This chapter is from the book
This chapter is from the book
This chapter is from the book
3.9 SLIP Initialization
The SLIP interface relies on a standard asynchronous serial device initialized within the call to cpu_startup. The SLIP pseudo-device is initialized when main calls slattach indirectly through the pdev_attach pointer in SLIP'S pdevinit structure.
Each SLIP interface is described by an sl_softc structure shown in Figure 3.28.
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Figure 3.28. sl_softc structure.
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As with all interface structures, sl_softc starts with an ifnet structure followed by device-specific information.
In addition to the output queue found in the ifnet structure, a SLIP device maintains a separate queue, sc_fastq, for packets requesting low-delay service—typically generated by interactive applications.
sc_ttyp points to the associated terminal device. The two pointers sc_buf and sc_ep point to the first and last bytes of the buffer for an incoming SLIP packet. sc_mp points to the location for the next incoming byte and is advanced as additional bytes arrive.
The four flags defined by the SLIP driver are shown in Figure 3.29.
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Figure 3.29. SLIP if_flags and sc_flags values.
SLIP defines the three interface flags reserved for the device driver in the ifnet structure and one additional flag defined in the sl_softc structure.
sc_escape is used by the IP encapsulation mechanism for serial lines (Section 5.3), while TCP header compression (Section 29.13) information is kept in sc_comp.
The BPF information for the SLIP device is pointed to by sc_bpf.
The sl_softc structure is initialized by slattach, shown in Figure 3.30.
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Figure 3.30. slattach function.
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Unlike leattach, which initializes only one interface at a time, the kernel calls slattach once and slattach initializes all the SLIP interfaces. Hardware devices are initialized as they are discovered by the kernel during cpu_startup, while pseudo-devices are initialized all at once when main calls the pdev_attach function for the device. if_mtu for a SLIP device is 296 bytes (SLMTU). This accommodates the standard 20-byte IP header, the standard 20-byte TCP header, and 256 bytes of user data (Section 5.3).
A SLIP network consists of two interfaces at each end of a serial communication line. slattach turns on IFF_POINTOPOINT, SC_AUTOCOMP, and IFF_MULTICAST in if_flags.
The SLIP interface limits the length of its output packet queue, if_snd, to 50 and its own internal queue, sc_fastq, to 32. Figure 3.42 shows that the length of the if_snd queue defaults to 50 (ifqmaxlen) if the driver does not select a length, so the initialization here is redundant.
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The Ethernet driver doesn't set its output queue length explicitly and relies on ifinit (Figure 3.42) to set it to the system default.
if_attach expects a pointer to an ifnet structure so slattach passes the address of sc_if, an ifnet structure and the first member of the sl_softc structure.
A special program, slattach, is run (from the /etc/netstart initialization file) after the kernel has been initialized and joins the SLIP interface and an asynchronous serial device by opening the serial device and issuing ioctl commands (Section 5.3).
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For each SLIP device, slattach calls bpfattach to register the interface with BPF.