OECG – Chapter 6 Definitions


passive mode FTP – an FTP mode where the server dynamically allocates a port and the client opens a socket to that port from a port he dynamically allocated.

active mode FTP – an FTP mode where the client dynamically allocates a port, and the server opens a socket to that port from his source port of 20.

SNMP agent – the agent running on a device that responds to SNMP queries against his MIB variables. Sends traps.
SNMP manager – a device that queries the MIB of an SNMP agent. Accepts traps.
Get – an SNMP command to retrieve a single MIB variable.

GetNext – an SNMP command to retrieve the next MIB variable after the one previously requested.

MRTT – measured round-trip time. A timer maintained by a TCP sender. If this timer expires, unacknowledged data will be resent.

GetBulk – an SNMP command that gets a whole lot of data at a whack, without having to ask for one variable at a time.

MIB-I – a standard management information base as defined in the SNMP v1 specification. RFC 1158.
MIB-II – a standard management information base as defined in the SNMP v2 specification. RFC 1213.
Response – the standard method an SNMP agent responses to SNMP manager get, getbulk, getnext and set requests.

trap – unsolicited SNMP information sent from an agent to a manager

set – an SNMP manager telling an SNMP agent what to set a particular MIB variable to

inform – 2 SNMP managers exchanging MIB data

SMI – structure of management information. Defines the syntax for creating a MIB.
MIB – management information base

CWND – congestion window, used by a TCP sender to send data without losing packets.

SSThresh – slow start threshold, half the value of the original CWND used when packet loss was first experienced

window – the amount of data in bytes that a receiver will accept before sending an acknowledgment

Slow Start – the algorithm used to exponentially slide the window open during successful TCP exchanges

Congestion avoidance – the algorithm used to linearly slide the window open after the SSThresh is reached

MSS – maximum segment size. The largest size you can make a packet, minus the TCP and IP headers of a TCP packet. A TCP host must support an MSS of at least 536.
MTU – maximum transmission unit. The largest size you can make a packet, including the TCP and IP headers. IP hosts must accept an MTU of at least 576.
socket – a formed connection between 2 TCP/IP hosts.

TCP code bits – a field of 6 bits or flags that can be set to identify important packet characteristics.

TCP flags – PSH, URG, SYN and ACK. (Same as code bits.)
Receiver’s advertised window – what the receiver first advertises to the sender he’ll accept for data before acknowledging.

MIB walk – what “getnext” and “getbulk” are doing – walking the MIB tree is a way to discover all the possible variables you can query without actually knowing what they are.

By Ethan Banks

Ethan Banks is a podcaster and writer with a BSCS and 20+ years in enterprise IT. He's operated data centers with a special focus on infrastructure — especially networking. He's been a CNE, MCSE, CEH, CCNA, CCNP, CCSP, and CCIE R&S #20655. He's the co-founder of Packet Pushers Interactive, LLC where he creates content for humans in the hot aisle.