SIPPING J. Rosenberg Internet-Draft Cisco Systems Expires: August 29, 2006 February 25, 2006 Requirements for Management of Overload in the Session Initiation Protocol draft-rosenberg-sipping-overload-reqs-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 29, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract Overload occurs in Session Initiation Protocol (SIP) networks when proxies and user agencies have insuffient resources to complete the processing of a request. SIP provides limited support for overload handling through its 503 response code, which tells an upstream element that it is overloaded. However, numerous problems have been identified with this mechanism. This draft summarizes the problems with the existing 503 mechanism, and provides some requirements for a solution. Rosenberg Expires August 29, 2006 [Page 1]
Internet-Draft Overload Requirements February 2006 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Causes of Overload . . . . . . . . . . . . . . . . . . . . . . 3 3. Current SIP Mechanisms . . . . . . . . . . . . . . . . . . . . 5 4. Problems with the Mechanism . . . . . . . . . . . . . . . . . 5 4.1 Load Amplification . . . . . . . . . . . . . . . . . . . . 5 4.2 The Off/On Retry-After Problem . . . . . . . . . . . . . . 9 4.3 Ambiguous Usages . . . . . . . . . . . . . . . . . . . . . 9 5. Solution Requirements . . . . . . . . . . . . . . . . . . . . 10 6. Security Considerations . . . . . . . . . . . . . . . . . . . 12 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 9. Informative References . . . . . . . . . . . . . . . . . . . . 12 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12 Intellectual Property and Copyright Statements . . . . . . . . 13 Rosenberg Expires August 29, 2006 [Page 2]
Internet-Draft Overload Requirements February 2006 1. Introduction Overload occurs in Session Initiation Protocol (SIP) [1] networks when proxies and user agencies have insuffient resources to complete the processing of a request or a response. SIP provides limited support for overload handling through its 503 response code, which tells an upstream element that it is overloaded. However, numerous problems have been identified with this mechanism. This draft describes the general problem of SIP overload, and then reviews the current SIP mechanisms for dealing with overload. It then explains some of the problems with these mechanisms. Finally, the document provides a set of requirements for fixing these problems. 2. Causes of Overload Overload occurs when an element, such as a SIP user agent or proxy, has insufficient resources to keep up with the volume of traffic it is receiving. Resources include all of the capabilities of the element used to process a request, including CPU processing, memory, I/O, or disk resources. It can also include external resources, such as a database or DNS server. Overload can occur for many reasons, including: Poor Capacity Planning: SIP networks need to be designed with sufficient numbers of servers, hardware, disks, and so on, in order to meet the needs of the subscribers they are expected to serve. Capacity planning is the process of determining these needs. It is based on the number of expected subscribers and the types of flows they are expected to use. If this work is not done properly, the network may have insufficient capacity to handle predictable usages, including regular usages and predictably high ones (such as high voice calling volumes on Mothers Day). Dependency Failures: A SIP element can become overloaded because a resource on which it is dependent has failed, greatly reducing its actual capacity. As such, even minimal traffic might cause the server to go into overload. Examples of such dependency failures include DNS servers, databases, disks and network interfaces. Component Failures: A SIP element can become overloaded when it is a member of a cluster of servers which each share the load of traffic, and one or more of the other memebers in the cluster fail. In this case, the remaining elements take over the work of the failed elements. Normally, capacity planning takes such failures into account, and servers are typically run with enough spare capacity to handle failure of another element. However, Rosenberg Expires August 29, 2006 [Page 3]
Internet-Draft Overload Requirements February 2006 unusual failure conditions can cause many elements to fail at once. This is often the case with software failures, where a bad packet or bad database entry hits the same bug in a set of elements in a cluster. Avalanche Restart: One of the most troubling sources of overload is avalanche restart. This happens when a large number of clients all simultaneously attempt to connect to the network with a SIP registration. Avalanche restart can be caused by several events. One is the "Manhattan Reboots" scenario, where there is a power failure in a large metropolitan area, such as Manhattan. When power is restored, all of the SIP phones, whether in PCs or standalone devices, simultaneously power on and begin booting. They will all then connect to the network and register, causing a flood of SIP REGISTER messages. Another cause of avalanche restart is failure of a large network connection, for example, the access router for an enterprise. When it fails, SIP clients will detect the failure rapidly using the mechanisms in [3]. When connectivity is restored, this is detected, and clients re- REGISTER, all within a short time period. Another source of avalanche restart is failure of a proxy server. If clients had all connected to the server with TCP, its failure will be detected, followed by re-connection and re-registratoin to another server. Note that [3] does provide some remedies to this case. Flash Crowds: A flash crowd occurs when an extremely large number of users all attempt to simultaneously make a call. One example of how this can happen is a television commercial that advertises a number to call to receive a free gift. If the gift is compelling and many people see the ad, many calls can be simultaneously made to the same number. This can send the system into overload. Unfortunately, the overload problem tends to compound itself. When a network goes into overload, this can frequently cause failures of the elements that are trying to process the traffic. This causes even more load on the remaining elements. Furthermore, during load, the overall capacity of functional elements goes down, since much of their resources are spent just rejecting or treating load that they cannot actually process. In addition, overload tends to cause SIP messages to delayed or be lost, which causes retransmissions to be sent, further increasing the amount of work in the network. This compounding factor can produce substantial multipliers on the load in the system. Indeed, with as many as 7 retransmits of an INVITE request prior to timeout, overload can multiply the already-heavy message volume by as much as seven! Rosenberg Expires August 29, 2006 [Page 4]
Internet-Draft Overload Requirements February 2006 3. Current SIP Mechanisms SIP provides very basic support for overload. It defines the 503 response code, which is sent by an element that is overloaded. RFC 3261 defines it thusly: The server is temporarily unable to process the request due to a temporary overloading or maintenance of the server. The server MAY indicate when the client should retry the request in a Retry-After header field. If no Retry-After is given, the client MUST act as if it had received a 500 (Server Internal Error) response. A client (proxy or UAC) receiving a 503 (Service Unavailable) SHOULD attempt to forward the request to an alternate server. It SHOULD NOT forward any other requests to that server for the duration specified in the Retry-After header field, if present. Servers MAY refuse the connection or drop the request instead of responding with 503 (Service Unavailable). Figure 1 The objective is to provide a mechanism to move the work of the overloaded server to another server, so that the request can be processed. The Retry-After header field, when present, is meant to allow a server to tell an upstream element to back off for a period of time, so that the overloaded server can work through its backlog of work. RFC3261 also instructs proxies to not forward 503 responses upstream, at SHOULD NOT strength. This is to avoid the upstream server of mistakingly concluding that the proxy is overloaded, when in fact the problem was an element further downstream. 4. Problems with the Mechanism At the surface, the 503 mechanism seems workable. Unfortunately, this mechanism has had numerous problems in actual deployment. These problems are described here. 4.1 Load Amplification The principal problem with the 503 mechanism is that it tends to substantially amplify the load in the network when the network is overloaded, causing further escalation of the problem and introducing the very real possibility of congestive collapse. Consider the following topology: Rosenberg Expires August 29, 2006 [Page 5]
Internet-Draft Overload Requirements February 2006 +------+ > | | / | S1 | / | | / +------+ / / / / +------+ / +------+ --------> | |/ | | | P1 |---------> | S2 | --------> | |\ | | +------+ \ +------+ \ \ \ \ \ \ +------+ \ | | > | S3 | | | +------+ Figure 2 Proxy P1 receives SIP requests from many sources, and acts solely as a load balancer, proxying the requests to servers S1, S2 and S3 for processing. The input load increases to the point where all three servers become overloaded. Server S1, when it receives its next request, generates a 503. However, because the server is loaded, it might take some time to generate the 503, causing request retransmissions which further increase the work on S1. When the 503 is received by P1, it retries the request on S2. S2 is also overloaded, and eventually generates a 503, but in the interim is also hit with many retransmits. P1 once again tries another server, this time S3, which also eventually rejects it with a, but only after many retransmits of the request. Thus, the processing of this request, which ultimately failed, involved four SIP transactions, each of which involved many retransmissions - up to 7. Thus, under unloaded conditions, a single request from a client would generate one request (to S1, S2 or S3) and two responses. How, a single request from the client, before timing out, could generate as many as 18 requests and as many responses! Each server had to expend resources to process these message. Thus, more messages and more work were sent into the Rosenberg Expires August 29, 2006 [Page 6]
Internet-Draft Overload Requirements February 2006 network at the point at which the elements became overloaded. The 503 mechanism works well when a single element is overloaded. But, when the problem is overall network load, the 503 mechanism actually generates more messages and more work for all servers, ultimately resulting in the rejection of the request anyway. The problem becomes amplified further if one considers proxies upstream from P1: Rosenberg Expires August 29, 2006 [Page 7]
Internet-Draft Overload Requirements February 2006 +------+ > | | < / | S1 | \\ / | | \\ / +------+ \\ / \ / \\ / \\ / \ +------+ / +------+ +------+ | | / | | | | | P1 | ---------> | S2 |<----------| P2 | | | \ | | | | +------+ \ +------+ +------+ ^ \ / ^ \ \ // / \ \ // / \ \ // / \ \ / / \ \ +------+ // / \ \ | | // / \ > | S3 | < / \ | | / \ +------+ / \ / \ / \ / \ / \ / \ / \ / \ / +------+ | | | PA | | | +------+ ^ ^ | | | | Figure 3 Here, proxy PA receives requests, and sends these to proxies P1 or P2. P1 and P2 both load balance across S1 through S3. Assuming again S1 through S3 are all overloaded, a request arrives at PA, which tries P1 first. P1 tries S1, S2 and then S3, and each transaction resulting in many request retransmits. Since P1 is Rosenberg Expires August 29, 2006 [Page 8]
Internet-Draft Overload Requirements February 2006 unable to eventually process the request, it rejects it. However, since all of its downstream dependencies are busy, it decides to send a 503. This propagates to PA, which tries P2, which tries S1 through S3 again, resulting in a 503 once more. Thus, in this case, we have doubled the number of SIP transactions and overall work in the network compared to the previous case. 4.2 The Off/On Retry-After Problem The Retry-After mechanism allows a server to tell an upstream element to stop sending traffic for a period of time. The work that would have otherwise been sent to that server is instead sent to another server. The mechanism is an all-or-nothing technique. A server can turn of all traffic towards it, or none of it. There is nothing in between. This tends to cause highly oscillatory behavior under even mild overload. Consider a proxy P1 which is balancing requests between two servers S1 and S2. The input load just reaches the point where both S1 and S2 are at 100% capacity. A request arrives at P1, and is sent to S1. S1 rejects this request with a 503 , and decides to use Retry-After to clear its backlog. P1 stops sending all traffic to S1. Now, S2 gets traffic, but it is seriously overloaded - at 200% capacity! It decides to reject a request with a 503 and a Retry-After, which now forces P1 to reject all traffic until S1's Retry-After timer expires. At that point, all load is shunted back to S1, which reaches overload, and the cycle repeats. Its important to observe that this problem is only observed for servers where there are a small number of upstream elements sending it traffic, as is the case in these examples. If a proxy was accessed by a large number of clients, each of which sends a small amount of traffic, the 503 mechanism with Retry-After is quite effective when utilized with a subset of the clients. This is because spreading the 503 out amongst the clients has the effect of providing the proxy more fine-grained controls on the amount of work it receives. 4.3 Ambiguous Usages Unfortunately, the specific instances under which a server is to send a 503 are ambiguous. The result is that implementations generate 503 for many reasons, only some of which are related to actual overload. For example, RFC 3398 [2], which specifies interworking from SIP to ISUP, defines the usage of 503 when the gateway receives certain ISUP cause codes from downstream switches. In these cases, the gateway has ample capacity; its just that this specific request could not be processed because of a downstream problem. This causes two problems. Firstly, during periods of overload, it Rosenberg Expires August 29, 2006 [Page 9]
Internet-Draft Overload Requirements February 2006 exacerbates the problems above because it causes additional 503 to be fed into the system, causing further work to be generated in conditions of overload. The other problem is that it becomes hard for an upstream element to know whether to retry when a 503 is received. There are classes of failures where trying on another server won't help, since the reason for the failure was that a common downstream resource is unavailable. For example, if servers S1 and S2 share a database, and the database fails. A request sent to S1 will result in a 503, but retrying on S2 won't help since the same database is unavailable. 5. Solution Requirements In this section, we propose requirements for an overload control mechanism for SIP which addresses these problems. REQ 1: The overload mechanism shall strive to maintain the throughput of a SIP at reasonable levels even when the incoming load on the network is far in excess of its capacity. The overall throughput under load is the ultimate measure of the value of an overload control mechanism. REQ 2: The failure, reduced processing capacity or overload of a single network element should be isolated from the remainder of the network, preventing a small-scale failure from becoming a widespread outage. REQ 3: The mechanism should seek to minimize the amount of configuration required in order to work. For example, it is better to avoid needing to configure a server with its SIP message throughput, as these kinds of quantities are hard to determine. REQ 4: The mechanism must be capable of dealing with elements which do not support it, so that a network can consist of a mix of ones which do and don't support it. Ideally, there should be incremental improvements in overall network throughput as increasing numbers of elements in the network support the mechanism. REQ 5: The mechanism should function in an environment where an upstream element is malicious and attempting to fool the system into believing it is overloaded when its not, and vice a versa. REQ 6: The mechanism shall provide a way to unambiguously inform an upstream element that it is overloaded, as distinct from other temporary failure conditions. Rosenberg Expires August 29, 2006 [Page 10]
Internet-Draft Overload Requirements February 2006 REQ 7: The mechanism shall provide a way for an element to throttle the amount of traffic it receives from an upstream element. This throttling shall provide the ability to reduce the traffic in incremental percentages from 0 to 100%. This recognizes the fact that "overload" is not a binary state, and there are degrees of overload. REQ 8: The mechanism shall ensure that, when a request has been rejected from an overloaded element, it is not sent to another overloaded element for processing. This requirement derives from REQ 1. REQ 9: When a request has been rejected from an overloaded element, it is not sent to another overloaded element for processing, but can be sent to one that is known to be available (i.e., not overloaded). This requirement derives from REQ 1. REQ 10: The mechanism should support servers that receive requests from a large number of different upstream elements, where the set of upstream elements is not enumerable. REQ 11: The mechanism should support servers that receive requests from a finite set of upstream elements, where the set of upstream elements is enumerable. REQ 12: The mechanism should work between servers in different domains. REQ 13: The mechanism must allow a proxy to prioritize requests, so that certain ones, such as call for emergency services, are still processed. REQ 14: The mechanism should provide unambigous directions to clients on when they should retry a request, and when they should not. This especially applies to TCP connection establishment and SIP registrations, in order to mitigate against avalanche restart. REQ 15: The mechanism shall take into account failures of downstream elements, detected either through SIP or through out-of-band means, in which case congestion indications will not be sent. REQ 16: The mechanism should attempt to minimize the overhead of the overload control messaging. REQ 17: The overload mechanism must not provide an avenue for malicious attack. Rosenberg Expires August 29, 2006 [Page 11]
Internet-Draft Overload Requirements February 2006 6. Security Considerations Like all protocol mechanisms, a solution for overload handling must prevent against malicious inside and outside attacks. This document includes requirements for such security functions. 7. IANA Considerations None. 8. Acknowledgements The author would like to thank Steve Mayer, Robert Whent, Mark Perkins and Joe Stone for their contributions to this document. 9. Informative References [1] 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. [2] Camarillo, G., Roach, A., Peterson, J., and L. Ong, "Integrated Services Digital Network (ISDN) User Part (ISUP) to Session Initiation Protocol (SIP) Mapping", RFC 3398, December 2002. [3] Jennings, C. and R. Mahy, "Managing Client Initiated Connections in the Session Initiation Protocol (SIP)", draft-ietf-sip-outbound-00 (work in progress), July 2005. Author's Address Jonathan Rosenberg Cisco Systems 600 Lanidex Plaza Parsippany, NJ 07054 US Phone: +1 973 952-5000 Email: jdrosen@cisco.com URI: http://www.jdrosen.net Rosenberg Expires August 29, 2006 [Page 12]
Internet-Draft Overload Requirements February 2006 Intellectual Property Statement 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. Disclaimer of Validity 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 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. Copyright Statement Copyright (C) The Internet Society (2006). 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Rosenberg Expires August 29, 2006 [Page 13]