[Snort-users] RE: Castor's use of "ECN" shut-off

cider at ...242... cider at ...242...
Fri Sep 15 13:57:30 EDT 2000


On Fri, Sep 15, 2000 at 08:49:09AM -0400, Novak, Judy H. wrote:
] As far as the false positives for the 2 high-order TCP reserved bits due to
] Explicit Congestion Notification (ECN) - looking at RFC 2481, it appears
] some validation can be done to determine if this is really ECN traffic and
] not "stealth" scanning.  In order for ECN to work, the ECN-Capable Transport
] (ECT) bit has to be set by the sender to indicate that the endpoints are ECN
] capable.  This bit is found in the TOS byte (0x02 bit - previously minimize
] cost bit).  So, when a TCP reserved bit is set, this IP bit should also be
] set in order for this to be a valid ECN indication.
] 
] Also, a couple of other conditions must exist too for this to be ECN.  The
] only time that both TCP reserved bits would be set is on the initial SYN.
] This indicates that the transport layer is ECN-capable.  After the three-way
] handshake, the 0x40 bit should be set alone to alert the sender that
] congestion was detected, and the 0x80 bit should be set alone to inform the
] receiver that the congestion window was reduced in response to the
] congestion notification.  So, these are additional conditions that could be
] used to determine if the reserved bits are set because of ECN or "stealth"
] scanning.  
] 
] 
] 
] Judy Novak
] Johns Hopkins University Applied Physics Lab
] 

i think you may have incorrectly interpreted RFC-2481.  it states:

	Bits 6 and 7 in the IPv4 TOS octet are designated as the
	ECN field.  Bit 6 is designated as the ECT bit, and bit 7 is
	designated as the CE bit.

both of these bits are unused bits in current TOS implementations for
IPv4.  TOS is defined as an 8-bit field; only the first five bits have
any significance; bit 0 must be zero, bits 1-5 are used to indicate one
of the four valid TOS values, and bits 6 and 7 are undefined (except by
RFC-2481, for ECN purposes).  the correct values for these bits are 0x40
and 0x80, not 0x02.

-- 
<cider at ...242...>



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