-----Original Message----- From: Thomas Narten [mailto:nar...@us.ibm.com] Sent: Friday, May 06, 2011 9:27 AM Subject: Re: Flow label drafts updated
Is the UDP port number mutable? Is the TCP sequence number immutable? [WEG] I think both are immutable because there's a checksum to detect changes. That doesn't make it impossible to change it in the middle, as you mention, but it makes it much less likely that it will be. There are ways of modifying them that are undetecable. Does that make them mutable? [WEG] Would you write similar language into the TCP and UDP spec like this that claims immutable but that implementers should assume that it'll be changed along the way? By my interpretation, it comes down to level of risk for modification. IMO the checksum is a significant difference and means that there is a reasonable assumption that things won't be changed along the way. At the very least it'll detect changes due to data corruption, which you can't say for the IPv6 header. The intention is that the Flow Label not get modified. Doing so can impact its usefulness. This is true of most fields in packets. If random actors start tweaking various fields in a packet, that tends to not be helpful (or worse). [WEG] Absolutely agree, but I don't think that calling it immutable is the only way to say it, especially if you bracket it with a qualifier that says it's likely to get changed anyway. I do think the document needs to say something about covert channels and border routers zeroing out the field (and not just stick its head in the sand and try to have it both ways, which the wording Wes quoted effectively does). But that doesn't mean we are declaring that the field is "mutable", implying that anyone can start doing with it what they want. [WEG] fair enough, but I think this is sort of like the line between MUST NOT and SHOULD NOT. What you're saying here is you SHOULD NOT change the value, while acknowledging that it's likely to happen in certain applications. To me, Immutable implies MUST NOT be changed, and while people may still ignore MUSTs it's less likely. To me, "anyone can start doing what they want" implies a MAY be changed Wes George
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