Network Working Group M. Barnes, Ed.
Request for Comments: 4097 Nortel Networks
Category: Informational June 2005
Middlebox Communications (MIDCOM) Protocol Evaluation
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 (2005).
Abstract
This document provides an evaluation of the applicability of SNMP
(Simple Network Management Protocol), RSIP (Realm Specific Internet
Protocol), Megaco, Diameter, and COPS (Common Open Policy Service) as
the MIDCOM (Middlebox Communications) protocol. A summary of each of
the proposed protocols against the MIDCOM requirements and the MIDCOM
framework is provided. Compliancy of each of the protocols against
each requirement is detailed. A conclusion summarizes how each of
the protocols fares in the evaluation.
Table of Contents
Overview.......................................................... 2
Conventions Used in This Document................................. 3
1. Protocol Proposals............................................ 3
1.1. SNMP.................................................... 3
1.2. RSIP.................................................... 5
1.3. Megaco.................................................. 7
1.4. Diameter................................................ 8
1.5. COPS.................................................... 10
2. Item Level Compliance Evaluation.............................. 11
2.1. Protocol Machinery...................................... 11
2.2. Protocol Semantics...................................... 20
2.3. General Security Requirements........................... 27
3. Conclusions................................................... 29
4. Security Considerations....................................... 30
5. References.................................................... 31
5.1. Normative References.................................... 31
5.2. Informative References.................................. 33
6. Acknowledgements.............................................. 33
Appendix A - SNMP Overview........................................ 34
Appendix B - RSIP with Tunneling.................................. 35
Appendix C - Megaco Modeling Approach............................. 37
Appendix D - Diameter IPFilter Rule............................... 39
Contributors ..................................................... 42
Overview
This document provides an evaluation of the applicability of SNMP
(Simple Network Management Protocol), RSIP (Realm Specific Internet
Protocol), Megaco, Diameter and COPS (Common Open Policy Service) as
the MIDCOM (Middlebox Communications) protocol. This evaluation
provides overviews of the protocols and general statements of
applicability based upon the MIDCOM framework [2] and requirements
[1] documents.
The process for the protocol evaluation was fairly straightforward as
individuals volunteered to provide an individual document evaluating
a specific protocol. Thus, some protocols that might be considered
as reasonably applicable as the MIDCOM protocol are not evaluated in
this document since there were no volunteers to champion the work.
The individual protocol documents for which there were volunteers
were submitted for discussion on the list with feedback being
incorporated into an updated document. The updated versions of these
documents formed the basis for the content of this WG document.
Section 1 contains a list of the proposed protocols submitted for the
purposes of the protocol evaluation with some background information
on the protocols and similarities and differences with regards to the
applicability to the framework [2] provided.
Section 2 provides the item level evaluation of the proposed
protocols against the Requirements [1].
Section 3 provides a summary of the evaluation. A table containing a
numerical breakdown for each of the protocols, with regards to its
applicability to the requirements, for the following categories is
provided: Fully met, Partially met through the use of extensions,
Partially met through other changes to the protocol, or Failing to be
met. This summary is not meant to provide a conclusive statement of
the suitability of the protocols, but rather to provide information
to be considered as input into the overall protocol decision process.
In order for this document to serve as a complete evaluation of the
protocols, some of the background information and more detailed
ASPects of the proposals documenting enhancements and applications of
the protocols to comply with the MIDCOM framework and requirements
are included in Appendices.
Conventions Used in this Document
The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [4].
1. Protocol Proposals
The following protocols were submitted to the MIDCOM WG for
consideration:
o SNMP
o RSIP
o Megaco
o Diameter
o COPS
The following provides an overview of each of the protocols and the
applicability of each protocol to the MIDCOM framework.
1.1. SNMP
This section provides a general statement with regards to the
applicability of SNMP as the MIDCOM protocol. A general overview and
some specific details of SNMP are provided in Appendix A. This
evaluation of SNMP is specific to SNMPv3, which provides the security
required for MIDCOM usage. SNMPv1 and SNMPv2c would be inappropriate
for MIDCOM since they have been declared Historic, and because their
messages have only trivial security. Some specifics with regards to
existing support for NAT and Firewall Control are provided in section
1.1.2. The differences between the SNMP framework and the MIDCOM
framework are addressed in section 1.1.3.
1.1.1. SNMP General Applicability
The primary advantages of SNMPv3 are that it is a mature, well
understood protocol, currently deployed in various scenarios, with
mature toolsets available for SNMP managers and agents.
Application intelligence is captured in MIB modules, rather than in
the messaging protocol. MIB modules define a data model of the
information that can be collected and configured for a managed
functionality. The SNMP messaging protocol transports the data in a
standardized format without needing to understand the semantics of
the data being transferred. The endpoints of the communication
understand the semantics of the data.
Partly due to the lack of security in SNMPv1 and SNMPv2c, and partly
due to variations in configuration requirements across vendors, few
MIB modules have been developed that enable standardized
configuration of managed devices across vendors. Since monitoring
can be done using only a least-common-denominator subset of
information across vendors, many MIB modules have been developed to
provide standardized monitoring of managed devices. As a result,
SNMP has been used primarily for monitoring rather than for
configuring network nodes.
SNMPv3 builds upon the design of widely-deployed SNMPv1 and SNMPv2c
versions. Specifically, SNMPv3 shares the separation of data
modeling (MIBs) from the protocol to transfer data, so all existing
MIBs can be used with SNMPv3. SNMPv3 also uses the SMIv2 standard,
and it shares operations and transport with SNMPv2c. The major
difference between SNMPv3 and earlier versions is the addition of
strong message security and controlled Access to data.
SNMPv3 uses the architecture detailed in RFC 3411 [5], where all SNMP
entities are capable of performing certain functions, sUCh as the
generation of requests, response to requests, the generation of
asynchronous notifications, the receipt of notifications, and the
proxy-forwarding of SNMP messages. SNMP is used to read and
manipulate virtual databases of managed-application-specific
operational parameters and statistics, which are defined in MIB
modules.
1.1.2. SNMP Existing Support for NAT and Firewall Control
For configuring NATs, a NAT MIB module [16] has been developed. The
NAT MIB module meets all of the MIDCOM requirements concerning NAT
control with the exception of grouping of policy rules (requirement
2.2.3.). In order to support this, an additional grouping table in
the NAT MIB module is required.
Existing work for firewall control with SNMP only considered the
monitoring of firewalls and not the configuration. Further work is
required towards the development of MIBs for configuring firewalls.
1.1.3. Architectural Differences between SNMP and MIDCOM
The SNMP management framework provides functions equivalent to those
defined by the MIDCOM framework, although there are a few
architectural differences.
Traditionally, SNMP entities have been called Manager and Agent.
Manager and agent are now recognized as entities designed to support
particular configurations of SNMPv3 functions. A traditional manager
is an entity capable of generating requests and receiving
notifications, and a traditional agent is an entity capable of
responding to requests and generating notifications. The SNMP use of
the term agent is different from its use in the MIDCOM framework: The
SNMP Manager corresponds to the MIDCOM agent and the SNMP Agent
corresponds to the MIDCOM PDP. The SNMP evaluation assumes that the
MIDCOM PDP (SNMP Agent) is physically part of the middlebox, which is
allowed by the MIDCOM framework as described in section 6.0 of [2].
Thus, for the purpose of this evaluation, the SNMP agent corresponds
to the Middlebox.
While this evaluation is based on the assumption that the SNMP agent
corresponds to the middlebox, SNMP does not force such a restriction.
Proxy means many things to many people. SNMP can be deployed using
intermediate entities to forward messages, or to help distribute
policies to the middlebox, similar to the proxy capabilities of the
other candidate protocols. Since proxy adds configuration and
deployment complexity and is not necessary to meet the specified
MIDCOM requirements, the use of a proxy agent or mid-level manager is
not considered in this evaluation. Further details on SNMP proxy
capabilities are provided in Appendix A.
Although the SNMP management framework does not have the concept of a
session, session-like associations can be established through the use
of managed objects. In order to implement the MIDCOM protocol based
on SNMP, a MIDCOM MIB module is required. All requests from the
MIDCOM agent to the Middlebox would be performed using write access
to managed objects defined in the MIDCOM MIB module. Replies to
requests are signaled by the Middlebox (SNMP agent), by modifying the
managed objects. The MIDCOM agent (SNMP manager) can receive this
information by reading or polling, if required, the corresponding
managed object.
1.2. RSIP
The RSIP framework and detailed protocol are defined in RFC 3102 [17]
and RFC 3103 [18] respectively.
1.2.1. Framework Elements in Common to MIDCOM and RSIP
The following framework elements are common to MIDCOM and RSIP listed
by their MIDCOM names, with the RSIP name indicated in parenthesis:
o Hosts
o Applications
o Middleboxes (RSIP gateways)
o Private domain (private realm)
o External domain (public realm)
o Middlebox communication protocol (RSIP)
o MIDCOM agent registration (host registration)
o MIDCOM session (RSIP session)
o MIDCOM Filter (local / remote address and port number(s) pairs)
1.2.2. MIDCOM Framework Elements Not Supported by RSIP
The following MIDCOM framework elements are not supported by RSIP:
o Policy actions and rules. RSIP always implicitly assumes a permit
action. To support MIDCOM, a more general and eXPlicit action
parameter would have to be defined. RSIP requests specifying
local / remote address and port number(s) pairs would have to be
extended to include an action parameter, in MIDCOM rules.
o MIDCOM agents. RSIP makes no distinction between applications and
agents; address assignment operations can be performed equally by
applications and agents.
o Policy Decision Points. RSIP assumes that middleboxes grant or
deny requests with reference to a policy known to them; the policy
could be determined jointly by the middlebox and a policy decision
point; such joint determination is not addressed by the RSIP
framework, nor is it specifically precluded.
1.2.3. RSIP Framework Elements Not Supported by MIDCOM
The following elements are unique to the RSIP framework. If RSIP
were adopted as the basis for the MIDCOM protocol, they could be
added to the MIDCOM framework:
o RSIP client: that portion of the application (or agent) that talks
to the RSIP gateway using RSIP.
o RSIP server: that portion of an RSIP gateway that talks to
applications using RSIP.
o Realm Specific Address IP (RSA-IP) and Realm Specific Address and
Port IP (RSAP-IP): RSIP distinguishes between filters that include
all ports on an IP address and those that do not.
o Demultiplexing Fields: Any set of packet header or payload fields
that an RSIP gateway uses to route an incoming packet to an RSIP
host. RSIP allows a gateway to perform, and an application to
control, packet routing to hosts in the private domain based on
more than IP header fields.
o Host-to-middlebox tunnels: RSIP assumes that data communicated
between a private realm host and a public realm host is
transferred through the private realm by a tunnel between the
inner host and the middle box, where it is converted to and from
native IP based communications to the public realm host.
1.2.4. Comparison of MIDCOM and RSIP Frameworks
RSIP with tunneling, has the advantage that the public realm IP
addresses and port numbers are known to the private realm host
application, thus no translation is needed for protocols such as SDP,
the FTP control protocol, RTSP, SMIL, etc. However, this does
require that an RSIP server and a tunneling protocol be implemented
in the middlebox and an RSIP client and the tunneling protocol be
implemented in the private realm host. The host modifications can
generally be made without modification to the host application or
requiring the implementation of a host application agent. This is
viewed as a significant advantage over NAT (Network Address
Translation).
Further details on the evaluation of RSIP with regards to tunneling
in the context of NAT support are available in Appendix B of this
document.
1.3. Megaco
1.3.1. Megaco Architectural Model
Megaco is a master-slave, transaction-oriented protocol defined in
RFC 3015 [20] in which Media Gateway Controllers (MGC) control the
operation of Media Gateways (MG). Originally designed to control IP
Telephony gateways, it is used between an application-unaware device
(the Media Gateway) and an intelligent entity (the Media Gateway
Controller) having application awareness.
The Megaco model includes the following key concepts:
1. Terminations: Logical entities on the MG that act as sources or
sink of packet streams. A termination can be physical or
ephemeral and is associated with a single MGC.
2. Context: An association between Terminations for sharing media
between the Terminations. Terminations can be added, suBTracted
from a Context and can be moved from one Context to another. A
Context and all of its Terminations are associated with a single
MGC.
3. Virtual Media Gateways: A physical MG can be partitioned into
multiple virtual MGs allowing multiple Controllers to interact
with disjoint sets of Contexts/Terminations within a single
physical device.
4. Transactions/Messages: Each Megaco command applies to one
Termination within a Context and generates a unique response.
Commands may be replicated implicitly so that they act on all
Terminations of a given Context through wildcarding of Termination
identifiers. Multiple commands addressed to different Contexts
can be grouped in a Transaction structure. Similarly, multiple
Transactions can be concatenated into a Message.
5. Descriptors/Properties: A Termination is described by a number of
characterizing parameters or Properties, which are grouped in a
set of Descriptors that are included in commands and responses.
6. Events and signals: A Termination can be programmed to perform
certain actions or to detect certain events and notify the Agent.
7. Packages: Packages are groups of properties, events, etc.
associated with a Termination. Packages are simple means of
extending the protocol to serve various types of devices or
Middleboxes.
1.3.2. Comparison of the Megaco and MIDCOM Architectural Frameworks
In the MIDCOM architecture, the Middlebox plays the role of an
application-unaware device being controlled by the application-aware
Agent. In the Megaco architecture, the Media Gateway controller
serves a role similar to the MIDCOM Agent (MA) and the Media Gateway
serves a role similar to the Middlebox (MB). One major difference
between the Megaco model and the MIDCOM protocol requirements is that
MIDCOM requires that the MIDCOM Agent establish the session.
Whereas, the Megaco definition is that a MG (Middlebox) establishes
communication with an MGC (MIDCOM Agent).
1.4. Diameter
1.4.1. Diameter Architecture
Diameter is designed to support AAA for network access. It is meant
to operate through networks of Diameter nodes, which both act upon
and route messages toward their final destinations. Endpoints are
characterized as either clients, which perform network access
control, or servers, which handle authentication, authorization and
accounting requests for a particular realm. Intermediate nodes
perform relay, proxy, redirect, and translation services. Design
requirements for the protocol include robustness in the face of
bursty message loads and server failures, resistance to specific DOS
attacks and protection of message contents, and extensibility
including support for vendor-specific attributes and message types.
The protocol is designed as a base protocol in RFC 3588 [24] to be
supported by all implementations, plus extensions devoted to specific
applications. Messages consist of a header and an aggregation of
"Attribute-Value Pairs (AVPs)", each of which is a tag-length-value
construct. The header includes a command code, which determines the
processing of the message and what other AVP types must or may be
present. AVPs are strongly typed. Some basic and compound types are
provided by the base protocol specification, while others may be
added by application extensions. One of the types provided in the
base is the IPFilterRule, which may be sufficient to express the
Policy Rules that MIDCOM deals with.
Messaging takes the form of request-answer exchanges. Some exchanges
may take multiple round-trips to complete. The protocol is
connection-oriented at both the transport and application levels. In
addition, the protocol is tied closely to the idea of sessions, which
relate sequences of message exchanges through use of a common session
identifier. Each application provides its own definition of the
semantics of a session. Multiple sessions may be open
simultaneously.
1.4.2. Comparison of Diameter With MIDCOM Architectural Requirements
The MIDCOM Agent does not perform the functions of a Diameter client,
nor does the Middlebox support the functions of a Diameter server.
Thus the MIDCOM application would introduce two new types of
endpoints into the Diameter architecture. Moreover, the MIDCOM
requirements do not at this time imply any type of intermediate node.
A general assessment might be that Diameter meets and exceeds MIDCOM
architectural requirements, however the connection orientation may be
too heavy for the number of relationships the Middlebox must support.
Certainly the focus on extensibility, request-response messaging
orientation, and treatment of the session, are all well-matched to
what MIDCOM needs. At this point, MIDCOM is focused on simple
point-to-point relationships, so the proxying and forwarding
capabilities provided by Diameter are not needed. Most of the
commands and AVPs defined in the base protocol are also surplus to
MIDCOM requirements.
1.5. COPS
Overall, COPS, defined in RFC 2748 [25], and COPS-PR, defined in RFC
3084 [26], have similar compliancy with regards to the MIDCOM
protocol requirements. In this document, references to COPS are
generally applicable to both COPS and COPS-PR. However, COPS-PR is
explicitly identified to meet two of the requirements. The only
other major difference between COPS-PR and COPS, as applied to the
MIDCOM protocol, would be the description of the MIDCOM policy rule
attributes with COPS-PR MIDCOM PIB attributes rather than COPS MIDCOM
client specific objects.
1.5.1. COPS Protocol Architecture
COPS is a simple query and response protocol that can be used to
exchange policy information between a policy server (Policy Decision
Point or PDP) and its clients (Policy Enforcement Points or PEPs).
COPS was defined to be a simple and extensible protocol. The main
characteristics of COPS include the following:
1. The protocol employs a client/server model. The PEP sends
requests, updates, and deletions to the remote PDP and the PDP
returns decisions back to the PEP.
2. The protocol uses TCP as its transport protocol for reliable
exchange of messages between policy clients and a server.
3. The protocol is extensible in that it is designed to leverage
self-identifying objects and can support diverse client specific
information without requiring modification of the COPS protocol.
4. The protocol was created for the general administration,
configuration, and enforcement of policies.
5. COPS provides message level security for authentication, replay
protection, and message integrity. COPS can make use of existing
protocols for security such as IPSEC [22] or TLS [21] to
authenticate and secure the channel between the PEP and the PDP.
6. The protocol is stateful in two main aspects:
(1) Request/Decision state is shared and kept synchronized in a
transactional manner between client and server. Requests from
the client PEP are installed or remembered by the remote PDP
until they are explicitly deleted by the PEP. At the same
time, Decisions from the remote PDP can be generated
asynchronously at any time for a currently installed request
state.
(2) State from various events (Request/Decision pairs) may be
inter-associated. The server may respond to new queries
differently because of previously installed, related
Request/Decision state(s).
7. The protocol is also stateful in that it allows the server to push
configuration information to the client, and then allows the
server to remove such state from the client when it is no longer
applicable.
1.5.2. Comparison of COPS and the MIDCOM Framework
In the MIDCOM framework, the Middlebox enforces the policy controlled
by an application-aware Agent. Thus, when compared to the COPS
architecture, the Middlebox serves as the PEP (COPS Client) and the
MIDCOM Agent serves as the PDP (COPS Policy Server). One major
difference between the COPS protocol model and the MIDCOM protocol
requirements is that MIDCOM requires that the MIDCOM Agent establish
the session. Whereas, the COPS definition is that a PEP (Middlebox)
establishes communication with a PDP (MIDCOM Agent).
2. Item Level Compliance Evaluation
This section contains a review of the protocol's level of compliance
to each of the MIDCOM Requirements [1]. The following key will be
used to identify the level of compliancy of each of the individual
protocols:
T = Total Compliance. Meets the requirement fully.
P+ = Partial Compliance+. Fundamentally meets the requirement
through the use of extensions (e.g., packages, additional
parameters, etc).
P = Partial Compliance. Meets some aspect of the requirement,
however, the necessary changes require more than an extension
and/or are inconsistent with the design intent of the
protocol.
F = Failed Compliance. Does not meet the requirement.
2.1. Protocol Machinery
This section describes the compliancy of the proposed protocols
against the protocol machinery requirements from section 2.1 of the
requirements document [1]. A short description of each of the
protocols is provided to substantiate the evaluation.
2.1.1. Ability to Establish Association Between Agent and Middlebox.
SNMP: T, RSIP: P+, Megaco: P, Diameter: T, COPS: P
SNMP: SNMPv3 provides mutual authentication at the user level
(where the user can be an application or a host if desired) via
shared secrets. Each authenticated principal is associated with a
group that has access rights that control the principals ability
to perform operations on specific subsets of data. Failure to
authenticate can generate a SNMP notification (administrator
configurable choice).
RSIP: RSIP allows sessions to be established between middleboxes
and applications and MIDCOM agents. Authorization credentials
would have to be added to the session establishment request to
allow the middlebox to authorize the session requestor.
Megaco: There is a directionality component implicit in this
requirement in that the MA initiates the establishment of the
authorized session. Megaco defines this association to be
established in the opposite direction, i.e., the Middlebox(MG)
initiates the establishment. If this restriction is not
considered, then Megaco makes the syntax and semantics available
for the endpoint to initiate the connection.
Diameter: Although this is out of scope, the Diameter specification
describes several ways to discover a peer. Having done so, a
Diameter node establishes a transport connection (TCP, TLS, or
SCTP) to the peer. The two peers then exchange Capability
Exchange Request/Answer messages to identify each other and
determine the Diameter applications each supports.
If the connection between two peers is lost, Diameter prescribes
procedures whereby it may be re-established. To ensure that loss
of connectivity is detected quickly, Diameter provides the
Device-Watchdog Request/Answer messages, to be used when traffic
between the two peers is low.
Diameter provides an extensive state machine to govern the
relationship between two peers.
COPS: COPS does not meet the directionality part of the
requirement. The definition of COPS allows a PEP (Middlebox) to
establish communication with a PDP (MIDCOM Agent). However,
nothing explicitly prohibits a PDP from establishing communication
with a PEP. The PEP could have local policies dictating what
action to take when it is contacted by an unknown PDP. These
actions, defined in the local policies, would ensure the proper
establishment of an authorized association.
2.1.2. Agent Can Relate to Multiple Middleboxes
SNMP: T, RSIP: P, Megaco: T, Diameter: T, COPS: T
SNMP: An SNMP manager can communicate simultaneously with several
Middleboxes.
RSIP: RSIP sessions are identified by their IP source and
destination addresses and their TCP / UDP port numbers. Thus each
RSIP client can communicate with multiple servers, and each server
can communicate with multiple clients. However, RSIP did not
explicitly include agents in its design. The architecture and
semantics of RSIP messages do not preclude agents, thus the RSIP
architecture could certainly be extended to explicitly include
agents; therefore RSIP is deemed partially compliant to this
requirement.
Megaco: Megaco allows an MA to control several Middleboxes. Each
message carries an identifier of the endpoint that transmitted the
message allowing the recipient to determine the source.
Diameter: Diameter allows connection to more than one peer (and
encourages this for improved reliability). Whether the Diameter
connection state machine is too heavy to support the number of
connections needed is a matter for discussion.
COPS: COPS PDPs are designed to communicate with several PEPs.
2.1.3. Middlebox Can Relate to Multiple Agents
SNMP: T, RSIP: P, Megaco: T, Diameter: T, COPS: T
SNMP: An SNMP agent can communicate with several SNMP managers
Simultaneously.
RSIP: Refer to 2.1.2.
Megaco: Megaco has the concept of Virtual Media Gateways (VMG),
allowing multiple MGCs to communicate simultaneously with the same
MG. Applying this model to MIDCOM would allow the same middlebox
(MG) to have associations with multiple MIDCOM Agents (MGCs).
Diameter: Diameter allows connection to more than one peer and
encourages this for improved reliability. Whether the Diameter
connection state machine is too heavy to support the number of
connections needed is a matter for discussion. The Middlebox and
Agent play symmetric roles as far as Diameter peering is
concerned.
COPS: The COPS-PR framework specifies that a PEP should have a
unique PDP in order to achieve effective policy control. The
COPS-PR protocol would allow the scenario whereby a PEP
establishes communication with multiple PDPs by creating a COPS
client instance per PDP.
2.1.4. Deterministic Outcome When Multiple Requests are Presented to
the Middlebox Simultaneously
SNMP: T, RSIP: T, Megaco: T, Diameter: T, COPS: T
SNMP: While the architectural design of SNMP can permit race
conditions to occur, there are mechanisms defined as part of the
SNMPv3 standard, such as view-based access control and advisory
locking that can be used to prevent the conditions, and MIB
modules may also contain special functionality, such as RMONs
OwnerString, to prevent conflicts. Deterministic behavior of SNMP
agents when being accessed by multiple managers is important for
several management applications and supported by SNMP.
RSIP: All RSIP requests are defined to be atomic. Near simultaneous
requests are executed as is they were sequential.
Megaco: Megaco supports the concept of VMGs to make these
interactions deterministic and to avoid resource access conflicts.
Each VMG has a single owner, in a MGC, and there can be no overlap
between the sets of Terminations belonging to multiple VMGs. The
Megaco protocol messages also include the identifier of the
sending entity, so that the MG can easily determine to whom to
send the response or asynchronously report certain events.
Diameter: Diameter depends partly upon the transport protocol to
provide flow control when the server becomes heavily loaded. It
also has application-layer messaging to indicate that it is too
busy or out of space (Diameter_TOO_BUSY and Diameter_OUT_OF_SPACE
result codes).
COPS: COPS has built-in support for clear state and policy
instances. This would allow the creation of well-behaved MIDCOM
state machines.
2.1.5. Known and Stable State
SNMP: T, RSIP: T, Megaco: T, Diameter: P, COPS: T
SNMP: Requests are atomic in SNMP. MIB modules can define which
data is persistent across reboots, so a known startup state can be
established. The manager can poll the agent to determine the
current state.
RSIP: RSIP assumes that on middlebox start-up no sessions are
defined, and thus no allocations have been made. In effect, all
resources are released upon restart after failure.
Megaco: Megaco has extensive audit capabilities to synchronize
states between the MG and the MGC. Megaco also provides the MGC
with the ability to do mass resets, as well as individual resets.
The MGC can always release resources in the MG. The MG can also
initiate the release of resources by the MGC.
Diameter: Diameter documentation does not discuss the degree of
atomicity of message processing, so this would have to be
specified in the MIDCOM extension.
COPS: The COPS protocol maintains synchronized states between
Middleboxes and MA hence all the states are known on both sides.
2.1.6. Middlebox Status Report
SNMP: T, RSIP: T, Megaco: T, Diameter: T, COPS: T
SNMP: The status of a middlebox can be reported using asynchronous
communications, or via polling.
RSIP: All RSIP client requests have explicit server responses.
Additionally, a client may explicitly request server status using
a QUERY request.
Megaco: Megaco has extensive audit capabilities for the MG to
report status information to the MGC. It can also report some
status updates using the ServiceChange command.
Diameter: Diameter provides a number of response codes by means of
which a server can indicate error conditions reflecting status of
the server as a whole. The Disconnect-Peer-Request provides a
means in the extreme case to terminate a connection with a peer
gracefully, informing the other end about the reason for the
disconnection.
COPS: The COPS Report message is designed to indicate any
asynchronous conditions/events.
2.1.7. Middlebox Can Generate Unsolicited Messages
SNMP: T, RSIP: T, Megaco: T, Diameter: T, COPS: T
SNMP: SNMPv3 supports both confirmed and unconfirmed asynchronous
notifications.
RSIP: An RSIP server will send an unsolicited DE_REGISTER_RESPONSE
to force an RSIP host to relinquish all of its bindings and
terminate its relationship with the RSIP gateway. An RSIP server
can send an asynchronous ERROR_RESPONSE to indicate less severe
conditions.
Megaco: Megaco supports the asynchronous notification of events
using the Notify command.
Diameter: The Diameter protocol permits either peer in a connection
to originate transactions. Thus the protocol supports Middlebox-
originated messages.
COPS: The COPS Report message is designed to indicate any
asynchronous conditions/events.
2.1.8. Mutual Authentication
SNMP: T, RSIP: T, Megaco: T, Diameter: T, COPS: T
SNMP: SNMPv3 meets this requirement. SNMPv3 supports user
authentication and explicitly supports symmetric secret key
encryption between MIDCOM agent (SNMP manager) and Middlebox (SNMP
agent), thus supporting mutual authentication. The default
authentication and encryption methods are specified in RFC 3414
[11] (MD5, SHA-1, and DES). Different users at the same
management application (MIDCOM agent) can authenticate themselves
with different authentication and encryption methods, and
additional methods can be added to SNMPv3 entities as needed.
RSIP: This requirement can be met by operating RSIP over IPSec as
described in RFC 3104 [19]. The RSIP framework recommends all
communication between an RSIP host and gateway be authenticated.
Authentication, in the form of a message hash appended to the end
of each RSIP protocol packet, can serve to authenticate the RSIP
host and gateway to one another, provide message integrity, and
avoid replay attacks with an anti-replay counter. However, the
message hash and replay counter parameters would need to be
defined for the RSIP protocol.
Megaco: Megaco provides for the use of IPSec [22] for all security
mechanisms including mutual authentication, integrity check and
encryption. Use of IKE is recommended with support of RSA
signatures and public key encryption.
Diameter: The Diameter base protocol assumes that messages are
secured by using either IPSec or TLS [21]. Diameter requires that
when using the latter, peers must mutually authenticate
themselves.
COPS: COPS has built-in message level security for authentication,
replay protection, and message integrity. COPS can also use TLS
or IPSec.
2.1.9. Termination of session by either party
SNMP: T, RSIP: T, Megaco: T, Diameter: T, COPS: T
SNMP: Each SNMPv3 message is authenticated and authorized, so each
message could be considered to have its own session, which
automatically terminates after processing. Processing may be
stopped for a number of reasons, such as security, and a response
is sent.
Either peer may stop operating, and be unavailable for further
operations. The authentication and/or authorization parameters of
a principal may be changed between operations if desired, to
prevent further authentication or authorization for security
reasons.
Additionally, managed objects can be defined for realizing
sessions that persist beyond processing of a single message. The
MIB module would need to specify the responsibility for cleanup of
the objects following normal/abnormal termination.
RSIP: An RSIP client may terminate a session with a
DE_REGISTER_REQUEST. An RSIP server may terminate a session with
an unsolicited DE_REGISTER_RESPONSE, and then respond to
subsequent requests on the session with a REGISTER_FIRST error.
Megaco: The Megaco protocol allows both peers to terminate the
association with proper reason code.
Diameter: Either peer in a connection may issue a Disconnect-Peer-
Request to end the connection gracefully.
COPS: COPS allows both the PEP and PDP to terminate a session.
2.1.10. Indication of Success or Failure
SNMP: T, RSIP: T, Megaco: T, Diameter: T, COPS: T
SNMP: Each operation request has a corresponding response message
that contains an error status to indicate success or failure. For
complex requests that the middlebox cannot complete immediately,
the corresponding MIB module may be designed to also provide
asynchronous notifications of the success or failure of the
complete transaction, and/or may provide pollable objects that
indicate the success or failure of the complete transaction. For
example, see ifAdminStatus and ifOperStatus in RFC 2863 [28].
RSIP: All RSIP requests result in a paired RSIP response if the
request was successful or an ERROR_RESPONSE if the request was not
successful.
Megaco: Megaco defines a special descriptor called an Error
descriptor that contains the error code and an optional
explanatory string.
Diameter: Every Diameter request is matched by a response, and this
response contains a result code as well as other information.
COPS: The COPS Report message directly fulfills this requirement.
2.1.11. Version Interworking
SNMP: T, RSIP: T, Megaco: T, Diameter: T, COPS: T
SNMP: SNMP has a separation of the protocol to carry data, and the
data that defines additional management functionality. Additional
functionality can be added easily through MIBs. Capability
exchange in SNMP is usually uni-directional. Managers can query
the middlebox (SNMP agent) to determine which MIBs are supported.
In addition, multiple message versions can be supported
simultaneously, and are identified by a version number in the
message header.
RSIP: Each RSIP message contains a version parameter.
Megaco: Version interworking and negotiation are supported both for
the protocol and any extension Packages.
Diameter: The Capabilities Exchange Request/Answer allows two peers
to determine information about what each supports, including
protocol version and specific applications.
COPS: The COPS protocol can carry a MIDCOM version number and
capability negotiation between the COPS client and the COPS
server. This capability negotiation mechanism allows the COPS
client and server to communicate the supported
features/capabilities. This would allow seamless version
interworking.
2.1.12. Deterministic Behaviour in the Presence of Overlapping
Rules
SNMP: T, RSIP: T, Megaco: P, Diameter: T, COPS: T
SNMP: Rulesets would be defined in MIBs. The priority of rulesets,
and the resolution of conflict, can be defined in the MIB module
definition. The SNMPConf policy MIB defines mechanisms to achieve
deterministic behavior in the presence of overlapping rule sets.
RSIP: All requests for allocation of IP addresses, or ports or both
resulting in rule overlap are rejected by an RSIP server with a
LOCAL_ADDR_INUSE error.
Megaco: This is met with the help of a model that separates Megaco
protocol elements from the overlapping Policy rules (see Appendix
C). However, new behavior for the Megaco protocol elements needs
to be specified as part of a new MIDCOM specific Package.
Diameter: The IPFilterRule type specification, which would probably
be used as the type of a Policy Rule AVP, comes with an extensive
semantic description providing a deterministic outcome, which the
individual Agent cannot know unless it knows all of the Policy
Rules installed on the Middlebox. Rules for the appropriate
direction are evaluated in order, with the first matched rule
terminating the evaluation. Each packet is evaluated once. If no
rule matches, the packet is dropped if the last rule evaluated was
a permit, and passed if the last rule was a deny. The
IPFilterRule format and further details on its applicability to
this requirement are provided in Appendix D.
COPS: The COPS protocol provides transactional-based communication
between the PEP and PDP, hence the behavior is totally
deterministic provided the middlebox state machine is designed
correctly. The COPS protocol features encourage and support good
state machine design.
2.2. Protocol Semantics
This section contains the individual protocols as evaluated against
the protocol semantic requirements from section 2.2 of the
requirements document [1]. A short description of each of the
protocols is provided to substantiate the evaluation.
2.2.1. Extensibility
SNMP: T, RSIP: T, Megaco: T, Diameter: T, COPS: T
SNMP: Extensibility is a basic feature of the SNMP management
Framework.
RSIP: All RSIP messages consist of three mandatory fields (protocol
version, message type, and message length) and a sequence of
parameterType / length / value 3-tuples. New messages may be
defined by defining new values for the message type field. New
parameter types may be defined, and existing messages may be
extended, by defining new parameterType values. If new messages,
parameters, or both are added in a non-backward compatible way, a
new value of the protocol version field may be defined. This may
be desirable even of the additions are backward compatible.
Megaco: Megaco is easily extensible through new Packages, which
allow definition of new attributes and behavior of a Termination.
Diameter: Diameter provides a great deal of flexibility for
extensions, including allowance for vendor-defined commands and
AVPs and the ability to flag each AVP as must-understand or
ignorable if not understood.
COPS: The COPS protocol is extensible, since it was designed to
separate the Protocol from the Policy Control Information.
2.2.2. Support of Multiple Middlebox Types
SNMP: T, RSIP: P+, Megaco: T, Diameter: P+, COPS: T
SNMP: SNMP explicitly supports managing different device types with
different capabilities. First the managed object called
sysObjectID from basic MIB-II [3] identifies the type of box. For
boxes with variable capabilities, SNMP can check the availability
of corresponding MIBs.
RSIP: All types of middleboxes are supported so long as the ruleset
action is permit. Other actions would require the definition of a
new RSIP message parameter with values for permit and the other
desired actions.
Megaco: Megaco can support multiple Middlebox types on the same
interface either by designing the properties representing the
Policy Rules to provide this support, or by using multiple
terminations in the same session, each representing one type of
action. In the latter case, the Megaco Context can be used as a
convenient means of managing the related terminations as a group.
However, the inherent idea of flow between terminations of a
context is irrelevant and would have to be discarded.
Diameter: Any necessary additional AVPs or values must be specified
as part of the MIDCOM application extension (see <2.2.8> below).
COPS: COPS allows a PDP to provide filters and actions to multiple
PEP functions through a single COPS session.
2.2.3. Ruleset Groups
SNMP: T, RSIP: P+, Megaco: T, Diameter: T, COPS: T
SNMP: This requirement can be realized via the SNMP management
framework by an appropriate definition of a MIB module. The
SNMPConf WG has already defined an SNMP Policy MIB that permits
the definitions of policy rulesets and grouping of rulesets.
RSIP: RSIP currently only allows one IP address, or address and
port range, to be assigned to a bind-ID. RSIP could implement
rulesets as required by adding an optional bind-ID parameter to
the ASSIGN_REQUESTs to extend an existing ruleset rather than
creating a new one. Similarly, the FREE_REQUESTs would have to be
extended by adding optional, local and remote, address and port
parameters.
Megaco: The Megaco context can be used to group terminations to be
managed together. For example, all of the terminations, each
representing an instantiation of a Policy Rule, can be deleted in
one command by doing a wildcarded Subtract from the context.
However, the inherent idea of media flows between terminations of
a context would be irrelevant in this application of the protocol.
Diameter: Diameter allows message syntax definitions where multiple
instances of the same AVP (for example, a Policy Rule AVP whose
syntax and low-level semantics are defined by the IPFilterRule
type definition) may be present. If a tighter grouping is
required, the set of Diameter base types includes the Grouped
type. MIDCOM can choose how to make use of these capabilities to
meet the ruleset group requirement when defining its application
extension to the Diameter protocol.
COPS: The COPS-PR Handle State may be used to associate the set of
closely related policy objects. As the Middlebox learns
additional requirements, the Middlebox adds these resource
requirements under the same handle ID, which constitutes the
required aggregation.
2.2.4. Lifetime Extension
SNMP: P+, RSIP: T, Megaco: T, Diameter: T, COPS: P+
SNMP: This requirement can be realized via the SNMP management
framework by an appropriate definition of a MIB module. The
SNMPConf WG has developed a Policy MIB module that includes a
pmPolicySchedule object with a modifiable lifetime.
RSIP: A client may request an explicit lease time when a request is
made to assign one or more IP addresses, ports or both. The
server may grant the requested lease time, or assign one if none
was requested. Subsequently, the lease time may be extended if a
client's EXTEND_REQUEST is granted by the server.
Megaco: The MG can report the imminent expiry of a policy rule to
the MGC, which can then extend or delete the corresponding
Termination.
Diameter: The Diameter concept of a session includes the session
lifetime, grace period, and lifetime extension. It may make sense
to associate the Diameter session with the lifetime of a MIDCOM
Policy Rule, in which case support for lifetime extension comes
ready-made.
COPS: COPS allows a PDP to send unsolicited decisions to the PEP.
However, the unsolicited events will be relevant to the COPS
MIDCOM specific client or the MIDCOM specific PIB which needs to
be defined. This would allow the PDP to extend the lifetime of an
existing ruleset.
2.2.5. Handling of Mandatory/Optional Nature of Unknown Attributes
SNMP: T, RSIP: T, Megaco: P+, Diameter: P+, COPS: T
SNMP: Unknown attributes in a read operation are flagged as
exceptions in the Response message, but the rest of the read
succeeds. In a write operation (a SET request), all attributes
are validated before the write is performed. If there are unknown
attributes, the request fails and no writes are done. Unknown
attributes are flagged as exceptions in the Response message, and
the error status is reported.
RSIP: All options of all requests are fully specified. Not
understood parameters must be reported by an ERROR_RESPONSE with
an EXTRA_PARM error value, with the entire request otherwise
ignored.
Megaco: Megaco entities provide Error codes in response messages.
If a command marked "Optional" in a transaction fails, the
remaining commands will continue. However, the specified
requirement deals with rules of processing properties that need
definition in new Package.
Diameter: Indication of the mandatory or optional status of AVPs is
fully supported, provided it is enabled in the AVP definition. No
guidance is imposed regarding the return of diagnostic information
for optional AVPs.
COPS: COPS provides for the exchange of capabilities and
limitations between the PEP and PDP to ensure well-known outcomes
are understood for scenarios with unknown attributes. There is
also clear error handling for situations when the request is
rejected.
2.2.6. Actionable Failure Reasons
SNMP: T, RSIP: P+, Megaco: T, Diameter: T, COPS: T
SNMP: The SNMPv3 protocol returns error codes and exception codes
in Response messages, to permit the requestor to modify their
request. Errors and exceptions indicate the attribute that caused
the error, and an error code identifies the nature of the error
encountered.
If desired, a MIB can be designed to provide additional data about
error conditions either via asynchronous notifications or polled
objects.
RSIP: RSIP defines a fairly large number of very specific error
values. It is anticipated that additional error values will also
have to be defined along with the new messages and parameters
required for MIDCOM.
Megaco: The MG can provide Error codes in response messages
allowing the MGC to modify its behavior. Megaco uses transaction
identifiers for correlation between a response and a command. If
the same transaction id is received more than once, the receiving
entity silently discards the message, thus providing some
protection against replay attacks.
Diameter: Diameter provides an extensive set of failure reasons in
the base protocol.
COPS: COPS uses an error object to identify a particular COPS
protocol error. The error sub-code field may contain additional
detailed COPS client (MIDCOM Middlebox) specific error codes.
2.2.7. Multiple Agents Operating on the Same Ruleset.
SNMP: T, RSIP: P, Megaco: P, Diameter: T, COPS: P
SNMP: The SNMP framework supports multiple managers working on the
same managed objects. The View-based Access Control Model (VACM,
RFC 3415 [14]) even offers means to customize the access rights of
different managers in a fine-grained way.
RSIP: RSIP neither explicitly permits nor precludes an operation on
a binding by a host that had not originally create the binding.
However, to support this requirement, the RSIP semantics must be
extended to explicitly permit any authorized host to request
operations on a binding; this does not require a change to the
protocol.
Megaco: If the Megaco state machine on the Middle Box is decoupled
from the Middle Box policy rule management, this requirement can
be met with local policies on the Middle Box. However, this
violates the spirit of the Megaco protocol, thus Megaco is
considered partially compliant to this requirement.
Diameter: The Diameter protocol, as currently defined, would allow
multiple agents to operate on the same ruleset.
COPS: It is possible to use COPS to operate the same resource with
multiple agents. An underlying resource management function,
separate from the COPS state machine, on the Middlebox will handle
the arbitration when resource conflicts happen.
2.2.8. Transport of Filtering Rules
SNMP: P+, RSIP: P+, Megaco: P+, Diameter: P+, COPS: P+
SNMP: This requirement can be met by an appropriate definition of a
MIDCOM MIB module. SMI, the language used for defining MIB
modules, is flexible enough to allow the implementation of a MIB
module to meet the semantics of this requirement.
RSIP: To support this requirement, a new optional enumeration
parameter, transportProtocol, can be added to the RSIP
ASSIGN_REQUESTs. When the parameter is included, the binding
created applies only to the use of the bound addresses and ports,
by the specific transportProtocol. When the parameter is not
included, the binding applies to the use of all the bound
addresses and ports, by any transport protocol, thus maintaining
backward compatibility with the current definition of RSIP.
Megaco: Megaco protocol can meet this requirement by defining a new
property for the transport of filtering rules.
Diameter: While Diameter defines the promising IPFilterRule data
type (see 2.1.12 above), there is no existing message, which would
convey this to a Middlebox along with other required MIDCOM
attributes. A new MIDCOM application extension of Diameter would
have to be defined.
COPS: The COPS protocol can meet this requirement by using a COPS
MIDCOM specific client or a MIDCOM specific PIB.
2.2.9. Mapped Port Parity
SNMP: P+, RSIP: P+, Megaco: P+, Diameter: P+, COPS: P+
SNMP: This requirement can be met by an appropriate definition of a
MIDCOM MIB module.
RSIP: To support this requirement, a new optional boolean
parameter, portOddity, can be added to the RSIP ASSIGN_REQUESTs.
If the parameter is TRUE, the remote port number of the binding
created would have the same oddity as the local port. If the
parameter is not specified, or is FALSE, the remote port's oddity
is independent of the local port's oddity, thus maintaining
backward compatibility with the current definition of RSIP.
Megaco: Megaco can be easily extended using a MIDCOM specific
Package to support this feature.
Diameter: This capability is not part of the current IPFilterRule
type definition. Rather than modify the IPFilterRule type, MIDCOM
could group it with other AVPs which add the missing information.
COPS: The COPS protocol has all the flexibility to meet this
requirement by using a COPS MIDCOM specific client or a MIDCOM
specific PIB.
2.2.10. Consecutive Range of Port Numbers
SNMP: P+, RSIP: T, Megaco: P+, Diameter: P+, COPS: P+
SNMP: This requirement can be met by an appropriate definition of a
MIDCOM MIB module. SMI, the language used for defining MIB
modules, is flexible enough to allow the implementation of a MIB
module to meet the semantics of this requirement.
RSIP: The ports parameter of the RSIP ASSIGN_REQUESTs specifically
allows multiple, consecutive port numbers to be specified.
Megaco: Megaco can be easily extended using a MIDCOM specific
Package to support this feature.
Diameter: This capability is not part of the current IPFilterRule
type definition. Rather than modify the IPFilterRule type, MIDCOM
could group it with other AVPs which add the missing information.
COPS: The COPS protocol has all the flexibility to meet this
requirement by using a COPS MIDCOM specific client or a MIDCOM
specific PIB.
2.2.11. More Precise Rulesets Contradicting Overlapping Rulesets
SNMP: P+, RSIP: P+, Megaco: P+, Diameter: T, COPS: P+
SNMP: This requirement can be met by an appropriate definition of a
MIDCOM MIB module.
RSIP: To support this requirement, a new optional boolean
parameter, overlapOK, can be added to the RSIP ASSIGN_REQUESTs.
If the parameter is TRUE, the binding may overlap with an existing
binding. If the parameter is unspecified, or is FALSE, the
binding will not overlap with an existing binding, thus
maintaining backward compatibility with the current definition of
RSIP.
Megaco: This requirement would be met if the policy in the
Middlebox allows contradictory, overlapping policy rules to be
installed.
Diameter: Allowed by the IPFilterRule semantics described in
Appendix D.
COPS: The COPS protocol has all the flexibility to meet this
requirement by using a COPS MIDCOM specific client or a MIDCOM
specific PIB.
2.3. General Security Requirements
This section contains the individual protocols as evaluated against
the General Security requirements from section 2.3 of the
requirements document [1]. A short description of each of the
protocols is provided to substantiate the evaluation.
2.3.1. Message Authentication, Confidentiality and Integrity
SNMP: T, RSIP: T, Megaco: T, Diameter: T, COPS: T
SNMP: SNMPv3 includes the User-based Security Model (USM,
RFC 3414 [11]), which defines three standardized methods for
providing authentication, confidentiality, and integrity.
Additionally, USM has specific built-in mechanisms for preventing
replay attacks including unique protocol engine IDs, timers and
counters per engine and time windows for the validity of messages.
RSIP: This requirement can be met by operating RSIP over IPSec. The
RSIP framework recommends all communication between an RSIP host
and gateway be authenticated. Authentication, in the form of a
message hash appended to the end of each RSIP protocol packet, can
serve to authenticate the RSIP host and gateway to one another,
provide message integrity, and avoid replay attacks with an anti-
replay counter. However, the message hash and replay counter
parameters would need to be defined for the RSIP protocol.
Megaco: Megaco provides for these functions with the combined usage
of IPSEC [22] or TLS [21].
Diameter: Diameter relies on either IPSEC or TLS for these
functions.
COPS: COPS has built-in message level security for authentication,
replay protection, and message integrity. COPS can also use TLS
or IPSec, thus reusing existing security mechanisms that have
interoperated in the markets.
2.3.2. Optional Confidentiality Protection
SNMP: T, RSIP: T, Megaco: T, Diameter: T, COPS: T
SNMP: SNMPv3 includes the User-based Security Model, which defines
three standardized methods for providing authentication,
confidentiality, and integrity, and is open to add further
methods. The method to use can be optionally chosen.
RSIP: Refer to 2.3.1.
Megaco: Refer to 2.3.1
Diameter: Implementation support of IPSEC ESP (RFC 2406 [23]) in
Diameter applications is not optional. Deployment of either IPSEC
or TLS is optional.
COPS: Refer to 2.3.1.
2.3.3. Operate Across Untrusted Domains
SNMP: T, RSIP: T, Megaco: T, Diameter: T, COPS: T
SNMP: The User-based Security Model of SNMPv3 defines three
standardized methods for providing authentication,
confidentiality, and integrity, and it is open to add further
methods. These methods operate securely across untrusted domains.
RSIP: Refer to 2.3.1.
Megaco: Refer to 2.3.1.
Diameter: The Diameter specification [24] recommends the use of
TLS [21] across untrusted domains.
COPS: Refer to 2.3.1
2.3.4. Mitigates Replay Attacks on Control Messages
SNMP: T, RSIP: T, Megaco: T, Diameter: T, COPS: T
SNMP: The User-based Security Model for SNMPv3 has specific built-
in mechanisms for preventing replay attacks including unique
protocol engine IDs, timers and counters per engine and time
windows for the validity of messages.
RSIP: Refer to 2.3.1
Megaco: Megaco commands and responses include matching transaction
identifiers. The recipient receiving the same transaction id
multiple times would discard the message, thus providing some
protection against replay attacks. If even stronger protection
against replay attack is needed, Megaco provides for the use of
IPSec or TLS.
Diameter: Diameter requires that implementations support the replay
protection mechanisms of IPSEC.
COPS: Refer to 2.3.1
3. Conclusions
The overall statistics with regards to the number of Fully Compliant,
Partially Compliant (P+ and P) and Failing Compliancy requirements
for each of the protocols is summarized in table 1.
T P+ P F
-----------------------------------------------------------------
SNMP 22 5 0 0
RSIP 17 7 3 0
Megaco 19 5 3 0
Diameter 21 5 1 0
COPS 20 5 2 0
Table 1: Totals across all Requirements
In considering the P+ category of compliancy, an important aspect is
the mechanism for support of extensibility. The extension mechanism
provided by SNMP and COPS-PR using MIBs and PIBs respectively,
provides extensions with no impact to the protocol. Diameter
extensions require protocol changes, thus has a higher impact,
although the extensions can be handled by other Diameter entities
without being understood. Megaco's extension mechanisms of packages
also requires protocol changes that must be understand by both
sending and receiving entities, also being considered higher impact.
The RSIP extension mechanism has the largest impact on the existing
protocol and is based upon defining the necessary new parameters.
The SNMP management framework meets all the specified MIDCOM protocol
requirements with the appropriate design of a MIDCOM MIB module.
SNMP is a proven technology with stable and proven development tools,
already has extensions defined to support NAT configuration and
policy-based management. SNMPv3 is a full standard, is more mature
and has undergone more validation than the other protocols in
the evaluation, and has been deployed to manage large-scale real-
world networks (e.g., DOCSIS cable modem networks). The
applicability of SNMP to the MIDCOM framework has a restriction in
that it assumes the MIDCOM PDP is part of the Middlebox.
RSIP fully meets many of the MIDCOM requirements. However, it does
require additions and extensions to meet several of the requirements.
RSIP would also require several framework elements to be added to the
MIDCOM framework as identified in section 1.2.3. In addition, the
tunneling required for RSIP as described in section 1.2.4, results in
RSIP not being acceptable by the WG as the MIDCOM protocol.
Megaco fully meets most of the key requirements for the MIDCOM
Protocol. Additional extensions in the form of a new Termination /
Package definition would be required for MIDCOM to meet several of
the requirements. In order to meet the remaining requirements,
modeling the underlying Middlebox resources (e.g., filters, policy
rules) as separate elements from the Megaco entities might allow the
usage of the protocol as-is, satisfying some of the resource access
control requirements.
The Diameter evaluation indicated a good overall fit. Some partially
met requirements were identified that could be addressed by a new
application extension. However, the Diameter architecture may be too
heavy for the MIDCOM application and clearly much of the Diameter
base is not needed. In addition, Diameter is the only protocol, at
the time of this evaluation, for which the RFCs had not yet been
published. Other than these reservations, the protocol is a good fit
to MIDCOM requirements.
The COPS evaluation indicates that the protocol meets the majority of
the MIDCOM protocol requirements by using the protocol's native
extension techniques, with COPS-PR being explicitly required to meet
requirements 2.1.3 and 2.2.3. In order to fully satisfy one
partially met requirement, 2.1.1, the COPS model would need to allow
a PDP to establish communication with a PEP. While not explicitly
prohibited by the COPS model, this would require additions, in the
form of local policy, to ensure the proper establishment of an
authorized association.
4. Security Considerations
Security considerations for the MIDCOM protocol are covered by the
comparison against the specific Security requirements in the MIDCOM
requirements document [1] and are specifically addressed by section
2.1.8 and section 2.3.
5. References
5.1. Normative References
[1] Swale, R., Mart, P., Sijben, P., Brim, S., and M. Shore,
"Middlebox Communications (MIDCOM) Protocol Requirements", RFC
3304, August 2002.
[2] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A.
Rayhan, "Middlebox Communications Architecture and Framework",
RFC 3303, August 2002.
[3] Rose, M. and K. McCloghrie, "Management Information Base for
Network Management of TCP/IP-based internets: MIB-II", STD 17,
RFC 1213, March 1991.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] Harrington, D., Presuhn, R., and B. Wijnen, "An Architecture for
Describing SNMP Management Frameworks", STD 62, RFC 3411,
December 2002.
[6] McCloghrie, K., Perkins, D., and J. Schoenwaelder, "Structure of
Management Information Version 2 (SMIv2)", STD 58, RFC 2578,
April 1999.
[7] McCloghrie, K., Perkins, D., and J. Schoenwaelder, "Textual
Conventions for SMIv2", STD 58, RFC 2579, April 1999.
[8] McCloghrie, K., Perkins, D., and J. Schoenwaelder, "Conformance
Statements for SMIv2", STD 58, RFC 2580, April 1999.
[9] Presuhn, R. (Ed.), "Transport Mappings for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3417, December 2002.
[10] Case, J., Harrington D., Presuhn R., and B. Wijnen, "Message
Processing and Dispatching for the Simple Network Management
Protocol (SNMP)", STD 62, RFC 3412, December 2002.
[11] Blumenthal, U. and B. Wijnen, "User-based Security Model(USM)
for version 3 of the Simple Network Management Protocol
(SNMPv3)", STD 62, RFC 3414, December 2002.
[12] Presuhn, R. (Ed.), "Version 2 of the Protocol Operations for the
Simple Network Management Protocol (SNMP)", STD 62, RFC 3416,
December 2002.
[13] Levi, D., Meyer, P., and B. Stewart, "SNMPv3 Applications", STD
62, RFC 3413, December 2002.
[14] Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based Access
Control Model (VACM) for the Simple Network Management Protocol
(SNMP)", STD 62, RFC 3415, December 2002.
[15] Case, J., Mundy, R., Partain, D., and B. Stewart, "Introduction
to Version 3 of the Internet-Standard Network Management
Framework", RFC 3410, December 2002.
[16] Rohit, R., Srisuresh, P., Raghunarayan, R., Pai, N., and C.
Wang, "Definitions of Managed Objects for Network Address
Translators (NAT)", RFC 4008, March 2005.
[17] Borella, M., Lo, J., Grabelsky, D., and G. Montenegro, "Realm
Specific IP: Framework", RFC 3102, October 2001.
[18] Borella, M., Grabelsky, D., Lo, J., and K. Taniguchi, "Realm
Specific IP: Protocol Specification", RFC 3103, October 2001.
[19] Montenegro, G. and M. Borella, "RSIP Support for End-to-end
Ipsec", RFC 3104, October 2001.
[20] Cuervo, F., Greene, N., Rayhan, A., Huitema, C., Rosen, B., and
J. Segers, "Megaco Protocol Version 1.0", RFC 3015, October
2001.
[21] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[22] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[23] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload",
RFC 2406, November 1998.
[24] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko,
"Diameter Base Protocol", RFC 3588, September 2003.
[25] Durham, D. (Ed.), Boyle, J., Cohen, R., Herzog, S., Rajan, R.,
and A. Sastry, "The COPS (Common Open Policy Service) Protocol",
RFC 2748, January 2000.
[26] Chan, K., Seligson, J., Durham, D., Gai, S., McCloghrie, K.,
Herzog, S., Reichmeyer, F., Yavatkar, R., and A. Smith, "COPS
Usage for Policy Provisioning", RFC 3084, March 2001.
5.2. Informative References
[27] Raz, D., Schoenwalder, J., and B. Sugla, "An SNMP Application
Level Gateway for Payload Address Translation", RFC 2962,
October 2000.
[28] McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB",
RFC 2863, June 2000.
6. Acknowledgements
The editor would like to acknowledge the constructive feedback
provided by Joel M. Halpern on the individual protocol evaluation
contributions. In addition, a thanks to Elwyn Davies, Christopher
Martin, Bob Penfield, Scott Brim and Martin Stiemerling for
contributing to the mailing list discussion on the document content.
Appendix A - SNMP Overview
The SNMP Management Framework presently consists of five major
components:
o An overall architecture, described in RFC 3411 [5]. A more
detailed introduction and applicability statements for the SNMP
Management Framework can be found in RFC 3410 [15].
o Mechanisms for describing and naming objects and events for the
purpose of management. The current version of this Structure of
Management Information (SMI) is called SMIv2 and described in RFC
2578 [6], RFC 2579 [7] and RFC 2580 [8].
o Message protocols for transferring management information. The
current version of the message protocol is called SNMPv3 and
described in RFC 3412 [10], RFC 3414 [11] and RFC 3417 [9].
o Protocol operations for accessing management information. The
current version of the protocol operations and associated PDU
formats is described in RFC 3416 [12].
o A set of fundamental applications described in RFC 3413 [13] and
the view-based access control mechanism described in RFC 3415
[14].
Managed objects are accessed via a virtual information store, termed
the Management Information Base or MIB. Objects in the MIB are
defined using the mechanisms defined in the SMI.
A.1 SNMPv3 Proxy Forwarding
SNMPv3 proxy forwarding (RFC 3413 [13]) provides a standardized
mechanism to configure an intermediate node to forward SNMP messages.
A command generating entity sends requests to a proxy forwarding
entity that forwards the request to a third entity.
One SNMP entity may serve both functions as the SNMP agent to monitor
and configure the node on which it is resident, and as an
intermediate node in a proxy relationship to permit monitoring and
configuration of additional entities.
Each entity is identified by a unique engineID value, specifically to
support proxy between addressing domains and/or trust domains. An
SNMPv3 message contains two engineIDs- one to identify the database
to be used for message security, and one to identify the source (or
target) of the contained data. Message security is applied between
the originator and the proxy, and then between the proxy and the
end-target. The PDU contains the engineID of the node whose data is
contained in the message, which passes end-to-end, unchanged by the
proxy.
SNMPv3 proxy was designed to provide a standard SNMP approach to
inserting an intermediate node in the middle of communications for a
variety of scenarios. SNMPv3 proxy can support crossing addressing
domains, such as IPv4 and IPv6, crossing SNMP version domains, such
as SNMPv3 and SNMPv1, crossing security mechanism domains, such as
DES and AES, and for providing a single point of management contact
for a subset of the network, such as managing a private network
through a NAT device or a VPN endpoint.
A.2 Proxies Versus Application Level Gateways
Proxies are generally preferred to Application Level Gateways for
SNMP. ALGs typically modify the headers and content of messages.
SNMP is a protocol designed for troubleshooting network (mis-)
configurations. Because an operator needs to understand the actual
configuration, the translation of addresses within SNMP data causes
confusion, hiding the actual configuration of a managed device from
the operator. ALGs also introduce security vulnerabilities, and
other complexities related to modifying SNMP data.
SNMP Proxies can modify message headers without modifying the
contained data. This avoids the issues associated with translating
the payload data, while permitting application level translation of
addresses.
The issues of ALGs versus proxies for SNMP Payload Address
Translation are discussed at length in RFC 2962 [27].
Appendix B - RSIP with Tunneling
NAT requires ALGs (Application Layer Gateways) in middleboxes without
MIDCOM, and application modifications or agents for middleboxes with
MIDCOM.
Support for NAT without tunneling could easily be added to the RSIP
control protocol. NAT would be defined as a new, null tunnel type.
Support for the NAT null tunnels could be implemented in hosts, or in
applications or application agents.
If support for NAT null tunnels were implemented in hosts, no
modifications to applications would be required, and no application
agents or ALGs would be required. This has obvious advantages. In
addition to the NAT null tunnel, the host would have to implement an
RSIP / MIDCOM client (or a STUN client) and the middlebox would have
to implement an RSIP / MIDCOM server, or a STUN server would have to
be available _beyond_ the middlebox. Note that the STUN client /
server approach may not work with all types of middleboxes.
If support for NAT null tunnels were NOT implemented in hosts, then
applications would have to be modified, or application agents or ALGs
would have to be implemented. This has the advantage over tunnels
(whether null or not) of not requiring modification to hosts, but
would require the modification of host applications or the
implementation of application agents, both of which would include an
RSIP / MIDCOM client, and the implementation of an RSIP/MIDCOM server
in the middlebox. Again, in some situations, STUN could be used
instead of RSIP / MIDCOM.
Tunneled or not, an RSIP / MIDCOM server is needed in the middlebox.
Tunneled, the host needs to be modified, but not the application.
Untunneled, an agent must be added or the application must be
modified, but there would be no host modifications. The
advantages/disadvantages of tunneling would need to be evaluated in
considering RSIP.
Appendix C - Megaco Modeling Approach
To model the Middlebox functions such as firewall, NAT etc., a new
Middlebox Termination type needs to be defined within Megaco. If
policy-rule overlap or modification by multiple Agents is NOT
required, then a policy rule is equivalent to a Termination (see
Figure 1). The various components of a Policy rule such as filter,
action, life-time, creator etc. are described as various properties
of a Termination. Use of the Virtual Media Gateway (VMG) concept
allows for conflict-free interaction of multiple MA's with the same
MB.
+-------+ +-------+
MA-1 MA-2
+-------+ IF2 +-------+
+-------------------------------------------+
+---------+ +-------------+
IF1 VMG1 +--+ +--+ +--+ VMG2 IF3
---------- Tx-------+ +---Ty--Tz----------------
+--+ +--+ +--+
.... +-------------+
+---------+
+---------------------------------
Middlebox IF4
+----------------------------------------------+
Tx: Termination x = Policy rule x
Ty: Termination y = Policy rule y
Tz: Termination z = Policy rule z
MA: MIDCOM Agent
IF: Interface
Figure 1.
If it is required to allow multiple agents manipulate the same
Middlebox resource (e.g., a Policy rule or a filter), the latter
needs to be kept separate from the Termination (the Policy rule is
manipulated by the MA by manipulating the properties of the
associated Termination). For example, if overlapping policy rule
manipulation is required, then a Termination shall be associated with
a single policy rule, but a policy rule may be associated with more
than one Termination. Thus, a Termination can share a policy rule
with another Termination, or have a policy rule partially overlapping
with that of another Termination. This model allows two MAs,
controlling two distinct Terminations (see Figure 2), manipulate the
same or overlapping policy rules. In Figure 2, policy rules 1 and 2
are overlapping and they are shared by MA-1 and MA-2.
+-------+ +-------+
MA-1 MA-2
+-------+ IF2 +-------+
MB
+-------------------------------------------+
+-----------+ +-------------+
IF1 VMG1 +--+ +--+ +--+ VMG2 IF3
------------------Ty----+ +---Tx--Tz----------------
+--+ +--+ +--+
.... +--/------\---+
+----------+ / \
+----/----------\------------------
+------+----+------+ +------+ IF4
Policy1 Policy2 Policy
3
+----+------+------+ +------+
+----------------------------------------------+
Tx: Termination x
Ty: Termination y
Tz: Termination z
MA: MIDCOM Agent
IF: Interface
MB: Middlebox
Figure 2.
This requires that the Agent and the Middlebox adhere to the
following principles:
(1) Only one Termination has read/write access to a filter at any
time.
(2) When the policy rule is being modified by a new agent (i.e., not
the one that created the policy) the Middlebox makes a policy
decision and decides whether to accept the requested modification
or not. In the case the modification is accepted the initial
MIDCOM agent may be notified.
Appendix D - Diameter IPFilter Rule
The IPFilterRule format is derived from the OctetString AVP Base
Format. It uses the UTF-8 encoding and has the same requirements as
the UTF8String. Packets may be filtered based on the following
information that is associated with it:
Direction (in or out)
Source and destination IP address (possibly masked)
Protocol
Source and destination port (lists or ranges)
TCP flags
IP fragment flag
IP options
ICMP types
Rules for the appropriate direction are evaluated in order, with the
first matched rule terminating the evaluation. Each packet is
evaluated once. If no rule matches, the packet is dropped if the
last rule evaluated was a permit, and passed if the last rule was a
deny.
IPFilterRule filters MUST follow the format:
action dir proto from src to dst [options]
action permit - Allow packets that match the rule.
deny - Drop packets that match the rule.
dir "in" is from the terminal, "out" is to the
terminal.
proto An IP protocol specified by number. The "ip"
keyword means any protocol will match.
src and dst [ports]
The may be specified as:
ipno An IPv4 or IPv6 number in dotted-
quad or canonical IPv6 form. Only
this exact IP number will match the
rule.
ipno/bits An IP number as above with a mask
width of the form 1.2.3.4/24. In
this case, all IP numbers from
1.2.3.0 to 1.2.3.255 will match.
The bit width MUST be valid for the
IP version and the IP number MUST
NOT have bits set beyond the mask.
For a match to occur, the same IP
version must be present in the
packet that was used in describing
the IP address. To test for a
particular IP version, the bits part
can be set to zero. The keyword
"any" is 0.0.0.0/0 or the IPv6
equivalent. The keyword "assigned"
is the address or set of addresses
assigned to the terminal. For IPv4,
a typical first rule is often
"deny in ip! assigned"
The sense of the match can be inverted by
preceding an address with the not modifier (!),
causing all other addresses to be matched
instead. This does not affect the selection of
port numbers.
With the TCP, UDP and SCTP protocols, optional
ports may be specified as:
{portport-port}[,ports[,...]]
The '-' notation specifies a range of ports
(including boundaries).
Fragmented packets that have a non-zero offset
(i.e., not the first fragment) will never match
a rule that has one or more port
specifications. See the frag option for
details on matching fragmented packets.
options:
frag Match if the packet is a fragment and this is not
the first fragment of the datagram. frag may not
be used in conjunction with either tcpflags or
TCP/UDP port specifications.
ipoptions spec
Match if the IP header contains the comma
separated list of options specified in spec. The
supported IP options are:
ssrr (strict source route), lsrr (loose source
route), rr (record packet route) and ts
(timestamp). The absence of a particular option
may be denoted with a '!'.
tcpoptions spec
Match if the TCP header contains the comma
separated list of options specified in spec. The
supported TCP options are:
mss (maximum segment size), window (tcp window
advertisement), sack (selective ack), ts (rfc1323
timestamp) and cc (rfc1644 t/tcp connection
count). The absence of a particular option may
be denoted with a '!'.
established
TCP packets only. Match packets that have the RST
or ACK bits set.
setup TCP packets only. Match packets that have the SYN
bit set but no ACK bit.
tcpflags spec
TCP packets only. Match if the TCP header
contains the comma separated list of flags
specified in spec. The supported TCP flags are:
fin, syn, rst, psh, ack and urg. The absence of a
particular flag may be denoted with a '!'. A rule
that contains a tcpflags specification can never
match a fragmented packet that has a non-zero
offset. See the frag option for details on
matching fragmented packets.
icmptypes types
ICMP packets only. Match if the ICMP type is in
the list types. The list may be specified as any
combination of ranges or individual types
separated by commas. Both the numeric values and
the symbolic values listed below can be used. The
supported ICMP types are:
echo reply (0), destination unreachable (3),
source quench (4), redirect (5), echo request
(8), router advertisement (9), router
solicitation (10), time-to-live exceeded (11), IP
