The Network Core
network core—the mesh of packet switches and links that interconnects the
Internet’s end systems
1.Packet Switching
To send a message from a source end system to a destination end
system, the source breaks long messages into smaller chunks of data known as pack-
ets. Between source and destination, each packet travels through communication
links and packet switches (for which there are two predominant types, routers and
link-layer switches).
1.1 Store-and-Forward Transmission
Store-and-forward transmission means that the packet switch must receive
the entire packet before it can begin to transmit the first bit of the packet onto the
outbound link.
consider the general case of sending one packet from source to des-
tination over a path consisting of N links each of rate R (thus, there are N-1 routers
between source and destination). Applying the same logic as above, we see that the
end-to-end delay is:
一个路由器在中间需要传3个数据包，会在4L/R时间传过去，延迟为1*L/R，1（路由器数量）=2条链路-1，
1.2 Queuing Delays and Packet Loss
Each packet switch has multiple links attached to it. For each attached link, the
packet switch has an output buffer (also called an output queue), which stores
packets that the router is about to send into that link. The output buffers play a key
role in packet switching. If an arriving packet needs to be transmitted onto a link but
finds the link busy with the transmission of another packet, the arriving packet must
wait in the output buffer. Thus, in addition to the store-and-forward delays, packets
suffer output buffer queuing delays.
Since the amount of buffer space is finite, an
arriving packet may find that the buffer is completely full with other packets waiting
for transmission. In this case, packet loss will occur—either the arriving packet or
one of the already-queued packets will be dropped.
1.3 Forwarding Tables and Routing Protocols 转发表和路由协议
a router uses a packet’s destination address to index a forwarding table and determine the appropriate outbound link.
A routing protocol may, for example, determine the shortest path from each router to each destination and use the shortest path results to configure the forwarding tables in the routers.
2.Circuit Switching
In circuit-switched networks, the resources needed along a path (buffers, link
transmission rate) to provide for communication between the end systems are
reserved for the duration of the communication session between the end systems.
满了就拒绝
In packet-switched networks, these resources are not reserved; a session’s messages
use the resources on demand and, as a consequence, may have to wait (that is, queue)
for access to a communication link.
不拒绝，满了就排队
2.1 Multiplexing in Circuit-Switched Networks 多路复用在电路交换网络中的应用
A circuit in a link is implemented with either frequency-division multiplexing
(FDM) or time-division multiplexing (TDM).
TDM计算：
通过电路交换网络将 640,000 位的文件从主机 A 发送到主机 B 需要多长时间。假设网络中的所有链路都使用具有 24 个slots的 TDM，比特率为 1.536 Mbps。此外，假设在主机 A 开始传输文件之前，需要 500 毫秒来建立端到端电路。发送文件需要多长时间？每个电路的传输速率为 （1.536 Mbps）/24 = 64 kbps，因此传输文件需要 （640,000 位）/（64 kbps） = 10 秒。在这 10 秒中，我们加上电路建立时间，有 10.5 秒来发送文件。请注意，传输时间与链路数量无关：如果端到端电路通过一个链路或一百个链路，则传输时间为 10 秒。
2.2 Packet Switching Vs Circuit Switching
Proponents of packet switching argue that
(1) it offers better sharing of transmission capacity than circuit switching and
(2) it is simpler, more efficient, and less costly to implement than circuit switching
3.A Network of Networks
oday’s Internet—a network of networks—is complex, consisting
of a dozen or so tier-1 ISPs and hundreds of thousands of lower-tier ISPs. The ISPs
are diverse in their coverage, with some spanning multiple continents and oceans,
and others limited to narrow geographic regions. The lower-tier ISPs connect to the
higher-tier ISPs, and the higher-tier ISPs interconnect with one another. Users and
content providers are customers of lower-tier ISPs, and lower-tier ISPs are customers
of higher-tier ISPs. In recent years, major content providers have also created their
own networks and connect directly into lower-tier ISPs where possible.
IXP为ISP提供了一个中立的交换平台，促进了它们之间的竞争与合作。通过IXP，ISP可以更容易地与其他网络建立连接，从而扩大其业务范围和服务范围。