|Title||TCP Sender Clarification for Persist Condition
|Author||M. Bashyam, M.
Jethanandani, A. Ramaiah
Internet Engineering Task Force (IETF) M. Bashyam
Request for Comments: 6429 Ocarina Networks, Inc.
Category: Informational M. Jethanandani
ISSN: 2070-1721 A. Ramaiah
TCP Sender Clarification for Persist Condition
This document clarifies the Zero Window Probes (ZWPs) described in
RFC 1122 ("Requirements for Internet Hosts -- Communication Layers").
In particular, it clarifies the actions that can be taken on
connections that are experiencing the ZWP condition. Rather than
making a change to the standard, this document clarifies what has
been until now a misinterpretation of the standard as specified in
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
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Table of Contents
1. Introduction ....................................................2
1.1. Requirements Language ......................................3
2. Discussion of RFC 1122 Requirement ..............................3
3. Description of One Simple Attack ................................4
4. Clarification Regarding RFC 1122 Requirements ...................5
5. Security Considerations .........................................5
6. Acknowledgments .................................................5
7. References ......................................................6
7.1. Normative References .......................................6
7.2. Informative References .....................................6
Section 220.127.116.11 of "Requirements for Internet Hosts -- Communication
Layers" [RFC1122] says:
"A TCP MAY keep its offered receive window closed indefinitely.
As long as the receiving TCP continues to send acknowledgments in
response to the probe segments, the sending TCP MUST allow the
connection to stay open.
It is extremely important to remember that ACK (acknowledgment)
segments that contain no data are not reliably transmitted by
Therefore, zero window probing needs to be supported to prevent a
connection from hanging forever if ACK segments that re-open the
window are lost. The condition where the sender goes into the Zero
Window Probe (ZWP) mode is typically known as the 'persist
This guidance is not intended to preclude resource management by the
operating system or application, which may request that connections
be aborted regardless of whether or not they are in the persist
condition. The TCP implementation needs to, of course, comply by
aborting such connections. If such resource management is not
performed external to the protocol implementation, TCP
implementations that misinterpret Section 18.104.22.168 of [RFC1122] have
the potential to make systems vulnerable to denial-of-service (DoS)
[RFC4732] scenarios where attackers tie up resources by keeping
connections in the persist condition.
Rather than making a change to the standard, this document clarifies
what has been until now a misinterpretation of the standard as
specified in RFC 1122 [RFC1122].
Section 2 of this document describes why implementations might not
close connections merely because they are in the persist condition,
yet need to still allow such connections to be closed on command.
Section 3 outlines a simple attack on systems that do not
sufficiently manage connections in this state. Section 4 concludes
with a requirements-language clarification to the RFC 1122
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
2. Discussion of RFC 1122 Requirement
Per [RFC1122], as long as the ACKs are being received for window
probes, a connection can continue to stay in the persist condition.
This is an important feature, because applications typically would
want the TCP connection to stay open unless an application explicitly
closes the connection.
For example, take the case of a user running a network print job
during which the printer runs out of paper and is waiting for the
user to reload the paper tray (user intervention). The printer may
not be reading data from the printing application during this time.
Although this may result in a prolonged ZWP state, it would be
premature for TCP to take action on its own and close the printer
connection merely due to its lack of progress. Once the printer's
paper tray is reloaded (which may be minutes, hours, or days later),
the print job needs to be able to continue uninterrupted over the
same TCP connection.
However, systems that misinterpret Section 22.214.171.124 of [RFC1122] may
fall victim to DoS attacks by not supporting sufficient mechanisms to
allow release of system resources tied up by connections in the
persist condition during times of resource exhaustion. For example,
take the case of a busy server where multiple (attacker) clients can
advertise a zero window forever (by reliably acknowledging the ZWPs).
This could eventually lead to resource exhaustion in the server
system. In such cases, the application or operating system would
need to take appropriate action on the TCP connection to reclaim
their resources and continue to maintain legitimate connections.
The problem is applicable to TCP and TCP-derived flow-controlled
transport protocols such as the Stream Control Transmission Protocol
Clearly, a system needs to be robust to such attacks and allow
connections in the persist condition to be aborted in the same way as
any other connection. Section 4 of this document provides the
requisite clarification to permit such resource management.
3. Description of One Simple Attack
To illustrate a potential DoS scenario, consider the case where many
client applications open TCP connections with an HTTP [RFC2616]
server, and each sends a GET request for a large page and stops
reading the response partway through. This causes the client's TCP
implementation to advertise a zero window to the server. For every
large HTTP response, the server is left holding on to the response
data in its sending queue. The amount of response data held will
depend on the size of the send buffer and the advertised window. If
the clients never read the data in their receive queues and therefore
do not clear the persist condition, the server will continue to hold
that data indefinitely. Since there may be a limit to the operating
system kernel memory available for TCP buffers, this may result in
DoS to legitimate connections by locking up the necessary resources.
If the above scenario persists for an extended period of time, it
will lead to starvation of TCP buffers and connection blocks, causing
legitimate existing connections and new connection attempts to fail.
A clever application needs to detect such attacks with connections
that are not making progress, and could close these connections.
However, some applications might have transferred all the data to the
TCP socket and subsequently closed the socket, leaving the
connections with no controlling process; such connections are
referred to as orphaned connections. These orphaned connections
might be left holding the data indefinitely in their sending queue.
The US Computer Emergency Readiness Team (CERT) has released an
advisory in this regard [VU723308] and is making vendors aware of
this DoS scenario.
4. Clarification Regarding RFC 1122 Requirements
As stated in [RFC1122], a TCP implementation MUST NOT close a
connection merely because it seems to be stuck in the ZWP or persist
condition. Though unstated in RFC 1122, but implicit for system
robustness, a TCP implementation needs to allow connections in the
ZWP or persist condition to be closed or aborted by their
applications or other resource management routines in the operating
An interface that allows an application to inform TCP on what to do
when the connection stays in the persist condition, or that allows an
application or other resource manager to query the health of the TCP
connection, is considered outside the scope of this document. All
such techniques, however, are in complete compliance with TCP
[RFC0793] and [RFC1122].
5. Security Considerations
This document discusses one system security consideration that is
listed in "Guidelines for Writing RFC Text on Security
Considerations" [RFC3552]. In particular, it describes an
inappropriate use of a system that is acting as a server for many
users. That use and a possible DoS attack are discussed in
This document limits itself to clarifying RFC 1122. It does not
discuss what can happen with orphaned connections and other possible
mitigation techniques, as these are considered outside the scope of
This document was inspired by the recent discussions that took place
regarding the TCP persist condition issue in the TCP Maintenance and
Minor Extensions (TCPM) Working Group mailing list [TCPM]. The
outcome of those discussions was to come up with a document that
would clarify the intentions of the ZWP as discussed in RFC 1122. We
would like to thank Mark Allman, Ted Faber, and David Borman for
clarifying the objective behind this document. Thanks also go to
Wesley Eddy for his extensive editorial comments and to Dan Wing,
Mark Allman, and Fernando Gont for providing feedback on this
7.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7.2. Informative References
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
[RFC4732] Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
Denial-of-Service Considerations", RFC 4732,
[TCPM] IETF, "TCP Maintenance and Minor Extensions (tcpm) -
[VU723308] Manion, A. and D. Warren, "TCP may keep its offered
receive window closed indefinitely (RFC 1122)",
November 2009, <http://www.kb.cert.org/vuls/id/723308>.
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