POP3 TCP or UDP?

[singh]

Hi

Fig 2.5 on page 80 shows POP3 port 110 to work over UDP. Exam cram book shows to be TCP along with wikipedia

http://en.wikipedia.org/wiki/Post_Office_Protocol


Can anyone clarify?

[lammle]

You're right. I loaded my trusty analyzer and this is what it showed:

Transmission Control Protocol, Src Port: pop3 (110), Dst Port: 52074 (52074), Seq: 215, Ack: 47, Len: 5


So, I am not sure how that got in there as UDP, but I'll get that fixed.
Thanks for the heads up!
Todd

[sukarabi]

Hi,

Yes and it's same for TFTP who was shown as a TCP protocol on page 70.
It's UDP right ?

[lammle]

Page 70 does not say that TFTP uses UDP

I think you were seeing UDP in the figure at the Application layer and assumed that I was saying it used UDP because UDP is shown below it at the Tranport layer. This is not what the figure is saying. It is just showing the protocols/applications that work at each layer.

Throughout that chapter, TFTP is referenced as UDP.

Todd

[sukarabi]

Ok, thanks for your answer.

[DragonJuice]

Per http://www.iana.org/assignments/port-numbers, POP3 can use port 110 on both TCP or UDP as shown below.

pop3 110/tcp Post Office Protocol - Version 3
pop3 110/udp Post Office Protocol - Version 3

I was reading the CCNA book and saw that it stated that POP3 was UDP, so I was curious if it was, which lead me to this forum posting. I thought it only used TCP for reliability, but now I know. It's probably a setting on the server that determines whether POP3 uses TCP or UDP.

[lazerqueen]

Am looking at tabl2 2.2 of the book and it lists Telnet: 23 as TCP but as per http://www.iana.org/assignments/port-numbers, telnet is also UDP.

The same is observed for SMTP:25, HTTP:80: FTP:21, HTTPS:443.

Am I missing something or did i misunderstand the IANA assignments?

[lazerqueen]

Hi!

As per the IANA post, in the study guide, chatper 2, table 2.2, all the protocols listed as TCP can actually do run on UDP as well. Infact. Can you actually provide any protocols that run on UDP alone.

lazerqueen.

[gabrielshorn]

DragonJuice - POP3 only uses TCP only. There's no such thing as a UDP POP3 server that I know of in a production environment. Maybe someone has written one as a project for a network programming class. IANA just reserves both TCP and UDP on port 110 for the sake of neatness. In other words, if someone were to come up with a good reason to run POP3 over UDP, the port is already reserved and is the same as the TCP port. Less confusion that way.

lazerqueen - TFTP and Echo come to mind as other protocols that only use UDP even though they have TCP port reservations as well. DNS is almost exclusively UDP, but there are still rare examples of DNS over TCP.