Computer Networks

As data packets move from lower to upper layers in the OSI model, headers are removed during decapsulation (encapsulation reversal). Each layer removes the header added by its corresponding layer on the sending side. Option A (added) describes the sending direction, while options C and D are incorrect.

The Session layer (Layer 5) provides dialog control, which manages the conversation between two applications. It establishes, maintains, and synchronizes communication sessions. Data representation is a Presentation layer function. Remote job execution is not a primary Session layer function. Creating logical connections is more associated with lower layers.

TCP does not provide addressing - that is handled by IP at the network layer. TCP provides flow control (managing data transmission rates), retransmission of lost packets, and proper sequencing of packets through sequence numbers. Addressing (IP addresses and port numbers) is handled by the IP layer and the concept of sockets, not TCP itself.

The /etc/nsswitch.conf file configures the Name Service Switch, determining the lookup sources and order of priority for system databases such as hosts, passwords, and groups. Distractors referring to backup configurations or pure DNS resolution describe unrelated system settings.

The /etc/services file maps human-readable service names to their corresponding port numbers and protocols. Modifying this mapping adjusts how services resolve their ports. Distractors like nmap are scanning tools, while /etc/hosts and resolv.conf handle host name resolution configurations.

An MX record is a Mail Exchanger resource record in the Domain Name System that specifies the mail server responsible for accepting email messages on behalf of a domain. Distractors like CNAME or Dynamic DNS represent host aliasing and dynamic updates.

In mail servers like sendmail, the access file (typically /etc/mail/access) is used to control mail relay and acceptance. You can block specific senders, domains, or IP addresses by adding reject entries to this database. This file is then compiled into a database format that the mail server uses to filter incoming connections.

Microsoft Message Queue (MSMQ) is specifically designed to handle message loss and ensure reliable delivery of packets/requests. It provides queuing, guaranteed delivery, and recovery mechanisms. Custom data queues (option A) would need to implement these features from scratch.

In Junos, inet.3 is reserved for BGP next-hop resolution using BGP-learned routes. When equal preference routes exist in both inet.0 and inet.3, BGP prefers the inet.3 version for next-hop selection.

When all OSPF routers on a broadcast network have equal priority, the router ID becomes the tiebreaker. The highest router ID is elected DR or BDR. Area ID, loopback address, and interface IP address are not used in this decision.

An Autonomous System Boundary Router (ASBR) redistributes routes between OSPF and other routing protocols. ABR connects OSPF areas. Internal routers stay within one area. Backbone routers are in area 0.

In Junos, 'show route receive-protocol bgp x.x.x.x' displays routes received from the specified peer. 'show bgp received routes' and 'show bgp advertised routes' are not valid syntax. 'advertising-protocol' shows what is sent, not received.

inet.0 is the default routing table for unicast IPv4 routes in Junos devices. The other routing tables serve specific purposes: inet.1 is used for the multicast forwarding cache, inet.2 for unicast routes used for multicast reverse-path forwarding (RPF) checks, and inet.3 for MPLS label-switched paths (LSPs).