UDP Checksums for Tunneled Packets
draft-ietf-6man-udpchecksums-04

Abstract

This document provides an update of the Internet Protocol version 6 (IPv6) specification (RFC2460) to improve the performance of IPv6 in the use case when a tunnel protocol uses UDP with IPv6 to tunnel packets. The performance improvement is obtained by relaxing the IPv6 UDP checksum requirement for suitable tunneling protocol where header information is protected on the "inner" packet being carried. This relaxation removes the overhead associated with the computation of UDP checksums on IPv6 packets used to carry tunnel protocols and thereby improves the efficiency of the traversal of firewalls and other network middleboxes by such protocols. We describe how the IPv6 UDP checksum requirement can be relaxed in the situation where the encapsulated packet itself contains a checksum, the limitations and risks of this approach, and defines restrictions on the use of this relaxation to mitigate these risks.

Status of This Memo

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Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Some Terminology
• 2.1. Requirements Language
• 3. Problem Statement
• 4. Discussion
• 5. The Zero-Checksum Update
• 6. Additional Observations
• 7. IANA Considerations
• 8. Security Considerations
• 9. Acknowledgements
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Authors' Addresses

1. Introduction

This work constitutes an update of the Internet Protocol Version 6 (IPv6) Specification [RFC2460], in the use case when a tunnel protocol uses UDP with IPv6 to tunnel packets. With the rapid growth of the Internet, tunneling protocols have become increasingly important to enable the deployment of new protocols. Tunneled protocols can be deployed rapidly, while the time to upgrade and deploy a critical mass of routers, switches and end hosts on the global Internet for a new protocol is now measured in decades. At the same time, the increasing use of firewalls and other security related middleboxes means that truly new tunnel protocols, with new protocol numbers, are also unlikely to be deployable in a reasonable time frame, which has resulted in an increasing interest in and use of UDP-based tunneling protocols. In such protocols, there is an encapsulated "inner" packet, and the "outer" packet carrying the tunneled inner packet is a UDP packet, which can pass through firewalls and other middleboxes filtering that is a fact of life on the current Internet.

Tunnel endpoints may be routers or middleboxes aggregating traffic from a large number of tunnel users, therefore the computation of an additional checksum on the outer UDP packet, may be seen as an unwarranted burden on nodes that implement a tunneling protocol, especially if the inner packet(s) are already protected by a checksum. In IPv4, there is a checksum on the IP packet itself, and the checksum on the outer UDP packet can be set to zero. However in IPv6 there is not a checksum on the IP packet and RFC 2460 [RFC2460] explicitly states that IPv6 receivers MUST discard UDP packets with a zero checksum. So, while sending a UDP packet with a zero checksum is permitted in IPv4 packets, it is explicitly forbidden in IPv6 packets. To improve support for IPv6 UDP tunnels, this document updates RFC 2460 to allow tunnel endpoints to use a zero UDP checksum under constrained situations (IPv6 tunnel transports that carry checksum-protected packets), following the considerations in [I-D.ietf-6man-udpzero].

Unicast UDP Usage Guidelines for Application Designers [RFC5405] should be consulted when reading this specification. It discusses both UDP tunnels (Section 3.1.3) and the usage of Checksums (Section 3.4).

While the origin of this specification is the problem raised by the draft titled "Automatic IP Multicast Without Explicit Tunnels", also known as "AMT," [I-D.ietf-mboned-auto-multicast] we expect it to have wide applicability. Since the first version of this document, the need for an efficient UDP tunneling mechanism has increased. Other IETF Working Groups, notably LISP [I-D.ietf-lisp] and Softwires [RFC5619] have expressed a need to update the UDP checksum processing in RFC 2460. We therefore expect this update to be applicable in future to other tunneling protocols specified by these and other IETF Working Groups.

2. Some Terminology

For the remainder of this document, we discuss only IPv6, since this problem does not exist for IPv4. Therefore all reference to 'IP' should be understood as a reference to IPv6.

The document uses the terms "tunneling" and "tunneled" as adjectives when describing packets. When we refer to 'tunneling packets' we refer to the outer packet header that provides the tunneling function. When we refer to 'tunneled packets' we refer to the inner packet, i.e. the packet being carried in the tunnel.

2.1. Requirements Language

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].

3. Problem Statement

This document provides an update for the case where a tunnel protocol transports tunnelled packets that already have a UDP header with a checksum, there is both a benefit and a cost to compute and check the UDP checksum of the outer (encapsulating) UDP transport header. In certain cases, where reducing the forwarding cost is important, such as for systems that perform the check in software, the cost may outweigh the benefit; this document describes a means to avoid that cost, in the case where there is an inner header with a checksum.

4. Discussion

IPv6 UDP Checksum Considerations [I-D.ietf-6man-udpzero] describes the issues related to allowing UDP over IPv6 to have a valid checksum of zero and is not repeated here.

Section 5.1 of [I-D.ietf-6man-udpzero], identifies 9 requirements that introduce constraints on the usage of a zero checksum for UDP over IPv6. This document is intended to satisfy these requirements.

[I-D.ietf-6man-udpzero] and mailing list discussions have noted there is still the possibility of deep-inspection firewall devices or other middleboxes checking the UDP checksum field of the outer packet and thereby discarding the tunneling packets. This would be an issue also for any legacy IPv6 system that has not implemented this update to the IPv6 specification. In this case, the system (according to RFC 2460) will discard the zero-checksum UDP packets, and should log this as an error.

The below discuss how path errors can be detected and handled in an UDP tunneling protocol when the checksum protection is disabled. Note that other (non-tunneling) protocols may have different approaches, but these are not the topic of this update. We propose the following approach to handle this problem:

• Context (i.e. tunneling state) should be established via application Protocol Data Units (PDUs) that are carried in checksummed UDP packets. That is, any control packets flowing between the tunnel endpoints should be protected by UDP checksums. The control packets can also contain any negotiation required to enable the endpoint/adapters to accept UDP packets with a zero checksum. The control packets may also carry any negotiation required to enable the endpoint/adapters to identify the set of ports that need to enable reception UDP datagrams with a zero checksum.
• A system shall not set the UDP checksum to zero in packets that do not contain tunneled packets.
• UDP keep-alive packets with checksum zero can be sent to validate paths, given that paths between tunnel endpoints can change and so middleboxes in the path may vary during the life of the association. Paths with middleboxes that are intolerant of a UDP checksum of zero will drop the keep-alives and the endpoints will discover that. Note that this need only be done per tunnel endpoint pair, not per tunnel context. Keep-alive traffic should include both packets with tunnel checksums and packets with checksums equal to zero to enable the remote end to distinguish between path failures and the blockage of packets with checksum equal to zero.
• Corruption of the encapsulating IPv6 source address, destination address and/or the UDP source port, destination port fields : If the 9 restrictions in [I-D.ietf-6man-udpzero] are followed, the inner packets (tunneled packets) should be protected and run the usual (presumably small) risk of having undetected corruption(s). If tunneling protocol contexts contain (at a minimum) source and destination IP addresses and source and destination ports, there are 16 possible corruption outcomes. We note that these outcomes are not equally likely. The possible corruption outcomes may be:
  • Half of the 16 possible corruption combinations have a corrupted destination address. If the incorrect destination is reached and the node doesn't have an application for the destination port, the packet will be dropped. If the application at the incorrect destination is the same tunneling protocol and if it has a matching context (which can be assumed to be a very low probability event) the inner packet will be decapsulated and forwarded. Application developers should verify the context of the packets they receive using UDP, as described in [RFC5405]. Applications that verify the context of a datagram are expected to have a high probability of discarding corrupted data. [I-D.ietf-6man-udpzero] presents examples of cases where corruption can inadvertently impact application state.
  • Half of the 8 possible corruption combinations with a correct destination address have a corrupted source address. If the tunnel contexts contain all elements of the address-port 4-tuple, then the likelihood is that this corruption will be detected.
  • Of the remaining 4 possibilities, with valid source and destination IPv6 addresses, 1 has all 4 fields valid, the other three have one or both ports corrupted. Again, if the tunneling endpoint context contains sufficient information, these error should be detected with high probability.
• Corruption of source-fragmented encapsulating packets: In this case, a tunneling protocol may reassemble fragments associated with the wrong context at the right tunnel endpoint, or it may reassemble fragments associated with a context at the wrong tunnel endpoint, or corrupted fragments may be reassembled at the right context at the right tunnel endpoint. In each of these cases, the IPv6 length of the encapsulating header may be checked (though [I-D.ietf-6man-udpzero] points out the weakness in this check). In addition, if the encapsulated packet is protected by a transport (or other) checksum, these errors can be detected (with some probability).

While they do not guarantee correctness, these mechanism can reduce the risks of relaxing the UDP checksum requirement for IPv6.

5. The Zero-Checksum Update

This specification updates IPv6 to allow a UDP checksum of zero for the outer encapsulating packet of a tunneling protocol. UDP endpoints that implement this update MUST change their behavior for any destination port explicitly configured for zero checksum and not discard UDP packets received with a checksum value of zero on the outer packet. When this is done, it requires the constraints in Section 5.1 of [I-D.ietf-6man-udpzero].

Specifically, the text in [RFC2460] Section 8.1, 4th bullet is updated. We refer to the following text:

"Unlike IPv4, when UDP packets are originated by an IPv6 node, the UDP checksum is not optional. That is, whenever originating a UDP packet, an IPv6 node must compute a UDP checksum over the packet and the pseudo-header, and, if that computation yields a result of zero, it must be changed to hex FFFF for placement in the UDP header. IPv6 receivers must discard UDP packets containing a zero checksum, and should log the error."

This item should be taken out of the bullet list and should be replaced by:

• Whenever originating a UDP packet, an IPv6 node SHOULD compute a UDP checksum over the packet and the pseudo-header, and, if that computation yields a result of zero, it must be changed to hex FFFF for placement in the UDP header. IPv6 receivers SHOULD discard UDP packets containing a zero checksum, and SHOULD log the error. However, some protocols, such as tunneling protocols that use UDP as a tunnel encapsulation, MAY omit computing the UDP checksum of the encapsulating UDP header and set it to zero, subject to the constraints described in RFCXXXX. In cases where the encapsulating protocol uses a zero checksum for UDP, the receiver of packets sent to a port enabled to receive zero-checksum packets MUST NOT discard packets solely for having a UDP checksum of zero. Note that these constraints apply only to encapsulating protocols that omit calculating the UDP checksum and set it to zero. An encapsulating protocol can always choose to compute the UDP checksum, in which case, its behavior is not updated and uses the method specified in RFC2460.
• 1. IPv6 protocol stack implementations SHOULD NOT by default allow the new method. The default node receiver behavior MUST discard all IPv6 packets carrying UDP packets with a zero checksum.
  2. Implementations MUST provide a way to signal the set of ports that will be enabled to receive UDP datagrams with a zero checksum. An IPv6 node that enables reception of UDP packets with a zero-checksum, MUST enable this only for a specific port or port-range. This may be implemented via a socket API call, or similar mechanism.
  3. RFC 2460 specifies that IPv6 nodes should log UDP datagrams with a zero-checksum. A port for which zero-checksum has been enabled MUST NOT log zero-checksum datagrams for that reason (of course, there might be other reasons to log such packets).
  4. A stack may separately identify UDP datagrams that are discarded with a zero checksum. It SHOULD NOT add these to the standard log, since the endpoint has not been verified.
  5. UDP Tunnels that encapsulate IP MAY rely on the inner packet integrity checks provided that the tunnel will not significantly increase the rate of corruption of the inner IP packet. If a significantly increased corruption rate can occur, then the tunnel MUST provide an additional integrity verification mechanism. An integrity mechanism is always recommended at the tunnel layer to ensure that corruption rates of the inner most packet are not increased.
  6. Tunnels that encapsulate Non-IP packets MUST have a CRC or other mechanism for checking packet integrity, unless the Non-IP packet specifically is designed for transmission over lower layers that do not provide any packet integrity guarantee. In particular, the application must be designed so that corruption of this information does not result in accumulated state or incorrect processing of a tunneled payload.
  7. UDP applications that support use of a zero-checksum, SHOULD NOT rely upon correct reception of the IP and UDP protocol information (including the length of the packet) when decoding and processing the packet payload. In particular, the application must be designed so that corruption of this information does not result in accumulated state or incorrect processing of a tunneled payload.
  8. If a method proposes recursive tunnels, it MUST provide guidance that is appropriate for all use-cases. Restrictions may be needed to the use of a tunnel encapsulations and the use of recursive tunnels (e.g. Necessary when the endpoint is not verified).
  9. IPv6 nodes that receive ICMPv6 messages that refer to packets with a zero UDP checksum MUST provide appropriate checks concerning the consistency of the reported packet to verify that the reported packet actually originated from the node, before acting upon the information (e.g. validating the address and port numbers in the ICMPv6 message body).
• Middleboxes MUST allow IPv6 packets with UDP checksum equal to zero to pass. Implementations of middleboxes MAY allow configuration of specific port ranges for which a zero UDP checksum is valid and may drop IPv6 UDP packets outside those ranges.
• The path between tunnel endpoints can change, thus also the middleboxes in the path may vary during the life of the association. Paths with middleboxes that are intolerant of a UDP checksum of zero will drop any keep-alives sent to validate the path using checksum zero and the endpoints will discover that. Therefore keep-alive traffic SHOULD include both packets with tunnel checksums and packets with checksums equal to zero to enable the remote end to distinguish between path failures and the blockage of packets with checksum equal to zero. Note that path validation need only be done per tunnel endpoint pair, not per tunnel context.
• RFC-Editor Note: Please replace RFCXXXX above with the RFC number this specification receives and remove this note.

6. Additional Observations

The existence of this issue among a significant number of protocols being developed in the IETF motivates this specified change. The authors would also like to make the following observations:

• An empirically-based analysis of the probabilities of packet corruptions (with or without checksums) has not (to our knowledge) been conducted since about 2000. It is now 2012. We strongly suggest that an empirical study is in order, along with an extensive analysis of IPv6 header corruption probabilities.
• A key cause to the increased usage of UDP in tunneling is the lack of protocol support in middleboxes. Specifically, new protocols, such as LISP [I-D.ietf-lisp], prefer to use UDP tunnels to traverse an end-to-end path successfully and avoid having their packets dropped by middleboxes. If this were not the case, the use of UDP-lite [RFC3828] might become more viable for some (but not necessarily all) tunneling protocols.
• Another issue is that the UDP checksum is overloaded with the task of protecting the IPv6 header for UDP flows (as is the TCP checksum for TCP flows). Protocols that do not use a pseudo-header approach to computing a checksum or CRC have essentially no protection from mis–delivered packets.

7. IANA Considerations

This document makes no request of IANA.

Note to RFC Editor: this section may be removed on publication as an RFC.

8. Security Considerations

It requires less work to generate zero-checksum attack packets than ones with full UDP checksums. However, this does not lead to any significant new vulnerabilities as checksums are not a security measure and can be easily generated by any attacker, as properly configured tunnels should check the validity of the inner packet and perform any needed security checks, regardless of the checksum status, and finally as most attacks are generated from compromised hosts which automatically create checksummed packets (in other words, it would generally be more, not less, effort for most attackers to generate zero UDP checksums on the host).

9. Acknowledgements

We would like to thank Brian Haberman and Gorry Fairhurst for discussions and reviews.

10. References

10.1. Normative References

10.2. Informative References

Authors' Addresses