Re: [Int-area] 答复: 答复: 答复: 答复: Is the UDP destination port number resource running out?// re: I-D Action: draft-ietf-intarea-gue-04.txt

[Joe Touch <[e-mail]>]

On 5/25/2017 11:47 AM, Tom Herbert wrote:
>> You can't put bare Ethernet inside GUE. You need to use EtherIP -
>> exactly because it has a 16-bit field, of which only the first 4 bits
>> are (already) defined.
>>
>> My point is that EtherIP burns 16 bits vs bare Ethernet, but those 16
>> bits allow it to be mapped to one of the IP versions (you picked IPv5).
>> The same trick works for UDP and TCP - just pick a different 16 bit
>> pattern for each one.
>>
> Inserting two bytes before the TCP header breaks four byte alignment
> of the header which is a bigger hit than the benefit of saving two
> bytes. A nice side effect of the two byte header in EtherIP is that it
> aligns the Ethernet payload (e.g. an IP header) to four bytes.
> Maintaining this four byte alignment is still important to some CPU
> architectures most notably Sparc, but can even be problematic to x86
> under certain circumstances.
>
> Tom
Sure - I'm not sure the 4-byte penalty is worth avoiding any nearly any
case, frankly -- even for IP.

Joe