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strongswan/doc/standards/rfc4301.txt

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Network Working Group S. Kent
Request for Comments: 4301 K. Seo
Obsoletes: 2401 BBN Technologies
Category: Standards Track December 2005
Security Architecture for the Internet Protocol
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes an updated version of the "Security
Architecture for IP", which is designed to provide security services
for traffic at the IP layer. This document obsoletes RFC 2401
(November 1998).
Dedication
This document is dedicated to the memory of Charlie Lynn, a long-time
senior colleague at BBN, who made very significant contributions to
the IPsec documents.
Kent & Seo Standards Track [Page 1]
RFC 4301 Security Architecture for IP December 2005
Table of Contents
1. Introduction ....................................................4
1.1. Summary of Contents of Document ............................4
1.2. Audience ...................................................4
1.3. Related Documents ..........................................5
2. Design Objectives ...............................................5
2.1. Goals/Objectives/Requirements/Problem Description ..........5
2.2. Caveats and Assumptions ....................................6
3. System Overview .................................................7
3.1. What IPsec Does ............................................7
3.2. How IPsec Works ............................................9
3.3. Where IPsec Can Be Implemented ............................10
4. Security Associations ..........................................11
4.1. Definition and Scope ......................................12
4.2. SA Functionality ..........................................16
4.3. Combining SAs .............................................17
4.4. Major IPsec Databases .....................................18
4.4.1. The Security Policy Database (SPD) .................19
4.4.1.1. Selectors .................................26
4.4.1.2. Structure of an SPD Entry .................30
4.4.1.3. More Regarding Fields Associated
with Next Layer Protocols .................32
4.4.2. Security Association Database (SAD) ................34
4.4.2.1. Data Items in the SAD .....................36
4.4.2.2. Relationship between SPD, PFP
flag, packet, and SAD .....................38
4.4.3. Peer Authorization Database (PAD) ..................43
4.4.3.1. PAD Entry IDs and Matching Rules ..........44
4.4.3.2. IKE Peer Authentication Data ..............45
4.4.3.3. Child SA Authorization Data ...............46
4.4.3.4. How the PAD Is Used .......................46
4.5. SA and Key Management .....................................47
4.5.1. Manual Techniques ..................................48
4.5.2. Automated SA and Key Management ....................48
4.5.3. Locating a Security Gateway ........................49
4.6. SAs and Multicast .........................................50
5. IP Traffic Processing ..........................................50
5.1. Outbound IP Traffic Processing
(protected-to-unprotected) ................................52
5.1.1. Handling an Outbound Packet That Must Be
Discarded ..........................................54
5.1.2. Header Construction for Tunnel Mode ................55
5.1.2.1. IPv4: Header Construction for
Tunnel Mode ...............................57
5.1.2.2. IPv6: Header Construction for
Tunnel Mode ...............................59
5.2. Processing Inbound IP Traffic (unprotected-to-protected) ..59
Kent & Seo Standards Track [Page 2]
RFC 4301 Security Architecture for IP December 2005
6. ICMP Processing ................................................63
6.1. Processing ICMP Error Messages Directed to an
IPsec Implementation ......................................63
6.1.1. ICMP Error Messages Received on the
Unprotected Side of the Boundary ...................63
6.1.2. ICMP Error Messages Received on the
Protected Side of the Boundary .....................64
6.2. Processing Protected, Transit ICMP Error Messages .........64
7. Handling Fragments (on the protected side of the IPsec
boundary) ......................................................66
7.1. Tunnel Mode SAs that Carry Initial and Non-Initial
Fragments .................................................67
7.2. Separate Tunnel Mode SAs for Non-Initial Fragments ........67
7.3. Stateful Fragment Checking ................................68
7.4. BYPASS/DISCARD Traffic ....................................69
8. Path MTU/DF Processing .........................................69
8.1. DF Bit ....................................................69
8.2. Path MTU (PMTU) Discovery .................................70
8.2.1. Propagation of PMTU ................................70
8.2.2. PMTU Aging .........................................71
9. Auditing .......................................................71
10. Conformance Requirements ......................................71
11. Security Considerations .......................................72
12. IANA Considerations ...........................................72
13. Differences from RFC 2401 .....................................72
14. Acknowledgements ..............................................75
Appendix A: Glossary ..............................................76
Appendix B: Decorrelation .........................................79
B.1. Decorrelation Algorithm ...................................79
Appendix C: ASN.1 for an SPD Entry ................................82
Appendix D: Fragment Handling Rationale ...........................88
D.1. Transport Mode and Fragments ..............................88
D.2. Tunnel Mode and Fragments .................................89
D.3. The Problem of Non-Initial Fragments ......................90
D.4. BYPASS/DISCARD Traffic ....................................93
D.5. Just say no to ports? .....................................94
D.6. Other Suggested Solutions..................................94
D.7. Consistency................................................95
D.8. Conclusions................................................95
Appendix E: Example of Supporting Nested SAs via SPD and
Forwarding Table Entries...............................96
References.........................................................98
Normative References............................................98
Informative References..........................................99
Kent & Seo Standards Track [Page 3]
RFC 4301 Security Architecture for IP December 2005
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 [Pos81a] and IPv6 [DH98]
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 Network Address
Translation (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
Kent & Seo Standards Track [Page 4]
RFC 4301 Security Architecture for IP December 2005
(end users or system administrators) also are part of the target
audience. A glossary is provided in Appendix A 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 interrelationship. They
include RFCs on the following topics:
a. security protocols -- RFCs describing the Authentication
Header (AH) [Ken05b] and Encapsulating Security Payload
(ESP) [Ken05a] protocols.
b. cryptographic algorithms for integrity and encryption -- one
RFC that defines the mandatory, default algorithms for use
with AH and ESP [Eas05], a similar RFC that defines the
mandatory algorithms for use with IKEv2 [Sch05] plus a
separate RFC for each cryptographic algorithm.
c. automatic key management -- RFCs on "The Internet Key
Exchange (IKEv2) Protocol" [Kau05] and "Cryptographic
Algorithms for Use in the Internet Key Exchange Version 2
(IKEv2)" [Sch05].
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 in a
standard fashion for all protocols that may be carried over IP
(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
Kent & Seo Standards Track [Page 5]
RFC 4301 Security Architecture for IP December 2005
via IKEv2.) The IPsec firewall function makes use of the
cryptographically-enforced authentication and integrity provided for
all IPsec traffic to offer better access control than could be
obtained through use of a firewall (one not privy to IPsec internal
parameters) plus separate cryptographic protection.
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 and 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.
To facilitate interoperability in the global Internet, a set of
default cryptographic algorithms for use with AH and ESP is specified
in [Eas05] and a set of mandatory-to-implement algorithms for IKEv2
is specified in [Sch05]. [Eas05] and [Sch05] will be periodically
updated to keep pace with computational and cryptologic advances. By
specifying these algorithms in documents that are separate from the
AH, ESP, and IKEv2 specifications, these algorithms can be updated or
replaced without affecting the standardization progress of the rest
of the IPsec document suite. 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 their implementation, which is
Kent & Seo Standards Track [Page 6]
RFC 4301 Security Architecture for IP December 2005
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
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
(SG), 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
4.4.1.1) matched against entries in the SPD. Each packet is either
PROTECTed using 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.
Kent & Seo Standards Track [Page 7]
RFC 4301 Security Architecture for IP December 2005
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"
(a) between a pair of hosts, (b) between a pair of security gateways,
or (c) between a security gateway and a host. A compliant host
implementation MUST support (a) and (c) and a compliant security
gateway must support all three of these forms of connectivity, since
under certain circumstances a security gateway acts as a host.
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 or emitted by the implementation on the
unprotected side of the boundary and directed towards the protected
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.
Kent & Seo Standards Track [Page 8]
RFC 4301 Security Architecture for IP December 2005
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 [SMPT01],
motivated in part by the observation that when encryption is employed
within IPsec, it prevents effective compression by lower protocol
layers.
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
[Ken05b, Ken05a]. 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) [Ken05b] offers integrity and
data origin authentication, with optional (at the discretion of
the receiver) anti-replay features.
o The Encapsulating Security Payload (ESP) protocol [Ken05a] offers
the same set of services, and also offers confidentiality. Use of
ESP to provide confidentiality without integrity is NOT
RECOMMENDED. When ESP is used with confidentiality enabled, there
are provisions for limited traffic flow confidentiality, i.e.,
provisions for concealing packet length, and for facilitating
efficient generation and discard of dummy packets. This
capability is likely to be effective primarily in virtual private
network (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
4.4.1).
These protocols may be applied individually or in combination with
each other to provide IPv4 and IPv6 security services. 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
Kent & Seo Standards Track [Page 9]
RFC 4301 Security Architecture for IP December 2005
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 4.1.
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 protocol (AH or ESP) to employ, the mode (transport
or tunnel), security service options, what cryptographic
algorithms to use, and in what combinations to use the specified
protocols and services, and
o the granularity at which protection should be applied.
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 [Kau05]) 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 local and remote 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 6; Section 4.4.1.1, last paragraph).
Kent & Seo Standards Track [Page 10]
RFC 4301 Security Architecture for IP December 2005
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 a 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.
This document often talks in terms of use of IPsec by a host or a
security gateway, 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.
A host implementation of IPsec may appear in devices that might not
be viewed as "hosts". For example, a router might employ IPsec to
protect routing protocols (e.g., BGP) and management functions (e.g.,
Telnet), without affecting subscriber traffic traversing the router.
A security gateway might employ separate IPsec implementations to
protect its management traffic and subscriber traffic. The
architecture described in this document is very flexible. For
example, a computer with a full-featured, compliant, native OS IPsec
implementation should be capable of being configured to protect
resident (host) applications and to provide security gateway
protection for traffic traversing the computer. Such configuration
would make use of the forwarding tables and the SPD selection
function described in Sections 5.1 and 5.2.
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 AH and ESP. 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 SAs. All implementations of AH or ESP MUST support
the concept of an SA as described below. The remainder of this
Kent & Seo Standards Track [Page 11]
RFC 4301 Security Architecture for IP December 2005
section describes various aspects of SA management, defining required
characteristics for SA policy management and SA management
techniques.
4.1. Definition and Scope
An 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 SAs (one in each
direction) is required. IKE explicitly creates SA pairs in
recognition of this common usage requirement.
For an SA used to carry unicast traffic, the Security Parameters
Index (SPI) by itself suffices to specify an SA. (For information on
the SPI, see Appendix A and the AH and ESP specifications [Ken05b,
Ken05a].) 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
below for mapping inbound IPsec datagrams to SAs. Implementations
that support only unicast traffic need not implement this de-
multiplexing algorithm.
In many secure multicast architectures, e.g., [RFC3740], a central
Group Controller/Key Server unilaterally assigns the Group Security
Association's (GSA's) SPI. This SPI assignment is not negotiated or
coordinated with the key management (e.g., IKE) subsystems that
reside in the individual end systems that constitute the group.
Consequently, it is possible that a GSA and a unicast SA can
simultaneously use the same SPI. A multicast-capable IPsec
implementation MUST correctly de-multiplex inbound traffic even in
the context of SPI collisions.
Each entry in the SA Database (SAD) (Section 4.4.2) must indicate
whether the SA lookup makes use of the destination IP address, or the
destination and source IP addresses, in addition to the SPI. For
multicast SAs, the protocol field is not employed for SA lookups.
For each inbound, IPsec-protected packet, an implementation must
conduct its search of the SAD such that it finds the entry that
matches the "longest" SA identifier. In this context, if two or more
SAD entries match based on the SPI value, then the entry that also
matches based on destination address, or destination and source
address (as indicated in the SAD entry) is the "longest" match. This
implies a logical ordering of the SAD search as follows:
Kent & Seo Standards Track [Page 12]
RFC 4301 Security Architecture for IP December 2005
1. Search the SAD for a match on the combination of SPI,
destination address, and source address. If an SAD entry
matches, then process the inbound packet with that
matching SAD entry. Otherwise, proceed to step 2.
2. Search the SAD for a match on both SPI and destination address.
If the SAD entry matches, then process the inbound packet
with that matching SAD entry. Otherwise, proceed to step 3.
3. Search the SAD for a match on only SPI if the receiver has
chosen to maintain a single SPI space for AH and ESP, and on
both SPI and protocol, otherwise. If an SAD entry matches,
then process the inbound packet with that matching SAD entry.
Otherwise, discard the packet and log an auditable event.
In practice, an implementation may choose any method (or none at all)
to accelerate this search, although its externally visible behavior
MUST be functionally equivalent to having searched the SAD in the
above order. For example, a software-based implementation could
index into a hash table by the SPI. The SAD entries in each hash
table bucket's linked list could be kept sorted to have those SAD
entries with the longest SA identifiers first in that linked list.
Those SAD entries having the shortest SA identifiers could be sorted
so that they are the last entries in the linked list. A
hardware-based implementation may be able to effect the longest match
search intrinsically, using commonly available Ternary
Content-Addressable Memory (TCAM) features.
The indication of whether source and destination address matching is
required to map inbound IPsec traffic to SAs MUST be set either as a
side effect of manual SA configuration or via negotiation using an SA
management protocol, e.g., IKE or Group Domain of Interpretation
(GDOI) [RFC3547]. Typically, Source-Specific Multicast (SSM) [HC03]
groups use a 3-tuple SA identifier composed of an SPI, a destination
multicast address, and source address. An Any-Source Multicast group
SA requires only an SPI and a destination multicast address as an
identifier.
If different classes of traffic (distinguished by Differentiated
Services Code Point (DSCP) bits [NiBlBaBL98], [Gro02]) are sent on
the same SA, and if the receiver is employing the optional
anti-replay feature available in both AH and ESP, this could result
in inappropriate discarding of lower priority packets due to the
windowing mechanism used by this feature. Therefore, a sender SHOULD
put traffic of different classes, but with the same selector values,
on different SAs to support Quality of Service (QoS) appropriately.
To permit this, the IPsec implementation MUST permit establishment
and maintenance of multiple SAs between a given sender and receiver,
Kent & Seo Standards Track [Page 13]
RFC 4301 Security Architecture for IP December 2005
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. These requirements apply to
both transport and tunnel mode SAs. In the case of tunnel mode SAs,
the DSCP values in question appear in the inner IP header. In
transport mode, the DSCP value might change en route, but this should
not cause problems with respect to IPsec processing since the value
is not employed for SA selection and MUST NOT be checked as part of
SA/packet validation. However, if significant re-ordering of packets
occurs in an SA, e.g., as a result of changes to DSCP values en
route, this may trigger packet discarding by a receiver due to
application of the anti-replay mechanism.
DISCUSSION: Although the DSCP [NiBlBaBL98, Gro02] and Explicit
Congestion Notification (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 an SA typically employed between a pair of
hosts to provide end-to-end security services. When security is
desired between two intermediate systems along a path (vs. end-to-end
use of IPsec), transport mode MAY be used between security gateways
or between a security gateway and a host. In the case where
transport mode is used between security gateways or between a
security gateway and a host, transport mode may be used to support
in-IP tunneling (e.g., IP-in-IP [Per96] or Generic Routing
Encapsulation (GRE) tunneling [FaLiHaMeTr00] or dynamic routing
[ToEgWa04]) over transport mode SAs. To clarify, the use of
transport mode by an intermediate system (e.g., a security gateway)
is permitted only when applied to packets whose source address (for
outbound packets) or destination address (for inbound packets) is an
address belonging to the intermediate system itself. 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.
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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 (e.g., TCP, UDP, Stream Control
Transmission Protocol (SCTP)). 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 [Ken05b].
A tunnel mode SA is essentially an SA applied to an IP tunnel, with
the access controls applied to the headers of the traffic inside the
tunnel. Two hosts MAY establish a tunnel mode SA between themselves.
Aside from the two exceptions below, 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.
The two exceptions are as follows.
o Where traffic is destined for a security gateway, e.g., Simple
Network Management Protocol (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.
o As noted above, security gateways MAY support a transport mode SA
to provide security for IP traffic between two intermediate
systems along a path, e.g., between a host and a security gateway
or between two security gateways.
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 a
security gateway, 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 prior to
cryptographic processing. 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
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a security gateway. (Use of an IP-in-IP tunnel in conjunction with
transport mode can also address these fragmentation issues. However,
this configuration limits the ability of IPsec to enforce access
control policies on traffic.)
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 IPv6, it would be feasible to carry a plaintext fragment
on a transport mode SA; however, for simplicity, this restriction
also applies to IPv6 packets. See Section 7 for more details on
handling plaintext fragments on the protected side of the IPsec
barrier.
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
network management, or to provide security between two
intermediate systems along a path.
4.2. SA 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 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 IP or extension
headers that may change in a fashion not predictable by the sender.
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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 SA. Moreover, the
authentication means employed by IPsec peers, e.g., during creation
of an IKE (vs. child) SA also affects 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.
4.3. Combining SAs
This document does not require support for nested security
associations or for what RFC 2401 [RFC2401] 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 capability 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 [RFC2401]. 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. (See
Appendix E for an example of how to configure nested SAs.)
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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 compliant.
There are three nominal databases in this model: the Security Policy
Database (SPD), the Security Association Database (SAD), and the Peer
Authorization Database (PAD). The first specifies the policies that
determine the disposition of all IP traffic inbound or outbound from
a host or security gateway (Section 4.4.1). The second database
contains parameters that are associated with each established (keyed)
SA (Section 4.4.2). The third database, the PAD, provides a link
between an SA management protocol (such as IKE) and the SPD (Section
4.4.3).
Multiple Separate IPsec Contexts
If an IPsec implementation acts as a security gateway for multiple
subscribers, it MAY implement multiple separate IPsec contexts.
Each context MAY have and MAY 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.
For example, a security gateway that provides VPN service to
multiple customers will be able to associate each customer's
traffic with the correct VPN.
Forwarding vs Security Decisions
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 the context in which IPsec is implemented
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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.
"Local" vs "Remote"
In this document, with respect to IP addresses and ports, the
terms "Local" and "Remote" are used for policy rules. "Local"
refers to the entity being protected by an IPsec implementation,
i.e., the "source" address/port of outbound packets or the
"destination" address/port of inbound packets. "Remote" refers to
a peer entity or peer entities. The terms "source" and
"destination" are used for packet header fields.
"Non-initial" vs "Initial" Fragments
Throughout this document, the phrase "non-initial fragments" is
used to mean fragments that do not contain all of the selector
values that may be needed for access control (e.g., they might not
contain Next Layer Protocol, source and destination ports, ICMP
message type/code, Mobility Header type). And the phrase "initial
fragment" is used to mean a fragment that contains all the
selector values needed for access control. However, it should be
noted that for IPv6, which fragment contains the Next Layer
Protocol and ports (or ICMP message type/code or Mobility Header
type [Mobip]) will depend on the kind and number of extension
headers present. The "initial fragment" might not be the first
fragment, in this context.
4.4.1. The Security Policy Database (SPD)
An SA 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 traffic
not protected by IPsec, that traverses the IPsec boundary. This
includes IPsec management traffic such as IKE. An IPsec
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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 [RFC2401].
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
identifier (SPD-ID).
The SPD is an ordered database, consistent with the use of Access
Control Lists (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.
Processing Choices: DISCARD, BYPASS, PROTECT
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 PROTECT using 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 cross the IPsec boundary
without 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.
SPD-S, SPD-I, SPD-O
An SPD is logically divided into three pieces. 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.
All three of these can be decorrelated (with the exception noted
above for native host implementations) to facilitate caching. If
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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, 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. In an ordered,
non-decorrelated SPD, the entries for the SPD-S, SPD-I, and SPD-O
are interleaved. So there is one lookup in the SPD.
SPD Entries
Each SPD entry specifies packet disposition as BYPASS, DISCARD, or
PROTECT. 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 4.4.1.1. 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 detailed structure of an SPD entry is described in
Section 4.4.1.2. Every SPD SHOULD have a nominal, final entry that
matches anything that is otherwise unmatched, and discards it.
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 to be protected via AH or ESP, controls on how to
create SAs based on these selectors, and the parameters needed
to effect this protection (e.g., algorithms, modes, etc.). Note
that an SPD-S entry also contains information such as "populate
from packet" (PFP) flag (see paragraphs below on "How To Derive
the Values for an SAD entry") and bits indicating whether the
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SA lookup makes use of the local and remote IP addresses in
addition to the SPI (see AH [Ken05b] or ESP [Ken05a]
specifications).
Representing Directionality in an SPD Entry
For traffic protected by IPsec, the Local and Remote address and
ports in an SPD entry are swapped to represent directionality,
consistent with IKE conventions. In general, the protocols that
IPsec deals with have the property of requiring symmetric SAs with
flipped Local/Remote IP addresses. However, for ICMP, there is
often no such bi-directional authorization requirement.
Nonetheless, for the sake of uniformity and simplicity, SPD
entries for ICMP are specified in the same way as for other
protocols. Note also that for ICMP, Mobility Header, and
non-initial fragments, there are no port fields in these packets.
ICMP has message type and code and Mobility Header has mobility
header type. Thus, SPD entries have provisions for expressing
access controls appropriate for these protocols, in lieu of the
normal port field controls. For bypassed or discarded traffic,
separate inbound and outbound entries are supported, e.g., to
permit unidirectional flows if required.
OPAQUE and ANY
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. ANY is a wildcard that matches any value in the
corresponding field of the packet, or that matches packets where
that field is not present or is obscured. OPAQUE indicates that
the corresponding selector field is not available for examination
because it may not be present in a fragment, it does not exist for
the given Next Layer Protocol, or prior application of IPsec may
have encrypted the value. The ANY value encompasses the OPAQUE
value. Thus, OPAQUE need be used only when it is necessary to
distinguish between the case of any allowed value for a field, vs.
the absence or unavailability (e.g., due to encryption) of the
field.
How to Derive the Values for an SAD Entry
For each selector in an SPD entry, the entry specifies how to
derive the corresponding values for a new SA Database (SAD, see
Section 4.4.2) 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. For outbound traffic, there are SPD-S cache
entries and SPD-O cache entries. For inbound traffic not
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protected by IPsec, there are SPD-I cache entries and there is the
SAD, which represents the cache for inbound IPsec-protected
traffic (see Section 4.4.2). If IPsec processing is specified for
an entry, a "populate from packet" (PFP) flag may be asserted for
one or more of the selectors in the SPD entry (Local IP address;
Remote IP address; Next Layer Protocol; and, depending on Next
Layer Protocol, Local port and Remote port, or ICMP type/code, or
Mobility Header type). If asserted for a given selector X, the
flag indicates that the SA to be created should take its value for
X from the value in the packet. Otherwise, the SA should take its
value(s) for X from the value(s) in the SPD entry. Note: In the
non-PFP case, the selector values negotiated by the SA management
protocol (e.g., IKEv2) may be a subset of those in the SPD entry,
depending on the SPD policy of the peer. Also, whether a single
flag is used for, e.g., source port, ICMP type/code, and Mobility
Header (MH) type, or a separate flag is used for each, is a local
matter.
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 Remote address
is a range of IPv4 addresses: 192.0.2.1 to 192.0.2.10. Suppose an
outbound packet arrives with a destination address of 192.0.2.3,
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:
PFP flag value example of new
for the Remote SAD dest. address
addr. selector selector value
--------------- ------------
a. PFP TRUE 192.0.2.3 (one host)
b. PFP FALSE 192.0.2.1 to 192.0.2.10 (range of hosts)
Note that if the SPD entry above had a value of ANY for the Remote
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.
Management Interface
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
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implementation making use of a socket interface, the SPD may not
need to be consulted on a per-packet basis, as noted at the end of
Section 4.4.1.1 and in Section 5.) The management interface for
the SPD MUST allow creation of entries consistent with the
selectors defined in Section 4.4.1.1, 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 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.
Decorrelation
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. Decorrelation
[CoSa04] is only a means of improving performance and simplifying
the processing description. This RFC does not require a compliant
implementation to make use of decorrelation. For example, native
host implementations typically make use of caching implicitly
because they bind SAs to socket interfaces, and thus there is no
requirement to be able to decorrelate SPD entries in these
implementations.
Note: Unless otherwise qualified, the use of "SPD" refers to the
body of policy information in both ordered or decorrelated
(unordered) state. 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 linked 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. For example, suppose one starts
with an entry A (from an ordered SPD) that when decorrelated,
yields entries A1, A2, and A3. When a packet comes along that
matches, say A2, and triggers the creation of an SA, the SA
management protocol (e.g., IKEv2) negotiates A. And all 3
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decorrelated entries, A1, A2, and A3, are placed in the
appropriate SPD-S cache and linked to the SA. The intent is that
use of a decorrelated SPD ought not to create more SAs than would
have resulted from use of a not-decorrelated SPD.
If a decorrelated SPD is employed, there are three options for
what an initiator sends to a peer via an SA management protocol
(e.g., IKE). By sending the complete set of linked, decorrelated
entries that were selected from the SPD, a peer is given the best
possible information to enable selection of the appropriate SPD
entry at its end, especially if the peer has also decorrelated its
SPD. However, if a large number of decorrelated entries are
linked, this may create large packets for SA negotiation, and
hence fragmentation problems for the SA management protocol.
Alternatively, the original entry from the (correlated) SPD may be
retained and passed to the SA management protocol. Passing the
correlated SPD entry keeps the use of a decorrelated SPD a local
matter, not visible to peers, and avoids possible fragmentation
concerns, although it provides less precise information to a
responder for matching against the responder's SPD.
An intermediate approach is to send a subset of the complete set
of linked, decorrelated SPD entries. This approach can avoid the
fragmentation problems cited above yet provide better information
than the original, correlated entry. The major shortcoming of
this approach is that it may cause additional SAs to be created
later, since only a subset of the linked, decorrelated entries are
sent to a peer. Implementers are free to employ any of the
approaches cited above.
A responder uses the traffic selector proposals it receives via an
SA management protocol to select an appropriate entry in its SPD.
The intent of the matching is to select an SPD entry and create an
SA that most closely matches the intent of the initiator, so that
traffic traversing the resulting SA will be accepted at both ends.
If the responder employs a decorrelated SPD, it SHOULD use the
decorrelated SPD entries for matching, as this will generally
result in creation of SAs that are more likely to match the intent
of both peers. If the responder has a correlated SPD, then it
SHOULD match the proposals against the correlated entries. For
IKEv2, use of a decorrelated SPD offers the best opportunity for a
responder to generate a "narrowed" response.
In all cases, when a decorrelated SPD is available, the
decorrelated entries are used to populate the SPD-S cache. If the
SPD is not decorrelated, caching is not allowed and an ordered
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search of SPD MUST be performed to verify that inbound traffic
arriving on an SA is consistent with the access control policy
expressed in the SPD.
Handling Changes to the SPD While the System Is Running
If a change is made to the SPD while the system is running, a
check SHOULD be made of the effect of this change on extant SAs.
An implementation SHOULD check the impact of an SPD change on
extant SAs and SHOULD provide a user/administrator with a
mechanism for configuring what actions to take, e.g., delete an
affected SA, allow an affected SA to continue unchanged, etc.
4.4.1.1. 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 Layer Protocol
and related fields, e.g., ports), 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 communicating host pair. The following selector
parameters MUST be supported by all IPsec implementations to
facilitate control of SA granularity. Note that both Local and
Remote addresses should either be IPv4 or IPv6, but not a mix of
address types. Also, note that the Local/Remote port selectors (and
ICMP message type and code, and Mobility Header type) may be labeled
as OPAQUE to accommodate situations where these fields are
inaccessible due to packet fragmentation.
- Remote IP Address(es) (IPv4 or IPv6): This is a list of ranges
of IP addresses (unicast, broadcast (IPv4 only)). This
structure allows expression of a single IP address (via a
trivial range), or a list of addresses (each a trivial range),
or a range of addresses (low and high values, inclusive), as
well as the most generic form of a list of ranges. Address
ranges are used to support more than one remote system sharing
the same SA, e.g., behind a security gateway.
- Local IP Address(es) (IPv4 or IPv6): This is a list of ranges of
IP addresses (unicast, broadcast (IPv4 only)). This structure
allows expression of a single IP address (via a trivial range),
or a list of addresses (each a trivial range), or a range of
addresses (low and high values, inclusive), as well as the most
generic form of a list of ranges. Address ranges are used to
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support more than one source system sharing the same SA, e.g.,
behind a security gateway. Local refers to the address(es)
being protected by this implementation (or policy entry).
Note: The SPD does not include support for multicast address
entries. To support multicast SAs, an implementation should
make use of a Group SPD (GSPD) as defined in [RFC3740]. GSPD
entries require a different structure, i.e., one cannot use the
symmetric relationship associated with local and remote address
values for unicast SAs in a multicast context. Specifically,
outbound traffic directed to a multicast address on an SA would
not be received on a companion, inbound SA with the multicast
address as the source.
- Next Layer Protocol: Obtained from the IPv4 "Protocol" or the
IPv6 "Next Header" fields. This is an individual protocol
number, ANY, or for IPv6 only, OPAQUE. The Next Layer Protocol
is whatever comes after any IP extension headers that are
present. To simplify locating the Next Layer Protocol, there
SHOULD be a mechanism for configuring which IPv6 extension
headers to skip. The default configuration for which protocols
to skip SHOULD include the following protocols: 0 (Hop-by-hop
options), 43 (Routing Header), 44 (Fragmentation Header), and 60
(Destination Options). Note: The default list does NOT include
51 (AH) or 50 (ESP). From a selector lookup point of view,
IPsec treats AH and ESP as Next Layer Protocols.
Several additional selectors depend on the Next Layer Protocol
value:
* If the Next Layer Protocol uses two ports (as do TCP, UDP,
SCTP, and others), then there are selectors for Local and
Remote Ports. Each of these selectors has a list of ranges
of values. Note that the Local and Remote ports may not be
available in the case of receipt of a fragmented packet or if
the port fields have been protected by IPsec (encrypted);
thus, a value of OPAQUE also MUST be supported. Note: In a
non-initial fragment, port values will not be available. If
a port selector specifies a value other than ANY or OPAQUE,
it cannot match packets that are non-initial fragments. If
the SA requires a port value other than ANY or OPAQUE, an
arriving fragment without ports MUST be discarded. (See
Section 7, "Handling Fragments".)
* 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. Note that the MH type may not be available
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in the case of receipt of a fragmented packet. (See Section
7, "Handling Fragments".) For IKE, the IPv6 Mobility Header
message type (MH type) is placed in the most significant
eight bits of the 16-bit local "port" selector.
* If the Next Layer Protocol value is ICMP, then there is a
16-bit selector 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 a single 8-bit
value that defines a specific subtype for an ICMP message.
For IKE, the message type is placed in the most significant 8
bits of the 16-bit selector and the code is placed in the
least significant 8 bits. This 16-bit selector can contain a
single type and a range of codes, a single type and ANY code,
and ANY type and ANY code. Given a policy entry with a range
of Types (T-start to T-end) and a range of Codes (C-start to
C-end), and an ICMP packet with Type t and Code c, an
implementation MUST test for a match using
(T-start*256) + C-start <= (t*256) + c <= (T-end*256) +
C-end
Note that the ICMP message type and code may not be available
in the case of receipt of a fragmented packet. (See Section
7, "Handling Fragments".)
- Name: This is not a selector like the others above. It is not
acquired from a packet. A name may be used as a symbolic
identifier for an IPsec Local or Remote address. Named SPD
entries are used in two ways:
1. A named SPD entry is used by a responder (not an initiator)
in support of access control when an IP address would not be
appropriate for the Remote IP address selector, e.g., for
"road warriors". The name used to match this field is
communicated during the IKE negotiation in the ID payload.
In this context, the initiator's Source IP address (inner IP
header in tunnel mode) is bound to the Remote IP address in
the SAD entry created by the IKE negotiation. This address
overrides the Remote IP address value in the SPD, when the
SPD entry is selected in this fashion. All IPsec
implementations MUST support this use of names.
2. A named SPD entry may be used by an initiator to identify a
user for whom an IPsec SA will be created (or for whom
traffic may be bypassed). The initiator's IP source address
(from inner IP header in tunnel mode) is used to replace the
following if and when they are created:
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- local address in the SPD cache entry
- local address in the outbound SAD entry
- remote address in the inbound SAD entry
Support for this use is optional for multi-user, native host
implementations and not applicable to other implementations.
Note that this name is used only locally; it is not
communicated by the key management protocol. Also, name
forms other than those used for case 1 above (responder) are
applicable in the initiator context (see below).
An SPD entry can contain both a name (or a list of names) and
also values for the Local or Remote IP address.
For case 1, responder, the identifiers employed in named SPD
entries are one of the following four types:
a. a fully qualified user name string (email), e.g.,
mozart@foo.example.com
(this corresponds to ID_RFC822_ADDR in IKEv2)
b. a fully qualified DNS name, e.g.,
foo.example.com
(this corresponds to ID_FQDN in IKEv2)
c. X.500 distinguished name, e.g., [WaKiHo97],
CN = Stephen T. Kent, O = BBN Technologies,
SP = MA, C = US
(this corresponds to ID_DER_ASN1_DN in IKEv2, after
decoding)
d. a byte string
(this corresponds to Key_ID in IKEv2)
For case 2, initiator, the identifiers employed in named SPD
entries are of type byte string. They are likely to be Unix
UIDs, Windows security IDs, or something similar, but could
also be a user name or account name. In all cases, this
identifier is only of local concern and is not transmitted.
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
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-O/SPD-S cache lookup based on the selectors.
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4.4.1.2. Structure of an SPD Entry
This section contains a prose description of an SPD entry. Also,
Appendix C provides an example of an ASN.1 definition of an SPD
entry.
This text describes the SPD in a fashion that is intended to map
directly into IKE payloads to ensure that the policy required by SPD
entries can be negotiated through IKE. Unfortunately, the semantics
of the version of IKEv2 published concurrently with this document
[Kau05] do not align precisely with those defined for the SPD.
Specifically, IKEv2 does not enable negotiation of a single SA that
binds multiple pairs of local and remote addresses and ports to a
single SA. Instead, when multiple local and remote addresses and
ports are negotiated for an SA, IKEv2 treats these not as pairs, but
as (unordered) sets of local and remote values that can be
arbitrarily paired. Until IKE provides a facility that conveys the
semantics that are expressed in the SPD via selector sets (as
described below), users MUST NOT include multiple selector sets in a
single SPD entry unless the access control intent aligns with the IKE
"mix and match" semantics. An implementation MAY warn users, to
alert them to this problem if users create SPD entries with multiple
selector sets, the syntax of which indicates possible conflicts with
current IKE semantics.
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".) Note that Remote/Local apply only to IP addresses
and ports, not to ICMP message type/code or Mobility Header type.
Also, if the reserved, symbolic selector value OPAQUE or ANY is
employed for a given selector type, only that value may appear in the
list for that selector, and it must appear only once in the list for
that selector. Note that ANY and OPAQUE are local syntax conventions
-- IKEv2 negotiates these values via the ranges indicated below:
ANY: start = 0 end = <max>
OPAQUE: start = <max> end = 0
An SPD is an ordered list of entries each of which contains the
following fields.
o Name -- a list of IDs. This quasi-selector is optional.
The forms that MUST be supported are described above in
Section 4.4.1.1 under "Name".
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o PFP flags -- one per traffic selector. A given flag, e.g.,
for Next Layer Protocol, applies to the relevant selector
across all "selector sets" (see below) contained in an SPD
entry. When creating an SA, each flag specifies for the
corresponding traffic selector whether to instantiate the
selector from the corresponding field in the packet that
triggered the creation of the SA or from the value(s) in
the corresponding SPD entry (see Section 4.4.1, "How to
Derive the Values for an SAD Entry"). Whether a single
flag is used for, e.g., source port, ICMP type/code, and
MH type, or a separate flag is used for each, is a local
matter. There are PFP flags for:
- Local Address
- Remote Address
- Next Layer Protocol
- Local Port, or ICMP message type/code or Mobility
Header type (depending on the next layer protocol)
- Remote Port, or ICMP message type/code or Mobility
Header type (depending on the next layer protocol)
o One to N selector sets that correspond to the "condition"
for applying a particular IPsec action. Each selector set
contains:
- Local Address
- Remote Address
- Next Layer Protocol
- Local Port, or ICMP message type/code or Mobility
Header type (depending on the next layer protocol)
- Remote Port, or ICMP message type/code or Mobility
Header type (depending on the next layer protocol)
Note: The "next protocol" selector is an individual value
(unlike the local and remote IP addresses) in a selector
set entry. This is consistent with how IKEv2 negotiates
the Traffic Selector (TS) values for an SA. It also makes
sense because one may need to associate different port
fields with different protocols. It is possible to
associate multiple protocols (and ports) with a single SA
by specifying multiple selector sets for that SA.
o Processing info -- which action is required -- PROTECT,
BYPASS, or DISCARD. There is just one action that goes
with all the selector sets, not a separate action for each
set. If the required processing is PROTECT, the entry
contains the following information.
- IPsec mode -- tunnel or transport
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- (if tunnel mode) local tunnel address -- For a
non-mobile host, if there is just one interface, this
is straightforward; if there are multiple
interfaces, this must be statically configured. For a
mobile host, the specification of the local address
is handled externally to IPsec.
- (if tunnel mode) remote tunnel address -- There is no
standard way to determine this. See 4.5.3, "Locating
a Security Gateway".
- Extended Sequence Number -- Is this SA using extended
sequence numbers?
- stateful fragment checking -- Is this SA using
stateful fragment checking? (See Section 7 for more
details.)
- Bypass DF bit (T/F) -- applicable to tunnel mode SAs
- Bypass DSCP (T/F) or map to unprotected DSCP values
(array) if needed to restrict bypass of DSCP values --
applicable to tunnel mode SAs
- IPsec protocol -- AH or ESP
- algorithms -- which ones to use for AH, which ones to
use for ESP, which ones to use for combined mode,
ordered by decreasing priority
It is a local matter as to what information is kept with regard to
handling extant SAs when the SPD is changed.
4.4.1.3. More Regarding Fields Associated with Next Layer Protocols
Additional selectors are often associated with fields in the Next
Layer Protocol header. A particular Next Layer Protocol can have
zero, one, or two selectors. There may be situations where there
aren't both local and remote selectors for the fields that are
dependent on the Next Layer Protocol. The IPv6 Mobility Header has
only a Mobility Header message type. AH and ESP have no further
selector fields. A system may be willing to send an ICMP message
type and code that it does not want to receive. In the descriptions
below, "port" is used to mean a field that is dependent on the Next
Layer Protocol.
A. If a Next Layer Protocol has no "port" selectors, then
the Local and Remote "port" selectors are set to OPAQUE in
the relevant SPD entry, e.g.,
Local's
next layer protocol = AH
"port" selector = OPAQUE
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Remote's
next layer protocol = AH
"port" selector = OPAQUE
B. Even if a Next Layer Protocol has only one selector, e.g.,
Mobility Header type, then the Local and Remote "port"
selectors are used to indicate whether a system is
willing to send and/or receive traffic with the specified
"port" values. For example, if Mobility Headers of a
specified type are allowed to be sent and received via an
SA, then the relevant SPD entry would be set as follows:
Local's
next layer protocol = Mobility Header
"port" selector = Mobility Header message type
Remote's
next layer protocol = Mobility Header
"port" selector = Mobility Header message type
If Mobility Headers of a specified type are allowed to be
sent but NOT received via an SA, then the relevant SPD
entry would be set as follows:
Local's
next layer protocol = Mobility Header
"port" selector = Mobility Header message type
Remote's
next layer protocol = Mobility Header
"port" selector = OPAQUE
If Mobility Headers of a specified type are allowed to be
received but NOT sent via an SA, then the relevant SPD
entry would be set as follows:
Local's
next layer protocol = Mobility Header
"port" selector = OPAQUE
Remote's
next layer protocol = Mobility Header
"port" selector = Mobility Header message type
C. If a system is willing to send traffic with a particular
"port" value but NOT receive traffic with that kind of
port value, the system's traffic selectors are set as
follows in the relevant SPD entry:
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Local's
next layer protocol = ICMP
"port" selector = <specific ICMP type & code>
Remote's
next layer protocol = ICMP
"port" selector = OPAQUE
D. To indicate that a system is willing to receive traffic
with a particular "port" value but NOT send that kind of
traffic, the system's traffic selectors are set as follows
in the relevant SPD entry:
Local's
next layer protocol = ICMP
"port" selector = OPAQUE
Remote's
next layer protocol = ICMP
"port" selector = <specific ICMP type & code>
For example, if a security gateway is willing to allow
systems behind it to send ICMP traceroutes, but is not
willing to let outside systems run ICMP traceroutes to
systems behind it, then the security gateway's traffic
selectors are set as follows in the relevant SPD entry:
Local's
next layer protocol = 1 (ICMPv4)
"port" selector = 30 (traceroute)
Remote's
next layer protocol = 1 (ICMPv4)
"port" selector = OPAQUE
4.4.2. Security Association Database (SAD)
In each IPsec implementation, there is a nominal Security Association
Database (SAD), in which each entry defines the parameters associated
with one SA. Each SA has an entry in the SAD. For outbound
processing, each SAD entry is pointed to by entries in the SPD-S part
of the SPD cache. For inbound processing, for unicast SAs, the SPI
is used either alone to look up an SA or in conjunction with the
IPsec protocol type. If an IPsec implementation supports multicast,
the SPI plus destination address, or SPI plus destination and source
addresses are used to look up the SA. (See Section 4.1 for details on
the algorithm that MUST be used for mapping inbound IPsec datagrams
to SAs.) The following parameters are associated with each entry in
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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 4.4.1.1, the entry for
an inbound SA in the SAD MUST be initially populated with the value
or values negotiated at the time the SA was created. (See the
paragraph in Section 4.4.1 under "Handling Changes to the SPD while
the System is Running" for guidance on the effect of SPD changes on
extant SAs.) For a receiver, these values are used to check that the
header fields of an inbound packet (after IPsec processing) match the
selector values negotiated for the SA. Thus, the SAD acts as a cache
for checking the selectors of inbound traffic arriving on SAs. 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 4.4.1.1,
"Selectors". Note also that there are a couple of situations in
which the SAD can have entries for SAs that do not have corresponding
entries in the SPD. Since this document does not mandate that the
SAD be selectively cleared when the SPD is changed, SAD entries can
remain when the SPD entries that created them are changed or deleted.
Also, if a manually keyed SA is created, there could be an SAD entry
for this SA that does not correspond to any SPD entry.
Note: The SAD can support multicast SAs, if manually configured. An
outbound multicast SA has the same structure as a unicast SA. The
source address is that of the sender, and the destination address is
the multicast group address. An inbound, multicast SA must be
configured with the source addresses of each peer authorized to
transmit to the multicast SA in question. The SPI value for a
multicast SA is provided by a multicast group controller, not by the
receiver, as for a unicast SA. Because an SAD entry may be required
to accommodate multiple, individual IP source addresses that were
part of an SPD entry (for unicast SAs), the required facility for
inbound, multicast SAs is a feature already present in an IPsec
implementation. However, because the SPD has no provisions for
accommodating multicast entries, this document does not specify an
automated way to create an SAD entry for a multicast, inbound SA.
Only manually configured SAD entries can be created to accommodate
inbound, multicast traffic.
Implementation Guidance: This document does not specify how an SPD-S
entry refers to the corresponding SAD entry, as this is an
implementation-specific detail. However, some implementations (based
on experience from RFC 2401) are known to have problems in this
regard. In particular, simply storing the (remote tunnel header IP
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address, remote SPI) pair in the SPD cache is not sufficient, since
the pair does not always uniquely identify a single SAD entry. For
instance, two hosts behind the same NAT could choose the same SPI
value. The situation also may arise if a host is assigned an IP
address (e.g., via DHCP) previously used by some other host, and the
SAs associated with the old host have not yet been deleted via dead
peer detection mechanisms. This may lead to packets being sent over
the wrong SA or, if key management ensures the pair is unique,
denying the creation of otherwise valid SAs. Thus, implementors
should implement links between the SPD cache and the SAD in a way
that does not engender such problems.
4.4.2.1. Data Items in the SAD
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 counter 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
rollover is permitted. The audit log entry for this event SHOULD
include the SPI value, current date/time, Local Address, Remote
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 as well.
o AH Authentication algorithm, key, etc. This is required only if
AH is supported.
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o ESP Encryption algorithm, key, mode, IV, etc. If a combined mode
algorithm is used, these fields will not be applicable.
o ESP integrity algorithm, keys, etc. If the integrity service is
not selected, these fields will not be applicable. If a combined
mode algorithm is used, these fields will not be applicable.
o ESP combined mode algorithms, key(s), etc. This data is used when
a combined mode (encryption and integrity) algorithm is used with
ESP. If a combined mode algorithm is not used, these fields are
not applicable.
o Lifetime of this SA: 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 Certificate Revocation
Lists (CRLs) used in the IKE exchange for the SA. Both initiator
and responder are responsible for constraining the 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 MUST be able to handle having the counters at
the ends of an SA get out of synch, e.g., because of packet
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 SA's
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.
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o Stateful fragment checking flag. Indicates whether or not
stateful fragment checking applies to this SA.
o Bypass DF bit (T/F) -- applicable to tunnel mode SAs where both
inner and outer headers are IPv4.
o DSCP values -- the set of DSCP values allowed for packets carried
over this SA. If no values are specified, no DSCP-specific
filtering is applied. If one or more values are specified, these
are used to select one SA among several that match the traffic
selectors for an outbound packet. Note that these values are NOT
checked against inbound traffic arriving on the SA.
o Bypass DSCP (T/F) or map to unprotected DSCP values (array) if
needed to restrict bypass of DSCP values -- applicable to tunnel
mode SAs. This feature maps DSCP values from an inner header to
values in an outer header, e.g., to address covert channel
signaling concerns.
o Path MTU: any observed path MTU and aging variables.
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.
4.4.2.2. Relationship between SPD, PFP flag, packet, and SAD
For each selector, the following tables show the relationship
between the value in the SPD, the PFP flag, the value in the
triggering packet, and the resulting value in the SAD. Note that
the administrative interface for IPsec can use various syntactic
options to make it easier for the administrator to enter rules.
For example, although a list of ranges is what IKEv2 sends, it
might be clearer and less error prone for the user to enter a
single IP address or IP address prefix.
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Value in
Triggering Resulting SAD
Selector SPD Entry PFP Packet Entry
-------- ---------------- --- ------------ --------------
loc addr list of ranges 0 IP addr "S" list of ranges
ANY 0 IP addr "S" ANY
list of ranges 1 IP addr "S" "S"
ANY 1 IP addr "S" "S"
rem addr list of ranges 0 IP addr "D" list of ranges
ANY 0 IP addr "D" ANY
list of ranges 1 IP addr "D" "D"
ANY 1 IP addr "D" "D"
protocol list of prot's* 0 prot. "P" list of prot's*
ANY** 0 prot. "P" ANY
OPAQUE**** 0 prot. "P" OPAQUE
list of prot's* 0 not avail. discard packet
ANY** 0 not avail. ANY
OPAQUE**** 0 not avail. OPAQUE
list of prot's* 1 prot. "P" "P"
ANY** 1 prot. "P" "P"
OPAQUE**** 1 prot. "P" ***
list of prot's* 1 not avail. discard packet
ANY** 1 not avail. discard packet
OPAQUE**** 1 not avail. ***
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If the protocol is one that has two ports, then there will be
selectors for both Local and Remote ports.
Value in
Triggering Resulting SAD
Selector SPD Entry PFP Packet Entry
-------- ---------------- --- ------------ --------------
loc port list of ranges 0 src port "s" list of ranges
ANY 0 src port "s" ANY
OPAQUE 0 src port "s" OPAQUE
list of ranges 0 not avail. discard packet
ANY 0 not avail. ANY
OPAQUE 0 not avail. OPAQUE
list of ranges 1 src port "s" "s"
ANY 1 src port "s" "s"
OPAQUE 1 src port "s" ***
list of ranges 1 not avail. discard packet
ANY 1 not avail. discard packet
OPAQUE 1 not avail. ***
rem port list of ranges 0 dst port "d" list of ranges
ANY 0 dst port "d" ANY
OPAQUE 0 dst port "d" OPAQUE
list of ranges 0 not avail. discard packet
ANY 0 not avail. ANY
OPAQUE 0 not avail. OPAQUE
list of ranges 1 dst port "d" "d"
ANY 1 dst port "d" "d"
OPAQUE 1 dst port "d" ***
list of ranges 1 not avail. discard packet
ANY 1 not avail. discard packet
OPAQUE 1 not avail. ***
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If the protocol is mobility header, then there will be a selector
for mh type.
Value in
Triggering Resulting SAD
Selector SPD Entry PFP Packet Entry
-------- ---------------- --- ------------ --------------
mh type list of ranges 0 mh type "T" list of ranges
ANY 0 mh type "T" ANY
OPAQUE 0 mh type "T" OPAQUE
list of ranges 0 not avail. discard packet
ANY 0 not avail. ANY
OPAQUE 0 not avail. OPAQUE
list of ranges 1 mh type "T" "T"
ANY 1 mh type "T" "T"
OPAQUE 1 mh type "T" ***
list of ranges 1 not avail. discard packet
ANY 1 not avail. discard packet
OPAQUE 1 not avail. ***
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If the protocol is ICMP, then there will be a 16-bit selector for
ICMP type and ICMP code. Note that the type and code are bound to
each other, i.e., the codes apply to the particular type. This
16-bit selector can contain a single type and a range of codes, a
single type and ANY code, and ANY type and ANY code.
Value in
Triggering Resulting SAD
Selector SPD Entry PFP Packet Entry
--------- ---------------- --- ------------ --------------
ICMP type a single type & 0 type "t" & single type &
and code range of codes code "c" range of codes
a single type & 0 type "t" & single type &
ANY code code "c" ANY code
ANY type & ANY 0 type "t" & ANY type &
code code "c" ANY code
OPAQUE 0 type "t" & OPAQUE
code "c"
a single type & 0 not avail. discard packet
range of codes
a single type & 0 not avail. discard packet
ANY code
ANY type & 0 not avail. ANY type &
ANY code ANY code
OPAQUE 0 not avail. OPAQUE
a single type & 1 type "t" & "t" and "c"
range of codes code "c"
a single type & 1 type "t" & "t" and "c"
ANY code code "c"
ANY type & 1 type "t" & "t" and "c"
ANY code code "c"
OPAQUE 1 type "t" & ***
code "c"
a single type & 1 not avail. discard packet
range of codes
a single type & 1 not avail. discard packet
ANY code
ANY type & 1 not avail. discard packet
ANY code
OPAQUE 1 not avail. ***
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If the name selector is used:
Value in
Triggering Resulting SAD
Selector SPD Entry PFP Packet Entry
--------- ---------------- --- ------------ --------------
name list of user or N/A N/A N/A
system names
* "List of protocols" is the information, not the way
that the SPD or SAD or IKEv2 have to represent this
information.
** 0 (zero) is used by IKE to indicate ANY for
protocol.
*** Use of PFP=1 with an OPAQUE value is an error and
SHOULD be prohibited by an IPsec implementation.
**** The protocol field cannot be OPAQUE in IPv4. This
table entry applies only to IPv6.
4.4.3. Peer Authorization Database (PAD)
The Peer Authorization Database (PAD) provides the link between the
SPD and a security association management protocol such as IKE. It
embodies several critical functions:
o identifies the peers or groups of peers that are authorized
to communicate with this IPsec entity
o specifies the protocol and method used to authenticate each
peer
o provides the authentication data for each peer
o constrains the types and values of IDs that can be asserted
by a peer with regard to child SA creation, to ensure that the
peer does not assert identities for lookup in the SPD that it
is not authorized to represent, when child SAs are created
o peer gateway location info, e.g., IP address(es) or DNS names,
MAY be included for peers that are known to be "behind" a
security gateway
The PAD provides these functions for an IKE peer when the peer acts
as either the initiator or the responder.
To perform these functions, the PAD contains an entry for each peer
or group of peers with which the IPsec entity will communicate. An
entry names an individual peer (a user, end system or security
gateway) or specifies a group of peers (using ID matching rules
defined below). The entry specifies the authentication protocol
(e.g., IKEv1, IKEv2, KINK) method used (e.g., certificates or pre-
shared secrets) and the authentication data (e.g., the pre-shared
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secret or the trust anchor relative to which the peer's certificate
will be validated). For certificate-based authentication, the entry
also may provide information to assist in verifying the revocation
status of the peer, e.g., a pointer to a CRL repository or the name
of an Online Certificate Status Protocol (OCSP) server associated
with the peer or with the trust anchor associated with the peer.
Each entry also specifies whether the IKE ID payload will be used as
a symbolic name for SPD lookup, or whether the remote IP address
provided in traffic selector payloads will be used for SPD lookups
when child SAs are created.
Note that the PAD information MAY be used to support creation of more
than one tunnel mode SA at a time between two peers, e.g., two
tunnels to protect the same addresses/hosts, but with different
tunnel endpoints.
4.4.3.1. PAD Entry IDs and Matching Rules
The PAD is an ordered database, where the order is defined by an
administrator (or a user in the case of a single-user end system).
Usually, the same administrator will be responsible for both the PAD
and SPD, since the two databases must be coordinated. The ordering
requirement for the PAD arises for the same reason as for the SPD,
i.e., because use of "star name" entries allows for overlaps in the
set of IKE IDs that could match a specific entry.
Six types of IDs are supported for entries in the PAD, consistent
with the symbolic name types and IP addresses used to identify SPD
entries. The ID for each entry acts as the index for the PAD, i.e.,
it is the value used to select an entry. All of these ID types can
be used to match IKE ID payload types. The six types are:
o DNS name (specific or partial)
o Distinguished Name (complete or sub-tree constrained)
o RFC 822 email address (complete or partially qualified)
o IPv4 address (range)
o IPv6 address (range)
o Key ID (exact match only)
The first three name types can accommodate sub-tree matching as well
as exact matches. A DNS name may be fully qualified and thus match
exactly one name, e.g., foo.example.com. Alternatively, the name may
encompass a group of peers by being partially specified, e.g., the
string ".example.com" could be used to match any DNS name ending in
these two domain name components.
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Similarly, a Distinguished Name may specify a complete Distinguished
Name to match exactly one entry, e.g., CN = Stephen, O = BBN
Technologies, SP = MA, C = US. Alternatively, an entry may encompass
a group of peers by specifying a sub-tree, e.g., an entry of the form
"C = US, SP = MA" might be used to match all DNs that contain these
two attributes as the top two Relative Distinguished Names (RDNs).
For an RFC 822 e-mail addresses, the same options exist. A complete
address such as foo@example.com matches one entity, but a sub-tree
name such as "@example.com" could be used to match all the entities
with names ending in those two domain names to the right of the @.
The specific syntax used by an implementation to accommodate sub-tree
matching for distinguished names, domain names or RFC 822 e-mail
addresses is a local matter. But, at a minimum, sub-tree matching of
the sort described above MUST be supported. (Substring matching
within a DN, DNS name, or RFC 822 address MAY be supported, but is
not required.)
For IPv4 and IPv6 addresses, the same address range syntax used for
SPD entries MUST be supported. This allows specification of an
individual address (via a trivial range), an address prefix (by
choosing a range that adheres to Classless Inter-Domain Routing
(CIDR)-style prefixes), or an arbitrary address range.
The Key ID field is defined as an OCTET string in IKE. For this name
type, only exact-match syntax MUST be supported (since there is no
explicit structure for this ID type). Additional matching functions
MAY be supported for this ID type.
4.4.3.2. IKE Peer Authentication Data
Once an entry is located based on an ordered search of the PAD based
on ID field matching, it is necessary to verify the asserted
identity, i.e., to authenticate the asserted ID. For each PAD entry,
there is an indication of the type of authentication to be performed.
This document requires support for two required authentication data
types:
- X.509 certificate
- pre-shared secret
For authentication based on an X.509 certificate, the PAD entry
contains a trust anchor via which the end entity (EE) certificate for
the peer must be verifiable, either directly or via a certificate
path. See RFC 3280 for the definition of a trust anchor. An entry
used with certificate-based authentication MAY include additional
data to facilitate certificate revocation status, e.g., a list of
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appropriate OCSP responders or CRL repositories, and associated
authentication data. For authentication based on a pre-shared
secret, the PAD contains the pre-shared secret to be used by IKE.
This document does not require that the IKE ID asserted by a peer be
syntactically related to a specific field in an end entity
certificate that is employed to authenticate the identity of that
peer. However, it often will be appropriate to impose such a
requirement, e.g., when a single entry represents a set of peers each
of whom may have a distinct SPD entry. Thus, implementations MUST
provide a means for an administrator to require a match between an
asserted IKE ID and the subject name or subject alt name in a
certificate. The former is applicable to IKE IDs expressed as
distinguished names; the latter is appropriate for DNS names, RFC 822
e-mail addresses, and IP addresses. Since KEY ID is intended for
identifying a peer authenticated via a pre-shared secret, there is no
requirement to match this ID type to a certificate field.
See IKEv1 [HarCar98] and IKEv2 [Kau05] for details of how IKE
performs peer authentication using certificates or pre-shared
secrets.
This document does not mandate support for any other authentication
methods, although such methods MAY be employed.
4.4.3.3. Child SA Authorization Data
Once an IKE peer is authenticated, child SAs may be created. Each
PAD entry contains data to constrain the set of IDs that can be
asserted by an IKE peer, for matching against the SPD. Each PAD
entry indicates whether the IKE ID is to be used as a symbolic name
for SPD matching, or whether an IP address asserted in a traffic
selector payload is to be used.
If the entry indicates that the IKE ID is to be used, then the PAD
entry ID field defines the authorized set of IDs. If the entry
indicates that child SAs traffic selectors are to be used, then an
additional data element is required, in the form of IPv4 and/or IPv6
address ranges. (A peer may be authorized for both address types, so
there MUST be provision for both a v4 and a v6 address range.)
4.4.3.4. How the PAD Is Used
During the initial IKE exchange, the initiator and responder each
assert their identity via the IKE ID payload and send an AUTH payload
to verify the asserted identity. One or more CERT payloads may be
transmitted to facilitate the verification of each asserted identity.
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When an IKE entity receives an IKE ID payload, it uses the asserted
ID to locate an entry in the PAD, using the matching rules described
above. The PAD entry specifies the authentication method to be
employed for the identified peer. This ensures that the right method
is used for each peer and that different methods can be used for
different peers. The entry also specifies the authentication data
that will be used to verify the asserted identity. This data is
employed in conjunction with the specified method to authenticate the
peer, before any CHILD SAs are created.
Child SAs are created based on the exchange of traffic selector
payloads, either at the end of the initial IKE exchange or in
subsequent CREATE_CHILD_SA exchanges. The PAD entry for the (now
authenticated) IKE peer is used to constrain creation of child SAs;
specifically, the PAD entry specifies how the SPD is searched using a
traffic selector proposal from a peer. There are two choices: either
the IKE ID asserted by the peer is used to find an SPD entry via its
symbolic name, or peer IP addresses asserted in traffic selector
payloads are used for SPD lookups based on the remote IP address
field portion of an SPD entry. It is necessary to impose these
constraints on creation of child SAs to prevent an authenticated peer
from spoofing IDs associated with other, legitimate peers.
Note that because the PAD is checked before searching for an SPD
entry, this safeguard protects an initiator against spoofing attacks.
For example, assume that IKE A receives an outbound packet destined
for IP address X, a host served by a security gateway. RFC 2401
[RFC2401] and this document do not specify how A determines the
address of the IKE peer serving X. However, any peer contacted by A
as the presumed representative for X must be registered in the PAD in
order to allow the IKE exchange to be authenticated. Moreover, when
the authenticated peer asserts that it represents X in its traffic
selector exchange, the PAD will be consulted to determine if the peer
in question is authorized to represent X. Thus, the PAD provides a
binding of address ranges (or name sub-spaces) to peers, to counter
such attacks.
4.5. SA and Key Management
All IPsec implementations MUST support 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
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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.
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 SA
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 [Kau05]. This document assumes the availability of
certain functions from the key management protocol that are not
supported by IKEv1. 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
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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
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 (PAD) described in Section 4.4 is the most likely candidate.
(Note: S* indicates a system that is running IPsec, e.g., SH1 and SG2
below.)
Consider a situation in which a remote host (SH1) 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 crossing the Internet to his home organization's firewall (SG2).
This situation raises several issues:
1. How does SH1 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 SH1 and verify that SH1 is authorized to
contact H2?
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4. How does SH1 know/learn about any additional gateways that provide
alternate paths to H2?
To address these problems, an IPsec-supporting 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 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. SAs and Multicast
The receiver-orientation of the SA 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 SAs to conflict with automatically configured
(e.g., via a key management protocol) SAs or for SAs 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 SPI) 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 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 consulted during the
processing of all traffic that crosses the IPsec protection 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
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notion of an SPD cache for all outbound traffic (SPD-O plus SPD-S),
and a cache for inbound, non-IPsec-protected traffic (SPD-I). (As
mentioned earlier, the SAD acts as a cache for checking the selectors
of inbound IPsec-protected traffic arriving on SAs.) There is
nominally one cache per SPD. For the purposes of this specification,
it is assumed that each cached entry will map to exactly one SA.
Note, however, exceptions arise when one uses multiple SAs to carry
traffic of different priorities (e.g., as indicated by distinct DSCP
values) but the same selectors. Note also, that there are a couple
of situations in which the SAD can have entries for SAs that do not
have corresponding entries in the SPD. Since this document does not
mandate that the SAD be selectively cleared when the SPD is changed,
SAD entries can remain when the SPD entries that created them are
changed or deleted. Also, if a manually keyed SA is created, there
could be an SAD entry for this SA that does not correspond to any SPD
entry.
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
match against a different entry. But, if the SPD entries are first
decorrelated, then the resulting entries can safely be cached. Each
cached entry will indicate that matching traffic should be bypassed
or discarded, appropriately. (Note: The original SPD entry might
result in multiple SAs, e.g., because of PFP.) Unless otherwise
noted, all references below to the "SPD" or "SPD cache" or "cache"
are to a decorrelated SPD (SPD-I, SPD-O, SPD-S) or the SPD cache
containing entries from the decorrelated SPD.
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 build a list of cache-able 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.
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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.
Unprotected Interface
^
|
(nested SAs) +----------+
-------------------|Forwarding|<-----+
| +----------+ |
| ^ |
| | BYPASS |
V +-----+ |
+-------+ | SPD | +--------+
...| SPD-I |.................|Cache|.....|PROCESS |...IPsec
| (*) | | (*) |---->|(AH/ESP)| boundary
+-------+ +-----+ +--------+
| +-------+ / ^
| |DISCARD| <--/ |
| +-------+ |
| |
| +-------------+
|---------------->|SPD Selection|
+-------------+
^
| +------+
| -->| ICMP |
| / +------+
|/
|
|
Protected Interface
Figure 2. Processing Model for Outbound Traffic
(*) = The SPD caches are shown here. If there
is a cache miss, then the SPD is checked.
There is no requirement that an
implementation buffer the packet if
there is a cache miss.
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IPsec MUST perform the following steps when processing outbound
packets:
1. When a packet arrives from the subscriber (protected) interface,
invoke the SPD selection function to obtain the SPD-ID needed to
choose 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 PROTECT using AH
or ESP. If IPsec processing is applied, there is a link from the
SPD cache entry to the relevant SAD entry (specifying the mode,
cryptographic algorithms, keys, SPI, PMTU, etc.). IPsec
processing is as previously defined, for tunnel or transport
modes and for AH or ESP, as specified in their respective RFCs
[Ken05b, Ken05a]. Note that the SA PMTU value, plus the value of
the stateful fragment checking flag (and the DF bit in the IP
header of the outbound packet) determine whether the packet can
(must) be fragmented prior to or after IPsec processing, or if it
must be discarded and an ICMP PMTU message is sent.
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 one or more new outbound SPD cache
entries and if BYPASS, create one or more new inbound SPD cache
entries. (More than one cache entry may be created since a
decorrelated SPD entry may be linked to other such entries that
were created as a side effect of the decorrelation process.) If
the SPD entry calls for PROTECT, i.e., 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 outbound and inbound SAD entries, 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 (and packet, if any PFP flags were
"true") 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
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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
inbound bypassing of the packet, otherwise the packet will be
discarded. If necessary, i.e., if there is more than one SPD-I,
the traffic being looped back MAY be tagged as coming from this
internal interface. This would allow the use of a different
SPD-I for "real" external traffic vs. looped traffic, if needed.
Note: With the exception of IPv4 and IPv6 transport mode, an SG,
BITS, or BITW implementation MAY fragment packets before applying
IPsec. (This applies only to IPv4. For IPv6 packets, only the
originator is allowed to fragment them.) 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 (or ICMP message type and code, or Mobility
Header type) will only match rules having port (or ICMP message type
and code, or MH type) selectors of OPAQUE or ANY. (See Section 7 for
more details.)
Note: With regard to determining and enforcing the PMTU of an SA, the
IPsec system MUST follow the steps described in Section 8.2.
5.1.1. Handling an Outbound Packet That Must Be Discarded
If an IPsec system receives an outbound packet that it finds it must
discard, it SHOULD be capable of generating and sending an ICMP
message to indicate to the sender of the outbound packet that the
packet was discarded. The type and code of the ICMP message will
depend on the reason for discarding 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 from the packet.
a. The selectors of the packet matched an SPD entry requiring the
packet to be discarded.
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 successfully reached the remote peer but was
unable to negotiate the SA required by the SPD entry matching the
packet because, for example, the remote peer is administratively
prohibited from communicating with the initiator, the initiating
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peer was unable to authenticate itself to the remote peer, the
remote peer was unable to authenticate itself to the initiating
peer, or the SPD at the remote peer did not have a suitable
entry.
IPv4 Type = 3 (destination unreachable) Code = 13
(Communication Administratively Prohibited)
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
denial of service (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 within 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.
Kent & Seo Standards Track [Page 55]
RFC 4301 Security Architecture for IP December 2005
(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 DS/ECN Fields. The inner IP header
otherwise remains unchanged during its delivery to the tunnel exit
point.