CS475 – Networks
Lecture 18
Chapter 5: End-to-End Protocols
Reminders
• Homework 5, Wireshark Project 3, and Programming Project 3 due today.
Chapter 5: End-to-End Protocols
We've covered connecting computers. The ________________ messages, synchronization, flow control, support for multiple
layer deals with connecting processes running on computers.
processes.
A transport protocol may be expected to provide: guaranteed
Lower layers may: drop messages, reorder messages, deliver
delivery, in-order delivery, no duplicates, support for large
duplicates, limit message size, deliver after a long delay.
5.1 Simple Demultiplexer (UDP)
UDP extends the host-to-host service of the network to a
process-to-process service but adds no other functionality.
UDP uses 16-bit port numbers to _______________________
between processes. (A process is identified with a port
number/IP number pair.)
Implementation of the port abstraction may vary from OS to OS.
UDP header format
Typically, each port is associated with a message queue. When a
process receives a message one is removed from the queue.
How does a client know which port to send a message to on the
process on a well-known port. The client communicated with
server? The server may use a well-known port (see /etc/services the port mapper to find the port number of the desired process.
on a UNIX machine).
UDP does employ a checksum to verify a message. Packets with
Alternatively, the server could run a _______________________ errors are dropped.
5.2 Reliable Byte Stream (TCP)
In addition to demultiplexing, TCP provides guaranteed, reliable, Flow control prevents the sender from overwhelming the
in-order delivery with flow and congestion control. TCP
receiver. ___________________________ control prevents the
connections are full-duplex.
sender from overwhelming the network (switches, links).
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5.2.1 End-to-End Issues
The sliding window algorithm used by TCP is like that used on a
3.
point-to-point link (Section 2.5.2), but there are important
lifetime or MSL is typically 120 s)
differences:
1.
4.
Setup (exchange of state so the sliding window
Resources are not tied to a single link and can not be
determined in advance (flow control needed),
algorithm can start) and teardown are needed
2.
Packets can be reordered (the maximum segment
5.
Congestion is possible (congestion control needed)
RTTs are variable, so timeouts must be ____________,
5.2.2 Segment Format
TCP is a ____________________ protocol. Bytes are normally
collected into segments before before being sent to the
The TCP header is shown above. A TCP connection is identified
destination.
by the 4-tuple (SrcPort, SrcIPAddr, DstPort, DstIPAddr).
The HdrLen is the size of the header in 32-bit words.
The Acknowledgment, SequenceNum and AdvertisedWindow The Flags field contains 6 bits: SYN, FIN, RESET, PUSH,
fields are used by the _________________________ algorithm. URG, and ACK.
Each transmitted byte has a corresponding SequenceNum.
Acknowledgment and AdvertisedWindow are associated with
•
SYN and FIN are used to set up a connection.
•
RESET indicates that the receiver is confused and
received data.
wants to abort the connection.
•
PUSH indicates that data should be send immediately.
•
URG signifies that the segment contains urgent data.
The UrgPtr field contains the number of urgent data
bytes.
•
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ACK is set when the Acknowledgment field is valid.
5.2.3 Connection Establishment
A three-way ______________________ is used to set up the
connection. Packets contain the initial sequence numbers to be
used by the client and the server (x and y) in subsequent packets.
The TCP specification requires that the initial sequence numbers
be random numbers.
A trans.diagram for TCP setup and tear down is shown above.
Rectangles show states. Arcs have tags of the form event/action.
Retransmissions due to __________________ are not shown.
5.2.4 Sliding Window Revisited
The sliding window algorithm discussed previously provided
while on the receiver:
reliable, in-order delivery. TCP's sliding window algorithm
LastByteRead < NextByteExpected ≤ LastByteRcvd + 1
extends the prior one by adding _______________________.
Flow control is achieved by having the receiver advertise a
window size to the sender instead of using a fixed-size window.
Assume the send and receive buffers are of size MaxSendBuffer
and MaxRcvBuffer. On the receive side TCP must keep:
LastByteRcvd – LastByteRead ≤ MaxRcvBuffer
The sender is limited to sending no more than
AdvertisedWindow bytes of unacknowledged data at any time.
The advertised window size is
AdvertisedWindow = MaxRcvBuffer –
((NextByteExpected – 1) – LastByteRead)
On the sending side TCP ensures:
LastByteSent–LastByteAcked ≤ AdvertisedWindow
while maintaining
LastByteWritten – LastByteAcked ≤ MaxSendBuffer
If the sending process tries to write n bytes in such a way that
The sender maintains three pointers where:
LastByteAcked ≤ LastByteSent ≤ LastByteWritten
this inequality would not be maintained then the process is
______________________.
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A 32-bit sequence number will wrap around in 57 minutes at a
should be large enough to allow for a full delay x BW product.
10 Mbps transmit rate, but in only _________ seconds at 1
A cross country delay of 100 ms at 10 Mbps corresponds to 122
Gbps. An extension to TCP extends the sequence number space. KB. The TCP extension increases the advertised window size
A 16-bit AdvertisedWindow field allows for a 64 KB window. It also.
5.2.5 Triggering Transmission
TCP will transmit a segment when (1) it has collected a
Nagle's Algorithm:
when there is data to send
if both the data and the window ≥ MSS
send a full segment
else if there is unACKed data in flight
buffer data until ACK arrives
else
send all data now
maximum segment size (MSS) number of bytes, (2) the sending
process tells it too (a ______________), or (3) a “timer” expires.
5.2.6 Adaptive Retransmission
Originally, a TimeOut value for retransmission was computed
using:
EstimatedRTT = α EstimatedRTT + (1 – α)SampleRTT
TimeOut = 2 x EstimatedRTT
where SampleRTT is the time between when a segment is sent
and its _____________________ arrives.
The original TCP spec recommended a value of α between 0.8
Unfortunately an ACK for a retransmission is identical to an
ACK for the original. This can lead to incorrect values for
and 0.9.
_______________________.
The Karn/Partridge algorithm fixed the problem quite simply.
The original algorithm did not handle situations in which the
SampleRTT was measured only for segments that have been
SampleRTT might _________________ a lot. The
sent once.
Jackobson/Karels algorithm was an improvement:
The new algorithm included a second change. After each
Difference = SampleRTT – EstimatedRTT
retransmit the next timeout value would be set to twice the
EstimatedRTT = EstimatedRTT+(δ x Difference)
previous timeout value (exponential backoff). This helped to
Deviation = Deviation+δ(|Difference|-Deviation)
alleviate problems due to network ______________________.
TimeOut = μ x EstimatedRTT + φ x Deviation
where μ was typically 1 and φ was 4.
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5.2.7 Record Boundaries
TCP has two features that allow record boundaries to be put into A TCP push operation can be used to indicate a complete record.
the byte stream.
(The sockets API does not provide access to the PUSH flag.)
TCP allows data to be flagged as urgent or _________________. It is usually simpler for record boundary markers to be inserted
Urgent data can be used to indicate the end of a record.
by the application.
5.2.8 TCP Extensions
There have been four optional extensions to TCP that are
to determine if the sequence number has wrapped
implemented using Options in the TCP header:
around.
1.
The sender places a 32-bit _______________________
3.
in the header. The receiver echoes the time stamp in the
ACK. This allows for accurate measurement of the
2.
larger than 64 KB.
4.
The receiver can respond with a selective
RTT.
acknowledgment (______________). This allows the
The sequence number and the time stamp are examined
sender to transmit just missing segments.
5.2.9 Performace
Now that we have a complete protocol graph, we can discuss
how to measure its performance as seen by applications.
In particular, as network speeds increase, can a protocol like TCP
provide enough data to keep the network full?
Simple host-to-host in a room. 2 2.4GHz dual cores; 2 Gbps
bandwidth.
TTCP benchmark using various sizes of messages.
Note: "perfect" network, measures TCP implementation and
workstation hardware/software only. Will see other issues like
____________________.
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A scaling factor can be included to advertise a window
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5.2.10 Alternative Design Choices
TCP is a stream-oriented protocol as opposed to a request/reply
TCP uses connection setup and teardown. It is possible to send
protocol. We will examine a request/reply protocol (_________) all connection parameters with the first data message. TCP setup
next time. (TCP can be used for request/reply applications, but
allows a receiver to reject a connection before any data is sent.
there are complications.)
TCP teardown means that “____________________” messages
TCP is a byte-stream rather than a message-stream service.
don't need to be sent.
(Record boundaries can however be inserted into the byte
TCP uses window-based versus rate-based flow control. There
stream.)
are similarities but also some interesting differences.
In Class Exercise
Log on locally under Linux or log on remotely to csserver to answer the following questions:
• How do we send out-of-band data via TCP? (man send)
• How do we receive out-of-band data? (man recv)
• Which of the four TCP extensions described in class are supported under Linux? (man tcp)
• What acronym is used for the TCP extension that helps to determine if the sequence number has wrapped around? What
does this acronym stand for?
• Is there a way to disable Nagle's algorithm so that segments are sent immediately? If so, how?
The Linux /proc pseudo-filesystem interface can be used to tune many of the TCP algorithms. Changing the parameters requires
system administration privileges. Use cat to examine appropriate /proc file contents (man tcp) and determine the answers to the
following:
• Is the optional SACK extension enabled?
• What is the default receive buffer size?
• Is the optional window scaling extension enabled?
• What is the default congestion control algorithm? Which algorithms are available for use?
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