AnycastAnycast

Jennifer RexfordJennifer Rexford

Advanced Computer NetworksAdvanced Computer Networkshttp://www.cs.princeton.edu/courses/archive/fall08/http://www.cs.princeton.edu/courses/archive/fall08/

cos561/cos561/Tuesdays/Thursdays 1:30pm-2:50pmTuesdays/Thursdays 1:30pm-2:50pm

Naming and Addressing

• Some definitions– Name: what we seek– Address: where it is located

• Examples– Wide area: www.cnn.com vs. 63.251.179.13– LAN: 63.251.179.13 vs. 00:16:CF:1C:D0:24

• What should we name and address?– Objects: http://www.cnn.com/election-

over.htm– Hosts: www.cnn.com or 63.251.179.13– ASes: 7018

Anycast:Application-Level vs. IP-Level

Many Services are Replicated

• Servers in many locations– Reliability: copies that fail independently– Performance: clients directed to nearby replicas

Anycast

• Anycast– One-to-many association of name to endpoints– Each destination represents a set of receivers– Only one receives information from a given

sender

• Questions– How to name the (replicated) service?

• URL, host name, IP address, …

– How to decide which instance receives traffic?• Network proximity, load balancing policies, …

– How “sticky” should the binding be?• Each packet independent? Connection-oriented?

IP Anycast

• Announce IP prefix in interdomain routing– At each replica location

• Rely on global routing to direct traffic– To the “nearest” replica 63.251.179.13

63.251.179.13

63.251.179.13

IP Anycast: Pros and Cons

• Advantages– Completely transparent to clients and routers– Scales well for a large group of replicas– End-to-end paths automatically efficient

• Disadvantages– Pollutes the global routing system– Separate /24 for each replicated service– Does not consider server load– Different packets may reach different replicas– Slow BGP convergence after a withdrawal

Application-Level Anycast

• URL rewriting– Server dynamically rewrites HTML page– E.g., image at foo23.bar.com vs.

foo46.bar.com

• Application-level redirection– Explicit redirection of a request to new

location– E.g., HTTP 302 “Moved Temporarily”

• DNS redirection– Change mapping of domain name to address– E.g., www.cnn.com to 8.15.7.117

Application-Layer Anycast: Pros and Cons

• Advantages– Fine-grain control of load across group members– Can easily incorporate variety of criteria– Successive packets delivered to the same replica

• Disadvantages– Need to identify location of the requesting client

• Especially difficult for DNS-based redirection

– Extra round-trip times for redirection– Small TTLs to prevent long DNS caching– Boot-strapping to find redirecting/lookup server

Anycast in Practice

• Content Distribution Networks (CDNS)– Direct Web clients to site replica– E.g., URL rewriting, HTTP redirection, or DNS

• Reliable root DNS servers– Direct DNS queries to nearby DNS server– E.g., IP anycast

Improving IP Anycast

• Improvements– Considering network and server load– Ensuring successive packets reach same replica

• Anycast proxies– Proxies announce common IP prefix– And tunnel packets to group members

• Route control platforms– Collects networks and server load information– Determines which replica receives the requests

Other Uses of Anycast-Like BGP Prefix Announcements

Evolvability: Opt-In to Partially Deployed Solution

• New protocol partially deployed as overlay– E.g., IPv6

• Nodes announce common IP prefix– To suck user traffic into the overlay

• Participating host tunnels its traffic

Mobility: Dynamic Announcements & Withdrawals

• Boeing Connexion service

Internet

12.78.3.0/24

http://www.nanog.org/mtg-0405/abarbanel.html

Security: Hijacking the Hijacker

• Prevent BGP route hijacking– Group of nodes collectively announce prefix– And form overlay to deliver to the

destination

12.1.0.0/16

Tries to hijack

http://www.cs.princeton.edu/research/techreps/TR-808-07

Discussion

• Application vs. IP anycast– Early binding vs. late binding?

• Granularity of naming and addressing?– Services, hosts, or ASes?

• Handling change in replica selection?– Unplanned: failure and physical mobility– Planned: maintenance, load balancing,

migration

• Protecting health of the Internet?– DNS abuse (with small, cache-busting TTLs)– BGP abuse (with many prefixes and updates)

Next Class, on Thursday

• I will be out of town– At NSF serving on a proposal review panel

• Guest lecture by Changhoon Kim– Scalable Ethernet architecture for large

enterprises– Flat addressing, separating host name and

location, route caching, reactive cache invalidation