Internet Engineering Task Force P. Savola
Internet-Draft CSC/FUNET
Expires: July 22, 2004 R. Lehtonen
TeliaSonera
D. Meyer
Jan 22, 2004
PIM-SM Multicast Routing Security Issues and Enhancements
draft-savola-mboned-mroutesec-00.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This memo describes security threats for the larger (intra-domain, or
inter-domain) multicast routing infrastructures. Only Protocol
Independent Multicast - Sparse Mode (PIM-SM) is analyzed, in its
three main operational modes: the traditional Any Source Multicast
(ASM) model, Source-Specific Multicast (SSM) model, and the ASM model
enhanced by the Embedded RP group-to-RP mapping mechanism. This memo
also describes enhancements to the protocol operations to mitigate
these threats.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Threats to Multicast Routing . . . . . . . . . . . . . . . . 4
3.1 Receiver-based Attacks . . . . . . . . . . . . . . . . . . . 4
3.1.1 Joins to Different Groups . . . . . . . . . . . . . . . . . 5
3.2 Source-based Attacks . . . . . . . . . . . . . . . . . . . . 6
3.2.1 Sending Multicast to Empty Groups . . . . . . . . . . . . . 6
3.2.2 Disturbing Existing Group by Sending to It . . . . . . . . . 7
3.3 Aggravating Factors to the Threats . . . . . . . . . . . . . 8
3.3.1 Distant RP/Source Problem . . . . . . . . . . . . . . . . . 8
3.3.2 RPF Considers Interface, Not Neighbor . . . . . . . . . . . 8
3.3.3 No Receiver Information in PIM Joins . . . . . . . . . . . . 9
3.3.4 Injecting a Bogus Route . . . . . . . . . . . . . . . . . . 9
4. Threat Analysis . . . . . . . . . . . . . . . . . . . . . . 9
4.1 Summary of the Threats . . . . . . . . . . . . . . . . . . . 9
4.2 Enhancements for Threat Mitigation . . . . . . . . . . . . . 10
5. PIM Security Enhancements . . . . . . . . . . . . . . . . . 11
5.1 Remote Routability Signalling . . . . . . . . . . . . . . . 11
5.2 RPF to Check Neighbor, not Interface . . . . . . . . . . . . 12
5.3 Rate-limiting Possibilities . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 13
Normative References . . . . . . . . . . . . . . . . . . . . 13
Informative References . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . 16
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1. Introduction
This memo describes security threats to the Protocol Independent
Multicast - Sparse Mode (PIM-SM) multicast routing infrastructures,
and suggests ways to make these architectures more resistant to the
described threats.
Only attacks which have an effect on the multicast routing (whether
intra- or inter-domain) are considered. For example, attacks where
the hosts are specifically targeting the Designated Router (DR) or
other routers of the link, or where hosts are disrupting other hosts
on the same link are out of scope. Similarly, ensuring
confidentiality, authentication and integrity of multicast groups and
traffic is out of the scope [9].
PIM builds on a model where Reverse Path Forwarding (RPF) check is
(among other things) used to ensure loop-free properties of the
multicast distribution trees. As a side effect, this limits the
effect of using forged source addresses, which is often as a
component in unicast-based attacks. However, a host can still spoof
an address within the same subnet, or spoof the source of a
unicast-encapsulated PIM Register messages, which a host may send on
its own.
We consider PIM-SM [1] operating in the traditional Any Souce
Multicast (ASM) model (including the use of Multicast Source
Discovery Protocol (MSDP) [2] for source discovery), in
Source-Specific Multicast [3] (SSM) model, and the Embedded-RP [4]
group-to-RP mapping mechanism in ASM model. If Bidirectional-PIM
enhancements are globally significant, and have implications, they
could also be considered.
2. Terminology
ASM
Term "ASM" [6] is used to refer to the traditional Any Source
Multicast model with multiple PIM domains and a signalling
mechanism (MSDP) to exchange information about active sources
between them.
SSM
Term "SSM" [7] is used to refer to Source-Specific Multicast.
Embedded-RP
Embedded-RP refers to ASM model where the Embedded-RP mapping
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mechanism is used to find the RP for a group, and MSDP is not
needed.
Target Router
Target Router is used to refer to either the RP processing a
packet (ASM or Embedded-RP), or DR close to the transmitting
source (SSM).
3. Threats to Multicast Routing
We make the broad assumption that the multicast routing networks are
reasonably trusted. That is, we assume that the multicast routers
themselves behave "well", in the same sense that unicast routers are
expected to behave well, and are not a significant source of abuse.
This assumption is not entirely correct, but it simplifies the
analysis of threat models. If seen important, the threats caused by
misbehaving multicast routers (including fake multicast routers) may
be considered separately.
As the threats described in this memo are mainly Denial of Service
(DoS) attacks, it may be useful to note that the attackers will try
to find a scarce resource anywhere in the control or data plane, as
described in [5].
3.1 Receiver-based Attacks
These attacks are often referred to as control plane threats and the
aim of the attacker is usually to increase the amount of multicast
state information in routers above a manageable level.
One should note that hosts can also originate PIM messages (e.g. PIM
Joins) as long as their source address passes the RPF checks. This
implies that a willful attacker will be able to circumvent many of
the potential rate-limiting functions performed at the DR -- as one
can always send the messages yourself. The PIM-SM specification,
however, states that these messages should only be accepted from
known PIM neighbors [1]; if these would be implemented, the hosts
would have to forge PIM Hello messages as well.
One should also note that even if a host joins to a group multiple
times, the DR only sends one PIM Join message, without waiting for
any acknowledgement; the next message is only sent after the timer
expires or the state changes at the DR.
Also, if the host uses IGMPv3 [10] or MLDv2 [11], it is able to join
multiple sources for the same group and each of these joins for the
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same group generates new PIM (Source, Group), or (S,G) Joins.
3.1.1 Joins to Different Groups
Description of the threat: Join Flooding. This happens when a host
tries to join, once or a couple of times, to a group or a channel,
and the DR generates a PIM Join towards the Target Router. The group/
channel or the Targer Router may or may not exist.
Example of this is a host trying to join different, non-existent
groups at a very rapid pace, trying to overload the routers on the
path with an excessive amount of (*/S,G) state (also refered to as
"PIM State"), or the Target Router with an excessive number of
packets.
This kind of joining causes PIM state to be created, but this state
is relatively short-lived (260 seconds by default, which is the
default time that the state is active at DR in the absence of IGMP/
MLD Reports/Leaves). It should also be noted that the host can join a
number of different channels with only one IGMPv3/MLDv2 Report as the
protocol allows to include multiple sources in the same message.
However, even short-lived state may be harmful, if the intent is to
cause as much state as possible. The host can continue to send IGMP/
MLD Reports to these groups to make the state attack more long-lived.
This results in:
o ASM: a (*,G) join is sent towards an intra-domain RP, causing
state on that path; in turn, that RP joins to the DR of the source
(if it exists). If the source address was specified by the host in
the IGMPv3/MLDv2 Report, a (S,G) Join is sent directly towards the
specified source.
o SSM: a (S,G) join is sent inter-domain to the DR of the source S,
causing state on that path. If the source does not exist, the
join goes to the closest router to S as possible.
o Embedded RP: a (*,G) join is sent towards an inter/intra-domain RP
embedded in the group G, causing state on that path. If the RP
does not exist, the join goes to the closest router to the RP as
possible.
If the source or RP does not exist, the multicast routing protocol
does not have any means to remove the distribution tree if the host
remains active. Worst case attack could be a host remaining active to
many different groups (containing either imaginary source or RP).
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3.2 Source-based Attacks
These attacks are often referred to as "data plane" threats; however,
with traditional ASM and MSDP, these also include an MSDP control
plane threat.
3.2.1 Sending Multicast to Empty Groups
Description of the threat: Data Flooding. This happens when a host
sends data packets to a multicast group or channel for which there
are no real subscribers.
Note that as unicast-encapsulation is not subject to RPF checks, the
hosts can also craft and send these packets themselves, also spoofing
the source address of the register messages unless ingress filtering
[12] has been deployed [13].
Examples of this are a virus/worm trying to propagate to multicast
addresses, an attacker trying to crash routers with excessive MSDP
state, or an attacker wishing to overload the RP with encapsulated
packets or different groups. This results in:
o ASM: the DR unicast-encapsulates the packets in Register messages
to the intra-domain RP, which may join to the source and issue a
Register-Stop, but continues to get the data. A notification
about the active source is sent (unless the group or source is
configured to be local) inter-domain with MSDP and propagated
globally.
o SSM: the DR receives the data, but the data does not propagate
from the DR unless someone joins the (S,G) channel.
o Embedded RP: the DR register-encapsulates the packets to the
intra/inter-domain RP, which may join to the source and issue a
Register-Stop. The data continues to be encapsulated.
This yields many potential attacks, especially if at least parts of
the multicast forwarding functions are implemented on a "slow" path
or with software in the routers, at least:
o The MSDP control plane traffic generated can cause a significant
amount of messages/data which may overload the routers receiving
it. The thorough analysis of MSDP vulnerabilities can be found
from [14]. This is only related to the ASM. However, this is the
most serious threat at the moment, because MSDP will flood the
multicast group information to all multicast domains in Internet
including the multicast packet encapsulated to MSDP source-active
message. This creates a lot of data and state to be shared by all
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multicast enabled routers and if the source remains active, the
flooding will be repeated every 60 seconds by default.
o As a large amount of data is forwarded on the multicast tree; if
multicast forwarding is performed on software, it may be a
performance bottleneck, and a way to perform DoS on the path.
Similarly, the DR must always be capable of processing (and
discarding, if necessary) the multicast packets received from the
source. These are potentially present in every model.
o If the encapsulation is performed on software, it may be a
performance bottleneck, and a way to perform DoS on the DR.
Similarly, if the decapsulation is performed on software, it may
be a performance bottleneck, and a way to perform DoS on the RP.
Note: the decapsulator may know, based on access configuration, a
rate-limit or something else, that it doesn't need to decapsulate
the packet, avoiding bottlenecks. These threats are related to
ASM and Embedded RP.
3.2.2 Disturbing Existing Group by Sending to It
Description of the threat: Group Integrity Violation. This happens
when a host sends packets to a group or channel, which already
exists, to disturb the users of the existing group/channel.
The SSM service model prevents injection of packets to (S,G)
channels, avoiding this problem. However, if the source address can
be spoofed to be a topologically-correct address, it's possible to
get the packet into the distribution tree -- typically only those
hosts which are on-link with the source are able to perform this, so
this is not really relevant in the scope of this memo.
With ASM and Embedded RP sources can inject bogus traffic through
RPs, which provide the source discovery for the group. The RP(s) send
the traffic over the shared tree towards receivers (routers with
(*,G) state). DR then forwards the bogus traffic to receivers unless
the legitimate recipients are able to filter out unwanted sources,
e.g., using MSF API [8]. Typically this is not used or supported by
the applications using these protocols.
Note that with ASM and Embedded RP, the RP may exert some form of
control on who can send to a group, as the first packets are
unicast-encapsulated in register packets to the RP -- if the RP drops
the packet based on access-list, rate-limiter or something else, it
doesn't get injected to an existing group.
With ASM this "source control" is distributed across all the PIM
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domains, which decreases it's applicability. Embedded RP enables
easier control, because source discovery is done through single RP
per group.
So, for this attack to succeed, the RP must decapsulate the packets,
and join to the source.
3.3 Aggravating Factors to the Threats
This section describes a few factors, which aggravate the threats
described in sections Section 3.1 and Section 3.2. These could also
be viewed as individual threats on their own.
There are multiple threats relating to the use of host-to-router
signalling protocols -- such as Internet Group Management Protocol
(IGMP) or Multicast Listener Discovery (MLD) -- but these are outside
the scope of this memo.
PIM-SM can also be abused in the cases where RPF checks are not
applicable, in particular, in the stub LAN networks -- as spoofing
the on-link traffic is very simple. For example, a host would get
elected to become DR for the subnet, but not perform any of its
functions. These are described at some length in [1], but are also
considered out of scope of this memo.
3.3.1 Distant RP/Source Problem
In the shared tree model, if the RP or a source is distant
(topologically), then joins will travel to the distant RP or source
and keep the state information in the path active, even if the data
is being delivered locally.
Note that this problem will be exacerbated if the RP/source space is
global; if a router is registering to a RP/source that is not in the
local domain (say, fielded by the site's direct provider), then the
routing domain is flat.
Also note that PIM assumes that the addresses used in PIM messages
are valid. However, there is no way to ensure this, and using
non-existent S or G in (*,G) or (S,G) -messages will cause the
signalling to be set up, even though one cannot reach the address.
This will be analysed at more length in Section 5.1.
3.3.2 RPF Considers Interface, Not Neighbor
In most current implementations, the RPF check considers only the
incoming interface, and not the upstream neighbor (RPF neighbor).
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This can result in accepting packets from the "wrong" RPF neighbor
(the neighbor is "wrong" since, while the RPF check succeeds and the
packet is forwarded, the unicast policy would not have forwarded the
packet).
This is a problem in the media where more than two routers can
connect to, in particular, Ethernet-based Internet Exchanges.
Therefore any neighbor on such a link could inject any PIM signalling
as long as a route matching the address used in the signalling is
going through the interface.
3.3.3 No Receiver Information in PIM Joins
Only DRs, which are directly connected to receivers, know the exact
receiver information (e.g. IP address). PIM does not forward that
information further in the multicast distribution tree. Therefore
individual routers (e.g. domain edge routers) are not able to make
policy decisions on who can be connected to the distribution tree.
3.3.4 Injecting a Bogus Route
Hosts that able to inject a bogus route can be used to "steal" PIM
Joins. This prevents the correct multicast tree forming. If the
injected route information changes, it causes route flapping and that
could have harmful effect on multicast routing and packet delivery
(depending on the group). The threat is similar to unicast case
meaning that by injecting a bogus route, routing does not work
correctly.
4. Threat Analysis
4.1 Summary of the Threats
Trying to summarize the severity of the major classes of threats with
respect to each multicast usage model, we have a matrix of resistance
to different kinds of threats:
+----------------+------------------+-----------------+
| Bogus Join | Being a Source | Group Integrity |
+-------------+----------------+------------------+-----------------+
| ASM | bad 1) | very bad | bad/mediocre |
+-------------+----------------+------------------+-----------------+
| SSM | bad | very good | very good |
+-------------+----------------+------------------+-----------------+
| Embedded RP |bad/mediocre 2) | good/mediocre 3) | good |
+-------------+----------------+------------------+-----------------+
Notes:
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1) in ASM host can directly join also (S,G) groups with IGMPv3/MLDv2
and thus have same characteristics as SSM (also allows inter-domain
shared state to be created).
2) allows inter-domain shared state to be created.
3) register messages can be sent to long-distance RPs with spoofed
source addresses and this could create unnecessary joins towards DRs
(from spoofed source address space).
4.2 Enhancements for Threat Mitigation
There are several desirable actions ("requirements") which could be
considered to mitigate these threats; these are listed below. A
future revision of this memo will describe the best methods and
parameters at more detail.
o Inter-domain MSDP (ASM) should be retired (or not introduced) to
avoid attacks; or, if this is not reasonable, the DRs should
rate-limit the unicast-encapsulation (note that the hosts can
avoid this) and (more importantly) the RPs should rate-limit the
unicast-decapsulation especially from different sources, or MSDP
must rate-limit the MSDP data generation for new sources.
o DRs should rate-limit PIM Joins and Prunes somehow; there are
multiple possibilities how exactly this should be considered
(i.e., which variables to take into the consideration).
o DRs could rate-limit unicast-encapsulation somehow; there are
multiple ways to perform this. Note that the hosts can avoid this
by performing the unicast-encapsulation themselves if so inclined.
o RPs could rate-limit unicast-decapsulation somehow; there are
multiple ways to perform this. Note that if the source of the
unicast packets is spoofed by the host, this may have an effect on
how e.g. rate-limiters behave.
o RPs should rate limit the MSDP SA messages coming from MSDP peers.
o RPs could limit or even disable the SA cache size. However, this
could have negative effects on normal operation.
o RPs should provide good interfaces to reject packets which are not
interesting; for example, if an Embedded RP group is not
configured to be allowed in the RP, the unicast-encapsulated
packets would not even be decapsulated.
o DRs could rate-limit the multicast traffic somehow to reduce the
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disturbing possibilities; there are multiple possibilities how
exactly this should be considered.
o DRs should rate-limit the number of groups that can be created by
a given source, S.
5. PIM Security Enhancements
This section includes in-depth description of the above-mentioned
rate-limiting etc. functions as well as description of the remote
routability signalling issue.
5.1 Remote Routability Signalling
As described in section Section 3.3.1, non-existent DRs or RPs may
cause some problems when setting up multicast state. There seem to
be a couple of different approaches to mitigate this, especially if
rate-limiting is not extensively deployed.
With ASM and Embedded RP, Register message delivery could be ensured
somehow. For example:
1) At the very least, receiving an ICMP unreachable message (of
any flavor) should cause the DR to stop the Register packets -- as
the RP will not be receiving them anyway.
2) An additional method could be implementing a timer on the RPs
so that unless nothing is heard back from the RP within a defined
time period, the flow of Register messages would stop. (Currently,
the RPs are not required to answer back, unless they want to join
to the source.)
3) An extreme case would be performing some form of return
routability check prior to starting the register messages: first a
packet would be sent to the RP, testing its existence and
willingness to serve, and also proving to the RP that the sender
of the "bubble" and the sender of the registers are the same and
the source address is not forged (i.e., the RP would insert a
cookie in the bubble, and it would have to be present in the
register message.)
With all the models, PIM Joins and other state management messages
could also be somehow managed. For example:
1) At the very least, receiving an ICMP unreachable message (of
any flavor) should cause the DR to stop the PIM messages toward
the destination, as the packets will not be received anyway.
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(Currently the sending of an ICMP error message in response to a
multicast packet, even though in this case the PIM message to only
received by one router, is prohibited.)
2) A possible method would be limiting the number of PIM messages
sent towards a destination until some response (e.g. other PIM
state messages).
5.2 RPF to Check Neighbor, not Interface
As described in Section 3.3.2, especially Ethernet-based Internet
Exchange Points (IXP) are susceptible to signalling attacks from any
member of the IXP, as the RPF considers the Interface, not a
Neighbor.
Consequently, PIM must be modified so that on non- point-to-point
links, the RPF must also consider whether the neighbor is correct.
Note that in case of IPv6, this requires (an already necessary) a
mapping between link-local and global addresses.
5.3 Rate-limiting Possibilities
There seem to be many ways to implement rate-limiting (for
signalling, data encapsulation and multicast traffic) at the DRs or
RPs -- the best approach likely depends on the threat model; factors
in the evaluation might be e.g.:
o Whether the host is willfully malicious, uncontrolled (e.g.,
virus/worm), or a regular user just doing something wrong.
o Whether the threat is aimed towards a single group, a single RP
handling the group, or the (multicast) routing infrastructure in
general.
o Whether the host on a subnet is spoofing its address (but still as
one which fulfills the RPF checks of the DR) or not.
o Whether the host may generate the PIM join (and similar) messages
itself to avoid rate-limiters at the DR if possible.
o Whether unicast RPF checks are applied on the link (i.e., whether
the host can send unicast-encapsulated register-messages on its
own).
o Whether blocking the misbehaving host on a subnet is allowed to
also block other, legitimate hosts on the same subnet.
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o Whether these mechanisms would cause false positives on links with
only properly working hosts if many of them are receivers or
senders.
As should be obvious, there are many different scenarios here which
seem to call for different kinds of solutions.
For example, the rate-limiting could be performed based on:
1. multicast address, or the RP where the multicast address maps to
2. source address
3. the (source address, multicast address) -pair (or the RP which
maps to the multicast address)
4. data rate in case of rate limiting the source
5. everything (multicast groups and sources would not be
distinguished at all)
In the above, we make an assumption that rate-limiting would be
performed per-interface (on DRs) if a more fine-grained filter is not
being used.
It should be noted that some of the rate limiting functions can be
used as a tool for DoS against legimate multicast users. Therefore
several parameters for rate limiting should be used to prevent such
operation.
The next revisions of this document (or separated in other documents,
if appropriate) will include more explicit discussion of the best
ways to perform rate-limiting, especially considering the effects on
the legimate traffic.
6. Security Considerations
This memo analyzes the security of PIM routing infrastructures in
some detail, and proposes enhancements to mitigate the observed
threats.
7. IANA Considerations
This memo is for informational purposes and does not introduce new
namespaces for the IANA to manage.
Normative References
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[1] Fenner, B., Handley, M., Holbrook, H. and I. Kouvelas, "Protocol
Independent Multicast - Sparse Mode PIM-SM): Protocol
Specification (Revised)", draft-ietf-pim-sm-v2-new-08 (work in
progress), October 2003.
[2] Fenner, B. and D. Meyer, "Multicast Source Discovery Protocol
(MSDP)", RFC 3618, October 2003.
[3] Holbrook, H. and B. Cain, "Source-Specific Multicast for IP",
draft-ietf-ssm-arch-04 (work in progress), October 2003.
[4] Savola, P. and B. Haberman, "Embedding the Address of RP in IPv6
Multicast Address", draft-ietf-mboned-embeddedrp-00 (work in
progress), October 2003.
[5] Barbir, A., Murphy, S. and Y. Yang, "Generic Threats to Routing
Protocols", draft-ietf-rpsec-routing-threats-04 (work in
progress), December 2003.
Informative References
[6] Deering, S., "Host extensions for IP multicasting", STD 5, RFC
1112, August 1989.
[7] Bhattacharyya, S., "An Overview of Source-Specific Multicast
(SSM)", RFC 3569, July 2003.
[8] Thaler, D., Fenner, B. and B. Quinn, "Socket Interface
Extensions for Multicast Source Filters",
draft-ietf-magma-msf-api-05 (work in progress), July 2003.
[9] Hardjono, T. and B. Weis, "The Multicast Security
Architecture", draft-ietf-msec-arch-05 (work in progress),
January 2004.
[10] Cain, B., Deering, S., Kouvelas, I., Fenner, B. and A.
Thyagarajan, "Internet Group Management Protocol, Version 3",
RFC 3376, October 2002.
[11] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2
(MLDv2) for IPv6", draft-vida-mld-v2-08 (work in progress),
December 2003.
[12] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[13] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Savola, et al. Expires July 22, 2004 [Page 14]
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Networks", draft-savola-bcp38-multihoming-update-03 (work in
progress), December 2003.
[14] Rajvaidya, P., Ramachandran, K. and K. Almeroth, "Detection and
Deflection of DoS Attacks Against the Multicast Source
Discovery Protocol", IEEE Infocom 2003.
Authors' Addresses
Pekka Savola
CSC/FUNET
Espoo
Finland
EMail: psavola@funet.fi
Rami Lehtonen
TeliaSonera
Hataanpaan valtatie 20
Tampere 33100
Finland
EMail: rami.lehtonen@teliasonera.com
David Meyer
EMail: dmm@1-4-5.net
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