Next-generation IP network transmission protocol-traffic control transmission protocol sctp

With the development of the IP network to the multi-service network, especially the development of VoIP in recent years, it is inevitable to transmit signaling messages in the IP network. But in the IP network cannot provide high-quality transmission services like mtp3 and ATM, so the ITU-T proposed an improved version of sscop-sscopmce in Multi-link and non-connection environment ), it is used to transmit signaling messages in an IP-based network. However, compared with sscop, sscopmce only defines an adaptation interface with IP and UDP, and does not address the high loss rate and latency of the IP network compared to the ATM network, what changes have been made in traffic control and error control mechanisms. As a result, sscopmce can only use some other research institutions on internal or dedicated IP networks with certain transmission quality assurance to propose a new protocol for transport Signaling on the IP network, such as reliable signaling transmission protocol RSTP.

The IETF signaling transmission Working Group (sigtran) is defining and improving another mechanism for transmitting signaling messages, which is just becoming the ietfrfc stream control transmission protocol (sctp ). This Protocol enables the exchange of signaling messages on an IP-based public group switching network. Traffic Control and error control are implemented end-to-end, to improve the effectiveness, you can use a cluster of applicationserver processes and multi-homed nodes. In this way, with sctp, business flows on the user plane and control plane can be transmitted over a single IP address. I. Overview of stream control transmission protocol (sctp)

Currently, the signaling message exchange in the IP network is usually completed through UDP or TCP. However, neither of these can fully meet the requirements of the carrier's Citic order.

UDP is message-based and provides fast connectionless services. This makes it suitable for signaling messages with sensitive transmission latency. However, UDP only provides unreliable datagram services. Error Control, including message sequence, repeated message detection, and re-transmission of lost messages, can only be implemented by upper-layer applications.

Although TCP provides error and traffic control, there are many defects in transmitting signaling messages:★TCP is for byte streams. This means that the description of the message must be completed by the application, and TCP should be explicitly notified at the end of the message to force it to immediately send the corresponding byte data;

★Many applications only need to order part of the signaling message, for example, messages belonging to the same call or session. However, TCP only provides strict data transmission in order, which will cause unnecessary header blocking and increase the transmission latency of messages;

★TCP connections are directly identified by a pair of transmission addresses (IP addresses and port numbers), which cannot provide transparent support for multi-host hosts;

★A typical TCP implementation does not allow high-level applications to set protocol control parameters. However, some applications may need to adjust the attributes of transmission protocols to meet their specific requirements. For example, some signaling protocols have high latency requirements, while other signaling protocols only require high reliability. Sctp and developed the strengths of UDP and TCP Protocols. On the one hand, it enhances UDP services and provides reliable data transmission; on the other hand, sctp protocol behavior is similar to TCP and attempts to overcome some limitations of TCP. As defined in ietfrfc2960, sctp is a reliable datagram transmission protocol that runs on a group network that provides unreliable transmission, such as an IP network. It provides users with the following services:

User data is transmitted without errors and duplicates. User data is segmented based on the detected MTU length to avoid IP layer segmentation) the options for orderly delivery of user messages and delivery of individual user messages in the order of arrival; optional, multiplexing of multiple user messages to sctp datagram; network-level fault tolerance is achieved by supporting the multi-host feature of one or both nodes. Sctp's design goals include appropriate congestion avoidance features and resistance to flood and disguised attacks.

Stream in the sctp name indicates that this protocol can process several user datagram streams in each association and provide ordered transmission for user datagram in each stream. The existence of the stream Sub-layer can prevent header blocking caused by data loss in other streams.

User data error-free and repeated validation transmission is achieved through the grouping sequence Gap Report and selective resending. Sctp adopts traffic control and congestion control algorithms similar to TCP, and references the slow start mechanism. Only in terms of the Data grouping validation and retransmission mechanism, selective validation is clearly part of the standard, and the mechanism for selective retransmission and rapid recovery is standardized.

1. Protocol Structure

RFC 2960 breaks down the sctp Protocol into functional independent modules and modeled interfaces (such as timers) with operating system functions ). This decomposition is good for structured program development during protocol implementation.

A sctp system instance contains the following modules:

(1) Message verification and distribution, verification of sctp datagram and identification of its corresponding Association.

(2) Manage routes to monitor the accessibility of different transport addresses on the associated peer. (3) reuse/demultiplexing: Reuse/demultiplexing several associated data blocks and control data blocks to/from a sctp datagram, this datagram is transmitted in an IP group.

(4) window and traffic control, implementing traffic control and congestion avoidance mechanisms similar to TCP.

(5) sctp control, used to control the association status (for example, the establishment and removal of association, see Figure 3 ).

(6) reliable transmission, buffering and storing the sent messages until they are confirmed and accepted by the associated peer end, and sending them again when necessary.

(7) receive control, monitor all incoming messages, and generate required validation control data blocks.

(8) The stream engine submits user datagram of each stream in sequence, and completes segmentation and reinstallation of user datagram of too long when necessary. 2. sctp Association

Similar to TCP, sctp is also connection-oriented. However, the Association concept in sctp has a wider meaning than the connection concept in TCP. The two associated sctp endpoints provide each other with a sctp port number and an IP address list, so that each association is identified by two sctp port numbers and two IP address lists. The congestion control mechanism in an association is similar to the TCP congestion control mechanism.

A "join" association is composed of multiple one-way "streams. Each stream is relatively independent. data can be sent independently without being influenced by other streams, and user data can be submitted in an orderly manner. The process of stream creation and removal is relatively independent and simple. The association process is relatively complicated. It is a "four-way handshake" process, and the concept of "cookie" is used. The so-called "cookie" is actually a data block containing the initial information and encrypted information of the endpoint. When the association is established, both parties are involved to process and exchange the information. This mechanism is introduced to enhance Protocol security and prevent potential attacks such as denial of service (denialof service) and disguise.

3. Fault Management

Sctp provides more powerful path status monitoring functions than TCP. The introduction of the "multi-host" concept enables a connection to select and switch between multiple transmission paths, improving the network-level fault tolerance capability. Sctp defines heartbeat messages. When a path is idle, a corresponding Heartbeat message is generated and sent to the peer end, and the peer end must immediately send back the corresponding confirmation message. This mechanism is used to accurately measure the loop latency RTT, and can monitor the availability of the link at any time and maintain the link activation status. The heartbeat message sending interval can be flexibly changed based on your needs.

In addition, the number of repeated data packets is counted at two levels. The path-level or stream-level retransmission count statistics are used to determine whether the path is inactive. Association-level statistics are used to determine the accessibility of a remote endpoint.

Ii. sctp Application

1. overall frame of IP network 7 signaling transmission

This article takes sctp as an example to discuss the application of SS7 in VoIP applications. The overall framework for IETF-Based IP network transmission signaling 7 is described in rfc2719. Sctp is mainly used to transmit ISUP messages between the signaling gateway (SG) and the Media Gateway controller (MGC. SG is the gateway between the SS7 network and the IP network. It processes all MTP-related tasks, but does not include SS7 users. This user part (typically ISUP) is located in MGC, where MGC communicates with SG and controls the media gateway (MG) through the IP network ). An MGC can be associated with multiple SG instances to achieve redundancy and possible load balancing. Under this framework, from the MTP-Based Network Perspective, SG can be considered as a signaling transmission point (STP), while MGC can be considered as a signaling endpoint (SEP ). By using SG, MGC can communicate transparently with SEP in the MTP network. IP-based signaling transmission makes it possible to establish a service control point (SCP) without an MTP protocol stack. In this way, ISUP messages can be transmitted between SG and SCP.

2. Application Server Cluster

Although sctp is designed as a universal transmission protocol to overcome some limitations of TCP, the signaling message transmission for IP networks is a major motivation for sctp development in the sigtran Working Group. Using only the characteristics of multi-host and sctp is not enough to meet the demanding reliability requirements of SS7, because it must avoid failure of a single node. Therefore, the upper-layer protocol (ULP) discussed in the signtan group should use the concept of "Application Server process (ASP)" cluster. This cluster is called "Application Server (AS)". ULP provides functional entities to manage these clusters. That is to say, a control message can suspend or activate an ASP. It is worth noting that it is possible to run multiple ASP of one as on different hosts. In addition, it is possible to achieve load balancing between multiple ASP files of one. In the interconnection structure mentioned above, an as corresponds to an MGC. By running the as multiple ASP on different hosts, you can get a distributed MGC.

3. sctp upper-Layer Protocol Adaptation Layer

The definition of the adaptation layer allows you to use the IP-based signaling Transport Protocol without affecting the upper interface. For example, to transmit an ISUP message over an IP network, the ISUP itself does not have to be modified. To transmit SCCP and ISUP messages, the primitives between the MTP Layer 3 (mtp3) and SS7 user sections must be transmitted. In addition to m3ua, the adaptation layer m2ua for mtp2, The iua for ISUP, and the SUA for SCCP have also been defined. M2ua provides the transmission of primitives between mtp2 and mtp3. iua and SUA enable the ISUP and SCCP user messages to be directly transmitted by sctp.

Iii. Outlook

Sctp is developed for the transmission signaling business flow. However, it has some advanced protocol mechanisms that are superior to TCP, such as selective validation, fast retransmission, unordered submission, and support for multiple network features, sctp can meet the needs of high-performance transmission to a certain extent. This will provide a wider application space for sctp. At the same time, because sctp adopts a traffic control mechanism similar to TCP, there is no problem of deteriorating interference and fairness caused by UDP-based Real-time media streams. This paves the way for smooth introduction of sctp. Some research and standardization organizations believe that sctp will replace TCP and become a reliable connection-oriented transmission protocol on the Next Generation IP network. Of course, sctp has just become the official recommendation of IETF. Although several organizations, such as Motorola, Cisco, Siemens, Nortel and Nokia, have completed their respective protocol implementation and interconnection tests, however, the performance has not been simulated and further work is required. In addition, the application and performance evaluation of sctp in wireless Internet still need further research.