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Zorluk: Çok zorEncapsulation and De-encapsulation

A network administrator analyzes a multi-hop traffic path between Host A (192.168.1.50/24192.168.1.50/24) and Server B (10.0.2.80/2410.0.2.80/24) separated by a default gateway Layer 3 router. Host A initiates a file transfer over TCP. As the data payload traverses the egress interface of the router toward Server B's local subnet, which structural transformations occur during the de-encapsulation and re-encapsulation process at Layer 2 and Layer 3?

  1. The router strips the incoming Layer 2 frame header and FCS trailer, decrements the Layer 3 IPv4 Time to Live (TTL) field, recalculates the IPv4 header checksum, and encapsulates the unchanged Layer 3 packet into a new Layer 2 frame with the router egress interface's MAC as the source and Server B's MAC as the destination.Cevap
  2. B
    The router strips both the Layer 2 frame header and Layer 3 IPv4 header, translates the TCP segment into a UDP datagram to minimize hop latency, and prepends a new Layer 2 header containing Server B's MAC address.
  3. C
    The router retains the original Layer 2 frame header intact, alters the source and destination IPv4 addresses within the Layer 3 IP packet header to reflect the router's egress interface and Server B, and recalculates the Layer 4 TCP checksum.
  4. D
    The router unwraps the frame to Layer 4, strips the TCP header and sequence numbers, replaces them with Layer 2 MAC addresses, and forwards the resulting PDU directly to Layer 1 for physical encoding.

Cevap

The router strips the incoming Layer 2 frame header and FCS trailer, decrements the IP TTL, recalculates the IP header checksum, and encapsulates the unchanged IP packet into a new Layer 2 frame using the egress MAC address as source and the destination host's MAC address as target.
When a packet traverses a Layer 3 boundary, the router de-encapsulates the incoming Layer 2 frame by stripping its header and trailer. It operates on the Layer 3 IP header by decrementing the TTL field (and recomputing the checksum). The router then re-encapsulates the original IP packet into a brand-new Layer 2 frame equipped with source MAC address of the router's egress interface and destination MAC address of the destination host (or next-hop router).

Adım Adım Çözüm

1
Analyze incoming frame processing at the router's ingress interface
The router receives the physical bits, processes the Layer 2 frame header, verifies the frame check sequence (FCS), and strips the Layer 2 header and trailer to extract the enclosed Layer 3 IPv4 packet PDU.
Layer 2 data frames are hop-by-hop units that terminate at the receiving Layer 3 interface.
2
Evaluate Layer 3 header processing and routing table lookup
The router inspects the IP header destination address (10.0.2.8010.0.2.80), decrements the Time to Live (TTL) field by 1, and recalculates the IPv4 header checksum.
Routers decrement TTL to prevent indefinite forwarding loops and must update the checksum due to modifying the TTL field.
3
Perform Layer 2 re-encapsulation for the egress network segment
The router queries its ARP cache for Server B's IP address (10.0.2.8010.0.2.80), constructs a new Layer 2 Ethernet frame header containing the egress interface's MAC as the source MAC and Server B's MAC as the destination MAC, appends a new FCS, and transmits the frame.
Re-encapsulation builds a new Data Link PDU specific to the media and hardware addresses of the target subnet.

Anahtar Kavram

Layer 3 Routing PDU Encapsulation and De-encapsulation Mechanics
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Encapsulation and De-encapsulation

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