Internet Engineering Task Force (IETF) M. Eubanks
Request for Comments: 6935 AmericaFree.TV LLC
Updates: 2460 P. Chimento
Category: Standards Track Johns Hopkins University Applied
ISSN: 2070-1721 Physics Laboratory
April 2013 IPv6 and UDP Checksums for Tunneled Packets
This document updates the IPv6 specification (RFC 2460) to improve
performance when a tunnel protocol uses UDP with IPv6 to tunnel
packets. The performance improvement is obtained by relaxing the
IPv6 UDP checksum requirement for tunnel protocols whose header
information is protected on the "inner" packet being carried.
Relaxing this requirement removes the overhead associated with the
computation of UDP checksums on IPv6 packets that carry the tunnel
protocol packets. This specification describes how the IPv6 UDP
checksum requirement can be relaxed when the encapsulated packet
itself contains a checksum. It also describes the limitations and
risks of this approach and discusses the restrictions on the use of
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
This document constitutes an update of the IPv6 specification
[RFC2460] for cases where a tunnel protocol uses UDP with IPv6 to
tunnel packets. With the rapid growth of the Internet, tunnel
protocols have become increasingly important to enable the deployment
of new protocols. Tunnel protocols can be deployed rapidly, while
the time to upgrade and deploy a new protocol on a critical mass of
routers, middleboxes, and hosts on the global Internet 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. The result is an increasing
interest in and use of UDP-based tunnel 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 that perform the
filtering that is a fact of life on the current Internet.
Tunnel endpoints may be routers or middleboxes aggregating traffic
from a 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 tunnel protocol,
especially if the inner packets are already protected by a checksum.
IPv4 has a checksum over the IP packet header, and the checksum on
the outer UDP packet may be set to zero. However, IPv6 has no
checksum in the IP header, and RFC 2460 [RFC2460] explicitly states
that IPv6 receivers MUST discard UDP packets with a zero checksum.
So, while sending a UDP datagram 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
endpoints to use a zero UDP checksum under constrained situations
(primarily for IPv6 tunnel transports that carry checksum-protected
packets), following the applicability statements and constraints in
When reading this document, the advice in "Unicast UDP Usage
Guidelines for Application Designers" [RFC5405] is applicable. It
discusses both UDP tunnels (Section 3.1.3) and the usage of checksums
While the origin of this specification is the problem raised by the
draft titled "Automatic Multicast Tunnels", also known as "AMT"
[AMT], we expect it to have wide applicability. Since the first
draft of this RFC was written, the need for an efficient UDP
tunneling mechanism has increased. Other IETF Working Groups,
notably LISP [RFC6830] 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 the future to other tunnel
protocols specified by these and other IETF Working Groups.
This document discusses only IPv6, because the problem being
addressed does not exist for IPv4. Therefore, all references to "IP"
should be understood as references 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
When using tunnel protocols based on UDP, there can be both a benefit
and a cost to computing and checking the UDP checksum of the outer
(encapsulating) UDP transport header. In certain cases, where
reducing the forwarding cost is important, the cost of the
computation may outweigh the benefit of the checksum. This document
provides an update for usage of the UDP checksum with IPv6. The
update is specified for use by a tunnel protocol that transports
packets that are themselves protected by a checksum.
"Applicability Statement for the Use of IPv6 UDP Datagrams with Zero
Checksums" [RFC6936] describes issues related to allowing UDP over
IPv6 to have a valid zero UDP checksum and is the starting point for
this discussion. Sections 4 and 5 of [RFC6936], respectively,
identify node implementation and usage requirements for datagrams
sent and received with a zero UDP checksum. These sections introduce
constraints on the usage of a zero checksum for UDP over IPv6. The
remainder of this section analyzes the use of general tunnels and
explains the motivations for why tunnel protocols are being permitted
to use the method described in this update. It also discusses issues
4.1. Analysis of Corruption in Tunnel Context
This section analyzes the impact of the different corruption modes in
the context of a tunnel protocol. It specifies what needs to be
considered by the designer and user of a tunnel protocol for the
protocol to be robust. It also summarizes why use of a zero UDP
checksum is thought to be safe for deployment.
o Context (i.e., tunneling state) should be established by
exchanging application Protocol Data Units (PDUs) carried in
checksummed UDP datagrams or by using other protocols that provide
integrity protection against corruption. These control packets
should also carry any negotiation required to enable the tunnel
endpoint to accept UDP datagrams with a zero checksum and identify
the set of ports that are used. It is important that the control
traffic is robust against corruption, because undetected errors
can lead to long-lived and significant failures that may affect
much more than the single packet that was corrupted.
o Keepalive datagrams with a zero UDP checksum should be sent to
validate the network path, because the path between tunnel
endpoints can change, and therefore, the set of middleboxes along
the path may change during the life of an association. Paths with
middleboxes that drop datagrams with a zero UDP checksum will drop
these keepalives. To enable the tunnel endpoints to discover and
react to this behavior in a timely way, the keepalive traffic
should include datagrams with a non-zero checksum and datagrams
with a zero checksum.
o Receivers should attempt to detect corruption of the address
information in an encapsulating packet. A robust tunnel protocol
should track tunnel context based on the 5-tuple (tunneled
protocol number, IPv6 source address, IPv6 destination address,
UDP source port, UDP destination port). A corrupted datagram that
arrives at a destination may be filtered based on this check.
* If the datagram header matches the 5-tuple and the node has
enabled the zero checksum for this port, the payload is matched
to the wrong context. The tunneled packet will then be
decapsulated and forwarded by the tunnel egress.
* If a corrupted datagram matches a different 5-tuple and the
node has enabled zero checksum for the port, the datagram
payload is matched to the wrong context and may be processed by
the wrong tunnel protocol, provided that it also passes the
verification of that protocol.
* If a corrupted datagram matches a 5-tuple and node has not
enabled the zero checksum for this port, the datagram will be
When only the source information is corrupted, the datagram could
arrive at the intended applications or protocol, which will
process the datagram and try to match it against an existing
tunnel context. The likelihood that a corrupted packet enters a
valid context is reduced when the protocol restricts processing to
only the source addresses with established contexts. When both
source and destination fields are corrupted, this also decreases
the likelihood of matching a context. However, the exception is
when errors replace one packet header with another, so both
packets could be tunneled, and therefore the corrupted packet
could match a previously defined context.
o Receivers should attempt to detect corruption of source-fragmented
encapsulating packets. A tunnel protocol may reassemble fragments
associated with the wrong context at the right tunnel endpoint, 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
(although [RFC6936] 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
o Compared to other applications, tunnel protocols using UDP have
some advantages that reduce the risk for a corrupted tunnel packet
reaching a destination that will receive it. These advantages
result from processing by the network of the inner (tunneled)
packet after it is forwarded from the tunnel egress using a wrong
* A tunneled packet may be forwarded to the wrong address domain,
for example, to a private address domain where the inner
packet's address is not routable, or it may fail a source
address check, such as Unicast Reverse Path Forwarding
[RFC2827], resulting in the packet being dropped.
* The destination address of a tunneled packet may not be
reachable at all from the delivered domain. An example is an
Ethernet frame where the destination MAC address is not present
on the LAN segment that was reached.
* The type of the tunneled packet may prevent delivery. For
example, an attempt to interpret an IP packet payload as an
Ethernet frame would likely to result in the packet being
dropped as invalid.
* The tunneled packet checksum or integrity mechanism may detect
corruption of the inner packet caused at the same time as
corruption to the outer packet header. The resulting packet
would likely be dropped as invalid.
Each of these checks significantly reduces the likelihood that a
corrupted inner tunneled packet is finally delivered to a protocol
listener that can be affected by the packet. While the methods do
not guarantee correctness, they can reduce the risks of relaxing the
UDP checksum requirement for a tunnel application using IPv6.
4.2. Limitation to Tunnel Protocols
This document describes the applicability of using a zero UDP
checksum to support tunnel protocols. There are good motivations
behind this, and the arguments are provided here.
o Tunnels carry inner packets that have their own semantics, which
may make any corruption less likely to reach the indicated
destination and be accepted as a valid packet. This is true for
IP packets with the addition of verification that can be made by
the tunnel protocol, the network processing of the inner packet
headers as discussed above, and verification of the inner packet
checksums. Non-IP inner packets are likely to be subject to
similar effects that may reduce the likelihood of a misdelivered
packet being delivered to a protocol listener that can be affected
by the packet.
o Protocols that directly consume the payload must have sufficient
robustness against misdelivered packets (from any context),
including ones that are corrupted in tunnels or corrupted by other
usage of the zero checksum. This will require an integrity
mechanism. Using a standard UDP checksum reduces the
computational load in the receiver that is necessary to verify
o The design for stateful protocols or protocols where corruption
causes cascade effects requires extra care. In tunnel usage, each
encapsulating packet provides no functions other than a transport
from tunnel ingress to tunnel egress. A corruption will commonly
affect only the single tunneled packet, not the established
protocol state. One common effect is that the inner packet flow
will see only a corruption and a misdelivery of the outer packet
as a lost packet.
o Some non-tunnel protocols operate with general servers that do not
know the source from which they will receive a packet. In such
applications, a zero UDP checksum is unsuitable, because it is
necessary to provide the first level of verification that the
packet was intended for the receiving server. A verification
prevents the server from processing the datagram payload; without
this, the server may spend significant resources processing the
packet, including sending replies or error messages.
Tunnel protocols that encapsulate IP will generally be safe for
deployment, because all IPv4 and IPv6 packets include at least one
checksum at either the network or transport layer. The network
delivery of the inner packet will then further reduce the effects of
corruption. Tunnel protocols carrying non-IP packets may offer
equivalent protection when the non-IP networks reduce the risk of
misdelivery to applications. However, further analysis is necessary
to understand the implications of misdelivery of corrupted packets
for each non-IP protocol. The analysis above suggests that non-
tunnel protocols can be expected to have significantly more cases
where a zero checksum would result in misdelivery or negative side
One unfortunate side effect of increased use of a zero checksum is
that it also increases the likelihood of acceptance when a datagram
with a zero UDP checksum is misdelivered. This requires all tunnel
protocols using this method to be designed to be robust in the face
"Applicability Statement for the Use of IPv6 UDP Datagrams with Zero
Checksums" [RFC6936] specifies requirements for middleboxes and
tunnels that need to traverse middleboxes. Tunnel protocols
intending to use a zero UDP checksum need to ensure that they have
defined a method for handling cases when a middlebox prevents the
path between the tunnel ingress and egress from supporting
transmission of datagrams with a zero UDP checksum. This is
especially important as middleboxes that conform to RFC 2460 are
likely to discard datagrams with a zero UDP checksum.
5. The Zero UDP Checksum Update
This specification updates IPv6 to allow a zero UDP checksum in the
outer encapsulating datagram of a tunnel protocol. UDP endpoints
that implement this update MUST follow the node requirements in
"Applicability Statement for the Use of IPv6 UDP Datagrams with Zero
The following text in [RFC2460], Section 8.1, fourth bullet should be
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 text should be replaced by:
An IPv6 node associates a mode with each used UDP port (for
sending and/or receiving packets).
Whenever originating a UDP packet for a port in the default mode,
an IPv6 node MUST compute a UDP checksum over the packet and the
pseudo-header, and, if that computation yields a result of zero,
the checksum MUST be changed to hex FFFF for placement in the UDP
header, as specified in [RFC2460]. IPv6 receivers MUST by default
discard UDP packets containing a zero checksum and SHOULD log the
As an alternative, certain protocols that use UDP as a tunnel
encapsulation MAY enable zero-checksum mode for a specific port
(or set of ports) for sending and/or receiving. Any node
implementing zero-checksum mode MUST follow the node requirements
specified in Section 4 of "Applicability Statement for the use of
IPv6 UDP Datagrams with Zero Checksums" [RFC6936].
Any protocol that enables zero-checksum mode for a specific port
or ports MUST follow the usage requirements specified in Section 5
of "Applicability Statement for the Use of IPv6 UDP Datagrams with
Zero Checksums" [RFC6936].
Middleboxes supporting IPv6 MUST follow requirements 9, 10, and 11
of the usage requirements specified in Section 5 of "Applicability
Statement for the Use of IPv6 UDP Datagrams with Zero Checksums"
6. Additional Observations
This update was motivated by the existence of a number of protocols
being developed in the IETF that are expected to benefit from the
change. The following observations are made:
o An empirically based analysis of the probabilities of packet
corruption (with or without checksums) has not, to our knowledge,
been conducted since about 2000. At the time of publication, it
is now 2013. We strongly suggest that a new empirical study be
performed, along with extensive analysis of the corruption
probabilities of the IPv6 header. This could potentially allow
revising the recommendations in this document.
o A key motivation for the increase in use of UDP in tunneling is a
lack of protocol support in middleboxes. Specifically, new
protocols, such as LISP [RFC6830], may prefer to use UDP tunnels
to traverse an end-to-end path successfully and avoid having their
packets dropped by middleboxes. If middleboxes were updated to
support UDP-Lite [RFC3828], UDP-Lite would provide better
protection than offered by this update. UDP-Lite may be suited to
a variety of applications and would be expected to be preferred
over this method for many tunnel protocols.
o 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 misdelivered packets.
7. Security Considerations
Less work is required to generate an attack using a zero UDP checksum
than one using a standard full UDP checksum. However, this does not
lead to significant new vulnerabilities, because checksums are not a
security measure and can be easily generated by any attacker.
In general, any user of zero UDP checksums should apply the checks
and context verification that are possible to minimize the risk of
unintended traffic to reach a particular context. This will,
however, not protect against an intentional attack that creates
packets with the correct information. Source address validation can
help prevent injection of traffic into contexts by an attacker.
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. This processing overhead can
become a security risk for designs that can handle a significantly
larger number of packets with zero UDP checksums compared to
datagrams with a non-zero checksum, such as a tunnel egress. An
attacker could attempt to inject non-zero checksummed UDP packets
into a tunnel forwarding zero checksum UDP packets and cause overload
in the processing of the non-zero checksums, e.g., if this happens in
a router's slow path. Therefore, protection mechanisms should be
employed when this threat exists. Protection may include source-
address filtering to prevent an attacker from injecting traffic, as
well as throttling the amount of non-zero checksum traffic. The
latter may impact the functioning of the tunnel protocol.
We would like to thank Brian Haberman, Dan Wing, Joel Halpern, David
Waltermire, J.W. Atwood, Peter Yee, Joe Touch, and the IESG of 2012
for discussions and reviews. Gorry Fairhurst has been very diligent
in reviewing and helping to ensure alignment between this document
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums",
RFC 6936, April 2013.
9.2. Informative References
[AMT] Bumgardner, G., "Automatic Multicast Tunneling", Work
in Progress, June 2012.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
G. Fairhurst, "The Lightweight User Datagram Protocol
(UDP-Lite)", RFC 3828, July 2004.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405,
[RFC5619] Yamamoto, S., Williams, C., Yokota, H., and F. Parent,
"Softwire Security Analysis and Requirements", RFC 5619,
[RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
Locator/ID Separation Protocol (LISP)", RFC 6830,
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