The Secret of Online Physical Examination Technology (Part 1)

1. Necessity of online physical examination

In daily life, we should be exposed to various inspections, except for physical inspections, vehicle inspections, and security inspections. If you are experienced, you should be able to pick out many examples of inspections. What about the network, which is closely related to us and is said to be as important as or even more important than water and electricity? Do we need to conduct a physical inspection on it? You may say, "What's the use of a physical inspection? A physical inspection can't make the network speed faster or better."

Indeed, for a long time we have chosen to accept the Internet, not because we have a good temper, but because we really don't seem to have a good way to check our network and determine which link is causing trouble when our network is acting like a spoiled child. If we can have some effective methods or handy tools to diagnose our network, we can determine the crux of the problem, and then call the relevant person in charge to solve the problem immediately, and our network will be restored to health in time. We can also continue to surf the Internet happily.

Through this article, I will share with you a new understanding of this topic. Maybe you will find that the "if" in the above text can actually be removed. In other words, we have some means to detect our network, otherwise how can we say that Ethernet is a NB network~

2. Commonly used methods of online physical examination

We have also mentioned in previous articles that learning technology is not easy, so we only learn mainstream (standard) technologies. It would be embarrassing if other non-mainstream technologies are eliminated before we even understand them.

1. Common methods of second-level physical examination

The main means of Layer 2 are basically defined and regulated in the three standards IEEE 802.3ah, IEEE 802.1ag, and ITU-T Y.1731 (MEF actually has a definition, which is similar). The most common ones are EFM and CFD. Let's first compare them in a table, and then introduce them separately.

Table 1 Comparison between EFM and CFD

(1) EFM technology

a. Protocol message

EFM OAM works at the data link layer, and its protocol messages are called OAMPDU (OAM Protocol Data Units). EFM OAM reports link status through regular exchange of OAMPDUs between devices, enabling network administrators to effectively manage the network.

Figure 1 OAMPDU message format diagram

Table 2 Meaning of important fields in OAMPDU

Table 3 Common OAMPDU

b. EFM OAM connection establishment

The implementation of EFM OAM functions is based on EFM OAM connections. The process of establishing an EFM OAM connection is also called the Discovery phase, which is the process in which the local OAM entity discovers the remote OAM entity and establishes a stable dialogue with it.

When an interface of a device is enabled with the EFM OAM function, if the EFM OAM working mode of the interface is active mode, the interface will initiate an EFM OAM connection to the remote end. In the process of establishing an EFM OAM connection, the connected OAM entities notify each other of their EFM OAM configuration information by exchanging Information OAMPDUs. After receiving the configuration parameters from the remote end, the OAM entity decides whether to establish an EFM OAM connection.

Figure 2 EFM OAM connection diagram

As shown in Figure 2, interface Ethernet 1/1 of Device A works in active mode. When EFM OAM is enabled on the interface:

• Device A sends an Information OAMPDU to Device B, which contains the EFM OAM configuration information of Device A.
• After receiving the OAMPDU, Device B matches it with its own EFM OAM configuration, and then replies to Device A with an Information OAMPDU, which contains not only the EFM OAM configuration information of both Device A and Device B, but also flag information indicating whether Device B matches Device A's EFM OAM configuration.
• After Device A receives the OAMPDU from Device B, it determines whether the EFM OAM configuration of Device B matches its own configuration.

Through the above process, if the EFM OAM configurations of both parties match, the EFM OAM connection is established. After the EFM OAM connection is established, the OAM entities at both ends will periodically send Information OAMPDU to detect whether the connection is normal. If one end of the OAM entity does not receive the Information OAMPDU sent by the remote end within the connection timeout period, it is considered that the EFM OAM connection is disconnected.

c. Link performance monitoring

When an OAM entity at one end monitors a general link event, it will send an Event Notification OAMPDU to its remote OAM entity for notification, and record the monitoring information in a log and report it to the network management system; after receiving the information, the remote OAM entity will also record it in a log and report it to the network management system. In this way, the administrator can dynamically grasp the status of the network by observing the log information.

• Remote fault detection: When an emergency link event occurs on a device and causes traffic interruption, the fault-side OAM entity notifies the remote OAM entity of the fault information (i.e., the type of emergency link event) through the Flag field in the Information OAMPDU, and records the fault information in the log and reports it to the network management system; after receiving the information, the remote OAM entity also records it in the log and reports it to the network management system. In this way, the administrator can dynamically understand the link status by observing the log information and handle the corresponding errors in a timely manner.
• Remote loopback: The remote loopback function means that when the OAM entity in active mode sends all other messages except OAMPDU to the remote end, the remote end does not forward the message according to its destination address after receiving it, but returns it to the local end along the original route. It can be used to locate link failures and detect link quality: network administrators can judge link performance (including packet loss rate, delay, jitter, etc.) by observing the return of non-OAMPDU messages.

Figure 3 Remote loopback diagram

As shown in Figure 3, interface Ethernet 1/1 of Device A works in active mode. After the EFM OAM connection between Device A and Device B is established, the remote loopback function is enabled on the interface:

• Device A sends a Loopback Control OAMPDU with enable information to Device B and waits for a reply.
• After receiving the OAMPDU, Device B replies to Device A with an Information OAMPDU indicating the status change and enters the loopback state (in this state, the device returns all non-OAMPDU messages received along the original path).
• After receiving the reply, Device A starts to send non-OAMPDU test packets to Device B.
• After receiving the test message, Device B returns it to Device A along the original path.
• When Device A needs to stop remote loopback, it sends a Loopback Control OAMPDU containing the disable information to Device B.
• After receiving the OAMPDU, Device B exits the loopback state and replies to Device A with an Information OAMPDU indicating the state change.