Overlay networks

Overlay networks

• application processing vs packet forwarding
• overlay nodes attach “outer headers” to route on the underlying network
• overlay networks don’t need to route based on the underlying network headers
  • can have their own headers

Routing Overlays

• simplest exists is to support alternate routing strategies
• overlay networks circumvent standard IP..allow for new testing

Experimental IP

• multicast internet
• used to be a multicast backbone
• MBone used IP tunnels+addresses, forwarding implemented in an new way
• 6-BONE, similar multicast overlay relying on IPv6

End System Multicast

• IP Multicast popular with some researchers…still limited deployment
• multicast-based applications like videoconferencing have turned to end system multicast
• end hosts participating in multicast implement their own multicast trees
• only hosts participate in the tree
• exchange via UDP tunnels
• can be partially solved by using cloud infrastructure with static hosts to have a partially static multicast tree where end users connect to the closest cloud
• constructing mesh overlay should roughly correspond to the physical topology of the underlying internet
• building a mesh
  • get information about neighbors
  • only add links to overlay when finding a new low-latency neighbor
  • nodes send “join mesh”/”leave mesh” messages notifying neighbors

Resilient Overlay Networks

• BGP only finds some route – it may not be the best
• sometimes routing through an intermediate node is faster
• BGP scales to large networks, but does so slowly and does not pick optimal paths
• RON showed it can improve performance and could adapt quickly by using an overlay
  • not scalable: O(N^2)
• RON in practice –> SD-WAN
• what happens when everyone is using RONs? does it impact net perf?

P2P Networks

• no server for downloads
• napster…not true p2p
• two processes
  • locate object of interest
  • download object of interest
  • ..how to achieve with no central authority?
    • technically, an overlay network can do this

Gnutella

• no centralized registry of objects
• gnutella floods the overlay to search for a particular item
• new nodes join via single link
• learn about other nodes from query response messages
• flooding does not scale

Structured Overlays

• form a structure which is reliable and efficient for object location
• challenges: how to map object onto node, how to route request to the right node?
• well known technique for mapping names to addresses: hashing
• hashing flaw: how many buckets?
  • also..how to get IP address from hash?
• consistent hashing
  • map objects and nodes across a single large ID space
• to consistently place nodes and objects:
  • hash(object) –> object ID
  • hash(nodeIP) –> node ID
• each object maintained on the node which is closest to its hashed ID
  • when leaving/joining, only a small amount of data needs to be moved
• to handle accessing the object for a particular hash: route!

BitTorrent

• avoid bottleneck of single file source
• each file shared by an independent network
• swarm nodes join+leave to download the file
• magnet file has file size, piece sizes, SHA, and URL of tracker
• trackerless torrents form a DHT using randomly generated 160-bit IDs
• depends on “good citizenship” of other providing upload/download (seeding)

CDNs

• four potential internet bottlenecks
  • first mile
  • last mile
  • server itself
  • peering points
• first mile, can’t do much about
  • the rest, solve with replication
• akamai – the original CDN
• cache pages
• only good for static content
• CDNs also need “redirectors” to route requests to best CDN server
• how does redirection happen?
  • 1: DNS
  • 2: request proxy
• redirectors can hash URLs to make requests go to a particular server (e.g. serving from the in-memory cache) -