第四章路由协议

Forwarding with classful addressing
Forwarding without subnetting A typical forwarding module of classing address can be designed using three tables, one for each unicast class(A,B,C).
Next Hop（下一跳）
IPA IP11 Net 1 Net 1 IP12 IP21 Net 2 Net 2 IP22 Net 3 Net 3 IPB
Src IP = IPA，Dst IP = IPB 主机A的默认网关 1st delivery： indirect，Next-hop IP = IP11 2nd delivery：indirect，Next-hop IP = IP21 3th delivery：direct， Dst IP = IP B
4.2.1 Routing Table and Routing Module
Policy:
Direct delivery
Host-specific Host-specific delivery
NetworkNetworkspecific delivery
Default route
最长匹配（longest match） 最长匹配（longest match）
Discussion
Network-specific routing
路由表的大小只与网络的个数有关，与每个网络的大 小（包含的主机数多少）无关
Next-hop routing
路由器独立选路，从Net 1到Net 2的路径可能与从Net 2 到Net 1的路径不是同一条 Net 1 A B C Net 2 A B C Net 2 Net 1 D E Net 1 A D E Net 2 只有最后一个路由器才知道目的主机是否存在
面向连接 通信过程 路由选择 传输路径 建立连接 交付分组 终止连接 无连接 交付分组 每个分组独立选路 可以不同
仅在建立连接时选路 相同
Discussion
IP分组传输环境
多个网络 差异：通信能力、分组长度、延时 多个路由器 独立选路
IP通信 —— 无连接通信
多种网络都能满足的通信方式 允许多种选路策略（如：带宽、延时）
longest match
This principle states that the routing table is sorted from the longest mask to the shortest mask If there are three masks,/27,/26,and /24, the mask /27 must be the first entry and /24 must be last
Net 1 Net 1
Net 2 Net 2
Method
Delivery
分组目的IP与其发送接口是否在同一IP网络 中？ 目的IP & 发送口掩码 == 发送口IP & 发送口掩码 ？ 相等：直接交付 不等：间接交付
Operation—— 地址映射：IP地址 址
物理地 ARP
直接交付：目的IP 地址 目的物理地址 间接交付：下一跳IP地址 目的物理地址 目的物理地址：标识本地链路上的通信对象
4.1.3 Forwarding (转发)
Forwarding means to place the packet in its route to its destination. Forwarding requires a host or a router to have a routing table.
The forwarding module
1. 2. 3.
4.
5.
The destination address of the packet is extracted. A copy of the destination address is used to find the class of the address. The result of step 2 for class A,B or C and the destination address are used to extract the network address. The class of the address and the network address are used to find next-hop information. The class determines the table to be searched. The ARP module used the next-hop address and the interface number to find the physical address of the next router.
3、Host-specific routing
A route for a specific host Feature
Host mask: all 1s
Dst. Network Next-hop IP Mask Address I1 Mask1 － IB /32 I11 I12 I3 Mask3 Net 1 I1, Mask1 I12 I21 Net 2 I2, Mask2 I22 Net 3 I11 I32 I31 Net 3 I3, Mask3 IA Host B IB
Chapter 4 Delivery, Forwarding, and Routing of IP Packets
Architecture of the Internet
Information: IP packet Node: router Select the path Channel: physical network Transmit IP packets
Router
Dst. Network Next-hop IP Mask Address Mask1 I1 － Mask2 I2 － 0.0.0.0 0.0.0.0 I13
4.2 Classful Addressing Routing (分类寻址路由表)
Routing Table and Routing Module Routing process
Function
Route check Security Management
4、Default routing
Use a default route if no entry for destination network in the routing table Feature
Destination network address = 0.0.0.0 Destination network mask = 0.0.0.0
尽最大努力交付，best-effort delivery
4.1.2 Direct vs. Indirect Delivery
Direct delivery（直接交付）
分组目的与分组的发送接口在同一IP网络中
Indirect delivery（间接交付）
分组目的与分组的发送接口在不同IP网络中 direct indirect direct delivery delivery D2 S delivery D1
Forwarding with subnetting
In classful addressing, subnetting happens inside the organization. The routers that handle subnetting are either at the border of the organization site or inside the site boundary. If the organization is using variable-length subnetting, we need several tables; otherwise, we need only one table.
Network Next-hopInterface address address number
Each routing table has a minimum of three columns: The network address of the destination network tells us where the destination host is located. The next-hop address tells us to which router the packet must be delivered for an indirect delivery. The interface number defines the outgoing port from which the packet is sent out.
2、Network-Specific Method
Instead of having an entry for every destination host connected to the same physical network, it have only one entry that defines the address of the destination network itself.
Forwarding Techniques
1. 2. 3. 4.
Next-Hop Method Network-Specific Method Host-Specific Method Default Method
1、Next-Hop Method