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diff --git a/doc/rfc/rfc5626.txt b/doc/rfc/rfc5626.txt new file mode 100644 index 0000000..44b8cbf --- /dev/null +++ b/doc/rfc/rfc5626.txt @@ -0,0 +1,2803 @@ + + + + + + +Network Working Group C. Jennings, Ed. +Request for Comments: 5626 Cisco Systems +Updates: 3261, 3327 R. Mahy, Ed. +Category: Standards Track Unaffiliated + F. Audet, Ed. + Skype Labs + October 2009 + + + Managing Client-Initiated Connections + in the Session Initiation Protocol (SIP) + +Abstract + + The Session Initiation Protocol (SIP) allows proxy servers to + initiate TCP connections or to send asynchronous UDP datagrams to + User Agents in order to deliver requests. However, in a large number + of real deployments, many practical considerations, such as the + existence of firewalls and Network Address Translators (NATs) or the + use of TLS with server-provided certificates, prevent servers from + connecting to User Agents in this way. This specification defines + behaviors for User Agents, registrars, and proxy servers that allow + requests to be delivered on existing connections established by the + User Agent. It also defines keep-alive behaviors needed to keep NAT + bindings open and specifies the usage of multiple connections from + the User Agent to its registrar. + +Status of This Memo + + This document specifies an Internet standards track protocol for the + Internet community, and requests discussion and suggestions for + improvements. Please refer to the current edition of the "Internet + Official Protocol Standards" (STD 1) for the standardization state + and status of this protocol. Distribution of this memo is unlimited. + +Copyright Notice + + Copyright (c) 2009 IETF Trust and the persons identified as the + document authors. All rights reserved. + + This document is subject to BCP 78 and the IETF Trust's Legal + Provisions Relating to IETF Documents + (http://trustee.ietf.org/license-info) in effect on the date of + publication of this document. Please review these documents + carefully, as they describe your rights and restrictions with respect + to this document. Code Components extracted from this document must + include Simplified BSD License text as described in Section 4.e of + + + + +Jennings, et al. Standards Track [Page 1] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + the Trust Legal Provisions and are provided without warranty as + described in the BSD License. + + This document may contain material from IETF Documents or IETF + Contributions published or made publicly available before November + 10, 2008. The person(s) controlling the copyright in some of this + material may not have granted the IETF Trust the right to allow + modifications of such material outside the IETF Standards Process. + Without obtaining an adequate license from the person(s) controlling + the copyright in such materials, this document may not be modified + outside the IETF Standards Process, and derivative works of it may + not be created outside the IETF Standards Process, except to format + it for publication as an RFC or to translate it into languages other + than English. + +Table of Contents + + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 + 2. Conventions and Terminology . . . . . . . . . . . . . . . . . 5 + 2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 5 + 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 + 3.1. Summary of Mechanism . . . . . . . . . . . . . . . . . . . 6 + 3.2. Single Registrar and UA . . . . . . . . . . . . . . . . . 7 + 3.3. Multiple Connections from a User Agent . . . . . . . . . . 8 + 3.4. Edge Proxies . . . . . . . . . . . . . . . . . . . . . . . 10 + 3.5. Keep-Alive Technique . . . . . . . . . . . . . . . . . . . 11 + 3.5.1. CRLF Keep-Alive Technique . . . . . . . . . . . . . . 12 + 3.5.2. STUN Keep-Alive Technique . . . . . . . . . . . . . . 12 + 4. User Agent Procedures . . . . . . . . . . . . . . . . . . . . 13 + 4.1. Instance ID Creation . . . . . . . . . . . . . . . . . . . 13 + 4.2. Registrations . . . . . . . . . . . . . . . . . . . . . . 14 + 4.2.1. Initial Registrations . . . . . . . . . . . . . . . . 14 + 4.2.2. Subsequent REGISTER Requests . . . . . . . . . . . . . 16 + 4.2.3. Third-Party Registrations . . . . . . . . . . . . . . 17 + 4.3. Sending Non-REGISTER Requests . . . . . . . . . . . . . . 17 + 4.4. Keep-Alives and Detecting Flow Failure . . . . . . . . . . 18 + 4.4.1. Keep-Alive with CRLF . . . . . . . . . . . . . . . . . 19 + 4.4.2. Keep-Alive with STUN . . . . . . . . . . . . . . . . . 21 + 4.5. Flow Recovery . . . . . . . . . . . . . . . . . . . . . . 21 + 5. Edge Proxy Procedures . . . . . . . . . . . . . . . . . . . . 22 + 5.1. Processing Register Requests . . . . . . . . . . . . . . . 22 + 5.2. Generating Flow Tokens . . . . . . . . . . . . . . . . . . 23 + 5.3. Forwarding Non-REGISTER Requests . . . . . . . . . . . . . 23 + 5.3.1. Processing Incoming Requests . . . . . . . . . . . . . 24 + 5.3.2. Processing Outgoing Requests . . . . . . . . . . . . . 24 + 5.4. Edge Proxy Keep-Alive Handling . . . . . . . . . . . . . . 25 + 6. Registrar Procedures . . . . . . . . . . . . . . . . . . . . . 25 + 7. Authoritative Proxy Procedures: Forwarding Requests . . . . . 27 + + + +Jennings, et al. Standards Track [Page 2] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 8. STUN Keep-Alive Processing . . . . . . . . . . . . . . . . . . 28 + 8.1. Use with SigComp . . . . . . . . . . . . . . . . . . . . . 29 + 9. Example Message Flow . . . . . . . . . . . . . . . . . . . . . 30 + 9.1. Subscription to Configuration Package . . . . . . . . . . 30 + 9.2. Registration . . . . . . . . . . . . . . . . . . . . . . . 32 + 9.3. Incoming Call and Proxy Crash . . . . . . . . . . . . . . 34 + 9.4. Re-Registration . . . . . . . . . . . . . . . . . . . . . 37 + 9.5. Outgoing Call . . . . . . . . . . . . . . . . . . . . . . 38 + 10. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 + 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40 + 11.1. Flow-Timer Header Field . . . . . . . . . . . . . . . . . 40 + 11.2. "reg-id" Contact Header Field Parameter . . . . . . . . . 40 + 11.3. SIP/SIPS URI Parameters . . . . . . . . . . . . . . . . . 41 + 11.4. SIP Option Tag . . . . . . . . . . . . . . . . . . . . . . 41 + 11.5. 430 (Flow Failed) Response Code . . . . . . . . . . . . . 41 + 11.6. 439 (First Hop Lacks Outbound Support) Response Code . . . 42 + 11.7. Media Feature Tag . . . . . . . . . . . . . . . . . . . . 42 + 12. Security Considerations . . . . . . . . . . . . . . . . . . . 43 + 13. Operational Notes on Transports . . . . . . . . . . . . . . . 44 + 14. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 44 + 15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 45 + 16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45 + 16.1. Normative References . . . . . . . . . . . . . . . . . . . 45 + 16.2. Informative References . . . . . . . . . . . . . . . . . . 47 + Appendix A. Default Flow Registration Backoff Times . . . . . . . 49 + Appendix B. ABNF . . . . . . . . . . . . . . . . . . . . . . . . 49 + + + + + + + + + + + + + + + + + + + + + + + + + +Jennings, et al. Standards Track [Page 3] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + +1. Introduction + + There are many environments for SIP [RFC3261] deployments in which + the User Agent (UA) can form a connection to a registrar or proxy but + in which connections in the reverse direction to the UA are not + possible. This can happen for several reasons, but the most likely + is a NAT or a firewall in between the SIP UA and the proxy. Many + such devices will only allow outgoing connections. This + specification allows a SIP User Agent behind such a firewall or NAT + to receive inbound traffic associated with registrations or dialogs + that it initiates. + + Most IP phones and personal computers get their network + configurations dynamically via a protocol such as the Dynamic Host + Configuration Protocol (DHCP) [RFC2131]. These systems typically do + not have a useful name in the Domain Name System (DNS) [RFC1035], and + they almost never have a long-term, stable DNS name that is + appropriate for use in the subjectAltName of a certificate, as + required by [RFC3261]. However, these systems can still act as a + Transport Layer Security (TLS) [RFC5246] client and form outbound + connections to a proxy or registrar that authenticates with a server + certificate. The server can authenticate the UA using a shared + secret in a digest challenge (as defined in Section 22 of RFC 3261) + over that TLS connection. This specification allows a SIP User Agent + who has to initiate the TLS connection to receive inbound traffic + associated with registrations or dialogs that it initiates. + + The key idea of this specification is that when a UA sends a REGISTER + request or a dialog-forming request, the proxy can later use this + same network "flow" -- whether this is a bidirectional stream of UDP + datagrams, a TCP connection, or an analogous concept in another + transport protocol -- to forward any incoming requests that need to + go to this UA in the context of the registration or dialog. + + For a UA to receive incoming requests, the UA has to connect to a + server. Since the server can't connect to the UA, the UA has to make + sure that a flow is always active. This requires the UA to detect + when a flow fails. Since such detection takes time and leaves a + window of opportunity for missed incoming requests, this mechanism + allows the UA to register over multiple flows at the same time. This + specification also defines two keep-alive schemes. The keep-alive + mechanism is used to keep NAT bindings fresh, and to allow the UA to + detect when a flow has failed. + + + + + + + + +Jennings, et al. Standards Track [Page 4] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + +2. Conventions and 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]. + +2.1. Definitions + + Authoritative Proxy: A proxy that handles non-REGISTER requests for + a specific Address-of-Record (AOR), performs the logical Location + Server lookup described in [RFC3261], and forwards those requests + to specific Contact URIs. (In [RFC3261], the role that is + authoritative for REGISTER requests for a specific AOR is a + Registration Server.) + + Edge Proxy: An edge proxy is any proxy that is located topologically + between the registering User Agent and the Authoritative Proxy. + The "first" edge proxy refers to the first edge proxy encountered + when a UA sends a request. + + Flow: A Flow is a transport-layer association between two hosts that + is represented by the network address and port number of both ends + and by the transport protocol. For TCP, a flow is equivalent to a + TCP connection. For UDP a flow is a bidirectional stream of + datagrams between a single pair of IP addresses and ports of both + peers. With TCP, a flow often has a one-to-one correspondence + with a single file descriptor in the operating system. + + Flow Token: An identifier that uniquely identifies a flow which can + be included in a SIP URI (Uniform Resource Identifier [RFC3986]). + + reg-id: This refers to the value of a new header field parameter + value for the Contact header field. When a UA registers multiple + times, each for a different flow, each concurrent registration + gets a unique reg-id value. + + instance-id: This specification uses the word instance-id to refer + to the value of the "sip.instance" media feature tag which appears + as a "+sip.instance" Contact header field parameter. This is a + Uniform Resource Name (URN) that uniquely identifies this specific + UA instance. + + "ob" Parameter: The "ob" parameter is a SIP URI parameter that has a + different meaning depending on context. In a Path header field + value, it is used by the first edge proxy to indicate that a flow + token was added to the URI. In a Contact or Route header field + value, it indicates that the UA would like other requests in the + same dialog to be routed over the same flow. + + + +Jennings, et al. Standards Track [Page 5] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + outbound-proxy-set: A set of SIP URIs (Uniform Resource Identifiers) + that represents each of the outbound proxies (often edge proxies) + with which the UA will attempt to maintain a direct flow. The + first URI in the set is often referred to as the primary outbound + proxy and the second as the secondary outbound proxy. There is no + difference between any of the URIs in this set, nor does the + primary/secondary terminology imply that one is preferred over the + other. + +3. Overview + + The mechanisms defined in this document are useful in several + scenarios discussed below, including the simple co-located registrar + and proxy, a User Agent desiring multiple connections to a resource + (for redundancy, for example), and a system that uses edge proxies. + + This entire section is non-normative. + +3.1. Summary of Mechanism + + Each UA has a unique instance-id that stays the same for this UA even + if the UA reboots or is power cycled. Each UA can register multiple + times over different flows for the same SIP Address of Record (AOR) + to achieve high reliability. Each registration includes the + instance-id for the UA and a reg-id label that is different for each + flow. The registrar can use the instance-id to recognize that two + different registrations both correspond to the same UA. The + registrar can use the reg-id label to recognize whether a UA is + creating a new flow or refreshing or replacing an old one, possibly + after a reboot or a network failure. + + When a proxy goes to route a message to a UA for which it has a + binding, it can use any one of the flows on which a successful + registration has been completed. A failure to deliver a request on a + particular flow can be tried again on an alternate flow. Proxies can + determine which flows go to the same UA by comparing the instance-id. + Proxies can tell that a flow replaces a previously abandoned flow by + looking at the reg-id. + + When sending a dialog-forming request, a UA can also ask its first + edge proxy to route subsequent requests in that dialog over the same + flow. This is necessary whether the UA has registered or not. + + UAs use a simple periodic message as a keep-alive mechanism to keep + their flow to the proxy or registrar alive. For connection-oriented + transports such as TCP this is based on carriage-return and line-feed + + + + + +Jennings, et al. Standards Track [Page 6] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + sequences (CRLF), while for transports that are not connection + oriented, this is accomplished by using a SIP-specific usage profile + of STUN (Session Traversal Utilities for NAT) [RFC5389]. + +3.2. Single Registrar and UA + + In the topology shown below, a single server is acting as both a + registrar and proxy. + + +-----------+ + | Registrar | + | Proxy | + +-----+-----+ + | + | + +----+--+ + | User | + | Agent | + +-------+ + + User Agents that form only a single flow continue to register + normally but include the instance-id as described in Section 4.1. + The UA also includes a "reg-id" Contact header field parameter that + is used to allow the registrar to detect and avoid keeping invalid + contacts when a UA reboots or reconnects after its old connection has + failed for some reason. + + For clarity, here is an example. Bob's UA creates a new TCP flow to + the registrar and sends the following REGISTER request. + + REGISTER sip:example.com SIP/2.0 + Via: SIP/2.0/TCP 192.0.2.2;branch=z9hG4bK-bad0ce-11-1036 + Max-Forwards: 70 + From: Bob <sip:bob@example.com>;tag=d879h76 + To: Bob <sip:bob@example.com> + Call-ID: 8921348ju72je840.204 + CSeq: 1 REGISTER + Supported: path, outbound + Contact: <sip:line1@192.0.2.2;transport=tcp>; reg-id=1; + ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-000A95A0E128>" + Content-Length: 0 + + The registrar challenges this registration to authenticate Bob. When + the registrar adds an entry for this contact under the AOR for Bob, + the registrar also keeps track of the connection over which it + received this registration. + + + + + +Jennings, et al. Standards Track [Page 7] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + The registrar saves the instance-id + ("urn:uuid:00000000-0000-1000-8000-000A95A0E128") and reg-id ("1") + along with the rest of the Contact header field. If the instance-id + and reg-id are the same as a previous registration for the same AOR, + the registrar replaces the old Contact URI and flow information. + This allows a UA that has rebooted to replace its previous + registration for each flow with minimal impact on overall system + load. + + When Alice sends a request to Bob, his authoritative proxy selects + the target set. The proxy forwards the request to elements in the + target set based on the proxy's policy. The proxy looks at the + target set and uses the instance-id to understand if two targets both + end up routing to the same UA. When the proxy goes to forward a + request to a given target, it looks and finds the flows over which it + received the registration. The proxy then forwards the request over + an existing flow, instead of resolving the Contact URI using the + procedures in [RFC3263] and trying to form a new flow to that + contact. + + As described in the next section, if the proxy has multiple flows + that all go to this UA, the proxy can choose any one of the + registration bindings for this AOR that has the same instance-id as + the selected UA. + +3.3. Multiple Connections from a User Agent + + There are various ways to deploy SIP to build a reliable and scalable + system. This section discusses one such design that is possible with + the mechanisms in this specification. Other designs are also + possible. + + In the example system below, the logical outbound proxy/registrar for + the domain is running on two hosts that share the appropriate state + and can both provide registrar and outbound proxy functionality for + the domain. The UA will form connections to two of the physical + hosts that can perform the authoritative proxy/registrar function for + the domain. Reliability is achieved by having the UA form two TCP + connections to the domain. + + + + + + + + + + + + +Jennings, et al. Standards Track [Page 8] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + +-------------------+ + | Domain | + | Logical Proxy/Reg | + | | + |+-----+ +-----+| + ||Host1| |Host2|| + |+-----+ +-----+| + +---\------------/--+ + \ / + \ / + \ / + \ / + +------+ + | User | + | Agent| + +------+ + + The UA is configured with multiple outbound proxy registration URIs. + These URIs are configured into the UA through whatever the normal + mechanism is to configure the proxy address and AOR in the UA. If + the AOR is alice@example.com, the outbound-proxy-set might look + something like "sip:primary.example.com" and "sip: + secondary.example.com". Note that each URI in the outbound-proxy-set + could resolve to several different physical hosts. The + administrative domain that created these URIs should ensure that the + two URIs resolve to separate hosts. These URIs are handled according + to normal SIP processing rules, so mechanisms like DNS SRV [RFC2782] + can be used to do load-balancing across a proxy farm. The approach + in this document does not prevent future extensions, such as the SIP + UA configuration framework [CONFIG-FMWK], from adding other ways for + a User Agent to discover its outbound-proxy-set. + + The domain also needs to ensure that a request for the UA sent to + Host1 or Host2 is then sent across the appropriate flow to the UA. + The domain might choose to use the Path header approach (as described + in the next section) to store this internal routing information on + Host1 or Host2. + + When a single server fails, all the UAs that have a flow through it + will detect a flow failure and try to reconnect. This can cause + large loads on the server. When large numbers of hosts reconnect + nearly simultaneously, this is referred to as the avalanche restart + problem, and is further discussed in Section 4.5. The multiple flows + to many servers help reduce the load caused by the avalanche restart. + If a UA has multiple flows, and one of the servers fails, the UA + delays a recommended amount of time before trying to form a new + + + + + +Jennings, et al. Standards Track [Page 9] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + connection to replace the flow to the server that failed. By + spreading out the time used for all the UAs to reconnect to a server, + the load on the server farm is reduced. + + Scalability is achieved by using DNS SRV [RFC2782] to load-balance + the primary connection across a set of machines that can service the + primary connection, and also using DNS SRV to load-balance across a + separate set of machines that can service the secondary connection. + The deployment here requires that DNS is configured with one entry + that resolves to all the primary hosts and another entry that + resolves to all the secondary hosts. While this introduces + additional DNS configuration, the approach works and requires no + additional SIP extensions to [RFC3263]. + + Another motivation for maintaining multiple flows between the UA and + its registrar is related to multihomed UAs. Such UAs can benefit + from multiple connections from different interfaces to protect + against the failure of an individual access link. + +3.4. Edge Proxies + + Some SIP deployments use edge proxies such that the UA sends the + REGISTER to an edge proxy that then forwards the REGISTER to the + registrar. There could be a NAT or firewall between the UA and the + edge proxy. + + +---------+ + |Registrar| + |Proxy | + +---------+ + / \ + / \ + / \ + +-----+ +-----+ + |Edge1| |Edge2| + +-----+ +-----+ + \ / + \ / + ----------------------------NAT/FW + \ / + \ / + +------+ + |User | + |Agent | + +------+ + + + + + + +Jennings, et al. Standards Track [Page 10] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + The edge proxy includes a Path header [RFC3327] so that when the + proxy/registrar later forwards a request to this UA, the request is + routed through the edge proxy. + + These systems can use effectively the same mechanism as described in + the previous sections but need to use the Path header. When the edge + proxy receives a registration, it needs to create an identifier value + that is unique to this flow (and not a subsequent flow with the same + addresses) and put this identifier in the Path header URI. This + identifier has two purposes. First, it allows the edge proxy to map + future requests back to the correct flow. Second, because the + identifier will only be returned if the user authenticates with the + registrar successfully, it allows the edge proxy to indirectly check + the user's authentication information via the registrar. The + identifier is placed in the user portion of a loose route in the Path + header. If the registration succeeds, the edge proxy needs to map + future requests (that are routed to the identifier value from the + Path header) to the associated flow. + + The term edge proxy is often used to refer to deployments where the + edge proxy is in the same administrative domain as the registrar. + However, in this specification we use the term to refer to any proxy + between the UA and the registrar. For example, the edge proxy may be + inside an enterprise that requires its use, and the registrar could + be from a service provider with no relationship to the enterprise. + Regardless of whether they are in the same administrative domain, + this specification requires that registrars and edge proxies support + the Path header mechanism in [RFC3327]. + +3.5. Keep-Alive Technique + + This document describes two keep-alive mechanisms: a CRLF keep-alive + and a STUN keep-alive. Each of these mechanisms uses a client-to- + server "ping" keep-alive and a corresponding server-to-client "pong" + message. This ping-pong sequence allows the client, and optionally + the server, to tell if its flow is still active and useful for SIP + traffic. The server responds to pings by sending pongs. If the + client does not receive a pong in response to its ping (allowing for + retransmission for STUN as described in Section 4.4.2), it declares + the flow dead and opens a new flow in its place. + + This document also suggests timer values for these client keep-alive + mechanisms. These timer values were chosen to keep most NAT and + firewall bindings open, to detect unresponsive servers within 2 + minutes, and to mitigate against the avalanche restart problem. + However, the client may choose different timer values to suit its + needs, for example to optimize battery life. In some environments, + + + + +Jennings, et al. Standards Track [Page 11] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + the server can also keep track of the time since a ping was received + over a flow to guess the likelihood that the flow is still useful for + delivering SIP messages. + + When the UA detects that a flow has failed or that the flow + definition has changed, the UA needs to re-register and will use the + back-off mechanism described in Section 4.5 to provide congestion + relief when a large number of agents simultaneously reboot. + + A keep-alive mechanism needs to keep NAT bindings refreshed; for + connections, it also needs to detect failure of a connection; and for + connectionless transports, it needs to detect flow failures including + changes to the NAT public mapping. For connection-oriented + transports such as TCP [RFC0793] and SCTP [RFC4960], this + specification describes a keep-alive approach based on sending CRLFs. + For connectionless transport, such as UDP [RFC0768], this + specification describes using STUN [RFC5389] over the same flow as + the SIP traffic to perform the keep-alive. + + UAs and Proxies are also free to use native transport keep-alives; + however, the application may not be able to set these timers on a + per-connection basis, and the server certainly cannot make any + assumption about what values are used. Use of native transport + keep-alives is outside the scope of this document. + +3.5.1. CRLF Keep-Alive Technique + + This approach can only be used with connection-oriented transports + such as TCP or SCTP. The client periodically sends a double-CRLF + (the "ping") then waits to receive a single CRLF (the "pong"). If + the client does not receive a "pong" within an appropriate amount of + time, it considers the flow failed. + + Note: Sending a CRLF over a connection-oriented transport is + backwards compatible (because of requirements in Section 7.5 of + [RFC3261]), but only implementations which support this + specification will respond to a "ping" with a "pong". + +3.5.2. STUN Keep-Alive Technique + + This approach can only be used for connection-less transports, such + as UDP. + + For connection-less transports, a flow definition could change + because a NAT device in the network path reboots and the resulting + public IP address or port mapping for the UA changes. To detect + this, STUN requests are sent over the same flow that is being used + + + + +Jennings, et al. Standards Track [Page 12] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + for the SIP traffic. The proxy or registrar acts as a limited + Session Traversal Utilities for NAT (STUN) [RFC5389] server on the + SIP signaling port. + + Note: The STUN mechanism is very robust and allows the detection + of a changed IP address and port. Many other options were + considered, but the SIP Working Group selected the STUN-based + approach. Approaches using SIP requests were abandoned because + many believed that good performance and full backwards + compatibility using this method were mutually exclusive. + +4. User Agent Procedures + +4.1. Instance ID Creation + + Each UA MUST have an Instance Identifier Uniform Resource Name (URN) + [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. + + + + + + +Jennings, et al. Standards Track [Page 13] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 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. + +4.2. Registrations + +4.2.1. Initial Registrations + + At configuration time, UAs obtain one or more SIP URIs representing + the default outbound-proxy-set. This specification assumes the set + is determined via any of a number of configuration mechanisms, and + future specifications can define additional mechanisms such as using + DNS to discover this set. How the UA is configured is outside the + scope of this specification. However, a UA MUST support sets with at + least two outbound proxy URIs and SHOULD support sets with up to four + URIs. + + + + + + +Jennings, et al. Standards Track [Page 14] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 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 + + + +Jennings, et al. Standards Track [Page 15] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 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. + +4.2.2. Subsequent REGISTER Requests + + Registrations for refreshing a binding and for removing a binding use + the same instance-id and reg-id values as the corresponding initial + registration where the binding was added. Registrations that merely + refresh an existing binding are sent over the same flow as the + original registration where the binding was added. + + If a re-registration is rejected with a recoverable error response, + for example by a 503 (Service Unavailable) containing a Retry-After + header, the UAC SHOULD NOT tear down the corresponding flow if the + flow uses a connection-oriented transport such as TCP. As long as + "pongs" are received in response to "pings", the flow SHOULD be kept + active until a non-recoverable error response is received. This + prevents unnecessary closing and opening of connections. + + + + + + + +Jennings, et al. Standards Track [Page 16] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + +4.2.3. Third-Party Registrations + + In an initial registration or re-registration, a UA MUST NOT include + a "reg-id" header field parameter in the Contact header field if the + registering UA is not the same instance as the UA referred to by the + target Contact header field. (This practice is occasionally used to + install forwarding policy into registrars.) + + A UAC also MUST NOT include an instance-id feature tag or "reg-id" + Contact header field parameter in a request to un-register all + Contacts (a single Contact header field value with the value of "*"). + +4.3. Sending Non-REGISTER Requests + + When a UAC is about to send a request, it first performs normal + processing to select the next hop URI. The UA can use a variety of + techniques to compute the route set and accordingly the next hop URI. + Discussion of these techniques is outside the scope of this document. + UAs that support this specification SHOULD include the outbound + option tag in a Supported header field in a request that is not a + REGISTER request. + + The UAC performs normal DNS resolution on the next hop URI (as + described in [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]. + + + + + +Jennings, et al. Standards Track [Page 17] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 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]. + +4.4. Keep-Alives and Detecting Flow Failure + + Keep-alives are used for refreshing NAT/firewall bindings and + detecting flow failure. Flows can fail for many reasons including + the rebooting of NATs and the crashing of edge proxies. + + As described in 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. + + + + + +Jennings, et al. Standards Track [Page 18] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 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. + +4.4.1. Keep-Alive with CRLF + + This approach MUST only be used with connection oriented transports + such as TCP or SCTP; it MUST NOT be used with connection-less + transports such as UDP. + + A User Agent that forms flows checks if the configured URI to which + the UA is connecting resolves to a connection-oriented transport + (e.g., TCP and TLS over TCP). + + For this mechanism, the client "ping" is a double-CRLF sequence, and + the server "pong" is a single CRLF, as defined in the ABNF below: + + CRLF = CR LF + double-CRLF = CR LF CR LF + CR = %x0D + LF = %x0A + + + + +Jennings, et al. Standards Track [Page 19] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 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. + + + +Jennings, et al. Standards Track [Page 20] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + +4.4.2. Keep-Alive with STUN + + This approach MUST only be used with connection-less transports, such + as UDP; it MUST NOT be used for connection-oriented transports such + as TCP and SCTP. + + A User Agent that forms flows checks if the configured URI to which + the UA is connecting resolves to use the UDP transport. The UA can + periodically perform keep-alive checks by sending STUN [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. + +4.5. Flow Recovery + + When a flow used for registration (through a particular URI in the + outbound-proxy-set) fails, the UA needs to form a new flow to replace + the old flow and replace any registrations that were previously sent + over this flow. Each new registration MUST have the same reg-id + value as the registration it replaces. This is done in much the same + way as forming a brand new flow as described in 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. + + + +Jennings, et al. Standards Track [Page 21] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 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. + +5. Edge Proxy Procedures + +5.1. Processing Register Requests + + When an edge proxy receives a registration request with a "reg-id" + header field parameter in the Contact header field, it needs to + determine if it (the edge proxy) will have to be visited for any + subsequent requests sent to the User Agent identified in the Contact + header field, or not. If the edge proxy is the first hop, as + + + +Jennings, et al. Standards Track [Page 22] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 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. + +5.2. Generating Flow Tokens + + A trivial but impractical way to satisfy the flow token requirement + in 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. + +5.3. Forwarding Non-REGISTER Requests + + When an edge proxy receives a request, it applies normal routing + procedures with the following additions. If the edge proxy receives + a request where the edge proxy is the host in the topmost Route + header field value, and the Route header field value contains a flow + token, the proxy follows the procedures of this section. Otherwise + the edge proxy skips the procedures in this section, removes itself + from the Route header field, and continues processing the request. + + + + +Jennings, et al. Standards Track [Page 23] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 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. + +5.3.1. Processing Incoming Requests + + If the Route header value contains an "ob" URI parameter, the Route + header was probably copied from the Path header in a registration. + If the Route header value contains an "ob" URI parameter, and the + request is a new dialog-forming request, the proxy needs to adjust + the route set to ensure that subsequent requests in the dialog can be + delivered over a valid flow to the UA instance identified by the flow + token. + + Note: A simple approach to satisfy this requirement is for the + proxy to add a Record-Route header field value that contains the + flow-token, by copying the URI in the Route header minus the "ob" + parameter. + + Next, whether the Route header field contained an "ob" URI parameter + or not, the proxy removes the Route header field value and forwards + the request over the 'logical flow' identified by the flow token, + that is known to deliver data to the specific target UA instance. If + the flow token has been tampered with, the proxy SHOULD send a 403 + (Forbidden) response. If the flow no longer exists, the proxy SHOULD + send a 430 (Flow Failed) response to the request. + + Proxies that used the example algorithm described in 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. + +5.3.2. Processing Outgoing Requests + + For mid-dialog requests to work with outbound UAs, the requests need + to be forwarded over some valid flow to the appropriate UA instance. + If the edge proxy receives an outgoing dialog-forming request, the + edge proxy can use the presence of the "ob" URI parameter in the + UAC's Contact URI (or topmost Route header field) to determine if the + edge proxy needs to assist in mid-dialog request routing. + + + + +Jennings, et al. Standards Track [Page 24] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 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. + +5.4. Edge Proxy Keep-Alive Handling + + All edge proxies compliant with this specification MUST implement + support for STUN NAT keep-alives on their SIP UDP ports as described + in 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. + +6. Registrar Procedures + + This specification updates the definition of a binding in [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. + + + + +Jennings, et al. Standards Track [Page 25] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 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 + + + + +Jennings, et al. Standards Track [Page 26] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 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. + +7. Authoritative Proxy Procedures: Forwarding Requests + + When a proxy uses the location service to look up a registration + binding and then proxies a request to a particular contact, it + selects a contact to use normally, with a few additional rules: + + o The proxy MUST NOT populate the target set with more than one + contact with the same AOR and instance-id at a time. + + o If a request for a particular AOR and instance-id fails with a 430 + (Flow Failed) response, the proxy SHOULD replace the failed branch + with another target (if one is available) with the same AOR and + instance-id, but a different reg-id. + + o If the proxy receives a final response from a branch other than a + 408 (Request Timeout) or a 430 (Flow Failed) response, the proxy + MUST NOT forward the same request to another target representing + the same AOR and instance-id. The targeted instance has already + provided its response. + + + + +Jennings, et al. Standards Track [Page 27] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 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. + +8. STUN Keep-Alive Processing + + This section describes changes to the SIP transport layer that allow + SIP and STUN [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. + + + + + +Jennings, et al. Standards Track [Page 28] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 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]. + +8.1. Use with SigComp + + When STUN is used together with SigComp [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 + + + +Jennings, et al. Standards Track [Page 29] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 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. + +9. Example Message Flow + + Below is an example message flow illustrating most of the concepts + discussed in this specification. In many cases, Via, Content-Length, + and Max-Forwards headers are omitted for brevity and readability. + + In these examples, "EP1" and "EP2" are outbound proxies, and "Proxy" + is the authoritativeProxy. + + The section is subdivided into independent calls flows; however, they + are structured in sequential order of a hypothetical sequence of call + flows. + +9.1. Subscription to Configuration Package + + If the outbound proxy set is already configured on Bob's UA, then + this subsection can be skipped. Otherwise, if the outbound proxy set + is learned through the configuration package, Bob's UA sends a + SUBSCRIBE request for the UA profile configuration package + [CONFIG-FMWK]. This request is a poll (Expires is zero). After + receiving the NOTIFY request, Bob's UA fetches the external + configuration using HTTPS (not shown) and obtains a configuration + file that contains the outbound-proxy-set "sip:ep1.example.com;lr" + and "sip:ep2.example.com;lr". + + [----example.com domain-------------------------] + Bob EP1 EP2 Proxy Config + | | | | | + 1)|SUBSCRIBE->| | | | + 2)| |---SUBSCRIBE Event: ua-profile ->| + 3)| |<--200 OK -----------------------| + 4)|<--200 OK--| | | | + 5)| |<--NOTIFY------------------------| + 6)|<--NOTIFY--| | | | + 7)|---200 OK->| | | | + 8)| |---200 OK ---------------------->| + | | | | | + + In this example, the DNS server happens to be configured so that sip: + example.com resolves to EP1 and EP2. + + + +Jennings, et al. Standards Track [Page 30] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + Example Message #1: + + SUBSCRIBE sip:00000000-0000-1000-8000-AABBCCDDEEFF@example.com + SIP/2.0 + Via: SIP/2.0/TCP 192.0.2.2;branch=z9hG4bKnlsdkdj2 + Max-Forwards: 70 + From: <anonymous@example.com>;tag=23324 + To: <sip:00000000-0000-1000-8000-AABBCCDDEEFF@example.com> + Call-ID: nSz1TWN54x7My0GvpEBj + CSeq: 1 SUBSCRIBE + Event: ua-profile ;profile-type=device + ;vendor="example.com";model="uPhone";version="1.1" + Expires: 0 + Supported: path, outbound + Accept: message/external-body, application/x-uPhone-config + Contact: <sip:192.0.2.2;transport=tcp;ob> + ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>" + Content-Length: 0 + + In message #2, EP1 adds the following Record-Route header: + + Record-Route: + <sip:GopIKSsn0oGLPXRdV9BAXpT3coNuiGKV@ep1.example.com;lr> + + In message #5, the configuration server sends a NOTIFY with an + external URL for Bob to fetch his configuration. The NOTIFY has a + Subscription-State header that ends the subscription. + + Message #5 + + NOTIFY sip:192.0.2.2;transport=tcp;ob SIP/2.0 + Via: SIP/2.0/TCP 192.0.2.5;branch=z9hG4bKn81dd2 + Max-Forwards: 70 + To: <anonymous@example.com>;tag=23324 + From: <sip:00000000-0000-1000-8000-AABBCCDDEEFF@example.com>;tag=0983 + Call-ID: nSz1TWN54x7My0GvpEBj + CSeq: 1 NOTIFY + Route: <sip:GopIKSsn0oGLPXRdV9BAXpT3coNuiGKV@ep1.example.com;lr> + Subscription-State: terminated;reason=timeout + Event: ua-profile + Content-Type: message/external-body; access-type="URL" + ;expiration="Thu, 01 Jan 2009 09:00:00 UTC" + ;URL="http://example.com/uPhone.cfg" + ;size=9999;hash=10AB568E91245681AC1B + Content-Length: 0 + + + + + + +Jennings, et al. Standards Track [Page 31] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + EP1 receives this NOTIFY request, strips off the Route header, + extracts the flow-token, calculates the correct flow, and forwards + the request (message #6) over that flow to Bob. + + Bob's UA fetches the configuration file and learns the outbound proxy + set. + +9.2. Registration + + Now that Bob's UA is configured with the outbound-proxy-set whether + through configuration or using the configuration framework procedures + of the previous section, Bob's UA sends REGISTER requests through + each edge proxy in the set. Once the registrations succeed, Bob's UA + begins sending CRLF keep-alives about every 2 minutes. + + Bob EP1 EP2 Proxy Alice + | | | | | + 9)|-REGISTER->| | | | + 10)| |---REGISTER-->| | + 11)| |<----200 OK---| | + 12)|<-200 OK---| | | | + 13)|----REGISTER---->| | | + 14)| | |--REG-->| | + 15)| | |<-200---| | + 16)|<----200 OK------| | | + | | | | | + | about 120 seconds later... | + | | | | | + 17)|--2CRLF--->| | | | + 18)|<--CRLF----| | | | + 19)|------2CRLF----->| | | + 20)|<------CRLF------| | | + | | | | | + + In message #9, Bob's UA sends its first registration through the + first edge proxy in the outbound-proxy-set by including a loose + route. The UA includes an instance-id and reg-id in its Contact + header field value. Note the option-tags in the Supported header. + + + + + + + + + + + + + +Jennings, et al. Standards Track [Page 32] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + Message #9 + + REGISTER sip:example.com SIP/2.0 + Via: SIP/2.0/TCP 192.0.2.2;branch=z9hG4bKnashds7 + Max-Forwards: 70 + From: Bob <sip:bob@example.com>;tag=7F94778B653B + To: Bob <sip:bob@example.com> + Call-ID: 16CB75F21C70 + CSeq: 1 REGISTER + Supported: path, outbound + Route: <sip:ep1.example.com;lr> + Contact: <sip:bob@192.0.2.2;transport=tcp>;reg-id=1 + ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>" + Content-Length: 0 + + Message #10 is similar. EP1 removes the Route header field value, + decrements Max-Forwards, and adds its Via header field value. Since + EP1 is the first edge proxy, it adds a Path header with a flow token + and includes the "ob" parameter. + + Path: <sip:VskztcQ/S8p4WPbOnHbuyh5iJvJIW3ib@ep1.example.com;lr;ob> + + Since the response to the REGISTER (message #11) contains the + outbound option-tag in the Require header field, Bob's UA will know + that the registrar used outbound binding rules. The response also + contains the currently active Contacts, and the Path for the current + registration. + + Message #11 + + SIP/2.0 200 OK + Via: SIP/2.0/TCP 192.0.2.15;branch=z9hG4bKnuiqisi + Via: SIP/2.0/TCP 192.0.2.2;branch=z9hG4bKnashds7 + From: Bob <sip:bob@example.com>;tag=7F94778B653B + To: Bob <sip:bob@example.com>;tag=6AF99445E44A + Call-ID: 16CB75F21C70 + CSeq: 1 REGISTER + Supported: path, outbound + Require: outbound + Contact: <sip:bob@192.0.2.2;transport=tcp>;reg-id=1;expires=3600 + ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>" + Path: <sip:VskztcQ/S8p4WPbOnHbuyh5iJvJIW3ib@ep1.example.com;lr;ob> + Content-Length: 0 + + The second registration through EP2 (message #13) is similar except + that the Call-ID has changed, the reg-id is 2, and the Route header + goes through EP2. + + + + +Jennings, et al. Standards Track [Page 33] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + Message #13 + + REGISTER sip:example.com SIP/2.0 + Via: SIP/2.0/TCP 192.0.2.2;branch=z9hG4bKnqr9bym + Max-Forwards: 70 + From: Bob <sip:bob@example.com>;tag=755285EABDE2 + To: Bob <sip:bob@example.com> + Call-ID: E05133BD26DD + CSeq: 1 REGISTER + Supported: path, outbound + Route: <sip:ep2.example.com;lr> + Contact: <sip:bob@192.0.2.2;transport=tcp>;reg-id=2 + ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>" + Content-Length: 0 + + Likewise in message #14, EP2 adds a Path header with flow token and + "ob" parameter. + + Path: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr;ob> + + Message #16 tells Bob's UA that outbound registration was successful, + and shows both Contacts. Note that only the Path corresponding to + the current registration is returned. + + Message #16 + + SIP/2.0 200 OK + Via: SIP/2.0/TCP 192.0.2.2;branch=z9hG4bKnqr9bym + From: Bob <sip:bob@example.com>;tag=755285EABDE2 + To: Bob <sip:bob@example.com>;tag=49A9AD0B3F6A + Call-ID: E05133BD26DD + Supported: path, outbound + Require: outbound + CSeq: 1 REGISTER + Contact: <sip:bob@192.0.2.2;transport=tcp>;reg-id=1;expires=3600 + ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>" + Contact: <sip:bob@192.0.2.2;transport=tcp>;reg-id=2;expires=3600 + ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>" + Path: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr;ob> + Content-Length: 0 + +9.3. Incoming Call and Proxy Crash + + In this example, after registration, EP1 crashes and reboots. Before + Bob's UA notices that its flow to EP1 is no longer responding, Alice + calls Bob. Bob's authoritative proxy first tries the flow to EP1, + + + + + +Jennings, et al. Standards Track [Page 34] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + but EP1 no longer has a flow to Bob, so it responds with a 430 (Flow + Failed) response. The proxy removes the stale registration and tries + the next binding for the same instance. + + Bob EP1 EP2 Proxy Alice + | | | | | + | CRASH X | | | + | Reboot | | | + | | | | | + 21)| | | |<-INVITE-| + 22)| |<---INVITE----| | + 23)| |----430------>| | + 24)| | |<-INVITE| | + 25)|<---INVITE-------| | | + 26)|----200 OK------>| | | + 27)| | |200 OK->| | + 28)| | | |-200 OK->| + 29)| | |<----------ACK----| + 30)|<---ACK----------| | | + | | | | | + 31)| | |<----------BYE----| + 32)|<---BYE----------| | | + 33)|----200 OK------>| | | + 34)| | |--------200 OK--->| + | | | | | + + + Message #21 + + INVITE sip:bob@example.com SIP/2.0 + To: Bob <sip:bob@example.com> + From: Alice <sip:alice@a.example>;tag=02935 + Call-ID: klmvCxVWGp6MxJp2T2mb + CSeq: 1 INVITE + + Bob's proxy rewrites the Request-URI to the Contact URI used in Bob's + registration, and places the path for one of the registrations + towards Bob's UA instance into a Route header field. This Route goes + through EP1. + + Message #22 + + INVITE sip:bob@192.0.2.2;transport=tcp SIP/2.0 + To: Bob <sip:bob@example.com> + From: Alice <sip:alice@a.example>;tag=02935 + Call-ID: klmvCxVWGp6MxJp2T2mb + CSeq: 1 INVITE + Route: <sip:VskztcQ/S8p4WPbOnHbuyh5iJvJIW3ib@ep1.example.com;lr;ob> + + + +Jennings, et al. Standards Track [Page 35] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + Since EP1 just rebooted, it does not have the flow described in the + flow token. It returns a 430 (Flow Failed) response. + + Message #23 + + SIP/2.0 430 Flow Failed + To: Bob <sip:bob@example.com> + From: Alice <sip:alice@a.example>;tag=02935 + Call-ID: klmvCxVWGp6MxJp2T2mb + CSeq: 1 INVITE + + The proxy deletes the binding for this path and tries to forward the + INVITE again, this time with the path through EP2. + + Message #24 + + INVITE sip:bob@192.0.2.2;transport=tcp SIP/2.0 + To: Bob <sip:bob@example.com> + From: Alice <sip:alice@a.example>;tag=02935 + Call-ID: klmvCxVWGp6MxJp2T2mb + CSeq: 1 INVITE + Route: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr;ob> + + In message #25, EP2 needs to add a Record-Route header field value, + so that any subsequent in-dialog messages from Alice's UA arrive at + Bob's UA. EP2 can determine it needs to Record-Route since the + request is a dialog-forming request and the Route header contained a + flow token and an "ob" parameter. This Record-Route information is + passed back to Alice's UA in the responses (messages #26, 27, and + 28). + + Message #25 + + INVITE sip:bob@192.0.2.2;transport=tcp SIP/2.0 + To: Bob <sip:bob@example.com> + From: Alice <sip:alice@a.example>;tag=02935 + Call-ID: klmvCxVWGp6MxJp2T2mb + CSeq: 1 INVITE + Record-Route: + <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr> + + + + + + + + + + + +Jennings, et al. Standards Track [Page 36] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + Message #26 + + SIP/2.0 200 OK + To: Bob <sip:bob@example.com>;tag=skduk2 + From: Alice <sip:alice@a.example>;tag=02935 + Call-ID: klmvCxVWGp6MxJp2T2mb + CSeq: 1 INVITE + Record-Route: + <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr> + + At this point, both UAs have the correct route-set for the dialog. + Any subsequent requests in this dialog will route correctly. For + example, the ACK request in message #29 is sent from Alice's UA + directly to EP2. The BYE request in message #31 uses the same route- + set. + + Message #29 + + ACK sip:bob@192.0.2.2;transport=tcp SIP/2.0 + To: Bob <sip:bob@example.com>;tag=skduk2 + From: Alice <sip:alice@a.example>;tag=02935 + Call-ID: klmvCxVWGp6MxJp2T2mb + CSeq: 1 ACK + Route: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr> + + Message #31 + + BYE sip:bob@192.0.2.2;transport=tcp SIP/2.0 + To: Bob <sip:bob@example.com>;tag=skduk2 + From: Alice <sip:alice@a.example>;tag=02935 + Call-ID: klmvCxVWGp6MxJp2T2mb + CSeq: 2 BYE + Route: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr> + +9.4. Re-Registration + + Somewhat later, Bob's UA sends keep-alives to both its edge proxies, + but it discovers that the flow with EP1 failed. Bob's UA re- + registers through EP1 using the same reg-id and Call-ID it previously + used. + + + + + + + + + + + +Jennings, et al. Standards Track [Page 37] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + Bob EP1 EP2 Proxy Alice + | | | | | + 35)|------2CRLF----->| | | + 36)|<------CRLF------| | | + 37)|--2CRLF->X | | | | + | | | | | + 38)|-REGISTER->| | | | + 39)| |---REGISTER-->| | + 40)| |<----200 OK---| | + 41)|<-200 OK---| | | | + | | | | | + + Message #38 + + REGISTER sip:example.com SIP/2.0 + From: Bob <sip:bob@example.com>;tag=7F94778B653B + To: Bob <sip:bob@example.com> + Call-ID: 16CB75F21C70 + CSeq: 2 REGISTER + Supported: path, outbound + Route: <sip:ep1.example.com;lr> + Contact: <sip:bob@192.0.2.2;transport=tcp>;reg-id=1 + ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>" + + In message #39, EP1 inserts a Path header with a new flow token: + + Path: <sip:3yJEbr1GYZK9cPYk5Snocez6DzO7w+AX@ep1.example.com;lr;ob> + +9.5. Outgoing Call + + Finally, Bob makes an outgoing call to Alice. Bob's UA includes an + "ob" parameter in its Contact URI in message #42. EP1 adds a Record- + Route with a flow-token in message #43. The route-set is returned to + Bob in the response (messages #45, 46, and 47), and either Bob or + Alice can send in-dialog requests. + + + + + + + + + + + + + + + + +Jennings, et al. Standards Track [Page 38] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + Bob EP1 EP2 Proxy Alice + | | | | | + 42)|--INVITE-->| | | | + 43)| |---INVITE---->| | + 44)| | | |-INVITE->| + 45)| | | |<--200---| + 46)| |<----200 OK---| | + 47)|<-200 OK---| | | | + 48)|--ACK----->| | | | + 49)| |-----ACK--------------->| + | | | | | + 50)|-- BYE---->| | | | + 51)| |-----------BYE--------->| + 52)| |<----------200 OK-------| + 53)|<--200 OK--| | | | + | | | | | + + Message #42 + + INVITE sip:alice@a.example SIP/2.0 + From: Bob <sip:bob@example.com>;tag=ldw22z + To: Alice <sip:alice@a.example> + Call-ID: 95KGsk2V/Eis9LcpBYy3 + CSeq: 1 INVITE + Route: <sip:ep1.example.com;lr> + Contact: <sip:bob@192.0.2.2;transport=tcp;ob> + + In message #43, EP1 adds the following Record-Route header. + + Record-Route: + <sip:3yJEbr1GYZK9cPYk5Snocez6DzO7w+AX@ep1.example.com;lr> + + When EP1 receives the BYE (message #50) from Bob's UA, it can tell + that the request is an "outgoing" request (since the source of the + request matches the flow in the flow token) and simply deletes its + Route header field value and forwards the request on to Alice's UA. + + Message #50 + + BYE sip:alice@a.example SIP/2.0 + From: Bob <sip:bob@example.com>;tag=ldw22z + To: Alice <sip:alice@a.example>;tag=plqus8 + Call-ID: 95KGsk2V/Eis9LcpBYy3 + CSeq: 2 BYE + Route: <sip:3yJEbr1GYZK9cPYk5Snocez6DzO7w+AX@ep1.example.com;lr> + Contact: <sip:bob@192.0.2.2;transport=tcp;ob> + + + + + +Jennings, et al. Standards Track [Page 39] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + +10. Grammar + + This specification defines a new header field "Flow-Timer", and new + Contact header field parameters, "reg-id" and "+sip.instance". The + grammar includes the definitions from [RFC3261]. Flow-Timer is an + extension-header from the message-header in the [RFC3261] ABNF. + + The ABNF [RFC5234] is: + + Flow-Timer = "Flow-Timer" HCOLON 1*DIGIT + + contact-params =/ c-p-reg / c-p-instance + + c-p-reg = "reg-id" EQUAL 1*DIGIT ; 1 to (2^31 - 1) + + c-p-instance = "+sip.instance" EQUAL + DQUOTE "<" instance-val ">" DQUOTE + + instance-val = 1*uric ; defined in RFC 3261 + + The value of the reg-id MUST NOT be 0 and MUST be less than 2^31. + +11. IANA Considerations + +11.1. Flow-Timer Header Field + + This specification defines a new SIP header field "Flow-Timer" whose + syntax is defined in Section 10. + + Header Name compact Reference + ----------------- ------- --------- + Flow-Timer [RFC5626] + +11.2. "reg-id" Contact Header Field Parameter + + This specification defines a new Contact header field parameter + called reg-id in the "Header Field Parameters and Parameter Values" + sub-registry as per the registry created by [RFC3968]. The syntax is + defined in Section 10. The required information is: + + Predefined + Header Field Parameter Name Values Reference + ---------------------- --------------------- ---------- --------- + Contact reg-id No [RFC5626] + + + + + + + +Jennings, et al. Standards Track [Page 40] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + +11.3. SIP/SIPS URI Parameters + + This specification augments the "SIP/SIPS URI Parameters" sub- + registry as per the registry created by [RFC3969]. The required + information is: + + Parameter Name Predefined Values Reference + -------------- ----------------- --------- + ob No [RFC5626] + +11.4. SIP Option Tag + + This specification registers a new SIP option tag, as per the + guidelines in Section 27.1 of [RFC3261]. + + Name: outbound + + Description: This option-tag is used to identify UAs and registrars + that support extensions for Client-Initiated Connections. A UA + places this option in a Supported header to communicate its + support for this extension. A registrar places this option-tag in + a Require header to indicate to the registering User Agent that + the registrar used registrations using the binding rules defined + in this extension. + +11.5. 430 (Flow Failed) Response Code + + This document registers a new SIP response code (430 Flow Failed), as + per the guidelines in Section 27.4 of [RFC3261]. This response code + is used by an edge proxy to indicate to the Authoritative Proxy that + a specific flow to a UA instance has failed. Other flows to the same + instance could still succeed. The Authoritative Proxy SHOULD attempt + to forward to another target (flow) with the same instance-id and + AOR. Endpoints should never receive a 430 response. If an endpoint + receives a 430 response, it should treat it as a 400 (Bad Request) + per normal procedures, as in Section 8.1.3.2 of [RFC3261]. This + response code is defined by the following information, which has been + added to the method and response-code sub-registry under the SIP + Parameters registry. + + Response Code Reference + ------------------------------------------ --------- + Request Failure 4xx + 430 Flow Failed [RFC5626] + + + + + + + +Jennings, et al. Standards Track [Page 41] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + +11.6. 439 (First Hop Lacks Outbound Support) Response Code + + This document registers a new SIP response code (439 First Hop Lacks + Outbound Support), as per the guidelines in Section 27.4 of + [RFC3261]. This response code is used by a registrar to indicate + that it supports the 'outbound' feature described in this + specification, but that the first outbound proxy that the user is + attempting to register through does not. Note that this response + code is only appropriate in the case that the registering User Agent + advertises support for outbound processing by including the outbound + option tag in a Supported header field. Proxies MUST NOT send a 439 + response to any requests that do not contain a "reg-id" parameter and + an outbound option tag in a Supported header field. This response + code is defined by the following information, which has been added to + the method and response-code sub-registry under the SIP Parameters + registry. + + Response Code Reference + ------------------------------------------ --------- + Request Failure 4xx + 439 First Hop Lacks Outbound Support [RFC&rfc.number;] + +11.7. Media Feature Tag + + This section registers a new media feature tag, per the procedures + defined in [RFC2506]. The tag is placed into the sip tree, which is + defined in [RFC3840]. + + Media feature tag name: sip.instance + + ASN.1 Identifier: 23 + + Summary of the media feature indicated by this tag: This feature tag + contains a string containing a URN that indicates a unique + identifier associated with the UA instance registering the + Contact. + + Values appropriate for use with this feature tag: String (equality + relationship). + + The feature tag is intended primarily for use in the following + applications, protocols, services, or negotiation mechanisms: + This feature tag is most useful in a communications application, + for describing the capabilities of a device, such as a phone or + PDA. + + Examples of typical use: Routing a call to a specific device. + + + + +Jennings, et al. Standards Track [Page 42] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + Related standards or documents: RFC 5626 + + Security Considerations: This media feature tag can be used in ways + which affect application behaviors. For example, the SIP caller + preferences extension [RFC3841] allows for call routing decisions + to be based on the values of these parameters. Therefore, if an + attacker can modify the values of this tag, they might be able to + affect the behavior of applications. As a result, applications + that utilize this media feature tag SHOULD provide a means for + ensuring its integrity. Similarly, this feature tag should only + be trusted as valid when it comes from the user or User Agent + described by the tag. As a result, protocols for conveying this + feature tag SHOULD provide a mechanism for guaranteeing + authenticity. + +12. Security Considerations + + One of the key security concerns in this work is making sure that an + attacker cannot hijack the sessions of a valid user and cause all + calls destined to that user to be sent to the attacker. Note that + the intent is not to prevent existing active attacks on SIP UDP and + TCP traffic, but to ensure that no new attacks are added by + introducing the outbound mechanism. + + The simple case is when there are no edge proxies. In this case, the + only time an entry can be added to the routing for a given AOR is + when the registration succeeds. SIP already protects against + attackers being able to successfully register, and this scheme relies + on that security. Some implementers have considered the idea of just + saving the instance-id without relating it to the AOR with which it + registered. This idea will not work because an attacker's UA can + impersonate a valid user's instance-id and hijack that user's calls. + + The more complex case involves one or more edge proxies. When a UA + sends a REGISTER request through an edge proxy on to the registrar, + the edge proxy inserts a Path header field value. If the + registration is successfully authenticated, the registrar stores the + value of the Path header field. Later, when the registrar forwards a + request destined for the UA, it copies the stored value of the Path + header field into the Route header field of the request and forwards + the request to the edge proxy. + + The only time an edge proxy will route over a particular flow is when + it has received a Route header that has the flow identifier + information that it has created. An incoming request would have + gotten this information from the registrar. The registrar will only + save this information for a given AOR if the registration for the AOR + has been successful; and the registration will only be successful if + + + +Jennings, et al. Standards Track [Page 43] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + the UA can correctly authenticate. Even if an attacker has spoofed + some bad information in the Path header sent to the registrar, the + attacker will not be able to get the registrar to accept this + information for an AOR that does not belong to the attacker. The + registrar will not hand out this bad information to others, and + others will not be misled into contacting the attacker. + + The Security Considerations discussed in [RFC3261] and [RFC3327] are + also relevant to this document. For the security considerations of + generating flow tokens, please also see Section 5.2. A discussion of + preventing the avalanche restart problem is in Section 4.5. + + This document does not change the mandatory-to-implement security + mechanisms in SIP. User Agents are already required to implement + Digest authentication while support of TLS is recommended; proxy + servers are already required to implement Digest and TLS. + +13. Operational Notes on Transports + + This entire section is non-normative. + + [RFC3261] requires proxies, registrars, and User Agents to implement + both TCP and UDP but deployments can chose which transport protocols + they want to use. Deployments need to be careful in choosing what + transports to use. Many SIP features and extensions, such as large + presence notification bodies, result in SIP requests that can be too + large to be reasonably transported over UDP. [RFC3261] states that + when a request is too large for UDP, the device sending the request + attempts to switch over to TCP. It is important to note that when + using outbound, this will only work if the UA has formed both UDP and + TCP outbound flows. This specification allows the UA to do so, but + in most cases it will probably make more sense for the UA to form a + TCP outbound connection only, rather than forming both UDP and TCP + flows. One of the key reasons that many deployments choose not to + use TCP has to do with the difficulty of building proxies that can + maintain a very large number of active TCP connections. Many + deployments today use SIP in such a way that the messages are small + enough that they work over UDP but they can not take advantage of all + the functionality SIP offers. Deployments that use only UDP outbound + connections are going to fail with sufficiently large SIP messages. + +14. Requirements + + This specification was developed to meet the following requirements: + + 1. Must be able to detect that a UA supports these mechanisms. + + 2. Support UAs behind NATs. + + + +Jennings, et al. Standards Track [Page 44] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + 3. Support TLS to a UA without a stable DNS name or IP address. + + 4. Detect failure of a connection and be able to correct for this. + + 5. Support many UAs simultaneously rebooting. + + 6. Support a NAT rebooting or resetting. + + 7. Minimize initial startup load on a proxy. + + 8. Support architectures with edge proxies. + +15. Acknowledgments + + Francois Audet acted as document shepherd for this document, tracking + hundreds of comments and incorporating many grammatical fixes as well + as prodding the editors to "get on with it". Jonathan Rosenberg, + Erkki Koivusalo, and Byron Campen provided many comments and useful + text. Dave Oran came up with the idea of using the most recent + registration first in the proxy. Alan Hawrylyshen co-authored the + document that formed the initial text of this specification. + Additionally, many of the concepts here originated at a connection + reuse meeting at IETF 60 that included the authors, Jon Peterson, + Jonathan Rosenberg, Alan Hawrylyshen, and Paul Kyzivat. The TCP + design team consisting of Chris Boulton, Scott Lawrence, Rajnish + Jain, Vijay K. Gurbani, and Ganesh Jayadevan provided input and text. + Nils Ohlmeier provided many fixes and initial implementation + experience. In addition, thanks to the following folks for useful + comments: Francois Audet, Flemming Andreasen, Mike Hammer, Dan Wing, + Srivatsa Srinivasan, Dale Worely, Juha Heinanen, Eric Rescorla, + Lyndsay Campbell, Christer Holmberg, Kevin Johns, Jeroen van Bemmel, + Derek MacDonald, Dean Willis, and Robert Sparks. + +16. References + +16.1. Normative References + + [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, March 1997. + + [RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997. + + [RFC2506] Holtman, K., Mutz, A., and T. Hardie, "Media Feature + Tag Registration Procedure", BCP 31, RFC 2506, + March 1999. + + + + + + +Jennings, et al. Standards Track [Page 45] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., + Johnston, A., Peterson, J., Sparks, R., Handley, M., + and E. Schooler, "SIP: Session Initiation Protocol", + RFC 3261, June 2002. + + [RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation + Protocol (SIP): Locating SIP Servers", RFC 3263, + June 2002. + + [RFC3327] Willis, D. and B. Hoeneisen, "Session Initiation + Protocol (SIP) Extension Header Field for Registering + Non-Adjacent Contacts", RFC 3327, December 2002. + + [RFC3581] Rosenberg, J. and H. Schulzrinne, "An Extension to the + Session Initiation Protocol (SIP) for Symmetric + Response Routing", RFC 3581, August 2003. + + [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO + 10646", STD 63, RFC 3629, November 2003. + + [RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, + "Indicating User Agent Capabilities in the Session + Initiation Protocol (SIP)", RFC 3840, August 2004. + + [RFC3841] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, + "Caller Preferences for the Session Initiation + Protocol (SIP)", RFC 3841, August 2004. + + [RFC3968] Camarillo, G., "The Internet Assigned Number Authority + (IANA) Header Field Parameter Registry for the Session + Initiation Protocol (SIP)", BCP 98, RFC 3968, + December 2004. + + [RFC3969] Camarillo, G., "The Internet Assigned Number Authority + (IANA) Uniform Resource Identifier (URI) Parameter + Registry for the Session Initiation Protocol (SIP)", + BCP 99, RFC 3969, December 2004. + + [RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally + Unique IDentifier (UUID) URN Namespace", RFC 4122, + July 2005. + + [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax + Specifications: ABNF", STD 68, RFC 5234, January 2008. + + [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, + "Session Traversal Utilities for NAT (STUN)", + RFC 5389, October 2008. + + + +Jennings, et al. Standards Track [Page 46] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + +16.2. Informative References + + [CONFIG-FMWK] Petrie, D. and S. Channabasappa, Ed., "A Framework for + Session Initiation Protocol User Agent Profile + Delivery", Work in Progress, February 2008. + + [NAT-SCEN] Boulton, C., Rosenberg, J., Camarillo, G., and F. + Audet, "Best Current Practices for NAT Traversal for + Client-Server SIP", Work in Progress, September 2008. + + [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, + August 1980. + + [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, + RFC 793, September 1981. + + [RFC1035] Mockapetris, P., "Domain names - implementation and + specification", STD 13, RFC 1035, November 1987. + + [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: + Keyed-Hashing for Message Authentication", RFC 2104, + February 1997. + + [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", + RFC 2131, 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. + + [RFC3320] Price, R., Bormann, C., Christoffersson, J., Hannu, + H., Liu, Z., and J. Rosenberg, "Signaling Compression + (SigComp)", RFC 3320, January 2003. + + [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. + + [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, + "Uniform Resource Identifier (URI): Generic Syntax", + STD 66, RFC 3986, January 2005. + + [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram + Congestion Control Protocol (DCCP)", RFC 4340, + March 2006. + + + + + +Jennings, et al. Standards Track [Page 47] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + + [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data + Encodings", RFC 4648, October 2006. + + [RFC4960] Stewart, R., "Stream Control Transmission Protocol", + RFC 4960, September 2007. + + [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer + Security (TLS) Protocol Version 1.2", RFC 5246, + August 2008. + + [RFC5627] Rosenberg, J., "Obtaining and Using Globally Routable + User Agent URIs (GRUUs) in the Session Initiation + Protocol (SIP)", RFC 5627, October 2009. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Jennings, et al. Standards Track [Page 48] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + +Appendix A. Default Flow Registration Backoff Times + + The base-time used for the flow re-registration backoff times + described in Section 4.5 are configurable. If the base-time-all-fail + value is set to the default of 30 seconds and the base-time-not- + failed value is set to the default of 90 seconds, the following table + shows the resulting amount of time the UA will wait to retry + registration. + + +-------------------+--------------------+---------------------+ + | # of reg failures | all flows unusable | > 1 non-failed flow | + +-------------------+--------------------+---------------------+ + | 0 | 0 s | 0 s | + | 1 | 30-60 s | 90-180 s | + | 2 | 1-2 min | 3-6 min | + | 3 | 2-4 min | 6-12 min | + | 4 | 4-8 min | 12-24 min | + | 5 | 8-16 min | 15-30 min | + | 6 or more | 15-30 min | 15-30 min | + +-------------------+--------------------+---------------------+ + +Appendix B. ABNF + + This appendix contains the ABNF defined earlier in this document. + + + CRLF = CR LF + double-CRLF = CR LF CR LF + CR = %x0D + LF = %x0A + + Flow-Timer = "Flow-Timer" HCOLON 1*DIGIT + + contact-params =/ c-p-reg / c-p-instance + + c-p-reg = "reg-id" EQUAL 1*DIGIT ; 1 to (2^31 - 1) + + c-p-instance = "+sip.instance" EQUAL + DQUOTE "<" instance-val ">" DQUOTE + + instance-val = 1*uric ; defined in RFC 3261 + + + + + + + + + + +Jennings, et al. Standards Track [Page 49] + +RFC 5626 Client-Initiated Connections in SIP October 2009 + + +Authors' Addresses + + Cullen Jennings (editor) + Cisco Systems + 170 West Tasman Drive + Mailstop SJC-21/2 + San Jose, CA 95134 + USA + + Phone: +1 408 902-3341 + EMail: fluffy@cisco.com + + + Rohan Mahy (editor) + Unaffiliated + + EMail: rohan@ekabal.com + + + Francois Audet (editor) + Skype Labs + + EMail: francois.audet@skypelabs.com + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Jennings, et al. Standards Track [Page 50] + |