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RFC4028 Session Timer in the Session Initiation Protocol

RFC4028 Session Timer in the Session Initiation Protocol. Speaker ： Ying Shun Lin Adviser ： Quincy Wu. Outline. Introduction Session-Expires Header field Definition Min-SE Header field Definition 422-Response Code Definition UAC / Proxy / UAS Behavior Security Considerations

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RFC4028 Session Timer in the Session Initiation Protocol

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  1. RFC4028Session Timer in theSession Initiation Protocol Speaker：Ying Shun Lin Adviser ：Quincy Wu

  2. Outline • Introduction • Session-Expires Header field Definition Min-SE Header field Definition 422-Response Code Definition • UAC / Proxy / UAS Behavior • Security Considerations • Example call Flow

  3. Introduction (1/3) • SIP does not define a keepalive mechanism for the sessions it establishes INVITE UAC 100 Trying BYE call stateful proxy will retain state for the call

  4. Introduction (2/3) • This extension defines a keepalive mechanism for SIP sessions. UAs send periodic (re-INVITE or UPDATE) requests (referred to as session refresh requests) to keep the session alive . • If a session refresh request is not received before the interval passes，the session is considered terminated. Both UAs are supposed to send a BYE，and call stateful proxies can remove any state for the call.

  5. Introduction (3/3) • Two new header fields (Session-Expires and Min-SE) and a new response code (422) are defined - Session-Expires：conveys the duration of the session - Min-SE ：conveys the minimum allowed value for the session expiration. - 422 response ：indicates that the session timer duration was too small.

  6. Define some terms • Session Interval • Session Expiration • Session Refresh Request • Initial Session Refresh Request • Subsequent Session Refresh Request • Refresh

  7. Session-Expires Header Field Definition • placed only in requests (INVITE or UPDATE), as well as in any 2xx response to request. • MUST be prepared to handle Session-Expires header field values of any duration greater than 90 ；1800 seconds (30 minutes) is RECOMMENDED. • insert the Session-Expires header field SHOULD NOT choose values of less than 30 minutes. Session-Expires:1800;refresher=uac

  8. Min-SE Header Field Definition • used in an request (INVITE or UPDATE) ， it indicates the smallest value of the session interval that can be used for that session . • MUST NOT be less than 90 seconds - When the header field is not present, its default value for is 90 seconds. • MUST NOT be used in responses except for those with a 422 response code . Min-SE:90

  9. 422 Response Code Definition • Session Interval Too Small - generated by a UAS or proxy when a request contains a Session-Expires header field with a duration below the minimum timer for the server . • MUST contain a Min-SE header field with the minimum timer for that server.

  10. Session-Expire & Min-SE Header Fields

  11. UAC Behavior • Generating an Initial Session Refresh Request • Processing a 2xx Response • Processing a 422 Response • Generating Subsequent Session Refresh Requests

  12. UAC /Proxy Behavior Supported :timer Session Expires: XX ; refresher =‘uac` Min-SE: XX INVITE UAC Require: timer Supported: timer Session-Expires : ; refresher= 200 proxy Min-SE : 422 Session Interval Too Small

  13. UAC Behavior UAC Min-SE :xx 422 proxy Supported :timer Session Expires: XX ; refresher =‘ uac/uas’ Min-SE: XX INVITE • If a UAC knows that its peer supports the UPDATE method • RECOMMENDED that UPDATE be used instead of a re-INVITE

  14. Proxy Behavior • The proxy processing rules require the proxy to remember information between the request and response, ruling out stateless proxies. - Processing of Requests - Processing of Responses - Session Expiration

  15. Proxy Behavior (Request) Supported :timer Session Expires: (small) Proxy 2 call failure Session Expires: XX Min-SE:XX Session Expires: XX Min-SE:XX INVITE INVITE Proxy 1

  16. Proxy Behavior (Response) (proxy remembers UAC did not support ) There is no session expiration for this session UAS did not support the session timer UAS Session Expires Session-Expires (from the forwarded request) refresher :`uac` (proxy remembers that the UAC did support the session timer)

  17. UAS Behavior Supported :timer Session Expires: Min-SE: INVITE UAS 422 Min-SE: Min-SE: proxy 200 ok Session Expires:

  18. UAS Behavior

  19. Security Considerations(1/3) • Inside Attacks Case 1: a rogue UAC that wishes to force a UAS to generate refreshes at a rapid rate - The UAS or any proxy that objects to this low timer will reject the request with a 422, thereby preventing the attack.

  20. Security Considerations(2/3) Case2: rogue UAS that wishes to force a UAC to generate refreshes at a rapid rate . - UAC copy the current session interval into the Session-Expires header field in the request. The proxies will reject this request and provide a Min-SE with a higher minimum, which the UAC will then use.

  21. Security Considerations(3/3) • Outside Attacks - An element that can observe and modify a request or response in transit can force rapid session refreshes . -proxies that record-route and request session timer SHOULD record-route with a SIPS URI . A UA that inserts a Session-Expires header into a request or response SHOULD include a Contact URI that is a SIPS URI.

  22. (1) INVITE sips:bob@biloxi.example.com SIP/2.0 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8 Supported: timer Session-Expires: 90 Max-Forwards: 70 To: Bob <sips:bob@biloxi.example.com> From: Alice <sips:alice@atlanta.example.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: <sips:alice@pc33.atlanta.example.com> Content-Type: application/sdp Content-Length: 142 Example Call Flow Proxy P2 Proxy P1 Alice Bob (1)INVITE SE:90 (4) INVITE sips:bob@biloxi.example.com SIP/2.0 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds9 Supported: timer Session-Expires: 3600 Min-SE: 3600 Max-Forwards: 70 To: Bob <sips:bob@biloxi.example.com> From: Alice <sips:alice@atlanta.example.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314160 INVITE Contact: <sips:alice@pc33.atlanta.example.com> Content-Type: application/sdp Content-Length: 142 (2) SIP/2.0 422 Session Interval Too Small Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8 ;received=192.0.2.1 Min-SE: 3600 To: Bob <sips:bob@biloxi.example.com>;tag=9a8kz From: Alice <sips:alice@atlanta.example.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq:314159 INVITE (2)422 MSE:3600 (3)ACK (4)INVITE SE:3600 MSE:3600

  23. Example Call Flow (10) INVITE sips:bob@biloxi.example.com SIP/2.0 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds10 Supported: timer Session-Expires: 4000 Min-SE: 4000 Max-Forwards: 70 To: Bob <sips:bob@biloxi.example.com> From: Alice <sips:alice@atlanta.example.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314161 INVITE Contact: <sips:alice@pc33.atlanta.example.com> Content-Type: application/sdp Content-Length: 142 Proxy P2 Proxy P1 Alice Bob (5)INVITE SE:3600 MSE:3600 (6)422 MSE:4000 (7)ACK (8)422 MSE:4000 (9)ACK (10)INVITESE:4000 MSE:4000

  24. (15) SIP/2.0 200 OK Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds10 ;received=192.0.2.1 Require: timer Supported: timer Record-Route: sips:p1.atlanta.example.com Session-Expires: 4000;refresher=uac To: Bob <sips:bob@biloxi.example.com>;tag=9as888nd From: Alice <sips:alice@atlanta.example.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314161 INVITE Contact: <sips:bob@192.0.2.4> Content-Type: application/sdp Content-Length: 142 Example Call Flow Proxy P2 Proxy P1 Alice (11)INVITESE:4000 MSE:4000 Bob (12)INVITESE:4000 MSE:4000 (13)200OKSE:4000 (14)200OK SE:4000 (15)200OK SE:4000 (16)ACK (17)ACK

  25. Example Call Flow Proxy P2 Proxy P1 Alice Bob (18)UPDATESE:4000 (19)UPDATESE:4000 (18) UPDATE sips:bob@192.0.2.4 SIP/2.0 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds12 Route: sips:p1.atlanta.example.com Supported: timer Session-Expires: 4000;refresher=uac Max-Forwards: 70 To: Bob <sips:bob@biloxi.example.com>;tag=9as888nd From: Alice <sips:alice@atlanta.example.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314162 UPDATE Contact: <sips:alice@pc33.atlanta.example.com> (20)200OK SE:4000 (21)200OK SE:4000 (22)BYE (23)BYE (24)408 (Request Timeout)

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