[e2e] Are we doing sliding window in the Internet?

Joe Touch touch at ISI.EDU
Thu Jan 4 10:40:16 PST 2007

Ted Faber wrote:
> On Thu, Jan 04, 2007 at 08:57:45AM -0800, Joe Touch wrote:
>> RFC1122 also states:
>>          *    "SHOULD"
>>               This word or the adjective "RECOMMENDED" means that there
>>               may exist valid reasons in particular circumstances to
>>               ignore this item, but the full implications should be
>>               understood and the case carefully weighed before choosing
>>               a different course.
> FWIW, I don't think SHOULDs should be thrown aside lightly, either.  But
> they're spots where the IETF consensus admits that designers and
> implementors can make a different decision without catastrophic
> interoperability problems.

That's not what's implied above, IMO, e.g., by using the terms "full"
and "carefully". Let's consider ma few of the SHOULDs in 1122 and
consider whether we can negate them without catastrophe:

- ARP would discard the first packet sent to each unresolved IP address
(Nagle saw this problem in 1986:

- ICMPs redirects could be used for arbitrary off-path diversion (

- packets could be forwarded to a gateway indefinitely in the absence of
positive information it is available

> For my money "bug" is much more derisive than even "wrong design"
> because it implies (to me) a level of obliviousness that doesn't seem
> present here.  Bugs are accidents; this seems like a conscious choice.

Bugs can be conscious choices too; they are just incorrect ones.

> I understand it's a choice you disagree with, but IMHO it's a choice
> that violates no RFC.

If 'violates' means obeys only MUSTs, then we agree. If 'violates' means
obeys all MUSTs and negates SHOULDs only in particular circumstances,
then we disagree.

> I think you're much better off debating the content of the design
> decision than wether it violates some unenforcable boundary.

I've already pointed out that it is likely to be unfair w.r.t. TCPs that
ACK every second packet all the time (excepting timeouts). Others seem
intent on finding ways to make their preferred OS behave better so long
as it's within the 'letter of the RFCs'; it is in that spirit that we
need to be clear on the conditions where SHOULDs are OK to skip.


Joe Touch
Sr. Network Engineer, USAF TSAT Space Segment

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