Transcript snom technology Presentation
Running SIP behind NAT
Dr. Christian Stredicke, snom technology AG
Tokyo, Japan, Oct 22 th 2002
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Overview 5 6 1 2 3 4 Problem Description STUN: Using Legacy Equipment TURN: Fixing Remaining Problems UPnP: Remote Control for Routers Application Layer Gateways Remaining Problems & Solutions 2
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3 Which information does a client has to set up for port forwarding in NAT equipment?
• •
Router needs information where to send packets in private network
– Map port to private address and port – By default packets will be rejected or sent to DMZ
Router needs hint for security checking
– Accept packets from any destination – Accept packets only from associated host – Accept packets only from associated host and port Router Client Client
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How did other applications solve the problem?
• • • •
HTTP, telnet, …
– Using TCP
DNS, others
– “Digging holes”: Set up association when client sends out packet from unmapped port for 15-60 seconds – Security policy hardwired by vendor – Some offer a DNS proxy (application layer gateway)
ftp
– Does not work!
– Inexperienced users use http instead – Some routers offer applications layer gateway
Heterogeneous environment
– Every vendor does it in a different way – “Digging holes” is common denominator
4
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5 snom STUN uses the digging hole trick to set up port associations
•
Initialization procedure checks environment
– Goal: Check if STUN is needed – Type of NAT does actually not really matter because user is not interested in failure reason •
SIP port kept alive by sending packets every 15-60 s
•
RTP ports are allocated dynamically when starting a call
– Otherwise keep-alive traffic would be double – RTCP port can not be allocated because next port allocation is unlikely – Long ringing and putting caller on hold is problematic (no port refresh during this time)
6 In cases when NAT is symmetrical, TURN could be a solution V1.0
STUN/TURN Server 3. SIP/Media Router Client Client
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7 TURN works in symmetrical NAT environment, but has too many problems
• • •
Scalability
– TURN server becomes “media server” – Every call generates about 50 packets per second
Delay
– Sending packets over media server increases transport delay significantly – E.g. local call in Tokyo when TURN server is in Frankfurt
TURN specification
– Needs rework (activation message not defined)
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UPnP is the right approach
• • •
Generic protocol to allocate ports on router
– Works with SIP, can be used with other applications as well – Can be integrated with firewalls – Not too hard to implement
Microsoft Messenger uses UPnP
– “De facto standard” – Virtually all DSL router vendors offer UPnP now
Problem: Old Equipment
– Use STUN – Maybe use TURN, even if call duration is terrible – Instruct customers to set up ports manually
8
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With the increasing availability of UPnP, most home customers can be addressed 2002 2003
STUN STUN UPnP • Software Updates • New Equipment UPnP
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10 Application layer gateways (ALG) solve the problem in the business area
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Business customers have different requirements than home users
– Many phones – Want to run proxies, media servers, application servers behind their firewall – These applications probably will not have UPnP or STUN •
Therefore, firewalls will probably include SIP-aware ALG
•
Sample SIP NAT ALG available from snom
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Calling phones in the same network requires ancillary information 11
1a) Phone A sends to public address of B 1b) Router will not forward packet, call will fail 2) A knows B is in the same NAT and sends packet to private address instead
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Ancillary information must be placed in contact URI and in SDP 12
INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP 218.230.0.59:5060;branch=z9hG4bK-6rms4e9tmtsz Max-Forwards: 70 From:
CSeq: 1 INVITE Contact:
s=SIP Call c=IN IP4 218.230.0.59
t=0 0 m=audio 10004 RTP/AVP 0 101 a=rtpmap:0 pcmu/8000 a=rtpmap:101 telephone-event/8000 a=fmtp:101 0-15 a=private:192.168.0.4:10004 218.230.0.59:10004
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13 Multi-tier NAT requires a list of private addresses
STUN When using STUN, a STUN server is required between the layers STUN 10.0.0.2
NAT2 192.168.0.1
123.123.123.123
10.0.0.1
10.0.0.3
NAT1 192.168.0.1
NAT3 Phone A 192.168.0.2
A has three identities: 1. 192.168.0.2:5060 2. 10.0.0.2:1234 3. 123.123.123.123:5678 Phone B 192.168.0.2
B has three identities: 1. 192.168.0.2:5060 2. 10.0.0.3:1234 3. 123.123.123.123:5679
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What is the status of snom products today?
• •
Latest snom 100 image includes UPnP and STUN
– Support for single-tier NAT – TURN has been put out again
snom 200 integration of UPnP and STUN
• • • •
snom 4S STUN/TURN server
– Includes STUN and TURN (with extensions) – Security and TCP not implemented
snom 4S proxy/location server adds STUN
– Integrated STUN server – Gives user agents hint when they are behind NAT
snom 4S media server
– Smooth call termination when no media available
snom 4S NAT gateway
– Available for Linux
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© 2002 snom technology Aktiengesellschaft Written by: Dr. Christian Stredicke Version: 1.0
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