UDPUser Datagram Protocol

1
UDPUser Datagram Protocol

• An unreliable, connectionless transport layer
  protocol
• UDP format. See picture
• Two additional functions beyond IP
• Demultiplexing deliver to different upper layer
  entities such as DNS, RTP, SNMP based on the
  destination port in the header. i.e., UDP can
  support multiple applications in the same end
  systems.
• (Optionally) check the integrity of entire UDP.
  (recall IP only checks the integrity of IP
  header.)
• If source does not want to compute checksum, fill
  checksum with all 0s.
• If compute checksum and the checksum happens to
  be 0s, then fill all 1s.
• UDP checksum computation is similar to IP
  checksum, with two more
• Add extra 0s to entire datagram if not multiple
  of 16 bits.
• Add pseudoheader to the beginning of datagram.
  UDP pseudoheader

2
Back to UDPUser Datagram Protocol
UDP datagram
0
16
31
Source Port
Destination Port
UDP Length
UDP Checksum
Data
Figure 8.16
3
Back to UDPUser Datagram Protocol
UDP pseudoheader
0 8
16
31
Source IP Address
Destination IP Address
0 0 0 0 0 0 0 0 Protocol 17
UDP Length
1.Pseudoheader is to ensure that the datagram has
indeed reached the correct destination host
and port. 2. The padding of 0s and pseudoheader
is only for the computation of checksum and
not be transmitted.
Figure 8.17
4
TCPtransmission control protocol

• TCP functionality
• Provides connection-oriented, reliable,
  in-sequence, byte-stream service
• Provides a logical full-duplex (two way)
  connection
• Provides flow-control by advertised window.
• Provides congestion control by congestion window.
• Support multiple applications in the same end
  systems.
• TCP establishes connection by setting up
  variables that are used in two peer TCP entities.
  Most important variables are initial sequence
  numbers.
• TCP uses Selective Repeat ARQ.
• TCP terminates each direction of connection
  independently, allowing data to continue flowing
  in one direction after closing the other
  direction.
• TCP does not keep messages boundaries and treats
  data as byte stream. e.g, when source sends out
  two chunks of data with length 400 and 600 bytes,
  the receiver may receive data in chunks of 300,
  400, and 300 bytes, or 100 and 900 bytes.

5
TCP operations

• TCP delivers byte stream.See picture
• TCP deals with old packets from old connections
  by several methods. See picture
• TCP uses sliding-window to implement reliable
  transfer of byte stream. See picture
• TCP uses advertised window for flow control.
• Adaptive timer
• tout tRTT4dRTT ,
• tRTT(new) ? tRTT(old) (1-?)?n ,
  dRTT(new)?dRTT(old) (1-?)(?n-tRTT)
• Where ?n is the time from transmitting a segment
  until receiving its ACK. ?, ? are in 0 to 1 with
  ? being 7/8 and ? being ¼ typically. tRTT is
  mean round-trip-time, dRTT is average of
  deviation.
• TCP uses congestion window for congestion
  control. See picture

6
TCP byte stream
Application
Application
byte stream
byte stream
segments
Transmitter
Receiver
Send buffer
Receive buffer
ACKs
Figure 8.18
7
Back to TCP operations
An old segment could not be distinguished from
current ones
Question How does TCP prevent old packets of old
connections?

• Using long (32 bit) sequence number

• Random initial sequence number

-- set a timer at the end of a connection to
clear all lost packets from this connection.
As a result, that an old packet from an old
connection conflicts with packets in current
connection is very low!!
Figure 8.23
8
Back to TCP operations
TCP uses Selective-Repeat ARQ
Receiver
Transmitter
Receive Window
Send Window
RlastWR1
Rlast
SlastWS-1



...
...
...
Octets transmitted and ACKed
Rnext
Rnew
SlastWA-1
Slast
Srecent
Advertised window
Rlast highest-numbered octet not yet read by the
application Rnext next expected octet Rnew
highest numbered octet received correctly RlastWR
-1 highest-numbered octet that can be
accommodated in receive buffer
Slast oldest unacknowledged octet Srecent
highest-numbered transmitted octet SlastWA-1
highest-numbered octet that can be
transmitted SlastWS-1 highest-numbered octet
that can be accepted from the application
Note 1. Rnew highest bytes received correctly,
which are out-of sequence bytes.
2. Advertised window WA Srecent Slast ? WA WR
( Rnew Rlast)
Figure 8.19
9
Back to TCP operations
Dynamics of TCP congestion window
Congestion occurs
Congestion
20
avoidance
15
Congestion
window
Threshold
10
Slow
start
5
0
Round-trip times
Figure 7.63
10
TCP protocol

• TCP segment See Segment format
• TCP pseudoheader. See pseudoheader
• TCP connection establishment. See establishment
• Client-server application See socket
• TCP Data transfer
• Sliding window with window sliding on byte basis
• Flow control and piggybacking See flow control
• TCP connection termination
• After receiving ACK for previous data, but no
  more data to send, the TCP will terminate the
  connection in its direction by issuing an FIN
  segment. Graceful termination
• TCP state transition diagram

11
Back to TCP protocol
TCP segment format
0 4 10
16
24 31
Source Port
Destination Port
Sequence Number
Acknowledgement Number
U
A
P
R
S
F
Header
R
C
S
S
Y
I
Reserved
(Advertised) Window Size
Length
G
K
H
T
N
N
Checksum
Urgent Pointer
Options
Padding
Data
1.SYN request to set a connection. 2. RST
tell the receiver to abort the connection. 3.
FIN tell receiver this is the final segment, no
more data, i.e, close the connection in this
direction. 4. ACK tell the receiver (or sender)
that the value is the field of acknowledgment
number is valid. 5. PSH tell the receiving TCP
entity to pass the data to the application
immediately. 6. URG tell the receiver that the
Urgent Pointer is valid. Urgent Pointer this
pointer added to the sequence number points to
the last byte of the Urgent Data, (the data
that needs immediately delivery).
Figure 8.20
12
Back to TCP protocol
TCP pseudoheader
0 8
16
31
Source IP Address
Destination IP Address
0 0 0 0 0 0 0 0 Protocol 6
TCP Segment Length
The padding of 0s and pseudoheader is only used
in computation of checksum but not be
transmitted, as in UDP checksum.
Figure 8.21
13
Back to TCP protocol
Host A
Host B

• Random initial SN
• Initial SNs in two
• directions are different
• 3. Initial SNs for two
• connections are different.
• 4. It should be clear here that
• what setting up connection
• means
• both A and B know that
• they will exchange data,
• and go into ready state to
• send and receive data.
• Most important is that
• they agree upon the
• initial SNs.

SYN, Seq_no x
SYN, Seq_no y, ACK, Ack_no x1
Seq_no x1, ACK, Ack_no y1
Three-way handshake to set up connection
Figure 8.22
14
Back to TCP protocol
Host B (Server)
Host A (Client)
socket bind listen accept (blocks)
socket connect (blocks)
SYN, Seq_no x
SYN, Seq_no y, ACK, Ack_no x1
connect returns
Seq_no x1, ACK, Ack_no y1
write read (blocks)
accept returns read (blocks)
request message
read returns
write read (blocks)
reply message
read returns
Figure 8.24
15
Back to TCP protocol
TCP window flow control
Host A
Host B
t0
Seq_no 1, Ack_no 2000, Win 2048, No Data
t1
Seq_no 2000, Ack_no 1, Win 1024, Data
2000-3023
t2
Seq_no 3024, Ack_no 1, Win 1024, Data
3024-4047
t3
Seq_no 1, Ack_no 4048, Win 512, Data 1-128
t4
Seq_no 4048, Ack_no 129, Win 1024, Data
4048-4559
Figure 8.25
16
Back to TCP protocol
TCP graceful termination
Host A
Host B
Question is termination easier than
establishment? Or to say, is it possible that a
connection is closed when both of two parties
confirm with each other?
FIN, seq 5086
ACK 5087
Data (150 bytes), seq. 303, ACK 5087
ACK 453
No, Saying goodbye is hard to do. Famous blue-red
armies problem.
FIN, seq. 453, ACK 5087
ACK 454
Figure 8.27
17
Back to TCP protocol
Thick lines normal client states Dashed lines
normal server states
CLOSED
passive open, create TCB
applic.close
active open,create TCB send SYN
LISTEN
receive SYN, send SYN, ACK
receive RST
send SYN
applic. close or timeout, delete TCB
SYN_SENT
SYN_RCVD
receive SYN, send ACK
receiveACK
receive SYN, ACK, send ACK
applic. close, send FIN
ESTABLISHED
receive FIN, send ACK
applic. close, send FIN
CLOSE_WAIT
receive FIN send ACK
applic. close send FIN
CLOSING
FIN_WAIT_1
receive ACK
LAST_ACK
receive ACK
receive ACK
receive FIN, ACK send ACK
receive FIN send ACK
2MSL timeout delete TCB
FIN_WAIT_2
TIME_WAIT
Figure 8.28
18
Sequence number wraparound and timestamps

• Original TCP specification for MSL (Maximum
  Segment Lifetime) is 2 minutes.
• How long will it take to wrap around 32 bit
  sequence number when 2324,294,967,296 bytes have
  been sent (maximum window size231)
• T-1 line, (232?8)/(1.544 ? 106) 6 hours
• T-3 line, (232?8)/(45 ? 106) 12 minutes
• OC-48 line, (232?8)/(2.4 ? 109) 14 seconds !!!
• When sequence number wrap around, the
  wraparounded sequence number will confuse with
  previous sequence number.
• Solution optional timestamp field (32 bits) in
  TCP header, thus, 232?232264 is big enough right
  now.

19
Internet routing protocols

• Autonomous system (AS)
• A set of routers or networks technically
  administrated by a single organization.
• No restriction that an AS must run a single
  routing protocol
• Only requirement is that from outside, an AS
  presents a consistent picture of which ASs are
  reachable through it.
• Three types of ASs
• Stub AS has only a single connection to outside.
• Multihomed AS has multiple connections to
  outside, but refuses to carry out transit traffic
• Transit AS multiple connections to outside and
  carry transit traffic.
• ASs need to be assigned globally unique AS number
  (ASN)

20
Classification of Internet routing protocols

• IGP (Interior Gateway Protocol)
• For routers to communicate within an AS and
  relies on IP address to construct paths.
• Provides a map of a county dealing with how to
  reach each building.
• RIP (Routing Information Protocol) distance
  vector
• OSPF (Open Shortest Path First) link state
• EGP (Exterior Gateway Protocol)
• For routers to communicate among different ASs
  and relies on AS numbers to construct AS paths.
• Provides a map of a country, connecting each
  county.
• BGP (Border Gateway Protocol) (distance) path
  vector

21
RIPRouting Information Protocol

• Distance vector
• On top of UDP with port 520
• Metric is number of hops
• Maximum number of hops is 15, 16 stands for
  infinity
• Using split-horizon with poisoned reverse.
• May speed up convergence by triggered updates.
• Routers exchange distance vector every 30 seconds
• If a router does not receive distance vector from
  its neighbor X within 180 seconds, the link to X
  is considered broken and the router sets the cost
  to X is 16 (infinity).
• RIP-2 contains more information subnet mask,
  next hop, routing domain, authentication, CIDR

22
RIP message format

• Command 1 request other routers to send routing
  information
• 2 a response containing its routing information

2. Version 1 or 2 3. Up to 25 routing
information message 3.1 Family identifier
only 2 for IP address 3.2 IP address can be a
host address or a network address 3.3 Metric
115. 16 indicates infinity
Problems of RIP not scalable, slow convergence,
counting-to-infinity, therefore replaced By OSPF
in 1979.
Figure 8.32
23
OSPFOpen Shortest Path First

• Flooding LSP to all routers
• Partitioning ASs into areas to improve
  scalability, thus two level hierarchical routing.
• Calculating multiple routes to a given
  destination.
• Supporting for variable-length subnetting
• A more flexible link cost 1 to 65535
• Balancing traffic over multiple paths having
  equal cost
• Supporting authentication
• Multicast rather than broadcast to reduce load on
  systems which do not understand OSPF
• Using designated router on multiaccess networks
  to reduce the number of OSPF messages

24
OSPF areas
To another AS
R1
N1
N5
R3
R6
R7
N2
N4
R2
N6
R4
R5
N3
Area 0.0.0.0
Area 0.0.0.1
R8
Area 0.0.0.2
N7
Areas like cities or towns in a county. Area
0.0.0.0 is called backbone area.
R router N network
Area 0.0.0.3
Internal routers just within an area, e.g,
R1,R3,R7 Area border routers connect to more
than one area, e.g., R3,R6,R8 Backbone routers
connect to the backbone, e.g., R2,R4,R5,R6,R8 Auto
nomous system boundary routers connect to other
ASs, e.g., R4.
Figure 8.33
25
OSPF routing (cont.)

• Neighbors the routers having an interface to a
  common network
• multiaccess networks a set of routers that can
  communicate directly with each other.
• Designated router in multiaccess networks the
  router responsible for routing information
  exchange on behalf of the entire multiaccess
  network.
• Adjacent if two routers are neighbors and
  connected by a link, then they are called
  adjacent. For a multiaccess network, the
  designated router and other routers are called
  adjacent. There is no adjacent relationship among
  non-designated routers of a multiaccess network.
  The purpose of adjacent relation is that OSPF
  only exchange (flood) routing information among
  adjacent routers to reduce the routing
  information exchange.

26
OSPF routing (cont.)

• OSPF runs over IP, with port number 89.
• Five types of OSPF packets hello, database
  description, link-state-request, link-state
  update, link-state ACK.
• OSPF operations
• Neighbors are discovered via the sending of hello
  messages and designated routers are elected in
  multiaccess networks
• Adjacent relationships are established and
  link-state database are synchronized
• Link-state advertisements (LSAs) are exchanged
• (flooded) among adjacent routers reliably (i.e.,
  the receiver of a routing information gives ACK.

27
OSPF routing (cont.)

• In summary
• 0. Using hello messages, neighbors, designated
  routers and adjacent relationship are established
• Using flooding among adjacent routers, each
  router informs all the other routers in its area
  of its neighbors and costs.
• Using these link state information, each internal
  router/area border router constructs a graph for
  its area/areas and computes its intraarea
  routes/interarea routes. The backbone routers do
  the same and compute interarea routes among all
  areas.
• As a result, intraarea routing and interarea
  routing.

28
Border Gateway Protocol

• InterAS routing protocol
• (Distance) Path-vector protocol not keep cost
  (distance) to each destination, but keep exact
  AS path to the destination
• In order to exchange routing information, the TCP
  connection was established
• TCP connection is 179.
• Routing decision is mainly based on policies, not
  the reachability
• Initially complete routing information are
  exchanged, then incremental updates are sent.

29
IPv6 (IPng)

• IPv4 is very successful but the victim of its own
  success.
• IPv6 Keeps IPv4 connectionless feature, with
  changes
• Longer address fields 16 bytes (128 bits)
• Simplified header format, no checksum,IHL,identifi
  cation,flag, offset
• Flexible support for options
• Flow label capacity identify certain QoS flow
• Security built-in authentication and
  confidentiality.
• Large packets longer 64Kbytes, called jumbo
  payload
• Fragmentation at source only routers not allowed
  to do fragment
• No checksum field since upper level (TCP, UDP)
  and lower level (Ethernet, token-ring) have
  error-check capacity, removing error check from
  IP layer is not a big problem.

30
Internet multicast

• A packet is to be sent to multiple hosts with the
  same multicast address
• Class D multicast addresses e.g.,
• 224.0.0.1 all systems on a LAN
• 224.0.0.2 all routers on a LAN
• 224.0.0.5 all OSPF routers on a LAN
• 224.0.0.6 all designated OSPF routers on a LAN
• It is not efficient to implement multicast by
  unicast, i.e., the source sends a separate copy
  for every destination.
• Reverse-path broadcasting / multicasting, each
  packet is transmitted once per link
• IGMP (Internet Group Management Protocol) allow
  a user to join a multicast group and let routers
  to collect multicast group membership
  information.

31
DHCP (Dynamic Host Configuration Protocol)

• A host broadcasts a DHCP discovery message in its
  physical network for an IP address.
• Server(s) reply with DHCP offer message
• The host selects one IP address and broadcasts a
  DHCP request message including the IP address
• The selected server allocates the IP address and
  sends back a DHCP ACK message with a lease time
  T, two thresholds T1 (0.5T), T2(0.875T)
• when T1 expires, the host asks the server for
  extension.
• If T2 expire, the host broadcasts DHCP request
  to any server on the network
• If T expires, the host relinquishes the IP
  address and reapply from scratch.

32
Mobile IP

• Mobile host, home agent, foreign agent
• If mobile host is currently at the same network
  with HA (home agent), the packet to the mobile
  host will be broadcast to it.
• If mobile host moves to another network,
• the mobile host will register itself with FA
  (foreign agent) and gets a new care-of IP
  address. Then packet is sent to HA, which will
  forward to the FA and FA continues to forward to
  destination.

33
Deliver packets to mobile host through home agent
and foreign agent
Foreign network
Home network
Foreign agent
Mobile host
2
Home agent
Internet
3
1
Correspondent host
Figure 8.29