OSI Layer 2 datagrams are called frames.
Layer 2 datagrams of the ATM protocol are called cells.
The PPP (Point-to-point protocol) is a somewhat of a partial Layer 2 protocol. It is always used in conjunction with a second Layer 2 protocol, working as a carrier.
|PPP Header or Field or Tag|
Such PPP frames are encapsulated into the carrier frame: common are Ethernet and ATM. So we talk about
- PPPoE Frame: TODO (or simply combine the already given information)
- PPPoA Cell: TODO (or simply combine the already given information)
But since ATM protocol is ever present, you only seldom have pure PPPoE over the whole distance from your Modem to the DSLAM. You have ATM as well as Ethernet. And this gives you two possibilities to combine the two protocols. You can either encapsulate or bridge:
- Encapsulating: PPPoEoA Frame: Point-to-Point_Protocol_over_Ethernet#How_PPPoE_fits_in_the_DSL_Internet_access_architecture
- Briding: PPPoE-2-PPPoA this means that the Modem, strips the PPPoE header of every frame and replaces it with a PPPoA header.
- Pure PPPoE: it is being claimed that many VDSL2 connections relinquish the ATM protocols, and use pure PPPoE.
Datagrams at OSI Layer 2 are called packets.
|Protocol||Header Size in Bytes||Total Size in Bytes|
|Ethernet frame||38–42||Size: 84–1542Bytes, Jumbo frame up to 9042|
|IPv4 Packet||20–60||Size: 20– (20-byte header + 0 bytes data) 65.535|
|IPv6 Packet||40 fixed, Optional Extension Header possible||up to 65.535|
|TCP Segment||20–60||up to 65.535|
|UDP Packet||4–8 with IPv4 and 6–8 with IPv6||up to 65.535|
TCP segment in IPv4 packet in Ethernet frame
|Start of frame delimiter||1|
|802.1Q tag (opt.)||4|
|Ethertype or length||2||IPv4||Bits|
|Differentiated Services Code Point||6|
|Explicit Congestion Notification||2|
|Time to Live||8|
|Source IP Address||32|
|Destination IP Address||32|
|Options ( if Header Length > 5 )||?|
|Options (if Data Offset > 5)||varies|
doc/networking/datagram.structures.txt · Last modified: 2012/11/30 16:13 by uvray313