One document matched: draft-ietf-6man-udpchecksums-06.xml
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<rfc category="std" docName="draft-ietf-6man-udpchecksums-06"
ipr="trust200902" updates="2460">
<front>
<title abbrev="udp-checksum">IPv6 and UDP Checksums for Tunneled
Packets</title>
<author fullname="Marshall Eubanks" initials="M." surname="Eubanks">
<organization>AmericaFree.TV LLC</organization>
<address>
<postal>
<street>P.O. Box 141</street>
<city>Clifton</city>
<region>Virginia</region>
<code>20124</code>
<country>USA</country>
</postal>
<phone>+1-703-501-4376</phone>
<facsimile/>
<email>marshall.eubanks@gmail.com</email>
</address>
</author>
<author fullname="P.F. Chimento" initials="P.F." surname="Chimento">
<organization>Johns Hopkins University Applied Physics
Laboratory</organization>
<address>
<postal>
<street>11100 Johns Hopkins Road</street>
<city>Laurel</city>
<region>MD</region>
<code>20723</code>
<country>USA</country>
</postal>
<phone>+1-443-778-1743</phone>
<email>Philip.Chimento@jhuapl.edu</email>
</address>
</author>
<author fullname="Magnus Westerlund" initials="M." surname="Westerlund">
<organization>Ericsson</organization>
<address>
<postal>
<street>Farogatan 6</street>
<city>SE-164 80 Kista</city>
<country>Sweden</country>
</postal>
<phone>+46 10 714 82 87</phone>
<email>magnus.westerlund@ericsson.com</email>
</address>
</author>
<date day="11" month="December" year="2012"/>
<abstract>
<t>This document provides an update of the Internet Protocol version 6
(IPv6) specification (RFC2460) to improve the performance 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. The specification describes how
the IPv6 UDP checksum requirement can be relaxed for the situation where
the encapsulated packet itself contains a checksum. The limitations and
risks of this approach are described, and restrictions specified on the
use of the method.</t>
</abstract>
</front>
<middle>
<section anchor="Intro" title="Introduction">
<t>This work constitutes an update of the <xref
target="RFC2460">Internet Protocol Version 6 (IPv6)
Specification</xref>, 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,
middleboxes and 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 that perform filtering that is a fact of life on
the current Internet.</t>
<t>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 tunneling protocol,
especially if the inner packet(s) are already protected by a checksum.
In IPv4, there is a checksum over the IP packet header, and the checksum
on the outer UDP packet may be set to zero. However in IPv6 there is no
checksum in the IP header and RFC 2460 <xref target="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 IPv6 tunnel transports that carry
checksum-protected packets), following the applicability statements and
constraints in <xref target="I-D.ietf-6man-udpzero"/>.</t>
<t><xref target="RFC5405">Unicast UDP Usage Guidelines for Application
Designers</xref> should be consulted when reading this specification. It
discusses both UDP tunnels (Section 3.1.3) and the usage of checksums
(Section 3.4).</t>
<t>While the origin of this specification is the problem raised by the
draft titled "Automatic IP Multicast Without Explicit Tunnels", also
known as "AMT," <xref target="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 <xref
target="I-D.ietf-lisp">LISP</xref> and <xref
target="RFC5619">Softwires</xref> 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.</t>
</section>
<section anchor="term" title="Some Terminology">
<t>This document discusses only IPv6, since this problem does not exist
for IPv4. Therefore all reference to 'IP' should be understood as a
reference to IPv6.</t>
<t>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.</t>
<section title="Requirements Language">
<t>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 <xref
target="RFC2119">RFC 2119</xref>.</t>
</section>
</section>
<section anchor="Prob" title="Problem Statement">
<t>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, such as for nodes that perform the
checksum in software, where the cost may outweigh the benefit. 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.</t>
</section>
<section anchor="alts" title="Discussion">
<t><xref target="I-D.ietf-6man-udpzero">Applicability Statement for the
use of IPv6 UDP Datagrams with Zero Checksums</xref> describes issues
related to allowing UDP over IPv6 to have a valid zero UDP checksum and
is the starting point for this discussion. Section 4 and 5 of <xref
target="I-D.ietf-6man-udpzero"/>, respectively identify node
implementation and usage requirements for datagrams sent and received
with a zero UDP checksum. These introduce constraints on the usage of a
zero checksum for UDP over IPv6. The remainder of this section analyses
the use of general tunnels and motivates why tunnel protocols are being
permitted to use the method described in this update. Issues with
middleboxes are also discussed.</t>
<section title="Analysis of Corruption in Tunnel Context">
<t>This section analyzes the impact of the different corruption modes
in the context of a tunnel protocol. It indicates what needs to be
considered by the designer and user of a tunnel protocol to be robust.
It also summarizes why use of a zero UDP checksum is thought safe for
deployment.</t>
<t><list style="symbols">
<t>Context (i.e. tunneling state) should be established by
exchanging application Protocol Data Units (PDUs) carried in
checksummed UDP datagrams or by other protocols with 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 affect not only the
single packet that was corrupted.</t>
<t>Keep-alive 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 keep-alives. To enable the tunnel endpoints to discover and
react to this behavior in a timely way, the keep-alive traffic
should include datagrams with both a non-zero checksum and ones
with a zero checksum.</t>
<t>Corruption of the address information in an encapsulating
packets, i.e. IPv6 source address, destination address and/or the
UDP source port, and destination port fields. A robust tunnel
protocol should track tunnel context based on the 5-tuple, i.e.
the protocol and both the address and port for both the source and
destination. A corrupted datagram that arrives at a destination
may be filtered based on this check. <list style="symbols">
<t>If the datagram header matches the 5-tuple with a zero
checksum enabled, the payload is matched to the wrong context.
The tunneled packet will then be decapsulated and forwarded by
the tunnel egress.</t>
<t>If a corrupted datagram matches a different 5-tuple with a
zero checksum enabled, the payload is matched to the wrong
context, and may be processed by the wrong tunneling protocol,
if it passes the verification of that protocol.</t>
<t>If a corrupted datagram matches a 5-tuple that does not
have a zero checksum enabled, it will be discarded.</t>
</list>When only the source information is corrupted, the
datagram could arrive at the intended applications/protocol which
will process it and try to match it against an existing tunnel
context. If the protocol restricts processing to only the source
addresses with established contexts the likelihood that a
corrupted packet enters a valid context is reduced. When both
source and destination fields are corrupted, this increases the
likelihood of failing to match a context, with the exception of
errors replacing one packet header with another one. In this case
it is possible that both are tunnels and thus the corrupted packet
can match a previously defined context.</t>
<t>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
<xref target="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).</t>
<t>Tunnel protocols using UDP have some advantages that reduce the
risk for a corrupted tunnel packet reaching a destination that
will receive it, compared to other applications. This results from
processing by the network of the inner (tunneled) packet after
being forwarded from the tunnel egress using a wrong context:<list
style="symbols">
<t>A tunneled packet may be forwarded to the wrong address
domain, for example a private address domain where the inner
packet's address is not routable, or may fail a source address
check, such as <xref target="RFC2827">Unicast Reverse Path
Forwarding</xref>, resulting in the packet being dropped.</t>
<t>The destination address of a tunneled packet may not at all
be reachable from the delivered domain. For example an
Ethernet packet where the destination MAC address is not
present on the LAN segment that was reached.</t>
<t>The type of the tunneled packet may prevent delivery for
example if an IP packet payload was attempted to be
interpreted as an Ethernet packet. This is likely to result in
the packet being dropped as invalid.</t>
<t>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.</t>
</list></t>
</list>These different examples each help to significantly reduce
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
risk of relaxing the UDP checksum requirement for a tunnel application
using IPv6.</t>
</section>
<section title="Limitation to Tunnel Protocols">
<t>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.<list style="symbols">
<t>Tunnels carry inner packets that have their own semantics that
makes 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 networks' processing of the inner packet
headers as discussed above, and verification of the inner packet
checksums. Also non-IP inner packets are likely to be subject to
similar effects that reduce the likelihood that an mis-delivered
packet are delivered.</t>
<t>Protocols that directly consume the payload must have
sufficient robustness against mis-delivered packets from any
context, including the ones that are corrupted in tunnels and any
other usage of the zero checksum. This will require an integrity
mechanism. Using a standard UDP checksum reduces the computational
load in the receiver to verify this mechanism.</t>
<t>Stateful protocols or protocols where corruption causes cascade
effects need to be extra careful. In tunnel usage each
encapsulating packet provides only a transport mechanism from
tunnel ingress to tunnel egress. A corruption will commonly only
effect the single packet, not established protocol state. One
common effect is that the inner packet flow will only see a
corruption and mis-delivery of the outer packet as a lost
packet.</t>
<t>Some non-tunnel protocols operate with general servers that do
not know from where they will receive a packet. In such
applications, the usage of a zero UDP checksum is especially
unsuitable because there is a need to provide the first level of
verification that the packet was intended for the server. This
verification prevents the server from processing the datagram
payload and spend any significant amount of resources on it,
including sending replies or error messages.</t>
</list></t>
<t>Tunnel protocols encapsulating IP this will generally be safe,
since all IPv4 and IPv6 packets include at least one checksum at
either the network or transport layer and the network delivery of the
inner packet will further reduce the effects of corruption. Tunnel
protocols carrying non-IP packets may provide equivalent protection
due to the non-IP networks reducing the risk of delivery to
applications. However, there is need for further analysis to
understand the implications of mis-delievery of corrupted packets for
that 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 mis-delivery or negative
side-effects.</t>
<t>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 mis-delivered. This requires all tunnel
protocols using this method to be designed to be robust to
mis-delivery.</t>
</section>
<section title="Middleboxes">
<t><xref target="I-D.ietf-6man-udpzero">Applicability Statement for
the use of IPv6 UDP Datagrams with Zero Checksums</xref> notes that
middlebox devices that conform to RFC 2460 will discard datagrams with
a zero UDP checksum and should log this as an error. Thus tunnel
protocols intending to use a zero UDP checksum needs 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.</t>
</section>
</section>
<section anchor="rec" title="The Zero-Checksum Update">
<t>This specification updates IPv6 to allow a zero UDP checksum in the
outer encapsulating datagram of a tunneling protocol. UDP endpoints that
implement this update MUST follow the node requirements <xref
target="I-D.ietf-6man-udpzero">"Applicability Statement for the use of
IPv6 UDP Datagrams with Zero Checksums"</xref>.</t>
<t>The following text in <xref target="RFC2460"/> Section 8.1, 4th
bullet should be deleted:</t>
<t>"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."</t>
<t>This text should be replaced by:<list style="empty">
<t>Whenever originating a UDP packet 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, it
MUST be changed to hex FFFF for placement in the UDP header. IPv6
receivers MUST by default discard UDP packets containing a zero
checksum, and SHOULD log the error. As an alternative usage for some
protocols, such as protocols that use UDP as a tunnel encapsulation,
MAY enable the zero-checksum mode for specific sets of ports. Any
node implementing the zero-checksum mode MUST follow the node
requirements specified in Section 4 of <xref
target="I-D.ietf-6man-udpzero">Applicability Statement for the use
of IPv6 UDP Datagrams with Zero Checksums</xref>.</t>
<t>Any protocol using the zero-checksum mode MUST follow the usage
requirements specified in Section 5 of <xref
target="I-D.ietf-6man-udpzero">Applicability Statement for the use
of IPv6 UDP Datagrams with Zero Checksums</xref>.</t>
<t>Middleboxes supporting IPv6 MUST follow the requirements 9, 10
and 11 of the usage requirements specified in Section 5 of <xref
target="I-D.ietf-6man-udpzero">Applicability Statement for the use
of IPv6 UDP Datagrams with Zero Checksums</xref>.</t>
</list></t>
</section>
<section title="Additional Observations">
<t>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: <list style="symbols">
<t>An empirically-based analysis of the probabilities of packet
corruptions (with or without checksums) has not (to our knowledge)
been conducted since about 2000. At the time of publication, it is
now 2012. We strongly suggest a new empirical study, along with an
extensive analysis of the corruption probabilities of the IPv6
header.</t>
<t>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 <xref target="I-D.ietf-lisp"/>, 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 <xref target="RFC3828"/>, this
would provide better protection than offered by this update. This
may be suited to a variety of applications and would be expected to
be preferred over this method for many tunnel protocols.</t>
<t>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.</t>
</list></t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>This document makes no request of IANA.</t>
<t>Note to RFC Editor: this section may be removed on publication as an
RFC.</t>
</section>
<section anchor="Security" title="Security Considerations">
<t>Less work is required 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. Properly
configured tunnels should check the validity of the inner packet and
perform security checks.</t>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>We would like to thank Brian Haberman, Dan Wing, Joel Halpern and the
IESG of 2012 for discussions and reviews. Gorry Fairhurst has been very
diligent in reviewing and help ensuring alignment between this document
and <xref target="I-D.ietf-6man-udpzero"/>.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include='reference.RFC.2119'?>
<?rfc include="reference.RFC.2460"?>
<?rfc include="reference.I-D.ietf-6man-udpzero"?>
</references>
<references title="Informative References">
<?rfc include="reference.I-D.ietf-mboned-auto-multicast"?>
<?rfc include="reference.I-D.ietf-lisp"?>
<?rfc include='reference.RFC.2827'?>
<?rfc include='reference.RFC.3828'?>
<?rfc include='reference.RFC.5405'?>
<?rfc include='reference.RFC.5619'?>
</references>
</back>
</rfc>
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