Network Working Group Eliot Lear
INTERNET-DRAFT Cisco Systems
Category: Informational
<draft-lear-middlebox-arch-00.txt>
January 3, 2001
A Middle Box Architectural Framework
1 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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2. Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
2 Abstract
It used to be reasonable to expect that any two points connected to
the Internet to have the ability to hold any communication. Such an
expectation has not be reasonable for quite some time, thanks to
firewalls, NATs, and other intermediate devices. Today, we
acknowledge a new architecture and we name the functional blocks of
that architecture as well as several ways to get ends to communicate,
and how two devices could expect to communicate with each other.
This document does not define the protocols involved.
3 Introduction
The IPv4 Internet consists of a network of interconnected networks
that may use public or private address space. The use of private
address space [BCP5] has broken the classic connection model that
applications use to speak to other devices. Similarly, many networks
are separated by firewalls.
Now we consider the components necessary for end to end
communications in this environment, as well as the forms of signaling
necessary for those communications to occur in a reliable way.
The reader should be familiar with RFCs 1918, 2663, and 2775. We do
not intend to fix all problems related to NATs or firewalls with this
architecture. For instance, one will not find below a way to save a
TCP connection in the face of a NAT failure.
3.1 Motivation
The currently envisioned sets of applications to make use of this
architecture are voice and video conferencing and telephony. Others
may follow. Today voice and video capture a small percentage of
total network usage. As demand for these uses grows it will become
more important that we have in place mechanisms that have proper
scaling properties.
3.2 Goals, Terms, and Limits
Here are our design goals:
1. Enable hosts within different administrative domains and address
realms to have end to end sessions.
2. The architecture must allow for multiple middle boxes that connect
one realm to multiple realms.
3. Enable this with a minimal amount of signaling from the end host,
and no new signaling beyond administratively defined boundaries.
4. The recommended methods must not radically alter the end host stack
below the application.
5. Work within the existing interior and exterior routing framework.
6. The mechanisms used must easily integrate with the existing
Internet mechanisms and services.
The three middle box processes we consider in this document are
discovery, diagnostics, and signaling. Discovery means determining
that one or more middle box sits between a host end the remote end of
a session. Once that box is discovered, either an end host or its
proxy will exchange information with it in order for the
communication to proceed. Diagnostics is the untimely discovery of a
problem involving a middle box. Discovery and diagnostics differ in
that an end host may receive a diagnostic message from one middle box
while it might need to discover a separate middle box in order for
the communication to proceed. The architecture we propose elides the
two functions.
No changes are made to the layer 3 routing mechanism, other than the
possible addition of a new ICMP message to be multiplexed back to the
responsible application. To do otherwise would dramatically increase
implementation cost and complexity, as well as the cost of
troubleshooting problems. While the host may signal for additional
information to and from the application layer, it cannot rely on new
layer 3 routing facilities within the network.
Nothing this architecture proposes may stop two oHOSTs from
communicating with each other without the help of a middle box.
Similarly, this architecture must not prevent two iHOSTS from
communicating, just as they would have previously. However, for all
the reasons listed in RFC 2775 it is not possible for a middle box to
enable all forms of communication between iHOSTs and oHOSTs. The
recommended method to clear at least some of these roadblocks is the
wide deployment of IP version 6.
4 Architectural Components and Terms
We now define components of the architecture based on the following
diagram:
______________________________
| |
| Private Realm |
| |
| [AC] |
| | ------- [oHOST]
| | / \
| [iHOST] [middle box] |Internet|
| | \ /
| [iHOST] | \------/
| |
-------------------------------
A private realm consists either of hosts that are numbered within the
space defined by BCP 5 or hosts that sit within a single security
domain. An iHOST is a host that sits within that realm. An oHOST
sits outside the private address realm. It may be assigned public
address space or private address space. In the latter case it would
sit within its own private address realm. An AC is an application
controller. An application controller may be an application layer
gateway, or it may merely arrange for communication between two
hosts, be they iHOSTs or oHOSTs. An AC may itself be a middle box.
A middle box is a device that sits on the edge of a private realm.
It is the responsibility of the middle box to police or transform
sessions from its private realm to the outside world. Middle boxes
consist of firewalls, NATs and other devices that sit within the flow
of packets and may impact a session from an iHOST to an oHOST. At
the border of a private realm there may be multiple middle boxes that
connect the realm to other realms.
The remainder of this document refers to communications between an
iHOST and an oHOST.
4.2 Communication Models
No matter the model we assume that iHOSTS may only signal to middle
boxes that exist within the same administrative domain.
There are two models of communication we will consider. The first
model may not require end host application changes, but rather
requires changes to servers that assist end hosts in communicating
with each other. The second model allows end hosts themselves to
exchange information with the intermediate devices. These two models
differ significantly.
4.2.1 Proxy Signaling
In the first case, iHOSTs use something akin to an application layer
gateway to first arrange a session. We will generically refer to
such a box as an application controller (AC). An example would be
H.225 signaling between an H.323 end point and a gate keeper. As the
gatekeeper processes call setup information it would communicate with
any middle box necessary for the end points to establish end to end
connectivity. This model is best employed when there already exists
some sort of application controller, since no additional signaling
may be necessary between the iHOST and the AC. Either the end
application is required to know of the AC or the AC must reside
within the data path between the iHOST and oHOST.
______________________________
| |
| Private Realm |
| |
| --- 1 --> [AC]->v |
| / v |
| / 2 |
| | v | ------- [oHOST]
| | v | / \ ^
| [iHOST]-----3-------->[middle box]--->|Internet|---3---->^
| | \ /
| [iHOST] | \------/
| |
-------------------------------
In order for the end hosts to continue sessions with some level of
reliability the AC must maintain topological awareness, so that it
can determine which middle boxes are involved. It might do this by
monitoring and examining link state information from the IGP.
However, no new signaling would be required between iHOSTs and middle
boxes.
4.2.2 Direct Signaling
In the second case, no AC is required. Instead, an iHOST must
determine that a middle box exists, signal to it for end to end
configuration information, and then proceed. Furthermore, the iHOST
must determine when the path to the oHOST has changed during a
session. The application should recover as circumstances dictate.
In this model it is important for the end host to determine that not
only has a failure occurred, but that the failure occurred due to
something the middle box could see or control.
______________________________
| |
| Private Realm |
| |
| |
| | ------- [oHOST]
| >------2--------v | / \
| [iHOST]<-----1--------[middle box] |Internet|
| ^<------3--------v | \ /
| | \------/
| |
-------------------------------
1. Diagnostic/discovery, a message or messages that indicate that the
middle box requires attention. This message is sent in response to an
attempt by iHOST to start a session with oHOST that the middle box knows
will not succeed.
2. Signaling request from iHOST to middle box.
3. Response from middle box.
Note that in this model there is no AC.
4.2.3 Current use of Application Layer Gateways
An alternative model would be for end hosts to use application layer
gateways to access external resources. This model requires no new
generic signaling, but a method for each iHOST to determine when it
should use an application layer gateway, and when it should
communicate directly with another iHOST.
In this case, because the ALG is a middle box, it follows that this
method requires the ALG to reside on the perimeter of the
administrative domain.
We mention this model not merely for completeness, but because it is
the operative model for many applications that would eventually use
one of the other two models. Its clear benefit is that it exists
today. Furthermore, there may be benefits to having ALGs reside on
the perimeter. For instance, these devices will be able to
statefully inspect each and every packet to and from an internal
network.
4.3 Which model should we build?
The astute reader will notice that the direct model is very close to
a superset of the proxy model. The proxy model needs a signaling
mechanism between the AC and the middle box. There is no reason that
signaling mechanism couldn't be the same one used by the direct
model.
Indeed as diagnostics are introduced they can enhance both the proxy
model and the ALG model by returning decent diagnostics to the end
user when iHOST not properly configured or the AC or ALG unexpectedly
falls outside the data stream.
5 Diagnostics and Discovery
As mentioned in the introduction, we elide the diagnostics and
discovery functions in this architecture. No matter which of the
above models is implemented diagnostics are required to indicate to
the users and application when a failure has occurred. These
failures can take numerous forms, but here are some examples:
o A middle box might reboot and lose state of which holes are
meant to have been open.
o The traffic may be rerouted to an unsuspecting middle box.
o The application on iHOST may be unaware of a middle box for
which an information exchange is required.
o An AC may be misconfigured to prevent the iHOST from
establishing a session with an oHOST.
The diagnostic mechanism used on the Internet is ICMP. When a middle
box detects an attempt by an iHOST to start a session to an oHOST
that will not succeed, it should send a message back to the iHOST
indicating that a failure is likely. The content of that ICMP
message is discussed separately in [BLOCKER].
Of the two models mentioned in Section 4, discovery may be different.
With the proxy model it is fairly easy to configure by hand a
relatively small number of ACs. Indeed either ACs must know about
the middle boxes or the middle boxes must know about them, since ACs
do not sit within the packet flow.
With the direct signaling model it is possible to piggyback discovery
on top of the diagnostic message discussed earlier.
6 Signaling
Once the application has determined that it must communicate with a
middle box in order for a communication to properly proceed, either
the AC or the iHOST initiates an exchange with the middle box. The
nature of this exchange depends on the function of the middle box.
For instance, if the middle box is a firewall, the application is in
essence asking permission from the firewall for the communication to
proceed. If, on the other hand, the box is a NAT or NAPT, the middle
box may merely need to know the mapping between the two addressing
realms for the communication. These two functions can be combined,
and it is reasonable to do so in order to reduce signaling overhead.
Any signaling requests to reserve address space or open pinholes must
be matched with similar requests to undo what was done. However,
firewalls will as a matter of policy not trust that all went well.
Indeed they should be fairly conservative to reduce the risk that
pinholes have not been left open beyond their legitimate purposes.
6.1 Firewalls
Firewalls require sufficient information about the communication to
determine whether or not it is authentic and whether or not it is
authorized. The firewall may query the application for specific
information necessary for authorization, but we can assume that
during the initial contact the firewall will need at least the
following information:
1. Protocol to be used during a session
2. IP address and source port of the iHOST
3. IP address and source port of the oHOST
4. One or more methods to indicate when the session has ended.
The firewall may also wish to know what person is initiating the
request, the application that is being used, or even perhaps a token
of some form. While it might wish to have arbitrary amounts of
information in order to make its decision, applications need to be
aware of the sorts of information the firewall will demand. Thus,
the names and formats of the values of the requested information must
be standardized, and should be managed by IANA.
If a firewall is going to respond to a request from an iHOST or AC it
SHOULD do so with a request for all the information it needs in
response to an initial request from either the iHOST or AC.
The format of the query the firewall makes must be standardized, as
should the names and formats of the individual attributes.
A firewall may accept, reject, or ignore such signaling requests. If
the firewall accepts a request it should respond with the protocol,
source and destination IP addresses and ports, confirming the way the
communication shall be terminated. In addition, it may return
additional information, as requested. This brings us to NATs.
Note that even if a communication is initially authorized, nothing we
state here should prevent or even discourage further stateful
inspection of any communication.
6.2 NATs
An AC or iHOST may request the port and IP address mapping between
two realms as part of the query discussed above. The middle box may
return the appropriate mappings. If it does so, it MUST also return
connection termination conditions.
6.3 Termination Conditions
It is important for the middle box and the application (AC or iHOST)
to agree on when a session has ended. In the case of a firewall it
is critical that it properly close the pinhole it opened. In the
case of a NAT, once a session has terminated the NAT may reallocate
addressees and ports to another iHOST.
Termination conditions can be one of several methods:
1. A period of time, similar to a TTL used by DHCP.
2. A requirement that the application tell the middle box when the
communication has ended.
3. An easily discernable in stream method to determine that the
session is over. For instance, a TCP session ends with the exchange of
FINs, followed by their acknowledgement. Such methods should be
described in an RFC.
While an application might request one or more method it is up to the
middle box to decide which method to employ. If more than one method
is contained within a request or a response, the termination
condition that occurs soonest will be used.
For example, an H.323 gateway might request a UDP connection from the
iHOST on port 7499 to oHOST on port 8233. The AC might also request
that it tell the middle box when the session is terminated. A
firewall might respond that it has opened a pinhole as described, and
the termination conditions are that the AC will indicate the
completion of the session AND the session will be considered closed
after five minutes.
Prior to the expiration of the TTL, if the call is still active, the
AC might further request additional time from the middle box.
6.4 Communication between middle boxes
Although it is theoretically possible for middle boxes to exchange
connection state amongst each other, the overhead for doing so may
well prove quite high, and the value is dubious. If a middle box
fails it is possible that a hot spare would be able to take over its
responsibilities. There exists at least one document that considers
this possibility [YAKOV et.al]. However, we choose not to
standardize this function at this time.
More likely the case will be that any existing connections will fail
due to a topological change, either the middle box failing or a route
to the middle box failing. Therefore, it is important that end hosts
be able to re-establish communications and retain state above the
transport layer, as is necessary and appropriate.
6.5 Signaling Protocol Choice
Returning to the discussion of the two different models discussed in
Section 4, we note that there are subtle differences in expected
protocol characteristics between the proxy and direct models. In the
case of the direct model an iHOST could expect to issue a single
request per session. However, in the case of a proxy, it is likely
to make many requests to the middle box on behalf many client iHOSTs.
The signaling protocol should allow for ease of failover. In
addition, the protocol should also take minimal resources on both
client and middle box. The client itself may be a middle box. In any
event the signaling end points - both middle box and client - MUST
MUST MUST implement appropriate congestion control mechanisms.
7 Multiple Middle Boxes
When used with the direct model, a diagnostic message such as ICMP
allows the application on an iHOST to determine not only that a
middle box is in its path, but also which middle box is in its path.
Once the iHOST identifies the middle box it can signal to it. Should
the data path change so that another middle box is chosen, the iHOST
will once again receive a notification.
Depending on the application or environment it may be possible for an
iHOST to fail over between two middle boxes that are sharing state.
Such failures must be transparent to the iHOST at all layers.
The matter of multiple middle boxes is somewhat more complex with the
proxy model. Because the AC is not in the data flow it must go to
some additional measures to determine that a middle box has failed.
Indeed once the AC has determined that a particular middle box has
failed, or that a path has changed, it must communicate appropriate
information back to the iHOST. Thus, unless the application already
anticipates appropriate failure and restart conditions, modifications
may be required, defeating the usefulness of the proxy model.
There is one additional case to be considered. When an iHOST attempts
to communicate across several concentric boundaries it might require
several rounds of signaling before a session could proceed. The
fastest way for signaling to proceed within the direct model is for
the middle box to forward a packet that has generated a diagnostic,
so that the very same packet could cause the next middle box to
generate a diagnostic as well, etc.
Note that there are a number of deployment scenarios one could create
in which the signaling itself could create a diagnostic from a middle
box. We declare such scenarios a matter of bad implementation and
deployment.
8 The Stack Simplification Act of 2001
Some mechanisms such as [RSIP] significantly complicate the host
stack by not giving sufficient guidance as to when the mechanism
should be used.
We propose that there are only three alternatives:
1. the application relies on the host's network layer to get the
packets directly to the other end;
2. the application communicates with a service that identifies
failures and optionally enables a flow;
3. the application is explicitly configured to use an application
layer gateway.
Option 2 is of great concern, in as much as the host must properly
multiplex any messages to an application, and those messages may need
to be received out of band of the normal communication.
9 Future Work
The first effort necessary is to conclude in fact whether or not
additional diagnostics are necessary. If so, we must next determine
the exact mechanisms to deliver those diagnostics. We must further
agree on whether or not additional discovery mechanisms should be
employed.
This document focuses on unicast based applications. We believe it
provides sufficient flexibility to allow for design of multicast
applications that take advantage of those building blocks, but more
study is clearly needed.
10 Security Considerations
There are numerous security considerations that middle boxes will
encounter, and the ones we list below should be viewed as far from
complete.
Any time a request is made for information or for a configuration
change it should be viewed with great suspicion. It is as of yet
unclear all the attacks that can be made using either the signaling
mechanism proposed in this document or the diagnostic messages
proposed in [BLOCKER].
To minimize the risk of attacks, this mechanism should only be used
in conjunction with strong authentication and a conservative
authorization model. The lower bound of risk is likely that of
today's model, where sites generally allow outbound TCP connections.
The signaling, diagnostics, and discovery discussed in this draft are
useful only within the boundaries of a single administrative domain.
The middle boxes on the borders of that domain should prevent
external devices from participating by not transmitting diagnostic
messages outside, and by not listening for signaling requests on
interfaces external to that domain.
Furthermore, intrusion detection systems would be well advised to
look for such requests as an indication of either configuration error
or a possible attack.
The period of time between the termination of a communication and the
termination of pinholes in firewalls may allow for mischief. End
hosts must be prepared to ignore unsolicited traffic to the ports
involved.
11 IANA Considerations
The names of attributes and the format of their contents that a
middle box can either furnish or request will need to be held in a
registry.
12 References
[1] Rekhter et. al., "Address Allocation for Private Internets", RFC
1918, February 1996.
[4] H.323
[6] Carpenter, B., ..., RFC 2775
[7] Postel, J., "Internet Control Message Protocol", RFC 792,
USC/Information Sciences Institute, September 1981.
[9] Postel, J., ed., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC 793, USC/Information Sciences
Institute, NTIS AD Number A111091, September 1981.
[11] Postel, J., "User Datagram Protocol", RFC 768, USC/Information
Sciences Institute, August 1980.
RFC 2663
BLOCKER
RSIP
DHCP
NAT-FAILOVER (Yakov, et. al).
13 Author's Address
Eliot Lear
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134-1706
Email: lear@cisco.com
Phone: +1 (408) 527 4020
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17 Expiration Date
This memo is filed as <draft-lear-middlebox-arch-00.txt>, and expires
July 3, 2001.