Network Working Group R. Rahman, Ed.
Internet-Draft Cisco
Intended status: BCP November 14, 2008
Expires: May 18, 2009
IP Router Alert Considerations and Usage
draft-rahman-rtg-router-alert-considerations-00
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Abstract
The IP Router Alert Option is an IP option that alerts transit
routers to more closely examine the contents of an IP packet. RSVP,
PGM and IGMP are some of the protocols which make use of the IP
Router Alert option. This document discusses security aspects and
common practices around the use of router alert and discusses
consequences on the use of router alert by existing or new
applications. Common practices in router alert implementation
facilitating router protection are also discussed. Finally a
possible enhancement to the current specification of Router Alert is
presented for feedback.
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Conventions Used in This Document . . . . . . . . . . . . 5
3. Guidelines for use of Router Alert . . . . . . . . . . . . . . 6
3.1. Reliance on Router Alert by Applications . . . . . . . . . 6
3.2. When Consenting Adults Exchange IP Router Alert Packets . 6
4. Example Protection Mechanisms in a Router Alert
Implementation . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Handling Packets Carrying the Router Alert Option . . . . 9
4.2. Applying Rate Limiting . . . . . . . . . . . . . . . . . . 9
4.3. Router Alert in Congested Systems . . . . . . . . . . . . 10
4.4. Handling Unknown Payload Protocols . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . . 15
Appendix A. A New Filtering Mechanism to Select IP RAO
Packets of Interest . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . . . 21
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1. Terminology
For readability, this document uses the following loosely defined
terms:
o Slow path : Software processing path for packets
o Fast path : ASIC/Hardware processing path for packets
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2. Introduction
[RFC2113] and [RFC2711] respectively define the IPv4 and IPv6 Router
Alert Option. In this document, we collectively refer to those as
the IP Router Alert option. RSVP ([RFC2205], [RFC3209]), PGM
([RFC3208]) and IGMP ([RFC3376]) are some of the protocols which make
use of the IP Router Alert option. Those protocols are used to
support critical elements of the Internet infrastructure (e.g.
RSVP-TE for traffic engineering within a service provider network)
and as such they need to be protected.
IP datagrams carrying the IP Router Alert option are usually examined
in a router's "slow path" and an excess of such datagrams can cause
performance degradation or packet drops in a router's "slow path".
(Note that a router's "slow path" can also be attacked with IP
packets destined to one of the router's local IP addresses.)
[RFC4081] and [RFC2711] mention the security risks associated with
the use of the IP Router Alert option: flooding a router with bogus
IP datagrams which contain the IP Router Alert option would cause a
performance degradation of the router's "slow path" and can also lead
to packet drops in the "slow path".
[RFC2711] mentions that limiting, by rate or some other means, the
use of 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 among router alert packet of interest from unwanted
router alert packet, a router alert attack could still severely
degrade operation of protocols of interest that depend on the use of
IP Router Alert. Section 4 discusses examples of protection
mechanisms that may be available from a router implementation of the
IP router alert.
In some environments where it was not possible to accurately and
reliably distinguish between router alert packet of interest and
unwanted router alert packets, operators have resorted to actively
protecting themselves against externally generated router alert
packets in ways that result in end to end router alert packets being
(occasionally or systematically) dropped and/or ignored on a segment.
The consequences of such practises on the use of router alert by
existing or new applications are discussed in Section 3 of the
present document. This section also discusses environments where IP
router alert can be used effectively.
This document also discusses in Appendix A a possible enhancements to
the current specification of Router Alert to ensure that risks
associated with unintentional interception of packets that are not of
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real interest to a given router are minimized (if not eliminated) by
facilitating identification in the fast path of the subset of packets
with router alert that are of interest to the router. The objective
of this appendix is to solicit feedback on the question of whether an
enhancement to the current router alert specification is justified,
and if yes, how to enhance it.
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 [RFC2119].
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3. Guidelines for use of Router Alert
3.1. Reliance on Router Alert by Applications
As mentioned earlier, some networks actively protect themselves
against externally generated router alert packets. This may be by
tunneling router alert packets [I-D.dasmith-mpls-ip-options] so that
the router alert option is hidden through that network or it may be
via mechanisms resulting in occasional (e.g. rate limiting) or
systematic drop of router alert packets.
Also, application protocols are usually carried within IP by a
transport protocol such as UDP, TCP, or SCTP. In these cases, the
application protocol is not visible in the IP header and could not be
determined without further "deep packet" inspection. In the event of
the Router Alert option being used for an application protocol
carried in a transport protocol, the intercepted IP packet would be
delivered to the transport protocol for processing in accordance with
[RFC2113]. However, the behavior of the transport protocol in these
circumstances is not defined, and may cause rejection of the packet
for various reasons.
As a consequence, in the general case of open networks, applications
can not safely rely on router alert packets being visible to all
nodes on the path today, and importantly can not even rely on router
alert packets being transported end to end.
[RFC2113] implies that the router may examine other fields of a
received packet that contains the IP Router Alert option to decide
whether that packet needs further processing, but no further advice
is given. Examination of other fields of the received IP packet that
carries the Router Alert to help determine what to do with the packet
would result in implementation-specific behavior that is
unpredictable to the sender of the packet. Therefore applications
cannot depend on router alert interception involving inspection of
fields in the packet outside the IP header.
Thus, creating an application or end-to-end protocol that uses the IP
Router Alert is currently considered harmful and is strongly
discouraged. A different mechanism should be used to decrease the
risk of impacting existing protocols that use the IP Router Alert
option.
3.2. When Consenting Adults Exchange IP Router Alert Packets
In some controlled environments, the network administrator can
determine that IP router alert packets will only be received from
trusted well-behaved devices or can establish that the protection
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mechanisms discussed in the present document against the plausible
RAO- based DoS attacks (e.g. RAO filtering and rate-limiting) are
sufficient. In that case, an application relying on exchange and
handling of RAO packets (e.g. RSVP) may be safely deployed within
the controlled network. In other words, a network that feels
appropriately protected against the risks associated with his
environment, may decide to freely and openly partake in router alert
message exchange with consenting entities. A private enterprise
network firewalled from the Internet may be an example of such
controlled environment.
In some environments, the network administrator can reliably ensure
that IP 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 may be achieved by ensuring
that routers straddling the trust boundary (e.g. edge routers) always
encapsulate those packets (without Router Alert) through the trusted
area (as discussed in [I-D.dasmith-mpls-ip-options]). In such
environments, the risks of DOS attacks through the IP router alert
vector is removed in the trusted area (or greatly reduced) even if IP
router alert is used inside the trusted area (say for RSVP). Thus an
application relying on Router Alert may be safely deployed within the
trusted area. In other words, the network protects itself from the
risks of partaking in IP Router-Alert exchange with strangers but
feels free to exchange router alert messages among trusted parties.
A Service Provider running RSVP-TE in his network may be an example
of such protected environment.
When a controlled environment requires RAO packet exchange across his
routers for some application (e.g. RSVP) and transits via a Service
Provider network (that is not part of the controlled environment),
the administrator of the controlled environment needs to ensure with
the Service Provider that the router alert messages will not be
dropped when transiting the Service Provider network. In other
words, the network ought to ensure that another network that needs to
be involved in exchange of router alert packet is consenting.
Since some existing applications would benefit from end-to-end
transport of router alert packets, it is desirable that a Service
Provider protects his network from attacks based on router alert
using mechanisms that minimize dropping of end to end router alert
packets. For example, using protection mechanisms such as those
described in Section 4 a Service Provider can safely protect
operation of a protocol depending on router alert within his network
(e.g. RSVP-TE) while at the same time safely transporting router
alert packets carrying another protocol that may be used end to end.
As another example, using mechanisms such as those discussed in
[I-D.dasmith-mpls-ip-options] a Service Provider can safely protect
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operation of a protocol depending on router alert within his network
(e.g. RSVP-TE) while at the same time safely transporting router
alert packets carrying another protocol that may be used end to end
(e.g. RSVP IPv4/6).
Where the Service Provider cannot transport end to end router alert
packets over his network (i.e. they choose to drop them), it is
desirable that the Service Provider carry these packets through their
network and remove the IP router alert option from the IP header on
ingress/receipt of the packet. This ensures that at least the packet
will make it through the Service Provider network allowing the packet
to be intercepted via other means than the IP router alert.
Where the Service Provider does not ensure transport of router alert
packets for an end to end protocol of interest to a Service Provider
user, the user may remove the IP router alert option from the IP
header before sending the packet to the Service Provider and may then
intercept the packet on the other side using some other interception
technique (and then possibly restore the IP Router alert option).
The authors of this document are seeking feedback on whether some of
the practices discussed in this section ought to be elevated to BCP
requirements or recommendations.
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4. Example Protection Mechanisms in a Router Alert Implementation
Implementations of Router Alert on routers generally include
mechanisms for protection against the associated security risk. This
section provides examples of behaviors that may be supported by
router implementations to help protect the router. The authors are
seeking feedback about whether such mechanisms (or a subset/superset
of those) ought to be elevated to BCP requirements and
recommendations instead of simply described as examples.
4.1. Handling Packets Carrying the Router Alert Option
o a router implementation may elect to perform packet inspection to
see whether they carry the Router Alert option in the "fast path".
This avoids punting all packets to the slow path when the router
is interested in some router alert packets.
o a router implementation may elect to not send to the "slow path"
IP packets carrying the Router Alert option unless router alert
interception is explicitly enabled on the router (or interface).
This avoids punting any router alert packet when the router is not
interested in any of those.
o a router implementation may elect to not send to the "slow path"
IP packets carrying the Router Alert option unless there is at
least one protocol explicitly enabled on the router (or interface)
which is defined to use the IP Router Alert option. This avoids
punting any router alert packet when the router is not interested
in any of those.
o a router implementation may elect to allow configuration of which
protocol is "of interest" for the Router Alert option interception
(on router or interface level). The router implementation may
then elect to not send packets carrying the Router Alert option to
the "slow path" unless the payload protocol carried in the packet
is configured as "protocol of interest". This avoids punting
router alert packet carrying a protocol in which the router is not
interested.
4.2. Applying Rate Limiting
o a router implementation may elect to support (in the fast path)
rate limiting of the number of Router Alert IP datagrams (e.g. at
router or interface level) which go to the "slow path". The
benefits of rate limiting is described in [RFC2711].
o a router implementation may elect to support (in the fast path)
separate rate limiting per payload protocol. This allows one
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protocol relying on router alert to be protected from DOS attacks
using router alert with a different protocol.
4.3. Router Alert in Congested Systems
o a router implementation may elect to support selective dropping of
packets carrying the Router Alert option (rather than pass them to
the "slow path") in preference to dropping other control plane
packets, in the face of control plane congestion. This protects
other control plane protocols from router alert attacks.
4.4. Handling Unknown Payload Protocols
If an IP packet contains the Router Alert option, but the payload
protocol is not explicitly identified as a Payload of interest by the
router examining the packet, the behavior is not defined by
[RFC2113]. However, the definition of RSVP in [RFC2205] assumes that
the packet will be forwarded using normal forwarding based on the
destination IP address.
o a router implementation may elect to forward within the "fast
path" (subject to all normal policies and forwarding rules) a
packet carrying the Router Alert option containing a payload that
is not a payload of interest to that router. The "not passing"
behavior protects the router from DOS attacks using router alert
packets of a protocol unknown to the router. The "forwarding"
behavior contributes to transparent end to end transport of router
alert packets (e.g. to facilitate their use by end to end
application).
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5. Security Considerations
This document discusses security risks associated with current usage
of the IP Router Alert Option and associated practices.
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6. IANA Considerations
None.
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7. Contributors
The contributors to this document (in addition to the editors) are:
o David Ward:
* Cisco Systems
* wardd@cisco.com
o Francois Le Faucheur:
* Cisco Systems
* flefauch@cisco.com
o Ashok Narayanan:
* Cisco Systems
* ashokn@cisco.com
o Adrian Farrell:
* OldDog Consulting
* adrian@olddog.co.uk
o Tony Li:
* tony.li@tony.li
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8. Acknowledgments
We would like to thank Dave Oran, Magnus Westerlund, John Scudder,
Ron Bonica and Ross Callon for their comments.
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9. References
9.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC2113] Katz, D., "IP Router Alert Option", RFC 2113,
February 1997.
[RFC2711] Partridge, C. and A. Jackson, "IPv6 Router Alert Option",
RFC 2711, October 1999.
9.2. Informative References
[I-D.dasmith-mpls-ip-options]
Jaeger, W., Mullooly, J., Scholl, T., and D. Smith,
"Requirements for Label Edge Router Forwarding of IPv4
Option Packets", draft-dasmith-mpls-ip-options-01 (work in
progress), October 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC3175] Baker, F., Iturralde, C., Le Faucheur, F., and B. Davie,
"Aggregation of RSVP for IPv4 and IPv6 Reservations",
RFC 3175, September 2001.
[RFC3208] 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.
[RFC3209] 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.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, October 2002.
[RFC4081] Tschofenig, H. and D. Kroeselberg, "Security Threats for
Next Steps in Signaling (NSIS)", RFC 4081, June 2005.
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[RFC5350] Manner, J. and A. McDonald, "IANA Considerations for the
IPv4 and IPv6 Router Alert Options", RFC 5350,
September 2008.
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Appendix A. A New Filtering Mechanism to Select IP RAO Packets of
Interest
This appendix discusses a possible enhancement to the current
specification of Router Alert to ensure that risks associated with
unintentional interception of packets that are not of real interest
to a given router are minimized (if not eliminated) by facilitating
identification in the fast path of the subset of packets with router
alert that are of interest to the router. A key aspect of the
proposal is to facilitate finer grain identification of router alert
packets of interest versus unwanted router alert packets while only
requiring inspection of the router alert header. In particular:
o the proposal allows router alert packets from different
application protocols to be easily distinguished even if they
share the same transport protocol (i.e. the have the same IP PID).
o the proposal allows router alert packets for the same application
protocol but associated with different contexts (e.g. end to end
RSVP vs internal RSVP-TE) to be easily distinguished.
The objective of this appendix is to solicit feedback on the question
of whether an enhancement to the current router alert specification
is justified, and if yes, how to enhance it.
[RFC2113] specifies no mechanism for identifying different users of
IP RAO, with the result that many fast switching implementations punt
most/all packets marked with IP RAO into the slow path. It is
desirable for fast switching implementations to easily identify which
packets marked with IP RAO are actually of interest to local
protocols, so that other packets marked with IP RAO may be
efficiently forwarded. In the past, router alert implementations
have also looked at the IP PID [RFC0791] as a discriminator for
different protocols using IP RAO. However, this has two drawbacks.
The first is that messages with the same IP PID may represent
different protocol operations for IP RAO processing (e.g. RSVP vs.
RSVP-TE), or even different contexts (e.g. different levels of RSVP
aggregation [RFC3175]), and it is desirable to distinguish these in
the fast path. The second drawback is that IP PID values are a
scarce resource, and it is likely that new IP datagram protocols will
be assigned UDP port numbers rather than IP PIDs. Any such future
protocol which desires to use IP RAO would require additional checks
in the fast path to select out the correct packets for local
processing. To solve these problems, we propose an extension to the
specification and processing behaviour of the IP RAO header.
[RFC2113] specifies a 2-octet value in the IP RAO option field.
[RFC5350] specifies creation of an IANA registry for managing this
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2-octet value, and proposes a unified IPv4/IPv6 usage as follows:
+-------------+--------------------------------------+-----------+
| Value | Description | Reference |
+-------------+--------------------------------------+-----------+
| 0 | Router shall examine packet | [RFC2113] |
| 1-32 | Aggregated Reservation Nesting Level | [RFC3175] |
| 33-65502 | Available for assignment by the IANA | |
| 65503-65534 | Available for experimental use | |
| 65535 | Reserved | |
+-------------+--------------------------------------+-----------+
We propose the following change to IP RAO processing:
o The following 2-octet field will now be used to identify the
protocol and context from an IP RAO perspective. For IANA
assignment purposes, this field will be split into two octets
+----------+----------+
+ protocol + context +
+ selector + selector +
+ (8 bits) + (8 bits) +
+----------+----------+
The protocol selector will be assigned for major protocols by IANA
and the context selector will be specific to the protocol.
Protocol selector 0 is reserved for backward compatibility and
protocol selector 255 is reserved for experimental use. New
protocol using IP RAO MUST allocate and use new protocol selector
and context selector values. For protocol selector 1-254, the
value of the protocol and context selector fields MUST be assigned
in a manner such that the content of the IP RAO option is
sufficient to determine whether a packet is of interest to a node,
with a reasonable level of granularity. For example, having the
[RFC3175] aggregate reservation nesting level in the context
selector allows P routers to quickly separate out RSVP messages
for aggregate vs. end-to-end flows. Or, a separate context
selector for RSVP-IPv4 vs. RSVP-TE sessions allows nodes to
efficiently ignore one session type while processing another.
o Fast path switching implementations SHOULD use this field to
determine whether they wish to select a packet with IP RAO for
local processing. A table of in-use protocol/context selector
values can be looked up during packet switching to determine
whether the packet is to be locally processed. For packets marked
with protocol selectors 1-254, the value of the IP RAO value field
is sufficient to rapidly determine whether the packet may be
forwarded unmodified or whether it should be punted to the "slow
path" for local processing.
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o The protocol selector 0 is reserved for backwards compatibility.
For packets marked with protocol selector 0, the packet MUST be
examined further to determine whether it is of local interest, in
compliance with current protocol requirements.
o All the requirements regarding protecting router control plane
resources from attacks based on IP RAO, and protecting different
protocols using IP RAO from each other, continue to apply in this
context. The protocol selector and context selector fields MAY be
used to differentiate between these protocols.
o Protocol and context selector values will be allocated for
existing users of IP RAO as well (e.g. RSVP, IGMPv2 and PGM).
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Author's Address
Reshad Rahman (editor)
Cisco Systems
2000 Innovation Dr.
Kanata, Ontario K2K 3E8
Canada
Email: rrahman@cisco.com
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