Behave S. Sivakumar
Internet-Draft Cisco Systems
Intended status: Informational March 5, 2012
Expires: September 6, 2012
Issues with End-point dependent mapping
draft-sivakumar-behave-edm-harmful-00
Abstract
Some NAT devices implement the End-point dependent mapping and
filtering behavior. This document describes the issues that would
arise with End-point dependent mapping and filtering behavior.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3
3. Why is End-point dependent mapping and filtering used . . . . . 4
4. Peer to Peer applications . . . . . . . . . . . . . . . . . . . 5
5. Issues with End-point dependent mapping and filtering . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Terminology
The usage of the term "NAT device" in this document refer to any
NAT44 and NAT64 devices.
Address dependent end-point mapping - A NAT device creates a mapping
taking into account the destination address and port. The same
mapping is reused if a packet from the same source address, source
port destined to the same destination address.
Address and port dependent end-point mapping - A NAT device creates a
mapping taking into account the destination address and port. The
same mapping is reused if a packet from the same source address,
source port destined to the same destination address and destination
port.
End-point dependent mapping - A NAT device that does either Address
dependent mapping and/or Address and port dependent mapping is
referred to follow End-point dependent mapping.
Address dependent end-point filtering - A NAT device that allows
inbound traffic to the internal hosts only if the internal host had
previously communicated to the external address.
Address and port dependent end-point filtering - A NAT device that
allow inbound traffic to the internal hosts only if the internal host
had previously communicated to the same external address and port.
End-point dependent filtering - A NAT device that filters the
incoming traffic based on the external hosts' source address and/or
port.
2. Introduction
End-point dependent mapping and filtering is still prevalent in many
NAT devices. Even though [RFC4787] and [RFC5382] recommends against
using end point dependent mapping and filtering, these two RFCs does
not go into the details of why it is end point dependent mapping is
bad for applications. This document focuses on the negative impacts
of End point dependent mapping and filtering on applications.
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 [RFC2119].
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3. Why is End-point dependent mapping and filtering used
There are two main reasons that End-point dependent mapping and
filtering is used. a. End-point independent mapping can be used to
extend the number of ports available per global address, often
referred to as port overloading. For example, if there are two hosts
behind the same NAT device
+---------+ X1,x1 P,p +---------+
| X1 |---------+ +---------| Y1 |
+---------+ | +--------+ | +---------+
|---| NAT |---|
+---------+ | +--------+ | P,p +---------+
| X2 |---------+ +---------| Y2 |
+---------+ X2, x2 +---------+
Figure 1
Host X1 (X1, x1) initiating communication to destination (Y1, y1) and
host X2 (Y1, y1) initiating communication to destination (Y2, y2).
NAT device will translate the source address and port (X1, x1) to
public address (P, p) and it will translate the source address and
port (X2, x2) also to public address and port (P, p), as long as both
the hosts are not destined to the same end point. In order to
correctly demultiplex the return packets, the NAT device will store
the destination information. So, when the return packet from (Y1,
y1) to (P, p) is seen the NAT device can translate the destination to
(X1, x1). Similarly, when the return packet from (Y2, y2) destined
to (P, p) is seen, the destination will be translated to (X2, x2).
As can be seen with this example, the same external port p can be
used for multiple flows thus by maximizing the use of a single
external IPv4 address.
This kind of implementation also gives a perception that the NAT
device is actually doing end-point dependent filtering.
The End-point dependent filtering is perceived to offer security.
Since the outbound packet was sent to the external address, it is
assumed that is a trusted entry. Hence the return packets from the
same external address is assumed to be trusted and doing filtering
based on the external address and port is perceived to offer
security.
Secondly, most of the client server based applications with End-point
dependent filtering will work fine. This leads one to believe that
End-point dependent filtering will work for all other applications as
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well.
4. Peer to Peer applications
Peer to Peer (P2P) applications use a variety of techniques to
traverse NATs like using Relay servers, TCP/UDP hole punching etc as
described in [RFC5128]. For P2P applications to work reliably across
NATs it is expected that the NAT devices does End-point independent
mapping and filtering.
Peer to Peer applications are becoming very common and some real life
examples of P2P applications are SIP used by VoIP service providers,
instant messaging, voice and video chat, Google Talk, Apple Facetime
and several famous gaming applications. The notion of not supporting
P2P applications is not just practical and NATs MUST be designed to
facilitate these applications.
While it is tempting to maximize the return on the investment by
maximizing the use of the existing IPv4 addresses, doing End point
dependent mapping, the harmful effects of this overrides any benefits
that it offers.
5. Issues with End-point dependent mapping and filtering
1. End point dependent mapping does not guarantee that the same
External address and port will be used regardless of the destination.
This would prevent the inbound connection from an application.
2. End point dependent filtering will allow inbound connections only
if a previous flow to the same host was previously initiated from an
internal host. This will prevent incoming connection requests from
an application.
6. Acknowledgements
Thanks to Dan Wing for providing the idea to write this document and
reviewing it.
7. IANA Considerations
None
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8. Security Considerations
None.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, October 2008.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, April 2011.
9.2. Informative References
[RFC5128] Srisuresh, P., Ford, B., and D. Kegel, "State of Peer-to-
Peer (P2P) Communication across Network Address
Translators (NATs)", RFC 5128, March 2008.
Author's Address
Senthil Sivakumar
Cisco Systems
7100-8 Kit Creek Road
Research Triangle Park, North Carolina 27709
USA
Phone: +1 919 392 5158
Email: ssenthil@cisco.com
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