Transcript Other adaptation functions

1.
2.
Reliable stream service--TCP
TCP accepts byte stream, segments
It is over IP, so out-of-sequence is very common
--lost or error frame results in out-of-sequence
-- It is not “wirelike”, even there is a TCP connection between sender and
receiver, but this connection has logical meaning and is not a real physical
path, i.e., packets ultimately travel along different routes, so out-
3.
of-sequence
Because of not “wirelike”, it is possible for old packets from
previous connections to arrive at a receiver. The old packets will
confuse the new packets with the same SN. TCP solves it by
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2.
4.
5.
using long (32 bit) SNs and selecting a random initial SN during connection
setup;
set a timer at end of a connection to clear old packets. So accepting an very
old packet is very unlikely.
TCP uses Selective Repeat ARQ with a mechanism to advertise
receive window size for flow control. Moreover, window size is not
based on number of packets, but number of bytes
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TCP can also be used for congestion control.
Application
Application
byte stream
byte stream
Segments
Transmitter
Receiver
Receive buffer
Send buffer
ACKs
TCP preview—reliable stream service
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Figure 5.32
Transmitter
Receiver
Send Window
Receive Window
Slast + Wa-1
...
...
octets
S
S
transmitted last recent
& ACKed
Rlast
Rlast + WR – 1
...
Slast + Ws – 1
Rnext Rnew
Rlast highest-numbered byte not
Slast oldest unacknowledged byte
yet read by the application
Srecent highest-numbered
Rnext next expected byte
transmitted byte
Rnew highest numbered byte
Slast+Wa-1 highest-numbered byte
received correctly
that can be transmitted
Rlast+WR-1 highest-numbered
Slast+Ws-1 highest-numbered byte
byte that can be accommodated
that can be accepted from the
in receive buffer
application
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Advertised window: Wa=Wr-(Rnew-Rlast), Srecent-Slast<=Wa
Data link protocols
• Framing: indicate the boundaries of frames
• Error control: ensure reliable transmission
• Flow control: prevent sender from overrunning
receiver
• Addressing information: is required when the
channel carries information for multiple users.
• Assumption: data link is “wirelike”.
• Two protocols: HDLC and PPP.
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HDLC data link control
• Was set by ISO
• Is connection-oriented: set up connection first,
then transfer frames using one of three ARQs,
finally tear down the connection (Note: LAN
generally provides unacknowledged
connectionless service)
• HLDC configurations and transfer modes
• HDLC frame format
• Typical frame exchange
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Data link layer
Network
Layer
NLPDU
Network
Layer
“packet”
DLSDU
DLSAP
DLSAP
Data Link
Layer
DLSDU
DLPDU
Data Link
Layer
“frame”
Physical
Layer
Physical
Layer
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Figure 5.32
HDLC configurations and transfer modes
Unbalanced Point-to-point link
Commands
Primary
Secondary
Responses
NRM:
Normal
Response
Mode
Unbalanced Multipoint link
Commands
Primary
Responses
Secondary
Secondary
Secondary
Balanced Point-to-point link between Combined Stations
Commands
Primary
Secondary
Responses
Responses
Secondary
Commands
Primary
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ABM: Asynchronous Balance Mode
Figure 5.33
HDLC frame format
Flag
Address
Control
Information
FCS
Flag
1. Framing: Flags at both ends define the frame boundaries.
Flag value is 01111110
2. Addressing: indicate the destination address
3. Control: various control fields to indicate different frames
4. Information: user information by bits, generally no length limit.
5. FCS: Frame Check Sum, using 16-or 32-bit CRC
Question: flag value 01111110 may appear in frame, what to do?
Solution: bit stuffing, a technique to prevent occurrence of flag.
sender inserts an extra 0 after each instance of five consecutive 1s.
receiver looks for five consecutive 1s, if followed by 0, then this 0
is stuffed and is removed; if followed by 10, then flag found.
Example: 0110111111111100 011011111011111000 where 0 is stuffed
011011111-11111-00 where –: removed stuff 0
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Figure 5.35
Control field format: three types of frames
Information Frame or I-frame
1
2-4
0
5
N(S)
6-8
P/F
N(R)
P/F
N(R)
Supervisory Frame or S-frame
1
0
S
S
Unnumbered Frame or U-frame
1
1
M
M
P/F
M
M
M
P/F bit: Poll/Final bit. If set, the frame is a poll frame from primary
or the last I-frame of all transmitted frames.
N(S): sender SN of I-frame, N(R): piggybacked ACK
Window size: 23 –1 = 7 for Stop-and-Wait and Go-back-N, 4 for Selective-Repeat
(in extended control field) 27 –1 = 127 for the first two ARQs, 64 for the last ARQ
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Control field format: S-frame and U-frame
Four types of S-frames:
SS=00: RR (Receive Ready) frame used when no I-frame for piggyback
SS=01: REJ (Reject) frame, i.e., NAK frame
SS=10: RNR (Receive Not Ready) frame, indicate unable to receive any more.
SS=11: SREJ (Selective Reject) frame, indicate the retransmission of specified frame
Supervisory Frame or S-frame
1
0
S
S
N(R)
P/F
Unnumbered Frame or U-frame
1
1
M
M
P/F
M
M
M
U-frames for setup or release of connection:
SABM (Set Asynchronous Balance Mode) SNRM (Set Normal Response Mode)
SABME: SABM Extended
SNRME: SNRM Extended
DISC (DISConnect) UA (Unnumbered acknowledgment) FRMR (Frame Reject)
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Figure 5.36
Typical frame exchange—for connection setup and release
SABM
UA
Data
transfer
DISC
UA
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Figure 5.37
Typical frame exchange— using normal response mode
Secondaries B, C
Primary A
B, RR, 0, P
X
B, I, 0, 0
B, I, 1, 0
B, I, 2, 0,F
B, SREJ, 1
C, RR, 0, P
C, RR, 0, F
B, SREJ, 1,P
B, I, 1, 0
B, I, 3, 0
B, I, 4, 0, F
B, I, 0, 5
What ARQ?
Selective repeat
(Des.-IP, frame-type, N(S) (for I-frame), N(R), P/F)
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Figure 5.38
Typical frame exchange— using asynchronous balanced mode
Combined Station A
Combined Station B
B, I, 0, 0
B, I, 1, 0
A, I, 0, 0
X
A, I, 1, 1
B, I, 2, 1
A, I, 2, 1
B, I, 3, 2
B, REJ, 1
B, I, 4, 3
A, I, 3, 1
B, I, 1, 3
B, I, 2, 4
B, I, 3, 4
What ARQ?
Go-back-N
B, RR, 2
B, RR, 3
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Figure 5.39
Point-to-Point protocol
• Used to connect:
– Router to router, or home PC to ISP.
• HDLC-like frame format
• Information by bytes, so char-stuffing in case a flag byte
appears in information
• Support multiple network protocols simultaneously, e.g, IP,
IPX( Novell NetWare) etc.
• LCP (Link Control Protocol): set up, configure, test, maintain,
and terminate a link connection
• NCP (Network Control Protocol): configure each network
protocol, specifically, NCP for IP performs dynamical IP
address assignment
• PAP (Password Authentication Protocol): login ID and
password
• CHAP (Challenge-Handshake Authentication Protocol): no
plain password goes between two peers.
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PPP frame format
Flag
Address
01111110 1111111
All stations are to
accept the frame
Control
00000011
Unnumbered
frame
Protocol
Information
CRC
flag
01111110
Specifies what kind of packet is contained in the
payload, e.g., LCP, NCP, IP, OSI CLNP, IPX
1. Framing flag: same as HDLC
2. Address: 1111111, broadcast address, no need for specific host address
3. Control: default for connectionless transfer, not use sequence number
4.
5.
Protocol: support multiple network protocols
Information: byte-based, not bit-based. So Char-stuffing,
see Problem 5-54.
6. CRC: check sum
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Figure 5.40
A Typical Scenario
1. Carrier
Detected
Dead
7. Carrier
Dropped
failed
Establish
Terminate
6. Done
failed
2. LCP Options
Negotiated
Authenticate
5. Open
4. NCP
Configuration
3. Authentication
Completed
Network
Home PC to Internet Service
Provider
1. PC calls router via modem.
2. PC and router exchange LCP
packets to negotiate PPP
parameters.
3. Check on identities.
4. NCP packets exchanged to
configure the network layer, e.g.,
TCP/IP ( requires IP address
assignment).
5. Data transport, e.g. send/receive IP
packets.
6. NCP used to tear down the network
layer connection (free up IP
address); LCP used to shut down
data link layer connection.
7. Modem hangs up.
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Figure 5.41