Transcript PPT slides
DNS and Naming Aditya Akella 03/16/2007 Supplemental slides Domain Name System Goals • Basically building a wide area distributed database • Scalability • Decentralized maintenance • Robustness • Global scope – Names mean the same thing everywhere • Don’t need – Atomicity – Strong consistency DNS Experience • 23% of lookups with no answer – Retransmit aggressively most packets in trace for unanswered lookups! – Correct answers tend to come back quickly/with few retries • 10 - 42% negative answers most = no name exists – Inverse lookups and bogus NS records • Worst 10% lookup latency got much worse – Median 8597, 90th percentile 4471176 • Increasing share of low TTL records what is happening to caching? DNS Experience • Hit rate for DNS = 80% 1-(#DNS/#connections) – Most Internet traffic is Web – What does a typical page look like? average of 4-5 imbedded objects needs 4-5 transfers accounts for 80% hit rate! • 70% hit rate for NS records i.e. don’t go to root/gTLD servers – NS TTLs are much longer than A TTLs – NS record caching is much more important to scalability • Name distribution = Zipf-like = 1/xa • A records TTLs = 10 minutes similar to TTLs = infinite • 10 client hit rate = 1000+ client hit rate Architectural Brittleness • Hosts are tied to IP addresses – Mobility and multi-homing pose problems • Services are tied to hosts – A service is more than just one host: replication, migration, composition • Packets might require processing at intermediaries before reaching destination – “Middleboxes” (NATs, firewalls, …) Naming Can Help • Thesis: proper naming can cure some ills – Layered naming provides layers of indirection and shielding • Many proposals advocate large-scale, overarching architectural change – Routers, end-hosts, services • Proposal: – Changes “only” hosts and name resolution – Synthesis of much previous work Internet Naming is Host-Centric • Two global namespaces: DNS and IP addresses • These namespaces are host-centric – IP addresses: network location of host – DNS names: domain of host – Both closely tied to an underlying structure – Motivated by host-centric applications The Trouble with Host-Centric Names • Host-centric names are fragile – If a name is based on mutable properties of its referent, it is fragile – Example: If Joe’s Web page www.berkeley.edu/~hippie moves to www.wallstreetstiffs.com/~yuppie, Web links to his page break • Fragile names constrain movement – IP addresses are not stable host names – DNS URLs are not stable data names Key Architectural Questions 1. Which entities should be named? 2. What should names look like? 3. What should names resolve to? Name Services and Hosts Separately • Service identifiers (SIDs) are hostindependent data names • End-point identifiers (EIDs) are location-independent host names • Protocols bind to names, and resolve them – Apps should use SIDs as bind data protocols handles only Binding principle: Names should Transportaspects connections should bind to EIDs to–relevant of underlying structure The Naming Layers User-level descriptors (e.g., search) Application App-specific search/lookup returns SID Use SID as handle App session App session Resolves SID to EID Opens transport conns Bind to EID Transport Transport Resolves EID to IP IP IP hdr EID TCP SID … IP Key Architectural Questions 1. Which entities should be named? 2. What should names look like? 3. What should names resolve to? SIDs and EIDs should be Flat 0xf436f0ab527bac9e8b100afeff394300 Stable-name principle: A stable name should not impose restrictions on the entity it names • Flat names impose no structure on entities –Structured names stable only if name structure matches natural structure of entities –Can be resolved scalably using, e.g., DHTs • Flat names can be used to name anything –Once you have a large flat namespace, you never need other global “handles” Flat Names Enable Flexible Migration • SID abstracts all object reachability information • Objects: any granularity (files, directories) • Benefit: Links (referrers) don’t break Domain H 10.1.2.3 <A HREF= http://f012012/pub.pdf >here is a paper</A> /docs/ (10.1.2.3,80, /docs/)(20.2.4.6,80, Resolution Service /~user/pubs/) Domain Y 20.2.4.6 /~user/pubs/ Flat Names are a Two-Edged Sword • Global resolution infrastructure needed – Perhaps as “managed DHT” infrastructure • Lack of local name control • Lack of locality • Not user-friendly – User-level descriptors are human-friendly Key Architectural Questions 1. Which entities should be named? 2. What should names look like? 3. What should names resolve to? Delegation • Names usually resolve to “location” of entity • Packets might require processing at intermediaries before reaching destination Delegation A network entity should be able • Such principle: processing today violates layering to direct resolutions of its name not only IP to its own – Only element identified by packet’s location, but also inspect to chosen delegates destination should higher layers Delegation Enables ArchitecturallySound Intermediaries Resolution svc Dest EID Mapping d f f ipf Packet structure (dests only) ipf EID d TCP hdr Firewall EID s EID f IP ipf • Delegate can be anywhere in the network, not necessarily on the IP path to d (ipd) • SID/EID can resolve to sequence of delegates EID d IP ipd