Internet DRAFT - draft-tsou-stateless-nat44
draft-tsou-stateless-nat44
Network Working Group T. Tsou
Internet-Draft Huawei Technologies (USA)
Intended status: Standards Track W. Liu
Expires: April 25, 2013 Huawei Technologies
S. Perreault
Viagenie
R. Penno
Cisco Systems, Inc.
M. Chen
FreeBit
October 22, 2012
Stateless IPv4 Network Address Translation
draft-tsou-stateless-nat44-02
Abstract
This memo describes a protocol for decentralizing IPv4 NAT to the
customer-premises equipment (CPE) such that no state information is
kept on the central NAT device. The CPE uses a restricted source
port set that is encoded in its provisioned IPv4 WAN address. The
NAT device performs only strictly stateless address (not port)
translation.
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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This Internet-Draft will expire on April 25, 2013.
Copyright Notice
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Address Formats . . . . . . . . . . . . . . . . . . . . . . . 4
4. Customer Provisioning . . . . . . . . . . . . . . . . . . . . 5
5. SLNAT44 Configuration . . . . . . . . . . . . . . . . . . . . 6
6. Port Set Computation . . . . . . . . . . . . . . . . . . . . . 6
7. CPE Operation . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. ALG Handling . . . . . . . . . . . . . . . . . . . . . . . 8
8. SLNAT44 Operation . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Internal to External . . . . . . . . . . . . . . . . . . . 8
8.2. External to Internal . . . . . . . . . . . . . . . . . . . 8
8.3. Fragment Handling . . . . . . . . . . . . . . . . . . . . 9
9. Address Mapping Example . . . . . . . . . . . . . . . . . . . 9
10. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
12.1. Normative References . . . . . . . . . . . . . . . . . . . 10
12.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
IPv4 address exhaustion has become world-wide reality. NAT is one of
the solutions to deal with the problem. The drawbacks of traditional
NAT include statefulness and the need to track transport-layer
sessions. This makes NAT complex, hard to scale up, and fragile.
This document describes a method of deploying stateless NAT as a
backwards-compatible evolution of an IPv4-only network.
The assumed topology is illustrated in Figure 1.
+-----+ +---------+
Home -----+ CPE |---<ISP network>---+ SLNAT44 +---- Internet
network +-----+ +---------+
^ ^
| |
+-- Internal Address +--- External Address
Figure 1: Stateless NAT44 topology
When CPE is configured working as a transparent bridge, internal
addresses are directly assigned to the end hosts in the home network,
as is shown in Figure 2.
+-----+ +---------+
Home -----+ CPE |---<ISP network>---+ SLNAT44 +---- Internet
network +-----+ +---------+
^ ^
| |
+-- Internal Addresses +--- External Address
Figure 2: Stateless NAT44 topology: CPE as bridge
Note that SLNAT44 has no IPv6 component. Any deployment of IPv6 is
unaffected by SLNAT44. Therefore, this document only describes IPv4
addresses and IPv4 packets. IPv6 is not discussed further.
2. Terminology
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].
The following terms are used throughout this document:
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Port set: Set of transport-layer ports that each CPE is assigned, to
be used as source ports by packets sent by the CPE.
Port Set ID: A value from which a unique port set is algorithmically
derived.
SLNAT44: Depending on the context, either the stateless NAT44
protocol or the stateless NAT44 device that translates between
internal and external addresses. NAT44 in turn stands for "IPv4-
to-IPv4 NAT".
Internal Address: The IPv4 address assignted to a CPE. It is used
in the ISP network between the CPE and the SLNAT44.
External Address: The IPv4 address used on the Internet and routed
to the SLNAT44.
Mapping rule: A set of parameters configured on the SLNAT44 (not on
the CPE) describing the relationship between internal and external
addresses.
3. Address Formats
Internal addresses have the format illustrated in Figure 3. The
addresses are simply made of three parts concatenated together: the
Internal Prefix, the External Suffix, and the Port Set ID.
|0 |a |32
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internal Prefix | External Suffix | Port Set ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Internal Address format
External Addresses have the format illustrated in Figure 4. It is
made of two parts: the External Prefix and the External Suffix.
|0 |b |32
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Prefix | External Suffix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: External Address format
The lengths of the Internal and External Prefixes, "a" and "b", are
mandatory parameters of SLNAT44. They are determined by the ISP.
They need not be communicated to the CPE. Other lengths can be
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computed from them as follows:
o Length of External Suffix: 32 - b
o Length of Port Set ID: b - a
4. Customer Provisioning
Customer Provisioning is applied to the CPE when the CPE serves a
gateway.
As part of its start up routine, the CPE is assigned an IPv4 address
by the ISP using regular means (DHCP, PPP, etc.). This is the
Internal Address.
In addition, using new provisioning options, the CPE is assigned a
Port Set ID.
Optionally, a Port Set Mask is also provisioned to the CPE. This
mask is of the same length as the Port Set ID (i.e., b-a bits). Its
purpose is to allow discontinuous port ranges. If no mask is
provided, a mask of all ones is assumed by default, which implies a
continuous port range. System ports (0-1023) should not to be
assigned to any CPE.
In summary, the CPE is provisioned with the following elements:
o IPv4 address (as usual)
o Port Set ID
o Port Set Mask (optional)
When the CPE is configured in the bridge mode, all the above features
are provisioned directly to the end host behind the CPE.
Note: no matter in which mode the CPE is running, the customer
provisioning could be either dynamic or static. Static provisioning
implies an address planning for the private IPv4 address (i.e.,
RFC1918 addresses) inside in the domain. Static provisioning enables
servers (passive daemons) at the home network being accessible within
the domain. CPE running as bridge makes this feature easy to deploy
while running as L3 gateway requires port redirection if an in-domain
server at a host is demanded.
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5. SLNAT44 Configuration
The SLNAT44 is configured with a set of mapping rules. Each rule
contains:
o Internal Prefix
o External Prefix
o Port Set Mask (optional)
Prefixes include their length. For simplicity, rule prefixes MUST
NOT overlap with other rules.
If it is absent, the Port Set Mask is assumed to be all ones by
default.
6. Port Set Computation
Given a Port Set ID and a Port Set Mask, both n bits in length, the
set of allowed ports is defined as the set of port numbers for which
the higher-order n bits of their binary expression whose
corresponding mask bits are 1 are equal to corresponding bits from
the Port Set ID.
|0 |5
+-+-+-+-+-+
|1 1 1 0 1| Port Set ID = 29 (length n = 5 bits)
+-+-+-+-+-+
& & & & &
+-+-+-+-+-+
|1 1 1 1 1| Port Set Mask
+-+-+-+-+-+
| | | | |
V V V V V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 0 1 x x x x x x x x x x x x| Port Set = 59392-61439
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 |16
Figure 5: Example Contiguous Port Set Computation
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|0 |8
+-+-+-+-+-+-+-+-+
|0 0 1 0 1 1 1 1| Port Set ID = 29 (length n = 8 bits)
+-+-+-+-+-+-+-+-+
& & & & & & & &
+-+-+-+-+-+-+-+-+
|0 0 1 1 1 1 1 1| Port Set Mask
+-+-+-+-+-+-+-+-+
| | | | | | | |
V V V V V V V V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x x 1 0 1 1 1 1 x x x x x x x x x| Port Set = 12032-12287, 28416-28671,
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 44800-45055, 61184-61439
|0 |16
Figure 6: Example Non-Contiguous Port Set Computation
It follows that the number of ports in the set is 2^(16-x), where x
is the number of ones in the Port Set Mask.
This computation is performed by the CPE as part of its provisioning
routine as well as by the SLNAT44 for dropping packets with ports
outside the allowed range.
For the purposes of SLNAT44, a "source port" corresponds to either a
TCP source port, a UDP source port, or an ICMPv4 identifier, while a
"destination port" corresponds to either a TCP destination port, a
UDP destination port, or an ICMPv4 identifier. Note that an ICMPv4
identifier plays the role of both source and destination port.
Transport protocols other than TCP and UDP, as well as ICMPv4 types
without an identifier field, are not supported.
7. CPE Operation
CPE can be configured as either a gateway or transparent bridge.
In the gateway mode, packets sent from the CPE MUST have the
provisioned IPv4 address as source and MUST have a source port that
is within the allowed set. This is usually accomplished by having
the CPE run a NAT44 configured with the provisioned address and
allowed port set and having it process all packets sent out the WAN
interface.
Packets received by the CPE on its WAN interface with a destination
port outside the allowed range MUST be dropped.
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In the bridge mode, however, CPE only transfers packets and therefore
the service of stateless NAT44 is performed by the SLNAT44 directly
towards end hosts that possibly running as in-domain servers.
Regardless of any mode of the CPE, the operation involves injecting
private addresses (or prefixes) into the intra-domain backbone
routing infrastructure. It is necessary to operationally ensure that
there are no private addresses/prefixes are leaking into the backbone
route tables unless they are assigned by the SLNAT44 to CPEs or
directly to hosts.
7.1. ALG Handling
If the CPE implements application level gateways (ALGs) such as FTP,
RSTP or PPTP, it must ensure that ports present in the payload when
tanslated fall within the allowed range.
8. SLNAT44 Operation
8.1. Internal to External
When it receives a packet on an internal interface, the SLNAT44 finds
the rule whose Internal Prefix matches the packet's source address.
It extracts the Port Set ID from the packet's source address. It
then checks if the packet's source port is within the allowed set,
using the rule's Port Set Mask. If it is not, the packet MUST be
dropped.
If the packet's source port is within the allowed set, the SLNAT44
builds the External Address by concatenating the rule's External
Prefix with the External Suffix extracted from the packet's source
address. It then replaces the packet's source address with this
External Address. The IPv4 and transport-layer checksums are updated
as necessary. The packet is then forwarded as usual.
8.2. External to Internal
When it receives a packet on an external interface, the SLNAT44 finds
the rule whose External Prefix matches the packet's destination
address. It then builds the Internal Address by concatenating the
rule's Internal Prefix, the External Suffix extracted from the
packet's destination address, and the Port Set ID computed by
applying the rule's Port Set Mask to the packet's destination port's
higher-order bits. It then replaces the packet's destination address
with this Internal Address. The IPv4 and transport-layer checksums
are updated as necessary. The packet is then forwarded as usual.
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8.3. Fragment Handling
If the incoming IP packet contains a fragment, then more processing
may be needed. This specification leaves open the exact details of
how a SLNAT44 handles incoming IP packets containing fragments, and
simply requires that the external behavior of the SLNAT444 be
compliant with the following conditions.
The SLNAT44 MUST handle fragments. In particular, SLNAT44 MUST
handle fragments arriving out of order, conditional on the following:
o The SLNAT44 MUST limit the amount of resources devoted to the
storage of fragmented packets in order to protect from DoS
attacks.
o As long as the SLNAT44 has available resources, the SLNAT44 MUST
allow the fragments to arrive over a time interval. The time
interval SHOULD be configurable and the default value MUST be of
at least 2 seconds.
o The SLNAT44 MAY require that the UDP, TCP, or ICMPv4 header be
completely contained within the fragment that contains fragment
offset equal to zero.
For incoming packets carrying TCP or UDP fragments with a non-zero
checksum, SLNAT44 MAY elect to queue the fragments as they arrive and
translate all fragments at the same time. In this case, the incoming
tuple is determined as documented above to the un-fragmented packets.
Alternatively, a SLNAT44 MAY translate the fragments as they arrive,
by storing information that allows it to compute the necessary port
number for fragments other than the first. In the latter case,
subsequent fragments may arrive before the first, and the rules (in
the bulleted list above) about how the SLNAT44 handles (out-of-order)
fragments apply.
Implementers of SLNAT44 should be aware that there are a number of
well-known attacks against IP fragmentation; see [RFC1858] and
[RFC3128]. Implementers should also be aware of additional issues
with reassembling packets at high rates, described in [RFC4963].
9. Address Mapping Example
An operator has two public ranges of size /18 and /19 called foo and
bar respectively. It plans to use 10/8 as its internal address
prefix and PSID (port range) of length 5. Two prefixes of the
internal network
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The internal prefixes lengths are:
o 32 - 18 - 5 = 13 (derived from foo)
o 32 - 19 - 5 = 14 (derived from bar)
This will give the following possible mappings:
o foo/18 <--> 10.0.0.0/13
o bar/19 <--> 10.128.0.0/14
Author note: Discuss the where internal prefixes are overlapping
10. Security Considerations
The security considerations related to IP address sharing documented
in RFC 6269 [RFC6269] and RFC 6056 [RFC6056] apply to SLNAT44.
11. Acknowledgements
Section 8.3 is adapted from [RFC6146].
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
12.2. Informative References
[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858,
October 1995.
[RFC3128] Miller, I., "Protection Against a Variant of the Tiny
Fragment Attack (RFC 1858)", RFC 3128, June 2001.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, July 2007.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
January 2011.
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[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.
[RFC6269] Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
Roberts, "Issues with IP Address Sharing", RFC 6269,
June 2011.
Authors' Addresses
Tina Tsou
Huawei Technologies (USA)
2330 Central Expressway
Santa Clara, CA 95050
USA
Phone: +1 408 330 4424
Email: tina.tsou.zouting@huawei.com
Will Liu
Huawei Technologies
Bantian, Longgang District
Shenzhen 518129
P.R. China
Email: liushucheng@huawei.com
Simon Perreault
Viagenie
246 Aberdeen
Quebec, QC G1R 2E1
Canada
Email: simon.perreault@viagenie.ca
Reinaldo Penno
Cisco Systems, Inc.
170 West Tasman Drivee
San Jose, California 95134
USA
Email: repenno@cisco.com
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Maoke Chen
FreeBit
3-6 Maruyama-cho
Shibuya-ku, Tokyo 150-0044
Japan
Email: fibrib@gmail.com
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