Internet Draft M. Stiemerling Document: draft-stiemerling-midcom-semantics-02.txt J. Quittek Expires: February 2003 NEC Europe Ltd. August 2002 MIDCOM Protocol Semantics Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Distribution of this document is unlimited. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. 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 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 a superset of it. The MIDCOM protocol semantics is derived from the MIDCOM requirements, from the MIDCOM framework, and from working group decisions. Stiemerling & Quittek [Page 1] Internet-Draft MIDCOM Protocol Semantics August 2002 Table of Contents 1 Introduction ................................................. 2 1.1 Terminology ................................................ 3 1.2 Transaction Definition Template ............................ 3 2 Semantics Specification ...................................... 5 2.1 General Protocol Design .................................... 5 2.1.1 Session, Policy, and Policy Group ........................ 5 2.1.2 Atomicity ................................................ 5 2.1.3 Access Control ........................................... 6 2.1.4 Conformance .............................................. 6 2.2 Session Control Transactions ............................... 7 2.2.1 Session Establishment (SE) ............................... 7 2.2.2 Session Termination (ST) ................................. 9 2.2.3 Asynchronous Session Termination (AST) ................... 10 2.2.4 Session Termination by Interruption of Connection ........ 11 2.2.5 Session State Machine .................................... 11 2.3 Policy Group Transactions .................................. 12 2.3.1 Group Establishment (GE) ................................. 13 2.3.2 Group Lifetime Change (GLC) .............................. 14 2.3.3 Group List (GL) .......................................... 15 2.3.4 Group Status (GS) ........................................ 16 2.3.5 Asynchronous Group Deletion (AGD) ........................ 17 2.3.6 Group State Machine ...................................... 18 2.4 Policy Rule Transactions ................................... 19 2.4.1 Policy Reserve Rule (PRR) ................................ 20 2.4.2 Policy Allow Rule (PAR) .................................. 23 2.4.3 Policy Lifetime Change (PLC) ............................. 27 2.4.4 Policy Status (PS) ....................................... 28 2.4.5 Asynchronous Policy Deletion (APD) ....................... 31 2.4.6 Policy Rule State Machine ................................ 31 3 Conformance Statements ....................................... 32 3.1 General Implementation Conformance ......................... 33 3.2 Middlebox Conformance ...................................... 33 3.3 Agent Conformance .......................................... 34 4 Transaction Usage Examples ................................... 34 4.1 Exploring Policies and Policy Groups ....................... 34 4.2 Enabling a SIP-Signaled Call ............................... 38 5 Compliance with MIDCOM Requirements .......................... 42 5.1 Protocol Machinery Requirements ............................ 42 5.1.1 Authorized Association ................................... 42 5.1.2 Agent connects to Multiple Middleboxes ................... 42 5.1.3 Multiple Agents connect to same Middlebox ................ 43 5.1.4 Deterministic Behavior ................................... 43 5.1.5 Known and Stable State ................................... 43 5.1.6 Status Report ............................................ 44 5.1.7 Unsolicited Messages (Asynchronous Notifications) ........ 44 5.1.8 Mutual Authentication .................................... 44 5.1.9 Session Termination by any Party ......................... 44 5.1.10 Request Result .......................................... 44 Stiemerling & Quittek [Page 2] Internet-Draft MIDCOM Protocol Semantics August 2002 5.1.11 Version Interworking .................................... 45 5.1.12 Deterministic Handling of Overlapping Rules ............. 45 5.2 Protocol Semantics Requirements ............................ 45 5.2.1 Extensible Syntax and Semantics .......................... 45 5.2.2 Policy Rules for Different Types of Middleboxes .......... 45 5.2.3 Ruleset Groups ........................................... 45 5.2.4 Policy Lifetime Extension ................................ 46 5.2.5 Robust Failure Modes ..................................... 46 5.2.6 Failure Reasons .......................................... 46 5.2.7 Multiple Agents Manipulating Same Policy ................. 46 5.2.8 Carrying Filtering Rules ................................. 46 5.2.9 Oddity of Port Numbers ................................... 46 5.2.10 Consecutive Range of Port Numbers ....................... 46 5.2.11 Contradicting Overlapping Policies ...................... 47 5.3 Security Requirements ...................................... 47 5.3.1 Authentication, Confidentiality, Integrity ............... 47 5.3.2 Optional Confidentiality of Control Messages ............. 47 5.3.3 Operation across Un-trusted Domains ...................... 47 5.3.4 Mitigate Replay Attacks .................................. 47 6 Security Considerations ...................................... 47 7 Acknowledgments .............................................. 48 8 Open Issues .................................................. 48 9 Acknowledgements ............................................. 48 10 References .................................................. 48 11 Authors' Address ............................................ 49 12 Full Copyright Statement .................................... 49 1. Introduction The MIDCOM working group has defined a framework [MDC-FRM] for the middle box communication as well as a list of requirements [MDC-REQ]. But for specifying a concrete protocol, the clear semantics need to be defined. The documents mentioned above are not completely sufficient for this purpose. Some required capabilities are not mentioned explicitly in the framework or requirements document, but are inherent to the problem. This memo suggests a semantics for the MIDCOM protocol. It is fully compliant with the requirements listed in [MDC-REQ] and with the working groups consensus on semantical issues. In conformance with the working group charter, the semantics is targeted at packet filters and network address translators (NATs) and it supports applications that require dynamic configuration of these middleboxes. The semantics are defined in terms of transactions. Two basic types of transactions are used: request-reply transactions and notification transactions. For each transaction the semantics is specified by Stiemerling & Quittek [Page 3] Internet-Draft MIDCOM Protocol Semantics August 2002 describing (1) the parameters of the transaction, (2) the processing (of request transactions) at the middlebox, and (3) the state transitions at the middlebox caused by the transactions. 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. Anyway, the semantics below will still be a subset of the implemented semantics. The document contains four major sections. Section 2 describes the protocol semantics. It is structured in four subsections: - General Protocol Issues (Section 2.1) - Session Control (Section 2.2) - Policy Groups (Section 2.3) - Policy Rules (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 message transfer from the agent and to the middlebox followed by a reply message transfer from the middlebox to the agent. notification transaction An asynchronous message transfer from the middlebox and to the agent. agent unique An agent unique value is unique in the context of the agent. This context includes all MIDCOM session 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 session the middlebox Stiemerling & Quittek [Page 4] Internet-Draft MIDCOM Protocol Semantics August 2002 participates in. A middlebox unique value is assigned by the middlebox. 1.2. Transaction Definition Template In the following sections semantics of the MIDCOM protocol is specified per transaction. A transaction specification contains the following entries. (Parameter entities are only specified if applicable.) transaction-name A description name for this type of transaction. transaction-type The transaction type is either 'request' or 'notification'. See above for the description of request transaction and notification transaction. transaction-compliance This entry contains either 'mandatory' or 'optional'. For details see Section 2.1.4. request-parameter This entry lists all parameters that are necessary for this request. A description for each parameter is given. reply-parameter (success) This entry lists all parameters that are sent back from the middlebox to the agent as positive response to the prior request. A description for each parameter is given. reply-parameter (failure) This entry lists all parameters that are sent back from the middlebox to the agent as negative response to the prior request. A description for each parameter is given. notification parameters This entry lists all parameters that are used by the middlebox to notify the agent about any asynchronous event. A description for each parameter is given. semantics This entry describes the actual semantics of the transaction. Stiemerling & Quittek [Page 5] Internet-Draft MIDCOM Protocol Semantics August 2002 2. Semantics Specification 2.1. General Protocol Design A major goal of the semantics is finding a good balance between properly support of applications that require dynamic configuration of middleboxes and simplicity of specification and implementation of the protocol. The MIDCOM protocol will be subdivided into three phases as specified in Section 4 of [MDC-FRM]: - session setup phase - run-time phase - session termination phase In all phases two kinds of state transitions may occur at the middlebox: State transactions either are initiated by a requests from the agent to the middlebox, or they are initiated by any other event. In the first case the middlebox informs the agent by sending a reply on the actual state transition, in latter case the middlebox sends a notification to the agent. Requests and replies contain an agent unique request identifier that allows the agent to determine to which sent request a received reply corresponds. 2.1.1. Session, Policy, and Policy Group An analysis of the requirements showed that three kinds of transactions are required: transactions for session control, transactions for controlling of policies, and transaction for controlling policy groups. Policy groups can be used to indicated relationships between policies and to simplify transactions on a set of policies by using a single one per group instead of one per policy. Requirement analysis also showed that session state, policy state, and policy group state can be separated. The separation simplifies the specification of the semantics as well as a protocol implementation. Therefore, the semantics specification is structured accordingly and we use three separated state machines to illustrate the semantics. Please note, that state machines of concrete protocol designs and implementations will most probably more complex than the state machines presented here. However, the protocol state machines are expected to be a superset of the state machines in this document. 2.1.2. Atomicity All request transactions are atomic with respect to each other. This means that processing a request at the middlebox is never interrupted by another arriving or already queued request. This particularly applies when the middlebox concurrently receives requests originating Stiemerling & Quittek [Page 6] Internet-Draft MIDCOM Protocol Semantics August 2002 in different session. However, asynchronous notification transactions may interrupt and terminate processing of a request at any time. All request transactions are atomic from the point of view of the agent. Processing of a request does not start before the complete request arrived 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 close to a minimal set that still meets the requirements. For a real implementation of the protocol, it might be required to split some of the transactions specified below into two or more transactions of the respective protocol. Reasons for this might be 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. 2.1.3. Access Control Access to policies and policy groups is based on ownership. When a policy or a group is created, a middlebox unique identifier is generated for identifying it in further transactions. Beyond the identifier, each group has an owner. The owner is the authenticated agent that established the policy or group. The middlebox uses the owner attribute of a policy or group for controlling access to it: each time an authenticated agent requests to modify an existing policy or group, the middlebox determines the owner of the policy or group and checks if the requesting agent is authorized to perform transactions on the owning agent's policies or groups. By configuring the middlebox, certain authenticated agents may get authorized to access and modify groups with certain owner. Certainly, a reasonable default configuration would be that each agent can access its own groups. Also, it might be a good idea, to have an agent identity configured to act as administrator being allowed to modify all policies owned by any agent. Anyway, the configuration of authorization is not subject of the MIDCOM protocol semantics. 2.1.4. Conformance The MIDCOM requirements in [MDC-REQ] demand certain capabilities of the MIDCOM protocol, which are met by the set of transactions specified below. However, an actual implementation of a middlebox may support only a subset of these transactions. Support limitation may be different for different authenticated agents. At session establishment, the middlebox informs the authenticated agent by capability exchange, which transactions the agent is authorized to perform. Some transactions need to be offered to every authenticated Stiemerling & Quittek [Page 7] Internet-Draft MIDCOM Protocol Semantics August 2002 agent. Each transaction definitions below has a conformance entry which contains either 'mandatory' or 'optional'. A mandatory transaction need to be implemented by every middlebox offering MIDCOM service. A mandatory request transaction must be offered to each of the authenticated agents. An optional transaction does not necessarily need to be implemented by a middlebox. An implemented optional request transaction does not necessarily need to be offered to every authenticated agent. Whether or not 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 groups is possible, a valid MIDCOM session must be established. A MIDCOM session is an authorized association between agent and middlebox. Sessions are initiated by agents and can be terminated by any party. Both agent and middlebox may participate in several sessions at the same time. For distinguishing different sessions each party uses local session identifiers. Session control is supported by three transactions: - Session Establishment (SE) - Session Termination (ST) - Asynchronous Session Termination (AST) The first two are request transactions initiated by the agent, the last one is a notification transaction initiated by the middlebox. 2.2.1. Session Establishment (SE) transaction-name: session establishment transaction-type: request 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 the middlebox authentication. Stiemerling & Quittek [Page 8] Internet-Draft MIDCOM Protocol Semantics August 2002 - agent authentication (aa): an authentication token to authenticate the agent to the middlebox. - encryption method: an identifier of an encryption method. Also 'no encryption' may be specified. reply-parameters (success): - request identifier: an identifier matching the identifier request. - middlebox authentication (ma): an authentication token to authenticate the middlebox to the agent. - agent challenge token (ac): an authentication challenge token for the agent authentication. - middlebox capabilities: a parameter set describing the middlebox's capabilities. The set includes - type of the middlebox for example: FW, NAT, NATFW, NAPT, NAPTFW, NAT-PT, NAT-PTFW, ...) - 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 - policy rule persistency: persistent or not persistent - maximum remaining lifetime of a policy rule or policy group reply-parameters (failure): - request identifier: an identifier matching the identifier request. - failure reason: the reason why the session establishment transaction failed. The list of possible reasons includes but is not restricted to: - authentication failed - no authorization - protocol version of agent and middlebox do not match - encryption method not supported - lack of resources semantics: This session establishment transaction is used to establish a MIDCOM session. For mutual authentication of both parties two Stiemerling & Quittek [Page 9] Internet-Draft MIDCOM Protocol Semantics August 2002 subsequent session establishment transactions are required as shown in Figure 1. agent middlebox | session establishment request | | (with middlebox challenge mc) | |-------------------------------------------->| | | | successful reply (with middlebox | | authentication ma and agent challenge ac) | |<--------------------------------------------| | | | session establishment request | | (with agent authentication aa) | |-------------------------------------------->| | | | successful reply | |<--------------------------------------------| | | 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 if the requesting agent is authorized to open a MIDCOM session. If not a negative reply with 'no authorization' as failure reason is generated by the middlebox. If authentication and authorization are successful, the session is established and the agent may start with requesting transactions on policy rules and policy groups. Part of the successful reply is an indication of the middlebox's capabilities. The list of capabilities to be included needs to be further elaborated. The agent specifies an encryption method for the session including the option of not using encryption. The middlebox can accept this suggestion or reject it. In case of rejection, the session establishment fails and an appropriate failure reason is indicated by the middlebox in the reply message. Then the agent may try session setup again with a different encryption method. 2.2.2. Session Termination (ST) transaction-name: session termination Stiemerling & Quittek [Page 10] Internet-Draft MIDCOM Protocol Semantics August 2002 transaction-type: request 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 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 groups and policy rules until their lifetime expires or until an event occurs on which the middlebox terminates them. The middlebox generates always 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, which it has received while it was processing the session termination request, or which it receives later. 2.2.3. Asynchronous Session Termination (AST) transaction-name: asynchronous session termination transaction-type: notification transaction-compliance: mandatory notification-parameters: - termination reason: The reason why the session is terminated without any request from the agent. semantics: The middlebox may decide at any point in time to terminate a MIDCOM session. Before terminating the actual session the middle box generates this notification transaction. After sending the notification, the middlebox will not process any further request by the agent, even if it is already queued at the middlebox. Stiemerling & Quittek [Page 11] Internet-Draft MIDCOM Protocol Semantics August 2002 After session termination the middlebox keeps all established policy groups and policy rules until their lifetime expires or until an event occurs on which the middlebox terminates them. 2.2.4. Session Termination by Interruption of Connection If a MIDCOM session is based on an underlying network connection, then 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 groups and 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 used transaction abbreviations can be found in the headings of the particular transaction section. All sessions start in state CLOSED. A successful SE transaction can cause a state transition to state OPEN, if mutual authentication is already provided by other means. Otherwise, it causes a transition to state NOAUTH. From this state a failed SE transaction returns to state closed, as well as a successful ST transaction. A successful SE transaction causes a transition to state OPEN. At any time an AST transaction may occur causing a transition to state CLOSED. 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 Stiemerling & Quittek [Page 12] Internet-Draft MIDCOM Protocol Semantics August 2002 2.3. Policy Group Transactions This section describes the semantics for transactions on groups of policies. The following transactions are specified: - Group Establishment (GE) - Group Lifetime Change (GLC) - Group List (GL) - Group Status (GS) - Asynchronous Group Deletion (AGD) The first four are request transactions initiated by the agent, the last one is a notification transaction initiated by the middlebox. The status information transactions (GL and GS) do not have any effect on the group state machine. Group transactions are redundant in the sense that a transaction on a group can be replaced by the corresponding transaction on each member of a group (except for the GE transaction). They can be removed easily from the semantics specification without changing the set of possible middlebox configurations an agent can request. Therefore all of them are declared as 'optional' by their respective compliance entry. Before any group request can be processed a valid MIDCOM session must have been established. The establishment of groups is a premise of any further policy establishment. However, there is a default group which is automatically established by the middlebox for every authenticated agent. This group has unlimited lifetime and cannot be controlled by a GE or GLC transaction. It has to be used by the agent if the middlebox does not offer group transactions. But it may be used by the agent at any time. It is addressed by a fixed group identifier value. Each policy is member of exactly one group, and membership does not change during policy lifetime. Each group that is not a default group has its individual lifetime. If the group lifetime expires, the group and all member policies will be deleted at the middlebox. A group lifetime change (GLC) transaction may extend the lifetime of the group up to the limit specified at session setup, when the middlebox informs the agent about its capabilities. Also a GLC transaction may be used for deleting a group by requesting a lifetime of 0. After a successful GLC transaction, all member policies have the same lifetime as the group. Please note that by policy-specific transactions, the lifetime of an individual policy may be set to other values than the group lifetime, but an individual policy lifetime may never exceed the group lifetime. Stiemerling & Quittek [Page 13] Internet-Draft MIDCOM Protocol Semantics August 2002 The status information transactions GL and GS can be used by the agent for exploring the state of the middlebox and for exploring its access rights. The GL transaction lists all groups that the agent may access, including groups owned by other agents. The GS transaction reports the status of an individual group and it lists all policies of this group by their policy identifers. The agent can explore the state of the individual policies by using the policy identifiers in a policy information transaction (see Section 2.4.4). 2.3.1. Group Establishment (GE) transaction-name: group establishment transaction-type: request transaction-compliance: optional request-parameters: - request identifier: an agent unique identifier for matching corresponding request and reply at the agent. - group lifetime: a lifetime proposal to the middlebox for the requested group. reply-parameters (success): - request identifier: an identifier matching the identifier request. - group identifier: a middlebox unique group identifier. It is assigned by the middlebox and used as group handle in further group transactions and in policy transactions adding policies to the group. - group lifetime: the group lifetime granted by the middlebox. reply-parameters (failure): - request identifier: an identifier matching the identifier request. - failure reason: the reason why the group establishment was rejected. The list of possible reasons includes but is not restricted to: - transaction not supported - agent not authorized for this transaction - lack of resources semantics: Stiemerling & Quittek [Page 14] Internet-Draft MIDCOM Protocol Semantics August 2002 This transaction creates an empty group with no policy being member of. The middlebox generates a middlebox unique identifier for the new group and assigns the requesting agent to be the group owner. The lifetime of the group is proposed by the agent. In case of a success reply, the middlebox chooses a lifetime value that is greater than zero and smaller than or equal to the proposed value. If the middlebox decides not to create a new group, a failure reply is generated containing a specification of the reason for failure. 2.3.2. Group Lifetime Change (GLC) transaction-name: group lifetime change transaction-type: request transaction-compliance: optional request-parameters: - request identifier: an agent unique identifier for matching corresponding request and reply at the agent. - group identifier: a reference to the group for which the lifetime is requested to be changed. - group lifetime: the new lifetime proposal for the group. reply-parameters (success): - request identifier: an identifier matching the identifier request. - group lifetime: The remaining group lifetime granted by the middlebox. reply-parameters (failure): - request identifier: an identifier matching the identifier request. - failure reason: the reason why the lifetime change was rejected. The list of possible reasons includes but is not restricted to: - transaction not supported - agent not authorized for this transaction - agent not authorized for changing lifetime of this group - no such group - this transaction cannot be applied to the default group - lifetime cannot be extended Stiemerling & Quittek [Page 15] Internet-Draft MIDCOM Protocol Semantics August 2002 semantics: The agent can use this transaction type to request an extension the lifetime of an already established group, to request shortening of the life time, or to request group termination which includes termination of all member policies. Group termination is requested by suggesting a new group lifetime of zero. The middlebox first checks whether or not the specified group exists and whether or not the agent is authorized to access this group. If one of the checks fails, an appropriate failure reply is generated. Also a failure reply is generated if the transaction is applied to the agent's default group. If the requested lifetime is longer than the current one, the middlebox also checks, whether or not the lifetime of the group may be extended and generates an appropriate failure message if not. A failure reply is implies that the lifetime of the group remains unchanged. A success reply is generated by the middlebox, if the lifetime of the group was changed in any way. The success reply contains the new lifetime of the group. The middlebox chooses the lifetime within the interval limited by the lifetime of the group at arrival of the request and by the suggested lifetime. The granted remaining lifetime must not exceed the maximum lifetime that the middlebox specified at session setup together with its other capabilities. A changed lifetime is applied to each member of the group. After sending a success reply with a lifetime of zero, the member policies will be deleted without any further notification to the agent, and the middlebox will consider the group and its members to be non-existent. It will not process any further transaction on this group or on any of its members. 2.3.3. Group List (GL) transaction-name: group list transaction-type: request transaction-compliance: optional request-parameters: - request identifier: an agent unique identifier for matching corresponding request and reply at the agent. reply-parameters (success): Stiemerling & Quittek [Page 16] Internet-Draft MIDCOM Protocol Semantics August 2002 - request identifier: an identifier matching the identifier request. - group list: list of all groups that the agent can access. For each listed group the identifier and the owner are indicated. reply-parameters (failure): - request identifier: an identifier matching the identifier request. - failure reason: the reason why the request for listing groups was rejected. The list of possible reasons includes but is not restricted to: - transaction not supported - agent not authorized for this transaction semantics: The agent can use this transaction type to list all groups which it can access, including default groups. Usually, the agent has this information already, but in special cases (for example after an agent failover) or for special agents (for example an administrating agent that can access all groups) this transaction can be helpful. The middlebox first checks whether or not the agent is authorized to request this transaction. If the checks fails, an appropriate failure reply is generated. Otherwise a list of all groups the agent can access is returned indicating the identifier and the owner each group. The shortest possible list to be replied contains just the requesting agent's default group. This transaction does not have any effect on the group state. 2.3.4. Group Status (GS) transaction-name: group status transaction-type: request transaction-compliance: optional request-parameters: - request identifier: an agent unique identifier for matching corresponding request and reply at the agent. - group identifier: a reference to the group for which status information is requested. Stiemerling & Quittek [Page 17] Internet-Draft MIDCOM Protocol Semantics August 2002 reply-parameters (success): - request identifier: an identifier matching the identifier request. - group owner: an identifier of the agent owning this policy group. - group lifetime: the remaining lifetime of the group. - member list: list of all policies that are members of the group. The policies are specified by their middlebox unique policy identifier. reply-parameters (failure): - request identifier: an identifier matching the identifier request. - failure reason: the reason why the request for a status report was rejected. The list of possible reasons includes but is not restricted to: - transaction not supported - agent not authorized for this transaction - no such group - agent not authorized for listing members of this group semantics: The agent can use this transaction type to list all member policies of a group. Usually, the agent has this information already, but in special cases (for example after an agent failover) or for special agents (for example an administrating agent that can access all groups) this transaction can be helpful. The middlebox first checks whether or not the specified group exists and whether or not the agent is authorized to access this group. If one of the checks fails, an appropriate failure reply is generated. Otherwise a list of all group members is returned indicating the identifier of each group. If the list of member policies is empty, a successful reply is returned containing an empty list. This transaction does not have any effect on the group state. 2.3.5. Asynchronous Group Deletion (AGD) transaction-name: asynchronous group deletion transaction-type: notification Stiemerling & Quittek [Page 18] Internet-Draft MIDCOM Protocol Semantics August 2002 transaction-compliance: optional notification-parameters: - group identifier: a reference to the group that will be deleted. - deletion reason: the reason why the middlebox will delete the group including all member policies. semantics: The middlebox may decide at any point in time to delete a group. Particularly, this transaction is triggered by lifetime expiration of the group. Among other events that may cause this transaction are changes in the policy decision point. If this notification is generated, it is sent to all agents that are in an open session with the middlebox and that are authorized to access the group. The notification is sent to the agents before the middlebox deletes the group and its member policies. The member policies will be deleted without any further notification to the agents. After sending the notification, the middlebox will consider the group and all its members to be non- existent. It will not process any further transaction on the group or on any of its members. 2.3.6. Group State Machine A state machine illustrating the semantics of the transactions on groups is shown in Figure 3. The used transaction abbreviations can be found in the headings of the particular transaction section. This state machine exists per group identifier. Initially, all groups are in state GROUP UNUSED, which means that the group does not exist. A successful GE transaction causes a transition to state GROUP INUSE. From there the state returns to GROUP UNUSED with a successful GLC transaction requesting a lifetime of zero and with an AGD transaction. After returning to state GROUP UNUSED, the group identifier is not anymore bound to an existing group and may be re- used by the middlebox. Stiemerling & Quittek [Page 19] Internet-Draft MIDCOM Protocol Semantics August 2002 GE/failure +--------+ | v +----------+ | GROUP | | UNUSED | +----------+ | ^ GE/success | | GLC(lt=0)/success | | AGD v | +----------+ | GROUP | | INUSE | +----------+ | ^ +--------+ GLC(lt>0)/ success lt = lifetime GLC/failure Figure 3: Group State Machine 2.4. Policy Rule Transactions This section describes the semantics for transactions on policies. The following transactions are specified: - Policy Reserve Rule (PRR) - Policy Allow Rule (PAR) - Policy Lifetime Change (PLC) - Policy Status (PS) - Asynchronous Policy Deletion (APD) The first four are request transactions initiated by the agent, the last one is a notification transaction initiated by the middlebox. The status information transaction (PS) does not have any effect on the policy state machine. Policy transactions PAR and PLC constitute the core of the MIDCOM protocol. Both are mandatory. They serve for - configuring NAT bindings (PAR) - configuring firewall pinholes (PAR) - extending the lifetime of established policies (PLC) - deleting policies (PLC) In some cases it is required to know in advance which IP address (and port number) would be chosen by NAT in a PAR transaction. This Stiemerling & Quittek [Page 20] Internet-Draft MIDCOM Protocol Semantics August 2002 information is required before sufficient information for performing a complete PAR transaction is required (see example in Section 4.2). For supporting such cases, the core transactions are extended by the Policy Reserve Rule (PRR) transaction serving for - reserving addresses and port numbers at NATs (PRR) A policy rule contains either a reserve action or an allow action. The reserve action allocates IP addresses and port numbers at a NAT. It does not have any function at a firewall. The allow action is interpreted as as bind action at a NAT for establishing bindings between internal and external addresses. At a firewall, the allow action is interpreted as one or more allow actions. The number of allow actions depends on the parameters of the request and the implementation of the firewall. For a more detailed description, see Sections 2.4.1. and 2.4.2. below. When a policy is established, it immediately becomes a member of one of the groups the agent may access. Each policy is member of exactly one group, and membership does not change during policy lifetime. If an agent does not need to group policies, it may just use its default group and have all policies being member of it. A default group is automatically generated by the middlebox for each authenticated agent. Each policy has its individual lifetime. If the policy lifetime expires, the policy will be deleted at the middlebox. A policy lifetime change (PLC) transaction may extend the lifetime of the policy up to the limit specified at session setup. Also a PLC transaction may be used for deleting a policy by requesting a lifetime of 0. Pease note that policy lifetime may also be modified by the group lifetime change transaction. The agent can explore the status of any policy by using the Policy Status (PS) transaction. 2.4.1. Policy Reserve Rule (PRR) transaction-name: policy reserve rule transaction-type: request transaction-compliance: optional request-parameters: - request identifier: an agent unique identifier for matching corresponding request and reply at the agent. Stiemerling & Quittek [Page 21] Internet-Draft MIDCOM Protocol Semantics August 2002 - group identifier: a reference to the group of which the reserve policy should be a member. - protocol identifier: identifies the protocol for which a reservation is requested. Examples are 'IP', 'UDP', and 'TCP'. - port range: the number of consecutive ports numbers to be reserved. This parameter is irrelevant, if the protocol identifier does not have the value 'TCP' or 'UDP'. - port oddity: the requested oddity of the port number to be reserved. Allowed values of this parameter are 'odd', 'even', and 'any'. - side of middlebox: the value of this parameter is either 'inside' or 'outside'. For an outside reservation, a reservation of an external address at the middlebox is requested, for an inside reservation, an internal address reservation at the middlebox is requested. - policy lifetime: a lifetime proposal to the middlebox for the requested policy. reply-parameters (success): - an identifier matching the identifier request. - policy identifier: a middlebox unique policy rule identifier. It is assigned by the middlebox and used as policy rule handle in further policy transactions. - reserved IP address: The reserved IPv4 or IPv6 address. - reserved port number: The reserved port number. In case of a port range greater than 1, it is the lowest port number of a consecutive sequence of reserved port numbers. This parameter is irrelevant, if the in the protocol identifier of the request parameters does not have the value 'TCP' or 'UDP'. - policy lifetime: the policy lifetime granted by the middlebox. reply-parameters (failure): - an identifier matching the identifier request. - failure reason: the reason why the reserve policy was rejected. The list of possible reasons includes but is not restricted to: - agent not authorized for this transaction - agent not authorized for adding members to this group - no such group Stiemerling & Quittek [Page 22] Internet-Draft MIDCOM Protocol Semantics August 2002 - no reservation of inside addresses supported - no reservation of outside addresses supported - lack of IP addresses - lack of port numbers - lack of resources semantics: The agent can use this transaction type to reserve an IP address or a combination of IP address, transport type, port number and port range at the middlebox. In some scenarios it is required to perform such a reservation before sufficient parameters for a complete policy allow rule transaction are available. See section 4.2 for an example. The reservation can be made for either side of the NAT but not for both of them. So far, not scenario has been found where reservation on both sides of the middlebox is required. Typically reservations will be requested for external addresses of a single-NAT. But for twice-NAT middleboxes, also reservations of internal addresses are supported. The middlebox first checks whether or not the specified group exists and whether or not the agent is authorized to add members to this group. If one of the checks fails, an appropriate failure reply is generated. In case of success, this transaction creates a new policy that becomes a member of the specified group. The middlebox generates a middlebox unique identifier for the new policy. The owner of the new policy is the owner of the group. The middlebox chooses a lifetime value that is greater than zero and smaller than or equal to the proposed value and that is smaller than or equal to the maximum lifetime specified at session setup. If the protocol identifier is 'IP', then the middlebox reserves an available internal or external IP address, depending on the specified direction. Depending on the specified side of the middlebox, either and internal address is reserved at the inside of the middlebox or an external address is reserved at the outside of the middlebox. The reserved address is returned to the agent. In this case the request-parameters port range and port oddity and the reply-parameter port number are irrelevant. If the protocol identifier is 'UDP' or 'TCP', then a combination of an IP address and a consecutive sequence of port numbers starting with the specified oddity is reserved. As for the protocol identifier 'IP', now the IP address is reserved as an internal one or an external one depending on the specified side of the middlebox. The IP address and the first reserved port number of the consecutive sequence are returned to the agent. Stiemerling & Quittek [Page 23] Internet-Draft MIDCOM Protocol Semantics August 2002 If the reservation fails because of lack of resources, such as available IP addresses, port numbers, or storage for further policies, then an appropriate failure reply is generated. 2.4.2. Policy Allow Rule (PAR) transaction-name: policy allow rule transaction-type: request transaction-compliance: mandatory request-parameters: - request identifier: an agent unique identifier for matching corresponding request and reply at the agent. - group identifier: a reference to the group of which the allow policy should be a member. - reservation identifier: a reference to an already existing reserve policy. As reference the policy identifier can be used. The reference may be empty. - protocol identifier: identifies the protocol for which a reservation is requested. Examples are 'IP', 'UDP', and 'TCP'. - port range: the number of consecutive ports numbers to be reserved. This parameter is irrelevant, if the protocol identifier does not have the value 'TCP' or 'UDP'. - port oddity: the requested oddity of the port number(s) to be mapped. Allowed values of this parameter are 'same' and 'any'. - topology: location of reservation or direction of communication. For the reserve action, this parameter specifies the side of the middlebox, either 'inside' or 'outside'. For allow actions, this parameter specifies the direction of allowed communication, either 'inbound', 'outbound', or 'bi-directional'. - internal IP address: the IP address of the internal communication endpoint (A0 in Fig. 4). The address may be wildcarded, for example by carrying a network mask. - internal port number: the port number of the internal communication endpoint (A0 in Fig. 4). The port number may be wildcarded. This parameter is irrelevant, if the in the protocol identifier does not have the value 'TCP' or 'UDP'. Stiemerling & Quittek [Page 24] Internet-Draft MIDCOM Protocol Semantics August 2002 - external IP address: the IP address of the external communication endpoint (A3 in Fig. 4). The address may be wildcarded, for example by carrying a network mask. - external port number: the port number of the external communication endpoint (A3 in Fig. 4). The port number may be wildcarded. This parameter is irrelevant, if the in the protocol identifier does not have the value 'TCP' or 'UDP'. - policy lifetime: a lifetime proposal to the middlebox for the requested policy. reply-parameters (success): - an identifier matching the identifier request. - policy identifier: a middlebox unique policy rule identifier. It is assigned by the middlebox and used as policy rule handle in further policy transactions. - inside IP address number: the internal IP address provided at the inside of the NAT (A1 in Fig. 4). The address may be wildcarded, for example by carrying a network mask. - inside port number: the internal port number provided at the inside of the NAT (A1 in Fig. 4). In case of a port range greater than 1, it is the lowest port number of a consecutive sequence of mapped port numbers. The port number may be wildcarded. This parameter is irrelevant, if the in the protocol identifier of the request parameters does not have the value 'TCP' or 'UDP'. - outside IP address number: the external IP address provided at the outside of the NAT (A2 in Fig. 4). The address may be wildcarded, for example by carrying a network mask. - outside port number: the external port number provided at the outside of the NAT (A2 in Fig. 4). In case of a port range greater than 1, it is the lowest port number of a consecutive sequence of mapped port numbers. The port number may be wildcarded. This parameter is irrelevant, if the in the protocol identifier of the request parameters does not have the value 'TCP' or 'UDP'. - policy lifetime: the policy lifetime granted by the middlebox. reply-parameters (failure): - an identifier matching the identifier request. Stiemerling & Quittek [Page 25] Internet-Draft MIDCOM Protocol Semantics August 2002 - failure reason: the reason why the allow policy was rejected. The list of possible reasons includes but is not restricted to: - agent not authorized for this transaction - no such group - agent not authorized for adding members to this group - no such reserve policy - the reserve policy is not member of the specified group - agent not authorized for accessing this reserve policy - mismatching protocol identifier in reserve policy - conflict with already existing policy rule - lack of IP addresses - lack of port numbers - lack of resources semantics: This transactions can be used by an agent for enabling communication between an internal endpoint and an external endpoint independent of the type of middlebox (NAT, NAPT, firewall, NAT-PT, combined devices, ... ) for uni-directional or bi-directional traffic. The agent sends an allow request specifying the endpoints (optionally including wildcards) and the direction of communication (inbound, outbound, bi-directional). The communication endpoints are displayed in Figure 4. They are addressed by the address tuples A0 and A3, respectively. An address tuple includes a protocol identifier, an IP address, and optionally a port number and a port number range. The middlebox replies to the allow request with a pair of communication address tuples A1 and A2 to be used by the partners for addressing each other. +----------+ +----------+ | internal | A0 A1 +-----------+ A2 A3 | external | | endpoint +----------+ middlebox +----------+ endpoint | +----------+ +-----------+ +----------+ Policy Allow Rule (PAR) Transaction: agent -> middlebox: A0, A3, direction middlebox -> agent: A1, A2 (in case of success) Figure 4: Communication endpoints in the PAR transaction In case of a pure packet filtering firewall, the returned address tuples are the same than the ones in the request: A2=A0 and A1=A3. Each partner uses the other one's real address. In case of a traditional NAT the internal endpoint may use the real address of the external endpoint (A1=A3), but the external endpoint uses an Stiemerling & Quittek [Page 26] Internet-Draft MIDCOM Protocol Semantics August 2002 address tuple provided by the NAT (A2!=A0). In case of a twice- NAT device, both endpoints uses address tuples provided by the NAT for addressing their communication partner (A3!=A1 and A2!=A0). If a firewall is combined with a NAT or a twice-NAT, the replied address tuples will be the same as for pure traditional NAT or twice-NAT, respectively, but the middlebox will configure its packet filter in addition to the performed NAT bindings. In case of a firewall combined with a traditional NAT, more than one allow action might be required for the firewall configuration, because incoming and outgoing packets use different source-destination pairs. The middlebox first checks whether or not the specified group exists and whether or not the agent is authorized to add members to this group. If the reservation identifier is not empty, then the middlebox also checks whether or not the reference policy exists whether or not it is member of the specified group, and whether or not the agent is authorized to modify this policy. If one of the checks fails, an appropriate failure reply is generated. In case of success, this transaction creates a new policy that becomes a member of the specified group. If a reservation policy was referenced, then the identifier of the reservation policy will be used for the new allow policy. Otherwise, the middlebox generates a middlebox unique identifier for the new policy. The owner of the new policy is the owner of the group. The middlebox chooses a lifetime value that is greater than zero and smaller than or equal to the proposed value and that is smaller than or equal to the maximum lifetime specified at session setup. If the protocol identifier is 'IP', then the middlebox allows communication between the specified external IP address and the specified internal IP address. The addresses to be used by the communication partners in order to address each other are returned to the agent as inside IP address and outside IP address. If the reservation identifier is not empty and if the reservation used the same protocol type, then the reserved IP address is used either as inside or as outside IP address (depending on the reservation). For the protocol identifiers 'UDP' and 'TCP' the middlebox acts analogously to 'IP' with additionally mapping ranges of port numbers and keeping the port oddity if requested. The configuration of the middlebox may fail because a specified reservation policy does not have a matching protocol identifier or because of lack of resources, such as available IP addresses, port numbers, or storage for further policies. Also it may fail Stiemerling & Quittek [Page 27] Internet-Draft MIDCOM Protocol Semantics August 2002 because of a conflict with an already established policy. In case of a conflict, the first come first serve mechanism is applied. Already existing policies remain unchanged and arriving new ones are rejected. However, in case of a non-conflicting overlap of policies (including identical policies), all policies are accepted. In each case of failure, an appropriate failure reply is generated. 2.4.3. Policy Lifetime Change (PLC) transaction-name: policy lifetime change transaction-type: request transaction-compliance: mandatory request-parameters: - request identifier: an agent unique identifier for matching corresponding request and reply at the agent. - policy identifier: identifying the policy for which the lifetime is requested to be changed. - policy lifetime: the new lifetime proposal for the policy. reply-parameters (success): - request identifier: an identifier matching the identifier request. - policy lifetime: The remaining policy lifetime granted by the middlebox. reply-parameters (failure): - request identifier: an identifier matching the identifier request. - failure reason: the reason why the lifetime change was rejected. The list of possible reasons includes but is not restricted to: - agent not authorized for this transaction - agent not authorized for changing lifetime of this policy - no such policy - lifetime cannot be extended semantics: Stiemerling & Quittek [Page 28] Internet-Draft MIDCOM Protocol Semantics August 2002 The agent can use this transaction type to request an extension the lifetime of an already established policy, to request shortening of the life time, or to request policy termination. Policy termination is requested by suggesting a new policy lifetime of zero. The middlebox first checks whether or not the specified policy exists and whether or not the agent is authorized to access this policy. If one of the checks fails, an appropriate failure reply is generated. If the requested lifetime is longer than the current one, the middlebox also checks, whether or not the lifetime of the policy may be extended and generates an appropriate failure message if not. A failure reply is implies that the lifetime of the policy remains unchanged. A success reply is generated by the middlebox, if the lifetime of the policy was changed in any way. The success reply contains the new lifetime of the policy. The middlebox chooses the lifetime within the interval limited by the lifetime of the policy at arrival of the request and by the suggested lifetime. The granted remaining lifetime must not exceed the maximum lifetime that the middlebox specified at session setup together with its other capabilities. it also must not exceed the lifetime of the group of which the policy is a member. After sending a success reply with a lifetime of zero, the middlebox will consider the policy to be non-existent. It will not process any further transaction on this policy. Please note, that policy lifetime may also be changed by the Group Lifetime Change (AGD) transaction if applied to the group of which the policy is a member. 2.4.4. Policy Status (PS) transaction-name: policy status transaction-type: request transaction-compliance: optional request-parameters: - request identifier: an agent unique identifier for matching corresponding request and reply at the agent. - policy identifier: the middlebox unique policy identifier. Stiemerling & Quittek [Page 29] Internet-Draft MIDCOM Protocol Semantics August 2002 reply-parameters (success): - request identifier: an identifier matching the identifier request. - policy owner: an identifier of the agent owning this policy. - group identifier: a reference to the group of which the policy is a member. - policy action: this parameter has either the value 'reserve' or the value 'allow'. - protocol identifier: identifies the protocol for which a reservation is requested. Examples are 'IP', 'UDP', and 'TCP'. - port range: the number of consecutive ports numbers. This parameter is irrelevant, if the protocol identifier does not have the value 'TCP' or 'UDP'. - direction: the direction of the communication allowed by the middlebox. The value of this parameter is either 'inbound', 'outbound', or 'bi-directional'. - internal IP address: the IP address of the internal communication endpoint (A0 in Fig. 4). The address may be wildcarded, for example by carrying a network mask. - internal port number: the port number of the internal communication endpoint (A0 in Fig. 4). The port number may be wildcarded. This parameter is irrelevant, if the in the protocol identifier does not have the value 'TCP' or 'UDP'. - external IP address: the IP address of the external communication endpoint (A3 in Fig. 4). The address may be wildcarded, for example by carrying a network mask. - external port number: the port number of the external communication endpoint (A3 in Fig. 4). The port number may be wildcarded. This parameter is irrelevant, if the in the protocol identifier does not have the value 'TCP' or 'UDP'. - inside IP address number: the internal IP address provided at the inside of the NAT (A1 in Fig. 4). The address may be wildcarded, for example by carrying a network mask. - inside port number: the internal port number provided at the inside of the NAT (A1 in Fig. 4). In case of a port range greater than 1, it is the lowest port number of a consecutive sequence of mapped port numbers. The port number may be Stiemerling & Quittek [Page 30] Internet-Draft MIDCOM Protocol Semantics August 2002 wildcarded. This parameter is irrelevant, if the in the protocol identifier of the request parameters does not have the value 'TCP' or 'UDP'. - outside IP address number: the external IP address provided at the outside of the NAT (A2 in Fig. 4). The address may be wildcarded, for example by carrying a network mask. - outside port number: the external port number provided at the outside of the NAT (A2 in Fig. 4). In case of a port range greater than 1, it is the lowest port number of a consecutive sequence of mapped port numbers. The port number may be wildcarded. This parameter is irrelevant, if the in the protocol identifier of the request parameters does not have the value 'TCP' or 'UDP'. - policy lifetime: the remaining lifetime of the policy. reply-parameters (failure): - request identifier: an identifier matching the identifier request. - failure reason: the reason why the request for a status report was rejected. The list of possible reasons includes but is not restricted to: - transaction not supported - agent not authorized for this transaction - no such policy - agent not authorized for accessing this policy semantics: The agent can use this transaction type to list all properties of a policy. Usually, the agent has this information already, but in special cases (for example after an agent failover) or for special agents (for example an administrating agent that can access all policies) this optional transaction can be helpful. The middlebox first checks whether or not the specified policy exists and whether or not the agent is authorized to access this group. If one of the checks fails, an appropriate failure reply is generated. Otherwise all properties of the policy are returned to the agent. Some of the returned parameters may be irrelevant, depending on the policy action ('reserve' or 'allow') and depending on other parameters, for example the protocol identifier. This transaction does not have any effect on the policy state. Stiemerling & Quittek [Page 31] Internet-Draft MIDCOM Protocol Semantics August 2002 2.4.5. Asynchronous Policy Deletion (APD) transaction-name: asynchronous policy deletion transaction-type: notification transaction-compliance: mandatory notification-parameters: - policy identifier: the policy that will be deleted. - deletion reason: the reason why the middlebox will delete the policy. semantics: The middlebox may decide at any point in time to delete a policy. Particularly, this transaction is triggered by lifetime expiration of the policy. Among other events that may cause this transaction are changes in the policy decision point. If this notification is generated, it is sent to all agents that are in an open session with the middlebox and that are authorized to access the policy. The notification is sent to the agents before the middlebox deletes the policy. After sending the notification, the middlebox will consider the policy to be non- existent. It will not process any further transaction on the policy. Please note that asynchronous policy termination may also be indicated by an Asynchronous Group Deletion (AGD) transaction without an individual APD for each member of the group. 2.4.6. Policy Rule State Machine The state machine for the policy rule transactions is shown in Figure 5 with all possible state transitions. You'll find the used transaction abbreviations in the headings of the particular transaction section. After returning to state POLICY UNUSED, the policy identifier is not anymore bound to an existing policy and may be re-used by the middlebox. Stiemerling & Quittek [Page 32] Internet-Draft MIDCOM Protocol Semantics August 2002 PRR/failure PAR/failure +-----------+ | v PRR/success +-+-------------+ +-----------------+ POLICY UNUSED |<-+ +----+ | +---------------+ | | | | ^ | | | v v APD | | | | +-------------+ PAR/failure| | PAR/ | APD | | RESERVED +------------+ | success | PLC(lt=0)/ | +-+----+------+ PLC(lt=0)/ | | success | | | success | | +----+ | v | PLC(lt>0)/ | PAR/success +---------------+ | success +---------------->| POLICY INUSE +--+ PLC/failure +-+-------------+ | ^ +-----------+ lt = lifetime PLC(lt>0)/success PLC/failure Figure 5: Policy Rule State Machine This state machine exists per policy identifier. Initially, all policies are in state POLICY UNUSED, which means that the policy does not exist or is not active. A successful PRR transaction causes a transition to state RESERVED, where an address reservation is established. From there, state POLICY INUSE can be entered by a PAR transaction. This transaction can also be used for entering state POLICY INUSE directly from state POLICY UNUSED without a reservation. In state POLICY INUSE the requested communication between the internal and the external endpoint is allowed. The states RESERVED and POLICY INUSE can be maintained by a successful PLC transactions with a requested lifetime greater than 0. Transitions from both of these states back to state POLICY UNUSED can be caused by an APD transaction or by a successful PLC transaction with a lifetime parameter of 0. Additionally, a failed PAR transaction causes a transition from state RESERVED to POLICY UNUSED. Please note, transitions initiated by APD transactions may also be initiated by AGD transactions. Analogously, transitions initiated by PLC transactions may also be initiated by GLC transactions. 3. Conformance Statements A protocol definition complies with the semantics defined in Section 2 if the protocol specification includes all specified transactions Stiemerling & Quittek [Page 33] Internet-Draft MIDCOM Protocol Semantics August 2002 with all their parameters. However, concrete implementations of the protocol may not support some of the optional transactions. Which transactions are required for compliancy is different for agent and middlebox. This section contains conformance statements for MIDCOM protocol implementations related to the semantics. Conformance is specified differently for agents and middleboxes. Most probably these conformance statements will be extended by a concrete protocol specification. However, such an extension is expected to extend the statements below in a way that all of them still hold. The following list shows the transaction-compliance property of all transactions as specified in the previous section: - Session Control Transactions - Session Establishment (SE) mandatory - Session Termination (ST) mandatory - Asynchronous Session Termination (AST) mandatory - Policy Group Transactions - Group Establishment (GE) optional - Group Lifetime Change (GLC) optional - Group List (GL) optional - Group Status (GS) optional - Asynchronous Group Deletion (AGD) optional - Policy Rule Transactions - Policy Reserve Rule (PRR) optional - Policy Allow Rule (PAR) mandatory - Policy Lifetime Change (PLC) mandatory - Policy Status (PS) optional - Asynchronous Policy Deletion (APD) mandatory 3.1. General Implementation Conformance A compliant implementation of a MIDCOM protocol must support all mandatory transactions. A compliant implementation of a MIDCOM protocol must support either the entire set of the group transactions GE, GLC, and AGD, or none of them. A compliant implementation of a MIDCOM protocol may support none, one, or more of the following transactions: GL, GS, PRR, PS. 3.2. Middlebox Conformance A middlebox implementation of a MIDCOM protocol supports a request transaction if it is able to receive and process all possible correct Stiemerling & Quittek [Page 34] Internet-Draft MIDCOM Protocol Semantics August 2002 message instances of the particular request transaction and if it generates a correct reply for any correct request it receives. A middlebox implementation o a MIDCOM protocol supports a notification transaction if it is able to to generate the corresponding notification message properly. A compliant middlebox implementation of a MIDCOM protocol must inform the agent about the list of supported transactions within the SE transaction. 3.3. Agent Conformance An agent implementation of a MIDCOM protocol supports a request transaction if it is able to generate the corresponding request message properly and if it is able to receive and process all possible correct replies to the particular request. An agent implementation of a MIDCOM protocol supports a notification transaction if it is able to receive and process all possible correct message instances of the particular transaction. A compliant agent implementation of a MIDCOM protocol must not use any optional transaction that is not supported by the middlebox. The middlebox informs the agent about the list of supported transactions within the SE transaction. 4. Transaction Usage Examples This section gives two usage examples of the transactions specified in Section 2. First it is shown, how an agent can explore all policies and policy groups, which it may access at a middlebox. Then the middlebox configuration for enabling a SIP-signaled call is demonstrated. 4.1. Exploring Policies and Policy Groups This example precludes an already established session. It shows how an agent can find out - which groups it may access and who owns these groups - the status and member list of all accessible groups - the status and properties of all accessible policies If there is just a single session, there is no need for any of these actions, because the middlebox informs the agent about each state transition of any policy or policy group. However, after the disruption of a session or after an intentional session termination, the agent might want to re-establish the session and explore, which Stiemerling & Quittek [Page 35] Internet-Draft MIDCOM Protocol Semantics August 2002 of the groups and policies it established are still in place. Also an agent system may fail and another one takes over. Then the other one need to find out what has already been configured by the failing system and what still needs to be done. A third situation where exploring policies and groups is useful is the case of an agent with 'administrator' authorization. This agent may access any policy or group created by any other agent and modify them. All of them probably will start their exploration with the Group List (GL) transaction, as shown in Figure 6. On this request, the middlebox returns a list of pairs each containing an agent identifier and a group identifier (GID). The agent gets informed which own group and which of other agents' groups it may access. agent middlebox | GL | |**********************************************>| |<**********************************************| | (agent1,GID1) (agent1,GID2) (agent2,GID3) | | | | GS GID2 | |**********************************************>| |<**********************************************| | agent1 lifetime PID1 PID2 PID3 PID4 | | | Figure 6: Using the GL and the GS transaction In Figure 6 three groups are accessible to the agent, and the agent retrieves information about the second group by using the Group Status (GS) transaction. It receives the owner of the group, the remaining lifetime, and the list of member policies, in this case containing four policy identifiers (PIDs). In the following, the agent explores these four policies. The example assumes the middlebox to be a traditional NAPT. Figure 7 shows the exploration of the first policy. As reply to a Policy Status (PS) transaction, the middlebox always returns the following list of parameters: - policy owner - group identifier - policy action (reserve or allow) - protocol type - port range - direction Stiemerling & Quittek [Page 36] Internet-Draft MIDCOM Protocol Semantics August 2002 - internal IP address - internal port number - external address - external port number - NAT inside IP address - NAT inside port number - NAT outside IP address - NAT outside port number agent middlebox | PS PID1 | |**********************************************>| |<**********************************************| | agent1 GID2 RESERVE UDP 1 OUTSIDE | | ANY ANY ANY ANY | | ANY ANY IPADR_OUT PORT_OUT1 | | | Figure 7: Status report for an outside reservation The policy with PID1 is a reserve policy for UDP traffic at the outside of the middlebox. Since there is no internal or external address involved yet, these four fields are wildcarded in the reply. The same holds for the inside NAT address and port number. The only address information given by the reply is the reserved outside IP address of the NAT (IPADDR_OUT) and the corresponding port number (PORT_OUT1). Note, that IPADR_OUT and PORT_OUT1 may not be wildcarded, because the reserve action does not support this. Applying PS to PID2 (Figure 8) shows that the second policy is an allow policy for inbound UDP packets. The internal destination is fixed concerning IP address, protocol and port number, but for the external source, the port number is wildcarded. The outside IP address and port number of the middlebox are the ones the external sender needs to use as destination in the original packet it sends. At the middlebox, the destination address is replaced with the internal address of the final receiver. During address translation, the source IP address and the source port numbers of the packets remain unchanged. This is indicated by the inside address which is identical to the external address. Stiemerling & Quittek [Page 37] Internet-Draft MIDCOM Protocol Semantics August 2002 agent middlebox | PS PID2 | |**********************************************>| |<**********************************************| | agent1 GID2 ALLOW UDP 1 IN | | IPADR_INT PORT_INT1 IPADR_EXT ANY | | IPADR_EXT ANY IPADR_OUT PORT_OUT2 | | | Figure 8: Status report for allowed inbound packets For traditional NATs the identity of the inside IP address and port number with the external IP address and port number always holds (A1=A3 in Figure 4). For a pure firewall, also the outside IP address and port number are always identical with the internal IP address and port number (A0=A2 in Figure 4). agent middlebox | PS PID3 | |**********************************************>| |<**********************************************| | agent1 GID2 ALLOW UDP 1 OUT | | IPADR_INT PORT_INT2 IPADR_EXT PORT_EXT1 | | IPADR_EXT PORT_EXT1 IPADR_OUT PORT_OUT3 | | | Figure 9: Status report for allowed outbound packets Figure 9 shows allowed outbound UDP communication between the same host. Here all port numbers are known. Since again A1=A3, the internal sender uses the external IP address and port number as destination in the original packets. At the firewall, the internal source IP address and port number are replaced by the shown outside IP address and port number of the middlebox. agent middlebox | PS PID4 | |**********************************************>| |<**********************************************| | agent1 GID2 ALLOW TCP 1 BI | | IPADR_INT PORT_INT3 IPADR_EXT PORT_EXT2 | | IPADR_EXT PORT_EXT2 IPADR_OUT PORT_OUT4 | | | Figure 10: Status report for bi-directional TCP traffic Finally, Figure 10 shows the status report for allowed bi-directional TCP traffic. Please note that still A1=A3: For outbound packets, only Stiemerling & Quittek [Page 38] Internet-Draft MIDCOM Protocol Semantics August 2002 the source IP address and port number are replaced at the middlebox, and for inbound packets, only the destination IP address and port number are replaced. 4.2. Enabling a SIP-Signaled Call This elaborated transaction usage example shows the interaction between a SIP proxy and a middlebox. The middlebox itself is a traditional NAPT and two user agents communciate with each other via the SIP proxy and NAPT as shown in figure 11. +----------+ |SIP Proxy | |for domain| | mb.com | +----------+ Private ^ ^ Public Network Network | | +----------+ | | +---------+ +----------+ |User Agent|<-+ +->|Middlebox|<------->|User Agent| | A |<#######>| NAPT |<#######>| B | +----------+ +---------+ +----------+ <--> SIP Signalling <##> RTP Traffic Figure 11: Example SIP Scenario For the below sequence charts we make these assumptions: - The NAPT is statically configured to forward SIP signalling from the outside to the SIP proxy server, i.e. traffic to the NAPT's external IP address and port 5060 is forwarded to the internal SIP proxy. - The user agent A, located inside the private network, is registered at the SIP proxy with its private IP address. - User A knows the general SIP URL of user B. The URL is B@b.de. However, the concrete URL of the SIP User Agent B, which user B currently uses, is not known. - Only the RTP paths are configured, but not the RTCP paths. - The middlebox and the SIP server share an already established MIDCOM session. Stiemerling & Quittek [Page 39] Internet-Draft MIDCOM Protocol Semantics August 2002 Furthermore these abbreviations are used: - IP_AI: Internal IP address of user agent A - P_AI: Internal port number of user agent A to receive RTP data - P_AE: External mapped port number of user agent A - IP_AE: External IP address of the middlebox - IP_B: IP address of user agent B - P_B: Port number of user agent B to receive RTP data - GID: Group identifier - PID: Policy rule identifier The abbreviations of the MIDCOM transactions can be found in the particular section headings. In our example, user A tries to call user B. Therefore, the user agent A sends an INVITE SIP message to the SIP proxy server (see Figure 13). The SDP part of the particular SIP message that is relevant for the middlebox configuration is shown in the sequence chart as: SDP: m=..P_AI.. c=IP_AI Whereas the m tag is the media tag which contains the receiving udp port number and the c tag contains the IP address of the terminal receiving the media stream. On receiving the SIP INVITE message, the SIP proxy server allocates a group for this call with the group establishment (GE) transaction. All following policy rules for this call will be bound to this group. The INVITE message forwarded to user agent B must contain a public IP address and a port number to which user agent B can send its RTP media stream. Therefore, the SIP proxy server needs an outside IP address and port number at the middlebox (the NAPT) to be available for this purpose. However, since the IP address of user agent B is not known yet (it will be sent by user agent B in the reply message), the porxy server cannot just request an address binding. Instead it just reserves an outside IP address and port number with the policy reserve rule (PRR). The PRR reply delivers the reserved IP address and port number. Now the SIP proxy server replaces in the SDP payload of the INVITE message the IP address and port number of user agent A by the reserved address and port (see Figure 12). Then the SIP INVITE message is forwarded to user agent B with a modified SDP body containing the outside address and port number, to which user agent B will send its RTP media stream. Stiemerling & Quittek [Page 40] Internet-Draft MIDCOM Protocol Semantics August 2002 User Agent SIP Middlebox User Agent A Proxy NAPT B | | | | | INIVTE B@B.DE | | | | SDP:m=..P_AI.. | | | | c=IP_AI | | | |--------------->| GE 600s | | | |*****************************>| | | |<*****************************| | | | GE OK GID 600s | | | | | | | | PRR GID UDP 1 EVEN IN 300s | | | |*****************************>| | | |<*****************************| | | | PRR OK PID1 IP_MB/P_AE 300s | | | | | | | | INVITE B@B.DE SDP:m=..P_AE.. c=IP_MB | | |-------------------------------------------->| | |<--------------------------------------------| | | 200 OK SDP:m=..P_B.. c=IP_B | Figure 12: Group establishment and rule reservation This SIP `200 OK' reply contains the IP address and port number, at which user agent B will receive a media stream. The IP address is assumed to be equal to the IP address from which user agwent B will send its media stream. Now, the SIP proxy server has sufficient information for estblishing the complete NAT binding with a policy allow rule (PAR) transaction, i.e. the UDP/RTP data of the call can flow from user agent B to user agent A. For the opposite direction, UDP/RTP data from user agent A to B, has to be allowed also. This is done by a second PAR transaction with all the necessary parameters (see figure 13). After having allowed both UDP/RTP streams the SIP proxy can forward the `200 OK' SIP message to user agent A to indicate that the telephone call can start. Stiemerling & Quittek [Page 41] Internet-Draft MIDCOM Protocol Semantics August 2002 User Agent SIP Middlebox User Agent A Proxy NAPT B | | | | | | PAR GID PID1 UDP 1 EVEN IN | | | | IP_AI P_AI IP_B ANY 300s | | | |*****************************>| | | |<*****************************| | | | PAR OK PID1 NONE NONE | | | | IP_MB P_AE1 300s | | | | | | ...media stream from user agent B to A allowed... | | | | | | PAR GID PID2 UDP 1 EVEN OUT | | | | IP_AI ANY IP_B P_B 300s | | | |*****************************>| | | |<*****************************| | | | PAR OK PID2 NONE NONE | | | | IP_MB P_AE2 300s | | | | | | ...media streams from both directions allowed... | | | | | 200 OK | | | |<---------------| | | | SDP:m=..P_B.. | | | | c=IP_B | | | Figure 13: Policy rule establishment for UDP flows User agent B decides to terminate the call and sends its `BYE' SIP message to user agent A. The SIP proxy forwards all SIP messages and deletes the group afterwards using a group lifetime change (GLC) transaction with a requested remaining lifetime of 0 seconds (see Figure 14). Deletion of the group includes deleting all member policies. Stiemerling & Quittek [Page 42] Internet-Draft MIDCOM Protocol Semantics August 2002 User Agent SIP Middlebox User Agent A Proxy NAPT B | | | | | BYE | BYE | |<---------------|<--------------------------------------------| | | | | | 200 OK | 200 OK | |--------------->|-------------------------------------------->| | | | | | | GLC GID 0s | | | |*****************************>| | | |<*****************************| | | | GLC OK 0s | | | | | | ...both NAT bindings for the media streams are removed... Figure 14: Deletion of Policy Groups 5. Compliance with MIDCOM Requirements This section explains the compliance of the specified semantics with the MIDCOM requirements. It is structured according to [MDC-REQ]: - Compliance with Protocol Machinery Requirements (Section 5.1) - Compliance with Protocol Semantics Requirements (Section 5.2) - Compliance with Security Requirements (Section 5.3) The requirements are referred to using the section number they are defined in: "requirement x.y.z" refers to the requirement specified in section x.y.z of [MDC-REQ]. 5.1. Protocol Machinery Requirements 5.1.1. Authorized Association The specified semantics enable a MIDCOM agent to establish an authorized association between itself and the middlebox. The agent identifies itself by the authentication mechanism of the Session Establishment transaction described in Section 2.2.1. Based on this authentication the middlebox can make a determination as to whether or not the agent will be permitted to request a service. Thus, requirement 2.1.1 is met. 5.1.2. Agent connects to Multiple Middleboxes As specified in Section 2.2, the MIDCOM protocol allows the agent to communicate with more than one middlebox simultaneously. The selection of a mechanism for separating different sessions is left to Stiemerling & Quittek [Page 43] Internet-Draft MIDCOM Protocol Semantics August 2002 the concrete protocol definition. It must provide a clear mapping of protocol messages to open sessions. Then requirement 2.1.2 is met. 5.1.3. Multiple Agents connect to same Middlebox As specified in Section 2.2, the MIDCOM protocol allows the middlebox to communicate with more than one agent simultaneously. The selection of a mechanism for separating different sessions is left to the concrete protocol definition. It must provide a clear mapping of protocol messages to open sessions. Then requirement 2.1.3 is met. 5.1.4. Deterministic Behavior Section 2.1.2 states, that processing a request of an agent may not be interrupted by any request of the same or another agent. This provides atomicity among request transactions. This avoids race conditions resulting in an unpredictable behavior of the middlebox. Anyway, the behavior of the middlebox can only be predictable in the view of its administrators. In the view of an agent, the middlebox behavior is unpredictable, because the administrator can, for example at any time modify the authorization of the agent without the agent being able to observe this change. Consequently, the behavior of the middlebox is not necessarily deterministic from the point of view of any agent. Since predictability of the middlebox behavior is given for its administrator, requirement 2.1.4 is met. 5.1.5. Known and Stable State Section 2.1.2 states that request transactions are atomic with respect to each other and from the point of view of an agent. All transactions are defined clearly as state transitions that either leave the current stable and well defined state and enter a new stable and well defined one or that remain in the current stable and well defined state. Section 2.1 clearly demands that intermediate states are not stable and not reported to any agent. Furthermore, for each state transition a message is sent to the corresponding agent, either a reply or a notification. The agent can uniquely map each reply to one of the requests that it sent to the middlebox, because request agent unique request identifiers are used for this purpose. Notifications are self-explanatory by their definition. Furthermore, the Group List transaction (Section 2.3.3), the Group Status transaction (Section 2.3.4), and the Policy Status transaction (Section 2.4.4) allow the agent at any time during a session to retrieve information about Stiemerling & Quittek [Page 44] Internet-Draft MIDCOM Protocol Semantics August 2002 - all policy groups it may access, - the status and member policies of all accessible groups, - and the status of all accessible policies. Therefore, the agent is precisely informed about the state of the middlebox (as far as the services requested by the agent are affected) and requirement 2.1.5 is met. 5.1.6. Status Report As argued in the previous section, the middlebox unambiguously informs the agent about every state transition related to any of the services requested by the agent. Also the agent can at any time retrieve full status information about all accessible policies and policy groups. Thus, requirement 2.1.6 is met. 5.1.7. Unsolicited Messages (Asynchronous Notifications) The semantics include asynchronous notifications from the middlebox to the agent, including Asynchronous Session Termination (Section 2.2.3), Asynchronous Group Deletion (Section 2.3.5), and Asynchronous Policy Deletion (Section 2.4.5). These notifications report every change of state, that was not explicitly requested by the agent. Thus, requirement 2.1.7 is met by the semantics specified above. 5.1.8. Mutual Authentication As specified in Section 2.2.1, the semantics require mutual authentication of agent and middlebox, either by using two subsequent Session Establishment transactions or by using mutual authentication provided on a lower protocol layer. Thus, requirement 2.1.8 is met. 5.1.9. Session Termination by any Party The semantics specification states in Section 2.2.2 that the agent may request session termination by generating the Session Termination request and that the middlebox may not reject this request. In turn, Session 2.2.3 states that the agent may send the Asynchronous Session Termination notification at any time and then terminate the session. Thus, requirement 2.1.9 is met. 5.1.10. Request Result Section 2.1 states that each request of an agent is followed by a reply of the middlebox indicating either success of failure. Thus, requirement 2.2.10 is met. Stiemerling & Quittek [Page 45] Internet-Draft MIDCOM Protocol Semantics August 2002 5.1.11. Version Interworking Section 2.2.1 states that the agent need to specify the protocol version number which it is going to use during the session. The middlebox may accept this and act according to this protocol version or reject the session if it does not support this version. If the session setup gets rejected, the agent may try again with another version. Thus, requirement 2.2.11 is met. 5.1.12. Deterministic Handling of Overlapping Rules The only policy rule actions specified are 'reserve' and 'allow'. For firewalls, overlapping allow actions or reserve actions do not create any conflict, so a firewall will always accept overlapping rules as specified in Sections 2.4.1 and 2.4.2 (assuming the required authorization is given). For NATs reserve and allow may conflict. If a conflicting request arrives, it is rejected, as stated in Sections 2.4.1 and 2.4.2. If an overlapping request arrives that does not conflict with the ones it overlaps, it is accepted (assuming the required authorization is given). Therefore, the behavior of the middlebox in the presence of overlapping rules can be predicted deterministically, and requirement 2.1.12 is met. 5.2. Protocol Semantics Requirements 5.2.1. Extensible Syntax and Semantics requirement 2.2.1 explicitly requests extensibility of protocol syntax. This needs to be addressed by the concrete protocol definition. The semantics specification is extensible anyway, because new transaction may be added. 5.2.2. Policy Rules for Different Types of Middleboxes Section 2.4 explains that the semantics use identical transactions for all middlebox types and that the same policy rule can be applied to all of them. Thus requirement 2.2.2 is met. 5.2.3. Ruleset Groups The semantics explicitly supports grouping of policies and transactions on policy groups, as described in Section 2.3. The group transactions can be used for lifetime extension and deletion of all policies being member of the particular group. Thus, requirement 2.2.3 is met. Stiemerling & Quittek [Page 46] Internet-Draft MIDCOM Protocol Semantics August 2002 5.2.4. Policy Lifetime Extension The semantics include a transaction for explicit lifetime extension of policies, as described in Section 2.4.3. Thus requirement 2.2.4 is met. 5.2.5. Robust Failure Modes The state transitions at the middlebox are clearly specified and communicated to the agent. There is no intermediate state reached by a partial processing of a request. All requests are always processed completely, either successful or unsuccessful. All request transaction include a list of failure reasons. These failure reasons cover indication of invalid parameters where applicable. In case of failure one of the specified reasons is returned from the middlebox to the agent. Thus requirement 2.2.5 is met. 5.2.6. Failure Reasons The semantics include a failure reason parameter in each failure reply. Thus requirement 2.2.6 is met. 5.2.7. Multiple Agents Manipulating Same Policy As specified in Sections 2.3 and 2.4, each installed policy rule and policy group has an owner, which is the authenticated agent that created the policy or group, respectively. The authenticated identity is input to authorization of access to policies and groups. If the middlebox is sufficiently configurable, its administrator can configure it such that one authenticated agent is is authorized to access and modify policies and groups owned by another agent. Because specified semantics does not preclude this, it meets requirement 2.2.7. 5.2.8. Carrying Filtering Rules The Policy Allow Rule transaction specified in Section 2.4.2 can carry 5-tuple filtering rules. It meets requirement 2.2.8. 5.2.9. Oddity of Port Numbers As specified in Section 2.4.2, the agent is able to request to keep the port oddity. Thus requirement 2.2.9 is met. 5.2.10. Consecutive Range of Port Numbers The Policy Allow Rule transaction (PAR, Section 2.4.2) allows the agent to specify a range of consecutive port numbers to be mapped. This can be used for mapping a consecutive range of external port Stiemerling & Quittek [Page 47] Internet-Draft MIDCOM Protocol Semantics August 2002 numbers to consecutive internal ports. Thus requirement 2.2.10 is met. 5.2.11. Contradicting Overlapping Policies requirement 2.2.11 is based on the assumption that contradicting policy actions, such as 'allow' and 'disallow' are supported. In conformance with decisions made by the working group after finalizing the requirements document, this requirement is not met by the semantics, because not 'disallow' action is supported. 5.3. Security Requirements 5.3.1. Authentication, Confidentiality, Integrity The semantics definition support mutual authentication of agent and middlebox and the selection of an encryption method in the Session Establishment transaction (Section 2.2.1). Encryption can be used for achieving confidentiality of messages as well as for ensuring integrity. Thus requirement 2.3.1 is met. 5.3.2. Optional Confidentiality of Control Messages The Session Establishment transaction (Section 2.2.1) allows the agent to suggest an encryption method (including 'no encryption'). Thus requirement 2.3.2 is met. 5.3.3. Operation across Un-trusted Domains Operation across un-trusted domains is supported by mutual authentication and by encryption. Thus requirement 2.3.3 is met. 5.3.4. Mitigate Replay Attacks The specified semantics mitigates replay attacks and meets requirement 2.3.4 by requiring mutual authentication of agent and middlebox, and by supporting message encryption. Further mitigation can be provided as part of a concrete MIDCOM protocol definition, for example by requiring consecutively increasing numbers for request identifiers. 6. Security Considerations The interaction between a middlebox and an agent is (see [MDC-FRM]) a very sensitive point with respect to security. The configuration of policy rules from a middlebox external entity appears very contradictive to the nature of a middlebox. Therefore, effective means have to be used to ensure: - mutual authentication between agent and middlebox Stiemerling & Quittek [Page 48] Internet-Draft MIDCOM Protocol Semantics August 2002 - authorization - message integrity - message confidentiality The semantics define a mechanism to ensure mutual authentication between agent and middlebox (see section 2.2.1). In combination with the authentication, the middlebox is able to decide wether an agent is authorized to request an action at the middlebox or not. The semantics rely on underlying protocols, like TLS or IPSEC, to keep the message integrity and confidentiality of the transfered data between both entities. 7. Acknowledgments We like to thank all the people contributing to the semantics discussion on the mailing list for a lot of valuable comments. 8. Open Issues Here is the list of open issues and to do issues: - The capability information send from the middlebox to the agent at session setup need to be modeled. What further capability items should be sent? - Define behavior for ICMP, IGMP, RSVP, ... - Complete section on security considerations. 9. Acknowledgements We like to thank all the people contributing to the semantics discussion on the mailling list and especially Tom Taylor. 10. References [MDC-FRM] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., Rayhan, A., "Middlebox Communication Architecture and framework", RFC 3303, August 2002 [MDC-REQ] Swale, R.P., Mart, P.A., Sijben, P., Brimm, S., Shore, M., "Middlebox Control (MIDCOM) Protocol Architecture and Requirements", RFC 3304, August 2002 [NAT-TERM] Srisuresh,P., and Holdrege, M., "IP Network Translator (NAT) Terminology and Considerations", RFC 2663, August 1999. Stiemerling & Quittek [Page 49] Internet-Draft MIDCOM Protocol Semantics August 2002 [RFC2246] Dierks, T., Allen, C., "The TLS Protocol Version 1.0", RFC 2246, January 1999. [RFC2402] Kent, S., and Atkinson, R., "IP Authentication Header", RFC 2402, November 1998. [RFC2406] Kent, S., and Atkinson, R., "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998. 11. Authors' Address Martin Stiemerling NEC Europe Ltd. Network Laboratories Adenauerplatz 6 69115 Heidelberg Germany Phone: +49 6221 90511-13 Email: stiemerling@ccrle.nec.de Juergen Quittek NEC Europe Ltd. Network Laboratories Adenauerplatz 6 69115 Heidelberg Germany Phone: +49 6221 90511-15 EMail: quittek@ccrle.nec.de 12. Full Copyright Statement Copyright (C) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be Stiemerling & Quittek [Page 50] Internet-Draft MIDCOM Protocol Semantics August 2002 followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Stiemerling & Quittek [Page 51]