Internet Draft -- IPsec Architecture
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Network Working Group Stephen Kent, BBN Corp Internet Draft Randall Atkinson, @Home Network draft-ietf-ipsec-arch-sec-01.txt 30 July 1997 Security Architecture for the Internet Protocol Status of this Memo This document is an Internet Draft. 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. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as "work in progress". Please check the I-D abstract listing contained in each Internet Draft directory to learn the current status of this or any other Internet Draft. This particular Internet Draft is a product of the IETF's IP Security (IPsec) working group. It is intended that a future version of this draft be submitted to the IESG for publication as a Draft Standard RFC. Comments about this draft may be sent to the authors or to the IPsec WG mailing list <ipsec@tis.com>. Distribution of this document is unlimited. Kent, Atkinson [Page 1] Internet Draft Security Architecture for IP 30 July 1997 Table of Contents 1. Introduction............................................................4 1.1 Summary of Contents of Document.....................................4 1.2 Audience -- assumptions about background knowledge..................4 1.3 Related Documents...................................................4 2. Design Objectives (how this system fits into the IP environment)........5 2.1 Goals/Objectives/Requirements/Problem Description...................5 2.2 Caveats and Assumptions.............................................5 3. System Overview ........................................................5 3.1 What IPSEC Does.....................................................6 3.2 How IPSEC Works.....................................................6 3.3 Where IPSEC May Be Implemented......................................7 4. Security Associations...................................................8 4.1 Definition and Scope................................................8 4.2 Security Association Functionality..................................9 4.3 Combining Security Associations....................................10 4.4 Security Association Processing....................................11 4.4.1 The Security Policy Database (SPD)............................11 4.4.2 Security Association Outbound Processing......................12 4.4.3 Selectors.....................................................13 4.4.4 Security Association Database (SAD)...........................14 4.5 Basic Combinations of Security Associations........................15 4.6 SA Establishment...................................................17 4.6.1 Manual Techniques.............................................17 4.6.2 Automatic Techniques -- Key Mgt Protocol Requirements.........18 4.6.3 Locating a security gateway...................................18 4.7 Security Associations and Multicast................................20 5. Processing IPSEC Traffic............................................... 5.1 Processing Outbound IPsec Traffic.................................. 5.1.1 Mapping to an SA or a bundle of SAs........................... 5.1.2 Header construction for tunnel mode........................... 5.1.2.1 IPv4 -- Header construction for tunnel mode.............. 5.1.2.2 IPv6 -- Header construction for tunnel mode.............. 5.2 Processing Inbound IPsec Traffic................................... 6. ICMP processing (relevant to IPsec).................................... 6.1 PMTU/DF processing................................................. 6.1.1 DF bit........................................................ 6.1.2 Path MTU Discovery (PMTU)..................................... 6.1.2.1 Propagation of PMTU...................................... 6.1.2.2 Calculation of PMTU...................................... 6.1.2.3 Granularity of PMTU processing........................... 6.1.2.4 PMTU Aging............................................... 7. Algorithm Descriptions................................................. 8. Usage Scenarios........................................................ 9. Auditing............................................................... 10. Use in systems supporting information flow security................... 11. Performance Issues.................................................... 12. Conformance Requirements.............................................. 13. Security Considerations............................................... 14. Differences from RFC 1825............................................. Kent, Atkinson [Page 2] Internet Draft Security Architecture for IP 30 July 1997 Acknowledgements.......................................................... Appendix A -- Glossary.................................................... A.1. Relevant Network Security Terminology............................. A.2 Requirements Terminology........................................... Appendix B -- Analysis/Discussion of PMTU/DF/Fragmentation Issues......... B.1 DF bit............................................................. B.2 Fragmentation...................................................... B.3 Path MTU Discovery................................................. B.3.1 Identifying the Originating Host(s)........................... B.3.2 Calculation of PMTU........................................... B.3.3 Granularity of Maintaining PMTU Data.......................... B.3.4 Per Socket Maintenance of PMTU Data........................... B.3.5 Delivery of PMTU Data to the Transport Layer.................. B.3.6 Aging of PMTU Data............................................ Appendix C - Sequence Space Window Code Example........................... References................................................................ Disclaimer................................................................ Author Information........................................................ Kent, Atkinson [Page 3] Internet Draft Security Architecture for IP 30 July 1997 1. Introduction 1.1 Summary of Contents of Document This memo specifies the architecture of a system aimed at providing security for traffic at the IP layer, both IPv4 and IPv6. This document describes the goals of the system, its components and how they fit together with each other and into the IP environment. It also describes the security services offered by the IPsec protocols, and how these services can be used in the IP environment. The following fundamental components of 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 automatic (Oakley/ISAKMP) d. Algorithms for authentication and encryption This document is not an overall 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. [This version of the document is a VERY ROUGH DRAFT and requires considerable additional work.] 1.2 Audience The target audience for this document includes implementers of this IP security technology and others interested in gaining a general background understanding of this system. In particular, prospective users of this technology (end users or system administrators) are part of the target audience. A glossary is provided as an appendix to help fill in gaps in background/vocabulary. This document assumes that the reader is familiar with the Internet Protocol, related networking technology, and general 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. "IP Security Document Roadmap" -- a document providing guidelines for specifications describing encryption and authentication algorithms used in this system. b. security protocols -- RFCs describing the Authentication Kent, Atkinson [Page 4] Internet Draft Security Architecture for IP 30 July 1997 Header (AH) and Encapsulating Security Payload (ESP) protocols. c. algorithms for authentication and encryption -- a separate RFC for each algorithm d. automatic key management, e.g., an RFC on Oakley/ISAKMP 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, protection against replays (a form of partial sequence integrity), confidentiality (encryption), and limited traffic flow confidentiality. These services are provided at the IP layer, offering protection for IP and/or upper layer protocols. These objectives are met through the 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 any context, and the ways in which they are employed, will be determined by the security and system requirements of users, applications, and/or sites/organizations When these mechanisms correctly implemented and deployed, they ought not adversely affect users, hosts, and other Internet components that do not employ these security mechanisms for protection of their traffic. These mechanisms also are designed to be algorithm- independent. This modularity permits selection of different sets of algorithms without affecting the other parts of the implementation. For example, different user communities may select different sets of algorithms (creating cliques) if required. A standard set of default algorithms is specified to facilitate interoperability in the global Internet. The use of these 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 [To be supplied] 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 Kent, Atkinson [Page 5] Internet Draft Security Architecture for IP 30 July 1997 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. 3.1 What IPSEC Does IPsec provides security services at the IP layer by enabling a system to select required security protocols, determine the algorithm(s) to use for the service(s), and put in place any cryptographic keys required to provide the requested services. IPsec can be used to protect paths between a pair of hosts, between a pair of security gateways, or between a security gateway and a host. (The term "security gateway" is used throughout the IPsec documents to refer to an intermediate system that implements IPsec protocols. For example, a router or a firewall implementing IPsec is a security gateway.) The set of security services that IPsec can provide includes access control connectionless integrity, data origin authentication, protection against replays (providing a form of partial sequence integrity), confidentiality (encryption), and limited traffic flow confidentiality. Because these services are provided at the IP layer, they can be used by any higher layer protocol, e.g., TCP, UDP, ICMP, BGP, etc. NOTE: When encryption is employed within IPsec, it prevents effective compression by lower protocol layers. However, IPsec does not provide its own compression services. Such services may be provided by existing higher layer protocols, or, in the future, in IP itself. The IETF working group, "IP Payload Compression Protocol (ippcp)" has the charter to "develop protocol specifications that make it possible to perform lossless compression on individual payloads before the payload is processed by a protocol that encrypts it. These specifications will allow for compression operations to be performed prior to the encryption of a payload by such protocols as IPSec." 3.2 How IPSEC Works IPsec uses two protocols to provide traffic security -- Authentication Header (AH) and Encapsulating Security Payload (ESP). Both protocols are described in more detail below in Section 5 ("Security Protocols") and in complete detail in their respective RFCs [KA97a, KA97b]. o The IP Authentication Header (AH) [KA97a] provides connectionless integrity, data origin authentication, and an optional anti-replay service (a form of partial sequence integrity). o The Encapsulating Security Payload (ESP) header/protocol provides confidentiality (encryption), and limited traffic flow confidentiality. It also may provide connectionless Kent, Atkinson [Page 6] Internet Draft Security Architecture for IP 30 July 1997 integrity, data origin authentication, an optional anti-replay service (a form of partial sequence integrity). o Both AH and ESP are vehicles for access control, based on the distribution of cryptographic keys and the management of traffic flows relative to these security protocols. These protocols may be applied alone or in combination with the each other to provide desired sets of security services in IPv4 and IPv6. They support two modes of use: transport mode and tunnel mode. In transport mode the protocols provide protection primarily for upper layer protocols; in tunnel mode, the protocols are applied to a tunneled IP packet. The differences between the two modes are discussed in Section 4. IPsec allows the user (or system administrator) to control the granularity at which a service is offered. For example, one can create a single encrypted tunnel to carry all 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 management incorporates facilities for specifying: o which security services to use and in what combinations o the granularity at which a given security protection should be applied o the algorithms used to effect cryptographic-based security Because these security services use shared secret values (cryptographic keys), IPsec relies on a separate set of mechanisms for putting these keys in place. (The keys are used for authentication and for encryption services.) This document requires support for both manual and automatic distribution of keys. It specifies a specific public-key based approach (Oakley/ISAKMP [Reference???]) for automatic key management, but other automated key distribution techniques could be used. For example, KDC-based systems such as Kerberos and other public-key systems such as SKIP could be employed. 3.3 Where IPSEC May Be Implemented There are several ways in which IPsec may be implemented in hosts or in conjunction with routers or firewalls (to create a security gateway). Several common examples are provided below: a. Integration of IPSEC into the native IP implementation. This requires access to the IP source code and is applicable to both hosts and security gateways. b. "Bump-in-the-stack" (BITS) implementations, where IPSEC is implemented "underneath" an existing implementation of an IP protocol stack, between the native IP and the local Kent, Atkinson [Page 7] Internet Draft Security Architecture for IP 30 July 1997 network drivers. Source code access for stack is not required in this context, making it appropriate for use with legacy systems. This is generally assumed to be implemented in hosts. c. The use of an outboard crypto processor is a common design feature of network security 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 (or both). Usually the device is IP addressable. When supporting a single host, it may be quite analogous to a BITS implementation, but in supporting a router or firewall, it is more like a security gateway. 4. Security Associations This section defines Security Association management requirements for all IPv6 implementations and for those IPv4 implementations that implement AH, ESP, or both. The concept of a "Security Association" (SA) is fundamental to IPsec. Both AH and ESP make use of SAs and a major function of Oakley/ISAKMP is the establishment and maintenance of Security Associations. All implementations of AH or ESP MUST support the concept of a Security Association as described below. The remainder of this section describes various aspects of Security Association management, defining required characteristics for SA policy management, traffic processing, and SA management techniques. 4.1 Definition and Scope A Security Association (SA) is a simplex "connection" that affords security services to the traffic carried by it. Security services are afforded to an SA by the use of AH, or of ESP, but not both. If both AH and ESP protection is applied to a traffic stream, then two (or more) SAs are created to afford protection to the traffic stream. To secure typical, bi-directional communication between two hosts (or between two security gateways), two Security Associations (one in each direction) are required. The combination of a Security Parameter Index (SPI), a Destination Address, and the security protocol identifier uniquely identifies a Security Association. In principle, the Destination Address may be a unicast address, an IP broadcast address, or a multicast group address. However, IPsec SA management mechanisms currently are defined only for unicast SAs. Hence, in the discussions that follow, SAs will be described in the context of point-to-point communication, even though the concept is applicable in the point-to-multipoint case as well. As noted above, two types of SAs are defined: transport mode and tunnel mode. A transport mode SA is a security association between Kent, Atkinson [Page 8] Internet Draft Security Architecture for IP 30 July 1997 two hosts. The security protocol header appears immediately after the IP header (and any options or extensions), and before any higher layer protocols (e.g., TCP or UDP). In the case of ESP, a tunnel mode SA provides security services only for these higher layer protocols, not for the IP header. In the case of AH, the protection is also extended to selected portions of the IP header (and options). For more details on the coverage afforded by AH, see the AH specification [KA97b]. A tunnel mode SA is essentially an SA applied to an IP tunnel. Whenever either end of a security association is a security gateway, the SA MUST be tunnel mode. So, an SA between two security gateways is always a tunel mode SA, as is an SA between a host and a security gateway. Two hosts MAY establish a tunnel mode SA between them. An SA involving a security gateway must be a tunnel SA to avoid potential problems with regard to fragmentation and reassembly, and in circumstances where multiple paths (e.g., via different routers or firewalls) exist to the same destination (behind the security gateway). For a tunnel mode SA, there is an "outer" IP header that specifies the IPsec processing destination, plus an "inner" IP header that specifies the (apparently) ultimate 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 higher layer protocols). If ESP is employed, the protection is afforded only to the tunneled packet, not to the outer header. 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, AH provides data origin authentication and connectionless integrity for IP datagrams (hereafter referred to as just "authentication"). The "precision" of the authentication service is a function of the granularity of the security association with which AH is employed, as discussed in Section 4.??. AH also offers an anti-replay (partial sequence integrity) service at the discretion of the receiver, to counter denial of service attacks. AH is an appropriate protocol to employ when confidentiality is not required (or is not permitted, e.g , due to government restrictions on encryption). AH also provides authentication for selected portions of the IP header, which may be necessary in some contexts. For example, if the integrity of an IP option or IPv6 extended header must be protected en route between sender and receiver, AH can provide this service. Kent, Atkinson [Page 9] Internet Draft Security Architecture for IP 30 July 1997 ESP always provides confidentiality for traffic. It also may optionally provide authentication (as defined above). If authentication is negotiated for an ESP SA, the receiver also may elect to enforce an anti-replay service with the same features as the AH anti-replay service. The scope of the authentication offered by ESP is narrower than for AH, i.e., the IP header "below" the ESP header is not protected. If only the upper layer protocols need to be authenticated, then ESP is an appropriate choice and is more space efficient than nested use of AH. 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). 4.3 Combining Security Associations The IP datagrams transmitted over an individual security association are afforded protection by exactly one security protocol, either AH or ESP. Sometimes a security policy may call for a combination of services and service sitings for a particular traffic flow that is not achievable with a single SA. In such instances it will be necessary to employ multiple SAs to implement the required security policy. The term "security association bundle" or "SA bundle" is applied to a sequence of SAs through which traffic must be processed to satisfy a security policy. (Note that the SAs that comprise a bundle need may terminate at different endpoints.) Security associations may be combined into bundles in two ways: transport adjacency and iterated tunneling. Transport adjacency refers to applying more than one security protocol to the same IP datagram, without invoking tunneling. This approach to combining AH and ESP allows for only one level of combination; further nesting yields no added benefit since the processing is performed at one IPsec instance the (ultimate) destination. Iterated tunneling refers to the application of multiple layers of security protocols effected through tunneling. This approach allows for multiple levels of nesting, since each tunnel can terminate at a different IPsec site along the path. These two approaches also can be combined, i.e., an SA bundle could be constructed from one tunnel mode SA and one or two transport mode SAs, applied in sequence. For transport mode SAs, only one ordering of security protocols seems appropriate. AH is applied to both the upper layer protocols and (parts of) the IP header. Thus if AH is used in a transport mode, in Kent, Atkinson [Page 10] Internet Draft Security Architecture for IP 30 July 1997 conjunction with ESP, AH should appear as the first header after IP, then ESP. In that context, AH is applied to the ciphertext output of ESP. In contrast, for tunnel mode SAs, one can imagine uses for various orderings of AH and ESP. 4.4 Security Association Databases Many of the details associated with processing IP traffic in an IPsec implementation are largely a local matter, not subject to standardization. However, some external aspects of the processing must be standardized, to ensure interoperability and to provide a minimum management capability that is essential for productive use of IPsec. This section describes a general model for processing IP traffic relative to security associations, in support of these interoperability and functionality goals. 4.4.1 The Security Policy Database (SPD) Ultimately, a security association is a management construct used to enforce a security policy in the IPsec environment. 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 does specify certain minimum management functionality that must be provided, to allow a user or system administrator to control how IPsec is applied to traffic transmitted or received by a host or transiting a security gateway. An SPD must discriminate among traffic that is afforded IPsec protection and traffic that is allowed to bypass IPsec. For any (outbound) datagram three processing choices are possible: discard, bypass, protect. The first choice refers to traffic that is not allowed to exit the host or traverse the security gateway, at all. The second choice refers to traffic that is allowed to pass without IPsec protection. The third choice refers to traffic that is afforded IPsec protection, and for such traffic the SPD must specify the security services to be provided, protocols to be employed, algorithms to be used, etc. For every IPsec implementation, there MUST be some form of administrative interface that allows a user or system administrator to manage the SPD. 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. The SPD contains an ordered list of policy entries that define the security services, protocols, and algorithms that will be employed Kent, Atkinson [Page 11] Internet Draft Security Architecture for IP 30 July 1997 for IP traffic that processed by the IPsec implementation. Each policy entry is keyed by one or more selectors that define the set of IP traffic encompassed by this policy entry. The selectors are the sort of information that could be used to create a socket in a host. These define the granularity of SAs. Each entry also includes an indication of whether traffic matching this policy will be bypassed, discarded, or subject to IPsec processing. Finally, the entry includes an SA (or SA bundle) specification, listing the IPsec protocols, modes, and algorithms to be employed, including nesting requirements. For example, an entry may call for all matching traffic to be protected by ESP in transport mode using 3DES-CBC with explicit IV, nested inside of AH in tunnel mode using HMAC/SHA-1. As described below in Section 4.4.3, selectors may include "wildcard" entries and hence the selectors for two entries may overlap. Thus, to ensure consistent, predictable processing, SPD entries must be ordered. Note that the SPD does not map traffic to specific SAs or SA bundles. Instead, it can be thought of as the reference database for security policy, to be consulted when no existing SA or SA bundle matches the requirements for traffic. In a host IPsec implementation based on sockets, the SPD will be consulted whenever a new socket was created, to determine what, if any, IPsec processing will be applied to the traffic that will flow on that socket. The SPD also will be consulted when any IPsec implementation is the target of an SA establishment request from another IPsec implementation, e.g., using Oakley/ISAKMP. An IPsec implementation in a security gateway, BITW or BITS context, it usually will be necessary to examine every outbound packet to determine what, Ipsec processing, if any, is needed. In these instances, a second database is required. The Security Association Map is the database that maps selectors to existing SAs (or SA bundles) and will be consulted on a per-packet basis (for outbound traffic). Section 4.4.2 defines the requirements for this database 4.4.2 Security Association Map (SAM) The Security Association Map (SAM) is a nominal database used to map outbound traffic IP to a security association (or to an SA bundle) when the IPsec implementation does not make use of a socket-based interface. This likely to be the sort of interface encountered for most security gateways, BITW and BITS IPsec implementations. This document does not specify a required form for the database nor an interface. It provides an illustration of database entries and entry contents as a guide for implementors. Like the SPD, this is an ordered database in which each entry is keyed by one or more selectors that define the granularity of SAs (or SA bundles). Unlike the SPD, entries in the SAM refer to existing Kent, Atkinson [Page 12] Internet Draft Security Architecture for IP 30 July 1997 SAs, or define traffic that is to be bypassed or discarded. Thus each entry that calls for IPsec processing points to an ordered list of SAs (to support SA bundles) that will be applied to the traffic. When SAs are created, an entry is made in the SAM, and when SAs expire or are otherwise explicitly terminated, entries in the SAM are deleted. The selectors used in the SAM are the same as those used in the SPD, and are defined below in Section 4.4.3. 4.4.3 Selectors An SA (or SA bundle) may be fine-grained or coarse-grained, depending on the selectors used to define the set of (outbound) traffic for the SA. For example, all traffic between two hosts may be carried via a single SA, and afforded a uniform set of security services. Alternatively, traffic between a pair of hosts might be spread over multiple SAs, depending on the applications being used (as defined by the Next Protocol and Port fields), with different security services offered for 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 for SA management to facilitate control of SA granularity: - Destination IP Address(es): this may be a single IP address (unicast or multicast group), an enumerated list of addresses, or a wildcard (mask) address. The last two are required to support more than one destination system sharing the same SA (behind a security gateway). [REQUIRED for all implementations] - Source IP Address(es): this may be a single IP address, an enumerated list of addresses, or a wildcard (mask) address. The last two are required to support more than one source system sharing the same SA (e.g., behind a security gateway or in a multihomed host). [REQUIRED for all implementations] - UserID: a user identifier from the operating system. (The use of a User ID as a SA selector is sometimes referred to as "user-oriented keying.") [REQUIRED for host implementations, unless the layering of the implementation precludes access to this information, e.g., a BITS implementation need not support this selector.] - Data sensitivity level: (IPSO/CIPSO labels) [REQUIRED for all systems providing label-based security, OPTIONAL for all other systems] - Transport Layer Protocol (formerly Next Protocol): Both the IPv4 "Protocol" and the IPv6 "Next Header" fields may not contain the Transport Protocol due to the presence of IP extension headers. Kent, Atkinson [Page 13] Internet Draft Security Architecture for IP 30 July 1997 These fields could contain a Routing Header, AH, ESP, Fragmentation Header, Destination Options, Hop-by-hop options, etc. To address the question of which Protocol/Next-Header to use when there is more than one, this selector has been defined to be the Transport Layer Protocol selector. This is based on the assumption that it is not necessary to allow use (as selectors) of Protocol or Next Header fields other than the one containing the Transport Protocol field. It is assumed to be unlikely that a policy administrator might want to map a security association to a communication association using a Protocol or Next Header field with an extension header value. This means, for example, it will not be possible to specify that "Any packet with a routing header (which defines a source route) must be authenticated so that the destination can tell whether or not to accept the packet." [REQUIRED for all implementations] NOTE: To locate the transport protocol, a system has to chain through the packet headers checking the "Next Protocol" field until it encounters either one it recognizes as a transport protocol or until it reaches one that isn't on its list of extension headers. - Source and Destination (TCP/UDP) Ports: These may be individual UPD or TCP port values, an enumerated list of ports, or a wildcard (mask) port. (The use of the Next Protocol field and the Source and/or Destination Port fields (in conjunction with the Source and/or Destination Address fields), as an SA selector is sometimes referred to as "session-oriented keying.") [REQUIRED for all implementations] - IPv6 Priority (from IP header): This may be expressed as ??? [REQUIRED for all systems that implement IPv6] - IPv6 Flow Label (from IP header): This may be expressed as ???. The IPv6 spec (RFC 1883) calls for all datagrams for a given IPv6 Flow Label to have the same Source Address, Destination Address, Hop-by-hop Options header, and Routing Header. The Flow Label may be assigned on a per socket basis. It would then be correlated with the Source/Destination and could be used to provide finer granularity selection of security association(s). [REQUIRED for all systems that implement IPv6] 4.4.4 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 entry in the SAD is indexed by a destination IP address,IPsec protocol type, and SPI, for use in inbound IPsec packet processing. For outbound processing, entries are pointed to by entries in the SAM. The following parameters are associated with Kent, Atkinson [Page 14] Internet Draft Security Architecture for IP 30 July 1997 each entry in the SAD. This description does not purport to be a MIB, but only a specification of the minimal data items required to support an SA in an IPsec implementation. - Destination IP address: the IPv4 or IPv6 address used as an index for SA lookup in this database. [REQUIRED for all implementations] - IPsec Protocol: AH or ESP. Specifies the IPsec protocol to be applied to the traffic on this SA. [REQUIRED for all implementations] - SPI: the 32-bit value used to distinguish among different SAs terminating at the same destination and using the same IPsec protocol. [REQUIRED for all implementations] - IPsec protocol mode: tunnel or transport. Indicates which mode of AH or ESP is applied to traffic on this SA. [REQUIRED for all implementations] - Replay Protection: selection/non-selection by receiver and window size. [REQUIRED for all implementations] - AH Authentication algorithm. [REQUIRED for AH implementations] - ESP Encryption algorithm and mode. [REQUIRED for ESP implementations] - ESP authentication algorithm. If the authentication service is not selected, this field will be null. [REQUIRED for ESP implementations] - Lifetime of this Security Association: a time interval after which an SA must be rekeyed or terminated, plus an indication of which of these actions should occur. [REQUIRED for all implementations] 4.5 Basic Combinations of Security Associations There are 4 obvious examples of combinations of security associations. Support for each of these is required. Note that there may be other uses of IPSEC; but these appear to be the most critical ones, ones that all compliant (host/security gateway) implementations are required to support. The diagrams and text below describe the basic cases. The legend for the diagrams is: Kent, Atkinson [Page 15] Internet Draft Security Architecture for IP 30 July 1997 ==== = security association (AH or ESP, transport or tunnel) ---- = connectivity (or if so labelled, administrative boundary) Hx = host x SGx = security gateway x X* = X supports IPSEC NOTE: The security associations below can be either AH or ESP. The mode (tunnel vs transport) is determined by the nature of the endpoints. For host-to-host SAs, the mode can be either transport or tunnel. For host-to-gateway SAs and gateway-to-gateway SAs the mode can ONLY be tunnel. Section 5.4, "Required Support for AH and ESP Combinations", provides additional detail on the required support for different combinations of IPsec protocols and modes. Case 1. The case of providing end-to-end security between 2 hosts across the Internet (or an Intranet). ==================================== | | H1* ------ (Inter/Intranet) ------ H2* Case 2. This case includes creating virtual private networks. =========================== | | ---------------------|---- ---|----------------------- | | | | | | | H1 -- (Local --- SG1* |--- (Internet) ---| SG2* --- (Local --- H2 | | Intranet) | | Intranet) | -------------------------- --------------------------- admin. boundary admin. boundary Case 3. This case takes case 2 and adds end-to-end security between the sending and receiving hosts =============================================================== | | | ========================= | | | | | ---|-----------------|---- ---|-------------------|--- | | | | | | | | | H1* -- (Local --- SG1* |-- (Internet) --| SG2* --- (Local --- H2* | | Intranet) | | Intranet) | -------------------------- --------------------------- admin. boundary admin. boundary Kent, Atkinson [Page 16] Internet Draft Security Architecture for IP 30 July 1997 Case 4. This covers the situation where a remote host (H1) is using the Internet to reach an organization's firewall (SG1) and to then gain access to some server or other machine (H2). The remote host could be a mobile host (H1) dialing up to a local PPP/ARA server (not shown) on the Internet and then crossing the Internet to the home organization's firewall (SG1), etc. The details of support for this case, (how H1 locates SG1, authenticates it, and verifies its authorization to represent H2) are discussed in Section 4.4.3, "Locating a Security Gateway". ====================================================== | | |============================== | || | | || ---|----------------------|--- || | | | | H1* ----- (Internet) ------| SG1* ---- (Local ----- H2* | ^ | Intranet) | | ------------------------------ could be dialup admin. boundary (optional) to PPP/ARA server 4.6 SA Establishment 4.6.1 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 quite practical in small, static environments but they do not scale well. It is not a viable medium-term or long-term approach, but might be appropriate and useful in some environments in the near-term. For example, a company could create a Virtual Private Internet (VPI) using IPsec in security gateways at several sites. If the number of sites is small, and since all the sites come under the purview of a single administrative domain, this is likely to 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 encrypting 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. Kent, Atkinson [Page 17] Internet Draft Security Architecture for IP 30 July 1997 4.6.2 Automatic Techniques -- Key Mgt Protocol Requirements Widespread deployment and use of IP security requires an Internet- standard, scalable, key management protocol. This protocol should not be limited to supporting IP security. This protocol should be compatible with the IETF's DNS Security work and should include the ability to obtain bootstrapping information (e.g. keys, addresses) from the Secure DNS as a mandatory-to-implement feature. Signed host keys to the Domain Name System [EK96] The DNS keys enable the originating party to authenticate key management messages with the other key management party using an asymmetric algorithm. A standards-track key management protocol for use with IP Security MUST provide the property of "Perfect Forward Secrecy" as a mandatory-to- implement feature. Further, any standards-track key management protocol MUST permit the secure negotiation or secure identification of the Security Association attributes to all parties of that Security Association. 4.6.3 Locating a Security Gateway This section discusses the 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. [NOTE: This topic is still under discussion so the text below describes the problem and some proposed approaches rather than a final agreed-upon solution.] Suppose you have a remote host (H1) which is using the Internet to gain access to a server or other machine (H2) and there is a primary security gateway (SG1), e.g., a firewall, through which the H1's traffic must pass. Suppose also that there is a secondary security gateway (SG2) available as a backup path. An example of this situation would be a mobile host (Road Warrior) dialing up to a local PPP/ARA server on the Internet and then crossing the Internet to the home organization's firewall (SG1), etc. The following discussion also applies to the situation where the remote entity setting up the security associations to SG1 (or SG2) is H1's security gateway (SG3) acting on behalf of H1. To support this kind of situation, H1 MUST be able to create a communication association to H2 that makes use of two SAs -- a tunnel mode SA from H1 to to SG1 and a transport mode SA from H1 to H2. The diagram below illustrates this. The legend for the diagram is: ==== = security association (AH or ESP, transport or tunnel) ---- = connectivity (or if so labelled, administrative boundary) Hx = host x SGx = security gateway x X* = X supports IPSEC Kent, Atkinson [Page 18] Internet Draft Security Architecture for IP 30 July 1997 ====================================================== | | |============================== | || | | || ---|----------------------|--- || | | | | H1* ----- (Internet) ------|-SG1* ---- (Local ----- H2* | ^ | | Intranet) | | | | | | | -----------|-SG2* --------- | | ------------------------------ could be dialup admin. boundary (optional) to PPP/ARA server This situation raises several questions: 1. How does H1 know/learn about the existence of the security gateway SG1? 2. How does it authenticate SG1, and once it has authenticated SG1, how does it confirm that SG1 has the "right" to represent H2? 3. How does SG1 authenticate H1 and verify that H1 is authorized to contact H2? 4. How does H1 know to use SG2 as an alternate path to H2 when something disrupts connectivity via SG1? There are appear to be 2 main instances where this situation would arise. 1. H1 is the system of an individual associated with the organization administering SG1/SG2/H2, e.g., an employee. H1 might be remotely accessing a home system H2 through the firewall SG1 and the H1 to SG1 connection could be part of a Virtual Private Network. In this case, it is reasonable for H1 to be pre-configured with the requisite information about SG1, SG2, and H2. To do this, H1 MUST have an administrative interface that allows the user/administrator to specify: o the SAs to use for a communication association to H2 -- a tunnel mode SA from H1 to SG1 and a transport mode SA from H1 to H2. o the SAs to use if the path to H2 via SG1 fails -- a tunnel mode SA from H1 to SG2 and a transport mode SA from H1 to H2. o the requisite information for locating, authenticating, and verifying the authorization of SG1 and SG2. 2. H1 is the system of a person who's been told about system H2 at the organization administering SG1/H2, but who's otherwise Kent, Atkinson [Page 19] Internet Draft Security Architecture for IP 30 July 1997 unconnected to that organization. When H1 tries to contact H2, several things will need to happen to dynamically provide H1 with the requisite information: a) H1 needs to find out about SG1. b) H1 must have a mechanism that allows it to authenticate SG1 and verify that SG1 is authorized to represent H2. c) SG1 has to have a mechanism to authenticate H1 and verify that H1 is authorized to contact H2. d) If the path via SG1 is unusable for some reason, (SG1 is down, source routing, etc.), then H1 must know to use SG2 and then (b), (c), and (d) apply to SG2. To address this situation, an approach has been proposed that uses a new "key exchange" record (KX) in the Secure Domain Name System (DNS) as a mechanism to allow a host/gateway to determine the set "of authorised remote key exchanger systems" for a given destination. (See Randall Atkinson's Internet Draft, "Key Exchange Delegation Record for the DNS" for details.) In both cases: 1. H1 MUST be able to use SG1's public key certificate to authenticate that the connection is to the real SG1. The same applies to H1 authenticating SG2. 2. SG1 MUST be able to use H1's public key certificate to authenticate H1. The same applies to SG2 authenticating H1. 3. SG1 and SG2 MUST be able to check H1's authorization to contact H2. NOTE: If H2 were outside the firewall/security gateway perimeter, it might be possible to handle this situation by use of SSL [need reference]. 4.7 Security Associations and Multicast The receiver-orientation of the Security Association implies that, in the case of unicast traffic, the destination system will normally select the SPI value. By having the destination select the SPI value, there is no potential for manually configured Security Associations that conflict with automatically configured (e.g. via a key management protocol) Security Associations. For multicast traffic, there are multiple destination systems but a single destination multicast group, so some system or person will need to select SPIs on behalf of that multicast group and then communicate the 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 Kent, Atkinson [Page 20] Internet Draft Security Architecture for IP 30 July 1997 that group when a symmetric cryptographic algorithm is in use. In that case, the receiver only knows that the message came from a system knowing the security association data for that multicast group. A receiver cannot generally authenticate which system sent the multicast traffic when symmetric algorithms (e.g. DES, IDEA) are in use. Multicast senders SHOULD use a separate Security Association for each sender to the multicast group when an asymmetric cryptographic algorithm is in use. In this last case, the receiver can know the specific system that originated the message. Multicast key distribution was an active research area in the published literature at the time this specification was published. For multicast groups having relatively few members, manual key distribution or multiple use of existing unicast key distribution algorithms such as modified Diffie-Hellman appears feasible. For very large groups, new scalable techniques will be needed. 5. IPSEC Traffic Processing 5.1 Outbound IPsec Traffic Processing 5.1.1 Selecting an SA or SA Bundle - socket-based host implementations - SAM-based mapping - sequential application of SAs to traffic - fragmentation 5.1.2 Header construction for tunnel mode [There are a variety of unresolved issues here. The text below is included as a starting place for further discussion. For example, RFC 1853 may be an appropriate basis for this discussion, for outbound processing] This section describes the handling of the inner and outer IP headers, extension headers, and options for AH and ESP tunnels. This includes how to construct the encapsulating (outer) IP header, how to handle fields in the inner IP header, and what other actions should be taken. This description is based on the situation below with H1 sending IP traffic to H2 and an IPsec tunnel between SG1 and SG2. ==== = security association (AH or ESP, tunnel) ---- = connectivity Hx = host x Gx = gateway x SGx = security gateway x X* = X supports IPSEC =========================== | | | | Kent, Atkinson [Page 21] Internet Draft Security Architecture for IP 30 July 1997 H1 -- G1 -- G2 - SG1* -- G3 -- G4 -- G5--- SG2* -- G6 -- G7 -- H2 This processing is a function of: a) which stage of processing is occurring: - outbound (the sender at the beginning of the tunnel) - inbound (the receiver at the end of the tunnel) b) IP version c) header/option fields d) security policy The tables in the following sub-sections show the handling for the different header/option fields using the following "actions": constructed-indep = the value in the outer field is constructed independently of the value in the inner field. constructed-calc = for outbound packets, the value in the outer field is computed from the inner field and possibly some other information. For inbound packets, the value in the inner field is computed from the outer field and possibly some other information. configured = the derivation of the value in the field is "configurable" by the administrator to one of several choices, e.g., outer header's TOS can be (a) "copied" from the inner field, (b) hardwired by the configuration to a particular value, (c) "filtered", i.e., the administrator defines a range such that within (or outside of) the range, the value in the inner field is used; and outside (or within) the range, a configuration-defined value is used. copied = the value in the outer field is always copied as is from the inner field. never copied = the value in the inner field is never copied to the outer field. consumed = the outer field is ignored/discarded. nc = no change. 5.1.2.1 IPv4 -- Header construction for tunnel mode Kent, Atkinson [Page 22] Internet Draft Security Architecture for IP 30 July 1997 <-------- How Outer Hdr Relates to Inner Hdr ---------> Outbound Inbound ------------------------------ -------------------- IPv4 Outer hdr Inner hdr Outer header Inner Header fields version constructed-indep(11) nc consumed nc header length constructed-calc nc consumed nc TOS configured (6) nc consumed nc total length constructed-calc nc consumed nc ID constructed-indep nc consumed nc flags (DF,MF) constructed-calc (8) nc consumed nc fragmt offset constructed-calc nc consumed nc TTL configured (7) nc consumed conf (5) protocol AH, ESP, routing hdr nc consumed nc checksum constructed-calc nc consumed constr-calc src address constructed-calc (9) nc consumed nc dest address constructed-calc (9) nc consumed nc Options sec option copied nc consumed nc loose src route configured (1) nc consumed conf (1) strict src route configured (1) nc consumed conf (1) record route configured (10) nc consumed cnstr-calc(10) timestamp copied nc consumed cnstr-calc (2) end constructed-calc (3) nc consumed nc nop constructed-calc (4) nc consumed nc (1) loose and strict source routing for IPv4 raise several issues: a) Should source routing information from the inner IP header be copied to the outer header? b) If yes, how does SG1 figure out how to construct the outer IP header, i.e., what part of the source route comes before SG2 and should be copied to the outer header? c) If yes, should SG2 copy the recorded route information from the outer header to the inner header? For IPv4, SG1 can be configured with: a) 2 choices for outbound processing: o outer header is constructed from remaining hops in inner routing header with SG2 as the last destination. If part of the source route is "beyond" SG2, then SG1 needs to construct an outer header containing just the part of the source route that extends up to SG2, inserting SG2 as the last hop (destination). [Need to specify how SG1 figures out how much of the source route belongs in the outer header, e.g., use an ICMP message from Kent, Atkinson [Page 23] Internet Draft Security Architecture for IP 30 July 1997 SG2] o outer routing header is constructed based on security policy specification for the tunnel. b) 3 choices for the inbound processing: o inner routing header is updated to know to skip over "used" hops in outer header but the recorded route information is not copied over and the corresponding information for the "used" hops is zero'd in the inner header. o inner routing header is updated to know to skip over "used" hops in outer header and the recorded route information is copied over to the inner header. o inner routing header is constructed based on security policy specification for the tunnel. For IPv6, SG1 can be configured with: a) 2 choices for outbound processing: o outer (version 0) routing header is constructed from remaining hops in inner routing header. o outer (version 0) routing header is constructed based on security policy specification for the tunnel. b) 3 choices for the inbound processing: o inner routing header is updated to know to skip over "used" hops in outer header but the recorded route information is not copied over and the locations where the recorded route information would normally be placed for the "used" hops is zero'd in the inner header. o inner routing header is updated to know to skip over "used" hops in outer header and the recorded route information is copied over to the inner header. o inner routing header is constructed based on security policy specification for the tunnel. (2) copy the inner fields to the outer fields. At the tunnel destination, the inner fields MUST be updated with any additional information recorded on outside header. (3) for outside field, this is inserted if needed based on whatever else was copied. At the tunnel destination, it is not changed as any changes made to the inner option fields cannot change the length of an option. (4) constructed, based on alignment and options copied (5) [needs to be coordinated between src endpoint and dst endpoint] The following steps assume that IPSEC does the decapsulating of the packet and then passes it to the IP forwarding code where the decrementing of TTL occurs. Accordingly no decrementing is done in IPSEC. (a) if the outer TTL was a configured number, leave inner TTL as is. Kent, Atkinson [Page 24] Internet Draft Security Architecture for IP 30 July 1997 (b) if inner TTL was copied to outer field, replace inner TTL with outer TTL. (6) [needs to be coordinated between src endpoint and dst endpoint] Choices = copy from inner header, use configured value, "filter" (administrator defines a range such that within (outside of) the range, the inner IP header value is used; and outside (inside) the range, a configuration-defined value is used.) (7) choices = copy from inner header, use configured value. MUST be consistent with (5). (8) see Section Y on PMTU/DF. (9) src and dst addresses depend on the SA, which is used to determine the dst address which in turn determines which src address (net interface) is used to forward the packet. (10) whether to copy the inner fields to outer fields is "configurable"; but "always" update the inner fields with the hops (if any) recorded in the outer fields. (11) the IP version in the encapsulating header can be different from the value in the inner header. 5.1.2.2 IPv6 -- Header construction for tunnel mode <-------- How Outer Hdr Relates to Inner Hdr ---------> Outbound Inbound ------------------------------ -------------------- IPv6 Outer hdr Inner hdr Outer header Inner Header fields version constr.-indep (11) nc consumed nc priority configured (6) nc consumed nc flow id configured (6) nc consumed nc len constructed-calc nc consumed nc next header AH,ESP,routing hdr nc consumed nc hop count configured (7) nc consumed conf (5) src address constructed-calc(9) nc consumed nc dest address constructed-calc(9) nc consumed nc Extension headers destination options pad 1 constructed-calc(4) nc consumed nc pad N constructed-calc(4) nc consumed nc EID never copied nc consumed nc hop by hop options pad 1 constructed-calc(4) nc consumed nc pad N constructed-calc(4) nc consumed nc jumbogram copied but adjusted nc consumed nc fragmentation never copied (12) nc consumed (12) nc routing configured (1) nc consumed conf (1) AH/ESP constr.-indep(13) nc consumed nc (1), (4)-(7), (9), (11) see table notes from Section 4.3.1.4.1, "IPv4 -- Header construction for tunnel mode" Kent, Atkinson [Page 25] Internet Draft Security Architecture for IP 30 July 1997 (12) in tunnelling, the new packet can be fragmented. At the tunnel end, the outer header should have been removed by re-assembly. (13) the outer header is constructed for the tunnel and is not derived from any inner header AH/ESP In IPv6 source routing, the system routes the packet (reading the IPv6 header) until the current router = the destination in the IPv6 header, then the current router processes the next header. At each router, the next hop in the routing header's chain of source routes is swapped with the IPv6 destination field; the Segments Left field is decremented by 1. The system routes the packet onward (goes no further up the stack) until the Segments Left field is 0. Any headers that are after the RH are processed only when Segments Left field is 0. Suppose, you have the sample headers/options below RH = routing header version 0 (processed by routers listed in RH): A B ---- ---- IPv6 IPv6 RH AH/ESP AH/ESP RH TCP TCP In case A, the AH/ESP header gets processed only after the packet reaches the final destination (RH Segments Left = 0). This is the typical case for end-to-end AH/ESP. In case B, the AH/ESP header will get processed at every router listed in the RH (they get copied to the IPv6 header). In a typical case, the AH/ESP header is validated and replaced with a different SA(s) at each hop listed in the source route. 5.2 Processing Inbound IPsec Traffic Processing of inbound IPsec traffic generally is easier that processing of outbound processing. This is because each inbound IP datagram to which IPsec processing will be applied is identified by the appearance of the AH or ESP values in the IP Next Protocol field (or of AH or ESP as an extension header in the IPv6 context). Moreover, mapping the IP datagram to the appropriate SA is simplified because of the presence of the SPI in the AH or ESP header. - mapping packets to a SAD entry - iterative processing for nested SAs - reassembly 6. ICMP processing (relevant to IPsec) - anything other than PMTU issues? Kent, Atkinson [Page 26] Internet Draft Security Architecture for IP 30 July 1997 6.1 PMTU/DF processing 6.1.1 DF bit In cases where a system (host or gateway) adds an encapsulating header (ESP or AH tunnel), it MUST support the option of copying the DF bit from the original packet to the encapsulating header (and processing ICMP PMTU messages). This means that it MUST be possible to configure the system's treatment of the DF bit (set, clear, copy from encapsulated header) for each interface. (See Appendix B for rationale.) 6.1.2 Path MTU Discovery (PMTU) [This section assumes PMTU processing based on inputs from possibly untrusted intermediate routers. We must consider whether such processing is optionally supported, with the alternative of processing based only on information from trusted routers (see Richardson I-D on this topic).] This section discusses IPsec handling for Path MTU Discovery messages. ICMP PMTU is used here to refer to an ICMP message for: IPv4: - Type = 3 (Destination Unreachable) - Code = 4 (Fragmentation needed and DF set) - Next-Hop MTU in the low-order 16 bits of the second word of the ICMP header (labelled "unused" in RFC 792), with high-order 16 bits set to zero IPv6 (RFC 1885): - Type = 2 (Packet Too Big) - Code = 0 (Fragmentation needed and DF set) - Next-Hop MTU in the 32 bit MTU field of the ICMP6 message 6.1.2.1 Propagation of PMTU The amount of information returned with the ICMP PMTU message (IPv4 or IPv6) is limited and this affects what selectors are available for use in further propagating the PMTU information. (See Appendix B for more detailed discussion of this topic.) o PMTU message with 64 bits of IPSEC header -- If the ICMP PMTU message contains only 64 bits of the IPSEC header (minimum for IPv4), then a security gateway MUST support the following options on a per SPI/SA basis: a. if the originating host(s) can be determined, send the PMTU information to all the possible originating hosts. Kent, Atkinson [Page 27] Internet Draft Security Architecture for IP 30 July 1997 b. if the originating host(s) cannot be determined, store the PMTU with the SA and wait until the next packet(s) arrive from the originating host(s) for the relevant security association. If the packet(s) are bigger than the PMTU, drop the packet(s), and compose ICMP PMTU message(s) with the new packet(s) and the updated PMTU, and send the ICMP message(s) about the problem to the originating host(s). Retain the PMTU information for any message that might arrive subsequently (until ???) o PMTU message with >64 bits of IPSEC header -- If the ICMP message contains more information from the original packet, e.g., the 576 byte minimum for IPv6, then there MAY be enough information to immediately determine to which host to propagate the ICMP/PMTU message and to provide that system with a 5-selector pointer for storing/updating the PMTU. Under such circumstances, a security gateway MUST generate an ICMP PMTU message immediately upon receipt of an ICMP PMTU from further down the path. o Distributing the PMTU to the Transport Layer -- The host mechanism for getting the updated PMTU to the transport layer is unchanged, as specified in RFC 1191 (Path MTU Discovery). 6.1.2.2 Calculation of PMTU The calculation of PMTU from an ICMP PMTU MUST take into account the addition of any IPSEC header -- ESP or AH transport, or ESP or AH tunnel. (See Appendix B for discussion of implementation issues.) 6.1.2.3 Granularity of PMTU processing In hosts, the granularity with which ICMP PMTU processing can be done differs depending on the implementation situation. Looking at a host, there are 3 situations that are of interest with respect to PMTU issues (See Appendix B for detailed discussion of this issue): a. Integration of IPSEC into the native IP implementation b. Bump-in-the-stack implementations, where IPSEC is implemented "underneath" an existing implementation of a TCP/IP protocol stack, between the native IP and the local network drivers c. No IPSEC implementation -- This case is included because it is relevant in cases where a security gateway is sending PMTU information back to a host. Only in case (a) can the PMTU data be maintained at the same granularity as communication associations. In (b) and (c), the IP layer will only be able to maintain PMTU data at the granularity of source and destination IP addresses (and optionally ToS), as described in RFC 1191. This is an important difference, because more than one communication association may map to the same source and Kent, Atkinson [Page 28] Internet Draft Security Architecture for IP 30 July 1997 destination IP addresses, and each communication association may have a different amount of IPSEC header overhead (e.g., due to use of different transforms or different algorithms). Implementation of the calculation of PMTU and support for PTMUs at the granularity of individual communication associations is a local matter. However, a socket-based implementation of IPSEC in a host SHOULD maintain the information on a per socket basis. Bump in the stack systems MUST pass an ICMP PMTU to the host IP implementation, after adjusting it for any IPSEC header overhead added by these systems. The calculation of the overhead SHOULD be determined by analysis of the SPI and any other selector information present in a returned ICMP PMTU message. 6.1.2.4 PMTU Aging In all systems (host or gateway) implementing IPSEC and maintaining PMTU information, the PMTU associated with a security association (transport or tunnel) MUST be "aged" and some mechanism put in place for updating the PMTU in a timely manner, especially for discovering if the PMTU is smaller than it needs to be. A given PMTU has to remain in place long enough for a packet to get from the source end of the security association to the system at the other end of the security association and propagate back an ICMP error message if the current PMTU is too big. Systems SHOULD use the approach described in the Path MTU Discovery document (RFC 1191, Section 6.3), which suggests periodically resetting the PMTU to the first-hop data-link MTU and then letting the normal PMTU Discovery processes update the PMTU as necessary. The period SHOULD be configurable. 7. Algorithm Descriptions [To be supplied -- refers to separate algorithm documents] 8. Usage Scenarios [To be supplied. including s subsection on special processing in an information flow security environment, e.g., MLS hosts and networks.] 9. Auditing Not all systems that implement IPsec will implement auditing. However, if a system supports auditing, then the IPsec implementation MUST also support auditing and MUST allow a system administrator to enable or disable auditing for IPsec. For the most part, the granularity of auditing is a local matter. However, several auditable events are identified in the AH and ESP specifications and for each of these events a minimum set of information that SHOULD be included in an audit log is defined. Additional information also MAY be included in the audit log for each of these events, and additional events, not explicitly called out in this specification, also MAY Kent, Atkinson [Page 29] Internet Draft Security Architecture for IP 30 July 1997 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. 10. Use in systems supporting information flow security [To be supplied] 11. Performance Issues The use of IPsec imposes computational performance costs on the hosts or security gateways that implement these protocols. These costs are associated with the computation of integrity check values, encryption and decryption,and added per-packet handling. These per-packet computational costs will be manifested by increased latency and, possibly, reduced throughout. Use of security association management protocols, especially ones that employ public key cryptography, also adds computational performance costs to use of IPsec. These per- association computational costs will be manifested in terms of increased latency in association establishment. For many hosts, it is anticipated that software-based cryptography will not appreciably reduce throughput, but hardware may be required for security gateways (since they represent aggregation points), and for some hosts. The use of IPsec also imposes bandwidth utilization costs on transmission, switching, and routing components of the Internet infrastructure, components not implementing IPsec. This is due to the increase in the packet size resulting from the addition of AH and/or ESP headers, ESP tunneling (which adds a second IP header), and the increased packet traffic associated with key management protocols. It is anticipated that, in most instances, this increased bandwidth demand will not noticeably affect the Internet infrastructure. However, in some instances, the effects may be significant, e.g., transmission of ESP encrypted traffic over a dialup link that otherwise would have compressed the traffic. Note: As discussed above, compression can still employed at layers above IP. There is an IETF working group (IP Payload Compression Protocol (ippcp)) working on "protocol specifications that make it possible to perform lossless compression on individual payloads before the payload is processed by a protocol that encrypts it. These specifications will allow for compression operations to be performed prior to the encryption of a payload by IPsec protocols. 12. Conformance Requirements [Will be a summary] Kent, Atkinson [Page 30] Internet Draft Security Architecture for IP 30 July 1997 13. Security Considerations [To be supplied] 14. Differences from RFC 1825 [To be supplied] Acknowledgements Many of the concepts embodied in this specification were derived from or influenced by the US Government's SP3 security protocol, ISO/IEC's NLSP, the proposed swIPe security protocol [SDNS, ISO, IB93, IBK93], and the work done for SNMP Security and SNMPv2 Security. For over 2 years, this document has evolved through multiple versions and iterations. During this time, many people have contributed significant ideas and energy to the process and the documents themselves. The authors would like to thank Karen Seo for providing extensive help in the review, editing, background research, and coordination for this version of the specification. The authors would also like to thank the members of the IPSEC and IPng working groups, with special mention of the efforts of (in alphabetic order): Steve Bellovin, Steve Deering, James Hughes, Phil Karn, Frank Kastenholz, Perry Metzger, David Mihelcic, Hilarie Orman, William Simpson, Harry Varnis, and Nina Yuan. Kent, Atkinson [Page 31] Internet Draft Security Architecture for IP 30 July 1997 Appendix A -- Glossary A.1. Relevant Network Security Terminology 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., [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 often is a network interface on a host security gateway. 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 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.) 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. Kent, Atkinson [Page 32] Internet Draft Security Architecture for IP 30 July 1997 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 the connectionless integrity service. 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 messages. Although authentication and integrity services often are cited separately, in practice they are intimately connected and almost always offered in tandem. 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 enforced through the use of AH or ESP. Security Gateway A security gateway is an intermediate system that acts as the communications interface between two networks. The set of hosts (and nets) on the external side of the security gateway is viewed as untrusted (or less trusted), while the networks and hosts and on the internal side are viewed as trusted (or more trusted). 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." The combination of an SPI, a destination address, and a security protocol uniquely identifies a security association (SA, see above). The SPI is carried in AH and ESP protocols 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 Kent, Atkinson [Page 33] Internet Draft Security Architecture for IP 30 July 1997 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] Trusted Subnetwork A subnetwork containing hosts and routers that trust each other not to engage in active or passive attacks. There also is an assumption that the underlying communications channel (e.g., a LAN or CAN) isn't being attacked by other means. A.2. Requirements Terminology In this document, the words that are used to define the significance of each particular requirement are usually capitalized. These words are: MUST This word or the adjective "REQUIRED" means that implementation of the item is an absolute requirement of the specification. SHOULD This word or the adjective "RECOMMENDED" means that there might exist valid reasons in particular circumstances to not implement this item, but the full implications should be understood and the case carefully weighed before taking a different course. MAY This word or the adjective "OPTIONAL" means that this item is truly optional to implement. For example, one vendor might choose to include the item because a particular marketplace requires it or because it enhances the product; another vendor might omit the same item. Kent, Atkinson [Page 34] Internet Draft Security Architecture for IP 30 July 1997 Appendix B -- Analysis/Discussion of PMTU/DF/Fragmentation Issues B.1 DF bit In cases where a system (host or gateway) adds an encapsulating header (e.g., ESP tunnel), should/must the DF bit in the original packet be copied to the encapsulating header? Fragmenting seems correct for some situations, e.g., it might be appropriate to fragment packets over a network with a very small MTU, e.g., a packet radio network, or a cellular phone hop to mobile node, rather than propagate back a very small PMTU for use over the rest of the path. In other situations, it might be appropriate to set the DF bit in order to get feedback from later routers about PMTU constraints which require fragmentation. The existence of both of these situations argues for enabling a system to decide whether or not to fragment over a particular network "link", i.e., for requiring an implementation to be able to copy the DF bit (and to process ICMP PMTU messages), but making it an option to be selected on a per interface basis. In other words, an administrator should be able to configure the router's treatment of the DF bit (set, clear, copy from encapsulated header) for each interface. B.2 Fragmentation Fragmentation MUST be done after outbound IPSEC processing. Reassembly MUST be done before inbound IPSEC processing. The general reasoning is shown below (delimited by the *******'s). NOTE: IPSEC always has to figure out what the encapsulating IP header fields are. This is independent of where you insert IPSEC and is intrinsic to the definition of IPSEC. Therefore any IPSEC implementation that is not integrated into an IP implementation must include code to construct the necessary IP headers (IP2): o AH-tunnel --> IP2-AH-IP1-Transport-Data o ESP-tunnel --> IP2-ESP_hdr-IP1-Transport-Data-ESP_trailer **************************************************************************** Overall, the fragmentation/reassembly approach described above works for all cases examined. Kent, Atkinson [Page 35] Internet Draft Security Architecture for IP 30 July 1997 AH Xport AH Tunnel ESP Xport ESP Tunnel Implementation approach IPv4 IPv6 IPv4 IPv6 IPv4 IPv6 IPv4 IPv6 ----------------------- ---- ---- ---- ---- ---- ---- ---- ---- Hosts (integr w/ IP stack) Y Y Y Y Y Y Y Y Hosts (betw/ IP and drivers) Y Y Y Y Y Y Y Y S. Gwy (integr w/ IP stack) Y Y Y Y Outboard crypto processor * * If the crypto processor system has its own IP address, then it is covered by the security gateway case. This box receives the packet from the host and performs IPSEC processing. It has to be able to handle the same AH, ESP, and related IPv4/IPv6 tunnel processing that a security gateway would have to handle. If it doesn't have it's own address, then it is similar to the bump-in-the stack implementation between IP and the network drivers. The following analysis assumes that: 1. There is only one IPSEC module in a given system's stack. There isn't an IPSEC module A (adding ESP/encryption and thus) hiding the transport protocol, SRC port, and DEST port from IPSEC module B. 2. There are several places where IPSEC could be implemented (as shown in the table above). a. Hosts with integration of IPSEC into the native IP implementation. Implementer has access to the source for the stack. b. Hosts with bump-in-the-stack implementations, where IPSEC is implemented between IP and the local network drivers. Source access for stack is not available; but there are well-defined interfaces that allows the IPSEC code to be incorporated into the system. c. Security gateways and outboard crypto processors with integration of IPSEC into the stack. 3. Not all of the above approaches are feasible in all hosts. But it was assumed that for each approach, there are some hosts for whom the approach is feasible. For each of the above 3 categories, there are IPv4 and IPv6, AH transport and tunnel modes, and ESP transport and tunnel modes -- for a total of 24 cases (3 x 2 x 4). Some header fields and interface fields are listed here for ease of reference -- they're not in the header order, but instead listed to allow comparison between the columns. (* = not covered by AH authentication. ESP authentication doesn't cover any headers that precede it.) Kent, Atkinson [Page 36] Internet Draft Security Architecture for IP 30 July 1997 IP/Transport Interface IPv4 IPv6 (RFC 1122 -- Sec 3.4) ---- ---- ---------------------- Version = 4 Version = 6 Header Len *TOS Prty,Flow Lbl TOS Packet Len Payload Len Len ID ID (optional) *Flags DF *Offset *TTL *Hop Limit TTL Protocol Next Header *Checksum Src Address Src Address Src Address Dst Address Dst Address Dst Address Options? Options? Opt ? = AH covers Option-Type and Option-Length, but not Option-Data. The results for each of the 24 cases is shown below ("works" = will work if system fragments after outbound IPSEC processing, reassembles before inbound IPSEC processing). Notes indicate implementation issues. a. Hosts (integrated into IP stack) o AH-transport --> (IP1-AH-Transport-Data) - IPv4 -- works - IPv6 -- works o AH-tunnel --> (IP2-AH-IP1-Transport-Data) - IPv4 -- works - IPv6 -- works o ESP-transport --> (IP1-ESP_hdr-Transport-Data-ESP_trailer) - IPv4 -- works - IPv6 -- works o ESP-tunnel --> (IP2-ESP_hdr-IP1-Transport-Data-ESP_trailer) - IPv4 -- works - IPv6 -- works b. Hosts (Bump-in-the-stack) -- put IPSEC between IP layer and network drivers. In this case, the IPSEC module would have to do something like one of the following for fragmentation and reassembly. - do the fragmentation/reassembly work itself and send/receive the packet directly to/from the network layer. In AH or ESP transport mode, this is fine. In AH or ESP tunnel mode where the tunnel is to the ultimate destination, this is fine. But in AH or ESP tunnel modes where the tunnel end is different from the ultimate destination and where the source host is multi-homed, this approach could result in sub-optimal Kent, Atkinson [Page 37] Internet Draft Security Architecture for IP 30 July 1997 routing because the IPSEC module may be unable to obtain the information needed (LAN interface and next-hop gateway) to direct the packet to the appropriate network interface. This is not a problem if the interface and next-hop gateway are the same for the ultimate destination and for the tunnel end. But if they are different, then IPSEC would need to know the LAN interface and the next-hop gateway for the tunnel end. (Note: The tunnel end (security gateway) is highly likely to be on the regular path to the ultimate destination. But there could also be more than one path to the destination, e.g., the host could be at an organization with 2 firewalls. And the path being used could involve the less commonly chosen firewall.) OR - pass the IPSEC'd packet back to the IP layer where an extra IP header would end up being pre-pended and the IPSEC module would have to check and let IPSEC'd fragments go by. OR - pass the packet contents to the IP layer in a form such that the IP layer recreates an appropriate IP header At the network layer, the IPSEC module will have access to the following selectors from the packet -- SRC address, DST address, TOS, Next Protocol, and if there's a transport layer header --> SRC port and DST port. One cannot assume IPSEC has access to the User ID. It is assumed that the available selector information is sufficient to figure out the relevant Security Association(s). o AH-transport --> (IP1-AH-Transport-Data) - IPv4 -- works - IPv6 -- works o AH-tunnel --> (IP2-AH-IP1-Transport-Data) - IPv4 -- works - IPv6 -- works o ESP-transport --> (IP1-ESP_hdr-Transport-Data-ESP_trailer) - IPv4 -- works - IPv6 -- works o ESP-tunnel --> (IP2-ESP_hdr-IP1-Transport-Data-ESP_trailer) - IPv4 -- works - IPv6 -- works c. Security gateways -- integrate IPSEC into the IP stack NOTE: The IPSEC module will have access to the following selectors from the packet -- SRC address, DST address, TOS, Next Protocol, and if there's a transport layer header --> Kent, Atkinson [Page 38] Internet Draft Security Architecture for IP 30 July 1997 SRC port and DST port. It won't have access to the User ID (only Hosts have access to User ID information.) It also won't have access to the transport layer information if there is an ESP header, or if it's not the first fragment of a fragmented message. It is assumed that the available selector information is sufficient to figure out the relevant Security Association(s). o AH-tunnel --> (IP2-AH-IP1-Transport-Data) - IPv4 -- works - IPv6 -- works o ESP-tunnel --> (IP2-ESP_hdr-IP1-Transport-Data-ESP_trailer) - IPv4 -- works - IPv6 -- works **************************************************************************** B.3 Path MTU Discovery As mentioned earlier, "ICMP PMTU" refers to an ICMP message used for Path MTU Discovery. The legend for the diagrams below in B.3.1 and B.3.3 (but not B.3.2) is: ==== = security association (AH or ESP, transport or tunnel) ---- = connectivity (or if so labelled, administrative boundary) .... = ICMP message (hereafter referred to as ICMP PMTU) for IPv4: - Type = 3 (Destination Unreachable) - Code = 4 (Fragmentation needed and DF set) - Next-Hop MTU in the low-order 16 bits of the second word of the ICMP header (labelled unused in RFC 792), with high-order 16 bits set to zero IPv6 (RFC 1885): - Type = 2 (Packet Too Big) - Code = 0 (Fragmentation needed and DF set) - Next-Hop MTU in the 32 bit MTU field of the ICMP6 Hx = host x Rx = router x SGx = security gateway x X* = X supports IPSEC B.3.1 Identifying the Originating Host(s) The amount of information returned with the ICMP message is limited and this affects what selectors are available to identify security Kent, Atkinson [Page 39] Internet Draft Security Architecture for IP 30 July 1997 associations, originating hosts, etc. for use in further propagating the PMTU information. In brief... An ICMP message must contain the following information from the "offending" packet: - IPv4 (RFC 792) -- IP header plus a minimum of 64 bits - IPv6 (RFC 1885) -- IP header plus a minimum of 576 bytes Accordingly, in the IPv4 context, an ICMP PMTU may identify only the first (outermost) security association. This is because the ICMP PMTU may contain only 64 bits of the "offending" packet beyond the IP header, which would capture only the first SPI from AH or ESP. In the IPv6 context, an ICMP PMTU will probably provide all the SPIs and the selectors in the IP header, but maybe not the SRC/DST ports (in the transport header) or the encapsulated (TCP, UDP, etc.) protocol. Moreover, if ESP is used, the transport ports and protocol selectors may be encrypted. Looking at the diagram below of a security gateway tunnel (as mentioned elsewhere, security gateways do not use transport mode)... H1 =================== H3 \ | | / H0 -- SG1* ---- R1 ---- SG2* ---- R2 -- H5 / ^ | \ H2 |........| H4 Suppose that the security policy for SG1 is to use a single SA to SG2 for all the traffic between hosts H0, H1, and H2 and hosts H3, H4, and H5. And suppose H0 sends a data packet to H5 which causes R1 to send an ICMP PMTU message to SG1. If the PMTU message has only the SPI, SG1 will be able to look up the SA and find the list of possible hosts (H0, H1, H2); but SG1 will have no way to figure out that H0 sent the traffic that triggered the ICMP PMTU message. Kent, Atkinson [Page 40] Internet Draft Security Architecture for IP 30 July 1997 original after IPSEC ICMP packet processing packet -------- ----------- ------ IP-3 header (S = R1, D = SG1) ICMP header (includes PMTU) IP-2 header IP-2 header (S = SG1, D = SG2) ESP header minimum of 64 bits of ESP hdr (*) IP-1 header IP-1 header TCP header TCP header TCP data TCP data ESP trailer (*) The 64 bits will include enough of the ESP (or AH) header to include the SPI. - ESP -- SPI (32 bits), unknown (32 bits) -- could be the optional Replay counter but one can't be sure. - AH -- Next header (8 bits), Payload Len (8 bits), Reserved (16 bits), SPI (32 bits) This limitation on the amount of information returned with an ICMP message creates a problem in identifying the originating hosts for the packet (so as to know where to further propagate the ICMP PMTU information). If the ICMP message contains only 64 bits of the IPSEC header (minimum for IPv4), then the 5 original IPSEC selectors will have been lost -- Source and Destination addresses, Next Protocol, Source and Destination ports. But the ICMP error message will still provide SG1 with the SPI, the PMTU information and the source and destination gateways for the relevant security association. The destination security gateway and SPI uniquely define a security association which in turn defines a set of possible originating hosts. At this point, SG1 could: a. send the PMTU information to all the possible originating hosts. This would not work well if the host list is a wild card or if many/most of the hosts weren't sending to SG1; but it might work if the SPI/destination/etc mapped to just one host. b. store the PMTU with the SPI/etc and wait until the next packet(s) arrive from the originating host(s) for the relevant security association. If it/they are bigger than the PMTU, drop the packet(s), and compose ICMP PMTU message(s) with the new packet(s) and the updated PMTU, and send the originating host(s) the ICMP message(s) about the problem. This involves a delay in notifying the originating host(s), but avoids the problems of (a). Since only the latter approach is feasible in all instances, a security gateway MUST provide such support, as an option. However, if the ICMP message contains more information from the original packet, e.g., the 576 byte minimum for IPv6, then there MAY be enough information to immediately determine to which host to propagate the ICMP/PMTU message and to provide that system with a 5-selector Kent, Atkinson [Page 41] Internet Draft Security Architecture for IP 30 July 1997 pointer for storing/updating the PMTU. Under such circumstances, a security gateway MUST generate an ICMP PMTU message immediately upon receipt of an ICMP PMTU from further down the path. NOTE: The Next Protocol field MAY not be contained in the 576 bytes and the use of ESP encryption MAY hide the selector fields that have been encrypted. B.3.2 Calculation of PMTU The calculation of PMTU from an ICMP PMTU has to take into account the addition of any IPSEC header by H1 -- ESP or AH transport, or ESP or AH tunnel. Within a single host, multiple applications may share an SPI and nesting of security associations may occur. The diagram below illustrates several possible combinations of security associations between a pair of hosts (as viewed from the perspective of one of the hosts.) (ESPt or AHt = tunnel mode; ESPx or AHx = transport mode) Socket 1 ----------------------------------------------- I | n Socket 2 (ESPt/SPI-A) ------------------------------- | t \| e Socket 3 (AHx/SPI-B, ESPt/SPI-C) --- AHx (SPI-D) --- ESPt (SPI-E)--r / n Socket 4 (ESPx/SPI-F, ESPt/SPI-G) -- ESPx (SPI-H) --- e t In order to figure out the PMTU for each socket that maps to SPI-E, it will be necessary to have backpointers from SPI-E to each of the 4 paths that lead to it -- Socket 1, SPI-A, SPI-D, and SPI-H. B.3.3 Granularity of Maintaining PMTU Data In hosts, the granularity with which PMTU ICMP processing can be done differs depending on the implementation situation. Looking at a host, there are 3 situations that are of interest with respect to PMTU issues: a. Integration of IPSEC into the native IP implementation b. Bump-in-the-stack implementations, where IPSEC is implemented "underneath" an existing implementation of a TCP/IP protocol stack, between the native IP and the local network drivers c. No IPSEC implementation -- This case is included because it is relevant in cases where a security gateway is sending PMTU information back to a host. Only in case (a) can the PMTU data be maintained at the same granularity as communication associations. In the other cases, the IP layer will maintain PMTU data at the granularity of Source and Destination IP addresses (and optionally ToS), as described in RFC 1191. This is an important difference, because more than one communication association may map to the same source and destination Kent, Atkinson [Page 42] Internet Draft Security Architecture for IP 30 July 1997 IP addresses, and each communication association may have a different amount of IPSEC header overhead (e.g., due to use of different transforms or different algorithms). The examples below illustrate this. In cases (a) and (b)... Suppose you have the following situation. H1 is sending to H2 and the packet to be sent from R1 to R2 exceeds the PMTU of the network hop between them. ================================== | | H1* --- R1 ----- R2 ---- R3 ---- H2* ^ | |.......| If R1 is configured to not fragment subscriber traffic, then R1 sends an ICMP PMTU message with the appropriate PMTU to H1. H1's processing would vary with the nature of the implementation. In case (a) (native IP), the security services are bound to sockets or the equivalent. Here the IP/IPSEC implementation in H1 can store/update the PMTU for the associated socket. In case (b), the IP layer in H1 can store/update the PMTU but only at the granularity of Source and Destination addresses and possibly ToS, as noted above. So the result may be sub-optimal, since the PMTU for a given SRC/DST/ToS will be the subtraction of the largest amount of IPSEC header used for any communication association between a given source and destination. In case (c), there has to be a security gateway to have any IPSEC processing. So suppose you have the following situation. H1 is sending to H2 and the packet to be sent from SG1 to R exceeds the PMTU of the network hop between them. ================ | | H1 ---- SG1* --- R --- SG2* ---- H2 ^ | |.......| As described above for case (b), the IP layer in H1 can store/update the PMTU but only at the granularity of Source and Destination addresses, and possibly ToS. So the result may be sub-optimal, since the PMTU for a given SRC/DST/ToS will be the subtraction of the largest amount of IPSEC header used for any communication association between a given source and destination. B.3.4 Per Socket Maintenance of PMTU Data Implementation of the calculation of PMTU (Section B.2.2) and support for PMTUs at the granularity of individual "communication associations" (Section B.2.3) is a local matter. However, a socket- Kent, Atkinson [Page 43] Internet Draft Security Architecture for IP 30 July 1997 based implementation of IPSEC in a host SHOULD maintain the information on a per socket basis. Bump in the stack systems MUST pass an ICMP PMTU to the host IP implementation, after adjusting it for any IPSEC header overhead added by these systems. The determination of the overhead SHOULD be determined by analysis of the SPI and any other selector information present in a returned ICMP PMTU message. B.3.5 Delivery of PMTU Data to the Transport Layer The host mechanism for getting the updated PMTU to the transport layer is unchanged, as specified in RFC 1191 (Path MTU Discovery). B.3.6 Aging of PMTU Data In all systems (host or gateway) implementing IPSEC and maintaining PMTU information, the PMTU associated with a security association (transport or tunnel) has to be "aged" and some mechanism put in place for updating the PMTU in a timely manner, especially for discovering if the PMTU is smaller than it needs to be. A given PMTU has to remain in place long enough for a packet to get from the source end of the security association to the system at the other end of the security association and propagate back an ICMP error message if the current PMTU is too big. Systems SHOULD use the approach described in the Path MTU Discovery document (RFC 1191, Section 6.3), which suggests periodically resetting the PMTU to the first-hop data-link MTU and then letting the normal PMTU Discovery processes update the PMTU as necessary. The period SHOULD be Configurable. Kent, Atkinson [Page 44] Internet Draft Security Architecture for IP 30 July 1997 Appendix C - Sequence Space Window Code Example This appendix contains a routine that implements a bitmask check for a 32 packet window. It was provided by James Hughes (jim_hughes@stortek.com) and Harry Varnis (hgv@anubis.network.com) and is intended as an implementation example. Note that this code both checks for a replay and updates the window. Thus the algorithm, as shown, should only be called AFTER the packet has been authenticated. Implementers might wish to consider splitting the code to do the check for replays before computing the ICV. If the packet is not a replay, the code would then compute the ICV, (discard any bad packets), and if the packet is OK, update the window. #include <stdio.h> #include <stdlib.h> typedef unsigned long u_long; enum { ReplayWindowSize = 32 }; u_long bitmap = 0; /* session state - must be 32 bits */ u_long lastSeq = 0; /* session state */ /* Returns 0 if packet disallowed, 1 if packet permitted */ int ChkReplayWindow(u_long seq); int ChkReplayWindow(u_long seq) { u_long diff; if (seq == 0) return 0; /* first == 0 or wrapped */ if (seq > lastSeq) { /* new larger sequence number */ diff = seq - lastSeq; if (diff < ReplayWindowSize) { /* In window */ bitmap <<= diff; while (diff > 1) bitmap &= ~(1 << --diff); bitmap |= 1; /* set bit for this packet */ } else bitmap = 1; /* This packet has a "way larger" */ lastSeq = seq; return 1; /* larger is good */ } diff = lastSeq - seq; if (diff >= ReplayWindowSize) return 0; /* too old or wrapped */ if (bitmap & (1 << diff)) return 0; /* this packet already seen */ bitmap |= (1 << diff); /* mark as seen */ return 1; /* out of order but good */ } char string_buffer[512]; #define STRING_BUFFER_SIZE sizeof(string_buffer) Kent, Atkinson [Page 45] Internet Draft Security Architecture for IP 30 July 1997 int main() { int result; u_long last, current, bits; printf("Input initial state (bits in hex, last msgnum):0); if (!fgets(string_buffer, STRING_BUFFER_SIZE, stdin)) exit(0); sscanf(string_buffer, "%lx %lu", &bits, &last); if (last != 0) bits |= 1; bitmap = bits; lastSeq = last; printf("bits:%08lx last:%lu0, bitmap, lastSeq); printf("Input value to test (current):0); while (1) { if (!fgets(string_buffer, STRING_BUFFER_SIZE, stdin)) break; sscanf(string_buffer, "%lu", ¤t); result = ChkReplayWindow(current); printf("%-3s", result ? "OK" : "BAD"); printf(" bits:%08lx last:%lu0, bitmap, lastSeq); } return 0; } Kent, Atkinson [Page 46] Internet Draft Security Architecture for IP 30 July 1997 References [BCCH94] R. Braden, D. Clark, S. Crocker, & C. Huitema, "Report of IAB Workshop on Security in the Internet Architecture", RFC-1636, DDN Network Information Center, June 1994. [Bel89] Steven M. Bellovin, "Security Problems in the TCP/IP Protocol Suite", ACM Computer Communications Review, Vol. 19, No. 2, March 1989. [Bel95] Steven M. Bellovin, Presentation at IP Security Working Group Meeting, Proceedings of the 32nd Internet Engineering Task Force, March 1995, Internet Engineering Task Force, Danvers, MA. [Bel96] Steven M. Bellovin, "Problem Areas for the IP Security Protocols", Proceedings of the Sixth Usenix Unix Security Symposium, July, 1996. [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. [CERT95] CA-95:01 [CB94] William R. Cheswick & Steven M. Bellovin, Firewalls & Internet Security, Addison-Wesley, Reading, MA, 1994. [CG96] Shu-jen Chang & Rob Glenn, "HMAC-SHA IP Authentication with Replay Prevention", Internet Draft, 1 May 1996. [DIA] US Defense Intelligence Agency, "Compartmented Mode Workstation Specification", Technical Report DDS-2600- 6243-87. [DoD85] US National Computer Security Center, "Department of Defense Trusted Computer System Evaluation Criteria", DoD 5200.28-STD, US Department of Defense, Ft. Meade, MD., December 1985. [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. [DH76] W. Diffie & M. Hellman, "New Directions in Cryptography", IEEE Transactions on Information Theory, Vol. IT-22, No. 6, November 1976, pp. 644-654. [DH95] Steve Deering & Bob Hinden, Internet Protocol version 6 Kent, Atkinson [Page 47] Internet Draft Security Architecture for IP 30 July 1997 (IPv6) Specification, RFC-1883, December 1995. [EK96] D. Eastlake III & C. Kaufman, "Domain Name System Protocol Security Extensions", Internet Draft, 30 January 1996. [GM93] J. Galvin & K. McCloghrie, Security Protocols for version 2 of the Simple Network Management Protocol (SNMPv2), RFC- 1446, DDN Network Information Center, April 1993. [HA94] N. Haller & R. Atkinson, "On Internet Authentication", RFC-1704, DDN Network Information Center, October 1994. [Hugh96] J. Hughes (Editor), "Combined DES-CBC, HMAC, and Replay Prevention Security Transform", Internet Draft, June 1996. [ISO] ISO/IEC JTC1/SC6, Network Layer Security Protocol, ISO-IEC DIS 11577, International Standards Organisation, Geneva, Switzerland, 29 November 1992. [IB93] John Ioannidis and Matt Blaze, "Architecture and Implementation of Network-layer Security Under Unix", Proceedings of USENIX Security Symposium, Santa Clara, CA, October 1993. [IBK93] John Ioannidis, Matt Blaze, & Phil Karn, "swIPe: Network- Layer Security for IP", presentation at the Spring 1993 IETF Meeting, Columbus, Ohio [KA97a] Steve Kent, Randall Atkinson, "IP Authentication Header", Internet Draft, ?? 1997. [KA97b] Steve Kent, Randall Atkinson, "IP Encapsulating Security Payload (ESP)", Internet Draft, ?? 1997. [Ken91] Steve Kent, US DoD Security Options for the Internet Protocol, RFC-1108, DDN Network Information Center, November 1991. [Ken93] Steve Kent, Privacy Enhancement for Internet Electronic Mail: Part II: Certificate-Based Key Management, RFC-1422, DDN Network Information Center, 10 February 1993. [KB93] J. Kohl & B. Neuman, The Kerberos Network Authentication Service (V5), RFC-1510, DDN Network Information Center, 10 September 1993. [NS78] R.M. Needham & M.D. Schroeder, "Using Encryption for Authentication in Large Networks of Computers", Communications of the ACM, Vol. 21, No. 12, December 1978, pp. 993-999. Kent, Atkinson [Page 48] Internet Draft Security Architecture for IP 30 July 1997 [NS81] R.M. Needham & M.D. Schroeder, "Authentication Revisited", ACM Operating Systems Review, Vol. 21, No. 1., 1981. [OG96] Mike Oehler & Rob Glenn, "HMAC-MD5 IP Authentication with Replay Prevention", Internet Draft, 1 May 1996. [OTA94] US Congress, Office of Technology Assessment, "Information Security & Privacy in Network Environments", OTA-TCT-606, Government Printing Office, Washington, DC, September 1994. [Sch94] Bruce Schneier, Applied Cryptography, Section 8.6, John Wiley & Sons, New York, NY, 1994. [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. [STD-1] J. Postel, "Internet Official Protocol Standards", STD-1, March 1996. [VK83] V.L. Voydock & S.T. Kent, "Security Mechanisms in High- level Networks", ACM Computing Surveys, Vol. 15, No. 2, June 1983. [ZDESZ93] Zhang, L., Deering, S., Estrin, D., Shenker, S., and Zappala, D., "RSVP: A New Resource ReSerVation Protocol", IEEE Network magazine, September 1993. Disclaimer The views and specification expressed in this document are those of the authors and are not necessarily those of their employers. The authors and their employers specifically disclaim responsibility for any problems arising from correct or incorrect implementation or use of this design. Kent, Atkinson [Page 49] Internet Draft Security Architecture for IP 30 July 1997 Author Information Stephen Kent BBN Corporation 70 Fawcett Street Cambridge, MA 02140 USA E-mail: kent@bbn.com Telephone: +1 (617) 873-3988 Randall Atkinson @Home Network 385 Ravendale Drive Mountain View, CA 94043 USA E-mail: rja@inet.org Kent, Atkinson [Page 50]
- Internet Draft -- IPsec Architecture Karen Seo
- Re: Internet Draft -- IPsec Architecture Robert Moskowitz
- Re: Internet Draft -- IPsec Architecture Stephen Kent