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PC-A Sending a TCP SYN to PC-B
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Now that PC-A knows the IP address of PC-B, the telnet application can proceed with the actual telnet Telnet uses TCP at the transport layer, so the three-way handshake must take place first The first step is for PC-A to send a TCP segment with the SYN code (commonly called a flag) set It uses a dynamic port above 49,151 and a destination, the well-known port of 23 for telnet PDU 1 in Table 10-2 shows the Ethernet frame, IP packet, and TCP header information Notice that the next-hop layer 2 address is Router-A, the default gateway When the router receives this and processes the information, at layer 3 the router notices that the destination IP address is not its own; so the router does a lookup in its routing table and sees that the next-hop at layer 3 is Router-B
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If the link is down between Router-A and Router-B, Router-A will send an ICMP destination unreachable message to PC-A with a source address of 10012 If the link between Router-B and PC-B is down, Router-B will send an ICMP destination unreachable message to PC-A with a source address of 10022 At
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least you know what network component is telling you where the problem is If your ping application doesn t display destination unreachable, just reply timed out, as some ping applications do, you can use traceroute (tracert on Windows) to troubleshoot the problem further to nd where the problem is located
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TABLE 10-2
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00001111BBBB 00002222BBBB 00003333BBBB
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Src MAC
00001111AAAA 00002222AAAA 00003333AAAA
Src IP
10011 10011 10011
Dest IP
10032 10032 10032
Proto
TCP TCP TCP
Src Port
50001 50001 50001
Dest TCP Port Code
23 23 23 SYN SYN SYN
Don t be surprised if you see a question on the exam that asks you to gure out, based on the link an Ethernet frame is traversing, the contents of the
elds in the Ethernet header, IP header, and TCP/UDP header, as shown in Tables 10-1, 10-2, and 10-3
If Router-A doesn t know the IP-to-MAC address mapping of Router-B, it will ARP for it Router-A then re-encapsulates the IP packet in a new Ethernet frame, shown in PDU 2 in Table 10-2: the IP and TCP headers remain the same, but a new frame header was generated to get it information across the 10020/24 subnet When Router-B receives the frame, it notices that the IP address doesn t match its own, so Router-B looks in its routing table to see where the packet should be forwarded In this case, Router-B is directly connected to the destination subnet, 10030/24 If Router-B doesn t know the IP-to-MAC address mapping for PC-B, Router-B will ARP for it During the ARP request process, Switch-B will learn about Router-B s MAC address and add it and port 1 to its CAM table (if it hasn t already done this) Likewise, Switch-B will learn PC-B s MAC address during the ARP reply process, if it doesn t know it already Router-B then encapsulates the IP packet in a new frame to get the data to PC-B The Ethernet frame header, IP packet header, and TCP segment header are shown in PDU 3 of Table 10-2 One important item to point out about the sending of the preceding SYN segment is that the only thing that changed from PC-A to PC-B was the Ethernet framing information three Ethernet frames were used, but the original IP packet stayed the same
10: Sending and Receiving TCP/IP Packets
PC-B Sending a TCP SYN/ACK to PC-A
At this point, PC-B is processing the frame, sending the IP packet up to layer 3, and then sending the TCP segment up to layer 4 At the transport layer, PC-B notices that this is a new connection based on the TCP SYN code and that the application that should handle it is telnet Assuming that a telnet server is running on the host, PC-B will add the connection to its local connection table and reply back with a TCP SYN/ACK segment You can see this in PDU 1 in Table 10-3 The source port is 23 (the server) and the destination port is 50001, PC-A The process basically works in reverse when sending the SYN/ACK back to PC-A: the source and destination addresses and ports are reversed Also, no ARPs need to be performed since this was already done in the PC-A-to-PC-B direction Also, both switches have the destination MAC addresses in their CAM tables, so no flooding will occur
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