RFC2141] that uniquely identifies the device. Usage of a URN provides a persistent and unique name for the UA instance. It also provides an easy way to guarantee uniqueness within the AOR. This URN MUST be persistent across power cycles of the device. The instance ID MUST NOT change as the device moves from one network to another. A UA SHOULD create a Universally Unique Identifier (UUID) URN [RFC4122] as its instance-id. The UUID URN allows for non- centralized computation of a URN based on time, unique names (such as a MAC address), or a random number generator. Note: A device like a "soft phone", when first installed, can generate a UUID [RFC4122] and then save this in persistent storage for all future use. For a device such as a "hard phone", which will only ever have a single SIP UA present, the UUID can include the MAC address and be generated at any time because it is guaranteed that no other UUID is being generated at the same time on that physical device. This means the value of the time component of the UUID can be arbitrarily selected to be any time less than the time when the device was manufactured. A time of 0 (as shown in the example in Section 3.2) is perfectly legal as long as the device knows no other UUIDs were generated at this time on this device. If a URN scheme other than UUID is used, the UA MUST only use URNs for which an RFC (from the IETF stream) defines how the specific URN needs to be constructed and used in the "+sip.instance" Contact header field parameter for outbound behavior.
To convey its instance-id in both requests and responses, the UA includes a "sip.instance" media feature tag as a UA characteristic [RFC3840]. This media feature tag is encoded in the Contact header field as the "+sip.instance" Contact header field parameter. One case where a UA could prefer to omit the "sip.instance" media feature tag is when it is making an anonymous request or some other privacy concern requires that the UA not reveal its identity. Note: [RFC3840] defines equality rules for callee capabilities parameters, and according to that specification, the "sip.instance" media feature tag will be compared by case- sensitive string comparison. This means that the URN will be encapsulated by angle brackets ("<" and ">") when it is placed within the quoted string value of the "+sip.instance" Contact header field parameter. The case-sensitive matching rules apply only to the generic usages defined in the callee capabilities [RFC3840] and the caller preferences [RFC3841] specifications. When the instance ID is used in this specification, it is "extracted" from the value in the "sip.instance" media feature tag. Thus, equality comparisons are performed using the rules for URN equality that are specific to the scheme in the URN. If the element performing the comparisons does not understand the URN scheme, it performs the comparisons using the lexical equality rules defined in [RFC2141]. Lexical equality could result in two URNs being considered unequal when they are actually equal. In this specific usage of URNs, the only element that provides the URN is the SIP UA instance identified by that URN. As a result, the UA instance has to provide lexically equivalent URNs in each registration it generates. This is likely to be normal behavior in any case; clients are not likely to modify the value of the instance ID so that it remains functionally equivalent to (yet lexicographically different from) previous registrations.
For each outbound proxy URI in the set, the User Agent Client (UAC) SHOULD send a REGISTER request using this URI as the default outbound proxy. (Alternatively, the UA could limit the number of flows formed to conserve battery power, for example). If the set has more than one URI, the UAC MUST send a REGISTER request to at least two of the default outbound proxies from the set. UAs that support this specification MUST include the outbound option tag in a Supported header field in a REGISTER request. Each of these REGISTER requests will use a unique Call-ID. Forming the route set for the request is outside the scope of this document, but typically results in sending the REGISTER such that the topmost Route header field contains a loose route to the outbound proxy URI. REGISTER requests, other than those described in Section 4.2.3, MUST include an instance-id media feature tag as specified in Section 4.1. A UAC conforming to this specification MUST include in the Contact header field, a "reg-id" parameter that is distinct from other "reg-id" parameters used in other registrations that use the same "+sip.instance" Contact header field parameter and AOR. Each one of these registrations will form a new flow from the UA to the proxy. The sequence of reg-id values does not have to be sequential but MUST be exactly the same sequence of reg-id values each time the UA instance power cycles or reboots, so that the reg-id values will collide with the previously used reg-id values. This is so the registrar can replace the older registrations. Note: The UAC can situationally decide whether to request outbound behavior by including or omitting the "reg-id" Contact header field parameter. For example, imagine the outbound-proxy-set contains two proxies in different domains, EP1 and EP2. If an outbound-style registration succeeded for a flow through EP1, the UA might decide to include 'outbound' in its Require header field when registering with EP2, in order to ensure consistency. Similarly, if the registration through EP1 did not support outbound, the UA might not register with EP2 at all. The UAC MUST support the Path header [RFC3327] mechanism, and indicate its support by including the 'path' option-tag in a Supported header field value in its REGISTER requests. Other than optionally examining the Path vector in the response, this is all that is required of the UAC to support Path. The UAC examines successful registration responses for the presence of an outbound option-tag in a Require header field value. Presence of this option-tag indicates that the registrar is compliant with this specification, and that any edge proxies which needed to participate are also compliant. If the registrar did not support
outbound, the UA has potentially registered an un-routable contact. It is the responsibility of the UA to remove any inappropriate Contacts. If outbound registration succeeded, as indicated by the presence of the outbound option-tag in the Require header field of a successful registration response, the UA begins sending keep-alives as described in Section 4.4. Note: The UA needs to honor 503 (Service Unavailable) responses to registrations as described in [RFC3261] and [RFC3263]. In particular, implementors should note that when receiving a 503 (Service Unavailable) response with a Retry-After header field, the UA is expected to wait the indicated amount of time and retry the registration. A Retry-After header field value of 0 is valid and indicates the UA is expected to retry the REGISTER request immediately. Implementations need to ensure that when retrying the REGISTER request, they revisit the DNS resolution results such that the UA can select an alternate host from the one chosen the previous time the URI was resolved. If the registering UA receives a 439 (First Hop Lacks Outbound Support) response to a REGISTER request, it MAY re-attempt registration without using the outbound mechanism (subject to local policy at the client). If the client has one or more alternate outbound proxies available, it MAY re-attempt registration through such outbound proxies. See Section 11.6 for more information on the 439 response code.
RFC3263]) to find a protocol, IP address, and port. For protocols that don't use TLS, if the UAC has an existing flow to this IP address, and port with the correct protocol, then the UAC MUST use the existing connection. For TLS protocols, there MUST also be a match between the host production in the next hop and one of the URIs contained in the subjectAltName in the peer certificate. If the UAC cannot use one of the existing flows, then it SHOULD form a new flow by sending a datagram or opening a new connection to the next hop, as appropriate for the transport protocol. Typically, a UAC using the procedures of this document and sending a dialog-forming request will want all subsequent requests in the dialog to arrive over the same flow. If the UAC is using a Globally Routable UA URI (GRUU) [RFC5627] that was instantiated using a Contact header field value that included an "ob" parameter, the UAC sends the request over the flow used for registration, and subsequent requests will arrive over that same flow. If the UAC is not using such a GRUU, then the UAC adds an "ob" parameter to its Contact header field value. This will cause all subsequent requests in the dialog to arrive over the flow instantiated by the dialog-forming request. This case is typical when the request is sent prior to registration, such as in the initial subscription dialog for the configuration framework [CONFIG-FMWK].
Note: If the UAC wants a UDP flow to work through NATs or firewalls, it still needs to put the 'rport' parameter [RFC3581] in its Via header field value, and send from the port it is prepared to receive on. More general information about NAT traversal in SIP is described in [NAT-SCEN]. Section 4.2, a UA that registers will begin sending keep-alives after an appropriate registration response. A UA that does not register (for example, a PSTN gateway behind a firewall) can also send keep-alives under certain circumstances. Under specific circumstances, a UAC might be allowed to send STUN keep-alives even if the procedures in Section 4.2 were not completed, provided that there is an explicit indication that the target first- hop SIP node supports STUN keep-alives. For example, this applies to a non-registering UA or to a case where the UA registration succeeded, but the response did not include the outbound option-tag in the Require header field. Note: A UA can "always" send a double CRLF (a "ping") over connection-oriented transports as this is already allowed by Section 7.5 of [RFC3261]. However a UA that did not register using outbound registration cannot expect a CRLF in response (a "pong") unless the UA has an explicit indication that CRLF keep- alives are supported as described in this section. Likewise, a UA that did not successfully register with outbound procedures needs explicit indication that the target first-hop SIP node supports STUN keep-alives before it can send any STUN messages. A configuration option indicating keep-alive support for a specific target is considered an explicit indication. If these conditions are satisfied, the UA sends its keep-alives according to the same guidelines as those used when UAs register; these guidelines are described below. The UA needs to detect when a specific flow fails. The UA actively tries to detect failure by periodically sending keep-alive messages using one of the techniques described in Sections 4.4.1 or 4.4.2. If a flow with a registration has failed, the UA follows the procedures in Section 4.2 to form a new flow to replace the failed one.
When a successful registration response contains the Flow-Timer header field, the value of this header field is the number of seconds the server is prepared to wait without seeing keep-alives before it could consider the corresponding flow dead. Note that the server would wait for an amount of time larger than the Flow-Timer in order to have a grace period to account for transport delay. The UA MUST send keep-alives at least as often as this number of seconds. If the UA uses the server-recommended keep-alive frequency it SHOULD send its keep-alives so that the interval between each keep-alive is randomly distributed between 80% and 100% of the server-provided time. For example, if the server suggests 120 seconds, the UA would send each keep-alive with a different frequency between 95 and 120 seconds. If no Flow-Timer header field was present in a register response for this flow, the UA can send keep-alives at its discretion. The sections below provide RECOMMENDED default values for these keep- alives. The client needs to perform normal [RFC3263] SIP DNS resolution on the URI from the outbound-proxy-set to pick a transport. Once a transport is selected, the UA selects the keep-alive approach that is recommended for that transport. Section 4.4.1 describes a keep-alive mechanism for connection- oriented transports such as TCP or SCTP. Section 4.4.2 describes a keep-alive mechanism for connection-less transports such as UDP. Support for other transports such as DCCP [RFC4340] is for further study.
The "ping" and "pong" need to be sent between SIP messages and cannot be sent in the middle of a SIP message. If sending over TLS, the CRLFs are sent inside the TLS protected channel. If sending over a SigComp [RFC3320] compressed data stream, the CRLF keep-alives are sent inside the compressed stream. The double CRLF is considered a single SigComp message. The specific mechanism for representing these characters is an implementation-specific matter to be handled by the SigComp compressor at the sending end. If a pong is not received within 10 seconds after sending a ping (or immediately after processing any incoming message being received when that 10 seconds expires), then the client MUST treat the flow as failed. Clients MUST support this CRLF keep-alive. Note: This value of 10-second timeout was selected to be long enough that it allows plenty of time for a server to send a response even if the server is temporarily busy with an administrative activity. At the same time, it was selected to be small enough that a UA registered to two redundant servers with unremarkable hardware uptime could still easily provide very high levels of overall reliability. Although some Internet protocols are designed for round-trip times over 10 seconds, SIP for real- time communications is not really usable in these type of environments as users often abandon calls before waiting much more than a few seconds. When a Flow-Timer header field is not provided in the most recent success registration response, the proper selection of keep-alive frequency is primarily a trade-off between battery usage and availability. The UA MUST select a random number between a fixed or configurable upper bound and a lower bound, where the lower bound is 20% less then the upper bound. The fixed upper bound or the default configurable upper bound SHOULD be 120 seconds (95 seconds for the lower bound) where battery power is not a concern and 840 seconds (672 seconds for the lower bound) where battery power is a concern. The random number will be different for each keep-alive "ping". Note on selection of time values: the 120-second upper bound was chosen based on the idea that for a good user experience, failures normally will be detected in this amount of time and a new connection will be set up. The 14-minute upper bound for battery- powered devices was selected based on NATs with TCP timeouts as low as 15 minutes. Operators that wish to change the relationship between load on servers and the expected time that a user might not receive inbound communications will probably adjust this time. The 95-second lower bound was chosen so that the jitter introduced will result in a relatively even load on the servers after 30 minutes.
RFC5389] Binding Requests over the flow as described in Section 8. Clients MUST support STUN-based keep-alives. When a Flow-Timer header field is not included in a successful registration response, the time between each keep-alive request SHOULD be a random number between 24 and 29 seconds. Note on selection of time values: the upper bound of 29 seconds was selected, as many NATs have UDP timeouts as low as 30 seconds. The 24-second lower bound was selected so that after 10 minutes the jitter introduced by different timers will make the keep-alive requests unsynchronized to evenly spread the load on the servers. Note that the short NAT timeouts with UDP have a negative impact on battery life. If a STUN Binding Error Response is received, or if no Binding Response is received after 7 retransmissions (16 times the STUN "RTO" timer -- where RTO is an estimate of round-trip time), the UA considers the flow failed. If the XOR-MAPPED-ADDRESS in the STUN Binding Response changes, the UA MUST treat this event as a failure on the flow. Section 4.2; however, if there is a failure in forming this flow, the UA needs to wait a certain amount of time before retrying to form a flow to this particular next hop. The amount of time to wait depends if the previous attempt at establishing a flow was successful. For the purposes of this section, a flow is considered successful if outbound registration succeeded, and if keep-alives are in use on this flow, at least one subsequent keep-alive response was received.
The number of seconds to wait is computed in the following way. If all of the flows to every URI in the outbound proxy set have failed, the base-time is set to a lower value (with a default of 30 seconds); otherwise, in the case where at least one of the flows has not failed, the base-time is set to a higher value (with a default of 90 seconds). The upper-bound wait time (W) is computed by taking two raised to the power of the number of consecutive registration failures for that URI, and multiplying this by the base-time, up to a configurable maximum time (with a default of 1800 seconds). W = min (max-time, (base-time * (2 ^ consecutive-failures))) These times MAY be configurable in the UA. The three times are: o max-time with a default of 1800 seconds o base-time (if all failed) with a default of 30 seconds o base-time (if all have not failed) with a default of 90 seconds For example, if the base-time is 30 seconds, and there were three failures, then the upper-bound wait time is min(1800, 30*(2^3)) or 240 seconds. The actual amount of time the UA waits before retrying registration (the retry delay time) is computed by selecting a uniform random time between 50 and 100% of the upper-bound wait time. The UA MUST wait for at least the value of the retry delay time before trying another registration to form a new flow for that URI (a 503 response to an earlier failed registration attempt with a Retry- After header field value may cause the UA to wait longer). To be explicitly clear on the boundary conditions: when the UA boots, it immediately tries to register. If this fails and no registration on other flows succeed, the first retry happens somewhere between 30 and 60 seconds after the failure of the first registration request. If the number of consecutive-failures is large enough that the maximum of 1800 seconds is reached, the UA will keep trying indefinitely with a random time of 15 to 30 minutes between each attempt.
indicated by the Via header field, it MUST insert its URI in a Path header field value as described in [RFC3327]. If it is not the first hop, it might still decide to add itself to the Path header based on local policy. In addition, if the edge proxy is the first SIP node after the UAC, the edge proxy either MUST store a "flow token" (containing information about the flow from the previous hop) in its Path URI or reject the request. The flow token MUST be an identifier that is unique to this network flow. The flow token MAY be placed in the userpart of the URI. In addition, the first node MUST include an "ob" URI parameter in its Path header field value. If the edge proxy is not the first SIP node after the UAC it MUST NOT place an "ob" URI parameter in a Path header field value. The edge proxy can determine if it is the first hop by examining the Via header field. Section 5.1 involves storing a mapping between an incrementing counter and the connection information; however, this would require the edge proxy to keep an infeasible amount of state. It is unclear when this state could be removed, and the approach would have problems if the proxy crashed and lost the value of the counter. A stateless example is provided below. A proxy can use any algorithm it wants as long as the flow token is unique to a flow, the flow can be recovered from the token, and the token cannot be modified by attackers. Example Algorithm: When the proxy boots, it selects a 20-octet crypto random key called K that only the edge proxy knows. A byte array, called S, is formed that contains the following information about the flow the request was received on: an enumeration indicating the protocol, the local IP address and port, the remote IP address and port. The HMAC of S is computed using the key K and the HMAC-SHA1-80 algorithm, as defined in [RFC2104]. The concatenation of the HMAC and S are base64 encoded, as defined in [RFC4648], and used as the flow identifier. When using IPv4 addresses, this will result in a 32-octet identifier.
The proxy decodes the flow token and compares the flow in the flow token with the source of the request to determine if this is an "incoming" or "outgoing" request. If the flow in the flow token identified by the topmost Route header field value matches the source IP address and port of the request, the request is an "outgoing" request; otherwise, it is an "incoming" request. Section 5.2 to form a flow token follow the procedures below to determine the correct flow. To decode the flow token, take the flow identifier in the user portion of the URI and base64 decode it, then verify the HMAC is correct by recomputing the HMAC and checking that it matches. If the HMAC is not correct, the request has been tampered with.
Implementation note: Specific procedures at the edge proxy to ensure that mid-dialog requests are routed over an existing flow are not part of this specification. However, an approach such as having the edge proxy add a Record-Route header with a flow token is one way to ensure that mid-dialog requests are routed over the correct flow. Section 8. When a server receives a double CRLF sequence between SIP messages on a connection-oriented transport such as TCP or SCTP, it MUST immediately respond with a single CRLF over the same connection. The last proxy to forward a successful registration response to a UA MAY include a Flow-Timer header field if the response contains the outbound option-tag in a Require header field value in the response. The reason a proxy would send a Flow-Timer is if it wishes to detect flow failures proactively and take appropriate action (e.g., log alarms, provide alternative treatment if incoming requests for the UA are received, etc.). The server MUST wait for an amount of time larger than the Flow-Timer in order to have a grace period to account for transport delay. RFC3261], Section 10 and [RFC3327], Section 5.3. Registrars that implement this specification MUST support the Path header mechanism [RFC3327]. When receiving a REGISTER request, the registrar MUST check from its Via header field if the registrar is the first hop or not. If the registrar is not the first hop, it MUST examine the Path header of the request. If the Path header field is missing or it exists but the first URI does not have an "ob" URI parameter, then outbound processing MUST NOT be applied to the registration. In this case, the following processing applies: if the REGISTER request contains the reg-id and the outbound option tag in a Supported header field, then the registrar MUST respond to the REGISTER request with a 439 (First Hop Lacks Outbound Support) response; otherwise, the registrar MUST ignore the "reg-id" parameter of the Contact header. See Section 11.6 for more information on the 439 response code.
A Contact header field value with an instance-id media feature tag but no "reg-id" header field parameter is valid (this combination will result in the creation of a GRUU, as described in the GRUU specification [RFC5627]), but one with a reg-id but no instance-id is not valid. If the registrar processes a Contact header field value with a reg-id but no instance-id, it simply ignores the reg-id parameter. A registration containing a "reg-id" header field parameter and a non-zero expiration is used to register a single UA instance over a single flow, and can also de-register any Contact header fields with zero expiration. Therefore, if the Contact header field contains more than one header field value with a non-zero expiration and any of these header field values contain a "reg-id" Contact header field parameter, the entire registration SHOULD be rejected with a 400 (Bad Request) response. The justification for recommending rejection versus making it mandatory is that the receiver is allowed by [RFC3261] to squelch (not respond to) excessively malformed or malicious messages. If the Contact header did not contain a "reg-id" Contact header field parameter or if that parameter was ignored (as described above), the registrar MUST NOT include the outbound option-tag in the Require header field of its response. The registrar MUST be prepared to receive, simultaneously for the same AOR, some registrations that use instance-id and reg-id and some registrations that do not. The registrar MAY be configured with local policy to reject any registrations that do not include the instance-id and reg-id, or with Path header field values that do not contain the "ob" URI parameter. If the Contact header field does not contain a "+sip.instance" Contact header field parameter, the registrar processes the request using the Contact binding rules in [RFC3261]. When a "+sip.instance" Contact header field parameter and a "reg-id" Contact header field parameter are present in a Contact header field of a REGISTER request (after the Contact header validation as described above), the corresponding binding is between an AOR and the combination of the instance-id (from the "+sip.instance" Contact header parameter) and the value of "reg-id" Contact header field parameter parameter. The registrar MUST store in the binding the Contact URI, all the Contact header field parameters, and any Path header field values. (Even though the Contact URI is not used for binding comparisons, it is still needed by the authoritative proxy to form the target set.) Provided that the UAC had included an outbound option-tag (defined in Section 11.4) in a Supported header field
value in the REGISTER request, the registrar MUST include the outbound option-tag in a Require header field value in its response to that REGISTER request. If the UAC has a direct flow with the registrar, the registrar MUST store enough information to uniquely identify the network flow over which the request arrived. For common operating systems with TCP, this would typically be just the handle to the file descriptor where the handle would become invalid if the TCP session was closed. For common operating systems with UDP this would typically be the file descriptor for the local socket that received the request, the local interface, and the IP address and port number of the remote side that sent the request. The registrar MAY store this information by adding itself to the Path header field with an appropriate flow token. If the registrar receives a re-registration for a specific combination of AOR, and instance-id and reg-id values, the registrar MUST update any information that uniquely identifies the network flow over which the request arrived if that information has changed, and SHOULD update the time the binding was last updated. To be compliant with this specification, registrars that can receive SIP requests directly from a UAC without intervening edge proxies MUST implement the same keep-alive mechanisms as edge proxies (Section 5.4). Registrars with a direct flow with a UA MAY include a Flow-Timer header in a 2xx class registration response that includes the outbound option-tag in the Require header.
The proxy uses the next-hop target of the message and the value of any stored Path header field vector in the registration binding to decide how to forward and populate the Route header in the request. If the proxy is co-located with the registrar and stored information about the flow to the UA that created the binding, then the proxy MUST send the request over the same 'logical flow' saved with the binding, since that flow is known to deliver data to the specific target UA instance's network flow that was saved with the binding. Implementation note: Typically this means that for TCP, the request is sent on the same TCP socket that received the REGISTER request. For UDP, the request is sent from the same local IP address and port over which the registration was received, to the same IP address and port from which the REGISTER was received. If a proxy or registrar receives information from the network that indicates that no future messages will be delivered on a specific flow, then the proxy MUST invalidate all the bindings in the target set that use that flow (regardless of AOR). Examples of this are a TCP socket closing or receiving a destination unreachable ICMP error on a UDP flow. Similarly, if a proxy closes a file descriptor, it MUST invalidate all the bindings in the target set with flows that use that file descriptor. RFC5389] Binding Requests to be mixed over the same flow. This constitutes a new STUN usage. The STUN messages are used to verify that connectivity is still available over a UDP flow, and to provide periodic keep-alives. These STUN keep-alives are always sent to the next SIP hop. STUN messages are not delivered end-to- end. The only STUN messages required by this usage are Binding Requests, Binding Responses, and Binding Error Responses. The UAC sends Binding Requests over the same UDP flow that is used for sending SIP messages. These Binding Requests do not require any STUN attributes. The corresponding Binding Responses do not require any STUN attributes except the XOR-MAPPED-ADDRESS. The UAS, proxy, or registrar responds to a valid Binding Request with a Binding Response that MUST include the XOR-MAPPED-ADDRESS attribute. If a server compliant to this section receives SIP requests on a given interface and UDP port, it MUST also provide a limited version of a STUN server on the same interface and UDP port.
Note: It is easy to distinguish STUN and SIP packets sent over UDP, because the first octet of a STUN Binding method has a value of 0 or 1, while the first octet of a SIP message is never a 0 or 1. Because sending and receiving binary STUN data on the same ports used for SIP is a significant and non-backwards compatible change to RFC 3261, this section requires a number of checks before sending STUN messages to a SIP node. If a SIP node sends STUN requests (for example, due to incorrect configuration) despite these warnings, the node could be blacklisted for UDP traffic. A SIP node MUST NOT send STUN requests over a flow unless it has an explicit indication that the target next-hop SIP server claims to support this specification. UACs MUST NOT use an ambiguous configuration option such as "Work through NATs?" or "Do keep- alives?" to imply next-hop STUN support. A UAC MAY use the presence of an "ob" URI parameter in the Path header in a registration response as an indication that its first edge proxy supports the keep-alives defined in this document. Note: Typically, a SIP node first sends a SIP request and waits to receive a 2xx class response over a flow to a new target destination, before sending any STUN messages. When scheduled for the next NAT refresh, the SIP node sends a STUN request to the target. Once a flow is established, failure of a STUN request (including its retransmissions) is considered a failure of the underlying flow. For SIP over UDP flows, if the XOR-MAPPED-ADDRESS returned over the flow changes, this indicates that the underlying connectivity has changed, and is considered a flow failure. The SIP keep-alive STUN usage requires no backwards compatibility with [RFC3489]. RFC3320] compressed SIP messages over the same flow, the STUN messages are simply sent uncompressed, "outside" of SigComp. This is supported by multiplexing STUN messages with SigComp messages by checking the two topmost bits of the message. These bits are always one for SigComp, or zero for STUN. Note: All SigComp messages contain a prefix (the five most significant bits of the first byte are set to one) that does not occur in UTF-8 [RFC3629] encoded text messages, so for
applications that use this encoding (or ASCII encoding) it is possible to multiplex uncompressed application messages and SigComp messages on the same UDP port. The most significant two bits of every STUN Binding method are both zeroes. This, combined with the magic cookie, aids in differentiating STUN packets from other protocols when STUN is multiplexed with other protocols on the same port.