Network Working Group S. Kent
Internet Draft K. Seo
draft-ietf-ipsec-rfc2401bis-05.txt BBN Technologies
Obsoletes: RFC 2401 December 2004
Expires June 2005
Security Architecture for the Internet Protocol
Dedicated to the memory of Charlie Lynn, a long time senior
colleague at BBN, who made very significant contributions to
the IPsec documents.
Status of this Memo
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patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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Copyright (C) The Internet Society (2004). All Rights Reserved.
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).
Comments should be sent to Stephen Kent (kent@bbn.com). [RFC Editor:
Please remove this line prior to publication as an RFC.]
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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...........................................11
4.2 SA Functionality...............................................15
4.3 Combining SAs..................................................16
4.4 Major IPsec Databases..........................................17
4.4.1 The Security Policy Database (SPD)........................18
4.4.1.1 Selectors............................................24
4.4.1.2 Structure of an SPD entry............................27
4.4.1.3 More re: Fields Associated with Next Layer Protocols.29
4.4.2 Security Association Database (SAD).......................31
4.4.2.1 Data Items in the SAD................................32
4.4.2.2 Relationship between SPD, PFP flag, packet, and SAD..34
4.4.3 Peer Authorization Database (PAD).........................38
4.5 SA and Key Management..........................................39
4.5.1 Manual Techniques.........................................39
4.5.2 Automated SA and Key Management...........................39
4.5.3 Locating a Security Gateway...............................40
4.6 SAs and Multicast..............................................41
5. IP Traffic Processing...............................................42
5.1 Outbound IP Traffic Processing (protected-to-unprotected)......43
5.1.1 Handling an Outbound Packet That Must Be Discarded........45
5.1.2 Header Construction for Tunnel Mode.......................46
5.1.2.1 IPv4 -- Header Construction for Tunnel Mode..........48
5.1.2.2 IPv6 -- Header Construction for Tunnel Mode..........49
5.2 Processing Inbound IP Traffic (unprotected-to-protected).......50
6. ICMP Processing ....................................................53
6.1 Processing ICMP Error Messages Directed to an IPsec
Implementation......................................54
6.1.1 ICMP Error Messages Received on the Unprotected
Side of the Boundary................................54
6.1.2 ICMP Error Messages Received on the Protected
Side of the Boundary................................55
6.2 Processing Protected, Transit ICMP Error Messages..............55
7. Handling Fragments (on the protected side of the IPsec boundary)....56
7.1 Tunnel Mode SAs that Carry Initial and Non-Initial Fragments...57
7.2 Separate Tunnel Mode SAs for Non-Initial Fragments.............58
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7.3 Stateful Fragment Checking.....................................59
7.4 BYPASS/DISCARD traffic.........................................59
8. Path MTU/DF Processing..............................................60
8.1 DF Bit.........................................................60
8.2 Path MTU (PMTU) Discovery......................................60
8.2.1 Propagation of PMTU.......................................60
8.2.2 PMTU Aging................................................61
9. Auditing............................................................61
10. Conformance Requirements...........................................62
11. Security Considerations............................................62
12. IANA Considerations................................................62
13. Differences from RFC 2401..........................................62
Acknowledgements.......................................................65
Appendix A -- Glossary.................................................66
Appendix B -- Decorrelation............................................69
Appendix C -- ASN.1 for an SPD Entry...................................72
Appendix D -- Fragment Handling Rationale..............................78
D.1 Transport Mode and Fragments...................................78
D.2 Tunnel Mode and Fragments......................................79
D.3 The Problem of Non-Initial Fragments...........................80
D.4 BYPASS/DISCARD traffic.........................................83
D.5 Just say no to ports?..........................................83
D.6 Other Suggested Solutions......................................84
D.7 Consistency....................................................85
D.8 Conclusions....................................................85
Appendix E -- Example of Supporting Nested SAs via SPD and Forwarding
Table Entries......................................86
References.............................................................88
Author Information.....................................................90
Notices................................................................91
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1. Introduction
1.1 Summary of Contents of Document
This document specifies the base architecture for IPsec compliant
systems. It describes how to provide a set of security services for
traffic at the IP layer, in both the IPv4 [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
(end users or system administrators) also are part of the target
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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 inter-relationship.
They include RFCs on the following topics:
a. security protocols -- RFCs describing the Authentication Header
(AH) [Ken04b] and Encapsulating Security Payload (ESP) [Ken04a]
protocols.
b. cryptographic algorithms for integrity and encryption -- one RFC
that defines the mandatory, default algorithms for use with AH
and ESP [Eas03], a similar RFC that defines the mandatory
algorithms for use with IKEv2 [Sch03] plus a separate RFC for
each cryptographic algorithm.
c. automatic key management -- RFCs on "The Internet Key Exchange
(IKEv2) Protocol" [Kau04] and "Cryptographic Algorithms for use
in the Internet Key Exchange Version 2" [Sch03]
2. Design Objectives
2.1 Goals/Objectives/Requirements/Problem Description
IPsec is designed to provide interoperable, high quality,
cryptographically-based security for IPv4 and IPv6. The set of
security services offered includes access control, connectionless
integrity, data origin authentication, detection and rejection of
replays (a form of partial sequence integrity), confidentiality (via
encryption), and limited traffic flow confidentiality. These
services are provided at the IP layer, offering protection 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
via IKEv2.) The IPsec firewall function makes use of the
cryptographically-enforced authentication and integrity provided for
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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 & 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 [Eas03] and a set of mandatory-to-implement algorithms for IKEv2
is specified in [Sch03]. [Eas03] and [Sch03] 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 the their implementation, which
is outside the scope of this set of standards. Moreover, the
security of a computer system or network is a function of many
factors, including personnel, physical, procedural, compromising
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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, 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 Security Policy Database (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. 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"
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(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.
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.
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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
[Ken04b, Ken04a]. IPsec implementations MUST support ESP and MAY
support AH. (Support for AH has been downgraded to MAY because
experience has shown that there are very few contexts in which ESP
cannot provide the requisite security services. Note that ESP can be
used to provide only integrity, without confidentiality, making it
comparable to AH in most contexts.)
o The IP Authentication Header (AH) [Ken04b] offers integrity and
data origin authentication, with optional (at the discretion of
the receiver) anti-replay features.
o The Encapsulating Security Payload (ESP) protocol [Ken04a] offers
the same set of services, and also offers 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 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
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
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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,
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 [Kau04]) for
automated key management, but other automated key distribution
techniques MAY be used.
Note: This document mandates support for several features for which
support is available in IKEv2 but not in IKEv1, e.g., negotiation of
an SA representing ranges of 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).
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
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"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.
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
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 SPI (Security Parameters
Index - see Appendix A and AH [Ken04b] and ESP [Ken04a]
specifications) by itself suffices to specify an SA. However, as a
local matter, an implementation may choose to use the SPI in
conjunction with the IPsec protocol type (AH or ESP) for SA
identification. If an IPsec implementation supports multicast, then
it MUST support multicast SAs using the algorithm below for mapping
inbound IPsec datagrams to SAs. Implementations that support only
unicast traffic need not implement this demultiplexing algorithm.
In many secure multicast (or anycast) architectures, e.g., [RFC3740],
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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:
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
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Memory (TCAM0 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 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 CodePoint (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 QoS appropriately. To permit this, the
IPsec implementation MUST permit establishment and maintenance of
multiple SAs between a given sender and receiver, with the same
selectors. Distribution of traffic among these parallel SAs to
support QoS is locally determined by the sender and is not negotiated
by IKE. The receiver MUST process the packets from the different SAs
without prejudice.
DISCUSSION: While the DSCP [NiBlBaBL98, Gro02] and 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 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
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(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.
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, 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 [Ken04b].
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., 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
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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 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 on the
election of optional services within the protocol.
For example, both AH and ESP offer integrity and authentication
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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.
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 effects the granularity of the access
control afforded.
If confidentiality is selected, then an ESP (tunnel mode) SA between
two security gateways can offer partial traffic flow confidentiality.
The use of tunnel mode allows the inner IP headers to be encrypted,
concealing the identities of the (ultimate) traffic source and
destination. Moreover, ESP payload padding also can be invoked to
hide the size of the packets, further concealing the external
characteristics of the traffic. Similar traffic flow confidentiality
services may be offered when a mobile user is assigned a dynamic IP
address in a dialup context, and establishes a (tunnel mode) ESP SA
to a corporate firewall (acting as a security gateway). Note that
fine granularity SAs generally are more vulnerable to traffic
analysis than coarse granularity ones that are carrying traffic from
many subscribers.
Note: A compliant implementation MUST NOT allow instantiation of an
ESP SA that employs both NULL encryption and no integrity algorithm.
An attempt to negotiate such an SA is an auditable event by both
initiator and responder. The audit log entry for this event SHOULD
include the current date/time, local IKE IP address, and remote IKE
IP address. The initiator SHOULD record the relevant SPD entry.
4.3 Combining SAs
This document does not require support for nested security
associations or for what RFC 2401 called "SA bundles." These features
still can be effected by appropriate configuration of both the SPD
and the local forwarding functions (for inbound and outbound
traffic), but this 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. 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
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example of how to configure nested SAs.)
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 Peer Authorization
Database (PAD) provides a link between an SA management protocol like
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
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may be complex, e.g., if there are multiple protected or
unprotected interfaces or if the context in which IPsec is
implemented employs a sophisticated forwarding function. IPsec
assumes only that outbound and inbound traffic that has passed
through IPsec processing is forwarded in a fashion consistent with
the context in which IPsec is implemented. Support for nested SAs
is optional; if required, it requires coordination between
forwarding tables and SPD entries to cause a packet to traverse
the IPsec boundary more than once.
"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) 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
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includes IPsec management traffic such as IKE. An IPsec
implementation MUST have at least one SPD, and it MAY support
multiple SPDs, if appropriate for the context in which the IPsec
implementation operates. There is no requirement to maintain SPDs on
a per interface basis, as was specified in RFC 2401. However, if an
implementation supports multiple SPDs, then it MUST include an
explicit SPD selection function, that is invoked to select the
appropriate SPD for outbound traffic processing. The inputs to this
function are the outbound packet and any local metadata (e.g., the
interface via which the packet arrived) required to effect the SPD
selection function. The output of the function is an SPD identifier
(SPD-ID).
The SPD is an ordered database, consistent with the use of ACLs or
packet filters in firewalls, routers, etc. The ordering requirement
arises because entries often will overlap due to the presence of
(non-trivial) ranges as values for selectors. Thus a user or
administrator MUST be able to order the entries to express a desired
access control policy. There is no way to impose a general, canonical
order on SPD entries, because of the allowed use of wildcards for
selector values and because the different types of selectors are not
hierarchically related.
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-SPD-S, SPD-SPD-I, SPD-SPD-O
An SPD is logically divided into three pieces. The SPD-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 an
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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 look up 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
SA lookup makes use of the local and remote IP addresses in
addition to the SPI (see AH [Ken04b] or ESP [Ken04a]
specifications).
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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, does not exist for
the given Next Layer Protocol, or because 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
protected by IPsec, there are SPD-I cache entries and there is the
SAD, which represents the cache for inbound IPsec-protected
traffic (See Figure 3 in Section 5.2). 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
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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 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: 192.168.2.1 to 192.168.2.10. Suppose
an outbound packet arrives with a destination address of
192.168.2.3, and there is no extant SA to carry this packet. The
value used for the SA created to transmit this packet could be
either of the two values shown below, depending on what the SPD
entry for this selector says is the source of the selector value:
PFP flag value example of new
for the Remote SAD dest. address
addr. selector selector value
--------------- ------------
a. PFP TRUE 192.168.2.3 (one host)
b. PFP FALSE 192.168.2.1 to 192.168.2.10 (range of hosts)
Note that if the SPD entry 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
implementation making use of a socket interface, the SPD may not
need to be consulted on a per packet basis, as noted above.) The
management interface for the SPD MUST allow creation of entries
consistent with the selectors defined in Section 4.4.1.1, and MUST
support (total) ordering of these entries, as seen via this
interface. The SPD entries' selectors are analogous to the ACL or
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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. ppendix 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
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. Note also that if a
decorrelated SPD is employed, the original entry from the
(correlated) SPD should be retained and passed to the SA
management protocol, e.g., IKE. Passing the correlated SPD entry
to the SA management protocol keeps the use of a decorrelated SPD
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a local matter, not visible to peers. When acting as a responder,
the peer uses a correlated SPD entry for matching, and for issuing
a "narrowed" response. Then the decorrelated entries are used to
populate the SPD-S cache.
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, anycast, broadcast (IPv4 only), or
multicast group). This structure allows expression of a single
IP address (via a trivial range), or a list of addresses (each a
trivial 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, anycast, broadcast (IPv4 only), or
multicast group). This structure allows expression of a single
IP address (via a trivial range), or a list of addresses (each a
trivial range), or a range of addresses (low and high values,
inclusive), as well as the most generic form of a list of
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ranges. Address ranges are used to 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).
- 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 (e.g., TCP, UDP,
SCTP, ...), 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
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.
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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,
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:
- 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.
An SPD entry can contain both a name (or a list of names) and
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also values for the Local or Remote IP address. 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.,
C = US, SP = MA,
O = BBN Technologies, CN = Stephen T. Kent
(this corresponds to ID_DER_ASN1_DN in IKEv2, after
decoding)
d. a byte string
(this corresponds to Key_ID in IKEv2)
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.
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 maps directly into IKE
payloads to ensure that the policy required by SPD entries can be
negotiated through IKE. 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
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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".
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 IKE v2 negotiates
the TS values for an SA. It also makes sense because one
may need to associate different port fields with different
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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
- (if tunnel mode) local tunnel address -- For a
non-mobile host, if there is just one interface, this
is straightforward; and 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 re: 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
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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
Remote's
next layer protocol = AH
"port" selector = OPAQUE
B. If a Next Layer Protocol has only one selector, e.g.,
Mobility Header type, then that field is placed in the
Local "port" selector in the relevant SPD entry, and the
Remote "port" selector is set to OPAQUE in the relevant
SPD entry, e.g.,
Local's
next layer protocol = Mobility Header
"port" selector = Mobility Header message type
Remote's
next layer protocol = Mobility Header
"port" selector = OPAQUE
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:
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
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"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 the SPI may be used 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 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 Section
4.4.1, paragraph on 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."
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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.
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.
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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 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 SAs
lifetime, the packet SHOULD be discarded.
o IPsec protocol mode: tunnel or transport. Indicates which mode of
AH or ESP is applied to traffic on this SA.
o 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
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
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
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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.
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 system- N/A packet from N/A
dependent user or
user or sys. system
names
* "List of protocols" is the information, not the way
that the SPD or SAD or IKv2 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 a link between an SA
management protocol like IKE and the SPD. The PAD indicates the range
of identities that an IPv4 or IPv6 peer is authorized to represent
when (child) SAs are negotiated with the peer. The identities may be
a list of IPv4 or IPv6 address ranges or a set of symbolic names.
The IP version of the identities does not have to be the same as that
of the IP version of the peer representing them. The fundamental
requirement associated with the PAD is that the traffic selectors
passed by the SA management protocol for comparison against the SPD
MUST be verified as authorized relative to the authenticated peer of
the SA management protocol. (See also Section 4.5.3, which levies
requirements on the PAD in support of locating security gateways.)
The PAD also specifies how to authenticate each peer, e.g., via
shared secret or use of a certificate. If a shared secret is used,
the secret is stored here. If a certificate is to be used, a trust
anchor for validating the certificate is available via the PAD. The
PAD also MAY include data in support of certificate revocation status
checking, if this information is not otherwise available from the
trust anchor or the peer's certificate. Because the PAD might be
incorporated into the SA management protocol implementation, it is
not discussed extensively in this document.
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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
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
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IPsec is IKEv2 [Kau04]. This document assumes the availability of
certain functions from the key management protocol which 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
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
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 (road warrior) 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?
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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?
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. The IP Security Policy
(IPSP) Working Group is a possible future source of guidance. One of
its goals is to produce an Internet Draft on a "Security Gateway
Discovery, Policy Exchange and Negotiation Protocol".
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.
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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
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.
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.
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
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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 [Ken04b
and Ken04a]. 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 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
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"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, e.g., because the remote peer is administratively
prohibited from communicating with the initiator, or the
initiating peer was unable to authenticate itself to the remote
peer, or the remote peer was unable to authenticate itself to the
initiating peer, or SPD at remote peer did not have a suitable
entry, etc.
IPv4 Type = 3 (destination unreachable) Code = 13
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(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
DoS attack among hosts behind a security gateway. To address this, a
security gateway SHOULD include a management control to allow an
administrator to configure an IPsec implementation to send or not
send the ICMP messages under these circumstances, and if this
facility is selected, to rate limit the transmission of such ICMP
responses.
5.1.2 Header Construction for Tunnel Mode
This section describes the handling of the inner and outer IP
headers, extension headers, and options for AH and ESP tunnels, with
regard to outbound traffic processing. This includes how to
construct the encapsulating (outer) IP header, how to process fields
in the inner IP header, and what other actions should be taken for
outbound, tunnel mode traffic. The general processing described here
is modeled after RFC 2003, "IP Encapsulation with IP" [Per96]:
o The outer IP header Source Address and Destination Address
identify the "endpoints" of the tunnel (the encapsulator and
decapsulator). The inner IP header Source Address and Destination
Addresses identify the original sender and recipient of the
datagram, (from the perspective of this tunnel), respectively.
(See footnote 3 after the table in 5.1.2.1 for more details on the
encapsulating source IP address.)
o The inner IP header is not changed except as noted below for TTL
(or Hop Limit) and the DS/ECN Fields. The inner IP header
otherwise remains unchanged during its delivery to the tunnel exit
point.
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o No change to IP options or extension headers in the inner header
occurs during delivery of the encapsulated datagram through the
tunnel.
Note: IPsec tunnel mode is different from IP-in-IP tunneling (RFC
2003) in several ways:
o IPsec offers certain controls to a security administrator to
manage covert channels (which would not normally be a concern for
tunneling) and to ensure that the receiver examines the right
portions of the received packet re: application of access
controls. An IPsec implementation MAY be configurable with regard
to how it processes the outer DS field for tunnel mode for
transmitted packets. For outbound traffic, one configuration
setting for the outer DS field will operate as described in the
following sections on IPv4 and IPv6 header processing for IPsec
tunnels. Another will allow the outer DS field to be mapped to a
fixed value, which MAY be configured on a per SA basis. (The value
might really be fixed for all traffic outbound from a device, but
per SA granularity allows that as well.) This configuration option
allows a local administrator to decide whether the covert channel
provided by copying these bits outweighs the benefits of copying.
o IPsec describes how to handle ECN or DS.
o IPsec allows the IP version of the encapsulating header to be
different from that of the inner header.
The tables in the following sub-sections show the handling for the
different header/option fields ("constructed" means that the value in
the outer field is constructed independently of the value in the
inner).
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5.1.2.1 IPv4 -- Header Construction for Tunnel Mode
<-- How Outer Hdr Relates to Inner Hdr -->
Outer Hdr at Inner Hdr at
IPv4 Encapsulator Decapsulator
Header fields: -------------------- ------------
version 4 (1) no change
header length constructed no change
DS Field copied from inner hdr (5) no change
ECN Field copied from inner hdr constructed (6)
total length constructed no change
ID constructed no change
flags (DF,MF) constructed, DF (4) no change
fragment offset constructed no change
TTL constructed (2) decrement (2)
protocol AH, ESP no change
checksum constructed constructed (2)(6)
src address constructed (3) no change
dest address constructed (3) no change
Options never copied no change
1. The IP version in the encapsulating header can be
different from the value in the inner header.
2. The TTL in the inner header is decremented by the
encapsulator prior to forwarding and by the decapsulator
if it forwards the packet. (The IPv4 checksum changes
when the TTL changes.)
Note: Decrementing the TTL value is a normal part of
forwarding a packet. Thus, a packet originating from
the same node as the encapsulator does not have its TTL
decremented, since the sending node is originating the
packet rather than forwarding it.
3. Local and Remote addresses depend on the SA, which is
used to determine the Remote address which in turn
determines which Local address (net interface) is used
to forward the packet.
Note: For multicast traffic, the destination address, or
source and destination addresses, may be required for
demuxing. In that case, it is important to ensure
consistency over the lifetime of the SA by ensuring that
the source address that appears in the encapsulating
tunnel header is the same as the one that was negotiated
during the SA establishment process. There is an
exception to this general rule, i.e., a mobile IPsec
implementation will update its source address as it
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moves.
4. Configuration determines whether to copy from the inner
header (IPv4 only), clear, or set the DF.
5. If the packet will immediately enter a domain for which
the DSCP value in the outer header is not appropriate,
that value MUST be mapped to an appropriate value for
the domain [RFC 2474]. See RFC 2475[BBCDWW98] for
further information.
6. If the ECN field in the inner header is set to ECT(0) or
ECT(1) and the ECN field in the outer header is set to
CE, then set the ECN field in the inner header to CE,
otherwise make no change to the ECN field in the inner
header. (The IPv4 checksum changes when the ECN
changes.)
Note: IPsec does not copy the options from the inner header into the
outer header, nor does IPsec construct the options in the outer
header. However, post-IPsec code MAY insert/construct options for the
outer header.
5.1.2.2 IPv6 -- Header Construction for Tunnel Mode
See previous section 5.1.2.1 for notes 1-6 indicated by (footnote
number).
<-- How Outer Hdr Relates Inner Hdr --->
Outer Hdr at Inner Hdr at
IPv6 Encapsulator Decapsulator
Header fields: -------------------- ------------
version 6 (1) no change
DS Field copied from inner hdr (5) no change
ECN Field copied from inner hdr constructed (6)
flow label copied or configured (8) no change
payload length constructed no change
next header AH,ESP,routing hdr no change
hop limit constructed (2) decrement (2)
src address constructed (3) no change
dest address constructed (3) no change
Extension headers never copied (7) no change
7. IPsec does not copy the extension headers from the inner
packet into outer headers, nor does IPsec construct
extension headers in the outer header. However, post-
IPsec code MAY insert/construct extension headers for
the outer header.
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8. See [RaCoCaDe04]. Copying is acceptable only for end
systems, not SGs. If an SG copied flow labels from the
inner header to the outer header, collisions might
result.
5.2 Processing Inbound IP Traffic (unprotected-to-protected)
Inbound processing is somewhat different from outbound processing,
because of the use of SPIs to map IPsec protected traffic to SAs. The
inbound SPD cache (SPD-I) is applied only to bypassed or discarded
traffic. If an arriving packet appears to be an IPsec fragment from
an unprotected interface, reassembly is performed prior to IPsec
processing. The intent for any SPD cache is that a packet that fails
to match any entry is then referred to the corresponding SPD. Every
SPD SHOULD have a nominal, final entry that catches anything that is
otherwise unmatched, and discards it. This ensures that non-IPsec
protected traffic that arrives and does not match any SPD-I entry
will be discarded.
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Unprotected Interface
|
V
+-----+ IPsec protected
------------------->|Demux|-------------------+
| +-----+ |
| | |
| Not IPsec | |
| | |
| V |
| +-------+ +---------+ |
| |DISCARD|<---|SPD-I (*)| |
| +-------+ +---------+ |
| | |
| |-----+ |
| | | |
| | V |
| | +------+ |
| | | ICMP | |
| | +------+ |
| | V
+---------+ | +--------+
....|SPD-O (*)|............|....................|PROCESS |...IPsec
+---------+ | |(AH/ESP)| Boundary
^ | +--------+
| | +---+ |
| BYPASS | +-->|IKE| |
| | | +---+ |
| V | V
| +----------+ +---------+ +----+
|--------<------|Forwarding|<---------|SAD Check|-->|ICMP|
nested SAs +----------+ +---------+ +----+
|
V
Protected Interface
Figure 3. Inbound Traffic Processing Model
(*) = The 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.
Prior to performing AH or ESP processing, any IP fragments that
arrive via the unprotected interface are reassembled (by IP). Each
inbound IP datagram to which IPsec processing will be applied is
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identified by the appearance of the AH or ESP values in the IP Next
Protocol field (or of AH or ESP as a next layer protocol in the IPv6
context).
IPsec MUST perform the following steps:
1. When a packet arrives, it may be tagged with the ID of the
interface (physical or virtual) via which it arrived, if necessary
to support multiple SPDs and associated SPD-I caches. (The
interface ID is mapped to a corresponding SPD-ID.)
2. The packet is examined and demuxed into one of two categories:
- If the packet appears to be IPsec protected and it is addressed
to this device, an attempt is made to map it to an active SA
via the SAD. Note that the device may have multiple IP
addresses that may be used in the SAD lookup, e.g., in the case
of protocols such as SCTP.
- Traffic not addressed to this device, or addressed to this
device and not AH or ESP, is directed to SPD-I lookup. (This
implies that IKE traffic MUST have an explicit BYPASS entry in
the SPD.) If multiple SPDs are employed, the tag assigned to
the packet in step 1 is used to select the appropriate SPD-I
(and cache) to search. SPD-I lookup determines whether the
action is DISCARD or BYPASS.
3a. If the packet is addressed to the IPsec device and AH or ESP is
specified as the protocol, the packet is looked up in the SAD. For
unicast traffic, use only the SPI (or SPI plus protocol). For
multicast traffic, use the SPI plus the destination or SPI plus
destination and source addresses, as specified in section 4.1. In
either case (unicast or multicast), if there is no match, discard
the traffic. This is an auditable event. The audit log entry for
this event SHOULD include the current date/time, SPI, source and
destination of the packet, IPsec protocol, and any other selector
values of the packet that are available. If the packet is found
in the SAD, process it accordingly (see step 4).
3b. If the packet is not addressed to the device or is addressed to
this device and is not AH or ESP, look up the packet header in the
(appropriate) SPD-I cache. If there is a match and the packet is
to be discarded or bypassed, do so. If there is no cache match,
look up the packet in the corresponding SPD-I and create a cache
entry as appropriate. (No SAs are created in response to receipt
of a packet that requires IPsec protection; only BYPASS or DISCARD
cache entries can be created this way.) If there is no match,
discard the traffic. This is an auditable event. The audit log
entry for this event SHOULD include the current date/time, SPI if
available, IPsec protocol if available, source and destination of
the packet, and any other selector values of the packet that are
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available.
3c. Processing of ICMP messages is assumed to take place on the
unprotected side of the IPsec boundary. Unprotected ICMP messages
are examined and local policy is applied to determine whether to
accept or reject these messages and, if accepted, what action to
take as a result. For example, if an ICMP unreachable message is
received, the implementation must decide whether to act on it,
reject it, or act on it with constraints. (See Section 6.)
4. Apply AH or ESP processing as specified, using the SAD entry
selected in step 3a above. Then match the packet against the
inbound selectors identified by the SAD entry to verify that the
received packet is appropriate for the SA via which it was
received.
If an IPsec system receives an inbound packet on an SA and the
packet's header fields are not consistent with the selectors for
the SA, it MUST discard the packet. This is an auditable event.
The audit log entry for this event SHOULD include the current
date/time, SPI, IPsec protocol(s), source and destination of the
packet, and any other selector values of the packet that are
available, and the selector values from the relevant SAD entry.
The system SHOULD also be capable of generating and sending an IKE
notification of INVALID_SELECTORS to the sender (IPsec peer),
indicating that the received packet was discarded because of
failure to pass selector checks.
To minimize the impact of a DoS attack, or a mis-configured peer,
the IPsec system SHOULD include a management control to allow an
administrator to configure the IPsec implementation to send or not
send this IKE notification, and if this facility is selected, to
rate limit the transmission of such notifications.
After traffic is bypassed or processed through IPsec, it is handed
to the inbound forwarding function for disposition. This function
may cause the packet to be sent (outbound) across the IPsec
boundary for additional inbound IPsec processing, e.g., in support
of nested SAs. If so, then as with ALL outbound traffic that is to
be bypassed, the packet MUST be matched against an SPD-O entry.
Ultimately, the packet should be forwarded to the destination host
or process for disposition.
6. ICMP Processing
This section describes IPsec handling of ICMP traffic. There are two
categories of ICMP traffic: error messages (e.g., type = destination
unreachable) and non-error messages (e.g., type = echo). This section
applies exclusively to error messages. Disposition of non-error,
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ICMP messages (that are not addressed to the IPsec implementation
itself) MUST be explicitly accounted for using SPD entries.
The discussion in this section applies to ICMPv6 as well as to
ICMPv4. Also, a mechanism SHOULD be provided to allow an
administrator to cause ICMP error messages (selected, all, or none)
to be logged as an aid to problem diagnosis.
6.1 Processing ICMP Error Messages Directed to an IPsec Implementation
6.1.1 ICMP Error Messages Received on the Unprotected Side of the
Boundary
Figure 3 in Section 5.2 shows a distinct ICMP processing module on
the unprotected side of the IPsec boundary, for processing ICMP
messages (error or otherwise) that are addressed to the IPsec device
and that are not protected via AH or ESP. An ICMP message of this
sort is unauthenticated and its processing may result in denial or
degradation of service. This suggests that, in general, it would be
desirable to ignore such messages. However, many ICMP messages will
be received by hosts or security gateways from unauthenticated
sources, e.g., routers in the public Internet. Ignoring these ICMP
messages can degrade service, e.g., because of a failure to process
PMTU message and redirection messages. Thus there is also a
motivation for accepting and acting upon unauthenticated ICMP
messages.
To accommodate both ends of this spectrum, a compliant IPsec
implementation MUST permit a local administrator to configure an
IPsec implementation to accept or reject unauthenticated ICMP
traffic. This control MUST be at the granularity of ICMP type and
MAY be at the granularity of ICMP type and code. Additionally, an
implementation SHOULD incorporate mechanisms and parameters for
dealing with such traffic. For example, there could be the ability to
establish a minimum PMTU for traffic (on a per destination basis), to
prevent receipt of an unauthenticated ICMP from setting the PMTU to a
trivial size.
If an ICMP PMTU message passes the checks above and the system is
configured to accept it, then there are two possibilities. If the
implementation applies fragmentation on the ciphertext side of the
boundary, then the accepted PMTU information is passed to the
forwarding module (outside of the IPsec implementation) which uses it
to manage outbound packet fragmentation. If the implementation is
configured to effect plaintext side fragmentation, then the PMTU
information is passed to the plaintext side and processed as
described in Section 8.2.
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6.1.2 ICMP Error Messages Received on the Protected Side of the Boundary
These ICMP messages are not authenticated, but they do come from
sources on the protected side of the IPsec boundary. Thus these
messages generally are viewed as more "trustworthy" than their
counterparts arriving from sources on the unprotected side of the
boundary. The major security concern here is that a compromised host
or router might emit erroneous ICMP error messages that could degrade
service for other devices "behind" the security gateway, or that
could even result in violations of confidentiality. For example, if a
bogus ICMP redirect were consumed by a security gateway, it could
cause the forwarding table on the protected side of the boundary to
be modified so as to deliver traffic to an inappropriate destination
"behind" the gateway. Thus implementers MUST provide controls to
allow local administrators to constrain the processing of ICMP error
messages received on the protected side of the boundary, and directed
to the IPsec implementation. These controls are of the same type as
those employed on the unprotected side, described above in Section
6.1.1.
6.2 Processing Protected, Transit ICMP Error Messages
When an ICMP error message is transmitted via an SA to a device
"behind" an IPsec implementation, both the payload and the header of
the ICMP message require checking from an access control perspective.
If one of these messages is forwarded to a host behind a security
gateway, the receiving host IP implementation will make decisions
based on the payload, i.e., the header of the packet that purportedly
triggered the error response. Thus an IPsec implementation MUST be
configurable to check that this payload header information is
consistent with the SA via which it arrives. (This means that the
payload header, with source and destination address and port fields
reversed, matches the traffic selectors for the SA.) If this sort of
check is not performed, then for example, anyone with whom the
receiving IPsec system (A) has an active SA could send an ICMP
destination dead message that refers to any host/net with which A is
currently communicating, and thus effect a highly efficient DoS
attack re: communication with other peers of A. Normal IPsec
receiver processing of traffic is not sufficient to protect against
such attacks. However, not all contexts may require such checks, so
it is also necessary to allow a local administrator to configure an
implementation to NOT perform such checks.
To accommodate both policies, the following convention is adopted. If
an administrator wants to allow ICMP error messages to be carried by
an SA without inspection of the payload, then configure an SPD entry
that explicitly allows for carriage of such traffic. If an
administrator wants IPsec to check the payload of ICMP error messages
for consistency, then do not create any SPD entries that accommodate
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carriage of such traffic based on the ICMP packet header. This
convention motivates the following processing description.
IPsec senders and receivers MUST support the following processing for
ICMP error messages that are sent and received via SAs.
If an SA exists that accommodates an outbound ICMP error message,
then the message is mapped to the SA and only the IP and ICMP headers
are checked upon receipt, just as would be the case for other
traffic. If no SA exists that matches the traffic selectors
associated with an ICMP error message, then the SPD is searched to
determine if such an SA can be created. If so, the SA is created and
the ICMP error message is transmitted via that SA. Upon receipt, this
message is subject to the usual traffic selector checks at the
receiver. This processing is exactly what would happen for traffic in
general, and thus does not represent any special processing for ICMP
error messages.
If no SA exists that would carry the outbound ICMP message in
question, and if no SPD entry would allow carriage of this outbound
ICMP error message, then an IPsec implementation MUST map the message
to the SA that would carry the return traffic associated with the
packet that triggered the ICMP error message. This requires an IPsec
implementation to detect outbound ICMP error messages that map to no
extant SA or SPD entry, and treat them specially with regard to SA
creation and lookup. The implementation extracts the header for the
packet that triggered the error (from the ICMP message payload),
reverses the source and destination IP address fields, extracts the
protocol field, and reverses the port fields (if accessible). It then
uses this extracted information to locate an appropriate, active
outbound SA, and transmits the error message via this SA. If no such
SA exists, no SA will be created, and this is an auditable event.
If an IPsec implementation receives an inbound ICMP error message on
an SA, and the IP and ICMP headers of the message do not match the
traffic selectors for the SA, the receiver MUST process the received
message in a special fashion. Specifically, the receiver must extract
the header of the triggering packet from the ICMP payload, and
reverse fields as described above to determine if the packet is
consistent with the selectors for the SA via which the ICMP error
message was received. If the packet fails this check, the IPsec
implementation MUST NOT forwarded the ICMP message to the
destination. This is an auditable event.
7. Handling Fragments (on the protected side of the IPsec boundary)
Earlier sections of this document describe mechanisms for (a)
fragmenting an outbound packet after IPsec processing has been
applied and reassembling it at the receiver before IPsec processing
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and (b) handling inbound fragments received from the unprotected side
of the IPsec boundary. This section describes how an implementation
should handle the processing of outbound plaintext fragments on the
protected side of the IPsec boundary. (See Appendix D for discussion
of Fragment Handling Rationale.) In particular, it addresses:
o mapping an outbound non-initial fragment to the right SA
(or finding the right SPD entry)
o verifying that a received non-initial fragment is
authorized for the SA via which it was received
o mapping outbound and inbound non-initial fragments to the
right SPD-O/SPD-I entry or the relevant cache entry, for
BYPASS/DISCARD traffic
Note: In Section 4.1, transport mode SAs have been defined to not
carry fragments (IPv4 or IPv6). Note also that in Section 4.4.1, two
special values, ANY and OPAQUE, were defined for selectors and that
ANY includes OPAQUE. The term "non-trivial" is used to mean that the
selector has a value other than OPAQUE or ANY.
Note: The term "non-initial fragment" is used here to indicate a
fragment that does not contain all the selector values that may be
needed for access control. As observed in Section 4.4.1, depending
on the Next Layer Protocol, in addition to Ports, the ICMP message
type/code or Mobility Header type could be missing from non-initial
fragments. Also, for IPv6, even the first fragment might NOT contain
the Next Layer Protocol or Ports (or ICMP message type/code, or
Mobility Header type) depending on the kind and number of extension
headers present. If a non-initial fragment contains the Port (or
ICMP type and code or Mobility header type) but not the Next Layer
Protocol, then unless there is an SPD entry for the relevant
Local/Remote addresses with ANY for Next Layer Protocol and Port (or
ICMP type and code or Mobility header type), the fragment would not
contain all the selector information needed for access control.
To address the above issues, three approaches have been defined:
o Tunnel mode SAs that carry initial and non-initial fragments
(See Section 7.1)
o Separate tunnel mode SAs for non-initial fragments (See
Section 7.2)
o Stateful fragment checking (See Section 7.3)
7.1 Tunnel Mode SAs that Carry Initial and Non-Initial Fragments
All implementations MUST support tunnel mode SAs that are configured
to pass traffic without regard to port field (or ICMP type/code or
Mobility Header type) values. If the SA will carry traffic for
specified protocols, the selector set for the SA MUST specify the
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port fields (or ICMP type/code or Mobility Header type) as ANY. An SA
defined in this fashion will carry all traffic including initial and
non-initial fragments for the indicated Local/Remote addresses and
specified Next Layer protocol(s). If the SA will carry traffic
without regard to a specific protocol value (i.e., ANY is specified
as the (Next Layer) protocol selector value), then the port field
values are undefined and MUST be set to ANY as well. (As noted in
4.4.1, ANY includes OPAQUE as well as all specific values.)
7.2 Separate Tunnel Mode SAs for Non-Initial Fragments
An implementation MAY support tunnel mode SAs that will carry only
non-initial fragments, separate from non-fragmented packets and
initial fragments. The OPAQUE value will be used to specify port (or
ICMP type/code or Mobility Header type) field selectors for an SA to
carry such fragments. Receivers MUST perform a minimum offset check
on IPv4 (non-initial) fragments to protect against overlapping
fragment attacks when SAs of this type are employed. Because such
checks cannot be performed on IPv6 non-initial fragments, users and
administrators are advised that carriage of such fragments may be
dangerous, and implementers may choose to NOT support such SAs for
IPv6 traffic. Also, an SA of this sort will carry all non-initial
fragments that match a specified Local/Remote address pair and
protocol value, i.e., the fragments carried on this SA belong to
packets that if not fragmented, might have gone on separate SAs of
differing security. Therefore users and administrators are advised
to protect such traffic using ESP (with integrity) and the
"strongest" integrity and encryption algorithms in use between both
peers. (Determination of the "strongest" algorithms requires
imposing an ordering of the available algorithms, a local
determination at the discretion of the initiator of the SA.)
Specific port (or ICMP type/code or Mobility header type) selector
values will be used to define SAs to carry initial fragments and non-
fragmented packets. This approach can be used if a user or
administrator wants to create one or more tunnel mode SAs between the
same Local/Remote addresses that discriminate based on port (or ICMP
type/code or Mobility header type) fields. These SAs MUST have non-
trivial protocol selector values, otherwise approach #1 above MUST be
used.
Note: In general, for the approach described in this section, one
needs only a single SA between two implementations to carry all non-
initial fragments. However, if one chooses to have multiple SAs
between the two implementations for QoS differentiation, then one
might also want multiple SAs to carry fragments-without-ports, one
for each supported QoS class. Since support for QoS via distinct SAs
is a local matter, not mandated by this document, the choice to have
multiple SAs to carry non-initial fragments should also be local.
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7.3 Stateful Fragment Checking
An implementation MAY support some form of stateful fragment checking
for a tunnel mode SA with non-trivial port (or ICMP type/code or MH
type) field values (not ANY or OPAQUE). Implementations that will
transmit non-initial fragments on a tunnel mode SA that makes use of
non-trivial port (or ICMP type/code or MH type) selectors MUST notify
a peer via the IKE NOTIFY NON_FIRST_FRAGMENTS_ALSO payload.
The peer MUST reject this proposal if it will not accept non-initial
fragments in this context. If an implementation does not successfully
negotiate transmission of non-initial fragments for such an SA, it
MUST NOT send such fragments over the SA. This standard does not
specify how peers will deal with such fragments, e.g., via reassembly
or other means, at either sender or receiver. However, a receiver
MUST discard non-initial fragments that arrive on an SA with non-
trivial port (or ICMP type/code or MH type) selector values unless
this feature has been negotiated. Also, the receiver MUST discard
non-initial fragments that do not comply with the security policy
applied to the overall packet. Discarding such packets is an
auditable event. Note that in network configurations where fragments
of a packet might be sent or received via different security gateways
or BITW implementations, stateful strategies for tracking fragments
may fail.
7.4 BYPASS/DISCARD traffic
All implementations MUST support DISCARDing of fragments using the
normal SPD packet classification mechanisms. All implementations MUST
support stateful fragment checking to accommodate BYPASS traffic for
which a non-trivial port range is specified. The concern is that
BYPASS of a cleartext, non-initial fragment arriving at an IPsec
implementation could undermine the security afforded IPsec-protected
traffic directed to the same destination. For example, consider an
IPsec implementation configured with an SPD entry that calls for
IPsec-protection of traffic between a specific source/destination
address pair, and for a specific protocol and destination port, e.g.,
TCP traffic on port 23 (Telnet). Assume that the implementation also
allows BYPASS of traffic from the same source/destination address
pair and protocol, but for a different destination port, e.g., port
119 (NNTP). An attacker could send a non-initial fragment (with a
forged source address) that, if bypassed, could overlap with IPsec-
protected traffic from the same source and thus violate the integrity
of the IPsec-protected traffic. Requiring stateful fragment checking
for BYPASS entries with non-trivial port ranges prevents attacks of
this sort.
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8. Path MTU/DF Processing
The application of AH or ESP to an outbound packet increases the size
of a packet and thus may cause a packet to exceed the PMTU for the SA
via which the packet will travel. An IPsec implementation also may
receive an unprotected ICMP PMTU message and, if it choose to act
upon it, the result will affect outbound traffic processing. This
section describes the processing required of an IPsec implementation
to deal with these two PMTU issues.
8.1 DF Bit
All IPsec implementations MUST support the option of copying the DF
bit from an outbound packet to the tunnel mode header that it emits,
when traffic is carried via a tunnel mode SA. This means that it MUST
be possible to configure the implementation's treatment of the DF bit
(set, clear, copy from inner header) for each SA.
8.2 Path MTU Discovery (PMTU)
This section discusses IPsec handling for unprotected Path MTU
Discovery messages. ICMP PMTU is used here to refer to an ICMP
message for:
IPv4 (RFC 792 [Pos81b]):
- Type = 3 (Destination Unreachable)
- Code = 4 (Fragmentation needed and DF set)
- Next-Hop MTU in the low-order 16 bits of the
second word of the ICMP header (labeled "unused"
in RFC 792), with high-order 16 bits set to zero)
IPv6 (RFC 2463 [CD98]):
- Type = 2 (Packet Too Big)
- Code = 0 (Fragmentation needed)
- Next-Hop MTU in the 32 bit MTU field of the ICMP6
message
8.2.1 Propagation of PMTU
When an IPsec implementation receives an unauthenticated PMTU
message, and it is configured to process (vs. ignore) such messages,
it maps the message to the SA to which it corresponds. This mapping
is effected by extracting the header information from the payload of
the PMTU message and applying the procedure described in Section 5.2.
The PMTU determined by this message is used to update the SAD PMTU
field, taking into account the size of the AH or ESP header that will
be applied, any crypto synchronization data, and the overhead imposed
by an additional IP header, in the case of a tunnel mode SA.
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In a native host implementation it is possible to maintain PMTU data
at the same granularity as for unprotected communication, so there is
no loss of functionality. Signaling of the PMTU information is
internal to the host. For all other IPsec implementation options, the
PMTU data must be propagated via a synthesized ICMP PMTU. In these
cases, the IPsec implementation SHOULD wait for outbound traffic to
be mapped to the SAD entry When such traffic arrives, if the traffic
would exceed the updated PMTU value the traffic MUST be handled as
follows:
Case 1: Original (cleartext) packet is IPv4 and has the DF
bit set. The implementation SHOULD discard the packet
and send a PMTU ICMP message.
Case 2: Original (cleartext) packet is IPv4 and has the DF
bit clear. The implementation SHOULD fragment (before or
after encryption per its configuration) and then forward
the fragments. It SHOULD NOT send a PMTU ICMP message.
Case 3: Original (cleartext) packet is IPv6. The implementation
SHOULD discard the packet and send a PMTU ICMP message.
8.2.2 PMTU Aging
In all IPsec implementations the PMTU associated with an SA MUST be
"aged" and some mechanism is required to update the PMTU in a timely
manner, especially for discovering if the PMTU is smaller than
required by current network conditions. A given PMTU has to remain
in place long enough for a packet to get from the source of the SA to
the peer, and to propagate an ICMP error message if the current PMTU
is too big.
Implementations SHOULD use the approach described in the Path MTU
Discovery document (RFC 1191 [MD90], Section 6.3), which suggests
periodically resetting the PMTU to the first-hop data-link MTU and
then letting the normal PMTU Discovery processes update the PMTU as
necessary. The period SHOULD be configurable.
9. Auditing
IPsec implementations are not required to support auditing. For the
most part, the granularity of auditing is a local matter. However,
several auditable events are identified in this document and for each
of these events a minimum set of information that SHOULD be included
in an audit log is defined. Additional information also MAY be
included in the audit log for each of these events, and additional
events, not explicitly called out in this specification, also MAY
result in audit log entries. There is no requirement for the
receiver to transmit any message to the purported transmitter in
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response to the detection of an auditable event, because of the
potential to induce denial of service via such action.
10. Conformance Requirements
All IPv4 IPsec implementations MUST comply with all requirements of
this document. All IPv6 implementations MUST comply with all
requirements of this document.
11. Security Considerations
The focus of this document is security; hence security considerations
permeate this specification.
If an IPsec implementation is configured to pass ICMP error messages
over SAs based on the ICMP header values, without checking the header
information from the ICMP message payload, serious vulnerabilities
may arise. Consider a scenario in which several sites (A, B, and C)
are connected to one another via ESP-protected tunnels: A-B, A-C, and
B-C. Also assume that the traffic selectors for each tunnel specify
ANY for protocol and port fields and IP source/destination address
ranges that encompass the address range for the systems behind the
security gateways serving each site. This would allow a host at site
B to send an ICMP destination dead message to any host at site A,
that declares all hosts on the net at site C to be unreachable. This
is a very efficient DoS attack that could have been prevented if the
ICMP error messages were subjected to the checks that IPsec provides,
if the SPD is suitably configured, as described in Section 6.2.
12. IANA Considerations
This document has no actions for IANA.
13. Differences from RFC 2401
This architecture document differs substantially from RFC 2401 in
detail and in organization, but the fundamental notions are
unchanged.
o The processing model has been revised to address new IPsec
scenarios, improve performance and simplify implementation. This
includes a separation between forwarding (routing) and SPD
selection, several SPD changes, and the addition of an outbound
SPD cache and an inbound SPD cache for bypassed or discarded
traffic. There is also a new database, the Peer Authorization
Database (PAD). This provides a link between an SA management
protocol like IKE and the SPD.
o There is no longer a requirement to support nested SAs or "SA
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bundles." Instead this functionality can be achieved through SPD
and forwarding table configuration. An example of a
configuration has been added in Appendix E.
o SPD entries were redefined to provide more flexibility. Each SPD
entry now consists of 1 to N sets of selectors, where each
selector set contains one protocol and a "list of ranges" can now
be specified for the Local IP address, Remote IP address, and
whatever fields (if any) are associated with the Next Layer
Protocol (Local Port, Remote Port, ICMP message type and code, and
Mobility Header Type). An individual value for a selector is
represented via a trivial range and ANY is represented via a range
than spans all values for the selector. An example of an ASN.1
description is included in Appendix C.
o TOS (IPv4) and Traffic Class (IPv6) have been replaced by DSCP and
ECN. The tunnel section has been updated to explain how to handle
DSCP and ECN bits.
o For tunnel mode SAs, an SG, BITS, or BITW implementation is now
allowed to fragment packets before applying IPsec. This applies
only to IPv4. For IPv6 packets, only the originator is allowed to
fragment them.
o When security is desired between two intermediate systems along a
path or between an intermediate system and an end system,
transport mode may now be used between security gateways and
between a security gateway and a host.
o This document clarifies that for all traffic that crosses the IPsec
boundary, including IPsec management traffic, the SPD or
associated caches must be consulted.
o This document defines how to handle the situation of a security
gateway with multiple subscribers requiring separate IPsec
contexts.
o A definition of reserved SPIs has been added.
o Text has been added explaining why ALL IP packets must be checked
-- IPsec includes minimal firewall functionality to support access
control at the IP layer.
o The tunnel section has been updated to clarify how to handle the IP
options field and IPv6 extension headers when constructing the
outer header.
o SA mapping for inbound traffic has been updated to be consistent
with the changes made in AH and ESP for support of unicast,
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anycast, and multicast SAs.
o Guidance has been added re: how to handle the covert channel
created in tunnel mode by copying the DSCP value to outer header.
o Support for AH in both IPv4 and IPv6 is no longer required.
o PMTU handling has been updated. The appendix on
PMTU/DF/Fragmentation has been deleted.
o Added text saying "The IP Security Policy (IPSP) Working Group is a
possible future source of guidance. One of their goals is to
produce a Internet Draft on a "Security Gateway Discovery, Policy
Exchange and Negotiation Protocol."
o Three approaches have been added for handling plaintext fragments
on the protected side of the IPsec boundary. Appendix D documents
the rationale behind them.
o Added revised text describing how to derive selector values for SAs
(from the SPD entry or from the packet, etc.)
o Added a new table describing the relationship between selector
values in an SPD entry, the PFP flag, and resulting selector
values in the corresponding SAD entry.
o Added Appendix B to describe decorrelation.
o Added text describing how to handle an outbound packet which must
be discarded.
o Added text describing how to handle a DISCARDED inbound packet,
i.e., one that does not match the SA upon which it arrived.
o IPv6 mobility header has been added as a possible Next Layer
Protocol. IPv6 mobility header message type has been added as a
selector.
o ICMP message type and code have been added as selectors.
o The selector "data sensitivity level" has been removed to simplify
things.
o Updated text describing handling ICMP error messages. The appendix
on "Categorization of ICMP messages" has been deleted.
o The text for the selector name has been updated and clarified.
o The "Next Layer Protocol" has been further explained and a default
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list of protocols to skip when looking for the Next Layer Protocol
has been added.
o The text has been amended to say that this document assumes use of
IKEv2 or an SA management protocol with comparable features.
o Text has been added clarifying the algorithm for mapping inbound
IPsec datagrams to SAs in the presence of multicast SAs.
o The appendix "Sequence Space Window Code Example" has been removed.
o With respect to IP addresses and ports, the terms "Local" and
"Remote" are used for policy rules (replacing source and
destination). "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 still used for packet header
fields.
Acknowledgements
The authors would like to acknowledge the contributions of Ran
Atkinson, who played a critical role in initial IPsec activities, and
who authored the first series of IPsec standards: RFCs 1825-1827; and
Charlie Lynn, who made significant contributions to the second series
of IPsec standards (RFCs 2401,2402,and 2406) and to the current
versions, especially with regard to IPv6 issues. The authors also
would like to thank the members of the IPsec and MSEC working groups
who have contributed to the development of this protocol
specification.
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Appendix A -- Glossary
This section provides definitions for several key terms that are
employed in this document. Other documents provide additional
definitions and background information relevant to this technology,
e.g., [Shi00, VK83, HA94]. Included in this glossary are generic
security service and security mechanism terms, plus IPsec-specific
terms.
Access Control
Access control is a security service that prevents unauthorized
use of a resource, including the prevention of use of a resource
in an unauthorized manner. In the IPsec context, the resource to
which access is being controlled is often:
o for a host, computing cycles or data
o for a security gateway, a network behind the gateway
or bandwidth on that network.
Anti-replay
[See "Integrity" below]
Authentication
This term is used informally to refer to the combination of two
nominally distinct security services, data origin authentication
and connectionless integrity. See the definitions below for each
of these services.
Availability
Availability, when viewed as a security service, addresses the
security concerns engendered by attacks against networks that deny
or degrade service. For example, in the IPsec context, the use of
anti-replay mechanisms in AH and ESP support availability.
Confidentiality
Confidentiality is the security service that protects data from
unauthorized disclosure. The primary confidentiality concern in
most instances is unauthorized disclosure of application level
data, but disclosure of the external characteristics of
communication also can be a concern in some circumstances.
Traffic flow confidentiality is the service that addresses this
latter concern by concealing source and destination addresses,
message length, or frequency of communication. In the IPsec
context, using ESP in tunnel mode, especially at a security
gateway, can provide some level of traffic flow confidentiality.
(See also traffic analysis, below.)
Data Origin Authentication
Data origin authentication is a security service that verifies the
identity of the claimed source of data. This service is usually
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bundled with connectionless integrity service.
Encryption
Encryption is a security mechanism used to transform data from an
intelligible form (plaintext) into an unintelligible form
(ciphertext), to provide confidentiality. The inverse
transformation process is designated "decryption". Oftimes the
term "encryption" is used to generically refer to both processes.
Integrity
Integrity is a security service that ensures that modifications to
data are detectable. Integrity comes in various flavors to match
application requirements. IPsec supports two forms of integrity:
connectionless and a form of partial sequence integrity.
Connectionless integrity is a service that detects modification of
an individual IP datagram, without regard to the ordering of the
datagram in a stream of traffic. The form of partial sequence
integrity offered in IPsec is referred to as anti-replay
integrity, and it detects arrival of duplicate IP datagrams
(within a constrained window). This is in contrast to connection-
oriented integrity, which imposes more stringent sequencing
requirements on traffic, e.g., to be able to detect lost or re-
ordered messages. Although authentication and integrity services
often are cited separately, in practice they are intimately
connected and almost always offered in tandem.
Protected vs Unprotected
"Protected" refers to the systems or interfaces that are inside
the IPsec protection boundary and "unprotected" refers to the
systems or interfaces that are outside the IPsec protection
boundary. IPsec provides a boundary through which traffic passes.
There is an asymmetry to this barrier, which is reflected in the
processing model. Outbound data, if not discarded or bypassed, is
protected via the application of AH or ESP and the addition of the
corresponding headers. Inbound data, if not discarded or
bypassed, is processed via the removal of AH or ESP headers. In
this document, inbound traffic enters an IPsec implementation from
the "unprotected" interface. Outbound traffic enters the
implementation via the "protected" interface, or is internally
generated by the implementation on the "protected" side of the
boundary and directed toward the "unprotected" interface. An IPsec
implementation may support more than one interface on either or
both sides of the boundary. The protected interface may be
internal, e.g., in a host implementation of IPsec. The protected
interface may link to a socket layer interface presented by the
OS.
Security Association (SA)
A simplex (uni-directional) logical connection, created for
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security purposes. All traffic traversing an SA is provided the
same security processing. In IPsec, an SA is an internet layer
abstraction implemented through the use of AH or ESP. State data
associated with an SA is represented in the SA Database (SAD).
Security Gateway
A security gateway is an intermediate system that acts as the
communications interface between two networks. The set of hosts
(and networks) on the external side of the security gateway is
termed unprotected (they are generally at least less protected
than those "behind" the SG), while the networks and hosts on the
internal side are viewed as protected. The internal subnets and
hosts served by a security gateway are presumed to be trusted by
virtue of sharing a common, local, security administration. (See
"Trusted Subnetwork" below.) In the IPsec context, a security
gateway is a point at which AH and/or ESP is implemented in order
to serve a set of internal hosts, providing security services for
these hosts when they communicate with external hosts also
employing IPsec (either directly or via another security gateway).
SPI
Acronym for "Security Parameters Index" (SPI). The SPI is an
arbitrary 32-bit value that is used by a receiver to identify the
SA to which an incoming packet should be bound. For a unicast SA,
the SPI can be used by itself to specify an SA, or it may be used
in conjunction with the IPsec protocol type. Additional IP
address information is used to identify multicast SAs. The SPI is
carried in AH and ESP protocols to enable the receiving system to
select the SA under which a received packet will be processed. An
SPI has only local significance, as defined by the creator of the
SA (usually the receiver of the packet carrying the SPI); thus an
SPI is generally viewed as an opaque bit string. However, the
creator of an SA may choose to interpret the bits in an SPI to
facilitate local processing.
Traffic Analysis
The analysis of network traffic flow for the purpose of deducing
information that is useful to an adversary. Examples of such
information are frequency of transmission, the identities of the
conversing parties, sizes of packets, flow identifiers, etc.
[Sch94]
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Appendix B - Decorrelation
This appendix is based on work done for caching of policies in the IP
Security Policy Working Group by Luis Sanchez, Matt Condell, and John
Zao.
Two SPD entries are correlated if there is a non-null intersection
between the values of corresponding selectors in each entry. Caching
correlated SPD entries can lead to incorrect policy enforcement. A
solution to this problem, that still allows for caching, is to remove
the ambiguities by decorrelating the entries. That is, the SPD
entries must be rewritten so that for every pair of entries there
exists a selector for which there is a null intersection between the
values in both of the entries. Once the entries are decorrelated,
there is no longer any ordering requirement on them, since only one
entry will match any lookup. The next section describes
decorrelation in more detail and presents an algorithm that may be
used to implement decorrelation.
B.1 Decorrelation Algorithm
The basic decorrelation algorithm takes each entry in a correlated
SPD and divides it up into a set of entries using a tree structure.
The nodes of the tree are the selectors that may overlap between the
policies. At each node, the algorithm creates a branch for each of
the values of the selector. It also creates one branch for the
complement of the union of all selector values. Policies are then
formed by traversing the tree from the root to each leaf. The
policies at the leaves are compared to the set of already
decorrelated policy rules. Each policy at a leaf is either completely
overridden by a policy in the already decorrelated set and is
discarded or is decorrelated with all the policies in the
decorrelated set and is added to it.
The basic algorithm does not guarantee an optimal set of decorrelated
entries. That is, the entries may be broken up into smaller sets
than is necessary, though they will still provide all the necessary
policy information. Some extensions to the basic algorithm are
described later to improve this and improve the performance of the
algorithm.
C A set of ordered, correlated entries (a correlated SPD)
Ci The ith entry in C.
U The set of decorrelated entries being built from C
Ui The ith entry in U.
Sik The kth selection for policy Ci
Ai The action for policy Ci
A policy (SPD entry) P may be expressed as a sequence of selector
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values and an action (BYPASS, DISCARD, or PROTECT):
Ci = Si1 x Si2 x ... x Sik -> Ai
1) Put C1 in set U as U1
For each policy Cj (j > 1) in C
2) If Cj is decorrelated with every entry in U, then add it to U.
3) If Cj is correlated with one or more entries in U, create a tree
rooted at the policy Cj that partitions Cj into a set of decorrelated
entries. The algorithm starts with a root node where no selectors
have yet been chosen.
A) Choose a selector in Cj, Sjn, that has not yet been chosen when
traversing the tree from the root to this node. If there are no
selectors not yet used, continue to the next unfinished branch
until all branches have been completed. When the tree is
completed, go to step D.
T is the set of entries in U that are correlated with the entry
at this node.
The entry at this node is the entry formed by the selector
values of each of the branches between the root and this node.
Any selector values that are not yet represented by branches
assume the corresponding selector value in Cj, since the values
in Cj represent the maximum value for each selector.
B) Add a branch to the tree for each value of the selector Sjn that
appears in any of the entries in T. (If the value is a superset
of the value of Sjn in Cj, then use the value in Cj, since that
value represents the universal set.) Also add a branch for the
complement of the union of all the values of the selector Sjn
in T. When taking the complement, remember that the universal
set is the value of Sjn in Cj. A branch need not be created
for the null set.
C) Repeat A and B until the tree is completed.
D) The entry to each leaf now represents an entry that is a subset
of Cj. The entries at the leaves completely partition Cj in
such a way that each entry is either completely overridden by
an entry in U, or is decorrelated with the entries in U.
Add all the decorrelated entries at the leaves of the tree to U.
4) Get next Cj and go to 2.
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5) When all entries in C have been processed, then U will contain an
decorrelated version of C.
There are several optimizations that can be made to this algorithm.
A few of them are presented here.
It is possible to optimize, or at least improve, the amount of
branching that occurs by carefully choosing the order of the
selectors used for the next branch. For example, if a selector Sjn
can be chosen so that all the values for that selector in T are equal
to or a superset of the value of Sjn in Cj, then only a single branch
needs to be created (since the complement will be null).
Branches of the tree do not have to proceed with the entire
decorrelation algorithm. For example, if a node represents an entry
that is decorrelated with all the entries in U, then there is no
reason to continue decorrelating that branch. Also, if a branch is
completely overridden by an entry in U, then there is no reason to
continue decorrelating the branch.
An additional optimization is to check to see if a branch is
overridden by one of the CORRELATED entries in set C that has already
been decorrelated. That is, if the branch is part of decorrelating
Cj, then check to see if it was overridden by an entry Cm, m < j.
This is a valid check, since all the entries Cm are already expressed
in U.
Along with checking if an entry is already decorrelated in step 2,
check if Cj is overridden by any entry in U. If it is, skip it since
it is not relevant. An entry x is overridden by another entry y if
every selector in x is equal to or a subset of the corresponding
selector in entry y.
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Appendix C -- ASN.1 for an SPD Entry
This appendix is included as an additional way to describe SPD
entries, as defined in Section 4.4.1. It uses ASN.1 syntax. Since it
describes encodings to be used with IKEv2, to express SPD entries,
using ASN.1 constraints, it will not compile as shown due to
"duplicate" tags. However it has been successfully complied when
augmented with appropriate compiler directives. This syntax is merely
illustrative and need not be employed in an implementation to achieve
compliance. The SPD description in Section 4.4.1 is normative.
SPDModule
-- need a module identifier to be assigned from the IPsec WG arc
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
Name
FROM PKIX1Explicit88
{ iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7)
id-mod(0) id-pkix1-explicit(18) } ;
-- An SPD is a list of policies in decreasing order of preference
SPD ::= SEQUENCE OF SPDEntry
SPDEntry ::= CHOICE {
iPsecEntry IPsecEntry, -- PROTECT traffic
bypassOrDiscard [0] BypassOrDiscardEntry } -- DISCARD/BYPASS
IPsecEntry ::= SEQUENCE { -- Each entry consists of:
name NameSets OPTIONAL,
pFPs PacketFlags, -- Populate from packet flags
-- Applies to ALL of the corresponding
-- traffic selectors in the SelectorLists
condition SelectorLists, -- Policy "condition"
processing Processing -- Policy "action"
}
BypassOrDiscardEntry ::= SEQUENCE {
bypass BOOLEAN, -- TRUE: BYPASS, FALSE: DISCARD
condition InOutBound }
InOutBound ::= CHOICE {
outbound [0] SelectorLists,
inbound [1] SelectorLists,
bothways [2] BothWays }
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BothWays ::= SEQUENCE {
inbound SelectorLists,
outbound SelectorLists }
NameSets ::= SEQUENCE {
passed SET OF Names, -- Matched to IKE ID
local SET OF Names } -- Used internally
Names ::= CHOICE { -- IKEv2 IDs:
dName DistinguishedName, -- ID_DER_ASN1_DN
fqdn FQDN, -- ID_FQDN
rfc822 [0] RFC822Name, -- ID_RFC822_ADDR
keyID OCTET STRING } -- KEY_ID
DistinguishedName ::= Name
FQDN ::= IA5String
RFC822Name ::= IA5String
PacketFlags ::= BIT STRING {
-- if set, take selector value from packet establishing SA
-- else use value in SPD entry
localAddr (0),
remoteAddr (1),
protocol (2),
localPort (3),
remotePort (4) }
SelectorLists ::= SET OF SelectorList
SelectorList ::= SEQUENCE {
localAddr AddrList,
remoteAddr AddrList,
protocol ProtocolChoice }
Processing ::= SEQUENCE {
extSeqNum BOOLEAN, -- TRUE: 64 bit counter, FALSE: 32 bit
seqOverflow BOOLEAN, -- TRUE: rekey, FALSE: terminate & audit
fragCheck BOOLEAN, -- TRUE: stateful fragment checking,
-- FALSE: no stateful fragment checking
lifetime SALifetime,
spi ManualSPI,
algorithms ProcessingAlgs,
tunnel TunnelOptions OPTIONAL } -- if absent, use transport mode
SALifetime ::= SEQUENCE {
seconds [0] INTEGER OPTIONAL,
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bytes [1] INTEGER OPTIONAL }
ManualSPI ::= SEQUENCE {
spi INTEGER,
keys KeyIDs }
KeyIDs ::= SEQUENCE OF OCTET STRING
ProcessingAlgs ::= CHOICE {
ah [0] IntegrityAlgs, -- AH
esp [1] ESPAlgs} -- ESP
ESPAlgs ::= CHOICE {
integrity [0] IntegrityAlgs, -- ESP integrity only
confidentiality [1] ConfidentialityAlgs, -- ESP confidentiality only
both [2] IntegrityConfidentialityAlgs,
combined [3] CombinedModeAlgs }
IntegrityConfidentialityAlgs ::= SEQUENCE { -- must have both
integrity IntegrityAlgs,
confidentiality ConfidentialityAlgs }
-- Integrity Algorithms, ordered by decreasing preference
IntegrityAlgs ::= SEQUENCE OF IntegrityAlg
-- Confidentiality Algorithms, ordered by decreasing preference
ConfidentialityAlgs ::= SEQUENCE OF ConfidentialityAlg
-- Integrity Algorithms
IntegrityAlg ::= SEQUENCE {
algorithm IntegrityAlgType,
parameters ANY DEFINED BY algorithm OPTIONAL }
IntegrityAlgType ::= INTEGER {
none (0),
auth-HMAC-MD5-96 (1),
auth-HMAC-SHA1-96 (2),
auth-DES-MAC (3),
auth-KPDK-MD5 (4),
auth-AES-XCBC-96 (5)
-- tbd (6..65535)
}
-- Confidentiality Algorithms
ConfidentialityAlg ::= SEQUENCE {
algorithm ConfidentialityAlgType,
parameters ANY DEFINED BY algorithm OPTIONAL }
ConfidentialityAlgType ::= INTEGER {
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encr-DES-IV64 (1),
encr-DES (2),
encr-3DES (3),
encr-RC5 (4),
encr-IDEA (5),
encr-CAST (6),
encr-BLOWFISH (7),
encr-3IDEA (8),
encr-DES-IV32 (9),
encr-RC4 (10),
encr-NULL (11),
encr-AES-CBC (12),
encr-AES-CTR (13)
-- tbd (14..65535)
}
CombinedModeAlgs ::= SEQUENCE OF CombinedModeAlg
CombinedModeAlg ::= SEQUENCE {
algorithm CombinedModeType,
parameters ANY DEFINED BY algorithm } -- as defined outside of
-- this document for AES modes.
CombinedModeType ::= INTEGER {
comb-AES-CCM (1),
comb-AES-GCM (2)
-- tbd (3..65535)
}
TunnelOptions ::= SEQUENCE {
dscp DSCP,
ecn BOOLEAN, -- TRUE: Copy CE to inner header
df DF
addresses TunnelAddresses }
TunnelAddresses ::= CHOICE {
ipv4 IPv4Pair,
ipv6 [0] IPv6Pair }
IPv4Pair ::= SEQUENCE {
local OCTET STRING (SIZE(4)),
remote OCTET STRING (SIZE(4)) }
IPv6Pair ::= SEQUENCE {
local OCTET STRING (SIZE(16)),
remote OCTET STRING (SIZE(16)) }
DSCP ::= SEQUENCE {
copy BOOLEAN, -- TRUE: copy from inner header, FALSE: do not
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mapping OCTET STRING OPTIONAL} -- pointer to table if no copy
DF ::= INTEGER {
clear (0),
set (1),
copy (2) }
ProtocolChoice::= CHOICE {
anyProt AnyProtocol, -- for ANY protocol
noNext [0] NoNextLayerProtocol, -- has no next layer items
oneNext [1] OneNextLayerProtocol, -- has one next layer item
twoNext [2] TwoNextLayerProtocol, -- has two next layer items
fragment FragmentNoNext } -- has no next layer info
AnyProtocol ::= SEQUENCE {
id INTEGER (0), -- ANY protocol
nextLayer AnyNextLayers }
AnyNextLayers ::= SEQUENCE { -- with either
first AnyNextLayer, -- ANY next layer selector
second AnyNextLayer } -- ANY next layer selector
NoNextLayerProtocol ::= INTEGER (2..254)
FragmentNoNext ::= INTEGER (44) -- Fragment identifier
OneNextLayerProtocol ::= SEQUENCE {
id INTEGER (1..254), -- ICMP, MH, ICMPv6
nextLayer NextLayerChoice } -- ICMP Type*256+Code
-- MH Type*256
TwoNextLayerProtocol ::= SEQUENCE {
id INTEGER (2..254), -- Protocol
local NextLayerChoice, -- Local and
remote NextLayerChoice } -- Remote ports
NextLayerChoice ::= CHOICE {
any AnyNextLayer,
opaque [0] OpaqueNextLayer,
range [1] NextLayerRange }
-- Representation of ANY in next layer field
AnyNextLayer ::= SEQUENCE {
start INTEGER (0),
end INTEGER (65535) }
-- Representation of OPAQUE in next layer field.
-- Matches IKE convention
OpaqueNextLayer ::= SEQUENCE {
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start INTEGER (65535),
end INTEGER (0) }
-- Range for a next layer field
NextLayerRange ::= SEQUENCE {
start INTEGER (0..65535),
end INTEGER (0..65535) }
-- List of IP addresses
AddrList ::= SEQUENCE {
v4List IPv4List OPTIONAL,
v6List [0] IPv6List OPTIONAL }
-- IPv4 address representations
IPv4List ::= SEQUENCE OF IPv4Range
IPv4Range ::= SEQUENCE { -- close, but not quite right ...
ipv4Start OCTET STRING (SIZE (4)),
ipv4End OCTET STRING (SIZE (4)) }
-- IPv6 address representations
IPv6List ::= SEQUENCE OF IPv6Range
IPv6Range ::= SEQUENCE { -- close, but not quite right ...
ipv6Start OCTET STRING (SIZE (16)),
ipv6End OCTET STRING (SIZE (16)) }
END
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Appendix D -- Fragment Handling Rationale
There are three issues that must be resolved re processing of
(plaintext) fragments in IPsec:
- mapping a non-initial, outbound fragment to the right SA
(or finding the right SPD entry)
- verifying that a received, non-initial fragment is authorized
for the SA via which it is received
- mapping outbound and inbound non-initial fragments to the
right SPD/cache entry, for BYPASS/DISCARD traffic.
The first and third issues arise because we need a deterministic
algorithm for mapping traffic to SAs (and SPD/cache entries). All
three issues are important because we want to make sure that non-
initial fragments that cross the IPsec boundary do not cause the
access control policies in place at the receiver (or transmitter) to
be violated.
D.1 Transport Mode and Fragments
First, we note that transport mode SAs have been defined to not
carry fragments. This is a carryover from RFC 2401, where transport
mode SAs always terminated at end points. This is a fundamental
requirement because, in the worst case, an IPv4 fragment to which
IPsec was applied, might then be fragmented (as a ciphertext packet),
en route to the destination. IP fragment reassembly procedures at the
IPsec receiver would not be able to distinguish between pre-IPsec
fragments and fragments created after IPsec processing.
For IPv6, only the sender is allowed to fragment a packet. As for
IPv4, an IPsec implementation is allowed to fragment tunnel mode
packets after IPsec processing, because it is the sender relative to
the (outer) tunnel header. However, unlike IPv4, it would be feasible
to carry a plaintext fragment on a transport mode SA, because the
fragment header in IPv6 would appear after the AH or ESP header, and
thus would not cause confusion at the receiver re reassembly.
Specifically, the receiver would not attempt reassembly for the
fragment until after IPsec processing. To keep things simple, this
specification prohibits carriage of fragments on transport mode SAs
for IPv6 traffic.
When only end systems used transport mode SAs, the prohibition on
carriage of fragments was not a problem, since we assumed that the
end system could be configured to not offer a fragment to IPsec. For
a native host implementation this seems reasonable, and, as someone
already noted, RFC 2401 warned that a BITS implementation might have
to reassemble fragments before performing an SA lookup. (It would
then apply AH or ESP and could re-fragment the packet after IPsec
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processing.) Because a BITS implementation is assumed to be able to
have access to all traffic emanating from its host, even if the host
has multiple interfaces, this was deemed a reasonable mandate.
In this specification, it is acceptable to use transport mode in
cases where the IPsec implementation is not the ultimate destination,
e.g., between two SGs. In principle, this creates a new opportunity
for outbound, plaintext fragments to be mapped to a transport mode SA
for IPsec processing. However, in these new contexts in which a
transport mode SA is now approved for use, it seems likely that we
can continue to prohibit transmission of fragments, as seen by IPsec,
i.e., packets that have an "outer header" with a non-zero fragment
offset field. For example, in an IP overlay network, packets being
sent over transport mode SAs are IP-in-IP tunneled and thus have the
necessary inner header to accommodate fragmentation prior to IPsec
processing. When carried via a transport mode SA, IPsec would not
examine the inner IP header for such traffic, and thus would not
consider the packet to be a fragment.
D.2 Tunnel Mode and Fragments
For tunnel mode SAs, it has always been the case that outbound
fragments might arrive for processing at an IPsec implementation. The
need to accommodate fragmented outbound packets can pose a problem
because a non-initial fragment generally will not contain the port
fields associated with a next layer protocol such as TCP, UDP, or
SCTP. Thus, depending on the SPD configuration for a given IPsec
implementation, plaintext fragments might or might not pose a
problem.
For example, if the SPD requires that all traffic between two address
ranges is offered IPsec protection (no BYPASS or DISCARD SPD entries
apply to this address range), then it should be easy to carry non-
initial fragments on the SA defined for this address range, since the
SPD entry implies an intent to carry ALL traffic between the address
ranges. But, if there are multiple SPD entries that could match a
fragment, and if these entries reference different subsets of port
fields (vs. ANY), then it is not possible to map an outbound non-
initial fragment to the right entry, unambiguously. (If we choose to
allow carriage of fragments on transport mode SAs for IPv6, the
problems arises in that context as well.)
This problem largely, though not exclusively, motivated the
definition of OPAQUE as a selector value for port fields in RFC 2401.
The other motivation for OPAQUE is the observation that port fields
might not be accessible due to the prior application of IPsec. For
example, if a host applied IPsec to its traffic and that traffic
arrived at an SG, these fields would be encrypted. The algorithm
specified for locating the "next layer protocol" described in RFC
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2401 also motivated use of OPAQUE to accommodate an encrypted next
layer protocol field in such circumstances. Nonetheless, the primary
use of the OPAQUE value was to match traffic selector fields in
packets that did not contain port fields (non-initial fragments), or
packets in which the port fields were already encrypted (as a result
of nested application of IPsec). RFC 2401 was ambiguous in discussing
the use of OPAQUE vs. ANY, suggesting in some places that ANY might
be an alternative to OPAQUE.
We gain additional access control capability by defining both ANY and
OPAQUE values. OPAQUE can be defined to match only fields that are
not accessible. We could define ANY as the complement of OPAQUE,
i.e., it would match all values but only for accessible port fields.
We have therefore simplified the procedure employed to locate the
next layer protocol in this document, so that we treat ESP and AH as
next layer protocols. As a result, the notion of an encrypted next
layer protocol field has vanished, and there is also no need to worry
about encrypted port fields either. And accordingly, OPAQUE will be
applicable only to non-initial fragments.
Since we have adopted the definitions above for ANY and OPAQUE, we
need to clarify how these values work when the specified protocol
does not have port fields, and when ANY is used for the protocol
selector. Accordingly, if a specific protocol value is used as a
selector, and if that protocol has no port fields, then the port
field selectors are to be ignored and ANY MUST be specified as the
value for the port fields. (In this context, ICMP TYPE and CODE
values are lumped together as a single port field (for IKEv2
negotiation), as is the IPv6 Mobility Header TYPE value.) If the
protocol selector is ANY, then this should be treated as equivalent
to specifying a protocol for which no port fields are defined, and
thus the port selectors should be ignored, and MUST be set to ANY.
D.3. The Problem of Non-Initial Fragments
For an SG implementation, it is obvious that fragments might arrive
from end systems behind the SG. A BITW implementation also may
encounter fragments from a host or gateway behind it. (As noted
earlier, native host implementations and BITS implementations
probably can avoid the problems described below.) In the worst case,
fragments from a packet might arrive at distinct BITW or SG
instantiations and thus preclude reassembly as a solution option.
Hence, in RFC 2401 we adopted a general requirement that fragments
must be accommodated in tunnel mode for all implementations. However,
RFC 2401 did not provide a perfect solution. The use of OPAQUE as a
selector value for port fields (a SHOULD in RFC 2401) allowed an SA
to carry non-initial fragments.
Using the features defined in RFC 2401, if one defined an SA between
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two IPsec (SG or BITW) implementations using the OPAQUE value for
both port fields, then all non-initial fragments matching the S/D
address and protocol values for the SA would be mapped to that SA.
Initial fragments would NOT map to this SA, if we adopt a strict
definition of OPAQUE. However, RFC 2401 did not provide detailed
guidance on this and thus it may not have been apparent that use of
this feature would essentially create a "non-initial fragment only"
SA.
In the course of discussing the "fragment-only" SA approach, it was
noted that some subtle problems, problems not considered in RFC 2401,
would have to be avoided. For example, an SA of this sort must be
configured to offer the "highest quality" security services for any
traffic between the indicated S/D addresses (for the specified
protocol). This is necessary to ensure that any traffic captured by
the fragment-only SA is not offered degraded security relative to
what it would have been offered if the packet were not fragmented. A
possible problem here is that we may not be able to identify the
"highest quality" security services defined for use between two IPsec
implementation, since the choice of security protocols, options, and
algorithms is a lattice, not a totally ordered set. (We might safely
say that BYPASS < AH < ESP w/integrity, but it gets complicated if we
have multiple ESP encryption or integrity algorithm options.) So, one
has to impose a total ordering on these security parameters to make
this work, but this can be done locally.
However, this conservative strategy has a possible performance down
side; if most traffic traversing an IPsec implementation for a given
S/D address pair (and specified protocol) is bypassed, then a
fragment-only SA for that address pair might cause a dramatic
increase in the volume of traffic afforded crypto processing. If the
crypto implementation cannot support high traffic rates, this could
cause problems. (An IPsec implementation that is capable of line rate
or near line rate crypto performance would not be adversely affected
by this SA configuration approach. Nonetheless, the performance
impact is a potential concern, specific to implementation
capabilities.)
Another concern is that non-initial fragments sent over a dedicated
SA might be used to effect overlapping reassembly attacks, when
combined with an apparently acceptable initial fragment. (This sort
of attack assumes creation of bogus fragments, and is not a side
effect of normal fragmentation.) This concern is easily addressed in
IPv4, by checking the fragment offset value to ensure that no non-
initial fragments have a small enough offset to overlap port fields
that should be contained in the initial fragment. Recall that the
IPv4 MTU minimum is 576 bytes, and the max IP header length is 60
bytes, so any ports should be present in the initial fragment. If we
require all non-initial fragments to have an offset of say 128 or
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greater, just to be on the safe side, this should prevent successful
attacks of this sort. If the intent is only to protect against this
sort of reassembly attack, this check need be implemented only by a
receiver.
IPv6 also has a fragment offset, carried in the fragmentation
extension header. However, IPv6 extension headers are variable in
length and there is no analogous max header length value that we can
use to check non-initial fragments, to reject ones that might be used
for an attack of the sort noted above. A receiver would need to
maintain state analogous to reassembly state, to provide equivalent
protection. So, only for IPv4 it is feasible to impose a fragment
offset check that would reject attacks designed to circumvent port
field checks by IPsec (or firewalls) when passing non-initial
fragments.
Another possible concern is that in some topologies and SPD
configurations this approach might result in an access control
surprise. The notion is that if we create an SA to carry ALL (non-
initial) fragments then that SA would carry some traffic that might
otherwise arrive as plaintext via a separate path, e.g., a path
monitored by a proxy firewall. But, this concern arises only if the
other path allows initial fragments to traverse it without requiring
reassembly, presumably a bad idea for a proxy firewall. Nonetheless,
this does represent a potential problem in some topologies and under
certain assumptions re: SPD and (other) firewall rule sets, and
administrators need to be warned of this possibility.
A less serious concern is that non-initial fragments sent over a non-
initial fragment-only SA might represent a DoS opportunity, in that
they could be sent when no valid, initial fragment will ever arrive.
This might be used to attack hosts behind an SG or BITW device.
However, the incremental risk posed by this sort of attack, which can
be mounted only by hosts behind an SG or BITW device, seems small.
If we interpret the ANY selector value as encompassing OPAQUE, then a
single SA with ANY values for both port fields would be able to
accommodate all traffic matching the S/D address and protocol traffic
selectors, an alternative to using the OPAQUE value. But, using ANY
here precludes multiple, distinct SAs between the same IPsec
implementations for the same address pairs and protocol. So, it is
not an exactly equivalent alternative.
Fundamentally, fragment handling problems arise only when more than
one SA is defined with the same S/D address and protocol selector
values, but with different port field selector values.
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D.4 BYPASS/DISCARD Traffic
We also have to address the non-initial fragment processing issue for
BYPASS/DISCARD entries, independent of SA processing. This is largely
a local matter for two reasons:
1) We have no means for coordinating SPD entries for such
traffic between IPsec implementations since IKE is not
invoked.
2) Many of these entries refer to traffic that is NOT
directed to or received from a location that is using
IPsec. So there is no peer IPsec implementation with
which to coordinate via any means.
However, this document should provide guidance here, consistent with
our goal of offering a well-defined, access control function for all
traffic, relative to the IPsec boundary. To that end, this document
says that implementations MUST support fragment reassembly for
BYPASS/DISCARD traffic when port fields are specified. An
implementation also MUST permit a user or administrator to accept
such traffic or reject such traffic using the SPD conventions
described in Secion 4.4.1. The concern is that BYPASS of a
cleartext, non-initial fragment arriving at an IPsec implementation
could undermine the security afforded IPsec-protected traffic
directed to the same destination. For example, consider an IPsec
implementation configured with an SPD entry that calls for IPsec-
protection of traffic between a specific source/destination address
pair, and for a specific protocol and destination port, e.g., TCP
traffic on port 23 (Telnet). Assume that the implementation also
allows BYPASS of traffic from the same source/destination address
pair and protocol, but for a different destination port, e.g., port
119 (NNTP). An attacker could send a non-initial fragment (with a
forged source address) that, if bypassed, could overlap with IPsec-
protected traffic from the same source and thus violate the integrity
of the IPsec-protected traffic. Requiring stateful fragment checking
for BYPASS entries with non-trivial port ranges prevents attacks of
this sort.
D.5 Just say no to ports?
It has been suggested that we could avoid the problems described
above by not allowing port field selectors to be used in tunnel mode.
But the discussion above shows this to be an unnecessarily stringent
approach, i.e., since no problems arise for the native OS and BITS
implementations. Moreover, some WG members have described scenarios
where use of tunnel mode SAs with (non-trivial) port field selectors
is appropriate. So the challenge is defining a strategy that can deal
with this problem in BITW and SG contexts. Also note that
BYPASS/DISCARD entries in the SPD that make use of ports pose the
same problems, irrespective of tunnel vs. transport mode notions.
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Some folks have suggested that a firewall behind an SG or BITW should
be left to enforce port level access controls, and the effects of
fragmentation. However, this seems to be an incongruous suggestion in
that elsewhere in IPsec (e.g., in IKE payloads) we are concerned
about firewalls that always discard fragments. If many firewalls
don't pass fragments in general, why should we expect them to deal
with fragments in this case? So, this analysis rejects the suggestion
of disallowing use of port field selectors with tunnel mode SAs.
D.6 Other Suggested Solutions
One suggestion is to reassemble fragments at the sending IPsec
implementation, and thus avoid the problem entirely. This approach is
invisible to a receiver and thus could be adopted as a purely local
implementation option.
A more sophisticated version of this suggestion calls for
establishing and maintaining minimal state from each initial fragment
encountered, to allow non-initial fragments to be matched to the
right SAs or SPD/cache entries. This implies an extension to the
current processing model (and the old one). The IPsec implementation
would intercept all fragments, capture Source/Destination IP
addresses, protocol, packet ID, and port fields from initial
fragments and then use this data to map non-initial fragments to SAs
that require port fields. If this approach is employed, the receiver
needs to employ an equivalent scheme, as it too must verify that
received fragments are consistent with SA selector values. A non-
initial fragment that arrives prior to an initial fragment could be
cached or discarded, awaiting arrival of the corresponding initial
fragment.
A downside of both approaches noted above is that they will not
always work. When a BITW device or SG is configured in a topology
that might allow some fragments for a packet to be processed at
different SGs or BITW devices, then there is no guarantee that all
fragments will ever arrive at the same IPsec device. This approach
also raises possible processing problems. If the sender caches non-
initial fragments until the corresponding initial fragment arrives,
buffering problems might arise, especially at high speeds. If the
non-initial fragments are discarded rather than cached, there is no
guarantee that traffic will ever pass, e.g., retransmission will
result in different packet IDs that cannot be matched with prior
transmissions. In any case, housekeeping procedures will be needed to
decide when to delete the fragment state data, adding some complexity
to the system. Nonetheless, this is a viable solution in some
topologies, and these are likely to be common topologies.
The Working Group rejected an earlier version of the convention of
creating an SA to carry only non-initial fragments, something that
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was supported implicitly under the RFC 2401 model via use of OPAQUE
port fields, but never clearly articulated in RFC 2401. The
(rejected) text called for each non-initial fragment to be treated as
protocol 44 (the IPv6 fragment header protocol ID) by the sender and
receiver. This approach has the potential to make IPv4 and IPv6
fragment handling more uniform, but it does not fundamentally change
the problem, nor does it address the issue of fragment handling for
BYPASS/DISCARD traffic. Given the fragment overlap attack problem
that IPv6 poses, it does not seem that it is worth the effort to
adopt this strategy.
D.7 Consistency
Earlier the WG agreed to allow an IPsec BITS, BITW or SG to perform
fragmentation prior to IPsec processing. If this fragmentation is
performed after SA lookup at the sender, there is no "mapping to the
right SA" problem. But, the receiver still needs to be able to verify
that the non-initial fragments are consistent with the SA via which
they are received. Since the initial fragment might be lost en route,
the receiver encounters all of the potential problems noted above.
Thus, if we are to be consistent in our decisions, we need to say how
a receiver will deal with the non-initial fragments that arrive.
D.8 Conclusions
There is no simple, uniform way to handle fragments in all contexts.
Different approaches work better in different contexts. Thus this
document offers 3 choices -- one MUST and two MAYs. At some point in
the future, if the community gains experience with the two MAYs, they
may become SHOULDs or MUSTs or other approaches may be proposed.
Kent & Seo [Page 85]
Internet Draft Security Architecture for IP December 2004
Appendix E - Example of Supporting Nested SAs via SPD and Forwarding
Table Entries
This appendix provides an example of how to configure the SPD and
forwarding tables to support a nested pair of SAs, consistent with
the new processing model. For simplicity, this example assumes just
one SPD-I.
The goal in this example is to support a transport mode SA from A to
C, carried over a tunnel mode SA from A to B. For example, A might be
a laptop connected to the public internet, B a firewall that protects
a corporate network, and C a server on the corporate network that
demands end-to-end authentication of A's traffic.
+---+ +---+ +---+
| A |=====| B | | C |
| |------------| |
| |=====| | | |
+---+ +---+ +---+
A's SPD contains entries of the form:
Next Layer
Rule Local Remote Protocol Action
---- ----- ------ ---------- -----------------------
1 C A ESP BYPASS
2 A C ICMP,ESP PROTECT(ESP,tunnel,integr+conf)
3 A C ANY PROTECT(ESP,transport,integr-only)
4 A B ICMP,IKE BYPASS
A's unprotected-side forwarding table is set so that outbound packets
destined for C are looped back to the protected side. A's protected
side forwarding table is set so that inbound ESP packets are looped
back to the unprotected side. A's forwarding tables contain entries
of the form:
Unprotected-side forwarding table
Rule Local Remote Protocol Action
---- ----- ------ -------- ---------------------------
1 A C ANY loop back to protected side
2 A B ANY forward to B
Protected-side forwarding table
Rule Local Remote Protocol Action
---- ----- ------ -------- -----------------------------
1 A C ESP loop back to unprotected side
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Internet Draft Security Architecture for IP December 2004
An outbound TCP packet from A to C would match SPD rule 3 and have
transport mode ESP applied to it. The unprotected-side forwarding
table would then loop back the packet. The packet is compared against
SPD-I (see Figure 2), matches SPD rule 1, and so it is BYPASSed. The
packet is treated as an outbound packet and compared against the SPD
for a third time. This time it matches SPD rule 2, so ESP is applied
in tunnel mode. This time the forwarding table doesn't loop back the
packet, because the outer destination address is B, so the packet
goes out onto the wire.
An inbound TCP packet from C to A, is wrapped in two ESP headers; the
outer header (ESP in tunnel mode) shows B as the source whereas the
inner header (ESP transport mode) shows C as the source. Upon arrival
at A, the packet would be mapped to an SA based on the SPI, have the
outer header removed, and be decrypted and integrity-checked. Then it
would be matched against the SAD selectors for this SA, which would
specify C as the source and A as the destination, derived from SPD
rule 2. The protected-side forwarding function would then send it
back to the unprotected side based on the addresses and the next
layer protocol (ESP), indicative of nesting. It is compared against
SPD-O (see figure 3) and found to match SPD rule 1, so it is
BYPASSed. The packet is mapped to an SA based on the SPI, integrity-
checked, and compared against the SAD selectors derived from SPD rule
3. The forwarding function then passes it up to the next layer,
because it isn't an ESP packet.
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Internet Draft Security Architecture for IP December 2004
References
Normative
[BBCDWW98]Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated Service",
RFC 2475, December 1998.
[Bra97] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Level", BCP 14, RFC 2119, March 1997.
[CD98] Conta, A. and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", RFC 2463, December 1998.
[DH98] Deering, S., and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[Eas03] Eastlake, D., "Cryptographic Algorithm Implementation
Requirements For ESP And AH", ???, ???? 2004.
[RFC Editor: Please update reference [Eas03] "Cryptographic
Algorithm Implementation Requirements For ESP And AH"
(draft-ietf-ipsec-esp-ah-algorithms-02.txt) with the RFC
number and month when it is issued.]
[Kau04] Kaufman, C., "The Internet Key Exchange (IKEv2) Protocol",
RFC ???, ???? 2004.
[RFC Editor: Please update the reference [Kau04] "The
Internet Key Exchange (IKEv2) Protocol" (draft-ietf-ipsec-
ikev2-17.txt) with the RFC number and month when it is
issued.]
[Ken04a] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
???, ???? 2004.
[RFC Editor: Please update the reference [Ken04a] "IP
Encapsulating Security Payload (ESP)" (draft-ietf-ipsec-
esp-v3-09.txt) with the RFC number and month when it is
issued.]
[Ken04b] Kent, S., "IP Authentication Header", RFC ???, ??? 2004.
[RFC Editor: Please update the reference [Ken04b] "IP
Authentication Header" (draft-ietf-ipsec-rfc2402bis-09.txt)
with the RFC number and month when it is issued.]
Kent & Seo [Page 88]
Internet Draft Security Architecture for IP December 2004
[MD90] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[Pos81a] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981
[Pos81b] Postel, J., "Internet Control Message Protocol", RFC 792,
September 1981
[Sch03] Schiller, J., "Cryptographic Algorithms for use in the
Internet Key Exchange Version 2", RFC ???, ???? 2003
[RFC Editor: Please update the reference [Sch03]
"Cryptographic Algorithms for use in the Internet Key
Exchange Version 2" (draft-ietf-ipsec-
ikev2-algorithms-05.txt) with the RFC number and month when
it is issued.]
Informative
[CoSa04] Condell, M., and Sanchez, L. On the Deterministic
Enforcement of Un-ordered Security Policies", BBN Technical
Memo 1346, March 2004
[FaLiHaMeTr00]Farinacci, D., Li, T., Hanks, S., Meyer, D., Traina,
P., "Generic Routing Encapsulation (GRE), RFC 2784, March
2000.
[Gro02] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, April 2002.
[HC03] Holbrook, H., and Cain, B., "Source Specific Multicast for
IP", WWork in Progress, November 3, 2002.
[HA94] Haller, N., and Atkinson, R., "On Internet Authentication",
RFC 1704, October 1994
[Mobip] Johnson, D., Perkins, C., Arkko, J., "Mobility Support in
IPv6", RFC 3775, June 2004
[NiBlBaBL98]Nichols, K., Blake, S., Baker, F., Black, D., "Definition
of the Differentiated Services Field (DS Field) in the IPv4
and IPv6 Headers", RFC2474, December 1998.
[Per96] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RaFlBl01]Ramakrishnan, K., Floyd, S., Black, D., "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168,
Kent & Seo [Page 89]
Internet Draft Security Architecture for IP December 2004
September 2001.
[RFC3547] Baugher, M., Weis, B., Hardjono, T., Harney, H., "The Group
Domain of Interpretation", RFC 3547, July 2003.
[RFC3740] Hardjono, T., Weis, B., "The Multicast Group Security
Architecture", RFC 3740, March 2004.
[RaCoCaDe04]Rajahalme, J., Conta, A., Carpenter, B., Deering, S.,
"IPv6 Flow Label Specification, RFC 3697, March 2004.
[Sch94] Schneier, B., Applied Cryptography, Section 8.6, John
Wiley & Sons, New York, NY, 1994.
[Shi00] Shirey, R., "Internet Security Glossary", RFC 2828, May
2000.
[SMPT01] Shacham, A., Monsour, B., Pereira, R., and M. Thomas, "IP
Payload Compression Protocol (IPComp)", RFC 3173, September
2001.
[ToEgWa04]Touch, J., Eggert, L., Wang, Y., Use of IPsec Transport
Mode for Dynamic Routing, RFC 3884, September 2004.
[VK83] V.L. Voydock & S.T. Kent, "Security Mechanisms in High-
level Networks", ACM Computing Surveys, Vol. 15, No. 2,
June 1983.
Author Information
Stephen Kent
BBN Technologies
10 Moulton Street
Cambridge, MA 02138
USA
Phone: +1 (617) 873-3988
EMail: kent@bbn.com
Karen Seo
BBN Technologies
10 Moulton Street
Cambridge, MA 02138
USA
Phone: +1 (617) 873-3152
EMail: kseo@bbn.com
Kent & Seo [Page 90]
Internet Draft Security Architecture for IP December 2004
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Internet Draft Security Architecture for IP December 2004
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