Visara LINCS Features - Manual (Page 17)

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Chapter 2. APPN Network Node
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HPR provides end-to-end connections, thereby obtaining a significant throughput increase
over ISR. HPR utilizes three new protocols to achieve additional throughput:
• Automatic Network Routing
• Rapid Transport Protocol
• Adaptive Rate Based congestion control
Automatic Network Routing (ANR)
ANR is a connectionless protocol that switches packets on a predetermined path. It improves
switching speed, because of its reduced number of instructions compared to ISR. ANR also
eliminates the 500-byte control block for each session, increasing the number of connections
that the LINCS node can support, similar to Source-Route-Bridging. Labels that represent the
full path between end node partners are carried in the header of each ANR packet. Because
there is no limit to the number of labels in the header, ANR is not limited to 7 hops, like Source
Route Bridging is. ANR selects the highest priority packet, gets the next link from and deletes
the first label, then sends the packet out on the identified link.
Some important features the ANR offers are:
• Fast packet switching – ANR is much faster than base APPN’s ISR. ANR operates at a
lower layer than ISR. (ANR operates at Layer 2; ISR operates at Layer 3.) That minimizes
the storage and processing required to route packets through intermediate nodes. ANR
performs the traffic prioritization and packet forwarding functions found in ISR. Functions
such as flow control, segmentation, link-level error recovery, and congestion control are
NOT performed in the intermediate node (as in HPR). Instead, these functions are executed
only at the endpoints of an RTP connection.
• Source Routing – ANR supports source routing. Each packet has a network layer header
with routing information at the beginning of the packet. This routing information consists
of a string of ANR labels. These labels describe the path of the packet through an HPR
subnet. ANR labels are locally assigned by each HPR node. When a HPR node receives a
packet, it:
1. looks at the first ANR label in the packet
2. selects the corresponding link over which to send the packet
3. deletes this first ANR label from the packet
4. forwards the packet out onto the selected link
• No session awareness – Intermediate HPR nodes have no knowledge of sessions. They
simply route the session traffic based on the routing information. Therefore, intermediate
nodes no longer have to store any session information (routing tables) for sessions that are
routed across it, as in base APPN.
Rapid Transport Protocol (RTP)
RTP is designed for fast high quality networks. It is a connection-oriented transport protocol
at OSI Layer 2. Before ANR takes place, RTP determines from the APPN topology and directory
databases what the largest packet size is that can be supported at each node across an entire
route. Before the first ANR hop, RTP segments packets to the minimum packet size, eliminating
any requirement for segmenting and reassembly within the network. The last NN in the path
reassembles and resequences the packets, if necessary. RTP retransmits only missing packets,