Common Tips

Here I will share some tips of a few headers that are used repeately in many parts on the explanation in this site. Most of the Tips in this note is about a specific header, not about large scope like a message, transaction, dialog etc. The descriptions or tips with larger scope (e,g,  message, transaction, dialog) are explained in other notes.


In SIP, while it is common and logical for the first message of a new sequence within a dialog to start with a CSeq number of 1, the SIP specification does not mandate that sequence numbers must start at 1. The important requirement, as per the SIP specification (RFC 3261), is that the CSeq number must increment for each new request within the same dialog.The starting CSeq number for the first request in a dialog is chosen by the User Agent Client (UAC) initiating the dialog and can technically be any positive integer. The subsequent requests within the same dialog should have CSeq numbers that are incrementally higher to ensure proper sequencing and to maintain the integrity of the dialog.

The specific value itself does not imply anything beyond its role in sequencing within the dialog. The starting value and the increments are chosen by the client initiating the requests. The relatively high value suggests that either the user agent prefers starting with high sequence numbers, or it has been part of a long sequence of transactions within this dialog. In practice, the absolute value of the CSeq is less important than its role in ensuring the orderly processing of requests and responses within the SIP dialog.


You would have seen this kind of comments ' prevent looping...' in the description in previous sections, what does it mean. This is some explanation for it.

In the context of SIP (Session Initiation Protocol), "looping" refers to a situation where a SIP request, such as an INVITE, NOTIFY, or SUBSCRIBE, circulates or traverses through the network repeatedly without reaching its intended destination. This can occur due to incorrect routing, misconfigurations, or the absence of mechanisms to limit the number of hops a request can make. Looping is undesirable because it can lead to unnecessary network congestion, degrade performance, and potentially cause call setup or message delivery failures.

In other words, "looping" in SIP is the undesired repetition of request routing within the network, and it is mitigated by using the Max-Forwards header to limit the number of hops, Via headers to trace and identify request paths, and Call-ID and CSeq headers to uniquely identify and sequence messages within dialogs.

To prevent looping, SIP utilizes several mechanisms as below:

  • Max-Forwards Header:This header is included in SIP requests and indicates the maximum number of hops (transfers from one node to another) that the request is allowed to make. Each time a request passes through a SIP proxy, the proxy decreases the value in the Max-Forwards header by one. When the value reaches zero, the request is not forwarded further, effectively preventing infinite loops. The Max-Forwards header is crucial for loop prevention and is also useful for tracing and debugging SIP messages.
  • Via Headers:The Via headers trace the path that the request has taken through the network by listing the sequence of proxies that have forwarded the request. Each proxy adds its own identifier to the Via headers when forwarding a request. By examining the Via headers, a proxy can detect if it has already seen the request, helping to identify and stop potential loops.
  • Call-ID and CSeq:While primarily used for identifying and sequencing SIP messages within a dialog, the combination of the Call-ID (which uniquely identifies a SIP dialog or session) and the CSeq (which sequences requests within a dialog) also contributes indirectly to loop prevention. By ensuring that each dialog and request within that dialog is uniquely identifiable, these fields help SIP entities manage and route messages correctly, reducing the risk of looping due to misrouted requests.


The Allow header in SIP (Session Initiation Protocol) messages lists the request methods (e.g., INVITE, ACK, BYE) that a User Agent supports, serving both to inform other entities of its capabilities and to facilitate interoperable SIP communications by clarifying which actions are possible within a given session or dialog. At first it was not clear to me on why almost every SIP message carries this header which are often almost same.

Why almost every method carries this kind of header "Allow" Shouldn't it be common to every method in the dialog ?

Here's why it's commonly included in many types of SIP messages:

  • Advertisement of Capabilities
    • Dynamic Capability Discovery: SIP entities (User Agents, Proxies, etc.) use the Allow header to advertise their supported SIP methods. This is crucial for interoperability, as it informs the communicating parties of the capabilities of their counterparts, allowing them to adjust their behavior accordingly. SIP implementations vary widely, and not all entities support the full suite of methods defined by the protocol and its extensions.
  • Protocol Flexibility and Extension
    • Support for Extensions: The SIP protocol is designed to be extensible. New methods can be defined in extensions to the SIP protocol. The Allow header enables SIP entities to advertise their support for these extended methods. This feature is essential for backward compatibility and for the graceful introduction of new features.
  • Session Management
    • Contextual Method Support: The methods supported by a SIP entity might change based on the state of a session. For example, certain methods may only be relevant or allowed once a call has been established. Including the Allow header in messages related to a dialog (a call or session) makes it clear which actions are currently appropriate.
  • Error Handling and Diagnostics
    • Improved Error Handling: When a SIP entity receives a request for a method it does not support, it can respond with a 405 Method Not Allowed response, which can optionally include an Allow header to inform the requester of the supported methods. This aids in debugging and interoperability testing.
  • Efficiency and Optimization
    • Avoiding Unnecessary Requests: By knowing the capabilities of the other party, SIP entities can avoid sending requests that will be rejected due to unsupported methods, thus optimizing network resource usage and improving the efficiency of the communication.
  • Compliance with the Standard
    • Specification Requirement: The inclusion of the Allow header in responses, especially in the context of certain responses like 405 Method Not Allowed, is recommended or required by the SIP specification (RFC 3261). While not mandatory in every message, its use in responses helps ensure that the communication is in line with the protocol's standards.

What is the scope of 'Allow' header ? Is it per dialog ? or per transaction ? or per entire call ?

The scope of the Allow header in SIP (Session Initiation Protocol) can be understood in the context of SIP dialogs, transactions, and calls, with its relevance and implications varying slightly depending on where and how it's used.

In other words, the Allow header's scope is effectively applicable to the entire duration of a dialog or call, providing a mechanism for advertising supported SIP methods to ensure interoperability and proper session management. Its inclusion in specific transactions within those dialogs or calls serves to communicate or reaffirm the capabilities at those particular points in time.

  • Per Dialog
    • A dialog in SIP is a peer-to-peer SIP relationship between two user agents that persists for some time and is established by SIP messages like INVITE. The Allow header's information is relevant to the entire dialog because it informs the other party of the methods supported throughout the dialog's lifecycle. However, the Allow header can be included in any SIP message within the dialog, potentially allowing for dynamic advertisement of supported methods if the capabilities of the user agent change during the dialog's lifetime (though such dynamic changes are rare in practice).
  • Per Transaction
    • A transaction consists of a single request and any messages sent in response to it. The Allow header can be included in responses within a transaction to inform or remind the counterpart of the supported methods at that point in time. While the scope of a transaction is more limited than a dialog, the Allow header in this context still serves to communicate the current capabilities regarding which requests the user agent can process.
  • Per Entire Call
    • In the context of a call (which can be considered a series of dialogs or transactions from call setup to termination), the Allow header reflects the methods supported for the duration of the call. Since a call might involve multiple dialogs (e.g., if a call is transferred or if multiple media sessions are established), the Allow header in initial and subsequent messages ensures that all parties are aware of the supported methods throughout the call's progression.
  • Practical Implications
    • Initial INVITE: Including the Allow header in the initial INVITE request or the early dialogs of a call helps set the stage for what subsequent requests can be supported during the call.
    • Responses: Responses, especially to INVITE requests or within early dialogs, often include the Allow header to ensure that the other party is aware of the supported methods as the call progresses.
    • Changing Capabilities: While the capabilities of a user agent typically do not change mid-dialog, the Allow header can be included in mid-dialog requests or responses if there is a need to clarify or remind the supported methods.

Would 'Allow' header in every message within a dialog be same ?

In theory, the Allow header in every message within a SIP (Session Initiation Protocol) dialog could be the same, reflecting the set of request methods that a User Agent (UA) supports throughout the duration of the dialog.

While the Allow header could theoretically vary within the same dialog under certain conditions, in most practical SIP deployments, the capabilities of a UA and thus the contents of the Allow header remain consistent throughout the dialog. Variations would be exceptional and driven by specific, dynamic conditions affecting the UA's capabilities.

there are a few nuances and scenarios where the Allow header might vary within the same dialog:

  • Consistency in Allow Header
    • Static Capabilities: For UAs with static capabilities that do not change over time or in response to different conditions, the Allow header should remain consistent across all messages within a dialog. This is the most common scenario, where the UA's method support does not dynamically change during the course of a dialog.
  • Potential Variations
    • Dynamic Capabilities: In more complex or advanced implementations, a UA's supported methods could theoretically change based on dynamic conditions, such as:
      • Session Progression: As a session progresses, the context or state might change (e.g., from pre-established to established), potentially affecting which methods are relevant or supported.
      • Resource Availability: Changes in resource availability or network conditions might lead a UA to adjust its capabilities, reflected in the Allow header.
      • Policy or Configuration Changes: Administrative actions or policy changes could temporarily alter the set of supported methods for a UA.
  • Practical Considerations
    • Rare in Practice: Dynamic changes to the Allow header within a dialog are rare in practice. Most SIP endpoints have a fixed set of capabilities that do not change frequently. Including the Allow header in each message could be redundant, especially if the capabilities remain constant.
    • Signaling Overhead: Repeatedly including the Allow header in every message adds to the signaling overhead. While informative, it may not be necessary if both parties have already established the capabilities at the beginning of the dialog.
    • Documentation and Debugging: Including the Allow header can be useful for documentation, debugging, and interoperability testing, providing a clear indication of the UA's capabilities at each step of the dialog.

Is 'Allow' header mandatory ?

The Allow header in SIP (Session Initiation Protocol) messages is not mandatory in all SIP messages. Its inclusion is recommended in specific scenarios to convey the set of request methods that a User Agent (UA) supports. The SIP specification (RFC 3261) provides guidelines on when and where the Allow header should be used, but it does not make it an absolute requirement for every message.

In other words, while the Allow header is not mandatory in all SIP messages, its inclusion is recommended in specific scenarios to enhance communication clarity and interoperability. The decision to include it depends on the context of the message, the nature of the transaction or dialog, and the preferences of the UA or the network administrators.

  • When Allow Header is Recommended:
    • Responses to OPTIONS Requests: When responding to an OPTIONS request, including the Allow header is recommended to inform the requester of the methods supported by the UA.
    • 405 Method Not Allowed Responses: When a UA receives a request method it does not support, it should respond with a 405 (Method Not Allowed) response, including an Allow header listing the methods it does support.
    • Informative Contexts: It's often useful to include the Allow header in other response messages (e.g., 200 OK to INVITE) as a way of informing or reminding the remote party of the request methods the UA supports. This can aid in debugging, interoperability, and ensuring smooth communication flows.
  • Not Mandatory:
    • General Use: There is no strict requirement to include the Allow header in every SIP message. Many SIP messages are sent without it, especially if the methods supported by the UA are already known or have not changed.
    • Performance Considerations: In environments where message size and network efficiency are critical, the Allow header might be omitted to reduce overhead, particularly in transactions or dialogs where the capabilities of the UAs are already established or understood.
  • Practical Implications:
    • Omission of Allow Header: Not including an Allow header does not inherently cause a SIP transaction to fail. However, omitting it when it would be particularly informative (e.g., in response to an OPTIONS request or in a 405 response) might lead to missed opportunities for optimizing communication or handling errors more gracefully.
    • Dynamic Capabilities: In systems where the capabilities of a UA might change dynamically (though this is relatively rare), including the Allow header in messages can help ensure that the remote party is always aware of the current set of supported methods.

Contact header and UA Capability Information

The original purpose/functionality of the Contact header in SIP (Session Initiation Protocol) is used to provide a URI (Uniform Resource Identifier) that represents the direct contact information for a User Agent (UA). This header is crucial for establishing where and how subsequent SIP messages should be sent to reach the UA, but through the informations carried by this header we can indirectly figure out various capabilities of the UA as described below :

  • Transport Protocol: The Contact header can include information about the transport protocol (like UDP, TCP, or TLS) that the UA prefers to use, indicating support for these protocols.
  • Feature Tags:The header can also carry feature tags that provide hints about the UA's capabilities. These tags can indicate support for specific extensions or services like video, instant messaging, and other media types.
  • Mobility and GRUU: With the use of a parameter like mobility="mobile", it can suggest the UA's mobility type. If a Globally Routable User Agent URI (GRUU) is provided, it tells us that the UA supports a persistent, globally routable contact address, which is a specific capability.
  • Instance Identifiers:Including a +sip.instance parameter, which contains a unique identifier for the UA instance, can help in managing multiple registrations and determining the UA's support for this mechanism.
  • Feature Tag : The contact header can carry the feagure tag as shown in the following example. From this feature tag, we can figure out various detailed information about UA's capability.

      Contact: <>;












    • Followings are detailed description
      • +g.3gpp.cs-voice: Indicates that the UA supports or is available for circuit-switched voice calls, which is a feature related to mobile networks.
        • video: Implies that the UA is capable of handling video media streams in addition to audio.
      • +g.3gpp.icsi-ref: This parameter includes one or more URNs that specify the UA's support for certain IMS (IP Multimedia Subsystem) services:
        • urn:urn-7:3gpp-service.ims.icsi.oma.cpm.msg: Indicates support for Converged IP Messaging (CPM) services.
        • urn:urn-7:3gpp-service.ims.icsi.oma.cpm.largemsg: Suggests the ability to handle large CPM messages.
        • urn:urn-7:3gpp-service.ims.icsi.mmtel: Refers to support for the Multimedia Telephony (MMTel) service, which includes voice, video, and supplementary services.
      • +g.3gpp.iari-ref: Contains a list of URNs that define support for RCS-e (Rich Communication Suite enhanced) features:
        • Support for instant messaging.
        • urn:urn-7:3gpp-application.ims.iari.rcs.fullsfgroupchat: Indicates the capability for full-store-and-forward group chat.
        • urn:urn-7:3gpp-application.ims.iari.rcs.ftthumb: The ability to send file transfer thumbnails.
        • urn:urn-7:3gpp-application.ims.iari.rcs.fthttp: File transfer over HTTP.
        • urn:urn-7:3gpp-application.ims.iari.rcse.sp: Social presence information.
        • urn:urn-7:3gpp-application.ims.iari.rcse.dp: Discovery of presence information.
        • urn:urn-7:3gpp-application.ims.iari.rcs.geopull: Geolocation pull functionality.
    • NOTE :  what is urn and how it differs from uri ?
      • A URN (Uniform Resource Name) is a type of URI (Uniform Resource Identifier) that is designed to serve as a persistent, location-independent resource identifier and is often used for long-term resource identification. URNs are a specific subset of URIs, but they differ in purpose and function.
      • In the context of SIP and IMS (IP Multimedia Subsystem), URNs are often used to uniquely identify specific services, features, or protocols, as you've seen in the +g.3gpp.icsi-ref and +g.3gpp.iari-ref parameters. These URNs help in identifying the capabilities or services supported by a user agent or in defining the types of media or sessions that can be handled in a SIP communication setup. They are useful for service discovery and negotiation in complex communication systems.
      • URI (Uniform Resource Identifier):
        • A URI is a generic term for all types of names and addresses that refer to objects on the web. It provides a simple and extensible means for identifying a resource.
        • A URI can be a name, location, or both, and it can take various forms, including URLs (Uniform Resource Locators) and URNs.
        • Example of a URI could be a URL like, which indicates both the location of the resource and a way to retrieve it.
      • URN (Uniform Resource Name):
        • A URN functions as a resource's name more than its location. Its primary purpose is to provide a globally unique identifier for a resource without implying its location or how to access it.
        • URNs are used when the exact location of a resource is either not available or not relevant. They remain consistent and valid even if the resource moves or changes its access protocol.
        • An example of a URN might be an ISBN number for a book (urn:isbn:0451450523), which uniquely identifies the book but doesn't tell you where to find it or how to access it.


I put the explanation on this header in a separate note here.


mode-change-capability is defined in RFC 4867 as follows.

mode-change-capability: Specifies if the client is capable to transmit with a restricted mode change period.  The parameter may take value of 1 or 2.  A value of 1 indicates that the client is not capable of restricting the mode change period to 2, and that the codec mode may be changed at any point.  A value of 2 indicates that the client has the capability to restrict the mode change period to 2, and thus that the client can correctly interoperate with a receiver requiring a mode-change-period=2.  If this parameter is not present, the mode-change restriction capability is not supported, i.e. mode-change-capability=1.  To be able to interoperate fully with gateways to circuit switched networks (for example, GSM networks), transmissions with restricted mode changes (mode-change-capability=2) are required. Thus, clients RECOMMENDED to have the capability to support transmission according to mode-change-capability=2.

The mode-change-capability parameter in the fmtp (format parameter) attributes of an SDP (Session Description Protocol) message is related to certain codecs that have the ability to change modes during a call. This capability is often associated with adaptive codecs, such as Adaptive Multi-Rate (AMR) codecs, which can switch between different bitrates and quality levels depending on network conditions or other factors.

In the context of VoIP and other real-time audio/video communication protocols, having a mode-change capability means that the quality of the communication can be maintained by adapting to network conditions. If, for example, the network conditions worsen and there is not enough bandwidth for a high-quality audio stream, the codec can switch to a lower bitrate mode that requires less bandwidth, thus maintaining the call without dropping it.

It's important to note that both parties in the communication need to support the same set of capabilities for this feature to work effectively. The negotiation of these capabilities is part of the call setup process, where each party indicates its capabilities and preferences in the SDP part of SIP messages.

Here's what mode-change-capability generally refers to:

  • Mode-Change Capability: This refers to the ability of the codec to switch modes or configurations dynamically during a call. For instance, an AMR codec may switch between different bitrate modes to adapt to varying network bandwidth or to conserve bandwidth.
  • Capability Indicators: The numerical value assigned to mode-change-capability indicates what kind of mode changes are supported. While the specific meanings of these values can vary by implementation, they often represent:
    • Whether the codec can change modes only at certain boundaries (like frame boundaries) or more freely.
    • If the mode change can be requested by the sender, the receiver, or both.
    • Whether such changes are limited to predefined sets of modes or if they can include a wider range of mode changes.