Internet Engineering Task Force G. Fairhurst
Internet-Draft University of Aberdeen
Intended status: Informational M. Westerlund
Expires: February 9, 2011 Ericsson Research
August 12, 2010
IPv6 UDP Checksum Considerations
draft-ietf-6man-udpzero-01
Abstract
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. The document describes issues
and design principles that need to be considered when UDP is used
with IPv6 to support tunnel encapsulations and provides
recommendations.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 9, 2011.
Copyright Notice
Copyright (c) 2010 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
Fairhurst & Westerlund Expires February 9, 2011 [Page 1]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Use of UDP Tunnels . . . . . . . . . . . . . . . . . . . . 5
1.2.1. Motivation for new approaches . . . . . . . . . . . . 5
1.2.2. Reducing forwarding cost . . . . . . . . . . . . . . . 6
1.2.3. Need to inspect the entire packet . . . . . . . . . . 7
1.2.4. Interactions with middleboxes . . . . . . . . . . . . 7
1.2.5. Support for load balancing . . . . . . . . . . . . . . 7
2. Standards-Track Transports . . . . . . . . . . . . . . . . . . 8
2.1. UDP with Standard Checksum . . . . . . . . . . . . . . . . 8
2.2. UDP-Lite . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.1. Using UDP-Lite as a Tunnel Encapsulation . . . . . . . 8
2.3. IP in IPv6 Tunnel Encapsulations . . . . . . . . . . . . . 9
3. Evaluation of proposal to update RFC 2460 to support zero
checksum . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Alternatives to the Standard Checksum . . . . . . . . . . 10
3.2. Applicability of method . . . . . . . . . . . . . . . . . 11
3.3. Effect of packet modification in the network . . . . . . . 12
3.3.1. Corruption of the destination IP address . . . . . . . 12
3.3.2. Corruption of the source IP address . . . . . . . . . 13
3.3.3. Delivery to an unexpected port . . . . . . . . . . . . 14
3.3.4. Validating the network path . . . . . . . . . . . . . 15
3.3.5. Requirements on the specification of transported
protocols . . . . . . . . . . . . . . . . . . . . . . 16
3.4. Comparision . . . . . . . . . . . . . . . . . . . . . . . 18
4. Requirements on the specification of transported protocols . . 18
4.1. Constraints required oin usage of a zero checksum . . . . 20
5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . . 23
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
9.1. Normative References . . . . . . . . . . . . . . . . . . . 23
9.2. Informative References . . . . . . . . . . . . . . . . . . 23
Appendix A. Document Change History . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
Fairhurst & Westerlund Expires February 9, 2011 [Page 2]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
1. Introduction
The User Datagram Protocol (UDP) transport was defined by RFC768
[RFC0768] for IPv4 RFC791 [RFC0791] and is defined in RFC2460
[RFC2460] for IPv6 hosts and routers. A UDP transport endpoint may
be either a host or a router. The UDP Usage Guidelines [RFC5405]
provides overall guidance for application designers, including the
use of UDP to support tunneling. These guidelines are applicable to
this discussion.
This section 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 evaluates proposals to update the UDP transport behaviour
to allow for better support of tunnel protocols. It focuses on a
proposal to eliminate the checksum for this use-case with IPv6 and
assess the trade-offs that would arise.
Section 4 reviews the trade offs and provides recommendations.
1.1. Background
An Internet transport endpoint should concern itself with the
following issues:
o Protection of the endpoint transport state from unnecessary extra
state (i.e. Invalid state from rogue packets).
o Protection of the endpoint transport state from corruption of
internal state.
o Pre-filtering by the endpoint of erroneous data, to protect the
transport from unnecessary processing and from corruption that it
can not itself reject.
o Pre-filter of incorrectly addressed destination packets, before
responding to a source address.
UDP, as defined in [RFC0768], supports two checksum behaviours when
used with IPv4. The normal behaviour is for the sender to calculate
a checksum over a block of data that includes a pseudo header and the
UDP datagram payload. The UDP header includes a 16-bit one's
complement checksum that provides a statistical guarantee that the
payload was not corrupted in transit. This also allows a receiver to
verify that the endpoint was the intended destination of the
Fairhurst & Westerlund Expires February 9, 2011 [Page 3]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
datagram, because the transport pseudo header covers the IP
addresses, port numbers, transport payload length, and Next Header/
Protocol value corresponding to the UDP transport protocol [RFC1071].
The length field verifies that the datagram is not truncated or
padded. The checksum therefore protects an application against
receiving corrupted payload data in place of, or in addition to, the
data that was sent. Although the IPv4 UDP [RFC0768] checksum may be
disabled, applications are recommended to enable UDP checksums
[RFC5405].
IPv4 UDP checksum control is often a kernel-wide configuration
control (e.g. In Linux and BSD), rather than a per socket call.
There are also Networking Interface Cards (NICs) that automatically
calculate TCP [RFC0793] and UDP checksums on transmission when a
checksum of zero is sent to the NIC, using a method known as checksum
offloading.
The network-layer fields that are validated by a transport checksum
are:
o Endpoint IP source address (always included in the pseudo header
of the checksum)
o Endpoint IP destination address (always included in the pseudo
header of the checksum)
o Upper layer payload type (always included in the pseudo header of
the checksum)
o IP length of payload (always included in the pseudo header of the
checksum)
o Length of the network layer extension headers (i.e. by correct
position of the checksum bytes)
The transport-layer fields that are validated by a transport checksum
are:
o Transport demultiplexing, i.e. ports (always included in the
checksum)
o Transport payload size (always included in the checksum)
Transport endpoints also need to verify the correctness of reassembly
of any fragmented datagram (unless the application using the payload
is corruption tolerant, as indicated by UDP-Lite's checksum coverage
field). For UDP, this is normally provided as a part of the
integrity check. Disabling the IPv4 checksum prevents this check. A
Fairhurst & Westerlund Expires February 9, 2011 [Page 4]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
lack of checksum can lead to issues in a translator or middlebox
(e.g. Many IPv4 Network Address Translators, NATs, rely on port
numbers to find the mappings, packet fragments do not carry port
numbers, so fragments get dropped). RFC2765 [RFC2765] provides some
guidance on the processing of fragmented IPv4 UDP datagrams that do
not carry a UDP checksum.
IPv6 does not provide a network-layer integrity check. The removal
of the header checksum from the IPv6 specification released routers
from a need to update a network-layer checksum for each router hop as
the IPv6 Hop Count is changed (in contrast to the checksum update
needed when an IPv4 router modifies the Time-To-Live (TTL)).
The IP header checksum calculation was seen as redundant for most
traffic (with UDP or TCP checksums enabled), and people wanted to
avoid this extra processing. However, there was concern that the
removal of the IP header checksum in IPv6 would lessen the protection
of the source/destination IP addresses and result in a significant (a
multiplier of ~32,000) increase in the number of times that a UDP
packet was accidentally delivered to the wrong destination address
and/or apparently sourced from the wrong source address when the UDP
checksum was set to zero. This would have had implications on the
detectability of mis-delivery of a packet to an incorrect endpoint/
socket, and the robustness of the Internet infrastructure. The use
of the UDP checksum is therefore required [RFC2460] when endpoint
application s transmit UDP datagrams over IPv6.
1.2. 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.
1.2.1. 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 currently being defined (e.g.
Fairhurst & Westerlund Expires February 9, 2011 [Page 5]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
Automated Multicast Tunnels, AMT [AMT], 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.
1.2.2. 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 Without Explicit Tunnels, known as AMT [AMT]
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). 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-
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. Relaxing RFC 2460 to minimise the
processing impact for existing hardware is a transition policy
decision, which seems undesirable if at the same time it yields a
Fairhurst & Westerlund Expires February 9, 2011 [Page 6]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
solution that may reduce stability and functionality in future
network scenarios.
1.2.3. 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.
1.2.4. Interactions with middleboxes
In IPv4, UDP-encapsulation may be desirable for NAT traversal, since
UDP support is commonly provided.
IPv6 NAT traversal does not necessarily present the same protocol
issues as for IPv4. It is not clear that NATs will work the same way
for IPv6. Any change to RFC 2460 would also require rewriting (or
defining) IPv6 NAT behaviour to achieve consistent widescale
deployment.
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 an updated IPv6
mode were to be defined for IPv6, this may also need firewalla to be
updated.
Key questions in this space include:
o What do IPv6 routers do today with zero-checksum UDP packets?
o What types of middleboxes does the tunnel protocol need to cross
(routers, NAT boxes, firewalls, etc.), and how will those
middleboxes deal with these packets?
o What other IPv6 middleboxes exist today, and what would they do?
1.2.5. Support for load balancing
The UDP port number fields have been used as a basis to design load-
balancing solutions for IPv4. This approach could also be leveraged
for IPv6. However, support for extension headers would increase the
complexity of providing standards-compliant solutions for IPv6.
Fairhurst & Westerlund Expires February 9, 2011 [Page 7]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
An alternate method could utilise the IPv6 Flow Label to perform load
balancing. This would release IPv6 load-balancing devices from the
need to assume semantics for the use of the transport port field.
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.
2. Standards-Track Transports
The IETF has defined a set of IPv6 transports that at be used with
IPv6. These are described in the following sections, followed by a
description of standards tunnel encapsulations.
2.1. UDP with Standard Checksum
UDP with standard checksum behaviour is defined in RFC 2460, and
should be the default choice. 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 RFC 5097 and
unicast usage guidelines in [RFC5405].
UDP-Lite provides a checksum with an 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). 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. Over radio access bearers). When the checksum covers
the entire packet, which should be the default.
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), 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.
In the LISP case, the bytes that would need to be "checksummed" for
Fairhurst & Westerlund Expires February 9, 2011 [Page 8]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
UDP-Lite would be the set of bytes that are added to the packet by
the LISP encapsulating router. When an IPv4/UDP header is per-pended
by a LISP router, the LISP ETR needs to calculate the IP header
checksum over 20 bytes (the IP header). If an IPv6/UDP-Lite header
were per-pended by a LISP router, the ETR would need to calculate an
IP header checksum over 48 bytes (the IP pseudo header and the UDP
header). This results in an increase in the number of bytes to be
the checksummed for IPv6 (48 bytes rather than 20), but this is not
thought to be a major additional processing overhead for a well-
optimized implementation where the pre-pended header bytes are
already in memory.
2.3. IP in IPv6 Tunnel Encapsulations
The IETF has defined a set of tunneling protocols. These do not
include a checksum, since tunnel encapsulations are typically layered
directly over the Internet layer (identified by the upper layer type
field) and are also not used as endpoint transport protocols. That
is, 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. Evaluation of proposal to update RFC 2460 to support zero checksum
This section evaluates a proposal to update IPv6 [RFC2460], to
provide the option that some nodes may suppress generation and
checking of the UDP transport checksum. The decision to omit an
integrity check at the IPv6 level means that the transport check is
overloaded with many functions including validating:
o the endpoint address was not corrupted within a router - i.e.
This packet was intended to be received by this destination and a
wrong header has not been 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;
Fairhurst & Westerlund Expires February 9, 2011 [Page 9]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
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
[RFC2463] 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. Alternatives to the Standard Checksum
There are several alternatives to the normal method for calculating
the UDP Checksum 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 (RFC 1624), 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). The process may be easier for IPv4 over IPv6
encapsulation, where the encapsulated IPv4 header checksum could
be used as a basis.
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
Fairhurst & Westerlund Expires February 9, 2011 [Page 10]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
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 flow. The value may be cached per
flow/destination to minimise per-packet processing. This proposal
is not discussed further in this document, since function is
nearly the same as for UDP-Lite.
o Use of a new IPv6 Extension Header that provides 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.
o UDP modified to disable checksum processing[UDPZ] (if progressed).
This requires no checksum calculation, but would require
constraints on appropriate usage.
These options are discussed further in the following sections.
3.2. Applicability of method
The expectation of the present proposal defined in [UDPZ] is that
this change would only apply to IPv6 router nodes that implement
specific protocols which 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.
There are additional requirements, e.g. fragmentation must not be
performed, since correct reassembly can not be verified at the
receiver when there is no checksum. Allowing fragmentation would
also open the receiver to a wide range of mis-behaviours. Host-based
fragmentation must therefore be disabled. Policing this, and
ensuring correct interactions with the stack, implies much more than
simply disabling the checksum algorithm for specific packets at the
transport interface.
There are also proposals to simply ignore a specific received UDP
checksum value, however this also can result in problems (e.g. when
used with a NAT that always adjusts the checksum value).
The IETF should carefully consider constraints on sanctioning the use
Fairhurst & Westerlund Expires February 9, 2011 [Page 11]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
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, also 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.
3.3. 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 (RFC 3819). 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 with UDP over IPv6, 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.
3.3.1. Corruption of the destination IP address
An IP endpoint destination address could be modified in the network
(e.g. corrupted by an error). This is not a concern for IPv4,
Fairhurst & Westerlund Expires February 9, 2011 [Page 12]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
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 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 this 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).
3.3.2. 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. 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 Prottocol
(RTP) flows commonly employ a source independent receiver port.
State is created for each received flow. Reception of a datagram
Fairhurst & Westerlund Expires February 9, 2011 [Page 13]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
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.
3.3.3. Delivery to an unexpected port
This section considers 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 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).
There are several possible outcomes for a packet that passes and 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 source address and the destination port of the
datagram (the source port is not always used in UDP) match those of
an existing connection. Unfortunately, such a match may be more
likely for UDP than for connection-oriented transports, because
Fairhurst & Westerlund Expires February 9, 2011 [Page 14]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
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. The
encapsulated tunnel protocol would need to perform its own integrity
checks on any control information and ensure an integrity check is
applied to the tunneled packet. It is not reasonable to assume that
it is safe for one application to use a zero checksum value and that
other applications will not. It is therefore important to consider
the possibility that a packet will be received by a different node to
that for which it was intended, or that it will arrive at the correct
tunnel destination with the wrong source address in the external
header.
3.3.4. Validating the network path
IP transports designed for use in the general Internet should not
assume specific characteristics. Network protocols may reroute
packets and change the set of routers and middleboxes along a path.
Therefore transports such as TCP, SCTP and DCCP are designed to
negotiate protocol parameters, adapt to different network path
characteristics, and receive feedback 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.
Any application/tunnel that seeks to make use of zero checksum must
include functionality to both negotiate and verify that the zero
checksum support is provided by the path and validate that this
continues to work (e.g., in the case of re-routing events) between
the intended parties. This increases the complexity of using such a
solution.
Fairhurst & Westerlund Expires February 9, 2011 [Page 15]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
3.3.5. Requirements on the specification of transported protocols
If the IETF were to revise the standard for UDP using IPv6 for
specific use-cases there are a set of questions that would need to be
answered. These include:
Is there a reason why IP in IP is not a reasonable choice for
encapsulation?
o Examples of arguments for requiring an encapsulation beyond
IP-in-IP include the need for NAT traversal and/or firewall
traversal. However, the use of any new or non-standard transport
protocol or variant would additionally require specific support in
middleboxes.
o Another example is a need to perform port-demultiplexing (e.g. for
load balancing or ECMP). This need could also be met using UDP,
UDP-Lite, or another supported transport, or by utilising the IPv6
flow label.
Is there a reason why UDP-Lite is not a reasonable choice for
encapsulation?
o One argument against using UDP-Lite is that this transport is not
implemented on all endpoints. However, there is at least one open
source implementation as a part of the Linux kernel since version
2.6.20.
o Another argument against using UDP-Lite is that it uses a
different IPv6 Next Header, which is currently not widely
supported in middleboxes.
o It has also been argued that UDP-Lite requires a checksum
computation. The UDP-Lite checksum, for instance includes the
length field, but need not include the UDP-Lite payload, and
therefore would not require access to the full datagram payload by
the tunnel endpoints.
If the IETF needs to revise the rationale for UDP checksums in RFC
2460, should we remove the checksum or replace it with one closer to
UDP-Lite ?
Additional topics to be considered in making this decision:
o In IPv6, a node selects the role of a router or host is selected
on a per interface basis. The role of a router and host are
therefore not fixed, and a consistent method must be specified
that can be used on all nodes. It can not be assumed that a
Fairhurst & Westerlund Expires February 9, 2011 [Page 16]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
particular protocol (or transport mode) will only be used on a
specific type of network node (e.g. permitting the UDP checksum to
be disabled only on a router). It is important to note that
protocol changes intended for one specific use are often re-used
for different applications.
o Behaviour of NAT/Middleboxes may need to be updated. This is the
case for a zero UDP checksum and also for use of an IPv6 Extension
Header carrying a transport checksum.
o The method needs to consider the impact of load balancing, and
whether this may be enabled for the chosen transport protocol.
If a zero checksum approach were to be adopted by the IETF, the
specification should consider adding the following constraints on
usage:
1. IPv6 protocol stack implementations should not by default allow
the new method. The default node behaviour must discard all IPv6
packets carrying UDP packets with no checksum. RFC 2460
specifies that IPv6 nodes should log discarded packets.
2. A method must be specified to verify the integrity of the inner
(tunneled) packet for each tunnel application that uses a zero-
checksum. This method must be robust to the use of other
applications that also use a zero-checksum.
3. Non-IP inner (tunneled) packets must have a CRC or other
mechanism for checking packet integrity.
4. If a method proposes selective ignoring of the checksum on
reception, it needs to provide guidance that is appropriate for
all use-cases, including defining how currently standardised
nodes handle any new use.
5. The tunneling protocol must not allow fragmentation of the inner
packets being carried. We suggest the following elaborations of
the above restrictions, if a change in the IPv6 specification
moves forward, the tunnel must not forward an inner (tunneled)
IPv4 packet that also has a UDP checksum equal to 0. This
includes not tunneling other tunneling protocols that also use a
UDP checksum equal to 0, even if more deeply encapsulated packets
have checksums or other integrity checking mechanisms.
6. If a method proposes recursive tunnels, it needs to provide
guidance that is appropriate for all use-cases. Restrictions may
be needed to the use of a tunnel encapsulations and the use of
recursive tunnels (e.g. Necessary when the endpoint is not
Fairhurst & Westerlund Expires February 9, 2011 [Page 17]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
verified).
7.
8. The new method should remain restricted to endpoints that
explicitly enable this mode and adopt the above procedures.
3.4. Comparision
This section compares different methods to support datagram
tunneling. This includes a proposal for updating the behaviour of
UDP. This is provided as an example, and does not seek to endorse
any specific method or suggest that these proposals are ready to be
standardised. The final column the expected functions if an
additional end-to-end IPv6 extension header were to be required in
combination with use of the zero checksum option.
Comparison of functions for selected methods
UDP UDPv4 UDPL IP IP UDPv6 UDPv6 UPv6
zero in in zero EH
IPv4 IPv6
Incremental cksum update? X - X N/A N/A X - ?
Verification of IP length? X X X X X X X X
Detect dest addr corruption? X X X X - X - X
Detect NH addr corruption? - - - X - - - X
Flow demux fields present? X X X - X X X -
Detect port corruption? X - X N/A N/A X - -
Detect illegal pay length? X X - N/A N/A X X X
Detect pay corruption? X - ? N/A N/A X - -
Static cksum per flow? - X - N/A N/A - X X
Partial/full midbox support? X * ? ? ? X ? ?
Restricted tunnel behaviour X * X X ? X - -
X = Provided/supported
- = Not provided/supported
N/A = Not applicable
? = Partial support
* = Supports a subset of functions (i.e. not all combinations)
Table 1
4. Requirements on the specification of transported protocols
If the IETF were to revise the standard for UDP using IPv6 for
specific use-cases there are a set of questions that would need to be
answered. These include:
Fairhurst & Westerlund Expires February 9, 2011 [Page 18]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
Is there a reason why IP in IP is not a reasonable choice for
encapsulation?
o Examples of arguments for requiring an encapsulation beyond
IP-in-IP include the need for NAT traversal and/or firewall
traversal. However, the use of any new or non-standard transport
protocol or variant would additionally require specific support in
middleboxes.
o Another example is a need to perform port-demultiplexing (e.g. for
load balancing or ECMP). This need could also be met using UDP,
UDP-Lite, or another supported transport, or by utilising the IPv6
flow label.
Is there a reason why UDP-Lite is not a reasonable choice for
encapsulation?
o One argument against using UDP-Lite is that this transport is not
implemented on all endpoints. However, there is at least one open
source implementation as a part of the Linux kernel since version
2.6.20.
o Another argument against using UDP-Lite is that it uses a
different IPv6 Next Header, which is currently not widely
supported in middleboxes.
o It has also been argued that UDP-Lite requires a checksum
computation. The UDP-Lite checksum, for instance includes the
length field, but need not include the UDP-Lite payload, and
therefore would not require access to the full datagram payload by
the tunnel endpoints.
If the IETF needs to revise the rationale for UDP checksums in RFC
2460, should we remove the checksum or replace it with one closer to
UDP-Lite ?
Additional topics to be considered in making this decision:
o In IPv6, a node selects the role of a router or host is selected
on a per interface basis. The role of a router and host are
therefore not fixed, and a consistent method must be specified
that can be used on all nodes. It can not be assumed that a
particular protocol (or transport mode) will only be used on a
specific type of network node (e.g. permitting the UDP checksum to
be disabled only on a router). It is important to note that
protocol changes intended for one specific use are often re-used
for different applications.
Fairhurst & Westerlund Expires February 9, 2011 [Page 19]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
o Behaviour of NAT/Middleboxes may need to be updated. This is the
case for a zero UDP checksum and also for use of an IPv6 Extension
Header carrying a transport checksum.
o The method needs to consider the impact of load balancing, and
whether this may be enabled for the chosen transport protocol.
o If a method proposes selective ignoring of the checksum on
reception, it needs to provide guidance that is appropriate for
all use-cases, including defining how currently standardised nodes
handle any new use.
4.1. Constraints required oin usage of a zero checksum
If a zero checksum approach were to be adopted by the IETF, the
specification should consider adding the following constraints on
usage:
1. IPv6 protocol stack implementations should not by default allow
the new method. The default node receiver behaviour must
discard all IPv6 packets carrying UDP packets with no checksum.
2. Implementations must provide a way to signal which ports will be
enabled to receive UDP datagrams with a zero checksum. An IPv6
node that enables reception of must enable this only for a
specific port or port-range. This may be implemented via a
socket API call, or similar mechanism.
3. RFC 2460 specifies that IPv6 nodes should log 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.
4. (that pass the checksum). A stack may separately identify 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.
5. A method must be specified to verify the integrity of the inner
(tunneled) packet for each tunnel application that uses a zero-
checksum. This method must be robust to the use of other
applications that also use a zero-checksum.
6. Non-IP inner (tunneled) packets must have a CRC or other
mechanism for checking packet integrity.
Fairhurst & Westerlund Expires February 9, 2011 [Page 20]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
7. UDP applications that support use of a zero-checksum, should not
rely upon correct reception of the IP and UDP protocol
information 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.
8. The tunnel must not forward an inner (tunneled) IPv4 packet that
also has a UDP checksum equal to 0. This includes not tunneling
other tunneling protocols that also use a UDP checksum equal to
0, even if more deeply encapsulated packets have checksums or
other integrity checking mechanisms.
9. The tunneling protocol must not allow fragmentation of the inner
packets being carried.
10. If a method proposes recursive tunnels, it needs to provide
guidance that is appropriate for all use-cases. Restrictions
may be needed to the use of a tunnel encapsulations and the use
of recursive tunnels (e.g. Necessary when the endpoint is not
verified).
11. IPv6 nodes that receive ICMPv6 messages that relate to packets
with a zero UDP checksum must provide appropriate checks
concerning the consistency of the reported packet was actually
originated by the node, before acting upon the information (e.g.
validating the address and port numbers in the ICMPv6 message
body).
Deployment of the new method should remain restricted to endpoints
that explicitly enable this mode and adopt the above procedures
5. 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 and ensure that an integrity check is included for each
tunneled packet. When correctly implemented, such a tunnel endpoint
will not be negatively impacted by omission of the transport-layer
Fairhurst & Westerlund Expires February 9, 2011 [Page 21]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
checksum. However, other applications at the intended destination
node or another IPv6 node can be impacted if they are allowed to
receive datagrams without a transport-layer checksum.
In particular, it is 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 without a UDP checksum.
The implications on firewalls, NATs and other middleboxes need to be
considered. It should not be expected that NATs handle IPv6 UDP
datagrams in the same way as they handle IPv4 UDP datagrams.
Firewalls are intended to be configured, and therefore may need to be
explicitly updated to allow new services or protocols.
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 applications
data stream 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.
Although the use of UDP over IPv6 with no checksum may have merits as
a tunnel encapsulation and is widely used in IPv4, there are dangers
for IPv6 nodes (hosts and routers). If the use of UDP transport
without a checksum were to become prevalent for IPv6 (e.g. tunnel and
host applications using this are widely deployed), there would also
be a significant danger of the Internet carrying an increased volume
of packets without a transport checksum for other applications,
potentially including applications that have traditionally used IPv4
UDP transport without a checksum. This result is highly undesirable.
Other solutions need to be found, such as the use of IPV6 with the
minimal checksum coverage for UDP-Lite. This requires that the IPv4
and IPv6 solutions to differ, since there are different deployed
infrastructures.
Guidance has also been provided to help evaluate the case for
disabling the checksum for specific applications
6. Acknowledgements
Brian Haberman, Brian Carpenter, Magaret Wasserman, Lars Eggert,
Magnus Westerlund, 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.
Fairhurst & Westerlund Expires February 9, 2011 [Page 22]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
7. IANA Considerations
This document does not require IANA considerations.
8. 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.
9. References
9.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1071] Braden, R., Borman, D., Partridge, C., and W. Plummer,
"Computing the Internet checksum", RFC 1071,
September 1988.
[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.
9.2. Informative References
[AMT] Internet draft, draft-ietf-mboned-auto-multicast-10,
"Automatic IP Multicast Without Explicit Tunnels (AMT)",
March 2010.
[ECMP] "Using the IPv6 flow label for equal cost multipath
routing in tunnels (draft-carpenter-flow-ecmp)".
[LISP] Internet draft, draft-farinacci-lisp-12.txt, "Locator/ID
Separation Protocol (LISP)", March 2009.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
Fairhurst & Westerlund Expires February 9, 2011 [Page 23]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
[RFC1141] Mallory, T. and A. Kullberg, "Incremental updating of the
Internet checksum", RFC 1141, January 1990.
[RFC2463] Conta, A. and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", RFC 2463, December 1998.
[RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm
(SIIT)", RFC 2765, February 2000.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
G. Fairhurst, "The Lightweight User Datagram Protocol
(UDP-Lite)", RFC 3828, July 2004.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405,
November 2008.
[Sigcomm2000]
http://conferences.sigcomm.org/sigcomm/2000/conf/abstract/
9-1.htm, "When the CRC and TCP Checksum Disagree", 2000.
[UDPTT] "The UDP Tunnel Transport mode", Feb 2010.
[UDPZ] "UDP Checksums for Tunneled Packets", (Oct 2009.
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).
Fairhurst & Westerlund Expires February 9, 2011 [Page 24]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
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.
**TO BE DONE **
* This version requires review from proponents and opponents to
the UDP zero checksum proposal.
* Work still to be done includes:
1. Text on issues with fragmentation needs to be updated to
provide more clarity on issues.
2. Need a recommendation on whether to permit a multicast
destination address with a zero UDP checksum.
3. Is it OK to send ICMPv6 messages in response to non-
delivered UDP datagrams with a zero checksum?
4. The final section may need to be reworked if this document
recommends a specific change to RFC 2460.
Fairhurst & Westerlund Expires February 9, 2011 [Page 25]
Internet-Draft IPv6 UDP Checksum Considerations August 2010
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 Research
Torshamgatan 23
Stockholm, SE-164 80
Sweden
Phone:
Fax:
Email: magnus.westerlund@ericsson.com
URI:
Fairhurst & Westerlund Expires February 9, 2011 [Page 26]
