TCP Maintenance and Minor M. Bashyam
Extensions Working Group Ocarina Networks, Inc
Internet-Draft M. Jethanandani
Intended status: Informational A. Ramaiah
Expires: May 14, 2011 Cisco Systems
November 10, 2010
Clarification of sender behaviour in persist condition.
draft-ietf-tcpm-persist-01.txt
Abstract
This document attempts to clarify the notion of the Zero Window
Probes (ZWP) described in RFC 1122 [RFC1122]. In particular, it
clarifies the actions that can be taken on connections which are
experiencing the ZWP condition. The motivation for this document
stems from the belief that TCP implementations strictly adhering to
the current RFC language have the potential to become vulnerable to
Denial of Service (DoS) scenarios.
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 http://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 May 14, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://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
Bashyam, et al. Expires May 14, 2011 [Page 1]
Internet-Draft TCP persist condition November 2010
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Discussion on RFC 1122 Requirement . . . . . . . . . . . . . . 4
3. Description of Attack . . . . . . . . . . . . . . . . . . . . 5
4. Clarification Regarding RFC 1122 Requirements . . . . . . . . 6
5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
7. Programming Considerations . . . . . . . . . . . . . . . . . . 9
8. Informative References . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Bashyam, et al. Expires May 14, 2011 [Page 2]
Internet-Draft TCP persist condition November 2010
1. Introduction
TCP implementations strictly adhering to Section 4.2.2.17 of
[RFC1122] have the potential to become vulnerable to Denial of
Service (DoS) scenarios. That section of [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."
DISCUSSION:
It is extremely important to remember that ACK (acknowledgment)
segments that contain no data are not reliably transmitted by
TCP.
Therefore zero window probing SHOULD be supported to prevent a
connection from hanging forever if ACK segments that re-opens the
window is lost. The condition where the sender goes into the Zero-
Window Probe (ZWP) mode is typically known as the 'persist
condition'. It is under this condition that the sending TCP can
become vulnerable to DoS.
Bashyam, et al. Expires May 14, 2011 [Page 3]
Internet-Draft TCP persist condition November 2010
2. Discussion on RFC 1122 Requirement
It needs to be emphasised that TCP MUST NOT take any action of its
own when a particular connection is in persist condition for a long
time. As per RFC 1122 as long as the ACK's are being received for
window probes, it can continue to stay in persist condition. This is
important because typically applications would want the TCP
connection to stay open unless it explicitly closes the connection.
For example take the case of user running a print job and the printer
ran out of paper waiting for the user intervention. It would be
premature for TCP to take action on its own. Hence TCP cannot act as
a resource manager and it is the system or application's
responsibility to take appropriate action.
At the same time, many existing TCP implementations that adhere
strictly to the above verbiage of RFC 1122 may fall victim to DOS
attacks, if appropriate measures are not followed. For example, if
we take the case of a busy server where multiple clients can
advertise a zero forever (by reliably acknowledging the ZWP's), it
could eventually lead to the resource exhaustion in the system. In
such cases the system would need to take appropriate action on the
TCP connection to reclaim the resources.
This document is not intended to provide any advice on any particular
resource management scheme that can be implemented to circumvent DOS
issues arising due to the connections stuck in the persist state.
The problem is applicable to TCP and TCP derived transport protocols
like SCTP.
In summary, TCP MUST NOT take any action on its own to abort a
connection in persist condition. Applications however can request
that a connection in persist condition be aborted. The resource
manager in the operating system when faced with depleted resources
can also ask TCP to abort a connection.
Bashyam, et al. Expires May 14, 2011 [Page 4]
Internet-Draft TCP persist condition November 2010
3. Description of Attack
If TCP implementations strictly follow RFC 1122 and there is no
instruction on what to do in persist condition, connections will
encounter an indefinite wait. To illustrate this, consider the case
where the client application opens a TCP connection with a HTTP
[RFC2616] server, sends a GET request for a large page and stops
reading the response. This would cause the client TCP 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 send queue. The
amount of response data held will depend on the size of the send
buffer and the advertised window. If the client never reads the data
in its receive queue or clears the persist condition, the server will
continue to hold that data indefinitely. Multiple such TCP
connections stuck in the same scenario on the server would cause
resource depletion resulting in a DoS situation on the server.
Applications on the sender can transfer all the data to the TCP
socket and subsequently close the socket leaving the connection with
no controlling process, hereby referred to as orphaned connection.
If the application on the receiver refuses to read the data, the
orphaned connection will be left holding the data indefinitely in its
send queue.
If the above scenario persists for an extended period of time, it
will lead to TCP buffers and connection blocks starvation causing
legitimate existing connections and new connection attempts to fail.
CERT has released an advisory in this regard[VU723308] and is making
vendors aware of this DoS scenario.
Bashyam, et al. Expires May 14, 2011 [Page 5]
Internet-Draft TCP persist condition November 2010
4. Clarification Regarding RFC 1122 Requirements
A consequence of adhering to the above requirement mandated by RFC
1122 is that multiple TCP receivers advertising a zero window to a
server could exhaust the connection and buffer resources of the
sender. In such cases, and specially when the receiver is reliably
acknowledging zero window probe, to achieve robustness, the system
should be able to take appropriate action on those TCP connections
and reclaim resources. A possible action could be to terminate the
connection and such an action is in the spirit of RFC 1122.
In order to accomplish this action, TCP MAY provide a feedback
regarding the persist condition to the application if requested to do
so or the application or the resource manager can query the health of
the TCP connection which would allow it to take the desired action.
All such actions are in complete compliance of RFC 793 and RFC 1122.
Bashyam, et al. Expires May 14, 2011 [Page 6]
Internet-Draft TCP persist condition November 2010
5. Conclusion
The document addresses the fact that terminating TCP connections
stuck in the persist condition does not violate RFC 1122 or RFC 793.
It also suggests that TCP must not abort any connection until
explicitly requested by the application or the operating system to do
so. The potential implementation guidelines of the request and the
action are documented in Section 7, and the details of mitigating the
DoS attack are left to the implementer.
Bashyam, et al. Expires May 14, 2011 [Page 7]
Internet-Draft TCP persist condition November 2010
6. Acknowledgments
This document was inspired by the recent discussions that took place
regarding the TCP persist condition issue in the TCPM WG mailing list
[TCPM]. The outcome of those discussions was to come up with a draft
that would clarify the intentions of the ZWP referred by RFC 1122.
We would like to thank Mark Allman and David Borman for clarifying
the objective behind this draft. To Dan Wing, Mark Allman and
Fernando Gont on providing feedback on the document.
Bashyam, et al. Expires May 14, 2011 [Page 8]
Internet-Draft TCP persist condition November 2010
7. Programming Considerations
As a potential implementation guideline, the authors are documenting
some of the programming considerations. This should not be in any
way construed as the only way that the mitigation against the DoS
condition can be achieved. Applications can choose their own
implementations on how to deal with this DoS sceanrio.
The key consideration in putting a solution together is to be able to
detect a connection that is in persist condition. The application
through the socket interface can inform TCP or kernel of how long
they are willing to wait in persist condition. When the connection
reaches that particular timeout value a EPERSISTTIMEOUT notification
will be sent to the application. The application on receiving the
notification can turn around and issue a close. In the case, the
application has terminated, TCP or kernel will go ahead and clear the
connection and reclaim the resoruces. Note, this persist condition
is mutually exclusive from a persist condition where we are not
getting zero windows acknowledgement for the probes.
PERSIST_TIMEOUT
Format:
int setsockopt (sockfd, SOL_TCP, SO_PERSISTTIMEO,
persist_timeout_value, length)
int getsockopt (sockfd, SOL_TCP, SO_PERSISTTIMEO,
persist_timeout_value, length)
where persist_timeout_value recorded in seconds is of type int and
the length is four.
The above interface allows applications to inform TCP that when the
local connection stays in persist condition it can be aborted after a
set time. Note that the default value of this option is infinite.
TCP sender will save the current time in the connection block when it
receives a zero window ACK. This time is referred to as the persist
entry time. Thereafter every time the probe timer expires and before
it sends another probe or an ACK carrying zero window is received a
check will be done to see how long the connection has been in persist
condition by comparing the current time to the persist entry time.
If the timeout has been exceeded, the connection will be aborted.
Any time a ACK is received that advertises a non-zero window, the
persist entry time is cleared to take the connection out of persist
condition.
Bashyam, et al. Expires May 14, 2011 [Page 9]
Internet-Draft TCP persist condition November 2010
8. Informative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1122] Braden, R., "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.
[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.
[TCPM] TCPM, "IETF TCPM Working Group and mailing list
http://www.ietf.org/html.charters/tcpm-charter.html".
[VU723308]
Manion, "Vulnerability in Web Servers
http://www.kb.cert.org/vuls/id/723308", July 2009.
Bashyam, et al. Expires May 14, 2011 [Page 10]
Internet-Draft TCP persist condition November 2010
Authors' Addresses
Murali Bashyam
Ocarina Networks, Inc
42 Airport parkway
San Jose, CA 95110
USA
Phone: +1 (408) 512-2966
Email: mbashyam@ocarinanetworks.com
Mahesh Jethanandani
Cisco Systems
170 Tasman Drive
San Jose, CA 95134
USA
Phone: +1 (408) 527-8230
Email: mahesh@cisco.com
Anantha Ramaiah
Cisco Systems
170 Tasman Drive
San Jose, CA 95134
USA
Phone: +1 (408) 525-6486
Email: ananth@cisco.com
Bashyam, et al. Expires May 14, 2011 [Page 11]