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How big is an IP packet frame including headers ?

Answer 1

+2 A:

Maybe this article helps: http://sd.wareonearth.com/~phil/net/overhead/

Lucero 2010-10-13 23:05:37

Thanks, a very useful link. The throughput rates for gig-e are useful. I'm achieving 760GB/s on non-server hardware so I think thats not too bad. But, if I set my packet data correctly it may improve.

Matt H 2010-10-13 23:16:45

Answer 2

+5 A:

------------------------------------------------------------------------------
|Ethernet  | IPv4         |UDP    | Data                   |Ethernet checksum|
------------------------------------------------------------------------------
  14 bytes    20 bytes     8 bytes    x bytes                4 bytes
           \ (w/o options)                               /
            \___________________________________________/
                              |
                             MTU

If your MTU is 1500, you have 1500-20-8 = 1472 bytes for your data.

If you exceed that, the packets will be fragmented ,i.e. split into more packets.
There might be more layers involved, e.g. 4 byte a vlan header if you're on top of a vlan ethernet.
Some routers inbetween you and the destination might add more layers.

nos 2010-10-13 23:11:42

AFAIK the MTU does NOT include the Ethernet header and the trailing CRC32 (which you omitted), but is the effective Ethernet payload, where you have to subtract the IP protocol overhead.

Lucero 2010-10-13 23:12:59

Thanks, thats what I suspected but wasn't certain.

Matt H 2010-10-13 23:15:22

@Lucero Yes, you might be right, depending on which layer you define the MTU as. MTU of an IP datagram in this case I guess.

nos 2010-10-13 23:18:12

@nos: The MTU for Ethernet is the one we're talking about, so there is little speculation to which layer it applies.

Lucero 2010-10-13 23:21:08

Notable extra layer is subnetting, 8 bytes for 802.2 SNAP LLC.

Steve-o 2010-10-16 02:39:15

Answer 3

+1 A:

Yes your example would not fit in one frame.

The ethernet data payload is 1500 bytes. IPv4 requires a minimum of 20 bytes for its header. Alternatively IPv6 requires a minimum of 40 bytes. UDP requires 8 bytes for its header. That leaves 1472 bytes (ipv4) or 1452 (ipv6) for your data.

More information:

teambob 2010-10-13 23:24:09

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How big is an IP packet frame including headers ?

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