Optimizing BFD
AuthenticationKloud ServicesUSAmjethanandani@gmail.comSES Networksmishra.ashesh@gmail.comCiena Corporation3939 N 1st StreetSan JoseCA95134USAankurpsaxena@gmail.comNokiaBangaloreIndiamanav.bhatia@nokia.comBFDauthenticationThis document describes an optimization to BFD Authentication as
described in Section 6.7 of BFD RFC 5880. This document updates RFC
5880.Authenticating every BFD control packet
with a Simple Password, or with a MD5
Message-Digest Algorithm , or Secure Hash Algorithm (SHA-1)
algorithms is a computationally intensive process. This makes it
difficult, if not impossible to authenticate every packet - particularly
at faster rates. Also, the recent escalating series of attacks on MD5
and SHA-1 described in Finding Collisions
in the Full SHA-1 and New Collision
Search for SHA-1 raise concerns about their remaining useful
lifetime as outlined in Updated Security
Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithm
and Security Considerations for the SHA-0
and SHA-1 Message-Digest Algorithm . If replaced by stronger
algorithms, the computational overhead, will make the task of
authenticating every packet even more difficult to achieve.This document proposes that only BFD control packets that signal a
state change, a demand mode change (to D bit) or a poll sequence change
(P or F bit change) in a BFD control packet be categorized as a
significant change. This document also proposes that all BFD control
packets which signal a significant change MUST be authenticated if the
session's bfd.AuthType is non-zero. Other BFD control packets MAY be
transmitted and received without the A bit set.Most packets that are transmitted and received have no state change
associated with them. Limiting authentication to packets that affect a
BFD session state allows more sessions to be supported with this
optimized method of authentication. Moreover, most BFD control packets
that signal a significant change are generally transmitted at a slower
interval of 1s, leaving enough time to compute the hash.To detect a Man In the Middle (MITM) attack, it is also proposed that
a BFD control packet without a significant change be authenticated
occasionally. The interval of the BFD control packets without a
significant change can be configured depending on the detect multiplier
and the capability of the system. As an example, this could be equal to
the detect multiplier number of packets.The rest of the document is structured as follows. Section 2 talks
about the changes to authentication mode as described in BFD. Section 3 goes into the details of the new
Authentication Type.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 BCP 14 when, and only
when, they appear in all capitals, as shown here.The following terms used in this document have been defined in
BFD.Detect MultiplierDetection TimeThe following terms are introduced in this document.TermMeaningsignificant changeState change, a demand model change (to D bit) or a poll sequence
change (P or F bit).configured intervalInterval at which BFD control packets are authenticated in the UP
state.The cryptographic authentication mechanisms specified in BFD describes enabling and disabling of
authentication as a one time operation. As a security precaution, it
mentions that authentication state be allowed to change at most once.
Once enabled, every packet must have Authentication Bit set and the
associated Authentication Type appended. In addition, it states that an
implementation SHOULD NOT allow the authentication state to be changed
based on the receipt of a BFD control packet.This document proposes that the authentication mode be modified to be
enabled on demand. Instead of authenticating every packet, BFD peers are
configured for which packets need to be authenticated, and authenticate
only those packets. Rest of the packets can be transmitted and received
without authentication. For example, the two ends can be configured such
that BFD control packets that indicate a significant change should be
authenticated and enable authentication on those packets only. If the
two ends have previously been configured as such, but at least one side
decides not to authenticate a significant change packet, then the BFD
session will fail to come up.This proposal outlines which BFD control packets need to be
authenticated (carry the A-bit), and which packets can be transmitted or
received without authentication enabled. A BFD control packet that fails
authentication is discarded, or a BFD control packet that was supposed
to be authenticated, but was not, e.g. a significant change packet, is
discarded. However, there is no change to the state machine for BFD, as
the decision of a significant change is still decided by how many valid
consecutive packets were received, authenticated or otherwise.The following table summarizes when the A bit should be set. The
table should be read with the column indicating the BFD state the
receiver is currently in, and the row indicating the BFD state the
receiver might transition to based on the BFD control packet received.
The interesection of the two indicates whether the received BFD control
packet should have the A bit set (Auth), no authentication is needed
(NULL), most packets are NULL AUTH (Select) or the state transition is
not applicable. The BFD state refers to the states in BFD state machine
described in Section 6.2 of BFD.If P or F bit changes value, the BFD control packet MUST be
authenticated. If the D bit changes value, the BFD control packet MUST
be authenticated.All packets already carry the sequence number. The NULL AUTH packets
MUST contain the Type specified in Section 3. This enables a
monotonically increasing sequence number to be carried in each packet,
and prevents man-in-the-middle from capturing and replaying the same
packet again. Since all packets still carry a sequence number, the logic
for sequence number maintenance remains unchanged from BFD . If at a later time, a different scheme is
adopted for changing sequence number, e.g. Secure BFD Sequence
Numbers, this method can use the updated scheme without any
impact.Most packets transmitted on a BFD session are BFD UP packets.
Authenticating a small subset of these packets, for example, a detect
multiplier number of packets per configured interval, significantly
reduces the computational demand for the system while maintaining
security of the session across the configured interval. A minimum of
Detect Multiplier packets MUST be transmitted per configured interval.
This ensures that the BFD session should see at least one authenticated
packet during that interval.This section describes a new Authentication Type as: where:Auth Type: The Authentication Type, which in this case is TBD (NULL,
to be assigned by IANA)Auth Len: The length of the NULL Auth Type, in bytes i.e. 8 bytesAuth Key ID: The authentication key ID in use for this packet. Must
be set to zero.Reserved: This byte MUST be set to zero on transmit and ignored on
receive.Sequence Number: The sequence number for this packet. Implementation
may use sequence numbers (bfd.XmitAuthSeq) as defined in BFD, or secure sequence numbers as defined in
Secure BFD Sequence
Numbers .The NULL Auth Type must be used for all packets that are not
authenticated. This protects against replay-attacks by allowing the
session to maintain an incrementing sequence number for all packets
(authenticated and un-authenticated).In the future, if a new scheme is adopted for changing the sequence
number, this method can adopt the new scheme without any impact.This document requests an update to the registry titled "BFD
Authentication Types". IANA is requested to assign a new BFD Auth Type
for "NULL" (see Section 3).Note to RFC Editor: this section may be removed on publication as an
RFC.The approach described in this document enhances the ability to
authenticate a BFD session by taking away the onerous requirement that
every BFD control packet be authenticated. By authenticating packets
that affect the state of the session, the security of the BFD session is
maintained. In this mode, packets that are a significant change but are
not authenticated, are dropped by the system. Therefore, a malicious
user that tries to inject a non-authenticated packet, e.g. with a Down
state to take a session down will fail. That combined with the proposal
of using sequence number defined in Secure BFD Sequence
Numbers further enhances the security of BFD sessions.Finding Collisions in the Full SHA-1New Collision Search for SHA-1