RMT (TBD) Brian. Adamson
Internet-Draft Naval Research Laboratory
Intended status: Informational Vincent. Roca
Expires: April 7, 2007 INRIA
October 4, 2006
Security and Reliable Multicast Transport Protocols: Discussions and
Guidelines
draft-adamson-roca-rmtsec-issues-00
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Abstract
This document describes some security risks of the Reliable Multicast
Transport (RMT) Working Group set of building blocks and protocols.
An emphasis is placed on risks that might be resolved in the scope of
transport protocol design. However, relevant security issues related
to IP Multicast control-plane and other concerns not strictly within
the scope of reliable transport protocol design are also discussed.
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The document also begins an exploration of approaches that could be
embraced to mitigate these risks. The purpose of this document is to
provide a consolidated security discussion and provide a basis for
further discussion and potential resolution of any significant
security issues that may exist in the current set of RMT standards.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Quick Introduction to RMT Protocols and their Use . . . . . . 5
2.1. The Two Families of CDP . . . . . . . . . . . . . . . . . 5
2.2. RMT Protocol Specificities . . . . . . . . . . . . . . . . 5
2.3. Target Use Case Specificities . . . . . . . . . . . . . . 6
3. Known Security Threats . . . . . . . . . . . . . . . . . . . . 7
3.1. Control-Plane Attacks . . . . . . . . . . . . . . . . . . 8
3.1.1. Control Plane Monitoring . . . . . . . . . . . . . . . 8
3.1.2. Unauthorized (or Malicious) Group Membership . . . . . 8
3.2. Data-Plane Attacks . . . . . . . . . . . . . . . . . . . . 9
3.2.1. Rogue Traffic Generation . . . . . . . . . . . . . . . 9
3.2.2. Sender Message Spoofing . . . . . . . . . . . . . . . 10
3.2.3. Receiver Message Spoofing . . . . . . . . . . . . . . 10
3.2.4. Replay Attacks . . . . . . . . . . . . . . . . . . . . 11
4. General Security Goals . . . . . . . . . . . . . . . . . . . . 12
4.1. Network Protection . . . . . . . . . . . . . . . . . . . . 12
4.2. Protocol Protection . . . . . . . . . . . . . . . . . . . 13
4.3. Content Protection . . . . . . . . . . . . . . . . . . . . 13
5. Elementary Security Services . . . . . . . . . . . . . . . . . 13
6. Technological Building Blocks . . . . . . . . . . . . . . . . 15
6.1. IPsec . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1.1. Benefits . . . . . . . . . . . . . . . . . . . . . . . 15
6.1.2. Requirements . . . . . . . . . . . . . . . . . . . . . 15
6.1.3. Limitations . . . . . . . . . . . . . . . . . . . . . 16
6.2. TESLA . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.2.1. Benefits . . . . . . . . . . . . . . . . . . . . . . . 16
6.2.2. Requirements . . . . . . . . . . . . . . . . . . . . . 16
6.2.3. Limitations . . . . . . . . . . . . . . . . . . . . . 17
6.3. SSM Multicast Routing . . . . . . . . . . . . . . . . . . 17
6.3.1. Benefits . . . . . . . . . . . . . . . . . . . . . . . 17
6.3.2. Requirements . . . . . . . . . . . . . . . . . . . . . 17
6.3.3. Limitations . . . . . . . . . . . . . . . . . . . . . 17
7. Global Security Infrastructure . . . . . . . . . . . . . . . . 17
7.1. Public Key Infrastructure . . . . . . . . . . . . . . . . 18
7.1.1. Benefits . . . . . . . . . . . . . . . . . . . . . . . 18
7.1.2. Requirements . . . . . . . . . . . . . . . . . . . . . 18
7.1.3. Limitations . . . . . . . . . . . . . . . . . . . . . 18
7.2. Group Key Management and Re-keying Protocols . . . . . . . 18
7.2.1. Benefits . . . . . . . . . . . . . . . . . . . . . . . 18
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7.2.2. Requirements . . . . . . . . . . . . . . . . . . . . . 18
7.2.3. Limitations . . . . . . . . . . . . . . . . . . . . . 18
8. New Threats Introduced by the Security Scheme Itself . . . . . 18
9. Consequences for the RMT and MSEC Working Group . . . . . . . 18
9.1. RMT Transport Message Security Encapsulation Header . . . 18
10. Security Considerations . . . . . . . . . . . . . . . . . . . 19
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
12. Normative References . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . . . 21
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1. Introduction
The Reliable Multicast Transport (RMT) Working Group has produced a
set of building blocks (BB) and protocol instantiation (PI)
specifications for reliable multicast data transport. The two PIs
defined within the scope of RMT: ALC
[RFC3450][draft-ietf-rmt-pi-alc-revised-03] and NORM [RFC3940], as
well as the FLUTE application [RFC3926] built on top of ALC, are
"Content Delivery Protocols" (CDP). In this document the term CDP
will refer indifferently to either ALC or NORM, with their associated
BBs.
The use of these BBs and PIs raises new security risks. For
instance, these protocols share a novel set of Forward Error
Correction (FEC) and congestion control building blocks that present
some new capabilities for Internet transport, but may also pose some
new security risks. Yet some security risks are not related to the
particular BBs used by the PIs, but are more general. Reliable
multicast transport sessions are expected to involve at least one
sender and multiple receivers. Thus, the risk of and avenues to
attack are implicitly greater than that of point-to-point (unicast)
transport sessions. Also the nature of IP multicast can expose other
coexistent network flows and services to risk if malicious users
exploit it. The classic any-source multicast (ASM) model of
multicast routing allows any host to join an IP multicast group and
send traffic to that group. This poses many potential security
challenges. And, while the emerging single-source multicast (SSM)
model that allows only a single sender to send traffic to a group
simplifies some challenges, there remain some specific issues. For
instance, possible areas of attack include those against the control
plane where malicious hosts join IP multicast groups to cause
multicast traffic to be directed to parts of the network where it is
not needed or desired. This can indirectly cause denial-of-service
(DoS) to other network flows. Also, attackers may transmit erroneous
or corrupt messages to the group or employ strategies such as replay
attack within the "data plane" of protocol operation.
The goals of this document are therefore:
1. to define the possible general security goals. Defining what we
want to protect, i.e. the network itself, and/or the protocol,
and/or the content, is the first step.
2. to list the possible elementary security services that will make
it possible to fullfil the general security goals. Some of these
services are generic (e.g. object and/or packet integrity), while
others are specific to RMT protocols (e.g. congestion control
specific security schemes).
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3. to list some technological building blocks and solutions that can
provide the desired security services.
4. last but not least, to highlight the CDP specificities that will
impact security and define some use-cases. Indeed, the set of
solutions proposed to fulfill the security goals will greatly be
impacted by the target use case.
In some cases, the existing RMT documents already discuss the risks
and outline approaches to solve them, at least partially. The
purpose of this document is to consolidate this content and provide a
basis for further discussion and potential resolution of any
significant security issues that may exist.
2. Quick Introduction to RMT Protocols and their Use
2.1. The Two Families of CDP
TODO: a quick introduction to the two families of solutions: ALC/LCT
and NORM.
2.2. RMT Protocol Specificities
This section focuses on the RMT protocol specificities that will
impact the choice of the technological building blocks, and the way
they can be applied.
Both ALC and NORM have been designed with scalability in mind (in
terms of the number of receivers). Yet if ALC targets massively
scalable sessions (e.g. with millions of receivers), NORM is less
ambitious, essentially because of the use of feedback messages to the
source. Ideally, the use of security mechanisms should not break
this scalability feature.
The ALC and NORM protocols differ in the communication paths:
o sender to receivers: ALC and NORM, for bulk data transfer and
signaling messages;
o receivers to sender: NORM only, for feedback messages;
o receivers to receivers: NORM only for control messages;
But the fact that ALC is capable of working on top of purely
unidirectional networks does not mean that no back-channel will be
available (see Section 2.3).
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ALC defines an on-demand content delivery model where receivers can
arrive at any time, at their own discretion, download the content and
leave the session. This delivery model is not supported by NORM The
push and streaming content delivery models are supported by both ALC
and NORM.
2.3. Target Use Case Specificities
This section focuses on the target use cases and their specificities.
These specificities will impact both the choice of the technological
building blocks and the way they can be applied. One can distinguish
the following use case features:
o Purely unidirectional transport versus symmetric bidirectional
transport versus asymmetric bidirectional transport. Most of the
time, the amount of traffic flowing to the source is limited, and
one can overlook whether the transport channel is symmetric or
not. The nature of the underlying transport channel is of
paramount importance, since many security building blocks will
require a bidirectional communication;
o Massively scalable versus moderately scalable session. Here we do
not define precisely what the terms "massively scalable" and
"moderately scalable" mean.
o Known set of receivers versus unknown set of receivers: does the
source know at any point of time the set of receivers or not? Of
course, knowing the set of receivers is usually not compatible
with massively scalable sessions;
o Dynamic set of receivers versus fixed set of receivers: does the
source know at some point of time the maximum set of receivers or
will it evolve dynamically? In case of a dynamic set of
receivers, the source might desire to provide
o High rate data flow versus small rate data flow: some security
building blocks are CPU intensive and are therefore incompatible
with high data rate sessions (e.g. this is the case of solutions
that digitally sign all the packets sent).
o Protocol stack available at both ends: a solution that requires
some non-usual features within the protocol stack will not always
be usable. Some target environments (e.g. embedded systems) only
provide a minimum set of features and extending them (e.g. to add
IPsec) is not necessarily realistic;
o Multicast routing other layer-3 protocols in use: for instance,
SSM multicast routing is often seen as one of the key service to
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improve the security within multicast sessions, and some security
building blocks require specialized versions of layer-3 protocols
(e.g. IGMP/MLD with security extensions). Depending on the
target use case, these assumptions might or not be realistic;
o (TBD) more items?
Depending on the target goal and the associated security building
block used, other features, related to the use case, might be of
importance. For instance TESLA requires a loose time synchronization
between the source and the receivers. Several possibilities exist to
that purpose, some of them being only feasible if the target use case
provide the appropriate features.
3. Known Security Threats
Reliable multicast transport sessions are expected to involve at
least one sender and multiple receivers. Thus, the risk of and
avenues to attack are implicitly greater than that of point-to-point
(unicast) transport sessions. Also the nature of IP multicast can
expose other coexistent network flows and services to risk if
malicious users exploit it. The classic any-source multicast (ASM)
model of multicast routing allows any host to join an IP multicast
group and send traffic to that group. This poses many potential
security challenges. And, while the emerging single-source multicast
(SSM) model that allows only a single sender to send traffic to a
group simplifies some challenges, there remain some specific issues
with respect to adopting standard Internet security mechanisms such
as IPSec[RFC2401]. Possible areas of attack include those against
the control plane where malicious hosts join IP multicast groups to
cause multicast traffic to be directed to parts of the network where
it is not needed or desired. This can indirectly cause denial-of-
service (DoS) to other network flows. Also, attackers may transmit
erroneous or corrupt messages to the group or employ strategies such
as replay attack within the "data plane" of protocol operation.
CDP integrate several BBs. The TCP-Friendly Multicast Congestion
Control (TFMCC) [RFC4654] building block specifies protocol
mechanisms where senders and receivers exchange messages to manage
sender transmission rate.
The IP architecture provides common access to notional control and
data planes to both end and intermediate systems. For the purposes
of discussion here, the "control plane" mechanisms are considered
those with message exchanges between end systems (typically
computers) and intermediate systems (typically routers) and while the
"data plane" encompasses messages exchanged among end systems,
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usually pertaining to the transfer of application data.
3.1. Control-Plane Attacks
While control-plane attacks may be considered outside of the scope of
the transport protocol specfications discussed here, it is important
to understand the potential impact of such attacks with respect to
the deployment and operation of these protocols. For example,
awareness of possible IP Multicast control-plane manipulation that
can lead to unauthorized (or unexpected) monitoring of data plane
traffic by malicious users may lead a transport application or
protocol implementation to support encryption to ensure data
confidentiality and/or privacy. Also, these types of attack also
have bearing on assessing the real risks of potentially more complex
attacks against the transport mechanisms themselves. In some cases,
the solutions to these control-plane risk areas may reduce the impact
or possibility of some data-plane attacks that are discussed in this
document.
3.1.1. Control Plane Monitoring
While this may not be a direct attack on the transport system, it may
be possible for an attacker to gain useful information in advancing
attack goals by monitoring IP Multicast control plane traffic
including group membership and multicast routing information.
Indentification of hosts and/or routers participating in specific
multicast groups may readily identify systems vulnerable to protocol-
specific exploitation. And, with regards to user privacy concerns,
such "side information" may be relevant to this emerging aspect of
network security.
3.1.2. Unauthorized (or Malicious) Group Membership
One of the simplest attacks is that where a malicious host joins an
IP multicast group so that potentially unwanted traffic is routed to
the host's network interface. This type of attack can turn a
legitimate source of IP traffic into a "attacker" without requiring
any access privileges to the source host or routers involved. This
type of attack can be used for denial-of-service purposes or for the
real attacker (the malicious joiner) to gain access to the
information content being sent. Similarly, some routing protocols
may permit any sender (whether joined to the specific group or not)
to transmit messages to a multicast group.
It is possible that malicious hosts could also spoof IGMP messages,
joining groups posing as legitimate hosts (or spoof source traffic
from legitimate hosts). This may be done at intermediate locations
in the network or by hosts co-resident with the authorized hosts on
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local area networks. Such spoofing could be done by raw packet
generation or with replay of previously-recorded control messages.
For the sake of completeness, it should be noted that multicast
routing protocol control messaging may be subject to similar threats
if insufficient protocol security mechanisms are enabled in the
routing infrastructure.
The presence of these types of attack may necessitate that policy-
based controls be emplaced in routers to limit the distribution
(including transmission and reception) of multicast traffic (on a
group-wise and/or traffic volume basis) to different parts of the
network. Such policy-based controls are beyond the scope of the RMT
protocol specifications. However, such network protection mechanisms
may reduce the opportunities for or effectiveness of of some of the
data-plane attacks discussed later. For example, reverse-path checks
can significantly limit opportunities for attackers to conduct replay
attacks when hosts actually do use IPSec. Also, future IP Multicast
control protocols may wish to consider providing security mechanism
to prevent unauthorized monitoring or manipulation of messages
related to group membership, routing, and activity.
3.2. Data-Plane Attacks
This section discusses some types of active attacks that might be
conducted "in-band" with respect to the reliable multicast transport
protocol operating within the data plane of network data transfer.
The passive attack of unauthorized data-plan monitoring is discussed
above since such activity might be made possible by the
vulnerabilities of the IP Multicast control plane. To cover the two
classes of RMT protocols, the active data-plane attacks are
categorized as 1) those where the attacker generates messages posing
as a data sender, and 2) those where the attacker generates messages
posing as a receiver providing feedback to the sender(s) or group.
Additionally, a common threat to protocol operation is that of brute-
force, rogue packet generation. This is discussed briefly below, but
the more subtle attacks that might be conducted are given more
attention as those fall within the scope of the RMT transport
protocol design. Additionally, special consideration is given to
that of the "replay attack" [see Section 3.2.4], as it can be applied
across these different categories.
3.2.1. Rogue Traffic Generation
If an attacker is able to successfully inject packets into the
multicast distribution tree, the most obvious denial-of-service
attack is for the attacker to generate a large volume of apparently
authenticate (when authentication mechanisms are used, a "replay"
attack strategy might be used and this is discussed in more in detail
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in ) traffic. The impact of this type of attack can be significant
since the potential for routers to relay the traffic to multiple
portions of a networks (as compared to a single unicast routing
path). However, other than the amplified negative impact to the
network, this type of attack is no different than what is possible
with rogue unicast packet generation and similar measures used to
protect the network from such attacks could be used to contain this
type of brute-force attack. Of course, the pragmatic question of
whether current implementations of such protection mechanisms support
IP Multicast should be considered.
3.2.2. Sender Message Spoofing
These types of attacks are applicable to both general types of RMT
protocols (e.g., ALC (sender-only transmission) and NORM (sender-
receiver exhanges)). Without an authentication mechanism, it is
easily possible for a computer to generate sender messages that could
disrupt a reliable multicast transfer session. And with FEC-based
transport mechanisms, a single packet with an apparently-correct FEC
payload identifier [RFC3452] but a corrupted FEC payload could
potentially render an entire block of transported data invalid.
Thus, a modest injection rate of corrupt traffic could cause severe
impairment of data transport. Additionallly, such invalid sender
packets could convey out-of-bound indices (payload symbol or block
identifiers, etc) that could lead to buffer overflow exploits in
receivers or other similar issues in implementations with
insufficient checks for invalid data.
An indirect use of sender message spoofing would be to generate
messages that would cause receivers to take inappropriate congestion-
control action. In the case of the layered congestion control
mechanisms proposed for ALC use, this could lead to the receivers
erroneusly leaving groups associated with higher bandwidth transport
layers and suffering unnecessarily low transport rates. Similarly,
receivers may be misled to join inappropriate groups directing
unwanted traffic to their part of the network. Attacks with similar
effect could be conducted against the TFMCC approach proposed for
NORM operation with spoofing of sender messages carrying congestion
control state to receivers.
3.2.3. Receiver Message Spoofing
These atacks are limited to RMT protocols that use feedback from
receivers in the group to influence sender and other receiver
operation. In the NORM protocol, this includes negative-
acknowledgement (NACK) messages fed back to the sender to achieve
reliable transfer, congestion control feedback content, and the
optional positive acknowledgement features of the specification. It
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is also important to note that for ASM operation, NORM receivers pay
attention to the messages of other receivers for the purpose of
suppression to avoid feedback implosion as group size grows large.
An attacker that can generate false feedback can manipulate the NORM
sender to unnecessarily transmit repair information and reduce the
goodput of the reliable transfer regardless of the sender's transmit
rate. Contrived congestion control feedback could also cause the
sender to transmit at an unfairly low rate.
As mentioned, spoofed receiver messaging may not be directed only at
senders, but also at receivers participating in the session. For
example, an attacker may direct phony receiver feedback messages to
selected receivers in the group causing those receivers to suppress
feedback that might have otherwise been transmitted. This attack
could compromise the ability of those receivers to achieve reliable
transfer. Also, suppressed congestion control feedback could cause
the sender to perhaps transmit at a rate unfair to those attacked
receivers if their fair congestion control rate were lower than other
receivers in the group.
3.2.4. Replay Attacks
The infamous "replay attack" (injection of a previously transmitted
packet (or at least its payload) into the reliable transport group or
directly to one or more of its participants) is given special
attention here because of the special consequences it can have on RMT
protocol operation. Without specific protection mechanisms against
replay (e.g. duplicate message detection), it is possible for these
attacks to be successful even when security mechanisms such as packet
authentication and/or encryption are employed.
3.2.4.1. Replay of Sender Messages
Generally, replay of recent protocol messages from the sender will
not harm transport, and could potentially assist it, unless it is of
sufficient volume to result in the same type of impact as the "rogue
traffic generation" described above. However, it is possible that
replay of sufficiently old messages may cause receivers to think they
are "out of sync" with the sender and reset state, compromising the
transfer. Also, if sender transport data identifiers are reused
(object identifiers, FEC payload identifiers, etc), it is possible
that replay of old messages could corrupt data of a current transfer.
3.2.4.2. Replay of Receiver Messages
Replay of receiver messages are problematic for the NORM protocol,
because replay of NACK messages could cause the sender unnecessarily
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transmit repair information for an FEC coding block. Similarly, the
sender transmission rate might be manipulated by replay of congestion
control feedback messages from receivers in the group. And the way
that NORM senders estimate group round-trip timing (GRTT) could allow
a replay attack to manipulate the senders' GRTT estimate to an
unnecessarily large value, adding latency to the reliable transport
process.
4. General Security Goals
The term "security" is extremely vast and encompasses many different
meanings. The goal of this section is to clarify what "security"
means when considering the reliable multicast transport (RMT)
protocols being defined in the IETF RMT working group. The scope can
also encompass additional group communication applications, for
instance streaming applications. This section only focuses on the
desired general goals. The following sections will then discuss the
possible elementary services that will be required to fulfill these
general goals, as well as the underlying technological building
blocks.
The possible final goals include, in decreasing order of importance:
o network protection: the goal is to protect the network from
attacks, no matter whether these attacks are voluntary (i.e.
launched by one or several attackers) or non voluntary (i.e.
caused by a misbehaving system, where "system" can designate a
building block, a protocol, an application, or a user);
o protocol protection: the goal is to protect the RMT protocol
itself, e.g. to avoid that a misbehaving receiver prevents other
receivers to get the content, no matter whether this is done
intentionally or not;
o and content protection: to goal is to protect the content itself,
for instance to guaranty the integrity of the content, or to make
sure that only authorized clients can access the content.
4.1. Network Protection
Protecting the network is of course of primary importance. An
attacker should not be able to damage the whole infrastructure by
exploiting some features of the RMT protocol. Unfortunately, recent
past has shown that the multicast routing infrastructure is
relatively fragile, as well as the applications built on top of it.
(TBD) develop...
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4.2. Protocol Protection
Protecting the protocols is also importance, since the higher the
number of clients, the more serious the consequences of an attack.
This is all the more true as scalability is often one of the desired
goals of RMT protocols. Ideally, receivers should be sufficiently
isolated from one another, so that a single misbehaving receiver does
not affect others. Similarly, an external attacker should not be
able to break the system.
(TBD) develop...
4.3. Content Protection
Finally, the content itself should be protected when meaningful.
This level of security is often the concern of the content provider
(and its responsibility). For instance, in case of confidential (or
non-free) content, the typical solution consists in encrypting the
content. It can be done within the upper application, i.e. above the
RMT protocol, or within the transport system.
But other requirements may exist, like verifying the integrity of a
received object, or authenticating the sender of the received
packets. To that goal, one can rely on the use of building blocks
integrated within, or above, or beneath the RMT protocol.
One may also consider that offering the packet sender authentication
and content integrity services are basic requirements that should
fulfill any RMT system that operates within an open network, where
any attacker can easily inject spurious traffic in an ongoing NORM or
ALC session. In that case this goal is not the responsibility of the
content provider but the responsibility of the administrator who
deploys the RMT system itself.
5. Elementary Security Services
The goals defined in Section 4 will be fulfilled by means of
underlying elementary services, provided by one or several
technological building blocks. This section only focuses on these
elementary services.
The services traditionally listed are:
o packet source authentication and integrity: the goal is to enable
a receiver to verify the source of a packet and that the packet
has not been modified in transit;
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o packet group authentication and integrity: the goal is to enable a
receiver to verify that a packet originated within the group and
has not been modified by nonmembers in transit;
o packet non-repudiation: the goal is to enable any third party to
verify the source of a packet. In that case the source cannot
repudiate having sent the packet;
o packet anti-replay: the goal is to enable a receiver to detect
that a given packet has already been received;
o object integrity: the goal is to enable a receiver to verify the
integrity of the whole object;
o object confidentiality: the goal is to enable a source to guaranty
that only authorized receivers can access the object data;
o (TBD) more items?
To this list, one must add the services specific to the RMT
protocols:
o congestion control security: the goal is to prevent an attacker
from modifying the congestion control protocol normal behavior.
Possible goals for an attacker can be to reduce the transmission
(NORM) and/or reception rate (ALC or NORM) up to a point where no/
little traffic will flow, or on the opposite, to increase this
rate up to a point where it will congest the network;
o group management: the goal is to make sure that only authorized
receivers (as defined by a certain group management policy), join
the RMT session and possibly inform the source;
o backward group secrecy: the goal is to prevent a new group member
to access the information in clear sent to the group before he
joined the group;
o forward group secrecy: the goal is to prevent a former group
member to access the information in clear sent to the group after
he left the group;
o (TBD) more items?
These services are usually achieved by means of one or several
technological building blocks. The target use case where the RMT
system will be deployed will greatly impact the choice of the
technological building block(s) used to provide these services, as
explained in Section 2.3.
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6. Technological Building Blocks
Here is a list of techniques and building blocks that are likely to
fulfill one or several of the goals listed above:
o IPsec;
o TESLA;
o use of SSM (Source Specific Multicast) multicast routing;
o Digital Signature;
o (TBD) add other BBs
Each of them is now quickly discussed. In particular we identify
what service it can offer, its limitations, and its field of
application (adequacy W.R.T. the CDP and the target use case).
6.1. IPsec
6.1.1. Benefits
One direct approach using existing standards is to apply IPSec
[RFC2401] to achieve the following properties for message
transmission:
1. Authentication (IPSec AH or ESP)
2. Confidentiality (IPSec ESP)
6.1.2. Requirements
It is expected that the approach to apply IPSec for reliable
multicast transport sessions is similar to that described for OSPFv3
security[RFC4552]. The following list proposes the IPSec
capabilities needed to support a similar approach to RMT protocol
security:
1. Mode - Transport mode IPSec security is required;
2. Selectors - source and destination addresses and ports, protocol.
3. For some uses, preplaced manual key support may be required to
support application deployment and operation. For automated key
management for group communication the Group Secure Association
Key Management Protocol (GSAKMP) described in [RFC4535] may be
used to emplace the keys for IPSec operation.
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Note that a periodic rekeying procedure similar to that described in
RFC 4552 can also be applied with the additional benefit that the
reliable transport aspects of the RMT protocols provide robustness to
any message loss that might occur due to ANY timing discrepencies
among the participants in the reliable multicast session.
6.1.3. Limitations
It should be noted that current IPSec implementations may not provide
the capability for anti-replay protection for multicast operation.
In the case of the NORM protocol, a sequence number is provided for
packet loss measurement to support congestion control operation.
This sequence number can also be used within a NORM implementation
for detecting duplicate (replayed) messages from sources (senders or
receivers) within the transport session group. In this way,
protection against replay attack can be achieved in conjunction with
the authentication and possibly confidentiality properties provided
by an IPSec encapsulation of NORM messages. NORM receivers generate
a very low volume of feedback traffic and it is expected that the 16-
bit sequence space provided by NORM will be sufficient for replay
attack protection. When a NORM session is long-lived, the limits of
the sender repair window are expected to provide protection from
replayed NACKs as they would typically be outside of the sender's
current repair window. It is suggested that IPSec implementations
that can provide anti-replay protection for IP Multicast traffic,
even when there are multiple senders within a group, be adopted. The
GSAKMP document has some discussion in this area.
6.2. TESLA
6.2.1. Benefits
TESLA [TESLA_4_ALC_NORM] offers a loss tolerant, lightweight,
authentication/integrity service for the packets generated by the
session's sender. Depending on the time synchronization method used,
TESLA is compatible with massively scalable sessions. TESLA CPU
processing remains limited, in particular because it essentially
relies on symmetric cryptographic building blocks.
6.2.2. Requirements
The security offered by TESLA relies heavily on time. Therefore the
session's sender and each receiver need to be time synchronized in a
secure way. To that purpose, several methods exist, depending on the
use case: direct time synchronization (which requires a bidirectional
transport channel); using a secure NTP infrastructure (which also
requires a bidirectional transport channel); or a GPS or Galileo
device; or a clock with a time-drift that is negligible in front of
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the TESLA time accuracy requirements. So TESLA can be used with both
bidirectional and unidirectional transport channels, in the later
case on condition an appropriate indirect time synchronization method
exists.
6.2.3. Limitations
TESLA does not consider the packets that will be generated by
receivers, for instance the feedback packets of NORM, which must be
protected by other means.
Another limitation is that TESLA requires some buffering capabilities
at the receivers in order to enable the delayed authentication
feature. This is not considered though as a major issue in the
general case (e.g. FEC decoding of objects within an ALC session
already requires some buffering capabilities, that often exceed that
of TESLA), but it might be one in case of embedded environments.
6.3. SSM Multicast Routing
6.3.1. Benefits
6.3.2. Requirements
6.3.3. Limitations
7. Global Security Infrastructure
Deploying the elementary technological building blocks often requires
that a global security infrastructure exists. This security
infrastructure:
o can provide a PKI (Public Key Infrastructure): this PKI provides
for trusted third party vetting of, and vouching for, user
identities. It also allows the binding of public keys to users,
usually by means of certificates.
o can provide a group key management service: this service usually
provides rekeying schemes, either periodic or triggered by some
higher level event. It is required in particular when the group
is dynamic and forward/backward secrecy are important. This is
also required to improve the scalability of the CDP (since key
management is done automatically, using a key server topology), or
the security provided by the CDP (since the underlying
cryptographic keys will be changed frequently).
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o (TBD): add more...
TBD: more information on group key management, etc.
7.1. Public Key Infrastructure
7.1.1. Benefits
7.1.2. Requirements
7.1.3. Limitations
7.2. Group Key Management and Re-keying Protocols
7.2.1. Benefits
7.2.2. Requirements
7.2.3. Limitations
8. New Threats Introduced by the Security Scheme Itself
Introducing a security scheme, as a side effect, can sometimes
introduce new security threats. For instance, signing all packets
with asymmetric cryptographic schemes (to provide a source
authentication/content integrity/anti-replay service) opens the door
to DoS attacks. Indeed, verifying asymmetric-based cryptographic
signatures is a CPU intensive task. Therefore an attacker can easily
overload a receiver (or a sender in case of NORM) by injecting a
significant number of faked packets.
XXX: TODO...
9. Consequences for the RMT and MSEC Working Group
(TBD) discuss here the possible recommendations for the RMT and MSEC
groups... Or remove this section altogether...
9.1. RMT Transport Message Security Encapsulation Header
An alternative approach to using IPSec to provide the necessary
properties to protect RMT protocol operation from the application
attacks described earlier, is to extend the RMT protocol message set
to include a message encapsulation option. This encapsulation header
could be used to provide authentication, confidientiality, and anti-
replay protection as needed. Since this would be independent of the
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IP layer, the header might need to provide a source identifier to be
used as a "selector" for recalling security state (including
authentication certificate(s), sequence state, etc) for a given
message. In the case of the NORM protocol, a NormNodeId field exists
that could be used for this purpose. In the case of ALC, the
security encapsulation mechanism would need to add this function.
The security encapsulation mechanism, although resident "above" the
IP layer, could use GSAKMP [RFC4535] or a similar approach for
automated key managment.
10. Security Considerations
Not applicable since this document does not introduce any new
technique.
11. Acknowledgments
12. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC3450] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., and J.
Crowcroft, "Asynchronous Layered Coding (ALC) Protocol
Instantiation", RFC 3450, December 2002.
[RFC3451] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., Handley,
M., and J. Crowcroft, "Layered Coding Transport (LCT)
Building Block", RFC 3451, December 2002.
[RFC3452] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley,
M., and J. Crowcroft, "Forward Error Correction (FEC)
Building Block", RFC 3452, December 2002.
[RFC3926] Paila, T., Luby, M., Lehtonen, R., Roca, V., and R. Walsh,
"FLUTE - File Delivery over Unidirectional Transport",
RFC 3926, October 2004.
[RFC3940] Adamson, B., Bormann, C., Handley, M., and J. Macker,
"Negative-acknowledgment (NACK)-Oriented Reliable
Multicast (NORM) Protocol", RFC 3940, November 2004.
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[RFC4535] Harney, H., Meth, U., Colegrove, A., and G. Gross,
"GSAKMP: Group Secure Association Key Management
Protocol", RFC 4535, June 2006.
[RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, June 2006.
[RFC4654] Widmer, J. and M. Handley, "TCP-Friendly Multicast
Congestion Control (TFMCC): Protocol Specification",
RFC 4654, August 2006.
[TESLA_4_ALC_NORM]
Roca, V., Francillon, A., and S. Faurite, "The Use of
TESLA in the ALC and NORM Protocols",
draft-ietf-msec-tesla-for-alc-norm-00.txt (work in
progress), June 2006.
[draft-ietf-rmt-pi-alc-revised-03]
Luby, M., Watson, M., and L. Vicisano, "Asynchronous
Layered Coding (ALC) Protocol Instantiation",
draft-ietf-rmt-pi-alc-revised-03.txt (work in progress),
April 2006.
Authors' Addresses
Brian Adamson
Naval Research Laboratory
Washington, DC 20375
USA
Email: adamson@itd.nrl.navy.mil
URI: http://cs.itd.nrl.navy.mil
Vincent Roca
INRIA
655, av. de l'Europe
Zirst; Montbonnot, ST ISMIER cedex 38334
France
Email: vincent.roca@inrialpes.fr
URI: http://planete.inrialpes.fr/~roca/
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