CODEWITHSUNDEEP
socketstcpnettynio
M.I
M.I

Reputation: 303

Netty : does it need to care TCP segments reassembly?

I have a question regarding to TCP segments reassembly. I learned the packet could be devided into multiple segments (this is something to do with MSS).

i.e) Message flow (Assumption):

  1. Client sends a packet that is passed from application layer
  2. In client side's TCP layer, the packet divided into 3 segments.
  3. the segments passed to Client's IP layer.
  4. the Server's IP layer receives the segments.
  5. In server side's TCP layer, it receives the 3 packets and reassembles it as one packet.
  6. Server's application layer receives the one packet.

My understaning is that TCP layer is where the divided segments get reassembled. Correct me if I am wrong.

Here is the thing what I really want to clarify.

When using Netty, Server side's "messageReceived()" method gets called only one time or 3 times? If the TCP layer is the place where the divided segments gets reassembled, the "messageReceived()" method gets called only once, correct?

Then, is it really neccesarry to use something like "ReplayingDecoder" to guarantee the number of bytes server is receiving?

Your help is greatly appreciated.

Additional Question:

If the server fails to reassemble the segments because one of them is lost or something, then the TCP layer pass the incomplete packet to the application layer?

Upvotes: 3

Views: 2393

Answers (1)

user207421
user207421

Reputation: 311048

the packet could be divided into multiple segments

Upside down, or bad terminology. TCP sends segments which are divided into packets and which may be further split into sub-packets en route.

My understanding is that the TCP layer is where divided segments get reassembled.

Packet reassembly takes places in the IP layer, not the application (or the TCP layer). Segment reassembly takes place in the TCP layer.

"messageReceived()" method gets called only one or 3 times?

It gets called any number of times from 1 to N where N is the length of the byte stream. There is no guaranteed 1::1 correspondence between sender sends and receiver receives.

If the server fails to reassemble the segments because one of them is lost or something, then the TCP layer pass the incomplete packet to the application layer?

Absolutely not. TCP doesn't pass packets to the application layer at all. It passes an intact, correctly sequenced byte stream, or nothing.

Wondering if i should handle the segment reassembly by myself

You don't, and can't, handle any of it yourself. TCP provides a byte stream to the application, not segments or packets.

Upvotes: 3

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