Network Working Group J. Touch
Request for Comments: 5387 USC/ISI
Category: Informational D. Black
November 2008 Problem and Applicability Statement
for Better-Than-Nothing Security (BTNS)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
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The Internet network security protocol suite, IPsec, requires
authentication, usually of network-layer entities, to enable access
control and provide security services. This authentication can be
based on mechanisms such as pre-shared symmetric keys, certificates
with associated asymmetric keys, or the use of Kerberos (via
Kerberized Internet Negotiation of Keys (KINK)). The need to deploy
authentication information and its associated identities can be a
significant obstacle to the use of IPsec.
This document explains the rationale for extending the Internet
network security protocol suite to enable use of IPsec security
services without authentication. These extensions are intended to
protect communication, providing "better-than-nothing security"
(BTNS). The extensions may be used on their own (this use is called
Stand-Alone BTNS, or SAB) or may be used to provide network-layer
security that can be authenticated by higher layers in the protocol
stack (this use is called Channel-Bound BTNS, or CBB). The document
also explains situations for which use of SAB and/or CBB extensions
Table of Contents
1. Introduction ....................................................31.1. Authentication .............................................31.2. IPsec Channels and Channel Binding .........................41.3. BTNS Methods ...............................................61.4. BTNS Scope .................................................61.5. Structure of This Document .................................72. Problem Statement ...............................................72.1. Network Layer ..............................................82.1.1. Authentication Identities ...........................82.1.2. Authentication Methods ..............................82.1.3. Current IPsec Limits on Unauthenticated Peers .......92.2. Higher Layer Issues ........................................92.2.1. Transport Protection from Packet Spoofing ...........92.2.2. Authentication at Multiple Layers ..................103. BTNS Overview and Threat Models ................................123.1. BTNS Overview .............................................123.2. BTNS and IPsec Security Services ..........................133.3. BTNS and IPsec Modes ......................................144. Applicability Statement ........................................154.1. Benefits ..................................................164.2. Vulnerabilities ...........................................164.3. Stand-Alone BTNS (SAB) ....................................174.3.1. Symmetric SAB ......................................174.3.2. Asymmetric SAB .....................................184.4. Channel-Bound BTNS (CBB) ..................................184.5. Summary of Uses, Vulnerabilities, and Benefits ............195. Security Considerations ........................................205.1. Threat Models and Evaluation ..............................205.2. Interaction with Other Security Services ..................205.3. MITM and Masquerader Attacks ..............................215.4. Denial of Service (DoS) Attacks and Resource
Consumptions ..............................................225.5. Exposure to Anonymous Access ..............................225.6. ICMP Attacks ..............................................225.7. Leap of Faith .............................................225.8. Connection Hijacking through Rekeying .....................245.9. Configuration Errors ......................................256. Related Efforts ................................................257. Acknowledgments ................................................258. Informative References .........................................26
Network security is provided by a variety of protocols at different
layers in the stack. At the network layer, the IPsec protocol suite
(consisting of IKE (Internet Key Exchange protocol), ESP
(Encapsulating Security Payload), and AH (Authentication Header)) is
used to secure IP traffic. IPsec, including IKE, offers high levels
of security that provide protection from a wide array of possible
threats, but authentication is required . In turn,
authentication requires deployment of authentication identities and
credentials, which can be an obstacle to IPsec usage. This document
discusses this dependency and introduces "Better-Than-Nothing
Security" (BTNS) as one solution, whose goal is to provide a
generally useful means of applying IPsec security services without
requiring network-layer authentication.
There are two primary architectural approaches to authentication:
employing out-of-band communications and using pre-deployed
information. Out-of-band authentication can be done via a trusted
third party, via a separate communication channel to the peer, or via
the same channel as the communications to be secured but at a higher
layer. Out-of-band authentication requires mechanisms and interfaces
to bind the authenticated identities to the secure communication
channels, and is out of scope for this document (although it may be
possible to extend the channel binding mode of BTNS to work with such
mechanisms). Pre-deployed information includes identities, pre-
shared secrets, and credentials that have been authenticated by
trusted authorities (e.g., a certificate and its corresponding
This form of authentication often requires manual deployment and
coordination among communicating peers. Furthermore, obtaining and
deploying credentials such as certificates signed by certification
authorities (CA) involves additional protocol and administrative
actions that may incur significant time and effort to perform.
These factors increase the work required to use IKE with IPsec for
peer authentication. Consequently, some users and applications do
not use IPsec to protect traffic at the network layer, but rely
instead on higher-layer security protocols (e.g., TLS ) or operate
without any security. As Section 2.2.1 describes, higher-layer
security protocols may not be enough to protect against some
To improve the situation, one could either reduce the hurdles to
obtain and configure authentication information or remove the
requirement for authentication in IPsec. The latter approach is the
core idea of BTNS, which provides anonymous (unauthenticated) keying
for IPsec to create security associations (SAs) with peers that do
not possess requisite authentication credentials. This requires
extensions to the IPsec architecture. As the new BTNS modes for
IPsec relax the authentication requirement, the impacts, tradeoffs,
and risks must be thoroughly understood before applying BTNS to any
communications. More specifically, this document addresses the
issues of whether and when network-layer authentication can be
omitted, the risks of using BTNS, and finally, the impacts to the
existing IPsec architecture.
BTNS employs a weaker notion of authenticated identity by comparison
to most authentication protocols; this weaker notion is bootstrapped
from the security association itself. This notion, called
"continuity of association", doesn't mean "Bill Smith" or "owner of
shared secret X2YQ", but means "the entity with which I have been
communicating on connection #23". Continuity of association is only
invariant within a single SA; it is not invariant across SAs, and
hence can only be used to provide protection during the lifetime of
an SA. This is a core notion used by BTNS, particularly in the
absence of higher-layer authentication. Continuity of association
can be viewed as a form of authentication in which an identity is not
authenticated across separate associations or out-of-band, but does
not change during the lifetime of the SA.
1.2. IPsec Channels and Channel Binding
When IPsec security services are used by higher-layer protocols, it
is important to bind those services to higher-layer protocol sessions
in order to ensure that the security services are consistently
applied to the higher-layer traffic involved. The result of this
binding is an "IPsec channel", and the act of creating an IPsec
channel is an instance of channel binding. Channel binding is
discussed in RFC 5056  and in an associated connection latching
document . This subsection summarizes the portions of these
documents that are essential to understanding certain aspects of
A secure channel is a packet, datagram, octet stream connection, or
sequence of connections between two endpoints that affords
cryptographic integrity and, optionally, confidentiality to data
exchanged over it . Applying this concept to IPsec, an "IPsec
channel" is a packet flow associated with a higher-layer protocol
session, such as a TCP connection, where all the packets are
protected by IPsec SAs such that:
o the peer's identity is the same for the lifetime of the packet
o the quality of IPsec protection used for the packet flow's
individual packets is the same for all of them for the lifetime of
the packet flow .
The endpoints of an IPsec channel are the higher-layer protocol
endpoints, which are beyond the endpoints of the IPsec SAs involved.
This creates a need to bind each IPsec SA to the higher-layer
protocol session and its endpoints. Failure to do this binding
creates vulnerabilities to man-in-the-middle (MITM) attacks, where
what appears to be a single IPsec SA for the higher-layer protocol
traffic is actually two separate SAs concatenated by the attacker
acting as a traffic-forwarding proxy.
The combination of connection latching  with channel binding 
creates IPsec channels and binds IPsec SAs to higher-layer protocols.
Connection latching creates an IPsec channel by associating IPsec SAs
to higher-layer protocol sessions, and channel binding enables a
higher-layer protocol to bind its authentication to the IPsec SAs.
Caching of this "latch" across higher-layer protocol sessions is
necessary to counter inter-session spoofing attacks, and the channel
binding authentication should be performed on each higher-layer
protocol session. Connection latching and channel binding are useful
not only for BTNS but also for IPsec SAs whose peers are fully
authenticated by IKE during creation of the SA.
Channel binding for IPsec is based on information obtained from the
SA creation process that uniquely identifies an SA pair. Channel
binding can be accomplished by adding this identifying information to
higher-layer authentication mechanisms based on one-way hashes, key
exchanges, or (public key) cryptographic signatures; in all three
cases, the resulting higher-layer authentication resists man-in-the-
middle attacks on SA creation. When each IKE peer uses a public-
private key pair for IKE authentication to create an SA pair, the
pair of public keys used (one for each peer) suffices for channel
binding; strong incorporation of this information into higher-layer
authentication causes that higher-layer authentication to fail when
an MITM attacker has concatenated separate SAs by acting as a
1.3. BTNS Methods
There are two classes of scenarios in which BTNS may be used to apply
IPsec services without network-layer authentication:
1. Protection of traffic for a higher-layer protocol that does not
use authentication. The resulting protection is "better than
nothing" because once an unauthenticated SA is successfully
created without an MITM, that SA's IPsec security services resist
subsequent MITM attacks even though the absence of authentication
allows the initial creation of the BTNS-based security association
(SA) to be subverted by an MITM. This method of using BTNS is
called Stand-Alone BTNS (SAB) because it does not rely on any
security services outside of IPsec.
2. Protection of traffic generated by a higher-layer protocol that
uses authentication. The "better-than-nothing" protection in this
case relies on the strength of the higher-layer protocol's
authentication and the channel binding of that authentication with
the BTNS-based SAs. The level of protection may be comparable to
the level afforded by the use of network-layer IKE authentication
when the higher-layer protocol uses strong authentication and
strong channel binding is employed to associate the BTNS-based SA
with that higher-layer authentication. This method of using BTNS
is called Channel-Bound BTNS (CBB) when the combination of the
higher-layer authentication and channel binding is sufficient to
detect an MITM attack on creation of a BTNS-based SA.
It is possible to combine IKE authentication for one end of an SA
pair with BTNS's absence of network-layer authentication for the
other end. The resulting asymmetric authentication creates
asymmetric modes of BTNS that are discussed further in Section 3.2
1.4. BTNS Scope
The scope of BTNS is to provide a generally useful means of applying
IPsec security services that does not require network-level
authentication credentials. The following areas are outside this
scope of BTNS and hence are not discussed further in this document:
1. Use of security frameworks other than IPsec to provide security
services for higher-layer protocols. There are a variety of
security service frameworks other than IPsec, such as TLS ,
Simple Authentication and Security Layer (SASL) , and Generic
Security Service Application Program Interface (GSS-API) , as
well as a variety of non-IPsec security mechanisms, such as TCP
MD5 , that are described in other documents. BTNS is based on
IPsec by design; it will not always be the most appropriate
2. Use of the Extensible Authentication Protocol (EAP) for IKE
authentication. Section 1.3 of RFC 3748 clearly restricts EAP's
applicability to network access protocols :
"EAP was designed for use in network access authentication,
where IP layer connectivity may not be available. Use of EAP
for other purposes, such as bulk data transport, is NOT
Hence, EAP authentication for IKE is only applicable to situations
where IKE is being used to establish network access (e.g., create
a Virtual Private Network (VPN) connection). In contrast, the
BTNS goal of general applicability encompasses many areas other
than network access and specifically includes protocols that
transfer large amounts of data, such as iSCSI  and NFSv4 .
3. Manual keying is not considered for BTNS because manual keying is
unsafe for protocols that transfer large amounts of data (e.g.,
RFC 3723 forbids use of manual keying with the IP Storage
protocols, including iSCSI, for this reason ).
1.5. Structure of This Document
The next section discusses the motivations for BTNS, primarily based
on the implications of IKE's requirements for network-layer
authentication. Section 3 provides a high level overview of BTNS,
both SAB and CBB. Section 3 also includes descriptions of the
security services offered and the BTNS modes of operation (based on
combinations of SAB, CBB, and/or IKE authentication). Section 4
explores the applicability of all of the modes of BTNS. This is
followed by a discussion of the risks and other security
considerations in Section 5. Section 6 briefly describes other
2. Problem Statement
This section describes the problems that motivated the development of
BTNS. The primary concern is that IPsec is not widely utilized
despite rigorous development effort and emphasis on network security
by users and organizations. There are also differing viewpoints on
which layer is best for securing network communications and how
security protocols at different layers should interact. The
following discussion roughly categorizes these issues by layers:
network layer and higher layers.
2.1. Network Layer
At the network layer, one of the hurdles is to satisfy the
authentication requirements of IPsec and IKE. This section discusses
some drawbacks of network-layer authentication and the results of
2.1.1. Authentication Identities
Current IPsec authentication supports several types of identities in
the Peer Authorization Database (PAD): IPv4 addresses, IPv6
addresses, DNS names, Distinguished Names, RFC 822 email addresses,
and Key IDs . All require either certificates or pre-shared
secrets to authenticate. The identities supported by the PAD can be
roughly categorized as network-layer identifiers or other
The first three types of identifiers -- IPv4 addresses, IPv6
addresses and DNS names -- are network-layer identifiers. The main
deficiency of IP addresses as identifiers is that they often do not
consistently represent the same physical systems due to the
increasing use of dynamic address assignments (DHCP) and system
mobility. The use of DNS names is also affected because the name to
address mapping is not always up to date as a result. Stale mapping
information can cause inconsistencies between the IP address recorded
in the DNS for a named system and the actual IP address of that
system, leading to problems if the DNS is used to cross-check the IP
address from which a DNS name was presented as an identifier. DNS
names are also not always under the control of the endpoint owner.
There are two main drawbacks with the other, non-network-layer
identifiers defined for the PAD. The PAD functionality can be overly
restrictive because there are other forms of identifiers not covered
by the PAD specification (EAP does not loosen these restrictions in
general; see Section 1.4). Use of any non-network-layer identifiers
for IPsec authentication may result in multiple authentications for
the same or different identifiers at different layers, creating a
need to associate authentications and new error cases (e.g., one of
two authentications for the same identifier fails). These issues are
also related to channel binding and are further discussed later in
2.1.2. Authentication Methods
As described earlier, certificates and pre-shared secrets are the
only methods of authentication accepted by current IPsec and IKE
specifications. Pre-shared secrets require manual configuration and
out-of-band communications. The verification process for
certificates is cumbersome, plus there are administrative and
potential monetary costs in obtaining certificates. These factors
are among the possible reasons why IPsec is not widely used outside
of environments with the highest security requirements.
2.1.3. Current IPsec Limits on Unauthenticated Peers
Pre-configuration of Security Policy Database (SPD) "bypass" entries
to enable communication with unauthenticated peers only works if the
peer IP addresses are known in advance. The lack of unauthenticated
IPsec modes often prevents secure communications at the network layer
with unauthenticated or unknown peers, even when they are
subsequently authenticated in a higher-layer protocol or application.
The lack of a channel binding API between IPsec and higher-layer
protocols may further force such communications to completely bypass
IPsec, leaving the network layer of such communications unprotected.
2.2. Higher-Layer Issues
For higher layers, the next subsection focuses on whether IPsec is
necessary if transport layer security is already in use. The use of
IPsec in the presence of transport security provides further
motivation for reducing the administrative burdens of using IPsec.
This is followed by a discussion of the implications of using
authentication at both the network layer and a higher layer for the
2.2.1. Transport Protection from Packet Spoofing
Consider the case of transport protocols. Increases in network
performance and the use of long-lived connections have resulted in
increased vulnerability of connection-oriented transport protocols to
certain forms of attacks. TCP, like many other protocols, is
susceptible to off-path third-party attacks, such as injection of a
TCP RST . The Internet lacks comprehensive ingress filtering to
discard such spoofed traffic before it can cause damage. These
attacks can affect BGP sessions between core Internet routers, and
are thus of significant concern . As a result, a number of
proposed solutions have been developed, most of which are at the
Some of these solutions augment the transport protocol by improving
its own security, e.g., TCP MD5 . Others modify the core TCP
processing rules to make it harder for off-path attackers to inject
meaningful packets either during the initial handshake (e.g., SYN
cookies) or after a connection is established (e.g., TCPsecure)
. Some of these approaches are new to TCP, but have already
been incorporated into other transport protocols (e.g., Stream
Control Transmission Protocol (SCTP) ) or intermediate (so-called
layer 3.5) protocols (e.g., Host Identity Protocol (HIP) ).
TCP MD5 and its potential successor, TCP Auth , are based on
authentication; TCP-specific modifications that lack authentication
are, at best, temporary patches to the ubiquitous vulnerability to
spoofing attacks. The obvious solution to spoofing is end-to-end
validation of the traffic, either at the transport layer or the
network layer. The IPsec suite already provides authentication of a
network-layer packet and its contents, but the costs of an
authentication infrastructure required for the use of IPsec can be
prohibitive. Similarly, TCP MD5 requires pre-shared keys, which can
likewise be prohibitive. TCP Auth is currently under development,
and may include a BTNS-like mode.
Protecting systems from spoofed packets is ultimately an issue of
authentication, ensuring that a receiver's interpretation of the
source of a packet is accurate. Authentication validates the
identity of the source of the packet. The current IPsec suite
assumes that identity is validated either by a trusted third party --
e.g., a certification authority -- or by a pre-deployed shared
secret. Such an identity is unique and invariant across associations
(pair-wise security configuration), and can be used to reject packets
that are not authentic.
With regard to BGP in particular, it has been understood that the use
of appropriate network- or transport-layer authentication is the
preferred protection from TCP spoofing attacks . Authentication
at one router by itself does not provide overall BGP security because
that router remains at the mercy of all routers it peers with, since
it depends on them to also support authentication . The reality
is that few Internet routers are configured to support authentication
at all, and the result is the use of unsecured TCP for sending BGP
packets. BTNS allows an individual router to relax the need for
authentication in order to enable the use of protected sessions that
are not authenticated. The latter is "better than nothing" in cases
where "nothing" is the alternative. Although the routing community
has chosen solutions other than BTNS for protection of BGP's TCP
connections (e.g., TCP MD5), the discussion of BGP remains in this
document because it was a motivation for the development of BTNS.
2.2.2. Authentication at Multiple Layers
Some existing protocols used on the Internet provide authentication
above the network and transport layers but rely on the IPsec suite
for packet-by-packet cryptographic integrity and confidentiality
services. Examples of such protocols include iSCSI  and the
remote direct data placement (RDDP) protocols . With the
current IPsec suite, the result is two authentication operations: one
at the IPsec layer using an identity for IKE and an associated secret
or key, and another by the higher-layer protocol using a higher-layer
identity and secret or key. With the current IPsec specifications,
this redundant authentication is necessary because the identity and
key formats differ between IPsec and the higher-layer protocol and/or
because there is no standard interface to pass authentication results
from IPsec up to the higher layer. End-node software is then
responsible for ensuring that the identities used for these two
authentication operations are consistent in some fashion; determining
whether these identities are consistent is an authorization policy
Failure of the end-node software to enforce appropriate consistency
across authentication operations at different layers creates man-in-
the-middle attack opportunities at the network layer. An attacker
may exploit this omission by interposing as a proxy; rather than
impersonate the attacked endpoints, the attacker need only
authenticate with identities that are acceptable to the attacked
endpoints. The resulting success enables the attacker to obtain full
access to the higher-layer traffic by passing the higher-layer
authentication operation through without modification. In the
complete absence of consistency checks on the identities used at
different layers, higher-layer traffic may be accessible to any
entity that can successfully authenticate at the network layer.
In principle, a single authentication operation should suffice to
protect the higher-layer traffic, removing the need for:
o the second authentication operation,
o configuration and management of the identities and secrets or keys
for the second authentication (even if the identities and secrets
or keys are the same, the two authentication operations may employ
different repositories for identities, secrets, and keys), and
o determining in some fashion that the two authenticated identities
are consistent. As noted above, there are significant potential
MITM vulnerabilities if this is not done.
IPsec may not always be present for these higher-layer protocols, and
even when present, may not always be used. Hence, if there is a
choice, the higher-layer protocol authentication is preferable as it
will always be available for use, independent of IPsec.
A "better-than-nothing" security approach to IPsec can address this
problem by setting up an IPsec security association without an
authentication, and then using an extended form of the higher-layer
authentication to establish that the higher-layer protocol session is
protected by a single IPsec SA. This counters man-in-the-middle
(MITM) attacks on BTNS IPsec session establishment by terminating the
higher-layer session via an authentication failure when such an
attack occurs. The result is that a single authentication operation
validates not only the higher-layer peer's identity but also
continuity of the security association to that peer. This higher-
layer check for a single IPsec SA is referred in this document as
"channel binding", thus the name Channel-Bound BTNS (CBB) .
3. BTNS Overview and Threat Models
This section provides an overview of BTNS and the IPsec security
services that are offered when BTNS is used. It also describes the
multiple operating modes of BTNS.
3.1. BTNS Overview
This is an overview of what is needed in IPsec to enable BTNS. The
detailed specifications of the extensions are addressed by the
relevant protocol specifications.
The main update to IPsec is adding extensions to security policy that
permit secure communications with unauthenticated peers. These
extensions are necessary for both IPsec and IKE. For IPsec, the
first extension applies to the PAD, which specifies the forms of
authentication allowed for each IKE peer. In addition to existing
forms of authentication, such as X.509 certificates and pre-shared
secrets, the extension adds an unauthenticated category in which the
public key presented by the peer serves as its identity (and is
authenticated by the peer demonstrating knowledge of the
corresponding private key) . The second extension is that a flag
is added to each SPD entry to indicate whether BTNS lack of
authentication is acceptable for that SPD entry.
The changes to enable channel binding between IPsec and higher-layer
protocols or applications are more complex than the policy extensions
above. They require specifying APIs and interactions between IPsec
and higher-layer protocols. This document assumes such provisions
will be developed, but does not address their details.
3.2. BTNS and IPsec Security Services
The changes and extensions of BTNS primarily affect IPsec policy as
described above. Other parts of IPsec and IKE specifications are
unchanged. BTNS does not require a separate IPsec implementation, as
BTNS can be integrated with any IPsec implementation in a system.
The scope of BTNS functionality applies only to the SAs matching the
policies that explicitly specify or enable BTNS modes in the PAD and
for which the corresponding SPD entries allow BTNS. All other non-
BTNS policy entries, including entries in the SPD and the PAD, and
non-BTNS SAs are not affected by BTNS.
In principle, the result of removing the requirement that all SAs be
authenticated is that BTNS can establish secure IPsec connections in
a fashion similar to fully authenticated IKE, but BTNS cannot verify
or authenticate the peer identities of these SAs. The following is a
list of security services offered by the IPsec protocol suite with
notes that address the differences created by the addition of BTNS.
1. Access Control
BTNS extends IPsec's access control services to allow
unauthenticated connections. These extensions are integrated with
the IPsec PAD and SPD in a fashion that does not affect the access
controls associated with entries that do not use the BTNS
extensions. For Channel-Bound BTNS, the authentication that
applies to the SA is performed at a higher layer in a fashion that
links higher-layer access control policy to IPsec's network-layer
access control mechanisms.
2. Data Origin Authentication
Stand-Alone BTNS weakens data origin authentication to continuity
of association, namely the assurance that traffic on an SA
continues to originate from the same unauthenticated source.
Channel-Bound BTNS relies on higher-layer authentication to
provide data origin authentication of protected network traffic.
3. Connectionless Integrity
4. Anti-Replay Protection
6. (Limited) Traffic Flow Confidentiality
For the security services offered by IPsec that are listed in
items 3 through 6, it is possible to establish secure IPsec
connections with rogue peers via BTNS because authentication is
not required. On the other hand, once a secure connection is
established, the communication is protected by these security
services in the same fashion as a connection established by
conventional IPsec means.
3.3. BTNS and IPsec Modes
The previous sections have described two ways of using BTNS: Stand-
Alone (SAB) and Channel-Bound (CBB). Both of these can also be used
either symmetrically, where neither party authenticates at the
network layer, or asymmetrically, where only one party does not
authenticate at the network layer. There are a number of cases to
consider, based on combinations of the endpoint security capabilities
of SAB, CBB, and conventional IKE authentication of an identity
(denoted as AUTH below). The following tables show all of the
combinations based on the capabilities of the two security endpoints:
| AUTH | SAB | | CB-AUTH | CBB |
| | | | | |
AUTH | AUTH | A-SAB | CB-AUTH| CB-AUTH | A-CBB |
| | | | | |
| | | | | |
SAB | A-SAB | S-SAB | CBB | A-CBB | S-CBB |
| | | | | |
No Channel Binding With Channel Binding
There are six operating modes that result from the combinations. The
first three modes consist of network-layer authentication schemes
used without channel binding to higher-layer authentication:
1. AUTH: both parties provide and authenticate conventional, IKE-
2. Symmetric SAB (S-SAB): neither party authenticates with a
conventional, IKE-supported identity.
3. Asymmetric SAB (A-SAB): one party does not authenticate with a
conventional, IKE-supported identity, but the other side does
authenticate with such an identity.
The following three modes combine the network-layer behaviors with
channel binding to higher-layer authentication credentials:
4. CB-AUTH: channel binding is used and both parties authenticate
with conventional, IKE-supported identities.
5. Symmetric CBB (S-CBB): neither party authenticates with a
conventional, IKE-supported identity, but channel binding is used
to bind the SAs to higher-layer authentication operations.
6. Asymmetric CBB (A-CBB): asymmetric SAB (A-SAB) used with channel
binding; at the network layer, one party does not authenticate
with a conventional, IKE-supported identity, but the other party
does authenticate with such an identity. Channel binding is used
to bind the SA to higher-layer authentication operations.
There are three security mechanisms involved in BTNS with channel
1. BTNS and IPsec at the network layer,
2. higher-layer authentication, and
3. the connection latching plus channel binding mechanisms that bind
the higher-layer authentication credentials with the secure IPsec
Authentication at both the network and higher layers can be either
bidirectional (both peers are authenticated) or unidirectional (one
of the two peers does not authenticate). In contrast, when channel
binding is used, it must be applied at both ends of the communication
to prevent MITM attacks. Existing channel binding mechanisms and
APIs for this purpose (e.g., as defined in GSS-API ) mandate the
exchange and verification of the channel binding values at both ends
to ensure that correct, non-spoofed channel characteristics are bound
to the higher-layer authentication.
Note: When any Stand-Alone BTNS (SAB) or Channel-Bound BTNS (CBB) is
used without being qualified as symmetric or asymmetric, the
symmetric mode is the intended default meaning.
4. Applicability Statement
BTNS is intended for services open to the public but for which
protected associations are desired, and for services that can be
authenticated at higher layers in the protocol stack. BTNS can also
provide some level of protection for private services when the
alternative BTNS is no protection at all.
BTNS uses the IPsec protocol suite, and therefore should not be used
in situations where IPsec and specifically IKE are unsuitable. IPsec
and IKE incur additional computation overhead, and IKE further
requires message exchanges that incur round-trip latency to setup
security associations. These may be undesirable in environments with
limited computational resources and/or high communication latencies.
This section provides an overview of the types of applications
suitable for various modes of BTNS. The next two sections describe
the overall benefits and vulnerabilities, followed by the
applicability analysis for each BTNS mode. The applicability
statement covers only the four BTNS-specific modes; the AUTH and
CB-AUTH modes are out of scope for this discussion.
BTNS protects security associations after they are established by
reducing vulnerability to attacks from parties that are not
participants in the association. BTNS-based SAs protect network and
transport layers without requiring network-layer authentication.
BTNS can be deployed without pre-deployment of authentication
material for IPsec or pre-shared information and can protect all
transport layer protocols using a common mechanism.
BTNS also helps protect systems from low-effort attacks on higher-
layer sessions or connections that disrupt valuable services or
resources. BTNS raises the level of effort for many types of
network- and transport-layer attacks. Simple transport layer packet
attacks are rejected because the malicious packet or packets are not
part of an IPsec SA. The attacker is instead forced to establish an
unauthenticated IPsec SA and a transport connection for SAB,
requiring the attacker to perform as much work as a host engaging in
the higher-layer communication. SAB thus raises the effort for a
DDoS (Distributed Denial of Service) attack to that of emulating a
flash crowd. For open services, there may be no way to distinguish
such a DDoS attack from an actual flash crowd.
BTNS also allows individual security associations to be established
for protection of higher-layer traffic without requiring pre-deployed
BTNS removes the requirement that every IPsec SA be authenticated.
Hosts connecting to BTNS hosts are vulnerable to communicating with a
masquerader throughout the association for SAB, or until higher
layers provide additional authentication for CBB. As a result,
authentication data (e.g., passwords) sent to a masquerading peer
could be disclosed to an attacker. This is a deliberate design
tradeoff; in BTNS, network- and transport-layer access is no longer
controlled by the identity presented by the other host, opening hosts
to potential masquerading and flash crowd attacks. Conversely, BTNS
can secure connections to hosts that are unable to authenticate at
the network layer, so the network and transport layers are more
protected than can be achieved via higher-layer authentication alone.
Lacking network-layer authentication information, other means must be
used to provide access control for local resources. Traffic
selectors for the BTNS SPD entries can be used to limit which
interfaces, address ranges, and port ranges can access BTNS-enabled
services. Rate limiting can further restrict resource usage. For
SAB, these protections need to be considered throughout associations,
whereas for CBB they need be present only until higher-layer
protocols provide the missing authentication. CBB also relies on the
effectiveness of the binding of higher-layer authentication to the
BTNS network association.
4.3. Stand-Alone BTNS (SAB)
SAB is intended for applications that are unable to use IKE-
compatible authentication credentials and do not employ higher-layer
authentication or other security protection. SAB is also suitable
when the identities of either party are not important or are
deliberately omitted, but IPsec security services are desired (see
Section 3.2). SAB is particularly applicable to long-lived
connections or sessions for which assurance that the entity at the
other end of the connection has not changed may be a good enough
substitute for the lack of authentication. This section discusses
symmetric and asymmetric SAB.
4.3.1. Symmetric SAB
Symmetric SAB (S-SAB) is applicable when both parties lack network-
layer authentication information and that authentication is not
available from higher-layer protocols. S-SAB can still provide some
forms of protection for network and transport protocols, but does not
provide authentication beyond continuity of association. S-SAB is
useful in situations where transfer of large files or use of other
long-lived connections would benefit from not being interrupted by
attacks on the transport connection (e.g., via a false TCP RST), but
the particular endpoint identities are not important.
Open services, such as web servers, and peer-to-peer networks could
utilize S-SAB when their identities need not be authenticated but
their communication would benefit from protection. Such services
might provide files that are either not validated or validated by
other means (e.g., published hashes). These transmissions present a
target for off-path attacks that could be mitigated by S-SAB. S-SAB
may also be useful for protecting voice-over-IP (VoIP) traffic
between peers, such as direct calls between VoIP clients.
S-SAB is also useful in protecting any transport protocol when the
endpoints do not deploy authentication, for whatever reason. This is
the case for BGP TCP connections between core routers, where the
protection afforded by S-SAB is better than no protection at all,
even though BGP is not intended as an open service.
S-SAB can also serve as an intermediate step towards S-CBB. S-SAB is
the effective result when an IPsec channel is used (via connection
latching), but the higher-layer authentication is not bound to the
IPsec SAs within the channel.
4.3.2. Asymmetric SAB
Asymmetric SAB (A-SAB) allows one party lacking network-layer
authentication information to establish associations with another
party that possesses authentication credentials for any applicable
IKE authentication mechanism.
Asymmetric SAB is useful for protecting transport connections for
open services on the Internet, e.g., commercial web servers, etc. In
these cases, the server is typically authenticated by a widely known
CA, as is done with TLS at the application layer, but the clients
need not be authenticated . Although this may result in IPsec and
TLS being used on the same connection, this duplication of security
services at different layers is necessary when protection is required
from the sorts of spoofing attacks described in Section 2 (e.g., TLS
cannot prevent a spoofed TCP RST, as the RST is processed by TCP
rather than being passed to TLS).
A-SAB can also secure transport for streaming media such as would be
used by webcasts for remote education and entertainment.
4.4. Channel-Bound BTNS (CBB)
CBB allows hosts without network-layer authentication information to
cryptographically bind BTNS-based IPsec SAs to authentication at
higher layers. CBB is intended for applications that employ higher-
layer authentication but that also benefit from additional network-
layer security. CBB provides network-layer security services without
requiring authentication at the network layer. This enables IPsec
security services for applications that have IKE-incompatible
authentication credentials. CBB allows IPsec to be used with
authentication mechanisms not supported by IKE and frees higher-layer
applications and protocols from duplicating security services already
available in IPsec.
Symmetric CBB integrates channel binding with S-SAB, as does
asymmetric CBB with A-SAB. In both cases, the target applications
have similar characteristics at the network layer to their non-
channel-binding counterparts. The only significant difference is the
binding of authentication credentials at a higher layer to the
resulting IPsec channels.
Although the modes of CBB refer to the authentication at the network
layer, higher-layer authentication can also be either asymmetric
(one-way) or symmetric (two-way). Asymmetric CBB can be used to
complement one-way authentication at a higher layer by providing one-
way authentication of the opposite direction at the network layer.
Consider an application with one-way, client-only authentication.
The client can utilize A-CBB where the server must present IKE-
authenticated credentials at the network layer. This form of A-CBB
achieves mutual authentication, albeit at separate layers. Many
remote file system protocols, such as iSCSI and NFS, fit into this
category and can benefit from channel binding with IPsec for better
network-layer protection, including prevention of MITM attacks.
Mechanisms and interfaces for BTNS channel binding with IPsec are
discussed in further detail in .
4.5. Summary of Uses, Vulnerabilities, and Benefits
The following is a summary of the properties of each type of BTNS,
based on the previous subsections:
Uses Open services Same as SAB but with
Peer-to-peer higher-layer auth.,
Zero-config Infrastructure e.g., iSCSI , NFSv4 
Vuln. Masqueraders Masqueraders until bound
Needs data rate limit Needs data rate limit
Load on IPsec Load on IPsec
Exposure to open access
Benefit Protects L3 & L4 Protects L3 & L4
Avoids all auth. keys Avoids L3 auth. keys
Full auth. once bound
Most of the potential vulnerabilities in the above table have been
discussed in previous sections of this document; some of the more
general issues, such as the increased load on IPsec processing, are
addressed in the Security Considerations section of this document.
5. Security Considerations
This section describes the threat models for BTNS and discusses other
security issues based on the threat models for different modes of
BTNS. Some of the issues were mentioned previously in the document
but are listed again for completeness.
5.1. Threat Models and Evaluation
BTNS is intended to protect sessions from a variety of threats,
including on-path, man-in-the-middle attacks after key exchange, and
off-path attacks. It is intended to protect the contents of a
session once established, but does not protect session establishment
itself. This protection has value because it forces the attacker to
target connection establishment as opposed to waiting for a more
convenient time; this is of particular value for long-lived sessions.
BTNS is not intended to protect the key exchange itself, so this
presents an opportunity for a man-in-the-middle attack or a well-
timed attack from other sources. Furthermore, Stand-Alone BTNS is
not intended to protect the endpoint from nodes masquerading as
legitimate clients of a higher-layer protocol or service. Channel-
Bound BTNS can protect from such masquerading, though at a later
point after the security association is established, as a masquerade
attack causes a client authentication failure at a higher layer.
BTNS is also not intended to protect from DoS (Denial of Service)
attacks that seek to overload a CPU performing authentication or
other security computations, nor is BTNS intended to provide
protection from configuration mistakes. These latter two threat
assumptions are also the case for IPsec.
The following sections discuss the implications of the threat models
in more details.
5.2. Interaction with Other Security Services
As with any aspect of network security, the use of BTNS must not
interfere with other security services. Within IPsec, the scope of
BTNS is limited to the SPD and PAD entries that explicitly specify
BTNS and to the resulting SAD entries. It is incumbent on system
administrators to deploy BTNS only where safe, preferably as an
alternative to the use of "bypass" SPD entries that exempt specified
traffic from IPsec cryptographic protection. In other words, BTNS
should be used only as a substitute for no security, rather than as a
substitute for stronger security. When the higher-layer
authentication required for CBB is not available, other methods, such
as IP address filtering, can help reduce the vulnerability of SAB to
exposure to anonymous access.
5.3. MITM and Masquerader Attacks
Previous sections have described how CBB can counter MITM and
masquerader attacks, even though BTNS does not protect key exchange
and does not authenticate peer identities at the network layer.
Nonetheless, there are some security issues regarding CBB that must
be carefully evaluated before deploying BTNS.
For regular IPsec/IKE, a man in the middle cannot subvert IKE
authentication, and hence an attempt to attack an IPsec SA via use of
two SAs concatenated by the attacker acting as a traffic-forwarding
proxy will cause an IKE authentication failure. On the other hand, a
man-in-the-middle attack on IPsec with CBB is discovered later. With
CBB, the IKE protocol will succeed because it is unauthenticated, and
the security associations will be set up. The man in the middle will
not be discovered until the higher-layer authentication fails. There
are two security concerns with this approach: possible exposure of
sensitive authentication information to the attackers, and resource
consumption before attacks are detected.
The exposure of information depends on the higher-layer
authentication protocols used in applications. If the higher-layer
authentication requires exchange of sensitive information (e.g.,
passwords or password-derived materials) that are directly useful or
can be attacked offline, an attacker can gain such information even
though the attack can be detected. Therefore, CBB must not be used
with higher-layer protocols that may expose sensitive information
during authentication exchange. For example, Kerberos V AP exchanges
would leak little other than the target's krb5 principal name, while
Kerberos V AS exchanges using PA-ENC-TIMESTAMP pre-authentication
would leak material that can then be attacked offline. The latter
should not be used with BTNS, even with Channel Binding. Further,
the ways in which BTNS is integrated with the higher-layer protocol
must take into consideration vulnerabilities that could be introduced
in the APIs between these two systems or in the information that they
The resource consumption issue is addressed in the next section on
5.4. Denial of Service (DoS) Attacks and Resource Consumptions
A consequence of BTNS deployment is that more traffic requires
cryptographic operations; these operations increase the computation
required in IPsec implementations that receive protected traffic
and/or verify incoming traffic. That additional computation raises
vulnerability to overloading, which may be the result of legitimate
flash crowds or a DoS or DDoS attack. Although this may itself
present a substantial impediment to deployment, it is an issue for
all cryptographically protected communication systems. This document
does not address the impact BTNS has on such increases in required
The effects of the increased resource consumption are twofold. The
consumption raises the level of effort for attacks such as MITM, but
also consumes more resources to detect such attacks and to reject
spoofed traffic. At the network layer, proper limits and access
controls for resources should be set up for all BTNS SAs. CBB SAs
may be granted increased resource access after the higher-layer
authentications succeed. The same principles apply to the higher-
layer protocols that use CBB SAs. Special care must be taken to
avoid excessive resource usage before authentication is established
in these applications.
5.5. Exposure to Anonymous Access
The use of SAB by a service implies that the service is being offered
for open access, since network-layer authentication is not performed.
SAB should not be used with services that are not intended to be
5.6. ICMP Attacks
This document does not consider ICMP attacks because the use of BTNS
does not change the existing IPsec guidelines on ICMP traffic
handling . BTNS focuses on the authentication part of
establishing security associations. BTNS does not alter the IPsec
traffic processing model and protection boundary. As a result, the
entire IPsec packet processing guidelines, including ICMP processing,
remain applicable when BTNS is added to IPsec.
5.7. Leap of Faith
BTNS allows systems to accept and establish security associations
with peers without authenticating their identities. This can enable
functionality similar to "Leap of Faith" authentication utilized in
other security protocols and applications such as the Secure Shell
Protocol (SSH) .
SSH implementations are allowed to accept unknown peer credentials
(host public keys) without authentication, and these unauthenticated
credentials may be cached in local databases for future
authentication of the same peers. Similar to BTNS, such measures are
allowed due to the lack of "widely deployed key infrastructure" 
and to improve ease of use and end-user acceptance.
There are subtle differences between SSH and BTNS regarding Leap of
Faith, as shown in the following table:
| SSH | BTNS |
Accept unauthenticated | Allowed | Allowed |
credentials | | |
Options/Warnings to reject | Yes | No |
unauthenticated credentials | | |
Cache unauthenticated |Required | Allowed |
credential for future refs | | |
SSH requires proper warnings and options in applications to reject
unauthenticated credentials, while BTNS accepts such credentials
automatically when they match the corresponding policy entries. Once
SSH accepts a credential for the first time, that credential should
be cached and can be reused automatically without further warnings.
BTNS credentials can be cached for future use, but there is no
security advantage to doing so, as a new unauthenticated credential
that is allowed by the policy entries will be automatically accepted.
In addition, BTNS does not require IPsec to reuse credentials in a
manner similar to SSH. When IPsec does reuse unauthenticated
credentials, there may be implementation advantages to caching them.
SSH-style credential caching for reuse with SAB could be addressed by
future extension(s) to BTNS; such extension(s) would need to provide
warnings about unauthenticated credentials and a mechanism for user
acceptance or rejection of them in order to establish a level of
authentication assurance comparable to SSH's "Leap of Faith". Such
extension(s) would also need to deal with issues caused by the
absence of identities in BTNS. At best, a cached BTNS credential
reauthenticates the network-layer source of traffic when the
credential is reused -- in contrast, SSH credential reuse
reauthenticates an identity.
Network-layer reauthentication for SAB is further complicated by:
o the ability of NATs to cause multiple independent network-layer
sources of traffic to appear to be one source (potentially
requiring acceptance and caching of multiple BTNS credentials),
o the ability of multihoming to cause one network-layer source of
traffic to appear to be multiple sources (potentially triggering
unexpected warnings and requiring re-acceptance of the same BTNS
o interactions with both mobility and address ownership changes
(potentially requiring controlled BTNS credential reassignment
These issues are left to be addressed by possible future work on the
addition of "Leap of Faith" functionality to BTNS.
In contrast, for CBB, credential caching and verification are usually
done at the higher-layer protocols or applications. Caching
credentials for CBB at the BTNS level is not as important because the
channel binding will bind whatever credentials are presented (new or
cached) to the higher-layer protocol identity.
5.8. Connection Hijacking through Rekeying
Each IPsec SA has a limited lifetime (defined as a time and/or byte
count) and must be rekeyed or terminated when the lifetime expires.
Rekeying an SA provides a small window of opportunity where an on-
path attacker can step in and hijack the new SA created by rekeying
by spoofing the victim during rekeying. BTNS, and particularly SAB,
simplify this attack by removing the need for the attacker to
authenticate as the victim or via the same non-BTNS PAD entry that
was used by the victim for the original SA. CBB, on the other hand,
can detect such attacks by detecting the changes in the secure
This vulnerability is caused by the lack of inter-session binding or
latching of IKE SAs with the corresponding credentials of the two
peers. Connection latching, together with channel binding, enables
such binding but requires higher-layer protocols or applications to
verify consistency of identities and authentication across the two
5.9. Configuration Errors
BTNS does not address errors of configuration that could result in
increased vulnerability; such vulnerability is already possible using
"bypass" SPD entries. SPD entries that allow BTNS must be explicitly
flagged, and hence can be kept separate from SPD entries that do not
allow BTNS, just as "bypass" SPD entries are separate from entries
that create SAs with more conventional, stronger security.
6. Related Efforts
There have been a number of related efforts in the IETF and elsewhere
to reduce the configuration effort of deploying the Internet security
The IETF PKI4IPsec effort focused on providing an automatic
infrastructure for the configuration of Internet security services,
e.g., to assist in deploying signed certificates and CA information
. The IETF KINK effort focused on adapting Kerberos  for IKE,
enabling IKE to utilize the Kerberos key distribution infrastructure
rather than requiring certificates or shared private keys . KINK
takes advantage of an existing architecture for automatic key
management in Kerberos. Opportunistic Encryption (OE) is a system
for automatic discovery of hosts willing to do a BTNS-like
encryption, with authentication being exchanged by leveraging
existing use of the DNS . BTNS differs from all three in that
BTNS is intended to avoid the need for such infrastructure
altogether, rather than to automate it.
This document was inspired by discussions on the IETF TCPM WG about
the spoofed RST attacks on BGP routers and various solutions, as well
as discussions in the NFSv4 and IPS WGs about how to better integrate
with IPsec. The concept of BTNS was the result of these discussions
as well as discussions with USC/ISI's T. Faber, A. Falk, and B. Tung,
and discussions on the IETF SAAG (Security Area open meeting) mailing
list and IPsec mailing list. The authors would like to thank the
members of those WGs and lists, as well as the IETF BTNS BOFs and WG
and its associated ANONsec mailing list
(http://www.postel.org/anonsec) for their feedback -- in particular,
Steve Kent, Sam Hartman, Nicolas Williams, and Pekka Savola.
This document was prepared using 2-Word-v2.0.template.dot.
8. Informative References
 Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
 Aboba, B., Tseng, J., Walker, J., Rangan, V., and F.
Travostino, "Securing Block Storage Protocols over IP", RFC
3723, April 2004.
 CERT Vulnerability Note VU#415294, "The Border Gateway Protocol
relies on persistent TCP sessions without specifying
authentication requirements", 4/20/2004.
 Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
Protocol Version 1.2", RFC 5246, August 2008.
 Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
 Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
 Kaufman, C., Ed., "Internet Key Exchange (IKEv2) Protocol", RFC
4306, December 2005.
 Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", RFC 4301, December 2005.
 Korver, B., "The Internet IP Security PKI Profile of
IKEv1/ISAKMP, IKEv2, and PKIX", RFC 4945, August 2007.
 Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
 Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple Authentication
and Security Layer (SASL)", RFC 4422, June 2006.
 Murphy, S., "BGP Security Vulnerabilities Analysis", RFC 4272,
 Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos
Network Authentication Service (V5)", RFC 4120, July 2005.
 Moskowitz, R., Nikander, P., Jokela, P., Ed., and T. Henderson,
"Host Identity Protocol", RFC 5201, April 2008.
 Ramaiah, A., R Stewart, M. Dalal, "Improving TCP's Robustness
to Blind In-Window Attacks", Work in Progress, January 2008.
 Recio, R., Metzler, B., Culley, P., Hilland, J., and D. Garcia,
"A Remote Direct Memory Access Protocol Specification", RFC
5040, October 2007.
 Richardson, M. and D. Redelmeier, "Opportunistic Encryption
using the Internet Key Exchange (IKE)", RFC 4322, December
 Sakane, S., Kamada, K., Thomas, M., and J. Vilhuber,
"Kerberized Internet Negotiation of Keys (KINK)", RFC 4430,
 Satran, J., Meth, K., Sapuntzakis, C., Chadalapaka, M., and E.
Zeidner, "Internet Small Computer Systems Interface (iSCSI)",
RFC 3720, April 2004.
 Shah, H., Pinkerton, J., Recio, R., and P. Culley, "Direct Data
Placement over Reliable Transports", RFC 5041, October 2007.
 Shepler, S., Callaghan, B., Robinson, D., Thurlow, R., Beame,
C., Eisler, M., and D. Noveck, "Network File System (NFS)
version 4 Protocol", RFC 3530, April 2003.
 Stewart, R., Ed., "Stream Control Transmission Protocol", RFC
4960, September 2007.
 TCP SYN-cookies, http://cr.yp.to/syncookies.html Touch, J., "Defending TCP Against Spoofing Attacks", RFC 4953,
 Touch, J., A. Mankin, R. Bonica, "The TCP Authentication
Option", Work in Progress, November 2007.
 Williams, N., "IPsec Channels: Connection Latching", Work in
Progress, April 2008.
 Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
 Williams, N. and M. Richardson, "Better-Than-Nothing Security:
An Unauthenticated Mode of IPsec", RFC 5386, November 2008.
 Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
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