Indirection. science can be solved by adding another level of indirection" -- Butler Lampson. "Every problem in computer

Transcription

1 Indirection Indirection: rather than reference an entity directly, reference it ( indirectly ) via another entity, which in turn can or will access the original entity A x B "Every problem in computer science can be solved by adding another level of indirection" -- Butler Lampson 1

2 Multicast: one sender to many receivers Multicast: Act of sending datagram to multiple receivers with single transmit operation Analogy: One teacher to many students Question: How to achieve multicast Multicast routers (red) duplicate and forward multicast datagrams Network multicast Router actively participate in multicast, making copies of packets as needed and forwarding towards multicast receivers 2

3 Internet Multicast Service Model multicast group multicast group concept: use of indirection hosts addresses IP datagram to multicast group routers forward multicast datagrams to hosts that have joined that multicast group 3

4 Multicast groups q Class D Internet addresses reserved for multicast: q Host group semantics: o anyone can join (receive) multicast group o anyone can send to multicast group o no network-layer identification to hosts of members q Needed: Infrastructure to deliver mcast-addressed datagrams to all hosts that have joined that multicast group 4

5 Joining a mcast group: Two-step process Local: Host informs local mcast router of desire to join group: IGMP (Internet Group Management Protocol) Wide area: Local router interacts with other routers to receive mcast datagram flow many protocols (e.g., DVMRP, MOSPF, PIM) IGMP IGMP wide-area multicast routing IGMP 5

6 Multicast via Indirection: Why? Don't need to individually address each member in the group: header savings Looks like unicast; application interface is simple, single group Abstraction, delegating works of implementation to the routers More scalable because, sender doesn't manage the group, as receivers are added, new receivers must do the work to add themselves 6

7 Mobility and Indirection How do you contact a mobile friend? Consider friend frequently changing addresses, how do you find her? Search all phone books? Call her parents? Expect her to let you know where he/she is? I wonder where Alice moved to? 7

8 Mobility and indirection: Mobile node moves from network to network Correspondents want to send packets to mobile node Two approaches: Indirect routing: Communication from correspondent to mobile goes through home agent, then forwarded to remote Direct routing: Correspondent gets foreign address of mobile, sends directly to mobile 8

9 Mobility: Vocabulary Home network: permanent home of mobile (e.g., /24) Home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote Permanent address: address in home network, can always be used to reach mobile e.g., wide area network correspondent 9

10 Mobility: more vocabulary Permanent address: remains constant (e.g., ) Visited network: network in which mobile currently resides (e.g., /24) Care-of-address: address in visited network. (e.g., 79, ) wide area network Correspondent: wants to communicate with mobile Foreign agent: entity in visited network that performs mobility functions on behalf of mobile. 10

11 Mobility: registration home network visited network 2 wide area network foreign agent contacts home agent home: this mobile is resident in my network 1 mobile contacts foreign agent on entering visited network End result: Foreign agent knows about mobile Home agent knows location of mobile 11

12 Mobility via Indirect Routing home network home agent intercepts packets, forwards to foreign agent foreign agent receives packets, forwards to mobile 3 visited network correspondent addresses packets using home address of mobile 1 wide area network 2 4 mobile replies directly to correspondent 12

13 Indirect Routing: comments Mobile uses two addresses: Permanent address: used by correspondent (hence mobile location is transparent to correspondent) Care-of-address: used by home agent to forward datagrams to mobile Foreign agent functions may be done by mobile itself Triangle routing: correspondent-home-network-mobile Inefficient when correspondent, mobile are in same network 13

14 Indirect Routing: moving between networks Suppose mobile user moves to another network Registers with new foreign agent New foreign agent registers with home agent Home agent update care-of-address for mobile Packets continue to be forwarded to mobile (but with new care-of-address) Mobility, changing foreign networks transparent: Ongoing connections can be maintained! 14

15 Mobility via Direct Routing home network correspondent forwards to foreign agent foreign agent receives packets, forwards to mobile 4 visited network correspondent requests, receives foreign address of mobile 2 1 wide area network 3 4 mobile replies directly to correspondent 15

16 Mobility via Direct Routing: comments Overcome triangle routing problem Non-transparent to correspondent: Correspondent must get care-of-address from home agent What happens if mobile changes networks? 16

17 Mobile IP RFC 3220 Has many features we ve seen: home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet) 3 components to standard: agent discovery registration with home agent indirect routing of datagrams 17

18 Mobility via indirection: why indirection? Transparency to correspondent Mostly transparent to mobile (except that mobile must register with foreign agent) transparent to routers, rest of infrastructure potential concerns if egress filtering is in place in origin networks (since source IP address of mobile is its home address): spoofing? 18

19 An Internet Indirection Infrastructure Motivation: Today s Internet is built around point-to-point communication abstraction: Send packet p from host A to host B One sender, one receiver, at fixed and well-known locations not appropriate for applications that require other communications primitives: multicast (one to many) mobility (one to anywhere) anycast (one to any) We ve seen indirection used to provide these services Idea: Make indirection a first-class object 19

20 Internet Indirection Infrastructure (I3) Change communication abstraction: Instead of point-to-point, exchange packets by name each packet has an identifier ID to receive packet with identifier ID, receiver R stores trigger (ID, R) into network triggers stored in network overlay nodes send(id, data) send(r, data) Sender trigger Receiver (R) ID R 20

21 Service Model API sendpacket(p); inserttrigger(t); removetrigger(t); // optional best-effort service model (like IP) triggers periodically refreshed by end-hosts reliability, congestion control, flow-control implemented at end hosts, and trigger-storing overlay nodes 21

22 Discussion Trigger is similar to routing table entry Essentially: Application layer publish-subscribe infrastructure Application-level overlay infrastructure Unlike IP, end hosts control triggers, i.e., end hosts responsible for setting and maintaining routing tables Provide support for mobility multicast anycast composable services 22

23 Mobility Receiver updates its trigger as it moves from one subnet to another mobility transparent to sender location privacy send(id,data) send(r1, data) Sender ID R1 Receiver (R1) 23

24 Mobility Receiver updates its trigger as it moves from one subnet to another mobility transparent to sender location privacy send(id,data) Sender ID R1 send(r2, data) Receiver (R2) 24

25 Multicast Unifies multicast and unicast abstractions multicast: receivers insert triggers with same ID Application naturally moves between multicast and unicast, as needed impossible in current IP model send(id,data) send(r1, data) Sender ID R1 ID R2 send(r2, data) Receiver (R1) Receiver (R2) 25

26 Anycast (cont d) Route to any one in set of receivers Receivers i in anycast group inserts same ID, with anycast qualifications send(r1,data) Sender send(id a,data) ID s 1 R1 ID s 2 R2 ID s 3 R3 Receiver (R1) Receiver (R2) Receiver (R3) 26

27 Composable Services Use stack of IDs to encode successive operations to be performed on data (e.g., transcoding) Don t need to configure path between services S_MPEG/JPEG send((id_mpeg/jpeg,id), data) send(id, data) send(r, data) Sender (MPEG) ID_MPEG/JPEG S_MPEG/JPEG ID R Receiver R (JPEG) 27

28 Composable Services (cont d) Both receivers and senders can specify operations to be performed on data send(id, data) S_MPEG/JPEG send(r, data) Sender (MPEG) ID_MPEG/JPEG S_MPEG/JPEG send((id_mpeg/jpeg, R), data) ID (ID_MPEG/JPEG, R) Receiver R (JPEG) 28

29 Discussion of I3 How would receiver signal ACK to sender? What is needed? Does many-to-one fit well in this paradigm? security, snooping, information gathering: what are the issues? 29

30 Content Delivery Networks: Indirection with DNS 30

31 Internet Content Content is static web pages and documents images and videos, streaming, Content becomes more and more important! 500 exabytes (10 18 ) created in 2008 alone [Jacobson] Estimated inter-domain traffic rate: 39.8 TB/s [Labovitz] Annual growth rate of Internet traffic: ~40%-60% [Labovitz] Much of web growth due to video (Flash, RTSP, RTP, YouTube, etc.) How to deliver content? How to cope with growth of content? Following slides adapted from Wolfgang Mühlbauer 31

32 Application mix Source: Alexandre Gerber and Robert Doverspike. Traffic Types and Growth in Backbone Networks. AT&T Labs Research

33 Application mix in 2009 HTTP dominates Inside HTTP Flash-video dominates Images and RAR files next Source: Gregor Maier et al. On Dominant Characteristics of Residential Broadband Internet Traffic. IMC

34 Prevalence of CDNs 30 (out of ~30000) ASes contribute 30% of inter-domain traffic July 2009: CDNs originate at least 10% of all inter-domain traffic Top ten origin ASes in terms of traffic Source: Craig Labovitz et al. Internet Inter-Domain Traffic. SIGCOMM

35 Why not Serving Content from One s Own Site? Enormous demand for popular content Cannot be served from single server Bad performance Due to large distance: TCP-througput depends on RTT! Bad connectivity? Single point of failure High demand leads to crashes or high response times (e.g., flash crowds) High costs Bandwidth and disk space to serve large volumes (e.g., videos) content AS B AS C Download AS A AS D consumer AS E 35

36 Approaches to Content Delivery Idea: replicate content and serve it locally Replicate Centralized hosting Download content from closest location Content distribution networks (CDN) Offload content delivery to large number of content servers Put content servers near end-users Peer-to-peer networks In theory: infinite scalability Yet: download capacity throttled by uplink capacity of end users 36

37 Akamai A Large CDN Akamai (Hawaiian: intelligent ) Evolved out of MIT research effort: handle flash crowds > Servers located in 72 countries, > 1000 ASs Customers: Yahoo!, Airbus, Audi, BMW, Apple, Microsoft, etc. Why using Akamai? Content consumer: Fast download Content provider: Reduce infrastructure cost, quick and easy deployment of network services Task of CDNs: Serve content Static web content: HTML pages, embedded images, binaries Dynamic content: break page into fragments; assemble on Akamai server, fetch only noncacheable content from origin website: Applications: audio and video streaming 37

38 Akamai: Is the Idea Really That Novel? Local server cluster Bad if data center or upstream ISP fails Mirroring Deploying clusters in a few locations Each mirror must be able to carry all the load Multihoming Using multiple ISPs to connect to the Internet Each connection must be able to carry all the load Akamai vastly increases footprint monitors and controls their worldwide distributed servers directs user requests to appropriate servers handles failures 38

39 Akamai Relies on DNS Redirection Example: Access of Apple webpage ( Pictures are hosted by Akamai: images.apple.com Type: dig images.apple.com into your Linux shell [ ] ;; ANSWER SECTION: images.apple.com IN CNAME images.apple.com.edgesuite.net. [more CNAME redirections] images.apple.com.edgesuite.net.globalredir.akadns.net IN CNAME a199.gi3.akamai.net. a199.gi3.akamai.net. 10 IN A a199.gi3.akamai.net. 10 IN A [ ] DNS redirects request to DNS servers controlled by Akamai! 39

40 Akamai Deployment Edge server organized as content cluster in many Autonomous Systems multiple servers local low-level DNS server Client directed to closest server Content cluster Content cluster Content cluster Content cluster 40

41 How does Akamai Work? (simplified) Root DNS Server Top-Level Domain DNS Server Apple Authoritative DNS Server IP address Local DNS Server Normal web request: First DNS resolval Then HTTP connection IP address Apple Web Server http request/response Web Client Slide adapted from Drafting Behind Akamai, Sigcomm

42 How does Akamai Work? (simplified) Root DNS Server Top-Level Domain DNS Server images.apple.com? DNS request for "Akamized" content: Results in CNAME Apple Authoritative Local DNS Server DNS Server CNAME: images.apple.com.edgesuite.net. images.apple.com? Apple Web Server Web Client Slide adapted from Drafting Behind Akamai, Sigcomm

43 How does Akamai Work? (simplified) Top-Level Domain DNS Server Root DNS Server images.apple.com.edgesuite.net? images.apple.com? Akamai High-Level DNS Server images.apple.com.edgesuite.net? CNAME: a199.gi3.akmai.net Apple Authoritative Local DNS Server DNS Server CNAME: images.apple.com.edgesuite.net. Akamai Low-Level DNS Server Apple Web Server images.apple.com? Web Client Akamai Edge Server Slide adapted from Drafting Behind Akamai, Sigcomm

44 How does Akamai Work? (simplified) Top-Level Domain DNS Server Root DNS Server a199.gi3.akmai.net? Akamai High-Level DNS Server images.apple.com.edgesuite.net? CNAME: a199.gi3.akmai.net images.apple.com? a199.gi3.akamai.net? Apple Authoritative DNS Server Apple Web Server CNAME: images.apple.com.edgesuite.net. images.apple.com? 2 IP addresses of Akamai Low-Level Local DNS Server Akamai edge servers DNS Server Web Client 2 IP addresses of Akamai edge servers fetch image files Akamai Edge Server Slide adapted from Drafting Behind Akamai, Sigcomm

45 Two-level server assignment Akamai top-level DNS server Anycasted Selects location of best content cluster Delegates to content cluster s low-level name server TTL 1 hour Akamai low-level server Return IP addresses of servers that can satisfy the request: consistent hashing TTL 20 seconds: quick adoption to load conditions Most CDNs use similar techniques Some CDNs rely on Anycast to send traffic to closest content server (e.g., Limelight) 45

46 What is the best location? Service requested Server must be able to satisfy the request (e.g., QuickTime stream) Server health Up and running without errors Server load Server s CPU, disk, and network utilization Network condition minimal packet loss to client, sufficient bandwidth to handle requests Client location Server should be close to client, e.g., in terms of RTT 46

47 Continuous measurement effort Number of hops between ASes Live network statistics (e.g., traceroute) Load of data centers/content servers Report load of content servers to local DNS servers Report content cluster load to the top-level DNS resolver to direct traffic away from overloaded content clusters Entire system s health end-to-end Agents that simulate end-user behavior by downloading web objects Measure failure rates and download times Monitor individual customers/services What is the busiest customer, etc.? 47

48 Inverse view: Impact of DNS resolver choice 48

49 Importance of DNS for CDN Redirection CDN cache CDN cache 2. CDN selects closest / best cache CDN cache DNS resolver 1. DNS query host CDN only learns IP address of DNS resolver, not of host 49

50 What Happens if Alternate DNS Resolver is Chosen? CDN cache CDN cache DNS resolver Alternate resolver,e.g. GoogleDNS: OpenDNS: CDN cache 1. DNS query host CDN thinks that host is in Google or OpenDNS network 50

51 What Happens if Alternate DNS Resolver is Chosen? 2. CDN selects closest, best cache CDN cache CDN cache DNS resolver Alternate resolver,e.g. GoogleDNS: OpenDNS: CDN cache 1. DNS query host CDN thinks that host is in Google or OpenDNS network 51

52 Data and Approach Provide custom script to friends of friends Run on 50 commercial ISPs All around the globe Query 10k+ top hostnames from Alexa DNS resolvers: local resolver, Google DNS, OpenDNS Collect DNS response times Returned IP addresses, which provide information about subnet (/24) Autonomous systems (AS) and country from where content is fetched Source: Bernhard Ager et al. DNS in the Wild. IMC

53 DNS response times q 3rd-party resolvers sometimes better performance Source: Bernhard Ager et al. DNS in the Wild. IMC

54 Local DNS vs. Google DNS: Where are we directed? For 2000 out of queries: subnets are different In half of these cases: different AS and country Source: Bernhard Ager et al. DNS in the Wild. IMC

55 Indirection may cause inefficiency Choice of DNS resolver is crucial It is a criteria for determining from where content is retrieved For content locality you have to use the local resolver Google DNS, OpenDNS etc. may lead to suboptimal choice of content server Recent IETF activity Include IP address of original host in DNS request 55

56 How to mitigate? So far we have seen CDNs rely on resolver location only Extensive measurements to find best client-server mappings 3rd-party resolvers mess up the system Provider-aided Distance Information System: PaDIS Knows network topology and conditions Finds better content servers: good for users Reduces network load: good for ISPs Following slides adapted from Georgios Smaragdakis and Anja Feldmann. 56

57 PaDIS External DNS Provider DNS Internet Service Provider (ISP) 5 4 PaDIS Client Host 57

58 PaDIS External DNS Provider DNS Internet Service Full View of the ISP Provider (ISP) Network 5 4 PaDIS Client Host 58

59 PaDIS External DNS Client Provider DNS Internet Service Full View of the ISP Provider (ISP) Network 5 Content can be downloaded from any eligible host! 4 PaDIS Host 59

60 PaDIS External DNS Provider DNS Host1 Host2 Internet Service Full View Host3 of the ISP Provider (ISP) Host4 Network 5 4 PaDIS Client Host 60

61 PaDIS External DNS Provider DNS Host1 Host2 Internet Service Full View Host3 of the ISP Provider (ISP) Host4 Network 5 Host2 4 PaDIS Host4 Host3 Host1 Client Host 61

62 PaDIS 7 External DNS Provider DNS Internet Service Full View of the ISP Provider (ISP) Network 5 4 PaDIS Client 7 Host 62

63 Case 1: Network load balancing Host A Host B Host C Client 63

64 Case 1: Network load balancing Host A Host B Host C Client 64

65 Case 1: Network load balancing Host A Host B Host C Client 65

66 Case 2: ISP-CDN collaboration 7 1 External DNS Provider DNS Internet Service Provider (ISP) 3 4 PaDIS Client 7 Host 66

67 Case 2: ISP-CDN Host1 collaboration 7 1 External DNS Provider DNS Host2 Host3 Host4 Internet Service Provider (ISP) 3 4 PaDIS Host2 Host4 Host3 Host1 Client 7 Host 67

68 Case 2: ISP-CDN Host1 collaboration 7 1 External DNS Provider DNS Host2 Host3 Host4 Internet Service Provider (ISP) 3 4 PaDIS Host2 Host3 Host1 Host4 Host2 Host4 Host3 Host1 Client 7 Host 68

69 Summary Alternative traffic engineering Do not change the routing Change the traffic matrix! Benefits ISPs: Regain control of network traffic User: Performance improvements PADIS à Win-win situation for ISPs and end-users à ISPs can share benefits with content and application providers Simple and easy to implement Prototype running 69

70 Secure Overlay Service SoS: An overlay network, using indirection and randomization to provide legitimate users (only) with denial-of-service free access to a server. Overlay network: Network or distributed infrastructure with common network services (e.g., routing) built on top of other networks Example: Distributed application in which application-layer nodes relay messages among themselves, using underlying IP routing to get from one site to another 70

71 Performing a DoS Attack 1. Select Target to attack 2. Break into accounts around the network 3. Have these accounts send packets toward the target 71

72 Goal of Secure Overlay Service (SoS) Pre-approved legitimate users communicate with target legit users may be mobile (IP addresses change) Un-approved (attackers ) packets don t reach target attackers legit user target 72

73 Step 1 Filtering Routers near target filter packets based on IP addr IP addresses from legitimate user allowed through IP addresses from illegitimate users are not Concerns: Bad users have same IP address as good user? Bad users know good user s IP address: spoofing? Good IP address changes frequently (mobility)? 73

74 Step 2 Indirection via a proxy Use proxy, outside filtered region Proxy, being a computer (rather than router) can perfom heavyweight authentication, access control Only packets from proxy permitted through filter Proxy only forwards verified packets from legitimate sources through filter w.x.y.z 74

75 Problems with a known Proxy Proxies introduce other problems Attacker can breach filter by attacking with spoofed proxy address Attacker can DoS attack proxy, again preventing legitimate user communication I m w.x.y.z w.x.y.z I m w.x.y.z I m w.x.y.z 75

76 Step 3 Multiple proxies with secret forwarding Create many proxies (too many to attack) Target specifies small set of proxies as secret forwarders Only secret-forwarder packets pass through filter Only secret forwarders know they are secret forwarders (other proxies unaware) To get host packet to target Host contacts any proxy (which checks legitimacy) Proxy randomly routes packet to another proxy If destination proxy is secret forwarder, packet forwarded to target, otherwise packet randomly routed to another proxy 76

77 SOS with Random routing? secret forwarder proxy With filters, multiple proxies, and secret forwarder(s), attacker cannot focus attack 77

78 SoS Why indirection? Ultimate destination address is unknown (hackers can not attack target, only attack proxies (?)) Address of target only known to small number of secret forwarders, which rotate and can change Issues: Why can t hacker just try all addresses of all proxies to get through? 78

79 Indirection: Summary We ve seen indirection used in many ways: multicast mobility Internet indirection CDNs SoS The uses of indirection: Sender does not need to know receiver id do not want sender to know intermediary identities Load balancing Beauty, grace, elegance Transparency of indirection is important Performance: is it more efficient? Security: Important issue for I3 79