RFC 3360 (rfc3360) - Page 1 of 19

Inappropriate TCP Resets Considered Harmful

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Network Working Group                                           S. Floyd
Request for Comments: 3360                                          ICIR
BCP: 60                                                      August 2002
Category: Best Current Practice


              Inappropriate TCP Resets Considered Harmful

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document is being written because there are a number of
   firewalls in the Internet that inappropriately reset a TCP connection
   upon receiving certain TCP SYN packets, in particular, packets with
   flags set in the Reserved field of the TCP header.  In this document
   we argue that this practice is not conformant with TCP standards, and
   is an inappropriate overloading of the semantics of the TCP reset.
   We also consider the longer-term consequences of this and similar
   actions as obstacles to the evolution of the Internet infrastructure.

1.  Introduction

   TCP uses the RST (Reset) bit in the TCP header to reset a TCP
   connection.  Resets are appropriately sent in response to a
   connection request to a nonexistent connection, for example.  The TCP
   receiver of the reset aborts the TCP connection, and notifies the
   application [RFC 793, RFC 1122, Ste94].

   Unfortunately, a number of firewalls and load-balancers in the
   current Internet send a reset in response to a TCP SYN packet that
   use flags from the Reserved field in the TCP header.  Section 3 below
   discusses the specific example of firewalls that send resets in
   response to TCP SYN packets from ECN-capable hosts.

   This document is being written to inform administrators of web
   servers and firewalls of this problem, in an effort to encourage the
   deployment of bug-fixes [FIXES].  A second purpose of this document
   is to consider the longer-term consequences of such middlebox
   behavior on the more general evolution of protocols in the Internet.



Floyd                    Best Current Practice
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