CS144-lab4
Checkpoint 4 Writeup
报文头格式
IPV4头
/*
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* |Version| IHL |Type of Service| Total Length |
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* | Identification |Flags| Fragment Offset |
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* | Time to Live | Protocol | Header Checksum |
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* | Source Address |
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* | Destination Address |
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* | Options | Padding |
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
*/
static constexpr size_t LENGTH = 20; // IPv4 header length, not including options, at least 20 bytes
static constexpr uint8_t DEFAULT_TTL = 128; // A reasonable default TTL value
static constexpr uint8_t PROTO_TCP = 6; // Protocol number for TCP
// IPv4 Header fields
uint8_t ver = 4; // IP version
uint8_t hlen = LENGTH / 4; // header length (multiples of 32 bits, that is 4 bytes)
uint8_t tos = 0; // type of service
uint16_t len = 0; // total length of packet
uint16_t id = 0; // identification number
bool df = true; // don't fragment flag
bool mf = false; // more fragments flag
uint16_t offset = 0; // fragment offset field
uint8_t ttl = DEFAULT_TTL; // time to live field
uint8_t proto = PROTO_TCP; // protocol field
uint16_t cksum = 0; // checksum field
uint32_t src = 0; // src address
uint32_t dst = 0; // dst address
ARP头包含
uint16_t hardware_type = TYPE_ETHERNET; // Type of the link-layer protocol (generally Ethernet/Wi-Fi)
uint16_t protocol_type = EthernetHeader::TYPE_IPv4; // Type of the Internet-layer protocol (generally IPv4)
uint16_t opcode {}; // Request or reply
EthernetAddress sender_ethernet_address {};
uint32_t sender_ip_address {};
EthernetAddress target_ethernet_address {};
uint32_t target_ip_address {};
发送时要加上以太网头
以太网头包含
EthernetAddress dst;
EthernetAddress src;
uint16_t type; //Frame type：TYPE_IPv4 or TYPE_ARP
相关类
本次实验相关文件里给出了几种不同的传输单位
- InternetDatagram 网络数据报(IP层)
- ARPMessage ARP报文
- EthernetFrame 以太网帧(链路层)
并且提供了将几种传输报文(姑且统称)序列化为报文内部有效载荷payload的函数serialize，和将payload解析为指定报文的函数parse，以便编写时方便地加上或去除头部。
NetworkInterface作为本地接口，负责以太网帧的加工、接受和转发，为此需要增添内部成员实现记录和排队
typedef uint32_t ip_addr_t;
typedef size_t time_t;
static constexpr time_t map_lmt = 30000;
static constexpr time_t arp_request_lmt = 5000;
std::map<ip_addr_t, std::pair<EthernetAddress,time_t>> arp_table{};
std::map<ip_addr_t, time_t> arp_request_table{};
std::queue<EthernetFrame> sending_queue{};
std::map<ip_addr_t, std::vector<EthernetFrame>> waiting_queue{};
- arp_table 维护从ip到mac地址的映射，由于映射是有map_lmt时间限制的，超时后删除，所以每个映射还要记录时间
- arp_request_table 为了防止ARP广播风暴，维护从ip到time_t的映射，在arp_request_lmt时间内接口对同一个ip的多次ARP REQUEST会被忽略
- sending_queue 以太网帧的等待队列，与Checkpoint 3的等待队列类似，将send_datagram产生的帧放入并等待may_send调用
- waiting_queue 存放由于本地无映射、必须等待ARP请求得到dst的帧，注意此处对同一个ip可能有多个以太网帧在等待，故需要使用vector
主要函数实现
void NetworkInterface::send_datagram( const InternetDatagram& dgram, const Address& next_hop )
查看本地map是否缓存了目标ip对应的mac地址
- 是，构建以太网帧(填写头部，并用InternetDatagram构造payload)，放入sending_queue
- 否，构建以太网帧，放入waiting_queue，如果最近无对目标ip的ARP请求，构建请求报文并放入sending_queue
optional
NetworkInterface::recv_frame( const EthernetFrame& frame )
只接受广播帧和mac地址是本地mac地址的帧
- 如果是TYPE_IPv4，说明是发送给自己的，去除头部将InternetDatagram交给上层
- 如果是TYPE_ARP，使用parse解析出ARP报文，并在本地map中添加报文发送方的ip和mac的映射，继续判断ARP类型，如果是OPCODE_REQUEST，就和本地ip比较，相同就发送ARP REPLY，否则忽略。完成ARP处理后还要把waiting_queue里发送到最近更新的ip的帧添加到sending_queue
void NetworkInterface::tick( const size_t ms_since_last_tick )
每次被调用时清除超时映射和超时ARP_REQUEST记录即可
optional
NetworkInterface::maybe_send()
取出sending_queue最早的帧发送
最后附上代码
#include "network_interface.hh"
#include "arp_message.hh"
#include "ethernet_frame.hh"
using namespace std;
// ethernet_address: Ethernet (what ARP calls "hardware") address of the interface
// ip_address: IP (what ARP calls "protocol") address of the interface
NetworkInterface::NetworkInterface( const EthernetAddress& ethernet_address, const Address& ip_address )
: ethernet_address_( ethernet_address ), ip_address_( ip_address )
{
cerr << "DEBUG: Network interface has Ethernet address " << to_string( ethernet_address_ ) << " and IP address "
<< ip_address.ip() << "\n";
}
// dgram: the IPv4 datagram to be sent
// next_hop: the IP address of the interface to send it to (typically a router or default gateway, but
// may also be another host if directly connected to the same network as the destination)
// Note: the Address type can be converted to a uint32_t (raw 32-bit IP address) by using the
// Address::ipv4_numeric() method.
void NetworkInterface::send_datagram( const InternetDatagram& dgram, const Address& next_hop )
{
uint32_t next_hop_ip = next_hop.ipv4_numeric();
if(arp_table.count(next_hop_ip)){
EthernetFrame frame;
frame.header.dst=arp_table[next_hop_ip].first;
frame.header.src=ethernet_address_;
frame.header.type=EthernetHeader::TYPE_IPv4;
frame.payload=serialize(dgram);
sending_queue.push(frame);
}else{
EthernetFrame frame;
frame.header.src=ethernet_address_;
frame.header.type=EthernetHeader::TYPE_IPv4;
frame.payload=serialize(dgram);
waiting_queue[next_hop_ip].push_back(frame);
if(!arp_request_table.count(next_hop_ip)){
ARPMessage arp;
arp.opcode=ARPMessage::OPCODE_REQUEST;
arp.sender_ethernet_address=ethernet_address_;
arp.sender_ip_address=ip_address_.ipv4_numeric();
arp.target_ip_address=next_hop_ip;
arp_request_table[next_hop_ip]=0;
frame.header.dst=ETHERNET_BROADCAST;
frame.header.src=ethernet_address_;
frame.header.type=EthernetHeader::TYPE_ARP;
frame.payload=serialize(arp);
sending_queue.push(frame);
}
}
}
// frame: the incoming Ethernet frame
optional<InternetDatagram> NetworkInterface::recv_frame( const EthernetFrame& frame )
{
if(frame.header.dst!=ethernet_address_ && frame.header.dst!=ETHERNET_BROADCAST){
return {};
}
if(frame.header.type==EthernetHeader::TYPE_IPv4){
InternetDatagram dgram;
if(parse(dgram,frame.payload)){
return dgram;
}
}else if(frame.header.type==EthernetHeader::TYPE_ARP){
ARPMessage arp;
if(parse(arp,frame.payload)){
arp_table[arp.sender_ip_address]={arp.sender_ethernet_address,0};
if(arp.opcode==ARPMessage::OPCODE_REQUEST){
if(arp.target_ip_address==ip_address_.ipv4_numeric()){
ARPMessage reply;
reply.opcode=ARPMessage::OPCODE_REPLY;
reply.sender_ethernet_address=ethernet_address_;
reply.sender_ip_address=ip_address_.ipv4_numeric();
reply.target_ethernet_address=arp.sender_ethernet_address;
reply.target_ip_address=arp.sender_ip_address;
EthernetFrame newframe;
newframe.header.dst=arp.sender_ethernet_address;
newframe.header.src=ethernet_address_;
newframe.header.type=EthernetHeader::TYPE_ARP;
newframe.payload=serialize(reply);
sending_queue.push(newframe);
}
// else if(frame.header.dst == ETHERNET_BROADCAST){
// sending_queue.push(frame);
// }
}
auto datagrams=waiting_queue[arp.sender_ip_address];
for(auto& i:datagrams){
i.header.dst=arp.sender_ethernet_address;
sending_queue.push(i);
}
waiting_queue.erase(arp.sender_ip_address);
}
}
return {};
}
// ms_since_last_tick: the number of milliseconds since the last call to this method
void NetworkInterface::tick( const size_t ms_since_last_tick )
{
for(auto it=arp_table.begin();it!=arp_table.end();){
it->second.second+=ms_since_last_tick;
if(it->second.second>map_lmt){
it=arp_table.erase(it);
}else{
it++;
}
}
for(auto it=arp_request_table.begin();it!=arp_request_table.end();){
it->second+=ms_since_last_tick;
if(it->second>arp_request_lmt){
it=arp_request_table.erase(it);
}else{
it++;
}
}
}
optional<EthernetFrame> NetworkInterface::maybe_send()
{
if(sending_queue.empty()){
return {};
}
EthernetFrame frame=sending_queue.front();
sending_queue.pop();
return frame;
}
