Internet DRAFT - draft-ovsienko-babel-hmac-authentication
draft-ovsienko-babel-hmac-authentication
Network Working Group D. Ovsienko
Internet-Draft Yandex
Updates: 6126 (if approved) April 18, 2014
Intended status: Experimental
Expires: October 20, 2014
Babel HMAC Cryptographic Authentication
draft-ovsienko-babel-hmac-authentication-09
Abstract
This document describes a cryptographic authentication mechanism for
Babel routing protocol, updating, but not superseding RFC 6126. The
mechanism allocates two new TLV types for the authentication data,
uses HMAC and is both optional and backward compatible.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on October 20, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 6
2. Cryptographic Aspects . . . . . . . . . . . . . . . . . . . . 6
2.1. Mandatory-to-Implement and Optional Hash Algorithms . . . 6
2.2. Definition of Padding . . . . . . . . . . . . . . . . . . 7
2.3. Cryptographic Sequence Number Specifics . . . . . . . . . 9
2.4. Definition of HMAC . . . . . . . . . . . . . . . . . . . . 9
3. Updates to Protocol Data Structures . . . . . . . . . . . . . 11
3.1. RxAuthRequired . . . . . . . . . . . . . . . . . . . . . . 11
3.2. LocalTS . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3. LocalPC . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.4. MaxDigestsIn . . . . . . . . . . . . . . . . . . . . . . . 12
3.5. MaxDigestsOut . . . . . . . . . . . . . . . . . . . . . . 12
3.6. ANM Table . . . . . . . . . . . . . . . . . . . . . . . . 13
3.7. ANM Timeout . . . . . . . . . . . . . . . . . . . . . . . 14
3.8. Configured Security Associations . . . . . . . . . . . . . 15
3.9. Effective Security Associations . . . . . . . . . . . . . 16
4. Updates to Protocol Encoding . . . . . . . . . . . . . . . . . 17
4.1. Justification . . . . . . . . . . . . . . . . . . . . . . 17
4.2. TS/PC TLV . . . . . . . . . . . . . . . . . . . . . . . . 19
4.3. HMAC TLV . . . . . . . . . . . . . . . . . . . . . . . . . 20
5. Updates to Protocol Operation . . . . . . . . . . . . . . . . 21
5.1. Per-Interface TS/PC Number Updates . . . . . . . . . . . . 21
5.2. Deriving ESAs from CSAs . . . . . . . . . . . . . . . . . 23
5.3. Updates to Packet Sending . . . . . . . . . . . . . . . . 25
5.4. Updates to Packet Receiving . . . . . . . . . . . . . . . 27
5.5. Authentication-Specific Statistics Maintenance . . . . . . 29
6. Implementation Notes . . . . . . . . . . . . . . . . . . . . . 30
6.1. Source Address Selection for Sending . . . . . . . . . . . 30
6.2. Output Buffer Management . . . . . . . . . . . . . . . . . 31
6.3. Optimizations of ESAs Deriving . . . . . . . . . . . . . . 32
6.4. Security Associations Duplication . . . . . . . . . . . . 32
7. Network Management Aspects . . . . . . . . . . . . . . . . . . 34
7.1. Backward Compatibility . . . . . . . . . . . . . . . . . . 34
7.2. Multi-Domain Authentication . . . . . . . . . . . . . . . 34
7.3. Migration to and from Authenticated Exchange . . . . . . . 35
7.4. Handling of Authentication Keys Exhaustion . . . . . . . . 36
8. Implementation Status . . . . . . . . . . . . . . . . . . . . 37
9. Security Considerations . . . . . . . . . . . . . . . . . . . 39
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 43
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 43
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 44
12.1. Normative References . . . . . . . . . . . . . . . . . . . 44
12.2. Informative References . . . . . . . . . . . . . . . . . . 45
Appendix A. Figures and Tables . . . . . . . . . . . . . . . . . 48
Appendix B. Test Vectors . . . . . . . . . . . . . . . . . . . . 52
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Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 55
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1. Introduction
[RFC Editor: before publication please remove the sentence below.]
Comments are solicited and should be addressed to the author.
Authentication of routing protocol exchanges is a common mean of
securing computer networks. Use of protocol authentication
mechanisms helps in ascertaining that only the intended routers
participate in routing information exchange, and that the exchanged
routing information is not modified by a third party.
[BABEL] ("the original specification") defines data structures,
encoding, and the operation of a basic Babel routing protocol
instance ("instance of the original protocol"). This document ("this
specification") defines data structures, encoding, and the operation
of an extension to the Babel protocol, an authentication mechanism
("this mechanism"). Both the instance of the original protocol and
this mechanism are mostly self-contained and interact only at
coupling points defined in this specification.
A major design goal of this mechanism is transparency to operators
that is not affected by implementation and configuration specifics.
A complying implementation makes all meaningful details of
authentication-specific processing clear to the operator, even when
some of the operational parameters cannot be changed.
The currently established (see [RIP2-AUTH], [OSPF2-AUTH],
[OSPF3-AUTH], [ISIS-AUTH-A], and [RFC6039]) approach to
authentication mechanism design for datagram-based routing protocols
such as Babel relies on two principal data items embedded into
protocol packets, typically as two integral parts of a single data
structure:
o A fixed-length unsigned integer, typically called a cryptographic
sequence number, used in replay attack protection.
o A variable-length sequence of octets, a result of the HMAC
construct (see [RFC2104]) computed on meaningful data items of the
packet (including the cryptographic sequence number) on one hand
and a secret key on the other, used in proving that both the
sender and the receiver share the same secret key and that the
meaningful data was not changed in transmission.
Depending on the design specifics either all protocol packets are
authenticated or only those protecting the integrity of protocol
exchange. This mechanism authenticates all protocol packets.
Although the HMAC construct is just one of many possible approaches
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to cryptographic authentication of packets, this mechanism makes use
of relevant prior experience by using HMAC too and its solution space
correlates with the solution spaces of the mechanisms above. At the
same time, it allows for a future extension that treats HMAC as a
particular case of a more generic mechanism. Practical experience
with the mechanism defined herein should be useful in designing such
future extension.
This specification defines the use of the cryptographic sequence
number in details sufficient to make replay attack protection
strength predictable. That is, an operator can tell the strength
from the declared characteristics of an implementation and, whereas
the implementation allows to change relevant parameters, the effect
of a reconfiguration.
This mechanism explicitly allows for multiple HMAC results per
authenticated packet. Since meaningful data items of a given packet
remain the same, each such HMAC result stands for a different secret
key and/or a different hash algorithm. This enables a simultaneous,
independent authentication within multiple domains. This
specification is not novel in this regard, e.g., L2TPv3 allows for 1
or 2 ([RFC3931] Section 5.4.1) and MANET protocols allow for several
([RFC7183] Section 6.1) results per authenticated packet.
An important concern addressed by this mechanism is limiting the
amount of HMAC computations done per authenticated packet,
independently for sending and receiving. Without these limits the
number of computations per packet could be as high as the number of
configured authentication keys (in the sending case) or as the number
of keys multiplied by the number of supplied HMAC results (in the
receiving case).
These limits establish a basic competition between the configured
keys and (in the receiving case) an additional competition between
the supplied HMAC results. This specification defines related data
structures and procedures in a way to make such competition
transparent and predictable for an operator.
Wherever this specification mentions the operator reading or changing
a particular data structure, variable, parameter, or event counter
"at runtime", it is up to the implementor how this is to be done.
For example, the implementation can employ an interactive CLI, or a
management protocol such as SNMP, or an inter-process communication
mean such as a local socket, or a combination of these.
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1.1. Requirements Language
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 [RFC2119].
2. Cryptographic Aspects
2.1. Mandatory-to-Implement and Optional Hash Algorithms
[RFC2104] defines HMAC as a construct that can use any cryptographic
hash algorithm with a known digest length and internal block size.
This specification preserves this property of HMAC by defining data
processing that itself does not depend on any particular hash
algorithm either. However, since this mechanism is a protocol
extension case, there are relevant design considerations to take into
account.
Section 4.5 of [RFC6709] suggests selecting one hash algorithm as
mandatory-to-implement for the purpose of global interoperability
(Section 3.2 ibid.) and selecting another of distinct lineage as
recommended for implementation for the purpose of cryptographic
agility. This specification makes the latter property guaranteed,
rather than probable, through an elevation of the requirement level.
There are two hash algorithms mandatory-to-implement, unambiguously
defined and generally available in multiple implementations each.
An implementation of this mechanism MUST include support for two hash
algorithms:
o RIPEMD-160 (160-bit digest)
o SHA-1 (160-bit digest)
Besides that, an implementation of this mechanism MAY include support
for additional hash algorithms, provided each such algorithm is
publicly and openly specified and its digest length is 128 bits or
more (to meet the constraint implied in Section 2.2). Implementors
SHOULD consider strong, well-known hash algorithms as additional
implementation options and MUST NOT consider hash algorithms for that
by the time of implementation meaningful attacks exist or that are
commonly viewed as deprecated.
In the latter case it is important to take into account
considerations both common (such as those made in [RFC4270]) and
specific to the HMAC application of the hash algorithm. E.g.,
[RFC6151] considers MD5 collisions and concludes that new protocol
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designs should not use HMAC-MD5, while [RFC6194] includes a
comparable analysis of SHA-1 that finds HMAC-SHA-1 secure for the
same purpose.
For example, the following hash algorithms meet these requirements at
the time of this writing (in alphabetical order):
o GOST R 34.11-94 (256-bit digest)
o SHA-224 (224-bit digest, SHA-2 family)
o SHA-256 (256-bit digest, SHA-2 family)
o SHA-384 (384-bit digest, SHA-2 family)
o SHA-512 (512-bit digest, SHA-2 family)
o Tiger (192-bit digest)
o Whirlpool (512-bit digest, 2nd rev., 2003)
The set of hash algorithms available in an implementation MUST be
clearly stated. When known weak authentication keys exist for a hash
algorithm used in the HMAC construct, an implementation MUST deny a
use of such keys.
2.2. Definition of Padding
Many practical applications of HMAC for authentication of datagram-
based network protocols (including routing protocols) involve the
padding procedure, a design-specific conditioning of the message that
both the sender and the receiver perform before the HMAC computation.
Specific padding procedure of this mechanism addresses the following
needs:
o Data Initialization
A design that places the HMAC result(s) computed for a message
inside the same message after the computation has to allocate in
the message some data unit(s) purposed for the result(s) (in this
mechanism it is the HMAC TLV(s), see Section 4.3). The padding
procedure sets respective octets of the data unit(s), in the
simplest case to a fixed value known as the padding constant.
Particular value of the constant is specific to each design. For
instance, in [RIP2-AUTH] as well as works derived from it
([ISIS-AUTH-B], [OSPF2-AUTH], and [OSPF3-AUTH]) the value is
0x878FE1F3. In many other designs (for instance, [RFC3315],
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[RFC3931], [RFC4030], [RFC4302], [RFC5176], and [ISIS-AUTH-A]) the
value is 0x00.
However, the HMAC construct is defined on the base of a
cryptographic hash algorithm, that is, an algorithm meeting
particular set of requirements made for any input message. Thus
any padding constant values, whether single- or multiple-octet, as
well as any other message conditioning methods, don't affect
cryptographic characteristics of the hash algorithm and the HMAC
construct respectively.
o Source Address Protection
In the specific case of datagram-based routing protocols the
protocol packet (that is, the message being authenticated) often
does not include network layer addresses, although the source and
(to a lesser extent) the destination address of the datagram may
be meaningful in the scope of the protocol instance.
In Babel the source address may be used as a prefix hext hop (see
Section 3.5.3 of [BABEL]). A well-known (see Section 2.3 of
[OSPF3-AUTH]) solution to the source address protection problem is
to set the first respective octets of the data unit(s) above to
the source address (yet setting the rest of the octets to the
padding constant). This procedure adapts this solution to the
specifics of Babel, which allows for exchange of protocol packets
using both IPv4 and IPv6 datagrams (see Section 4 of [BABEL]).
Even though in the case of IPv6 exchange a Babel speaker currently
uses only link-local source addresses (Section 3.1 ibid.), this
procedure protects all octets of an arbitrary given source address
for the reasons of future extensibility. The procedure implies
that future Babel extensions will never use an IPv4-mapped IPv6
address as a packet source address.
This procedure does not protect the destination address, which is
currently considered meaningless (ibid.) in the same scope. A
future extension that looks to add such protection would likely
use a new TLV or sub-TLV to include the destination address into
the protocol packet (see Section 4.1).
Description of the padding procedure:
1. Set the first 16 octets of the Digest field of the given HMAC TLV
to:
* the given source address, if it is an IPv6 address, or
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* the IPv4-mapped IPv6 address (per Section 2.5.5.2 of
[RFC4291]) holding the given source address, if it is an IPv4
address.
2. Set the remaining (TLV Length - 18) octets of the Digest field of
the given HMAC TLV to 0x00.
For an example of a Babel packet with padded HMAC TLVs see Table 3.
2.3. Cryptographic Sequence Number Specifics
Operation of this mechanism may involve multiple local and multiple
remote cryptographic sequence numbers, each essentially being a
48-bit unsigned integer. This specification uses a term "TS/PC
number" to avoid confusion with the route's (Section 2.5 of [BABEL])
or node's (Section 3.2.1 ibid.) sequence numbers of the original
Babel specification and to stress the fact that there are two
distinguished parts of this 48-bit number, each handled in its
specific way (see Section 5.1):
0 1 2 3 4
0 1 2 3 4 5 6 7 8 9 0 // 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS // | PC |
+-+-+-+-+-+-+-+-+-+-//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//
The high-order 32 bits are called "timestamp" (TS) and the low-order
16 bits are called "packet counter" (PC).
This mechanism stores, updates, compares, and encodes each TS/PC
number as two independent unsigned integers, TS and PC respectively.
Such comparison of TS/PC numbers performed in item 3 of Section 5.4
is algebraically equivalent to comparison of respective 48-bit
unsigned integers. Any byte order conversion, when required, is
performed on TS and PC parts independently.
2.4. Definition of HMAC
The algorithm description below uses the following nomenclature,
which is consistent with [FIPS-198]:
Text Is the data on which the HMAC is calculated (note item (b) of
Section 9). In this specification it is the contents of a
Babel packet ranging from the beginning of the Magic field of
the Babel packet header to the end of the last octet of the
Packet Body field, as defined in Section 4.2 of [BABEL] (see
Figure 2).
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H Is the specific hash algorithm (see Section 2.1).
K Is a sequence of octets of an arbitrary, known length.
Ko Is the cryptographic key used with the hash algorithm.
B Is the block size of H, measured in octets rather than bits.
Note that B is the internal block size, not the digest length.
L Is the digest length of H, measured in octets rather than
bits.
XOR Is the bitwise exclusive-or operation.
Opad Is the hexadecimal value 0x5C repeated B times.
Ipad Is the hexadecimal value 0x36 repeated B times.
The algorithm below is the original, unmodified HMAC construct as
defined in both [RFC2104] and [FIPS-198], hence it is different from
the algorithms defined in [RIP2-AUTH], [ISIS-AUTH-B], [OSPF2-AUTH],
and [OSPF3-AUTH] in exactly two regards:
o The algorithm below sets the size of Ko to B, not to L (L is not
greater than B). This resolves both ambiguity in XOR expressions
and incompatibility in handling of keys that have length greater
than L but not greater than B.
o The algorithm below does not change value of Text before or after
the computation. Both padding of a Babel packet before the
computation and placing of the result inside the packet are
performed elsewhere.
The intent of this is to enable the most straightforward use of
cryptographic libraries by implementations of this specification. At
the time of this writing implementations of the original HMAC
construct coupled with hash algorithms of choice are generally
available.
Description of the algorithm:
1. Preparation of the Key
In this application, Ko is always B octets long. If K is B
octets long, then Ko is set to K. If K is more than B octets
long, then Ko is set to H(K) with the necessary amount of zeroes
appended to the end of H(K), such that Ko is B octets long. If K
is less than B octets long, then Ko is set to K with zeroes
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appended to the end of K, such that Ko is B octets long.
2. First-Hash
A First-Hash, also known as the inner hash, is computed as
follows:
First-Hash = H(Ko XOR Ipad || Text)
3. Second-Hash
A second hash, also known as the outer hash, is computed as
follows:
Second-Hash = H(Ko XOR Opad || First-Hash)
4. Result
The resulting Second-Hash becomes the authentication data that is
returned as the result of HMAC calculation.
Note that in the case of Babel the Text parameter will never exceed a
few thousands of octets in length. In this specific case the
optimization discussed in Section 6 of [FIPS-198] applies, namely,
for a given K that is more than B octets long the following
associated intermediate results may be precomputed only once: Ko,
(Ko XOR Ipad), and (Ko XOR Opad).
3. Updates to Protocol Data Structures
3.1. RxAuthRequired
RxAuthRequired is a boolean parameter, its default value MUST be
TRUE. An implementation SHOULD make RxAuthRequired a per-interface
parameter, but MAY make it specific to the whole protocol instance.
The conceptual purpose of RxAuthRequired is to enable a smooth
migration from an unauthenticated to an authenticated Babel packet
exchange and back (see Section 7.3). Current value of RxAuthRequired
directly affects the receiving procedure defined in Section 5.4. An
implementation SHOULD allow the operator to change RxAuthRequired
value at runtime or by means of Babel speaker restart. An
implementation MUST allow the operator to discover the effective
value of RxAuthRequired at runtime or from the system documentation.
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3.2. LocalTS
LocalTS is a 32-bit unsigned integer variable, it is the TS part of a
per-interface TS/PC number. LocalTS is a strictly per-interface
variable not intended to be changed by the operator. Its
initialization is explained in Section 5.1.
3.3. LocalPC
LocalPC is a 16-bit unsigned integer variable, it is the PC part of a
per-interface TS/PC number. LocalPC is a strictly per-interface
variable not intended to be changed by the operator. Its
initialization is explained in Section 5.1.
3.4. MaxDigestsIn
MaxDigestsIn is an unsigned integer parameter conceptually purposed
for limiting the amount of CPU time spent processing a received
authenticated packet. The receiving procedure performs the most CPU-
intensive operation, the HMAC computation, only at most MaxDigestsIn
(Section 5.4 item 7) times for a given packet.
MaxDigestsIn value MUST be at least 2. An implementation SHOULD make
MaxDigestsIn a per-interface parameter, but MAY make it specific to
the whole protocol instance. An implementation SHOULD allow the
operator to change the value of MaxDigestsIn at runtime or by means
of Babel speaker restart. An implementation MUST allow the operator
to discover the effective value of MaxDigestsIn at runtime or from
the system documentation.
3.5. MaxDigestsOut
MaxDigestsOut is an unsigned integer parameter conceptually purposed
for limiting the amount of a sent authenticated packet's space spent
on authentication data. The sending procedure adds at most
MaxDigestsOut (Section 5.3 item 5) HMAC results to a given packet,
concurring with the output buffer management explained in
Section 6.2.
The MaxDigestsOut value MUST be at least 2. An implementation SHOULD
make MaxDigestsOut a per-interface parameter, but MAY make it
specific to the whole protocol instance. An implementation SHOULD
allow the operator to change the value of MaxDigestsOut at runtime or
by means of Babel speaker restart, in a safe range. The maximum safe
value of MaxDigestsOut is implementation-specific (see Section 6.2).
An implementation MUST allow the operator to discover the effective
value of MaxDigestsOut at runtime or from the system documentation.
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3.6. ANM Table
The ANM (Authentic Neighbours Memory) table resembles the neighbour
table defined in Section 3.2.3 of [BABEL]. Note that the term
"neighbour table" means the neighbour table of the original Babel
specification, and the term "ANM table" means the table defined
herein. Indexing of the ANM table is done in exactly the same way as
indexing of the neighbour table, but purpose, field set and
associated procedures are different.
The conceptual purpose of the ANM table is to provide longer term
replay attack protection than it would be possible using the
neighbour table. Expiry of an inactive entry in the neighbour table
depends on the last received Hello Interval of the neighbour and
typically stands for tens to hundreds of seconds (see Appendix A and
Appendix B of [BABEL]). Expiry of an inactive entry in the ANM table
depends only on the local speaker's configuration. The ANM table
retains (for at least the amount of seconds set by ANM timeout
parameter defined in Section 3.7) a copy of TS/PC number advertised
in authentic packets by each remote Babel speaker.
The ANM table is indexed by pairs of the form (Interface, Source).
Every table entry consists of the following fields:
o Interface
An implementation-specific reference to the local node's interface
that the authentic packet was received through.
o Source
The source address of the Babel speaker that the authentic packet
was received from.
o LastTS
A 32-bit unsigned integer, the TS part of a remote TS/PC number.
o LastPC
A 16-bit unsigned integer, the PC part of a remote TS/PC number.
Each ANM table entry has an associated aging timer, which is reset by
the receiving procedure (Section 5.4 item 9). If the timer expires,
the entry is deleted from the ANM table.
An implementation SHOULD use a persistent memory (NVRAM) to retain
the contents of ANM table across restarts of the Babel speaker, but
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only as long as both the Interface field reference and expiry of the
aging timer remain correct. An implementation MUST make it clear, if
and how persistent memory is used for ANM table. An implementation
SHOULD allow the operator to retrieve the current contents of ANM
table at runtime. An implementation SHOULD allow the operator to
remove some or all of ANM table entries at runtime or by means of
Babel speaker restart.
3.7. ANM Timeout
ANM timeout is an unsigned integer parameter. An implementation
SHOULD make ANM timeout a per-interface parameter, but MAY make it
specific to the whole protocol instance. ANM timeout is conceptually
purposed for limiting the maximum age (in seconds) of entries in the
ANM table standing for inactive Babel speakers. The maximum age is
immediately related to replay attack protection strength. The
strongest protection is achieved with the maximum possible value of
ANM timeout set, but it may not provide the best overall result for
specific network segments and implementations of this mechanism.
In the first turn, implementations unable to maintain local TS/PC
number strictly increasing across Babel speaker restarts will reuse
the advertised TS/PC numbers after each restart (see Section 5.1).
The neighbouring speakers will treat the new packets as replayed and
discard them until the aging timer of respective ANM table entry
expires or the new TS/PC number exceeds the one stored in the entry.
Another possible, but less probable, case could be an environment
using IPv6 for Babel datagrams exchange and involving physical moves
of network interfaces hardware between Babel speakers. Even
performed without restarting the speakers, these would cause random
drops of the TS/PC number advertised for a given (Interface, Source)
index, as viewed by neighbouring speakers, since IPv6 link-local
addresses are typically derived from interface hardware addresses.
Assuming that in such cases the operators would prefer to use a lower
ANM timeout value to let the entries expire on their own rather than
having to manually remove them from the ANM table each time, an
implementation SHOULD set the default value of ANM timeout to a value
between 30 and 300 seconds.
At the same time, network segments may exist with every Babel speaker
having its advertised TS/PC number strictly increasing over the
deployed lifetime. Assuming that in such cases the operators would
prefer using a much higher ANM timeout value, an implementation
SHOULD allow the operator to change the value of ANM timeout at
runtime or by means of Babel speaker restart. An implementation MUST
allow the operator to discover the effective value of ANM timeout at
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runtime or from the system documentation.
3.8. Configured Security Associations
A Configured Security Association (CSA) is a data structure
conceptually purposed for associating authentication keys and hash
algorithms with Babel interfaces. All CSAs are managed in finite
sequences, one sequence per interface ("interface's sequence of CSAs"
hereafter). Each interface's sequence of CSAs, as an integral part
of the Babel speaker configuration, MAY be intended for a persistent
storage as long as this conforms with the implementation's key
management policy. The default state of an interface's sequence of
CSAs is empty, which has a special meaning of no authentication
configured for the interface. The sending (Section 5.3 item 1) and
the receiving (Section 5.4 item 1) procedures address this convention
accordingly.
A single CSA structure consists of the following fields:
o HashAlgo
An implementation-specific reference to one of the hash algorithms
supported by this implementation (see Section 2.1).
o KeyChain
A finite sequence of elements ("KeyChain sequence" hereafter)
representing authentication keys, each element being a structure
consisting of the following fields:
* LocalKeyID
An unsigned integer of an implementation-specific bit length.
* AuthKeyOctets
A sequence of octets of an arbitrary, known length to be used
as the authentication key.
* KeyStartAccept
The time that this Babel speaker will begin considering this
authentication key for accepting packets with authentication
data.
* KeyStartGenerate
The time that this Babel speaker will begin considering this
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authentication key for generating packet authentication data.
* KeyStopGenerate
The time that this Babel speaker will stop considering this
authentication key for generating packet authentication data.
* KeyStopAccept
The time that this Babel speaker will stop considering this
authentication key for accepting packets with authentication
data.
Since there is no limit imposed on the number of CSAs per interface,
but the number of HMAC computations per sent/received packet is
limited (through MaxDigestsOut and MaxDigestsIn respectively), only a
fraction of the associated keys and hash algorithms may appear used
in the process. The ordering of elements within a sequence of CSAs
and within a KeyChain sequence is important to make the association
selection process deterministic and transparent. Once this ordering
is deterministic at the Babel interface level, the intermediate data
derived by the procedure defined in Section 5.2 will be
deterministically ordered as well.
An implementation SHOULD allow an operator to set any arbitrary order
of elements within a given interface's sequence of CSAs and within
the KeyChain sequence of a given CSA. Regardless if this requirement
is or isn't met, the implementation MUST provide a mean to discover
the actual element order used. Whichever order is used by an
implementation, it MUST be preserved across Babel speaker restarts.
Note that none of the CSA structure fields is constrained to contain
unique values. Section 6.4 explains this in more detail. It is
possible for the KeyChain sequence to be empty, although this is not
the intended manner of CSAs use.
The KeyChain sequence has a direct prototype, which is the "key
chain" syntax item of some existing router configuration languages.
Whereas an implementation already implements this syntax item, it is
suggested to reuse it, that is, to implement a CSA syntax item
referring to a key chain item instead of reimplementing the latter in
full.
3.9. Effective Security Associations
An Effective Security Association (ESA) is a data structure
immediately used in sending (Section 5.3) and receiving (Section 5.4)
procedures. Its conceptual purpose is to determine a runtime
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interface between those procedures and the deriving procedure defined
in Section 5.2. All ESAs are temporary data units managed as
elements of finite sequences that are not intended for a persistent
storage. Element ordering within each such finite sequence
("sequence of ESAs" hereafter) MUST be preserved as long as the
sequence exists.
A single ESA structure consists of the following fields:
o HashAlgo
An implementation-specific reference to one of the hash algorithms
supported by this implementation (see Section 2.1).
o KeyID
A 16-bit unsigned integer.
o AuthKeyOctets
A sequence of octets of an arbitrary, known length to be used as
the authentication key.
Note that among the protocol data structures introduced by this
mechanism ESA is the only one not directly interfaced with the system
operator (see Figure 1), it is not immediately present in the
protocol encoding either. However, ESA is not just a possible
implementation technique, but an integral part of this specification:
the deriving (Section 5.2), the sending (Section 5.3), and the
receiving (Section 5.4) procedures are defined in terms of the ESA
structure and its semantics provided herein. ESA is as meaningful
for a correct implementation as the other protocol data structures.
4. Updates to Protocol Encoding
4.1. Justification
Choice of encoding is very important in the long term. The protocol
encoding limits various authentication mechanism designs and
encodings, which in turn limit future developments of the protocol.
Considering existing implementations of Babel protocol instance
itself and related modules of packet analysers, the current encoding
of Babel allows for compact and robust decoders. At the same time,
this encoding allows for future extensions of Babel by three (not
excluding each other) principal means defined by Section 4.2 and
Section 4.3 of [BABEL] and further discussed in
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[I-D.chroboczek-babel-extension-mechanism]:
a. A Babel packet consists of a four-octet header followed by a
packet body, that is, a sequence of TLVs (see Figure 2). Besides
the header and the body, an actual Babel datagram may have an
arbitrary amount of trailing data between the end of the packet
body and the end of the datagram. An instance of the original
protocol silently ignores such trailing data.
b. The packet body uses a binary format allowing for 256 TLV types
and imposing no requirements on TLV ordering or number of TLVs of
a given type in a packet. [BABEL] allocates TLV types 0 through
10 (see Table 1), defines TLV body structure for each and
establishes the requirement for a Babel protocol instance to
ignore any unknown TLV types silently. This makes it possible to
examine a packet body (to validate the framing and/or to pick
particular TLVs for further processing) considering only the type
(to distinguish between a Pad1 TLV and any other TLV) and the
length of each TLV, regardless if and how many additional TLV
types are eventually deployed.
c. Within each TLV of the packet body there may be some "extra data"
after the "expected length" of the TLV body. An instance of the
original protocol silently ignores any such extra data. Note
that any TLV types without the expected length defined (such as
PadN TLV) cannot be extended with the extra data.
Considering each principal extension mean for the specific purpose of
adding authentication data items to each protocol packet, the
following arguments can be made:
o Use of the TLV extra data of some existing TLV type would not be a
solution, since no particular TLV type is guaranteed to be present
in a Babel packet.
o Use of the TLV extra data could also conflict with future
developments of the protocol encoding.
o Since the packet trailing data is currently unstructured, using it
would involve defining an encoding structure and associated
procedures, adding to the complexity of both specification and
implementation and increasing the exposure to protocol attacks
such as fuzzing.
o A naive use of the packet trailing data would make it unavailable
to any future extension of Babel. Since this mechanism is
possibly not the last extension and since some other extensions
may allow no other embedding means except the packet trailing
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data, the defined encoding structure would have to enable
multiplexing of data items belonging to different extensions.
Such a definition is out of the scope of this work.
o Deprecating an extension (or only its protocol encoding) that uses
purely purpose-allocated TLVs is as simple as deprecating the
TLVs.
o Use of purpose-allocated TLVs is transparent for both the original
protocol and any its future extensions, regardless of the
embedding mean(s) used by the latter.
Considering all of the above, this mechanism neither uses the packet
trailing data nor uses the TLV extra data, but uses two new TLV
types: type 11 for a TS/PC number and type 12 for an HMAC result (see
Table 1).
4.2. TS/PC TLV
The purpose of a TS/PC TLV is to store a single TS/PC number. There
is exactly one TS/PC TLV in an authenticated Babel packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 11 | Length | PacketCounter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type Set to 11 to indicate a TS/PC TLV.
Length The length in octets of the body, exclusive of the
Type and Length fields.
PacketCounter A 16-bit unsigned integer in network byte order, the
PC part of a TS/PC number stored in this TLV.
Timestamp A 32-bit unsigned integer in network byte order, the
TS part of a TS/PC number stored in this TLV.
Note that the ordering of PacketCounter and Timestamp in the TLV
structure is opposite to the ordering of TS and PC in "TS/PC" term
and the 48-bit equivalent (see Section 2.3).
Considering the "expected length" and the "extra data" in the
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definition of Section 4.3 of [BABEL], the expected length of a TS/PC
TLV body is unambiguously defined as 6 octets. The receiving
procedure correctly processes any TS/PC TLV with body length not less
than the expected, ignoring any extra data (Section 5.4 items 3 and
9). The sending procedure produces a TS/PC TLV with body length
equal to the expected and Length field set respectively (Section 5.3
item 3).
Future Babel extensions (such as sub-TLVs) MAY modify the sending
procedure to include the extra data after the fixed-size TS/PC TLV
body defined herein, making necessary adjustments to Length TLV
field, "Body length" packet header field and output buffer management
explained in Section 6.2.
4.3. HMAC TLV
The purpose of an HMAC TLV is to store a single HMAC result. To
assist a receiver in reproducing the HMAC computation, LocalKeyID
modulo 2^16 of the authentication key is also provided in the TLV.
There is at least one HMAC TLV in an authenticated Babel packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 12 | Length | KeyID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Digest...
+-+-+-+-+-+-+-+-+-+-+-+-
Fields:
Type Set to 12 to indicate an HMAC TLV.
Length The length in octets of the body, exclusive of the
Type and Length fields.
KeyID A 16-bit unsigned integer in network byte order.
Digest A variable-length sequence of octets, which is at
least 16 octets long (see Section 2.2).
Considering the "expected length" and the "extra data" in the
definition of Section 4.3 of [BABEL], the expected length of an HMAC
TLV body is not defined. The receiving and the padding procedures
process every octet of the Digest field, deriving the field boundary
from the Length field value (Section 5.4 item 7 and Section 2.2
respectively). The sending procedure produces HMAC TLVs with Length
field precisely sizing the Digest field to match digest length of the
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hash algorithm used (Section 5.3 items 5 and 8).
The HMAC TLV structure defined herein is final, future Babel
extensions MUST NOT extend it with any extra data.
5. Updates to Protocol Operation
5.1. Per-Interface TS/PC Number Updates
The LocalTS and LocalPC interface-specific variables constitute the
TS/PC number of a Babel interface. This number is advertised in the
TS/PC TLV of authenticated Babel packets sent from that interface.
There is only one property mandatory for the advertised TS/PC number:
its 48-bit equivalent (see Section 2.3) MUST be strictly increasing
within the scope of a given interface of a Babel speaker as long as
the protocol instance is continuously operating. This property
combined with ANM tables of neighbouring Babel speakers provides them
with the most basic replay attack protection.
Initialization and increment are two principal updates performed on
an interface TS/PC number. The initialization is performed when a
new interface becomes a part of a Babel protocol instance. The
increment is performed by the sending procedure (Section 5.3 item 2)
before advertising the TS/PC number in a TS/PC TLV.
Depending on particular implementation method of these two updates
the advertised TS/PC number may possess additional properties
improving the replay attack protection strength. This includes, but
is not limited to the methods below.
a. The most straightforward implementation would use LocalTS as a
plain wrap counter, defining the updates as follows:
initialization Set LocalPC to 0, set LocalTS to 0.
increment Increment LocalPC by 1. If LocalPC wraps (0xFFFF
+ 1 = 0x0000), increment LocalTS by 1.
In this case the advertised TS/PC numbers would be reused after
each Babel protocol instance restart, making neighbouring
speakers reject authenticated packets until the respective ANM
table entries expire or the new TS/PC number exceeds the old (see
Section 3.6 and Section 3.7).
b. A more advanced implementation could make a use of any 32-bit
unsigned integer timestamp (number of time units since an
arbitrary epoch) such as the UNIX timestamp, whereas the
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timestamp itself spans a reasonable time range and is guaranteed
against a decrease (such as one resulting from network time use).
The updates would be defined as follows:
initialization Set LocalPC to 0, set LocalTS to 0.
increment If the current timestamp is greater than LocalTS,
set LocalTS to the current timestamp and LocalPC
to 0, then consider the update complete.
Otherwise increment LocalPC by 1 and, if LocalPC
wraps, increment LocalTS by 1.
In this case the advertised TS/PC number would remain unique
across the speaker's deployed lifetime without the need for any
persistent storage. However, a suitable timestamp source is not
available in every implementation case.
c. Another advanced implementation could use LocalTS in a way
similar to the "wrap/boot counter" suggested in Section 4.1.1 of
[OSPF3-AUTH], defining the updates as follows:
initialization Set LocalPC to 0. If there is a TS value stored
in NVRAM for the current interface, set LocalTS
to the stored TS value, then increment the stored
TS value by 1. Otherwise set LocalTS to 0 and
set the stored TS value to 1.
increment Increment LocalPC by 1. If LocalPC wraps, set
LocalTS to the TS value stored in NVRAM for the
current interface, then increment the stored TS
value by 1.
In this case the advertised TS/PC number would also remain unique
across the speaker's deployed lifetime, relying on NVRAM for
storing multiple TS numbers, one per interface.
As long as the TS/PC number retains its mandatory property stated
above, it is up to the implementor, which TS/PC number updates
methods are available and if the operator can configure the method
per-interface and/or at runtime. However, an implementation MUST
disclose the essence of each update method it includes, in a
comprehensible form such as natural language description, pseudocode,
or source code. An implementation MUST allow the operator to
discover, which update method is effective for any given interface,
either at runtime or from the system documentation. These
requirements are necessary to enable the optimal (see Section 3.7)
management of ANM timeout in a network segment.
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Note that wrapping (0xFFFFFFFF + 1 = 0x00000000) of LastTS is
unlikely, but possible, causing the advertised TS/PC number to be
reused. Resolving this situation requires replacing all
authentication keys of the involved interface. In addition to that,
if the wrap was caused by a timestamp reaching its end of epoch,
using this mechanism will be impossible for the involved interface
until some different timestamp or update implementation method is
used.
5.2. Deriving ESAs from CSAs
Neither receiving nor sending procedures work with the contents of
interface's sequence of CSAs directly, both (Section 5.4 item 4 and
Section 5.3 item 4 respectively) derive a sequence of ESAs from the
sequence of CSAs and use the derived sequence (see Figure 1). There
are two main goals achieved through this indirection:
o Elimination of expired authentication keys and deduplication of
security associations. This is done as early as possible to keep
subsequent procedures focused on their respective tasks.
o Maintenance of particular ordering within the derived sequence of
ESAs. The ordering deterministically depends on the ordering
within the interface's sequence of CSAs and the ordering within
KeyChain sequence of each CSA. The particular correlation
maintained by this procedure implements a concept of fair
(independent of number of keys contained by each) competition
between CSAs.
The deriving procedure uses the following input arguments:
o input sequence of CSAs
o direction (sending or receiving)
o current time (CT)
The processing of input arguments begins with an empty output
sequence of ESAs and consists of the following steps:
1. Make a temporary copy of the input sequence of CSAs.
2. Remove all expired authentication keys from each KeyChain
sequence of the copy, that is, any keys such that:
* for receiving: KeyStartAccept is greater than CT or
KeyStopAccept is less than CT
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* for sending: KeyStartGenerate is greater than CT or
KeyStopGenerate is less than CT
Note well that there are no special exceptions. Remove all
expired keys, even if there are no keys left after that (see
Section 7.4).
3. Use the copy to populate the output sequence of ESAs as follows:
1. When the KeyChain sequence of the first CSA contains at least
one key, use its first key to produce an ESA with fields set
as follows:
HashAlgo Set to HashAlgo of the current CSA.
KeyID Set to LocalKeyID modulo 2^16 of the current
key of the current CSA.
AuthKeyOctets Set to AuthKeyOctets of the current key of the
current CSA.
Append this ESA to the end of the output sequence.
2. When the KeyChain sequence of the second CSA contains at
least one key, use its first key the same way and so forth
until all first keys of the copy are processed.
3. When the KeyChain sequence of the first CSA contains at least
two keys, use its second key the same way.
4. When the KeyChain sequence of the second CSA contains at
least two keys, use its second key the same way and so forth
until all second keys of the copy are processed.
5. And so forth until all keys of all CSAs of the copy are
processed, exactly once each.
In the description above the ordinals ("first", "second", and so
on) with regard to keys stand for an element position after the
removal of expired keys, not before. For example, if a KeyChain
sequence was { Ka, Kb, Kc, Kd } before the removal and became
{ Ka, Kd } after, then Ka would be the "first" element and Kd
would be the "second".
4. Deduplicate the ESAs in the output sequence, that is, wherever
two or more ESAs exist that share the same (HashAlgo, KeyID,
AuthKeyOctets) triplet value, remove all of these ESAs except the
one closest to the beginning of the sequence.
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The resulting sequence will contain zero or more unique ESAs, ordered
in a way deterministically correlated with ordering of CSAs within
the original input sequence of CSAs and ordering of keys within each
KeyChain sequence. This ordering maximizes the probability of having
equal amount of keys per original CSA in any N first elements of the
resulting sequence. Possible optimizations of this deriving
procedure are outlined in Section 6.3.
5.3. Updates to Packet Sending
Perform the following authentication-specific processing after the
instance of the original protocol considers an outgoing Babel packet
ready for sending, but before the packet is actually sent (see
Figure 1). After that send the packet regardless if the
authentication-specific processing modified the outgoing packet or
left it intact.
1. If the current outgoing interface's sequence of CSAs is empty,
finish authentication-specific processing and consider the packet
ready for sending.
2. Increment TS/PC number of the current outgoing interface as
explained in Section 5.1.
3. Add to the packet body (see the note at the end of this section)
a TS/PC TLV with fields set as follows:
Type Set to 11.
Length Set to 6.
PacketCounter Set to the current value of LocalPC variable of
the current outgoing interface.
Timestamp Set to the current value of LocalTS variable of
the current outgoing interface.
Note that the current step may involve byte order conversion.
4. Derive a sequence of ESAs using procedure defined in Section 5.2
with the current interface's sequence of CSAs as the input
sequence of CSAs, the current time as CT and "sending" as the
direction. Proceed to the next step even if the derived sequence
is empty.
5. Iterate over the derived sequence using its ordering. For each
ESA add to the packet body (see the note at the end of this
section) an HMAC TLV with fields set as follows:
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Type Set to 12.
Length Set to 2 plus digest length of HashAlgo of the current
ESA.
KeyID Set to KeyID of the current ESA.
Digest Size exactly equal to the digest length of HashAlgo of
the current ESA. Pad (see Section 2.2) using the source
address of the current packet (see Section 6.1).
As soon as there are MaxDigestsOut HMAC TLVs added to the current
packet body, immediately proceed to the next step.
Note that the current step may involve byte order conversion.
6. Increment the "Body length" field value of the current packet
header by the total length of TS/PC and HMAC TLVs appended to the
current packet body so far.
Note that the current step may involve byte order conversion.
7. Make a temporary copy of the current packet.
8. Iterate over the derived sequence again, using the same order and
number of elements. For each ESA (and respectively for each HMAC
TLV recently appended to the current packet body) compute an HMAC
result (see Section 2.4) using the temporary copy (not the
original packet) as Text, HashAlgo of the current ESA as H, and
AuthKeyOctets of the current ESA as K. Write the HMAC result to
the Digest field of the current HMAC TLV (see Table 4) of the
current packet (not the copy).
9. After this point, allow no more changes to the current packet
header and body and consider it ready for sending.
Note that even when the derived sequence of ESAs is empty, the packet
is sent anyway with only a TS/PC TLV appended to its body. Although
such a packet would not be authenticated, the presence of the sole
TS/PC TLV would indicate authentication key exhaustion to operators
of neighbouring Babel speakers. See also Section 7.4.
Also note that it is possible to place the authentication-specific
TLVs in the packet's sequence of TLVs in a number of different valid
ways so long as there is exactly one TS/PC TLV in the sequence and
the ordering of HMAC TLVs relative to each other, as produced in step
5 above, is preserved.
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For example, see Figure 2. The diagrams represent a Babel packet
without (D1) and with (D2, D3, D4) authentication-specific TLVs. The
optional trailing data block that is present in D1 is preserved in
D2, D3, and D4. Indexing (1, 2, ..., n) of the HMAC TLVs means the
order in which the sending procedure produced them (and respectively
the HMAC results). In D2 the added TLVs are appended: the previously
existing TLVs are followed by the TS/PC TLV, which is followed by the
HMAC TLVs. In D3 the added TLVs are prepended: the TS/PC TLV is the
first and is followed by the HMAC TLVs, which are followed by the
previously existing TLVs. In D4 the added TLVs are intermixed with
the previously existing TLVs and the TS/PC TLV is placed after the
HMAC TLVs. All three packets meet the requirements above.
Implementors SHOULD use appending (D2) for adding the authentication-
specific TLVs to the sequence, this is expected to result in more
straightforward implementation and troubleshooting in most use cases.
5.4. Updates to Packet Receiving
Perform the following authentication-specific processing after an
incoming Babel packet is received from the local network stack, but
before it is acted upon by the Babel protocol instance (see
Figure 1). The final action conceptually depends not only upon the
result of the authentication-specific processing, but also on the
current value of RxAuthRequired parameter. Immediately after any
processing step below accepts or refuses the packet, either deliver
the packet to the instance of the original protocol (when the packet
is accepted or RxAuthRequired is FALSE) or discard it (when the
packet is refused and RxAuthRequired is TRUE).
1. If the current incoming interface's sequence of CSAs is empty,
accept the packet.
2. If the current packet does not contain exactly one TS/PC TLV,
refuse it.
3. Perform a lookup in the ANM table for an entry having Interface
equal to the current incoming interface and Source equal to the
source address of the current packet. If such an entry does not
exist, immediately proceed to the next step. Otherwise, compare
the entry's LastTS and LastPC field values with Timestamp and
PacketCounter values respectively of the TS/PC TLV of the
packet. That is, refuse the packet, if at least one of the
following two conditions is true:
* Timestamp is less than LastTS
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* Timestamp is equal to LastTS and PacketCounter is not greater
than LastPC
Note that the current step may involve byte order conversion.
4. Derive a sequence of ESAs using procedure defined in Section 5.2
with the current interface's sequence of CSAs as the input
sequence of CSAs, current time as CT and "receiving" as the
direction. If the derived sequence is empty, refuse the packet.
5. Make a temporary copy of the current packet.
6. Pad (see Section 2.2) every HMAC TLV present in the temporary
copy (not the original packet) using the source address of the
original packet.
7. Iterate over all the HMAC TLVs of the original input packet (not
the copy) using their order of appearance in the packet. For
each HMAC TLV look up all ESAs in the derived sequence such that
2 plus digest length of HashAlgo of the ESA is equal to Length
of the TLV and KeyID of the ESA is equal to value of KeyID of
the TLV. Iterate over these ESAs in the relative order of their
appearance on the full sequence of ESAs. Note that nesting the
iterations the opposite way (over ESAs, then over HMAC TLVs)
would be wrong.
For each of these ESAs compute an HMAC result (see Section 2.4)
using the temporary copy (not the original packet) as Text,
HashAlgo of the current ESA as H, and AuthKeyOctets of the
current ESA as K. If the current HMAC result exactly matches the
contents of Digest field of the current HMAC TLV, immediately
proceed to the next step. Otherwise, if the number of HMAC
computations done for the current packet so far is equal to
MaxDigestsIn, immediately proceed to the next step. Otherwise
follow the normal order of iterations.
Note that the current step may involve byte order conversion.
8. Refuse the input packet unless there was a matching HMAC result
in the previous step.
9. Modify the ANM table, using the same index as for the entry
lookup above, to contain an entry with LastTS set to the value
of Timestamp and LastPC set to the value of PacketCounter fields
of the TS/PC TLV of the current packet. That is, either add a
new ANM table entry or update the existing one, depending on the
result of the entry lookup above. Reset the entry's aging timer
to the current value of ANM timeout.
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Note that the current step may involve byte order conversion.
10. Accept the input packet.
An implementation SHOULD before the authentication-specific
processing above perform those basic procedures of the original
protocol that don't take any protocol actions upon the contents of
the packet but discard it unless the packet is sufficiently well-
formed for further processing. Although exact composition of such
procedures belongs to the scope of the original protocol, it seems
reasonable to state that a packet SHOULD be discarded early,
regardless if any authentication-specific processing is due, unless
its source address conforms to Section 3.1 of [BABEL] and is not the
receiving speaker's own address (see item (e) of Section 9).
Note that RxAuthRequired affects only the final action, but not the
defined flow of authentication-specific processing. The purpose of
this is to preserve authentication-specific processing feedback (such
as log messages and event counters updates) even with RxAuthRequired
set to FALSE. This allows an operator to predict the effect of
changing RxAuthRequired from FALSE to TRUE during a migration
scenario (Section 7.3) implementation.
5.5. Authentication-Specific Statistics Maintenance
A Babel speaker implementing this mechanism SHOULD maintain a set of
counters for the following events, per protocol instance and per
interface:
a. Sending of an unauthenticated Babel packet through an interface
having an empty sequence of CSAs (Section 5.3 item 1).
b. Sending of an unauthenticated Babel packet with a TS/PC TLV but
without any HMAC TLVs due to an empty derived sequence of ESAs
(Section 5.3 item 4).
c. Sending of an authenticated Babel packet containing both TS/PC
and HMAC TLVs (Section 5.3 item 9).
d. Accepting of a Babel packet received through an interface having
an empty sequence of CSAs (Section 5.4 item 1).
e. Refusing of a received Babel packet due to an empty derived
sequence of ESAs (Section 5.4 item 4).
f. Refusing of a received Babel packet that does not contain exactly
one TS/PC TLV (Section 5.4 item 2).
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g. Refusing of a received Babel packet due to the TS/PC TLV failing
the ANM table check (Section 5.4 item 3). In the view of future
extensions this event SHOULD leave out some small amount, per
current (Interface, Source, LastTS, LastPC) tuple, of the packets
refused due to Timestamp value being equal to LastTS and
PacketCounter value being equal to LastPC.
h. Refusing of a received Babel packet missing any HMAC TLVs
(Section 5.4 item 8).
i. Refusing of a received Babel packet due to none of the processed
HMAC TLVs passing the ESA check (Section 5.4 item 8).
j. Accepting of a received Babel packet having both TS/PC and HMAC
TLVs (Section 5.4 item 10).
k. Delivery of a refused packet to the instance of the original
protocol due to RxAuthRequired parameter set to FALSE.
Note that terms "accepting" and "refusing" are used in the sense of
the receiving procedure, that is, "accepting" does not mean a packet
delivered to the instance of the original protocol purely because the
RxAuthRequired parameter is set to FALSE. Event counters readings
SHOULD be available to the operator at runtime.
6. Implementation Notes
6.1. Source Address Selection for Sending
Section 3.1 of [BABEL] allows for exchange of protocol datagrams
using IPv4 or IPv6 or both. The source address of the datagram is a
unicast (link-local in the case of IPv6) address. Within an address
family used by a Babel speaker there may be more than one addresses
eligible for the exchange and assigned to the same network interface.
The original specification considers this case out of scope and
leaves it up to the speaker's network stack to select one particular
address as the datagram source address. But the sending procedure
requires (Section 5.3 item 5) exact knowledge of packet source
address for proper padding of HMAC TLVs.
As long as a network interface has more than one addresses eligible
for the exchange within the same address family, the Babel speaker
SHOULD internally choose one of those addresses for Babel packet
sending purposes and make this choice to both the sending procedure
and the network stack (see Figure 1). Wherever this requirement
cannot be met, this limitation MUST be clearly stated in the system
documentation to allow an operator to plan network address management
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accordingly.
6.2. Output Buffer Management
An instance of the original protocol buffers produced TLVs until the
buffer becomes full or a delay timer has expired. This is performed
independently for each Babel interface with each buffer sized
according to the interface MTU (see Sections 3.1 and 4 of [BABEL]).
Since TS/PC and HMAC TLVs and any other TLVs, in the first place
those of the original protocol, share the same packet space (see
Figure 2) and respectively the same buffer space, a particular
portion of each interface buffer needs to be reserved for 1 TS/PC TLV
and up to MaxDigestsOut HMAC TLVs. The amount (R) of this reserved
buffer space is calculated as follows:
R = St + MaxDigestsOut * Sh =
= 8 + MaxDigestsOut * (4 + Lmax)
St Is the size of a TS/PC TLV.
Sh Is the size of an HMAC TLV.
Lmax Is the maximum digest length in octets possible for a
particular interface. It SHOULD be calculated based on
particular interface's sequence of CSAs, but MAY be taken as
the maximum digest length supported by particular
implementation.
An implementation allowing for per-interface value of MaxDigestsOut
or Lmax has to account for different value of R across different
interfaces, even having the same MTU. An implementation allowing for
runtime change of the value of R (due to MaxDigestsOut or Lmax) has
to take care of the TLVs already buffered by the time of the change,
especially when the value of R increases.
The maximum safe value of MaxDigestsOut parameter depends on the
interface MTU and maximum digest length used. In general, at least
200-300 octets of a Babel packet should be always available to data
other than TS/PC and HMAC TLVs. An implementation following the
requirements of Section 4 of [BABEL] would send packets sized 512
octets or larger. If, for example, the maximum digest length is 64
octets and MaxDigestsOut value is 4, the value of R would be 280,
leaving less than a half of a 512-octet packet for any other TLVs.
As long as the interface MTU is larger or digest length is smaller,
higher values of MaxDigestsOut can be used safely.
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6.3. Optimizations of ESAs Deriving
The following optimizations of the ESAs deriving procedure can reduce
amount of CPU time consumed by authentication-specific processing,
preserving an implementation's effective behaviour.
a. The most straightforward implementation would treat the deriving
procedure as a per-packet action. But since the procedure is
deterministic (its output depends on its input only), it is
possible to significantly reduce the number of times the
procedure is performed.
The procedure would obviously return the same result for the same
input arguments (sequence of CSAs, direction, CT) values.
However, it is possible to predict when the result will remain
the same even for a different input. That is, when the input
sequence of CSAs and the direction both remain the same but CT
changes, the result will remain the same as long as CT's order on
the time axis (relative to all critical points of the sequence of
CSAs) remains unchanged. Here, the critical points are
KeyStartAccept and KeyStopAccept (for the "receiving" direction)
and KeyStartGenerate and KeyStopGenerate (for the "sending"
direction) of all keys of all CSAs of the input sequence. In
other words, in this case the result will remain the same as long
as both none of the active keys expire and none of the inactive
keys enter into operation.
An implementation optimized this way would perform the full
deriving procedure for a given (interface, direction) pair only
after an operator's change to the interface's sequence of CSAs or
after reaching one of the critical points mentioned above.
b. Considering that the sending procedure iterates over at most
MaxDigestsOut elements of the derived sequence of ESAs
(Section 5.3 item 5), there would be little sense in the case of
"sending" direction in returning more than MaxDigestsOut ESAs in
the derived sequence. Note that a similar optimization would be
relatively difficult in the case of "receiving" direction, since
the number of ESAs actually used in examining a particular
received packet (not to be confused with the number of HMAC
computations) depends on additional factors besides just
MaxDigestsIn.
6.4. Security Associations Duplication
This specification defines three data structures as finite sequences:
a KeyChain sequence, an interface's sequence of CSAs, and a sequence
of ESAs. There are associated semantics to take into account during
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implementation, in that the same element can appear multiple times at
different positions of the sequence. In particular, none of CSA
structure fields (including HashAlgo, LocalKeyID, and AuthKeyOctets)
alone or in a combination has to be unique within a given CSA, or
within a given sequence of CSAs, or within all sequences of CSAs of a
Babel speaker.
In the CSA space defined this way, for any two authentication keys
their one field (in)equality would not imply their another field
(in)equality. In other words, it is acceptable to have more than one
authentication key with the same LocalKeyID or the same AuthKeyOctets
or both at a time. It is a conscious design decision that CSA
semantics allow for duplication of security associations.
Consequently, ESA semantics allow for duplication of intermediate
ESAs in the sequence until the explicit deduplication (Section 5.2
item 4).
One of the intentions of this is to define the security association
management in a way that allows the addressing of some specifics of
Babel as a mesh routing protocol. For example, a system operator
configuring a Babel speaker to participate in more than one
administrative domain could find each domain using its own
authentication key (AuthKeyOctets) under the same LocalKeyID value,
e.g., a "well-known" or "default" value like 0 or 1. Since
reconfiguring the domains to use distinct LocalKeyID values isn't
always feasible, the multi-domain Babel speaker using several
distinct authentication keys under the same LocalKeyID would make a
valid use case for such duplication.
Furthermore, if in this situation the operator decided to migrate one
of the domains to a different LocalKeyID value in a seamless way,
respective Babel speakers would use the same authentication key
(AuthKeyOctets) under two different LocalKeyID values for the time of
the transition (see also item (f) of Section 9). This would make a
similar use case.
Another intention of this design decision is to decouple security
association management from authentication key management as much as
possible, so that the latter, be it manual keying or a key management
protocol, could be designed and implemented independently (as
respective reasoning made in Section 3.1 of [RIP2-AUTH] still
applies). This way the additional key management constraints, if
any, would remain out of scope of this authentication mechanism. A
similar thinking justifies LocalKeyID field having bit length in ESA
structure definition, but not in that of CSA.
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7. Network Management Aspects
7.1. Backward Compatibility
Support of this mechanism is optional, it does not change the default
behaviour of a Babel speaker and causes no compatibility issues with
speakers properly implementing the original Babel specification.
Given two Babel speakers, one implementing this mechanism and
configured for authenticated exchange (A) and another not
implementing it (B), these would not distribute routing information
uni-directionally or form a routing loop or experience other protocol
logic issues specific purely to the use of this mechanism.
The Babel design requires a bi-directional neighbour reachability
condition between two given speakers for a successful exchange of
routing information. Apparently, in the case above neighbour
reachability would be uni-directional. Presence of TS/PC and HMAC
TLVs in Babel packets sent by A would be transparent to B. But lack
of authentication data in Babel packets send by B would make them
effectively invisible to the instance of the original protocol of A.
Uni-directional links are not specific to use of this mechanism, they
naturally exist on their own and are properly detected and coped with
by the original protocol (see Section 3.4.2 of [BABEL]).
7.2. Multi-Domain Authentication
The receiving procedure treats a packet as authentic as soon as one
of its HMAC TLVs passes the check against the derived sequence of
ESAs. This allows for packet exchange authenticated with multiple
(hash algorithm, authentication key) pairs simultaneously, in
combinations as arbitrary as permitted by MaxDigestsIn and
MaxDigestsOut.
For example, consider three Babel speakers with one interface each,
configured with the following CSAs:
o speaker A: (hash algorithm H1; key SK1), (hash algorithm H1; key
SK2)
o speaker B: (hash algorithm H1; key SK1)
o speaker C: (hash algorithm H1; key SK2)
Packets sent by A would contain 2 HMAC TLVs each, packets sent by B
and C would contain 1 HMAC TLV each. A and B would authenticate the
exchange between themselves using H1 and SK1; A and C would use H1
and SK2; B and C would discard each other's packets.
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Consider a similar set of speakers configured with different CSAs:
o speaker D: (hash algorithm H2; key SK3), (hash algorithm H3; key
SK4)
o speaker E: (hash algorithm H2; key SK3), (hash algorithm H4, keys
SK5 and SK6)
o speaker F: (hash algorithm H3; keys SK4 and SK7), (hash algorithm
H5, key SK8)
Packets sent by D would contain 2 HMAC TLVs each, packets sent by E
and F would contain 3 HMAC TLVs each. D and E would authenticate the
exchange between themselves using H2 and SK3; D and F would use H3
and SK4; E and F would discard each other's packets. The
simultaneous use of H4, SK5, and SK6 by E, as well as use of SK7, H5,
and SK8 by F (for their own purposes) would remain insignificant to
A.
An operator implementing a multi-domain authentication should keep in
mind that values of MaxDigestsIn and MaxDigestsOut may be different
both within the same Babel speaker and across different speakers.
Since the minimum value of both parameters is 2 (see Section 3.4 and
Section 3.5), when more than 2 authentication domains are configured
simultaneously it is advised to confirm that every involved speaker
can handle sufficient number of HMAC results for both sending and
receiving.
The recommended method of Babel speaker configuration for multi-
domain authentication is not only using a different authentication
key for each domain, but also using a separate CSA for each domain,
even when hash algorithms are the same. This allows for fair
competition between CSAs and sometimes limits the consequences of a
possible misconfiguration to the scope of one CSA. See also item (f)
of Section 9.
7.3. Migration to and from Authenticated Exchange
It is common in practice to consider a migration to authenticated
exchange of routing information only after the network has already
been deployed and put to an active use. Performing the migration in
a way without regular traffic interruption is typically demanded, and
this specification allows a smooth migration using the RxAuthRequired
interface parameter defined in Section 3.1. This measure is similar
to the "transition mode" suggested in Section 5 of [OSPF3-AUTH].
An operator performing the migration needs to arrange configuration
changes as follows:
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1. Decide on particular hash algorithm(s) and key(s) to be used.
2. Identify all speakers and their involved interfaces that need to
be migrated to authenticated exchange.
3. For each of the speakers and the interfaces to be reconfigured
first set RxAuthRequired parameter to FALSE, then configure
necessary CSA(s).
4. Examine the speakers to confirm that Babel packets are
successfully authenticated according to the configuration
(supposedly, through examining ANM table entries and
authentication-specific statistics, see Figure 1) and address any
discrepancies before proceeding further.
5. For each of the speakers and the reconfigured interfaces set the
RxAuthRequired parameter to TRUE.
Likewise, temporarily setting RxAuthRequired to FALSE can be used to
migrate smoothly from an authenticated packet exchange back to
unauthenticated one.
7.4. Handling of Authentication Keys Exhaustion
This specification employs a common concept of multiple authenticaion
keys co-existing for a given interface, with two independent lifetime
ranges associated with each key (one for sending and another for
receiving). It is typically recommended to configure the keys using
finite lifetimes, adding new keys before the old keys expire.
However, it is obviously possible for all keys to expire for a given
interface (for sending or receiving or both). Possible ways of
addressing this situation raise their own concerns:
o Automatic switching to unauthenticated protocol exchange. This
behaviour invalidates the initial purposes of authentication and
is commonly viewed as "unacceptable" ([RIP2-AUTH] Section 5.1,
[OSPF2-AUTH] Section 3.2, [OSPF3-AUTH] Section 3, [OSPF3-AUTH-BIS]
Section 3).
o Stopping routing information exchange over the interface. This
behaviour is likely to impact regular traffic routing and is
commonly viewed as "not advisable" ([RIP2-AUTH], [OSPF2-AUTH],
[OSPF3-AUTH]), although [OSPF3-AUTH-BIS] is different in this
regard.
o Use of the "most recently expired" key over its intended lifetime
range. This behaviour is recommended for implementation in
[RIP2-AUTH], [OSPF2-AUTH], [OSPF3-AUTH], but not in
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[OSPF3-AUTH-BIS]. The use may become a problem due to an offline
cryptographic attack (see item (f) of Section 9) or a compromise
of the key. In addition, telling a recently expired key from a
key never ever been in a use may be impossible after a router
restart.
Design of this mechanism prevents the automatic switching to
unauthenticated exchange and is consistent with similar
authentication mechanisms in this regard. But since the best choice
between two other options depends on local site policy, this decision
is left up to the operator rather than the implementor (in a way
resembling the "fail secure" configuration knob described in Section
5.1 of [RIP2-AUTH]).
Although the deriving procedure does not allow for any exceptions in
expired keys filtering (Section 5.2 item 2), the operator can
trivially enforce one of the two remaining behaviour options through
local key management procedures. In particular, when using the key
over its intended lifetime is more preferred than regular traffic
disruption, the operator would explicitly leave the old key expiry
time open until the new key is added to the router configuration. In
the opposite case the operator would always configure the old key
with a finite lifetime and bear associated risks.
8. Implementation Status
[RFC Editor: before publication please remove this section and the
reference to [RFC6982], along the offered experiment of which this
section exists to assist document reviewers.]
At the time of this writing the original Babel protocol is available
in two free, production-quality implementations, both of which
support IPv4 and IPv6 routing but exchange Babel packets using IPv6
only:
o The "standalone" babeld, a BSD-licensed software with source code
publicly available [1].
That implementation does not support this authentication
mechanism.
o The integrated babeld component of Quagga-RE, a work derived from
Quagga routing protocol suite, a GPL-lisensed software with source
code publicly available [2].
That implementation supports this authentication mechanism as
defined in revision 09 of this document. It supports both
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mandatory-to-implement hash algorithms (RIPEMD-160 and SHA-1) and
a few additional algorithms (SHA-224, SHA-256, SHA-384, SHA-512
and Whirlpool). It does not support more than one link-local IPv6
address per interface. It does not distinguish refused replayed
packets for purpose of logging in the sense of item (g) of
Section 5.5 and does not check the packet source address before
the authentication-specific processing as suggested in
Section 5.4. It implements authentication-specific parameters,
data structures and methods as follows (whether a parameter can be
"changed at runtime", it is done by means of CLI and can also be
set in a configuration file):
* MaxDigestsIn value is fixed to 4.
* MaxDigestsOut value is fixed to 4.
* RxAuthRequired value is specific to each interface and can be
changed at runtime.
* ANM Table contents is not retained across speaker restarts, can
be retrieved and reset (all entries at once) by means of CLI.
* ANM Timeout value is specific to the whole protocol instance,
has a default value of 300 seconds and can be changed at
runtime.
* Ordering of elements within each interface's sequence of CSAs
is arbitrary as set by operator at runtime. CSAs are
implemented to refer to existing key chain syntax items.
Elements of an interface's sequence of CSAs are constrained to
be unique reference-wise, but not contents-wise, that is, it is
possible to duplicate security associations using a different
key chain name to contain the same keys.
* Ordering of elements within each KeyChain sequence is fixed to
the sort order of LocalKeyID. LocalKeyID is constrained to be
unique within each KeyChain sequence.
* TS/PC number updates method can be configured at runtime for
the whole protocol instance to one of two methods standing for
items (a) and (b) of Section 5.1. The default method is (b).
* Most of the authentication-specific statistics counters listed
in Section 5.5 are implemented (per protocol instance and per
each interface) and their readings are available by means of
CLI with an option to log respective events into a file.
No other implementations of this authentication mechanism are
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known to exist, thus interoperability can only be assessed on
paper. The only existing implementation has been tested to be
fully compatible with itself regardless of a speaker CPU
endianness.
9. Security Considerations
Use of this mechanism implies requirements common to a use of shared
authentication keys, including, but not limited to:
o holding the keys secret,
o including sufficient amounts of random bits into each key,
o rekeying on a regular basis, and
o never reusing a used key for a different purpose
That said, proper design and implementation of a key management
policy is out of scope of this work. Many publications on this
subject exist and should be used for this purpose (BCP 107 [RFC4107],
BCP 132 [RFC4962], and [RFC6039] may be suggested as starting
points).
It is possible for a network that exercises authentication keys
rollover to experience accidental expiration of all the keys for a
network interface as discussed at greater length in Section 7.4.
With that and the guidance of Section 5.1 of [RIP2-AUTH] in mind, in
such an event the Babel speaker MUST send a "last key expired"
notification to the operator (e.g. via syslog, SNMP, and/or other
implementation-specific means), most likely in relation to the item
(b) of Section 5.5. Also, any actual occurrence of an authentication
key expiration MUST cause a security event to be logged by the
implementation. The log item MUST include at least a note that the
authentication key has expired, the Babel routing protocol
instance(s) affected, the network interface(s) affected, the
LocalKeyID that is affected, and the current date/time. Operators
are encouraged to check such logs as an operational security
practice.
Considering particular attacks being in-scope or out of scope on one
hand and measures taken to protect against particular in-scope
attacks on the other, the original Babel protocol and this
authentication mechanism are in line with similar datagram-based
routing protocols and their respective mechanisms. In particular,
the primary concerns addressed are:
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a. Peer Entity Authentication
The Babel speaker authentication mechanism defined herein is
believed to be as strong as is the class itself that it belongs
to. This specification is built on fundamental concepts
implemented for authentication of similar routing protocols: per-
packet authentication, use of HMAC construct, use of shared keys.
Although this design approach does not address all possible
concerns, it is so far known to be sufficient for most practical
cases.
b. Data Integrity
Meaningful parts of a Babel datagram are the contents of the
Babel packet (in the definition of Section 4.2 of [BABEL]) and
the source address of the datagram (Section 3.5.3 ibid.). This
mechanism authenticates both parts using the HMAC construct, so
that making any meaningful change to an authenticated packet
after it has been emitted by the sender should be as hard as
attacking the HMAC construct itself or successfully recovering
the authentication key.
Note well that any trailing data of the Babel datagram is not
meaningful in the scope of the original specification and does
not belong to the Babel packet. Integrity of the trailing data
is respectively not protected by this mechanism. At the same
time, although any TLV extra data is also not meaningful in the
same scope, its integrity is protected, since this extra data is
a part of the Babel packet (see Figure 2).
c. Denial of Service
Proper deployment of this mechanism in a Babel network
significantly increases the efforts required for an attacker to
feed arbitrary Babel PDUs into protocol exchange (with an intent
of attacking a particular Babel speaker or disrupting exchange of
regular traffic in a routing domain). It also protects the
neighbour table from being flooded with forged speaker entries.
At the same time, this protection comes with a price of CPU time
being spent on HMAC computations. This may be a concern for low-
performance CPUs combined with high-speed interfaces, as
sometimes seen in embedded systems and hardware routers. The
MaxDigestsIn parameter, which is used to limit the maximum amount
of CPU time spent on a single received Babel packet, addresses
this concern to some extent.
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d. Reflection Attacks
Given the approach discussed in item (b), the only potential
reflection attack on this mechanism could be replaying exact
copies of Babel packets back to the sender from the same source
address. The mitigation in this case is straightforward and is
discussed in Section 5.4.
The following in-scope concern is only partially addressed:
e. Replay Attacks
This specification establishes a basic replay protection measure
(see Section 3.6), defines a timeout parameter affecting its
strength (see Section 3.7), and outlines implementation methods
also affecting protection strength in several ways (see
Section 5.1). The implementor's choice of the timeout value and
particular implementation methods may be suboptimal due to, for
example, insufficient hardware resources of the Babel speaker.
Furthermore, it may be possible that an operator configures the
timeout and the methods to address particular local specifics and
this further weakens the protection. An operator concerned about
replay attack protection strength should understand these factors
and their meaning in a given network segment.
That said, a particular form of replay attack on this mechanism
remains possible anyway. Whether there are two or more network
segments using the same CSA and there is an adversary that
captures Babel packets on one segment and replays on another (and
vice versa due to the bi-directional reachability requirement for
neighbourship), some of the speakers on one such segment will
detect the "virtual" neighbours from another and may prefer them
for some destinations. This applies even more so as Babel
doesn't require a common pre-configured network prefix between
neighbours.
A reliable solution to this particular problem, which Section 4.5
of [RFC7186] discusses as well, is not currently known. It is
recommended that the operators use distinct CSAs for distinct
network segments.
The following in-scope concerns are not addressed:
f. Offline Cryptographic Attacks
This mechanism is obviously subject to offline cryptographic
attacks. As soon as an attacker has obtained a copy of an
authenticated Babel packet of interest (which gets easier to do
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in wireless networks), he has got all the parameters of the
authentication-specific processing performed by the sender,
except authentication key(s) and choice of particular hash
algorithm(s). Since digest lengths of common hash algorithms are
well-known and can be matched with those seen in the packet,
complexity of this attack is essentially that of the
authentication key attack.
Viewing the cryptographic strength of particular hash algorithms
as a concern of its own, the main practical means of resisting
offline cryptographic attacks on this mechanism are periodic
rekeying and use of strong keys with a sufficient number of
random bits.
It is important to understand that in the case of multiple keys
being used within single interface (for a multi-domain
authentication or during a key rollover) the strength of the
combined configuration would be that of the weakest key, since
only one successful HMAC test is required for an authentic
packet. Operators concerned about offline cryptographic attacks
should enforce the same strength policy for all keys used for a
given interface.
Note that a special pathological case is possible with this
mechanism. Whenever two or more authentication keys are
configured for a given interface such that all keys share the
same AuthKeyOctets and the same HashAlgo, but LocalKeyID modulo
2^16 is different for each key, these keys will not be treated as
duplicate (Section 5.2 item 4), but an HMAC result computed for a
given packet will be the same for each of these keys. In the
case of sending procedure this can produce multiple HMAC TLVs
with exactly the same value of the Digest field, but different
values of KeyID field. In this case the attacker will see that
the keys are the same, even without the knowledge of the key
itself. Reuse of authentication keys is not the intended use
case of this mechanism and should be strongly avoided.
g. Non-repudiation
This specification relies on a use of shared keys. There is no
timestamp infrastructure and no key revocation mechanism defined
to address a shared key compromise. Establishing the time that a
particular authentic Babel packet was generated is thus not
possible. Proving that a particular Babel speaker had actually
sent a given authentic packet is also impossible as soon as the
shared key is claimed compromised. Even with the shared key not
being compromised, reliably identifying the speaker that had
actually sent a given authentic Babel packet is not possible any
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better than proving the speaker belongs to the group sharing the
key (any of the speakers sharing a key can impose any other
speaker sharing the same key).
h. Confidentiality Violations
The original Babel protocol does not encrypt any of the
information contained in its packets. The contents of a Babel
packet is trivial to decode, revealing network topology details.
This mechanism does not improve this situation in any way. Since
routing protocol messages are not the only kind of information
subject to confidentiality concerns, a complete solution to this
problem is likely to include measures based on the channel
security model, such as IPSec and WPA2 at the time of this
writing.
i. Key Management
Any authentication key exchange/distribution concerns are left
out of scope. However, the internal representation of
authentication keys (see Section 3.8) allows for diverse key
management means, manual configuration in the first place.
j. Message Deletion
Any message deletion attacks are left out of scope. Since a
datagram deleted by an attacker cannot be distinguished from a
datagram naturally lost in transmission and since datagram-based
routing protocols are designed to withstand a certain loss of
packets, the currently established practice is treating
authentication purely as a per-packet function without any added
detection of lost packets.
10. IANA Considerations
[RFC Editor: please do not remove this section.]
At the time of this publication Babel TLV Types namespace did not
have an IANA registry. TLV types 11 and 12 were assigned (see
Table 1) to the TS/PC and HMAC TLV types by Juliusz Chroboczek,
designer of the original Babel protocol. Therefore, this document
has no IANA actions.
11. Acknowledgements
Thanks to Randall Atkinson and Matthew Fanto for their comprehensive
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work on [RIP2-AUTH] that initiated a series of publications on
routing protocols authentication, including this one. This
specification adopts many concepts belonging to the whole series.
Thanks to Juliusz Chroboczek, Gabriel Kerneis, and Matthieu Boutier.
This document incorporates many technical and editorial corrections
based on their feedback. Thanks to all contributors to Babel,
because this work would not be possible without the prior works.
Thanks to Dominic Mulligan for editorial proofreading of this
document. Thanks to Riku Hietamaki for suggesting the test vectors
section.
Thanks to Joel Halpern, Jim Schaad, Randall Atkinson, and Stephen
Farrell for providing (in chronological order) valuable feedback on
draft versions of this document.
Thanks to Jim Gettys and Dave Taht for developing CeroWrt wireless
router project and collaborating on many integration issues. A
practical need for Babel authentication emerged during a research
based on CeroWrt that eventually became the very first use case of
this mechanism.
Thanks to Kunihiro Ishiguro and Paul Jakma for establishing GNU Zebra
and Quagga routing software projects respectively. Thanks to Werner
Koch, the author of Libgcrypt. The very first implementation of this
mechanism was made on base of Quagga and Libgcrypt.
This document was produced using the xml2rfc ([RFC2629]) authoring
tool.
12. References
12.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[FIPS-198]
US National Institute of Standards & Technology, "The
Keyed-Hash Message Authentication Code (HMAC)", FIPS
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PUB 198-1, July 2008.
[BABEL] Chroboczek, J., "The Babel Routing Protocol", RFC 6126,
April 2011.
12.2. Informative References
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
[RFC4030] Stapp, M. and T. Lemon, "The Authentication Suboption for
the Dynamic Host Configuration Protocol (DHCP) Relay Agent
Option", RFC 4030, March 2005.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, June 2005.
[RFC4270] Hoffman, P. and B. Schneier, "Attacks on Cryptographic
Hashes in Internet Protocols", RFC 4270, November 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RIP2-AUTH]
Atkinson, R. and M. Fanto, "RIPv2 Cryptographic
Authentication", RFC 4822, February 2007.
[RFC4962] Housley, R. and B. Aboba, "Guidance for Authentication,
Authorization, and Accounting (AAA) Key Management",
BCP 132, RFC 4962, July 2007.
[RFC5176] Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
Aboba, "Dynamic Authorization Extensions to Remote
Authentication Dial In User Service (RADIUS)", RFC 5176,
January 2008.
[ISIS-AUTH-A]
Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, October 2008.
[ISIS-AUTH-B]
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Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, February 2009.
[OSPF2-AUTH]
Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
Authentication", RFC 5709, October 2009.
[RFC6039] Manral, V., Bhatia, M., Jaeggli, J., and R. White, "Issues
with Existing Cryptographic Protection Methods for Routing
Protocols", RFC 6039, October 2010.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, March 2011.
[RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, March 2011.
[OSPF3-AUTH]
Bhatia, M., Manral, V., and A. Lindem, "Supporting
Authentication Trailer for OSPFv3", RFC 6506,
February 2012.
[RFC6709] Carpenter, B., Aboba, B., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709,
September 2012.
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982,
July 2013.
[I-D.chroboczek-babel-extension-mechanism]
Chroboczek, J., "Extension Mechanism for the Babel Routing
Protocol", draft-chroboczek-babel-extension-mechanism-00
(work in progress), June 2013.
[OSPF3-AUTH-BIS]
Bhatia, M., Manral, V., and A. Lindem, "Supporting
Authentication Trailer for OSPFv3", RFC 7166, March 2014.
[RFC7183] Herberg, U., Dearlove, C., and T. Clausen, "Integrity
Protection for the Neighborhood Discovery Protocol (NHDP)
and Optimized Link State Routing Protocol Version 2
(OLSRv2)", RFC 7183, April 2014.
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[RFC7186] Yi, J., Herberg, U., and T. Clausen, "Security Threats for
the Neighborhood Discovery Protocol (NHDP)", RFC 7186,
April 2014.
URIs
[1] <https://github.com/jech/babeld>
[2] <https://github.com/Quagga-RE/quagga-RE>
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Appendix A. Figures and Tables
+-------------------------------------------------------------+
| authentication-specific statistics |
+-------------------------------------------------------------+
^ | ^
| v |
| +-----------------------------------------------+ |
| | system operator | |
| +-----------------------------------------------+ |
| ^ | ^ | ^ | ^ | ^ | |
| | v | | | | | | | v |
+---+ +---------+ | | | | | | +---------+ +---+
| |->| ANM | | | | | | | | LocalTS |->| |
| R |<-| table | | | | | | | | LocalPC |<-| T |
| x | +---------+ | v | v | v +---------+ | x |
| | +----------------+ +---------+ +----------------+ | |
| p | | MaxDigestsIn | | | | MaxDigestsOut | | p |
| r |<-| ANM timeout | | CSAs | | |->| r |
| o | | RxAuthRequired | | | | | | o |
| c | +----------------+ +---------+ +----------------+ | c |
| e | +-------------+ | | +-------------+ | e |
| s | | Rx ESAs | | | | Tx ESAs | | s |
| s |<-| (temporary) |<----+ +---->| (temporary) |->| s |
| i | +-------------+ +-------------+ | i |
| n | +------------------------------+----------------+ | n |
| g | | instance of | output buffers |=>| g |
| |=>| the original +----------------+ | |
| | | protocol | source address |->| |
+---+ +------------------------------+----------------+ +---+
/\ | ||
|| v \/
+-------------------------------------------------------------+
| network stack |
+-------------------------------------------------------------+
/\ || /\ || /\ || /\ ||
|| \/ || \/ || \/ || \/
+---------+ +---------+ +---------+ +---------+
| speaker | | speaker | ... | speaker | | speaker |
+---------+ +---------+ +---------+ +---------+
Flow of control data : --->
Flow of Babel datagrams/packets: ===>
Figure 1: Interaction Diagram
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P
|<---------------------------->| (D1)
| B |
| |<------------------------->|
| | |
+--+-----+-----+...+-----+-----+--+ P: Babel packet
|H |some |some | |some |some |T | H: Babel packet header
| |TLV |TLV | |TLV |TLV | | B: Babel packet body
| | | | | | | | T: optional trailing data block
+--+-----+-----+...+-----+-----+--+
P
|<----------------------------------------------------->| (D2)
| B |
| |<-------------------------------------------------->|
| | |
+--+-----+-----+...+-----+-----+------+------+...+------+--+
|H |some |some | |some |some |TS/PC |HMAC | |HMAC |T |
| |TLV |TLV | |TLV |TLV |TLV |TLV 1 | |TLV n | |
| | | | | | | | | | | |
+--+-----+-----+...+-----+-----+------+------+...+------+--+
P
|<----------------------------------------------------->| (D3)
| B |
| |<-------------------------------------------------->|
| | |
+--+------+------+...+------+-----+-----+...+-----+-----+--+
|H |TS/PC |HMAC | |HMAC |some |some | |some |some |T |
| |TLV |TLV 1 | |TLV n |TLV |TLV | |TLV |TLV | |
| | | | | | | | | | | |
+--+------+------+...+------+-----+-----+...+-----+-----+--+
P
|<------------------------------------------------------------>| (D4)
| B |
| |<--------------------------------------------------------->|
| | |
+--+-----+------+-----+------+...+-----+------+...+------+-----+--+
|H |some |HMAC |some |HMAC | |some |HMAC | |TS/PC |some |T |
| |TLV |TLV 1 |TLV |TLV 2 | |TLV |TLV n | |TLV |TLV | |
| | | | | | | | | | | | |
+--+-----+------+-----+------+...+-----+------+...+------+-----+--+
Figure 2: Babel Datagram Structure
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+-------+-------------------------+---------------+
| Value | Name | Reference |
+-------+-------------------------+---------------+
| 0 | Pad1 | [BABEL] |
| 1 | PadN | [BABEL] |
| 2 | Acknowledgement Request | [BABEL] |
| 3 | Acknowledgement | [BABEL] |
| 4 | Hello | [BABEL] |
| 5 | IHU | [BABEL] |
| 6 | Router-Id | [BABEL] |
| 7 | Next Hop | [BABEL] |
| 8 | Update | [BABEL] |
| 9 | Route Request | [BABEL] |
| 10 | Seqno Request | [BABEL] |
| 11 | TS/PC | this document |
| 12 | HMAC | this document |
+-------+-------------------------+---------------+
Table 1: Babel TLV Types 0 through 12
+--------------+-----------------------------+-------------------+
| Packet field | Packet octets (hexadecimal) | Meaning (decimal) |
+--------------+-----------------------------+-------------------+
| Magic | 2a | 42 |
| Version | 02 | version 2 |
| Body length | 00:14 | 20 octets |
| [TLV] Type | 04 | 4 (Hello) |
| [TLV] Length | 06 | 6 octets |
| Reserved | 00:00 | no meaning |
| Seqno | 09:25 | 2341 |
| Interval | 01:90 | 400 (4.00 s) |
| [TLV] Type | 08 | 8 (Update) |
| [TLV] Length | 0a | 10 octets |
| AE | 00 | 0 (wildcard) |
| Flags | 40 | default router-id |
| Plen | 00 | 0 bits |
| Omitted | 00 | 0 bits |
| Interval | ff:ff | infinity |
| Seqno | 68:21 | 26657 |
| Metric | ff:ff | infinity |
+--------------+-----------------------------+-------------------+
Table 2: A Babel Packet without Authentication TLVs
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+---------------+-------------------------------+-------------------+
| Packet field | Packet octets (hexadecimal) | Meaning (decimal) |
+---------------+-------------------------------+-------------------+
| Magic | 2a | 42 |
| Version | 02 | version 2 |
| Body length | 00:4c | 76 octets |
| [TLV] Type | 04 | 4 (Hello) |
| [TLV] Length | 06 | 6 octets |
| Reserved | 00:00 | no meaning |
| Seqno | 09:25 | 2341 |
| Interval | 01:90 | 400 (4.00 s) |
| [TLV] Type | 08 | 8 (Update) |
| [TLV] Length | 0a | 10 octets |
| AE | 00 | 0 (wildcard) |
| Flags | 40 | default router-id |
| Plen | 00 | 0 bits |
| Omitted | 00 | 0 bits |
| Interval | ff:ff | infinity |
| Seqno | 68:21 | 26657 |
| Metric | ff:ff | infinity |
| [TLV] Type | 0b | 11 (TS/PC) |
| [TLV] Length | 06 | 6 octets |
| PacketCounter | 00:01 | 1 |
| Timestamp | 52:1d:7e:8b | 1377664651 |
| [TLV] Type | 0c | 12 (HMAC) |
| [TLV] Length | 16 | 22 octets |
| KeyID | 00:c8 | 200 |
| Digest | fe:80:00:00:00:00:00:00:0a:11 | padding |
| | 96:ff:fe:1c:10:c8:00:00:00:00 | |
| [TLV] Type | 0c | 12 (HMAC) |
| [TLV] Length | 16 | 22 octets |
| KeyID | 00:64 | 100 |
| Digest | fe:80:00:00:00:00:00:00:0a:11 | padding |
| | 96:ff:fe:1c:10:c8:00:00:00:00 | |
+---------------+-------------------------------+-------------------+
Table 3: A Babel Packet with Each HMAC TLV Padded Using IPv6 Address
fe80::0a11:96ff:fe1c:10c8
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+---------------+-------------------------------+-------------------+
| Packet field | Packet octets (hexadecimal) | Meaning (decimal) |
+---------------+-------------------------------+-------------------+
| Magic | 2a | 42 |
| Version | 02 | version 2 |
| Body length | 00:4c | 76 octets |
| [TLV] Type | 04 | 4 (Hello) |
| [TLV] Length | 06 | 6 octets |
| Reserved | 00:00 | no meaning |
| Seqno | 09:25 | 2341 |
| Interval | 01:90 | 400 (4.00 s) |
| [TLV] Type | 08 | 8 (Update) |
| [TLV] Length | 0a | 10 octets |
| AE | 00 | 0 (wildcard) |
| Flags | 40 | default router-id |
| Plen | 00 | 0 bits |
| Omitted | 00 | 0 bits |
| Interval | ff:ff | infinity |
| Seqno | 68:21 | 26657 |
| Metric | ff:ff | infinity |
| [TLV] Type | 0b | 11 (TS/PC) |
| [TLV] Length | 06 | 6 octets |
| PacketCounter | 00:01 | 1 |
| Timestamp | 52:1d:7e:8b | 1377664651 |
| [TLV] Type | 0c | 12 (HMAC) |
| [TLV] Length | 16 | 22 octets |
| KeyID | 00:c8 | 200 |
| Digest | c6:f1:06:13:30:3c:fa:f3:eb:5d | HMAC result |
| | 60:3a:ed:fd:06:55:83:f7:ee:79 | |
| [TLV] Type | 0c | 12 (HMAC) |
| [TLV] Length | 16 | 22 octets |
| KeyID | 00:64 | 100 |
| Digest | df:32:16:5e:d8:63:16:e5:a6:4d | HMAC result |
| | c7:73:e0:b5:22:82:ce:fe:e2:3c | |
+---------------+-------------------------------+-------------------+
Table 4: A Babel Packet with Each HMAC TLV Containing an HMAC Result
Appendix B. Test Vectors
The test vectors below may be used to verify the correctness of some
procedures performed by an implementation of this mechanism, namely:
o appending of TS/PC and HMAC TLVs to the Babel packet body,
o padding of the HMAC TLV(s),
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o computation of the HMAC result(s), and
o placement of the result(s) in the TLV(s).
This verification isn't exhaustive, there are other important
implementation aspects that would require testing methods of their
own.
The test vectors were produced as follows.
1. A Babel speaker with a network interface with IPv6 link-local
address fe80::0a11:96ff:fe1c:10c8 was configured to use two CSAs
for the interface:
* CSA1={HashAlgo=RIPEMD-160, KeyChain={{LocalKeyID=200,
AuthKeyOctets=Key26}}}
* CSA2={HashAlgo=SHA-1, KeyChain={{LocalKeyId=100,
AuthKeyOctets=Key70}}}
The authentication keys above are:
* Key26 in ASCII:
ABCDEFGHIJKLMNOPQRSTUVWXYZ
* Key26 in hexadecimal:
41:42:43:44:45:46:47:48:49:4a:4b:4c:4d:4e:4f:50
51:52:53:54:55:56:57:58:59:5a
* Key70 in ASCII:
This=key=is=exactly=70=octets=long.=ABCDEFGHIJKLMNOPQRSTUVWXYZ01234567
* Key70 in hexadecimal:
54:68:69:73:3d:6b:65:79:3d:69:73:3d:65:78:61:63
74:6c:79:3d:37:30:3d:6f:63:74:65:74:73:3d:6c:6f
6e:67:2e:3d:41:42:43:44:45:46:47:48:49:4a:4b:4c
4d:4e:4f:50:51:52:53:54:55:56:57:58:59:5a:30:31
32:33:34:35:36:37
The length of each key was picked to relate (in the terms of
Section 2.4) with the properties of respective hash algorithm as
follows:
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* the digest length (L) of both RIPEMD-160 and SHA-1 is 20
octets,
* the internal block size (B) of both RIPEMD-160 and SHA-1 is 64
octets,
* the length of Key26 (26) is greater than L but less than B,
and
* the length of Key70 (70) is greater than B (and thus greater
than L).
KeyStartAccept, KeyStopAccept, KeyStartGenerate and
KeyStopGenerate were set to make both authentication keys valid.
2. The instance of the original protocol of the speaker produced a
Babel packet (PktO) to be sent from the interface. Table 2
provides a decoding of PktO, contents of which is below:
2a:02:00:14:04:06:00:00:09:25:01:90:08:0a:00:40
00:00:ff:ff:68:21:ff:ff
3. The authentication mechanism appended one TS/PC TLV and two HMAC
TLVs to the packet body, updated the "Body length" packet header
field and padded the Digest field of the HMAC TLVs using the
link-local IPv6 address of the interface and necessary amount of
zeroes. Table 3 provides a decoding of the resulting temporary
packet (PktT), contents of which is below:
2a:02:00:4c:04:06:00:00:09:25:01:90:08:0a:00:40
00:00:ff:ff:68:21:ff:ff:0b:06:00:01:52:1d:7e:8b
0c:16:00:c8:fe:80:00:00:00:00:00:00:0a:11:96:ff
fe:1c:10:c8:00:00:00:00:0c:16:00:64:fe:80:00:00
00:00:00:00:0a:11:96:ff:fe:1c:10:c8:00:00:00:00
4. The authentication mechanism produced two HMAC results,
performing the computations as follows:
* For H=RIPEMD-160, K=Key26, and Text=PktT the HMAC result is:
c6:f1:06:13:30:3c:fa:f3:eb:5d:60:3a:ed:fd:06:55
83:f7:ee:79
* For H=SHA-1, K=Key70, and Text=PktT the HMAC result is:
df:32:16:5e:d8:63:16:e5:a6:4d:c7:73:e0:b5:22:82
ce:fe:e2:3c
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5. The authentication mechanism placed each HMAC result into
respective HMAC TLV, producing the final authenticated Babel
packet (PktA), which was eventually sent from the interface.
Table 4 provides a decoding of PktA, contents of which is below:
2a:02:00:4c:04:06:00:00:09:25:01:90:08:0a:00:40
00:00:ff:ff:68:21:ff:ff:0b:06:00:01:52:1d:7e:8b
0c:16:00:c8:c6:f1:06:13:30:3c:fa:f3:eb:5d:60:3a
ed:fd:06:55:83:f7:ee:79:0c:16:00:64:df:32:16:5e
d8:63:16:e5:a6:4d:c7:73:e0:b5:22:82:ce:fe:e2:3c
Interpretation of this process is to be done in the view of Figure 1,
differently for the sending and the receiving directions.
For the sending direction, given a Babel speaker configured using the
IPv6 address and the sequence of CSAs as described above, the
implementation SHOULD (see notes in Section 5.3) produce exactly the
temporary packet PktT if the original protocol instance produces
exactly the packet PktO to be sent from the interface. If the
temporary packet exactly matches PktT, the HMAC results computed
afterwards MUST exactly match respective results above and the final
authenticated packet MUST exactly match the PktA above.
For the receiving direction, given a Babel speaker configured using
the sequence of CSAs as described above (but a different IPv6
address), the implementation MUST (assuming the TS/PC check didn't
fail) produce exactly the temporary packet PktT above if its network
stack receives through the interface exactly the packet PktA above
from the source IPv6 address above. The first HMAC result computed
afterwards MUST match the first result above. The receiving
procedure doesn't compute the second HMAC result in this case, but if
the implementor decides to compute it anyway for the verification
purpose, it MUST exactly match the second result above.
Author's Address
Denis Ovsienko
Yandex
16, Leo Tolstoy St.
Moscow, 119021
Russia
Email: infrastation@yandex.ru
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