NAT Working Group P. Srisuresh
INTERNET-DRAFT Campio Communications
Category: Informational July, 2000
Expires on January 12, 2001
Framework for interfacing with Network Address Translator
<draft-ietf-nat-interface-framework-01.txt>
Status of this Memo
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Abstract
NAT provides routing transparency for hosts in disparate address
realms to communicate with each other. However, external agents
such as Application Level Gateways (ALGs), Realm Specific IP
(RSIP) clients and Management applications need to interact with
NAT and influence its operations. The document identifies NAT
managed resources and ways by which these resources may be
controlled from external agents. The resource control mechanism
is illustrated functionally through an Application Programming
Interface (API). However, it is not the intent of this document
to standardize the API. Rather, use the API as basis to
illustrate and provide a basis for the development of one or
more protocols by which external agents could interact with NAT.
Identification of NAT controlled resources also provides a basis
to generate NAT Management Information Base (MIB).
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1. Introduction
NAT provides routing transparency for hosts in disparate address
realms to communicate with each other. [Ref 1] details the various
flavors of NAT that abound. Many internet applications use IP
address as host identifier rather than just as a way to locate a
host. For this reason, routing transparency by NAT alone is not
sufficient to provide end-to-end transparency for applications
operating across realms. Application specific ALGs are required
in conjunction with NAT to provide end-to-end transparency for
some applications.
In addition to ALGs, there are other kinds of external agents that
may need to influence NAT operation. Section 3 below identifies a
list of external agents that may likely interface with NAT.
Section 2 below is devoted to describing the resources controlled
by NAT. The requirements of external agents, combined with the
nature of NAT resources provide the basis to illustrate the
resource control mechanism functionally through an API in
section 4. Section 5 illustrates how an external agent could use
the resource control mechanism to influence NAT operation.
The intent of the document is two-fold. First, the resource
control mechanism, illustrated through an API, provides a basis
for the development of one or more protocols by which external
agents (specified in section 3 below) could communicate with
NAT. Such a protocol would need to authenticate clients, locate
NAT devices and exchange data elements. The API illustration
assumes a trusted environment and does not address
authentication and Service location. It is also important to
note that the illustartion does not assume or require external
agents to reside on the same physical device as NAT, even
though assuming they reside on the same physical device might
help in understanding. In reality, some agents may be
co-located with NAT on the same device and others reside
on external devices. Discussion of a communication protocol
that may be used by external agents to interface with NAT
is outside the purview of this document.
Second, the document identifies the NAT controlled resources.
This may be used as a basis to develop NAT Management
Information Base (MIB).
2. Elements of NAT operation
In order to identify an API for use by external agents, it is
important to understand the resources and other elments managed
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by NAT. This would help identify the extent to which an external
agent may influence NAT operation. This section describes objects
within NAT, that could be externalized via Management Information
Base (MIB).
2.1. NAT Descriptor
All flavors of NAT are designed to provide routing transparency
to hosts in disparate address realms. A physical device may have
multiple NAT instances or there may be multiple NAT devices
associated with a specific realm. The following list of attributes
identify a specific instance of NAT.
a. NAT IDentifier:
A NAT Identifier uniquely identifies a NAT instantiation.
The External interface address may be one way to uniquely
describe NAT Identifier.
b. Private and External realm types:
Every NAT device will have a minimum of two routing
interfaces, one connecting to a private realm and one
connecting to external realm. An IPv4 NAT device will
have both its realm types set to IPv4.
c. NAT type
NAT type could be one of Basic-NAT, NAPT, Bi-directional-NAT,
Twice-NAT, RSA-IP server, RSAP-IP-server or a combination
of the above. NAT type is an indication of the direction in
which NAT sessions are allowed and the extent of translation
within the IP and transport headers. [Ref 1] has a discussion
on the nature of various NAT flavors and the extent of their
translations.
d. Address(and Transport-ID) maps
Address map on a NAT device could consist of one or more of
static and dynamic Address maps. Likewise, Transport-ID
mapping could consists of one or more of static and dynamic
transport-ID maps. Transport-ID mapping is more specific
than address mapping in that a specific TCP/UDP port (or
port range) pertaining to an address in external realm is
mapped to a specific TCP/UDP port (or port range) in private
realm or vice versa. Address (and Transport-ID) maps may be
defined for both inbound and outbound directions. Outbound
address map refers to mapping a selected set of addresses
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from private realm to a selected set of addresses in
external realm; whereas inbound address map refers to
mapping a set of addresses from the external realm to
private realm.
e. Miscellaneous parameters
NAT may optionally provide TCP, UDP and other types of session
Idle-times used to terminate sessions. It may also provide the
current range (and, the maximum range) of session IDs and
Bind IDs (to be covered in the follow on sub-sections); and
the actual count of session IDs and BIND IDs. Specifically,
this information will be of relevance to another NAT (backup
NAT) that intends to emulate this NAT, in case of failure.
Lastly, NAT may choose to supply any other vendor specific
parameters such as log options, session direction failure
actions and so forth.
f. Realm Specific IP (RSIP) parameters
A NAT device offering RSIP-Server capability may specify the
RSIP tunnel types it supports.
2.2. Address (and Transport-ID) BINDing Descriptor
These bindings can be static or dynamic. Hereafter, the term BIND
will be used in place of BINDing, for ease of use. When external
agents do not intervene, dynamic address(and transport-ID) binding
is determined by NAT based on the first packet of a session, as
described in [Ref 1]. Address binding is between an address in
private realm and an address from external realm. Transport-ID BIND
is an extension of the same concept to the tuple of Address and
transport ID (such as TCP/UDP port no.). The following list of
attributes describe the BIND object(s) maintained by a NAT device.
a. Bind ID
A number (say, in the range of 1 through 0xFFFFFFFF) assigned
to BIND to uniquely identify this BIND from a different BIND
on the same NAT.
b. Direction of Bind
A bind can be uni-directional or bi-directional, same as the
orientation of address map based on which this BIND is formed.
As before, the direction is with reference to private realm.
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c. Bind type
Indicates whether the BIND is Address-BIND (between a pair of
addresses) or Transport-ID-Bind (between a pair of Address,
transport ID tuples). Further, this also indicates if the Bind
is static or dynamically generated.
d. Private and External addresses (and Transport-IDs)
Theese parameters specify the BINDing items in private and
external realms.
e. Maximum lease time
The validity of a BIND may be limited by the duration of lease
time it is allowed. Unless the lease time is renewed, a BIND
will not be valid past the lease time. As a special case, a
value of 0 may be assumed to indicate no lease time limit.
Typically, this attribute is of relevance only in conjunction
with Realm-Specific-IP(RSIP) operation.
f. Available lease time
This parameter is of relevance only when Maximum lease time is
a non-zero value. At any given instance of time, this parameter
indicates the real-time left for a BIND to remain valid.
Typically, this attribute is of relevance only in conjunction
with Realm-Specific-IP(RSIP) operation.
g. Maximum Idle time
This parameter indicates maximum amount of time a dynamic BIND
is allowed to remain valid, with no NAT session hanging off this
BIND. Typically, a dynamic Bind is established when NAT notices
the first session that needs such a binding. Subsequent to
this, multiple NAT sessions can be maintained using the same
binding. When the last of these sessions is terminated, the
bind is also terminated. In other words, Maximum Idle time is 0,
by default, for native NAT. External agents could control this
parameter differently. Static Binds and lease time limited BINDs
are not effected by this parameter.
h. Current Idle time
This parameter is of relevance only when Maximum Idle time is
set to a non-zero value. At any given instance of time, this
parameter indicates the real-time the BIND has been idle with
no sessions attached to it.
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i. Controlling Agent IDentification
This indicates the last external Agent who has tried to
control (i.e., set) parameters for this BIND. A value of 0
indicates that native NAT is the responsible agent.
2.3. Session State descriptor
NAT maintains soft-state for the sessions it tracks. Typically, these
states are created dynamically during NAT operation and are
referenced for translation of packets pertaining to the session. The
translation of a packet is based on the bind(two binds in case of
twice-nat) the session state points to. The following list of
attributes identify a session state (or, simply session) within NAT.
a. Session IDentifier
A number (say, in the range of 1 through 0xFFFFFFFF) assigned
to session to uniquely identify this from other sessions on
the same NAT.
b. Direction of Session.
Direction of first packet of the session. As specified
earlier, direction is with reference to private realm.
c. Bind IDentifier
Identifies the Bind based on which this session is created.
The Direction of BIND must be same as that of the session,
if the BIND is uni-directional. Typically, if a Bind supporting
the session translation does not already exist, a Bind is
created prior to creating new session state. However, this
Identifier may be set to 0, when BIND creation is unnecessary
for the session. For example, there can be no more than one
ICMP Query session using am ICMP Query based transport-ID-bind.
In such a case, it suffices to do away with BIND and keep all
requisite information within the session state itself.
d. Second Bind IDentifier
This is of relevance only to Twice-NAT. For all other flavors
of NAT, this parameter may be set to zero. In the case of
Twice-NAT, the Primary Bind Identifier refers to the binding
of source address (of the first packet) and the Second Bind
Identifier refers to the binding of the destination address.
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e. Original Session parameters
These parameters identify the session level parameters as
they appear in the first packet of session. These parameters
include src and dest IP addresses, IP protocol and transport
IDentifier info (such as TCP/UDP port numbers or ICMP Query
Identifier).
f. Translated Session parameters
These parameters identify the session level parameters as
the first packet of session is translated. These parameters
are derived from the BIND ID(s) off which this session hangs.
g. Session tag
NAT managed sessions are assigned a session tag, so that
sessions bearing the same tag (session bundle) are handled
the same way. A session tag may be identified as a tuple of
(<IP-protocol>, <session-Port>). This tag value is of
significance to NAT or an external agent controlling the
session. NAT retains control of all sessions, unless an
agent registers to control the session. For example, an
FTP-ALG may choose to take control of all sessions with
an FTP (TCP port 21) session tag.
h. Session Termination heuristic
Session-Idle-time is typically used as a heuristic means by NAT
to determine if the session has ended. There may other heuristic
approaches. A value of zero is an indication that NAT would not
use any heuristic to session termination, unless it is a TCP
session and the session has noticeable ended with FIN or RST
options. The agent may take the responsibility for terminating
the session.
i. Maximum Idle time
This parameter indicates maximum amount of time this session
is allowed to remain valid, even as there is no activity.
Idle time is typically used as a heuristic means to determine
session termination. There may be other heuristic approaches.
As a special case, a value of 0 implies that NAT should run
the same timer as used for native sessions.
j. Current Idle Time
This parameter is of relevance only when session termination
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heuristic is set to session-idle-time. Typically, NAT would
examine the idle time on the sessions it manages periodically
and updates this variable. When the idle time exceeds the
maximum allowed idle time, the session is terminated.
k. Packet modifier functions
Typically, NAT modifies IP header and sometimes the transport
header. External agents may choose to assume responsibility
for payload modification alone, or the entire packet
modification. In the case an external agent assumes
responsibility for the entire packet modification, NAT will
simply redirect the original packet as is to external
translation agent. Otherwise, NAT will perform its share of
translation (i.e., IP and transport header translation) and
direct the translated packet to external agent.
l. Bundle ID
Applications that deal with a bundle of sessions may cause
multiple sessions to be managed by NAT. Even though these
sessions constitute a single session from application stand
point, NAT is not congnizant of the relation. In such cases,
it is not uncommon for external agents to store a unique
application ID (say, the session ID of the first NAT session
the application originated) in all sessions it spawns in its
incarnation. By default, this would be same as the session-id.
m. Controlling Agent IDentification
This indicates the last external Agent who has tried to
control parameters for this session. A value of 0 indicates
that native NAT is the responsible agent.
3. External agents interfacing with NAT
Many network applications assume the IP address of their host to be
host Identifier and embed the Identifier information in application
specific payload. When packets from such an application traverse
NAT, the IP address of private host remains uncorrected in the
payload, as the packet is delivered to hosts in external realm. An
Application Level Gateway (ALG) is required to re-interpret such a
payload as the payload traverses realms.
In addition, there are applications such as H.323 that use
out-of-band signaling to dynamically create newer sessions. While
a signaling session itself may be directed to a well-known port,
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sessions created by it need not be that way. Once again, an ALG may
be required to process payload in the signaling sessions and notify
NAT to recognize the newly created sessions.
There may be other instances where an ALG may be required to
provide application level transparency. In all cases, there is a
need for the ALGs to interface with NAT. The ALGs may reside
on the NAT device or on an external device. Independent of where
an ALG resides, NAT interface requirements remain the same.
In a multi-homed NAT configuration, there is a need for a backup NAT
to communicate with the primary and keep in sync, so that when the
primary goes away, the backup NAT could instantly assume support for
the sessions that primary NAT was responsible for. This is yet
another case where an external agent (i.e., backup NAT) has a need
to interface with NAT.
A NAT device is uniquely qualified to serve as Realm-Specific-IP
Server (i.e., RSA-IP-Server or RSAP-IP-Server) for Realm-Specific-IP
clients (i.e., RSA-IP clients or RSAP-IP clients). [Ref 1] has a
description of RSIP terminology. RSA-IP clients and RSAP-IP clients
need to interface with the server node to obtain an external address
(or a tuple of address and TCP/UDP port) while communicating with
hosts in external realms. In addition, if NAT were to act as tunnel
end-point, RSIP clients will need to interface with NAT to setup
tunnel state for the lifetime of RSIP-client address assignment.
So, once again, there is a need for an API for use by an external
agent(i.e., RSIP client) to communicate with NAT, acting as
RSIP-server.
Lastly, a mangement utility would be useful to interface with NAT
for configuration and monitor purposes and to enforce NAT policies.
For example, reconfigure a NAT device to switch over from NAPT to
Basic-NAT configuration or vice versa. Or, add, terminate and
monitor ALGs and other external agents on a NAT box. Such a program
would also be useful to notify NAT about the status and setup
information concerning ALGs, backup NATs and RSIP clients.
Clearly, agents such as RSIP clients and Backup-NATs are likely
to reside on a different physical device than the NAT device. Some
of the ALG agents may also reside on an external device. The API
presented in the follow-on section will provide a base to identify
requirements for the development of one or more protocols by which
each of these external agents could communicate with NAT. It may be
a single protocol applicable to all external agents (or) multiple
protocols, specific to each agent type.
The following diagram identifies a selected list of external agents
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that might interact with NAT via an API or an yet to be devised
protocol.
+--------------+ +------+ +-------------+ +------------------+
| RSIP Clients | | ALGs | | Pri/Sec NAT | | Management Appl. |
+--------------+ +------+ +-------------+ +------------------+
^ ^ ^ ^
| | | |
| | | |
v v v v
+---------------------------------------------+
| NAT Application Program Interface (NAT-API) |
+---------------------------------------------+
| N A T |
+---------------------------------------------+
figure 1. External agents interfacing with NAT
The following list of attributes uniquely identify an external
agent with reference to a NAT.
a. Agent IDentifier
A number (say, in the range of 1 through 0xFFFFFFFF) assigned
to the agent by the NAT device to distinguish from other
agents. Typically, this handle may be assigned when the
agent registers with NAT.
b. Agent type
Based on the categories of external agents described thus far,
it is clear that the API requirements differ considerably
amongst them. A native NAT API may or may not be able to
support the requirements of all these agents. It is beneficial
for NAT to know the agent type to be one of ALG or
RSIP Client or Backup-NAT or Management Application or
something else, so it can accept or deny registration.
c. Agent call-back requirements
An agent will typically require NAT to invoke a call-back
function supplied by the agent upon the occurrence of
specific events. However, events for which an agent
wants to be notified of varies based on agent type.
An ALG will require NAT to call back when a data packet is
received on a session with a certain session-tag (say, FTP
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session). Management applications and Backup-NAT might
require NAT to periodically invoke a call-back function.
Events all agents might require to be notified of (through
a call-back function) would be - termination of a session
with certain session-tag or session-ID, termination of a
Bind and termination of NAT itself.
d. Agent call-back functions
Depending upon call-back requirements, the agent will be
required to register one or more call-back function entry
points with NAT. Below are three different call-back
function prototypes.
Event notification - void agent_callback_event(nat_id,
agent_id, event, event_info)
Periodic notification - void agent_callback_periodic(nat_id,
agent_id, info_type, info_length,
information)
Packet notification - void agent_callback_packet(nat_id,
agent_id, session_id,
pkt_direction, packet)
e. Periodic Notification interval
This parameter would be required only when the agent calls
for periodic notification. This may be specified in units of
seconds.
f. RSIP Server tunnel type requirement
An RSIP client may have a requirement for NAT, acting as
RSIP server to support a certain type of tunneling. In
such a case, the agent will specify the tunneling
requirement through this parameter.
g. Agent access information
In the case the agent is resident on a different physical
device than NAT, this parameter is used by the agent to
specify a means by which NAT can access the agent. This
will include a combination of Agent's IP address,
IP protocol (e.g., TCP or UDP), well-known port etc.
As a special case, a value of 0 to agent_ip_address would
indicate that the agent is on the same device as NAT and
a proprietary mechanism may be assumed to exist to access
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the agent.
4. NAT Application Programming Interface (NAT API)
A resource control mechanism by which external agents could
interface with a NAT device is illustrated in this section
through an Application Programming Interface (API) in pseudo
C language. The API is by no means exhaustive in coverage.
This section is divided into two sub-sections. The first
sub-section lists function calls available to external agents.
These calls are synchronous and require NAT to return back
a value. The second sub-section lists functions required to be
provided by external agents in order for NAT to call-back upon
some events.
4.1. NAT API functions
4.1.1. int nat_enquire_IDentity(nat_type, **natid_info)
Purpose:
This function is used by external agents to obtain NAT-ID
and its characteristics, as described in section 2.1
Input parameters:
nat_type - This parameter is specified to verify if NAT
device supports a certain flavor of NAT.
A value of 0 requires all instances of NAT
to be reported
Output Parameters:
natid_info - NAT will fill up a descriptor block with its
characteristics (as described in section 2.1)
for the matching nat_type and return pointer
to the descriptor block. The descriptor
block would specifically include an identifier
(nat_id) that uniquely identifies NAT instance.
Multiple pieces of this information may be returned,
if NAT supports multiple instances of the same NAT
type. Multiple instances of NAT descriptor blocks
may also be returned when nat_type is set to 0 and
the NAT device supports multiple NAT instances.
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Return Value:
No-Error(0) - A return value of 0 implies success
and that natid_info may be examined
for NAT description.
NAT-TYPE-NOT-SUPPORTED - Notify the client that the
requested NAT device does not
support the specified NAT type.
4.1.2. int nat_enquire_address_bind (nat_id, pvt_address,
ext_address, &bind_info)
Purpose:
This function is used by external agents to obtain
Address BIND information.
Input parameters:
nat_id - The identifier that uniquely identifies the NAT instance.
pvt_address, ext_address - The caller might specify both or just
one of either private address or external address and
set the other to zero.
Output Parameters:
bind_info - NAT will fill up the bind_info data structure
with info as described in section 2.2, if NAT were
to find a match for the addresses specified.
Return Value:
No-Error(0) - A return value of 0 implies success
in finding a match.
NO-MATCHING_BIND - Notify the client that there isn't a BIND
matching the specified addresses.
INVALID-NAT-ID - The specified NAT-ID is not operational
or is incorrect.
4.1.3. int nat_enquire_transport_bind(nat_id, pvt_address, pvt_port,
transport_protocol, ext_address, ext_port, &bind_info)
Purpose:
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This function is used by external agents to obtain
Transport ID BIND information.
Input parameters:
nat_id - The identifier that uniquely identifies the NAT instance.
pvt_address, pvt_port,
ext_address, ext_port - The caller might specify both or just
one of either (private address and the port no.) or
external address and the port number.
transport_protocol - This must be one of TCP, UDP or ICMP Query
Output Parameters:
bind_info - NAT will fill up the bind_info data structure
with info as described in section 2.2, if NAT were
to find a match for the addresses specified.
Return Value:
No-Error(0) - A return value of 0 implies success
in finding a match.
NO-MATCHING_BIND - Notify the client that there isn't a BIND
matching the specified addresses.
INVALID-NAT-ID - The specified NAT-ID is not operational
or is incorrect.
4.1.4. int nat_enquire_sess_range(nat_id, agent_id, sessid_min,
sessid_max, &sess_count, &sess_info)
Purpose:
This function is used by external agents to request NAT to
send valid session information for all sessions with an ID
in the range of sessid_min through sessid_max. As a special
case, this will return session descriptor block for a
single session when sessid_min and sessid_max are the same.
Input parameters:
nat_id - The identifier that uniquely identifies the NAT
instance.
agent_id - The agent Identifier that uniquely identifies the
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agent to NAT.
sessid_min, sessid_max - The range of session IDs that the
agent is interested in knowing about.
Output Parameters:
sess_count - Number of sessions being returned through
sess_info pointer.
sess_info - Return one or more sessions maintained by NAT,
with an ID in the given range.
Return Value:
No-Error(0) - A return value of 0 implies successful
session termination.
INVALID-NAT-ID - The specified NAT-ID is not operational
or is incorrect.
INVALID-AGENT-ID - The specified Agent-ID is not currently
registered with NAT.
4.1.5. int nat_register_agent (nat_id, &agent_info)
Purpose:
This function is used by external agents to register with NAT.
Input parameters:
nat_id - The identifier that uniquely identifies the NAT
instance.
agent_info - The agent is required to provide all the requisite
information (with the exception of agent_id) as
described in section 3.0. This ID may be used by
the caller to influence NAT operation.
Output Parameters:
agent_info - NAT will return the agent_id in agent_info structure
when registration is successful.
Return Value:
No-Error(0) - A return value of 0 implies successful
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registration.
AGENT-TYPE-NOT-SUPPORTED - Notify the caller that NAT does not
support API requirements of the agent.
TUNNEL-TYPE-NOT-SUPPORTED - Notify caller that NAT does not
support the RSIP tunnel type
requested.
INVALID-NAT-ID - The specified NAT-ID is not operational
or is incorrect.
4.1.6. int nat_set_bind (nat_id, agent_id, &bind_info)
Purpose:
This function is used by external agents to create a new Address
Bind or set certain parameters of an existing Bind.
Input parameters:
nat_id - The identifier that uniquely identifies the NAT
instance.
agent_id - The agent Identifier that uniquely identifies the
agent to NAT.
bind_info - The caller supplies the specifics of a new BIND or
sets a selected number of parameters of an existing
BIND to influence NAT operation. The BIND can be
an address BIND or transport BIND. A new BIND
request is made by setting the BIND ID within
bind_info structure to 0. A non-Zero Bind-ID would
be interpreted by NAT to mean that the agent is
attempting to set some BIND parameters.
Output Parameters:
bind_info - If the caller requested for a BIND creation and NAT
was successful in creating a new BIND, NAT will
fill the structure with the assigned BIND ID and
any other NAT assigned parameter values. If the
caller requested to set some BIND parameters and
NAT succeeded in doing so, the bind_info would
be filled with the values that NAT holds.
Return Value:
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No-Error(0) - A return value of 0 implies successful
BIND creation or parameter setting.
BIND-MAKE-FAILED - When NAT was unable to create BIND
or was unable to set the requested
parameter(s).
INVALID-BIND-INFO - When NAT finds that one or all of the
parameters specified is not valid.
INVALID-NAT-ID - The specified NAT-ID is not operational
or is incorrect.
INVALID-AGENT-ID - The specified Agent-ID is not currently
registered with NAT.
4.1.7. int nat_set_sess(nat_id, agent_id, &sess_info)
Purpose:
This function is used by external agents to create a new session
state or set certain parameters of an existing session.
Input parameters:
nat_id - The identifier that uniquely identifies the NAT
instance.
agent_id - The agent Identifier that uniquely identifies the
agent to NAT.
sess_info - The caller supplies the specifics of a new session
parameters or sets a selected number of parameters
of an existing session to influence NAT operation.
A new session request is made by setting the
session-ID within sess_info structure to 0. A
non-Zero session-ID would be interpreted by NAT to
mean that the agent is attempting to set some
session specific parameters.
Output Parameters:
sess_info - If the caller requested for a session creation and
NAT was successful in creating a new session, NAT
will fill the structure with the assigned session-ID
and any other NAT assigned parameter values. If the
caller requested to set some session parameters and
NAT succeeded in doing so, the sess_info would
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be filled with the values that NAT holds.
Return Value:
No-Error(0) - A return value of 0 implies successful
session creation or parameter setting.
SESS-MAKE-FAILED - When NAT was unable to create session
or was unable to set the requested
parameter(s).
INVALID-SESS-INFO - When NAT finds that one or all of the
parameters specified is not valid.
INVALID-NAT-ID - The specified NAT-ID is not operational
or is incorrect.
INVALID-AGENT-ID - The specified Agent-ID is not currently
registered with NAT.
4.1.8. int nat_free_bind(nat_id, agent_id, bind_id)
Purpose:
This function is used by external agents to terminate
the specified BIND and any sessions that are based on
this BIND.
Input parameters:
nat_id - The identifier that uniquely identifies the NAT
instance.
agent_id - The agent Identifier that uniquely identifies the
agent to NAT.
bind_id - The ID of the BIND that needs to be terminated.
Output Parameters:
none.
Return Value:
No-Error(0) - A return value of 0 implies successful
BIND termination.
INVALID-BIND-ID - The specified BIND ID does not exist.
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INVALID-NAT-ID - The specified NAT-ID is not operational
or is incorrect.
INVALID-AGENT-ID - The specified Agent-ID is not currently
registered with NAT.
4.1.9. int nat_free_sess(nat_id, agent_id, sess_id)
Purpose:
This function is used by external agents to terminate
the specified session.
Input parameters:
nat_id - The identifier that uniquely identifies the NAT
instance.
agent_id - The agent Identifier that uniquely identifies the
agent to NAT.
sess_id - The ID of the session that needs to be terminated.
Output Parameters:
none.
Return Value:
No-Error(0) - A return value of 0 implies successful
session termination.
INVALID-SESS-ID - The specified session ID does not exist.
INVALID-NAT-ID - The specified NAT-ID is not operational
or is incorrect.
INVALID-AGENT-ID - The specified Agent-ID is not currently
registered with NAT.
4.1.10. int nat_free_sess_bundle(nat_id, agent_id, bundle_id)
Purpose:
This function is used by external agents to terminate
a bundle of sessions identified by the same bundle ID.
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Input parameters:
nat_id - The identifier that uniquely identifies the NAT
instance.
agent_id - The agent Identifier that uniquely identifies the
agent to NAT.
bundle_id - The ID of the session bundle (group of sessions)
that needs to be terminated.
Output Parameters:
none.
Return Value:
No-Error(0) - A return value of 0 implies successful
session termination.
INVALID-BUNDLE-ID - The specified bundle ID does not exist.
INVALID-NAT-ID - The specified NAT-ID is not operational
or is incorrect.
INVALID-AGENT-ID - The specified Agent-ID is not currently
registered with NAT.
4.2. Call-back functions within an external agent
4.2.1. void agent_callback_event(nat_id, agent_id, event_type,
&event_info)
Purpose:
This function is used by NAT to notify an agent of an
event status.
Input parameters:
nat_id - The identifier that uniquely identifies the NAT
instance.
agent_id - The agent Identifier that uniquely identifies the
agent to NAT.
event_type - The event can be one of BIND creation, BIND
termination, session Creation, and session
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termination.
event_info - This will return the BIND or session description
structure that contains the specific instance
identifier and other pertinent information.
4.2.2. void agent_callback_periodic(nat_id, agent_id, info_type,
info_length, &periodic_info)
Purpose:
This function is used by NAT to notify an agent of a
certain piece of information periodically.
Input parameters:
nat_id - The identifier that uniquely identifies the NAT
instance.
agent_id - The agent Identifier that uniquely identifies the
agent to NAT.
info_type - NAT may have been requested to periodically
notify the agent many types of information.
Possible values for this parameter would be
statistics update, Incremental BIND update
Incremental session update, Incremental
BIND termination, Incremental session
termination etc..
info_length- Number of bytes included in periodic info block.
periodic_info - This point to the actual periodic information
being sent to the agent.
4.2.3. void agent_callback_packet(nat_id, agent_id, sess_id,
pkt_direction, packet)
Purpose:
This function is used by NAT to notify an agent of a
data packet for processing. The agent is expected to
process the packet and forward to the actual destination
in the first-in-first-out (FIFO) order. The processing
performed by the agent may be limited to just the payload
or the entire packet, as set by the agent at session
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setup time.
Input parameters:
nat_id - The identifier that uniquely identifies the NAT
instance.
agent_id - The agent Identifier that uniquely identifies the
agent to NAT.
sess_id - The Identifier if NAT session to which the packet
belongs.
pkt_direction - This can be inbound or outbound.
packet - IP packet that needs to be processed by the agent.
If NAT was required to perform header translation,
this packet is post-NAT-translated version of
the packet. In the case the agent selected to
perform the entire translation, the original
packet is sent as is to the agent, without any
NAT transformation.
5. An illustration of the use of NAT Resource Control Mechanism
The following is an illustration of how an FTP-ALG could use
the API specified to interface with NAT and provide
application level transparency for FTP application. Note,
this is not meant to be a detailed description of how an
FTP-ALG would work. But, rather an illustration of how the
FTP-ALG could use the resource control mechanims to interface
with NAT. The section is divided into three sub-sections to
illustrate (a) ALG registration with NAT, (b) NAT interface
with ALG while an FTP session is active, and (c) Notification
to ALG when the FTP session terminates.
5.1. FTP-ALG registration with NAT
FTP-ALG will first probe NAT device to understand the type of
service provided by NAT and obtain NAT-ID. Once the service
type is agreeable, the ALG will register itself as a client
with the NAT device with callback functions (as described
below) and obtain an agent-ID from the NAT. The tuple of
(nat-id, agent-id) uniquely identifies the interface
between NAT and ALG.
ftp_alg_pkt_notify() will be registered to process FTP
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session (TCP port 21) traffic. ftp_alg_event_notify() will
be registered to process session or NAT termination.
FTP-ALG NAT
------- ---
1. Obtain NAT descriptor Info
nat_enquire_IDentity(
0, **nat_descriptor)
------------------------>
NAT will fill a descriptor
block with pertinent information,
specifically NAT-type and NAT-ID
and supply the pointer to the
descriptor block.
OK
<------------------------------
2. Register with NAT as ALG for
FTP (TCP port 21) and obtain
agent-ID from the NAT.
nat_register_agent(nat_id,
&ftp_alg_info)
------------------------>
NAT will assign an agent-ID.
OK
<------------------------------
5.2. NAT interface with the ALG during FTP session operation
When NAT sees the first packet of an FTP session, it sets up
a BIND descriptor and a session descriptor and tags the
session descriptor as FTP-Type (i.e., TCP port 21). NAT
will then redirect the packet to FTP-ALG by invoking the
ALG supplied callback function - ftp_alg_pkt_notify().
The ALG will obtain session descriptor and BIND descriptor
info from the NAT.
Subsequent to this, when NAT redirects FTP packets, the
ALG would parse the payload for PORT command or response to
"PASV" to determine ensuing data sessions and interact with
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NAT, as necessary, to obtain the requisite translation
parameters. The ALG may modify the FTP packet with
translation parameters prior to resending to NAT for
forwarding.
FTP-ALG NAT
------- ---
1. NAT sees the first packet
of an FTP session. NAT will
set up a session state and
notify the agent as follows.
ftp_alg_pkt-notify(nat-id,
agent-ID, session-ID,
packet-direction, pkt)
<------------------------
The ALG may optionally make
calls to the NAT to find out
about the session and BIND
characteristics of the FTP.
Further, additional calls may
be made to change the control
parameters in these blocks.
nat_enquire_sess_range(
nat_id, agent-id,
session_id, session_id,
&sess_count, **sess_info)
------------------------------>
Find the session descriptor
block matching session_id and
return Pointer to this. Bind-id
is one of the items in the block.
OK
<------------------------------
...
nat_enquire_address_bind(
nat_id, pvt_address,
external_address, &bind_info)
------------------------------>
...
nat_set_bind(
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nat_id, agent_id, &bind_info)
------------------------------>
.............
n. NAT will forward all
subsequent FTP packets to
the agent as follows.
ftp_alg_pkt-notify(nat-id,
agent-ID, session-ID,
packet-direction, pkt)
<------------------------
The ALG will parse for PORT
command and PASV response in
the payload and track any deltas
to TCP sequence and acknowledge
numbers. The ALG will interact
with NAT, as necessary, to obtain
BIND parameters for the data
session, setup data session state
ahead of time and modify the FTP
packet (as necessary) prior to
resending to NAT for forwarding.
Request BIND parameters for the
new data session such that there
is no leased-time set for it.
nat_set_bind(nat_id, agent-id,
&bind_info)
------------------------------>
....
Setup a new state for the data
session such that the Bundle-ID
is set to be the session ID of
the controlling FTP session.
nat_set_sess(nat-id, agent-id,
&sess-info)
---------------------------->
5.3. Session termination notification
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When the FTP control session is ready to be terminated
by the NAT, NAT will notifiy the event to FTP-ALG as follows.
FTP-ALG NAT
------- ---
1. NAT determines the FTP
session is to be
terminated.
ftp_alg_notify(nat-id,
agent-id,
SESSION-TERMINATED,
session-ID)
<------------------------
The ALG will in turn clean up any
data sessions that may be based on
the FTP session prior to freeing
the control session itself.
nat_free_sess(nat-id, agent-id,
sess-id)
---------------------------->
6. Acknowledgement
The author would like to express sincere appreciation and thanks
to Yakov Rekhter for his valuable advice and contribution in the
presentation of this document.
7. Security considerations.
The security considerations described in [Ref 1] for all variations
of NATs are applicable here.
REFERENCES
[1] P. Srisuresh, M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663
[2] Y. Rekhter, B. Moskowitz, D. Karrenberg, G. de Groot, and,
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Internet Draft NAT Interface Framework July 2000
E. Lear, "Address Allocation for Private Internets", RFC 1918
[3] J. Reynolds and J. Postel, "Assigned Numbers", RFC 1700
[4] R. Braden, "Requirements for Internet Hosts -- Communication
Layers", RFC 1122
[5] R. Braden, "Requirements for Internet Hosts -- Application
and Support", RFC 1123
[6] F. Baker, "Requirements for IP Version 4 Routers", RFC 1812
[7] J. Postel, J. Reynolds, "FILE TRANSFER PROTOCOL (FTP)",
RFC 959
[8] "TRANSMISSION CONTROL PROTOCOL (TCP) SPECIFICATION", RFC 793
[9] J. Postel, "INTERNET CONTROL MESSAGE (ICMP) SPECIFICATION",
RFC 792
[10] J. Postel, "User Datagram Protocol (UDP)", RFC 768
[11] J. Mogul, J. Postel, "Internet Standard Subnetting Procedure",
RFC 950
[12] Brian carpenter, Jon Crowcroft, Yakov Rekhter, "IPv4 Address
Behaviour Today", RFC 2101
Author's Address:
Pyda Srisuresh
Campio Communications
630 Alder Drive
Milpitas, CA 95035
U.S.A.
Voice: (408) 519-3849
EMail: srisuresh@yahoo.com
Srisuresh [Page 27]
