hosts - Communication Layers This RFC is an official specification for the Internet community. It incorporates by reference, amends, corrects, and supplements the primary protocol standards documents relating to hosts. [STANDARDS-TRACK] Requirements for IP Version 4 Routers This memo defines and discusses requirements for devices that perform the network layer forwarding function of the Internet protocol suite. [STANDARDS-TRACK] TCP Problems with Path MTU Discovery This memo catalogs several known Transmission Control Protocol (TCP) implementation problems dealing with Path Maximum Transmission Unit Discovery (PMTUD), including the long-standing black hole problem, stretch acknowlegements (ACKs) due to confusion between Maximum Segment Size (MSS) and segment size, and MSS advertisement based on PMTU. This memo provides information for the Internet community. Datagram Congestion Control Protocol (DCCP) The Datagram Congestion Control Protocol (DCCP) is a transport protocol that provides bidirectional unicast connections of congestion-controlled unreliable datagrams. DCCP is suitable for applications that transfer fairly large amounts of data and that can benefit from control over the tradeoff between timeliness and reliability. [STANDARDS-TRACK] Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification This document describes the format of a set of control messages used in ICMPv6 (Internet Control Message Protocol). ICMPv6 is the Internet Control Message Protocol for Internet Protocol version 6 (IPv6). [STANDARDS-TRACK] Packetization Layer Path MTU Discovery This document describes a robust method for Path MTU Discovery (PMTUD) that relies on TCP or some other Packetization Layer to probe an Internet path with progressively larger packets. This method is described as an extension to RFC 1191 and RFC 1981, which specify ICMP-based Path MTU Discovery for IP versions 4 and 6, respectively. [STANDARDS-TRACK] Recommendations for Filtering ICMPv6 Messages in Firewalls In networks supporting IPv6, the Internet Control Message Protocol version 6 (ICMPv6) plays a fundamental role with a large number of functions, and a correspondingly large number of message types and options. ICMPv6 is essential to the functioning of IPv6, but there are a number of security risks associated with uncontrolled forwarding of ICMPv6 messages. Filtering strategies designed for the corresponding protocol, ICMP, in IPv4 networks are not directly applicable, because these strategies are intended to accommodate a useful auxiliary protocol that may not be required for correct functioning. This document provides some recommendations for ICMPv6 firewall filter configuration that will allow propagation of ICMPv6 messages that are needed to maintain the functioning of the network but drop messages that are potential security risks. This memo provides information for the Internet community.
Revision Notes Note to RFC-Editor: please remove this entire section prior to publication. Individual draft -00:
  • Comments and corrections are welcome directly to the authors or via the IETF TSVWG working group mailing list.
  • This update is proposed for WG comments.
Individual draft -01:
  • Contains the first representation of the algorithm, showing the states and timers
  • This update is proposed for WG comments.
Individual draft -02:
  • Contains updated representation of the algorithm, and textual corrections.
  • The text describing when to set the effective PMTU has not yet been validated by the authors
  • To determine security to off-path-attacks: We need to decide whether a received PTB message SHOULD/MUST be validated? The text on how to handle a PTB message indicating a link MTU larger than the probe has yet not been validated by the authors
  • No text currently describes how to handle inconsistent results from arbitrary re-routing along different parallel paths
  • This update is proposed for WG comments.
Working Group draft -00:
  • This draft follows a successful adoption call for TSVWG
  • There is still work to complete, please comment on this draft.
Working Group draft -01:
  • This draft includes improved introduction.
  • The draft is updated to require ICMP validation prior to accepting PTB messages - this to be confirmed by WG
  • Section added to discuss Selection of Probe Size - methods to be evlauated and recommendations to be considered
  • Section added to align with work proposed in the QUIC WG.
Working Group draft -02:
  • The draft was updated based on feedback from the WG, and a detailed review by Magnus Westerlund.
  • The document updates RFC 4821.
  • Requirements list updated.
  • Added more explicit discussion of a simpler black-hole detection mode.
  • This draft includes reorganisation of the section on IETF protocols.
  • Added more discussion of implementation within an application.
  • Added text on flapping paths.
  • Replaced 'effective MTU' with new term PLPMTU.
Working Group draft -03:
  • Updated figures
  • Added more discussion on blackhole detection
  • Added figure describing just blackhole detection
  • Added figure relating MPS sizes
Working Group draft -04:
  • Described phases and named these consistently.
  • Corrected transition from confirmation directly to the search phase (Base has been checked).
  • Redrawn state diagrams.
  • Renamed BASE_MTU to BASE_PMTU (because it is a base for the PMTU).
  • Clarified Error state.
  • Clarified supsending DPLPMTUD.
  • Verified normative text in requirements section.
  • Removed duplicate text.
  • Changed all text to refer to /packet probe/probe packet/ /validation/verification/ added term /Probe Confirmation/ and clarified BlackHole detection.
Working Group draft -05:
  • Updated security considerations.
  • Feedback after speaking with Joe Touch helped improve UDP-Options description.
Working Group draft -06:
  • Updated description of ICMP issues in section 1.1
  • Update to description of QUIC.
Working group draft -07:
  • Moved description of the PTB processing method from the PTB requirements section.
  • Clarified what is performed in the PTB validation check.
  • Updated security consideration to explain PTB security without needing to read the rest of the document.
  • Reformatted state machine diagram
Working group draft -08:
  • Moved to rfcxml v3+
  • Rendered diagrams to svg in html version.
  • Removed Appendix A. Event-driven state changes.
  • Removed section on DPLPMTUD with UDP Options.
  • Shortened the dsecription of phases.
Authors' Addresses University of Aberdeen
School of Engineering Fraser Noble Building Aberdeen AB24 3UE UK gorry@erg.abdn.ac.uk
University of Aberdeen
School of Engineering Fraser Noble Building Aberdeen AB24 3UE UK tom@erg.abdn.ac.uk
Muenster University of Applied Sciences
Stegerwaldstrasse 39 48565 Steinfurt DE tuexen@fh-muenster.de
Muenster University of Applied Sciences
Stegerwaldstrasse 39 48565 Steinfurt DE i.ruengeler@fh-muenster.de
Muenster University of Applied Sciences
Stegerwaldstrasse 39 48565 Steinfurt DE timo.voelker@fh-muenster.de