TCP Problems with Path MTU Discovery
RFC 2923

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Network Working Group                                        K. Lahey
Request for Comments: 2923                            dotRocket, Inc.
Category: Informational                                September 2000

                  TCP Problems with Path MTU Discovery

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This memo catalogs several known Transmission Control Protocol (TCP)
   implementation problems dealing with Path Maximum Transmission Unit
   Discovery (PMTUD), including the long-standing black hole problem,
   stretch acknowlegements (ACKs) due to confusion between Maximum
   Segment Size (MSS) and segment size, and MSS advertisement based on
   PMTU.

1. Introduction

   This memo catalogs several known TCP implementation problems dealing
   with Path MTU Discovery [RFC1191], including the long-standing black
   hole problem, stretch ACKs due to confusion between MSS and segment
   size, and MSS advertisement based on PMTU.  The goal in doing so is
   to improve conditions in the existing Internet by enhancing the
   quality of current TCP/IP implementations.

   While Path MTU Discovery (PMTUD) can be used with any upper-layer
   protocol, it is most commonly used by TCP;  this document does not
   attempt to treat problems encountered by other upper-layer protocols.
   Path MTU Discovery for IPv6 [RFC1981] treats only IPv6-dependent
   issues, but not the TCP issues brought up in this document.

   Each problem is defined as follows:

   Name of Problem
      The name associated with the problem.  In this memo, the name is
      given as a subsection heading.

Lahey                        Informational                      [Page 1]
RFC 2923          TCP Problems with Path MTU Discovery    September 2000

   Classification
      One or more problem categories for which the problem is
      classified:  "congestion control", "performance", "reliability",
      "non-interoperation -- connectivity failure".

   Description
      A definition of the problem, succinct but including necessary
      background material.

   Significance
      A brief summary of the sorts of environments for which the problem
      is significant.

   Implications
      Why the problem is viewed as a problem.

   Relevant RFCs
      The RFCs defining the TCP specification with which the problem
      conflicts.  These RFCs often qualify behavior using terms such as
      MUST, SHOULD, MAY, and others written capitalized.  See RFC 2119
      for the exact interpretation of these terms.

   Trace file demonstrating the problem
      One or more ASCII trace files demonstrating the problem, if
      applicable.

   Trace file demonstrating correct behavior
      One or more examples of how correct behavior appears in a trace,
      if applicable.

   References
      References that further discuss the problem.

   How to detect
      How to test an implementation to see if it exhibits the problem.
      This discussion may include difficulties and subtleties associated
      with causing the problem to manifest itself, and with interpreting
      traces to detect the presence of the problem (if applicable).

   How to fix
      For known causes of the problem, how to correct the
      implementation.

Lahey                        Informational                      [Page 2]
RFC 2923          TCP Problems with Path MTU Discovery    September 2000

2. Known implementation problems

2.1.

   Name of Problem
      Black Hole Detection

   Classification
      Non-interoperation -- connectivity failure

   Description
      A host performs Path MTU Discovery by sending out as large a
      packet as possible, with the Don't Fragment (DF) bit set in the IP
      header.  If the packet is too large for a router to forward on to
      a particular link, the router must send an ICMP Destination
      Unreachable -- Fragmentation Needed message to the source address.
      The host then adjusts the packet size based on the ICMP message.

      As was pointed out in [RFC1435], routers don't always do this
      correctly -- many routers fail to send the ICMP messages, for a
      variety of reasons ranging from kernel bugs to configuration
      problems.  Firewalls are often misconfigured to suppress all ICMP
      messages.  IPsec [RFC2401] and IP-in-IP [RFC2003] tunnels
      shouldn't cause these sorts of problems, if the implementations
      follow the advice in the appropriate documents.

      PMTUD, as documented in [RFC1191], fails when the appropriate ICMP
      messages are not received by the originating host.  The upper-
      layer protocol continues to try to send large packets and, without
      the ICMP messages, never discovers that it needs to reduce the
      size of those packets.  Its packets are disappearing into a PMTUD