CODEWITHSUNDEEP
network-programmingip
Mark
Mark

Reputation: 6484

RFC791, MTU and a minimum packet size

I was reading rfc791 and trying to understand the relations with MTU as well as with the minimum packet size for IPv4. Here are two quotes from the rfc:

"All hosts must be prepared to accept datagrams of up to 576 octets (whether they arrive whole or in fragments)."

And

"Every internet module must be able to forward a datagram of 68 octets without further fragmentation.  This is because an internet header may be up to 60 octets, and the minimum fragment is 8 octets."

Do I understand correctly that first related only to hosts, i.e. only hosts must be able to process minimum packet size of 576 bytes, while the second statement defines the mi packet size for a router? But of so, then it is possible to have a router not being able to receive a packet of 68 bytes for himself ?

Or I'm missing something very fundamental?

Thanks. Mark

Upvotes: 1

Views: 6871

Answers (3)

Ashish Sagar
Ashish Sagar

Reputation: 11

By standard, 576 Bytes is "minimum MTU" supported over IP infrastructure. This means, any host/router must support this value and any IP packets can be smaller than 576 Bytes (atleast 68 Bytes) which can move IP world without fragmentation.

HTH

Upvotes: 1

Chris Ryding
Chris Ryding

Reputation: 1533

The 576 octet is a "least maximum". In other words, a host needs to be prepared to have a maximum packet size of no less than 576 octets. It can be bigger than that, such as the 1518 limit used by most (non-jumbo) Ethernet devices, but not any smaller.

Anything that is set up to forward packets must not split them into chunks of less than 68 octets.

Upvotes: 1

user207421
user207421

Reputation: 311008

The first relates to accepting; the second relates to forwarding.

then it is possible to have a router not being able to receive a packet of 68 bytes for himself

That doesn't make any sense. A host must be able to accept datagrams of up to 576 octets.

Upvotes: 0

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