* Diagnosis of failures and misconfigurations is more complex. Middleboxes may also require configuration and management. * Configuration is no longer limited to the two ends of a session The fate of sessions involving crashed middleboxes must also be * Middleboxes introduce new failure modes rerouting of IP packetsДЄround crashed routers is no longer the only case to consider. Unpredictably, in the presence of middleboxes. Without consideration of middleboxes may fail, predictably or

* New middleboxes challenge old protocols. This is a matter of concern for several reasons: Instead ofД¬oncentrating diversity and function at the end systems, they spreadДiversity and function throughout the network. Single necessary feature of all communications sessions. Layer does not go away, middleboxes dilute its significance as the Way, they are a challenge to the hourglass model: although the IP To the transparency of the network layer. Viewed in one way, these boxes are a challenge In the network of numerous middleboxes performing functions other Today, we observe deviations from this model, caused by the insertion IP routers, and their only function is to determine routes andДЇorward packets (while also updating fields necessary for theДЇorwarding process). In this architecture, the only boxes in the neck of the hourglass are Performance enhancement, but not as a substitute for the end-to-end

The end-to-Д®nd principle notes that providing an incomplete version of suchДЇunctions in the network itself can sometimes be useful as a Security and reliability) can only be implemented completely andД¬orrectly end-to-end, with the help of the end points. The end-to-end principle asserts that some functions (such as Riding over a single IP protocol, which itself rides over a variety The hourglass model depicts the protocolДЄrchitecture as a narrow-necked hourglass, with all upper layers The classical description of the Internet architecture is basedДЄround the hourglass model and the end-to-end principle In this document, the traditional sense is always qualified as 'IPДЎ.2. Not the mere forwarding of a packet in the only direction available. In either case it implies a choice of outbound direction, Others use it in the sense of higher layerДecision-making (based perhaps on a URL or other applications layer Of path selection based on IP address, i.e., the decision-makingДЄction of an IP router. Some people use it in the narrow, traditional sense There is some discrepancy in the way the word 'routing' is used in Taking place within the IP layer may be considered to be middleboxДЇunctions, but functions below the IP layer are excluded from the Router is essentially transparent to IP packets. IPv6) are not considered to be middlebox functions a standard IP Selection functions described in, and their equivalent for Normal, standard IP routing functions (i.e., the route discovery and May be a device that terminates one IP packet flow and originatesДЄnother, or a device that transforms or diverts an IP packet flow in Interpreted as necessarily referring to a separate physical box. I.e., an embedded function of some other box. In some discussions, especially those concentrating on HTTP traffic, The normal, standard functions of an IP router on the datagram pathД«etween a source host and destination host. A middlebox isДefined as any intermediary device performing functions other than

The phrase 'middlebox' was coined by Lixia Zhang as a graphicДescription of a recent phenomenon in the Internet. The end to end principle under challenge. Content and applications distribution boxes. Load balancers that divert/munge packets. Packet classifiers, markers and schedulers. RFC 3234 Middleboxes: Taxonomy and Issues February 2002ДЎ.

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