EBU6502_cloud_computing_notes/4-2-cdn.md

# Content Delivery Networks

## DNS

### Definition:

- Domain name system
- Intended use: to translate domain name to IP addresses
- Other uses: load distribution: replicated web server has many IPs, use DNS to
  redirect client to closest place
- Distributed system, that servers are interconnected
    - Centralizing is hard, because of the huge traffic, and distance, and
      single point of failure
- Many applications rely on DNS

### Hierarchy

- Root DNS Server: Root name server
    - First point of contact
    - Directly query authoritative name server
    - Get Domain-name - IP mapping
    - Query for IP address for TLD DNS servers
- TLD (Top Level Domain) `.com`, `.org`, `.edu` DNS server
    - Query for IP address to Authoritative DNS Server
- Authoritative DNS Server: Owned by site owner like `amazon.com`

### Local DNS Server:

- Actually a client, not in a part of the Hierarchy
- Each ISP (Internet Service Provider) has one
- Workings:
    - When host makes DNS query, it's sent to local DNS server
    - The Server may have local cache of name-to-address pair
    - Otherwise forward the query to the DNS hierarchy

### DNS Caching

- Once the server knows about the mapping, it is **cached**
- Cache entry timeout after time (TTL): on the other hand it may be out of date
- TLD servers are typically cached in local, since root names are not frequently
  visited
- Benefits
    - Reduce network traffic on: **Root servers**, **across the internet**
    - This increases network performance because DNS response is much faster.

## P2P

### Definition

- A **Distributed** network architecture
- Every node is both the **Client** and the **Server**
- Advantages:
    - Scalable:
        - As the number of clients increase, the number of servers also
          increases
        - Both consume and donate resource
    - Less cost: Cost at the edge of network
    - More privacy: No centralized source of data
    - Reliability:
        - Distributed geographically
        - Has Replicas
        - No single point of failure
    - All of above made it easy to share content

### Categories

- Unstructured:
    - No restriction on overlay structures and data placement
    - Examples:
        - Napster, BitTorrent, FreeNet
- Structured
    - Uses Distributed Hash Table, that use an interface like `put(k, v)`, and
      `get(k)`
    - Has restriction on overlay structure, and data placement
    - Examples:
        - Chord, Pastery and CAN

### Server Selection

- For BitTorrent, a Tracker is used, which informs the clients about the peers
  available
    - TODO: See diagram at page 26

### Issues with P2P

- Reliability
- Performance
- Control: have a lot of copyrighted content

## Content Delivery Networks

### History of Content Delivery

- Web 1.0: Pre-CDN, Infrastructure development
- CDN 1.0: First generation of CDN, replication, intelligent routing, edge
  computing
- CDN 2.0: P2P, Cloud Computing, Energy Awareness
- CDN 3.0: Autonomic composition

### Web Caches

- The precursor to CDN
- Improve efficiency by caching
- Caching proxy:
    - Receive HTTP request from client
    - If object in cache, then send cached content
    - Otherwise request the object from origin server
- Works as both client and server:
    - Client: request content from origin
    - Server: serve content to downstream client
- Usually installed by ISP
- Reason:
    - Reduce response time for client request
    - Reduce traffic across network
- Problem:
    - Can't serve all of the web users, since the web is too large, and
    - Web content is dynamic and customized, which means many of them are not
      cacheable
    - Origin upstream web servers shouldn't rely on downstream caching proxy
    - Upstream web servers can't see the real statistics of their site, since
      the user data is not sent to their servers

### Definition

- Also called _Content Distribution Network_
- **Infra**: large distributed system of servers deployed in multiple data
  centers across the internet
- **Goal**: distribute content to end users on a large scale with high
  **availability** and high **performance**
- Is a mechanism to **replicate** content on multiple servers on the internet,
  providing client a way to choose server that can provide content fast.
- Content providers are the CDN customers:
    - They pay CDN companies to deliver their content
    - CDN pays ISPs, carriers, and network operators for hosting their servers
- Usually used by large web platforms

### What CDN do

- Serve a large fraction of internet content
    - Web objects (Text, JavaScript, graphics)
    - Downloadable objects
    - Applications
    - Stream media
- Most of the web uses CDN

### The model

- TODO: See the slide p41

### CDN Deployment

- CDN company deploy hundreds of servers around the world, often inside ISP
  networks, so that it's close to users
- CDN Customer side:
    - Replicates customer's content in CDN servers
    - When provider update content, CDN update server with their content
- User side:
    - Send request to origin server
    - Intercepted by redirection service
    - Forward user's request to best CDN server
    - Content served from CDN server

### Companies

- Akamai
- Limelight
- ChinaCache
- Edgecast

### Benefits

- Reduce latency to users
- Reduce load on original server
    - Increase security against Denial of Service Attacks
- Scalability
- Cheaper, easier to manage
- Bypass traffic jams on the web:
    - Requested data is close to clients
    - Avoid bottleneck links

### Optimizations in CDN side

- Content is cached at various locations, for faster access
- Use data compression
- Use load balancing to reduce traffic
- Security features like DDoS protection
- Use network peering, for shorter data paths

### Examples and Usage

- Netflix:
    - Low latency and high defiition media can be played
    - Handles peak traffic
    - Content has consistent quality
- Alibaba:
    - Rapid page loads for product listing
    - Support large scale events
    - Stability and scalability

### CDN Routing

#### Server Selection

- Load: To balance load
- Performance: improve client performance, based on:
    - Geography
    - RTT
    - Throughput
    - Load
- Any Node Alive: provide fault tolerance

#### Ways of redirecting

- As a part of routing: anycast (Single IP address is shared by many devices in
  multiple locations), cluster, load balancing
    - Pros: transparent to clients, works when browser cached failed addresses,
      circumvents many routing problems
    - Cons: Little control over selection of server, complex, scalability, and
      can't recover TCP
- Part of application: HTTP Redirect
    - Pros: Application level, has more control
    - Cons: Has Additional load and RTT, and is hard to cache
- Part of naming: DNS
    - Pros: Suitable for caching, dns redirect to any IP
    - Cons: This is implemented in resolver, requesting for a domain not URL,
      and hidden load factor for resolver's population
        - Can estimate the stats

#### More on DNS redirection

- DNS redirection is used to redirect client to a nearby server.
- Based on:
    - Latency to client
    - Load balancing
        - Try to balance client across many servers to avoid hotspot
    - Available servers
- Process:
    - Client's DNS request come to CDN's nameserver ( See below to how it's
      accessed. )
    - DNS request is being resolved to a nearby server, by accessing CDN
      controlled name servers
    - CDN measures the state of network in the infrastructure
- Two types of DNS redirection
    - Full:
        - the origin server is controlled by CDN
        - Pro: All requests are automatically redirected
        - Cons: May send a lot of traffic to CDN, so it's expensive
    - Partial:
        - Content provider mark what to provide to CDN
        - usually larger objects
        - Refer to images as `<img src=http://cdn.com/foo/bar/img.gif>`
        - Accessing the website, CDN serve the data
        - Pros: Better control
        - Cons: Have to mark content

## Deployment

### Hosting your stuff

- Where: rely on measures
    - Sample popular hostnames on alexa.com
    - Ask DNS from multiple vantage points
    - Categorize by type:
        - Hostnames
        - Files
        - Unpopular

### Examples

- ChinaCache

## Future

### Challenges

- Mobile networks: latency to cell is higher, opaque internal network structure
- Video: Large bandwidth,
    - 16M - 30M bps compressed
    - When Combined can be 25K TBps
    - Even data centers don't have that much
    - Using multicast from end systems as potential solution

### CDN2.0

- Hybrid CDN: Akamai
- Cloud Based Video: NetFlix
- Meta CDN: Conviva
- Virtual CDN: ISP micro-datacenters