IMS/SIP中流程中我们经常看到订阅(SUSCRIBE)和通知（NOTIFY）消息，它们的定义、用途及示例分别如下：
1.消息定义
订阅(SUSCRIBE)在无线协议中类似“测量控制”或“信息请求”，要求对方对特定事件或状态进行报告；
通知（NOTIFY）在无线协议中类似“测量报告”或“信息反馈”，其是对订阅（SUSCRIBE）事件的信息反馈（具体见RFC3265中SIP事件定义节）；
2.订阅(SUSCRIBE)支持的事件
订阅是经常见的事件就是注册“reg”(注册状态检查)，另有很多其他的事件；详见下表：
Event | Description | Reference |
reg | Request for IMS/SIP Registration Status | RFC 3680 |
presence | Request for presence information (commonly used in RCS) | RFC 3856 |
winfo | Request for Watcher information (commonly used in RCS) | RFC 3857 |
xcap-diff | Request for Xcap-Diff information (commonly used in RCS) | RFC 3875 |
3.通知（NOTIFY)支持的事件
通知（通常是CSCF）有错误出现（如481，400，500）；除在消息正文之外提供的详细（除已订阅）信息，还会发送带有特定订阅状态信息的通知消息（具体参阅RFC3265 3.2.4）；
Subscription-State | Description |
active（激活） | It means the subscription is accepted (in most case, authorized as well) |
pending （暂缓） | It means the subscription has been received by the notifier, but there is insufficient policy information to grant or deny the subscription yet |
terminated （结束） | It means the subscription is terminated and this can specify as a reason code as follows |
| deactivated （去激活） | This mean that the subscription ended (terminated) but the subscriber (UE) SHOULD retry with a new subscription immediately. |
| probation （稍后再试） | This mean that the subscription ended (terminated) but the client SHOULD retry at some time later. |
| rejected （拒绝） | This mean that the subscription ended (terminated) due to change in authorization policy. The client SHOULD NOT retry. |
| timeout （超时） | This mean that the subscription ended (terminated) because it is not refreshed before it expired. The client MAY retry immediately. |
| giveup （放弃） | This mean that the subscription ended (terminated) because the notifier could not obtain authorization in time |
| noresource （无资源） | This mean that the subscription ended (terminated) because the resource being monitored is no longer available. |
4.订阅和通知流程
订阅(SUSCRIBE)一般由终端（UA）发起，服务器应答 通知（NOTIFY），其流程简化如下：
5.订阅和通知消息内容
以注册“reg”(注册状态检查)为例的“订阅”（ SUBSCRIBE）和“通知”（NOTIFY ）消息内容如下：
Step 1 : SUBSCRIBE -----------------------------------
SUBSCRIBE sip:+11234567890@test.3gpp.com SIP/2.0
Via: SIP/2.0/UDP 10.133.202.46:50997;branch=z9hG4bK2968d27245f17c7bcae38c31991bfdaa
Max-Forwards: 70
Contact: <sip:+11234567890@10.133.202.46:50997>;+sip.instance="<urn:gsma:imei:00440113-904785-0>"
To: <sip:+11234567890@test.3gpp.com>
From: <sip:+11234567890@test.3gpp.com>;tag=210a54
Call-ID: d57a0b04-785ba328-13a4d876@10.133.202.46
CSeq: 14534 SUBSCRIBE
Expires: 600000
User-Agent: IM-client/OMA1.0 DUT-IMS
Event: reg
Accept: application/reginfo+xml
P-Access-Network-Info: 3GPP-E-UTRAN-FDD;utran-cell-id-3gpp="0010100010000000"
P-Preferred-Identity: <sip:+11234567890@test.3gpp.com>
Content-Length: 0
Step 2 : 200 OK -----------------------------------
SIP/2.0 200 OK
Via: SIP/2.0/UDP 10.133.202.46:50997;branch=z9hG4bK2968d27245f17c7bcae38c31991bfdaa
From: <sip:+11234567890@test.3gpp.com>;tag=210a54
To: <sip:+11234567890@test.3gpp.com>;tag=987654321
Call-ID: d57a0b04-785ba328-13a4d876@10.133.202.46
CSeq: 14534 SUBSCRIBE
Expires: 600000
Contact: <sip:10.133.202.47:5060>
Record-Route: <sip:10.133.202.47;lr>
Content-Length: 0
Step 3 : NOTIFY -----------------------------------
NOTIFY sip:+11234567890@test.3gpp.com SIP/2.0
Via: SIP/2.0/UDP 10.133.202.47:5060;branch=z9hG4bK-d1e4c4961ca9d523ae76b67e088589cd
Call-ID: d57a0b04-785ba328-13a4d876@10.133.202.46
From: <sip:+11234567890@test.3gpp.com>;tag=987654321
To: <sip:+11234567890@test.3gpp.com>;tag=210a54
Subscription-State: active;expires=600000
Event: reg
CSeq: 14534 NOTIFY
Contact: <sip:10.133.202.47:5060>
Max-Forwards: 70
Content-Type: application/reginfo+xml
Content-Length: 340
<?xml version="1.0" encoding="UTF-8"?>
<reginfo xmlns="urn:ietf:params:xml:ns:reginfo" version="0" state="full">
<registration aor="sip:+11234567890@test.3gpp.com" id="12345" state="active">
<contact id="100" state="active" event="registered" expires="600000">
<uri>sip:+11234567890@10.133.202.46:50997</uri>
</contact>
</registration>
</reginfo>
Step 4 : 200 OK -----------------------------------
SIP/2.0 200 OK
Via: SIP/2.0/UDP 10.133.202.47:5060;branch=z9hG4bK-d1e4c4961ca9d523ae76b67e088589cd
Max-Forwards: 70
Contact: <sip:+11234567890@10.133.202.46:50997>;+sip.instance="<urn:gsma:imei:00440113-904785-0>"
To: <sip:+11234567890@test.3gpp.com>;tag=210a54
From: <sip:+11234567890@test.3gpp.com>;tag=987654321
Call-ID: d57a0b04-785ba328-13a4d876@10.133.202.46
CSeq: 14534 NOTIFY
Allow: NOTIFY,SUBSCRIBE
Content-Length: 0
原文来自：http://www.sharetechnote.com
整理：kangguoying20201023
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