Are You Smarter Than a CCENT? Transport Layer

Here is a query from a current project – enjoy!

  1. You are examining a packet capture from your local network. What two determinations can you make from this output? (Choose 2)

100-105a. This is a UDP packet
b. This is a TCP packet
c. The target destination is FTP
d. The target destination is HTTP

Correct Answer: b, d

Explanation: Due to its use of reliable transmissions, Transmission Control Protocol (TCP) uses many fields in its packet structure. These include:

  • source TCP port number
  • destination TCP port number
  • sequence number
  • acknowledgement number
  • TCP data offset
  • reserved data
  • control flags
  • window size
  • TCP checksum
  • urgent pointer
  • TCP optional data

In contrast, the unreliable User Datagram Protocol (UDP) packets feature these simple fields:

  • source port number
  • destination port number
  • length
  • checksum

From the destination TCP port number here of 80, you can determine the intended target service is the Hypertext Transfer Protocol (HTTP). Know the following common ports for this exam:

Port Number TCP or UDP Protocol
20 TCP FTP Data
21 TCP FTP Control
22 TCP SSH
23 TCP Telnet
25 TCP SMTP
53 UDP DNS
67, 68 UDP DHCP
69 UDP TFTP
80 TCP HTTP
110 TCP POP3
161 UDP SNMP
443 TCP SSL/TLS
514 UDP Syslog
520 UDP RIP

20 thoughts on “Are You Smarter Than a CCENT? Transport Layer

  1. you can determinate that this is TCP because UPD dose not do ACK’s
    and you can assume that it should be HTTP because 80 is a well known port number for HTTP, but it could be anything that runs on tcp 🙂

  2. Answer:
    b. This is a TCP packet
    d. The target destination is HTTP

    [Source port 49993 (FTP Server web Interface Port) & Destination port 80 (HTTP), sequence numbers & acknowledgement number represent it’s a TCP packet & target destination is HTTP.]

  3. Its already quite obvious since everybody let the cat out of the bag that the answer is b and d. My justification would be that the destianation port 80 is a well known TCP port number which happens to represent HTTP so even without looking at the ACK packet, you can tell that this is a TCP packet With an HTTP target destination

  4. B and D. The destination port of 80 is a well known port for HTTP. Also you can see that there is an acknowledgement in this packet, which means that it is a TCP packet and not UDP.

  5. Answer:

    b. This is a TCP packet – TCP because i see an ACK.
    d. The target destination is HTTP – The Destination Port is 80 (well known port for HTTP)

  6. B. TCP packet
    D. target destination is HTTP

    Between ACKs and windowing, it is utilizing TCP. And it’s likely HTTP due to the well known port 80.

  7. B. TCP packet
    D. target destination is HTTP

    Between ACKs and windowing, it is utilizing TCP. And it’s likely HTTP due to the well known port 80.

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