| Internet Engineering Task Force | G. Fairhurst |
| Internet-Draft | University of Aberdeen |
| Intended status: Standards Track | M. Westerlund |
| Expires: June 14, 2013 | Ericsson |
| December 11, 2012 |
Applicability Statement for the use of IPv6 UDP Datagrams with Zero Checksums
draft-ietf-6man-udpzero-08
This document provides an applicability statement for the use of UDP transport checksums with IPv6. It defines recommendations and requirements for the use of IPv6 UDP datagrams with a zero UDP checksum. It describes the issues and design principles that need to be considered when UDP is used with IPv6 to support tunnel encapsulations and examines the role of the IPv6 UDP transport checksum. An appendix presents a summary of the trade-offs that were considered in evaluating the safety of the update to RFC 2460 that updates use of the UDP checksum with IPv6.
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Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.
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The User Datagram Protocol (UDP) [RFC0768] transport is defined for the Internet Protocol (IPv4) [RFC0791] and is defined in Internet Protocol, Version 6 (IPv6) [RFC2460] for IPv6 hosts and routers. The UDP transport protocol has a minimal set of features. This limited set has enabled a wide range of applications to use UDP, but these application do need to provide many important transport functions on top of UDP. The UDP Usage Guidelines [RFC5405] provides overall guidance for application designers, including the use of UDP to support tunneling. The key difference between UDP usage with IPv4 and IPv6 is that RFC 2460 mandates use of a calculated UDP checksum, i.e. a non-zero value, due to the lack of an IPv6 header checksum.
The lack of a possibility to use an IPv6 datagram with a zero UDP checksum has been observed as a real problem for certain classes of application, primarily tunnel applications. This class of application has been deployed with a zero UDP checksum using IPv4. The design of IPv6 raises different issues when considering the safety of using a UDP checksum with IPv6. These issues can significantly affect applications, both when an endpoint is the intended user and when an innocent bystander (i.e. a packet received by a different endpoint to that intended).
This document examines the issues and an appendix compares the strengths and weaknesses of a number of proposed solutions. This identifies a set of issues that must be considered and mitigated to be able to safely deploy IPv6 applications that use a zero UDP checksum. The provided comparison of methods is expected to also be useful when considering applications that have different goals from the ones that initiated the writing of this document, especially the use of already standardized methods. The analysis concludes that using a zero UDP checksum is the best method of several proposed alternatives to meet the goals for certain tunnel applications.
This document defines recommendations and requirements for use of IPv6 datagrams with a zero UDP checksum. This usage is expected to have initial deployment issues related to middleboxes, limiting the usability more than desired in the currently deployed internet. However, this limitation will be largest initially and will reduce as updates are provided in middleboxes that support the zero UDP checksum for IPv6. The document therefore derives a set of constraints required to ensure safe deployment of a zero UDP checksum.
Finally, the document also identifies some issues that require future consideration and possibly additional research.
Section 1 provides a background to key issues, and introduces the use of UDP as a tunnel transport protocol.
Section 2 describes a set of standards-track datagram transport protocols that may be used to support tunnels.
Section 3 discusses issues with a zero UDP checksum for IPv6. It considers the impact of corruption, the need for validation of the path and when it is suitable to use a zero UDP checksum.
Section 4 is an applicability statement that defines requirements and recommendations on the implementation of IPv6 nodes that support the use of a zero UDP checksum.
Section 5 provides an applicability statement that defines requirements and recommendations for protocols and tunnel encapsulations that are transported over an IPv6 transport flow that does not perform a UDP checksum calculation to verify the integrity at the transport endpoints.
Section 6 provides the recommendations for standardization of zero UDP checksum with a summary of the findings and notes remaining issues needing future work.
Appendix A evaluates the set of proposals to update the UDP transport behaviour and other alternatives intended to improve support for tunnel protocols. It concludes by assessing the trade-offs of the various methods identifying advantages and disadvantages for each method.
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 [RFC2119].
One increasingly popular use of UDP is as a tunneling protocol, where a tunnel endpoint encapsulates the packets of another protocol inside UDP datagrams and transmits them to another tunnel endpoint. Using UDP as a tunneling protocol is attractive when the payload protocol is not supported by the middleboxes that may exist along the path, because many middleboxes support transmission using UDP. In this use, the receiving endpoint decapsulates the UDP datagrams and forwards the original packets contained in the payload [RFC5405]. Tunnels establish virtual links that appear to directly connect locations that are distant in the physical Internet topology and can be used to create virtual (private) networks.
A number of tunnel encapsulations deployed over IPv4 have used the UDP transport with a zero checksum. Users of these protocols expect a similar solution for IPv6.
A number of tunnel protocols are also currently being defined (e.g. Automated Multicast Tunnels, AMT [I-D.ietf-mboned-auto-multicast], and the Locator/Identifier Separation Protocol, LISP [I-D.ietf-lisp]). These protocols motivated an update to IPv6 UDP checksum processing to benefit from simpler checksum processing for various reasons:
It is a common requirement to terminate a large number of tunnels on a single router/host. The processing cost per tunnel includes both state (memory requirements) and per-packet processing.
Automatic IP Multicast Tunneling, known as AMT [I-D.ietf-mboned-auto-multicast] currently specifies UDP as the transport protocol for packets carrying tunneled IP multicast packets. The current specification for AMT requires that the UDP checksum in the outer packet header should be 0 (see Section 6.6 of [I-D.ietf-mboned-auto-multicast]). This argues that the computation of an additional checksum is an unwarranted burden on nodes implementing lightweight tunneling protocols when an inner packet is already adequately protected, . The AMT protocol needs to replicate a multicast packet to each gateway tunnel. In this case, the outer IP addresses are different for each tunnel and therefore require a different pseudo header to be built for each UDP replicated encapsulation.
The argument concerning redundant processing costs is valid regarding the integrity of a tunneled packet. In some architectures (e.g. PC-based routers), other mechanisms may also significantly reduce checksum processing costs: There are implementations that have optimised checksum processing algorithms, including the use of checksum-offloading. This processing is readily available for IPv4 packets at high line rates. Such processing may be anticipated for IPv6 endpoints, allowing receivers to reject corrupted packets without further processing. However, there are certain classes of tunnel end-points where this off-loading is not available and unlikely to become available in the near future.
The currently-deployed hardware in many routers uses a fast-path processing that only provides the first n bytes of a packet to the forwarding engine, where typically n <= 128. This prevents fast processing of a transport checksum over an entire (large) packet. Hence the currently defined IPv6 UDP checksum is poorly suited to use within a router that is unable to access the entire packet and does not provide checksum-offloading. Thus enabling checksum calculation over the complete packet can impact router design, performance improvement, energy consumption and/or cost.
In IPv4, UDP-encapsulation may be desirable for NAT traversal, since UDP support is commonly provided. It is also necessary due to the almost ubiquitous deployment of IPv4 NATs. There has also been discussion of NAT for IPv6, although not for the same reason as in IPv4. If IPv6 NAT becomes a reality they hopefully do not present the same protocol issues as for IPv4. If NAT is defined for IPv6, it should take into consideration the use of a zero UDP checksum.
The requirements for IPv6 firewall traversal are likely be to be similar to those for IPv4. In addition, it can be reasonably expected that a firewall conforming to RFC 2460 will not regard datagrams with a zero UDP checksum as valid. Use of a zero UDP checksum with IPv6 requires firewalls to be updated before the full utility of the change is available.
It can be expected that datagrams with zero UDP checksum will initially not have the same middlebox traversal characteristics as regular UDP (RFC 2460). However when implementations follow the requirements specified in this document, we expect the traversal capabilities to improve over time. We also note that deployment of IPv6-capable middleboxes is still in its initial phases. Thus, it might be that the number of non-updated boxes quickly become a very small percentage of the deployed middleboxes.
The UDP port number fields have been used as a basis to design load-balancing solutions for IPv4. This approach has also been leveraged for IPv6. An alternate method would be to utilise the IPv6 Flow Label as a basis for entropy for load balancing. This would have the desirable effect of releasing IPv6 load-balancing devices from the need to assume semantics for the use of the transport port field and also works for all type of transport protocols.
This use of the flow-label is consistent with the intended use, although further clarity may be needed to ensure the field can be consistently used for this purpose, (e.g. the updated IPv6 Flow Label Specification [RFC6437] and Equal-Cost Multi-Path routing, ECMP [RFC6438]). Router vendors could be encouraged to start using the IPv6 Flow Label as a part of the flow hash, providing support for ECMP without requiring use of UDP.
However, the method for populating the outer IPv6 header with a value for the flow label is not trivial: If the inner packet uses IPv6, then the flow label value could be copied to the outer packet header. However, many current end-points set the flow label to a zero value (thus no entropy). The ingress of a tunnel seeking to provide good entropy in the flow label field would therefore need to create a random flow label value and keep corresponding state, so that all packets that were associated with a flow would be consistently given the same flow label. Although possible, this complexity may not be desirable in a tunnel ingress.
The end-to-end use of flow labels for load balancing is a long-term solution. Even if the usage of the flow label is clarified, there would be a transition time before a significant proportion of end-points start to assign a good quality flow label to the flows that they originate, with continued use of load balancing using the transport header fields until any widespread deployment is finally achieved.
The IETF has defined a set of transport protocols that may be applicable for tunnels with IPv6. There are also a set of network layer encapsulation tunnels such as IP-in-IP and GRE. These already standardized solutions are discussed here prior to the issues, as background for the issue description and some comparison of where the issue may already occur.
UDP [RFC0768] with standard checksum behaviour, as defined in RFC 2460, has already been discussed. UDP usage guidelines are provided in [RFC5405].
UDP-Lite [RFC3828] offers an alternate transport to UDP, specified as a proposed standard, RFC 3828. A MIB is defined in RFC 5097 and unicast usage guidelines in [RFC5405]. There is at least one open source implementation as a part of the Linux kernel since version 2.6.20.
UDP-Lite provides a checksum with optional partial coverage. When using this option, a datagram is divided into a sensitive part (covered by the checksum) and an insensitive part (not covered by the checksum). When the checksum covers the entire packet, UDP-Lite is fully equivalent with UDP, with the exception that it uses a different value in the Next Header field in the IPv6 header. Errors/corruption in the insensitive part will not cause the datagram to be discarded by the transport layer at the receiving endpoint. A minor side-effect of using UDP-Lite is that this was specified for damage-tolerant payloads and some link-layers may employ different link encapsulations when forwarding UDP-Lite segments (e.g. radio access bearers). Most link-layers will cover the insensitive part with the same strong layer 2 frame CRC that covers the sensitive part.
Tunnel encapsulations can use UDP-Lite (e.g. Control And Provisioning of Wireless Access Points, CAPWAP [RFC5415]), since UDP-Lite provides a transport-layer checksum, including an IP pseudo header checksum, in IPv6, without the need for a router/middlebox to traverse the entire packet payload. This provides most of the verification required for delivery and still keeps the complexity of the checksumming operation low. UDP-Lite may set the length of checksum coverage on a per packet basis. This feature could be used if a tunnel protocol is designed to only verify delivery of the tunneled payload and uses full checksumming for control information.
There is currently poor support for middlebox traversal using UDP-Lite, because UDP-Lite uses a different IPv6 network-layer Next Header value to that of UDP, and few middleboxes are able to interpret UDP-Lite and take appropriate actions when forwarding the packet. This makes UDP-Lite less suited to protocols needing general Internet support, until such time that UDP-Lite has achieved better support in middleboxes and end-points.
The IETF has defined a set of tunneling protocols or network layer encapsulations, e.g., IP-in-IP and GRE. These either do not include a checksum or use a checksum that is optional, since tunnel encapsulations are typically layered directly over the Internet layer (identified by the upper layer type in the IPv6 Next Header field) and are also not used as endpoint transport protocols. There is little chance of confusing a tunnel-encapsulated packet with other application data that could result in corruption of application state or data.
From the end-to-end perspective, the principal difference is that the network-layer Next Header field identifies a separate transport, which reduces the probability that corruption could result in the packet being delivered to the wrong endpoint or application. Specifically, packets are only delivered to protocol modules that process a specific Next Header value. The Next Header field therefore provides a first-level check of correct demultiplexing. In contrast, the UDP port space is shared by many diverse applications and therefore UDP demultiplexing relies solely on the port numbers.
This informative section evaluates issues around the proposal to update IPv6 [RFC2460], to enable the UDP transport checksum to be set to zero. Some of the identified issues are shared with other protocols already in use. The section also provides background to the requirements and recommendations that follow.
The decision by RFC 2460 to omit an integrity check at the network level meant that the IPv6 transport checksum was overloaded with many functions, including validating:
In IPv4, the first four checks are performed using the IPv4 header checksum.
In IPv6, these checks occur within the endpoint stack using the UDP checksum information. An IPv6 node also relies on the header information to determine whether to send an ICMPv6 error message [RFC4443] and to determine the node to which this is sent. Corrupted information may lead to mis-delivery to an unintended application socket on an unexpected host.
IP packets may be corrupted as they traverse an Internet path. Evidence has been presented [Sigcomm2000] to show that this was once an issue with IPv4 routers, and occasional corruption could result from bad internal router processing in routers or hosts. These errors are not detected by the strong frame checksums employed at the link-layer [RFC3819]. There is no current evidence that such cases are rare in the modern Internet, nor that they may not be applicable to IPv6. It therefore seems prudent not to relax this constraint. The emergence of low-end IPv6 routers and the proposed use of NAT with IPv6 further motivate the need to protect from this type of error.
Corruption in the network may result in:
When a checksum is used, this significantly reduces the impact of errors, reducing the probability of undetected corruption of state (and data) on both the host stack and the applications using the transport service.
The following sections examine the impact of modifying each of these header fields.
An IPv6 endpoint destination address could be modified in the network (e.g. corrupted by an error). This is not a concern for IPv4, because the IP header checksum will result in this packet being discarded by the receiving IP stack. Such modification in the network can not be detected at the network layer when using IPv6.
There are two possible outcomes:
This section examines what happens when the source address is corrupted in transit. This is not a concern in IPv4, because the IP header checksum will normally result in this packet being discarded by the receiving IP stack.
Corruption of an IPv6 source address does not result in the IP packet being delivered to a different endpoint protocol or destination address. If only the source address is corrupted, the datagram will likely be processed in the intended context, although with erroneous origin information. When using Unicast Reverse Path Forwarding [RFC2827], a change in address may result in the router discarding the packet when the route to the modified source address is different to that of the source address of the original packet.
The result will depend on the application or protocol that processes the packet. Some examples are:
In general, the effect of corrupting the source address will depend upon the protocol that processes the packet and its robustness to this error. For the case where the packet is received by a tunnel endpoint, the tunnel application is expected to correctly handle a corrupted source address.
The impact of source address modification is more difficult to quantify when the receiving application is not that originally intended and several fields have been modified in transit.
This section describes what happens if one or both of the UDP port values are corrupted in transit. This can also happen with IPv4 when used with a zero UDP checksum, but not when UDP checksums are calculated or with UDP-Lite. If the ports carried in the transport header of an IPv6 packet were corrupted in transit, packets may be delivered to the wrong application process (on the intended machine) and/or responses or errors sent to the wrong application process (on the intended machine).
If one combines the corruption effects, such as destination address and ports, there is a number of potential outcomes when traffic arrives at an unexpected port. This section discusses these possibilities and their outcomes for a packet that does not use the UDP checksum validation:
The probability of each outcome depends on the statistical probability that the address or the port information for the source or destination becomes corrupt in the datagram such that they match those of an existing flow or server port. Unfortunately, such a match may be more likely for UDP than for connection-oriented transports, because:
While these rules could, in principle, be revised to declare naive applications as "Historic". This remedy is not realistic: the transport owes it to the stack to do its best to reject bogus datagrams.
If checksum coverage is suppressed, the application therefore needs to provide a method to detect and discard the unwanted data. A tunnel protocol would need to perform its own integrity checks on any control information if transported in datagrams with a zero UDP checksum. If the tunnel payload is another IP packet, the packets requiring checksums can be assumed to have their own checksums provided that the rate of corrupted packets is not significantly larger due to the tunnel encapsulation. If a tunnel transports other inner payloads that do not use IP, the assumptions of corruption detection for that particular protocol must be fulfilled, this may require an additional checksum/CRC and/or integrity protection of the payload and tunnel headers.
A protocol that uses a zero UDP checksum can not assume that it is the only protocol using a zero UDP checksum. Therefore, it needs to gracefully handle mis-delivery. It must be robust to reception of malformed packets received on a listening port and expect that these packets may contain corrupted data or data associated with a completely different protocol.
The fragmentation information in IPv6 employs a 32-bit identity field, compared to only a 16-bit field in IPv4, a 13-bit fragment offset and a 1-bit flag, indicating if there are more fragments. Corruption of any of these field may result in one of two outcomes:
The erroneous reassembly of packets is a general concern and such packets should be discarded instead of being passed to higher layer processes. The primary detector of packet length changes is the IP payload length field, with a secondary check by the transport checksum. The Upper-Layer Packet length field included in the pseudo header assists in verifying correct reassembly, since the Internet checksum has a low probability of detecting insertion of data or overlap errors (due to misplacement of data). The checksum is also incapable of detecting insertion or removal of all zero-data that occurs in a multiple of a 16-bit chunk.
The most significant risk of corruption results following mis-association of a fragment with a different packet. This risk can be significant, since the size of fragments is often the same (e.g. fragments resulting when the path MTU results in fragmentation of a larger packet, common when addition of a tunnel encapsulation header expands the size of a packet). Detection of this type of error requires a checksum or other integrity check of the headers and the payload. Such protection is anyway desirable for tunnel encapsulations using IPv4, since the small fragmentation ID can easily result in wrap-around [RFC4963], this is especially the case for tunnels that perform flow aggregation [I-D.ietf-intarea-tunnels].
Tunnel fragmentation behavior matters. There can be outer or inner fragmentation "Tunnels in the Internet Architecture" [I-D.ietf-intarea-tunnels]. If there is inner fragmentation by the tunnel, the outer headers will never be fragmented and thus a zero UDP checksum in the outer header will not affect the reassembly process. When a tunnel performs outer header fragmentation, the tunnel egress needs to perform reassembly of the outer fragments into an inner packet. The inner packet is either a complete packet or a fragment. If it is a fragment, the destination endpoint of the fragment will perform reassembly of the received fragments. The complete packet or the reassembled fragments will then be processed according to the packet Next Header field. The receiver may only detect reassembly anomalies when it uses a protocol with a checksum. The larger the number of reassembly processes to which a packet has been subjected, the greater the probability of an error.
In conclusion, fragmentation of datagrams with a zero UDP checksum does not worsen the performance compared to some other commonly used tunnel encapsulations. However, caution is needed for recursive tunneling without any additional verification at the different tunnel layers.
Corruption of IP packets can occur at any point in the transmission chain, during packet generation, in the transmission link, in the process of routing and switching, etc. Some steps have checksum or Cyclic Redundancy Check (CRC), which reduces the probability for erroneous packets being used, but there still exists some probability for errors to propagate undetected. Unfortunately we lack solid information about what the most common functions or equipment that generate packet corruption are. However we have indications that there are significant variations in where corruption may occur. Thus there is a risk in applying evidence from one domain of usage onto another. Anyone intending general Internet usage must unfortunately assume that corruption will occur and cope with it.
IP transports designed for use in the general Internet should not assume specific path characteristics. Network protocols may reroute packets that change the set of routers and middleboxes along a path. Therefore transports such as TCP, SCTP and DCCP have been designed to negotiate protocol parameters, adapt to different network path characteristics, and receive feedback to verify that the current path is suited to the intended application. Applications using UDP and UDP-Lite need to provide their own mechanisms to confirm the validity of the current network path.
A zero value in the UDP checksum field is explicitly disallowed in RFC2460. Thus it may be expected that any device on the path that has a reason to look beyond the IP header will consider such a packet as erroneous or illegal and may discard it, unless the device is updated to support the new behavior. A pair of end-points intending to use a new behavior will therefore not only need to ensure support at each end-point, but also that the path between them will deliver packets with the new behavior. This may require negotiation or an explicit mandate to use the new behavior by all nodes needed to support the use of a new protocol.
Enabling the use of a zero checksum places new requirements on equipment deployed within the network, such as middleboxes. A middlebox (e.g. Firewalls, Network Address and Port Translation (NAPT)) may enable zero checksum usage for a particular range of ports. Note that checksum off-loading and operating system design may result in all IPv6 UDP traffic being sent with a calculated checksum. This requires middleboxes that are configured to enable a zero UDP checksum to continue to work with bidirectional UDP flows that use a zero UDP checksum in only one direction, and therefore they must not maintain separate state for a UDP flow based on its checksum usage.
Support along the path between end points can be guaranteed in limited deployments by appropriate configuration. In general, it can be expected to take time for deployment of any updated behaviour to become ubiquitous.
A sender will need to probe the path to verify the expected behavior. Path characteristics may change, and usage therefore should be robust and able to detect a failure of the path under normal usage and re-negotiate. Note that a bidirectional path does not necessarily support the same checksum usage in both the forward and return directions: Receipt of a datagram with a zero UDP checksum, does not imply that the remote endpoint can also receive a datagram with a zero UDP checksum. This will require periodic validation of the path, adding complexity to any solution using the new behavior.
The IPv6 specification update defined in [I-D.ietf-6man-udpchecksums] only modifies IPv6 nodes that implement specific protocols designed to permit omission of a UDP checksum. This document therefore provides an applicability statement for the updated method indicating when the mechanism can (and can not) be used. Enabling this, and ensuring correct interactions with the stack, implies much more than simply disabling the checksum algorithm for specific packets at the transport interface.
When the method is widely available, it may be expected to be used by applications that are perceived to gain benefit. Any solution that uses an end-to-end transport protocol, rather than an IP-in-IP encapsulation, needs to minimise the possibility that application processes could confuse a corrupted or wrongly delivered UDP datagram with that of data addressed to the application running on their endpoint.
First of all the using protocol or application must ensure that this doesn't significantly affect themselves. That includes receiving packets from other protocols or contexts as an effect of the corruption of destination or source address and port values. That also includes considering what additional implicit protection mechanisms that exist due to the usage the payload of the UDP packet with a zero checksum have.
It is important to consider the potential impact of using a zero UDP checksum on end-point devices or applications that are not modified to support the new behavior or by default or preference, use the regular behavior. These applications must not be significantly impacted by the update.
To illustrate why this necessary, consider the implications of a node enabling the use of a zero UDP checksum at the interface level: This would result in all applications that listen to a UDP socket receiving datagrams where the checksum was not verified. This could have a significant impact on an application that was not designed with the additional robustness needed to handle received packets with corruption, creating state or destroying existing state in the application.
The use of a zero UDP checksum therefore needs to be enabled only for individual ports by an explicit request by the application. In this case, applications using other ports would maintain the current IPv6 behavior, discarding incoming datagrams with a zero UDP checksum. These other applications would not be affected by this changed behavior. An application that allows the changed behavior should be aware of the risk of corruption and the increased level of misdirected traffic, and can be designed robustly to handle this risk.
This section is an applicability statement that defines requirements and recommendations on the implementation of IPv6 nodes that support use of a zero value in the checksum field of a UDP datagram.
All implementations that support this zero UDP checksum method MUST conform to the requirements defined below.
This section is an applicability statement that identifies requirements and recommendations for protocols and tunnel encapsulations that are transported over an IPv6 transport flow that does not perform a UDP checksum calculation to verify the integrity at the transport endpoints.
This document examines the role of the UDP transport checksum when used with IPv6. It presents a summary of the trade-offs in evaluating the safety of updating RFC 2460 to permit an IPv6 endpoint to use a zero UDP checksum field to indicate that no checksum is present.
The use of UDP with a zero UDP checksum has merits for some applications, such as tunnel encapsulation, and is widely used in IPv4. However, there are different dangers for IPv6: There is an increased risk of corruption and mis-delivery when using zero UDP checksum in IPv6 compared to IPv4, due to the lack of an IPv6 header checksum. Thus, applications need to re-evaluate the risks of enabling use of a zero UDP checksum and consider a solution that at least provides the same delivery protection as for IPv4, for example by utilizing UDP-Lite, or by enabling the UDP checksum. The use of checksum off-loading may help alleviate the checksum processing cost and permit use of a checksum using method defined in RFC 2460.
Tunnel applications using UDP for encapsulation can in many cases use a zero UDP checksum without significant impact on the corruption rate. A well-designed tunnel application should include consistency checks to validate the header information encapsulated with a received packet. In most cases, tunnels encapsulating IP packets can rely on the inner packets' own integrity protection. When correctly implemented, such a tunnel endpoint will not be negatively impacted by omission of the transport-layer checksum. Recursive tunneling and fragmentation is a potential issue that can raise corruption rates significantly, and requires careful consideration.
Other UDP applications at the intended destination node or another node can be impacted if they are allowed to receive datagrams that have a zero UDP checksum. It is important that already deployed applications are not impacted by a change at the transport layer. If these applications execute on nodes that implement RFC 2460, they will discard (and log) all datagrams with a zero UDP checksum. This is not an issue.
In general, UDP-based applications need to employ a mechanism that allows a large percentage of the corrupted packets to be removed before they reach an application, both to protect the data stream of the application and the control plane of higher layer protocols. These checks are currently performed by the UDP checksum for IPv6, or the reduced checksum for UDP-Lite when used with IPv6.
Recursive tunneling and fragmentation is a difficult issue relating to tunnels in general. There is an increased risk of an error in the inner-most packet when fragmentation results from several layers of tunneling and several different reassembly processes are run without verification of correctness. This issue requires extra thought and careful consideration.
The use of the updated method must consider the implications on firewalls, NATs and other middleboxes. It is not expected that IPv6 NATs handle IPv6 UDP datagrams in the same way that they handle IPv4 UDP datagrams. This possibly reduces the need to update the checksum. Firewalls are intended to be configured, and therefore may need to be explicitly updated to allow new services or protocols. IPv6 middlebox deployment is not yet as prolific as it is in IPv4, and therefore new devices are expected to follow the methods specified in this document.
Each application should consider the implications of choosing an IPv6 transport that uses a zero UDP checksum, and consider whether other standard methods may be more appropriate, and may simplify application design.
Brian Haberman, Brian Carpenter, Magaret Wasserman, Lars Eggert, others in the TSV directorate.
Thanks also to: Rémi Denis-Courmont, Pekka Savola, Glen Turner, and many others who contributed comments and ideas via the 6man, behave, lisp and mboned lists.
Barry Leiba, Ronald Bonica and Stewart Bryant are thanked for resulting in a document with much greater applicability.
A Special thanks to P.F. Chimento for review and editorial corrections.
This document does not require any actions by IANA.
Transport checksums provide the first stage of protection for the stack, although they can not be considered authentication mechanisms. These checks are also desirable to ensure packet counters correctly log actual activity, and can be used to detect unusual behaviours.
Depending on the hardware design, the processing requirements may differ for tunnels that have a zero UDP checksum and those that calculate a checksum. This processing overhead may need to be considered when deciding whether to enable a tunnel and to determine an acceptable rate for transmission.
Transmission of IPv6 packets with a zero UDP checksum could reveal additional information to an on-path attacker to identify the operating system or configuration of a sending node. There is a need to probe the network path to determine whether the path supports using IPv6 packets with a zero UDP checksum. The details of the probing mechanism may differ for different tunnel encapsulations and if visible in the network (e.g. if not using IPsec in encryption mode) could reveal additional information to an on-path attacker to identify the type of tunnel being used.
IP-in-IP or GRE tunnels offer good traversal of middleboxes that have not been designed for security, e.g. firewalls. However, firewalls may be expected to be configured to block general tunnels as they present a large attack surface. This applicability statement therefore permits this method to be enabled only for specific ranges of ports.
| [RFC0791] | Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. |
| [RFC0768] | Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. |
| [RFC2460] | Deering, S.E. and R.M. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [I-D.ietf-6man-udpchecksums] | Eubanks, M, Chimento, P and M Westerlund, "UDP Checksums for Tunneled Packets", Internet-Draft draft-ietf-6man-udpchecksums-05, October 2012. |
This informative appendix documents the evaluation of the proposal to update IPv6 [RFC2460], to provide the option that some nodes may suppress generation and checking of the UDP transport checksum. It also compares the proposal with other alternatives, and notes that for a particular application some standard methods may be more appropriate than using IPv6 with a zero UDP checksum.
There are several alternatives to the normal method for calculating the UDP Checksum [RFC1071] that do not require a tunnel endpoint to inspect the entire packet when computing a checksum. These include (in decreasing order of complexity):
These options are compared and discussed further in the following sections.
This section compares the above listed methods to support datagram tunneling. It includes proposals for updating the behaviour of UDP.
While this comparison focuses on applications that are expected to execute on routers, the distinction between a router and a host is not always clear, especially at the transport level. Systems (such as unix-based operating systems) routinely provide both functions. There is no way to identify the role of the receiving node from a received packet.
Regular UDP with a standard checksum or the delta encoded optimization for creating correct checksums have the best possibilities for successful traversal of a middlebox. No new support is required.
A method that ignores the UDP checksum on reception is expected to have a good probability of traversal, because most middleboxes perform an incremental checksum update. UDPTT may also traverse a middlebox with this behaviour. However, a middlebox on the path that attempts to verify a standard checksum will not forward packets using either of these methods, preventing traversal. A method that ignores the checksum has an additional downside in that it prevents improvement of middlebox traversal, because there is no way to identify UDP datagrams that use the modified checksum behaviour.
IP-in-IP or GRE tunnels offer good traversal of middleboxes that have not been designed for security, e.g. firewalls. However, firewalls may be expected to be configured to block general tunnels as they present a large attack surface.
A new IPv6 Destination Options header will suffer traversal issues with middleboxes, especially Firewalls and NATs, and will likely require them to be updated before the extension header is passed.
Datagrams with a zero UDP checksum will not be passed by any middlebox that validates the checksum using RFC 2460 or updates the checksum field, such as NAT or firewalls. This would require an update to correctly handle a datagram with a zero UDP checksum.
UDP-Lite will require an update of almost all type of middleboxes, because it requires support for a separate network-layer protocol number. Once enabled, the method to support incremental checksum update would be identical to that for UDP, but different for checksum validation.
The usefulness of solutions for load balancers depends on the difference in entropy in the headers for different flows that can be included in a hash function. All the proposals that use the UDP protocol number have equal behavior. UDP-Lite has the potential for equally good behavior as for UDP. However, UDP-Lite is currently unlikely to be supported by deployed hashing mechanisms, which may cause a load balancer to not use the transport header in the computed hash. A load balancer that only uses the IP header will have low entropy, but could be improved by including the IPv6 the flow label, providing that the tunnel ingress ensures that different flow labels are assigned to different flows. However, a transition to the common use of good quality flow labels is likely to take time to deploy.
IP-in-IP tunnels are often considered efficient, because they introduce very little processing and low data overhead. The other proposals introduce a UDP-like header incurring associated data overhead. Processing is minimised for the method that uses a zero UDP checksum, ignoring the UDP checksum on reception, and only slightly higher for UDPTT, the extension header and UDP-Lite. The delta-calculation scheme operates on a few more fields, but also introduces serious failure modes that can result in a need to calculate a checksum over the complete datagram. Regular UDP is clearly the most costly to process, always requiring checksum calculation over the entire datagram.
It is important to note that the zero UDP checksum method, ignoring checksum on reception, the Option Header, UDPTT and UDP-Lite will likely incur additional complexities in the application to incorporate a negotiation and validation mechanism.
The major factors influencing deployability of these solutions are a need to update both end-points, a need for negotiation and the need to update middleboxes. These are summarised below:
The standard UDP checksum and the delta checksum can both provide some verification at the tunnel egress. This can significantly reduce the probability that a corrupted inner packet is forwarded. UDP-Lite, UDPTT and the extension header all provide some verification against corruption, but do not verify the inner packet. They only provide a strong indication that the delivered packet was intended for the tunnel egress and was correctly delimited. The methods using a zero UDP checksum, ignoring the UDP checksum on reception and IP-and-IP encapsulation all provide no verification that a received datagram was intended to be processed by a specific tunnel egress or that the inner encapsulated packet was correct.
The comparisons above may be summarised as "there is no silver bullet that will slay all the issues". One has to select which down side(s) can best be lived with. Focusing on the existing solutions, this can be summarized as:
The delta-checksum is an optimization in the processing of UDP, as such it exhibits some of the drawbacks of using regular UDP.
The remaining proposals may be described in similar terms:
There is no clear single optimum solution. If the most important need is to traverse middleboxes, then the best choice is to stay with regular UDP and consider the optimizations that may be required to perform the checksumming. If one can live with limited middlebox traversal, low complexity is necessary and one does not require load balancing, then IP-in-IP tunneling is the simplest. If one wants strengthened error detection, but with currently limited middlebox traversal and load-balancing. UDP-Lite is appropriate. Zero UDP checksum addresses another set of constraints, low complexity and a need for load balancing from the current Internet, providing it can live with currently limited middlebox traversal.
Techniques for load balancing and middlebox traversal do continue to evolve. Over a long time, developments in load balancing have good potential to improve. This time horizon is long since it requires both load balancer and end-point updates to get full benefit. The challenges of middlebox traversal are also expected to change with time, as device capabilities evolve. Middleboxes are very prolific with a larger proportion of end-user ownership, and therefore may be expected to take long time cycles to evolve.
One potential advantage is that the deployment of IPv6-capable middleboxes are still in its initial phase and the quicker a new method becomes standardized, the fewer boxes will be non-compliant.
Thus, the question of whether to permit use of datagrams with a zero UDP checksum for IPv6 under reasonable constraints, is therefore best viewed as a trade-off between a number of more subjective questions:
The analysis concludes that the IETF should carefully consider constraints on sanctioning the use of any new transport mode. The 6man working group of the IETF has determined that the answer to the above questions are sufficient to update IPv6 to standardise use of a zero UDP checksum for use by tunnel encapsulations for specific applications.
Each application should consider the implications of choosing an IPv6 transport that uses a zero UDP checksum. In many cases, standard methods may be more appropriate, and may simplify application design. The use of checksum off-loading may help alleviate the checksum processing cost and permit use of a checksum using method defined in RFC 2460.
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