This is an introduction to technologies used in CDN-Content Distribution Networks. For more details click on the blue banner for additional terms and tutorials on this subject.
IPv4-to-IPv6 or Internet Protocol Version 4 to Version 6 also known as 6to4 relay router anycast routers advertise anycast address prefix into IPv4 network where IPv4 packets are routed to nearest 6to4 relay router and then to IPv6 network.
MSDP-Multicast Source Discovery Protocol is a process that connects PIM SP-Sparse Mode (pull down the data when asked for) domains and allows RP-Rendezvous Points or Shared Root to share information about active sources or shared trees. RP-Rendezvous Points in a rooted Shared Tree distribution system provide sub-optimal routing but are created as a result of user receiver densities such as rural areas or sub-continental geographic limitations. Content is distributed first to RP and then to receivers. RP-Rendezvous Points in a Bidirectional Tree distributes traffic content at any “point” in the network reducing data delays. SSM-Source Specific Multicast is a single-source-rooted distribution tree with no RP-Rendezvous Points using PIM-Protocol Independent Multicasting and IGMP-Internet Group Management Protocol. Receivers are explicitly-joined existing members which reduces complexity, billing, security, etc. MBGP-Multi-protocol Border Gateway Protocol is used to carry unicast prefixes of source networks across AS-Autonomous Systems boundaries. RP in different AS establishes peer arrangement to exchange MBPG updates called MSDP-Multicast Source Discovery Protocol SA-Source Address AM-Active Messages. AM are used to know about active sources in other AS-Autonomous Systems to build inter-domain (AS) Source trees. IP Anycast is defined, by Cisco, as the point-to-point flow of IP-Internet Protocol packets between a single client and the “nearest” destination server identified by any IP Anycast address. Criteria for selecting “nearest” server determines the type of anycast content. Also referred to a Virtual IP address with mirrored (duplicated or load-balanced disk) servers sharing same IP address. That is, the DNS-Domain Name Server maps (identifies) unique IP address which then distributes content to local or remote content servers.
Some background on IP Anycast:
Advantages:
– IP routing will deliver packets over the shortest path to the nearest host
– Client needs to be configured with only a single Anycast host address
– Upon failure, routing will forward to the next nearest server with the same Anycast address
– Distributed Anycast servers will effectively “load balance” traffic
Addressing
– Works with IPv4 and IPv6
Routing
– Works with standard IP routing protocols
– Changes in network topology (configuration) will automatically change to the nearest Anycast server
– Route Summarization may increase Route Table size
– IPv6 use can add QoS-Quality of Service (policy-based) routing
Mesh-First Tree Networks are created on the basis that a mesh (interconnected) network is formed BEFORE a routing protocol is determined. Then Source-Rooted Trees are selected based on the routing protocol such as DVMRP-Distance Vector Multicast Routing Protocol. DV-Distance Vector routing protocols maintain a routing table which consists of destination nodes, corresponding costs and next hops (routers). Routing Tables analyze the routes and determine the SPF-Shortest Path First. Then for multicast routing, RPF-Reverse Path Forwarding is used to determine SRT-Source Rooted Trees for content distribution. SPT-Shortest Path Tree is used by only ONE source (the root of the tree) to reach all receivers. Also known as a Directed Tree, packets only flow from the root (source) to the leaves (receivers). SFT content can also be distributed across different trees. ST-Shared Tree is where there are MANY sources to reach all receivers. That is, each tree (source) is also a root (receiver). This is also referred to as SCT-Shared Center Tree as the content is at the center of the distribution tree. There is research that suggests that Shared Trees have 25% less delay than SPT as content is closer to the receiver.
Two other factors are important in a multicast network – Dense and Spare modes.
DM-Dense Mode uses a “push” model to flood multicast traffic (broadcast)to every device. That is, every device gets the data whether they asked for it or not.
SP-Sparse Mode uses “pull” or “pull down” model where the data is retrieved when asked for (on demand). SP allows RP-Rendezvous Points or Shared Root to share information about active sources or shared trees.
Dense and Spare mode selection is also based on the amount and urgency (real-time events versus archival) of the content.
The basic concept of IP Protocol Multicast is to provide data to multiple receivers (like PCs, wireless devices, internet-equipped televisions or radios) with the least bandwidth and router processing (routing and storing large files).
This is a complex challenge with more and more uses involved with broadcasting of radio, television, teleconferences, music and two-way multicasting. That is, the internet is powerful medium for broadcasting of video, audio, etc.
The basic concept of internet Protocol Independent Multicast works with any interior (internal to AS-Autonomous System such as RIP-Routing Information Protocol, OSPF-Open Shortest Path First) or exterior routing protocols (external such as BGP-Border Gateway Protocol, MBGP-Multiprotocol BGP (multicast extensions of BGP)) and others. Multicasting routing protocols (how packets get from one place to another) portend (possibly) to provide fault tolerance (alternative delivery methods), error correction, load balancing (to avoid traffic jams or congestion) and improve performance in an increasing complex internet web of wired, wireless and any media communications.
Multicasting is not always used within a single network or AS-Autonomous System. An AS-Autonomous System is a group of IP-Internet Protocol networks operated by one or more network operators which has a single and clearly defined external routing policy. An AS has a unique number associated with it which is used both in exchange of exterior routing information and as an identifier of the AS itself. Exterior routing protocols such as BGP-Border Gateway Protocol and EGP-Exterior Gateway Protocol are used to exchange routing information between ASes. In routing terms an AS will normally use one or more IGB-Interior Gateway Protocols in conjunction with some sort of common agreed metrics when exchanging network information within its own AS.
- BGPs come in two flavors – interior and exterior:
- Interior border (patrol) gateway protocols are RIP-Routing Information Protocol, OSPF-Open Shortest Path First and others
- Exterior border (patrol) gateway protocols are BGP.
The End
