trans_design.nr

早期freebsd实现

tab(%);
l l l l.
.\" The next line sets the spacing for the table: 1+3 17+3 17+3 13+3
.\"456789 123456789- 123456789 123456-789 123456789 1234567890
.\"
.T&
l l l s.
 %%/* log2(the negotiated max size) */% 
.T&
l l l l.
 %int%tp_l_tpdusize;%/* # bytes */
.\" *************************************** 
 %struct timeval%tp_rtt;% 
.T&
l l l s.
 % %/* smoothed avg round-trip time */% 
 %struct timeval%tp_rtv;% 
 % %/* std deviation of round-trip time */% 
%struct timeval%tp_rttemit[ TP_RTT_NUM + 1 ];%
%%/* times that the last TP_RTT_NUM %
%% * DT_TPDUs were transmitted %
%% */%
.\" *************************************** 
 %unsigned % % 
%  tp_sendfcc:1,%/* shall next ack %
% %include flow control conf. param? */%
.\" *************************************** 
.T&
l l l s.
 %  tp_trace:1,%/* is this pcb being traced?% 
%% * (not used yet) %
%% */%
.\" *************************************** 
%  tp_perf_on:1,%/* statistics being kept? */% 
.\" *************************************** 
%  tp_reneged:1,%/* have we reneged on credit%
%% * since the last AK TPDU was sent? %
%% */%
%  tp_decbit:4,%/* congestion experienced? */%
%  tp_flags:8,%/* see #defines below */%
.\" *************************************** 
%  tp_unused:16;%%
.T&
l s s l.
#define  TPF_XPD_PRESENT%TPFLAG_XPD_PRESENT
#define  TPF_NLQOS_PDN%TPFLAG_NLQOS_PDN
#define  TPF_PEER_ON_SAMENET%TPFLAG_PEER_ON_SAMENET
%%%
.\" *************************************** 
.T&
l l l l.
 %struct tp_pmeas%*tp_p_meas;% 
.T&
l l l s.
 % %/* ptr to mbuf to hold the perf.% 
%% * statistics structure %
%% */%
.\" *************************************** 
};
.TE
\fR
.\"
.\" end of tpcb structure (thank you)
.\"
.)b
.fi
.sh 1 "Sequence Number Arithmetic"
.pp
Sequence numbers in TP can be either 7 bits 
(\*(lqnormal format\*(rq)
or 31 bits
(\*(lqextended format\*(rq).
Sequence numbers are unsigned integers,
regardless of their format.
Three fields are kept in the pcb to manage the sequence
number arithmetic:
.(b
\fC
.TS
tab(+);
l l l l.
 +u_int+tp_seqmask;+/* mask for seq space */
 +u_int+tp_seqbit;+/* bit for seq # wraparound */
 +u_int+tp_seqhalf;+/* half the seq space */
.TE
\fR
.)b
.lp
\fITp_seqmask\fR 
is a bit mask indicating which bits are legitimate 
for a sequence number of either format.
It takes the value 0x7f if 7-bit sequence numbers are in use,
and 0x7fffffff if 31-bit sequence numbers are in use.
\fITp_seqbit\fR 
is the bit that becomes set when a sequence number wraps around
while being incremented.
Its value is 0x80 for normal format, 0x80000000 for extended format.
\fITp_seqhalf\fR 
takes the value which is in the middle of the sequence space,
0x40 for normal format,
and
0x40000000 for extended format.
.(b
.nf
The macro 
.fi
\fC
.TS
tab(+);
l l l l.
     SEQ(tpcb, x)
.TE
\fR
.)b
.lp
extracts a sequence number from the location
in which it is stored.
.pp
The macros
.(b
\fC
.TS
tab(+);
l l s s l.
 +SEQ_GT(tpcb, seq, t)+is seq > t?
 +SEQ_GEQ(tpcb, seq, t)+is seq >= t?
 +SEQ_LT(tpcb, seq, t)+is seq < t?
 +SEQ_LEQ(tpcb, seq, t)+is seq <= t?
 +SEQ_INC(tpcb, seq)+seq\+\+
 +SEQ_DEC(tpcb, seq)+seq--
 +SEQ_SUB(tpcb, seq, amt)+seq -= amt
 +SEQ_ADD(tpcb, seq, amt)+seq \+= amt
.TE
\fR
.)b
.lp
perform the indicated comparisons and arithmetic
on their arguments.
.pp
An example of how these macros
are used is as follows.
To determine if a sequence
number \fIseq\fR is in a receive window
bounded by
\fIlwe\fR and \fIuwe\fR,
we define the
macro
.(b
\fC
.TS
tab(+);
l l.
#define+IN_RWINDOW(tpcb, seq, lwe, uwe)\\
+( SEQ_GEQ(tpcb, seq, lwe) && SEQ_LT(tpcb, seq, uwe) )
.TE
\fR
.)b
.sh 1 "TP Implementation Options"
.pp
The transport protocol specification leaves several
things to the discretion of the implementor,
some of which may affect the performance
of individual connections and
aggregate performance.
Wherever different strategies are likely to favor
the performance of
individual connections to the detriment of aggregate performance
or vice versa, the
various strategies are under the control of options via the
\fIgetsockopt()\fR and
\fIsetsockopt()\fR system calls (see the manual pages
\fIgetsockopt(2)\fR,
\fIsetsockopt(2)\fR  
and
\fItp(4p)\fR  
for details).
In some cases the preferred strategies differ for the different
subnetworks, so the strategies chosen will be determined
by the subnetwork in use.
.sh 2 "TPDU size"
.pp
The limitation of the maximum TPDU size to a power of two is
unfortunate in the LAN environment.
For example, if the maximum NSDU size is around 1500, as in the case of an
Ethernet,
using a maximum TPDU size of 1024 reduces
the possible throughput by approximately 30%.
TP negotiates a maximum TPDU size of 2048 and
generates TPDUs of size around 1500.
Obviously this works well only when the peer is known to be 
using the same scheme (so that the peer
doesn't send TPDUs of size 2048 and cause its
network layer to fragment the TPDUs).
This is likely to be the case in a LAN where
all protocol entities are under the same administrative
control.
The maximum TPDU size negotiated is under the control of the user,
so
it is possible to prevent this scheme from being used
by default
when the peer is not on the same LAN, by
setting the \fItp.tpdusize\fR parameter in the ARGO directory service
file to
something less than the network's maximum transmission
unit.
.\"***********************************************************
.sh 2 "Congestion Window Strategy"
.pp
The congestion window strategy from the
DoD Internet 
was adapted for use with TP.
The strategy is intended to minimize the 
adverse effect
of transport's retransmission on an
already congested network.
.pp
A TP entity keeps two notions of the peer's window:
the real window, which is that advertised by the peer
in AK TPDUs, and the congestion window, which is a locally
controlled window.
TP uses the smaller of the two windows when transmitting.
The congestion window starts small, which keeps a
new connection from overloading the network with a sudden
burst of packets
immediately after connection establishement.
This is called \fIslow start\fR. 
For each successful acknowledgment received, the congestion
window grows by one, until eventually the real window
is the one in use.
If a retransmission timer expires, the congestion window
is reset to size one.
.pp
The congestion window strategy is used for class 4 unless
the transport user requests that it not be used.
The slow start strategy is used for traffic over a PDN
unless
the transport user requests that it not be used.
Slow start is not used for traffic over a LAN unless
its use is requested by the transport user.
.\"***********************************************************
.sh 2 "Retransmission strategies"
.pp
A retransmission timer is invoked for each set of DT TPDUs
sent in one send operation (call to \fItp_send()\fR).
This set of packets is called the \fIsend window\fR for the purpose
of this discusssion.
.pp
The number of TPDUs 
in a send window
depends on the remote credit and the amount of data
in the local send buffers.
When a retransmission timer goes off, the lower
window edge 
is reevaluated but the upper window edge is not reevaluated.
.pp
There are several retransmission strategies implemented in
ARGO TP.
The choice of strategies is the user's, and is made with the 
\fIsetsockopt()\fR system call.
The strategies are summarized here:
.ip "Retransmit LWE TPDU only:" 5
Only the TPDU representing the new lower window edge 
is retransmitted.
This is the default retransmission strategy.
.ip "Retransmit whole send window:" 5
Retransmission begins with the new lower window edge
and continues up to the old upper window edge.
.pp
The value of the data retransmission timer
adapts to the average round trip time and the standard deviation of
the round trip time.
A round trip time is the time that passes between
the moment of a packet's first transmission and 
the moment it is first acknowledged.
The average round trip time
is kept by the sending side of TP, using
a formula for 
smoothing the average:
.(b
\fC
.TS
tab(+);
l l l l.
#define+TP_RTT_ALPHA+3
#define+TP_RTV_ALPHA+2
+++
#define+SMOOTH(alpha, old, new) \\
+(((new-old) >> alpha ) \+ (old) )
.TE
\fR
.)b
.lp
The times included in the average are chosen as follows.
The time of 
each packet's initial transmission is kept (for the last
\fIN\fR packets, where \fIN\fR is a defined constant).
When an AK TPDU arrives, ARGO TP subtracts the initial transmission
time for the lowest unacknowledged sequence number that was
acknowledged by this AK TPDU from the current time,
and apply the resulting time to the average.
Hence, not all packets are included in this average,
which is as it should be since
the purpose of this measurement is 
to find a good value for the retransmission timer.
.pp
Each time part of a window is retransmitted,
the retransmission timer for that window is increased.
This does not affect the retransmission timers for other windows.
.\"***********************************************************
.sh 2 "Acknowledgment strategies"
.pp
The transport protocol specification
requires acknowledgments to be sent immediately
upon receipt
of  CC TPDUs (in class 4), XPD TPDUs, and DT TPDUs containing an
EOT marker, and at other times as required for flow control,
otherwise acknowledgments may be delayed.
In addition to the times when an acknowledgment is required,
ARGO TP transmits an AK TPDU whenever the user receives some data,
thereby increasing the size of the window.
For those times when
immediate acknowledgment is optional,
ARGO TP offers two acknowledgment strategies:
.ip "     Acknowledge each TPDU" 10
Upon receipt of a DT TPDU and AK TPDU is sent.
.ip "     Acknowledge full window" 10
Acknowledgment is issued
upon receipt of enough data to
consume the last advertised credit.
.pp
The latter strategy
requires a timer to trigger an acknowledgment
in case the peer doesn't send the entire window 
quickly.
This timer is called the
\fIsendack timer\fR.
The upper bound on the value of this timer 
is called the \fIlocal acknowledgment time\fR.
The local acknowledgment time may be "advertised" to the 
peer during connection establishment, and the
peer may choose to use this value to
adjust its retransmission timers.
The ARGO TP entity advertises its local acknowledgment time
on a CR TPDU, but it is not 
constrained by 
the remote acknowledge time, should the peer 
advertise it.
Instead,
ARGO TP adapts its sendack timer
to the behavior of the connection.
.pp
Under the assumption that the round trip time is
often 
symmetric,
and lacking 
a method to measure
the round trip time in the other direction,
ARGO TP uses the measured average round trip time
to adjust the the sendack timer.
.pp
The choice of strategies is made with the
\fIsetsockopt()\fR system call.
The default strategy is
to
delay acknowledgments until the most recently advertised window is filled.