RFC 5189
Middlebox Communication (MIDCOM) Protocol Semantics
2.3. Policy Rule Transactions
This section describes the semantics for transactions on policy rules. The following transactions are specified: - Policy Reserve Rule (PRR) - Policy Enable Rule (PER) - Policy Rule Lifetime Change (RLC) - Policy Rule List (PRL) - Policy Rule Status (PRS) - Asynchronous Policy Rule Event (ARE) The first three transactions (PRR, PER, RLC) are configuration transactions initiated by the agent. The fourth and fifth (PRL, PRS) are monitoring transactions. The last one (ARE) is an asynchronous transaction. The PRL and PRS transactions do not have any effect on the policy rule state machine. Before any transaction can start, a valid MIDCOM session must be established.
2.3.1. Configuration Transactions
Policy rule transactions PER and RLC constitute the core of the MIDCOM protocol. Both are mandatory, and they serve for - configuring NAT bindings (PER) - configuring firewall pinholes (PER) - extending the lifetime of established policy rules (RLC) - deleting policy rules (RLC) Some cases require knowing in advance which IP address (and port number) would be chosen by NAT in a PER transaction. This information is required before sufficient information for performing a complete PER transaction is available (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)2.3.2. Establishing Policy Rules
Both PRR and PER establish a policy rule. The action within the rule is 'reserve' if set by PRR and 'enable' if set by PER. The Policy Reserve Rule (PRR) transaction is used to establish an address reservation on neither side, one side, or both sides of the middlebox, depending on the middlebox configuration. The transaction returns the reserved IP addresses and the optional ranges of port numbers to the agent. No address binding or pinhole configuration is performed at the middlebox. Packet processing at the middlebox remains unchanged. On pure firewalls, the PRR transaction is successfully processed without any reservation, but the state transition of the MIDCOM protocol engine is exactly the same as on NATs. On a traditional NAT (see [NAT-TRAD]), only an external address is reserved; on a twice-NAT, an internal and an external address are reserved. The reservation at a NAT is for required resources, such as IP addresses and port numbers, for future use. How the reservation is exactly done depends on the implementation of the NAT. In both cases, the reservation concerns either an IP address only or a combination of an IP address with a range of port numbers. The Policy Enable Rule (PER) transaction is used to establish a policy rule that affects packet processing at the middlebox. Depending on its input parameters, it may make use of the reservation established by a PRR transaction or create a new rule from scratch.
On a NAT, the enable action is interpreted as a bind action establishing bindings between internal and external addresses. At a firewall, the enable action is interpreted as one or more allow actions configuring pinholes. The number of allow actions depends on the parameters of the request and the implementation of the firewall. On a combined NAT/firewall, the enable action is interpreted as a combination of bind and allow actions. The PRR transaction and the PER transaction are described in more detail in sections 2.3.8 and 2.3.9 below.2.3.3. Maintaining Policy Rules and Policy Rule Groups
Each policy rule has a middlebox-unique identifier. Each policy rule has an owner. Access control to the policy rule is based on ownership (see section 2.1.5). Ownership of a policy rule does not change during lifetime of the policy rule. Each policy rule has an individual lifetime. If the policy rule lifetime expires, the policy rule will be terminated at the middlebox. Typically, the middlebox indicates termination of a policy rule by an ARE transaction. A Policy Rule Lifetime Change (RLC) transaction may extend the lifetime of the policy rule up to the limit specified by the middlebox at session setup. Also, an RLC transaction may be used for shortening a policy rule's lifetime or deleting a policy rule by requesting a lifetime of zero. (Please note that policy rule lifetimes may also be modified by the Group Lifetime Change (GLC) transaction.) Each policy rule is a member of exactly one policy rule group. Group membership does not change during the lifetime of a policy rule. Selecting the group is part of the transaction establishing the policy rule. This transaction implicitly creates a new group if the agent does not specify one. The new group identifier is chosen by the middlebox. New members are added to an existing group if the agent's request designates one. A group only exists as long as it has member policy rules. As soon as all policies belonging to the group have reached the ends of their lifetimes, the group does not exist anymore. Agents can explore the properties and status of all policy rules they are allowed to access by using the Policy Rule Status (PRS) transaction.
2.3.4. Policy Events and Asynchronous Notifications
If a policy rule changes its state or if its remaining lifetime is changed in ways other than being decreased by time, then all agents that can access this policy rule and that participate in an open session with the middlebox are notified by the middlebox. If the state or lifetime change was requested explicitly by a request message, then the middlebox notifies the requesting agent by returning the corresponding reply. All other agents that can access the policy are notified by a Policy Rule Event Notification (REN) message. Note that a middlebox can serve multiple agents at the same time in different parallel sessions. Between these agents, the sets of policy rules that can be accessed by them may overlap. For example, there might be an agent that authenticates as administrator and that can access all policies of all agents. Or there could be a backup agent running a session in parallel to a main agent and authenticating itself as the same entity as the main agent. In case of a PER, PRR, or RLC transaction, the requesting agent receives a PER, PRR, or RLC reply, respectively. To all other agents that can access the created, modified, or terminated policy rule (and that participate in an open session with the middlebox), the middlebox sends a REN message carrying the policy rule identifier (PID) and the remaining lifetime of the policy rule. In case of a rule termination by lifetime truncation or other events not triggered by an agent, the middlebox sends a REN message to each agent that can access the particular policy rule and that participates in an open session with the middlebox. This ensures that an agent always knows the most recent state of all policy rules it can access.2.3.5. Address Tuples
Request and reply messages of the PRR, PER, and PRS transactions contain address specifications for IP and transport addresses. These parameters include - IP version - IP address - IP address prefix length - transport protocol - port number - port parity - port range
Additionally, the request message of PER and the reply message of PRS
contain a direction of flow parameter. This direction of flow
parameter indicates for UDP and IP the direction of packets
traversing the middlebox. For 'inbound', the UDP packets are
traversing from outside to inside; for 'outbound', from inside to
outside. In both cases, the packets can traverse the middlebox only
unidirectionally. A bidirectional flow is enabled through
'bidirectional' as direction of flow parameter. For TCP, the packet
flow is always bidirectional, but the direction of the flow parameter
is defined as
- inbound: bidirectional TCP packet flow. First packet, with TCP
SYN flag set and ACK flag not set, must arrive at the middlebox
at the outside interface.
- outbound: bidirectional TCP packet flow. First packet, with TCP
SYN flag set and ACK flag not set, must arrive at the middlebox
at the inside interface.
- bidirectional: bidirectional TCP packet flow. First packet,
with TCP SYN flag set and ACK flag not set, may arrive at inside
or outside interface.
We refer to the set of these parameters as an address tuple. An
address tuple specifies either a communication endpoint at an
internal or external device or allocated addresses at the middlebox.
In this document, we distinguish four kinds of address tuples, as
shown in Figure 3.
+----------+ +----------+
| internal | A0 A1 +-----------+ A2 A3 | external |
| endpoint +----------+ middlebox +----------+ endpoint |
+----------+ +-----------+ +----------+
Figure 3: Address Tuples A0 - A3
- A0 - internal endpoint: Address tuple A0 specifies a
communication endpoint of a device within the internal network,
with respect to the middlebox.
- A1 - middlebox inside address: Address tuple A1 specifies a
virtual communication endpoint at the middlebox within the
internal network. A1 is the destination address for packets
passing from the internal endpoint to the middlebox and is the
source for packets passing from the middlebox to the internal
endpoint.
- A2 - middlebox outside address: Address tuple A2 specifies a
virtual communication endpoint at the middlebox within the
external network. A2 is the destination address for packets
passing from the external endpoint to the middlebox and is the
source for packets passing from the middlebox to the external
endpoint.
- A3 - external endpoint: Address tuple A3 specifies a
communication endpoint of a device within the external network,
with respect to the middlebox.
For a firewall, the inside and outside endpoints are identical to the
corresponding external or internal endpoints, respectively. In this
case, the installed policy rule sets the same value in A2 as in A0
(A0=A2) and sets the same value in A1 as in A3 (A1=A3).
For a traditional NAT, A2 is given a value different from that of A0,
but the NAT binds them. As for the firewall, it is also as it is at
a traditional NAT: A1 has the same value as A3.
For a twice-NAT, there are two bindings of address tuples: A1 and A2
are both assigned values by the NAT. The middlebox outside address
A2 is bound to the internal endpoint A0, and the middlebox inside
address A1 is bound to the external endpoint A3.
2.3.6. Address Parameter Constraints
For transaction parameters belonging to an address tuple, some
constraints exist that are common for all messages using them.
Therefore, these constraints are summarized in the following and are
not repeated again when describing the parameters in the transaction
descriptions are presented.
The MIDCOM semantics defined in this document specifies the handling
of IPv4 and IPv6 as network protocols, and of TCP and UDP (over IPv4
and IPv6) as transport protocols. The handling of any other
transport protocol, e.g., Stream Control Transmission Protocol
(SCTP), is not defined within the semantics but may be supported by
concrete protocol specifications.
The IP version parameter has either the value 'IPv4' or 'IPv6'. In a
policy rule, the value of the IP version parameter must be the same
for address tuples A0 and A1, and for A2 and A3.
The value of the IP address parameter must conform with the specified
IP version.
The IP address of an address tuple may be wildcarded. Whether IP address wildcarding is allowed or in which range it is allowed depends on the local policy of the middlebox; see also section 6, "Security Considerations". Wildcarding is specified by the IP address prefix length parameter of an address tuple. In line with the common use of a prefix length, this parameter indicates the number of high significant bits of the IP address that are fixed, while the remaining low significant bits of the IP address are wildcarded. The value of the transport protocol parameter can be either 'TCP', 'UDP', or 'ANY'. If the transport protocol parameter has the value 'ANY', only IP headers are considered for packet handling in the middlebox -- i.e., the transport header is not considered. The values of the parameters port number, port range, and port parity are irrelevant if the protocol parameter is 'ANY'. In a policy rule, the value of the transport protocol parameter must be the same for all address tuples A0, A1, A2, and A3. The value of the port number parameter is either zero or a positive integer. A positive integer specifies a concrete UDP or TCP port number. The value zero specifies port wildcarding for the protocol specified by the transport protocol parameter. If the port number parameter has the value zero, then the value of the port range parameter is irrelevant. Depending on the value of the transport protocol parameter, this parameter may truly refer to ports or may refer to an equivalent concept. The port parity parameter is differently used in the context of Policy Reserve Rules (PRRs) and Policy Enable Rules (PERs). In the context of a PRR, the value of the parameter may be 'odd', 'even', or 'any'. It specifies the parity of the first (lowest) reserved port number. In the context of a PER, the port parity parameter indicates to the middlebox whether port numbers allocated at the middlebox should have the same parity as the corresponding internal or external port numbers, respectively. In this context, the parameter has the value 'same' or 'any'. If the value is 'same', then the parity of the port number of A0 must be the same as the parity of the port number of A2, and the parity of the port number of A1 must be the same as the parity of the port number of A3. If the port parity parameter has the value 'any', then there are no constraints on the parity of any port number. The port range parameter specifies a number of consecutive port numbers. Its value is a positive integer. Like the port number parameter, this parameter defines a set of consecutive port numbers
starting with the port number specified by the port number parameter as the lowest port number and having as many elements as specified by the port range parameter. A value of 1 specifies a single port number. The port range parameter must have the same value for each address tuple A0, A1, A2, and A3. A single policy rule P containing a port range value greater than one is equivalent to a set of policy rules containing a number n of policies P_1, P_2, ..., P_n where n equals the value of the port range parameter. Each policy rule P_1, P_2, ..., P_n has a port range parameter value of 1. Policy rule P_1 contains a set of address tuples A0_1, A1_1, A2_1, and A3_1, each of which contains the first port number of the respective address tuples in P; policy rule P_2 contains a set of address tuples A0_2, A1_2, A2_2, and A3_2, each of which contains the second port number of the respective address tuples in P; and so on.2.3.7. Interface-Specific Policy Rules
Usually, agents request policy rules with the knowledge of A0 and A3 only, i.e., the address tuples (see section 2.3.5). But in very special cases, agents may need to select the interfaces to which the requested policy rule is bound. Generally, the middlebox is careful about choosing the right interfaces when reserving or enabling a policy rule, as it has the overall knowledge about its configuration. For agents that want to select the interfaces, optional parameters are included in the Policy Reserve Rule (PRR) and Policy Enable Rule (PER) transactions. These parameters are called - inside interface: The selected interface at the inside of the middlebox -- i.e., in the private or protected address realm. - outside interface: The selected interface at the outside of the middlebox -- i.e., in the public address realm. The Policy Rule Status (PRS) transactions include these optional parameters in their replies when they are supported. Agents can learn at session startup whether interface-specific policy rules are supported by the middlebox, by checking the middlebox capabilities (see section 2.1.6).2.3.8. Policy Reserve Rule (PRR)
transaction-name: policy reserve rule transaction-type: configuration
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 policy
reserve rule should be a member. As indicated in section 2.3.3,
if this value is not supplied, the middlebox assigns a new group
for this policy reserve rule.
- service: The requested NAT service of the middlebox. Allowed
values are 'traditional' or 'twice'.
- internal IP version: Requested IP version at the inside of the
middlebox; see section 2.3.5.
- internal IP address: The IP address of the internal
communication endpoint (A0 in Figure 3); see section 2.3.5.
- internal port number: The port number of the internal
communication endpoint (A0 in Figure 3); see section 2.3.5.
- inside interface (optional): Interface at the inside of the
middlebox; see section 2.3.7.
- external IP version: Requested IP version at the outside of the
middlebox; see section 2.3.5.
- outside interface (optional): Interface at the outside of the
middlebox; see section 2.3.7.
- transport protocol: See section 2.3.5.
- port range: The number of consecutive port numbers to be
reserved; see section 2.3.5.
- port parity: The requested parity of the first (lowest) port
number to be reserved; allowed values for this parameter are
'odd', 'even', and 'any'. See also section 2.3.5.
- policy rule lifetime: A lifetime proposal to the middlebox for
the requested policy rule.
reply-parameters (success):
- request identifier: An identifier matching the identifier of the
request.
- policy rule identifier: A middlebox-unique policy rule
identifier. It is assigned by the middlebox and used as policy
rule handle in further policy rule transactions, particularly to
refer to the policy reserve rule in a subsequent PER
transaction.
- group identifier: A reference to the group of which the policy
reserve rule is a member.
- reserved inside IP address: The reserved IPv4 or IPv6 address on
the internal side of the middlebox. For an outbound flow, this
will be the destination to which the internal endpoint sends its
packets (A1 in Figure 3). For an inbound flow, it will be the
apparent source address of the packets as forwarded to the
internal endpoint (A0 in Figure 3). The middlebox reserves and
reports an internal address only in the case where twice-NAT is
in effect. Otherwise, an empty value for the addresses
indicates that no internal reservation was made. See also
section 2.3.5.
- reserved inside port number: See section 2.3.5.
- reserved outside IP address: The reserved IPv4 or IPv6 address
on the external side of the middlebox. For an inbound flow,
this will be the destination to which the external endpoint
sends its packets (A2 in Figure 3). For an outbound flow, it
will be the apparent source address of the packets as forwarded
to the external endpoint (A3 in Figure 3). If the middlebox is
configured as a pure firewall, an empty value for the addresses
indicates that no external reservation was made. See also
section 2.3.5.
- reserved outside port number: See section 2.3.5.
- policy rule lifetime: The policy rule lifetime granted by the
middlebox, after which the reservation will be revoked if it has
not been replaced already by a policy enable rule in a PER
transaction.
failure reason:
- agent not authorized for this transaction
- agent not authorized to add members to this group
- lack of IP addresses
- lack of port numbers
- lack of resources
- specified inside/outside interface does not exist
- specified inside/outside interface not available for specified
service
notification message type: Policy Rule Event Notification (REN)
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 neither side, one side, or both sides of the
middlebox as required to support the enabling of a flow.
Typically, the PRR will be used in scenarios where it is required
to perform such a reservation before sufficient parameters for a
complete policy enable rule transaction are available. See
section 4.2 for an example.
When receiving the request, the middlebox determines how many
address (and port) reservations are required based on its
configuration. If it provides only packet filter services, it
does not perform any reservation and returns empty values for the
reserved inside and outside IP addresses and port numbers. If it
is configured for twice-NAT, it reserves both inside and outside
IP addresses (and an optional range of port numbers) and returns
them. Otherwise, it reserves and returns an outside IP address
(and an optional range of port numbers) and returns empty values
for the reserved inside address and port range.
The A0 parameter (inside IP address version, inside IP address,
and inside port number) can be used by the middlebox to determine
the correct NAT mapping and thus A2 if necessary. Once a PRR
transaction has reserved an outside address (A2) for an internal
endpoint (A0) at the middlebox, the middlebox must ensure that
this reserved A2 is available in any subsequent PER and PRR
transactions.
For middleboxes supporting interface-specific policy rules, as
defined in section 2.3.7, the optional inside and outside
interface parameters must both be included in the request, or
neither of them should be included. In the presence of these
parameters, the middlebox uses the outside interface parameter to
select the interface at which the outside address tuple (outside
IP address and port number) is reserved, and the inside interface
parameter to select the interface at which the inside address
tuple (inside IP address and port number) is reserved. Without
the presence of these parameters, the middlebox selects the
particular interfaces based on its internal configuration.
If there is a lack of resources, such as available IP addresses,
port numbers, or storage for further policy rules, then the
reservation fails, and an appropriate failure reply is generated.
If a non-existing policy rule group was specified, or if an
existing policy rule group was specified that is not owned by the
requesting agent, then no new policy rule is established, and an
appropriate failure reply is generated.
In case of success, this transaction creates a new policy reserve
rule. If an already existing policy rule group is specified, then
the new policy rule becomes a member of it. If no policy group is
specified, a new group is created with the new policy rule as its
only member. The middlebox generates a middlebox-unique
identifier for the new policy rule. The owner of the new policy
rule is the authenticated agent that sent the request. The
middlebox chooses a lifetime value that is greater than zero and
less than or equal to the minimum of the requested value and the
maximum lifetime specified by the middlebox at session startup,
i.e.,
0 <= lt_granted <= MINIMUM(lt_requested, lt_maximum)
where
- lt_granted is the lifetime actually granted by the middlebox
- lt_requested is the lifetime the agent requested
- lt_maximum is the maximum lifetime specified at session
setup
A middlebox with NAT capability always reserves a middlebox
external address tuple (A2) in response to a PRR request. In the
special case of a combined twice-NAT/NAT middlebox, the agent can
request only NAT service or twice-NAT service by choosing the
service parameter 'traditional' or 'twice'. An agent that does
not have any preference chooses 'twice'. The 'traditional' value
should only be used to select traditional NAT service at
middleboxes offering both traditional NAT and twice-NAT. In the
'twice' case, the combined twice-NAT/NAT middlebox reserves A2 and
A1; the 'traditional' case results in a reservation of A2 only.
An agent must always use the PRR transaction for choosing NAT only
or twice-NAT service in the special case of a combined twice-
NAT/NAT middlebox. A firewall middlebox ignores this parameter.
If the protocol identifier is 'ANY', then the middlebox reserves
available inside and/or outside IP address(es) only. The reserved
address(es) are returned to the agent. In this case, the
request-parameters "port range" and "port parity" as well as the
reply-parameters "inside port number" and "outside 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 parity, is reserved, on neither side,
one side, or both sides of the middlebox, as appropriate. The IP
address(es) and the first (lowest) reserved port number(s) of the
consecutive sequence are returned to the agent. (This also
applies to other protocols supporting ports or the equivalent.)
After a new policy reserve rule is successfully established and
the reply message has been sent to the requesting agent, the
middlebox checks whether there are other authenticated agents
participating in open sessions, which can access the new policy
rule. If the middlebox finds one or more of these agents, then it
sends a REN message reporting the new policy rule to each of them.
MIDCOM agents use the policy enable rule (PER) transaction to enable
policy reserve rules that have been established beforehand by a
policy reserve rule (PRR) transaction. See also section 2.3.2.
2.3.9. Policy Enable Rule (PER)
transaction-name: policy enable rule
transaction-type: configuration
transaction-compliance: mandatory
request-parameters:
- request identifier: An agent-unique identifier for matching
corresponding request and reply at the agent.
- policy reserve rule identifier: A reference to an already
existing policy reserve rule created by a PRR transaction. The
reference may be empty, in which case the middlebox must assign
any necessary addresses and port numbers within this PER
transaction. If it is not empty, then the following request
parameters are irrelevant: group identifier, transport protocol,
port range, port parity, internal IP version, external IP
version.
- group identifier: A reference to the group of which the policy
enable rule should be a member. As indicated in section 2.3.3,
if this value is not supplied, the middlebox assigns a new group
for this policy reserve rule.
- transport protocol: See section 2.3.5.
- port range: The number of consecutive port numbers to be
reserved; see section 2.3.5.
- port parity: The requested parity of the port number(s) to be
mapped. Allowed values of this parameter are 'same' and 'any'.
See also section 2.3.5.
- direction of flow: This parameter specifies the direction of
enabled communication, either 'inbound', 'outbound', or
'bidirectional'.
- internal IP version: Requested IP version at the inside of the
middlebox; see section 2.3.5.
- internal IP address: The IP address of the internal
communication endpoint (A0 in Figure 3); see section 2.3.5.
- internal port number: The port number of the internal
communication endpoint (A0 in Figure 3); see section 2.3.5.
- inside interface (optional): Interface at the inside of the
middlebox; see section 2.3.7.
- external IP version: Requested IP version at the outside of the
middlebox; see section 2.3.5.
- external IP address: The IP address of the external
communication endpoint (A3 in Figure 3); see section 2.3.5.
- external port number: The port number of the external
communication endpoint (A3 in Figure 3), see section 2.3.5.
- outside interface (optional): Interface at the outside of the
middlebox; see section 2.3.7.
- policy rule lifetime: A lifetime proposal to the middlebox for
the requested policy rule.
reply-parameters (success):
- request identifier: An identifier matching the identifier of the
request.
- policy rule identifier: A middlebox-unique policy rule
identifier. It is assigned by the middlebox and used as policy
rule handle in further policy rule transactions. If a policy
reserve rule identifier was provided in the request, then the
returned policy rule identifier has the same value.
- group identifier: A reference to the group of which the policy
enable rule is a member. If a policy reserve rule identifier
was provided in the request, then this parameter identifies the
group of which the policy reserve rule was a member.
- inside IP address: The IP address provided at the inside of the
middlebox (A1 in Figure 3). In case of a twice-NAT, this
parameter will be an internal IP address reserved at the inside
of the middlebox. In all other cases, this reply-parameter will
be identical with the external IP address passed with the
request. If the policy reserve rule identifier parameter was
supplied in the request and the respective PRR transaction
reserved an inside IP address, then the inside IP address
provided in the PER response will be the identical value to that
returned by the response to the PRR request. See also section
2.3.5.
- inside port number: The internal port number provided at the
inside of the middlebox (A1 in Figure 3); see also section
2.3.5.
- outside IP address: The external IP address provided at the
outside of the middlebox (A2 in Figure 3). In case of a pure
firewall, this parameter will be identical with the internal IP
address passed with the request. In all other cases, this
reply-parameter will be an external IP address reserved at the
outside of the middlebox. See also section 2.3.5.
- outside port number: The external port number provided at the
outside of the NAT (A2 in Figure 3); see section 2.3.5..
- policy rule lifetime: The policy rule lifetime granted by the
middlebox.
failure reason:
- agent not authorized for this transaction
- agent not authorized to add members to this group
- no such policy reserve rule
- agent not authorized to replace this policy reserve rule
- conflict with already existing policy rule (e.g., the same
internal address-port is being mapped to different outside
address-port pairs)
- lack of IP addresses
- lack of port numbers
- lack of resources
- no internal IP wildcarding allowed
- no external IP wildcarding allowed
- specified inside/outside interface does not exist
- specified inside/outside interface not available for specified
service
- reserved A0 to requested A0 mismatch
notification message type: Policy Rule Event Notification (REN)
semantics:
This transaction can be used by an agent to enable communication
between an internal endpoint and an external endpoint
independently of the type of middlebox (NAT, NAPT, firewall, NAT-
PT, combined devices), for unidirectional or bidirectional
traffic.
The agent sends an enable request specifying the endpoints
(optionally including wildcards) and the direction of
communication (inbound, outbound, bidirectional). The
communication endpoints are displayed in Figure 3. The basic
operation of the PER transaction can be described by
1. the agent sending A0 and A3 to the middlebox,
2. the middlebox reserving A1 and A2 or using A1 and A2 from a
previous PRR transaction,
3. the middlebox enabling packet transfer between A0 and A3 by
binding A0-A2 and A1-A3 and/or by opening the corresponding
pinholes, both according to the specified direction, and
4. the middlebox returning A1 and A2 to the agent.
In case of a pure packet filtering firewall, the returned address
tuples are the same as those in the request: A2=A0 and A1=A3.
Each partner uses the other'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
address tuple provided by the NAT (A2!=A0). In case of a twice-
NAT device, both endpoints use 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, the policy rule may
imply more than one enable action for the firewall configuration,
as incoming and outgoing packets may use different source-
destination pairs.
For middleboxes supporting interface-specific policy rules, as
defined in section 2.3.7, the optional inside and outside
interface parameters must both be included in the request, or
neither of them should be included. In the presence of these
parameters, the middlebox uses the outside interface parameter to
select the interface at which the outside address tuple (outside
IP address and port number) is bound, and the inside interface
parameter to select the interface at which the inside address
tuple (inside IP address and port number) is bound. Without the
presence of these parameters, the middlebox selects the particular
interfaces based on its internal configuration.
Checking the Policy Reservation Rule Identifier
If the parameter specifying the policy reservation rule
identifier is not empty, then the middlebox checks whether the
referenced policy rule exists, whether the agent is authorized
to replace this policy rule, and whether this policy rule is a
policy reserve rule.
In case of success, this transaction creates a new policy
enable rule. If a policy reserve rule was referenced, then the
policy reserve rule is terminated without an explicit
notification sent to the agent (other than the successful PER
reply).
The PRR transaction sets the internal endpoint A0 during the
reservation process. In the process of creating a new policy
enable rule, the middlebox may check whether the requested A0
is equal to the reserved A0. The middlebox may reject a PER
request with a requested A0 not equal to the reserved A0 and
must then send an appropriate failure message. Alternatively,
the middlebox may change A0 due to the PER request.
The middlebox generates a middlebox-unique identifier for the
new policy rule. If a policy reserve rule was referenced, then
the identifier of the policy reserve rule is reused.
The owner of the new policy rule is the authenticated agent
that sent the request.
Checking the Policy Rule Group Identifier
If no policy reserve rule was specified, then the policy rule
group parameter is checked. If a non-existing policy rule
group is specified, or if an existing policy rule group is
specified that is not owned by the requesting agent, then no
new policy rule is established, and an appropriate failure
reply is generated.
If an already existing policy rule group is specified, then the
new policy rule becomes a member. If no policy group is
specified, then a new group is created with the new policy rule
as its only member.
If the transport protocol parameter value is 'ANY', then the
middlebox enables communication between the specified external IP
address and the specified internal IP address. The addresses to
be used by the communication partners 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 transport protocol type, then the reserved IP
addresses are used.
For the transport protocol parameter values 'UDP' and 'TCP', the
middlebox acts analogously as for 'ANY' but also maps ranges of
port numbers, keeping the port parity, if requested.
The configuration of the middlebox may fail because of lack of
resources, such as available IP addresses, port numbers, or
storage for further policy rules. It may also fail because of a
conflict with an established policy rule. In case of a conflict,
the first-come first-served mechanism is applied. Existing policy
rules remain unchanged and arriving new ones are rejected.
However, in case of a non-conflicting overlap of policy rules
(including identical policy rules), all policy rules are accepted.
The middlebox chooses a lifetime value that is greater than zero
and less than or equal to the minimum of the requested value and
the maximum lifetime specified by the middlebox at session
startup, i.e.,
0 <= lt_granted <= MINIMUM(lt_requested, lt_maximum)
where
- lt_granted is the lifetime actually granted by the middlebox
- lt_requested is the lifetime the agent requested
- lt_maximum is the maximum lifetime specified at session setup
In each case of failure, an appropriate failure reply is generated.
The policy reserve rule that is referenced in the PER transaction is
not affected in case of a failure within the PER transaction -- i.e.,
the policy reserve rule remains.
After a new policy enable rule is successfully established and the
reply message has been sent to the requesting agent, the middlebox
checks whether there are other authenticated agents participating in
open sessions that can access the new policy rule. If the middlebox
finds one or more of these agents, then it sends a REN message
reporting the new policy rule to each of them.
2.3.10. Policy Rule Lifetime Change (RLC)
transaction-name: policy rule lifetime change
transaction-type: configuration
transaction-compliance: mandatory
request-parameters:
- request identifier: An agent-unique identifier for matching
corresponding request and reply at the agent.
- policy rule identifier: Identifying the policy rule for which
the lifetime is requested to be changed. This may identify
either a policy reserve rule or a policy enable rule.
- policy rule lifetime: The new lifetime proposal for the policy
rule.
reply-parameters (success):
- request identifier: An identifier matching the identifier of the
request.
- policy rule lifetime: The remaining policy rule lifetime granted
by the middlebox.
failure reason:
- agent not authorized for this transaction
- agent not authorized to change lifetime of this policy rule
- no such policy rule
- lifetime cannot be extended
notification message type: Policy Rule Event Notification (REN)
semantics:
The agent can use this transaction type to request the extension
of an established policy rule's lifetime, the shortening of the
lifetime, or policy rule termination. Policy rule termination is
requested by suggesting a new policy rule lifetime of zero.
The middlebox first checks whether the specified policy rule
exists and whether the agent is authorized to access this policy
rule. 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 the lifetime of the policy
rule may be extended and generates an appropriate failure message
if it may not.
A failure reply implies that the new lifetime was not accepted,
and the policy rule remains unchanged. A success reply is
generated by the middlebox if the lifetime of the policy rule was
changed in any way.
The success reply contains the new lifetime of the policy rule.
The middlebox chooses a lifetime value that is greater than zero
and less than or equal to the minimum of the requested value and
the maximum lifetime specified by the middlebox at session
startup, i.e.,
0 <= lt_granted <= MINIMUM(lt_requested, lt_maximum)
where
- lt_granted is the lifetime actually granted by the middlebox
- lt_requested is the lifetime the agent requested
- lt_maximum is the maximum lifetime specified at session setup
After sending a success reply with a lifetime of zero, the middlebox
will consider the policy rule non-existent. Any further transaction
on this policy rule results in a negative reply, indicating that this
policy rule does not exist anymore.
Note that policy rule lifetime may also be changed by the Group Lifetime Change (GLC) transaction, if applied to the group of which the policy rule is a member. After the remaining policy rule lifetime was successfully changed and the reply message has been sent to the requesting agent, the middlebox checks whether there are other authenticated agents participating in open sessions that can access the policy rule. If the middlebox finds one or more of these agents, then it sends a REN message reporting the new remaining policy rule lifetime to each of them.2.3.11. Policy Rule List (PRL)
transaction-name: policy rule list transaction-type: monitoring transaction-compliance: mandatory request-parameters: - request identifier: An agent-unique identifier for matching corresponding request and reply at the agent. reply-parameters (success): - request identifier: An identifier matching the identifier of the request. - policy list: List of policy rule identifiers of all policy rules that the agent can access. failure reason: - transaction not supported - agent not authorized for this transaction
semantics:
The agent can use this transaction type to list all policies that
it can access. Usually, the agent has this information already,
but in special cases (for example, after an agent fail-over) or
for special agents (for example, an administrating agent that can
access all policies) this transaction can be helpful.
The middlebox first checks whether the agent is authorized to
request this transaction. If the check fails, an appropriate
failure reply is generated. Otherwise, a list of all policies the
agent can access is returned indicating the identifier and the
owner of each policy.
This transaction does not have any effect on the policy rule
state.
2.3.12. Policy Rule Status (PRS)
transaction-name: policy rule status
transaction-type: monitoring
transaction-compliance: mandatory
request-parameters:
- request identifier: An agent-unique identifier for matching
corresponding request and reply at the agent.
- policy rule identifier: The middlebox-unique policy rule
identifier.
reply-parameters (success):
- request identifier: An identifier matching the identifier of the
request.
- policy rule owner: An identifier of the agent owning this policy
rule.
- group identifier: A reference to the group of which the policy
rule is a member.
- policy rule action: This parameter has either the value
'reserve' or the value 'enable'.
- transport protocol: Identifies the protocol for which a
reservation is requested; see section 2.3.5.
- port range: The number of consecutive port numbers; see section
2.3.5.
- direction: The direction of the communication enabled by the
middlebox. Applicable only to policy enable rules.
- internal IP address version: The version of the internal IP
address (IP version of A0 in Figure 3).
- external IP address version: The version of the external IP
address (IP version of A3 in Figure 3).
- internal IP address: The IP address of the internal
communication endpoint (A0 in Figure 3); see section 2.3.5.
- internal port number: The port number of the internal
communication endpoint (A0 in Figure 3); see section 2.3.5.
- external IP address: The IP address of the external
communication endpoint (A3 in Figure 3); see section 2.3.5.
- external port number: The port number of the external
communication endpoint (A3 in Figure 3); see section 2.3.5.
- inside interface (optional): The inside interface at the
middlebox; see section 2.3.7.
- inside IP address: The internal IP address provided at the
inside of the NAT (A1 in Figure 3); see section 2.3.5.
- inside port number: The internal port number provided at the
inside of the NAT (A1 in Figure 3); see section 2.3.5.
- outside interface (optional): The outside interface at the
middlebox; see section 2.3.7.
- outside IP address: The external IP address provided at the
outside of the NAT (A2 in Figure 3); see section 2.3.5.
- outside port number: The external port number provided at the
outside of the NAT (A2 in Figure 3); see section 2.3.5.
- port parity: The parity of the allocated ports.
- service: The selected service in the case of mixed traditional
and twice-NAT middlebox (see section 2.3.8).
- policy rule lifetime: The remaining lifetime of the policy rule.
failure reason:
- transaction not supported
- agent not authorized for this transaction
- no such policy rule
- agent not authorized to access this policy rule
semantics:
The agent can use this transaction type to list all properties of
a policy rule. Usually, the agent has this information already,
but in special cases (for example, after an agent fail-over) or
for special agents (for example, an administrating agent that can
access all policy rules) this transaction can be helpful.
The middlebox first checks whether the specified policy rule
exists and whether 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 rule are
returned to the agent. Some of the returned parameters may be
irrelevant, depending on the policy rule action ('reserve' or
'enable') and depending on other parameters -- for example, the
protocol identifier.
This transaction does not have any effect on the policy rule
state.
2.3.13. Asynchronous Policy Rule Event (ARE)
transaction-name: asynchronous policy rule event
transaction-type: asynchronous
transaction-compliance: mandatory
notification message type: Policy Rule Event Notification (REN)
semantics:
The middlebox may decide at any point in time to terminate a
policy rule. This transaction is triggered most frequently by
lifetime expiration of the policy rule. Among other events that
may cause this transaction are changes in the policy rule decision
point.
The middlebox sends a REN message to all agents that participate
in an open session with the middlebox and that are authorized to
access the policy rule. The notification is sent to the agents
before the middlebox changes the policy rule's lifetime. The
change of lifetime may be triggered by any other authorized agent
and results in shortening (lt_new < lt_existing), extending
(lt_new > lt_existing), or terminating the policy rule
(lt_new = 0).
The ARE transaction corresponds to the REN message handling described
in section 2.3.4 for multiple agents.
2.3.14. Policy Rule State Machine
The state machine for the policy rule transactions is shown in Figure
4 with all possible state transitions. The used transaction
abbreviations may be found in the headings of the particular
transaction section.
PRR/success +---------------+
+-----------------+ PRID UNUSED |<-+
+----+ | +---------------+ |
| | | ^ | |
| v v | | |
| +-------------+ ARE | | PER/ | ARE
| | RESERVED +------------+ | success | RLC(lt=0)/
| +-+----+------+ RLC(lt=0)/ | | success
| | | success | |
+----+ | v |
RLC(lt>0)/ | PER/success +---------------+ |
success +---------------->| ENABLED +--+
+-+-------------+
| ^
lt = lifetime +-----------+
RLC(lt>0)/success
Figure 4: Policy Rule State Machine
This state machine exists per policy rule identifier (PRID).
Initially, all policy rules are in state PRID UNUSED, which means
that the policy rule does not exist or is not active. After
returning to state PRID UNUSED, the policy rule identifier is no
longer bound to an existing policy rule and may be reused by the
middlebox.
A successful PRR transaction causes a transition from the initial state PRID UNUSED to the state RESERVED, where an address reservation is established. From there, state ENABLED can be entered by a PER transaction. This transaction can also be used for entering state ENABLED directly from state PRID UNUSED without a reservation. In state ENABLED, the requested communication between the internal and the external endpoint is enabled. The states RESERVED and ENABLED can be maintained by successful RLC transactions with a requested lifetime greater than 0. Transitions from both of these states back to state PRID UNUSED can be caused by an ARE transaction or by a successful RLC transaction with a lifetime parameter of 0. A failed request transaction does not change state at the middlebox. Note that transitions initiated by RLC transactions may also be initiated by GLC transactions.2.4. Policy Rule Group Transactions
This section describes the semantics for transactions on groups of policy rules. These transactions are specified as follows: - Group Lifetime Change (GLC) - Group List (GL) - Group Status (GS) All are request transactions initiated by the agent. GLC is a configuration transaction. GL and GS are monitoring transactions that do not have any effect on the group state machine.2.4.1. Overview
A policy rule group has only one attribute: the list of its members. All member policies of a single group must be owned by the same authenticated agent. Therefore, an implicit property of a group is its owner, which is the owner of the member policy rules. A group is implicitly created when its first member policy rule is established. A group is implicitly terminated when the last remaining member policy rule is terminated. Consequently, the lifetime of a group is the maximum of the lifetimes of all member policy rules. A group has a middlebox-unique identifier.
Policy rule group transactions are declared as 'optional' by their respective compliance entry in section 3. However, they provide some functionalities, such as convenience for the agent in sending only one request instead of several, that is not available if only mandatory transactions are available. The Group Lifetime Change (GLC) transaction is equivalent to simultaneously performed Policy Rule Lifetime Change (RLC) transactions on all members of the group. The result of a successful GLC transaction is that all member policy rules have the same lifetime. As with the RLC transaction, the GLC transaction can be used to delete all member policy rules by requesting a lifetime of zero. The monitoring transactions Group List (GL) and Group Status (GS) can be used by the agent to explore the state of the middlebox and to explore 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 on an individual group and lists all policy rules of this group by their policy rule identifiers. The agent can explore the state of the individual policy rules by using the policy rule identifiers in a policy rule status (PRS) transaction (see section 2.3.12). The GL and GS transactions are particularly helpful in case of an agent fail-over. The agent taking over the role of a failed one can use these transactions to retrieve whichever policies have been established by the failed agent. Notifications on group events are generated analogously to policy rule events. To notify agents about group events, the Policy Rule Group Event Notification (GEN) message type is used. GEN messages contain an agent-unique notification identifier, the policy rule group identifier, and the remaining lifetime of the group.2.4.2. Group Lifetime Change (GLC)
transaction-name: group lifetime change transaction-type: configuration 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 of the
request.
- group lifetime: The group lifetime granted by the middlebox.
failure reason:
- transaction not supported
- agent not authorized for this transaction
- agent not authorized to change lifetime of this group
- no such group
- lifetime cannot be extended
notification message type: Policy Rule Group Event Notification (GEN)
semantics:
The agent can use this transaction type to request an extension of
the lifetime of all members of a policy rule group, to request
shortening the lifetime of all members, or to request termination
of all member policies (which implies termination of the group).
Termination is requested by suggesting a new group lifetime of
zero.
The middlebox first checks whether the specified group exists and
whether the agent is authorized to access this group. 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 the lifetime of the group may be
extended and generates an appropriate failure message if it may
not.
A failure reply 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 common lifetime of all member
policy rules of the group. The middlebox chooses the new lifetime
less than or equal to the minimum of the requested lifetime and
the maximum lifetime that the middlebox specified at session setup
along with its other capabilities, i.e.,
0 <= lt_granted <= MINIMUM(lt_requested, lt_maximum)
where
- lt_granted is the lifetime actually granted by the middlebox
- lt_requested is the lifetime the agent requested
- lt_maximum is the maximum lifetime specified at session setup
After sending a success reply with a lifetime of zero, the middlebox
will terminate the member policy rules without any further
notification to the agent, and will consider the group and all of its
members non-existent. Any further transaction on this policy rule
group or on any of its members results in a negative reply,
indicating that this group or policy rule, respectively, does not
exist anymore.
After the remaining policy rule group lifetime is successfully
changed and the reply message has been sent to the requesting agent,
the middlebox checks whether there are other authenticated agents
participating in open sessions that can access the policy rule group.
If the middlebox finds one or more of these agents, it sends a GEN
message reporting the new remaining policy rule group lifetime to
each of them.
2.4.3. Group List (GL)
transaction-name: group list
transaction-type: monitoring
transaction-compliance: optional
request-parameters:
- request identifier: An agent-unique identifier for matching
corresponding request and reply at the agent.
reply-parameters (success):
- request identifier: An identifier matching the identifier of the
request.
- group list: List of all groups that the agent can access. For
each listed group, the identifier and the owner are indicated.
failure reason:
- transaction not supported
- agent not authorized for this transaction
semantics:
The agent can use this transaction type to list all groups that it
can access. Usually, the agent has this information already, but
in special cases (for example, after an agent fail-over) or for
special agents (for example, an administrating agent that can
access all groups) this transaction can be helpful.
The middlebox first checks whether the agent is authorized to
request this transaction. If the check 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 of each group.
This transaction does not have any effect on the group state.
2.4.4. Group Status (GS)
transaction-name: group status
transaction-type: monitoring
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.
reply-parameters (success):
- request identifier: An identifier matching the identifier of the
request.
- group owner: An identifier of the agent owning this policy rule
group.
- group lifetime: The remaining lifetime of the group. This is
the maximum of the remaining lifetimes of all members' policy
rules.
- member list: List of all policy rules that are members of the
group. The policy rules are specified by their middlebox-unique
policy rule identifier.
failure reason:
- transaction not supported
- agent not authorized for this transaction
- no such group
- agent not authorized to list members of this group
semantics:
The agent can use this transaction type to list all member policy
rules of a group. Usually, the agent has this information
already, but in special cases (for example, after an agent fail-
over) or for special agents (for example, an administrating agent
that can access all groups) this transaction can be helpful.
The middlebox first checks whether the specified group exists and
whether 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.
This transaction does not have any effect on the group state.
(page 48 continued on part 3)
