TCP: Reliable Stream Transport

Chapter 13

Introduction

• Application programs often need to send large volumes of data from one computer to another

• An unreliable connectionless delivery system for large volume transfers is tedious and annoying– Why?

• There is a need for reliable stream delivery which– isolates the application programs from the

details of networking and– defines a uniform interface for stream transfer

Properties of a Reliable Delivery Service• Stream Orientation - data is a stream of bytes• Virtual Circuit Connection - a connection is agreed

upon, data is transferred, disconnection• Buffered Transfer - sending application decides data

size, the size may change at a lower layer, then data is available to destination application

• Unstructured Stream - structure boundaries are not preserved

• Full Duplex Connection - concurrent transfer in both directions

Providing Reliability

• Reliable protocols usually require the receiver to send an acknowledgement (ACK) or a PAR

• The sender keeps a copy of the packet that was sent and waits for an ACK– if an ACK is received, next packet is sent– if an ACK is not received, after a timer expires, the packet is

retransmitted• Figure 13.1 is a timeline for normal case

• Figure 13.2 is a timeline showing packet loss

• What to do about duplicate packets or ACKs?– Remember the sequence # field in IP header?

Sliding Windows

• In the case of Figure 13.1, the network is idle during machine delays

• We can allow the sender to transmit multiple packets before waiting for an ACK

• A protocol can place a small window over the sequence of packets to send and transmit all packets in that window as in Figure 13.3

• The window slides forward as ACKs are sent for packets in the window

Sliding Windows

• The performance of sliding window protocols depends on:– the window size – the speed at which the network accepts packets

• See Figure 13.4 which timelines 3 packets• A window of size one is just like a positive

acknowledgement with retransmission, or PAR• A steady state is reached when the sender can transmit

packets as fast as the network can - the network is kept busy

Sliding Windows

• A sliding window protocol must:– remember which packets have been acknowledged

– keep a separate timer for each unacknowledged packet• if a packet is lost, timer expires and sender retransmits

• When the sender slides its window, it moves past all acknowledged packets

• Similar software exists in the receiver• The window partitions the packets into:

– those sent and ACKed, those being transmitted and those not yet transmitted

The Transmission Control Protocol• TCP is a transmission protocol, not software• TCP defines the concepts, it is not the implementation• What does TCP specify?

– the format of data and acknowledgements

– procedures to ensure that data arrives correctly

– how software distinguishes among multiple destinations on a given machine

– how to recover from errors

– how to set up a connection to transfer data

The Transmission Control Protocol• What does the protocol not provide?

– does not dictate the details of the interface between TCP and the application layer

• TCP can be used with a variety of packet delivery systems, not just IP

• TCP can use dial-up lines, local area networks, high speed fiber, long haul, etc.

Ports, Connections and Endpoints• TCP resides above the IP layer, along with UDP

as shown in Figure 13.5• TCP

– allows multiple application programs to communicate concurrently

– demultiplexes incoming TCP traffic among applications

– uses protocol port numbers to identify the ultimate destination within a machine

Ports, Connections and Endpoints• TCP ports identify virtual circuit connections, not a

single object like the UDP port identifies • Connections are identified by a pair of endpoints

– An endpoint is a pair of integers (host, port) where host is the IP address for a host and port is a TCP port on that host

• Example:– A connection between a machine at MIT (18.26.0.36)

and a machine at Purdue (128.10.2.3) is defined by (18.26.0.36, 1069) and (128.10.2.3, 25)

Ports, Connections and Endpoints• Multiple connections may be made to another

protocol port on a given machine• Multiple connections may share a complete endpoint• Because TCP identifies a connection by a pair of

endpoints, a given TCP port number can be shared by multiple connections on the same machine– this means that a program can provide concurrent service

to multiple connections simultaneously without giving unique port numbers for each one (email)

Passive and Active Opens

• TCP is connection-oriented and requires both endpoints to agree to participate– Thus, the application program on one end performs a

passive open by telling its operating system that it is willing to accept an incoming connection

– At that time, the O.S. assigns a TCP port number for this end

– The application program at the other end issues an active open to its O.S. to request a connection

– The two establish and verify a connection

Segments, Streams and Sequence Numbers• The data stream is viewed as a sequence of octets that

are divided into segments for transmission• Each segment transmitted in a single IP datagram• Octets in the TCP data stream are numbered

sequentially, and a sender keeps 3 pointers for each connection (see Figure 13.6):– the left of the sliding window:ack’d vs. unack’d– the right of the sliding window, marks the highest sequence

# that can be sent before ACKs required– octets sent and octets not sent

Segments, Streams and Sequence Numbers• The receiver keeps similar window information• Because TCP connections are full-duplex, the

TCP software maintains window information at each end for both sending and receiving – thus, four windows per connection

Variable Window Size and Flow Control• TCP allows the window size to vary over time• Each ACK indicates how many octets have been

received– It also indicates a window advertisement which states

how many additional octets the receiver is willing to receive (specifies the receiver’s current buffer size)

• In response to an increased window advertisement, the sender increases the size of its sliding window

• In response to a decreased window advertisement, the sender decreases the size of its sliding window

Variable Window Size and Flow Control• A sliding window

– provides flow control

– provides reliable transfer

• To stop all transfer, the receiver can send a window advertisement of zero– A sender is allowed to transmit a segment with an

urgent bit set

– To avoid deadlock, the sender can probe periodically, by asking if buffer space is available

Variable Window Size and Flow Control• Two flow problems

– internet flow control between source and ultimate destination (called end-to-end flow control)

– flow control for intermediate systems (routers)

• When intermediate systems become overloaded, we have a condition called congestion– We will look at ways to handle congestion later

TCP Segment Format

• The unit of transfer between the TCP software on two machines is called a segment

• Segments are exchanged to:– establish connections– transfer data– send ACKs– advertise window sizes– close connections

• Using piggybacking, an ACK from one machine to another may travel in the same segment with data travelling in the opposite direction (in reality, this does not happen often)

TCP Segment Format

• A segment is made up of a header and data• The TCP header consists of:

– source and destination ports which identify applications– a sequence number identifying this data’s position in the

sender’s byte stream (stream flowing this direction)

– an ACK number identifying which octet the source is expecting next (stream flowing opposite direction of this stream)

– header length ( in 32 bit multiples) to cover options– 6 code bits to indicate purpose and contents of segment

(URG, ACK, PSH, RST, SYN, FIN)

TCP Segment Format

– how much data it is willing to accept, buffer size • window advertisements accompany each segment, and are

piggybacking on ACKs

– checksum

– urgent pointer which specifies the end of the urgent data

• what marks the beginning of the urgent data?

Out of Band Data

• It may be necessary to send messages that are not a part of the regular data in the stream, out of band data– Example: sending a keyboard sequence that interrupts or

aborts the program at the other end

• This data is specified as urgent and is indicated by the URG bit and is in the data portion of this segment– its end is known by the number in the Urgent Pointer

field

Maximum Segment Size Option

• Both ends need to agree on a maximum segment size they will transfer– If the two endpoints are on the same network, this will be

the network’s MTU

– If not on the same network, they will go through a process of MTU discovery, or choose 536 (default size of IP datagram, 576, plus TCP and IP headers)

• What happens when:– segment size is too small? network utilization is low

– segment size is too big? fragmentation

TCP Checksum Computation

• A pseudo header is prepended • Zeroes are added to make the segment a multiple

of 16 bits• Takes the one’s complement of sum of all 16-bit

entities, assuming the original checksum is zero• On the receiving side, IP passes source and

destination IP addresses when it passes the segment

Acknowledgements and Retransmission• The receiver reconstructs the original stream sent by

the sender by piecing together segments

• Some segments may be lost, delayed, or arrive out of order

• The receiver uses the sequence numbers to reconstruct the stream

• The receiver acknowledges the longest contiguous prefix of the stream that has arrived correctly– It sends the sequence number of the segment it is expecting

next

Acknowledgements and Retransmission• When an acknowledgement is not received within

a given timeout period, the segments may be retransmitted– only the first unacknowledged segment

• worst case is to have to retransmit one at a time

– all segments in the window

• worst case is only the first one was needed

Timeout and Retransmission

• Every time a segment is sent, TCP starts a timer and waits for an ACK

• If the timer expires before an ACK is received, it assumes the segment was lost or corrupted and retransmits it

• How long should the timeout be? • See Figure 13.10 for roundtrip times of 100

successive IP datagrams on the Internet

Timeout and Retransmission

• An adaptive retransmission algorithm for TCP monitors a connection and determines a reasonable timeout period (RTT - Round Trip Time) for that connection– TCP records the time that a segment was sent, and the time

that an ACK was received for it– the elapsed time is a round trip sample– a new sample is obtained and the RTT is modified by

RTT = (alpha * oldRTT) + ((1-alpha) * newSample)

• When performance changes, the value of RTT is modified accordingly

Timeout and Retransmission

• When the value for alpha is close to:– zero: the weighted average responds to changes in

delay very quickly

– one: the weighted average does not change significantly when a temporary change is noticed

Accurate Measurement of Round Trip Samples• In measuring the round trip samples, we could

have acknowledgement ambiguity if we had to retransmit– When we receive an ACK is it for the original

datagram or for the retransmitted datagram?

– Problems with assuming the ACK for the original, and with assuming the ACK is for the most recent transmission

Karn’s Algorithm

• Karn’s algorithm for handling ambiguous acknowledgements is to not update the round trip estimate for retransmitted segments

• If retransmission times are completely ignored, a large delay could not be noticed and the problem of spirally retransmissions could occur

• Karn’s algorithm suggests a timer backoff– if the timer expires and retransmission is done, TCP

increases the timeout up to a point that is larger than the delay along any path in the internet

Responding to High Variance in Delay• Research shows that previous algorithms do not

respond well to a wide range of variation in delay• Queuing theory suggests that the variation in

round trip time varies proportional to 1/(1-L) where L is the current network load

• A 1989 specification of TCP requires estimating the average round trip time and the variance

Responding to Congestion

• When congestion occurs, delays increase and routers queue datagrams

• Routers have finite buffer space and when that limit is reached, datagrams will be discarded

• Delay causes retransmissions and retransmissions cause more delay ... until congestion collapse

• To avoid congestion, transmission rates must be reduced– Routers watch queue length and use ICMPsource quench

Responding to Congestion

• TCP maintains a congestion window which is the smaller of the receiver’s advertised window and the current congestion window – multiplicative decrease - reduces the congestion

window by half each time a segment is lost

• if loss continues, TCP limits transmission to one datagram and doubles timeout values before retransmitting

• provides significant and fast response to congestion

• lets routers clear datagrams in their queues

Responding to Congestion

• How does TCP recover when congestion ends?– slow start

• start with setting the congestion window to the size of one segment

• increase the congestion window by one segment each time an acknowledgement arrives

– thus, one segment, two segments, four segments, eight …

• once the congestion window is half of its original size before congestion occurred, it slows down and increases by one segment only, if all goes well

Congestion, Tail Drop and TCP

• An early policy, called Tail Drop discarded a datagram if the input queue at a router was full

• Since datagrams are typically multiplexed, with successive datagrams from a different source, the router might discard one segment from N connections rather than N segments from one connection

• So, what’s wrong with that?– All N instances of TCP will enter slow start at the same

time

Random Early Discard (RED)

• A router uses Tmin and Tmax to mark positions in the queue

• If the queue currently contains fewer than Tmin datagrams, add the new datagram to the queue

• If the queue contains more than Tmax datagrams, discard the new datagram

• If the queue contains between Tmin and Tmax datagrams, randomly discard the datagram with a probability p

Random Early Discard (RED)

• A router slowly and randomly drops datagrams as congestion increases

• How are Tmin and Tmax determined?

• How is the value of p determined?

Establishing a TCP Connection

• To establish a connection, TCP uses a three-way handshake as shown in Figure 13.13 (consider right side the server and left side the client - see handout)

– the first segment has the SYN bit set, port number of the server that the client wants to connect to, and an initial sequence number (ISN)

– the second segment has both the SYN and ACK bits set, server sends its own ISN, the ACK contains the client’s ISN plus one

– the third segment is an ACK with the server’s ISN plus one

Establishing a TCP Connection

• Usually the TCP software on one machine waits passively for a handshake, the other initiates it

• However, if both request a connection at the same time, this can be done, and is called a simultaneous open

Initial Sequence Numbers

• Each machine in a connection chooses an ISN at random

• These numbers are shared with the other machine in the connection establishment handshake

• Segments that follow in the data transfer period will be numbered sequentially from the initial numbers

Closing a TCP Connection

• When one side of the connection has no more data to send, it will close the connection in one direction

• At the end of its data transfer, this side will send a segment with the FIN bit set

• The other side will acknowledge the FIN segment and may continue sending until it has no more data, when it sends its own FIN segment

• See Figure 13.14 - third step is ACK

TCP Connection Reset

• Normally, connections are released as in the previous section

• But, sometimes a connection is broken by accident• To completely close the connection, it can be reset• This is done to release resources like buffers • One side sends a segment with the RST bit set

TCP State Machine

• See Figure 13.15• Circles represent states• Arrows represent transitions between states• Labels show what causes the transition and what it

sends in response• Notice the syn, ack , rst and fin indicators that are

in the segment header

Forcing Data Delivery

• We don’t always want to wait until a buffer is full in order to send it

• If an application is gathering keystrokes, we want them to appear on the screen as they are entered, not as a block of keystrokes

• TCP provides a push operation that forces delivery of octets without waiting for the buffer to fill - it uses the psh code

TCP Reserved Port Numbers

• Well-known ports were originally those under 256• But some over 1024 have been assigned for such• TCP and UDP have some commonly numbered

ports• See some of the currently assigned TCP port

numbers in Figure 13.16

Silly Window Syndrome

• The problem that is presented for this discussion is one that arises when an application reads incoming data one octet at a time– When a connection is established, the receiving TCP allocates a

buffer of K octets

– If the sender generates data quickly, the sending TCP transmits segments to fill the entire window

– Then the receiver indicates that it has no buffer space available

– When the receiver reads one octet, it can advertise that it has one octet of space available

– Thus, very small segments are generated with large overhead (41 bytes/segment) and much processing for little data

Avoiding Silly Window Syndrome• Receive Side

– The receiver maintains the actual available buffer space, but will not advertise an increase until the window can be advanced by:

• half of the receiver’s buffer• or the maximum size of a segment

• Delayed Acknowledgements– The receiver delays sending an ACK when the window is

not sufficiently large enough to advertise (!> 500ms)

• advantage: delayed ACKs decrease traffic, increase throughput• disadvantage: the sender may retransmit

Avoiding Silly Window Syndrome• Send Side

– New data is placed in the buffer, but the sender does not send until a maximum size segment is filled

– If still waiting to send when an ACK arrives, send all data that has accumulated in the buffer

– Apply the rule even if the user has requested a push

– This is called the Nagle algorithm and requires little computation

• In general, the receiver avoids advertising a small window and the sender delays transmission

Summary

• TCP provides – reliable stream delivery– full-duplex connections between two machines allowing for

exchange of large volumes of data

• TCP uses sliding windows for efficient use of the network

• TCP provides flow control and allows systems of varying speeds to communicate

• Segments are used to transfer data or control information

Summary

• TCP implements flow control by letting the receiver advertise the amount of data it is willing to accept, yet supports out of band messages

• Current TCP uses:– exponential backoff for retransmission timers

– congestion avoidance algorithms

– heuristics to avoid transferring small packets

For Next Time

• Several chapters on routing• Read Chapter 14