CODEWITHSUNDEEP
udptcp
Jichao
Jichao

Reputation: 41845

Why UDP does not employ the path MTU?

According to my knowledge, UDP does not use the path MTU to avoid fragmentation which however TCP does. I am trying to come up a reason to this particular design.

TCP needs to avoid fragmentation because it has to retransmit the whole datagram even if just one fragment is dropped. On the contrary, UDP does not need to retransmit datagram, it's up to the application layer to ensure the integrity.

In conclusion, fragment will slow the TCP transport protocol but not UDP transport protocol.

Here comes the problem, for communication that need integrity, whether employ TCP which natrually guarantee the integrity, or develop an application layer retransmit protocol upon UDP, it will need to resend the whole datagram if not ACKed. Then, fragmentation will slow down application layer retransmit protocol upon UDP just the same as TCP.

What's wrong with my reasoning?

Upvotes: 3

Views: 2420

Answers (2)

user207421
user207421

Reputation: 311028

  1. Because there's nothing useful it can do with the MTU. It's a datagram protocol.
  2. Because there is no path. It's an connectionless protocol.

Upvotes: 1

Steffen Ullrich
Steffen Ullrich

Reputation: 123531

UDP is a datagram protocol where each packet means a single entity, independent of the other packets (UDP does not detected duplication, reordering etc). TCP instead is a stream protocol, that is the whole transfer consists of a single unstructured octet stream similar to a large file. To make the transfer of this stream more efficient it makes sense to detect the MTU of the connection and try to send mostly packets which max out this MTU, thus reducing the overhead of the transfer. To further reduce the overhead TCP will merge multiple consecutive writes into as few packets (with max MTU) as possible.

UDP instead can not avoid fragmentation by itself because it transmits the datagram as it is, that is datagram boundary is determined by packet boundary. Any kinds of optimization to reduce overhead have to be done by the application itself.

Thus TCP is best suited for applications where its features like guaranteed and ordered delivery and efficient use of bandwidth are needed. Unfortunately these features come with drawbacks like comparable slow connection setup, higher latency (in case of packet loss) etc. But there are applications which don't need all the good parts but have to avoid the bad parts. For example real time audio and video can deal with packet loss but needs a low latency, i.e. it does not matter if all data arrive but they have to arrive fast. In these cases the more simple UDP protocol is better suited.

Upvotes: 2

Related Questions