Internet DRAFT - draft-gont-tcpm-tcp-soft-errors
draft-gont-tcpm-tcp-soft-errors
TCP Maintenance and Minor F. Gont
Extensions (tcpm) UTN/FRH
Internet-Draft September 12, 2005
Expires: March 16, 2006
TCP's Reaction to Soft Errors
draft-gont-tcpm-tcp-soft-errors-02.txt
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Copyright (C) The Internet Society (2005).
Abstract
This document discusses the problem of long delays between connection
establishment attempts that may arise in a number of scenarios,
including that in which dual stack nodes that have IPv6 enabled by
default are deployed in IPv4 or mixed IPv4 and IPv6 environments.
Additionaly, it describes a modification to TCP's reaction to soft
errors that has been implemented in a variety of TCP/IP stacks to
help overcome this problem.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Error Handling in TCP . . . . . . . . . . . . . . . . . . . . 3
2.1. Reaction to Hard Errors . . . . . . . . . . . . . . . . . 4
2.2. Reaction to Soft Errors . . . . . . . . . . . . . . . . . 4
3. Problems that may arise from TCP's reaction to soft errors . . 5
3.1. General Discussion . . . . . . . . . . . . . . . . . . . . 5
3.2. Problems that may arise with Dual Stack IPv6 on by
Default . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. A workaround for long delays between
connection-establishment attempts . . . . . . . . . . . . . . 6
5. Possible drawbacks . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Non-deterministic transient network failures . . . . . . . 7
5.2. Deterministic transient network failures . . . . . . . . . 7
6. Future work . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Other possible solutions . . . . . . . . . . . . . . 9
A.1. A more conservative approach . . . . . . . . . . . . . . . 10
A.2. Asynchronous Application Notification . . . . . . . . . . 10
A.3. Issuing several connection requests in parallel . . . . . 11
Appendix B. Changes from draft-gont-tcpm-tcp-soft-errors-01 . . . 11
Appendix C. Changes from draft-gont-tcpm-tcp-soft-errors-00 . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
The handling of network failures can be separated into two different
actions: fault isolation and fault recovery. Fault isolation is the
actions that hosts and routers take to determine that there is some
network failure. Fault recovery, on the other hand, is the actions
that hosts and routers will perform to isolate and survive a network
failure.[8]
In the Internet architecture, the Internet Control Message Protocol
(ICMP) [1] is used to perform the fault isolation function, that is,
to report network error conditions to the hosts sending datagrams
over the network.
When a host is signalled of a network error, there is still the issue
of what to do to let communication survive, if possible, the network
failure. The fault recovery strategy may depend on the type of
network failure taking place, and the time the error condition is
detected.
This document analyzes the fault recovery policy of TCP [2], and the
problems that may arise due to TCP's policy of reaction to soft
errors. Among others, it analyzes the problems that may arise in
scenarios where dual stack nodes that have IPv6 enabled by default
are deployed in IPv4 or mixed IPv4 and IPv6 environments.
Additionaly, it documents a modification to TCP's policy of reaction
to ICMP messages indicating "soft errors", that has been implemented
in a variety of TCP/IP stacks to help overcome the problems discussed
in this document.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [3].
2. Error Handling in TCP
Network errors can be divided into soft and hard errors. Soft errors
are considered to be transient network failures, which will hopefully
be solved in the near term. Hard errors, on the other hand, are
considered to reflect permanent network error conditions, which are
unlikely to be solved in the near future.
Therefore, it may make sense for the fault recovery action to be
different depending on the type of error being detected.
When there is a network failure that's not signalled to the sending
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host, such as a gateway corrupting packets, TCP's fault recovery
action is to repeatedly retransmit the segment until either it gets
acknowledged, or the connection times out. In case the connection
times out before the segment is acknowledged, TCP won't be able to
provide more information than the timeout condition.
In case a host does receive an ICMP error message meant for an
ongoing TCP connection, the IP layer will pass this message up to
correspoding TCP instance to raise awareness of the network failure.
[4]
TCP's reaction to ICMP messages will depend on the type of error
being signalled.
2.1. Reaction to Hard Errors
When receiving a segment with the RST bit set, or an ICMP error
message indicating a hard error condition, TCP will simply abort the
corresponding connection, regardless of the state the connection is
in.
The "Requirements for Internet Hosts -- Communication Layers" RFC [4]
states, in section 4.2.3.9, that TCP SHOULD abort connections when
receiving ICMP error messages that indicate hard errors. This policy
is based on the premise that, as hard errors indicate network error
conditions that won't change in the near term, it will not be
possible for TCP to recover from this type of network failure.
2.2. Reaction to Soft Errors
If an ICMP error message is received that indicates a soft error, TCP
will just record this information [9], and repeatedly retransmit the
data until either they get acknowledged or the connection times out.
The "Requirements for Internet Hosts -- Communication Layers" RFC [4]
states, in section 4.2.3.9, that TCP MUST NOT abort connections when
receiving ICMP error messages that indicate soft errors. This policy
is based on the premise that, as soft errors are transient network
failures that will hopefully be solved in the near term, one of the
retransmissions will succeed.
In case the connection timer expires, and an ICMP error message had
been received before the timeout, TCP will use this information to
provide the user with a more specific error message. [9]
This handling of soft errors exploits the valuable feature of the
Internet that for many network failures, the network can be
dynamically reconstructed without any disruption of the endpoints.
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3. Problems that may arise from TCP's reaction to soft errors
3.1. General Discussion
Even though TCP's fault recovery strategy in the presence of soft
errors allows for TCP connections to survive transient network
failures, there are scenarios in which this policy may cause
undesirable effects.
For example, consider the case in which an application on a local
host is trying to communicate with a destination whose name resolves
to several IP addresses. The application on the local host will try
to establish a connection with the destination host, cycling through
the list of IP addresses, until one succeeds [5]. Suppose that some
(but not all) of the addresses in the returned list are permanently
unreachable. If they are the first IP addresses in the list, the
application will usually try to use these addresses first.
As discussed in Section 2, this unreachability condition may or may
not be signalled to the sending host. If the local TCP is not
signalled of the error condition, it will repeatedly retransmit the
SYN segment, until the connection times out. If unreachability is
signalled by some intermediate router to the local TCP by means of an
ICMP error message, the local TCP will just record the error message
and will still repeatedly retransmit the SYN segment until the
connection timer expires. The "Requirements For Internet Hosts --
Communication Layers" RFC [4] states that this timer MUST be large
enough to provide retransmission of the SYN segment for at least 3
minutes. This would mean that the application on the local host
would spend several minutes for each unreachable address it tries to
use for a connection attempt. These long delays between connection
establishment attempts would be inappropriate for interactive
applications such as the web. [10] [11]
3.2. Problems that may arise with Dual Stack IPv6 on by Default
Another scenario in which this type of problem may occur is that
where dual stack nodes that have IPv6 enabled by default are deployed
in IPv4 or mixed IPv4 and IPv6 environments, and the IPv6
connectivity is non-existent [6].
As discussed in [6], there are two possible variants of this
scenario, which differ in whether the lack of connectivity is
signalled to the sending node, or not.
In cases where packets sent to a destination are silently dropped and
no ICMPv6 [7] errors are generated, there is very little that can be
done other than waiting for the existing connection timeout mechanism
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in TCP, or an aplication timeout, to be triggered.
In cases where a node has no default routers and Neighbor
Unreachability Detection (NUD) fails for destinations assumed to be
on-link, or where firewalls or other systems that enforce scope
boundaries send ICMPv6 errors, the sending node will be signalled of
the unreachability problem. However, as discussed in Section 2.2,
TCP implementations will not abort connections when receiving ICMP
error messages that indicate soft errors.
4. A workaround for long delays between connection-establishment
attempts
As discussed in Section 1, it may make sense for the fault recovery
action to depend not only on the type of error being reported, but
also on the time the error is reported. For example, one could infer
that when an error arrives in response to opening a new connection,
it is probably caused by opening the connection improperly, rather
than by a transient network failure. [8]
A variety of TCP/IP stacks have modified TCP's reaction to soft
errors, to make it abort a connection in the SYN-SENT or the SYN-
RECEIVED state if it receives an ICMP "Destination Unreachable"
message that indicates a soft error about that connection.
The "Requirements for Internet Hosts -- Communication Layers" RFC [4]
states, in section 4.2.3.9., that the ICMP "Destination Unreachable"
messages that indicate soft errors are ICMP codes 0 (network
unreachable), 1 (host unreachable), and 5 (source route failed).
Even though ICMPv6 didn't exist when [4] was written, one could
extrapolate the concept of soft errors to ICMPv6 Type 1 Codes 0 (no
route to destination) and 3 (address unreachable).
It must be noted that this behaviour violates section 4.2.3.9 of [4],
since it states that as these Unreachable messages indicate soft
error conditions, TCP MUST NOT abort the corresponding connection.
This workaround has been implemented, for example, in the Linux
kernel since version 2.0.0 (released in 1996) [12]. Appendix A.1
discusses a more conservative approach than the one introduced in
this section.
5. Possible drawbacks
The following subsections discuss some of the possible drawbacks
arising from the use of the modification to TCP's reaction to soft
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errors described in Section 4.
5.1. Non-deterministic transient network failures
In case there's a transient network failure affecting all of the
addresses returned by the name-to-address translation function, all
destinations could be unreachable for some short period of time. In
such a scenario, the application could quickly cycle through all the
IP addresses in the list and return an error, when it could have let
TCP retry a destination a few seconds later, when the transient
problem could have disappeared.
However, it must be noted that non-interactive applications, such as
a Mail Transfer Agent (MTA), usually must implement application-layer
retry mechanisms, and thus are able to handle these scenarios
appropriately. For interactive applications, the user would likely
not be satisfied with a connection attempt that succeeds only after
several seconds, anyway. [13]
5.2. Deterministic transient network failures
There are some scenarios in which transient network failures could be
deterministic. For example, consider the case in which upstream
network connectivity is triggered by network use. In this scenario,
the connection triggering the upstream connectivity would
deterministically receive ICMP Destination Unreachables while the
upstream connectivity is being activated, and thus would be aborted.
As discussed in Section 5.1, applications usually implement mechanims
to handle these scenarios appropriately. Also, connection attempts
are usually preceded by a UDP-based DNS name-to-address lookup.
Thus, unless the name-to-address mapping has been cached by a local
nameserver or resolver, it will be the DNS query that will trigger
the upstream network connectivity, and thus the corresponding
connection will not be aborted.
6. Future work
A Higher-Level API would be useful for isolating applications from
protocol details. The API could contain the intelligence required to
resolve the hostname, try each destination address, etc. One could
even argue that this document wouldn't have existed if application
programmers had been using a Higher-Level API. However, such an API
would need to be designed, standardized, implemented, deployed, and
documented even before application programmers start (if ever) to use
it.
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7. Security Considerations
This document describes a modification to TCP's reaction to soft
errors that has been implemented in a variety of TCP/IP stacks. This
modification makes TCP abort a connection in the SYN-SENT or the SYN-
RECEIVED states when it receives an ICMP "Destination Unreachable"
message that indicates a "soft error" about that connection. While
this modification could be exploited to reset valid connections, it
must be noted that this behaviour is meant only for connections in
the SYN-SENT or the SYN-RECEIVED states, and thus the window of
exposure is very short.
In any case, it must be noted that the workaround discussed in this
document neither strengthens nor weakens TCP's resistance to attack.
An attacker wishing to reset ongoing TCP connections could perform
the attack by sending any of the ICMP error messages that indicate
"hard errors", not only for connections in the SYN-SENT or the SYN-
RECEIVED states, but for connections in any state.
A discussion of the use of ICMP to perform a variety of attacks
against TCP, and a number of proposed counter-measures that eliminate
or greatly minimize the impact of these attacks can be found in [14].
A discussion of the security issues arising from the use of ICMPv6
can be found in [7].
8. Acknowledgements
The author wishes to thank Michael Kerrisk, Eddie Kohler, Mika
Liljeberg, Pasi Sarolahti, Pekka Savola, and Joe Touch, for
contributing many valuable comments.
9. Contributors
Mika Liljeberg was the first to describe how their implementation
treated soft errors. Based on that, the solution discussed in
Section 4 was documented in [6] by Sebastien Roy, Alain Durand and
James Paugh.
10. References
10.1. Normative References
[1] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
September 1981.
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[2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989.
[5] Braden, R., "Requirements for Internet Hosts - Application and
Support", STD 3, RFC 1123, October 1989.
[6] Roy, S., Durand, A., and J. Paugh, "Issues with Dual Stack IPv6
on by Default", draft-ietf-v6ops-v6onbydefault-03 (work in
progress), July 2004.
[7] Conta, A. and S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC 2463, December 1998.
10.2. Informative References
[8] Clark, D., "Fault isolation and recovery", RFC 816, July 1982.
[9] "TCP/IP Illustrated, Volume 1: The Protocols", Addison-Wesley ,
1994.
[10] Shneiderman, B., "Response Time and Display Rate in Human
Performance with Computers", ACM Computing Surveys , 1984.
[11] Thadani, A., "Interactive User Productivity", IBM Systems
Journal No. 1, 1981.
[12] The Linux Project, "http://www.kernel.org".
[13] Guynes, J., "Impact of System Response Time on State Anxiety",
Communications of the ACM , 1988.
[14] Gont, F., "ICMP attacks against TCP",
draft-gont-tcpm-icmp-attacks-04 (work in progress),
September 2005.
[15] Wright, G. and W. Stevens, "TCP/IP Illustrated, Volume 2: The
Implementation", Addison-Wesley , 1994.
Appendix A. Other possible solutions
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A.1. A more conservative approach
A more conservative approach would be to abort a connection in the
SYN-SENT or SYN-RECEIVED states only after a ICMP Destination
Unreacheable has been received a specified number of times, and the
SYN segment has been retransmitted more than some specified number of
times.
Two new parameters would have to be introduced to TCP, to be used
only during the connection-establishment phase: MAXSYNREXMIT and
MAXSOFTERROR. MAXSYNREXMIT would specify the number of times the SYN
segment would have to be retransmitted before a connection is
aborted. MAXSOFTERROR would specify the number of ICMP messages
indicating soft errors that would have to be received before a
connection is aborted.
Two additional variables would need to be introduced to store
additional state information during the connection-establishment
phase: "nsynrexmit" and "nsofterror". Both would be initialized to
zero. "nsynrexmit" would be incremented by one every time the SYN
segment is retransmitted. "nsofterror" would be incremented by one
every time an ICMP message that indicates a soft error is received.
A connection in the SYN-SENT or SYN-RECEIVED states would be aborted
if nsynrexmit was greater than MAXSYNREXMIT and "nsofterror" was
simultaneously greater than MAXSOFTERROR.
This approach would give the network more time to solve the
connectivity problem. However, it should be noted that depending on
the values chosen for the MAXSYNREXMIT and MAXSOFTERROR parameters,
this approach could still lead to long delays in connection
establishment attempts. For example, BSD systems abort connections
in the SYN-SENT or the SYN-RECEIVED state when a second ICMP error is
received, and the SYN segment has been retransmitted more than three
times. They also set up a "connection-establishment timer" that
imposes an upper limit on the time the connection establishment
attempt has to succeed, which expires after 75 seconds [15]. Even
when this policy may be better than the three-minutes timeout policy
specified in [4], it may still be inappropriate for handling the
potential problems described in this document. This more
conservative approach has been implemented in BSD systems since, at
least, 1994 [15].
A.2. Asynchronous Application Notification
In section 4.2.4.1, [4] states that there MUST be a mechanism for
reporting soft TCP error conditions to the application. Such a
mechanism (assuming one is implemented) could be used by applications
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to cycle through the destination IP addresses. However, this
approach would increase application complexity, and would take a long
time to kick in, as would require every existing applications to be
modified.
A.3. Issuing several connection requests in parallel
For those scenarios in which a domain name maps to several IP
addresses, several connection requests could be issued in parallel,
each one to a different destination IP address. The host would then
use the first connection attempt to succeed, eliminating the
potential delay in establishing a connection with the destination
host. However, this would mean that every attempt to connect to a
multihomed host would imply sending several SYN segments, making it
hard for network operators to distinguish valid connection attempts
from those performing Denial of Service (DoS) attacks.
An alternative approach would be as follows. A host would issue a
connection request to the first IP address in the list returned by
the name-to-address mapping function. If this connection request
doesn't succeed in some time, a connection request to the second IP
address in the list would be issued in parallel. If none of these
connection requests succeeds in some time, and there are still more
addresses left in the list, they would be tried in the same way.
While this approach would, in principle, avoid the problems of the
previous approach, it might be hard to define the time interval to
wait before issuing each parallel connection request. A short time
interval would lead to the problems caused by the previous approach,
while a long time interval would likely still lead to long delays in
establishing a connection with the destination host.
In any case, it must be noted that both approachs have the same
drawbacks as the solution described in Appendix A.2: they would
increase application complexity, and would take too long to begin to
be used by applications.
Appendix B. Changes from draft-gont-tcpm-tcp-soft-errors-01
o Changed wording to describe the mechanism, rather than proposing
it
o Miscellaneous editorial changes
Appendix C. Changes from draft-gont-tcpm-tcp-soft-errors-00
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o Added reference to the Linux implementation in Section 4
o Added Section 5
o Added Section 6
o Added Appendix A.1
o Moved section "Asynchronous Application Notification" to
Appendix A.2
o Added a Appendix A.3
o Miscellaneous editorial changes
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Author's Address
Fernando Gont
Universidad Tecnologica Nacional
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
Email: fernando@gont.com.ar
URI: http://www.gont.com.ar
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