Ethernet Padding

Hi All,

This post is related to Ethernet padding. When the payload is small, Ethernet adds padding to make the frame size to 64 bytes. (Ethernet's minimum frame size is 64 bytes).

This padding, will it removed by anybody or will it just propagate through the network? Is this padding different in Etherenet II and 802.3?

Reply to
Anbarasan S
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The padding is applied to the packet that will be placed "on the wire". As the packet flows to all the nodes on the local subnet, each node will receive the padding with the packet, so the padding will "propagate through the network". Each node will ignore the padding (silently discarding it) when receiving and evaluating the packet address information, so the padding "will be removed" by all receiving nodes.

The padding brings the size of the smallest datagram up to the size of the smallest physical packet. If Ethernet II and/or 802.3 have different minimum physical packet sizes from Ethernet, then the amount of padding will be different. But remember, the amount of padding in /any/ packet depends on the difference in size between the logical data and the physical packet; the amount of padding *will* vary, packet to packet, as the amount of logical data varies.

Reply to
Lew Pitcher

=A0 =A0 ------

how does the receive ethernet will remove the padding? Etherenet header has no information about the length of padding it added while transmitting. or in the receive side, will the ethernet pass the payload to IP and IP, based on total header length, will ignore padding.

assume a router in a network. how this router will see the padding? when it receives with padding in the ethernet interface and routes the IP to some other interface say PPP, will the padding added by ethernet also transfered to PPP or will the padding be removed and only IP will be sent in PPP link.

Ethernet =3D=3D=3D=3D=3D=3D Router ------ PPP

Regards Anbarasan

Reply to
Anbarasan S

Some higher-level protocol incorporated the pad at the sending end. At the receiving end, the same protocol will remove the pad. How that happens is protocol-specific, but by necessity both the sender and the receiver are using the same higher-level protocol.

If IP is the higher-level protocol, then yes, it will know how much (if any) pad is present based on the appropriate length field.

If it is an IP router, then it is the same as if it were an IP station, above. If it is some other protocol, whatever protocol- specific mechanism is used will be invoked.

Since there is generally no need to pad on a PPP link, the router will not include the pad. The router routes the *payload*, which by definition does not include any Ethernet padding.

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

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Reply to
Rich Seifert

[snip]

No, but the protocol datagram carried within the packet carries a lengt= h field. The protocol datagram is left-aligned (that is to say, comes fir= st within the frame), and any padding that has been added follows the prot= ocol datagram. Thus, any excess octets in the frame (trailing octets receive= d in the frame, but not accounted for by the protocol datagram's lengty fiel= d) are silently ignored. And thus "removed".

--=20 Lew Pitcher Master Codewright & JOAT-in-training | Registered Linux User #112576 Me:

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Reply to
Lew Pitcher

Warning:

Lew Pitcher, who posts to this newsgroup, is a domain thief.

Read the full story at

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Lew Pitcher

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