Re: [Int-area] Kathleen Moriarty's Discuss on draft-ietf-intarea-gre-mtu-04: (with DISCUSS)

[Ronald Bonica <[e-mail]>]

Kathleen,

The following is an updated Security Considerations Section. Does this work?

                                                                                Ron

Security Considerations
In the GRE fragmentation solution described above, either the GRE payload or the GRE delivery packet can be fragmented.  If the GRE payload is fragmented, it is typically reassembled at its ultimate destination.  If the GRE delivery packet is fragmented, it is typically reassembled at the GRE egress node.

The packet reassembly process is resource intensive and vulnerable to several denial of service attacks.  In the simplest attack, the attacker sends fragmented packets more quickly than the victim can reassemble them.  In a variation on that attack, the first fragment of each packet is missing, so that no packet can ever be reassembled.


Given that the packet reassembly process is resource intensive and vulnerable to denial of service attacks, operators should decide where reassembly process is best performed.  Having made that decision, they should decide whether to fragment the GRE payload or GRE delivery packet, accordingly.


Some IP implementations are vulnerable to the Overlapping Fragment Attack [RFC 1858]. This vulnerability is not specific to GRE and needs to be considered in all environments where IP fragmentation is present. [RFC 3128] describes a procedure by which IPv4 implementations can partially mitigate the vulnerability. [RFC 5722] mandates a procedure by which IPv6-compliant implementations are required to mitigate the vulnerability. The procedure described in RFC 5722 completely mitigates the vulnerability. Operators SHOULD ensure that the vulnerability is mitigated to their satisfaction on equipment that they deploy.

PMTU Discovery is vulnerable to two denial of service attacks (see Section 8 of [RFC1191]<https://tools.ietf.org/html/rfc1191#section-8> for details).  Both attacks are based upon on a malicious party sending forged ICMPv4 Destination Unreachable or ICMPv6 Packet Too Big messages to a host.  In the first attack, the forged message indicates an inordinately small PMTU.  In the second attack, the forged message indicates an inordinately large MTU.  In both cases, throughput is adversely affected.  On order to mitigate such attacks, GRE implementations include a configuration option to disable PMTU discovery on GRE tunnels.  Also, they can include a configuration option that conditions the behavior of PMTUD to establish a minimum PMTU.

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