E pluribus unum matchmaking in halo 3
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E Pluribus Unum Matchmaking in Halo 3. Chris Butcher Bungie Studios butcher@bungie.com Game Developers Conference 2008. Overview. What Is Matchmaking? Matchmaking Basics Lessons from Halo 2 Halo 3 Design Goals Voice, Identity, Community Reinforcement

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E Pluribus Unum

Matchmaking in Halo 3

Chris Butcher

Bungie Studios

butcher@bungie.com

Game Developers Conference 2008


Overview

  • What Is Matchmaking?

  • Matchmaking Basics

  • Lessons from Halo 2

  • Halo 3 Design Goals

    • Voice, Identity, Community Reinforcement

    • Skill Measurement and Reward Systems

  • Technical Design

    • TrueSkill

    • Matchmaking Algorithms

  • Recommendations

  • Results from Halo 3 Live Operation


What Is Matchmaking?


Manual Game Browsing

  • User is presented with a list of possible games

  • Tries to find an open slot

  • Tries to find a fair game

  • Inconsistent experience

  • Not good for casual gamers

  • “I Just Want To Play!”


Terminology

  • Manual game browsing is a standard technique

  • Host Game / Join Game options in UI are common

  • Xbox LIVE refers to this as the “Matchmaking” API

    • Quick Match, Custom Match

  • In this presentation, “matchmaking” means an automated peer-to-peer system that organizes players into groups based on user preference

  • Game could still be client / server once the game starts

    • Could even use dedicated servers


Vision of Matchmaking

  • Provide an experience that is:

    Fast

    Reliable

    Consistent

  • Continuous stream of enjoyable games

  • Reward skilled play and also investment of time

  • Don’t give players a reason to stop!


Matchmaking Basics


Matchmaking Ecosystem

  • Continuous stream of groups entering matchmaking

    • Some groups decide to start gathering a game

    • The remainder search for games to join

  • Each group can be multiple machines and players


Xbox LIVE Matchmaking Service

  • Gatherers register with XBL service

  • Each group has unique matchmaking desires

    • Type of game, skill level, spoken language, etc

  • Searchers query service with parameter filters

  • Service returns matching candidates


Candidate Evaluation

  • Searcher evaluates all candidates in parallel, best matches first

    • Ping network connectivity, get current group state

    • Measure quality of connection using Xbox QoS probes

    • Group-to-group XNetConnect

    • Group-to-group session join

  • Network layer handles as asynchronous processes


Matchmaking Life Cycle

  • Groups enter Matchmaking continuously

  • Each group chooses to gather or search

    • Gather: register session with XBL service

    • Search: query service for candidates

  • Search, evaluate candidates, try to join

  • If no suitable candidates, search again

  • Halo specific game flow:

    • Gatherer waits until game is full

    • Determine game settings, host selection

    • Start game


Lessons from Halo 2


Halo 2 has had good longevity

  • Year-on-Year retention is > 80%


Game is well suited to Matchmaking

  • Small-group gameplay (2-5 per team)

  • Interact with friends in your group

  • Both coordinated effort and individual skill required

  • Opponents are anonymous and interchangeable

  • Long term goals are self-driven rather than peer-driven

    • I want to reach Level 30

    • Not: I want to be the best on my server


Lessons Learned - Matchmaking

  • Received well by the majority of players

  • Always something to do, a mix of novelty and the familiar

  • Configurable experience allows longevity

  • Required several early updates to operate robustly

  • DLC maps locking people out was a problem

  • People don’t like feeling they have no control

  • International experience was poor


Lessons Learned - Skill System

  • Modified ELO rating system

  • Both a skill measurement and also reward for investment

    • Non-zero-sum for levels 1-20 to give a “hill-climbing” experience

    • Was abused through boosting

    • Zero-sum competition for advancement

    • Skill level achievement is always in jeopardy

    • Leads to anxiety, anger and frustration in players

    • “WTF I lost my level 30, my team sucks”


Ranked Matchmaking in Halo 2

Hyper-Competition

+

Anonymity

+

Loss Anxiety

=

Negative Emotional Pressure


Ranked Matchmaking in Halo 2


Design Goals for Halo 3


Overall Goals

  • Make the online experience approachable

  • Provide accountability and identity

  • Give players a reason to keep coming back

  • Tools:

    • Voice

    • Identity

    • Skill System

    • Reward System

    • New Player Experience


Voice Design

  • Can’t predict how players will use voice

    • Give listeners control over what they hear

  • Remove temptation to use voice negatively

  • Allow time for socialization that isn’t under pressure

    • Make it easy for players to opt out or mute

  • Positive: Chatting idly with friendly strangers

  • Negative: Being abused by hostile anonymous bigots


Identity Design

  • Every player has a public Service Record

  • Persistent individual identity reduces anonymity

    • Goal is to reduce anonymity and provide long-term identification

    • Publicly accessible in-game to everyone

    • Reduce sock-puppeting that was prevalent in Halo 2

  • Rewards are individual

    • Success recognized directly, or via social comparison with friends

    • No global leaderboards!

    • Primarily competing with yourself


Skill System Design

  • Range 1-50; everyone starts at level 1

    • Almost everyone gains levels quickly, providing positive feedback

  • After 50-100 games, skill level stabilizes

    • Needs to still feel dynamic and not stagnant.

    • But shouldn’t “lose a level” from one bad game.

  • Skill should be a statistic, not a reward


Reward System Design

  • Reward for playing

    • “Experience points” (XP)

    • Only for wins, to prevent boosting

    • Penalty for quitting games early

  • Experience rating hierarchy

    • Ratings require both skill and XP

  • Emphasized in UI over skill

  • Permanent; no loss anxiety


New Player Experience

  • Separate “Boot Camp” playlist for new players only

    • Limited set of maps and weapons to ease players in

    • Small groups for socialization

  • Reward early and often!

  • Move skilled players out quickly

    • 5 wins triggers ‘graduation’


Technical Design (Skill)


Xbox LIVE Skill System – TrueSkill

  • A mathematical library implemented on XBL back end

    • Bayesian estimation techniques developed by Microsoft Research Cambridge

  • Models player skills as probability density functions [µ, σ]

    • µ is mean (current estimate), σ is standard deviation (uncertainty)

  • TrueSkill is stored and updated invisibly by XBL back end


Using TrueSkill in Halo 3

  • Don’t show players the raw mathematics of [µ, σ]

    • Use skill lower bound: s = µ - kσ (we chose k=4)

    • Transform by remap function into range 1-50 for display in UI


Customizing TrueSkill

  • Mathematical configuration variables

    • β (performance factor),  (dynamics factor), draw probability

    • Left β alone: dangerous, affects final skill distribution

    • Increased  so that players’ skill never fully converges

    • Draw probability must be accurate, if it is set too low then ties will be considered highly significant

  • Update Weight – modifies rate of change of [µ, σ]

  • We used this to give players a “hill-climbing” experience by initially decreasing their TrueSkill update weight

    • Weights start out small and return to normal over 50-100 games in a playlist

    • Even though we can identify good or bad players after 8 games, it is more satisfying for them to feel they earned their skill over time


TrueSkill Summary

  • Advantages

    • Already implemented for you by Xbox LIVE

    • Converges quickly

    • Provides good estimate of player skill for matchmaking

    • TrueSkill developers are very helpful and knowledgeable

  • Disadvantages

    • Complex mathematics, takes an expert to understand and tweak

    • Hard to predict overall convergence of system

    • Default behavior does not fit our ideals for a skill system

    • Vulnerable to exploitation, both real and perceived (ties increase rank)

  • This is a hard problem with no clear solution


Technical Design (Matchmaking)


Search Criteria

  • Use precise initial query parameters to find an ideal match

    • Skill, Experience, Network Connection Quality

  • Initial queries are less likely to find a match

  • Allows tight matches in large populations

  • Query parameters must include all selection criteria

    • Halo 2 had some criteria that were not stored in XBL service

    • Searcher spent time querying candidates that they would never want to join e.g. due to spoken language

    • Wastes bandwidth and also wastes precious search time


Search Expansion

  • ‘Fuzzy match’ in many dimensions

    • Analog parameters (skill, experience, network connection)

    • Binary parameters (language, country, DLC maps)

  • Treat binary parameters as “soft filters”

  • Expansion has multiple phases

    • Look for ideal match, expand analog filter a bit

    • Remove binary “soft filters”

    • Expand analog filter out to max, relax connection quality

    • Keep trying intermittently, switch to gathering


Ecosystem Balance

  • Must have good balance of searchers and gatherers

  • Halo 2: Easy to model in theory, impossible in practice

    • Global Internet network properties

    • Latency in Live service updating

    • Network engine internals (time to discover, time to join)

  • Expire lists of candidates quickly

    • Searchers are in a race to join limited set of active games

  • Make the ecosystem adaptive

    • Gatherers can also search

    • If nobody is joining you, you have a chance to join someone

    • Ecosystem can adaptively balance for low-population scenario


Recommendations


Is Matchmaking Right For You?

  • Works with different genres

  • Works with different game models

    • Could match into games in progress

    • Could use dedicated servers

  • Scales to wide range of user populations

    • Halo 3 playlists range from < 1k to 100k concurrent users

  • Significant investment in client software

    • 2-3 developers for project lifecycle

  • Back end functionality optional but helps a lot

  • Payoff comes from building a lasting community


Design For Security

  • Users have no control so you must provide safe games for them

  • Every aspect of your game will be attacked

    • Hardware attacks, network attacks (bridging, standby, DoS), game attacks (modified content, LSP interception), exploits (skill de-leveling, out-of-map), many more

  • Halo 2 required five updates over three years for security

  • This is an entire talk in itself


Test Early, Test Often

  • MS-Internal Alpha and Beta (10k) – 11/06 and 4/07

  • Rich text data mining as primary feedback

    • Searchable centralized logging system with event severities

    • Find hard bugs in client (edge cases, crashes, network protocol)

    • Transcontinental Matchmaking and network testing

  • Public Beta (900k) – 5/07

    • PR boost, some gameplay feedback also

    • Tune TrueSkill distribution curves on real player skill mix

    • Load balancing of LSP servers to avoid Day 1 meltdown

  • High population games must involve XBL in testing

    • Easy to create a scalability problem on back end


Collect Data From Production

  • You will need retail environment instrumentation

    • Can’t use data mining – too much data to store and transmit

    • For Halo 2 launch we had no alternative

  • Halo 3 uses special-purpose binary uploads

    • End-of-game report for bungie.net analysis

    • Matchmaking status report for ecosystem diagnosis

    • Network QoS report for research

  • Volume of data is massive, we discard 90%+

    • Slim fire-and-forget stateless HTTP-over-XLSP upload

    • Per-machine settings for deep investigation


Results


Results – Overall

  • Deployed successfully, no client update needed

    • Some normal LSP scalability balancing in first few days

  • Reviews mention MP as “streamlined, transparent”

  • High penetration of online multiplayer

    • 5.9M unique users observed on Xbox LIVE

    • 5.2M have played in Matchmaking (88%)

  • Approximately 3x Halo 2 peak concurrency

    • 560k peak concurrent users in 15 playlists

  • Longevity is an open question

    • Tracking steady at 1.2M unique users per week


Results – Launch


Results – Player Community

  • Skill numbers that you can actually believe in!

  • Perception is that level 50 means skilled, not a cheater

  • Some experience boosting

    • We assumed it would be possible to circle boost XP

    • There were ways that let you do it many times faster than normal

  • No way to advance to higher ratings without ranked play

    • This was probably a mistake

  • Player identity features very well received

  • Social community seems to be better than Halo 2


Results – MP Game Selection

Custom Games: 16%Matchmaking: 84%


Results – Player Retention


Results – Skill in Team Slayer


Results – Overall Skill


Results – Experience Rating


Future Design Thoughts

  • New player experience was good, not great

    • Implemented late, needs goal-driven UI flow

    • 58% of players went on to play 100 games or more

    • But 19% of players stopped after < 20 games

  • Online model is focused on skill improvement

    • But most players don’t care about skill, they like reward better

  • Negative behavior was reduced somewhat

    • Tremendous amount of room for improvement

    • Reputation and social history as part of public player identity

    • Empowering players to change their experience is the right path


Future Technical Thoughts

  • Starting to feel like a solved problem technically

  • Ecosystem could be more self adjusting

    • Still some search / gather balance issues

  • Move more of the ecosystem to a centralized service?

  • Ubiquitous Matchmaking?

    • Not just as explicit UI

    • Invisible fabric of online experience

    • Peer-to-peer is the future


Credits

  • Bungie

    • This system is the work of many people

    • Design, Networking, UI, bungie.net, more

  • Microsoft Research

    • MSR Cambridge Applied Games Group (TrueSkill)

    • MSR Networking Research Group (QoS data analysis)

  • Microsoft Game Studios

  • Xbox Platform

    • XDC (XNA Developer Connection)

    • Xbox LIVE Team

    • Xbox LIVE Operations Team


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