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IETF RFC 6398
Last modified on Thursday, October 27th, 2011
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Internet Engineering Task Force (IETF) F. Le Faucheur, Ed.
Request for Comments: 6398 Cisco
BCP: 168 October 2011
Updates: 2113, 2711
Category: Best Current Practice
ISSN: 2070-1721
IP Router Alert Considerations and Usage
 Abstract
The IP Router Alert Option is an IP option that alerts transit
routers to more closely examine the contents of an IP packet. The
Resource reSerVation Protocol (RSVP), Pragmatic General Multicast
(PGM), the Internet Group Management Protocol (IGMP), Multicast
Listener Discovery (MLD), Multicast Router Discovery (MRD), and
General Internet Signaling Transport (GIST) are some of the protocols
that make use of the IP Router Alert Option. This document discusses
security aspects and usage guidelines around the use of the current
IP Router Alert Option, thereby updating RFC 2113 and RFC 2711.
Specifically, it provides recommendations against using the Router
Alert in the end-to-end open Internet and identifies controlled
environments where protocols depending on Router Alert can be used
safely. It also provides recommendations about protection approaches
for service providers. Finally, it provides brief guidelines for
Router Alert implementation on routers.
Status of This Memo
This memo documents an Internet Best Current Practice.
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
BCPs 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/RFC 6398.
Le Faucheur Best Current Practice PAGE 1 
RFC 6398 Router Alert Considerations October 2011
Copyright Notice
Copyright (c) 2011 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.
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................4
2.1. Conventions Used in This Document ..........................4
3. Security Concerns of Router Alert ...............................5
4. Guidelines for Use of Router Alert ..............................7
4.1. Use of Router Alert End to End in the Internet
(Router Alert in Peer Model) ...............................7
4.2. Use of Router Alert in Controlled Environments .............9
4.2.1. Use of Router Alert within an Administrative
Domain ..............................................9
4.2.2. Use of Router Alert in Overlay Model ...............11
4.3. Router Alert Protection Approaches for Service Providers ..13
5. Guidelines for Router Alert Implementation .....................15
6. Security Considerations ........................................16
7. Contributors ...................................................16
8. Acknowledgments ................................................16
9. References .....................................................17
9.1. Normative References ......................................17
9.2. Informative References ....................................17
Le Faucheur Best Current Practice PAGE 2
RFC 6398 Router Alert Considerations October 2011
1. Introduction
[RFC 2113] and [RFC 2711] define the IPv4 and IPv6 Router Alert Options
(RAOs), respectively. In this document, we collectively refer to
those options as the IP Router Alert. The IP Router Alert Option is
an IP option that alerts transit routers to more closely examine the
contents of an IP packet.
Some of the protocols that make use of the IP Router Alert are the
Resource reSerVation Protocol (RSVP) ([RFC 2205], [RFC 3175],
[RFC 3209]), Pragmatic General Multicast (PGM) ([RFC 3208]), the
Internet Group Management Protocol (IGMP) ([RFC 3376]), Multicast
Listener Discovery (MLD) ([RFC 2710], [RFC 3810]), Multicast Router
Discovery (MRD) ([RFC 4286]), and Next Steps in Signaling (NSIS)
General Internet Signaling Transport (GIST) ([RFC 5971]).
Section 3 describes the security concerns associated with the use of
the Router Alert Option.
Section 4 provides guidelines for the use of Router Alert. More
specifically, Section 4.1 recommends that Router Alert not be used
for end-to-end applications over the Internet, while Section 4.2
presents controlled environments where applications/protocols relying
on IP Router Alert can be deployed effectively and safely.
Section 4.3 provides recommendations on protection approaches to be
used by service providers in order to protect their network from
Router-Alert-based attacks.
Finally, Section 5 provides generic recommendations for router
implementation of Router Alert, aiming at increasing protection
against attacks.
This document discusses considerations and practices based on the
current specifications of IP Router Alert ([RFC 2113], [RFC 2711]).
Possible future enhancements to the specifications of IP Router Alert
(in view of reducing the security risks associated with the use of IP
Router Alert) are outside the scope of this document. One such
proposal is discussed in [RAO-EXT], but at the time of this writing,
the IETF has not adopted any extensions for this purpose.
The IPv6 base specification [RFC 2460] defines the hop-by-hop options
extension header. The hop-by-hop options header is used to carry
optional information that must be examined by every node along a
packet's delivery path. The IPv6 Router Alert Option is one
particular hop-by-hop option. Similar security concerns to those
discussed in this document for the IPv6 Router Alert apply more
generically to the concept of the IPv6 hop-by-hop options extension
header. However, thoroughly addressing the broader concept of the
Le Faucheur Best Current Practice PAGE 3
RFC 6398 Router Alert Considerations October 2011
IPv6 hop-by-hop option would require additional material so as to
cover additional considerations associated with it (e.g., the
effectiveness of the attack could depend on how many options are
included and on the range to which the option-type value belongs), so
this is kept outside the scope of this document. A detailed
discussion about security risks and proposed remedies associated with
the IPv6 hop-by-hop option can be found in [IPv6-HOPBYHOP].
The IPv4 base specification [RFC 791] defines a general notion of
IPv4 options that can be included in the IPv4 header (without
distinguishing between the hop-by-hop and end-to-end options). The
IPv4 Router Alert Option is one particular IPv4 option. Security
concerns similar to those discussed in this document for the IPv4
Router Alert apply more generically to the concept of the IPv4
option. However, thoroughly addressing the security concerns of the
broader concept of the IPv4 option is kept outside the scope of this
document, because it would require additional material so as to cover
additional considerations associated with it (such as lack of option
ordering, etc.), and because other IPv4 options are often blocked in
firewalls and not very widely used, so the practical risks they
present are largely nonexistent.
2. Terminology
For readability, this document uses the following loosely defined
terms:
o Fast path: Hardware or Application-Specific Integrated Circuit
(ASIC) processing path for packets. This is the nominal
processing path within a router for IP datagrams.
o Slow path: Software processing path for packets. This is a sub-
nominal processing path for packets that require special
processing or differ from assumptions made in fast-path
heuristics.
o Next level protocol: The protocol transported in the IP datagram.
In IPv4 [RFC 791], the next level protocol is identified by the
IANA protocol number conveyed in the 8-bit "Protocol" field in the
IPv4 header. In IPv6 [RFC 2460], the next level protocol is
identified by the IANA protocol number conveyed in the 8-bit "Next
Header" field in the IPv6 header.
2.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
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RFC 6398 Router Alert Considerations October 2011
3. Security Concerns of Router Alert
The IP Router Alert Option is defined ([RFC 2113], [RFC 2711]) as a
mechanism that alerts transit routers to more closely examine the
contents of an IP packet. [RFC 4081] and [RFC 2711] mention the
security risks associated with the use of the IP Router Alert:
flooding a router with bogus (or simply undesired) IP datagrams that
contain the IP Router Alert could impact operation of the router in
undesirable ways. For example, if the router punts the datagrams
containing the IP Router Alert Option to the slow path, such an
attack could consume a significant share of the router's slow path
and could also lead to packet drops in the slow path (affecting
operation of all other applications and protocols operating in the
slow path), thereby resulting in a denial of service (DoS)
([RFC 4732]).
Furthermore, [RFC 2113] specifies no (and [RFC 2711] specifies a very
limited) mechanism for identifying different users of IP Router
Alert. As a result, many fast switching implementations of IP Router
Alert punt most/all packets marked with IP Router Alert into the slow
path (unless configured to systematically ignore or drop all Router
Alert packets). However, some existing deployed IP routers can and
do process IP packets containing the Router Alert Option inside the
fast path.
Some IP Router Alert implementations are able to take into account
the next level protocol as a discriminator for the punting decision
for different protocols using IP Router Alert. However, this still
only allows very coarse triage among various protocols using IP
Router Alert, for two reasons. First, the next level protocol is the
same when IP Router Alert is used for different applications of the
same protocol (e.g., RSVP vs. RSVP - Traffic Engineering (RSVP-TE)),
or when IP Router Alert is used for different contexts of the same
application (e.g., different levels of RSVP aggregation [RFC 3175]).
Thus, it is not always possible to achieve the necessary triage in
the fast path across IP Router Alert packets from different
applications or from different contexts of an application. Secondly,
some protocols requiring punting might be carried over a transport
protocol (e.g., TCP or UDP), possibly because (1) they require the
services of that transport protocol, (2) the protocol does not
justify allocation of a scarce next level protocol value, or (3) not
relying on a very widely deployed transport protocol is likely to
result in deployment issues due to common middlebox behaviors (e.g.,
firewalls or NATs discarding packets of "unknown" protocols). Thus,
considering the next level protocol alone in the fast path is not
sufficient to allow triage in the fast path of IP Router Alert
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RFC 6398 Router Alert Considerations October 2011
packets from different protocols sharing the same transport protocol.
Therefore, it is generally not possible to ensure that only the IP
Router Alert packets for next level protocols of interest are punted
to the slow path while other IP Router Alert packets are efficiently
forwarded (i.e., in the fast path).
Some IP Router Alert implementations are able to take into account
the Value field inside the Router Alert Option. However, only one
value (zero) was defined in [RFC 2113], and no IANA registry for IPv4
Router Alert values was available until recently ([RFC 5350]). So
this did not allow most IPv4 Router Alert implementations to support
useful classification based on the Value field in the fast path.
Also, while [RFC 2113] states that unknown values should be ignored
(i.e., the packets should be forwarded as normal IP traffic), it has
been reported that some existing implementations simply ignore the
Value field completely (i.e., process any packet with an IPv4 Router
Alert regardless of its option value). An IANA registry for further
allocation of IPv4 Router Alert values has been introduced recently
([RFC 5350]), but this would only allow coarse-grain classification,
if supported by implementations. For IPv6, [RFC 2711] states that
"the Value field can be used by an implementation to speed processing
of the datagram within the transit router" and defines an IANA
registry for these values. But again, this only allows coarse-grain
classification. Besides, some existing IPv6 Router Alert
implementations are reported to depart from that behavior.
[RFC 2711] mentions that limiting, by rate or some other means, the
use of the IP Router Alert Option is a way of protecting against a
potential attack. However, if rate limiting is used as a protection
mechanism, but if the granularity of the rate limiting is not fine
enough to distinguish IP Router Alert packets of interest from
unwanted IP Router Alert packets, an IP Router Alert attack could
still severely degrade operation of protocols of interest that depend
on the use of IP Router Alert.
In a nutshell, the IP Router Alert Option does not provide a
convenient universal mechanism to accurately and reliably distinguish
between IP Router Alert packets of interest and unwanted IP Router
Alert packets. This, in turn, creates a security concern when the IP
Router Alert Option is used, because, short of appropriate router-
implementation-specific mechanisms, the router slow path is at risk
of being flooded by unwanted traffic.
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RFC 6398 Router Alert Considerations October 2011
Note that service providers commonly allow external parties to
communicate with a control plane application in their routers, such
as with BGP peering. Depending on the actual environment and BGP
security practices, with BGP peering, the resulting DoS attack vector
is similar to or somewhat less serious than it would be with the
Router Alert Option for a number of reasons, including the following:
o With BGP, edge routers only exchange control plane information
with pre-identified peers and can easily filter out any control
plane traffic coming from other peers or non-authenticated peers,
while the Router Alert Option can be received in a datagram with
any source address and any destination address. However, we note
that the effectiveness of such BGP filtering is dependent on
proper security practices; poor BGP security practices (such as
infrequent or nonexistent update of BGP peers' authentication
keys) create vulnerabilities through which the BGP authentication
mechanisms can be compromised.
o With BGP peering, the control plane hole is only open on the edge
routers, and core routers are completely isolated from any direct
control plane exchange with entities outside the administrative
domain. Thus, with BGP, a DoS attack would only affect the edge
routers, while with the Router Alert Option, the attack could
propagate to core routers. However, in some BGP environments, the
distinction between edge and core routers is not strict, and many/
most/all routers act as both edge and core routers; in such BGP
environments, a large part of the network is exposed to direct
control plane exchanges with entities outside the administrative
domain (as it would be with Router Alert).
o With BGP, the BGP policy control would typically prevent re-
injection of undesirable information out of the attacked device,
while with the Router Alert Option, the non-filtered attacking
messages would typically be forwarded downstream. However, we
note that there have been real-life occurrences of situations
where incorrect information was propagated through the BGP system,
causing widespread problems.
4. Guidelines for Use of Router Alert
4.1. Use of Router Alert End to End in the Internet (Router Alert in
Peer Model)
Because of the security concerns associated with Router Alert
discussed in Section 3, network operators SHOULD actively protect
themselves against externally generated IP Router Alert packets.
Because there are no convenient universal mechanisms to triage
between desired and undesired Router Alert packets, network operators
Le Faucheur Best Current Practice PAGE 7
RFC 6398 Router Alert Considerations October 2011
currently often protect themselves in ways that isolate them from
externally generated IP Router Alert packets. This might be achieved
by tunneling IP Router Alert packets [RFC 6178] so that the IP Router
Alert Option is hidden through that network, or it might be achieved
via mechanisms resulting in occasional (e.g., rate limiting) or
systematic drop of IP Router Alert packets.
Thus, applications and protocols SHOULD NOT be deployed with a
dependency on processing of the Router Alert Option (as currently
specified) across independent administrative domains in the Internet.
Figure 1 illustrates such a hypothetical use of Router Alert end to
end in the Internet. We refer to such a model of Router Alert Option
use as a "Peer Model" Router Alert Option use, since core routers in
different administrative domains would partake in processing of
Router Alert Option datagrams associated with the same signaling
flow.
-------- -------- -------- --------
/ A \ / B \ / C \ / D \
| (*) | | (*) | | (*) | | (*) |
| | |<============>| |<=============>| |<=============>| | |
| - | | - | | - | | - |
\ / \ / \ / \ /
-------- -------- -------- --------
(*) closer examination of Router Alert Option datagrams
<==> flow of Router Alert Option datagrams
Figure 1: Use of Router Alert End to End in the Open Internet
(Router Alert in Peer Model)
While this recommendation is framed here specifically in the context
of Router Alert, the fundamental security risk that network operators
want to preclude is to allow devices/protocols that are outside of
their administrative domain (and therefore not controlled) to tap
into the control plane of their core routers. Similar security
concerns would probably result whether this control plane access is
provided through the Router Alert Option or provided by any other
mechanism (e.g., deep packet inspection). In other words, the
fundamental security concern is associated with the notion of end-to-
end signaling in a Peer Model across domains in the Internet. As a
result, it is expected that network operators would typically not
want to have their core routers partake in end-to-end signaling with
external uncontrolled devices through the open Internet, and
therefore prevent deployment of end-to-end signaling in a Peer Model
through their network (regardless of whether that signaling uses
Router Alert or not).
Le Faucheur Best Current Practice PAGE 8
RFC 6398 Router Alert Considerations October 2011
4.2. Use of Router Alert in Controlled Environments
4.2.1. Use of Router Alert within an Administrative Domain
In some controlled environments, such as within a given
administrative domain, the network administrator can determine that
IP Router Alert packets will only be received from trusted well-
behaved devices or can establish that specific protection mechanisms
(e.g., RAO filtering and rate limiting) against the plausible RAO-
based DoS attacks are sufficient. In that case, an application
relying on exchange and handling of RAO packets (e.g., RSVP) can be
safely deployed within the controlled network. A private enterprise
network firewalled from the Internet and using RSVP reservations for
voice and video flows might be an example of such a controlled
environment. Such an environment is illustrated in Figure 2.
------------------------- -------- --------
/ A \ / B \ / C \
| (*) (*) | -- | | | |
| | |<============>| | |--|FW|--| |--------| |
| - - | -- | | | |
\ / \ / \ /
------------------------- -------- --------
(*) closer examination of Router Alert Option datagrams
<==> flow of Router Alert Option datagrams
FW: Firewall
Figure 2: Use of Router Alert within an Administrative Domain -
Private Enterprise Network Firewalled from the Internet
and Using RSVP Reservations
In some controlled environments, several administrative domains have
a special relationship whereby they cooperate very tightly and
effectively operate as a single trust domain. In that case, one
domain is willing to trust another with respect to the traffic
injected across the boundary. In other words, a downstream domain is
willing to trust that the traffic injected at the boundary has been
properly validated/filtered by the upstream domain. Where it has
been established that such trust can be applied to Router Alert
Option packets, an application relying on exchange and handling of
RAO packets (e.g., RSVP) can be safely deployed within such a
controlled environment. The entity within a company responsible for
operating multimedia endpoints and the entity within the same company
Le Faucheur Best Current Practice PAGE 9
RFC 6398 Router Alert Considerations October 2011
responsible for operating the network might be an example of such a
controlled environment. For example, they might collaborate so that
RSVP reservations can be used for video flows from endpoints to
endpoints through the network.
In some environments, the network administrator can reliably ensure
that Router Alert packets from any untrusted device (e.g., from
external routers) are prevented from entering a trusted area (e.g.,
the internal routers). For example, this might be achieved by
ensuring that routers straddling the trust boundary (e.g., edge
routers) always encapsulate those packets (without setting IP Router
Alert -or equivalent- in the encapsulating header) through the
trusted area (as discussed in [RFC 6178]). In such environments, the
risks of DoS attacks through the IP Router Alert vector are removed
(or greatly reduced) in the trusted area even if IP Router Alert is
used inside the trusted area (say, for RSVP-TE). Thus, an
application relying on IP Router Alert can be safely deployed within
the trusted area. A service provider running RSVP-TE within its
network might be an example of such a protected environment. Such an
environment is illustrated in Figure 3.
-------- -------------------------- --------
/ A \ / B \ / C \
| | | (*) (*) | | |
| |-------TT | |<=============>| | TT------- | |
| | | - - | | |
\ / \ / \ /
-------- -------------------------- --------
(*) closer examination of Router Alert Option datagrams
<==> flow of Router Alert Option datagrams
TT: Tunneling of Router Alert Option datagrams
Figure 3: Use of Router Alert within an Administrative Domain -
Service Provider Running RSVP-TE within Its Network
Le Faucheur Best Current Practice PAGE 10
RFC 6398 Router Alert Considerations October 2011
4.2.2. Use of Router Alert in Overlay Model
In some controlled environment:
o The sites of a network A are interconnected through a service
provider network B.
o The service provider network B protects itself from IP Router
Alert messages without dropping those messages when they transit
over the network (for example, using mechanisms discussed in
[RFC 6178]).
In such a controlled environment, an application relying on exchange
and handling of RAO packets (e.g., RSVP) in the network A sites (but
not inside network B) can be safely deployed. We refer to such a
deployment as a use of Router Alert in a Water-Tight Overlay --
"Overlay", because Router Alert Option datagrams are used in network
A on top of, and completely transparently to, network B; and
"Water-Tight", because Router Alert Option datagrams from network A
cannot leak inside network B. A private enterprise intranet realized
as a Virtual Private Network (VPN) over a service provider network
and using RSVP to perform reservations within the enterprise sites
for voice and video flows might be an example of such a controlled
environment. Such an environment is illustrated in Figure 4.
-------- --------
/ A \ / A \
| (*) | | (*) |
| | |<=====================================>| | |
| - | | - |
\ / \ /
-------- --------
\ /
\ ------------------------- /
\ / B \ /
\| |/
TT TT
| |
\ /
-------------------------
(*) closer examination of Router Alert Option datagrams
<==> flow of Router Alert Option datagrams
TT: Tunneling of Router Alert Option datagrams
Figure 4: Use of Router Alert in Water-Tight Overlay
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RFC 6398 Router Alert Considerations October 2011
In the controlled environment described above, an application relying
on exchange and handling of RAO packets (e.g., RSVP-TE) in the
service provider network B (but not in network A) can also be safely
deployed simultaneously. Such an environment with independent,
isolated deployment of Router Alert in overlay at two levels is
illustrated in Figure 5.
-------- --------
/ A \ / A \
| (*) | | (*) |
| | |<=====================================>| | |
| - | | - |
\ / \ /
-------- --------
\ /
\ ------------------------- /
\ / B \ /
\| (*) (*) |/
TT | |<============>| | TT
| - - |
\ /
-------------------------
(*) closer examination of Router Alert Option datagrams
<==> flow of Router Alert Option datagrams
TT: Tunneling of Router Alert Option datagrams
Figure 5: Use of Router Alert in Water-Tight Overlay at Two Levels
In some controlled environment:
o The sites of a network A are interconnected through a service
provider network B.
o The service provider B processes Router Alert packets on the edge
routers and protects these edge routers against RAO-based attacks
using mechanisms such as (possibly per port) RAO rate limiting and
filtering.
o The service provider network B protects its core routers from
Router Alert messages without dropping those messages when they
transit over the network (for example, using mechanisms discussed
in [RFC 6178]).
Le Faucheur Best Current Practice PAGE 12
RFC 6398 Router Alert Considerations October 2011
In such a controlled environment, an application relying on exchange
and handling of RAO packets (e.g., RSVP) in the network A sites and
in network B's edges (but not in the core of network B) can be safely
deployed. We refer to such a deployment as a use of Router Alert in
a Leak-Controlled Overlay -- "Overlay", because Router Alert Option
datagrams are used in network A on top of, and completely
transparently to, network B's core; and "Leak-Controlled", because
Router Alert Option datagrams from network A leak inside network B's
edges but not inside network B's core. A private enterprise
intranet, whose sites are interconnected through a service provider
network, using RSVP for voice and video within network A sites as
well as on network B's edge to extend the reservation onto the
attachment links between networks A and B (as specified in
[RFC 6016]), might be an example of such a controlled environment.
Such an environment is illustrated in Figure 6.
-------- --------
/ A \ / A \
| | | |
| | ------------------------ | |
| (*) | /(*) (*) \ | (*) |
| | |<======>| |<============>| |<=========>| | |
| - | | - - | | - |
\ / | \ - - / | \ /
-------- | TT-| | | |-TT | --------
| - - |
\ /
------------------------
(*) closer examination of Router Alert Option datagrams
<==> flow of Router Alert Option datagrams
TT: Tunneling of Router Alert Option datagrams
Figure 6: Use of Router Alert in Leak-Controlled Overlay
4.3. Router Alert Protection Approaches for Service Providers
Section 3 discusses the security risks associated with the use of the
IP Router Alert and how it opens up a DoS vector in the router
control plane. Thus, a service provider MUST implement strong
protection of its network against attacks based on IP Router Alert.
As discussed in Section 4.2.2, some applications can benefit from the
use of IP Router Alert packets in an Overlay Model (i.e., where
Router Alert packets are exchanged transparently on top of a service
provider). Thus, a service provider protecting its network from
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RFC 6398 Router Alert Considerations October 2011
attacks based on IP Router Alert SHOULD use mechanisms that avoid (or
at least minimize) the dropping of end-to-end IP Router Alert packets
(other than those involved in an attack).
For example, if the service provider does not run any protocol
depending on IP Router Alert within its network, it might elect to
simply turn off punting/processing of IP Router Alert packets on its
routers; this will ensure that end-to-end IP Router Alert packets
transit transparently and safely through its network.
As another example, using protection mechanisms such as selective
filtering and rate limiting (which Section 5 suggests be supported by
IP Router Alert implementations), a service provider can protect the
operation of a protocol depending on IP Router Alert within its
network (e.g., RSVP-TE) while at the same time transporting IP Router
Alert packets carrying another protocol that might be used end to
end. Note that the service provider might additionally use protocol-
specific mechanisms that reduce the dependency on Router Alert for
operation of this protocol inside the service provider environment;
use of RSVP refresh reduction mechanisms ([RFC 2961]) would be an
example of such mechanisms in the case where the service provider is
running RSVP-TE within its network, since this allows the refresh of
existing Path and Resv states without the use of the IP Router Alert
Option.
As yet another example, using mechanisms such as those discussed in
[RFC 6178], a service provider can safely protect the operation of a
protocol depending on IP Router Alert within its network (e.g.,
RSVP-TE) while at the same time safely transporting IP Router Alert
packets carrying another protocol that might be used end to end
(e.g., IPv4/IPv6 RSVP). We observe that while tunneling of Router
Alert Option datagrams over an MPLS backbone as discussed in
[RFC 6178] is well understood, tunneling Router Alert Option datagrams
over a non-MPLS IP backbone presents a number of issues (in
particular, for determining where to forward the encapsulated
datagram) and is not common practice at the time of writing this
document.
As a last resort, if the service provider does not have any means to
safely transport end-to-end IP Router Alert Option packets over its
network, the service provider can drop those packets. It must be
noted that this has the undesirable consequence of preventing the use
of the Router Alert Option in the Overlay Model on top of that
network, and therefore prevents users of that network from deploying
a number of valid applications/protocols in their environment.
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RFC 6398 Router Alert Considerations October 2011
5. Guidelines for Router Alert Implementation
A router implementation of the IP Router Alert Option SHOULD include
protection mechanisms against Router-Alert-based DoS attacks as
appropriate for their targeted deployment environments. For example,
this can include the ability of an edge router to "tunnel" received
IP Router Alert Option packets when forwarding those packets over the
core, as discussed in [RFC 6178]. As another example, although not
always available from current implementations, new implementations
MAY include protection mechanisms such as selective (possibly
dynamic) filtering and rate limiting of IP Router Alert Option
packets.
In particular, router implementations of the IP Router Alert Option
SHOULD offer the configuration option to simply ignore the presence
of "IP Router Alert" in IPv4 and IPv6 packets. As discussed in
Section 4.3, that permits IP Router Alert packets to transit a
network segment without presenting an adverse operational security
risk to that particular network segment, provided the operator of
that network segment does not ever use the IP Router Alert messages
for any purpose.
If an IP packet contains the IP Router Alert Option, but the next
level protocol is not explicitly identified as a protocol of interest
by the router examining the packet, the behavior is not explicitly
defined by [RFC 2113]. However, the behavior is implied, and, for
example, the definition of RSVP in [RFC 2205] assumes that the packet
will be forwarded using normal forwarding based on the destination IP
address. Thus, a router implementation SHOULD forward within the
"fast path" (subject to all normal policies and forwarding rules) a
packet carrying the IP Router Alert Option containing a next level
protocol that is not a protocol of interest to that router. The "not
punting" behavior protects the router from DoS attacks using IP
Router Alert packets of a protocol unknown to the router. The
"forwarding" behavior contributes to transparent end-to-end transport
of IP Router Alert packets (e.g., to facilitate their use by end-to-
end applications).
Similarly, an implementation MAY support selective forwarding within
the fast path (subject to all normal policies and forwarding rules)
or punting of a packet with the IP Router Alert Option, based on the
Value field of the Router Alert Option. This would allow router
protection against DoS attacks using IP Router Alert packets with a
value that is not relevant for that router (e.g., nesting levels of
aggregated RSVP reservation [RFC 5350]).
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RFC 6398 Router Alert Considerations October 2011
6. Security Considerations
This document expands the security considerations of [RFC 2113] and
[RFC 2711], which define the IPv4 and IPv6 RAOs, respectively, by
discussing security risks associated with usage of the current IP
Router Alert Option and associated practices. See [RFC 4081] for
additional security considerations.
7. Contributors
The contributors to this document (in addition to the editor) are:
Reshad Rahman
Cisco Systems
rrahman@cisco.com
David Ward
Juniper Networks
dward@juniper.net
Ashok Narayanan
Cisco Systems
ashokn@cisco.com
Adrian Farrel
OldDog Consulting
adrian@olddog.co.uk
Tony Li
Cisco Systems
tony.li@tony.li
8. Acknowledgments
The editor and contributors would like to thank Dave Oran, Magnus
Westerlund, John Scudder, Ron Bonica, Ross Callon, Alfred Hines,
Carlos Pignataro, Roland Bless, Jari Arkko, and Ran Atkinson for
their comments. This document also benefited from discussions with
Jukka Manner and Suresh Krishnan. The discussion about use of the
Value field in the IPv4 Router Alert is borrowed from a similar
discussion in [RFC 5971].
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RFC 6398 Router Alert Considerations October 2011
9. References
9.1. Normative References
[RFC 791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC 2113] Katz, D., "IP Router Alert Option", RFC 2113,
February 1997.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC 2711] Partridge, C. and A. Jackson, "IPv6 Router Alert Option",
RFC 2711, October 1999.
[RFC 5350] Manner, J. and A. McDonald, "IANA Considerations for the
IPv4 and IPv6 Router Alert Options", RFC 5350,
September 2008.
9.2. Informative References
[IPv6-HOPBYHOP]
Krishnan, S., "The case against Hop-by-Hop options", Work
in Progress, October 2010.
[RAO-EXT] Narayanan, A., Le Faucheur, F., Ward, D., and R. Rahman,
"IP Router Alert Option Extension", Work in Progress,
March 2009.
[RFC 2205] Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC 2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710,
October 1999.
[RFC 2961] Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F.,
and S. Molendini, "RSVP Refresh Overhead Reduction
Extensions", RFC 2961, April 2001.
[RFC 3175] Baker, F., Iturralde, C., Le Faucheur, F., and B. Davie,
"Aggregation of RSVP for IPv4 and IPv6 Reservations",
RFC 3175, September 2001.
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RFC 6398 Router Alert Considerations October 2011
[RFC 3208] Speakman, T., Crowcroft, J., Gemmell, J., Farinacci, D.,
Lin, S., Leshchiner, D., Luby, M., Montgomery, T., Rizzo,
L., Tweedly, A., Bhaskar, N., Edmonstone, R.,
Sumanasekera, R., and L. Vicisano, "PGM Reliable Transport
Protocol Specification", RFC 3208, December 2001.
[RFC 3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC 3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol,
Version 3", RFC 3376, October 2002.
[RFC 3810] Vida, R., Ed., and L. Costa, Ed., "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
June 2004.
[RFC 4081] Tschofenig, H. and D. Kroeselberg, "Security Threats for
Next Steps in Signaling (NSIS)", RFC 4081, June 2005.
[RFC 4286] Haberman, B. and J. Martin, "Multicast Router Discovery",
RFC 4286, December 2005.
[RFC 4732] Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
Denial-of-Service Considerations", RFC 4732,
December 2006.
[RFC 5971] Schulzrinne, H. and R. Hancock, "GIST: General Internet
Signalling Transport", RFC 5971, October 2010.
[RFC 6016] Davie, B., Le Faucheur, F., and A. Narayanan, "Support for
the Resource Reservation Protocol (RSVP) in Layer 3 VPNs",
RFC 6016, October 2010.
[RFC 6178] Smith, D., Mullooly, J., Jaeger, W., and T. Scholl, "Label
Edge Router Forwarding of IPv4 Option Packets", RFC 6178,
March 2011.
Le Faucheur Best Current Practice PAGE 18
RFC 6398 Router Alert Considerations October 2011
Author's Address
Francois Le Faucheur (editor)
Cisco Systems
Greenside, 400 Avenue de Roumanille
Sophia Antipolis 06410
France
Phone: +33 4 97 23 26 19
EMail: flefauch@cisco.com
Le Faucheur Best Current Practice PAGE 19
RFC TOTAL SIZE: 44914 bytes
PUBLICATION DATE: Thursday, October 27th, 2011
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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