Network Working Group S. Kent
Internet Draft K. Seo
draft-ietf-ipsec-rfc2401bis-02.txt BBN Technologies
Obsoletes: RFC 2401 April 2004
Expires October 2004
Security Architecture for the Internet Protocol
Status of this Memo
This document is an Internet Draft and is subject to all provisions
of Section 10 of RFC2026. Internet Drafts are working documents of
the Internet Engineering Task Force (IETF), its areas, and its
working groups. Note that other groups may also distribute working
documents as Internet Drafts
Internet Drafts are draft documents valid for a maximum of 6 months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet Drafts as reference
material or to cite them other than as a "work in progress".
The list of current Internet Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
Copyright (C) The Internet Society (2004). All Rights Reserved.
Kent & Seo [Page 1]
Internet Draft Security Architecture for IP April 2004
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 May Be Implemented.................................10
4. Security Associations...............................................11
4.1 Definition and Scope...........................................11
4.2 Security Association Functionality.............................15
4.3 Combining Security Associations................................16
4.4 Major IPsec Databases..........................................16
4.4.1 The Security Policy Database (SPD)........................19
4.4.1.1 Selectors............................................24
4.4.1.2 Structure of an SPD entry............................27
4.4.2 Security Association Database (SAD).......................29
4.5 SA and Key Management..........................................35
4.5.1 Manual Techniques.........................................35
4.5.2 Automated SA and Key Management...........................36
4.5.3 Locating a Security Gateway...............................37
4.6 Security Associations and Multicast............................38
5. IP Traffic Processing...............................................38
5.1 Outbound IP Traffic Processing (protected-to-unprotected)......39
5.1.1 Handling an Outbound Packet That Must Be Discarded........41
5.1.2 Header Construction for Tunnel Mode.......................42
5.1.2.1 IPv4 -- Header Construction for Tunnel Mode..........43
5.1.2.2 IPv6 -- Header Construction for Tunnel Mode..........45
5.2 Processing Inbound IP Traffic (unprotected-to-protected).......45
6. ICMP Processing ....................................................49
7. Handling Fragments (on the protected side of the IPsec boundary)....49
8. Auditing............................................................51
9. Conformance Requirements............................................52
10. Security Considerations............................................52
11. Differences from RFC 2401..........................................52
Acknowledgements.......................................................57
Appendix A -- Glossary.................................................58
Appendix B -- Decorrelation............................................61
Appendix C -- Categorization of ICMP messages [May be deleted].........64
Appendix D -- ASN.1 for an SPD entry...................................67
Appendix E -- Fragment Handling Rationale..............................72
References.............................................................73
Author Information.....................................................74
Kent & Seo [Page 2]
Internet Draft Security Architecture for IP April 2004
Notices................................................................77
Kent & Seo [Page 3]
Internet Draft Security Architecture for IP April 2004
1. Introduction
1.1 Summary of Contents of Document
This document specifies the base architecture for IPsec compliant
systems. It describes how to provide a set of security services for
traffic at the IP layer, in both the IPv4 and IPv6 environments.
This document describes the requirements for systems that implement
IPsec, the fundamental elements of such systems, and how the elements
fit together and fit into the IP environment. It also describes the
security services offered by the IPsec protocols, and how these
services can be employed in the IP environment. This document does
not address all aspects of the IPsec architecture. Other documents
address additional architectural details in specialized environments,
e.g., use of IPsec in NAT environments and more comprehensive support
for IP multicast. The fundamental components of the IPsec security
architecture are discussed in terms of their underlying, required
functionality. Additional RFCs (see Section 1.3 for pointers to
other documents) define the protocols in (a), (c), and (d).
a. Security Protocols -- Authentication Header (AH) and
Encapsulating Security Payload (ESP)
b. Security Associations -- what they are and how they work,
how they are managed, associated processing
c. Key Management -- manual and automated (The Internet Key
Exchange (IKE))
d. Cryptographic algorithms for authentication and encryption
This document is not a Security Architecture for the Internet; it
addresses security only at the IP layer, provided through the use of
a combination of cryptographic and protocol security mechanisms.
The spelling "IPsec" is preferred and used throughout this and all
related IPsec standards. All other capitalizations of IPsec (e.g.,
IPSEC, IPSec, ipsec) are deprecated. However, any capitalization of
the sequence of letters "IPsec" should be understood to refer to the
IPsec protocols.
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in RFC 2119 [Bra97].
1.2 Audience
The target audience for this document is primarily individuals who
implement this IP security technology or who architect systems that
will use this technology. Technically adept users of this technology
(end users or system administrators) also are part of the target
Kent & Seo [Page 4]
Internet Draft Security Architecture for IP April 2004
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" [Kau03] and "Cryptographic Algorithms for use
in the Internet Key Exchange Version 2" [Sch03]
2. Design Objectives
2.1 Goals/Objectives/Requirements/Problem Description
IPsec is designed to provide interoperable, high quality,
cryptographically-based security for IPv4 and IPv6. The set of
security services offered includes access control, connectionless
integrity, data origin authentication, detection and rejection of
replays (a form of partial sequence integrity), confidentiality (via
encryption), and limited traffic flow confidentiality. These
services are provided at the IP layer, offering protection for all
protocols that may be carried over IP in a standard fashion
(including IP itself).
IPsec includes a specification for minimal firewall functionality,
since that is an essential aspect of access control at the IP layer.
Implementations are free to provide more sophisticated firewall
mechanisms, and to implement the IPsec-mandated functionality using
those more sophisticated mechanisms. (Note that interoperability may
suffer if additional firewall constraints on traffic flows are
imposed by an IPsec implementation but cannot be negotiated based on
the traffic selector features defined in this document and negotiated
via IKEv2.) The IPsec firewall function makes use of the
Kent & Seo [Page 5]
Internet Draft Security Architecture for IP April 2004
cryptographically-enforced authentication and integrity provided for
all IPsec traffic to offer better access control than could be
obtained through use of a firewall (one not privy to IPsec internal
parameters) plus separate cryptographic protection.
Most of the security services are provided through use of two traffic
security protocols, the Authentication Header (AH) and the
Encapsulating Security Payload (ESP), and through the use of
cryptographic key management procedures and protocols. The set of
IPsec protocols employed in a context, and the ways in which they are
employed, will be determined by the users/administrators in that
context. It is the goal of the IPsec architecture to ensure that
compliant implementations include the services and management
interfaces needed to meet the security requirements of a broad user
population.
When IPsec is correctly implemented and deployed, it ought not
adversely affect users, hosts, and other Internet components that do
not employ IPsec for traffic protection. IPsec security protocols
(AH & ESP, and to a lesser extent, IKE) are designed to be
cryptographic algorithm-independent. This modularity permits
selection of different sets of cryptographic algorithms as
appropriate, without affecting the other parts of the implementation.
For example, different user communities may select different sets of
cryptographic algorithms (creating cryptographically-enforced
cliques) if required.
A set of default cryptographic algorithms for use with AH and ESP is
specified [Eas03] to facilitate interoperability in the global
Internet. The use of these cryptographic algorithms, in conjunction
with IPsec traffic protection and key management protocols, is
intended to permit system and application developers to deploy high
quality, Internet layer, cryptographic security technology.
2.2 Caveats and Assumptions
The suite of IPsec protocols and associated default cryptographic
algorithms are designed to provide high quality security for Internet
traffic. However, the security offered by use of these protocols
ultimately depends on the quality of the their implementation, which
is outside the scope of this set of standards. Moreover, the
security of a computer system or network is a function of many
factors, including personnel, physical, procedural, compromising
emanations, and computer security practices. Thus IPsec is only one
part of an overall system security architecture.
Finally, the security afforded by the use of IPsec is critically
dependent on many aspects of the operating environment in which the
Kent & Seo [Page 6]
Internet Draft Security Architecture for IP April 2004
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 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"
(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
Kent & Seo [Page 7]
Internet Draft Security Architecture for IP April 2004
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. The term "outbound" refers to traffic entering
the implementation via the protected interface, or emitted by the
implementation on the protected side of the boundary and directed
toward the unprotected interface. An IPsec implementation may
support more than one interface on either or both sides of the
boundary.
Note the facilities for discarding traffic on either side of the
IPsec boundary, the BYPASS facility that allows traffic to transit
the boundary without cryptographic protection, and the reference to
IKE as a protected-side key and security management function.
IPsec optionally supports negotiation of IP compression [SMPT98],
motivated in part by the observation that when encryption is employed
within IPsec, it prevents effective compression by lower protocol
layers.
Kent & Seo [Page 8]
Internet Draft Security Architecture for IP April 2004
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 confidentially. Use of
ESP in a confidentiality-only mode is discouraged. When ESP is
used with confidentiality enabled, there are provisions for
limited traffic flow confidentiality, i.e., provisions for
concealing packet length, and to facilitate efficient generation
and discard of dummy packets. This capability is likely to be
effective primarily in VPN and overlay network contexts.
o Both AH and ESP offer access control, enforced through the
distribution of cryptographic keys and the management of traffic
flows as dictated by the Security Policy Database (SPD, Section
4.4.1).
These protocols may be applied individually or in combination with
each other to provide security services in IPv4 and IPv6. However,
most security requirements can be met through the use of ESP by
itself. Each protocol supports two modes of use: transport mode and
tunnel mode. In transport mode, AH and ESP provide protection
primarily for next layer protocols; in tunnel mode, AH and ESP are
applied to tunneled IP packets. The differences between the two
modes are discussed in Section 4.1.
IPsec allows the user (or system administrator) to control the
granularity at which a security service is offered. For example, one
can create a single encrypted tunnel to carry all the traffic between
two security gateways or a separate encrypted tunnel can be created
for each TCP connection between each pair of hosts communicating
across these gateways. IPsec, through the SPD management paradigm,
incorporates facilities for specifying:
o which security protocols (AH, ESP) to employ, their mode
Kent & Seo [Page 9]
Internet Draft Security Architecture for IP April 2004
(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
"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
Kent & Seo [Page 10]
Internet Draft Security Architecture for IP April 2004
gateway.
This document often talks in terms of host or security gateway use of
IPsec, without regard to whether the implementation is native, BITS
or BITW. When the distinctions among these implementation options are
significant, the document makes reference to specific implementation
approaches.
4. Security Associations
This section defines Security Association management requirements for
all IPv6 implementations and for those IPv4 implementations that
implement AH, ESP, or both 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 Security Associations. All implementations of AH or
ESP MUST support the concept of a Security Association as described
below. The remainder of this section describes various aspects of
Security Association management, defining required characteristics
for SA policy management and SA management techniques.
4.1 Definition and Scope
affords security services to the traffic carried by it. Security
services are afforded to an SA by the use of AH, or ESP, but not
both. If both AH and ESP protection are applied to a traffic stream,
then two SAs must be created and coordinated to effect protection
through iterated application of the security protocols. To secure
typical, bi-directional communication between two IPsec-enabled
systems, a pair of Security Associations (one in each direction) are
required. IKE explicitly creates SA pairs in recognition of this
common usage requirement.
For an SA used to carry unicast (or anycast) traffic, the SPI
(Security Parameters Index - see Appendix A and AH [Ken04b] and ESP
[Ken04a] specifications) by itself suffices to specify an SA.
However, as a local matter, an implementation may choose to use the
SPI in conjunction with the IPsec protocol type (AH or ESP) for SA
identification. If an IPsec implementation supports multicast, then
it MUST support multicast SAs using the algorithm below for mapping
inbound IPsec datagrams to SAs. Implementations that support only
unicast traffic need not implement this demultiplexing algorithm.
In many secure multicast architectures, e.g., [RFC3740], a central
Group Controller/Key Server unilaterally assigns the group security
association'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 comprise the group.
Kent & Seo [Page 11]
Internet Draft Security Architecture for IP April 2004
Consequently, it is possible that a group security association and a
unicast security association 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 Security Association Database (SAD) [Section 4.4.2]
must indicate whether the SA lookup makes use of the destination, or
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, or destination and source, address comparison (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 {SPI, destination address, source
address}. If a SAD entry matches then process the inbound ESP
packet with that matching SAD entry. Otherwise, proceed to step
2.
2. Search the SAD for a match on {SPI, destination address}. If the
SAD entry matches then process the inbound ESP 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
{SPI, protocol} otherwise. If an SAD entry matches then process
the inbound ESP packet with that matching SAD entry. Otherwise,
discard the packet and log an auditable event.
In practice, an implementation MAY choose any method 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 are 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 are 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 TCAM features.
The indication of whether source and destination address matching is
required to map inbound IPsec traffic to SAs MUST be set either as a
Kent & Seo [Page 12]
Internet Draft Security Architecture for IP April 2004
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 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 appropriately support
QoS. To permit this, the IPsec implementation MUST permit
establishment and maintenance of multiple SAs between a given sender
and receiver, with the same selectors. Distribution of traffic among
these parallel SAs to support QoS is locally determined by the sender
and is not negotiated by IKE. The receiver MUST process the packets
from the different SAs without prejudice.
DISCUSSION: While the DSCP [NiBlBaBL98, Gro02] and ECN [RaFlBL01]
fields are not "selectors", as that term in used in this
architecture, the sender will need a mechanism to direct packets with
a given (set of) DSCP values to the appropriate SA. This mechanism
might be termed a "classifier".
As noted above, two types of SAs are defined: transport mode and
tunnel mode. IKE creates pairs of SAs, so for simplicity, we choose
to require that both SAs in a pair be of the same mode, transport or
tunnel.
A transport mode SA is a security association typically employed
between a pair of hosts to provide end-to-end security services. When
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
latter case, transport mode may be used to support in-IP tunneling
(e.g., IP-in-IP [Per96] or GRE tunneling [FaLiHaMeTr00]) over
transport mode SAs. To further clarify, the use of transport mode by
an intermediate system (e.g., a security gateway) is permitted only
when applied to packets whose source address (for outbound packets)
or destination address (for inbound packets) is an address belonging
to the intermediate system itself. The access control functions that
are an important part of IPsec are significantly limited in this
context, as they cannot be applied to the end-to-end headers of the
packets that traverse a transport mode SA used in this fashion. Thus
this way of using transport mode should be evaluated carefully before
Kent & Seo [Page 13]
Internet Draft Security Architecture for IP April 2004
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 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
multiple security gateways, it is important that an IPsec packet be
sent to the security gateway with which the SA was negotiated.
Similarly, a packet that might be fragmented en-route must have all
the fragments delivered to the same IPsec instance for reassembly
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
Kent & Seo [Page 14]
Internet Draft Security Architecture for IP April 2004
are several reasons for employing tunnel mode when either end of an
SA is a security gateway.
Note: AH and ESP cannot be applied using transport mode to IPv4
packets that are fragments. Only tunnel mode can be employed in such
cases. For 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 Security Association Functionality
The set of security services offered by an SA depends on the security
protocol selected, the SA mode, the endpoints of the SA, and on the
election of optional services within the protocol.
For example, both AH and ESP offer integrity and authentication
services, but the coverage differs for each protocol and differs for
transport vs. tunnel mode. If the integrity of an IPv4 option or IPv6
extension header must be protected en-route between sender and
receiver, AH can provide this service, except for the mutable (non-
predictable) parts of the IP or extension headers. However, the same
security may be achieved in some contexts by applying ESP to a tunnel
carrying a packet.
Kent & Seo [Page 15]
Internet Draft Security Architecture for IP April 2004
The granularity of access control provided is determined by the
choice of the selectors that define each security association.
Moreover, the authentication means employed by IPsec peers, e.g.,
during creation of an IKE (vs. child) SA also effects the granularity
of the access control afforded.
If confidentiality is selected, then an ESP (tunnel mode) SA between
two security gateways can offer partial traffic flow confidentiality.
The use of tunnel mode allows the inner IP headers to be encrypted,
concealing the identities of the (ultimate) traffic source and
destination. Moreover, ESP payload padding also can be invoked to
hide the size of the packets, further concealing the external
characteristics of the traffic. Similar traffic flow confidentiality
services may be offered when a mobile user is assigned a dynamic IP
address in a dialup context, and establishes a (tunnel mode) ESP SA
to a corporate firewall (acting as a security gateway). Note that
fine granularity SAs generally are more vulnerable to traffic
analysis than coarse granularity ones that are carrying traffic from
many subscribers.
NOTE: A compliant implementation MUST NOT allow instantiation of an
ESP SA that employs both NULL encryption and no integrity algorithm.
An attempt to negotiate such an SA is an auditable event by both
initiator and responder. The audit log entry for this event SHOULD
include the current date/time, local IKE IP address, and remote IKE
IP address. The initiator SHOULD record the relevant SPD entry.
4.3 Combining Security Associations
This document does not require support for nested security
associations or for what RFC 2401 called "SA bundles." These features
still can be effected by appropriate configuration of both the SPD
and the local forwarding functions (for inbound and outbound
traffic), but this 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.
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
Kent & Seo [Page 16]
Internet Draft Security Architecture for IP April 2004
standardization. However, some external aspects of the processing
must be standardized, to ensure interoperability and to provide a
minimum management capability that is essential for productive use of
IPsec. This section describes a general model for processing IP
traffic relative to IPsec functionality, in support of these
interoperability and functionality goals. The model described below
is nominal; implementations need not match details of this model as
presented, but the external behavior of implementations MUST
correspond to the externally observable characteristics of this model
in order to be deemed compliant.
There are 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)
security association (Section 4.4.2).
Peer Authorization Database (PAD)
The third database, 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 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 used, the trust
anchor for the certificate is part of the PAD. Because the PAD
might be incorporated into the SA management protocol
implementation, it is not discussed extensively in this document.
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 use completely independent identities,
policies, key management SAs, and/or IPsec SAs. This is for the
Kent & Seo [Page 17]
Internet Draft Security Architecture for IP April 2004
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~Os
traffic with the correct VPN.
Forwarding vs Security Decisions
The IPsec model described here embodies a clear separation between
forwarding (routing) and security decisions, to accommodate a wide
range of contexts where IPsec may be employed. Forwarding may be
trivial, in the case where there are only two interfaces, or it
may be complex, e.g., if 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
Kent & Seo [Page 18]
Internet Draft Security Architecture for IP April 2004
"initial" fragment might not be the first fragment.
4.4.1 The Security Policy Database (SPD)
A security association is a management construct used to enforce
security policy for traffic crossing the IPsec boundary. Thus an
essential element of SA processing is an underlying Security Policy
Database (SPD) that specifies what services are to be offered to IP
datagrams and in what fashion. The form of the database and its
interface are outside the scope of this specification. However, this
section specifies minimum management functionality that must be
provided, to allow a user or system administrator to control whether
and how IPsec is applied to traffic transmitted or received by a host
or transiting a security gateway. The SPD, or relevant caches, must
be consulted during the processing of ALL traffic (inbound and
outbound), including traffic not protected by IPsec, that traverses
the IPsec boundary. This includes IPsec management traffic such as
IKE. An IPsec implementation MUST have at least one SPD, and it MAY
support multiple SPDs, if appropriate for the context in which the
IPsec implementation operates. There is no requirement to maintain
SPDs on a per interface basis, as was specified in RFC 2401. However,
if an implementation supports multiple SPDs, then it MUST include an
explicit SPD selection function, that is invoked to select the
appropriate SPD for outbound traffic processing. The inputs to this
function are the outbound packet and any local metadata (e.g., the
interface via which the packet arrived) required to effect the SPD
selection function. The output of the function is an SPD ID.
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
Kent & Seo [Page 19]
Internet Draft Security Architecture for IP April 2004
first choice refers to traffic that is not allowed to traverse the
IPsec boundary (in the specified direction). The second choice
refers to traffic that is allowed to cross the IPsec boundary
without IPsec protection. The third choice refers to traffic that
is afforded IPsec protection, and for such traffic the SPD must
specify the security protocols to be employed, their mode,
security service options, and the cryptographic algorithms to be
used.
SPD-S, SPD-I, SPD-O
An SPD is logically divided into three pieces. The SPD-S (secure
traffic) contains entries for all traffic subject to IPsec
protection. SPD-O (outbound) contains entries for all outbound
traffic that is to be bypassed or discarded. SPD-I (inbound) is
applied to inbound traffic that will be bypassed or discarded. All
three of these can be decorrelated (with the exception noted above
for native host implementations) to facilitate caching. If an
IPsec implementation supports only one SPD, then the SPD consists
of all three parts. If multiple SPDs are supported, some of them
may be partial, e.g., some SPDs might contain only SPD-I entries,
to control inbound bypassed traffic on a per-interface basis. The
split allows SPD-I to be consulted without having to consult SPD-
S, for such traffic. Since the SPD-I is just a part of the SPD,
the same rule applies here, i.e., if a packet that is looked up in
the SPD-I cannot be matched to an entry there, then the packet
MUST be discarded. Note that for outbound traffic, if a match is
not found in SPD-S, then SPD-O must be checked to see if the
traffic should be bypassed. Similarly, if SPD-O is checked first
and no match is found, then SPD-S must be checked.
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
Kent & Seo [Page 20]
Internet Draft Security Architecture for IP April 2004
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).
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 or does not exist for
the given Next Layer Protocol. ANY includes OPAQUE.
How To Derive the Values for an SAD entry
Kent & Seo [Page 21]
Internet Draft Security Architecture for IP April 2004
For each selector in an SPD entry, the entry specifies how to
derive the corresponding values for a new Security Association
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. If IPsec processing is
specified for an entry, a "populate from packet" (PFP) flag may be
asserted for one or more of the selectors in the SPD entry (Local
IP address, Remote IP address, Next Layer Protocol, and depending
on Next Layer Protocol -- Local port, ICMP type/code or Mobility
Header type; Remote port). If asserted for a given selector, the
flag indicates that the SA to be created should take its value for
that selector from the value in the packet. Otherwise, the SA
should take its value(s) for that selector 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
Kent & Seo [Page 22]
Internet Draft Security Architecture for IP April 2004
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
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. (Native host
implementations have an implicit form of caching available, due to
the use of, for example, socket interfaces for applications, and
thus there is no requirement to be able to decorrelate SPD entries
in these implementations.)
Note: Decorrelation is a means of improving performance and
simplifying the processing description; it is not a requirement
for a compliant implementation. In this section, unless otherwise
noted, the use of "SPD" refers to the body of policy information
in both ordered or decorrelated (unordered) state.
Appendix B provides an algorithm that can be used to decorrelate
SPD entries, but any algorithm that produces equivalent output may
be used. Note that when an SPD entry is decorrelated all the
resulting entries MUST be linked together, so that all members of
the group derived from an individual, SPD entry (prior to
decorrelation) can all be placed into caches and into the SAD at
the same time. For example, suppose one starts with an entry A
(from an ordered SPD) that when decorrelated, yields entries A1,
Kent & Seo [Page 23]
Internet Draft Security Architecture for IP April 2004
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 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 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 affect of this change on extant SAs.
This document does not impose a requirement to do this, but an
implementation MAY choose to check the impact of an SPD change on
extant SAs and to 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
Kent & Seo [Page 24]
Internet Draft Security Architecture for IP April 2004
multicast group). This structure allows expression of a single
IP address (via a trivial range), or a list of addresses (each a
trivial ranges), or a range of addresses (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
destination 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 ranges), 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 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, or ANY. 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 IP 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,
these selectors is 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, 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.
* 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
Kent & Seo [Page 25]
Internet Draft Security Architecture for IP April 2004
message. Note that the MH type may not be available in the case
of receipt of a fragmented packet, thus a value of OPAQUE MUST be
supported.
* 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. 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, thus a value of
OPAQUE MUST be supported.
- Name: 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). 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
also values for the Local or Remote IP address. If a name is
used in the IKE exchange, that name is matched against the SPD,
Kent & Seo [Page 26]
Internet Draft Security Architecture for IP April 2004
rather than matching the corresponding address in the SPD, and
the IP address in the corresponding SAD entry is derived from
that of the named entity (Use initiator's source address as the
"Remote" address for case 1, and use the user's source address
as the "Local" address for case 2).
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, an
ASN.1 definition of an SPD entry is provided in Appendix D.
This text describes the SPD in a fashion that maps directly into IKE
payloads. One should not create SPD entries that cannot be mapped
into something that IKE can negotiate. The management GUI can offer
the user other forms of data entry and display, e.g., the option of
using address prefixes as well as ranges, and symbolic names for
protocols, ports, etc. (Do not confuse the use of symbolic names in a
Kent & Seo [Page 27]
Internet Draft Security Architecture for IP April 2004
management interface with the SPD selector "Name".) If the reserved,
symbolic selector value OPAQUE or ANY is employed for a given
selector type, only it may appear in the list for that selector, and
it must appear only once in the list for that selector. Note that
ANY and OPAQUE are local syntax conventions -D IKEv2 negotiates these
values via ranges. Also, Remote/Local only applies to ports, not to
ICMP message type/code or Mobility Header type.
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 selector is optional.
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.
- 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)
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.
Kent & Seo [Page 28]
Internet Draft Security Architecture for IP April 2004
If the required processing is PROTECT, the entry contains
the following information.
- IPsec mode -- tunnel or transport
- 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)
- IPsec protocol(s) -- AH, ESP
- algorithms -- which ones to use for AH, which ones to
use for ESP, ordered by decreasing priority
4.4.2 Security Association Database (SAD)
In each IPsec implementation there is a nominal Security Association
Database, in which each entry defines the parameters associated with
one SA. Each SA has an entry in the SAD. For outbound processing,
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.)
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 contain the value or values negotiated
at the time the SA was created. For a receiver, these values are used
to check that the header fields of an inbound packet match the
selector values negotiated for the SA. For the receiver, this is part
of verifying that a packet arriving on an SA is consistent with the
policy for the SA. (See Section 6 for rules for ICMP messages.)
These fields can have the form of specific values, ranges, ANY, or
OPAQUE, as described in section 4.4.1.1, "Selectors."
The following data items MUST be in the SAD:
o Security Parameter Index (SPI): a 32-bit value selected by the
receiving end of an SA to uniquely identify the SA. In an SAD
entry for an outbound SA, the SPI is used to construct the
packet's AH or ESP header. In an SAD entry for an inbound SA, the
SPI is used to map traffic to the appropriate SA (see text on
unicast/multicast in Section 4.1).
Kent & Seo [Page 29]
Internet Draft Security Architecture for IP April 2004
o Sequence Number Counter: a 64-bit 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.
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.
o Lifetime of this Security Association: a time interval after which
an SA must be replaced with a new SA (and new SPI) or terminated,
plus an indication of which of these actions should occur. This
may be expressed as a time or byte count, or a simultaneous use of
both with the first lifetime to expire taking precedence. A
compliant implementation MUST support both types of lifetimes, and
must support a simultaneous use of both. If time is employed, and
if IKE employs X.509 certificates for SA establishment, the SA
lifetime must be constrained by the validity intervals of the
certificates, and the NextIssueDate of the CRLs used in the IKE
Kent & Seo [Page 30]
Internet Draft Security Architecture for IP April 2004
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 Path MTU: any observed path MTU and aging variables.
o Tunnel header IP source and destination address - both addresses
must be either IPv4 or IPv6 addresses. The version implies the
type of IP header to be used. Only used when the IPsec protocol
mode is tunnel.
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.
Kent & Seo [Page 31]
Internet Draft Security Architecture for IP April 2004
Value in
Triggering Resulting SAD
Selector SPD Entry PFP Packet Entry
-------- ---------------- --- ------------ --------------
loc addr list of ranges 0 IP addr "S" list of ranges
or ANY or ANY
list of ranges 1 IP addr "S" "S"
or ANY
rem addr list of ranges 0 IP addr "D" list of ranges
or ANY or ANY
list of ranges 1 IP addr "D" "D"
or ANY
protocol list of prot's* 0 prot. "P" list of prot's*
or ANY** or ANY
list of prot's* 1 prot. "P" "P"
or ANY**
OPAQUE 0 not avail. "undefined"
OPAQUE 1 not avail. ***
Kent & Seo [Page 32]
Internet Draft Security Architecture for IP April 2004
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
or ANY or ANY
list of ranges 0 no src port discard packet
or ANY
OPAQUE 0 not avail. OPAQUE
OPAQUE 1 not avail. ***
list of ranges 1 src port "s" "s"
or ANY
list of ranges 1 no src port discard packet
or ANY
rem port list of ranges 0 dst port "d" list of ranges
or ANY or ANY
list of ranges 0 no dst port discard packet
or ANY
OPAQUE 0 not avail. OPAQUE
OPAQUE 1 not avail. ***
list of ranges 1 dst port "d" "d"
or ANY
list of ranges 1 no dst port discard packet
or ANY
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
or ANY or ANY
list of ranges 0 no mh type discard packet
or ANY
OPAQUE 0 not avail. OPAQUE
OPAQUE 1 not avail. ***
list of ranges 1 mh type "T" "T"
or ANY
list of ranges 1 no mh type discard packet
or ANY
Kent & Seo [Page 33]
Internet Draft Security Architecture for IP April 2004
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, ANY type and ANY code.
Value in
Triggering Resulting SAD
Selector SPD Entry PFP Packet Entry
--------- ---------------- --- ------------ --------------
ICMP type a single type 0 type "t" list of ranges
or ANY or ANY
a single type 0 no type discard packet
or ANY
OPAQUE 0 not avail. OPAQUE
OPAQUE 1 not avail. ***
a single type 1 type "t" "t"
or ANY
a single type 1 no type discard packet
or ANY
ICMP code list of ranges 0 type "c" list of ranges
or ANY or ANY
list of ranges 0 no code discard packet
or ANY
OPAQUE 0 not avail. OPAQUE
OPAQUE 1 not avail. ***
list of ranges 1 type "c" "c"
or ANY
list of ranges 1 no code discard packet
or ANY
Kent & Seo [Page 34]
Internet Draft Security Architecture for IP April 2004
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.
4.5 SA and Key Management
IPsec mandates support for both manual and automated SA and
cryptographic key management. The IPsec protocols, AH and ESP, are
largely independent of the associated SA management techniques,
although the techniques involved do affect some of the security
services offered by the protocols. For example, the optional anti-
replay service available for AH and ESP requires automated SA
management. Moreover, the granularity of key distribution employed
with IPsec determines the granularity of authentication provided. In
general, data origin authentication in AH and ESP is limited by the
extent to which secrets used with the integrity algorithm (or with a
key management protocol that creates such secrets) are shared among
multiple possible sources.
The following text describes the minimum requirements for both types
of SA management.
4.5.1 Manual Techniques
The simplest form of management is manual management, in which a
person manually configures each system with keying material and
security association management data relevant to secure communication
with other systems. Manual techniques are practical in small, static
environments but they do not scale well. For example, a company
could create a Virtual Private Network (VPN) using IPsec in security
gateways at several sites. If the number of sites is small, and
since all the sites come under the purview of a single administrative
Kent & Seo [Page 35]
Internet Draft Security Architecture for IP April 2004
domain, this might be a feasible context for manual management
techniques. In this case, the security gateway might selectively
protect traffic to and from other sites within the organization using
a manually configured key, while not protecting traffic for other
destinations. It also might be appropriate when only selected
communications need to be secured. A similar argument might apply to
use of IPsec entirely within an organization for a small number of
hosts and/or gateways. Manual management techniques often employ
statically configured, symmetric keys, though other options also
exist.
4.5.2 Automated SA and Key Management
Widespread deployment and use of IPsec requires an Internet-standard,
scalable, automated, SA management protocol. Such support is required
to facilitate use of the anti-replay features of AH and ESP, and to
accommodate on-demand creation of SAs, e.g., for user- and session-
oriented keying. (Note that the notion of "rekeying" an SA actually
implies creation of a new SA with a new SPI, a process that generally
implies use of an automated SA/key management protocol.)
The default automated key management protocol selected for use with
IPsec is IKEv2 [Kau04]. Other automated SA management protocols MAY
be employed.
When an automated SA/key management protocol is employed, the output
from this protocol is used to generate multiple keys for a single SA.
This also occurs because distinct keys are used for each of the two
SAs created by IKE. If both integrity and confidentiality are
employed, then a minimum of four keys are required. Additionally,
some cryptographic algorithms may require multiple keys, e.g., 3DES.
The Key Management System may provide a separate string of bits for
each key or it may generate one string of bits from which all keys
are extracted. If a single string of bits is provided, care needs to
be taken to ensure that the parts of the system that map the string
of bits to the required keys do so in the same fashion at both ends
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
Kent & Seo [Page 36]
Internet Draft Security Architecture for IP April 2004
cryptographic algorithm.
4.5.3 Locating a Security Gateway
This section discusses issues relating to how a host learns about the
existence of relevant security gateways and once a host has contacted
these security gateways, how it knows that these are the correct
security gateways. The details of where the required information is
stored is a local matter, but the Peer Authorization Database
described in Section 4.4 is the most likely candidate.
Consider a situation in which a remote host (H1) is using the
Internet to gain access to a server or other machine (H2) and there
is a security gateway (SG2), e.g., a firewall, through which H1's
traffic must pass. An example of this situation would be a mobile
host (road warrior) crossing the Internet to his home organization's
firewall (SG2). This situation raises several issues:
1. How does H1 know/learn about the existence of the security gateway
SG2?
2. How does it authenticate SG2, and once it has authenticated SG2,
how does it confirm that SG2 has been authorized to represent H2?
3. How does SG2 authenticate H1 and verify that H1 is authorized to
contact H2?
4. How does H1 know/learn about any additional gateways that provide
alternate paths to H2?
To address these problems, a host or security gateway MUST have an
administrative interface that allows the user/administrator to
configure the address of one or more security gateways for ranges of
destination addresses that require its use. This includes the
ability to configure information for locating and authenticating one
or more security gateways and verifying the authorization of these
gateways to represent the destination host. (The authorization
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".
Kent & Seo [Page 37]
Internet Draft Security Architecture for IP April 2004
4.6 Security Associations and Multicast
The receiver-orientation of the Security Association implies that, in
the case of unicast traffic, the destination system will select the
SPI value. By having the destination select the SPI value, there is
no potential for manually configured Security Associations to
conflict with automatically configured (e.g., via a key management
protocol) Security Associations or for Security Associations from
multiple sources to conflict with each other. For multicast traffic,
there are multiple destination systems associated with a single SA.
So some system or person will need to coordinate among all multicast
groups to select an SPI or SPIs on behalf of each multicast group and
then communicate the group's IPsec information to all of the
legitimate members of that multicast group via mechanisms not defined
here.
Multiple senders to a multicast group SHOULD use a single Security
Association (and hence Security Parameter Index) for all traffic to
that group when a symmetric key encryption or integrity algorithm is
employed. In such circumstances, the receiver knows only that the
message came from a system possessing the key for that multicast
group. In such circumstances, a receiver generally will not be able
to authenticate which system sent the multicast traffic.
Specifications for other, more general multicast approaches are
deferred to the IETF's Multicast Security Working Group.
5. IP Traffic Processing
As mentioned in Section 4.4.1 "The Security Policy Database (SPD)",
the SPD (or associated caches) must be 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). There
is nominally one cache per SPD. Since SPD entries may overlap, one
cannot safely cache these entries in general. Simple caching might
result in a match against a cache entry whereas an ordered search of
the SPD would have resulted in a match against a different entry.
But, if the SPD entries are first decorrelated, then the resulting
entries can safely be cached, and each cached entry will map to
exactly one SA, or 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"
Kent & Seo [Page 38]
Internet Draft Security Architecture for IP April 2004
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.
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 | |PROTECT |
| SPD-I | |DISCARD|<---|Cache|---->|(AH/ESP)|
+-------+ +-------+ +-----+ +--------+
: ^
: |
: +-------------+
:................>|SPD Selection|
+-------------+
^
|
Protected Interface
Kent & Seo [Page 39]
Internet Draft Security Architecture for IP April 2004
Figure 2. Processing Model for Outbound Traffic
IPsec MUST perform the following steps when processing outbound
packets:
1. When a packet arrives from the subscriber (protected) interface,
invoke the SPD selection function to obtain the SPD-ID needed to
choose the appropriate SPD. (If the implementation uses only one
SPD, this step is a no-op.)
2. Match the packet headers against the cache for the SPD specified
by the SPD-ID from step 1. Note that this cache contains entries
from SPD-O and SPD-S.
3a. If there is a match, then process the packet as specified by the
matching cache entry, i.e., BYPASS, DISCARD, or PROTECT using AH
or ESP. If IPsec processing is applied, there is a link from the
SPD cache entry to the relevant SAD entry (specifying the mode,
cryptographic algorithms, keys, SPI, etc.). IPsec processing is
as previously defined, for tunnel or transport modes and for AH or
ESP, as specified in their respective RFCs [Ken04b and Ken04a].
3b. If no match is found in the cache, search the SPD (SPD-S and SPD-
O parts) specified by SPD-ID. If the SPD entry calls for BYPASS or
DISCARD, create new outbound SPD cache entries and if BYPASS,
create new inbound SPD cache entries. 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.
Kent & Seo [Page 40]
Internet Draft Security Architecture for IP April 2004
NOTE: With the exception of IPv4 and IPv6 transport mode, an SG,
BITS, or BITW implementation MAY fragment packets before applying
IPsec. The device SHOULD have a configuration setting to disable
this. The resulting fragments are evaluated against the SPD in the
normal manner. Thus, fragments not containing port numbers (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.)
5.1.1 Handling an Outbound Packet That Must Be Discarded
If an IPsec system receives an outbound packet which 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 was unable to set up the SA required by the SPD
entry matching the packet because the IPsec peer at the other end
of the exchange is administratively prohibited from communicating
with the initiator.
IPv4 Type = 3 (destination unreachable) Code = 13
(Communication Administratively Prohibited)
IPv6 Type = 1 (destination unreachable) Code = 1
(Communication with destination administratively
prohibited)
b2. The IPsec system was unable to set up the SA required by the SPD
entry matching the packet because the IPsec peer at the other end
of the exchange could not be contacted.
IPv4 Type = 3 (destination unreachable) Code = 1 (host
unreachable)
Kent & Seo [Page 41]
Internet Draft Security Architecture for IP April 2004
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.
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
Kent & Seo [Page 42]
Internet Draft Security Architecture for IP April 2004
portions of the received packet re: application of access
controls. An IPsec implementation MAY be configurable with regard
to how it processes the DS field for tunnel mode for transmitted
packets. For outbound traffic, one configuration setting for DSCP
will operate as described in the following sections on IPv4 and
IPv6 header processing for IPsec tunnels. Another will allow the
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).
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.
Kent & Seo [Page 43]
Internet Draft Security Architecture for IP April 2004
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
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] 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.
Kent & Seo [Page 44]
Internet Draft Security Architecture for IP April 2004
5.1.2.2 IPv6 -- Header Construction for Tunnel Mode
See previous section 5.1.2.1 for notes 1-6 indicated by (footnote
number).
<-- How Outer Hdr Relates Inner Hdr --->
Outer Hdr at Inner Hdr at
IPv6 Encapsulator Decapsulator
Header fields: -------------------- ------------
version 6 (1) no change
DS Field copied from inner hdr (5) no change
ECN Field copied from inner hdr constructed (6)
flow label copied or configured no change
payload length constructed no change
next header AH,ESP,routing hdr no change
hop limit constructed (2) decrement (2)
src address constructed (3) no change
dest address constructed (3) no change
Extension headers never copied (7) no change
7. IPsec does not copy the extension headers from the inner
packet into the outer header, nor does IPsec construct
extension headers in the outer header. However, post-
IPsec code MAY insert/construct extension headers for
the outer header.
5.2 Processing Inbound IP Traffic (unprotected-to-protected)
Inbound processing is somewhat different from outbound processing,
because of the use of SPIs to map IPsec protected traffic to SAs. The
inbound SPD cache (SPD-I) is applied only to bypassed or discarded
traffic. If an arriving packet appears to be an IPsec fragment from
an unprotected interface, reassembly is performed prior to the IPsec
processing. The intent for any SPD cache is that a packet that fails
to match any entry is then referred to the corresponding SPD. Every
SPD SHOULD have a nominal, final entry that catches anything that is
otherwise unmatched, and discards it. This ensures that non-IPsec
protected traffic that arrives and does not match any SPD-I entry
will be discarded.
Kent & Seo [Page 45]
Internet Draft Security Architecture for IP April 2004
Unprotected Interface
|
V
+-----+ IPsec protected
------------------->|Demux|-------------------+
| +-----+ |
| | |
| Not IPsec | |
| | |
| V |
| +-------+ +-------+ +------+ |
| |DISCARD|<---| SPD-I |-->| ICMP | |
| +-------+ +-------+ +------+ |
| | V
+-----+ | +--------+
....|SPD-O|................|....................|PROTECT |...IPsec
+-----+ | |(AH/ESP)| Boundary
^ | +--------+
| | +---+ |
| BYPASS | +-->|IKE| |
| | | +---+ |
| V | V
| +----------+ +---------+
|--------<------|Forwarding|<------------|SAD Check|
+----------+ +---------+
|
V
Protected Interface
Figure 3. Inbound Traffic Processing Model
Prior to performing AH or ESP processing, any IP fragments that
arrive via the unprotected interface are reassembled (by IP). Each
inbound IP datagram to which IPsec processing will be applied is
identified by the appearance of the AH or ESP values in the IP Next
Protocol field (or of AH or ESP as 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 with different SPD-I entries. (The
interface ID is mapped to a corresponding SPD-ID.)
Kent & Seo [Page 46]
Internet Draft Security Architecture for IP April 2004
2. The packet is examined and demuxed into one of three categories:
- If the packet appears to be IPsec protected and it is addressed
to this device, an attempt is made to map it to an active SA
via the SAD.
- Traffic not addressed to this device, or addressed to this
device and not AH, ESP, or ICMP, is directed to BYPASS/DISCARD
lookup. (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.
- ICMP traffic directed to this device is directed to
"unprotected" ICMP processing (see Section 6).
3a. If the packet is addressed to the IPsec device and AH or ESP is
specified as the protocol, the packet is looked up in the SAD
identified by the SPD-ID from step 1. For unicast traffic, use
only the SPI (or SPI plus protocol). For multicast traffic, use
the SPI plus the destination and/or source addresses, as specified
in the SAD. If there is no match, discard the traffic. This is an
auditable event. The audit log entry for this event SHOULD include
the current date/time, SPI, source and destination of the packet,
IPsec protocol, and any other selector values of the packet that
are available. If the packet is found in the SAD, process it
accordingly (see step 4).
3b. If the packet is not addressed to the device or is addressed to
this device and is not AH, ESP, or ICMP, look up the packet header
in the (appropriate) SPD-I cache. If there is a match and the
packet is to be discarded or bypassed, do so. If there is no cache
match, look up the packet in the corresponding SPD-I and create a
cache entry as appropriate. (No SAs are created in response to
receipt of a packet that requires IPsec protection; only BYPASS or
DISCARD entries can be created this way.) If there is no match,
discard the traffic. This is an auditable event. The audit log
entry for this event SHOULD include the current date/time, SPI if
available, IPsec protocol if available, source and destination of
the packet, and any other selector values of the packet that are
available.
3c. Unprotected ICMP processing is assumed to take place on the
unprotected side of the IPsec boundary. Unprotected ICMP messages
are examined and local policy is applied to determine whether to
accept or reject these messages and, if accepted, what action to
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
Kent & Seo [Page 47]
Internet Draft Security Architecture for IP April 2004
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 to the sender (IPsec peer), indicating that the
received packet was discarded because of failure to pass selector
checks.
IKEv2 NOTIFY MESSAGES - ERROR TYPES Value
----------------------------------- -----
INVALID_SELECTORS iana-tbd
This error indication MAY be sent in an IKE INFORMATIONAL
exchange when a node receives an ESP or AH packet whose
selectors do not match those of the SA on which it was
delivered (and which caused the packet to be discarded).
The Notification Data contains the start of the offending
packet (as in ICMP messages) and the SPI field of the
notification is set to match the SPI of the IPsec SA.
To minimize the impact of a DoS attack or a mis-configured peer, the
IPsec system SHOULD include a management control to allow an
administrator to configure the IPsec implementation to send or not
send this IKE notification, and if this facility is selected, to rate
limit the transmission of such notifications.
After traffic is bypassed or processed through IPsec, it is handed to
the inbound forwarding function for disposition. This function may
cause the packet to be sent (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.
Kent & Seo [Page 48]
Internet Draft Security Architecture for IP April 2004
6. ICMP Processing
[This section will be filled in when IPsec issue # 91 is resolved.]
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
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 E 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.
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 an initial 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 requirements, three approaches have been
defined:
Kent & Seo [Page 49]
Internet Draft Security Architecture for IP April 2004
1. 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 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.)
2. All implementations MAY/SHOULD 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, because an SA of this sort
will carry ALL non-initial fragments that match a specified
Local/Remote address pair and protocol value, users and
administrators are advised to protect such traffic using ESP (with
integrity) and the "strongest" integrity and encryption algorithms
available at 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 approach 2, 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
Kent & Seo [Page 50]
Internet Draft Security Architecture for IP April 2004
2401bis, the choice to have multiple SAs to carry non-initial
fragments should also be local.
3. An implementation MAY/SHOULD 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 payload:
IKEv2 NOTIFY MESSAGES - ERROR TYPES Value
----------------------------------- -----
NON FIRST FRAGMENTS ALSO iana-tbd
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. Also note that stateful fragment checking may create DoS
opportunities that may be exploitable by hosts on a protected network
behind a security gateway.
An implementation MAY/SHOULD choose to support stateful fragment
checking for BYPASS/DISCARD traffic for a tunnel mode SA with non-
trivial port field values (not ANY or OPAQUE) (Approach 3 above). An
implementation also MUST permit a user or administrator to accept or
reject BYPASS/DISCARD traffic using the SPD conventions described in
approaches 1 and 2 above.
8. Auditing
Not all systems that implement IPsec will implement auditing. For
the most part, the granularity of auditing is a local matter.
However, several auditable events are identified in this document and
for each of these events a minimum set of information that SHOULD be
Kent & Seo [Page 51]
Internet Draft Security Architecture for IP April 2004
included in an audit log is defined. Additional information also MAY
be included in the audit log for each of these events, and additional
events, not explicitly called out in this specification, also MAY
result in audit log entries. There is no requirement for the
receiver to transmit any message to the purported transmitter in
response to the detection of an auditable event, because of the
potential to induce denial of service via such action.
9. Conformance Requirements
All IPv4 systems that claim to implement IPsec MUST comply with all
requirements of this document. All IPv6 systems that claim to
implement IPsec MUST comply with all requirements of this document.
10. Security Considerations
The focus of this document is security; hence security considerations
permeate this specification.
11. Differences from RFC 2401
[This section will be further updated when things have settled down.
Issue numbers, status, rejected items, and "proposed changes", etc.
will be removed in final version. Only the text describing the
differences from 2401 will remain.]
This architecture document differs substantially from RFC 2401 in
detail and in organization, but the fundamental notions are
unchanged.
o [Issues 40,44,45]
- 40 [closed] "Interface SPD selector vs. per-interface SPD"
- 44 [pending] "Proposed change: forwarding table lookup to
select virtual interface ID"
- 45 [pending] "Proposed change: use of cache with
decorrelated SPD"
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.
o [Issue #46] [closed] "Proposed change: no need for iterated
processing" -- There is no longer a requirement to support nested SAs
Kent & Seo [Page 52]
Internet Draft Security Architecture for IP April 2004
or "SA bundles." Instead this functionality can be achieved through
SPD and forwarding table configuration.
o [Issue #47] [closed] "Proposed change: all selectors can be a list
of ranges, per IKEv2 spec"
SPD entries were redefined to provide more flexibility. Each SPD
entry now consists of a set 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, Local Port, Remote Port, and
ICMP message type and code. 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 ASN.1 description is
included in Appendix D.
o [Issue #48] [closed] "Proposed change: add ToS traffic selector
option" -- TOS (IPv4) and Traffic Class (IPv6) have been replaced by
DSCP and ECN.
o [Issues #49 and #88]
- 49 [closed] "Proposed change: red-side fragmentation option"
- 88 [accepted] "Lift the prohibition on red-side
fragmentation by SG, BITS, BITW"
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 it.
o [Issue #50 and #87]
- 50 [closed] "Proposed change: tunnel vs. transport mode"
- 87 [closed] "Permit Security Gateways to use transport mode
when they are the endpoints of the communication"
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 [Issue #57] [closed] "ECN support"
The tunnel section has been updated to explain how to handle DSCP and
ECN bits.
o [Issue #67] [closed] "IPsec management traffic"
2401bis clarifies that for all traffic that crosses the IPsec
boundary, including IPsec management traffic, the SPD or associated
caches must be consulted.
Kent & Seo [Page 53]
Internet Draft Security Architecture for IP April 2004
o [Issue #68] [closed] "VPNs with overlapping IP address ranges"
2401bis now defines how to handle the situation of a security gateway
with multiple subscribers requiring separate IPsec contexts.
o [Issue #69] [closed] "Multiple protocols per SPD entry" -- Covered
by resolution of Issue #47
o [Issue #70] [closed] "Add diffserv (IPv4) and class (IPv6) as
selectors -- Covered by Issues #48 and #57
o [Issue #71] [closed] "Add definition of reserved SPIs"
A definition of reserved SPIs has been added.
o [Issue #72] [closed] "Explain why ALL IP packets must be checked"
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 [Issue #73] [closed] "IP Option & Ext Hdr handling in Tunnel Mode"
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 [Issue #74] [closed] "Inbound SA lookup -- multicast & unicast"
SA mapping for inbound traffic has been updated to be consistent with
the changes made in AH and ESP for support of unicast, anycast, and
multicast SAs.
o [Issue #75] [closed] "TOS (now ECN) copying in tunnel mode"
Guidance has been added re: how to handle the covert channel created
in tunnel mode by copying the DSCP value to outer header.
o [Issue #76] [accepted] "More explanation re: ESPv3 TFC padding &
dummy packets"
Modified ESP -- added more explanation re: ESPv3 TFC padding. IKEv2
to be modified to support negotiation of use of TFC padding.
o [Issue #77] [closed] "Should AH be mandatory?"
Support for AH in both IPv4 and IPv6 is now a MAY.
Kent & Seo [Page 54]
Internet Draft Security Architecture for IP April 2004
o [Issue #78] [closed] "PMTU issues" -- Ongoing discussion: PMTU
discovery (Ravi Kumar (9/30/03), Michael Richardson (11/14/03 and
11/17/03) Will be updated based on conclusion of discussion on ICMP
handling.
o [Issue #79] [closed] "Detection of dead peers and dead SAs -- No
change required; IKEv2 handles dead peer/SA detection
o [Issue #80] [closed] "Security gateway discovery" -- The IPSP
working group was supposed to produce an ID on "SG discovery, Policy
Exchange and Negotiation Protocol" in June 2003, but has not yet
posted this draft.
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 [Issue #81] [closed] "Handling outbound red fragments (e.g., on
separate SA)" The Issues Tracking Database lists this as Closed
(rejected). The working group rejected creation of a separate SA for
fragments. Based on a subsequent discussion on the mailing list,
2401bis was amended with 3 approaches.
Three approaches have been added for handling plaintext fragments on
the protected side of the IPsec boundary. An appendix has been added
documenting the rationale behind them.
o [Issue #82] [closed] "Creation of SAs"
Current 2401bis draft has revised text re: how to derive selector
values for SAs (from the SPD entry or from the packet, etc.). 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. Also, an appendix on decorrelation has been
added.
o [Issue #83] [rejected] "DROP'd inbound packet -- missing required
IPsec protection"
o [Issue #84] [closed] "DROP'd outbound packet"
If an IPsec system receives an outbound packet which 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.
o [Issue #85] [closed] "DROP'd inbound packet -- does not match SA"
Kent & Seo [Page 55]
Internet Draft Security Architecture for IP April 2004
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 SHOULD be able to send an IKE notification to the sender of
the packet.
o [Issue #86] [closed] "Add IPv6 mobility header message type as
selector"
IPv6 mobility header has been added as a possible Next Layer
Protocol. IPv6 mobility header message type has been added as a
selector.
o [Issue #87] [closed] "Permit Security Gateways to use transport
mode when they are the endpoints of the communication" -- See Issue
50.
o [Issue #88] [accepted] "Lift the prohibition on red-side
fragmentation by SG, BITS, BITW" -- See Issue 49.
o [Issue #89] [closed] " Remove the selector "name"" -- Rejected.
See issue 93.
o [Issue #90] [closed] "Remove the selector "data sensitivity level"
-- The selector "data sensitivity level" has been removed to simplify
things.
o [Issue #91] [pending] "Handling ICMP error messages" -- Ongoing
discussion
o [Issue #93] [pending] "Clarification re: the selector "name""
The text for the selector name has been updated and clarified.
o [ na ] "Next Layer Protocol" has been further explained and a
default list of protocols to skip when looking for the Next Layer
Protocol has been added.
o [ na ] The text has been amended to say that 2401bis assumes use of
IKEv2 or an SA management protocol with comparable features.
o [ na ] Text has been added clarifying the algorithm for mapping
inbound IPsec datagrams to SAs in the presence of multicast SAs
o [ na ] Text and an ASN.1 description have been added to clarify the
structure of an SPD entry and its alignment with what can be
negotiated in IKE.
Kent & Seo [Page 56]
Internet Draft Security Architecture for IP April 2004
Acknowledgements
The authors would like to acknowledge the contributions of Ran
Atkinson, who played a critical role in initial IPsec activities, and
who authored the first series of IPsec standards: RFCs 1825-1827.
Also a contributor who wishes to remain nameless, deserves special
thanks for providing extensive help in the editing of this
specification. The authors also would like to thank the members of
the IPsec and MSEC working groups who have contributed to the
development of this protocol specification.
Kent & Seo [Page 57]
Internet Draft Security Architecture for IP April 2004
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.)
Kent & Seo [Page 58]
Internet Draft Security Architecture for IP April 2004
Data Origin Authentication
Data origin authentication is a security service that verifies the
identity of the claimed source of data. This service is usually
bundled with connectionless integrity service.
Encryption
Encryption is a security mechanism used to transform data from an
intelligible form (plaintext) into an unintelligible form
(ciphertext), to provide confidentiality. The inverse
transformation process is designated "decryption". Oftimes the
term "encryption" is used to generically refer to both processes.
Integrity
Integrity is a security service that ensures that modifications to
data are detectable. Integrity comes in various flavors to match
application requirements. IPsec supports two forms of integrity:
connectionless and a form of partial sequence integrity.
Connectionless integrity is a service that detects modification of
an individual IP datagram, without regard to the ordering of the
datagram in a stream of traffic. The form of partial sequence
integrity offered in IPsec is referred to as anti-replay
integrity, and it detects arrival of duplicate IP datagrams
(within a constrained window). This is in contrast to connection-
oriented integrity, which imposes more stringent sequencing
requirements on traffic, e.g., to be able to detect lost or re-
ordered messages. Although authentication and integrity services
often are cited separately, in practice they are intimately
connected and almost always offered in tandem.
Protected vs Unprotected
"Protected" refers to the systems or interfaces that are inside
the IPsec protection boundary and "unprotected" refers to the
systems or interfaces that are outside the IPsec protection
boundary. IPsec provides a 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
Kent & Seo [Page 59]
Internet Draft Security Architecture for IP April 2004
OS.
Security Association (SA)
A simplex (uni-directional) logical connection, created for
security purposes. All traffic traversing an SA is provided the
same security processing. In IPsec, an SA is an internet layer
abstraction implemented through the use of AH or ESP. State data
associated with an SA is represented in the Security Association
Database (SAD).
Security Gateway
A security gateway is an intermediate system that acts as the
communications interface between two networks. The set of hosts
(and networks) on the external side of the security gateway is
termed unprotected (they are 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]
Kent & Seo [Page 60]
Internet Draft Security Architecture for IP April 2004
Appendix B - Decorrelation
This section 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 resulting entries that are decorrelated with the decorrelated set
of entries are then added to that decorrelated set.
The basic algorithm does not guarantee an optimal set of decorrelated
entries. That is, the entries may be broken up into smaller sets
than is necessary, though they will still provide all the necessary
policy information. Some extensions to the basic algorithm are
described later to improve this and improve the performance of the
algorithm.
C A set of ordered, correlated entries (a correlated SPD)
Ci The ith entry in C.
U The set of decorrelated entries being built from C
Ui The ith entry in U.
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
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
Kent & Seo [Page 61]
Internet Draft Security Architecture for IP April 2004
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.
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.
Kent & Seo [Page 62]
Internet Draft Security Architecture for IP April 2004
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.
Kent & Seo [Page 63]
Internet Draft Security Architecture for IP April 2004
Appendix C -- Categorization of ICMP messages [May be deleted]
The tables below characterize ICMP messages as being either host
generated, router generated, both, unassigned/unknown. The first set
of messages are for IPv4. The second set of messages are for IPv6.
IPv4
Type Name/Codes Reference
========================================================================
HOST GENERATED:
3 Destination Unreachable
2 Protocol Unreachable [RFC792]
3 Port Unreachable [RFC792]
8 Source Host Isolated [RFC792]
14 Host Precedence Violation [RFC1812]
10 Router Selection [RFC1256]
Type Name/Codes Reference
========================================================================
ROUTER GENERATED:
3 Destination Unreachable
0 Net Unreachable [RFC792]
4 Fragmentation Needed, Don't Fragment was Set [RFC792]
5 Source Route Failed [RFC792]
6 Destination Network Unknown [RFC792]
7 Destination Host Unknown [RFC792]
9 Comm. w/Dest. Net. is Administratively Prohibited [RFC792]
11 Destination Network Unreachable for Type of Service[RFC792]
5 Redirect
0 Redirect Datagram for the Network (or subnet) [RFC792]
2 Redirect Datagram for the Type of Service & Network[RFC792]
9 Router Advertisement [RFC1256]
18 Address Mask Reply [RFC950]
Kent & Seo [Page 64]
Internet Draft Security Architecture for IP April 2004
IPv4
Type Name/Codes Reference
========================================================================
BOTH ROUTER AND HOST GENERATED:
0 Echo Reply [RFC792]
3 Destination Unreachable
1 Host Unreachable [RFC792]
10 Comm. w/Dest. Host is Administratively Prohibited [RFC792]
12 Destination Host Unreachable for Type of Service [RFC792]
13 Communication Administratively Prohibited [RFC1812]
15 Precedence cutoff in effect [RFC1812]
4 Source Quench [RFC792]
5 Redirect
1 Redirect Datagram for the Host [RFC792]
3 Redirect Datagram for the Type of Service and Host [RFC792]
6 Alternate Host Address [JBP]
8 Echo [RFC792]
11 Time Exceeded [RFC792]
12 Parameter Problem [RFC792,RFC1108]
13 Timestamp [RFC792]
14 Timestamp Reply [RFC792]
15 Information Request [RFC792]
16 Information Reply [RFC792]
17 Address Mask Request [RFC950]
30 Traceroute [RFC1393]
31 Datagram Conversion Error [RFC1475]
32 Mobile Host Redirect [Johnson]
39 SKIP [Markson]
40 Photuris [Simpson]
Type Name/Codes Reference
========================================================================
UNASSIGNED TYPE OR UNKNOWN GENERATOR:
1 Unassigned [JBP]
2 Unassigned [JBP]
7 Unassigned [JBP]
19 Reserved (for Security) [Solo]
20-29 Reserved (for Robustness Experiment) [ZSu]
33 IPv6 Where-Are-You [Simpson]
34 IPv6 I-Am-Here [Simpson]
35 Mobile Registration Request [Simpson]
36 Mobile Registration Reply [Simpson]
37 Domain Name Request [Simpson]
38 Domain Name Reply [Simpson]
41-255 Reserved [JBP]
Kent & Seo [Page 65]
Internet Draft Security Architecture for IP April 2004
IPv6
Type Name/Codes Reference
========================================================================
HOST GENERATED:
1 Destination Unreachable [RFC 1885]
4 Port Unreachable
Type Name/Codes Reference
========================================================================
ROUTER GENERATED:
1 Destination Unreachable [RFC1885]
0 No Route to Destination
1 Comm. w/Destination is Administratively Prohibited
2 Not a Neighbor
3 Address Unreachable
2 Packet Too Big [RFC1885]
0
3 Time Exceeded [RFC1885]
0 Hop Limit Exceeded in Transit
1 Fragment reassembly time exceeded
Type Name/Codes Reference
========================================================================
BOTH ROUTER AND HOST GENERATED:
4 Parameter Problem [RFC1885]
0 Erroneous Header Field Encountered
1 Unrecognized Next Header Type Encountered
2 Unrecognized IPv6 Option Encountered
Kent & Seo [Page 66]
Internet Draft Security Architecture for IP April 2004
Appendix D -- ASN.1 for an SPD entry (work in progress)
This appendix uses ASN.1 syntax to describe the information that is
contained in an SPD. Since it describes encodings that are to be
used with the key management protocol, e.g., IKEv2, using ASN.1
constraints, it will not compile as shown due to "duplicate" tags.
-- An SPD is a list of policies in decreasing order of preference
SPD ::= SEQUENCE OF SPDEntry
-- An entry describes either traffic to be afforded IPsec protection
-- or traffic that is to be bypassed or discarded
SPDEntry ::= CHOICE {
iPsecEntry IPsecEntry, -- PROTECT traffic
bypassOrDiscard BypassOrDiscardEntry } -- DISCARD/BYPASS
-- traffic
-- A "selector set"
IPsecEntry ::= SEQUENCE { -- Each entry consist of:
name SEQUENCE {
passed SET OF Names, -- Matched to IKE ID
local SET OF Names }, -- Used internally
-- Populate from packet flags
pFPs BIT STRING { -- applies to ALL of the correspond-
pfpLocalAddr (0), -- ing traffic selectors;
pfpRemoteAddr (1), -- one does not want to
pfpProtocol (2), -- allow some SelectorSet
pfpLocalNext (3), -- items to use one value
pfpRemoteNext (4)}, -- and some to use another
-- Policy "condition"
condition SET OF SelectorList,
-- Policy "action"
processing SEQUENCE {
mode BOOLEAN, -- TRUE: transport, FALSE: tunnel
extSeqNum BOOLEAN, -- TRUE: 64 bit, FALSE: 32 bit
fragCheck BOOLEAN, -- TRUE: stateful fragment checking,
-- FALSE: no stateful fragment
-- checking
[need to add fields/etc. for the following
SEQ counter overflow
SA lifetime
manual SPI
Kent & Seo [Page 67]
Internet Draft Security Architecture for IP April 2004
DS
ECN
DF
Flow Label]
CHOICE {
aH IntegrityAlgs,
eSP SEQUENCE {
IntegrityAlgs,
ConfidentialityAlgs } } } }
Names ::= CHOICE { -- IKEv2 IDs:
DistinguishedName, -- ID_DER_ASN1_DN
FQDN, -- ID_FQDN
RFC822Name } OPTIONAL, -- ID_RFC822_ADDR
BypassOrDiscardEntry ::= SEQUENCE {
action BOOLEAN, -- TRUE: BYPASS, FALSE: DISCARD
outbound SET OF SelectorList OPTIONAL, -- one or both may
inbound SET OF SelectorList OPTIONAL } -- be present
-- A "selector set"
SelectorList ::= SEQUENCE {
localAddr AddrList,
remoteAddr AddrList,
protocol CHOICE {
-- Representation for ANY protocol
anyProt SEQUENCE {
INTEGER (0), -- ANY protocol
SEQUENCE { -- with either
ANY, -- ANY next layer selector
ANY }, -- ANY next layer selector
-- Protocols that have no next layer items
noNext SEQUENCE {
INTEGER (2..254),
SEQUENCE { -- if protocol has no next
OPAQUE,
OPAQUE } },
-- Fragments that have no next layer information
frag SEQUENCE {
INTEGER (44), -- Fragment identifier
SEQUENCE {
OPAQUE,
OPAQUE } },
Kent & Seo [Page 68]
Internet Draft Security Architecture for IP April 2004
-- Protocols that have one next layer item
oneNext SEQUENCE {
INTEGER (1..254), -- ICMP, MH, ICMPv6
SEQUENCE { -- ICMP Type*256+Code
type NextChoice, -- MH Type*256
OPAQUE } },
-- Protocols that have two next layer items
twoNext SEQUENCE {
INTEGER (2..254), -- Protocol
SEQUENCE {
local NextChoice, -- Local and
remote NextChoice }}} -- Remote ports
}
NextChoice ::= CHOICE {
aNY ANY,
oPAQUE OPAQUE,
range Next }
-- Representation of ANY in next layer field
ANY ::= SEQUENCE {
start INTEGER (0),
end INTEGER (65535) }
-- Representation of OPAQUE in next layer field
OPAQUE ::= SEQUENCE {
start INTEGER (65535),
end INTEGER (0) }
-- Range for a next layer field
Next ::= SEQUENCE {
start INTEGER (0..65535),
end INTEGER (0..65535) }
-- List of IP addresses
AddrList ::= SEQUENCE {
IPv4List OPTIONAL,
IPv6List OPTIONAL }
Kent & Seo [Page 69]
Internet Draft Security Architecture for IP April 2004
-- IPv4 address representations
IPv4List ::= CHOICE {
anyIPv4 SEQUENCE {
OCTET STRING ('00000000'H) (SIZE (4)),
OCTET STRING ('FFFFFFFF'H) (SIZE (4)) },
ipv4List SET OF CHOICE {
ipv4Addr OCTET STRING (SIZE (4)),
ipv4Range SEQUENCE { -- close, but not quite right ...
ipv4Start OCTET STRING ('00000001'H..'FFFFFFFE'H)
(SIZE (4)),
ipv4End OCTET STRING ('00000001'H..'FFFFFFFE'H)
(SIZE (4)) } } }
-- IPv6 address representations
IPv6List ::= CHOICE {
anyIPv6 SEQUENCE {
OCTET STRING
('00000000000000000000000000000000'H)
(SIZE (16)),
OCTET STRING
('FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF'H)
(SIZE (16)) },
ipv6List SET OF CHOICE {
ipv6Addr OCTET STRING (SIZE (16)),
ipv6Range SEQUENCE { -- close, but not quite right ...
ipv6Start OCTET STRING
('00000000000000000000000000000001'H
..'FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFE'H)
(SIZE (16)),
ipv6End OCTET STRING
('00000000000000000000000000000001'H
..'FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFE'H)
(SIZE (16))
} } }
-- Integrity Algorithms, ordered by decreasing preference
IntegrityAlgs ::= SEQUENCE OF IntegAlg
-- Confidentiality Algorithms, ordered by decreasing preference
ConfidentialityAlgs ::= SEQUENCE OF ConfAlg
Kent & Seo [Page 70]
Internet Draft Security Architecture for IP April 2004
-- Integrity Algorithms
IntegAlg ::= SEQUENCE {
algorithm ENUMERATED {
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) },
parameters ANY DEFINED BY algorithm OPTIONAL }
-- Confidentiality Algorithms
ConfAlg ::= SEQUENCE {
algorithm ENUMERATED {
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) },
parameters ANY DEFINED BY algorithm OPTIONAL }
Kent & Seo [Page 71]
Internet Draft Security Architecture for IP April 2004
Appendix E -- Fragment Handling Rationale
[Will be added in next draft -- based on write up Steve distributed
on the list plus subsequent discussion.]
Kent & Seo [Page 72]
Internet Draft Security Architecture for IP April 2004
References
[Will be updated after the text settles down]
Normative
[Bra97] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Level", BCP 14, RFC 2119, March 1997.
[DH98] Deering, S., and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[Eas03] Eastlake, D., "Cryptographic Algorithm Implementation
Requirements For ESP And AH", draft-ietf-ipsec-esp-ah-
algorithms-00.txt, December 2003.
[HC03] Holbrook, H., and Cain, B., "Source Specific Multicast for
IP", Internet Draft, draft-ietf-ssm-arch-01.txt, November
3, 2002.
[Kau03] Kaufman, C., "The Internet Key Exchange (IKEv2) Protocol",
draft-ietf- ipsec-ikev2-11.txt, October 2003
[Ken04a] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
???, ???? 2004.
[Ken04b] Kent, S., "IP Authentication Header", RFC ???, ??? 2004.
[Mobip] Johnson, D., Perkins, C., Arkko, J., "Mobility Support in
IPv6", Internet Draft, draft-ietf-mobileip-ipv6-24.txt,
June 2003
[Pos81] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981
[Sch03] Schiller, J., "Cryptographic Algorithms for use in the
Internet Key Exchange Version 2", draft-ietf-ipsec-
ikev2-algorithms-04.txt, September 2003
Informative
[BL73] Bell, D.E. & LaPadula, L.J., "Secure Computer Systems:
Mathematical Foundations and Model", Technical Report
M74-244, The MITRE Corporation, Bedford, MA, May 1973.
[DoD85] US National Computer Security Center, "Department of
Defense Trusted Computer System Evaluation Criteria", DoD
Kent & Seo [Page 73]
Internet Draft Security Architecture for IP April 2004
5200.28-STD, US Department of Defense, Ft. Meade, MD.,
December 1985.
[DoD87] US National Computer Security Center, "Trusted Network
Interpretation of the Trusted Computer System Evaluation
Criteria", NCSC-TG-005, Version 1, US Department of
Defense, Ft. Meade, MD., 31 July 1987.
[FaLiHaMeTr00]Farinacci, D., Li, T., Hanks, S., Meyer, D., Traina,
P., "Generic Routing Encapsulation (GRE), RFC 2784, March
2000.
[Gro02] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, April 2002.
[HA94] Haller, N., and Atkinson, R., "On Internet Authentication",
RFC 1704, October 1994
[ISO] ISO/IEC JTC1/SC6, Network Layer Security Protocol, ISO-IEC
DIS 11577, International Standards Organisation, Geneva,
Switzerland, 29 November 1992.
[IB93] Ioannidis, J. and Blaze, M., "Architecture and
Implementation of Network-layer Security Under Unix",
Proceedings of USENIX Security Symposium, Santa Clara, CA,
October 1993.
[IBK93] Ioannidis, J., Blaze, M., and Karn, P., "swIPe: Network-
Layer Security for IP", presentation at the Spring 1993
IETF Meeting, Columbus, Ohio
[Ken91] Kent, S., "US DoD Security Options for the Internet
Protocol", RFC 1108, November 1991.
[MSST97] Maughan, D., Schertler, M., Schneider, M., and J. Turner,
"Internet Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, November 1998.
[NiBlBaBL98]Nichols, K., Blake, S., Baker, F., Black, D., "Definition
of the Differentiated Services Field (DS Field) in the IPv4
and IPv6 Headers", RFC2474, December 1998.
[Orm97] Orman, H., "The OAKLEY Key Determination Protocol", RFC
2412, November 1998.
[Per96] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
Kent & Seo [Page 74]
Internet Draft Security Architecture for IP April 2004
[Pip98] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMP", RFC 2407, November 1998.
[RaFlBL01]Ramakrishnan, K., Floyd, S., Black, D., "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168,
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.
[Sch94] Schneier, B., Applied Cryptography, Section 8.6, John
Wiley & Sons, New York, NY, 1994.
[Shi00] Shirey, R., "Internet Security Glossary", RFC 2828, May
2000.
[SDNS] SDNS Secure Data Network System, Security Protocol 3, SP3,
Document SDN.301, Revision 1.5, 15 May 1989, published in
NIST Publication NIST-IR-90-4250, February 1990.
[SMPT98] Shacham, A., Monsour, R., Pereira, R., and M. Thomas, "IP
Payload Compression Protocol (IPComp)", RFC 2393, August
1998.
[VK83] V.L. Voydock & S.T. Kent, "Security Mechanisms in High-
level Networks", ACM Computing Surveys, Vol. 15, No. 2,
June 1983.
Author Information
Stephen Kent
BBN Technologies
10 Moulton Street
Cambridge, MA 02138
USA
Phone: +1 (617) 873-3988
EMail: kent@bbn.com
Kent & Seo [Page 75]
Internet Draft Security Architecture for IP April 2004
Karen Seo
BBN Technologies
10 Moulton Street
Cambridge, MA 02138
USA
Phone: +1 (617) 873-3152
EMail: kseo@bbn.com
Kent & Seo [Page 76]
Internet Draft Security Architecture for IP April 2004
Notices
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards- related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
Kent & Seo [Page 77]
Internet Draft Security Architecture for IP April 2004
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Expires October 2004
Kent & Seo [Page 78]
