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1.1 root 1: .\" Copyright (c) 1983, 1986 The Regents of the University of California.
2: .\" All rights reserved.
3: .\"
4: .\" Redistribution and use in source and binary forms are permitted
5: .\" provided that the above copyright notice and this paragraph are
6: .\" duplicated in all such forms and that any documentation,
7: .\" advertising materials, and other materials related to such
8: .\" distribution and use acknowledge that the software was developed
9: .\" by the University of California, Berkeley. The name of the
10: .\" University may not be used to endorse or promote products derived
11: .\" from this software without specific prior written permission.
12: .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
13: .\" IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
14: .\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
15: .\"
16: .\" @(#)9.t 6.4 (Berkeley) 3/7/89
17: .\"
18: .nr H2 1
19: .\".ds RH "Protocol/network-interface
20: .br
21: .ne 2i
22: .NH
23: \s+2Protocol/network-interface interface\s0
24: .PP
25: The lowest layer in the set of protocols which comprise a
26: protocol family must interface itself to one or more network
27: interfaces in order to transmit and receive
28: packets. It is assumed that
29: any routing decisions have been made before handing a packet
30: to a network interface, in fact this is absolutely necessary
31: in order to locate any interface at all (unless, of course,
32: one uses a single ``hardwired'' interface). There are two
33: cases with which to be concerned, transmission of a packet
34: and receipt of a packet; each will be considered separately.
35: .NH 2
36: Packet transmission
37: .PP
38: Assuming a protocol has a handle on an interface, \fIifp\fP,
39: a (struct ifnet\ *),
40: it transmits a fully formatted packet with the following call,
41: .DS
42: error = (*ifp->if_output)(ifp, m, dst)
43: int error; struct ifnet *ifp; struct mbuf *m; struct sockaddr *dst;
44: .DE
45: The output routine for the network interface transmits the packet
46: \fIm\fP to the \fIdst\fP address, or returns an error indication
47: (a UNIX error number). In reality transmission may
48: not be immediate or successful; normally the output
49: routine simply queues the packet on its send queue and primes
50: an interrupt driven routine to actually transmit the packet.
51: For unreliable media, such as the Ethernet, ``successful''
52: transmission simply means that the packet has been placed on the cable
53: without a collision. On the other hand, an 1822 interface guarantees
54: proper delivery or an error indication for each message transmitted.
55: The model employed in the networking system attaches no promises
56: of delivery to the packets handed to a network interface, and thus
57: corresponds more closely to the Ethernet. Errors returned by the
58: output routine are only those that can be detected immediately,
59: and are normally trivial in nature (no buffer space,
60: address format not handled, etc.).
61: No indication is received if errors are detected after the call has returned.
62: .NH 2
63: Packet reception
64: .PP
65: Each protocol family must have one or more ``lowest level'' protocols.
66: These protocols deal with internetwork addressing and are responsible
67: for the delivery of incoming packets to the proper protocol processing
68: modules. In the PUP model [Boggs78] these protocols are termed Level
69: 1 protocols,
70: in the ISO model, network layer protocols. In this system each such
71: protocol module has an input packet queue assigned to it. Incoming
72: packets received by a network interface are queued for the protocol
73: module, and a VAX software interrupt is posted to initiate processing.
74: .PP
75: Three macros are available for queuing and dequeuing packets:
76: .IP "IF_ENQUEUE(ifq, m)"
77: .br
78: This places the packet \fIm\fP at the tail of the queue \fIifq\fP.
79: .IP "IF_DEQUEUE(ifq, m)"
80: .br
81: This places a pointer to the packet at the head of queue \fIifq\fP
82: in \fIm\fP
83: and removes the packet from the queue.
84: A zero value will be returned in \fIm\fP if the queue is empty.
85: .IP "IF_DEQUEUEIF(ifq, m, ifp)"
86: .br
87: Like IF_DEQUEUE, this removes the next packet from the head of a queue
88: and returns it in \fIm\fP.
89: A pointer to the interface on which the packet was received
90: is placed in \fIifp\fP, a (struct ifnet\ *).
91: .IP "IF_PREPEND(ifq, m)"
92: .br
93: This places the packet \fIm\fP at the head of the queue \fIifq\fP.
94: .PP
95: Each queue has a maximum length associated with it as a simple form
96: of congestion control. The macro IF_QFULL(ifq) returns 1 if the queue
97: is filled, in which case the macro IF_DROP(ifq) should be used to
98: increment the count of the number of packets dropped, and the offending
99: packet is dropped. For example, the following code fragment is commonly
100: found in a network interface's input routine,
101: .DS
102: ._f
103: if (IF_QFULL(inq)) {
104: IF_DROP(inq);
105: m_freem(m);
106: } else
107: IF_ENQUEUE(inq, m);
108: .DE
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