Network Working Group P. Srisuresh
INTERNET-DRAFT Jasmine Networks
Expires as of November 15, 2001 J. Kuthan
GMD Fokus
J. Rosenberg
Dynamicsoft
A. Molitor
Aravox Technologies
A. Rayhan
Consultant
May, 2001
Middlebox Communication Architecture and framework
<draft-ietf-midcom-framework-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
There are a variety of intermediate devices in the Internet today
that require application intelligence for their operation.
Datagrams pertaining to real-time streaming applications such
as SIP and H.323 and peer-to-peer applications such as Napster
and NetMeeting cannot be identified by merely examining packet
headers. Middleboxes implementing Firewall and Network Address
Translator services typically embed application intelligence
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within the device for their operation. The document specifies an
architecture and framework in which trusted third parties can
be delegated to assist the middleboxes to perform their operation
without resorting to embedding application intelligence. Doing
this will allow a middlebox to continue to provide the services,
while keeping the middlebox application agnostic. A principal
objective of this document is to enable complex applications
through the middleboxes seamlessly using a trusted third party.
1. Introduction
Intermediate devices requiring application intelligence are the
subject of this document. These devices are referred as
middleboxes throughout the document. Many of these devices enforce
application specific policy based functions such as packet
filtering, differentiated Quality of Service, tunneling, Intrusion
detection, security and so forth. Network Address Translator
service, on the other hand, provides routing transparency across
address realms (within IPv4 routing network or across V4 and V6
routing realms). Application Level gateways (ALGs) are used in
conjunction with NAT to provide end-to-end transparency for many of
the applications. There may be other types of services requiring
embedding application intelligence in middleboxes for their
operation. The discussion scope of this document is however limited
to middleboxes implementing Firewall and NAT services only.
Nonetheless, the middlebox framework is designed to be extensible
to support the deployment of new services.
Tight coupling of application intelligence with middleboxes makes
maintenance of middleboxes hard with the advent of new applications.
Built-in application awareness typically requires updates of
operating systems with new applications or newer versions of
existing applications. Operators requiring support for newer
applications will not be able to use third party software/hardware
specific to the application and are at the mercy of their
middlebox vendor to make the necessary upgrade. Further, embedding
intelligence for a large number of application protocols within
the same middlebox increases complexity of the middlebox and is
likely to be error prone and degrade in performance.
This document describes a framework in which application
intelligence can be moved from middleboxes into external MIDCOM
agents. The premise of the framework is to devise a MIDCOM
protocol that is application independent, so the middleboxes
can stay focussed on services such firewall and NAT. MIDCOM
agents with application intelligence can, in turn, assist the
middleboxes through the MIDCOM protocol in permitting applications
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such as FTP, SIP and H.323. The communication between a MIDCOM
agent and a middlebox will be transparent to the end-hosts that
take part in the application, unless one of the end-hosts assumes
the role of a MIDCOM agent. Discovery of middleboxes in the path
of an application instance and communication amongst middleboxes
is outside the scope of this document.
This document describes the framework in which middlebox
communication takes place and the various elements that constitute
the framework. Section 2 describes the terms used in the document.
Section 3 defines the architectural framework of a middlebox for
communication with MIDCOM agents. The remaining sections cover the
components of the framework, illustration using sample flows and
operational considerations with the MIDCOM architecture. Section 4
describes the nature of MIDCOM protocol. Section 5 identifies
entities that could potentially host the MIDCOM agent function.
Section 6 considers the role of Policy server and its function
with regard to communicating MIDCOM agent authorization policies.
Sections 7 and 8 are illustration of MIDCOM framework with sample
flows using In-Path and out-of-path agents respectively. Section 9
addresses operational considerations in deploying a protocol
adhering to the framework described here. Section 10 is an
applicability statement, scoping the location of middleboxes.
Section 12 outlines security considerations for the middlebox
in view of the MIDCOM framework.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALLNOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119.
Below are the definitions for the terms used throughout the
document.
2.1. MiddleBox function/service
A middlebox function or a middlebox service is an operation or
method performed on a network intermediary that requires application
specific intelligence for its operation. Policy based packet
filtering (a.k.a. firewall), Network address translation (NAT),
Intrusion detection, Load balancing, Policy based tunneling and
IPsec security are all examples of a middlebox function (or
service).
2.2. MiddleBox
Middlebox is a network intermediate device that implements one or
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more of the middlebox services. A NAT middlebox is a middlebox
implementing NAT service. A firewall middlebox is a middlebox
implementing firewall service.
2.3. Firewall
Firewall is a policy based packet filtering Middlebox function,
typically used for restricting access to/from specific devices and
applications. The policies are often termed Access Control
Lists (ACLs).
2.4. NAT
Network Address Translation is a method by which IP addresses are
mapped from one address realm to another, providing transparent
routing to end-hosts. This is achieved by modifying end node
addresses en-route and maintaining state for these updates so
that datagrams pertaining to a session are forwarded to the right
end-host in either realm. Refer [NAT-TERM] for the definition of
various NAT types and the associated terms in use.
The term NAT in this document is very similar to the IPv4 NAT
described in [NAT-TERM], but is extended beyond IPv4 networks
to include the IPv4-v6 NAT-PT described in [NAT-PT]. While the
IPv4 NAT [NAT-TERM] translates one IPv4 address into another IPv4
address to provide routing between private V4 and external V4
address realms, IPv4-v6 NAT-PT [NAT-PT] translates an IPv4 address
into an IPv6 address and vice versa to provide routing between a
V6 address realm and an external V4 address realm.
Unless specified otherwise, NAT in this document is a middlebox
function referring to both IPv4 NAT as well as IPv4-v6 NAT-PT.
2.5. Proxy
A proxy is an intermediate relay agent between clients and servers
of an application, relaying application messages between the two.
Proxies use special protocol mechanisms to communicate with proxy
clients and relay client data to servers and vice versa. A Proxy
terminates sessions with both the client and the server, acting as
server to the end-host client and as client to the end-host server.
Applications such as FTP, SIP and RTSP use a control connection to
establish data sessions. These control and data sessions can take
divergent paths. While a proxy can intercept both the control and
data connections, it might intercept only the control connection.
This is often the case with real-time streaming applications such
as SIP and RTSP.
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2.6. ALG
Application Level Gateways (ALGs) are agents that possess the
application specific intelligence and knowledge of an associated
middlebox function. An ALG examines application traffic in transit
and assists middlebox in carrying out its function.
An ALG may be co-resident with a middlebox or reside externally,
communicating through a middlebox communication protocol. It
interacts with a middlebox to set up state, access control
filters, use middlebox state information, modify application
specific payload or perform whatever else is necessary to enable
the application to run through the middlebox.
ALGs are different from proxies. ALGs are transparent to
end-hosts, unlike the proxies which are relay agents terminating
sessions with both end-hosts. ALGs do not terminate session with
either end-host. Instead, ALGs examine and optionally modify
application payload content to facilitate the flow of application
traffic through a middlebox. ALGs are middlebox centric, in that
they assist the middleboxes in carrying out their function.
Whereas, the proxies act as focal point for application servers,
relaying traffic between application clients and servers.
ALGs are similar to Proxies, in that, both ALGs and proxies
facilitate Application specific communication between clients
and servers.
2.7. End-Hosts
End-hosts are entities that are party to a networked application
instance. End-hosts referred in this document are specifically
those terminating Real-time streaming Voice-over-IP
applications such as SIP and H.323 and peer-to-peer applications
such as Napster and NetMeeting.
2.8. MIDCOM Agents
MIDCOM agents are entities performing ALG function, logically
external to a middlebox. MIDCOM agents possess a combination of
application awareness and knowledge of the middlebox function.
A MIDCOM agent may communicate with one or more middleboxes.
MIDCOM agents may be located either In-Path or Out-of-path of
an application instance. In-Path MIDCOM agents are those in
which the MIDCOM agent function is co-resident on devices that
are naturally within the message path of the application they
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are associated with. This may be an application proxy, gateway,
or in the extreme case, one of the end-hosts, that is party to
the application. Out-of-Path MIDCOM agents are those that are
not necessarily resident (or co-resident) on entities that are
in the path of application flows.
2.9. Policy Server
Policy Server is a management entity that interfaces with a
middlebox to communicate policies concerning authorization of
MIDCOM agents gaining access to middlebox resources. A MIDCOM
agent may be pre-configured on a middlebox as a trusted entity.
In the case where a MIDCOM agent is not pre-configured, the
middlebox will consult Policy Server Out-of-band for validating
the authorization to accept requests from the agent. A policy
server might add or delete MIDCOM agents on a middlebox.
The protocol facilitating the communication between a middlebox
and Policy Server need not be part of MIDCOM protocol.
2.10. Middlebox Communication (MIDCOM) protocol
The protocol between a MIDCOM agent and a middlebox that allows
the MIDCOM agent to gain access to middlebox resources and
allows the middlebox to delegate application specific processing
to MIDCOM agent. The MIDCOM protocol allows the middlebox to
perform its operation with the aid of MIDCOM agents, without
resorting to embedding application intelligence. The principal
motivation behind architecting this protocol is to enable complex
applications through middleboxes seamlessly using a trusted third
party, i.e., a MIDCOM agent.
This is a protocol yet to be devised.
3.0 Architectural framework for Middleboxes
A middlebox may implement one or more of the middlebox functions
selectively on multiple interfaces of the device. There can be a
variety of MIDCOM agents interfacing with the middlebox to
communicate with one or more of the middlebox functions on an
interface. As such, the Middlebox communication protocol MUST
allow for selective communication between a specific MIDCOM agent
and one or more middlebox functions on the interface. The following
diagram identifies a possible layering of the service supported
by a middlebox and a list of MIDCOM agents that might interact
with it.
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+---------------+ +---------------+ +-------------+
| MIDCOM agent | | MIDCOM agent | | Stand-alone |
| co-resident on| | co-resident on| | MIDCOM agent|
| Proxy Server | | Application GW| | |
+---------------+ +---------------+ +-------------+
^ ^ ^
| | | +--------+
| | | | Policy |
| | | +-| Server |
| | | / +--------+
| | MIDCOM | /\/
+-------------+ | | Protocol | / +-------------+
| MIDCOM agent| | | | / | MIDCOM agent|
| co-resident | | | | / | co-resident |
| on End-hosts|<-+ | | | / +->| on End-hosts|
+-------------+ | | | | | | +-------------+
v v v v v v
+-------------------------------------------+
| Middlebox Communication Protocol (MIDCOM) |
| Interface |
+----------+--------+-----------+-----------+
Middlebox | | | | |
Functions | Firewall | NAT | DiffServ- | Intrusion |
| | | QOS | Detection |
+----------+--------+-----------+-----------+
Middlebox | Firewall ACLs, Session-descriptors, |
Managed | NAT-BINDs, NAT Address-Maps and other |
Resources | other Middlebox function attributes |
+-------------------------------------------+
Figure 1: MIDCOM agents interfacing with a middlebox
Resources such as a Session-Descriptor may be shared across
middlebox functions. A Session-Descriptor may uniquely identify
a session denoted by the tuple of (SessionDirection,
SourceAddress, DestinationAddress, Protocol, SourcePort,
DestinationPort). An aggregated Session-Descriptor, on the other
hand, may have one of the tuple elements denoted by a regular
expression (ex: Any source port). The attributes associated
with a Session-Descriptor may be specific to the individual
middlebox function. As Session-Descriptors may be shared across
middlebox functions, a Session-Descriptor may be created by a
function, and terminated by a different function. For example, a
session-descriptor may be created by the firewall function, but
terminated by the NAT function, when a session timer expires.
A middlebox may also have function specific resources such as
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Address maps and Address binds to enforce NAT function and
application based policies to enforce firewall function.
Application specific MIDCOM agents (co-resident on the middlebox
or external to the middlebox) would examine the IP datagrams and
help identify the application the datagram belongs to and assist
the middlebox in performing functions unique to the application
and the middlebox service. For example, a MIDCOM agent assisting
a NAT middlebox might perform payload translations; whereas a
MIDCOM agent assisting a firewall middlebox might request the
firewall to permit access to application specific dynamically
generated session traffic.
4. MIDCOM Protocol
The MIDCOM protocol between a MIDCOM agent and a middlebox allows
the MIDCOM agent to gain access to middlebox resources and
allows the middlebox to delegate application specific processing
to MIDCOM agent. The protocol will allow MIDCOM agents to signal
the middleboxes to let complex applications using dynamic port
based sessions through them (i.e., middleboxes) seamlessly.
It is important to note that an agent and a middlebox can be on
the same physical device. In such a case, it is not desirable
for them to communicte using MIDCOM protocol. They may communicate
using a MIDCOM protocol message formats, but using a non-IP based
transport such as IPC messaging (or) they may communicate using a
well-defined API/DLL (or) the application intelligence is fully
embedded into the middlebox service (as it is done today in many
stateful inspection firewall devices and NAT devices).
The MIDCOM protocol will consist of a connection setup phase,
run-time connection phase and a connection termination phase.
Connection setup must be preceded by registration of the
MIDCOM agent with the middlebox. The MIDCOM agent access and
authorization profile may either be pre-configured on the
middlebox (or) listed on a Policy Server the middlebox is
configured to consult. MIDCOM is a peer-to-peer protocol. As
such, either the agent or the middlebox may choose to initiate
the connection.
A MIDCOM session may be terminated by either of the parties.
Alternately, a MIDCOM session termination may be triggered by
one of (a) agent going out of service and not being available
for further MIDCOM operations, or (b) a policy server notifying
the middlebox that a particular MIDCOM agent is no longer
authorized for a particular set of sessions any longer.
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The MIDCOM protocol data exchanged during run-time is governed
principally by the middleboxbox services the protocol supports.
Firewall and NAT middlebox services are considered in this
document. Nonetheless, the MIDCOM protocol will be designed to
be extensible to support deployment of other services as well.
Few of the middlebox services are stateless. There are many that
are stateful and maintain per-connection state in the system.
Firewall service may be implemented as a stateless list of ACLs.
Many firewall implementations, however, are stateful. NAT
service, on the other hand, is inherently stateful. As such,
support of the MIDCOM protocol will require a middlebox to be
stateful. Here is why.
Let us consider the case of a middlebox implementing firewall
service. With the advent of MIDCOM protocol, MIDCOM agents ought to
be able to set a sessions timer that is launched when a pinhole
is opened for a dynamically permitted session. When the session
timer reaches it's expiration, the middlebox will notify the MIDCOM
agent to refresh it, else the pinhole will be closed. Explicit
pinhole closing is done when the application session is ended and
Midcom agent will request to close it. Session timer is also
required so the pinhole doesnt stay open forever, just in case
the MIDCOM agent suddenly diappears (or terminates for whatever
reason). This goes to show that firewall function will also
necessarily need to maintain per-connection state, as a requirement
to support the MIDCOM protocol.
5.0. MIDCOM Agents
MIDCOM agents are logical entities which may reside physically
on nodes external to a middlebox, possessing a combination of
application awareness and knowledge of middlebox function. A
MIDCOM agent may communicate with one or more middleboxes. The
issues of middleboxes discovering agents or vice versa are
outside the scope of this document. The focus of the document
is the framework in which a MIDCOM agent communicates with a
middlebox using MIDCOM protocol, which is yet to be devised.
We will examine two types of MIDCOM agents in the following
sub-sections.
5.1. In-path MIDCOM agents
In-Path MIDCOM agents are entities that have a native role in the
path of the application traversal (with prior knowledge to one of
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the application end-hosts), independent of their MIDCOM function.
Bundled session applications such as H.323, SIP and RTSP which
have separate control and data sessions may have their
sessions take divergent paths. In those scenarios, In-Path MIDCOM
agents are those that find themselves in the control path.
In majority of cases, a middlebox will likely require the
assistance of a single agent for an application in the control
path alone. However, it is possible that a middlebox function
might require the intervention of more than a single MIDCOM
agent for the same application, one for each sub-session of the
application.
Application Proxies and gateways are a good choice for In-Path
MIDCOM agents, as these entities, by definition, are in the path
of an application between a client and server. In addition to
hosting the MIDCOM agent function, these natively in-path
application specific entities may also enforce
application-specific choices locally, such as dropping messages
infected with known viruses, or lacking user authentication.
These entities can be interjecting both the control and data
connections. For example, FTP control and Data sessions are
interjected by an FTP proxy server. However, proxies may also be
interjecting just the control connection and not the data
connections, as is the case with real-time streaming applications
such as SIP and RTSP. Note, applications may not always traverse
a proxy and some applications may not have a proxy server
available.
SIP proxies and H.323 gatekeepers may be used to host MIDCOM
agent function to control middleboxes implementing firewall and
NAT functions. The advantage of using in-path entities as opposed
to creating an entirely new agent is that the in-path entities
already possess application intelligence. You will need to merely
enable the use of MIDCOM protocol to be an effective MIDCOM
agent. Figure 2 below illustrates a scenario where the in-path
MIDCOM agents interface with the middlebox. Let us say, the
policy Server has pre-configured the in-path proxies as trusted
MIDCOM agents on the middlebox and the packet filter
implements 'default-deny' packet filtering policy. Proxies use
their application-awareness knowledge to control the firewall
function and selectively permit a certain number of voice stream
sessions dynamically using MIDCOM protocol.
In the illustration below, the proxiess and the policy server are
shown inside a private domain. The intent however is not to imply
that they be inside the private boundary alone. The proxies may
also reside external to the domain. The only requirement is that
there be a trust relationship with the middlebox.
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+-----------+
| Middlebox |
| Policy |
| Server |~~~~~~~~~~~~|
+-----------+ \
\
+---------+ \
| SIP |___ \
________| Proxy | \ \ Middlebox
/ +---------+.. | +-----------------+
| : | MIDCOM | |
| RSTP +----------+ :..|........| MIDCOM |
SIP | ____| RSTP |.....|........| PROTOCOL |
| / | Proxy |___ | | INTERFACE |
| | +----------+ \ \ |-----------------|
| | \ \------| |
| | \--------| |
| | -----| FIREWALL |-->--
+-----------+ /-----| |--<--
+-----------+| Data streams // +-----------------+
+-----------+||----------->----// |
|end-hosts ||------------<----- .
+-----------+ (RTP, RSTP data, etc.) |
. Outside the
Within a private domain | private domain
Legend: ---- Application data path datagrams
____ Application control path datagrams
.... Middlebox Communication Protocol (MIDCOM)
~~~~ MIDCOM Policy Server Interface
|
. private domain Boundary
|
Figure 2: In-Path MIDCOM Agents for Middlebox Communication
5.1.1. End-hosts as In-Path MIDCOM agents
End-hosts are another variation of In-Path MIDCOM agents. Unlike
Proxies, End-hosts are direct party to the application and
possess all the end-to-end application intelligence there is to
it. End-hosts terminate both the control and data paths of an
application. Unlike other entities hosting MIDCOM agents, end-host
is able to process secure datagrams. However, the problem
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would be one of manageability - upgrading all the end-hosts
running a specific application.
5.2. Out-of-Path MIDCOM agents
Out-of-Path MIDCOM agents (a.k.a. OOP agents) are entities that
are not natively in the transport path of an application.
OOP Agents have a role in the application traversal, only by
virtue of their MIDCOM function - No native role otherwise. It
would be safe to assume that OOP agents are not in the path
of application traversal. Out-of-Path agents have a few
benefits. Out-of-Path agents can be implemented in a system,
independent of any pre-existing application-aware entity. Unlike
In-path agents, there are no topological restrictions to where
the agents can be located. Further, multiple application
specific agents can be grouped together on the same node.
There is however a significant difference between in-path MIDCOM
agents and Out-of-path MIDCOM agents in the way the middlebox
directs application specific traffic for processing by the
registered MIDCOM agents. The MIDCOM protocol provides a means
for MIDCOM agents to gain access to middlebox resources and for
the middlebox to direct selective application specific traffic
(ex: Control path datagrams in the case of bundled session
applications) to MIDCOM agents. When a MIDCOM agent is
naturally in the transport path of the application (as is the
case with an In-Path MIDCOM agent), there is no additional effort
required of the middlebox in redirecting the application
traffic. However, Out-of-Path MIDCOM agent is not natively in the
transport path of an application and hence will need to instruct
the middlebox to explicitly redirect datagrams to itself, as
appropriate, on a per-session basis. The middlebox must be able
to redirect the selective application traffic toward the
MIDCOM agent. The out-of-path MIDCOM agent should in turn be
capable of returning the processed traffic to the middlebox
point of origin or forwarding to the destination.
In essence, a middlebox provides to an Out-of-Path MIDCOM agent
the ability to transparently "snoop" and modify the control
traffic.
It is reasonable to further classify Out-of-Path agents into
those which modify control traffic, and those which do not.
For example, if an Out-of-Path agent is used simply to manage
firewall policy for SIP-based telephony, it is enough to simply
forward SIP messages to the agent for examination. On the
other hand, if the agent must also manage NAT bindings, the
agent needs to modify the SIP messages, and re-inject them into
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the control path.
In order to support Out-of-Path agents, the middlebox will require
an additional "Datagram diverter" functional component. This
function is strictly to support the Out-of-path MIDCOM agents and
is independent of any middlebox service or application. The
diverter function on the middlebox would be required to do the
following.
1. When a datagram is received by the middlebox, the middlebox
will subject the datagram to the standard middlebox services as
appropriate. However, if the datagram is designated for diversion
(by the application specific out-of-path MIDCOM agent), the
middlebox will redirect the datagram to the diversion target.
The datagram will have been directed to an application specific
payload processing entity. As such, this may be accomplished using
some type of tunneling mechanism (or) Remote procedure Call (RPC)
(or) some other proprietary mechanism.
2. The recipient of the diverted datagram (i.e., the OOP agent) will
snoop and optionally modify the payload (as appropriate to the
middlebox service) and does one of the following. Of these, the
safe thing to assume would be the first option.
(a) Send the processed datagram right back to the middlebox
using the same diversion approach the middlebox used.
(or)
(b) Forward the datagram to the appropriate destination
(i.e., one of the end-hosts that is party to the
application). This however assumes that the OOP agent
has routing/forwarding capability.
3. When the middlebox receives a diverted (i.e., co-processed)
datagram from the middlebox, the middlebox will simply forward
the processed datagram to the appropriate destination (i.e., one
of the end-hosts that is party to the application). Note, the
middlebox will not subject the datagram to any of the middlebox
services (i.e., NAT or firewall) this time around.
Note, Step 2a followed by step 3 would be same as going with
step 2b by itself. Below is an illustration of a scenario where
Out-of-path MIDCOM agents interface with the middlebox. The
middlebox is assumed to implement firewall service on it. Let us
say, the Out-of-path agents are pre-configured as trusted MIDCOM
agents on the middlebox and the packet filter implements
'default-deny' packet filtering policy. The OOP agents register
themselves as the diversion traffic targets for the applications
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they support. They snoop the payload of the diverted traffic and
use application-awareness knowledge to control the firewall
function and selectively permit a certain number of FTP or voice
stream sessions dynamically using the MIDCOM protocol.
+---------+ Snooped ftp-control traffic
| FTP OOP |============>=====================\
| Agent |++++++++++++<++++++++++++++++++++ ||
| | Diverted ftp-control traffic + ||
+---------+ + ||
+-----------+ : + ||
| Middlebox | : +----------+ Snooped SIP traffic + ||
| Policy | : | SIP OOP |=========>==============\ + ||
| Server | : | Agent |+++++++++<+++++++++++++ || + ||
+-----------+ : | | Diverted SIP traffic + || + ||
| : +----------+ +-----------+------------+
| : :.............| | |
| : MIDCOM | MIDCOM | MIDDLEBOX |
| :....................| PROTOCOL | DATAGRAM |
| | INTERFACE | DIVERTER |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~| | |
+------------+ +-----------+------------+
+------------+|----------->---------| FIREWALL |->-
+------------+||-----------<---------| |-<-
|end-hosts || Data & Control +------------------------+
+------------+ (SIP, RTP, ftp-control, |
ftp-data, etc.) .
|
Within a private domain . Outside the
| private domain
Legend: ---- Application data & control path datagrams
.... Middlebox Communication Protocol (MIDCOM)
~~~~ MIDCOM Policy Server Interface
++++ Control traffic diverted To a MIDCOM agent
==== Snooped and optionally modified application specific
control traffic returning FROM the Out-of-Path agent
|
. private domain Boundary
|
Figure 3: Out-of-Path MIDCOM Agents for Middlebox Communication
6.0. Policy Server functions
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The functional decomposition of the MIDCOM architecture assumes
the existence of a logical entity known as Policy Server,
responsible for performing authorization and related provisioning
services for the middlebox as depicted in figure 1. For example,
a policy server has the ability to add or delete MIDCOM agents on
a middlebox and to notify a middlebox about the status and
security requirements to allow accessibility to MIDCOM agents.
The primary objective of a policy server is to ensure that the
security and integrity of a middlebox is not jeopardized.
Specifically, the policy server should associate a trust level
with each agent attempting to connect to a middlebox and provide
a security profile. Since some agents may be less secure than
others, the policy server should be able to determine the
profile for each agent based on the trust and access security
permitted to the agent. The policy server should be capable of
addressing cases when end-hosts are agents to the middle-box.
6.1. Authentication, Integrity and Confidentiality
Host authenticity and individual message authentication are two
distinct types of authentications to consider. Host authentication
refers to credentials required of a MIDCOM agent to authenticate
itself to the middlebox and vice versa. When authentication fails,
the middlebox MUST not process signaling requests received from
the agent that failed authentication.
To protect MIDCOM messages from being tampered with, individual
message authentication may be used [IPsec-AH] in addition to
host authentication. Further, message confidentiality may
be administered by employing IPsec ESP protocol [IPsec-ESP] for
the MIDCOM messages, when a higher level of security is required.
Alternatively, there are other security options instead of the
IPsec protocols. TLS based transport layer security is one option.
There are also many application-layer security mechanisms
available. Simple Source-address based security is the least form
of security in a trusted domain and should be permitted only to
the most trusted hosts.
Clearly, the middlebox should be able to perform host level
authentication, and be able to authenticate individual messages
(using IPsec or TLS based security).
6.2. Registration and deregistration with a middlebox
Prior to giving MIDCOM agents access to the middlebox resources,
a registration process MUST take place. Registration is a
different process than establishing a transport connection.
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The former requires exchanging the agent profile information
to the middlebox. The latter refers to establishing a MIDCOM
transport connection and exchanging security credentials
adhering to the registered profile.
MIDCOM agents, their trust level and accessibility (i.e.,
the MIDCOM agent profile) may be pre-registered with the
middlebox while provisioning the middlebox function. Either
the agent or the middlebox can choose to initiate a connection
prior to any data traffic. Alternately, either party (middlebox
or the MIDCOM agent) may choose to initiate a connection only
upon noticing the application specific traffic.
Coupling MIDCOM agents with the middlebox resources requires
a means of reflecting that into the resource description table
of the middlebox. In the case of a firewall, for example, the
ACL tuple may me altered to reflect the optional ALG presence.
The revised ACL may look something like the following.
(<Session-Direction>, <Source-Address>, <Destination-Address>,
<IP-Protocol>, <Source-Port>, <Destination-Port>, <ALG>)
The reader should note that this is an illustrative example and
not necessarily the actual definition of an ACL tuple. The formal
description of the ACL is yet to be devised. Agent accessibility
information should also be provisioned. For a MIDCOM agent,
accessibility information includes the IP address, trust level,
host authentication parameters and message authentication
parameters. Once a connection is established between a middlebox
and a MIDCOM agent, that connection should be usable with multiple
instances of the application(s), as appropriate. Note, all of this
could be captured in an agent profile for ease of management.
The technique described above is necessary for the
pre-registration of MIDCOM agents with the middlebox. However,
it is possible to retain the provisioning on middlebox
unchanged, by requiring MIDCOM agents to initiate the
connection to middlebox. When Middlebox notices an incoming
MIDCOM connection, and the middlebox has no prior profile
of the MIDCOM agent, the middlebox will consult its Policy
Server for authenticity, authorization and trust guidelines
for the connection.
At the end of the MIDCOM session, it should be possible for either
the middlebox or the agent to disconnect. MIDCOM session
disconnection may be prompted by a successful termination or
failure of some sort.
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It should be possible for the agent to deregister itself from the
middlebox, which means that the agent is going out of service and
will not be available for further MIDCOM operations. Alternately,
a policy server may notify a middlebox that a particular MIDCOM
agent is no longer authorized for a particular set of sessions
any longer.
7.0. MIDCOM Framework Illustration using an In-Path agent
In figure 3 below, we consider SIP application (Refer [SIP]) to
illustrate the operation of MIDCOM protocol. Specifically, the
application assumes a caller, external to a private domain,
initiates the call. Middlebox is assumed to be located at the
edge of the private domain. A SIP phone (SIP User Agent
Client/Server) inside the private domain is capable of receiving
calls from external SIP phones. The caller uses a SIP Proxy
node, located external to the private domain, as its outbound
proxy. No interior proxy is assumed for the callee. Lastly, the
external SIP proxy node is designated to host the MIDCOM agent
function.
Arrows 1 and 4 in the figure below refer to SIP call setup
exchange between the external SIP phone and the SIP proxy.
Arrows 6 and 7 refer to SIP call setup exchange between the SIP
proxy and the interior SIP phone and are assumed to be
traversing the middlebox. Arrows 2 and 3 below between the SIP
proxy and the middlebox refer to MIDCOM communication. Na and Nb
represent RTP/RTCP media traffic (Refer [RTP]) path in the
external network. Nc and Nd represent media traffic inside the
private domain.
_________
--->| SIP |<-----\
/ | Proxy | \
| |_________| |
1| | | 6|
| | | |
|4 |2 |3 |7
______________ | | | | _____________
| |<-/ _v_____^___ \->| |
| External | Na | | Nc | SIP Phone |
| SIP phone |>------->| MiddleBox |>------>| within |
| |<-------<|___________|<------<| Pvt. domain|
|____________| Nb Nd |____________|
Figure 4: MIDCOM framework illustration with In-Path SIP Proxy
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As for the SIP application, we make the assumption that the
middlebox is pre-configured to accept SIP calls into the
private SIP phone. Specifically, this would imply that the
middlebox implementing firewall service is pre-configured to
permit SIP calls (destination TCP or UDP port number set to
5060) into the private phone. Likewise, middlebox implementing
NAPT service would have been pre-configured to provide a port
binding to permit incoming SIP calls to be redirected to the
specific private SIP phone. I.e., the INVITE from the external
caller is not made to the private IP address, but to the NAPT
external address.
The objective of the MIDCOM agent in the following illustration
is to merely permit the RTP/RTSP media stream (Refer [RTP],
[RTSP]) through the middlebox, using the MIDCOM protocol
architecture outlined in the document. RTP/RTSP media stream,
When used in conjunction with SIP will typically result in two
independent media sessions - one from the caller to the callee
and another from the callee to the caller. These media sessions
are UDP based and will use dynamic ports. The dynamic ports
used for the media stream are specified in the SDP section
(Refer [SDP]) of SIP payload message. The MIDCOM agent will
parse the SDP section and use the MIDCOM protocol to
(a) open pinholes (i.e., permit RTP/RTSP session tuples) in a
middlebox implementing firewall service, or (b) create PORT
bindings and appropriately modify the SDP content to permit
the RTP/RTSP streams through a middlebox implementing NAT
service. The MIDCOM protocol should be sufficiently rich and
expressive to support the operations described under the
timelines.
Midcom agent Registration and connectivity between the
Midcom agent and the middlebox are not shown in the interest
of restricting the focus of the MIDCOM transactions to
enabling the middlebox to let the media stream through.
Policy server is also not shown in the diagram below or
on the timelines for the same reason.
The following subsections illustrate a typical timeline
sequence of operations that transpire with the various
elements involved in a SIP telephony application path.
Each subsection is devoted to a specific instantiation of a
middlebox service - NAPT (refer [NAT-TERM], [NAT-TRAD]),
firewall and a combination of both NAPT and firewall
are considered.
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7.1. Timeline flow - Middlebox implementing firewall service
In the following example, we will assume a middlebox implementing
a simple, stateless firewall service. We further assume that the
middlebox is pre-configured to permit SIP calls (destination TCP
or UDP port number set to 5060) into the private phone. The
following timeline will illustrate the operations that need to be
performed by the MIDCOM agent to permit the RTP/RTSP media stream
through the middlebox.
SIP Phone SIP Proxy Middlebox SIP Phone
(External) (In-Path (FIREWALL (private)
MIDCOM agent) Service) |
| | | |
|----INVITE------>| | |
| | | |
| |--------INVITE---------------------->|
|<---100Trying----| | |
| | | |
| |<-----180Ringing---------------------|
|<--180Ringing----| | |
| |<-------200 OK-----------------------|
| | | |
| Identify end-2-end session| |
| parameters for the two | |
| RTP/RTCP sessions - | |
| Ext-to-Pri(RTP1, RTCP1) & | |
| Pri-to-ext(RTP2, RTCP2). | |
| | | |
| |+Permit RTP1, RTCP1 +>| |
| |<+RTP1, RTCP1 OKed++++| |
| |+Permit RTP2, RTCP2 +>| |
| |<+RTP2, RTCP2 OKed++++| |
| | | |
|<---200 OK ------| | |
|-------ACK------>| | |
| |-----------ACK---------------------->|
| | | |
|<===================RTP/RTCP==========================>|
| | | |
|-------BYE------>| | |
| |--------------------------BYE------->|
| | | |
| |<----------200 OK--------------------|
| | | |
| |++Remove permits to | |
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| | RTP1, RTP2 etc.++++>| |
| |<+Removed permits | |
| | to RTP1, RTP2 etc.++| |
| | | |
|<---200 OK-------| | |
| | | |
Legend: ++++ MIDCOM control traffic
---- SIP control traffic
==== RTP/RCTP media traffic
7.2. Timeline flow - Middlebox implementing NAPT service
In the following example, we will assume a middlebox implementing
NAPT service. We make the assumption that the middlebox is
pre-configured to redirect SIP calls to the specific private SIP
phone application. I.e., the INVITE from the external caller is
not made to the private IP address, but to the NAPT external
address. Let us say, the external phone's IP address is Ea, NAPT
middlebox external Address is Ma and the internal SIP phone's
private address is Pa. SIP calls to the private SIP phone will
arrive as TCP/UDP sessions with destination address and port set
to Ea and 5060 respectively. The middlebox will redirect these
datagrams to the internal SIP phone. The following timeline
will illustrate the operations necessary to be performed by the
MIDCOM agent to permit the RTP/RTCP media stream through the
middlebox.
Unlike firewall, NAT is stateful and strictly session oriented.
The reader may refer [NAT-FRAMEWORK] for a detailed
discussion of NAT managed stateful resources, including that of
NAT session-descriptor and NAT BIND. [NAT-Framework] also has
illustration of how an Out-of-path Midcom agent could interface
with NAT middlebox to gain access to middelbox resources and
request redirecting application specific traffic to the agent.
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SIP Phone SIP Proxy Middlebox SIP Phone
(External) (In-Path (NAPT (Private)
IP Addr:Ea MIDCOM agent) Service) IP addr:Pa
| | IP addr:Ma |
| | | |
|----INVITE------>| | |
| |++ Query Port-BIND | |
| | for (Ma, 5060) +++>| |
| |<+ Port-BIND reply | |
| | for (Ma, 5060) ++++| |
| | | |
| Determine the Internal | |
| IP address (Pa) of the | |
| callee. | |
| | |..redirected..|
| |--------INVITE--------|------------->|
|<---100Trying----| | |
| | | |
| |<-----180Ringing---------------------|
| | | |
| |++ Query NAT Session | |
| | Descriptor for | |
| | Ea-to-Pa SIP flow+>| |
| |<+ Ea-to-Pa SIP flow | |
| | Session Descriptor+| |
| | | |
|<--180Ringing----| | |
| |<-------200 OK-----------------------|
| | | |
| Identify end-2-end session| |
| parameters for the two | |
| RTP/RTCP sessions - | |
| Ext-to-Pri(RTP1, RTCP1) & | |
| Pri-to-ext(RTP2, RTCP2). | |
| Identify UDP port numbers | |
| on Pa for RTP1 (Port1) | |
| and for RTP2 (Port2) | |
| | | |
| |++Create port BINDs | |
| | for (pa, port1), | |
| | (Pa, Port2) ++++++>| |
| |<+Port BINDs created++| |
| | | |
| |++Create NAT Session | |
| | descriptors for | |
| | RTP1, RTP2 pointing | |
| | to SIP session ++++>| |
| |<+RTP1, RTP2 session | |
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| | descriptors created+| |
| | | |
| Modify the SDP | |
| parameters in "200 OK" | |
| with NAPT PORT-BINDs | |
| for port1 & port2. | |
| | | |
|<---200 OK ------| | |
| | | |
|-------ACK------>| | |
| | | |
| Modify the SDP payload | |
| parameters in "ACK" | |
| | |..redirected..|
| |-----------ACK--------|------------->|
| | | |
| | | |
|<===================RTP/RTCP============|=============>|
| | | |
|-------BYE------>| | |
| | | |
| Modify SDP payload | |
| parameters in BYE | |
| | | |
| |----------------------|-----BYE----->|
| | | |
| |<----------200 OK--------------------|
| | | |
| |+++Terminate the SIP | |
| | Session bundle +++>| |
| |<++SIP Session bundle | |
| | terminated ++++++++| |
| | | |
| Modify SDP | |
| parameters in "200 OK" | |
| | | |
|<---200 OK-------| | |
| | | |
Legend: ++++ MIDCOM control traffic
---- SIP control traffic
==== RTP/RTCP media traffic
7.3. Timeline flow - Middlebox implementing NAPT and firewall
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In the following example, we will assume a middlebox
implementing a combination of a stateless firewall and a
stateful NAPT service. We make the assumption that the
middlebox is configured to translate the IP and TCP headers
of the initial SIP session into the private SIP phone and,
the firewall is configured to permit the initial SIP session.
In the following time line, it may be noted that the firewall
description is based on packet fields on the wire (ex: as seen
on the external interface of the middlebox). In order to
ensure correct behavior of the individual services, you will
notice that NAT specific MIDCOM operations precede firewall
specific operations on the MIDCOM agent. This is noticeable in
the time line below when the MIDCOM agent processes the
"200 OK" from the private SIP phone. The MIDCOM agent initially
requests the NAT service on the middlebox to set up port-BIND
and session-descriptors for the media stream in both directions.
Subsequent to that, the MIDCOM agent determines the session
parameters (i.e, the dynamic UDP ports) for the media stream,
as viewed by the external interface and requests the firewall
service on the middlebox to permit those sessions through.
SIP Phone SIP Proxy Middlebox SIP Phone
(External) (In-Path (NAPT & (Private)
IP Addr:Ea MIDCOM agent) firewall IP addr:Pa
| | Services) |
| | IP addr:Ma |
| | | |
|----INVITE------>| | |
| |++ Query Port-BIND | |
| | for (Ma, 5060) +++>| |
| |<+ Port-BIND reply | |
| | for (Ma, 5060) ++++| |
| | | |
| Determine the Internal | |
| IP address (Pa) of the | |
| callee. | |
| | |..redirected..|
| |--------INVITE--------|------------->|
|<---100Trying----| | |
| | | |
| |<-----180Ringing---------------------|
| | | |
| |++ Query NAT Session | |
| | Descriptor for | |
| | Ea-to-Pa SIP flow+>| |
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| |<+ Ea-to-Pa SIP flow | |
| | Session Descriptor+| |
| | | |
|<--180Ringing----| | |
| |<-------200 OK-----------------------|
| | | |
| Identify end-2-end session| |
| parameters for the two | |
| RTP/RTCP sessions - | |
| Ext-to-Pri(RTP1) | |
| and Pri-to-ext(RTP2). | |
| Identify UDP port numbers | |
| on Pa for RTP1 (Port1) | |
| and for RTP2 (Port2) | |
| | | |
| |+Create NAT port-BINDs| |
| | for (Pa, Port1), | |
| | (Pa, Port2) ++++++++>| |
| |<+Port BINDs created++| |
| | | |
| |++Create NAT Session | |
| | descriptors for | |
| | RTP1, RTP2;Set their| |
| | parent session to | |
| | SIP ctrl session ++>| |
| |<+RTP1, RTP2 session | |
| | descriptors created+| |
| | | |
| Extract session parameters| |
| for the two media streams | |
| as viewed on the NAPT | |
| external interface. Say, | |
| these are F-RTP1 and | |
| F-RTP2, reflecting RTP1 | |
| and RTP2 respectively. | |
| | | |
| |++Permit F-RTP1, | |
| | F-RTP2 sessions +++>| |
| |<+F-RTP1,F-RTP2 OKed++| |
| | | |
| Modify the SDP | |
| parameters in "200 OK" | |
| with NAPT PORT-BINDs | |
| for port1 & port2. | |
| | | |
|<---200 OK ------| | |
| | | |
|-------ACK------>| | |
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| | | |
| | |..redirected..|
| |-----------ACK--------|------------->|
| | | |
| | | |
|<===================RTP/RTCP============|=============>|
| | | |
|-------BYE------>| | |
| | | |
| Modify SDP payload | |
| parameters in BYE | |
| | | |
| |----------------------|-----BYE----->|
| | | |
| |<----------200 OK--------------------|
| | | |
| |+++Terminate the SIP | |
| | Session bundle +++>| |
| |<++SIP Session bundle | |
| | terminated ++++++++| |
| | | |
| |++Remove permits to | |
| | F-RTP1, F-RTP2 ++++>| |
| |<+Removed permits | |
| | to F-RTP1, F-RTP2+++| |
| | | |
| Modify SDP | |
| parameters in "200 OK" | |
| | | |
|<---200 OK-------| | |
| | | |
Legend: ++++ MIDCOM control traffic
---- SIP control traffic
==== RTP/RCTP media traffic
8.0. MIDCOM framework illustration with an Out-Of-path FTP Agent
In the following figure, an FTP client inside a private domain
connects via a middlebox to an external FTP server. The middlebox
is assumed to implement NAPT and firewall functions. The FTP
traffic is addressed directly to the external FTP server. The Arrow
labeled 1 indicates a registration via the MIDCOM protocol in
which the Out-of-Path FTP agent indicates that it would like to
receive TCP traffic directed to or from port 21 (FTP control). The
OOP agent may be located either inside the private domain or
external to the domain.
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The FTP control traffic traversing the middlebox is diverted by
the middlebox to the Out-of-Path FTP agent for FTP control payload
processing. Diverted control traffic is indicated by Arrow 2. The
OOP agent parses the FTP control commands and responses and possibly
modifies, as appropriate and forwards the traffic over to the server
and/or the client. Neither of the end-hosts is aware of the OOP Agent
or the middlebox in transit.
At some point, the Client sends a PORT command to the Server,
indicating that the Server should create a TCP connection from the
Server to the Client. This port command specifies an IP address and
port number to which the Server should connect. The IP address may
be a private IP address, if the client is located in a privately
addressed domain.
The OOP agent parses the PORT command, and carries out appropriate
MIDCOM transactions (Arrow 4) to discover any changes to the IP
address required, to request a new NAPT port binding if necessary,
and to open a suitable pinhole allowing the connection from the
Server to the dynamically allocated port number on the Client to
succeed. The (perhaps modified) PORT command is then sent on to the
Server, which responds by connecting to the indicated IP address and
port, which will now flow through the middlebox to the Client.
---------------
| Out-of-Path |
| (OOP) FTP |
| Agent |
|_____________|
| ^ | |
| | | |
|1 |2 |3 |4
______________ | | | | _____________
| | _v__|__v__v_ | |
| FTP client | Ctrl | | Ctrl | External |
| within the |<------->| MiddleBox |<------>| FTP Server |
| Pvt. domain|<------->|___________|<------>| |
|____________| Data Data |____________|
Ctrl - indicates the FTP control traffic, which
is transparently diverted to the OOP agent (2 and 3)
Data - indicates the FTP data traffic, which flows
directly through the middlebox between the FTP
end hosts (i.e., FTP client and Server)
Figure 5: MIDCOM framework illustration Out-of-Path FTP agent
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8.1. Timeline Flow - Middlebox implementing NAPT and Firewall
In the following figure, an end-host inside the private network
at address(pa) 10.0.0.4 wishes to communicate with an external FTP
server with an IP address Ea. The Middlebox provides public IP
address(Ma) 209.46.41.66 for external communication by private
hosts.
The middlebox diverts the FTP control traffic to the OOP agent.
The OOP agent, in turn, reviews the datagrams and optionally
modify as appropriate and redirects the datagrams right back to
the middlebox. The OOP agent may need to update even the TCP
SYNs and ACKs (i.e., datagrams with no application specific
payload) in the event the agent had to rewrite the address
content in the payload and the payload length changed as a result.
FTP-client OOP FTP Middlebox (NAPT & FTP Server
(Private) Agent Firewall Services) (External)
IP addr(Pa): | IP addr(Ma): IP addr: Ea
10.0.0.4 | 209.46.41.66 |
| | | |
| | | |
| |++Attach as FTP ALG+++>| |
| | | |
| |<+++++ OK +++++++++++++| |
| | | |
| The OOP FTP Agent attaches with middlebox & |
| is authorized to process FTP control |
| traffic from private hosts (or any set |
| of hosts adhering to a certain policy) |
| | | |
| | | |
The FTP client connects to the external FTP server. The middlebox
would have created a PORT BIND and an FTP control session resource
with the appropriate translation parameters.
| | | |
| PORT 10,0,0,4,4,9 | |
|--------------------------------------| |
| | | |
| |<## Ctrl-Pkt diverted #| |
| | | |
| |++Query NAT Session | |
| | Descriptor for | |
| | Pa-to-Ea FTP flow+++>| |
| | | |
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| |<+Pa-to-Ea FTP flow | |
| | Session Descriptor+++| |
| | | |
| |++Create NAT port-BIND | |
| | for (Pa, 1033) +++++>| |
| | | |
| |<+Port BINDs created | |
| | with (Ma, 15324)+++++| |
| | | |
| |++Create NAT Session | |
| | descriptor for the | |
| | Data session from Ea | |
| | to (Ma, 15324);Set | |
| | Parent session to | |
| | FTP-Ctrl session +++>| |
| | | |
| |<+FTP-Data session | |
| | descriptor created+++| |
| | | |
| |++Permit FTP data | |
| | session from Ea to | |
| | (Ma, 15324)+++++++++>| |
| | | |
| |<+Data session OKed++++| |
| | | |
| |### Modifed Control | |
| | Pkt forwarded #######################>|
| | | |
|<===FTP Data traffic between Pa & Ea==|=================>|
| | | |
| | | |
Legend: ++++ MIDCOM control traffic
##### Diverted datagrams between
---- FTP control traffic
==== FTP data traffic
The above flow does not indicate all packets as diverted, only
the important ones (e.g. the datagram with the PORT command in
the payload). It is safe to assume that all control packets are
diverted from the middlebox to the OOP Agent via the datagram
diversion component of the middlebox.
Note that the FTP data traffic is not diverted to the OOP Agent.
This is because the OOP agent does not assign a diversion function
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associated with the data session while at the instance creating the
FTP-Data session. This is an essential feature, since we allow the
middlebox to move the data about, while the Agent intervention is
limited just to the control session.
9.0. Operational considerations
9.1. Multiple MIDCOM connections between agents and middlebox
A middlebox cannot be assumed to be a simple device
implementing just one middlebox function and no more than a
couple of interfaces. Middleboxes often combine multiple
intermediate functions into the same device and have the
ability to provision individual interfaces of the same device
with different sets of functions and varied provisioning for
the same function across the interfaces.
As such, a MIDCOM agent ought to be able to have a single
MIDCOM connection with a middlebox and use the MIDCOM
interface on the middlebox to interface with different
services on the same middlebox interface.
9.2. MIDCOM agent registration with a middlebox
A MIDCOM agent may be pre-configured on a middlebox as a
trusted entity. In the case where a MIDCOM agent is not
pre-configured, a policy server should be made available
to the middlebox, so the middlebox can consult the Policy
Server for authorization to accept requests from the agent.
A middlebox should be capable of connecting to more than
a single MIDCOM agent.
9.3. Asynchronous notification to MIDCOM agents
Asynchronous notification by the middlebox to a MIDCOM agent
can be useful for events such as Session creation, Session
termination, MIDCOM protocol failure, Middlebox function
failure or any other significant event. Independently, ICMP
error codes can also be useful to notify transport layer
failures to the agents.
In addition, periodic notification of statistics update would
also be a useful function that would be beneficial to
certain types of agents.
9.4. Packet redirection
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Middleboxes should address the cases when MIDCOM agents are not
in-path of communication of the traffic in question. The agents
should be capable of returning the processed traffic to the
middlebox point of origin or forward it to the destination.
The middlebox should act accordingly when the traffic forwarded
earlier is received.
Packet forwarding by the agent might necessitate the packet to
traverse the middlebox for the second time. The middlebox should
simply forward the packet the second time around without
redirecting to the agent once again. Failing this, the packet
would simply be recycling between the two entities. The
progressing mechanisms to avoid such pitfalls should be addressed
by the MIDCOM protocol. A mechanism that maybe considered would
be to adopt a tunneling approach for packet redirection between
the agent and the middlebox.
9.5. Middleboxes supporting multiple services
A middlebox could be implementing a variety of services (e.g. NAT
and firewall) in the same box. Some of these services might have
inter-dependency on shared resources and sequence of operation.
Others may be independent of each other. Generally speaking,
the sequence in which these function operations may be performed
on datagrams is not within the scope of this document.
In the case of a middlebox implementing NAT and firewall
services, it is safe to state that the NAT operation will precede
firewall on the egress and will follow firewall on the ingress.
Further, firewall access control lists used by a firewall are
assumed to be based on session parameters as seen on the
interface supporting firewall service.
9.6. Signaling and Data traffic
The class of applications the MIDCOM architecture is addressing
focus around applications that have a combination of one or more
signaling and data traffic sessions. The signaling
may be done out-of-band using a dedicated stand-alone session
or may be done in-band with data session. Alternately, signaling
may also be done as a combination of both stand-alone and
in-band sessions.
SIP is an example of an application based on distinct signaling
and data sessions. SIP signaling session is used for call setup
between a caller and a callee. MIDCOM agent may be required to
examine/modify SIP payload content to administer the middlebox
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so as to let the media streams (RTP/RTSP based) through. MIDCOM
agent is not required to intervene in the data traffic.
Signaling and context specific Header information is sent in-band
within the same data stream for applications such as HTTP embedded
applications, sun-RPC (embedding a variety of NFS apps), Oracle
transactions (embedding oracle SQL+, MS ODBC, Peoplesoft) etc.
H.323 is an example of application that sends signaling in both
dedicated stand-alone session as well as in conjunction with data.
Q.931 traffic traverses middleboxes by virtue of static policy,
no MIDCOM control needed. Q.931 also negotiates ports for an
H.245 TCP stream. A MIDCOM agent is required to examine/modify
the contents of the H.245 so that H.245 can traverse it.
H.245 traverses the middlebox and also carries Open Logical
Channel information for media data. So the MIDCOM agent is once
again required to examine/modify the payload content needs to
let the media traffic flow.
The MIDCOM architecture takes into consideration, supporting
applications with independent signaling and data sessions as
well as applications that have signaling and data communicated
over the same session.
In the cases where signaling is done on a single stand-alone
session, it is desirable to have a MIDCOM agent interpret the
signaling stream and program the middlebox (that transits the
data stream) so as to let the data traffic through uninterrupted.
10. Applicability Statement
Middleboxes may be stationed in a number of topologies. However, the
signaling framework outlined in this document may be limited to only
those middleboxes that are located in a DMZ (De-Militarized Zone) at
the edge of a private domain, connecting to the Internet.
Specifically, the assumption is that you have a single middlebox
(running NAT or firewall) along the application route. Discovery of
middlebox along application route is outside the scope of this
document. It is conceivable to have middleboxes located between
departments within the same domain or inside service provider's
domain and so forth. However, care must be taken to review each
individual scenario and determine the applicability on a
case-by-case basis.
The applicability may also be illustrated as follows. Real-time and
streaming applications such as Voice-Over-IP and peer-to-peer
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applications require administering firewall and NAT middleboxes to
let their media streams reach hosts inside a private domain. The
requirements are in the form of establishing a "pin-hole" to permit
a TCP/UDP session (the port parameters of which are dynamically
determined) through a firewall or retain an address/port bind in
the NAT device to permit connections to a port. These requirements
are met by current generation middleboxes using adhoc methods, such
as embedding application intelligence within a middlebox to
identify the dynamic session parameters and administering the
middlebox internally as appropriate. The objective of the MIDCOM
architecture is to create a unified, standard way to exercise this
functionality, currently existing in an ad-hoc fashion in some of
the middleboxes.
By adopting MIDCOM architecture, middleboxes will be able to
support newer applications they have not been able to support thus
far. MIDCOM architecture does not and MUST not, in anyway, change
the fundamental characteristic of the services supported on the
middlebox.
Typically, organizations shield a majority of their corporate
resources (such as end-hosts) from visibility to the external
network by the use of a De-Militarized Zone (DMZ) at the domain
edge. Only a portion of these hosts are allowed to be accessed by
the external world. The remaining hosts and their names are unique
to the private domain. Hosts visible to the external world and the
authoritative name server that maps their names to network
addresses are often configured within a DMZ (De-Militarized Zone)
in front of a firewall. Hosts and middleboxes within DMZ are
referred to as DMZ nodes.
Figure 4 below illustrates configuration of a private domain with
a DMZ at its edge. Actual configurations may vary. Internal hosts
are accessed only by users inside the domain. Middleboxes,
located in the DMZ may be accessed by agents inside or outside
the domain.
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\ | /
+-----------------------+
|Service Provider Router|
+-----------------------+
WAN |
Stub A .........|\|....
|
+---------------+
| NAT Middlebox |
+---------------+
|
| DMZ - Network
------------------------------------------------------------
| | | | |
+--+ +--+ +--+ +--+ +-----------+
|__| |__| |__| |__| | Firewall |
/____\ /____\ /____\ /____\ | Middlebox |
DMZ-Host1 DMZ-Host2 ... DMZ-Name DMZ-Web +-----------+
Server Server etc. |
|
Internal Hosts (inside the private domain) |
------------------------------------------------------------
| | | |
+--+ +--+ +--+ +--+
|__| |__| |__| |__|
/____\ /____\ /____\ /____\
Int-Host1 Int-Host2 ..... Int-Hostn Int-Name Server
Figure 6: DMZ network configuration of a private domain.
11. Acknowledgements
The authors wish to express their thanks and gratitude to the
following for their valuable critique, advice and input on an
earlier rough version of this document. Christian Huitema,
Joon Maeng, Jon Peterson, Mike Fisk, Matt Holdrege, Melinda
Shore, Paul Sijben, Philip Mart, Scott Brim and Richard Swale.
The authors owe special thanks to Eliot Lear for kick-starting
the e-mail discussion on used-case scenarios with a SIP
application flow diagram through a middlebox. Much thanks to Bob
Penfield, George Michaelson, Christopher Martin and others in the
MIDCOM work group for continuing with timeline discussion to better
understand the MIDCOM operations vis-a-vis application flows.
Last, but not the least, the authors owe much thanks to Melinda
Shore for her constant support, critique and unbiased feedback
throughout in making this document a better read.
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12. Security Considerations
[SEC-GUIDE] defines security goals as either communication
security related or systems security related. While the latter
is important and should be addressed as part of a comprehensive
security solution which could be based on the features and
behavior of the middlebox to such threats, it is considered to
be outside the scope of this document. A middlebox performing
packet filtering or NAT services requires secure access to its
controlled internal resources. This requirement falls under the
former goal. This section predominantly addresses what is
required to ensure secure access to the middlebox.
The secure access has a number of requirements: authorization,
authentication, integrity and confidentiality. Authorization
refers to whether a particular agent is authorized to signal
middlebox with requests for one or more applications adhering to
a certain policy profile. Failing the authorization process might
indicate resource theft attempt or failure due to administrative
and/or credential deficiencies. In either case, the middlebox
should take the proper measures to audit/log such attempts and
consult its designated policy server for the required action if
the middlebox is configured with one. Alternatively, the middlebox
may resort to a default service deny policy when a midcom agent
fails to prompt the required credentials. Section 6 discusses the
middlebox-policy server interactions in view of policy decisions.
Authentication refers to whether a particular agent can provide
enough credentials to authenticate itself to the middlebox and
if the middlebox has enough credentials to authenticate itself
to the agent. Since the middlebox is implementing a security
function as a service for a midcom agent, it needs to be sure
of the identity of the agent. Likewise, the agent needs to
confirm that a signaling request is served by the middlebox
supposed to render the service in order to provide a level
of service reliability to its customers. Credentials are used
to establish the identify of the endpoint and consequently an
authorization decision is drawn as to whether allowing the
midcom process to proceed. Failing to submit the required/valid
credentials once challenged may indicate a replay attack and in
which case a proper action is required by the middlebox such as
auditing, logging, consulting its designated policy server to
reflect such failure. Mechanisms based on secret keys
(public-key based or shared) or certificates through some
Certificate authority can be utilized to facilitate the
authentication process. Lack of strong credentials during the
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authentication process can seriously jeopardize the fundamental
service rendered by the middlebox.
Integrity refers to the messages carrying the midcom signaling
requests in order to ensure that integrity is maintained and
has not been accidentally or maliciously altered or destroyed.
While authentication and message integrity are two distinct
functionalities, they are closely related as most algorithms
require a secret key (public-based or shared) to complete the
authentication process and carry out the integrity checks of
exchanged messages.
To accommodate the authentication and integrity constraints of
the midcom signaling and to reuse existing transport-based
security solutions such as Authentication Header [RFC2402] MAY
be used when the threat environment requires strong authentication
and integrity protections, but does not require confidentiality.
Confidentiality refers to the messages carrying the midcom
signaling requests in order to ensure that the signaling requests
are accessible only to the authorized entity. When a middlebox
agent is deployed in an untrusted environment or to satisfy
stronger security policy requirements, confidentiality SHOULD be
applied to the signaling messages. When confidentiality is not
administered properly, the domains protected by the middlebox can
be at a serious risk due to the sensitivity of the midcom
signaling. To accommodate that, a transport-based encryption such
as ESP tunneling [RFC2406] MAY be deployed between the middlebox
and the agent. This will ensure the confidentiality and integrity
of midcom communications.
Lastly, there can be security vulnerability to the applications
traversing a middlebox when a resource on a middlebox is controlled
by multiple external agents. A middlebox service may be abruptly
disrupted due to malicious manipulation or incorrect implementation
of the middlebox or its agents of a certain shared resource by an
agent purporting to offer ALG service for a different middlebox
function. Care must be taken in the protocol design to ensure that
agents for one function do not abruptly step over resources impacting
a different function. Alternately, the severity of such
manifestations could be lessened when a single MIDCOM agent is
responsible for supporting all the middlebox services for an
application due to the reduced complexity and synchronization effort
in managing the middlebox resources.
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REFERENCES
[IETF-STD] Bradner, S., " The Internet Standards Process --
Revision 3", RFC 1602, IETF, October 1996.
[SIP] Handley, M., H. Schulzrinne, E. Schooler, and
J. Rosenberg, "SIP: Session Initiation Protocol",
RFC 2543, IETF, March 1999.
[SDP] Handley, M., and Jacobson, V., "SDP: session
description protocol", RFC 2327, IETF, April 1998.
[H.323] ITU-T Recommendation H.323. "Packet-based Multimedia
Communications Systems," 1998.
[RTP] Schulzrinne, H., S. Casner, R. Frederick, and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 1889, IETF, January 1996.
[RTSP] Schulzrinne, H., A. Rao, R. Lanphier: "Real Time
Streaming Protocol", RFC 2326, IETF, April 1998.
[FTP] J. Postel, J. Reynolds, "FILE TRANSFER PROTOCOL (FTP)",
RFC 959
[NAT-TERM] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[NAT-TRAD] Srisuresh, P. and Egevang, K., "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[NAT-COMP] Holdrege, M. and Srisuresh, P., "Protocol Complications
with the IP Network Address Translator", RFC 3027,
January 2001.
[NAT-PT] Tsirtsis, G. and Srisuresh, P., "Network Address
Translation - Protocol Translation (NAT-PT)",
RFC 2766, February 2000.
[NAT-FRAMEWORK] Srisuresh, P., "Framework for interfacing with
Network Address Translator", Work in progress, April
2001, <draft-ietf-nat-interface-framework-03.txt>
[MIDCOM-REQ] Swale, R.P., Mart, P.A. and Sijben, P., "Requirements
for the MIDCOM protocol", work in progress, April 2001,
<draft-ietf-midcom-requirements-01.txt>
Srisuresh, et al. [Page 36]
Internet-Draft MIDCOM Architecture & Framework May 2001
[APPL-ID] Bernet, Y. and Pabbati, R., "Application and Sub
Application Identity Policy Element for Use with
RSVP", RFC 2872, June 2000.
[RFC 1918] Rekhter, Y., Moskowitz, B., Karrenberg, D.,
de Groot, G. and E. Lear, "Address Allocation for
Private Internets", BCP 5, RFC 1918, February 1996.
[RFC 1700] J. Reynolds and J. Postel, "Assigned Numbers",
RFC 1700
[IPsec-AH] Kent, S., and R. Atkinson, "IP Authentication
Header", RFC 2402, November 1998.
[IPsec-ESP] Kent, S., and R. Atkinson, "IP Encapsulating
Security Payload (ESP)", RFC 2406, November 1998.
[TLS] Dierks, T., and Allen, C., "The TLS Protocol
Version 1.0", RFC 2246, January 1999.
[SEC-GUIDE] Rescorla, E., and B. Korver, "Guidlines for Writing
RFC Text on Security Considerations", Work in Progress,
March 2001, <draft-rescorla-sec-cons-03.txt>
Authors' Addresses
Pyda Srisuresh
Jasmine Networks
3061 Zanker Road, Suite B
San Jose, CA 95134
U.S.A.
EMail: srisuresh@yahoo.com
Jiri Kuthan
GMD Fokus
Kaiserin-Augusta-Allee 31
D-10589 Berlin, Germany
E-mail: kuthan@fokus.gmd.de
Jonathan Rosenberg
dynamicsoft
200 Executive Drive
Suite 120
Srisuresh, et al. [Page 37]
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West Orange, NJ 07052
U.S.A.
email: jdrosen@dynamicsoft.com
Andrew Molitor
Aravox technologies
4201 Lexington Avenue North, Suite 1105
Arden Hills, MN 55126
U.S.A.
voice: (651) 256-2700
email: amolitor@visi.com
Abdallah Rayhan
P.O. Box 3511 Stn C
Ottawa, ON, Canada K1Y 4H7
eMail: ar_rayhan@yahoo.ca
Srisuresh, et al. [Page 38]
