Selecting Sequence Numbers
R a y m o n d S. T o m l i n s o n
Bolt B e r a n e k
Cambridge,
and N e w m a n Inc.
Massachusetts
Introduction
A c h a r a c t e r i s t i c of almost all
communication
protocols
is
the
use of u n i q u e n u m b e r s to i d e n t i f y i n d i v i d u a l pieces of data.
These i d e n t i f i e r s permit error
control
through
acknowledgement
and r e t r a n s m i s s i o n techniques.
U s u a l l y s u c c e s s i v e pieces of data
are
identified
with
s e q u e n t i a l n u m b e r s and the i d e n t i f i e r s are
thus c a l l e d s e q u e n c e numbers.
This
paper
discusses
techniques
for
selecting
and
synchronizing
s e q u e n c e n u m b e r s such that no errors will occur if
c e r t a i n n e t w o r k c h a r a c t e r i s t i c s can be b o u n d e d
and
if
adequate
data
error
detection
measures
are
taken.
The
discussion
s p e c i f i c a l l y focuses on the p r o t o c o l d e s c r i b e d by Cerf and
Kahn,
(I)
but the ideas are a p p l i c a b l e to other s i m i l a r protocols.
The P r o b l e m
One
of the p r o b l e m s with the p r o t o c o l d e s c r i b e d by Cerf and
Kahn w h i c h was brought out by our e x p e r i m e n t s at BBN is
"How
do
you
identify
duplicate
packets
from
a
previous
use
of
a
particular connection?".
What is n e c e s s a r y is a means for either
the r e c e i v e r of the late packet to i d e n t i f y the packet as late or
a means for the o r i g i n a t o r of the packet
to
tell
the
receiver
that the packet was late.
The
protocol
as
described
provides
a fine m e c h a n i s m for
i d e n t i f y i n g late packets while data are a c t i v e l y f l o w i n g
between
two
ports.
It w o u l d also work fine if hosts never went down and
had large a m o u n t s of storage.
But hosts do go down and
have
to
be
restarted
and
hosts
don't
have an i n e x h a u s t i b l e supply of
storage.
The S o l u t i o n
The e s s e n c e of the s o l u t i o n is that s e q u e n c e numbers must be
chosen such that a p a r t i c u l a r s e q u e n c e
number
never
refers
to
more
than
one
byte
at
any
one
time
~nd the valid range of
sequence numbers
must
be
positively
synchronized
whenever
a
c o n n e c t i o n is used.
The former r e q u i r e s c a r e f u l a t t e n t i o n to the
method
used
for s e l e c t i n g initial s e q u e n c e numbers.
The latter
r e q u i r e s a more i n v o l v e d h a n d s h a k e
than
that
provided
by
the
protocol.
(I) Cerf, V . G . and Kahn, R. E., "A P r o t o c o l for
Packet
Network
Intercommunication,"
IEEE
Transactions
on C o m m u n i c a t i o n s , Vol.
C O M - 2 2 #5, May 1974, pp. 637-648.
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Positive
Synchronization
Achieving
positive
synchronization
requires
a
three way
h a n d s h a k e for SYN and a
two-way
handshake
for
REL.
This
is
necessary
because
the
passive side of the c o n n e c t i o n must have
p o s i t i v e a s s u r a n c e from the active side that the packet
received
is
current.
Simply
receiving
a
packet does not p r o v i d e this
assurance.
A s s u m i n g one end as the i n i t i a t o r and the
other
end
as
responder,
the
normal
p r o c e d u r e for s y n c h r o n i z i n g s e q u e n c e
n u m b e r s is:
I) I n i t i a t o r
(one that
sends a SYN with
a
unique
sequence
cannot be o u t s t a n d i n g in the net).
number
2) R e s p o n d e r r e c e i v e s this packet but does not p r o c e s s the
data
(if
any) because it does not k n o w w h e t h e r or not
the packet is a late duplicate.
3) R e s p o n d e r r e t u r n s a packet w h i c h ACK's
number
and
SYN's
the
responder's
number.
the
initiator's
unique
sequence
4) The i n i t i a t o r r e c e i v e s the r e s p o n d e r ' s SYN and b e l i e v e s
it b e c a u s e it ACK's an a p p r o p r i a t e s e q u e n c e n u m b e r
and
SYN's
a
p r e v i o u s l y u n s y n c h r o n i z e d (in that d i r e c t i o n )
connection.
The i n i t i a t o r now knows that the r e s p o n d e r
is w i l l i n g to go ahead and knows w h e r e the r e s p o n d e r is
going to start but the r e s p o n d e r does not yet
know
if
the
initiator
was
r e a l l y trying to s y n c h r o n i z e or if
the packet r e c e i v e d was a late duplicate.
5) The i n i t i a t o r sends
responder's sequence
packet
back
a
number.
which
ACK's
the
6) The r e s p o n d e r r e c e i v e s
this
packet
and
believes
it
b e c a u s e it ACK's an a p p r o p r i a t e s e q u e n c e n u m b e r and has
a
sequence
number
of
its
own
which
is
in
the
a p p r o p r i a t e range.
The o r i g i n a l packet
and
this
one
may
now
be
processed
further, data delivered, REL's
p r o c e s s e d etc.
The h a n d s h a k e for REL needs to be only t w o - w a y because valid
sequence numbers
have
been
established
and
may
be
used
to
acknowledge
the
REL.
Since the need to send a REL may occur at
times when there are no data bytes
to
transmit,
a
dummy
byte
which
is
not
delivered
to the user p r o c e s s is sent to p r o v i d e
s o m e t h i n g to be a c k n o w l e d g e d .
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12
-
A c o n s e q u e n c e of this need for p o s i t i v e
synchronization
is
that
any
data
in
the
initial
packet with the SYN may not be
d e l i v e r e d to the user process until the v a l i d i t y of the packet is
verified.
It is u s e f u l to include data in
the
initial
packet,
however,
since
that
data
may
be
acknowledged
in
the first
r e s p o n s e packet e l i m i n a t i n g the need for a s u b s e q u e n t packet
for
acknowledging
the data.
A m i n i m u m e x c h a n g e for data flow in one
d i r e c t i o n is four packets.
This is i l l u s t r a t e d by example below.
The fourth packet is r e q u i r e d to i n f o r m the i n i t i a t o r that it may
forget the c o n n e c t i o n i n f o r m a t i o n .
It is also n e c e s s a r y to
provide a m e c h a n i s m
to
negatively
acknowledge
r e s p o n s e s to s p u r i o u s packets.
A REJ bit or command
s h o u l d be p r o v i d e d for this purpose.
Selecting
Unique
Sequence
Numbers
The a s s u m p t i o n that n e t w o r k s may g e n e r a t e d u p l i c a t e
packets
with
possibly
long
l i f e t i m e s renders the task of p r o v i d i n g for
u n i q u e s e q u e n c e number n o n - t r i v i a l .
The
period
of
time
over
w h i c h a p a r t i c u l a r s e q u e n c e n u m b e r may refer to a p a r t i c u l a r byte
requires
knowing
an
upper
bound on the packet l i f e t i m e in the
network.
This bound must be m o d e r a t e l y tight
because,
as
this
bound becomes looser, either packet o v e r h e a d must i n c r e a s e due to
the
need
for
more bits of s e q u e n c e n u m b e r or t r a n s m i s s i o n rate
must be r e s t r i c t e d so that s e q u e n c e n u m b e r are not reused
before
packets
die
out
in
the
network.
The
p e r i o d during which a
p a r t i c u l a r s e q u e n c e number refers to a p a r t i c u l a r
byte
is
then
some
sma~l
multiple
of
this packet l i f e t i m e ( d e p e n d i n g on how
many r e v e r b e r a t i o n s the protocol will support).
I will use T
as
a
parameter
of
the protocol design w h i c h d e s i g n a t e s this small
m u l t i p l e of the packet lifetime.
The size of the w i n d o w plays a part in d e t e r m i n i n g w h e t h e r a
late packet might be c o n f u s e d
with
current
packets.
A
large
window
increases
the
m a x i m u m s e q u e n c e n u m b e r the r e c e i v e r will
accept as valid.
The m a x i m u m w i n d o w size is a
design
parameter
of
the
protocol
and
must
be s p e c i f i e d b e f o r e the protocol is
complete.
For our purposes, the w i n d o w size can be
conveniently
t h o u g h t of as e x t e n d i n g the a p p a r e n t packet lifetime (T) and will
not be d i s c u s s e d f u r t h e r here.
Also
subsumed
into
the
parameter
T
is the m a x i m u m host
s e r v i c e i n t e r r u p t i o n time.
If
a
host
stops
executing
for
a
period of time and then resumes with no loss, any packets held in
that
host
during
that period, w h e t h e r they are data packets or
a c k n o w l e d g e m e n t packets, are e f f e c t i v e l y d e l a y e d by the
duration
of
the
service
interruption.
This delay is i n d i s t i n g u i s h a b l e
from any delay c a u s e d by the n e t w o r k itself.
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A n o t h e r p a r a m e t e r of the p r o t o c o l d e s i g n is the m a x i m u m data
rate.
The m a x i m u m data rate design p a r a m e t e r may be less than or
greater
than
the
data
rate
achievable
by
the
network
and
associated
hardware.
The
design
parameter
will
probably
ultimately
be
less
than
the
hardware/network
limitation
as
technology
improves.
If this is the case, s o f t w a r e c o n t r o l will
have to be used to limit the a c t u a l data rate.
Given the m a x i m u m data
rate
(R)
and
the
maximum
lifetime
(T)
and the m a x i m u m s e q u e n c e n u m b e r (M), we can
the m i n i m u m s e q u e n c e n u m b e r cycle time (C) as
C
:
packet
define
M/R
and state
lifetime
that
this
must
be
larger
than
the
maximum
packet
C > T
This
inequality
must
same s e q u e n c e numbers,
in the network.
be
g u a r a n t e e d o t h e r w i s e p a c k e t s with the
but from an e a r l i e r cycle, may still exist
The s e l e c t i o n of values for
some
of
these
parameters
is
arbitrary.
One
could select a field w i d t h for s e q u e n c e n u m b e r s
thus d e t e r m i n i n g M and compute R from that.
T is
determined
by
the
network
characteristics.
The
a m o u n t by w h i c h C e x c e e d s T
must
also
be
selected.
The
amount
by
which
C
exceeds
T
d e t e r m i n e s how f r e q u e n t l y s e q u e n c e n u m b e r s must be r e s y n c h r o n i z e d
when
activity
is
low
(see
below).
Therefore,
C
should be
s u b s t a n t i a l l y g r e a t e r than T.
Since the c h o i c e
of
sequence
numbers
is
directly
under
c o n t r o l of the sender, it is best to place the r e s p o n s i b i l i t y for
selecting
unique
sequence
n u m b e r s on the sender.
The r e c e i v e r
then a c c e p t s a packet on the basis of w h e t h e r the s e q u e n c e n u m b e r
falls w i t h i n the c u r r e n t w i n d o w or not.
If the r e c e i v e r
has
no
current
window
then
the
handshake
d e s c r i b e d above is used to
e s t a b l i s h one.
To p r e v e n t r e u s i n g a p a r t i c u l a r s e q u e n c e n u m b e r too soon
it
is
necessary
for
the
sender to have k n o w l e d g e about when that
sequence
number
was
last
used.
Figure
I illustrates
the
s i t u a t i o n when c o m p l e t e k n o w l e d g e of this sort is a v a i l a b l e .
The
curve
of
actual
sequence
numbers
used
as a f u n c t i o n of time
permits a region of f o r b i d d e n s e q u e n c e n u m b e r vs. time points
to
be
defined.
The
curve
of
actual
sequence
numbers
is
not
p e r m i t t e d to r e e n t e r this region.
Complete
knowledge
of
this
kind r e q u i r e s a p r o h i b i t i v e amount of storage.
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Another
p o s s i b i l i t y is to retain a few n u m b e r s w h i c h permit
the s e q u e n c e n u m b e r curve to be bounded.
This
scheme
and
the
previous
one r e q u i r e a m e m o r y w h i c h p e r s i s t s at least as long as
T even in the event of s y s t e m c r a s h e s or other m e m o r y
destroying
events.
The
method
I will
describe
uses
a t i m e - o f - d a y clock to
govern the s e l e c t i o n
of
sequence
numbers.
The
choice
of
a
time-of-day
clock
for this p u r p o s e is n a t u r a l since time is the
factor w h i c h d e t e r m i n e s w h e t h e r d u p l i c a t e s might still exist.
If
the t i m e - o f - d a y clock has its own n a t u r a l period such as
seconds
since midnight, then it is n e c e s s a r y to c h o o s e the s e q u e n c e cycle
time (C) such that the clock period is an i n t e g r a l m u l t i p l e of C.
There
are
several
mappings
of
clock to s e q u e n c e number.
The
mapping which
maintains
maximum
resolution
is
given
by
the
expression
(V mod
C)*M/C
w h e r e V is the clock value, C is the s e q u e n c e cycle time in units
of
V, and M is the m a x i m u m s e q u e n c e number.
The s e q u e n c e n u m b e r
e q u i v a l e n t given by this f o r m u l a is c a l l e d ISN (initial
sequence
number) in the f o l l o w i n g d i s c u s s i o n .
ISN is used for the i n i t i a l
sequence
n u m b e r for e s t a b l i s h i n g or r e - e s t a b l i s h i n g a c o n n e c t i o n
in the a b s e n c e of other i n f o r m a t i o n .
A plot of ISN as a f u n c t i o n of time is shown
in
figure
2.
The
step size d e p e n d s on the r e s o l u t i o n of the t i m e - o f - d a y clock
and the range of s e q u e n c e n u m b e r s required.
The
step
size
is
exaggerated
in
the
figure.
Also on figure 2 are drawn a n o t h e r
s t a i r c a s e p r e c e e d i n g the ISN
curve
by
one
step,
a
staircase
delayed
from
that
by C-T, and a h o r i z o n t a l line l a b e l l e d "last
used seq no."
These lines delimit a r e g i o n of a l l o w e d
sequence
numbers.
If p a c k e t s are never given s e q u e n c e n u m b e r s o u t s i d e of
this region, there will never be any p r o b l e m w i t h c o n f u s i n g
late
arriving
packets
with
current
packets.
This
is b e c a u s e the
s e q u e n c e n u m b e r of a late a r r i v i n g packet
will
be
outside
the
allowed
region
e i t h e r b e c a u s e the c u r r e n t s e q u e n c e n u m b e r s have
gone beyond it thus r a i s i n g the lower b o u n d a r y or b e c a u s e
it
is
impossible
for
enough
time
to
have p a s s e d to place it in the
a l l o w e d r e g i o n of the next cycle.
R e m e m b e r that the
next
cycle
will
not occur until an amount of time g r e a t e r than T has passed
and, by definition, the packet cannot p e r s i s t for that l e n g t h
of
time.
The
follows:
sequence
number
constraints
-
16
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summarized
as
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a.
The c u r r e n t s e q u e n c e n u m b e r m u s t not "get a h e a d " of the
ISN c l o c k by m o r e than one step
because
packets
with
those
sequence
numbers
from
the
previous
sequence
n u m b e r c y c l e m a y be left in the n e t w o r k .
b.
The c u r r e n t s e q u e n c e n u m b e r m u s t not "fall b e h i n d "
the
ISN
clock
by
more
than C - T - < o n e c l o c k s t e p > b e c a u s e
d u p l i c a t e s of
packets
generated
with
such
sequence
n u m b e r s m a y a p p e a r l a t e r and be c o n f u s e d w i t h the (then
c u r r e n t ) ISN.
c.
Potential sequence numbers must increase
monotonically
e v e n w h e n the c o n n e c t i o n is i n a c t i v e b e c a u s e t h e r e w i l l
a l m o s t c e r t a i n l y be a c o n f l i c t if t h e r e are any p a c k e t s
r e m a i n i n g in the n e t w o r k .
Referring
to f i g u r e 2 again, note that one of the b o u n d s of
the a l l o w e d r e g i o n is the l a r g e s t (last)
used
sequence
number.
This
would
seem
to
require
that that n u m b e r be r e m e m b e r e d at
least as long as its r e u s e m i g h t c a u s e c o n f u s i o n (T).
This w o u l d
r e q u i r e e x c e s s i v e a m o u n t s of s t o r a g e in all e x c e p t a few
special
cases.
It
is n e c e s s a r y to r e m e m b e r it for a s h o r t time b e c a u s e
it may e x c e e d the ISN curve.
This c o u l d be a v o i d e d by m o v i n g the
ISN c u r v e to the e x t r e m e left edge of the a l l o w e d r e g i o n .
Doing
this,
however,
makes
it
immediately
impossible
to
use
the
s e q u e n c e n u m b e r s e l e c t e d from the
ISN
curve
for
communication
until
the
next t i c k of the clock.
This is, in fact, the r e a s o n
w h y the ISN c u r v e in f i g u r e 2 is s h o w n d i s p l a c e d
from
the
left
edge of the a l l o w e d region.
The
solution
to
this
p o s s i b l e d i l e m m a is to r e m e m b e r the
last u s e d s e q u e n c e n u m b e r if it is g r e a t e r
than
ISN
until
the
next
clock
tick.
At this p o i n t the ISN c u r v e w i l l n e c e s s a r i l y
e x c e e d the last u s e d s e q u e n c e n u m b e r since, w h e n
the
connection
was a c t i v e , the s e q u e n c e n u m b e r s w o u l d not h a v e b e e n p e r m i t t e d to
exceed
the
ISN c u r v e by m o r e than one t i c k s i n c e d o i n g so w o u l d
have p l a c e d the s e q u e n c e n u m b e r s o u t s i d e of the
allowed
region.
In
the
event
of
a
memory
d e s t r o y i n g event, such as a s y s t e m
r e s t a r t , it is n e c e s s a r y to wait for one t i c k of the ISN
because
that
insures
that
an i n i t i a l s e q u e n c e n u m b e r s e l e c t i o n w i l l be
g r e a t e r than the s e q u e n c e n u m b e r s in use just p r i o r to the m e m o r y
lo~s.
The r u l e s for s e l e c t i n g i n i t i a l s e q u e n c e
numbers
are
as
follows:
I. R e m e m b e r
the next
2.
the last u s e d
tick of ISN.
sequence
number
at
least
until
Inhibit initial
sequence
number
selection
until
at
least one ISN tick has o c c u r r e d f o l l o w i n g a m e m o r y loss
(system restart).
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18
-
3. If the last used
(incremented
by
sequence
number
is
known,
I) for a new i n i t i a l s e q u e n c e
use
it
number.
4. If no last used s e q u e n c e n u m b e r is being r e m e m b e r e d and
rule 2 is satisfied, use ISN for a new i n i t i a l s e q u e n c e
number.
Once an i n i t i a l s e q u e n c e number is selected,
packets
are
given
sequence
n u m b e r s as s p e c i f i e d by the protocol.
S e q u e n c e numbers
so s e l e c t e d must not equal or exceed ISN plus one step.
If data flows at less than m a x i m u m rate for
a
long
enough
time,
the c u r r e n t s e q u e n c e n u m b e r w o u l d cross into the f o r b i d d e n
r e g i o n on the right.
Packets e m i t t e d w i t h these s e q u e n c e n u m b e r s
and d e l a y e d by time T w o u l d conflict w i t h a later value
of
ISN.
This must be p r e v e n t e d by r e s y n c h r o n i z i n g the s e q u e n c e n u m b e r s to
a
larger value.
In order to r e s y n c h r o n i z e s e q u e n c e numbers, the
current s e q u e n c e n u m b e r s must be r e l e a s e d and then a new s e q u e n c e
(using s e q u e n c e n u m b e r ISN) e s t a b l i s h e d .
Examples
of C o n n e c t i o n
Synchronization
The dialogs below i l l u s t r a t e the i n t e r c h a n g e of letters that
occurs in v a r i o u s situations.
Each line of the
dialog
consists
of
a
packet
label
in
p a r e n t h e s e s f o l l o w e d by the a c t i v i t y at
process A w h e r e "<--" s i g n i f i e s the packet being r e c e i v e d
by
A,
"-->"
signifies
the
packet
being
transmitted
by A and "..."
s i g n i f i e s that A is u n a w a r e of the packet
at
that
time.
Next
appears
a
description
of
the
packet
in
the
form
<SEQ
n><control><data>.
Next appears the a c t i v i t y at process B
where
"-->"
s i g n i f i e s the packet is r e c e i v e d by B, "<--" s i g n i f i e s the
of
packet is t r a n s m i t t e d by B, " ... " s i g n i f i e s that B is u n a w a r e
the packet.
The label is a handle on the packet for i l l u s t r a t i o n
purposes.
When
a packet is d u p l i c a t e d or r e t r a n s m i t t e d it will
be given the same label.
When the delay b e t w e e n t r a n s m i s s i o n and
r e c e p t i o n of a packet is u n i m p o r t a n t , the packet will be shown to
be t r a n s m i t t e d and r e c e i v e d on the same line.
Unidirectional
Connection
with
Lbl
A
Packet
(I)
-->
<SEQ 0 > < S Y N > < >
A synchronizes
(2)
<--
<SEQ
O><SYN,
Sender
Initiating
B
-->
to O.
ACK 0><>
-
19
-
<--
B syn@hronizes
0.
(3)
-->
(4)
__
(5)
-->
to 0
and
acknowledges
<SEQ 0><ACK 0><10 Data Bytes>
A acknowledges
B's
0 and sends
data.
-->
some
__
<SEQ 0><ACK 10><>
B a c k n o w l e d g e s A's data.
,.
(6)
<SEQ
10><ACK
<SEQ 0><ACK
-->
0><6 Data Bytes>
16><>
---->
(7)
---->
<SEQ 16><ACK 0, REL><20
A sends last data bytes
(8)
<--
<SEQ 0><ACK 36, REL, C T L > < D u m m y Byte>
B a c k n o w l e d g e s all A's data
(and
the
REL) and sends its own REL.
(9)
-->
<SEQ 36><ACK I><>
A a c k n o w l e d g e s B's REL.
Unidirectional
(1)
(2)
-->
<--
with
Receiver
Data Bytes>
and REL.
-->
Initiating
<SEQ 0><SYN><>
A synchronizes
-->
to 0.
<SEQ 0><SYN, ACK 0><14 Data Bytes>
B
synchronizes
to 0, a c k n o w l e d g e s
and sends data.
(3)
-->
<SEQ 0><ACK 14><>
A a c k n o w l e d g e s B's data.
(4)
<--
<SEQ
(5)
-->
<SEQ 0><ACK
(6)
<--
<SEQ 24><ACK
(7)
-->
<SEQ 0><ACK
(8)
<--
<SEQ 29><ACK
14><ACK
__
0><10
__
A,
-->
Data Bytes>
---->
24><>
0, REL><5
29, CTL,
I><>
-
20
-
Data Bytes>
REL><Dummy
Byte>
---->
Minimal
(I)
(2)
Late
-->
<--
Sender
Initiating
<SEQ 0><SYN><21 Data B y t e s >
A s y n c h r o n i z e s to 0 and sends
<SEQ 0><SYN, ACK 21><>
B a c k n o w l e d g e s A's data and
own
sequence number.
Data
d e l i v e r e d to user process.
-->
data.
__
SYN's
its
is not yet
(3)
-->
<SEQ 21><REL, ACK 0><>
A a c k n o w l e d g e s B's s e q u e n c e n u m b e r and
REL's the connection.
B d e l i v e r s data
to user process.
__>
(4)
-->
<SEQ 2 2 > < A C K I>
A a c k n o w l e d g e s B's REL and
drops
the
connection.
Note
that if this final
ACK gets lost, B will r e s e n d
its
REL
and
A will
respond
with
an
error
reply.
B must
interpret
this
as
e q u i v a l e n t to the ACK.
_->
Duplicate
SYN
(I)
<SEQ x > < S Y N > < 2 3 Data Bytes>
B receives a late d u p l i c a t e
(2)
(3)
Late
Exchange
__
-->
<SEQ 0><SYN, ACK x+23><>
B
responds
synchronizing
s e q u e n c e numbers.
A is not
the connection.
-->
packet.
__
its
aware
<SEQ x+23><REJ, ACK 0><>
A rejects the response.
B throws away
the data and resets the c o n n e c t i o n .
Duplicate
SYN Plus
(I)
<SEQ x><SYN><22~ Data Bytes>
B
receives
the late d u p l i c a t e
c o n t a i n i n g SYN.
(2)
__
own
of
Late
<SEQ 0><SYN,
B responds.
-->
Response
ACK x+22><>
-
2 1
-
-->
packet
- _
(3)
(4)
-->
Simultaneous
(I)
-->
(2)
<SEQ x + 2 2 > < A C K y><31 Data Bytes>
B r e c e i v e s a d u p l i c a t e of the o r i g i n a l
r e s p o n s e to
the
response
to
packet
(I).
Note
that
it ACK's y w h i c h is
o u t s i d e B's current s e q u e n c e
numbers.
The
packet
is
rejected
since
A's
sequence
numbers
have
not
been
verified
and the ACK is out of range.
-->
<SEQ x+22><REJ, ACK 0><>
A
rejects
the
response
p r e v i o u s example.
-->
as
in
the
Initiation
<SEQ 0 > < S Y N > < I 5 Data B y t e s >
A s y n c h r o n i z e s to 0.
<SEQ 0 > < S Y N > < >
Meanwhile, B s y n c h r o n i z e s
__
too.
-->
(I)
<SEQ 0 > < S Y N > < I 5 Data Bytes>
A's packet arrives.
(3)
<SEQ 0><SYN, ACK 0><>
B a c k n o w l e d g e s A's s e q u e n c e n u m b e r and
resynchronizes
its
own
since it has
not yet r e c e i v e d v e r i f i c a t i o n from
A.
(2)
<--
(4)
-->
(3)
__
<SEQ O > < S Y N > < >
B's o r i g i n a l
packet a r r i v e s
at A.
<SEQ 0><SYN, ACK 0><15 Data B y t e s >
A
sends
its
first
packet again but
this time
acknowledges
B's
sequence
number.
B
is
now
completely
synchronized.
<SEQ 0><SYN, ACK 0><>
B's first r e s p o n s e a r r i v e s at A.
now
completely
synchronized.
c o n t i n u e s as in the above cases.
-
22
-
--
A is
This
-->
Conclusion
The
methods
described
above
for m a n a g i n g use of s e q u e n c e
numbers on a c o n n e c t i o n assure that each byte of
information
is
u n i q u e l y i d e n t i f i e d in spite of packet d u p l i c a t i o n and long delay
in
the
network, and in spite of service d i s r u p t i o n s or c o m p l e t e
m e m o r y loss by either or both c o m m u n i c a t i n g
hosts.
It
is
our
belief
that
any
lesser schemes will lead to u n r e l i a b l e n e t w o r k
p e r f o r m a n c e and are, therefore, u n s a t i s f a c t o r y .
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