路由器运作原理以及网络互连技术

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Introduction

What is IP ?

• IP stands for Internet Protocol.
• Key tool used today to build scalable(可升级的), heterogeneous(混杂) internetworks.
• It runs on all the nodes in a collection of networks
• Defines the infrastructure(基础设施) that allows these nodes and networks to function as a single logical internetwork

IP Service Model

Packet Delivery Model

• Connectionless model for data delivery.
• Best-effort delivery (unreliable service).

Global Addressing Scheme

• Provides a way to identify all hosts in the network.

How Layer 3 Routers Work ?

• Layer 3 router uses store and forward scheme to forward incoming IP packets (datagrams).
  • IP Address Lookup (Forwarding Table constructed by routing protocols, such as RIP, OSPF, BGP, etc).
  • IP/MAC mapping table.
• Forward IP packet into next hop if the destination IP is found in the Forwarding Table. Otherwise, forward to default port.
• Wire-speed forwarding design.
• Not Plug-and-Play.
• Forward Table:

• Mapping Table:

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IP and Routers

IP Datagram Header Format

• Type of Service of IP)

Precedence	Delay	Throughput	Reliability	O	O	bits
3	1	1	1

• Precedence
  • 111 Network Control
  • 110 Internetwork Control 101 CRITIC/ECP
  • 100 Flash Override
  • 011 Flash
  • 010 Immediate
  • 001 Priority
  • 000 Routine
• Delay
  • 0 Normal
  • 1 Low
• Throughput
  • 0 Normal
  • 1 High
• Reliability

.

• Flags

O	DF	MF	bits
	1	1

• DF
  • 0 May Fragment
  • 1 Don’t Fragment
• MF
  • 0 Last Fragment
  • 1 More Fragment

IP Addresses

• Properties

  • Globally unique 32 bits address
  • Hierarchical(分级): network + host
  • Class A type (1/2)

  

  • Class B type (1⁄4)

  

  • Class C type (1/8)

  

Intra-LAN and Inter-LAN Communications

• B -> Y (Intra LAN):
  • Send the frame to the destination directly.

• B -> A (Inter-LAN):
  • Send the frame to attached Router first.
  • Router will forward to the destination.

IP Datagram Forwarding

• Strategy
  • every datagram contains destination’s address.
  • if directly connected to destination network, then forward to host.
  • if not directly connected to destination network, then forward to some router.
  • forwarding table maps network number(网络号) into next hop.
  • each host has a default router.
  • each router maintains a forwarding table.

IP Fragmentation and Reassembly(重组)

• MTU (Maximum Transmission Unit)

  • Ethernet (1518 bytes)
  • IEEE 802.11 Wireless (2312 bytes)
  • FDDI (4500 bytes)

• Strategy

  • Fragmentation occurs in a router when it receives a datagram that it wants to forward over a network which has MTU < datagram.
  • Reassembly is done at the receiving host.
  • All the fragments carry the same identifier.
  • Fragments are self-contained datagrams.
  • IP does not recover from missing fragments.

Router Characteristics

• Network Layer Routing
  • Network layer protocol dependent.
  • Filter MAC broadcast and multicast packets.
  • Easy to support mixed media.
  • Packet fragmentation and reassembly.
  • Filtering on network (IP) addresses and information.
  • Accounting.
• Direct Communication Between Endpoints and Routers
  • Highly configurable and hard to get right.
  • Handle speed mismatch.
  • Congestion(拥塞) control and avoidance.
• Routing Protocols
  • Interconnect layer 3 networks and exploit arbitrary topologies.(利用随机拓扑)
  • Determine which route to take.
  • Static routing.
  • Dynamic routing protocol support：
    • RIP: Routing Information Protocol.
    • OSPF: Open Shortest Path First.
  • Provides reliability with alternate(备用) routes.
• Router Management
  • Troubleshooting capabilities(故障排除功能).

Differences Between Bridges and Routers

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IP Subnetting

Add another level to address/routing hierarchy(等级制度): subnet.

Subnet masks define variable partition(可变区域) of host part of class A and B addresses.

Forwarding Algorithm:

D = destination IP address
for each entry < SubnetNum, SubnetMask, NextHop>
    D1 = (SubnetMask) AND (D)
    if D1 = SubnetNum
        if NextHop is an interface
            deliver datagram directly to destination
        else
            deliver datagram to NextHop (a router)

• A default router is used if nothing matches。
• Not necessary for all ones in subnet mask to be contiguous(邻近).
• Can put multiple subnets on one physical network.
• Subnets not visible from the rest of the Internet.

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Classless Addressing

Classless Inter-Domain Routing (CIDR, 无类别域间路由): A technique that addresses(解决) two scaling concerns(扩展问题) in the Internet:

• The growth of backbone routing table(骨干路由表) as more and more network numbers need to be stored in them.
• Potential exhaustion(耗尽) of the 32-bit address space.

CIDR uses aggregate routes(聚合路由):

• Uses a single entry in the forwarding table to tell the router how to reach a lot of different networks.
• Breaks the rigid boundaries(刚性边界) between address classes.

The convention(惯例) is to place a “/X” after the prefix where X is the prefix length in bits.

• For example, the 20-bit prefix for all the networks 192.4.16 through 192.4.31 is represented as 192.4.16/20(16个 Class C).
• By contrast(对比), if we wanted to represent a single class C network number 192.4.16, which is 24 bits long, we would write it 192.4.16/24(一个 Class C).

Routing protocols to handle this classless address

• It must understand that the network number may be of any length.
• Represent network number with a single pair <length, value>.
• All routers must understand CIDR addressing.
• CIDR means that prefixes may be of any length, from 2 to 32 bits.
• Longest prefix matching.
  • It is also possible to have prefixes in the forwarding tables that overlap.
  • For example, we might find both
    • 171.69/16 (a 16 bit prefix) and
    • 171.69.10/24 (a 24 bit prefix)
  • in the forwarding table of a single router.

Address Resolution Protocol (ARP)

• Map IP addresses into physical (MAC) addresses.
  • destination host, or next hop router.
• ARP (Address Resolution Protocol)
  • table of IP to physical address bindings.
  • broadcast request if IP address not in table.
  • target machine responds with its physical address.
  • table entries are discarded(丢弃) if not refreshed.
• ARP Packet Format
  • HardwareType: type of physical network (e.g., Ethernet)
  • ProtocolType: type of higher layer protocol (e.g., IP)
  • HLEN & PLEN: length of physical and protocol addresses
  • Operation: request or response
  • Source/Target Physical/Protocol addresses

Host Configurations

• Most host Operating Systems provide a way to manually configure the IP information(手动配置) for the host.
• Automated Configuration Process is required.

Dynamic Host Configuration Protocol (DHCP)

• DHCP server is responsible for providing configuration information to hosts.
• There is at least one DHCP server for an administrative domain.
• DHCP server maintains a pool of available addresses.
• DHCP:
  • Newly booted or attached host sends DHCP DISCOVER
    message to a special IP address (255.255.255.255, 广播)
  • DHCP relay agent unicasts the message to DHCP server and waits for the response. (DHCP 中继代理将消息单播到 DHCP 服务器并等待回应。)

Internet Control Message Protocol (ICMP)

• Defines a collection of error messages that are sent back to the source host whenever a router or host is unable to process an IP datagram successfully.
  • Destination host unreachable due to link /node failure.
  • Reassembly process failed.
  • TTL had reached 0 (so datagrams don’t cycle forever).
  • IP header checksum failed.
• ICMP-Redirect
  • From router to a source host
  • With a better route information
  • 需要路由器向源发送ICMP重定向的情况有两种：
    1. 当路由器从某个接口收到数据包后，还要将数据包从同一个接口发往目的地，就是路由器收到数据包的接口正是去往目的地的出口时，则会向源发送ICMP重定向，通告对方直接将数据包发向自己的下一跳即可，不要再发给自己。
    2. 数据包的源IP和自己转发时的下一跳IP地址是同网段时，则会向源发送ICMP重定向，通告对方直接将数据包发向自己的下一跳。
    • 注：路由器在向数据源发送ICMP重定向的同时，也会正常转发收到的数据包，并不会中断网络。
    • 说明:ICMP重定向是基于接口配置的，默认为开启状态。

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Routing protocols

Forwarding:

• to select an output port based on destination address and routing table.
• Forwarding table
  • Used when a packet is being forwarded.
  • An entry in the forwarding table contains the mapping from a network number to an outgoing interface and some MAC information, such as Ethernet Address of the next hop.

Routing:

• process to build the routing table.
• Routing table
  • Built by the routing algorithm.
  • Generally contains mapping from network numbers to next hops.

The basic problem of routing is to find the lowest-cost path between any two nodes.
- Where the cost of a path equals the sum of the costs of all the edges that make up the path.

Distributed and dynamic protocol:

• Distance Vector
• Link State

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Distance Vector protocol

Each node constructs a one dimensional array (a vector) containing the “distances” (costs) to all other nodes and distributes that vector to its immediate neighbors.

• Example Network:

• Assume that each node knows the cost of the link to each of its directly connected neighbors, Initial distances stored at each node (global view):

• Initial routing table at node A:

• Final routing table at node A:

• Final distances stored at each node (global view):

• Every T seconds each router sends its routing
  table to its neighbors.
• Each router then updates its routing table based
  on the new information.
• Problems include:
  • fast response to good news.
  • slow response to bad news.
  • Too many messages to update.
• When a node detects a link failure:
  • it may recover,
  • or encounter a Count-to-infinity problem.
  • In fact, some relatively small number is used to approximate the infinity, For example, the maximum number of hops to get across a certain network is less than 16.
  • One technique to improve the time to stabilize(稳定) routing is called split horizon(水平分割):
    • When a node sends a routing update to its neighbors, it does not send those routes it learned from each neighbor back to that neighbor.

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Link State protocol

Strategy: Send to all nodes (not just neighbors) information about directly connected links (not entire routing table).

Link State Packet (LSP)

• ID of the node that created the LSP.
• Cost of link to each directly connected neighbor.
• Sequence number (SEQNO).
• Time-to-live (TTL) for this packet.

Reliable Flooding

• Store most recent LSP from each node.
• Forward LSP to all nodes but one that sent it.
• Generate new LSP periodically; increment SEQNO.
• Start SEQNO at 0 when reboot.
• Decrement TTL of each stored LSP; discard when TTL=0.

Example of reliable flooding of LSP packets From node A

Shortest Path Routing

• OSPF (Open Shortest Path First).
• Each router computes its routing table directly from the LSP’s it has collected using the Dijkstra’s algorithm.
• Find the shortest path from the router to each other node.

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