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UDP—User 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
1
UDP datagram
0
16
31
Source Port
Destination Port
UDP Length
UDP Checksum
Data
Back to UDP—User Datagram Protocol
2
Figure 8.16
Back to UDP—User Datagram Protocol
UDP pseudoheader
0
8
16
31
Source IP Address
Destination IP Address
00000000
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.
3
Figure 8.17
TCP—transmission 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.
4
TCP operations
1. TCP delivers byte stream.See picture
2. TCP deals with old packets from old connections by
several methods. See picture
3. TCP uses sliding-window to implement reliable transfer of
byte stream. See picture
4. TCP uses advertised window for flow control.
5. Adaptive timer:
1.
2.
3.
tout = tRTT+4dRTT ,
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 roundtrip-time, dRTT is average of deviation.
6. TCP uses congestion window for congestion control. See
picture
5
TCP byte stream
Application
Application
byte stream
byte stream
segments
Transmitter
Receiver
Send buffer
Receive buffer
ACKs
6
Figure 8.18
An old segment could not be distinguished from current ones
Host A
Host B
Delayed segment with
Seq_no = n+2
will be accepted
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!!
Back to TCP operations
7
Figure 8.23
TCP uses Selective-Repeat ARQ
Receiver
Transmitter
Receive Window
Send Window
...
Octets
transmitted Slast
and ACKed
...
Srecent
Slast+WS1
...
Slast+WA-1
Rlast+WR+1
Rlast
…
…
Rnext
…
Rnew
Advertised window
Rlast highest-numbered octet not yet read
Slast oldest unacknowledged octet
by the application
Srecent highest-numbered transmitted octet
Rnext next expected octet
Slast+WA-1 highest-numbered octet that
Rnew highest numbered octet received
can be transmitted
correctly
Slast+WS-1 highest-numbered octet that
Rlast+WR-1 highest-numbered octet that
can be accepted from the application
can be accommodated in receive buffer
Note: 1. Rnew highest bytes received correctly, which are out-of sequence bytes.
2. Advertised window WA: Srecent – Slast  WA =WR – ( Rnew – Rlast)
Back to TCP operations
8
Figure 8.19
Dynamics of TCP congestion window
20
Congestion occurs
Congestion
avoidance
15
Congestion
window
Threshold
10
5
Slow
start
0
Round-trip times
Back to TCP operations
9
Figure 7.63
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
10
TCP segment format
Back to TCP protocol
0
4
10
16
24
Source Port
31
Destination Port
Sequence Number
Acknowledgement Number
Header
Length
Reserved
U A P R S F
R C S S Y I
GKH T N N
Checksum
(Advertised) Window Size
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).
11
Figure 8.20
Back to TCP protocol
TCP pseudoheader
0
8
16
31
Source IP Address
Destination IP Address
00000000
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.
12
Figure 8.21
Back to TCP protocol
Host A
Host B
1.
2.
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.
Three-way handshake to set up connection
13
Figure 8.22
Back to TCP protocol
Host A (Client)
socket
connect (blocks)
Host B (Server)
socket
bind
listen
accept (blocks)
connect returns
write
read (blocks)
accept returns
read (blocks)
read returns
write
read (blocks)
read returns
14
Figure 8.24
Host A
TCP window flow control
Host B
t0
t1
t2
t3
t4
Back to TCP protocol
15
Figure 8.25
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?
No, Saying goodbye
is hard to do.
Famous blue-red
armies problem.
16
Figure 8.27
Thick lines: normal client states
Dashed lines: normal server states
CLOSED
passive open,
create TCB
applic.
close
LISTEN
SYN_RCVD
applic.
close,
send
FIN
receive SYN,
send ACK
SYN_SENT
applic. close
or timeout,
delete TCB
ESTABLISHED
CLOSE_WAIT
FIN_WAIT_1
CLOSING
LAST_ACK
FIN_WAIT_2
Back to TCP protocol
TIME_WAIT
2MSL timeout
delete TCB
17
Figure 8.28
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 232=4,294,967,296 bytes
have been sent (maximum window size=231)
– 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232=264 is big enough right now.
18
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)
19
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
20
RIP—Routing 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
21
RIP message format
0
8
Command
16
31
Version
Zero
Address Family Identifier
Zero
IP Address
Zero
Zero
Metric
...
1. 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: 1—15. 16 indicates infinity
Problems of RIP: not scalable, slow convergence, counting-to-infinity
22
therefore replaced By OSPF in 1979.
Figure 8.32
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 collect multicast group membership
information.
23
Multicasting
G1
G1
1
2
3
4
2 4
1
1
5
5
3
2
3
3
G1
1 8
4
2
S
2
7 2
4
1 1 3
G1
5 4
2
2
1 2
4
1
3
3
G3
3
1
6 3
4
G2
G3
• Source S sends packets to multicast group G124
Multicast Routing
• Multicast routing useful when a source wants to
transmit its packets to several destinations
simultaneously
• Relying on unicast routing by transmitting each
copy of packet separately works, but can be very
inefficient if number of destinations is large
• Typical applications is multi-party conferencing
over the Internet
• Example: Multicast Backbone (MBONE) uses
reverse path multicasting
25
Reverse-Path Broadcasting (RPB)
• Fact: Set of shortest paths to the source node S forms a tree that spans the
network
– Approach: Follow paths in reverse direction
• Assume each router knows current shortest path to S
– Upon receipt of a multicast packet, router records the packet’s source
address and the port it arrives on
– If shortest path to source is through same port (“parent port”), router
forwards the packet to all other ports
– Else, drops the packet
• Loops are suppressed; each packet forwarded by a router exactly once
• Implicitly assume shortest path to source S is same as shortest path from
source
– If paths asymmetric, need to use link state info to compute shortest paths
from S
26
Example: Shortest Paths from S
G1
G1
1
2
3
4
2 4
1
1
5
1
5
3
2
3
4
2
S
2
7 2
1
3
G1
8
1 3
4
G1
5 4
1 2
2 4
3
1
3
3
G3
2
1
6 3
4
G2
G3
• Spanning tree of shortest paths to node S and parent
27
ports are shown in blue
Example: S sends a packet

G1
2
G1
1
3
4
2 4
1
1
5
1
5
3
2
3
4
2
S
2
7 2
1
3
G1
8
1 3
4
G1
5 4
1 2
2 4
3
1
3
3
G3
2
1
6 3
4
G2
G3
• S sends a packet to node 1
• Node 1 forwards to all ports, except parent port
28
Example: Hop 1 nodes broadcast

G1
2
G1
1
3
4
2 4
1
1
5
1
5
7 2
3
2
3
4
2
S
2

1
3
G1

G1

8
1 3
4
5 4
1 2
2 4
3
1
3
3
G3
2
1
6 3
4
G2
G3
• Nodes 2, 3, 4, and 5 broadcast, except on parent ports
29
• All nodes, not only G1, receive packets
Example: Broadcast continues
G1
G1
1
2
3
4
2 4
1
1
5
1
5
3
2
3
4
2
S
2
7 2
1
3
G1
8
1 3
4
G1
5 4
1 2
2 4
3
1
3
3
G3
2
1
6 3
4
G2
G3
• Truncated RPB (TRPB): Leaf routers do not broadcast if none
of its attached hosts belong to packet’s multicast group 30
Internet Group Management
Protocol (IGMP)
• Internet Group Management Protocol:
– Host can join a multicast group by sending an IGMP
message to its router
• Each multicast router periodically sends an IGMP
query message to check whether there are hosts
belonging to multicast groups
– Hosts respond with list of multicast groups they belong
to
– Hosts randomize response time; cancel response if
other hosts reply with same membership
• Routers determine which multicast groups are
associated with a certain port
• Routers only forward packets on ports that have
hosts belonging to the multicast group
31
Multicast programming
• 2.1 Multicast addresses.
– 224.0.0.0---247.255.255.255
• 2.2 Levels of conformance.
– 0: no, 1: sending, 2: receiving
• 2.3 Sending Multicast Datagrams.
– Open UDP socket, and send to multicast address
– TTL
•
•
•
•
•
•
0 Restricted to the same host.
1 Restricted to the same subnet.
<32 Restricted to the same site, organization or department.
<64 Restricted to the same region.
<128 Restricted to the same continent.
<255 Unrestricted in scope. Global.
• 2.4 Receiving Multicast Datagrams.
– Joining multicast group
– Drop multicast group
• Mapping of IP Multicast Addresses to Ethernet/FDDI addresses.
32
Multicast functions
• int getsockopt(int s, int level, int optname, void* optval,
int* optlen);
• int setsockopt(int s, int level, int optname, const void*
optval, int optlen);
•
setsockopt() getsockopt()
•
•
•
•
•
IP_MULTICAST_LOOP
IP_MULTICAST_TTL
IP_MULTICAST_IF
IP_ADD_MEMBERSHIP yes
IP_DROP_MEMBERSHIP yes
yes
yes
yes
no
no
yes
yes
yes
• http://www.ibiblio.org/pub/Linux/docs/HOWTO/o
ther-formats/html_single/MulticastHOWTO.html#ss2.1
33
IPv6 (IPng): IPv4 is very successful but the victim of its own success.
• Longer address field:
– 128 bits can support up to 3.4 x 1038 hosts
• Simplified header format:
– Simpler format to speed up processing of each header
– All fields are of fixed size
– IPv4 vs IPv6 fields:
• Same: Version
• Dropped: Header length, ID/flags/frag offset, header checksum
• Replaced:
– Datagram length by Payload length
– Protocol type by Next header
– TTL by Hop limit
– TOS by traffic class
• New: Flow label
34
Other IPv6 Features
• Flexible support for options: more efficient and
flexible options encoded in optional extension
headers
• Flow label capability: “flow label” to identify a
packet flow that requires a certain QoS
• Security: built-in authentication and confidentiality
• Large packets: supports payloads that are longer
than 64 K bytes, called jumbo payloads.
• Fragmentation at source only: source should check
the minimum MTU along the path
• No checksum field: removed to reduce packet 35
processing time in a router
IPv6 Header Format
0
4
Version
12
16
24
Traffic Class
Payload Length
31
Flow Label
Next Header
Hop Limit
Source Address
Destination Address
• Version field same size, same location
• Traffic class to support differentiated services
• Flow: sequence of packets from particular source to particular
36
destination for which source requires special handling
IPv6 Header Format
0
4
Version
12
16
24
Traffic Class
Payload Length
31
Flow Label
Next Header
Hop Limit
Source Address
Destination Address
• Payload length: length of data excluding header, up to 65535 B
• Next header: type of extension header that follows basic header
• Hop limit: # hops packet can travel before being dropped by a router
37
IPv6 Addressing
• Address Categories
– Unicast: single network interface
– Multicast: group of network interfaces, typically at different
locations. Packet sent to all.
– Anycast: group of network interfaces. Packet sent to only one
interface in group, e.g. nearest.
• Hexadecimal notation
– Groups of 16 bits represented by 4 hex digits
– Separated by colons
• 4BF5:AA12:0216:FEBC:BA5F:039A:BE9A:2176
– Shortened forms:
• 4BF5:0000:0000:0000:BA5F:039A:000A:2176
• To 4BF5:0:0:0:BA5F:39A:A:2176
• To 4BF5::BA5F:39A:A:2176
– Mixed notation:
• ::FFFF:128.155.12.198
38
Example
39
Address Types based on Prefixes
Binary prefix
Types
Percentage of address space
0000 0000
Reserved
0.39
0000 0001
Unassigned
0.39
0000 001
ISO network addresses
0.78
0000 010
IPX network addresses
0.78
0000 011
Unassigned
0.78
0000 1
Unassigned
3.12
0001
Unassigned
6.25
001
Unassigned
12.5
010
Provider-based unicast addresses
12.5
011
Unassigned
12.5
100
Geographic-based unicast addresses
12.5
101
Unassigned
12.5
110
Unassigned
12.5
1110
Unassigned
6.25
1111 0
Unassigned
3.12
1111 10
Unassigned
1.56
1111 110
Unassigned
0.78
1111 1110 0
Unassigned
0.2
1111 1110 10
Link local use addresses
0.098
1111 1110 11
Site local use addresses
0.098
40
Special Purpose Addresses
n bits
010 Registry ID
•
•
m bits
Provider ID
o bits
Subscriber ID
p bits
Subnet ID
(125-m-n-o-p) bits
Interface ID
Provider-based Addresses: 010 prefix
– Assigned by providers to their customers
– Hierarchical structure promotes aggregation
• Registry ID: ARIN, RIPE, APNIC
• ISP
• Subscriber ID: subnet ID & interface ID
Local Addresses: do not connect to global Internet
– Link-local: for single link
– Site-local: for single site
– Designed to facilitate transition to connection to Internet
41
Special Purpose Addresses
• Unspecified Address: 0::0
– Used by source station to learn own address
• Loopback Address: ::1
• IPv4-compatible addresses: 96 0’s + IPv4
– For tunneling by IPv6 routers connected to
IPv4 networks
– ::135.150.10.247
• IP-mapped addresses: 80 0’s + 16 1’s +
IPv4
– Denote IPv4 hosts & routers that do not support
IPv6
42
Migration from IPv4 to IPv6
• Gradual transition from IPv4 to IPv6
• Dual IP stacks: routers run IPv4 & IPv6
– Type field used to direct packet to IP version
• IPv6 islands can tunnel across IPv4
networks
– Encapsulate user packet insider IPv4 packet
– Tunnel endpoint at source host, intermediate
router, or destination host
– Tunneling can be recursive
43
Migration from IPv4 to IPv6
Source
Tunnel tail-end
Tunnel head-end
Destination
Tunnel
(a)
IPv6 network
IPv6 header
IPv4 header
IPv6 network
IPv4 network
Destination
Source
Link
(b)
IPv6 network
IPv6 network
44
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.
45
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.
46
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
47
Figure 8.29