]> Avoiding Registration Storms by adapted Registration Behavior for Voice Cloud Applications Deutsche Telekom
Ida-Rhodes-Str. 2 Darmstadt 64295 Germany roland.schott@telekom.de https://www.telekom.de
Deutsche Telekom
Nürnberg 90441 Germany michael.kreipl@telekom.de
Deutsche Telekom
Hamburg 20359 Germany Bastian.Dreyer@telekom.de
Deutsche Telekom
Darmstadt 64295 Germany r.jesske@telekom.de
Applications and Real-Time Area ART application, cloud, re-registration, sip, stateless, resumption This document describes the AVORS (Avoiding Registration Storms) concept that allows the resumption of active registrations. The concept can be mapped on any architecture having a distributed structure and could work for different protocols. The concept is exemplary explained here regarding an architecture for voice and is mapped on a 3GPP (3rd Generation Partnership Project) architecture. This document describes the AVORS (Avoiding Registration Storms) concept that allows the resumption of active UE (User Equipment) registrations on other Outbound Proxies (P-CSCF) within the SIP voice architecture. The AVORS concept increases service continuity, improves network resilience, and offers savings potential. Additionally, this document gives an outlook regarding stateless voice architectures, load calculation aspects, and Service Based Interfaces (SBI) in context data base interworking. Security aspects are considered in the security chapter. As stated above the AVORS principle is not only limited to the SIP protocol and could be adopted by other protocols.
Introduction The AVORS "Avoiding Registration Storms" concept in context of SIP and IMS (IP-Multimedia Subsystem) respectively helps, as the name suggests, to reduce registration storms in case of an outage especially site outages. Nowadays, registration storms are prevented by overcapacity. This overcapacity can be used for other applications, if available or it is idling until an outage occurs. The idling is causing electricity cost even in the case of intelligent power management. In stateless architectures the registration context is stored in a session data base and normally all instances could access this session data base. According to Service Based Architecture (SBA) and Service Based Interfaces (SBI) offer in principal access to the session data base for IMS cloud based applications. Regarding the current standardized registration behavior the IMS UE (User Equipment) MUST initiate a new initial registration. This registration needs to pass the outbound proxy (P-CSCF) and the registrar (S-CSCF) before reaching the data base. With the AVORS principle the outbound proxy (P-CSCF) has a dip into the data base and recognizes that a UE is already registered and is able to resume the registration. Resuming of a register of a session is feasible because the registration session context can be stored in a session data base. Instead of sending an initial registration in case of an outage the UE will send a re-register message to the secondary outbound proxy namely a failover P-CSCF. The latter is able to retrieve the session information out of the session data base and is able to resume the registration without sending the message via the registrar or S-CSCF. This works especially when the registrar (S-CSCF) is fully stateless and shortens the amount of messages being sent in case of a failover scenario. The idea of resumption of a registration or a session is working also for other protocols than SIP e.g., TLS or . AVORS use the idea of session resumption for SIP via a data base dip from the P-CSCF as an alternative approach to optimize registration behavior especially in case of heavy outages and registration storms. The mechanism does not obsolete the original or classical registration behavior and is complementary. The UE or end devices can run either in classical or AVORS mode and the SIP core or IMS core systems can have both options implemented, classical mode and AVORS mode. The AVORS mode is also working in the case that the failover P-CSCF (secondary outbound proxy) uses a different IP address compared to the primary one. The mechanism can be combined with TLS resumption in case of a wireline Residential Gateway (RG). In wireless or mobile context where IPSec is used for authentication the SIP registration resumption works similar to the wireline case. The aim of this document is to specify how resumption for SIP registration works in combination with a session data base. The focus of this document is on the aspect of registrations and recovery time in case of outages e.g., site outages. A fully session resumption including resumption of media streams needs to be analyzed in an additional work. This principle is described above for the geo-redundancy use case and also works for local redundant instances or in combination.
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 .
Rationale of the usage of AVORS mode for SIP sessions By using the AVORS mode for SIP sessions the overcapacity within the IMS Core or voice systems can be reduced because the amount of message flows is reduced when the P-CSCF is able to resume the registration directly. The approach fits into the SBA of 3GPP and the introduction of a session data base in context 5G. Recovery time of the system is also optimized and shortened which is beneficial for the users. In future cloud based SIP applications will run as virtual instances or in containers and will have a context data base anyway. Therefore, it is reasonable to provide an additional registration or re-registration mechanism when migrating the SIP or IMS systems into the cloud. Regarding the UE there are two paths possible:
  • UE agnostic approach requiring that the voice or IMS core respectively behaves accordingly. No special indication is given by the UE that it works in either the classical or the AVORS mode.
  • UE AVORS approach where the UE sends an AVORS header or parameter indicating that it is working in AVORS mode. Without sending this AVORS parameter, it is assumed that the UE is working in classical registration mode.
Depending on the installed basis of the operator or the operational requirements both options are valid approaches. In case the UE indicates that it operates in AVORS mode an AVORS header needs to be specified. A suggestion how such a header or parameter could look like is described in the Appendix.
Architecture Overview of AVORS Concept This section gives an overview of the registration resumption concept for SIP sessions. As stated above the classical registration mechanism in case of a failure of a site or a P-CSCF failure is to start a new initial registration towards the secondary outbound proxy and P-CSCF respectively. The registration process invokes P-CSCF, S-CSCF and the data base. With AVORS the P-CSCF is able to resume a registration by handling a normal re-register message of the UE. Instead of starting a new initial registration the UE sends the normal re-registration message to the secondary outbound proxy namely P-CSCF. The P-CSCF gets the session registration context out of the data base where the session context is stored, i.e., session data base. In case of using a TLS session the session resumption process can be implemented for both protocols, TLS and SIP, making the recovery and failover process more efficient. The spare overcapacity needed for processing the initial registration process can be reduced and the recovery mechanism is kept efficient. This document has not the ambition to investigate a TCP session resumption. The AVORS architecture is described in the following figures. describes the geo-redundancy use case.
AVORS Geo-Redundancy.
describes the local-redundancy use case.
AVORS Local-Redundancy.
Functions of Registration Resumption Feature This document is focusing on the introduction of registration resumption in a SIP (Session Initiation Protocol) environment. The method can be used for SIP-Proxies and SIP-Registrars or according to 3GPP IMS (IP-Multimedia Subsystem) for P-CSCF and S-CSCF architecture. It also works in context of 3GPP SBA (Service Based Architecture). In a second step, one can consider using a similar advanced mechanism for complete session resumption. The procedure and functions described here are part of a stateless voice architecture and are suitable for use in a cloud environment.
P-CSCF related Functions
Function Description
AVORS Option Tag P-CSCF shall on receipt of the AVORS Option Tag in the Supported Header of the Initial Register confirm the AVORS feature support by including the AVORS Option Tag in the 200 OK message.
P-CSCF - Database Interface P-CSCF shall provide an interface to the Database according Udsf Specification of 3GPP.
Storage of Security Data P-CSCF shall store after each successful TLS based Initial Registration the tls session context in the Database according to the related Datamodel and additionally locally (in order to save database requests).
Storage of TCP Fast Open (TFO) Data P-CSCF shall store after each successful TCP based Initial Registration or TLS based Initial Registration the TFOData in the Database according to the used Datamodel and additionally locally (in order to save database requests).
TCP Fast Open Support P-CSCF shall support TCP session resumption (TFO) at the Gm interface according to .
TFO parameter For the TCP session resumption P-CSCF shall obtain from the Database TFOData the required TFO session cookie based on the UE IP address.
TLS Session Resumption P-CSCF shall support TLS session resumption at the Gm interface according to for TLS 1.2 and for TLS 1.3.
TLS Session Resumption parameter For the TLS session resumption P-CSCF shall obtain from the Database Security Data View the TLS session context based on the TLS SessioID. Note: Be aware of the possible TLS Resumption timeout that shall be avoided.
Handling Register messages of users not registered at the P-CSCF On receipt of a Register message (Initial or Re-Register) not belonging to a registered UE on this P-CSCF instance, P-CSCF shall obtain from Database Registration data the required UE Registration and NBA (NASS Bundled Authentication) data based on the UE IP address and PAU. Note: PAU may contain 1-3 IMPUs that might be implicitly registered.
Initial Registration initiation Initial Registration shall be initiated when no Registration Data is available on the Database.
Generation of Registration Data and storage in Database After receipt of 200 OK, P-CSCF shall store after each successful UDP based Initial Registration, TCP based Initial Registration, TLS based Initial Registration, or IPSec based Initial Registration the Registration and NBA Data (fixed access) in the Database according to used Datamodel AVORS Registration View and additionally locally (in order to save Database requests). - For fixed access in IMPU table - For mobile access in IMSI table
Generation of IMSI/IMPU correlation table and storage in Database After receipt of 200 OK, P-CSCF generates IMSI/IMPU value pair and and stores it in the Database in IMSI/IMPU correlation table and if suitable locally. Note: IMSI/IMPU correlation table mighjt be used for in the terminating Invite use case, when the S-CSCF or terminating P-CSCF requires to obtain called user Registration Data stored under the record identifier "IMSI", but only knows the IMPU i the To-header.
Registration Resumption on new P-CSCF instance for fixed network access Registration of IMPUs shall be resumed (no Initial Registration) on P-CSCF instance previously not assigned during Initial Registration phase in case of available Registration Data the on
  - UDP based Initial Register message with or without supported SIP InstanceID (no nonce)
  - UDP based Re-Register message AVORS header, same IP address, same SIP InstanceID, same CallID, Cseq >1, valid nonce. The contact header may contain no port number or the port number 5060.
  - TFO based Re-Register message with AVORS header, same IP address, same SIP InstanceID, same CallID, Cseq >1, valid nonce. The contact header may contain no port number or the port number 5060.
  - TLS Session Resumption based Re-Register message with AVORS header, same IP address, same SIP InstanceID, same CallID, Cseq >1, valid nonce. The contact header may contain no port number or the port number 5061.
  Note: UDP based Initial Registration message for Registration Resumption is allowed due to IAD agnostic approach.
Cascaded registration timer for fixed network access For fixed network access cascaded registration timer shall be supported. In case of switch over to another P-CSCF instance, Re-Register message counter starts with “1 to 4 randomly”. Note: The 5th Re-Register message counter number is not used in order to avoid forwarding to S-CSCF in failure case. The numbers are examples here and should be configurable.
Registration Resumption confirmation P-CSCF shall confirm the successful Registration Resumption with a 200 OK message to the UE when Register message is not forwarded to S-CSCF.
Registration Resumption on new P-CSCF instance for mobile network access Registration of IMSI shall be resumed (no Initial Registration) on P-CSCF instance previously not assigned during Initial Registration phase in case of available Registration Data in the Database on
  - IPSec based Initial Register message
AA Request on registration resumption for mobile network access When a mobile network access registration resumption on a new P-CSCF instance is conducted, new P-CSCF shall execute a AA Request to PCRF via Rx interface. Note: The AA Request is required to update session information in the PDN Gateway, that a new P-CSCF instance has taken over.
Forwarded Register message to S-CSCF for mobile access during registration resumption For mobile access forwarded Register message to S-CSCF in registration resumption uses case shall comply with a Re-Register message, containing the previous Call-ID and the nonce in the Authorization header retrieved from the Database. Note: This applies in the Registration resumption use case (switchover to another node) when initiated by an Initial Register message.
Update of pcscfPath After successful Registration Resumption P-CSCF shall update the pcscfPath attribute of the Database Registration Data with it's own name.
Reset for registration Timer in fixed network P-CSCF must update the Registration Data in the Database with every 5th Re-Register, in order to reset the registration timer. Note: In fixed network cascaded registration timer is introduced. P-CSCF maintains for example a 12 min registration timer, S-CSCF a one-hour registration timer. When in an AVORS deployment Registration data gets stored locally in P-CSCF, P-CSCF must update the Registration Data in the Database with every 5th Re-Register, in order to reset the registration timer. The numbers are examples here and should be configurable.
Reset of registration Timer in mobile network P-CSCF must update the Registration Data in the Database with every Re-Register, in order to reset the registration timer. Note: In mobile network every Re-Register initiates an Registration Data update in the Database in order to reset the registration timer.
TLS based Re-Register message at new P-CSCF instance On receipt of a TLS based Re-Register message not belonging to a registered UE on this P-CSCF instance, P-CSCF shall initiate an Initial Registration.
No TFO Data in the Database If on a TCP session resumption request no TFO Data can be found in the Database, P-CSCF must reject the TCO session resumption request.
No Security Data in the Database If on a TLS Session Resumption request no Security Data can be found in the Database, P-CSCF must initiate a new TLS session establishment and Initial Registration.
No Registration Data in the Database If on an Initial Registration or Re-Registration message no Registration Data can be found in the Database, P-CSCF must initiate an Initial Registration for the UE.
No NBA Data in NDL If on an Initial Registration or Re-Registration message no NBA Data can be found in the Database, P-CSCF must initiate an Initial Registration for the UE.
UE initiated De-Registration On the receipt of a 200 OK messsage belonging to a De-Registration message from a UE, P-CSCF shall delete the Security data, TFO data, Registration Data and NBA Data of the concerned IMPU from the Database.
P-CSCF Registration timer expiry After the P-CSCF Registration Timer of a UE registration has expired, P-CSCF shall terminate the UE binding. Note: Originating and terminating calls must be rejected after binding termination. P-CSCF must not send a network initiated De-Registration.
SIP request forwarding to local S-CSCF Generally, every S-CSCF instance can be selected by P-CSCF for SIP request forwarding. However, the SIP request forwarding to the local S-CSCF is the prioritized option and shall be selected whenever possible.
Switch back to primary P-CSCF For the initiation of the intentional switch back to the primary P-CSCF, the Graceful Shutdown feature shall be utilized.
System wide Disaster Restoration mechanism In case of system wide outage with lost of registration data or new IP address assignment it shall be possible to deactivate the P-CSCF AVORS feature and and to allow step by step start up of single sites
Registration Data generation in case of activated Implicit Registration state (IRS) When IRS is activated P-CSCF shall generate on the first user's Initial Registration Registration Data for all IMPUs of the user and store them in the Database and locally. Note: The user's assigned IMPUs can be derived from the PAU header in the 200 OK response of the first Initial Registration.
Application Server instance Assignment at first IMPU/IMSI registration of a user P-CSCF shall assign an Application Server instance at the first IMPU registration of a user. After the receipt of the 200 OK response for the Initial Registration, P-CSCF shall query Database for Registration Data for all IMPUs of the user provided in the PAU of the 200 OK response. Since no Registration Data is available, P-CSCF selects an Application Server instance (preferably the local AS) and writes the selected AS instance into the new generated Registration Data of this IMPU.
Application Server instance identification for second or following IMPU/IMSI registration of a user or same IMPU at second or following IP address P-CSCF shall assign an Application Server instance for the first IMPU registration of a user. After the receipt of the 200 OK response for the Initial Registration, P-CSCF shall query Database for Registration Data for all IMPUs of the user provided in the PAU of the 200 OK response. Since already an IMPU is registered, Database will provide the Registration Data of this IMPU. P-CSCF identifies the already assigned AS for this user and inserts the assigned AS into the Registration Data of the IMPU that gets registered.
S-CSCF related Functions
Function Description
Stateless S-CSCF S-CSCF shall be provided in a stateless implementation with respect to Registration. Assignment of dedicated S-CSCF shall be abdicated.
Session Data Stateless S-CSCF only concerns Registration related features. Session related features shall remain stateful.
S-CSCF Registration Timer Since the S-CSCF is operated in stateless operation mode, the S-CSCF Registration timer shall be apdicated in S-CSCF.
  Note: Regitration timer is moved to the UDSF chapter 5.3 Nudsf Timer Service.
S-CSCF Restoration Procedure In stateless S-CSCF mode S-CSCF is not assigned to users anymore, thereby the S-CSCF Restoration Procedure shall be abdicated.
  Note: Stateless S-CSCF is able to process the SIP requests of any user.
S-CSCF User Re-Distribution In stateless S-CSCF mode S-CSCF is not assigned to users anymore, thereby the S-CSCF User Re-Distribution feature shall be abdicated.
  Note: Stateless S-CSCF can be selected freely after registration at new P-CSCF.
Initial Registration Call Flow mobile network access Deviant from the standard Initial Registration call flow the
  - S-CSCF Register authorization with UAR request shall be configurable
  - S-CSCF assignment with UAR request shall be abdicated
  - S-CSCF shall assign in case of stateful AS the responsible AS instance for the user and write the assigned AS instance into the Registration Data in the Database
Initial Registration Call Flow mobile network access Deviant from the standard Initial Registration call flow the
  - S-CSCF Register authorization with UAR request shall be configurable for foreign network access and own network access
  - S-CSCF assignment with UAR request shall be abdicated
  - 3rd Party Registration shall be abdicted in case of stateless Application Server
  - S-CSCF shall assign in case of stateful AS the responsible AS instance for the user and write the assigned AS instance into the Registration Data in the Database
  Note: When the Registration request is sent by a visited network, that can be derived from the P-Visited-Network-ID of the Register message sent by P-CSCF, a UAR query shall be sent.
  Note: When the Registration request is sent by the home network, that can be derived from the P-Visited-Network-ID of the Register message sent by P-CSCF, a UAR query shall be abdicated.
Service-Route header Due to the stateless S-CSCF implementation, P-CSCFs do not need to be informed about the assigned S-CSCF by the Service-Route header in the 200 OK response from the S-CSCF on Initial Registrations. The Service-Route header shall not be supported.
  Note: In a stateless S-CSCF deployment any S-CSCF instance can be selected for SIP requests processing. Usually the local S-CSCF instance gets selected.
Re-Registration On an incoming Re-Registration message and locally unavailable registration
  - S-CSCF shall query NDL AVORS Registration Data for active Registration
  - In case of active Registration, the Registration shall be resumed
  - In case no active Registration, initial registration process shall be initiated
Re-Registration Call Flow Deviant from the standard Re-Registration call flow the
  - assigned S-CSCF determination shall be abdicated
  - S-CSCF shall not include S-CSCF URI in SAR request towards HSS
  - User Data shall be requested with SAR request when the User Data is not available locally (switchover to Failover site)
Deletion of De-Registered Registration Data on Network initiated De-Registration The S-CSCF shall delete the Security Data, the TFO Data, the Registration Data, and NBA Data of an UE in the Database after network initiated De-Registration of the UE.
Deletion of De-Registered Registration Data in case of activated IRS If in an IRS environment an user de-registers an IMPU from one IP address, all other IMPUs belonging to the user and registered with the same IP address shall be de-registered and the S-CSCF shall delete the Security Data, the TFO Data, the Registration Data, and NBA Data of an UE in the Database. Note: The user's IMPUs registered with different IP addresses remain registered.
User initiated De-Registration On receipt of a De-Registration message from the UE, S-CSCF shall send an according Delete request to Database
Database initiated De-Regstration HSS notice When S-CSCF receives a De-Registration notification, S-CSCF sends a SAR message to HSS. Note: S-CSCF informs HSS about a network initiated De-Registration
Database initiated De-Regstration UE notice When S-CSCF receives a De-Registration notification, S-CSCF sends a Notfy (De-Registered) message to P-CSCF. Note: S-CSCF informs HSS about a network initiated De-Registration
HSS initiated De-Regstration Database notice On receipt of a Registration Termination Request (RTR) from HSS, S-CSCF shall send an according Delete request to Database
Invite Call Flow Deviant from the standard Invite call flow the
  - for Onnet Calls the selected S-CSCF instance shall act as originating and terminating S-CSCF
  - S-CSCF shall retrieve the user data of User B with SAR request
  - S-CSCF shall determine the P-CSCF instance of User B with retrieving the Registration Data from Database
Stateful Application Server selection In case the Application Server operates in stateless mode, the assigned ASs of User A and User B shall be determined by querrying the Registration Data of User A and User B locally or in Database
System wide Disaster Restoration mechanism In case of system wide outage with lost of registration data or new IAD IP address assignment it shall be possible to deactivate the S-CSCF AVORS feature and to enforce step by step start up of single sites.
Registration Timer Deactivation Parameter In case of disaster causing outage longer as the S-CSCF Registration time, it shall be possible to deactivate the Registration Timer validation in the Re-Registration process.
Identification of user / IMPU assigned AS instance for Invite request S-CSCF shall identfy the assigned AS to the user from the Registration Data of the IMPU, to which the application services shall be applied. The S-CSCF shall forward the Invite message to the assigned AS instance.
Data Base related Functions
Function Description
Storage of Registration Data Database shall store Registration Data according to Datamodel.
Registration Timer for each data record For each stored Registration Data (IMPU/IMSI), a registration timer must be applied.
Registration Data deletion After Reguistration Timer expiry, Database shall delete the according Registration Data and all correlated data.
Database Registration timer configuration The Database Registration Timer shall be configurable from Minutes to Days.
  Note: In case of a system wide outage resulting in long servie outages, the Registration Timer must be able to be increased in order not to loose the registrations.
Reset of registration Timer Each Registration Data record update initiates a reset of the registration timer
Network based De-Registration After registration timer expirey and data record deletion, Database must inform a S-CSCF instance about the De-Registration. Note: Registration timer expiery
Related work Related work can be found in the referenced standards.
Security and operational considerations Registration or session resumption leads to a situation where security plays a role to avoid unauthenticated and unauthorized access to the platform. The security can be hardened in case the sip session resumption is combined with a TLS session resumption. The AVORS mechanism helps in special failure situations to increase the recovery of the platform. It is up to the implementation to request a new initial registration after a longer time interval. Such kind of mechanism would increase security. In case of TCP or TLS the IP address spoofing is not or difficult to achieve. In case of UDP a nonce and next-nonce mechanism with short re-registration timer ensures security. This is also valid in case of using AVORS. Other security considerations will be addressed in future versions of the document.
Abbreviations
IAD Integrated Access Device
P-CSCF Proxy Session Control Function
RG Residential Gateway
S-CSCF Serving Session Control Function
UE User Equipment
Annex A AVORS (Avoidance of Registration Storms): The following sections describes the procedures for AVORS which allows a seamless switch over of IAD in case of a faulty connection towards the first SIP proxy where the IAD is connected. When changing the SIP proxy a simple (Re-) REGISTER is needed to reconnect instead of an challenged initial registration procedure. The following SIP option tag shall apply: This amendment specifies a single option tag, avors. The required information for this registration, as specified in , is: Description: This option tag is for the procedure used to send a re-register instead of a register when changing the first network proxy due to network failure/proxy failure. To allow this procedure the network will indicate if this procedure is implemented. The next part describes a possible process. It is requested to include the SIP Instance-ID in the Contact-Header. For TCP based protocols TFO (TCP Fast Open) according to shall be supported. Latter is relevant for the P-CSCF instances. For TLS based tansport protocols TLS session resumption according to is used at the failover P-CSCF instance. Additionally, the "avors" option tag in order to query P-CSCF support for Registration Recovery Procedure or registration resumption, respectively, is introduced.
IANA Considerations TBD
Acknowledgements This work has been supported by various contributors. Special thanks to TBD.
References Normative References The TLS Protocol Version 1.0 This document specifies Version 1.0 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications privacy over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. SIP: Session Initiation Protocol This document describes Session Initiation Protocol (SIP), an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences. [STANDARDS-TRACK] The Transport Layer Security (TLS) Protocol Version 1.2 This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK] Indication of Support for Keep-Alive This specification defines a new Session Initiation Protocol (SIP) Via header field parameter, "keep", which allows adjacent SIP entities to explicitly negotiate usage of the Network Address Translation (NAT) keep-alive mechanisms defined in SIP Outbound, in cases where SIP Outbound is not supported, cannot be applied, or where usage of keep-alives is not implicitly negotiated as part of the SIP Outbound negotiation. [STANDARDS-TRACK] TCP Fast Open This document describes an experimental TCP mechanism called TCP Fast Open (TFO). TFO allows data to be carried in the SYN and SYN-ACK packets and consumed by the receiving end during the initial connection handshake, and saves up to one full round-trip time (RTT) compared to the standard TCP, which requires a three-way handshake (3WHS) to complete before data can be exchanged. However, TFO deviates from the standard TCP semantics, since the data in the SYN could be replayed to an application in some rare circumstances. Applications should not use TFO unless they can tolerate this issue, as detailed in the Applicability section. The Transport Layer Security (TLS) Protocol Version 1.3 This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. Informative References IP Multimedia Subsystem (IMS); Stage 2 IP Multimedia Call Control Protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP); Stage 3 tbd. Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.