Relaxed Packet Counter Verification for Babel MAC Authentication
IRIF, University of Paris-Cité
Case 7014
Paris CEDEX 13
75205
France
jch@irif.fr
Red Hat
toke@toke.dk
This document relaxes packet verification rules defined in the Babel
MAC Authentication protocol in order to make it more robust in the
presence of packet reordering.
Introduction
The design of the Babel MAC authentication mechanism assumes that packet reordering is an exceptional
occurrence, and the protocol drops any packets that arrive out-of-order.
This assumption is generally correct on wired links, but turns out to be
incorrect on some kinds of wireless links.
In particular, IEEE 802.11 (Wi-Fi) defines
a number of power-saving modes that allow stations (mobile nodes) to
switch their radio off for extended periods of time, ranging in the
hundreds of milliseconds. The access point (network switch) buffers all
multicast packets, and only sends them out after the power-saving interval
ends. The result is that multicast packets are delayed by up to a few
hundred milliseconds with respect to unicast packets, which, under some
traffic patterns, causes the PC verification procedure in RFC 8967 to
systematically fail for multicast packets.
This document defines two ways to relax the PC validation: using two
separate receiver-side states, one for unicast and one for multicast
packets (), which allows arbitrary reordering
between unicast and multicast packets, and using a window of previously
received PC values (), which allows a bounded
amount of reordering between arbitrary packets. Usage of the former is
RECOMMENDED, while usage of the latter is OPTIONAL. The two MAY be used
simultaneously (). This document updates RFC
8967.
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.
Relaxing PC validation
The Babel MAC authentication mechanism prevents replay by decorating
every sent packet with a strictly increasing value, the Packet Counter
(PC). Notwithstanding the name, the PC does not actually count packets:
a sender is permitted to increment the PC by more than one between two
packets.
A receiver maintains the highest PC received from each neighbour. When
a new packet is received, the receiver compares the PC contained in the
packet with the highest received PC; if the new value is smaller or equal,
the packet is discarded; otherwise, the packet is accepted, and the
highest PC value for that neighbour is updated.
Note that there does not exist a one-to-one correspondence between
sender states and receiver states: multiple receiver states track a single
sender state. The receiver states corresponding to single sender state
are not necessarily identical, since only a subset of receiver states are
updated when a packet is sent to a unicast address or when a multicast
packet is received by a subset of the receivers.
Multiple highest PC values
Instead of a single highest PC value maintained for each neighbour, an
implementation of the procedure described in this section uses two values,
the highest unicast PC and the highest multicast PC. More precisely, the
(Index, PC) pair contained in the neighbour table () is replaced by:
- a triple (Index, PCm, PCu), where Index is an arbitrary string of 0 to
32 octets, and PCm and PCu are 32-bit (4-octet) integers.
When a challenge reply is successful, both highest PC values are updated
to the value contained in the PC TLV from the packet containing the
successful challenge. More precisely, the last sentence of the fourth
bullet point of is replaced
by:
- If the packet contains a successful Challenge Reply, then the Index
contained in the PC TLV MUST be stored in the Index field of the neighbour
table entry corresponding to the sender (which already exists in this
case), the PC contained in the TLV MUST be stored in both the PCm and
PCu fields of the neighbour table entry, and the packet is accepted.
When a packet that does not contain a successful challenge reply is
received, the PC value that it contains is compared to either the PCm or
the PCu field of the corresponding neighbour entry, depending on whether
the packet was sent to a unicast or a multicast address. If the
comparison is successful, then the same value (PCm or PCu) is updated.
More precisely, the last bullet point of is replaced by:
- 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 either the PCm field (if the packet was sent to a multicast
address) or the PCu field (otherwise) in the neighbour table; if the
received PC is smaller or equal than the value 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 PCm (if the packet was sent to
a multicast address) or the PCu (otherwise) field contained in the
neighbour table entry is set to the received PC, and the packet is
accepted.
Generalisations
Modern networking hardware tends to maintain more than just two queues,
and it might be tempting to generalise the approach taken to more than
just two last PC values. For example, one might be tempted to use
distinct last PC values for packets received with different values of the
Type of Service (ToS) field, or with different IEEE 802.11 access categories. However, choosing a highest PC
field by consulting a value that is not protected by the MAC () would no longer protect against replay.
In effect, this means that only the destination address and port number
and data stored in the packet body may be used for choosing the highest PC
value, since these are the only fields that are protected by the MAC (in
addition to the source address and port number, which are already used
when choosing the neighbour table entry and therefore provide no
additional information). Since Babel implementations do not usually send
packets using different ToS values or IEEE 802.11 access categories, this
is not an issue in practice.
The following example shows why it would be unsafe to select the highest
PC depending on the ToS field. Suppose that a node B were to maintain
distinct highest PC values for different values T1 and T2 of the ToS field,
and that initially all of the highest PC fields at B have value 42. Suppose
now that a node A sends a packet P1 with ToS equal to T1 and PC equal to
43; when B receives the packet, it sets the highest PC value associated with
ToS T1 to 43. If an attacker were now to send an exact copy of P1 but
with ToS equal to T2, B would consult the highest PC value associated with
T2, which is still equal to 42, and accept the replayed packet.
Window-based validation
Window-based validation is similar to what is described in . When using window-based validation,
in addition to retaining within its neighbour table the highest PC value
PCh seen from every neighbour, an implementation maintains a fixed-size
window of booleans corresponding to PC values directly below PCh. More
precisely, the (Index, PC) pair contained in the neighbour table () is replaced by:
- a triple (Index, PCh, Window), where Index is an arbitrary string of
0 to 32 octets, PCh is a 32-bit (4-octet) integer, and Window is a vector
of booleans of size S (the default value S=128 is RECOMMENDED).
The window is a vector of S boolean values numbered from 0 (the "left
edge" of the window) up to S-1 (the "right edge"); the boolean associated
with the index i indicates whether a packet with PC value (PCh - (S-1) + i)
has been seen before. Shifting the window to the left by an integer
amount k is defined as moving all values so that the value previously at
index n is now at index (n - k); k values are discarded at the left edge,
and k new unset values are inserted at the right edge.
Whenever a packet is received, the receiver computes its index
i = (PC - PCh + S - 1). It then proceeds as follows:
- If the index i is negative, the packet is considered too old,
and MUST be discarded.
- If the index i is non-negative and strictly less than the
window size S, the window value at the index is checked; if this
value is already set, the received PC has been seen before and the
packet MUST be discarded. Otherwise, the corresponding window value is
marked as set, and the packet is accepted.
- If the index i is larger or equal to the window size (i.e., PC
is strictly larger than PCh), the window MUST be shifted to the left by
(i - S + 1) values (or, equivalently, by the difference PC - PCh) and
the highest PC value PCh MUST be set to the received PC. The value at
the right of the window (the value with index S - 1) MUST be set, and
the packet is accepted.
When receiving a successful Challenge Reply, the remembered highest PC
value PCh MUST be set to the value received in the challenge reply, and
all of the values in the window MUST be reset except the value at index
S - 1, which MUST be set.
Combining the two techniques
The two techniques described above serve complementary purposes:
splitting the state allows multicast packets to be reordered with respect
to unicast ones by an arbitrary number of PC values, while the
window-based technique allows arbitrary packets to be reordered but only
by a bounded number of PC values. Thus, they can profitably be combined.
An implementation that uses both techniques MUST maintain, for every
entry of the neighbour table, two distinct windows, one for multicast and
one for unicast packets. When a successful challenge reply is received,
both windows MUST be reset. When a packet that does not contain
a challenge reply is received, then if the packet's destination address is
a multicast address, the multicast window MUST be consulted and possibly
updated, as described in ; otherwise, the unicast
window MUST be consulted and possibly updated.
Security considerations
The procedures described in this document do not change the security
properties described in Section 1.2 of RFC 8967. While they do slightly
increase the amount of per-neighbour state maintained by each node, this
increase is marginal (between 4 and 36 octets, depending on implementation
choices), and should not significantly impact the ability of nodes to
survive denial-of-service attacks.
This document requires no IANA actions.
The authors are indebted to Daniel Gröber, who first identified the
problem that the procedures in this document aim to solve and suggested
the solution described in .
Normative references
MAC Authentication for the Babel Routing Protocol
Key words for use in RFCs to Indicate Requirement Levels
Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words
Informative references
IEEE Standard for Information Technology — Telecommunications and
information exchange between systems Local and metropolitan area
networks — Specific requirements — Part 11: Wireless LAN Medium Access
Control (MAC) and Physical Layer (PHY) Specifications.
IP Encapsulating Security Payload (ESP)