Advanced network planning notes (IX)

DASH

Dynamic Adaptive Streaming over HTTP

HTTP Fluidization problem

All clients request the same code stream regardless of network conditions

The server

• Divide the video into blocks
• Block independent storage , Different bit rate coding
• List of goods ： Provide different blocks URL

client

• Get the goods list
• Periodically measure server to client bandwidth
• Check the goods list , Request a block at a time ,HTTP The header specifies the byte range
  • Enough bandwidth , Select the maximum bit rate video block
  • Switch between different coding blocks at different times in the session （ Depends on the current bandwidth ）

intelligence

The client adaptively decides

• When to request a block
  —— The cache is not empty and does not overflow
• What encoding rate video block is requested
  —— Enough bandwidth to request high quality video blocks
• Where to request blocks
  —— Ask the server close to you for or Request from the highly available bandwidth server

advantage

• Dynamically estimate bandwidth , Current cache condition , Usually it can be played continuously
• Lighten the load on the server , Strong expandability

CDN

Challenge

Servers stream video content to millions of users at the same time through the network

programme

A single super service center

• There are many hops from server to client , The bottleneck link bandwidth is small, resulting in a pause
• This principle ： The network is full of multiple copies of the same video , Inefficient
• A single point of failure , Performance bottleneck
• Peripheral network congestion
  Simple but not expandable

CDN

Network wide deployment nodes , Storage service content , Provide services to users nearby , Improve user experience

• enter deep： Deep access network
  • Closer to users
  • A large number
  • Close to users
  • Management is difficult
• bring home： Deployed in a few key locations
  • Leased lines connect server clusters

Content replication policy

• There is no need to copy everything to all cache nodes
• Decide what is stored in the cache node
  • Content ＋ Cache nodes
• Non hot content will be replaced , Make room （ Replacement strategy ）
  • The content on the node changes over time

Cluster selection policy

Challenge

CDN DNS How to choose good CDN Nodes provide customers with streaming services

Strategy

• Close to the physical location of the client
  • CDN Operators maintain databases
  • problem ： Network fluctuation is not considered
• The network is in good condition
  • For example, on a regular basis ping Access network ISP Report to DNS
• IP Any broadcast anycast
  • CDN The server cluster is in a anycast In the group , There are group addresses
  • Every CDN The server cluster advertises the standard anycast group address to the outside
  • BGP The router receives multiple notifications belonging to a group , Keep a good one （AS Less hops ） The path of
  • Use stage
    The customer wants to see a video ,CDN The domain name system returns any multicast address , Packets are routed to the nearest CDN Server cluster
• The client decides
  • Give the client a CDN Server list
  • client ping, Choose the best

Considerations

• The content is in some CDN cluster
• CDN Cluster server to client network status ： Delay , bandwidth , The loss of
• load ： Cannot direct to a heavily loaded server
• ISP Delivery price （ Traffic contract ）