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A Linux capture can show a TLS handshake’s TCP data grouped into larger units than the frames that crossed the network. Generic Receive Offload (GRO) combines compatible received packets for stack processing, so an endpoint capture alone may not reveal the original wire-frame boundaries. To determine what was actually transmitted, inspect an independent capture point such as a switch mirror or suitable network TAP, and interpret the TLS exchange separately from its packetization.
What GRO changes in a capture
GRO is a receive-side optimization: Linux can combine compatible incoming packets before passing data onward for stack processing. As a result, a host-side capture may present a larger TCP payload unit than an independent observer saw on the link. That difference does not mean the TCP byte stream or TLS handshake changed; it means the capture points can observe data at different stages of packet processing.
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Linux describes GRO as complementary to Generic Segmentation Offload (GSO): ideally, frames assembled by GRO can be segmented by GSO back into the original frame sequence. On transmit, GSO and TCP Segmentation Offload (TSO) can defer segmentation until later in the send path. A large unit visible in a host capture therefore does not, by itself, establish the size or boundaries of frames on the wire. See the Linux Kernel documentation on segmentation offloads and the Wireshark User’s Guide, version 4.7.0, on offloading.
Keep TCP, TLS records, handshake messages, and frames separate
TCP provides a byte stream, not a promise that each packet corresponds to one TLS record or handshake message. A TLS record or handshake message can span TCP segments, and a segment can carry data that does not align with a handshake-message boundary. Packet boundaries are therefore not a dependable way to infer TLS message boundaries.
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- TCP stream: the ordered bytes delivered by TCP, accounting for sequence numbers and retransmissions.
- TLS record and handshake message: protocol structures interpreted from that byte stream by TLS-aware dissection.
- Wire frame: the link-layer unit observed at a particular point in the network.
Use TCP stream reassembly and TLS dissection to understand the handshake. Use an appropriate independent capture point to answer which frames crossed a link. Do not treat a display’s packet rows or reassembled view as interchangeable evidence.
Why the capture location matters
A capture on an endpoint may observe traffic before or after relevant host processing, depending on the interface, driver, direction, and capture path. GRO can affect receive-side grouping; GSO or TSO can affect transmit-side segmentation. A switch mirror or suitable network TAP offers evidence from an independent point on the link, though it may not match an endpoint capture row-for-row. Compare TCP sequence ranges and payload coverage rather than expecting packet-for-packet identity.
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Linux documentation also distinguishes TLS handshake service from kernel TLS record handling. The kernel project’s In-Kernel TLS Handshake documentation says, “As of this writing, there is no TLS handshake implementation in the Linux kernel,” and describes a handshake agent, typically in user space. That does not imply that the kernel generated a special handshake packet split. Separately, the Linux Kernel TLS offload documentation describes specialized handling of TLS records and says decrypted and non-decrypted segments are not coalesced, for example by GRO or the socket layer. This kTLS detail should not be generalized to all TLS traffic: it does not mean ordinary TLS bypasses GRO or that every connection uses hardware offload.
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Diagnose a capture that looks different from the wire
- Record the context. Note the interface, operating system and kernel, NIC and driver, capture location, traffic direction, and whether the capture is on the sender, receiver, virtual interface, or an intermediate device.
- Save the current offload settings. On Linux,
ethtool -k <interface>commonly reports interface features. Keep the output before changing anything; supported features and their behavior depend on the driver and kernel. - Make a controlled host-side comparison. If supported, temporarily disable receive GRO with
sudo ethtool -K <interface> gro off, repeat the capture, then compare TCP sequence ranges and payload groupings. Restore the original setting afterward. This is a diagnostic approach, not a guaranteed procedure for every interface; feature names and behavior vary. - Capture independently if wire frames are the question. Use an available switch mirror port or suitable network TAP, then compare sequence coverage and retransmissions across captures. Do not assume rows will align one-to-one.
- Analyze TLS separately. Reassemble the TCP stream and dissect TLS records and handshake messages. Use the independent capture to examine frame sizes and splits.
How to compare captures without drawing the wrong conclusion
| What to compare | Why it matters |
|---|---|
| Capture point: endpoint, mirror, or TAP | Each point may observe traffic at a different processing stage; an endpoint view alone may not establish link frame boundaries. |
| Direction: receive or transmit | GRO is receive-side coalescing; GSO and TSO concern transmit-side segmentation. |
| Offload state and interface | Features are interface-, driver-, and kernel-dependent, so the relevant setting must be checked on the interface being captured. |
| TCP sequence coverage and retransmissions | These help establish whether captures represent the same stream data even when packet groupings differ. |
| Frame view versus reassembled view | A protocol analyzer’s reassembly or offload-aware display is not necessarily a literal list of wire frames. |
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