Path MTU Algorithms for QUIC
draft-pskim-quic-pmtu-algorithms-00

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

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Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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.

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   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.

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   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]

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     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.

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   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. 

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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.

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6.  References

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 
              DOI 10.17487/RFC1191, November 1990, 
              <https://www.rfc-editor.org/info/rfc1191>.

   [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, 
              <https://www.rfc-editor.org/info/rfc8201>.

   [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, <https://www.rfc-editor.org/info/rfc8899>.

   [UDP-PMTUD]  
              Work in Progress, Internet-Draft, 
              draft-ietf-tsvwg-udp-options-dplpmtud-10, 3 July 
              2023, <https://www.ietf.org/archive/id/draft-ietf-tsvwg-
              udp-options-dplpmtud-10.txt>.

   [RFC9000]  J. Iyengar, Ed., M. Thomson, Ed., "QUIC: A UDP-Based 
              Multiplexed and Secure Transport", RFC 9000, 
              DOI 10.17487/RFC9000, May 2021, 
              <https://www.rfc-editor.org/info/rfc9000>.

   [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, <https://datatracker.ietf.org/doc/draft-
              lee-optimal-detect-pmtu>.

   [RFC9268]  B. Hinden, G. Fairhurs, "IPv6 Minimum Path MTU Hop-by-Hop 
              Option", RFC 9268, August 2022, <https://datatracker.ietf.
              org/doc/rfc9268/>.

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Authors' Addresses

   Pyung Soo Kim
   Tech University of Korea
   Siheung, Gyeonggi
   South Korea
   Email: pskim@tukorea.ac.kr
   

 

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