RFC 3989
Middlebox Communications (MIDCOM) Protocol Semantics
Network Working Group M. Stiemerling Request for Comments: 3989 J. Quittek Category: Informational NEC T. Taylor Nortel February 2005 Middlebox Communications (MIDCOM) Protocol Semantics Status of This Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2005).Abstract
This memo specifies semantics for a Middlebox Communication (MIDCOM) protocol to be used by MIDCOM agents for interacting with middleboxes such as firewalls and Network Address Translators (NATs). The semantics discussion does not include any specification of a concrete syntax or a transport protocol. However, a concrete protocol is expected to implement the specified semantics or, more likely, a superset of it. The MIDCOM protocol semantics is derived from the MIDCOM requirements, from the MIDCOM framework, and from working group decisions.Table of Contents
1. Introduction ................................................. 3 1.1. Terminology ............................................ 4 1.2. Transaction Definition Template ........................ 6 2. Semantics Specification ...................................... 7 2.1. General Protocol Design ................................ 7 2.1.1. Protocol Transactions .......................... 8 2.1.2. Message Types .................................. 9 2.1.3. Session, Policy Rule, and Policy Rule Group .... 9 2.1.4. Atomicity ...................................... 10 2.1.5. Access Control ................................. 11 2.1.6. Middlebox Capabilities ......................... 11 2.1.7. Agent and Middlebox Identifiers ................ 12 2.1.8. Conformance .................................... 12 2.2. Session Control Transactions ........................... 13
2.2.1. Session Establishment (SE) ..................... 13
2.2.2. Session Termination (ST) ....................... 15
2.2.3. Asynchronous Session Termination (AST) ......... 16
2.2.4. Session Termination by Interruption of
Connection ..................................... 17
2.2.5. Session State Machine .......................... 17
2.3. Policy Rule Transactions ............................... 18
2.3.1. Configuration Transactions ..................... 19
2.3.2. Establishing Policy Rules ...................... 19
2.3.3. Maintaining Policy Rules and Policy Rule Groups 20
2.3.4. Policy Events and Asynchronous Notifications ... 21
2.3.5. Address Tuples ................................. 21
2.3.6. Address Parameter Constraints .................. 23
2.3.7. Interface-specific Policy Rules ................ 25
2.3.8. Policy Reserve Rule (PRR) ...................... 26
2.3.9. Policy Enable Rule (PER) ....................... 30
2.3.10. Policy Rule Lifetime Change (RLC) .............. 36
2.3.11. Policy Rule List (PRL) ......................... 38
2.3.12. Policy Rule Status (PRS) ....................... 39
2.3.13. Asynchronous Policy Rule Event (ARE) ........... 41
2.3.14. Policy Rule State Machine ...................... 42
2.4. Policy Rule Group Transactions ......................... 43
2.4.1. Overview ....................................... 43
2.4.2. Group Lifetime Change (GLC) .................... 44
2.4.3. Group List (GL) ................................ 46
2.4.4. Group Status (GS) .............................. 47
3. Conformance Statements ....................................... 48
3.1. General Implementation Conformance ..................... 49
3.2. Middlebox Conformance .................................. 50
3.3. Agent Conformance ...................................... 50
4. Transaction Usage Examples ................................... 50
4.1. Exploring Policy Rules and Policy Rule Groups .......... 50
4.2. Enabling a SIP-Signaled Call ........................... 54
5. Compliance with MIDCOM Requirements .......................... 59
5.1. Protocol Machinery Requirements ........................ 59
5.1.1. Authorized Association ......................... 59
5.1.2. Agent Connects to Multiple Middleboxes ......... 60
5.1.3. Multiple Agents Connect to Same Middlebox ...... 60
5.1.4. Deterministic Behavior ......................... 60
5.1.5. Known and Stable State ......................... 60
5.1.6. Status Report .................................. 61
5.1.7. Unsolicited Messages (Asynchronous
Notifications).................................. 61
5.1.8. Mutual Authentication .......................... 61
5.1.9. Session Termination by Any Party ............... 62
5.1.10. Request Result ................................. 62
5.1.11. Version Interworking ........................... 62
5.1.12. Deterministic Handling of Overlapping Rules .... 62
5.2. Protocol Semantics Requirements ........................ 63
5.2.1. Extensible Syntax and Semantics ................ 63
5.2.2. Policy Rules for Different Types of Middleboxes 63
5.2.3. Ruleset Groups ................................. 63
5.2.4. Policy Rule Lifetime Extension ................. 63
5.2.5. Robust Failure Modes ........................... 63
5.2.6. Failure Reasons ................................ 63
5.2.7. Multiple Agents Manipulating Same Policy Rule .. 64
5.2.8. Carrying Filtering Rules ....................... 64
5.2.9. Parity of Port Numbers ......................... 64
5.2.10. Consecutive Range of Port Numbers .............. 64
5.2.11. Contradicting Overlapping Policy Rules ......... 64
5.3. Security Requirements .................................. 65
5.3.1. Authentication, Confidentiality, Integrity ..... 65
5.3.2. Optional Confidentiality of Control Messages ... 65
5.3.3. Operation across Untrusted Domains ............. 65
5.3.4. Mitigate Replay Attacks ........................ 65
6. Security Considerations ...................................... 65
7. IAB Considerations on UNSAF .................................. 66
8. Acknowledgments .............................................. 67
9. References ................................................... 67
9.1. Normative References ................................... 67
9.2. Informative References ................................. 67
Authors' Addresses ............................................... 69
Full Copyright Statement ......................................... 70
1. Introduction
The MIDCOM working group has defined a framework [MDC-FRM] and a list
of requirements [MDC-REQ] for middlebox communication. The next step
toward a MIDCOM protocol is the specification of protocol semantics
that is constrained, but not completely implied, by the documents
mentioned above.
This memo suggests a semantics for the MIDCOM protocol. It is fully
compliant with the requirements listed in [MDC-REQ] and with the
working group's consensus on semantic issues.
In conformance with the working group charter, the semantics
description is targeted at packet filters and network address
translators (NATs), and it supports applications that require dynamic
configuration of these middleboxes.
The semantics is defined in terms of transactions. Two basic types
of transactions are used: request-reply transactions and asynchronous
transactions. For each transaction, the semantics is specified by
describing (1) the parameters of the transaction, (2) the processing
of request messages at the middlebox, and (3) the state transitions
at the middlebox caused by the request transactions or indicated by
the asynchronous transactions, respectively, and (4) the reply and
notification messages sent from the middlebox to the agent in order
to inform the agent about the state change.
The semantics can be implemented by any protocol that supports these
two transaction types and that is sufficiently flexible concerning
transaction parameters. Different implementations for different
protocols might need to extend the semantics described below by
adding further transactions and/or adding further parameters to
transactions and/or splitting single transactions into a set of
transactions. Regardless of such extensions, the semantics below
provides a minimum necessary subset of what must be implemented.
The remainder of this document is structured as follows. Section 2
describes the protocol semantics. It is structured in four
subsections:
- General Protocol Issues (section 2.1)
- Session Control (section 2.2)
- Policy Rules (section 2.3)
- Policy Rule Groups (section 2.4)
Section 3 contains conformance statements for MIDCOM protocol
definitions and MIDCOM protocol implementations with respect to the
semantics defined in section 2. Section 4 gives two elaborated usage
examples. Finally, section 5 explains how the semantics meets the
MIDCOM requirements.
1.1. Terminology
The terminology in this memo follows the definitions given in the
framework [MDC-FRM] and requirements [MDC-REQ] document.
In addition, the following terms are used:
request transaction A request transaction consists of a
request message transfer from the agent to
the middlebox, processing of the message
at the middlebox, a reply message transfer
from the middlebox to the agent, and the
optional transfer of notification messages
from the middlebox to agents other than
the one requesting the transaction. A
request transaction might cause a state
transition at the middlebox.
configuration transaction A configuration transaction is a request
transaction containing a request for state
change in the middlebox. If accepted, it
causes a state change at the middlebox.
monitoring transaction A monitoring transaction is a request
transaction containing a request for state
information from the middlebox. It does
not cause a state transition at the
middlebox.
asynchronous transaction An asynchronous transaction is not
triggered by an agent. It may occur
without any agent participating in a
session with the middlebox. Potentially,
an asynchronous transaction includes the
transfer of notification messages from the
middlebox to agents that participate in an
open session. A notification message is
sent to each agent that needs to be
notified about the asynchronous event.
The message indicates the state transition
at the middlebox.
agent-unique An agent-unique value is unique in the
context of the agent. This context
includes all MIDCOM sessions the agent
participates in. An agent-unique value is
assigned by the agent.
middlebox-unique A middlebox-unique value is unique in the
context of the middlebox. This context
includes all MIDCOM sessions the middlebox
participates in. A middlebox-unique value
is assigned by the middlebox.
policy rule In general, a policy rule is "a basic
building block of a policy-based system.
It is the binding of a set of actions to a
set of conditions -- where the conditions
are evaluated to determine whether the
actions are performed." [RFC3198]. In
the MIDCOM context the condition is a
specification of a set of packets to which
rules are applied. The set of actions
always contains just a single element per
rule, either action "reserve" or action
"enable".
policy reserve rule A policy rule containing a reserve action.
The policy condition of this rule is
always true. The action is the
reservation of just an IP address or a
combination of an IP address and a range
of port numbers on neither side, one side,
or both sides of the middlebox, depending
on the middlebox configuration.
policy enable rule A policy rule containing an enable action.
The policy condition consists of a
descriptor of one or more unidirectional
or bidirectional packet flows, and the
policy action enables packets belonging to
this flow to traverse the middlebox. The
descriptor identifies the protocol, the
flow direction, and the source and
destination addresses, optionally with a
range of port numbers.
NAT binding The term NAT binding as used in this
document does not necessarily refer to a
NAT bind as defined in [NAT-TERM]. A NAT
binding in the MIDCOM semantics refers to
an abstraction that enables communication
between two end points through the NAT-
type middlebox. An enable action may
result in a NAT bind or a NAT session,
depending on the request and its
parameters.
1.2. Transaction Definition Template
In the following sections, the semantics of the MIDCOM protocol is
specified per transaction. A transaction specification contains the
following entries. Parameter entries, failure reason, and
notification message type are only specified if applicable.
transaction-name
A description name for this type of transaction.
transaction-type
The transaction type is either 'configuration', 'monitoring', or
'asynchronous'. See section 1.1 for a description of transaction
types.
transaction-compliance
This entry contains either 'mandatory' or 'optional'. For details
see section 2.1.8.
request-parameters
This entry lists all parameters necessary for this request. A
description for each parameter is given.
reply-parameters (success)
This entry lists all parameters sent back from the middlebox to
the agent as positive response to the prior request. A
description for each parameter is given.
failure reason
All negative replies have two parameters: a request identifier
identifying the request on which the reply is sent and a parameter
indicating the failure reason. As these parameters are
compulsory, they are not listed in the template. But the template
contains a list of potential failure reasons that may be indicated
by the second parameter. The list is not exhaustive. A concrete
protocol specification may extend the list.
notification message type
The type of the notification message type that may be used by this
transaction.
semantics
This entry describes the actual semantics of the transaction.
Particularly, it describes the processing of the request message
by the middlebox, and middlebox state transitions caused by or
causing the transaction, respectively.
2. Semantics Specification
2.1. General Protocol Design
The semantics specification aims at a balance between proper support
of applications that require dynamic configuration of middleboxes and
simplicity of specification and implementation of the protocol.
Protocol interactions are structured into transactions. The state of
middleboxes is described by state machines. The state machines are
defined by states and state transitions. A single transaction may
cause or be caused by state transitions in more than one state
machine, but per state machine there is no more than one transition
per transaction.
2.1.1. Protocol Transactions
State transitions are initiated either by a request message from the agent to the middlebox or by some other event at the middlebox. In the first case, the middlebox informs the agent by sending a reply message on the actual state transition; in the second, the middlebox sends an unsolicited asynchronous notification message to each agent affected by the transaction (if it participates in an open session with the middlebox). Request and reply messages contain an agent-unique request identifier that allows the agent to determine to which sent request a received reply corresponds. An analysis of the requirements showed that four kinds of transactions are required: - Configuration transactions allowing the agent to request state transitions at the middlebox. - Asynchronous transactions allowing the middlebox to change state without a request by an agent. - Monitoring transactions allowing the agent to request state information from the middlebox. - Convenience transactions combining a set of configuration transactions. Configuration transactions and asynchronous transactions provide the basic MIDCOM protocol functionality. They are related to middlebox state transitions, and they concern establishment and termination of MIDCOM sessions and of policy rules. Monitoring transactions are not related to middlebox state transitions. They are used by agents to explore the number, status, and properties of policy rules established at the middlebox. Convenience transactions simplify MIDCOM sessions by combining a set of configuration transactions into a single one. They are not necessary for MIDCOM protocol operation. As specified in detail in section 3, configuration transactions and asynchronous transactions are mandatory. They must be implemented by a compliant middlebox. All convenience transactions are optional, and some of the monitoring transactions are optional.
2.1.2. Message Types
The MIDCOM protocol supports three kinds of messages: request messages, reply messages, and notification messages. For each kind, different message types exist. In this semantics document, message types are only defined by the list of parameters. The order of the parameters and their encoding is left to a concrete protocol definition. A protocol definition may also add further parameters to a message type or combine several parameters into one, as long as the information contained in the parameters defined in the semantics is still present. For request messages and positive reply messages there exists one message type per request transaction. Each reply transaction defines the parameter list of the request message and of the positive (successful) reply message by using the transaction definition template defined in section 1.2. In case of a failed request transaction, a negative reply message is sent from the middlebox to the agent. This message is the same for all request transactions; it contains the request identifier identifying the request to which the reply is sent and a parameter indicating the failure reason. There are three notification message types: the Session Termination Notification (STN), the Policy Rule Event Notification (REN), and the Group Event Notification (GEN). All of these contain a middlebox- unique notification identifier. STN The Session Termination Notification message additionally contains a single parameter indicating the reason for session termination by the middlebox. REN The Policy Rule Event Notification message contains the notification identifier, a policy rule identifier, and the remaining policy lifetime. GEN The Group Event Notification message contains the notification identifier, a policy rule group identifier, and the remaining policy rule group lifetime.2.1.3. Session, Policy Rule, and Policy Rule Group
All transactions can be further grouped into transactions concerning sessions, transactions concerning policy rules, and transactions concerning policy rule groups. Policy rule groups can be used to
indicate relationships between policy rules and to simplify transactions on a set of policy rules by using a single transaction per group instead of one per policy rule. Sessions and policy rules at the middlebox are stateful. Their states are independent of each other, and their state machines (one per session and one per policy rule) can be separated. Policy rule groups are also stateful, but the middlebox does not need to maintain state for policy rule groups, because the semantics were chosen so that the policy rule group state is implicitly defined by the state of all policy rules belonging to the group (see section 2.4). The separation of session state and policy rule state simplifies the specification of the semantics as well as a protocol implementation. Therefore, the semantics specification is structured accordingly and we use two separated state machines to illustrate the semantics. Please note that state machines of concrete protocol designs and implementations will probably be more complex than the state machines presented here. However, the protocol state machines are expected to be a superset of the semantics state machines in this document.2.1.4. Atomicity
All request transactions are atomic with respect to each other. This means that processing of a request at the middlebox is never interrupted by another request arriving or already queued. This particularly applies when the middlebox concurrently receives requests originating in different sessions. However, asynchronous transactions may interrupt and/or terminate processing of a request at any time. All request transactions are atomic from the point of view of the agent. The processing of a request does not start before the complete request arrives at the middlebox. No intermediate state is stable at the middlebox, and no intermediate state is reported to any agent. The number of transactions specified in this document is rather small. Again, for simplicity, we reduced it to a minimal set that still meets the requirements. A real implementation of the protocol might require splitting some of the transactions specified below into two or more transactions of the respective protocol. Reasons for this might include constraints of the particular protocol or the desire for more flexibility. In general this should not be a problem. However, it should be considered that this might change atomicity of the affected transactions.
2.1.5. Access Control
Ownership determines access to policy rules and policy rule groups. When a policy rule is created, a middlebox-unique identifier is generated to identify it in further transactions. Beyond the identifier, each policy rule has an owner. The owner is the authenticated agent that established the policy rule. The middlebox uses the owner attribute of a policy rule to control access to it; each time an authenticated agent requests to modify an existing policy rule, the middlebox determines the owner of the policy rule and checks whether the requesting agent is authorized to perform transactions on the owning agent's policy rules. All policy rules belonging to the same policy rule group must have the same owner. Therefore, authenticated agents have access either to all members of a policy rule group, or to none of them. The middlebox may be configured to allow specific authenticated agents to access and modify policy rules with certain specific owners. Certainly, a reasonable default configuration would let each agent access its own policy rules. Also, it might be good to configure an agent identity to act as administrator, allowing modification of all policy rules owned by any agent. However, the configuration of authorization at the middlebox is out of scope of the MIDCOM semantics and protocol.2.1.6. Middlebox Capabilities
For several reasons it is useful that at session establishment the agent learns about particular capabilities of the middlebox. Therefore, the session establishment procedure described in section 2.2.1 includes a transfer of capability information from the middlebox to the agent. The list of covered middlebox capabilities includes the following: - Support of firewall function - List of supported NAT functions, perhaps including - address translation - port translation - protocol translation - twice-NAT - Internal IP address wildcard support - External IP address wildcard support - Port wildcard support - Supported IP version(s) for internal network: IPv4, IPv6, or both
- Supported IP version(s) for external network:
IPv4, IPv6, or both
- List of supported optional MIDCOM protocol transactions
- Optional interface-specific policy rule support: not
supported or supported
- Policy rule persistence: persistent or non-persistent
(a rule is persistent when the middlebox can save the rule to
a non-volatile memory, e.g., a hard disk or flash memory)
- Maximum remaining lifetime of a policy rule or policy rule
group
- Idle-timeout of policy rules in the middlebox
(reserved and enabled policy rules not used by any
data traffic for the time of this idle-timeout are deleted
automatically by the middlebox; for the deletion of policy
rules by middleboxes, see section 2.3.13 about Asynchronous
Policy Rule Event).
- Maximum number of simultaneous MIDCOM sessions
The list of middlebox capabilities may be extended by a concrete
protocol specification with further information useful for the agent.
2.1.7. Agent and Middlebox Identifiers
To allow both agents and middleboxes to maintain multiple sessions,
each request message contains a parameter identifying the requesting
agent, and each reply message and each notification message contains
a parameter identifying the middlebox. These parameters are not
explicitly listed in the description of the individual transactions,
because they are common to all of them. They are not further
referenced in the individual semantics descriptions. Although, they
are not necessarily passed explicitly as parameters of the MIDCOM
protocol, they might be provided by the underlying (secure) transport
protocol being used. Agent identifiers at the middlebox are
middlebox-unique, and middlebox identifiers at the agent are agent-
unique, respectively.
2.1.8. Conformance
The MIDCOM requirements in [MDC-REQ] demand capabilities of the
MIDCOM protocol that are met by the set of transactions specified
below. However, an actual implementation of a middlebox may support
only a subset of these transactions. The set of announced supported
transactions may be different for different authenticated agents.
The middlebox informs the authenticated agent with the capability
exchange at session establishment about the transactions that the
agent is authorized to perform. Some transactions need to be offered
to every authenticated agent.
Each transaction definition below has a conformance entry that contains either 'mandatory' or 'optional'. A mandatory transaction needs to be implemented by every middlebox offering MIDCOM service and must be must be offered to each of the authenticated agents. An optional transaction does not necessarily need to be implemented by a middlebox; it may offer these optional transactions only to certain authenticated agents. The middlebox may offer one, several, all, or no optional transactions to the agents. Whether an agent is allowed to use an optional request transaction is determined by the middlebox's authorization procedure, which is not further specified by this document.2.2. Session Control Transactions
Before any transaction on policy rules or policy rule groups is possible, a valid MIDCOM session must be established. A MIDCOM session is an authenticated and authorized association between agent and middlebox. Sessions are initiated by agents and can be terminated by either the agent or the middlebox. Both agent and middlebox may participate in several sessions (with different entities) at the same time. To distinguish different sessions, each party uses local session identifiers. All transactions are transmitted within this MIDCOM session. Session control is supported by three transactions: - Session Establishment (SE) - Session Termination (ST) - Asynchronous Session Termination (AST) The first two are configuration transactions initiated by the agent, and the last one is an asynchronous transaction initiated by the middlebox.2.2.1. Session Establishment (SE)
transaction-name: session establishment transaction-type: configuration transaction-compliance: mandatory request-parameters: - request identifier: An agent-unique identifier for matching corresponding request and reply at the agent.
- version: The version of the MIDCOM protocol.
- middlebox authentication challenge (mc): An authentication
challenge token for authentication of the middlebox. As seen
below, this is present only in the first iteration of the
request.
- agent authentication (aa): An authentication token
authenticating the agent to the middlebox. As seen below, this
is updated in the second iteration of the request with material
responding to the middlebox challenge.
reply-parameters (success):
- request identifier: An identifier matching the identifier
request.
- middlebox authentication (ma): An authentication token
authenticating the middlebox to the agent.
- agent challenge token (ac): An authentication challenge token
for the agent authentication.
- middlebox capabilities: A list describing the middlebox's
capabilities. See section 2.1.6 for the list of middlebox
capabilities.
failure reason:
- authentication failed
- no authorization
- protocol version of agent and middlebox do not match
- lack of resources
semantics:
This session establishment transaction is used to establish a
MIDCOM session. For mutual authentication of both parties two
subsequent session establishment transactions are required as
shown in Figure 1.
agent middlebox | session establishment request | | (with middlebox challenge mc) | CLOSED |-------------------------------------------->| | | | successful reply (with middlebox | | authentication ma and agent challenge ac) | |<--------------------------------------------| | | NOAUTH | session establishment request | | (with agent authentication aa) | |-------------------------------------------->| | | | successful reply | |<--------------------------------------------| | | OPEN | | Figure 1: Mutual authentication of agent and middlebox Session establishment may be simplified by using only a single transaction. In this case, server challenge and agent challenge are omitted by the sender or ignored by the receiver, and authentication must be provided by other means, for example by TLS [RFC2246] or IPsec [RFC2402][RFC2406]. The middlebox checks with its policy decision point whether the requesting agent is authorized to open a MIDCOM session. If it is not, the middlebox generates a negative reply with 'no authorization' as failure reason. If authentication and authorization are successful, the session is established, and the agent may start with requesting transactions on policy rules and policy rule groups. Part of the successful reply is an indication of the middlebox's capabilities.2.2.2. Session Termination (ST)
transaction-name: session termination transaction-type: configuration transaction-compliance: mandatory request-parameters:
- request identifier: An agent-unique identifier for matching
corresponding request and reply at the agent.
reply-parameters (success only):
- request identifier: An identifier matching the identifier of the
request.
semantics:
This transaction is used to close the MIDCOM session on behalf of
the agent. After session termination, the middlebox keeps all
established policy rules until their lifetime expires or until an
event occurs that causes the middlebox to terminate them.
The middlebox always generates a successful reply. After sending
the reply, the middlebox will not send any further messages to the
agent within the current session. It also will not process any
further request within this session that it received while
processing the session termination request, or that it receives
later.
2.2.3. Asynchronous Session Termination (AST)
transaction-name: asynchronous session termination
transaction-type: asynchronous
transaction-compliance: mandatory
notification message type: Session Termination Notification (STN)
reply-parameters (success only):
- termination reason: The reason why the session is terminated.
semantics:
The middlebox may decide to terminate a MIDCOM session at any
time. Before terminating the actual session the middlebox
generates a STN message and sends it to the agent. After sending
the notification, the middlebox will not process any further
request by the agent, even if it is already queued at the
middlebox.
After session termination, the middlebox keeps all established
policy rules until their lifetime expires or until an event occurs
for which the middlebox terminates them.
Unlike in other asynchronous transactions, no more than one
notification is sent, because there is only one agent affected by
the transaction.
2.2.4. Session Termination by Interruption of Connection
If a MIDCOM session is based on an underlying network connection, the
session can also be terminated by an interruption of this connection.
If the middlebox detects this, it immediately terminates the session.
The effect on established policy rules is the same as for the
Asynchronous Session Termination.
2.2.5. Session State Machine
A state machine illustrating the semantics of the session
transactions is shown in Figure 2. The transaction abbreviations
used can be found in the headings of the particular transaction
section.
All sessions start in state CLOSED. If mutual authentication is
already provided by other means, a successful SE transaction can
cause a state transition to state OPEN. Otherwise, it causes a
transition to state NOAUTH. From this state a failed second SE
transaction returns to state CLOSED. A successful SE transaction
causes a transition to state OPEN. At any time, an AST transaction
or a connection failure may occur, causing a transition to state
CLOSED. A successful ST transaction from either NOAUTH or OPEN also
causes a return to CLOSED. The parameters of the transactions are
explained in Figure 2; the value mc=0 represents an empty middlebox
challenge.
mc = middlebox challenge SE/failure ma = middlebox authentication +-------+ ac = agent challenge | v aa = agent authentication +----------+ | CLOSED |----------------+ +----------+ | SE(mc!=0)/ | ^ ^ | success(ma,ac) SE(mc=0, | | | AST | aa=OK)/ | | | SE/failure v success | | | ST/success +----------+ | | +------------| NOAUTH | | | +----------+ | | AST | SE(mc=0, v | ST/success | aa=OK)/ +----------+ | success | OPEN |<---------------+ +----------+ Figure 2: Session State Machine
(page 18 continued on part 2)
