CFWorx

Peer-to-peer communication and PDUs

· in Cisco U

Peer-to-peer (horizontal) communication

A peer is an equal, so peer-to-peer communication means each layer logically talks to the same layer on the other host. A layer only physically interacts with the layers directly above and below it, but the data it produces is meant for its counterpart on the receiving host.

Every layer except physical is implemented in software, so this is logical communication. The processes never talk directly, and the hosts usually aren’t even directly connected.

Don’t confuse this with “peer-to-peer” application architecture (workload split equally among peers, vs. client-server where it isn’t). Different concept, same name.

Vertical communication

Data passing down or up the stack is vertical communication, and horizontal communication can’t happen without it.

On the sender, application data passes down the stack, each layer processing and altering it, until it’s ready for physical transmission. What goes on the wire is not the original application data.

On the receiver, the link layer takes the signal off the media, the data climbs the stack, and each layer strips its part. What the receiving application gets is the original data the sender’s application created.

PDUs

PDU (protocol data unit) is the generic term for the data unit at any layer. The name changes by layer; there’s no universal convention, but these are standard:

Wireshark

A free, open-source packet analyzer (aka sniffer). It captures every PDU on a chosen interface, interprets it, and displays it. Uses: troubleshooting, traffic analysis, protocol and software development, learning, and verifying QoS and that the correct traffic flows from source to destination.

Three panes:

  1. Packet list: a table of all captured frames, filterable (filter dns to see only DNS traffic).
  2. Details: one selected frame, broken out header by header, lower layers first.
  3. Bytes: the raw captured bytes of whatever is selected in details.

The details pane maps directly to header fields: the names shown correspond to actual field names (Source and Destination are the address fields, TTL is TTL, and so on).

What to look for per layer:

OSI vs. TCP/IP mapping