Network Working Group H. Tschofenig Request for Comments: 4081 D. Kroeselberg Category: Informational Siemens June 2005 Security Threats for Next Steps in Signaling (NSIS) Status of This Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2005).
AbstractThis threats document provides a detailed analysis of the security threats relevant to the Next Steps in Signaling (NSIS) protocol suite. It calls attention to, and helps with the understanding of, various security considerations in the NSIS Requirements, Framework, and Protocol proposals. This document does not describe vulnerabilities of specific parts of the NSIS protocol suite. 1. Introduction ....................................................2 2. Communications Models ...........................................3 3. Generic Threats .................................................7 3.1. Man-in-the-Middle Attacks ..................................8 3.2. Replay of Signaling Messages ..............................11 3.3. Injecting or Modifying Messages ...........................11 3.4. Insecure Parameter Exchange and Negotiation ...............12 4. NSIS-Specific Threat Scenarios .................................12 4.1. Threats during NSIS SA Usage ..............................13 4.2. Flooding ..................................................13 4.3. Eavesdropping and Traffic Analysis ........................15 4.4. Identity Spoofing .........................................15 4.5. Unprotected Authorization Information .....................17 4.6. Missing Non-Repudiation ...................................18 4.7. Malicious NSIS Entity .....................................19 4.8. Denial of Service Attacks .................................20 4.9. Disclosing the Network Topology ...........................21 4.10. Unprotected Session or Reservation Ownership .............21 4.11. Attacks against the NTLP .................................23
5. Security Considerations ........................................23 6. Contributors ...................................................24 7. Acknowledgements ...............................................24 8. References .....................................................25 8.1. Normative References ......................................25 8.2. Informative References ....................................25 RSVP-SEC] and [SIG-ANAL]) Security Threats for NSIS NSIS Requirements (see [RFC3726]) NSIS Framework (see [RFC4080]) NSIS Protocol Suite (see GIMPS [GIMPS], NAT/Firewall NSLP [NATFW-NSLP] and QoS NSLP [QOS-NSLP]) This document identifies the basic security threats that need to be addressed during the design of the NSIS protocol suite. Even if the base protocol is secure, certain extensions may cause problems when used in a particular environment. This document cannot provide detailed threats for all possible NSIS Signaling Layer Protocols (NSLPs). QoS [QOS-NSLP], NAT/Firewall [NATFW-NSLP], and other NSLP documents need to provide a description of their trust models and a threat assessment for their specific application domain. This document aims to provide some help for the subsequent design of the NSIS protocol suite. Investigations of security threats in a specific architecture or context are outside the scope of this document. We use the NSIS terms defined in [RFC3726] and in [RFC4080].
Figure 1, and in Figure 2 the relationship between NSIS entities along the path is shown. For illustrative reasons, only end-to-end NSIS signaling is depicted, yet it might be used in other variations as well. Signaling can start at any place and might terminate at any other place within the network. Depending on the trust relationship between NSIS entities and the traversed network parts, different security problems arise. The notion of trust and trust relationship used in this document is informal and can best be captured by the definition provided in Section 1.1 of [RFC3756]. For completeness we include the definition of a trust relationship, which denotes a mutual a priori relationship between the involved organizations or parties wherein the parties believe that the other parties will behave correctly even in the future. An important observation for NSIS is that a certain degree of trust has to be placed into intermediate NSIS nodes along the path between an NSIS Initiator and an NSIS Responder, specifically so that they perform message processing and take the necessary actions. A complete lack of trust between any of the participating entities will cause NSIS signaling to fail. Note that it is not possible to describe a trust model completely without considering the details and behavior of the NTLP, the NSLP (e.g., QoS NSLP), and the deployment environment. For example, securing the communication between an end host (which acts as the NSIS Initiator) and the first NSIS node (which might be in the attached network or even a number of networks away) is impacted by the trust relationships between these entities. In a corporate network environment, a stronger degree of trust typically exists than in an unmanaged network. Figure 1 introduces convenient abbreviations for network parts with similar properties: first-peer, last-peer, intra-domain, or inter-domain.
+------------------+ +---------------+ +------------------+ | | | | | | | Administrative | | Intermediate | | Administrative | | Domain A | | Domains | | Domain B | | | | | | | | (Inter-domain Communication) | | +-------->+---+<------------->+---+<--------+ | | (Intra-domain | | | | (Intra-domain | | Communication) | | | | Communication) | | | | | | | | | | v | | | | v | +--------+---------+ +---------------+ +---------+--------+ ^ ^ | | First Peer Communication Last Peer Communication | | v v +-----+-----+ +-----+-----+ | NSIS | | NSIS | | Initiator | | Responder | +-----------+ +-----------+ Figure 1: Communication patterns in NSIS First-Peer/Last-Peer Communication: The end-to-end communication scenario depicted in Figure 1 includes the communication between the end hosts and their nearest NSIS hops. "First-peer communications" refers to the peer-to-peer interaction between a signaling message originator, the NSIS Initiator (NI), and the first NSIS-aware entity along the path. This "first-peer communications" commonly comes with specific security requirements that are especially important for addressing security issues between the end host (and a user) and the network it is attached to. To illustrate this, in roaming environments, it is difficult to assume the existence of a pre-established security association directly available for NSIS peers involved in first-peer communications, because these peers cannot be assumed to have any pre-existing relationship with each other. In contrast, in enterprise networks usually there is a fairly strong (pre-established) trust relationship between the peers. Enterprise network administrators usually have some degree of freedom to select the appropriate security protection and to enforce it. The choice of selecting a security mechanism is therefore often influenced by the infrastructure already
available, and per-session negotiation of security mechanisms is often not required (although, in contrast, it is required in a roaming environment). Last-Peer communication is a variation of First-Peer communication in which the roles are reversed. Intra-Domain Communication: After verification of the NSIS signaling message at the border of an administrative domain, an NSIS signaling message traverses the network within the same administrative domain to which the first peer belongs. It might not be necessary to repeat the authorization procedure of the NSIS initiator again at every NSIS node within this domain. Key management within the administrative domain might also be simpler. Security protection is still required to prevent threats by non-NSIS nodes in this network. Inter-Domain Communication: Inter-Domain communication deals with the interaction between administrative domains. For some NSLPs (for example, QoS NSLP), this interaction is likely to take place between neighboring domains, whereas in other NSLPs (such as the NAT/Firewall NSLP), the core network is usually not involved. If signaling messages are conveyed transparently in the core network (i.e., if they are neither intercepted nor processed in the core network), then the signaling message communications effectively takes place between access networks. This might place a burden on authorization handling and on the key management infrastructure required between these access networks, which might not know of each other in advance. To refine the above differentiation based on the network parts that NSIS signaling may traverse, we subsequently consider relationships between involved entities. Because a number of NSIS nodes might actively participate in a specific protocol exchange, a larger number of possible relationships need to be analyzed than in other protocols. Figure 2 illustrates possible relationships between the entities involved in the NSIS protocol suite.
**************************************** * * +----+-----+ +----------+ +----+-----+ +-----+ NSIS +-------+ NSIS +--------+ NSIS +-----+ | | Node 1 | | Node 2 | | Node 3 | | | +----------+ +----+-----+ +----------+ | | ~ | | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | | ~ | +--+--+-----+ +---------+-+ | NSIS +//////////////////////////////////////////+ NSIS | | Initiator | | Responder | +-----------+ +-----------+ Legend: -----: Peer-to-Peer Relationship /////: End-to-End Relationship *****: Middle-to-Middle Relationship ~~~~~: End-to-Middle Relationship Figure 2: Possible NSIS Relationships End-to-Middle Communications: The scenario in which one NSIS entity involved is an end-entity (Initiator or Responder) and the other entity is any intermediate hop other than the immediately adjacent peer is typically called the end-to-middle scenario (see Figure 2). A motivation for including this scenario can, for example, be found in SIP [RFC3261]. An example of end-to-middle interaction might be an explicit authorization from the NSIS Initiator to some intermediate node. Threats specific to this scenario may be introduced by some intermediate NSIS hops that are not allowed to eavesdrop or modify certain objects. Middle-to-Middle Communications: Middle-to-middle communication refers to the exchange of information between two non-neighboring NSIS nodes along the path. Intermediate NSIS hops may have to deal with specific security threats that do not involve the NSIS Initiator or the NSIS Responder directly.
End-to-End Communications: NSIS aims to signal information from an Initiator to some NSIS nodes along the path to a data receiver. In the case of end-to-end NSIS signaling, the last node is the NSIS Responder, as it is the data receiver. The NSIS protocol suite is not an end-to-end protocol used to exchange information purely between end hosts. Typically, it is not required to protect NSIS messages cryptographically between the NSIS Initiator and the NSIS Responder. Protecting the entire signaling message end-to-end might not be feasible since intermediate NSIS nodes need to add, inspect, modify, or delete objects from the signaling message.
Unilateral Authentication: In the case of unilateral authentication, the NSIS entity that does not authenticate its peer is unable to discover a man-in- the-middle adversary. Although mutual authentication of signaling messages should take place between each peer participating in the protocol operation, special attention is given here to first-peer communications. Unilateral authentication between an end host and the first peer (just authenticating the end host) is still common today, but it opens up many possibilities for man-in-the-middle attackers impersonating either the end host or the (administrative domain represented by the) first peer. Missing or unilateral authentication, as described above, is part of a general problem of network access with inadequate authentication, and it should not be considered something unique to the NSIS signaling protocol. Obviously, there is a strong need to address this correctly in a future NSIS protocol suite. The signaling protocols addressed by NSIS are different from other protocols in which only two entities are involved. Note that first-peer authentication is especially important because a security breach there could impact nodes beyond the entities directly involved (or even beyond a local network). Finally, note that the signaling protocol should be considered a peer-to-peer protocol, wherein the roles of Initiator and Responder can be reversed at any time. Thus, unilateral authentication is not particularly useful for such a protocol. However, some form of asymmetry might be needed in the authentication process, whereby one entity uses an authentication mechanism different from that of the other one. As an example, the combination of symmetric and asymmetric cryptography should be mentioned. Weak Authentication: In the case of weak authentication, the threat can be carried out because information transmitted during the NSIS SA establishment process may leak passwords or allow offline dictionary attacks. This threat is applicable to NSIS for the process of selecting certain security mechanisms. Finally, we conclude with a description of a man-in-the-middle (MITM) attack during the discovery phase. This attack benefits from the fact that NSIS nodes are likely to be unaware of the network
topology. Furthermore, an authorization problem might arise if an NSIS QoS NSLP node pretends to be an NSIS NAT/Firewall-specific node or vice versa. An adversary might inject a bogus reply message, forcing the discovery message initiator to start a messaging association establishment with either an adversary or with another NSIS node that is not along the path. Figure 3 describes the attack in more detail for peer-to-peer addressed messages with a discovery mechanism. For end-to-end addressed messages, the attack is also applicable, particularly if the adversary is located along the path and able to intercept the discovery message that traverses the adversary. The man-in-the-middle adversary might redirect to another legitimate NSIS node. A malicious NSIS node can be detected with the corresponding security mechanisms, but a legitimate NSIS node that is not the next NSIS node along the path cannot be detected without topology knowledge. +-----------+ Messaging Association Message | Adversary | Establishment Association +--->+ +<----------------+ Establish- | +----+------+ |(4) ment | IPx | | (3)| |Discovery Reply v | | (IPx) +---+-------+ v | (2) | NSIS | +------+-----+ | /----------->+ Node B +-------- | NSIS +<--+ / Discovery +-----------+ | Node A +---------/ Request IPr +------------+ (1) IPi Figure 3: MITM Attack during the Discovery Exchange This attack assumes that the adversary is able to eavesdrop on the initial discovery message sent by the sender of the discovery message. Furthermore, we assume that the discovery reply message by the adversary returns to the discovery message initiator faster than the real response. This represents some race condition characteristics if the next NSIS node is very close (in IP-hop terms) to the initiator. Note that the problem is self-healing since the discovery process is periodically repeated. If an adversary is unable to mount this attack with every discovery message, then the correct next NSIS node along the path will be discovered again. A ping-pong behavior might be the consequence.
As shown in message step (2) in Figure 3, the adversary returns a discovery reply message with its own IP address as the next NSIS- aware node along the path. Without any additional information, the discovery message initiator has to trust this information. Then a messaging association is established with an entity at a given IP address IPx (i.e., with the adversary) in step (3). The adversary then establishes a messaging association with a further NSIS node and forwards the signaling message. Note that the adversary might just modify the Discovery Reply message to force NSIS Node A to establish a messaging association with another NSIS node that is not along the path. This can then be exploited by the adversary. The interworking with NSIS-unaware NATs in particular might cause additional unexpected problems. As a variant of this attack, an adversary not able to eavesdrop on transmitted discovery requests could flood a node with bogus discovery reply messages. If the discovery message sender accidentally accepts one of those bogus messages, then a MITM attack as described in Figure 3 is possible.
spoofing, it is possible to carry out fraud. This attack is only feasible in the absence of authentication and signaling message protection. Some threats directly related to these are described in Sections 4.4, 4.7, and 4.8.
Section 4.7. Replay attacks may be possible when an NSIS node crashes, restarts, and performs state re-establishment. Proper re-synchronization of the security mechanism must therefore be provided to address this problem. Section 4.11.
Force NTLP to Do More Processing: Some protocol fields might allow an adversary to force an NTLP node to perform more processing. Additionally it might be possible to interfere with the flow control or the congestion control procedure. These types of threats are also addressed in Section 4.11. Furthermore, it might be possible to force the NTLP node to perform some computations or signaling message exchanges by injecting "trigger" events (which are unprotected). Force NSLP to Do More Processing: An adversary might benefit from flooding an NSLP node with messages that must be stored (e.g., due to fragmentation handling) before verifying the correctness of signaling messages. Furthermore, causing memory allocation and computational efforts might allow an adversary to harm NSIS entities. If a signaling message contains, for example, a digital signature, then some additional processing is required for the cryptographic verification. An adversary can easily create a random bit sequence instead of a digital signature to force an NSIS node into heavy computation. Idempotent signaling messages are particularly vulnerable to this type of attack. The term "idempotent" refers to messages that contain the same amount of information as the original message. An example would be a refresh message that is equivalent to a create message. This property allows a refresh message to create state along a new path, where no previous state is available. For this to work, specific classes of cryptographic mechanisms supporting this behavior are needed. An example is a scheme based on digital signatures, which, however, should be used with care due to possible denial of service attacks. Problems with the usage of public-key-based cryptosystems in protocols are described in [AN97] and in [ALN00]. In addition to the threat scenario described above, an incoming signaling message might trigger communication with third-party nodes such as policy servers, LDAP servers, or AAA servers. If an adversary is able to transmit a large number of signaling messages (for example, with QoS reservation requests) with invalid credentials, then the verifying node may not be able to process other reservation messages from legitimate users.
Section 3.2. The eavesdropper might learn QoS parameters, communication patterns, policy rules for firewall traversal, policy information, application identifiers, user identities, NAT bindings, authorization objects, network configuration and performance information, and more. An adversary's capability to eavesdrop on signaling messages might violate a user's preference for privacy, particularly if unprotected authentication or authorization information (including policies and profile information) is exchanged. Because the NSIS protocol signals messages through a number of nodes, it is possible to differentiate between nodes actively participating in the NSIS protocol and those that do not. For certain objects or messages, it might be desirable to permit actively participating intermediate NSIS nodes to eavesdrop. On the other hand, it might be desirable that only the intended end points (NSIS Initiator and NSIS Responder) be able to read certain other objects. RFC1809] and [RFC3697]). Modification of these flow identifiers allows adversaries to exploit or to render ineffective quality of service
reservations or policy rules at middleboxes. An adversary could mount an attack by modifying the flow identifier of a signaling message. In the third case, an adversary may spoof data traffic. NSIS signaling messages contain some sort of flow identifier that is associated with a specified behavior (e.g., a particular flow experiences QoS treatment or allows packets to traverse a firewall). An adversary might, therefore, use IP spoofing and inject data packets to benefit from previously installed flow identifiers. We will provide an example of the latter threat. After NSIS nodes along the path between the NSIS initiator and the NSIS receiver processes a properly protected reservation request, transmitted by the legitimate user Alice, a QoS reservation is installed at the corresponding NSIS nodes (for example, the edge router). The flow identifier is used for flow identification and allows data traffic originated from a given source to be assigned to this QoS reservation. The adversary Eve now spoofs Alice's IP address. In addition, Alice's host may be crashed by the adversary with a denial of service attack or may lose connectivity (for example, because of mobility). If Eve is able to perform address spoofing, then she is able to receive and transmit data (for example, RTP data traffic) that receives preferential QoS treatment based on the previous reservation. Depending on the installed flow identifier granularity, Eve might have more possibilities to exploit the QoS reservation or a pin-holed firewall. Assuming the soft state paradigm, whereby periodic refresh messages are required, Alice's absence will not be detected until a refresh message is required, forcing Eve to respond with a protected signaling message. Again, this attack is applicable not only to QoS traffic, but also to a Firewall control protocol, with a different consequence. The ability for an adversary to inject data traffic that matches a certain flow identifier established by a legitimate user and to get some benefit from injecting that traffic often also requires the ability to receive the data traffic or to have one's correspondent receive it. For example, an adversary in an unmanaged network observes a NAT/Firewall signaling message towards a corporate network. After the signaling message exchange was successful, the user Alice is allowed to traverse the company firewall based on the establish packet filter in order to contact her internal mail server. Now, the adversary Eve, who was monitoring the signaling exchange, is able to build a data packet towards this mail server that will pass the company firewall. The packet will hit the mail server and cause some actions, and the mail server will reply with some response messages. Depending on the exact location of the adversary and the
degree of routing asymmetry, the adversary might even see the response messages. Note that for this attack to work, Alice does not need to participate in the exchange of signaling messages. We could imagine using attributes of a flow identifier that is not related to source and destination addresses. For example, we could think of a flow identifier for which only the 21-bit Flow ID is used (without source and destination IP address). Identity spoofing and injecting traffic is much easier since a packet only needs to be marked and an adversary can use a nearly arbitrary endpoint identifier to achieve the desired result. Obviously, though, the endpoint identifiers are not irrelevant, because the messages have to hit some nodes in the network where NSIS signaling messages installed state (in the above example, they would have to hit the same firewall). Data traffic marking based on DiffServ is such an example. Whenever an ingress router uses only marked incoming data traffic for admission control procedures, various attacks are possible. These problems have been known in the DiffServ community for a long time and have been documented in various DiffServ-related documents. The IPsec protection of DiffServ Code Points is described in Section 6.2 of [RFC2745]. Related security issues (for example denial of service attacks) are described in Section 6.1 of the same document. RFC3182], for example). Depending on the chosen authentication protocol, certain threats may exist. Section 3 discusses a number of issues related to this approach when the authentication and key exchange protocol is used to establish session keys for signaling message protection. Another approach is to use some sort of authorization token. The functionality and structure of such an authorization token for RSVP is described in [RFC3520] and [RFC3521]. Achieving secure interaction between different protocols based on authorization tokens, however, requires some care. By using such an authorization token, it is possible to link state information between different protocols. Returning an unprotected authorization token to the end host might allow an adversary (for example, an eavesdropper)
to steal resources. An adversary might also use the token to monitor communication patterns. Finally, an untrustworthy end host might also modify the token content. The Session/Reservation Ownership problem can also be regarded as an authorization problem. Details are described in Section 4.10. In enterprise networks, authorization is often coupled with membership in a particular class of users or groups. This type of information either can be delivered as part of the authentication and key agreement procedure or has to be retrieved via separate protocols from other entities. If an adversary manages to modify information relevant to determining authorization or the outcome of the authorization process itself, then theft of service might be possible.
occur. If a signaling protocol with the non-repudiation property is desired for establishing QoS reservations, then it certainly impacts the protocol design. Non-repudiation functionality places additional requirements on the security mechanisms. Thus, a solution would normally increase the overhead of a security solution. Threats related to missing non- repudiation are only considered relevant in certain specific scenarios and for specific NSLPs.
For an adversary to mount this attack, either an existing NSIS-aware node along the path has to be attacked successfully, or an adversary must succeed in convincing another NSIS node to make it the next NSIS peer (man-in-the-middle attack).
Faked Error or Response Messages: An adversary may be able to inject false error or response messages as part of a DoS attack. This could be at the signaling message protocol layer (NTLP), the layer of each client layer protocol (e.g., QoS NSLP or NAT/Firewall NSLP), or the transport protocol layer. An adversary might cause unexpected protocol behavior or might succeed with a DoS attack. The discovery protocol, especially, exhibits vulnerabilities with regard to this threat scenario (see the above discussion on discovery). If no separate discovery protocol is used and signaling messages are addressed to end hosts only (with a Router Alert Option to intercept message as NSIS aware nodes), an error message might be used to indicate a path change. Such a design combines a discovery protocol with a signaling message exchange protocol. RFC2745] for a description of diagnostic message functionality for RSVP), and query messages, in addition to record route and route objects, provide potential assistance to an adversary. Thus, the requirement of not disclosing a network topology might conflict with other requirements to provide means for discovering NSIS-aware nodes automatically or to provide diagnostic facilities (used for network monitoring and administration). Figure 4 shows an NSIS Initiator that has established state information at NSIS nodes along a path as part of the signaling procedure. As a result, Access Router 1, Router 3, and Router 4 (and other nodes) have stored session-state information, including the Session Identifier SID-x.
Session ID(SID-x) +--------+ +-----------------+ Router +------------> Session ID(SID-x)| | 4 | +---+----+ +--------+ | Router | +------+ 3 +******* | +---+----+ * | * | Session ID(SID-x) * Session ID(SID-x) +---+----+ +---+----+ | Access | | Access | | Router | | Router | | 1 | | 2 | +---+----+ +---+----+ | * | Session ID(SID-x) * Session ID(SID-x) +----+------+ +----+------+ | NSIS | | Adversary | | Initiator | | | +-----------+ +-----------+ Figure 4: Session or Reservation Ownership The Session Identifier is included in signaling messages to reference to the established state. If an adversary were able to obtain the Session Identifier (for example, by eavesdropping on signaling messages), it would be able to add the same Session Identifier SID-x to a new signaling message. When the new signaling message hits Router 3 (as shown in Figure 4), existing state information can be modified. The adversary can then modify or delete the established reservation and cause unexpected behavior for the legitimate user. The source of the problem is that Router 3 (a cross-over router) is unable to decide whether the new signaling message was initiated from the owner of the session or reservation. In addition, nodes other than the initial signaling message originator are allowed to signal information during the lifetime of an established session. As part of the protocol, any NSIS-aware node along the path (and the path might change over time) could initiate a signaling message exchange. It might, for example, be necessary to provide mobility support or to trigger a local repair procedure. If only the initial signaling message originator were allowed to trigger signaling message exchanges, some protocol behavior would not be possible.
If this threat scenario is not addressed, an adversary can launch DoS, theft of service, and various other attacks. 2LEVEL], a two-level architecture is proposed, that would split an NSIS protocol into layers: a signaling message transport-specific layer and an application-specific layer. This is further developed in the NSIS Framework [RFC4080]. Most of the threats described in this threat analysis are applicable to the NSLP application-specific part (e.g., QoS NSLP). There are, however, some threats that are applicable to the NTLP. Network and transport layer protocols lacking protection mechanisms are vulnerable to certain attacks, such as header manipulation, DoS, spoofing of identities, session hijacking, unexpected aborts, etc. Malicious nodes can attack the congestion control mechanism to force NSIS nodes into a congestion avoidance state. Threats that address parts of the NTLP that are not related to attacks against the use of transport layer protocols are covered in various sections throughout this document, such as Section 4.2. If existing transport layer protocols are used for exchanging NSIS signaling messages, security vulnerabilities known for these protocols need to be considered. A detailed threat description of these protocols is outside the scope of this document.
protocol. Denial of service, for example, is covered in the NSIS-specific section, not because it cannot be carried out against other protocols, but because the methods used to carry out denial of service attacks tend to be protocol specific. Numerous illustrative examples provide insight into what can happen if these threats are not mitigated. This document repeatedly points out that not all of the threats are equally serious in every context. It does attempt to identify the scenarios in which security failures may have the highest impact. However, it is difficult for the protocol designer to foresee all the ways in which NSIS protocols will be used or to anticipate the security concerns of a wide variety of likely users. Therefore, the protocol designer needs to offer a full range of security capabilities and ways for users to negotiate and select what they need, on a case-by-case basis. To counter these threats, security requirements have been listed in [RFC3726].
[RFC4080] Hancock, R., Karagiannis, G., Loughney, J., and S. van den Bosch, "Next Steps in Signaling (NSIS): Framework", RFC 4080, June 2005. [RFC3726] Brunner, M., "Requirements for Signaling Protocols", RFC 3726, April 2004. [ALN00] Aura, T., Leiwo, J., and P. Nikander, "Towards Network Denial of Service Resistant Protocols, In Proceedings of the 15th International Information Security Conference (IFIP/SEC 2000), Beijing, China", August 2000. [AN97] Aura, T. and P. Nikander, "Stateless Connections", In Proceedings of the International Conference on Information and Communications Security (ICICS'97), Lecture Notes in Computer Science 1334, Springer", 1997. [2LEVEL] Braden, R. and B. Lindell, "A Two-Level Architecture for Internet Signaling", Work in Progress, November 2002. [RFC3697] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering, "IPv6 Flow Label Specification", RFC 3697, March 2004. [NATFW-NSLP] Stiemerling, M., "A NAT/Firewall NSIS Signaling Layer Protocol (NSLP)", Work in Progress, February 2005. [GIMPS] Schulzrinne, H., "GIMPS: General Internet Messaging Protocol for Signaling", Work in Progress, February 2005. [QOS-NSLP] Bosch, S., Karagiannis, G., and A. McDonald, "NSLP for Quality-of-Service signaling", Work in Progress, February 2005. [RSVP-SEC] Tschofenig, H., "RSVP Security Properties", Work in Progress, February 2005.
[SIG-ANAL] Manner, J. and X. Fu, "Analysis of Existing Quality- of-Service Signaling Protocols", RFC 4094, May 2005. [RFC1809] Partridge, C., "Using the Flow Label Field in IPv6", RFC 1809, June 1995. [RFC2745] Terzis, A., Braden, B., Vincent, S., and L. Zhang, "RSVP Diagnostic Messages", RFC 2745, January 2000. [RFC3182] Yadav, S., Yavatkar, R., Pabbati, R., Ford, P., Moore, T., Herzog, S., and R. Hess, "Identity Representation for RSVP", RFC 3182, October 2001. [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [RFC3520] Hamer, L-N., Gage, B., Kosinski, B., and H. Shieh, "Session Authorization Policy Element", RFC 3520, April 2003. [RFC3521] Hamer, L-N., Gage, B., and H. Shieh, "Framework for Session Set-up with Media Authorization", RFC 3521, April 2003. [RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.
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