One document matched: draft-ietf-karp-design-guide-00.txt
KARP Working Group G. Lebovitz
Internet Draft Juniper
Intended status: Informational M. Bhatia
Expires: August, 2010 Alcatel-Lucent
February 2010
Keying and Authentication for Routing Protocols (KARP)
Design Guidelines
draft-ietf-karp-design-guide-00.txt
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Abstract
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In the March of 2006 the IAB held a workshop on the topic of "Unwanted
Internet Traffic". The report from that workshop is documented in RFC
4948 [RFC4948]. Section 8.2 of RFC 4948 calls for [t]ightening the
security of the core routing infrastructure." Four main steps were
identified for improving the security of the routing infrastructure.
One of those steps was "securing the routing protocols' packets on the
wire." One mechanism for securing routing protocol packets on the wire
is the use of per-packet cryptographic message authentication,
providing both peer authentication and message integrity. Many
different routing protocols exist and they employ a range of different
transport subsystems. Therefore there must necessarily be various
methods defined for applying cryptographic authentication to these
varying protocols. Many routing protocols already have some method for
accomplishing cryptographic message authentication. However, in many
cases the existing methods are dated, vulnerable to attack, and/or
employ cryptographic algorithms that have been deprecated. This
document is one of a series concerned with defining a roadmap of
protocol specification work for the use of modern cryptographic
mechanisms and algorithms for message authentication in routing
protocols. In particular, it defines the framework for a key
management protocol that may be used to create and manage session keys
for message authentication and integrity. The overall roadmap reflects
the input of both the security area and routing area in order to form a
jointly agreed upon and prioritized work list for the effort.
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. [RFC2119]
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Table of Contents
1. Introduction..................................................3
2. Categorizing Routing Protocols................................4
2.1. Category: Message Transaction Type.......................4
2.2. Category: Peer vs Group Keying...........................5
3. Consider the future existence of a KMP........................5
3.1. Consider Asymmetric Keys.................................6
3.2. Cryptographic Keys Life Cycle............................6
4. RoadMap.......................................................7
4.1. Work Phases on any Particular Protocol...................7
4.2. Work Items Per Routing Protocol..........................9
5. Routing Protocols in Categories..............................11
6. Gap Analysis.................................................14
7. Security Considerations......................................16
7.1. Use Strong Keys.........................................16
7.2. Internal vs. External Operation.........................18
7.3. Unique versus Shared Keys...............................18
7.4. Out-of-Band vs. In-line Key Management..................20
8. Acknowledgments..............................................21
9. IANA Considerations..........................................21
10. References..................................................22
10.1. Normative References...................................22
10.2. Informative References.................................22
1. Introduction
In March 2006 the Internet Architecture Board (IAB) held a workshop on
the topic of "Unwanted Internet Traffic". The report from that
workshop is documented in RFC 4948 [RFC4948]. Section 8.1 of that
document states that "A simple risk analysis would suggest that an
ideal attack target of minimal cost but maximal disruption is the core
routing infrastructure." Section 8.2 calls for "[t]ightening the
security of the core routing infrastructure." Four main steps were
identified for that tightening:
o More secure mechanisms and practices for operating routers.
This work is being addressed in the OPSEC Working Group.
o Cleaning up the Internet Routing Registry repository [IRR], and
securing both the database and the access, so that it can be used
for routing verifications. This work should be addressed through
liaisons with those running the IRR's globally.
o Specifications for cryptographic validation of routing message
content. This work will likely be addressed in the SIDR Working
Group.
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o Securing the routing protocols' packets on the wire
This document addresses the last bullet, securing the packets on the
wire of the routing protocol exchanges.
2. Categorizing Routing Protocols
For the purpose of this security roadmap definition, we will categorize
the routing protocols into groups and have design teams focus on the
specification work within those groupings. It is believed that the
groupings will have like requirements for their authentication
mechanisms, and that reuse of authentication mechanisms will be
greatest within these grouping. The work items placed on the roadmap
will be defined and assigned based on these categorizations. It is
also hoped that, down the road in the Phase 2 work, we can create one
Key Management Protocol (KMP) per category (if not for several
categories) so that the work can be easily leveraged by the various
Routing Protocol teams. KMPs are useful for allowing simple, automated
updates of the traffic keys used in a base protocol. KMPs replace the
need for humans, or OSS routines, to periodically replace keys on
running systems. It also removes the need for a chain of manual keys
to be chosen or configured. When configured properly, a KMP will
enforce the key freshness policy of two peers by keeping track of the
key lifetime and negotiating a new key at the defined interval.
2.1. Category: Message Transaction Type
The first categorization defines four types of messaging transactions
used on the wire by the base Routing Protocol. They are:
One-to-One
One peer router directly and intentionally delivers a route update
specifically to one other peer router. Examples are BGP [RFC4271], LDP
[RFC5036] [RFC3036], BFD [ I-D.ietf-bfd-base] and RSVP [RFC2205].
Point-to-point modes of both IS-IS [RFC1195] and OSPF [RFC2328], when
sent over both traditional point-to-point links and when using multi-
access layers, may both also fall into this category.
One-to-Many
A router peers with multiple other routers on a single network segment
-- i.e. on link local -- such that it creates and sends one route
update message which is intended for consumption by multiple peers.
Examples would be OSPF and IS-IS in their broadcast, non-point-to-point
mode and Routing Information Protocol (RIP) [RFC2453].
Multicast
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Multicast protocols have unique security properties because of the fact
that they are inherently group-based protocols and thus have group
keying requirements at the routing level where link-local routing
messages are multicasted. Also, at least in the case of PIM-SM
[RFC4601], some messages are sent unicast to a given peer(s), as is the
case with router-close-to-sender and the "Rendezvous Point". Some work
for application layer message security has been done in the Multicast
Security working group (MSEC, http://www.ietf.org/html.charters/msec-
charter.html) and may be helpful to review, but is not directly
applicable.
2.2. Category: Peer vs Group Keying
The second axis of categorization groups protocols by the keying
mechanism that will be necessary for distributing session keys to the
actual Routing Protocol transports. They are:
Peer keying
One router sends the keying messages directly and only to one other
router, such that a one-to-one, unique keying security association (SA)
is established between the two routers. This would be employed by
protocols like BGP, BFD, LDP, etc.
Group Keying
One router creates and distributes a single keying message to multiple
peers. In this case a group SA will be established and used between
multiple peers simultaneously. Group keying exists for protocols like
OSPF [RFC2328], and also for multicast protocols like PIM-SM [RFC4601].
3. Consider the future existence of a KMP
When it comes time for the KARP WG to design the re-usable model for a
KMP, [RFC4107] should be consulted.
However, when conducting the design work on a manual keyed version of a
routing protocol's authentication, consideration must be made for the
eventual use of a KMP. In particular, design teams must consider what
parameters would need to be handed down to the Routing Protocol by the
KMP.
Consider: some sort of security association identifier (e.g. IPsec
ESP's SPI, or TCP-AO's KeyID), key life times which may be represented
either in bytes or seconds, the cryptographic algorithms being used,
the keys themselves, and the direction of the keys (i.e. receiveKey,
sendKey).
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3.1. Consider Asymmetric Keys
The use of asymmetric keys can be a very powerful way to authenticate
machine peers as are found in routing protocol peer exchanges. If
generated on the machine, and never moved off the machine, these keys
will be very secret, and will not be subject to change if an
administrator leaves the organization. Since the keys are totally
random, and very long, they are far less susceptible to off-line
dictionary and guessing attacks.
An easy and simple way to use asymmetric keys is to start by having the
router generate a public/private key pair. At the time of this writing,
the keys in the pair SHOULD be no less than 2048bits long (though this
length will grow over time). Many routers have the ability to be
remotely managed over the SSH [RFC4252] and [RFC4253]. As such, they
will also have the ability to generate and store an asymmetric key
pair, because this is the commonly used method that users authenticate
the SSH service when connecting to the router for management sessions.
Once asymmetric key pair is generated, the KMP generating security
association parameters and keys for routing protocol may use the
machine's asymmetric keys for the identity proof. The form of the
identity proof could be either raw keys, the more easily administrable
self-signed certificate format, or a PKI issued certificate credential.
Regardless which form we eventually standardize, the proof of this
identity presentation can be as simple as the SHA-1 fingerprint, which
is represented in a very human readable and transferable form of 20
pairs of ASCII characters. More complexly, but also more securely, the
identity proof could be verified through the use of a PKI system's
revocation checking mechanism, (e.g. Certificate Revocation List (CRL)
or OCSP responder). If the SHA-1 fingerprint is used, the solution
could be as simple as loading a set of neighbor routers' peer ID
strings into a table and listing the associated fingerprint string for
each ID string. In most organizations or peering points, this list will
not be longer than a thousand or so routers, and often the list will be
much much shorter. In other words, the entire list for a given
organization's router ID & SHA-1 fingerprints could easily be held in a
router's configuration file, uploaded, downloaded and move about at
will. And it doesn't matter who sees or gains access to these
fingerprint strings, because they are meant to be distributed publicly.
3.2. Cryptographic Keys Life Cycle
Cryptographic keys must have a limited lifetime so that they are
vulnerable against cryptanalysis attacks. Each time a key is employed,
it generates a cipher text. In case of routing protocols the cipher
text is the authentication data that is carried by the protocol
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packets. Using the same key repetitively allows an attacker to build up
a store of cipher texts which can prove sufficient for a successful
cryptanalysis of the key value. It is also worthwhile to note that if
the routing protocol is transmitting packets at a high rate then the
"long life" may be in order of a few hours. Thus it's the amount of
traffic that has been put on the wire using a specific key for
authentication and not necessarily the duration for which the key has
been in use.
Another reason for limiting the lifetime of a key is to minimize the
damage from a compromised key. It is unlikely a user will discover an
attacker has compromised his or her key if the attacker remains
"passive." Relatively frequent key changes will limit any potential
damage from compromised keys.
Thus it is strongly recommended that routing and security protocols do
not directly use the long-lived key, but should instead use a key
derivation function to derive a short-lived key from the long-lived
key.
The long-lived cryptographic keys used by the routing protocols can be
either inserted manually in a database or can make use of an automated
key management protocol to do this. In this future environment, we do
not anticipate an environment where the automated key management
protocol will be used to create short-lived cryptographic session keys
for the security of routing protocols.
The cryptographic keying material for individual sessions is derived
from the keying material stored in the database of long-lived
cryptographic keys [I-D.housley-saag-crypto-key-table]. A key
derivation function (KDF) and its inputs are also specified in the
database of long-lived cryptographic keys; session specific values
based on the routing protocol are input to the KDF. Protocol specific
key identifiers may be assigned to the cryptographic keying material
for individual sessions if needed.
4. RoadMap
4.1. Work Phases on any Particular Protocol
The desired end state for the KARP work contains several items. First,
the people desiring to deploy securely authenticated and integrity
validated packets between routing peers have the tools specified,
implemented and shipping in order to deploy. These tools should be
fairly simple to implement, and not more complex than the security
mechanisms to which the operators are already accustomed. (Examples of
security mechanisms to which router operators are accustomed include:
the use of asymmetric keys for authentication in SSH for router
configuration, the use of pre-shared keys (PSKs) in TCP MD5 for BGP
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protection, the use of self-signed certificates for HTTPS access to
device Web-based user interfaces, the use of strongly constructed
passwords and/or identity tokens for user identification when logging
into routers and management systems.) While the tools that we intend
to specify may not be able to stop a deployment from using "foobar" as
an input key for every device across their entire routing domain, we
intend to make a solid, modern security system that is not too much
more difficult than that. In other words, simplicity and deployability
are keys to success. The Routing Protocols will specify modern
cryptographic algorithms and security mechanisms. Routing peers will
be able to employ unique, pair-wise keys per peering instance, with
reasonable key lifetimes, and updating those keys on a somewhat regular
basis will be operationally easy, causing no service interruption.
Achieving the above described end-state using manual keys may only be
pragmatic in very small deployments. In larger deployments, this end
state will be much more operationally difficult to reach with only
manual keys. Thus, there will be a need for key life cycle management,
in the form of a key management protocol, or KMP. We expect that the
two forms, manual key usage and KMP usage, will co-exist in the real
world. For example, a provider's edge router at a public exchange
peering point will want to use a KMP for ensuring unique and fresh keys
with external peers, while a manual key may be used between a
provider's access edge router and each of the same provider's customer
premise routers with which it peers.
In accordance with the desired end state just described, we define two
main work phases for each Routing Protocol:
1. Enhance the Routing Protocol's current authentication mechanism.
This work involves enhancing a Routing Protocol's current security
mechanisms in order to achieve a consistent, modern level of
security functionality within its existing keying framework. It is
understood and accepted that the existing keying frameworks are
largely based on manual keys. Since many operators have already
built operational support systems (OSS) around these manual key
implementations, there is some automation available for an operator
to leverage in that way, if the underlying mechanisms are themselves
secure. In this phase, we explicitly exclude embedding or creating
a KMP. Refer to [I-D.ietf-karp-threats-req] for the list of the
requirements for Phase 1 work.
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2. Develop an automated keying framework. The second phase will focus
on the development of an automated keying framework to facilitate
unique pair-wise (or perhaps group-wise, where applicable) keys per
peering instance. This involves the use of a KMP. A KMP is helpful
because it negotiates unique, pair wise, random keys without
administrator involvement. It also negotiates several of the SA
parameters required for the secure connection, including key life
times. It keeps track of those lifetimes using counters, and
negotiates new keys and parameters before they expire, again,
without administrator interaction. Additionally, in the event of a
breach, changing the KMP key will immediately cause a rekey to occur
for the Traffic Key, and those new Traffic Keys will be installed
and used in the current connection. In summary, a KMP provides a
protected channel between the peers through which they can negotiate
and pass important data required to exchange proof of key
identifiers, derive Traffic Keys, determine re-keying, synchronize
their keying state, signal various keying events, notify with error
messages, etc. To address brute force attacks [RFC3562] recommends
a key management practice to minimize the possibility of successful
attack-- frequent key rotation, limited key sharing, key length
restrictions, etc. Advances in computational power due to Moore's
law are making that management burden untenable-- keys must be of a
size and composition that makes configuration and maintenance
difficult or keys must be rotated with an unreasonable frequency. A
KMP will help immensely with this growing problem.
The framework for any one Routing Protocol will fall under, and be able
to leverage, the generic framework described in [I-D.ietf-karp-
framework]
4.2. Work Items Per Routing Protocol
Each Routing Protocol will have a team (the [Routing_Protocol]-KARP
team) working on incrementally improving their Routing Protocol's
security. These teams will have the following main work items:
PHASE 1:
Characterize the RP
Assess the Routing Protocol to see what authentication mechanisms it
has today. Does it needs significant improvement to its existing
mechanisms or not? This will include determining if modern, strong
security algorithms and parameters are present.
Define Optimal State
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List the requirements for the Routing Protocol's session key usage
and format to contain to modern, strong security algorithms and
mechanisms, per the Requirements document [I-D.ietf-karp-threats-
req]. The goal here is to determine what is needed for the Routing
Protocol alone to be used securely with at least manual keys.
Gap Analysis
Enumerate the requirements for this protocol to move from its
current security state, the first bullet, to its optimal state, as
listed just above.
Transition and Deployment Considerations
Document the operational transition plan for moving from the old to
the new security mechanism. Will adjacencies need to bounce? What
new elements/servers/services in the infrastructure will be
required? What is an example work flow that an operator will take?
The best possible case is if the adjacency does not break, but this
may not always be possible.
Define, Assign, Design
Create a deliverables list of the design and specification work,
with milestones. Define owners. Release a document(s)
PHASE 2:
KMP Analysis
Review requirements for KMPs. Identify any nuances for this
particular protocol's needs and its use cases for KMP. List the
requirements that this Routing Protocol has for being able to be use
in conjunctions with a KMP. Define the optimal state.
Gap Analysis
Enumerate the requirements for this protocol to move from its
current security state to its optimal state.
Define, Assign, Design
Create a deliverables list of the design and specification work,
with miletsones. Define owners. Do the design and document work
for a KMP to be able to generate the Routing Protocol's session keys
for the packets on the wire. These will be the arguments passed in
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the API to the KMP in order to bootstrap the session keys to the
Routing Protocol.
There will also be a team formed to work on the base framework
mechanisms for each of the main categories, i.e. the blocks and
API's represented in [I-D.ietf-karp-framework].
5. Routing Protocols in Categories
This section groups the Routing Protocols into like categories,
according to attributes set forth in Categories Section (Section 2).
Each group will have a design team tasked with improving the security
of the Routing Protocol mechanisms and defining the KMP requirements
for their group, then rolling both into a roadmap document upon which
they will execute.
BGP, LDP and MSDP
The Routing Protocols that fall into the category of the one-to-one
peering messages, and will use peer keying protocols. BGP [RFC4271]
and MSDP [RFC3618] are transmitted over TCP, while LDP [RFC5036]
uses UDP. A team will work on one mechanism to cover these TCP
unicast protocols. Much of the work on the Routing Protocol update
for its existing authentication mechanism is already occuring in the
TCPM Working Group, on the TCP-AO [I-D.ietf-tcpm-tcp-auth-opt]
document, as well as its cryptography-helper document, TCP-AO-CRYPTO
[I-D.ietf-tcpm-tcp-ao-crypto]. However, this cannot be used for LDP
as LDP runs over UDP. A separate team might want to look at LDP.
Another exception is the mode where LDP is used directly on the LAN.
The work for this may go into the Group keying category (along with
OSPF) as mentioned below.
OSPF, ISIS, and RIP
The Routing Protocols that fall into the category Group keying with
one-to-many peering messages includes OSPF [RFC2328], ISIS [RFC1195]
and RIP [RFC2453]. Not surprisingly, all these routing protocols
have two other things in common. First, they are run on a
combination of the OSI datalink layer 2, and the OSI network layer
3. By this we mean that they have a component of how the routing
protocol works which is specified in Layer 2 as well as in Layer 3.
Second, they are all internal gateway protocols, or IGPs. The
keying mechanisms and use will be much more complicated to define
for these than for a one-to-one messaging protocol.
BFD
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Because it is less of a routing protocol, per se, and more of a peer
aliveness detection mechanism, Bidirectional Forwarding Detection
(BFD) will have its own team. BFD is also different from the other
protocols covered here as it works on millisecond timers and would
need separate considerations to mitigate the potential for DoS
attacks. It also raises interesting issues with respect to the
sequence number scheme that is generally deployed to protect against
the replay attacks as this space can rollover quite frequently
because of the rate at which BFD packets are generated.
RSVP and RSVP-TE
The Resource reSerVation Protocol [RFC2205] allows hop-by-hop
authentication of RSVP neighbors, as specified in [RFC2747]. In this
mode, an integrity object is attached to each RSVP message to
transmit a keyed message digest. This message digest allows the
recipient to verify the authenticity of the RSVP node that sent the
message, and to validate the integrity of the message. Through the
inclusion of a sequence number in the scope of the digest, the
digest also offers replay protection.
[RFC2747] does not dictate how the key for the integrity operation
is derived. Currently, most implementations of RSVP use a
statically configured key, per interface or per neighbor.
RSVP relies on per peer authentication mechanism, where each hop
authenticates its neighbor with a shared key or certificate.
Trust in this model is transitive. Each RSVP node trusts explicitly
only its RSVP next hop peers, through the message digest contained
in the INTEGRITY object. The next hop RSVP speaker in turn trusts
its own peers and so on. See also the document "RSVP security
properties" [RFC4230] for more background.
The keys used for generating the RSVP messages can, in particular,
be group keys (for example distributed via GDOI [RFC3547], as
discussed in [I-D.weis-gdoi-mac-tek]).
The trust an RSVP node has to another RSVP node has an explicit and
an implicit component. Explicitly the node trusts the other node to
maintain the RSVP messages intact or confidential, depending on
whether authentication or encryption (or both) is used. This means
only that the message has not been altered or seen by another, non-
trusted node. Implicitly each node trusts each other node with
which it has a trust relationship established via the mechanisms
here to adhere to the protocol specifications laid out by the
various standards. Note that in any group keying scheme like GDOI a
node trusts all the other members of the group.
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RSVP TE [RFC3209] [RFC3473] [RFC4726] [RFC5151] is an extension of
the RSVP protocol for traffic engineering. It supports the
reservation of resources across an IP network and is used for
establishing MPLS LSPs, taking into consideration network constraint
parameters such as available bandwidth and explicit hops. RSVP-TE
signaling is used to establish both intra and inter-domain TE LSPs.
When signaling an inter-domain RSVP-TE LSP, folks MAY make use of
the security features already defined for RSVP-TE [RFC3209]. This
may require some coordination between the domains to share the keys
(see [RFC2747] and [RFC3097]), and care is required to ensure that
the keys are changed sufficiently frequently. Note that this may
involve additional synchronization, should the domain border nodes
be protected with Fast ReRoute, since the merge point (MP) and point
of local repair (PLR) should also share the key.
For inter-domain signaling for MPLS-TE, the administrators of
neighboring domains MUST satisfy themselves as to the existence of a
suitable trust relationship between the domains. In the absence of
such a relationship, the administrators SHOULD decide not to deploy
inter-domain signaling, and SHOULD disable RSVP-TE on any inter-
domain interfaces.
These protocols will be handled together
PIM-SM and PIM-DM
Finally, the multicast protocols of PIM-SM [RFC4601] and PIM-DM
[RFC3973] will be handled together. PIM-SM multicasts routing
information (Hello, Join/Prune, Assert) on a link-local basis, using
a defined multicast address. In addition, it specifies unicast
communication for exchange of information (Register, Register-Stop)
between the router closest to a group sender and the "rendezvous
point" (RP). The RP is typically not "on-link" for a particular
router. While much work has been done on multicast security for
application-layer groups, little has been done to address the
problem of managing hundreds or thousands of small one-to-many
groups with link-local scope. Such an authentication mechanism
should be considered along with the router-to-Rendezvous Point
authentication mechanism. The most important issue is ensuring that
only the "authorized neighbors" get the keys for (S,G), so that
rogue routers cannot participate in the exchanges. Another issue is
that some of the communication may occur intra-domain, e.g. the
link-local messages in an enterprise, while others for the same
(*,G) may occur inter-domain, e.g. the router-to-Rendezvous Point
messages may be from one enterprise's router to another. One
possible solution proposes a region-wide "master" key server
(possibly replicated), and one "local" key server per speaking
router. There is no issue with propagating the messages outside the
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link, because link-local messages, by definition, are not forwarded.
This solution is offered only as an example of how work may
progress; further discussion should occur in this work team.
Specification of a link-local protection mechanism for PIM-SM
occurred in RFC 4601 [RFC4601], and this work is being updated in
PIM-SM-LINKLOCAL [I-D.ietf-pim-sm-linklocal]. However, the KMP part
is completely unspecified, and will require work outside the
expertise of the PIM working group to accomplish, which is why this
roadmap is being created.
6. Gap Analysis
The [I-D.ietf-karp-threats-req] document lists the generic requirements
for the security and authentication mechanisms that must exist for the
various routing and signaling protocols that come under the purview of
KARP. There will be different design teams working for each of the
categories of routing protocols defined.
To start, design teams must review the "Threats and Requirements for
Authentication of Routing Protocols" document [I-D.ietf-karp-threats-
req]. This document contains detailed descriptions of the threat
analysis for routing protocol authentication in general. Note that it
will not contain all the authentication-related threats for any one
routing protocol, or category of routing protocol. The design team must
conduct a threat analysis to determine if specific threats beyond those
in the [I-D.ietf-karp-threats-req] document exist, and to describe
those threats.
The [I-D.ietf-karp-threats-req] document also contains many
requirements around security matters. The different routing protocol
design teams must walk through each section of the requirements and
determine one by one how their protocol either does or does not address
each requirement. Examples include modern, strong cryptographic
algorithms, with at least one such algorithm listed as a MUST;
algorithm agility; secure use of simple PSKs; intra-connection replay
protection; inter-connection replay protection, etc.
When doing the gap analysis we must first identify the elements of each
routing protocol that we wish to protect. In case of protocols riding
on top of IP, we might want to protect the IP header and the protocol
headers, while for those that work on top of TCP, it will be the TCP
header and the protocol payload. There is patently value in protecting
the IP header and the TCP header if the routing protocols rely on these
headers for some information (for example, identifying the neighbor
which originated the packet).
Then there will be a set of Cryptography requirements that we might
want to look at. For example, there MUST be at least on set of
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cryptography algorithms or constructions whose use is supported by all
implementations and can be safely assumed to be supported by any
implementation of the authentication option. The design teams should
look for this for the protocol that they are working on. If such
algorithms or constructions are not available then some should be
defined to support interoperability by having a single default.
Design teams MUST ensure that the default cryptographic algorithms and
constructions supported by the routing protocols are accepted by the
community. This means that the protocols MUST NOT rely on non-standard
or ad-hoc hash functions, keyed-hash constructions, signature schemes,
or other functions, and MUST use published and standard schemes.
Care should also be taken to ensure that the routing protocol
authentication scheme is capable of supporting algorithms other than
its defaults, in order to adapt to future discoveries.
Ideally, authentication MUST be performed on routing protocols packets
oblivious to the order in which they have arrived, so that it does not
get influenced by packets loss and reordering.
Design teams should ensure that their protocols authentication
mechanism is able to accommodate rekeying. This is essential since its
well known that keys must periodically be changed. Also what the
designers must ensure is that this rekeying event MUST NOT affect the
functioning of the routing protocol. For example, OSPF rekeying
requires coordination among the adjacent routers, while ISIS requires
coordination among routers in the entire domain.
Design teams while defining the new authentication and security
mechanisms MUST design in such a manner that the routing protocol
authentication mechanism remains oblivious of how the keying material
is derived. This decouples the authentication mechanism from the key
management system that is employed.
Design teams should also note that many routing protocols require
prioritized treatment of certain protocol packets and authentication
mechanisms should honor this.
Not all routing protocol authentication mechanisms provide support for
replay attacks, and the design teams should identify such
authentication mechanisms and work on them so that this can get fixed.
The design teams must look at the protocols that they are working on
and see if packets captured from the previous/stale sessions can be
replayed.
What might also influence the design is the rate at which the protocol
packets are originated. In case of protocols like BFD, where packets
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are originated at millisecond intervals, there are some special
considerations that must be kept in mind when defining the new
authentication and security mechanisms.
It is imperative that the new authentication and security mechanisms
defined support incremental deployment, as it is not feasible to deploy
a new routing protocol authentication mechanism throughout the network
instantaneously. It may also not be possible to deploy such a mechanism
to all routers in a large AS at one time. This means that the designers
must work on this aspect of authentication mechanism for the routing
protocol that they are working on. The mechanisms must provide backward
compatibility in the message formatting, transmission, and processing
of routing information carried through a mixed security environment.
The designers should also consider whether the current authentication
mechanisms impose considerable processing overhead on a router that's
doing authentication. Most currently deployed routers do not have
hardware accelerators for cryptographic processing and these operations
can impose a significant processing burden under some circumstances.
The proposed solutions should be evaluated carefully with regard to the
processing burden that they will impose, since deployment may be
impeded if network operators perceive that a solution will impose a
processing burden which either entails substantial capital expenses or
threatens to destabilize the routers.
7. Security Considerations
As mentioned in the Introduction, RFC4948 [RFC4948] identifies
additional steps needed to achieve the overall goal of improving the
security of the core routing infrastructure. Those include validation
of route origin announcements, path validation, cleaning up the IRR
databases for accuracy, and operational security practices that prevent
routers from becoming compromised devices. The KARP work is but one
step in a necessary system of security improvements.
The security of cryptographic-based systems depends on both the
strength of the cryptographic algorithms chosen and the strength of the
keys used with those algorithms. The security also depends on the
engineering of the protocol used by the system to ensure that there are
no non-cryptographic ways to bypass the security of the overall system.
7.1. Use Strong Keys
Care should be taken to ensure that the selected key is unpredictable,
avoiding any keys known to be weak for the algorithm in use. [RFC4086]
contains helpful information on both key generation techniques and
cryptographic randomness.
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In addition to using a strong key/PSK of appropriate length and
randomness, deployers of KARP protocols SHOULD use different keys
between different routing peers whenever operationally possible.
[RFC3562] provides some very sound guidance. It was meant specifically
for the use of TCP MD5 for BGP, but it is more or less applicable to
Routing Protocol authentication work that would result from KARP. It
states three main points: (1) key lengths SHOULD be between 12 and 24
bytes (this will vary depending on the MAC/KDF in use), with larger
keys having effectively zero additional computational costs when
compared to shorter keys, (2) key sharing SHOULD be limited so that
keys aren't shared among multiple BGP peering arrangements, and (3)
Keys SHOULD be changed at least every 90 days (this could be longer for
stronger MAC algorithms, but it is generally a wise idea).
This is especially true when the Routing Protocol takes a static
Traffic Key as opposed to a Traffic Key derived per-connection by a
KDF. The burden for doing so is understandable much higher than for
using the same static Traffic Key across all peering routers. This is
why use of a KMP network-wide increases peer-wise security so greatly,
because now each set of peers can enjoys a unique Traffic Key, and if
an attacker sitting between two routers learns or guesses the Traffic
Key for that connection, she doesn't gain access to all the other
connections as well.
However, whenever using manual keys, it is best to design a system
where a given PSK will be used in a KDF, mixed with connection specific
material, in order to generate session unique -- and therefore peer-
wise -- Traffic Keys. Doing so has the following advantages: the
Traffic Keys used in the per-message MAC operation are peer-wise
unique, it provides inter-connection replay protection, and, if the
per-message MAC covers some connection counter, intra-connection replay
protection.
Note that in the composition of certain key derivation functions (e.g.
KDF_AES_128_CMAC, as used in TCP-AO [I-D.ietf-tcpm-tcp-ao-crypto], the
pseudorandom function (PRF) used in the KDF may require a key of a
certain fixed size as an input. For example, AES_128_CMAC requires a
128 bit (16 byte) key as the seed. However, for convenience to the
administrators/deployers, a specification may not want to force the
deployer to enter a PSK of exactly 16 bytes. Instead, a specification
may call for a sub-key routine that could handle a variable length PSK,
one that might be less or more than 16 bytes (see [RFC4615], section 3,
as an example). That sub-key routine would act as a key extractor to
derive a second key of exactly the required length and thus suitable as
a seed to the PRF. This does NOT mean that administrators are safe to
use weak keys. Administrators are encouraged to follow [RFC4086] as
listed above. We simply attempted to "put a fence around stupidity", in
as much as possible.
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A better option, from a security perspective, is to use some
representation of a device-specific asymmetric key pair as the identity
proof, as described in section "UniqueVsSharedKeys" section.
7.2. Internal vs. External Operation
The designers must consider whether the protocol is an internal routing
protocol or an external one, i.e. Does it primarily run between peers
within a single domain of control or between two different domains of
control? Some protocols may be used in both cases, internally and
externally, and as such various modes of authentication operation may
be required for the same protocol. While it is preferred that all
routing exchanges run with the utmost security mechanisms enabled in
all deployments, this exhortation is greater for those protocols
running on inter-domain point-to-point links, and greatest for those on
shared access link layers with several different domains interchanging
together, because the volume of attackers are greater from the outside.
Note however that the consequences of internal attacks maybe no less
severe -- in fact they may be quite a bit more severe -- than an
external attack. An example of this internal versus external
consideration is BGP which has both EBGP and IBGP modes. Another
example is a multicast protocol where the neighbors are sometimes
within a domain of control and sometimes at an inter-domain exchange
point. In the case of PIM-SM running on an internal multi-access link,
it would be acceptable to give up some security to get some convenience
by using a group key between the peers on the link. On the other hand,
in the case of PIM-SM running over a multi-access link at a public
exchange point, operators may favor security over convenience by using
unique pair-wise keys for every peer. Designers must consider both
modes of operation and ensure the authentication mechanisms fit both.
Operators are encouraged to run cryptographic authentication on all
their adjacencies, but to work from the outside in, i.e. The EBGP
links are a higher priority than the IBGP links because they are
externally facing, and, as a result, more likely to be targeted in an
attack.
7.3. Unique versus Shared Keys
This section discusses security considerations regarding when it is
appropriate to use the same authentication key inputs for multiple
peers and when it is not. This is largely a debate of convenience
versus security. It is often the case that the best secured mechanism
is also the least convenient mechanism. For example, an air gap between
a host and the network absolutely prevents remote attacks on the host,
but having to copy and carry files using the "sneaker net" is quite
inconvenient and unscalable.
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Operators have erred on the side of convenience when it comes to
securing routing protocols with cryptographic authentication. Many do
not use it at all. Some use it only on external links, but not on
internal links. Those that do use it often use the same key for all
peers across their entire network. It is common to see the same key in
use for years, and that being the same key that was entered when
authentication was originally configured, or the routing gear deployed.
The goal for designers is to create authentication mechanisms that are
easy for the operators to deploy and manage, and still use unique keys
between peers (or small groups on multi-access links), and within
between sessions. Operators have the impression that they NEED one key
shared across the network, when in fact they do not. What they need is
the relative convenience they experience from deploying cryptographic
authentication with one (or few) key, compared to the inconvenience
they would experience if they deployed the same authentication
mechanism using unique pair-wise keys. An example is BGP Route
Reflectors. Here operators often use the same authentication key
between each client and the route reflector. The roadmaps defined from
this guidance document will allow for unique keys to be used between
each client and the peer, without sacrificing much convenience.
Designers should strive to deliver peer-wise unique keying mechanisms
with similar ease-of-deployment properties as today's one-key method.
Operators must understand the consequences of using the same keys
across many peers. Unique keys are more secure than shared keys because
they reduce both the attack target size and the attack consequence
size. In this context, the attack target size represents the number of
unique routing exchanges across a network that an attacker may be able
to observe in order to gain security association credentials, i.e.
crack the keys. If a shared key is used across the entire internal
domain of control, then the attack target size is very large. The
larger the attack target, the easier it is for the attacker to gain
access to analysis data, and greater the volume of analysis data he can
access in a given time frame, both of which make his job easier. Using
the same key across the network makes the attack vulnerability surface
more penetrable than unique keys. Consider also the attack consequence
size, the amount of routing adjacencies that can be negatively affected
once a breach has occurred, i.e. once the keys have been acquired by
the attacker.
Again, if a shared key is used across the internal domain, then the
consequence size is the whole network. Ideally, unique key pairs would
be used for each adjacency.
In some cases designers may need to use shared keys in order to solve
the given problem space. For example, a multicast packet is sent once
but then observed and consumed by several routing neighbors. If unique
keys were used per neighbor, the benefit of multicast would be erased
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because the casting peer would have to create a different announcement
packet/stream for each listening peer. Though this may be desired and
acceptable in some small amount of use cases, it is not the norm.
Shared group keys are an acceptable solution here, and much work has
been done already in this area (see MSEC working group).
7.4. Out-of-Band vs. In-line Key Management
This section discusses the security and use case considerations for
keys placed on devices through out-of-band configurations versus
through one routing peer-to-peer key management protocol exchanges.
Note, when we say here "Peer-to-Peer KMP" we do not mean in-band to the
Routing Protocol. Instead, we mean that the exchange occurs in-line,
over IP, between the two routing peers directly. In in-line KMP the
peers themselves handle the key and security association negotiation
between themselves directly, whereas in an out-of-band system the keys
are placed onto the device through some other configuration or
management method or interface.
An example of an out-of-band mechanism could be an administrator who
makes a remote management connection (e.g. using SSH) to a router and
manually enters the keying information -- like the algorithm, the
key(s), the lifetimes, etc. Another example could be an OSS system
which inputs the same information via a script over an SSH connection,
or by pushing configuration through some other management connection,
standard (Netconf-based) or proprietary.
The drawbacks of an out-of-band mechanism include: lack of scale-
ability, complexity and speed of changing if a breach is suspected. For
example, if an employee who had access to keys was terminated, or if a
machine holding those keys was believed to be compromised, then the
system would be considered insecure and vulnerable until new keys were
defined by a human. Those keys then need to be placed into the OSS
system, manually, and the OSS system then needs to push the change --
often during a very limited change window -- into the relevant devices.
If there are multiple organizations involved in these connections, this
process is greatly complicated.
The benefits of out-of-band mechanism is that once the new
keys/parameters are set in OSS system they can be pushed automatically
to all devices within the OSS's domain of control. Operators have
mechanisms in place for this already. In small environments with few
routers, a manual system is not difficult to employ.
We further define an in-line key exchange as using cryptographically
protected identity verification, session key negotiation, and security
association parameter negotiation between the two routing peers. The
KMP between the two peers may also include the negotiation of
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parameters, like algorithms, cryptographic inputs (e.g. initialization
vectors), key life-times, etc.
The benefits an in-line KMP are several. An in-line KMP results in
key(s) that are privately generated, and not recorded permanently
anywhere. Since the traffic keys used in a particular connection are
not a fixed part of a device configuration no steal-able data exists
anywhere else in the operator's systems which can be stolen, e.g. in
the case of a terminated or turned employee. If a server or other data
store is stolen or compromised, the thieves gain no access to current
traffic keys. They may gain access to key derivation material, like a
PSK, but not current traffic keys in use. In this example, these PSKs
can be updated into the device configurations (either manually or
through an OSS) without bouncing or impacting the existing session at
all. In the case of using raw asymmetric keys or certificates, instead
of PSKs, the data theft would likely not even result in any compromise,
as the key pairs would have been generated on the routers, and never
leave those routers. In such a case no changes are needed on the
routers; the connections will continue to be secure, uncompromised.
Additionally, with a KMP regular re-keys operations occur without any
operator involvement or oversight. This keeps keys fresh.
The drawbacks to using a KMP are few. First, a KMP requires more
cryptographic processing for the router at the very beginning of a
connection. This will add some minor start-up time to connection
establishment versus a purely manual key approach. Once a connection
with traffic keys have been established via a KMP, the performance is
the same in the KMP and the out-of-band case. KMPs also add another
layer of protocol and configuration complexity which can fail or be
mis-configured. This was more of an issue when these KMPs were first
deployed, but less so as these implementations and operational
experience with them has matured.
The desired end goal is in-line KMPs.
8. Acknowledgments
Much of the text for this document came originally from draft-lebovitz-
karp-roadmap, authored by Gregory M. Lebovitz.
We would like to thank Russ White, Michael Barnes and Vishwas Manral
for their comments on the draft.
9. IANA Considerations
This document places no requests to IANA.
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10. References
10.1. Normative References
[RFC2119] Bradner, S.,"Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4948] Andersson, L., et. al, "Report from the IAB workshop on
Unwanted Traffic March 9-10, 2006", RFC 4948, August 2007.
10.2. Informative References
[RFC1195] Callon, R. , "Use of OSI IS-IS for Routing in TCP/IP and Dual
Environments", RFC 1195, December 1990.
[RFC2205] Braden, R., et. al, "Resource ReSerVation Protocol (RSVP)
Version 1 Functional Specification", RFC 2205, September
1197.
[RFC2328] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[RFC2453] Malkin, G., "RIP Version 2", RFC 2453, November 1998.
[RFC2747] Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic
Authentication", RFC 2747, January 2000.
[RFC3036] Andersson, L., et. al, "LDP Specification", RFC 3036, January
2001.
[RFC3097] Braden, R, and Zhang, L., "RSVP Cryptographic Authentication
-- Updated Message Type Value", RFC 3097, April 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.
[RFC3473] Berger, L., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Resource ReserVation Protocol-Traffic
Engineering (RSVP-TE) Extensions", RFC 3473, January 2003.
[RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
Group Domain of Interpretation", RFC 3547, July 2003.
[RFC3562] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option", RFC 3562, July 2003.
[RFC3618] Fenner, B. and D. Meyer, "Multicast Source Discovery Protocol
(MSDP)", RFC 3618, October 2003.
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[RFC3973] Adams, A., Nicholas, J., and W. Siadak, "Protocol Independent
Multicast - Dense Mode (PIM-DM): Protocol Specification
(Revised)", RFC 3973, January 2005.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, June 2005.
[RFC4230] Tschofenig, H. and R. Graveman, "RSVP Security Properties",
RFC 4230, December 2005.
[RFC4252] Ylonen, T., et. al, "The Secure Shell (SSH) Authentication
Protocol", RFC 4252, January 2006.
[RFC4253] Ylonen, T., et. al, "The Secure Shell (SSH) Transport Layer
Protocol", RFC 4253, January 2006
[RFC4271] Rekhter, Y., Li, T. and Hares, S.,"A Border Gateway Protocol
4 (BGP-4)", RFC 4271, January 2006.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I.
Kouvelas,"Protocol Independent Multicast - Sparse Mode (PIM-
SM): Protocol Specification (Revised)", RFC 4601, August
2006.
[RFC4615] Song, J., Poovendran, R., Lee, J., and T. Iwata, "The
Advanced Encryption Standard-Cipher-based Message
Authentication Code-Pseudo-Random Function-128 (AES-CMAC-PRF-
128) Algorithm for the Internet Key Exchange Protocol (IKE)",
RFC 4615, August 2006.
[RFC4726] Farrel, A., et. al.,"A Framework for Inter-Domain
Multiprotocol Label Switching Traffic Engineering", RFC 4726,
November 2006.
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP Specification",
RFC 5036, October 2007.
[RFC5151] Farrel, A., et. al.,"Inter-Domain MPLS and GMPLS Traffic
Engineering -- Resource Reservation Protocol-Traffic
Engineering (RSVP-TE) Extensions", February 2008.
[ I-D.ietf-bfd-base] Katz, D. and Ward, D., "Bidirectional Forwarding
Detection", Work in Progress, January 2010.
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[I-D.ietf-tcpm-tcp-ao-crypto] Lebovitz, G., "Cryptographic Algorithms,
Use and Implementation Requirements for TCP Authentication
Option", Work in Progress, March 2009.
[I-D.ietf-karp-threats-req] Lebovitz, G., "KARP Threats and
Requirements", Work in Progress, February 2010.
[I-D.ietf-karp-framework] Lebovitz, G., "Framework for Cryptographic
Authentication of Routing Protocol Packets on the Wire", Work
in Progress, February 2010.
[I-D.ietf-pim-sm-linklocal] Atwood, W., Islam, S., and M. Siami,
"Authentication and Confidentiality in PIM-SM Link-local
Messages", Work in Progress, December 2009.
[I-D.ietf-tcpm-tcp-auth-opt] Touch, J., Mankin, A., and R. Bonica, "The
TCP Authentication Option", Work in Progress), October 2009.
[I-D.housley-saag-crypto-key-table] Housley, R. and Polk, T., "Database
of Long-Lived Cryptographic Keys" , Work in Progress,
September 2009
[I-D.weis-gdoi-mac-tek] Weis, B. and S. Rowles, "GDOI Generic Message
Authentication Code Policy", Work in Progress, July 2008.
[IRR] Merit Network Inc , "Internet Routing Registry Routing Assets
Database", 2006, http://www.irr.net/.
Author's Addresses
Gregory M. Lebovitz
Juniper Networks, Inc.
1194 North Mathilda Ave.
Sunnyvale CA 94089-1206
USA
Phone:
Email: gregory.ietf@gmail.com
Manav Bhatia
Alcatel-Lucent
Bangalore
India
Phone:
Email: manav.bhatia@alcatel-lucent.com
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