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Transcript
Reliable Distributed Systems Overlay Networks Resilient Overlay Networks A hot new idea from MIT Shorthand name: RON Today: What’s a RON? Are these a good idea, or just antisocial? What next: Underlay networks What is an overlay network? A network whose links are based on (end-to-end) IP hops And that has multi-hop forwarding I.e. a path through the network will traverse multiple IP “links” As a “network” service (i.e. not part of an application per se) I.e. we are not including Netnews, IRC, or caching CDNs in our definition Why do we have overlay networks? Historically (late 80’s, early 90’s): to get functionality that IP doesn’t provide as a way of transitioning a new technology into the router infrastructure mbone: IP multicast overlay 6bone: IPv6 overlay Why do we have overlay networks? More recently (mid-90’s): to overcome scaling and other limitations of “infrastructure-based” networks (overlay or otherwise) Yoid and End-system Multicast Two “end-system” or (nowadays) “peer-to-peer” multicast overlays Customer-based VPNs are kind-of overlay networks If they do multi-hop routing, which most probably don’t Why do we have overlay networks? Still more recently (late-90’s, early 00’s): to improve the performance and reliability of native IP!!! RON (Resilient Overlay Network) Work from MIT (Andersen, Balakrishnan) Based on results of Detour measurements of Savage et.al., Univ of Washington, Seattle End-to-end effects of Internet Path Selection Savage et. al., Univ of Washington Seattle Compared path found by internet routing with alternates Alternates composed by gluing together two internetrouted paths Roundtrip time, loss rate, bandwidth Data sets: Paxson, plus new ones from UW Found improvements in all metrics with these “Detour” routes BGP and other studies Paxson (ACIR), Labovitz (U Mich), Chandra (U Texas) Show that outages are frequent (>1%) BGP can take minutes to recover RON Rational BGP cannot respond to congestion Because of information hiding and policy, BGP cannot always find best path Private peering links often cannot be discovered BGP cannot respond quickly to link failures However, a small dynamic overlay network can overcome these limitations BGP lack of response to congestion Very hard for routing algorithms to respond to congestion (route around it) Problem is oscillations Traffic moved from congested link to lightly-loaded link, then lightly-load link becomes congestions, etc. ARPANET (~70 node network) struggled with this for years Khanna and Zinky finally solved this (SIGCOMM ’89) Heavy damping of responsiveness BGP information hiding Internet 30.1/16 20.1/16 ISP1 ISP2 30.1.3/24 30.1.3/24 Site1 20.1.5/24 Site2 20.1.5/24 Private peering link. Site1 and Site2 can exchange traffic, but Site2 cannot receive internet traffic via ISP1 (even if policy allows it). Acceptable Use Policies Why might Cornell hide a link Perhaps, Cornell has a great link to the Arecebo telescope in Puerto Rico but doesn’t want all the traffic to that island routing via Cornell E.g. we pay for it, and need it for scientific research But any Cornell traffic to Puerto Rico routes on our dedicated link This is an example of an “AUP” “Cornell traffic to 123.45.00.00/16 can go via link x, but non-Cornell traffic is prohibited” BGP information hiding Internet 30.1/16 ISP1 ISP2 20.1/16 X 30.1.3/24 Site1 Site2 20.1.5/24 RON can bypass BGP information hiding RON3 Internet 30.1/16 ISP1 ISP2 20.1/16 X 30.1.3/24 Site1 RON1 30.1.3.5 20.1.5/24 Site2 RON2 20.1.5.7 …but in doing so may violate AUP RON test network had private peering links BGP link failure response BGP cannot respond quickly to changes in AS path Hold down to prevent flapping Policy limitations But BGP can respond locally to link failures And, local topology can be engineered for redundancy Local router/link redundancy eBGP and/or iBGP can respond to peering failure without requiring an AS path change ISP ISP R R R R R R R R AS1 AS2 Intra-domain routing (i.e. OSPF) can respond to internal ISP failures AS path responsiveness is not strictly necessary to build robust internets with BGP. Note: the telephone signalling network (SS7, a data network) is built this way. Goals of RON Small group of hosts cooperate to find better-than-nativeIP paths Multiple criteria, application selectable per packet 10-20 seconds Policy routing Latency, loss rate, throughput Better reliability too Fast response to outages or performance changes ~50 hosts max, though working to improve Avoid paths that violate the AUP (Acceptable Usage Policy) of the underlying IP network General-purpose library that many applications may use C++ Some envisioned RON applications Multi-media conference Customer-provided VPN High-performance ISP Basic approach Small group of hosts All ping each other---a lot Run a simplified link-state algorithm over the N2 mesh to find best paths Order every 10 seconds 50 nodes produces 33 kbps of traffic per node! Metric and policy based Route over best path with specialized metricand policy-tagged header Use hysteresis to prevent route flapping Major results (tested with 12 and 16 node RONs) Recover from most complete outages and all periods of sustained high loss rates of >30% 18 sec average to route around failures Routes around throughput failures, doubles throughput 5% of time 5% of time, reduced loss probability by >0.5 Single-hop detour provides almost all the benefit RON Architecture Simple send(), recv(callback) API Router is itself a RON client Local or shared among nodes. Relational DB allows a rich set of query types. Conduit, Forwarder, and Router RON Header (inspired by IPv6!…but not IPv6) Runs under IP Performance metrics IPv4 Unique per flow. Cached to speed up (3-phase) forwarding decision Runs over UDP Selects conduit Link evaluation Defaults: Latency (sum of): Loss rate (product of): average of last 100 samples Throughput (minimum of): lat1 •lat1 + (1 - ) • new_sample1 exponential weighted moving average ( = 0.9) Noisy, so look for at least 50% improvement Use simple TCP throughput formula: √1.5 / (rtt • √p) p=loss probability Plus, application can run its own Responding to failure Probe interval: 12 seconds Probe timeout: 3 seconds Routing update interval: 14 seconds RON overhead 10 nodes 20 nodes 30 nodes 40 nodes 50 nodes 1.8 Kbps 5.9 Kbps 12 Kbps 21 Kbps 32 Kbps Probe overhead: 69 bytes RON routing overhead: 60 + 20 (N-1) 50: allows recovery times between 12 and 25 s Two policy mechanisms Exclusive cliques Only member of clique can use link Good for “Internet2” policy General policies No commercial endpoints went over Internet2 links BPF-like (Berkeley Packet Filter) packet matcher and list of denied links Note: in spite of this, AUPs may easily, even intentionally, be violated RON deployment (19 sites) To vu.nl Lulea.se OR-DSL CMU MIT MA-Cable Cisco Cornell CA-T1 CCI Aros Utah NYU To vu.nl lulea.se ucl.uk To kaist.kr, .ve .com (ca), .com (ca), dsl (or), cci (ut), aros (ut), utah.edu, .com (tx) cmu (pa), dsl (nc), nyu , cornell, cable (ma), cisco (ma), mit, vu.nl, lulea.se, ucl.uk, kaist.kr, univ-in-venezuela AS view Latency CDF ~20% improvement? Same latency data, but as scatterplot Banding due to different host pairs RON greatly improves lossrate 1 "loss.jit" 0.8 0.6 0.4 0.2 0 30-min average loss rate on Internet 0 0.2 0.4 0.6 0.8 1 RON loss rate never more than 30% 13,000 samples 30-min average loss rate with RON An order-of-magnitude fewer failures 30-minute average loss rates Loss Rate 10% 20% 30% 50% 80% 100% RON Better 479 127 32 20 14 10 No Change 57 4 0 0 0 0 RON Worse 47 15 0 0 0 0 Resilience Against DoS Attacks Some unanswered questions How much benefit comes from smaller or larger RONs? Would a 4-node RON buy me much? Do results apply to other user communities? Testbed consisted mainly of highbandwidth users (3 home broadband) Research networks may have more private peering than residential ISPs Some concerns Modulo unanswered questions, clearly RON provides an astonishing benefit However . . . Some concerns Is RON TCP-unfriendly? RON path change looks like a nonslowstarted TCP connection On the other hand, RON endpoints (TCP) would back off after failure Some concerns Would large-scale RON usage result in route instabilities? Small scale probably doesn’t because a few RONs are not enough to saturate a link Note: internet stability is built on congestion avoidance within a stable path, not rerouting Some concerns RON’s ambient overhead is significant Lots of RONs would increase overall internet traffic, lower performance This is not TCP-friendly overhead 32 Kbps (50-node RON) is equivalent to highquality audio, low-quality video! Clearly the internet can’t support much of this RON folks are working to improve overhead RON creators’ opinion on overhead “Not necessarily excessive” “Our opinion is that this overhead is not necessarily excessive. Many of the packets on today’s Internet are TCP acknowledgments, typically sent for every other TCP data segment. These “overhead” packets are necessary for reliability and congestion control; similarly, RON’s active probes may be viewed as “overhead” that help achieve rapid recovery from failures.” Some concerns RONs break AUPs (Acceptable Usage Policies) RON has its own policies, but requires user cooperation and diligence Underlay Networks Idea here is that the Internet has a lot of capacity So suppose we set some resources to the side and constructed a “dark network” It would lack the entire IP infrastructure but could carry packets, like Ethernet Could we build a new Internet on it with better properties? The vision: Side by side Internets The Internet MediaNet SecureNet ReliableNet Shared HW, but not “internet” Doing it on the edges We might squeak by doing it only at the edges After all, core of Internet is “infinitely fast” and loses no packets (usually) So if issues are all on the edge, we could offer these new services just at the edge Moreover, if a data center owns a fat pipe to the core, we might be able to do this just on the consumer side, and just on last few hops… Pros and cons Pros: With a free hand, we could build new and much “stronger” solutions, at least in desired dimensions New revenue opportunities for ISPs Cons These “SuperNets” might need non-standard addressing or non-IP interfaces, which users might then reject And ISPs would need to agree on a fee model and how to split the revenue Summary The Internet itself has become a serious problem for at least some applications To respond, people have started to hack the network with home-brew routing But a serious response might need to start from the ground up, and would face political and social challenges!