TCPNewReno Class Reference
#include <TCPNewReno.h>
Inheritance diagram for TCPNewReno:
Public Member Functions | |
| TCPNewReno () | |
| virtual void | receivedDataAck (uint32 firstSeqAcked) |
| virtual void | receivedDuplicateAck () |
Protected Member Functions | |
| virtual TCPStateVariables * | createStateVariables () |
| virtual void | recalculateSlowStartThreshold () |
| virtual void | processRexmitTimer (TCPEventCode &event) |
Protected Attributes | |
| TCPNewRenoStateVariables *& | state |
Detailed Description
Implements TCP NewReno.
Definition at line 34 of file TCPNewReno.h.
Constructor & Destructor Documentation
Ctor
Definition at line 26 of file TCPNewReno.cc.
: TCPTahoeRenoFamily(), state((TCPNewRenoStateVariables *&)TCPAlgorithm::state) { }
Member Function Documentation
| virtual TCPStateVariables* TCPNewReno::createStateVariables | ( | ) | [inline, protected, virtual] |
Create and return a TCPNewRenoStateVariables object.
Implements TCPAlgorithm.
Definition at line 40 of file TCPNewReno.h.
{
return new TCPNewRenoStateVariables();
}
| void TCPNewReno::processRexmitTimer | ( | TCPEventCode & | event | ) | [protected, virtual] |
Redefine what should happen on retransmission
Reimplemented from TCPBaseAlg.
Definition at line 51 of file TCPNewReno.cc.
{
TCPTahoeRenoFamily::processRexmitTimer(event);
if (event==TCP_E_ABORT)
return;
// RFC 3782, page 6:
// "6) Retransmit timeouts:
// After a retransmit timeout, record the highest sequence number
// transmitted in the variable "recover" and exit the Fast Recovery
// procedure if applicable."
state->recover = (state->snd_max - 1);
tcpEV << "recover=" << state->recover << "\n";
state->lossRecovery = false;
state->firstPartialACK = false;
tcpEV << "Loss Recovery terminated.\n";
// After REXMIT timeout TCP NewReno should start slow start with snd_cwnd = snd_mss.
//
// If calling "retransmitData();" there is no rexmit limitation (bytesToSend > snd_cwnd)
// therefore "sendData();" has been modified and is called to rexmit outstanding data.
//
// RFC 2581, page 5:
// "Furthermore, upon a timeout cwnd MUST be set to no more than the loss
// window, LW, which equals 1 full-sized segment (regardless of the
// value of IW). Therefore, after retransmitting the dropped segment
// the TCP sender uses the slow start algorithm to increase the window
// from 1 full-sized segment to the new value of ssthresh, at which
// point congestion avoidance again takes over."
// begin Slow Start (RFC 2581)
recalculateSlowStartThreshold();
state->snd_cwnd = state->snd_mss;
if (cwndVector) cwndVector->record(state->snd_cwnd);
tcpEV << "Begin Slow Start: resetting cwnd to " << state->snd_cwnd
<< ", ssthresh=" << state->ssthresh << "\n";
state->afterRto = true;
conn->retransmitOneSegment(true);
}
| void TCPNewReno::recalculateSlowStartThreshold | ( | ) | [protected, virtual] |
Utility function to recalculate ssthresh
Definition at line 31 of file TCPNewReno.cc.
Referenced by processRexmitTimer(), and receivedDuplicateAck().
{
// RFC 2581, page 4:
// "When a TCP sender detects segment loss using the retransmission
// timer, the value of ssthresh MUST be set to no more than the value
// given in equation 3:
//
// ssthresh = max (FlightSize / 2, 2*SMSS) (3)
//
// As discussed above, FlightSize is the amount of outstanding data in
// the network."
// set ssthresh to flight size/2, but at least 2 SMSS
// (the formula below practically amounts to ssthresh=cwnd/2 most of the time)
uint32 flight_size = std::min(state->snd_cwnd, state->snd_wnd); // FIXME TODO - Does this formula computes the amount of outstanding data?
// uint32 flight_size = state->snd_max - state->snd_una;
state->ssthresh = std::max(flight_size/2, 2*state->snd_mss);
if (ssthreshVector) ssthreshVector->record(state->ssthresh);
}
| void TCPNewReno::receivedDataAck | ( | uint32 | firstSeqAcked | ) | [virtual] |
Redefine what should happen when data got acked, to add congestion window management
Reimplemented from TCPBaseAlg.
Definition at line 93 of file TCPNewReno.cc.
{
TCPTahoeRenoFamily::receivedDataAck(firstSeqAcked);
// RFC 3782, page 5:
// "5) When an ACK arrives that acknowledges new data, this ACK could be
// the acknowledgment elicited by the retransmission from step 2, or
// elicited by a later retransmission.
//
// Full acknowledgements:
// If this ACK acknowledges all of the data up to and including
// "recover", then the ACK acknowledges all the intermediate
// segments sent between the original transmission of the lost
// segment and the receipt of the third duplicate ACK. Set cwnd to
// either (1) min (ssthresh, FlightSize + SMSS) or (2) ssthresh,
// where ssthresh is the value set in step 1; this is termed
// "deflating" the window. (We note that "FlightSize" in step 1
// referred to the amount of data outstanding in step 1, when Fast
// Recovery was entered, while "FlightSize" in step 5 refers to the
// amount of data outstanding in step 5, when Fast Recovery is
// exited.) If the second option is selected, the implementation is
// encouraged to take measures to avoid a possible burst of data, in
// case the amount of data outstanding in the network is much less
// than the new congestion window allows. A simple mechanism is to
// limit the number of data packets that can be sent in response to
// a single acknowledgement; this is known as "maxburst_" in the NS
// simulator. Exit the Fast Recovery procedure."
if (state->lossRecovery)
{
if (seqGE(state->snd_una-1, state->recover))
{
// Exit Fast Recovery: deflating cwnd
//
// option (1): set cwnd to min (ssthresh, FlightSize + SMSS)
uint32 flight_size = state->snd_max - state->snd_una;
state->snd_cwnd = std::min (state->ssthresh, flight_size + state->snd_mss);
tcpEV << "Fast Recovery - Full ACK received: Exit Fast Recovery, setting cwnd to " << state->snd_cwnd << "\n";
// option (2): set cwnd to ssthresh
// state->snd_cwnd = state->ssthresh;
// tcpEV << "Fast Recovery - Full ACK received: Exit Fast Recovery, setting cwnd to ssthresh=" << state->ssthresh << "\n";
// TODO - If the second option (2) is selected, take measures to avoid a possible burst of data (maxburst)!
if (cwndVector) cwndVector->record(state->snd_cwnd);
state->lossRecovery = false;
state->firstPartialACK = false;
tcpEV << "Loss Recovery terminated.\n";
}
else
{
// RFC 3782, page 5:
// "Partial acknowledgements:
// If this ACK does *not* acknowledge all of the data up to and
// including "recover", then this is a partial ACK. In this case,
// retransmit the first unacknowledged segment. Deflate the
// congestion window by the amount of new data acknowledged by the
// cumulative acknowledgement field. If the partial ACK
// acknowledges at least one SMSS of new data, then add back SMSS
// bytes to the congestion window. As in Step 3, this artificially
// inflates the congestion window in order to reflect the additional
// segment that has left the network. Send a new segment if
// permitted by the new value of cwnd. This "partial window
// deflation" attempts to ensure that, when Fast Recovery eventually
// ends, approximately ssthresh amount of data will be outstanding
// in the network. Do not exit the Fast Recovery procedure (i.e.,
// if any duplicate ACKs subsequently arrive, execute Steps 3 and 4
// above).
//
// For the first partial ACK that arrives during Fast Recovery, also
// reset the retransmit timer. Timer management is discussed in
// more detail in Section 4."
tcpEV << "Fast Recovery - Partial ACK received: retransmitting the first unacknowledged segment\n";
// retransmit first unacknowledged segment
conn->retransmitOneSegment(false);
// deflate cwnd by amount of new data acknowledged by cumulative acknowledgement field
state->snd_cwnd -= state->snd_una - firstSeqAcked;
if (cwndVector) cwndVector->record(state->snd_cwnd);
tcpEV << "Fast Recovery: deflating cwnd by amount of new data acknowledged, new cwnd=" << state->snd_cwnd << "\n";
// if the partial ACK acknowledges at least one SMSS of new data, then add back SMSS bytes to the cwnd
if (state->snd_una - firstSeqAcked >= state->snd_mss)
{
state->snd_cwnd += state->snd_mss;
if (cwndVector) cwndVector->record(state->snd_cwnd);
tcpEV << "Fast Recovery: inflating cwnd by SMSS, new cwnd=" << state->snd_cwnd << "\n";
}
// try to send a new segment if permitted by the new value of cwnd
sendData();
// reset REXMIT timer for the first partial ACK that arrives during Fast Recovery
if (state->lossRecovery)
{
if (!state->firstPartialACK)
{
state->firstPartialACK = true;
tcpEV << "First partial ACK arrived during recovery, restarting REXMIT timer.\n";
restartRexmitTimer();
}
}
}
}
else
{
//
// Perform slow start and congestion avoidance.
//
if (state->snd_cwnd < state->ssthresh)
{
tcpEV << "cwnd<=ssthresh: Slow Start: increasing cwnd by SMSS bytes to ";
// perform Slow Start. RFC 2581: "During slow start, a TCP increments cwnd
// by at most SMSS bytes for each ACK received that acknowledges new data."
state->snd_cwnd += state->snd_mss;
// Note: we could increase cwnd based on the number of bytes being
// acknowledged by each arriving ACK, rather than by the number of ACKs
// that arrive. This is called "Appropriate Byte Counting" (ABC) and is
// described in RFC 3465. This RFC is experimental and probably not
// implemented in real-life TCPs, hence it's commented out. Also, the ABC
// RFC would require other modifications as well in addition to the
// two lines below.
//
// int bytesAcked = state->snd_una - firstSeqAcked;
// state->snd_cwnd += bytesAcked*state->snd_mss;
if (cwndVector) cwndVector->record(state->snd_cwnd);
tcpEV << "cwnd=" << state->snd_cwnd << "\n";
}
else
{
// perform Congestion Avoidance (RFC 2581)
uint32 incr = state->snd_mss * state->snd_mss / state->snd_cwnd;
if (incr==0)
incr = 1;
state->snd_cwnd += incr;
if (cwndVector) cwndVector->record(state->snd_cwnd);
//
// Note: some implementations use extra additive constant mss/8 here
// which is known to be incorrect (RFC 2581 p5)
//
// Note 2: RFC 3465 (experimental) "Appropriate Byte Counting" (ABC)
// would require maintaining a bytes_acked variable here which we don't do
//
tcpEV << "cwnd>ssthresh: Congestion Avoidance: increasing cwnd linearly, to " << state->snd_cwnd << "\n";
}
// RFC 3782, page 13:
// "When not in Fast Recovery, the value of the state variable "recover"
// should be pulled along with the value of the state variable for
// acknowledgments (typically, "snd_una") so that, when large amounts of
// data have been sent and acked, the sequence space does not wrap and
// falsely indicate that Fast Recovery should not be entered (Section 3,
// step 1, last paragraph)."
state->recover = (state->snd_una - 2);
}
sendData();
}
| void TCPNewReno::receivedDuplicateAck | ( | ) | [virtual] |
Redefine what should happen when dupAck was received, to add congestion window management
Reimplemented from TCPBaseAlg.
Definition at line 257 of file TCPNewReno.cc.
{
TCPTahoeRenoFamily::receivedDuplicateAck();
if (state->dupacks==DUPTHRESH) // DUPTHRESH = 3
{
if (!state->lossRecovery)
{
// RFC 3782, page 4:
// "1) Three duplicate ACKs:
// When the third duplicate ACK is received and the sender is not
// already in the Fast Recovery procedure, check to see if the
// Cumulative Acknowledgement field covers more than "recover". If
// so, go to Step 1A. Otherwise, go to Step 1B."
//
// RFC 3782, page 6:
// "Step 1 specifies a check that the Cumulative Acknowledgement field
// covers more than "recover". Because the acknowledgement field
// contains the sequence number that the sender next expects to receive,
// the acknowledgement "ack_number" covers more than "recover" when:
// ack_number - 1 > recover;"
if (state->snd_una - 1 > state->recover)
{
tcpEV << "NewReno on dupAck=DUPTHRESH(=3): perform Fast Retransmit, and enter Fast Recovery:";
// RFC 3782, page 4:
// "1A) Invoking Fast Retransmit:
// If so, then set ssthresh to no more than the value given in
// equation 1 below. (This is equation 3 from [RFC2581]).
// ssthresh = max (FlightSize / 2, 2*SMSS) (1)
// In addition, record the highest sequence number transmitted in
// the variable "recover", and go to Step 2."
recalculateSlowStartThreshold();
state->recover = (state->snd_max - 1);
state->firstPartialACK = false;
state->lossRecovery = true;
tcpEV << " set recover=" << state->recover;
// RFC 3782, page 4:
// "2) Entering Fast Retransmit:
// Retransmit the lost segment and set cwnd to ssthresh plus 3*SMSS.
// This artificially "inflates" the congestion window by the number
// of segments (three) that have left the network and the receiver
// has buffered."
state->snd_cwnd = state->ssthresh + 3*state->snd_mss;
if (cwndVector) cwndVector->record(state->snd_cwnd);
tcpEV << " , cwnd=" << state->snd_cwnd << ", ssthresh=" << state->ssthresh << "\n";
conn->retransmitOneSegment(false);
// RFC 3782, page 5:
// "4) Fast Recovery, continued:
// Transmit a segment, if allowed by the new value of cwnd and the
// receiver's advertised window."
sendData();
}
else
{
tcpEV << "NewReno on dupAck=DUPTHRESH(=3): not invoking Fast Retransmit and Fast Recovery\n";
// RFC 3782, page 4:
// "1B) Not invoking Fast Retransmit:
// Do not enter the Fast Retransmit and Fast Recovery procedure. In
// particular, do not change ssthresh, do not go to Step 2 to
// retransmit the "lost" segment, and do not execute Step 3 upon
// subsequent duplicate ACKs."
}
}
tcpEV << "NewReno on dupAck=DUPTHRESH(=3): TCP is already in Fast Recovery procedure\n";
}
else if (state->dupacks > DUPTHRESH) // DUPTHRESH = 3
{
if (state->lossRecovery)
{
// RFC 3782, page 4:
// "3) Fast Recovery:
// For each additional duplicate ACK received while in Fast
// Recovery, increment cwnd by SMSS. This artificially inflates the
// congestion window in order to reflect the additional segment that
// has left the network."
state->snd_cwnd += state->snd_mss;
if (cwndVector) cwndVector->record(state->snd_cwnd);
tcpEV << "NewReno on dupAck>DUPTHRESH(=3): Fast Recovery: inflating cwnd by SMSS, new cwnd=" << state->snd_cwnd << "\n";
// RFC 3782, page 5:
// "4) Fast Recovery, continued:
// Transmit a segment, if allowed by the new value of cwnd and the
// receiver's advertised window."
sendData();
}
}
}
Member Data Documentation
TCPNewRenoStateVariables*& TCPNewReno::state [protected] |
Reimplemented from TCPTahoeRenoFamily.
Definition at line 37 of file TCPNewReno.h.
Referenced by processRexmitTimer(), recalculateSlowStartThreshold(), receivedDataAck(), and receivedDuplicateAck().
The documentation for this class was generated from the following files:
