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PROPOSED STANDARD
Internet Engineering Task Force (IETF) W. Atwood
Request for Comments: 5796 Concordia University/CSE
Updates: 4601 S. Islam
Category: Standards Track IRS-EMT
ISSN: 2070-1721 M. Siami
Concordia University/CIISE
March 2010
Authentication and Confidentiality in
Protocol Independent Multicast Sparse Mode (PIM-SM) Link-Local Messages
Abstract
RFC 4601 mandates the use of IPsec to ensure authentication of the
link-local messages in the Protocol Independent Multicast - Sparse
Mode (PIM-SM) routing protocol. This document specifies mechanisms
to authenticate the PIM-SM link-local messages using the IP security
(IPsec) Encapsulating Security Payload (ESP) or (optionally) the
Authentication Header (AH). It specifies optional mechanisms to
provide confidentiality using the ESP. Manual keying is specified as
the mandatory and default group key management solution. To deal
with issues of scalability and security that exist with manual
keying, optional support for an automated group key management
mechanism is provided. However, the procedures for implementing
automated group key management are left to other documents. This
document updates RFC 4601.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5796.
Atwood, et al. Standards Track [Page 1]
RFC 5796 PIM-SM Link-local Security March 2010
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Atwood, et al. Standards Track [Page 2]
RFC 5796 PIM-SM Link-local Security March 2010
Table of Contents
1. Introduction ....................................................4
1.1. Goals and Non-Goals ........................................4
2. Terminology .....................................................5
3. Transport Mode versus Tunnel Mode ...............................5
4. Authentication ..................................................5
5. Confidentiality .................................................6
6. IPsec Requirements ..............................................6
7. Key Management ..................................................8
7.1. Manual Key Management ......................................8
7.2. Automated Key Management ...................................8
7.3. Communications Patterns ....................................9
7.4. Neighbor Relationships ....................................10
8. Number of Security Associations ................................11
9. Rekeying .......................................................12
9.1. Manual Rekeying Procedure .................................13
9.2. KeyRolloverInterval .......................................14
9.3. Rekeying Interval .........................................14
10. IPsec Protection Barrier and SPD/GSPD .........................14
10.1. Manual Keying ............................................14
10.1.1. SAD Entries .......................................14
10.1.2. SPD Entries .......................................14
10.2. Automatic Keying .........................................15
10.2.1. SAD Entries .......................................15
10.2.2. GSPD Entries ......................................15
10.2.3. PAD Entries .......................................15
11. Security Association Lookup ...................................16
12. Activating the Anti-Replay Mechanism ..........................16
13. Implementing a Security Policy Database per Interface .........17
14. Extended Sequence Number ......................................17
15. Security Considerations .......................................18
16. Acknowledgements ..............................................18
17. References ....................................................19
17.1. Normative References .....................................19
17.2. Informative References ...................................19
Atwood, et al. Standards Track [Page 3]
RFC 5796 PIM-SM Link-local Security March 2010
1. Introduction
All the PIM-SM [RFC4601] control messages have IP protocol number
103. Some control messages are unicast; the rest are multicast with
Time to Live (TTL) = 1. The source address used for unicast messages
is a domain-wide reachable address. For the multicast messages, a
link-local address of the interface on which the message is being
sent is used as the source address and a special multicast address,
ALL_PIM_ROUTERS (224.0.0.13 in IPv4 and ff02::d in IPv6) is used as
the destination address. These messages are called link-local
messages. Hello, Join/Prune, and Assert messages are included in
this category. A forged link-local message may be sent to the
ALL_PIM_ROUTERS multicast address by an attacker. This type of
message affects the construction of the distribution tree [RFC4601].
The effects of these forged messages are outlined in Section 6.1 of
[RFC4601]. Some of the effects are very severe, whereas some are
minor.
PIM-SM version 2 was originally specified in RFC 2117 [RFC2117], and
revised in RFC 2362 [RFC2362] and RFC 4601. RFC 4601 obsoletes RFC
2362, and corrects a number of deficiencies. The "Security
Considerations" section of RFC 4601 is based primarily on the
Authentication Header (AH) specification described in RFC 4302
[RFC4302].
Securing the unicast messages can be achieved by the use of a normal
unicast IPsec Security Association (SA) between the two communicants.
This document focuses on the security issues for link-local messages.
It provides some guidelines to take advantage of the new permitted AH
functionality in RFC 4302 and the new permitted ESP functionality in
RFC 4303 [RFC4303], and to bring the PIM-SM specification into
alignment with the new AH and ESP specifications. In particular, in
accordance with RFC 4301, the use of ESP is made mandatory and AH is
specified as optional. This document specifies manual key management
as mandatory to implement, i.e., that all implementations MUST
support, and provides the necessary structure for an automated key
management protocol that the PIM routers may use.
1.1. Goals and Non-Goals
The primary goal for link-local security is to provide data origin
authentication for each link-local message. A secondary goal is to
ensure that communication only happens between legitimate peers
(i.e., adjacent routers). An optional goal is to provide data
confidentiality for the link-local messages.
Atwood, et al. Standards Track [Page 4]
RFC 5796 PIM-SM Link-local Security March 2010
The first goal implies that each router has a unique identity. It is
possible (but not mandatory) that this identity will be based on the
unicast identity of the router. (The unicast identity could be, for
example, based on some individually configured property of the
router, or be part of a region-wide public key infrastructure.) The
existence of this unique identity is assumed in this specification,
but procedures for establishing it are out of scope for this
document.
The second goal implies that there is some form of "adjacency matrix"
that controls the establishment of Security Associations among
adjacent multicast routers. For manual keying, this control will be
exercised by the Administrator of the router(s), through the setting
of initialization parameters. For automated keying, the existence of
this control will be reflected by the contents of the Peer
Authorization Database (PAD) (see RFC 4301 [RFC4301]) or the Group
Security Policy Database (GSPD) (see RFC 5374 [RFC5374]) in each
router. Procedures for controlling the adjacency and building the
associated PAD and GSPD are out of scope for this document.
2. Terminology
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 RFC 2119 [RFC2119].
They indicate requirement levels for compliant PIM-SM
implementations.
3. Transport Mode versus Tunnel Mode
All implementations conforming to this specification MUST support SA
in transport mode to provide required IPsec security to PIM-SM link-
local messages. They MAY also support SA in tunnel mode to provide
required IPsec security to PIM-SM link-local messages. If tunnel
mode is used, both destination address preservation and source
address preservation MUST be used, as described in Section 3.1 of RFC
5374 [RFC5374].
4. Authentication
Implementations conforming to this specification MUST support
authentication for PIM-SM link-local messages. Implementations
conforming to this specification MUST support HMAC-SHA1 [RFC2404].
Atwood, et al. Standards Track [Page 5]
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In order to provide authentication of PIM-SM link-local messages,
implementations MUST support ESP [RFC4303] and MAY support AH
[RFC4302].
If ESP in transport mode is used, it will only provide authentication
to PIM-SM protocol packets excluding the IP header, extension
headers, and options.
If AH in transport mode is used, it will provide authentication to
PIM-SM protocol packets, selected portions of the IP header,
extension headers and options.
Note: when authentication for PIM-SM link-local messages is enabled,
o PIM-SM link-local packets that are not protected with AH or ESP
will be silently discarded by IPsec, although the implementation
of IPsec may maintain a counter of such packets.
o PIM-SM link-local packets that fail the authentication checks will
be silently discarded by IPsec, although the implementation of
IPsec may maintain a counter of such packets.
5. Confidentiality
Implementations conforming to this specification SHOULD support
confidentiality for PIM-SM. Implementations supporting
confidentiality MUST support AES-CBC [RFC3602] with a 128-bit key.
If confidentiality is provided, ESP MUST be used.
Since authentication MUST be supported by a conforming
implementation, an implementation MUST NOT generate the combination
of NON-NULL Encryption and NULL Authentication.
Note: when confidentiality for PIM-SM link-local packets is enabled,
o PIM-SM packets that are not protected with ESP will be silently
discarded by IPsec, although the implementation of IPsec may
maintain a counter of such packets.
6. IPsec Requirements
In order to implement this specification, the following IPsec
capabilities are required.
Transport mode
IPsec in transport mode MUST be supported.
Atwood, et al. Standards Track [Page 6]
RFC 5796 PIM-SM Link-local Security March 2010
Multiple Security Policy Databases (SPDs)
The implementation MUST support multiple SPDs with an SPD
selection function that provides an ability to choose a specific
SPD based on interface.
Selectors
The implementation MUST be able to use source address, destination
address, protocol, and direction as selectors in the SPD.
Interface ID tagging
The implementation MUST be able to tag the inbound packets with
the ID of the interface (physical or virtual) on which they
arrived.
Manual key support
It MUST be possible to use manually configured keys to secure the
specified traffic.
Encryption and authentication algorithms
Encryption and authentication algorithm requirements described in
RFC 4835 [RFC4835] apply when ESP and AH are used to protect
PIM-SM. Implementations MUST support ESP-NULL, and if providing
confidentiality, MUST support the ESP transforms providing
confidentiality required by [RFC4835]. However, in any case,
implementations MUST NOT allow the user to choose a stream cipher
or block mode cipher in counter mode for use with manual keys.
Encapsulation of ESP packets
IP encapsulation of ESP packets MUST be supported. For
simplicity, UDP encapsulation of ESP packets SHOULD NOT be used.
If the automatic keying features of this specification are
implemented, the following additional IPsec capabilities are
required:
Group Security Policy Database (GSPD)
The implementation MUST support the GSPD that is described in RFC
5374 [RFC5374].
Multiple Group Security Policy Databases
The implementation MUST support multiple GSPDs with a GSPD
selection function that provides an ability to choose a specific
GSPD based on interface.
Selectors
The implementation MUST be able to use source address, destination
address, protocol and direction as selectors in the GSPD.
Atwood, et al. Standards Track [Page 7]
RFC 5796 PIM-SM Link-local Security March 2010
7. Key Management
All the implementations MUST support manual configuration of the
Security Associations (SAs) that will be used to authenticate PIM-SM
link-local messages. This does not preclude the use of a negotiation
protocol such as the Group Domain Of Interpretation (GDOI) [RFC3547]
or Group Secure Association Key Management Protocol (GSAKMP)
[RFC4535] to establish these SAs.
7.1. Manual Key Management
To establish the SAs at PIM-SM routers, manual key configuration will
be feasible when the number of peers (directly connected routers) is
small. The Network Administrator will configure a router manually.
At that time, the authentication method and the choice of keys SHOULD
be configured. The parameters for the Security Association Database
(SAD) will be entered. The Network Administrator will also configure
the Security Policy Database of a router to ensure the use of the
associated SA while sending a link-local message.
7.2. Automated Key Management
All the link-local messages of the PIM-SM protocol are sent to the
destination address, ALL_PIM_ROUTERS, which is a multicast address.
By using the sender address in conjunction with the destination
address for Security Association lookup, link-local communication
turns into a Source-Specific Multicast (SSM) or "one-to-many"
communication.
The procedures for automated key management are not specified in this
document.
One option is to use Group Domain Of Interpretation (GDOI) [RFC3547],
which enables a group of users or devices to exchange encrypted data
using IPsec data encryption. GDOI has been developed to be used in
multicast applications, where the number of end users or devices may
be large and the end users or devices can dynamically join/leave a
multicast group. However, a PIM router is not expected to join/leave
very frequently, and the number of routers is small when compared to
the possible number of users of a multicast application. Moreover,
most of the PIM routers will be located inside the same
administrative domain and are considered to be trusted parties. It
is possible that a subset of GDOI functionalities will be sufficient.
Another option is to use the Group Secure Association Key Management
Protocol (GSAKMP) [RFC4535].
Atwood, et al. Standards Track [Page 8]
RFC 5796 PIM-SM Link-local Security March 2010
7.3. Communications Patterns
Before discussing the set of Security Associations that are required
to properly manage a multicast region that is under the control of a
single administration, it is necessary to understand the
communications patterns that will exist among the routers in this
region. From the perspective of a speaking router, the information
from that router is sent (multicast) to all of its neighbors. From
the perspective of a listening router, the information coming from
each of its neighbors is distinct from the information coming from
every other router to which it is directly connected. Thus, an
administrative region contains many (small) distinct groups, all of
which happen to be using the same multicast destination address
(e.g., ALL_PIM_ROUTERS, see Section 11), and each of which is
centered on the associated speaking router.
Consider the example configuration as shown in Figure 1.
R2 R3 R4 R5 R6
| | | | |
| | | | |
--------- ---------------
| |
| |
\ /
R1
/ \
| |
| |
--------- --------------------
| | | | |
| | | | |
R7 R8 R9 R10 R11
| | | | |
|
|
-------------
| | |
| | |
R12 R13 R14
Figure 1: Set of router interconnections
In this configuration, router R1 has four interfaces, and is the
speaking router for a group whose listening routers are routers R2
through R11. Router R9 is the speaking router for a group whose
listening routers are routers R1, R8, and R10-R14.
Atwood, et al. Standards Track [Page 9]
RFC 5796 PIM-SM Link-local Security March 2010
From the perspective of R1 as a speaking router, if a Security
Association SA1 is assigned to protect outgoing packets from R1, then
it is necessary to distribute the key for this association to each of
the routers R2 through R11. Similarly, from the perspective of R9 as
a speaking router, if a Security Association is assigned to protect
the outgoing packets from R9, then it is necessary to distribute the
key for this association to each of the routers R1, R8, and R10
through R14.
From the perspective of R1 as a listening router, all packets
arriving from R2 through R11 need to be distinguished from each
other, to permit selecting the correct Security Association in the
SAD. (Packets from each of the peer routers (R2 through R11)
represent communication from a different speaker, with a separate
sequence-number space, even though they are sent using the same
destination address.) For a multicast Security Association, RFC 4301
permits using the source address in the selection function. If the
source addresses used by routers R2 through R11 are globally unique,
then the source addresses of the peer routers are sufficient to
achieve the differentiation. If the sending routers use link-local
addresses, then these addresses are unique only on a per-interface
basis, and it is necessary to use the Interface ID tag as an
additional selector, i.e., either the selection function has to have
the Interface ID tag as one of its inputs or separate SADs have to be
maintained for each interface.
If the assumption of connectivity to the key server can be made
(which is true in the PIM-SM case), then the Group Controller/Key
Server (GC/KS) that is used for the management of the keys can be
centrally located (and duplicated for reliability). If this
assumption cannot be made (i.e., in the case of adjacencies for a
unicast router), then some form of "local" key server must be
available for each group. Given that the listening routers are never
more than one hop away from the speaking router, the speaking router
is the obvious place to locate the "local" key server. As such, this
may be a useful approach even in the PIM-SM case. This approach has
the additional advantage that there is no need to duplicate the local
key server for reliability, since if the key server is down, it is
very likely that the speaking router is also down.
7.4. Neighbor Relationships
Each distinct group consists of one speaker, and the set of directly
connected listeners. If the decision is made to maintain one
Security Association per speaker (see Section 8), then the key server
will need to be aware of the adjacencies of each speaker. Procedures
for managing and distributing these adjacencies are out of scope for
this document.
Atwood, et al. Standards Track [Page 10]
RFC 5796 PIM-SM Link-local Security March 2010
8. Number of Security Associations
The number of Security Associations to be maintained by a PIM router
depends on the required security level and available key management.
This SHOULD be decided by the Network Administrator. Two different
ways are shown in Figures 2 and 3. It is assumed that A, B, and C
are three PIM routers, where B and C are directly connected with A,
and there is no direct link between B and C.
|
+++++ |
+ B + SAb ------------>|
+ + SAa <------------|
+++++ |
|
+++++ SAb <------------|
+ + ---->|
+ + /
+ A + SAa -------
+ + \
+ + ---->|
+++++ SAc <------------|
|
+++++ |
+ C + SAc ------------>|
+ + SAa <------------|
+++++ |
|
Directly connected network
Figure 2: Activate unique Security Association for each peer
The first method, shown in Figure 2, SHOULD be supported by every
implementation. In this method, each node will use a unique SA for
its outbound traffic. A, B, and C will use SAa, SAb, and SAc,
respectively, for sending any traffic. Each node will include the
source address when searching the SAD for a match. Router A will use
SAb and SAc for packets received from B and C, respectively. The
number of SAs to be activated and maintained by a PIM router will be
equal to the number of directly connected routers, plus one for
sending its own traffic. Also, the addition of a PIM router in the
network will require the addition of another SA on every directly
connected PIM router. This solution will be scalable and practically
feasible with an automated key management protocol. However, it MAY
be used with manual key management, if the number of directly
connected routers is small.
Atwood, et al. Standards Track [Page 11]
RFC 5796 PIM-SM Link-local Security March 2010
|
+++++ |
+ B + SAo ------------>|
+ + SAi <------------|
+++++ |
|
+++++ SAi <------------|
+ + ---->|
+ + /
+ A + SAo -------
+ + \
+ + ---->|
+++++ SAi <------------|
|
+++++ |
+ C + SAo ------------>|
+ + SAi <------------|
+++++ |
|
Directly connected network
Figure 3: Activate two Security Associations
The second method, shown in Figure 3, MUST be supported by every
implementation. In this simple method, all the nodes will use two
SAs, one for sending (SAo) and the other for receiving (SAi) traffic.
Thus, the number of SAs is always two and will not be affected by
addition of a PIM router. Although two different SAs (i.e., SAo and
SAi) are used in this method, the SA parameters (keys, Security
Parameter Index (SPI), etc.) for the two SAs are identical, i.e., the
same information is shared among all the routers in an administrative
region. This document RECOMMENDS this second method for manual key
configuration. However, it MAY also be used with automated key
configuration.
9. Rekeying
An analysis of the considerations for key management is provided in
RFC 4107 [RFC4107].
In PIM-SM deployments it is expected that secure sessions will be
relatively long-lived, and it is not expected that keys will be
significantly exposed through normal operational activity. Manual
keying is judged acceptable in the light of the relatively low rate
of change that is required.
Atwood, et al. Standards Track [Page 12]
RFC 5796 PIM-SM Link-local Security March 2010
To maintain the security of a link, the authentication and encryption
key values SHOULD be changed periodically, to limit the risk of
undetected key disclosure. Keys SHOULD also be changed when there is
a change of trusted personnel.
Manual keying offers the ability to change keys in a coordinated way,
but it has several drawbacks in PIM-SM systems. Some of these are
listed in Section 15 ("Security Considerations") of this document.
According to an analysis in line with RFC 4107 [RFC4107], PIM-SM
would benefit from automated key management and roll over because all
the disadvantages of manual keys listed in Section 15 would be
eliminated. However, suitable techniques for automated key
management do not currently exist. Work is in hand in the IETF to
develop suitable solutions. In the meantime, implementations MUST
support manual rekeying as described below. Implementers and
deployers need to be aware of the requirement to upgrade to support
automated key management as soon as suitable techniques are
available.
9.1. Manual Rekeying Procedure
In accordance with the requirements of RFC 4107 [RFC4107], the
following three-step procedure provides a possible mechanism to rekey
the routers on a link without dropping PIM-SM protocol packets or
disrupting the adjacency, while ensuring that it is always clear
which key is being used.
1. For every router on the link, create an additional inbound SA for
the interface being rekeyed using a new SPI and the new key.
2. For every router on the link, replace the original outbound SA
with one using the new SPI and key values. The SA replacement
operation MUST be atomic with respect to sending PIM-SM packets
on the link, so that no PIM-SM packets are sent without
authentication/encryption
3. For every router on the link, remove the original inbound SA.
Note that all routers on the link MUST complete step 1 before any
begin step 2. Likewise, all the routers on the link MUST complete
step 2 before any begin step 3.
One way to control the progression from one step to another is for
each router to have a configurable time constant KeyRolloverInterval.
After the router begins step 1 on a given link, it waits for this
interval and then moves to step 2. Likewise, after moving to step 2,
it waits for this interval and then moves to step 3.
Atwood, et al. Standards Track [Page 13]
RFC 5796 PIM-SM Link-local Security March 2010
In order to achieve smooth key transition, all routers on a link MUST
use the same value for KeyRolloverInterval and MUST initiate the key
rollover process within this time period.
At the end of this time period, all the routers on the link will have
a single inbound and outbound SA for PIM-SM with the new SPI and key
values.
9.2. KeyRolloverInterval
The configured value of KeyRolloverInterval needs to be long enough
to allow the Administrator to change keys on all the PIM-SM routers.
As this value can vary significantly depending on the implementation
and the deployment, it is left to the Administrator to choose an
appropriate value.
9.3. Rekeying Interval
In keeping with the goal of reducing key exposure, the encryption and
authentication keys SHOULD be changed at least every 90 days.
10. IPsec Protection Barrier and SPD/GSPD
10.1. Manual Keying
10.1.1. SAD Entries
The Administrator must configure the necessary Security Associations.
Each SA entry has the source address of an authorized peer, and a
Destination Address of ALL_PIM_ROUTERS. Unique SPI values for the
manually configured SAs MUST be assigned by the Administrator to
ensure that the SPI does not conflict with existing SPI values in the
SAD.
10.1.2. SPD Entries
The Administrator must configure the necessary SPD entries. The SPD
entry must ensure that any outbound IP traffic packet traversing the
IPsec boundary, with PIM as its next layer protocol and sent to the
Destination Address of ALL_PIM_ROUTERS, is protected by ESP or AH.
Note that this characterization includes all the link-local messages
(Hello, Join/Prune, Bootstrap, Assert).
Atwood, et al. Standards Track [Page 14]
RFC 5796 PIM-SM Link-local Security March 2010
10.2. Automatic Keying
When automatic keying is used, the SA creation is done dynamically
using a group key management protocol. The GSPD and PAD tables are
configured by the Administrator. The PAD table provides the link
between the IPsec subsystem and the group key management protocol.
For automatic keying, the implementation MUST support the multicast
extensions described in [RFC5374].
10.2.1. SAD Entries
All PIM routers participate in an authentication scheme that
identifies permitted neighbors and achieves peer authentication
during SA negotiation, leading to child SAs being established and
saved in the SAD.
10.2.2. GSPD Entries
The Administrator must configure the necessary GSPD entries for
"sender only" directionality. This rule MUST trigger the group key
management protocol for a registration exchange. This exchange will
set up the outbound SAD entry that encrypts the multicast PIM control
message. Considering that this rule is "sender only", no inbound SA
is created in the reverse direction.
In addition, the registration exchange will trigger the installation
of the GSPD entries corresponding to each legitimate peer router,
with direction "receiver only". Procedures for achieving the
registration exchange are out of scope for this document.
A router SHOULD NOT dynamically detect new neighbors as the result of
receiving an unauthenticated PIM-SM link-local message or an IPsec
packet that fails an SAD lookup. An automated key management
protocol SHOULD provide a means of notifying a router of new,
legitimate neighbors.
10.2.3. PAD Entries
The PAD will be configured with information to permit identification
of legitimate group members and senders (i.e., to control the
adjacency). Procedures for doing this are out of scope for this
document.
Atwood, et al. Standards Track [Page 15]
RFC 5796 PIM-SM Link-local Security March 2010
11. Security Association Lookup
For an SA that carries unicast traffic, three parameters (SPI,
destination address, and security protocol type (AH or ESP)) are used
in the Security Association lookup process for inbound packets. The
SPI is sufficient to specify an SA. However, an implementation may
use the SPI in conjunction with the IPsec protocol type (AH or ESP)
for the SA lookup process. According to RFC 4301 [RFC4301], for
multicast SAs, in conjunction with the SPI, the destination address
or the destination address plus the sender address may also be used
in the SA lookup. This applies to both ESP and AH. The security
protocol field is not employed for a multicast SA lookup.
Given that, from the prospective of a receiving router, each peer
router is an independent sender and given that the destination
address will be the same for all senders, the receiving router MUST
use SPI plus destination address plus sender address when performing
the SA lookup. In effect, link-local communication is an SSM
communication that happens to use an Any-Source Multicast (ASM)
address (which is shared among all the routers).
Given that it is always possible to distinguish a connection using
IPsec from a connection not using IPsec, it is recommended that the
address ALL_PIM_ROUTERS be used, to maintain consistency with present
practice.
Given that the sender address of an incoming packet may be only
locally unique (because of the use of link-local addresses), it is
necessary for a receiver to use the interface ID tag to determine the
associated SA for that sender. Therefore, this document mandates
that the interface ID tag, the SPI, and the sender address MUST be
used in the SA lookup process.
12. Activating the Anti-Replay Mechanism
Although link-level messages on a link constitute a multiple-sender,
multiple-receiver group, the use of the interface ID tag and sender
address for SA lookup essentially resolves the communication into a
separate SA for each sender/destination pair, even for the case where
only two SAs (with identical SA parameters) are used for the entire
administrative region. Therefore, the statement in the AH RFC
(Section 2.5 of [RFC4302]) that "for a multi-sender SA, the anti-
replay features are not available" becomes irrelevant to the PIM-SM
link-local message exchange.
To activate the anti-replay mechanism in a unicast communication, the
receiver uses the sliding window protocol and it maintains a sequence
number for this protocol. This sequence number starts from zero.
Atwood, et al. Standards Track [Page 16]
RFC 5796 PIM-SM Link-local Security March 2010
Each time the sender sends a new packet, it increments this number by
one. In a multi-sender multicast group communication, a single
sequence number for the entire group would not be enough.
The whole scenario is different for PIM link-local messages. These
messages are sent to local links with TTL = 1. A link-local message
never propagates through one router to another. The use of the
sender address and the interface ID tag for SA lookup converts the
relationship from a multiple-sender group to multiple single-sender
associations. This specification RECOMMENDS activation of the anti-
replay mechanism only if the SAs are assigned using an automated key
management procedure. If manual key management is used, the anti-
replay SHOULD NOT be activated.
If an existing router has to restart, in accordance with RFC 4303
[RFC4303], the sequence-number counter at the sender MUST be
correctly maintained across local reboots, etc., until the key is
replaced.
13. Implementing a Security Policy Database per Interface
RFC 4601 suggests that it may be desirable to implement a separate
Security Policy Database (SPD) for each router interface. The use of
link-local addresses in certain circumstances implies that
differentiation of ambiguous speaker addresses requires the use of
the interface ID tag in the SA lookup. One way to do this is through
the use of multiple SPDs. Alternatively, the interface ID tag may be
a specific component of the selector algorithm. This is in
conformance with RFC 4301, which explicitly removes the requirement
for separate SPDs that was present in RFC 2401 [RFC2401].
14. Extended Sequence Number
In the [RFC4302], there is a provision for a 64-bit Extended Sequence
Number (ESN) as the counter of the sliding window used in the anti-
replay protocol. Both the sender and the receiver maintain a 64-bit
counter for the sequence number, although only the lower order 32
bits are sent in the transmission. In other words, it will not
affect the present header format of AH. If ESN is used, a sender
router can send 2^64 -1 packets without any intervention. This
number is very large, and from a PIM router's point of view, a PIM
router can never exceed this number in its lifetime. This makes it
reasonable to permit manual configuration for a small number of PIM
routers, since the sequence number will never roll over. For this
reason, when manual configuration is used, ESN SHOULD be deployed as
the sequence number for the sliding window protocol. In addition,
Atwood, et al. Standards Track [Page 17]
RFC 5796 PIM-SM Link-local Security March 2010
when an ESN is used with a manually keyed SA, it MUST be saved over a
reboot, along with an indication of which sequence numbers have been
used.
15. Security Considerations
The whole document considers the security issues of PIM link-local
messages and proposes a mechanism to protect them.
Limitations of manual keys:
The following are some of the known limitations of the usage of
manual keys.
o If replay protection cannot be provided, the PIM routers will not
be secured against all the attacks that can be performed by
replaying PIM packets.
o Manual keys are usually long lived (changing them often is a
tedious task). This gives an attacker enough time to discover the
keys.
o As the Administrator is manually configuring the keys, there is a
chance that the configured keys are weak (there are known weak
keys for DES/3DES at least).
Impersonation attacks:
The usage of the same key on all the PIM routers connected to a link
leaves them all insecure against impersonation attacks if any one of
the PIM routers is compromised, malfunctioning, or misconfigured.
Detailed analysis of various vulnerabilities of routing protocols is
provided in RFC 4593 [RFC4593]. For further discussion of PIM-SM and
multicast security, the reader is referred to RFC 5294 [RFC5294], RFC
4609 [RFC4609], and the Security Considerations section of RFC 4601
[RFC4601].
16. Acknowledgements
The structure and text of this document draw heavily from RFC 4552
[RFC4552]. The authors of this document thank M. Gupta and N. Melam
for permission to do this.
The quality of this document was substantially improved after SECDIR
pre-review by Brian Weis, and after AD review by Adrian Farrel.
Atwood, et al. Standards Track [Page 18]
RFC 5796 PIM-SM Link-local Security March 2010
17. References
17.1. Normative References
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4835] Manral, V., "Cryptographic Algorithm Implementation
Requirements for Encapsulating Security Payload (ESP) and
Authentication Header (AH)", RFC 4835, April 2007.
[RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast
Extensions to the Security Architecture for the Internet
Protocol", RFC 5374, November 2008.
[RFC2404] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within
ESP and AH", RFC 2404, November 1998.
[RFC3602] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
Algorithm and Its Use with IPsec", RFC 3602,
September 2003.
17.2. Informative References
[RFC2117] Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering,
S., Handley, M., Jacobson, V., Liu, C., Sharma, P., and L.
Wei, "Protocol Independent Multicast-Sparse Mode (PIM-SM):
Protocol Specification", RFC 2117, June 1997.
[RFC2362] Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering,
S., Handley, M., and V. Jacobson, "Protocol Independent
Multicast-Sparse Mode (PIM-SM): Protocol Specification",
RFC 2362, June 1998.
Atwood, et al. Standards Track [Page 19]
RFC 5796 PIM-SM Link-local Security March 2010
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC4535] Harney, H., Meth, U., Colegrove, A., and G. Gross,
"GSAKMP: Group Secure Association Key Management
Protocol", RFC 4535, June 2006.
[RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
Group Domain of Interpretation", RFC 3547, July 2003.
[RFC4593] Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
Routing Protocols", RFC 4593, October 2006.
[RFC5294] Savola, P. and J. Lingard, "Host Threats to Protocol
Independent Multicast (PIM)", RFC 5294, August 2008.
[RFC4609] Savola, P., Lehtonen, R., and D. Meyer, "Protocol
Independent Multicast - Sparse Mode (PIM-SM) Multicast
Routing Security Issues and Enhancements", RFC 4609,
October 2006.
[RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, June 2006.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, June 2005.
Atwood, et al. Standards Track [Page 20]
RFC 5796 PIM-SM Link-local Security March 2010
Authors' Addresses
J. William Atwood
Concordia University/CSE
1455 de Maisonneuve Blvd. West
Montreal, QC H3G 1M8
Canada
Phone: +1(514)848-2424 ext3046
EMail: bill@cse.concordia.ca
URI: http://users.encs.concordia.ca/~bill
Salekul Islam
INRS Energie, Materiaux et Telecommunications
800 de La Gauchetiere, Suite 800
Montreal, QC H5A 1K6
Canada
EMail: Salekul.Islam@emt.inrs.ca
URI: http://users.encs.concordia.ca/~salek_is
Maziar Siami
Concordia University/CIISE
1455 de Maisonneuve Blvd. West
Montreal, QC H3G 1M8
Canada
EMail: mzrsm@yahoo.ca
Atwood, et al. Standards Track [Page 21]
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