[ntp:questions] Source address in response always the same as target address in request?

David L. Mills mills at udel.edu
Fri Dec 14 18:06:29 UTC 2007


Brian,

I recall the SHOULD was inserted to support the anycast model where any 
of a number of servers could respond to a specific request. NTP along 
with many others in the late 1970s did not anticipate such a model and, 
even if they did, as later in NTP manycast, the addresses are still 
necessary for operation sugbsequent to discovery.

Dave

Brian Utterback wrote:

> 
> Danny Mayer wrote:
> 
>>Brian,
>>
>>UDP is stateless. There is absolutely no way that the UDP protocol
>>developers could require that that a reply go back to the same address
>>from which the packet was sent or that it be sent from the same IP
>>address. No reply is ever required of a datagram. It would be a protocol
>>layering violation to do so. 
> 
> 
> And yet the host requirements RFC does so require it, at least to
> the "SHOULD" level:
> 
>        When the local host is multihomed, a UDP-based request/response
>        application SHOULD send the response with an IP source address
>        that is the same as the specific destination address of the UDP
>        request datagram.
> 
> My contention is that although it is a SHOULD, the sockets API did
> not provide a way to actually accomplish it, so very few UDP
> protocols do not deal with the failure to do so. Except NTP.




More information about the questions mailing list