Networking models
OSI layering provides for standard interfaces between layers and has these benefits:
- higher layers are shielded from the complexity of the lower layers
- application “thinks” it’s directly communicating with another application through the standard sockets API
- one vendor writes SW implementing higher layers (e.g. web browser), another vendor writes SW implementing lower layers (e.g. M$’s built-in TCP/IP SW)
OSI and TCP/IP models:
OSI TCP/IP TCP/IP Sample
original updated protocol
+------------+ +------------+ +------------+ +----------------+ The "language" that
7 |Application | | | | | |Telnet HTTP | apps and servers use
|------------+ | | | | |FTP SMTP | to communicate.
6 |Presentation| |Application | 5-7 |Application | |POP3 VoIP |
|------------+ | | | | |SNMP DNS Halo 3 |
5 |Session | | | | | |arp traceroute |
|------------+- - -+------------+- - -+------------+- - -+----------------+ Data transmission
4 |Transport | |Transport | 4 |Transport | |TCP UDP | characteristics.
|------------+- - -+------------+- - -+------------+- - -+----------------+ How to move packets
3 |Network | |Internetwork| 3 |Internetwork| |IP ICMP | from src to dst.
|------------+- - -+------------+- - -+------------+- - -+----------------+
2 |Data link | |Network | 2 |Data link | |Eth HDLC PPP ARP|
|------------+ | |- - -|------------|- - -|----------------| How to send raw data
1 |Physical | |access | 1 |Physical | |Eth RJ-45 V.35 | across a physical medium.
+------------+ +------------+ +------------+ +----------------+
- Please Do Not Throw Sausage Pizza Away
TCP error recovery service provided to HTTP:
TCP/IP network access layer encapsulation:
Encapsulation and data:
Binary math
Binary to hexadecimal conversion:
01101110 Binary (representation of) byte
+ +
| |
v v
Higher order 0110 1110 Lower order
nibble nibble
+ +
| |
v v
6 14 Decimal byte
+ +
| |
v v
6 E Hexadecimal byte
LANs
Most common Ethernet types
| Name | Speed (Mbps) | Alt. name | Standard | Cable (max) |
|---|---|---|---|---|
| Ethernet | 10 | 10BASE-T | IEEE 802.3 | Copper (100m) |
| Fast ethernet | 100 | 100BASE-TX | IEEE 802.3u | Copper (100m) |
| Gigabit ethernet | 1000 | 1000BASE-LX, 1000BASE-SX | IEEE 802.3z | Fiber (550m, 5km) |
| Gigabit ethernet | 1000 | 1000BASE-T | IEEE 802.3ab | Copper (100m) |
- “T” in alt. names – twisted pair
CSMA/CD algorithm (required by hubs)
- a device that wants to send a frame waits until the LAN is silent
- if a collission occurs, the devices that caused the collision wait a random amount of time and then try again
TIA standard Ethernet cabling pinouts
- straight-through cable – both ends of the cable use the same standard (devices use the opposite pins when transmitting) - PC <=> Hub
- crossover cable – devices use the same pins to transmit - Hub <=> Hub
- devices that transmit on 1,2 and receive on 3,6: PC NICs, routers
- devices that transmit on 3,6 and receive on 1,2: hubs, switches
- auto-mdix – Cisco switch feature that readjusts the standard logic when wrong cables are used
Half duplex vs. full duplex
- HDX – device either sends or receivs, but not both at the same time (imposed by CSMA/CD)
- FDX – possible if only one device is cabled to each switch’s port (full use of bandwidth)
Ethernet addressing terminology
- MAC = Ethernet address = NIC address = LAN address – 6-byte (48 bit, 12 hex digits) address usually burned in a ROM chip
- 3 bytes – Organizationally Unique Identifier (OUI)
- 3 bytes – vendor assigned part
- Unicast address – MAC address representing a single LAN interface (
FFFF.FFFF.FFFF) - Multicast address – subset of Ethernet devices (
0100.5exx.xxxx)
LAN headers
DIX
+--------+-----------+------+------+----------+---+
|Preamble|Destination|Source| Type |Data + pad|FCS|
Bytes| 8 | 6 | 6 | 2 | 46-1500 | 4 |
+--------+-----------+------+------+----------+---+
IEEE 802.3 (orig)
+----+---+-----------+------+------+----------+---+
|Pre.|SFD|Destination|Source|Length|Data + pad|FCS|
| 7 | 1 | 6 | 6 | 2 | 46-1500 | 4 |
+----+---+-----------+------+------+----------+---+
IEEE 802.3 (rev. 1997)
+----+---+-----------+------+------+----------+---+
|Pre.|SFD|Destination|Source|Len./ |Data + pad|FCS|
| 7 | 1 | 6 | 6 |type 2| 46-1500 | 4 |
+----+---+-----------+------+------+----------+---+
- IEEE 802.3 Ethernet header/trailer fields
- Preamble – synchronization
- Start Field Delimiter – tells that next byte is destination MAC address
- Length/Type – lenght/type of data field (either length or type is present, not both)
- Data and padding – data from a higher layer (ex. L3 PDU - IP packet)
- Frame Check Sequence – used by NIC to check the frame integrity
WANs
- WAN standards and protocols – networking spanning relatively large distances compared to LANs
Point-to-Point WANs - OSI L1
Leased line – a WAN circuit usually not owned by the data owner but by a telco (telecommunications company)
- (leased line = leased circuit = link = serial link = serial line = point-to-point link = circuit)
LEASED LINE COMPONENTS
| |
| T E L C O N E T |
| |
| CO |
+-------+ +---+| +----------+ +----------+ |+---+ +-------+
|Router1+----+CSU+-------+WAN switch+----------+WAN switch+--------+CSU+----+Router2|
+-------+ +---+| +----------+ +----------+ ^ |+---+ ^ +-------+
^ ^ | | | |
| | | | | Short cable
+----------+ | | | (max 15m)
| | | |
| Demarc | Demarc
| |
CPE Long cable (KMs)
WAN connectors:
Terminology
- Synchronous – both devices use (roughly) the same speed when transfering the bits over serial link
- Clock source – time source for devices using a synchronous serial link
- CSU/DSU – in U.S., digital links interface to telco
WAN links speeds
- DS0 - 64 kbps
- DS1 (T1) - 1.544 Mbps (24 DS0 + 8 kbps overhead)
- DS3 (T3) - 44.736 Mbps (28 DS1s + mngt. overhead)
- E1 - 2.048 Mbps (32 DS0s)
- E3 - 34.368 Mbps (16 E1s + mngt. overhead)
- J1 (Y1) - 2.048 Mbps (32 DS0s, Japanese standard)
Point-to-Point WANs - OSI L2
Most popular protocols: HDLC, PPP
HDLC
HDLC framing
Standard
+----+-------+-------+--------+---+
|Flag|Address|Control| Data |FCS|
Bytes | 1 | 1 | 1 |Variable| 2 |
+----+-------+-------+--------+---+
Cisco (PPP)
+----+-------+-------+----+--------+---+
|Flag|Address|Control|Type| Data |FCS|
| 1 | 1 | 1 | 2 |Variable| 2 |
+----+-------+-------+----+--------+---+
- Address field is not really needed
- since point-to-point links are relatively simple, HDLC only does
- error checking
- packet type determination
Point-to-point protocol
- framing is identical to Cisco framing (above)
- defined later than HDLC => more features => more popular
Packet switching
- most popular services: ATM, Frame relay (much more common today)
Frame relay
- multiaccess network similar to LANs
- access links - leased lines between routers (DTE) and FR switches (DCE)
- each FR header holds DLCI - based on DLCI switch forwards the frame to the destination
- FR creates logical path (VC) between two FR DTE devices
- each VC has a committed information rate (CIR)
IPv4 addressing and routing
- the only widely used L3 protocol - IP
Standard 20-byte IPv4 header:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| IHL | DS Field | Packet Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | Protocol | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- IHL – IP header length (including optional fields)
- DS Field – differentiated services field (QoS)
- Packet Length – entire packet length, including data
- Identification – used by IP fragmentation process, all fragments have the same ID
- Flags – used by IP fragmentation process (3 bits)
- TTL – to prevent routing loops
- Protocol – contents of the data portion of the IP packet (ex. 6 means TCP header is first thing in data field)
IP addressing
- all IP addresses in the same group (class) must not be separated by a router
Router logic
- use FCS to check for errors, if error occurred discard the frame and repeat this step
- discard the data-link header and trailer, leaving the IP packet
- use destination IP address to look up the outgoing interface in routing table
- encapsulate IP packet inside a data-link header and trailer appropriate for outgoing interface and forward the frame
L3 utilities
DNS
- What is the foo’s IP address?
- Foo’s IP is 10.1.1.2
ARP
- Hey everybody, if you are 10.1.1.2 tell me your MAC address!
- I’m 10.1.1.2 and my MAC is 0200.2222.222
- Client – DHCP discover message (LAN broadcast)
- Server – DHCP offer message (LAN broadcast)
- Client – DHCP request message (to server)
- Server – DHCP acknowledgement (to client)
Transmission Control protocol (TCP)
Connection-oriented protocol – requires an exchange of message (or preestablished correlation) before data transfer
Features:
- multiplexing using ports
- error recovery
- flow control using windowing
- connection establishment and termination
- ordered data transfer and segmentation
TCP header format (fields):
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -----
| Source Port | Destination Port | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Sequence Number | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Acknowledgment Number | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Required
| Data | |U|A|P|R|S|F| | |
| Offset| Reserved |R|C|S|S|Y|I| Window | |
| | |G|K|H|T|N|N| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Checksum | Urgent Pointer | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -----
| Options | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Window field – maximum number of unacknowledged bytes
- starts small, then grows until errors occur (dynamic or sliding window)
Multiplexing relies on sockets
- IP address (ex. 10.1.1.2)
- transport protocol (ex. TCP)
- port number (ex. 80)
TCP connection establishment (three-way handshake):
+-------+ SEQ=200 +------+
|Web | SYN, DPORT=80, SPORT=1027 |Web |
|browser| ----------------------------------> |server|
+-------+ +------+
SEQ=1450, ACK=201
SYN, ACK, DPORT=1027, SPORT=80
<----------------------------------
SEQ=201, ACK=1451
ACK, DPORT=80, SPORT=1027
---------------------------------->
Maximum transmission unit (MTU) - size of the largest L3 packet that can sit inside a frame’s data field (it’s 1500 bytes for many L2 protocols, including Ethernet)
- because IP and TCP headers are 20 bytes each, TCP typically segments large data into 1460-byte chunks
- TCP segment (L4PDU) = TCP header + data field
User Datagram Protocol (UDP)
Connectionless protocol – does not require an exchange of message (or preestablished correlation) before data transfer
UDP adds just two features to IP:
- port numbers for multiplexing
- data checksum for error detection
apps using UDP are tolerant of the data loss or have some application mechanism for lost data recovery
- VoiP – recovery wouldn’t help anyway, it would be too late
- DNS – will retry if DNS resolution fails
- NFS – recovery done by application layer code
UDP header format:
0 7 8 15 16 23 24 31
+--------+--------+--------+--------+
| Source | Destination |
| Port | Port |
+--------+--------+--------+--------+
| | |
| Length | Checksum |
+--------+--------+--------+--------+
|
| data octets ...
+---------------- ...
TCP/IP applications
QoS – application’s requirements from the network service
Before mid 1990s video and voice used totally separate networking facilities, today - VoIP.
VoIP QoS demands
- low delay (< 200 ms)
- low jitter (< 30 ms) - variation in delay
- loss - if VoIP packet is lost, it’s not retransmitted
HTTP commands and responses
- GET request – request from client to get a file from a web server
- server sends GET response with code 200 (meaning OK) together with file contents
- 404 – file not found
See HTTP protocol for more.
Sources
- W. Odom: CCENT/CCNA ICDN1 (2012)
- ULSAH, Ch. 14: TCP/IP Networking
- RFC 971 - IP
- RFC 793 - TCP
- RFC 768 - UDP