TCP/IP Protocol

Application layer - HTTP, FTP, Telnet, SMTP, DNS, etc.
Transport layer - TCP (reliable stream, ports, flow control, congestion control), UDP
Network layer - IP, ICMP, IPv6 (routing)
Data Link layer - PPP, Ethernet, ATM, etc. (channel sharing)

Packet structure


  +------------------+----------------+------------------+---------------+
  | Data Link header | Network header | Transport header | Appl. Data... |
  +------------------+----------------+------------------+---------------+

Transport Layer

UDP

User Datagram Protocol

Provides an unreliable datagram service between applications given an unreliable datagram service between hosts

Header (8 bytes)

Source port, 16 bits (0-65535)
Destination port, 16 bits (0-65535)
Length in bytes, 16 bits (0-65535)
Checksum, XOR of header, 16 bits

If the header checksum fails, the packet is discarded. The data is not error checked.

TCP

Transport Control Protocol.

Provides a reliable 2-way data stream between remote applications.

TCP header (20 bytes)

Source port, 16 bits (0-65535)
Destination port, 16 bits (0-65535)
Sequence number, 32 bits, number of bytes sent
Acknowledgment number, 32 bits, number of bytes received
Header length, 8 bits = 40 unless options are used
Unused, 2 bits
URG, 1 bit, unused
ACK, 1 bit, 1 = received sequence through acknowledgment number
PSH, 1 bit, unused
RST, 1 bit
SYN, 1 bit, 1 = opening connection
FIN, 1 bit, 1 = closing connection
Receiver window size, 16 bits (bytes free in buffer, scaled using option in setup)
Checksum, 16 bits (XOR of header only)
Urgent pointer, 16 bits, unused
Options, variable length (usually 0)
Data, variable length (usually 0-1500)

Underlying network layer provides an unreliable packet delivery service. Packets may be lost, duplicated, or delivered out of order.

Reliable stream

Protocol to ensure that all packets are received, and in the correct order.

Sender sends packet, waits for ACK
Receiver replies with ACK, seq. number
Sender marks data as sent
If checksum fails, packet is discarded
If ACK times out, sender retransmits all unmarked data
If same packet is ACKed 3 times, sender resends next packet (fast resend)
Receiver ACKs all packets, including duplicates

Connection setup

Protocol to open and close connections.

  Action             Client state        Server state
  ------             ------------        ------------
                     CLOSED              LISTEN
  SYN --->           SYN_SENT            SYN_RCVD
  <--- SYN+ACK       ESTABLISHED         ESTABLISHED
                     ...                 ...
  FIN --->           FIN_WAIT_1          CLOSE_WAIT
  <--- ACK           FIN_WAIT_2
  <--- FIN           TIME_WAIT           LAST_ACK
  ACK --->                               CLOSED
  Wait 30 seconds    CLOSED

Flow control

Protocol to ensure that the sender does not send data faster than the receiver can receive it.

Receiver sends window size in ACK
If window is small, sender waits
If window size is 0, sender sends 1 byte (to get updated window in ACK)

Congestion control

Protocol to ensure that the sender does not send data faster than the network can transmit it.

Tahoe
- Send 1 pack, wait for ACK (slow start)
- Send 2, 4, 8, ... up to threshold, wait for ACK
- If timeout, divide threshold by 2 and go back to 1, 2, 4, 8...
Reno (most commonly used)
- Fast retransmission after 3 ACKs
- Fast recovery, cancel slow start after 3 ACKs
Vegas
- Uses round trip delays to predict congestion

Network Layer

Routing algorithms

Global - Link state
- Each node has full knowledge of network
- Each node computes shortest path to destination
Decentralized - Distance Vector
- Each node knows only its neighbors
- Each node tells its neighbors the distance to all other nodes
Both algorithms result in global routing tables

The Internet uses a hierarchical network.

Autonomous System (AS) - a local area network (LAN) using its own routing protocol
Wide area network (WAN) - connects AS's
Gateway - connects a LAN to a WAN
Router - has 2 or more IP addresses
- LAN - usually a host with more than one network interface
- WAN - usually specialized hardware (no transport layer or above)

IP

Internet Protocol

Provides an unreliable packet delivery service between hosts (routing).

Header (20 bytes)

Version, 4 bits, IPv4 = 4
Header length, 4 bits, 4 = 20 bytes
TOS (Type of Service), 8 bits, unused
Packet length, 16 bits, (0-65535)
Identifier for fragment reassembly, 16 bits
Flags (3 bits), 1 = more fragments follow, 0 = last/only fragment
Fragmentation offset, 13 bits (0-8191)
TTL, Time To Live, 8 bits (255-0), decrement each hop, discard if 0
Upper layer protocol, 8 bits, 6 = TCP, 17 = UDP
Header checksum, 16 bits
Source IP address, 32 bits
Destination IP address, 32 bits
Options (if any), 0-10 32-bit words
Data, 0-65515 bytes

Addresses

Class A: 0-127.x.x.x
Class B: 128-191.0-255.x.x
Class C: 192-223.0-255.0-255.x
Multicast: 224-239.x.x.x
Localhost: 127.0.0.1
Broadcast: x.x.x.255

Packet fragmentation

Sender breaks up IP packet into MTU sized units. Receiver reassembles.

MTU = Maximum Transfer Unit
Ethernet: MTU = 1500
Wide area networks: MTU = 576


   +----------------+     +----------+  +-----------+  +------------+
   |  Len = 1500    |     | Len = 500|  | Len = 500 |  | Len = 500  |
   |  ID = 123      |  =  | ID = 123 |  | ID = 123  |  | ID = 123   |
   |  Flags = 0     |     | Flags = 1|  | Flags = 1 |  | Flags = 0  |
   |  Frag = 0      |     | Frag = 0 |  | Frag = 500|  | Frag = 1000|
   +----------------+     +----------+  +-----------+  +------------+

Routing protocols

RIP - Routing Internet Protocol

Distance Vector algorithm
Uses number of hops as cost metric
Widely used within AS's
Uses UDP to communicate with neighbors

OSPF - Open Shortest Path First

Successor to RIP
Link State algorithm
Secure (authenticated) transmission of routing tables
Hierarchical
Used within AS's

IGRP - Internal Gateway Routing Protocol

Proprietary (Cisco)
Distance Vector algorithm
Used within AS's

BGP - Border Gateway Protocol (or BGP4)

Used between AS's
Successor to EGP - Exterior Gateway Protocol
Distance vector algorithm

ICMP

Internet Control Message Protocol

Format

In data portion of IP packet.

  ICMP
  type   Code
  0      0      Echo reply to ping
  3      0      Destination network unreachable
  3      1      Destination host unreachable
  3      2      Destination protocol unreachable
  3      3      Destination port unreachable
  3      6      Destination network unknown
  3      7      Destination host unknown
  4      0      Source quench (unused, TCP provides congestion control)
  8      0      Echo request by ping
  9      0      Router advertisement (used by RIP)
  10     0      Router discovery
  11     0      TTL expired (router loop, also used by traceroute)
  12     0      IP header bad

IPv6

Internet Protocol version 6. Proposed replacement for IPv4. We will run out of IP addresses in 2005-2010.

128 bit IP addresses
No checksum (provided by TCP and data link layers)
No packet fragmentation (packets too long are discarded)
No option fields (uses next header field)

Header format (40 bytes)

Version, 4 bits, IPv6 = 6
Priority, 4 bits, replaces TOS
Flow, 24 bits, identifier for special data types
Payload length, 16 bits (0-65535)
Next header, 8 bits (TCP or UDP)
Hop limit, 8 bits (same as TTL)
Source address, 128 bits
Destination address, 128 bits
Data (cannot exceed MTU)

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