Transactions on Petri Nets and Other Models of Concurrency III

AccordingtoHolzmann [14], protocol speci?cationscomprise ?veelements: the service the protocol provides toits users; the set of messages that are exchanged between protocol entities; the format of each message; the rules governingm- sage exchange (procedures); and the assumptionsabout the environment in which the protocol is intended tooperate. In protocol standards documents, information related to the operatingenvironment isusually writteninformally andmayoccur in several di?erentplaces [37]. This informal speci?cation style canlead to misunderstandings andpossibly incompatible implementations. In contrast,executableformalmodelsrequireprecisespeci?cations oftheoperating environment. Ofparticularsigni?canceisthecommunicationmediumorchannel over which the protocol operates. Channelscan havedi?erent characteristics depending on the physical media (e. g. optical ?bre, copper, cable orunguided media (radio)) they employ. The characteristics also depend on the levelof the protocol inacomputer protocol architecture.
Forexample, the link-leveloperates over a singlemedium,whereas the network, transport andapplication levelsmayoperate over a network,or network of networks such as the Internet,which couldemploy several di?erent physical media. Channels (such as satellite links) can be noisy resulting in bit errors in packets. To correct biterrors in packets, many importantprotocols (such the Internet's TransmissionControl Protocol [27]) use CyclicRedundancy Checks (CRCs)[28] to detect errors. On detectingan error,the receiver discards the packet andrelies on the sender to retransmit itforrecovery,known as Au- maticRepeatreQuest(ARQ)[28]. Thisisachievedbythereceiveracknowledging the receipt of good packets, andby the transmitter maintainingatimer. When the timer expires before an acknowledgementhasbeen received, the transmitter retransmits packets that havebeen sent but are as yet notacknowledged. It may also be possibleforpacketsto be lost due to routers in networks discarding packets when congested.