DNS
- Human easily identifiable URL mapped to IP addresses
- DNS protocol runs over UDP, using port 53
- And HTTP / FTP / SMTP, like, DNS is an application layer protocol
- The site will be busy public tolerance is distributed across multiple servers, clients always want the IP address at the top of the server sends HTTP request messages, DNS between these redundant web server load distribution cycle
- Calling the application point of view from the user's host, DNS is to provide a simple and / direct conversion server black box.
- DNS uses a distributed design
A large number of DNS servers used to hierarchically organized and distributed worldwide, including the root DNS server / top-level domain DNS server / authoritative DNS server.
- Root DNS: There are more than 400 servers all over the world the name root server root name servers provide IP addresses TLD (top level domain) server by 13 different organizations management.
- Top-level domain DNS server: For each top-level domain (com / org / net / gov, etc.) as well as top-level domain for all countries (cn / jp / uk, etc.)
- Each organization has access to the host (such as web / mail, etc.) must provide a publicly accessible DNS records on the Internet, the host name mapped to IP. An organization's authoritative DNS server for collection of these DNS records.
- In addition, it is also remaining in the local DNS server, when the host DNS queries, DNS acts as a local proxy, the proxy host DNS query, the results are returned to the host
- Queries host and the local DNS server is recursive, iterative when the rest of the query.
- Local caching DNS server will be saved, these caches is not permanent (usually save 2 days)
- All DNS servers together to achieve DNS distributed database storage resource record (RR), RR provides the host name to IP address mapping.
Each record contains the following four parts (Name, Value, Type, TTL)
- If Type = A, Name is the host name, Value is the IP address.
- If Type = NS, then Name is a domain name, value is to know how to get the host name of the authoritative DNS servers in the domain IP address of the host.
- If Type = CNAME, then Value Name is an alias for the hostname of the host specification corresponds.
- If Type = MX, then Value is a canonical host name of the individual named Name of the mail server.
F2f
- With the increase of the number of parties peer, P2P and distribution time is significantly less than the customer service - the server distribution time
- Most scarce priority: BitTorrent using the most scarce priority of technology, not the host for this block will determine the most scarce blocks (all the neighbors in the most sparsely block) in his neighborhood, priority download the most scarce blocks.
Video streaming and content distribution network
- DASH (dynamic adaptation via the HTTP streaming): in DASH, the video encoder is divided into several different versions, each version having a different bit rates, corresponding to different levels of quality.
- After using DASH, each video version is stored in the HTTP server, each version has a different URL.HTTP server also has a sign document provides a URL and bit rate for each version.
- Content distribution network (CDN) into a dedicated CDN (content providers own) and third-party CDN
CDN server placement strategy:
- Deep. Deep into the ISP access network. The goal is to close to the end user, and reduce the number of routers link between the end user and CDN cluster, thereby improving the user experience of latency and throughput. Maintenance and management of clusters of tasks It will become a challenge.
- Invite a guest key position to build a cluster play to invite a guest to your ISP usually be placed in clusters of Internet exchange points. Lower maintenance and administrative overhead, but may be sacrificed end user latency and throughput.
Most cdn to pick and redirect the request through the DNS.
- CDN able to perform real-time measurements of its periodic delay and packet loss performance between the cluster and customers.
Original: Big Box Computer Network study notes 3