Network Working Group S. Kent
Internet Draft K. Seo
draft-ietf-ipsec-rfc2401bis-01.txt BBN Technologies
Obsoletes: RFC 2401 January 2004
Expires July 2004
Security Architecture for the Internet Protocol
Status of this Memo
This document is an Internet Draft and is subject to all provisions
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Copyright (C) The Internet Society (2004). All Rights Reserved.
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Table of Contents
1. Introduction.........................................................3
1.1 Summary of Contents of Document.................................3
1.2 Audience........................................................3
1.3 Related Documents...............................................4
2. Design Objectives....................................................4
2.1 Goals/Objectives/Requirements/Problem Description...............4
2.2 Caveats and Assumptions.........................................5
3. System Overview .....................................................6
3.1 What IPsec Does.................................................6
3.2 How IPsec Works.................................................8
3.3 Where IPsec May Be Implemented..................................9
4. Security Associations...............................................10
4.1 Definition and Scope...........................................10
4.2 Security Association Functionality.............................13
4.3 Combining Security Associations................................14
4.4 Major IPsec Databases..........................................14
4.4.1 The Security Policy Database (SPD)........................15
4.4.2 Selectors.................................................19
4.4.3 Security Association Database (SAD).......................22
4.5 SA and Key Management..........................................24
4.5.1 Manual Techniques.........................................25
4.5.2 Automated SA and Key Management...........................25
4.5.3 Locating a Security Gateway...............................26
4.6 Security Associations and Multicast............................27
5. IP Traffic Processing...............................................27
5.1 Outbound IP Traffic Processing (protected-to-unprotected)......28
5.1.1 Handling an Outbound Packet That Must Be Dropped..........30
5.1.2 Header Construction for Tunnel Mode.......................31
5.1.2.1 IPv4 -- Header Construction for Tunnel Mode..........33
5.1.2.2 IPv6 -- Header Construction for Tunnel Mode..........34
5.2 Processing Inbound IP Traffic (unprotected-to-protected).......35
6. ICMP Processing (to be filled in when IPsec issue #91 is resolved)..38
7. Auditing............................................................38
8. Conformance Requirements............................................38
9. Security Considerations.............................................38
10. Differences from RFC 2401..........................................38
Acknowledgements.......................................................38
Appendix A -- Glossary.................................................40
Appendix B -- Decorrelation............................................43
Appendix C -- Categorization of ICMP messages [May be deleted].........46
References.............................................................49
Author Information.....................................................51
Notices................................................................52
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1. Introduction
1.1 Summary of Contents of Document
This document specifies the base architecture for IPsec compliant
systems. It describes how to provide a set of security services for
traffic at the IP layer, in both the IPv4 and IPv6 environments.
This document describes the requirements for systems that implement
IPsec, the fundamental elements of such systems, and how the elements
fit together and fit into the IP environment. It also describes the
security services offered by the IPsec protocols, and how these
services can be employed in the IP environment. This document does
not address all aspects of the IPsec architecture. Other documents
address additional architectural details in specialized environments,
e.g., use of IPsec in NAT environments and more comprehensive support
for IP multicast. The fundamental components of the IPsec security
architecture are discussed in terms of their underlying, required
functionality. Additional RFCs (see Section 1.3 for pointers to
other documents) define the protocols in (a), (c), and (d).
a. Security Protocols -- Authentication Header (AH) and
Encapsulating Security Payload (ESP)
b. Security Associations -- what they are and how they work,
how they are managed, associated processing
c. Key Management -- manual and automated (The Internet Key
Exchange (IKE))
d. Cryptographic algorithms for authentication and encryption
This document is not a Security Architecture for the Internet; it
addresses security only at the IP layer, provided through the use of
a combination of cryptographic and protocol security mechanisms.
The spelling "IPsec" is preferred and used throughout this and all
related IPsec standards. All other capitalizations of IPsec (e.g.,
IPSEC, IPSec, ipsec) are deprecated. However, any capitalization of
the sequence of letters "IPsec" should be understood to refer to the
IPsec protocols.
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in RFC 2119 [Bra97].
1.2 Audience
The target audience for this document is primarily individuals who
implement this IP security technology or who architect systems that
will use this technology. Technically adept users of this technology
(end users or system administrators) also are part of the target
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audience. A glossary is provided as an appendix to help fill in gaps
in background/vocabulary. This document assumes that the reader is
familiar with the Internet Protocol (IP), related networking
technology, and general information system security terms and
concepts.
1.3 Related Documents
As mentioned above, other documents provide detailed definitions of
some of the components of IPsec and of their inter-relationship.
They include RFCs on the following topics:
a. security protocols -- RFCs describing the Authentication Header
(AH) [Ken04b] and Encapsulating Security Payload (ESP) [Ken04a]
protocols.
b. cryptographic algorithms for integrity and encryption -- one RFC
that defines the mandatory, default algorithms for use with AH
and ESP [Eas03], a similar RFC that defines the mandatory
algorithms for use with IKEv2 [Sch03] plus a separate RFC for
each cryptographic algorithm.
c. automatic key management -- RFCs on "The Internet Key Exchange
(IKEv2) Protocol" [Kau03] and "Cryptographic Algorithms for use
in the Internet Key Exchange Version 2" [Sch03]
2. Design Objectives
2.1 Goals/Objectives/Requirements/Problem Description
IPsec is designed to provide interoperable, high quality,
cryptographically-based security for IPv4 and IPv6. The set of
security services offered includes access control, connectionless
integrity, data origin authentication, detection and rejection of
replays (a form of partial sequence integrity), confidentiality (via
encryption), and limited traffic flow confidentiality. These
services are provided at the IP layer, offering protection for all
protocols that may be carried over IP in a standard fashion
(including IP itself).
IPsec includes a specification for minimal firewall functionality,
since that is an essential aspect of access control at the IP layer.
Implementations are free to provide more sophisticated firewall
mechanisms, and to implement the IPsec- mandated functionality using
those more sophisticated mechanisms. (Note that interoperability may
suffer if additional firewall constraints on traffic flows are
imposed by an IPsec implementation but cannot be negotiated based on
the traffic selector features defined in this document and negotiated
via IKEv2.) The IPsec firewall function makes use of the
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cryptographically-enforced authentication and integrity provided for
all IPsec traffic to offer better access control than could be
obtained through use of an independent firewall (one not privy to
IPsec internal parameters).
Most of the security services are provided through use of two traffic
security protocols, the Authentication Header (AH) and the
Encapsulating Security Payload (ESP), and through the use of
cryptographic key management procedures and protocols. The set of
IPsec protocols employed in a context, and the ways in which they are
employed, will be determined by the users/administrators in that
context. It is the goal of the IPsec architecture to ensure that
compliant implementations include the services and management
interfaces needed to meet the security requirements of a broad user
population.
When IPsec is correctly implemented and deployed, it ought not
adversely affect users, hosts, and other Internet components that do
not employ IPsec for traffic protection. IPsec security protocols
(AH & ESP, and to a lesser extent, IKE) are designed to be
cryptographic algorithm-independent. This modularity permits
selection of different sets of cryptographic algorithms as
appropriate, without affecting the other parts of the implementation.
For example, different user communities may select different sets of
cryptographic algorithms (creating cryptographically-enforced
cliques) if required.
A set of default cryptographic algorithms for use with AH and ESP is
specified [Eas03] to facilitate interoperability in the global
Internet. The use of these cryptographic algorithms, in conjunction
with IPsec traffic protection and key management protocols, is
intended to permit system and application developers to deploy high
quality, Internet layer, cryptographic security technology.
2.2 Caveats and Assumptions
The suite of IPsec protocols and associated default cryptographic
algorithms are designed to provide high quality security for Internet
traffic. However, the security offered by use of these protocols
ultimately depends on the quality of the their implementation, which
is outside the scope of this set of standards. Moreover, the
security of a computer system or network is a function of many
factors, including personnel, physical, procedural, compromising
emanations, and computer security practices. Thus IPsec is only one
part of an overall system security architecture.
Finally, the security afforded by the use of IPsec is critically
dependent on many aspects of the operating environment in which the
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IPsec implementation executes. For example, defects in OS security,
poor quality of random number sources, sloppy system management
protocols and practices, etc. can all degrade the security provided
by IPsec. As above, none of these environmental attributes are
within the scope of this or other IPsec standards.
3. System Overview
This section provides a high level description of how IPsec works,
the components of the system, and how they fit together to provide
the security services noted above. The goal of this description is
to enable the reader to "picture" the overall process/system, see how
it fits into the IP environment, and to provide context for later
sections of this document, which describe each of the components in
more detail.
An IPsec implementation operates in a host, as a security gateway, or
as an independent device, affording protection to IP traffic. (A
security gateway is an intermediate system implementing IPsec, e.g.,
a firewall or router that has been IPsec-enabled.) More detail on
these classes of implementations is provided later, in Section 3.3.
The protection offered by IPsec is based on requirements defined by a
Security Policy Database (SPD) established and maintained by a user
or system administrator, or by an application operating within
constraints established by either of the above. In general, packets
are selected for one of three processing actions based on IP and next
layer header information (Selectors, Section 4.4.2) matched against
entries in the database (SPD). Each packet is either afforded IPsec
security services, discarded, or allowed to bypass IPsec protection,
based on the applicable SPD policies identified by the Selectors.
3.1 What IPsec Does
IPsec creates a boundary between unprotected and protected
interfaces, for a host or a network (See Figure 1 below). Traffic
traversing the boundary is subject to the access controls specified
by the user or administrator responsible for the IPsec configuration.
These controls indicate whether packets cross the boundary unimpeded,
are afforded security services via AH or ESP, or are discarded. IPsec
security services are offered at the IP layer through selection of
appropriate security protocols, cryptographic algorithms, and
cryptographic keys. IPsec can be used to protect one or more "paths"
between a pair of hosts, between a pair of security gateways, or
between a security gateway and a host. Compliant implementations MUST
support all three forms of connectivity noted here.
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Unprotected
^ ^
| |
+-------------|-------|-------+
| +-------+ | | |
| |Discard|<--| V |
| +-------+ |B +--------+ |
................|y..| AH/ESP |..... IPsec Boundary
| +---+ |p +--------+ |
| |IKE|<----|a ^ |
| +---+ |s | |
| +-------+ |s | |
| |Discard|<--| | |
| +-------+ | | |
+-------------|-------|-------+
| |
V V
Protected
Figure 1. Top Level IPsec Processing Model
In this diagram, "unprotected" refers to an interface that might also
be described as "black" or "ciphertext." Here, "protected" refers to
an interface that might also be described as "red" or "plaintext."
The protected interface noted above may be internal, e.g., in a host
implementation of IPsec; the protected interface may link to a socket
layer interface presented by the OS. In this document, the term
"inbound" refers to traffic entering an IPsec implementation via the
unprotected interface. The term "outbound" refers to traffic entering
the implementation via the protected interface, or emitted by the
implementation on the protected side of the boundary and directed
toward the unprotected interface. An IPsec implementation may
support more than one interface on either or both sides of the
boundary.
Note the facilities for discarding traffic on either side of the
IPsec boundary, the bypass facility that allows traffic to transit
the boundary without cryptographic protection, and the reference to
IKE as a protected-side key and security management function.
IPsec optionally supports negotiation of IP compression [SMPT98],
motivated in part by the observation that when encryption is employed
within IPsec, it prevents effective compression by lower protocol
layers.
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3.2 How IPsec Works
IPsec uses two protocols to provide traffic security services --
Authentication Header (AH) and Encapsulating Security Payload (ESP).
Both protocols are described in detail in their respective RFCs
[KA98a, KA98b]. IPsec implementations MUST support ESP and MAY
support AH. (Support for AH has been downgraded to MAY because
experience has shown that there are very few contexts in which ESP
cannot provide the requisite security services. Note that ESP can be
used to provide only integrity, without confidentiality, making it
comparable to AH in most contexts.)
o The IP Authentication Header (AH) [Ken04b] offers integrity and
data origin authentication, with optional (at the discretion of
the receiver) anti-replay features.
o The Encapsulating Security Payload (ESP) protocol [Ken04a] offers
the same set of services, and also offers confidentially. Use of
ESP in a confidentiality-only mode is discouraged. When ESP is
used with confidentiality enabled, there are provisions for
limited traffic flow confidentiality, i.e., provisions for
concealing packet length, and to facilitate efficient generation
and discard of dummy packets. This capability is likely to be
effective primarily in VPN and overlay network contexts.
o Both AH and ESP offer access control, enforced through the
distribution of cryptographic keys and the management of traffic
flows as dictated by the Security Policy Database (SPD, Section
4.4).
These protocols may be applied individually or in combination with
each other to provide security services in IPv4 and IPv6. However,
most security requirements can be met through the use of ESP by
itself. Each protocol supports two modes of use: transport mode and
tunnel mode. In transport mode, AH and ESP provide protection
primarily for next layer protocols; in tunnel mode, AH and ESP are
applied to tunneled IP packets. The differences between the two
modes are discussed in Section 4.
IPsec allows the user (or system administrator) to control the
granularity at which a security service is offered. For example, one
can create a single encrypted tunnel to carry all the traffic between
two security gateways or a separate encrypted tunnel can be created
for each TCP connection between each pair of hosts communicating
across these gateways. IPsec, through the SPD management paradigm,
incorporates facilities for specifying:
o which security services to use and in what combinations
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o the granularity at which protection should be applied
o the cryptographic algorithms used to effect cryptographic-based
security
Because most of the security services provided by IPsec require the
use of cryptographic keys, IPsec relies on a separate set of
mechanisms for putting these keys in place. This document requires
support for both manual and automated distribution of keys. It
specifies a specific public-key based approach (IKEv2 [KAU04]) for
automated key management, but other automated key distribution
techniques MAY be used.
Note: This document mandates support for several features for which
support is available in IKEv2 but not in IKEv1, e.g., negotiation of
an SA representing ranges of source and destination ports or
negotiation of multiple SAs with the same selectors. Therefore this
document assumes use of IKEv2 or a key and security association
management system with comparable features.
3.3 Where IPsec Can Be Implemented
There are many ways in which IPsec may be implemented in a host, or
in conjunction with a router or firewall to create a security
gateway, or as an independent security device.
a. IPsec may be integrated into the native IP stack. This requires
access to the IP source code and is applicable to both hosts and
security gateways, although native host implementations benefit
the most from this strategy, as explained later (Section 4.4.1,
paragraph 4; Section 4.4.2, last paragraph)
b. In a "bump-in-the-stack" (BITS) implementation, IPsec is
implemented "underneath" an existing implementation of an IP
protocol stack, between the native IP and the local network
drivers. Source code access for the IP stack is not required in
this context, making this implementation approach appropriate for
use with legacy systems. This approach, when it is adopted, is
usually employed in hosts.
c. The use of an dedicated, inline security protocol processor is a
common design feature of systems used by the military, and of some
commercial systems as well. It is sometimes referred to as a
"bump-in-the-wire" (BITW) implementation. Such implementations
may be designed to serve either a host or a gateway. Usually the
BITW device is itself IP addressable. When supporting a single
host, it may be quite analogous to a BITS implementation, but in
supporting a router or firewall, it must operate like a security
gateway.
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This document often talks in terms of host or security gateway use of
IPsec, without regard to whether the implementation is native, BITS
or BITW. When the distinctions among these implementation options are
significant, the document makes reference to specific implementation
approaches.
4. Security Associations
This section defines Security Association management requirements for
all IPv6 implementations and for those IPv4 implementations that
implement AH, ESP, or both. The concept of a "Security Association"
(SA) is fundamental to IPsec. Both AH and ESP make use of SAs and a
major function of IKE is the establishment and maintenance of
Security Associations. All implementations of AH or ESP MUST support
the concept of a Security Association as described below. The
remainder of this section describes various aspects of Security
Association management, defining required characteristics for SA
policy management, traffic processing, and SA management techniques.
4.1 Definition and Scope
A Security Association (SA) is a simplex "connection" that affords
security services to the traffic carried by it. Security services
are afforded to an SA by the use of AH, or ESP, but not both. If
both AH and ESP protection are applied to a traffic stream, then two
SAs must be created and coordinated to effect protection through
iterated application of the security protocols. To secure typical,
bi-directional communication between two IPsec-enabled systems, a
pair of Security Associations (one in each direction) are required.
IKE explicitly creates SA pairs in recognition of this common usage
requirement.
For an SA used to carry unicast (or anycast) traffic, the SPI
(Security Parameters Index - see Appendix A and AH [Ken04b] and ESP
[Ken04a] specifications) by itself suffices to specify an SA.
However, as a local matter, an implementation may choose to use the
SPI in conjunction with the IPsec protocol type (AH or ESP) for SA
identification. If an IPsec implementation supports multicast, then
it MUST support multicast SAs using the algorithm described in AH and
ESP for mapping inbound IPsec protected datagrams to SAs.
(Implementations that support only unicast traffic need not implement
that demultiplexing algorithm.)
Note: If different classes of traffic (distinguished by DSCP bits
[NiBlBaBL98], [Gro02]) are sent on the same SA, this could result in
inappropriate discarding of lower priority packets due to the
windowing mechanism used by receivers to reject replays. Therefore a
sender SHOULD put traffic of different classes, but with the same
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selector values, on different SAs to appropriately support QoS. To
permit this, the IPsec implementation MUST permit establishment and
maintenance of multiple SAs between a given sender and receiver, with
the same selectors. Distribution of traffic among these parallel SAs
to support QoS is locally determined by the sender and is not
negotiated by IKE. The receiver MUST process the packets from the
different SAs without prejudice.
DISCUSSION: While the DSCP [NiBlBaBL98, Gro02] and ECN [RaFlBL01]
fields are not "selectors", as that term in used in this
architecture, the sender will need a mechanism to direct packets with
a given (set of) DSCP values to the appropriate SA. This mechanism
might be termed a "classifier".
As noted above, two types of SAs are defined: transport mode and
tunnel mode. IKE creates pairs of SAs, so for simplicity, we choose
to require that both SAs in a pair be of the same mode, transport or
tunnel.
A transport mode SA is a security association typically employed
between a pair of hosts to provide end-to-end security services. When
link (vs. end-to-end) security is desired between two intermediate
systems along a path, transport mode MAY be used between security
gateways or between a security gateway and a host. In the latter
case, transport mode may be used to support IP-in-IP [Per96] or GRE
tunneling [FaLiHaMeTr00] over transport mode SAs. The access control
functions that are an important part of IPsec are significantly
limited in this context, as they cannot be applied to the end-to-end
headers of the packets that traverse a transport mode SA used in this
fashion. Thus this way of using transport mode should be evaluated
carefully before being employed in a specific context.
In IPv4, a transport mode security protocol header appears
immediately after the IP header and any options, and before any next
layer protocols (e.g., TCP or UDP). In IPv6, the security protocol
header appears after the base IP header and selected extension
headers, but may appear before or after destination options; it MUST
appear before next layer protocols. In the case of ESP, a transport
mode SA provides security services only for these next layer
protocols, not for the IP header or any extension headers preceding
the ESP header. In the case of AH, the protection is also extended
to selected portions of the IP header preceding it, selected portions
of extension headers, and selected options (contained in the IPv4
header, IPv6 Hop-by-Hop extension header, or IPv6 Destination
extension headers). For more details on the coverage afforded by AH,
see the AH specification [Ken04b].
A tunnel mode SA is essentially an SA applied to an IP tunnel, with
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the access controls applied to the headers of the traffic inside the
tunnel. In general, whenever either end of a security association is
a security gateway, the SA MUST be tunnel mode. Thus an SA between
two security gateways is typically a tunnel mode SA, as is an SA
between a host and a security gateway. Note that for the case where
traffic is destined for a security gateway, e.g., SNMP commands, the
security gateway is acting as a host and transport mode is allowed.
In this case, the SA terminates at a host (management) function
within a security gateway and thus merits different treatment. Also,
as noted above, security gateways MAY support a transport mode SA to
provide link security for IP traffic. Two hosts MAY establish a
tunnel mode SA between themselves. Several concerns motivate the use
of tunnel mode for an SA involving a security gateway. For example,
if there are multiple paths (e.g., via different security gateways)
to the same destination behind security gateways, it is important
that an IPsec packet be sent to the security gateway with which the
SA was negotiated. Similarly, a packet that might be fragmented en
route must have all the fragments delivered to the same IPsec
instance for reassembly. Also, when a fragment is processed by IPsec
and transmitted, then fragmented en route, it is critical that there
be inner and outer headers to retain the fragmentation state data for
the pre- and post-IPsec packet formats. Hence there are several
reasons for employing tunnel mode when either end of an SA is a
security gateway.
Note: AH and ESP cannot be applied using transport mode to IPv4
packets that are fragments. Only tunnel mode can be employed in such
cases.
For a tunnel mode SA, there is an "outer" IP header that specifies
the IPsec processing source and destination, plus an "inner" IP
header that specifies the (apparently) ultimate source and
destination for the packet. The security protocol header appears
after the outer IP header, and before the inner IP header. If AH is
employed in tunnel mode, portions of the outer IP header are afforded
protection (as above), as well as all of the tunneled IP packet
(i.e., all of the inner IP header is protected, as well as next layer
protocols). If ESP is employed, the protection is afforded only to
the tunneled packet, not to the outer header.
In summary,
a) A host implementation of IPsec MUST support both transport and
tunnel mode. This is true for native, BITS, and BITW
implementations for hosts.
b) A security gateway MUST support tunnel mode and MAY support
transport mode. If it supports transport mode, that should be
used only when the security gateway is acting as a host, e.g., for
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network management, or to provide link security.
4.2 Security Association Functionality
The set of security services offered by an SA depends on the security
protocol selected, the SA mode, the endpoints of the SA, and on the
election of optional services within the protocol.
For example, both AH and ESP offer integrity and authentication
services, but the coverage differs for each protocol and differs for
transport vs. tunnel mode. If the integrity of an IPv4 option or IPv6
extension header must be protected en route between sender and
receiver, AH can provide this service, except for the mutable (non-
predictable) parts of the IP or extension headers. However, the same
security may be achieved in some contexts by applying ESP to a tunnel
carrying a packet.
The granularity of access control provided is determined by the
choice of the selectors that define each security association.
Moreover, the authentication means employed by IPsec peers, e.g.,
during creation of an IKE (vs. child) SA also effects the granularity
of the access control afforded.
If confidentiality is selected, then an ESP (tunnel mode) SA between
two security gateways can offer partial traffic flow confidentiality.
The use of tunnel mode allows the inner IP headers to be encrypted,
concealing the identities of the (ultimate) traffic source and
destination. Moreover, ESP payload padding also can be invoked to
hide the size of the packets, further concealing the external
characteristics of the traffic. Similar traffic flow confidentiality
services may be offered when a mobile user is assigned a dynamic IP
address in a dialup context, and establishes a (tunnel mode) ESP SA
to a corporate firewall (acting as a security gateway). Note that
fine granularity SAs generally are more vulnerable to traffic
analysis than coarse granularity ones that are carrying traffic from
many subscribers.
NOTE: A compliant implementation MUST NOT allow instantiation of an
ESP SA that employs both NULL encryption and no integrity algorithm.
An attempt to negotiate such an SA is an auditable event by both
initiator and responder. The audit log entry for this event SHOULD
include the current date/time, local IKE IP address, and remote IKE
IP address. The initiator SHOULD record the relevant SPD entry.
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4.3 Combining Security Associations
This document does not require support for nested security
associations or for what RFC 2401 called "SA bundles." These features
still can be effected by appropriate configuration of both the SPD
and the local forwarding functions (for inbound and outbound
traffic), but this function is outside of the IPsec module and thus
the scope of this specification. As a result, management of
nested/bundled SAs is potentially more complex and less assured than
under the model implied by RFC 2401. An implementation that provides
support for nested SAs SHOULD provide a management interface that
enables a user or administrator to express the nesting requirement,
and then create the appropriate SPD entries and forwarding table
entries to effect the requisite processing.
4.4 Major IPsec Databases
Many of the details associated with processing IP traffic in an IPsec
implementation are largely a local matter, not subject to
standardization. However, some external aspects of the processing
must be standardized, to ensure interoperability and to provide a
minimum management capability that is essential for productive use of
IPsec. This section describes a general model for processing IP
traffic relative to IPsec functionality, in support of these
interoperability and functionality goals. The model described below
is nominal; implementations need not match details of this model as
presented, but the external behavior of implementations MUST
correspond to the externally observable characteristics of this model
in order to be deemed compliant.
There are two nominal databases in this model: the Security Policy
Database and the Security Association Database. The first specifies
the policies that determine the disposition of all IP traffic inbound
or outbound from a host or security gateway. The second database
contains parameters that are associated with each established (keyed)
security association.
A third database, the Peer Authorization Database (PAD) is also
required. The PAD provides a link between an SA management protocol
like IKE and the SPD. The PAD indicates the range of identities that
a peer is authorized to represent when (child) SAs are negotiated
with the peer. The identities may be a list of IP address ranges or
symbolic names. The fundamental requirement associated with the PAD
is that the traffic selectors passed by the SA management protocol
for comparison against the SPD MUST be verified as authorized
relative to the authenticated peer of the SA management protocol.
(See also Section 4.5.3, which levies requirements on the PAD in
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support of locating security gateways.)
The PAD also specifies how to authenticate each peer, e.g., via
shared secret or use of a certificate. If a shared secret is used,
the secret is stored here. If a certificate is used, the trust anchor
for the certificate is part of the PAD. Because the PAD might be
incorporated into the SA management protocol implementation, it is
not discussed extensively in this document.
If an IPsec implementation acts as a security gateway for multiple
subscribers, it MAY implement multiple separate IPsec contexts. Each
context MAY have and use completely independent identities, policies,
key management SAs, and/or IPsec SAs. This is for the most part a
local implementation matter. However, a means for associating inbound
(SA) proposals with local contexts is required. To this end, if
supported by the key management protocol in use, context identifiers
MAY be conveyed from initiator to responder in the signaling
messages, with the result that IPsec SAs are created with a binding
to a particular context.
The IPsec model described here embodies a clear separation between
forwarding (routing) and security decisions, to accommodate a wide
range of contexts where IPsec may be employed. Forwarding may be
trivial, in the case where there are only two interfaces, or it may
be complex, e.g., if there are multiple protected or unprotected
interfaces or if the context in which IPsec is implemented employs a
sophisticated forwarding function. IPsec assumes only that outbound
and inbound traffic that has passed through IPsec processing is
forwarded in a fashion consistent with the context in which IPsec is
implemented. Support for nested SAs is optional; if required, it
requires coordination between forwarding tables and SPD entries to
cause a packet to traverse the IPsec boundary more than once.
4.4.1 The Security Policy Database (SPD)
A security association is a management construct used to enforce
security policy for traffic crossing the IPsec boundary. Thus an
essential element of SA processing is an underlying Security Policy
Database (SPD) that specifies what services are to be offered to IP
datagrams and in what fashion. The form of the database and its
interface are outside the scope of this specification. However, this
section specifies minimum management functionality that must be
provided, to allow a user or system administrator to control whether
and how IPsec is applied to traffic transmitted or received by a host
or transiting a security gateway. The SPD, or relevant caches, must
be consulted during the processing of ALL traffic (inbound and
outbound), including non-IPsec traffic, that traverses the IPsec
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boundary. This includes IPsec management traffic such as IKE. An
IPsec implementation MUST have at least one SPD, and it MAY support
multiple SPDs, if appropriate for the context in which the IPsec
implementation operates. There is no requirement to maintain SPDs on
a per interface basis, as was specified in RFC 2401. However, if an
implementation supports multiple SPDs, then it MUST include an
explicit SPD selection function, that is invoked to select the
appropriate SPD for outbound traffic processing. The inputs to this
function are the outbound packet and any local metadata (e.g., the
interface via which the packet arrived) required to effect the SPD
selection function. The output of the function is an SPD ID.
Each SPD entry is either implicitly or explicitly directional. For
traffic protected by IPsec, the source and destination address and
ports are swapped to represent directionality, consistent with IKE
conventions. For bypassed or discarded traffic, separate inbound and
outbound entries are supported, e.g., to permit unidirectional flows
if required.
The SPD is an ordered database, consistent with the use of ACLs or
packet filters in firewalls, routers, etc. The ordering requirement
arises because entries often will overlap due to the presence of
(non-trivial) ranges as values for selectors. Thus a user or
administrator MUST be able to order the entries to express a desired
access control policy. There is no way to impose a general, canonical
order on SPD entries, because of the allowed use of wildcards for
selector values and because the different types of selectors are not
hierarchically related.
The processing model described in this document assumes the ability
to decorrelate overlapping SPD entries to permit caching, which
enables more efficient processing of outbound traffic in security
gateways and BITS/BITW implementations. (Native host implementations
have an implicit form of caching available, due to the use of, for
example, socket interfaces for applications, and thus there is no
requirement to be able to decorrelate SPD entries in these
implementations.) Decorrelation is a means of improving performance
and simplifying the processing description; it is not a requirement
for a compliant implementation.
Appendix B provides an algorithm that can be used to decorrelate SPD
entries, but any algorithm that produces equivalent output may be
used. Note that when an SPD entry is decorrelated all the resulting
entries MUST be grouped together, so that all members of the group
derived from an individual, SPD entry (prior to decorrelation) can
all be placed into caches and into the SAD at the same time. The
intent is that use of a decorrelated SPD ought not create more SAs
than would have resulted from use of a not-decorrelated SPD.
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An SPD must discriminate among traffic that is afforded IPsec
protection and traffic that is allowed to bypass IPsec. This applies
to the IPsec protection to be applied by a sender and to the IPsec
protection that must be present at the receiver. For any outbound or
inbound datagram, three processing choices are possible: discard,
bypass IPsec, or apply IPsec. The first choice refers to traffic
that is not allowed to traverse the IPsec boundary (in the specified
direction). The second choice refers to traffic that is allowed to
pass without additional IPsec protection. The third choice refers to
traffic that is afforded IPsec protection, and for such traffic the
SPD must specify the security protocols to be employed, their mode,
security service options, and the cryptographic algorithms to be
used. An SPD is logically divided into three pieces, all of which
should be decorrelated (with the exception noted above for native
host implementations) to facilitate caching. The SPD-S (secure
traffic) contains entries for all traffic subject to IPsec
protection. SPD-O (outbound) contains entries for all outbound
traffic that is to be bypassed or discarded. SPD-I (inbound) is
applied to inbound traffic that will be bypassed or discarded. If an
IPsec implementation supports only one SPD, then the SPD consists of
all three parts. If multiple SPDs are supported, some of them may be
partial, e.g., some SPDs might contain only SPD-I entries, to control
inbound bypassed traffic on a per-interface basis. The split allows
SPD-I to be consulted without having to consult SPD-S, for such
traffic. Since the SPD-I is just a part of the SPD, the same rule
applies here, i.e., if a packet that is looked up in the SPD-I cannot
be matched to an entry there, then the packet MUST be discarded.
Note that for outbound traffic, if a match is not found in SPD-S,
then SPD-O must be checked to see if the traffic should be bypassed.
Similarly, if SPD-O is checked first and no match is found, then SPD-
S must be checked.
For every IPsec implementation, there MUST be a management interface
that allows a user or system administrator to manage the SPD. The
interface must allow the user (or administrator) to specify the
security processing to be applied to every packet that traverses the
IPsec boundary. (In a native host IPsec implementation making use of
a socket interface, the SPD may not need to be consulted on a per
packet basis, as noted above.) The management interface for the SPD
MUST allow creation of entries consistent with the selectors defined
in Section 4.4.2, and MUST support (total) ordering of these entries,
as seen via this interface. The SPD entries' selectors are analogous
to the ACL or packet filters commonly found in a stateless firewall
or packet filtering router and which are currently managed this way.
In host systems, applications MAY be allowed to create SPD entries.
(The means of signaling such requests to the IPsec implementation are
outside the scope of this standard.) However, the system
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administrator MUST be able to specify whether or not a user or
application can override (default) system policies. The form of the
management interface is not specified by this document and may differ
for hosts vs. security gateways, and within hosts the interface may
differ for socket-based vs. BITS implementations. However, this
document does specify a standard set of SPD elements that all IPsec
implementations MUST support.
Each SPD entry specifies packet disposition as BYPASS, DISCARD, or
IPsec. The entry is keyed by a list of one or more selectors. The SPD
contains an ordered list of these entries. The required selector
types are defined in Section 4.4.2. These selectors are used to
define the granularity of the SAs that are created in response to an
outbound packet or in response to a proposal from a peer.
The SPD MUST permit a user or administrator to specify policy entries
as follows:
- SPD-I: For inbound traffic that is to be bypassed or discarded,
the entry consists of the values of the selectors that apply to
the traffic to be bypassed or discarded.
- SPD-O: For outbound traffic that is to be bypassed or discarded,
the entry consists of the values of the selectors that apply to
the traffic to be bypassed or discarded.
- SPD-S: For traffic that is to be protected using IPsec, the entry
consists of the values of the selectors that apply to the traffic
that the initiator will send or receive and the values that apply
to the traffic that the responder will receive or send.
- The selector types are defined in Section 4.2.2 below.
For each selector in an SPD entry, in addition to the literal values
that define a match, there are two special values: ANY and OPAQUE.
The former value is a wildcard that matches any value in the
corresponding field of the packet, whereas the latter value indicates
that the corresponding selector field is not examined, e.g., because
it may be obscured by encryption already applied to the packet or may
not be present in a fragment.
For each selector in an SPD entry, the policy entry specifies how to
derive the corresponding values for a new Security Association
Database (SAD, see Section 4.4.3) entry from those in the SPD and the
packet. The goal is to allow an SAD entry and an SPD cache entry to
be created based on specific selector values from the packet, or from
the matching SPD entry. If IPsec processing is specified for an
entry, a "populate from packet" (PFP) flag may be asserted for one or
more of the selector types in the SPD entry. If present, the flag
applies to all selectors of the indicated type in the outbound
element of the pair. (PFP does not apply to inbound traffic.)
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Note that this text describes the representation in the SPD that maps
into IKE payloads, i.e., one should not create SPD entries that
cannot be mapped into what IKE can negotiate. The management GUI can
offer the user other forms of data entry and display, e.g., the
option of using address prefixes as well as ranges, and symbolic
names for protocols, ports, etc. (Do not confuse the use of symbolic
names in a management interface with the SPD selector "name".) If
the reserved, symbolic selector value OPAQUE or ANY is employed for a
given selector type, only it may appear in the list for that type,
and it must appear only once in the list for that type. Note that
"ANY" is a local syntax convention - IKE handles this concept via
ranges.
The following example illustrates the use of the PFP flag in the
context of a security gateway or a BITS/BITW implementation. Consider
an SPD entry where the allowed value for destination address is a
range of IPv4 addresses: 192.168.2.1 to 192.168.2.10. Suppose an
outbound packet arrives with a destination address of 192.168.2.3,
and there is no extant SA to carry this packet. The value used for
the SA created to transmit this packet could be either of the two
values shown below, depending on what the SPD entry for this selector
says is the source of the selector value:
source for the example of new
value to be SAD destination address
used in the SA selector value
--------------- ------------
a. PFP TRUE 192.168.2.3 (one host)
b. PFP FALSE 192.168.2.1 to 192.168.2.10 (range of hosts)
Note that if the SPD entry had a value of ANY for the destination
address, then the SAD selector value would have to be ANY for case
(b), but would still be as illustrated for case (a). Thus the PFP
flag can be used to prohibit sharing of an SA, even among packets
that match the same SPD entry.
4.4.2 Selectors
An SA may be fine-grained or coarse-grained, depending on the
selectors used to define the set of traffic for the SA. For example,
all traffic between two hosts may be carried via a single SA, and
afforded a uniform set of security services. Alternatively, traffic
between a pair of hosts might be spread over multiple SAs, depending
on the applications being used (as defined by the Next Protocol and
Port fields), with different security services offered by different
SAs. Similarly, all traffic between a pair of security gateways could
be carried on a single SA, or one SA could be assigned for each
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communicating host pair. The following selector parameters MUST be
supported by all IPsec implementations to facilitate control of SA
granularity. Note that both Source and Destination addresses should
either be IPv4 or IPv6, but not a mix of address types. Also, note
that the source/destination port selectors may be labeled as "OPAQUE"
to accommodate situations where these fields are inaccessible because
of prior encryption or due to packet fragmentation.
- Destination IP Address (IPv4 or IPv6): this is a list of ranges
of IP addresses (unicast, anycast, broadcast (IPv4 only), or
multicast group). This structure allows expression of a single
IP address (via a trivial range), or a list of addresses (each a
trivial ranges), or a range of addresses (high and low values,
inclusive), as well as the most generic form of a list of
ranges. Address ranges are used to support more than one
destination system sharing the same SA, e.g., behind a security
gateway.
- Source IP Address(es) (IPv4 or IPv6): this is a list of ranges
of IP addresses (unicast, anycast, broadcast (IPv4 only), or
multicast group). This structure allows expression of a single
IP address (via a trivial range), or a list of addresses (each a
trivial ranges), or a range of addresses (high and low values,
inclusive), as well as the most generic form of a list of
ranges. Address ranges are used to support more than one source
system sharing the same SA, e.g., behind a security gateway.
- Next Layer Protocol: Obtained from the IPv4 "Protocol" or the
IPv6 "Next Header" fields. This is an individual protocol
number, or ANY. The Next Layer Protocol is whatever comes after
any IP extension headers that are present. To simplify locating
the Next Layer Protocol in the IPv6 context, there SHOULD be a
mechanism for configuring which IP extension headers to skip,
e.g., Destination Options, Routing Header, Fragmentation Header,
Mobility Header, Hop-by-hop options, etc.
Several additional selectors depend on the Next Layer Protocol
value:
* If the Next Layer Protocol uses ports (e.g., TCP, UDP, SCTP,
...), then there are selectors for Source and Destination
Ports: Each of these selectors is a list of ranges of
values. Note that the source and destination ports may not
be available in the case of receipt of a fragmented packet,
thus a value of "OPAQUE" also MUST be supported. Note: In a
non-initial fragment, port values will not be available. If
the SA requires a non-OPAQUE port value, an arriving
fragment MUST be discarded.
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* If the Next Layer Protocol is a Mobility Header, then there
is a selector for IPv6 Mobility Header Message Type (MH
type) [Mobip]. This is an 8-bit value that identifies a
particular mobility message.
* If the Next Layer Protocol value is ICMP then there are
selectors for the ICMP message type and code. The message
type is a single 8-bit value, which defines the type of an
ICMP message, or ANY. The ICMP code is single 8-bit value
that defines a specific subtype for an ICMP message. This
selector can be a single value, or ANY.
- Name: A name is used as a symbolic identifier for an IPsec
source or destination address. Thus an SPD entry that has a non-
null Name selector MUST set either the source or destination IP
address selector to NULL in the corresponding, directional SPD
entry.
a. an RFC 822 address, e.g., mozart@foo.example.com
b. X.500 distinguished name
c. a fully qualified DNS name, e.g., foo.example.com
Use of this selector is different from all the other selectors
described above. Names do not appear in packets, so it is not
possible to match a packet against an SPD entry based on a Name
selector. Name selectors are used to trigger creation of SPD cache
(SPD-S and SPD-O) (and SAD) entries, which are then populated with
specific IP source or destination addresses provided by the SA
management protocol. For a native host implementation, a Name may be
used in an SPD entry to provide finer granularity access control that
would be otherwise be available on multi-user systems. In this case,
the entry may be consulted when SA creation is initiated by the host,
or when the host is a responder. The Name refers to an entity at the
host in question, and the implementation relies on its integration
into the host OS to ensure appropriate linking to the named entity's
process. The other use for the Name selector occurs when any IPsec
implementation (native host, BITW, BITS, or security gateway) is
contacted by a peer whose address cannot be known a priori, e.g., a
road warrior. In this context, the Name is used in lieu of the IP
address of the peer, who must be an initiator of the SA creation.
[This selector description may change based on discussion of some
name/identity issues that haven't yet been posted to the list.]
The IPsec implementation context determines how selectors are used.
For example, a native host implementation typically makes use of a
socket interface. When a new connection is established the SPD can
be consulted and an SA bound to the socket. Thus traffic sent via
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that socket need not result in additional lookups to the SPD (SPD-O
and SPD-S) cache. In contrast, a BITS, BITW, or security gateway
implementation needs to look at each packet and perform an SPD/SPD-S
cache lookup based on the selectors.
4.4.3 Security Association Database (SAD)
In each IPsec implementation there is a nominal Security Association
Database, in which each entry defines the parameters associated with
one SA. Each SA has an entry in the SAD. For outbound processing,
entries are pointed to by entries in the SPD-S part of the SPD cache.
For inbound processing, each entry in the SAD is indexed by an SPI
(from the AH or ESP protocol header), plus source and/or destination
address for multicast SAs, as noted earlier. The following parameters
are associated with each entry in the SAD. They should all be
present except where otherwise noted, e.g., AH Authentication
algorithm. This description does not purport to be a MIB, only a
specification of the minimal data items required to support an SA in
an IPsec implementation.
For each of the selectors defined in Section 4.4.2, the entry for an
inbound SA in the SAD MUST contain the value or values negotiated at
the time the SA was created. For a receiver, these values are used to
check that the header fields of an inbound packet match the selector
values negotiated for the SA. For the receiver, this is part of
verifying that a packet arriving on an SA is consistent with the
policy for the SA. (See Section 6 for rules for ICMP messages.)
These fields can have the form of specific values, ranges, ANY, or
"OPAQUE" as described in section 4.4.2, "Selectors."
The following data items MUST be in the SAD:
o Security Parameter Index (SPI): a 32-bit value selected by the
receiving end of an SA to uniquely identify the SA. In an SAD
entry for an outbound SA, the SPI is used to construct the
packet's AH or ESP header. In an SAD entry for an inbound SA, the
SPI is used to map traffic to the appropriate SA (see text on
unicast/multicast in Section 4.1).
o Sequence Number Counter: a 64-bit or 32-bit value used to generate
the Sequence Number field in AH or ESP headers. 64-bit sequence
numbers are the default, but 32-bit sequence numbers are also
supported if negotiated.
o Sequence Counter Overflow: a flag indicating whether overflow of
the Sequence Number Counter should generate an auditable event and
prevent transmission of additional packets on the SA, or whether
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rollover is permitted. The audit log entry for this event SHOULD
include the SPI value, current date/time, Source Address,
Destination Address, and the selectors from the relevant SAD
entry.
o Anti-Replay Window: a 64-bit counter and a bit-map (or equivalent)
used to determine whether an inbound AH or ESP packet is a replay.
NOTE: If anti-replay has been disabled by the receiver for an SA,
e.g., in the case of a manually keyed SA, then the Anti-Replay
Window is ignored for the SA in question. 64-bit sequence numbers
are the default, but this counter size accommodates 32-bit
sequence numbers.
o AH Authentication algorithm, key, etc. This is required only if AH
is supported.
o ESP Encryption algorithm, key, mode, IV, etc.
o ESP integrity algorithm, keys, etc. If the integrity service is
not selected, these fields will be null.
o ESP combined mode algorithms, key(s), etc. This data is used when
a combined mode (encryption and integrity) algorithm is used with
ESP.
o Lifetime of this Security Association: a time interval after which
an SA must be replaced with a new SA (and new SPI) or terminated,
plus an indication of which of these actions should occur. This
may be expressed as a time or byte count, or a simultaneous use of
both with the first lifetime to expire taking precedence. A
compliant implementation MUST support both types of lifetimes, and
must support a simultaneous use of both. If time is employed, and
if IKE employs X.509 certificates for SA establishment, the SA
lifetime must be constrained by the validity intervals of the
certificates, and the NextIssueDate of the CRLs used in the IKE
exchange for the SA. Both initiator and responder are responsible
for constraining SA lifetime in this fashion. NOTE: The details
of how to handle the refreshing of keys when SAs expire is a local
matter. However, one reasonable approach is:
(a) If byte count is used, then the implementation SHOULD count the
number of bytes to which the IPsec cryptographic algorithm is
applied. For ESP, this is the encryption algorithm (including
Null encryption) and for AH, this is the authentication
algorithm. This includes pad bytes, etc. Note that
implementations SHOULD be able to handle having the counters at
the ends of an SA get out of synch, e.g., because of packet
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loss or because the implementations at each end of the SA
aren't doing things the same way.
(b) There SHOULD be two kinds of lifetime -- a soft lifetime that
warns the implementation to initiate action such as setting up
a replacement SA; and a hard lifetime when the current SA ends
and is destroyed.
(c) If the entire packet does not get delivered during the SAs
lifetime, the packet SHOULD be discarded.
o IPsec protocol mode: tunnel or transport. Indicates which mode of
AH or ESP is applied to traffic on this SA.
o Path MTU: any observed path MTU and aging variables. See Section
6.1.2.4
o Tunnel header IP source and destination address - both addresses
must be either IPv4 or IPv6 addresses. The version implies the
type of IP header to be used. Only used when the IPsec protocol
mode is tunnel.
The following table summarizes the kinds of entries that one needs to
be able to express in the SPD and SAD. It also shows how they relate
to the fields in data traffic being subjected to IPsec screening.
[Table to be added in a future draft.]
4.5 SA and Key Management
IPsec mandates support for both manual and automated SA and
cryptographic key management. The IPsec protocols, AH and ESP, are
largely independent of the associated SA management techniques,
although the techniques involved do affect some of the security
services offered by the protocols. For example, the optional anti-
replay service available for AH and ESP requires automated SA
management. Moreover, the granularity of key distribution employed
with IPsec determines the granularity of authentication provided. In
general, data origin authentication in AH and ESP is limited by the
extent to which secrets used with the integrity algorithm (or with a
key management protocol that creates such secrets) are shared among
multiple possible sources.
The following text describes the minimum requirements for both types
of SA management.
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4.5.1 Manual Techniques
The simplest form of management is manual management, in which a
person manually configures each system with keying material and
security association management data relevant to secure communication
with other systems. Manual techniques are practical in small, static
environments but they do not scale well. For example, a company
could create a Virtual Private Network (VPN) using IPsec in security
gateways at several sites. If the number of sites is small, and
since all the sites come under the purview of a single administrative
domain, this might be a feasible context for manual management
techniques. In this case, the security gateway might selectively
protect traffic to and from other sites within the organization using
a manually configured key, while not protecting traffic for other
destinations. It also might be appropriate when only selected
communications need to be secured. A similar argument might apply to
use of IPsec entirely within an organization for a small number of
hosts and/or gateways. Manual management techniques often employ
statically configured, symmetric keys, though other options also
exist.
4.5.2 Automated SA and Key Management
Widespread deployment and use of IPsec requires an Internet-standard,
scalable, automated, SA management protocol. Such support is required
to facilitate use of the anti-replay features of AH and ESP, and to
accommodate on-demand creation of SAs, e.g., for user- and session-
oriented keying. (Note that the notion of "rekeying" an SA actually
implies creation of a new SA with a new SPI, a process that generally
implies use of an automated SA/key management protocol.)
The default automated key management protocol selected for use with
IPsec is IKEv2 [Kau04]. Other automated SA management protocols MAY
be employed.
When an automated SA/key management protocol is employed, the output
from this protocol is used to generate multiple keys for a single SA.
This also occurs because distinct keys are used for each of the two
SAs created by IKE. If both integrity and confidentiality are
employed, then a minimum of four keys are required. Additionally,
some cryptographic algorithms may require multiple keys, e.g., 3DES.
The Key Management System may provide a separate string of bits for
each key or it may generate one string of bits from which all keys
are extracted. If a single string of bits is provided, care needs to
be taken to ensure that the parts of the system that map the string
of bits to the required keys do so in the same fashion at both ends
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of the SA. To ensure that the IPsec implementations at each end of
the SA use the same bits for the same keys, and irrespective of which
part of the system divides the string of bits into individual keys,
the encryption keys MUST be taken from the first (left-most, high-
order) bits and the integrity keys MUST be taken from the remaining
bits. The number of bits for each key is defined in the relevant
cryptographic algorithm specification RFC. In the case of multiple
encryption keys or multiple integrity keys, the specification for the
cryptographic algorithm must specify the order in which they are to
be selected from a single string of bits provided to the
cryptographic algorithm.
4.5.3 Locating a Security Gateway
This section discusses issues relating to how a host learns about the
existence of relevant security gateways and once a host has contacted
these security gateways, how it knows that these are the correct
security gateways. The details of where the required information is
stored is a local matter, but the Peer Authorization database
described in Section 4.4 is the most likely candidate.
Consider a situation in which a remote host (H1) is using the
Internet to gain access to a server or other machine (H2) and there
is a security gateway (SG2), e.g., a firewall, through which H1's
traffic must pass. An example of this situation would be a mobile
host (road warrior) crossing the Internet to his home organization's
firewall (SG2). This situation raises several issues:
1. How does H1 know/learn about the existence of the security gateway
SG2?
2. How does it authenticate SG2, and once it has authenticated SG2,
how does it confirm that SG2 has been authorized to represent H2?
3. How does SG2 authenticate H1 and verify that H1 is authorized to
contact H2?
4. How does H1 know/learn about any additional gateways that provide
alternate paths to H2?
To address these problems, a host or security gateway MUST have an
administrative interface that allows the user/administrator to
configure the address of one or more security gateways for ranges of
destination addresses that require its use. This includes the
ability to configure information for locating and authenticating one
or more security gateways and verifying the authorization of these
gateways to represent the destination host. (The authorization
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function is implied in the PAD.) This document does not address the
issue of how to automate the discovery/verification of security
gateways.
4.6 Security Associations and Multicast
The receiver-orientation of the Security Association implies that, in
the case of unicast traffic, the destination system will select the
SPI value. By having the destination select the SPI value, there is
no potential for manually configured Security Associations to
conflict with automatically configured (e.g., via a key management
protocol) Security Associations or for Security Associations from
multiple sources to conflict with each other. For multicast traffic,
there are multiple destination systems associated with a single SA.
So some system or person will need to coordinate among all multicast
groups to select an SPI or SPIs on behalf of each multicast group and
then communicate the group's IPsec information to all of the
legitimate members of that multicast group via mechanisms not defined
here.
Multiple senders to a multicast group SHOULD use a single Security
Association (and hence Security Parameter Index) for all traffic to
that group when a symmetric key encryption or integrity algorithm is
employed. In such circumstances, the receiver knows only that the
message came from a system possessing the key for that multicast
group. In such circumstances, a receiver generally will not be able
to authenticate which system sent the multicast traffic.
Specifications for other, more general multicast approaches are
deferred to the IETF's Multicast Security Working Group.
5. IP Traffic Processing
As mentioned in Section 4.4.1 "The Security Policy Database (SPD)",
the SPD (or associated caches) must be be consulted during the
processing of all traffic that crosses the IPsec boundary, including
IPsec management traffic. If no policy is found in the SPD that
matches a packet (for either inbound or outbound traffic), the packet
MUST be discarded.
To simplify processing, and to allow for very fast SA lookups (for
SG/BITS/BITW), this document introduces the notion of an SPD cache
for all outbound traffic (SPD-O plus SPD-S), and a cache for inbound,
non-IPsec traffic (SPD-I). There is nominally one cache per SPD.
Since SPD entries may overlap, one cannot safely cache these entries
in general. Simple caching might result in a match against a cache
entry whereas an ordered search of the SPD would have resulted in a
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match against a different entry. But, if the SPD entries are first
decorrelated, then the resulting entries can safely be cached, and
each cached entry will map to an SA (or multiple SAs if "populate
from packet" (PFP) is specified), or indicate that matching traffic
should be bypassed or discarded, appropriately.
Note: In a host IPsec implementation based on sockets, the SPD will
be consulted whenever a new socket is created, to determine what, if
any, IPsec processing will be applied to the traffic that will flow
on that socket. This provides an implicit caching mechanism and the
portions of the preceding discussion that address caching can be
ignored in such implementations.
Note: It is assumed that one starts with a correlated SPD, because
that is how users and administrators are accustomed to managing these
sorts of access control lists or firewall filter rules. Then the
decorrelation algorithm is applied, to allow caching of SPD entries.
The decorrelation is invisible at the management interface.
For inbound IPsec traffic, the SAD entry selected by the SPI serves
as the cache for the selectors to be matched against arriving IPsec
packets, after AH or ESP processing has been performed.
5.1 Outbound IP Traffic Processing (protected-to-unprotected)
First consider the path for traffic entering the implementation via a
protected interface and exiting via an unprotected interface.
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Unprotected Interface
^
|
+----------+
...................|Forwarding|<-----+
: +----------+ |
: ^ |
: | Bypass |
: +-----+ |
+-------+ +-------+ | SPD | +--------+
| SPD-I | |Discard|<---|Cache|---->| AH/ESP |
+-------+ +-------+ +-----+ +--------+
: ^
: |
: +-------------+
:................>|SPD Selection|
+-------------+
^
|
Protected Interface
Figure 2. Processing Model for Outbound Traffic
IPsec MUST perform the following steps when processing outbound
packets:
1. when a packet arrives from the subscriber (protected) interface,
invoke the SPD lookup function to select the appropriate SPD. (If
the implementation uses only one SPD, this step is a no-op.)
2. Match the packet headers against the cache for the SPD specified
by the SPD-ID from step 1. Note that this cache contains entries
from SPD-O and SPD-S.
3a. If there is a match, then process the packet as specified by the
matching cache entry, i.e., bypass, discard, or apply AH or ESP in
the specified mode. If IPsec processing is applied, there is a
link from the SPD cache entry to the relevant SAD entry
(specifying the cryptographic algorithms, keys, SPI, etc.). IPsec
processing is as previously defined, for tunnel or transport modes
and for AH or ESP, as specified in their respective RFCs [Ken04b
and Ken04a].
3b. If no match is found in the cache, search the SPD (SPD-S and SPD-
O parts) specified by SPD-ID. If the SPD entry calls for bypass or
discard, create new outbound and inbound SPD cache entries. If the
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SPD entry calls for creation of an SA, the key management
mechanism (e.g., IKEv2) is invoked to create the SA. If SA
creation succeeds, a new outbound (SPD-S) cache entry is created,
along with an SAD entry, otherwise the packet is discarded. (A
packet that triggers an SPD lookup MAY be discarded by the
implementation, or it may be processed against the newly created
cache entry, if one is created.) Since SAs are created in pairs,
an SAD entry for the corresponding inbound SA also is created, and
it contains the selector values derived from the SPD entry used to
create the inbound SA, for use in checking inbound traffic
delivered via the SA .
4. The packet is passed to the outbound forwarding function
(operating outside of the IPsec implementation), to select the
interface to which the packet will be directed. This function may
cause the packet to be passed back across the IPsec boundary, for
additional IPsec processing, e.g., in support of nested SAs. If
so, there MUST be an entry in SPD-I database that permits bypass
of the packet.
5.1.1 Handling an Outbound Packet That Must Be Dropped
If an IPsec system receives an outbound packet which it finds it must
drop, it SHOULD be capable of generating and sending an ICMP message
to indicate to the sender of the outbound packet that the packet was
dropped. The type and code of the ICMP message will depend on the
reason for dropping the packet, as specified below. The reason
SHOULD be recorded in the audit log. The audit log entry for this
event SHOULD include the reason, current date/time, and the selector
values of the packet.
a. the selectors of the packet matched an SPD entry requiring the
packet to be dropped -->
IPv4 Type = 3 (destination unreachable) Code = 13
(Communication Administratively Prohibited)
IPv6 Type = 1 (destination unreachable) Code = 1
(Communication with destination administratively
prohibited)
b1. the IPsec system was unable to set up the SA required by the SPD
entry matching the packet because the IPsec peer at the other end
of the exchange is administratively prohibited from communicating
with the initiator and rejects the negotiation.
IPv4 Type = 3 (destination unreachable) Code = 13
(Communication Administratively Prohibited)
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IPv6 Type = 1 (destination unreachable) Code = 1
(Communication with destination administratively
prohibited)
b2. the IPsec system was unable to set up the SA required by the SPD
entry matching the packet because the IPsec peer at the other end
of the exchange could not be contacted.
IPv4 Type = 3 (destination unreachable) Code = 1 (host
unreachable)
IPv6 Type = 1 (destination unreachable) Code = 3 (address
unreachable)
Note that an attacker behind a security gateway could send packets
with a spoofed source address, W.X.Y.Z, to an IPsec entity causing it
to send ICMP messages to W.X.Y.Z. This creates an opportunity for a
DoS attack among hosts behind a security gateway. To address this, a
security gateway SHOULD include a management control to allow an
administrator to configure an IPsec implementation to send or not
send the ICMP messages under these circumstances, and if this
facility is selected, to rate limit the transmission of such ICMP
responses.
5.1.2 Header Construction for Tunnel Mode
This section describes the handling of the inner and outer IP
headers, extension headers, and options for AH and ESP tunnels, with
regard to outbound traffic processing. This includes how to
construct the encapsulating (outer) IP header, how to process fields
in the inner IP header, and what other actions should be taken for
outbound, tunnel mode traffic. The general processing described here
is modeled after RFC 2003, "IP Encapsulation with IP" [Per96]:
o The outer IP header Source Address and Destination Address
identify the "endpoints" of the tunnel (the encapsulator and
decapsulator). The inner IP header Source Address and Destination
Addresses identify the original sender and recipient of the
datagram, (from the perspective of this tunnel), respectively.
(see footnote 3 after the table in 5.1.2.1 for more details on the
encapsulating source IP address.)
o The inner IP header is not changed except as noted below for TTL
(or Hop Limit) and the ECN Field. The inner IP header otherwise
remains unchanged during its delivery to the tunnel exit point.
o No change to IP options or extension headers in the inner header
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occurs during delivery of the encapsulated datagram through the
tunnel.
Note: IPsec tunnel mode is different from IP-in-IP tunneling (RFC
2003) in several ways:
o IPsec offers certain controls to a security administrator to
manage covert channels (which would not normally be a concern for
tunneling) and to ensure that the receiver examines the right
portions of the received packet re: application of access
controls. An IPsec implementation MAY be configurable with regard
to how it processes the DSCP field for tunnel mode for transmitted
packets. For outbound traffic, one configuration setting for DSCP
will operate as described in the following sections on IPv4 and
IPv6 header processing for IPsec tunnels. Another will allow the
DSCPfield to be mapped to a fixed value, which MAY be configured
on a per SA basis. (The value might really be fixed for all
traffic outbound from a device, but per SA granularity allows that
as well.) This configuration option allows a local administrator
to decide whether the covert channel provided by copying these
bits outweighs the benefits of copying.
o IPsec describes how to handle ECN or DSCP.
o IPsec allows the IP version of the encapsulating header to be
different from that of the inner header.
The tables in the following sub-sections show the handling for the
different header/option fields ("constructed" means that the value in
the outer field is constructed independently of the value in the
inner).
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5.1.2.1 IPv4 -- Header Construction for Tunnel Mode
<-- How Outer Hdr Relates to Inner Hdr -->
Outer Hdr at Inner Hdr at
IPv4 Encapsulator Decapsulator
Header fields: -------------------- ------------
version 4 (1) no change
header length constructed no change
DS Field copied from inner hdr (5) no change
ECN Field copied from inner hdr constructed (6)
total length constructed no change
ID constructed no change
flags (DF,MF) constructed, DF (4) no change
fragment offset constructed no change
TTL constructed (2) decrement (2)
protocol AH, ESP no change
checksum constructed constructed (2)(6)
src address constructed (3) no change
dest address constructed (3) no change
Options never copied no change
1. The IP version in the encapsulating header can be
different from the value in the inner header.
2. The TTL in the inner header is decremented by the
encapsulator prior to forwarding and by the decapsulator
if it forwards the packet. (The checksum changes when
the TTL changes.)
Note: Decrementing the TTL value is a normal part of
forwarding a packet. Thus, a packet originating from
the same node as the encapsulator does not have its TTL
decremented, since the sending node is originating the
packet rather than forwarding it.
3. src and dest addresses depend on the SA, which is used
to determine the dest address which in turn determines
which src address (net interface) is used to forward the
packet.
Note: The source address that appears in the
encapsulating tunnel header MUST be the one that was
negotiated during the SA establishment process. In
principle, the encapsulating IP source address can be
any of the encapsulator's interface addresses or even an
address different from any of the encapsulator's IP
addresses, (e.g., if it's acting as a NAT box) so long
as the address is reachable through the encapsulator
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from the environment into which the packet is sent.
4. configuration determines whether to copy from the inner
header (IPv4 only), clear or set the DF.
5. If the packet will immediately enter a domain for which
the DSCP value in the outer header is not appropriate,
that value MUST be mapped to an appropriate value for
the domain [RFC 2474]. See [RFC 2475] for further
information.
6. If the ECN field in the inner header is set to ECT(0) or
ECT(1) and the ECN field in the outer header is set to
CE, then set the ECN field in the inner header to CE,
otherwise make no change to the ECN field in the inner
header. The checksum changes when the ECN changes.)
Note: IPsec does not copy the options from the inner header into the
outer header, nor does IPsec construct the options in the outer
header. However, post-IPsec code MAY insert/construct options for the
outer header.
5.1.2.2 IPv6 -- Header Construction for Tunnel Mode
See previous section 5.1.2.1 for notes 1-6 indicated by (footnote
number).
<-- How Outer Hdr Relates Inner Hdr --->
Outer Hdr at Inner Hdr at
IPv6 Encapsulator Decapsulator
Header fields: -------------------- ------------
version 6 (1) no change
DS Field copied from inner hdr (5) no change
ECN Field copied from inner hdr constructed (6)
flow label copied or configured no change
payload length constructed no change
next header AH,ESP,routing hdr no change
hop limit constructed (2) decrement (2)
src address constructed (3) no change
dest address constructed (3) no change
Extension headers never copied no change
Note: IPsec does not copy the extension headers from the inner header
into the outer header, nor does IPsec construct extension headers in
the outer header. However, post-IPsec code MAY insert/construct
extension headers for the outer header.
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5.2 Processing Inbound IP Traffic (unprotected-to-protected)
Inbound processing is somewhat different from outbound processing,
because of the use of SPIs to map IPsec protected traffic to SAs. The
inbound SPD cache (SPD-I) is applied only to bypassed or discarded
traffic. If an arriving packet appears to be an IPsec fragment from
an unprotected interface, reassembly is performed prior to the IPsec
processing. The intent for any SPD cache is that a packet that fails
to match any entry is then referred to the corresponding SPD. Every
SPD SHOULD have a nominal, final entry that catches anything that is
otherwise unmatched, and discards it. This ensures that non-IPsec
protected traffic that arrives and does not match any SPD-I entry
will be discarded.
Unprotected Interface
|
V
+-----+ IPsec protected
...................>|Demux|-------------------+
: +-----+ |
: | |
: | Not IPsec |
: |-----------+ |
: V | |
: +-------+ V V
+-----+ +-------+ |Bypass/| +------+ +------+
|SPD-O| |Discard|<---|Discard| | ICMP | |AH/ESP|
+-----+ +-------+ +-------+ +------+ +------+
^ | |
: | +---+ |
: Bypass | +-->|IKE| |
: | | +---+ |
: V | V
: +----------+ +---------+
:...............|Forwarding|<------------|SAD Check|
+----------+ +---------+
|
V
Protected Interface
Figure 3. Inbound Traffic Processing Model
Prior to performing AH or ESP processing, any IP fragments that
arrive via the unprotected interface are reassembled (by IP). Each
inbound IP datagram to which IPsec processing will be applied is
identified by the appearance of the AH or ESP values in the IP Next
Protocol field (or of AH or ESP as an extension header in the IPv6
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context).
IPsec MUST perform the following steps:
1. When a packet arrives, it may be tagged with the ID of the
interface (physical or virtual) via which it arrived, if necessary
to support multiple SPDs with different SPD-I entries.
2. The packet is examined and demuxed into one of three categories:
- If the packet appears to be IPsec protected and it is addressed
to this device, an attempt is made to map it to an active SA
via the SAD.
- Traffic not addressed to this device is directed to
BYPASS/DISCARD lookup. If multiple SPDs are employed, the tag
assigned to the packet in step 1 is used to select the
appropriate SPD-I (and cache) to search.
- ICMP traffic directed to this device is directed to
"unprotected" ICMP processing (see Section 6).
3a. If the packet is addressed to the IPsec device and AH or ESP is
specified as the protocol, the packet is looked up in the SAD
identified by the SPD-ID from step 1. For unicast traffic, use
only the SPI. For multicast traffic, use the SPI plus the source
and/or destination addresses, as specified in the SAD. If there is
no match, discard the traffic. This is an auditable event. The
audit log entry for this event SHOULD include the current
date/time, SPI, source and destination of the packet, IPsec
protocol, and any other selector values of the packet that are
available. If the packet is found in the SAD, process it
accordingly (see step 4).
3b. If the packet is not addressed to the device, look up the packet
header in the (appropriate) SPD-I cache. If there is a match and
the packet is to be discarded or bypassed, do so. If there is no
cache match, look up the packet in the corresponding SPD-I and
create a cache entry as appropriate. (No SAs are created in
response to receipt of a packet that requires IPsec protection;
only bypass or discard entries can be created this way.) If there
is no match, discard the traffic. This is an auditable event. The
audit log entry for this event SHOULD include the current
date/time, SPI if available, IPsec protocol if available, source
and destination of the packet, and any other selector values of
the packet that are available.
3c. Unprotected ICMP processing is assumed to take place on the
unprotected side of the IPsec boundary. Unprotected ICMP messages
are examined and local policy is applied to determine whether to
accept or reject these messages and, if accepted, what action to
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take as a result. For example, if an ICMP unreachable message is
received, the implementation must decide whether to act on it,
reject it, or act on it with constraints. [See Section 6.]
4. Apply AH or ESP processing as specified, using the SAD entry
selected in step 2a above. Then match the packet against the
inbound selectors identified by the SAD entry to verify that the
received packet is appropriate for the SA via which it was
received.
If an IPsec system receives an inbound packet on an SA and the
packet's header fields are not consistent with the selectors for
the SA, it MUST drop the packet. This is an auditable event. The
audit log entry for this event SHOULD include the current
date/time, SPI, IPsec protocol(s), source and destination of the
packet, and any other selector values of the packet that are
available, and the selector values from the relevant SAD entry.
The system SHOULD also be capable of generating and sending an IKE
notification to the sender (IPsec peer), indicating that the
received packet was dropped because of failure to pass selector
checks.
NOTIFY MESSAGES - ERROR TYPES Value
----------------------------- -----
INVALID_SELECTORS iana-tbd
MAY be sent in an IKE INFORMATIONAL Exchange when a node
receives an ESP or AH packet whose selectors do not match
those of the SA on which it was delivered (and which
caused the packet to be dropped). The Notification Data
contains the start of the offending packet (as in ICMP
messages) and the SPI field of the notification is set to
match the SPI of the IPsec SA.
To minimize the impact of a DoS attack or a mis-configured peer, the
IPsec system SHOULD include a management control to allow an
administrator to configure the IPsec implementation to send or not
send this IKE notification, and if this facility is selected, to rate
limit the transmission of such notifications.
After traffic is bypassed or processed through IPsec, it is handed to
the inbound forwarding function for disposition. This function may
cause the packet to be sent across the IPsec boundary for additional
inbound IPsec processing, e.g., in support of nested SAs. If so, then
as with ALL outbound traffic that is to be bypassed, the packet MUST
be matched against an SPD-O entry. Ultimately, the packet should be
forwarded to the destination host or process for disposition.
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6. ICMP Processing [This section will be filled in when IPsec issue # 91
is resolved. The following text needs to be inserted somewhere,
possibly this section.]
NOTE: With the exception of IPv4 transport mode, an SG, BITS, or BITW
implementation MAY fragment packets before applying IPsec. The
device SHOULD have a configuration setting to disable this. The
resulting fragments are evaluated against the SPD in the normal
manner. Thus, fragments not containing port numbers may only match
rules having port selectors of OPAQUE or "ANY".
7. Auditing
Not all systems that implement IPsec will implement auditing. For
the most part, the granularity of auditing is a local matter.
However, several auditable events are identified in this document and
for each of these events a minimum set of information that SHOULD be
included in an audit log is defined. Additional information also MAY
be included in the audit log for each of these events, and additional
events, not explicitly called out in this specification, also MAY
result in audit log entries. There is no requirement for the
receiver to transmit any message to the purported transmitter in
response to the detection of an auditable event, because of the
potential to induce denial of service via such action.
8. Conformance Requirements
All IPv4 systems that claim to implement IPsec MUST comply with all
requirements of this document. All IPv6 systems that claim to
implement IPsec MUST comply with all requirements of this document.
9. Security Considerations
The focus of this document is security; hence security considerations
permeate this specification.
10. Differences from RFC 2401 [Will be updated when things have settled
down.]
This architecture document differs substantially from RFC 2401 in
detail and in organization, but the fundamental notions are
unchanged.
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Acknowledgements
The authors would like to acknowledge the contributions of Ran
Atkinson, who played a critical role in initial IPsec activities, and
who authored the first series of IPsec standards: RFCs 1825-1827.
Also a contributor who wishes to remain nameless, deserves special
thanks for providing extensive help in the editing of this
specification. The authors also would like to thank the members of
the IPsec and MSEC working groups who have contributed to the
development of this protocol specification.
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Appendix A -- Glossary
This section provides definitions for several key terms that are
employed in this document. Other documents provide additional
definitions and background information relevant to this technology,
e.g., [Shi00, VK83, HA94]. Included in this glossary are generic
security service and security mechanism terms, plus IPsec-specific
terms.
Access Control
Access control is a security service that prevents unauthorized
use of a resource, including the prevention of use of a resource
in an unauthorized manner. In the IPsec context, the resource to
which access is being controlled is often:
o for a host, computing cycles or data
o for a security gateway, a network behind the gateway
or bandwidth on that network.
Anti-replay
[See "Integrity" below]
Authentication
This term is used informally to refer to the combination of two
nominally distinct security services, data origin authentication
and connectionless integrity. See the definitions below for each
of these services.
Availability
Availability, when viewed as a security service, addresses the
security concerns engendered by attacks against networks that deny
or degrade service. For example, in the IPsec context, the use of
anti-replay mechanisms in AH and ESP support availability.
Confidentiality
Confidentiality is the security service that protects data from
unauthorized disclosure. The primary confidentiality concern in
most instances is unauthorized disclosure of application level
data, but disclosure of the external characteristics of
communication also can be a concern in some circumstances.
Traffic flow confidentiality is the service that addresses this
latter concern by concealing source and destination addresses,
message length, or frequency of communication. In the IPsec
context, using ESP in tunnel mode, especially at a security
gateway, can provide some level of traffic flow confidentiality.
(See also traffic analysis, below.)
Data Origin Authentication
Data origin authentication is a security service that verifies the
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identity of the claimed source of data. This service is usually
bundled with connectionless integrity service.
Encryption
Encryption is a security mechanism used to transform data from an
intelligible form (plaintext) into an unintelligible form
(ciphertext), to provide confidentiality. The inverse
transformation process is designated "decryption". Oftimes the
term "encryption" is used to generically refer to both processes.
Integrity
Integrity is a security service that ensures that modifications to
data are detectable. Integrity comes in various flavors to match
application requirements. IPsec supports two forms of integrity:
connectionless and a form of partial sequence integrity.
Connectionless integrity is a service that detects modification of
an individual IP datagram, without regard to the ordering of the
datagram in a stream of traffic. The form of partial sequence
integrity offered in IPsec is referred to as anti-replay
integrity, and it detects arrival of duplicate IP datagrams
(within a constrained window). This is in contrast to connection-
oriented integrity, which imposes more stringent sequencing
requirements on traffic, e.g., to be able to detect lost or re-
ordered messages. Although authentication and integrity services
often are cited separately, in practice they are intimately
connected and almost always offered in tandem.
Protected vs Unprotected
"Protected" refers to the systems or interfaces that are inside
the IPsec protection boundary and "unprotected" refers to the
systems or interfaces that are outside the IPsec protection
boundary. IPsec provides a barrier through which traffic passes.
There is an asymmetry to this barrier, which is reflected in the
processing model. Outbound data, if not discarded or bypassed, is
protected via the application of AH or ESP and the addition of the
corresponding headers. Inbound data, if not discarded or
bypassed, is processed via the removal of AH or ESP headers. In
this document, inbound traffic enters an IPsec implementation from
the "unprotected" interface. Outbound traffic enters the
implementation via the "protected" interface, or is emitted by the
implementation on the "protected" side of the boundary and
directed toward the "unprotected" interface. An IPsec
implementation may support more than one interface on either or
both sides of the boundary. The protected interface may be
internal, e.g., in a host implementation of IPsec. The protected
interface may link to a socket layer interface presented by the
OS.
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Security Association (SA)
A simplex (uni-directional) logical connection, created for
security purposes. All traffic traversing an SA is provided the
same security processing. In IPsec, an SA is an internet layer
abstraction implemented through the use of AH or ESP. State data
associated with an SA is represented in the Security Association
Database (SAD).
Security Gateway
A security gateway is an intermediate system that acts as the
communications interface between two networks. The set of hosts
(and networks) on the external side of the security gateway is
termed unprotected (they are at generally at least less protected
than those "behind the SG), while the networks and hosts and on
the internal side are viewed as protected. The internal subnets
and hosts served by a security gateway are presumed to be trusted
by virtue of sharing a common, local, security administration.
(See "Trusted Subnetwork" below.) In the IPsec context, a
security gateway is a point at which AH and/or ESP is implemented
in order to serve a set of internal hosts, providing security
services for these hosts when they communicate with external hosts
also employing IPsec (either directly or via another security
gateway).
SPI
Acronym for "Security Parameters Index" (SPI). The combination of
a destination address, a security protocol, and an SPI uniquely
identifies a security association (SA, see above) in the context
of unicast or anycast traffic. Additional IP address information
is used to identify multicast SAs. The SPI is carried in AH and
ESP protocols to enable the receiving system to select the SA
under which a received packet will be processed. An SPI has only
local significance, as defined by the creator of the SA (usually
the receiver of the packet carrying the SPI); thus an SPI is
generally viewed as an opaque bit string. However, the creator of
an SA may choose to interpret the bits in an SPI to facilitate
local processing.
Traffic Analysis
The analysis of network traffic flow for the purpose of deducing
information that is useful to an adversary. Examples of such
information are frequency of transmission, the identities of the
conversing parties, sizes of packets, flow identifiers, etc.
[Sch94]
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Appendix B - Decorrelation
This section is based on work done in the IP Security Policy Working
Group by Luis Sanchez, Matt Condell, and John Zao.
Two SPD entries are correlated if there is a non-null intersection
between the values of corresponding selectors in each entry. Caching
correlated SPD entries can lead to incorrect policy enforcement. A
solution to this problem, that still allows for caching, is to remove
the ambiguities by decorrelating the entries. That is, the SPD
entries must be rewritten so that for every pair of entries there
exists a selector for which there is a null intersection between the
values in both of the entries. Once the entries are decorrelated,
there is no longer any ordering requirement on them, since only one
entry will match any lookup. The next section describes
decorrelation in more detail and presents an algorithm that may be
used to implement decorrelation.
B.1 Decorrelation Algorithm
The basic decorrelation algorithm takes each entry in a correlated
SPD and divides it up into a set of entries using a tree structure.
Those of the resulting entries that are decorrelated with the
decorrelated set of entries are then added to that decorrelated set.
The basic algorithm does not guarantee an optimal set of decorrelated
entries. That is, the entries may be broken up into smaller sets
than is necessary, though they will still provide all the necessary
policy information. Some extensions to the basic algorithm are
described later to improve this and improve the performance of the
algorithm.
C A set of ordered, correlated entries (a correlated SPD)
Ci The ith entry in C.
U The set of decorrelated entries being built from C
Ui The ith entry in U.
A policy (SPD entry) P may be expressed as a sequence of selector
values and an action (Bypass, Discard, or apply IPsec):
Pi = Si1 x Si2 x ... x Sik -> Ai
1) Put C1 in set U as U1
For each policy Cj (j > 1) in C
2) If Cj is decorrelated with every entry in U, then add it to U.
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3) If Cj is correlated with one or more entries in U, create a tree
rooted at the policy Cj that partitions Cj into a set of decorrelated
entries. The algorithm starts with a root node where no selectors
have yet been chosen.
A) Choose a selector in Cj, Scjn, that has not yet been chosen when
traversing the tree from the root to this node. If there are no
selectors not yet used, continue to the next unfinished branch
until all branches have been completed. When the tree is
completed, go to step D.
T is the set of entries in U that are correlated with the entry
at this node.
The entry at this node is the entry formed by the selector
values of each of the branches between the root and this node.
Any selector values that are not yet represented by branches
assume the corresponding selector value in Cj, since the values
in Cj represent the maximum value for each selector.
B) Add a branch to the tree for each value of the selector Scjn that
appears in any of the entries in T. (If the value is a superset
of the value of Scjn in Cj, then use the value in Cj, since that
value represents the universal set.) Also add a branch for the
complement of the union of all the values of the selector Scjn
in T. When taking the complement, remember that the universal
set is the value of Scjn in Cj. A branch need not be created
for the null set.
C) Repeat A and B until the tree is completed.
D) The entry to each leaf now represents an entry that is a subset
of Cj. The entries at the leaves completely partition Cj in
such a way that each entry is either completely overridden by
an entry in U, or is decorrelated with the entries in U.
Add all the decorrelated entries at the leaves of the tree to U.
4) Get next Cj and go to 2.
5) When all entries in C have been processed, then U will contain an
decorrelated version of C.
There are several optimizations that can be made to this algorithm.
A few of them are presented here.
It is possible to optimize, or at least improve, the amount of
branching that occurs by carefully choosing the order of the
Kent & Seo [Page 44]
Internet Draft Security Architecture for IP January 2004
selectors used for the next branch. For example, if a selector Scjn
can be chosen so that all the values for that selector in T are equal
to or a superset of the value of Scjn in Cj, then only a single
branch needs to be created (since the complement will be null).
Branches of the tree do not have to proceed with the entire
decorrelation algorithm. For example, if a node represents an entry
that is decorrelated with all the entries in U, then there is no
reason to continue decorrelating that branch. Also, if a branch is
completely overridden by an entry in U, then there is no reason to
continue decorrelating the branch.
An additional optimization is to check to see if a branch is
overridden by one of the CORRELATED entries in set C that has already
been decorrelated. That is, if the branch is part of decorrelating
Cj, then check to see if it was overridden by an entry Cm, m < j.
This is a valid check, since all the entries Cm are already expressed
in U.
Along with checking if an entry is already decorrelated in step 2,
check if Cj is overridden by any entry in U. If it is, skip it since
it is not relevant. An entry x is overridden by another entry y if
every selector in x is equal to or a subset of the corresponding
selector in entry y.
Kent & Seo [Page 45]
Internet Draft Security Architecture for IP January 2004
Appendix C -- Categorization of ICMP messages [May be deleted]
The tables below characterize ICMP messages as being either host
generated, router generated, both, unassigned/unknown. The first set
of messages are for IPv4. The second set of messages are for IPv6.
IPv4
Type Name/Codes Reference
========================================================================
HOST GENERATED:
3 Destination Unreachable
2 Protocol Unreachable [RFC792]
3 Port Unreachable [RFC792]
8 Source Host Isolated [RFC792]
14 Host Precedence Violation [RFC1812]
10 Router Selection [RFC1256]
Type Name/Codes Reference
========================================================================
ROUTER GENERATED:
3 Destination Unreachable
0 Net Unreachable [RFC792]
4 Fragmentation Needed, Don't Fragment was Set [RFC792]
5 Source Route Failed [RFC792]
6 Destination Network Unknown [RFC792]
7 Destination Host Unknown [RFC792]
9 Comm. w/Dest. Net. is Administratively Prohibited [RFC792]
11 Destination Network Unreachable for Type of Service[RFC792]
5 Redirect
0 Redirect Datagram for the Network (or subnet) [RFC792]
2 Redirect Datagram for the Type of Service & Network[RFC792]
9 Router Advertisement [RFC1256]
18 Address Mask Reply [RFC950]
Kent & Seo [Page 46]
Internet Draft Security Architecture for IP January 2004
IPv4
Type Name/Codes Reference
========================================================================
BOTH ROUTER AND HOST GENERATED:
0 Echo Reply [RFC792]
3 Destination Unreachable
1 Host Unreachable [RFC792]
10 Comm. w/Dest. Host is Administratively Prohibited [RFC792]
12 Destination Host Unreachable for Type of Service [RFC792]
13 Communication Administratively Prohibited [RFC1812]
15 Precedence cutoff in effect [RFC1812]
4 Source Quench [RFC792]
5 Redirect
1 Redirect Datagram for the Host [RFC792]
3 Redirect Datagram for the Type of Service and Host [RFC792]
6 Alternate Host Address [JBP]
8 Echo [RFC792]
11 Time Exceeded [RFC792]
12 Parameter Problem [RFC792,RFC1108]
13 Timestamp [RFC792]
14 Timestamp Reply [RFC792]
15 Information Request [RFC792]
16 Information Reply [RFC792]
17 Address Mask Request [RFC950]
30 Traceroute [RFC1393]
31 Datagram Conversion Error [RFC1475]
32 Mobile Host Redirect [Johnson]
39 SKIP [Markson]
40 Photuris [Simpson]
Type Name/Codes Reference
========================================================================
UNASSIGNED TYPE OR UNKNOWN GENERATOR:
1 Unassigned [JBP]
2 Unassigned [JBP]
7 Unassigned [JBP]
19 Reserved (for Security) [Solo]
20-29 Reserved (for Robustness Experiment) [ZSu]
33 IPv6 Where-Are-You [Simpson]
34 IPv6 I-Am-Here [Simpson]
35 Mobile Registration Request [Simpson]
36 Mobile Registration Reply [Simpson]
37 Domain Name Request [Simpson]
38 Domain Name Reply [Simpson]
41-255 Reserved [JBP]
Kent & Seo [Page 47]
Internet Draft Security Architecture for IP January 2004
IPv6
Type Name/Codes Reference
========================================================================
HOST GENERATED:
1 Destination Unreachable [RFC 1885]
4 Port Unreachable
Type Name/Codes Reference
========================================================================
ROUTER GENERATED:
1 Destination Unreachable [RFC1885]
0 No Route to Destination
1 Comm. w/Destination is Administratively Prohibited
2 Not a Neighbor
3 Address Unreachable
2 Packet Too Big [RFC1885]
0
3 Time Exceeded [RFC1885]
0 Hop Limit Exceeded in Transit
1 Fragment reassembly time exceeded
Type Name/Codes Reference
========================================================================
BOTH ROUTER AND HOST GENERATED:
4 Parameter Problem [RFC1885]
0 Erroneous Header Field Encountered
1 Unrecognized Next Header Type Encountered
2 Unrecognized IPv6 Option Encountered
Kent & Seo [Page 48]
Internet Draft Security Architecture for IP January 2004
References [Will be updated after the text settles down]
Normative
[Bra97] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Level", BCP 14, RFC 2119, March 1997.
[DH98] Deering, S., and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[Eas03] Eastlake, D., "Cryptographic Algorithm Implementation
Requirements For ESP And AH", draft-ietf-ipsec-esp-ah-
algorithms-00.txt, December 2003.
[HC03] Holbrook, H., and Cain, B., "Source Specific Multicast for
IP", Internet Draft, draft-ietf-ssm-arch-01.txt, November
3, 2002.
[Kau03] Kaufman, C., "The Internet Key Exchange (IKEv2) Protocol",
draft-ietf- ipsec-ikev2-11.txt, October 2003
[Ken04a] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
???, ???? 2004.
[Ken04b] Kent, S., "IP Authentication Header", RFC ???, ??? 2004.
[Mobip] Johnson, D., Perkins, C., Arkko, J., "Mobility Support in
IPv6", Internet Draft, draft-ietf-mobileip-ipv6-24.txt,
June 2003
[Pos81] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981
[Sch03] Schiller, J., "Cryptographic Algorithms for use in the
Internet Key Exchange Version 2", draft-ietf-ipsec-
ikev2-algorithms-04.txt, September 2003
Informative
[BL73] Bell, D.E. & LaPadula, L.J., "Secure Computer Systems:
Mathematical Foundations and Model", Technical Report
M74-244, The MITRE Corporation, Bedford, MA, May 1973.
[DoD85] US National Computer Security Center, "Department of
Defense Trusted Computer System Evaluation Criteria", DoD
5200.28-STD, US Department of Defense, Ft. Meade, MD.,
December 1985.
Kent & Seo [Page 49]
Internet Draft Security Architecture for IP January 2004
[DoD87] US National Computer Security Center, "Trusted Network
Interpretation of the Trusted Computer System Evaluation
Criteria", NCSC-TG-005, Version 1, US Department of
Defense, Ft. Meade, MD., 31 July 1987.
[FaLiHaMeTr00]Farinacci, D., Li, T., Hanks, S., Meyer, D., Traina,
P., "Generic Routing Encapsulation (GRE), RFC 2784, March
2000.
[Gro02] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, April 2002.
[HA94] Haller, N., and Atkinson, R., "On Internet Authentication",
RFC 1704, October 1994
[ISO] ISO/IEC JTC1/SC6, Network Layer Security Protocol, ISO-IEC
DIS 11577, International Standards Organisation, Geneva,
Switzerland, 29 November 1992.
[IB93] Ioannidis, J. and Blaze, M., "Architecture and
Implementation of Network-layer Security Under Unix",
Proceedings of USENIX Security Symposium, Santa Clara, CA,
October 1993.
[IBK93] Ioannidis, J., Blaze, M., and Karn, P., "swIPe: Network-
Layer Security for IP", presentation at the Spring 1993
IETF Meeting, Columbus, Ohio
[Ken91] Kent, S., "US DoD Security Options for the Internet
Protocol", RFC 1108, November 1991.
[MSST97] Maughan, D., Schertler, M., Schneider, M., and J. Turner,
"Internet Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, November 1998.
[NiBlBaBL98]Nichols, K., Blake, S., Baker, F., Black, D., "Definition
of the Differentiated Services Field (DS Field) in the IPv4
and IPv6 Headers", RFC2474, December 1998.
[Orm97] Orman, H., "The OAKLEY Key Determination Protocol", RFC
2412, November 1998.
[Per96] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[Pip98] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMP", RFC 2407, November 1998.
Kent & Seo [Page 50]
Internet Draft Security Architecture for IP January 2004
[RaFlBL01]Ramakrishnan, K., Floyd, S., Black, D., "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001.
[Sch94] Schneier, B., Applied Cryptography, Section 8.6, John
Wiley & Sons, New York, NY, 1994.
[Shi00] Shirey, R., "Internet Security Glossary", RFC 2828, May
2000.
[SDNS] SDNS Secure Data Network System, Security Protocol 3, SP3,
Document SDN.301, Revision 1.5, 15 May 1989, published in
NIST Publication NIST-IR-90-4250, February 1990.
[SMPT98] Shacham, A., Monsour, R., Pereira, R., and M. Thomas, "IP
Payload Compression Protocol (IPComp)", RFC 2393, August
1998.
[VK83] V.L. Voydock & S.T. Kent, "Security Mechanisms in High-
level Networks", ACM Computing Surveys, Vol. 15, No. 2,
June 1983.
Author Information
Stephen Kent
BBN Technologies
10 Moulton Street
Cambridge, MA 02138
USA
Phone: +1 (617) 873-3988
EMail: kent@bbn.com
Karen Seo
BBN Technologies
10 Moulton Street
Cambridge, MA 02138
USA
Phone: +1 (617) 873-3152
EMail: kseo@bbn.com
Kent & Seo [Page 51]
Internet Draft Security Architecture for IP January 2004
Notices
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
Kent & Seo [Page 52]
Internet Draft Security Architecture for IP January 2004
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Expires July 2004
Kent & Seo [Page 53]
