Which signature engine is recommended for creating a custom signature for packet header matching?

A.
MULTI-STRING

B.
FLOOD.HOST

C.
ATOMIC.IP

D.
SERVICE

E.
SWEEP

F.
META

Explanation:
http://www.cisco.com/en/US/docs/security/ips/6.1/configuration/guide/cli/cli_signature_engines.
html#wp1141808
Atomic IP Engine
The Atomic IP engine defines signatures that inspect IP protocol headers and associated Layer 4 transport protocols (TCP, UDP, and ICMP) and payloads.