Independent Submission T. Mizrahi
Request for Comments: 8367 Marvell
Category: Informational J. Yallouz
ISSN: 2070-1721 Intel
1 April 2018
Wrongful Termination of Internet Protocol (IP) Packets
Abstract
Routers and middleboxes terminate packets for various reasons. In
some cases, these packets are wrongfully terminated. This memo
describes some of the most common scenarios of wrongful termination
of Internet Protocol (IP) packets and presents recommendations for
mitigating them.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not candidates for any level of Internet Standard;
see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8367.
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Table of Contents
1. Introduction ................................................... 2
2. Abbreviations .................................................. 2
3. Wrongful Termination Scenarios ................................. 3
3.1. Color-Based Termination ................................... 3
3.2. Age-Based Termination ..................................... 3
3.3. Origin-Based Termination .................................. 4
3.4. Length-Based Termination .................................. 4
3.5. IP-Version-Based Termination .............................. 5
3.6. Flag-Based Termination .................................... 5
4. Security Considerations ........................................ 5
5. IANA Considerations ............................................ 5
6. Conclusion ..................................................... 6
7. References ..................................................... 6
7.1. Normative References ...................................... 6
7.2. Informative References .................................... 6
Authors' Addresses ................................................ 6
1. Introduction
IP packets are often terminated by network devices. In some cases,
control-plane packets are terminated and processed by the local
device, while in other cases packets are terminated (discarded) due
to a packet filtering mechanism. Packet filtering is widely employed
in network devices for sanity checking, policy enforcement, and
security. IP routers and middleboxes, such as firewalls, often
terminate packets that do not comply with a predefined policy.
Unfortunately, some filtering policies cause false positive or
unnecessary packet termination. Moreover, these wrongful
terminations are sometimes biased and discriminate against packets
based on their color, age, origin, length, or IP version.
This memo discusses some of the most common scenarios of wrongful
termination of IP packets and presents recommendations for preventing
such discrimination.
2. Abbreviations
IP Internet Protocol
TTL Time To Live
OAM Operations, Administration, and Maintenance
3. Wrongful Termination Scenarios
3.1. Color-Based Termination
Synopsis
IP packets are terminated due to their color.
Description
Routers often employ metering mechanisms [RFC4115]. These
mechanisms often support a color-aware mode, in which the packet's
color (green, yellow, or red) is used as a criterion in the
metering algorithm. This mode has been known to prefer green
packets over red and yellow packets.
Recommendation
Use of color-blind metering is recommended, as it allows equal
opportunity for packets of different colors.
3.2. Age-Based Termination
Synopsis
IP packets are terminated based on their TTL.
Description
The IPv4 TTL field [RFC791] and the IPv6 Hop Limit field [RFC8200]
are used for loop prevention. These fields essentially represent
the packet's age. A router that receives an IP packet with a TTL
value of 0 or 1 typically terminates the packet. In this
document, packets with a TTL or Hop Limit of 0 or 1 are referred
to as 'senior packets'.
Recommendation
When possible, the practice of reverse discrimination is
recommended. Notably, senior packets have been known to be highly
effective for OAM tasks, such as Hello [RFC2328] and Traceroute
[RFC2151]. Therefore, senior packets should not be easily
dismissed; to the extent possible, senior packets should be used
in control-plane protocols.
3.3. Origin-Based Termination
Synopsis
IP packets are terminated based on their origin (source IP address
prefix).
Description
Routers and middleboxes often perform IP address filtering.
Packets are often discarded based on the prefix of their source IP
address. In this memo, prefix-based source address filtering is
referred to as origin-based filtering. While source IP address
filtering is an acceptable technique for preventing security
attacks performed by known attackers, filtering an entire prefix
may lead to unnecessary termination of legitimate traffic.
Recommendation
Origin-based filtering should be limited, to the extent possible,
so as not to punish an entire autonomous system for the crime of a
single host. Individual address-based filtering should be
preferred in cases where the address of the potential threat is
well known.
3.4. Length-Based Termination
Synopsis
Short IP packets are wrongfully terminated due to their length.
Description
The minimum permissible size of an IPv4 [RFC791] packet is 20
octets, and the minimum size of an IPv6 [RFC8200] packet is 40
octets. However, due to the size limits of an Ethernet, it is
often the case that IP packets that are shorter than 46 octets are
discarded. This is because the minimal Ethernet frame size is 64
octets, the minimal Ethernet header size is 14 octets, and the
Ethernet Frame Check Sequence is 4 octets long (i.e., 64 - 14 - 4
= 46). In the context of this memo, legitimate IP packets that are
less than 46 octets long are referred to as 'short IP packets'.
Recommendation
Short IP packets should not be discarded. The Ethernet frame
length should be enforced at the Ethernet layer, while the IP
layer should avoid discrimination of short IP packets.
3.5. IP-Version-Based Termination
Synopsis
IPv6 packets are terminated due to their version.
Description
Many routers and middleboxes are configured to process only IPv4
[RFC791] packets and to reject IPv6 [RFC8200] packets.
Recommendation
It is quite unsettling that there are still networks in which IPv6
packets are deemed unwanted in the second decade of the 21st
century. Indeed, IPv6 packets have a slightly shorter payload
than IPv4 packets. However, they bare the potential for the
future growth of the Internet. It is time for operators to
finally give IPv6 its well-deserved opportunity.
3.6. Flag-Based Termination
Synopsis
IPv4 packets are terminated because their More Fragments (MF) flag
is set.
Description
Many routers and middleboxes are configured to discard fragmented
packets.
Recommendation
A packet should not be discarded on the grounds of a flag it
supports. All flags should be respected, as well as the features
they represent.
4. Security Considerations
This memo proposes to practice liberality with respect to IP packet
filtering in routers and middleboxes. Arguably, such a liberal
approach may compromise security in some cases. However, we argue
that security must not only be done, but must be seen to be done.
5. IANA Considerations
This document has no IANA actions.
6. Conclusion
This memo recommends that every router and middlebox be an Equal
Opportunity Device, which does not discriminate on the basis of
actual or perceived rate, color, age, origin, length, IP version,
fragmentation characteristics, higher-layer protocols, or any other
IP characteristic.
7. References
7.1. Normative References
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
7.2. Informative References
[RFC2151] Kessler, G. and S. Shepard, "A Primer On Internet and
TCP/IP Tools and Utilities", FYI 30, RFC 2151,
DOI 10.17487/RFC2151, June 1997,
<https://www.rfc-editor.org/info/rfc2151>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC4115] Aboul-Magd, O. and S. Rabie, "A Differentiated Service
Two-Rate, Three-Color Marker with Efficient Handling of
in-Profile Traffic", RFC 4115, DOI 10.17487/RFC4115,
July 2005, <https://www.rfc-editor.org/info/rfc4115>.
Authors' Addresses
Tal Mizrahi
Marvell
Email: talmi@marvell.com
Jose Yallouz
Intel
Email: jose@alumni.technion.ac.il