ALIAS

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					  The Trouble with
          Triggers
                    Bernard Aboba
                          Microsoft
     Draft-ietf-dhc-ipv4-dna-04.txt
                       ALIAS BOF
                           IETF 58
                  Minneapolis, MN
    Tuesday, November 10, 2003
http://www.drizzle.com/~aboba/IETF58/ALIAS/
    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
        misleading “hints”
Bad Hint, No Donut
   Using SSID as a “hint” in moving from one private network to
    another
     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!
   “Link down”
     Media Sense implemented in Windows 2000
     Some applications (foolishly) consumed the “hint”
     Result: tear down of TCP connections after momentary
       connectivity loss
   “Link up”
     When does link becomes usable for IP connectivity?
     IEEE 802.11-1999: Reassociation Response
     IEEE 802.11i: Completion of 4-way handshake
     Result:
         DHCP packets go out prior to completion of L2 auth are dropped
         TCP connections send, go into slow start
Importance of Robustness
   Questions:
       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?
   Example: DNA
       Reachability test required regardless
       If reachability test disagrees with “hint”, “hint”
        loses
Some Questions
   For purposes of ALIAS, do we know which
    hints are “strong” or “weak”?
   How does ALIAS verify hints?
   How does ALIAS recover from misleading
    hints?
Concept: Strong vs. Weak
Hints
   DNAv4 draft talks about “Strong” vs. “Weak”
    hints
   Distinction refers to probability of the hint
    being misleading
       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
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