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 Feedback?
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