Re: [tcpm] Is ECN a valid TCP header flag?

tom petch <ietfa@btconnect.com> Fri, 24 September 2021 15:42 UTC

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From: tom petch <ietfa@btconnect.com>
To: Jonathan Morton <chromatix99@gmail.com>
CC: tcpm <tcpm@ietf.org>
Thread-Topic: [tcpm] Is ECN a valid TCP header flag?
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Date: Fri, 24 Sep 2021 15:42:26 +0000
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Subject: Re: [tcpm] Is ECN a valid TCP header flag?
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From: Jonathan Morton <chromatix99@gmail.com>
Sent: 24 September 2021 16:02

> On 24 Sep, 2021, at 1:05 pm, t petch <ietfa@btconnect.com> wrote:
>
>  base tcp-flags;
>    identity cwr {
>    identity ecn {
>    identity urg {
>    identity ack {
>    identity psh {
>    identity rst {
>    identity syn {
>    identity fin {
> with descriptions and references.  My knowledge of this is limited but I suspect that the reference for 'ecn' should be RFC3168 and not RFC793 but then should it be 'ece' and not 'ecn'?  I suspect that a TCP expert might see rather more idiosyncrasies.

I agree, it should definitely be ECE and not ECN.  The abbreviation is for "Echo Congestion Experienced", where "Congestion Experienced" refers to a codepoint of the ECN field in the IP header.  RFC-3168 describes the ECE and CWR (Congestion Window Reduced) flags and how they must be used.

The remaining established TCP flags are named here with their usual and familiar abbreviations.  I don't see any cause for concern there.

There are several additional TCP flags, in an adjacent byte, which are reserved for future use - effectively "should be zero" at origin and "should be ignored" upon receipt.  One of them was, for example, historically defined as NS (Nonce Sum), and other uses may arise in future.  Are these flags accounted for in the model?

<tp>

Thank you for the information; yes, I think that the I-D should reference RFC3168.  The model does not cater for additional flags but then it is not really that sort of model.  It is about security, about monitoring and action, so that it models the sort of things that a firewall might be configured to be interested in, such as SYN, port ranges, POP3 and so on; but as I said, there is much overlap with all sorts of IETF WG and I think that users would find it simpler if the terminology were to be the same   (and a transport AD might not feel the need to tear his hair out - one change the authors made at my request was to remove ICMP from the list of layer-4 protocols).

Tom Petch

 - Jonathan Morton