Re: [lwip-users] Help in getting maximum throughput with LWIP.

[David Empson]

1500 is the normal limit on size for the body of an Ethernet frame. Subtract 20 bytes for the IP header and 20 for the TCP header, and you are left with 1460 bytes of data delivered via TCP for a standard Ethernet frame.

 

There is a mechanism known as "jumbo frames", which allows up to 9000 bytes in an Ethernet frame. It can only be used if it is supported by both the sender and receiver (and any switches between them). Jumbo frames tend to be associated with high speed network interfaces like Gigabit Ethernet (1000Base-T) but there may be some 100Mbit Ethernet controllers which support jumbo frames. On PCs, jumbo frames may only be available if enabled via a system configuration setting (which should only be set if your network infrastructure is compatible with jumbo frames).

 

If your packets are travelling beyond a LAN, then you are rarely able to achieve packet sizes larger than 1500 bytes. Many Internet connections have at least one hop with a maximum frame size somewhat less than 1500, and the smallest possible limit is 576.

 

Routers will typically fragment packets that are too large to send over the next hop, which reduces efficiency further.

 

 

----- Original Message -----

From: Dany Thiffeault

To: Mailing list for lwIP users

Sent: Wednesday, March 31, 2010 9:19 AM

Subject: Re: [lwip-users] Help in getting maximum throughput with LWIP.

hehe, another question.

I'm using Wireshark to see what's in flight. Whatever packet I choose to write in the stack, I get packets of size 1460bytes maximum on the wire.

...

1902 6.432931 192.168.0.72 192.168.0.68 TCP 1008 > lanyon-lantern [ACK] Seq=1709449 Ack=49 Win=17968 Len=1460

1904 6.434920 192.168.0.72 192.168.0.68 TCP 1008 > lanyon-lantern [ACK] Seq=1710909 Ack=49 Win=17968 Len=1460

...

Having configured a MSS size of 9216, I was thinking I would see that as the maximum. Why is it topped at 1460?