Information Technology Reference
In-Depth Information
Directed broadcasts deliver packets to all members of a specified network. Global multicasts deliver
packets to all members of the group within the entire internetwork, whereas global broadcasts deliver
packets to all internetwork addresses. One bit in the host number indicates a single versus a multicast
address. All ones in the host field indicate a broadcast address.
To route packets in an internetwork, XNS uses the dynamic routing scheme RIP. Today, RIP is still in
use, but has largely been replaced by more scalable protocols, such as Open Shortest Path First (OSPF)
and Border Gateway Protocol (BGP).
The Transport Layer
OSI transport-layer functions are implemented by several protocols. Each of the following protocols is
described in the XNS specification as a Layer 2 protocol.
The Sequenced Packet Protocol (SPP) provides reliable, connection-based, flow-controlled packet
transmission on behalf of client processes. It is similar in function to the Internet Protocol suite's TCP
and the OSI protocol suite's Transport Protocol 4 (TP4).
Each SPP packet includes a sequence number, which is used to order packets and to determine whether
any have been duplicated or missed. SPP packets also contain two 16-bit connection identifiers. One
connection identifier is specified by each end of the connection. Together, the two connection identifiers
uniquely identify a logical connection between client processes.
SPP packets cannot be longer than 576 bytes. Client processes can negotiate use of a different packet
size during connection establishment, but SPP does not define the nature of this negotiation.
The Packet Exchange Protocol (PEP) is a request-response protocol designed to have greater reliability
than simple datagram service (as provided by IDP, for example), but less reliability than SPP. PEP is
functionally similar to the Internet Protocol suite's User Datagram Protocol (UDP). PEP is single-packet
based, providing retransmissions but no duplicate packet detection. As such, it is useful in applications
where request-response transactions can be repeated without damaging data, or where reliable transfer
is executed at another layer.
The Error Protocol (EP) can be used by any client process to notify another client process that a network
error has occurred. This protocol is used, for example, in situations where an SPP implementation has
identified a duplicate packet.
Upper-Layer Protocols
XNS offers several upper-layer protocols. The Printing Protocol provides print services. The Filing
Protocol provides file-access services. The Clearinghouse Protocol provides name services. Each of
these three protocols runs on top of the Courier Protocol, which provides conventions for data structuring
and process interaction.
XNS also defines Level 4 protocols. These are application protocols but, because they have little to do
with actual communication functions, the XNS specification does not include any pertinent definitions
for them.
The Level 2 Echo Protocol is used to test the reachability of XNS network nodes and to support functions
such as that provided by the
ping
command found in UNIX and other environments.
