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Merged
christophefontaine merged 4 commits into from
Feb 26, 2026
Merged

srv6: add compressed SID (NEXT-CSID) support #509

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00dacf4
srv6: add compressed SID support
rjarry Feb 18, 2026
8bedd0f
smoke: add compressed CSID transit check to srv6_test
rjarry Feb 18, 2026
138fc0c
frr: accept NEXT_CSID flavor for SRv6 local SIDs
rjarry Feb 18, 2026
066c62f
smoke: support uN behavior in FRR smoke test helpers
rjarry Feb 18, 2026
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12 changes: 10 additions & 2 deletions frr/rt_grout.c
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Original file line number Diff line number Diff line change
Expand Up @@ -249,6 +249,11 @@ static int grout_gr_nexthop_to_frr_nexthop(
SET_SRV6_FLV_OP(ctx.flv.flv_ops, ZEBRA_SEG6_LOCAL_FLV_OP_PSP);
if (sr6->flags & GR_SR_FL_FLAVOR_USD)
SET_SRV6_FLV_OP(ctx.flv.flv_ops, ZEBRA_SEG6_LOCAL_FLV_OP_USD);
if (sr6->flags & GR_SR_FL_FLAVOR_NEXT_CSID) {
SET_SRV6_FLV_OP(ctx.flv.flv_ops, ZEBRA_SEG6_LOCAL_FLV_OP_NEXT_CSID);
ctx.flv.lcblock_len = sr6->block_bits;
ctx.flv.lcnode_func_len = sr6->csid_bits;
}

switch (sr6->behavior) {
case SR_BEHAVIOR_END:
Expand Down Expand Up @@ -723,8 +728,11 @@ grout_add_nexthop(uint32_t nh_id, gr_nh_origin_t origin, const struct nexthop *n
gr_log_debug("USP always enabled, ignoring flag");
if (CHECK_SRV6_FLV_OP(flv, ZEBRA_SEG6_LOCAL_FLV_OP_USD))
sr6_local->flags |= GR_SR_FL_FLAVOR_USD;
if (CHECK_SRV6_FLV_OP(flv, ZEBRA_SEG6_LOCAL_FLV_OP_NEXT_CSID))
gr_log_debug("next-c-sid not supported, ignoring flag");
if (CHECK_SRV6_FLV_OP(flv, ZEBRA_SEG6_LOCAL_FLV_OP_NEXT_CSID)) {
sr6_local->flags |= GR_SR_FL_FLAVOR_NEXT_CSID;
sr6_local->block_bits = nh->nh_srv6->seg6local_ctx.flv.lcblock_len;
sr6_local->csid_bits = nh->nh_srv6->seg6local_ctx.flv.lcnode_func_len;
}

break;
case GR_NH_T_SR6_OUTPUT:
Expand Down
3 changes: 3 additions & 0 deletions modules/srv6/api/gr_srv6.h
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Original file line number Diff line number Diff line change
Expand Up @@ -85,6 +85,7 @@ static inline const char *gr_srv6_behavior_name(gr_srv6_behavior_t b) {
typedef enum : uint8_t {
GR_SR_FL_FLAVOR_PSP = GR_BIT8(0), // Penultimate Segment Popping.
GR_SR_FL_FLAVOR_USD = GR_BIT8(1), // Ultimate Segment Decapsulation.
GR_SR_FL_FLAVOR_NEXT_CSID = GR_BIT8(2), // Compressed SID (RFC 9800).
} gr_srv6_flags_t;

// SRv6 local nexthop information for Local SID processing.
Expand All @@ -93,4 +94,6 @@ struct gr_nexthop_info_srv6_local {
uint16_t out_vrf_id;
gr_srv6_behavior_t behavior;
gr_srv6_flags_t flags;
uint8_t block_bits; // Locator-block length in bits (default 32).
uint8_t csid_bits; // Compressed SID length in bits (default 16).
};
80 changes: 68 additions & 12 deletions modules/srv6/cli/localsid.c
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Original file line number Diff line number Diff line change
Expand Up @@ -74,9 +74,18 @@ static cmd_status_t srv6_localsid_add(struct gr_api_client *c, const struct ec_p
sr6->flags |= GR_SR_FL_FLAVOR_PSP;
if (!strcmp(str, "usd"))
sr6->flags |= GR_SR_FL_FLAVOR_USD;
if (!strcmp(str, "next-csid"))
sr6->flags |= GR_SR_FL_FLAVOR_NEXT_CSID;
}
}

if (sr6->flags & GR_SR_FL_FLAVOR_NEXT_CSID) {
if (arg_u8(p, "BLOCK_BITS", &sr6->block_bits) < 0 && errno != ENOENT)
goto out;
if (arg_u8(p, "CSID_BITS", &sr6->csid_bits) < 0 && errno != ENOENT)
goto out;
}

n = ec_pnode_find(p, "BEHAVIOR");
if (n == NULL || ec_pnode_len(n) < 1)
goto out;
Expand All @@ -97,25 +106,54 @@ static cmd_status_t srv6_localsid_add(struct gr_api_client *c, const struct ec_p

static ssize_t format_nexthop_info_srv6_local(char *buf, size_t len, const void *info) {
const struct gr_nexthop_info_srv6_local *sr6 = info;
ssize_t n = 0;
char vrf[64];
char flavors[64], vrf[64];
ssize_t n = 0, f = 0;

SAFE_BUF(snprintf, len, "behavior=%s", gr_srv6_behavior_name(sr6->behavior));
vrf[0] = 0;
if (sr6->out_vrf_id != GR_VRF_ID_UNDEF)
snprintf(vrf, sizeof(vrf), "out_vrf=%d", sr6->out_vrf_id);

flavors[0] = 0;
if (sr6->flags & GR_SR_FL_FLAVOR_PSP)
f += snprintf(flavors + f, sizeof(flavors) - f, "psp,");
if (sr6->flags & GR_SR_FL_FLAVOR_USD)
f += snprintf(flavors + f, sizeof(flavors) - f, "usd,");
if (sr6->flags & GR_SR_FL_FLAVOR_NEXT_CSID)
f += snprintf(flavors + f, sizeof(flavors) - f, "next-csid,");
if (f > 0)
flavors[f - 1] = 0; // trim trailing comma

switch (sr6->behavior) {
case SR_BEHAVIOR_END:
SAFE_BUF(snprintf, len, " flavor=%#02x", sr6->flags);
break;
case SR_BEHAVIOR_END_T:
SAFE_BUF(snprintf, len, " flavor=%#02x %s", sr6->flags, vrf);
SAFE_BUF(snprintf, len, " flavor=%s", flavors[0] ? flavors : "none");
if (sr6->flags & GR_SR_FL_FLAVOR_NEXT_CSID)
SAFE_BUF(
snprintf,
len,
" block-bits=%u csid-bits=%u",
sr6->block_bits,
sr6->csid_bits
);
if (vrf[0])
SAFE_BUF(snprintf, len, " %s", vrf);
break;
case SR_BEHAVIOR_END_DT6:
case SR_BEHAVIOR_END_DT4:
case SR_BEHAVIOR_END_DT46:
SAFE_BUF(snprintf, len, " %s", vrf);
if (flavors[0])
SAFE_BUF(snprintf, len, " flavor=%s", flavors);
if (sr6->flags & GR_SR_FL_FLAVOR_NEXT_CSID)
SAFE_BUF(
snprintf,
len,
" block-bits=%u csid-bits=%u",
sr6->block_bits,
sr6->csid_bits
);
if (vrf[0])
SAFE_BUF(snprintf, len, " %s", vrf);
break;
}
return n;
Expand All @@ -135,23 +173,33 @@ static int ctx_init(struct ec_node *root) {

flavor_node = EC_NODE_CMD(
"FLAVORS",
"(psp,usd)",
"(psp,usd,next-csid)",
with_help("Penultimate Segment Pop of the SRH", ec_node_str("psp", "psp")),
with_help("Ultimate Segment Decapsulation of the SRH", ec_node_str("usd", "usd"))
with_help("Ultimate Segment Decapsulation of the SRH", ec_node_str("usd", "usd")),
with_help("NEXT-CSID uSID flavor (RFC 9800)", ec_node_str("next-csid", "next-csid"))
);
if (flavor_node == NULL)
return -1;
beh_node = EC_NODE_OR(
"BEHAVIOR",
EC_NODE_CMD(
EC_NO_ID,
"end [flavor FLAVORS]",
"end [(flavor FLAVORS),(block-bits BLOCK_BITS),(csid-bits CSID_BITS)]",
with_help("Transit endpoint.", ec_node_str("end", "end")),
with_help("Endpoint flavor(s).", ec_node_clone(flavor_node))
with_help("Endpoint flavor(s).", ec_node_clone(flavor_node)),
with_help(
"Locator-block length in bits.",
ec_node_uint("BLOCK_BITS", 8, 120, 10)
),
with_help(
"Compressed SID length in bits.",
ec_node_uint("CSID_BITS", 8, 64, 10)
)
),
EC_NODE_CMD(
EC_NO_ID,
"end.t [flavor FLAVORS] table TABLE",
"end.t [(flavor FLAVORS),(block-bits BLOCK_BITS),(csid-bits CSID_BITS)]"
" table TABLE",
with_help(
"L3 routing domain name.",
ec_node_dyn("TABLE", complete_vrf_names, NULL)
Expand All @@ -160,7 +208,15 @@ static int ctx_init(struct ec_node *root) {
"Transit endpoint with specific IPv6 table lookup.",
ec_node_str("end.t", "end.t")
),
with_help("Endpoint flavor(s).", flavor_node)
with_help("Endpoint flavor(s).", flavor_node),
with_help(
"Locator-block length in bits.",
ec_node_uint("BLOCK_BITS", 8, 120, 10)
),
with_help(
"Compressed SID length in bits.",
ec_node_uint("CSID_BITS", 8, 64, 10)
)
),
EC_NODE_CMD(
EC_NO_ID,
Expand Down
18 changes: 17 additions & 1 deletion modules/srv6/control/localsid.c
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Original file line number Diff line number Diff line change
Expand Up @@ -14,14 +14,30 @@ static bool srv6_local_nh_equal(const struct nexthop *a, const struct nexthop *b
struct nexthop_info_srv6_local *bd = nexthop_info_srv6_local(b);

return ad->behavior == bd->behavior && ad->out_vrf_id == bd->out_vrf_id
&& ad->flags == bd->flags;
&& ad->flags == bd->flags && ad->block_bits == bd->block_bits
&& ad->csid_bits == bd->csid_bits;
}

static int srv6_local_nh_import_info(struct nexthop *nh, const void *info) {
struct nexthop_info_srv6_local *priv = nexthop_info_srv6_local(nh);
const struct gr_nexthop_info_srv6_local *pub = info;
uint8_t block_bits = pub->block_bits;
uint8_t csid_bits = pub->csid_bits;

if (pub->flags & GR_SR_FL_FLAVOR_NEXT_CSID) {
if (block_bits == 0)
block_bits = 32;
if (priv->csid_bits == 0)
csid_bits = 16;
if (block_bits % CHAR_BIT != 0 || csid_bits % CHAR_BIT != 0)
return errno_set(EDOM);
if (block_bits + csid_bits > RTE_IPV6_MAX_DEPTH)
return errno_set(ERANGE);
Comment on lines +24 to +35
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@coderabbitai coderabbitai Bot Feb 24, 2026

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⚠️ Potential issue | 🟠 Major

Fix NEXT‑CSID defaulting check to avoid zero CSID length.

The defaulting uses priv->csid_bits, so a request with CSID_BITS=0 on an existing nexthop can leave csid_bits at 0. That passes validation and makes csid_shift() return true without changing the DA, which can trap packets in a local re-lookup loop. Use the incoming value for the defaulting check (or explicitly reject 0).

🐛 Proposed fix 
-		if (priv->csid_bits == 0)
-			csid_bits = 16;
+		if (csid_bits == 0)
+			csid_bits = 16;
🤖 Prompt for AI Agents 
Verify each finding against the current code and only fix it if needed.

In `@modules/srv6/control/localsid.c` around lines 24 - 35, The NEXT‑CSID
defaulting uses priv->csid_bits which allows an incoming CSID_BITS=0 to remain
zero and bypass validation; update the logic in the GR_SR_FL_FLAVOR_NEXT_CSID
branch to check the incoming pub->csid_bits (not priv->csid_bits) when deciding
to apply the default (e.g., if (pub->csid_bits == 0) csid_bits = 16), and ensure
after defaulting that csid_bits != 0 (or explicitly return errno_set(EDOM)) so
csid_shift() cannot get a zero CSID length.

}

priv->base = *pub;
priv->block_bits = block_bits;
priv->csid_bits = csid_bits;

return 0;
}
Expand Down
60 changes: 60 additions & 0 deletions modules/srv6/datapath/srv6_local.c
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Original file line number Diff line number Diff line change
Expand Up @@ -279,6 +279,49 @@ static int process_behav_decap(
return edge;
}

//
// Compressed SID shift-and-lookup (RFC 9800, Section 4.1.1).
//
// A CSID container packs multiple compressed SIDs after the locator block:
//
// |<-- block -->|<-- csid -->|<--- argument (remaining csids) --->|
// 0 ^ ^ 15
// block_end arg_off
//
// If the argument portion is non-zero, shift it left into the active CSID
// position and zero the vacated tail. This exposes the next CSID in the
// container as the new destination for FIB lookup. E.g.:
//
// Before: fd00:0202 : 0300 : 0100 : 0000 : 0000 : 0000 : 0000
// block csid ~~~~ argument (next csid) ~~~~~~~
//
// After: fd00:0202 : 0100 : 0000 : 0000 : 0000 : 0000 : 0000
// block csid ~~~~~~~~ zeroed tail ~~~~~~~~~~~~
//
// Returns true if the shift was performed, false if the container is exhausted
// (argument is all zeros, fall through to standard End).
//
static inline bool csid_shift(struct rte_ipv6_addr *da, uint8_t block_bits, uint8_t csid_bits) {
uint8_t block_end = block_bits / CHAR_BIT;
uint8_t csid_len = csid_bits / CHAR_BIT;
uint8_t arg_off = block_end + csid_len;
uint8_t arg = 0;

assert(arg_off <= ARRAY_DIM(da->a));

for (uint8_t i = arg_off; i < ARRAY_DIM(da->a); i++)
arg |= da->a[i];
if (arg == 0)
return false; // argument portion is all zeros

// shift argument left into the active CSID position
memmove(&da->a[block_end], &da->a[arg_off], ARRAY_DIM(da->a) - arg_off);
// zero the vacated tail
memset(&da->a[ARRAY_DIM(da->a) - csid_len], 0, csid_len);

return true;
}

//
// End behavior
//
Expand All @@ -290,6 +333,23 @@ static int process_behav_end(
struct rte_ipv6_routing_ext *sr = ip6_info->sr;
const struct iface *iface;

// NEXT-CSID processing (RFC 9800)
if (sr_d->flags & GR_SR_FL_FLAVOR_NEXT_CSID) {
struct rte_ipv6_hdr *ip6 = ip6_info->ip6_hdr;

if (csid_shift(&ip6->dst_addr, sr_d->block_bits, sr_d->csid_bits)) {
if (sr_d->out_vrf_id != GR_VRF_ID_UNDEF) {
iface = get_vrf_iface(sr_d->out_vrf_id);
if (iface == NULL)
return DEST_UNREACH;
mbuf_data(m)->iface = iface;
}
eth_input_mbuf_data(m)->domain = ETH_DOMAIN_LOCAL;
return IP6_INPUT;
}
// container exhausted, fall through to standard End
}

// at the end of the tunnel
if (sr == NULL || sr->segments_left == 0) {
// 4.16.3 USD
Expand Down
11 changes: 8 additions & 3 deletions smoke/_init_frr.sh
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Original file line number Diff line number Diff line change
Expand Up @@ -127,16 +127,21 @@ set_srv6_localsid() {
local count=0

# ---- translate behaviour aliases --------------------------------------
# map: end.dt4 -> uDT4, end.dt6 -> uDT6, end.dt46 -> uDT46
local frr_behavior
case "${grout_behavior,,}" in # ,, = lower-case
end) frr_behavior="uN" ;;
end.dt4) frr_behavior="uDT4" ;;
end.dt6) frr_behavior="uDT6" ;;
end.dt46) frr_behavior="uDT46" ;;
*) echo "Unsupported behavior '${grout_behavior}'. Use end.dt4, end.dt6, end.dt46."; exit 1 ;;
*) echo "Unsupported behavior '${grout_behavior}'."; exit 1 ;;
esac

# --- push the config into FRR ------------------------------------------
local vrf_clause="vrf default"
case "${frr_behavior}" in
uN) vrf_clause="" ;;
esac

vtysh <<-EOF
configure terminal
segment-routing
Expand All @@ -147,7 +152,7 @@ set_srv6_localsid() {
exit
exit
static-sids
sid ${sid_local}/48 locator ${locator} behavior ${frr_behavior} vrf default
sid ${sid_local}/48 locator ${locator} behavior ${frr_behavior} ${vrf_clause}
exit
exit
EOF
Expand Down
20 changes: 20 additions & 0 deletions smoke/srv6_test.sh
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Expand Up @@ -64,3 +64,23 @@ grcli route add fd00:202:100::/48 via id 666
ip netns exec n0 ping -i0.01 -c3 -n 192.168.60.1
# check that sid is reachable
ip netns exec n1 ping6 -i0.01 -c3 -n fd00:202:100::

#
# NEXT-CSID transit test
#
# A CSID container fd00:202:0300:0100:: packs two CSIDs (0300 and 0100)
# with block-bits=32, csid-bits=16. Grout matches fd00:202:0300::/48 as
# a NEXT-CSID End transit node, shifts the DA to fd00:202:0100::
# (= fd00:202:100::), and the second lookup hits the existing End.DT4
# endpoint.
#

# NEXT-CSID End transit node
grcli nexthop add srv6-local behavior end flavor next-csid id 700
grcli route add fd00:202:300::/48 via id 700

# linux sends to the CSID container instead of the single SID
ip -n n1 route replace 192.168.61.0/24 encap seg6 mode encap segs fd00:202:0300:0100:: dev x-p1

# test: ping goes through the NEXT-CSID transit node
ip netns exec n0 ping -i0.01 -c3 -n 192.168.60.1