Table of Contents
ICMP (Internet Control Message Protocol)
The Internet Control Message Protocol (ICMP) is a network protocol used to report errors and exchange diagnostic information between network devices.
Unlike TCP and UDP, ICMP is not used to transport application data. Instead, it helps devices determine whether communication is working correctly.
What Is ICMP Used For?
ICMP is commonly used for:
- Network diagnostics
- Error reporting
- Reachability testing
- Path discovery
Many familiar networking tools depend on ICMP.
Common ICMP Messages
Some of the most frequently used ICMP message types include:
| ICMP Message | Purpose |
|---|---|
| Echo Request | Sent by Ping |
| Echo Reply | Returned by Ping |
| Destination Unreachable | Destination cannot be reached |
| Time Exceeded | TTL expired (used by Traceroute) |
| Redirect | Suggests a better route |
| Parameter Problem | Invalid IP header |
Ping
The ping command uses ICMP.
When you execute:
ping 8.8.8.8
the computer sends an ICMP Echo Request.
If the destination is reachable, it returns an ICMP Echo Reply.
Example:
Computer
│
Echo Request
▼
Server
▲
Echo Reply
│
Computer
Ping measures:
- Reachability
- Round-trip time (RTT)
- Packet loss
Traceroute
Traceroute also relies on ICMP.
It discovers the path to a destination by sending packets with increasing Time To Live (TTL) values.
Example:
TTL = 1 ↓ Router 1 ↓ ICMP Time Exceeded TTL = 2 ↓ Router 2 ↓ ICMP Time Exceeded TTL = 3 ↓ Router 3 ↓ ICMP Time Exceeded
Eventually the packet reaches the destination.
This allows Traceroute to identify every router along the path.
Destination Unreachable
When a router or host cannot deliver a packet, it may return:
ICMP Destination Unreachable
Possible reasons include:
- Network unavailable
- Host unavailable
- Port unreachable
- Administrative filtering
ICMP Is Not TCP or UDP
ICMP is carried directly inside IP packets.
Unlike TCP and UDP, ICMP:
- Has no port numbers.
- Does not establish connections.
- Does not carry application data.
Is ICMP Secure?
Many firewalls restrict or block ICMP traffic.
Reasons include:
- Preventing network discovery
- Reducing abuse
- Limiting denial-of-service attacks
As a result:
Ping fails
does not always mean:
The host is offline.
The host may simply ignore ICMP Echo Requests.
ICMP and Telenegar
Several Telenegar services use or interpret ICMP.
Ping Check
Uses ICMP Echo Request and Echo Reply to determine:
- Reachability
- Latency
- Packet loss
Traceroute
Uses ICMP Time Exceeded messages returned by routers to discover the network path.
Example:
1 Home Router 2 ISP Gateway 3 Regional Router 4 ... Destination
Common Misconceptions
- ICMP is used to transfer files.
- False. ICMP is used only for control and diagnostic messages.
- If Ping fails, the server is down.
- False. Many servers intentionally block ICMP.
- Traceroute uses only ICMP.
- Not always. Depending on the operating system, Traceroute may use ICMP, UDP, or TCP probes.