Packet Format Overview

Transactions consist of Requests and Completions, which are communicated using packets. Figure 2-2 shows a high level serialized view of a Transaction Layer Packet (TLP), consisting of one or more optional TLP Prefixes, a TLP header, a data payload (for some types of packets), and an optional TLP digest. Figure 2-3 shows a more detailed view of the TLP. The following sections of this chapter define the detailed structure of the packet headers and digest.

Serial View of a TLP

PCI Express conceptually transfers information as a serialized stream of bytes as shown in Figure 2-2. Note that at the byte level, information is transmitted/received over the interconnect with the leftmost byte of the TLP as shown in Figure 2-2 being transmitted/received first (byte 0 if one or more optional TLP Prefixes are present else byte H). Refer to Section 4.2 for details on how individual bytes of the packet are encoded and transmitted over the physical media.

Detailed layouts of the TLP Prefix, TLP Header and TLP Digest (presented in generic form in Figure 2-3) are drawn with the lower numbered bytes on the left rather than on the right as has traditionally been depicted in other PCI specifications. The header layout is optimized for performance on a serialized interconnect, driven by the requirement that the most time critical information be transferred first. For example, within the TLP header, the most significant byte of the address field is transferred first so that it may be used for early address decode.

Generic TLP Format

Payload data within a TLP is depicted with the lowest addressed byte (byte J in Figure 2-3) shown to the upper left. Detailed layouts depicting data structure organization (such as the Configuration Space depictions in Chapter 7) retain the traditional PCI byte layout with the lowest addressed byte shown on the right. Regardless of depiction, all bytes are conceptually transmitted over the Link in increasing byte number order.
Depending on the type of a packet, the header for that packet will include some of the following types of fields:

• Format of the packet
• Type of the packet
• Length for any associated data
• Transaction Descriptor, including:
  1. Transaction ID
  2. Attributes
  3. Traffic Class

• Address/routing information
• Byte Enables
• Message encoding
• Completion status