SIPPING Working Group V. Hilt Internet-Draft I. Widjaja Intended status: Standards Track Bell Labs/Alcatel-Lucent Expires: January 14, 2009 H. Schulzrinne Columbia University July 13, 2008 Session Initiation Protocol (SIP) Overload Control draft-hilt-sipping-overload-05 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on January 14, 2009. Abstract Overload occurs in Session Initiation Protocol (SIP) networks when SIP servers have insufficient resources to handle all SIP messages they receive. Even though the SIP protocol provides a limited overload control mechanism through its 503 (Service Unavailable) response code, SIP servers are still vulnerable to overload. This document defines an overload control mechanism for SIP. Hilt, et al. Expires January 14, 2009 [Page 1] Internet-Draft Overload Control July 2008 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Design Considerations . . . . . . . . . . . . . . . . . . . . 4 3.1. SIP Mechanism . . . . . . . . . . . . . . . . . . . . . . 4 3.1.1. SIP Response Header Field . . . . . . . . . . . . . . 4 3.1.2. SIP Event Package . . . . . . . . . . . . . . . . . . 5 3.2. Backwards Compatibility . . . . . . . . . . . . . . . . . 6 3.3. Load Status . . . . . . . . . . . . . . . . . . . . . . . 6 3.4. Responding to an Overload Indication . . . . . . . . . . . 7 3.5. Message Prioritization . . . . . . . . . . . . . . . . . . 7 4. Via Header Parameters for Overload Control . . . . . . . . . . 7 4.1. The 'oc_accept' Parameter . . . . . . . . . . . . . . . . 7 4.2. Creating the 'oc' Parameter . . . . . . . . . . . . . . . 8 4.3. Determining the 'oc' Parameter Value . . . . . . . . . . . 9 4.4. Processing the 'oc' Parameter . . . . . . . . . . . . . . 10 4.5. Using the 'oc' Parameter Value . . . . . . . . . . . . . . 10 4.6. Rejecting Requests . . . . . . . . . . . . . . . . . . . . 11 4.7. Self-Limiting . . . . . . . . . . . . . . . . . . . . . . 11 4.8. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5. Security Considerations . . . . . . . . . . . . . . . . . . . 13 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 14 Appendix B. 'Overload-Control' Event Package . . . . . . . . . . 14 B.1. Event Package Name . . . . . . . . . . . . . . . . . . . . 14 B.2. Event Package Parameters . . . . . . . . . . . . . . . . . 14 B.3. SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 14 B.4. Subscription Duration . . . . . . . . . . . . . . . . . . 15 B.5. NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . . 15 B.6. Subscriber generation of SUBSCRIBE requests . . . . . . . 16 B.7. Notifier processing of SUBSCRIBE requests . . . . . . . . 16 B.8. Notifier generation of NOTIFY requests . . . . . . . . . . 16 B.9. Subscriber processing of NOTIFY requests . . . . . . . . . 17 B.10. Handling of forked requests . . . . . . . . . . . . . . . 17 B.11. Rate of notifications . . . . . . . . . . . . . . . . . . 17 B.12. State Agents . . . . . . . . . . . . . . . . . . . . . . . 18 B.13. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 18 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 7.1. Normative References . . . . . . . . . . . . . . . . . . . 18 7.2. Informative References . . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19 Intellectual Property and Copyright Statements . . . . . . . . . . 21 Hilt, et al. Expires January 14, 2009 [Page 2] Internet-Draft Overload Control July 2008 1. Introduction As with any network element, a Session Initiation Protocol (SIP) [RFC3261] server can suffer from overload when the number of SIP messages it receives exceeds the number of messages it can process. Overload can pose a serious problem for a network of SIP servers. During periods of overload, the throughput of a network of SIP servers can be significantly degraded. In fact, overload may lead to a situation in which the throughput drops down to a small fraction of the original processing capacity. This is often called congestion collapse. Overload is said to occur if a SIP server does not have sufficient resources to process all incoming SIP messages. These resources may include CPU processing capacity, memory, network bandwidth, input/ output, or disk resources. For overload control, we only consider failure cases where SIP servers are unable to process all SIP requests due to resource constraints. There are other cases where a SIP server can successfully process incoming requests but has to reject them due to other failure conditions. For example, a PSTN gateway that runs out of trunk lines but still has plenty of capacity to process SIP messages should reject incoming INVITEs using a 488 (Not Acceptable Here) response [RFC4412]. Similarly, a SIP registrar that has lost connectivity to its registration database but is still capable of processing SIP messages should reject REGISTER requests with a 500 (Server Error) response [RFC3261]. Overload control does not apply to these cases and SIP provides appropriate response codes for them. The SIP protocol provides a limited mechanism for overload control through its 503 (Service Unavailable) response code. However, this mechanism cannot prevent overload of a SIP server and it cannot prevent congestion collapse. In fact, the use of the 503 (Service Unavailable) response code may cause traffic to oscillate and to shift between SIP servers and thereby worsen an overload condition. A detailed discussion of the SIP overload problem, the problems with the 503 (Service Unavailable) response code and the requirements for a SIP overload control mechanism can be found in [I-D.rosenberg-sipping-overload-reqs]. 2. 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 RFC 2119 [RFC2119]. Hilt, et al. Expires January 14, 2009 [Page 3] Internet-Draft Overload Control July 2008 3. Design Considerations General design considerations for SIP overload control are discussed in [I-D.hilt-sipping-overload-design]. The following section discusses additional design considerations for a SIP extension for overload control. 3.1. SIP Mechanism A SIP mechanism is needed to convey overload feedback from the receiving to the sending SIP entity. A number of different alternatives exist to implement such a mechanism. 3.1.1. SIP Response Header Field Overload control information can be transmitted using a new Via header field parameter for overload control. A SIP server can add this header parameter to the responses it is sending upstream to inform its upstream neighbors about the current overload status. A detailed description of this header is provided in Section 4. This approach has the following characteristics: o A Via header parameter is light-weight and creates very little overhead. It does not require the transmission of additional messages for overload control and does not increase traffic or processing burdens in an overload situation. o Overload control status can frequently be reported to upstream neighbors since it is a part of a SIP response. This enables the use of this mechanism in scenarios where the overload status needs to be adjusted frequently. o With a Via header parameter, overload control status is inherent in SIP signaling and is automatically conveyed to all relevant upstream neighbors, i.e., neighbors that are currently contributing traffic. There is no need for a SIP server to specifically track the set of current upstream or downstream neighbors with which it should exchange overload feedback. o Overload status is not conveyed to inactive senders. This avoids the transmission of overload feedback to inactive senders, which do not contribute traffic. If an inactive sender starts to transmit while the receiver is in overload it will receive overload feedback in the first response and can adjust the amount of traffic forwarded accordingly. o A SIP server can limit the distribution of overload control information by only inserting it into responses to known upstream neighbors. A SIP server can use transport level authentication (e.g., via TLS) with its upstream neighbors. Hilt, et al. Expires January 14, 2009 [Page 4] Internet-Draft Overload Control July 2008 3.1.2. SIP Event Package Overload control information can also be conveyed from a receiver to a sender using a new event package. This event package enables a sending entity to subscribe to the overload status of its downstream neighbors and receive notifications of overload control status changes in NOTIFY requests. A description of such an event package is provided in Appendix B. This approach has the following characteristics: o Overload control information is conveyed outside of the SIP signaling flow and can be decoupled from SIP signaling. For example, a separate overload control manager can be responsible for monitoring the load on all servers in a server farm and provide overload control feedback to all SIP servers that have set up subscriptions to this controller. o With an event package, a receiver can send updates to senders that are currently inactive. Inactive senders will receive a notification about the overload and can refrain from sending traffic to this neighbor until the overload condition is resolved. The receiver can also notify all potential senders once they are permitted to send traffic again. However, these notifications do generate additional traffic, which adds to the overall load. o A SIP entity needs to set up and maintain overload control subscriptions with all upstream and downstream neighbors. A new subscription needs to be set up before/while a request is transmitted to a new downstream neighbor. Servers can be configured to subscribe at boot time. However, this would require additional protection to avoid the avalanche restart problem for overload control. Subscriptions need to be terminated when they are not needed any more, which can be done, for example, using a timeout mechanism. o A receiver needs to send NOTIFY messages to all subscribed upstream neighbors in a timely manner when the control algorithm requires a change in the control variable (e.g., when a SIP server is in an overload condition). This includes active as well as inactive neighbors. These NOTIFYs add to the amount of traffic that needs to be processed. To ensure that these requests will not be dropped due to overload, a priority mechanism needs to be implemented in all servers these request will pass through. o A SIP server can limit the set of senders that can receive overload control information by authenticating subscriptions to this event package. o This approach requires each proxy to implement a UAS/UAC to manage the subscriptions. Hilt, et al. Expires January 14, 2009 [Page 5] Internet-Draft Overload Control July 2008 OPEN ISSUE: We need to decide about one SIP mechanism for conveying overload control information. Section 4 and Appendix B provide details for the header and the event package alternative. 3.2. Backwards Compatibility An new overload control mechanism needs to be backwards compatible so that it can be gradually introduced into a network and functions properly if only a fraction of the servers support it. Hop-by-hop overload control does not require that all SIP entities in a network support it. It can be used effectively between two adjacent SIP servers if both servers support overload control and does not depend on the support from any other server or user agent. The more SIP servers in a network support hop-by-hop overload control, the better protected the network is against occurrences of overload. A SIP server may have multiple upstream neighbors from which only some may support overload control. If a server would simply use this overload control mechanism, only those that support it would reduce traffic. Others would keep sending at the full rate and benefit from the throttling by the servers that support overload control. In other words, upstream neighbors that do not support overload control would be better off than those that do. A SIP server should therefore use 5xx responses towards upstream neighbors that do not support overload control. The server should reject the same amount of requests with 5xx responses that would be otherwise be rejected/redirected by the upstream neighbor if it would support overload control. If the load condition on the server does not permit the creation of 5xx responses, the server should drop all requests from servers that do not support overload control. 3.3. Load Status It may be useful for a SIP server to frequently report its current load status to upstream neighbors. The load status indicates to which degree the resources needed by a SIP server to process SIP messages are utilized. An upstream neighbor can use load status to balance load between alternative SIP servers and to find under- utilized servers. Reporting load is not intended to replace specialized load balancing mechanisms. OPEN ISSUE: reporting load status seems useful but somewhat orthogonal to overload control. Should this be a separate mechanism? Hilt, et al. Expires January 14, 2009 [Page 6] Internet-Draft Overload Control July 2008 3.4. Responding to an Overload Indication An element may receive overload control feedback indicating that it needs to reduce the traffic it sends to its downstream neighbor. An element can accomplish this task by sending some of the requests that would have gone to the overloaded element to a different destination. It needs to ensure, however, that this destination is not in overload and capable of processing the extra load. An element can also buffer requests in the hope that the overload condition will resolve quickly and the requests still can be forwarded in time. Finally, it can reject these requests. 3.5. Message Prioritization Overload control can require a SIP server to prioritize messages and select messages that need to be rejected or redirected. The selection is largely a matter of local policy. A SIP server SHOULD honor the Resource-Priority header field as defined in RFC4412 [RFC4412] if it is present in a SIP request. The Resource-Priority header field enables a proxy to identify high- priority requests, such as emergency service requests, and preserve them as much as possible during times of overload. 4. Via Header Parameters for Overload Control This section defines new parameters for the SIP Via header for overload control. These parameter provide a SIP mechanism for conveying overload control information between SIP entities. 4.1. The 'oc_accept' Parameter A SIP server that supports this specification MUST add an "oc_accept" parameter to the Via headers it inserts into SIP requests. This provides an indication to downstream neighbors that this server supports overload control. OPEN ISSUE: To throttle upstream neighbors in a fair way, it is important that a SIP server can estimate the load each upstream neighbor receives for this server before it is throttled. This enables the server to throttle each upstream neighbor in the same way and thus provides each request the same chance of succeeding. In rate- and window-based overload control systems, a SIP server does not know how many messages each upstream neighbor had received for the server before throttling took place. A solution to this problem is to allow servers to report the load received for a downstream neighbor in the 'oc_accept' parameter. Hilt, et al. Expires January 14, 2009 [Page 7] Internet-Draft Overload Control July 2008 4.2. Creating the 'oc' Parameter A SIP server can provide overload control feedback to its upstream neighbors by adding the 'oc' parameter to the topmost Via header field of a SIP response. The 'oc' parameter is a new Via header parameter defined in this specification. When an 'oc' parameter is added to a response, it MUST be inserted into the topmost Via header. It MUST NOT be added to any other Via header in the response. The topmost Via header is determined after the SIP server has removed its own Via header. It is the Via header that was generated by the next upstream neighbor. Since the topmost Via header of a response will be removed by an upstream neighbor after processing it, overload control feedback contained in the 'oc' parameter will not travel beyond the next SIP server. A Via header parameter therefore provides hop-by-hop semantics for overload control feedback even if the next hop neighbor does not support this specification. A SIP server SHOULD add an 'oc' parameter to those responses, that contain an 'oc_accept' parameter in the topmost Via header. In this case, the SIP server MUST remove the 'oc_accept' parameter from the Via header and replace it with an 'oc' parameter. The 'oc' parameter can be used in all response types, including provisional, success and failure responses. A SIP server MAY generally add the 'oc' parameter to all responses it is sending. A SIP server MUST add an 'oc' parameter to responses when the transmission of overload control feedback is required by the overload control algorithm to limit the traffic received by the server. I.e., a SIP server MUST insert the 'oc' parameter when the overload control algorithm sets the 'oc' parameter to a value different from the default value. A SIP server that has added an 'oc' parameter to Via header SHOULD also add a 'oc_validity' parameter to the same Via header. The 'oc_validity' parameter defines the time in milliseconds during which the content (i.e., the overload control feedback) of the 'oc' parameter is valid. The default value of the 'oc_validity' parameter is 500. A SIP server SHOULD use a shorter 'oc_validity' time if its overload status varies quickly and MAY use a longer 'oc_validity' time if this status is more stable. If the 'oc_validity' parameter is not present, its default value is used. The 'oc_validity' parameter MUST NOT be used in a Via header without an 'oc' parameter and MUST be ignored if it appears in a Via header without 'oc' parameter. A SIP server MAY forward the content of an 'oc' parameter it has Hilt, et al. Expires January 14, 2009 [Page 8] Internet-Draft Overload Control July 2008 received from a downstream neighbor on to its upstream neighbor. However, forwarding the content of the 'oc' parameter is generally NOT RECOMMENDED and should only be performed if permitted by the configuration of SIP servers. For example, a SIP server that only relays messages between exactly two SIP servers could forward an 'oc' parameter. The 'oc' parameter is forwarded by copying it from the Via in which it was received into the next Via header (i.e., the Via header that will be on top after processing the response). If an 'oc_validity' parameter is present, MUST be copied along with the 'oc' parameter. The 'oc' and 'oc_validity' Via header parameters are only defined in SIP responses and MUST NOT be used in SIP requests. These parameters are only useful to the upstream neighbor of a SIP server (i.e., the entity that is sending requests to the SIP server) since this is the entity that can offload traffic by redirecting/rejecting new requests. If requests are forwarded in both directions between two SIP servers (i.e., the roles of upstream/downstream neighbors change), there are also responses flowing in both directions. Thus, both two SIP servers can exchange overload information. While adding 'oc' and 'oc_validity' parameters to requests may increase the frequency with which overload information is exchanged in these scenarios, this increase will rarely provide benefits and does not justify the added overhead and complexity needed. A SIP server MAY decide to add 'oc' and 'oc_validity' parameters only to responses that are sent via a secured transport channel such as TLS. The SIP server can use transport level authentication to identify the SIP servers, to which responses with these parameters are sent. This enables a SIP server to protect overload control information and ensure that it is only visible to trusted parties. Since overload control protects a SIP server from overload, it is RECOMMENDED that a SIP server generally inserts 'oc' and 'oc_validity' parameters into responses to all SIP servers. 4.3. Determining the 'oc' Parameter Value The value of the 'oc' parameter is determined by an overload control algorithm (see [I-D.hilt-sipping-overload-design]). This specification does not mandate the use of a specific overload control algorithm. However, the output of an overload control algorithm MUST be compliant to the semantics of this Via header parameter. OPEN ISSUE: the semantics of the 'oc' parameter depends on the overload control method used. It may contain a loss rate for loss-based overload control, a target rate for rate-based overload control or message confirmations and window-size for window-based overload control. It might be possible to allow multiple Hilt, et al. Expires January 14, 2009 [Page 9] Internet-Draft Overload Control July 2008 mechanisms to co-exist (e.g., by defining different parameters for the different feedback types). However, for interoperability purposes it seems preferable to agree on one mechanism. Further studies are needed to evaluate the different methods. 4.4. Processing the 'oc' Parameter A SIP entity compliant to this specification SHOULD remove 'oc' and 'oc_validity' parameters in all Via headers of a response received, except for the topmost Via header. This prevents 'oc'/'oc_validity' parameters that were accidentally or maliciously inserted into Via headers by a downstream SIP server from traveling upstream. A SIP server maintains the 'oc' parameter values received along with the address of the SIP servers from which they were received for the duration specified in the 'oc_validity' parameter or the default duration. Each time a SIP server receives a response with an 'oc' parameter from a SIP server, it overwrites the 'oc' value it has currently stored for this server with the new value received. The SIP server restarts the validity period of an 'oc' parameter each time a response with an 'oc' parameter is received from this server. A stored 'oc' parameter value MUST be discarded once it has reached the end of its validity. 4.5. Using the 'oc' Parameter Value A SIP server compliant to this specification MUST honor 'oc' parameter values it receives from downstream neighbors. The SIP server MUST NOT forward more messages to a SIP server than allowed by the current 'oc' parameter value from this server. When forwarding a SIP request, a SIP entity uses the SIP procedures to determine the next hop SIP server as, e.g., described in [RFC3261] and [RFC3263]. After selecting the next hop server, the SIP server MUST determine if it has an 'oc' parameter value for this server. If it has a non-expired 'oc' parameter value, the SIP server MUST determine if it can or cannot forward the current request within the current conditions. OPEN ISSUE: the specific mechanisms to throttle traffic depend on the type of feedback conveyed in the 'oc' parameter value. It needs to be defined depending on whether a loss-based, rate-based or window-based feedback is used. The treatment of SIP requests that cannot be forwarded to the selected SIP Server is a matter of local policy. A SIP entity MAY try to find an alternative target or it MAY reject the request (see Section 4.6). Hilt, et al. Expires January 14, 2009 [Page 10] Internet-Draft Overload Control July 2008 4.6. Rejecting Requests A SIP server that rejects a request because of overload MUST reject this request with the 5xx response code defined for overload control (e.g., 503 (Service Unavailable) or 507 (Server Overload) [I-D.hilt-sip-correction-503]). This response code indicates that the request did not succeed because the SIP servers processing the request are under overload. A SIP server that is under overload and has started to throttle incoming traffic SHOULD use 5xx response to reject a fraction of requests from upstream neighbors that do not include the 'oc_accept' parameter in their Via headers. These neighbors do not support this specification and will not respond to overload control feedback in the 'oc' parameter. The fraction of requests rejected SHOULD be equivalent to the fraction of requests the upstream server would reject/redirect if it did support this specification. This is to ensure that SIP servers, which do not support this specification, don't receive an unfair advantage over those that do. A SIP server that has reached overload (i.e., a load close to 100) SHOULD start using 5xx responses in addition to using the 'oc' parameter for all upstream neighbors. If the proxy has reached a load close to 100, it needs to protect itself against overload. Also, it is likely that upstream proxies have ignored overload feedback and do not support this specification. 4.7. Self-Limiting In some cases, a SIP server may not receive a response from a downstream neighbor when sending a request. RFC3261 [RFC3261] defines that when a timeout error is received from the transaction layer, it MUST be treated as if a 408 (Request Timeout) status code has been received. If a fatal transport error is reported by the transport layer, it MUST be treated as a 503 (Service Unavailable) status code. In these cases, a SIP server SHOULD stop sending requests to this server. The SIP server SHOULD occasionally forward a single request to probe if the downstream neighbor is alive. Once a SIP server has successfully transmitted a request to the downstream neighbor, it can resume normal transmission of requests. It should, of course, honor an 'oc' parameters it may receive. This avoids that a SIP server, which is unable to respond to incoming requests, is overloaded with additional requests. Hilt, et al. Expires January 14, 2009 [Page 11] Internet-Draft Overload Control July 2008 OPEN ISSUE: waiting for a timeout to occur seems a long time before starting to throttle back. It could make sense to throttle back earlier if no response is received for requests transmitted. 4.8. Syntax This section defines the syntax of three new Via header parameters: 'oc', 'oc_validity' and 'oc_accept'. These Via header parameters are used to implement an overload control feedback loop between neighboring SIP servers. The 'oc' and 'oc_validity' parameters are only defined in the topmost Via header of a response. They MUST NOT be used in the Via headers of requests and MUST NOT be used in other Via headers of a response. The 'oc' and 'oc_validity' parameters MUST be ignored if received outside of the topmost Via header of a response. The 'oc_accept' parameter MAY appear in all Via headers. OPEN ISSUE: the syntax of the 'oc' Via header parameter depends on the overload control method (i.e., loss-based, rate-based or window-based) in use. The following syntax definition defines a rate-based 'oc' header. This syntax needs to be adjusted if rate- based or window-based overload control is used. The 'oc_validity' Via header parameter contains the time during which the corresponding 'oc' Via header parameter is valid. The 'oc_validity' parameter can only be present in a Via header in conjunction with an 'oc' parameter. The 'oc_accept' Via header parameter indicates that the SIP server, which has created this Via header, supports overload control. oc-throttle = "oc" [EQUAL 0-100] oc-validity = "oc_validity" [EQUAL delta-ms] oc-accept = "oc_accept" This extends the existing definition of the Via header field parameters, so that its BNF now looks like: via-params = via-ttl / via-maddr / via-received / via-branch / oc-throttle / oc-validity / oc-accept / via-extension Example: Hilt, et al. Expires January 14, 2009 [Page 12] Internet-Draft Overload Control July 2008 Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1 ;received=192.0.2.111 ;oc=20;oc_validity=500 5. Security Considerations Overload control mechanisms can be used by an attacker to conduct a denial-of-service attack on a SIP entity if the attacker can pretend that the SIP entity is overloaded. When such a forged overload indication is received by an upstream SIP entity, it will stop sending traffic to the victim. Thus, the victim is subject to a denial-of-service attack. An attacker can create forged overload feedback by inserting itself into the communication between the victim and its upstream neighbors. The attacker would need to add overload feedback indicating a high load to the responses passed from the victim to its upstream neighbor. Proxies can prevent this attack by communicating via TLS. Since overload feedback has no meaning beyond the next hop, there is no need to secure the communication over multiple hops. Another way to conduct an attack is to send a message containing a high overload feedback value through a proxy that does not support this extension. If this feedback is added to the second Via headers (or all Via headers), it will reach the next upstream proxy. If the attacker can make the recipient believe that the overload status was created by its direct downstream neighbor (and not by the attacker further downstream) the recipient stops sending traffic to the victim. A precondition for this attack is that the victim proxy does not support this extension since it would not pass through overload control feedback otherwise. A malicious SIP entity could gain an advantage by pretending to support this specification but never reducing the amount of traffic it forwards to the downstream neighbor. If its downstream neighbor receives traffic from multiple sources which correctly implement overload control, the malicious SIP entity would benefit since all other sources to its downstream neighbor would reduce load. OPEN ISSUE: the solution to this problem depends on the overload control method. For rate-based and window-based overload control, it is very easy for a downstream entity to monitor if the upstream neighbor throttles traffic forwarded as directed. For percentage throttling this is not always obvious since the load forwarded depends on the load received by the upstream neighbor. Hilt, et al. Expires January 14, 2009 [Page 13] Internet-Draft Overload Control July 2008 6. IANA Considerations [TBD.] Appendix A. Acknowledgements Many thanks to Rich Terpstra, Daryl Malas, Jonathan Rosenberg and Charles Shen for their contributions to this specification. Appendix B. 'Overload-Control' Event Package This section defines a new SIP event package for overload control. This event package provides a SIP mechanism for conveying overload control information between SIP entities. The following sections provide the details for defining a SIP event package as required by RFC 3265 [RFC3265]. B.1. Event Package Name The name of this event package is "overload-control". This package name is carried in the Event and Allow-Events header fields, as defined in RFC 3265 [RFC3265]. B.2. Event Package Parameters No package specific Event header field parameters are defined for this event package. B.3. SUBSCRIBE Bodies A SUBSCRIBE request for overload control information MAY contain a body. This body would serve the purpose of filtering the overload control subscription. The definition of such a body is outside the scope of this specification. For example, the body might provide a threshold for reporting overload control information or it might indicate that overload control information should be reported as a loss-percentage or a request rate. A SUBSCRIBE request for the overload control package MAY be sent without a body. This implies that the default subscription filtering policy as described in Appendix B.8 has been requested. Hilt, et al. Expires January 14, 2009 [Page 14] Internet-Draft Overload Control July 2008 B.4. Subscription Duration A subscription to the overload control event package is usually established when a SIP server first sends a request to another SIP server and terminated when this server stops sending requests and overload control is not needed any more. The duration of a subscription is related to the time a signaling relationship exists between two servers. In a static SIP server configuration (e.g., two SIP servers are configured to exchange messages in a service provider's network) this relationship can last for days or weeks as long as both servers are running. In this scenario, the subscription duration is largely irrelevant. In a dynamic configuration (e.g., two SIP servers in different domains) the duration of the signaling relationship can be in the range of minutes or hours and might only last for the duration of a single session. Since it is unknown a priori when the next SIP request will be transmitted from the subscriber to the notifier, subscriber and notifier MAY terminate a subscription to overload control after a period of inactivity. The duration of a subscription to the overload control event package SHOULD be longer than the duration of a typical session. The default subscription duration for this event package is set to two hours. B.5. NOTIFY Bodies In this event package, the body of a notification contains the current overload status of the notifier. All subscribers and notifiers MUST support the format application/ overload-info+xml. The SUBSCRIBE request MAY contain an Accept header field. If no such header field is present, it has a default value of application/overload-info+xml. If the header field is present, it MUST include application/overload-info+xml, and MAY include any other MIME type capable of representing overload status information. As defined in RFC 3265 [RFC3265], the body of notifications MUST be in one of the formats defined in the Accept header of the SUBSCRIBE request or in the default format. TBD: An document format for the above placeholder application/ overload-info+xml needs to be defined. The following document snippet is an example for such a format: 200 Hilt, et al. Expires January 14, 2009 [Page 15] Internet-Draft Overload Control July 2008 B.6. Subscriber generation of SUBSCRIBE requests The subscriber follows the general rules for generating SUBSCRIBE requests defined in RFC 3265 [RFC3265]. B.7. Notifier processing of SUBSCRIBE requests It is RECOMMENDED that a notifier provides overload control status information to all subscribers and that the notifier accepts all subscriptions to this event package. By denying a subscription to overload control, a notifier would disable overload control to this subscriber. Since this subscriber would not know the current overload status of the notifier, it would not reduce the traffic forwarded when the notifier enters an overload condition. Thus, denying a subscription to this event package can leave the notifier vulnerable to SIP overload. A notifier MAY authenticate and authorize subscriptions to this event package. This is useful if the notifier wants to provide extended overload status information to certain subscribers. For example, a notifier can provide detailed resource usage information to authenticated subscribers and only provide the current throttle status to all other subscribers. The details of the authorization policy are at the discretion of the administrator. B.8. Notifier generation of NOTIFY requests A notifier sends a notification in response to SUBSCRIBE requests as defined in RFC 3265 [RFC3265]. In addition, a notifier MAY send a notification at any time during the subscription. Typically, the notifier will send a notification every time the overload control status has changed. For example, the notifier can create a notify every time the overload control value (e.g., the rate limit) changes. Overload status information is expressed in the format negotiated for the NOTIFY body (e.g., "application/overload-info+xml"). The overload status in a NOTIFY body MUST be complete. Notifications that contain the deltas to previous overload status or a partial overload status are not supported in this event package. It is RECOMMENDED that the notifier returns an initial NOTIFY that contains at least the current overload control value immediately after receiving a SUBSCRIBE request. It is RECOMMENDED that the notifier returns such an initial NOTIFY even if the notifier is still waiting for an authorization decision. Once the subscription is authorized, the notifier MAY send another notification that then contains all information the subscriber is authorized to receive. It is RECOMMENDED that the notifier accepts a subscription and creates a Hilt, et al. Expires January 14, 2009 [Page 16] Internet-Draft Overload Control July 2008 NOTIFY with at least the current overload control value even if the subscriber is not authorized to receive more information. The timely delivery of overload control notifications is important for overload control. It is therefore RECOMMENDED that NOTIFY messages for this event package are sent with highest priority. I.e., the transmission of NOTIFY messages for this event package ought not to be delayed by other tasks. B.9. Subscriber processing of NOTIFY requests A subscriber MUST use the overload control state contained in a NOTIFY body and apply this state to all subsequent SIP messages it is intending to send to the respective SIP server. The subscriber MUST NOT forward a higher number of SIP messages to the server than allowed by the current overload control state. A subscriber MUST use the overload state it has received for a SIP server until the subscriber receives another NOTIFY with an updated state or until the subscription is terminated. The subscriber SHOULD stop using the reported overload state once the subscription is terminated. It is RECOMMENDED that the subscriber processes incoming NOTIFY messages for this event package with highest priority. I.e., NOTIFY messages for this event package ought to be processed before other messages are processed. This is to ensure that a subscriber can react quickly to changes in the overload control status even if the subscriber is currently receiving a high volume of messages. B.10. Handling of forked requests This event package allows the creation of only one dialog as a result of an initial SUBSCRIBE request. The techniques to achieve this behavior are described in [RFC3265]. B.11. Rate of notifications Keeping the rate of notifications low is important for an overload control mechanism to avoid creating additional traffic in an overload condition. However, it is also important that an overload control algorithm can quickly adjust the overload control value as needed. Ideally, the overload control algorithm would generate a stable control value that rarely needs to be adjusted. The notifier SHOULD NOT generate NOTIFY messages at a rate faster once every 1 second for notifications that are triggered by a change in the control value. The notifier SHOULD NOT generate a NOTIFY Hilt, et al. Expires January 14, 2009 [Page 17] Internet-Draft Overload Control July 2008 message at a rate faster than once every 5 seconds for all other notifications (i.e., for any additional information included in the subscription). B.12. State Agents State agents play no role in this package. B.13. Examples The following message flow illustrates how proxy A can subscribe to overload control status of proxy B. The flow assumes that proxy A does not have an active subscription to the overload control status of proxy B and has received an INVITE request it needs to forward to B. Proxy A Proxy B | | |(1) SUBSCRIBE | |------------------>| |(2) 200 OK | |<------------------| |(3) NOTIFY | |<------------------| |(4) 200 OK | |------------------>| |(5) INVITE | |------------------>| |(6) 200 OK | |<------------------| |(7) ACK | |------------------>| | | Message Details TBD. 7. References 7.1. Normative References [I-D.hilt-sip-correction-503] Hilt, V. and I. Widjaja, "Essential Correction to the Session Initiation Protocol (SIP) 503 (Service Hilt, et al. Expires January 14, 2009 [Page 18] Internet-Draft Overload Control July 2008 Unavailable) Response", draft-hilt-sip-correction-503-01 (work in progress). [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [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. [RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific Event Notification", RFC 3265, June 2002. [RFC4412] Schulzrinne, H. and J. Polk, "Communications Resource Priority for the Session Initiation Protocol (SIP)", RFC 4412, February 2006. 7.2. Informative References [I-D.hilt-sipping-overload-design] Hilt (Ed.), V., "Design Considerations for Session Initiation Protocol (SIP) Overload Control", draft-hilt-sipping-overload-design-00 (work in progress). [I-D.rosenberg-sipping-overload-reqs] Rosenberg, J., "Requirements for Management of Overload in the Session Initiation Protocol", draft-rosenberg-sipping-overload-reqs-02 (work in progress), October 2006. Authors' Addresses Volker Hilt Bell Labs/Alcatel-Lucent 791 Holmdel-Keyport Rd Holmdel, NJ 07733 USA Email: volkerh@bell-labs.com Hilt, et al. Expires January 14, 2009 [Page 19] Internet-Draft Overload Control July 2008 Indra Widjaja Bell Labs/Alcatel-Lucent 600-700 Mountain Avenue Murray Hill, NJ 07974 USA Email: iwidjaja@alcatel-lucent.com Henning Schulzrinne Columbia University/Department of Computer Science 450 Computer Science Building New York, NY 10027 USA Phone: +1 212 939 7004 Email: hgs@cs.columbia.edu URI: http://www.cs.columbia.edu Hilt, et al. Expires January 14, 2009 [Page 20] Internet-Draft Overload Control July 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Hilt, et al. Expires January 14, 2009 [Page 21]