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Internetworking with TCP/IP- P34: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | Sec. 15.20 BGP NOTIFICATION Message 289 For each possible ERR CODE the ERR SUBCODE field contains a further explanation. Figure 15.15 lists the possible values. Subcodes For Message Header Errors 1 2 3 Connection not synchronized Incorrect message length Incorrect message type Subcodes For OPEN Message Errors 1 Version number unsupported 2 Peer AS invalid 3 BGP identifier invalid 4 Unsupported optional parameter 5 Authentication failure 6 Hold time unacceptable Subcodes For UPDATE Message Errors 1 Attribute list malformed 2 Unrecognized attribute 3 Missing attribute 4 Attribute flags error 5 Attribute length error 6 Invalid ORIGIN attribute 7 AS routing loop 8 Next hop invalid 9 Error in optional attribute 10 Invalid network field 11 Malformed AS path Figure 15.15 The meaning of the ERR SUBCODE field in a BGP NOTIFICATION message. 15.21 Decentralization Of Internet Architecture Two important architecture questions remain unanswered. The first focuses on centralization how can the Internet architecture be modified to remove dependence on a centralized router system The second concerns levels of trust can an internet architecture be expanded to allow closer cooperation trust between some autonomous systems than among others 290 Routing Exterior Gateway Protocols And Autonomous Systems BGP Chap. 15 Removing all dependence on a central system and adding trust are not easy. Although TCP IP architectures continue to evolve centralized roots are evident in many protocols. Without some centralization each ISP would need to exchange reachability information with all ISPs to which it attached. Consequently the volume of routing traffic would be significantly higher than with a routing arbiter scheme. Finally centralization fills an important role in rationalizing routes and guaranteeing trust in addition to storing the reachability database the routing arbiter system guarantees global consistency and provides a trusted source of information. 15.22 Summary Routers must be .
