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.6).These fields wereknown as service access points (SAPs).Included in the 802 header are SSAP (sourceservice access point) and DSAP (destination service access point).This arrangement givesthe flexibility of assigning numbers to protocols differently in each machine.Figure 2.6.SAP multiplexingThe SAP fields are 8 bits long.However, as with the LAN addresses, two of those bits arereserved.One of the reserved bits is for "global/local," which, as with the similar bit in the LANaddress field, indicates whether the 802 committee has assigned the number (to guarantee itsuniqueness) or whether the owner of the network or system manages the number.The other reserved bit is for "group/individual." This lets you send a packet that would bereceived by multiple higher-layer protocols within a system.The reason for reserving a bit inthe LAN address has to do with convenience in making chips.This reasoning is not applicablein the case of the SAP because filtering based on SAP is not done in the hardware.Anotherreason that the group/individual address makes sense in the LAN address but does not makeas much sense in the SAP is that it seems likely that many different stations on the same LANwould implement the same protocol and thus would want to receive the same message.Itseems less likely that a common protocol would be implemented by multiple upper-layerprocesses within the same station.However, with an entire 8 bits to play with, why notreserve one for the purpose, even though the justification for its need is slight? (Hint: Thepreceding statement is sarcastic, with a touch of bitterness.In fact, 8 bits are not nearlysufficient to make the scheme usable.Only because of the invention of the SNAP SAPkludge, described in the next few paragraphs, is it possible for protocols other than ISO orIEEE protocols to use the 802-defined LLC.)The SAP consisting of all 1's is reserved to mean "all SAPs," as with the broadcast address.The SAP consisting of all 0's (except for the "G/L" bit) is reserved to mean the data link layeritself and not a user of the data link layer.Figure 2.7 shows the SAP structure. Figure 2.7.Structure of SAPWith only 6 bits of globally assigned individual SAP numbers, the 802 committee cannot grantnumbers to every organization that might want to design a protocol.Rather than assignnumbers on a first-come, first-served basis until they are all gone, the 802 committee hasstrict rules for the sorts of organizations and protocols that can be granted a SAP number.Tobe eligible, a protocol must have been designed by a standards body approved by the 802committee.For those protocols privileged to receive a global SAP value, the SAP fields are used like asingle protocol type field because the SSAP and DSAP will be equal (they will equal theglobal SAP value assigned to that protocol).Other protocols (those without globally assigned SAP values) could use locally assigned SAPnumbers, and the manager of a system could ensure that each protocol had a unique numberwithin that system.However, this approach makes conversation start-up difficult because it ishard to send a protocol message to another machine when the SAP numbering within theforeign machine is unknown.A plan to make the SAP system usable was proposed within 802.It consisted of requesting asingle globally assigned SAP.When the DSAP was set to that value, it would indicate that theheader was expanded to include a "protocol type" field.The protocol type field could then belarge enough so that a global authority could ensure that every protocol was granted aglobally assigned number.Originally, the plan was that the protocol type field be 2 octets, on the theory that it was 2octets in the original Ethernet, and Xerox must have known what it was doing.But thensomeone noticed that the 802 header contained an odd number of octets.If the protocol typefield were an odd number of octets, it would make the entire header an even number ofoctets, and that would enhance performance on machines that like fields to be 16-bit aligned.Then someone noticed that if the protocol type field were longer than 3 octets, the field couldbe administered "for free" by linking the administration of the protocol type field with theadministration of the addresses.In other words, when a vendor bought a block of addresses,it received 3 octets of constant (including the group/individual bit), with the remaining 3 octetsassignable by the vendor.The vendor could use those same 3 octets of constant as thehigher-order portion of protocol types, which the vendor could then assign.So, for example, ifthe protocol type field were 4 octets long, then when a vendor purchased a block ofaddresses, it also received a block of 256 protocol types.The agreed-upon size of the protocol type field was 5 octets because 5 is the smallest oddnumber greater than 3.The globally assigned SAP value that indicates the presence of the protocol type field isknown as the SNAP (subnetwork access protocol) SAP.It is equal to 10101010 (binary),which equals aa hex.When the SNAP SAP is used, both DSAP and SSAP are set to aa hex.The protocols that have a globally assigned SAP set DSAP and SSAP to the same valuenamely, the value of the globally assigned SAP.And the protocols that do not have a globallyassigned SAP also set DSAP and SSAP to the same value in this case, the SNAP SAP, aahex.(In my opinion, all the fields defined by 802.2 that is, the SAPs and LLC are a wasteof space in the header and make the specifications much longer and more difficult tounderstand.The only thing needed was a protocol type field [ Pobierz całość w formacie PDF ] |
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