Interactive Connectivity Establishment

Abstract

Interactive Connectivity Establishment

Authors

Walter Fan

Status

WIP as draft

Updated

2024-08-21

Ovewview

RFC8445 Interactive Connectivity Establishment (ICE) 交互式连接建立是一个网络地址转换 NAT 穿透的协议

NAT

NAT 是指路由器把本地私有子网IP地址转换称公网 IP 地址的过程。 ICE/STUN/TURN 主要是解决如何在各自内网中的客户端之间如何连接

根据最少限制性到最多限制性来排序可以分为：

  • Full cone（全锥型）

  • Address-restricted cone （地址限制型锥形）

  • Port-restricted cone（端口限制型锥型）

  • Symmetric（对称型）

ICE procedure

1. Candidate gathering * STUN * TURN 2. Prioritisation 3. Exchange 4. Connectivity checks 5. Coordination 6. Communication

ICE candidae

ICE candiate type

  • host: Host Candidate

    A candidate obtained by binding to a specific port from an IP address on the host. This includes IP addresses on physical interfaces and logical ones, such as ones obtained through VPNs.

  • srflx: Server-Reflexive Candidate

    A candidate whose IP address and port are a binding allocated by a NAT for an ICE agent after it sends a packet through the NAT to a server, such as a STUN server.

  • prflx: Peer-Reflexive Candidate

    A candidate whose IP address and port are a binding allocated by a NAT for an ICE agent after it sends a packet through the NAT to its peer.

  • relay: Relayed Candidate

    A candidate obtained from a relay server, such as a TURN server.

TCP Candidates with Interactive Connectivity Establishment (ICE)

ICE defines a mechanism for NAT traversal for multimedia communication protocols based on the offer/answer model of session negotiation.

The ICE Candidate can also be TCP-based , refer to RFC6544 and RFC4571

           +----------+
           |          |
           |    App   |
+----------+----------+     +----------+----------+
|          |          |     |          |          |
|   STUN   |  (D)TLS  |     |   STUN   |    App   |
+----------+----------+     +----------+----------+
|                     |     |                     |
|      RFC 4571       |     |      RFC 4571       |
+---------------------+     +---------------------+
|                     |     |                     |
|         TCP         |     |         TCP         |
+---------------------+     +---------------------+
|                     |     |                     |
|         IP          |     |         IP          |
+---------------------+     +---------------------+

  Figure 1: ICE TCP Stack with and without (D)TLS

Framing RTP and RTCP Packets over TCP

参见 RFC4571

  • The Framing Method

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 ---------------------------------------------------------------
|             LENGTH            |  RTP or RTCP packet ...       |
 ---------------------------------------------------------------

     Figure 1: The bit field definition of the framing method

The session descriptions in Figures 3 and 4 define a TCP RTP/AVP session.

v=0
o=first 2520644554 2838152170 IN IP4 first.example.net
s=Example
t=0 0
c=IN IP4 192.0.2.105
m=audio 9 TCP/RTP/AVP 11
a=setup:active
a=connection:new

       Figure 3: TCP session description for the first participant

v=0
o=second 2520644554 2838152170 IN IP4 second.example.net
s=Example
t=0 0
c=IN IP4 192.0.2.94
m=audio 16112 TCP/RTP/AVP 10 11
a=setup:passive
a=connection:new

       Figure 4: TCP session description for the second participant

Reference