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Copyright Notice

Copyright (C) The Internet Society (2004).

Abstract

This document describes the state machines for the General Internet Messaging Protocol for Signaling (GIMPS). The states of GIMPS nodes for a given flow and their transitions are presented in order to illustrate how GIMPS may be implemented.

Table of Contents

1.  Introduction
2.  Terminology
3.  Notational conventions used in state diagrams
4.  State Machine Symbols
5.  Common Rules
    5.1  Common Procedures
    5.2  Common Variables
    5.3  Constants
6.  Default state machines for soft state refreshes and teardowns
    6.1  State machine for soft state refreshes
    6.2  State machine for soft state teardowns
7.  State machine for the first GIMPS node in the flow path
8.  State machine for intermediate GIMPS nodes
9.  State machine for the last GIMPS node in the flow path
10.  Security Considerations
11.  Open Issues
12.  Acknowledgments
13.  References
13.1  Normative References
13.2  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1. Introduction

This document describes the state machines for GIMPS [1]Schulzrinne, H., GIMPS: General Internet Messaging Protocol for Signaling, October 2004., trying to show how GIMPS can be implemented to support its deployment. The state machines described in this document are illustrative of how the GIMPS protocol defined in [1]Schulzrinne, H., GIMPS: General Internet Messaging Protocol for Signaling, October 2004. may be implemented for the GIMPS nodes in different locations of a flow path. Where there are differences [1]Schulzrinne, H., GIMPS: General Internet Messaging Protocol for Signaling, October 2004. are authoritative. The state machines are informative only. Implementations may achieve the same results using different methods.

There are 3 types of possible entities for GIMPS signaling:

The first node that speaks GIMPS in the path from FlowSender to the FlowReceiver (it is assumed that it has only downstream RoutingState or MessagingAssociation for the given flow);

Intermediate GIMPS node;

The last node that speaks GIMPS in the path from the FlowSender to the FlowReceiver (it is assumed that it has only upstream RoutingState or MessagingAssociation for the given flow).

We describe a set of state machines for these entities to illustrate how GIMPS may be implemented.

2. 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 [2]Bradner, S., Key words for use in RFCs to Indicate Requirement Levels, March 1997..

3. Notational conventions used in state diagrams

The following text is reused from [3]Vollbrecht, J., Eronen, P., Petroni, N. and Y. Ohba, State Machines for Extensible Authentication Protocol (EAP) Peer and Authenticator, September 2004. and the state diagrams are based on the conventions specified in [4]Institute of Electrical and Electronics Engineers, DRAFT Standard for Local and Metropolitan Area Networks: Port-Based Network Access Control (Revision), January 2004., Section 8.2.1. Additional state machine details are taken from [5]Ohba, Y., State Machines for Protocol for Carrying Authentication for Network Access (PANA), July 2004..

The complete text is reproduced here:

State diagrams are used to represent the operation of the protocol by a number of cooperating state machines each comprising a group of connected, mutually exclusive states. Only one state of each machine can be active at any given time.

All permissible transitions between states are represented by arrows, the arrowhead denoting the direction of the possible transition. Labels attached to arrows denote the condition(s) that must be met in order for the transition to take place. All conditions are expressions that evaluate to TRUE or FALSE; if a condition evaluates to TRUE, then the condition is met. The label UCT denotes an unconditional transition (i.e., UCT always evaluates to TRUE). A transition that is global in nature (i.e., a transition that occurs from any of the possible states if the condition attached to the arrow is met) is denoted by an open arrow; i.e., no specific state is identified as the origin of the transition. When the condition associated with a global transition is met, it supersedes all other exit conditions including UCT. The special global condition BEGIN supersedes all other global conditions, and once asserted remains asserted until all state blocks have executed to the point that variable assignments and other consequences of their execution remain unchanged.

On entry to a state, the procedures defined for the state (if any) are executed exactly once, in the order that they appear on the page. Each action is deemed to be atomic; i.e., execution of a procedure completes before the next sequential procedure starts to execute. No procedures execute outside of a state block. The procedures in only one state block execute at a time, even if the conditions for execution of state blocks in different state machines are satisfied, and all procedures in an executing state block complete execution before the transition to and execution of any other state block occurs, i.e., the execution of any state block appears to be atomic with respect to the execution of any other state block and the transition condition to that state from the previous state is TRUE when execution commences. The order of execution of state blocks in different state machines is undefined except as constrained by their transition conditions. A variable that is set to a particular value in a state block retains this value until a subsequent state block executes a procedure that modifies the value.

On completion of all of the procedures within a state, all exit conditions for the state (including all conditions associated with global transitions) are evaluated continuously until one of the conditions is met. The label ELSE denotes a transition that occurs if none of the other conditions for transitions from the state are met (i.e., ELSE evaluates to TRUE if all other possible exit conditions from the state evaluate to FALSE). Where two or more exit conditions with the same level of precedence become TRUE simultaneously, the choice as to which exit condition causes the state transition to take place is arbitrary.

In addition to the above notation, there are a couple of clarifications specific to this document. First, all boolean variables are initialized to FALSE before the state machine execution begins. Second, the following notational shorthand is specific to this document:

<variable> = <expression1> | <expression2> | ...

Execution of a statement of this form will result in <variable> having a value of exactly one of the expressions. The logic for which of those expressions gets executed is outside of the state machine and could be environmental, configurable, or based on another state machine such as that of the method.

4. State Machine Symbols

*_up

means the message is sent in upstream direction or received from upstream direction

*_down

means the message is sent in downstream direction or received from downstream direction

MA

means "Messaging Association"

RS

means "Routing State"

( )

Used to force the precedence of operators in Boolean expressions and to delimit the argument(s) of actions within state boxes.

;

Used as a terminating delimiter for actions within state boxes. Where a state box contains multiple actions, the order of execution follows the normal language conventions for reading text.

=

Assignment action. The value of the expression to the right of the operator is assigned to the variable to the left of the operator. Where this operator is used to define multiple assignments, e.g., a = b = X the action causes the value of the expression following the right-most assignment operator to be assigned to all of the variables that appear to the left of the right-most assignment operator.

!

Logical NOT operator.

&&

Logical AND operator.

||

Logical OR operator.

if...then...

Conditional action. If the Boolean expression following the if evaluates to TRUE, then the action following the then is executed.

\{ statement 1, ... statement N \}

Compound statement. Braces are used to group statements that are executed together as if they were a single statement.

!=

Inequality. Evaluates to TRUE if the expression to the left of the operator is not equal in value to the expression to the right.

==

Equality. Evaluates to TRUE if the expression to the left of the operator is equal in value to the expression to the right.

>

Greater than. Evaluates to TRUE if the value of the expression to the left of the operator is greater than the value of the expression to the right.

<=

Less than or equal to. Evaluates to TRUE if the value of the expression to the left of the operator is either less than or equal to the value of the expression to the right.

++

Increment the preceding integer operator by 1.

5. Common Rules

Throughout the document we use terms defined in the [1]Schulzrinne, H., GIMPS: General Internet Messaging Protocol for Signaling, October 2004., such as Query, Response, Confirm.

5.1 Common Procedures

Tg_SendMsg:

NSLP/GIMPS API message that request transmission of a NSLP message

Tg_SetStateLifetime(time_period):

NSLP/GIMPS API message providing info for the Lifetime of an RS, required by the application. "Time_period = 0" represents the cancellation of established RSs/MAs (invoked by NSLP application).

Tg_MessageDeliveryError:

NSLP/GIMPS API message informing NSLP application of unsuccessful delivery of a message

Tg_RecvMsg:

NSLP/GIMPS API message that provides received message to the NSLP

Tg_NetworkNotification:

NSLP/GIMPS API message that informs NSLP for change in RS

Tx_Query_Dmode:

Transmit of Query message in Dmode

Tx_Response_Dmode:

Transmit of Response message in Dmode

Rx_Response_Dmode:

Receive of Response message in Dmode

Tx_Confirm_Cmode:

Transmit of Confirm message in Cmode (via MA)

Rx_Query_Dmode:

Receive of Query message in Dmode

Rx_Confirm_Cmode:

Receive of Confirm message in Cmode (via MA)

Queue NSLP msg info:

Save NLSP messages in a queue until a required MA association is established

Tx_Msg_Cmode:

Transmit message in Cmode (via MA)

TIMEOUT_MAState:

Expiration of an established MA state lifetime timer.

TIMEOUT_RSState:

Expiration of an established RS state lifetime timer.

TIMEOUT_Refresh:

Refresh interval timer expiration

ESTABLISH RS:

Establish Downstream/Upstream Routing State at the node

ESTABLISH MA:

Establish Downstream/Upstream Message Association at the node

REFRESH RS:

Refreshes established RS

5.2 Common Variables

It is assumed that the type of mode and destination info (which need to be taken from the application parameters and local GIMPS policy) is provided. This is represented by the common variables Dmode, Cmode, MArequest, MAinfo, MApresent and Refresh.

Dmode:

The message MUST be transmitted in Dmode. This is specified by "Message transfer attributes" set to the following values:

Reliability:

is set to FALSE

Security:

is set to values that do not request special security handling of a message.

Local processing:

is set to values that do not require services offered by Cmode [1]Schulzrinne, H., GIMPS: General Internet Messaging Protocol for Signaling, October 2004.

Cmode:

The message MUST be transmitted in Cmode. This is specified by "Message transfer attributes" set to any of the following values:

Reliability:

is set to TRUE

Security:

is set to values that request secure handling of a message.

Local processing:

is set to values that require services offered by Cmode (e.g., congestion control) [1]Schulzrinne, H., GIMPS: General Internet Messaging Protocol for Signaling, October 2004.

MApresent:

Used for re-use of MAs. It shows that there is a perfect match between the Node Addressing Object (NAO) of an existing association and the NAO provided in the GIMPS Query Dmode message (section 4.4.2 of [1]Schulzrinne, H., GIMPS: General Internet Messaging Protocol for Signaling, October 2004.).

MArequest:

Request for MA establishment. Its mapping to specific GIMPS message parameters is left for future version of the document.

MAinfo:

Information for establishment of the requested MA. Its mapping to specific GIMPS message parameters is left for future version of the document.

NSLPinfo:

NSLP application information

Refresh:

This variable specifies that the message is for refresh purposes

Downstream:

Specifies that message must be sent in downstream direction

Upstream:

Specifies that message must be sent in upstream direction.

5.3 Constants

6. Default state machines for soft state refreshes and teardowns

This section presents the state machines for any "established state" in a GIMPS nodes. Whenever possible, they are not repeated in subsequent sections for simplicity.

Note: since in GIMPS there can be different combinations of RS (established or not) and MA (etablished or not) states in both DIRECTIONs (up- and downstream), there are 8 sub-states in total, all being soft states (subject to expiration unless refreshes), thus yield the state machine presented in this section (and the combination of different state variables, which are not fully elaborated here for simplicity).

6.1 State machine for soft state refreshes

This subsection presents the single state for a given "established state" and transitions to itself under refresh conditions. By default, "refresh the established state" action is performed for all these transitions.

   -------------------
   State: State_Estab (any established MA/RS state)
   -------------------

   Condition                Action                    State
   ------------------------+-------------------------+------------
   (tg_SendMsg) && (Cmode) |tx_Msg_Cmode_up          |State_Estab
      && (Upstream)        |                         |
                           |                         |
   (tg_SendMsg) && (Cmode) |tx_Msg_Cmode_down        |State_Estab
      && (Downstream)      |                         |
                           |                         |
   (tg_SendMsg) && (Dmode) |tx_Msg_Dmode_up          |State_Estab
      && (Upstream)        |                         |
                           |                         |
   (tg_SendMsg) && (Dmode) |tx_Msg_Dmode_down        |State_Estab
      && (Downstream)      |                         |
                           |                         |
   rx_Response_Dmode       | Refresh RS              |State_Estab
     (Refresh)             |                         |
   ------------------------+-------------------------+------------
                

6.2 State machine for soft state teardowns

This subsection presents the state machine for transitions for any established MA state and RS state under their timer teardown conditions.

   ----------------------------------
   STATE: RS_Established in DIRECTION (DIRECTION = {up, down})
   ----------------------------------

   Condition                Action                     State
   ------------------------+-------------------------+------------
   (TIMEOUT_RSstate)||     |Clear RS,                |IDLE in
   (tg_SetStateLifetime(0))| tg_Network Notification |DIRECTION
                           |                         |(Note 1)
   ------------------------+-------------------------+------------
                

   ----------------------------------
   STATE: MA_Established in DIRECTION (DIRECTION = {up, down})
   ----------------------------------

   Condition                Action                     State
   ------------------------+-------------------------+------------
   (TIMEOUT_MAstate)||     |(Clear MS)||             |IDLE in
   (tg_SetStateLifetime(0))|(Stop_using_existing MA) |DIRECTION
                           |tg_Network Notification  |(Note 1)
   ------------------------+-------------------------+------------
   Note 1: If both downstream and upstream states are established,
           then this transition goes to RS/MA Established state 
           only for the other DIRECTION.
               

7. State machine for the first GIMPS node in the flow path

   -----------
   State: INIT
   -----------

   Condition                Action                    State
   ------------------------+-------------------------+------------
   UCT                     | initialize variables    |IDLE
   ------------------------+-------------------------+------------
                

   -----------
   State: IDLE
   -----------

   Condition                Action                    State      
   ------------------------+-------------------------+-------------
   (tg_SendMsg) && (Dmode) |tx_Query_Dmode(NSLPinfo) |WAITRESP2
                           |                         |           
   (tg_SendMsg) && (Cmode) |tx_Query_Dmode(MArequest)|WAITRESP1
                           |  Queue NSLP msg info    |           
   ------------------------+-------------------------+-------------
               

   ----------------
   State: WAITRESP1
   ----------------

   Condition                Action                    State
   ------------------------+-------------------------+------------
   (TIMEOUT_Refresh) &&    |tx_Query_Dmode(MArequest)|WAITRESP1
      (!MaxRetry)          |                         |           
                           |                         |           
   (TIMEOUT_Refresh) &&    |tg_MessageDeliveryError  |IDLE
      (MaxRetry)           |                         |           
                           |                         |           
   (rx_RESPONSE_Dmode      |tx_Confirm_Cmode,        |DownMA_Estab
      (MAinfo)) ||         |  tx_queued_Msg_cmode,   |            
      (rx_RESPONSE_Cmode   |  (Establish MA) ||      |           
      (MAinfo))            |  (Re-use existing MA)   |           
   ------------------------+-------------------------+------------
                

   -------------------
   State: DownMA_Estab (Downstream MA Established)
   -------------------

   Condition                Action                    State
   ------------------------+-------------------------+------------
   rx_Msg_Cmode            |tg_RecvMsg to Application|DownMA_Estab
                           |                         |
   (tg_SendMsg) && (Cmode) |tx_Msg_Cmode             |DownMA_Estab
   ------------------------+-------------------------+------------
                

   ----------------
   State: WAITRESP2
   ----------------

   Condition                Action                    State
   ------------------------+-------------------------+------------
   (TIMEOUT_Refresh) &&    |tx_Query_Dmode(NSLPinfo) |WAITRESP2
      (!MaxRetry)          |                         |           
                           |                         |
   (TIMEOUT_Refresh) &&    |tg_MessageDeliveryError  |IDLE
      (MaxRetry)           |                         |        
                           |                         |
   rx_Response_Dmode       |(establish downstream RS)|DownRS_Estab
   ------------------------+-------------------------+------------
              

   -------------------
   State: DownRS_Estab (Downstream RS Established)
   -------------------

   Condition                Action                    State
   ------------------------+-------------------------+------------
   TIMEOUT_Refresh         |tx_Query_Dmode(Refresh)  |DownRS_Estab
                           |                         |
   rx_Response_Dmode       |Refresh RS               |DownRS_Estab
     (Refresh)             |                         |
                           |                         |
   (tg_SendMsg) && (Dmode) |tx_Msg_Dmode             |DownRS_Estab
   ------------------------+-------------------------+------------
                

8. State machine for intermediate GIMPS nodes

   -----------
   State: INIT
   -----------

   Condition                Action                    State
   ------------------------+-------------------------+------------
   UCT                     | initialize variables    |IDLE
   ------------------------+-------------------------+------------
                

   -----------
   State: IDLE
   -----------

   Condition                Action                    State      
   ------------------------+-------------------------+-------------
   (rx_Query_Dmode_up      |tg_RecvMsg to App.,      |UpRS_Estab
     (MArequest)) &&       |  tx_Response_Dmode_up   |           
     (!MApresent)          |  (MAinfo), establish RS |           
                           |                         |           
   rx_Query_Dmode_up       |tg_RecvMsg(NSLPinfo) to  |UpRS_Estab
     (NSLPinfo)            |  Application,           |           
                           |  tx_Response_Dmode_up,  |
                           |  establish RS           |           
                           |                         |           
   (rx_Query_Dmode_up      |tg_RecvMsg to Application|UpMA_Estab
     (MArequest)) &&       |  , tx_Response_Cmode_up |           
     (MApresent)           |  (MAinfo), re-use       |           
     (MApresent)           |  existing MA            |           
   ------------------------+-------------------------+-------------
               

   -----------------
   State: UpRS_Estab (UPstream RS Established)
   -----------------

   Condition                Action                    State      
   ------------------------+-------------------------+-------------
   (tg_SendMsg) && (Dmode) |tx_Query_Dmode_down      |UpRS_Estab + 
                           |  (NSLPinfo)             | WaitResp for
                           |                         | DownRS
                           |                         |
   (tg_SendMsg) && (Cmode) |tx_Query_Dmode_down      |UpRS_Estab + 
                           |  (MArequest), queue NSLP| WaitResp for
                           |  msg info               | DownRS
                           |                         |
   rx_Confirm_Cmode_up     |Estabsh MA               |UpMA_Estab
   ------------------------+-------------------------+-------------
               

   ---------------------------------------
   State: UpRS_Estab + WaitResp for DownRS
   ---------------------------------------

   Condition                Action                    State      
   ------------------------+-------------------------+-------------
   (rx_Response_Dmode_down |tx_Confirm_Cmode_down,   |UpRS_Estab +
    (MAinfo)) ||           | tx_queued_Msg_Cmode_down| DownMA_Estab
    (rx_Response_Cmode_down| estblish MA             | 
    (MAinfo))              |                         | 
                           |                         | 
   rx_Response_Dmode_down  |Establish RS             |UpRS_Estab + 
                           |                         | DownRS_Estab
   ------------------------+-------------------------+-------------
               

   --------------------------------
   State: UpRS_Estab + DownRS_Estab
   --------------------------------

   Condition                Action                    State      
   ------------------------+-------------------------+-------------
   rx_Confirm_Cmode_up     |Establish MA             |DownRS_Estab +
                           |                         | UpMA_Estab
   ------------------------+-------------------------+-------------
               

   --------------------------------
   State: DownRS_Estab + UpMA_Estab
   --------------------------------

   Condition                Action                    State      
   ------------------------+-------------------------+-------------
   TIMEOUT_Refresh_down    |tx_Query_Dmode_down      |DownRS_Estab +
                           |  (Refresh)              |  UpMA_Estab
   ------------------------+-------------------------+-------------
               

   --------------------------------
   State: UpRS_Estab + DownMA_Estab
   --------------------------------

   Condition                Action                    State      
   ------------------------+-------------------------+-------------
   rx_Confirm_Cmode_up     |Establish MA,            |DownMA_Estab + 
                           |  rx_Confirm_Cmode_up    | UpMA_Estab
   ------------------------+-------------------------+-------------
               

   --------------------------------
   State: DownMA_Estab + UpMA_Estab
   --------------------------------
   (Refer Section "State machine for soft state")
               

   -----------------
   State: UpMA_Estab
   -----------------

   Condition                Action                    State      
   ------------------------+-------------------------+-------------
   (tg_SendMsg) && (Dmode) |tx_Query_Dmode_down      |UpMA_Estab + 
                           |  (NSLPinfo)             | WaitResp for
                           |                         | DownRS
                           |                         |
   (tg_SendMsg) && (Cmode) |tx_Query_Dmode_down      |UpMA_Estab + 
                           |  (MArequest), queue NSLP| WaitResp for
                           |  msg info               | DownRS
                           |                         |
   rx_Confirm_Cmode_up     |                         |UpMA_Estab
   ------------------------+-------------------------+-------------
               

   ---------------------------------------
   State: UpMA_Estab + WaitResp for DownRS
   ---------------------------------------

   Condition                Action                    State      
   ------------------------+-------------------------+-------------
   (rx_Response_Dmode_down |tx_Confirm_Cmode_down,   |UpMA_Estab +
    (MAinfo)) ||           | tx_queued_Msg_Cmode_down| DownMA_Estab
    (rx_Response_Cmode_down| , estblish MA           | 
    (MAinfo))              |                         | 
                           |                         | 
   rx_Response_Dmode_down  |Establish RS             |UpMA_Estab + 
                           |                         | DownRS_Estab
   ------------------------+-------------------------+-------------
               

9. State machine for the last GIMPS node in the flow path

   -----------
   State: INIT
   -----------

   Condition                Action                    State
   ------------------------+-------------------------+------------
   UCT                     | initialize variables    |IDLE
   ------------------------+-------------------------+------------
                

   -----------
   State: IDLE
   -----------

   Condition                Action                    State      
   ------------------------+-------------------------+-------------
   (rx_Query_Dmode_up      |tg_RecvMsg to App.,      |UpRS_Estab
     (MArequest)) &&       |  tx_Response_Dmode_up   |           
     (!MApresent)          |  (MAinfo), Establish RS |           
                           |                         |           
   rx_Query_Dmode          |tg_RecvMsg(NSLP info) to |UpRS_Estab
     (NSLPinfo)            | Appl., tx_Response_Dmode|           
                           | establish RS            |           
                           |                         |           
   rx_Query_Dmode          |tg_RecvMsg to Application|UpMA_Estab
     (MArequest)           | , tx_Response_Dmode_up  |           
                           | (MAinfo), Re-use        |    
                           | existing MA             |           
   ------------------------+-------------------------+-------------
               

   -----------------
   State: UpRS_Estab
   -----------------

   Condition                Action                    State
   ------------------------+-------------------------+------------
   (TIMEOUT_RSstate) ||    |Clear RS,                |IDLE
   (tg_SetStateLifeTime(0))|  tg_NetworkNotification |           
                           |                         |           
   rx_Query_Dmode(Refresh) |Refresh RS,              |UpRS_Estab
                           |  tx_Response(Refresh)   |           
                           |                         |           
   rx_Confirm_Cmode        |Establish MA             |UpMA_Estab
   ------------------------+-------------------------+------------
                

   -----------------
   State: UpMA_Estab (Upstream MA Established)
   -----------------

   Condition                Action                    State
   ------------------------+-------------------------+------------
   rx_Confirm_Cmode        |Refresh MA               |UpMA_Estab
   ------------------------+-------------------------+------------
                

10. Security Considerations

This document does not raise new security considerations. Any security concerns with GIMPS are likely reflected in security related NSIS work already (such as [1]Schulzrinne, H., GIMPS: General Internet Messaging Protocol for Signaling, October 2004. or [6]Tschofenig, H. and D. Kroeselberg, Security Threats for NSIS, October 2004.).

For the time being, the state machines described in this document do not consider the security aspect of GMIPS protocol itself. A future versions of this document will add security relevant states and state transitions.

11. Open Issues

At this stage, normal GIMPS messages that carry NSLP data are not presented for all states. A default rules for handling of such messages at any "ESTABLISHED STATE" is provided. Messages exchanged through NSLP/GIMPS API and Refresh state messages are not completely described, too. These issues needs further consideration in the future version of the document.

Route change and local repair need further consideration.

Refreshment of Messaging Association (MA) state is currently not elaborated in the document (according to current GIMPS spec, there can be an idle timer but tearing down a MA can depend on a node's local policy).

State machine represents handling of GIMPS messages that match the node address and NSLPID for the given flow.

Bypass handling is left for future version of the document.

There are further unclear issues with processing rules and message definition, e.g., soft state handling for both RS and MA states (and their combinations), which will be described in more detail in a future version of this document.

12. Acknowledgments

The authors would like to thank Andreas Westermaier and Alex Zrim for helpful discussions with them.

13. References

13.1 Normative References

13.2 Informative References

Authors' Addresses

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