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Fallback on 1320 if ICMP is blocked

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This commit is contained in:
Quentin McGaw
2025-10-03 13:55:59 +00:00
parent 5b1dc295fe
commit ccc2f306b9
3 changed files with 26 additions and 32 deletions
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36
internal/pmtud/pmtud.go
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@@ -12,12 +12,13 @@ import (
"golang.org/x/net/icmp"
)
var ErrICMPAllMTUsFailed = errors.New("all MTU sizes tested failed")
var ErrMTUNotFound = errors.New("path MTU discovery failed to find MTU")
// PathMTUDiscover discovers the maximum MTU for the path to the given ip address.
// If the physicalLinkMTU is zero, it defaults to 1500 which is the ethernet standard MTU.
// If the pingTimeout is zero, it defaults to 1 second.
// If the logger is nil, a no-op logger is used.
// It returns [ErrMTUNotFound] if the MTU could not be determined.
func PathMTUDiscover(ctx context.Context, ip netip.Addr,
physicalLinkMTU int, pingTimeout time.Duration, logger Logger) (
mtu int, err error,
@@ -90,7 +91,7 @@ func pmtudMultiSizes(ctx context.Context, ip netip.Addr,
}
mtusToTest := makeMTUsToTest(minMTU, maxPossibleMTU)
if len(mtusToTest) == 0 {
if len(mtusToTest) == 1 { // only minMTU because minMTU == maxPossibleMTU
return minMTU, nil
}
logger.Debugf("testing the following MTUs: %v", mtusToTest)
@@ -139,13 +140,8 @@ func pmtudMultiSizes(ctx context.Context, ip netip.Addr,
}
}
if tests[0].mtu == minMTU+1 { // All MTUs failed.
return 0, fmt.Errorf("%w", ErrICMPAllMTUsFailed)
}
// Re-test with MTUs between the minimum MTU
// and the smallest next MTU we tested.
return pmtudMultiSizes(ctx, ip, minMTU, tests[0].mtu-1,
pingTimeout, logger)
// All MTUs failed.
return 0, fmt.Errorf("%w: ICMP might be blocked", ErrMTUNotFound)
case err != nil:
return 0, fmt.Errorf("collecting ICMP echo replies: %w", err)
default:
@@ -154,12 +150,11 @@ func pmtudMultiSizes(ctx context.Context, ip netip.Addr,
}
// Create the MTU slice of length 11 such that:
// - the first element is the minMTU plus the step
// - the first element is the minMTU
// - the last element is the maxMTU
// - elements in-between are separated as close to each other
// - Don't make the minMTU part of the MTUs to test since
// it's assumed it's already working.
// The number 11 is chosen to find the final MTU in 3 searches;
// The number 11 is chosen to find the final MTU in 3 searches,
// with a total search space of 1728 MTUs which is enough;
// to find it in 2 searches requires 37 parallel queries which
// could be blocked by firewalls.
func makeMTUsToTest(minMTU, maxMTU int) (mtus []int) {
@@ -170,21 +165,14 @@ func makeMTUsToTest(minMTU, maxMTU int) (mtus []int) {
panic("minMTU > maxMTU")
case diff <= mtusLength:
mtus = make([]int, 0, diff)
for mtu := minMTU + 1; mtu <= maxMTU; mtu++ {
for mtu := minMTU; mtu <= maxMTU; mtu++ {
mtus = append(mtus, mtu)
}
case diff < mtusLength*2:
step := float64(diff) / float64(mtusLength)
mtus = make([]int, 0, mtusLength)
for mtu := float64(minMTU) + step; len(mtus) < mtusLength-1; mtu += step {
mtus = append(mtus, int(math.Round(mtu)))
}
mtus = append(mtus, maxMTU) // last element is the maxMTU
default:
step := diff / mtusLength
step := float64(diff) / float64(mtusLength-1)
mtus = make([]int, 0, mtusLength)
for mtu := minMTU + step; len(mtus) < mtusLength-1; mtu += step {
mtus = append(mtus, mtu)
for mtu := float64(minMTU); len(mtus) < mtusLength-1; mtu += step {
mtus = append(mtus, int(math.Round(mtu)))
}
mtus = append(mtus, maxMTU) // last element is the maxMTU
}