The IP protocol corresponds to the network layer (Layer 3) of
the Open Systems Interconnection (OSI) reference model. Its function is to
transmit a data block, called a datagram, from a
source to a destination. Part of its functionality includes fragmentation and
reassembly. As a protocol, IP does not provide for data integrity, reliability
of transmission, or sequencing.
 | |||
| The IP Datagram Format |
The nature of how IP behaves and the
functions of some of the fields illustrated directly impact the resiliency of
the network, as follows:
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IP cannot handle signal degradation during transmission. For example, when the quality of a pair of fiber has degraded and is causing a high burst error rate, IP continues to transmit packets, even though it is sending garbage. Therefore, selection of Layer 2 technology is important because the right one can complement this deficiency. For example, a Packet over SONET (PoS) interface is always preferred as compared to Gigabit Ethernet. A PoS interface monitors the quality of its transmission and can declare itself unusable so that a reroute can take place.
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IP does not retransmit data in the event of a missing packet. It treats every packet as an independent entity and has no knowledge of whether other related packets are transmitted successfully. It is the job of the upper-layer protocol to deal with this. Therefore, selecting the right upper-layer protocol is crucial for applications that want resilient capability.
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Because IP treats every packet independently, packets for an application might not arrive at the destination in sequence. This happens when multiple paths are made available to a router along the transmission. Although it is the duty of the destination host to handle this situation, this might not be ideal for applications such as voice. Therefore, the forwarding decisions the router makes, whether per-destination load balancing or per-packet load balancing, directly impact applications.
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IP can fragment and reassemble packets. And it can work with different transmission technologies, such as Ethernet, PoS, and so on. These technologies have different maximum transmission unit (MTU) sizes, and traversing from one to another might require the IP protocol to break bigger packets into numerous smaller ones. However, if in the case of a misconfiguration, fragmentation taxes the processing power of the router, this might impact the performance of the network. When critical resources such as CPU of the router is taxed, resiliency is compromised. Therefore, fragmentation is one area that you want to avoid.
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The Type of Service (ToS) field has mostly been ignored until recently when the concept of quality of service (QoS) has become critical. QoS is the basis of identifying different classes of traffic within a network. In the event of network congestion, certain classes of traffic might be deemed less important and be dropped by the network. In this case, network service is affected for some users.
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The Options field is also one that is mostly ignored but has an impact on the performance of the network, which ultimately affects network services. Because of the specification of the IP protocol, any IP packet that has an Options field filled must be examined for further action. The packet has to be sent to the router's CPU for processing before being forwarded. In this case, the CPU is being hit by extra work and might potentially slow down the router.
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The length of the IP header is fixed, but the data portion is variable. This means that its efficiency might not be great when the actual payload is small. What this means is that the size of the payload traversing the network does impact the efficiency of the network. Although a larger payload is always preferred in applications such as a file transfer, some applications (Voice over IP [VoIP], for example) send traffic in small packet sizes. Therefore, it is important to have a traffic matrix utilizing the network so that a more accurate capacity can be planned for.
As you might have realized by now, one just needs to review the
protocol more thoroughly to realize its impact on network resiliency. This is
especially so when some of the fields are used to implement features such as
QoS. If you are thinking that the IP protocol is dated and no more work is being
done on it, you just might be proven wrong. One just needs to trace the
development work from the various RFCs to know that experts have worked on it
continuously. To stay informed of the latest development, visit the Internet
Engineering Task Force (IETF) website at http://www.ietf.org.
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