SCTP的信令连接建立过程

admin

本文翻译自一篇国外的技术文档，提供原文如下：

“Stream Control Transmission Protocol (SCTP) is a relatively new transport layer in the IP Protocol Stack.

SCTP belongs to the SIGTRAN protocol family and has been used as the transport layer for carrying telecom signaling over IP.

SCTP provides a reliable transport service that operates at message level (unlike TCP that provides a byte stream interface with no message boundaries).

SCTP allows multiple message streams to be exchanged on a single SCTP connection. Data from multiple streams can be sent in a single SCTP message as chunks. Selective acknowledgements are supported at individual chunk level.   

Recent additions to the SCTP protocol allow dynamic configuration of the IP addresses. Similar to SS7, SCTP can be switched over from one link to another. SCTP supports a make-before-break changeover, i.e. the packet stream is moved before removing the link that needs to be taken out of service.

In this sequence diagram we will be examining some of the features of SCTP.
(1) SCTP Connection establishment.
(2) SCTP data exchange and selective acknowledgement.
(3) Addition of a new IP address to an SCTP connection.
(4) Switching over to the new IP address.
(5) Removing the old IP address.
(6) SCTP connection release.

上述文字就不翻译了，关键是下面的这些信令过程。（比较多，拆分成多图介绍）。

1 An association setup request is received in the closed state. The server analyzes the received INIT chunk and generates all the values needed at its side to enter an established association, and generates a secure hash of these values and a secret key (e.g. with the MD5 or SHA-1 algorithms).

1 在关闭状态下收到了一个SCTP关联建立请求。服务器分析了接收到的INIT chunk并且在本端生成所有需要的值并进入到一个建立了的关联中，并且生成这些值的安全哈希值以及安全秘钥（用于鉴权对等体，例如MD5或者SHA-1算法）

2 The secure hash values and a message authentication code (MAC) are stored in a COOKIE parameter. This information is sent by in the INIT-ACK message.

2 安全哈希值并且鉴权的计算结果MAC被存放在COOKIUE参数字段中并进行传送，该参数字段通过INIT-ACK消息传送给对方。

3 SCTP Endpoint B copies the COOKIE into the COOKIE-ECHO message.

SCTP End Point A validates the COOKIE-ECHO by checking it against the included MAC,The COOKIE-ECHO validation had passed, so an SCTP instance is created.

SCTP Endpoint A replies with a COOKIE-ACK, indicating that the COOKIE-ECHO has been accepted.

3 SCTP端点B从COOKIE-ECHO消息中复制COOKIE的相关信息。

SCTP端点A通过检查COOKIE-ECHO消息中包含的MAC对该消息进行验证，如果COOK-ECHO的验证通过，那么一个SCTP的实例就创建了。

SCTP端点A将响应一个COOKIE-ACK消息，通知对方COOKIE-ECHO已经成功的接受了。

4 Endpoint B sends stream data using the DATA chunk.The Transport Sequence Number (TSN) is assigned to the chunk. The acknowledgements sent from the receiver to the sender are based on these sequence numbers.

4 SCTP端点B通过DATA chunk发送上层的stream数据部分。每个chunk都被分配了一个TSN。接收端将根据这些TSN序列号来发送确认给发送端。

admin

本部分描述的是如果在已经建立的SCTP连接中新增加一个IP地址以及设置主用IP地址的信令过程：

主要信令过程如下：

1 SCTP Endpoint B sends an Address configuration change chunk (ASCONF) with an "Add IP Address" indication for IP_Address_B_2.

2 SCTP Endpoint A acknowledges the data by sending a Cumulative TSN ACK. SCTP supports a powerful selective acknowledgement interface. Gaps in received sequence numbers can be signaled efficiently.

3 The SCTP Endpoint A acknowledges the addip request.The ack is sent within the Address Configuration Ack (ASCONF_ACK) chunk.

4 A new IP address has been added but this has no impact on the data communication.

5 A message containing the SACK and the DATA chunk.

6 The Set Primary Address command is used to indicate that henceworth IP_Address_B_2 should be the primary IP address for SCTP Endpoint B.

7 The SCTP Endpoint A acknowledges the message on the new primary IP address (i.e. IP_Address_B_2).

8 Pending data chunks are still exchanged using IP_Address_B_1.

1 SCTP端点B发送一个地址配置变更chunk（ASCONF）并且带有希望增加IP_Address_B_2的“add ip add”的指示。

2 SCTP端点A发送一个累计的TSN ACK进行确认。SCTP支持强大的选择性确认机制。通过确认报文中的gap字段可使得确认更有效率。

3 SCTP端点A确认了B的添加IP请求，并通过地址配置确认消息（ASCONF_ACK）chunk消息进行确认。

4 新的IP地址被添加了进来但是对数据的传输没有影响。

5 一个消息同时包含了SACK和DATA chunk。

6 set primary address命令将用于指示IP_Address_B_2应该作为SCTP端点B的主用IP地址。

7 SCTP端点A确认了新的主用IP地址（也就是IP_Address_B_2）。

8 临时缓存中的数据仍然通过IP_Address_B_1来传送。

完整的信令过程如下图，是接着楼上的信令流程图来的。如下所示：

xuanshao

有一个问题，在SCTP信令建立过程中，2条path和4条path在消息中有什么区别么？或者是那些字段可以看出来呢？多谢！