971031-4 Discussions often consists of a series of messages by different people, which are replies to each other. Which information in (a) e-mail, (b) netnews is used to specify the place of a message in such a chain? In which header fields is this information placed, and how is it made up?

Correct response:

Every message has a "Message-ID". The reply to a message has an "In-Reply-To" header with the Message-ID of the replied-to message. Following the chain of In-Reply-To headers allows a user to traverse a sequence of replies.

A variant of this, which is the proper procedure in netnews, but is getting more common also in e-mail, is to use the References header. It is used in similar ways to the In-Reply-To header, but it contains not only the Message-ID of the immediate replied-to message, but also all or most of the Message-ID-s of earlier messages in the thread.

A third variant is th use the Subject, which contains the same text in all messages in a thread, expect that the four characters "Re: " are added in all but the first message. This has the problem that responses sometimes change the subject, and that it gives no information about the place of a message within a thread, except indicating which is the message which started the whole thread.

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