Policy Based Routing; General - ELSA LANCOM Business Manuel De L'utilisateur

Online Trace 'Packet dump'
The 'packet dump' online trace supplements the trace outputs, which are generated by
the IP router. The first 64 bytes of a packet is output in hexadecimal format.

Policy Based Routing

General

The term 'policy based routing' describes the option of using additional routing methods
to the standard routing procedure for IP packets (these "policies").
To make the in-band configuration easier on wide-area networks with heavy data traffic
and to improve the cooperation of ELSA LANCOM with 'ping' and 'traceroute'
mechanisms, two methods for the IP routing have been introduced. Both methods are
based on the evaluation of the 'Type of Service' field in the IP header.
The 'Type of Service' field (for short TOS) describes how IP packets should preferably be
treated (but need not be), i.e. it reflects the preferred processing procedure intended by
the generator of this IP packet. TOS has the following structure in this context:
Bit 7, 6 Bit 5
Unused Reliable transmission
The R and the D bit are evaluated and the behavior adapted to its circumstances by the
routing methods.
A set R bit requires secured transmission of the associated IP packet. Packets identified
as such are always transmitted over a "secured" queue corresponding to their reception
sequence. In an extreme case this can result in a "normal" packet that is already in a
transmission queue being removed and placed back into the heap to make room for the
packet to be sent. This occurs if the maximum number of buffers for the associated
connection has already been used. However, the transmission sequence between
packets with a set R bit and 'normal' bits is not changed by this mechanism.
The secured transmission can be activated for all ICMP packets independently of the
entry in the 'Type of Service' field. Because an ICMP packet identified as such is sent
without changing the transmission sequence, the throughput delays of a ELSA LANCOM
can be determined by 'ping' or 'traceroute'.
With a set D bit the generator requests the fastest possible forwarding of an IP packet.
IP packets identified as such are transmitted over an 'urgent' queue before the send
queue packets corresponding to their reception sequence. On one hand, this results in
changes in the transmission sequence, because an IP packet identified as last received
ELSA LANCOM Business Internal
Bit 4 Bit 3
High Throughput Low delay
ELSA LANCOM Business
R10 5
Bit 2, 1, 0
Precedence