CSE 534 Content Distribu3on Networks

Transcription

1 CSE 534 Content Distribu3on Networks Vyas Sekar Stony Brook University Based on content from: Srini Seshan 1

2 Assigned reading The Akamai Network Nygren, Sitaraman, Sun Improving Performance on the Internet Tom Leighton Globally distributed content delivery Dilley et al 2

3 Likes/Dislikes? Like? This is real Interes3ng use of overlay/dns to innovate Actual deployable applica3ons for research we do Lot of web/video growth due to CDN Dislike? Low on technical details What was challenging? Lessons? Open research ques3ons? 3

4 Outline CDN Overview and mo3va3on How does a CDN work? Lessons from Akamai Network 4

5 Mo3va3on for CDN Quality expecta3ons rising Web, video Scalability Flash crowds Network is unreliable Infrastructure is sta3c Hard to change end hosts + routers 5

6 Content Distribution Networks Content providers are the CDN customers. CDN installs hundreds of servers Close to users origin server in North America CDN distribution node CDN servers replicate its customers content in CDN servers. When provider updates content, CDN updates servers CDN server in S. America CDN server in Europe CDN server in Asia 6

7 Builds on Old Idea Web Proxy Caches User configures browser: Web accesses via cache Browser sends all HTTP requests to cache Object in cache: cache returns object Else cache requests object from origin server, then returns object to client client client Proxy server origin server 7

8 Alterna3ve CDN architectures? Centralized hos3ng middle mile Big data center CDNs P2P latency Management, upload constrained Distributed CDNs like Akamai 8

9 Benefits to Ecosystem Content providers Performance Scalability Economies of scale End users Performance (low latency, higher bandwidth) Network service providers Reduced transit costs 9

10 Outline CDN Overview and mo3va3on How does a CDN work? Lessons from Akamai Network 10

11 Challenges in CDN design How/where to replicate content? On demand? Proactive/prefetch? everywhere or close to demands? How to choose/direct replicas What metrics? Latency, Load DNS, HTTP 304 response, anycast, etc. 11

12 How is content replicated? Akamai only replicates static content Serves a significant fraction of Internet traffic 15 30% of all web traffic! Akamai server is asked for content First checks local cache If not in cache, requests file from primary server and caches file Also: akamai has tiered hierarchy Further reduce load on origin server 12

13 Server Selection Service is replicated in many places in network Which server? Lowest load à to balance load on servers Best performance à to improve client performance Based on Geography? RTT? Throughput? Load? Any alive node à to provide fault tolerance How to direct clients to a particular server? As part of routing à anycast, cluster load balancing As part of application à HTTP redirect As part of naming à DNS 13

14 Cons of rou3ng + applica3on Rou3ng based Cannot incorporate applica3on policy, metrics E.g., load, latency? Stateful connec3ons? Applica3on based Extra RTT Could interact poorly with some browser implementa3ons 14

15 Naming Based Client does name lookup for service Name server chooses appropriate server address What information can it base decision on? Server load/location à must be collected Name service client Typically the local name server for client 15

16 How Akamai Works Clients fetch html document from primary server E.g. fetch index.html from cnn.com URLs for replicated content are replaced in html E.g. <img src= > replaced with <img src= x.gif > Client is forced to resolve axyz.g.akamai.net hostname 16

17 How Akamai Works Root server gives NS record for akamai.net Akamai.net name server returns NS record for g.akamai.net Name server in region of client s name server TTL is large G.akamai.net NS choses server in region Maps axyz name to a physical server Should try to chose server that has file in cache How to choose? TTL is small 17

18 Akamai E2E Walkthrough cnn.com (content provider) DNS root server End-user Get foo.jpg Get 10 index. html Get /cnn.com/foo.jpg Akamai high-level DNS server Akamai low-level DNS server Akamai server 18

19 Akamai Subsequent Requests cnn.com (content provider) DNS root server Get index. html 1 2 Akamai high-level DNS server End-user Get /cnn.com/foo.jpg Akamai low-level DNS server Akamai server 19

20 Digression: How to choose server.. Some cool theory underlying CDN Consistent hashing Has other applica3ons as well Key- value stores DHTs (not covered, check out Chord) P2P lookup (not covered) 20

21 Simple Hashing Given document XYZ, we need to choose a server to use Suppose we use modulo Number servers from 1 n Place document XYZ on server (XYZ mod n) What happens when a servers fails? n à n-1 Why might this be bad? 21

22 Desired Properties view = subset of all hash buckets that are visible Smoothness little impact on hash bucket contents when buckets are added/removed Spread small set of hash buckets that may hold an object regardless of views Load balance across all views # of objects assigned to hash bucket is small 22

23 Consistent Hashing map both keys and nodes to the same (metric) identifier space find a rule how to assign keys to nodes Ring is one option. 23

24 Consistent Hash Example Assign each of C hash buckets to random points on mod 2 n circle, where, hash key size = n. Map object to random position on unit interval Hash of object = closest bucket Smooth à addition of bucket does not cause movement between most existing buckets Spread à small set of buckets that lie near object Balance à no bucket is responsible for large number of objects Bucket 8 4

25 Outline CDN Overview and mo3va3on How does a CDN work? Lessons from Akamai Network 25

26 Akamai driving design principles Reliability Borrow from distributed systems literature Scalability Always growing Minimize human involvement Automated management, fault tolerance Performance Many network, server, protocol level op3miza3ons 26

27 Common op3miza3ons Improve transport layer Custom TCP op3miza3ons Beger rou3ng Exploit overlay proper3es to avoid BGP inefficiency Prefetch Assemble/Offload to edge Compression + delta encoding 27

28 Other akamai services Security (ddos defense) Netsession (hybrid cdn- p2p) Media delivery Mobile op3miza3ons 28

29 Open ques3ons.. Does video change things? Not much dynamic But more bandwidth? Do you really need global reach? Why cant ISPs do the same thing? Who pays for bandwidth? How does cloud change things? 29

30 Key Takeaways What s a CDN? Globally distributed delivery infrastructure Different candidate architectures Centralized, data centers, Akamai- style, P2P How do CDNs work in prac3ce? Extensive use of DNS based redirec3on Intelligent server selec3on techniques Lots of op3miza3ons Not just sta3c content dynamic web hos3ng Where are we headed next? Provider CDN, ISP federa3on, Cloud CDN 30