Network Working Group P. S. Kim Internet-Draft TU Korea Intended status: Informational Expires: 8 January 2025 7 July 2024 Path MTU Algorithms for QUIC draft-pskim-quic-pmtu-algorithms-00 Abstract This draft consider a couple of algorithms for Path MTU (PMTU) in QUIC to discovery optimal PMTU and resolve PMTU black hole problem. Fistly, a passive probing approach is adopted to discover the PMTU. The process of discovering the PMTU is not performed separately, but is performed simultaneously in the actual application data communication. That is, the actual application data is allowed to be carried in the process of discovering the PMTU. A probe packet is defined newly using 1-RTT packet which includes actual application data as well as a short packet header and a PING_EXT frame. Until the optimal PMTU is discovered, the size of the probe packet is changed according to the size of the PMTU candidate. Secondly, a PMTU black hole problem in secure and reliable transport protocol is discussed and a possible solution can be suggested from existing researches. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 8 January 2025. Copyright Notice Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved. P.S. Kim Expires 8 January 2025 [Page 1] Internet-Draft Path MTU Algorithms for QUIC July 2024 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 2. Passive Probing for PMTUD with QUIC . . . . . . . . . . . . . 4 2.1. Active Probing for PMTUD . . . . . . . . . . . . . . . . 4 2.2. A New PMTU Probe Packet . . . . . . . . . . . . . . . . . 4 2.2. Passive Probing . . . . . . . . . . . . . . . . . . . . . 5 3. Resolving PMTU Black Hole Problem . . . . . . . . . . . . . . 7 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction The maximum transmission unit (MTU) is the largest size frame or packet - in bytes or octets - that can be transmitted across a data link. It is most used in reference to packet size on an Ethernet network using the Internet Protocol (IP). The Path MTU (PMTU) is the smallest MTU of all involved network interfaces for a network path and limits the size of IP packets. A PMTU Discovery (PMTUD) is a standardized technique in computer networking for determining the PMTU size on the network path between two IP hosts, usually with the goal of avoiding IP fragmentation for IPv4[RFC1191] and for IPv6[RFC8201]. When a packet too large for the path was sent, the PMTUD expects to receive a Packet Too Big (PTB) message. However, there are multiple reasons why a PTB message might not arrive at the sender. Therefore, the PMTUD for the Packetization Layer (PL) that selects the size of IP packets is specified recently in [RFC8899]. RFC8899 works without a signal from the network and covers generic PL protocols such as QUIC of [RFC9000]. Meanwhile, [UDP-PMTUD] complements RFC8899 by specifying how a UDP Options sender implements Datagram PL PMTUD(DPLPMTUD). It allows a datagram application to discover the largest size of datagram that can be sent across a specific network path. However, [RFC8899] does not contain details about how to discovery for the optimal PMTU. P.S. Kim Expires 8 January 2025 [Page 2] Internet-Draft Path MTU Algorithms for QUIC July 2024 Recently, therefore, [Q-PMTUD] complements RFC8899 by presenting a discovery algorithm with QUIC. Using the discovery algorithm with a set of possible PMTU candidates and their possible probing sequences, the optimal PMTU is obtained. However, to discover the optimal PMTU, some probe packets which have no semantic value might be injecting into network, which is called active probing or active measurement. The active probing approach can increase a network load and perturb the network. Based on [Q-PMTUD] and [UDP-PMTUD], this draft consider an alternative PMTUD for QUIC. To discover the optimal PMTU, the passive probing approach is adopted. The process of discovering the optimal PMTU is not carried out separately, but is carried out simultaneously in the actual application data communication. A probe packet is defined newly using 1-RTT packet which includes actual application data as well as a short packet header and a PING_EXT frame. The PING_EXT frame is also defined newly. Until the optimal PMTU is discovered, the size of the probe packet is changed according to the size of the PMTU candidate. A simple discovery algorithm using only the PMTU candidate sequence with linear upward is described in this draft. Other rather complex discovery algorithms that consider various PMTU candidate sequences will be dealt with in the future. Meanwhile, in classical PMTU discovery, a PMTU black hole problem could arise when the PTB messages are not sent back to the sender for some reason. As an extreme case such as the secure and reliable transport protocol QUIC, a sender that trusts only cryptographically secured information will not use PTB messages. Thus, a possible solution can be suggested from existing researches. 1.1. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 2. Passive Probing for PMTUD with QUIC The specification of QUIC in RFC9000 recommends to use the PMTUD framework of RFC8899. RFC 8899 DPLPMTUD has the following phases: - Base: Send the probe in its basic size first. QUIC assumes that the specified 1280 bytes pass, so it starts from the next phase. - Search: Search for candidate PMTUs while sending probes. Once the optimal PMTU is detected, proceed to the next phase. - Search Complete: Since PMTU may change due to route changes, check if it is still optimal. P.S. Kim Expires 8 January 2025 [Page 3] Internet-Draft Path MTU Algorithms for QUIC July 2024 There are three possible ways to create a PMTU probe packet as follows[RFC8899]: - Probing using padding data - Probing using application data and padding data - Probing using application data [UDP-PMTUD] describes "Probe Packets that include Application Data" to implement "Probing using application data" of [RFC8899]. However, RFC8899 does not contain details about how to discovery for the optimal PMTU. 2.1. Active Probing for PMTUD with QUIC[Q-PMTUD] [Q-PMTUD] complements the specification, RFC8899, by presenting a discovery algorithm with QUIC. From a practical point of view, it might be a good choice to consider only a set of common PMTU values. However, the PMTU value may usually change over time. Thus, [Q-PMTUD] considers a set of possible PMTU candidates. PMTU candidates are values every 4 bytes from 1280 bytes to 1500 bytes. Then, a discovery algorithm is proposed, which probes one PMTU candidate after the other. This means, it starts the probe for the next candidate not before the probe for the current candidate either succeeded or failed. Then endpoint uses this discovery algorithm that repeatedly chooses PMTU candidates to probe. The candidate sequence is required to specify the order in which the discovery algorithm probes PMTU candidates. The endpoint must choose a PMTU candidate larger than the largest successfully probed candidate and smaller than any other probed candidate with a lost probe packet. Seven candidate sequences are considered, evaluated, and compared in [Q-PMTUD]. To probe one PMTU candidate, according to RFC9000, the endpoint builds a probe packet with a short packet header, a PING frame and PADDING frames. The endpoint controls the size of the probe packet by the number of PADDING frames, whose size is one byte each. The PING frame makes the packet ack-eliciting. However, to discover the optimal PMTU, some probe packets which have no semantic value might be injecting into network, which is thus called active measurement or active probing. This active probing approach can increase a network load and perturb the network. 2.2. A New PMTU Probe Packet (1-RTT packet format) (1) Probe packet format for active probing [Q-PMTUD] P.S. Kim Expires 8 January 2025 [Page 4] Internet-Draft Path MTU Algorithms for QUIC July 2024 IP header + UDP header + Short header(QUIC header) + PING frame + PADDING frames The size of the probe packet is controlled by the number of PADDING frames. (2) Probe packet format for passive probing In this drfat, a probe packet is defined newly using 1-RTT packet including actual application data as well as a PING_EXT frame as follows: IP header + UDP header + Short header(QUIC Header) + PING_EXT frame + Actual application data - PING_EXT frame (defined newly) . Frame Type Name : PING_EXT . Type Value : 0x20 . The PING_EXT frame makes the packet ack-eliciting. In addition, the PING_EXT frame indicates that the current 1-RTT packet is now discovering the optimal PMTU as well as transmitting actual application data. - Application data . Actual application data controls the size of the probe packet by a multiple of four bytes. The size of probe packet is changed according to PMTU candidates as follows: . 1280 + incremental where, for example, incremental can be a multiple of four as shown in [Q-PMTUD]. 2.3. Passive Probing Through the new probe packet, it is possible not only to discovery the optimal PMTU, but also to transmit actual application data. That is, to discover the optimal PMTU size and carry actual application data, the endpoint expand the payload of all UDP datagrams. (1) A simple algorithm for discovering the optimal PMTU As specified in RFC9000, QUIC must send QUIC packets with the smallest allowed maximum datagram size when validating a path during connection initiation or migration. Thus, the endpoint sets the probe packet initially to the smallest allowed maximum datagram size of 1280 bytes including actual application data as well as a short packet header, a PING_EXT frame. P.S. Kim Expires 8 January 2025 [Page 5] Internet-Draft Path MTU Algorithms for QUIC July 2024 As mentioned, until the optimal PMTU is discovered, the size of the probe packet is changed successively according to the size of the PMTU candidate. The size of the probe packet is controlled with the size of actual application data. The size of actual application data is a multiple of four. In the active probing approach [Q-PMTUD], the endpoint uses a simple discovery algorithm that repeatedly chooses PMTU candidates to probe. Thus, seven PMTU candidate sequences are considered and each candidate sequence specifies the order in which the discovery algorithm probes PMTU candidates. In addition, four metrics such as number of probed PMTU candidates, time to discover the optimal PMTU, network load, average PMTU estimation are defined for performance evaluations of seven sequences. However, because the process of discovering the optimal PMTU is carried out simultaneously in the actual application data communication, only the PMTU candidate sequence with linear upward is adopted first in this draft. The linear upward sequence selects one candidate after the other from a list of candidates in ascending order, starting with the second one (the first one was probed with the smallest allowed maximum datagram size of 1280 bytes). Other rather complex discovery approaches that consider various PMTU candidate sequence will be dealt with in the future. Until the optimal PMTU is discovered, the endpoint repeats a series of probing steps. In absence of a PTB message, the discovery algorithm considers a probe for a PMTU candidate as failed, only if the probe packet of the size of the candidate were detected as lost. A probe for a PMTU candidate that fails, lets all other probes for larger candidates fail as well. Therefore, the optimal PMTU is the PMTU candidate that succeeded just before the failure. (2) Discovery complete and PMTU cache When the algorithm determines that it has discovered the optimal PMTU, the endpoint terminates the probing. Then, the endpoint sets the 1-RTT packet finally to the optimal datagram size using the optimal PMTU discovered. From now on, the 1-RTT packet does not include a PING_EXT frame. QUIC can cache the optimal PMTU discovered and use it for future connections to the same endpoint. (3) Other rather complex discovery algorithms Other rather complex discovery algorithms that consider various PMTU candidate sequences will be dealt with in the future. P.S. Kim Expires 8 January 2025 [Page 6] Internet-Draft Path MTU Algorithms for QUIC July 2024 3. Resolving PMTU Black Hole Problem Classical PMTU discovery is subject to protocol failures. One failure arises when traffic using a packet size larger than the actual PMTU is black-holed. That is, all datagrams larger than the actual PMTU are discarded, which is known as PMTU black hole problem. This could arise when the PTB messages are not sent back to the sender for some reason. In extreme cases, such as the secure and reliable transport protocol QUIC, a sender who only trusts encrypted security information will not use PTB messages. The main idea of [RFC8899] is to prevent an endpoint from unintentionally sending packets that are too big by limiting their size using a PMTU estimation that is equal or smaller than the actual PMTU. The discovery begins with a PMTU search that provides successively increasing estimates of the actual PMTU. This process does not require PTB messages. However, a PMTU can change. A decrease in PMTU may cause the endpoint to transmit packets that are too large. [RFC8899] does not describe how to detect this without a PTB message. Recently, [Q-PMTUBH] presents a new parameterizable PTB detection algorithm for a secure and reliable transport protocol that does not depend on PTB messages. [Q-PMTUBH] chooses QUIC as an example to illustrate how to integrate the new parameterizable PTB detection algorithm into a transport protocol and elaborate it with different parameter values using the QUIC simulation model. Therefore, this new PTB detection algorithm can be a solution to the PMTU black hole problem in QUIC. Additionally, when applying QUIC in an IPv6 environment, the new IPv6 Hop-by-Hop (HBH) Option, which is used to record or cache the minimum PMTU along the forwarding path between the source and destination hosts, can be a solution. The concept of recording or caching the minimum PMTU was originated by [O-PMTUD] and standardized by [RFC9268]. 4. IANA Considerations This memo includes no request to IANA. 5. Security Considerations The same security considerations as those described in RFC7880 will apply to this document. P.S. Kim Expires 8 January 2025 [Page 7] Internet-Draft Path MTU Algorithms for QUIC July 2024 6. References [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, November 1990, . [RFC8201] McCann, J., S. Deering, J. Mogul, R. Hinden, Ed. "Path MTU Discovery for IP version 6", RFC 8201, DOI 10.17487/RFC8201, July 2017, . [RFC8899] Fairhurst, G., T. Jones, M. Tuxen, I. Rungeler, T. Volker, "Packetization Layer Path MTU Discovery for Datagram Transports", RFC 8899, DOI 10.17487/RFC8899, September 2020, . [UDP-PMTUD] Work in Progress, Internet-Draft, draft-ietf-tsvwg-udp-options-dplpmtud-10, 3 July 2023, . [RFC9000] J. Iyengar, Ed., M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, May 2021, . [Q-PMTUD] Timo Volker, Michael Tuxen, "The search of the path MTU with QUIC", EPIQ '21: Proceedings of the 2021 Workshop on Evolution, Performance and Interoperability of QUIC, December 2021 [Q-PMTUBH] Timo Volker, Michael Tuxen, "Packet Too Big Detection and its Integration into QUIC", 2023 16th International Conference on Signal Processing and Communication System (ICSPCS), September 2023 [O-PMTUD] Expired, Internet-Draft, draft-lee-optimal-detect-pmtu-00, 8 October 2002, . [RFC9268] B. Hinden, G. Fairhurs, "IPv6 Minimum Path MTU Hop-by-Hop Option", RFC 9268, August 2022, . P.S. Kim Expires 8 January 2025 [Page 8] Internet-Draft Path MTU Algorithms for QUIC July 2024 Authors' Addresses Pyung Soo Kim Tech University of Korea Siheung, Gyeonggi South Korea Email: pskim@tukorea.ac.kr P.S. Kim Expires 8 January 2025 [Page 9]