Network Working Group
Request for Comments: 5390
Category: Informational
J. Rosenberg
December 2008

Requirements for Management of Overload in the

Session Initiation Protocol

Status of This Memo

This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

Copyright Notice

Copyright © 2008 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 ( 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.


Overload occurs in Session Initiation Protocol (SIP) networks when proxies and user agents have insufficient 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 document summarizes the problems with the existing 503 mechanism, and provides some requirements for a solution.

Table of Contents

   1. Introduction ....................................................2
   2. Causes of Overload ..............................................2
   3. Terminology .....................................................4
   4. Current SIP Mechanisms ..........................................4
   5. Problems with the Mechanism .....................................5
      5.1. Load Amplification .........................................5
      5.2. Underutilization ...........................................9
      5.3. The Off/On Retry-After Problem .............................9
      5.4. Ambiguous Usages ..........................................10
   6. Solution Requirements ..........................................10
   7. Security Considerations ........................................13
   8. Acknowledgements ...............................................13
   9. References .....................................................14
      9.1. Normative Reference .......................................14
      9.2. Informative References ....................................14

1. Introduction

Overload occurs in Session Initiation Protocol (SIP) [RFC3261] networks when proxies and user agents have insufficient resources to complete the processing of a request or a response. SIP provides limited support for overload handling through its 503 response code. This code allows a server to tell an upstream element that it is overloaded. However, numerous problems have been identified with this mechanism.

This document describes the general problem of SIP overload and 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 successfully process all of the 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, in which case the CPU, processing, memory, I/O, and disk resources of those servers are effectively part of the logical element processing the request. 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 Mother's Day).
   Dependency Failures:  A SIP element can become overloaded because a
      resource on which it is dependent has failed or become overloaded,
      greatly reducing the logical capacity of the element.  In these
      cases, even minimal traffic might cause the server to go into
      overload.  Examples of such dependency overloads 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 that each share the load of
      traffic, and one or more of the other members 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, 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
   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 [OUTBOUND].
      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-registration to another server. Note that [OUTBOUND] 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.
   Denial of Service (DoS) Attacks:  An attacker, wishing to disrupt
      service in the network, can cause a large amount of traffic to be
      launched at a target server.  This can be done from a central
      source of traffic or through a distributed DoS attack.  In all
      cases, the volume of traffic well exceeds the capacity of the
      server, sending 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 overload, 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 be delayed or 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, in the case of UDP, with as many as seven retransmits of an INVITE request prior to timeout, overload can multiply the already-heavy message volume by as much as seven!

3. 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].

4. 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 thus:

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).

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.

RFC 3261 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.

5. 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.

5.1. 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 topology in Figure 1.

                                       > |      |
                                      /  |  S1  |
                                     /   |      |
                                    /    +------+
                      +------+ /         +------+
            --------> |      |/          |      |
                      |  P1  |---------> |  S2  |
            --------> |      |\          |      |
                      +------+ \         +------+
                                     \   +------+
                                      \  |      |
                                       > |  S3  |
                                         |      |

Figure 1

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. If SIP is being run over UDP, this may result in request retransmissions, which further increase the work on S1. Even in the case of TCP, if the server is loaded and the kernel cannot send TCP acknowledgements fast enough, TCP retransmits may occur. 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 may also be hit with retransmits. P1 once again tries another server, this time S3, which also eventually rejects it with a 503.

Thus, the processing of this request, which ultimately failed, involved four SIP transactions (client to P1, P1 to S1, P1 to S2, P1 to S3), each of which may have involved many retransmissions -- up to seven in the case of UDP. Thus, under unloaded conditions, a single request from a client would generate one request (to S1, S2, or S3) and two responses (from S1 to P1, then P1 to the client). When the

   network is overloaded, a single request from the client, before
   timing out, could generate as many as 18 requests and as many
   responses when UDP is used!  The situation is better with TCP (or any
   reliable transport in general), but even if there was never a TCP
   segment retransmitted, a single request from the client can generate
   three requests and four responses.  Each server had to expend
   resources to process these messages.  Thus, more messages and more
   work were sent into the 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

The problem becomes amplified further if one considers proxies upstream from P1, as shown in Figure 2.

                              > |      | <
                             /  |  S1  |  \\
                            /   |      |    \\
                           /    +------+      \\
                          /                     \
                         /                       \\
                        /                          \\
                       /                             \
            +------+  /         +------+           +------+
            |      | /          |      |           |      |
            |  P1  | ---------> |  S2  |<----------|  P2  |
            |      | \          |      |           |      |
            +------+  \         +------+           +------+
                ^      \                             / ^
                 \      \                          // /
                  \      \                       //  /
                   \      \                    //   /
                    \      \                  /    /
                     \      \   +------+    //    /
                      \      \  |      |  //     /
                       \      > |  S3  | <      /
                        \       |      |       /
                         \      +------+      /
                          \                  /
                           \                /
                            \              /
                             \            /
                              \          /
                               \        /
                                \      /
                                 \    /
                                |      |
                                |  PA  |
                                |      |
                                 ^   ^
                                 |   |
                                 |   |

Figure 2

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 results in many request retransmits if UDP is used. Since P1 is 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. The problem here is that the fact that S1 through S3 were overloaded was known to P1, but this information was not passed back to PA and through to P2, so that P2 retries S1 through S3 again.

5.2. Underutilization

   Interestingly, there are also examples of deployments where the
   network capacity was greatly reduced as a consequence of the overload
   mechanism.  Consider again Figure 1.  Unfortunately, RFC 3261 is
   unclear on the scope of a 503.  When it is received by P1, does the
   proxy cease sending requests to that IP address?  To the hostname?
   To the URI?  Some implementations have chosen the hostname as the
   scope.  When the hostname for a URI points to an SRV record in the
   DNS, which, in turn, maps to a cluster of downstream servers (S1, S2,
   and S3 in the example), a 503 response from a single one of them will
   make the proxy believe that the entire cluster is overloaded.
   Consequently, proxy P1 will cease sending any traffic to any element
   in the cluster, even though there are elements in the cluster that
   are underutilized.

5.3. 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 off all traffic towards it, or none.  There is nothing in
   between.  This tends to cause highly oscillatory behavior under even
   mild overload.  Consider a proxy P1 that 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.

It's 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 is 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.

5.4. 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 [RFC3398], which specifies interworking from SIP to ISDN User Part (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; it's just that this specific request could not be processed because of a downstream problem. All subsequent requests might succeed if they take a different route in the Public Switched Telephone Network (PSTN).

This causes two problems. First, during periods of overload, it 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. Second, 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.

6. Solution Requirements

In this section, we propose requirements for an overload control mechanism for SIP that addresses these problems.

   REQ 1:  The overload mechanism shall strive to maintain the overall
      useful throughput (taking into consideration the quality-of-
      service needs of the using applications) of a SIP server 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
   REQ 2:  When a single network element fails, goes into overload, or
      suffers from reduced processing capacity, the mechanism should
      strive to limit the impact of this on other elements in the
      network.  This helps to prevent 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 that
      do not support it, so that a network can consist of a mix of
      elements that do and don't support it.  In other words, the
      mechanism should not work only in environments where all elements
      support it.  It is reasonable to assume that it works better in
      such environments, of course.  Ideally, there should be
      incremental improvements in overall network throughput as
      increasing numbers of elements in the network support the
   REQ 5:  The mechanism should not assume that it will only be deployed
      in environments with completely trusted elements.  It should seek
      to operate as effectively as possible in environments where other
      elements are malicious; this includes preventing malicious
      elements from obtaining more than a fair share of service.
   REQ 6:  When overload is signaled by means of a specific message, the
      message must clearly indicate that it is being sent because of
      overload, as opposed to other, non overload-based failure
      conditions.  This requirement is meant to avoid some of the
      problems that have arisen from the reuse of the 503 response code
      for multiple purposes.  Of course, overload is also signaled by
      lack of response to requests.  This requirement applies only to
      explicit overload signals.
   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 be graded so that it is not all-or-nothing as
      with the current 503 mechanism.  This recognizes the fact that
      "overload" is not a binary state and that there are degrees of
   REQ 8:  The mechanism shall ensure that, when a request was not
      processed successfully due to overload (or failure) of a
      downstream element, the request will not be retried on another
      element that is also overloaded or whose status is unknown.  This
      requirement derives from REQ 1.
   REQ 9:  That a request has been rejected from an overloaded element
      shall not unduly restrict the ability of that request to be
      submitted to and processed by an element that is 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
   REQ 13:  The mechanism must not dictate a specific algorithm for
      prioritizing the processing of work within a proxy during times of
      overload.  It must permit a proxy to prioritize requests based on
      any local policy, so that certain ones (such as a call for
      emergency services or a call with a specific value of the
      Resource-Priority header field [RFC4412]) are given preferential
      treatment, such as not being dropped, being given additional
      retransmission, or being processed ahead of others.
   REQ 14:  The mechanism should provide unambiguous 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:  In cases where a network element fails, is so overloaded
      that it cannot process messages, or cannot communicate due to a
      network failure or network partition, it will not be able to
      provide explicit indications of the nature of the failure or its
      levels of congestion.  The mechanism must properly function in
      these cases.
   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, including DoS and DDoS attacks.
   REQ 18:  The overload mechanism should be unambiguous about whether a
      load indication applies to a specific IP address, host, or URI, so
      that an upstream element can determine the load of the entity to
      which a request is to be sent.
   REQ 19:  The specification for the overload mechanism should give
      guidance on which message types might be desirable to process over
      others during times of overload, based on SIP-specific
      considerations.  For example, it may be more beneficial to process
      a SUBSCRIBE refresh with Expires of zero than a SUBSCRIBE refresh

with a non-zero expiration (since the former reduces the overall amount of load on the element), or to process re-INVITEs over new INVITEs.

   REQ 20:  In a mixed environment of elements that do and do not
      implement the overload mechanism, no disproportionate benefit
      shall accrue to the users or operators of the elements that do not
      implement the mechanism.
   REQ 21:  The overload mechanism should ensure that the system remains
      stable.  When the offered load drops from above the overall
      capacity of the network to below the overall capacity, the
      throughput should stabilize and become equal to the offered load.
   REQ 22:  It must be possible to disable the reporting of load
      information towards upstream targets based on the identity of
      those targets.  This allows a domain administrator who considers
      the load of their elements to be sensitive information, to
      restrict access to that information.  Of course, in such cases,
      there is no expectation that the overload mechanism itself will
      help prevent overload from that upstream target.
   REQ 23:  It must be possible for the overload mechanism to work in
      cases where there is a load balancer in front of a farm of

7. 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.

Any mechanism that improves the behavior of SIP elements under load will result in more predictable performance in the face of application-layer denial-of-service attacks.

8. Acknowledgements

The author would like to thank Steve Mayer, Mouli Chandramouli, Robert Whent, Mark Perkins, Joe Stone, Vijay Gurbani, Steve Norreys, Volker Hilt, Spencer Dawkins, Matt Mathis, Juergen Schoenwaelder, and Dale Worley for their contributions to this document.

9. References

9.1. Normative Reference

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

9.2. Informative References

   [OUTBOUND]  Jennings, C. and R. Mahy, "Managing Client Initiated
               Connections in the Session Initiation Protocol (SIP)",
               Work in Progress, October 2008.
   [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.
   [RFC3398]   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.
   [RFC4412]   Schulzrinne, H. and J. Polk, "Communications Resource
               Priority for the Session Initiation Protocol (SIP)",
               RFC 4412, February 2006.

Author's Address

Jonathan Rosenberg
Edison, NJ