Toward Formal Modelling and Analysis of SCTP Connection Managment Somsak Vanit-Anunchai School of Telecommunication Engineering Institute of Engineering Suranaree University of Technology Nakhon Ratchasima Thailand 22 October 2008

CPN' /10/2008 Outline Introduction to SCTP Motivation SCTP-Packet and VTAG Message sequence chart Tie Tags An error in RFC 4960 Procedure-based modelling approach SCTP-CPN model Analysis Problems Discussion Conclusions and Future Work

CPN' /10/2008 What is Stream Control Transmission Protocol (SCTP)? A transport protocol originally developed by SIGTRANS group, Internet Engineering Task Force (IETF). It became Request For Comments (RFC) 2960 in October Aims to overcome the weakness of TCP. Using four-way handshake and a cookie mechanism to prevent the Denial of Service Attacks (DoS). Internet Protocol (IP) Transmission Control Protocol (TCP) Datagram Congestion Control Protocol User Datagram Protocol (UDP) Network Layer Transport Layer SCTP

CPN' /10/2008 Motivations Discrepancies between RFC 2960 and Implementation Guide (IG). SCTP Errata published in RFC 4460 (Sep. 2007). Revised SCTP spec. – RFC 4960 published in Sep  Q1. Are there any defects left? Q2. Are new deflect introduced in the new spec? Experiment with the Procedure-based modelling approach.

CPN' /10/2008 SCTP Packet Format An SCTP Packet comprises a header and a number of chunks.

CPN' /10/2008 Verification Tag (VTAG) Verification Tag is used to protect the association from blind attacks. An endpoint keeps two values of verification tag: “My Verification Tag” and “Peer’s Verification Tag”. In general, Any received packets containing a verification tag di ff ering from “My Verification Tag” will be discarded.

CPN' /10/2008 Typical message sequence: Connection Setup ESTABLISHED COOKIE-ECHOED CLOSED Init (vtag=0, itag=Ax) InitAck (vtag=Ax,itag=Zx, CK[Zx,Ax]) [ASSOCIATE] COOKIE-WAIT CookieEcho (vtag=Zx,CK[Zx,Ax]) ESTABLISHED CookieAck (vtag=Ax) Endpoint A Initial Verification Tag = Ax Endpoint Z Initial Verification Tag = Zx

CPN' /10/2008 Typical message sequence: Connection Closedown CLOSED No more outstanding data SHUTDOWN- RECEIVED ESTABLISHED Shutdown (vtag=Zx) ShutdownAck (vtag=Ax) SHUTDOWN-PENDING SHUTDOWN-SENT ShutdownComplete (vtag=Zx) CLOSED Endpoint AEndpoint Z [SHUTDOWN] No more outstanding data SHUTDOWN- ACK-SENT

CPN' /10/2008 Tie-Tag Modeling Tie-Tags is a main contribution of this paper. Tie-Tags are copies of two verification tags. RFC2960  Tie Tags being stored in the cookie. RFC4960  Tie Tags stored in both cookie and TCB. In TCB  “Local Tag” and “ Peer’sTag”. (definitions) In cookie  “Local Tie-Tag”and “Peer’s Tie-Tag”. Thus a cookie contains a pair of VTAG and a pair of Tie-tag. TCB contains a pair of VTAG and a pair of Tie-tag. The Tie-Tags are used to tie the received cookie of the new association with the old association.  Table 2 section of RFC 4960 TCB = Transmission Control Block containing state variables for SCTP connection.

CPN' /10/2008 An error in section of RFC 4960 (but the implementation is correct) Local VTAG in Cookie Peer’s VTAG in Cookie

CPN' /10/2008 Motivations Discrepancies between RFC 2960 and Implementation Guide (IG). SCTP Errata published in RFC 4460 (Sep. 2007). Revised SCTP spec. – RFC 4960 published in Sep  Q1. Are there any defects left?   don’t know yet Q2. Are new deflect introduced in the new spec?  A: yes! Experiment with the Procedure-based modelling approach.

CPN' /10/2008 What is the Procedure-based modelling approach? A CPN model usually divided into several CPN subpages according to protocol’s state  state-based.  the model is easy to read. For a protocol procedure, an event is when an endpoint receives a packet or user command. Events in difference states may cause the endpoint acting in the same way regardless of states. Event-processing style groups the similar events into the same CPN subpage.  the model is very compact but difficult to read.

CPN' /10/2008 What is the Procedure-based modelling approach? In order to develop a CPN model which is not only easy to read but also small. Billington proposed the procedure-based approach in [FI08] “Coloured Petri Nets Modelling of an Evolving Internet Standard: the Datagram Congestion Control Protocol. Fundamenta Informaticae, In Press, 2008” Following the Procedure-based style, we group events according its functionalities, e.g. Typical procedures; Error handling procedures (Unexpected events). In FI08 we built an event-processing CPN model from a state- based CPN model. Then a procedure-based CPN model was developed from the event-processing CPN model. Q3. What if we develop a procedure-based CPN model directly from the narrative specification?

CPN' /10/2008 SCTP_Procedure Normal Event Unexpected Event RetransmissionAbort Check InvalidVTAG EstablishShutDown Init_InitAckCookieEcho _CookieAck Shutdown RestartSimultaneous Open Delayed Cookie Tag_Match Hierarchy – SCTP-CPN Model 4-level, 2 ML functions 6 places 54 executable transitions

CPN' /10/2008

Top-level page

CPN' /10/2008 Typical message sequence: Connection Setup

CPN' /10/2008 One side opens  Simultaneous  Open One side closes  Simultaneous  Closed One side aborts  Analysis Results Number of retransmission - Init, InitAck, CookieEcho, CookieAck

CPN' /10/2008 Potential Problem 1-Case A Open side opens Source of the problem : CookieAck is so delayed

CPN' /10/2008 Potential Problem 2 – Case B Simultaneous Open

CPN' /10/2008 Discussion This paper focuses on modelling. Analysis is used to debug the model. It took me two months – part time to study the protocol, create and debug the model. Why the problems is called the potential problems.  We are not so sure if they are really problem.  We do not model time-stamp and user behavior. While developing the model, we find an error in Table 2 section of RFC This was confirmed by IETF. 

CPN' /10/2008 Conclusions The difficulty of designing a protocol is again witnessed by the defect list in RFC This paper presents a CPN model of SCTP connection management. We still need more exhaustive work on the analysis part. The procedure-based style suites SCTP specification. One error and two potential problems were found. ModellingAnalysis

CPN' /10/2008 Further work Investigate complex scenarios when unexpected CookieEcho chunks received. Investigate the user interface, time stamp, stale packets, and cookie authentication. Future work Multi-homing Security attacks against SCTP

CPN' /10/2008 Thankyou! Any questions?

CPN' /10/2008 Chunk - Declaration

CPN' /10/2008 TCB - Declaration