Previous

Content

Next 

6.0.- OSPF Protocol Extensions

  

Currently, the OSPF standard specifies the following 5 bits in the Options Field:

The least significant bit (T-bit) was used in the older OSPF specification to indicate TOS routing capability, but it was removed in the current specification. We propose to reclaim the T-bit as an indicator of router's QoS capability and refer to it as the Q-bit. A router sets this bit in its Hello packets to indicate it is QoS routing capable. In router or summary LSAs, this means that there are QoS fields to process in the packet. In network LSAs, this means that the network being described in the advertisement is QoS capable.
Encoding resources as Extended TOS
Introduction of QoS should ideally not influence the compatibility with existing OSPFv2 routers. For QoS resources, 32 combinations are available through the use of the fifth bit in TOS fields contained in different LSAs. Since the original TOS specification defines TOS as being four bits long, the QoS definition never conflicts with existing values. The definition of the "QoS encoding" is aligned in their semantic with the TOS encoding. Only bandwidth and delay are specified in this document and their values map onto "maximize throughput" and "minimize delay" if the most significant bit is not taken into account. Link reliability and jitter could be defined later if necessary.
The original TOS encoding and the new QoS encoding are shown in the following table:
Encoding bandwith resources
The actual OSPF metric field only provides 16 bits to encode the value used, and having into account that links supporting bandwitdh ranging into Gbps are becoming reality, linear representation of the available resource is not feasible. The solution is exponential encoding following the next rule:  

Represent available bandwidth in 16-bit field as a 3-bit exponent assuming a base of 8, followed by a 13-bit mantissa as shown in figure below, and advertise 2's complement of this representation.

Thus, the above encoding advertise a numeric value that is:

2^16 - 1 - (exponential encoding of the available bandwidth)
This has the property of advertising a higher numeric value for lower available bandwidth, a notion that is consistent with that of cost.
The following table shows bandwidth ranges covered when using different exponents and the granularity of possible reservations:

One example to clear what we are trying to explain: suppose a link with bandwidth of 8 Gbps:

Then, being the link idle, the bandwidth cost (advertised value) is 12287.
Assuming now a current reservation of 6400 Mbps, the remaing available bandwidth will be 1600 Mbps. The encoding of this value is as follows:

Then, being the link 80% loaded, the bandwidth cost (advertised value) is 18175.
The cost function behaves as it should, i.e., the less bandwidth is available on a link, the higher the cost and the less attractive the link becomes. Furthermore, the targeted property of better granularity for links with less bandwidth available is also achieved.
Encoding Delay
Delay is encoded in microseconds using the same previous method described for bandwidth, except that the base is defined as 4, instead of 8. The maximum delay that can be expressed is:

(2^13 - 1) * 4^7 ~ 134 seconds
Packet Format
No changes in packet format are necessary except for the reintroduction of the T-bit as the Q-bit in the Options Field. A router supporting QoS sets the Q-bit in the Options Field of its LSAs. A receiver that support QoS, receiving either Router-LSAs or Summary-LSAs set as QoS-enable, should parse the QoS metric encoded in the received advertisement.
Other issues
The problem of providing support for area address ranges and QoS metric aggregation is not discussed in this document, and is left for further studies.
The problem of providing support for AS External Links, Virtual Links, and incremental updates for Summary-LSAs are not addressed in this document and are left for further studies.
This document does not address the issue of non-QoS routers within a QoS domain.
   

Previous

Content

Next