--- 1/draft-ietf-tsvwg-datagram-plpmtud-06.txt 2019-02-18 05:13:14.689140981 -0800 +++ 2/draft-ietf-tsvwg-datagram-plpmtud-07.txt 2019-02-18 05:13:14.785143293 -0800 @@ -1,37 +1,38 @@ Internet Engineering Task Force G. Fairhurst Internet-Draft T. Jones Updates: 4821 (if approved) University of Aberdeen Intended status: Standards Track M. Tuexen -Expires: May 24, 2019 I. Ruengeler +Expires: August 22, 2019 I. Ruengeler + T. Voelker Muenster University of Applied Sciences - November 20, 2018 + February 18, 2019 Packetization Layer Path MTU Discovery for Datagram Transports - draft-ietf-tsvwg-datagram-plpmtud-06 + draft-ietf-tsvwg-datagram-plpmtud-07 Abstract This document describes a robust method for Path MTU Discovery (PMTUD) for datagram Packetization Layers (PLs). The document 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, and no ICMP message is received). The method can also probe a network path with progressively larger packets to find 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, + 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 @@ -41,25 +42,25 @@ 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 May 24, 2019. + This Internet-Draft will expire on August 22, 2019. Copyright Notice - Copyright (c) 2018 IETF Trust and the persons identified as the + Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (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 @@ -70,73 +71,73 @@ 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Classical Path MTU Discovery . . . . . . . . . . . . . . 4 1.2. Packetization Layer Path MTU Discovery . . . . . . . . . 6 1.3. Path MTU Discovery for Datagram Services . . . . . . . . 7 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7 3. Features Required to Provide Datagram PLPMTUD . . . . . . . . 9 4. DPLPMTUD Mechanisms . . . . . . . . . . . . . . . . . . . . . 12 4.1. PLPMTU Probe Packets . . . . . . . . . . . . . . . . . . 12 4.2. Confirmation of Probed Packet Size . . . . . . . . . . . 13 4.3. Detection of Black Holes . . . . . . . . . . . . . . . . 14 - 4.4. Response to PTB Messages . . . . . . . . . . . . . . . . 14 + 4.4. Response to PTB Messages . . . . . . . . . . . . . . . . 15 4.4.1. Validation of PTB Messages . . . . . . . . . . . . . 15 - 4.4.2. Use of PTB Messages . . . . . . . . . . . . . . . . . 15 - 5. Datagram Packetization Layer PMTUD . . . . . . . . . . . . . 16 - 5.1. DPLPMTUD Components . . . . . . . . . . . . . . . . . . . 17 - 5.1.1. Timers . . . . . . . . . . . . . . . . . . . . . . . 17 - 5.1.2. Constants . . . . . . . . . . . . . . . . . . . . . . 18 + 4.4.2. Use of PTB Messages . . . . . . . . . . . . . . . . . 16 + 5. Datagram Packetization Layer PMTUD . . . . . . . . . . . . . 17 + 5.1. DPLPMTUD Components . . . . . . . . . . . . . . . . . . . 18 + 5.1.1. Timers . . . . . . . . . . . . . . . . . . . . . . . 18 + 5.1.2. Constants . . . . . . . . . . . . . . . . . . . . . . 19 5.1.3. Variables . . . . . . . . . . . . . . . . . . . . . . 19 - 5.2. DPLPMTUD Phases . . . . . . . . . . . . . . . . . . . . . 19 - 5.2.1. Path Confirmation Phase . . . . . . . . . . . . . . . 21 - 5.2.2. Search Phase . . . . . . . . . . . . . . . . . . . . 21 - 5.2.2.1. Resilience to inconsistent path information . . . 22 - 5.2.3. Search Complete Phase . . . . . . . . . . . . . . . . 22 + 5.2. DPLPMTUD Phases . . . . . . . . . . . . . . . . . . . . . 20 + 5.2.1. BASE_PMTU Confirmation Phase . . . . . . . . . . . . 22 + 5.2.2. Search Phase . . . . . . . . . . . . . . . . . . . . 22 + 5.2.2.1. Resilience to Inconsistent Path Information . . . 22 + 5.2.3. Search Complete Phase . . . . . . . . . . . . . . . . 23 5.2.4. PROBE_BASE Phase . . . . . . . . . . . . . . . . . . 23 - 5.2.5. ERROR Phase . . . . . . . . . . . . . . . . . . . . . 23 - 5.2.5.1. Robustness to inconsistent path . . . . . . . . . 23 + 5.2.5. ERROR Phase . . . . . . . . . . . . . . . . . . . . . 24 + 5.2.5.1. Robustness to Inconsistent Path . . . . . . . . . 24 5.2.6. DISABLED Phase . . . . . . . . . . . . . . . . . . . 24 5.3. State Machine . . . . . . . . . . . . . . . . . . . . . . 24 5.4. Search to Increase the PLPMTU . . . . . . . . . . . . . . 27 - 5.4.1. Probing for a larger PLPMTU . . . . . . . . . . . . . 27 + 5.4.1. Probing for a Larger PLPMTU . . . . . . . . . . . . . 27 5.4.2. Selection of Probe Sizes . . . . . . . . . . . . . . 28 - 5.4.3. Resilience to inconsistent Path information . . . . . 29 - 6. Specification of Protocol-Specific Methods . . . . . . . . . 29 + 5.4.3. Resilience to Inconsistent Path Information . . . . . 28 + 6. Specification of Protocol-Specific Methods . . . . . . . . . 28 6.1. Application support for DPLPMTUD with UDP or UDP-Lite . . 29 - 6.1.1. Application Request . . . . . . . . . . . . . . . . . 30 - 6.1.2. Application Response . . . . . . . . . . . . . . . . 30 + 6.1.1. Application Request . . . . . . . . . . . . . . . . . 29 + 6.1.2. Application Response . . . . . . . . . . . . . . . . 29 6.1.3. Sending Application Probe Packets . . . . . . . . . . 30 6.1.4. Validating the Path . . . . . . . . . . . . . . . . . 30 6.1.5. Handling of PTB Messages . . . . . . . . . . . . . . 30 - 6.2. DPLPMTUD with UDP Options . . . . . . . . . . . . . . . . 31 + 6.2. DPLPMTUD with UDP Options . . . . . . . . . . . . . . . . 30 6.2.1. UDP Probe Request Option . . . . . . . . . . . . . . 32 - 6.2.2. UDP Probe Response Option . . . . . . . . . . . . . . 33 + 6.2.2. UDP Probe Response Option . . . . . . . . . . . . . . 32 6.3. DPLPMTUD for SCTP . . . . . . . . . . . . . . . . . . . . 33 6.3.1. SCTP/IPv4 and SCTP/IPv6 . . . . . . . . . . . . . . . 33 6.3.1.1. Sending SCTP Probe Packets . . . . . . . . . . . 33 6.3.1.2. Validating the Path with SCTP . . . . . . . . . . 34 6.3.1.3. PTB Message Handling by SCTP . . . . . . . . . . 34 6.3.2. DPLPMTUD for SCTP/UDP . . . . . . . . . . . . . . . . 34 6.3.2.1. Sending SCTP/UDP Probe Packets . . . . . . . . . 34 - 6.3.2.2. Validating the Path with SCTP/UDP . . . . . . . . 35 - 6.3.2.3. Handling of PTB Messages by SCTP/UDP . . . . . . 35 - 6.3.3. DPLPMTUD for SCTP/DTLS . . . . . . . . . . . . . . . 35 + 6.3.2.2. Validating the Path with SCTP/UDP . . . . . . . . 34 + 6.3.2.3. Handling of PTB Messages by SCTP/UDP . . . . . . 34 + 6.3.3. DPLPMTUD for SCTP/DTLS . . . . . . . . . . . . . . . 34 6.3.3.1. Sending SCTP/DTLS Probe Packets . . . . . . . . . 35 6.3.3.2. Validating the Path with SCTP/DTLS . . . . . . . 35 6.3.3.3. Handling of PTB Messages by SCTP/DTLS . . . . . . 35 6.4. DPLPMTUD for QUIC . . . . . . . . . . . . . . . . . . . . 35 - 6.4.1. Sending QUIC Probe Packets . . . . . . . . . . . . . 36 + 6.4.1. Sending QUIC Probe Packets . . . . . . . . . . . . . 35 6.4.2. Validating the Path with QUIC . . . . . . . . . . . . 36 6.4.3. Handling of PTB Messages by QUIC . . . . . . . . . . 36 - 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 37 - 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37 - 9. Security Considerations . . . . . . . . . . . . . . . . . . . 37 + 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36 + 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36 + 9. Security Considerations . . . . . . . . . . . . . . . . . . . 36 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 38 10.1. Normative References . . . . . . . . . . . . . . . . . . 38 10.2. Informative References . . . . . . . . . . . . . . . . . 39 Appendix A. Event-driven state changes . . . . . . . . . . . . . 40 Appendix B. Revision Notes . . . . . . . . . . . . . . . . . . . 43 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45 1. Introduction The IETF has specified datagram transport using UDP, SCTP, and DCCP, @@ -163,29 +164,29 @@ 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 - silently discarded without the sender receiving ICMP PTB messages). - This could arise when the PTB messages are not delivered back to the - sender for some reason [RFC2923]). + silently discarded without the sender receiving PTB messages). This + could arise when the PTB messages are not delivered back to the + sender for some reason (see for example [RFC2923]). Examples where PTB messages are not delivered include: o The generation of ICMP messages is usually rate limited. This may result in no PTB messages being sent to the sender (see section - 2.4 of [RFC4443] + 2.4 of [RFC4443]) o ICMP messages are increasingly filtered by middleboxes (including firewalls) [RFC4890]. A stateful firewall could be configured with a policy to block incoming ICMP messages, which would prevent reception of PTB messages to endpoints behind this firewall. o 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 @@ -204,21 +205,22 @@ router, then any resulting ICMP message needs to also be directed by the ECMP router towards the same server (i.e., ICMP messages need to follow the same path as the flows to which they correspond). Failure to do this results in black-holing. o There are 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 in a PTB to be sent, and result in consequent black-holing. + cause a PTB message to be sent, and result in consequent black- + holing. Another failure could result if a node that is not on the network path sends a PTB message 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 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. @@ -277,65 +279,66 @@ application MPS. 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 black-holing. For instance,the - information available at the PL, or higher layers, makes PTB + information available at the PL, or higher layers, makes PTB message validation more straight forward. 1.3. Path MTU Discovery for Datagram Services Section 5 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 Section 3 and applies to transport protocols operating over IPv4 and IPv6. It does not require - cooperation from the lower layers, although it can utilise ICMP PTB + cooperation from the lower layers, although it can utilise PTB messages when these received messages are made available to the PL. The UDP Usage 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 that can be used on a network path. Prior to this document, PLPMTUD had not been specified for UDP. Section 10.2 of [RFC4821] recommends a PLPMTUD probing method for the - Stream Control Transport Protocol (SCTP). SCTP utilises heartbeat - messages as probe packets, but RFC4821 does not provide a complete - specification. The present document provides the details to complete - that specification. + Stream Control Transport Protocol (SCTP). SCTP utilises probe + packets consisting of a minimal sized HEARTBEAT chunk bundled with a + PAD chunk as defined in [RFC4820], but RFC4821 does not provide a + complete specification. The present document provides the details to + complete that specification. The Datagram Congestion Control Protocol (DCCP) [RFC4340] 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. Section 6 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. 2. 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 [RFC2119] [[RFC8174]] when, and only when, they appear in all + 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. Other terminology is directly copied from [RFC4821], and the definitions in [RFC1122]. 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 Holed: Packets are Black holed when the sender is unaware that @@ -373,26 +376,29 @@ 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. In DPLPMTUD this quantity is derived from the PLPMTU by taking into consideration the size of the lower protocol layer headers. - MIN_PMTU: The MIN_PMTU is the smallest size of PLPMTU that DPLPTMUD + 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. @@ -434,89 +439,83 @@ 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 is REQUIRED to be able to choose - the size of datagrams sent to the network, up to the PLPMTU, or a + 2. PLPMTU: A datagram application using a transport layer 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 [RFC1191]) is equivalent to the EMTU_S (specified in [RFC1122]). 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 [RFC8201]). 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 [RFC8201]. 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. - When the PTB_SIZE is indicated in the PTB message, this MAY be - used by DPLPMTUD to reduce the probe size but MUST NOT be used to - increase the PLPMTU ([RFC8201]). This validation SHOULD utilise - information that can not be simply determined by an off-path - attacker, for example, by checking the value of a protocol header - field known only to the two PL endpoints. (Some datagram - applications use well-known source and destination ports and - therefore this check needs to rely on other information.) + before the PLPMTU discovery method reacts to the PTB message. A + PTB message MUST NOT be used to increase the PLPMTU [RFC8201]. 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-side DPLPMTUD 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 SHOULD NOT directly trigger a congestion control - reaction [RFC4821]. + 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. Probe loss recovery: If the data block carried by a probe packet - needs to be sent reliably, the PL (or layers above) are REQUIRED - to arrange any retransmission/repair of any resulting loss. This - method is REQUIRED to be robust in the case where probe packets - are lost due to other reasons (including link transmission error, - congestion). The DPLPMTUD sender treats isolated loss of a probe - packet (with or without an PTB message) as a potential indication - of a PMTU limit for the path, but not as an indication of - congestion, see Paragraph 6. + 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 [RFC4821]. 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 [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 MUST be robust to the wide variety of - underlying network forwarding behaviours, PLPMTU adjustments - based on shared PLPMTU values should be incorporated in the - search algorithms. Section 5.2 of [RFC8201] provides guidance on - the caching of PMTU information and also the relation to IPv6 - flow labels. + make use of the new MTU". Such methods MUST be robust to the + wide variety of underlying network forwarding behaviours, PLPMTU + adjustments based on shared PLPMTU values should be incorporated + in the search algorithms. 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 principles are stated for design of a DPLPMTUD method: o 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 @@ -595,42 +594,42 @@ 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 utilise 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 ICMP PTB + being robust to reordering and replay of probe response and PTB messages. 4.2. 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 [I-D.ietf-tsvwg-udp-options]. - Section Section 5 specifies this function for a set of IETF-specified + Section 5 specifies this function for a set of IETF-specified protocols. 4.3. Detection of Black Holes A PL sender needs to reduce the PLPMTU when it discovers the actual PMTU supported by a network path is less than the PLPMTU (i.e. to detect that traffic is being black holed). 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 @@ -643,27 +642,31 @@ sender detect that the current PLPMTU is not sustained by the path (i.e., to detect a black hole): o A PL can rely upon a mechanisms implemented within the PL protocol 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 [RFC4821]). o A PL can use the probing mechanism to send confirmation probe packets of the size of the current PLPMTU and a timer track - whether acknowledgments are received (e.g., The number of probe + whether acknowledgments are received (e.g., the number of probe packets sent without receiving an acknowledgement, PROBE_COUNT, becomes greater than the MAX_PROBES). These messages need to be generated periodically (e.g., using the confirmation timer - Section 5.1.1), and should be suppressed when the PL is not - actively sending data. Successive loss of probes is an indication - that the current path no longer supports the PLPMTU. + Section 5.1.1), and 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, MAY choose to continue PMTU discovery for each path. + However, this may result in additional packets being sent. + Successive loss of probes is an indication that the current path + no longer supports the PLPMTU. When the method detects the current PLPMTU is not supported (a black hole is found), DPLPMTUD sets a lower MPS. The PL then confirms that the updated PLPMTU can be successfully used across the path. This can need the PL 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. 4.4. Response to PTB Messages @@ -672,65 +675,82 @@ 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. Section 4.4.1 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. 4.4.1. Validation of PTB Messages - A PL that receives a PTB message from a router or middlebox, MUST - perform ICMP validation as specified in Section 5.2 of [RFC8085]. - This needs the PL to check the protocol information in the quoted - payload to validate the message originated from the sending node. - This check includes determining the appropriate port and IP - information - necessary for the PTB message to be passed to the PL. - In addition, the PL SHOULD validate information from the ICMP payload - to determine that the quoted packet was sent by the PL. These checks - are intended to provide protection from packets that originate from a - node that is not on the network path. PTB messages are discarded if - they fail to pass these checks, or where there is insufficient ICMP - payload to perform the checks + This section specifies utlisation of PTB messages. - PTB messages that have been validated can be utilised by the DPLPMTUD - algorithm. A method that utilises these PTB messages can improve the - speed at the which the algorithm detects an appropriate PLPMTU, - compared to one that relies solely on probing. + o A simple implementation MAY ignore received PTB messages and in + this case the PLPMTU is not updated when a PTB message is + received. + + o 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 [RFC8085][RFC8201]. + 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 the 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 utilise 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 utilised by the DPLPMTUD method. + + PTB messages that have been validated MAY be utilised by the DPLPMTUD + algorithm, but MUST NOT be used directly to set the PLPMTU. A method + that utilises these PTB messages can improve the speed at the which + the algorithm detects an appropriate PLPMTU, compared to one that + relies solely on probing. Section 4.4.2 describes this processing. 4.4.2. Use of PTB Messages A set of checks are intended to provide protection from a router that reports an unexpected PTB_SIZE. The PL 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 an informative summary of how PTB messages can - be utilised. - - Validating PTB Messages: - - * A simple implementation is permitted to ignore received PTB - messages and therefore the PLPMTU is not updated when a PTB - message is received. + This section provides a summary of how PTB messages can be utilised. + This processing depends on the PTB_SIZE and the current value of a + set of variables: - * An implementation that supports PTB messages MUST validate - messages before they are processed. + MIN_PMTU < PTB_SIZE < BASE_PMTU - MIN_PMTU < PTB_SIZE < BASE_MTU * A robust PL MAY enter the PROBE_ERROR state for an IPv4 path - when the PTB_SIZE reported in the PTB message >= 576B and when - this is less than the BASE_MTU. + when the PTB_SIZE reported in the PTB message >= 68 bytes and + when this is less than the BASE_PMTU. * A robust PL MAY enter the PROBE_ERROR state for an IPv6 path - when the PTB_SIZE reported in the PTB message >= 1280B and when - this is less than the BASE_MTU. + when the PTB_SIZE reported in the PTB message >= 1280 bytes and + when this is less than the BASE_PMTU. PTB_SIZE = PLPMTU * Transition to SEARCH_COMPLETE. PTB_SIZE > PROBED_SIZE * The PTB_SIZE > PROBED_SIZE, inconsistent network signal. These PTB messages ought to be discarded without further processing (the PLPMTU not updated). @@ -749,112 +769,124 @@ 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 + 5. Datagram Packetization Layer PMTUD This section specifies Datagram PLPMTUD (DPLPMTUD). The method can - be introduced at various points in the IP protocol stack to discover - the PLPMTU so that an application can utilise an appropriate MPS for - the current network path. + 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 utilise 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. +----------------------+ - | APP* | + | Application* | +-+-------+----+---+---+ | | | | +---+--+ +--+--+ | +-+---+ | QUIC*| |UDPO*| | |SCTP*| +---+--+ +--+--+ | ++--+-+ | | | | | +-------+-+ | | | | | | | ++-+--++ | | UDP | | +---+--+ | | | +--------------+-----+-+ | Network Interface | +----------------------+ Figure 1: Examples where DPLPMTUD can be implemented The central idea of DPLPMTUD is probing by a sender. Probe packets - are sent to find the maximum size of user message that is completely - transferred across the network path from the sender to the + 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. This section identifies the components needed for implementation, the phases of operation, the state machine and search algorithm. 5.1. DPLPMTUD Components This section describes components of DPLPMTUD. 5.1.1. Timers - The method utilises three timers: + The method utilises 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 be larger 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 - [RFC8085]. + 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 [RFC8085]. 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 [RFC4821]. - DPLPMTUD SHOULD inhibit sending probe packets when no application - data has been sent since the previous probe packet. + 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 could in sending additional packets. - CONFIRMATION_TIMER: 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[RFC8085]. + 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 [RFC8085]. - DPLPMTUD SHOULD inhibit sending probe packets when no application - data has been sent since the previous probe packet. + 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 process. + timer. 5.1.2. Constants The following constants are defined: - MAX_PROBES: MAX_PROBES is the maximum value of the - PROBE_ERROR_COUNTER. The default value of MAX_PROBES is 10. + MAX_PROBES: MAX_PROBES is the maximum value of the PROBE_COUNT + counter. The default value of MAX_PROBES is 10. MIN_PMTU: The MIN_PMTU is smallest allowed probe packet size. For IPv6, this value is 1280 bytes, as specified in [RFC2460]. For IPv4, the minimum value is 68 bytes. (An IPv4 router is required - to be able to forward a datagram of 68 octets without further + 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 octets. In addition, receivers are + the minimum fragment size of 8 bytes. In addition, receivers are required to be able to reassemble fragmented datagrams at least up - to 576B, as stated in section 3.3.3 of [RFC1122])) + to 576 bytes, as stated in section 3.3.3 of [RFC1122])) 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 @@ -875,26 +907,26 @@ size of PROBED_SIZE is first attempted. The figure below illustrates the relationship between the packet size constants and variables, in this case when the DPLPMTUD algorithm performs path probing to increase the size of the PLPMTU. The MPS is less than the PLPMTU. A probe packet has been sent of size PROBED_SIZE. When this is acknowledged, the PLPMTU will be raised to PROBED_SIZE allowing the PROBED_SIZE to be increased towards the actual PMTU. - MIN_PMTU PMTU_MAX - <------------------------------------------------------> - | | | | | - V | | | V - BASE_PMTU V | V Actual PMTU - MPS | PROBED_SIZE + MIN_PMTU MAX_PMTU + <--------------------------------------------------> + | | | | + V | | V + BASE_PMTU | V Actual PMTU + | PROBED_SIZE V PLPMTU Figure 2: Relationships between probe and packet sizes 5.2. DPLPMTUD Phases The Datagram PLPMTUD algorithm moves through several phases of operation. @@ -903,42 +935,42 @@ 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). - Black hole detection, see Section 4.3 and PTB processing Section 4.4 + Black hole detection (Section 4.3) and PTB processing (Section 4.4) proceed in parallel with these phases of operation. - +-------------------+ - | Path Confirmation +-- Connectivity - +--------+----------+ \----- or BASE_PMTU - | /\ \/ Confirmation Fails + +------------------------+ + | BASE_PMTU Confirmation +-- Connectivity + +------------+-----------+ \----+ or BASE_PMTU + | ^ V Confirmation Fails Connectivity and | | +-------+ - BASE_PMTU confirmed | ---------+ Error | + BASE_PMTU confirmed | +---------+ Error | | +-------+ | CONFIRMATION_TIMER | Fires - \/ + V +----------------+ +--------------+ | Search Complete|<---------+ Search | +----------------+ +--------------+ Search Algorithm Completes Figure 3: DPLPMTUD Phases - Path Confirmation + BASE_PMTU Confirmation * Connectivity is confirmed. * DPLPMTUD confirms the BASE_PMTU is supported across the network path. * DPLPMTUD then enters the search phase. Search @@ -958,61 +990,61 @@ to discover if the PLPMTU can be raised. Error * Inconsistent or invalid network signals cause DPLPMTUD to be unable to progress. * This causes the algorithm to lower the MPS until the path is shown to support the BASE_PMTU, or to suspend DPLPMTUD. -5.2.1. Path Confirmation Phase +5.2.1. BASE_PMTU Confirmation Phase - DPLPMTUD starts in the Path confirmation phase. Path confirmation is - performed in two stages: + DPLPMTUD starts in the BASE_PMTU confirmation phase. BASE_PMTU + confirmation is performed in two stages: 1. Connectivity to the remote peer is first confirmed. When a connection-oriented PL is used, this stage is implicit. It is performed as part of the normal PL connection handshake. In contrast, an connectionless PL MUST send an acknowledged probe packet to confirm that the remote peer is reachable. 2. In the second stage, the PL confirms it can successfully send a datagram of the BASE_PMTU size across the current path. A PL that does not wish to support a network path with a PLPMTU less than BASE_PMTU can simplify the phase into a single step by performing connectivity checks with probes of the BASE_PMTU size. A PL MAY respond to PTB messages while in this phase, see Section 4.4. - Once path confirmation has completed, DPLPMTUD can advertise an MPS - to an upper layer. + Once BASE_PMTU confirmation has completed, DPLPMTUD can advertise an + MPS to an upper layer. If DPLPMTUD fails to complete these tests it enters the PROBE_DISABLED phase, see Section 5.2.6, and ceases using DPLPTMUD. 5.2.2. Search Phase The search phase utilises a search algorithm in attempt to increase the PLPMTU (see Section 5.4.1). The PL sender increases the MPS each time a packet probe confirms a larger PLPMTU is supported by the path. The algorithm concludes by entering the SEARCH_COMPLETE phase, see Section 5.2.3. A PL MAY respond to PTB messages while in this phase, using the PTB to advance or terminate the search, see Section 4.4. Similarly black hole detection can terminate the search by entering the PROBE_BASE phase, see Section 5.2.4. -5.2.2.1. Resilience to inconsistent path information +5.2.2.1. Resilience to Inconsistent Path Information Sometimes a PL sender is able to detect inconsistent results from the sequence of PLPMTU probes that it sends or the sequence of PTB messages that it receives. This could be manifested as excessive fluctuation of the MPS. When inconsistent path information is detected, a PL sender can enable an alternate search mode that clamps the offered MPS to a smaller value for a period of time. This avoids unnecessary black- holing of packets. @@ -1073,188 +1105,169 @@ 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. Note: MIN_PMTU may be identical to BASE_PMTU, simplifying the actions in this phase. If no acknowledgement is received for PROBE_COUNT probes of size MIN_PMTU, the method suspends DPLPMTUD, see Section 5.2.5. -5.2.5.1. Robustness to inconsistent path +5.2.5.1. Robustness to Inconsistent Path Robustness to paths unable to sustain the BASE_PMTU. Some paths could be unable to sustain packets of the BASE_PMTU size. These paths could use an alternate algorithm to implement the PROBE_ERROR phase that allows fallback to a smaller than desired PLPMTU, rather than suffer connectivity failure. This could also utilise methods such as endpoint IP fragmentation to enable the PL sender to communicate using packets smaller than the BASE_PMTU. 5.2.6. DISABLED Phase This phase suspends operation of DPLPMTUD. It disables probing for the PLPMTU until action is taken by the PL or application using the PL. 5.3. State Machine A state machine for DPLPMTUD is depicted in Figure 4. If multihoming - is supported, a state machine is needed for each active path. + is supported, a state machine is needed for each path. - PROBE_TIMER expiry - (PROBE_COUNT = MAX_PROBES) - +-------------------+ +--------------+ - | PROBE_START +------>|PROBE_DISABLED| - +-------------------+ +--------------+ - | ^ - | Path confirmed | - v | - MAX_PMTU acked or +--------------+-+ (PROBE_COUNT | - PTB (BASE_PMTU <= +---------| PROBE_SEARCH | | < MAX_PROBES) | - PTB_SIZE | +--> +--------------+<+ or Probe acked | - | PROBE_BASE |<-------| PROBE_ERROR | - +------+--------+ +--------------+ +-------------+ - /\ | Black hole detected ^ | | BASE_PMTU Probe acked: ^ - | | or | | | | - | | (PTB_SIZE < PLPMTU) | | | Probe BASE_PMTU: | - | | | | | (PROBE_COUNT = MAX_PROBES)| - | | | | +---------------------------+ - +----+ +--+ - Confirmation: PROBE_TIMER expiry: - (PROBE_COUNT < MAX_PROBES) (PROBE_COUNT < MAX_PROBES) - or - PLPMTU Probe acked + | | +-----------------------------------------+ | | + | | MAX_PMTU Probe acked or | | + | | PTB (BASE_PMTU <= PTB_SIZE < PROBED_SIZE) or | | + +----+ PROBE_COUNT = MAX_PROBES +----+ + CONFIRMATION_TIMER expiry: PROBE_TIMER expiry: + PROBE_COUNT < MAX_PROBES or PROBE_COUNT < MAX_PROBES or + PLPMTU Probe acked Probe acked Figure 4: State machine for Datagram PLPMTUD. Note: Some state changes are not show to simplify the diagram. The following states are defined: - PROBE_START: The PROBE_START state is the initial state before - probing has started. The state confirms connectivity to the - remote PL. + 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. - The PLPMTU is set to the BASE_PMTU size. Probing ought to start - immediately after connection setup to prevent the prevent the loss - of user data. PLPMTUD is not performed in this state. The state - transitions to PROBE_SEARCH, when a network path has been - confirmed, i.e., when a sent packet has been acknowledged on this - network path and the BASE_PMTU is confirmed to be supported. If - the network path cannot be confirmed this state transitions to - PROBE_DISABLED. + 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. - PROBE_SEARCH: The PROBE_SEARCH state is the main probing state. - This state is entered when probing for the BASE_PMTU was - successful. + 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, and 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; a + 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 PTB message is validated; a probe of size - PMTU_MAX is acknowledged or black hole detection is triggered. + MAX_PMTU is acknowledged or black hole detection is triggered. SEARCH_COMPLETE: The SEARCH_COMPLETE state indicates a successful end to the PROBE_SEARCH state. DPLPMTUD remains in this state until either the PMTU_RAISE_TIMER expires; a received PTB message is validated; or black hole detection is triggered. 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 PROBE_BASE state. When - used with an acknowledged PL (e.g., SCTP), DPLPMTUD SHOULD NOT - continue to generate PLPMTU probes in this state. - - PROBE_BASE: The PROBE_BASE state is used to confirm whether 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, and the PROBE_TIMER is started. - The state is exited when the probe packet is acknowledged, and the - PL sender enters the PROBE_SEARCH state. - - The state is also left when the PROBE_COUNT reaches MAX_PROBES; a - PTB message is validated. This causes the PL sender to enter the - PROBE_ERROR state. - - PROBE_ERROR: The PROBE_ERROR state represents the case where the - network path is not known to support a PLPMTU of at least the - BASE_PMTU size. It is entered when either a probe of size - BASE_PMTU has not been acknowledged or a validated PTB message - indicates a smaller PTB_SIZE smaller than the BASE_PMTU. + 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. - On entry, the PROBE_COUNT is set to zero and the PROBED_SIZE is - set to the MIN_PMTU size, and the PLPMTU 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 of at least size BASE_PMTU. Robust - implementations may validate the BASE_PMTU several times before - transition to the PROBE_SEARCH. + 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 PROBE_DISABLED. - PROBE_DISABLED: The PROBE_DISABLED state indicates that connectivity - could not be established. DPLPMTUD MUST NOT probe in this state. - Appendix A contains an informative description of key events. 5.4. Search to Increase the PLPMTU This section describes the algorithms used by DPLPMTUD to search for a larger PLPMTU. -5.4.1. Probing for a larger PLPMTU +5.4.1. 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 PMTU_MAX. PMTU_MAX is the minimum of the local MTU - and EMTU_R (learned from the remote endpoint). The PMTU_MAX MAY be + 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 initialised 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 Section 4.1. Each time a probe packet is sent to the destination, the PROBE_TIMER @@ -1287,21 +1300,21 @@ 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. xxx Author Note: A future version of this section will detail example methods for selecting probe size values, but does not plan to mandate a single method. xxx -5.4.3. Resilience to inconsistent Path information +5.4.3. 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 (this could happen when probe packets are lost due to other reasons, or some of the packets in a flow are forwarded along a portion of the path that supports a different actual PMTU). Frequent path changes could occur due to unexpected "flapping" - where some packets from a flow pass along one path, but other packets follow a different path with different properties. DPLPMTUD can be @@ -1308,41 +1321,42 @@ made resilient to these anomalies by introducing hysteresis into the search decision to increase the MPS. 6. 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 subsection describe how DPLPMTUD can + 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. 6.1. Application support for DPLPMTUD with UDP or 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, 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 [RFC8085]. 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 ICMP PTB messages). + interpret received PTB messages). In addition, it is desirable that PMTU discovery is not performed by multiple protocol layers. An application SHOULD avoid implementing DPLPMTUD when the underlying transport system provides this capability. Using a common method for managing the PLPMTU has benefits, both in the ability to share state between different processes and opportunities to coordinate probing. 6.1.1. Application Request @@ -1377,34 +1391,35 @@ CONFIRMATION_TIMER to periodically send probe packets while in the SEARCH_COMPLETE state. 6.1.5. 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 [RFC8085]. 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. A validated PTB message MAY be used as input to the - DPLPMTUD algorithm, but MUST NOT be used directly to set the PLPMTU. + probed size (see Section 4.4.2). A validated PTB message MAY be used + as input to the DPLPMTUD algorithm, but MUST NOT be used directly to + set the PLPMTU. 6.2. DPLPMTUD with UDP Options UDP Options[I-D.ietf-tsvwg-udp-options] can supply the additional functionality required to implement DPLPMTUD within the UDP transport - service. Implementing DPLPMTU using UDP Options avoids the need for + service. Implementing DPLPMTUD using UDP Options avoids the need for each application to implement the DPLPMTUD method. Section 5.6 of[I-D.ietf-tsvwg-udp-options] defines the Maximum Segment Size (MSS) option, which allows the local sender to indicate the EMTU_R to the peer. The value received in this option can be - used to initialise PMTU_MAX. + used to initialise MAX_PMTU. UDP Options enables padding to be added to UDP datagrams that are used as Probe Packets. Feedback confirming reception of each Probe Packet is provided by two new UDP Options: o The Probe Request Option (Section 6.2.1) is set by a sending PL to solicit a response from a remote endpoint. A four-byte token identifies each request. o The Probe Response Option (Section 6.2.2 is generated by the UDP @@ -1613,21 +1628,21 @@ Quick UDP Internet Connection (QUIC) [I-D.ietf-quic-transport] 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 9.2 of [I-D.ietf-quic-transport] 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 illict an acknowledgement. Padding + padding only packet that will elicit an acknowledgement. Padding only frames enable probing the without affecting the transfer of other QUIC frames. The recommendation for QUIC endpoints implementing DPLPMTUD is therefore that a MPS is maintained for each combination of local and remote IP addresses [I-D.ietf-quic-transport]. 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 @@ -1690,29 +1705,30 @@ 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-initialised sequence number. A description of one way to do this when using UDP is provided in section 5.1 of [RFC8085]). - There are cases where PTB messages are not delivered due to policy, - configuration or equipment design (see Section 1.1), this method - therefore does not rely upon PTB messages being received, but is able - to utilise 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). + There are cases where ICMP Packet Too Big (PTB) messages are not + delivered due to policy, configuration or equipment design (see + Section 1.1), this method therefore does not rely upon PTB messages + being received, but is able to utilise 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 Section 4.4.1). 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 PROBE_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 @@ -2060,22 +2076,33 @@ o Feedback after speaking with Joe Touch helped improve UDP-Options description. Working Group draft -06: o Updated description of ICMP issues in section 1.1 o Update to description of QUIC. -Authors' Addresses + Working group draft -07: + o Moved description of the PTB processing method from the PTB + requirements section. + + o Clarified what is performed in the PTB validation check. + + o Updated security consideration to explain PTB security without + needing to read the rest of the document. + + o Reformatted state machine diagram + +Authors' Addresses Godred Fairhurst University of Aberdeen School of Engineering Fraser Noble Building Aberdeen AB24 3UE UK Email: gorry@erg.abdn.ac.uk Tom Jones @@ -2079,25 +2106,34 @@ Email: gorry@erg.abdn.ac.uk Tom Jones University of Aberdeen School of Engineering Fraser Noble Building Aberdeen AB24 3UE UK Email: tom@erg.abdn.ac.uk + Michael Tuexen Muenster University of Applied Sciences Stegerwaldstrasse 39 - Stein fart 48565 + Steinfurt 48565 DE Email: tuexen@fh-muenster.de Irene Ruengeler Muenster University of Applied Sciences Stegerwaldstrasse 39 - Stein fart 48565 + Steinfurt 48565 DE Email: i.ruengeler@fh-muenster.de + + Timo Voelker + Muenster University of Applied Sciences + Stegerwaldstrasse 39 + Steinfurt 48565 + DE + + Email: timo.voelker@fh-muenster.de