Advanced Computer Networks: Lecture 30 - Dr. Amir Qayyum
Bạn đang xem bản rút gọn của tài liệu. Xem và tải ngay bản đầy đủ của tài liệu tại đây (307.49 KB, 16 trang )
CS716
Advanced Computer Networks
By Dr. Amir Qayyum
1
1
Lecture No. 30
2
TCP Outline
•
•
•
•
•
•
•
•
TCP vs a sliding window on a direct link
Model of use
Segment header format and options
States and state diagram (thinkpairshare)
Sliding window implementation details
Flow control issues
Bit allocation limitations
Adaptive retransmission algorithms
3
TCP vs Sliding Window on Direct Link
• RTT varies
– Among peers (hosts at other end of
connections)
– Over time
– Requires adaptive approach to
retransmission (and window sizes)
4
TCP vs Sliding Window on Direct Link
• Packets can be
– Delayed for long periods (assume 60
seconds)
– Reordered in the network
– Must be prepared for arrival of very
old packets
5
TCP vs Sliding Window on Direct Link
• Peer capabilities vary
– Minimum link speed on route
– Buffering capacity at destination
– Requires adaptive approach to
window sizes
6
TCP vs Sliding Window on Direct Link
• Network capacity varies
– Other traffic competes for most links
– Requires (global) congestion control
strategy
• Why not implement more
functionality in IP, e.g. ordering
guarantees or congestion control ?
7
EndtoEnd Argument
• A function should not be provided at a given
layer unless it can be completely and correctly
implemented at that layer
• Inorder delivery: hopbyhop ordering guarantee
is not robust to path changes or multiple paths
F
A
B
C
E
D
8
EndtoEnd Argument
• Congestion control
– Should be stopped at source
– But network can provide feedback
A
100Mbps
5Mbps
100Mbps
100Mbps
B
1Mbps
100Mbps
1Mbps
10Mbps
C
D
5Mbps
E
10Mbps
F
10Mbps
blue should get 9Mbps, but gets only 5Mbps with hopbyhop drops
9
TCP Model of Use
• Connection setup via 3way handshake
• Data transport
– Sender writes some data
– TCP
• Breaks data into segments
• Sends each segment over IP
• Retransmits, reorders, and removes
duplicates as necessary
– Receiver reads some data
• Teardown through 4step exchange
10
TCP Connection Setup
• TCP Connection Setup via 3way Handshake
– J and K are (different) sequence numbers for messages
– Sequence numbers need not start at zero
SYN J
Active
participant
(client)
SYN K
ACK J+1
Passive
participant
(server)
ACK K+1
11
TCP Data Transport Model
• Data broken into segments
–
–
–
–
Limited by maximum segment size (MSS)
Defaults to 536 bytes
Negotiable during connection setup
Typically set to MTU of directly connected network
minus size of IP and TCP headers (40 bytes)
• Three events send segment
– >= MSS bytes of data ready to be sent
– Explicit PUSH operation by application
– Periodic timeout
12
TCP Connection Teardown
• TCP connection teardown in 4 steps
– Either client or server can initiate connection teardown
– FIN is associated with sequence number space
active close
FIN J
ACK J+1
passive close
closes connection
FIN K
Client
ACK K+1
Server
13
TCP Segment Header & PseudoHeader
0
16
4
10
source port
destination port
sequence number
ACK sequence number
0
hdrlen
flags
advertised window
TCP checksum
urgent pointer
options (variable)
0
8
0
16
source IP address
destination IP address
6 (TCP)
TCP length
14
31
31
TCP Segment Header
• 16bit source and destination ports
• 32bit send and ACK sequence
numbers
• 4bit header length in 4byte words
(minimum 5)
15
TCP Segment Header
• Six 1bit flags
– URG: segment contains urgent data
– ACK: ACK sequence number is valid
– PSH: do not delay delivery of data
– RST: reset connection (rejection or
abnormal termination); no sequence
number associated
– SYN: synchronize segment for setup
– FIN: final segment for teardown
16
