Internet Engineering Task Force G. Fairhurst
Internet-Draft University of Aberdeen
Intended status: Standards Track M. Westerlund
Expires: April 25, 2013 Ericsson
October 22, 2012
Applicability Statement for the use of IPv6 UDP Datagrams with Zero
Checksums
draft-ietf-6man-udpzero-07
Abstract
This document provides an applicability statement for the use of UDP
transport checksums when used with IPv6. This defines
recommendations and requirements for use of IPv6 UDP datagrams with a
zero checksum. It examines the role of the IPv6 UDP transport
checksum, as defined in RFC2460 and presents a summary of the trade-
offs for evaluating the safety of updating RFC 2460 to permit an IPv6
UDP endpoint to use a zero value in the checksum field as an
indication that no checksum is present. This method is compared with
some other possibilities. The document also describes the issues and
design principles that need to be considered when UDP is used with
IPv6 to support tunnel encapsulations.
XXX NOTE - This revision is a partial response to comments received
during IESG review. There are additional comments to be incorporated
- and updates anticipated to the related PS that updates IPv6. This
is therefore an interim version. XXX
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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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on April 25, 2013.
Copyright Notice
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Copyright (c) 2012 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
(http://trustee.ietf.org/license-info) in effect on the date of
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 . . . . . . . . . . . . . . . . . . . . 4
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 . . . . . . . . . . . . . . . . . . 8
2.1. UDP with Standard Checksum . . . . . . . . . . . . . . . . 8
2.2. UDP-Lite . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1. Using UDP-Lite as a Tunnel Encapsulation . . . . . . . 9
2.3. General Tunnel Encapsulations . . . . . . . . . . . . . . 9
3. Issues Requiring Consideration . . . . . . . . . . . . . . . . 10
3.1. Effect of packet modification in the network . . . . . . . 11
3.1.1. Corruption of the destination IP address . . . . . . . 12
3.1.2. Corruption of the source IP address . . . . . . . . . 12
3.1.3. Corruption of Port Information . . . . . . . . . . . . 13
3.1.4. Delivery to an unexpected port . . . . . . . . . . . . 13
3.1.5. Corruption of Fragmentation Information . . . . . . . 15
3.2. Validating the network path . . . . . . . . . . . . . . . 17
3.3. Applicability of method . . . . . . . . . . . . . . . . . 17
3.4. Impact on non-supporting devices or applications . . . . . 18
4. Evaluation of proposal to update RFC 2460 to support zero
checksum . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.1. Alternatives to the Standard Checksum . . . . . . . . . . 19
4.2. Comparison . . . . . . . . . . . . . . . . . . . . . . . . 20
4.2.1. Middlebox Traversal . . . . . . . . . . . . . . . . . 20
4.2.2. Load Balancing . . . . . . . . . . . . . . . . . . . . 21
4.2.3. Ingress and Egress Performance Implications . . . . . 21
4.2.4. Deployability . . . . . . . . . . . . . . . . . . . . 22
4.2.5. Corruption Detection Strength . . . . . . . . . . . . 22
4.2.6. Comparison Summary . . . . . . . . . . . . . . . . . . 23
5. Constraints on implementation of IPv6 nodes supporting
zero checksum . . . . . . . . . . . . . . . . . . . . . . . . 25
6. Requirements on the specification of transported protocols . . 25
7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
10. Security Considerations . . . . . . . . . . . . . . . . . . . 29
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
11.1. Normative References . . . . . . . . . . . . . . . . . . . 29
11.2. Informative References . . . . . . . . . . . . . . . . . . 29
Appendix A. Document Change History . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
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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 IPv6 mandates use of the UDP checksum, i.e. a non-
zero value, due to the lack of an IPv6 header checksum.
The lack of a possibility to use UDP with a zero-checksum in IPv6 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 checksum using IPv4. The design of IPv6 raises
different issues when considering the safety of using a zero checksum
for UDP with IPv6. These issues can significantly affect
applications, both when an endpoint is the intended user and when an
innocent bystander (received by a different endpoint to that
intended). The document examines these issues and compares the
strengths and weaknesses of a number of proposed solutions. This
analysis presents a set of issues that must be considered and
mitigated to be able to safely deploy UDP with a zero checksum over
IPv6. 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 UDP with a zero checksum is the
best method of the proposed alternatives to meet the goals for
certain tunnel applications. Unfortunately, this usage is expected
to have some 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 for support of UDP zero checksum for IPv6 are provided to
middleboxes. The document therefore derives a set of constraints
required to ensure safe deployment of zero checksum in UDP. It 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.
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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 checksum in UDP for IPv6. It
considers the impact of corruption, the need for validation of the
path and when it is suitable to use a zero checksum.
Section 4 evaluates a set of proposals to update the UDP transport
behaviour and other alternatives intended to improve support for
tunnel protocols. It focuses on a proposal to allow a zero checksum
for this use-case with IPv6 and assesses the trade-offs that would
arise.
Section 5 is an applicability statement that defines requirements and
recommendations on the implementation of IPv6 nodes that support the
use of a UDP zero value in the checksum of a UDP datagram.
Section 6 provides an applicability statement that identifies
requirements and recommendations for protocols and tunnel
encapsulations that are transported over an IPv6 transport connection
that does not perform a UDP checksum calculation to verify the
integrity at the transport endpoints.
Section 7 provides the recommendations for standardization of zero-
checksum with a summary of the findings and notes remaining issues
needing future work.
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,
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.
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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 have proposed an update to IPv6 UDP checksum processing.
These tunnel protocols could 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 middleboxes, especially Network
Address Translators, NATs.
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. Processing per tunnel concerns both state
(memory requirements) and per-packet processing costs.
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 requires that the UDP
checksum in the outer packet header should be 0 (see Section 6.6 of
[I-D.ietf-mboned-auto-multicast]). It argues that the computation of
an additional checksum, when an inner packet is already adequately
protected, is an unwarranted burden on nodes implementing lightweight
tunneling protocols. 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 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-
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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. This 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
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 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 UDP zero checksum into consideration.
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 UDP
datagrams with a zero checksum as valid packets. If a zero-checksum
for UDP were to be allowed for IPv6, this would need firewalls to be
updated before full utility of the change is available.
It can be expected that UDP with zero-checksum will initially not
have the same middlebox traversal characteristics as regular UDP.
However, if standardized we can expect an improvement over time of
the traversal capabilities. 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.
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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 as basis for entropy for the 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 is consistent with the intended use, although further
clarity may be needed to ensure the field can be consistently used
for this purpose, (e.g. Equal-Cost Multi-Path routing, ECMP [ECMP]).
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 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
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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 RFC 5097 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. 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.
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/middelbox to
traverse the entire packet payload. This provides most of the
verification required for delivery and still keeps the complexity of
the checksumming operation low. 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 full checksumming 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
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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.
3. Issues Requiring Consideration
This informative section evaluates issues around the proposal to
update IPv6 [RFC2460], to provide the option of using a UDP transport
checksum set to zero. Some of the identified issues are shared with
other protocols already in use.
The decision by IPv6 to omit an integrity check at the network level
has meant that the transport check 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);
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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.
3.1. Effect of packet modification in the network
IP packets may be corrupted as they traverse an Internet path.
Evidence has been presented [Sigcomm2000] to show that this was once
an issue 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]. There is no current evidence that such cases
are rare in the modern Internet, nor that they may not be applicable
to IPv6. It therefore seems prudent not to relax this constraint.
The emergence of low-end IPv6 routers and the proposed use of NAT
with IPv6 further motivate the need to protect from this type of
error.
Corruption in the network may result in:
o A datagram being mis-delivered 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.
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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.
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
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.
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
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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).
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 in
the zero checksum case, but not when UDP checksums are enabled or
with UDP-Lite. If the ports carried in the transport header of an
IPv6 packet were corrupted in transit, packets may be delivered to
the wrong 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).
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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:
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 is given only the data associated
with a 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 UDP with zero-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 using UDP zero-checksum can never assume that it is the
only protocol using a zero 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
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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
and discarded.
Erroneous reassembly: The re-assemblied 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 requires 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 recently 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
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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-
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 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-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 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 checksum both vulnerable to undetected errors.
In conclusion fragmentation of packets with a zero-checksum does not
worsen the situation compared to some other commonly used tunnel
encapsulations. However, caution is needed for recursive tunneling
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without any additional verification at the different tunnel layers.
3.2. 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.
The zero-checksum in UDP 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 will consider such a packet as erroneous or
illegal and may likely discard it, unless the device is updated to
support a new behavior. 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 negotiation or an explicit
mandate to use the new behavior by all nodes intended to use a new
protocol.
Support along the path between end points may 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. This will require periodic
validation of the path, adding complexity to any solution using the
new behavior.
3.3. Applicability of method
The expectation of the present proposal defined in
[I-D.ietf-6man-udpchecksums] is that this change would only apply to
IPv6 router nodes that implement specific protocols that permit
omission of UDP checksums. However, 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 also no way to identify the role of a receiver
from a received packet.
Any new method would therefore need a specific applicability
statement indicating when the mechanism can (and can not) be used.
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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.
The IETF should carefully consider constraints on sanctioning the use
of any new transport mode. If this is specified and 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 end-hosts could confuse a corrupted or
wrongly delivered packet with that of data addressed to an
application running on their endpoint unless they accept that
behavior.
3.4. Impact on non-supporting devices or applications
It is important to consider what potential impact the zero-checksum
behavior may have on end-points, devices or applications that are not
modified to support the new behavior or by default or preference, use
the regular behavior. These applications must not be significantly
impacted by the changes.
To illustrate a potential issue, consider the implications of a node
that were to enable use of a zero-checksum at the interface level:
This would result in all applications that listen to a UDP socket
receiving datagram 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.
In contrast, the use of a zero-checksum could 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 UDP datagrams with a zero-
checksum. These other applications would not be effected by this
changed behavior. An application that allows the changed behavior
should be aware of the risk for corruption and the increased level of
misdirected traffic, and can be designed robustly to handle this
risk.
4. Evaluation of proposal to update RFC 2460 to support zero checksum
This informative section evaluates 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.
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4.1. 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).
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 [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
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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 requires no 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.
4.2. Comparison
This section compares the above listed methods to support datagram
tunneling. It includes proposals for updating the behaviour of UDP.
4.2.1. 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 may also 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 packets 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
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require them to be updated before the extension header is passed.
Packets using UDP with a zero 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 the zero checksum packets.
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
validation.
4.2.2. 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
likely to not be supported by deployed hashing mechanisms, which may
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.
4.2.3. 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 zero-checksum method,
ignoring the 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 packet. Regular UDP is clearly the most costly to
process, always requiring checksum calculation over the entire
packet.
It is important to note that the zero-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.
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4.2.4. 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
tunnel ingress.
o IP-in-IP tunnels should not require changes to the end-points, but
raise issues when traversing firewalls and other security-type
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 tunneling control protocol to
negotiate support and validate the path.
o The remaining solutions offer similar deployability. UDP-Lite
requires support at both end-points and in middleboxes. UDPTT and
Zero-checksum with or without an Extension header require support
at both end-points and in middleboxes. UDP-Lite, UDPTT, and Zero-
checksum and Extension header may additionally require changes or
additions to the tunneling control protocol to negotiate support
and path validation.
4.2.5. 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
Zero-checksum, ignoring the checksum on reception and IP-and-IP
encapsulation provide no verification that a received packet was
intended to be processed by a specific tunnel egress or that the
inner packet was correct.
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4.2.6. 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:
Regular UDP: 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
and validation.
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.
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 and validation.
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 application level negotiation and
validation.
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 application level negotiation and validation.
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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 application level negotiation and validation.
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. UDP Zero-
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 zero-checksum becomes standardized the
fewer boxes will be non-compliant.
Thus, the question of whether to allow UDP with a zero-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 zero-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 authors do think the answer to the above questions are such that
zero-checksum should be standardized for use by tunnel
encapsulations.
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5. 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
the use of a UDP zero value in the checksum of a UDP datagram.
1. IPv6 nodes SHOULD by default NOT allow the zero checksum method
for transmission or reception.
2. The default node receiver behaviour MUST discard all IPv6 packets
carrying UDP datagrams with a zero checksum. IPv6 nodes MUST
provide a way for the application/protocol to indicate the set of
ports that will be enabled to send UDP datagrams with a zero
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.
3. IPv6 nodes MUST provide a way for the application/protocol to
indicate the set of ports that will be enabled to receive UDP
datagrams with a zero checksum.
4. RFC 2460 specifies that IPv6 nodes SHOULD log received UDP
datagrams with a zero-checksum. This should remain the case for
any datagram received on a port that does not explicitly enable
zero-checksum processing. A port for which zero-checksum has
been enabled MUST NOT log the datagram solely because the
checksum is zero, but MAY log this to support other functions
(such as a security policy).
5. IPv6 nodes MAY separately identify received UDP datagrams that
are discarded with a zero checksum. It SHOULD NOT add these to
the standard log, since the endpoint has not been verified.
6. 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).
6. Requirements on the specification of transported protocols
This section is an applicability statement that identifies
requirements and recommendations for protocols and tunnel
encapsulations that are transported over an IPv6 transport connection
that does not perform a UDP checksum calculation to verify the
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integrity at the transport endpoints.
1. UDP Tunnels that enable the use of zero checksum MUST only enable
this only for a specific port or port-range.
2. UDP Tunnels that encapsulate IP MAY rely on the inner packet
integrity checks provided that the tunnel will not significantly
increase the rate of corruption of the inner IP packet. If a
significantly increased corruption rate can occur, then the
tunnel MUST provide an additional integrity verification
mechanism. Early detection is desirable to avoid wasting
unneccessary computation/storage for packets that will
subsequently be discarded.
3. An integrity mechanisms is always RECOMMENDED at the tunnel layer
to ensure that corruption rates of the inner-most packet are not
increased. A mechanism that isolates the causes of corruption
(e.g. identifying mis-delivery, 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).
4. UDP Tunnels that encapsulate non-IP packets MUST have a CRC or
other mechanism for checking packet integrity, unless the non-IP
packet specifically is designed for transmission over lower
layers that do not provide any packet integrity guarantee. In
particular, the tunnel endpoint MUST be designed so that
corruption of this information does not result in accumulated
state or incorrect processing of a tunneled payload.
5. UDP Tunnels that support use of a zero-checksum, SHOULD NOT rely
upon correct reception of the IP and UDP protocol information
(including the length of the packet) when decoding and processing
the packet payload. In particular, the application MUST be
designed so that corruption of this information does not result
in accumulated state or incorrect processing of a tunneled
payload.
6. A UDP Tunnel egress that supports a zero UDP checksum MUST also
allow reception using a standard UDP checksum. The encapsulating
endpoint may choose to compute the UDP checksum, or the sending
endpoint IPv6 stack may enable this by default. In either case,
the remote endpoint uses the reception method specified in
RFC2460.
7. UDP Tunnels with control feedback need to be robust to changes in
network path. The set of middleboxes on a path may vary during
the life of an association. Endpoints need to discover paths
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with middleboxes that drop packets with a zero UDP checksum.
Therefore keep-alive messages SHOULD include both UDP datagrams
with a checksum and UDP datagrams with a zero checksum. This
will enable the remote endpoint to distinguish between a path
failure and dropping of UDP datagrams with a zero checksum. Note
that path validation need only be performed for each pair of
tunnel endpoints, not for each tunnel context.
8. Middleboxes implementations MUST allow IPv6 packets forward both
a zero and standard UDP checksum. A middlebox MAY configure
specific port ranges that forward UDP datagrams with a zero UDP
checksum. These middleboxes MUST forward both standard and zero
checksum UDP datagrams within the configured range, but may drop
IPv6 UDP datagrams with a zero checksum that are outside the
configured ranges.
7. Summary
This document examines the role of the transport checksum when used
with IPv6, as defined in RFC2460.
It presents a summary of the trade-offs for evaluating the safety of
updating RFC 2460 to permit an IPv6 UDP endpoint to use a zero value
in the checksum field to indicate that no checksum is present. A
decision not to include a UDP checksum in received IPv6 datagrams
could impact a tunnel application that receives these packets.
However, a well-designed tunnel application should include
consistency checks to validate any header information encapsulated
with a packet. In most cases tunnels encapsulating IP packets can
rely on the inner packets own integrity protection. When correctly
implemented, such a tunnel 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 applications at the intended destination node or another IPv6
node can be impacted if they are allowed to receive datagrams that do
not have a transport-layer checksum. It is particularly important
that already deployed applications are not impacted by any change at
the transport layer. If these applications execute on nodes that
implement RFC 2460, they will reject all datagrams with a zero UDP
checksum, thus this is not an issue. For nodes that implement
support for zero-checksum it is important to ensure that only UDP
applications that desire zero-checksum can receive and originate
zero-checksum packets. Thus, the enabling of zero-checksum needs to
be at a port level, not for the entire host or for all use of an
interface.
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The implications on firewalls, NATs and other middleboxes need to be
considered. It is not expected that IPv6 NATs handle IPv6 UDP
datagrams in the same way that they handle IPv4 UDP datagrams. This
possibly reduces the need to update the 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. Thus, relatively
few current middleboxes may actually block IPv6 UDP with a zero
checksum.
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 use of UDP with no checksum has merits for some applications,
such as tunnel encapsulation, and is widely used in IPv4. However,
there are dangers for IPv6: There is a bigger risk of corruption and
miss-delivery when using zero-checksum in IPv6 compared to IPv4 due
to the removed IP header checksum. Thus, applications need to make a
new evaluation of the risks of enabling a zero-checksum. Some
applications will need to re-consider their usage of zero-checksum,
and possibly 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. Tunnel applications using UDP for
encapsulation can in many case use zero-checksum without significant
impact on the corruption rate. In some cases, the use of checksum
off-loading may help alleviate the checksum processing cost.
Recursive tunneling and fragmentation is a difficult issue relating
to tunnels in general. 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 issue requires future thought and
consideration.
The conclusion is that UDP zero checksum in IPv6 should be
standardized, as it satisfies usage requirements that are currently
difficult to address. We do note that a safe deployment of zero-
checksum will need to follow a set of constraints listed in
Section 5.
8. Acknowledgements
Brian Haberman, Brian Carpenter, Magaret Wasserman, Lars Eggert,
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others in the TSV directorate.
Thanks also to: Remi Denis-Courmont, Pekka Savola and many others who
contributed comments and ideas via the 6man, behave, lisp and mboned
lists.
9. IANA Considerations
This document does not require any actions by IANA.
10. 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.
11. References
11.1. Normative References
[I-D.ietf-6man-udpchecksums]
Eubanks, M., Chimento, P., and M. Westerlund, "UDP
Checksums for Tunneled Packets",
draft-ietf-6man-udpchecksums-04 (work in progress),
September 2012.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, 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.
11.2. Informative References
[ECMP] "Using the IPv6 flow label for equal cost multipath
routing in tunnels (draft-carpenter-flow-ecmp)".
[I-D.ietf-intarea-tunnels]
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Internet-Draft Applicability of IPv6 UDP Zero Checksum October 2012
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),
June 2012.
[LISP] D. Farinacci et al, "Locator/ID Separation Protocol
(LISP)", March 2009.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[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.
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[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.
[RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", RFC 6145, April 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. 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.
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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.
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).
WoIt that UDP with a zero checksum in IPv6 can safely be used for
this purpose, provided that this usage is governed by a set of
constraints.rking Group Draft 07
* Resubmission after IESG Feedback
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* This document becomes a PS Applicability Statement
Authors' Addresses
Godred Fairhurst
University of Aberdeen
School of Engineering
Aberdeen, AB24 3UE,
Scotland, UK
Phone:
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
Fax:
Email: magnus.westerlund@ericsson.com
URI:
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