SOC Working Group Eric Noel Internet-Draft AT&T Labs Intended status: Standards Track Philip M Williams Expires: March 5 2012 BT Innovate & Design Janet Gunn CSC September 2, 2011 Session Initiation Protocol (SIP) Rate Control draft-noel-soc-overload-rate-control-01.txt Abstract The prevalent use of Session Initiation Protocol (SIP) [RFC3261] in Next Generation Networks necessitates that SIP networks provide adequate control mechanisms to optimize transaction throughput and prevent congestion collapse during traffic overloads. Already [draft-ietf-soc-overload-control-03] proposes a loss-based solution to remedy known vulnerabilities of the [RFC3261] SIP 503 (service unavailable) overload control mechanism. This document proposes a rate-based control solution to complement the loss-based control defined in [draft-ietf-soc-overload-control-03]. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on February 2, 2012. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. Noel, et al. Expires February 2, 2012 [Page 1] Internet-Draft SIP Rate Control September 2011 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 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction...................................................2 2. Terminology....................................................3 3. Rate-based algorithm scheme....................................3 3.1. Overview..................................................3 3.2. Client and server rate-control algorithm selection........4 3.3. Server operation..........................................4 3.4. Client operation (default algorithm)......................5 4. Example........................................................6 5. Syntax.........................................................7 6. Security Considerations........................................7 7. IANA Considerations............................................8 8. References.....................................................8 8.1. Normative References......................................8 8.2. Informative References....................................8 Appendix A. Acknowledgments.......................................9 1. Introduction The use of SIP in large scale Next Generation Networks requires that SIP based networks provide adequate control mechanisms for handling traffic growth. In particular, SIP networks must be able to handle traffic overloads gracefully, optimizing transaction throughput without causing congestion collapse. A promising SIP based overload control solution has been proposed in [draft-ietf-soc-overload-control-03]. That solution includes a default loss-based overload control algorithm that makes it possible for a set of clients to limit offered load towards an overloaded server. However, such loss control algorithm is sensitive to variations in load so that any increase in load would be directly reflected by the clients in the offered load presented to the overloaded servers. In Noel, et al. Expires March 2, 2012 [Page 2] Internet-Draft SIP Rate Control September 2011 other words, a loss-based control cannot guarantee clients to produce a constant offered load towards an overloaded server. This document proposes a rate-based control that guarantees clients produce a constant offered load towards an overloaded server. The penalty for such a benefit is in terms of algorithmic complexity, since the overloaded server must estimate a target offered load and allocate a portion to each conversing client. The proposed rate-based overload control algorithm mitigates congestion in SIP networks while adhering to the overload signaling scheme in [draft-ietf-soc-overload-control-03] and proposing a rate control in addition to the default loss-based control in [draft- ietf-soc-overload-control-03]. 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]. The normative statements in this specification as they apply to SIP clients and SIP servers assume that both the SIP clients and SIP servers support this specification. If, for instance, only a SIP client supports this specification and not the SIP server, then follows that the normative statements in this specification pertinent to the behavior of a SIP server do not apply to the server that does not support this specification. 3. Rate-based algorithm scheme 3.1. Overview The server is what the overload control algorithm defined here protects and the client is what throttles traffic towards the client. Following the procedures defined in [draft-ietf-soc-overload- control-03], the server and clients signal one another support for rate-based overload control. Noel, et al. Expires March 2, 2012 [Page 3] Internet-Draft SIP Rate Control September 2011 Then periodically, the server relies on internal measurements (e.g. CPU utilization, queueing delay...) to evaluate its overload state and estimate a target SIP request rate (as opposed to target percent loss in the case of loss-based control). When in overload, the server uses [draft-ietf-soc-overload-control- 03] via header oc parameters of SIP responses to inform the clients of its overload state and of the target SIP request rate. Upon receiving the oc parameters with a target SIP request rate, each client throttles new SIP requests towards the overloaded server. 3.2. Client and server rate-control algorithm selection Per [draft-ietf-soc-overload-control-03], new clients indicate supported overload control algorithms to servers by inserting oc and oc-algo in Via header of SIP requests destined to servers. While servers notify clients of selected overload control algorithm through the oc-algo parameter in the Via header of SIP responses to clients. Support of rate-based control MUST be indicated by clients and servers by setting oc-algo to "rate". 3.3. Server operation The actual algorithm used by the server to determine its overload state and estimate a target SIP request rate is beyond the scope of this document. However, the server MUST be able to evaluate periodically its overload state and estimate a target SIP request rate beyond which it would become overloaded. The server must allocate a portion of the target SIP request rate to each of its client. Upon detection of overload, the server MUST follow the specifications in [draft-ietf-soc-overload-control-03] to notify its clients of its overload state and of the allocated target SIP request rate. Noel, et al. Expires March 2, 2012 [Page 4] Internet-Draft SIP Rate Control September 2011 The server MUST use [draft-ietf-soc-overload-control-03] oc parameter to send a target SIP request rate to each of its client. 3.4. Client operation (default algorithm) To throttle new SIP requests at the rate specified in the oc value sent by the server to its clients, the client MAY use the proposed default algorithm for rate-based control or any other equivalent algorithm. The default Leaky Bucket algorithm presented here is based on [ITU-T Rec. I.371] Appendix A.2. The algorithm makes it possible for client to deliver SIP requests at a rate specified in the oc value with tolerance TAU. Conceptually, the Leaky Bucket algorithm can be viewed as a finite capacity bucket whose real-valued content drains out at a continuous rate of 1 unit of content per time unit and whose content increases by the increment T for each forwarded SIP request. T is computed as the inverse of the rate specified in the oc value, namely T = 1 / oc-value. If at a new SIP request arrival the content of the bucket is less than or equal to the limit value TAU, then the SIP request is forwarded to the server; otherwise, the SIP request is rejected. Note that the capacity of the bucket (the upper bound of the counter) is (T + TAU). At the arrival time of the k-th new SIP request ta(k), the content of the bucket is provisionally updated to the value X' = X - ([ta(k) - LCT]) where X is the content of the bucket after arrival of the last forwarded SIP request, and LCT is the time at which the last SIP request was forwarded. Noel, et al. Expires March 2, 2012 [Page 5] Internet-Draft SIP Rate Control September 2011 If X' is less than or equal to the limit value TAU, then the new SIP request is forwarded and the bucket content X is set to X' (or to 0 if X' is negative) plus the increment T, and LCT is set to the current time ta(k). If X' is greater than the limit value tau, then the new SIP request is rejected and the values of X and LCT are unchanged. At the arrival time of the first new SIP request ta(1), the content of the bucket X is set to zero and LCT is set to ta(1). Note that specification of a value for TAU is beyond the scope of this document. 4. Example Adapting [draft-ietf-soc-overload-control-03] example in section 6.2 where SIP client P1 sends requests to a downstream server P2: INVITE sips:user@example.com SIP/2.0 Via: SIP/2.0/TLS p1.example.net; branch=z9hG4bK2d4790.1;received=192.0.2.111; oc;oc-algo="loss,rate" ... SIP/2.0 100 Trying Via: SIP/2.0/TLS p1.example.net; branch=z9hG4bK2d4790.1;received=192.0.2.111; oc=0;oc-algo="rate";oc-validity=500; oc-seq=1282321615.781 ... Noel, et al. Expires March 2, 2012 [Page 6] Internet-Draft SIP Rate Control September 2011 In the messages above, the first line is sent by P1 to P2. This line is a SIP request; because P1 supports overload control, it inserts the "oc" parameter in the topmost Via header that it created. P1 supports two overload control algorithms: loss and rate. The second line --- a SIP response --- shows the topmost Via header amended by P2 according to this specification and sent to P1. Because P2 also supports overload control, it chooses the "rate" based scheme and sends that back to P1 in the "oc-algo" parameter. It also sets the value of "oc" parameter to 0. At some later time, P2 starts to experience overload. It sends the following SIP message indicating P1 should send SIP requests at a rate no greater than or equal to 150 SIP requests per seconds. SIP/2.0 180 Ringing Via: SIP/2.0/TLS p1.example.net; branch=z9hG4bK2d4790.1;received=192.0.2.111; oc=150;oc-algo="rate";oc-validity=1000; oc-seq=1282321615.782 ... 5. Syntax This specification extends the existing definition of the Via header field parameters of [RFC3261] as follows: oc = "oc" EQUAL oc-value oc-value = "NaN" / oc-num oc-num = 1*DIGIT 6. Security Considerations None. Noel, et al. Expires March 2, 2012 [Page 7] Internet-Draft SIP Rate Control September 2011 7. IANA Considerations None. 8. References 8.1. Normative References [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. 8.2. Informative References [draft-ietf-soc-overload-control-03] Gurbani, V., Hilt, V., Schulzrinne, H., "Session Initiation Protocol (SIP) Overload Control", draft-ietf- soc-overload-control-03. [ITU-T Rec. I.371] "Traffic control and congestion control in B-ISDN", ITU-T Recommendation I.371. Noel, et al. Expires March 2, 2012 [Page 8] Internet-Draft SIP Rate Control September 2011 Appendix A. Acknowledgments Many thanks for the contributions, comments and feedback on this document to: This document was prepared using 2-Word-v2.0.template.dot. Authors' Addresses Eric Noel AT&T Labs 200s Laurel Avenue Middletown, NJ, 07747 USA Philip M Williams BT Innovate & Design UK Janet Gunn CSC USA Noel, et al. Expires March 2, 2012 [Page 9]