SOC Working Group                                        V. Gurbani, Ed.
Internet-Draft                         Bell Laboratories, Alcatel-Lucent
Intended status: Standards Track                                 V. Hilt
Expires: February 24, 2011                      Bell Labs/Alcatel-Lucent
                                                          H. Schulzrinne
                                                     Columbia University
                                                         August 23, 2010

           Session Initiation Protocol (SIP) Overload Control


   Overload occurs in Session Initiation Protocol (SIP) networks when
   SIP servers have insufficient resources to handle all SIP messages
   they receive.  Even though the SIP protocol provides a limited
   overload control mechanism through its 503 (Service Unavailable)
   response code, SIP servers are still vulnerable to overload.  This
   document defines an overload control mechanism for SIP.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   described in the BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Overview of operations . . . . . . . . . . . . . . . . . . . .  4
   4.  Via Header Parameters for Overload Control . . . . . . . . . .  5
     4.1.  The 'oc' Parameter . . . . . . . . . . . . . . . . . . . .  5
     4.2.  Creating the Overload Control Parameters . . . . . . . . .  5
     4.3.  Determining the 'oc' Parameter Value . . . . . . . . . . .  7
     4.4.  Processing the Overload Control Parameters . . . . . . . .  7
     4.5.  Using the Overload Control Parameter Values  . . . . . . .  8
     4.6.  Forwarding the overload control parameters . . . . . . . .  9
     4.7.  Self-Limiting  . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Responding to an Overload Indication . . . . . . . . . . . . .  9
     5.1.  Message prioritization at the hop before the
           overloaded server  . . . . . . . . . . . . . . . . . . . . 10
     5.2.  Rejecting requests at an overloaded server . . . . . . . . 10
   6.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  Design Considerations  . . . . . . . . . . . . . . . . . . . . 12
     7.1.  SIP Mechanism  . . . . . . . . . . . . . . . . . . . . . . 12
       7.1.1.  SIP Response Header  . . . . . . . . . . . . . . . . . 12
       7.1.2.  SIP Event Package  . . . . . . . . . . . . . . . . . . 13
     7.2.  Backwards Compatibility  . . . . . . . . . . . . . . . . . 14
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     10.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16

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1.  Introduction

   As with any network element, a Session Initiation Protocol (SIP)
   [RFC3261] server can suffer from overload when the number of SIP
   messages it receives exceeds the number of messages it can process.
   Overload can pose a serious problem for a network of SIP servers.
   During periods of overload, the throughput of a network of SIP
   servers can be significantly degraded.  In fact, overload may lead to
   a situation in which the throughput drops down to a small fraction of
   the original processing capacity.  This is often called congestion

   Overload is said to occur if a SIP server does not have sufficient
   resources to process all incoming SIP messages.  These resources may
   include CPU processing capacity, memory, network bandwidth, input/
   output, or disk resources.

   For overload control, we only consider failure cases where SIP
   servers are unable to process all SIP requests due to resource
   constraints.  There are other cases where a SIP server can
   successfully process incoming requests but has to reject them due to
   failure conditions unrelated to the SIP server being overloaded.  For
   example, a PSTN gateway that runs out of trunk lines but still has
   plenty of capacity to process SIP messages should reject incoming
   INVITEs using a 488 (Not Acceptable Here) response [RFC4412].
   Similarly, a SIP registrar that has lost connectivity to its
   registration database but is still capable of processing SIP 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 to
   shift between SIP servers and thereby worsen an overload condition.
   A detailed discussion of the SIP overload problem, the problems with
   the 503 (Service Unavailable) response code and the requirements for
   a SIP overload control mechanism can be found in [RFC5390].

2.  Terminology

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

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3.  Overview of operations

   We now explain the 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

   Because overload control is best performed hop-by-hop, the Via
   parameter is attractive since it allows two adjacent SIP entities to
   indicate support for, and exchange information associated with
   overload control.  This document defines three new parameters for the
   SIP Via header for overload control.  These parameters provide a SIP
   mechanism for conveying overload control information between adjacent
   SIP entities.)  These parameters are:

   1.  oc: This parameter serves a dual purpose; when inserted by a SIP
       entity in a request going downstream, the parameter indicates
       that the SIP entity supports overload control.  When the
       downstream SIP server sends a response, the downstream SIP server
       will add a value to the parameter that indicates a loss rate (in
       percentage) by which the requests arriving at the downstream SIP
       server should be reduced. (c.f.  Section 4.2, Section 4.3,
       Section 4.4 and Section 4.5.)
   2.  oc-validity: Inserted by the SIP entity sending a response
       upstream.  This parameter contains a value that indicates the
       time (in ms) that the load reduction specified by the "oc"
       parameter should be in effect (c.f.  Section 4.2.)
   3.  oc-seq: Inserted by the SIP entity sending a response upstream.
       This is an optional parameter.  This parameter contains a value
       that indicates the sequence number associated with the "oc"
       parameter defined above (c.f.  Section Section 4.2).

   Consider a SIP entity, 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 SIP/2.0
          Via: SIP/2.0/TLS;

          SIP/2.0 100 Trying
          Via: SIP/2.0/TLS;

   In the messages above, the first line is sent by P1 to P2.  This line

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   is a SIP request; because P1 supports overload control, it inserts
   the "oc" parameter in the topmost Via header that it created.

   The second line --- a SIP response --- shows the topmost Via header
   amended by P2 according to this specification and sent to P1.
   Because P2 also supports overload control, it sends back further
   overload control parameters towards P1 requesting that P1 reduce the
   incoming traffic by 20% for 500ms.  P2 updates the "oc" parameter to
   add a value and inserts the remaining two parameters, "oc-validity"
   and "oc-seq".

4.  Via Header Parameters for Overload Control

4.1.  The 'oc' Parameter

   A SIP entity that supports this specification MUST add an "oc"
   parameter to the Via headers it inserts into SIP requests.  This
   provides an indication to downstream neighbors that this server
   supports overload control.  When inserted into a request by a SIP
   entity to indicate support for overload control, there MUST NOT be a
   value associated with the parameter.

4.2.  Creating the Overload Control Parameters

   A SIP server can provide overload control feedback to its upstream
   neighbors by providing a value for the "oc" parameter to the topmost
   Via header field of a SIP response.  The topmost Via header is
   determined after the SIP server has removed its own Via header; i.e.,
   it is the Via header that was generated by the upstream neighbor.

   Since the topmost Via header of a response will be removed by an
   upstream neighbor after processing it, overload control feedback
   contained in the "oc" parameter will not travel beyond the upstream
   SIP entity.  A Via header parameter therefore provides hop-by-hop
   semantics for overload control feedback (see
   [I-D.ietf-soc-overload-design]) 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.  A SIP server MAY update
   the "oc" parameter to all responses it is sending.  A SIP server MUST
   update the "oc" parameter to responses when the transmission of
   overload control feedback is required by the overload control
   algorithm to limit the traffic received by the server.  I.e., a SIP
   server MUST update the "oc" parameter when the overload control
   algorithm sets the value of an "oc" parameter to a value different
   than the default value.

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   A SIP server that has updated the "oc" parameter to Via header SHOULD
   also add a "oc_validity" parameter to the same Via header.  The
   "oc_validity" parameter defines the time in milliseconds during which
   the content (i.e., the overload control feedback) of the "oc"
   parameter is valid.  The default value of the "oc_validity" parameter
   is 500 (millisecond).  A SIP server SHOULD use a shorter
   "oc_validity" time if its overload status varies quickly and MAY use
   a longer "oc_validity" time if this status is more stable.  If the
   "oc_validity" parameter is not present, its default value is used.
   The "oc_validity" parameter MUST NOT be used in a Via header that did
   not originally contain an "oc" parameter when received.  Furthermore,
   when a SIP server receives a request with the topmost Via header
   containing only an "oc-validity" parameter without the accompanying
   "oc" parameter. it MUST ignore the "oc-validity" parameter.

   When a SIP server retransmits a response, it SHOULD use the "oc"
   parameter value and "oc-validity" parameter value consistent with the
   overload state at the time the retransmitted response is sent.  This
   implies that the values in the "oc" and "oc-validity" parameters may
   be different then 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 for 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 for 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

   The "oc", "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
   this is the entity that can offload traffic by redirecting/rejecting
   new requests.  If requests are forwarded in both directions between
   two SIP servers (i.e., the roles of upstream/downstream neighbors
   change), there are also responses flowing in both directions.  Thus,
   both SIP servers can exchange overload information.

   While adding "oc" and "oc_validity" parameters to requests may

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   increase the frequency with which overload information is exchanged
   in these scenarios, this increase will rarely provide benefits and
   does not justify the added overhead and complexity needed.

   Since overload control protects a SIP server from overload, it is
   RECOMMENDED that a SIP server use the mechanisms described in this
   specification.  However, if a SIP server wanted to limit its overload
   control capability for privacy reasons, it MAY decide to perform
   overload control only for requests that are received on a secure
   transport channel, such as TLS.  This enables a SIP server to protect
   overload control information and ensure that it is only visible to
   trusted parties.

4.3.  Determining the 'oc' Parameter Value

   The value of the "oc" parameter is determined by an overload control
   algorithm (see [I-D.ietf-soc-overload-design]).  This specification
   does not mandate the use of a specific overload control algorithm.
   However, the output of an overload control algorithm MUST be
   compliant to the semantics of this Via header parameter.

   The "oc" parameter value specifies the percentage by which the load
   forwarded to this SIP server should be reduced.  Possible values
   range from 0 (the traffic forwarded is reduced by 0%, i.e., all
   traffic is forwarded) to 100 (the traffic forwarded is reduced by
   100%, i.e., no traffic forwarded).  The default value of this
   parameter is 0.

      OPEN ISSUE 1: The "oc" parameter value specified in this document
      is defined to contain a loss rate.  However, other types of
      overload control feedback exist, for example, a target rate for
      rate-based overload control or message confirmations and window-
      size for window-based overload control.

      While it would in theory be possible to allow multiple types of
      overload control feedback to co-exist (e.g., by using different
      parameters for the different feedback types) it is very
      problematic for interoperability purposes and would require SIP
      servers to implement multiple overload control mechanisms.

4.4.  Processing the Overload Control Parameters

   A SIP entity compliant to this specification SHOULD remove "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

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   A SIP entity maintains the "oc" parameter 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 entity receives a response
   with an "oc" parameter from a downstream SIP server, it overwrites
   the "oc" value it has currently stored for this server with the new
   value received.  The SIP entity restarts the validity period of an
   "oc" parameter each time a response with an "oc" parameter is
   received from this server.  A stored "oc" parameter value MUST be
   discarded once it has reached the end of its validity.

4.5.  Using the Overload Control Parameter Values

   A SIP entity compliant to this specification MUST honor overload
   control values it receives from downstream neighbors.  The SIP entity
   MUST NOT forward more requests to a SIP server than allowed by the
   current "oc" parameter value from a particular downstream server.

   When forwarding a SIP request, a SIP entity uses the SIP procedures
   of [RFC3263] to determine the next hop SIP server.  The procedures of
   [RFC3263] take as input a SIP URI, extract the domain portion of that
   URI for use as a lookup key, and 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, the
   SIP entity MUST determine if it already has overload control
   parameter values for the server chosen from the Expected Output.  If
   the SIP entity has a non-expired "oc" parameter value for the server
   chosen from the Expected Output, and this chosen server is operating
   in overload control mode.  Thus, the SIP entity MUST determine if it
   can or cannot forward the current request to the SIP server depending
   on the nature of the request and the prevailing overload conditions.

   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 generate the same number and rate of SIP requests
   as the default algorithm (to be determined), which treats all
   requests as equal.

   In the absence of a different local policy, the SIP entity SHOULD use
   the following default algorithm to determine if it can forward the
   request downstream (TODO: Need to devise an algorithm.  The original
   simple algorithm based on random number generation does not suffice
   for all cases.)

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4.6.  Forwarding the overload control parameters

   A SIP entity MAY forward the content of an "oc" parameter it has
   received from a downstream neighbor on to its upstream neighbor.
   However, forwarding the content of the "oc" parameter is generally
   NOT RECOMMENDED and should only be performed if permitted by the
   configuration of SIP servers.  For example, a SIP server that only
   relays messages between exactly two SIP servers may forward an "oc"
   parameter.  The "oc" parameter is forwarded by copying it from the
   Via in which it was received into the next Via header (i.e., the Via
   header that will be on top after processing the response).  If an
   "oc_validity" parameter is present, MUST be copied along with the
   "oc" parameter.

4.7.  Self-Limiting

   In some cases, a SIP entity may not receive a response from a
   downstream server after sending a request.  RFC3261 [RFC3261] defines
   that when a timeout error is received from the transaction layer, it
   MUST be treated as if a 408 (Request Timeout) status code has been
   received.  If a fatal transport error is reported by the transport
   layer, it MUST be treated as a 503 (Service Unavailable) status code.

   In these cases, the SIP entity MUST stop sending requests to this
   server.  The SIP entity SHOULD occasionally forward a single request
   to probe if the downstream server is alive.  Once a SIP entity has
   successfully transmitted a request to the downstream server, the SIP
   entity can resume normal traffic rates.  It should, of course, honor
   any "oc" parameters it may receive subsequent to resuming normal
   traffic rates.

      OPEN ISSUE 2: If a downstream neighbor does not respond to a
      request at all, the upstream SIP entity will stop sending requests
      to the downstream neighbor.  The upstream SIP entity will
      periodically forward a single request to probe the health of its
      downstream neighbor.  It has been suggested --- see http://
      --- that we have a notification mechanism in place for the
      downstream neighbor to signal to the upstream SIP entity that it
      is ready to receive requests.  This notification scheme has
      advantages, but comes with obvious disadvantages as well.  Need
      some more discussion around this.

5.  Responding to an Overload Indication

   A SIP entity can receive overload control feedback indicating that it
   needs to reduce the traffic it sends to its downstream server.  The

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   entity can accomplish this task by sending some of the requests that
   would have gone to the overloaded element to a different destination.
   It needs to ensure, however, that this destination is not in overload
   and capable of processing the extra load.  An entity can also buffer
   requests in the hope that the overload condition will resolve quickly
   and the requests still can be forwarded in time.  In many cases,
   however, it will need to reject these requests.

5.1.  Message prioritization at the hop before the overloaded server

   During an overload condition, a SIP entity needs to prioritize
   requests and select those requests that need to be rejected or
   redirected.  While this selection is largely a matter of local
   policy, certain heuristics can be suggested.  One, during overload
   control, the SIP entity should preserve existing dialogs as much as
   possible.  This suggests that mid-dialog requests MAY be given
   preferential treatment.  Similarly, requests that result in releasing
   resources (such as a BYE) MAY also be given preferential treatment.

   A SIP entity SHOULD honor the local policy for prioritizing SIP
   requests such as policies based on the content of the Resource-
   Priority header (RPH, RFC4412 [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.  Other indicators, such as the SOS URN
   [RFC5031] indicating an emergency request, may also be used for

   Local policy could also include giving precedence to mid- dialog SIP
   requests (re-INVITEs, UPDATEs, BYEs etc.) in times of overload.  A
   local policy can be expected to combine both the SIP request type and
   the prioritization markings, and SHOULD be honored when overload
   conditions prevail.

5.2.  Rejecting requests at an overloaded server

   If the upstream SIP entity to the overloaded server does not support
   overload control, it will continue to direct requests to the
   overloaded server.  Thus, the overloaded server must bear the cost of
   rejecting some session requests as well as the cost of processing
   other 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 as the overloaded server would devote to
   processing requests from an upstream SIP entity that supported
   overload control.  This is to ensure that SIP servers that do not
   support this specification don't receive an unfair advantage over
   those that do.

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   A SIP server that is under overload and has started to throttle
   incoming traffic MUST reject this request with a "503 (Service
   Unavailable)" response without Retry-After header to reject a
   fraction of requests from upstream neighbors that do not support
   overload control.

6.  Syntax

   This section defines the syntax of new Via header parameters: "oc",
   "oc_validity", and "oc-seq".

   The "oc" Via header parameter, when it has a value, MUST restrain
   that value to a number between 0 and 100.  This value describes the
   percentage by which the traffic (SIP requests) to the SIP server from
   which the response has been received should be reduced.  The default
   value for this parameter is 0.

   The "oc_validity" Via header parameter contains the time during which
   the corresponding "oc" Via header parameter is valid.  The
   "oc_validity" parameter can only be present in a Via header in
   conjunction with an "oc" parameter.

   The "oc-seq" Via header parameter contains a sequence number.  Those
   implementations that are capable of providing finer-grained overload
   control information may do so, however, each response that contains
   the updated overload control information MUST have an increasing
   value in this parameter.  This is to allow the upstream server to
   properly order out-of-order responses that contain overload control

   This specification extends the existing definition of the Via header
   field parameters of [RFC3261] as follows:

       via-params        =  via-ttl / via-maddr
                         / via-received / via-branch
                         / oc / oc-validity
                         / oc-seq / via-extension

      oc = "oc" [EQUAL 0-100]

      oc-validity = "oc_validity" [EQUAL delta-ms]

      oc-seq = (1*12DIGIT "." 1*5DIGIT)


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     Via: SIP/2.0/TCP

7.  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

7.1.  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.

7.1.1.  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

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      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.

7.1.2.  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 request 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.

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   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.

7.2.  Backwards Compatibility

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

   Hop-by-hop overload control (see [I-D.ietf-soc-overload-design]) 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 use 5xx responses towards upstream
   neighbors that do not support overload control.  The server should
   reject the same amount of requests with 5xx responses that would be
   otherwise be rejected/redirected by the upstream neighbor if it would
   support overload control.  If the load condition on the server does
   not permit the creation of 5xx responses, the server should drop all
   requests from servers that do not support overload control.

8.  Security Considerations

   Overload control mechanisms can be used by an attacker to conduct a
   denial-of-service attack on a SIP entity if the attacker can pretend
   that the SIP entity is overloaded.  When such a forged overload
   indication is received by an upstream SIP entity, it will stop
   sending traffic to the victim.  Thus, the victim is subject to a
   denial-of-service attack.

   An attacker can create forged overload feedback by inserting itself
   into the communication between the victim and its upstream neighbors.

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   The attacker would need to add overload feedback indicating a high
   load to the responses passed from the victim to its upstream
   neighbor.  Proxies can prevent this attack by communicating via TLS.
   Since overload feedback has no meaning beyond the next hop, there is
   no need to secure the communication over multiple hops.

   Another way to conduct an attack is to send a message containing a
   high overload feedback value through a proxy that does not support
   this extension.  If this feedback is added to the second Via headers
   (or all Via headers), it will reach the next upstream proxy.  If the
   attacker can make the recipient believe that the overload status was
   created by its direct downstream neighbor (and not by the attacker
   further downstream) the recipient stops sending traffic to the
   victim.  A precondition for this attack is that the victim proxy does
   not support this extension since it would not pass through overload
   control feedback otherwise.

   A malicious SIP entity could gain an advantage by pretending to
   support this specification but never reducing the amount of traffic
   it forwards to the downstream neighbor.  If its downstream neighbor
   receives traffic from multiple sources which correctly implement
   overload control, the malicious SIP entity would benefit since all
   other sources to its downstream neighbor would reduce load.

      The solution to this problem depends on the overload control
      method.  For rate-based and window-based overload control, it is
      very easy for a downstream entity to monitor if the upstream
      neighbor throttles traffic forwarded as directed.  For percentage
      throttling this is not always obvious since the load forwarded
      depends on the load received by the upstream neighbor.

9.  IANA Considerations


10.  References

10.1.  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.

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   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              June 2002.

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

10.2.  Informative References

              Hilt, V., Noel, E., Shen, C., and A. Abdelal, "Design
              Considerations for Session Initiation Protocol (SIP)
              Overload Control", draft-ietf-soc-overload-design-01 (work
              in progress), August 2010.

   [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.

Appendix A.  Acknowledgements

   Many thanks to Rich Terpstra, Daryl Malas, Jonathan Rosenberg,
   Charles Shen, Padma Valluri, Janet Gunn, Shaun Bharrat, and Paul
   Kyzivat for their contributions to this specification.

Authors' Addresses

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


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   Volker Hilt
   Bell Labs/Alcatel-Lucent
   791 Holmdel-Keyport Rd
   Holmdel, NJ  07733


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

   Phone: +1 212 939 7004

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