However, a connection is usually not 100% reliable, particularly not if it is in fact done via a modem over a public telephone line. In a 100% reliable serial connection this would be tolerable, because length information in the IP packets could be used to separate packets from the serial data stream. The receiving side would not be able to explicitly identify the start and end of a packet. In cases where a 'naked' serial connection is used as the link layer, this creates a problem, because a simple serial connection does not provide any framing facilities. The link layer is supposed to do this, so when packets are placed in some data stream they can be properly extracted from the stream. This means, the link layer is, among other tasks, responsible for properly separating each packet and identifying an IP packet's start and end when it moves the packet over some communication link. One of the services the IP protocol expects from the link layer is the framing, deframing of IP packets. Higher layers expect particular services from their lower layers.
All mentioned problems are explained as it follows.ĭifferent layers have different tasks in the TCP/IP protocol stack. Of which lack of framing is the most serious one. Transmitting IP data over a serial (RS-232) line is haunted by several problems, among them:
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