构建可靠的网络：BGP边界网关协议详解

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"BGP: 构建可靠的网络边界网关协议" 本文将深入探讨BGP（边界网关协议），这是一种用于互联网上自治系统（AS）之间交换路由信息的关键协议。由Iljitsch van Beijnum编写的这本书详细介绍了如何利用BGP构建可靠且高效的网络。 BGP是Internet Protocol (IP)路由选择中的核心协议之一，它允许不同的网络（如ISP或企业网络）相互通告可达性信息，从而决定数据包应如何在这些网络之间传输。该书详细阐述了BGP的工作原理，包括其路径选择策略、路由反射器、联盟以及多路径负载均衡等概念。在构建可靠网络方面，BGP提供了多种机制来确保网络的稳定性和容错性。例如，通过使用路径属性，BGP可以避免环路并选择最佳路径。此外，BGP还支持路由前缀的撤销，这有助于在网络故障或配置错误时快速恢复。书中详细讲解了这些机制，并给出了实际部署的案例和建议。书中的内容涵盖了BGP的基本概念，如AS号、BGP会话建立、路由宣告和撤销，以及更高级的主题，如路由策略的制定和优化。读者将学习如何配置和管理BGP，以实现高效的数据传输和网络故障隔离。作者还讨论了BGP的安全问题，如路由劫持和路径注入，这些都是现代网络环境中必须关注的重要议题。书中提到了防止这些威胁的措施，包括使用路由验证技术和安全策略。此外，书中还涉及了BGP与其它网络协议（如OSPF和IS-IS）的交互，以及在云计算和大型数据中心环境中的应用。对于希望深入了解BGP在网络架构中作用的网络管理员、系统工程师和IT专业人员来说，这本书是一份宝贵的参考资料。 "BGP: building reliable networks with the border gateway protocol"提供了一个全面的指南，帮助读者理解BGP的工作方式，并利用这一强大的工具来设计和维护可靠的互联网基础设施。通过阅读这本书，读者能够掌握构建和维护复杂网络所需的知识，确保网络的稳定性和安全性。

to either suffices. However, it isn't possible to bridge easily from Ethernet or FDDI to

ATM and vice versa. Over the past several years, the importance of the NAPs has

diminished as the main interconnect locations for Internet traffic. Large networks are

showing a tendency to interconnect privately, and smaller networks are looking more

and more at regional public interconnect locations. There are now numerous small

Internet Exchanges in the United States, and in addition to Worldcom, two other

companies now operate Internet Exchanges as a commercial service: Equinix and

PAIX. Figure 1-1 shows the distribution of NAPs, MAEs, Equinix Internet Business

Exchanges, and PAIX exchanges.

Figure 1-1. Distribution of interconnect locations in the United States

The Rest of the World

The traffic volumes for the Internet Exchanges in Europe and the Asia/Pacific region

were much lower at the time the NAPs were being created, so these exchange were

not forced to adopt expensive (FDDI) or then still immature (ATM) technologies as

the American NAPs were. Because Ethernet is cheap, easier to configure than ATM,

and conveniently available in several speeds, most of the non-NAP and non-MAE

Internet Exchanges use Ethernet. There are also a few that use frame relay, SMDS, or

SRP, usually when the Internet Exchange isn't limited to a single location or a small

number of locations but allows connections to any ISP office or point of presence

(POP) within a metropolitan area.

In Europe, most countries have an Internet Exchange. From an international per-

spective, the main ones are the London Internet Exchange (LINX), the Amsterdam

Internet Exchange (AMS-IX), and the Deutsche Commercial Internet Exchange

(DE-CIX) in Frankfurt. Internet Exchanges in the rest of the world haven't yet

Chapter 1: The Internet, Routing, and BGP

reached the scale of those in the United States and Europe and are used mainly to

exchange national traffic.

Transit and Peering

When a customer connects to an Internet service provider (ISP), the customer pays.

This seems natural. Because the customer pays, the ISP has to carry packets to and

from all possible destinations worldwide for this customer. This is called transit ser-

vice. Smaller ISPs buy transit from larger ISPs, just as end-user organizations do. But

ISPs of roughly similar size also interconnect in a different way: they exchange traffic

as equals. This is called peering, and typically, there is no money exchanged. Unlike

transit, peering traffic always has one network (or one of its customers) as the source

and the other network (or one of its customers) as its destination. Chapter 12 offers

more details on interconnecting with other networks and peering.

Classification of ISPs

All ISPs aren't created equal: they range from huge, with worldwide networks, to

tiny, with only a single Ethernet as their "backbone." Generally, ISPs are categorized

in three groups:

Tier-1

Tier-1 ISPs are so large they don't pay anyone else for transit. They don't have

to, because they peer with all other tier-1 networks. All other networks pay at

least one tier-1 ISP for transit, so peering with all tier-1 ISPs ensures connectivity

to the entire Internet.

Tier-2

Tier-2 ISPs have a sizable network of their own, but they aren't large enough to

convince all tier-1 networks to peer with them, so they get transit service from at

least one tier-1 ISP.

Tier-3

Tier-3 ISPs don't have a network to speak of, so they purchase transit service

from one or more tier-1 or tier-2 ISPs that operate in the area. If they peer with

other networks, it's usually at just a single exchange point. Many don't even

multihome. I

The line between tier-1 networks and the largest tier-2 is somewhat blurred, with

some tier-2 networks doing "paid peering" with tier-1 networks and calling them-

selves tier-1. The real difference is that tier-2 networks generally have a geographi-

*cally limited presence. For instance, even some very large European networks with

trans-Atlantic connections of their own pay a U.S. network for transit, rather than

interconnecting with a large number of other networks at NAPs throughout the

United States. Because tier-1 networks see these regional ISPs as potential custom-

ers, they are less likely to peer with them. This goes double for tier-3 networks.

Topology of the Internet

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

it, which seems like a lot at first glance. The function of each field, except perhaps

the Type of Service and fragmentation-related fields, is simple enough, however.

Figure 1-2. The IP header as defined in RFC 791

The first 32 bits of the header are mainly for housekeeping: the Version field indi-

cates the IP version (4), the Internet Header Length ("IHL"), and the length of the

header (usually 5 32-bit words); the Total Length is the length of the entire IP

packet, including the header, in bytes. The Type of Service field can be used by appli-

cations to indicate that they desire a nonstandard service level or quality of service

(QoS). In most networks, the contents of this field are ignored.

The next 32 bits are used when the IP packet needs to be fragmented. This happens

when the maximum packet size on a network link isn't enough to transmit the

packet whole. The router breaks up the packet in smaller packets, and the receiving

host can later reassemble the original packet using the information in the Identifier,

Flags, and Fragment Offset fields.

The middle 32 bits contains the Time to Live (TTL), Protocol, and Header Check-

sum fields. The TTL is initialized at a sufficiently high value (usually 60) by the

source host and then decremented by each router. When the TTL reaches zero, the

router throws away the packet. This is done to prevent packets from circling the Net

indefinitely when there are routing loops.' The Protocol field indicates what's inside

the IP packet: usually TCP or UDP data, or an ICMP control message. The Header

Checksum is just that, and it's used to protect the header from inadvertent changes

en route. As with all checksums, the receiver performs the checksum calculation over

the received information, and if the computed checksum is different from the

received checksum, the packet contains invalid information and is discarded. The

final two 32-bit words contain the address of the source system that generated the

packet and the destination system to which the packet is addressed.

This happens when router A thinks a certain destination is reachable over router B, but router B thinks this

destination is reachable over router A. The packet is then forwarded back and forth between the two routers.

A routing loop is usually caused by incorrect configuration or by temporary inconsistencies when there is a

change in the network.

TCP/IP Design Philosophy

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