Download Routing

Lecture 6 Overview
TCP: Transmission Control Protocol
• TCP is an alternative transport layer protocol
supported by TCP/IP.
• TCP provides:
– Connection-oriented
– Reliable
– Full-duplex
– Byte-Stream
CPE 401/601 Lecture 6 : TCP/IP model
2
TCP Segment Format
1 byte
1 byte
1 byte
1 byte
Source Port
Destination Port
Sequence Number
Request Number (ACK)
Control
Window Size
offset Rsrvd
Checksum
Urgent Pointer
Options (if any)
Data
CPE 401/601 Lecture 6 : TCP/IP model
3
TCP Segment Format
• Sequence Number refers to the first byte of
data included in the segment
• Request Number (Acknowledgement Number)
indicates the byte number of the next data
that is expected to be received
– All bytes up through this number have been
received
• Window: tells sender how many bytes it can
send before receiver will have to toss it away
– due to fixed buffer size
CPE 401/601 Lecture 6 : TCP/IP model
4
Control Flags
• URG: urgent data included
• ACK: this segment is (among other things) an
acknowledgement
• RST: error - abort the session
• SYN: synchronize Sequence Numbers (setup)
• FIN: polite connection termination
CPE 401/601 Lecture 6 : TCP/IP model
5
TCP Connection Creation
Client
SYN
ISN=X
1
ACK=Y+1
CPE 401/601 Lecture 6 : TCP/IP model
time
2
SYN
ISN=Y, ACK=X+1
Server
3
6
TCP Data and ACK
• Once the connection is established, data can
be sent
• Each data segment includes a sequence
number identifying the first byte in the
segment
• Each segment (data or empty) includes a
request number indicating what data has been
received
CPE 401/601 Lecture 6 : TCP/IP model
7
TCP Buffers
• The TCP layer doesn’t know when the
application will ask for any received data
– buffers incoming data so it’s ready when
application asks for it
• Both client and server allocate buffers to hold
incoming and outgoing data
• Both client and server announce with every
ACK how much buffer space remains
– Window field in a TCP segment
CPE 401/601 Lecture 6 : TCP/IP model
8
Send Buffers
• The application gives the TCP layer some data
to send
• The data is put in a send buffer, where it stays
until the data is ACK’d
– it has to stay, as it might need to be sent again!
• TCP layer won’t accept data from application
unless (or until) there is buffer space
CPE 401/601 Lecture 6 : TCP/IP model
9
ACKs
• A receiver doesn’t have to ACK every segment
– it can ACK many segments with a single ACK
segment
• Each ACK can also contain outgoing data
– piggybacking
• If a sender doesn’t get an ACK after some time
limit (MSL) it resends the data.
CPE 401/601 Lecture 6 : TCP/IP model
10
TCP Segment Order
• IP delivers TCP segments and IP in not reliable
– IP datagrams can be lost or arrive out of order
• Most TCP implementations will accept out-oforder segments
– if there is room in the buffer
• Once the missing segments arrive, a single
ACK can be sent for the whole thing
TCP/IP model
11
Termination
• The TCP layer can send a RST segment that
terminates a connection if something is wrong
• Usually the application tells TCP to terminate
the connection politely with a FIN segment
CPE 401/601 Lecture 6 : TCP/IP model
12
Connection Termination
App1
FIN
SN=X
2
...
ACK=X+1
1
App2
CPE 401/601 Lecture 6 : TCP/IP model
FIN
SN=Y
3
ACK=Y+1
4
13
TCP TIME_WAIT
• Once a TCP connection has been terminated
there is some unfinished business:
– What if the ACK is lost?
• The last FIN will be resent and it must be ACK’d.
– What if there are lost or duplicated segments that
finally reach the destination after a long delay?
• TCP hangs out for a while to handle these
situations
CPE 401/601 Lecture 6 : TCP/IP model
14
Lecture 7
Routing
CPE 401 / 601
Computer Network Systems
slides
modified
from
Dave Hollinger
slides
are are
modified
from
J. Kurose
& K. Ross
Two Key Network-Layer Functions
• forwarding: move packets from router’s input
to appropriate router output
• routing: determine route taken by packets
from source to dest.
– routing algorithms
• Analogy
– routing: process of planning trip from source to
destination
– forwarding: process of getting through single
interchange
CPE 401/601 Lecture 7 : Routing
16
routing and forwarding
routing algorithm
local forwarding table
header value output link
0100
0101
0111
1001
3
2
2
1
value in arriving
packet’s header
0111
1
3 2
CPE 401/601 Lecture 7 : Routing
17
Connection setup
• 3rd important function in some architectures
– ATM, frame relay, X.25
• before datagrams flow, two end hosts and
intervening routers establish virtual connection
– routers get involved
• network vs transport layer connection service:
– network: between two hosts
• may also involve intervening routers in case of VCs
– transport: between two processes
CPE 401/601 Lecture 7 : Routing
18
Network service model
• Q: What service model for channel transporting
datagrams from sender to receiver?
• Example services for individual datagrams:
– guaranteed delivery
– guaranteed delivery with less than 40 msec delay
• Example services for a flow of datagrams:
– in-order datagram delivery
– guaranteed minimum bandwidth to flow
– restrictions on changes in inter-packet spacing
CPE 401/601 Lecture 7 : Routing
19
Network layer service models
Network
Architecture
Internet
Service
Model
Guarantees ?
Congestion
Bandwidth Loss Order Timing feedback
best effort none
ATM
BR
ATM
VBR
ATM
ABR
ATM
UBR
CPE 401/601 Lecture 7 : Routing
constant
rate
guaranteed
rate
guaranteed
minimum
none
no
no
no
yes
yes
yes
yes
yes
yes
no
yes
no
no (inferred
via loss)
no
congestion
no
congestion
yes
no
yes
no
no
20
Connection and connection-less service
• Datagram network provides network-layer
connectionless service
• Virtual Circuit network provides network-layer
connection service
• analogous to the transport-layer services, but:
– service: host-to-host
– no choice: network provides one or the other
– implementation: in network core
CPE 401/601 Lecture 7 : Routing
21
Virtual circuits
• src-to-dst path behaves much like telephone circuit
– performance-wise
– network actions along source-to-dest path
• call setup, teardown for each call before data can flow
• each packet carries VC identifier
– not destination host address
• every router on source-dest path maintains “state” for
each passing connection
• link, router resources (bandwidth, buffers) may be
allocated to VC
– dedicated resources = predictable service)
CPE 401/601 Lecture 7 : Routing
22
VC implementation
• A VC consists of:
– path from source to destination
– VC numbers
• one number for each link along path
– entries in forwarding tables in routers along path
• packet belonging to VC carries VC number
– rather than dest address
• VC number can be changed on each link
– New VC number comes from forwarding table
CPE 401/601 Lecture 7 : Routing
23
Forwarding table
VC number
22
12
1
3
interface
number
Forwarding table in
northwest router:
Incoming interface Incoming VC #
1
2
3
1
…
2
32
12
63
7
97
…
Outgoing interface Outgoing VC #
3
1
2
3
22
18
17
87
…
…
Routers maintain connection state information!
CPE 401/601 Lecture 7 : Routing
24
Virtual circuits: signaling protocols
• used to setup, maintain teardown VC
• used in ATM, frame-relay, X.25
• not used in today’s Internet
application
transport 5. Data flow begins
network 4. Call connected
data link 1. Initiate call
physical
CPE 401/601 Lecture 7 : Routing
6. Receive data application
3. Accept call transport
2. incoming call network
data link
physical
25
Datagram networks
• no call setup at network layer
• routers: no state about end-to-end connections
– no network-level concept of “connection”
• packets forwarded using destination host address
– packets between same src-dst pair may take different paths
application
transport
network
data link
physical
1. Send data
CPE 401/601 Lecture 7 : Routing
application
transport
2. Receive data network
data link
physical
26
Forwarding table
4 billion
possible entries
Destination Address Range
Link Interface
11001000 00010111 00010000 00000000
through
11001000 00010111 00010111 11111111
0
11001000 00010111 00011000 00000000
through
11001000 00010111 00011000 11111111
1
11001000 00010111 00011001 00000000
through
11001000 00010111 00011111 11111111
2
otherwise
CPE 401/601 Lecture 7 : Routing
3
27
Longest prefix matching
Prefix Match
11001000 00010111 00010
11001000 00010111 00011000
11001000 00010111 00011
otherwise
Link Interface
0
1
2
3
Examples
DA: 11001000 00010111 00010110 10100001
Which interface?
DA: 11001000 00010111 00011000 10101010
Which interface?
Network Layer
4-28
Datagram or VC network: why?
• Internet (datagram)
– data exchange among computers
• “elastic” service, no strict timing requirement
– “smart” end systems (computers)
• can adapt, perform control, error recovery
• simple inside network, complexity at “edge”
– many link types
• different characteristics
• uniform service difficult
CPE 401/601 Lecture 7 : Routing
29
Datagram or VC network: why?
• ATM (VC)
– evolved from telephony
– human conversation:
• strict timing, reliability requirements
• need for guaranteed service
– “dumb” end systems
• telephones
• complexity inside network
CPE 401/601 Lecture 7 : Routing
30