Point-to-Point Protocol (PPP) is a data link (Layer 2) protocol for carrying IP and other network traffic over a direct link between two devices. Besides framing the data, it negotiates the link, can authenticate the connecting device, and assigns addresses. PPP was standardized by the IETF in the early 1990s, and its current core specification, RFC 1661, dates from 1994. It became the standard for dial-up internet, and today it survives mainly as PPP over Ethernet (PPPoE) on DSL and some fiber broadband, and on older serial and T1 links.
At a glance
- PPP frames traffic on a link between two devices and handles setup, login and address assignment.
- PPPoE, which runs PPP over Ethernet, is still used by many DSL and some fiber broadband providers.
- If your provider uses PPPoE, your router or firewall must support it and be configured with the provider’s login.
- PPPoE reduces the maximum packet size to 1492 bytes, a common cause of odd connection problems.
- PPP’s login methods are weak by modern standards and are not a substitute for encryption.
What problem it solves
In the early days of the internet, computers and routers needed a standard way to send IP traffic over phone lines and serial links. An earlier method, SLIP, carried only IP and had no error checking, login or address negotiation. PPP added all of those in one standard that worked across many kinds of links and equipment.
That made it the basis of dial-up internet: the provider could verify each subscriber, hand out an IP address and run the session. When broadband arrived, providers kept the same model by running PPP over DSL and Ethernet, which let them reuse their login, addressing and billing systems.
How it works
- Framing. PPP wraps each packet with a header that says which protocol it carries and a check value for error detection.
- Link Control Protocol (LCP). Sets up and tests the link and agrees options such as maximum packet size and authentication method.
- Authentication. Optionally checks the connecting device with PAP, which sends the password in clear text, or CHAP, which uses a challenge and response. PPP can also use EAP, a framework for other authentication methods.
- Network Control Protocols. Configure each network protocol on the link, for example IPCP for IPv4 addresses and IPv6CP for IPv6.
Variations extend PPP to other links. PPPoE carries PPP sessions over Ethernet and is how many DSL and fiber providers log routers in. PPPoA does the same over ATM-based DSL. Multilink PPP bonds several links, such as two T1 lines, into one larger logical link. Older VPN protocols such as PPTP and L2TP also carried PPP sessions; PPTP is considered insecure and should not be used.
When it matters for buyers
- Replacing a router, firewall or SD-WAN device. If the broadband line uses PPPoE, the new device needs PPPoE support and the provider’s username and password. Find them before the cutover, not during it.
- Odd broadband problems. Pages or VPN tunnels that hang on large transfers can come from the reduced packet size on PPPoE. Setting the MTU or TCP MSS on your edge device often resolves them.
- Static IP addresses. Some providers deliver static addresses over a PPPoE session, tied to the login. Moving or replacing equipment without the right settings can break them.
- Retiring legacy circuits. T1 and serial links running PPP or Multilink PPP are often candidates for replacement as copper is retired. Plan the move to Ethernet-based services before the provider forces it.
- Choosing a service type. DSL and some broadband services use PPPoE; dedicated internet and Ethernet services usually don’t.
Our internet access page covers how to compare broadband and dedicated options.
Questions to ask vendors
- Does this service use PPPoE, DHCP or static addressing on the hand-off?
- If PPPoE, what credentials and settings does our equipment need, and who keeps them?
- What maximum packet size (MTU) does the service support?
- Are static IP addresses delivered over the PPP session or routed separately?
- Can you provide a modem or ONT in bridge mode so our own firewall handles the connection?
- For legacy T1 or serial links: what is the retirement timeline, and which Ethernet service replaces them?
How it differs from a point-to-point circuit
PPP is a protocol: a set of rules that two devices use to carry traffic over a link. A point-to-point circuit is a service a provider sells to connect two locations, such as an Ethernet private line. The names overlap because both involve exactly two ends, but buying a point-to-point circuit today rarely involves PPP. Most are Ethernet services. PPP is more likely to appear on a broadband internet line using PPPoE, or on an older T1 link between sites.
Problems this helps with
- A new firewall or SD-WAN box won't connect to our DSL or fiber line
- Websites or VPNs half-loading over a broadband connection
- Static IP addresses that stopped working after an equipment swap
- Old T1 or serial links nobody wants to touch before copper is retired
