Session Initiation Protocol (SIP) Overload Control
DOCNAME

Abstract

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

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This Internet-Draft will expire on January 13, 2011.

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

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Table of Contents

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

1.  Introduction

As with any network element, a Session Initiation Protocol (SIP) [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) 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 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, 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] (Schulzrinne, H. and J. Polk, “Communications Resource Priority for the Session Initiation Protocol (SIP),” February 2006.). Similarly, a SIP registrar that has lost connectivity to its registration database but is still capable of processing SIP messages should reject REGISTER requests with a 500 (Server Error) response [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.). 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] (Rosenberg, J., “Requirements for Management of Overload in the Session Initiation Protocol,” December 2008.).

2.  Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

3.  Overview of operations

We now explain the overview of how the overload control mechanism operates by introducing the overload control parameters. Section 4 (Via Header Parameters for Overload Control) provides more details and normative behavior on the parameters listed below.

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 five 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-accept: Inserted by a SIP entity sending a request downstream. This parameter indicates that the SIP entity inserting (or removing) this parameter supports overload control as specified by this document (c.f. Section 4.1 (The 'oc_accept' Parameter). This parameter does not have an associated value.
2. oc: Inserted by the SIP entity sending a response upstream. This parameter contains a value that indicates a loss rate (in percentage) by which the load arriving at the SIP entity should be reduced (c.f. Section 4.2 (Creating the Overload Control Parameters), Section 4.3 (Determining the 'oc' Parameter Value), Section 4.4 (Processing the Overload Control Parameters) and Section 4.5 (Using the Overload Control Parameter Values).)
3. 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 (Creating the Overload Control Parameters).)
4. 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 (Creating the Overload Control Parameters)).
5. oc-port: Inserted by the SIP entity sending a response upstream. This is an optional parameter and does not have an associated value. This parameter indicates that overload control should be performed by the upstream SIP server only on subsequent requests destined to the same port that the earlier request carrying the "oc-accept" parameter was sent to. The absence of this parameter in the response indicates that overload control is to be applied to SIP traffic destined to the IP address irrespective of any specific port on that IP address (c.f. Section Section 4.2 (Creating the Overload Control Parameters).)

Consider a proxy P1 that is sending requests to another downstream proxy, 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;
         branch=z9hG4bK2d4790.1;received=192.0.2.111;oc-accept
       ...

       SIP/2.0 100 Trying
       Via: SIP/2.0/TLS p1.example.net;
         branch=z9hG4bK2d4790.1;received=192.0.2.111;
         oc=20;oc-validity=500;oc-seq=87165
       ...

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-accept" 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.

4.  Via Header Parameters for Overload Control

4.1.  The 'oc_accept' Parameter

A SIP server that supports this specification MUST add an "oc_accept" parameter to the Via headers it inserts into SIP requests. This provides an indication to downstream neighbors that this server supports overload control.

A SIP server MUST remove the 'oc_accept' parameter from the topmost Via header in a response before forwarding the response. The topmost Via header is determined after the SIP server has removed its own Via header. The topmost Via header corresponds to the upstream neighbor of the SIP server processing the response, and the 'oc-accept' parameter was added by the upstream neighbor when the request was proxied by the upstream neighbor.

Removing the 'oc-accept' parameter from the topmost Via header before forwarding the response upstream has the property that the upstream entity knows that the downstream SIP server is capable of supporting overload control as specified by this document. This is true even if the downstream SIP server is currently not experiencing an overload situation and therefore did not insert other overload related parameters.

4.2.  Creating the Overload Control Parameters

A SIP server can provide overload control feedback to its upstream neighbors by adding the 'oc' parameter to the topmost Via header field of a SIP response. The 'oc' parameter is a new Via header parameter defined in this specification. When an 'oc' parameter is added to a response, it MUST be inserted into the topmost Via header. It MUST NOT be added to any other Via header in the 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 server. A Via header parameter therefore provides hop-by-hop semantics for overload control feedback (see [I‑D.ietf‑soc‑overload‑design] (Hilt, V., Noel, E., Shen, C., and A. Abdelal, “Design Considerations for Session Initiation Protocol (SIP) Overload Control,” June 2010.)) 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 add the 'oc' parameter to all responses it is sending. A SIP server SHOULD add an 'oc' parameter to responses, that contain an 'oc_accept' parameter in the topmost Via header. A SIP server MUST add an '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 insert the 'oc' parameter when the overload control algorithm sets the 'oc' parameter to a value different than the default value.

A SIP server that has added an '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. 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 without an 'oc' parameter and MUST be ignored if it appears in a Via header without 'oc' 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. One way to achieve this is to use a monotonically increasing sequence number that is less than 2**31 and wraps to 0 when the maximum number is generated. Another way to generate the value of the 'oc-seq' parameter is to use the current timestamp with millisecond precision.

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 retransmissions.

The 'oc-port' parameter is inserted by a SIP server in the response if it is only interested in throttling traffic to a specific destination port. This is an optional parameter and does not have any value associated with it.

A node identified by a certain IP address may be hosting multiple SIP servers listening on different ports. In some architectures, the same IP address is shared by different blades and a SIP server listens on a distinct port on a separate blade. In yet other architectures with multi-core CPUs or multiple CPUs, an administrator may assign a CPU to a specific SIP server, but the node itself has more capacity in form of unused CPUs or unused blades. Consequently, throttling traffic destined to a specific port seems reasonable.

The semantics of the 'oc-port' parameter --- its absence or presence in a SIP response --- on the upstream SIP server are further outlined in Section 4.5 (Using the Overload Control Parameter Values).

The 'oc', 'oc_validity', 'oc-port' 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 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 generally inserts 'oc' and 'oc_validity' parameters into responses to all SIP servers. However, if a SIP server wanted to limit its overload control capability for privacy reasons, it MAY decide to add 'oc' and 'oc_validity' parameters only to responses that are sent via a secured transport channel such as TLS. The SIP server can use transport level authentication to identify the SIP servers, to which responses with these parameters are sent. 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] (Hilt, V., Noel, E., Shen, C., and A. Abdelal, “Design Considerations for Session Initiation Protocol (SIP) Overload Control,” June 2010.)). 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', 'oc-seq' and if present, the 'oc-port' 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.

A SIP server 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 server 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 server 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 server compliant to this specification MUST honor 'oc' parameter values it receives from downstream neighbors. The SIP server MUST NOT forward more messages to a SIP server than allowed by the current 'oc' and 'oc-port' parameter values from this server.

When forwarding a SIP request, a SIP entity uses the SIP procedures of [RFC3263] (Rosenberg, J. and H. Schulzrinne, “Session Initiation Protocol (SIP): Locating SIP Servers,” June 2002.) to determine the next hop SIP server. The procedures of [RFC3263] (Rosenberg, J. and H. Schulzrinne, “Session Initiation Protocol (SIP): Locating SIP Servers,” June 2002.) 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 server MUST determine if it already has overload control parameter values for the server chosen from the Expected Output. If the SIP server has a non-expired 'oc' parameter value for the server chosen from the Expected Output, and this chosen server had restricted throttling to a specific port (by setting the 'oc-port' parameter earlier), then the SIP server MUST determine if it can or cannot forward the current request within the current conditions.

The SIP server SHOULD use the following algorithm to determine if it can forward the request. The SIP server draws a random number between 1 and 100 for the current request. If the random number is less than or equal to the 'oc' parameter value, the request is not forwarded. Otherwise, the request is forwarded (note that this algorithm does not prioritize the requests it is dropping --- c.f. OPEN ISSUE 3 in Section 5.1 (Message Prioritization).) Any other algorithms that approximate the random number algorithm may be used as well.

4.6.  Forwarding the overload control parameters

A SIP server 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 server may not receive a response from a downstream neighbor when sending a request. RFC3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [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, a SIP server MUST stop sending requests to this server. The SIP server SHOULD occasionally forward a single request to probe if the downstream neighbor is alive. Once a SIP server has successfully transmitted a request to the downstream neighbor, it can resume normal transmission of requests. It should, of course, honor an 'oc' parameters it may receive. This avoids that a SIP server, which is unable to respond to incoming requests, is overloaded with additional requests.

OPEN ISSUE 2: If a downstream neighbor does not respond to a request at all, the upstream SIP server will stop sending requests to the downstream neighbor. The upstream SIP server will periodically forward a single request to probe the health of its downstream neighbor. It has been suggested --- see http://www.ietf.org/mail-archive/web/sip-overload/current/msg00229.html --- that we have a notification mechanism in place for the downstream neighbor to signal to the upstream SIP server 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

An element can receive overload control feedback indicating that it needs to reduce the traffic it sends to its downstream neighbor. An element 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 element 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

During an overload condition, a SIP server needs to prioritize messages and select messages 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 server 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 server SHOULD honor a request containing the Resource-Priority header field RFC4412 (Schulzrinne, H. and J. Polk, “Communications Resource Priority for the Session Initiation Protocol (SIP),” February 2006.) [RFC4412]. Resource-Priority header field enables a proxy to identify high-priority requests, such as emergency service requests, and preserve them as much as possible during times of overload.

OPEN ISSUE 3: The process by which the upstream server selects messages to be rejected to reduce load needs to be discussed further. Clearly, SIP messages that contain the Resource-Priority header should not be rejected. Also, mid- dialog requests (re-INVITEs, UPDATEs, BYEs etc.) should be honored, if possible. This can be left as a policy decision with guidelines provided --- example, if a request has both the To tag and From tag, do not drop it since it is a mid- dialog request; do not drop requests with the Resource-Priority header, etc. Some discussion on this is captured in http://www.ietf.org/mail-archive/web/sip-overload/current/msg00272.html.

This issue also has a bearing on the algorithm outlined in Section Section 4.5 (Using the Overload Control Parameter Values).

5.2.  Rejecting Requests

A SIP server that rejects a request because of overload MUST reject this request with a 503 (Service Unavailable) response code. This response code indicates that the request did not succeed because the SIP servers processing the request are under overload.

A SIP server that is under overload and has started to throttle incoming traffic SHOULD use 503 responses without Retry-After header to reject a fraction of requests from upstream neighbors that do not include the 'oc_accept' parameter in their Via headers. These neighbors do not support this specification and will not respond to overload control feedback in the 'oc' parameter. If the upstream server does not support "oc" then the downstream 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 overload compliant upstream server (note to editor: reword later.) This is to ensure that SIP servers, which do not support this specification, don't receive an unfair advantage over those that do.

A SIP server that has reached overload (i.e., a load close to 100) SHOULD start using 503 responses in addition to using the 'oc' parameter for all upstream neighbors. If the proxy has reached a load close to 100, it needs to protect itself against overload. Also, it is likely that upstream proxies have ignored overload feedback and do not support this specification.

6.  Syntax

This section defines the syntax of new Via header parameters: 'oc', 'oc_validity', 'oc_accept', 'oc-port' and 'oc-seq'. These Via header parameters are used to implement an overload control feedback loop between neighboring SIP servers.

The 'oc', 'oc_validity', 'oc-port' and 'oc-seq' parameters are only defined in the topmost Via header of a response. They MUST NOT be used in the Via headers of requests and MUST NOT be used in other Via headers of a response. The 'oc', 'oc_validity', 'oc-port' and 'oc-seq' parameters MUST be ignored if received outside of the topmost Via header of a response. The 'oc_accept' parameter MAY appear in all Via headers.

The 'oc' Via header parameter contains a number between 0 and 100. It describes the percentage by which the traffic 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_accept' Via header parameter indicates that the SIP server, which has created this Via header, supports overload control.

The 'oc-port' Via header parameter does not contain a value. It is used as placeholder to impart to the upstream entity the specific port number on which overload control is desired (i.e., perform overload control only on requests destined for a specific port.)

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 information.

This specification extends the existing definition of the Via header field parameters of [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) as follows:

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

oc = "oc" [EQUAL 0-100]

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

oc-accept = "oc_accept"

oc-port = "oc_port"

oc-seq = (1*DIGIT) / (1*DIGIT "." 1*DIGIT)

Example:

  Via: SIP/2.0/TCP ss1.atlanta.example.com:5060
    ;branch=z9hG4bK2d4790.1
    ;received=192.0.2.111
    ;oc=20;oc_validity=500;oc-seq=87165

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 [I‑D.ietf‑soc‑overload‑design] (Hilt, V., Noel, E., Shen, C., and A. Abdelal, “Design Considerations for Session Initiation Protocol (SIP) Overload Control,” June 2010.).

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:

• 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.
• 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.
• 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 feedback.
• 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.
• 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 characteristics:

• 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.
• 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.
• 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.
• 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.
• As overload feedback is sent to all senders in separate messages, this mechanism is not suitable when frequent overload control feedback is needed.
• A SIP server can limit the set of senders that can receive overload control information by authenticating subscriptions to this event package.
• 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] (Hilt, V., Noel, E., Shen, C., and A. Abdelal, “Design Considerations for Session Initiation Protocol (SIP) Overload Control,” June 2010.)) 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. 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

[TBD.]

10.  References

10.1. Normative References

10.2. Informative References

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