P-Early-Media 是 ETSI TISPAN 為了在 3GPP IMS authorizing early media flows 所採用的 private SIP P-header，也用在其它 SIP 網路。
- ETSI
- European Telecommunications Standards Institute
- TISPAN
- Telecommunications Internet-converged Services and Protocols for Advanced Networks
- 3GPP
- Third Generation Partnership Project
- IMS
- IP Multimedia Subsystems
- Early Media
- 應答前的 media，目的是協助用戶完成呼叫，通常不是兩端用戶間直接交換資訊，包括彩鈴、或告訴主叫端「您撥的號碼通話中」等聲音訊息。
Unfortunately, a SIP network interconnected with another SIP network may have no means of assuring that the interconnected network is implementing a compatible early media policy, thus allowing the exchange of user data within early media under some circumstances. For example, if a network "A" allows all early media with user equipment as UAC and an interconnected network "B" allows all early media with user equipment as UAS, any session established between user equipment as UAC in "A" and user equipment as UAS in "B" will allow bidirectional user data exchange as early media. Other combinations of early media policies may also produce similar undesirable results. The purpose of the extension is to allow a SIP network interconnected to other SIP networks with different early media policies to correctly identify and enable authorized early media according to its policies. - Backward Early Media
- 由 terminating switch 往主叫端方向，例如提供回鈴音。
- Forward Early Media
- 由主叫端往 terminating switch 方向，例如二次撥號的 DTMF digit collection。
authorize the cut-through of backward and/or forward early media when permitted by the early media policies of the networks involved. 用在有下列特性的 SIP 網路：兩端用戶間透過其它 SIP 網路連接，兩端用戶間無法直接交換 early media，但可以 "gate" (enable/disable) the flow of early media to/from user equipment.
在一個 isolated SIP 網路，可以自己決定是否開啟內部兩個用戶間的 early media。但如果兩個用戶間需要透過其它 SIP 網路連接，只有連到外部 SIP 網路的 boundary node 可以決定 which early media policy。The P-Early-Media header field provides a means for this boundary node to communicate this early media policy decision to other nodes within the network.
P-Early-Media 只應用 inside 定義在 RFC 3325 的 "Trust Domain". Nodes in such a Trust Domain are explicitly trusted by its users and end-systems to authorize early media requests only when allowed by early media policy within the Trust Domain.
This document does NOT offer a general early media authorization model suitable for inter-domain use or use in the Internet at large. Furthermore, since the early media requests are not cryptographically certified, they are subject to forgery, replay, and falsification in any architecture that does not meet the requirements of the Trust Domain.
An early media request also lacks an indication of who specifically is making or modifying the request, and so it must be assumed that the Trust Domain is making the request. Therefore, the information is only meaningful when securely received from a node known to be a member of the Trust Domain.
Although this extension can be used with parallel forking, it does not improve on the known problems with early media and parallel forking, as described in RFC 3960, unless one can assume the use of symmetric RTP.
待整理
Section 4.1 以後
| P-Access-Network-Info | [RFC7315] |
| P-Answer-State | [RFC4964] |
| P-Asserted-Identity | [RFC3325] |
| P-Asserted-Service | [RFC6050] |
| P-Called-Party-ID | [RFC7315] |
| P-Charge-Info | [RFC8496] |
| P-Charging-Function-Addresses | [RFC7315] |
| P-Charging-Vector | [RFC7315] |
| P-DCS-Trace-Party-ID | [RFC5503] |
| P-DCS-OSPS | [RFC5503] |
| P-DCS-Billing-Info | [RFC5503] |
| P-DCS-LAES | [RFC5503] |
| P-DCS-Redirect | [RFC5503] |
| P-Media-Authorization | [RFC3313] |
| P-Preferred-Identity | [RFC3325] |
| P-Preferred-Service | [RFC6050] |
| P-Private-Network-Indication | [RFC7316] |
| P-Profile-Key | [RFC5002] |
| P-Refused-URI-List | [RFC5318] |
| P-Served-User | [RFC5502] [RFC8498] |
| P-User-Database | [RFC4457] |
| P-Visited-Network-ID | [RFC7315] |
- P-Served-User: <sip:user@example.com>; sescase=orig; regstate=reg
- RFC 5502 The SIP P-Served-User Private-Header (P-Header) for the 3GPP IP Multimedia (IM) Core Network (CN) Subsystem
- sescase 可為 orig 或 term
- regstate 可為 reg 或 unreg
- P-Served-User 只用在 Trust Domain 內，如果來自非 Trust Domain、或要送到非 Trust Domain，則必須移除 P-Server-User。
SIP proxy 需要知道 request 的 served user 才能插入 P-Served-User。
serverd user 的定義是什麼？
Trust Domain 的定義是什麼？
