Git Server Protocol

Transport

The Git protocol operates over pipes or TCP/IP. When a client connects over TCP/IP, it sends a header that tells the server which program to run and what parameters to use. When invoked over SSH, git will run a program with the parameters as command line arguments.

Protocols

Basics

Git communicates with a server by piping data between a local program and a remote program.

A common way of sending a unit of information is a pkt_line. This is a 4 byte size as human encoded hex (i.e. totally underusing the 4 bytes…) that tells you the size of the payload, followed by the payload. The size includes the 4 bytes used by the size itself.

0009ABCDn

Git can also multiplex data using the sideband. As well as 4 bytes size, there would be a 1 byte channel number. This is in binary, so 1 will be \x01.

Typically Git will piggyback a list of capabilities on the first pkt_line it sends. It will also look for capabilities in the first pkt_like it receives. Git will degrade as much as possible when encountering a server or client with differing capabilities.

git-upload-pack

git-upload pack is used by git-ls-remote, git-clone, git-fetch and git-pull. And i’m sure others. Typically a client will connect a local git-fetch-pack to a remote git-upload-pack.

Capabilities for this protocol include multi_ack, thin-pack, ofs-delta, sideband and sideband-64k A thin pack can reference objects not in the current pack.

The server tells the client what refs it has. The client states which of those SHA1’s it would like. It then starts to report which SHA1’s it has. The server ACKs these allowing the client to work out when to stop sending SHA1’s. This saves a lot of transfer because the client can make decisions like “well if it has this SHA, then it has all its parents so I don’t need to care about those”. When the client stops sending shas, the server can work out an optimal pack and then send it to the client.

git-receive-pack

git-receive-pack is used by git push. Typically a client connects a local git-send-pack to a remote git-receive-pack.

Capabilities include report-status and delete-ref.