Peer-to-peer Communication Services Henning Schulzrinne, Jae Woo Lee, Salman Baset Columbia...
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Transcript of Peer-to-peer Communication Services Henning Schulzrinne, Jae Woo Lee, Salman Baset Columbia...
Peer-to-peer Communication Services
Henning Schulzrinne, Jae Woo Lee, Salman BasetColumbia University
Wolfgang Kellerer, Zoran DespotovicDoCoMo Communications Laboratories Europe
Outline
• Research overview• Conceptual framework
– Four stages of p2p systems
• Zeroconf: solution for bootstrapping– Overview and example
• z2z: Zeroconf-to-Zeroconf interconnection– Overview, design and implementation
• Zeroconf for SIP– Motivation and overview of the Internet Draft
• P2P systems for VoIP• P2P-SIP
– Background concepts and overview of current proposals
• Next step– DHT discovery– DHT initialization
Current results• Conceptual framework: 4 stages of p2p systems
– Bootstrapping– Interconnection– Structure formation– Growth
• Zeroconf: solution for bootstrapping– Detailed study of Bonjour, Apple’s Zeroconf implementation– Internet Draft published on using Zeroconf for SIP
• z2z: Zeroconf-to-Zeroconf Toolkit– Interconnect Zeroconf networks using OpenDHT– C++ prototype for proof of concept– z2z v1.0: open-source Java implementation on SourceForge– Paper submitted to IEEE Globecom’07 Workshop on Service Discovery
• P2PP: generic P2P transport protocol• Next step: DHT discovery and initialization
– How to discover an existing DHT?– How to construct a DHT efficiently from scratch?
Four stages of dynamic p2p systems
1. Bootstrapping• Formation of small private p2p islands
2. Interconnection• Connectivity and service discovery between the
p2p islands (each represented by a leader)
3. Structure formation• DHT construction among the leaders
4. Growth• Merger of multiple such DHTs
Zeroconf: solution for bootstrapping
• Three requirements for zero configuration networks:1) IP address assignment without a DHCP server
2) Host name resolution without a DNS server
3) Local service discovery without any rendezvous server
• Solutions and implementations:– RFC3927: Link-local addressing standard for 1)– DNS-SD/mDNS: Apple’s protocol for 2) & 3)– Bonjour: DNS-SD/mDNS implementation by Apple – Avahi: DNS-SD/mDNS implementation for Linux and BSD
DNS-SD/mDNS overview
• DNS-Based Service Discovery (DNS-SD) adds a level of indirection to SRV using PTR:_daap._tcp.local. PTR Tom’s Music._daap._tcp.local._daap._tcp.local. PTR Joe’s Music._daap._tcp.local.
Tom’s Music._daap._tcp.local. SRV 0 0 3689 Toms-machine.local.
Tom’s Music._daap._tcp.local. TXT "Version=196613" "iTSh Version=196608" "Machine ID=6070CABB0585" "Password=true”
Toms-machine.local. A 160.39.225.12
• Multicast DNS (mDNS)– Run by every host in a local link– Queries & answers are sent via multicast– All record names end in “.local.”
1:n mapping
z2z: Zeroconf-to-Zeroconf interconnection
rendezvous point - OpenDHT
z2z
Import/exportservices
Zeroconf subnet A
z2z
Import/exportservices
Zeroconf subnet B
Demo: global iTunes sharing
• Exporting iTunes shares under key “columbia”:$ z2z --export:opendht _daap._tcp --key “columbia”
• Importing services stored under key “columbia”:$ z2z --import:opendht --key “columbia”
How z2z works (exporting)
OpenDHT
z2z
Send browse request (i.e., PTR query) for service type: _daap._tcp
1)
Tom’s Music._daap._tcp.local
Joe’s Music._daap._tcp.local
Send resolve request (i.e., SRV, A, and TXT query) for each service
2)
160.39.225.12Tom’s ComputerPassword=true……
160.39.225.13Joe’s ComputerPassword=false……
Export them by putting into OpenDHT
3)
put:key= z2z._daap._tcp.columbiavalue= Tom’s Music 160.39.225.12:3689 Password=true ……
How z2z works (importing)
OpenDHT
z2z
Issue get call into OpenDHT
1)
Add “A” record into mDNS
2)
Import services by registering them (i.e., add PTR, SRV, TXT records to the local mDNS)
3)
get:key=z2z._daap._tcp.columbiavalue=Tom’s Music
160.39.225.12:3689……
value=Joe’s Music…… mDNS
“A” record for 160.39.225.12
Tom’s Music._daap._tcp.local_remote-160.39.225.12.local……
z2z implementation
• C++ Prototype using xmlrpc-c for OpenDHT access– Proof of concept– Porting problem due to Bonjour and Cygwin incompatibility
• z2z v1.0 released – Rewritten in Java from scratch– Open-source (BSD license)– Available in SourceForge (https://sourceforge.net/projects/z2z)
• Paper describing design and implementation detail– z2z: Discovering Zeroconf Services Beyond Local Link
• Lee, Schulzrinne, Kellerer, and Despotovic
– Submitted to IEEE Globecom’07 Workshop on Service Discovery
Zeroconf for SIP
• Enable SIP communication when proxy and registrar are not available– Good use case for z2z– Fill in the gap of P2P-SIP effort:
• local & small scale (10s to 100s)• high mobility• avoid construction of DHT
• Internet Draft published and presented at IETF-68– SIP URI Service Discovery using DNS-SD
• Lee, Schulzrinne, Kellerer, and Despotovic• http://tools.ietf.org/html/draft-lee-sip-dns-sd-uri-01
SIP URI advertisement• Example
_sipuri._udp.local. PTR sip:[email protected]._sipuri._udp.local. _sipuri._udp.local. PTR sip:[email protected]._sipuri._udp.local. sip:[email protected]._sipuri._udp.local. SRV
0 0 5060 bobs-host.local. sip:[email protected]._sipuri._udp.local. TXT txtvers=1 name=Bob contact=sip:[email protected].
• Service instance name: Instance.Service.Domain– Instance = ( SIP-URI / SIPS-URI ) [ SP description ]– Service = “_sipuri._udp” / “_sipuri._tcp” / “_sipuri._sctp”– E.g.) sip:[email protected] - PDA._sipuri._udp.local.
• Contact TXT record attribute– Similar to Contact SIP header except:
• It contains only a single URI• Non-SIP URIs are not allowed
– UA capabilities advertised via field parameters (RFC3840)
Next step: DHT discovery and initialization
• DHT discovery (prospective peer to overlay)– How to discover an existing DHT to join– Current mechanisms:
• Well-known bootstrap server• Expanding ring multicast• Server selection infrastructure: overlay anycast, LoST• Meta-DHT
• DHT initialization– How to construct a DHT efficiently from scratch
• first time or after major disruption• deal with network partition?• avoid creating multiple islands
– Comparison between different DHT architectures• Ring vs prefix-based• Flat vs hierarchical
– Cost considerations: time and network bandwidth– Especially timely with recent Skype failure
P2P for Voice - Open Issues
VoIP functions
• All subject to distribution:• call routing• media server (mixing, transcoding, recognition)• media storage• credentialing• authorization• PSTN gateway
Performance
• Look-up performance for N peers is O(log N)– affects call setup delay– e.g., Skype delay much higher than C-S calls
• ==> use combination of peers and clients• media generally not routed through overlay• spare capacity => more resilient to overload• harder to compensate for hot spots
Economics
• Operator saves on– bandwidth
• minimal for SIP signaling• interesting for media (TURN, relay, mixing)
– servers• single SIP server can handle > 100,000 users ==>
$0.10/month• except for NAT traversal (heartbeat)• except for media processing
Reliability
• CW: “P2P systems are more reliable”• Catastrophic failure vs. partial failure
– single data item vs. whole system
• Node reliability– correlated failures of servers (power, access,
DOS)– lots of very unreliable servers (95%?)
• Natural vs. induced replication of data items
Security & privacy
• Security much harder– user authentication and credentialing
• usually now centralized
– sybil attacks– byzantine failures
• Privacy– storing user data on somebody else’s machine
• Distributed nature doesn’t help much– one attack likely to work everywhere
• CALEA?
OA&M
• No real peer-to-peer management systems– system loading (CPU, bandwidth)
• automatic splitting of hot spots
– user experience (signaling delay, data path)– call failures
• P2PP adds mechanism to query nodes for characteristics
• Who gathers and evaluates the overall system health?
Locality
• Most P2P systems location-agnostic– each “hop” half-way across the globe
• Locality matters– media servers, STUN servers, relays, ...
• Working on location-aware systems– keep successors in close proximity– AS-local STUN servers
Mobility
• Mobile nodes are poor peer candidates– power consumption– unreliable links– asymmetric links
• But no problem as clients
Peer-to-Peer Protocol (P2PP)
Salman Abdul Baset, Henning SchulzrinneColumbia University
Overview
• Objective: key (opaque) data– distributed data structure with O(log N) or O(1) [rarely]
• Practical issues in peer-to-peer systems• Peer-to-peer systems
– file sharing– VoIP– streaming
• P2PSIP architecture• Peer-to-peer protocol (P2PP)• P2PP design issues• Implementation
Practical issues in peer-to-peer systems
• Bootstrap / service discovery
• NAT and firewall traversal• TCP or UDP?
• Routing-table management• Operation during churn
• Availability and replication
• Identity and trust management
Peer-to-peer systems
File sharing VoIP Streaming
Low
Medium
High
NAT
NAT
NAT
Data size
Data size
Data size
Pe
rfo
rman
ce im
pa
ct /
req
uire
me
nt
Service discovery
Replication
Replication
Replication
P2PSIP: Concepts
• Decentralized SIP– Replace SIP proxy and registrar with
p2p endpoints
• Supernode architecture– P2PSIP peers
• participate in the p2p overlay
– P2PSIP clients• use peers to locate users and resources
P2PSIP architecture
SIP
P2P STUN
TLS / SSL
A peer in P2PSIP
NAT
NAT
A client
[ Bootstrap / authentication server ]
Overlay1
Overlay2
Peer-to-Peer Protocol (P2PP)
• P2P applications have common requirements such as discovery, NAT traversal, relay selection, replication, and churn management.
• Goals– A protocol to potentially implement any structured or
unstructured protocol.– Not dependent on a single DHT or p2p protocol
• Not a new DHT!
• It is hard!– Too many structured and unstructured p2p protocols– Too many design choices!
• Lets consider DHTs
DHTs
DHT GeometryDistance function
Lookup correctness (neighbor
table)
Lookup performance
(routing table)
ChordAccordion
RingModulo numeric
differenceSuccessor list Finger table
Tapestry, Pastry,
Bamboo
Hybrid =
Tree + Ring
Prefix match. If fails, then modulo numeric
difference
Leaf-set (Pastry) Routing table
Kademlia XOR XOR of two IDs None Routing table
Kademlia
XOR
Finger table
Parallel requests
Recursive routing Pastry
Bamboo
ChordSuccessor
Modulo additionPrefix-match
Leaf-set
Routing-table stabilization
Lookup correctness
Lookup performanceProximity neighbor selection
Proximity route selection
Routing-table size
Strict vs. surrogate routing
OneHop
Bootstrapping
Updating routing-table from lookup requests
Tapestry Ring
Tree
HybridReactive recovery
Periodic recovery Accordion
Routing-table exploration
How to design P2PP?
• Structured– Identify commonalities in DHTs
• Routing table (finger table)• Neighbor table (successor list, leaf-set)
– Separate core routing mechanisms from from DHT-independent issues.
• Unstructured– may not always find all keys
• Incorporate mechanisms for– discovery– NAT / firewall traversal– churn, identity and trust management– request routing (recursive / iterative / parallel)
Parallel requests
Recursive routing
Routing-table stabilization
Proximity neighbor selection
Proximity route selection
Bootstrapping
Reactive vs. periodic recovery
DHT-independent
Kademlia
XOR
Pastry
Bamboo Chord
Modulo addition
Prefix-match
OneHop
Tapestry
Ring
Tree
Hybrid
DHT-specific
Accordion
Finger table / routingtable
Successor / leaf-set
Lookup correctness
Lookup performance
Routing-table size
Strict vs. surrogate routing
Updating routing-table from lookup requests
DHT-specific Not restricted toone DHT
Routing-table exploration
Geometry
How to design P2PP?
Chord (Strict routing-table management)
Neighbor table(successor)
Node
x+2i
x+2i+1
x+2i+2
x+2i+3
id=x
Routing table
Immediately succeeds routing-table id
Chord (flexible routing-table management)
Neighbor table
Node
x+2i
x+2i+1
x+2i+2
x+2i+3
id=x
Routing table
Any node inthe interval
Kademlia(XOR)
Node
2i
2i+1
2i+2
2i+3
id=x
Routing table
No neighbor table
Peer-to-Peer Protocol (P2PP)
• A binary protocol• Geared towards IP telephony but equally applicable
to file sharing, streaming, and p2p-VoD• Multiple DHT and unstructured p2p protocol support• Application API• NAT traversal
– using STUN, TURN and ICE– ICE encoding in P2PP
• Request routing– recursive, iterative, parallel– per message
• Supports hierarchy (super nodes [peers], ordinary nodes [clients])
• Reliable or unreliable transport (TCP or UDP)
Peer-to-Peer Protocol (P2PP)
• Security– DTLS, TLS, signatures
• Multiple hash function support– SHA1, SHA256, MD4, MD5
• Diagnostics– churn rate, messages sent/received
• Node capabilities– bw determination, CPU utilization, number of
neighbors, mobility
JoinJP BS P5 P7
1. Query
2. 200
P5, P30, P2P-Options
4. Join
9. 200
N(P9, P15)
5. Join
7. 200
P9
JP(P10)
8. Join
6. 200
N(P9, P15)
10. Transfer
11. 200
3+. STUN (ICE candidate gathering)
Call establishmentP1 P3 P5 P7
1. Lookup-Peer (P7)
5. 200 (P7 Peer-Info)
2. Lookup-Peer (P7) 3. Lookup-Peer (P7)
4. 200 (P7 Peer-Info)
6. 200 (P7 Peer-Info)
7. INVITE
8. 200 Ok
9. ACK
Media
Peer-to-Peer Protocol (P2PP)
HT = host | NAT-address | relayed
Peer-Info
P2P-Options
Implementation
• Chord, Kademlia, Bamboo (in-progress)• SHA1, SHA256, MD5, MD4• Windows, Linux• Integrated with OpenWengo (VoIP phone)
• Available for download (Linux + Windows)http://www1.cs.columbia.edu/~salman/p2pp/setupp2pp.html
Implementation
Transport / timers
Node
BigInt
Parser / encoder
UDP TCP
Transactions
ClientBootstrap ChordPeer KadPeer OtherPeer
Sys
insert (key, value, callback)callback (resp)
lookup (key, callback)
Routing table
Neighbor table
Distance
Screen snapshot
• Alice and Bob are part of Kademlia network• Alice calls Bob• The lookup is performed using P2PP• Call is established using SIP
Conclusion
• P2P techniques now becoming mainstream– motivated by low opex, ease of deployment– building block, rather than application
• Many operational issues– interconnection: z2z– local peering: Bonjour for SIP– start-up and recovery: cf. Skype failure
• P2PP: Common platform protocol– application-neutral– extensible mechanism