BEHAVE D. Wing Internet-Draft J. Rosenberg Intended status: Standards Track Cisco Systems Expires: January 9, 2008 H. Tschofenig Nokia Siemens Networks July 8, 2007 Discovering, Querying, and Controlling Firewalls and NATs using STUN draft-wing-behave-nat-control-stun-usage-03 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on January 9, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract Simple Traversal Underneath NAT (STUN) is a mechanism for traversing NATs. STUN requests are transmitted through a NAT to external STUN servers. While this works very well, its two primary drawbacks are the inability to modify the properties of a NAT binding and the need to query a public STUN server for every new NAT binding (e.g., every phone call). These drawbacks require frequent messages which present Wing, et al. Expires January 9, 2008 [Page 1] Internet-Draft STUN Control July 2007 a load on servers (like SIP servers and STUN servers) and are bad for low speed access networks, such as cellular access. This document describes two mechanisms to discover NATs and firewalls and a mechanism to query and control them. With these mechanisms, binding discovery and keepalive traffic can be reduced to involve only the necessary NATs or firewalls. At the same time, backwards compatibility with NATs and firewalls that do not support this document is retained, which allows for incremental deployment of these mechanisms. This document is discussed on the SAFE mailing list, . Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Wing, et al. Expires January 9, 2008 [Page 2] Internet-Draft STUN Control July 2007 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 5 4. Discovery of Middleboxes (NATs and Firewalls) . . . . . . . . 6 4.1. Outside-In . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1.1. Nested NATs . . . . . . . . . . . . . . . . . . . . . 8 4.2. Tagging . . . . . . . . . . . . . . . . . . . . . . . . . 10 5. Query and Control . . . . . . . . . . . . . . . . . . . . . . 11 5.1. Client Procedures . . . . . . . . . . . . . . . . . . . . 11 5.2. Server Procedures . . . . . . . . . . . . . . . . . . . . 11 6. New Attributes . . . . . . . . . . . . . . . . . . . . . . . . 12 6.1. REFRESH-INTERVAL Attribute . . . . . . . . . . . . . . . . 12 6.2. XOR-INTERNAL-ADDRESS Attribute . . . . . . . . . . . . . . 12 6.3. PLEASE-TAG Attribute . . . . . . . . . . . . . . . . . . . 13 6.4. TAG Attribute . . . . . . . . . . . . . . . . . . . . . . 13 6.5. BOOTNONCE Attribute . . . . . . . . . . . . . . . . . . . 14 7. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7.1. Simple Security Model . . . . . . . . . . . . . . . . . . 15 7.2. Incremental Deployment . . . . . . . . . . . . . . . . . . 15 7.3. Optimize SIP-Outbound . . . . . . . . . . . . . . . . . . 15 7.4. Optimize ICE . . . . . . . . . . . . . . . . . . . . . . . 16 7.4.1. Candidate Gathering . . . . . . . . . . . . . . . . . 16 7.4.2. Keepalive . . . . . . . . . . . . . . . . . . . . . . 16 7.4.3. Learning STUN Servers without Configuration . . . . . 16 8. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 17 8.1. Overlapping IP Addresses with Nested NATs . . . . . . . . 17 8.2. Address Dependent NAT on Path . . . . . . . . . . . . . . 17 8.3. Address Dependent Filtering . . . . . . . . . . . . . . . 18 8.4. Interacting with Legacy NATs . . . . . . . . . . . . . . . 18 9. Security Considerations . . . . . . . . . . . . . . . . . . . 19 9.1. Authorization . . . . . . . . . . . . . . . . . . . . . . 19 9.2. Resource Exhaustion . . . . . . . . . . . . . . . . . . . 19 9.3. Comparison to Other NAT Control Techniques . . . . . . . . 19 9.4. Rogue STUN Server . . . . . . . . . . . . . . . . . . . . 20 10. Open Issues and Discussion Points . . . . . . . . . . . . . . 20 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 13.1. Normative References . . . . . . . . . . . . . . . . . . . 22 13.2. Informational References . . . . . . . . . . . . . . . . . 23 Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . . 24 A.1. Changes between -03 and 02 . . . . . . . . . . . . . . . . 24 Appendix B. Block Diagram of Internal NAT Operation . . . . . . . 24 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25 Intellectual Property and Copyright Statements . . . . . . . . . . 27 Wing, et al. Expires January 9, 2008 [Page 3] Internet-Draft STUN Control July 2007 1. Introduction Two common usages of Simple Traversal Underneath NAT (STUN) ([I-D.ietf-behave-rfc3489bis],[RFC3489]) are Binding Discovery and NAT Keepalive. The Binding Discovery usage allows a STUN client to learn its public IP address (from the perspective of the STUN server it contacted) and the NAT keepalive usage allows a STUN client to keep an active NAT binding alive. Unlike some other techniques (e.g., UPnP [UPnP], MIDCOM [RFC3303], Bonjour [Bonjour]), STUN does not interact directly with the NAT. Thus, STUN cannot request additional services from the NAT, such as longer lifetimes which would reduce keepalive messages. Furthermore, allocating new NAT bindings (e.g., each phone call) requires communication with a STUN server located somewhere on the Internet. This document describes three mechanisms for the STUN client to discover NATs and firewalls that are on path with its STUN server. After discovering the NATs and firewalls, the STUN client can query and control those devices using STUN. The STUN client needs to only ask those STUN servers (embedded in the NATs and firewalls) for public IP addresses and UDP ports, thereby offloading that traffic from the STUN server on the Internet. Additionally, the STUN client can ask the NAT's embedded STUN server to extend the NAT binding for the flow, and the STUN client can learn the IP address of the next- outermost NAT. By repeating this procedure with the next-outermost NAT, all of the NATs along that path can have their bindings extended. By learning all of the STUN servers on the path between the public Internet and itself, an endpoint can optimize the path of peer-to-peer communications. 2. Motivation There are a number of problems with existing NAT traversal techniques, such as STUN [RFC3489], [UPnP], and [Bonjour]): nested NATs: Today, many ISPs provide their subscribers with modems that have embedded NATs, often with only one physical Ethernet port. These subscribers then install NATs behind those devices to provide additional features, such as wireless access. Another example is a NAT in the basement of an apartment building or a campus dormitory, which combined with a NAT within the home or dormitory room also result in nested NATs. In both of these situations, UPnP and Bonjour no longer function at all, as those protocols can only control the first NAT closest to the host. STUN continues to function, but is unable to optimize network traffic behind those nested NATs (e.g., traffic that stays within the same house or Wing, et al. Expires January 9, 2008 [Page 4] Internet-Draft STUN Control July 2007 within the same apartment building). One technique to avoid nested NATs is to disable one of the NATs, if it obtains an RFC 1918 address on its WAN interface. This merely sidesteps the problem. This technique is also ineffective if the ISP is NATting its subscribers and the ISP restricts each subscriber to one IP address. The technique described in this document allows optimization of the traffic behind those NATs so that the traffic can traverse the fewest NATs possible. chattiness: To perform its binding discovery, a STUN client communicates to a server on the Internet. This consumes bandwidth across the user's access network, which in some cases is bandwidth constrained (e.g., wireless, satellite). STUN's chattiness is often cited as a reason to use other NAT traversal techniques such as UPnP or Bonjour. The technique described in this document provides a significant reduction in STUN's chattiness, to the point that the only time a STUN client needs to communicate with the STUN server on the public Internet is when the STUN client is initialized. incremental deployment: Many other NAT traversal techniques require the endpoint and its NAT to both support the new feature or else NAT traversal is not possible at all. The technique described in this document allows incremental deployment of local endpoints and NATs that support STUN Control. If the local endpoint, or its NATs, does not support the STUN Control functionality, then STUN (see [I-D.ietf-behave-rfc3489bis]) and ICE [I-D.ietf-mmusic-ice] procedures are used to traverse the NATs without the optimizations described in this document. 3. Overview of Operation This document describes three functions, which are all implemented using the STUN protocol: Wing, et al. Expires January 9, 2008 [Page 5] Internet-Draft STUN Control July 2007 Discovery of Middleboxes (NATs and Firewalls): This document describes two techniques for finding NATs or firewalls (see Section 4). These two approaches are: Outside-In: Uses STUN to find the outer-most NAT and works itself towards the host. Tagging: Send a STUN Request packet to your STUN server, and asks for compliant firewalls along the path to indicate their presence by adding an IP address to the STUN Response packet. Querying Discovered Middleboxes: After discovering a NAT or a firewall, it is useful to determine characteristics of the NAT binding or the firewall pinhole. Two of the most useful things to learn is the duration the NAT binding or firewall pinhole will remain open if there is no traffic, and the filtering applied to that binding or pinhole. This is described in Section 5. Controlling Discovered Middleboxes: A NAT or a firewall might default to a more restrictive behavior than desired by an application (e.g., aggressive timeout, filtering). Requesting the NAT or firewall to change its default behavior is useful for traffic optimization (e.g., reduce keepalive traffic) and network optimization (e.g., adjust filters to eliminate the need for a media relay device [I-D.ietf-behave-turn]). A discussion of this functionality can be found in Section 5. 4. Discovery of Middleboxes (NATs and Firewalls) This document investigates two techniques to discover a NAT and a firewall: outside-in and by tagging. Ideally, a single technique could be selected as an outcome of the standardization process. However, it is possible to combine these two techniques. 4.1. Outside-In When a STUN client sends a STUN Request to a STUN server, it receives a STUN Response that indicates the IP address and UDP port seen by the STUN server. If the IP address and UDP port differs from the IP address and UDP port of the socket used to send the request, the STUN client knows there is at least one NAT between itself and the STUN Wing, et al. Expires January 9, 2008 [Page 6] Internet-Draft STUN Control July 2007 server, and knows the 'public' IP address (and port) allocated by the outermost NAT. For example, in the following diagram, the STUN client learns the public IP address of its NAT is 192.0.2.1: +--------+ +---------------+ | STUN | | 192.0.2.1 +--------+ | Client +-------------+ +------+ STUN | | 10.1.1.2/4193 10.1.1.1 | | Server | +--------+ | | +--------+ | NAT with | | Embedded STUN | | Server | +---------------+ Figure 1: One NAT with embedded STUN server After learning the public IP address of its outer-most NAT, the STUN client attempts to communicate with the STUN server embedded in that outer-most NAT. The STUN client does this by sending a STUN Binding Request to the NAT's public IP address. The NAT will return a STUN Binding Response message including the XOR-INTERNAL-ADDRESS attribute, which will indicate the IP address and UDP port seen on the *internal* side of the NAT for that translation (see Figure 14 in Appendix B). In the example above, the IP address and UDP port indicated in XOR-INTERNAL-ADDRESS are the same as that used by the STUN client (10.1.1.2/4193), which indicates there are no other NATs between the STUN client and that outer-most NAT. STUN Client NAT STUN Server | | | 1. |-----Binding Request (UDP)--------------->| 2. |<----Binding Response (UDP)---------------| | | | 3. |--Binding Request (UDP)------->| | 4. |<-Binding Response (UDP)-------| | | | | Figure 2: Communication Flow In the call flow above, steps 1 and 2 correspond to the STUN behavior described in [I-D.ietf-behave-rfc3489bis]: Wing, et al. Expires January 9, 2008 [Page 7] Internet-Draft STUN Control July 2007 1: The STUN client sends a UDP Binding Request to its STUN server that is located on the Internet. 2: The STUN server on the Internet responds with a UDP Binding Response. The next steps are the additional steps performed by a STUN client that has implemented the STUN Control functionality: 3: The STUN client sends a UDP Binding Request to the IP address learned from the STUN server on the Internet. This will be received by the STUN server embedded in the outer-most NAT. 4: The STUN server (embedded in the NAT) responds with a UDP Binding Response. The response obtained in message (4) contains the XOR-MAPPED-ADDRESS attribute, which will have the same value as when the STUN server on the Internet responded (in step 2). The STUN client can perform steps (3) and (4) for any new UDP communication, without needing to repeat steps (1) and (2). This meets the desire to reduce chattiness. The STUN client also only needs to send keepalives towards the outer-most NAT's IP address, as well (reduces chatter for SIP outbound [I-D.ietf-sip-outbound]). The response obtained in message (4) will also contain the XOR- INTERNAL-ADDRESS, which allows the STUN client to repeat steps (3) and (4) in order to query or control those on-path NATs between itself and its STUN server on the Internet. This is described in detail in Section 4.1.1. This functionality meets the need to optimize traffic between nested NATs, without requiring configuration of intermediate STUN servers. The STUN client can request each NAT to increase the binding lifetime, as described in Section 6.1. The STUN client receives positive confirmation that the binding lifetime has been extended, allowing the STUN client to significantly reduces its NAT keepalive traffic. Additionally, as long as the NAT complies with [RFC4787] (which is indicated by its support of this document), the STUN client's keepalive traffic need only be sent to the outer-most NAT's IP address. This functionality meets the need to reduce STUN's chattiness. 4.1.1. Nested NATs Nested NATs are controlled individually. The nested NATs are discovered, from outer-most NAT to the inner-most NAT, using the XOR- INTERNAL-ADDRESS attribute. Wing, et al. Expires January 9, 2008 [Page 8] Internet-Draft STUN Control July 2007 The example in Figure 3 shows how a STUN client iterates over the NATs to communicate with all of the NATs in the path. The following figure shows two nested NATs: +------+ +--------+ +--------+ | 192.168.1.2 | 10.1.1.2 | 192.0.2.1 +-----------+ | STUN +------+ NAT-B +-----+ NAT-A +------+STUN Server| |Client| 192.168.1.1 | 10.1.1.1 | +-----------+ +------+ +--------+ +--------+ Figure 3: Two nested NATs with embedded STUN servers First, the STUN client would learn the outer-most NAT's IP address by performing the steps shown in Figure 2. In the case below, however, the IP address and UDP port indicated by the XOR-INTERNAL-ADDRESS will not be the STUN client's own IP address and UDP port -- rather, it is the IP address and UDP port on the *outer* side of the NAT-B -- 10.1.1.2. Because of this, the STUN client repeats the procedure and sends another STUN Binding Request to that newly-learned address (the *outer* side of NAT-B). NAT-B will respond with a STUN Binding Response containing the XOR-INTERNAL-ADDRESS attribute, which will match the STUN client's IP address and UDP port. The STUN client knows there are no other NATs between itself and NAT-B, and finishes. The message flow with two nested NATs is shown below: STUN Client NAT-B NAT-A STUN Server | | | | 1. |-----Binding Request (UDP)--------------->| 2. |<----Binding Response (UDP)---------------| | | | | 3. |--Binding Request (UDP)------->| | } 4. |<-Binding Response (UDP)-------| | } NAT Control | | | | } STUN Usage 5. |--Binding Req (UDP)-->| | | } 6. |<-Binding Resp (UDP)--| | | } | | | | Figure 4: Message Flow for Outside-In with Two NATs Once a shared secret has been obtained with each of the on-path NATs, the STUN client no longer needs the TLS/TCP connection -- all subsequent bindings for individual UDP streams (that is, for each Wing, et al. Expires January 9, 2008 [Page 9] Internet-Draft STUN Control July 2007 subsequent call) are obtained by just sending a Binding Request to each of the NATs. By sending a Binding Request to both NAT-A and NAT-B, the STUN client has the opportunity to optimize the packet flow if their UDP peer is also behind the same NAT. 4.2. Tagging To discover an on-path firewall, the PLEASE-TAG attribute is used with a STUN Binding Request (a STUN packet sent to UDP/3478) message. A firewall would inspect bypassing Binding Request messages and determine whether there is a PLEASE-TAG attribute. When the firewall sees the associated Binding Response, the firewall appends a TAG attribute as the last attribute of the Binding Response. This TAG attribute contains the firewall's management IP address and UDP port. Each on-path firewall would be able to insert its own TAG attribute. In this way, the STUN Response would contain a pointer to each of the on-path firewalls between the client and that STUN server. Motivation for developing the Tagging mechanism: The Outside-In discovery technique (Section 4.1) uses the public IP address of the NAT to find the outer-most NAT that supports STUN Control. Firewalls do not translate packets and hence a different technique is needed to identify firewalls. Note that tagging is similar to how NSIS [I-D.ietf-nsis-nslp-natfw], TIST [I-D.shore-tist-prot], and NLS [I-D.shore-nls-tl] function. This figure shows how tagging functions. STUN Client firewall STUN Server | | | 1. |--Binding Request->|------------------>| 2. | |<-Binding Response-| 3. | [inserts tag] | 4. |<-Binding Response-| | 5. [firewall discovered] | | Figure 5: Tagging Message Flow 1. A Binding Request, containing the PLEASE-TAG attribute, is sent to the IP address of the STUN server that is located somewhere on the Internet. This is seen by the firewall, and the firewall remembers the STUN transaction id, and permits the STUN Binding Request packet. 2. When the firewall observes a STUN Binding Response packet it checks its cache for the previously stored STUN transaction id. Wing, et al. Expires January 9, 2008 [Page 10] Internet-Draft STUN Control July 2007 If a previous STUN transaction id was found then the firewall inserts the TAG attribute, which contains the firewall's management address. 3. The firewall sends the (modified) STUN Binding Response towards the STUN client. 4. The STUN client has now discovered the firewall, and can query it or control it. 5. Query and Control This section describes how to use STUN to query and control a NAT that was discovered using the technique described in Section 4. 5.1. Client Procedures After discovering on-path NATs and firewalls with the procedure described in Section 4, the STUN client begins querying and controlling those devices. To modify an existing NAT mapping's attributes, or to request a new NAT mapping for a new UDP port, the STUN client can now send a STUN Binding Request to the IP address of address of the respective NAT or firewall (at STUN UDP port (3478)). Client produces for handling the BOOTNONCE attribute can be found in Section 6.5. 5.2. Server Procedures When receiving a STUN Binding Request the STUN controlled NAT will respond with a STUN Binding Response containing an XOR-MAPPED-ADDRESS attribute (which points at the NAT's public IP address and port -- just as if the STUN Binding Request had been sent to a STUN server on the public Internet) and an XOR-INTERNAL-ADDRESS attribute (which points to the source IP address and UDP port the packet STUN Binding Request packet had prior to being NATted). For example, looking at Figure 1, the XOR-INTERNAL-ADDRESS is the IP address and UDP port prior to the NAPT operation. If there is only one NAT, as shown in Figure 1, XOR-INTERNAL-ADDRESS would contain the STUN client's own IP address and UDP port. If there are multiple NATs, XOR-INTERNAL-ADDRESS would indicate the IP address of the next NAT (that is, the next NAT closer to the STUN client). Wing, et al. Expires January 9, 2008 [Page 11] Internet-Draft STUN Control July 2007 When receiving a STUN Binding Request the STUN controlled firewall will respond with a STUN Binding Response containing an XOR-MAPPED- ADDRESS attribute (which points at the public IP address and port) and an XOR-INTERNAL-ADDRESS attribute (which points to the source IP address of the interface and UDP port where the packet STUN Binding Request packet was received, i.e., the internal interface. Server produces for handling the BOOTNONCE attribute can be found in Section 6.5. STUN Binding Requests, which arrived from its public interface(s), MAY be handled as if the server is not listening on that port (e.g., return an ICMP error) -- this specification does not need them. 6. New Attributes 6.1. REFRESH-INTERVAL Attribute In a STUN request per [I-D.ietf-behave-rfc3489bis], the REFRESH- INTERVAL attribute indicates the number of milliseconds that the client wants the NAT binding (or firewall pinhole) to be opened. When the NAT Keepalive usage is being used, the server may become overloaded with keepalive messages (Binding Requests or other application-level keepalive messages). The REFRESH-INTERVAL provies a mechanism for the client to learn and adjust the NAT's binding lifetime and thus reduce the frequency of client-initiated keepalive messages. In a STUN response, the same attribute indicates how long the STUN controlled NAT (or a STUN controlled firewall) is willing to allocate the binding (or to create the pinhole). REFRESH-INTERVAL is specified as an unsigned 32 bit integer, and represents an interval measured in milliseconds (thus the maximum value is approximately 50 days). This attribute can be present in Binding Requests and in Binding Responses. 6.2. XOR-INTERNAL-ADDRESS Attribute This attribute MUST be present in a Binding Response and is necessary to allow a STUN client to perform the outside-in discovery technique, in order to discover all of the STUN Control-aware NATs along the path. Wing, et al. Expires January 9, 2008 [Page 12] Internet-Draft STUN Control July 2007 The format of the XOR-INTERNAL-ADDRESS attribute is: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |x x x x x x x x| Family | X-Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | X-Address (32 bits or 128 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: XOR-INTERNAL-ADDRESS Attribute The meaning of Family, X-Port, and X-Address are exactly as in [I-D.ietf-behave-rfc3489bis]. The length of X-Address depends on the address family (IPv4 or IPv6). 6.3. PLEASE-TAG Attribute If a STUN client wants to discover on-path firewalls, it MUST include this attribute in its Binding Response when performing the Binding Discovery usage. STUN servers are not expected to understand this attribute; if they return this attribute as an unknown attribute, it does not affect the operation described in this document. The format of the PLEASE-TAG attribute is: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Mech.|x x x x x x x x x x x x x x x x x x x x x x x x x x x x x| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: PLEASE-TAG Attribute The 3-bit Mechanism field indicates the control mechanism desired. Currently, the only defined mechanism is STUN Control, and is indicated with all zeros. The intent of this field is to allow additional control mechanisms (e.g., UPnP, Bonjour, MIDCOM). 6.4. TAG Attribute The TAG attribute contains the XOR'd management transport address of the middlebox. Typically, a firewall as well as a NAT may find this technique useful as well. Wing, et al. Expires January 9, 2008 [Page 13] Internet-Draft STUN Control July 2007 If the associated STUN Request contained the PLEASE-TAG attribute, a middlebox MUST append this attribute as the last attribute of the STUN Response (with that same transaction-id). After appending this attribute, the STUN length field MUST be also be adjusted. The format of the TAG attribute is: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Mech.|M|x x x x| Family | X-Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | X-Address (32 bits or 128 bit) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: TAG Attribute Mech: The 3-bit Mechanism field indicates the control mechanism supported on the described port. Currently, the only defined mechanism is STUN Control, and is indicated with 0x0. The intent of this field is to allow additional control mechanisms (e.g., UPnP, Bonjour, MIDCOM). The one-bit M field indicates if this firewall permits Mobility Header packets to flow through it ([RFC3775]). The meaning of Family, X-Port, and X-Address are exactly as in [I-D.ietf-behave-rfc3489bis]. The length of X-Address depends on the address family (IPv4 or IPv6). 6.5. BOOTNONCE Attribute The BOOTNONCE attribute protects against the attack described in Section 9.4. Client procedures: The STUN client expects each NAT to return the same BOOTNONCE value each time that NAT is contacted. If a NAT returns a different value, the STUN client MUST NOT use any information returned in the Binding Response and MUST re-run the NAT Control procedures from the beginning (i.e., obtain its public IP address from the STUN server on the Internet). This would only occur if an attack is in progress or if the NAT rebooted. If the NAT rebooted, it is good practice to re-run the NAT Control procedures anyway, as the network topology could be different as well. Server procedures: This attribute's value is a hash of the STUN client's IP address and a value that is randomly-generated each time Wing, et al. Expires January 9, 2008 [Page 14] Internet-Draft STUN Control July 2007 the NAT is initialized. The STUN client's IP address is included in this hash to thwart an attacker attaching to the NAT's internal network and learning the BOOTNONCE value. The format of the BOOTNONCE attribute is: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Boot Nonce value (32 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: BOOTNONCE Attribute 7. Benefits 7.1. Simple Security Model Unlike other middlebox control techniques which have relatively complex security models because a separate control channel is used, STUN Control's is simple. It's simple because only the flow being used can be controlled (e.g., have its NAT timeout queried or extended). Other flows cannot be created, queried, or controlled via STUN Control. 7.2. Incremental Deployment NAT Control can be incrementally deployed. If the outer-most NAT does not support it, the STUN client behaves as normal. In this case, the tagging procedure described in Section 4.2, will still allow to gain some optimizations. Where the outer-most STUN NAT does support it, the STUN client can gain some significant optimizations as described in the following sections. Likewise, there is no change required to applications if NATs are deployed which support NAT Control: such applications will be unaware of the additional functionality in the NAT, and will not be subject to any worse security risks due to the additional functionality in the NAT. 7.3. Optimize SIP-Outbound In SIP outbound [I-D.ietf-sip-outbound], the SIP proxy is also the STUN server. STUN Control as described in this document enables two optimizations of SIP-Outbound's keepalive mechanism: Wing, et al. Expires January 9, 2008 [Page 15] Internet-Draft STUN Control July 2007 1. STUN keepalive messages need only be sent to the outer-most NAT, rather than across the access link to the SIP proxy, which vastly reduces the traffic to the SIP proxy, and; 2. all of the on-path NATs can explicitly indicate their timeouts, reducing the frequency of keepalive messages. 7.4. Optimize ICE The NAT Control usage provides several opportunities to optimize ICE [I-D.ietf-mmusic-ice], as described in this section. 7.4.1. Candidate Gathering During its candidate gathering phase, an ICE endpoint normally contacts a STUN server on the Internet. If an ICE endpoint discovers that its outer-most NAT runs a STUN server, the ICE endpoint can use the outer-most NAT's STUN server rather than using the STUN server on the Internet. This saves access bandwidth and reduces the reliance on the STUN server on the Internet -- the STUN server on the Internet need only be contacted once -- when the ICE endpoint first initializes. 7.4.2. Keepalive ICE uses STUN Indications as its primary media stream keepalive mechanism. This document enables two optimizations of ICE's keepalive technique: 1. STUN keepalive messages need only be sent to the outer-most NAT, rather than across the access link to the remote peer, and; 2. all of the on-path NATs can explicitly indicate their timeouts, which allows reducing the keepalive frequency. 7.4.3. Learning STUN Servers without Configuration ICE allows endpoints to have multiple STUN servers, but it is difficult to configure all of the STUN servers in the ICE endpoint -- it requires some awareness of network topology. By using the 'walk backward' technique described in this document, all the on-path NATs and their embedded STUN servers can be learned without additional configuration. By knowing the STUN servers at each address domain, ICE endpoints can optimize the network path between two peers. Wing, et al. Expires January 9, 2008 [Page 16] Internet-Draft STUN Control July 2007 For example, if endpoint-1 is only configured with the IP address of the STUN server on the left, endpoint-1 can learn about NAT-B and NAT-A. Utilizing the STUN server in NAT-A, endpoint-1 and endpoint-2 can optimize their media path so they make the optimal path from endpoint-1 to NAT-A to endpoint-2: +-------+ +-------+ +-------------+ endpoint-1---| NAT-A +--+--+ NAT-B +-------| STUN Server | +-------+ | +-------+ +-------------+ | endpoint-2 8. Limitations 8.1. Overlapping IP Addresses with Nested NATs If nested NATs have overlapping IP address space, there will be undetected NATs on the path. When this occurs, the STUN client will be unable to detect the presence of NAT-A if NAT-A assigns the same UDP port. For example, in the following figure, NAT-A and NAT-B are both using 10.1.1.x as their 'private' network. +------+ +--------+ +--------+ | 10.1.1.2 | 10.1.1.2 | 192.0.2.1 | STUN +-------+ NAT-A +-----+ NAT-B +------ |client| 10.1.1.1 | 10.1.1.1 | +------+ +--------+ +--------+ Figure 11: Overlapping Addresses with Nested NATs When this situation occurs, the STUN client can only learn the outer- most address. This is not a problem -- the STUN client is still able to communicate with the outer-most NAT and is still able to avoid consuming access network bandwidth and avoid communicating with the public STUN server. All that is lost is the ability to optimize paths within the private network that has overlapped addresses. Of course when such an overlap occurs the end host (STUN client) cannot successfully establish bi-directional communication with hosts in the overlapped network, anyway. 8.2. Address Dependent NAT on Path In order to utilize the mechanisms described in this document, a STUN Request is sent from the same source IP address and source port as the original STUN Binding Discovery message, but is sent to a different destination IP address -- it is sent to the IP address of Wing, et al. Expires January 9, 2008 [Page 17] Internet-Draft STUN Control July 2007 an on-path NAT. If there is an on-path NAT, between the STUN client and the STUN server, with 'address dependent' or 'address and port- dependent' mapping behavior (as described in Section 4.1 of [RFC4787]), that NAT will prevent a STUN client from taking advantage of the technique described in this document. When this occurs, the ports indicated by XOR-MAPPED-ADDRESS from the public STUN server and the NAT's embedded STUN server will differ. An example of such a topology is shown in the following figure: +------+ +--------+ +--------+ | STUN | | 10.1.1.2 | 192.0.2.1 |client+-----+ NAT-A +---+ NAT-B +------ | | 10.1.1.1 | 10.1.1.1 | +------+ +--------+ +--------+ In this figure, NAT-A is a NAT that has address dependent mapping. Thus, when the STUN client sends a STUN Binding Request to 192.0.2.1 on UDP/3478, NAT-A will choose a new public UDP port for that communication. NAT-B will function normally, returning a different port in its XOR-MAPPED-ADDRESS, which indicates to the STUN client that a symmetric NAT exists between the STUN client and the STUN server it just queried (NAT-B, in this example). Figure 12: Address Dependant NAT on Path 8.3. Address Dependent Filtering If there is an NAT along the path that has address dependent filtering (as described in section 5 of [RFC4787]), and the STUN client sends a STUN packet directly to any of the on-path NATs public addresses, the address-dependent filtering NAT will filter packets from the remote peer. Thus, after communicating with all of the on- path NATs the STUN client MUST send a UDP packet to the remote peer, if the remote peer is known. 8.4. Interacting with Legacy NATs There will be cases where the STUN client attempts to communicate with an on-path NAT, which does not support the outside-in usage. There are two cases: o the NAT does not run a STUN server on its public interface (this will be the most common) o the NAT does run a STUN server on its public interface, but does not return the XOR-INTERNAL-ADDRESS attribute defined in this document Wing, et al. Expires January 9, 2008 [Page 18] Internet-Draft STUN Control July 2007 In both cases the optimizations described in this section will not be available to the STUN client. This is no worse than the condition today. This allows incremental upgrades of applications and NATs that implement the technique described in this document. 9. Security Considerations This security considerations section will be expanded in a subsequent version of this document. So far, the authors have identified the following considerations: 9.1. Authorization Only hosts that are 'inside' a NAT, which a NAT is already providing services for, can query or adjust the timeout of a NAT mapping. A discussion of additional authorization mechanisms that might be needed for firewall traversal can be found at [I-D.wing-session-auth]. 9.2. Resource Exhaustion A malicious STUN client could ask for absurdly long NAT bindings (days) for many UDP sessions, which would exhaust the resources in the NAT. The same attack is possible (without considering this document and without considering STUN or other UNSAF [RFC3424] NAT traversal techniques) -- a malicious TCP (or UDP) client can open many TCP (or UDP) connections, and keep them open, causing resource exhaustion in the NAT. 9.3. Comparison to Other NAT Control Techniques Like UPnP, Bonjour, and host-initiated MIDCOM, the STUN usage described in this document allows a host to learn its public IP address and UDP port mapping, and to request a specific lifetime for that mapping. However, unlike those technologies, the NAT Control usage described in this document only allows each UDP port on the host to create and adjust the mapping timeout of its own NAT mappings. Specifically, an application on a host can only adjust the duration of a NAT bindings for itself, and not for another application on that same host, and not for other hosts. This provides security advantages over other NAT control mechanisms where malicious software on a host can surreptitiously create NAT mappings to another application or to another host. Wing, et al. Expires January 9, 2008 [Page 19] Internet-Draft STUN Control July 2007 9.4. Rogue STUN Server Using the mechanisms described in this document, a STUN client learns the public IP addresses of its NATs which support the mechanisms described in this document. However, without the STUN client's knowledge, those NATs may acquire a new IP address (e.g., DHCP lease expiration, a moving network). When any of those NAT acquire a new IP address without the STUN client's knowledge, the STUN client will send a STUN Binding Request to the NAT's previous public IP address. If an attacker were to run a rogue STUN server on that address, the attacker will have effectively compromised the STUN server, as described in Section 12.2.1 of [RFC3489]. The attacker, upon receiving STUN Binding Requests, will reply with STUN Binding Responses indicating an IP address the attacker controls. The attacker will thus ensure access to whatever media stream is being established by the STUN client (e.g., RTP traffic). When such an attack occurs, the STUN client is unable to distinguish the attacker's replies from legitimate replies from the STUN server embedded in the STUN client's NAT. To defend against this attack, the STUN server embedded in the NAT returns a BOOTNONCE value. The STUN client validates that it receives the same BOOTNONCE value in each STUN Binding Response from that NAT. A weakness of this approach is that an attacker can learn the BOOTNONCE value if the attacker is able to connect to the NAT's internal network prior to initiating the attack; this is plausible if the internal network has no security (e.g., public WiFi network). For this reason, it is RECOMMENDED that the BOOTNONCE value is hashed with the STUN client's IP address. Doing so means the attacker must acquire the same IP address as the victim from behind the NAT (to learn the BOOTNONCE), and must also acquire the NAT's previous public IP address. 10. Open Issues and Discussion Points o Discussion Point: After discovering NATs and firewalls, controlling those devices might also be done with a middlebox control protocol (e.g., by using standard or slightly modified versions of SIMCO, UPnP, MIDCOM, or Bonjour). This is open for discussion as this document is scoped within the IETF. o Discussion Point: Tagging would also be useful for the Connectivity Check usage (which is used by ICE), especially considering that a different firewall may be traversed for media than for the initial Binding Discovery usage. In such a Wing, et al. Expires January 9, 2008 [Page 20] Internet-Draft STUN Control July 2007 situation, the new on-path firewall's policy might not allow a binding request to leave the network or allow a binding response to return. In this case, the firewall would need to indicate its presence to the STUN client in another way. An ICMP error message may be appropriate, and an ICMP extension [RFC4884] could indicate the firewall is controllable. o Open issue: We could resolve the problem of address dependant NATs along the path by introducing a new STUN attribute which indicates the UDP port the STUN client wants to control. However, this changes the security properties of STUN Control, so this seems undesirable. Open issue: When the STUN client detects an address dependant NAT, should we recommend it abandon the STUN Control usage, and revert to operation as if it doesn't support the STUN Control usage? o Open issue: How many filter entries are in address dependent filtering NATs? If only one, this does become a real limitation if NATs are nested; if they're not nested, the outer-most NAT can avoid overwriting its own address in its address dependent filter. o Discussion: One way to thwart a resource consumption attack is to challenge the STUN client. This would allow the STUN server to delay the establishment of resources before a return-routability test is performed. This functionality is currently not provided by this specification. The NONCE attribute [I-D.ietf-behave-rfc3489bis] could be useful to provide this function. However, the mere sending of a UDP packet across a NAT creates a binding (for ~2 minutes), and there isn't a return- routability check for that. o The inside-out discovery technique was removed with version -03 of this document. The procedure worked as follows: The STUN client sends a STUN request to UDP/3478 of the IP address of its default router. If there is a STUN server listening there, it will respond, and will indicate its default route via the new DEFAULT- ROUTE attribute. With that information, the STUN client can discover the next-outermost NAT by repeating the procedure. More feedback is needed to determine whether the functionality is needed. Wing, et al. Expires January 9, 2008 [Page 21] Internet-Draft STUN Control July 2007 11. IANA Considerations This section registers new STUN attributes per the procedures in [I-D.ietf-behave-rfc3489bis]: Mandatory range: 0x0029 XOR-INTERNAL-ADDRESS 0x00.. BOOTNONCE Optional range: 0x8024 REFRESH-INTERVAL 0x80.. PLEASE-TAG 0x80.. TAG 12. Acknowledgements Thanks to Remi Denis-Courmont, Bajko Gabor, Markus Isomaki, Cullen Jennings, and Philip Matthews for their suggestions which have improved this document. 13. References 13.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [I-D.ietf-behave-rfc3489bis] Rosenberg, J., "Session Traversal Utilities for (NAT) (STUN)", draft-ietf-behave-rfc3489bis-06 (work in progress), March 2007. [RFC4787] Audet, F. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, January 2007. [RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN - Simple Traversal of User Datagram Protocol (UDP) Through Network Address Translators (NATs)", RFC 3489, March 2003. [RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in IPv6", RFC 3775, June 2004. Wing, et al. Expires January 9, 2008 [Page 22] Internet-Draft STUN Control July 2007 13.2. Informational References [I-D.ietf-behave-turn] Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal Underneath NAT (STUN)", draft-ietf-behave-turn-03 (work in progress), March 2007. [UPnP] UPnP Forum, "Universal Plug and Play", 2000, . [Bonjour] Apple Computer, "Bonjour", 2005, . [RFC3303] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A. Rayhan, "Middlebox communication architecture and framework", RFC 3303, August 2002. [I-D.ietf-mmusic-ice] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", draft-ietf-mmusic-ice-16 (work in progress), June 2007. [I-D.ietf-sip-outbound] Jennings, C. and R. Mahy, "Managing Client Initiated Connections in the Session Initiation Protocol (SIP)", draft-ietf-sip-outbound-09 (work in progress), June 2007. [I-D.ietf-nsis-nslp-natfw] Stiemerling, M., "NAT/Firewall NSIS Signaling Layer Protocol (NSLP)", draft-ietf-nsis-nslp-natfw-14 (work in progress), March 2007. [RFC4884] Bonica, R., Gan, D., Tappan, D., and C. Pignataro, "Extended ICMP to Support Multi-Part Messages", RFC 4884, April 2007. [I-D.shore-tist-prot] Shore, M., "The TIST (Topology-Insensitive Service Traversal) Protocol", draft-shore-tist-prot-00 (work in progress), May 2002. [I-D.shore-nls-tl] Shore, M., "Network-Layer Signaling: Transport Layer", draft-shore-nls-tl-05 (work in progress), June 2007. [RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-Address Fixing (UNSAF) Across Network Address Wing, et al. Expires January 9, 2008 [Page 23] Internet-Draft STUN Control July 2007 Translation", RFC 3424, November 2002. [I-D.wing-session-auth] Wing, D., "Media Session Authorization", draft-wing-session-auth-00 (work in progress), February 2006. Appendix A. Changes A.1. Changes between -03 and 02 o Removed TLS from normal STUN operation (as few use it, and ICE makes it unnecessary anyway) o BOOTNONCE attribute replaces STUN Control's previous use of TLS. o Added "MIP-capable" bit to TAG attribute o Removed "inside-out" discovery technique. Appendix B. Block Diagram of Internal NAT Operation Wing, et al. Expires January 9, 2008 [Page 24] Internet-Draft STUN Control July 2007 Internally, the NAT can be diagrammed to function like this, where the NAT operation occurs before the STUN server: | | outside interface | +---------+---------------+ | | | | | +--------+ | | |----+ STUN | | | | | Server | | | | +--------+ | | | | | +-------+-------------+ | | | NAT Function | | | +-------+-------------+ | | | | +---------+---------------+ | | inside interface | | Figure 14: Block Diagram of Internal NAT Operation Authors' Addresses Dan Wing Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 USA Email: dwing@cisco.com Jonathan Rosenberg Cisco Systems, Inc. Edison, NJ 07054 USA Email: jdrosen@cisco.com Wing, et al. Expires January 9, 2008 [Page 25] Internet-Draft STUN Control July 2007 Hannes Tschofenig Nokia Siemens Networks Otto-Hahn-Ring 6 Munich, Bavaria 81739 Germany Email: Hannes.Tschofenig@nsn.com URI: http://www.tschofenig.com Wing, et al. Expires January 9, 2008 [Page 26] Internet-Draft STUN Control July 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 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The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Wing, et al. Expires January 9, 2008 [Page 27]