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Network Layer ​

Internet Protocol ​

 An IP datagram consists of a header part and a text part.

IPv4 ​

Ipv4

  • IPv4 Header
    • The header has a 20-byte fixed part and a variable length optional part.
      • Fixed Part include:
        • Version
        • IHL
        • TOS
        • Total Length
        • Identification
        • DF and MF
        • Fragment offset
        • TTL
        • Protocol
        • Header checksum
        • Source address and destination address

IPv4 Header Part ​

Version ​

Version: to keep track of which version of the protocol the datagram belongs to. 4 bit

IHL ​

IHL: to provide the length of the header, in 32-bit words. 4 bit - When the value is 5, which applies no options are present.

TOS ​

TOS: (Type of service) to distinguish between different classes of services. 8 bit - Three-bit precedence field and three flags, D, T, and R {Delay, Throughput and Reliability}. 3 bit - 三位优先字段（precedence field）和三标志位（D、T、R）用于指示数据报的服务质量需求和优先级。这些字段通常位于IPv4首部的服务类型（Type of Service, TOS）字段中。

TOS BitsDescription
0000Normal (default)
0001Minimize cost
0010Maximize reliability
0100Maximize throughput
1000Minimize delay

这个表格展示了服务类型字段的不同比特组合及其对应的描述。每种组合表示数据报在传输过程中不同的服务质量要求，例如默认服务、最小化成本、最大化可靠性、最大化吞吐量和最小化延迟。

Total Length ​

Total Length: to provide the length of the datagram (including header and data). 16 bit

Identification ​

  • Identification: to identify each datagram. 16 bit

Fragment and Fragment offset ​

  • DF and MF: don’t fragment, and more fragments. 3 bit
  • Fragment offset: To tell where in the current datagram this fragment belongs. 13 bit

TTL ​

  • Time to live: a counter used to limit packet lifetimes. 8 bit

Protocol and Header Checksum ​

  • Protocol: to tell which protocol process to give it to (e.g., TCP or UDP). 8 bit
  • Header checksum: to detect errors in the datagram. 16 bit

Source address and Destination address ​

Address Classification

  • Source address and destination address 32 bit

  • Unique: no two machines on the Internet have the same IP address.

32-bit long.

IPV4 Summary ​

字段比特数功能描述
版本 (Version)4标识IP协议的版本，IPv4的版本字段值为4
首部长度 (IHL)4指示IP数据报首部的长度，以32位（4字节）为单位
服务类型 (TOS)8指定数据报的优先级和服务质量
总长度16指示整个IP数据报的总长度，包括首部和数据部分，以字节为单位
标识 (Identification)16唯一标识数据报，以便在需要时进行分片和重组
标志 (Flags)3控制和识别数据报的分片，包括不分片位（DF）和更多分片位（MF）
片偏移 (Fragment Offset)13指示数据报片在原始数据报中的相对位置，以8字节为单位
生存时间 (TTL)8限制数据报在网络中的生存时间，防止无限循环
协议 (Protocol)8指示数据报携带的数据所使用的协议，如TCP、UDP等
首部校验和 (Header Checksum)16验证IP首部的完整性
源地址 (Source Address)32表示数据报发送方的IP地址
目的地址 (Destination Address)32表示数据报接收方的IP地址
选项 (Options)可变支持其他的控制功能和服务，长度可变，最多可达40字节

总结：IPv4首部的基本部分是20字节（160位），但如果包含选项字段，长度可能会超过20字节。选项字段的长度是可变的，最大可达40字节，因此整个IPv4首部的最大长度是60字节。

IPv6 ​

IPv6

Transition from IPv4 to IPv6 ​

  • Three transition strategies
    • Dual stack
    • Tunneling
    • Header translation

Dual Stack ​

Dual Stack

Tunneling ​

Tunneling

Header Translation ​

Header Translation

Network Routing ​

Delivery ​

  • The network layer supervises the handling of the packets by the underlying physical networks.
  • We define this handling as the delivery of a packet.
  • Direct and indirect delivery

Delivery

Forwarding ​

  • Forwarding means to place the packet in its route to its destination. 转发
  • Forwarding requires a host or a router to have a routing table.
  • When a host has a packet to send or when a router has received a packet to be forwarded, it looks at this table to find the route to the final destination.

Routing Algorithms ​

Shortest Path Routing

  • To study the routing algorithms, a graph is commonly used to represent a subnet.
  • Each node of the graph represents a router and each arc of the graph represents a communication line.
  • The path length can be measured in terms of the number of hops, the geographic distance, etc.

Dijkstra Algorithm ​

[!abstract]+ Dijkstra Algorithm

  1. Set the value of the initial node to zero and set all other nodes to infinity.
  2. Mark all nodes unvisited. Set the initial node as starting point.
  3. For the current node, consider all of its unvisited neighbors and calculate their tentative distances.
  4. Mark the current node as visited and remove it from the unvisited set. A visited node will never be checked again.
  5. If the destination node has been marked visited or if the smallest tentative distance among the nodes in the unvisited set is infinity, then stop. The algorithm has finished.
  6. Set the unvisited node marked with the smallest tentative distance as the next current node and go back to step 3.

Flooding Algorithm ​

  1. Every incoming packet is sent out on every outgoing line except the one it arrived on.
  2. To solve the vast numbers of duplicate packets, a hop counter is introduced.
  3. The hop counter is contained in the header of each packet.
  4. It decremented at each hop. The packet will be discarded when the counter reached zero.

Distance Vector Routing ​

  1. It is a dynamic routing algorithm.
  2. Each router maintains a table giving the best known distance to each destination and which line to use to get there.
  3. These tables are updated by exchanging information with their neighbors.
  4. An entry of routing tables contains two parts: the preferred outgoing line to be used for the destination, and an estimate of the distance to that destination.