![](https://csdnimg.cn/release/download_crawler_static/10297101/bg12.jpg)
Tier-2 networks, on the other hand, may not peer with many tier-1 networks, but
they often peer with all other tier-2 networks operating in the same region and with
many tier-3 networks.
TCP/IP Design Philosophy
The fact that TCP/IP runs well over all kinds of underlying networks is no coinci-
dence. Today, every imaginable kind of computer is connected to the Net, even
though those connected over the fastest links, such as Gigabit Ethernet, can transfer
more data in a second than the slowest, connected through wireless modems, can
transfer in a day. This flexibility is the result of the philosophy that network failures
shouldn't impede communication between two hosts and that no assumptions
should be made about the underlying communications channels. Any kind of circuit
that can carry packets from one place to another with some reasonable degree of reli-
ability may be used.*
This philosophy makes it necessary to move all the decision-making to the source
and destination hosts: it would be very hard to survive the loss of a router some-
where along the way if this router holds important, necessary information about the
connection. This way of doing things is very different from the way telephony and
virtual circuit-oriented networks such as X.25 work: they go through a setup phase,
in which a path is configured at central offices or telephone switches along the way
before any communication takes place. The problem with this approach is that when
a switch fails, all paths that use this switch fail, disrupting ongoing communication.
In a network built on an unreliable datagram service, such as the Internet, packets
can simply be diverted around the failure and still be delivered. The price to be paid
for this flexibility is that end hosts have to do more work. Packets that were on their
way over the broken circuit may be lost; some packets may be diverted in the wrong
direction at first, so that they arrive after subsequent packets have already been
received; or the new route may be of a different speed or capacity. The networking
software in the end hosts must be able to handle any and all of these eventualities.
The IP Protocol
Because the TCP protocol takes care of the most complex tasks, IP processing along
the way becomes extremely simple: basically, just take the destination address, look
it up in the routing table to find the next-hop address and/or interface, and send the
packet on its way to this next hop over the appropriate interface. This isn't immedi-
ately obvious by looking at the IP header (Figure 1-2), because there are 12 fields in
"The Design Philosophy of the DARPA Internet Protocols" contains a good overview; it can be found at http:
//www. cs.umd.edu/dass/falll999/cmsc711/papers/design-philosophy.pdf.
6 | Chapter 1: The Internet, Routing, and BGP