RFC 3421 (rfc3421) - Page 2 of 8
Select and Sort Extensions for the Service Location Protocol (SLP)
Alternative Format: Original Text Document
RFC 3421 Select and Sort Extensions for SLP November 2002 Using the Select extension, a UA can opt for finding just a few (not necessarily all) matching services, which is useful if the UA uses a low-bandwidth channel. Using the Sort extension, a UA can ask the DA/SA to sort matching URL entries, which helps the UA to choose a service from multiple candidates. Sorting by the server is more efficient than sorting by the client since for sorting purposes, the former does not need to pass the attributes of matching URLs to the client. Furthermore, using the Select and Sort extensions together can facilitate discovering the best match, such as finding a service that has the maximum speed or the minimum load, or has a speed closest to a reference value. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted according to in BCP 14, RFC 2119 [RFC 2119]. 2. Select Extension 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Select Extension ID = 0x4002 | Next Extension Offset (NEO) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NEO, cont'd | Number of URL Entries | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1. Select Extension The format of the Select extension is shown in Figure 1. A UA uses this extension in a Service Request (SrvRqst) to limit the maximum number (say, n) of URL entries to be returned. When a DA/SA receives a SrvRqst with a Select extension, it MUST use a Select extension in the corresponding SrvRply to indicate the total number (say, m) of matching URL entries if the DA/SA supports this extension, otherwise the DA/SA MUST set the error code in the corresponding SrvRply to OPTION_NOT_UNDERSTOOD [RFC 2608]. If n < m, then only the first n matching URL entries are returned, else all m matching URL entries are returned. As a special case, a UA may set n to zero to obtain the number of matching URL entries without retrieving the entries themselves. We denote a Select extension as Select(number). Thus, Select(3) means that the corresponding SrvRply can have at most three URL entries. Zhao, et. al. Experimental
