Network Working Group P. Srisuresh
Request for Comments: 3303 Kuokoa Networks
Category: Informational J. Kuthan
Fraunhofer Institute FOKUS
J. Rosenberg
dynamicsoft
A. Molitor
Aravox Technologies
A. Rayhan
Ryerson University
August 2002
Middlebox communication architecture and framework
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
A principal objective of this document is to describe the underlying
framework of middlebox communications (MIDCOM) to enable complex
applications through the middleboxes, seamlessly using a trusted
third party. This document and a companion document on MIDCOM
requirements ([REQMTS]) have been created as a precursor to
rechartering the MIDCOM working group.
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 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.
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1. Introduction
Intermediate devices requiring application intelligence are the
subject of this document. These devices are referred to as
middleboxes throughout the document. Many of these devices enforce
application specific policy based functions such as packet filtering,
VPN (Virtual Private Network) 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),
independent of applications. Application Level Gateways (ALGs) are
used in conjunction with NAT to examine and optionally modify
application payload so the end-to-end application behavior remains
unchanged for many of the applications traversing NAT middleboxes.
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 Firewall and NAT
services. Nonetheless, the MIDCOM 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 focused on
services such as firewall and NAT. The framework document includes
some explicit and implied requirements for the MIDCOM protocol.
However, it must be noted that these requirements are only a subset.
A separate requirements document lists the requirements in detail.
MIDCOM agents with application intelligence can assist the
middleboxes through the MIDCOM protocol in permitting applications
such as FTP, SIP and H.323. The communication between a MIDCOM agent
and a middlebox will not be noticeable 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 or MIDCOM agents in the
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path of an application instance is outside the scope of this
document. Further, any communication amongst middleboxes is also
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. Section
7 is an illustration of SIP flows using a MIDCOM framework in which
the MIDCOM agent is co-resident on a SIP proxy server. Section 8
addresses operational considerations in deploying a protocol adhering
to the framework described here. Section 9 is an applicability
statement, scoping the location of middleboxes. Section 11 outlines
security considerations for the middlebox in view of the MIDCOM
framework.
2. Terminology
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 by a network intermediary that may require 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
A Middlebox is a network intermediate device that implements one or
more of the middlebox services. A NAT middlebox is a middlebox
implementing NAT service. A firewall middlebox is a middlebox
implementing firewall service.
Traditional middleboxes embed application intelligence within the
device to support specific application traversal. Middleboxes
supporting the MIDCOM protocol will be able to externalize
application intelligence into MIDCOM agents. In reality, some of the
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middleboxes may continue to embed application intelligence for
certain applications and depend on MIDCOM protocol and MIDCOM agents
for the support of remaining applications.
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. Transparent routing here refers to modifying
end-node addresses en-route and maintaining state for these updates
so that when a datagram leaves one realm and enters another,
datagrams pertaining to a session are forwarded to the right end-host
in either realm. Refer to [NAT-TERM] for the definition of
Transparent routing, various NAT types, and the associated terms in
use. Two types of NAT are most common. Basic-NAT, where only an IP
address (and the related IP, TCP/UDP checksums) of packets is altered
and NAPT (Network Address Port Translation), where both an IP address
and a transport layer identifier, such as a TCP/UDP port (and the
related IP, TCP/UDP checksums), are altered.
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.
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Applications such as FTP, SIP, and RTSP use a control session to
establish data sessions. These control and data sessions can take
divergent paths. While a proxy can intercept both the control and
data sessions, it might intercept only the control session. This is
often the case with real-time streaming applications such as SIP and
RTSP.
2.6. ALG
Application Level Gateways (ALGs) are entities that possess the
application specific intelligence and knowledge of an associated
middlebox function. An ALG examines application traffic in transit
and assists the middlebox in carrying out its function.
An ALG may be a 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 not visible to end-hosts,
unlike the proxies which are relay agents terminating sessions with
both end-hosts. ALGs do not terminate sessions 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 a 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 to 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 functions, logically
external to a middlebox. MIDCOM agents possess a combination of
application awareness and knowledge of the middlebox function. This
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combination enables the agents to facilitate traversal of the
middlebox by the application's packets. A MIDCOM agent may interact
with one or more middleboxes.
Only "In-Path MIDCOM agents" are considered in this document. In-
Path MIDCOM agents are agents which are within the path of those
datagrams that the agent needs to examine and/or modify in fulfilling
its role as a MIDCOM agent. "Within the path" here simply means that
the packets in question flow through the node that hosts the agent.
The packets may be addressed to the agent node at the IP layer.
Alternatively they may not be addressed to the agent node, but may be
constrained by other factors to flow through it. In fact, it is
immaterial to the MIDCOM protocol which of these is the case. Some
examples of In-Path MIDCOM agents are application proxies, gateways,
or even end-hosts that are party to the application.
Agents not resident on nodes that are within the path of their
relevant application flows are referred to as "Out-of-Path (OOP)
MIDCOM agents" and are out of the scope of this document.
2.9. MIDCOM PDP
MIDCOM Policy Decision Point (PDP) is primarily a Policy Decision
Point(PDP), as defined in [POL-TERM]; and also acts as a policy
repository, holding MIDCOM related policy profiles in order to make
authorization decisions. [POL-TERM] defines a PDP as "a logical
entity that makes policy decisions for itself or for other network
elements that request such decisions"; and a policy repository as "a
specific data store that holds policy rules, their conditions and
actions, and related policy data".
A middlebox and a MIDCOM PDP may communicate further if the MIDCOM
PDP's policy changes or if a middlebox needs further information.
The MIDCOM PDP may, at anytime, notify the middlebox to terminate
authorization for an agent.
The protocol facilitating the communication between a middlebox and
MIDCOM PDP need not be part of the MIDCOM protocol. Section 6 in the
document addresses the MIDCOM PDP interface and protocol framework
independent of the MIDCOM framework.
Application specific policy data and policy interface between an
agent or application endpoint and a MIDCOM PDP is out of bounds for
this document. The MIDCOM PDP issues addressed in the document are
focused at an aggregate domain level as befitting the middlebox. For
example, a SIP MIDCOM agent may choose to query a MIDCOM PDP for the
administrative (or corporate) domain to find whether a certain user
is allowed to make an outgoing call. This type of application
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specific policy data, as befitting an end user, is out of bounds for
the MIDCOM PDP considered in this document. It is within bounds,
however, for the MIDCOM PDP to specify the specific end-user
applications (or tuples) for which an agent is permitted to be an
ALG.
2.10. Middlebox Communication (MIDCOM) protocol
The protocol between a MIDCOM agent and a middlebox allows the MIDCOM
agent to invoke services of the middlebox and allow the middlebox to
delegate application specific processing to the 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.
2.11. MIDCOM agent registration
A MIDCOM agent registration is defined as the process of provisioning
agent profile information with the middlebox or a MIDCOM PDP. MIDCOM
agent registration is often a manual operation performed by an
operator rather than the agent itself.
A MIDCOM agent profile may include agent authorization policy (i.e.,
session tuples for which the agent is authorized to act as ALG),
agent-hosting-entity (e.g., Proxy, Gateway, or end-host which hosts
the agent), agent accessibility profile (including any host level
authentication information), and security profile (for the messages
exchanged between the middlebox and the agent).
2.12. MIDCOM session
A MIDCOM session is defined to be a lasting association between a
MIDCOM agent and a middlebox. The MIDCOM session is not assumed to
imply any specific transport layer protocol. Specifically, this
should not be construed as referring to a connection-oriented TCP
protocol.
2.13. Filter
A filter is packet matching information that identifies a set of
packets to be treated a certain way by a middlebox. This definition
is consistent with [POL-TERM], which defines a filter as "A set of
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terms and/or criteria used for the purpose of separating or
categorizing. This is accomplished via single- or multi-field
matching of traffic header and/or payload data".
5-Tuple specification of packets in the case of a firewall and 5-
tuple specification of a session in the case of a NAT middlebox
function are examples of a filter.
2.14. Policy action (or) Action
Policy action (or Action) is a description of the middlebox
treatment/service to be applied to a set of packets. This definition
is consistent with [POL-TERM], which defines a policy action as
"Definition of what is to be done to enforce a policy rule, when the
conditions of the rule are met. Policy actions may result in the
execution of one or more operations to affect and/or configure
network traffic and network resources".
NAT Address-BIND (or Port-BIND in the case of NAPT) and firewall
permit/deny action are examples of an Action.
2.15. Policy rule(s)
The combination of one or more filters and one or more actions.
Packets matching a filter are to be treated as specified by the
associated action(s). The Policy rules may also contain auxiliary
attributes such as individual rule type, timeout values, creating
agent, etc.
Policy rules are communicated through the MIDCOM protocol.
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 |
| co-resident on| | co-resident |
| Proxy Server | | on Appl. GW |
+---------------+ +--------------+
^ ^
| | +--------+
MIDCOM | | | MIDCOM |
Protocol | | +-| PDP |
| | / +--------+
+-------------+ | | /
| MIDCOM agent| | | /
| co-resident | | | /
| on End-hosts|<-+ | | /
+-------------+ | | | |
v v v v
+-------------------------------------------+
| Middlebox Communication |Policy |
| Protocol (MIDCOM) Interface |Interface |
+----------+--------+-----------+-----------+
Middlebox | | | | |
Functions | Firewall | NAT | VPN | Intrusion |
| | | tunneling | Detection |
+----------+--------+-----------+-----------+
Middlebox | Middlebox function specific policy rule(s)|
Managed | and other attributes |
Resources | |
+-------------------------------------------+
Figure 1: MIDCOM agents interfacing with a middlebox
Firewall ACLs, NAT-BINDs, NAT address-maps and Session-state are a
few of the middlebox function specific policy rules. A session state
may include middlebox function specific attributes, such as timeout
values, NAT translation parameters (i.e., NAT-BINDS), and so forth.
As Session-state may be shared across middlebox functions, a
Session-state may be created by a function, and terminated by a
different function. For example, a session-state may be created by
the firewall function, but terminated by the NAT function, when a
session timer expires.
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
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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 invoke services of the middlebox and allow the
middlebox to delegate application specific processing to the 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, 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 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 session setup phase, run-time
session phase, and a session termination phase.
Session setup must be preceded by registration of the MIDCOM agent
with either the middlebox or the MIDCOM PDP. The MIDCOM agent access
and authorization profile may either be pre-configured on the
middlebox (or) listed on a MIDCOM PDP; the middlebox is configured to
consult. MIDCOM shall be a client-server protocol, initiated by the
agent.
A MIDCOM session may be terminated by either of the parties. A
MIDCOM session termination may also be triggered by (a) the middlebox
or the agent going out of service and not being available for further
MIDCOM operations, or (b) the MIDCOM PDP notifying the middlebox that
a particular MIDCOM agent is no longer authorized.
The MIDCOM protocol data exchanged during run-time is governed
principally by the middlebox services the protocol supports.
Firewall and NAT middlebox services are considered in this document.
Nonetheless, the MIDCOM framework is designed to be extensible to
support the deployment of other services as well.
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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. Specifically, the focus is
restricted to just the In-Path agents.
In-Path MIDCOM agents are MIDCOM agents that are located naturally
within the message path of the application(s) they are associated
with. 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 a
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, or a specific
application traversing the middlebox 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 sessions. For example, FTP control and Data sessions are
interjected by an FTP proxy server.
However, proxies may also be interjecting just the control session
and not the data sessions, 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
functions 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 the MIDCOM protocol to be an effective MIDCOM agent. Figure 2
below illustrates a scenario where the in-path MIDCOM agents
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interface with the middlebox. Let us say, the MIDCOM PDP has pre-
configured the in-path proxies as trusted MIDCOM agents on the
middlebox and the packet filter implements a '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 proxies and the MIDCOM PDP 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.
+-----------+
| MIDCOM |
| PDP |~~~~~~~~~~~~~|
+-----------+ \
\
+--------+ \
| SIP |___ \
________| Proxy | \ Middlebox \
/ +--------+.. | +--------------------+
| : | MIDCOM | | |
| RTSP +---------+ :..|........| MIDCOM | POLICY |
SIP | ____| RTSP |.....|........| PROTOCOL | INTER- |
| / | Proxy |___ | | INTERFACE | FACE |
| | +---------+ \ \ |--------------------|
| | \ \______| |__SIP
| | \________| |__RTSP
| | ---| FIREWALL |--->--
+-----------+ /---| |---<--
+-----------+| Data streams // +--------------------+
+-----------+||---------->----// |
|end-hosts ||-----------<----- .
+-----------+ (RTP, RTSP data, etc.) |
. Outside the
Within a private domain | private domain
Legend: ---- Application data path datagrams
____ Application control path datagrams
.... Middlebox Communication Protocol (MIDCOM)
~~~~ MIDCOM PDP Interface
|
. private domain Boundary
|
Figure 2: In-Path MIDCOM Agents for middlebox Communication
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5.1. End-hosts as In-Path MIDCOM agents
End-hosts are another variation of In-Path MIDCOM agents. Unlike
Proxies, End-hosts are a direct party to the application and possess
all the end-to-end application intelligence there is to it. End-
hosts presumably 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 would be
one of manageability - upgrading all the end-hosts running a specific
application.
6.0. MIDCOM PDP functions
The functional decomposition of the MIDCOM architecture assumes the
existence of a logical entity, known as MIDCOM PDP, responsible for
performing authorization and related provisioning services for the
middlebox as depicted in figure 1. The MIDCOM PDP is a logical
entity which may reside physically on a middlebox or on a node
external to the middlebox. The protocol employed for communication
between the middlebox and the MIDCOM PDP is unrelated to the MIDCOM
protocol.
Agents are registered with a MIDCOM PDP for authorization to invoke
services of the middlebox. The MIDCOM PDP maintains a list of agents
that are authorized to connect to each of the middleboxes the MIDCOM
PDP supports. In the context of the MIDCOM Framework, the MIDCOM PDP
does not assist a middlebox in the implementation of the services it
provides.
The MIDCOM PDP acts in an advisory capacity to a middlebox, to
authorize or terminate authorization for an agent attempting
connectivity to the middlebox. The primary objective of a MIDCOM PDP
is to communicate agent authorization information, so as to ensure
that the security and integrity of a middlebox is not jeopardized.
Specifically, the MIDCOM PDP should associate a trust level with each
agent attempting to connect to a middlebox and provide a security
profile. The MIDCOM PDP should be capable of addressing cases when
end-hosts are agents to the middlebox.
6.1. Authentication, Integrity and Confidentiality
Host authenticity and individual message security are two distinct
types of security considerations. 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. Two-way authentication should be supported.
In some cases, the 2-way authentication may be tightly linked to the
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establishment of keys to protect subsequent traffic. Two-way
authentication is often required to prevent various active attacks on
the MIDCOM protocol and secure establishment of keying material.
Security services such as authentication, data integrity,
confidentiality and replay protection may be adapted to secure MIDCOM
messages in an untrusted domain. Message authentication is the same
as data origin authentication and is an affirmation that the sender
of the message is who it claims to be. Data integrity refers to the
ability to ensure that a message has not been accidentally,
maliciously or otherwise altered or destroyed. Confidentiality is
the encryption of a message with a key, so that only those in
possession of the key can decipher the message content. Lastly,
replay protection is a form of sequence integrity, so when an
intruder plays back a previously recorded sequence of messages, the
receiver of the replay messages will simply drop the replay messages
into bit-bucket. Certain applications of the MIDCOM protocol might
require support for non-repudiation as an option of the data
integrity service. Typically, support for non-repudiation is
required for billing, service level agreements, payment orders, and
receipts for delivery of service.
IPsec AH ([IPSEC-AH]) offers data-origin authentication, data
integrity and protection from message replay. IPsec ESP ([IPSEC-
ESP]) provides data-origin authentication to a lesser degree (same as
IPsec AH if the MIDCOM transport protocol turns out to be TCP or
UDP), message confidentiality, data integrity and protection from
replay. Besides the IPsec based protocols, there are other security
options as well. TLS based transport layer security is one option.
There are also many application-layer security mechanisms available.
Simple Source-address based security is a minimal form of security
and should be relied on only in the most trusted environments, where
those hosts will not be spoofed.
The MIDCOM message security shall use existing standards, whenever
the existing standards satisfy the requirements. Security shall be
specified to minimize the impact on sessions that do not use the
security option. Security should be designed to avoid introducing
and to minimize the impact of denial of service attacks. Some
security mechanisms and algorithms require substantial processing or
storage, in which case the security protocols should protect
themselves as well as against possible flooding attacks that
overwhelm the endpoint (i.e., the middlebox or the agent) with such
processing. For connection oriented protocols (such as TCP) using
security services, the security protocol should detect premature
closure or truncation attacks.
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6.2. Registration and deregistration of MIDCOM agents
Prior to allowing MIDCOM agents to invoke services of the middlebox,
a registration process must take place. Registration is a different
process than establishing a MIDCOM session. The former requires
provisioning agent profile information with the middlebox or a MIDCOM
PDP. Agent registration is often a manual operation performed by an
operator rather than the agent itself. Setting up MIDCOM session
refers to establishing a MIDCOM transport session and exchanging
security credentials between an agent and a middlebox. The transport
session uses the registered information for session establishment.
Profile of a MIDCOM agent includes agent authorization policy (i.e.,
session tuples for which the agent is authorized to act as ALG),
agent-hosting-entity (e.g., Proxy, Gateway or end-host which hosts
the agent), agent accessibility profile (including any host level
authentication information) and security profile (i.e., security
requirements for messages exchanged between the middlebox and the
agent).
MIDCOM agent profile may be pre-configured on a middlebox.
Subsequent to that, the agent may choose to initiate a MIDCOM session
prior to any data traffic. For example, MIDCOM agent authorization
policy for a middlebox service may be preconfigured by specifying the
agent in conjunction with a filter. In the case of a firewall, for
example, the ACL tuple may be altered to reflect the optional Agent
presence. The revised ACL may look something like the following.
(<Session-Direction>, <Source-Address>, <Destination-Address>, <IP-
Protocol>, <Source-Port>, <Destination-Port>, <Agent>)
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 session is established between a middlebox and a MIDCOM agent,
that session 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. The middlebox provisioning may
remain unchanged, if the middlebox learns of the registered agents
through a MIDCOM PDP. In either case, the MIDCOM agent should
initiate the session prior to the start of the application. If the
agent session is delayed until after the application has started, the
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agent might be unable to process the control stream to permit the
data sessions. When a middlebox notices an incoming MIDCOM session,
and the middlebox has no prior profile of the MIDCOM agent, the
middlebox will consult its MIDCOM PDP for authenticity,
authorization, and trust guidelines for the session.
7.0. MIDCOM Framework Illustration using an In-Path agent
In figure 3 below, we consider SIP applications (Refer [SIP]) to
illustrate the operation of the MIDCOM protocol. Specifically, the
application assumes that a caller, external to a private domain,
initiates the call. The 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 8 in the figure below refer to a SIP call setup exchange
between the external SIP phone and the SIP proxy. Arrows 4 and 5
refer to a SIP call setup exchange between the SIP proxy and the
interior SIP phone, and are assumed to be traversing the middlebox.
Arrows 2, 3, 6 and 7 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| |^ ^| 4|
| || || |
|8 2||3 7||6 |5
______________ | || || | _____________
| |<-/ _v|____|v___ \->| |
| External | Na | | Nc | SIP Phone |
| SIP phone |>------->| Middlebox |>------>| within |
| |<-------<|___________|<------<| Pvt. domain|
|____________| Nb Nd |____________|
Figure 3: MIDCOM framework illustration with In-Path SIP Proxy
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
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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/RTCP media stream (Refer [RTP]) through the
middlebox, when using the MIDCOM protocol architecture outlined in
the document. A SIP session typically establishes two RTP/RTCP media
streams - one from the callee to the caller and another from the
caller to the callee. 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 the SIP payload
message. The MIDCOM agent will parse the SDP section and use the
MIDCOM protocol to (a) open pinholes (i.e., permit RTP/RTCP session
tuples) in a middlebox implementing firewall service, or (b) create
PORT bindings and appropriately modify the SDP content to permit the
RTP/RTCP streams through a middlebox implementing NAT service. The
MIDCOM protocol should be sufficiently rich and expressive to support
the operations described under the timelines. The examples do not
show the timers maintained by the agent to keep the middlebox policy
rule(s) from timing out.
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. MIDCOM PDP 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.
7.1. Timeline flow - Middlebox implementing firewall service
In the following example, we will assume a middlebox implementing a
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
illustrates the operations performed by the MIDCOM agent, to permit
RTP/RTCP media stream through the middlebox.
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The INVITE from the caller (external) is assumed to include the SDP
payload. You will note that the MIDCOM agent requests the middlebox
to permit the Private-to-external RTP/RTCP flows before the INVITE is
relayed to the callee. This is because, in SIP, the calling party
must be ready to receive the media when it sends the INVITE with a
session description. If the called party (private phone) assumes
this and sends "early media" before sending the 200 OK response, the
firewall will have blocked these packets without this initial MIDCOM
signaling from the agent.
SIP Phone SIP Proxy Middlebox SIP Phone
(External) (MIDCOM agent) (FIREWALL (private)
| | Service) |
| | | |
|----INVITE------>| | |
| | | |
|<---100Trying----| | |
| | | |
| Identify end-2-end | |
| parameters (from Caller's | |
| SDP) for the pri-to-Ext | |
| RTP & RTCP sessions. | |
| (RTP1, RTCP1) | |
| | | |
| |+Permit RTP1, RTCP1 +>| |
| |<+RTP1, RTCP1 OKed++++| |
| | | |
| |--------INVITE---------------------->|
| | | |
| |<-----180 Ringing--------------------|
|<--180Ringing----| | |
| |<-------200 OK-----------------------|
| | | |
| Identify end-2-end | |
| parameters (from callee's | |
| SDP) for the Ext-to-Pri | |
| RTP and RTCP sessions. | |
| (RTP2, RTCP2) | |
| | | |
| |+Permit RTP2, RTCP2 +>| |
| |<+RTP2, RTCP2 OKed++++| |
| | | |
|<---200 OK ------| | |
|-------ACK------>| | |
| |-----------ACK---------------------->|
| | | |
|<===================RTP/RTCP==========================>|
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| | | |
|-------BYE------>| | |
| |--------------------------BYE------->|
| | | |
| |<----------200 OK--------------------|
| | | |
| |++Cancel permits to | |
| | RTP1, RTCP1, RTP2, | |
| | and RTCP2 +++++++++>| |
| |<+RTP1, RTP2, RTCP1 & | |
| | RTCP2 cancelled ++++| |
| | | |
|<---200 OK-------| | |
| | | |
Legend: ++++ MIDCOM control traffic
---- SIP control traffic
==== RTP/RTCP 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 the destination address and port set to Ma 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.
As with the previous example (section 7.1), the INVITE from the
caller (external) is assumed to include the SDP payload. You will
note that the MIDCOM agent requests the middlebox to create NAT
session descriptors for the private-to-external RTP/RTCP flows before
the INVITE is relayed to the private SIP phone (for the same reasons
as described in section 7.1). If the called party (private phone)
sends "early media" before sending the 200 OK response, the NAPT
middlebox will have blocked these packets without the initial MIDCOM
signaling from the agent. Also, note that after the 200 OK is
received by the proxy from the private phone, the agent requests the
middlebox to allocate NAT session descriptors for the external-to-
private RTP2 and RTCP2 flows, such that the ports assigned on the Ma
for RTP2 and RTCP2 are contiguous. The RTCP stream does not happen
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with a non-contiguous port. Lastly, you will note that even though
each media stream (RTP1, RTCP1, RTP2 and RTCP2) is independent, they
are all tied to the single SIP control session, while their NAT
session descriptors were being created. Finally, when the agent
issues a terminate session bundle command for the SIP session, the
middlebox is assumed to delete all associated media stream sessions
automagically.
SIP Phone SIP Proxy Middlebox SIP Phone
(External) (MIDCOM agent) (NAPT (Private)
IP Addr:Ea | Service) IP addr:Pa
| | IP addr:Ma |
| | | |
|----INVITE------>| | |
| | | |
|<---100Trying----| | |
| | | |
| |++ Query Port-BIND | |
| | for (Ma, 5060) +++>| |
| |<+ Port-BIND reply | |
| | for (Ma, 5060) ++++| |
| | | |
| |++ Query NAT Session | |
| | Descriptor for | |
| | Ea-to-Pa SIP flow+>| |
| |<+ Ea-to-Pa SIP flow | |
| | Session Descriptor+| |
| | | |
| Determine the Internal | |
| IP address (Pa) | |
| of the callee. | |
| | | |
| Identify UDP port numbers | |
| on Ea (Eport1, Eport1+1) | |
| for pri-to-ext RTP & RTCP | |
| sessions (RTP1, RTCP1) | |
| | | |
| |++Create NAT Session | |
| | descriptors for | |
| | RTP1, RTCP1; Set | |
| | parent session to | |
| | SIP-ctrl session ++>| |
| |<+RTP1, RTCP1 session | |
| | descriptors created+| |
| | | |
| | |..redirected..|
| |--------INVITE--------|------------->|
| | | |
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| |<-----180Ringing---------------------|
| | | |
|<--180Ringing----| | |
| |<-------200 OK-----------------------|
| | | |
| Identify UDP port numbers | |
| on Pa (Pport2, Pport2+1) | |
| for ext-to-pri RTP & RTCP | |
| sessions (RTP2, RTCP2) | |
| | | |
| |++Create consecutive | |
| | port BINDs on Ma | |
| | for (Pa, Pport2), | |
| | (Pa, Pport2+1) ++++>| |
| |<+Port BINDs created++| |
| | | |
| |++Create NAT Session | |
| | descriptors for | |
| | RTP2, RTCP2; Set | |
| | parent session to | |
| | SIP-ctrl session ++>| |
| |<+RTP2, RTCP2 session | |
| | descriptors created+| |
| | | |
| Modify the SDP | |
| parameters in "200 OK" | |
| with NAPT PORT-BIND | |
| for the RTP2 port on Ma. | |
| | | |
|<---200 OK ------| | |
| | | |
|-------ACK------>| | |
| | | |
| Modify IP addresses | |
| appropriately in the SIP | |
| header (e.g., To, from, | |
| Via, contact fields) | |
| | |..redirected..|
| |-----------ACK--------|------------->|
| | | |
| | | |
|<===================RTP/RTCP============|=============>|
| | | |
|-------BYE------>| | |
| | | |
| |----------------------|-----BYE----->|
| | | |
| |<----------200 OK--------------------|
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| | | |
| |+++Terminate the SIP | |
| | Session bundle +++>| |
| |<++SIP Session bundle | |
| | terminated ++++++++| |
| | | |
|<---200 OK-------| | |
| | | |
Legend: ++++ MIDCOM control traffic
---- SIP control traffic
==== RTP/RTCP media traffic
7.3. Timeline flow - Middlebox implementing NAPT and firewall
In the following example, we will assume a middlebox implementing a
combination of a firewall and a stateful NAPT service. We make the
assumption that the NAPT function is configured to translate the IP
and TCP headers of the initial SIP session into the private SIP
phone, and the firewall function 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) (MIDCOM agent) (NAPT & (Private)
IP Addr:Ea | firewall IP addr:Pa
| | Services) |
| | IP addr:Ma |
| | | |
|----INVITE------>| | |
| | | |
|<---100Trying----| | |
| | | |
| |++ Query Port-BIND | |
| | for (Ma, 5060) +++>| |
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RFC 3303 MIDCOM Architecture and Framework August 2002
| |<+ Port-BIND reply | |
| | for (Ma, 5060) ++++| |
| | | |
| |++ Query NAT Session | |
| | Descriptor for | |
| | Ea-to-Pa SIP flow+>| |
| |<+ Ea-to-Pa SIP flow | |
| | Session Descriptor+| |
| | | |
| Determine the Internal | |
| IP address (Pa) | |
| of the callee. | |
| | | |
| Identify UDP port numbers | |
| on Ea (Eport1, Eport1+1) | |
| for pri-to-ext RTP & RTCP | |
| sessions (RTP1, RTCP1) | |
| | | |
| |++Create NAT Session | |
| | descriptors for | |
| | RTP1, RTCP1; Set the| |
| | parent session to | |
| | point to SIP flow++>| |
| |<+RTP1, RTCP1 session | |
| | descriptors created+| |
| | | |
| |++Permit RTP1 & RTCP1 | |
| | sessions External to| |
| | middlebox, namely | |
| | Ma to Ea:Eport1, | |
| | Ma to Ea:Eport1+1 | |
| | sessions ++++++++++>| |
| |<+Ma to Ea:Eport1, | |
| | Ma to Ea:Eport1+1 | |
| | sessions OKed ++++++| |
| | | |
| | |..redirected..|
| |--------INVITE--------|------------->|
| | | |
| |<-----180Ringing---------------------|
| | | |
|<--180Ringing----| | |
| |<-------200 OK-----------------------|
| | | |
| Identify UDP port numbers | |
| on Pa (Pport2, Pport2+1) | |
| for ext-to-pri RTP & RTCP | |
| sessions (RTP2, RTCP2) | |
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| | | |
| |++Create consecutive | |
| | port BINDs on Ma | |
| | for (Pa, Pport2), | |
| | (Pa, Pport2+1) ++++>| |
| |<+Port BINDs created | |
| | on Ma as (Mport2, | |
| | Mport2+1) ++++++++++| |
| | | |
| |++Create NAT Session | |
| | descriptors for | |
| | RTP2, RTCP2; Set the| |
| | parent session to | |
| | point to SIP flow++>| |
| |<+RTP2, RTCP2 session | |
| | descriptors created+| |
| | | |
| Modify the SDP | |
| parameters in "200 OK" | |
| with NAPT PORT-BIND | |
| for RTP2 port on Ma. | |
| | | |
| |++Permit RTP2 & RTCP2 | |
| | sessions External | |
| | middlebox, namely | |
| | Ea to Ma:Mport2, | |
| | Ea to Ma:Mport2+1 | |
| | sessions ++++++++++>| |
| |<+Ea to Ma:Mport2, | |
| | Ea to Ma:Mport2 | |
| | sessions OKed ++++++| |
| | | |
|<---200 OK ------| | |
| | | |
|-------ACK------>| | |
| | |..redirected..|
| |-----------ACK--------|------------->|
| | | |
| | | |
|<===================RTP/RTCP============|=============>|
| | | |
|-------BYE------>| | |
| | | |
| |----------------------|-----BYE----->|
| | | |
| |<----------200 OK--------------------|
| | | |
| |+++Terminate the SIP | |
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| | Session bundle +++>| |
| |<++SIP Session bundle | |
| | terminated ++++++++| |
| | | |
| |++Cancel permits to | |
| | sessions External | |
| | middlebox, namely | |
| | Ma to Ea:Eport1, | |
| | Ma to Ea:Eport1+1 | |
| | Ea to Ma:Mport2, | |
| | Ea to Ma:Mport2+1 | |
| | sessions ++++++++++>| |
| |<+Removed permits to | |
| | sessions listed ++++| |
| | | |
|<---200 OK-------| | |
| | | |
Legend: ++++ MIDCOM control traffic
---- SIP control traffic
==== RTP/RTCP media traffic
8.0. Operational considerations
8.1. Multiple MIDCOM sessions 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
session with a middlebox and use the MIDCOM interface on the
middlebox to interface with different services on the same middlebox.
8.2. 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 various forms of data, such as
statistics update, would also be a useful function that would be
beneficial to certain types of agents.
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8.3. Timers on middlebox considered useful
When supporting the MIDCOM protocol, the middlebox is required to
allocate dynamic resources, as specified in policy rule(s), upon
request from agents. Explicit release of dynamically allocated
resources happens when the application session is ended or when a
MIDCOM agent requests the middlebox to release the resource.
However, the middlebox should be able to recover the dynamically
allocated resources, even as the agent that was responsible for the
allocation is not alive. Associating a lifetime for these dynamic
resources and using a timer to track the lifetime can be a good way
to accomplish this.
8.4. 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 on an interface will precede
a firewall on the egress and will follow a 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.
8.5. Signaling and Data traffic
The class of applications the MIDCOM architecture addresses 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, within a 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. A SIP signaling session is used for call setup
between a caller and a callee. A MIDCOM agent may be required to
examine/modify SIP payload content to administer the middlebox so as
to let the media streams (RTP/RTCP based) through. A MIDCOM agent is
not required to intervene in the data traffic.
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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 an application that sends signaling in both
dedicated stand-alone sessions, as well as in conjunction with data.
H.225.0 call signaling traffic traverses middleboxes by virtue of
static policy, no MIDCOM control needed. H.225.0 call signaling 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.
9. 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
a middlebox along an application route is outside the scope of this
document. It is conceivable to have middleboxes located between
departments within the same domain or inside the 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
applications, such as Napster and Netmeeting, require administering
firewalls and NAT middleboxes to let their media streams reach hosts
inside a private domain. The requirements are in the form of
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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 sessions 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 the 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 4: DMZ network configuration of a private domain.
10. Acknowledgements
The authors wish to thank Christian Huitema, Joon Maeng, Jon
Peterson, Mike Fisk, Matt Holdrege, Melinda Shore, Paul Sijben,
Philip Mart, Scott Brim and Richard Swale for their valuable
critique, advice and input on an earlier rough version of this
document. The authors owe special thanks to Eliot Lear for kick-
starting the e-mail discussion on use-case scenarios with a SIP
application flow diagram through a middlebox. Much thanks to Bob
Penfield, Cedric Aoun, Christopher Martin, Eric Fleischman, George
Michaelson, Wanqun Bao, and others in the MIDCOM work group for their
very detailed feedback on a variety of topics and adding clarity to
the discussion. Last, but not the least, the authors owe much thanks
to Mark Duffy, Scott Brim, Melinda Shore and others for their help
with terminology definition and discussing the embedded requirements
within the framework document.
Srisuresh, et al. Informational [Page 29]
RFC 3303 MIDCOM Architecture and Framework August 2002
11. Security Considerations
Discussed below are security considerations in accessing a middlebox.
Without MIDCOM protocol support, the premise of a middlebox operation
fundamentally requires the data to be in the clear, as the middlebox
needs the ability to inspect and/or modify packet headers and
payload. This compromises the confidentiality requirement in some
environments. Further, updating transport headers and rewriting
application payload data, in some cases, by NAT prevents the use of
integrity protection on some data streams traversing NAT middleboxes.
Clearly, this can pose a significant security threat to the
application in an untrusted transport domain.
The MIDCOM protocol framework removes the need for a middlebox to
inspect or manipulate transport payload. This allows applications to
better protect themselves end-to-end with the aid of a trusted MIDCOM
agent. This is especially the case when the agent is a resident on
the end-host. When an agent has the same end-to-end ability as the
end-host to interpret encrypted and integrity protected data,
transiting a middlebox can be encrypted and integrity protected. The
MIDCOM agent will still be able to interpret the data and simply
notify the middlebox of open holes, install NAT table entries, etc.
Note, however, the MIDCOM framework does not help with the problem of
NAT breaking IPsec since in this case the middlebox still modifies IP
and transport headers.
Security between a MIDCOM agent and a middlebox has a number of
components. Authorization, authentication, integrity and
confidentiality. Authorization refers to whether a particular agent
is authorized to signal a middlebox with requests for one or more
applications, adhering to a certain policy profile. Failing the
authorization process might indicate a 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 MIDCOM PDP 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 to MIDCOM PDP interactions in view
of policy decisions.
Authentication refers to confirming the identity of an originator for
all datagrams received from the originator. Lack of strong
credentials for authentication of MIDCOM messages between an agent
and a middlebox can seriously jeopardize the fundamental service
rendered by the middlebox. A consequence of not authenticating an
agent would be that an attacker could spoof the identity of a
"legitimate" agent and open holes in the firewall. Another would be
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RFC 3303 MIDCOM Architecture and Framework August 2002
that it could otherwise manipulate the state on a middlebox, creating
a denial-of-service attack by closing needed pinholes or filling up a
NAT table. A consequence of not authenticating the middlebox to an
agent is that an attacker could pose as a middlebox and respond to
NAT requests in a manner that would divert data to the attacker.
Failing to submit the required/valid credentials, once challenged,
may indicate a replay attack, in which case a proper action is
required by the middlebox such as auditing, logging, or consulting
its designated MIDCOM PDP to reflect such failure. A consequence of
not protecting the middlebox against replay attacks would be that a
specific pinhole may be reopened or closed by an attacker at will,
thereby bombarding end hosts with unwarranted data or causing denial
of service.
Integrity is required to ensure that a MIDCOM message has not been
accidentally or maliciously altered or destroyed. The result of a
lack of data integrity enforcement in an untrusted environment could
be that an imposter will alter the messages sent by an agent and
bring the middlebox to a halt or cause a denial of service for the
application the agent is attempting to enable.
Confidentiality of MIDCOM messages ensure that the signaling data is
accessible only to the authorized entities. When a middlebox agent
is deployed in an untrusted environment, lack of confidentiality will
allow an intruder to perform traffic flow analysis and snoop the
middlebox. The intruder could cannibalize a lesser secure MIDCOM
session and destroy or compromise the middlebox resources he
uncovered on other sessions. Needless to say, the least secure
MIDCOM session will become the achilles heel and make the middlebox
vulnerable to security attacks.
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 disrupted
due to conflicting directives from multiple agents associated with
different middlebox functions but applied to the same application
session. 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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RFC 3303 MIDCOM Architecture and Framework August 2002
References
[SIP] Rosenberg, J., Shulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., Schooler, E.,
"SIP: Session Initiation Protocol", RFC 3261, June 2002.
[SDP] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[H.323] ITU-T Recommendation H.323. "Packet-based Multimedia
Communications Systems," 1998.
[RTP] Schulzrinne, H., Casner, S., Frederick, R. and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 1889, January 1996.
[RTSP] Schulzrinne, H., Rao, A. and R. Lanphier: "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[FTP] Postel, J. and J. Reynolds, "File Transfer Protocol", STD
9, RFC 959, October 1985.
[NAT-TERM] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999.
[NAT-TRAD] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
2001.
[NAT-PT] Tsirtsis, G. and P. Srisuresh, "Network Address
Translation - Protocol Translation (NAT-PT)", RFC 2766,
February 2000.
[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 C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[POL-TERM] Westerinen, A., Schnizlein, J., Strassner, J., Scherling,
M., Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry,
J. and S. Waldbusser, "Terminology for Policy-Based
Management", RFC 3198, November 2001.
Srisuresh, et al. Informational [Page 32]
RFC 3303 MIDCOM Architecture and Framework August 2002
[REQMTS] Swale, R. P., Mart, P. A., Sijben, P., Brim, S. and M.
Shore, "Middlebox Communications (midcom) Protocol
Requirements", RFC 3304, August 2002.
Authors' Addresses
Pyda Srisuresh
Kuokoa Networks, Inc.
475 Potrero Ave.
Sunnyvale, CA 94085
EMail:
[email protected]
Jiri Kuthan
Fraunhofer Institute FOKUS
Kaiserin-Augusta-Allee 31
D-10589 Berlin, Germany
EMail:
[email protected]
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
U.S.A.
EMail:
[email protected]
Andrew Molitor
Aravox technologies
4201 Lexington Avenue North, Suite 1105
Arden Hills, MN 55126
U.S.A.
voice: (651) 256-2700
EMail:
[email protected]
Abdallah Rayhan
WINCORE Lab
Electrical and Computer Engineering
Ryerson University
350 Victoria Street
Toronto, ON M5B 2K3
EMail:
[email protected],
[email protected]
Srisuresh, et al. Informational [Page 33]
RFC 3303 MIDCOM Architecture and Framework August 2002
Full Copyright Statement
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Srisuresh, et al. Informational [Page 34]
