The Trouble with
Tuesday, November 10, 2003
What is a “Trigger”?
A lower layer indication
Also known as a “hint” in DNA
What’s in a word?
A “hint” can be wrong
A trigger implies action
Thought for the day
Implementations MUST be robust in the face of
Bad Hint, No Donut
Using SSID as a “hint” in moving from one private network to
SSID names the ESS, not the network prefix
Result: host moves from one “default” SSID to another,
concludes it is on the same link. Wrong!
Media Sense implemented in Windows 2000
Some applications (foolishly) consumed the “hint”
Result: tear down of TCP connections after momentary
When does link becomes usable for IP connectivity?
IEEE 802.11-1999: Reassociation Response
IEEE 802.11i: Completion of 4-way handshake
DHCP packets go out prior to completion of L2 auth are dropped
TCP connections send, go into slow start
Importance of Robustness
What happens if the “hint” is wrong?
Can the host test the veracity of the “hint”?
Can the host recover if the “hint” is wrong?
Reachability test required regardless
If reachability test disagrees with “hint”, “hint”
For purposes of ALIAS, do we know which
hints are “strong” or “weak”?
How does ALIAS verify hints?
How does ALIAS recover from misleading
Concept: Strong vs. Weak
DNAv4 draft talks about “Strong” vs. “Weak”
Distinction refers to probability of the hint
Goal: encourage creation of “strong” hints
Question: does host behavior change
regardless of the strength of the hint?
DNA: No, reachability detection required anyway
Motto: Trust but Verify