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RFC 4638 / START HERE

PPPoE larger payloads, before the details

PPPoE means Point-to-Point Protocol over Ethernet. MTU is a maximum transmit size; MRU is the maximum receive size advertised to a peer.

It lets broadband session endpoints signal support for a PPP payload above the usual 1492-octet limit, then use normal PPP negotiation within the allowed limits.

Start the guide See the 5 chapters ↓

The idea in one picture

  1. Count the overhead

  2. Signal a maximum

  3. Check the session

A simplified picture of the idea; the chapters explain the actual mechanisms.

Why it exists

PPP over Ethernet spends six octets on a PPPoE header and two on a PPP Protocol field. This Informational publication explains an optional extension for networks that can carry extra Ethernet payload; agreement does not guarantee the whole path.

A familiar example

A home gateway can ask for a 1500-octet PPP payload, which needs 1508 octets of Ethernet payload. The access server must also respect its interface and configuration; a session-open test can reveal a size problem and lead to a 1492 send cap.

Helpful background

You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.

IPv4RFC 791 · beginner overview ↗IPv4 Path MTURFC 1191 · beginner overview ↗IPv6 Path MTURFC 8201 · beginner overview ↗

What you’ll learn next

Begin with chapter one, or choose a topic.

  1. Make room for PPPoE overheadPoint-to-Point Protocol over Ethernet (PPPoE) adds link information around a PPP payload. MTU is a transmit limit; MRU is what a peer says it can receive.
  2. Advertise and echo support during discoveryA client discovers an access server before the PPP session. A larger-payload client carries PPP-Max-Payload in its initiation and request; a supporting server echoes it in offer and confirmation.
  3. Read the real PPP-Max-Payload tagDiscovery carries a type-length-value tag. Its binary value is a maximum PPP payload in octets, directly comparable to the MRU value, rather than an Ethernet frame size.
  4. Use a cap before ordinary MRU negotiationThe Link Control Protocol (LCP) negotiates PPP options. Discovery permits considering a larger MRU; a selected server interface can still impose a smaller maximum.
  5. Probe the opened session and bound a failureLarge discovery values describe endpoint capability. An intermediate Ethernet segment can still drop a large packet, so the sender can offer a session-open size test.
Start with chapter one →

This introduction explains RFC 4638 (2006). Read the original RFC ↗