# What is Time to Live (TTL)?

> Time to live (TTL) is an eight-bit Internet Protocol version 4 (IPv4) header field that limits packet travel.

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## Content

Each forwarding router reduces it by at least one; at zero, the packet is discarded and the router normally returns an Internet Control Message Protocol (ICMP) Time Exceeded message. Internet Protocol version 6 (IPv6) provides the same function through Hop Limit.

Although IPv4 defined time-based semantics, forwarding makes TTL effectively a hop-count limit. Its purpose is to stop packets from circulating indefinitely in routing loops. Senders choose an initial value, and traceroute deliberately varies it to elicit responses from successive routers.

### Key points

- **At the sender:** The operating system or application places an initial TTL or Hop Limit in each packet; the value must be high enough for the intended path, while higher values allow more forwarding hops before expiry.
- **At a router:** Forwarding decrements the field. Expiry stops the packet and may generate an ICMP error, subject to ICMP rules and rate limits.
- **Diagnostic value:** A received packet’s remaining value and expiry responses can support path troubleshooting, but tunnels, middleboxes, differing initial values, and route changes complicate interpretation.
- **Important limitation:** TTL is not an authentication, integrity, or authorization control. A sender can choose the initial value, so observed TTL alone does not reliably identify an operating system, establish proximity, or prove that traffic is trustworthy.

### Related terms

[Traceroute](<https://yellowcube.eu/glossary/traceroute/>) · [Internet Control Message Protocol (ICMP)](<https://yellowcube.eu/glossary/internet-control-message-protocol/>) · [TCP/IP protocol suite](<https://yellowcube.eu/glossary/tcp-ip-protocol-suite/>) · [Domain Name System (DNS)](<https://yellowcube.eu/glossary/domain-name-system/>) · [IP addressing: static and dynamic addresses](<https://yellowcube.eu/glossary/ip-addressing-static-and-dynamic-addresses/>)

### Sources

[RFC 1812: Requirements for IP Version 4 Routers](https://www.rfc-editor.org/rfc/rfc1812) · [RFC 8200: Internet Protocol, Version 6 (IPv6) Specification](https://www.rfc-editor.org/rfc/rfc8200)

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