BEHAVE D. MacDonald Internet-Draft CounterPath Solutions, Inc. Intended status: Standards Track B. Lowekamp Expires:
August 26,December 29, 2007 SIPeerior Technologies and William & Mary February 22,June 27, 2007 NAT Behavior Discovery Using STUN draft-ietf-behave-nat-behavior-discovery-00draft-ietf-behave-nat-behavior-discovery-01 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 August 26,December 29, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract This specification defines a usage of the Simple Traversal Underneath Network Address Translators (NAT) (STUN) Protocol that allows applications to discoverdiscovers the presence and current behaviour of NATs and firewalls between themthe STUN client and the STUN server. Requirements Language 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 RFC 2119 [RFC2119]. Table of Contents 1. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Overview of Operations . . . . . . . . . . . . . . . . . . . . 5 3.1. Determining NAT Mapping . . . . . . . . . . . . . . . . . 5 3.2. Determining NAT Filtering . . . . . . . . . . . . . . . . 6 3.3. Binding Lifetime Discovery . . . . . . . . . . . . . . . . 6 3.4. Diagnosing NAT Hairpinning . . . . . . . . . . . . . . . . 6 3.5. Determining Fragment Handling . . . . . . . . . . . . . . 67 3.6. Detecting Generic ALGs . . . . . . . . . . . . . . . . . . 7 4. Discovery Process . . . . . . . . . . . . . . . . . . . . . . 7 4.1. Checking if UDP is Blocked . . . . . . . . . . . . . . . . 7 4.2. Determining NAT Mapping Behavior . . . . . . . . . . . . . 78 4.3. Determining NAT Filtering Behavior . . . . . . . . . . . . 8 4.4. Combining and Ordering Tests . . . . . . . . . . . . . . . 89 4.5. Binding Lifetime Discovery . . . . . . . . . . . . . . . . 9 5. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 10 5.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 11 5.2. Security . . . . . . . . . . . . . . . . . . . . . . . . . 11 6. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 12 6.1. Preparing the Response . . . . . . . . . . . . . . . . . . 12 7. New Attributes . . . . . . . . . . . . . . . . . . . . . . . . 14 7.1. Representing Transport Addresses . . . . . . . . . . . . . 15 7.2. RESPONSE-ADDRESS . . . . . . . . . . . . . . . . . . . . . 15 7.3.CHANGE-REQUEST . . . . . . . . . . . . . . . . . . . . . . 15 126.96.36.199. SOURCE-ADDRESS . . . . . . . . . . . . . . . . . . . . . . 16 188.8.131.52. OTHER-ADDRESS . . . . . . . . . . . . . . . . . . . . . . 16 7.6. REFLECTED-FROM . . . . . . . . . . . . . . . . . . . . . . 16 184.108.40.206. XOR-REFLECTED-FROM . . . . . . . . . . . . . . . . . . . . 17 7.8.16 7.6. XOR-RESPONSE-ADDRESS . . . . . . . . . . . . . . . . . . . 17 7.9.16 7.7. PADDING . . . . . . . . . . . . . . . . . . . . . . . . . 17 220.127.116.11. CACHE-TIMEOUT . . . . . . . . . . . . . . . . . . . . . . 17 8. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 1817 8.1. Problem Definition . . . . . . . . . . . . . . . . . . . . 18 8.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 18 8.3. Brittleness Introduced by STUN NAT Behavior Discovery . . 1918 8.4. Requirements for a Long Term Solution . . . . . . . . . . 19 8.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . . 2019 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 10. Security Considerations . . . . . . . . . . . . . . . . . . . 20 11. AcknowledgementsOpen Issues . . . . . . . . . . . . . . . . . . . . . . . . . 21 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 18.104.22.168. Normative References . . . . . . . . . . . . . . . . . . . 21 22.214.171.124. Informative References . . . . . . . . . . . . . . . . . . 22 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 22 A.1. from draft-macdonald-behave-nat-behavior-diagnostics-00 . 22 A.2. from draft-ietf-behave-nat-behavior-discovery-00 . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23 Intellectual Property and Copyright Statements . . . . . . . . . . 24 1. Applicability This STUN usage does not allow an application behind a NAT to make an absolute determination of the NAT's characteristics. NAT devices do not behave consistently enough to predict future behaviour with any guarantee. This STUN usage provides information about observable transient behavior; it only truly determines a NAT's behavior with regard to the STUN server used at the instant the test is run. Applications requiring reliable reach must establish a communication channel through a NAT using another technique such as ICE [I-D.ietf-mmusic-ice] or OUTBOUND [I-D.ietf-sip-outbound]. 2. Introduction The Simple Traversal Underneath Network Address Translators (NAT) (STUN) [I-D.ietf-behave-rfc3489bis] provides a mechanism to discover the reflexive transport address toward the STUN server, using the Binding Request. This specification defines a usage of STUN, calledthe NAT Behavior Discovery STUN usage, which allows applicationsa STUN client to probe the current behaviour of the NAT/FW devices with respect tobetween the client and the STUN server. This usage defines new STUN attributes for the Binding Request and Binding Response. Many NAT/FW devices do not behave consistently and will change their behaviour under load and over time. Applications requiring high reliability must be prepared for the NAT's behaviour to become more restrictive. Specifically, it has been found that under load NATs may transition to the most restrictive filtering and mapping behaviour and shorten the lifetime of new and existing bindings. In short, applications can discover how bad things currently are, but not how bad things will get. In principle, an application might base an adaptation decision based on the results of the Behavior Discovery usage. For example, a P2P application could use some of these tests to deduce if it is a reasonable supernode candidate, meaning that its NAT(s) offer(s) Address Independent Filtering. It might periodically re-run tests and would remove itself as a supernode if its NAT/FW chain lost this characteristic. However, automatic adaptation based only on the results of the Behavior Discovery usage may fail to account for its inherent limitations in indicating only the current behavior of the NAT(s) with regard to a particular destination address at a particular instant in time. More importantly, it assumes the application selects a STUN server that is appropriately located with regards to its future communication partners. In general, an application is unable to make such determinations. Consequently, usage of the NAT Behavior Discovery STUN usage by applications to select operating modes or optimizations is discouraged; only experience with establishing connections with real communication partners using a mechanism such as ICE or OUTBOUND can reliably indicate the behavior an application will experience from the NAT. Despite these limitations, instantaneous observations are often quite useful in troubleshooting network problems, and repeated tests over time, or in known load situations, may be used to characterize a NAT's behavior. In particular, in the hands of a person knowledgeable about the needs of an application and the nodes an application needs to communicate with, it can be a powerful tool. 3. Overview of Operations In a typical configuration, a STUN client is connected to a private network and through one or more NATs to the public Internet. The client is configured with the address of a STUN server on the public Internet. The Behavior Discovery usage makes use of SRV records so that a server may use a different transport address for this usage than for other usages. BackwardsThis usage does not provide backward compatibility with RFC3489 [RFC3489] is supported.for either clients or servers. Implementors of clients that wish to be compliant with RFC3489 servers should see that specification. Implementors of servers SHOULD NOT include support for RFC3489 clients as the original uses of that protocol have been deprecated. The STUN NAT Behavior Discovery usage defines new attributes on the STUN Binding Request and STUN Binding Response that allow these messages to be used to diagnose the current behavior of the NAT(s) between the client and server. This section provides a descriptive overview of the typical use of these attributes. Normative behavior is described in Sections 5, 6, and 7. 3.1. Determining NAT Mapping A client behind a NAT wishes to determine if the NAT it is behind is currently using independent, address dependent, or port dependent mapping[RFC4787]. The client performs a series of tests that make use of the OTHER-ADDRESS attribute; these tests are described in detail in Section 4. These tests send binding requests to the alternate address and port of the STUN server to determine mapping behaviour. These tests can be used for UDP, TCP, or TCP/TLS connections. This usage renames RFC3489's CHANGED-ADDRESS attribute to OTHER- ADDRESS. Experience with 3489 indicated that many found use of the word "changed" to be confusing. In all respects, OTHER- ADDRESS is identical to CHANGED-ADDRESS, it is the same attribute (including its attribute type number) with a new name. 3.2. Determining NAT Filtering A client behind a NAT wishes to determine if the NAT it is behind is currently using independent, address dependent, or port dependent filtering[RFC4787]. The client performs a series of tests that make use of the OTHER-ADDRESS and CHANGE-REQUEST attributes; these tests are described in Section 4. These tests request responses from the alternate address and port of the STUN server; a precondition to these tests is that no binding be established to the alternate address and port. Because the NAT does not know that the alternate address and port belong to the same server as the primary address and port, it treats these responses the same as it would those from any other host on the Internet. Therefore, the success of the binding responses sent from the alternate address and port indicate whether the NAT is currently performing independent filtering, address dependent filtering, or address and port dependent filtering. This test applies only to UDP datagrams. 3.3. Binding Lifetime Discovery Many systems, such as VoIP, rely on being able to keep a connection open between a client and server or between peers of a P2P system. Because NAT bindings expire over time, keepalive messages must be sent across the connection to preserve it. Because keepalives impose some overhead on the network and servers, reducing the frequency of keepalives can be useful. Binding lifetime can be discovered by performing timed tests that use XOR-RESPONSE-ADDRESS. The client uses a second port and the STUN server's alternate address to check if an existing binding that hasn't had traffic sent on it is still open after time T. This approach is described in detail in Section 4.5. This test applies only to UDP datagrams. 3.4. Diagnosing NAT Hairpinning STUN Binding Requests allow a client to determine whether it is behind a NAT that supports hairpinning of connections. To perform this test, the client first sends a Binding Request to its STUN server to determine its mapped address. The client then sends a STUN Binding Request to this mapped address from a different port. If the client receives its own request, the NAT hairpins connections. This test applies to UDP, TCP, or TCP/TLS connections. 3.5. Determining Fragment Handling Some NATs exhibit different behavior when forwarding fragments than when forwarding a single-frame datagram. In particular, some NATs do not hairpin fragments at all and some platforms discard fragments under load. To diagnose this behavior, STUN messages may be sent with the PADDING attribute, which simply inserts additional space into the message. By forcing the STUN message to be divided into multiple fragments, the NAT's behavior can be observed. All of the previous tests can be performed with PADDING if a NAT's fragment behavior is important for an application, or only those tests which are most interesting to the application can be retested. PADDING only applies to UDP datagrams. PADDING can not be used with XOR-RESPONSE-ADDRESS. 4. Discovery Process The NAT Behavior Discovery usage provides primitives that allow STUN checks to be made to determine the current behaviour3.6. Detecting Generic ALGs A number of theNAT or NATs an application is behind. These tests can only giveboxes are now being deployed into the instantaneous behaviour of a NAT;market which try to provide "generic" ALG functionality. These generic ALGs hunt for IP addresses, either in text or binary form within a packet, and rewrite them if they match a binding. This behavior can be detected because the STUN server returns both the MAPPED-ADDRESS and XOR- MAPPED-ADDRESS in the same response. If the result in the two does not match, there a NAT with a generic ALG in the path. 4. Discovery Process The NAT Behavior Discovery usage provides primitives that allow STUN checks to be made to determine the current behaviour of the NAT or NATs an application is behind. These tests can only give the instantaneous behaviour of a NAT; it has been found that NATs can change behaviour under load and over time. An application must assume that NAT behaviour can become more restrictive at any time. The tests described here are for UDP connectivity, NAT mapping behaviour, and NAT filtering behaviour; additional tests could be designed using this usage's mechanisms. Definitions for NAT filtering and mapping behaviour are from [RFC4787]. 4.1. Checking if UDP is Blocked The client sends a STUN Binding Request to a server. This causes the server to send the response back to the address and port that the request came from. If this test yields no response, the client knows right away that it is not capable of UDP connectivity. This test requires only RFC3489-bis [I-D.ietf-behave-rfc3489bis] functionality. 4.2. Determining NAT Mapping Behavior This will require at most three tests. In test I, the client performs the UDP connectivity test. The server will return its alternate address and port in OTHER-ADDRESS in the binding response. If OTHER-ADDRESS is not returned, the server does not support this usage and this test cannot be run. The client examines the XOR- MAPPED-ADDRESS attribute. If this address and port are the same as the local IP address and port of the socket used to send the request, the client knows that it is not NATed and the effective mapping will be Endpoint Independent. In test II, the client sends a Binding Request to the alternate address, but primary port. If the XOR-MAPPED-ADDRESS in the Binding Response is the same as test I the NAT currently has Endpoint Independent Mapping. If not, test III is performed: the client sends a Binding Request to the alternate address and port. If the XOR- MAPPED-ADDRESS matches test II, the NAT currently has Address Dependent Mapping; if it doesn't match it currently has Address and Port Dependent Mapping. 4.3. Determining NAT Filtering Behavior This will also require at most three tests. These tests should be performed using a port that wasn't used for mapping or other tests as packets sent during those tests may affect results. In test I, the client performs the UDP connectivity test. The server will return its alternate address and port in OTHER-ADDRESS in the binding response. If OTHER-ADDRESS is not returned, the server does not support this usage and this test cannot be run. In test II, the client sends a binding request to the primary address of the server with the CHANGE-REQUEST attribute set to change-port and change-IP. This will cause the server to send its response from its alternate IP address and alternate port. If the client receives a response the current behaviour of the NAT is Address Independent Filtering. If no response is received, test III must be performed to distinguish between Address Dependent Filtering and Address and Port Dependent Filtering. In test III, the client sends a binding request to the original server address with CHANGE-REQUEST set to change-port. If the client receives a response the current behaviour is Address Dependent Filtering; if no response is received the current behaviour is Address and Port Dependent Filtering. 4.4. Combining and Ordering Tests Clients may wish to combine and parallelize these tests to reduce the number of packets sent and speed the discovery process. For example, test I of the filtering and mapping tests also checks if UDP is blocked. Furthermore, an application or user may not need as much detail as these sample tests provide. For example, establishing connectivity between nodes becomes significantly more difficult if a NAT has any behavior other than endpoint independent mapping, which requires only test I and II of Section 4.2. An application determining its NAT does not always provide independent mapping might notify thatthe user if no relay is configured, whereas an application behind a NAT that provides endpoint independent mapping might not notify the user until a subsequent connection actually fails or might provide a less urgent notification that no relay is configured. Such a test does not alleviate the need for ICE [I-D.ietf-mmusic-ice], but it does provide some information regarding whether ICE is likely to be successful establishing connections. Care must be taken when parallelizing tests, as some NAT devices have an upper limit on how quickly bindings will be allocated. 4.5. Binding Lifetime Discovery STUN can also be used to probe the lifetimes of the bindings created by the NAT. For many NAT devices, an absolute refresh interval cannot be determined; bindings might be closed quicker under heavy load or might not behave as the tests suggest. For this reason applications that require reliable bindings must send keep-alives as frequently as required by all NAT devices that will be encountered. Suggested refresh intervals are outside the scope of this document. ICE [I-D.ietf-mmusic-ice] and OUTBOUND [I-D.ietf-sip-outbound] have suggested refresh intervals. To determine the binding lifetime, the client first sends a Binding Request to the server from a particular socket, X. This creates a binding in the NAT. The response from the server contains a MAPPED- ADDRESS attribute, providing the public address and port on the NAT. Call this Pa and Pp, respectively. The client then starts a timer with a value of T seconds. When this timer fires, the client sends another Binding Request to the server, using the same destination address and port, but from a different socket, Y. This request contains an XOR-RESPONSE-ADDRESS address attribute, set to (Pa,Pp). This will create a new binding on the NAT, and cause the STUN server to send a Binding Response that would match the old binding, if it still exists. If the client receives the Binding Response on socket X, it knows that the binding has not expired. If the client receives the Binding Response on socket Y (which is possible if the old binding expired, and the NAT allocated the same public address and port to the new binding), or receives no response at all, it knows that the binding has expired. Because some NATs only refresh bindings when outbound traffic is sent, the client must resend a binding request on the original port before beginning a second test with a different value of T. The client can find the value of the binding lifetime by doing a binary search through T, arriving eventually at the value where the response is not received for any timer greater than T, but is received for any timer less than T. This discovery process takes quite a bit of time and is something that will typically be run in the background on a device once it boots. It is possible that the client can get inconsistent results each time this process is run. For example, if the NAT should reboot, or be reset for some reason, the process may discover a lifetime than is shorter than the actual one. For this reason, implementations are encouraged to run the test numerous times and be prepared to get inconsistent results. Like the other diagnostics, this test is inherently unstable. In particular, an overloaded NAT might reduce binding lifetime to shed load. A client might find this diagnostic useful at startup, for example setting the initial keepalive interval on its connection to the server to 10 seconds while beginning this check. After determining the current lifetime, the keepalive interval used by the connection to the server can be set to this appropriate value. Subsequent checks of the binding lifetime can then be performed using the keepalives in the server connection. The STUN Keepalive Usage [I-D.ietf-behave-rfc3489bis]provides a response that confirms the connection is open and allows the client to check that its mapped address has not changed. As that provides both the keepalive action and diagnostic that it is working, it should be preferred over any attempt to characterize the connection by a secondary technique. 5. Client Behavior Unless otherwise specified here, all procedures for preparing, sending, and processing messages as described in the STUN Binding Usage [I-D.ietf-behave-rfc3489bis] are followed. If a client intends to utilize an XOR-RESPONSE-ADDRESS attribute in future transactions, as described in Section 4.5, then it MUST include a CACHE-TIMEOUT attribute in the Request with the value set greater than the longest time duration it intends to cache.test. The server will also include this attribute in its Response, modified with its estimate of how long it will be able to cache this connection. Because 3489 implementations do not support this attribute and because anythe returned value is only an estimate, the client must be prepared to not receive a CACHE-TIMEOUT value orfor the value to be wrong, and therefore to receive a 430 response to its subsequent Requests with XOR-RESPONSE-ADDRESS.XOR-RESPONSE- ADDRESS. OPEN ISSUE: is 430 the best approach to this (which should force the client to get a new shared secret and retry the transaction) or should a new return-type be used. Also, if shared secret is not required, it's not at all appropriate. Support for XOR-RESPONSE-ADDRESS is optional; it has a state cost on the server and requires short-term credentials, which many implementations don't support. Therefore, a client MUST be prepared for receiving a 420 (Unknown Attribute) error to requests that include XOR-RESPONSE-ADDRESS. Support for OTHER-ADDRESS and CHANGE- REQUEST is optional, but MUST be supported by servers advertised via SRV, as described below. This is to allow the use of PADDING and XOR-RESPONSE-ADDRESS in P2P situations where peers do not have multiple IP addresses. Clients MUST be prepared to receive a 420 for requests that include CHANGE-REQUEST when OTHER-ADDRESS was not received in Binding Response messages from the server. 5.1. Discovery Unless the user or application is aware of the transport address of a STUN server supporting the NAT Behavior Discovery usage through other means, a client is configured with the domain name of the provider of the STUN servers. The domain is resolved to a transport address using SRV procedures [RFC2782]. The mechanism for configuring the client with the domain name of the STUN servers or of acquiring a specific transport address is out of scope for this document. For the Behavior Discovery Usage the service name is "stun-behavior". The protocol can be "udp", "tcp" or "tls". For backward compatibility with an RFC3489Other aspects of handling failures and default ports are followed as described in STUN server, if there is no SRV entry for "stun-behavior" the client SHOULD next search for the service name "stun".[I-D.ietf-behave-rfc3489bis]. 5.2. Security If there is an entry,the client willis interested in performing a Binding Lifetime Discovery test or other test requiring use these entries. However, because the lackof a "stun-behavior" entry could indicate a STUN [I-D.ietf-behave-rfc3489bis] compliant server that does not supportthe Behavior Discovery usage, a client SHOULD NOT send a request with a Behavior Discovery attribute if it has received a 420 response to a requests with that attribute previously or if there is no OTHER-ADDRESS received in the response to a Binding Request. Rationale: When combining the issues of backward compatibility with 3489 as well as servers compliant with both this specification and 3489 allowed to support only some of the attributes in each specification, there is no way to be certain what attributes a server supports without trying them. 3489 compliant servers alway return OTHER-ADDRESS, aka CHANGED-ADDRESS, but do not support PADDING or XOR-RESPONSE-ADDRESS. Other aspects of handling failures and default ports are followed as described in STUN [I-D.ietf-behave-rfc3489bis]. 5.2. Security If the client is interested in performing a Binding Lifetime Discovery test or other test requiring use of the XOR-RESPONSE- ADDRESS attribute, it MUST obtainXOR-RESPONSE- ADDRESS attribute, it MUST obtain a shared secret prior to beginning the test, and that shared secret must be used for all transactions during the test. If the client receives a 430 (Stale Credentials) Response to a Request containing a XOR-RESPONSE-ADDRESS, then it must acquire a new short-term credential and begin the test again. Procedures for obtaining a shared secret are described in STUN [I-D.ietf-behave-rfc3489bis]. OPEN ISSUE: We would like to remove the MUST requirement for shared secret in favor of allowing servers to do rate limiting. 6. Server Behavior Unless otherwise specified here, all procedures for preparing, sending, and processing messages as described for the STUN Binding Usage of STUN [I-D.ietf-behave-rfc3489bis] are followed. A server implementing the NAT Behavior Discovery usage SHOULD be configured with two separate IP addresses on the public Internet. On startup, the server SHOULD allocate two UDP ports, such that it can send and receive datagrams using the same ports on each IP address (normally a wildcard binding accomplishes this). If a server cannot allocate the same ports on two different IP address, then it MUST NOT include an OTHER-ADDRESS attribute in any Response and MUST respond with a 420 (Unknown Attribute) to any Request with a CHANGE-REQUEST attribute. A server with only one IP address MUST NOT be advertised using SRV.the SRV service name "stun-behavior". 6.1. Preparing the Response After performing all authentication and verification steps and after adding the MAPPED-ADDRESS or XOR-MAPPED-ADDRESS,the server begins processing specific to this Usage if the Request contains any request attributes defined in this document: RESPONSE-ADDRESS, XOR-RESPONSE- ADDRESSXOR-RESPONSE-ADDRESS, CHANGE-REQUEST, or PADDING. If the Request does not contain any attributes from this document, OTHER-ADDRESS and SOURCE-ADDRESS are still included in the response as specified below. The server MUST include both MAPPED-ADDRESS and XOR-MAPPED-ADDRESS in its Response. If the Request contains CHANGE-REQUEST attribute and the server does not have an alternate address and port as described above, the server MUST generate an error response of type 420. If the Request contains a CACHE-TIMEOUT attribute, then the server SHOULD include a CACHE-TIMEOUT attribute in its response indicating the duration (in seconds) it anticipates being able to cache this binding request in anticipation of a future Request using the XOR- RESPONSE-ADDRESS attribute. The CACHE-TIMEOUT response value can be greater or less than the value in the request. If the server is not prepared to provide such an estimate, it SHOULD NOT include the CACHE-TIMEOUT attribute in its Response. If the Request contains a XOR-RESPONSE-ADDRESS attribute, but the message does not contain a MESSAGE-INTEGRITY attribute and a USERNAME, the server MUST generate an error response of type 401. If XOR-RESPONSE-ADDRESS is included, then the server must verify that it has previously received a binding request from the same address as is specified in XOR-RESPONSE-ADDRESS. If it has not, or if sufficient time has passed that it no longer has a record of having received such a request due to limited state, it MUST respond with an error response of type 430. The source address and port of the Binding Response depend on the value of the CHANGE-REQUEST attribute and on the address and port the Binding Request was received on, and are summarized in Table 1. Let Da represent the destination IP address of the Binding Request (which will be either A1 or A2), and Dp represent the destination port of the Binding Request (which will be either P1 or P2). Let Ca represent the other address, so that if Da is A1, Ca is A2. If Da is A2, Ca is A1. Similarly, let Cp represent the other port, so that if Dp is P1, Cp is P2. If Dp is P2, Cp is P1. If the "change port" flag was set in CHANGE-REQUEST attribute of the Binding Request, and the "change IP" flag was not set, the source IP address of the Binding Response MUST be Da and the source port of the Binding Response MUST be Cp. If the "change IP" flag was set in the Binding Request, and the "change port" flag was not set, the source IP address of the Binding Response MUST be Ca and the source port of the Binding Response MUST be Dp. When both flags are set, the source IP address of the Binding Response MUST be Ca and the source port of the Binding Response MUST be Cp. If neither flag is set, or if the CHANGE-REQUEST attribute is absent entirely, the source IP address of the Binding Response MUST be Da and the source port of the Binding Response MUST be Dp. +--------------------+----------------+-------------+---------------+ | Flags | Source Address | Source Port | OTHER-ADDRESS | +--------------------+----------------+-------------+---------------+ | none | Da | Dp | Ca:Cp | | Change IP | Ca | Dp | Ca:Cp | | Change port | Da | Cp | Ca:Cp | | Change IP and | Ca | Cp | Ca:Cp | | Change port | | | | +--------------------+----------------+-------------+---------------+ Table 1: Impact of Flags on Packet Source and OTHER-ADDRESS The server MUST add a SOURCE-ADDRESS attribute to the Binding Response, containing the source address and port used to send the Binding Response. OPEN ISSUE: 3489bis MUST allow SOURCE-ADDRESS and OTHER-ADDRESS in any Binding Response, to allow 3489bis clients to use 3489 servers, and to allow multiplexing of this usage on the same port of other stun usages without adding a discovery mechanism. We decided that this made sense for OTHER-ADDRESS in San Diego, but we forgot about SOURCE-ADDRESS. This would be accomplished by adding SOURCE-ADDRESS and OTHER-ADDRESS as known attributes to 3489bis...IANA registration of the attributes would also move there. If the server supports an alternate address and port the server MUST add an OTHER-ADDRESS attribute to the Binding Response. This contains the source IP address and port that would be used if the client had set the "change IP" and "change port" flags in the Binding Request. As summarized in Table 1, these are Ca and Cp, respectively, regardless of the value of the CHANGE-REQUEST flags. Next the server inspects the Request for a XOR-RESPONSE-ADDRESS attribute. If the XOR-RESPONSE-ADDRESS attribute is included, then the thenit includes an XOR-REFLECTED-FROM attribute with the source address the Request was received from. If the request contains RESPONSE-ADDRESS(indicating a legacy 3489 client) it includes a REFLECTED-FROM attribute. OPEN Issue: should this document really describe server and client behaviour for the old functionality from 3489(RESPONSE-ADDRESS on client and server)? I am leaning towards removing this for simplicity as it is confusing to and implementors none of it is mandatory. If theRequest contained a PADDING attribute, then the server SHOULD insert a PADDING attribute of the same length into its response, but no longer than 64K. If the Request also contains the XOR-RESPONSE- ADDRESS attribute the server MUST return an error response of type 400. Following that, the server completes the remainder of the processing from STUN [I-D.ietf-behave-rfc3489bis], including adding the SERVER, MESSAGE-INTEGRITY, and FINGERPRINT attributes as appropriate. When it sends the Response, it is sent from the source address as determined above and to the destination address determined from the XOR-RESPONSE-ADDRESS or RESPONSE-ADDRESS,XOR-RESPONSE-ADDRESS, or to the source address of the Request if not specified. 7. New Attributes This document defines several STUN attributes that are required for NAT Behavior Discovery. These attributes are all used only with Binding Requests and Binding Responses. The majority of these attributes were originally defined in RFC3489 [RFC3489], but are redefined here as that document is obsoleted by RFC3489bis [I-D.ietf-behave-rfc3489bis]. 0x0002: RESPONSE-ADDRESS0x0003: CHANGE-REQUEST 0x0004: SOURCE-ADDRESS 0x0005: OTHER-ADDRESS 0x000b: REFLECTED-FROM0x0023: XOR-REFLECTED-FROM 0x0027: XOR-RESPONSE-ADDRESS 0x8026: PADDING 0x8027: CACHE-TIMEOUT 7.1. Representing Transport Addresses Whenever an attribute contains a transport address, it has the same format as MAPPED-ADDRESS. Similarly, the XOR- attributes have the same format as XOR-MAPPED-ADDRESS[I-D.ietf-behave-rfc3489bis]. 7.2. RESPONSE-ADDRESS The RESPONSE-ADDRESS attribute contains an IP address and port. The RESPONSE-ADDRESS attribute can be present in the Binding Request and indicates where the Binding Response is to be sent. When not present, the server sends the Binding Response to the source IP address and port of the Binding Request. The server MUST NOT process a request containing a RESPONSE-ADDRESS that does not contain MESSAGE-INTEGRITY. The RESPONSE-ADDRESS attribute is optional in the Binding Request. RESPONSE-ADDRESS is for compatibility with legacy RFC3489 implementations only. New implementations MUST use XOR-RESPONSE- ADDRESS unless that attribute is rejected by the server with a 420 error code, in which case they SHOULD fall back to RESPONSE-ADDRESS. 7.3.CHANGE-REQUEST The CHANGE-REQUEST attribute contains two flags to control the IP address and port the server uses to send the response. These flags are called the "change IP" and "change port" flags. The CHANGE- REQUEST attribute is allowed only in the Binding Request. The "change IP" and "change port" flags are useful for determining the current filtering behavior of a NAT. They instruct the server to send the Binding Responses from the alternate source IP address and/or alternate port. The CHANGE-REQUEST attribute is optional in the Binding Request. The attribute is 32 bits long, although only two bits (A and B) are used: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 A B 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The meanings of the flags are: A: This is the "change IP" flag. If true, it requests the server to send the Binding Response with a different IP address than the one the Binding Request was received on. B: This is the "change port" flag. If true, it requests the server to send the Binding Response with a different port than the one the Binding Request was received on. 126.96.36.199. SOURCE-ADDRESS The SOURCE-ADDRESS attribute is inserted by the server and indicates the source IP address and port the response was sent from. It is useful for detecting twice NAT configurations. It is only present in Binding Responses. SOURCE-ADDRESS MUST NOT be inserted into a Binding Response unless the Binding Request contained an attribute defined in this specification. 188.8.131.52. OTHER-ADDRESS The OTHER-ADDRESS attribute is used in Binding Responses. It informs the client of the source IP address and port that would be used if the client requested the "change IP" and "change port" behavior. OTHER-ADDRESS MUST NOT be inserted into a Binding Response unless the server has a second IP address. OTHER-ADDRESS uses the same attribute as CHANGED-ADDRESS from RFC3489 because it is simply a new name with the same semantics as CHANGED- ADDRESS. It has been renamed to more clearly indicate its function. 7.6. REFLECTED-FROM7.5. XOR-REFLECTED-FROM The REFLECTED-FROMXOR-REFLECTED-FROM attribute is present only in Binding Responses when the Binding Request contained a RESPONSE-ADDRESSXOR-RESPONSE-ADDRESS attribute. The attribute contains the transport address of the source where the request came from. Its purpose is to provide traceability, so that a STUN server cannot be used as a reflector for denial-of-serviceanonymous denial-of- service attacks. REFLECTED-FROM is included for compatibility with legacy applications, only. New implementations should use XOR-REFLECTED- FROM. 7.7. XOR-REFLECTED-FROMThe XOR-REFLECTED-FROM attribute is used in place of the REFLECTED- FROMRFC3489's REFLECTED-FROM attribute. It provides the same information, but because the NAT's public address is obfuscated through the XOR function, It can pass through a NAT that would otherwise attempt to translate it to the private network address. 184.108.40.206. XOR-RESPONSE-ADDRESS The XOR-RESPONSE-ADDRESS attribute contains an IP address and port. The XOR-RESPONSE-ADDRESS attribute can be present in the Binding Request and indicates where the Binding Response is to be sent. When not present, the server sends the Binding Response to the source IP address and port of the Binding Request. The server MUST NOT process a request containing a XOR-RESPONSE-ADDRESS that does not contain MESSAGE-INTEGRITY. The XOR-RESPONSE-ADDRESS attribute is optional in the Binding Request. XOR-RESPONSE-ADDRESS is used in place of theRFC3489's RESPONSE-ADDRESS. It provides the same information, but because the NAT's public address is obfuscated through the XOR function, It can pass through a NAT that would otherwise attempt to translate it to the private network address. 220.127.116.11. PADDING The PADDING attribute allows for the entire message to be padded to force the STUN message to be divided into UDPIP fragments. PADDING consists entirely of a freeform string, the value of which does not matter. When PADDING is used, it SHOULD be 1500 bytes long, unless a more appropriate length is known based on the MTU of the path. PADDING can be used in either Binding Requests or Binding Responses. If PADDING is present in the Binding Request and the server supports it, PADDING MUST be present in the Binding Response. The server SHOULD use the same length PADDING as was used in the Binding Request, but it MAY use another length if it knows what length is required to cause fragmentation along the return path, orpath. If the server supports PADDING (i.e. doesn't return a 420 in response to a Request containing PADDING), then it MAYMUST use either the requested length or a length of zero to indicate that the field is understood but the serverit knows is ignoring it.sufficient to cause fragmentation. PADDING MUST be no longer than 64K and SHOULD be an even multiple of four bytes. 18.104.22.168. CACHE-TIMEOUT The CACHE-TIMEOUT is used in Binding Requests and Responses. It indicates the time duration (in seconds) that the server will cache the source address and USERNAME of an original binding request that will later by followed by a request from a different source address with a RESPONSE-ADDRESSXOR-RESPONSE-ADDRESS asking that a response be reflected to the source address of the original binding request. A server SHOULD NOT send a response to a target address requested with RESPONSE-ADDRESSXOR-RESPONSE- ADDRESS unless it has cached that the same USERNAME made a previous binding request from that target address. The client inserts a value in CACHE-TIMEOUT into the Binding Request indicating the amount of time it would like the server to cache that information. The server responds with a CACHE-TIMEOUT in its Binding Response providing a prediction of how long it will cache that information. The response value can be greater than, equal to, or less than the requested value. If the server is not able to provide such an estimate or the information in the response would be meaningless, the server should not include a CACHE-TIMEOUT attribute in its response. 8. IAB Considerations The IAB has studied the problem of ``Unilateral Self Address Fixing'', which is the general process by which a client attempts to determine its address in another realm on the other side of a NAT through a collaborative protocol reflection mechanism RFC 3424 [RFC3424]. The STUN NAT Behavior Discovery usage is an example of a protocol that performs this type of function. The IAB has mandated that any protocols developed for this purpose document a specific set of considerations. This section meets those requirements. 8.1. Problem Definition From RFC 3424 [RFC3424], any UNSAF proposal must provide: Precise definition of a specific, limited-scope problem that is to be solved with the UNSAF proposal. A short term fix should not be generalized to solve other problems; this is why "short term fixes usually aren't". The specific problem being solved by the STUN NAT Behavior Discovery usage is for a client, which may be located behind a NAT of any type, to determine the characteristics of that NAT in order to either diagnose the cause of problems experienced by that or other applications or for an application to modify its behavior based on the current behavior of the NAT. 8.2. Exit Strategy From [RFC3424], any UNSAF proposal must provide: Description of an exit strategy/transition plan. The better short term fixes are the ones that will naturally see less and less use as the appropriate technology is deployed. The STUN NAT Behavior Discovery usage does not itself provide an exit strategy. Instead, that is provided by other protocols. Specifically, the Interactive Connectivity Establishment (ICE) [I-D.ietf-mmusic-ice] mechanism allows two cooperating clients to interactively determine the best addresses to use when communicating, regardless of the type of NAT involved. BEHAVE is currently considering proposals for protocols that allow clients to determine the location of and control the behavior of NATs through direct interaction with the NAT. STUN NAT Behavior Discovery is no longer needed once NATs that can be communicated with directly are in use. Finally, as NATs phase out and as IPv6 is deployed, STUN NAT Behavior Discovery will no longer be of any interest. 8.3. Brittleness Introduced by STUN NAT Behavior Discovery From [RFC3424], any UNSAF proposal must provide: Discussion of specific issues that may render systems more "brittle". For example, approaches that involve using data at multiple network layers create more dependencies, increase debugging challenges, and make it harder to transition. The STUN NAT Behavior Discovery usage allows a client to determine the current behavior of a NAT. This information can be quite useful to a developer or network administrator outside of an application, and as such can be used to diagnose the brittleness induced in another application. When used within an application itself, STUN NAT Behavior Discovery allows the application to adjust its behavior according to the current behavior of the NAT. While this can be helpful in improving the performance of an application, an improperly written application could use information from this usage and assume that the NAT will always behave in the same manner, and thus failing to work properly when the NAT changes its behavior. Regardless of whether an application makes use of NAT Behavior Discovery or not, if it does not use techniques such as ICE [I-D.ietf-mmusic-ice] or OUTBOUND [I-D.ietf-sip-outbound] it exposes itself to the inherent instability of NAT. 8.4. Requirements for a Long Term Solution From [RFC3424]}, any UNSAF proposal must provide: Identify requirements for longer term, sound technical solutions -- contribute to the process of finding the right longer term solution. Our experience with STUN NAT Behavior Discovery continues to validate our belief in the requirements outlined in Section 14.4 of STUN [I-D.ietf-behave-rfc3489bis]. 8.5. Issues with Existing NAPT Boxes >From [RFC3424], any UNSAF proposal must provide: Discussion of the impact of the noted practical issues with existing, deployed NA[P]Ts and experience reports. A number of NAT boxes are now being deployed into the market which try and provide "generic" ALG functionality. These generic ALGs hunt for IP addresses, either in text or binary form within a packet, and rewrite them if they match a binding. This usage avoids that problem by using the XOR-REFLECTED-FROM and XOR-RESPONSE-ADDRESS attributes instead of the older REFLECTED-FROM and RESPONSE-ADDRESS attributes. This usage provides a set of generic attributes that can be assembled to test many types of NAT behavior. While tests for the most commonly known NAT box behaviors are described, the BEHAVE mailing list regularly has descriptions of new behaviors, some of which may not be readily detected using the tests described herein. However, the techniques described in this usage can be assembled in different combinations to test NAT behaviors not now known or envisioned. 9. IANA Considerations This specification defines several new STUN attributes. This section directs IANA to add these new protocol elements to the IANA registry of STUN protocol elements. The code for OTHER-ADDRESS renames this code from CHANGED-ADDRESS to OTHER-ADDRESS for clarity, the semantics remain the same. OPEN ISSUE: does IANA consider these new attributes or are they in forever from original 3489? 0x0002: RESPONSE-ADDRESS0x0003: CHANGE-REQUEST 0x0004: SOURCE-ADDRESS 0x0005: OTHER-ADDRESS 0x000b: REFLECTED-FROM0x0023: XOR-REFLECTED-FROM 0x0027: XOR-RESPONSE-ADDRESS 0x0026: PADDING 0x8026: CACHE-TIMEOUT 10. Security Considerations This usage inherits the security considerations of STUN [I-D.ietf-behave-rfc3489bis]. This usage adds several new attributes; security considerations for those are detailed here. OTHER-ADDRESS does not permit any new attacks; it provides another place where an attacker can impersonate a STUN server but it is not an interesting attack. An attacker positioned where it can compromise the Binding Request can completely hide the STUN server from the client. RESPONSE-ADDRESSXOR-RESPONSE-ADDRESS allows a STUN server to be used as a reflector for denial-of-service attacks. The XOR-REFLECTED-FROM mitigates this by providing the identity (in terms of IP address) of the source where the request came from. Its purpose is to provide traceability, so that a STUN server cannot be used as an anonymous reflector for denial-of-service attacks. Authenticating the RESPONSE-ADDRESSXOR-RESPONSE-ADDRESS using shared secrets alleviates this threat. Server caching previous contacts before directing a response to a RESPONSE-ADDRESSXOR-RESPONSE-ADDRESS further eliminates the threat, although it introduces the complexity of state into a STUN server. CACHE-TIMEOUT is used to reduce the amount of additional state required. The only attack possible with the PADDING attribute is to have a large padding length which could cause a server to allocate a large amount of memory. As servers will ignore any padding length greater than 64k so the scope of this attack is limited. In general, servers should not allocate more memory than the size of the received datagram. This attack would only affect non-compliant implementations. 11. Open Issues In Section 5: Use 430 for CACHE-TIMEOUT errors or introduce a new error code? In Section 6.1 need to confirm final solution for 3489bis clients to not error out on responses with SOURCE-ADDRESS and OTHER-ADDRESS. Does IANA consider attributes that were in 3489 but not in 3489bis to have been removed from the registry and should be re-registered by this document, or are there forever in the registry from 3489? We would like to remove the MUST requirement for shared secret when used with XOR-RESPONSE-ADDRESS in favor of allowing servers to do rate limiting. 12. Acknowledgements The authors would like to thank the authors of the original STUN specification [RFC3489] from which many of the ideas, attributes, and description in this document originated. 12.13. References 22.214.171.124. Normative References [I-D.ietf-behave-rfc3489bis] Rosenberg, J., "SimpleHuitema, C., Mahy, R., Matthews, P., and D. Wing, "Session Traversal Underneath Network Address TranslatorsUtilities for (NAT) (STUN)", draft-ietf-behave-rfc3489bis-05draft-ietf-behave-rfc3489bis-07 (work in progress), October 2006.July 2007. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000. [RFC4787] Audet, F. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, January 2007. 126.96.36.199. Informative References [I-D.ietf-mmusic-ice] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A MethodologyProtocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", draft-ietf-mmusic-ice-13draft-ietf-mmusic-ice-16 (work in progress), JanuaryJune 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-07draft-ietf-sip-outbound-09 (work in progress), JanuaryJune 2007. [RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-Address Fixing (UNSAF) Across Network Address Translation", RFC 3424, November 2002. [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. Appendix A. Change Log RFC-EDITOR: Please remove this entire Change Log section while formatting this document for publication. A.1. from draft-macdonald-behave-nat-behavior-diagnostics-00 o Only OTHER-ADDRESS, CHANGE-ADDRESS, RESPONSE-ADDRESSand XOR- RESPONSE-ADDRESSXOR-RESPONSE-ADDRESS support is optional; support for PADDING and SOURCE-ADDRESS is now mandatory o PADDING is now a mandatory attribute o OTHER-ADDRESS is returned in all binding responses if the server has a second IP address A.2. from draft-ietf-behave-nat-behavior-discovery-00 o Clarified that only servers with two IP addresses should have an SRV entry o Removed support for backward compatibility with 3489 clients by removing non-XOR forms of attributes. Language states that backward compatiblity with 3489 clients is SHOULD NOT. Compatibility with 3489 servers is left unspecified. o PADDING is mandatory and language has been changed to indicate that if a server supports PADDING it must either actually provide the padding or return an error (can't support it but refuse to do it) o Require both MAPPED-ADDRESS and XOR-MAPPED-ADDRESS to be returned to support detection of generic ALGs Authors' Addresses Derek C. MacDonald CounterPath Solutions, Inc. Suite 300, One Bentall Centre, 505 Burrard St Vancouver, BC V7X1M3 Canada Phone: +1-604-320-3344 Email: email@example.com Bruce B. Lowekamp SIPeerior Technologies and William & Mary 3000 Easter Circle Williamsburg, Virginia 23188 USA Phone: +1-757-565-0101 Email: firstname.lastname@example.org 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. 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