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                     16: .\"    @(#)2.t 6.4 (Berkeley) 3/7/89
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                     18: .nr H2 1
                     19: .\".ds RH Overview
                     20: .br
                     21: .ne 2i
                     22: .NH
                     23: \s+2Overview\s0
                     24: .PP
                     25: If we consider
                     26: the International Standards Organization's (ISO)
                     27: Open System Interconnection (OSI) model of
                     28: network communication [ISO81] [Zimmermann80],
                     29: the networking facilities
                     30: described here correspond to a portion of the
                     31: session layer (layer 3) and all of the transport and
                     32: network layers (layers 2 and 1, respectively).
                     33: .PP
                     34: The network layer provides possibly imperfect
                     35: data transport services with minimal addressing
                     36: structure.
                     37: Addressing at this level is normally host to host,
                     38: with implicit or explicit routing optionally supported
                     39: by the communicating agents. 
                     40: .PP
                     41: At the transport
                     42: layer the notions of reliable transfer, data sequencing,
                     43: flow control, and service addressing are normally
                     44: included.  Reliability is usually managed by 
                     45: explicit acknowledgement of data delivered.  Failure
                     46: to acknowledge a transfer results in retransmission of
                     47: the data.  Sequencing may be handled by tagging
                     48: each message handed to the network layer by a
                     49: \fIsequence number\fP and maintaining
                     50: state at the endpoints of communication to utilize
                     51: received sequence numbers in reordering data which
                     52: arrives out of order.
                     53: .PP
                     54: The session layer facilities may provide forms of
                     55: addressing which are mapped into formats required
                     56: by the transport layer, service authentication
                     57: and client authentication, etc.  Various systems
                     58: also provide services such as data encryption and
                     59: address and protocol translation.
                     60: .PP
                     61: The following sections begin by describing some of the common
                     62: data structures and utility routines, then examine
                     63: the internal layering.  The contents of each layer
                     64: and its interface are considered.  Certain of the
                     65: interfaces are protocol implementation specific.  For
                     66: these cases examples have been drawn from the Internet [Cerf78]
                     67: protocol family.  Later sections cover routing issues,
                     68: the design of the raw socket interface and other
                     69: miscellaneous topics.

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