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. In 1981 I published a paper called "Incorporation of service classes into a networkarchitecture." In it I described three categories of service classes:" Handling directives (for example, use link encryption, priority, try not to reorder, usehop-by-hop error recovery to increase the reliability)" Metrics (for example, delay, bandwidth)" Constraints (for example, don't go through a particular domain perhaps it's in anunfriendly country or a domain that does not want to carry traffic between you and thedestination for some reason don't go on links that cost money, go only through linksthat can reserve me a particular chunk of bandwidth)Constraints can be considered a special case of metrics, where the cost is binary.People are confused about service classes, and they use terminology in conflicting ways.Basically, it is easy to implement handling directives.It doesn't affect the computed routes,but each switch does what it can to provide that special handling for your special packet.Metrics are more difficult, and it's not clear that they are worth the effort.Constraints mightunfortunately be necessary, although I suspect that if the world didn't provide them it wouldlearn to live without them.They are difficult, and perhaps impossible to provide for if there aretoo many of them and if the network is hierarchical.10.4.20 OptionsIPv4, IPv6, and CLNP have a provision for "options." Each has several options defined, andany subset of them (including none) can appear in any packet.The other protocols (IPX,DECnet, AppleTalk) do not have options.10.4.20.1 Options in CLNPIn CLNP, each option is defined as shown in Figure 10.12.Figure 10.12.CLNP format for optionsIn CLNP, options can appear in any order, but no option is allowed to appear more than once.For the CLNP options security and quality of service, the most significant 2 bits of the firstoctet of value have the following meaning." 00 = reserved (this combination is not used)." 01 = source-address-specific.(Brace yourself for a mind-boggling definition!) Thismeans that the interpretation of the option is not given in the spec but rather isdefined by the authority from which the source got its address." 10 = destination-address-specific.(Why not?) This means that the interpretation ofthe option is defined by the authority from which the destination got its address." 11 = globally unique.This means that the interpretation of the option is given in thespec.I am not aware of any implementations that use anything other than the globally uniqueoptions.The source- and destination-address-specific options are not useful unless therouters are coded to understand and implement them. The source- and destination-address-specific options might be of use in a private network,with personalized routers (routers specially coded to handle options as specified by thecustomers).ISO, being a standards body, was reluctant to specify how to define optionsthus, it provided this level of "flexibility." However, just to show that it does have the clout tomake rules, it decreed that no option should appear twice.This restriction places a burden onrouters (are they supposed to check that no option appears twice every time they forward apacket?) and reduces flexibility.For example, it precludes using padding in multiple places orusing both the globally unique form of an option and the source-specified version in the samepacket.10.4.20.2 Options in IPIn IP, options are also coded as code, length, and value but with the following differences.1.In the code octet, the copy flag specifies whether a router, upon fragmenting thepacket, should copy the option into every fragment (if set) or only leave the option inthe first fragment.2.Instead of defining the bottom 7 bits as the code, the IP designers chose to sub-divide these bits into two fields, giving the first octet of an option the format shown inFigure 10.13.Figure 10.13.Format of first byte of IP optionFor convenience, I refer to the copy/class/number octet as the "option code," eventhough it also contains the copy flag.There is no particular reason that class and number could not be considered asingle 7-bit field identifying the option.But the IP designers decided to mark optionsas being either control (class 0) or debugging and measurement (class 2).Classes 1and 3 are reserved.Class 2 is used only for a single option: Internet timestamp.Allthe other defined options have the class field equal to 0.3.Two options in IP are only a single octet long that is, the only octet is the optioncode.Rather than waste an octet specifying that the option has 0 length, IP definesthose options as not having the length octet because a length of 0 is implied by theoption code.The options that are just 1 octet long are as follows.End of option list:IP option 0.(CLNP does not have, nor does it need, an equivalent option.)This indicates that the option list is ended.If the IP header length were inunits of octets, this option would not be required; however, because theheader length is in units of 32-bit chunks, this option indicates where theheader ends.Note that because the padding is required to be transmitted as0, the transmitter automatically adds this option when the header length is anonintegral number of 32-bit chunks.This option may be introduced by arouter after a packet is fragmented.(The router may fragment the packet, andthe header length may change because of an option's not being copied; thatmight result in the header's being a nonintegral number of 32-bit chunks [ Pobierz całość w formacie PDF ]

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