draft-ietf-tcpm-tcp-soft-errors-03.txt   draft-ietf-tcpm-tcp-soft-errors-04.txt 
TCP Maintenance and Minor F. Gont TCP Maintenance and Minor F. Gont
Extensions (tcpm) UTN/FRH Extensions (tcpm) UTN/FRH
Intended status: Informational Intended status: Informational
Expires: July 29, 2007 Expires: September 19, 2007
TCP's Reaction to Soft Errors TCP's Reaction to Soft Errors
draft-ietf-tcpm-tcp-soft-errors-03.txt draft-ietf-tcpm-tcp-soft-errors-04.txt
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Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
Abstract Abstract
This document discusses the problem of long delays between connection This document describes a non-standard, but widely implemented,
modification to TCP's handling of ICMP soft error messages that
rejects connections experiencing those errors immediately. This
behavior reduces the likelihood of long delays between connection
establishment attempts that may arise in a number of scenarios, establishment attempts that may arise in a number of scenarios,
including one in which dual stack nodes that have IPv6 enabled by including one in which dual stack nodes that have IPv6 enabled by
default are deployed in IPv4 or mixed IPv4 and IPv6 environments. default are deployed in IPv4 or mixed IPv4 and IPv6 environments.
Additionally, this document describes a non-standard, but widely
implemented modification to TCP's reaction to ICMP "soft errors" that
can help overcome this problem.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Error Handling in TCP . . . . . . . . . . . . . . . . . . . . 3 2. Error Handling in TCP . . . . . . . . . . . . . . . . . . . . 3
2.1. Reaction to ICMP error messages that indicate hard 2.1. Reaction to ICMP error messages that indicate hard
errors . . . . . . . . . . . . . . . . . . . . . . . . . . 4 errors . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Reaction to ICMP error messages that indicate soft 2.2. Reaction to ICMP error messages that indicate soft
errors . . . . . . . . . . . . . . . . . . . . . . . . . . 4 errors . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Problems that may arise from TCP's reaction to soft errors . . 5 3. Problems that may arise from TCP's reaction to soft errors . . 5
skipping to change at page 2, line 28 skipping to change at page 2, line 28
connection-establishment attempts . . . . . . . . . . . . . . 6 connection-establishment attempts . . . . . . . . . . . . . . 6
5. A more conservative approach . . . . . . . . . . . . . . . . . 7 5. A more conservative approach . . . . . . . . . . . . . . . . . 7
6. Possible drawbacks . . . . . . . . . . . . . . . . . . . . . . 8 6. Possible drawbacks . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Non-deterministic transient network failures . . . . . . . 8 6.1. Non-deterministic transient network failures . . . . . . . 8
6.2. Deterministic transient network failures . . . . . . . . . 8 6.2. Deterministic transient network failures . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 9 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . . 9 10.1. Normative References . . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . . 10 10.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Change log (to be removed before publication of Appendix A. Change log (to be removed before publication of
the document as an RFC) . . . . . . . . . . . . . . . 10 the document as an RFC) . . . . . . . . . . . . . . . 10
A.1. Changes from draft-ietf-tcpm-tcp-soft-errors-02 . . . . . 10 A.1. Changes from draft-ietf-tcpm-tcp-soft-errors-03 . . . . . 10
A.2. Changes from draft-ietf-tcpm-tcp-soft-errors-01 . . . . . 11 A.2. Changes from draft-ietf-tcpm-tcp-soft-errors-02 . . . . . 10
A.3. Changes from draft-ietf-tcpm-tcp-soft-errors-00 . . . . . 11 A.3. Changes from draft-ietf-tcpm-tcp-soft-errors-01 . . . . . 11
A.4. Changes from draft-gont-tcpm-tcp-soft-errors-02 . . . . . 11 A.4. Changes from draft-ietf-tcpm-tcp-soft-errors-00 . . . . . 11
A.5. Changes from draft-gont-tcpm-tcp-soft-errors-01 . . . . . 11 A.5. Changes from draft-gont-tcpm-tcp-soft-errors-02 . . . . . 11
A.6. Changes from draft-gont-tcpm-tcp-soft-errors-00 . . . . . 11 A.6. Changes from draft-gont-tcpm-tcp-soft-errors-01 . . . . . 11
A.7. Changes from draft-gont-tcpm-tcp-soft-errors-00 . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 13 Intellectual Property and Copyright Statements . . . . . . . . . . 13
1. Introduction 1. Introduction
The handling of network failures can be separated into two different The handling of network failures can be separated into two different
actions: fault isolation and fault recovery. Fault isolation actions: fault isolation and fault recovery. Fault isolation
consists of the actions that hosts and routers take to determine that consists of the actions that hosts and routers take to determine that
there is a network failure. Fault recovery, on the other hand, there is a network failure. Fault recovery, on the other hand,
consists of the actions that hosts and routers perform in an attempt consists of the actions that hosts and routers perform in an attempt
to survive a network failure.[RFC0816] to survive a network failure [RFC0816].
In the Internet architecture, the Internet Control Message Protocol In the Internet architecture, the Internet Control Message Protocol
(ICMP) [RFC0792] is one fault isolation technique to report network (ICMP) [RFC0792] is one fault isolation technique to report network
error conditions to the hosts sending datagrams over the network. error conditions to the hosts sending datagrams over the network.
When a host is signaled of a network error, there is still the issue When a host is notified of a network error its network stack will
of what to do to let communication survive, if possible, the network attempt to continue communications, if possible, in the presence of
failure. The fault recovery strategy may depend on the type of the network failure. The fault recovery strategy may depend on the
network failure taking place, and the time the error condition is type of network failure taking place, and the time the error
detected. condition is detected.
This document analyzes the fault recovery strategy of TCP [RFC0793], This document analyzes the fault recovery strategy of TCP [RFC0793],
and the problems that may arise due to TCP's reaction to ICMP soft and the problems that may arise due to TCP's reaction to ICMP soft
errors. Among others, it analyzes the problems that may arise in errors. Among others, it analyzes the problems that may arise in
scenarios where dual stack nodes that have IPv6 enabled by default scenarios where dual stack nodes that have IPv6 enabled by default
are deployed in IPv4 or mixed IPv4 and IPv6 environments. are deployed in IPv4 or mixed IPv4 and IPv6 environments.
Additionally, we document a modification to TCP's reaction to ICMP Additionally, we document a modification to TCP's reaction to ICMP
messages indicating "soft errors" during connection startup, that has messages indicating soft errors during connection startup, that has
been implemented in a variety of TCP/IP stacks to help overcome the been implemented in a variety of TCP/IP stacks to help overcome the
problems outlined below. We stress that this modification runs problems outlined below. We stress that this modification runs
contrary to the standard behavior and this document unambiguously contrary to the standard behavior and this document unambiguously
does not change the standard reaction. does not change the standard reaction.
2. Error Handling in TCP 2. Error Handling in TCP
Network errors can be divided into soft and hard errors. Soft errors Network errors can be divided into soft and hard errors. Soft errors
are considered to be transient network failures, which are likely to are considered to be transient network failures, which are likely to
be solved in the near term. Hard errors, on the other hand, are be solved in the near term. Hard errors, on the other hand, are
considered to reflect network error conditions which are unlikely to considered to reflect network error conditions that are unlikely to
be solved in the near future. be solved in the near future.
The "Requirements for Internet Hosts -- Communication Layers" RFC The Host Requirements RFC [RFC1122] states, in section 4.2.3.9., that
[RFC1122] states, in section 4.2.3.9., that the ICMP "Destination the ICMP "Destination Unreachable" messages that indicate soft errors
Unreachable" messages that indicate soft errors are ICMP codes 0 are ICMP codes 0 (network unreachable), 1 (host unreachable), and 5
(network unreachable), 1 (host unreachable), and 5 (source route (source route failed). Even though ICMPv6 didn't exist when
failed). Even though ICMPv6 didn't exist when [RFC1122] was written, [RFC1122] was written, one could extrapolate the concept of soft
one could extrapolate the concept of soft errors to ICMPv6 Type 1 errors to ICMPv6 Type 1 Codes 0 (no route to destination) and 3
Codes 0 (no route to destination) and 3 (address unreachable). (address unreachable).
When there is a network failure that's not signaled to the sending When there is a network failure that's not signaled to the sending
host, such as a gateway corrupting packets, TCP's fault recovery host, such as a gateway corrupting packets, TCP's fault recovery
action is to repeatedly retransmit the segment until either it gets action is to repeatedly retransmit the segment until either it gets
acknowledged, or the connection times out. acknowledged, or the connection times out.
In case a host does receive an ICMP error message referring to an In the case that a host does receive an ICMP error message referring
ongoing TCP connection, the IP layer will pass this message up to to an ongoing TCP connection, the IP layer will pass this message up
corresponding TCP instance to raise awareness of the network failure. to corresponding TCP instance to raise awareness of the network
[RFC1122] failure [RFC1122].
TCP's reaction to ICMP messages will depend on the type of error TCP's reaction to ICMP messages will depend on the type of error
being signaled. being signaled.
2.1. Reaction to ICMP error messages that indicate hard errors 2.1. Reaction to ICMP error messages that indicate hard errors
When receiving an ICMP error message that indicates a hard error When receiving an ICMP error message that indicates a hard error
condition, TCP will simply abort the corresponding connection, condition, TCP will simply abort the corresponding connection,
regardless of the state the connection is in. regardless of the connection state.
The "Requirements for Internet Hosts -- Communication Layers" RFC The Host Requirements RFC [RFC1122] states, in section 4.2.3.9, that
[RFC1122] states, in section 4.2.3.9, that TCP SHOULD abort TCP SHOULD abort connections when receiving ICMP error messages that
connections when receiving ICMP error messages that indicate hard indicate hard errors. This policy is based on the premise that, as
errors. This policy is based on the premise that, as hard errors hard errors indicate network error conditions that won't change in
indicate network error conditions that won't change in the near term, the near term, it will not be possible for TCP to usefully recover
it will not be possible for TCP to usefully recover from this type of from this type of network failure.
network failure.
2.2. Reaction to ICMP error messages that indicate soft errors 2.2. Reaction to ICMP error messages that indicate soft errors
If an ICMP error message is received that indicates a soft error, TCP If an ICMP error message is received that indicates a soft error, TCP
will repeatedly retransmit the packet until it either gets will repeatedly retransmit the packet until it either gets
acknowledged or the connection times out. In addition, the TCP may acknowledged or the connection times out. In addition, the TCP
record the information for possible later use [Stevens]. sender may record the information for possible later use [Stevens]
(pp. 317-319).
The "Requirements for Internet Hosts -- Communication Layers" RFC The Host Requirements RFC [RFC1122] states, in section 4.2.3.9, that
[RFC1122] states, in section 4.2.3.9, that TCP MUST NOT abort TCP MUST NOT abort connections when receiving ICMP error messages
connections when receiving ICMP error messages that indicate soft that indicate soft errors. This policy is based on the premise that,
errors. This policy is based on the premise that, as soft errors are as soft errors are transient network failures that will hopefully be
transient network failures that will hopefully be solved in the near solved in the near term, one of the retransmissions will succeed.
term, one of the retransmissions will succeed.
In case the connection timer expires, and an ICMP soft error message When the connection timer expires, and an ICMP soft error message has
has been received before the timeout, TCP can use this information to been received before the timeout, TCP can use this information to
provide the user with a more specific error message. [Stevens] provide the user with a more specific error message [Stevens] (pp.
317-319).
This reaction to soft errors exploits the valuable feature of the This reaction to soft errors exploits the valuable feature of the
Internet that for many network failures, the network can be Internet that for many network failures, the network can be
dynamically reconstructed without any disruption of the endpoints. dynamically reconstructed without any disruption of the endpoints.
3. Problems that may arise from TCP's reaction to soft errors 3. Problems that may arise from TCP's reaction to soft errors
3.1. General Discussion 3.1. General Discussion
Even though TCP's fault recovery strategy in the presence of soft Even though TCP's fault recovery strategy in the presence of soft
errors allows for TCP connections to survive transient network errors allows for TCP connections to survive transient network
failures, there are scenarios in which this policy may cause failures, there are scenarios in which this policy may cause
undesirable effects. undesirable effects.
For example, consider the case in which an application on a local For example, consider a scenario in which an application on a local
host is trying to communicate with a destination whose name resolves host is trying to communicate with a destination whose name resolves
to several IP addresses. The application on the local host will try to several IP addresses. The application on the local host will try
to establish a connection with the destination host, cycling through to establish a connection with the destination host, cycling through
the list of IP addresses, until one succeeds [RFC1123]. Suppose that the list of IP addresses, until one succeeds [RFC1123]. Suppose that
some (but not all) of the addresses in the returned list are some (but not all) of the addresses in the returned list are
permanently unreachable. If such a permanently unreachable address permanently unreachable. If such a permanently unreachable address
is the first in the list, the application will likely try to use the is the first in the list, the application will likely try to use the
permanently unreachable address first and block waiting for a timeout permanently unreachable address first and block waiting for a timeout
before trying alternate addresses. before trying alternate addresses.
As discussed in Section 2, this unreachability condition may or may As discussed in Section 2, this unreachability condition may or may
not be signaled to the sending host. If the local TCP is not not be signaled to the sending host. If the local TCP is not
signaled concerning the error condition, there is very little that signaled concerning the error condition, there is very little that
can be done other than repeatedly retransmit the SYN segment, and can be done other than repeatedly retransmit the SYN segment, and
wait for the existing timeout mechanism in TCP, or an application wait for the existing timeout mechanism in TCP, or an application
timeout, to be triggered. However, even if unreachability is timeout, to be triggered. However, even if unreachability is
signaled by some intermediate router to the local TCP by means of an signaled by some intermediate router to the local TCP by means of an
ICMP soft error message, the local TCP will still repeatedly ICMP soft error message, the local TCP will still repeatedly
retransmit the SYN segment until the connection timer expires (in the retransmit the SYN segment until the connection timer expires (in the
hopes that the error is transient). The "Requirements For Internet hopes that the error is transient). The Host Requirements RFC
Hosts -- Communication Layers" RFC [RFC1122] states that this timer [RFC1122] states that this timer MUST be large enough to provide
MUST be large enough to provide retransmission of the SYN segment for retransmission of the SYN segment for at least 3 minutes. This would
at least 3 minutes. This would mean that the application on the mean that the application on the local host would spend several
local host would spend several minutes for each unreachable address minutes for each unreachable address it uses for trying to establish
it uses for trying to establish a TCP connection. These long delays a TCP connection. These long delays between connection establishment
between connection establishment attempts would be inappropriate for attempts would be inappropriate for many interactive applications
many interactive applications such as the web. ([Shneiderman] and such as the web. ([Shneiderman] and [Thadani] offer some insight
[Thadani] offer some insight into the interactive systems.) This into the interactive systems.) This highlights that there is no one
highlights that there is no one definition of a "transient error" and definition of a "transient error" and that the level of persistence
that the level of persistence in the face of failure represents a in the face of failure represents a tradeoff.
tradeoff.
3.2. Problems that may arise with Dual Stack IPv6 on by Default 3.2. Problems that may arise with Dual Stack IPv6 on by Default
A particular scenario in which the above sketched type of problem may A particular scenario in which the above sketched type of problem may
occur regularly is that where dual stack nodes that have IPv6 enabled occur regularly is that where dual stack nodes that have IPv6 enabled
by default are deployed in IPv4 or mixed IPv4 and IPv6 environments, by default are deployed in IPv4 or mixed IPv4 and IPv6 environments,
and the IPv6 connectivity is non-existent and the IPv6 connectivity is non-existent
[I-D.ietf-v6ops-v6onbydefault]. [I-D.ietf-v6ops-v6onbydefault].
As discussed in [I-D.ietf-v6ops-v6onbydefault], there are two As discussed in [I-D.ietf-v6ops-v6onbydefault], there are two
possible variants of this scenario, which differ in whether the lack possible variants of this scenario, which differ in whether the lack
of connectivity is signaled to the sending node, or not. of connectivity is signaled to the sending node, or not.
In cases where packets sent to a destination are silently dropped and In those scenarios in which packets sent to a destination are
no ICMPv6 [RFC4443] errors are generated, there is little that can be silently dropped and no ICMPv6 [RFC4443] errors are generated, there
done other than waiting for the existing connection timeout mechanism is little that can be done other than waiting for the existing
in TCP, or an application timeout to be triggered. connection timeout mechanism in TCP, or an application timeout, to be
triggered.
In cases where a node has no default routers and Neighbor In scenarios where a node has no default routers and Neighbor
Unreachability Detection (NUD) fails for destinations assumed to be Unreachability Detection (NUD) fails for destinations assumed to be
on-link, or where firewalls or other systems that enforce scope on-link, or where firewalls or other systems that enforce scope
boundaries send ICMPv6 errors, the sending node will be signaled of boundaries send ICMPv6 errors, the sending node will be signaled of
the unreachability problem. However, as discussed in Section 2.2, the unreachability problem. However, as discussed in Section 2.2,
standard TCP implementations will not abort connections when standard TCP implementations will not abort connections when
receiving ICMP error messages that indicate soft errors. receiving ICMP error messages that indicate soft errors.
4. A workaround for long delays between connection-establishment 4. A workaround for long delays between connection-establishment
attempts attempts
As discussed in Section 1, it may make sense for the fault recovery 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 action to depend not only on the type of error being reported, but
also on the state of the connection against which the error is also on the state of the connection against which the error is
reported. For example, one could infer that when an error arrives in reported. For example, one could infer that when an error arrives in
response to opening a new connection, it is probably caused by response to opening a new connection, it is probably caused by
opening the connection improperly, rather than by a transient network opening the connection improperly, rather than by a transient network
failure. [RFC0816] failure [RFC0816].
A number of TCP implementations have modified their reaction to soft A number of TCP implementations have modified their reaction to soft
errors, to treat the errors as hard errors in the SYN-SENT or SYN- errors, to treat the errors as hard errors in the SYN-SENT or SYN-
RECEIVED states. It must be noted that this change violates section RECEIVED states. However, this change violates section 4.2.3.9 of
4.2.3.9 of [RFC1122], which states that these Unreachable messages [RFC1122], which states that these Unreachable messages indicate soft
indicate soft error conditions and TCP MUST NOT abort the error conditions and TCP MUST NOT abort the corresponding connection.
corresponding connection.
This workaround has been implemented, for example, in the Linux This workaround has been implemented, for example, in the Linux
kernel since version 2.0.0 (released in 1996) [Linux]. Section 5 kernel since version 2.0.0 (released in 1996) [Linux]. Section 5
discusses a more conservative approach than that sketched above, that discusses a more conservative approach than that sketched above that
is implemented in FreeBSD. is implemented in FreeBSD.
We note that the TCPM WG could not arrive at consensus on allowing We note that the TCPM WG could not arrive at consensus on allowing
the above described behavior as part of the standard. Therefore, the above described behavior as part of the standard. Therefore,
treating soft errors as hard errors during connection establishment, treating soft errors as hard errors during connection establishment,
while widespread, is not part of standard TCP behavior and this while widespread, is not part of standard TCP behavior and this
document does not change that state of affairs. document does not change that state of affairs.
5. A more conservative approach 5. A more conservative approach
skipping to change at page 7, line 31 skipping to change at page 7, line 31
Two additional state variables would need to be introduced to store Two additional state variables would need to be introduced to store
additional state information during the connection-establishment additional state information during the connection-establishment
phase: "nsynrexmit" and "nsofterror". Both would be initialized to phase: "nsynrexmit" and "nsofterror". Both would be initialized to
zero when a connection attempt is initiated, with "nsynrexmit" being zero when a connection attempt is initiated, with "nsynrexmit" being
incremented by one every time the SYN segment is retransmitted and incremented by one every time the SYN segment is retransmitted and
"nsofterror" being incremented by one every time an ICMP message that "nsofterror" being incremented by one every time an ICMP message that
indicates a soft error is received. indicates a soft error is received.
A connection in the SYN-SENT or SYN-RECEIVED states would be aborted A connection in the SYN-SENT or SYN-RECEIVED states would be aborted
if nsynrexmit was greater than MAXSYNREXMIT and "nsofterror" was if "nsynrexmit" was greater than MAXSYNREXMIT and "nsofterror" was
simultaneously greater than MAXSOFTERROR. simultaneously greater than MAXSOFTERROR.
This approach would give the network more time to solve the This approach would give the network more time to solve the
connectivity problem than simply aborting a connection attempt upon connectivity problem than simply aborting a connection attempt upon
reception of the first soft error. However, it should be noted that reception of the first soft error. However, it should be noted that
depending on the values chosen for the MAXSYNREXMIT and MAXSOFTERROR depending on the values chosen for the MAXSYNREXMIT and MAXSOFTERROR
parameters, this approach could still lead to long delays between parameters, this approach could still lead to long delays between
connection establishment attempts, thus not solving the problem. For connection establishment attempts, thus not solving the problem. For
example, BSD systems abort connections in the SYN-SENT or the SYN- example, BSD systems abort connections in the SYN-SENT or the SYN-
RECEIVED state when a second ICMP error is received, and 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 segment has been retransmitted more than three times. They also set
up a "connection-establishment timer" that imposes an upper limit on up a "connection-establishment timer" that imposes an upper limit on
the time the connection establishment attempt has to succeed, which the time the connection establishment attempt has to succeed, which
expires after 75 seconds [Stevens2]. Even when this policy may be expires after 75 seconds [Stevens2] (pp. 828-829). Even when this
better than the three-minutes timeout policy specified in [RFC1122], policy may be better than the three-minutes timeout policy specified
it may still be inappropriate for handling the potential problems in [RFC1122], it may still be inappropriate for handling the
described in this document. This more conservative approach has been potential problems described in this document. This more
implemented in BSD systems since, at least, 1994 [Stevens2]. conservative approach has been implemented in BSD systems since, at
least, 1994 [Stevens2].
We also note that the approach given in this section is a generalized We also note that the approach given in this section is a generalized
version of the approach sketched in the previous section. In version of the approach sketched in the previous section. In
particular, with MAXSOFTERROR set to 1 and MAXSYNREXMIT set to zero particular, with MAXSOFTERROR set to 1 and MAXSYNREXMIT set to zero
the schemes are identical. the schemes are identical.
6. Possible drawbacks 6. Possible drawbacks
The following subsections discuss some of the possible drawbacks The following subsections discuss some of the possible drawbacks
arising from the use of the non-standard modifications to TCP's arising from the use of the non-standard modifications to TCP's
reaction to soft errors described in Section 4 and Section 5. reaction to soft errors described in Section 4 and Section 5.
6.1. Non-deterministic transient network failures 6.1. Non-deterministic transient network failures
In case a transient network failure affects all of the addresses In scenarios where a transient network failure affects all of the
returned by the name-to-address translation function, all addresses returned by the name-to-address translation function, all
destinations could be unreachable for some short period of time. In destinations could be unreachable for some short period of time. In
such a scenario, the application could quickly cycle through all the 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 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 TCP retry a destination a few seconds later, when the transient
problem could have disappeared. problem could have disappeared.
6.2. Deterministic transient network failures 6.2. Deterministic transient network failures
There are some scenarios in which transient network failures could be There are some scenarios in which transient network failures could be
deterministic. For example, consider the case in which upstream deterministic. For example, consider a scenario in which upstream
network connectivity is triggered by network use. That is, network network connectivity is triggered by network use. That is, network
connectivity is instantiated only on an "as needed" basis. In this connectivity is instantiated only on an "as needed" basis. In this
scenario, the connection triggering the upstream connectivity would scenario, the connection triggering the upstream connectivity would
deterministically receive ICMP Destination Unreachables while the deterministically receive ICMP Destination Unreachables while the
upstream connectivity is being activated, and thus would be aborted. upstream connectivity is being activated, and thus would be aborted.
7. Security Considerations 7. Security Considerations
This document describes a non-standard modification to TCP's reaction This document describes a non-standard modification to TCP's reaction
to soft errors that has been implemented in a variety of TCP to soft errors that has been implemented in a variety of TCP
implementations. This modification makes TCP abort a connection in implementations. This modification makes TCP abort a connection in
the SYN-SENT or the SYN-RECEIVED states when it receives an ICMP the SYN-SENT or the SYN-RECEIVED states when it receives an ICMP
"Destination Unreachable" message that indicates a "soft error". "Destination Unreachable" message that indicates a soft error.
Therefore, the modification could be exploited to reset valid Therefore, the modification could be exploited to reset valid
connections during the connection-establishment phase. connections during the connection-establishment phase.
The non-standard workaround described in this document makes TCP more The non-standard workaround described in this document makes TCP more
vulnerable to attack---even if only slightly. However, we note that vulnerable to attack---even if only slightly. However, we note that
an attacker wishing to reset ongoing TCP connections could send any an attacker wishing to reset ongoing TCP connections could send any
of the ICMP hard error messages in any connection state. of the ICMP hard error messages in any connection state.
A discussion of the use of ICMP to perform a variety of attacks A discussion of the use of ICMP to perform a variety of attacks
against TCP, and a number of counter-measures that minimize the against TCP, and a number of counter-measures that minimize the
impact of these attacks can be found in [I-D.ietf-tcpm-icmp-attacks]. impact of these attacks can be found in [I-D.ietf-tcpm-icmp-attacks].
A discussion of the security issues arising from the use of ICMPv6 A discussion of the security issues arising from the use of ICMPv6
can be found in [RFC4443]. can be found in [RFC4443].
8. Acknowledgements 8. Acknowledgements
The author wishes to thank Mark Allman, Ron Bonica, Sally Floyd, The author wishes to thank Mark Allman, Ron Bonica, Ted Faber, Gorry
Guillermo Gont, Michael Kerrisk, Eddie Kohler, Mika Liljeberg, Pasi Fairhurst, Sally Floyd, Guillermo Gont, Michael Kerrisk, Eddie
Sarolahti, Pekka Savola, and Joe Touch, for contributing many Kohler, Mika Liljeberg, Pasi Sarolahti, Pekka Savola, and Joe Touch,
valuable comments on earlier versions of this document. for contributing many valuable comments on earlier versions of this
document.
9. Contributors 9. Contributors
Mika Liljeberg was the first to describe how their implementation Mika Liljeberg was the first to describe how their implementation
treated soft errors. Based on that, the solution discussed in treated soft errors. Based on that, the solution discussed in
Section 4 was documented in [I-D.ietf-v6ops-v6onbydefault] by Section 4 was documented in [I-D.ietf-v6ops-v6onbydefault] by
Sebastien Roy, Alain Durand and James Paugh. Sebastien Roy, Alain Durand and James Paugh.
10. References 10. References
skipping to change at page 9, line 39 skipping to change at page 9, line 41
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, [RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981. RFC 793, September 1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts - [RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989. Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application [RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989. and Support", STD 3, RFC 1123, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006. Version 6 (IPv6) Specification", RFC 4443, March 2006.
10.2. Informative References 10.2. Informative References
[Guynes] Guynes, J., "Impact of System Response Time on State [Guynes] Guynes, J., "Impact of System Response Time on State
Anxiety", Communications of the ACM , 1988. Anxiety", Communications of the ACM , 1988.
[I-D.ietf-tcpm-icmp-attacks] [I-D.ietf-tcpm-icmp-attacks]
skipping to change at page 10, line 29 skipping to change at page 10, line 24
[Linux] The Linux Project, "http://www.kernel.org". [Linux] The Linux Project, "http://www.kernel.org".
[RFC0816] Clark, D., "Fault isolation and recovery", RFC 816, [RFC0816] Clark, D., "Fault isolation and recovery", RFC 816,
July 1982. July 1982.
[Shneiderman] [Shneiderman]
Shneiderman, B., "Response Time and Display Rate in Human Shneiderman, B., "Response Time and Display Rate in Human
Performance with Computers", ACM Computing Surveys , 1984. Performance with Computers", ACM Computing Surveys , 1984.
[Stevens] "TCP/IP Illustrated, Volume 1: The Protocols", Addison- [Stevens] Stevens, W., "TCP/IP Illustrated, Volume 1: The
Wesley , 1994. Protocols", Addison-Wesley , 1994.
[Stevens2] [Stevens2]
Wright, G. and W. Stevens, "TCP/IP Illustrated, Volume 2: Wright, G. and W. Stevens, "TCP/IP Illustrated, Volume 2:
The Implementation", Addison-Wesley , 1994. The Implementation", Addison-Wesley , 1994.
[Thadani] Thadani, A., "Interactive User Productivity", IBM Systems [Thadani] Thadani, A., "Interactive User Productivity", IBM Systems
Journal No. 1, 1981. Journal No. 1, 1981.
Appendix A. Change log (to be removed before publication of the Appendix A. Change log (to be removed before publication of the
document as an RFC) document as an RFC)
A.1. Changes from draft-ietf-tcpm-tcp-soft-errors-02 A.1. Changes from draft-ietf-tcpm-tcp-soft-errors-03
o Addresses feedback sent by Ted Faber and Gorry Fairhurst
(miscellaneous editorial changes).
A.2. Changes from draft-ietf-tcpm-tcp-soft-errors-02
o Moved appendix on FreeBSD's approach to the body of the draft. o Moved appendix on FreeBSD's approach to the body of the draft.
o Removed rest of the appendix, as suggested by Ron Bonica and Mark o Removed rest of the appendix, as suggested by Ron Bonica and Mark
Allman. Allman.
o Reworded some parts of the document to make the text more neutral. o Reworded some parts of the document to make the text more neutral.
o Miscellaneous editorial changes. o Miscellaneous editorial changes.
A.2. Changes from draft-ietf-tcpm-tcp-soft-errors-01 A.3. Changes from draft-ietf-tcpm-tcp-soft-errors-01
o Addressed feedback posted by Sally Floyd (remove sentence in o Addressed feedback posted by Sally Floyd (remove sentence in
Section 2.1 regarding processing of RST segments) Section 2.1 regarding processing of RST segments)
A.3. Changes from draft-ietf-tcpm-tcp-soft-errors-00 A.4. Changes from draft-ietf-tcpm-tcp-soft-errors-00
o Miscellaneous editorial changes o Miscellaneous editorial changes
A.4. Changes from draft-gont-tcpm-tcp-soft-errors-02 A.5. Changes from draft-gont-tcpm-tcp-soft-errors-02
o Draft resubmitted as draft-ietf. o Draft resubmitted as draft-ietf.
o Miscellaneous editorial changes o Miscellaneous editorial changes
A.5. Changes from draft-gont-tcpm-tcp-soft-errors-01 A.6. Changes from draft-gont-tcpm-tcp-soft-errors-01
o Changed wording to describe the mechanism, rather than proposing o Changed wording to describe the mechanism, rather than proposing
it it
o Miscellaneous editorial changes o Miscellaneous editorial changes
A.6. Changes from draft-gont-tcpm-tcp-soft-errors-00 A.7. Changes from draft-gont-tcpm-tcp-soft-errors-00
o Added reference to the Linux implementation in Section 4 o Added reference to the Linux implementation in Section 4
o Added Section 6 o Added Section 6
o Added section on Higher-Level API o Added section on Higher-Level API
o Added Section 5 o Added Section 5
o Moved section "Asynchronous Application Notification" to Appendix o Moved section "Asynchronous Application Notification" to Appendix
 End of changes. 39 change blocks. 
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