LTE Tutorial Long-Term Evolution

VoLTE SIP Methods, Response Codes and Details

VoLTE IMS Protocols

In IMS subsystem, the critical protocols are the Session Initiation Protocol (SIP), SigComp, Real-time Transport Protocol (RTP), RTP Control Protocol (RTCP) and IP Security (IPSec) and Diameter. These protocols are very important because these are the ones impacted by the UE or have a direct impact on UE operation.

In a VoLTE call, SIP protocol is used to create, modify and terminate sessions, essentially negotiating a session between two users. SIP does not perform transport layer (delivering data) those are done by RTP/RTCP.

SIP is a sequential protocol with request/response similar to HTTP both in functionality and format. Every SIP request begins with a starting line that includes the name of a request type and also called a metho. The table below lists all request methods used for SIP.

SIP MethodDescriptionReference
INVITEINVITE indicates that a client is being invited to participate in a call sessionRFC 3261
ACKACK Confirms that the client has received a final response to an INVITE requestRFC 3261
BYETerminates a call; It can be sent by either the caller or the called partyRFC 3261
CANCELCancels any pending requestRFC 3261
OPTIONSIt Queries the capabilities of serversRFC 3261
PRACKProvisional acknowledgmentRFC 3262
SUBSCRIBESubscribes to event notificationRFC 3265
NOTIFYNotifies the subscriber of a new EventRFC 3265
PUBLISHPublishes an event to the ServerRFC 3903
INFOSends mid-session information that does not modify the session stateRFC 6086
REFERAsks the recipient to issue a SIP request (call transfer)RFC 3515
MESSAGETransports instant messages using SIPRFC 3428
UPDATEModifies the state of a session without changing the state of the dialogRFC 3311

SIP Message Codes and It’s Meaning

SIP message responses are maintained in an Internet Assigned Numbers Authority (IANA) list called Session Initiation Protocol (SIP) Parameters. They always begin with a response code. These Response Codes are divided into the following categories:

  • 1XX Codes (Informational/Provisional):These codes are for the request received and being processed e.g. 100 Trying, 180 Ringing
  • 2xx Codes (Successful Action):The action was successfully received, understood, and accepted e.g. 200 OK, 202 Accepted
  • 3XX Codes (Redirection):Further action needs to be taken (typically by the sender) to complete the request e.g. 301 Moved Permanently, 302 Moved Temporarily
  • 4XX Codes (Client Failure):The request contains bad syntax or cannot be fulfilled at the server e.g. 401 Unauthorized, 403 Forbidden
  • 5XX (Codes Server Failure):The server failed to fulfill an apparently valid request e.g. 500 Server Internal Error, 504 Server Time-out
  • 6XX (Global Failure):The request cannot be fulfilled at any server e.g. 600 Busy Everywhere, 604 Does Not Exist Anywhere

The detailed list of all Response code is given in below tables.

1XX Codes

SIP Message CodeDescriptionReference
181Call Is Being Forwarded
183Session Progress
199Early Dialog TerminatedRFC6228

2XX Codes

SIP Message CodeDescriptionReference
204No NotificationRFC5839

3XX Codes

SIP Message CodeDescriptionReference
300Multiple Choices
301Moved Permanently
302Moved Temporarily
305Use Proxy
380Alternative Service

4XX Codes

SIP Message CodeDescriptionReference
400Bad Request
402Payment Required
404Not Found
405Method Not Allowed
406Not Acceptable
407Proxy Authentication Required
408Request Timeout
412Conditional Request FailedRFC3903
413Request Entity Too Large
414Request-URI Too Long
415Unsupported Media Type
416Unsupported URI Scheme
417Unknown Resource-PriorityRFC4412
420Bad Extension
421Extension Required
422Session Interval Too SmallRFC4028
423Interval Too Brief
424Bad Location InformationRFC6442
428Use Identity HeaderRFC4474
429Provide Referrer IdentityRFC3892
430Flow FailedRFC5626
433Anonymity DisallowedRFC5079
436Bad Identity-InfoRFC4474
437Unsupported CertificateRFC4474
438Invalid Identity HeaderRFC4474
439First Hop Lacks Outbound SupportRFC5626
440Max-Breadth ExceededRFC5393
469Bad Info PackageRFC6086
470Consent NeededRFC5360
480Temporarily Unavailable
481Call/Transaction Does Not Exist
482Loop Detected
483Too Many Hops
484Address Incomplete
486Busy Here
487Request Terminated
488Not Acceptable Here
489Bad EventRFC3265
491Request Pending
494Security Agreement RequiredRFC3329

5XX Codes

SIP Message CodeDescriptionReference
500Server Internal Error
501Not Implemented
502Bad Gateway
503Service Unavailable
504Server Time-out
505Version Not Supported

6XX Codes

SIP Message CodeDescriptionReference
600Busy Everywhere
604Does Not Exist Anywhere
606Not Acceptable