1. 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

2. Back to UDP—User Datagram Protocol
UDP datagram
Source Port Destination Port
UDP Length UDP Checksum
Data
Back to UDP—User Datagram Protocol
Figure 8.16

3. Back to UDP—User Datagram Protocol
UDP pseudoheader
Source IP Address
Destination IP Address
Protocol = 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. 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.

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 = 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 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. An old segment could not be distinguished from current ones
Host A
Host B
SYN, Seq_no = n
SYN, Seq_no = n, ACK, Ack_no = n+1
Seq_no = n+1, ACK, Ack_no = n+1
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
Figure 8.23

8. Back to TCP operations TCP uses Selective-Repeat ARQ … … …
Receiver
Transmitter
Receive Window
Send Window
Rlast+WR+1
Slast+WS-1
Rlast … … …
...
...
...
Octets
transmitted
and ACKed
Rnext
Rnew
Slast
Slast+WA-1
Srecent
Advertised window
Rlast highest-numbered octet not yet read by the application
Rnext next expected octet
Rnew highest numbered octet received
correctly
Rlast+WR-1 highest-numbered octet that can be accommodated in receive buffer
Slast oldest unacknowledged octet
Srecent highest-numbered transmitted octet
Slast+WA-1 highest-numbered octet that can be transmitted
Slast+WS-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)
Back to TCP operations
Figure 8.19

9. Dynamics of TCP congestion window
Congestion occurs
Congestion 20
avoidance 15
Congestion
window
Threshold 10
Slow
start 5
Round-trip times
Back to TCP operations
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. TCP segment format Back to TCP protocol
Source Port Destination Port
Sequence Number
Acknowledgement Number U A P R S F
Header
Reserved R C S S Y I
(Advertised) Window Size
Length G K H T N N
Checksum Urgent Pointer
Options Padding
Data
1.SYN: request to set a connection 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
Source IP Address
Destination IP Address
Protocol = 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. Three-way handshake to set up connection
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 = x+1
Seq_no = x+1, ACK, Ack_no = y+1
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 = x+1
connect returns
Seq_no = x+1, ACK, Ack_no = y+1
write
read (blocks)
accept returns
read (blocks)
request message
read returns
write
read (blocks)
reply message
read returns
Figure 8.24

15. 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 = t2
Seq_no = 3024, Ack_no = 1, Win = 1024, Data = t3
Seq_no = 1, Ack_no = 4048, Win = 512, Data = 1-128 t4
Seq_no = 4048, Ack_no = 129, Win = 1024, Data =
Back to TCP protocol
Figure 8.25

16. TCP graceful termination
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. Thick lines: normal client states Dashed lines: normal server states
CLOSED
passive open,
create TCB
applic.close
active open,create TCB
send SYN
receive SYN,
send SYN, ACK
LISTEN
receive
RST
send SYN
applic. close
or timeout,
delete TCB
SYN_RCVD
receive SYN,
send ACK
SYN_SENT
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
FIN_WAIT_1
CLOSING
receive
ACK
LAST_ACK
receive
ACK
receive
ACK
receive FIN, ACK
send ACK
receive FIN
send ACK
Back to TCP protocol
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 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.

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. 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

22. RIP message format
Command Version
Zero
Address Family Identifier Zero
IP Address
Metric
. . .
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— indicates infinity
Problems of RIP: not scalable, slow convergence, counting-to-infinity,
therefore replaced By OSPF in 1979.
Figure 8.32

23. Internet multicast
A packet is to be sent to multiple hosts with the same multicast address
Class D multicast addresses: e.g.,
all systems on a LAN
all routers on a LAN
all OSPF routers on a LAN
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.

24. Multicasting Source S sends packets to multicast group G1 G1 7 2 5 8 S3 4 5 6 7 8 G2
Source S sends packets to multicast group G1

25. 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

26. 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

27. Example: Shortest Paths from SG1 G3 1 2 3 4 5 6 7 8 G2
Spanning tree of shortest paths to node S and parent ports are shown in blue

28. Example: S sends a packet G1 G1 1 7 2 3 2 4 2 3 4 2 1 1 5 2 5 3 3 G1 4 8 2 1 S 1 1 4 G1 3 5 4 2 2 4 1 3 6 3 2 1 1 3 4 G2 3 G3 G3
S sends a packet to node 1
Node 1 forwards to all ports, except parent port

29. Example: Hop 1 nodes broadcast  G1 G1 1 7 2 3 2 4 2 3 4 2 1 1 5 2  5 3 3 G1 4 8 2 1 S 1 4  1 G1 3 5 4 2 2 4 1 3 6 3 2 1 1 3 4 G2 3 G3 G3
Nodes 2, 3, 4, and 5 broadcast, except on parent ports
All nodes, not only G1, receive packets

30. Example: Broadcast continuesG1 G1 1 7 2 3 2 4 2 3 4 2 1 1 5 2 5 3 3 G1 4 8 2 1 S 1 1 4 G1 3 5 4 2 2 4 1 3 6 3 2 1 1 3 4 G2 3 G3 G3
Truncated RPB (TRPB): Leaf routers do not broadcast if none of its attached hosts belong to packet’s multicast group

31. Internet Group Management Protocol (IGMP)
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

32. Multicast programming
2.1 Multicast addresses.
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.

33. 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 yes yes
IP_MULTICAST_TTL yes yes
IP_MULTICAST_IF yes yes
IP_ADD_MEMBERSHIP yes no
IP_DROP_MEMBERSHIP yes no

34. Simplified header format:
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

35. 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 processing time in a router

36. Payload Length Next Header Hop Limit
IPv6 Header Format
Version Traffic Class
Flow Label
Payload Length 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 destination for which source requires special handling

37. Payload Length Next Header Hop Limit
IPv6 Header Format
Version Traffic Class
Flow Label
Payload Length Next Header Hop Limit
Source Address
Destination Address
Payload length: length of data excluding header, up to B
Next header: type of extension header that follows basic header
Hop limit: # hops packet can travel before being dropped by a router

38. IPv6 Addressing Address Categories Hexadecimal notation
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:

39. Example

40. Address Types based on Prefixes
Binary prefix
Types
Percentage of address space
Reserved
0.39
Unassigned
ISO network addresses
0.78
IPX network addresses
0000 1
3.12
0001
6.25
001
12.5
010
Provider-based unicast addresses
011
100
Geographic-based unicast addresses
101
110
1110
1111 0
1.56
0.2
Link local use addresses
0.098
Site local use addresses
Multicast addresses

41. Special Purpose Addresses
010 Registry ID Provider ID Subscriber ID Subnet ID Interface ID
n bits
m bits
o bits
p bits
(125-m-n-o-p) bits
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

42. 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 ::
IP-mapped addresses: 80 0’s ’s + IPv4
Denote IPv4 hosts & routers that do not support IPv6

43. 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

44. Migration from IPv4 to IPv6
Source
Destination
IPv6 network
IPv4 network
Tunnel
Tunnel head-end
Tunnel tail-end
IPv6 header
IPv4 header
(a)
Source
Destination
IPv6 network
Link
(b)

45. 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.

46. 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.

47. 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