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RFC 7339 - Session Initiation Protocol (SIP) Overload Control

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Internet Engineering Task Force (IETF)                   V. Gurbani, Ed.
Request for Comments: 7339                                       V. Hilt
Category: Standards Track                      Bell Labs, Alcatel-Lucent
ISSN: 2070-1721                                           H. Schulzrinne
                                                     Columbia University
                                                          September 2014

           Session Initiation Protocol (SIP) Overload Control


   Overload occurs in Session Initiation Protocol (SIP) networks when
   SIP servers have insufficient resources to handle all the 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 the behavior of SIP servers involved
   in overload control and also specifies a loss-based overload scheme
   for SIP.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................4
   2. Terminology .....................................................5
   3. Overview of Operations ..........................................6
   4. Via Header Parameters for Overload Control ......................6
      4.1. The "oc" Parameter .........................................6
      4.2. The "oc-algo" Parameter ....................................7
      4.3. The "oc-validity" Parameter ................................8
      4.4. The "oc-seq" Parameter .....................................8
   5. General Behavior ................................................9
      5.1. Determining Support for Overload Control ..................10
      5.2. Creating and Updating the Overload Control Parameters .....10
      5.3. Determining the "oc" Parameter Value ......................12
      5.4. Processing the Overload Control Parameters ................12
      5.5. Using the Overload Control Parameter Values ...............13
      5.6. Forwarding the Overload Control Parameters ................14
      5.7. Terminating Overload Control ..............................14
      5.8. Stabilizing Overload Algorithm Selection ..................15
      5.9. Self-Limiting .............................................15
      5.10. Responding to an Overload Indication .....................16
           5.10.1. Message Prioritization at the Hop before
                   the Overloaded Server .............................16
           5.10.2. Rejecting Requests at an Overloaded Server ........17
      5.11. 100 Trying Provisional Response and Overload
            Control Parameters .......................................17
   6. Example ........................................................18
   7. The Loss-Based Overload Control Scheme .........................19
      7.1. Special Parameter Values for Loss-Based Overload Control ..19
      7.2. Default Algorithm for Loss-Based Overload Control .........20
   8. Relationship with Other IETF SIP Load Control Efforts ..........23
   9. Syntax .........................................................24
   10. Design Considerations .........................................24
      10.1. SIP Mechanism ............................................24
           10.1.1. SIP Response Header ...............................24
           10.1.2. SIP Event Package .................................25
      10.2. Backwards Compatibility ..................................26
   11. Security Considerations .......................................27
   12. IANA Considerations ...........................................29
   13. References ....................................................29
      13.1. Normative References .....................................29
      13.2. Informative References ...................................30
   Appendix A. Acknowledgements ......................................31
   Appendix B. RFC 5390 Requirements .................................31

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 where the retransmissions of dropped SIP messages may
   overwhelm the capacity of the network.  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, input/output, or disk

   For overload control, this document only addresses 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
   failure conditions unrelated to the SIP server being overloaded.  For
   example, a Public Switched Telephone Network (PSTN) gateway that runs
   out of trunks 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 requests 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 shift
   between SIP servers, thereby worsening 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 [RFC5390].

   This document defines the protocol for communicating overload
   information between SIP servers and clients so that clients can
   reduce the volume of traffic sent to overloaded servers, avoiding
   congestion collapse and increasing useful throughput.  Section 4
   describes the Via header parameters used for this communication.  The

   general behavior of SIP servers and clients involved in overload
   control is described in Section 5.  In addition, Section 7 specifies
   a loss-based overload control scheme.

   This document specifies the loss-based overload control scheme
   (Section 7), which is mandatory to implement for this specification.
   In addition, this document allows other overload control schemes to
   be supported as well.  To do so effectively, the expectations and
   primitive protocol parameters common to all classes of overload
   control schemes are specified in this document.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   In this document, the terms "SIP client" and "SIP server" are used in
   their generic forms.  Thus, a "SIP client" could refer to the client
   transaction state machine in a SIP proxy, or it could refer to a user
   agent client (UAC).  Similarly, a "SIP server" could be a user agent
   server (UAS) or the server transaction state machine in a proxy.
   Various permutations of this are also possible, for instance, SIP
   clients and servers could also be part of back-to-back user agents

   However, irrespective of the context these terms are used in (i.e.,
   proxy, B2BUA, UAS, UAC), "SIP client" applies to any SIP entity that
   provides overload control to traffic destined downstream.  Similarly,
   "SIP server" applies to any SIP entity that is experiencing overload
   and would like its upstream neighbor to throttle incoming traffic.

   Unless otherwise specified, all SIP entities described in this
   document are assumed to support this specification.

   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 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

3.  Overview of Operations

   This section provides an overview of how the overload control
   mechanism operates by introducing the overload control parameters.
   Section 4 provides more details and normative behavior on the
   parameters listed below.

   Because overload control is performed hop-by-hop, the Via header
   parameter is attractive since it allows two adjacent SIP entities to
   indicate support for, and exchange information associated with,
   overload control [RFC6357].  Additional advantages of this choice are
   discussed in Section 10.1.1.  An alternative mechanism using SIP
   event packages was also considered, and the characteristics of that
   choice are further outlined in Section 10.1.2.

   This document defines four new parameters for the SIP Via header for
   overload control.  These parameters provide a mechanism for conveying
   overload control information between adjacent SIP entities.  The "oc"
   parameter is used by a SIP server to indicate a reduction in the
   number of requests arriving at the server.  The "oc-algo" parameter
   contains a token or a list of tokens corresponding to the class of
   overload control algorithms supported by the client.  The server
   chooses one algorithm from this list.  The "oc-validity" parameter
   establishes a time limit for which overload control is in effect, and
   the "oc-seq" parameter aids in sequencing the responses at the
   client.  These parameters are discussed in detail in the next

4.  Via Header Parameters for Overload Control

   The four Via header parameters are introduced below.  Further context
   about how to interpret these under various conditions is provided in
   Section 5.

4.1.  The "oc" Parameter

   This parameter is inserted by the SIP client and updated by the SIP

   A SIP client MUST add an "oc" parameter to the topmost Via header it
   inserts into every SIP request.  This provides an indication to
   downstream neighbors that the client supports overload control.
   There MUST NOT be a value associated with the parameter (the value
   will be added by the server).

   The downstream server MUST add a value to the "oc" parameter in the
   response going upstream to a client that included the "oc" parameter
   in the request.  Inclusion of a value to the parameter represents two

   things.  First, upon the first contact (see Section 5.1), addition of
   a value by the server to this parameter indicates (to the client)
   that the downstream server supports overload control as defined in
   this document.  Second, if overload control is active, then it
   indicates the level of control to be applied.

   When a SIP client receives a response with the value in the "oc"
   parameter filled in, it MUST reduce, as indicated by the "oc" and
   "oc-algo" parameters, the number of requests going downstream to the
   SIP server from which it received the response (see Section 5.10 for
   pertinent discussion on traffic reduction).

4.2.  The "oc-algo" Parameter

   This parameter is inserted by the SIP client and updated by the SIP

   A SIP client MUST add an "oc-algo" parameter to the topmost Via
   header it inserts into every SIP request, with a default value of

   This parameter contains names of one or more classes of overload
   control algorithms.  A SIP client MUST support the loss-based
   overload control scheme and MUST insert at least the token "loss" as
   one of the "oc-algo" parameter values.  In addition, the SIP client
   MAY insert other tokens, separated by a comma, in the "oc-algo"
   parameter if it supports other overload control schemes such as a
   rate-based scheme [RATE-CONTROL].  Each element in the comma-
   separated list corresponds to the class of overload control
   algorithms supported by the SIP client.  When more than one class of
   overload control algorithms is present in the "oc-algo" parameter,
   the client may indicate algorithm preference by ordering the list in
   a decreasing order of preference.  However, the client cannot assume
   that the server will pick the most preferred algorithm.

   When a downstream SIP server receives a request with multiple
   overload control algorithms specified in the "oc-algo" parameter
   (optionally sorted by decreasing order of preference), it chooses one
   algorithm from the list and MUST return the single selected algorithm
   to the client.

   Once the SIP server has chosen a mutually agreeable class of overload
   control algorithms and communicated it to the client, the selection
   stays in effect until the algorithm is changed by the server.
   Furthermore, the client MUST continue to include all the supported
   algorithms in subsequent requests; the server MUST respond with the
   agreed-to algorithm until the algorithm is changed by the server.

   The selection SHOULD stay the same for a non-trivial duration of time
   to allow the overload control algorithm to stabilize its behavior
   (see Section 5.8).

   The "oc-algo" parameter does not define the exact algorithm to be
   used for traffic reduction; rather, the intent is to use any
   algorithm from a specific class of algorithms that affect traffic
   reduction similarly.  For example, the reference algorithm in
   Section 7.2 can be used as a loss-based algorithm, or it can be
   substituted by any other loss-based algorithm that results in
   equivalent traffic reduction.

4.3.  The "oc-validity" Parameter

   This parameter MAY be inserted by the SIP server in a response; it
   MUST NOT be inserted by the SIP client in a request.

   This parameter contains a value that indicates an interval of time
   (measured in milliseconds) that the load reduction specified in the
   value of the "oc" parameter should be in effect.  The default value
   of the "oc-validity" parameter is 500 (milliseconds).  If the client
   receives a response with the "oc" and "oc-algo" parameters suitably
   filled in, but no "oc-validity" parameter, the SIP client should
   behave as if it had received "oc-validity=500".

   A value of 0 in the "oc-validity" parameter is reserved to denote the
   event that the server wishes to stop overload control or to indicate
   that it supports overload control but is not currently requesting any
   reduction in traffic (see Section 5.7).

   A non-zero value for the "oc-validity" parameter MUST only be present
   in conjunction with an "oc" parameter.  A SIP client MUST discard a
   non-zero value of the "oc-validity" parameter if the client receives
   it in a response without the corresponding "oc" parameter being
   present as well.

   After the value specified in the "oc-validity" parameter expires and
   until the SIP client receives an updated set of overload control
   parameters from the SIP server, overload control is not in effect
   between the client and the downstream SIP server.

4.4.  The "oc-seq" Parameter

   This parameter MUST be inserted by the SIP server in a response; it
   MUST NOT be inserted by the SIP client in a request.

   This parameter contains an unsigned integer value that indicates the
   sequence number associated with the "oc" parameter.  This sequence
   number is used to differentiate two "oc" parameter values generated
   by an overload control algorithm at two different instants in time.
   "oc" parameter values generated by an overload control algorithm at
   time t and t+1 MUST have an increasing value in the "oc-seq"
   parameter.  This allows the upstream SIP client to properly collate
   out-of-order responses.

      Note: A timestamp can be used as a value of the "oc-seq"

   If the value contained in the "oc-seq" parameter overflows during the
   period in which the load reduction is in effect, then the "oc-seq"
   parameter MUST be reset to the current timestamp or an appropriate
   base value.

      Note: A client implementation can recognize that an overflow has
      occurred when it receives an "oc-seq" parameter whose value is
      significantly less than several previous values.  (Note that an
      "oc-seq" parameter whose value does not deviate significantly from
      the last several previous values is symptomatic of a tardy packet.
      However, overflow will cause the "oc-seq" parameter value to be
      significantly less than the last several values.)  If an overflow
      is detected, then the client should use the overload parameters in
      the new message, even though the sequence number is lower.  The
      client should also reset any internal state to reflect the
      overflow so that future messages (following the overflow) will be

5.  General Behavior

   When forwarding a SIP request, a SIP client uses the SIP procedures
   of [RFC3263] to determine the next-hop SIP server.  The procedures of
   [RFC3263] take a SIP URI as input, extract the domain portion of that
   URI for use as a lookup key, query the Domain Name Service (DNS) to
   obtain an ordered set of one or more IP addresses with a port number
   and transport corresponding to each IP address in this set (the
   "Expected Output").

   After selecting a specific SIP server from the Expected Output, a SIP
   client determines whether overload controls are currently active with
   that server.  If overload controls are currently active (and the "oc-
   validity" period has not yet expired), the client applies the
   relevant algorithm to determine whether or not to send the SIP
   request to the server.  If overload controls are not currently active
   with this server (which will be the case if this is the initial
   contact with the server, the last response from this server had

   "oc-validity=0", or the time period indicated by the "oc-validity"
   parameter has expired), the SIP client sends the SIP message to the
   server without invoking any overload control algorithm.

5.1.  Determining Support for Overload Control

   If a client determines that this is the first contact with a server,
   the client MUST insert the "oc" parameter without any value and MUST
   insert the "oc-algo" parameter with a list of algorithms it supports.
   This list MUST include "loss" and MAY include other algorithm names
   approved by IANA and described in corresponding documents.  The
   client transmits the request to the chosen server.

   If a server receives a SIP request containing the "oc" and "oc-algo"
   parameters, the server MUST determine if it has already selected the
   overload control algorithm class with this client.  If it has, the
   server SHOULD use the previously selected algorithm class in its
   response to the message.  If the server determines that the message
   is from a new client or a client the server has not heard from in a
   long time, the server MUST choose one algorithm from the list of
   algorithms in the "oc-algo" parameter.  It MUST put the chosen
   algorithm as the sole parameter value in the "oc-algo" parameter of
   the response it sends to the client.  In addition, if the server is
   currently not in an overload condition, it MUST set the value of the
   "oc" parameter to be 0 and MAY insert an "oc-validity=0" parameter in
   the response to further qualify the value in the "oc" parameter.  If
   the server is currently overloaded, it MUST follow the procedures in
   Section 5.2.

      Note: A client that supports the rate-based overload control
      scheme [RATE-CONTROL] will consider "oc=0" as an indication not to
      send any requests downstream at all.  Thus, when the server
      inserts "oc-validity=0" as well, it is indicating that it does
      support overload control, but it is not under overload mode right
      now (see Section 5.7).

5.2.  Creating and Updating the Overload Control Parameters

   A SIP server provides overload control feedback to its upstream
   clients by providing a value for the "oc" parameter to the topmost
   Via header field of a SIP response, that is, the Via header added by
   the client before it sent the request to the server.

   Since the topmost Via header of a response will be removed by an
   upstream client after processing it, overload control feedback
   contained in the "oc" parameter will not travel beyond the upstream

   SIP client.  A Via header parameter therefore provides hop-by-hop
   semantics for overload control feedback (see [RFC6357]) even if the
   next-hop neighbor does not support this specification.

   The "oc" parameter can be used in all response types, including
   provisional, success, and failure responses (please see Section 5.11
   for special consideration on transporting overload control parameters
   in a 100 Trying response).  A SIP server can update the "oc"
   parameter in a response, asking the client to increase or decrease
   the number of requests destined to the server or to stop performing
   overload control altogether.

   A SIP server that has updated the "oc" parameter SHOULD also add a
   "oc-validity" parameter.  The "oc-validity" parameter defines the
   time in milliseconds during which the overload control feedback
   specified in the "oc" parameter is valid.  The default value of the
   "oc-validity" parameter is 500 (milliseconds).

   When a SIP server retransmits a response, it SHOULD use the "oc" and
   "oc-validity" parameter values consistent with the overload state at
   the time the retransmitted response was sent.  This implies that the
   values in the "oc" and "oc-validity" parameters may be different than
   the ones used in previous retransmissions of the response.  Due to
   the fact that responses sent over UDP may be subject to delays in the
   network and arrive out of order, the "oc-seq" parameter aids in
   detecting a stale "oc" parameter value.

   Implementations that are capable of updating the "oc" and "oc-
   validity" parameter values during retransmissions MUST insert the
   "oc-seq" parameter.  The value of this parameter MUST be a set of
   numbers drawn from an increasing sequence.

   Implementations that are not capable of updating the "oc" and "oc-
   validity" parameter values during retransmissions -- or
   implementations that do not want to do so because they will have to
   regenerate the message to be retransmitted -- MUST still insert a
   "oc-seq" parameter in the first response associated with a
   transaction; however, they do not have to update the value in
   subsequent retransmissions.

   The "oc-validity" and "oc-seq" 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
   the client is the entity that can offload traffic by redirecting or
   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 SIP servers can exchange overload

   This specification provides a good overload control mechanism that
   can protect a SIP server from overload.  However, if a SIP server
   wants to limit advertisements of overload control capability for
   privacy reasons, it might decide to perform overload control only for
   requests that are received on a secure transport, such as Transport
   Layer Security (TLS).  Indicating support for overload control on a
   request received on an untrusted link can leak privacy in the form of
   capabilities supported by the server.  To limit the knowledge that
   the server supports overload control, a server can adopt a policy of
   inserting overload control parameters in only those requests received
   over trusted links such that these parameters are only visible to
   trusted neighbors.

5.3.  Determining the "oc" Parameter Value

   The value of the "oc" parameter is determined by the overloaded
   server using any pertinent information at its disposal.  The only
   constraint imposed by this document is that the server control
   algorithm MUST produce a value for the "oc" parameter that it expects
   the receiving SIP clients to apply to all downstream SIP requests
   (dialogue forming as well as in-dialogue) to this SIP server.  Beyond
   this stipulation, the process by which an overloaded server
   determines the value of the "oc" parameter is considered out of the
   scope of this document.

      Note: This stipulation is required so that both the client and
      server have a common view of which messages the overload control
      applies to.  With this stipulation in place, the client can
      prioritize messages as discussed in Section 5.10.1.

   As an example, a value of "oc=10" when the loss-based algorithm is
   used implies that 10% of the total number of SIP requests (dialogue
   forming as well as in-dialogue) are subject to reduction at the
   client.  Analogously, a value of "oc=10" when the rate-based
   algorithm [RATE-CONTROL] is used indicates that the client should
   send SIP requests at a rate of 10 SIP requests or fewer per second.

5.4.  Processing the Overload Control Parameters

   A SIP client SHOULD remove the "oc", "oc-validity", and "oc-seq"
   parameters from all Via headers of a response received, except for
   the topmost Via header.  This prevents overload control parameters
   that were accidentally or maliciously inserted into Via headers by a
   downstream SIP server from traveling upstream.

   The scope of overload control applies to unique combinations of IP
   and port values.  A SIP client maintains the overload control values
   received (along with the address and port number 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 client
   receives a response with an overload control parameter from a
   downstream SIP server, it compares the "oc-seq" value extracted from
   the Via header with the "oc-seq" value stored for this server.  If
   these values match, the response does not update the overload control
   parameters related to this server, and the client continues to
   provide overload control as previously negotiated.  If the "oc-seq"
   value extracted from the Via header is larger than the stored value,
   the client updates the stored values by copying the new values of the
   "oc", "oc-algo", and "oc-seq" parameters from the Via header to the
   stored values.  Upon such an update of the overload control
   parameters, the client restarts the validity period of the new
   overload control parameters.  The overload control parameters now
   remain in effect until the validity period expires or the parameters
   are updated in a new response.  Stored overload control parameters
   MUST be reset to default values once the validity period has expired
   (see Section 5.7 for the detailed steps on terminating overload

5.5.  Using the Overload Control Parameter Values

   A SIP client MUST honor overload control values it receives from
   downstream neighbors.  The SIP client MUST NOT forward more requests
   to a SIP server than allowed by the current "oc" and "oc-algo"
   parameter values from that particular downstream server.

   When forwarding a SIP request, a SIP client uses the SIP procedures
   of [RFC3263] to determine the next-hop SIP server.  The procedures of
   [RFC3263] take a SIP URI as input, extract the domain portion of that
   URI for use as a lookup key, query the DNS to obtain an ordered set
   of one or more IP addresses with a port number and transport
   corresponding to each IP address in this set (the Expected Output).

   After selecting a specific SIP server from the Expected Output, the
   SIP client determines if it already has overload control parameter
   values for the server chosen from the Expected Output.  If the SIP
   client has a non-expired "oc" parameter value for the server chosen
   from the Expected Output, then this chosen server is operating in
   overload control mode.  Thus, the SIP client determines if it can or
   cannot forward the current request to the SIP server based on the
   "oc" and "oc-algo" parameters and any relevant local policy.

   The particular algorithm used to determine whether or not to forward
   a particular SIP request is a matter of local policy and may take
   into account a variety of prioritization factors.  However, this
   local policy SHOULD transmit the same number of SIP requests as the
   sample algorithm defined by the overload control scheme being used.
   (See Section 7.2 for the default loss-based overload control

5.6.  Forwarding the Overload Control Parameters

   Overload control is defined in a hop-by-hop manner.  Therefore,
   forwarding the contents of the overload control parameters is
   generally NOT RECOMMENDED and should only be performed if permitted
   by the configuration of SIP servers.  This means that a SIP proxy
   SHOULD strip the overload control parameters inserted by the client
   before proxying the request further downstream.  Of course, when the
   proxy acts as a client and proxies the request downstream, it is free
   to add overload control parameters pertinent to itself in the Via
   header it inserted in the request.

5.7.  Terminating Overload Control

   A SIP client removes overload control if one of the following events

   1.  The "oc-validity" period previously received by the client from
       this server (or the default value of 500 ms if the server did not
       previously specify an "oc-validity" parameter) expires.

   2.  The client is explicitly told by the server to stop performing
       overload control using the "oc-validity=0" parameter.

   A SIP server can decide to terminate overload control by explicitly
   signaling the client.  To do so, the SIP server MUST set the value of
   the "oc-validity" parameter to 0.  The SIP server MUST increment the
   value of "oc-seq" and SHOULD set the value of the "oc" parameter to

      Note that the loss-based overload control scheme (Section 7) can
      effectively stop overload control by setting the value of the "oc"
      parameter to 0.  However, the rate-based scheme [RATE-CONTROL]
      needs an additional piece of information in the form of "oc-

   When the client receives a response with a higher "oc-seq" number
   than the one it most recently processed, it checks the "oc-validity"
   parameter.  If the value of the "oc-validity" parameter is 0, this
   indicates to the client that overload control of messages destined to

   the server is no longer necessary and the traffic can flow without
   any reduction.  Furthermore, when the value of the "oc-validity"
   parameter is 0, the client SHOULD disregard the value in the "oc"

5.8.  Stabilizing Overload Algorithm Selection

   Realities of deployments of SIP necessitate that the overload control
   algorithm may be changed upon a system reboot or a software upgrade.
   However, frequent changes of the overload control algorithm must be
   avoided.  Frequent changes of the overload control algorithm will not
   benefit the client or the server as such flapping does not allow the
   chosen algorithm to stabilize.  An algorithm change, when desired, is
   simply accomplished by the SIP server choosing a new algorithm from
   the list in the client's "oc-algo" parameter and sending it back to
   the client in a response.

   The client associates a specific algorithm with each server it sends
   traffic to, and when the server changes the algorithm, the client
   must change its behavior accordingly.

   Once the server selects a specific overload control algorithm for a
   given client, the algorithm SHOULD NOT change the algorithm
   associated with that client for at least 3600 seconds (1 hour).  This
   period may involve one or more cycles of overload control being in
   effect and then being stopped depending on the traffic and resources
   at the server.

      Note: One way to accomplish this involves the server saving the
      time of the last algorithm change in a lookup table, indexed by
      the client's network identifiers.  The server only changes the
      "oc-algo" parameter when the time since the last change has
      surpassed 3600 seconds.

5.9.  Self-Limiting

   In some cases, a SIP client may not receive a response from a server
   after sending a request.  RFC 3261 [RFC3261] states:

      Note: 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 ..., the condition MUST be treated as a 503
      (Service Unavailable) status code.

   In the event of repeated timeouts or fatal transport errors, the SIP
   client MUST stop sending requests to this server.  The SIP client
   SHOULD periodically probe if the downstream server is alive using any

   mechanism at its disposal.  Clients should be conservative in their
   probing (e.g., using an exponential back-off) so that their liveness
   probes do not exacerbate an overload situation.  Once a SIP client
   has successfully received a normal response for a request sent to the
   downstream server, the SIP client can resume sending SIP requests.
   It should, of course, honor any overload control parameters it may
   receive in the initial, or later, responses.

5.10.  Responding to an Overload Indication

   A SIP client can receive overload control feedback indicating that it
   needs to reduce the traffic it sends to its downstream server.  The
   client 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 is capable of processing the extra load.  A client can also
   buffer requests in the hope that the overload condition will resolve
   quickly and the requests can still be forwarded in time.  In many
   cases, however, it will need to reject these requests with a "503
   (Service Unavailable)" response without the Retry-After header.

5.10.1.  Message Prioritization at the Hop before the Overloaded Server

   During an overload condition, a SIP client needs to prioritize
   requests and select those requests that need to be rejected or
   redirected.  This selection is largely a matter of local policy.  It
   is expected that a SIP client will follow local policy as long as the
   result in reduction of traffic is consistent with the overload
   algorithm in effect at that node.  Accordingly, the normative
   behavior in the next three paragraphs should be interpreted with the
   understanding that the SIP client will aim to preserve local policy
   to the fullest extent possible.

   A SIP client SHOULD honor the local policy for prioritizing SIP
   requests such as policies based on message type, e.g., INVITEs versus
   requests associated with existing sessions.

   A SIP client SHOULD honor the local policy for prioritizing SIP
   requests based on the content of the Resource-Priority header (RPH)
   [RFC4412].  Specific (namespace.value) RPH contents may indicate
   high-priority requests that should be preserved as much as possible
   during overload.  The RPH contents can also indicate a low-priority
   request that is eligible to be dropped during times of overload.

   A SIP client SHOULD honor the local policy for prioritizing SIP
   requests relating to emergency calls as identified by the SOS URN
   [RFC5031] indicating an emergency request.  This policy ensures that

   when a server is overloaded and non-emergency calls outnumber
   emergency calls in the traffic arriving at the client, the few
   emergency calls will be given preference.  If, on the other hand, the
   server is overloaded and the majority of calls arriving at the client
   are emergency in nature, then no amount of message prioritization
   will ensure the delivery of all emergency calls if the client is to
   reduce the amount of traffic as requested by the server.

   A local policy can be expected to combine both the SIP request type
   and the prioritization markings, and it SHOULD be honored when
   overload conditions prevail.

5.10.2.  Rejecting Requests at an Overloaded Server

   If the upstream SIP client to the overloaded server does not support
   overload control, it will continue to direct requests to the
   overloaded server.  Thus, for the non-participating client, the
   overloaded server must bear the cost of rejecting some requests from
   the client as well as the cost of processing the non-rejected
   requests to completion.  It would be fair to devote the same amount
   of processing at the overloaded server to the combination of
   rejection and processing from a non-participating client as the
   overloaded server would devote to processing requests from a
   participating client.  This is to ensure that SIP clients that do not
   support this specification don't receive an unfair advantage over
   those that do.

   A SIP server that is in overload and has started to throttle incoming
   traffic MUST reject some requests from non-participating clients with
   a 503 (Service Unavailable) response without the Retry-After header.

5.11.  100 Trying Provisional Response and Overload Control Parameters

   The overload control information sent from a SIP server to a client
   is transported in the responses.  While implementations can insert
   overload control information in any response, special attention
   should be accorded to overload control information transported in a
   100 Trying response.

   Traditionally, the 100 Trying response has been used in SIP to quench
   retransmissions.  In some implementations, the 100 Trying message may
   not be generated by the transaction user (TU) nor consumed by the TU.
   In these implementations, the 100 Trying response is generated at the
   transaction layer and sent to the upstream SIP client.  At the
   receiving SIP client, the 100 Trying is consumed at the transaction
   layer by inhibiting the retransmission of the corresponding request.
   Consequently, implementations that insert overload control
   information in the 100 Trying cannot assume that the upstream SIP

   client passed the overload control information in the 100 Trying to
   their corresponding TU.  For this reason, implementations that insert
   overload control information in the 100 Trying MUST re-insert the
   same (or updated) overload control information in the first non-100
   Trying response being sent to the upstream SIP client.

6.  Example

   Consider a SIP client, P1, which is sending requests to another
   downstream SIP server, P2.  The following snippets of SIP messages
   demonstrate how the overload control parameters work.

           INVITE sips:user@example.com SIP/2.0
           Via: SIP/2.0/TLS p1.example.net;

           SIP/2.0 100 Trying
           Via: SIP/2.0/TLS p1.example.net;

   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 an algorithm
   called "A".

   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 and chooses the loss-based
   scheme, it sends "loss" back to P1 in the "oc-algo" parameter.  It
   also sets the value of the "oc" and "oc-validity" parameters to 0
   because it is not currently requesting overload control activation.

   Had P2 not supported overload control, it would have left the "oc"
   and "oc-algo" parameters unchanged, thus allowing the client to know
   that it did not support overload control.

   At some later time, P2 starts to experience overload.  It sends the
   following SIP message indicating that P1 should decrease the messages
   arriving to P2 by 20% for 0.5 seconds.

          SIP/2.0 180 Ringing
          Via: SIP/2.0/TLS p1.example.net;
   After some time, the overload condition at P2 subsides.  It then
   changes the parameter values in the response it sends to P1 to allow
   P1 to send all messages destined to P2.

          SIP/2.0 183 Queued
          Via: SIP/2.0/TLS p1.example.net;

7.  The Loss-Based Overload Control Scheme

   Under a loss-based approach, a SIP server asks an upstream neighbor
   to reduce the number of requests it would normally forward to this
   server by a certain percentage.  For example, a SIP server can ask an
   upstream neighbor to reduce the number of requests this neighbor
   would normally send by 10%.  The upstream neighbor then redirects or
   rejects 10% of the traffic originally destined for that server.

   This section specifies the semantics of the overload control
   parameters associated with the loss-based overload control scheme.
   The general behavior of SIP clients and servers is specified in
   Section 5 and is applicable to SIP clients and servers that implement
   loss-based overload control.

7.1.  Special Parameter Values for Loss-Based Overload Control

   The loss-based overload control scheme is identified using the token
   "loss".  This token appears in the "oc-algo" parameter list sent by
   the SIP client.

   Upon entering the overload state, a SIP server that has selected the
   loss-based algorithm will assign a value to the "oc" parameter.  This
   value MUST be in the range of [0, 100], inclusive.  This value
   indicates to the client the percentage by which the client is to
   reduce the number of requests being forwarded to the overloaded
   server.  The SIP client may use any algorithm that reduces the
   traffic it sends to the overloaded server by the amount indicated.

   Such an algorithm should honor the message prioritization discussion
   in Section 5.10.1.  While a particular algorithm is not subject to
   standardization, for completeness, a default algorithm for loss-based
   overload control is provided in Section 7.2.

7.2.  Default Algorithm for Loss-Based Overload Control

   This section describes a default algorithm that a SIP client can use
   to throttle SIP traffic going downstream by the percentage loss value
   specified in the "oc" parameter.

   The client maintains two categories of requests.  The first category
   will include requests that are candidates for reduction, and the
   second category will include requests that are not subject to
   reduction except when all messages in the first category have been
   rejected and further reduction is still needed.  Section 5.10.1
   contains directives on identifying messages for inclusion in the
   second category.  The remaining messages are allocated to the first

   Under overload condition, the client converts the value of the "oc"
   parameter to a value that it applies to requests in the first
   category.  As a simple example, if "oc=10" and 40% of the requests
   should be included in the first category, then:

      10 / 40 * 100 = 25

   Or, 25% of the requests in the first category can be reduced to get
   an overall reduction of 10%.  The client uses random discard to
   achieve the 25% reduction of messages in the first category.
   Messages in the second category proceed downstream unscathed.  To
   affect the 25% reduction rate from the first category, the client
   draws a random number between 1 and 100 for the request picked from
   the first category.  If the random number is less than or equal to
   the converted value of the "oc" parameter, the request is not
   forwarded; otherwise, the request is forwarded.

   A reference algorithm is shown below.

cat1 := 80.0         // Category 1 -- Subject to reduction
cat2 := 100.0 - cat1 // Category 2 -- Under normal operations,
// only subject to reduction after category 1 is exhausted.
// Note that the above ratio is simply a reasonable default.
// The actual values will change through periodic sampling
// as the traffic mix changes over time.

while (true) {
  // We're modeling message processing as a single work
  // queue that contains both incoming and outgoing messages.
  sip_msg := get_next_message_from_work_queue()

  update_mix(cat1, cat2)  // See Note below

  switch (sip_msg.type) {

    case outbound request:
      destination := get_next_hop(sip_msg)
      oc_context := get_oc_context(destination)

      if (oc_context == null)  {
          send_to_network(sip_msg) // Process it normally by
          // sending the request to the next hop since this
          // particular destination is not subject to overload.
      else  {
         // Determine if server wants to enter in overload or is in
         // overload.
         in_oc := extract_in_oc(oc_context)
         oc_value := extract_oc(oc_context)
         oc_validity := extract_oc_validity(oc_context)

         if (in_oc == false or oc_validity is not in effect)  {
            send_to_network(sip_msg) // Process it normally by sending
            // the request to the next hop since this particular
            // destination is not subject to overload.  Optionally,
            // clear the oc context for this server (not shown).
         else  {  // Begin performing overload control.
            r := random()
            drop_msg := false

            category := assign_msg_to_category(sip_msg)

            pct_to_reduce_cat1 = oc_value / cat1 * 100

            if (oc_value <= cat1)  {  // Reduce all msgs from category 1
                if (r <= pct_to_reduce_cat1 && category == cat1)  {
                   drop_msg := true
            else  { // oc_value > category 1.  Reduce 100% of msgs from
                    // category 1 and remaining from category 2.
               pct_to_reduce_cat2 = (oc_value - cat1) / cat2 * 100
               if (category == cat1)  {
                  drop_msg := true
               else  {
                  if (r <= pct_to_reduce_cat2)  {
                      drop_msg := true;

            if (drop_msg == false) {
                send_to_network(sip_msg) // Process it normally by
               // sending the request to the next hop.
            else  {
               // Do not send request downstream; handle it locally by
               // generating response (if a proxy) or treating it as
               // an error (if a user agent).

         }  // End perform overload control.

    end case // outbound request

    case outbound response:
      if (we are in overload) {

    end case // outbound response

    case inbound response:

       if (sip_msg has oc parameter values)  {
           create_or_update_oc_context()  // For the specific server
           // that sent the response, create or update the oc context,
           // i.e., extract the values of the oc-related parameters
           // and store them for later use.


    end case // inbound response
    case inbound request:

      if (we are not in overload)  {
      else {  // We are in overload.
         if (sip_msg has oc parameters)  {  // Upstream client supports
            process_msg(sip_msg)  // oc; only sends important requests.
         else {  // Upstream client does not support oc
            if (local_policy(sip_msg) says process message)  {
            else  {
               send_response(sip_msg, 503)
    end case // inbound request

   Note: A simple way to sample the traffic mix for category 1 and
   category 2 is to associate a counter with each category of message.
   Periodically (every 5-10 seconds), get the value of the counters, and
   calculate the ratio of category 1 messages to category 2 messages
   since the last calculation.

   Example: In the last 5 seconds, a total of 500 requests arrived at
   the queue.  450 out of the 500 were messages subject to reduction,
   and 50 out of 500 were classified as requests not subject to
   reduction.  Based on this ratio, cat1 := 90 and cat2 := 10, so a
   90/10 mix will be used in overload calculations.

8.  Relationship with Other IETF SIP Load Control Efforts

   The overload control mechanism described in this document is reactive
   in nature, and apart from the message prioritization directives
   listed in Section 5.10.1, the mechanisms described in this document
   will not discriminate requests based on user identity, filtering
   action, and arrival time.  SIP networks that require pro-active
   overload control mechanisms can upload user-level load control
   filters as described in [RFC7200].  Local policy will also dictate
   the precedence of different overload control mechanisms applied to

   the traffic.  Specifically, in a scenario where load control filters
   are installed by signaling neighbors [RFC7200] and the same traffic
   can also be throttled using the overload control mechanism, local
   policy will dictate which of these schemes shall be given precedence.
   Interactions between the two schemes are out of the scope of this

9.  Syntax

   This specification extends the existing definition of the Via header
   field parameters of [RFC3261].  The ABNF [RFC5234] syntax is as

       via-params  =/ oc / oc-validity / oc-seq / oc-algo
       oc          = "oc" [EQUAL oc-num]
       oc-num      = 1*DIGIT
       oc-validity = "oc-validity" [EQUAL delta-ms]
       oc-seq      = "oc-seq" EQUAL 1*12DIGIT "." 1*5DIGIT
       oc-algo     = "oc-algo" EQUAL DQUOTE algo-list *(COMMA algo-list)
       algo-list   = "loss" / *(other-algo)
       other-algo  = %x41-5A / %x61-7A / %x30-39
       delta-ms    = 1*DIGIT

10.  Design Considerations

   This section discusses specific design considerations for the
   mechanism described in this document.  General design considerations
   for SIP overload control can be found in [RFC6357].

10.1.  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.

10.1.1.  SIP Response Header

   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
   provide overload control feedback to its upstream neighbors.  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.  It also enables the use of overload
      control mechanisms that use regular feedback, such as window-based
      overload control.

   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 and manage the set of current upstream or
      downstream neighbors with which it should exchange overload

   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.

10.1.2.  SIP Event Package

   Overload control information can also be conveyed from a receiver to
   a sender using a new event package.  Such an 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.  This approach has the following

   o  Overload control information is conveyed decoupled from SIP
      signaling.  It enables an overload control manager, which is a
      separate entity, to monitor the load on other servers and provide
      overload control feedback to all SIP servers that have set up
      subscriptions with the 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 requests will pass through.

   o  As overload feedback is sent to all senders in separate messages,
      this mechanism is not suitable when frequent overload control
      feedback is needed.

   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 user agent
      functionality (UAS and UAC) to manage the subscriptions.

10.2.  Backwards Compatibility

   A new overload control mechanism needs to be backwards compatible so
   that it can be gradually introduced into a network and function
   properly if only a fraction of the servers support it.

   Hop-by-hop overload control (see [RFC6357]) has the advantage that it
   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 follow the behavior outlined in
   Section 5.10.2 to handle clients that do not support overload

11.  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 client, it will stop
   sending traffic to the victim.  Thus, the victim is subject to a
   denial-of-service attack.

   To better understand the threat model, consider the following

         Pa -------                    ------ Pb
                   \                  /
         :  ------ +-------- P1 ------+------ :
                   /    L1        L2  \
         :  -------                    ------ :

         -----> Downstream (requests)
         <----- Upstream (responses)

   Here, requests travel downstream from the left-hand side, through
   Proxy P1, towards the right-hand side; responses travel upstream from
   the right-hand side, through P1, towards the left-hand side.  Proxies
   Pa, Pb, and P1 support overload control.  L1 and L2 are labels for
   the links connecting P1 to the upstream clients and downstream

   If an attacker is able to modify traffic between Pa and P1 on link
   L1, it can cause a denial-of-service attack on P1 by having Pa not
   send any traffic to P1.  Such an attack can proceed by the attacker
   modifying the response from P1 to Pa such that Pa's Via header is
   changed to indicate that all requests destined towards P1 should be
   dropped.  Conversely, the attacker can simply remove any "oc", "oc-
   validity", and "oc-seq" markings added by P1 in a response to Pa.  In

   such a case, the attacker will force P1 into overload by denying
   request quenching at Pa even though Pa is capable of performing
   overload control.

   Similarly, if an attacker is able to modify traffic between P1 and Pb
   on link L2, it can change the Via header associated with P1 in a
   response from Pb to P1 such that all subsequent requests destined
   towards Pb from P1 are dropped.  In essence, the attacker mounts a
   denial-of-service attack on Pb by indicating false overload control.
   Note that it is immaterial whether Pb supports overload control or
   not; the attack will succeed as long as the attacker is able to
   control L2.  Conversely, an attacker can suppress a genuine overload
   condition at Pb by simply removing any "oc", "oc-validity", and "oc-
   seq" markings added by Pb in a response to P1.  In such a case, the
   attacker will force P1 into sending requests to Pb even under
   overload conditions because P1 would not be aware that Pb supports
   overload control.

   Attacks that indicate false overload control are best mitigated by
   using TLS in conjunction with applying BCP 38 [RFC2827].  Attacks
   that are mounted to suppress genuine overload conditions can be
   similarly avoided by using TLS on the connection.  Generally, TCP or
   WebSockets [RFC6455] in conjunction with BCP 38 makes it more
   difficult for an attacker to insert or modify messages but may still
   prove inadequate against an adversary that controls links L1 and L2.
   TLS provides the best protection from an attacker with access to the
   network links.

   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 header
   (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 that correctly implement
   overload control, the malicious SIP entity would benefit since all
   other sources to its downstream neighbor would reduce load.

      Note: The solution to this problem depends on the overload control
      method.  With rate-based, window-based, and other similar overload
      control algorithms that promise to produce no more than a
      specified number of requests per unit time, the overloaded server
      can regulate the traffic arriving to it.  However, when using
      loss-based overload control, such policing is not always obvious
      since the load forwarded depends on the load received by the

   To prevent such attacks, servers should monitor client behavior to
   determine whether they are complying with overload control policies.
   If a client is not conforming to such policies, then the server
   should treat it as a non-supporting client (see Section 5.10.2).

   Finally, a distributed denial-of-service (DDoS) attack could cause an
   honest server to start signaling an overload condition.  Such a DDoS
   attack could be mounted without controlling the communications links
   since the attack simply depends on the attacker injecting a large
   volume of packets on the communication links.  If the honest server
   attacked by a DDoS attack has a long "oc-validity" interval and the
   attacker can guess this interval, the attacker can keep the server
   overloaded by synchronizing the DDoS traffic with the validity
   period.  While such an attack may be relatively easy to spot,
   mechanisms for combating it are outside the scope of this document
   and, of course, since attackers can invent new variations, the
   appropriate mechanisms are likely to change over time.

12.  IANA Considerations

   This specification defines four new Via header parameters as detailed
   below in the "Header Field Parameter and Parameter Values" sub-
   registry as per the registry created by [RFC3968].  The required
   information is:

       Header Field  Parameter Name  Predefined Values  Reference
       Via           oc                 Yes             [RFC7339]
       Via           oc-validity        Yes             [RFC7339]
       Via           oc-seq             Yes             [RFC7339]
       Via           oc-algo            Yes             [RFC7339]

13.  References

13.1.  Normative References

   [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

   [RFC3968]  Camarillo, G., "The Internet Assigned Number Authority
              (IANA) Header Field Parameter Registry for the Session
              Initiation Protocol (SIP)", BCP 98, RFC 3968, December

   [RFC4412]  Schulzrinne, H. and J. Polk, "Communications Resource
              Priority for the Session Initiation Protocol (SIP)", RFC
              4412, February 2006.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

13.2.  Informative References

              Noel, E. and P. Williams, "Session Initiation Protocol
              (SIP) Rate Control", Work in Progress, July 2014.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

   [RFC5031]  Schulzrinne, H., "A Uniform Resource Name (URN) for
              Emergency and Other Well-Known Services", RFC 5031,
              January 2008.

   [RFC5390]  Rosenberg, J., "Requirements for Management of Overload in
              the Session Initiation Protocol", RFC 5390, December 2008.

   [RFC6357]  Hilt, V., Noel, E., Shen, C., and A. Abdelal, "Design
              Considerations for Session Initiation Protocol (SIP)
              Overload Control", RFC 6357, August 2011.

   [RFC6455]  Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC
              6455, December 2011.

   [RFC7200]  Shen, C., Schulzrinne, H., and A. Koike, "A Session
              Initiation Protocol (SIP) Load-Control Event Package", RFC
              7200, April 2014.

Appendix A.  Acknowledgements

   The authors acknowledge the contributions of Bruno Chatras, Keith
   Drage, Janet Gunn, Rich Terpstra, Daryl Malas, Eric Noel, R.
   Parthasarathi, Antoine Roly, Jonathan Rosenberg, Charles Shen, Rahul
   Srivastava, Padma Valluri, Shaun Bharrat, Paul Kyzivat, and Jeroen
   Van Bemmel to this document.

   Adam Roach and Eric McMurry helped flesh out the different cases for
   handling SIP messages described in the algorithm in Section 7.2.
   Janet Gunn reviewed the algorithm and suggested changes that led to
   simpler processing for the case where "oc_value > cat1".

   Richard Barnes provided invaluable comments as a part of the Area
   Director review of the document.

Appendix B.  RFC 5390 Requirements

   Table 1 provides a summary of how this specification fulfills the
   requirements of [RFC5390].  A more detailed view on how each
   requirements is fulfilled is provided after the table.

                    | Requirement | Meets requirement |
                    | REQ 1       | Yes               |
                    | REQ 2       | Yes               |
                    | REQ 3       | Partially         |
                    | REQ 4       | Yes               |
                    | REQ 5       | Partially         |
                    | REQ 6       | Not applicable    |
                    | REQ 7       | Yes               |
                    | REQ 8       | Partially         |
                    | REQ 9       | Yes               |
                    | REQ 10      | Yes               |
                    | REQ 11      | Yes               |
                    | REQ 12      | Yes               |
                    | REQ 13      | Yes               |
                    | REQ 14      | Yes               |
                    | REQ 15      | Yes               |
                    | REQ 16      | Yes               |
                    | REQ 17      | Partially         |
                    | REQ 18      | Yes               |
                    | REQ 19      | Yes               |
                    | REQ 20      | Yes               |
                    | REQ 21      | Yes               |
                    | REQ 22      | Yes               |
                    | REQ 23      | Yes               |

           Table 1: Summary of Meeting Requirements in RFC 5390

   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 mechanism.

      Meets REQ 1: Yes.  The overload control mechanism allows an
      overloaded SIP server to maintain a reasonable level of throughput
      as it enters into congestion mode by requesting the upstream
      clients to reduce traffic destined downstream.

   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

      Meets REQ 2: Yes.  When a SIP server enters overload mode, it will
      request the upstream clients to throttle the traffic destined to
      it.  As a consequence of this, the overloaded SIP server will
      itself generate proportionally less downstream traffic, thereby
      limiting the impact on other elements in the network.

   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.

      Meets REQ 3: Partially.  On the server side, the overload
      condition is determined monitoring "S" (cf., Section 4 of
      [RFC6357]) and reporting a load feedback "F" as a value to the
      "oc" parameter.  On the client side, a throttle "T" is applied to
      requests going downstream based on "F".  This specification does
      not prescribe any value for "S" nor a particular value for "F".
      The "oc-algo" parameter allows for automatic convergence to a
      particular class of overload control algorithm.  There are
      suggested default values for the "oc-validity" parameter.

   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 mechanism.

      Meets REQ 4: Yes.  The mechanism is designed to reduce congestion
      when a pair of communicating entities support it.  If a downstream
      overloaded SIP server does not respond to a request in time, a SIP
      client will attempt to reduce traffic destined towards the non-
      responsive server as outlined in Section 5.9.

   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.

      Meets REQ 5: Partially.  Since overload control information is
      shared between a pair of communicating entities, a confidential
      and authenticated channel can be used for this communication.
      However, if such a channel is not available, then the security
      ramifications outlined in Section 11 apply.

   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.

      Meets REQ 6: Not applicable.  Overload control information is
      signaled as part of the Via header and not in a new header.

   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 overload.

      Meets REQ 7: Yes.  Please see Sections 5.5 and 5.10.

   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.

      Meets REQ 8: Partially.  A SIP client that has overload
      information from multiple downstream servers will not retry the
      request on another element.  However, if a SIP client does not
      know the overload status of a downstream server, it may send the
      request to that server.

   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.

      Meets REQ 9: Yes.  A SIP client conformant to this specification
      will send the request to a different element.

   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.

      Meets REQ 10: Yes.  There are no constraints on the number of
      upstream clients.

   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.

      Meets REQ 11: Yes.  There are no constraints on the number of
      upstream clients.

   REQ 12: The mechanism should work between servers in different

      Meets REQ 12: Yes.  There are no inherent limitations on using
      overload control between domains.  However, interconnections
      points that engage in overload control between domains will have
      to populate and maintain the overload control parameters as
      requests cross domains.

   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.

      Meets REQ 13: Yes.  Please see Section 5.10.

   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 an avalanche restart.

      Meets REQ 14: Yes.  Section 5.9 provides normative behavior on
      when to retry a request after repeated timeouts and fatal
      transport errors resulting from communications with a non-
      responsive downstream SIP server.

   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.

      Meets REQ 15: Yes.  Section 5.9 provides normative behavior on
      when to retry a request after repeated timeouts and fatal
      transport errors resulting from communications with a non-
      responsive downstream SIP server.

   REQ 16: The mechanism should attempt to minimize the overhead of the
   overload control messaging.

      Meets REQ 16: Yes.  Overload control messages are sent in the
      topmost Via header, which is always processed by the SIP elements.

   REQ 17: The overload mechanism must not provide an avenue for
   malicious attack, including DoS and DDoS attacks.

      Meets REQ 17: Partially.  Since overload control information is
      shared between a pair of communicating entities, a confidential
      and authenticated channel can be used for this communication.
      However, if such a channel is not available, then the security
      ramifications outlined in Section 11 apply.

   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.

      Meets REQ 18: Yes.  Please see discussion in Section 5.5.

   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.

      Meets REQ 19: Yes.  Please see Section 5.10.

   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.

      Meets REQ 20: Yes.  An element that does not implement overload
      control does not receive any measure of extra benefit.

   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.

      Meets REQ 21: Yes.  The overload control mechanism described in
      this document ensures the stability of the system.

   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.

      Meets REQ 22: Yes.  An operator of a SIP server can configure the
      SIP server to only report overload control information for
      requests received over a confidential channel, for example.
      However, note that this requirement is in conflict with REQ 3 as
      it introduces a modicum of extra configuration.

   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 proxies.

      Meets REQ 23: Yes.  Depending on the type of load balancer, this
      requirement is met.  A load balancer fronting a farm of SIP
      proxies could be a SIP-aware load balancer or one that is not SIP-
      aware.  If the load balancer is SIP-aware, it can make conscious
      decisions on throttling outgoing traffic towards the individual
      server in the farm based on the overload control parameters
      returned by the server.  On the other hand, if the load balancer
      is not SIP-aware, then there are other strategies to perform
      overload control.  Section 6 of [RFC6357] documents some of these
      strategies in more detail (see discussion related to Figure 3(a)
      of that document).

Authors' Addresses

   Vijay K. Gurbani (editor)
   Bell Labs, Alcatel-Lucent
   1960 Lucent Lane, Rm 9C-533
   Naperville, IL  60563

   EMail: vkg@bell-labs.com

   Volker Hilt
   Bell Labs, Alcatel-Lucent
   Lorenzstrasse 10
   70435 Stuttgart

   EMail: volker.hilt@bell-labs.com

   Henning Schulzrinne
   Columbia University/Department of Computer Science
   450 Computer Science Building
   New York, NY  10027

   Phone: +1 212 939 7004
   EMail: hgs@cs.columbia.edu
   URI:   http://www.cs.columbia.edu


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