What is meant by record or data boundaries in the sense of TCP & UDP protocol?

Question

I am learning to sockets and found the word Data OR Record Boundaries in SOCK_SEQPACKET communication protocol? Can anyone explain in simple words what is Data boundary and how the SOCK_SEQPACKET is different from SOCK_STREAM & SOCK_DGRAM ?

Answer 1

This answer https://stackoverflow.com/a/9563694/1076479 has a good succinct explanation of message boundaries (a different name for "record boundaries").

Extending that answer to SOCK_SEQPACKET:

• SOCK_STREAM provides reliable, sequenced communication of streams of data between two peers. It does not maintain message (record) boundaries, which means the application must manage its own boundaries on top of the stream provided.

• SOCK_DGRAM provides unreliable transmission of datagrams. Datagrams are self-contained capsules and their boundaries are maintained. That means if you send a 20 byte buffer on peer A, peer B will receive a 20 byte message. However, they can be dropped, or received out of order, and it's up to the application to figure that out and handle it.

• SOCK_SEQPACKET is a newer technology that is not yet widely used, but tries to marry the benefits of both of the above. That is, it provides reliable, sequenced communication that also transmits entire "datagrams" as a unit (and hence maintains message boundaries).

It's easiest to demonstrate the concept of message boundaries by showing what happens when they're neglected. Beginners often post client code like this here on SO (using python for convenience):

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('192.168.4.122', 9000))
s.send(b'FOO')        # Send string 1
s.send(b'BAR')        # Send string 2
reply = s.recv(128)   # Receive reply

And server code similar to this:

lsock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
lsock.bind(('', 9000))
lsock.listen(5)
csock, caddr = lsock.accept()
string1 = csock.recv(128)    # Receive first string
string2 = csock.recv(128)    # Receive second string <== XXXXXXX
csock.send(b'Got your messages') # Send reply

They don't understand then why the server hangs on the second recv call, while the client is hung on its own recv call. That happens because both strings the client sent (may) get bundled together and received as a single unit in the first recv on the server side. That is, the message boundary between the two logical messages was not preserved, and so string1 will often contain both chunks run together: 'FOOBAR'

(Often there are other timing-related aspects to the code that influence when/whether that actually happens or not.)