x/net/quic: INITIAL is not RFC 9000 compliant

[ripatel-fd]

Go version

golang.org/x/net v0.31.0

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/ripatel/.cache/go-build'
GOENV='/home/ripatel/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/ripatel/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/ripatel/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/golang'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/usr/lib/golang/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.21.11 (Red Hat 1.21.11-1.module+el8.10.0+21986+2112108a)'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/data/ripatel/quictool/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1610345316=/tmp/go-build -gno-record-gcc-switches'

What did you do?

Create an outgoing connection with the QUIC library

What did you see happen?

The first UDP packet sent by the QUIC client is padded up to 1200 bytes.
The padding is done by appending 'raw' zeros past the Initial packet, which is not spec compliant.

[ UDP Header ]
[ Initial Packet ]
[ Zero Padding ... ]

RFC 9000 says:

A client MUST expand the payload of all UDP datagrams carrying Initial packets to at least the smallest allowed maximum datagram size of 1200 bytes by adding PADDING frames to the Initial packet or by coalescing the Initial packet;

x/net/quic currently uses coalescing (the Initial Packet ends before the UDP datagram payload ends). Rules on coalescing:

Using the Length field, a sender can coalesce multiple QUIC packets into one UDP datagram.

QUIC makes no specific allowance to coalesce a QUIC packet with zeros into a UDP datagram.

Many QUIC servers try to parse the zeros sent by x/net/quic, and interpret it as garbage data.

What did you expect to see?

x/net/quic should append padding frames to increase the size of the Initial Packet to 1200.
On the wire, this would look like random bytes (the zero padding is encrypted)

[cherrymui]

cc @neild

[ripatel-fd]

I missed this note in RFC 9000 Section 14.1. Initial Datagram Size.
x/net/quic is doing the correct thing here and this bug report is invalid.

Initial packets can even be coalesced with invalid packets, which a receiver will discard.