Network Working Group F. Gont
Request for Comments: 5461 UTN/FRH
Category: Informational February 2009
TCP's Reaction to Soft Errors
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (http://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
This document describes a non-standard, but widely implemented,
modification to TCP's handling of ICMP soft error messages that
rejects pending connection-requests when those error messages are
received. This behavior reduces the likelihood of long delays
between connection-establishment attempts that may arise in a number
of scenarios, including one in which dual-stack nodes that have IPv6
enabled by default are deployed in IPv4 or mixed IPv4 and IPv6
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Error Handling in TCP . . . . . . . . . . . . . . . . . . . . 3
2.1. Reaction to ICMP Error Messages That Indicate Hard
Errors . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Reaction to ICMP Error Messages That Indicate Soft
Errors . . . . . . . . . . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Deployed Workarounds for Long Delays between
Connection-Establishment Attempts . . . . . . . . . . . . . . 7
4.1. Context-Sensitive ICMP/TCP Interaction . . . . . . . . . . 7
4.2. Context-Sensitive ICMP/TCP Interaction with Repeated
Confirmation . . . . . . . . . . . . . . . . . . . . . . . 8
5. Possible Drawbacks of Changing ICMP Semantics . . . . . . . . 9
5.1. Non-Deterministic Transient Network Failures . . . . . . . 9
5.2. Deterministic Transient Network Failures . . . . . . . . . 10
5.3. Non-Compliant Network Address Translators (NATs) . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . . 12
The handling of network failures can be separated into two different
actions: fault isolation and fault recovery. Fault isolation
consists of the actions that hosts and routers take to determine that
there is a network failure. Fault recovery, on the other hand,
consists of the actions that hosts and routers perform in an attempt
to survive a network failure [RFC0816].
In the Internet architecture, the Internet Control Message Protocol
(ICMP) [RFC0792] is one fault isolation technique to report network
error conditions to the hosts sending datagrams over the network.
When a host is notified of a network error, its network stack will
attempt to continue communications, if possible, in the presence of
the network failure. The fault recovery strategy may depend on the
type of network failure taking place and the time at which the error
condition is detected.
This document analyzes the problems that may arise due to TCP's fault
recovery reactions to ICMP soft errors. It analyzes the problems
that may arise when a host tries to establish a TCP connection with a
multihomed host that has some unreachable addresses. 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.
Finally, we document a modification to TCP's reaction to ICMP
messages indicating soft errors during connection startup that has
been implemented in a variety of TCP/IP stacks to help overcome the
problems outlined below. We stress that this modification runs
contrary to the standard behavior and this document unambiguously
does not change the standard reaction.
[Gont] describes alternative approaches for dealing with the problem
of long delays between connection-establishment attempts in TCP.
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
Network errors can be divided into soft and hard errors. Soft errors
are considered to be transient network failures that are likely to be
solved in the near term. Hard errors, on the other hand, are
considered to reflect network error conditions that are unlikely to
be solved in the near future.
The Host Requirements RFC [RFC1122] states, in Section 184.108.40.206, that
the ICMP messages that indicate soft errors are ICMP "Destination
Unreachable" codes 0 (network unreachable), 1 (host unreachable), and
5 (source route failed); ICMP "Time Exceeded" codes 0 (time to live
exceeded in transit) and 1 (fragment reassembly time exceeded); and
ICMP "Parameter Problem". Even though ICMPv6 did not exist when
[RFC1122] was written, one could extrapolate the concept of soft
errors to ICMPv6 "Destination Unreachable" codes 0 (no route to
destination) and 3 (address unreachable); ICMPv6 "Time Exceeded"
codes 0 (hop limit exceeded in transit) and 1 (fragment reassembly
time exceeded); and ICMPv6 "Parameter Problem" codes 0 (erroneous
header field encountered), 1 (unrecognized Next Header type
encountered), and 2 (unrecognized IPv6 option encountered) [RFC4443].
| 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
action is to repeatedly retransmit the corresponding data until
either they get acknowledged or the connection times out.
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 the corresponding TCP instance to raise awareness of the network
failure [RFC1122]. 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
When receiving an ICMP error message that indicates a hard error
condition, compliant TCP implementations will simply abort the
corresponding connection, regardless of the connection state.
The Host Requirements RFC [RFC1122] states, in Section 220.127.116.11, 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 will not change in
the near term, it will not be possible for TCP to usefully recover
from this type of network failure.
It should be noted that virtually none of the current TCP
implementations follow the advice in [RFC1122], and they do not abort
the corresponding connection when an ICMP hard error is received for
a connection that is in any of the synchronized states
2.2. Reaction to ICMP Error Messages That Indicate Soft Errors
If an ICMP error message is received that indicates a soft error, TCP
will repeatedly retransmit the corresponding data until either they
get acknowledged or the connection times out. In addition, the TCP
sender may record the information for possible later use (see
[Stevens], pp. 317-319).
The Host Requirements RFC [RFC1122] states, in Section 18.104.22.168, 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.
When the connection timer expires and an ICMP soft error message has
been received before the timeout, TCP can use this information to
provide the user with a more specific error message (see [Stevens],
This reaction to soft errors exploits a valuable feature of the
Internet -- that, for many network failures, the network can be
dynamically reconstructed without any disruption of the endpoints.
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
For example, consider a scenario 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, usually cycling
through the list of IP addresses until one succeeds [RFC1123].
Suppose that some (but not all) of the addresses in the returned list
are permanently unreachable. If such a permanently unreachable
address is the first in the list, the application will likely try to
use it first and block waiting for a timeout before trying an
As discussed in Section 2, this unreachability condition may or may
not be signaled to the sending host. If the local TCP is not
signaled concerning the error condition, there is very little that
can be done other than to repeatedly retransmit the SYN segment and
wait for the existing timeout mechanism in TCP, or an application
timeout, to be triggered. However, even if unreachability is
signaled by some intermediate router to the local TCP by means of an
ICMP soft error message, the local TCP will still repeatedly
retransmit the SYN segment until the connection timer expires (in the
hopes that the error is transient). The Host Requirements RFC
[RFC1122] 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 with which it tries to establish
the TCP connection. These long delays between connection-
establishment attempts would be inappropriate for many interactive
applications, such as the Web. [Shneiderman] and [Thadani] offer some
insight into interactive systems (e.g., how the response time affects
the usability of an application). This highlights that there is no
one definition of a "transient error" and that the level of
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 [Gont].
3.2. Problems That May Arise with Dual-Stack IPv6 on by Default
A particular scenario in which the above type of problem may occur
regularly 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 [RFC4943].
As discussed in [RFC4943], there are two possible variants of this
scenario, which differ in whether or not the lack of connectivity is
signaled to the sending node.
In those scenarios in which packets sent to a destination are
silently dropped and no ICMPv6 [RFC4443] errors are generated, there
is little that can be done other than to wait for the existing
connection-timeout mechanism in TCP, or an application timeout, to be
In scenarios where a legacy node has no default routers and Neighbor
Unreachability Detection (NUD) [RFC4861] 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
signaled of the unreachability problem. However, as discussed in
Section 2.2, compliant TCP implementations will not abort connections
when receiving ICMP error messages that indicate soft errors.
4. Deployed Workarounds for Long Delays between Connection-
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 consensus of the
TCPM WG (TCP Maintenance and Minor Extensions Working Group) 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
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
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
A number of TCP implementations have modified their reaction to all
ICMP soft errors and treat them as hard errors when they are received
for connections in the SYN-SENT or SYN-RECEIVED states. For example,
this workaround has been implemented in the Linux kernel since
version 2.0.0 (released in 1996) [Linux]. However, it should be
noted that this change violates section 22.214.171.124 of [RFC1122], which
states that these ICMP error messages indicate soft error conditions
and that, therefore, TCP MUST NOT abort the corresponding connection.
[RFC3168] states that a host that receives a RST in response to the
transmission of an ECN (Explicit Congestion Notification)-setup SYN
packet MAY resend a SYN with the CWR (Congestion Window Reduced) and
ECE (ECN-Echo) bits cleared. This is meant to deal with faulty
middle-boxes that reject connections when a SYN segment has the ECE
and CWR bits set. Some faulty middle-boxes (e.g., firewalls) may
reject these connection requests with an ICMP soft error of type 3
(Destination Unreachable), code 0 (net unreachable) or 1 (host
unreachable), instead of a RST. Therefore, a system that processes
ICMP soft error messages as hard errors when they are received for a
connection in any of the non-synchronized states could resend the SYN
segment with the ECE and CWR bits cleared when an ICMP "net
unreachable" (type 3, code 0) or "host unreachable" (type 3, code 1)
error message is received in response to a SYN segment that had these
Section 4.2 discusses a more conservative approach than that sketched
above, which is implemented in FreeBSD.
4.2. Context-Sensitive ICMP/TCP Interaction with Repeated Confirmation
A more conservative approach than simply treating soft errors as hard
errors (as described above) would be to abort a connection in the
SYN-SENT or SYN-RECEIVED states only after an ICMP soft error 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 state 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 when a connection attempt is initiated, with "nsynrexmit" being
incremented by one every time the SYN segment is retransmitted and
"nsofterror" being 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 than does simply aborting a connection attempt
upon reception of the first soft error. 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 between connection-establishment attempts, thus not solving
the problem. 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 (see [Stevens2], pp. 828-
829). Even when this policy may be better than the three-minute
timeout policy specified in [RFC1122], it may still be inappropriate
for handling the potential problems described in this document. This
more conservative approach has been implemented in BSD systems for
more than ten years [Stevens2].
We also note that the approach given in this section is a generalized
version of the approach sketched in the previous section. In
particular, with MAXSOFTERROR set to 1 and MAXSYNREXMIT set to zero,
the schemes are identical.
5. Possible Drawbacks of Changing ICMP Semantics
The following subsections discuss some possible drawbacks that could
arise from use of the non-standard modifications to TCP's reaction to
soft errors, which are described in Section 4.1 and Section 4.2.
5.1. Non-Deterministic Transient Network Failures
In scenarios where a transient network failure affects all of the
addresses returned by the name-to-address translation function, all
destinations could be unreachable for some short period of time. For
example, a mobile system consisting of a cell and a repeater may pass
through a tunnel, leading to a loss of connectivity at the repeater,
with the repeater sending ICMP soft errors back to the cell. Also, a
transient routing problem might lead some intervening router to drop
a SYN segment that was meaning to establish a TCP connection and send
an ICMP soft error back to the host. Finally, a SYN segment carrying
data might get fragmented and some of the resulting fragments might
get lost, with the destination host timing out the reassembly process
and sending an ICMP soft error back to the sending host (although
this particular scenario is unlikely because, while [RFC0793] allows
SYN segments to carry data, in practice they do not). In such
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.
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
deterministic. For example, consider a scenario in which upstream
network connectivity is triggered by network use. That is, network
connectivity is instantiated only on an "as needed" basis. In this
scenario, the connection triggering the upstream connectivity could
deterministically receive ICMP Destination Unreachables while the
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.
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 from succeeding through
the NAT. However, it must be stressed that those NATs described in
this section are not BEHAVE-compliant and therefore should implement
REQ-4 of [RFC5382] instead.
In those scenarios in which such a non-BEHAVE-compliant NAT is
deployed, TCP simultaneous opens could fail. While undesirable, this
is tolerable in many situations. For instance, a number of host
implementations of TCP do not support TCP simultaneous opens
6. Security Considerations
This document describes a non-standard modification to TCP's reaction
to soft errors that has been implemented in a variety of TCP
implementations. This modification makes TCP abort a connection in
the SYN-SENT or the SYN-RECEIVED states when it receives an ICMP
error message that indicates a soft error. Therefore, the
modification could be exploited to reset valid connections during the
The non-standard workaround described in this document makes TCP more
vulnerable to attack, even if only slightly. However, we note that
an attacker wishing to reset ongoing TCP connections could send any
of the ICMP hard error messages in any connection state.
Generally, TCP backs off its retransmission timer each time it
retransmits the SYN segment for the same connection. If a TCP
implements the modification described in this document, that is,
tries the next address in the list upon receipt of an ICMP error
message, it might end up injecting more packets into the network than
if it had simply retried the same address a number of times.
However, compliant TCP implementations might already incur this
behavior (e.g., as a result of cycling through the list of IP
addresses in response to RST segments) as there are currently no
recommendations on methods for limiting the rate at which SYN
segments are sent for connecting to a specific destination.
A discussion of the use of ICMP to perform a variety of attacks
against TCP, and a number of counter-measures that minimize the
impact of these attacks, can be found in [ICMP-ATTACKS].
A discussion of the security issues arising from the use of ICMPv6
can be found in [RFC4443].
The author wishes to thank Mark Allman, Jari Arkko, David Black, Ron
Bonica, Ted Faber, Gorry Fairhurst, Sally Floyd, Juan Fraschini,
Tomohiro Fujisaki, Guillermo Gont, Saikat Guha, Alfred Hoenes,
Michael Kerrisk, Eddie Kohler, Mika Liljeberg, Arifumi Matsumoto,
Sandy Murphy, Carlos Pignataro, Pasi Sarolahti, Pekka Savola, Pyda
Srisuresh, Jinmei Tatuya, and Joe Touch for contributing many
valuable comments on earlier versions of this document.
The author wishes to thank Secretaria de Extension Universitaria at
Universidad Tecnologica Nacional and Universidad Tecnologica
Nacional/Facultad Regional Haedo for their support in this work.
Finally, the author wishes to express deep and heartfelt gratitude to
Jorge Oscar Gont and Nelida Garcia for their precious motivation and
Mika Liljeberg was the first to describe how their implementation
treated soft errors. Based on that, the solution discussed in
Section 4.1 was documented in [v6-ON] by Sebastien Roy, Alain Durand,
and James Paugh.
9.1. Normative References
[RFC0792] Postel, J., "Internet Control Message Protocol",
STD 5, RFC 792, September 1981.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1123] Braden, R., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The
Addition of Explicit Congestion Notification (ECN) to
IP", RFC 3168, September 2001.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", RFC 4443,
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H.
Soliman, "Neighbor Discovery for IP version 6
(IPv6)", RFC 4861, September 2007.
9.2. Informative References
[Gont] Gont, F., "On the problem of long delays between
connection-establishment attempts in TCP", Work
in Progress, January 2009.
[ICMP-ATTACKS] Gont, F., "ICMP attacks against TCP", Work
in Progress, October 2008.
[Linux] The Linux Project, "http://www.kernel.org".
[RFC0816] Clark, D., "Fault isolation and recovery", RFC 816,
[RFC4943] Roy, S., Durand, A., and J. Paugh, "IPv6 Neighbor
Discovery On-Link Assumption Considered Harmful",
RFC 4943, September 2007.
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP",
BCP 142, RFC 5382, October 2008.
[Shneiderman] Shneiderman, B., "Response Time and Display Rate in
Human Performance with Computers", ACM
Computing Surveys, 1984.
[Stevens] Stevens, W., "TCP/IP Illustrated, Volume 1: The
Protocols", Addison-Wesley, 1994.
[Stevens2] Wright, G. and W. Stevens, "TCP/IP Illustrated,
Volume 2: The Implementation", Addison-Wesley, 1994.
[Thadani] Thadani, A., "Interactive User Productivity", IBM
Systems Journal, No. 1, 1981.
[Zuquete] Zuquete, A., "Improving the functionality of SYN
cookies", 6th IFIP Communications and Multimedia
Security Conference (CMS 2002), 2002.
[v6-ON] Roy, S., Durand, A., and J. Paugh, "Issues with Dual
Stack IPv6 on by Default", Work in Progress,
Universidad Tecnologica Nacional / Facultad Regional Haedo
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Phone: +54 11 4650 8472