Inside a Router
📅
Router architecture overview 🔗
Routing processor
- The central component responsible for the control plane.
- Runs routing algorithms and manages routing tables.
High-speed switching fabric
- A core data-plane component that transfers packets at high speed.
The forwarding table is copied from the routing processor to line cards over a separate bus, such as PCI.
Each line card has a shadow copy, allowing its input ports to make forwarding decisions locally without consulting the central processor.
Input-port functions 🔗
Physical layer
- Bit-level reception.
- Line termination.
Data-link layer
- Ethernet, for example.
- Protocol reception.
Decentralized switching
Use header-field values to look up an output port in the forwarding table stored in input-port memory: match plus action
.- Destination-based forwarding: Uses only the destination IP address, the traditional approach.
- Generalized forwarding: Uses any set of header fields.
Goal
: Complete input-port processing at line speed.
Queuing
: Occurs when datagrams arrive faster than they can be forwarded through the switching fabric.Other input-port functions 🔗
- Physical- and link-layer processing.
- Checking packet version numbers, checksums, and TTL. TTL and checksum must be rewritten.
- Updating counters used for network management.
Switching fabric 🔗
Transfers packets from input buffers to the appropriate output buffers.
Switching rate: The rate at which packets move from input to output.
- Measured as a multiple of the input/output line rate.
- For N inputs, a switching rate of N times the line rate is appropriate.
Types of switching fabrics 🔗
- Switching through memory.
- Switching through a bus.
- Crossbar switching.
Input-port queuing 🔗
If the switching fabric is slower than the combined rate of all input ports, queues may form at the inputs.
- Input-buffer overflow can cause queuing delay and loss.
Head-of-the-line (HOL) blocking occurs when the first packet in a queue prevents other packets from proceeding.
- The lower red packet is blocked.
- The lower green packet is also blocked → It experiences HOL blocking.
Output ports 🔗
Send packets delivered by the switching fabric onto an output link.
Datagram buffering and queuing
- Packets arriving from the switching fabric are stored in the output buffer and queued.
- Queuing can occur when packets arrive faster than the output port can transmit them.
Output-port queuing and buffering 🔗
Packets move from input to output through the switching fabric.
- Buffering is necessary when the arrival rate exceeds the output line rate.
- Output-buffer overflow causes queuing delay and loss.
Output-port scheduling 🔗
Determines which packet to transmit first among packets arriving from multiple input ports.
FIFO scheduling
- Transmit packets in arrival order.
Drop policies:
- Tail drop: Discard newly arriving packets when the queue is full.
- Priority drop: Discard priority packets first.
- Random early drop: Randomly discard packets before the queue fills.
Priority scheduling
- Transmit higher-priority packets first.
Round-robin scheduling
- Cycle through multiple queues.
- Send one packet from a queue, then move to the next queue.
Weighted scheduling
- A generalized form of round-robin scheduling.
- Each class has a weight, and higher-weight classes transmit packets first.
