802.11r Requirements Discussion Month 2000 doc.: IEEE 802.11-00/xxx May 2004 802.11r Requirements Discussion Bob Beach, Symbol Technologies Nancy Cam-Winget, Cisco Systems Darwin Engwer, Nortel Networks Haixiang He, Nortel Networks Robert Love, LAN Connect Consultants Jesse Walker, Intel Corporation Jesse Walker et al John Doe, His Company

May 2004 What is Fast Roaming? Actually the first question is: what is roaming in general? 802.11 really does not talk about the roaming process at all Protocols are defined but their usage is not Roaming is a complex problem, fast roaming is even harder We need a model for roaming before we can talk about requirements. Jesse Walker et al

Roaming in the “Old Days” May 2004 Roaming in the “Old Days” Find an AP by listening to beacons and/or doing probes The number of APs is small and all lead to the same wired network Just pick one, maybe with the best RSSI value Do Assoc Request/Response handshake The new AP sends out an empty message that unassociates STA with the old AP if STA didn’t do it first (and no one did) Security is “free”: Open or shared WEP keys Don’t do “Authenticate” Sequence All APs are trustworthy If STA gets into trouble, start over again Jesse Walker et al

Roaming now Find APs like before, but now: May 2004 Roaming now Find APs like before, but now: There could be hundreds of them Connected to different networks Many of which have the same ESS Some of which are rogues or honey pots Getting “associated” is the easy part But then comes “11i”, 802.1x, authentication,… And then comes “11e”, TSPECs, …. And after all that maybe STA changes its mind and decides that the old AP was better after all And now it has to start over again on the old one Who may not want you back! Don’t even ask what happened to your data in this process 802.11r exists because 802.11i broke roaming Jesse Walker et al

May 2004 The Plan In order to make sense of “fast roaming” we need to define a model for roaming in general. By having such a model we can partition the problem and hence discover what makes roaming fast or slow and how we can improve the roaming Jesse Walker et al

May 2004 A Model for Roaming The roaming process consists of four independent but related activities Some run all the time, others run only on certain occasions The model works for both unassociated and associated mobile units Jesse Walker et al

The Roaming “Process” Build a “Data Base” of APs May 2004 The Roaming “Process” Build a “Data Base” of APs Evaluate the current AP against all of the possibilities in the AP data base Try to transition to a “better” AP Post transition cleanup Jesse Walker et al

Building the AP Data Base May 2004 Building the AP Data Base Mobile STAs build a data base of known APs There could be hundreds of them STA maintains a separate state machine for each AP State machines not independent – STA can be associated with only one AP at a time They get this information from several sources Listening to beacons Doing probe/probe response sequences Configuration information Information from “third parties” The amount of information about any given AP can vary but the more the better But the amount of data not be the same for all APs The type of data may not be uniform for all APs Jesse Walker et al

Interesting things about an AP May 2004 Interesting things about an AP ESSid, BSSid, Channel/Frequency Security options, keys Provider Last/average received RSSI Last Contact Time (local time and TSF) Association “State”/History Load information Other useful things “Bad experiences” (e.g. it’s a rogue, it refused my association) Things that disqualify it (wrong security, wrong network) Jesse Walker et al

The Information will change May 2004 The Information will change As the mobile STA moves, the set of available APs may change, as will their RSSI The APs themselves can also change Load will vary as Mobile STAs do different things Channel/output power may change The data base must be updated on a regular basis The frequency depends on the state of the device as well as the what applications may be running Jesse Walker et al

Evaluating the State of the World May 2004 Evaluating the State of the World Given the Mobile STA has built the AP data base, it must evaluate its current association against all the possibilities present in the AP data base “Is the grass greener over there?” The evaluation criteria is not constant but varies over time based on several factors such as: State of current association Application requirements Jesse Walker et al

The most basic criteria May 2004 The most basic criteria Is there a current association? If not, Did things just change? Was STA in the middle of something and really need one Maybe its not important and STA should just give up for now But when should STA try again?? Jesse Walker et al

Given STA is associated May 2004 Given STA is associated What does STA need from the association? Basic connectivity (nothing going on, just listening to beacons) Doing “plain old data transfer things” Transactions, web browsing, file transfer Want good transfer rate but will accept what I can get Doing time critical things that are all or nothing Voice is a good example – 95% of needed throughput is useless Jesse Walker et al

How painful is it to roam? May 2004 How painful is it to roam? Which is more painful? Staying where STA is now, or Going through the roaming process? If some target AP is in another SSID, then should STA expect a full authentication be required, and transition will not be fast? If STA roams Need to go through the process It will take time Will that disrupt current activity? It may not succeed STA might get a new IP address Jesse Walker et al

May 2004 The Roaming Process Having made the decision to roam, we now have to do it. It will take time and it may not succeed The process consists of at least three substeps Failure is possible at any step Jesse Walker et al

The Steps to a Successful Roam May 2004 The Steps to a Successful Roam The new AP accepts by association request May get rejected for one reason or another Successfully authenticate and get valid key information Lots of failures possible here Obtain the level of service needed May be a non-issue or big issue Jesse Walker et al

Its not over until its over May 2004 Its not over until its over A roam is not successful unless all steps are successful A failure after basic association sequence can leave both AP and mobile STA in odd states Am I associated or not?? 802.11 is a little ambiguous on this point “Association” is complete after initial handshake But its not usable until after 11i and 11e is done Jesse Walker et al

The final step: Cleanup May 2004 The final step: Cleanup After the step 3 is done, the Mobile STA is connected to the new AP We still need to do things Let the old AP know so that it stops forwarding data over the air interface to the STA Let the old AP know to stop billing the STA STA updates the local AP data base to reflect the new association Jesse Walker et al

STA Model Per-AP Pre-Transition Discovery Per-AP Data Base May 2004 STA Model Per-AP Pre-Transition Discovery Per-AP Data Base Per-AP Transition Per-AP Post-Transition Jesse Walker et al

Scope APs in the same ESS (same SSID) May 2004 Scope APs in the same ESS (same SSID) APs might support multiple media, viz .11a, .11b, .11g Jesse Walker et al

Requirements we will not address May 2004 Requirements we will not address Algorithms for determining which AP to chose or when to transition to another AP. Except to exclude pathological solutions that lead network to collapse. PHY layer specific issues Others? Jesse Walker et al

Requirements we will address May 2004 Requirements we will address Reliability of the transition process QoS Fall-Back mechanisms (to alternate slower mechanisms) Handover algorithm between APs. Define infrastructure conditions for fast transition Requirement on Transition Time Information collection flexibility (scanning) PHY awareness and its implications Define security implications and insure preservation of security level if required Characterize transition to understand when the transition is complete Others? Jesse Walker et al

May 2004 Feedback? Jesse Walker et al