Can We Pay for What We Get in 3G Data Access? ACM MOBICOM 2012 Istanbul, Turkey Chunyi Peng, Guan-Hua Tu, Chi-Yu Li, Songwu Lu University of California, Los Angeles

Mobile Data Access is Popular Internet Core Cellular Network 62% US broadband users with wireless data plans; 1.2 billion global users for mobile web. C PENG MOBICOM'12 2

Mobile Data Accounting Internet Cellular Network $$$ Usage-based charging based on data volume e.g., $15 for 200MB for AT&T iPhone Accounting: How much data is actually used? C PENG MOBICOM'12 3

Accounting in 3G Networks Internet 3G Cellular Network BS RNC SGSNGGSN UE V OP_RAW V OP Alice Policy C PENG MOBICOM'12 4 Accounting done at SGSN/GGSN Accounting policy defined by carriers

Internet BS RNC SGSNGGSN UE V OP_RAW Alice Policy 2 Issues in 3G Accounting Question: V UE = V OP ? 1.V UE V OP_RAW ?(accounting architecture) V UE 2. V OP_RAW V OP ? (policy practice) V OP C PENG MOBICOM'12 5

Contributions First work to assess mobile data accounting Largely successful, but pathological cases do exist Study accounting discrepancy between the operators log and the users record Identify 2 extreme cases WE PAY FOR WHAT WE DO NOT GET WE GET WHAT WE DO NOT PAY FOR Explore root causes limitation in accounting architecture Loopholes in policy practice Suggest remedies C PENG MOBICOM'12 6

Methodology Conduct experiments over 2 US carriers Partial validation with 3rd US carrier and 2 operators in China and Taiwan Both extreme and common cases Use Android phones for mobile data access in various test scenarios Accessing accounting records V OP from operators #1: Dial-in for the remaining monthly data usage #2: Online itemized data usage BillAudit: logging usage V C PENG MOBICOM'12 7

The Rest of Talk Overcharging Extreme cases Average cases Root cause: limitation in 3G accounting architecture Undercharging Root cause: Loopholes in policy enforcement Gray areas Discussion and summary C PENG MOBICOM'12 8

WE PAY FOR WHAT WE DO NOT GET 9

Extreme Case: No Signal DL-NS experiment over UDP (1) Issue a UDP-based service (2) Move to a blind zone (3) UDP traffic for t mins (rate: s) V UE V OP V SR Server 3G Network V OP_RAW V UE Result: s = 50Kbps, t = 10 mins V OP V SR = 50K x10 x 60/8 = 3.75MB V UE 0 UEs PAY FOR WHAT THEY DO NOT GET. C PENG MOBICOM'12 10

S = 50 Kbps Time (hours) How Bad the Gap Can Be? Gap = V OP – V UE S × T UDP source S: 50Kbps ~ 8Mbps Duration T: 1min ~ 6 hours lasts at least three hours! Observed gap reaches 450MB (t = 1h, s = 1Mbps)! Operator-I, t = 1min Source Rate (Mbps) C PENG MOBICOM'12 11

Root Cause 12 RNC SGSNGGSN V UE --- V OP 3G accounting decision takes local view at SGSN/GGSN, w/o using feedback from end-host. C PENG MOBICOM'12 12

Still-Bad Case: Even With Signals DL-NS experiments with different signal strength (1) Issue a UDP-based service ( 2) Move to a blind zone (3) UDP traffic for t mins (rate: s) V UE V OP V SR (2) Stay in different zones Server 3G Network RSSI (dBm) Strong-Signal (SS-zone) Weak-Signal (W-zone) Weaker-Signal (WR-zone) No-Signal (NS-zone) C PENG MOBICOM'12 13

Gap Exists Even With Signals! S, Gap RSSI, Gap Cause: Packet drops over radio link. Source Rate (Kbps) UEs PAY FOR WHAT THEY DO NOT GET, though wireless link exists! (Kbps) C PENG MOBICOM'12 14

Still-Bad Case: Intermittent Signals When users lose signals for a while but recover them shortly The gap exists with transient lost links Buffering and retransmission over radio links may reduce the gap (see the paper) UEs PAY FOR WHAT THEY DO NOT GET, when they temporarily (10+ seconds) lose wireless links! C PENG MOBICOM'12 15

So Terrible In Reality ? Good news: Probably not! TCP/App control will teardown it (adjust its incoming rate) Gap for DL-NS over TCP: 2.9 ~ 50KB V OP RNC SGSNGGSN V UE --- V OP C PENG MOBICOM'12 16

Application Behaviors DL-NS tests with 5 applications: Web, Skype, YouTube, PPS streaming, VLC streaming over VPN WebSkypeYouTubePPSVLC Med (MB) Min (MB) Max (MB) Mobile accounting is largely successful in practice. Users may occasionally be overcharged It depends on when and how app control works. C PENG MOBICOM'12 17

Real User Performance Two-week usage for 7 users Operator-IOperator-II User (1day) Apps.MapStock Game Skype, PPS etc. YouTu be, PPS Ebook-YouTub e, PPS V UE V OP Gap %-0.1%3.9%5.3%-0.6%0.4%7.2% YouTube on the train to NYC. C PENG MOBICOM'12 18

3 Views on Overcharging Optimistic view: not too bad in reality, no fix Built-in TCP/application control is sufficient Alternative (Operators) view: not to intend to account the data volume to end-hosts, but the one traversing the core network, no need to fix Security: What if that the data is not what users want? Audit: How to guarantee that inside accounting is correct? Conservative view: need to fix it Users should pay for what they get 3G accounting architecture should not depend on external control C PENG MOBICOM'12 19

Proposals Exploit feedback from devices in accounting decision E.g., using info already collected by cellular networks V RNC_unsent V OP V OP - V RNC_unsent Packet drops 20 RNC SGSNGGSN C PENG MOBICOM'12 20

WE GET WHAT WE DO NOT PAY FOR 21

Loopholes in Accounting Policy Practice BS RNC SGSNGGSN V OP_RAW V OP Policy Policy: Free DNS Service V OP (DNS) = 0 Loophole: A DNS flow should be identified by five tuples (src_addr, dest_addr, src_port, dest_port, protocol ID) But only dest_port (+ protocol ID) is used in practice Policy + Loophole any fake DNS message, or any real data packet using DNS port (53), can be free of charge! V OP (ANY-over-DNS) = 0 C PENG MOBICOM'12 22

Our Findings Free DNS policy enforcement Operator-I: Packets via port 53 are free Operator-II: Packets via UDP+Port 53 are free Exploit DNS tunneling for free data access Proxy server (outside 3G network) relays packets to/from UE via Port-53 Observed: Free data access > 200MB, V OP = 0 No sign to limit free data volume C PENG MOBICOM'12 23

More on Operator Policy Other carriers 3 rd US carrier: free DNS by June 2012, no free after July China/Taiwan carriers: no free DNS service at all Accounting policy is operator specific Other free or differential-pricing policies Free Internet access to a given website Hack: web redirection for free Internet access Free access via a specific Access Point Name (APN) Hack: use this APN, not the default APN Unlimited plans/discounts for Facebook access Similar to web redirection if we can evade Facebook (probably not) C PENG MOBICOM'12 24

Discussion and Proposals Operators have freedom to define their own policy Flexibility to compete in the market Gap between policy and policy enforcement Should be conflict free Otherwise, policy may open loopholes unanticipated Simplest fix: stop free DNS service Negligible DNS traffic volume in normal cases Other options: DNS server authentication Quota Message integrity check Policy C PENG MOBICOM'12 25

GRAY ACCOUNTING AREAS 26

Effect of Middle-boxes Middle-boxes lead to inconsistent accounting views at the core network and the end host Pay for the uplink to a non-existing host due to FTP/HTTP proxy Invalid link V OP > 0 RNC SGSNGGSN Middl e-box C PENG MOBICOM'12 27

Packet Drops over the Internet Misbehaviors over the Internet can incur extra mobile data charging Packet drops over the internet increases volume within cellular networks 28 Packet drops V OP TCP ReTX RNC SGSNGGSN C PENG MOBICOM'12 28

Overhead for Wanted Content V OP covers protocol overhead and app. signaling HTTP redirection: #redirection, V OP significant protocol overhead for sending a short Skype: significant protocol management overhead V OP covers Ads, or whatever users may not expect Hidden cost for free-version applications with more Ads? Security issue? Content-centric charging? C PENG MOBICOM'12 29

Beyond Accounting Revisit charging/accounting design principles Cooperate with Internet? Segmented charging for one data service? Who should pay? Receiver-based, sender-based, or both (current practice)? For what? Volume? Content? Part of content? What if using different pricing schemes? C PENG MOBICOM'12 30

Discussion and Future Work Revisit accounting architecture What failures and losses should be handled? What mechanisms are indispensable for given failures? When and how does the end host report delivery losses? How to ensure that the feedback information is secure and trustworthy? How many mechanisms should be placed into the future cellular network standards? Policy and policy enforcement C PENG MOBICOM'12 31

Summary First assessment of mobile data accounting system over operational 3G networks Largely successful, but also exceptions Accounting discrepancy between the operators log and the users record Identify two extreme cases: WE PAY FOR WHAT WE DO NOT GET WE GET WHAT WE DO NOT PAY FOR Explore root cause in accounting architecture & policy Propose remedy suggestions Many research issues ahead e.g., security, auditing, pricing, … C PENG MOBICOM'12 32