Internet DRAFT - draft-ietf-6man-udpzero
draft-ietf-6man-udpzero
Internet Engineering Task Force G. Fairhurst
Internet-Draft University of Aberdeen
Intended status: Standards Track M. Westerlund
Expires: August 29, 2013 Ericsson
February 25, 2013
Applicability Statement for the use of IPv6 UDP Datagrams with Zero
Checksums
draft-ietf-6man-udpzero-12
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 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 updates use of
the UDP checksum with IPv6.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on August 29, 2013.
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
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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 . . . . . . . . . . . . . . . . . . . . 5
1.3.1. Motivation for new approaches . . . . . . . . . . . . 6
1.3.2. Reducing forwarding cost . . . . . . . . . . . . . . . 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. Relation to UDP-Lite and UDP with checksum . . . . . . . . 10
3. Issues Requiring Consideration . . . . . . . . . . . . . . . . 12
3.1. Effect of packet modification in the network . . . . . . . 13
3.1.1. Corruption of the destination IP address . . . . . . . 14
3.1.2. Corruption of the source IP address . . . . . . . . . 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 . . . . . . . 17
3.2. Where Packet Corruption Occurs . . . . . . . . . . . . . . 19
3.3. Validating the network path . . . . . . . . . . . . . . . 20
3.4. Applicability of method . . . . . . . . . . . . . . . . . 21
3.5. Impact on non-supporting devices or applications . . . . . 21
4. Constraints on implementation of IPv6 nodes supporting
zero checksum . . . . . . . . . . . . . . . . . . . . . . . . 22
5. Requirements on usage of the zero UDP checksum . . . . . . . . 24
6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
9. Security Considerations . . . . . . . . . . . . . . . . . . . 28
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
10.1. Normative References . . . . . . . . . . . . . . . . . . . 29
10.2. Informative References . . . . . . . . . . . . . . . . . . 29
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Appendix A. Evaluation of proposal to update RFC 2460 to
support zero checksum . . . . . . . . . . . . . . . . 31
A.1. Alternatives to the Standard Checksum . . . . . . . . . . 31
A.2. Comparison . . . . . . . . . . . . . . . . . . . . . . . . 33
A.2.1. Middlebox Traversal . . . . . . . . . . . . . . . . . 33
A.2.2. Load Balancing . . . . . . . . . . . . . . . . . . . . 34
A.2.3. Ingress and Egress Performance Implications . . . . . 34
A.2.4. Deployability . . . . . . . . . . . . . . . . . . . . 34
A.2.5. Corruption Detection Strength . . . . . . . . . . . . 35
A.2.6. Comparison Summary . . . . . . . . . . . . . . . . . . 35
Appendix B. Document Change History . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
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1. Introduction
The User Datagram Protocol (UDP) [RFC0768] transport is defined for
the Internet Protocol (IPv4) [RFC0791] and is defined in "Internet
Protocol, Version 6 (IPv6) [RFC2460] for IPv6 hosts and routers. The
UDP transport protocol has a minimal set of features. This limited
set has enabled a wide range of applications to use UDP, but these
application do need to provide many important transport functions on
top of UDP. The UDP Usage Guidelines [RFC5405] provides overall
guidance for application designers, including the use of UDP to
support tunneling. The key difference between UDP usage with IPv4
and IPv6 is that RFC 2460 mandates use of a calculated UDP checksum,
i.e. a non-zero value, due to the lack of an IPv6 header checksum.
The 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 lack of a possibility to use an IPv6 datagram with a zero UDP
checksum has been observed as a real problem for certain classes of
application, primarily tunnel applications. This class of
application has been deployed with a zero UDP checksum using IPv4.
The design of IPv6 raises different issues when considering the
safety of using a UDP checksum with IPv6. These issues can
significantly affect applications, both when an endpoint is the
intended user and when an innocent bystander (when a packet is
received by a different endpoint to that intended).
This document examines the issues and an appendix compares the
strengths and weaknesses of a number of proposed solutions. This
identifies a set of issues that must be considered and mitigated to
be able to safely deploy IPv6 applications that use a zero UDP
checksum. The provided comparison of methods is expected to also be
useful when considering applications that have different goals from
the ones that initiated the writing of this document, especially the
use of already standardized methods. The analysis concludes that
using a zero UDP checksum is the best method of the proposed
alternatives to meet the goals for certain tunnel applications.
This document defines recommendations and requirements for use of
IPv6 datagrams with a zero UDP checksum. This usage is expected to
have initial deployment issues related to middleboxes, limiting the
usability more than desired in the currently deployed Internet.
However, this limitation will be largest initially and will reduce as
updates are provided in middleboxes that support the zero UDP
checksum for IPv6. The document therefore derives a set of
constraints required to ensure safe deployment of a zero UDP
checksum.
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Finally, the document also 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 remaining
issues needing future work.
Appendix A evaluates the set of proposals to update the UDP transport
behaviour and other alternatives intended to improve support for
tunnel protocols. It concludes by assessing the trade-offs of the
various methods, identifying advantages and disadvantages for each
method.
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].
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,
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because many middleboxes support transmission using UDP. In this
use, the receiving endpoint decapsulates the UDP datagrams and
forwards the original packets contained in the payload [RFC5405].
Tunnels establish virtual links that appear to directly connect
locations that are distant in the physical Internet topology and can
be used to create virtual (private) networks.
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 [I-D.ietf-mboned-auto-multicast],
and the Locator/Identifier Separation Protocol, LISP [LISP]). These
protocols motivated an update to IPv6 UDP checksum processing to
benefit from simpler checksum processing for various reasons:
o Reducing forwarding costs, motivated by redundancy present in the
encapsulated packet header, since in tunnel encapsulations,
payload integrity and length verification may be provided by
higher layer encapsulations (often using the IPv4, UDP, UDP-Lite,
or TCP checksums).
o Eliminating a need to access the entire packet when forwarding the
packet by a tunnel endpoint.
o Enhancing 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 cost
It is a common requirement to terminate a large number of tunnels on
a single router/host. The processing cost per tunnel includes both
state (memory requirements) and per-packet processing at the tunnel
ingress and egress.
Automatic IP Multicast Tunneling, known as AMT
[I-D.ietf-mboned-auto-multicast] currently specifies UDP as the
transport protocol for packets carrying tunneled IP multicast
packets. The current specification for AMT states that the UDP
checksum in the outer packet header should be zero (see Section 6.6
of [I-D.ietf-mboned-auto-multicast]). This argues that the
computation of an additional checksum is an unwarranted burden on
nodes implementing lightweight tunneling protocols when an inner
packet is already adequately protected, . The AMT protocol needs to
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replicate a multicast packet to each gateway tunnel. In this case,
the outer IP addresses are different for each tunnel and therefore
require a different pseudo header to be built for each UDP replicated
encapsulation.
The argument concerning redundant processing costs is valid regarding
the integrity of a tunneled packet. In some architectures (e.g. PC-
based routers), other mechanisms may also significantly reduce
checksum processing costs: There are implementations that have
optimised checksum processing algorithms, including the use of
checksum-offloading. This processing is readily available for IPv4
packets at high line rates. Such processing may be anticipated for
IPv6 endpoints, allowing receivers to reject corrupted packets
without further processing. However, there are certain classes of
tunnel end-points where this off-loading is not available and
unlikely to become available in the near future.
1.3.3. Need to inspect the entire packet
The currently-deployed hardware in many routers uses a fast-path
processing that only provides the first n bytes of a packet to the
forwarding engine, where typically n <= 128.
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 to use within a router that is unable to access the
entire packet and does not provide checksum-offloading. Thus
enabling checksum calculation over the complete packet can impact
router design, performance improvement, energy consumption and/or
cost.
1.3.4. Interactions with middleboxes
Many paths in the Internet include one or more middleboxes of various
types. There exist large classes of middleboxes that will handle
zero UDP checksum packets, which would not support UDP-Lite or the
other investigated proposals. These middleboxes includes load
balancers (see Section 1.3.5) including Equal Cost Multipath Routing,
traffic classifiers and other functions that reads some fields in the
UDP headers but does not validate the UDP checksum.
There are also middleboxes that either validates or modify the UDP
checksum. The two most common classes are Firewalls and NATs. In
IPv4, UDP-encapsulation may be desirable for NAT traversal, since UDP
support is commonly provided. It is also necessary due to the almost
ubiquitous deployment of IPv4 NATs. There has also been discussion
of NAT for IPv6, although not for the same reason as in IPv4. If
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IPv6 NAT becomes a reality they hopefully do not present the same
protocol issues as for IPv4. If NAT is defined for IPv6, it should
take into consideration the use of a zero UDP checksum.
The requirements for IPv6 firewall traversal are likely be to be
similar to those for IPv4. In addition, it can be reasonably
expected that a firewall conforming to RFC 2460 will not regard
datagrams with a zero UDP checksum as valid. Use of a zero UDP
checksum with IPv6 requires firewalls to be updated before the full
utility of the change is available.
It can be expected that datagrams with zero UDP checksum will
initially not have the same middlebox traversal characteristics as
regular UDP (RFC 2460). However when implementations follow the
requirements specified in this document, we expect the traversal
capabilities to improve over time. We also note that deployment of
IPv6-capable middleboxes is still in its initial phases. Thus, it
might be that the number of non-updated boxes quickly become a very
small percentage of the deployed middleboxes.
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 utilise the IPv6 Flow
Label [RFC6437] as a basis for entropy for load balancing. This
would have the desirable effect of releasing IPv6 load-balancing
devices from the need to assume semantics for the use of the
transport port field and also works for all type of transport
protocols.
This use of the flow-label 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 Equal-Cost Multi-Path routing usage, (ECMP)
[RFC6438] was produced. Router vendors could be encouraged to start
using the IPv6 Flow Label as a part of the flow hash, providing
support for ECMP without requiring use of UDP.
However, the method for populating the outer IPv6 header with a value
for the flow label is not trivial: If the inner packet uses IPv6,
then the flow label value could be copied to the outer packet header.
However, many current end-points set the flow label to a zero value
(thus no entropy). The ingress of a tunnel seeking to provide good
entropy in the flow label field would therefore need to create a
random flow label value and keep corresponding state, so that all
packets that were associated with a flow would be consistently given
the same flow label. Although possible, this complexity may not be
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desirable in a tunnel ingress.
The end-to-end use of flow labels for load balancing is a long-term
solution. Even if the usage of the flow label is clarified, there
would be a transition time before a significant proportion of end-
points start to assign a good quality flow label to the flows that
they originate, with continued use of load balancing using the
transport header fields until any widespread deployment is finally
achieved.
2. Standards-Track Transports
The IETF has defined a set of transport protocols that may be
applicable for tunnels with IPv6. There are also a set of network
layer encapsulation tunnels such as IP-in-IP and GRE. These already
standardized solutions are discussed here prior to the issues, as
background for the issue description and some comparison of where the
issue may already occur.
2.1. UDP with Standard Checksum
UDP [RFC0768] with standard checksum behaviour, 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, specified as
a proposed standard, RFC 3828. A MIB is defined in [RFC5097] and
unicast usage guidelines in [RFC5405]. There is at least one open
source implementation as a part of the Linux kernel since version
2.6.20.
UDP-Lite provides a checksum with optional partial coverage. When
using this option, a datagram is divided into a sensitive part
(covered by the checksum) and an insensitive part (not covered by the
checksum). When the checksum covers the entire packet, UDP-Lite is
fully equivalent with UDP, with the exception that it uses a
different value in the Next Header field in the IPv6 header. Errors/
corruption in the insensitive part will not cause the datagram to be
discarded by the transport layer at the receiving endpoint. A minor
side-effect of using UDP-Lite is that this was specified for damage-
tolerant payloads and some link-layers may employ different link
encapsulations when forwarding UDP-Lite segments (e.g. radio access
bearers). Most link-layers will cover the insensitive part with the
same strong layer 2 frame CRC that covers the sensitive part.
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2.2.1. Using UDP-Lite as a Tunnel Encapsulation
Tunnel encapsulations can use UDP-Lite (e.g. Control And
Provisioning of Wireless Access Points, CAPWAP [RFC5415]), since UDP-
Lite provides a transport-layer checksum, including an IP pseudo
header checksum, in IPv6, without the need for a router/middlebox to
traverse the entire packet payload. This provides most of the
verification required for delivery and still keeps 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 only verify delivery of the
tunneled payload and uses a calculated checksum for control
information.
There is currently poor support for middlebox traversal using UDP-
Lite, because UDP-Lite uses a different IPv6 network-layer Next
Header value to that of UDP, and few middleboxes are able to
interpret UDP-Lite and take appropriate actions when forwarding the
packet. This makes UDP-Lite less suited to protocols needing general
Internet support, until such time that UDP-Lite has achieved better
support in middleboxes and end-points.
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, since tunnel
encapsulations are typically layered directly over the Internet layer
(identified by the upper layer type in the IPv6 Next Header field)
and are also not used as endpoint transport protocols. There is
little chance of confusing a tunnel-encapsulated packet with other
application data that could result in corruption of application state
or data.
From the end-to-end perspective, the principal difference is that the
network-layer Next Header field identifies a separate transport,
which reduces the probability that corruption could result in the
packet being delivered to the wrong endpoint or application.
Specifically, packets are only delivered to protocol modules that
process a specific Next Header value. The Next Header field
therefore provides a first-level check of correct demultiplexing. In
contrast, the UDP port space is shared by many diverse applications
and therefore UDP demultiplexing relies solely on the port numbers.
2.4. Relation 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
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be fooled into thinking the two are the same. The requirements below
list a set of additional considerations.
Where possible, existing general tunnel encapsulations, such as GRE,
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. The 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 can incur significant processing cost (e.g. requiring
processing in the router slow-path) and can hence reduce capacity or
fail to function.
UDP-Lite has the advantage of using a checksum that is 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, only requiring access to the part to be
protected. A drawback is that UDP-Lite has currently limited support
in both end-points (i.e. is not supported on all operating system
platforms) and middleboxes (that require support for the UDP-Lite
header type). A path verification method is therefore 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
currently limited, and this may result in datagrams with a zero UDP
checksum being discarded, therefore a path verification method is
recommended.
There are sets of constrains for which no solution exist: A protocol
designer that needs to originate or receive datagrams on a device
that can not 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 does not support a zero UDP
checksum, can not use the zero UDP checksum method. Similarly, one
that originates datagrams on a device with UDP-Lite support, but
needs the packets to pass through a middlebox that does not support
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UDP-Lite, can not use UDP-Lite. For such cases, there is no optimal
solution and 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 around the proposal to
update IPv6 [RFC2460], to enable the UDP transport checksum to be set
to zero. Some of the identified issues are shared with other
protocols already in use. The section also provides background to
the requirements and recommendations that follow.
The decision 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 the endpoint address was not corrupted within a router, i.e., a
packet was intended to be received by this destination and
validate 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.
In IPv4, the first four checks are performed using the IPv4 header
checksum.
In IPv6, these checks occur within the endpoint stack using the UDP
checksum information. An IPv6 node also relies on the header
information to determine whether to send an ICMPv6 error message
[RFC4443] and to determine the node to which this is sent. Corrupted
information may lead to misdelivery to an unintended application
socket on an unexpected host.
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3.1. Effect of packet modification in the network
IP packets may be corrupted as they traverse an Internet path. Older
evidence in "When the CRC and TCP Checksum Disagree" [Sigcomm2000]
show that this was once an issue in year 2000 with IPv4 routers, and
occasional corruption could result from bad internal router
processing in routers or hosts. These errors are not detected by the
strong frame checksums employed at the link-layer [RFC3819]. During
the development of this document in 2009, individuals provided
reports of observed rates for received UDP datagrams using IPv4 where
the UDP checksum had been detected as corrupt. These rates where as
high as 1.39E-4 for some paths, but also close to zero for some other
paths.
There is extensive experience of deployment using tunnel protocols in
well-managed networks (e.g. corporate networks or service provider
core networks). This has shown the robustness of methods such as PWE
and MPLS that do not employ a transport protocol checksum and 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 form the majority of the inner traffic carried by these tunnel.
Hence from a transport perspective, endpoint verification is
already being performed when processing a received IPv4 packet or
by the transport pseudo-header for an IPv6 packet. This update to
UDP does not change this behaviour.
o In certain cases, a combination of additional filtering (e.g.
filter of a MAC destination address in a L2 tunnel) significantly
reduces the probability of final mis-delivery to the IP stack.
o The tunnel protocols did not use a UDP transport header, any
corruption is therefore unlikely to result in misdelivery to
another UDP-based application. This concern is specific to the
use of UDP with IPv6.
While this experience can guide the present recommendations, any
update to UDP must preserve operation in the general Internet. This
is heterogeneous and can include links and systems of very 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.
For the general Internet, there is no current evidence that
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corruption is rare, nor that this may not be applicable to IPv6. It
therefore seems prudent not to relax checks on misdelivery . The
emergence of low-end IPv6 routers and the proposed use of NAT with
IPv6 further motivate the need 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.
When a checksum is used, this significantly reduces the impact of
errors, reducing the probability of undetected corruption of state
(and data) on both the host stack and the applications using the
transport service.
The following sections examine the impact of modifying each of these
header fields.
3.1.1. Corruption of the destination IP address
An IPv6 endpoint destination address could be modified in the network
(e.g. corrupted by an error). This is not a concern for IPv4,
because the IP header checksum will result in this packet being
discarded by the receiving IP stack. Such modification in the
network can not be detected at the network layer when using IPv6.
Detection of this corruption by a UDP receiver relies on the IPv6
pseudo header 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 could result in an error report);
o Delivery to a different destination address. This modification
will normally be detected by the transport checksum, resulting in
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 the subsequent section on
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port processing).
3.1.2. Corruption of the source IP address
This section examines what happens when the source address is
corrupted in transit. This is not a concern in IPv4, because the IP
header checksum will normally result in this packet being discarded
by the receiving IP stack. Detection of this corruption by a UDP
receiver relies on the IPv6 pseudo header 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
to that of the source address of the original packet.
The result will depend on the application or protocol that processes
the packet. Some examples are:
o An application that requires a per-established context may
disregard the datagram as invalid, or could map this to another
context (if a context for the modified source address was 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.
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.
Reception of a datagram with a corrupted source address will
therefore result in 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.
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In general, the effect of corrupting the source address will depend
upon the protocol that processes the packet and its robustness to
this error. For the case where the packet is received by a tunnel
endpoint, the tunnel application is expected to correctly handle a
corrupted source address.
The impact of source address modification is more difficult to
quantify when the receiving application is not that originally
intended and several fields have been modified in transit.
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 with 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 were corrupted in transit, packets
may be delivered to the wrong application process (on the intended
machine) and/or responses or errors sent to the wrong application
process (on the intended machine).
3.1.4. Delivery to an unexpected port
If one combines the corruption effects, such as destination address
and ports, there is a number of potential outcomes when traffic
arrives at an unexpected port. This section discusses these
possibilities and their outcomes for a packet that does not use the
UDP checksum validation:
o Delivery to a port that is not in use. The packet is discarded,
but could generate an ICMPv6 message (e.g. port unreachable).
o It could be delivered to a different node that implements the same
application, where the packet may be accepted, generating side-
effects or accumulated state.
o It could be delivered to an application that does not implement
the tunnel protocol, where the packet may be incorrectly parsed,
and may be misinterpreted, generating side-effects or 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 corrupt in the datagram such that they match
those of an existing flow or server port. Unfortunately, such a
match may be more likely for UDP than for connection-oriented
transports, because:
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1. There is no handshake prior to communication and no sequence
numbers (as in TCP, DCCP, or SCTP). Together, 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 wild-card values in endpoint
identifiers and do not always validate 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 therefore needs
to provide a method to detect and discard the unwanted data. A
tunnel protocol would need to perform its own integrity checks on any
control information if transported in datagrams with a zero UDP
checksum. If the tunnel payload is another IP packet, the packets
requiring checksums can be assumed to have their own checksums
provided that the rate of corrupted packets is not significantly
larger due to the tunnel encapsulation. If a tunnel transports other
inner payloads that do not use IP, the assumptions of corruption
detection for that particular protocol must be fulfilled, this may
require an additional checksum/CRC and/or integrity protection of the
payload and tunnel headers.
A protocol that uses a zero UDP checksum can not assume that it is
the only protocol using a zero UDP checksum. Therefore, it needs to
gracefully handle misdelivery. It must be robust to reception of
malformed packets received on a listening port and expect that these
packets may contain corrupted data or data associated with a
completely different protocol.
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 if there are more fragments.
Corruption of any of these field may result in one of two outcomes:
Reassembly failure: An error in the "More Fragments" field for the
last fragment will for example result in the packet never being
considered complete and 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 incomplete, unless that packet has been duplicated prior
to corruption. The incomplete packet will eventually be timed out
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and discarded.
Erroneous reassembly: The re-assembled 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 completion of reassembly is much less probable,
since this would require consistent corruption of the IPv6 headers
payload length field and the offset field. The possibility of
mis-assembly requires the reassembling stack to provide strong
checks that detect overlap or 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 by the transport
checksum. The Upper-Layer Packet length field included in the pseudo
header assists in verifying correct reassembly, since the Internet
checksum has a low probability of detecting insertion of data or
overlap errors (due to misplacement of data). The checksum is also
incapable of detecting insertion or removal of all zero-data that
occurs in a multiple of a 16-bit chunk.
The most significant risk of corruption results following mis-
association of a fragment with a different packet. This risk can be
significant, since the size of fragments is often the same (e.g.
fragments resulting when the path MTU results in fragmentation of a
larger packet, common when addition of a tunnel encapsulation header
expands the size of a packet). Detection of this type of error
requires a checksum or other integrity check of the headers and the
payload. Such protection is anyway desirable for tunnel
encapsulations using IPv4, since the small fragmentation ID can
easily result in wrap-around [RFC4963], this is especially the case
for tunnels that perform flow aggregation [I-D.ietf-intarea-tunnels].
Tunnel fragmentation behavior matters. There can be outer or inner
fragmentation "Tunnels in the Internet Architecture"
[I-D.ietf-intarea-tunnels]. If there is inner fragmentation by the
tunnel, the outer headers will never be fragmented and thus a zero
UDP checksum in the outer header will not affect the reassembly
process. When a tunnel performs outer header fragmentation, the
tunnel egress needs to perform reassembly of the outer fragments into
an inner packet. The inner packet is either a complete packet or a
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fragment. If it is a fragment, the destination endpoint of the
fragment will perform reassembly of the received fragments. The
complete packet or the reassembled fragments will then be processed
according to the packet Next Header field. The receiver may only
detect reassembly anomalies when it uses a protocol with a checksum.
The larger the number of reassembly processes to which a packet has
been subjected, the greater the probability of an error.
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 inner fragmentation unless it goes to the
inner-most IP header.
o Unless there is verification at each reassembly, the probability
for undetected error will increase with the number of times
fragmentation is recursively applied, making IP-in-IP and UDP with
zero UDP checksum both 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 without any 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 Cyclic Redundancy Check (CRC) that reduces the
probability for corrupted packets being forwarded, but there still
exists a probability that errors may propagate undetected.
Unfortunately the community lacks reliable information to identify
the most common functions or equipment that result in packet
corruption. However, there are indications that the place where
corruption occurs can vary significantly from one path to another.
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There is therefore a risk in applying evidence from one domain of
usage to infer characteristics for another. Methods intended for
general Internet usage must therefore assume that corruption can
occur and deploy mechanisms to mitigate the effect of corruption
and/or 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 that change the set of routers and middleboxes along a path.
Therefore transports such as TCP, SCTP and DCCP have been designed to
negotiate protocol parameters, adapt to different network path
characteristics, and receive feedback to verify that the current path
is suited to the intended application. Applications using UDP and
UDP-Lite need to provide their own mechanisms to confirm the validity
of the current network path.
A zero value in the UDP checksum field is explicitly disallowed in
RFC2460. Thus it may be expected that any device on the path that
has a reason to look beyond the IP header, 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
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 a
non-zero by an incremental update.
A pair of end-points intending to use a new behavior will therefore
not only need to ensure support at each end-point, but also that the
path between them will deliver packets with the new behavior. This
may require 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. Firewalls, Network Address Translators) 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.
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Support along the path between end points can be guaranteed in
limited deployments by appropriate configuration. In general, it can
be expected to take time for deployment of any updated behaviour to
become ubiquitous.
A sender will need to probe the path to verify the expected behavior.
Path characteristics may change, and usage therefore should be robust
and able to detect a failure of the path under normal usage and re-
negotiate. Note that a bidirectional path does not necessarily
support the same checksum usage in both the forward and return
directions: Receipt of a datagram with a zero UDP checksum, does not
imply that the remote endpoint can also receive a datagram with a
zero UDP checksum. This will require periodic validation of the
path, adding complexity to any solution using the new behavior.
3.4. Applicability of method
The update to the IPv6 specification defined in
[I-D.ietf-6man-udpchecksums] only modifies IPv6 nodes that implement
specific protocols designed to permit omission of a UDP checksum.
This document therefore provides an applicability statement for the
updated method indicating when the mechanism can (and can not) be
used. Enabling this, and ensuring correct interactions with the
stack, implies much more than simply disabling the checksum algorithm
for specific packets at the transport interface.
When the method is widely available, it may be expected to be used by
applications that are perceived to gain benefit. Any solution that
uses an end-to-end transport protocol, rather than an IP-in-IP
encapsulation, needs to minimise the possibility that application
processes could confuse a corrupted or wrongly delivered UDP datagram
with that of data addressed to the application running on their
endpoint.
The protocol or application that uses the zero checksum method must
ensure that the lack of checksum does not affect the protocol
operation. This includes being robust to receiving a 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 end-point devices or applications that are not modified
to support the new behavior or by default or preference, use the
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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.
A zero UDP checksum therefore 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.
4. Constraints on implementation of IPv6 nodes supporting zero checksum
This section is an applicability statement that defines requirements
and recommendations on the implementation of IPv6 nodes that support
use of a zero value in the checksum field of a UDP datagram.
All implementations that support this zero UDP checksum method MUST
conform to the requirements defined below.
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 offloading to insert an updated
UDP checksum value in all UDP datagrams that it forwards,
however note that sending a calculated checksum requires the
receiver to also perform the checksum calculation. Checksum
offloading 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
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transport mode using a socket API call when the socket is
established, or 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 keep-alive
datagrams), not just when a socket is established in the zero
checksum mode.
5. The default IPv6 node receiver behaviour MUST 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 similar mechanism. It may also be
implemented by enabling the method for a pre-assigned static
port used by a specific tunnel protocol.
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. encapsulating ingress, may choose to compute the
UDP checksum, or may calculate this 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. It SHOULD NOT add these
to the standard log, since 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
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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.
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 loose the integrity verification provided, 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:
1. Transported protocols that enable the use of zero UDP checksum
MUST only enable this 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 is not increased (e.g. the inner-most packet of a UDP
tunnel). A mechanism that isolates the causes of corruption
(e.g. identifying misdelivery, IPv6 header corruption, tunnel
header corruption) is expected to also 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, then 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 use of a zero UDP checksum,
MUST be designed so that corruption of this information does not
result in accumulated state for the protocol.
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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, since 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 keep-alive messages with a zero UDP
checksum. An endpoint that discovers an appreciable loss rate
for keep-alive packets MAY terminate the UDP flow (e.g. tunnel).
Section 3.1.3 of RFC 5405 describes requirements for congestion
control when using a UDP-based transport.
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, keep-alive
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 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 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 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
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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 offloading 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
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
by using 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, applications 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
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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 omission of the
transport-layer checksum. Recursive tunneling and fragmentation is a
potential issue that can raise corruption rates significantly, and
requires careful consideration.
Other UDP applications at the intended destination node or another
node can be impacted if they are allowed to receive datagrams that
have a zero UDP checksum. It is important that already deployed
applications are not impacted by a change at the transport layer. If
these applications execute on nodes that implement RFC 2460, they
will discard (and log) all datagrams with a zero UDP checksum. This
is not an issue.
In general, UDP-based applications need to employ a mechanism that
allows a large percentage of the corrupted packets to be removed
before they reach an application, both to protect the data stream of
the application and the control plane of higher layer protocols.
These checks are currently performed by the UDP checksum for IPv6, or
the reduced checksum for UDP-Lite when used with IPv6.
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
inner-most packet when fragmentation when 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 on
firewalls, NATs and other middleboxes. It is not expected that IPv6
NATs handle IPv6 UDP datagrams in the same way that they handle IPv4
UDP datagrams. In many deployed cases this will require an update to
support an IPv6 zero UDP checksum. Firewalls are intended to be
configured, and therefore may need to be explicitly updated to allow
new services or protocols. IPv6 middlebox deployment is not yet as
prolific as it is in IPv4, and therefore new devices are expected to
follow the methods specified in this document.
Each application should consider the implications of choosing an IPv6
transport that uses a zero UDP checksum, and consider whether other
standard methods may be more appropriate, and may simplify
application design.
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7. Acknowledgements
Brian Haberman, Brian Carpenter, Margaret Wasserman, Lars Eggert,
others in the TSV directorate. Barry Leiba, Ronald Bonica, Pete
Resnick, and Stewart Bryant are thanked for resulting in a document
with much 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.
8. IANA Considerations
This document does not require any actions by IANA.
9. Security Considerations
Transport checksums provide the first stage of protection for the
stack, although they can not be considered authentication mechanisms.
These checks are also desirable to ensure packet counters correctly
log actual activity, and can be used to detect unusual behaviours.
Depending on the hardware design, the processing requirements may
differ for tunnels that have a zero UDP checksum and those that
calculate a checksum. This processing overhead may need to be
considered when deciding whether to enable a tunnel and to determine
an acceptable rate for transmission. 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 tunnel egress. An attacker could attempt to inject
non-zero checksummed UDP packets into a tunnel forwarding zero
checksum UDP packets and cause overload in the processing of the non-
zero checksums, e.g. if this happens in a routers slow path.
Protection mechanisms should therefore be employed when this threat
exists. Protection may include source address filtering to prevent
an attacker injecting traffic, as well as throttling the amount of
non-zero checksum traffic. The latter may impact the function of the
tunnel protocol.
Transmission of IPv6 packets with a zero UDP checksum could reveal
additional information to an on-path attacker to identify the
operating system or configuration of a sending node. There is a need
to probe the network path to determine whether the current path
supports using IPv6 packets with a zero UDP checksum. The details of
the probing mechanism may differ for different tunnel encapsulations
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and if visible in the network (e.g. if not using IPsec in encryption
mode) could reveal additional information to an on-path attacker to
identify the type of tunnel being used.
IP-in-IP or GRE tunnels offer good traversal of middleboxes that have
not been designed for security, e.g. firewalls. However, firewalls
may be expected to be configured to block general tunnels as they
present a large attack surface. This applicability statement
therefore permits this method to be enabled only for specific ranges
of ports.
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.
10. References
10.1. Normative References
[I-D.ietf-6man-udpchecksums]
Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
UDP Checksums for Tunneled Packets",
draft-ietf-6man-udpchecksums-08 (work in progress),
February 2013.
[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.
10.2. Informative References
[I-D.ietf-intarea-tunnels]
Touch, J. and M. Townsley, "Tunnels in the Internet
Architecture", draft-ietf-intarea-tunnels-00 (work in
progress), March 2010.
[I-D.ietf-mboned-auto-multicast]
Bumgardner, G., "Automatic Multicast Tunneling",
draft-ietf-mboned-auto-multicast-14 (work in progress),
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June 2012.
[LISP] D. Farinacci et al, "Locator/ID Separation Protocol
(LISP)", November 2012.
[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.
[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
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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.
[Sigcomm2000]
Jonathan Stone and Craig Partridge , "When the CRC and TCP
Checksum Disagree", 2000.
[UDPTT] G Fairhurst, "The UDP Tunnel Transport mode", Feb 2010.
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], to provide the option that some nodes may
suppress generation and checking of the UDP transport checksum. It
also compares the proposal with other alternatives, and notes that
for a particular application some standard methods may be more
appropriate than using IPv6 with a zero UDP checksum.
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
(in decreasing order of complexity):
o Delta computation of the checksum from an encapsulated checksum
field. Since 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 on each packet. The method would only work
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).
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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 minimise 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 The proposed UDP Tunnel Transport [UDPTT] 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 minimise per-packet processing.
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.
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 end point, 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 UDP modified to disable checksum processing
[I-D.ietf-6man-udpchecksums]. This eliminates the need for a
checksum calculation, but would require constraints on appropriate
usage and updates to end-points and middleboxes.
o IP-in-IP tunneling. As 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 no ports to perform
demultiplexing between different tunnel types. This reduces the
information available upon which a load balancer may act.
These options are compared and discussed further in the following
sections.
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A.2. Comparison
This section compares the above listed methods to support datagram
tunneling. It includes proposals for updating the behaviour of UDP.
While this comparison focuses on applications that are expected to
execute on routers, the distinction between a router and a host is
not always clear, especially at the transport level. Systems (such
as unix-based operating systems) routinely provide both functions.
There is no way to identify the role of the receiving node from a
received packet.
A.2.1. Middlebox Traversal
Regular UDP with a standard checksum or the delta encoded
optimization for creating correct checksums have the best
possibilities for successful traversal of a middlebox. No new
support is required.
A method that ignores the UDP checksum on reception is expected to
have a good probability of traversal, because most middleboxes
perform an incremental checksum update. UDPTT would also have been
able to traverse a middlebox with this behaviour. However, a
middlebox on the path that attempts to verify a standard checksum
will not forward packets using either of these methods, preventing
traversal. A method that ignores the checksum has an additional
downside in that it prevents improvement of middlebox traversal,
because there is no way to identify UDP datagrams that use the
modified checksum behaviour.
IP-in-IP or GRE tunnels offer good traversal of middleboxes that have
not been designed for security, e.g. firewalls. However, firewalls
may be expected to be configured to block general tunnels as they
present a large attack surface.
A new IPv6 Destination Options header will suffer traversal issues
with middleboxes, especially Firewalls and NATs, and will likely
require them to be updated before the extension header is passed.
Datagrams with a zero UDP checksum will not be passed by any
middlebox that validates the checksum using RFC 2460 or updates the
checksum field, such as NAT or firewalls. This would require an
update to correctly handle a datagram with a zero UDP checksum.
UDP-Lite will require an update of almost all type of middleboxes,
because it requires support for a separate network-layer protocol
number. Once enabled, the method to support incremental checksum
update would be identical to that for UDP, but different for checksum
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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
equally good behavior as for UDP. However, UDP-Lite is currently
unlikely to be supported by deployed hashing mechanisms, which could
cause a load balancer to not use the transport header in the computed
hash. A load balancer that only uses the IP header will have low
entropy, but could be improved by including the IPv6 the flow label,
providing that the tunnel ingress ensures that different flow labels
are assigned to different flows. However, a transition to the common
use of good quality flow labels is likely to take time to deploy.
A.2.3. Ingress and Egress Performance Implications
IP-in-IP tunnels are often considered efficient, because they
introduce very little processing and low data overhead. The other
proposals introduce a UDP-like header incurring associated data
overhead. Processing is minimised for the method that uses a zero
UDP checksum, ignoring the UDP checksum on reception, and only
slightly higher for UDPTT, the extension header and UDP-Lite. The
delta-calculation scheme operates on a few more fields, but also
introduces serious failure modes that can result in a need to
calculate a checksum over the complete datagram. Regular UDP is
clearly the most costly to process, always requiring checksum
calculation over the entire datagram.
It is important to note that the zero UDP checksum method, ignoring
checksum on reception, the Option Header, UDPTT and UDP-Lite will
likely incur additional complexities in the application to
incorporate a negotiation and validation mechanism.
A.2.4. Deployability
The major factors influencing deployability of these solutions are a
need to update both end-points, a need for negotiation and the need
to update middleboxes. These are summarised below:
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 only needs changes in
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tunnel ingress.
o IP-in-IP tunnels should not require changes to the end-points, but
raise issues when traversing firewalls and other security devices,
which are expected to require updates.
o Ignoring the checksum on reception will require changes at both
end-points. The never ceasing risk of path failure requires
additional checks to ensure this solution is robust and will
require changes or additions to the tunnel control protocol to
negotiate support and validate the path.
o The remaining solutions (including the zero checksum method) offer
similar deployability. UDP-Lite requires support at both end-
points and in middleboxes. UDPTT and the zero UDP checksum method
with or without an extension header require support at both end-
points and in middleboxes. UDP-Lite, UDPTT, and the zero UDP
checksum method and use of extension headers may additionally
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 do not verify the inner packet.
They only provide a strong indication that the delivered packet was
intended for the tunnel egress and was correctly delimited.
The methods using a zero UDP checksum, ignoring the UDP checksum on
reception and IP-and-IP encapsulation all provide no verification
that a received datagram was intended to be processed by a specific
tunnel egress or that the inner encapsulated packet was correct.
Section 3.1 discusses experience using specific protocols in well-
managed networks.
A.2.6. Comparison Summary
The comparisons above may be summarised as "there is no silver bullet
that will slay all the issues". One has to select which down side(s)
can best be lived with. Focusing on the existing solutions, this can
be summarized as:
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Regular UDP: The method defined in RFC 2460 has good middlebox
traversal and load balancing and multiplexing, requiring a
checksum in the outer headers covering the whole packet.
IP in IP: A low complexity encapsulation, with limited middlebox
traversal, no multiplexing support, and currently poor load
balancing support that could improve over time.
UDP-Lite: A medium complexity encapsulation, with good multiplexing
support, limited middlebox traversal, but possible to improve over
time, currently poor load balancing support that could improve
over time, in most cases requiring application level negotiation
to select the protocol and validation to confirm the path forwards
UDP-Lite.
The delta-checksum is an optimization in the processing of UDP, as
such it exhibits some of the drawbacks of using regular UDP.
The remaining proposals may be described in similar terms:
Zero-Checksum: A low complexity encapsulation, with good
multiplexing support, limited middlebox traversal that could
improve over time, good load balancing support, in most cases
requiring application level negotiation and validation to confirm
the path forwards a zero UDP checksum.
UDPTT: A medium complexity encapsulation, with good multiplexing
support, limited middlebox traversal, but possible to improve over
time, good load balancing support, in most cases requiring
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,
with no multiplexing support, limited middlebox traversal,
currently poor load balancing support that could improve over
time, in most cases requiring negotiation to confirm the option is
supported and validation to confirm the path forwards the option.
IPv6 Destination Option combined with UDP Zero-checksuming: A medium
complexity encapsulation, with good multiplexing support, limited
load balancing support that could improve over time, in most cases
requiring 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,
with good multiplexing support, medium middlebox traversal that
never can improve, good load balancing support, in most cases
requiring negotiation to confirm the option is supported by the
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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, then the best choice is to stay with
regular UDP and consider the optimizations that may be required to
perform the checksumming. If one can live with limited middlebox
traversal, low complexity is necessary and one does not require load
balancing, then IP-in-IP tunneling is the simplest. If one wants
strengthened error detection, but with currently limited middlebox
traversal and load-balancing. UDP-Lite is appropriate. Zero UDP
checksum addresses another set of constraints, low complexity and a
need for load balancing from the current Internet, providing it can
live with currently limited middlebox traversal.
Techniques for load balancing and middlebox traversal do continue to
evolve. Over a long time, developments in load balancing have good
potential to improve. This time horizon is long since it requires
both load balancer and end-point updates to get full benefit. The
challenges of middlebox traversal are also expected to change with
time, as device capabilities evolve. Middleboxes are very prolific
with a larger proportion of end-user ownership, and therefore may be
expected to take long time cycles to evolve.
One potential advantage is that the deployment of IPv6-capable
middleboxes are still in its initial phase and the quicker a new
method becomes standardized, the fewer boxes will be non-compliant.
Thus, the question of whether to permit use of datagrams with a zero
UDP checksum for IPv6 under reasonable constraints, is therefore best
viewed as a trade-off between a number of more subjective questions:
o Is there sufficient interest in using a zero UDP checksum with the
given constraints (summarised 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 answer to the
above questions are sufficient to update IPv6 to standardise use of a
zero UDP checksum for use by tunnel encapsulations for specific
applications.
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Each application should consider the implications of choosing an IPv6
transport that uses a zero UDP checksum. In many cases, standard
methods may be more appropriate, and may simplify application design.
The use of checksum off-loading may help alleviate the checksum
processing cost and permit use of a checksum using method defined in
RFC 2460.
Appendix B. Document Change History
{RFC EDITOR NOTE: This section must be deleted prior to publication}
Individual Draft 00 This is the first DRAFT of this document - It
contains a compilation of various discussions and contributions
from a variety of IETF WGs, including: mboned, tsv, 6man, lisp,
and behave. This includes contributions from Magnus with text on
RTP, and various updates.
Individual Draft 01
* This version corrects some typos and editorial NiTs and adds
discussion of the need to negotiate and verify operation of a
new mechanism (3.3.4).
Individual Draft 02
* Version -02 corrects some typos and editorial NiTs.
* Added reference to ECMP for tunnels.
* Clarifies the recommendations at the end of the document.
Working Group Draft 00
* Working Group Version -00 corrects some typos and removes much
of rationale for UDPTT. It also adds some discussion of IPv6
extension header.
Working Group Draft 01
* Working Group Version -01 updates the rules and incorporates
off-list feedback. This version is intended for wider review
within the 6man working group.
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Working Group Draft 02
* This version is the result of a major rewrite and re-ordering
of the document.
* A new section comparing the results have been added.
* The constraints list has been significantly altered by removing
some and rewording other constraints.
* This contains other significant language updates to clarify the
intent of this draft.
Working Group Draft 03
* Editorial updates
Working Group Draft 04
* Resubmission only updating the AMT and RFC2765 references.
Working Group Draft 05
* Resubmission to correct editorial NiTs - thanks to Bill Atwood
for noting these.Group Draft 05.
Working Group Draft 06
* Resubmission to keep draft alive (spelling updated from 05).
Working Group Draft 07
* Interim Version
* Submission after IESG Feedback Added
* Updates to enable the document to become a PS Applicability
Statement
Working Group Draft 08
* First Version written as a PS Applicability Statement
* Changes to reflect decision to update RFC 2460, rather than
recommend decision
* Updates to requirements for middleboxes
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* Inclusion of requirements for security, API, and tunnel
* Move of the rationale for the update to an Annex (former
section 4)
Working Group Draft 09
* Submission after second WGLC (note mistake corrected in -09).
* Clarified role of API for supporting full checksum.
* Clarified that full checksum is required in security
considerations, and therefore noting that full checksum should
not be treated as an attack - consistent with remainder of
document.
* Added mention that API can set a mode in transport stack - to
link to similar statement in RFC 2460 update.
* Fixed typos.
Working Group Draft 10
* Submission to correct unwanted removal of text from section 5
bullets 5-7 by GF.
* Replaced section 5 text with the text from 08, and reapplied
the editorial correction.
* Note to reviewers: Please compare this revision with -08 used
in the IETF LC).
Working Group Draft 11
* Added REF for 5097 (Noted by S.Turner)
* Added text in response to P. Resnick on place where checksum is
calculated.
* Added text to note experience with MPLS/PWE; Appendix updated
to refer to this (S. Bryant)
* Added text in response to P.Resnick's 2nd comments.
* Request to make UDP-Lite more clearly recommended (J Touch,
P.Resnick)
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* Added considerations around usage of zero checksum in routers.
* Added text in response to Stewart Bryant's comments on router
requirements.
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
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