Content Distribution Networks

Improving scalability and reliability of content delivery
- local clustering (but, correlated failures)
- multi-homing (but, site can fail, and BGP convergence is slow)
- geo-replication (but, need to synchronize the site among the mirrors, and do smart DNS)
----> and all need excess capacity, well in excess of average demand ("flash crowds")

Smart DNS is hard
- know location/performance of clients as it changes (e.g., end-to-end performance monitoring)
- know capacity of each site as it changes (e.g., health monitoring)
- adjust as you make more copies of the content
- ensure the DNS service itself is reliable
- update DNS entries, use appropriate TTLs,etc.

Proxy caching by client ISP
- advantages: improves performance and reduces peering traffic
- but, some content is not cacheable (e.g., dynamic content)
- but, doesn't help with authorization
- but, doesn't help with reliability
- but, doesn't give content owners visibility into usage
- but, cannot revoke/invalidate content

Akamai
- edge caching (~2200 sites)
     * edge servers with DNS names like www.example.com.edgesuite.net
     * consistent hashing for edge caching
     * inter-cache protocol to get data from other caches
     * on miss, can go to origin (including using overlay route)
- cooperation with content provider
     * content provider names content (e.g., www.example.com.edgesuite.net)
     * on-demand purges of cached content
     * pass-through of authorization
     * customization or blocking of content based on geography
     * cooperation on generating dynamic content
     * change object name when content changes
- DNS
     * content provider uses Akamai name, e.g., www.example.com.edgesuite.net 
     * returns as CNAME for www.example.com DNS request
     * DNS resolution based on user location, performance, load, etc.
     * returns two IP addresses (for failover) for Akamai servers that handle that content
- TCP enhancements
     * improves even uncacheable content due to split TCP connection
     * start with higher initial window, increase faster, decrease less, ...
- BGP and DNS security
     * less risk of BGP hijack due to proximity to client
     * Akamai has deployed DNSSEC
     * large DNS server deployment can handle DoS on DNS