lwIP Ethernet examples definition constant values

Patrick Ruf

Hi,

I have a question relating the definition constants in the Ethernet examples of StarterWare for AM335x (V2.00.01.01).
The examples define the following values:

- TCP_MSS (in lwipopts.h): 1500
- MAX_TRANSFER_UNITS (in cpswif.c): 1500
- MIN_PKT_LEN (in cpswif.c): 60
- PBUF_LEN_MAX (in cpswif): 1520
- PBUF_POOL_BUFSIZE (in lwipopts.h): 1520

I'm wondering how this values evolved. I'm fine with the following values:

- MAX_TRANSFER_UNITS (in cpswif.c): 1500
- MIN_PKT_LEN (in cpswif.c): 60

For standard Ethernet (IP V4) I see the following values:

- TCP_MSS (in lwipopts.h): 1460 (as lwIP WIKI suggest for Ethernet)
- PBUF_POOL_BUFSIZE (in lwipopts.h): 1514 or 1518 (VLAN) (1518 or 1522, if FCS is part of the data)
- PBUF_LEN_MAX (in cpswif): same as PBUF_POOL_BUFSIZE

If FCS is part of the received data from CPSW module is an other open question in this forum.

Any ideas from TI side for this values? Why did TI choose PBUF_LEN_MAX with 1520? Why did TI choose TCP_MSS with 1500?

Thanks and best regards,
Patrick

over 10 years ago

0 Rogerio Almeida over 10 years ago

TI__Mastermind 26205 points

Patrick,

I am checking my notes from my visit and I am escalating this...sorry for taking so long...

0 Ramesh Dandamudi over 10 years ago

TI__Intellectual 1480 points

Patrick,

The constants are assigned with the max value so as to cover all combinations. TCP_MSS is recommended bythe LWIP. PBUF_LEN_MAX is set as maximum length.

Regards,

Ramesh D

0 Patrick Ruf over 10 years ago in reply to Ramesh Dandamudi

Genius 4965 points

Ramesh,

What I still not understand is the value 1500 for TCP_MSS. As the examples from TI are for Ethernet this value should be 1460 as recommended by lwIP here:

http://lwip.wikia.com/wiki/Tuning_TCP

In my opinion also the value for PBUF_LEN_MAX (1520) is wrong. This value should be the sum of header length (14) and Maximum Transfer Unit (1500) and VLAN tag size (4) that results in 1518.

As CPSW module in AM335x also copy the FCS (4 Bytes) the PBUF_LEN_MAX should in my opinion be 1522 to cover all combinations for Ethernet.

Can you explain how the value 1520 evolved for PBUF_LEN_MAX?

You can call this a "detail". But as someone comes into this examples as a novice things should be comprehensible.

It would help a lot if you can explain how the values in the examples evolved. Thanks.

Best regards,
Patrick

0 Ramesh Dandamudi over 10 years ago in reply to Patrick Ruf

TI__Intellectual 1480 points

Patrick,

TCP_MSS has to be 1460 as per LwIP recommendation. PBUF_LEN_MAX has to be 1522. Same will be incorporated for next release.

Regards,

Ramesh D

0 Patrick Ruf over 10 years ago in reply to Ramesh Dandamudi

Genius 4965 points

Ramesh,

Thanks for clarification. I hope your decision is not only based on my opinion. If you came to this result with your own investigations than I'm shure this are the right values to choose.

Thanks and best regards,
Patrick

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lwIP Ethernet examples definition constant values