Internet DRAFT - draft-ietf-6man-udpchecksums
draft-ietf-6man-udpchecksums
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: August 25, 2013 Physics Laboratory
M. Westerlund
Ericsson
February 21, 2013
IPv6 and UDP Checksums for Tunneled Packets
draft-ietf-6man-udpchecksums-08
Abstract
This document provides an update of the Internet Protocol version 6
(IPv6) specification (RFC2460) to improve the performance in the use
case where a tunnel protocol uses UDP with IPv6 to tunnel packets.
The performance improvement is obtained by relaxing the IPv6 UDP
checksum requirement for any suitable tunnel 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. The
specification describes how the IPv6 UDP checksum requirement can be
relaxed for the situation where the encapsulated packet itself
contains a checksum. The limitations and risks of this approach are
described, and restrictions specified on the use of the method.
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 August 25, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
Eubanks, et al. Expires August 25, 2013 [Page 1]
Internet-Draft udp-checksum February 2013
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
4.1. Analysis of Corruption in Tunnel Context . . . . . . . . . 5
4.2. Limitation to Tunnel Protocols . . . . . . . . . . . . . . 7
4.3. Middleboxes . . . . . . . . . . . . . . . . . . . . . . . 8
5. The Zero-Checksum Update . . . . . . . . . . . . . . . . . . . 8
6. Additional Observations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Eubanks, et al. Expires August 25, 2013 [Page 2]
Internet-Draft udp-checksum February 2013
1. Introduction
This work constitutes an update of the Internet Protocol Version 6
(IPv6) Specification [RFC2460], in the use case where a tunnel
protocol uses UDP with IPv6 to tunnel packets. With the rapid growth
of the Internet, tunnel protocols have become increasingly important
to enable the deployment of new protocols. Tunnel protocols can be
deployed rapidly, while the time to upgrade and deploy a critical
mass of routers, middleboxes and 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 tunnel
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
that perform filtering that is a fact of life on the current
Internet.
Tunnel endpoints may be routers or middleboxes aggregating traffic
from a 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 tunnel protocol,
especially if the inner packet(s) are already protected by a
checksum. In IPv4, there is a checksum over the IP packet header,
and the checksum on the outer UDP packet may be set to zero. However
in IPv6 there is no checksum in the IP header and RFC 2460 [RFC2460]
explicitly states that IPv6 receivers MUST discard UDP packets with a
zero checksum. So, while sending a UDP datagram 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 endpoints to use a zero UDP checksum under
constrained situations (primarily IPv6 tunnel transports that carry
checksum-protected packets), following the applicability statements
and constraints 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 Multicast 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 [RFC6830] and Softwires [RFC5619] have
Eubanks, et al. Expires August 25, 2013 [Page 3]
Internet-Draft udp-checksum February 2013
expressed a need to update the UDP checksum processing in RFC 2460.
We therefore expect this update to be applicable in the future to
other tunnel protocols specified by these and other IETF Working
Groups.
2. Some Terminology
This document discusses 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
When using tunnel protocols based on UDP, there can be both a benefit
and a cost to computing and checking the UDP checksum of the outer
(encapsulating) UDP transport header. In certain cases, reducing the
forwarding cost is important, e.g., for nodes that perform the
checksum in software the cost may outweigh the benefit. This
document provides an update for usage of the UDP checksum with IPv6.
The update is specified for use by a tunnel protocol that transports
packets that are themselves protected by a checksum.
4. Discussion
"Applicability Statement for the use of IPv6 UDP Datagrams with Zero
Checksums" [I-D.ietf-6man-udpzero] describes issues related to
allowing UDP over IPv6 to have a valid zero UDP checksum and is the
starting point for this discussion. Sections 4 and 5 of
[I-D.ietf-6man-udpzero], respectively identify node implementation
and usage requirements for datagrams sent and received with a zero
UDP checksum. These introduce constraints on the usage of a zero
checksum for UDP over IPv6. The remainder of this section analyses
the use of general tunnels and motivates why tunnel protocols are
Eubanks, et al. Expires August 25, 2013 [Page 4]
Internet-Draft udp-checksum February 2013
being permitted to use the method described in this update. Issues
with middleboxes are also discussed.
4.1. Analysis of Corruption in Tunnel Context
This section analyzes the impact of the different corruption modes in
the context of a tunnel protocol. It indicates what needs to be
considered by the designer and user of a tunnel protocol to be
robust. It also summarizes why use of a zero UDP checksum is thought
to be safe for deployment.
1. Context (i.e., tunneling state) should be established by
exchanging application Protocol Data Units (PDUs) carried in
checksummed UDP datagrams or by other protocols with integrity
protection against corruption. These control packets should also
carry any negotiation required to enable the tunnel endpoint to
accept UDP datagrams with a zero checksum and identify the set of
ports that are used. It is important that the control traffic is
robust against corruption because undetected errors can lead to
long-lived and significant failures that may affect much more
than the single packet that was corrupted.
2. Keep-alive datagrams with a zero UDP checksum should be sent to
validate the network path, because the path between tunnel
endpoints can change and therefore the set of middleboxes along
the path may change during the life of an association. Paths
with middleboxes that drop datagrams with a zero UDP checksum
will drop these keep-alives. To enable the tunnel endpoints to
discover and react to this behavior in a timely way, the keep-
alive traffic should include datagrams with a non-zero checksum
and datagrams with a zero checksum.
3. Receivers should attempt to detect corruption of the address
information in an encapsulating packet. A robust tunnel protocol
should track tunnel context based on the 5-tuple (tunneled
protocol number, IPv6 source address, IPv6 destination address,
UDP source port, UDP destination port). A corrupted datagram
that arrives at a destination may be filtered based on this
check.
* If the datagram header matches the 5-tuple and the node has
the zero checksum enabled for this port, the payload is
matched to the wrong context. The tunneled packet will then
be decapsulated and forwarded by the tunnel egress.
* If a corrupted datagram matches a different 5-tuple and the
zero checksum was enabled for the port, the datagram payload
is matched to the wrong context, and may be processed by the
Eubanks, et al. Expires August 25, 2013 [Page 5]
Internet-Draft udp-checksum February 2013
wrong tunnel protocol, if it also passes the verification of
that protocol.
* If a corrupted datagram matches a 5-tuple and the zero
checksum has not been enabled for this port, the datagram will
be discarded.
When only the source information is corrupted, the datagram could
arrive at the intended applications/protocol, which will process
the datagram and try to match it against an existing tunnel
context. The likelihood that a corrupted packet enters a valid
context is reduced when the protocol restricts processing to only
the source addresses with established contexts. When both source
and destination fields are corrupted, this increases the
likelihood of failing to match a context, with the exception of
errors replacing one packet header with another one. In this
case, it is possible that both packets are tunnelled and
therefore the corrupted packet could match a previously defined
context.
4. Receivers should attempt to detect corruption of source-
fragmented encapsulating packets. A tunnel 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).
5. Tunnel protocols using UDP have some advantages that reduce the
risk for a corrupted tunnel packet reaching a destination that
will receive it, compared to other applications. This results
from processing by the network of the inner (tunneled) packet
after being forwarded from the tunnel egress using a wrong
context:
* A tunneled packet may be forwarded to the wrong address
domain, for example, a private address domain where the inner
packet's address is not routable, or may fail a source address
check, such as Unicast Reverse Path Forwarding [RFC2827],
resulting in the packet being dropped.
* The destination address of a tunneled packet may not at all be
reachable from the delivered domain. For example, an Ethernet
Eubanks, et al. Expires August 25, 2013 [Page 6]
Internet-Draft udp-checksum February 2013
frame where the destination MAC address is not present on the
LAN segment that was reached.
* The type of the tunneled packet may prevent delivery. For
example, an attempt to interpret an IP packet payload as an
Ethernet frame, would likely to result in the packet being
dropped as invalid.
* The tunneled packet checksum or integrity mechanism may detect
corruption of the inner packet caused at the same time as
corruption to the outer packet header. The resulting packet
would likely be dropped as invalid.
These checks each significantly reduce the likelihood that a
corrupted inner tunneled packet is finally delivered to a protocol
listener that can be affected by the packet. While the methods do
not guarantee correctness, they can reduce the risk of relaxing the
UDP checksum requirement for a tunnel application using IPv6.
4.2. Limitation to Tunnel Protocols
This document describes the applicability of using a zero UDP
checksum to support tunnel protocols. There are good motivations
behind this and the arguments are provided here.
o Tunnels carry inner packets that have their own semantics, which
may make any corruption less likely to reach the indicated
destination and be accepted as a valid packet. This is true for
IP packets with the addition of verification that can be made by
the tunnel protocol, the network processing of the inner packet
headers as discussed above, and verification of the inner packet
checksums. Non-IP inner packets are likely to be subject to
similar effects that may reduce the likelihood of a misdelivered
packet being delivered to a protocol listener that can be affected
by the packet.
o Protocols that directly consume the payload must have sufficient
robustness against misdelivered packets from any context,
including the ones that are corrupted in tunnels and any other
usage of the zero checksum. This will require an integrity
mechanism. Using a standard UDP checksum reduces the
computational load in the receiver to verify this mechanism.
o The design for stateful protocols or protocols where corruption
causes cascade effects requires extra care. In tunnel usage, each
encapsulating packet provides only a transport mechanism from
tunnel ingress to tunnel egress. A corruption will commonly only
affect the single tunneled packet, not the established protocol
Eubanks, et al. Expires August 25, 2013 [Page 7]
Internet-Draft udp-checksum February 2013
state. One common effect is that the inner packet flow will only
see a corruption and misdelivery of the outer packet as a lost
packet.
o Some non-tunnel protocols operate with general servers that do not
know the source from which they will receive a packet. In such
applications, a zero UDP checksum is unsuitable because there is a
need to provide the first level of verification that the packet
was intended for the receiving server. A verification prevents
the server from processing the datagram payload and without this
it may spend significant resources processing the packet,
including sending replies or error messages.
Tunnel protocols that encapsulate IP will generally be safe for
deployment, since all IPv4 and IPv6 packets include at least one
checksum at either the network or transport layer. The network
delivery of the inner packet will then further reduce the effects of
corruption. Tunnel protocols carrying non-IP packets may offer
equivalent protection when the non-IP networks reduce the risk of
misdelivery to applications. However, there is a need for further
analysis to understand the implications of misdelievery of corrupted
packets for that each non-IP protocol. The analysis above suggests
that non-tunnel protocols can be expected to have significantly more
cases where a zero checksum would result in misdelivery or negative
side-effects.
One unfortunate side-effect of increased use of a zero-checksum is
that it also increases the likelihood of acceptance when a datagram
with a zero UDP checksum is misdelivered. This requires all tunnel
protocols using this method to be designed to be robust to
misdelivery.
4.3. Middleboxes
"Applicability Statement for the use of IPv6 UDP Datagrams with Zero
Checksums" [I-D.ietf-6man-udpzero] notes that middleboxes that
conform to RFC 2460 will discard datagrams with a zero UDP checksum
and should log this as an error. Tunnel protocols intending to use a
zero UDP checksum need to ensure that they have defined a method for
handling cases when a middlebox prevents the path between the tunnel
ingress and egress from supporting transmission of datagrams with a
zero UDP checksum.
5. The Zero-Checksum Update
This specification updates IPv6 to allow a zero UDP checksum in the
outer encapsulating datagram of a tunnel protocol. UDP endpoints
Eubanks, et al. Expires August 25, 2013 [Page 8]
Internet-Draft udp-checksum February 2013
that implement this update MUST follow the node requirements in
"Applicability Statement for the use of IPv6 UDP Datagrams with Zero
Checksums" [I-D.ietf-6man-udpzero].
The following text in [RFC2460] Section 8.1, 4th bullet should be
deleted:
"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 text should be replaced by:
An IPv6 node associates a mode with each used UDP port (for
sending and/or receiving packets).
Whenever originating a UDP packet for a port in the default mode,
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 as
specified in [RFC2460]. IPv6 receivers MUST by default discard
UDP packets containing a zero checksum, and SHOULD log the error.
As an alternative, certain protocols that use UDP as a tunnel
encapsulation, MAY enable the zero-checksum mode for a specific
port (or set of ports) for sending and/or receiving. Any node
implementing the zero-checksum mode MUST follow the node
requirements specified in Section 4 of "Applicability Statement
for the use of IPv6 UDP Datagrams with Zero Checksums"
[I-D.ietf-6man-udpzero].
Any protocol that enables the zero-checksum mode for a specific
port or ports MUST follow the usage requirements specified in
Section 5 of "Applicability Statement for the use of IPv6 UDP
Datagrams with Zero Checksums" [I-D.ietf-6man-udpzero].
Middleboxes supporting IPv6 MUST follow requirements 9, 10 and 11
of the usage requirements specified in Section 5 of "Applicability
Statement for the use of IPv6 UDP Datagrams with Zero Checksums"
[I-D.ietf-6man-udpzero].
Eubanks, et al. Expires August 25, 2013 [Page 9]
Internet-Draft udp-checksum February 2013
6. Additional Observations
This update was motivated by the existence of a number of protocols
being developed in the IETF that are expected to benefit from the
change. The following observations are made:
o An empirically-based analysis of the probabilities of packet
corruption (with or without checksums) has not (to our knowledge)
been conducted since about 2000. At the time of publication, it
is now 2012. We strongly suggest a new empirical study, along
with an extensive analysis of the corruption probabilities of the
IPv6 header. This can potentially allow revising the
recommendations in this document.
o A key motivation for the increase in use of UDP in tunneling is a
lack of protocol support in middleboxes. Specifically, new
protocols, such as LISP [RFC6830], may prefer to use UDP tunnels
to traverse an end-to-end path successfully and avoid having their
packets dropped by middleboxes. If middleboxes were updated to
support UDP-Lite [RFC3828], UDP-Lite would provide better
protection than offered by this update. This may be suited to a
variety of applications and would be expected to be preferred over
this method for many tunnel 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 misdelivered 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.
8. Security Considerations
Less work is required to generate an attack using a zero UDP checksum
than one using a standard full UDP checksum. However, this does not
lead to significant new vulnerabilities because checksums are not a
security measure and can be easily generated by any attacker.
In general any user of zero UDP checksums should apply the checks and
context verification that are possible to minimize the risk of
Eubanks, et al. Expires August 25, 2013 [Page 10]
Internet-Draft udp-checksum February 2013
unintended traffic to reach a particular context. This will however
not protect against an intended attack that create packet with the
correct information. Source address validation can help prevent
injection of traffic into contexts by an attacker.
Depending on the hardware design, the processing requirements may
differ for tunnels that have a zero UDP checksum and those that
calculate a checksum. This processing overhead may need to be
considered when deciding whether to enable a tunnel and to determine
an acceptable rate for transmission. This can become a security risk
for designs that can handle a significantly larger number of packets
with zero UDP checksums compared to datagrams with a non-zero
checksum, such as tunnel egress. An attacker could attempt to inject
non-zero checksummed UDP packets into a tunnel forwarding zero
checksum UDP packets and cause overload in the processing of the non-
zero checksums, e.g. if this happens in a routers slow path.
Protection mechanisms should therefore be employed when this threat
exists. Protection may include source address filtering to prevent
an attacker injecting traffic, as well as throttling the amount of
non-zero checksum traffic. The latter may impact the function of the
tunnel protocol.
9. Acknowledgements
We would like to thank Brian Haberman, Dan Wing, Joel Halpern, David
Waltermire, J.W. Atwood, Peter Yee, Joe Touch and the IESG of 2012
for discussions and reviews. Gorry Fairhurst has been very diligent
in reviewing and help ensuring alignment between this document and
[I-D.ietf-6man-udpzero].
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-10 (work in progress),
January 2013.
[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.
Eubanks, et al. Expires August 25, 2013 [Page 11]
Internet-Draft udp-checksum February 2013
10.2. Informative References
[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.
[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.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405,
November 2008.
[RFC5619] Yamamoto, S., Williams, C., Yokota, H., and F. Parent,
"Softwire Security Analysis and Requirements", RFC 5619,
August 2009.
[RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
Locator/ID Separation Protocol (LISP)", RFC 6830,
January 2013.
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
Eubanks, et al. Expires August 25, 2013 [Page 12]
Internet-Draft udp-checksum February 2013
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
Magnus Westerlund
Ericsson
Farogatan 6
SE-164 80 Kista
Sweden
Phone: +46 10 714 82 87
Email: magnus.westerlund@ericsson.com
Eubanks, et al. Expires August 25, 2013 [Page 13]
