SIP Glossary for SIP Trunk Operations
SIP Glossary
for SIP Trunk
Operations
Developed for network engineers and administrators, this SIP glossary is based on the SIP standard (RFC 3261). It serves as a practical reference for reliable SIP trunk deployment, operation, and troubleshooting. Use it to understand key SIP methods, headers, response codes, transactions, dialogs, and common operational issues encountered during session establishment and call processing.
A
Accept
A header field specifying media types acceptable to the User Agent Client (UAC) for the message body (e.g., in responses or subsequent requests).
Troubleshooting/Operational Context
If media negotiation fails with a 415 (Unsupported Media Type), check this field to see which formats the UA accepts.
Accept-Encoding
A header field indicating acceptable content encodings (compression) in the response.
Troubleshooting/Operational Context
Check this if a compressed message body fails to decode; an empty field means only identity (no encoding) is permissible.
Accept-Language
A header field indicating preferred languages for reason phrases, session descriptions, or status responses carried in the message body.
Troubleshooting/Operational Context
Used to confirm language settings if the server’s descriptive phrase is unexpected.
ACK
An acknowledgment request sent to confirm the receipt of a final response to an INVITE request. If the response is 2xx, it is a separate transaction; if non-2xx, it is part of the INVITE transaction.
Troubleshooting/Operational Context
Essential for completing the three-way session setup handshake (INVITE/200 OK/ACK). A missing ACK after 200 OK causes the UAS to generate a BYE.
ACKm
The BNF notation for the ACK method name in all capital letters.
Troubleshooting/Operational Context
Used for validating the syntax of the ACK request line.
addr-spec
An address specification consisting of a SIP URI, SIPS URI, or absoluteURI, typically without a display name.
Troubleshooting/Operational Context
Used for syntax validation in headers like From, To, and Contact.
Address-of-Record (AOR)
A SIP or SIPS URI (the “public address”) pointing to a domain with a Location Service capable of mapping it to a current Contact URI.
Troubleshooting/Operational Context
Used in the To header of REGISTER requests to identify the user being registered.
Address Incomplete (484)
A 4xx response indicating that the Request-URI received by the server was incomplete.
Troubleshooting/Operational Context
Investigate incomplete dialing strings or issues related to address formatting.
algorithm
The hashing algorithm used in Digest authentication (e.g., MD5).
Troubleshooting/Operational Context
If authentication fails (401/407), ensure the client and server agree on the hashing algorithm.
Alert-Info
A header field that specifies an alternative ring tone to the UAS when present in an INVITE request or a 180 (Ringing) response.
Troubleshooting/Operational Context
Debugging customized alerting features (e.g., different ring tones for specific calls).
Allow
A header field listing the methods (functions) supported by the User Agent Server (UAS).
Troubleshooting/Operational Context
Check the response to an OPTIONS request to ensure the UAS supports methods required for the call flow (e.g., INVITE, BYE, CANCEL).
Ambiguous (485)
A 4xx response indicating the Request-URI resolved to several choices, and the proxy could not decide where to route.
Troubleshooting/Operational Context
Occurs when the Location Service returns multiple conflicting contacts. The proxy should usually return 485 or redirect (3xx).
application/pkcs7-mime
A MIME type used in S/MIME bodies for confidentiality and integrity.
Troubleshooting/Operational Context
Used when the message body (e.g., SDP) is encrypted.
application/sdp
The media type used for Session Description Protocol (SDP) content in the message body.
Troubleshooting/Operational Context
Essential for successful media negotiation. Check Content-Type header when media establishment fails.
Authorization
Credentials provided by the client to the UAS (after a 401 challenge) to authenticate the user.
Troubleshooting/Operational Context
UAC sends this in response to a 401 (Unauthorized). Check if the credentials match the realm/nonce provided.
Authentication-Info
A header field (2xx response) that may contain parameters such as nextnonce.
Troubleshooting/Operational Context
Used in Digest authentication exchange after successful authentication.
B
Back-to-Back User Agent (B2BUA)
A logical entity that receives a request and processes it as a UAS, then creates a new request as a UAC.
Troubleshooting/Operational Context
Check if the B2BUA is properly maintaining or translating dialog state, as it acts as two separate endpoints.
Bad Extension (420)
A 4xx response indicating the request contained a Require or Proxy-Require header field listing an option tag not supported by a proxy or UAS.
Troubleshooting/Operational Context
Occurs when mandatory extensions are missing. The response includes an Unsupported header listing the problematic tags.
Bad Gateway (502)
A 5xx error indicating a gateway or proxy received an invalid response from an upstream server.
Troubleshooting/Operational Context
Indicates an issue with upstream connectivity or a non-SIP protocol translation error.
Bad Request (400)
A 4xx response indicating the request could not be understood due to poor syntax or mandatory header field errors.
Troubleshooting/Operational Context
The most basic syntax error. Review message formatting against the SIP Augmented BNF (Section 25).
Basic Rules (BNF)
The Augmented Backus-Naur Form grammar used to specify the SIP protocol’s syntax and encoding.
Troubleshooting/Operational Context
Referenced when troubleshooting 400 Bad Request errors to confirm syntax compliance.
Basic SIP Trapezoid
The typical call flow arrangement involving a UAC, UAS, and two proxy servers that facilitate session establishment.
Troubleshooting/Operational Context
Used for visualizing and debugging call routing paths, ensuring proxies are correctly involved (or bypassed).
Binding
The association created by a Registrar in the Location Service, mapping an AOR to one or more Contact addresses.
Troubleshooting/Operational Context
Verify successful registration by checking the 200 OK response to REGISTER, which should contain the new binding in the Contact header.
branch parameter
A parameter in the Via header field that uniquely identifies a single transaction.
Troubleshooting/Operational Context
Crucial for matching requests to responses and detecting routing loops (must start with “z9hG4bK”).
Busy Everywhere (600)
A 6xx global failure response indicating the request cannot be fulfilled at any server.
Troubleshooting/Operational Context
Used when a forked request fails everywhere. Indicates a definitive, permanent failure across all contacts.
Busy Here (486)
A 4xx response indicating the callee is currently unavailable (e.g., busy on another call).
Troubleshooting/Operational Context
Check the UAS availability/capacity. May include a Retry-After header indicating when to try again.
BYE
A method used to terminate an established dialog/session.
Troubleshooting/Operational Context
Sent by either UA within the dialog. Triggers session termination and requires a 200 OK response.
C
Call-ID
A globally unique identifier that groups all messages (requests/responses) belonging to a particular call or dialog.
Troubleshooting/Operational Context
Essential for correlating logs across different servers and devices for a single session.
Call/Transaction Does Not Exist (481)
A 4xx response indicating the UAS received a request that does not match any existing dialog or pending transaction.
Troubleshooting/Operational Context
Usually means the dialog state is inconsistent (e.g., trying to send BYE on an already terminated call).
CANCEL
A method used to cancel a pending INVITE request that has not yet resulted in a final (non-1xx) response.
Troubleshooting/Operational Context
Used when the caller hangs up before the callee answers.
Client
Any network element (UAC or Proxy) that sends SIP requests and receives responses.
Troubleshooting/Operational Context
Understanding the client role is key to validating mandatory headers (To, From, Call-ID, CSeq, Max-Forwards, Via).
Client Error (4xx)
The response class (400-499) indicating a failure caused by the client (e.g., authentication failure, bad syntax, requested resource unavailable).
Troubleshooting/Operational Context
The most common class of failure; requires checking client configuration, authentication, or address resolution logic.
Client Transaction
The UAC component responsible for ensuring reliable request delivery, handling retransmissions, and matching responses.
Troubleshooting/Operational Context
Monitor transaction timers (A, B, E, F) and retransmission attempts when using unreliable transport (UDP).
cnonce
A client-generated nonce value used in Digest authentication for replay protection.
Troubleshooting/Operational Context
Verifies the Authorization header field’s authenticity.
Confidentiality
The security property ensuring that messages cannot be read by unauthorized entities.
Troubleshooting/Operational Context
Achieved via SIPS/TLS for hop-by-hop signaling, or S/MIME for end-to-end body content.
Contact
A header field containing a SIP or SIPS URI representing a direct route to contact the specific UA instance.
Troubleshooting/Operational Context
Used by endpoints to bypass proxies for subsequent dialog requests (ACK, BYE, re-INVITE).
Content-Disposition
A header field describing how the message body should be interpreted (e.g., “session” for SDP or “render” for display).
Troubleshooting/Operational Context
If SDP is sent without this header, it defaults to “session”.
Content-Length
A header field indicating the size of the message body in octets.
Troubleshooting/Operational Context
Critical for message framing, especially over stream transports (TCP). Check to avoid fragmentation/loss over UDP.
Content-Type
A header field specifying the media type of the message body (e.g., application/sdp).
Troubleshooting/Operational Context
Ensures the receiving UA knows how to parse the payload (e.g., media description).
Core
The functional layer specific to a type of SIP entity (UA, Proxy, Registrar) that handles request logic and acts as a Transaction User (TU).
Troubleshooting/Operational Context
Used to define the specific behavior rules for a device based on its role.
CRLF
Carriage Return and Line Feed sequence, used as the line break in text-based SIP messages.
Troubleshooting/Operational Context
Essential for proper message parsing, especially at the end of the Request-Line/Status-Line and after headers.
CSeq (Command Sequence)
A mandatory header field containing an integer and a method name, used for sequencing requests within a dialog.
Troubleshooting/Operational Context
Check CSeq number integrity for mid-dialog messages (re-INVITE, BYE) to ensure they are processed in order.
D
Date
A header field indicating the time and date when the message was first sent (MUST use RFC 1123 GMT format).
Troubleshooting/Operational Context
Used for logging, time synchronization, and security purposes (tunneling integrity).
Decline (603)
A 6xx global failure response indicating the user does not wish to participate in the call and no alternative end system will accept it.
Troubleshooting/Operational Context
Indicates a definitive rejection by the user (not just busy).
Dialog
A peer-to-peer SIP relationship between two UAs that persists for some time, facilitating sequencing and routing of mid-dialog requests.
Troubleshooting/Operational Context
Defined by Call-ID, From tag, and To tag. All subsequent requests (BYE, re-INVITE) must be within this context.
Dialog ID
The unique identifier composed of Call-ID, Local Tag (From tag), and Remote Tag (To tag).
Troubleshooting/Operational Context
Used to disambiguate multiple sessions resulting from a single forked INVITE.
Digest Authentication Scheme
The challenge-based authentication mechanism mandated for SIP (Basic authentication is deprecated), based on HTTP Digest.
Troubleshooting/Operational Context
Check the realm, nonce, and qop parameters in 401/407 challenges.
DNS (Domain Name Service)
Used by SIP elements (especially proxies) to resolve a host name in a URI to an IP address, port, and transport protocol.
Troubleshooting/Operational Context
Troubleshoot routing issues by checking DNS SRV records for SIP (A/AAAA records for hosts).
Downstream
The direction of message forwarding from the User Agent Client to the User Agent Server.
Troubleshooting/Operational Context
Used to track the path of requests through a proxy chain.
E
Error-Info
A header field providing a pointer (URI) to additional information about an error status response.
Troubleshooting/Operational Context
Check this for diagnostic URIs, which might point to a recorded announcement or detailed error page.
Escaped
A character encoding format (“%” followed by two hexadecimal digits) used to represent reserved characters in URIs.
Troubleshooting/Operational Context
Used when special characters appear in the user part of a URI, or in header field names/values within the URI.
Expires
A header field indicating the relative time (in seconds) after which a registration binding or content is valid.
Troubleshooting/Operational Context
Crucial for REGISTER requests (setting binding lifetime) and 3xx responses (setting URI validity).
Extension Required (421)
A 4xx response indicating the UAS cannot process the request without a specific SIP extension listed in the Require header field of the response.
Troubleshooting/Operational Context
Indicates a functional gap where a required capability is missing.
extension-method
A SIP method other than the six defined standard methods (INVITE, ACK, CANCEL, BYE, REGISTER, OPTIONS).
Troubleshooting/Operational Context
Used for custom or extended SIP functions; check Allowed/Supported headers for compatibility.
F
Final Response
A response (2xx, 3xx, 4xx, 5xx, 6xx) that terminates a SIP transaction.
Troubleshooting/Operational Context
Used to confirm the definitive outcome (success or failure) of a transaction.
Final Response
A response (2xx, 3xx, 4xx, 5xx, 6xx) that terminates a SIP transaction.
Troubleshooting/Operational Context
Used to confirm the definitive outcome (success or failure) of a transaction.
Forking
The action of a proxy server sending a single incoming request to multiple potential destinations simultaneously.
Troubleshooting/Operational Context
Explains why a UAC may receive multiple 1xx or 2xx responses for a single INVITE.
Forbidden (403)
A 4xx response indicating the server refuses to fulfill the request, typically due to security or policy constraints, without requesting authentication.
Troubleshooting/Operational Context
Indicates the request violated an access rule (e.g., source IP filtering).
From
A mandatory header field indicating the logical identity of the request initiator (caller), containing a URI and a mandatory tag parameter.
Troubleshooting/Operational Context
Essential for establishing the Dialog ID (Local Tag) and is subject to authentication.
Framing
The mechanism used to determine the boundaries of SIP messages, relying on Content-Length for stream-oriented transports (TCP).
Troubleshooting/Operational Context
Crucial for messages sent over TCP; check if Content-Length is present and accurate.
FQDN (Fully Qualified Domain Name)
A complete host name used in URIs (e.g., in Contact or host fields).
Troubleshooting/Operational Context
Ensure that the host portion of URIs is resolvable via DNS.
G
Global Failure (6xx)
The response class (600-699) indicating that the request cannot be fulfilled by any server.
Troubleshooting/Operational Context
Represents permanent failure, often resulting from failed forking attempts.
Globally Unique Identifier
The requirement that the Call-ID header field must be unique across space and time.
Troubleshooting/Operational Context
Ensures that session logs are distinct and transactions can be reliably matched.
H
Header Field
A named attribute in a SIP message that conveys specific information about the message or session.
Troubleshooting/Operational Context
Check headers for correctness during troubleshooting (e.g., CSeq ordering, Via path, Call-ID matching).
Header Field Format
The syntax and structure defining how a header field’s value must be expressed.
Troubleshooting/Operational Context
Used for validating message construction (400 Bad Request).
Home Domain
The domain providing service to a SIP user, typically reflected in the URI of the AOR.
Troubleshooting/Operational Context
Used by proxies to apply local routing and authentication policies.
host
The fully-qualified domain name (FQDN) or numeric IP address providing the SIP resource.
Troubleshooting/Operational Context
The key component of a URI used for DNS resolution to find the next hop.
hostport
The host followed by an optional port number (host [ “:” port ]).
Troubleshooting/Operational Context
The target address used for establishing connections.
HTTP-like request/response model
The fundamental SIP design philosophy, where interactions consist of requests and at least one response.
Troubleshooting/Operational Context
Understanding this model is crucial for debugging SIP state and transaction behavior.
I
IANA
Internet Assigned Numbers Authority, which manages SIP parameter registries (e.g., Option Tags, Methods, Response Codes).
Troubleshooting/Operational Context
Consulted when dealing with unknown extensions, methods, or response codes.
In-Reply-To
A header field listing Call-IDs that this call references or returns to.
Troubleshooting/Operational Context
Used by call distribution systems to route return calls to the original initiator.
Informational Response (1xx)
Provisional responses (100-199) that indicate processing is ongoing but do not terminate the transaction.
Troubleshooting/Operational Context
Confirms request receipt and provides call progress (Trying, Ringing).
Integrity
The security property ensuring a message has not been modified in transit.
Troubleshooting/Operational Context
SIP headers integrity is compromised if non-modifiable headers are changed by proxies; bodies are protected by S/MIME.
Internal Server Error (500)
A 5xx error indicating the server encountered an unexpected condition.
Troubleshooting/Operational Context
Requires investigation of the local UAS/Proxy application logs for unhandled exceptions.
INVITE
The method used to initiate a session, establish a dialog, and carry the initial session description (SDP offer).
Troubleshooting/Operational Context
The most complex SIP transaction due to required reliability and dialog setup.
INVITE Client Transaction
The transaction state machine specific to INVITE requests, handling extended timers (Timer B/D) for reliable delivery and session setup.
Troubleshooting/Operational Context
Monitor Timer B (64*T1) to ensure the transaction doesn’t time out prematurely.
INVITE Server Transaction
The transaction state machine specific to processing INVITE requests, managing 1xx responses and handling the special ACK logic.
Troubleshooting/Operational Context
Ensures reliable delivery of the final response by managing retransmissions until ACK is received.
IPv4address
The BNF notation for an IPv4 address.
Troubleshooting/Operational Context
Used for verifying the syntax of IP addresses in URI host fields or Via received parameters.
IPv6reference
The BNF notation for an IPv6 address, including mandatory square brackets.
Troubleshooting/Operational Context
Used for verifying the syntax of IPv6 addresses in URI host fields.
L
LAQUOT
Left Angle Quote (<).
Troubleshooting/Operational Context
Used to enclose URIs within name-addr formats (e.g., From, To, Contact).
Location Service
An abstract database or service mapping an AOR to its current Contact URI(s).
Troubleshooting/Operational Context
Managed by the Registrar and consulted by Proxies/Redirect servers for routing.
Local Tag
The tag parameter in the From header field of a request, used by the UAC to generate half of the Dialog ID.
Troubleshooting/Operational Context
Used with the Remote Tag and Call-ID to uniquely identify the dialog.
Loop Detected (482)
A 4xx response indicating the server detected a routing loop for the request.
Troubleshooting/Operational Context
Check Via header entries for circular paths or incorrect proxy configuration.
Loose Router
A proxy compliant with RFC 3261 that processes the Route header field without replacing the Request-URI (unless the Route URI lacks the lr parameter).
Troubleshooting/Operational Context
Standard behavior for modern SIP proxies. Crucial for maintaining the Request-URI throughout the path.
lr parameter
The Loose Routing parameter, mandatory in Record-Route URIs added by RFC 3261 compliant proxies.
Troubleshooting/Operational Context
Its presence indicates the proxy uses loose routing logic.
LWS (Linear White Space)
Whitespace (SP or HTAB) that may span multiple lines (folding).
Troubleshooting/Operational Context
Used for formatting SIP headers, though LWS is not allowed in Request-Line elements.
M
maddr parameter
A SIP URI parameter specifying the server address to be contacted, overriding the host field, often used for multicast.
Troubleshooting/Operational Context
Used in complex routing logic; check if the request should be routed to this specific address instead of the host resolved by DNS.
Max-Forwards
A mandatory header field (SHOULD be 70) decremented at each hop to limit the number of proxy traversals.
Troubleshooting/Operational Context
When it reaches 0, the request is rejected with 483 (Too Many Hops). Used to prevent loops.
Message Too Large (513)
A 5xx error indicating the message size exceeded the server’s capacity.
Troubleshooting/Operational Context
Indicates a message size limit violation, common when using UDP where message bodies are large.
message/sip
A MIME type used in S/MIME tunneling to encapsulate an entire SIP message (including headers and body) for integrity and privacy.
Troubleshooting/Operational Context
Used for end-to-end SIP header protection when intermediaries cannot be fully trusted.
Method
The primary function a request is intended to invoke on a server (e.g., INVITE, BYE, REGISTER).
Troubleshooting/Operational Context
Defines the action and dictates which transaction rules (INVITE or Non-INVITE) apply.
MIME-Version
A header field describing the MIME version of the message body.
Troubleshooting/Operational Context
Used when the message includes complex multi-part MIME bodies.
Min-Expires
A header field used in a 423 (Interval Too Brief) response to specify the minimum acceptable expiration time for a registration.
Troubleshooting/Operational Context
UACs must adhere to this value to successfully REGISTER.
Moved Permanently (301)
A 3xx redirection response indicating the user has permanently moved to the URI(s) specified in the Contact header.
Troubleshooting/Operational Context
The client should update its address book/configuration for future attempts.
Moved Temporarily (302)
A 3xx redirection response indicating the user has temporarily moved to the URI(s) specified in the Contact header.
Troubleshooting/Operational Context
The client should retry the request using the contact URI but not update permanent records.
Multiple Choices (300)
A 3xx redirection response indicating the Request-URI resolved to multiple choices, listed in the Contact header.
Troubleshooting/Operational Context
The client must choose the preferred contact URI and retry.
N
name-addr
A header field value structure consisting of an optional display name and the URI enclosed in angle brackets ().
Troubleshooting/Operational Context
Standard format for To, From, and Contact headers.
nonce
A server-generated unique value used in Digest authentication challenges (401/407).
Troubleshooting/Operational Context
Must be included in the WWW-Authenticate/Proxy-Authenticate challenge.
Non-INVITE Client Transaction
The transaction state machine used for all methods other than INVITE, ACK, and CANCEL.
Troubleshooting/Operational Context
Uses faster transaction timers (Timer E, F) than the INVITE transaction.
Not Acceptable (606)
A 6xx global failure response indicating the UA cannot accept the session parameters in the SDP.
Troubleshooting/Operational Context
Used when media negotiation fails globally across all possible destinations.
Not Acceptable Here (488)
A 4xx response indicating the UAS cannot accept the session parameters (SDP) due to local limitations.
Troubleshooting/Operational Context
The primary code for codec or media incompatibility failure; often accompanied by a Warning header.
Not Acceptable Here (488)
A 4xx response indicating the UAS cannot accept the session parameters (SDP) due to local limitations.
Troubleshooting/Operational Context
The primary code for codec or media incompatibility failure; often accompanied by a Warning header.
Not Found (404)
A 4xx response indicating the Request-URI user/resource could not be found.
Troubleshooting/Operational Context
Most common routing failure. Check if the user is registered in the Location Service.
Not Implemented (501)
A 5xx error indicating the server does not support the requested method.
Troubleshooting/Operational Context
Check if the server’s capabilities (Allow header) support the attempted method (e.g., OPTIONS).
O
Offer/Answer Model
The mechanism, typically relying on SDP, where two UAs exchange session descriptions to agree on media characteristics.
Troubleshooting/Operational Context
Troubleshoot media setup failures by examining the SDP exchange flow (Offer in INVITE, Answer in 200 OK, or vice versa).
OK (200)
A 2xx success response indicating the request was successfully processed.
Troubleshooting/Operational Context
The desired response for INVITE (session establishment), BYE (session termination), and REGISTER (successful binding).
opaque-part
The non-hierarchical part of an absolute URI.
Troubleshooting/Operational Context
Used for syntax checking non-SIP URIs.
opaque-part
The non-hierarchical part of an absolute URI.
Troubleshooting/Operational Context
Used for syntax checking non-SIP URIs.
Option Tags
Unique identifiers (tokens) used in Supported, Require, and Unsupported headers to designate SIP extension capabilities.
Troubleshooting/Operational Context
Used to negotiate optional/required features (e.g., 100rel) between UAs.
OPTIONS
A method used to query a UA or proxy server for its supported capabilities (methods, extensions, media types).
Troubleshooting/Operational Context
Use for pre-call testing to confirm reachability (200 OK or 486) and feature support.
Organization
A header field conveying the name of the organization to which the SIP element belongs.
Troubleshooting/Operational Context
Used for identification and logging purposes.
Outbound Proxy
A proxy manually configured by a UA to receive all outgoing requests, regardless of the Request-URI.
Troubleshooting/Operational Context
Verify that client devices are correctly configured to send traffic to the designated trunk endpoint.
P
password
An optional field in the URI userinfo part. Its use is NOT RECOMMENDED due to security risks.
Troubleshooting/Operational Context
Should generally be avoided for authentication; Digest challenge/response mechanism is preferred.
Payment Required (402)
A 4xx response reserved for future use.
Troubleshooting/Operational Context
Placeholder, currently not used in standard SIP operation.
port
The transport port number where a SIP host listens for requests (default 5060 for UDP/TCP SIP, 5061 for TLS/SIPS).
Troubleshooting/Operational Context
Check firewall settings and network connectivity to ensure the correct port is open.
Priority
A header field indicating the urgency of the request.
Troubleshooting/Operational Context
Used for policy enforcement on resource prioritization.
Provisional Response (1xx)
Also known as an Informational Response. Indicates progress without terminating the transaction.
Troubleshooting/Operational Context
Confirms activity to the UAC (e.g., 100 Trying stops retransmissions).
Proxy-Authenticate
A challenge header field included in a 407 response, requiring the client to authenticate with the proxy.
Troubleshooting/Operational Context
Used to generate the necessary Proxy-Authorization header.
Proxy-Authorization
Credentials provided by the client to a proxy server (after a 407 challenge).
Troubleshooting/Operational Context
Essential for UACs to authenticate themselves to an intermediary proxy.
Proxy-Require
A header field used by the UAC to demand that proxies (not the UAS) along the path support specific extensions.
Troubleshooting/Operational Context
If mandatory proxy extensions are missing, the proxy must return 420 (Bad Extension).
Proxy Server
An intermediary entity that routes requests on behalf of clients, performing functions like routing, policy enforcement, and authentication.
Troubleshooting/Operational Context
Trace the path of requests (Via, Route, Record-Route) to identify failed hops.
Proxy Authentication Required (407)
A 4xx response requiring the client to authenticate itself to the proxy server.
Troubleshooting/Operational Context
UAC must respond with Proxy-Authorization header.
PSTN (Public Switched Telephone Network)
The traditional circuit-switched telephone network.
Troubleshooting/Operational Context
SIP trunks often interface with the PSTN via gateways (MEGACO/H.323), requiring protocol conversion.
Q
q parameter
A parameter in the Contact header field indicating the preference (quality value, 0-1) for a URI.
Troubleshooting/Operational Context
Determines the order in which multiple contacts for an AOR should be attempted for routing.
qop (Quality of Protection)
A parameter in Digest authentication challenges (e.g., “auth” or “auth-int”).
Troubleshooting/Operational Context
Determines the security level of the authentication; servers MUST always send this parameter.
Queued (182)
A provisional response indicating the request has been temporarily queued at the destination.
Troubleshooting/Operational Context
Informs the client that the callee is currently busy but the call is waiting (queue status).
quoted-string
A string enclosed in double quotes.
Troubleshooting/Operational Context
Used for display names or values that may contain reserved characters (like commas or semicolons).
R
RAQUOT
Right Angle Quote (>).
Troubleshooting/Operational Context
Used to terminate URIs within name-addr formats.
Reason Phrase
The descriptive textual phrase accompanying the numeric Status Code (e.g., “OK”, “Bad Request”).
Troubleshooting/Operational Context
Provides context during manual log analysis.
received parameter
A Via header parameter added by a proxy, indicating the actual IP address from which the request was received.
Troubleshooting/Operational Context
Essential for debugging NAT traversal, confirming the previous hop’s public IP.
Record-Route
A header field added by proxies that wish to remain in the signaling path (Route Set) for the duration of the ensuing dialog.
Troubleshooting/Operational Context
Check this header to ensure mandatory proxies are included in the subsequent routing path.
Redirect Server
A server that responds to a request by consulting the Location Service and returning a 3xx redirection response.
Troubleshooting/Operational Context
Triggers the UAC to try a different URI.
Redirection (3xx)
The response class (300-399) providing new locations or alternative services.
Troubleshooting/Operational Context
Requires the UAC to retry the request at the URI(s) provided in the Contact header.
REGISTER
The method used by UAs to create, query, or remove address bindings (AOR to Contact URI) at a registrar.
Troubleshooting/Operational Context
Used frequently in SIP trunk environments where endpoints dynamically register their location.
Registrar
A server accepting REGISTER requests and updating the Location Service for the domain it handles.
Troubleshooting/Operational Context
Responsible for authenticating users and maintaining up-to-date user locations.
Remote Tag
The tag parameter in the To header field of a response, generated by the UAS to complete the Dialog ID.
Troubleshooting/Operational Context
Used to distinguish between multiple dialogs created by forking.
Reply-To
A header field specifying an address where responses should be directed.
Troubleshooting/Operational Context
Primarily used for non-dialog-establishing requests.
Request
A SIP message sent from a client to a server to invoke a specific operation (Method).
Troubleshooting/Operational Context
The starting point of any SIP transaction.
Request-Line
The first line of a Request message, containing the Method, Request-URI, and SIP-Version.
Troubleshooting/Operational Context
Must be syntactically correct for the request to be processed.
Request-URI
The URI indicating the user or resource to which the request is addressed.
Troubleshooting/Operational Context
Determines the initial routing target for proxies.
Require
A header field used by UACs to mandate that the UAS supports specific Option Tags (extensions) needed to process the request.
Troubleshooting/Operational Context
Failure to meet these requirements results in a 420 (Bad Extension) error.
Response
A SIP message sent from a server to a client, indicating the Status Code of a request.
Troubleshooting/Operational Context
Used to confirm transaction status and progress.
Retry-After
A header field included in some failure responses (e.g., 503, 486, 600, 603) indicating the recommended time (in seconds) before the request should be retried.
Troubleshooting/Operational Context
Used for congestion control to prevent flooding the server.
Ringing (180)
A provisional response indicating the callee is currently being alerted.
Troubleshooting/Operational Context
Triggers the UAC to generate a ringback tone.
Route
A header field listing a sequence of proxies that must be visited; it dictates the request path, overriding normal Request-URI lookup.
Troubleshooting/Operational Context
Used primarily for mid-dialog requests (BYE, re-INVITE) based on the stored Route Set.
Route Set
An ordered collection of URIs (derived from Record-Route headers) stored by the UAs for routing all subsequent dialog requests.
Troubleshooting/Operational Context
Ensures that required proxies remain in the signaling path.
RTP (Real-time Transport Protocol)
A protocol typically used to transport real-time media data (audio/video).
Troubleshooting/Operational Context
Though separate from SIP signaling, media flow failure (RTP not starting) is a major trunk operational issue, often caused by incorrect SDP parameters negotiated by SIP.
S
S/MIME
A MIME security mechanism used for securing message bodies (encryption/integrity) and optionally tunneling SIP headers.
Troubleshooting/Operational Context
Used to ensure end-to-end security of session descriptions (SDP) or message headers.
SCTP (Stream Control Transmission Protocol)
A transport protocol that SIP MAY use, suitable for reliable connections.
Troubleshooting/Operational Context
An alternative reliable transport to TCP.
SDP (Session Description Protocol)
A protocol (RFC 2327) used in SIP message bodies to describe the multimedia session characteristics (codecs, ports, IP addresses).
Troubleshooting/Operational Context
Errors in SDP format or content cause 488 (Not Acceptable Here) and prevent media flow.
SDP (Session Description Protocol)
A protocol (RFC 2327) used in SIP message bodies to describe the multimedia session characteristics (codecs, ports, IP addresses).
Troubleshooting/Operational Context
Errors in SDP format or content cause 488 (Not Acceptable Here) and prevent media flow.
Server Error (5xx)
The response class (500-599) indicating a failure caused by the server.
Troubleshooting/Operational Context
Requires diagnosing infrastructure issues (network, application, database, gateway).
Server Time-out (504)
A 5xx error indicating the server (acting as a proxy/gateway) did not receive a timely response from an upstream element.
Troubleshooting/Operational Context
Indicates a failure in the upstream communication path.
Session
A collection of participants and media streams established for communication purposes, typically resulting in one or more SIP dialogs.
Troubleshooting/Operational Context
The multimedia conversation that SIP controls.
Session Progress (183)
A provisional response providing information about the session’s progression (e.g., early media or call processing status).
Troubleshooting/Operational Context
Used when a call is handled by a gateway or voicemail system before ringing.
SIP Message
The basic unit of SIP data transfer, either a Request or a Response.
Troubleshooting/Operational Context
Text-based and structured using Augmented BNF (Section 25).
SIPS URI
A secure URI scheme (sips:) that mandates the use of secure transport (TLS) from the UAC up to the destination domain.
Troubleshooting/Operational Context
Use SIPS to enforce security policies; requires implementation of TLS.
SIP Version
The SIP protocol version used, typically “SIP/2.0”.
Troubleshooting/Operational Context
Must be included in the Request-Line and Status-Line. Failure to support the version results in 505 error.
Spiral
A SIP request routed back to a proxy it has already visited, often leading to loops if not checked by Max-Forwards.
Troubleshooting/Operational Context
Indicates complex or potentially flawed routing logic.
Stateless Proxy
A proxy that maintains no transaction state, only forwarding requests and responses based on the Via header.
Troubleshooting/Operational Context
Faster, but cannot perform reliable delivery or handle complex logic. Transparent to transactions.
Status Code
The 3-digit integer result code in a response indicating its class and specific outcome.
Troubleshooting/Operational Context
The primary diagnostic tool for determining call failure reason.
Status Line
The first line of a Response message, containing the SIP-Version, Status-Code, and Reason-Phrase.
Troubleshooting/Operational Context
Must be syntactically correct for the response to be processed.
Strict Router
A proxy that implements older routing logic (RFC 2543), where the Request-URI is replaced by the first Route header entry.
Troubleshooting/Operational Context
If interacting with legacy equipment, be aware this behavior changes the Request-URI.
Subject
A header field indicating the topic of the session.
Troubleshooting/Operational Context
Can be used for call filtering or display purposes.
Success (2xx)
The response class (200-299) indicating successful processing of the request.
Troubleshooting/Operational Context
Indicates successful outcome (e.g., 200 OK).
Supported
A header field enumerating SIP extension Option Tags that the UA supports.
Troubleshooting/Operational Context
Used in conjunction with Require to negotiate capabilities.
T
tag parameter
A parameter in the To/From headers that, along with Call-ID, defines the Dialog ID.
Troubleshooting/Operational Context
The To tag is generated by the UAS in the response and is crucial for matching subsequent dialog messages.
TCP (Transmission Control Protocol)
A reliable, connection-oriented transport protocol SIP MUST implement.
Troubleshooting/Operational Context
Mandatory for SIPS URIs and necessary when message sizes exceed UDP MTU.
tel URL
A URI scheme (RFC 2806) used to represent telephone numbers, which SIP elements MAY support and translate.
Troubleshooting/Operational Context
Check URI translation logic when dialing PSTN numbers.
Terminated
The final state of a SIP transaction.
Troubleshooting/Operational Context
Indicates the transaction has completed (successfully or failed) and the state machine can be destroyed.
Timer A
The timer controlling INVITE request retransmissions when using unreliable transport (UDP).
Troubleshooting/Operational Context
If Timer A fires repeatedly, it indicates network loss or that the proxy/UAS failed to respond with 100 Trying.
Timer B
The timer controlling the overall INVITE transaction timeout (value 64*T1 seconds).
Troubleshooting/Operational Context
If Timer B fires, the transaction fails with a 408 (Request Timeout).
Timer E
The timer controlling Non-INVITE request retransmissions (starts at T1).
Troubleshooting/Operational Context
Similar to Timer A, but for Non-INVITE methods.
Timer F
The timer controlling the overall Non-INVITE transaction timeout (value 64*T1 seconds).
Troubleshooting/Operational Context
If Timer F fires, the Non-INVITE transaction fails with a 408 (Request Timeout).
TLS (Transport Layer Security)
The standard protocol used to encrypt transport (typically over TCP, port 5061).
Troubleshooting/Operational Context
Mandatory for SIPS URIs and required for secure trunk communication.
To
A mandatory header field specifying the desired logical recipient of the request (callee).
Troubleshooting/Operational Context
Used for routing; in the response, it carries the Remote Tag.
Too Many Hops (483)
A 4xx response indicating Max-Forwards reached 0 before the request reached its destination.
Troubleshooting/Operational Context
Directly indicates a routing loop or overly long path.
Transaction
The fundamental message exchange unit in SIP (one request and all responses).
Troubleshooting/Operational Context
Used to define the boundaries for reliability, timers, and retransmissions.
transport parameter
A SIP URI parameter determining the transport mechanism (UDP, TCP, TLS, SCTP) to be used for sending messages.
Troubleshooting/Operational Context
Used to override the default transport protocol derived from DNS.
Trying (100)
A provisional response indicating the request was received and processing is underway.
Troubleshooting/Operational Context
The first 1xx response. It stops the UAC’s Timer A retransmission.
ttl parameter
A SIP URI parameter specifying the Time-To-Live for multicast requests.
Troubleshooting/Operational Context
Used for managing multicast communication.
TU (Transaction User)
The layer above the Transaction layer (UAC Core or Proxy Core) that utilizes transaction services.
Troubleshooting/Operational Context
The logic layer responsible for interpreting the outcome of a transaction.
U
UAC (User Agent Client)
The logical SIP endpoint that generates requests.
Troubleshooting/Operational Context
The initiating device in the SIP trunk; responsible for request construction and managing client transactions.
UAS (User Agent Server)
The logical SIP endpoint that receives requests and generates responses.
Troubleshooting/Operational Context
The terminating device; responsible for final response generation and dialog state maintenance.
UDP (User Datagram Protocol)
An unreliable datagram transport protocol SIP MUST implement.
Troubleshooting/Operational Context
Most common SIP transport, requiring internal reliability mechanisms (retransmissions/timers).
Unauthorized (401)
A 4xx response requiring user authentication (sent by a UAS or Registrar).
Troubleshooting/Operational Context
Triggers the UAC to resend the request with an Authorization header field.
Uniform Resource Identifier (URI)
A string identifying a communications resource (e.g., SIP, SIPS, tel).
Troubleshooting/Operational Context
The fundamental addressing scheme in SIP.
URI Comparison
The strict rules used to determine whether two SIP or SIPS URIs are equivalent (case-sensitive userinfo, case-insensitive host, parameter significance).
Troubleshooting/Operational Context
Critical for registrar functions and dialog matching.
Unsupported URI Scheme (416)
A 4xx response indicating the Request-URI scheme (e.g., mailto) is not supported by the proxy/server.
Troubleshooting/Operational Context
UAC should retry using a SIP URI.
Unsupported
A header field included in 420 (Bad Extension) responses, listing the unsupported Option Tags.
Troubleshooting/Operational Context
Used to communicate missing capabilities back to the UAC.
Upstream
The direction of message forwarding from the User Agent Server to the User Agent Client (responses travel upstream).
Troubleshooting/Operational Context
Used to track the path of responses through a proxy chain.
uri-parameters
Parameters appended to the hostport portion of a URI (e.g., transport, lr, maddr).
Troubleshooting/Operational Context
Used to convey specific handling instructions for the URI.
User Agent (UA)
A SIP endpoint, representing an end system containing a UAC and a UAS.
Troubleshooting/Operational Context
Refers to the physical or logical device handling the call.
User-Agent
A header field providing information about the UAC’s software.
Troubleshooting/Operational Context
Used for compatibility checks and tracing issues related to specific device models.
userinfo
The optional identifier and password portion of a SIP URI (user:password@).
Troubleshooting/Operational Context
Used for user identification; password use is discouraged.
Use Proxy (305)
A 3xx redirection response instructing the client to use the proxy specified in the Contact header.
Troubleshooting/Operational Context
Enforces a specific routing policy via a designated proxy.
V
Via
A mandatory header field identifying the transport used, the client’s address, and where the response should be sent.
Troubleshooting/Operational Context
Critical for response routing. Each proxy adds its own Via header entry upon forwarding a request.
via-branch
The branch parameter within the Via header field.
Troubleshooting/Operational Context
Must start with “z9hG4bK” for transaction matching and loop detection.
via-received
The received parameter within the Via header field.
Troubleshooting/Operational Context
The IP address from which the request was actually received by the proxy.
W
Warning
A header field providing additional information (Warn-codes) about the message status or transformation.
Troubleshooting/Operational Context
Used to detail technical reasons for session failure (e.g., 488 Not Acceptable Here).
Warn-code
A 3-digit code within the Warning header field, typically starting with ‘3’ for SIP-specific warnings.
Troubleshooting/Operational Context
Used to specify the exact type of media or network related warning.
WWW-Authenticate
A challenge header field included in a 401 response, requiring user authentication.
Troubleshooting/Operational Context
Provides the necessary realm and nonce for the UAC to construct the Authorization header.
Z
z9hG4bK
The mandatory “magic cookie” prefix for the branch parameter in the Via header.
Troubleshooting/Operational Context
Verifies RFC 3261 compliance and is essential for transaction matching and loop detection logic.
Key Response Codes (Operational Focus)
| Code | Term | Class |
|---|---|---|
| 100 | Trying | Provisional |
| 183 | Session Progress | Provisional |
| 408 | Request Timeout | Client Error |
| 423 | Interval Too Brief | Client Error |
| 480 | Temporarily Unavailable | Client Error |
| 487 | Request Terminated | Client Error |
| 503 | Service Unavailable | Server Error |
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