1. TCP/IP TCP/IP architecture
IP (Internet Protocol) and ICMP (Internet Control Message Protocol)
TCP (Transmission Control Protocol)/UDP (Used Datagram Protocol)
Internet Routing
Multicast routing
IPV6, DHCP (Dynamic Host Configuration Protocol) and mobile IP (if time is allowed)

2. TCP/IP is a suite of protocols
Application protocols: HTTP, FTP, SMTP
(over TCP) and SNMP, DNS (over UDP)
Application
Application
2. TCP: connection-oriented while
UDP: connectionless
3. IP: connectionless datagram, routing and
possible fragmentation, congestion control
TCP
UDP
4. Complementary protocols with IP: ICMP, ARP.
5. Different physical networks.
ICMP IP
ARP
RARP
6. IP provides best-effort connectionless service
7. (Hierarchical) IP address. Intradomain and
interdomain routing, subnet, supernet.
Physical
network
Figure 8.1

3. 4. In general, data link frames changes each hop.
Header may contain other information
such as sequence number in TCP PDUs.
HTTP Request
2. Upper layer PDUs encapsulated
in lower layer PDUs, generally
keep unchanged during their traveling
Header contains source and destination port numbers
TCP Header
3. Some fields of IP datagram packets are modified
during its way to destination.
Header contains: source and destination IP addresses; transport protocol type
IP Header
4. In general, data link frames changes each hop.
Header contains: source and destination physical addresses; network protocol type
FCS
Ethernet Header
Figure 8.2

4. Machine A
Machine B
Application
Application
Transport
Router/Gateway
Transport
Internet
Internet
Internet
Network Interface
Network Interface
Network Interface
Network 1
Network 2
1. Upper layer is just based on lower layer’s service, not implementation.
2. Routers/gateways route packets from source to destination
3. PDUs are passed down in sender, ups and downs in routers, up in receiver.
Figure 8.3

5. IP protocol—the heart of TCP/IP architecture
IP packet
IP addressing
Subnet addressing
IP routing
Supernetting –CIDR (Classless Interdomain Routing)
Address resolution / Reverse address resolution
Fragmentation / /reassembly
ICMP (Internet Control Message Protocol).

6. IP version 4 header
Version IHL Type of Service Total Length
Identification Flags Fragment Offset
Time to Live Protocol Header Checksum
Source IP Address
Destination IP Address
Options Padding
0.Fixed length of 20 bytes and variable-length up to 40 byte options then data.
1. Version: 4: current version, 5: real time stream packet (ST2), 6: IPng or IPv6.
2. IHL (Internet Header Length) in unit of 32-bit words, 5 if no options.
3. Type of Service: 3-bit priorities with 0 (normal) to 7 (control packet), 4 bits tags of requirements: delay, throughput, reliability, and cost. 1 unused bit.
4. Total Length: header + data, maximum packet length:216-1 = bytes.
5. Identification, Flags, Fragment Offset: used for fragmentation and reassembly.
6. TTL (Time-to-Live): amount of time for a packet to live, mostly interpret as # of hops.
7. Protocol: 6: TCP, 17: UDP, 1: ICMP 8. Header checksum: internet Checksum algorithm, only on header, whenever TTL change, checksum needed to be recomputed.
9. Source & destination address. 10: options: security level, source routing, timestamps
11: padding: make the header a multiple of 32-bit words.
Figure 8.4

7. Router’s work for IP
When network interface passes over IP packet to IP layer
Compute the header checksum which is compared with the checksum in the header.
If valid, IP entity updates TTL and recompute checksum
Look up routing table to find next hop
Forward to the next hop.

8. IP addressing A computer may have multiple IP addresses
32 bit address with two level hierarchies:
network ID + host ID
Routing could be based on network IDs to reduce the size of routing table
Network Ids are assigned by Internet Network Information Center while host IDs are assigned by local network administrator.
Five classes of networks and different number of bits for network IDs and host IDs.

9. Five classes of IP addresses
Bit position:
Class A
Net ID
Host ID
Class B
1 0
Net ID
Host ID
Class C
Net ID
Host ID
Class D
Multicast address
Class E
Reserved for experiments
1.Class A: 7 bit network ID and 24 bit host ID, so 126 networks & 16 million hosts per network.
2.Class B: 14 bit network ID and 16 bit host ID, so networks & hosts per network.
3. Class C: 21 bit network ID and 8 bit host ID, so 2 million networks and 254 hosts per network
4. Class D: multicast addresses
6. Class E: reserved.
7. ID with all 1s or all 0s has special purpose. All 1s means broadcast, all 0s means this.
Figure 8.5

10. IP address --IDs with all 1s and all 0s
Network ID host ID
all 1s all 1s broadcast to local network
not all 1s all 1s broadcast to the specified network
all 1s not all 1s broadcast to the specified host on all networks (?)
all 0s all 0s this host
all 0s not all 0s the specified host on this network
not all 0s all 0s the specified network
127.x.y.z: x, y, z could be anything. Used for loopback.
When a host send a packet with this address, the packet will be returned to the host. Two usages: interprocess communication on a local host via TCP/IP ; debugging purpose.

11. IP address—dotted decimal notation
Four bytes denoted as four decimal numbers
Class A: to
Class B: to
Class C: to to
Class D:
Class E: to

12. Subnet addressing
Original
address
Subnetted
Net ID
Host ID
1 0
Subnet ID
Class B problems: a class B network (typical a university network) has about 64000
hosts. Big burden on administrator. Moreover a campus may need multiple local
networks.Therefore another hierarchical level: subnet.
Advantage of subneting is that outside hosts still see the hosts within the network
as two levels.
The length of subnet ID is variable. The subnet ID and its length is determined by
subnet mask.
A subnet mask consists of all 1s in network ID and subnet ID but all 0s in host ID.
Figure 8.6

13. Design of subnets
Suppose a university is assigned class B network:
Assume this university has many LANs but each LAN has no more than 100 hosts.
Therefore 7 bits for hosts are enough.
As a result, 9 bits for subnetworks.
The subnet mask will be
, i.e.,
If given a IP address: , find its subnet ID:
i.e.,
and ==
i.e.,
The hosts within this subnet have IP addresses from to

14. Address assignment with subnets and subnet routing
To the rest of
the Internet
1.A site with class B IP address:
2. Outside see all packets to any host within
the network is to get the packets to
network
3. Suppose a packet with arrive at R1 from outside:
R1 finds the subnet first by doing the following:
& =
&
= i.e.,
R1 looks up its routing table for , which has the next hop: (R2)
R1 forwards the packet to R2. R2 does similarly to find it is H5 and forwards to H5.
Figure 8.7

15. IP routing
IP entities in sender and routers work together to route a packet from source to destination.
Sender’s IP entity first looks up its routing table. If the destination is directly connected to it by a link or a LAN, the sender sends the packet to the destination directly
Otherwise, the sender will send the packet to default router, which is directly connected to the sender.
The router first check whether a packet is for itself. If yes, pass up to its upper layer. Otherwise look up its routing table to find the next hop and forward to the next hop.
The routing table:
Destination IP, next hop ID, outgoing interface, flags
Flags: H flag, H=1 to a host, H=0: to a network
G flag: G=1 to gateway, G=0: to destination

16. The order to search routing table
For complete destination address, if found, then forward a packet according to next hop IP and flag G. Otherwise
For destination network ID. otherwise
For default router, otherwise
An ICMP “host unreachable error” is sent back to the sender.

17. Routing table example: H5
(send an IP packet to ) H1 H2 R1
To the rest of H3 H4
the Internet R2 H5
Destination next-hop flags interface
H lo0
Default G emd0
emd0
Figure 8.7

18. Routing example Suppose H5’s routing table
Destination next-hop flags interface
H lo0
Default G emd0
emd0
H5 will find default router and forward to it
R2’s routing table
R2 uses default router to forward the packet.
Destination next-hop flags interface
H lo0
Default G emd0
emd1
emd1

19. Routing example (cont.)
R1’s routing table
Destination next-hop flags interface
H lo0
emd0
emd1
G emd1
R2 find the entry for destination IP: and sends out to emd0.

20. CIDR: classless interdomain routing
Historical story:
1987, a few visionaries predicted: 100,000 networks someday, most experts pooh-poohed.
100,000th network connected in 1996.
Problem: IP addresses soon run out of availability.
classful addresses wastes millions of address space
Three bears problem: A too large, C too small, need class B
But class B is still too large for many organizations.
Solution: CIDR (Classless InterDomain Routing)
Allocate remaining class C networks in variable-size blocks.
Examples: if a organization needs 2000 hosts, then allocate it a block of addresses, i.e., 8 contiguous class C networks. If need 8000 hosts, then allocate a block of 8192 addresses, i.e., 32 contiguous class C networks.

21. CIDR: classless interdomain routing (cont.)
Suppose an organization is allocated four contiguous class C networks:
, , ,
Question: how to treat these four contiguous networks as one from outside?
Network mask which will mask out one common prefix for these four networks.
Question: what is the network mask for these four networks?
The common prefix:
Therefore, network mask: , i.e.,
In routing table, instead of putting all four networks entries, just put one entry:
/22, where 22 indicates the network mask is 22 bits.
CIDR is also called supernetting because it “supernets” multiple networks into one.

22. A few discussions about CIDR
it is possible that both supernet /22 and /20 appear in the routing table.
Therefore, the IP address will match both of them.
Solution: longest prefix match will match /22.
A prefix of arbitrary length, along with the network mask of the same length, indicates a network number.
The multiple contiguous networks can not begin at a random class C network address but must begin at certain boundary.
E.g., 16 contiguous networks (i.e., 4096 addresses) can not begin at Instead, they must lie on a 4096-byte boundary. Such as begin from through
Go to Tanenbaum for more discussion.

23. Fragmentation and Reassembly
MTU (maximum transmission unit): a network generally has its MTU
Ethernet: 1500 bytes, FDDI: 4464 bytes
Fragmentation an IP packet if it is larger than MTU
Reassembly all packets of an IP packet into the original IP packet at receiver.

24. Packet fragmentation Fragmentation takes place at the sender / routers
Source
Router
Destination IP IP
Network
Network
Fragmentation takes place at the sender / routers
Reassembly takes place at the receiver.
Figure 8.9

25. Fragmentation fields in IP header
Identification (16 bits): identify which IP packet a particular fragment belongs to. Unique regarding IP packets destined to the same host.
Flags (3 bits): one unused, one is “don’t fragment” (DF) bit, another is “more fragment” (MF). When DF is 1, then the IP packet is not allowed to fragment. At this time, if the packet is longer than the underlying MTU, then the packet will be discarded and an error will be sent to source.
Offset (13 bits): indicate the location of a fragment in a packet. Since maximum IP packet length is 216, the offset is in the unit of 216/213 =8 bytes.
In summary: an IP packet with DF=0 is fragmented into several fragments with the same identification, all of which set MF=1 except the last fragment which keeps its IP packet’s MF. The receiver will reassembly all these segments with the same identification into its original IP packet based on the three fields in these fragments.

26. Example—fragment a IP packet
Suppose a packet of 20 header plus 1484 data arrives at an X.25
network having an MTU of 576 bytes, do the fragmentation.
Analysis: maximum fragment=576-20=556 bytes.
556 is not the multiple of 8, thus set data length to 552.
break 1484 into Therefore 3 fragments.
Total length ID MF Offset x x x x
Original packet
Fragment 1
Fragment 2
Fragment 3

27. Further segmentation Suppose the next network has MTU 324.
So data is =304, which is a multiple of 8
Fragment 1: x
Fragment 11: x
Fragment 12: x
Fragment 2: x
Fragment 21: x
Fragment 22: x
Fragment 3: x
Fragment 31: x
Fragment 32: x

28. A few words about fragmentation
Question: Do all the fragments travel along the same path?
NO.
Question: can reassembly be done by routers, not by receiver?
NO.
Because fragments will travel along possible different paths,
a router can not get all the fragments corresponding a packet
2. Even it happens that all the fragments arrive at the same router which
can reassembly them, it is possible that the reassembled packet be
fragmented again.
3. Fragmentation is easy but reassembly is difficult because the fragments
may arrive out of order, letting routers do reassembly will make routers
complicated, thus violate IP motivation and end-to-end argument.
Fragmentation results in subtle performance penalty: if any fragment lost or error,
All fragments needs to be retransmitted.

29. Address Resolution –ARP (Address Resolution Protocol)H1 H2 H3 H4
ARP request (what is the MAC address of ?)
ARP response (my MAC address is A-C5-3B-94)
The way for a host to find the physical address for a IP address.
Suppose H1 wants to find the physical address of H3:
1. H1 broadcasts a ARP request: what is the MAC address of ?
2. (only) H3 gives ARP response: my MAC address is A-C5-3B-94.
H1 will cache H3’s MAC address for future use. Timer is needed to remove the cached
pair of IP and MAC addresses after certain time in case of MAC address change.
Figure 8.8

30. RARP---Reverse Address Resolution Protocol
The MAC address of a host may be known, but the IP address is not.
E.g., a diskless computer at boot time can read its MAC address but does not know its IP address, which is stored at a server.
RARP: getting IP address of a MAC address.
The host broadcasts a RARP request: what is the IP address of such a MAC address
The server containing the IP address gives RARP response: the IP address of such a MAC address is so so.

31. ICMP: Intenet Control Message Protocol
Deal with error and control messages
ICMP messages are encapsulated in IP packet, but ICMP is considered in the same layer as IP.
Some message types: echo request/reply, timestamp request/reply, destination unreachable, source quench, redirect, time exceeded, parameter problem
Examples:
echo request/reply is used by ping program.
Time exceeded message is used in traceroute program:
Source host (running traceroute) sends messages to the destination with TTL incremented by one per message, beginning from 1.
When a message arrive at a router and its TTL =0, the router will send a time exceeded message back to source host, with the router included in the message.