Network Protocols
Andy Wang
Operating Systems
COP 4610 / CGS 5765
Protocol


An agreement between two parties as
to how information is to be transmitted
A network protocol abstracts packets
into messages
Physical Reality vs. Abstraction
Physical reality: packets
Limited size
Unordered
Unreliable
Machine-to-machine
Only on local area network
Asynchronous
Insecure
Abstraction: messages
Arbitrary size
Ordered
Reliable
Process-to-process
Routed anywhere
Synchronous
Secure
Arbitrary-Size Messages

Can be built on top of limited-size ones


By splitting a message into fix-sized
packets
Checksum can be computed on each
fragment or the whole message
Internet Protocol (IP)

Provides unreliable, unordered,
machine-to-machine transmission of
arbitrary-size messages
Process-to-Process
Communications


Built on top of machine-to-machine
communications through the use of
port addresses
Each message contains the destination
port to talk to the correct process
Unreliable Data Protocol
(UDP)


Provides unreliable, unordered, user-touser communication
Built on the top of IP
Ordered Messages


Built on top of unordered ones
Use sequence numbers to indicate
the order of arrival



Specific to a connection
If packet 3 arrives before packet 2, wait
for packet 2.
Always deliver packets in order, to user
applications
Reliable Message Delivery


Built on top of unreliable delivery
Problem: Network infrastructure can
garble messages

Packets can be dropped if network buffers
are full
Solution




Checksum each message
At a receiver, discard messages with
mismatching checksums
A receiver acknowledges if a packet is
received properly
A sender resends the same message
after not hearing the acknowledgment
for some time (a timeout period)
A Minor Problem


A sender may send twice, if the first
acknowledge is lost
The receiver needs to discard duplicate
packets
Implications


A sender needs to buffer messages that
are not yet acknowledged
The receiver must track messages that
could be duplicates
Transmission Control
Protocol (TCP)

Provides a reliable byte stream between
two processes on different machines
over the Internet
sequence number: 1
checksum:
fa73cd10
Transmission Control Protocol

Fragments the byte stream into packets
and hands them to IP
TCP Message Categories

Sender




Sent and acknowledged
Sent and not acknowledged
Not yet sent
Receiver



Forwarded to application
Received and buffered
Not yet received
More on the Sequence
Number

Need a way to recycle sequence
numbers

Each TCP packet has a time-to-live field

If the packet is not delivered in X seconds



The packet is dropped
Sequence numbers can be reused
An epoch number used to identify which
set of sequence numbers is being used


Incremented at each boot
Stored on disk
Congestion

Implications of timeout period at a
sender



Too long  unnecessary waiting
Too short  a message is transmitted
when an acknowledgement is in transit
Network congestion  delayed
acknowledgement  timeout  data
retransmission  more congestion
TCP Solution

Slow start: TCP starts by sending a
small amount of data


If no timeout, more data is sent
If timeout, TCP reduces the amount of
data being sent
The Byzantine Generals’
Problem

Two generals are on the tops of two
mountains…


They communicate only through
messengers…
They need to coordinate the attack…


If they attack at the same time, they win…
If they attack at different times, they
will…die…
The Byzantine Generals’
Problem

Question: can they guarantee a
synchronized attack?
The Byzantine Generals’
Problem Illustrated
General X
11am OK?
General Y
11am sounds good
So, 11am it is.
Yeah, what if you
don’t get this ack?
The Byzantine Generals’
Problem
Over an unreliable network, we cannot
guarantee that two computers will
synchronize
Distributed Transaction


Multiple machines agree to do
something atomically, but not
necessarily at exactly the same time
Mechanism: two-phase commit
Two-Phase Commit
Account X
Account Y
Phase 1: ask if each can commit
1. Begin transaction
Ask Y for $1
Enough cash
2. Write “Y = Y - $1”
Ready to commit
Phase 2: commit
3. Write “X = X + $1”
4. Commit
Ask Y to commit
5. Commit
Scenarios

If X crashes between 1 and 2



If X crashes before step 4


Y will wake up and do nothing
X will timeout and abort the transaction
X will wake up and abort the transaction
If X crashes between 4 and 5

Y will timeout and ask X for the transaction
Scenarios

If Y crashes between 2 and 5



Y will wake up and check the log
When X sends Y the commit message, Y
will commit
Y can also timeout and ask X the current
status