Request for Comments: 5461
TCP's Reaction to Soft Errors
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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 environments.
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 220.127.116.11, 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 18.104.22.168, 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 [ICMP-ATTACKS].
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 22.214.171.124, 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], pp. 317-319).
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 undesirable effects.
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 alternate address.
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 triggered.
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 failure [RFC0816].
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 126.96.36.199 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 bits set.
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 [Zuquete].
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 connection-establishment phase.
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 guidance.
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, October 1989. [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, March 2006. [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, July 1982. [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, July 2004.
Fernando Gont Universidad Tecnologica Nacional / Facultad Regional Haedo Evaristo Carriego 2644 Haedo, Provincia de Buenos Aires 1706 Argentina Phone: +54 11 4650 8472 EMail: firstname.lastname@example.org URI: http://www.gont.com.ar