MAC authentication for the
Babel routing protocol
IRIF, University of Paris-Diderot
75205 Paris Cedex 13
- spoof a Babel packet, and redirect traffic by announcing a route with
a smaller metric, a larger sequence number, or a longer prefix;
- spoof a malformed packet, which could cause an insufficiently robust
implementation to crash or interfere with the rest of the network;
- replay a previously captured Babel packet, which could cause traffic to
be redirected or otherwise interfere with the network.
Protecting a Babel network is challenging due to the fact that the
Babel protocol uses both unicast and multicast communication. One
possible approach, used notably by the Babel over Datagram Transport Layer
Security (DTLS) protocol , is to use
unicast communication for all semantically significant communication, and
then use a standard unicast security protocol to protect the Babel
traffic. In this document, we take the opposite approach: we define
a cryptographic extension to the Babel protocol that is able to protect
both unicast and multicast traffic, and thus requires very few changes to
the core protocol. This document obsoletes .
Applicability
The protocol defined in this document assumes that all interfaces on
a given link are equally trusted and share a small set of symmetric keys
(usually just one, and two during key rotation). The protocol is inapplicable
in situations where asymmetric keying is required, where the trust
relationship is partial, or where large numbers of trusted keys are
provisioned on a single link at the same time.
This protocol supports incremental deployment (where an insecure Babel
network is made secure with no service interruption), and it supports
graceful key rotation (where the set of keys is changed with no service
interruption).
This protocol does not require synchronised clocks, it does not require
persistently monotonic clocks, and it does not require persistent storage
except for what might be required for storing cryptographic keys.
Assumptions and security properties
The correctness of the protocol relies on the following assumptions:
- that the Message Authentication Code (MAC) being used is invulnerable
to forgery, i.e., that an attacker is unable to generate a packet with
a correct MAC without access to the secret key;
- that a node never generates the same index or nonce twice over the
lifetime of a key.
The first assumption is a property of the MAC being used. The second
assumption can be met either by using a robust random number generator
and sufficiently large indices and nonces, by
using a reliable hardware clock, or by rekeying often enough that
collisions are unlikely.
If the assumptions above are met, the protocol described in this
document has the following properties:
- it is invulnerable to spoofing: any Babel packet accepted as authentic
is the exact copy of a packet originally sent by an authorised node;
- locally to a single node, it is invulnerable to replay: if a node has
previously accepted a given packet, then it will never again accept a copy
of this packet or an earlier packet from the same sender;
- among different nodes, it is only vulnerable to immediate replay: if
a node A has accepted an authentic packet from C, then a node B will only
accept a copy of that packet if B has accepted an older packet from C and
B has received no later packet from C.
While this protocol makes efforts to mitigate the effects of a denial
of service attack, it does not fully protect against such attacks.
Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP 14
when, and only when,
they appear in all capitals, as shown here.
Conceptual overview of the protocol
When a node B sends out a Babel packet through an interface that is
configured for MAC cryptographic protection, it computes one or more MACs
(one per key) which it appends to the packet. When a node A receives
a packet over an interface that requires MAC cryptographic protection, it
independently computes a set of MACs and compares them to the MACs
appended to the packet; if there is no match, the packet is discarded.
In order to protect against replay, B maintains a per-interface 32-bit
integer known as the "packet counter" (PC). Whenever B sends a packet
through the interface, it embeds the current value of the PC within the
region of the packet that is protected by the MACs and increases the PC
by at least one. When A receives the packet, it compares the value of the
PC with the one contained in the previous packet received from B, and
unless it is strictly greater, the packet is discarded.
By itself, the PC mechanism is not sufficient to protect against
replay. Consider a peer A that has no information about a peer
B (e.g., because it has recently rebooted). Suppose that A receives
a packet ostensibly from B carrying a given PC; since A has no information
about B, it has no way to determine whether the packet is freshly
generated or a replay of a previously sent packet.
In this situation, A discards the packet and challenges B to prove that
it knows the MAC key. It sends a "challenge request", a TLV containing
a unique nonce, a value that has never been used before and will never be
used again. B replies to the challenge request with a "challenge reply",
a TLV containing a copy of the nonce chosen by A, in a packet protected by
MAC and containing the new value of B's PC. Since the nonce has never
been used before, B's reply proves B's knowledge of the MAC key and the
freshness of the PC.
By itself, this mechanism is safe against replay if B never resets its
PC. In practice, however, this is difficult to ensure, as persistent
storage is prone to failure, and hardware clocks, even when available, are
occasionally reset. Suppose that B resets its PC to an earlier value, and
sends a packet with a previously used PC n. A challenges B,
B successfully responds to the challenge, and A accepts the PC equal to
n + 1. At this point, an attacker C may send a replayed packet
with PC equal to n + 2, which will be accepted by A.
Another mechanism is needed to protect against this attack. In this
protocol, every PC is tagged with an "index", an arbitrary string of
octets. Whenever B resets its PC, or whenever B doesn't know whether its
PC has been reset, it picks an index that it has never used before (either
by drawing it randomly or by using a reliable hardware clock) and starts
sending PCs with that index. Whenever A detects that B has changed its
index, it challenges B again.
With this additional mechanism, this protocol is invulnerable to replay
attacks (see above).
Data Structures
Every Babel node maintains a set of conceptual data structures
described in Section 3.2 of . This protocol
extends these data structures as follows.
The Interface Table
Every Babel node maintains an interface table, as described in Section
3.2.3 of . Implementations of this protocol MUST
allow each interface to be provisioned with a set of one or more MAC keys
and the associated MAC algorithms (see
for suggested algorithms, and for
suggested methods for key generation). In order to allow incremental
deployment of this protocol (see ),
implementations SHOULD allow an interface to be configured in a mode in
which it participates in the MAC authentication protocol but accepts
packets that are not authenticated.
This protocol extends each entry in this table that is associated with
an interface on which MAC authentication has been configured with two new
pieces of data:
- a set of one or more MAC keys, each associated with a given MAC
algorithm;
- a pair (Index, PC), where Index is an arbitrary string of 0 to 32
octets, and PC is a 32-bit (4-octet) integer.
We say that an index is fresh when it has never been used before with any
of the keys currently configured on the interface. The Index field is
initialised to a fresh index, for example by drawing a random string of
sufficient length (see for
suggested sizes), and the PC is initialised to an arbitrary value
(typically 0).
The Neighbour table
Every Babel node maintains a neighbour table, as described in
Section 3.2.4 of . This protocol extends each
entry in this table with two new pieces of data:
- a pair (Index, PC), where Index is a string of 0 to 32 octets, and PC
is a 32-bit (4-octet) integer;
- a Nonce, which is an arbitrary string of 0 to 192 octets, and an
associated challenge expiry timer.
The Index and PC are initially undefined, and are managed as described in
. The Nonce and challenge expiry timer are
initially undefined, and used as described in
.
Protocol Operation
MAC computation
A Babel node computes the MAC of a Babel packet as follows.
First, the node builds a pseudo-header that will participate in MAC
computation but will not be sent. If the packet is carried over IPv6,
the pseudo-header has the following format:
If the packet is carried over IPv4, the pseudo-header has the following
format:
Fields :
- Src address
- The source IP address of the packet.
- Src port
- The source UDP port number of the packet.
- Dest address
- The destination IP address of the packet.
- Src port
- The destination UDP port number of the packet.
The node takes the concatenation of the pseudo-header and the Babel
packet including the packet header but excluding the packet trailer (from
octet 0 inclusive up to (Body Length + 4) exclusive) and
computes a MAC with one of the implemented algorithms. Every
implementation MUST implement HMAC-SHA256 as defined in
and Section 2 of ,
SHOULD implement keyed BLAKE2s , and MAY implement
other MAC algorithms.
Packet Transmission
A Babel node might delay actually sending TLVs by a small amount, in
order to aggregate multiple TLVs in a single packet up to the
interface MTU (Section 4 of ). For an
interface on which MAC protection is configured, the TLV aggregation
logic MUST take into account the overhead due to PC TLVs (one in each
packet) and MAC TLVs (one per configured key).
Before sending a packet, the following actions are performed:
- a PC TLV containing the PC and Index associated with the
outgoing interface MUST be appended to the packet body; the PC MUST be
incremented by a strictly positive amount (typically just 1); if the
PC overflows, a fresh index MUST be generated (as defined in
); a node MUST NOT include multiple PC
TLVs in a single packet;
- for each key configured on the interface, a MAC is computed as
specified in above, and stored in a
MAC TLV that MUST be appended to the packet trailer (see Section 4.2 of
).
Packet Reception
When a packet is received on an interface that is configured for MAC
protection, the following steps are performed before the packet is passed
to normal processing:
- First, the receiver checks whether the trailer of the received packet
carries at least one MAC TLV; if not, the packet MUST be immediately dropped
and processing stops. Then, for each key configured on the receiving
interface, the receiver computes the MAC of the packet. It then
compares every generated MAC against every MAC included in the packet;
if there is at least one match, the packet passes the MAC test; if there
is none, the packet MUST be silently dropped and processing stops at this
point. In order to avoid memory exhaustion attacks, an entry in the
Neighbour Table MUST NOT be created before the MAC test has passed
successfully. The MAC of the packet MUST NOT be computed for each MAC
TLV contained in the packet, but only once for each configured key.
- If an entry for the sender does not exist in the Neighbour Table, it
MAY be created at this point (or, alternatively, its creation can be
delayed until a challenge needs to be sent, see below);
- The packet body is then parsed a first time. During this "preparse"
phase, the packet body is traversed and all TLVs are ignored except PC,
Challenge Request and Challenge Reply TLVs. When a PC TLV is
encountered, the enclosed PC and Index are saved for later processing; if
multiple PCs are found (which should not happen, see
above), only the first one is
processed, the remaining ones MUST be silently ignored. If a Challenge
Request is encountered, a Challenge Reply MUST be scheduled, as described
in . If a Challenge Reply is
encountered, it is tested for validity as described in
and a note is made of the result of
the test.
- The preparse phase above has yielded two pieces of data: the PC and
Index from the first PC TLV, and a bit indicating whether the packet
contains a successful Challenge Reply. If the packet does not contain
a PC TLV, the packet MUST be dropped and processing stops at this point.
If the packet contains a successful Challenge Reply, then the PC and Index
contained in the PC TLV MUST be stored in the Neighbour Table entry
corresponding to the sender (which already exists in this case), and the
packet is accepted.
- Otherwise, if there is no entry in the Neighbour Table corresponding to
the sender, or if such an entry exists but contains no Index, or if the
Index it contains is different from the Index contained in the PC TLV,
then a challenge MUST be sent as described in
, the packet MUST be dropped, and
processing stops at this stage.
- At this stage, the packet contains no successful challenge reply and
the Index contained in the PC TLV is equal to the Index in the Neighbour
Table entry corresponding to the sender. The receiver compares the
received PC with the PC contained in the Neighbour Table; if the received
PC is smaller or equal than the PC contained in the Neighbour Table, the
packet MUST be dropped and processing stops (no challenge is sent in this
case, since the mismatch might be caused by harmless packet reordering on
the link). Otherwise, the PC contained in the Neighbour Table entry is
set to the received PC, and the packet is accepted.
In the algorithm described above, challenge requests are processed and
challenges are sent before the PC/Index pair is verified against the
neighbour table. This simplifies the implementation somewhat (the node
may simply schedule outgoing requests as it walks the packet during the
preparse phase), but relies on the rate-limiting described in
to avoid sending too many challenges
in response to replayed packets. As an optimisation, a node MAY ignore
all challenge requests contained in a packet except the last one, and it
MAY ignore a challenge request in the case where it is contained in
a packet with an Index that matches the one in the Neighbour Table and
a PC that is smaller or equal to the one contained in the Neighbour Table.
Since it is still possible to replay a packet with an obsolete Index, the
rate-limiting described in is
required even if this optimisation is implemented.
The same is true of challenge replies. However, since validating
a challenge reply has minimal additional cost (it's just a bitwise
comparison of two strings of octets), a similar optimisation for challenge
replies is not worthwhile.
After the packet has been accepted, it is processed as normal, except
that any PC, Challenge Request and Challenge Reply TLVs that it contains
are silently ignored.
Challenge requests and replies
During the preparse stage, the receiver might encounter a mismatched
Index, to which it will react by scheduling a Challenge Request. It might
encounter a Challenge Request TLV, to which it will reply with a Challenge
Reply TLV. Finally, it might encounter a Challenge Reply TLV, which it
will attempt to match with a previously sent Challenge Request TLV in
order to update the Neighbour Table entry corresponding to the sender of
the packet.
Sending challenges
When it encounters a mismatched Index during the preparse phase, a node
picks a nonce that it has never used with any of the keys currently
configured on the relevant interface, for example by drawing
a sufficiently large random string of bytes or by consulting a strictly
monotonic hardware clock. It MUST then store the nonce in the entry of
the Neighbour Table associated to the neighbour (the entry might need to
be created at this stage), initialise the neighbour's challenge expiry
timer to 30 seconds, and send a Challenge Request TLV to the unicast
address corresponding to the neighbour.
A node MAY aggregate a Challenge Request with other TLVs; in other
words, if it has already buffered TLVs to be sent to the unicast address
of the neighbour, it MAY send the buffered TLVs in the same packet as the
Challenge Request. However, it MUST arrange for the Challenge Request to
be sent in a timely manner, as any packets received from that neighbour
will be silently ignored until the challenge completes.
A node MUST impose a rate limitation to the challenges it sends; the
limit SHOULD default to one challenge request every 300ms, and MAY be
configurable. This rate limiting serves two purposes. First, since
a challenge may be sent in response to a packet replayed by an attacker,
it limits the number of challenges that an attacker can cause a node to
send. Second, it limits the number of challenges sent when there are
multiple packets in flight from a single neighbour.
Replying to challenges
When it encounters a Challenge Request during the preparse phase,
a node constructs a Challenge Reply TLV by copying the Nonce from the
Challenge Request into the Challenge Reply. It MUST then send the
Challenge Reply to the unicast address from which the Challenge Request
was sent. A challenge sent to a multicast address MUST be silently ignored.
A node MAY aggregate a Challenge Reply with other TLVs; in other words,
if it has already buffered TLVs to be sent to the unicast address of the
sender of the Challenge Request, it MAY send the buffered TLVs in the same
packet as the Challenge Reply. However, it MUST arrange for the Challenge
Reply to be sent in a timely manner (within a few seconds), and SHOULD NOT
send any other packets over the same interface before sending the
Challenge Reply, as those would be dropped by the challenger.
Since a challenge reply might be caused by a replayed challenge request,
a node MUST impose a rate limitation to the challenge replies it sends;
the limit SHOULD default to one challenge reply for each peer every
300ms and MAY be configurable.
Receiving challenge replies
When it encounters a Challenge Reply during the preparse phase, a node
consults the Neighbour Table entry corresponding to the neighbour that
sent the Challenge Reply. If no challenge is in progress, i.e., if
there is no Nonce stored in the Neighbour Table entry or the challenge
timer has expired, the Challenge Reply MUST be silently ignored and the
challenge has failed.
Otherwise, the node compares the Nonce contained in the Challenge Reply
with the Nonce contained in the Neighbour Table entry. If the two are
equal (they have the same length and content), then the challenge has
succeeded and the nonce stored in the Neighbour Table for this neighbour
SHOULD be discarded; otherwise, the challenge has failed (and the nonce is
not discarded).
Expiring per-neighbour state
The per-neighbour (Index, PC) pair is maintained in the neighbour
table, and is normally discarded when the neighbour table entry expires.
Implementations MUST ensure that an (Index, PC) pair is discarded within
a finite time since the last time a packet has been accepted. In
particular, unsuccessful challenges MUST NOT prevent an (Index, PC) pair
from being discarded for unbounded periods of time.
A possible implementation strategy for implementations that use a Hello
history (Appendix A of ) is to discard the
(Index, PC) pair whenever the Hello history becomes empty. Another
implementation strategy is to use a timer that is reset whenever a packet
is accepted, and to discard the (Index, PC) pair whenever the timer
expires. If the latter strategy is being used, the timer SHOULD default
to a value of 5 min, and MAY be configurable.
Incremental deployment and key rotation
In order to perform incremental deployment, the nodes in the network
are first configured in a mode where packets are sent with authentication
but not checked on reception. Once all the nodes in the network are
configured to send authenticated packets, nodes are reconfigured to reject
unauthenticated packets.
In order to perform key rotation, the new key is added to all the
nodes; once this is done, both the old and the new key are sent in all
packets, and packets are accepted if they are properly signed by either of
the keys. At that point, the old key is removed.
In order to support the procedures described above, implementations of
this protocol SHOULD support an interface configuration in which packets
are sent authenticated but received packets are accepted without
verification, and they SHOULD allow changing the set of keys associated
with an interface without a restart.
Packet Format
MAC TLV
Fields :
- Type
- Set to 16 to indicate a MAC TLV.
- Length
- The length of the body, in octets, exclusive of the
Type and Length fields. The length depends on the MAC algorithm being
used.
- MAC
- The body contains the MAC of the packet, computed as
described in .
This TLV is allowed in the packet trailer (see Section 4.2 of
), and MUST be ignored if it is found in the
packet body.
PC TLV
Fields :
- Type
- Set to 17 to indicate a PC TLV.
- Length
- The length of the body, in octets, exclusive of the
Type and Length fields.
- PC
- The Packet Counter (PC), a 32-bit (4-octet) unsigned
integer which is increased with every packet sent over this interface.
A fresh index (as defined in ) MUST be
generated whenever the PC overflows.
- Index
- The sender's Index, an opaque string of 0 to 32
octets.
Indices are limited to a size of 32 octets: a node MUST NOT send a TLV
with an index of size strictly larger than 32 octets, and a node MAY
ignore a PC TLV with an index of length strictly larger than 32 octets.
Indices of length 0 are valid: if a node has reliable stable storage and
the packet counter never overflows, then only one index is necessary, and
the value of length 0 is the canonical choice.
Challenge Request TLV
Fields :
- Type
- Set to 18 to indicate a Challenge Request TLV.
- Length
- The length of the body, in octets, exclusive of the
Type and Length fields.
- Nonce
- The nonce uniquely identifying the challenge, an
opaque string of 0 to 192 octets.
Nonces are limited to a size of 192 octets: a node MUST NOT send
a Challenge Request TLV with a nonce of size strictly larger than 192
octets, and a node MAY ignore a nonce that is of size strictly larger than
192 octets. Nonces of length 0 are valid: if a node has reliable stable
storage, then it may use a sequential counter for generating nonces which
get encoded in the minimum number of octets required; the value 0 is then
encoded as the string of length 0.
Challenge Reply TLV
Fields :
- Type
- Set to 19 to indicate a Challenge Reply TLV.
- Length
- The length of the body, in octets, exclusive of the
Type and Length fields.
- Nonce
- A copy of the nonce contained in the corresponding
challenge request.
Security Considerations
This document defines a mechanism that provides basic security
properties for the Babel routing protocol. The scope of this protocol is
strictly limited: it only provides authentication (we assume that routing
information is not confidential), it only supports symmetric keying, and
it only allows for the use of a small number of symmetric keys on every
link. Deployments that need more features, e.g., confidentiality or
asymmetric keying, should use a more featureful security mechanism such as
the one described in .
This mechanism relies on two assumptions, as described in . First, it assumes that the MAC being used
is invulnerable to forgery (Section 1.1 of ); at
the time of writing, HMAC-SHA256, which is mandatory to implement
(), is believed to be safe against
practical attacks.
Second, it assumes that indices and nonces are generated uniquely over
the lifetime of a key used for MAC computation (more precisely, indices
must be unique for a given (key, source) pair, and nonces must be unique
for a given (key, source, destination) triple). This property can be
satisfied either by using a cryptographically secure random number
generator to generate indices and nonces that contain enough entropy
(64-bit values are believed to be large enough for all practical
applications), or by using a reliably monotonic hardware clock. If
uniqueness cannot be guaranteed (e.g., because a hardware clock has been
reset), then rekeying is necessary.
The expiry mechanism mandated in is required to
prevent an attacker from delaying an authentic packet by an unbounded
amount of time. If an attacker is able to delay the delivery of a packet
(e.g., because it is located at a layer 2 switch), then the packet will be
accepted as long as the corresponding (Index, PC) pair is present at the
receiver. If the attacker is able to cause the (Index, PC) pair to
persist for arbitrary amounts of time (e.g., by repeatedly causing failed
challenges), then it is able to delay the packet by arbitrary amounts of
time, even after the sender has left the network, which could allow it to
redirect or blackhole traffic to destinations previously advertised by the
sender.
This protocol exposes large numbers of packets and their MACs to an
attacker that is able to capture packets; it is therefore vulnerable to
brute-force attacks. Keys must be chosen in a manner that makes them
difficult to guess. Ideally, they should have a length of 32 octets (both
for HMAC-SHA256 and Blake2s), and be chosen randomly. If, for some
reason, it is necessary to derive keys from a human-readable passphrase,
it is recommended to use a key derivation function that hampers dictionary
attacks, such as PBKDF2 , bcrypt
or scrypt . In that case,
only the derived keys should be communicated to the routers; the original
passphrase itself should be kept on the host used to perform the key
generation (e.g., an administator's secure laptop computer).
While it is probably not possible to be immune against denial of
service (DoS) attacks in general, this protocol includes a number of
mechanisms designed to mitigate such attacks. In particular, reception of
a packet with no correct MAC creates no local Babel state
(). Reception of a replayed packet with
correct MAC, on the other hand, causes a challenge to be sent; this is
mitigated somewhat by requiring that challenges be rate-limited
().
Receiving a replayed packet with an obsolete index causes an entry to
be created in the Neighbour Table, which, at first sight, makes the
protocol susceptible to resource exhaustion attacks (similarly to the
familiar "TCP SYN Flooding" attack ). However,
the MAC computation includes the sender address (), and thus the amount of storage that an
attacker can force a node to consume is limited by the number of distinct
source addresses used with a single MAC key (see also Section 4 of
, which mandates that the source address is
a link-local IPv6 address or a local IPv4 address).
In order to make this kind of resource exhaustion attacks less
effective, implementations may use a separate table of uncompleted
challenges that is separate from the Neighbour Table used by the core
protocol (the data structures described in Section 3.2 of are conceptual, and any data structure that yields the
same result may be used). Implementers might also consider using the fact
that the nonces included in challenge requests and replies can be fairly
large (up to 192 octets), which should in principle allow encoding the
per-challenge state as a secure "cookie" within the nonce itself; note
however that any such scheme will need to prevent cookie replay.
IANA Considerations
IANA has allocated the following values in the Babel TLV Types
registry:
| Type |
Name |
Reference |
| 16 |
MAC |
this document |
| 17 |
PC |
this document |
| 18 |
Challenge Request |
this document |
| 19 |
Challenge Reply |
this document |
Acknowledgments
The protocol described in this document is based on the original HMAC
protocol defined by Denis Ovsienko . The use of
a pseudo-header was suggested by David Schinazi. The use of an index to
avoid replay was suggested by Markus Stenberg. The authors are also
indebted to Antonin Decimo, Donald Eastlake, Toke Hoiland-Jorgensen,
Florian Horn, Benjamin Kaduk, Dave Taht and Martin Vigoureux.
References
Normative References
Informational References
A Future-Adaptable Password Scheme
In Proceedings of the 1999 USENIX Annual Technical
Conference.
Changes from previous versions
[RFC Editor: please remove this section before publication.]
Changes since draft-ietf-babel-hmac-00
- Changed the title.
- Removed the appendix about the packet trailer, this is now in
rfc6126bis.
- Removed the appendix with implicit indices.
- Clarified the definitions of acronyms.
- Limited the size of nonces and indices.
Changes since draft-ietf-babel-hmac-01
- Made BLAKE2s a recommended HMAC algorithm.
- Added requirement to expire per-neighbour crypto state.
Changes since draft-ietf-babel-hmac-02
- Clarified that PCs are 32-bit unsigned integers.
- Clarified that indices and nonces are of arbitrary size.
- Added reference to RFC 4086.
Changes since draft-ietf-babel-hmac-03
- Use the TLV values allocated by IANA.
- Fixed an issue with packets that contain a successful challenge
reply: they should be accepted before checking the PC value.
- Clarified that keys are the exact value of the HMAC hash size, and not
subject to preprocessing; this makes management more deterministic.
Changes since draft-ietf-babel-hmac-04
- Use normative language in more places.
Changes since draft-ietf-babel-hmac-05
- Do not update RFC 6126bis.
- Clarify that indices and nonces of length 0 are valid.
- Clarify that multiple PC TLVs in a single packet are not allowed.
- Allow discarding challenge requests when they carry an old PC.
Changes since draft-ietf-babel-hmac-06
- Do not update RFC 6126bis, for real this time.
Changes since draft-ietf-babel-hmac-07
- Clarify that a Neighbour Table entry may be created just after the HMAC
has been computed.
- Clarify that a Neighbour Table entry already exists when a successful
Challenge Reply has been received.
- Expand the Security Considerations section with information about
resource exhaustion attacks.
Changes since draft-ietf-babel-hmac-08
- Fix the size of the key to be equal to the block size, not the hash size.
- Moved the information about incremental deployment to the body.
- Clarified the double purpose of rate limitation.
- Editorial changes.
Changes since draft-ietf-babel-hmac-09
- Renamed HMAC to MAC throughout, relevant rewording.
- Made it mandatory to rate-limit challenge replies in addition to
requests.
- Added discussion of key generation.
- Added discussion of the consequences of delaying packets.
Changes since draft-ietf-babel-hmac-10
Changes since draft-ietf-babel-hmac-11
- Clarified that the state SHOULD be discarded after a successful challenge.
- Replaced "pre-image attack" with "forgery", this is more accurate.
- Minor editorial changes.