DHCP - Dynamic Host Configuration Protocol (RFC 2131)
DHCP is a network application, which is used by tools - DHCP-clients - to do the necessary sets for the usage of an IP network automatically, making thus easier the work of the administrator of the network. It is worth using even in small networks, because it accelarates essentially the assignment of newer network terminals.
The network tool configured as a DHCP-client, explores the DHCP server with a broadcast message after switching it on. On one hand, server has an IP address field to be assigned, and on the other hand, it has further information to be shared with the occurring clients. E.g. "default gateway", "domain name", address of the DNS-server, or perhaps accessibiliy of the "time server", etc. If the DHCP-server gets a valid request, then it assigns an IP-address to the client, and encloses the other necessary network parameters. Assignment of the client and the IP number is for a determined time interval, it is called "lease time".
Assignment of the IP-address can be dynamic, automatic, or static. In the first case the address belongs to the client only during the lease time, and in the second case the server records the assignments, and gives again the earlier address to a repeatedly occurring tool. In case of a static allocation it assigns an IP-address set in advance to a tool with a given MAC-address.
DHCP uses the UDP port 67 on the server side and UDP port 68 on the client side.
Communication will happen by the following:
Further DHCP messages:
DNS - Domain Name System (RFC 1034 and RFC 1035)
DNS is a hierarchic name system used at the Internet. We can mark computers, services, and other resource with symbolic names, however, these tools are handled by network tools and protocols on the basis of numeric identifiers, as well as route control of packets going through the Internet. DNS system makes the translation between the numeric identifier (IP number) and the name, that is in connection with the given tool, resource, service, and can be noted by people easier. In DNS system there are further information recorded in a Resource Record (RR) beyond the record of the assignment between names and binary numbers, e.g. the type of a record, and time of validity. We can imagine in the simplest way the space of the domain names in a tree structure: there are records belonging to every node or leaf, even several ones. The tree can be divided to zones. Such a zone consists of a group of interconnected nodes, that is served by an authoritative name server.
If we examine the domain name, we will see, that it consists of two or more parts (these are called labels), that we write in a common way, separated with dots. Label at the right side bears the domain with the highest level (top level domain - TLD). For example, in domain name of www.example.com the "com" is the top level domain. From the left of it every label represents a subdomain. It is a relative denomination: example.com is a subdomain of the com, and www.example.com is a subdomain of the example.com. Theoretically 127 levels of subdomains can be used, with the lenght of 63 octets by labels, and the whole domain name does not exceed the lenght of 253 octets. Host name is a domain name, which is assigned with one or more IP-addresses.
DNS operates on the basis of a distributed database, and uses a client server model. Peak of the hierarchy is served by root name servers. Under them the authoritative DNS servers of the individual subdomains can be found. Client side (DNS resolver) initiates the series of queries, which finally results a whole resolution. DNS query can be recursive or non-recursive. In the latter case DNS server itself is responsible for the resolution of the given record. In the recursive case DNS server can answer the question after other DNS-queries.
DNS queries generate a significant traffic at the Internet. Usage of the local DNS-databases gives a possibility for the reduction of this traffic (DNS caching). After the successful queries result will be stored in the local DNS-server which will give back the stored record in the case of a repeated query, indicating, that it is from the cache, not from an actual query. Duration of storage is set by the administrator of the given DNS server, from some seconds till some days. Application of caching means also, that change of a DNS RR cannot be perceived immediately at the whole network. This phenomenon often describes with using the "propagation" expression, however, it does not describe this phenomenon correctly, because actually the new information is immediately available for every DNS-server, that has not cached it earlier.
We should mention in connection with the Domain Name System the possibility of Reverse lookup which gives the name assigned to it to the IP-number.
Services
RTP packets serve for the real-time transmission of digitalized sounds and pictures through the Internet.
bytes (20) from UDP frame
+--------------------------------+
2 | Control |
+--------------------------------+
2 | Sequence Number |
+--------------------------------+
4 | Timestamp |
+--------------------------------+
4 | SSRC identifier |
+--------------------------------+
4 | CSRC identifiers (optional) |
+--------------------------------+
Control bits:
Record of an RTP packet (an IP packet transmitting an RTP frame):
No. Time Source Destination Protocol Info
61 16.024875 152.66.245.171 152.66.245.226 RTP PT=ITU-T G.711 PCMU, SSRC=0xD47B2821, Seq=15740, Time=2624576
Frame 61 (214 bytes on wire, 214 bytes captured)
Ethernet II, Src: Cisco_cd:70:64 (00:60:70:cd:70:64), Dst: Standard_9f:09:2f (00:e0:29:9f:09:2f)
Internet Protocol, Src: 152.66.245.171 (152.66.245.171), Dst: 152.66.245.226 (152.66.245.226)
User Datagram Protocol, Src Port: 18096 (18096), Dst Port: avt-profile-1 (5004)
Real-Time Transport Protocol
[Stream setup by H245 (frame 41)]
10.. .... = Version: RFC 1889 Version (2)
..0. .... = Padding: False
...0 .... = Extension: False
.... 0000 = Contributing source identifiers count: 0
0... .... = Marker: False
Payload type: ITU-T G.711 PCMU (0)
Sequence number: 15740
[Extended sequence number: 81276]
Timestamp: 2624576
Synchronization Source identifier: 0xd47b2821 (3564840993)
Payload: F9F7FCFF7B7DFD7C7C787AFC7C7A7FFDFD7EFFFDFBF8F3F0...
(160 byte, mely 20 msec-es beszédszegmenst hordoz.)
0000 00 e0 29 9f 09 2f 00 60 70 cd 70 64 08 00 <-MAC
IP -> 45 b8 ..)../.`p.pd..E.
0010 00 c8 9e 11 00 00 3e 11 c1 48 98 42 f5 ab 98 42 ......>..H.B...B
0020 f5 e2
UDP-> 46 b0 13 8c 00 b4 00 00
RTP-> 80 00 3d 7c 00 28 ..F.........=|.(
0030 0c 40 d4 7b 28 21
speech samples -> f9 f7 fc ff 7b 7d fd 7c 7c 78 .@.{(!....{}.||x
0040 7a fc 7c 7a 7f fd fd 7e ff fd fb f8 f3 f0 f3 f7 z.|z...~........
0050 fc fb f8 fa 7d 76 78 79 77 77 78 7a 7b 7b 7c 7d ....}vxywwxz{{|}
0060 7e fe fe fd f9 f7 fb fc f7 f9 fb fb fc fa fa fd ~...............
0070 fd fb fb fa fd fe fd fc f8 fa fd fb fc fd fb fa ................
0080 f8 f8 fe fe fa fa fb fd 7e 7b 79 78 79 7f fa fe ........~{yxy...
0090 7c 7f 7e 7c 7e 7f 7b 7b 7c 7a 78 78 78 74 75 79 |.~|~.{{|zxxxtuy
00a0 7c 7c 7c 7f fc f9 fa fc fa f9 f9 fc 7d 7e fb f9 |||.........}~..
00b0 f7 f9 7f 7e fe fd fb fb fc fe 7d fc f9 fe 7e 7b ...~......}...~{
00c0 7c 7b 7b 7a 79 7b 7a 7c 7c 79 7c 7a 79 7b 7c 7b |{{zy{z||y|zy{|{
00d0 7b 7b 7b 7c 7b 7b {{{|{{
exchange (server) terminal (client)
message
----INVITE--------> initializing a call
<---Trying--100--
<---Ringing-180--
<---OK------200--
----ACK---------->
<-------RTP--------> speech transmission
-------BYE--------> release
<-------OK---200---