Friendships that last Peer lifespan and its role

Friendships that last Peer lifespan and its role in P 2 P protocols Fabián E. Bustamante & Yi Qiao Department of Computer Science Northwestern University {fabianb, yqiao}cs. northwestern. edu www. aqualab. cs. northwestern. edu

P 2 P and heterogeneity P 2 P computing: sharing of computer resources & services by direct exchange between participants Purest form … all peers are equal Problem: clash between assumption and reality - peer populations show high variations on storage, bandwidth, latency, degree of sharing, uptime, … Dept. of Computer Science Northwestern University 2

Transient peers and P 2 P systems Peers defined an overlay network – Set of connections to other peers (their “friends”) – Maintenance protocol that repairs the overlay Degree of peer transiency – Median up-time ~ 70’ – Implications 1. Maintenance-related messages 2. Plus degree of replication, effectiveness of caches, spread of queries, overall system scalability, … Dept. of Computer Science Northwestern University 3

Our approach Part of the problem is whom one befriends – One solution: pick those that will live/stay long Without knowing the future, can we predict it? – Yes; peer lifespan follows a Pareto distribution! Given a good prediction - how should it be used in P 2 P protocols? Can it really help? Dept. of Computer Science Northwestern University 4

Determining lifespan distribution In Gnutella, using a modified client, between March 1 st-8 th, 2003 Some details: – Attempt a Gnutella connection setup – 20 monitoring peers for fine probe granularity – First-time found peers only recorded with Time. When-Found – Peer considered dead when • Connection attempt fails 3 rd time • Unexpected response is received Dept. of Computer Science Northwestern University 5

Peer lifespan distribution 500, 000 peers, ~1 million peers’ lifespans Create-based method for sample limited scope Figures show RCDF of peers with lifespan in [1, 300 sec, 3. 5 days] Pareto distribution of the form λTk (k < 0) Dept. of Computer Science Northwestern University 6

Peer Lifespan and P 2 P protocols Choosing among “acquaintances”: – When deciding whom to befriend – Responding to requests for references In most P 2 P protocols – random selection Peer lifespan fits a Pareto distributions Є UBNE class (Used Better than New in Expectation) Peer’s expected remaining lifetime directly proportional to current age Dept. of Computer Science Northwestern University 7

Some of the questions … How could we incorporate lifespan-based ideas into P 2 P systems? Potential gains in reduced maintenance overhead Effects on application performance … Dept. of Computer Science Northwestern University 8

Lifespan-based protocols Increased dependency as commitment to the community becomes clear Protocol Connect? Recommend? LSPAN-1 Oldest Random LSPAN-2 Oldest LSPAN-3 Oldest & more available connections Random Dept. of Computer Science Northwestern University 9

Experimental setup Trace-driven simulation – P 2 P simulator includes membership management and various query distribution, cache and replication strategies Runs of one of the 20 collected traces for a period of 510, 000 sec. , ~36, 577 peers Cold start, warm-up ~80, 000 sec. excluded ~1, 000 peers under stable conditions Dept. of Computer Science Northwestern University 10

Alternative protocols compared Unstructured Decentralized Protocol (UDP) – ~ early Gnutella – Separate pools for cached pongs (per connection) – Pong replies include random set of entries from cache Hierarchical Decentralized Protocol (HDP) – ~ new Gnutella, Ka. Zaa – Leaf- and ultra-peers: leafs can only connect to ultras; ultras to anybody – To decide a peer’s role – trace information Dept. of Computer Science Northwestern University 11

Comparing connection breakdowns Indicator of stability √ Lifespan-based protocols More selective → fewer breakdowns Reductions – 42 -43% -LSPAN-2 – 26 -30% -LSPAN-1 and LSPAN-3 Saw-tooth shape → time -of-day patterns Dept. of Computer Science Northwestern University 12

Comparing connection rejections Does preference for long-lived peers have to mean high rejection rates? True for LSPAN-2 – although may be a reasonable “cost” Still, for LSPAN-1 and LSPAN-3 low enough to be ignored LSPAN -3 ~ 1/17. 58 hrs! Dept. of Computer Science Northwestern University 13

Comparing number of connections … not just rejections, what about number of connections? LSPAN-1 and LSPAN 3 – higher ratio of connections per peer Little benefit from checking available connections Dept. of Computer Science Northwestern University 14

A preview: Effects on applications Gains in scalability With random-walkers & NCU (Neighboring Caching) Lifespan-based: 5 and random topology: 16 walkers Dept. of Computer Science Northwestern University 15

Conclusions and future work Peer lifespan fits a Pareto distribution – current age to predict lifespan Illustrative lifespan-based protocols Advantages of considering peers’ age in P 2 P protocols Possible research paths – – Effect on query distribution and cache strategies Lifespan-based strategies Determining a peer’s age in decentralized P 2 P systems Lifespan and DHTs Dept. of Computer Science Northwestern University 16