KARP Working Group M. Bhatia
Internet-Draft Alcatel-Lucent
Intended status: Standards Track S. Hartman
Expires: August 18, 2011 Painless Security
D. Zhang
Huawei Technologies co., LTD.
February 14, 2011

Security Extension for OSPFv2 when using Manual Key Management
draft-bhatia-karp-ospf-ip-layer-protection-03

Abstract

The current OSPFv2 cryptographic authentication mechanism as defined
in the OSPF standards is vulnerable to both inter-session and intra-
session replay attacks when its uses manual keying. Additionally,
the existing cryptographic authentication schemes do not cover the IP
header. This omission can be exploited to carry out various types of
attacks.

This draft proposes an authentication scheme based on a challenge-
response mechanism that will protect OSPFv2 from both inter and intra
replay attacks when its using manual keys for securing its protocol
packets. For comparison, an approach based on making sequence
numbers unique is presented. Later we also describe some changes in
the cryptographic hash computation so that we eliminate most attacks
that result because of OSPFv2 not protecting the IP header.

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
Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."

This Internet-Draft will expire on August 18, 2011.

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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5

2. A Challenge and Response Solution . . . . . . . . . . . . . . 6
2.1. Neighbor State Required . . . . . . . . . . . . . . . . . 11
2.2. Receiver Behavior . . . . . . . . . . . . . . . . . . . . 12
2.3. Nonce Triggers . . . . . . . . . . . . . . . . . . . . . . 13

3. Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 14
3.1. Extensions to OSPF packets . . . . . . . . . . . . . . . . 14
3.2. Extension of Hello Packet . . . . . . . . . . . . . . . . 15

4. Key Selection in Processing OSPF Packets . . . . . . . . . . . 17
4.1. Key Selection in Sending Unicast OSPF Packets . . . . . . 17
4.2. Key Selection in Sending Multicast OSPF Packets . . . . . 17
4.3. Key Selection on Receiving OSPF Packets . . . . . . . . . 18

5. Existing Cryptographic Authentication Mechanism . . . . . . . 19

6. Mechanism to secure the IP header . . . . . . . . . . . . . . 20

7. Alternative Boot Count Approach . . . . . . . . . . . . . . . 21

8. Security Considerations . . . . . . . . . . . . . . . . . . . 22

9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23

10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24

11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
11.1. Normative References . . . . . . . . . . . . . . . . . . . 25
11.2. Informative References . . . . . . . . . . . . . . . . . . 25

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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26

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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 RFC2119 [RFC2119].

When used in lower case, these words convey their typical use in
common language, and are not to be interpreted as described in
RFC2119 [RFC2119].

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1. Introduction

The OSPFv2 cryptographic authentication mechanism as described in
[[RFC2328]] uses per-packet sequence numbers to provide protection
against replay attacks. The sequence numbers increase monotonically
so that the attempts to replay the stale packets can be thwarted.
The sequence number values are maintained as a part of adjacency
states. Therefore, if an adjacency is broken down, the associated
sequence numbers get re-initiated and the neighbors start all over
again. Additionally, the cryptographic authentication mechanism does
not specify how to deal with the rollover of a sequence number when
it reaches its maximum limit. These omissions can be taken advantage
of by attackers to implement various replay attacks ([RFC6039]). In
order to address these issues, we propose a challenge/ response
mechanism that introduces two additional random numbers to help
routers generate distinguishable new states when the sequence numbers
need to be re-initiated. Compared with the cryptographic
authentication mechanism proposed in [RFC5709], the solution proposed
does not impose any more security presumption.

The cryptographic authentication as described in [RFC2328] and later
updated in [RFC5709] does not include the IP header. This also can
be exploited to launch several attacks as the source address in the
IP header is no longer protected. The OSPF specification, in certain
cases, requires the implementations to look at the source address
carried in the IP header to determine the neighbor the packet was
recieved from. Changing the source address of a packet can thus,
confuse the reciever which can be exploited to produce a number of
denial of service attacks [RFC6039]. If the packet is interpreted as
coming from a different neighbor, the sequence number received from
the neighbor may be updated. This may disrupt communication with the
legitimate neighbor. Hello packets may be reflected to cause a
neighbor to appear to have one-way communication. Old Database
descriptions may be reflected in cases where the per-packet sequence
numbers are sufficiently divergent in order to disrupt an adjacency
[I-D.hartman-ospf-analysis]. This is referred to as the IP layer
issue in [I-D.ietf-karp-threats-reqs].

[RFC2328] states that implementations MUST offer keyed MD5
authentication. It is likely that this will be deprecated in favor
of the stronger algorithms described in [RFC5709] in future
deployments [RFC6094].

This draft proposes a simple change in the cryptographic
authentication mechanism, as currently described in [RFC5709], to
prevent such IP layer attacks.

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2. A Challenge and Response Solution

In OSPFv2, a non-decreasing sequence member is associated with each
OSPF packet sent from a router in order to prevent replay attacks.
However, as illustrated in [I-D.hartman-ospf-analysis] and [RFC6039],
in the circumstances where automatic key management mechanisms are
unavailable, any re-initiation of sequence numbers can potentially be
taken advantage of to perform replay attacks. In this section, we
introduce an extension of the OSPFv2 protocol, which uses challenge/
response to benefit the verification of the freshness of OSPF packets
when the sequence numbers of routers are re-initiated. This solution
eliminates the reliance on automatic key management mechanisms.
However, it is assumed that a traffic key is shared between two
communicating routers so that an attacker can play antique packets
but lacks the capability to modify packets without being detected.

In this protocol, two random numbers (Session ID and Nonce) are
introduced. The session ID is used to identify the session a packet
is within and thus makes inter-session replay attacks difficult. The
nonce is used to challenge the liveness of communicating routers so
that states need not be maintained with routers that are not
currently neighbors. In combination with the sequence number, the
session ID can effectively resist intra-session replay attacks. When
the sequence space is exhausted, a router simply chooses a new
session ID.

Figure 1 illustrates how two routers A and B, challenge each other's
liveness when they are initially connected to a link. First, A
selects a new session ID (X1) and a new nonce (N1), and sends them
out within a hello packet (see step 1). Particularly, X1and N1 are
encapsulated in the OSPF header of the packet. Note that if A is on
a multicast LAN, the packet is sent using multicast. Similarly, B
sends a hello packet with its new session ID (X2) and Nonce (N2)
(step 2). Upon receiving the hello packet from B, A sends a hello
packet with X1 and N1. In the neighbor field of the packet, the
router ID of B, X2, and N2 is encapsulated (Step 3). Upon receiving
the packet sent in step 3, B can ensure the freshness of the packet
if the attached session ID and nonce values of both routers are
correct.

In the same way, after receiving the hello packet from A, B sends a
hello packet with X2 and N2 in the OSPF header, and in the neighbor
field of the packet, the router ID of A, X1, and N1 is listed to
identify that A has been discovered. After receiving the packet, A
can make sure the packet is fresh if the session IDs and the nonce of
both routers contained in the packet are correct. After A and B
discover each other, they start exchanging their database information
(steps 5 and 6). During the exchange, every packet from Router A is

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associated with X1 and N1, while every packet from Router B is
associated with X2 and N2. Each of these packets also contains a
sequence number as part of the cryptographic authentication option.
The sequence number MUST increase for every packet sent.

+---------------+ +---------------+
| router A | | router B |
+---------------+ +---------------+
---------------------------------------------------------------

OSPF Hdr: Session ID = X1; Nonce = N1; OSPF HELLO: Neighbor = 0
(1) ------------------------------------>

OSPF Hdr: Session ID = X2; Nonce = N2; OSPF HELLO: Neighbor = 0
(2) <------------------------------------

OSPF Hdr: Session ID = X1; Nonce = N1; OSPF HELLO: Neighbor = B;
Session ID = X2; Nonce = N2
(3) ------------------------------------>

OSPF Hdr: Session ID = X2; Nonce = N2; OSPF HELLO: Neighbor = A;
Session ID = X1; Nonce = N1
(4) <------------------------------------

OSPF Hdr: Session ID = X1; Nonce = N1; Normal OSPF DD packet
(5) ------------------------------------>

OSPF Hdr: Session ID = X2; Nonce = N2 Normal OSPF DD packet
(6) <------------------------------------

*
*
*

OSPF Hdr: Session ID = X1; Nonce = N1; OSPF HELLO: Neighbor = B;
Session ID = X2; Nonce = N2
(7) ------------------------------------>

Figure 1 Scenario: two Routers coming up on a LAN

After A and B have generated a neighbor relationship, assume another
router, C, is connected to the link. C finds the existence of A and
intends to become a neighbor of A. The packets exchanged during this
process are illustrated in Figure 2. Firstly, C selects a new
session ID (X3) and a new nonce value (N3), and sends them out within
a blank hello packet (see the second step of Figure 2). After
receiving this packet, A sends out a hello packet with the
information of C (router ID, X3, and N3) in the neighbor field.

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Because A is challenging the liveness of a new neighbor, A selects a
new nonce N1' and encapsulates it in the OSPF header of the hello
packet to challenge whether the packet sent in step 2 is really from
C. After receiving the packet from A, C can make sure the packet is
valid since it consists of its current session ID and nonce (e.g., X3
and N3). Thus, C replies to A with a hello packet including the
information of A (e.g., X1 and N1') in the neighbor field. After
receiving this packet and checking the correctness of X1 and N1', A
can ensure that the packet is fresh and C is currently online.

It worthwhile to note that during the challenge and response the
hello packets sent immediately amnogst routers.

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+---------------+ +---------------+
| router A | | router C |
+---------------+ +---------------+
--------------------------------------------------------------

OSPF Hdr: Session ID = X1; Nonce = N1; OSPF HELLO: Neighbor = B;
Session ID = X2; Nonce = N2
(1) ------------------------------------>

OSPF Hdr: Session ID = X3; Nonce = N3; OSPF HELLO: Neighbor = 0
(2) <------------------------------------

OSPF Hdr: Session ID = X1; Nonce = N1'; OSPF HELLO: Neighbor = B;
Session ID = X2; Nonce = N2; Neighbor = C;
Session ID = X3; Nonce = N3
(3) ------------------------------------>

OSPF Hdr: Session ID = X3; Nonce = N3; OSPF HELLO: Neighbor = A;
Session ID = X1; Nonce = N1'
(4) <------------------------------------

OSPF Hdr: Session ID = X1; Nonce = N1'; Normal OSPF DD packet
(5) ------------------------------------>

OSPF Hdr: Session ID = X3; Nonce = N3 Normal OSPF DD packet
(6) <------------------------------------

*
*
*

OSPF Hdr: Session ID = X1; Nonce = N1'; OSPF HELLO: Neighbor = B;
Session ID = X2; Nonce = N2; Neighbor = C;
Session ID = X3; Nonce = N3
(7) ------------------------------------>

Figure 2. Scenario: another Router C comes up on that LAN

Figure 3 illustrates the scenario in which router A is rebooted.
After the reboot, A lost its state and selects a new session ID (X4)
and a new nonce value (N4). However, B still maintains the earlier
session ID and nonce values of A (X1 and N1). In step 1, A sends a
blank hello packet out with its new session ID and nonce value.
After receiving the hello packet, B realizes that the session ID and
the nonce value of A in the OSPF header are different from the ones
maintained in its database. In order to distinguish a reboot from a
replay of an old packet, B selects a new nonce value, N2', and

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transports it as well as its session ID (X2) in a hello packet to
check whether the packet is from A. In the neighbor field of this
packet, B continues listing A with the earlier session ID and nonce
values (i.e., X1 and N1). Therefore, if an attacker attempts to send
an antique packet to masquerade as A, A would update its database
with the new nonce of B and send a hello packet with its existing
Session ID and nonce values (X1 and N1). In step 3, B receives a new
hello packet consisting of B's new nonce value from A. Since this
packet lists B with the new nonce value in the neighbors field of the
hello and since the nonce is new, this packet cannot be a replay.
Now, B can safely assume that A has indeed restarted and can start
using the new session ID and the nonce values sent by A in the
neighbor field of its hellos.

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+---------------+ +---------------+
| router A | | router B |
+---------------+ +---------------+
---------------------------------------------------------------

OSPF Hdr: Session ID = X4; Nonce = N4; OSPF HELLO: Neighbor = 0
(1) ------------------------------------>

OSPF Hdr: Session ID = X2; Nonce = N2'; OSPF HELLO: Neighbor = A;
Session ID = X1; Nonce = N1
(2) <------------------------------------

OSPF Hdr: Session ID = X4; Nonce = N4; OSPF HELLO: Neighbor = B;
Session ID = X2; Nonce = N2'
(3) ------------------------------------>

OSPF Hdr: Session ID = X2; Nonce = N2'; OSPF HELLO: Neighbor = A;
Session ID = X4; Nonce = N4
(4) <------------------------------------

OSPF Hdr: Session ID = X4; Nonce = N4; Normal OSPF DD packet
(5) ------------------------------------>

OSPF Hdr: Session ID = X2; Nonce = N2' Normal OSPF DD packet
(6) <------------------------------------

*
*
*

OSPF Hdr: Session ID = X4; Nonce = N4; OSPF HELLO: Neighbor = B;
Session ID = X2; Nonce = N2'
(7) ------------------------------------>

Figure 3. Scenario: Router A Reboot

2.1. Neighbor State Required

This authentication type requires the following additional fields be
stored per neighbor:

o The session ID most recently received from a neighbor

o The nonce most recently received from a neighbor; this only needs
to be kept up-to-date when the session ID changes or when
establishing an adjacency

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o A set of sequence numbers for the neighbor; if packets are
sometimes processed out of order, then a sequence number MAY be
maintained for each type of packet

2.2. Receiver Behavior

This section describes how OSPF receivers will handle the reception
of packets with the nonce and session ID.

If a packet is received for a neighbor in at least the 2-way state,
then the session ID is compared to the one stored in the neighbor
table. If the session ID does not match the session ID recorded with
the neighbor, and the packet is not a hello, the packet is discarded.
If the packet is a hello, then rules for hellos in following
paragraphs apply. Otherwise, if the session ID matches, then if the
sequence number in the cryptographic authentication option is not
strictly greater than the sequence number associated with the
neighbor for this type of packet, then the packet is discarded. If
the cryptographic verification of the checksum fails, the packet is
discarded. Otherwise, the packet is accepted by the cryptographic
authentication and the sequence number associated with the neighbor
for this packet type is updated to be the sequence number in the
packet. The router MAY update the nonce associated with the neighbor
to a nonce in a received hello packet. Updating the nonce is
optional because the adjacency is already established. One case
where a router implementationd would want to update nonces is where
the router has recently changed session IDs without dropping all
adjacencies. Such a session ID change is likely to be rare, either
the result of a reboot that preserved adjacencies but might not
preserve sequence numbers or running out of sequence number space.

If a hello is received for a neighbor that is not found or that has
not reached 2-way state the following steps are performed. If a
neighbor structure exists for the neighbor and the session ID match
that stored in the neighbor structure, then the packet is processed
as follows. The sequence number is checked and MUST be strictly
greater than the sequence number in the neighbor structure. The
cryptographic authentication is verified. If this router is listed
in the set of neighbors in the hello packet, the nonce and session ID
MUST match this router's current nonce and session ID. If any of
these checks fail, the packet is discarded. Otherwise the packet is
accepted past cryptographic processing.

By this point, the router has received a hello packet. Either no
neighbor structure exists or the session ID has changed. Before
permitting communication with this router, its liveness needs to be
challenged. If a neighbor has been deleted (because of a timeout)
since the last nonce trigger, then a nonce trigger (see Section 2.3is

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performed and the packet is discarded. If this router is listed in
the list of neighbors, it MUST be listed with its current session ID
and nonce otherwise the packet is discarded. If verification of the
cryptographic checksum fails, the packet is discarded. If the
neighbor is already in 2-way state or greater and this router is not
listed in the set of neighbors, the packet is discarded. Otherwise,
the session ID, nonce and all sequence numbers associated with the
neighbor are updated from the packet and the packet is accepted by
cryptographic authentication processing.

2.3. Nonce Triggers

The router keeps track of whether a nonce trigger has happened since
the last time a neighbor is deleted.

In order to test liveness, a router updates its current nonce to a
new value. As a side effect, all routers on the link that do not
already have an adjacency with this router will update the nonce
associated with this router. More importantly, though, the router we
are testing liveness with will update the nonce in its hello entry
for this router. That will allow this router to confirm that the
session ID is correct and corresponds to current replay state.

As part of a nonce trigger, the router updates its current nonce. If
a hello has not been sent too recently, then a hello is sent with the
new nonce. The nonce trigger state is updated to indicate that no
new neighbors have been deleted since the last nonce trigger.

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3. Packet Format

In the challenge/ response mechanism, every OSPFv2 packet MUST carry
the current Session ID and the associated Nonce value. This section
describes how this information is carried in the OSPFv2 packets.

The OSPF packet header includes an authentication type field, and 64-
bits of data for use by the appropriate authentication scheme
(determined by the type field). Authentication types 0, 1 and 2 are
defined in [RFC2328]. This document defines Authentication type 3.

When using this authentication scheme the 64 bit Authentication field
in the OSPF packet header remains unchanged and is the same as
defined in Section D.3 of [RFC2328]. NOTE to the WG: We can also
increase the size of the Key ID. Currently it has been kept as, but
nothing prevents us from changing this.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | Key ID | Auth Data Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cryptographic sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4.Usage of the Authentication field in the OSPF header when this mechanism is employed

The Session ID and the Nonce information is placed before the message
digest that is appended to the OSPF packet. In this case too, the
final Authentication data is not actually considered part of the OSPF
protocol packet.

3.1. Extensions to OSPF packets

This section describes the new OSPFv2 packet format when this
authentication scheme is being used.

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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -
| Version # | Type | Packet length | ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Router ID | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Area ID | |OSPF
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |HEADER
| Checksum | AuType | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| 0 | Key ID | Auth Data Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Cryptographic sequence number | V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -
| | ^
| OSPF Protocol Packet | |
~ ~ |BODY
| | |
| | V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -
| Session ID | ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Nonce Value | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AUTH
| | |DIGEST
~ Authentication Data ~ |
| | V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -

Figure 5.OSPFv2 Packet view

3.2. Extension of Hello Packet

The following figure shows an OSPF HELLO packet when this
authentication scheme is being used. The HELLO payload has been
modified to include each neighbor's Session ID and the Nonce value.
The authentication data, as described above, carries the router's
current Session ID and the Nonce value.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -
| Version # | 1 | Packet length | ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Router ID | |

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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |OSPF
| Area ID | |HDR
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Checksum | AuType = 3 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| 0 | Key ID | Auth Data Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Cryptographic sequence number | V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -
| Network Mask | ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| HelloInterval | Options | Rtr Pri | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| RouterDeadInterval | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Designated Router | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Backup Designated Router | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |HELLO
| Neighbor 1 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Session ID of Neighbor 1 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Nonce of Neighbor 1 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Neighbor 2 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Session ID of Neighbor 2 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Nonce of Neighbor 2 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ... | V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -
| Session ID | ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Nonce Value | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AUTH
| | |DATA
~ Authentication Data ~ |
| | V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -

Figure 6.Extension of Protocol Packet

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4. Key Selection in Processing OSPF Packets

This section introduces how the proposed security solution looks up
long lived keys from key tables [I-D.ietf-karp-crypto-key-table].
Generally, a proper key selected to process an OSPFv2 packet should
satisfy the requirements listed as follows:

the key is in its valid period; and

the key can be used for the desired security algorithm.

In the remainder of this section, other requirements that a selected
key should particularly satisfy are depicted in different scenarios.

4.1. Key Selection in Sending Unicast OSPF Packets

Assume that a router R1 tries to send a unicast OSPF packet from its
interface I1 to the interface R2 of a remote router R2 using security
protocol P via interface I at time T. Firstly consider the
circumstances where R1 and R2 are not connected with a virtual link.
R1 then needs to select a long long-lived symmetric key from its key
table. Because the key should be shared by the by both R1 and R2 to
protect the communication between I1 and I2, the key should satisfy
the following requirements:

the Peer field includes the router ID of R2;

the PeerKeyID field is not "unknown";

the Interfaces field includes I1; and

the Direction field is either "out" or "both".

When R1 and R2 are at the ends of a virtual link, the condition is a
little more complex. Because the virtual link can be regarded as an
unnumbered point-to-point network, the IP address of the interface
actually used to send the packet (i.e., I1) is discovered during the
routing table build process. Therefore, when the system operator
deploys the keys to protect the virtual link, I1 has not been
specified yet. Therefore, the key should be identified by the router
IDs rather than by the interface originating the packet, and the
third requirement introduced above should be changed to "the
Interface field includes the router ID".

4.2. Key Selection in Sending Multicast OSPF Packets

If a router R1 sends an OSPF packet from its interface I1 to a
multicast address (e.g., AllSPFRouters, AllDRouters), it needs to

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select a key according to the following requirements:

the Peer field includes the multicast address;

the PeerKeyID field is "group";

the Interfaces field includes I1; and

the Direction field is either "out" or "both".

4.3. Key Selection on Receiving OSPF Packets

When Cryptographic Authentication is employed, the ID of the adopted
key is encapsulated within the authentication field of an OSPF packet
header. Using this ID, it is relatively easy for a receiver to
locate the key. The requirement is relatively simple:

the Peer field includes the router ID of the sender; and

the PeerKeyID field includes the key ID obtained from the
authentication field

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5. Existing Cryptographic Authentication Mechanism

The overall cryptographic authentication process defined in [RFC5709]
remains unchanged. To reduce the potential for confusion, this
section minimises the repetition of text from RFC 5709 and is
incorporated here by reference [RFC5709].

RFC 5709, Section 3.3, describes how the cryptographic authentication
must be computed. It requires OSPFv2 packet's Authentication Trailer
(which is the appendage described in RFC 2328, Section D.4.3, Page
233, items (6)(a) and (6)(d)) to be filled with the value Apad where
Apad is a hexadecimal constant value 0x878FE1F3 repeated (L/4) times,
where L is the length of the hash being used and is measured in
octets rather than bits.

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6. Mechanism to secure the IP header

This document updates the definition of Apad which is currently a
constant defined in [RFC5709] to the source address thats carried in
the IP header of the OSPFv2 protocol packet. Routers at the sending
side must initialize Apad to a value of the source address that would
be used when sending out the OSPFv2 packet, repeated L/4 times, where
L is the length of the hash, measured in octets. The basic idea is
to incorporate the source address from the IP header in the
cryptographic authentication computation so that any change there can
be detected.

At the recieving end implementations MUST initialize Apad as the
source address that exists in the IP Header of the incoming OSPFv2
protocol packet, repeated L/4 times, instead of the constant that's
currently defined in [RFC5709]. Besides changing the value of Apad
this document does not introduce any other changes to the
authentication mechanism described in [RFC5709].

This would prevent all attacks where a rogue OSPF router changes the
source address of the protocol packet and reflects it on some other
interface as the authentication check would fail and all such packets
would get rejected.

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7. Alternative Boot Count Approach

During discussion of the challenge/response authentication approach,
a desire was expressed to have a simpler alternative to consider.
This section presents an alternative that obtains most advantages of
the challenge/response mechanism. Instead of adding nonces and
session IDs, OSPF implementations are required to keep a count of the
number of times they have booted in non-volatile storage. This
requirement is also placed on agents by the SNMPv3 security
architecture; the same boot count can be used both for SNMP and for
this OSPF mechanism.

The OSPF sequence number is extended to be 64-bits rather than 32-
bits. The most significant 32-bits are the boot count. The least
significant 32-bits is a counter that increases for every packet
sent.

A receiver verifies that the sequence number on a received packet is
strictly greater than the sequence number of the previous packet
received.

Requiring that each packet have a strictly greater sequence number is
a change from the current OSPF security model. However this change
is required for a number of the security guarantees.

This mechanism requires fewer changes to the OSPF packet than the
challenge/response mechanism. Also, the implementation complexity is
somewhat less.

However there are disadvantages. First, this mechanism requires that
the boot count be maintained succesfully in nonvolatile storage. If
the boot count ever goes backwards without changing the encryption
key, then all the attacks against the current OSPF protocol become
possible against this protocol until the time that the boot count
reaches a value greater than the largest value ever used for this
client. This can be particularly problematic if equipment is
replaced, using a router ID that has been used previously on a link
but with a fresh boot count.

Another disadvantage is that the boot count mechanism does not
protect against a session replayed while a router is down. If a
router crashes or is taken out of service, then an attacker can
replay packets as soon as the adjacencies with the router time out.
The vulnerabilities of this have not been fully analyzed. Potential
vulnerabilities include attacks on the designated router election
process and replays of complete sessions. So far it looks like it is
not likely that an attacker could bring up a replayed session far
enough to inject routes from a down router.

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8. Security Considerations

This document attempts to fix the manual key management procedure
that currently exists within OSPFv2, as part of the Phase 1 of the
KARP Working Group. This therefore, only considers manual key
management mechanism to be used for OSPFv2. Any solution that takes
advantage of the automatic key management mechanism is beyond the
scope of this document.

This document also provides a solution to prevent certain denial of
service attacks that can be launched by changing the source address
in the IP header of the OSPFv2 protocol packet.

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9. IANA Considerations

This document requests a new Auth Type to be defined for OSPFv2. It
currently uses 3 to foster pre-standard deployments.

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10. Acknowledgements

The authors would like to thank Acee Lindem for valuable
contributions and helping to understand the tradeoffs surrounding
various solutions.

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11. References

11.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

[RFC5709] Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
Authentication", RFC 5709, October 2009.

11.2. Informative References

[I-D.hartman-ospf-analysis]
Hartman, S. and D. Zhang, "Analysis of OSPF Security
According to KARP Design Guide",
draft-hartman-ospf-analysis-02 (work in progress),
December 2010.

[I-D.ietf-karp-crypto-key-table]
Housley, R. and T. Polk, "Database of Long-Lived Symmetric
Cryptographic Keys", draft-ietf-karp-crypto-key-table-00
(work in progress), November 2010.

[I-D.ietf-karp-threats-reqs]
Lebovitz, G., Bhatia, M., and R. White, "The Threat
Analysis and Requirements for Cryptographic Authentication
of Routing Protocols' Transports",
draft-ietf-karp-threats-reqs-01 (work in progress),
October 2010.

[RFC6039] Manral, V., Bhatia, M., Jaeggli, J., and R. White, "Issues
with Existing Cryptographic Protection Methods for Routing
Protocols", RFC 6039, October 2010.

[RFC6094] Bhatia, M. and V. Manral, "Summary of Cryptographic
Authentication Algorithm Implementation Requirements for
Routing Protocols", RFC 6094, February 2011.

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Authors' Addresses

Manav Bhatia
Alcatel-Lucent
Bangalore,
India

Phone:
Email: manav.bhatia@alcatel-lucent.com

Sam Hartman
Painless Security

Email: hartmans@painless-security.com

Dacheng Zhang
Huawei Technologies co., LTD.
Beijing,
China

Phone:
Fax:
Email: zhangdacheng@huawei.com
URI:

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