Internet Engineering Task Force (IETF) G. Fairhurst
Request for Comments: 6936 University of Aberdeen
Category: Standards Track M. Westerlund
ISSN: 2070-1721 Ericsson
April 2013
Applicability Statement for the Use of IPv6 UDP Datagrams
with Zero Checksums
Abstract
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 it examines the role of the IPv6 UDP transport
checksum. The document also identifies issues and constraints for
deployment on network paths that include middleboxes. An appendix
presents a summary of the trade-offs that were considered in
evaluating the safety of the update to RFC 2460 that changes the use
of the UDP checksum with IPv6.
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
http://www.rfc-editor.org/info/rfc6936.
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Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Document Structure . . . . . . . . . . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Use of UDP Tunnels . . . . . . . . . . . . . . . . . . . . 6
1.3.1. Motivation for New Approaches . . . . . . . . . . . . 6
1.3.2. Reducing Forwarding Costs . . . . . . . . . . . . . . 6
1.3.3. Need to Inspect the Entire Packet . . . . . . . . . . 7
1.3.4. Interactions with Middleboxes . . . . . . . . . . . . 7
1.3.5. Support for Load Balancing . . . . . . . . . . . . . . 8
2. Standards-Track Transports . . . . . . . . . . . . . . . . . . 9
2.1. UDP with Standard Checksum . . . . . . . . . . . . . . . . 9
2.2. UDP-Lite . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1. Using UDP-Lite as a Tunnel Encapsulation . . . . . . . 10
2.3. General Tunnel Encapsulations . . . . . . . . . . . . . . 10
2.4. Relationship of Zero UDP Checksum to UDP-Lite and UDP
with Checksum . . . . . . . . . . . . . . . . . . . . . . 11
3. Issues Requiring Consideration . . . . . . . . . . . . . . . . 12
3.1. Effect of Packet Modification in the Network . . . . . . . 13
3.1.1. Corruption of the Destination IP Address Field . . . . 14
3.1.2. Corruption of the Source IP Address Field . . . . . . 15
3.1.3. Corruption of Port Information . . . . . . . . . . . . 16
3.1.4. Delivery to an Unexpected Port . . . . . . . . . . . . 16
3.1.5. Corruption of Fragmentation Information . . . . . . . 18
3.2. Where Packet Corruption Occurs . . . . . . . . . . . . . . 20
3.3. Validating the Network Path . . . . . . . . . . . . . . . 20
3.4. Applicability of the Zero UDP Checksum Method . . . . . . 21
3.5. Impact on Non-Supporting Devices or Applications . . . . . 22
4. Constraints on Implementation of IPv6 Nodes Supporting
Zero Checksum . . . . . . . . . . . . . . . . . . . . . . . . 23
5. Requirements on Usage of the Zero UDP Checksum . . . . . . . . 24
6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7. Security Considerations . . . . . . . . . . . . . . . . . . . 28
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
9.1. Normative References . . . . . . . . . . . . . . . . . . . 30
9.2. Informative References . . . . . . . . . . . . . . . . . . 30
Appendix A. Evaluation of Proposal to Update RFC 2460 to
Support Zero Checksum . . . . . . . . . . . . . . . . 33
A.1. Alternatives to the Standard Checksum . . . . . . . . . . 33
A.2. Comparison of Alternative Methods . . . . . . . . . . . . 34
A.2.1. Middlebox Traversal . . . . . . . . . . . . . . . . . 34
A.2.2. Load Balancing . . . . . . . . . . . . . . . . . . . . 35
A.2.3. Ingress and Egress Performance Implications . . . . . 36
A.2.4. Deployability . . . . . . . . . . . . . . . . . . . . 36
A.2.5. Corruption Detection Strength . . . . . . . . . . . . 37
A.2.6. Comparison Summary . . . . . . . . . . . . . . . . . . 37
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1. Introduction
The User Datagram Protocol (UDP) [RFC0768] transport is defined for
IPv4 [RFC0791], and it 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 applications do
need to provide many important transport functions on top of UDP.
The UDP usage guidelines [RFC5405] provide 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 inclusion of
the pseudo-header in the checksum computation provides a statistical
check that datagrams have been delivered to the intended IPv6
destination node. Algorithms for checksum computation are described
in [RFC1071].
The inability to use an IPv6 datagram with a zero UDP checksum has
been found to be 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, whether an endpoint is the intended user or an innocent
bystander (i.e., when a packet is received by a different endpoint to
that intended).
This document identifies a set of issues that must be considered and
mitigated to enable safe deployment of IPv6 applications that use a
zero UDP checksum. The appendix compares the strengths and
weaknesses of a number of proposed solutions. The comparison of
methods provided in this document is also expected to be useful when
considering applications that have different goals from the ones
whose needs led to the writing of this document, especially
applications that can use existing standardized transport protocols.
The analysis concludes that using a zero UDP checksum is the best
method of the proposed alternatives to meet the goals of 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 decrease
as updates are provided in middleboxes that support the zero UDP
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checksum for IPv6. Therefore, in this document, we derive a set of
constraints required to ensure safe deployment of a zero UDP
checksum.
Finally, the document identifies some issues that require future
consideration and possibly additional research.
1.1. Document Structure
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 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 the remaining
issues that need future work.
Appendix A evaluates the set of proposals to update the UDP transport
behavior and other alternatives intended to improve support for
tunnel protocols. It concludes by assessing the trade-offs of the
various methods and by identifying advantages and disadvantages for
each method.
1.2. Terminology
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].
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1.3. Use of UDP Tunnels
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
they can be used to create virtual (private) networks.
1.3.1. Motivation for New Approaches
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] and Locator/Identifier Separation
Protocol (LISP) [RFC6830]). These protocols provided several
motivations to update IPv6 UDP checksum processing so that it would
benefit from simpler checksum processing, including:
o Reducing forwarding costs, motivated by redundancy present in the
encapsulated packet header, because in tunnel encapsulations,
payload integrity and length verification may be provided by
higher-layer encapsulations (often using the IPv4, UDP, UDP-Lite
[RFC3828], or TCP checksums [RFC0793]).
o Eliminating the need to access the entire packet when a tunnel
endpoint forwards the packet.
o Enhancing the ability to traverse and function with middleboxes.
o A desire to use the port number space to enable load sharing.
1.3.2. Reducing Forwarding Costs
It is a common requirement to terminate a large number of tunnels on
a single router or host. The processing cost per tunnel includes
both state (memory requirements) and per-packet processing at the
tunnel ingress and egress.
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Automatic IP Multicast Tunneling, known as AMT [AMT], currently
specifies UDP as the transport protocol for packets carrying tunneled
IP multicast packets. The current specification for AMT states that
the UDP checksum in the outer packet header should be zero (see
Section 6.6 of [AMT]). That section 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; therefore, a different
pseudo-header must be built to form the header for each tunnel egress
that receives replicated multicast packets.
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. For example, there are implementations
that have optimized 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, for certain classes of tunnel
endpoints, this off-loading is not available and is unlikely to
become available in the near future.
1.3.3. Need to Inspect the Entire Packet
The currently deployed hardware in many routers uses a fast-path
processing that provides only the first n bytes of a packet to the
forwarding engine, where typically n <= 128.
When this design is used to support a tunnel ingress and egress, it
prevents fast processing of a transport checksum over an entire
(large) packet. Hence, the currently defined IPv6 UDP checksum is
poorly suited for use within a router that is unable to access the
entire packet and does not provide checksum off-loading. Thus,
enabling checksum calculation over the complete packet can impact
router design, performance, energy consumption, and cost.
1.3.4. Interactions with Middleboxes
Many paths in the Internet include one or more middleboxes of various
types. Large classes of middleboxes will handle zero UDP checksum
packets, but do not support UDP-Lite or the other investigated
proposals. These middleboxes include load balancers (see
Section 1.3.5) including equal-cost multipath (ECMP) routing, traffic
classifiers, and other functions that reads some fields in the UDP
headers but does not validate the UDP checksum.
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There are also middleboxes that either validate or modify the UDP
checksum. The two most common classes are firewalls and NATs. In
IPv4, UDP encapsulation may be desirable for NAT traversal, because
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, it hopefully will 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 becomes 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 becomes a very
small percentage of the deployed middleboxes.
1.3.5. Support for Load Balancing
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 utilize the IPv6 flow
label [RFC6437] as a basis for entropy for load balancing. This
would have the desirable effect of freeing IPv6 load-balancing
devices from the need to assume semantics for the use of the
transport port field, and also, it works for all types of transport
protocols.
This use of the Flow Label for load balancing is consistent with the
intended use, although further clarity was needed to ensure the field
can be consistently used for this purpose. Therefore, an updated
IPv6 flow label [RFC6437] and ECMP routing [RFC6438] usage were
specified. 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,
the flow label value could be copied to the outer packet header.
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However, many current endpoints 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 has been clarified,
there will be a transition time before a significant proportion of
endpoints start to assign a good quality flow label to the flows that
they originate. The use of load balancing using the transport header
fields would continue until any widespread deployment is finally
achieved.
2. Standards-Track Transports
The IETF has defined a set of transport protocols that may be
applicable for tunnels with IPv6. There is also a set of network-
layer encapsulation tunnels, such as IP-in-IP and Generic Routing
Encapsulation (GRE). These solutions, which are already
standardized, are discussed first, before discussing the issues,
because they provide background for the description of the issues and
allow some comparison with existing issues.
2.1. UDP with Standard Checksum
UDP [RFC0768] with standard checksum behavior, as defined in RFC
2460, has already been discussed. UDP usage guidelines are provided
in [RFC5405].
2.2. UDP-Lite
UDP-Lite [RFC3828] offers an alternate transport to UDP and is
specified as a proposed standard, RFC 3828. A MIB is defined in
[RFC5097], and unicast usage guidelines are defined in [RFC5405].
There has been at least one open-source implementation of UDP-Lite as
a part of the Linux kernel since version 2.6.20.
UDP-Lite provides a checksum with an option for 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
or corruption in the insensitive part will not cause the datagram to
be discarded by the transport layer at the receiving endpoint. A
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minor side effect of using UDP-Lite is that it 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 Cyclic Redundancy Check (CRC) that
covers the sensitive part.
2.2.1. Using UDP-Lite as a Tunnel Encapsulation
Tunnel encapsulations, such as Control And Provisioning of Wireless
Access Points (CAPWAP) [RFC5415], can use UDP-Lite, because it
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 a low complexity
for the checksumming operation. 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 verify only delivery of the
tunneled payload and uses a calculated checksum for control
information.
Currently, support for middlebox traversal using UDP-Lite is poor,
because UDP-Lite uses a different IPv6 network-layer Next Header
value than that used for UDP; therefore, 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 as UDP-Lite has achieved better
support in middleboxes and endpoints.
2.3. General Tunnel Encapsulations
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, because tunnel
encapsulations are typically layered directly over the Internet layer
(identified by the upper layer type in the IPv6 Next Header field)
and because they are not used as endpoint transport protocols. There
is little chance of confusing a tunnel-encapsulated packet with other
application data. Such confusion could result in corruption of
application state or data.
From an end-to-end perspective, the principal difference between an
endpoint transport and a tunnel encapsulation is the value of the
network-layer Next Header field. In the former, it identifies a
transport protocol that supports endpoint applications. In the
latter, it identifies a tunnel protocol egress. This separation of
function reduces the probability that corruption of a tunneled packet
could result in the packet being erroneously delivered to an
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application. Specifically, packets are delivered only 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.
2.4. Relationship of Zero UDP Checksum to UDP-Lite and UDP with
Checksum
The operation of IPv6 with UDP with a zero checksum is not the same
as IPv4 with UDP with a zero checksum. Protocol designers should not
be fooled into thinking that the two are the same. The requirements
below list a set of additional considerations for IPv6.
Where possible, existing general tunnel encapsulations, such as GRE
and IP-in-IP, should be used. This section assumes that such
existing tunnel encapsulations do not offer the functionally required
to satisfy the protocol designer's goals. This section considers the
standardized alternative solutions rather than the full set of ideas
evaluated in Appendix A. The alternatives to UDP with a zero
checksum are UDP with a (calculated) checksum and UDP-Lite.
UDP with a checksum has the advantage of close to universal support
in both endpoints and middleboxes. It also provides statistical
verification of delivery to the intended destination (address and
port). However, some classes of device have limited support for
calculation of a checksum that covers a full datagram. For these
devices, this limited support can incur significant processing costs
(e.g., requiring processing in the router's slow path) and hence can
reduce capacity or fail to function.
UDP-Lite has the advantage of using a checksum that can be calculated
only over the pseudo-header and the UDP header. This provides a
statistical verification of delivery to the intended destination
(address and port). The checksum can be calculated without access to
the datagram payload, requiring access only to the part that is to be
protected. A drawback is that UDP-Lite currently has limited support
in both endpoints (i.e., is not supported on all operating system
platforms) and middleboxes (which must support the UDP-Lite header
type). Therefore, using a path verification method is recommended.
IPv6 and UDP with a zero checksum can also be used by nodes that do
not permit calculation of a payload checksum. Many existing classes
of middleboxes do not verify or change the transport checksum. For
these middleboxes, IPv6 with a zero UDP checksum is expected to
function where UDP-Lite would not. However, support for the zero UDP
checksum in middleboxes that do change or verify the checksum is
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currently limited, and this may result in datagrams with a zero UDP
checksum being discarded. Therefore, using a path verification
method is recommended.
For some sets of constraints, no solution exists. For example, a
protocol designer who needs to originate or receive datagrams on a
device that cannot efficiently calculate a checksum over a full
datagram and also needs these packets to pass through a middlebox
that verifies or changes a UDP checksum, but that does not support a
zero UDP checksum, cannot use the zero UDP checksum method.
Similarly, a protocol designer who needs to originate datagrams on a
device with UDP-Lite support, but needs the packets to pass through a
middlebox that does not support UDP-Lite, cannot use UDP-Lite. For
such cases, there is no optimal solution. The current recommendation
is to use or fall back to using UDP with full checksum coverage.
3. Issues Requiring Consideration
This informative section evaluates issues about the proposal to
update IPv6 [RFC2460] to enable the UDP transport checksum to be set
to zero. Some of the identified issues are common to other protocols
already in use. This section also provides background to help in
understanding the requirements and recommendations that follow.
The decision in RFC 2460 to omit an integrity check at the network
level meant that the IPv6 transport checksum was overloaded with many
functions, including validating:
o That the endpoint address was not corrupted within a router, i.e.,
a packet was intended to be received by this destination, and that
the packet does not consist of a wrong header spliced to a
different payload.
o That extension header processing is correctly delimited, i.e., the
start of data has not been corrupted. In this case, reception of
a valid Next Header value provides some protection.
o Reassembly processing, when used.
o The length of the payload.
o The port values, i.e., the correct application receives the
payload. (Applications should also check the expected use of
source ports/addresses.)
o The payload integrity.
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In IPv4, the first four of these 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 misdelivery to an unintended application
socket on an unexpected host.
3.1. Effect of Packet Modification in the Network
IP packets may be corrupted as they traverse an Internet path. Older
evidence presented in "When the CRC and TCP Checksum Disagree"
[Sigcomm2000] shows that this was an issue with IPv4 routers in the
year 2000 and that 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]. During the development of this document in 2009, a number
of individuals provided reports of observed rates for received UDP
datagrams using IPv4 where the UDP checksum had been detected as
corrupt. These rates were as high as 1.39E-4 for some paths, but
close to zero for other paths.
There is extensive experience with deployments using tunnel protocols
in well-managed networks (e.g., corporate networks and service
provider core networks). This has shown the robustness of methods
such as Pseudowire Emulation Edge-to-Edge (PWE3) and MPLS that do not
employ a transport protocol checksum and that have not specified
mechanisms to protect from corruption of the unprotected headers
(such as the VPN Identifier in MPLS). Reasons for the robustness may
include:
o A reduced probability of corruption on paths through well-managed
networks.
o IP forms the majority of the inner traffic carried by these
tunnels. Hence, from a transport perspective, endpoint
verification is already being performed when a received IPv4
packet is processed or by the transport pseudo-header for an IPv6
packet. This update to UDP does not change this behavior.
o In certain cases, a combination of additional filtering (e.g.,
filtering a MAC destination address in a Layer 2 tunnel)
significantly reduces the probability of final misdelivery to the
IP stack.
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o The tunnel protocols did not use a UDP transport header.
Therefore, any corruption is unlikely to result in misdelivery to
another UDP-based application. This concern is specific to UDP
with IPv6.
While this experience can guide the present recommendations, any
update to UDP must preserve operation in the general Internet, which
is heterogeneous and can include links and systems of widely varying
characteristics. Transport protocols used by hosts need to be
designed with this in mind, especially when there is need to traverse
edge networks, where middlebox deployments are common.
Currently, for the general Internet, there is no evidence that
corruption is rare, nor is there evidence that corruption in IPv6 is
rare. Therefore, it seems prudent not to relax checks on
misdelivery. The emergence of low-end IPv6 routers and the proposed
use of NAT with IPv6 provide further motivation to protect from
misdelivery.
Corruption in the network may result in:
o A datagram being misdelivered to the wrong host/router or the
wrong transport entity within an endpoint. Such a datagram needs
to be discarded.
o A datagram payload being corrupted, but still delivered to the
intended host/router transport entity. Such a datagram needs to
be either discarded or correctly processed by an application that
provides its own integrity checks.
o A datagram payload being truncated by corruption of the length
field. Such a datagram needs to be discarded.
Using a checksum 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 effect of modifications to the
destination and source IP address fields, the port fields, and the
fragmentation information.
3.1.1. Corruption of the Destination IP Address Field
An IPv6 endpoint destination address could be modified in the
network; for example, it could be 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. When using
IPv6, however, such modification in the network cannot be detected at
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the network layer. Detection of this corruption by a UDP receiver
relies on the IPv6 pseudo-header that is incorporated in the
transport checksum.
There are two possible outcomes:
o Delivery to a destination address that is not in use. The packet
will not be delivered, but an error report could be generated.
o Delivery to a different destination address. This modification
will normally be detected by the transport checksum, resulting in
a silent discard. Without a computed checksum, the packet would
be passed to the endpoint port demultiplexing function. If an
application is bound to the associated ports, the packet payload
will be passed to the application. (See Section 3.1.4 on port
processing.)
3.1.2. Corruption of the Source IP Address Field
This section examines what happens when the source IP 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. Detection of this corruption by a UDP
receiver relies on the IPv6 pseudo-header that is incorporated in the
transport checksum.
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
from 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:
o An application that requires a pre-established context may
disregard the datagram as invalid or could map it to another
context (if a context for the modified source address were already
activated).
o A stateless application will process the datagram outside of any
context. A simple example is the ECHO server, which will respond
with a datagram directed to the modified source address. This
would create unwanted additional processing load and generate
traffic to the modified endpoint address.
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o Some datagram applications build state using the information from
packet headers. A previously unused source address would result
in receiver processing and the creation of unnecessary transport-
layer state at the receiver. For example, Real-time Protocol
(RTP) [RFC3550] sessions commonly employ a source-independent
receiver port. State is created for each received flow.
Therefore, reception of a datagram with a corrupted source address
will result in the accumulation of unnecessary state in the RTP
state machine, including collision detection and response (since
the same synchronization source (SSRC) value will appear to arrive
from multiple source IP addresses).
o ICMP messages relating to a corrupted packet can be misdirected to
the wrong source node.
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 the one originally
intended and several fields have been modified in transit.
3.1.3. Corruption of Port Information
This section describes what happens if one or both of the UDP port
values are corrupted in transit. This can also happen when IPv4 is
used with a zero UDP checksum, but not when UDP checksums are
calculated or when UDP-Lite is used. If the ports carried in the
transport header of an IPv6 packet are corrupted in transit, packets
may be delivered to the wrong application process (on the intended
machine), responses or errors may be sent to the wrong application
process (on the intended machine), or both may occur.
3.1.4. Delivery to an Unexpected Port
If one combines the corruption effects, such as a corrupted
destination address and corrupted ports, there are a number of
potential outcomes when traffic arrives at an unexpected port. The
following are the possibilities and their outcomes for a packet that
does not use UDP checksum validation:
o The packet could be delivered to a port that is not in use. The
packet is discarded, but could generate an ICMPv6 message (e.g.,
port unreachable).
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o The packet could be delivered to a different node that implements
the same application, so the packet may be accepted, but side
effects could occur or accumulated state could be generated.
o The packet could be delivered to an application that does not
implement the tunnel protocol, so the packet may be incorrectly
parsed and may be misinterpreted, causing side effects or
generating accumulated state.
The probability of each outcome depends on the statistical
probability that the address or the port information for the source
or destination becomes corrupted 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:
1. There is no handshake prior to communication and no sequence
numbers (as in TCP, Datagram Congestion Control Protocol (DCCP),
and Stream Control Transmission Protocol (SCTP)). This makes it
hard to verify that an application process is given only the
application data associated with a specific transport session.
2. Applications writers often bind to wildcard values in endpoint
identifiers and do not always validate the correctness of
datagrams they receive. (Guidance on this topic is provided in
[RFC5405].)
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 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 it is 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 cannot assume that it is the
only protocol using a zero UDP checksum. Therefore, it needs to
handle misdelivery gracefully. It must be robust when malformed
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packets are received on a listening port, and it must expect that
these packets may contain corrupted data or data associated with a
completely different protocol.
3.1.5. Corruption of Fragmentation Information
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 whether there are more fragments.
Corruption of any of these fields may result in one of two outcomes:
o Reassembly failure: An error in the "More Fragments" field for the
last fragment will, for example, result in the packet never being
considered complete, so it will eventually be timed out and
discarded. A corruption in the ID field will result in the
fragment not being delivered to the intended context, thus leaving
the rest of the packet incomplete, unless that packet has been
duplicated before the corruption. The incomplete packet will
eventually be timed out and discarded.
o Erroneous reassembly: The reassembled packet did not match the
original packet. This can occur when the ID field of a fragment
is corrupted, resulting in a fragment becoming associated with
another packet and taking the place of another fragment.
Corruption in the offset information can cause the fragment to be
misaligned in the reassembly buffer, resulting in incorrect
reassembly. Corruption can cause the packet to become shorter or
longer; however, completing the reassembly is much less probable,
because this would require consistent corruption of the IPv6
header's payload length and offset fields. To prevent erroneous
assembly, the reassembling stack must provide strong checks that
detect overlap and missing data. Note, however, that this is not
guaranteed and has been clarified in "Handling of Overlapping IPv6
Fragments" [RFC5722].
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 provided by the
transport checksum. The Upper-Layer Packet length field included in
the pseudo-header assists in verifying correct reassembly, because
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 data that is
all-zero in a chunk that is a multiple of 16 bits.
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The most significant risk of corruption results following mis-
association of a fragment with a different packet. This risk can be
significant, because the size of fragments is often the same (e.g.,
fragments that form when the path MTU results in fragmentation of a
larger packet, which is common when addition of a tunnel
encapsulation header increases the size of a packet). Detection of
this type of error requires a checksum or other integrity check of
the headers and the payload. While such protection is desirable for
tunnel encapsulations using IPv4, because the small fragmentation ID
can easily result in wraparound [RFC4963], this is especially
desirable for tunnels that perform flow aggregation [TUNNELS].
Tunnel fragmentation behavior matters. There can be outer or inner
fragmentation tunnels in the Internet Architecture [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 detect reassembly anomalies only
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. The following list describes
some tunnel fragmentation behaviors:
o An IP-in-IP tunnel that performs inner fragmentation has similar
properties to a UDP tunnel with a zero UDP checksum that also
performs inner fragmentation.
o An IP-in-IP tunnel that performs outer fragmentation has similar
properties to a UDP tunnel with a zero UDP checksum that performs
outer fragmentation.
o A tunnel that performs outer fragmentation can result in a higher
level of corruption due to both inner and outer fragmentation,
enabling more chances for reassembly errors to occur.
o Recursive tunneling can result in fragmentation at more than one
header level, even for fragmentation of the encapsulated packet,
unless the fragmentation is performed on the innermost IP header.
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o Unless there is verification at each reassembly, the probability
of undetected errors will increase with the number of times
fragmentation is recursively applied, making both IP-in-IP and UDP
with zero UDP checksum vulnerable to undetected errors.
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 that offers no additional verification at the different
tunnel layers.
3.2. Where Packet Corruption Occurs
Corruption of IP packets can occur at any point along a network path:
during packet generation, during transmission over the link, in the
process of routing and switching, etc. Some transmission steps
include a checksum or CRC that reduces the probability for corrupted
packets being forwarded, but there still exists a probability that
errors may propagate undetected.
Unfortunately, the Internet community lacks reliable information to
identify the most common functions or equipment that results in
packet corruption. However, there are indications that the place
where corruption occurs can vary significantly from one path to
another. However, there is a risk in taking evidence from one usage
domain and using it to infer characteristics for another. Methods
intended for general Internet usage must therefore assume that
corruption can occur, and mechanisms must be deployed to mitigate the
effects of corruption and any resulting misdelivery.
3.3. Validating the Network Path
IP transports designed for use in the general Internet should not
assume specific path characteristics. Network protocols may reroute
packets, thus changing 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
RFC 2460. Thus, it may be expected that any device on the path that
has a reason to look beyond the IP header, for example, to validate
the UDP checksum, will consider such a packet as erroneous or illegal
and may discard it, unless the device is updated to support the new
behavior. Any middlebox that modifies the UDP checksum, for example,
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a NAT that changes the values of the IP and UDP header in such a way
that the checksum over the pseudo-header changes value, will need to
be updated to support this behavior. Until then, a zero UDP checksum
packet is likely to be discarded, either directly in the middlebox or
at the destination, when a zero UDP checksum has been modified to be
non-zero by an incremental update.
A pair of endpoints intending to use the new behavior will therefore
need not only to ensure support at each endpoint, but also to ensure
that the path between them will deliver packets with the new
behavior. This may require using negotiation or an explicit mandate
to use the new behavior by all nodes that support the 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., a firewall or NAT) 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 endpoints can be guaranteed in limited
deployments by appropriate configuration. In general, it can be
expected to take time for deployment of any updated behavior 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
should be able to renegotiate. 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 behavior will require
periodic validation of the path, adding complexity to any solution
using the new behavior.
3.4. Applicability of the Zero UDP Checksum Method
The update to the IPv6 specification defined in [RFC6935] modifies
only IPv6 nodes that implement specific protocols designed to permit
omission of a UDP checksum. This document provides an applicability
statement for the updated method, indicating when the mechanism can
(and cannot) be used. Enabling a zero UDP checksum, and ensuring
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RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
correct interactions with the stack, implies much more than simply
disabling the checksum algorithm for specific packets at the
transport interface.
When the zero UDP checksum method is widely available, we expect that
it will be used by applications that perceive to gain benefit from
it. Any solution that uses an end-to-end transport protocol rather
than an IP-in-IP encapsulation needs to minimize 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.
A protocol or application that uses the zero UDP checksum method must
ensure that the lack of checksum does not affect the protocol
operation. This includes being robust to receiving an unintended
packet from another protocol or context following corruption of a
destination or source address and/or port value. It also includes
considering the need for additional implicit protection mechanisms
required when using the payload of a UDP packet received with a zero
checksum.
3.5. Impact on Non-Supporting Devices or Applications
It is important to consider the potential impact of using a zero UDP
checksum on endpoint devices and applications that are not modified
to support the new behavior or, by default or preference, do not 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
that enables 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.
Therefore, a zero UDP checksum needs to be enabled only for
individual ports using 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.
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RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
4. Constraints on Implementation of IPv6 Nodes Supporting Zero Checksum
This section is an applicability statement that defines requirements
and recommendations for the implementation of IPv6 nodes that support
the use of a zero value in the checksum field of a UDP datagram.
All implementations that support the zero UDP checksum method MUST
conform to the requirements defined below:
1. An IPv6 sending node MAY use a calculated RFC 2460 checksum for
all datagrams that it sends. This explicitly permits an
interface that supports checksum off-loading to insert an
updated UDP checksum value in all UDP datagrams that it
forwards. Note, however, that sending a calculated checksum
requires the receiver to also perform the checksum calculation.
Checksum off-loading can normally be switched off for a
particular interface to ensure that datagrams are sent with a
zero UDP checksum.
2. IPv6 nodes SHOULD, by default, NOT allow the zero UDP checksum
method for transmission.
3. IPv6 nodes MUST provide a way for the application/protocol to
indicate the set of ports that will be enabled to send datagrams
with a zero UDP checksum. This may be implemented by enabling a
transport mode using a socket API call when the socket is
established, or by a similar mechanism. It may also be
implemented by enabling the method for a pre-assigned static
port used by a specific tunnel protocol.
4. IPv6 nodes MUST provide a method to allow an application/
protocol to indicate that a particular UDP datagram is required
to be sent with a UDP checksum. This needs to be allowed by the
operating system at any time (e.g., to send keepalive
datagrams), not just when a socket is established in zero
checksum mode.
5. The default IPv6 node receiver behavior MUST be to discard all
IPv6 packets carrying datagrams with a zero UDP checksum.
6. IPv6 nodes MUST provide a way for the application/protocol to
indicate the set of ports that will be enabled to receive
datagrams with a zero UDP checksum. This may be implemented via
a socket API call or by a similar mechanism. It may also be
implemented by enabling the method for a pre-assigned static
port used by a specific tunnel protocol.
Fairhurst & Westerlund Standards Track [Page 23]RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
7. IPv6 nodes supporting usage of zero UDP checksums MUST also
allow reception using a calculated UDP checksum on all ports
configured to allow zero UDP checksum usage. (The sending
endpoint, e.g., the encapsulating ingress, may choose to compute
the UDP checksum or may calculate it by default.) The receiving
endpoint MUST use the reception method specified in RFC2460 when
the checksum field is not zero.
8. RFC 2460 specifies that IPv6 nodes SHOULD log received datagrams
with a zero UDP checksum. This remains the case for any
datagram received on a port that does not explicitly enable
processing of a zero UDP checksum. A port for which the zero
UDP checksum has been enabled MUST NOT log the datagram solely
because the checksum value is zero.
9. IPv6 nodes MAY separately identify received UDP datagrams that
are discarded with a zero UDP checksum. They SHOULD NOT add
these to the standard log, because the endpoint has not been
verified. This may be used to support other functions (such as
a security policy).
10. 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).
5. Requirements on Usage of the Zero UDP Checksum
This section is an applicability statement that identifies
requirements and recommendations for protocols and tunnel
encapsulations that are transported over an IPv6 transport flow
(e.g., a tunnel) that does not perform a UDP checksum calculation to
verify the integrity at the transport endpoints. Before deciding to
use the zero UDP checksum and lose the integrity verification
provided by non-zero checksumming, a protocol developer should
seriously consider if they can use checksummed UDP packets or UDP-
Lite [RFC3828], because IPv6 with a zero UDP checksum is not
equivalent in behavior to IPv4 with zero UDP checksum.
The requirements and recommendations for protocols and tunnel
encapsulations using an IPv6 transport flow that does not perform a
UDP checksum calculation to verify the integrity at the transport
endpoints are:
Fairhurst & Westerlund Standards Track [Page 24]RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
1. Transported protocols that enable the use of zero UDP checksum
MUST enable this only for a specific port or port range. This
needs to be enabled at the sending and receiving endpoints for a
UDP flow.
2. An integrity mechanism is always RECOMMENDED at the transported
protocol layer to ensure that corruption rates of the delivered
payload are not increased (e.g., at the innermost packet of a
UDP tunnel). A mechanism that isolates the causes of corruption
(e.g., identifying misdelivery, IPv6 header corruption, or
tunnel header corruption) is also expected to provide additional
information about the status of the tunnel (e.g., to suggest a
security attack).
3. A transported protocol that encapsulates Internet Protocol (IPv4
or IPv6) packets MAY rely on the inner packet integrity checks,
provided that the tunnel protocol will not significantly
increase the rate of corruption of the inner IP packet. If a
significantly increased corruption rate can occur, the tunnel
protocol MUST provide an additional integrity verification
mechanism. Early detection is desirable to avoid wasting
unnecessary computation, transmission capacity, or storage for
packets that will subsequently be discarded.
4. A transported protocol that supports the use of a zero UDP
checksum MUST be designed so that corruption of any header
information does not result in accumulation of incorrect state
for the protocol.
5. A transported protocol with a non-tunnel payload or one that
encapsulates non-IP packets MUST have a CRC or other mechanism
for checking packet integrity, unless the non-IP packet is
specifically designed for transmission over a lower layer that
does not provide a packet integrity guarantee.
6. A transported protocol with control feedback SHOULD be robust to
changes in the network path, because the set of middleboxes on a
path may vary during the life of an association. The UDP
endpoints need to discover paths with middleboxes that drop
packets with a zero UDP checksum. Therefore, transported
protocols SHOULD send keepalive messages with a zero UDP
checksum. An endpoint that discovers an appreciable loss rate
for keepalive packets MAY terminate the UDP flow (e.g., a
tunnel). Section 3.1.3 of RFC 5405 describes requirements for
congestion control when using a UDP-based transport.
Fairhurst & Westerlund Standards Track [Page 25]RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
7. A protocol with control feedback that can fall back to using UDP
with a calculated RFC 2460 checksum is expected to be more
robust to changes in the network path. Therefore, keepalive
messages SHOULD include both UDP datagrams with a checksum and
datagrams with a zero UDP checksum. This will enable the remote
endpoint to distinguish between a path failure and the dropping
of datagrams with a zero UDP checksum.
8. A middlebox implementation MUST allow forwarding of an IPv6 UDP
datagram with both a zero and a standard UDP checksum using the
same UDP port.
9. A middlebox MAY configure a restricted set of specific port
ranges that forward UDP datagrams with a zero UDP checksum. The
middlebox MAY drop IPv6 datagrams with a zero UDP checksum that
are outside a configured range.
10. When a middlebox forwards an IPv6 UDP flow containing datagrams
with both a zero and a standard UDP checksum, the middlebox MUST
NOT maintain separate state for flows, depending on the value of
their UDP checksum field. (This requirement is necessary to
enable a sender that always calculates a checksum to communicate
via a middlebox with a remote endpoint that uses a zero UDP
checksum.)
Special considerations are required when designing a UDP tunnel
protocol where the tunnel ingress or egress may be a router that may
not have access to the packet payload. When the node is acting as a
host (i.e., sending or receiving a packet addressed to itself), the
checksum processing is similar to other hosts. However, when the
node (e.g., a router) is acting as a tunnel ingress or egress that
forwards a packet to or from a UDP tunnel, there may be restricted
access to the packet payload. This prevents calculating (or
verifying) a UDP checksum. In this case, the tunnel protocol may use
a zero UDP checksum and must:
o Ensure that tunnel ingress and tunnel egress router are both
configured to use a zero UDP checksum. For example, this may
include ensuring that hardware checksum off-loading is disabled.
o The tunnel operator must ensure that middleboxes on the network
path are updated to support use of a zero UDP checksum.
o A tunnel egress should implement appropriate security techniques
to protect from overload, including source address filtering to
prevent traffic injection by an attacker and rate-limiting of any
packets that incur additional processing, such as UDP datagrams
used for control functions that require verification of a
Fairhurst & Westerlund Standards Track [Page 26]RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
calculated checksum to verify the network path. Usage of common
control traffic for multiple tunnels between a pair of nodes can
assist in reducing the number of packets to be processed.
6. Summary
This document provides an applicability statement for the use of UDP
transport checksums with IPv6.
It examines the role of the UDP transport checksum when used with
IPv6 and 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.
Application designers should first examine whether their transport
goals may be met using standard UDP (with a calculated checksum) or
UDP-Lite. 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 misdelivery when using zero UDP
checksum in IPv6 compared to using IPv4 due to the lack of an IPv6
header checksum. Thus, application designers need to 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 cost of checksum
processing 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 integrity protection provided by the transported protocol
(or tunneled inner packet). When correctly implemented, such an
endpoint will not be negatively impacted by the omission of the
transport-layer checksum. Recursive tunneling and fragmentation are
potential issues that can raise corruption rates significantly, and
they require careful consideration.
Other UDP applications at the intended destination node or another
node can be impacted if the nodes 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.
Fairhurst & Westerlund Standards Track [Page 27]RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
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, to protect both 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
by the reduced checksum for UDP-Lite when used with IPv6.
The transport of recursive tunneling and the use of fragmentation
pose difficult issues that need to be considered in the design of
tunnel protocols. There is an increased risk of an error in the
innermost packet when fragmentation occurs across several layers of
tunneling and several different reassembly processes are run without
verification of correctness. This requires extra thought and careful
consideration in the design of transported tunnels.
Any use of the updated method must consider the implications for
firewalls, NATs, and other middleboxes. It is not expected that IPv6
NATs will handle IPv6 UDP datagrams in the same way that they handle
IPv4 UDP datagrams. In many deployed cases, an update to support an
IPv6 zero UDP checksum will be required. Firewalls are intended to
be configured, and therefore, they may need to be explicitly updated
to allow new services or protocols. Deployment of IPv6 middleboxes
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 should consider whether
other standard methods may be more appropriate and may simplify
application design.
7. Security Considerations
Transport checksums provide the first stage of protection for the
stack, although they cannot be considered authentication mechanisms.
These checks are also desirable to ensure that packet counters
correctly log actual activity, and they can be used to detect unusual
behaviors.
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 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 that is
forwarding zero checksum UDP packets and cause overload in the
Fairhurst & Westerlund Standards Track [Page 28]
RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
processing of the non-zero checksums, e.g., if it happens in a
router's slow path. Protection mechanisms should therefore 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.
Transmission of IPv6 packets with a zero UDP checksum could reveal
additional information to help an on-path attacker identify the
operating system or configuration of a sending node. There is a need
to probe the network path to determine whether the current path
supports the use of IPv6 packets with a zero UDP checksum. The
details of the probing mechanism may differ for different tunnel
encapsulations, and if they are visible in the network (e.g., if not
using IPsec in encryption mode), they could reveal additional
information to help an on-path attacker 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, because
they present a large attack surface. This applicability statement
therefore permits this method to be enabled only for specific port
ranges.
When the zero UDP checksum mode is enabled for a range of ports,
nodes and middleboxes must forward received UDP datagrams that have
either a calculated checksum or a zero checksum.
8. Acknowledgments
We would like to thank Brian Haberman, Brian Carpenter, Margaret
Wasserman, Lars Eggert, and others in the TSV directorate. Barry
Leiba, Ronald Bonica, Pete Resnick, and Stewart Bryant helped to make
this document one with greater applicability. Thanks to P.F.
Chimento for careful review and editorial corrections.
Thanks also to Remi Denis-Courmont, Pekka Savola, Glen Turner, and
many others who contributed comments and ideas via the 6man, behave,
lisp, and mboned lists.
Fairhurst & Westerlund Standards Track [Page 29]
RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
9. References
9.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[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.
[RFC6935] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
UDP Checksums for Tunneled Packets", RFC 6935,
April 2013.
9.2. Informative References
[AMT] Bumgardner, G., "Automatic Multicast Tunneling", Work
in Progress, June 2012.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1071] Braden, R., Borman, D., Partridge, C., and W. Plummer,
"Computing the Internet checksum", RFC 1071,
September 1988.
[RFC1141] Mallory, T. and A. Kullberg, "Incremental updating of
the Internet checksum", RFC 1141, January 1990.
[RFC1624] Rijsinghani, A., "Computation of the Internet Checksum
via Incremental Update", RFC 1624, May 1994.
[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.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
Fairhurst & Westerlund Standards Track [Page 30]RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and
L. Wood, "Advice for Internet Subnetwork Designers",
BCP 89, RFC 3819, July 2004.
[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.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4
Reassembly Errors at High Data Rates", RFC 4963,
July 2007.
[RFC5097] Renker, G. and G. Fairhurst, "MIB for the UDP-Lite
protocol", RFC 5097, January 2008.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage
Guidelines for Application Designers", BCP 145,
RFC 5405, November 2008.
[RFC5415] Calhoun, P., Montemurro, M., and D. Stanley, "Control
And Provisioning of Wireless Access Points (CAPWAP)
Protocol Specification", RFC 5415, March 2009.
[RFC5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments",
RFC 5722, December 2009.
[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J.
Rajahalme, "IPv6 Flow Label Specification", RFC 6437,
November 2011.
[RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
for Equal Cost Multipath Routing and Link Aggregation
in Tunnels", RFC 6438, November 2011.
[RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
"The Locator/ID Separation Protocol (LISP)", RFC 6830,
January 2013.
[Sigcomm2000] Stone, J. and C. Partridge, "When the CRC and TCP
Checksum Disagree", 2000,
<http://conferences.sigcomm.org/sigcomm/2000/conf/
abstract/9-1.htm>.
Fairhurst & Westerlund Standards Track [Page 31]RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
[TUNNELS] Touch, J. and M. Townsley, "Tunnels in the Internet
Architecture", Work in Progress, March 2010.
[UDPTT] Fairhurst, G., "The UDP Tunnel Transport mode", Work in
Progress, February 2010.
Fairhurst & Westerlund Standards Track [Page 32]RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
Appendix A. Evaluation of Proposal to Update RFC 2460 to Support Zero
Checksum
This informative appendix documents the evaluation of the proposal to
update IPv6 [RFC2460] such that it provides the option that some
nodes may suppress generation and checking of the UDP transport
checksum. It also compares this 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.
A.1. Alternatives to the Standard 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:
o IP-in-IP tunneling. Because this method completely dispenses with
a transport protocol in the outer layer, it has reduced overhead
and complexity, but also reduced functionality. There is no outer
checksum over the packet, and also there are no ports to perform
demultiplexing among different tunnel types. This reduces the
available information upon which a load balancer may act.
o UDP-Lite with the checksum coverage set to only the header portion
of a packet. This requires a pseudo-header checksum calculation
only on the encapsulating packet header. The computed checksum
value may be cached (before adding the Length field) for each
flow/destination and subsequently combined with the Length of each
packet to minimize per-packet processing. This value is combined
with the UDP payload length for the pseudo-header. However, this
length is expected to be known when performing packet forwarding.
o Delta computation of the checksum from an encapsulated checksum
field. Because the checksum is a cumulative sum [RFC1624], an
encapsulating header checksum can be derived from the new pseudo-
header, the inner checksum, and the sum of the other network-layer
fields not included in the pseudo-header of the encapsulated
packet, in a manner resembling incremental checksum update
[RFC1141]. This would not require access to the whole packet, but
does require fields to be collected across the header and
arithmetic operations to be performed on each packet. The method
would work only for packets that contain a 2's complement
transport checksum (i.e., it would not be appropriate for SCTP or
when IP fragmentation is used).
Fairhurst & Westerlund Standards Track [Page 33]RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
o UDP has been modified to disable checksum processing (Zero UDP
Checksum) [RFC6935]. This eliminates the need for a checksum
calculation, but would require constraints on appropriate usage
and updates to endpoints and middleboxes.
o The proposed UDP Tunnel Transport [UDPTT] protocol suggested a
method where UDP would be modified to derive the checksum only
from the encapsulating packet protocol header. This value does
not change between packets in a single flow. The value may be
cached per flow/destination to minimize per-packet processing.
o A method has been proposed that uses a new (to-be-defined) IPv6
Destination Options Header to provide an end-to-end validation
check at the network layer. This would allow an endpoint to
verify delivery to an appropriate endpoint, but would also require
IPv6 nodes to correctly handle the additional header and would
require changes to middlebox behavior (e.g., when used with a NAT
that always adjusts the checksum value).
o There has been a proposal to simply ignore the UDP checksum value
on reception at the tunnel egress, allowing a tunnel ingress to
insert any value, correct or false. For tunnel usage, a non-
standard checksum value may be used, forcing an RFC 2460 receiver
to drop the packet. The main downside is that it would be
impossible to identify a UDP datagram (in the network or an
endpoint) that is treated in this way compared to a packet that
has actually been corrupted.
These options are compared and discussed further in the following
sections.
A.2. Comparison of Alternative Methods
This section compares the methods listed above to support datagram
tunneling. It includes proposals for updating the behavior 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.
From a received packet, there is no way to identify the role of the
receiving node.
A.2.1. Middlebox Traversal
Regular UDP with a standard checksum or the delta-encoded
optimization for creating correct checksums has the best possibility
for successful traversal of a middlebox. No new support is required.
Fairhurst & Westerlund Standards Track [Page 34]RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
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 would also be able to
traverse a middlebox with this behavior. However, a middlebox on the
path that attempts to verify a standard checksum will not forward
packets using either of these methods, thus preventing traversal. A
method that ignores the checksum has the additional downside that it
prevents improvement of middlebox traversal, because there is no way
to identify UDP datagrams that use the modified checksum behavior.
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, because
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 types of middleboxes,
because it requires support for a separate network-layer protocol
number. Once enabled, the method to support incremental checksum
updates would be identical to that for UDP, but different for
checksum validation.
A.2.2. Load Balancing
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
behavior that is equally as good as UDP. However, UDP-Lite is
currently unlikely to be supported by deployed hashing mechanisms,
which could cause a load balancer not to use the transport header in
the computed hash. A load balancer that uses only the IP header will
have low entropy, but this could be improved by including the IPv6
the flow label, provided 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.
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A.2.3. Ingress and Egress Performance Implications
IP-in-IP tunnels are often considered efficient, because they
introduce very little processing and have low data overhead. The
other proposals introduce a UDP-like header, which incurs an
associated data overhead. Processing is minimized for the method
that uses a zero UDP checksum and for the method that ignores the UDP
checksum on reception, and processing is 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.
A.2.4. Deployability
The major factors influencing deployability of these solutions are a
need to update both endpoints, a need for negotiation, and the need
to update middleboxes. These are summarized below:
o The solution with the best deployability is regular UDP. This
requires no changes and has good middlebox traversal
characteristics.
o The next easiest to deploy is the delta checksum solution. This
does not modify the protocol on the wire and needs changes only in
the tunnel ingress.
o IP-in-IP tunnels should not require changes to the endpoints, but
they raise issues regarding the traversal of firewalls and other
security devices, which are expected to require updates.
o Ignoring the checksum on reception will require changes at both
endpoints. The never-ceasing risk of path failure requires
additional checks to ensure that this solution is robust, and it
will require changes or additions to the tunnel control protocol
to negotiate support and validate the path.
o The remaining solutions (including the zero UDP checksum method)
offer similar deployability. UDP-Lite requires support at both
endpoints and in middleboxes. UDPTT and the zero UDP checksum
method, with or without an extension header, require support at
Fairhurst & Westerlund Standards Track [Page 36]RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
both endpoints and in middleboxes. UDP-Lite, UDPTT, and the zero
UDP checksum method and the use of extension headers may also
require changes or additions to the tunnel control protocol to
negotiate support and path validation.
A.2.5. Corruption Detection Strength
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 they do not verify the inner
packet. They provide only 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.
Section 3.1 discusses experience using specific protocols in well-
managed networks.
A.2.6. Comparison Summary
The comparisons above may be summarized as, "there is no silver
bullet that will slay all the issues". One has to select which
downsides can best be lived with. Focusing on the existing
solutions, they can be summarized as:
Regular UDP: The method defined in RFC 2460 has good middlebox
traversal and load balancing and multiplexing, and requires a
checksum in the outer headers to cover the whole packet.
IP-in-IP: A low-complexity encapsulation that has limited middlebox
traversal, no multiplexing support, and poor load-balancing
support that could improve over time.
UDP-Lite: A medium-complexity encapsulation that has good
multiplexing support, limited middlebox traversal that may
possibly improve over time, and poor load-balancing support that
could improve over time, and that, in most cases, requires
application-level negotiation to select the protocol and
validation to confirm that the path forwards UDP-Lite.
Delta computation of a tunnel checksum: The delta checksum is an
optimization in the processing of UDP, and, as such, it exhibits
some of the drawbacks of using regular UDP.
Fairhurst & Westerlund Standards Track [Page 37]RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
The remaining proposals may be described in similar terms:
Zero Checksum: A low-complexity encapsulation that has good
multiplexing support, limited middlebox traversal that could
improve over time, and good load-balancing support, and that, in
most cases, requires application-level negotiation and validation
to confirm that the path forwards a zero UDP checksum.
UDPTT: A medium-complexity encapsulation that has good multiplexing
support, limited middlebox traversal that may possibly improve
over time, and good load-balancing support, and that, in most
cases, requires application-level negotiation to select the
transport and validation to confirm the path forwards UDPTT
datagrams.
IPv6 Destination Option IP-in-IP Tunneling: A medium-complexity
encapsulation that has no multiplexing support, limited middlebox
traversal, and poor load-balancing support that could improve over
time, and that, in most cases, requires negotiation to confirm
that the option is supported and validation to confirm the path
forwards the option.
IPv6 Destination Option Combined with Zero UDP Checksum: A medium-
complexity encapsulation that has good multiplexing support,
limited load-balancing support that could improve over time, and
that, in most cases, requires negotiation to confirm the option is
supported and validation to confirm the path forwards the option.
Ignore the Checksum on Reception: A low-complexity encapsulation
that has good multiplexing support, medium middlebox traversal
that can never improve, and good load-balancing support, and that,
in most cases, requires negotiation to confirm that the option is
supported by the remote endpoint and validation to confirm the
path forwards a zero UDP checksum.
There is no clear single optimum solution. If the most important
need is to traverse middleboxes, 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, if low complexity is necessary, and one does not require
load balancing, IP-in-IP tunneling is the simplest. If one wants
strengthened error detection, but with the 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,
provided that the usage can accept the currently limited support for
middlebox traversal.
Fairhurst & Westerlund Standards Track [Page 38]RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
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, because it requires
both load balancer and endpoint 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 a long time to evolve.
However, we note that the deployment of IPv6-capable middleboxes is
still in its initial phase, and if a new method becomes standardized
quickly, 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 best viewed as
a trade-off among a number of more subjective questions:
o Is there sufficient interest in using a zero UDP checksum with the
given constraints (summarized below)?
o Are there other avenues of change that will resolve the issue in a
better way and sufficiently quickly ?
o Do we accept the complexity cost of having one more solution in
the future?
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 answers to the
above questions are sufficient to update IPv6 to standardize 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 the method defined
in RFC 2460.
Fairhurst & Westerlund Standards Track [Page 39]RFC 6936 Applicability of Zero UDP Checksum with IPv6 April 2013
Authors' Addresses
Godred Fairhurst
University of Aberdeen
School of Engineering
Aberdeen, AB24 3UE
Scotland, UK
EMail: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk/users/gorry
Magnus Westerlund
Ericsson
Farogatan 6
Stockholm, SE-164 80
Sweden
Phone: +46 8 719 0000
EMail: magnus.westerlund@ericsson.com
Fairhurst & Westerlund Standards Track [Page 40]
