Internet Engineering Task Force (IETF) J. Chroboczek

Internet Engineering Task Force (IETF) J. Chroboczek
Request for Comments: 9467 IRIF, University of Paris-Cité
Updates: 8967 T. Høiland-Jørgensen
Category: Standards Track Red Hat
ISSN: 2070-1721 January 2024

Relaxed Packet Counter Verification for Babel MAC Authentication

Abstract

This document relaxes the packet verification rules defined in "MAC
Authentication for the Babel Routing Protocol" (RFC 8967) in order to
make the protocol more robust in the presence of packet reordering.
This document updates RFC 8967.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9467.

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Table of Contents

1. Introduction
2. Specification of Requirements
3. Relaxing PC Verification
3.1. Multiple Highest PC Values
3.1.1. Generalisations
3.2. Window-Based Verification
3.3. Combining the Two Techniques
4. Security Considerations
5. IANA Considerations
6. Normative References
7. Informative References
Acknowledgments
Authors' Addresses

1. Introduction

The design of the Babel MAC authentication mechanism [RFC8967]
assumes that packet reordering is an exceptional occurrence, and the
protocol drops any packets that arrive out-of-order. The assumption
that packets are not routinely reordered 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) [IEEE80211] 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 Packet Counter (PC)
verification procedure in RFC 8967 to systematically fail for
multicast packets.

This document defines two distinct ways to relax the PC validation:

* using two separate receiver-side states, one for unicast and one
for multicast packets (Section 3.1), which allows arbitrary
reordering between unicast and multicast packets, and

* using a window of previously received PC values (Section 3.2),
which allows a bounded amount of reordering between arbitrary
packets.

We assume that reordering between arbitrary packets only happens
occasionally, and, since Babel is designed to gracefully deal with
occasional packet loss, usage of the former mechanism is RECOMMENDED,
while usage of the latter is OPTIONAL. The two mechanisms MAY be
used simultaneously (Section 3.3).

This document updates RFC 8967 by relaxing the packet verification
rules defined therein. It does not change the security properties of
the protocol.

2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Relaxing PC Verification

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 successively transmitted 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, then 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 a 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.

3.1. Multiple Highest PC Values

Instead of maintaining a single highest PC value for each neighbour,
an implementation of the procedure described in this section uses two
values: the highest multicast value PCm and the highest non-multicast
(unicast) value PCu. More precisely, the (Index, PC) pair contained
in the neighbour table (Section 3.2 of [RFC8967]) 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 Section 4.3 of [RFC8967] is
replaced as follows:

OLD:

| 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.

NEW:

| 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 PCu
or the PCm field of the corresponding neighbour entry, depending on
whether or not the packet was sent to a multicast address. If the
comparison is successful, then the same value (PCm or PCu) is
updated. More precisely, the last bullet point of Section 4.3 of
[RFC8967] is replaced as follows:

OLD:

| 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.

NEW:

| 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 IP address) or the PCu field (otherwise)
| in the neighbour table. If the received PC is smaller than or
| equal to the value contained in the neighbour table, the packet
| MUST be dropped and processing stops. Note that 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.

3.1.1. 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 the 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 Message
Authentication Code (MAC) (Section 4.1 of [RFC8967]) would no longer
protect against replay. In effect, this means that only the
destination address and port number as well as the 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 with differing TOS values or IEEE 802.11 access
categories, this is unlikely to be 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.

3.2. Window-Based Verification

Window-based verification is similar to what is described in
Section 3.4.3 of [RFC4303]. When using window-based verification, 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 (Section 3.2 of [RFC8967]) 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 a PC
value of (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:

1. If the index i is negative, the packet is considered too old, and
it MUST be discarded.

2. 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.

3. 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.

3.3. 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, if the packet's destination
address is a multicast address, the multicast window MUST be
consulted and possibly updated, as described in Section 3.2.
Otherwise, the unicast window MUST be consulted and possibly updated.

4. Security Considerations

The procedures described in this document do not change the security
properties described in Section 1.2 of [RFC8967]. In particular, the
choice between the multicast and the unicast packet counter is made
by examining a packet's destination IP address, which is included in
the pseudo-header and therefore participates in MAC computation.
Hence, an attacker cannot change the destination address without
invalidating the MAC, and therefore cannot replay a unicast packet as
a multicast one or vice versa.

While these procedures do slightly increase the amount of per-
neighbour state maintained by each node, this increase is marginal
(between 4 and 36 octets per neighbour, depending on implementation
choices), and should not significantly impact the ability of nodes to
survive denial-of-service attacks.

5. IANA Considerations

This document has no IANA actions.

6. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8967] Dô, C., Kolodziejak, W., and J. Chroboczek, "MAC
Authentication for the Babel Routing Protocol", RFC 8967,
DOI 10.17487/RFC8967, January 2021,
<https://www.rfc-editor.org/info/rfc8967>.

7. Informative References

[IEEE80211]
IEEE, "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",
DOI 10.1109/IEEESTD.2021.9363693, IEEE Std 802.11-2020,
February 2021,
<https://ieeexplore.ieee.org/document/9363693>.

[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.

Acknowledgments

The authors are greatly indebted to Daniel Gröber, who first
identified the problem that this document aims to solve and first
suggested the solution described in Section 3.1.

Authors' Addresses

Juliusz Chroboczek
IRIF, University of Paris-Cité
Case 7014
75205 Paris CEDEX 13
France
Email: [email protected]

Toke Høiland-Jørgensen
Red Hat
Email: [email protected]