draft-ietf-tcpm-tcp-soft-errors-05.txt   draft-ietf-tcpm-tcp-soft-errors-06.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: October 4, 2007 Expires: December 26, 2007
TCP's Reaction to Soft Errors TCP's Reaction to Soft Errors
draft-ietf-tcpm-tcp-soft-errors-05.txt draft-ietf-tcpm-tcp-soft-errors-06.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 describes a non-standard, but widely implemented, This document describes a non-standard, but widely implemented,
modification to TCP's handling of ICMP soft error messages received modification to TCP's handling of ICMP soft error messages, that
in any of the non-synchronized states, that rejects connections rejects pending connection-requests when those error messages are
experiencing those errors immediately. This behavior reduces the received. This behavior reduces the likelihood of long delays
likelihood of long delays between connection establishment attempts between connection establishment attempts that may arise in a number
that may arise in a number of scenarios, including one in which dual of scenarios, including one in which dual stack nodes that have IPv6
stack nodes that have IPv6 enabled by default are deployed in IPv4 or enabled by default are deployed in IPv4 or mixed IPv4 and IPv6
mixed IPv4 and IPv6 environments. environments.
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 . . . . . . . . . . . . . . . . . . . . . . . . . . 5
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
3.1. General Discussion . . . . . . . . . . . . . . . . . . . . 5 3.1. General Discussion . . . . . . . . . . . . . . . . . . . . 5
3.2. Problems that may arise with Dual Stack IPv6 on by 3.2. Problems that may arise with Dual Stack IPv6 on by
Default . . . . . . . . . . . . . . . . . . . . . . . . . 6 Default . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. A workaround for long delays between 4. Deployed workarounds for long delays between
connection-establishment attempts . . . . . . . . . . . . . . 6 connection-establishment attempts . . . . . . . . . . . . . . 7
5. A more conservative approach . . . . . . . . . . . . . . . . . 7 4.1. Context-sensitive ICMP/TCP interaction . . . . . . . . . . 7
6. Possible drawbacks . . . . . . . . . . . . . . . . . . . . . . 8 4.2. Context-sensitive ICMP/TCP interaction with repeated
6.1. Non-deterministic transient network failures . . . . . . . 8 confirmation . . . . . . . . . . . . . . . . . . . . . . . 7
6.2. Deterministic transient network failures . . . . . . . . . 8 5. Possible drawbacks of changing ICMP semantics . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 5.1. Non-deterministic transient network failures . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 5.2. Deterministic transient network failures . . . . . . . . . 9
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.3. Non-compliant Network Address Translators (NATs) . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . . 9 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . . 10 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . . 11
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) . . . . . . . . . . . . . . . 12
A.1. Changes from draft-ietf-tcpm-tcp-soft-errors-04 . . . . . 10 A.1. Changes from draft-ietf-tcpm-tcp-soft-errors-05 . . . . . 12
A.2. Changes from draft-ietf-tcpm-tcp-soft-errors-03 . . . . . 11 A.2. Changes from draft-ietf-tcpm-tcp-soft-errors-04 . . . . . 12
A.3. Changes from draft-ietf-tcpm-tcp-soft-errors-02 . . . . . 11 A.3. Changes from draft-ietf-tcpm-tcp-soft-errors-03 . . . . . 12
A.4. Changes from draft-ietf-tcpm-tcp-soft-errors-01 . . . . . 11 A.4. Changes from draft-ietf-tcpm-tcp-soft-errors-02 . . . . . 13
A.5. Changes from draft-ietf-tcpm-tcp-soft-errors-00 . . . . . 11 A.5. Changes from draft-ietf-tcpm-tcp-soft-errors-01 . . . . . 13
A.6. Changes from draft-gont-tcpm-tcp-soft-errors-02 . . . . . 11 A.6. Changes from draft-ietf-tcpm-tcp-soft-errors-00 . . . . . 13
A.7. Changes from draft-gont-tcpm-tcp-soft-errors-01 . . . . . 11 A.7. Changes from draft-gont-tcpm-tcp-soft-errors-02 . . . . . 13
A.8. Changes from draft-gont-tcpm-tcp-soft-errors-00 . . . . . 11 A.8. Changes from draft-gont-tcpm-tcp-soft-errors-01 . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12 A.9. Changes from draft-gont-tcpm-tcp-soft-errors-00 . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 13 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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 notified of a network error its network stack will When a host is notified of a network error, its network stack will
attempt to continue communications, if possible, in the presence of attempt to continue communications, if possible, in the presence of
the network failure. The fault recovery strategy may depend on the the network failure. The fault recovery strategy may depend on the
type of network failure taking place, and the time the error type of network failure taking place, and the time the error
condition is 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. It analyzes the problems that may arise when a host tries to
scenarios where dual stack nodes that have IPv6 enabled by default establish a TCP connection with a multihomed host for which some of
are deployed in IPv4 or mixed IPv4 and IPv6 environments. its addresses are unreachable. Additionally, it analyzes the
problems that may arise in the specific scenario where dual stack
nodes that have IPv6 enabled by default are deployed in IPv4 or mixed
IPv4 and IPv6 environments.
Additionally, we document a modification to TCP's reaction to ICMP Finally, 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.
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 [RFC2119].
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 that 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 Host Requirements RFC [RFC1122] states, in Section 4.2.3.9., that The Host Requirements RFC [RFC1122] states, in Section 4.2.3.9., that
the ICMP messages that indicate soft errors are ICMP "Destination the ICMP messages that indicate soft errors are ICMP "Destination
Unreachable" codes 0 (network unreachable), 1 (host unreachable), and Unreachable" codes 0 (network unreachable), 1 (host unreachable), and
5 (source route failed), ICMP "Time Exceeded" codes 0 (time to live 5 (source route failed), ICMP "Time Exceeded" codes 0 (time to live
exceeded in transit) and 1 (fragment reassembly time exceeded), and exceeded in transit) and 1 (fragment reassembly time exceeded), and
ICMP "Parameter Problem". Even though ICMPv6 didn't exist when ICMP "Parameter Problem". Even though ICMPv6 did not exist when
[RFC1122] was written, one could extrapolate the concept of soft [RFC1122] was written, one could extrapolate the concept of soft
errors to ICMPv6 "Destination Unreachable" codes 0 (no route to errors to ICMPv6 "Destination Unreachable" codes 0 (no route to
destination) and 3 (address unreachable), ICMPv6 "Time Exceeded" destination) and 3 (address unreachable), ICMPv6 "Time Exceeded"
codes 0 (Hop limit exceeded in transit) and 1 (Fragment reassembly codes 0 (Hop limit exceeded in transit) and 1 (Fragment reassembly
time exceeded), and ICMPv6 "Parameter Problem" codes 0 (Erroneous time exceeded), and ICMPv6 "Parameter Problem" codes 0 (Erroneous
header field encountered), 1 (Unrecognized Next Header type header field encountered), 1 (Unrecognized Next Header type
encountered) and 2 (Unrecognized IPv6 option encountered). encountered) and 2 (Unrecognized IPv6 option encountered) [RFC4443].
When there is a network failure that's not signaled to the sending +----------------------------------+--------------------------------+
| ICMP | ICMPv6 |
+----------------------------------+--------------------------------+
| Destination Unreachable (codes | Destination Unreachable (codes |
| 0, 1, and 5) | 0 and 3) |
+----------------------------------+--------------------------------+
| Time Exceeded (codes 0 and 1) | Time exceeded (codes 0 and 1) |
+----------------------------------+--------------------------------+
| Parameter Problem | Parameter Problem (codes 0, 1, |
| | and 2) |
+----------------------------------+--------------------------------+
Table 1: Extrapolating the concept of soft errors to ICMPv6
When there is a network failure that is 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 the case that a host does receive an ICMP error message referring In the case that a host does receive an ICMP error message referring
to an ongoing TCP connection, the IP layer will pass this message up to an ongoing TCP connection, the IP layer will pass this message up
to corresponding TCP instance to raise awareness of the network to the corresponding TCP instance to raise awareness of the network
failure [RFC1122]. failure [RFC1122]. TCP's reaction to ICMP messages will depend on
the type of error being signaled.
TCP's reaction to ICMP messages will depend on the type of error
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 connection state. regardless of the connection state.
The Host Requirements RFC [RFC1122] states, in Section 4.2.3.9, that The Host Requirements RFC [RFC1122] states, in Section 4.2.3.9, that
TCP SHOULD abort connections when receiving ICMP error messages that TCP SHOULD abort connections when receiving ICMP error messages that
indicate hard errors. This policy is based on the premise that, as indicate hard errors. This policy is based on the premise that, as
hard errors indicate network error conditions that won't change in hard errors indicate network error conditions that will not change in
the near term, it will not be possible for TCP to usefully recover the near term, it will not be possible for TCP to usefully recover
from this type of network failure. from this type of 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 segment until it either gets will repeatedly retransmit the segment until it either gets
acknowledged or the connection times out. In addition, the TCP acknowledged or the connection times out. In addition, the TCP
sender may record the information for possible later use [Stevens] sender may record the information for possible later use [Stevens]
(pp. 317-319). (pp. 317-319).
skipping to change at page 5, line 24 skipping to change at page 5, line 43
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 a scenario 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, usually cycling
the list of IP addresses, until one succeeds [RFC1123]. Suppose that through the list of IP addresses, until one succeeds [RFC1123].
some (but not all) of the addresses in the returned list are Suppose that some (but not all) of the addresses in the returned list
permanently unreachable. If such a permanently unreachable address are permanently unreachable. If such a permanently unreachable
is the first in the list, the application will likely try to use the address is the first in the list, the application will likely try to
permanently unreachable address first and block waiting for a timeout use the permanently unreachable address first and block waiting for a
before trying alternate addresses. timeout before trying an alternate address.
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 Host Requirements RFC hopes that the error is transient). The Host Requirements RFC
[RFC1122] states that this timer MUST be large enough to provide [RFC1122] states that this timer MUST be large enough to provide
retransmission of the SYN segment for at least 3 minutes. This would retransmission of the SYN segment for at least 3 minutes. This would
mean that the application on the local host would spend several mean that the application on the local host would spend several
minutes for each unreachable address it uses for trying to establish minutes for each unreachable address it uses for trying to establish
a TCP connection. These long delays between connection establishment the TCP connection. These long delays between connection
attempts would be inappropriate for many interactive applications establishment attempts would be inappropriate for many interactive
such as the web. ([Shneiderman] and [Thadani] offer some insight applications such as the web ([Shneiderman] and [Thadani] offer some
into the interactive systems). This highlights that there is no one insight into interactive systems). This highlights that there is no
definition of a "transient error" and that the level of persistence one definition of a "transient error" and that the level of
in the face of failure represents a tradeoff. persistence in the face of failure represents a tradeoff.
It is worth noting that while most applications try the addresses
returned by the name-to-address function in serial, this is certainly
not the only possible approach. For example, applications could try
multiple addresses in parallel until one succeeds, possibly avoiding
the problem of long delays between connection establishment attempts
described in this document.
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 those scenarios in which packets sent to a destination are In those scenarios in which packets sent to a destination are
silently dropped and no ICMPv6 [RFC4443] errors are generated, there silently dropped and no ICMPv6 [RFC4443] errors are generated, there
is little that can be done other than waiting for the existing is little that can be done other than waiting for the existing
connection timeout mechanism in TCP, or an application timeout, to be connection timeout mechanism in TCP, or an application timeout, to be
triggered. triggered.
In scenarios 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) [RFC2461] fails for destinations
on-link, or where firewalls or other systems that enforce scope assumed to be on-link, or where firewalls or other systems that
boundaries send ICMPv6 errors, the sending node will be signaled of enforce scope boundaries send ICMPv6 errors, the sending node will be
the unreachability problem. However, as discussed in Section 2.2, signaled of the unreachability problem. However, as discussed in
standard TCP implementations will not abort connections when Section 2.2, standard TCP implementations will not abort connections
receiving ICMP error messages that indicate soft errors. when receiving ICMP error messages that indicate soft errors.
4. A workaround for long delays between connection-establishment 4. Deployed workarounds for long delays between connection-
attempts establishment attempts
The following subsections describe a number of workarounds for the
problem of long delays between connection-establishment attempts that
have been implemented in a variety of TCP/IP stacks. We note that
treating soft errors as hard errors during connection establishment,
while widespread, is not part of standard TCP behavior and this
document does not change that state of affairs. The TCPM WG
consensus was to document this widespread implementation of
nonstandard TCP behavior, but to not change the TCP standard.
4.1. Context-sensitive ICMP/TCP interaction
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. However, this change violates section 4.2.3.9 of RECEIVED states. However, this change violates section 4.2.3.9 of
[RFC1122], which states that these Unreachable messages indicate soft [RFC1122], which states that these Unreachable messages indicate soft
error conditions and TCP MUST NOT abort the corresponding connection. error conditions and TCP MUST NOT abort the 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 4.2
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 4.2. Context-sensitive ICMP/TCP interaction with repeated confirmation
the above described behavior as part of the standard. Therefore,
treating soft errors as hard errors during connection establishment,
while widespread, is not part of standard TCP behavior and this
document does not change that state of affairs.
5. A more conservative approach
A more conservative approach than simply treating soft errors as hard A more conservative approach than simply treating soft errors as hard
errors as described above would be to abort a connection in the SYN- errors as described above would be to abort a connection in the SYN-
SENT or SYN-RECEIVED states only after an ICMP Destination SENT or SYN-RECEIVED states only after an ICMP Destination
Unreachable has been received a specified number of times, and the Unreachable has been received a specified number of times, and the
SYN segment has been retransmitted more than some specified number of SYN segment has been retransmitted more than some specified number of
times. times.
Two new parameters would have to be introduced to TCP, to be used Two new parameters would have to be introduced to TCP, to be used
only during the connection-establishment phase: MAXSYNREXMIT and only during the connection-establishment phase: MAXSYNREXMIT and
skipping to change at page 8, line 10 skipping to change at page 8, line 40
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] (pp. 828-829). Even when this expires after 75 seconds [Stevens2] (pp. 828-829). Even when this
policy may be better than the three-minutes timeout policy specified policy may be better than the three-minutes timeout policy specified
in [RFC1122], it may still be inappropriate for handling the in [RFC1122], it may still be inappropriate for handling the
potential problems described in this document. This more potential problems described in this document. This more
conservative approach has been implemented in BSD systems since, at conservative approach has been implemented in BSD systems for more
least, 1994 [Stevens2]. than ten years [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 5. Possible drawbacks of changing ICMP semantics
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 4.1 and Section 4.2.
6.1. Non-deterministic transient network failures 5.1. Non-deterministic transient network failures
In scenarios where a transient network failure affects all of the In scenarios where a transient network failure affects all of the
addresses 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. For
such a scenario, the application could quickly cycle through all the example, a mobile system consisting of a cell and a repeater may pass
IP addresses in the list and return an error, when it could have let through a tunnel, leading to a loss of connectivity at the repeater,
TCP retry a destination a few seconds later, when the transient with the repeater sending ICMP soft errors back to the cell. In such
problem could have disappeared. scenarios, 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. In this case, the modifications described
here make TCP less robust than a standards-compliant implementation.
6.2. Deterministic transient network failures Additionally, in many cases a domain name maps to a single IP
address. In such a case, it might be better to try that address
persistently according to normal TCP rules, instead of just aborting
the pending connection upon receipt of an ICMP soft error.
5.2. 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 a scenario 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 could scenario, the connection triggering the upstream connectivity could
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.
Again, in this case, the modifications described here make TCP less
robust than a standards-compliant implementation.
7. Security Considerations 5.3. Non-compliant Network Address Translators (NATs)
Some NATs respond to an unsolicited inbound SYN segment with an ICMP
soft error message. If the system sending the unsolicited SYN
segment implements the workaround described in this document, it will
abort the connection upon receipt of the ICMP error message, thus
probably preventing TCP's simultaneous open through the NAT from
succeeding. However, it must be stressed that those NATs described
in this section are not BEHAVE-compliant, and therefore should
implement REQ-4 of [I-D.ietf-behave-tcp] instead.
6. 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 7. Acknowledgements
The author wishes to thank Mark Allman, Ron Bonica, Ted Faber, Gorry The author wishes to thank Mark Allman, Ron Bonica, Ted Faber, Gorry
Fairhurst, Sally Floyd, Guillermo Gont, Michael Kerrisk, Eddie Fairhurst, Sally Floyd, Tomohiro Fujisaki, Guillermo Gont, Saikat
Kohler, Mika Liljeberg, Carlos Pignataro, Pasi Sarolahti, Pekka Guha, Alfred Hoenes, Michael Kerrisk, Eddie Kohler, Mika Liljeberg,
Savola, and Joe Touch, for contributing many valuable comments on Arifumi Matsumoto, Carlos Pignataro, Pasi Sarolahti, Pekka Savola,
earlier versions of this document. Pyda Srisuresh, and Joe Touch, for contributing many valuable
comments on earlier versions of this document.
9. Contributors The author wishes to express deep and heartfelt gratitude to Jorge
Oscar Gont and Nelida Garcia, for their precious motivation and
guidance.
8. 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 4.1 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 9. References
10.1. Normative References 9.1. Normative References
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981. RFC 792, September 1981.
[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.
[RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461,
December 1998.
[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 9.2. 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-behave-tcp]
Guha, S., "NAT Behavioral Requirements for TCP",
draft-ietf-behave-tcp-07 (work in progress), April 2007.
[I-D.ietf-tcpm-icmp-attacks] [I-D.ietf-tcpm-icmp-attacks]
Gont, F., "ICMP attacks against TCP", Gont, F., "ICMP attacks against TCP",
draft-ietf-tcpm-icmp-attacks-01 (work in progress), draft-ietf-tcpm-icmp-attacks-02 (work in progress),
October 2006. May 2007.
[I-D.ietf-v6ops-v6onbydefault] [I-D.ietf-v6ops-v6onbydefault]
Roy, S., Durand, A., and J. Paugh, "Issues with Dual Stack Roy, S., Durand, A., and J. Paugh, "Issues with Dual Stack
IPv6 on by Default", draft-ietf-v6ops-v6onbydefault-03 IPv6 on by Default", draft-ietf-v6ops-v6onbydefault-03
(work in progress), July 2004. (work in progress), July 2004.
[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.
skipping to change at page 10, line 47 skipping to change at page 12, line 18
[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-04 A.1. Changes from draft-ietf-tcpm-tcp-soft-errors-05
o Miscellaneous edits, clarifications, and reorganization of both
workarounds into a single top-level section, as suggested by Pasi
Sarolahti.
o Added note on non-compliant NATs, as suggested by Ted Faber and
Saikat Guha
o Miscellaneous edits suggested by Gorry Fairhurst
o Added a table to clarify how to extrapolate the concept of ICMPv4
"soft errors" to ICMPv6 (as suggested by Arifumi Matsumoto and
Gorry Fairhurst).
o Miscellaneous edits, clarification on alternative approach by
sending connection requests in parallel, example of mobile system
(for non-deterministic errors), and note on the possible impact of
the workarounds on TCP's robusteness (as suggested by Joe Touch)
A.2. Changes from draft-ietf-tcpm-tcp-soft-errors-04
o Addresses feedback sent by Carlos Pignataro (adds missing error o Addresses feedback sent by Carlos Pignataro (adds missing error
codes in Section 2, and fixes a number of typos/writeos). codes in Section 2, and fixes a number of typos/writeos).
A.2. Changes from draft-ietf-tcpm-tcp-soft-errors-03 A.3. Changes from draft-ietf-tcpm-tcp-soft-errors-03
o Addresses feedback sent by Ted Faber and Gorry Fairhurst o Addresses feedback sent by Ted Faber and Gorry Fairhurst
(miscellaneous editorial changes). (miscellaneous editorial changes).
A.3. Changes from draft-ietf-tcpm-tcp-soft-errors-02 A.4. 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.4. Changes from draft-ietf-tcpm-tcp-soft-errors-01 A.5. 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.5. Changes from draft-ietf-tcpm-tcp-soft-errors-00 A.6. Changes from draft-ietf-tcpm-tcp-soft-errors-00
o Miscellaneous editorial changes o Miscellaneous editorial changes
A.6. Changes from draft-gont-tcpm-tcp-soft-errors-02 A.7. 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.7. Changes from draft-gont-tcpm-tcp-soft-errors-01 A.8. 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.8. Changes from draft-gont-tcpm-tcp-soft-errors-00 A.9. 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 4.1
o Added Section 5
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 4.2
o Moved section "Asynchronous Application Notification" to Appendix o Moved section "Asynchronous Application Notification" to Appendix
o Added section on parallel connection requests o Added section on parallel connection requests
o Miscellaneous editorial changes o Miscellaneous editorial changes
Author's Address Author's Address
Fernando Gont Fernando Gont
Universidad Tecnologica Nacional / Facultad Regional Haedo Universidad Tecnologica Nacional / Facultad Regional Haedo
Evaristo Carriego 2644 Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706 Haedo, Provincia de Buenos Aires 1706
Argentina Argentina
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