Lollipop sequence numbering is a
numbering scheme used in
routing protocols
A routing protocol specifies how routers communicate with each other to distribute information that enables them to select routes between nodes on a computer network. Routers perform the traffic directing functions on the Internet; data packets ...
. In this numbering scheme, sequence numbers start at a negative value, increase until they reach zero, then cycle 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
A lollipop is a type of sugar candy usually consisting of hard candy mounted on a stick and intended for sucking or licking. Different informal terms are used in different places, including lolly, sucker, sticky-pop, etc. Lollipops are ava ...
.
Lollipop sequence numbering was originally believed to resolve the ambiguity problem in cyclic sequence numbering schemes, and was used in
OSPF
Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR) algorithm and falls into the group of interior gateway protocols (IGPs), operating within a single autonomous syst ...
version 1 for this reason. Later work showed that this was not the case, like in the
ARPANET sequence bug, and OSPF version 2 replaced it with a linear numbering space, with special rules for what happens when the sequence numbers reach the end of the numbering space.
[Jon T. Moy "Link State Advertisements (LSAs)." ''OSPF: Anatomy of an Internet Routing Protocol'' p. 77.]
References
* R. Perlman. "Fault-Tolerant Broadcasting of Routing Information." ''Computer Networks'', Vol. 7, December 1983, pp. 395–405.
External links
Powerpoint presentation on routing protocolsCisco press article on routing protocols, contains discussion of lollipop numbering, and its failure to resolve ambiguity
Identifiers
Routing protocols