NNRP provides the article, body, group, head, newgroups, post and stat commands in the same form as they are provided by NNTP. The list command is available in an extended form. As in the previous section we do not describe all of the commands available. A comprehensive discussion of the commands available can be found in [KL86] and in [Bar97].
One extension of NNRP is the Overview Database . The overview database [Bar97] stores data for each article, like the author, the subject, message-id, etc. This is basically all the information that is necessary for a news reader to display a summary of the articles available in a newsgroup. The overview database is transferred to a client in a fixed format without field identifiers. The format of the overview database can be requested using the list overview.fmt command. Figure B.2 shows the overview record for the article shown in Figure 2.2 ( indicates the tabulator character).
The overview database has approximately half the size of the head of its article and reduces the transmission time for the newsgroup by about 50%. Since the overview database is smaller than the sum of all article heads, the news reader can reduce the network bandwidth and transmission time. To display the summary of available articles the news reader requests the overview database instead of all the articles' heads.
The overview database does not reduce the required network bandwidth if the whole group is read and the news reader caches the articles' heads. However, this is rarely the case.
The List command is extended. In addition to the format explained in section B.2.1 the command may have one of the following arguments. In all cases the response codes of the list command without arguments are valid responses.
Returns the list of newsgroups available on the news server along with the numbers of the first and last article and the newsgroups type (e.g., public, moderated, etc.). As optional argument a wildmat pattern can be specified that allows to limit the newsgroups to those that match the pattern.
Returns the list of newsgroups with their creation date and creator.
Returns a short description for each newsgroup.
Returns the format of the overview database.
This command returns a list of numbers of available articles within the currently selected newsgroup. If a name of the newsgroup is given as argument the given newsgroup will be selected and the article numbers of this group will be returned.
|211 list of article numbers follows|
|412 not currently in a newsgroup|
|502 no permission|
The XOVER command returns information from the overview database for the article(s) specified. If no argument is specified, then the overview record for the current article is returned. Alternatively an argument in the form first -last may be specified. If first or last is omitted, then this is equivalent with the first or the last article number respectively.
On success this command returns a 224 response code along with the overview records for the requested articles.
|224 Overview information follows|
|412 No news group current selected|
|420 No article(s) selected|
|502 no permission|