Lollipop sequence numbering

Lollipop sequence numbering is a numbering scheme used in routing protocols. In this numbering scheme, sequence numbers start at a negative value, increase until they reach zero, then cycles through a finite set of positive numbers indefinitely. When a system is rebooted, the sequence is restarted from a negative number again. This allows recently rebooted systems to be distinguished from systems which have simply looped around their numbering space. This path can be visualized as a line with a circle at the end; hence a lollipop.

Lollipop sequence numbering was originally believed to resolve the ambiguity problem in cyclic sequence numbering schemes, and was used in OSPF version 1 for this reason. Later work showed that this was not the case, and OSPF version 2 no longer uses it, instead replacing it with a linear numbering space, with special rules for what happens when the sequence numbers reach the end of the numbering space.

References

* R. Perlman. "Fault-Tolerant Broadcasting of Routing Information." "Computer Networks", Vol. 7, December 1983, pp. 395–405.

External links

* [http://www.it.iitb.ac.in/~it605/dep/slides/08_ospf.ppt Powerpoint presentation on routing protocols]
* [http://www.ciscopress.com/articles/article.asp?p=24090&seqNum=4 Cisco press article on routing protocols, contains discussion of lollipop numbering, and its failure to resolve ambiguity]
* [http://archives.neohapsis.com/archives/microsoft/various/ospf/2001-q3/0353.html Mailing list post on vulnerabilities of lollipop approach]