[U-Boot-Users] Multicast TFTP problems

[Ben Warren]

Rafal,

Rafal Jaworowski wrote:
> Ben Warren wrote:
> [...]
>   
>> TFTP packets are structured as follows:
>>
>> | Eth header (14 bytes) | IP header (20 bytes) | UDP header (8 bytes) | 
>> TFTP message | Eth FCS (4 bytes) |
>>
>> You can see that the total overhead is 46 bytes. If your network port's 
>> maximum MTU size is 1500 bytes (the default), the math works out for a 
>> maximum TFTP PDU size of 1454 bytes.
>>
>> However... the Ethernet header/trailer is not supposed to be included in 
>> the MTU. I suspect we have a terminology problem here. Let me dig a bit 
>> further to find out what's going on.
>>
>>     
>
> Hi Ben,
>
> Did you manage to look at this any further?
>   
The size of 1468 works fine for me on an MPC8349EMDS board. I captured 
the transfer using Wireshark (Ethereal) and the numbers match exactly as 
I've shown above.
Here are the first few transfers.

-----------------------------------------------------------------------------------------------
No.     Time        Source                Destination           Protocol Info
      1 0.000000    10.69.69.201          10.69.69.23           TFTP     Read Request, File: zImage, Transfer type: octet

Frame 1 (80 bytes on wire, 80 bytes captured)
Ethernet II, Src: Motorola_00:7d:20 (00:e0:0c:00:7d:20), Dst: WwPcbaTe_a3:fc:9c (00:0f:1f:a3:fc:9c)
Internet Protocol, Src: 10.69.69.201 (10.69.69.201), Dst: 10.69.69.23 (10.69.69.23)
User Datagram Protocol, Src Port: 2714 (2714), Dst Port: tftp (69)
Trivial File Transfer Protocol
    Opcode: Read Request (1)
    Source File: zImage
    Type: octet
    Option: timeout = 5
    Option: blksize = 1468

*****
***** You can see we've requested a block size of 1468 bytes
*****

No.     Time        Source                Destination           Protocol Info
      2 0.000891    10.69.69.23           10.69.69.201          TFTP     Option Acknowledgement

Frame 2 (67 bytes on wire, 67 bytes captured)
Ethernet II, Src: WwPcbaTe_a3:fc:9c (00:0f:1f:a3:fc:9c), Dst: Motorola_00:7d:20 (00:e0:0c:00:7d:20)
Internet Protocol, Src: 10.69.69.23 (10.69.69.23), Dst: 10.69.69.201 (10.69.69.201)
User Datagram Protocol, Src Port: 33454 (33454), Dst Port: 2714 (2714)
Trivial File Transfer Protocol
    Opcode: Option Acknowledgement (6)
    Option: timeout = 5
    Option: blksize = 1468

*****
***** The server acknowledges this option
*****

No.     Time        Source                Destination           Protocol Info
      3 0.000945    10.69.69.201          10.69.69.23           TFTP     Acknowledgement, Block: 0

Frame 3 (60 bytes on wire, 60 bytes captured)
Ethernet II, Src: Motorola_00:7d:20 (00:e0:0c:00:7d:20), Dst: WwPcbaTe_a3:fc:9c (00:0f:1f:a3:fc:9c)
Internet Protocol, Src: 10.69.69.201 (10.69.69.201), Dst: 10.69.69.23 (10.69.69.23)
User Datagram Protocol, Src Port: 2714 (2714), Dst Port: 33454 (33454)
Trivial File Transfer Protocol
    Opcode: Acknowledgement (4)
    Block: 0

No.     Time        Source                Destination           Protocol Info
      4 0.001605    10.69.69.23           10.69.69.201          TFTP     Data Packet, Block: 1

Frame 4 (1514 bytes on wire, 1514 bytes captured)
Ethernet II, Src: WwPcbaTe_a3:fc:9c (00:0f:1f:a3:fc:9c), Dst: Motorola_00:7d:20 (00:e0:0c:00:7d:20)
Internet Protocol, Src: 10.69.69.23 (10.69.69.23), Dst: 10.69.69.201 (10.69.69.201)
User Datagram Protocol, Src Port: 33454 (33454), Dst Port: 2714 (2714)
Trivial File Transfer Protocol
    Opcode: Data Packet (3)
    Block: 1
    Data (1468 bytes)

*****
***** The first block transfer.  Notice the total size is 1514, indicating that this fits in an Ethernet MTU of 1500 bytes.
*****

-----------------------------------------------------------------------------------------------


The only thing I can think of is that something on your network is 
limiting the MTU size to less than 1500 bytes. Are you using VLANs or 
PPPoE? Maybe your server has its MTU limited to 1486?

Please repeat what I've done here and let me know what you see.

regards,
Ben