Network Working Group M. Eubanks
Internet-Draft AmericaFree.TV LLC
Updates: 2460 (if approved) P. Chimento
Intended status: Standards Track Johns Hopkins University Applied
Expires: April 25, 2013 Physics Laboratory
M. Westerlund
Ericsson
October 22, 2012
UDP Checksums for Tunneled Packets
draft-ietf-6man-udpchecksums-05
Abstract
This document provides an update of the Internet Protocol version 6
(IPv6) specification (RFC2460) to improve the performance of IPv6 in
the use case when a tunnel protocol uses UDP with IPv6 to tunnel
packets. The performance improvement is obtained by relaxing the
IPv6 UDP checksum requirement for suitable tunneling protocol where
header information is protected on the "inner" packet being carried.
This relaxation removes the overhead associated with the computation
of UDP checksums on IPv6 packets used to carry tunnel protocols and
thereby improves the efficiency of the traversal of firewalls and
other network middleboxes by such protocols. We describe how the
IPv6 UDP checksum requirement can be relaxed in the situation where
the encapsulated packet itself contains a checksum, the limitations
and risks of this approach, and define restrictions on the use of
this relaxation to mitigate these risks.
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
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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 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Some Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. The Zero-Checksum Update . . . . . . . . . . . . . . . . . . . 7
6. Additional Observations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
This work constitutes an update of the Internet Protocol Version 6
(IPv6) Specification [RFC2460], in the use case when a tunnel
protocol uses UDP with IPv6 to tunnel packets. With the rapid growth
of the Internet, tunneling protocols have become increasingly
important to enable the deployment of new protocols. Tunneled
protocols can be deployed rapidly, while the time to upgrade and
deploy a critical mass of routers, switches and end hosts on the
global Internet for a new protocol is now measured in decades. At
the same time, the increasing use of firewalls and other security
related middleboxes means that truly new tunnel protocols, with new
protocol numbers, are also unlikely to be deployable in a reasonable
time frame, which has resulted in an increasing interest in and use
of UDP-based tunneling protocols. In such protocols, there is an
encapsulated "inner" packet, and the "outer" packet carrying the
tunneled inner packet is a UDP packet, which can pass through
firewalls and other middleboxes filtering that is a fact of life on
the current Internet.
Tunnel endpoints may be routers or middleboxes aggregating traffic
from a large number of tunnel users, therefore the computation of an
additional checksum on the outer UDP packet, may be seen as an
unwarranted burden on nodes that implement a tunneling protocol,
especially if the inner packet(s) are already protected by a
checksum. In IPv4, there is a checksum on the IP packet itself, and
the checksum on the outer UDP packet can be set to zero. However in
IPv6 there is not a checksum on the IP packet and RFC 2460 [RFC2460]
explicitly states that IPv6 receivers MUST discard UDP packets with a
zero checksum. So, while sending a UDP packet with a zero checksum
is permitted in IPv4 packets, it is explicitly forbidden in IPv6
packets. To improve support for IPv6 UDP tunnels, this document
updates RFC 2460 to allow tunnel endpoints to use a zero UDP checksum
under constrained situations (IPv6 tunnel transports that carry
checksum-protected packets), following the considerations in
[I-D.ietf-6man-udpzero].
Unicast UDP Usage Guidelines for Application Designers [RFC5405]
should be consulted when reading this specification. It discusses
both UDP tunnels (Section 3.1.3) and the usage of Checksums (Section
3.4).
While the origin of this specification is the problem raised by the
draft titled "Automatic IP Multicast Without Explicit Tunnels", also
known as "AMT," [I-D.ietf-mboned-auto-multicast] we expect it to have
wide applicability. Since the first version of this document, the
need for an efficient UDP tunneling mechanism has increased. Other
IETF Working Groups, notably LISP [I-D.ietf-lisp] and Softwires
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[RFC5619] have expressed a need to update the UDP checksum processing
in RFC 2460. We therefore expect this update to be applicable in
future to other tunneling protocols specified by these and other IETF
Working Groups.
2. Some Terminology
For the remainder of this document, we discuss only IPv6, since this
problem does not exist for IPv4. Therefore all reference to 'IP'
should be understood as a reference to IPv6.
The document uses the terms "tunneling" and "tunneled" as adjectives
when describing packets. When we refer to 'tunneling packets' we
refer to the outer packet header that provides the tunneling
function. When we refer to 'tunneled packets' we refer to the inner
packet, i.e., the packet being carried in the tunnel.
2.1. Requirements Language
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 RFC 2119 [RFC2119].
3. Problem Statement
This document provides an update for the case where a tunnel protocol
transports tunneled packets that already have a transport header with
a checksum. There is both a benefit and a cost to computing and
checking the UDP checksum of the outer (encapsulating) UDP transport
header. In certain cases, where reducing the forwarding cost is
important, such as for systems that perform the check in software,
the cost may outweigh the benefit; this document describes a means to
avoid that cost. In the case where there is an inner header with a
checksum.
4. Discussion
IPv6 UDP Checksum Considerations [I-D.ietf-6man-udpzero] describes
the issues related to allowing UDP over IPv6 to have a valid checksum
of zero and is not repeated here.
Section 5 and 6 of [I-D.ietf-6man-udpzero], identifies node and inner
protocol requirements respectively that introduce constraints on the
usage of a zero checksum for UDP over IPv6. This document is
intended to satisfy these requirements.
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[I-D.ietf-6man-udpzero] and mailing list discussions have noted there
is still the possibility of deep-inspection firewall devices or other
middleboxes checking the UDP checksum field of the outer packet and
thereby discarding the tunneling packets. This would be an issue
also for any legacy IPv6 system that has not implemented this update
to the IPv6 specification. In this case, the system (according to
RFC 2460) will discard the zero-checksum UDP packets, and should log
this as an error.
The points below discuss how path errors can be detected and handled
in an UDP tunneling protocol when the checksum protection is
disabled. Note that other (non-tunneling) protocols may have
different approaches, but these are not the topic of this update. We
propose the following approach to handle this problem:
o Context (i.e. tunneling state) should be established via
application Protocol Data Units (PDUs) that are carried in
checksummed UDP packets. That is, any control packets flowing
between the tunnel endpoints should be protected by UDP checksums.
The control packets can also contain any negotiation required to
enable the endpoint/adapters to accept UDP packets with a zero
checksum. The control packets may also carry any negotiation
required to enable the endpoint/adapters to identify the set of
ports that need to enable reception of UDP datagrams with a zero
checksum.
o A system never sets the UDP checksum to zero in packets that do
not contain tunneled packets.
o UDP keep-alive packets with checksum zero can be sent to validate
paths, given that paths between tunnel endpoints can change and so
middleboxes in the path may vary during the life of the
association. Paths with middleboxes that are intolerant of a UDP
checksum of zero will drop the keep-alives and the endpoints will
discover that. Note that this need only be done per tunnel
endpoint pair, not per tunnel context. Keep-alive traffic can
include both packets with tunnel checksums and packets with
checksums equal to zero to enable the remote end to distinguish
between path failures and the blockage of packets with checksum
equal to zero.
o Corruption of the encapsulating IPv6 source address, destination
address and/or the UDP source port, and destination port fields :
If the restrictions in [I-D.ietf-6man-udpzero] are followed, the
inner packets (tunneled packets) will be protected and run the
usual (presumably small) risk of having undetected corruption(s).
If tunneling protocol contexts contain (at a minimum) source and
destination IP addresses and source and destination ports, there
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are 16 possible corruption outcomes. We note that these outcomes
are not equally likely. The possible corruption outcomes may be:
* Half of the 16 possible corruption combinations have a
corrupted destination address. If the incorrect destination is
reached and the node doesn't have an application for the
destination port, the packet will be dropped. If the
application at the incorrect destination is the same tunneling
protocol and if it has a matching context (which can be assumed
to be a very low probability event) the inner packet will be
decapsulated and forwarded. Application developers can verify
the context of the packets they receive using UDP, as described
in [RFC5405]. Applications that verify the context of a
datagram are expected to have a high probability of discarding
corrupted data. [I-D.ietf-6man-udpzero] presents examples of
cases where corruption can inadvertently impact application
state.
* Half of the 8 possible corruption combinations with a correct
destination address have a corrupted source address. If the
tunnel contexts contain all elements of the address-port
4-tuple, then the likelihood is that this corruption will be
detected. It may in fact be discarded on route due to source
address validation techniques, such as Unicast Reverse Path
Forwarding [RFC2827].
* Of the remaining 4 possibilities, with valid source and
destination IPv6 addresses, one has all 4 fields valid, the
other three have one or both ports corrupted. Again, if the
tunneling endpoint context contains sufficient information,
these errors should be detected with high probability.
o Corruption of source-fragmented encapsulating packets: In this
case, a tunneling protocol may reassemble fragments associated
with the wrong context at the right tunnel endpoint, or it may
reassemble fragments associated with a context at the wrong tunnel
endpoint, or corrupted fragments may be reassembled at the right
context at the right tunnel endpoint. In each of these cases, the
IPv6 length of the encapsulating header may be checked (though
[I-D.ietf-6man-udpzero] points out the weakness in this check).
In addition, if the encapsulated packet is protected by a
transport (or other) checksum, these errors can be detected (with
some probability).
While they do not guarantee correctness, these mechanism can reduce
the risks of relaxing the UDP checksum requirement for IPv6.
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5. The Zero-Checksum Update
This specification updates IPv6 to allow a UDP checksum of zero for
the outer encapsulating packet of a tunneling protocol. UDP
endpoints that implement this update MUST change their behavior for
any destination port explicitly configured for zero checksum and MUST
NOT discard UDP packets received with a checksum value of zero on the
outer packet. When this is done, it requires the constraints in
Section 5 and 6 of [I-D.ietf-6man-udpzero].
Specifically, the text in [RFC2460] Section 8.1, 4th bullet is
updated. We refer to the following text:
"Unlike IPv4, when UDP packets are originated by an IPv6 node, the
UDP checksum is not optional. That is, whenever originating a UDP
packet, an IPv6 node must compute a UDP checksum over the packet and
the pseudo-header, and, if that computation yields a result of zero,
it must be changed to hex FFFF for placement in the UDP header. IPv6
receivers must discard UDP packets containing a zero checksum, and
should log the error."
This item should be taken out of the bullet list and should be
replaced by:
Whenever originating a UDP packet, an IPv6 node SHOULD compute a
UDP checksum over the packet and the pseudo-header, and, if that
computation yields a result of zero, it must be changed to hex
FFFF for placement in the UDP header. IPv6 receivers SHOULD
discard UDP packets containing a zero checksum, and SHOULD log the
error. However, some protocols, such as tunneling protocols that
use UDP as a tunnel encapsulation, MAY omit computing the UDP
checksum of the encapsulating UDP header and set it to zero,
subject to the constraints described in Applicability Statement
for the use of IPv6 UDP Datagrams with Zero Checksums
[I-D.ietf-6man-udpzero]. In cases where the encapsulating
protocol uses a zero checksum for UDP, the receiver of packets
sent to a port enabled to receive zero-checksum packets MUST NOT
discard packets solely for having a UDP checksum of zero. Note
that these constraints apply only to encapsulating protocols that
omit calculating the UDP checksum and set it to zero. An
encapsulating protocol can always choose to compute the UDP
checksum, in which case, its behavior is not updated and uses the
method specified in Section 8.1 of RFC2460.
Middleboxes MUST allow IPv6 packets with UDP checksum equal to
zero to pass. Implementations of middleboxes MAY allow
configuration of specific port ranges for which a zero UDP
checksum is valid and may drop IPv6 UDP packets outside those
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ranges.
The path between tunnel endpoints can change, thus also the
middleboxes in the path may vary during the life of the
association. Paths with middleboxes that are intolerant of a UDP
checksum of zero will drop any keep-alives sent to validate the
path using checksum zero and the endpoints will discover that.
Therefore keep-alive traffic SHOULD include both packets with
tunnel checksums and packets with checksums equal to zero to
enable the remote end to distinguish between path failures and the
blockage of packets with checksum equal to zero. Note that path
validation need only be done per tunnel endpoint pair, not per
tunnel context.
6. Additional Observations
The existence of this issue among a significant number of protocols
being developed in the IETF motivates this specified change. The
authors would also like to make the following observations:
o An empirically-based analysis of the probabilities of packet
corruptions (with or without checksums) has not (to our knowledge)
been conducted since about 2000. It is now 2012. We strongly
suggest that an empirical study is in order, along with an
extensive analysis of IPv6 header corruption probabilities.
o A key cause to the increased usage of UDP in tunneling is the lack
of protocol support in middleboxes. Specifically, new protocols,
such as LISP [I-D.ietf-lisp], prefer to use UDP tunnels to
traverse an end-to-end path successfully and avoid having their
packets dropped by middleboxes. If this were not the case, the
use of UDP-lite [RFC3828] might become more viable for some (but
not necessarily all) tunneling protocols.
o Another issue is that the UDP checksum is overloaded with the task
of protecting the IPv6 header for UDP flows (as is the TCP
checksum for TCP flows). Protocols that do not use a pseudo-
header approach to computing a checksum or CRC have essentially no
protection from mis-delivered packets.
7. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
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8. Security Considerations
It requires less work to generate zero-checksum attack packets than
ones with full UDP checksums. However, this does not lead to any
significant new vulnerabilities as checksums are not a security
measure and can be easily generated by any attacker. Properly
configured tunnels should check the validity of the inner packet and
perform any needed security checks, regardless of the checksum
status. Most attacks are generated from compromised hosts which
automatically create checksummed packets (in other words, it would
generally be more, not less, effort for most attackers to generate
zero UDP checksums on the host).
9. Acknowledgements
We would like to thank Brian Haberman and Gorry Fairhurst for
discussions and reviews.
10. References
10.1. Normative References
[I-D.ietf-6man-udpzero]
Fairhurst, G. and M. Westerlund, "Applicability Statement
for the use of IPv6 UDP Datagrams with Zero Checksums",
draft-ietf-6man-udpzero-07 (work in progress),
October 2012.
[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.
[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.
[RFC5619] Yamamoto, S., Williams, C., Yokota, H., and F. Parent,
"Softwire Security Analysis and Requirements", RFC 5619,
August 2009.
10.2. Informative References
[I-D.ietf-lisp]
Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
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"Locator/ID Separation Protocol (LISP)",
draft-ietf-lisp-23 (work in progress), May 2012.
[I-D.ietf-mboned-auto-multicast]
Bumgardner, G., "Automatic Multicast Tunneling",
draft-ietf-mboned-auto-multicast-14 (work in progress),
June 2012.
[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.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405,
November 2008.
Authors' Addresses
Marshall Eubanks
AmericaFree.TV LLC
P.O. Box 141
Clifton, Virginia 20124
USA
Phone: +1-703-501-4376
Fax:
Email: marshall.eubanks@gmail.com
P.F. Chimento
Johns Hopkins University Applied Physics Laboratory
11100 Johns Hopkins Road
Laurel, MD 20723
USA
Phone: +1-443-778-1743
Email: Philip.Chimento@jhuapl.edu
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Magnus Westerlund
Ericsson
Farogatan 6
SE-164 80 Kista
Sweden
Phone: +46 10 714 82 87
Email: magnus.westerlund@ericsson.com
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